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-The Project Gutenberg eBook of The Trouvelot astronomical drawings
-manual, by Étienne Léopold Trouvelot
-
-This eBook is for the use of anyone anywhere in the United States and
-most other parts of the world at no cost and with almost no restrictions
-whatsoever. You may copy it, give it away or re-use it under the terms
-of the Project Gutenberg License included with this eBook or online at
-www.gutenberg.org. If you are not located in the United States, you
-will have to check the laws of the country where you are located before
-using this eBook.
-
-Title: The Trouvelot astronomical drawings manual
-
-Author: Étienne Léopold Trouvelot
-
-Release Date: June 24, 2022 [eBook #68394]
-
-Language: English
-
-Produced by: Laura Natal Rodrigues (Images generously made available by
-             Hathi Trust Digital Library and Wikipedia Commons.)
-
-*** START OF THE PROJECT GUTENBERG EBOOK THE TROUVELOT ASTRONOMICAL
-DRAWINGS MANUAL ***
-
-
-THE
-TROUVELOT
-ASTRONOMICAL DRAWINGS
-MANUAL
-
-
-
-
-BY
-
-E. L. TROUVELOT,
-
-
-FORMERLY CONNECTED WITH THE OBSERVATORY OF HARVARD COLLEGE; FELLOW OF THE
-AMERICAN ACADEMY OF ARTS AND SCIENCES, AND MEMBER OF THE SELENO-GRAPHICAL
-SOCIETY OF GREAT BRITAIN; IN CHARGE OF A
-GOVERNMENT EXPEDITION TO OBSERVE THE
-TOTAL SOLAR ECLIPSE OF 1878.
-
-
-
-
-NEW YORK
-
-CHARLES SCRIBNER'S SONS
-
-1882
-
-
-
-
-INTRODUCTION
-
-
-During a study of the heavens, which has now been continued for more
-than fifteen years, I have made a large number of observations
-pertaining to physical astronomy, together with many original drawings
-representing the most interesting celestial objects and phenomena.
-
-With a view to making these observations more generally useful, I was
-led, some years ago, to prepare, from this collection of drawings, a
-series of astronomical pictures, which were intended to represent the
-celestial phenomena as they appear to a trained eye and to an
-experienced draughtsman through the great modern telescopes, provided
-with the most delicate instrumental appliances. Over two years were
-spent in the preparation of this series, which consisted of a number of
-large drawings executed in pastel. In 1876, these drawings were
-displayed at the United States Centennial Exhibition at Philadelphia,
-forming a part of the Massachusetts exhibit, in the Department of
-Education and Science.
-
-The drawings forming the present series comprise only a part of those
-exhibited at Philadelphia; but, although fewer in number, they are quite
-sufficient to illustrate the principal classes of celestial objects and
-phenomena.
-
-While my aim in this work has been to combine scrupulous fidelity and
-accuracy in the details, I have also endeavored to preserve the natural
-elegance and the delicate outlines peculiar to the objects depicted; but
-in this, only a little more than a suggestion is possible, since no
-human skill can reproduce upon paper the majestic beauty and radiance of
-the celestial objects.
-
-The plates were prepared under my supervision, from the original pastel
-drawings, and great care has been taken to make the reproduction exact.
-
-The instruments employed in the observations, and in the delineation of
-the heavenly bodies represented in the series, have varied in aperture
-from 6 to 26 inches, according to circumstances, and to the nature of
-the object to be studied. The great Washington refractor, kindly placed
-at my disposal by the late Admiral C. H. Davis, has contributed to this
-work, as has also the 26 inch telescope of the University of Virginia,
-while in the hands of its celebrated constructors, Alvan Clark & Sons.
-The spectroscope used was made by Alvan Clark & Sons. Attached to it is
-an excellent diffraction grating, by Mr. L. M. Rutherfurd, to whose
-kindness I am indebted for it.
-
-Those unacquainted with the use of optical instruments generally suppose
-that all astronomical drawings are obtained by the photographic process,
-and are, therefore, comparatively easy to procure; but this is not true.
-Although photography renders valuable assistance to the astronomer in
-the case of the Sun and Moon, as proved by the fine photographs of these
-objects taken by M. Janssen and Mr. Rutherfurd; yet, for other subjects,
-its products are in general so blurred and indistinct that no details of
-any great value can be secured. A well-trained eye alone is capable of
-seizing the delicate details of structure and of configuration of the
-heavenly bodies, which are liable to be affected, and even rendered
-invisible, by the slightest changes in our atmosphere.
-
-The method employed to secure correctness in the proportions of the
-original drawings is simple, but well adapted to the purpose in view. It
-consists in placing a fine reticule, cut on glass, at the common focus
-of the objective and the eye-piece, so that in viewing an object, its
-telescopic image, appearing projected on the reticule, can be drawn very
-accurately on a sheet of paper ruled with corresponding squares. For a
-series of such reticules I am indebted to the kindness of Professor
-William A. Rogers, of the Harvard College Observatory.
-
-The drawings representing telescopic views are inverted, as they appear
-in a refracting telescope--the South being upward, the North downward,
-the East on the right, and the West on the left. The Comet, the
-Milky-Way, the Eclipse of the Moon, the Aurora Borealis, the Zodiacal
-Light and the Meteors are represented as seen directly in the sky with
-the naked eye. The Comet was, however, drawn with the aid of the
-telescope, without which the delicate structure shown in the drawing
-would not have been visible.
-
-The plate representing the November Meteors, or so-called "Leonids," may
-be called an ideal view, since the shooting stars delineated, were not
-observed at the same moment of time, but during the same night. Over
-three thousand Meteors were observed between midnight and five o'clock
-in the morning of the day on which this shower occurred; a dozen being
-sometimes in sight at the same instant. The paths of the Meteors,
-whether curved, wavy, or crooked, and also their delicate colors, are in
-all cases depicted as they were actually observed.
-
-In the Manual, I have endeavored to present a general outline of what is
-known, or supposed, on the different subjects and phenomena illustrated
-in the series. The statements made are derived either from the best
-authorities on physical astronomy, or from my original observations,
-which are, for the most part, yet unpublished.
-
-The figures in the Manual relating to distance, size, volume, mass,
-etc., are not intended to be strictly exact, being only round numbers,
-which can, therefore, be more easily remembered.
-
-It gives me pleasure to acknowledge that the experience acquired in
-making the astronomical drawings published in Volume VIII. of the Annals
-of the Harvard College Observatory, while I was connected with that
-institution, has been of considerable assistance to me in preparing this
-work; although no drawings made while I was so connected have been used
-for this series.
-
-
-                             E. L. TROUVELOT.
-
-_Cambridge, March, 1882._
-
-
-
-
-CONTENTS
-
-
-INTRODUCTION
-
-LIST OF PLATES
-
-_THE SUN_
-
-GENERAL REMARKS ON THE SUN
-
-SUN-SPOTS AND VEILED SPOTS
-
-SOLAR PROTUBERANCES
-
-TOTAL ECLIPSE OF THE SUN
-
-_THE AURORAL AND ZODIACAL LIGHTS_
-
-THE AURORA BOREALIS
-
-THE ZODIACAL LIGHT
-
-_THE MOON_
-
-THE MOON
-
-ECLIPSES OF THE MOON
-
-_THE PLANETS_
-
-THE PLANETS
-
-THE PLANET MARS
-
-THE PLANET JUPITER
-
-THE PLANET SATURN
-
-_COMETS AND METEORS_
-
-COMETS
-
-SHOOTING-STARS AND METEORS
-
-_THE STELLAR SYSTEMS_
-
-THE MILKY-WAY OR GALAXY
-
-THE STAR-CLUSTERS
-
-THE NEBULÆ
-
-APPENDIX
-
-
-
-
-LIST OF PLATES[1]
-
-PLATE
-
-I. GROUP OF SUN-SPOTS AND VEILED SPOTS.
-
-_Observed June 17, 1875, at 7 h. 30m. A. M._
-
-II. SOLAR PROTUBERANCES.
-
-_Observed May 5, 1873, at 9h. 40m. A. M._
-
-III. TOTAL ECLIPSE OF THE SUN.
-
-_Observed July 29, 1878, at Creston, Wyoming Territory._
-
-IV. AURORA BOREALIS.
-
-_As observed March 1, 1872, at 9h. 25m. P. M._
-
-V. THE ZODIACAL LIGHT.
-
-_Observed February 20, 1876._
-
-VI. MARE HUMORUM.
-
-_From a study made in 1875._
-
-VII. PARTIAL ECLIPSE OF THE MOON.
-
-_Observed October 24, 1874._
-
-VIII. THE PLANET MARS.
-
-_Observed September 3, 1877, at 11h. 55m. P. M._
-
-IX. THE PLANET JUPITER.
-
-_Observed November 1, 1880, at 9h. 30m. P. M._
-
-X. THE PLANET SATURN.
-
-_Observed November 30, 1874, at 5th. 50m. P. M._
-
-XI. THE GREAT COMET OF 1881.
-
-_Observed on the night of June 25-26, at 1h. 30m. A. M._
-
-XII. THE NOVEMBER METEORS.
-
-_As observed between midnight and 3 o'clock A. M., on the night
-of November 13-14, 1868._
-
-XIII. PART OF THE MILKY-WAY.
-
-_From a study made during the years 1874, 1875 and 1876._
-
-XIV. STAR-CLUSTER IN HERCULES.
-
-_From a study made in June, 1877._
-
-XV. THE GREAT NEBULA IN ORION.
-
-_From a study made in the years 1873-76._
-
-
-[Footnote 1: For Key to the Plates, see Appendix.]
-
-
-_Reproduced from the Original Drawings, by Armstrong & Company,
-Riverside Press, Cambridge, Mass._
-
-
-
-
-GENERAL REMARKS ON THE SUN
-
-
-The Sun, the centre of the system which bears its name, is a
-self-luminous sphere, constantly radiating heat and light.
-
-Its apparent diameter, as seen at its mean from the Earth, subtends an
-angle of 32', or a little over half a degree. A dime, placed about six
-feet from the eye, would appear of the same proportions, and cover the
-Sun's disk, if projected upon it.
-
-That the diameter of the Sun does not appear larger, is due to the great
-distance which separates us from that body. Its distance from the Earth
-is no less than 92,000,000 miles. To bridge this immense gap, would
-require 11,623 globes like the Earth, placed side by side, like beads on
-a string.
-
-The Sun is an enormous sphere whose diameter is over 108 times the
-diameter of our globe, or very nearly 860,000 miles. Its radius is
-nearly double the distance from the Earth to the Moon. If we suppose,
-for a moment, the Sun to be hollow, and our globe to be placed at the
-centre of this immense spherical shell, not only could our satellite
-revolve around us at its mean distance of 238,800 miles, as now, but
-another satellite, placed 190,000 miles farther than the Moon, could
-freely revolve likewise, without ever coming in contact with the solar
-envelope.
-
-The circumference of this immense sphere measures 2,800,000 miles. While
-a steamer, going at the rate of 300 miles a day, would circumnavigate
-the Earth in 83 days, it would take, at the same rate, nearly 25 years
-to travel around the Sun.
-
-The surface of the Sun is nearly 12,000 times the surface of the Earth,
-and its volume is equal to 1,300,000 globes like our own. If all the
-known planets and satellites were united in a single mass, 600 such
-compound masses would be needed to equal the volume of our luminary.
-
-Although the density of the Sun is only one-quarter that of the Earth,
-yet the bulk of this body is so enormous that, to counterpoise it, no
-less than 314,760 globes like our Earth would be required.
-
-The Sun uniformly revolves around its axis in about 25½ days. Its
-equator is inclined 7° 15' to the plane of the ecliptic, the axis of
-rotation forming, therefore, an angle of 82° 45' with the same plane.
-As the Earth revolves about the Sun in the same direction as that of the
-Sun's rotation, the apparent time of this rotation, as seen by a
-terrestrial spectator, is prolonged from 25½ days to about 27 days and
-7 hours.
-
-The rotation of the Sun on its axis, like that of the Earth and the
-other planets, is direct, or accomplished from West to East. To an
-observer on the Earth, looking directly at the Sun, the rotation of this
-body is from left to right, or from East to West.
-
-The general appearance of the Sun is that of an intensely luminous disk,
-whose limb, or border, is sharply defined on the heavens. When its
-telescopic image is projected on a screen, or fixed on paper by
-photography, it is noticed that its disk is not uniformly bright
-throughout, but is notably more luminous in its central parts. This
-phenomenon is not accidental, but permanent, and is due in reality to a
-very rare but extensive atmosphere which surrounds the Sun, and absorbs
-the light which that body radiates, proportionally to its thickness,
-which, of course, increases towards the limb, to an observer on the
-Earth.
-
-
-THE ENVELOPING LAYERS OF THE SUN
-
-
-The luminous surface of the Sun, or that part visible at all times, and
-which forms its disk, is called the _Photosphere_, from the property it
-is supposed to possess of generating light. The photosphere does not
-extend to a great depth below the luminous surface, but forms a
-comparatively thin shell, 3,000 or 4,000 miles thick, which is distinct
-from the interior parts, above which it seems to be kept in suspense by
-internal forces. From the observations of some astronomers it would
-appear that the diameter of the photosphere is subject to slight
-variations, and, therefore, that the solar diameter is not a constant
-quantity. From the nature of this envelope, such a result does not seem
-at all impossible, but rather probable.
-
-Immediately above the photosphere lies a comparatively thin stratum,
-less than a thousand miles in thickness, called the _Reversing Layer_.
-This stratum is composed of metallic vapors, which, by absorbing the
-light of particular refrangibilities emanating from the photosphere
-below, produces the dark Fraunhofer lines of the solar spectrum.
-
-Above the reversing layer, and resting immediately upon it, is a
-shallow, semi-transparent gaseous layer, which has been called the
-_Chromosphere_, from the fine tints which it exhibits during total
-eclipses of the Sun, in contrast with the colorless white light radiated
-by the photosphere below. Although visible to a certain extent on the
-disk, the chromosphere is totally invisible on the limb, except with the
-spectroscope, and during eclipses, on account of the nature of its
-light, which is mainly monochromatic, and too feeble, compared with that
-emitted by the photosphere, to be seen.
-
-The chromospheric layer, which has a thickness of from 3,000 to 4,000
-miles, is uneven, and is usually upheaved in certain regions, its matter
-being transported to considerable elevations above its general surface,
-apparently by some internal forces. The portions of the chromosphere
-thus lifted up, form curious and complicated figures, which are known
-under the names of _Solar Protuberances, or Solar Flames_.
-
-Above the chromosphere, and rising to an immense but unknown height, is
-the solar atmosphere proper, which is only visible during total eclipses
-of the Sun, and which then surrounds the dark body of the Moon with the
-beautiful rays and glorious nimbus, called the _Corona_.
-
-These four envelopes: the photosphere, the reversing layer, the
-chromosphere, and the corona, constitute the outer portions of our
-luminary.
-
-Below the photosphere little can be seen, although it is known, as will
-appear below, that at certain depths cloud-like forms exist, and freely
-float in an interior atmosphere of invisible gases. Beyond this all is
-mystery, and belongs to the domain of hypothesis.
-
-
-STRUCTURE OF THE PHOTOSPHERE AND CHROMOSPHERE
-
-
-The apparent uniformity of the solar surface disappears when it is
-examined with a telescope of sufficient aperture and magnifying powers.
-Seen under good atmospheric conditions, the greater part of the solar
-surface appears mottled with an infinite number of small, bright
-granules, irregularly distributed, and separated from each other by a
-gray-tinted background.
-
-These objects are known under different names. The terms granules and
-granulations answer very well for the purpose, as they do not imply
-anything positive as to their form and true nature. They have also been
-called _Luculœ_, _Rice Grains_, _Willow Leaves_, etc., by different
-observers.
-
-Although having different shapes, the granulations partake more or less
-of the circular or slightly elongated form. Their diameter, which varies
-considerably, has been estimated at from 0".5 to 3", or from 224 to
-1,344 miles. The granulations which attain the largest size appear,
-under good atmospheric conditions, to be composed of several granules,
-closely united and forming an irregular mass, from which short
-appendages protrude in various directions.
-
-The number of granulations on the surface of the Sun varies considerably
-under the action of unknown causes. Sometimes they are small and very
-numerous, while at other times they are larger, less numerous, and more
-widely separated. Other things being equal, the granulations are better
-seen in the central regions of the Sun than they are near the limb.
-
-Usually the granulations are very unstable; their relative position,
-form, and size undergoing continual changes. Sometimes they are seen to
-congregate or to disperse in an instant, as if acting under the
-influence of attractive and repulsive forces; assembling in groups or
-files, and oftentimes forming capricious figures which are very
-remarkable, but usually of short duration. In an area of great solar
-disturbances, the granulations are often stretched to great distances,
-and form into parallel lines, either straight, wavy, or curved, and they
-have then some resemblance to the flowing of viscous liquids.
-
-The granulations are usually terminated either by rounded or sharply
-pointed summits, but they do not all rise to the same height, as can be
-ascertained with the spectroscope when they are seen sidewise on the
-limb. In the regions where they are most abundant, they usually attain
-greater elevations, and when observed on the limb with the spectroscope,
-they appear as slender acute flames.
-
-The granulations terminated by sharply-pointed crests, although observed
-in all latitudes, seem to be characteristic of certain regions. A daily
-study of the chromosphere, extending over a period of ten years, has
-shown me that the polar regions are rarely ever free from these objects,
-which are less frequent in other parts of the Sun. In the polar regions
-they are sometimes so abundant that they completely form the solar limb.
-These forms of granulation are comparatively rare in the equatorial
-zones, and when seen there, they never have the permanency which they
-exhibit in the polar regions. When observed in the equatorial regions,
-they usually appear in small groups, in the vicinity of sun spots, or
-they are at least enclosed in areas of disturbances where such spots are
-in process of formation. In these regions they often attain greater
-elevations than those seen in high latitudes.
-
-As we are certain that in the equatorial zones these slender flames (_i.
-e._, granulations) are a sure sign of local disturbance, it may be
-reasonably supposed that the same kind of energy producing them nearly
-always prevails in the polar regions, although it is there much weaker,
-and never reaches beyond certain narrow limits.
-
-Studied with the spectroscope, the granulations are found to be composed
-in the main of incandescent hydrogen gas, and of an unknown substance
-provisionally called "helium." Among the most brilliant of them are
-found traces of incandescent metallic vapors, belonging to various
-substances found on our globe.
-
-The chromosphere is not fixed, but varies considerably in thickness in
-its different parts, from day to day. Its thickness is usually greater
-in the polar regions, where it sometimes exceeds 6,700 miles. In the
-equatorial regions the chromosphere very rarely attains this height, and
-when it does, the rising is local and occupies only a small area. In
-these regions it is sometimes so shallow that its depth is only a few
-seconds, and is then quite difficult to measure. These numbers give, of
-course, the extreme limits of the variations of the chromosphere; but,
-nearly always, it is more shallow in the equatorial regions; and, as far
-as my observations go, the difference in thickness between the polar and
-equatorial zones is greater in years of calm than it is in years of
-great solar activity. But ten years of observation are not sufficient to
-warrant any definite conclusions on this subject.
-
-There is undoubtedly some relation between the greater thickness of the
-chromosphere in the polar regions, and the abundance and permanence of
-the sharply-pointed granulations observed in the same regions. This
-becomes more evident when we know that the appearance of
-similarly-pointed flames in the equatorial zones is always accompanied
-with a local thickening of the chromosphere. The thickening in the polar
-regions may be only apparent, and not due to a greater accumulation of
-chromospheric gases there; but may be caused by some kind of repulsive
-action or polarity, which lifts up and extends the summit of the
-granulations in a manner similar to the well-known mode of electric
-repulsion and polarity.
-
-As it seems very probable that the heat and light emanating from the Sun
-are mainly generated at the base of the granulations, in the filamentary
-elements composing the chromosphere and photosphere, it would follow
-that, as the size and number of these objects constantly vary, the
-amount of heat and light emitted by the Sun should also vary in the same
-proportion.
-
-The granulations of the solar surface are represented on Plate I., and
-form the general background to the group of Sun-spots forming the
-picture.
-
-
-THE FACULÆ
-
-
-Although the solar surface is mainly covered with the luminous
-granulations and the grayish background above described, it is very rare
-that its appearance is so simple and uniform as already represented. For
-the most part, on the contrary, it is diversified by larger, brighter,
-and more complicated forms, which are especially visible towards the
-border of the Sun. Owing to their extraordinary brilliancy, these
-objects have been called _Faculœ_ (torches).
-
-Although the faculæ are very seldom seen well beyond 50 heliocentric
-degrees from the limb, yet they exist, and are as numerous in the
-central parts of the disk as they are towards the border; since they
-form a part of the solar surface, and participate in its movement of
-rotation. Their appearance near the limb has been attributed to the
-effect of absorption produced by the solar atmosphere on the
-light from the photosphere; but this explanation seems inadequate,
-and does not solve the problem. The well-known fact that the solar
-protuberances--which are in a great measure identical with the
-faculæ--are much brighter at the base than they are at the summit,
-perhaps gives a clue to the explanation of the phenomenon; especially
-since we know that, in general, the summit of the protuberances is
-considerably broader than their base. When these objects are observed in
-the vicinity of the limb, they present their brightest parts to the
-observer, since, in this position, they are seen more or less sidewise;
-and, therefore, they appear bright and distinct. But as the faculæ
-recede from the limb, their sides, being seen under a constantly
-decreasing angle, appear more and more foreshortened; and, therefore,
-these objects grow less bright and less distinct, until they finally
-become invisible, when their bases are covered over by the broad, dusky
-summit generally terminating the protuberances.
-
-The faculæ appear as bright and luminous masses or streaks on the
-granular surface of the Sun, but they differ considerably in form and
-size. Two types at least are distinguishable. In their simplest form
-they appear either as isolated white spots, or as groups of such spots
-covering large surfaces, and somewhat resembling large flakes of snow.
-In their most characteristic types they appear as intensely luminous,
-heavy masses, from which, in most cases, issue intricate ramifications,
-sometimes extending to great distances. Generally, the ramifications
-issuing from the masses of faculæ have their largest branches directed
-in the main towards the eastern limb of the Sun. Some of these branches
-have gigantic proportions. Occasionally they extend over 60° and even
-80° of the solar surface, and, therefore, attain a length of from
-450,000 to 600,000 miles.
-
-Although the faculæ may be said to be seen everywhere on the surface of
-the Sun, there is a vast difference in different regions, with regard to
-their size, number, and brilliancy. They are largest, most abundant, and
-brightest on two intermediate zones parallel to the solar equator, and
-extending 35° or 40° to the north and to the south of this line. The
-breadth of these zones varies considerably with the activity of the
-solar forces. When they are most active, the faculæ spread on either
-side, but especially towards the equator, where they sometimes nearly
-meet those of the other zone. In years of little solar activity the
-belts formed by the faculæ are very narrow--the elements composing them
-being very few and small, although they never entirely disappear.
-
-The faculæ are very unstable, and are constantly changing: those of the
-small types sometimes form and vanish in a few minutes. When an area of
-disturbance of the solar surface is observed for some time, all seems in
-confusion; the movements of the granulations become unusually violent;
-they congregate in all sorts of ways, and thus frequently form temporary
-faculæ. Action of this kind is, for the most part, peculiar to the
-polar regions of the Sun.
-
-The larger faculæ have undoubtedly another origin, as they seem to be
-mainly formed by the ejection of incandescent gases and metallic vapors
-from the interior of the photosphere. In their process of development
-some of the heavy masses of faculæ are swollen up to great heights,
-being torn in all sorts of ways, showing large rents and fissures
-through which the sight can penetrate.
-
-Very few faculæ are represented in Plate I.; several streaks are shown
-at the upper left-hand corner, some appearing as whitish ramifications
-among the granulations representing the general solar surface.
-
-
-
-
-SUN-SPOTS AND VEILED SPOTS
-
-PLATE I
-
-
-Besides the brilliant faculæ already described, much more conspicuous
-markings, though of a totally different character, are very frequently
-observed on the Sun. On account of their darkish appearance, which is in
-strong contrast with the white envelope of our luminary, these markings
-were called _Maculæ_, or Sun-spots, by their earlier observers.
-
-The Sun-spots are not equally distributed on the solar surface; but like
-the faculæ, to which they are closely related, they occupy two
-zones--one on each side of the equator. These zones are comprised
-between 10° and 35° of north latitude, and 10° and 35° of south
-latitude. Between these two zones is a belt 20° in width, where the
-Sun-spots are rarely seen.
-
-Above the latitudes 35° north and south, the Sun-spots are rare, and it
-is only occasionally, and during years of great solar activity, that
-they appear in these regions; in only a few cases have spots of
-considerable size been seen there. A few observers, however, have seen
-spots as far as 40° and 50 from the equator; and La Hire even observed
-one in 70° of north latitude; but these cases are exceedingly rare. It
-is not uncommon, however, to see very small spots, or groups of such
-spots, within 8° or 10° from the poles.
-
-The activity of the Sun is subject to considerable fluctuation, and
-accordingly the Sun-spots vary in size and number in different years.
-During some years they are large, complicated, and very numerous; while
-in others they are small and scarce, and are sometimes totally absent
-for weeks and months together. The fluctuations in the frequency of
-Sun-spots are supposed to be periodical in their character, although
-their periods do not always appear to recur at exactly regular
-intervals. Sometimes the period is found to be only nine years, while at
-other times it extends to twelve years. The period generally adopted now
-is 11⅒ years, nearly; but further investigations are needed to
-understand the true nature of the phenomenon.
-
-The number of Sun-spots does not symmetrically augment and diminish, but
-the increase is more rapid than the diminution.
-
-The period of increase is only about four years, while that of decrease
-is over seven years; each period of Sun-spot maximum being nearer the
-preceding period of Sun-spot minimum than it is to that next following.
-
-The cause of these fluctuations in the solar energy is at present wholly
-unknown. Some astronomers, however, have attributed it to the influence
-of the planets Venus and Jupiter, the period of revolution of the latter
-planet being not much longer than the Sun-spot period; but this
-supposition lacks confirmation from direct observations, which, so far,
-do not seem to be in favor of the hypothesis. At the present time the
-solar activity is on the increase, and the Sun-spots will probably reach
-their maximum in 1883. The last minimum occurred in 1879, when only
-sixteen small groups of spots were observed during the whole year.
-
-Sun-spots vary in size and appearance; but, unless they are very small,
-in which case they appear as simple black dots, they generally consist
-of two distinct and well-characterized parts, nearly always present.
-There is first, a central part, much darker than the other, and sharply
-divided from it, called the "_Umbra_;" second, a broad, irregular
-radiated fringe of lighter shade, completely surrounding the first, and
-called the "_Penumbra_."
-
-Reduced to its simplest expression, a Sun-spot is a funnel-shaped
-opening through the chromosphere and the photosphere. The inner end of
-the funnel, or opening, gives the form to the umbra, while its sloping
-sides form the penumbra.
-
-The umbra of Sun-spots, whose outlines approximately follow the
-irregularities of the penumbral fringe, has a diameter which generally
-exceeds the width of the penumbral ring. Sometimes it appears uniformly
-black throughout; but it is only so by contrast, as is proved when
-either Mercury or Venus passes near a spot during a transit over the
-Sun's disk. The umbra then appears grayish, when compared with the
-jet-black disk of the planet.
-
-The umbra of spots is rarely so simple as just described; but it is
-frequently occupied, either partly or wholly, by grayish and rosy forms,
-somewhat resembling loosely-entangled muscular fibres. These forms have
-been called the _Gray and Rosy Veils_. Frequently these veils appear as
-if perforated by roundish black holes, improperly called _Nuclei_, which
-permit the sight to penetrate deeper into the interior. To all
-appearance the gray and rosy veils are of the same nature as the
-chromosphere and the faculæ, and are therefore mainly composed of
-hydrogen gas.
-
-Whatever can be known about the interior of the Sun, must be learned
-from the observations of these openings, which are comparatively small.
-But whatever this interior may be, we certainly know that it is not
-homogeneous. Apparently, the Sun is a gigantic bubble, limited by a very
-thin shell. Below this shell exists a large open space filled with
-invisible gases, in which, through the openings constituting the
-Sun-spots, the gray and rosy veils described above are occasionally seen
-floating.
-
-The fringe forming the penumbra of spots is much more complicated than
-the umbra. In its simpler form, it is composed of a multitude of bright,
-independent filaments of different forms and sizes, partly projecting
-one above the other, on the sloping wall of the penumbra, from which
-they seem to proceed. Seen from the Earth, these filaments have somewhat
-the appearance of thatched straw, converging towards the centre of the
-umbra. It is very rare, however, that the convergence of the penumbral
-filaments is regular, and great confusion sometimes arises from the
-entanglement of these filaments. Some of these elements appear straight,
-others are curved or loop-shaped; while still others, much larger and
-brighter than the rest, give a final touch to this chaos of filaments,
-from which results the general thatched and radiating appearance of the
-penumbra.
-
-The extremities of the penumbral filaments, especially of those forming
-the border of the umbra, are usually club-shaped and appear very
-brilliant, as if these elements had been superheated by some forces
-escaping through the opening of the spots.
-
-Besides these characteristics, the Sun-spots have others, which,
-although not always present, properly belong to them. Comparatively few
-spots are so simple as the form just described. Very frequently a spot
-is accompanied by brilliant faculæ, covering part of its umbra and
-penumbra, and appearing to form a part of the spot itself.
-
-When seen projected over Sun-spots, the faculæ appear intensely bright,
-and from these peculiarities they have been called _Luminous Bridges_.
-They are, in fact, bridges, but in most cases they are at considerable
-heights above the spots, kept there by invisible forces. When such spots
-with luminous bridges approach the Sun's limb, it is easy to see, by the
-rapid apparent displacement which they undergo, that they are above the
-general level.
-
-When the spots are closing up, the inverse effect is sometimes observed.
-On several occasions, I have seen huge masses of faculæ advance slowly
-over the penumbra of a spot and fall into the depths of the umbra,
-resembling gigantic cataracts. I have seen narrow branches of faculæ,
-which, after having fallen to great depths in the umbra, floated across
-it and disappeared under the photosphere on the opposite side. I have
-also seen luminous bridges, resembling cables, tightly stretched across
-the spots, slackening slowly, as if loosened at one end, and gently
-curving into the umbra, where they formed immense loops, large enough to
-receive our globe.
-
-It is to be remarked that, in descending under the photospheric shell,
-the bright faculæ and the luminous bridges gradually lose their
-brilliancy. At first they appear grayish, but in descending farther they
-assume more and more the pink color peculiar to the rosy veils. The
-pinkish color acquired by the faculæ when they reach a certain depth
-under the photosphere, is precisely the color of the chromosphere and of
-the solar protuberances, as seen during total eclipses of the Sun--a
-fact which furnishes another proof that the faculæ are of the same
-nature as the protuberances.
-
-I record here an observation which, at first sight, may appear
-paradoxical; but which seems, however, to be of considerable importance,
-as it shows unmistakably that the solar light is mainly, if not
-entirely, generated on its surface, or at least very near to it. On May
-26, 1878, I observed a large group of Sun-spots at a little distance
-from the east limb of the Sun. The spot nearest to the limb was partly
-covered over on its eastern and western sides by bright and massive
-faculæ which concealed about two-thirds of the whole spot, only a
-narrow opening, running from north to south, being left across the
-middle of the spot. Owing to the rotundity of the Sun, the penumbra of
-this spot, although partly covered by the faculæ, could, however, be
-seen on its eastern side, since the sight of the observer could there
-penetrate sidewise under the faculæ. Upon that part of the penumbra
-appeared a strong shadow, representing perfectly the outline of the
-facular mass situated above it. The phenomenon was so apparent that no
-error of observation was possible, and a good drawing of it was secured.
-If this faculæ had been as bright beneath as it was above, it is
-evident that no shadow could have been produced; hence the light of
-these faculæ must have been mainly generated on or very near their
-exterior surfaces. This, with the well-proved fact that the bright
-faculæ lose their light in falling into the interior of the Sun, seems
-to suggest the idea that the bright light emitted by the faculæ, and
-very probably all the solar light, can be generated only on its surface;
-the presence of the coronal atmosphere being perhaps necessary to
-produce it. Several times before this observation, I had suspected that
-some faculæ were casting a shadow, but as this seemed so improbable, my
-attention was not awakened until the phenomenon became so prominent that
-it could not escape notice.
-
-With due attention, some glimpses of the phenomenon can frequently be
-observed through the openings of some of the faculæ projecting over the
-penumbra of Sun-spots. It is very seldom that the structure of the
-penumbra is seen through such openings, which usually appear as dark as
-the umbra of the large spots, although they do not penetrate through the
-photosphere like the latter. It is only when the rents in the faculæ
-are numerous and quite large, that the penumbral structure is recognized
-through them. Since these superficial rents in the faculæ do not extend
-through the photosphere, and appear black, it seems evident that the
-penumbra seen through them cannot be as bright as it is when no faculæ
-are projected upon it, and therefore that the faculæ intercept light
-from the exterior surface, which would otherwise reach the penumbra.
-
-While the matter forming the faculæ sometimes falls into the interior
-of the Sun, the same kind of matter is frequently ejected in enormous
-quantities, and with great force, from the interior, through the visible
-and invisible openings of the photosphere, and form the protuberances
-described in the following section of this manual (Solar protuberances)
-It is not only the incandescent hydrogen gas or the metallic vapors
-which are thus ejected, but also cooler hydrogen gas, which sometimes
-appears as dark clouds on the solar surface. On December 12, 1875, I
-observed such a cloud of hydrogen issuing from the corner of a small
-Sun-spot. It traveled several thousand miles on the solar surface, in a
-north-easterly direction, before it became invisible.
-
-Solar spots are formed in various ways; but, for the most part, the
-apparition of a spot is announced beforehand, by a great commotion of
-the solar surface at the place of its appearance, and by the formation
-of large and bright masses of faculæ, which are usually swollen into
-enormous bubbles by the pressure of the internal gases. These bubbles
-become visible in the spectroscope while they are traversing the solar
-limb, as they are then presented to us sidewise. Under the action of the
-increasing pressure, the base of the faculæ is considerably stretched,
-and, its weakest side finally giving way, the facular mass is torn in
-many places from the solar surface, and is perforated by holes of
-different sizes and forms. The holes thus made along the border of the
-faculæ appear as small black spots, separated more or less by the
-remaining portion of the lacerated faculæ, and they enlarge more and
-more at the expense of the intervening portions, which thus become very
-narrow. This perforated side of the faculæ, offering less resistance,
-is gradually lifted up, as would be the cover of a box, for example,
-while its opposite side remains attached to the surface. The facular
-matter separating the small black holes is greatly stretched during this
-action, and forms long columns and filaments. These appear as luminous
-bridges upon the large and perfectly-formed spot, which is then seen
-under the lifted facular masses. The spots thus made visible are soon
-freed from the facular masses, which are gradually shifted towards the
-opposite side.
-
-In such cases the spots are undoubtedly formed under the faculæ before
-they can be seen. This becomes evident when such spots, not yet cleared
-from the faculæ covering them, are observed near the east limb; since
-in this position the observer can see through the side-openings of the
-faculæ, and sometimes recognize the spots under their cover.
-
-It frequently occurs that the spots thus formed under the faculæ
-continue to be partly covered by the facular clouds, the forces at work
-in them being apparently too feeble to shift them aside. In such cases
-these spots are visible when they are in the vicinity of the limb, where
-they are seen sidewise; but when observed in the east, in being carried
-forward by the solar rotation towards the centre of the disk, they
-gradually diminish in size, and finally become invisible. The
-disappearance of these spots, however, is only apparent, being due to
-the fact that, as they advance towards the centre of the disk, our
-lateral view of them is gradually lost, and they are finally hidden from
-sight by the overhanging faculæ which then serve as a screen between
-the observer and the spot. This class of spots may be called _Lateral
-Spots_, from the fact that they can only be seen laterally, and near the
-Sun's limb.
-
-Solar spots are also formed in various other ways. Some, like those
-represented in Plate I., appearing without being announced by any
-apparent disturbance of the surface, or by the formation of any faculæ,
-form and develop in a very short time. Others, appearing at first as
-very small spots having an umbra and a penumbra, slowly and gradually
-develop into very large spots. This mode of formation, which would seem
-to be the most natural, is, however, quite rare. Spots of this class
-have a duration and permanence not observed in those of any other type.
-These spots of slow and regular development are never accompanied by
-faculæ or luminous bridges, nor have they any gray or rosy veils in
-their interior; a fact which may, perhaps, account for their permanent
-character.
-
-Another class of spots, which is also rare, appear as long and narrow
-crevasses showing the penumbral structure of the ordinary spots; but
-these rarely have any umbra. These long, and sometimes exceedingly
-narrow fissures of the solar envelopes, with their radiated penumbral
-structure, strongly suggest the idea that the photosphere is composed of
-a multitude of filamentary elements having the granulations for summits.
-Such a crevasse is represented on Plate I., and unites the two spots
-which form the group.
-
-The duration of Sun-spots varies greatly. Some last only for a few
-hours; while others continue for weeks and even months at a time, but
-not without undergoing changes.
-
-The modes of disappearance of Sun-spots are as various as those of their
-apparition. The spots rarely close up by a gradual diminution or
-contraction of their umbra and penumbra. This mode of disappearance
-belongs exclusively to the spots deprived of faculæ and veils. One of
-the most common modes of the disappearance of a spot is its invasion by
-large facular masses, which slowly advance upon its penumbra and umbra
-and finally cover it entirely. It is a process precisely the reverse of
-that in which spots are formed by the shifting aside of the faculæ, as
-above described. In other types, the spots close up by the gradual
-enlargement of the luminous bridges traversing them, which are slowly
-transformed into branches of the photosphere, all of the characteristics
-of which they have acquired. In many cases, the spots covered over by
-the faculæ continue to exist for some time, hidden under these masses,
-as is often proved, either by the appearance of small spots on the
-facular mass left at the place they occupied, or even by the
-reappearance of the same spot.
-
-Apart from the general movement of rotation of the solar surface, some
-of the spots seem to be endowed with a proper motion of their own, which
-becomes greater the nearer the spots are to the solar equator. According
-to the observations of Mr. Carrington, the period of rotation of the
-Sun, as deduced from the observations of the solar spots during a period
-of seven years, is 25 days at the equator; while at 50° of heliocentric
-latitude it is 27 days. But the period of rotation, as derived from the
-observations of spots occupying the same latitude, is far from being
-constant, as it varies at different times, with the frequency of the
-spots and with the solar activity, so that at present the law of these
-variations is not well known. From the character of the solar envelope,
-it seems very natural that the rotation should differ in the different
-zones and at different times, since this envelope is not rigid, but very
-movable, and governed by forces which are themselves very variable.
-
-Although it is a general law that the spots near the equator have a more
-rapid motion than those situated in higher latitudes, yet, in many
-cases, the proper motion of the spots is more apparent than real. For
-the most part, the changes of form and the rapid displacements observed
-in some spots are only apparent, and due to the fact that the large
-masses of faculæ which are kept in suspense above them are very
-unstable, and change position with the slightest change in the forces
-holding them in suspension. Since in these cases we view the spots
-through the openings of the faculæ situated above them, the slightest
-motion of these objects produces an apparent motion in the spots,
-although they have remained motionless. Accordingly, it has been
-remarked that of all the spots, those which have the greater proper
-motion are precisely those which have the most faculæ and luminous
-bridges; while the other spots in the same regions, but not attended by
-similar phenomena, are comparatively steady in their movement. These
-last spots are undoubtedly better adapted than any others to exhibit the
-rotation of the Sun; but it is probable that this period of rotation
-will never be known with accuracy, simply because the solar surface is
-unstable, and does not rotate uniformly.
-
-The Sun-spots have a remarkable tendency to form into groups of various
-sizes, but whatever may be the number of spots thus assembled, the group
-is nearly always composed of two principal spots, to which the others
-are only accessories. The tendency of the Sun-spots to assemble in pairs
-is general, and is observed in all latitudes, even among the minute
-temporary groups formed in the polar regions. Whenever several are
-situated quite close together, those belonging to the same group can be
-easily recognized by this character. Whatever may be the position of the
-axis of the two principal spots of a group when it is first formed, this
-axis has a decided tendency to place itself parallel to the solar
-equator, no matter to what latitude the group belongs; and if it is
-disturbed from this position, it soon returns to it when the disturbance
-has ceased.
-
-It is also remarkable that the spots observed at the same time remain in
-nearly the same parallel of latitude for a greater or less period of
-time; but they keep changing their position from year to year, their
-latitude decreasing with the activity of the solar forces.
-
-Among the Sun-spots, those associated with faculæ form the groups which
-attain the largest proportions. When such groups acquire an apparent
-diameter of 1' or more, they are plainly visible to the naked eye, since
-for a spot to be visible to the naked eye on the Sun, it need only
-subtend an angle of 50". I have sometimes seen such groups through a
-smoky atmosphere, when the solar light was so much reduced that the disk
-could be observed directly and without injury to the sight.
-
-The largest spot which ever came under my observation was seen during
-the period from the 13th to the 19th of November, 1870. This spot, which
-was on the northern hemisphere of the Sun, was conspicuous among the
-smaller spots constituting the group to which it belonged, and followed
-them on the east. On November 16th, when it attained its largest size,
-the diameter of its penumbra occupied fully one-fifth of the diameter of
-the Sun; its real diameter being, therefore, not less than 172,000
-miles, or nearly 22 times the diameter of the Earth. As the umbra of
-this spot occupied a little more than one-third of its whole diameter,
-seven globes like our own, placed side by side on a straight line, could
-easily have passed through this immense gap. To fill the area of this
-opening, about 45 such globes would have been needed. This spot was, of
-course, very easily seen with the naked eye, its diameter being almost
-eight times that required for a spot to be visible without a telescope.
-
-Ancient historians often speak of obscurations of the Sun, and it has
-been supposed by some astronomers that this phenomenon might have been
-due in some cases to the apparition of large spots. A few spots on the
-surface of the Sun, like that just described, would sensibly reduce its
-light.
-
-Besides the ordinary Sun-spots already described, others are at times
-observed on the surface of the Sun, which show some of the same
-characteristics, but never attain so large proportions. They always
-appear as if seen through a fog, or veil, between the granulations of
-the solar surface. On account of their vagueness and ill-defined
-contours, I have proposed for these objects the term, "_Veiled Spots_."
-Veiled spots have a shorter duration than the ordinary spots, the
-smaller types sometimes forming and vanishing in a few minutes. Some of
-the larger veiled spots, however, remain visible for several days in
-succession, and show the characteristics of other spots in regard to the
-arrangement of their parts.
-
-The veiled spots have no umbra or penumbra, although they are usually
-accompanied by faculæ resembling those seen near the ordinary spots.
-They are frequently seen in the polar regions, but are there always of
-small size and of short duration. The veiled spots are larger, and more
-apt to arrange themselves into groups, in the regions occupied by the
-ordinary spots, and it is not rare to observe such spots transform
-themselves into ordinary spots, and vice versa. The veiled spots,
-therefore, seem to be ordinary spots filled up, or covered over by the
-granulations and semi-transparent gases composing the chromospheric
-layer. That it is so, becomes more evident, from the fact that large
-Sun-spots in process of diminution are sometimes gradually covered with
-faint and scattered granules which descend in long, narrow filaments,
-and become less and less distinguishable as they attain greater depth.
-This phenomenon, associated with the fact that the luminous bridges seen
-over the Sun-spots which are closing up are sometimes transformed into
-branches which show the characteristic structure of the photosphere,
-goes far to prove that the solar envelopes are mainly composed of an
-innumerable quantity of radial filaments of varying height.
-
-
-[PLATE I.--GROUP OF SUN-SPOTS AND VEILED SPOTS.
-
-Observed on June 17th 1875 at 7 h. 30 m. A.M.]
-
-
-The group of Sun-spots represented in Plate I., was observed and drawn
-on June 17th, 1875, at 7h. 30m. A. M. The first traces of this group
-were seen on June 15th, at noon, and consisted of three small black dots
-disseminated among the granulations. At that time, no disturbance of the
-surface was noticeable, and no faculæ were seen in the vicinity of these
-spots. On June 16th, at 8 o'clock A. M., the three small spots had
-become considerably enlarged, and, as usual, the group consisted of two
-principal spots. Between these two spots all was in motion: the
-granulations, stretched into long, wavy, parallel lines, had somewhat
-the appearance of a liquid in rapid motion. At 1 o'clock, P. M., on the
-same day, the group had considerably enlarged; the faculæ, the
-granulations, and the penumbral filaments being interwoven in an
-indescribable manner. On the morning of the 17th, these spots had
-assumed the complicated form and development represented in the drawing;
-while at the same time two conspicuous veiled spots were seen on the
-left hand, at some distance above the group.
-
-Some luminous bridges are visible upon the left hand spot, traversing
-the penumbra and umbra of this spot in various directions. The umbra of
-one of the spots is occupied, and partly filled with gray and rosy
-veils, similar to those above described, and the granulations of the
-solar surface form a background to the group of spots.
-
-This group of spots was not so remarkable for its size as for its
-complicated structure. The diameter of the group from east to west was
-only 2½ minutes of arc, or about 67,000 miles. The upper part of the
-umbra of the spot situated on the right hand side of the group was
-nearly 7,000 miles in diameter, or less by 1,000 miles than the diameter
-of the Earth. Some of the long filaments composing that part of the
-penumbra, situated on the left hand side of the same spot, were 17,000
-miles in length. One of these fiery elements would be sufficient to
-encircle two-thirds of the circumference of the Earth.
-
-
-
-
-SOLAR PROTUBERANCES
-
-PLATE II
-
-
-The chromosphere forming the outlying envelope of the Sun, is subject,
-as has been shown above, to great disturbances in certain regions,
-causing considerable upheavals of its surface and violent outbursts of
-its gases. From these upheavals and outbursts of the chromosphere result
-certain curious and very interesting forms, which are known under the
-name of "_Solar Protuberances_," "_Prominences_," or "_Flames_."
-
-These singular forms, which could, until recently, be observed only
-during the short duration of the total eclipses of the Sun, can now be
-seen on every clear day with the spectroscope, thanks to Messrs. Janssen
-and Lockyer, to whose researches solar physics is so much indebted.
-
-
-[PLATE II.--SOLAR PROTUBERANCES.
-
-Observed on May 5, 1873 at 9h, 40m. A.M.]
-
-
-The solar protuberances, the Sun-spots, and the faculæ to which they
-are closely related, are confined within the same general regions of the
-Sun, although the protuberances attain higher heliocentric latitudes.
-
-There is certainly a very close relation between the faculæ and the
-solar protuberances, since when a group of the faculæ traverses the
-Sun's limb, protuberances are always seen at the same place. It seems
-very probable that the faculæ and the protuberances are in the main
-identical. The faculæ may be the brighter portion of the protuberances,
-consisting of gases which are still undergoing a high temperature and
-pressure; while the gases which have been relieved from this pressure
-and have lost a considerable amount of their heat, may form that part of
-the protuberances which is only visible on the Sun's limb.
-
-A daily study of the solar protuberances, continued for ten years, has
-shown me that these objects are distributed on two zones which are
-equidistant from the solar equator, and parallel with it. The zone
-arrangement of the protuberances is more easily recognized during the
-years of minimum solar activity, as in these years the zones are very
-narrow and widely separated. During these years the belt of
-protuberances is situated between 40° and 45° of latitude, north and
-south. In years of great solar activity the zones spread considerably on
-either side of these limits, especially towards the equator, which they
-nearly reach, only a narrow belt, usually free from protuberances,
-remaining between them. Towards the poles the zones do not spread so
-much, and there the space free from protuberances is considerably
-greater than it is at the equator.
-
-During years of maximum solar activity, the protuberances, like the
-Sun-spots and the faculæ, are very numerous, very large, and very
-complicated--sometimes occupying a great part of the whole solar limb.
-As many as twenty distinct flames are sometimes observed at one time. In
-years of minimum solar activity, on the contrary, the prominences are
-very few in number, and they are of small size; but, as far as my
-observations go, they are never totally absent.
-
-In general, the solar flames undergo rapid changes, especially those
-which are situated in the vicinity of Sun-spots, although they
-occasionally remain unchanged in appearance and form for several hours
-at a time. The protuberances situated in higher latitudes are less
-liable to great and sudden changes, often retaining the same form for
-several days. The changes observed in the protuberances of the
-equatorial regions are due in part to the comparatively great changes in
-their position with respect to the spectator, which are occasioned by
-the rotation of the Sun. This rotation, of course, has a greater angular
-velocity on the equator than in higher latitudes. In most cases,
-however, the changes of the equatorial protuberances are too great and
-too sudden to be thus explained. They are, in fact, due to the greater
-solar activity developed in the equatorial zones, and wherever spots are
-most numerous.
-
-The solar protuberances appear under various shapes, and are often so
-complicated in appearance that they defy description. Some resemble huge
-clumsy masses having a few perforations on their sides; while others
-form a succession of arches supported by pillars of different styles.
-Others form vertical or inclined columns, often surmounted by cloud-like
-masses, or by various appendages, which sometimes droop gracefully,
-resembling gigantic palm leaves. Some resemble flames driven by the
-wind; others, which are composed of a multitude of long and narrow
-filaments, appear as immense fiery bundles, from which sometimes issue
-long and delicate columns surmounted by torch-like objects of the most
-fantastic pattern. Some others resemble trees, or animal forms, in a
-very striking manner; while still others, apparently detached from the
-solar limb, float above it, forming graceful streamers or clouds of
-various shapes. Some of the protuberances are very massive, while others
-are so thin and transparent as to form a mere veil, through which more
-distant flames can easily be seen.
-
-Notwithstanding this variety of form, two principal classes of solar
-protuberances may be recognized: the cloud-like or quiescent, and the
-eruptive or metallic protuberances.
-
-The first class, which is the most common, comprises all the cloud-like
-protuberances resting upon the chromosphere or floating about it. The
-protuberances of this type often obtain enormous horizontal proportions,
-and it is not rare to see some among them occupying 20° and 30° of the
-solar limb. The height attained by protuberances of this class does not
-correspond in general to their longitudinal extent; although some of
-their branches attain considerable elevations. These prominences very
-seldom have the brilliancy displayed by the other type, and are
-sometimes so faint as to be seen with difficulty. Although it is
-generally stated by observers that some of the protuberances belonging
-to this class are detached from the solar surface, and kept in
-suspension above the surface, like the clouds in our atmosphere, yet it
-seems to me very doubtful whether protuberances are ever disconnected
-from the chromosphere, since, in an experience of ten years, I have
-never been able to satisfy myself that such a thing has occurred. Many
-of them have appeared to me at first sight to be detached from the
-surface, but with a little patience and attention I was always able to
-detect faint traces of filamentary elements connecting them with the
-chromosphere. Quite often I have seen bright protuberances gradually
-lose their light and become invisible, while soon after they had
-regained it, and were as clearly visible as before. Observations of this
-kind seem to show that while the prominences are for the most part
-luminous, there are also a few which are non-luminous and invisible to
-the eye. These dark and invisible forms are most generally found in the
-vicinity of Sun-spots in great activity. When observing such regions
-with the spectroscope, it is not rare to encounter them in the form of
-large dark spots projecting on the solar spectrum near the hydrogen
-lines. On July 28th, 1872, I observed with the spectroscope a dark spot
-of this kind issuing from the vicinity of a large Sun-spot, and
-extending over one-fifth of the diameter of the Sun. This object had
-been independently observed in France a little earlier by M. Chacornac
-with the telescope, in which it appeared as a bluish streak.
-
-The second class of solar protuberances, comprising the eruptive type,
-is the most interesting, inasmuch as it conveys to us a conception of
-the magnitude and violence of the solar forces. The protuberances of
-this class, which are always intensely bright, appear for the most part
-in the immediate vicinity of Sun-spots or faculæ. These protuberances,
-which seem to be due to the outburst of the chromosphere, and to the
-violent ejection of incandescent gases and metallic vapors from the
-interior of the Sun, sometimes attain gigantic proportions and enormous
-heights.
-
-While the spectrum of the protuberances of the cloudy type is simple,
-and usually composed of four hydrogen lines and the yellow line D{3}, that
-of the eruptive class is very complicated, and, besides the hydrogen
-lines and D{3}, it often exhibits the bright lines of sodium, magnesium,
-barium, titanium, and iron, and occasionally, also, a number of other
-bright lines.
-
-The phenomena of a solar outburst are grand and imposing. Suddenly
-immense and acute tongues and jets of flames of a dazzling brilliancy
-rise up from the solar limb and extend in various directions. Some of
-these fiery jets appear perfectly rigid, and remain apparently
-motionless in the midst of the greatest disorder. Immense straps and
-columns form and rise in an instant, bending and waving in all sorts of
-ways and assuming innumerable shapes. Sometimes powerful jets resembling
-molten metal spring up from the Sun, describing graceful parabolas,
-while in their descent they form numerous fiery drops which acquire a
-dazzling brilliancy when they approach the surface.
-
-The upward motion of the protuberances in process of formation is
-sometimes very rapid. Some protuberances have been observed to ascend in
-the solar atmosphere at the rate of from 120 to 497 miles a second.
-Great as this velocity may appear, it is nevertheless insignificant when
-compared with that sometimes attained by protuberances moving in the
-line of sight instead of directly upwards. Movements of this kind are
-indicated by the displacement of the bright or dark lines in the
-spectrum. A remarkable instance of this kind occurred on the 26th of
-June, 1874. On that day I observed a displacement of the hydrogen C line
-corresponding to a velocity of motion of 1,600 miles per second. The
-mass of hydrogen gas in motion producing such a displacement was,
-according to theory, moving towards the Earth at this incredible rate,
-when it instantly vanished from sight as if it had been annihilated, and
-was seen no more.
-
-Until recently the protuberances had not been observed to rise more than
-200,000 miles above the solar surface; but, on October 7th, 1880, a
-flame, which had an elevation of 80,000 miles when I observed it at 8h.
-55m. A. M., had attained the enormous altitude of 350,000 miles when it
-was observed at noon by Professor C. A. Young. If we had such a
-protuberance on the Earth, its summit would be at a height sufficient
-not merely to reach, but to extend 100,000 miles beyond the Moon.
-
-Although the solar protuberances represented in Plate II. have not the
-enormous proportions attained by some of these objects, yet they are as
-characteristic as any of the largest ones, and afford a good
-illustration of the purely eruptive type of protuberances. The height of
-the largest column in the group equals 4' 43", or a little over 126,000
-miles. A large group of Sun-spots was in the vicinity of these
-protuberances when they were observed and delineated.
-
-
-
-
-TOTAL ECLIPSE OF THE SUN
-
-PLATE III
-
-
-A solar eclipse is due to the passage of the Moon directly between the
-observer and the Sun. Such an eclipse can only occur at New Moon, since
-it is only at that time that our satellite passes between us and the
-Sun. The Moon's orbit does not lie precisely in the same plane as the
-orbit of the Earth, but is inclined about five degrees to it, otherwise
-an eclipse of the Sun would occur at every New Moon, and an eclipse of
-the Moon at every Full Moon.
-
-Since the Moon's orbit is inclined to that of the Earth, it must
-necessarily intersect this orbit at two opposite points. These points
-are called the nodes of the Moon's orbit. When our satellite passes
-through either of the nodes when the Moon is new, it appears interposed
-to some extent between the Sun and the Earth, and so produces a solar
-eclipse; while if it passes a node when the Moon is full, it is more or
-less obscured by the Earth's shadow, which then produces an eclipse of
-the Moon. But, on the other hand, when the New Moon and the Full Moon do
-not coincide with the passage of our satellite through the nodes of its
-orbit, no eclipse can occur, since the Moon is not then on a line with
-the Sun and the Earth, but above or below that line.
-
-Owing to the ellipticity of the Moon's orbit, the distance of our
-satellite from the Earth varies considerably during each of its
-revolutions around us, and its apparent diameter is necessarily subject
-to corresponding changes. Sometimes it is greater, sometimes it is less,
-than the apparent diameter of the Sun. If it is greater at the time of a
-solar eclipse, the eclipse will be total to a terrestrial observer
-stationed nearly on the line of the centres of the Sun and Moon, while
-it will be only partial to another observer stationed further from this
-line. But the Moon's distance from the Earth may be so great and its
-apparent diameter consequently so small that even those observers
-nearest the central line of the eclipse see the border of the Sun all
-round the black disk of the Moon; the eclipse is then annular. Even
-during the progress of one and the same eclipse the distance of the Moon
-from the parts of the Earth towards which its shadow is directed may
-vary so much that, while the eclipse is total to some observers, others
-equally near the central line, but stationed at a different place, will
-see it as annular.
-
-The shadow cast by the Moon on the Earth during total eclipses, travels
-along upon the surface of the Earth, in consequence of the daily
-movement of rotation of our globe combined with the movements of the
-Earth and Moon in their orbits. The track of the Moon's shadow over the
-Earth's surface has a general eastward course, so that the more westerly
-observers see it earlier than those east of them. An eclipse may
-continue total at one place for nearly eight minutes, but in ordinary
-cases the total phase is much shorter.
-
-The nodes of the Moon's orbit do not invariably occupy the same
-position, but move nearly uniformly, their position with regard to the
-Sun, Earth, and Moon being at any time approximately what it formerly
-was at a series of times separated by equal intervals from each other.
-Each interval comprises 223 lunations, or 18 years, 11 days, and 7 or 8
-hours. The eclipses which occur within this interval are almost exactly
-repeated during the next similar interval. This period, called the
-"Saros," was well known to the ancients, who were enabled by its means
-to predict eclipses with some certainty.
-
-
-[PLATE III.--TOTAL ECLIPSE OF THE SUN.
-
-Observed July 29, 1878, at Creston, Wyoming Territory]
-
-
-A total eclipse of the Sun is a most beautiful and imposing phenomenon.
-At the predicted time the perfectly round disk of the Sun becomes
-slightly indented at its western limb by the yet invisible Moon. This
-phenomenon is known as the "first contact."
-
-The slight indentation observed gradually increases with the advance of
-the Moon from west to east, the irregularities of the surface of our
-satellite being plainly visible on the border of the dark segment
-advancing on the Sun's disk. With the advance of the Moon on the Sun,
-the light gradually diminishes on the Earth. Every object puts on a dull
-and gloomy appearance, as when night is approaching; while the bright
-sky, losing its light, changes its pure azure for a livid grayish color.
-
-Two or three minutes before totality begins, the solar crescent, reduced
-to minute proportions, gives comparatively so little light that faint
-traces of the Sun's atmosphere appear on the western side behind the
-dark body of the Moon, whose limb then becomes visible outside of the
-Sun. I observed this phenomenon at Creston during the eclipse of 1878.
-From 15 to 20 seconds before totality, the narrow arc of the Sun's disk
-not yet obscured by the Moon seems to break and separate towards the
-extremities of its cusps, which, thus divided, form independent points
-of light, which are called "_Baily's beads_." A moment after, the whole
-solar crescent breaks into numerous beads of light, separated by dark
-intervals, and, suddenly, they all vanish with the last ray of Sunlight,
-and totality has begun with the "second contact." This phenomenon of
-Baily's beads is undoubtedly caused by the irregularities of the Moon's
-border, which, on reaching the solar limb, divide the thin solar
-crescent into as many beads of light and dark intervals as there are
-peaks and ravines seen sidewise on that part of the Moon's limb.
-
-With the disappearance of the last ray of light, the planets and the
-stars of the first and second magnitude seem to light up and become
-visible in the sky. The darkness, which had been gradually creeping in
-with the progress of the eclipse, is then at its maximum. Although
-subject to great variations in different eclipses, the darkness is never
-so great as might be expected from the complete obscuration of our
-luminary, as the part of our atmosphere which is still exposed to the
-direct rays of the Sun, reflects to us some of that light, which thus
-diminishes the darkness resulting from the disappearance of the Sun.
-Usually the darkness is sufficient to prevent the reading of common
-print, and to deceive animals, causing them to act as if night was
-really approaching. During totality the temperature decreases, while the
-humidity of the atmosphere augments.
-
-Simultaneously with the disappearance of Baily's beads, a pale, soft,
-silvery light bursts forth from behind the Moon, as if the Sun, in
-disappearing, had been vaporized and expanded in all directions into
-soft phosphorescent rays and streamers. This pale light is emitted by
-gases constituting the solar atmosphere surrounding the bright nucleus
-now obscured by the dark body of our satellite. This solar atmosphere is
-called _Corona_, from its distant resemblance to the aureola, or glory,
-represented by ancient painters around the heads of saints.
-
-With the bursting forth of the corona, a very thin arc of bright white
-light is seen along the Moon's limb, where the solar crescent has just
-disappeared. This thin arc of light is the reversing layer, which, when
-observed with the spectroscope at that moment, exhibits bright lines
-answering to the dark lines of the ordinary solar spectrum. Immediately
-above this reversing layer, and concentric with it, appears the
-pink-colored chromospheric layer, with its curiously shaped flames and
-protuberances. During totality, the chromosphere and protuberances are
-seen without the aid of the spectroscope, and appear of their natural
-color, which, although somewhat varying in their different parts, is, on
-the whole, pinkish, and similar to that of peach-blossoms; yet it is
-mixed here and there with delicate prismatic hues, among which the pink
-and straw colors predominate.
-
-The color of the corona seems to vary in every eclipse, but as its tints
-are very delicate, it may depend, in a great measure, upon the vision of
-the observer; although there seems to be no doubt that there are real
-variations. At Creston, in 1878, it appeared to both Professor W.
-Harkness and myself of a decided pale greenish hue.
-
-The corona appears under different forms, and has never been observed
-twice alike. Its dimensions are also subject to considerable variations.
-Sometimes it appears regular and very little extended, its distribution
-around the Sun being almost uniform; although in general it spreads a
-little more in the direction of the ecliptic, or of the solar equator.
-At other times it appears much larger and more complicated, and forms
-various wings and appendages, which in some cases, as in 1878, extend to
-immense distances; while delicate rays radiate in straight or curved
-lines from the spaces left in the polar regions between the wings. The
-corona has sometimes appeared as if divided by immense dark gaps,
-apparently free from luminous matter, and strongly resembling the dark
-rifts seen in the tails of comets. This was observed in Spain and Sicily
-during the total eclipse of the Sun in 1870. Different structures,
-forming wisps and streamers of great length, and interlaced in various
-ways, are sometimes present in the corona, while faint but more
-complicated forms, distantly resembling enormous solar protuberances
-with bright nuclei, have also been observed.
-
-As the Moon continues its eastward progress, it gradually covers the
-chromosphere and the solar protuberances on the eastern side of the Sun;
-while, at the same time, the protuberances and the chromosphere on the
-opposite limb gradually appear from under the retreating Moon. Then, the
-thin arc of the reversing layer is visible for an instant, and is
-instantly followed by the appearance of a point of dazzling white light,
-succeeded immediately by the apparition of Daily's beads on each side,
-and totality is over, with this third contact. The corona continues to
-be visible on the eastern side of the Sun for several minutes longer,
-and then rapidly vanishes.
-
-The thin solar crescent increases in breadth as the Moon advances;
-while, at the same time, the darkness and gloom spread over nature
-gradually disappear, and terrestrial objects begin to resume their
-natural appearance. Finally the limb of the Moon separates from that of
-the Sun at the instant of "fourth contact," and the eclipse is over.
-
-The phenomena exhibited by the corona in different eclipses are very
-complex, and, so far, they have not been sufficiently studied to enable
-us to understand the true nature of the solar atmosphere. From the
-spectral analysis of the corona, and the phenomena of polarization, it
-has been learned, at least, that while the matter composing the upper
-part of the solar atmosphere is chiefly composed of an unknown
-substance, producing the green line 1474, its lower part is mainly
-composed of hydrogen gas at different temperatures, a part of which is
-self-luminous, while the other part only reflects the solar light. But
-the proportion of the gaseous particles emitting light, to those simply
-reflecting it, is subject to considerable variations in different
-eclipses. At present it would seem that in years of great solar
-disturbances, the particles emitting light are found in greater quantity
-in the corona than those reflecting it; but further observations will be
-required to confirm these views.
-
-It is very difficult to understand how the corona, which in certain
-eclipses extends only one diameter of the Sun, should, in other cases,
-as in 1878, extend to the enormous distance of twelve times the same
-diameter. Changes of such magnitude in the solar atmosphere, if due to
-the operation of forces with which we are acquainted, cannot yet be
-accounted for by what is known of such forces. Their causes are still as
-mysterious as those concerned in the production of the monstrous tails
-displayed by some comets on their approach to the Sun.
-
-Plate 3, representing the total eclipse of the Sun of July 29th, 1878,
-was drawn from my observations made at Creston, Wyoming Territory, for
-the Naval Observatory. The eclipse is represented as seen in a
-refracting telescope, having an aperture of 6⅓ inches, and as it
-appeared a few seconds before totality was over, and when the
-chromosphere was visible on the western limb of the Sun. The two long
-wings seen on the east and west side of the Sun, appeared considerably
-larger in the sky than they are represented in the picture.
-
-
-
-
-THE AURORA BOREALIS
-
-PLATE IV
-
-
-The name of Polar Auroras is given to certain very remarkable luminous
-meteoric phenomena which appear at intervals above the northern or the
-southern horizons of both hemispheres of the Earth. When the phenomenon
-is produced in our northern sky, it is called "Aurora Borealis," or
-"Northern Lights;" and when it appears in the southern sky, it is called
-"Aurora Australis," or southern aurora.
-
-Marked differences appear in the various auroras observed from our
-northern latitudes. While some simply consist in a pale, faint
-luminosity, hardly distinguishable from twilight, others present the
-most gorgeous and remarkable effects of brightness and colors.
-
-A great aurora is usually indicated in the evening soon after twilight,
-by a peculiar grayish appearance of the northern sky just above the
-horizon. The grayish vapors giving that appearance, continuing to form
-there, soon assume a dark and gloomy aspect, while they gradually take
-the form of a segment of a circle resting on the horizon. At the same
-time that this dark segment is forming, a soft pearly light, which seems
-to issue from its border, spreads up in the sky, where it gradually
-vanishes, being the brightest at its base. This arc of light, gradually
-increasing in extent as well as in brightness, reaches sometimes as far
-as the polar star. On some rare occasions, one or two, and even three,
-concentric arches of bright light form one above the other over the dark
-segment, where they appear as brilliant concentric rainbows. While the
-aurora continues to develop and spread out its immense arc, the border
-of the dark segment loses its regularity and appears indented at several
-places by patches of light, which soon develop into long, narrow,
-diverging rays and streamers of great beauty. For the most part the
-auroral light is either whitish or of a pale, greenish tint; but in some
-cases it exhibits the most beautiful colors, among which the red and
-green predominate. In these cases the rays and streamers, which are
-usually of different colors, produce the most magnificent effects by
-their continual changes and transformations.
-
-The brightness and extent of the auroral rays are likewise subject to
-continual changes. An instant suffices for their development and
-disappearance, which may be succeeded by the sudden appearance of others
-elsewhere, as though the original streamers had been swiftly transported
-to a new place while invisible. It frequently happens that all the
-streamers seem to move sidewise, from west to east, along the arch,
-continuing meanwhile to exhibit their various changes of form and color.
-For a time, these appearances of motion continue to increase, a
-succession of streamers alternately shooting forth and again fading,
-when a sudden lull occurs, during which all motion seems to have ceased.
-The stillness then prevailing is soon succeeded by slight pulsations of
-light, which seem to originate on the border-of the dark segment, and
-are propagated upwards along the streamers, which have now become more
-numerous and active. Slow at first, these pulsations quicken by degrees,
-and after a few minutes the whole northern sky seems to be in rapid
-vibration. The lively upward and downward movement of these streamers
-entitles them to the name of "merry dancers" given them in northern
-countries where they are frequent.
-
-Long waves of light, quickly succeeded by others, are propagated in an
-instant from the horizon to the zenith; these, in their rapid passage,
-cause bends and curves in the streamers, which then, losing their
-original straightness, wave and undulate in graceful folds, resembling
-those of a pennant in a gentle breeze. Although the coruscations add to
-the grandeur of the spectacle, they tend to destroy the diverging
-streamers, which, being disconnected from the dark segment, or torn in
-various ways, are, as it were, bodily carried up towards the zenith.
-
-In this new phase the aurora is transformed into a glorious crown of
-light, called the "Corona." From this corona diverge in all directions
-long streamers of different colors and forms, gracefully undulating in
-numerous folds, like so many banners of light. Some of the largest of
-these streamers appear like fringes composed of short transverse rays of
-different intensity and colors, producing the most fantastic effects,
-when traversed by the pulsations and coruscations which generally run
-across these rays during the great auroral displays.
-
-The aurora has now attained its full development and beauty. It may
-continue in this form for half an hour, but usually the celestial fires
-begin to fade at the end of fifteen or twenty minutes, reviving from
-time to time, but gradually dying out. The northern sky usually appears
-covered by gray and luminous streaks and patches after a great aurora,
-these being occasionally rekindled, but more often they gradually
-disappear, and the sky resumes its usual appearance.
-
-The number of auroras which develop a corona near the zenith is
-comparatively small in our latitudes; but many of them, although not
-exhibited on so grand a scale, are nevertheless very interesting. On
-some very rare occasions the auroral display has been confined almost
-exclusively to the dark segment, which appeared then as if pierced along
-its border by many square openings, like windows, through which appeared
-the bright auroral light.
-
-
-[PLATE IV.--AURORA BOREALIS.
-
-As observed March 1, 1872, at 9h. 25m. P.M.]
-
-
-Among the many auroras which I have had occasion to observe, none are
-more interesting, excepting the type first described, than those which
-form an immense arch of light spanning the heavens from East to West.
-This form of aurora, which is quite rare, I last observed on September
-12th, 1881. All the northern sky was covered with light vapors, when a
-small auroral patch appeared in the East at about 20° above the
-horizon. This patch of light, gradually increasing westward, soon
-reached the zenith, and continued its onward progress until it arrived
-at about 20° above the western horizon, where it stopped. The aurora
-then appeared as a narrow, wavy band of light, crossed by numerous
-parallel rays of different intensity and color. These rays seemed to
-have a rapid motion from West to East along the delicately-fringed
-streamer, which, on the whole, moved southward, while its extremities
-remained undisturbed. Aside from the apparent displacement of the
-fringes, a singular vibrating motion was observed in the auroral band,
-which was traversed by pulsations and long waves of light. The phenomena
-lasted for about twenty minutes, after which the arch was broken in many
-places, and it slowly vanished.
-
-The aurora usually appears in the early part of the evening, and attains
-its full development between ten and eleven o'clock. Although the
-auroral light may have apparently ceased, yet the phenomenon is not at
-an end, as very often a solitary ray is visible from time to time; and
-even towards morning these rays sometimes become quite numerous. On some
-occasions the phenomenon even continues through the following day, and
-is manifested by the radial direction of the cirrus-clouds in the
-heights of our atmosphere. In 1872 I, myself, observed an aurora which
-apparently continued for two or three consecutive days and nights. In
-August, 1859, the northern lights remained visible in the United States
-for a whole week.
-
-The height attained by these meteors is considerable, and it is now
-admitted that they are produced in the rarefied air of the upper regions
-of our atmosphere. From the researches of Professor Elias Loomis on the
-great auroras observed in August and September, 1859, it was ascertained
-that the inferior part of the auroral rays had an altitude of 46 miles,
-while that of their summits was 428 miles. These rays had, therefore, a
-length of 382 miles. From the observation of thirty auroral displays, it
-has been found that the mean height attained by the summit of these
-streamers above the Earth's surface was 450 miles.
-
-But if the auroral streamers are generally manifested at great heights
-in our atmosphere, it would appear from the observations of persons
-living in the regions where the auroras are most frequent, as also from
-those who have been stationed in high northern and southern latitudes,
-that the phenomenon sometimes descends very low. Both Sabine and Parry
-saw the auroral rays projected on a distant mountain; Ross saw them
-almost at sea-level projected on the polar ice; while Wrangel, Franklin,
-and others observed similar phenomena. Dr. Hjaltalin, who has lived in
-latitude 64° 46' north, and has made a particular study of the aurora,
-on one occasion saw the aurora much below the summit of a hill 1,600
-feet high, which was not very far off.
-
-The same aurora is sometimes observed on the same night at places very
-far distant from one another. The great aurora borealis of August 28th,
-1859, for instance, was seen over a space occupying 150° in
-longitude--from California to the Ural Mountains in Russia. It even
-appears now very probable that the phenomenon is universal on our globe,
-and that the northern lights observed in our hemisphere are simultaneous
-with the aurora australis of the southern hemisphere. The aurora of
-September 2d, 1859, was observed all through North and South America,
-the Sandwich Islands, Australia, and Africa; the streamers and
-pulsations of light of the north pole responding to the rays and
-coruscations of the south pole. Of thirty-four auroras observed at
-Hobart Town, in Tasmania, twenty-nine corresponded with aurora borealis
-observed in our hemisphere.
-
-The auroral phenomena, although sometimes visible within the tropics,
-are, however, quite rare in these regions. For the most part they are
-confined within certain zones situated in high latitudes north and
-south. The zone where they are most frequent in our hemisphere forms an
-ellipse, which has the north pole at one of its foci; while the other is
-situated somewhere in North America, in the vicinity of the magnetic
-pole. The central line of the zone upon which the auroras seem to be
-most frequent passes from the northern coast of Alaska through Hudson's
-Bay and Labrador to Iceland, and then follows the northern coast of
-Europe and Asia. The number of auroras diminishes as the observer
-recedes from this zone, and it is only in exceptional cases that they
-are seen near the equator. Near the pole the phenomenon is less frequent
-than it is in the region described. In North America we occupy a
-favorable position for the observation of auroras, as we are nearer the
-magnetic poles than are the Europeans and Asiatics, and we consequently
-have a greater number of auroras in corresponding latitudes.
-
-The position of the dark auroral segment varies with the place occupied
-by the observer, and its centre always corresponds with the magnetic
-meridian. In our Eastern States the auroral segment appears a little to
-the west of the north point; but as the observer proceeds westward it
-gradually approaches this point, and is due north when seen from the
-vicinity of Lake Winnipeg. At Point Barrow, in the extreme north-west of
-the United States, the aurora is observed in the east. In Melville
-Islands, Parry saw it in the south; while in Greenland it is directly in
-the west.
-
-It is stated that auroras are more numerous about the equinoxes than
-they are at any other seasons; and also, when the earth is in perigee,
-than when it is in apogee. An examination which I have made of a
-catalogue by Professor Loomis, comprising 4,137 auroras observed in the
-temperate zone of our hemisphere from 1776 to 1873, sustains this
-statement. During this period, one hundred more auroras were recorded
-during each of the months comprising the equinoxes, than during any
-other months of the year; while eighty more auroras were observed when
-the earth was in perigee, than when it was in apogee. But to establish
-the truth of this assertion on a solid basis, more observations in both
-hemispheres will be required.
-
-The aurora is not simply a terrestrial phenomenon, but is associated in
-some mysterious way with the conditions of the Sun's surface. It is a
-well-known fact that terrestrial magnetism is influenced directly by the
-Sun, which creates the diurnal oscillations of the magnetic needle.
-Between sunrise and two o'clock, the north pole of the needle moves
-towards the west in our northern hemisphere, and in the afternoon and
-evening it moves the other way. These daily oscillations of the needle
-are not uniform in extent; they have a period of regular increase and
-decrease. At a given place the daily oscillations of the magnetic needle
-increase and decrease with regularity during a period which is equal to
-10⅓ years. As this period closely coincides with the Sun-spot period,
-the connection between the variation of the needle and these solar
-disturbances has been recognized.
-
-Auroral phenomena generally accompany the extraordinary perturbations in
-the oscillations of the magnetic needle, which are commonly called
-"magnetic storms," and the greater the auroral displays, the greater are
-the magnetic perturbations. Not only is the needle subject to unusual
-displacements during an aurora, but its movements seem to be
-simultaneous with the pulsations and waving motions of the delicate
-auroral streamers in the sky. When the aurora sends forth a coruscation,
-or a streamer in the sky, the magnetic needle responds to it by a
-vibration. The inference that the auroral phenomena are connected with
-terrestrial magnetism is further supported by the fact that the centre
-of the corona is always situated exactly in the direction of that point
-in the heavens to which the dipping needle is directed.
-
-It has been found that the aurora is a periodical phenomenon, and that
-its period corresponds very closely with those of the magnetic needle
-and Sun-spots. The years which have the most Sun-spots and magnetic
-disturbances have also the most auroras. There is an almost perfect
-similarity between the courses of the three sets of phenomena, from
-which it is concluded that the aurora is connected in some mysterious
-way with the action of the Sun, as well as with the magnetic condition
-of the earth.
-
-A very curious observation, which has been supposed to have some
-connection with this subject, was made on Sept. 1st, 1859, by Mr.
-Carrington and Mr. Hodgson, in England. While these observers, who were
-situated many miles from one another, were both engaged at the same time
-in observing the same Sun-spot, they suddenly saw two luminous spots of
-dazzling brilliancy bursting into sight from the edge of the Sun-spot.
-These objects moved eastward for about five minutes, after which they
-disappeared, having then traveled nearly 34,000 miles. Simultaneously
-with these appearances, a magnetic disturbance was registered at Kew by
-the self-registering magnetic instruments. The very night that followed
-these observations, great magnetic perturbations, accompanied by
-brilliant auroral displays, were observed in Europe. A connection
-between the terrestrial magnetism and the auroral phenomena is further
-proved by the fact that, before the appearance of an aurora, the
-magnetic intensity of our globe considerably increases, but diminishes
-as soon as the first flashes show themselves.
-
-The auroral phenomena are also connected in some way with electricity,
-and generate serious disturbances in the electric currents traversing
-our telegraphic lines, which are thus often rendered useless for the
-transmission of messages during great auroral displays. It sometimes
-happens, however, during such displays, that the telegraphic lines can
-be operated for a long distance, without the assistance of a battery;
-the aurora, or at least its cause, furnishing the necessary electric
-current for the working of the line. During auroras, the telephonic
-lines are also greatly affected, and all kinds of noises and
-crepitations are heard in the instruments.
-
-Two observations of mine, which may have a bearing on the subject,
-present some interest, as they seem to indicate the action of the aurora
-on some of the clouds of our atmosphere. On January 6th, 1872, after I
-had been observing a brilliant aurora for over one hour, an isolated
-black cumulus cloud appeared at a little distance from the western
-extremity of the dark auroral segment. This cloud, probably driven by
-the wind, rapidly advanced eastward, and was soon followed by a
-succession of similar clouds, all starting from the same point. All
-these black clouds apparently followed the same path, which was not a
-straight line, but parallel to and concentric with the border of the
-dark auroral segment. When the first cloud arrived in the vicinity of
-the magnetic meridian passing through the middle of the auroral arc, it
-very rapidly dissolved, and on reaching this meridian became invisible.
-The same phenomenon was observed with the succession of black clouds
-following, each rapidly dissolving as it approached the magnetic
-meridian. This phenomenon of black clouds vanishing like phantoms in
-crossing the magnetic meridian, was observed for nearly an hour. On June
-17th, 1879, I observed a similar phenomenon during a fine auroral
-display. About midway between the horizon and the polar star, but a
-little to the west of the magnetic meridian, there was a large black
-cumulo-stratus cloud which very slowly advanced eastward. As it
-progressed in that direction, its eastern extremity was dissolved in
-traversing the magnetic meridian; while, at the same time, several short
-and quite bright auroral rays issued from its western extremity, which
-in its turn dissolved rapidly, as if burned or melted away in the
-production of the auroral flame.
-
-It seems to be a well observed fact, that during auroras, a strong
-sulphurous odor prevails in high northern latitudes. According to Dr.
-Hjaltalin, during these phenomena, "the ozone of the atmosphere
-increases considerably, and men and animals exposed out of doors emit a
-sulphurous odor when entering a heated room." The Esquimaux and other
-inhabitants of the northern regions assert that great auroras are
-sometimes accompanied by crepitations and crackling noises of various
-sorts. Although these assertions have been denied by several travelers
-who have visited the regions of these phenomena, they are confirmed by
-many competent observers. Dr. Hjaltalin, who has heard these noises
-about six times in a hundred observations, says that they are especially
-audible when the weather is clear and calm; but that when the atmosphere
-is agitated they are not heard. He compares them to the peculiar sound
-produced by a silk cloth when torn asunder, or to the crepitations of
-the electric machine when its motion is accelerated. "When the auroral
-light is much agitated and the streamers show great movements, it is
-then that these noises are heard at different places in the atmosphere."
-
-The spectrum of the auroral light, although it varies with almost every
-aurora, always shows a bright green line on a faint continuous spectrum.
-In addition to this green line I have frequently observed four broad
-diffused bands of greater refrangibility in the spectra of some auroras.
-In two cases, when the auroras appeared red towards the west, the
-spectrum showed a bright red line, in addition to the green line and the
-broad bands described. These facts evidently show that the light of the
-aurora is due to the presence of luminous vapors in our atmosphere; and
-it may reasonably be supposed that these vapors are rendered luminous by
-the passage of electric discharges through them.
-
-
-
-
-THE ZODIACAL LIGHT
-
-PLATE V
-
-
-In our northern latitudes may be seen, on every clear winter and spring
-evening, a column of faint, whitish, nebulous light, rising obliquely
-above the western horizon. A similar phenomenon may also be observed in
-the east, before day-break, on any clear summer or autumn night. To this
-pale, glimmering luminosity the name of "Zodiacal Light" has been given,
-from the fact that it lies in the zodiac along the ecliptic.
-
-In common with all the celestial bodies, the zodiacal light participates
-in the diurnal motion of the sky, and rises and sets with the
-constellations in which it appears. Aside from this apparent motion, it
-is endowed with a motion of its own, accomplished from west to east, in
-a period of a year. In its motion among the stars, the zodiacal light
-always keeps pace with the Sun, and appears as if forming two faint
-luminous wings, resting on opposite sides of this body. In reality it
-extends on each side of the Sun, its axis lying very nearly in the plane
-of the ecliptic.
-
-In our latitudes the phenomena can be observed most advantageously
-towards the equinoxes, in March and September, when twilight is of short
-duration. As we proceed southward it becomes more prominent, and
-gradually increases in size and brightness. It is within the tropical
-regions that the zodiacal light acquires all its splendor: there it is
-visible all the year round, and always appears very nearly perpendicular
-to the horizon, while at the same time its proportions and brilliancy
-are greatly increased.
-
-
-[PLATE V.--THE ZODIACAL LIGHT.
-
-Observed February 20, 1876]
-
-
-The zodiacal light appears under the form of a spear-head, or of a
-narrow cone of light whose base apparently rests on the horizon, while
-its summit rises among the zodiacal constellations. In general
-appearance it somewhat resembles the tail of a large comet whose head is
-below the horizon. The most favorable time to observe this phenomenon in
-the evening, is immediately after the last trace of twilight has
-disappeared; and in the morning, one or two hours before twilight
-appears. When observed with attention, it is seen that the light of the
-zodiacal cone is not uniform, but gradually increases in brightness
-inwardly, especially towards its base, where it sometimes surpasses in
-brilliancy the brightest parts of the Milky-Way. In general, its
-outlines are vague and very difficult to make out, so gradually do they
-blend with the sky. On some favorable occasions, the luminous cone
-appears to be composed of several distinct concentric conical layers,
-having different degrees of brightness, the inner cone being the most
-brilliant of all. There is a remarkable distinction between the evening
-and morning zodiacal light. In our climate, the morning light is pale,
-and never so bright nor so extended as the evening light.
-
-In general, the zodiacal light is whitish and colorless, but in some
-cases it acquires a warm yellowish or reddish tint. These changes of
-color may be accidental and due to atmospheric conditions, and not to
-actual change in the color of the object. Although the zodiacal light is
-quite bright, and produces the impression of having considerable depth,
-yet its transparency is great, since all the stars, except the faint
-ones, can be seen through its substance.
-
-The zodiacal light is subject to considerable variations in brightness,
-and also varies in extent, the apex of its cone varying in distance from
-the Sun's place, from 40 to 90 degrees. These variations cannot be
-attributed to atmospheric causes alone, some of them being due to real
-changes in the zodiacal light itself, whose light and dimensions
-increase or decrease under the action of causes at present unknown. From
-the discussion of a series of observations on the zodiacal light made at
-Paris and Geneva, it appears certain that its light varies from year to
-year, and sometimes even from day to day, independently of atmospheric
-causes. Some of my own observations agree with these results, and one of
-them, at least, seems to indicate changes even more rapid. On December
-18th, 1875, I observed the zodiacal light in a clear sky free from any
-vapors, at six o'clock in the evening. At that time, the point of its
-cone was a little to the north of the ecliptic, at a distance of about
-90 degrees from the Sun's place. Ten minutes later, its summit had sunk
-down 35 degrees, the cone then being reduced to nearly one-half of its
-original dimensions. Ten minutes later, it had risen 25 degrees, and was
-then 80 degrees from the Sun's place, where it remained all the evening.
-On March 22d, 1878, the sky was very clear and the zodiacal light was
-bright when I observed it, at eight o'clock. At that moment the apex of
-the cone of light was a little to the south of the Pleiades, but this
-cone presented an unusual appearance never noticed by me before, its
-northern border appearing much brighter and sharper than usual, while at
-the same time its axis of greatest brightness appeared to be much nearer
-to this northern border than it was to the southern. After a few minutes
-of observation it became evident that the northern border was extending
-itself, as stars which were at some distance from it became gradually
-involved in its light. At the same time that this border spread
-northward, it seemed to diffuse itself, and after a time the cone
-presented its usual appearance, having its southern border brighter and
-better defined than the other. It would have been impossible to
-attribute this sudden change to an atmospheric cause, since only one of
-the borders of the cone participated in it, and since some very faint
-stars near this northern border were not affected in the least while the
-phenomenon occurred. Besides these observations, Cassini, Mairan,
-Humboldt, and many other competent observers have seen pulsations,
-coruscations and bickerings in the light of the cone, which they thought
-could not be attributed to atmospheric causes. It has also been observed
-that at certain periods the zodiacal light has shone with unusual
-intensity for months together.
-
-When this phenomenon is observed from the tropical regions, it is found
-that its axis of symmetry always corresponds with its axis of greatest
-brightness, and that both lie in the plane of the ecliptic, which
-divides its cone into two equal parts. But when the zodiacal light is
-observed in our latitude, the axis of symmetry does not correspond with
-the axis of greatest brightness, and both axes are a little to the north
-of this plane, the axis of symmetry being the farther removed.
-Furthermore, as already stated, the southern border of the cone always
-appears better defined and brighter than the corresponding northern
-margin. It is very probable, if not absolutely certain, that these
-phenomena are exactly reversed when the zodiacal light is observed from
-corresponding latitudes in the southern hemisphere, and that there, its
-axes, both of symmetry and of greatest brightness, appear south of the
-ecliptic, while the northern margin is the brightest. This seems to be
-established by the valuable observations of Rev. George Jones, made on
-board the U. S. steam frigate Mississippi, in California, Japan, and the
-Southern Ocean. "When I was north of the ecliptic," says this observer,
-"the greatest part of the light of the cone appeared to the north of
-this line; when I was to the south of the ecliptic, it appeared to be
-south of it; while when my position was on the ecliptic, or in its
-vicinity, the zodiacal cone was equally divided by this line."
-
-Besides the zodiacal light observed in the East and West, some observers
-have recognized an exceedingly faint, luminous, gauzy band, about 10 or
-12 degrees wide, stretching along the ecliptic from the summit of the
-western to that of the eastern zodiacal cone. This faint narrow belt has
-been called the Zodiacal Band. It has been recognized by Mr. H. C.
-Lewis, who has made a study of this phenomenon, that the zodiacal band
-has its southern margin a little brighter and a little sharper than the
-northern border. This observation is in accordance with similar
-phenomena observed in the zodiacal light, and may have considerable
-importance.
-
-In 1854, Brorsen recognized a faint, roundish, luminous spot in a point
-of the heavens exactly opposite to the place occupied by the Sun, which
-he has called "Gegenschein," or counter-glow. This luminous spot has
-sometimes a small nucleus, which is a little brighter than the rest.
-Night after night this very faint object shifts its position among the
-constellations, keeping always at 180 degrees from the Sun. The position
-of the counter-glow, like that of the zodiacal light and zodiacal band,
-is not precisely on the plane of the ecliptic, but a little to the north
-of this line. It is very probable that near the equator the phenomenon
-would appear different and there would correspond with this plane.
-
-There seems to be some confusion among observers in regard to the
-spectrum of the zodiacal light. Some have seen a bright green line in
-its spectrum, corresponding to that of the aurora borealis; while others
-could only see a faint grayish continuous spectrum, which differs,
-however, from that of a faint solar light, by the fact that it presents
-a well-defined bright zone, gradually blending on each side with the
-fainter light of the continuous spectrum. I have, myself, frequently
-observed the faint continuous spectrum of the zodiacal light, and on one
-occasion recognized the green line of the aurora; but it might have been
-produced by the aurora itself, as yet invisible to the eye, and not by
-the zodiacal light, since, later in the same evening, there was a
-brilliant auroral display. If it were demonstrated that this green line
-exists in the spectrum of the zodiacal light, the fact would have
-importance, as tending to show that the aurora and the zodiacal light
-have a common origin.
-
-Rev. Geo. Jones describes a very curious phenomenon which he observed
-several times a little before the moon rose above the horizon. The
-phenomenon consisted in a short, oblique, luminous cone rising from the
-Moon's place in the direction of the ecliptic. This phenomenon he has
-called the Moon Zodiacal Light. In 1874, I had an opportunity to observe
-a similar phenomenon when the Moon was quite high in the sky. By taking
-the precaution to screen the Moon's disk by the interposition of some
-buildings between it and my eye, I saw two long and narrow cones of
-light parallel to the ecliptic issuing from opposite sides of our
-satellite. The phenomenon could not possibly be attributed to vapors in
-our atmosphere, since the sky was very clear at the moment of the
-observation. Later on, these appendages disappeared with the formation
-of vapors near the Moon, but they reappeared an hour later, when the sky
-had cleared off, and continued visible for twenty minutes longer, and
-then disappeared in a clear sky.
-
-Although the zodiacal light has been studied for over two centuries, no
-wholly satisfactory explanation of the phenomenon has yet been given.
-Now, as in Cassini's time, it is generally considered by astronomers to
-be due to a kind of lens-shaped ring surrounding the Sun, and extending
-a little beyond the Earth's orbit. This ring is supposed to lie in the
-plane of the ecliptic, and to be composed of a multitude of independent
-meteoric particles circulating in closed parallel orbits around the Sun.
-But many difficulties lie in the way of this theory. It seems as
-incompetent to explain the slow and rapid changes in the light of this
-object as it is to explain the contractions and extensions of its cone.
-It fails, moreover, to explain the flickering motions, the coruscations
-observed in its light, or the displacement of its cone and of its axes
-of brightness and symmetry by a mere change in the position of the
-observer. Rev. Geo. Jones, unable to explain by this theory the
-phenomena which came under his observation, has proposed another, which
-supposes the zodiacal light to be produced by a luminous ring
-surrounding the Earth, this ring not extending as far as the orbit of
-the Moon. But this theory also fails in many important points, so that
-at present no satisfactory explanation of the phenomenon can be given.
-
-As the phenomenon is connected in some way with the Sun, and as we have
-many reasons to believe this body to be always more or less electrified,
-it might be supposed that the Sun, acting by induction on our globe,
-develops feeble electric currents in the rarefied gases of the superior
-regions of our atmosphere, and there forms a kind of luminous ridge
-moving with the Sun in a direction contrary to the diurnal motion, and
-so producing the zodiacal light. On this hypothesis, the counter-glow
-would be the result of a smaller cone of light generated by the solar
-induction on the opposite point of the Earth.
-
-Plate 5, which sufficiently explains itself, represents the zodiacal
-light as it appeared in the West on the evening of February 20th, 1876.
-All the stars are placed in their proper position, and their relative
-brightness is approximately shown by corresponding variations in
-size--the usual and almost the only available means of representation.
-Of course, it must be remembered that a star does not, in fact, show any
-disk even in the largest telescopes, where it appears as a mere point of
-light, having more or less brilliancy. The cone of light rises obliquely
-along the ecliptic, and the point forming its summit is found in the
-vicinity of the well-known group of stars, called the Pleiades, in the
-constellation of Taurus, or the Bull.
-
-
-
-
-THE MOON
-
-PLATE VI
-
-
-In its endless journey through space, our globe is not solitary, like
-some of the planets, but is attended by the Moon, our nearest celestial
-neighbor. Although the Moon does not attain to the dignity of a planet,
-and remains a secondary body in the solar system, yet, owing to its
-proximity to our globe, and to the great influence it exerts upon it by
-its powerful attraction, it is to us one of the most important celestial
-bodies.
-
-While the Moon accompanies the Earth around the Sun, it also revolves
-around the Earth at a mean distance of 238,800 miles. For a celestial
-distance this is only a trifling one; the Earth in advancing on its
-orbit travels over such a distance in less than four hours. A cannon
-ball would reach our satellite in nine days; and a telegraphic dispatch
-would be transmitted there in 1½ seconds of time, if a wire could be
-stretched between us and the Moon.
-
-Owing to the ellipticity of the Moon's orbit, its distance from the
-Earth varies considerably, our satellite being sometimes 38,000 miles
-nearer to us than it is at other times. These changes in the distance of
-the Moon occasion corresponding changes from 29' to 33' in its apparent
-diameter. The real diameter of the Moon is 2,160 miles, or a little over
-one-quarter the diameter of our globe; our satellite being 49 times
-smaller than the Earth.
-
-The mean density of the materials composing the Moon is only
-⁶⁄₁₀ that of the materials composing the Earth, and the force of
-gravitation at the surface of our satellite is six times less than it is
-at the surface of our globe. If a person weighing 150 lbs. on our Earth
-could be transported to the Moon, his weight there would be only 25 lbs.
-
-The Moon revolves around the Earth in about 27⅓ days, with a mean
-velocity of one mile per second, the revolution constituting its
-sidereal period. If the Earth were motionless, the lunar month would be
-equal to the sidereal period; but owing to its motion in space, the Sun
-appears to move with the Moon, though more slowly, so that after having
-accomplished one complete revolution, our satellite has yet to advance
-2¼ days before reaching the same apparent position in regard to the
-Earth and the Sun that it had at first. The interval of time comprised
-between two successive New Moons, which is a little over 29½ days,
-constitutes the synodical period of the Moon, or the lunar month.
-
-The Moon is not a self-luminous body, but, like the Earth and the
-planets, it reflects the light which it receives from the Sun, and so
-appears luminous. That such is the case is sufficiently demonstrated by
-the phases exhibited by our satellite in the course of the lunar month.
-Every one is familiar with these phases, which are a consequence of the
-motion of the Moon around the Earth. When our satellite is situated
-between us and the Sun, it is New Moon; since we cannot see its
-illuminated side, which is then turned away from us towards the Sun.
-When, on the contrary, it reaches that point of its orbit which, in
-regard to us, is opposite to the Sun's place, it is Full Moon; since
-from the Earth we can only see the fully illuminated side of our
-satellite. Again, when the Moon arrives at either of the two opposite
-points of its orbit, the direction of which from the Earth is at right
-angles with that of the Sun, it is either the First or the Last Quarter;
-since in these positions we can only see one-half of its illuminated
-disk.
-
-The curve described by the Moon around the Earth lies approximately in a
-plane, this plane being inclined about 5° to the ecliptic. Since our
-satellite, in its motion around us and the Sun, closely follows the
-ecliptic, which is inclined 23½° to the equator, it results that when
-this plane is respectively high or low in the sky, the moon is also high
-or low when crossing the meridian of the observer. In winter that part
-of the ecliptic occupied by the Sun is below the equator, and,
-consequently, the New Moons occurring in that season are low in the sky,
-since at New Moon our satellite must be on the same side of the ecliptic
-with the Sun. But the Full Moons in the same season are necessarily high
-in the sky, since a Full Moon can only occur when our satellite is on
-the opposite side of the ecliptic from the Sun, in which position it is,
-of course, as many degrees above the equator as the Sun is below. The
-Full Moon which happens nearest to the autumnal equinox is commonly
-called the Harvest Moon, from the fact that, after full, its delays in
-rising on successive evenings are very brief and therefore favorable for
-the harvest work in the evening. The same phenomenon occurs in every
-other lunar month, but not sufficiently near the time of Full Moon to be
-noticeable. When, in spring, a day or two after New Moon, our satellite
-begins to show its thin crescent, its position on the ecliptic is north
-as well as east of that occupied by the Sun; hence, its horns are nearly
-upright in direction, and give it a crude resemblance to a tipping bowl,
-from which many people who are unaware of its cause, and that this
-happens every year, draw conclusions as to the amount of rain to be
-expected.
-
-One of the most remarkable features of the Moon's motions is that our
-satellite rotates on its axis in exactly the same period of time
-occupied by its revolution around the Earth, from which it results that
-the Moon always presents to us the same face. To explain this
-peculiarity, astronomers have supposed that the figure of our satellite
-is not perfectly spherical, but elongated, so that the attraction of the
-Earth, acting more powerfully upon its nearest portions, always keeps
-them turned toward us, as if the Moon were united to our globe by a
-string. It is not exactly true, however, that the Moon always presents
-its same side to us, although its period of rotation exactly equals that
-of its revolution; since in consequence of the inclination of its axis
-of rotation to its orbit, combined with the irregularities of its
-orbital motion about us, apparent oscillations in latitude and in
-longitude, called librations, are created, from which it results that
-nearly ⁶⁄₁₀ of the Moon's surface is visible from the Earth at
-one time or another.
-
-The Moon is a familiar object, and every one is aware that our
-satellite, especially when it is fully illuminated, presents a variety
-of bright and dark markings, which, from their distant resemblance to a
-human face, are popularly known as "the man in the moon." A day or two
-after New Moon, when the thin crescent of our satellite is visible above
-the western horizon after sunset, the dark portion of its disk is
-plainly visible, and appears of a pale, ashy gray color, although not
-directly illuminated by the Sun. This phenomenon is due to the
-Earth-shine, or to that portion of solar light which the illuminated
-surface of our globe reflects to the dark side of the Moon, exactly in
-the same manner that the Moon-shine, on our Earth, is due to the solar
-light reflected to our globe by the illuminated Moon.
-
-Seen with a telescope of moderate power, or even with a good
-opera-glass, the Moon presents a peculiar mottled appearance, and has a
-strong resemblance to a globe made of plaster of Paris, on the surface
-of which numerous roundish, saucer-shaped cavities of various sizes are
-scattered at random. This mottled structure is better seen along the
-boundary line called the _terminator_, which divides the illuminated
-from the dark side of the Moon. The line of the terminator always
-appears jagged, and it is very easy to recognize that this irregularity
-is due to the uneven and rugged structure of the surface of our
-satellite.
-
-A glance at the Moon through a larger telescope shows that the bright
-spots recognized with the naked eye belong to very uneven and
-mountainous regions of our satellite, while the dark ones belong to
-comparatively smooth, low surfaces, comparable to those forming the
-great steppes and plains of the Earth. When examined with sufficient
-magnifying power, the white, rugged districts of the Moon appear covered
-over by numerous elevated craggy plateaus, mountain-chains, and deep
-ravines; by steep cliffs and ridges; by peaks of great height and
-cavities of great depth. This rugged formation, which is undoubtedly of
-volcanic origin, gives our satellite a desolate and barren appearance.
-The rugged tract occupies more than one-half of the visible surface of
-the Moon, forming several distinct masses, the principal of which occupy
-the south and south-western part of the disk. That this formation is
-elevated above the general level is proved by the fact that the
-mountains, peaks, and other objects which compose it, all cast a shadow
-opposite to the Sun; and further, that the length of these shadows
-diminishes with the elevation of the Sun above the lunar horizon.
-
-Since Galileo's time the surface of the Moon has been studied by a host
-of astronomers, and accurate maps of its topographical configuration
-have been made, and names given to all features of any prominence. It
-may even be said that in its general features, the visible surface of
-our satellite is now better known to us than is the surface of our own
-Earth.
-
-One of the most striking and common features of the mountainous
-districts of the Moon, is the circular, ring-like disposition of their
-elevated parts, which form numerous crater-like objects of different
-sizes and depths. Many thousands of crater-like objects are visible on
-the Moon through a good telescope, and, considering how numerous the
-small ones are, there is, perhaps, no great exaggeration in fixing their
-number at 50,000, as has been done by some astronomers. These volcanic
-regions of the Moon cannot be compared to anything we know, and far
-surpass in extent those of our globe. The number and size of the craters
-of our most important volcanic regions in Europe, in Asia, in North and
-South America, in Java, in Sumatra, and Borneo, are insignificant when
-compared with those of the Moon. The largest known craters on the Earth
-give only a faint idea of the magnitude of some of the lunar craters.
-The great crater Haleakala, in the Sandwich Islands, probably the
-largest of the terrestrial volcanoes, has a circumference of thirty
-miles, or a diameter of a little less than ten miles. Some of the great
-lunar craters, called walled plains, such as Hipparchus, Ptolemæus,
-etc., have a diameter more than ten times larger than that of Haleakala,
-that of the first being 115 miles and that of the last 100 miles. These
-are, of course, among the largest of the craters of the Moon, although
-there are on our satellite a great number of craters above ten miles in
-diameter.
-
-The crater-forms of the Moon have evidently appeared at different
-periods of time, since small craters are frequently found on the walls
-of larger ones; and, indeed, still smaller craters are not rarely seen
-on the walls of these last. The walls of the lunar craters are usually
-quite elevated above the surrounding surface, some of them attaining
-considerable elevations, especially at some points, which form peaks of
-great height. Newton, the loftiest of all, rises at one point to the
-height of 23,000 feet, while many others range from ten to twenty
-thousand feet in height. Several craters have their floor above the
-general surface--Plato, for instance. Wargentin has its floor nearly on
-a level with the summit of its walls, showing that at some period of its
-history liquid lavas, ejected from within, have filled it to the brim
-and then solidified. The floors of some of the craters are smooth and
-flat, but in general they are occupied by peaks and abrupt mountainous
-masses, which usually form the centre. Many of their outside walls are
-partly or wholly covered by numerous ravines and gullies, winding down
-their steep declivities, branching out and sometimes extending to great
-distances from their base. It would seem that these great volcanic
-mouths have at some time poured out torrents of lavas, which, in their
-descent, carved their passage by the deep gullies now visible.
-Sometimes, also, the crater slopes are strewn with debris, giving them a
-peculiar volcanic appearance.
-
-Notwithstanding their many points of similarity with the volcanoes of
-the Earth, the lunar craters differ from them in many particulars,
-showing that volcanic forces acting on different globes may produce
-widely different results. For example, the floors of terrestrial craters
-are usually situated at considerable elevations above the general
-surface, while those of the lunar craters are generally much depressed,
-the height of their walls being only about one-half the depth of their
-cavities. Again, while on the Earth the mass of the volcanic cones far
-exceeds the capacity of their openings, on the Moon it is not rare to
-see the capacity of the crater cavities exceeding the mass of the
-surrounding walls. On the Earth, the volcanic cones and mouths are
-comparatively regular and smooth, and are generally due to the
-accumulation of the ashes and the debris of all kinds which are ejected
-from the volcanic mouths. On the Moon, very few craters show this
-character, and for the most part their walls have a very different
-structure, being irregular, very rugged, and composed of a succession of
-concentric ridges, rising at many points to great elevations, and
-forming peaks of stupendous height. Again, many of the larger
-terrestrial craters have their interior occupied by a central cone, or
-several such cones, having a volcanic mouth on their summits; on the
-Moon such central cones are very rare. Although many of the large lunar
-craters have their interior occupied by central masses which have been
-often compared to the central cones of our great volcanoes, yet these
-objects have a very different character and origin. For the most part,
-they are mountainous masses of different forms--having very rarely any
-craters on them--and seem to have resulted from the crowding and lifting
-up of the crater floor by the phenomena of subsidence, of which these
-craters show abundant signs. Besides, the terrestrial craters are
-characterized by large and important lava streams, while on the Moon the
-traces of such phenomena are quite rare, and when they are shown, they
-generally differ from those of the Earth by their numerous and
-complicated ramifications, and also by the fact that many of these lava
-streamlets take their origin at a considerable distance from the crater
-slopes, and are grooved and depressed as if the burning liquids which
-are supposed to have produced them had subsequently disappeared, by
-evaporation or otherwise, leaving the furrow empty.
-
-The dark spots of the Moon, when viewed through a telescope, exhibit a
-totally different character, and show that they belong to a different
-formation from that of the brighter portions. These darker tracts do not
-seem to have had a direct volcanic origin like the latter, but rather
-appear to have resulted from the solidification of semi-fluid materials,
-which have overflowed vast areas at different times. The surface of this
-system is comparatively smooth and uniform, only some small craters and
-low ridges being seen upon it. The level and dark appearance of these
-areas led the ancient astronomers to the belief that they were produced
-by a liquid strongly absorbing the rays of light, and were seas like our
-seas. Accordingly, these dark surfaces were called _Maria_, or Seas, a
-name which it is convenient to retain, although it is well known to have
-originated in an error. The so-called seas of the Moon are evidently
-large flat surfaces similar to the deserts, steppes, pampas, and
-prairies of the Earth in general appearance. The great plains of the
-Moon are at a lower level than that of the other formation, and that
-which first attracts the observer's attention is the fact that they are
-surrounded almost on all sides by an irregular line of abrupt cliffs and
-mountain chains, showing phenomena of dislocation. This character of
-dislocation, which is general, and is visible everywhere upon the
-contours of the plains, seems to indicate that phenomena of subsidence,
-either slow or rapid, have occurred on the Moon; while, at the same
-time, the sunken surfaces were overflowed by a semi-fluid liquid, which
-solidified afterwards. The evidences of subsidence and overflowing
-become unmistakable when we observe that, along the borders of the gray
-plains, numerous craters are more or less embedded in the gray
-formation, only parts of the summit of their walls remaining visible, to
-attest that once large craters existed there. The farther from the
-border of the plain the vestiges of these craters are observed, the
-deeper they are embedded in the gray formation. That phenomena of
-subsidence have occurred on a grand scale on the Moon, is further
-indicated by the fact that the singular systems of fractures called
-clefts and rifts generally follow closely the outside border of the gray
-plains, often forming parallel lines of dislocation and fractures. In
-the interior regions of the gray formation, these fractures are
-comparatively rare.
-
-The gray, lava-like formation is obviously of later origin than the
-mountainous system to which belong the embedded craters above described.
-Its comparatively recent origin might also be inferred from the
-smallness of its craters and its low ridges. The few large craters
-observed on this formation evidently belong to the earlier system.
-
-The color of this system of gray plains is far from being uniform. In
-general appearance it is of a bluish gray, but when observed
-attentively, large areas appear tinted with a dusky olive-green, while
-others are slightly tinged with yellow. Some patches appear brownish,
-and even purplish. A remarkable example of the first case is seen on the
-surface, which encloses within a large parallelogram the two conspicuous
-craters, Aristarchus and Herodotus. This surface evidently belongs to a
-different system from that of the Oceanus Procellarum surrounding it,
-as, besides its color, which totally differs from that of the gray
-formation, its surface shows the rugged structure of the volcanic
-formation.
-
-When the Moon is full, some very curious white, luminous streaks are
-seen radiating from different centres, which, for the most part, are
-important craters, occupied by interior mountains. The great crater
-Tycho is the centre of the most imposing of the systems of white
-streaks. Some of the diverging rays of this great centre extend to a
-distance equal to one-quarter of the Moon's circumference, or about
-1,700 miles. The true nature of these luminous streaks is unknown, but
-it seems certain that they have their origin in the crater from which
-they diverge. They do not form any relief on the surface, and are seen
-going up over the mountains and steep walls of the crater, as well as
-down the ravines and on the floors of craters.
-
-The Moon seems to be deprived of an atmosphere; or, if it has any, it
-must be so excessively rare that its density is less than of the density
-of the Earth's atmosphere, since delicate tests afforded by the
-occultation of stars have failed to reveal its presence. Although no
-atmosphere of any consequence exists on the Moon, yet phenomena which I
-have observed seem to indicate the occasional presence there of vapors
-of some sort. On several occasions, I have seen a purplish light over
-some parts of the Moon, which prevented well-known objects being as
-distinctly seen as they were at other times, causing them to appear as
-if seen through a fog. One of the most striking of these observations
-was made on January 4th, 1873, on the crater Kant and its vicinity,
-which then appeared as if seen through luminous purplish vapors. On one
-occasion, the great crater Godin, which was entirely involved in the
-shadow of its western wall, appeared illuminated in its interior by a
-faint purplish light, which enabled me to recognize the structure of
-this interior. The phenomenon could not be attributed in this case to
-reflection, since the Sun, then just rising on the western wall of the
-crater, had not yet grazed the eastern wall, which was invisible. It is
-not impossible that a very rare atmosphere composed of such vapors
-exists in the lower parts of the Moon.
-
-If the Moon has no air, and no liquids of any sort, it seems impossible
-that its surface can maintain any form of life, either vegetable or
-animal, analogous to those on the Earth. In fact, nothing indicating
-life has been detected on the Moon--our satellite looking like a barren,
-lifeless desert. If life is to be found there at all, it must be of a
-very elementary nature. Aside from the want of air and water to sustain
-it, the climatic conditions of our satellite are very unfavorable for
-the development of life. The nights and days of the Moon are each equal
-to nearly fifteen of our days and nights. For fifteen consecutive
-terrestrial days the Sun's light is absent from one hemisphere of the
-Moon; while for the same number of days the Sun pours down on the other
-hemisphere its light and heat, the effects of which are not in any way
-mitigated by an atmosphere. During the long lunar nights the temperature
-must at least fall to that of our polar regions, while during its long
-days it must be far above that of our tropical zone. It has been
-calculated that during the lunar nights the temperature descends to 23°
-below zero, while during the days it rises to 468°, or 256° above the
-boiling point.
-
-It has been a question among astronomers whether changes are still
-taking place at the surface of the Moon. Aside from the fact that
-change, not constancy, is the law of nature, it does not seem doubtful
-that changes occur on the Moon, especially in view of the powerful
-influences of contraction and dilatation to which its materials are
-submitted by its severe alternations of temperature. From the distance
-at which we view our satellite, we cannot expect, of course, to be able
-to see changes, unless they are produced on a large scale. Theoretically
-speaking, the largest telescopes ever constructed ought to show us the
-Moon as it would appear to the naked eye from a distance of 40 miles;
-but in practice it is very different. The difficulty is in the fact
-that, while we magnify the surface of a telescopic image, we are unable
-to increase its light; so that, practically, in magnifying an object, we
-weaken its light proportionally to the magnifying power employed. The
-light of the Moon, especially near the terminator, where we almost
-always make our observations, is not sufficiently bright to bear a very
-high magnifying power, and only moderate ones can be applied to its
-study. What we gain by enlarging an object, we more than lose by the
-weakening of its light. Besides, a high magnifying power, by increasing
-the disturbances generally present in our atmosphere, renders the
-telescopic image unsteady and very indistinct. On the whole, the largest
-telescopes now in existence do not show us our satellite better than if
-we could see it with the naked eye from a distance of 300 miles or more.
-At such a distance only considerable changes would be visible.
-
-Notwithstanding these difficulties, it is believed that changes have
-been detected in Linné, Marius, Messier, and several other craters. An
-observation of mine seems to indicate that changes have recently taken
-place in the great crater Eudoxus. On February 20th, 1877, between 9h.
-30m. and 10h. 30m., I observed a straight, narrow wall crossing this
-crater from east to west, a little to the south of its centre. This wall
-had a considerable elevation, as was proved by the shadow it cast on its
-northern side. Towards its western end this wall appeared as a brilliant
-thread of light on the black shadow cast by the western wall of the
-crater. The first time I had occasion to observe this crater again,
-after this observation, was a year later, on February 17th, 1878; no
-traces of the wall were then detected. Many times since I have tried to
-find this narrow wall again, when the Moon presented the same phase and
-the same illumination, but always with negative results. It seems
-probable that this structure has crumbled down, yet it is very singular
-that so prominent a feature should not have been noticed before.
-
-
-[PLATE VI.--MARE HUMORUM.
-
-From a study made in 1875]
-
-
-The "Mare Humorum," or sea of moisture, as it is called, which is
-represented on Plate VI., is one of the smaller gray lunar plains. Its
-diameter, which is very nearly the same in all directions, is about 270
-miles, the total area of this plain being about 50,000 square miles. It
-is one of the most distinct plains of the Moon, and is easily seen with
-the naked eye on the left-hand side of the disk. The floor of the plain
-is, like that of the other gray plains, traversed by several systems of
-very extended but low hills and ridges, while small craters are
-disseminated upon its surface. The color of this formation is of a dusky
-greenish gray along the border, while in the interior it is of a lighter
-shade, and is of brownish olivaceous tint. This plain, which is
-surrounded by high clefts and rifts, well illustrates the phenomena of
-dislocation and subsidence. The double-ringed crater Vitello, whose
-walls rise from 4,000 to 5,000 feet in height, is seen in the upper
-left-hand corner of the gray plain. Close to Vitello, at the east, is
-the large broken ring-plain Lee, and farther east, and a little below,
-is a similarly broken crater called Doppelmayer. Both of these open
-craters have mountainous masses and peaks on their floor, which is on a
-level with that of the Mare Humorum. A little below, and to the left of
-these objects, is seen a deeply embedded oval crater, whose walls barely
-rise above the level of the plain. On the right-hand side of the great
-plain, is a long _fault_, with a system of fracture running along its
-border. On this right-hand side, may be seen a part of the line of the
-terminator, which separates the light from the darkness. Towards the
-lower right-hand corner, is the great ring-plain Gassendi, 55 miles in
-diameter, with its system of fractures and its central mountains, which
-rise from 3,000 to 4,000 feet above its floor. This crater slopes
-southward towards the plain, showing the subsidence to which it has been
-submitted. While the northern portion of the wall of this crater rises
-to 10,000 feet, that on the plain is only 500 feet high, and is even
-wholly demolished at one place where the floor of the crater is in
-direct communication with the plain. In the lower part of the _mare_,
-and a little to the west of the middle line, is found the crater
-Agatharchides, which shows below its north wall the marks of rills
-impressed by a flood of lava, which once issued from the side of the
-crater. On the left-hand side of the plain, is seen the half-demolished
-crater Hippalus, resembling a large bay, which has its interior strewn
-with peaks and mountains. On this same side can be seen one of the most
-important systems of clefts and fractures visible on the Moon, these
-clefts varying in length from 150 to 200 miles.
-
-
-
-
-ECLIPSES OF THE MOON
-
-PLATE VII
-
-
-Since the Moon is not a self-luminous body, but shines by the light
-which it borrows from the Sun, it follows that when the Sun's light is
-prevented from reaching its surface, our satellite becomes obscured. The
-Earth, like all opaque bodies exposed to sunlight, casts a shadow in
-space, the direction of which is always opposite to the Sun's place. The
-form of the Earth's shadow is that of a long, sharply-pointed cone,
-which has our globe for its base. Its length, varying with the distance
-of the Earth from the Sun, is, on an average, 855,000 miles, or 108
-times the terrestrial diameter. This conical shadow of the Earth,
-divided longitudinally by the plane of the ecliptic, lies half above and
-half below that plane, on which the summit of the shadow describes a
-whole circumference in the course of a year. If the Moon's orbit were
-not inclined to the ecliptic, our satellite would pass at every Full
-Moon directly through the Earth's shadow; but, owing to that
-inclination, it usually passes above or below the shadow. Twice,
-however, during each of its revolutions, it must cross the plane of the
-ecliptic, the points of its orbit where this happens being called nodes.
-Accordingly, if it is near a node at the time of Full Moon, it will
-enter the shadow of the Earth, and become either partly or wholly
-obscured, according to the distance of its centre from the plane of the
-ecliptic. The partial or total obscuration of the Moon's disk thus
-produced constitutes a partial or total eclipse of the Moon. The
-essential conditions for an eclipse of the Moon are, therefore, that our
-satellite must not only be full, but must also be at or very near one of
-its nodes.
-
-Although inferior in importance to the eclipses of the Sun, the eclipses
-of the Moon are, nevertheless, very interesting and remarkable
-phenomena, which never fail to produce a deep impression on the mind of
-the observer, inasmuch as they give him a clear insight into the silent
-motions of the planetary bodies.
-
-At the mean distance of the Moon from the Earth, the diameter of the
-conical shadow cast in space by our globe is more than twice as large as
-that of our satellite. But, besides this pure dark shadow of the Earth,
-its cone is enveloped by a partial shadow called "Penumbra," which is
-produced by the Sun's light being partially, but not wholly, cut off by
-our globe.
-
-While the Moon is passing into the penumbra, a slight reduction of the
-light of that part of the disk which has entered it, is noticeable. As
-the progress of the Moon continues, the reduction becomes more
-remarkable, giving the impression that rare and invisible vapors are
-passing over our satellite. Some time after, a small dark-indentation,
-marking the instant of first contact, appears on the eastern or
-left-hand border of the Moon, which is always the first to encounter the
-Earth's shadow, since our satellite is moving from west to east. The
-dark indentation slowly and gradually enlarges with the onward progress
-of the Moon into the Earth's shadow, while the luminous surface of its
-disk diminishes in the same proportion. The form of the Earth's shadow
-on the Moon's disk clearly indicates the rotundity of our globe by its
-circular outline. Little by little the dark segment covers the Moon's
-disk, and its crescent, at last reduced to a mere thread of light,
-disappears at the moment of the second contact. With this the phase of
-totality begins, our satellite being then completely involved in the
-Earth's shadow.
-
-The Moon remains so eclipsed for a period of time which varies with its
-distance from the Earth, and with the point of its orbit where it
-crosses the conical shadow. When it passes through the middle of this
-shadow, while its distance from our globe is the least, the total phase
-of an eclipse of the Moon may last nearly two hours. The left-hand
-border of our satellite having gone first into the Earth's shadow, is
-also the first to emerge, and, at the moment of doing so, it receives
-the Sun's light, and totality ends with the third contact. The lunar
-crescent gradually increases in breadth after its exit from the shadow,
-and finally the Moon recovers its fully illuminated disk as before, at
-the moment its western border leaves the Earth's shadow. Soon after, it
-passes out of the penumbra, and the eclipse is over. In total eclipses,
-the interval of time from the first to last contact may last 5h. 30m,
-but it is usually shorter.
-
-Soon after the beginning of an eclipse, the dark segment produced by the
-Earth's shadow on the Moon's disk generally appears of a dark grayish
-opaque color, but with the progress of the phenomenon, this dark tint is
-changed into a dull reddish color, which, gradually increasing, attains
-its greatest intensity when the eclipse is total. At that moment the
-color of the Moon is of a dusky, reddish, coppery hue, and the general
-features of the Moon's surface are visible as darker and lighter tints
-of the same color. It sometimes happens, however, that our satellite
-does not exhibit this peculiar coppery tint, but appears either blackish
-or bluish, in which case it is hardly distinguishable from the sky.
-
-It is very rare for the Moon to disappear completely during totality,
-and even when involved in the deepest part of the Earth's shadow, our
-satellite usually remains visible to the naked eye, or, at least, to the
-telescope. This phenomenon is to be attributed to the fact that the
-portion of the solar rays which traverse the lower strata of our
-atmosphere are strongly refracted, and bend inward in such a manner that
-they fall on the Moon, and sufficiently illuminate its surface to make
-it visible. The reddish color observed is caused by the absorption of
-the blue rays of light by the vapors which ordinarily-saturate the lower
-regions of our atmosphere, leaving only red rays to reach the Moon's
-surface. Of course, these phenomena are liable to vary with every
-eclipse, and depend almost exclusively on the meteorological conditions
-of our atmosphere.
-
-In some cases the phase of totality lasts longer than it should,
-according to calculation. This can be attributed to the fact that the
-Earth is enveloped in a dense atmosphere, in which opaque clouds of
-considerable extent are often forming at great elevations. Such strata
-of clouds, in intercepting the Sun's light, would have, of course, the
-effect of increasing the diameter of the Earth's shadow, in a direction
-corresponding to the place they occupy, and, if the Moon were moving in
-this direction, would increase the phase of total obscuration.
-
-The eclipses of the Moon, like those of the Sun, as shown above, have a
-cycle of 18 years, 11 days and 7 hours, and recur after this period of
-time in nearly the same order. They can, therefore, be approximately
-predicted by adding 18y. 11d. 7h. to the date of the eclipses which have
-occurred during the preceding period. During this cycle 70 eclipses will
-occur--41 being eclipses of the Sun and 29 eclipses of the Moon. At no
-time can there ever be more than seven eclipses in a year, and there are
-never less than two. When there are only two eclipses in a year, they
-are both eclipses of the Sun.
-
-Although the number of solar eclipses occurring at some point or other
-of the Earth's surface is greater than that of the eclipses of the Moon,
-yet at any single terrestrial station the eclipses of the Moon are the
-more frequent. While an eclipse of the Sun is only visible on a narrow
-belt, which is but a very small fraction of the hemisphere then
-illuminated by the Sun, an eclipse of the Moon is visible from all the
-points of the Earth which have the Moon above their horizon at the time.
-Furthermore, an eclipse of the Sun is not visible at one time over the
-whole length of its narrow tract, but moves gradually from one end of it
-to the other; while, on the contrary, an eclipse of the Moon begins and
-ends at the very same instant for all places from which it can be seen,
-but, of course, not at the same local time, which varies with the
-longitude of the place.
-
-
-[PLATE VII.--PARTIAL ECLIPSE OF THE MOON.
-
-Observed October 24, 1874]
-
-
-The partial eclipse of the Moon, represented on Plate VII., shows quite
-plainly the configuration of our satellite as seen with the naked eye
-during the eclipse, with its bright and dark spots, and its radiating
-streaks. This eclipse was observed on October 24th, 1874.
-
-
-
-
-THE PLANETS
-
-
-Around the Sun circulate a number of celestial bodies, which are called
-"_Planets_." The planets are opaque bodies, and appear luminous because
-their surfaces reflect the light they receive from the Sun.
-
-The planets are situated at various distances from the Sun, and revolve
-around this body in widely different periods of time, which are,
-however, constant for each planet, so far as ascertained, and doubtless
-are so in the other cases.
-
-The ideal line traced in space by a planet in going around the Sun, is
-called _the orbit_ of the planet; while the period of time employed by a
-planet to travel over its entire orbit and return to its starting point,
-is called _the sidereal revolution_, _or year_ of the planet. The
-dimensions of the orbits of the different planets necessarily vary with
-the distance of these bodies from the Sun, as does also the length of
-their sidereal revolution.
-
-The distance of a planet from the Sun does not remain constant, but is
-subject to variations, which in certain cases are quite large. These
-variations result from the fact that the planetary orbits are not
-perfect circles having the Sun for centre, but curves called
-"_Ellipses_," which have two centres, or foci, one of which is always
-occupied by the Sun. This is in accordance with Kepler's first law.
-
-The ideal point situated midway between the two foci is called _the
-centre of the ellipse_, or _orbit_; while the imaginary straight line
-which passes through both foci and the centre, with its ends at opposite
-points of the ellipse, is called "_the major axis_" of the orbit. It is
-also known as "_the line of the apsides_." The ideal straight line
-which, in passing through the centre of the orbit, cuts the major axis
-at right angles, and is prolonged on either side to opposite points on
-the ellipse, is called "_the minor axis_" of the orbit.
-
-When a planet reaches that extremity of the major axis of its orbit
-which is the nearest to the Sun, it is said to be in its "_perihelion_;"
-while, when it arrives at the other extremity, which is farthest from
-this body, it is said to be in its "_aphelion_." When a planet reaches
-either of the two opposite points of its orbit situated at the
-extremities of its minor axis, it is said to be at its _mean distance_
-from the Sun.
-
-The rapidity with which the planets move on their orbits varies with
-their distance from the Sun; the farther they are from this body, the
-more slowly they move. The rapidity of their motion is greatest when
-they are in perihelion, and least when they are in aphelion, having its
-mean rate when these bodies are crossing either of the extremities of
-the minor axes of their orbits.
-
-The imaginary line which joins the Sun to a planet at any point of its
-orbit, and moves with this planet around the Sun, is called "_the radius
-vector_." According to Kepler's second law, whatever may be the distance
-of a planet from the Sun, the radius vector sweeps over equal areas of
-the plane of the planet's orbit in equal times.
-
-There is a remarkable relation between the distance of the planets from
-the Sun and their period of revolution, in consequence of which the
-squares of their periodic times are respectively equal to the cubes of
-their mean distances from the Sun. From this third law of Kepler, it
-results that the mere knowledge of the mean distance of a planet from
-the Sun enables one to know its period of revolution, and _vice versa_.
-
-The orbit described by the Earth around the Sun in a year, or the
-apparent path of the Sun in the sky, is called "_the ecliptic_." Like
-that of all the planetary orbits, the plane of the ecliptic passes
-through the Sun's centre. The ecliptic has a great importance in
-astronomy, inasmuch as it is the fundamental plane to which the orbits
-and motions of all planets are referred.
-
-The orbits of the larger planets are not quite parallel to the ecliptic,
-but more or less inclined to this plane; although the inclination is
-small, and does not exceed eight degrees. On account of this inclination
-of the orbits, the planets, in accomplishing their revolutions around
-the Sun, are sometimes above and sometimes below the plane of the
-ecliptic. A belt extending 8° on each side of the ecliptic, and,
-therefore, 16° in width, comprises within its limits the orbits of all
-the principal planets. This belt is called "_the Zodiac_."
-
-Since all the planets have the Sun for a common centre, and have their
-orbits inclined to the ecliptic, it follows that each of these orbits
-must necessarily intersect the plane of the ecliptic at two opposite
-points situated at the extremities of a straight line passing through
-the Sun's centre. The two opposite points on a planetary orbit where its
-intersections with the ecliptic occur, are called "_the Nodes_," and the
-imaginary line joining them, which passes through the Sun's centre, is
-called "the line of the nodes." The node situated at the point where a
-planet crosses the ecliptic from the south to the north, is called "_the
-ascending node_" while that situated where the planet crosses from north
-to south, is called "_the descending node_."
-
-The planets circulating around the Sun are eight in number, but, beside
-these, there is a multitude of very small planets, commonly called
-"asteroids," which also revolve around our luminary. The number of
-asteroids at present known surpasses two hundred, and constantly
-increases by new discoveries. In their order of distance from the Sun
-the principal planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn,
-Uranus and Neptune. The orbits of the asteroids are comprised between
-the orbits of Mars and Jupiter.
-
-When the principal planets are considered in regard to their differences
-in size, they are separated into two distinct groups of four planets
-each, viz.: the small planets and the large planets. The orbits of the
-small planets are wholly within the region occupied by the orbits of the
-asteroids, while those of the large planets are wholly without this
-region.
-
-When the planets are considered in regard to their position with
-reference to the Earth, they are called "inferior planets" and "superior
-planets." The inferior planets comprise those whose orbits are within
-the orbit of our globe; while the superior planets are those whose
-orbits lie beyond the orbit of the Earth.
-
-Since the orbits of the inferior planets lie within the orbit of the
-Earth, the angular distances of these bodies from the Sun, as seen from
-the Earth, must always be included within fixed limits; and these
-planets must seem to oscillate from the east to the west, and from the
-west to the east of the Sun during their sidereal revolution. In this
-process of oscillation these planets sometimes pass between the Earth
-and the Sun, and sometimes behind the Sun. When they pass between us and
-the Sun they are said to be in "inferior conjunction," while, when they
-pass behind the Sun, they are said to be in "superior conjunction." When
-such a planet reaches its greatest distance, either east or west, it is
-said to be at its greatest elongation east or west, as the case may be,
-or in quadrature.
-
-The superior planets, whose orbits lie beyond that of the Earth and
-enclose it, present a different appearance. A superior planet never
-passes between the Earth and the Sun, since its orbit lies beyond that
-of our globe, and, therefore, no inferior conjunction of such a planet
-can ever occur. When one of these planets passes beyond the Sun, just
-opposite to the place occupied by the Earth, the planet is said to be in
-"conjunction;" while, when it is on the same side of the Sun with our
-globe, it is said to be in "opposition." While occupying this last
-position, the planet is most advantageously situated for observation,
-since it is then nearer to the Earth. The period comprised between two
-successive conjunctions, or two successive oppositions of a planet, is
-called its "synodical period." This period differs for every planet.
-
-It is supposed that all the planets rotate from west to east, like our
-globe; although no direct evidence of the rotation of Mercury Uranus,
-and Neptune has yet been obtained, it is probable that these planets
-rotate like the others. It results from the rotation of the planets that
-they have their days and nights, like our Earth, but differing in
-duration for every planet.
-
-The axes of rotation of the planets are more or less inclined to their
-respective orbits, and this inclination varies but little in the course
-of time. From the inclination of the axes of rotation of the planets to
-their orbits, it results that these bodies have seasons like those of
-the Earth; but, of course, they differ from our seasons in duration and
-intensity, according to the period of revolution and the inclination of
-the axis of each separate planet.
-
-
-
-
-THE PLANET MARS
-
-PLATE VIII
-
-
-Mars is the fourth of the planets in order of distance from the sun;
-Mercury, Venus and the Earth being respectively the first, second and
-third.
-
-Owing to the great eccentricity of its orbit, the distance of Mars from
-the Sun is subject to considerable variations. When this planet is in
-its aphelion, its distance from the Sun is 152,000,000 miles, but at
-perihelion it is only 126,000,000 miles distant, the planet being
-therefore 26,000,000 miles nearer the Sun at perihelion than at
-aphelion. The mean distance of Mars from the Sun is 139,000,000 miles.
-Light, which travels at the rate of 185,000 miles a second, occupies
-12½ minutes in passing from the Sun to this planet.
-
-While the distance of Mars from the Sun varies considerably, its
-distance from the Earth varies still more. When Mars comes into
-opposition, its distance from our globe is comparatively small,
-especially if the opposition occurs in August, as the two planets are
-then as near together as it is possible for them to be, their distance
-apart being only 33,000,000 miles. But if the opposition occurs in
-February, the distance may be nearly twice as great, or 62,000,000
-miles. On the other hand, when Mars is in conjunction in August, the
-distance between the two planets is the greatest possible, or no less
-than 245,000,000 miles; while, when the conjunction occurs in February,
-it is only 216,000,000 miles. Hence the distance between Mars and the
-Earth varies from .33 to 245 millions of miles; that is, this planet may
-be 212 million miles nearer to us at its nearest oppositions than at its
-most distant conjunctions.
-
-From these varying distances of Mars from the Earth, necessarily result
-great variations in the brightness and apparent size of the planet, as
-seen from our globe. When nearest to us it is a very conspicuous object,
-appearing as a star of the first magnitude, and approaching Jupiter in
-brightness; but when it is farthest it is much reduced, and is hardly
-distinguishable from the stars of the second and even third magnitude.
-In the first position, the apparent diameter of Mars is 26", in the last
-it is reduced to 3" only.
-
-The orbit of Mars has the very small inclination of 1° 51' to the plane
-of the ecliptic. The planet revolves around the Sun in a period of 687
-days, which constitutes its sidereal year, the year of Mars being only
-43 days less than two of our years.
-
-Mars travels along its orbit with a mean velocity of 15 miles per
-second, being about ⁸⁄₁₀ of the velocity of our globe in its
-orbit. The synodical period of Mars is 2 years and 48 days, during which
-the planet passes through all its degrees of brightness.
-
-Mars is a smaller planet than the Earth, its diameter being only 4,200
-miles, and its circumference 13,200 miles. It seems well established
-that it is a little flattened at its poles, but the actual amount of
-this flattening is difficult to obtain. According to Prof. Young, the
-polar compression is ¹⁄₂₁₉.
-
-The surface of this planet is a little over ²⁸⁄₁₀₀ of the
-surface of our globe, and its volume is 6½ times less than that of the
-Earth. Its mass is only about ⅒ while its density is about ¾ that of
-the Earth. The force of gravitation at its surface is nearly ¾ of what
-it is at the surface of our globe.
-
-The planet Mars rotates on an axis inclined 61° 18' to the plane of its
-orbit, so that its equator makes an angle of 28° 42' with the same
-plane. The period of rotation of this planet, which constitutes its
-sidereal day, is 24 h. 37 m. 23 s.
-
-The year of Mars, which is composed of 669⅔ of these Martial days,
-equals 687 of our days, this planet rotating 669⅔ times upon its axis
-during this period. But owing to the movement of Mars around the Sun,
-the number of solar days in the Martial year is only 668⅔, while,
-owing to the same cause, the solar day of Mars is a little longer than
-its sidereal day, and equals 24 h. 39 m. 35 s.
-
-The days and nights on Mars are accordingly nearly of the same length as
-our days and nights, the difference being a little less than
-three-quarters of an hour. But while the days and nights of Mars are
-essentially the same as ours, its seasons are almost twice as long as
-those of the Earth. Their duration for the northern hemisphere,
-expressed in Martial days, is as follows: Spring, 191; Summer, 181;
-Autumn, 149; Winter, 147. While the Spring and Summer of the northern
-hemisphere together last 372 days, the Autumn and Winter of the same
-hemisphere last only 296 days, or 76 days less. Since the summer seasons
-of the northern hemisphere correspond to the winter seasons of the
-southern hemisphere, and vice versa, the northern hemisphere, owing to
-its longer summer, must accumulate a larger quantity of heat than the
-last. But on Mars, as on the Earth, there is a certain law of
-compensation resulting from the eccentricity of the planet's orbit, and
-from the fact that the middle of the summer of the southern hemisphere
-of this planet, coincides with its perihelion. From the greater
-proximity of Mars to the Sun at that time, the southern hemisphere then
-receives more heat in a given time than does the northern hemisphere in
-its summer season. When everything is taken into account, however, it is
-found that the southern hemisphere must have warmer summers and colder
-winters than the northern hemisphere.
-
-Seen with the naked eye, Mars appears as a fiery red star, whose
-intensity of color is surpassed by no other star in the heavens. Seen
-through the telescope, it retains the same red tint, which, however,
-appears less intense, and gradually fades away toward the limb, where it
-is replaced by a white luminous ring.
-
-
-[PLATE VIII.--THE PLANET MARS.
-
-Observed September 3, 1877, at 11h. 55m. P.M.]
-
-
-Mars is a very difficult object to observe, the atmosphere surrounding
-it being sometimes so cloudy and foggy that the sight can hardly
-penetrate through its vapors. When this planet is observed under
-favorable atmospheric conditions, and with sufficient magnifying power,
-its surface, which is of a general reddish tint, is found to be
-diversified by white, gray and dark markings. The dark markings, which
-are the most conspicuous, almost completely surround the planet. They
-are of different forms and sizes, and very irregular, as can be seen on
-Plate VIII., which represents one of the hemispheres of this planet.
-Many of them, especially those situated in the tropical regions of the
-planet, form long narrow bands, whose direction is in the main parallel
-to the Martial equator.
-
-The dusky spots differ very much, both from one another and in their
-several parts, as regards intensity of shade. Some appear almost black,
-while others which appear grayish, are so faint, that they can seldom be
-seen. In the southern hemisphere, the darkest part of the spots is
-generally found along their northern border; especially where there are
-deep indentations.
-
-Some observers have described these spots as being greenish or bluish,
-but I have never been able to see the faintest trace of these colors in
-them, except when they were observed close to the limb, and involved in
-the greenish tinted ring which is always to be seen there. It is
-probably an effect of contrast, since green and red are complementary
-colors, and since this greenish tinge around the limb covers all kinds
-of spots, whether white or dark. When such dark spots, involved in the
-greenish tint, are carried by the rotation towards the centre of the
-disk, they no longer show this greenish color. To me, these spots have
-always appeared dark, and of such tints as would result from a mixture
-of white and black in different proportions; except that on their
-lighter portions they show some of the prevalent reddish tint of the
-Martial surface. It is to be remarked that in moments of superior
-definition of the telescopic image, the intensity of darkness of all the
-spots is considerably increased--some of them appearing almost perfectly
-black.
-
-The markings on the surface of Mars are now tolerably well
-known--especially those of its southern hemisphere, which, owing to the
-greater proximity of the planet to our globe when this hemisphere is
-inclined towards the earth, have been better studied. Those of the
-northern hemisphere are not so well known, since when this hemisphere is
-inclined towards us, the distance of Mars from the Earth is 26,000,000
-miles greater, so that the occasions for observing them are not so
-favorable.
-
-Several charts of Mars are in existence, but as the same nomenclature
-has not been employed in all of them, some confusion has arisen in
-regard to the names given to the most remarkable features of the
-planet's surface. In order to give clearness to the subject, it will be
-necessary here to give a brief description of the principal markings
-represented on Plate VIII. In this the nomenclature will be employed
-which has been adopted by the English observers in the fine chart of Mr.
-Nath. Green. The large dark spot represented on the left-hand side of
-the plate is called De La Rue Ocean. The dark oval spot, isolated in the
-vicinity of the centre of the disk, is called Terby Sea; while the dark,
-irregular form on the right, near the border, represents the western
-extremity of Maraldi Sea.
-
-The dusky spots of Mars seem to be permanent, and to form a part of the
-general surface of the planet. That several among them, at least, are
-permanent, is proved by the fact that they have been observed in the
-same position, and with the same general form, for over two centuries.
-Yet, if we are to depend upon the drawings made fifty years ago by Beer
-and Maedler, it would seem that the permanency of some of them does not
-exist, since a very large spot represented by these astronomers on their
-chart of Mars is not visible now. This object, which, on their map, has
-its middle at 270°, should be precisely under the prominent dark oval
-spot called Terby Sea, seen near the centre of the picture, and would
-extend down almost as far as the northern limb. This can hardly be
-attributed to an error of observation, since these observers were both
-careful, and had great experience in this class of work. It is a very
-singular fact that, at the very same place where Maedler represented the
-spot in question, I found a conspicuous dark mark on December 16, 1881,
-which was certainly not visible in 1877, during one of the most
-favorable oppositions which can ever occur. The object, which is still
-visible (Feb., 1882), consists of an isolated spot situated a little to
-the north of Terby Sea. During the memorable opposition of 1877, I
-investigated thoroughly the markings of Mars, and made over 200 drawings
-of its disk, 32 of which represent the Terby Sea; but this isolated spot
-was not to be seen, unless it be identified with the faint mark,
-represented on the plate, which occupied its place. There cannot be the
-slightest doubt that a change has occurred at that place. Changes in the
-markings have also been suspected on the other hemisphere of the planet.
-
-The well-known fact that the continents, and especially the mountainous
-and denuded districts of our globe, reflect much more light than the
-surfaces covered by water, has led astronomers to suppose that the dark
-spots on Mars are produced by a liquid strongly absorbing the rays of
-light, like the liquids on the surface of the Earth. According to this
-theory the dark spots observed are supposed to be lakes, seas, and
-oceans, similar to our own seas and oceans, while the reddish and
-whitish surfaces separating these dark spots, are supposed to be
-islands, peninsulas, and continents. This supposition seems certainly to
-have a great deal of probability in its favor, although some of the
-lighter markings may have a different origin, and perhaps be due to
-vegetation; but no observer has yet seen in them any of the changes
-which ought to result from change of seasons. Some of the changes in the
-dark spots might also be attributed to the flooding or drying up of
-marshes and low land. The change which I have observed lately might be
-attributed to such a cause, especially as my observation was made
-shortly after the spring equinox of the northern hemisphere of Mars,
-which occurred on December 8th.
-
-Besides the dark spots just described, there are markings of a different
-character and appearance. Among the most conspicuous are two very
-brilliant white oval spots, which always occupy opposite sides of the
-planet. These two bright spots, which correspond very closely with the
-poles of rotation of Mars, have been called "polar spots."
-
-On account of the inclination of the axis of rotation of this planet to
-the ecliptic, it is rare that both of these spots are visible on the
-disk at the same time; and when this occurs, they are seen considerably
-foreshortened, as they are then both on the limb of the planet. Usually
-only one spot is visible, and it appears to its best advantage when the
-region to which it belongs attains its maximum of inclination towards
-the Earth.
-
-The polar spots change considerably in size, as they do also in form.
-Sometimes they occupy nearly one-third of the disk, as is proved by many
-of my observations; while at other times they are so much reduced as to
-be totally invisible. It is to be remarked that the reduction of these
-spots generally corresponds with the summer seasons, and their
-enlargement with the winter seasons of the hemispheres to which they
-respectively belong. From these well-observed facts it would appear that
-a relation exists between the temperature of the two hemispheres of Mars
-and the variations of the white spots observed at its poles. A similar
-relation is known to exist on our globe between the progress of the
-seasons and the melting away and the accumulation of snow in the polar
-regions. Astronomers have been led, accordingly, to attribute the polar
-spots of Mars, with all their variations, to the alternate accumulation
-and melting of snows. On this account, the polar spots of Mars are
-sometimes also called "snow-spots."
-
-Errors have certainly been made by astronomers in some of their
-observations of the so-called polar snow-spots, other objects occupying
-their place having been mistaken for them. A regular series of
-observations on this planet, which I have now continued for seven years,
-has revealed the fact that during the winter seasons of the southern
-hemisphere of Mars, the polar spots are most of the time invisible,
-being covered over by white, opaque, cloud-like forms, strongly
-reflecting light. In 1877, during more than a month, I, myself, mistook
-for the polar spots such a canopy of clouds, which covered at least
-one-fifth of the surface of the whole disk. I only became aware of my
-error when the opaque cloud, beginning to dissolve at the approach of
-the Martial summer, allowed the real polar spot to be seen through its
-vapors, as through a mist at first, and afterwards with great
-distinctness. In this particular case, the snow-spot was considerably
-smaller than the cloudy cap which covered it, and it is to be remarked
-that it was not situated at the centre of this cloudy cap, but was east
-of that centre; a fact which may account for the so-called polar spots
-not being always observed on exactly opposite sides of the disk. From my
-observations of 1877, 1878 and 1880, it appears that at the approach of
-the autumnal equinox of the southern hemisphere of Mars, large, opaque
-masses, like cumulus clouds in form, began to gather in the polar
-regions of that hemisphere, and continued through autumn and winter,
-dissolving only at the approach of spring. These clouds, which varied in
-form and extent, were very unsteady at first, but as the winter drew
-nearer they enlarged and became more permanent, covering large surfaces
-for months at a time.
-
-That the large white spots under consideration are real clouds in the
-atmosphere of Mars, and are not due to a fall of snow, is proved by the
-fact that these spots covered both seas and continents with equal
-facility, even in the equatorial regions of the planet. Snow, of course,
-could not cover the seas of Mars, unless these were all frozen over,
-even in the equatorial zones; therefore, if the dark spots of Mars are
-assumed to be due to water, these large white spots cannot well be
-ascribed to snow.
-
-The real polar spots of Mars seem to be in relief on the surface of the
-planet, since the southern spot often appeared slightly shaded on the
-side opposite to the Sun during my observations in 1877. In certain
-cases, when they are on or very near the limb, they have been observed,
-both by others and by myself, to project from the disk slightly.
-
-The polar spots of Mars are doubtless composed of a material which, like
-our snow or ice, melts under the rays of the Sun; although it seems
-difficult to admit that the Martial snow is identical with our
-terrestrial snow, and that it melts at a like temperature. The south
-polar spot of Mars entirely disappeared from sight in its summer season
-in 1877, although the planet receives less than one-half as much heat as
-we receive from the Sun; yet on our globe the arctic or antarctic ices
-and snow are perpetual--never melting entirely. An important fact
-disclosed by the melting away of the southern polar spot is, that in
-melting it is always surrounded with a very dark surface, which takes
-the place of the melted portion of the spot, as observed by myself in
-1877-78. When the polar spot had entirely disappeared, its place was
-occupied by a very dark spot. Now, if the polar spot is really ice, and
-the dark spots are actual seas, this polar spot must be situated in
-mid-ocean, since, on melting away, it is replaced by a dark spot. If the
-polar spots are composed of a white substance melting under the rays of
-the Sun, as seems altogether probable, its melting point must be above
-that of terrestrial snow.
-
-Many of the dark spots of Mars, and especially those whose northern
-border forms an irregular belt upon the equatorial regions of this
-planet, are bordered on that side by a white luminous belt, following
-all their sinuosities. These white borders are variable. Sometimes they
-are very prominent and intensely bright, especially at some points,
-which occasionally almost equal the polar spots in brilliancy; while at
-other times they are so faint, that they can hardly be distinguished, or
-are even invisible; although the atmosphere is clear and the dark spots
-appear perfectly well defined. While these white borders were invisible,
-I have sometimes watched for several hours at a time to see if I could
-detect any traces of them in places where they usually appear the most
-prominent, but generally without success. On a few occasions, however, I
-had the good fortune to see some of these spots forming gradually in the
-course of one or two hours, at places where nothing of them could be
-seen before.
-
-These whitish fringes forming and vanishing along the coasts of the
-Martial seas have been very little studied by astronomers. From my
-observations made during the last seven years, it appears very probable
-that this belt and its white spots are mainly due to the condensation of
-vapors around, and over high peaks, and extensive mountain chains,
-forming the Martial sea-coasts, as the Andes and Rocky Mountains form
-the sea-coasts of the Pacific Ocean. These high mountains on Mars,
-condensing the vapors into fogs or clouds above them, or at their sides,
-as often happens in our mountainous districts, would certainly suffice
-to produce the phenomena observed. Some of the highest peaks among these
-mountain chains may even have their summits covered with perpetual snow,
-or some substance partaking of the nature of snow. The temporary
-visibility and invisibility of the white spots seen on Mars, as well as
-the rapid transformations they sometimes undergo, may be explained as
-caused by clouds having a high reflective power and a liability to form
-and disappear quickly.
-
-The assumption that these irregular whitish bands and spots are formed
-by the condensation of vapors on mountain chains, and elevated table
-lands, is supported by my observations made in 1877 and 1879. When such
-white spots were traversing the terminator at sunrise, they very often
-projected far into the night side, thus indicating that they were at a
-higher level than that of the general surface. Indentations in the
-terminator, corresponding to large dark spots crossing its line, also
-clearly indicated the depression of the dark spots below the general
-surface. The highest mountainous districts thus observed on Mars, are
-situated between 60° and 70° of south latitude, towards the western
-extremity of Gill Land. The mountain chain, which almost completely
-forms the surface of this land, is so elevated at some points, that they
-not only change the form of the terminator when they are seen upon it,
-but also the limb of the planet, as seen by myself. They then appear so
-brilliant, that the principal summit among them has been mistaken by
-several observers for the polar spot itself, as proved by the wrong
-position assigned to it on their drawings. It seems probable that this
-high peak, which appears always white, is constantly covered with snow,
-or the similar material replacing it on Mars. This high region is
-situated between longitudes 180° and 190°.
-
-The highest mountainous parts belonging to the hemisphere represented on
-Plate VIII., which are nearly always more or less visible as whitish
-spots and bands, form a coast line along the northern (lower) border of
-De La Rue Ocean. This great spot, which is not so simple as it has been
-represented by observers, is in fact divided by two narrow isthmuses,
-one in the north, the other in the east, both joining, in the interior
-of the great ocean, a peninsula heretofore known as Hall Island. Upon
-the south-eastern extremity of this peninsula, a white spot, called
-Dawes Ice Island, was observed in 1865, but it soon disappeared, and was
-after that seen only now and then. It is very probable that this
-so-called Ice Island was due to clouds forming around the summit of some
-high peak of this peninsula.
-
-On the opposite hemisphere to that represented on Plate VIII., the white
-fringes bordering the dark spots are much more conspicuous than they are
-on this side. On the eastern side of a remarkable dark spot called
-Kaiser Sea, they are very bright, and almost always present, although
-they vary considerably, both in brightness and in extent. To the south
-of Kaiser Sea, they are very conspicuous on the eastern border of
-Lockyer Land, forming an elevated and deeply indented coast-line along
-Lambert Sea. There the white spots never disappear entirely, being
-always visible on the north side, where they turn westward along Dawes'
-Ocean--the mountain chain attaining there its greatest altitude. Very
-frequently Lockyer Land, which seems to be a vast plateau, appears
-throughout white and brilliant, this occurring usually towards the
-sunrise or sunset of that region, probably from the condensation of
-vapors and the formation of fogs, but generally this whiteness gradually
-disappears with the progress of the sun above this plateau. Inside of
-the great continents of Mars these temporary white spots are not so
-frequent, but when visible they occupy always the same positions--a fact
-which probably indicates that they occupy the culminating points of
-these continents. One of these temporary white spots inside of the
-continents is represented on Plate VIII., on the left-hand side, below
-De La Rue Ocean, on Maedler Continent.
-
-Although large, opaque, cumulus-shaped, cloud-like forms are seen in the
-polar regions of Mars, such forms are very seldom seen in the tropical
-zones, or, at least, it appears so, from the fact that my observations,
-continued during the last seven years, have disclosed no real opaque
-cloudy forms there. Although the Martial sky is frequently overcast by
-dense vapors or thick fogs in these regions, yet no real opaque clouds
-were ever seen; the most prominent among the dusky spots being faintly
-visible through the vapory veil, when they approached the centre of the
-disk.
-
-Besides these phenomena, which prove that Mars is surrounded by an
-atmosphere having a great deal of similarity to our own, a further proof
-is afforded by the fact that the dark spots, which appear sharply
-defined and black when they are seen near the centre, become less and
-less visible as they advance towards the limb, and are totally invisible
-before they reach it. Moreover, the spectroscope also indicates the
-existence of an atmosphere, and even the presence of watery vapor in it.
-A very curious state of the Martial atmosphere is revealed by my
-observations of 1877-78. During eight consecutive weeks, from December
-12th to February 6th, a whole hemisphere of the planet--precisely that
-represented on Plate VIII.--was completely covered by dense vapors, or a
-thick fog which barely allowed the dark spots to be seen through it,
-even when they were in the centre of the disk. The opposite hemisphere
-of Mars appeared just as clear and calm as possible; there all the spots
-and their minutest details could be seen, and when the planet was
-observed at the proper time, the line separating the foggy from the
-clear side was plainly visible.
-
-The reddish tint observed on the continents of Mars has been supposed by
-some astronomers to be the real color of the atmosphere of this planet.
-But, for many reasons, this explanation is not acceptable. Besides the
-fact that the border of the planet appears white, while it should be
-more red than the other part, owing to the greater depth of atmosphere
-there presented to us, the polar spots, the white bands along the
-sea-coasts, and the cloud-like forms appear perfectly white, not the
-slightest tint of red being visible on them, as would be the case if
-these objects were seen through an atmosphere tinted red. Other
-astronomers have supposed that the vegetation of this planet has a
-reddish color; but this is not supported by observation. It has been
-again supposed, with much more probability, that the surface of Mars is
-composed of an ochreous material which gives the planet its predominant
-ruddy color.
-
-Until lately Mars was supposed to be without a satellite, but in August,
-1877, Professor Hall, of the Washington Observatory, made one of the
-most remarkable discoveries of the time, and found two satellites
-revolving around this planet. These satellites are among the smallest
-known heavenly bodies, their diameter having been estimated at from 6 to
-10 miles for the outer satellite, and from 10 to 40 miles for the inner
-one.
-
-The most extraordinary feature of these bodies is the proximity of the
-inner satellite to the planet, and the consequent rapidity of its
-motion. The distance of the inner satellite from the centre of Mars is
-about 6,000 miles, and from surface to surface it is less than 4,000
-miles, or a little more than the distance from New York to San
-Francisco. The shortest period of revolution of any satellite previously
-known, is that of the inner satellite of Saturn, which is a little more
-than 22½ hours; but the inner satellite of Mars accomplishes its
-revolution in 7h. 38m., or in 17 hours less than the period of rotation
-of the planet upon its axis. The period of revolution of the outer
-satellite is greater, of course, and equals 30h. 7m.
-
-From this rapidity of motion of the inner satellite of Mars, a very
-curious result follows, which at first sight may appear in contradiction
-with the fact that this body has a direct motion, like that of all the
-planets of the solar system, and moves around Mars from west to east.
-While the outer satellite of this planet, in company with all the stars
-and planets, rises in the east and sets in the west, the inner
-satellite, on the contrary, rises in the west and sets in the east.
-Since the period of rotation of Mars is greater than is the period of
-revolution of this satellite, it necessarily follows that this last body
-must constantly be gaining on the rotation, and, consequently, that the
-satellite sets in the east and rises in the west, compassing the whole
-heavens around Mars three times a day, passing through all its phases in
-11 hours, each quarter of this singular Moon lasting less than 3 hours.
-
-It has been shown above that Mars has many points of resemblance to the
-Earth. It has an atmosphere constituted very nearly like ours; it has
-fogs, clouds, rains, snows, and winds. It has water, or at least some
-liquids resembling it; it has rivers, lakes, seas and oceans. It has
-also islands, peninsulas, continents, mountains and valleys. It has two
-Moons, which must create great and rapid tides in the waters of its seas
-and oceans. It has its days and nights, its warm and cold seasons, and
-very likely its vegetation, its prairies and forests, like the Earth. On
-the other hand, its year and seasons are double those of the Earth, and
-its distance from the Sun is greater.
-
-Is this planet, which is certainly constituted very nearly like our
-globe, and seems so nearly fitted for the wants of the human race,
-inhabited by animals and intelligent beings?
-
-To answer this question, either in the negative or in the affirmative,
-would be to step out of the pure province of science, and enter the
-boundless domain of speculation, since no observer has ever seen
-anything indicating that animal life exists on Mars, or on any other
-planet or satellite. So far as observation goes, Mars seems to be a
-planet well suited to sustain animal life, and we may reason from
-analogy that if animal life can exist at all outside of the Earth, Mars
-must have its flora and fauna; it must have its fishes and birds, its
-mammalia and men; although all these living beings must inevitably be
-very different in appearance from their representatives on the Earth, as
-can easily be imagined from the differences existing between the two
-planets. Although all this is possible, and even very probable, yet it
-must be remembered that we have not the slightest evidence that it is
-so; and until we have acquired this evidence, we may only provisionally
-accept this idea as a pleasing hypothesis, which, after all, may be
-wrong and totally unfounded.
-
-
-
-
-THE PLANET JUPITER
-
-PLATE IX
-
-
-Jupiter, the giant of the planetary world, is the fifth in order of
-distance from the Sun, and is next to Mars, our ruddy neighbor. To the
-naked eye, Jupiter appears as a very brilliant star, whose magnitude,
-changing with the distance of this planet from the Earth, sometimes
-approaches that of Venus, our bright morning and evening star.
-
-The mean distance of Jupiter from the Sun is 475,000,000 miles, but
-owing to the eccentricity of its orbit, its distance varies from 452 to
-498 millions of miles. The distance of this planet from the Earth varies
-still more. When nearest to our globe, or in opposition, its distance is
-reduced to 384,000,000 miles, and its apparent diameter increased to
-50"; while when it is farthest, or in conjunction, its distance is
-increased to 567,000,000 miles, and its apparent diameter reduced to
-30"; Jupiter being thus 183,000,000 miles nearer our globe while in
-opposition than when it is in conjunction.
-
-This planet revolves around the Sun in 11 years, 10 months and 17 days,
-or in only 50 days less than 12 terrestrial years. Such is the year of
-this planet. The plane of its orbit is inclined 1° 19' to the ecliptic.
-No planet, except Uranus, has an orbit exhibiting a smaller inclination.
-The planet advances in its orbit at the mean rate of 8 miles a second;
-which is a little less than half the orbital velocity of the Earth.
-
-Jupiter is of enormous proportions. Its equatorial diameter measures
-88,000 miles, and its circumference no less than 276,460 miles, these
-dimensions being 11 times greater than those of the Earth. This planet,
-notwithstanding its huge size, rotates on its axis in not far from 9h.
-55m. 36s., which period constitutes its day. Owing, however, to the
-changeable appearance of its surface, this period cannot be ascertained
-with very great exactitude. In consequence of its rapid rotation, the
-planet is far from spherical, its polar diameter being shorter than the
-equatorial by about ¹⁄₁₆, or 5,500 miles. Its surface is 124
-times the surface of the earth; while its volume is 1,387 times as
-great. If Jupiter occupied the place of our satellite in the sky, it
-would appear 40 times as large as the Moon appears to us, and would
-cover a surface of the heavens 1,600 times that covered by the full
-Moon, and would subtend an angle of 21°. Jupiter's mass does not
-correspond with its great bulk, and is only ¹⁄₁₀₄₇ of the
-mass of the Sun, and 310 times the mass of the earth; its density being
-only ¼ of that of our globe. The force of gravitation at the surface of
-this planet is over 2½ times what it is on the Earth, so that a
-terrestrial object carried to the surface of Jupiter would weigh over
-two and a half times as much as on our globe.
-
-Observed with a telescope, even of moderate aperture, Jupiter, with its
-four attending satellites and its dazzling brilliancy, appears as one of
-the most magnificent objects in the sky. The general appearance of the
-disk is white; but unlike that of Mars, it is brightest towards its
-central parts, and a little darker around the limb, especially on the
-side opposite to the Sun. Although an exterior planet, and so far from
-us, Jupiter shows faint traces of phases when observed near its
-quadratures, but this gibbosity of its disk is very slight, and is
-indicated only by a kind of penumbral shadow on the limb.
-
-When observed with adequate power, the disk of Jupiter is found to be
-highly diversified. The principal features consist of a series of
-alternate light and dark streaks or bands, disposed most of the time
-parallel with the Jovian equator. These bands differ from each other in
-intensity as well as in breadth; those near the equator being usually
-much more prominent than those situated in higher latitudes north and
-south.
-
-The equatorial zone of Jupiter is occupied most of the time by a broad,
-prominent belt 20° or 30° wide, limited on each side by a very dark
-narrow streak. Between these two dark borders, but seldom occupying the
-whole space between them, appears an irregular white belt, apparently
-composed of dense masses of clouds strongly reflecting the Sun's light,
-some of these cloudy masses being very brilliant. The spaces left
-between the cloudy belt and the dark borders, usually exhibit a delicate
-pink or rosy color, which produces a very harmonious effect with the
-varying grayish and bluish shades of some of the belts and streaks seen
-on the disk. Quite often the cloudy belt is broken up, and consists of
-independent cloudy masses, separated by larger or smaller intervals,
-these intervals disclosing the rosy background of this zone.
-
-On each side of the equatorial belt there is usually a broad whitish
-belt, succeeded by a narrow gray band; the space left on each hemisphere
-between these last bands and the limb being usually occupied by two or
-three alternate white and gray bands. A uniform gray segment usually
-forms a sort of polar cap to Jupiter.
-
-When observed under very favorable conditions, all the lighter belts
-appear as if composed of masses of small cloudlets, resembling the white
-opaque clouds seen in our atmosphere. This, as already stated, is
-particularly noticeable on the equatorial belt. It is not unusual, when
-Jupiter is in quadrature, to see some of the most conspicuous white
-spots casting a shadow opposite to the Sun; a fact which sufficiently
-indicates that these spots are at different levels. They probably form
-the summits of vast banks of clouds floating high up in the atmosphere
-of Jupiter.
-
-What we see of Jupiter is chiefly a vaporous, cloudy envelope. If our
-sight penetrates anywhere deeper into the interior, it can only be
-through the narrow fissures of this envelope, which appear as gray or
-dark streaks or spots. That most of the visible surface of Jupiter is
-simply a cloudy covering, is abundantly proved by the proper motion of
-its spots, which sometimes becomes very great.
-
-In periods of calm, very few changes are noticeable in the markings of
-the planet, except, perhaps, some slight modifications of form in the
-cloudy, equatorial belt which, in general, is much more liable to
-changes than the other belts. But the Jovian surface is not always so
-tranquil, great changes being observed during the terrific storms which
-sometimes occur on this mighty planet, when all becomes disorder and
-confusion on its usually calm surface; and nothing on the Earth can give
-us a conception of the velocity with which some of its clouds and spots
-are animated. New belts quickly form, while old ones disappear. The
-usual parallelism of the belts no more exists. Huge, white, cumulus-like
-masses advance and spread out, the rosy equatorial belt enlarges
-sometimes to two or three times its usual size, and occupies two-thirds,
-or more, of the disk, the rosy tint spreading out in a very short time.
-At times very dark bands extending across the disk are transformed into
-knots or dark spots, which encircle the planet with a belt, as it were,
-of jet black beads. Sometimes, also, a secondary but narrower rosy belt
-forms either in the northern or the southern hemisphere, and remains
-visible for a few days or for years at a time.
-
-On May 25, 1876, I witnessed one of the grandest commotions which can be
-conceived as taking place in an atmosphere. All the southern hemisphere
-of Jupiter, from equator to pole, was in rapid motion, the belts and
-spots being transported entirely across the disk, from the eastern to
-the western limb, in one hour's time, during which the equatorial belt
-swelled to twice its original breadth, towards the south.
-
-Now, when one stops for a moment to think what is signified by that
-motion of the dark spots across the little telescopic disk of Jupiter in
-an hour's time, he may arrive at some conception of the magnitude of the
-Jovian storms, compared with those of our globe. The circumference of
-Jupiter's equator, as stated above, is 276,460 miles; half this number,
-or 138,230 miles, represents the length of the equatorial line seen from
-the Earth. Now, after taking into account the rotation of the planet,
-which somewhat diminishes the apparent motion, we arrive at the
-astonishing result that the spots and markings were carried along by
-this Jovian storm, at the enormous rate of 110,584 miles an hour, or
-over 30.7 miles a second. On our globe, a hurricane or tornado, which
-blows at the rate of 100 miles an hour, sweeps everything before it.
-What, then, must be expected from a velocity over 1,105 times as great?
-Enormous as this motion may appear, its occurrence cannot be doubted,
-since it is disclosed by direct observation.
-
-The surface of Jupiter, it would seem, has its periods of calm and
-activity like that of the Sun, although it is not yet known, as it is
-for the latter, that they recur with approximate regularity.
-
-My observations of this planet, which embrace a period of ten years,
-seem to point in that direction, for they show, at least, that Jupiter
-has its years of calm and its years of disturbances. The year 1876 was a
-year of extraordinary disturbance on Jupiter. Changes in the markings
-were going on all the time, and no one form could be recognized the next
-day, or even sometimes the next hour, as shown above. The cloudy
-envelope of the planet was in constant motion, the equatorial belt,
-especially, showing the signs of greatest disturbance, being, for the
-most part, two or three times as wide as in other years. After 1876 the
-calm was very great on the planet, only a slight change now and then
-being noticeable, the same forms being recognized day after day, month
-after month, and even year after year. In one case the same marking has
-been observed for seventeen consecutive months, and in another for
-twenty-eight months. This state of quietude lasted until October, 1880,
-when considerable commotion occurred on the northern hemisphere, where
-large round black spots, somewhat resembling the Sun-spots, formed in
-the cloudy atmosphere, and finally changed, towards the end of December,
-into a narrow pink belt, which still exists.
-
-
-
-[PLATE IX.--THE PLANET JUPITER.
-
-Observed November 1, 1880, at 9h. 30m. P.M.]
-
-
-The most curious marking ever seen on Jupiter is undoubtedly the great
-Red Spot, observed on the southern hemisphere of this planet for the
-last three years. This interesting object, seen first in July, 1878,
-disappeared for a time, reappeared on September 25 of the same year, and
-has remained visible until now. When seen by me in September, it was
-much elongated, and sharply pointed on one side, like a spear-head, but
-it subsequently acquired an irregular form, with short appendages
-protruding from its northern border. At first, the changes were great
-and frequent, but at length it acquired the regular oval form, which,
-with but slight modifications, it has retained until now. During the
-month of November, 1880, I noticed two small black specks upon this Red
-Spot, and they were seen again in January of the succeeding year, by Mr.
-Alvan Clark, Jr. When the spot had attained its oval shape, it appeared
-part of the time surrounded with a white luminous ring of cloudy forms
-which, however, was changing more or less all the time, being sometimes
-invisible. The color of this curious spot is a brilliant rosy red,
-tinged with vermilion, and altogether different in shade from the
-pinkish color of the equatorial belt. The size of the spot varies, but
-of late its changes have been slight. Its longer diameter may be
-estimated at 8,000 miles, and the shorter at 2,200 miles. The Red Spot
-is represented on Plate IX. with its natural color, and as it appears at
-the moments most favorable for observation. In ordinary cases its color
-does not appear so brilliant, but paler.
-
-It is difficult to account for the color of the equatorial belt and that
-of the Red Spot; but it is known, at least, that the material to which
-they are due cannot be situated at the level of the general surface
-visible to us, and especially that of the cloudy forms of the equatorial
-zone. Undoubtedly the red layer lies deeper than the superficial
-envelope of the planet, although it does not seem to be very deeply
-depressed.
-
-Jupiter is attended by four satellites, which revolve around the planet
-at various distances, and shine like stars of the 6th and 7th magnitude.
-It is said that under very favorable circumstances, and in a very clear
-sky, the satellites can be seen with the naked eye, but this requires
-exceptionally keen eyes, since the glare of the planet is so strong as
-to overpower the comparatively faint light of the satellites. However, I
-myself have sometimes seen, without the aid of the telescope, two or
-three of the satellites as a single object, when they were closely
-grouped on the same side of Jupiter.
-
-The four moons of Jupiter are all larger than our Moon, except the
-first, which has about the same diameter. They range in size from 2,300
-to 3,400 miles in diameter, the third being the largest; the
-determination of their diameter is by no means accurate, however, as it
-is difficult to measure such small objects with precision. Their mean
-distance from the centre of Jupiter varies from 267,000 to 1,192,000
-miles, the first satellite, the nearest to the planet, being a little
-farther from Jupiter then our satellite is from us. The four satellites
-revolve around the planet in orbits whose planes have a slight
-inclination to the equator of Jupiter, and consequently to the ecliptic.
-The diameter of the largest satellite is nearly half that of the Earth,
-or 3,436 miles; while its volume is five times that of our Moon. The
-period of revolution of these satellites varies from 1d. 18h. for the
-first, to 16d. 16h. for the last.
-
-Owing to the slight inclination of the plane of their orbits to that of
-the planet, the three first satellites, and generally the fourth, pass
-in front of the disk and also through the shadow of the planet at every
-revolution, and are accordingly eclipsed. Their passages behind
-Jupiter's disk are called occultations; those in front of it, transits.
-The eclipses, the occultations and the transits of the moons of Jupiter
-are interesting and important phenomena; the eclipses being sometimes
-observed for the rough determination of longitudes at sea.
-
-The satellites in transit present some curious phenomena. When they
-enter the disk, they appear intensely luminous upon its grayish border;
-but as they advance, they seem by degrees to lose their brightness,
-until they finally become undistinguishable from the luminous surface of
-Jupiter. It sometimes happens, however, that the first, the third and
-the fourth satellites, after ceasing to appear as bright spots, continue
-to be visible as dark spots upon the bright central portions of the
-planet's disk; but in these cases their disks appear smaller than the
-shadows they cast. Undoubtedly these satellites have extensive
-atmospheres, since they sometimes pass unperceived across the central
-parts of Jupiter, this being probably when their atmospheres are
-condensed into clouds, strongly reflecting light; while when these
-clouds are absent, we can see their actual surface, with traces of the
-dark spots upon them similar to those on Mars.
-
-From the variation in the brightness of these satellites, which is said
-to be always observed in the same part of their orbit, William Herschel
-was led to suppose that these bodies, like our Moon, rotate upon their
-axes in the same period in which they move round the planet, so that
-they always present the same face to Jupiter; but these conclusions have
-been denied. From my observations it is apparent, however, that the
-light reflected by them varies in intensity as well as in color. But
-this is rather to be attributed to the presence of an atmosphere
-surrounding these bodies, which when cloudy reflects more light than
-when clear, with corresponding changes in the color of the light.
-
-The satellites in transit are sometimes preceded or followed, according
-to the position of the Sun, by a round black spot having about the same
-size as the satellite itself. This black spot is the shadow of the
-satellite cast on the vapory envelope of Jupiter, similar to the shadow
-cast by the Moon on the Earth, during eclipses of the Sun; in fact, all
-the Jovian regions traversed by these shadows have the Sun totally
-eclipsed. Sometimes it happens that the shadow appears elliptical. This
-occurs either when it is observed very near the limb, or when entering
-upon a round, cloud-like spot. This effect is attributable to the
-perspective under which the shadow is seen on the spherical globe or
-spot.
-
-The proper motion of the satellites in the Jovian sky is much more rapid
-than that of the Moon in our sky. During one Jovian day of ten hours,
-the first satellite advances 84°; the second, 42°; the third, 20° and
-the fourth, 9°. The first satellite passes from New Moon to its first
-quarter in a little more than a Jovian day, while the fourth occupies
-ten such days in attaining the same phase.
-
-In density, as well as in physical constitution, Jupiter differs widely
-from the interior planets, and especially from the Earth; and, as has
-been shown, it is surrounded by a dense, opaque, cloudy layer, which is
-almost always impenetrable to the sight, and hides from view the
-nucleus, which we may conceive to exist under this vaporous envelope. In
-1876, the year of the great Jovian disturbances, I observed frequently
-in the northern hemisphere of the planet a very curious phenomenon,
-which seems to prove that its cloudy envelope is at times partially
-absent in some places, its vapors being apparently either condensed, or
-transported to other parts of its surface, and that, therefore, a
-considerable part of the real globe of the planet was visible at these
-places. The phenomenon consisted in the deformation of the northern
-limb, which had a much shorter radius on all of this hemisphere situated
-northward of the white belt which adjoins the equatorial zone. The
-deformation of the limb on both sides, where it passed from a longer to
-a shorter radius, was abrupt, and at right angles to the limb, forming
-there a steplike indentation which was very prominent. The polar segment
-having a smaller radius, appeared unusually dark, and was not striped,
-as usual, but uniform in tint throughout. On September 27th, the third
-satellite passed over this dark segment, and emerged from the western
-border, a little below the place where the limb was abruptly deformed,
-as above described. When the satellite had fully emerged from this limb,
-it was apparent that if the portion of the limb having a longer radius
-had been prolonged a little below, and as far as the satellite, it would
-have enclosed it within its border, and thus retarded the time of
-emersion. The depth of deformation of the limb was accordingly greater
-than the diameter of the third satellite, and certainly more than 4,000
-miles. That the phenomenon was real, is proved by the fact that the
-egress of this satellite occurred at least four minutes sooner than the
-time predicted for it in the American Ephemeris. Other observations seem
-to point in the same direction, since some of the satellites which were
-occulted have been seen through the limb of Jupiter by different
-astronomers, as if this limb was sometimes semi-transparent. Another
-observation of mine seems to confirm these conclusions. On April 24th,
-1877, at 15h. 25m. the shadow of the first satellite was projected on
-the dark band forming the northern border of the equatorial belt, the
-shadow being then not far from the east limb. Close to this shadow, and
-on its western side, it was preceded by a secondary shadow, which was
-fainter, but had the same apparent size. This round dark spot was not
-the satellite itself, as I had supposed at first, since this object was
-yet outside of the planet, on the east, and entered upon it only at 16h.
-4m. I watched closely this strange phenomenon, and at 16h. 45m., when
-the shadow had already crossed about ¾ of the disk, it was still
-preceded by the secondary, or mock shadow, as it may be called; the same
-relative distance having been kept all the while between the two
-objects, which had therefore traveled at the same rate. It is obvious
-that this dark spot could not be one of the planet's markings, since the
-shadow of the first satellite moves more quickly on the surface of
-Jupiter than a spot on the same surface travels by the effect of
-rotation, so that in this case the shadow would soon have passed over
-this marking, and left it behind, during the time occupied by the
-observation. From these observations it seems very probable that Jupiter
-has a nucleus, either solid or liquid, which lies several thousand
-mites below the surface of its cloudy envelope. It is also probable that
-the uniformly shaded dark segment seen in 1876, was a portion of the
-surface of this nucleus itself. When the cloudy envelope is
-semi-transparent at the place situated on a line with an occulted
-satellite and the eye of an observer, this satellite may accordingly
-remain visible for a time through the limb, as shown by observation. The
-phenomenon of the mock shadow may also be attributed to a similar cause,
-where semi-transparent vapors receive the shadow of a satellite at their
-surface, while at the same time part of this shadow, passing through the
-semi-transparent vapors, may be seen at the surface of the nucleus, or
-of a layer of opaque clouds situated at some distance below the surface.
-
-Some astronomers are inclined to think that Jupiter is at a high
-temperature, and self-luminous to a certain extent. If this planet is
-self-luminous to any degree, we might expect that some light would be
-thrown upon the satellites when they are crossing the shadow cast into
-space by the planet; but when they cross this shadow they are totally
-invisible in the best telescopes, a proof that they do not receive much
-light from the non-illuminated side of Jupiter. It would, indeed, seem
-probable that some of the intensely white spots occasionally seen on the
-equatorial belt of the planet are self-luminous in a degree, yet not
-enough to render the satellites visible while they are immersed in
-Jupiter's shadow. It does not seem impossible that the planet should
-have the high temperature attributed to it, when we remember the
-terrific storms observed in its atmosphere, which, owing to the great
-distance of Jupiter from the Sun, do not seem to be attributable to this
-body, but rather to some local cause within the envelope of the planet.
-
-Astronomy, which is a science of observation, is naturally silent with
-regard to the inhabitants of Jupiter. If there are any such inhabitants,
-they are confined to the domain of conjecture, under the dense cloudy
-envelope of the planet. The conditions of habitability on Jupiter must
-differ very widely from those of our globe. Comparatively little direct
-light from the Sun reaches the surface of the globe of Jupiter, except
-that which passes through the narrow openings forming the dark clouds.
-All the rest of the planet's surface, being covered perpetually by
-opaque clouds, receives only diffused light. On Jupiter there are
-practically no seasons, since its axis is nearly perpendicular to its
-orbit. The force of gravity on the surface of Jupiter being more than
-double what it is on the Earth, living bodies would there have more than
-double the weight of similar bodies on the Earth. Furthermore, Jupiter
-only receives 0.011 of the light and heat which we receive from the Sun;
-and its year is nearly equal to 12 of our years. If there are living
-beings on Jupiter, they must, then, be entirely different from any known
-to us, and they may have forms never dreamed of in our most fantastic
-conceptions.
-
-The two round black spots represented towards the central parts of Plate
-IX. are the shadows of the first and second satellite; while the two
-round white spots seen on the left of the disk, are the satellites
-themselves, as they appeared at the moment of the observation. The first
-satellite and its shadow are the nearest to the equator; while the
-second satellite and its shadow are higher, the last being projected on
-the Great Red Spot.[2] The row of dark circular spots represented on the
-northern, or lower hemisphere, when they first appeared, had some
-resemblance to Sun-spots without a penumbra, with bright markings around
-them, resembling faculæ. These round spots subsequently enlarged
-considerably, until they united along the entire line, encircling the
-planet, and finally forming a narrow pink belt, which is still visible.
-
-
-[Footnote 2: By an accidental error in enlarging the original drawing,
-the satellites and shadows appear in Plate IX. of double their actual
-size. The error is one easy of mental correction.]
-
-
-
-
-THE PLANET SATURN
-
-PLATE X
-
-
-Saturn, which is next to Jupiter in order of distance from the Sun,
-while not the largest, is certainly the most beautiful and interesting
-of all the planets, with his grand and unique system of rings, and his
-eight satellites, which, like faithful servants, attend the planet's
-interminable journey through space.
-
-Seen with the naked eye, Saturn shines in the night like a star of the
-first magnitude, whose dull, soft whiteness is, however, far from
-attaining the brilliancy of Venus or Jupiter, although it sometimes
-approaches Mars in brightness. Saturn hardly ever exhibits the
-phenomenon of scintillation, or twinkling, a peculiarity which makes it
-easily distinguishable among the stars and planets of the heavens.
-
-The synodical period of Saturn occupies 1 year and 13 days, so that
-every 378 days, on an average, this planet holds the same position in
-the sky relatively to the Sun and the Earth.
-
-The mean distance of Saturn from the Sun is a little over 9½ times that
-of our globe, or 872,000,000 miles. Owing to the orbital eccentricity,
-this distance may increase to 921,000,000 miles, when the planet is in
-aphelion; or decrease to 823,000,000 miles, when it is in perihelion;
-Saturn being therefore 98,000,000 miles nearer to the Sun when in
-perihelion than in aphelion. If gravitation were free to exert its
-influence alone, Saturn would fall into the Sun in 5 years and 2 months.
-
-The distance of Saturn from the Earth varies, according to the position
-of the two planets in their respective orbits. At the time of
-opposition, when the Earth lies between the Sun and Saturn, this
-distance is smallest; while, on the contrary, at the time of
-conjunction, when the Sun lies between the Earth and Saturn, it is
-greatest. Owing, however, to the eccentricity of the orbits of Saturn
-and our globe, and the inclination of their planes to each other, and
-owing also to the variable heliocentric longitude of the perihelion, the
-distance of the two planets from each other at their successive
-conjunctions and oppositions is rendered extremely variable. At present
-it is when the oppositions of Saturn occur in December that this planet
-comes nearest to us; while when the conjunctions take place in June, the
-distance of Saturn from the Earth is the greatest possible. In the
-former case the distance of the planet from our globe is only
-730,000,000 miles; while in the last it is 1,014,000,000 miles, the
-difference between the nearest and farthest points of Saturn's approach
-to us being no less than 284,000,000 miles, or over three times the mean
-distance of the Sun from the Earth.
-
-From the great variations in the distance of Saturn from the Earth,
-necessarily result corresponding changes in the brightness and apparent
-diameter of this body. When it is farthest from us, its angular diameter
-measures but 14"; while, when it is nearest, it measures 20".
-
-The orbit of Saturn is inclined 2°30' to the ecliptic, and its
-eccentricity, which equals 0,056, is over three times that of the
-Earth's orbit.
-
-This planet revolves around the Sun in a period of 29 years and 5½
-months, or 10,759 terrestrial days, which constitutes its sidereal year.
-The extension of the immense curve forming the orbit of this planet, is
-no less than 5,505,000,000 miles, which is traversed by the planet with
-a mean velocity of a little less than 6 miles per second, or three times
-less than the motion of our globe in space.
-
-The real dimensions of the globe of Saturn are not yet known with
-accuracy, and the equatorial diameter has been variously estimated by
-observers, at from 71,000 to 79,000 miles. If we adopt the mean of these
-numbers, 75,000 miles, the circumference of the Saturnian equator would
-measure 235,620 miles, or 9½ times the circumference of our globe; the
-surface of Saturn would be 86 times, and its volume over 810 times that
-of the Earth.
-
-However great the volume of Saturn, its mass is proportionally small,
-being only 90 times greater than that of our globe; the mean density of
-the materials composing this planet being less than that of cork, and
-only 0.68 the density of water. The force of gravitation at the surface
-of Saturn is greater, by a little over ⅑, than it is at the surface of
-the Earth; a body falling in a vacuum at its surface, would travel 17.59
-feet during the first second.
-
-From observations of markings seen on the surface of Saturn, and from
-the study of their apparent displacements on the disk, William Herschel
-found that the planet rotated upon its axis in 10h. 16m. 0.24s. Since
-Herschel's determination, new researches have been made, and lately,
-Professor Hall, noticing a bright spot, followed it for nearly a month,
-observing its transits across the central meridian of the disk. From
-these observations he has obtained for the rotation period 10h. 14m.
-23.8s., a result which agrees very closely with that obtained 82 years
-earlier by Herschel, considering the fact that the markings from which
-the period of rotation is ascertained are not fixed on the planet, but
-are always more or less endowed with proper motion. The velocity of
-rotation at the equator is 21,538 miles per hour, or nearly 6 miles per
-second.
-
-The axis of rotation of Saturn is inclined 64° 18' to the plane of the
-orbit, so that its equator makes an angle of 25° 42' with the same
-plane. The seasons of this planet therefore present greater extremes of
-temperature than those of the Earth, but not quite so great variations
-as the seasons of Mars.
-
-The globe of Saturn is not a perfect sphere, but its figure is that of
-an oblong spheroid, flattened at the poles. The polar compression of
-Saturn is greater than that of any other planet, surpassing even that of
-Jupiter. Though not yet determined with a great degree of accuracy, the
-compression is known to be between ²⁄₁₈ and ⅒ of the equatorial
-diameter; that is, a flattening of about 3,894 miles, at each pole, the
-polar diameter being 7,788 miles shorter than the equatorial.
-
-The internal condition of the planet Saturn, whether solid, liquid or
-gaseous, cannot be discovered from the examination of its surface, as
-its globe is enwrapped in a dense opaque layer of vapors and cloud-like
-forms, through which the sight fails to penetrate. The appearance of
-this vapory envelope is like that of _cumulus_ clouds, and one of its
-characteristics is to arrange itself into alternate bright and dark
-parallel belts, broader than those seen on Jupiter, and also more
-regular and dark. These belts, which are parallel to the equator of the
-planet, vary in curvature with the inclination of its axis of rotation
-to the line of sight.
-
-The belts of Saturn, like those of Jupiter, are not permanent, but keep
-changing more or less rapidly. Sometimes they have been observed to be
-quite numerous; while at other times they are few. Occasionally
-conspicuous white or dark spots are seen on the surface, although the
-phenomenon is quite rare. It is from the observation of such spots that
-Saturn's period of rotation has been determined, as stated above. The
-equatorial zone of Saturn always appears more white and brilliant than
-the other parts, as it also appears more mottled and cloud-like. In late
-years the globe has been characterized, and much adorned, by a pale
-pinkish tint on its equatorial belt, resembling that of Jupiter, but
-somewhat fainter. On either side of the equatorial belt there is a
-narrower band, upon which the mottled appearance is visible. Below
-these, one or two dark belts, separated by narrow white bands, are
-usually seen; but, of late, the bands have been less numerous, being
-replaced in high latitudes by a dark segment, which forms a polar cap to
-Saturn. The globe of Saturn does not anywhere appear perfectly white,
-and when compared with its ring, it looks of a smoky yellowish tint,
-which becomes an ashy gray on its shaded parts. It usually appears
-darker near the limb than in its central portions; although on some
-occasions I have seen portions of the limb appear brighter, as if some
-white spots were traversing it.
-
-Some observers have seen the limb deformed and flattened at different
-places, and W. Herschel even thought such a deformation to be a
-permanent feature of this globe, which he termed diamond-shaped, or
-"square shouldered." But this was evidently an illusion, since the
-planet's limb usually appears perfectly elliptical, although it
-occasionally appears as if flattened at some points, especially where it
-comes in apparent contact with the shadow cast by the globe on the ring,
-as observed by myself many times. But with some attention, it is
-generally found that this deformation is apparent rather than real, and
-is caused by the passage of some large dark spots over the limb, which
-is thus rendered indistinguishable from the dark background upon which
-it is projected.
-
-What distinguishes Saturn from all known planets, or heavenly bodies,
-and makes it unique in our universe, is the marvelous broad flat ring
-which encircles its equator at a considerable distance from it. With a
-low magnifying power this flat ring appears single, but when carefully
-examined with higher powers, it is found to consist of several distinct
-concentric rings and zones, all lying nearly in the same plane with the
-planet's equator.
-
-
-[PLATE X.--THE PLANET SATURN.
-
-Observed on November 30, 1874, at 5h. 30m. P.M.]
-
-
-At first sight only two concentric rings are recognized, the _outer_ and
-the _middle_, or _intermediary_, which are separated by a wide and
-continuous black line, called the _principal division_. This line, and
-indeed all the features of the surface of the rings are better seen, and
-appear more prominent on that part of the ring on either side called the
-_ansa_, or handle. Besides these two conspicuous rings, a third, of very
-dark bluish or purplish color, lies between this middle ring, to which
-it is contiguous, and the planet. This inner ring, which is quite wide,
-is called the _gauze_ or _dusky ring_. Closer examination shows that the
-outer ring is itself divided by a narrow, faint, grayish line called the
-pencil line, which, from its extreme faintness, is only visible on the
-ansæ. Moreover, the middle ring is composed of three concentric zones,
-or belts, which, although not apparently divided by any interval of
-space, are distinguished by the different shadings of the materials
-composing them. The outer zone of this compound middle ring is, by far,
-the brightest of all the system of rings and belts, especially close to
-its external border, where, on favorable occasions, I have seen it
-appear on the ansæ as if mottled over, and covered throughout with
-strongly luminous cloud-like masses. On the ansæ of the double outer
-ring, similar cloudy forms have also been seen at different times. The
-second zone of the middle ring is darker than the first, the innermost
-being darker still. All the characteristic points which have thus been
-described, are shown in Plate X.
-
-Although suspected in 1838, the dusky ring was not recognized before
-1850, when G. P. Bond discovered it with the 15-inch refractor of the
-Cambridge Observatory. It was also independently discovered the same
-year in England by Dawes and Lassell. The dusky ring differs widely in
-appearance and in constitution from the other rings, inasmuch as these
-last are opaque, and either white or grayish, while the former is very
-dark, and yet so transparent that the limb of the planet is plainly seen
-through its substance. On particularly favorable occasions, the
-appearance of this ring resembles that of the fine particles of dust
-floating in a ray of light traversing a dark chamber. Whatever may be
-the material of which this ring is composed, it must be quite rarefied,
-especially towards its inner border, which appears as if composed of
-distinct and minute particles of matter feebly reflecting the solar
-light. That the inner part of the dusky ring is composed of separate
-particles, is proved by the fact that the part of the ring which is seen
-in front of the globe of Saturn has its inner border abruptly deflected
-and curved inward on entering upon the disk, causing it to appear
-considerably narrower than it must be in reality, a peculiarity which is
-shown in the Plate. This phenomenon may be attributed to an effect of
-irradiation, due to the strong light reflected by the central parts of
-the ball, which so reduces the apparent diameter of the individual
-particles that they become invisible to us, especially those near the
-inner border, which are more scattered and less numerous than elsewhere.
-
-The dusky ring, which was described by Bond, Lassell and other
-astronomers as being equally transparent throughout all its width, has
-not been found so by me in later years. The limb of the planet, seen by
-these observers through the whole width of the dusky ring in 1850, could
-not be traced through its outer half by myself in 1872 and 1874, and
-this with the very same instrument used by Bond in his observations of
-1848 and 1850. Moreover, I have plainly seen that its transparency was
-not everywhere equal, but greatest on the inner border, from which it
-gradually decreases, until it becomes opaque, as proved by the gradual
-loss of distinctness of the limb, which vanishes at about the middle of
-the dusky ring. These facts, which have been well ascertained, prove
-that the particles composing this ring are not permanently located, and
-are undergoing changes of relative position. It will be shown that the
-surface of the other rings is also subject to changes, which are
-sometimes very rapid.
-
-The globe of Saturn is not self-luminous, but opaque. It shines by the
-solar light, as is proved by the shadow it casts opposite the Sun upon
-the ring. Although receiving its light from the Sun, Saturn does not
-exhibit any traces of phases, like the other planets nearer to the Sun,
-owing to its great distance from the Earth. When near its quadratures,
-however, the limb opposite to the Sun appears much darker, and shows
-traces of twilight. As far as can be ascertained, the rings, with the
-exception of the inner one, are opaque, as proved by the strong shadow
-which they cast on the globe of Saturn.
-
-The shadows cast by the planet on the ring, and by the ring on the
-planet, are very interesting phenomena, inasmuch as they enable the
-astronomer to recognize the form of the surface which receives them. The
-shadow cast by the ring on the ball is not quite so interesting as the
-other, although it has served to prove that the surface of this globe is
-not smooth, as is likewise suggested by its mottled appearance. I have
-sometimes found, as have also other observers, that the outline of this
-shadow upon the ball was irregular and indented, an observation which
-proves either that the surface of the ball is irregular, or that the
-border of the ring casting the shadow was jagged. The shadow of the
-globe on the rings has much more interest, as it enables us to get at
-some knowledge of the form of the surface of the rings, which otherwise
-is very difficult to discover, owing to the oblique position in which we
-always see them.
-
-In general, the shadow of the ball on the middle ring has its outline
-concave towards the planet; while on the outer ring it is usually
-slanting, and at a greater distance from the limb than on the middle,
-and dusky rings. This form of the shadow evidently proves that the
-middle ring stands at a higher level than the two others, especially
-towards its outer margin. The system seems to increase gradually in
-thickness from the inner border of the dusky ring to the vicinity of the
-outer margin of the middle ring, after which it rapidly diminishes on
-this border, while the surface of the outer ring is almost level.
-
-But this surface is by no means fixed, as its form sometimes changes, as
-proved by my observations and those of others. As may be noticed on
-Plate X., the outline of the shadow of the planet on the rings is
-strongly deviated towards the planet, near the outer margin of the
-middle ring; the notch indicating an abrupt change of level, and a rise
-of the surface at that point. Some observers have endeavored to explain
-these deviations by the phenomena of irradiation, from which it would
-follow that the maximum effect of deviation should be observed where the
-ring is the brightest, which does not accord with observation; as the
-deepest depression in the shadow is not to be found usually at the
-brightest part, which is towards the outer border of the middle ring,
-but occurs near its centre. From these observations it is undoubtedly
-established that the surface of the rings is far from being flat
-throughout, and is, besides, not permanent, but changes, as would, for
-instance, the surface of a large mass of clouds seen from the top of a
-high mountain. In general, the system is thickest not very far from the
-outer border of the intermediary ring.
-
-Some interesting phenomena which I had occasion to observe before and
-after the passage of the Sun through the plane of the rings, on February
-6th, 1878, conclusively show that the surface of this system cannot be
-of a uniform level, but must be thicker towards the outer border of the
-middle ring, thence gradually sloping towards the planet. Many of my
-observations irresistibly lead to this conclusion. As it would, however,
-be out of place to have them recorded here in detail, I will simply give
-one of the most characteristic among them.
-
-From December 18th, 1877, when the Sun was about 41' above the plane of
-the rings, to February 6th, 1878, the day of its passage through their
-plane, the illuminated surface of this system gradually decreased in
-breadth with the lowering of the Sun, until it was lost sight of,
-February 5th, on the eve of the passage of the Sun through their plane.
-The phenomenon in question consisted in the gradual invasion of their
-illuminated surface by what appeared to be a black shadow, apparently
-cast by the front part of the outer portion of the middle ring the
-nearest to the Sun. On January 25th, when the elevation of the Sun above
-the plane of the rings was reduced to 15', the shadow thus cast had
-extended so far on their surface that it reached the shadow cast by the
-globe on the opposite part of the ring in the east, and accordingly the
-remaining portion of the illuminated surface of the eastern ansa then
-appeared entirely disconnected from the ball, by a large dark gap,
-corresponding in breadth to that of the globe's shadow on the rings. On
-February 4th, when the Sun was only 5' above the plane of the rings,
-their illuminated and only visible surface was reduced to a mere thread
-of light, which on the 5th appeared broken into separate points. It is
-evident that the phenomenon was not caused by the obliquity of the ring
-as seen from our globe, since the elevation of the Earth above the plane
-of the rings--which on December 18th was 3° 20'--was still 1° 20' on
-the 4th of February. In ordinary circumstances, when the Sun is a little
-more elevated, and the rings seen at this last angle, they appear quite
-broad and conspicuous, and even the dark open space separating the dusky
-ring from the planet is perfectly visible on the ansæ, where the
-Earth's elevation above their plane is reduced to 40'. It is also
-evident that the phenomenon was not to be attributed to the reduction of
-the light which they received from the Sun, although the illumination in
-February might be expected to be comparatively feeble, since the Sun
-then shone upon the rings so obliquely; yet (on the supposition that
-their surface is flat) they should have been illuminated throughout, and
-if not very brightly, sufficiently so, at least, to make them visible
-and as bright as was the narrow thread of light observed on the 4th of
-February. The phenomenon actually observed may be explained most readily
-by assuming, as other phenomena also indicate, that the surface of the
-ring is not flat, but more elevated towards, or in the vicinity of its
-outer border, from which place it slopes inwardly towards the planet. On
-this assumption, it is evident that the elevated part of the ring the
-nearest to the Sun would cast a shadow, which, with the increasing
-obliquity of the Sun, would gradually cover the whole surface comprised
-within the elevated part, and thus become invisible to us. Several
-observations made by Bond and other observers undoubtedly show the same
-phenomenon, and do not seem to be intelligible on any other supposition.
-From my observations made in 1881 it would appear, however, that the
-opposite surfaces of the rings do not exactly correspond in form, but
-this may not be a permanent feature, as the surface of this system is
-subject to changes, as already shown.
-
-The dimensions of the rings are great, the diameter of the outer one
-being no less than 172,982 miles, the distance from the centre of the
-globe to the outer border of the system being, therefore, 86,491 miles.
-The breadth of the outer ring is 9,941 miles; that of the principal
-division, 2,131 miles; that of the middle ring, 19,902 miles, and that
-of the dusky ring, 8,772 miles. The breadth of all the rings taken
-together is, therefore, 40,746 miles. The interval between the surface
-of Saturn and the inner border of the dusky ring is 7,843 miles.
-
-The thickness of the system of rings has been variously estimated by
-astronomers, on account of the great difficulties attending its
-determination. While Sir John Herschel estimated it at more than 250
-miles, G. P. Bond reduces it to 40 miles. Both of these numbers are
-evidently too small, as so slight a thickness cannot explain the
-observed phenomenon of the shadow cast by a portion of the ring on its
-own surface, when the Sun is very low in its horizon, as shown above.
-
-The plane of the system of rings is inclined 27° to the planet's orbit,
-and is parallel, or at least very nearly so, with the equator of the
-planet, passing, therefore, through its centre, and dividing its globe
-into northern and southern hemispheres. Seen from the Earth, a portion
-of the ring always appears projected in front of the planet, thus
-concealing a small part of its globe, while the opposite portion passes
-behind the globe, which hides it from sight.
-
-As the plane of the ring is not affected by the motion of the planet
-around the Sun, but always remains parallel to itself, it follows that
-as Saturn advances in its orbit the rings must successively present
-themselves to us under various angles of inclination, appearing,
-therefore, more or less elliptical, and presenting two maxima and two
-minima of inclination in the course of one of its revolutions. As the
-revolution of Saturn is accomplished in 29½ years, the maxima and the
-minima must recur every 14 years and 9 months; the maxima being
-separated from the minima by an interval of 7 years and 4½ months.
-
-When Saturn arrives at the two opposite points of its orbit, where the
-major axis of its ring is at right angles to the line joining its centre
-to that of the Sun, the ring, which is then viewed at an inclination of
-27°, the greatest angle at which it can ever be seen, has reached its
-maximum opening, the smaller diameter of its ellipse being then about
-half that of the larger. At this moment the outer ring projects north
-and south beyond the globe, which is then completely enclosed in its
-ellipse. The maximum opening of the northern surface of the ring takes
-place, at present, when Saturn arrives in longitude 262°, in the
-constellation Sagittarius, and that of the southern surface when it
-arrives in longitude 82° in the constellation Taurus. When, on the
-contrary, Saturn reaches the two opposite points of its orbit, where the
-plane of its ring is parallel to the line joining its centre to that of
-the Sun, the opening vanishes, as only the thin edge of the ring is then
-presented to the Sun and receives its light, the rest being in darkness.
-At this moment the ring disappears, except in the largest telescopes,
-where it is seen as an exceedingly thin thread of light; and the
-Saturnian globe, having apparently lost its ring, appears solitary in
-the sky, like the other planets. The disappearance of the ring from this
-cause occurs now when Saturn arrives at 90° from either of the
-positions of maximum inclination, that is, in longitude 352° in the
-constellation Pisces, and in longitude 172° in the constellation Leo.
-
-When the planet is in any other position than one of these last two,
-either the northern or the southern surface of the ring is illuminated
-by the Sun, while the opposite surface is in the night, and does not
-receive any direct sunlight. At the time of the passage of the plane of
-the ring through the Sun's centre, a change takes place in the
-illumination of the ring. If it is the northern surface which has
-received the rays of the Sun during the previous half of the Saturnian
-year, at the moment the plane has passed the centre of the Sun, the
-southern surface, after having been buried in darkness for 14¾ years,
-sees the dawn of its long day of the same length. Such a phenomenon will
-not occur until 1892, when the passage of the Sun from the northern to
-the southern side of the ring will close in twilight the day commenced
-in 1878.
-
-Aside from the periodic disappearance of the ring, resulting from the
-passage of the Sun through its plane, the ring may also disappear from
-other reasons. Just before or just after the time of the passage of the
-Sun through the plane of the ring, the Earth and the Sun may occupy such
-positions, that while the one is north of the plane of the ring, the
-other is south of it, or vice versa, in which event the ring becomes
-invisible, because its dark and non-illuminated surface is presented to
-us. The ring may also become invisible to us when the Earth passes
-through its plane.
-
-Since the distance from Saturn to the Sun is to the distance of the
-Earth from this last body as 9.54 is to 1; and since the circumference
-of a circle increases in the same proportion as its radius, it follows
-that the diameter of the Earth's orbit projected on the orbit of Saturn
-would occupy only part of the latter, or about 12° 2', this being 6°
-1' on either side of the nodes of the rings. To describe such an arc on
-its orbit, it takes Saturn almost 360 days on an average, or almost a
-complete year; the Earth describing therefore almost a whole revolution
-around the Sun during the time it takes Saturn to advance 12° 2' on its
-orbit. Then, when Saturn occupies a position comprised within an arc 6°
-1' from either side of the nodes of its ring, the Earth, by its motion,
-is liable to encounter the plane of the ring, when therefore it will
-only present its thin edge to us, and becomes invisible. At least one
-such encounter is unavoidable within the time during which Saturn
-occupies either of these positions on its orbit; while three frequently
-happen, and two are possible.
-
-The natural impression received by looking at the rings, while seeing
-the ponderous globe of Saturn enclosed in its interior, is that this
-gigantic, but very delicate structure, in order to avoid destruction,
-must be endowed with a swift movement of rotation on an axis
-perpendicular to its plane, and that the centrifugal force thence
-arising counterbalances the powerful attraction of the planet, and thus
-keeps the system in equilibrium.
-
-Theoretically, the rotation of the rings is admitted by every
-astronomer, as being an essential condition to the existence of the
-system, which otherwise, it is thought, would fall upon the planet.
-Although the rotation of the rings seems so probable that it is
-theoretically considered as certain, yet its existence has not been
-satisfactorily demonstrated by direct observation, which alone can
-establish it on a firm basis as a matter of scientific knowledge.
-
-The determination of the period of rotation of the rings, which is
-supposed to be 10h. 32m. 15s., rests only on the observations of W.
-Herschel, made in 1790, from the apparent displacement of irregularities
-on the ring; but his results have been contradicted by other
-observations, and even by those of Herschel himself, made in later
-years.
-
-Although the system of rings is very nearly concentric with the globe of
-Saturn, yet the coincidence is not considered as mathematically exact.
-It seems to have been satisfactorily demonstrated by direct observations
-that the centre of gravity of the system oscillates around that of the
-planet, thus describing a minute orbit. This peculiarity is in
-accordance with theory, which has shown it to be essential to the
-stability of the system.
-
-Besides its system of rings, which makes Saturn the most remarkable
-planet of the solar system, this globe is attended by eight satellites,
-moving in orbits whose planes very nearly coincide with the plane of the
-rings, except that of the most distant one, which has an inclination of
-about 12° 14'. In the order of their distance from the planets, the
-satellites of Saturn are as follows: Mimas, Enceladus, Tethys, Dione,
-Rhea, Titan, Hyperion and Iapetus. The three first satellites are nearer
-to Saturn than the Moon is to the Earth; while Iapetus, the farthest, is
-9½ times the distance of our satellite from us. All the satellites,
-with the exception of the farthest, move more rapidly around Saturn than
-the Moon moves around the Earth; while Iapetus, on the contrary, takes
-almost three times as long to make one revolution.
-
-The period of revolution of the four inner satellites is accomplished in
-less than three days, that of Mimas being only a little more than 22
-hours. From such swiftness of motion, it is easily understood how short
-must be the intervals between the different phases of these satellites.
-Mimas, for instance, passes from New Moon to First Quarter in less than
-6 hours.
-
-The distance of the nearest satellite from the planet's surface is
-84,000 miles, and its distance from the outer ring only 36,000 miles. It
-is difficult to determine the diameter of objects so faint and distant
-as are some of these satellites, but the diameter of Titan, the largest
-of all, is pretty well known, and estimated to be ¹⁄₁₆ the
-diameter of the planet, or more than half the diameter of our globe.
-
-Iapetus is subject to considerable variations in brilliancy, and as the
-maxima and minima always occur when this satellite occupies the same
-parts of its orbit, it was conjectured by W. Herschel that, like our
-Moon, it turns once upon its axis during each of its revolutions about
-the planet. It has been shown by my observations, that Iapetus attains
-its maximum brightness a little before it reaches its greatest western
-elongation, and its minimum on the opposite side.
-
-As the planes of the orbits of the satellites are inclined to the
-planet's orbit, it follows that their transits, occultations and
-eclipses, are only possible when Saturn is near its equinoxes. Passages
-of the satellites and their shadows across the disk, although rare, have
-been observed, and they somewhat resemble the phenomena exhibited by the
-satellites of Jupiter in transit. When the Earth is very near the plane
-of the rings, the satellites, except the farthest, appear to be in a
-straight line nearly coincident with the plane of the rings, and are
-seen occasionally moving along the thin edge of the rings, appearing as
-luminous beads moving on a thread of light.
-
-Owing to the considerable inclination of the axis of rotation of Saturn
-to its orbit, the seasons of this planet must have greater extremes of
-temperature than those of the Earth. As the year of Saturn consists of
-25,217 Saturnian days, each season, on the average, is composed of 6,304
-Saturnian days.
-
-To an observer on Saturn, the immense arches formed by its rings would
-appear as objects of great magnificence, spanning the sky like soft
-colorless rainbows. Moreover, the eight moons, several of which are
-always visible, would be of the highest interest, with their swift
-motions and rapid phases. Mimas, traveling in its orbit at the rate of
-16 of arc per minute of time, moves over a space equal to the apparent
-diameter of our Moon in two minutes, or at the rate of 16° an hour.
-
-Owing to the globular form of Saturn, the rings would be invisible in
-latitudes situated above 65° from its equator, and their apparent form
-and breadth would naturally vary with the latitude. At 63° only a very
-small portion of the outer ring would be visible above the equatorial
-horizon, where it would appear as a small segment of a circle. At 62°
-the principal division would just graze the horizon. At 46° the outer
-portion of the dusky ring would become visible, while at 35° its inner
-edge would appear above the horizon. From 65° of latitude down to the
-equator, the arches of the rings would be seen more and more elevated
-above the equatorial horizon, but at the same time that they are seen
-higher up, their apparent breadth gradually diminishes, owing to the
-effect of foreshortening, and at the equator itself the system would
-only present its thin edge to view.
-
-During the summer seasons of either hemisphere of Saturn, the surface of
-the rings turned towards such hemisphere, being fully illuminated by the
-Sun, is visible from these regions. In the day time its light must be
-feeble and similar to the light reflected by our Moon during sunshine;
-but at night the system would display all its beauty, and the different
-rings, with their divisions and their various reflective powers, must
-present a magnificent sight.
-
-During the nights of the long winter seasons on Saturn, on the contrary,
-the surface of the rings turned towards the hemisphere undergoing
-winter, receives no light from the Sun, and is invisible, or very nearly
-so, except towards morning and evening, when it may be faintly
-illuminated by the secondary light which it receives from the
-illuminated globe of Saturn. Although dark and invisible, the rings may
-make their form apparent at night by the absence of stars from the
-region which they occupy in the sky. Again, in other seasons, the days
-present very curious phenomena. In consequence of the diurnal rotation
-of the planet, the Sun seems to move in circular arcs, which, owing to
-the inclination of Saturn's axis, are more or less elevated above its
-horizon, according to the position of the planet in its orbit. As such
-arcs described by the Sun in the sky of Saturn are liable to encounter
-the rings, the Sun in passing behind them becomes eclipsed. It must be a
-magnificent spectacle to witness the gradual disappearance of the fiery
-globe behind the outer ring, and its early reappearance, but for a
-moment only, through the narrow gap of the principal division; to see it
-vanish again behind the middle ring, to reappear a little later through
-the semi-transparent dusky ring, but very faint and red colored at
-first; and then, gradually brighten up, and finally emerge in all its
-beauty from the inner edge of the dusky ring.
-
-It is in latitude 23° that the rings produce the most prolonged
-eclipses of the Sun. During a period equivalent to ten of our
-terrestrial years, such eclipses continually succeed each other with but
-very short periods of interruption; and even during a long series of
-rotations of Saturn, the Sun remains completely invisible in those
-regions where the apparent arcs which it describes coincide with the
-arcs of the rings. In neighboring latitudes, the eclipses of the Sun,
-although still frequent, would have a shorter and shorter duration as
-the observer should travel north or south. These eclipses of the Sun
-must produce a partial darkness of the regions involved in the shadow of
-the rings, which may be compared to the darkness produced on our globe
-by a total eclipse of the Sun. The frequent recurrence of these
-eclipses, and their comparatively long duration in some regions, must
-still further reduce the duration of the short Saturnian days.
-
-The globe of Saturn, as already shown, casts a shadow on the rings,
-which, according to the position of the planet in its orbit, either
-extends across their whole breadth, or covers only a part of their
-surface. The shadow on the rings rising in the east after sunset,
-ascends to the culminating point of their arcs in the sky, in 2h. 34m.,
-and as rapidly descends on the western horizon, to disappear with
-sunrise. This shadow, when projected on the rings in the sky, must be
-hardly distinguishable from the dark background of the heavens, except
-from the absence of stars in the regions which it occupies. It must
-appear as a large dark gap, separating the rings into two parts, and
-constantly moving from east to west. Possibly the refraction of the
-solar rays, in passing through Saturn's atmosphere, may cast some
-colored light on the rings, similar to that observed on the Moon during
-its eclipses.
-
-An observer on the rings would behold phenomena still more curious, a
-long day of 14¾ years being followed by a long night of 14¾ years. The
-long days of Saturn's rings are, however, diversified by numerous
-eclipses of the Sun, which regularly occur every 10¼ hours; the
-phenomenon being due to the interposition of the globe of Saturn between
-the rings and the Sun. These eclipses produce partial obscurations of
-their surface, lasting from 1½ to 2 hours at a time. Although the
-surface of the rings never receives direct sunlight during their long
-nights, yet they are not plunged all the time in total darkness, as they
-receive some reflected light from that part of the globe of Saturn which
-is illuminated by the Sun. To the supposed observer on the rings, during
-every 10¼ hours, the immense globe would exhibit continually changing
-phases. At first he would see a point of light rapidly ascending from
-the horizon, and appearing under the form of a half crescent of
-considerable radius; 5⅛ hours later, the crescent having gradually
-increased, would appear as a half circle, covering ⅛ of the visible
-heavens, its surface being more than 20,000 times as large as the
-surface of the Moon. Upon this brilliantly illuminated semi-circle would
-be projected the shadows of the rings, appearing as black belts
-separated by a narrow luminous band.
-
-It is very difficult for one to conceive how such a delicate structure,
-as the system of rings appears to be, can keep together in equilibrium
-and avoid destruction from the powerful attraction of the planet on one
-side and the disturbing influence of the satellites on the other. To
-explain it, several hypotheses have been advanced. The rings were first
-supposed to be solid, and upon this supposition Laplace determined the
-necessary conditions for their equilibrium; the most important of which
-require that the cross section of the rings should be an ellipse of
-irregular curvature, and having its major axis directed towards the
-centre of the planet, and also that the system should rotate upon an
-axis perpendicular to the plane of the rings. This theory was superseded
-by another, which supposed the rings to be fluid. This one was soon
-rejected for a third, assuming the system to be composed of vapors or
-gases; and more recently, all these theories were considered untenable,
-and replaced by a fourth, which supposes the system of rings to be made
-up of a congregation of innumerable small, independent bodies, revolving
-around Saturn in concentric zones. Naturally, such a divergence of
-opinion can only result from our comparative ignorance of the subject,
-and sufficiently indicates our inability to explain the phenomena; and
-it must be admitted that, so far, nothing is certainly known about this
-strange system. We shall probably remain in the same uncertainty until
-the rotation of the rings is ascertained by direct observations. It is
-pretty certain, however, that none of these theories account for the
-observed phenomena in their details, although a partial explanation may
-be obtained by borrowing something from each hypothesis.
-
-It has been conjectured, and a theory has been advanced, that the
-breadth of the whole ring system is gradually increasing inwards, and
-that it will come in contact with the planet in about 2,150 years; but
-the question seems to have been settled in the negative by the elaborate
-measurements of the English observers. It is likely that the increase is
-only in the defining power of the instruments.
-
-
-
-
-COMETS
-
-PLATE XI
-
-
-Among the celestial phenomena, none are more interesting than those
-mysterious apparitions from the depths which unexpectedly display their
-strange forms in our familiar constellations, through which they wander
-for a time, until they disappear like phantoms.
-
-A comet, with its luminous diffused head, whence proceeds a long vapory
-appendage gradually fading away in the sky, presents an extraordinary
-aspect, which may well astonish and deeply impress the observer.
-Although these visitors from infinite space do not now inspire dread, as
-in by-gone times, yet, owing to the mystery in which the phenomenon is
-still involved, the apparition of a large comet, even in our days, never
-fails to create a profound sensation, and in some cases that unconscious
-fear which results from the unknown.
-
-The effect of such a spectacle largely depends upon its rarity; but
-since the telescope has been applied to the sounding of the heavens, it
-has been found that the appearance of comets is by no means an unusual
-occurrence. If so few comets, comparatively, are seen, it is because
-most of them are telescopic objects, and are therefore invisible to the
-naked eye. Most of the telescopic comets are not only too faint to be
-perceived by the unaided eye, but are insignificant objects, even when
-observed through the largest telescopes.
-
-It was Kepler's opinion that comets are as numerous in the sky as fishes
-are in the ocean. Undoubtedly the number of these bodies must be great,
-considering that we can only see them when they come into the
-neighborhood of the Earth, and that many even here remain invisible, or
-at least pass unperceived. That many of them have passed unperceived
-heretofore, is proved by the fact that the number of those observed
-becomes greater every year, with the increase of the number of
-instruments used in their search. The number of comets observed with the
-naked eye during historic times is nearly 600, and that of telescopic
-comets, which, of course, all belong to the last few centuries, is more
-than 200, so that we have a total number of about 800 comets of which
-records have been kept. From theoretical considerations, Lambert and
-Arago estimated their entire number at several millions, but such
-speculations have generally no real value, since they cannot be
-established on a firm basis.
-
-Comets remain visible for more or less time, according to their size and
-the nature and position of their orbits, but in general, the large ones
-can be followed with the telescope for several months after they have
-become invisible to the naked eye. The comet of 1861, for example,
-remained telescopically visible for a year, and that of 1811, for 17
-months after disappearing from ordinary sight.
-
-While a comet remains visible, it appears to revolve daily about us like
-the stars in general; but it also moves among the constellations, and
-from this movement its orbit may be computed like that of a planet. From
-the apparent diurnal motion of a comet with the heavens, result the
-changes of position which it seems to undergo in the course of a night.
-The direction of the head and tail of a comet, of course, has only
-changed in regard to the horizon, but not in regard to the sky, in which
-they occupy very nearly the same position throughout a given night, and
-even for many nights in succession.
-
-The movements of the comets in their orbits are, like those of the
-planets, in accordance with Kepler's laws, the Sun occupying one of the
-foci of the orbit they describe; but the orbits of comets differ,
-however, in several points from those of the planets. Their eccentricity
-is always great, being sometimes apparently infinite, in which case the
-orbit is said to be parabolic, or hyperbolic; but the smallness of the
-portion of a cometary orbit which can ordinarily be observed, makes it
-difficult to determine this with certainty. Again, while the planetary
-orbits are usually near the plane of the ecliptic, those of comets
-frequently have great inclinations to that plane, and even when the
-inclination is less than 90°, the comet may have a retrograde movement,
-or, in other words, a movement contrary to the course in which all the
-planets revolve about the Sun.
-
-Notwithstanding these differences between the elements of the orbits of
-the comets and those of the planets, the fact that each has the Sun in
-one focus indicates that the body moving in it is a member of the solar
-system, either for the time, or permanently, according to the nature of
-its orbit.
-
-A distinction may accordingly be made between the comets which are
-permanent members of our solar system and those which are only
-accidental or temporary visitors. Those moving in elliptical orbits
-around the Sun, like the planets, and therefore having a determinate
-period of revolution, from which the time of their successive returns
-may be predicted, are permanent members of our system, and are called
-periodic comets. All comets moving in parabolical or hyperbolical
-curves, are only temporary members of the solar system, being apparently
-strangers who have been diverted from their courses by some disturbing
-influence. No comet is classed as periodical which does not follow a
-perceptibly elliptical orbit. Any comet passing around the Sun at the
-mean distance of the Earth from this body, with a velocity of 26 miles
-per second, will fly off into infinite space, to return to us no more.
-
-The time of revolution of the different periodic comets thus far
-observed varies greatly, as do also the distances to which they recede
-from the Sun at aphelion. Whilst the period of revolution of Encke's
-comet, the shortest thus far known, is only 3½ years, that of the comet
-of 1844, II., is 102,000 years; and whilst the orbit of the first is
-comprised within the orbit of Jupiter, that of the last extends to a
-distance equal to 147 times the distance of Neptune from the Sun. But so
-vast an orbit cannot be accurately determined from the imperfect data at
-our disposal.
-
-The periodic comets are usually divided into two classes. The comets
-whose orbits are within the orbit of Neptune are called interior comets,
-while those whose orbits extend beyond that of Neptune are called
-exterior comets. The known interior periodic comets are twelve in
-number, while, including all the cases in which there is some slight
-evidence of elliptic motion, the number of exterior comets observed is
-six or seven times as great. The periodic comets of short period are
-very interesting objects, inasmuch as by their successive returns they
-afford an opportunity to calculate their motions and to observe the
-physical changes which they undergo in their intervals of absence.
-
-From observation of the periodic comets, it has been learned that the
-same comet never presents twice the same physical appearance at its
-different returns, its size, shape and brilliancy varying so greatly
-that a comet can never be identified by its physical characters alone.
-It is only when its elements have been calculated, and are found to
-agree with those of a cometary orbit previously known, that the two
-comets can be identified one with the other. There are reasons to
-believe that, in general, comets decrease in brightness and size at each
-of their successive returns, and that they are also continually losing
-some of their matter as they traverse their orbits.
-
-When very far away from us, all comets appear nearly alike, consisting
-of a faint nebulosity, of varying dimensions. When a comet first appears
-in the depths of space, and travels towards the Sun, it generally
-resembles a faint, uniformly luminous nebulosity, either circular or
-slightly elongated in form. As it approaches nearer to the Sun, a slight
-condensation of light appears towards its centre, and as it draws still
-nearer, it becomes brighter and brighter, and in condensing forms a kind
-of diffused luminous nucleus. At the same time that the comet acquires
-this concentration of light, the nebulosity gradually becomes elongated
-in the direction of the Sun. These effects generally go on increasing so
-long as the comet is approaching the Sun; the condensation of light
-sometimes forms a bright nucleus, comparable to a very brilliant star,
-while the elongation becomes an immense appendage or tail. When the
-comet has passed its perihelion and recedes from the Sun, the inverse
-phenomena are observed; the comet, decreasing in brightness, gradually
-loses its nucleus and tail, resumes its nebulous aspect, and finally
-vanishes in space, to appear again in due course, if it chance to be a
-periodic comet. While all comets become brighter in approaching the Sun,
-they do not all, however, develop a large tail, some of them showing
-only a slight elongation.
-
-When a comet is first discovered with the telescope at a great distance
-from the Sun, it is difficult to predict whether it will become visible
-to the naked eye, or will remain a telescopic object, as it is only in
-approaching the Sun that these singular bodies acquire their full
-development. Thus, Donati's comet, whose tail became so conspicuous an
-object at its full appearance in 1858, remained two months after its
-discovery by the telescope without any indication of a tail. The comet
-of Halley, which before and after its return in 1759, remained five
-years inside of the orbit of Saturn, showed not the least trace of its
-presence during the greater part of this time. Nothing but calculation
-could then indicate the position in the sky of this invisible object,
-which was so prominent when it approached the Sun.
-
-Another curious phenomenon exhibited by comets, and first noticed by
-Valz, is that in approaching the Sun the nebulosity composing these
-bodies contracts, instead of dilating, as would be naturally supposed
-from the greater amount of solar heat which they must then receive. In
-receding from the Sun, on the contrary, they expand gradually. As comets
-approach the Sun, the tail and nucleus are developed, while the
-nebulosity originally constituting these comets contracts, as if its
-material had been partly consumed in this development. In a certain
-sense it may be said that the comets are partly created by the Sun; in
-more exact terms, the changes of form which they undergo are induced by
-the Sun's action upon them at different distances and under varying
-conditions. Moreover, they are rendered visible by its influence,
-without which they would pass unperceived in our sky. When a comet
-disappears from view, it is not because its apparent diameter is so much
-reduced by the distance that it vanishes, but rather on account of the
-diminution of its light, both that which it receives from the Sun, and
-its own light; these bodies being in some degree self-luminous, as will
-be shown below.
-
-The large comets, such as can be seen with the naked eye, always show
-the following characteristics, on examination with the telescope. A
-condensation of light resembling a diffused star forms the brightest
-part of the comet, this condensation being situated towards the
-extremity the nearest to the Sun. It is this starlike object which is
-called the _nucleus_. The nucleus seems to be entirely enclosed in a
-luminous vapory envelope of the same general texture, called the _coma_.
-This envelope, which is quite variable in brightness and form, is
-brightest next to the nucleus, and gradually fades away as it recedes
-from it. The _nucleus_ and the _coma_, considered as a whole, constitute
-the _head_ of a comet. From the head of a comet proceeds a long trail of
-pale nebulous light, which usually grows wider, but fainter, as it
-recedes from the nucleus, and insensibly vanishes in the sky. This
-delicate appendage, or tail, as it is commonly called, varies very much
-in size and shape, not only in different comets, but in the very same
-comet, at different times. Its direction is generally opposite to that
-of the Sun from the head of the comet.
-
-The nuclei vary very much in brightness, in size and in shape; and while
-in some telescopic comets they are either absent or barely
-distinguishable as a small condensation of light, in bright comets they
-may become plainly visible to the naked eye, and they sometimes even
-surpass in brightness the most brilliant stars of the heavens. But
-whatever may be the size of cometary nuclei, they are subject to sudden
-and rapid changes, and vary from day to day. Sometimes they appear
-exceedingly brilliant and sharply outlined, while at other times they
-are so dim and diffused that they are hardly distinguishable from the
-coma of which they seem then to form a part.
-
-
-[PLATE XI.--THE GREAT COMET OF 1881.
-
-Observed on the night of June 25-26 at 1h. 30m. A.M.]
-
-
-From my observations upon the comets which have appeared since the year
-1873, it is apparent that the changes in the nucleus, coma and tail, are
-due to a solar action, which contracts or expands these objects in such
-a manner that the nuclei become either bright and star-like, or dim and
-diffused, in a very short time. I had excellent opportunity, especially
-in the two large comets of 1881, to observe some of these curious
-changes, a description of which will give an idea of their extent and
-rapidity. On July 2d, 1881, at 9 o'clock, the nucleus of comet 1881,
-III., which is represented on Plate XI., appeared sharply defined,
-bright and considerably flattened crosswise; but half an hour later it
-had considerably enlarged and had become so diffused that it could
-hardly be distinguished from the coma, with which it gradually blended.
-It is perhaps worth mention that, at the time this last observation was
-made, an aurora borealis was visible. This comet 1881, III., underwent
-other very important changes of its nucleus, coma and tail. On June
-25th, the nucleus, which was bright and clearly defined, was ornamented
-with four bright diverging conical wings of light, as shown on Plate XI.
-On the 26th these luminous wings had gone, and the nucleus appeared
-one-third smaller. On the 28th it had enlarged, but on the 29th its
-shape was considerably altered, the nucleus extending in one direction
-to three or four times its diameter on previous nights, and being
-curved, so as to resemble a comma. On the 6th of July the nucleus of
-this comet showed the greatest disturbances. The nucleus, which had
-appeared perfectly round on the evening of the 5th, was found much
-elongated at 10 o'clock on the 6th, forming then a straight, acute, and
-well-defined wedge of light, inclined upwards to the left. The length of
-the nucleus, at this time, was three or four times its ordinary
-diameter. At the same time rapid changes occurred; the strangely shaped
-nucleus soon became unsteady, extending and contracting alternately, and
-varying greatly in brightness. At 10h. 45m., the elongated nucleus, then
-gently curved, took the shape of a succession of luminous knots, which
-at times became so brilliant and distinct that they seemed to be about
-to divide and form separate nuclei; but such a separation did not
-actually occur, at least while I was observing. While these important
-changes were going on in the comet, a bright auroral arch appeared in
-the north, which lasted only a short time. On July 7th, the sky being
-cloudy, no observations were made, but on the 8th I observed the comet
-again. The nucleus had then resumed its circular form, but it was yet
-very unsteady, being sometimes small, bright and sharp, while a few
-seconds later it appeared twice as large, but dim in outlines; and
-sometimes an ill-defined secondary nucleus appeared at its centre. On
-several occasions the nucleus appeared as if it were double, one nucleus
-being apparently projected partly upon the other.
-
-The nuclei of comets are sometimes very small, and in other cases very
-large. Among those which have been measured, the nucleus of the comet of
-1798, I., was only 28 miles in diameter, but that of Donati's comet, in
-1858, was 5,600 miles, and that of the comet of 1845 was 8,000 miles in
-diameter.
-
-The coma of comets is found to be even more variable than the nucleus.
-The changes observed in the coma are generally in close connection with
-those of the nucleus and tail, the same perturbations affecting
-simultaneously the whole comet. While the coma of the comet of 1847 was
-only 18,000 miles in diameter, that of Halley's comet, in 1835, was
-357,000 miles, and that of the comet of 1811 was 1,125,000 miles in
-diameter. In general, as already stated, the coma of a comet decreases
-in size in approaching the Sun. That of Encke's comet, which, on October
-9th, 1838, had a diameter of 281,000 miles, gradually decreased at a
-daily mean rate of 4,088 miles in going towards the Sun; so that, on
-December 17th, when the distance of the comet from the Sun was more than
-four times less than it was on the first date, its diameter was reduced
-to 3,000 miles.
-
-The form of the coma, in that part which is free from the tail, is in
-general a portion of a circle, but is sometimes irregular, with its
-border deformed. Thus, the border of the coma of Halley's comet was
-depressed at one point towards the Sun. I observed a similar phenomenon
-in Coggia's comet, with the great refractor of the Harvard College
-Observatory, on July 13th, 1874, when its border appeared deeply
-depressed on the side nearest to the Sun, as if repelled by this body.
-The coma of comet 1881, III., showed also very singular outlines on the
-nights of the 25th and 26th of June, when its border was so deeply
-depressed that the coma appeared as if it were double. Luminous rays and
-jets often radiate from the nucleus across the coma, and describe
-graceful lateral curves, falling backwards and gradually fading away
-into the tail, of which they then form a part. The rays and jets emitted
-by the nucleus seem at first to obey the solar attraction and travel
-towards the Sun; but they are soon repelled, and move backward towards
-the tail. It is a mystery, as yet unexplained, how these cometary jets,
-which at first seem to obey to the laws of attraction, are compelled to
-retreat apparently by superior opposing forces. Among the forces of
-nature, we know of no other than those of an electrical sort, which
-would act in a similar manner; but this explanation would require us to
-assume some direct electrical communication between the comet and the
-Sun. Considering the distance between the two bodies, and the probable
-absence or great tenuity of the gaseous material in interstellar space,
-such an assumption is a difficult one.
-
-Under the action of the solar forces, the coma also very frequently
-forms itself into concentric luminous arcs, separated by comparatively
-dark intervals. These luminous semi-circles vary in number, but
-sometimes there are as many as four or five at a time. All great comets
-show these concentric curves more or less, but sometimes only a portion
-is visible, the rest of the coma having a different structure. When
-great comets approach near the Sun, their coma is generally composed of
-two distinct parts, an inner and an outer coma, the inner one being due
-to the luminous jets issuing from the nucleus, which, never extending
-very far, form a distinct, bright zone within the fainter exterior coma.
-
-The tails of comets, which are in fact a prolongation of the coma, are
-likewise extremely variable in form. They are sometimes straight like a
-rod; again, are curved like a sabre, or even crooked like an S, as was
-that of the comet of 1769. They are also fan-shaped, pointed, or of the
-same width throughout. Many of these appendages appear longitudinally
-divided through their middle by a narrow, darkish rift, extending from
-the nucleus to the extremity. This peculiarity appears in the comet
-shown on Plate XI. Sometimes the dark rift does not commence near the
-nucleus, but at some distance from it, as I observed in the case of
-comet 1881, III., on June 26th. This dark rift is not a permanent
-feature of a comet's tail, but may be visible one day and not at all the
-next. Comet 1881, III., which had shown a dark rift towards the end of
-June, did not exhibit any such rift during July and August, when, on the
-contrary, its tail appeared brighter in the middle. Coggia's comet,
-which showed so prominent a dark rift in July, 1874, had none on June
-10th. On the contrary, the tail was on that date very bright along its
-middle, as also along each of its edges.
-
-The tail of a comet does not invariably point directly away from the
-Sun, as above mentioned, and sometimes the deviation is considerable;
-for instance, the tail of the comet of 1577 deviated 21° from the point
-opposite to the Sun.
-
-In general, the tail inclines its extremity towards the regions of space
-which it has just left, always presenting its convex border to the
-regions towards which it is moving. It is also a remarkable fact that
-this convex border, moving first in space, always appears brighter and
-sharper than the opposite one, which is often diffused. From these
-peculiarities it would seem that in moving about the Sun the comets
-encounter some resistance to their motion, from the medium through which
-they pass, and that this resistance is sufficient to curve their tails
-away from the course in which they move, and to crowd their particles
-together on the forward side. It is especially when they approach their
-perihelion, and move more rapidly on a curve of a shorter radius, that
-the comets' tails show the greatest curvature, unless their position in
-regard to the observer prevents their being advantageously seen. The
-tail of Donati's comet presented a fair illustration of this
-peculiarity, its curvature having augmented with the velocity of the
-comet's motion about the Sun. But possibly this phenomenon has another
-cause, and may be found rather in the solar repulsion which acts on
-comets and is not instantaneously propagated throughout their mass.
-
-Although, in general, comets have but one tail, it is not very rare to
-see them with multiple tails. The comets of 1807 and 1843 had each a
-double tail; Donati's comet, in 1858, showed several narrow, long
-rectilinear rays, issuing from its abruptly curved tail. The comet of
-1825 had five branches, while that of 1744 exhibited no less than six
-distinct tails diverging from the coma at various angles. In general
-character the multiple and single tails are similar. When a comet has
-two tails, it is not rare for the second to extend in the general
-direction of the Sun, as was the case with the great comet of 1881,
-III., represented on Plate XI. From July 14th to the 21st it exhibited
-quite an extended conical tail, starting obliquely downwards from the
-right side of the coma, and directed towards the Sun. From the 24th of
-July to the 2d of August this secondary tail was exactly opposite in its
-direction from that of the primary tail, and gave to the head a very
-elongated appearance. Comet 1881, IV., also exhibited a secondary
-appendage, not directed towards the Sun, but making an angle of about
-45° with the main tail.
-
-These cometary appendages sometimes attain prodigious dimensions. The
-comets of 1680 and 1769 had tails so extended that, after their heads
-had set under the horizon, the extremities of these immense appendages
-were still seen as far up as the zenith. In a single day the tail of the
-comet of 1843 extended 100°, and it was thrust from the comet "as a
-dart of light" to the enormous distance of 48,500,000 miles, and yet of
-this immense appendage nothing was left on the following day. The tail
-of Donati's comet, in 1858, attained a real length of 42,000,000 miles,
-while that of the great comet of 1843 had the enormous length of
-200,000,000 miles. If this last comet had occupied the position of the
-Sun, which it approached very nearly for a moment, the extremity of its
-tail would have extended 60,000,000 miles beyond the orbit of Mars.
-
-In some cases the tails of comets have been seen undulating and
-vibrating in a manner similar to the undulations and coruscations of
-light characteristic of some auroras. Many observers report having seen
-such phenomena. The comet of 1769 was traversed by luminous waves and
-pulsations, comparable to those seen in the aurora borealis. I myself
-observed these curious undulations in Coggia's comet in 1874, while the
-head of this object was below the horizon. For an hour the undulations
-rapidly succeeded each other, and ran along the whole length of the
-tail.
-
-Some of the brightest comets have shone with such splendor that they
-could be observed easily in full sunshine. Many comets, such as those of
-1577 and 1744, have equaled Sirius and Venus in brilliancy. The great
-comet of 1843, which suddenly appeared in our sky, was so brilliant that
-it was seen by many observers at noon time, within a few degrees from
-the Sun. I remember that I myself saw this remarkable object in the day
-time, with a number of persons, who were gazing at the wonderful
-apparition. So brilliant was this comet, that besides its nucleus and
-head, a portion of its tail was also visible in the day time, provided
-the observer screened his eyes from the full sunlight by standing in the
-shadow of some building.
-
-Of all the bodies revolving around the Sun, none have been known to
-approach so near its surface as did the comet of 1843. When it arrived
-at perihelion, the distance from the centre of its nucleus to the
-surface of the Sun's photosphere was only 96,000 miles, while the
-distance from surface to surface was less than 60,000 miles. This comet,
-then, went through the solar atmosphere, and in traversing it with its
-tremendous velocity of 366 miles per second, may very possibly have
-swept through some solar protuberances, many of which attain much higher
-elevations than that at which the comet passed. The comet of 1680 also
-approached quite near the surface of the Sun, and near enough to
-encounter some of the high solar protuberances, its distance at
-perihelion being about two-thirds of the Moon's distance from the Earth.
-The rapidity of motion of the comet of 1843 was such, when it approached
-the Sun, that it swept through all that part of its orbit which is
-situated north of the plane of the ecliptic in a little more than two
-hours, moving in this short time from one node to the other, or 1800.
-
-But if some comets have a very short perihelion distance, that of others
-is considerable. Such a comet was that of 1729, whose perihelion
-distance was 383,000,000 miles, the perihelion point being situated
-between the orbits of Mars and Jupiter.
-
-While some comets come near enough to the Sun at perihelion to be
-volatilized by its intense heat, others recede so far from it at
-aphelion that they may be said to be frozen. The shortest cometary
-aphelion distance known is that of Encke's comet, whose greatest
-distance from the sun is 388,000,000 miles. But that of the comet of
-1844 is 406,000,000,000 miles from the Sun. The comets of 1863 and 1864
-are so remote in space when they reach their aphelion points that light,
-with its velocity of 185,500 miles a second, would require 171 days in
-the first case, and 230 in the last, to pass from them to the Earth.
-
-The period of revolution of different comets also varies immensely.
-While that of Encke's comet is only 3½ years, that of comet 1864, II.,
-is 280,000 years.
-
-Among the periodic comets of short period, some have exhibited highly
-interesting phenomena. Encke's comet, discovered in 1818, is remarkable
-for the fact that its period of revolution diminishes at each of its
-successive returns, and consequently this comet, with each revolution,
-approaches nearer and nearer to the Sun. The decrease of the period is
-about 2½ hours at each return. Although the decrease is small, if it go
-on in future as it does at present, the inevitable consequence will be
-that this comet will finally fall into the Sun. This curious phenomenon
-of retardation has been attributed by astronomers to the existence of a
-resisting medium filling space, but so rare and ethereal that it does
-not produce any sensible effect on the movements of the planets. But
-some other causes may retard this comet, as similar retardations have
-not been observed in the case of other periodic comets of short period.
-These, however, are not so near to the Sun, and perhaps our luminary may
-be surrounded by matter of extreme tenuity, which does not exist at a
-greater distance from it.
-
-Another of the periodic comets which has exhibited a very remarkable
-phenomenon of transformation is Biela's comet, which divided into two
-distinct parts, moving together in the same direction. When this comet
-was first detected at its return in 1845, it presented nothing unusual,
-but in the early part of 1846 it was noticed by several astronomers to
-be divided into two parts of unequal brightness, forming thus a twin
-comet. At its next return in 1852, the two sister comets were still
-traveling in company, but their distance apart, which in 1846 was
-157,000 miles, had increased to 1,500,000 miles. At the two next returns
-in 1859 and 1865, their position not being very favorably situated for
-observation, the comets were not seen. In 1872 the position should have
-been favorable for observation, and they were consequently searched for,
-but in vain; neither comet was found. An astronomer in the southern
-hemisphere, however, found a comet on the track of Biela's, but
-calculation has shown that the two objects are probably not identical,
-since this comet was two months behind the computed position for
-Biela's. It will be shown in the following chapter that our globe
-probably crossed the orbit of Biela's comet on November 27th, 1872, and
-the phenomena resulting from this passage will be there described.
-
-It is seen from these observations that comets may be lost or dissipated
-in space by causes entirely unknown to us. Biela's comet is not the only
-one which has been thus disintegrated. Ancient historians speak of the
-separation of large comets into two or more parts. In 1661 Hevelius
-observed the apparent division of the comet of that year and its
-reduction to fragments. The return of this comet, calculated for 1790,
-was vainly waited for; the comet was not seen.
-
-Other comets, whose periods of revolution were well known, have
-disappeared, probably never to return. Such is Lexell's comet, whose
-period was 5⁶⁄₁₀ years; also De Vico's comet, both of which are
-now lost. It is supposed that Lexell's comet, which passed twice very
-near the giant planet Jupiter, had its orbit changed from an ellipse to
-a parabola, by the powerful disturbing influence of this planet, and was
-thus lost from our system. Several other comets, in traveling over their
-different orbits, have approached near enough to Saturn, Jupiter and the
-Earth to have their orbits decidedly altered by the powerful attraction
-of these bodies.
-
-But since comets are liable to pass near the planets, and several have
-orbits which approach that of the Earth, it becomes important for us to
-know whether an encounter of such a body with our globe is possible, and
-what would then be the result for us. Although that knowledge would not
-enable us to modify the possibilities of an encounter, yet it is better
-to know the dangers of our navigation through space than to ignore them.
-This question of a collision of the Earth with a comet has been answered
-in different ways, according to the ideas entertained in regard to the
-mass of these bodies. While some have predicted calamities of all kinds,
-such as deluges, conflagrations, or the reduction of the Earth to
-incandescent gases, others have asserted that it would produce no more
-effect than does a fly on encountering a railroad train. In our days
-astronomers entertain very little fears from such an encounter, because
-the probabilities of danger from an occurrence of this sort are very
-slight, the mass of an ordinary comet being so small compared with that
-of our globe. We know with certainty that the Earth has never had an
-encounter with a comet _by which it has been transformed into gases_, at
-least within the several millions of years during which animal and
-vegetable life have left their marks upon the stony pages of its
-history, otherwise these marks would not now be seen. If, then, such an
-accident has not happened during this long period, the chances for its
-occurring must be very small, so small indeed that they might almost be
-left out of the question. It is true that our globe shows signs of great
-perturbations of its surface, but we have not the slightest proofs that
-they resulted from an encounter with a celestial body. It seems very
-probable that our globe passed through the tail of the comet of 1861,
-before it was first seen on June 29th; but nothing unusual was observed,
-except perhaps some phosphorescent light in the atmosphere, which was
-afterwards attributed to this cause.
-
-The density and mass of comets must be comparatively very small. Their
-tails consist of matter of such extreme tenuity that it affects but very
-little the light of the small stars over which they pass. The coma and
-nucleus, however, are not quite so transparent, and may have greater
-masses. On several occasions I have seen the light of stars reduced by
-the interposition of cometary matter, comet 1881, III., presenting
-remarkable cases of this sort. On July 8th, at 10h. 50m., several small
-stars were involved in this comet, one of which passed quite near the
-nucleus through the bright inner coma. At that time the comet was
-greatly disturbed, its nucleus was contracting and enlarging rapidly,
-and becoming bright and again faint in an instant. Every time that the
-nucleus grew larger, the star became invisible, but reappeared the
-moment the nucleus was reduced in size. This phenomenon could not be
-attributed to an atmospheric effect, since, while the nucleus was
-enlarging, a very small inner nucleus was visible within the large
-diffused one, the matter of which had apparently spread over the part of
-the coma in which the star was involved, making it invisible.
-
-That the mass of comets is small, is proved by the fact that they have
-sometimes passed near the planets without disturbing them in any
-sensible manner. Lexell's comet, which in 1770 remained four months very
-near Jupiter, did not affect in the least the orbits, or the motions of
-its satellites. The same comet also came within less than 1,500,000
-miles from the Earth, and on this occasion it was calculated that its
-mass could not have been the ¹⁄₅₀₀₀ part of that of our
-globe, since otherwise the perturbations which it would have caused in
-the elements of the Earth's orbit would have been sensible. There was,
-however, no change. If this comet's mass had been equal to that of our
-globe, the length of our year would have been increased by 2h. 47m. The
-comet of 1837 remained four days within 3,500,000 miles of the Earth,
-with no sensible effect.
-
-It seems quite difficult to admit that the denser part of a comet
-forming the nucleus is solid, as supposed by some physicists, since it
-is so rapidly contracted and dilated by the solar forces, while the
-comet is yet at a too great distance from the Sun to allow these effects
-to be attributed to solar heat alone. This part of a comet, as indeed
-the other parts, seems rather to be in the gaseous than in the solid
-state; the changes observed in the intensity of its light and in its
-structure may be conceived as due to some solar action partaking of the
-nature of electricity.
-
-It has been a question whether comets are self-luminous, or whether they
-simply reflect the solar light. When their light is analyzed by the
-spectroscope, it is found that the nucleus of a comet generally gives a
-continuous spectrum, while the coma and tail give a spectrum consisting
-of several bright diffused bands. The spectrum given by the nucleus is
-rarely bright enough to allow the dark lines of the solar spectrum to be
-discerned upon it; but such lines were reported in the spectrum of comet
-1881, III., a fact proving that this nucleus at least reflected some
-solar light. The nucleus of a comet may be partly self-luminous, and
-either solid, liquid, or composed of incandescent gases submitted to a
-great pressure. As to the coma and tail, they are evidently gaseous, and
-partly, if not entirely, self-luminous, as is proved by the band
-spectrum which they give. The position of these bands, moreover,
-indicates that the luminous gases of which they are composed contain
-carbon. The phenomena of polarization, however, seem to prove that these
-parts of comets also reflect some solar light.
-
-No theory so far proposed, to explain comets and the strange phenomena
-they exhibit, seems to have been successful in its attempts, and the
-mystery in which these bodies have been involved from the beginning of
-their apparition, seems to be now nearly as great as ever. It has been
-supposed that their tails have no real existence, but are due to an
-optical illusion. Prof. Tyndall has endeavored to explain cometary
-phenomena by supposing these bodies to be composed of vapors subject to
-decomposition by the solar radiations, and thus made visible, the head
-and tail being an actinic cloud due to such decompositions. According to
-this view, the tails of comets would not consist of matter projected
-into spacer but simply of matter precipitated by the solar rays in
-traversing the cometary nebulosity. The endeavor has also been made to
-explain the various phenomena presented by comets by an electrical
-action of the Sun on the gases composing these objects. Theories taking
-this as a base seem to us to be more likely to lead to valuable results.
-M. Faye, who has devoted much time and learning to this subject, assumes
-a real repulsive force of the Sun, acting inversely to the square of the
-distance and proportionally to the surface, and not to the mass as
-attraction does. He supposes, however, that this repulsive force is
-generated by the solar heat, and not by electricity. Prof. Wm. Harkness
-says that many circumstances seem to indicate that the comets' tails are
-due, in a great measure, to electrical phenomena.
-
-The fact that the tails of comets are better defined and brighter on the
-forward side, associated with the other fact that they curve the most
-when their motion is most rapid, sufficiently indicates that these
-appendages are material, and that they either encounter some resistance
-from the medium in which they move, or from a solar repulsion. The
-phenomena of condensation and extension, which I have observed in the
-comets of 1874 and 1881, added to the curious behavior exhibited by the
-jets issuing from the nucleus, seem to indicate the action of electrical
-forces rather than of heat. The main difficulty encountered in the
-framing of a theory of comets consists in explaining how so delicate and
-extended objects as their tails seem to be, can be transported and
-whirled around the Sun at their perihelion with such an enormous
-velocity, always keeping opposite to the Sun, and, as expressed by Sir
-John Herschel, "in defiance of the law of gravitation, nay, even of the
-received laws of motion."
-
-To consider the direction of the comets' tails as an indirect effect of
-attraction, seems out of the question; the phenomenon of repulsion so
-plainly exhibited by these objects seems to point to a positive solar
-repulsion, as alone competent to produce these great changes. The
-repulsive action of the Sun on comets' tails might be conceived, for
-instance, as acting in a manner similar to that of a powerful current of
-wind starting from the Sun, and constantly changing in direction, but
-always keeping on a line with the comet. Such a current, acting on a
-comet's tail as if it were a pennant, would drive it behind the nucleus
-just as observed. If it could once be ascertained that the great
-disturbances on comets correspond with the magnetic disturbances on our
-globe and with the display of the auroral light, the electric nature of
-the forces acting so strangely on the comets would be substantially
-demonstrated. I have shown that some of the great disturbances observed
-in the comets of 1874 and 1881 have coincided with auroral displays, and
-it will be shown hereafter that similar displays have also coincided
-with the passage of meteoric showers through our atmosphere. Whether
-these simultaneous phenomena were simple coincidences having no
-connection, or whether they are the result of a common cause, can only
-be ascertained by long continued future observations.
-
-
-
-
-SHOOTING-STARS AND METEORS
-
-PLATE XII
-
-
-While contemplating the heavens on a clear moonless night, we
-occasionally witness the sudden blazing forth of a star-like meteor,
-which glides swiftly and silently across some of the constellations, and
-as suddenly disappears, leaving sometimes along its track a
-phosphorescent trail, which remains visible for a while and gradually
-vanishes. These strange apparitions of the night are called _Falling_ or
-_Shooting-stars_.
-
-There is certainly no clear night throughout the year during which some
-of these meteors do not make their appearance, but their number is quite
-variable. In ordinary nights only four or five will be observed by a
-single person in the course of an hour; but on others they are so
-numerous that it becomes impossible to count them. When the falling
-stars are only a few in number, and appear scattered in the sky, they
-are called _Sporadic Meteors_, and when they appear in great numbers
-they constitute _Meteoric Showers_ or _Swarms_.
-
-Probably there is no celestial phenomenon more impressive than are these
-wonderful pyrotechnic displays, during which the heavens seem to break
-open and give passage to fiery showers, whose luminous drops describe
-fantastic hieroglyphics in the sky. While observing them, one can fully
-realize the terror with which they have sometimes filled beholders, to
-whom it seemed that the stability of the universe had come to an end,
-and that all the stars of the firmament were pouring down upon the Earth
-in deluges of fire.
-
-The ancients have left record of many great meteoric displays, and the
-manner in which they describe them sufficiently indicates the fear
-caused by these mysterious objects. Among the many meteoric showers
-recorded by ancient historians may be mentioned one observed in
-Constantinople, in the month of November, 472, when all the sky appeared
-as if on fire with meteors. In the year 599, meteors were seen on a
-certain night flying in all directions like fiery grasshoppers, and
-giving much alarm to the people. In March, 763, "the stars fell
-suddenly, and in such crowded number that people were much frightened,
-and believed the end of the world had come." On April 10th, 1095, the
-stars fell in such enormous quantity from midnight till morning that
-they were as crowded as are the hail stones during a severe storm.
-
-In modern times the fall of the shooting-stars in great number has been
-frequently recorded. One of the most remarkable meteoric showers of the
-eighteenth century occurred on the night of November 13th, 1799, and was
-observed throughout North and South America and Europe. On this
-memorable night thousands of falling stars were seen traversing the sky
-between midnight and morning. Humboldt and Boupland, then traveling in
-South America, observed the phenomena at Cumana, between two and five
-o'clock in the morning. They saw an innumerable number of shooting-stars
-going from north to south, appearing like brilliant fire-works. Several
-of these meteors left long phosphorescent trails in the sky, and had
-nuclei whose apparent diameter, in some cases, surpassed that of the
-Moon.
-
-The shower of November 13th, 1833, was still more remarkable for the
-great number of meteors which traversed the heavens, and was visible
-over the whole of North and South America. On that occasion the falling
-stars were far too numerous to be counted, and they fell so thickly that
-Prof. Olmsted, of New Haven, who observed them carefully, compared their
-number at the moment of their maximum fall to half that of the flakes of
-snow falling during a heavy storm. This observer estimated at 240,000
-the number of meteors which must have traversed the heavens above the
-horizon during the seven hours while the display was visible.
-
-
-
-[PLATE XII.--THE NOVEMBER METEORS.
-
-As observed between midnight and 5 o'clock A.M. on the night of November
-13-14 1868.]
-
-
-In the years 1866, 1867 and 1868, there were also extraordinary meteoric
-displays on the night of November 13th. It was on the last mentioned
-date that I had the opportunity to observe the remarkable shower of
-shooting-stars of which I have attempted to represent all the
-characteristic points in Plate XII. My observations were begun a little
-after midnight, and continued without interruption till sunrise. Over
-three thousand meteors were observed during this interval of time in the
-part of the sky visible from a northern window of my house. The maximum
-fall occurred between four and five o'clock, when they appeared at a
-mean rate of 15 in a minute.
-
-In general, the falling stars were quite large, many being superior to
-Jupiter in brightness and apparent size, while a few even surpassed
-Venus, and were so brilliant that opaque objects cast a strong shadow
-during their flight. A great many left behind them a luminous train,
-which remained visible for more or less time after the nucleus had
-vanished. In general, these meteors appeared to move either in straight
-or slightly curved orbits; but quite a number among them exhibited very
-extraordinary motions, and followed very complicated paths, some of
-which were quite incomprehensible.
-
-While some moved either in wavy or zig-zag lines, strongly accentuated,
-others, after moving for a time in a straight line, gradually changed
-their course, curving upward or downward, thus moving in a new
-direction. Several among them, which were apparently moving in a
-straight line with great rapidity, suddenly altered their course,
-starting at an abrupt angle in another direction, with no apparent
-slackening in their motion. One of them, which was a very conspicuous
-object, was moving slowly in a straight course, when of a sudden it made
-a sharp turn and continued to travel in a straight line, at an acute
-angle with the first, retreating, and almost going back towards the
-regions from which it originally came. As nearly all the meteors which
-exhibited these extraordinary motions left the trace of their passage in
-the sky by a luminous trail, it was easily ascertained that these
-appearances were not deceptive. On one occasion I noticed that the
-change of direction in the orbit corresponded with the brightening up of
-the meteor thus disturbed in its progress.
-
-Among these meteors, some traveled very slowly, and a few seemed to
-advance as if by jerks, but in general they moved very rapidly. One of
-the meteors thus appearing to move by jerks left a luminous trail, upon
-which the various jerks seemed to be left impressed by a succession of
-bright and faint spaces along the train. Some of the largest meteors
-appeared to rotate upon an axis as they advanced, and most of these
-revolving meteors, as also a great number of the others, seemed to
-explode just before they disappeared, sending bright fiery sparks of
-different colors in all directions, although no sound was at any time
-heard. The largest and most brilliant meteor observed on that night
-appeared at 5h. 30m., a little before sunrise. It was very bright, and
-appeared considerably larger than Venus, having quite a distinct disk.
-This meteor moved very slowly, leaving behind a large phosphorescent
-trail, which seemed to issue from the inside of the nucleus as it
-advanced. For a moment the train increased in size and brightness close
-to the nucleus, which then appeared as an empty transparent sphere,
-sprinkled all over with minute fiery sparks; the nucleus then suddenly
-burst out into luminous particles, which immediately vanished, only the
-luminous trail of considerable dimensions being left.
-
-Many of the trails thus left by the meteors retained their luminosity
-for several minutes, and sometimes for over a quarter of an hour. These
-trails slowly changed their form and position; but it is perhaps
-remarkable that almost all those which I observed on that night assumed
-the same general form--that of an open, irregular ring, or horse-shoe,
-somewhat resembling the letter C. This ring form was subsequently
-transformed into an irregular, roundish cumulus-like cloud. The trail
-left by a very large meteor, which I observed on the evening of
-September 5th, 1880, also exhibited the same general character of
-transformation.
-
-While I was observing a long brilliant trail left by a meteor on the
-night of November 13th, 1868, it was suddenly crossed by another bright
-shooting-star. The latter apparently went through the luminous substance
-forming the trail, which was suddenly altered in form, and considerably
-diminished in brightness simultaneously with this passage, although
-electrical action at some distance might perhaps as well explain the
-sudden change observed.
-
-In the majority of cases the meteors appeared white; but many,
-especially the largest, exhibited a variety of brilliant colors, among
-which the red, blue, green, yellow and purple were the most common. In
-general the trails exhibited about the same color as the nucleus, but
-much fainter, and they were usually pervaded by a greenish tint. In some
-instances the trails were of quite a different color from the nucleus.
-
-The luminous cloud observed at 5h. 30m. on the morning of November 14th,
-1868, after having passed through the series of transformations above
-described, remained visible for a long while after sunrise, appearing
-then as a small cirrus cloud, exactly similar in appearance to the
-hundreds of small cirrus clouds then visible in the sky, which had
-probably the same meteoric origin. For over three hours after sunrise,
-these cirrus clouds remained visible in the sky, moving all together
-with the wind in the high regions of the atmosphere.
-
-Although Plate XII. is intended to represent all the characteristics
-exhibited by the meteors observed on that night, every form represented
-having been obtained by direct observation, yet the number is much
-greater than it was at any single moment during the particular shower of
-1868. As regards number, the intention was to give an idea of a great
-meteoric shower, such as that of 1833, for instance. Although many of
-the falling stars seem to be close to the Earth's surface, yet this is
-only an effect of perspective due to their great distance, very few of
-these meteors ever coming into the lower regions of our atmosphere at
-all.
-
-The phenomena exhibited during other great meteoric showers have been
-similar to those presented by the shower just described, the only
-differences consisting in variations of size and brightness in the
-meteors, and also in the trails, which sometimes are not so numerous as
-they were in 1868.
-
-While some shooting-stars move so rapidly that they can hardly be
-followed in their orbits, others move so slowly that the sight can
-easily follow them, and even remark the peculiarities of their
-movements, some remaining visible for half a minute. Some of the falling
-stars move at the rapid rate of 100 miles a second, but others only 10
-miles a second, and even less. In general, they move about half as fast
-again as the Earth in its orbit. The arcs described by the meteors in
-the sky are variable. While some extend 80° and even 100°, others are
-hardly half a degree in length. While some shooting-stars are so faint
-that they can hardly be seen through the largest telescopes, others are
-so large and brilliant that they can be seen in the day-time. In
-general, a shooting-star of average brightness resembles a star of the
-third or fourth magnitude.
-
-Whatever may be the origin of the shooting-stars, they are, when we see
-them, not in the celestial spaces, like the planets, the comets, or the
-stars, but in our atmosphere, through which they travel as long as they
-remain visible. The height at which they appear and disappear is
-variable, but in general they are about 80 miles above the surface of
-our globe when they are first seen, and at about 55 miles when they
-disappear. In many cases, however, they have been observed at greater
-elevations, as also at smaller. A meteor simultaneously observed at two
-different stations first appeared at the height of 285 miles, and was
-last seen at 192 miles above the Earth's surface; but in rare cases the
-falling stars have been seen below a layer of clouds completely covering
-the sky. I myself saw one such shooting-star a few years since. The fact
-that the meteors are visible at so great elevations, proves that our
-atmosphere extends much farther than was formerly supposed, although at
-these great heights it must be extremely rarefied, and very different
-from what it is in its lower regions.
-
-There is a remarkable difference between the sporadic meteors seen in
-the sky on every night, and the meteoric showers observed only at
-comparatively rare intervals. While the first appear from different
-points in the sky and travel in all directions, being perfectly
-independent, the meteors of a shower all come from the same point of the
-heavens, from which they apparently diverge in all directions. This
-point of divergence of the meteors is called the _radiant point_ of the
-shower. Although the meteors seem to diverge in all directions from the
-radiant point, yet they all move in approximately parallel lines, the
-divergence being an effect of perspective.
-
-Whatever may be the position of the radiant point in the constellations,
-it remains as fixed in the sky as the stars themselves, and participates
-with them in the apparent motion which they undergo by the effect of the
-diurnal motion, and thus rises and sets with the constellation to which
-it belongs. This fact is sufficient to prove that the orbits of these
-meteors are independent of the Earth's motion, and that consequently
-they do not originate in our atmosphere. It has been shown by Encke that
-the radiant point of the meteoric shower of November 13th is precisely
-the point towards which our globe moves in space on November 13th; a
-tangent to the Earth's orbit would pass through this radiant point.
-
-The meteoric showers are particularly remarkable, not merely because of
-the large number of meteors which are visible and the fact that they all
-follow a common orbit, but chiefly because they have a periodic return,
-either after an interval of a year, or after a lapse of several years.
-At the beginning of the present century only two meteoric showers were
-known, those of August 10th and of November 13th, and their periodicity
-had not yet been recognized, although it had begun to be suspected. It
-was only in 1836 that Quetelet and Olbers ventured to predict the
-reappearance of the November meteors in the year 1867. Having made
-further investigations, Prof. Newton, of Yale College, announced their
-return in the year 1866. In both of these years, as also in 1868, the
-meteors were very numerous, and were observed in Europe and in America
-on the night of November 13th. The predictions having thus been
-fulfilled, the periodicity of the meteors was established. Since then,
-other periodic showers have been recognized, although they are much less
-important in regard to number than those of August and November, except
-that of November 27th, which exhibited so brilliant a display in Europe
-in 1872. These successive appearances have established the main fact
-that meteoric showers are more or less visible every year when the Earth
-occupies certain positions in its orbit.
-
-The meteoric shower of the 10th of August has its radiant point situated
-in the vicinity of the variable star Algol, in the constellation
-Perseus, from which its meteors have received the name of Perseids.
-Although varying in splendor, this meteoric swarm never fails to make
-its appearance every year. The Perseids move through our atmosphere at
-the rate of 37 miles per second. The shower usually lasts about six
-hours.
-
-The meteoric shower of November 13th has its radiant point situated in
-the vicinity of the star Gamma, in the constellation Leo, from which its
-meteors have been called Leonids. But while the August meteors recur
-regularly every year, with slight variations, the shower of November
-does not occur with the same regularity. During several years it is
-hardly noticeable, and is even totally absent, while in other years it
-is very remarkable. Every 33 years an extraordinary meteoric shower
-occurs on the 13th of November, and the phenomenon is repeated on the
-two succeeding years at the same date, but with a diminution in its
-splendor at each successive return. The Leonids move in an opposite
-direction to that of the Earth, and travel in our atmosphere with an
-apparent velocity of 45 miles per second, this being about the maximum
-velocity observed in falling stars. But when the motion of our globe is
-taken into account, and a deduction is made of the 18 miles which it
-travels per second, it is found that these meteors move at an actual
-mean rate of 27 miles a second.
-
-In a meteoric shower the stars do not fall uniformly throughout the
-night, there being a time when they appear in greater numbers. Usually
-it is towards morning, between 4 and 6 o'clock, that the maximum occurs.
-The probable cause of this phenomenon will be explained in its place
-hereafter.
-
-The orbits of the meteoric showers are not all approximately in the same
-plane, like those of the planets, but rather resemble those of comets,
-and have all possible inclinations to the ecliptic. Like the comets,
-too, the different meteoric showers have either direct or retrograde
-motion.
-
-The shooting-stars were formerly considered as atmospheric meteors,
-caused by the combustion of inflammable gases generated at the surface
-of the Earth, and transported to the high regions of our atmosphere by
-their low specific gravity. But the considerable height at which they
-usually appear, the great velocity of their motion, the common orbit
-followed by the meteors of the same shower, and the periodicity of their
-recurrence, do not permit us now to entertain these ideas, or to doubt
-their cosmical origin. But what is their nature?
-
-It is now generally admitted that innumerable minute bodies, moving in
-various directions around the Sun, are scattered in the interplanetary
-spaces through which our globe travels. It has been supposed that
-congregations of such minute bodies form elliptical rings, within which
-they are all moving in close parallel orbits around the Sun. On the
-supposition that such rings intersect the orbit of the Earth at the
-proper places, it was practicable to account for the shooting-stars by
-the passage through our atmosphere of the numerous minute cosmical
-bodies composing the rings, and the Leonid and Perseid showers were so
-explained. But when the elements of the orbits of these two last swarms
-came to be better known, and were compared with those of other celestial
-bodies, it was found necessary to alter this theory.
-
-It had for a long while been suspected that some kind of relation
-existed between the shooting-stars and the comets. This idea, vaguely
-formulated by Kepler more than two centuries ago, more clearly expressed
-by Chladni, and still more by Mr. Grey, before the British Association,
-at Liverpool, in 1855, has recently received a brilliant confirmation by
-the researches of Professor Schiaparelli, Director of the Observatory of
-Milan. A thorough investigation of the orbits of the August and November
-meteors led Schiaparelli to the discovery of a remarkable relation
-between meteoric and cometary orbits. By comparing the elements of these
-meteoric orbits with those of comets, he found a very close resemblance
-between the orbit of the August meteors and that of the comet 1862,
-III., and again between the orbit of the November meteors and that of
-Tempel's comet, 1866, I. These resemblances were too striking to be the
-result of mere chance, and demonstrated the identity of these cometary
-orbits with those of the Perseid and Leonid showers. In accordance with
-these new facts, it is now admitted that the meteoric showers result
-from the passage of our globe through swarms of meteoric particles
-following the orbits of comets, which intersect the orbit of the Earth.
-
-Professor Schiaparelli has attempted to show how these meteoric swarms
-were originally scattered along the orbits of comets, by supposing these
-bodies to originate from nebulous masses, which, in entering the sphere
-of attraction of the Sun, are gradually scattered along their orbits,
-and finally form comets followed by long trails of meteoric particles.
-
-It has been shown that in approaching the Sun the comets become
-considerably elongated, their particles being disseminated over immense
-distances by the solar repulsion. It seems probable that, owing to its
-feeble attractive power, the nucleus is incompetent to recall the
-scattered cometary particles and retain them in its grasp when they are
-relieved from the solar repulsion, so that they remain free from the
-nucleus, although they continue to move along its orbit. It is
-supposable that these cometary particles will scatter more and more in
-course of time. Forming at first an elongated meteoric cloud, they will
-finally spread along the whole orbit, and thus form a ring of meteoric
-particles. Since our globe constantly moves in its orbit and daily
-occupies a different position, it follows that at any point where such a
-cometary orbit happens to cross that of the Earth, our globe will
-necessarily encounter the cometary particles as a shower of meteors.
-This encounter will take place at a certain time of the year, either
-yearly, if they form a continuous ring, or after a succession of years,
-if they simply form an elongated cloud. Such meteoric clouds or rings
-would not be visible in ordinary circumstances, even through the largest
-telescopes, except on penetrating the upper regions of our atmosphere,
-when they would appear as showers of falling stars. It is supposed that
-in penetrating our atmosphere, even in its most rarefied regions, these
-meteors are heated by the resistance offered by the air to their motion,
-first becoming luminous and then being finally vaporized and burnt
-before they can reach the surface of the Earth.
-
-The orbit of the comet of 1862, III., which so closely corresponds with
-that of the Perseid meteors, is much more extended than that of Tempel's
-comet corresponding with that of the Leonids. While the first extends
-far beyond the orbit of Neptune, the latter only goes a little beyond
-that of Uranus. The former orbit makes a considerable angle with the
-plane of the Earth's orbit, but the latter is much nearer to parallelism
-with it. The period of revolution of the first is 108 years, and that of
-the last about 33¼ years.
-
-From the fact that the Perseid shower occurs yearly on the 10th of
-August, when the Earth crosses the orbit of the comet of 1862, III., it
-is supposed that the cometary particles producing this shower are
-disseminated along the whole orbit, and form a ring encircling the Sun
-and Earth. To explain the yearly variations in the number of the
-shooting-stars observed, these particles are supposed to be unequally
-distributed over the orbit, being more crowded at one place than they
-are at another. In order to explain the meteoric shower of Leonids,
-which appears in all its splendor every 33 years, and then with
-diminished intensity for two successive years, after which it is without
-importance, it is supposed that the cometary particles of the comet of
-1866, I., have not as yet spread all along the orbit, a sufficient time
-not having been allowed, but form an elongated meteoric cloud, more
-dense in its front than in its rear part. From these considerations it
-has been supposed also that the comet of 1866, I., is of a more recent
-date than that of 1862, III. While Tempel's comet makes its revolution
-around the Sun in about 33 years, this meteoric cloud, which has the
-same period and returns to the same point of its orbit every 33 years,
-encounters our globe for three successive years. The first year we are
-passing through its densest parts, and the two following years in less
-and less crowded parts, from which result the observed phenomena. An
-idea of the extent of this meteoric cloud may be formed from the fact
-that, with its cometary velocity of motion, it takes this cloud three
-years at least to cross the Earth's orbit. From recent researches it
-would appear that the Leonid cloud is not single, but that at least two
-others of smaller importance exist, and have periods of 33¼ years.
-
-Biela's comet, which was divided into two parts in 1846, is another of
-the few comets whose orbit approaches that of the Earth. Possessing this
-knowledge, and knowing then the close connection existing between
-meteors and comets, astronomers supposed that there were sufficient
-reasons to expect a meteoric shower when this comet was passing near the
-Earth. They consequently expected a meteoric display in 1872, when our
-globe was to cross its orbit. Their anticipation was plainly fulfilled,
-and on the night of November 27th, 1872, a splendid meteoric display,
-having its radiant point in the constellation Andromeda, was observed in
-Europe, and also in America, but the meteors seen here were not so
-numerous as in Europe. Other meteoric showers of less importance, such
-as that of April 20th, for instance, have also been identified with
-cometary orbits, so that now no doubt seems to remain as to the identity
-of cometary particles and shooting-stars.
-
-The fact that the maximum number of meteors is always observed in the
-morning hours, supports the hypothesis of the cosmic origin of the
-shooting-stars, since the regions of the Earth where it is morning are
-precisely those fronting the regions towards which our globe is moving
-in space, and accordingly encounter more directly the meteors moving in
-their orbit. The greater abundance of falling stars at that time may
-thus be accounted for.
-
-The number of meteors penetrating our atmosphere must be very great;
-there is not an hour and probably not a minute during which none fall.
-From various considerations, some astronomers have estimated at from
-65,000,000,000 to 146,000,000,000 the total number of shooting-stars
-yearly penetrating in our atmosphere. The actual number is undoubtedly
-great, yet the fact that the meteors are rarely seen through the
-telescope while employed in observing various celestial objects, does
-not indicate that they are so numerous as these figures imply. It is
-only occasionally that one is seen traversing the field of the
-instrument. Even when the sky is observed with a low power eye-piece for
-several hours in succession, many nights may pass without disclosing
-one, although an observer, sweeping the sky more freely with the naked
-eye, may often perceive four or five during an ordinary night.
-
-About the true nature of these bodies nothing is known with certainty.
-From spectrum analysis it seems to be established that most of them
-contain sodium and magnesium, while a few indicate the presence of
-strontium and iron, and in some rare cases there are traces of coal-gas.
-Some of the nuclei give a continuous spectrum, and others a spectrum of
-lines. The trail always gives a spectrum of bright lines which indicates
-its gaseous state. The traces of coal-gas rarely seen in meteors are,
-however, of great importance, as it identifies them more closely with
-the comets, which generally show a similar spectrum. The continuous
-spectra exhibited by some nuclei would indicate that they are
-incandescent and either solid or liquid; but it is difficult to conclude
-from their spectra what is their true nature, since we do not know
-exactly what part the terrestrial atmosphere may play in producing the
-results.
-
-The mass of the shooting-stars is not known with certainty, but the fact
-that during great meteoric showers, none are seen to reach the surface
-of the Earth, all being consumed in a few seconds, sufficiently
-indicates that it must be very small. It has been calculated that those
-equal to Venus in apparent size and brilliancy may weigh several pounds,
-while the faint ones would weigh only a few grains.
-
-If the shooting-stars have even such a mass as that here attributed to
-some of them, the extraordinary motions which I have described above
-seem to be unaccountable. The change of direction of a heavy mass moving
-swiftly cannot be sudden. The semi-circular, the wavy and the angular
-orbits observed could not be described, it would seem, by such a mass
-animated with a great velocity. Although the meteors are said to be
-ignited by the transformation of part of their progressive motion into
-molecular motion, yet it is not observed that the velocity of the
-falling stars diminishes when they are about to disappear. The luminous
-trails they leave in the atmosphere do not appear to be endowed with any
-motion, but remain for a time in their original positions. These facts
-are apparently opposed to the hypothesis that such meteors have any
-appreciable mass. The extraordinary motions exhibited by some meteors
-seem to indicate that some unsuspected force resides in these bodies,
-and causes them to deviate from the laws of ordinary motion.
-
-Although it is very probable that the ordinary shooting-stars have no
-appreciable mass, yet it is known that very heavy meteoric masses
-sometimes fall at the surface of the Earth. Such falls are generally
-preceded by the sudden apparition in the sky of a large, and usually
-very brilliant fire-ball, which traverses the air at a great speed,
-sometimes leaving behind it a luminous trail, after which it explodes
-with a loud sound, and heavy fiery meteoric fragments, diverging in all
-directions, fall at the surface of the Earth. The name of _Aerolites_ or
-_Meteorolites_ is given to these ponderous fragments. As these meteors,
-before they explode and fall to the ground, have many points of
-resemblance with the shooting-stars, they are generally supposed to be
-connected with them, and to have a similar cometary origin. The fact
-that the aerolites differ widely from each other in constitution, and
-are all composed of substances found on the Earth, associated with other
-facts given below, would rather seem to indicate a terrestrial than a
-celestial origin.
-
-If the aerolites belong to the same class of bodies as the falling
-stars, differing from them only in size and mass, it is difficult to see
-why so very few should fall upon the Earth during the great meteoric
-showers, when thousands of shooting-stars traverse our atmosphere. In
-Prof. Kirkwood's "Meteoric Astronomy" are given catalogues of all the
-falls of aerolites and fire-balls which have been observed at the time
-of the periodic meteoric showers of the 10th of August and the 13th of
-November, during a period of 221 years for the Perseids, or August
-showers, and of 318 years for the Leonids, or November showers. During
-221 years, 10 falls of aerolites have been witnessed simultaneously with
-the fall of the Perseids; while during 318 years, only 4 such falls have
-been recorded as having occurred at the time of the Leonid shower. If
-there is any close connection between the shooting-stars and the
-aerolites, we should expect to find a maximum in their fall at the time
-of the great meteoric displays. So far, no maxima or minima have yet
-been discovered in the fall of aerolites; they do not seem, like
-meteoric showers, to be governed by a law of periodicity.
-
-A very remarkable peculiarity of the aerolites is that they seem to have
-a tendency to fall in certain regions. Such are the southern part of
-France, the north of Italy, Hindostan, the central states of North
-America, and Mexico and Brazil. There is a curious contrast existing
-between the quick cometary motion of the aerolites before their
-explosion, and the comparatively slow motion of their fragments as they
-reach the Earth; motion which seems to be no greater than that
-corresponding to their natural fall impeded by the resistance of the
-air. In general, their penetration into the soil upon which they fall
-does not at all correspond to the great velocity with which they move in
-the atmosphere. The fragmentary structures of the aerolites, their
-identity of substance with that of our globe, their great resemblance to
-the volcanic minerals of the Earth, and the fractures and faults which
-some of them exhibit, do not correspond at all with the idea that they
-are cometary particles fallen on the Earth. As far as their structure
-and appearance is concerned, they seem rather to be a volcanic product
-of the interior of the Earth than parts of disintegrated comets. It must
-be admitted that their identity with the shooting-stars is far from
-established, and that they are still involved in mystery.
-
-The so-called meteoric dust gathered at sea and on high mountains may
-have various origins, and may be partly furnished by volcanic dust
-carried to great distances in the atmosphere.
-
-Since millions of shooting-stars penetrate our atmosphere every year and
-remain in it, becoming definitively a part of the Earth, it follows
-that, no matter how small may be the quantity of matter of which they
-are composed, they must gradually increase the volume and mass of our
-globe, although the increase may be exceedingly slow. Supposing every
-one of the shooting-stars penetrating our atmosphere to contain one
-cubic millimeter of matter, it has been calculated that it would take
-nearly 35,000 years to make a deposit one centimeter in thickness all
-over the surface of our globe. Insignificant as this may appear, it is
-probable that the quantity of matter of meteoric origin which is added
-to our globe is much less than has just been supposed.
-
-
-
-
-THE MILKY-WAY OR GALAXY
-
-PLATE XIII
-
-
-During clear nights, when the Moon is below the horizon, the starry
-vault is greatly adorned by an immense belt of soft white light,
-spanning the heavens from one point of the horizon to the opposite
-point, and girdling the celestial sphere in its delicate folds. Every
-one is familiar with this remarkable celestial object, called the
-_Milky-way_ or _Galaxy_.
-
-Seen with the naked eye, the Galaxy appears as an irregular, narrow,
-nebulous belt, apparently composed of cloud-like luminous masses of
-different forms and sizes, separated by comparatively dark intervals.
-These cloud-like masses vary much in luminous intensity, and while some
-among them are very bright and conspicuous, others are so faint that
-they are hard to recognize. In general, the brightest parts of the
-Milky-way are situated along the middle of its belt, while its borders,
-which are usually very faint, gradually vanish in the sky. Some parts of
-the Galaxy, however, show very little of the cloudy structure so
-characteristic of other parts, being almost uniform throughout, except
-towards the borders, which are always fainter. These parts showing
-greater uniformity are also the faintest.
-
-Such is the general appearance of the Milky-way on ordinary nights, but
-on rare occasions, when the atmosphere is particularly pure, it presents
-one of the grandest sights that can be imagined. At such favorable
-moments I have seen the Galaxy gleaming with light, and appearing as if
-composed of star-dust or of precious stones. The strange belt then
-appeared all mottled over and fleecy, its large cloud-like masses being
-subdivided into numerous small, irregular cloudlets of great brilliancy,
-which appeared projected upon a soft luminous background.
-
-The width of the Galaxy is far from being uniform; while in some places
-it is only 4° or 5°, in others it is 15° and even more. In some
-places it appears wavy in outline, at others quite straight; then it
-contracts, to expand a few degrees distant; while at other places it
-sends off branches and loops, varying in form, size and direction, some
-of which are quite prominent, while others are very faint.
-
-Although very irregular in form, the general appearance of the galactic
-belt is that of a regular curve occupying one of the great circles of
-the celestial sphere. The Milky-way completely encircles the heavens,
-but, of course, only one-half is visible at any one moment, since our
-globe prevents the other half from being seen. If, for a moment, we
-imagine ourselves left in space, our globe having vanished from under
-our feet, we should then see the whole Galaxy forming a continuous belt
-in the heavens, at the centre of which we should apparently be situated.
-
-While only one-half of the galactic belt can be seen at once from any
-point on the Earth, yet, according to the position of the observer, a
-larger or smaller portion of the whole can be seen at different times.
-In high northern or southern latitudes but little more than half can be
-seen even by continuous observations; but as we approach the equatorial
-regions, more and more of it becomes visible, until the whole may be
-seen at different hours and seasons. In the latitudes of the northern
-states, about two-thirds of the Galaxy is visible, the rest remaining
-hidden below the horizon; but from the southern states very nearly the
-whole can be seen. The half of the Milky-way visible at any one time
-from any latitude on the Earth never entirely sets below the horizon,
-although in some places it may be so near the horizon as to be rendered
-invisible by vapors. In the latitude of Cambridge, when in its lowest
-position, the summit of its arc is still about 12° or 15° above the
-northern horizon. The great circle of the celestial sphere, occupied by
-the galactic belt, is inclined at an angle of about 63° to the
-celestial equator, and intersects this great circle on one side in the
-constellation Monoceros in 6h. 47m., and on the opposite side in the
-constellations Aquila and Ophiuchus in 18h. 47m. of right ascension; so
-that its northern pole is situated in the constellation Coma Berenices
-in R. A. 12h. 47m., declination N. 27°, and the southern in the
-constellation Cetus in R. A. 0h. 47m., declination S. 27°.
-
-According to the seasons and to the hours of the night at which it is
-observed, the galactic arch presents different inclinations in the sky.
-Owing to its inclination to the equator of the celestial sphere, its
-opposite parts exhibit opposite inclinations when they pass the meridian
-of a place. That part of the Galaxy which is represented on Plate XIII.,
-and which intersects the celestial equator in the constellation Aquila,
-is inclined to the left or towards the east, when it is on the meridian;
-while the opposite part, situated in Monoceros, is inclined to the
-right, or towards the west, when it reaches the meridian. The former
-passes the meridian in the evening in the summer and autumn months; the
-latter, in the winter and spring months.
-
-
-[PLATE XIII.--PART OF THE MILKY WAY.
-
-From a study made during the years 1874, 1875 and 1876]
-
-
-By beginning at its northernmost part, represented at the upper part of
-Plate XIII., situated in "the chair" of the constellation Cassiopeia,
-and descending southwardly, and continuing in the same direction until
-the whole circle is completed, the course of the Milky-way through the
-constellations may be briefly described as follows: From Cassiopeia's
-chair, the Galaxy, forming two streams, descends south, passing partly
-through Lacerta on the left, and Cepheus on the right; at this last
-point it approaches nearest to the polar star. Then it enters Cygnus,
-where it becomes very complicated and bright, and where several large
-cloudy masses are seen terminating its left branch, which passes to the
-right, near the bright star Deneb, the leader of this constellation.
-Below Deneb, the Galaxy is apparently disconnected and separated from
-the northern part by a narrow, irregular dark gap. From this rupture,
-the Milky-way divides into two great streams separated by an irregular
-dark rift. An immense branch extends to the right, which, after having
-formed an important luminous mass between the stars _γ_ and _β_,
-continues its southward progress through parts of Lyra, Vulpecula,
-Hercules, Aquila and Ophiuchus, where it gradually terminates a few
-degrees south of the equator. The main stream on the left, after having
-formed a bright mass around _ε_ Cygni, passes through Vulpecula and
-then Aquila, where it crosses the equinoctial just below the star _η_
-after having involved in its nebulosity the bright star Altair, the
-leader of Aquila. In the southern hemisphere the Galaxy becomes very
-complicated and forms a succession of very bright, irregular masses, the
-upper one being in Scutum Sobieskii, while the others are respectively
-situated in Sagittarius and in Scorpio; the last, just a little above
-our horizon, being always considerably dimmed by vapors. From Scutum
-Sobieskii, the Galaxy expands considerably on the right, and sends a
-branch into Scorpio, in which the fiery red star Antares is somewhat
-involved.
-
-Continuing its course below our horizon, the Milky-way enters Ara and
-Norma, and then, passing partly through Circinus, Centaurus and Musca,
-it reaches the Southern Cross, after having been divided by the large
-dark pear-shaped spot known to navigators as the "Coal-Sack." In Ara and
-Crux the Milky-way attains its maximum of brightness, which there
-surpasses its brightest parts in Cygnus. In Musca, it makes its nearest
-approach to the south pole of the heavens. It then enters Carina and
-Vela, where it spreads out like a fan, and terminates in this last
-constellation, before reaching _λ_, being once more interrupted by a
-dark and very irregular gap, on a line with the two stars _γ_ and _λ_.
-It is noteworthy that this second rupture of continuity of the Galaxy in
-Vela is very nearly opposite, or at about 180° from the break near
-Deneb in Cygnus.
-
-Continuing its course on the other side of the break, the Milky-way
-again spreads out into the shape of a fan, grows narrower in entering
-Puppis, where it is longitudinally divided by darkish channels. It then
-passes above our southern horizon, becoming visible to us, passing
-through part of Canis Major, where its border just grazes the brilliant
-star Sirius. But from Puppis it gradually diminishes in brightness and
-complication, becoming faint and uniform. It enters Monoceros and Orion,
-where it again crosses the equator a little above _δ_, the northernmost
-of the three bright stars in the belt of Orion. Continuing its northward
-course it passes through Gemini, extending as far as Castor and Pollux,
-and then entering Auriga, where it begins to increase in brightness and
-in complication of structure. It passes partly through Camelopardus and
-into Perseus, where an important branch proceeds from its southern
-border.
-
-This branch beginning near the star _θ_, advances towards the
-celebrated variable star Algol, around which it is quite bright and
-complicated. Continuing its course in the same direction, the branch
-rapidly loses its brightness, becoming very faint a little below Algol,
-and passing through _ζ_ Persei, it enters Taurus, leaving the Pleiades
-on its extreme southern margin; and after having passed through _ε_
-where it branches off, it rapidly curves towards the main stream, which
-it joins near _ζ_ Tauri, thus forming an immense loop. The ramification
-projecting near _ε_ Tauri involves in its nebulosity the ruddy star
-Aldebaran and the scattered group of the Hyades. It then advances
-towards the three bright stars _δ_, _ε_ and _ζ_ of the belt of Orion,
-which, together with the sextuple star _θ_ Orionis, are involved in its
-faint nebulosity, and joins the main stream on the equinoctial, having
-thus formed a second loop, whose interior part is comparatively free
-from nebulosity, and contains the fine stars Betelgeuse and Bellatrix.
-
-That portion of the main galactic stream which is comprised between the
-star Deneb in Cygnus, and Capella in Auriga, is divided longitudinally
-by a very irregular, narrow, darkish cleft, comparatively devoid of
-nebulosity, which, however, is interrupted at some points. This dark gap
-sends short branches north and south, the most important of which are
-situated near _ζ_ Cephei and _β_ Cassiopeiæ. Another branch runs from
-_γ_ beyond _ε_ of the constellation last mentioned. The main stream of
-the Galaxy after leaving Perseus, enters Cassiopeia, and sending short
-branches into Andromeda, it completes its immense circle in Cassiopeia's
-chair, where this description was begun.
-
-When examined through the telescope, the appearance of the Milky-way
-completely changes, and its nebulous light is resolved into an immense
-number of stars, too faint to be individually seen with the naked eye.
-When Galileo first directed the telescope to the galactic belt, its
-nebulous, cloud-like masses were at once resolved into stars, even by
-the feeble magnifying power of his instrument. When, much later, Sir
-William Herschel undertook his celebrated star-gaugings of the Galaxy,
-millions of stars blazed out in his powerful telescopes. The stars
-composing this great nebulous belt are so numerous that it is impossible
-to arrive at any definite idea as to their number. From his soundings
-Herschel estimated at 116,000 the number of stars which, on one
-occasion, passed through the field of his telescope in 15 minutes, by
-the simple effect of the diurnal motion of the heavens; and on another
-occasion, a number estimated at 250,000 crossed the field in 41 minutes.
-In a space of 50, comprised between _β_ and _γ_ Cygni, shown on Plate
-XIII., he found no less than 331,000 stars. Prof. Struve has estimated
-at 20,500,000 the number of stars seen in the Milky-way through the
-twenty-foot telescope employed by Herschel in his star-gaugings. Great
-as this number may seem, it is yet far below the truth; as the great
-modern telescopes, according to Professor Newcomb, would very probably
-double the number of stars seen through Herschel's largest telescope,
-and detect from thirty to fifty millions of stars in the Milky-way.
-
-Although the telescope resolves the Galaxy into millions of stars, yet
-the largest instruments fail to penetrate its immense depths. The
-forty-foot telescope of Herschel, and even the giant telescope of Lord
-Rosse, have failed to resolve the Milky-way entirely into stars, the
-most distant ones appearing in them as nebulosities upon which the
-nearer stars are seen projected, the galactic stratum being unfathomable
-by the largest telescopes yet made.
-
-The stars composing the Milky-way are very unevenly distributed, as
-might easily be supposed from the cloud-like appearance of this belt. In
-some regions they are loosely scattered, forming long rows or streams of
-various figures, while in others they congregate into star groups and
-clusters having all imaginable forms, some being compressed into very
-dense globular masses. The intervals left between the clustering masses
-are poorer in stars, and indeed some of them are even totally devoid of
-stars or nebulosity. Such are the great and small "coal-sacks" in the
-southern Galaxy. I have myself detected such a dark space devoid of
-stars and nebulosity in one of the brightest parts of the Milky-way, in
-the constellation Sagittarius, in about 17h. 45m. right ascension, and
-27° 35' south declination. It is a small miniature coal-sack or opening
-in the Galaxy, through which the sight penetrates beyond this great
-assemblage of stars. Close to this, is another narrow opening near a
-small, loose cluster.
-
-Although lacking the optical resources which now enable us to recognize
-the structure of the Milky-way, some of the ancient philosophers had
-succeeded tolerably well in their speculations regarding its nature. It
-was the opinion of Democritus, Pythagoras and Manilius, that the Galaxy
-was nothing else but a vast and confused assemblage of stars, whose
-faint light was the true cause of its milky appearance.
-
-Before the invention of the telescope, no well-founded theory in regard
-to the structure of the Milky-way could, of course, be attempted.
-Although Kepler entertained different ideas in regard to the structure
-of this great belt from those now generally admitted, yet in them may be
-found the starting point of the modern conception of the structure of
-the Galaxy and of the visible universe. In the view of this great mind,
-the Milky-way, with all its stars, formed a vast system, the centre of
-which, and of the universe, was occupied by our Sun. Kepler reasoned
-that the place of the Sun must be near the centre of the galactic belt,
-from the fact this last object appears very nearly as a great circle of
-the celestial sphere, and that its luminous intensity is about the same
-in all its parts.
-
-Half a century later, another attempt to explain the Milky-way was made
-by Wright, of Durham, who rejected the idea of an accidental and
-confused distribution of the stars as inconsistent with the appearance
-of the Galaxy, and regarded them as arranged along a fundamental plane
-corresponding to that of the Milky-way. These ideas which were
-subsequently developed and enlarged by Kant, and then by Lambert,
-constitute what is now known as Kant's theory. According to this theory,
-the stars composing the Galaxy are conceived as being uniformly arranged
-between two flat planes of considerable extension, but which are
-comparatively near together, the Sun occupying a place not very far from
-the centre of this immense starry stratum. As we view this system
-crosswise through its thinnest parts, the stars composing it appear
-scattered and comparatively few in number, but when we view it
-lengthwiser through its most extended parts, they appear condensed and
-extremely numerous, thus giving the impression of a luminous belt
-encircling the heavens. In the conception of Kant, each star was a sun,
-forming the centre of a planetary system. These systems are not
-independent, but are kept together by the bonds of universal
-gravitation. The Galaxy itself is one of these great systems, its
-principal plane being the equivalent of the zodiac in our planetary
-system, while a preponderant body, which might be Sirius, is the
-equivalent of our Sun, and keeps the galactic system together. In the
-universe there are other galaxies, but as they are too distant to be
-resolved into stars, they appear as elliptical nebulæ. Such are, in
-brief, the grand speculations of Kant and Lambert on the Milky-way, and
-the structure of the universe.
-
-Kant's theory rested more on conjectures than on observed facts, and
-needed therefore the sanction of direct observations to be established
-on a firm basis. With this view, Sir William Herschel investigated the
-subject, by a long and laborious series of observations. His plan, which
-was that of "star-gauging," consisted in counting all the stars visible
-in his twenty-foot telescope, comprised in a wide belt cutting the
-Galaxy at right angles, and extending from one of its sides to the
-opposite one, thus embracing 180° of the celestial sphere. In this belt
-he executed 3,400 telescopic star-gaugings of a quarter of a degree
-each, from which he obtained 683 mean gaugings giving the stellar
-density of the corresponding regions.
-
-The general result derived from this immense labor was that the stars
-are fewest in regions the most distant from the galactic belt; while
-from these regions, which correspond to the pole of the Galaxy, they
-gradually increase in number in approaching the Milky-way. The star
-density was found to be extremely variable, and while some of the
-telescopic gaugings detected either no star at all, or only one or two,
-other gaugings gave 500 stars and even more. The average number of stars
-in a field of view of his telescope, obtained for the six zones, each of
-15°, into which Herschel divided up the portion of his observing belt,
-extending from the Galaxy to its pole, is as follows: In the first zone,
-commencing at 90° from the galactic belt and extending towards it, 4
-stars per telescopic field were found; 5 in the second; 8 in the third;
-14 in the fourth; 24 in the fifth and 53 in the sixth, which terminated
-in the Galaxy itself. Very nearly similar results were afterwards found
-by Sir John Herschel, for corresponding regions in the southern
-hemisphere.
-
-From these studies, Herschel concluded that the stellar system is of the
-general form supposed by the Kantian theory, and that its diameter must
-be five times as extended in the direction of the galactic plane, as it
-is in a direction perpendicular to it. To explain the great branch sent
-out by the Galaxy in Cygnus, he supposed a great cleft dividing the
-system edgewise, about half way from its circumference to its centre.
-From suppositions founded on the apparent magnitude and arrangement of
-stars, he estimated that it would take light about 7,000 years to reach
-us from the extremities of the Galaxy, and therefore 14,000 years to
-travel across the system, from one border to the opposite one.
-
-But Herschel's theory concerning the Milky-way rested on the erroneous
-assumption that the stars are uniformly distributed in space, and also
-that his telescopes penetrated through the entire depth of the Galaxy.
-Further study showed him that his telescope of twenty feet, and even his
-great forty-foot telescope, which was estimated to penetrate to a
-distance 2,300 times that of stars of the first magnitude, failed to
-resolve some parts of the Galaxy into stars. Meanwhile, the structure of
-the Milky-way being better known, the irregular condensation of its
-stars became apparent, while the mutual relation existing between binary
-and multiple systems of stars, as also between the stars which form
-clusters, was recognized, as showing evidence of closer association
-between certain groups of stars than between the stars in general.
-Herschel's system, which rested on the assumption of the uniform
-distribution of the stars in space, and on the supposition that the
-telescopes used for his gauges penetrated through the greater depths of
-the Galaxy, being thus found to contradict the facts, was gradually
-abandoned by its author, who adopted another method of estimating the
-relative distances of the stars observed in his gaugings.
-
-This method, founded on photometric principles, consisted in judging the
-penetrating power of his telescope by the brightness of the stars, and
-not, as formerly, by the number which they brought into view. He then
-studied by this new method the structure of the Milky-way and the
-probable distance of the clustering masses of which it is formed,
-concluding that the portion of the Galaxy traversing the constellation
-Orion is the nearest to us. This last result seems indicated by the fact
-that this portion of the Milky-way is the faintest and the most uniform
-of all the galactic belt.
-
-More recently Otto Struve investigated the same subject, and arrived at
-very nearly similar conclusions, which may be briefly stated as follows:
-The galactic system is composed of a countless number of stars,
-spreading out on all sides along a very extended plane. These stars,
-which are very unevenly distributed, show a decided tendency to cluster
-together into individual groups of different sizes and forms, separated
-by comparatively vacant spaces. This layer where the stars congregate in
-such vast numbers may be conceived as a very irregular flat disk,
-sending many branches in various directions, and having a diameter eight
-or ten times its thickness. The size of this starry disk cannot be
-determined, since it is unfathomable in some directions, even when
-examined with the largest telescopes. The Sun, with its attending
-planets, is involved in this immense congregation of suns, of which it
-forms but a small particle, occupying a position at some distance from
-the principal plane of the Galaxy. According to Struve, this distance is
-approximately equal to 208,000 times the radius of the Earth's orbit.
-The Milky-way is mainly composed of star-clusters, two-thirds, perhaps,
-of the whole number visible in the heavens being involved in this great
-belt. In conclusion, our Sun is only one of the individual stars which
-constitute the galactic system, and each of these stars itself is a sun
-similar to our Sun. These individual suns are not independent, but are
-associated in groups varying in number from a few to several thousands,
-the Galaxy itself being nothing but an immense aggregation of such
-clusters, whose whole number of individual suns probably ranges between
-thirty and fifty millions. In this vast system our globe is so
-insignificant that it cannot even be regarded as one of its members.
-According to Dr. Gould, there are reasons to believe that our Sun is a
-member of a small, flattened, bifid cluster, composed of more than 400
-stars, ranging between the first and seventh magnitude, its position in
-this small system being eccentric, but not very far from the galactic
-plane.
-
-The study of the Milky-way, of which Plate XIII. is only a part, was
-undertaken to answer a friendly appeal made by Mr. A. Marth, in the
-Monthly Notices of the Royal Astronomical Society, in 1872. I take
-pleasure in offering him my thanks for the suggestion, and for the
-facility afforded me in this study by his "List of Co-ordinates of Stars
-within and near the Milky-way," which was published with it.
-
-
-
-
-THE STAR-CLUSTERS
-
-PLATE XIV
-
-
-It is a well-known fact that the stars visible to the naked eye are very
-unequally distributed in the heavens, and that while they are loosely
-scattered in some regions, in others they are comparatively numerous,
-sometimes forming groups in which they appear quite close together.
-
-In our northern sky are found a few such agglomerations of stars, which
-are familiar objects to all observers of celestial objects. In the
-constellation Coma Berenices, the stars are small, but quite condensed,
-and form a loosely scattered, faint group. In Taurus, the Hyades and the
-Pleiades, visible during our winter nights, are conspicuous and familiar
-objects which cannot fail to be recognized. In the last group, six stars
-may be easily detected by ordinary eyes on any clear night, but more can
-sometimes be seen; on rare occasions, when the sky was especially
-favorable, I have detected eleven clearly and suspected several others.
-The six stars ordinarily visible, are in order of decreasing brightness,
-as follows: Alcyone, Electra, Atlas, Maia, Taygeta and Merope. Glimpses
-of Celano and Pleione are sometimes obtained.
-
-When the sky is examined with some attention on any clear, moonless
-night, small, hazy, luminous patches, having a cometary aspect, are
-visible here and there to the naked eye. In the constellation Cancer is
-found one of the most conspicuous, called Præsepe, which forms a small
-triangle with the two stars _γ_ and _δ_. In Perseus, and involved in
-the Milky-way, is found another luminous cloud, situated in the
-sword-handle, and almost in a line with the two stars _γ_ and _δ_ of
-Cassiopeia's Chair. In the constellation Hercules, another nebulous mass
-of light, but fainter, is also visible between the stars _η_ and _ζ_
-where it appears as a faint comet, in the depths of space. In Ophiuchus
-and Monoceros are likewise found hazy, luminous patches. In the southern
-sky, several such objects are also visible to the naked eye, being found
-in Sagittarius, in Canis Major and in Puppis; but the most conspicuous
-are those in Centaurus and Toucan. That in Centaurus involves the star
-_ω_ in its pale diffused nebulosity, and that in Toucan is involved in
-the lesser Magellanic cloud.
-
-When the telescope is directed to these nebulous objects, their hazy,
-ill-defined aspect disappears, and they are found to consist of
-individual stars of different magnitudes, which being more or less
-closely grouped together, apparently form a system of their own. These
-groups, which are so well adapted to give us an insight into the
-structure and the vastness of the stellar universe, are called
-_Star-clusters_.
-
-Star-clusters are found of all degrees of aggregation, and while in some
-of them, such as in the Pleiades, in Præsepe and in Perseus, the stars
-are so loosely scattered that an opera glass, and even the naked eye,
-will resolve them; in others, such as in those situated in Hercules,
-Aquarius, Toucan and Centaurus, they are so greatly compressed that even
-in the largest telescopes they appear as a confused mass of blazing
-dust, in which comparatively few individual stars can be distinctly
-recognized. Although only about a dozen Star-clusters can be seen in the
-sky with the naked eye, yet nearly eleven hundred such objects visible
-through the telescope, have been catalogued by astronomers.
-
-The stars composing the different clusters visible in the heavens vary
-greatly in number, and while in some clusters there are only a few, in
-others they are so numerous and crowded that it would be idle to try to
-count them, their number amounting to several thousands. It has been
-calculated by Herschel that some clusters are so closely condensed, that
-in an area not more than ⅒ part of that covered by the Moon, at least
-5,000 stars are agglomerated.
-
-When the group in the Pleiades is seen through the telescope it appears
-more important than it does to the naked eye, and several hundreds of
-stars are found in it. In a study of Tempel's nebula, which is involved
-in the Pleiades, I have mapped out 250 stars, mostly comprised within
-this nebula, with the telescope of 6⅓ inches aperture, which I have
-used for this study.
-
-As a type of a loose, coarse cluster, that in Perseus is one of the
-finest of its class. It appears to the naked eye as a single object, but
-in the telescope it has two centres of condensation, around which
-cluster a great number of bright stars, forming various curves and
-festoons of great beauty. Among its components are found several yellow
-and red stars, which give a most beautiful contrast of colors in this
-gorgeous and sparkling region. In a study which I have made of this twin
-cluster, I have mapped out 664 stars belonging to it, among which are
-two yellow and five red stars.
-
-While some clusters, like those just described, are very easily
-resolvable into stars with the smallest instruments, others yield with
-the greatest difficulty, even to the largest telescopes, in which their
-starry nature is barely suspected. Owing to this peculiarity,
-star-clusters are usually divided into two principal classes. In the
-first class are comprised all the clusters which have been plainly
-resolved into stars, and in the second all those which, although not
-plainly resolvable with the largest instruments now at our disposal,
-show a decided tendency to resolvability, and convey the impression that
-an increase of power in telescopes is the only thing needed to resolve
-them into stars. Of course this classification, which depends on the
-power of telescopes to decide the nature of these objects, is arbitrary,
-and a classification based on spectrum analysis is now substituted for
-it.
-
-The star-clusters are also divided into globular and irregular clusters,
-according to their general form and appearance. The globular clusters,
-which are the most numerous, are usually well-defined objects, more or
-less circular in their general outlines. The rapid increase of
-brightness towards their centres, where the stars composing them are
-greatly condensed, readily conveys the impression that the general form
-of these sparkling masses is globular. The irregular clusters are not so
-rich in stars as the former. Usually their stars are less condensed
-towards the centre, and are, for the most part, so loosely and
-irregularly distributed, that it is impossible to recognize the outlines
-of these clusters or to decide where they terminate. The globular
-clusters are usually quite easily resolvable into stars, either partly
-or wholly, although some among them do not show the least traces of
-resolvability, even in the largest instruments. This may result from
-different causes, and may be attributed either to the minuteness of
-their components or to their great distance from the Earth, many
-star-clusters being at such immense distances that they are beyond our
-means of measurement.
-
-As has been shown in the preceding section, the star-clusters are found
-in great number in the Galaxy; indeed, it is in this region and in its
-vicinity that the greater portion of them are found. In other regions,
-with the exception of the Magellanic clouds, where they are found in
-great number and in every stage of resolution, the clusters are few and
-scattered.
-
-
-[PLATE XIV.--STAR-CLUSTER IN HERCULES.
-
-From a study made in June, 1877]
-
-
-The star-cluster in the constellation Hercules, designated as No. 4,230
-in Sir J. Herschel's catalogue, and which is represented on Plate XIV.,
-is one of the brightest and most condensed in the northern hemisphere,
-although it is not so extended as several others, its angular diameter
-being only 7' or 8'. This object, which was discovered by Halley in
-1714, is one of the most beautiful of its class in the heavens.
-According to Herschel, it is composed of thousands of stars between the
-tenth and fifteenth magnitudes. Undoubtedly the stars composing this
-group are very numerous, although those which can be distinctly seen as
-individual stars, and whose position can be determined, are not so many
-as a superficial look at the object would lead us to suppose. From a
-long study of this cluster, which I have made with instruments of
-various apertures, I have not been able to identify more stars than are
-represented on the plate, although the nebulosity of which this object
-mainly consists, and especially the region situated towards its centre,
-appeared at times granular and blazing with countless points of light,
-too faint and too flickering to be individually recognized. Towards its
-centre there is quite an extended region, whose luminous intensity is
-very great, and which irresistibly conveys the impression of the
-globular structure of this cluster. Besides several outlying appendages,
-formed by its nebulosity, the larger stars recognized in this cluster
-are scattered and distributed in such a way that they form various
-branches, corresponding with those formed by the irresolvable
-nebulosity. At least six or seven of these branches and wings are
-recognized, some of which are curved and bent in various ways, thus
-giving this object a distant resemblance to some crustacean forms.
-Although I have looked for it with care, I have failed to recognize the
-spiral structure attributed to this object by several observers. Among
-the six appendages which I have recognized, some are slightly curved;
-but their curves are sometimes in opposite directions, and two branches
-of the upper portion make so short a bend that they resemble a claw
-rather than a spiral wing. The spectrum of this cluster, like that of
-many objects of its class, is continuous, with the red end deficient.
-
-A little to the north-east of this object is found the cluster No.
-4,294, which, although smaller and less bright than the preceding, is
-still quite interesting. It appears as a distinctly globular cluster
-without wings, and much condensed towards its centre. The stars
-individually recognized in it, although less bright than those of the
-other cluster, are so very curiously distributed in curved lines that
-they give a peculiar appearance to this condensed region.
-
-A little to the north of _γ_ Centauri may be found the great _ω_
-Centauri cluster, No. 3,531, already referred to above. This magnificent
-object, which appears as a blazing globe 20' in diameter, is, according
-to Herschel, the richest in the sky, and is resolved into a countless
-number of stars from the twelfth to the fifteenth magnitude, which are
-greatly compressed towards the centre. The larger stars are so arranged
-as to form a sort of net-work, with two dark spaces in the middle.
-
-The great globular cluster No. 52, involved in the lesser Magellanic
-cloud, in the constellation Toucan, is a beautiful and remarkable
-object. It is composed of three distinct, concentric layers of stars,
-varying in brightness and in degree of condensation in each layer. The
-central mass, which is the largest and most brilliant, is composed of an
-immense number of stars greatly compressed, whose reddish color gives to
-this blazing circle a splendid appearance. Around the sparkling centre
-is a broad circle, composed of less compressed stars, this circle being
-itself involved in another circular layer, where the stars are fainter
-and more scattered and gradually fade away.
-
-Many other great globular clusters are found in various parts of the
-heavens, among which may be mentioned the cluster No. 4,678, in
-Aquarius. This object is composed of several thousand stars of the
-fifteenth magnitude, greatly condensed towards the centre, and, as
-remarked by Sir J. Herschel, since the brightness of this cluster does
-not exceed that of a star of the sixth magnitude, it follows that in
-this case several thousand stars of the fifteenth magnitude equal only a
-star of the sixth magnitude. In the constellation Serpens the globular
-clusters No. 4,083 and No. 4,118 are both conspicuous objects, also No.
-4,687 in Capricornus. In Scutum Sobieskii the cluster No. 4,437 is one
-of the most remarkable of this region. The stars composing it, which are
-quite large and easily made out separately, form various figures, in
-which the square predominates.
-
-Among the loose irregular clusters, some are very remarkable for the
-curious arrangement of their stars. In the constellation Gemini the
-cluster No. 1,360, which is visible to the naked eye, is a magnificent
-object seen through the telescope, in which its sparkling stars form
-curves and festoons of great elegance. The cluster No. 1,467, of the
-same constellation, is remarkable for its triangular form. In the
-constellation Ara the cluster No. 4,233, composed of loosely scattered
-stars, forming various lines and curves, is enclosed on three sides by
-nearly straight single lines of stars. In Scorpio the cluster No. 4,224
-is still more curious, being composed of a continuous ring of loosely
-scattered stars, inside of which is a round, loose cluster, which is
-divided into four parts by a dark cross-shaped gap, in which no stars
-are visible.
-
-Among the 1,034 objects which are now classified as clusters more or
-less resolvable, 565 have been absolutely resolved into stars, and 469
-have been only partly resolved, but are considered as belonging to this
-class of objects. In Sir J. Herschel's catalogue there are 102 clusters
-which are Considered as being globular; among them 30 have been
-positively resolved into stars.
-
-The agglomeration of thousands of stars into a globular cluster cannot
-be conceived, of course, to be simply the result of chance. This
-globular form seems clearly to indicate the existence of some bond of
-union, some general attractive force acting between the different
-members of these systems, which keeps them together, and condenses them
-towards the centre. Herschel regards the loose, irregular clusters as
-systems in a less advanced stage of condensation, but gradually
-concentrating by their mutual attraction into the globular form.
-Although the stars of some globular clusters appear very close together,
-they are not necessarily so, and may be separated by great intervals of
-space. It has been shown that the clusters are agglomeration of suns,
-and that our Sun itself is a member of a cluster composed of several
-hundreds of suns, although, from our point of observation, these do not
-seem very close together. So far as known, the nearest star to us is a
-Centauri, but its distance from the Earth equals 221,000 times the
-distance of the Sun from our globe, a distance which cannot be traversed
-by light in less than three years and five months. It seems very
-probable that if the suns composing the globular clusters appear so near
-together, it is because, in the first place, they are at immense
-distances from us, and in the second, because they appear nearly in a
-line with other suns, which are at a still greater distance from us, and
-on which they accordingly are nearly projected. If one should imagine
-himself placed at the centre of the cluster in Hercules, for instance,
-the stars, which from our Earth seems to be so closely grouped, would
-then quite likely appear very loosely scattered around him in the sky,
-and would resemble the fixed stars as seen from our terrestrial station.
-
-Judging by their loose and irregular distribution, the easily resolvable
-clusters would appear, in general, to be the nearer to us. It is
-probable that the globular clusters do not possess, to a very great
-degree, the regular form which they ordinarily present to us. It seems
-rather more natural to infer that they are irregular, and composed of
-many wings and branches, such as are observed in the cluster in
-Hercules; but as these appendages would necessarily be much poorer in
-stars than the central portions, they would be likely to become
-invisible at a great distance, and therefore the object would appear
-more or less globular; the globular form being simply given by the close
-grouping of the stars in the central portion. It would seem, then, that
-in general, the most loosely scattered and irregular clusters are the
-nearest to us, while the smallest globular clusters and those resolvable
-with most difficulty are the most distant.
-
-In accordance with the theory that the clusters are composed of stars,
-the spectrum of these objects is in general continuous; although, in
-many cases, the red end of the spectrum is either very faint or
-altogether wanting. Many objects presenting in a very high degree the
-principal characteristics exhibited by the true star-clusters, namely, a
-circular or oval mass, whose luminous intensity is greatly condensed
-toward the centre, have not yielded, however, to the resolving power of
-the largest telescopes, although their continuous spectrum is in close
-agreement with their general resemblance to the star-clusters. Although
-such objects may remain irresolvable forever, yet it is highly probable
-that they do not materially differ from the resolvable and partly
-resolvable clusters, except by their enormous distance from us, which
-probably reaches the extreme boundary of our visible universe.
-
-
-
-
-THE NEBULÆ
-
-PLATE XV
-
-
-Besides the foggy, luminous patches which have just been described, a
-few hazy spots of a different kind are also visible to the naked eye on
-any clear, moonless night. These objects mainly differ from the former
-in this particular, that when viewed through the largest telescopes in
-existence they are not resolved into stars, but still retain the same
-cloudy appearance which they present to the unassisted eye. On account
-of the misty and vaporous appearance which they exhibit, these objects
-have been called _Nebulæ_.
-
-Of the 26 nebulous objects visible to the naked eye in the whole
-heavens, 19 belong to the class of star-clusters, and 7 to the class of
-nebulæ. Among the most conspicuous nebulæ visible to the unassisted
-eye, are those in the constellations Argo Navis, Andromeda and Orion.
-
-Besides the seven nebulæ visible to the naked eye, a great number of
-similar objects are visible through the telescope. In Sir John
-Herschel's catalogue of nebulæ and clusters, are found 4,053
-irresolvable nebulæ, and with every increase of the aperture of
-telescopes, new nebulæ, invisible in smaller instruments, are found.
-Notwithstanding their irresolvability it is probable, however, that many
-among them have a stellar structure, which their immense distance
-prevents us from recognizing, and are not therefore true nebulæ. The
-giant telescope of Lord Rosse has shown nebulæ so remote that it has
-been estimated that it takes their light 30 million years to reach the
-Earth.
-
-The nebulæ are very far from being uniformly distributed in space. In
-some regions they are rare, while in others they are numerous and
-crowded together, forming many small, irregular groups, differing in
-size and in richness of aggregation. The grouping of the nebulæ does
-not occur at random in any part of the heavens, as might naturally be
-supposed, but, on the contrary, it is chiefly confined to certain
-regions. Outside of these regions nebulæ are rare and are separated
-from each other by immense intervals; so that these isolated objects
-appear as if they were lost wanderers from the great nebulous systems.
-
-The regions where the nebulæ congregate in great number are very
-extensive, and in a general view there are two vast systems of nebular
-agglomeration, occupying almost opposite points of the heavens, whose
-centres are not very distant from the poles of the Milky-way. In the
-northern hemisphere, the nebulous system is much richer and more
-condensed than in the southern hemisphere. The northern nebulæ are
-principally contained in the constellations Ursa Minor and Major, in
-Draco, Canes Venatici, Bootes, Leo Major and Minor, Coma Berenices, and
-Virgo. In this region, which occupies about ⅛ of the whole surface of
-the heavens, ⅓ of the known nebulæ are assembled. The southern
-nebulæ are more evenly distributed and less numerous, with the
-exception of two comparatively small, but very remarkable centres of
-condensation which, together with many star-clusters, constitute the
-Magellanic clouds.
-
-These two vast nebular groups are by no means regular in outline, and
-send various branches toward each other. They are separated by a wide
-and very irregular belt, comparatively free from nebulæ, which
-encircles the celestial sphere, and whose medial line approximately
-coincides with that of the galactic belt. The Milky-way, so rich in
-star-clusters, is very barren in nebulæ; but it is a very remarkable
-fact, nevertheless, that almost all the brightest, largest, and most
-complicated nebulæ of the heavens are situated either within it, or in
-its immediate vicinity. Such are the great nebulæ in Orion and
-Andromeda; the nebula of _ζ_ Orionis; the Ring nebula in Lyra; the
-bifurcate nebula in Cygnus; the Dumb-bell nebula in Vulpecula; the Fan,
-Horse-shoe, Trifid and Winged nebulæ in Sagittarius; the great nebula
-around _η_ Argus Navis, and the Crab nebula in Taurus.
-
-Aside from the discovery of some of the largest nebulæ by different
-observers, and their subsequent arrangement in catalogues by Lacaille
-and Messier, very little had been done towards the study of these
-objects before 1779, when Sir W. Herschel began to observe them with the
-earnestness of purpose which was one of the distinctive points of the
-character of this great man. He successively published three catalogues
-in 1786, 1789, and 1802, in which the position of 2,500 nebulous objects
-was given. This number was more than doubled before 1864, when Sir John
-Herschel published his catalogue of 5,079 nebulæ and star-clusters. To
-this long list must be added several hundred similar objects, since
-discovered by D'Arrest, Stephan, Gould and others. But, as has been
-shown above, among the so-called nebulæ are many star-clusters which do
-not properly belong to the same class of objects, it being sometimes
-impossible in the present state of our knowledge to know whether a
-nebulous object belongs to one class or to the other.
-
-The nebulæ exhibit a great variety of forms and appearances, and, in
-accordance with their most typical characters, they are usually divided
-into several classes, which are: the Nebulous stars, the Circular, or
-Planetary, the Elliptical, the Annular, the Spiral and Irregular
-nebulæ.
-
-
-[PLATE XV.--THE GREAT NEBULA IN ORION.
-
-From a study made in the years 1875-76]
-
-
-The so-called nebulous stars consist of a faint nebulosity, usually
-circular, surrounding a bright and sharp star, which generally occupies
-its centre. The nebulosity surrounding these stars varies in brightness
-as well as in extent, and while, in general, its light gradually fades
-away, it sometimes terminates quite suddenly. Such nebulosities are
-usually brighter and more condensed towards the central star. The stars
-thus surrounded do not seem, however, to be distinguished from others by
-any additional peculiarity. Some nebulæ of this kind are round, with
-one star in the centre; others are oval and have two stars, one at each
-of their foci. The nebulous star, _τ_ Orionis, represented at the upper
-part of Plate XV., above the great nebula, has a bright star at its
-centre and two smaller ones on the side. The association of double stars
-with nebulæ is very remarkable, and may in some cases indicate a mutual
-relation between them.
-
-The so-called planetary nebulæ derive their name from their likeness to
-the planets, which they resemble in a more or less equable distribution
-of light and in their round or slightly oval form. While some of them
-have edges comparatively sharp and well defined, the outlines of others
-are more hazy and diffused. These nebulæ, which are frequently of a
-bluish tint, are comparatively rare objects, and most of those known
-belong to the southern hemisphere. When seen through large telescopes,
-however, they present a different aspect, and their apparent uniformity
-changes. The largest of these objects, No. 2,343 of the General
-Catalogue, is situated in the Great Bear, close to the star _β_. Its
-apparent diameter is, according to Sir J. Herschel, 2', 40", and "its
-light is equable, except at the edge, where it is a little hazy." In a
-study which I made of this object in 1876, with a refractor 6⅓ inches
-in aperture, I found it decidedly brighter on the preceding side, where
-the brightest part is crescent shaped. In Lord Rosse's telescope its
-disk is transformed into a luminous ring with a fringed border, and two
-small star-like condensations are found within. Another planetary
-nebula, near _χ_ Andromedæ, has also shown an annular structure in
-Rosse's telescope.
-
-The elliptical nebulæ, as their name implies, are elongated, elliptical
-objects; but while some of them are only slightly elongated ovals,
-others form ellipses whose eccentricity is so great that they appear
-almost linear. In all these objects the light is more or less condensed
-towards the centre; but while in some of them the condensation is
-gradual and slight, in others it is so great and sudden that the centre
-of the nebula appears as a large diffused star, somewhat resembling the
-nucleus of a comet. From the general appearance of these objects, it is
-not unlikely that some of them are either flattish, nebulous disks, like
-the planetary nebulæ, or nebulous rings, seen more or less sidewise.
-The condensation of light at their centres does not appear to be
-stellar, but nebulous like the rest, and it is a remarkable fact that
-very few, if any, of these objects are resolvable into stars.
-
-Several elliptical nebulæ are remarkable for having a star at or near
-each of their foci, or at each of their extremities. Such are the
-elliptical nebulæ in Draco, Centaurus, and Sagittarius, Nos. 4,419,
-3,706 and 4,395 of the General Catalogue, the last of which is in the
-vicinity of the triple star _μ_ Sagitarii. Each of these nebulæ has a
-star at each of its foci, while No. 1, in Cetus, has a star at each of
-its extremities.
-
-Among the most remarkable elliptic nebulæ may be mentioned Nos. 1,861
-and 2,373 of Sir J. Herschel's catalogue, both situated in the
-constellation Leo. The first is one of Lord Rosse's spiral nebulæ, and
-the last, which is a very elongated object, is formed of concentric oval
-rings, which are especially visible towards its central part. The
-constellation Draco is particularly remarkable for the number of
-elliptical nebulæ found within its boundaries. Among them are Nos.
-3,939, 4,058, 4,064, 4,087, 4,415, etc., which are quite remarkable
-objects of their class. No. 4,058, of which I have made a study, is
-bright, and has a decided lenticular form with a condensation in the
-centre. Its following edge is better defined than the preceding. In Lord
-Rosse's telescope this object exhibits a narrow, dark, longitudinal, gap
-in its interior.
-
-By far the largest and the finest object of this class is the great
-nebula in Andromeda. Although this object belongs rather to the class of
-irregular nebulæ, yet it is generally considered as an elliptic nebula,
-since its complicated structure, being less prominent, was not
-recognized until 1848, when it was perceived by George P. Bond, Director
-of the Harvard College Observatory. This, the first nebula discovered,
-was found in 1612 by Simon Marius. It is situated in the constellation
-Andromeda, in the vicinity of the star _ν_, and almost in a line with
-the stars _μ_ and _β_ of the same constellation. It is visible to the
-naked eye, and appears as a faint comet-like object. It is represented
-at the upper left hand corner of Plate XIII., on the border of the
-Milky-way, as it appears to the naked eye.
-
-The nebula in Andromeda is one of the brightest in the heavens, and is
-closely attended by two smaller nebulæ. Perhaps it would be rather more
-correct to say that it has three centres of condensation, as the two
-small nebulæ referred to are entirely involved in the same faint and
-extensive nebulosity. Its general form is that of an irregular oval,
-upwards of one degree in breadth and two and a half degrees in length.
-Its brightest and most prominent part, which alone was seen by the
-earlier observers, consists in a very elongated lenticular mass, which
-gradually condenses towards its centre into a blazing, star-like
-nucleus, surrounded by a brilliant nebulous mass. At a little distance
-to the south of this central condensation is found one of the lesser
-centres of condensation noted above, which is globular in appearance,
-with a bright, star-like nucleus like the former. The other centre of
-condensation is found to the north-west of the centre of the principal
-mass, and is quite elongated, with a centre of condensation towards its
-southern extremity, but it is not so bright as the others. Close to the
-western edge of the bright lenticular mass first described, and making a
-very slight angle with its longer axis, are found two narrow and nearly
-rectilinear dark rifts, running almost parallel to each other, and both
-terminating in a slender point in the south. These dark rifts, which are
-almost totally devoid of nebulous matter, are quite rare in nebulæ, and
-afford a good opportunity to watch the changes which this part of the
-nebulæ may undergo.
-
-This nebula has never been positively resolved into stars, although
-Prof. Geo. Bond and others have strongly suspected its resolvability. In
-a study which I have made of it, with the same instrument employed by
-Bond, and also with the great Washington telescope, I detected a decided
-mottled appearance in several places, which might be attributable to a
-beginning of resolvability; but I do not consider this a conclusive
-indication that the nebula is resolvable. The continuous spectrum given
-by this nebula, showing that it is not in the gaseous state which its
-appearance seems to indicate, warrants the conclusion, however, that it
-will ultimately be found to be resolvable. This object, being situated
-on the edge of the Galaxy and involved in its diffused light, has a
-great number of small stars belonging to this belt projected upon it.
-During my observations I have mapped out 1,323 of these stars, none of
-which seems to be in physical connection with the nebula.
-
-Among the circular and elliptical nebulæ a few exhibit a very
-remarkable structure, being apparently perforated, and forming either
-round, slightly oval, or elongated rings of great beauty. These Annular
-nebulæ are among the rarest objects in the heavens. In Scorpio, two
-such nebulæ are found involved in the Milky-way, and also one in
-Cygnus. One of those in Scorpio has two stars involved within its ring,
-at the extremities of its smallest interior diameter. A very elongated
-nebula in the vicinity of the fine triple star _γ_ Andromedæ is also
-annular, and has two stars symmetrically placed at the extremities of
-its greatest interior axis. Another elongated annular nebula is also
-found north of _η_ Pegasi.
-
-The grandest and most remarkable of the annular nebulæ is found in the
-constellation Lyra, about midway between the two stars _β_ and _γ_. It
-is slightly elliptical in form, and according to Prof. E. S. Holden, its
-major axis is 77".3 and the minor 58". From a study and several drawings
-which I have made of this object, with instruments of various apertures,
-I have found it decidedly brighter towards its outer border, at the
-extremities of its minor axis, than at the ends of the major axis. On
-very favorable occasions, some of its brightest parts have appeared
-decidedly, but very faintly mottled, and I have recognized three small
-centres of condensation. Its interior, in which Professor Holden has
-detected a very faint star, is quite strongly nebulous. In Lord Rosse's
-telescope, this nebula is completely surrounded by wisps and appendages
-of all sorts of forms, which I have failed to trace, however, both with
-the refractor of the Harvard College Observatory and with that of the
-Naval Observatory at Washington; Rosse, Secchi and Chacornac, have seen
-this nebula glittering as if it were a "heap of star dust," although its
-spectrum indicates that it is gaseous.
-
-The nebula No. 1,541, in Camelopardus, of which I have also made a study
-and a drawing, is closely allied to the class of annular nebulæ. This
-object, which is quite bright, has a remarkable appearance. It consists
-principally of somewhat more than half of an oval ring, surrounding a
-bright, nebulous mass which condenses around a star; this mass being
-separated from the imperfect ring by a dark interval. Upon the bright
-portion of the ring, and on opposite points, are found two bright stars,
-between which lies the star occupying the central mass. The central mass
-extends at some distance outside of the ring on its open side. Several
-stars are involved in this object.
-
-The Spiral nebulæ are very curious and complicated objects, but they
-are visible only in the largest telescopes. Prominent above all is the
-double spiral nebula No. 3,572, in Canes Venatici, which is not far from
-_η_ Ursa Majoris. In Lord Rosse's telescope, this object presents a
-wonderful spiral disposition, looking somewhat like one of the
-fire-works called pin-wheels, and forming long, curved wisps, diverging
-from two bright centres. The spectrum of this object, however, is not
-that of a gas. In the constellation Virgo, Rosse has detected another
-such nebula. In Cepheus, Triangulum, and Ursa Major, are found other
-spiral nebulæ of smaller size. Lord Rosse has recognized 40 spiral
-nebulæ and suspected a similar structure in 30 others.
-
-The class of the Irregular nebulæ, which will be now considered,
-differs greatly in character from the others, and includes the largest,
-the brightest and the most extraordinary nebulæ in the heavens. The
-nebulæ of this class differ from those belonging to the other classes
-by a want of symmetry in their form and in the distribution of their
-light, as well as by their capricious shapes, and their very complicated
-structure. Another and perhaps the principal difference between them and
-the objects above described, consists in the remarkable fact already
-stated, that they are not, except in rare cases, to be found in the
-regions where the other nebulæ abound. On the contrary, they are found
-in or very near the Milky-way, precisely where the other nebulæ are the
-most rare. This fact, recognized by Sir J. Herschel, led him to consider
-them as "outlying, very distant, and as it were detached fragments of
-the great stratum of the Galaxy." It seems very probable that the reason
-why these objects differ so greatly from the other nebulæ in size,
-brightness and complication of structure, is simply because they are
-much nearer to us than are most of the others. They are perhaps nebulous
-members of our Galaxy. The same remark which has been made of
-star-clusters may be applied to nebulæ. The nearer they are to us, the
-larger, the brighter and the more complicated they will appear, while
-the farther they are removed, the more simple and regular and round they
-will appear, only their brightest and deepest parts being then visible.
-
-The Crab Nebula of Lord Rosse, near _ζ_ Tauri, No. 1,157, is one of the
-interesting objects of this class. It has curious appendages streaming
-off from an oval, luminous mass, which give it a distant resemblance to
-the animal from which it derives its name. The Bifid nebula in Cygnus,
-Nos. 4,400 and 4,616, is another object of this class. It consists of a
-long, narrow, crooked streak, forking out at several places, and passing
-through _χ_ Cygni. Observers, having failed to recognize the connection
-existing between its different centres of brightness, have made distinct
-nebulæ of this extended object.
-
-The Dumb-bell nebula in Vulpecula, No. 4,532, is a bright and curious
-object, with a general resemblance to the instrument from which it
-derives its name. Lord Rosse's telescope has shown many stars in it,
-projected on a nebulous background, and Prof. Bond seems to have thought
-that it showed traces of resolvability, although in the study which I
-made of this nebula with the same instrument used by the latter
-observer, I failed to perceive any such traces. Dr. Huggins finds its
-spectrum gaseous.
-
-The star-cluster, No. 4,400, in Scutum Sobieskii, which is described by
-Sir J. Herschel as a loose cluster of at least 100 stars, I have found
-to be involved in an extensive, although not very bright, nebula, which
-would seem to have escaped his scrutiny. In a study and drawing of this
-nebula made in 1876, its general form is that of an open fan, with the
-exception that the handle is wanting, with deeply indented branches on
-the preceding side, where the brightest stars of the cluster are
-grouped. From its peculiar form, this object might appropriately be
-called the Fan nebula.
-
-The Omega or Horse-shoe nebula, in Sagittarius, No. 4,403, of which I
-have made a study and two drawings, one with a refractor 6⅓ inches in
-aperture, and the other conjointly with Prof. Holden, with the great
-telescope of the Naval Observatory, is a bright and very complicated
-object. Its general appearance in small instruments, with low power, is
-that of a long, narrow pisciform mass of light, from which proceeds on
-the preceding side, the great double loop from which it derives its
-name. But in the great Washington refractor its structure becomes very
-complicated, forming various bright nebulous masses and wisps of great
-extension. Prof. Holden, who has made a careful, comparative study of
-the published drawings of this object, thinks there are reasons to
-believe that its western branch has moved relatively to the stars found
-within its loop. The spectrum of this nebula is gaseous.
-
-The Trifid nebula, No. 4,355, in the same constellation, is also a very
-remarkable object, although it is not so bright as the last. This
-nebula, which I have studied with the refractor of the Cambridge
-Observatory, consists of four principal masses of light, separated by a
-wide and irregular gap branching out in several places. These masses,
-which are brighter along the dark gap, gradually fade away externally. A
-group of stars, two of which are quite bright, is found near the centre
-of the nebula, on the inner edge of the following mass, and close to the
-principal branch of the dark channel. A little to the north, and
-apparently forming a part of this nebula, is a globular-looking nebula,
-having a pale yellow star at its centre. Prof. Holden's studies on this
-nebula show that the triple star, which was centrally situated in the
-dark gap from 1784 to 1833, was found involved in the border of the
-nebulous mass following it, from 1839 to 1877; the change, he thinks,
-is attributable either to the proper motion of the group of stars or to
-that of the nebula itself.
-
-In the same vicinity is found the splendid and very extensive nebula No.
-4,361, in which is involved a loose, but very brilliant star-cluster.
-This nebula and cluster, which I have studied and drawn with a 6⅓ inch
-telescope, is very complicated in structure, and divided by a dark
-irregular gap into three principal masses of light, condensing at one
-point around a star, and at others forming long, bright, gently-curved
-branches, which give to this object a strong resemblance to the wings of
-a bird when extended upwards in the action of flying. From this
-peculiarity this object might appropriately be called the Winged nebula.
-Its spectrum is that of a gas.
-
-The variable star _η_ Argus is completely surrounded by the great
-nebula of the same name, No. 2,197, first delineated by Sir J. Herschel,
-during his residence at the Cape of Good Hope, in 1838. This object,
-which covers more than ⁴⁄₇ of a square degree, is divided into
-three unequal masses, separated by dark oval spots, comparatively free
-from nebulosity, and is suspected to have undergone changes since
-Herschel's time.
-
-In the same field with the double star, _ζ_ Orionis, the most easterly
-of the three bright stars in the belt of Orion, is found another
-irregular nebula of the Trifid type. From the drawings which I have made
-of this object, it appears to be composed of three principal unequal
-masses, separated by a wide, irregular, dark channel, two of the masses
-being quite complicated in structure, and forming curved, nebulous
-streams of considerable length and breadth. This nebula, like the next
-to be described, seems to be connected with the Galaxy by the great
-galactic loop described in another section.
-
-By far the most conspicuous irregular nebula visible from our northern
-States, is the great nebula in Orion, No. 1,179, represented on Plate
-XV. This object, visible to the naked eye, is the brightest and the most
-wonderful nebula in the heavens. It is situated a little to the south of
-the three bright stars in the belt of Orion, and may be readily detected
-surrounding the star _θ_, situated between and in a line with two faint
-stars, the three being in a straight line which points directly towards
-_ε_, the middle star of the three in Orion's belt. The area occupied by
-this nebula is about equal to that occupied by the Moon.
-
-In its brightest parts the nebula in Orion appears as a luminous cloud
-of a pentagonal form, from which issue many luminous appendages of
-various shapes and lengths. This principal mass is divided into
-secondary masses, separated by darkish, irregular intervals. These
-secondary masses in their turn appear mottled and fleecy. Towards the
-lower part of the pentagonal mass is found a roundish dark space,
-comparatively devoid of nebulosity, in which are involved four bright
-stars forming a trapezium, and several fainter ones. The four bright
-stars of the trapezium constitute the quadruple star _θ_ Orionis, from
-which the nebula has received its name. The cloud-like pentagonal form
-is brightest on the north-west of the trapezium, and is surrounded on
-three sides by long, soft, curved wisps, fading insensibly into the
-outer nebulous mass in which they are involved. On the east a broad,
-wavy wing spreads out, and sends an important branch southward.
-South-east of the trapezium are found several curious dark spaces,
-comparatively devoid of nebulosity, especially those on the east, which
-give to this nebula a singular character. Close to the north-eastern
-part of the nebula, or rather in contact with it, is found a small,
-curiously-shaped nebula, condensing around a bright star into a blazing
-nucleus. From this centre it continues northward in a narrow diffused
-stream, which spreads out in passing over the stars _c^1_ and _c^2_; and
-after having sent short branches northward, it curves back to the south
-and joins the main nebula on the west of its starting point, having thus
-formed a great loop which is not shown on the Plate. The nebula also
-forms a loop towards the south, which is partly shown on Plate XV., a
-small branch of which, passing through _τ_ Orionis, the nebulous star
-shown at the top of the Plate, and extending southward, is not here
-represented.
-
-On ordinary nights the nebula in Orion is a splendid object, and
-inspires the observer with amazement; but this is as nothing compared
-with the grand and magnificent sight which it presents during the very
-rare moments when our atmosphere is perfectly clear and steady. I have
-seen this nebula but once under these favorable circumstances, and I was
-surprised by the grandeur of the scene. Then could be detected features
-to be seen at no other time, and its fleecy, floculent, cloud-like
-masses glittered with such intensity that it seemed as if thousands of
-stars were going to blaze out the next moment. Although I observed the
-nebula under such favorable conditions, and with the fifteen-inch
-refractor of the Cambridge Observatory, yet I was disappointed in my
-expectations, and distinguished no new stars or points of light, and
-nothing more than a very bright mass, finely divided into minute blazing
-cloudlets. Although I failed to resolve this nebula into stars, yet Lord
-Rosse, Bond and Secchi thought they had caught glimpses of star dust.
-Its spectrum, however, proves to be mainly that of incandescent gases,
-probably hydrogen and nitrogen. In the curved wisps found in this
-nebula, Lord Rosse and others saw indications of a spiral structure.
-
-Several bright stars are found scattered over this nebula, and besides
-those forming the trapezium, there are three in a row, a little to the
-south-east of that group, which are quite bright and remarkable. Among
-the stars involved in this nebula, few show signs of having a physical
-connection with it, although it seems probable that the group of the
-trapezium is so connected. Some of these stars are variable. The small
-stars represented on this Plate, as on others of the series, are
-somewhat exaggerated in size, as was unavoidable with any process of
-reproduction which could be adopted.
-
-In 1811, W. Herschel was led to suspect that some changes had occurred
-in this nebula, but changes in such complicated and delicate objects are
-not easily ascertained, since, for the most part, we have for comparison
-with our later observations only coarse drawings made by hands unskilled
-in delineation.
-
-Although comparatively rare, double and multiple nebulæ may be found in
-the sky. When this occurs, their constituents most commonly belong to
-the class of spherical nebulæ. Sometimes the components are separated
-and distinct, at other times one of them is projected upon the other,
-either really or by the effect of perspective. Sometimes one is round
-and the other elongated. It is probable that while some of these nebulæ
-are physically associated and form a system, others appear to be so only
-because they happen to be almost in a line with the observer. A double
-nebula in Draco, Nos. 4,127 and 4,128, which I have drawn, is a fair
-type of those which are separated. The first is a globular nebula, and
-the last an oval one, with a star at its centre. The double nebula, Nos.
-858 and 859, in Taurus, which I have also studied, is a type of the
-cases in which one nebula is partly projected on another. In this
-instance both the nebulæ are globular.
-
-The nebulæ in general show very little color in their light, which is
-ordinarily whitish and pale. Some, however, present a decided bluish or
-greenish tint. The great nebula in Orion has a greenish cast, and we
-have seen that some planetary nebulæ are bluish.
-
-It has been a question whether nebulæ are changing. It has already been
-stated that Prof. Holden believes there is ground to suspect that the
-Trifid and Horse-shoe nebulæ have undergone some changes. A nebula near
-_ε_ Tauri has been lost and found again several times. Two other
-nebulæ in the same constellation have presented curious variations.
-One, near a star of the tenth magnitude, exhibited variations of
-brightness like those of the star itself, and for a time disappeared.
-The other, near _ζ_ Tauri, increased in brightness for three months,
-after which it disappeared. In 1859 Tempel discovered a nebula in the
-Pleiades, which has shown some fluctuations. In 1875 I made a long study
-of this object, and drew it carefully a dozen times, but I was not able
-to see any changes in it within the two or three months during which my
-observations were continued. But on Nov. 24, 1876, it was found of a
-different color, being purplish and very faint. On Dec. 23, 1880, it was
-found just as bright and visible as when I drew it in 1875, and on Oct.
-20, 1881, it appeared faint and purplish again, as in 1876. On this last
-night, and on those which followed it, it was impossible for me to trace
-the nebulosity as far as in 1875. I consider this as due to a variation
-in the light of this object, which in 1875 was bright enough to be well
-seen while the Moon after her First Quarter was within ten or fifteen
-degrees from the Pleiades.
-
-From the observations of M. Laugier, it appears that some nebulæ have a
-proper motion, comparable to that of stars. From the displacement of the
-lines of their spectra by their motion in the line of sight, Dr. Huggins
-found that no nebula observed by him has a proper motion surpassing 25
-miles per second. The Ring nebula in Lyra appears to move from us at the
-rate of 3 miles per second, and that in Orion recedes about 17 miles per
-second.
-
-The important question arises, are all the irresolvable nebulæ in the
-heavens to be considered as so many star-clusters, differing only from
-them by the minuteness of their components, or their immense distance
-from us; or are they cosmical clouds, composed of luminous vapors,
-similar to the matter composing the heads and tails of comets?
-Originally, W. Herschel, with many astronomers, thought that all these
-objects were stellar aggregations, too distant to be resolved into
-stars; but he subsequently modified his opinion, and accepted the idea
-that some of them are of a gaseous nature.
-
-No direct proof that the nebulæ are gaseous could be obtained, however,
-before the spectroscope was known. The attempt to analyze the light of
-the nebulæ with this instrument was made in 1864, by Dr. Huggins, who
-directed his spectroscope to the planetary nebula, No. 4,373, in Draco.
-Its spectrum was found to consist of three bright, distinct lines, the
-brightest of which corresponded with the strongest nitrogen line, and
-the feeblest with the hydrogen C line. Besides these lines, it gave also
-a very faint, continuous spectrum, apparently due to a central point of
-condensation. By this observation, the gaseous nature of a nebula was
-for the first time demonstrated. Dr. Huggins thus analyzed 70 nebulæ,
-of which one-third gave a gaseous spectrum, consisting of several bright
-lines, the brightest of which invariably corresponded with the lines of
-nitrogen. The others gave a continuous spectrum, with the red end
-usually deficient. These results indicate that if some of the so-called
-nebulæ are due to an aggregation of stars, either too minute or too
-remote in space to be individually resolved, others are in a gaseous
-state. Yet the faint, continuous spectrum, given by some nebulæ, in
-addition to their gaseous spectra, seems to show that these nebulæ have
-some stars or matter in a different state, either involved in them or
-projected on their surface.
-
-The idea of diffused matter distributed here and there in space, and
-gradually condensing into stars, is by no means new. As early as 1572,
-Tycho Brahé proposed such an hypothesis, to explain the sudden
-apparition of a new star in Cassiopeia, which he considered as formed by
-the recent agglomeration of the "celestial matter" diffused in space.
-Kepler adopted the same idea to explain the new star which appeared in
-Ophiuchus, in 1604. Halley, Lacaille, Mairan and others, entertained the
-same opinion. The hypothesis of a self-luminous, nebulous matter
-diffused in space, and forming here and there immense masses, has been
-proposed from the origin of the telescope, and was adopted by Sir
-William Herschel, who in his grand speculations on the universe
-considered the nebulæ as immense masses of phosphorescent vapors,
-gradually condensing around one or several centres into stars or
-clusters of stars. The evidence afforded by the spectroscope seems to be
-in favor of such an hypothesis, and shows us that gaseous agglomerations
-exist in space.
-
-According to our modern conception, the visible universe is but an
-infinitely small portion of the infinite universe perceived by our mind.
-The great blazing centre around which our little, non-luminous globe
-pursues its endless journey, is only an humble member of a cluster
-comprising four hundred equally powerful suns, as they are believed to
-be, although they appear to us as little twinkling stars. The nearest of
-these stars is 221,000 times as far from the Sun as the Sun is from the
-Earth, and yet this entire cluster is only one among the several hundred
-Star-clusters composing the great galactic nebula in which we are
-involved, comprising thirty or fifty millions of such suns. Among the
-4,000 irresolvable nebulæ in the sky, perhaps over one-half are
-supposed to be galaxies, like our own galaxy, composed of star-clusters,
-and millions of stars. Besides these remote galaxies, vast
-agglomerations of yet uncondensed, nebulous matter exist in space, and
-form the nebulæ proper, in which the genesis of suns is slowly
-elaborated. Although the visible universe is limited by the penetrating
-power of our instruments, yet we see in imagination the infinite
-universe stretching farther and farther; but we know not whether this
-invisible universe is totally devoid of matter, or whether it also is
-filled with millions and millions of suns and galaxies.
-
-
-
-
-APPENDIX
-
-KEY TO THE PLATES
-
-
-PLATE I.--GROUP OF SUN-SPOTS AND VEILED SPOTS.
-
-_Observed June_ 17, 1875, _at 7h. 30m. A. M._
-
-
-The background shows the sun's visible surface, or _photosphere_, as
-seen with a telescope of high power at the most favorable moments,
-composed of innumerable light markings, or granules, separated by a
-network of darker gray. The granules, each some hundreds of miles in
-width, are thought to be the flame-like summits of the radial filaments
-or columns of gas and vapor which compose the photospheric shell. The
-two principal sun-spots of the group here represented show the
-characteristic dark _umbra_ in the centre, overhung by the thatch-like
-_penumbra_, composed of whitish gray filaments. The penumbral filaments
-are not supposed to differ in their nature from those constituting the
-ordinary photosphere, save that they are seen here elongated and
-violently disturbed by the force of gaseous currents. Both spots are
-traversed partly or wholly by bright overlying _faculæ_, or so-called
-_luminous bridges_, depressed portions of which, in the left-hand spot,
-form the _gray and rosy veils_ commonly attendant upon this class of
-spots. In each of these spots, also, the inner ends of projecting
-penumbral filaments have fallen so far within the umbra as to appear
-much darker than the rest. At the right of the upper portion of the
-left-hand spot, is a mass of white facular clouds, honey-combed by dark
-spaces, through which are seen traces of the undeveloped third spot of
-the triple group first observed. If seen upon the sun's limb, this would
-have presented the appearance of a _lateral spot_. Above the right-hand
-spot is a small black "dot," or incipient spot, without distinct
-penumbra. The irregular dark rift below the two large spots and
-connecting them is a spot of the crevasse type, with very slight umbra,
-a still better example of which is seen in a westward
-prolongation of the penumbra of the left-hand spot. In the upper
-left-hand corner of the Plate are seen several small _faculæ_,
-appearing as irregular whitish streaks amongst the granules. In the
-pear-shaped darkening of the solar surface below and at their left, is
-seen a veiled spot, two of which attended this group.
-
-_Approximate scale, 2500 miles--1 inch._
-
-
-PLATE II.--SOLAR PROTUBERANCES.
-
-_Observed May_ 5, 1873, _at 9h. 40m. A. M._
-
-
-A view of an upheaval of the _chromosphere_, or third outlying envelope
-of the sun, as observed with the tele-spectroscope, or telescope with
-spectroscope attached.
-
-The _method of the observation_ requires a word of explanation. Save on
-the rare occasions of a total solar eclipse, no direct telescopic view
-of the solar prominences or flames is possible, owing to the fact that
-the intense white light from the sun's main disk entirely obscures
-the feeble pink light of the chromosphere. A few years ago Messrs.
-Jannsen and Lockyer found that a spectroscope of high dispersive power
-so weakens the spectrum of ordinary sun-light as to show the spectrum of
-bright lines given by the chromosphere, on any clear day. The telescope
-is adjusted so that a portion of the sun's limb, usually near a group of
-active sun-spots, shall be presented before the opened slit of the
-spectroscope. The light of the chromosphere thus admitted along with
-some diffused sun-light from the earth's atmosphere, produces a spectrum
-of intensely bright lines, widely separated, on the fainter background
-of the strongly dispersed spectrum of sun-light. The most prominent of
-these bright lines are those known as the C line (_scarlet_), F line
-(_blue_), which with several others are due to the hydrogen present in
-the chromosphere, the D{3} line (_orange_) ascribed to a little known
-substance called "helium", and occasionally the sodium lines D{1}, D{3},
-(_yellow_). By adjusting the slit upon the scarlet C line, the
-appearances represented in Plate II. were observed as through an
-atmosphere of scarlet light: in the D or F lines identical appearances
-may be seen, but somewhat less clearly defined, as through yellow or
-blue light respectively. Hence the solar times, as here observed with
-the spectroscope in the hydrogen C line, are seen through a portion only
-(the scarlet rays) of the light coming from but one substance (hydrogen)
-of the companion incandescent substances present in the chromosphere.
-The color of the collective chromospheric light is seen directly with
-the telescope during an eclipse (See Plate III.) to be a delicate rosy
-pink.
-
-
-_Description of the Plate._--The black background represents the general
-darkness of the eye-piece to the spectroscope. The broad red stripe
-stretching from top to bottom of the Plate is a portion of the red band
-of the spectrum, magnified about 100 times as compared with the actual
-spectroscopic view. The upper and lower edges of the cross-section of
-dusky red correspond with the edges of the slit, opened widely enough to
-admit a view of the chromospheric crest and of the whole height of the
-protuberances at once. With a narrower opening of the slit this
-background would have been nearly black, its reddish cast increasing
-with the amount of opening and consequent admission of diffused
-sun-light. Rising above the lower edge of the opening is seen a small
-outer segment of the chromosphere, which, as a portion of the sun's
-eastern limb, should be imagined as moving directly towards the
-beholder. The seams and rifts by which its surface is broken, as well as
-the distorted forms of the huge protuberances show the chromosphere to
-be in violent agitation. Some of the most characteristic shapes of the
-_eruptive protuberances_ are presented, as also _cloud-like_ forms
-overtopping the rest. In the immediate foreground the bases of two
-towering columns appear deeply depressed below the general horizon of
-the segment observed, showing an extraordinary velocity of motion of the
-whole uplifted mass toward the observer. The highest of these
-protuberances was 126,000 miles in height at the moment of observation.
-The triple protuberance at the left with two drooping wings and a tall
-swaying spire tipped with a very bright flame, shows by its more
-brilliant color the higher temperature (and possibly compression) to
-which its gases have been subjected. The irregular black bands behind
-this protuberance indicate the presence there of less condensed and
-cooler clouds of the same gases. The dimmer jets of dame rising from the
-chromosphere are either vanishing protuberances, or, as in the case of
-the smallest jet shown at the extreme right of the horizon, are the tops
-of protuberances just coming into view.
-
-_Approximate scale, 6000 miles--1 inch._
-
-
-PLATE III.--TOTAL ECLIPSE OF THE SUN.
-
-_Observed July_ 29, 1878, _at Creston, Wyoming Territory._
-
-
-A telescopic view of the sun's _corona_ or extreme outer atmosphere and
-of the _solar flames_ or _prominences_ during a total eclipse. At the
-moment of observation the dark disk of the moon, while still hiding the
-sun's main body, had passed far enough eastward to allow the rosy pink
-chromospheric prominences to be seen on its western border. On all sides
-of the sun's hidden disk, the _corona_ shows its pale greenish light
-extending in halo-like rays and streamers, and two very remarkable wings
-stretch eastward and westward very nearly in the plane of the ecliptic
-and in the direction of the positions of Mercury and Venus respectively
-at the time of observation. The full extent of these wings could not be
-shown in the Plate without reducing its scale materially, since the
-westerly wing extended no less than twelve times the sun's diameter, and
-the easterly wing nearly as far, or over ten million miles. A circlet of
-bright light immediately bordering the moon's disk is the so-called
-_inner corona_, next to which the wings and streamers arc brightest,
-thence shading off imperceptibly into the twilight sky of the eclipse.
-Other noteworthy peculiarities of the corona, as observed during this
-eclipse, are the varying angles at which the radiating streamers are
-seen to project, the comparatively dark intervals between them, and the
-curved, wisp-like projections seen upon the wings. An especially
-noticeable gap appears where the most westerly of the upward streamers
-abruptly cuts off the view of the long wing. The largest and brightest
-of the curving streamers on the westerly wing coincides with the highest
-flame-like protuberance. To some observers of this eclipse the upward
-and downward streamers seemed pointed at their outer extremities and
-less regular in form.
-
-_Approximate scale, 135,000 miles--1 inch._
-
-
-PLATE IV.--AURORA BOREALIS.
-
-_As observed March_ 1, 1872, _at 9h. 25m. P. M._
-
-
-The view presents the rare spectacle of an aurora spanning the sky from
-east to west in concentric arches. The Polar Star is nearly central in
-the background, the constellation of the Great Bear on the right and
-Cassiopeia's chair on the left. The large star at some distance above
-the horizon on the right is Arcturus. The almost black inner segment of
-the aurora resting upon the horizon, has its summit in the magnetic
-meridian, which was in this case a little west of north, its arc being
-indented by the bases of the ascending streamers. Both streamers and
-arches were, when observed, tremulous with upward pulsations and there
-was also a wave-like movement of the streamers from west to east. The
-prevailing color of this aurora is a pale whitish green and the
-complementary red appears especially at the west end of the auroral
-arch. The summits of the streamers are from four hundred to live hundred
-miles above the earth and the aurora is therefore a phenomenon of the
-terrestrial atmosphere rather than of astronomical observation proper.
-
-
-PLATE V.--THE ZODIACAL LIGHT.
-
-_Observed February_ 20, 1876.
-
-
-An observation of the cone of light whose axis lies along the Zodiac,
-whence it derives its name. It is drawn as seen in the west, with its
-base in the constellation Pisces, and its apex near the familiar group
-of the Pleiades in the constellation Taurus. The first bright star above
-the horizon in the base of the cone is the planet _Venus_ and at some
-distance above is the reddish disk of _Mars_, the two being in rare
-companionship as evening stars. Above the constellation Pisces, two
-bright stars of Aries lie just outside the cone at the right. The
-nearest bright star above these at the right is _Beta_, the leading star
-of the constellation Triangula. Further at the right the three prominent
-stars nearly in a line are, in ascending order, _Delta_, _Beta_ and
-_Gamma_ of the constellation Andromeda. Above these at the left, the
-brightest star of a quadrangular group of four is the remarkable
-variable star _Algol_ (_Beta_) of the constellation Perseus, which
-changes from the second to the fourth magnitude in a period of less than
-three days. At the left and a little above the Pleiades is the ruddy
-star _Aldebaran_, one of the Hyades and chief star in the constellation
-Taurus. These are the principal stars visible in this portion of the sky
-at the time of the observation. Their relative positions are represented
-as seen in the sky and not by the common method of star-atlases, which
-allows for the change from a spherical to a plane surface. Their
-magnitude in the order of brightness is indicated only approximately.
-
-
-PLATE VI.--MARE HUMORUM.
-
-_From a study made in 1875._
-
-
-A view of one of the lunar plains, or so-called _seas_ (_Maria_), with
-an encircling mountainous wall consisting of volcano-like craters in
-various stages of subsidence and dislocation. The sun-light coming from
-the west casts strong shadows from all the elevations eastward, and is
-just rising on the _terminator_, where the rugged structure of the
-Moon's surface is best seen. The lighter portions are the more elevated
-mountainous tracts and crater summits. The detailed description of this
-Plate given in the body of the Manual is repeated here for convenience
-of reference: The Mare Humorum, or sea of moisture, as it is called, is
-one of the smaller gray lunar plains. Its diameter, which is very nearly
-the same in all directions, is about 270 miles, the total area of this
-plain being about 50,000 square miles. It is one of the most distinct
-plains of the Moon, and is easily seen with the naked eye on the
-left-hand side of the disk. The floor of the plain is, like that of the
-other gray plains, traversed by several systems of very extended but low
-hills and ridges, while small craters are disseminated upon its surface.
-The color of this formation is of a dusky greenish gray along the
-border, while in the interior it is of a lighter shade, and is of
-brownish olivaceous tint. This plain, which is surrounded by high clefts
-and rifts, well illustrates the phenomena of dislocation and subsidence.
-The double-ringed crater Vitello, whose walls rise from 4,000 to 5,000
-feet in height, is seen in the upper left-hand corner of the gray plain.
-Close to Vitello at the east is the large broken ring-plain Lee, and
-farther east, and a little below, is a similarly broken crater called
-Doppelmayer. Both of these open craters have mountainous masses and
-peaks on their door, which is on a level with that of the Mare Humorum.
-A little below, and to the left of these objects, is dimly seen a deeply
-imbedded oval crater, whose walls barely rise above the level of the
-plain. On the right-hand side of the great plain is a long _fault_, with
-a system of fracture running along its border. On this right-hand side
-may be seen a part of the line of the terminator, which separates the
-light from the darkness. Towards the lower right-hand corner is the
-great ring-plain Gassendi, 55 miles in diameter, with its system of
-fractures and its central mountains, which rise from 3,000 to 4,000 feet
-above its floor. This crater slopes towards the plain, showing the
-subsidence to which it has been submitted. While the northern portion of
-the wall of this crater rises to 10,000 feet, that on the plain is only
-500 feet high, and is even wholly demolished at one place where the
-floor of the crater is in direct communication with the plain. In the
-lower part of the sea, and a little to the west of the middle line, is
-found the crater Agatharchides, which shows below its north wall the
-marks of rills impressed by a flood of lava, which once issued from the
-side of the crater. On the left-hand side of the plain is seen the
-half-demolished crater Hippalus resembling a large bay, which has its
-interior strewn with peaks and mountains. On this same side can be seen
-one of the most important systems of clefts and fractures visible on the
-Moon, these clefts varying in length from 150 to 200 miles.
-
-_Approximate scale, 15 miles--1 inch._
-
-
-PLATE VII.--PARTIAL ECLIPSE OF THE MOON.
-
-_Observed October_ 24, 1874.
-
-
-A view of the Moon partially obscured by the Earth's shadow, whose
-outline gives ocular proof of the earth's rotundity of form. The
-shadowed part of the Moon's surface is rendered visible by the diffused
-sun-light refracted upon it from the earth's atmosphere. Its reddish
-brown color is due to the absorption, by vapors present near the earth's
-surface, of a considerable part of this dim light. On both the obscured
-and illuminated tracts the configurations of the Moon's surface are seen
-as with the naked eye. The craters appear as distinct patches of lighter
-color, and the noticeably darker areas are the depressed plains or
-_Maria_. The large crater _Tycho_, at the lower part of the disk, is the
-most prominent of these objects, with its extensive system of _radiating
-streaks_. The largest crater above is _Copernicus_, at the left of which
-is _Kepler_ and still above are _Aristarchus_ and _Herodotus_ appearing
-as if blended in one. Above and at the left of the great crater _Tycho_,
-the first dark tract is the _Mare Humorum_ of Plate VI., seen in its
-natural position, with the crater _Gassendi_ at its northern (upper)
-extremity and _Vitello_ on its southern (lower) border. The advancing
-border of the shadow appears, as always, noticeably darker than the
-remainder, an effect probably of contrast. The illuminated segment of
-the Moon's disk has its usual appearance, the lighter portions being the
-more elevated mountainous surfaces, and the dark spaces the floors of
-extensive plains.
-
-_Approximate scale, 140 miles--1 inch._
-
-
-PLATE VIII.--THE PLANET MARS.
-
-_Observed September_ 3, 1877, _at 11h. 55m. P. M._
-
-A view of the southern hemisphere of Mars, when in the most favorable
-position for observation, and when exceptionally free from the clouds,
-which very frequently hide its surface configurations. Since, of all the
-planets, Mars is most like the earth, Plate VIII. may give a fair idea
-of the appearance of our globe to a supposed observer on Mars. The dark
-gray and black markings, are regarded as tracts of water, or of some
-liquid with similar powers of absorbing light; and, for the same reason,
-the lighter portions, of a prevailing reddish tint, are supposed to be
-bodies of land, while the bright white portions are variously due to
-clouds, to polar snow or ice, and the bright rim of white along the
-limb, to the depth of the atmosphere through which the limb is seen. The
-chief permanent features of the planet's surface have been named in
-honor of various astronomers.
-
-The large dark tract on the left is _De La Rue Ocean_, the isolated oval
-spot near the centre is _Terby Sea_, and on the right is the western end
-of _Maraldi Sea_, with strongly indented border. Directly north of
-(below) De La Rue Ocean, is _Maedler Continent_; above it stretches
-_Jacob Land_; and surrounding Terby Sea is _Secchi Continent_. Extending
-into the centre of De La Rue Ocean is a curious double peninsula,
-called, in consequence of the dimness of former observations, _Hall
-Island_. The sharply defined, white-crested northern borders of De La
-Rue Ocean and Maraldi Sea may indicate the existence there of lofty
-coast ranges, more or less constantly covered with opaque clouds
-strongly reflecting light. The white spot in the centre of Maedler
-Continent, of a temporary nature, has a similar explanation. The
-intervals of olivaceous gray on Secchi Continent and elsewhere may
-perhaps be ascribed to the flooding and drying up of marshes and
-lowlands, as these markings have been observed to vary somewhat in
-connection with the change of seasons on Mars. The greenish tints
-observed along the planet's limb, alike on the darker and lighter
-surfaces, are probably due to an optical effect, the green being
-complementary to the prevailing red of the disk. The brilliant oval
-white spot near the southern (upper) pole of the planet is a so-called
-polar spot, in all probability consisting of a material similar to snow
-or ice and here observed in the midst of a dark open sea.
-
-_Approximate scale, 300 miles--1 inch._
-
-
-PLATE IX.--THE PLANET JUPITER.
-
-_Observed November_ 1, 1880, _at 9h. 30m. P. M._
-
-This planet is perpetually wrapped in dense clouds which hide its inner
-globe from view. The drawing shows Jupiter's outer clouded surface with
-its usual series of alternate light and dark belts, the disk as a whole
-appearing brighter in the centre than near the limb. The darker gray and
-black markings indicate in general the lower cloud-levels; that is,
-partial breaks or rifts in the cloudy envelope, whose prevailing depth
-apparently exceeds four thousand miles. While the deepest depression in
-the cloudy envelope is within the limits of the Great Red Spot, the
-vision may not even here penetrate very deeply. Two of Jupiter's four
-moons present bright disks near the planet's western limb, and cast
-their shadows far eastward on the disk, that of the "second satellite"
-falling upon the Red Spot. On the Red Spot are seen in addition two
-small black spots, no explanation of which can yet be offered. The broad
-white ring of clouds bordering the Red Spot appeared in constant motion.
-The central, or equatorial belt, shows brilliant cloudy masses of both
-the _cumulus_ and _stratus_ types, and the underlying gray and black
-cloudy surfaces are pervaded with the pinkish color characteristic of
-this belt. The dark circular spots on the wide white belt next north
-showed in their mode of formation striking resemblances to sun-spots.
-They afterward coalesced into a continuous pink belt. The diffusion of
-pinkish color over the three northernmost dark bands, as here observed,
-is unusual. About either pole is seen the uniform gray segment or polar
-cap. The equatorial diameter is noticeably longer than the polar
-diameter, a consequence of the planet's extraordinary swiftness of
-rotation. To the same cause may also be due chiefly the distribution of
-the cloudy belts parallel to the planet's equator, though the analogy of
-the terrestrial trade-winds fails to explain all the observed phenomena.
-
-_Approximate scale, 5,500 miles--1 inch._
-
-
-PLATE X.--THE PLANET SATURN.
-
-_Observed November_ 30, 1874, _at 5h. 50m. P. M._
-
-
-Saturn is unique amongst the planets in that its globe is encircled by a
-series of concentric rings, which lie in the plane of its equator, and
-consist, according to present theories, of vast throngs of minute bodies
-revolving about the planet, like so many satellites, in closely parallel
-orbits. The globe of Saturn, like that of Jupiter, is surrounded by
-cloudy belts parallel to its equator. The broad equatorial belt, of a
-delicate pinkish tint, is both brighter and more mottled than the
-narrower yellowish white belts, which alternate with darker belts of
-ashy gray on both the north and south sides, but are seen here only on
-the northern side. The disk has an oval shape, owing to the extreme
-polar compression of the globe.
-
-The outer, middle and inner rings, with their various subdivisions, are
-clearly shown in Plate X., and are best seen on the so-called _ansæ_,
-or handles, projecting on either side. The gray _outer ring_ is
-separated by the dusky pencil line into two divisions, both of which
-appear slightly mottled on the ansæ, as if with clouds. The _middle
-ring_ has three sub-divisions which are clearly distinguishable,
-although separated by no dark interval, viz., a brilliant white outer
-zone, distinctly mottled, as seen on the extremities of the ansæ, and
-two interior zones of gradually diminishing brightness. The _gauze_ or
-_dusky ring_ is seen at its full width on the ansæ, but on the
-background of the strongly illuminated globe only its outer and
-presumably denser border is visible. The shadow of the globe on the
-rings is seen on the lower portion of the eastern ansæ. The shadow on
-the dusky ring is with difficulty perceptible; the shadow on the middle
-ring is slightly concave toward the planet, which concavity is abruptly
-increased on the outer zone of this ring; while the shadow on the outer
-ring slants away from the globe. These appearances are fully accounted
-for by supposing a general increase of level from the inner edge of the
-dusky ring to the outer margin of the middle ring, and a uniform lower
-level on the outer ring. Other observers have regarded the deflection of
-the shadow as an effect of irradiation. The inner margin of the double
-outer ring presents on both ansæ a number of slight indentations,
-which, if not actual irregularities in the contour of this ring, may be
-explained as shadows caused by elevations on the outer border of the
-middle ring, or possibly by over-hanging clouds.
-
-_Approximate scale, 6,500 miles--1 inch._
-
-
-PLATE XI.--THE GREAT COMET OF 1881.
-
-_Observed on the night of June_ 25-26, _at 1h. 30m. A. M._
-
-
-A view of the comet 1881, III., drawn as if seen with the naked eye, the
-minute details, however, being reproduced as seen with the telescope.
-The star-like _nucleus_ is attended by four conical wings which cause it
-to appear diamond-shaped. The _coma_ appears double, the brilliant inner
-coma, immediately enveloping the nucleus, being surrounded by a fainter
-exterior coma, which has a noticeable depression corresponding to that
-of the inner edge of the principal coma. The tail is divided lengthwise
-by a dark rift and is brightest on its convex or forward side. An inner
-portion of the tail, brighter than the rest, is more strongly curved, as
-if by solar repulsion. Stars are seen through the brighter parts of the
-tail, as they may be seen even through the coma and nucleus, with little
-diminution of their light.
-
-
-PLATE XII.--THE NOVEMBER METEORS.
-
-_As observed on the night of November_ 13-14, 1868.
-
-
-A partially ideal view of the November Meteors, combining forms observed
-at different times during the night of Nov. 13th, 1868. It is not,
-however, a fanciful view, since a much larger number of meteors were
-observed falling at once during the shower of November, 1833, and at
-other times. The locality of the observation is shown by the Polar Star
-seen near the centre of the Plate, and Cassiopeia's Chair at the left.
-The general direction of the paths of the meteors is from the
-north-east, the _radiant point_ of the shower having been in the
-constellation Leo, beyond and above Ursa Major. While the orbits of the
-meteors are, in general, curved regularly and slightly, several are
-shown with very eccentric paths, among them one which changed its course
-at a sharp angle. In the upper left-hand corner appear two vanishing
-trails of the "ring-form," and several others still further transformed
-into faint luminous patches of cloud. Red, yellow, green, blue and
-purple tints were observed in the meteors and their trails, as
-represented in the Plate.
-
-
-PLATE XIII.--PART OF THE MILKY WAY.
-
-_From a study made during the years_ 1874, 1875 _and_ 1876.
-
-
-The course of the portion of the Galaxy represented in Plate XIII. is as
-follows: From Cassiopeia's chair, three bright stars of which appear at
-the upper edge of the Plate, the Galaxy, forming two streams, descends
-south, passing partly through Lacerta on the left, and Cepheus on the
-right; at this last point it approaches nearest to the Polar Star, which
-is itself outside of the field of view. Then it enters Cygnus, where it
-becomes very complicated and bright, and where several large cloudy
-masses are seen terminating its left branch, which passes to the right,
-near the bright star _Deneb_, the leader of this constellation. Below
-_Deneb_, the Galaxy is apparently disconnected and separated from the
-northern part by a narrow, irregular dark gap. From this rupture, the
-Milky-way divides into two great streams, separated by an irregular dark
-rift. An immense branch extends to the right, which, after having formed
-an important luminous mass between the stars _Gamma_ and _Beta_,
-continues its southward progress through parts of Lyra, Vulpecula,
-Hercules, Aquila and Ophiuchus, where it gradually terminates a few
-degrees south of the equator. The main stream on the left, after having
-formed a bright mass around _Epsilon Cygni_, passes through Vulpecula
-and then Aquila, where it crosses the equinoctial just below the star
-_Eta_, after having involved in its nebulosity the bright star _Altair_,
-the leader of Aquila. In the southern hemisphere the Galaxy becomes very
-complicated and forms a succession of very bright, irregular masses, the
-upper one being in Scutum Sobieskii, while the others are respectively
-situated in Sagittarius and in Scorpio; the last, just a little above
-our horizon, being always considerably dimmed by vapors. From Scutum
-Sobieskii, the Galaxy expands considerably on the right, and sends a
-branch into Scorpio, in which the fiery red star _Antares_ is somewhat
-involved. In the upper left-hand corner of the Plate, at some distance
-from the Milky-way, is seen dimly the Nebula in Andromeda, which becomes
-so magnificent an object to telescopic view.
-
-
-PLATE XIV.--STAR-CLUSTER IN HERCULES.
-
-_From a study made in June_, 1877.
-
-
-In the constellation Hercules, a small nebulous mass is faintly visible
-to the eye, a telescopic view of which is presented in Plate XIV. It is
-one of the most beautiful of the easily resolvable globular clusters.
-The brilliancy of the centre gives the cluster a distinctly globular
-appearance, while the several wings curving in various directions, have
-suggested to some observers an irregularly spiral structure. The large
-stars of the cluster are arranged in several groups which correspond, in
-a general way, with the faintly luminous wings.
-
-
-PLATE XV.--THE GREAT NEBULA IN ORION.
-
-_From a study made in the years_ 1875-76.
-
-
-This nebula, which is one of the most brilliant and wonderful of
-telescopic objects, readily visible to the naked eve as a patch of
-nebulous light immediately surrounding the middle star of the three
-which form the sword of Orion, and a little south of the three
-well-known stars forming the belt. The small stars in this, as in other
-Plates of the series, are somewhat exaggerated in size, as was
-unavoidable with any mode of reproduction that could be employed. The
-bright pentagonal centre of the nebula is traversed by less luminous
-rifts, the several subdivisions thus outlined being irregularly mottled
-as if by bright fleecy clouds. Toward the lower part of this bright
-pentagonal centre is a comparatively dark space containing four bright
-stars which form a trapezium and together constitute the quadruple star
-_Theta Orionis_, which, to the naked eye, appears as the single star in
-the centre of the sword. On three sides of the central mass extend long
-bright wisps, whose curves fail, however, to reveal the spiral structure
-often attributed to this nebula. On the east a broad wing, with
-wave-shaped inner border, stretches southward. East of the trapezium are
-two especially noticeable dark spaces. Close to the main nebula on the
-north-east, a small faint nebula surrounds a bright star, and a branch
-from another faint stream of nebulous matter forming a loop to the
-southward, encloses the nebulous star (_Iota Orionis_) shown at the top
-of the Plate.
-
-*** END OF THE PROJECT GUTENBERG EBOOK THE TROUVELOT ASTRONOMICAL
-DRAWINGS MANUAL ***
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+*** START OF THE PROJECT GUTENBERG EBOOK 68394 ***
+
+THE
+TROUVELOT
+ASTRONOMICAL DRAWINGS
+MANUAL
+
+
+
+
+BY
+
+E. L. TROUVELOT,
+
+
+FORMERLY CONNECTED WITH THE OBSERVATORY OF HARVARD COLLEGE; FELLOW OF THE
+AMERICAN ACADEMY OF ARTS AND SCIENCES, AND MEMBER OF THE SELENO-GRAPHICAL
+SOCIETY OF GREAT BRITAIN; IN CHARGE OF A
+GOVERNMENT EXPEDITION TO OBSERVE THE
+TOTAL SOLAR ECLIPSE OF 1878.
+
+
+
+
+NEW YORK
+
+CHARLES SCRIBNER'S SONS
+
+1882
+
+
+
+
+INTRODUCTION
+
+
+During a study of the heavens, which has now been continued for more
+than fifteen years, I have made a large number of observations
+pertaining to physical astronomy, together with many original drawings
+representing the most interesting celestial objects and phenomena.
+
+With a view to making these observations more generally useful, I was
+led, some years ago, to prepare, from this collection of drawings, a
+series of astronomical pictures, which were intended to represent the
+celestial phenomena as they appear to a trained eye and to an
+experienced draughtsman through the great modern telescopes, provided
+with the most delicate instrumental appliances. Over two years were
+spent in the preparation of this series, which consisted of a number of
+large drawings executed in pastel. In 1876, these drawings were
+displayed at the United States Centennial Exhibition at Philadelphia,
+forming a part of the Massachusetts exhibit, in the Department of
+Education and Science.
+
+The drawings forming the present series comprise only a part of those
+exhibited at Philadelphia; but, although fewer in number, they are quite
+sufficient to illustrate the principal classes of celestial objects and
+phenomena.
+
+While my aim in this work has been to combine scrupulous fidelity and
+accuracy in the details, I have also endeavored to preserve the natural
+elegance and the delicate outlines peculiar to the objects depicted; but
+in this, only a little more than a suggestion is possible, since no
+human skill can reproduce upon paper the majestic beauty and radiance of
+the celestial objects.
+
+The plates were prepared under my supervision, from the original pastel
+drawings, and great care has been taken to make the reproduction exact.
+
+The instruments employed in the observations, and in the delineation of
+the heavenly bodies represented in the series, have varied in aperture
+from 6 to 26 inches, according to circumstances, and to the nature of
+the object to be studied. The great Washington refractor, kindly placed
+at my disposal by the late Admiral C. H. Davis, has contributed to this
+work, as has also the 26 inch telescope of the University of Virginia,
+while in the hands of its celebrated constructors, Alvan Clark & Sons.
+The spectroscope used was made by Alvan Clark & Sons. Attached to it is
+an excellent diffraction grating, by Mr. L. M. Rutherfurd, to whose
+kindness I am indebted for it.
+
+Those unacquainted with the use of optical instruments generally suppose
+that all astronomical drawings are obtained by the photographic process,
+and are, therefore, comparatively easy to procure; but this is not true.
+Although photography renders valuable assistance to the astronomer in
+the case of the Sun and Moon, as proved by the fine photographs of these
+objects taken by M. Janssen and Mr. Rutherfurd; yet, for other subjects,
+its products are in general so blurred and indistinct that no details of
+any great value can be secured. A well-trained eye alone is capable of
+seizing the delicate details of structure and of configuration of the
+heavenly bodies, which are liable to be affected, and even rendered
+invisible, by the slightest changes in our atmosphere.
+
+The method employed to secure correctness in the proportions of the
+original drawings is simple, but well adapted to the purpose in view. It
+consists in placing a fine reticule, cut on glass, at the common focus
+of the objective and the eye-piece, so that in viewing an object, its
+telescopic image, appearing projected on the reticule, can be drawn very
+accurately on a sheet of paper ruled with corresponding squares. For a
+series of such reticules I am indebted to the kindness of Professor
+William A. Rogers, of the Harvard College Observatory.
+
+The drawings representing telescopic views are inverted, as they appear
+in a refracting telescope--the South being upward, the North downward,
+the East on the right, and the West on the left. The Comet, the
+Milky-Way, the Eclipse of the Moon, the Aurora Borealis, the Zodiacal
+Light and the Meteors are represented as seen directly in the sky with
+the naked eye. The Comet was, however, drawn with the aid of the
+telescope, without which the delicate structure shown in the drawing
+would not have been visible.
+
+The plate representing the November Meteors, or so-called "Leonids," may
+be called an ideal view, since the shooting stars delineated, were not
+observed at the same moment of time, but during the same night. Over
+three thousand Meteors were observed between midnight and five o'clock
+in the morning of the day on which this shower occurred; a dozen being
+sometimes in sight at the same instant. The paths of the Meteors,
+whether curved, wavy, or crooked, and also their delicate colors, are in
+all cases depicted as they were actually observed.
+
+In the Manual, I have endeavored to present a general outline of what is
+known, or supposed, on the different subjects and phenomena illustrated
+in the series. The statements made are derived either from the best
+authorities on physical astronomy, or from my original observations,
+which are, for the most part, yet unpublished.
+
+The figures in the Manual relating to distance, size, volume, mass,
+etc., are not intended to be strictly exact, being only round numbers,
+which can, therefore, be more easily remembered.
+
+It gives me pleasure to acknowledge that the experience acquired in
+making the astronomical drawings published in Volume VIII. of the Annals
+of the Harvard College Observatory, while I was connected with that
+institution, has been of considerable assistance to me in preparing this
+work; although no drawings made while I was so connected have been used
+for this series.
+
+
+                             E. L. TROUVELOT.
+
+_Cambridge, March, 1882._
+
+
+
+
+CONTENTS
+
+
+INTRODUCTION
+
+LIST OF PLATES
+
+_THE SUN_
+
+GENERAL REMARKS ON THE SUN
+
+SUN-SPOTS AND VEILED SPOTS
+
+SOLAR PROTUBERANCES
+
+TOTAL ECLIPSE OF THE SUN
+
+_THE AURORAL AND ZODIACAL LIGHTS_
+
+THE AURORA BOREALIS
+
+THE ZODIACAL LIGHT
+
+_THE MOON_
+
+THE MOON
+
+ECLIPSES OF THE MOON
+
+_THE PLANETS_
+
+THE PLANETS
+
+THE PLANET MARS
+
+THE PLANET JUPITER
+
+THE PLANET SATURN
+
+_COMETS AND METEORS_
+
+COMETS
+
+SHOOTING-STARS AND METEORS
+
+_THE STELLAR SYSTEMS_
+
+THE MILKY-WAY OR GALAXY
+
+THE STAR-CLUSTERS
+
+THE NEBULÆ
+
+APPENDIX
+
+
+
+
+LIST OF PLATES[1]
+
+PLATE
+
+I. GROUP OF SUN-SPOTS AND VEILED SPOTS.
+
+_Observed June 17, 1875, at 7 h. 30m. A. M._
+
+II. SOLAR PROTUBERANCES.
+
+_Observed May 5, 1873, at 9h. 40m. A. M._
+
+III. TOTAL ECLIPSE OF THE SUN.
+
+_Observed July 29, 1878, at Creston, Wyoming Territory._
+
+IV. AURORA BOREALIS.
+
+_As observed March 1, 1872, at 9h. 25m. P. M._
+
+V. THE ZODIACAL LIGHT.
+
+_Observed February 20, 1876._
+
+VI. MARE HUMORUM.
+
+_From a study made in 1875._
+
+VII. PARTIAL ECLIPSE OF THE MOON.
+
+_Observed October 24, 1874._
+
+VIII. THE PLANET MARS.
+
+_Observed September 3, 1877, at 11h. 55m. P. M._
+
+IX. THE PLANET JUPITER.
+
+_Observed November 1, 1880, at 9h. 30m. P. M._
+
+X. THE PLANET SATURN.
+
+_Observed November 30, 1874, at 5th. 50m. P. M._
+
+XI. THE GREAT COMET OF 1881.
+
+_Observed on the night of June 25-26, at 1h. 30m. A. M._
+
+XII. THE NOVEMBER METEORS.
+
+_As observed between midnight and 3 o'clock A. M., on the night
+of November 13-14, 1868._
+
+XIII. PART OF THE MILKY-WAY.
+
+_From a study made during the years 1874, 1875 and 1876._
+
+XIV. STAR-CLUSTER IN HERCULES.
+
+_From a study made in June, 1877._
+
+XV. THE GREAT NEBULA IN ORION.
+
+_From a study made in the years 1873-76._
+
+
+[Footnote 1: For Key to the Plates, see Appendix.]
+
+
+_Reproduced from the Original Drawings, by Armstrong & Company,
+Riverside Press, Cambridge, Mass._
+
+
+
+
+GENERAL REMARKS ON THE SUN
+
+
+The Sun, the centre of the system which bears its name, is a
+self-luminous sphere, constantly radiating heat and light.
+
+Its apparent diameter, as seen at its mean from the Earth, subtends an
+angle of 32', or a little over half a degree. A dime, placed about six
+feet from the eye, would appear of the same proportions, and cover the
+Sun's disk, if projected upon it.
+
+That the diameter of the Sun does not appear larger, is due to the great
+distance which separates us from that body. Its distance from the Earth
+is no less than 92,000,000 miles. To bridge this immense gap, would
+require 11,623 globes like the Earth, placed side by side, like beads on
+a string.
+
+The Sun is an enormous sphere whose diameter is over 108 times the
+diameter of our globe, or very nearly 860,000 miles. Its radius is
+nearly double the distance from the Earth to the Moon. If we suppose,
+for a moment, the Sun to be hollow, and our globe to be placed at the
+centre of this immense spherical shell, not only could our satellite
+revolve around us at its mean distance of 238,800 miles, as now, but
+another satellite, placed 190,000 miles farther than the Moon, could
+freely revolve likewise, without ever coming in contact with the solar
+envelope.
+
+The circumference of this immense sphere measures 2,800,000 miles. While
+a steamer, going at the rate of 300 miles a day, would circumnavigate
+the Earth in 83 days, it would take, at the same rate, nearly 25 years
+to travel around the Sun.
+
+The surface of the Sun is nearly 12,000 times the surface of the Earth,
+and its volume is equal to 1,300,000 globes like our own. If all the
+known planets and satellites were united in a single mass, 600 such
+compound masses would be needed to equal the volume of our luminary.
+
+Although the density of the Sun is only one-quarter that of the Earth,
+yet the bulk of this body is so enormous that, to counterpoise it, no
+less than 314,760 globes like our Earth would be required.
+
+The Sun uniformly revolves around its axis in about 25½ days. Its
+equator is inclined 7° 15' to the plane of the ecliptic, the axis of
+rotation forming, therefore, an angle of 82° 45' with the same plane.
+As the Earth revolves about the Sun in the same direction as that of the
+Sun's rotation, the apparent time of this rotation, as seen by a
+terrestrial spectator, is prolonged from 25½ days to about 27 days and
+7 hours.
+
+The rotation of the Sun on its axis, like that of the Earth and the
+other planets, is direct, or accomplished from West to East. To an
+observer on the Earth, looking directly at the Sun, the rotation of this
+body is from left to right, or from East to West.
+
+The general appearance of the Sun is that of an intensely luminous disk,
+whose limb, or border, is sharply defined on the heavens. When its
+telescopic image is projected on a screen, or fixed on paper by
+photography, it is noticed that its disk is not uniformly bright
+throughout, but is notably more luminous in its central parts. This
+phenomenon is not accidental, but permanent, and is due in reality to a
+very rare but extensive atmosphere which surrounds the Sun, and absorbs
+the light which that body radiates, proportionally to its thickness,
+which, of course, increases towards the limb, to an observer on the
+Earth.
+
+
+THE ENVELOPING LAYERS OF THE SUN
+
+
+The luminous surface of the Sun, or that part visible at all times, and
+which forms its disk, is called the _Photosphere_, from the property it
+is supposed to possess of generating light. The photosphere does not
+extend to a great depth below the luminous surface, but forms a
+comparatively thin shell, 3,000 or 4,000 miles thick, which is distinct
+from the interior parts, above which it seems to be kept in suspense by
+internal forces. From the observations of some astronomers it would
+appear that the diameter of the photosphere is subject to slight
+variations, and, therefore, that the solar diameter is not a constant
+quantity. From the nature of this envelope, such a result does not seem
+at all impossible, but rather probable.
+
+Immediately above the photosphere lies a comparatively thin stratum,
+less than a thousand miles in thickness, called the _Reversing Layer_.
+This stratum is composed of metallic vapors, which, by absorbing the
+light of particular refrangibilities emanating from the photosphere
+below, produces the dark Fraunhofer lines of the solar spectrum.
+
+Above the reversing layer, and resting immediately upon it, is a
+shallow, semi-transparent gaseous layer, which has been called the
+_Chromosphere_, from the fine tints which it exhibits during total
+eclipses of the Sun, in contrast with the colorless white light radiated
+by the photosphere below. Although visible to a certain extent on the
+disk, the chromosphere is totally invisible on the limb, except with the
+spectroscope, and during eclipses, on account of the nature of its
+light, which is mainly monochromatic, and too feeble, compared with that
+emitted by the photosphere, to be seen.
+
+The chromospheric layer, which has a thickness of from 3,000 to 4,000
+miles, is uneven, and is usually upheaved in certain regions, its matter
+being transported to considerable elevations above its general surface,
+apparently by some internal forces. The portions of the chromosphere
+thus lifted up, form curious and complicated figures, which are known
+under the names of _Solar Protuberances, or Solar Flames_.
+
+Above the chromosphere, and rising to an immense but unknown height, is
+the solar atmosphere proper, which is only visible during total eclipses
+of the Sun, and which then surrounds the dark body of the Moon with the
+beautiful rays and glorious nimbus, called the _Corona_.
+
+These four envelopes: the photosphere, the reversing layer, the
+chromosphere, and the corona, constitute the outer portions of our
+luminary.
+
+Below the photosphere little can be seen, although it is known, as will
+appear below, that at certain depths cloud-like forms exist, and freely
+float in an interior atmosphere of invisible gases. Beyond this all is
+mystery, and belongs to the domain of hypothesis.
+
+
+STRUCTURE OF THE PHOTOSPHERE AND CHROMOSPHERE
+
+
+The apparent uniformity of the solar surface disappears when it is
+examined with a telescope of sufficient aperture and magnifying powers.
+Seen under good atmospheric conditions, the greater part of the solar
+surface appears mottled with an infinite number of small, bright
+granules, irregularly distributed, and separated from each other by a
+gray-tinted background.
+
+These objects are known under different names. The terms granules and
+granulations answer very well for the purpose, as they do not imply
+anything positive as to their form and true nature. They have also been
+called _Luculœ_, _Rice Grains_, _Willow Leaves_, etc., by different
+observers.
+
+Although having different shapes, the granulations partake more or less
+of the circular or slightly elongated form. Their diameter, which varies
+considerably, has been estimated at from 0".5 to 3", or from 224 to
+1,344 miles. The granulations which attain the largest size appear,
+under good atmospheric conditions, to be composed of several granules,
+closely united and forming an irregular mass, from which short
+appendages protrude in various directions.
+
+The number of granulations on the surface of the Sun varies considerably
+under the action of unknown causes. Sometimes they are small and very
+numerous, while at other times they are larger, less numerous, and more
+widely separated. Other things being equal, the granulations are better
+seen in the central regions of the Sun than they are near the limb.
+
+Usually the granulations are very unstable; their relative position,
+form, and size undergoing continual changes. Sometimes they are seen to
+congregate or to disperse in an instant, as if acting under the
+influence of attractive and repulsive forces; assembling in groups or
+files, and oftentimes forming capricious figures which are very
+remarkable, but usually of short duration. In an area of great solar
+disturbances, the granulations are often stretched to great distances,
+and form into parallel lines, either straight, wavy, or curved, and they
+have then some resemblance to the flowing of viscous liquids.
+
+The granulations are usually terminated either by rounded or sharply
+pointed summits, but they do not all rise to the same height, as can be
+ascertained with the spectroscope when they are seen sidewise on the
+limb. In the regions where they are most abundant, they usually attain
+greater elevations, and when observed on the limb with the spectroscope,
+they appear as slender acute flames.
+
+The granulations terminated by sharply-pointed crests, although observed
+in all latitudes, seem to be characteristic of certain regions. A daily
+study of the chromosphere, extending over a period of ten years, has
+shown me that the polar regions are rarely ever free from these objects,
+which are less frequent in other parts of the Sun. In the polar regions
+they are sometimes so abundant that they completely form the solar limb.
+These forms of granulation are comparatively rare in the equatorial
+zones, and when seen there, they never have the permanency which they
+exhibit in the polar regions. When observed in the equatorial regions,
+they usually appear in small groups, in the vicinity of sun spots, or
+they are at least enclosed in areas of disturbances where such spots are
+in process of formation. In these regions they often attain greater
+elevations than those seen in high latitudes.
+
+As we are certain that in the equatorial zones these slender flames (_i.
+e._, granulations) are a sure sign of local disturbance, it may be
+reasonably supposed that the same kind of energy producing them nearly
+always prevails in the polar regions, although it is there much weaker,
+and never reaches beyond certain narrow limits.
+
+Studied with the spectroscope, the granulations are found to be composed
+in the main of incandescent hydrogen gas, and of an unknown substance
+provisionally called "helium." Among the most brilliant of them are
+found traces of incandescent metallic vapors, belonging to various
+substances found on our globe.
+
+The chromosphere is not fixed, but varies considerably in thickness in
+its different parts, from day to day. Its thickness is usually greater
+in the polar regions, where it sometimes exceeds 6,700 miles. In the
+equatorial regions the chromosphere very rarely attains this height, and
+when it does, the rising is local and occupies only a small area. In
+these regions it is sometimes so shallow that its depth is only a few
+seconds, and is then quite difficult to measure. These numbers give, of
+course, the extreme limits of the variations of the chromosphere; but,
+nearly always, it is more shallow in the equatorial regions; and, as far
+as my observations go, the difference in thickness between the polar and
+equatorial zones is greater in years of calm than it is in years of
+great solar activity. But ten years of observation are not sufficient to
+warrant any definite conclusions on this subject.
+
+There is undoubtedly some relation between the greater thickness of the
+chromosphere in the polar regions, and the abundance and permanence of
+the sharply-pointed granulations observed in the same regions. This
+becomes more evident when we know that the appearance of
+similarly-pointed flames in the equatorial zones is always accompanied
+with a local thickening of the chromosphere. The thickening in the polar
+regions may be only apparent, and not due to a greater accumulation of
+chromospheric gases there; but may be caused by some kind of repulsive
+action or polarity, which lifts up and extends the summit of the
+granulations in a manner similar to the well-known mode of electric
+repulsion and polarity.
+
+As it seems very probable that the heat and light emanating from the Sun
+are mainly generated at the base of the granulations, in the filamentary
+elements composing the chromosphere and photosphere, it would follow
+that, as the size and number of these objects constantly vary, the
+amount of heat and light emitted by the Sun should also vary in the same
+proportion.
+
+The granulations of the solar surface are represented on Plate I., and
+form the general background to the group of Sun-spots forming the
+picture.
+
+
+THE FACULÆ
+
+
+Although the solar surface is mainly covered with the luminous
+granulations and the grayish background above described, it is very rare
+that its appearance is so simple and uniform as already represented. For
+the most part, on the contrary, it is diversified by larger, brighter,
+and more complicated forms, which are especially visible towards the
+border of the Sun. Owing to their extraordinary brilliancy, these
+objects have been called _Faculœ_ (torches).
+
+Although the faculæ are very seldom seen well beyond 50 heliocentric
+degrees from the limb, yet they exist, and are as numerous in the
+central parts of the disk as they are towards the border; since they
+form a part of the solar surface, and participate in its movement of
+rotation. Their appearance near the limb has been attributed to the
+effect of absorption produced by the solar atmosphere on the
+light from the photosphere; but this explanation seems inadequate,
+and does not solve the problem. The well-known fact that the solar
+protuberances--which are in a great measure identical with the
+faculæ--are much brighter at the base than they are at the summit,
+perhaps gives a clue to the explanation of the phenomenon; especially
+since we know that, in general, the summit of the protuberances is
+considerably broader than their base. When these objects are observed in
+the vicinity of the limb, they present their brightest parts to the
+observer, since, in this position, they are seen more or less sidewise;
+and, therefore, they appear bright and distinct. But as the faculæ
+recede from the limb, their sides, being seen under a constantly
+decreasing angle, appear more and more foreshortened; and, therefore,
+these objects grow less bright and less distinct, until they finally
+become invisible, when their bases are covered over by the broad, dusky
+summit generally terminating the protuberances.
+
+The faculæ appear as bright and luminous masses or streaks on the
+granular surface of the Sun, but they differ considerably in form and
+size. Two types at least are distinguishable. In their simplest form
+they appear either as isolated white spots, or as groups of such spots
+covering large surfaces, and somewhat resembling large flakes of snow.
+In their most characteristic types they appear as intensely luminous,
+heavy masses, from which, in most cases, issue intricate ramifications,
+sometimes extending to great distances. Generally, the ramifications
+issuing from the masses of faculæ have their largest branches directed
+in the main towards the eastern limb of the Sun. Some of these branches
+have gigantic proportions. Occasionally they extend over 60° and even
+80° of the solar surface, and, therefore, attain a length of from
+450,000 to 600,000 miles.
+
+Although the faculæ may be said to be seen everywhere on the surface of
+the Sun, there is a vast difference in different regions, with regard to
+their size, number, and brilliancy. They are largest, most abundant, and
+brightest on two intermediate zones parallel to the solar equator, and
+extending 35° or 40° to the north and to the south of this line. The
+breadth of these zones varies considerably with the activity of the
+solar forces. When they are most active, the faculæ spread on either
+side, but especially towards the equator, where they sometimes nearly
+meet those of the other zone. In years of little solar activity the
+belts formed by the faculæ are very narrow--the elements composing them
+being very few and small, although they never entirely disappear.
+
+The faculæ are very unstable, and are constantly changing: those of the
+small types sometimes form and vanish in a few minutes. When an area of
+disturbance of the solar surface is observed for some time, all seems in
+confusion; the movements of the granulations become unusually violent;
+they congregate in all sorts of ways, and thus frequently form temporary
+faculæ. Action of this kind is, for the most part, peculiar to the
+polar regions of the Sun.
+
+The larger faculæ have undoubtedly another origin, as they seem to be
+mainly formed by the ejection of incandescent gases and metallic vapors
+from the interior of the photosphere. In their process of development
+some of the heavy masses of faculæ are swollen up to great heights,
+being torn in all sorts of ways, showing large rents and fissures
+through which the sight can penetrate.
+
+Very few faculæ are represented in Plate I.; several streaks are shown
+at the upper left-hand corner, some appearing as whitish ramifications
+among the granulations representing the general solar surface.
+
+
+
+
+SUN-SPOTS AND VEILED SPOTS
+
+PLATE I
+
+
+Besides the brilliant faculæ already described, much more conspicuous
+markings, though of a totally different character, are very frequently
+observed on the Sun. On account of their darkish appearance, which is in
+strong contrast with the white envelope of our luminary, these markings
+were called _Maculæ_, or Sun-spots, by their earlier observers.
+
+The Sun-spots are not equally distributed on the solar surface; but like
+the faculæ, to which they are closely related, they occupy two
+zones--one on each side of the equator. These zones are comprised
+between 10° and 35° of north latitude, and 10° and 35° of south
+latitude. Between these two zones is a belt 20° in width, where the
+Sun-spots are rarely seen.
+
+Above the latitudes 35° north and south, the Sun-spots are rare, and it
+is only occasionally, and during years of great solar activity, that
+they appear in these regions; in only a few cases have spots of
+considerable size been seen there. A few observers, however, have seen
+spots as far as 40° and 50 from the equator; and La Hire even observed
+one in 70° of north latitude; but these cases are exceedingly rare. It
+is not uncommon, however, to see very small spots, or groups of such
+spots, within 8° or 10° from the poles.
+
+The activity of the Sun is subject to considerable fluctuation, and
+accordingly the Sun-spots vary in size and number in different years.
+During some years they are large, complicated, and very numerous; while
+in others they are small and scarce, and are sometimes totally absent
+for weeks and months together. The fluctuations in the frequency of
+Sun-spots are supposed to be periodical in their character, although
+their periods do not always appear to recur at exactly regular
+intervals. Sometimes the period is found to be only nine years, while at
+other times it extends to twelve years. The period generally adopted now
+is 11⅒ years, nearly; but further investigations are needed to
+understand the true nature of the phenomenon.
+
+The number of Sun-spots does not symmetrically augment and diminish, but
+the increase is more rapid than the diminution.
+
+The period of increase is only about four years, while that of decrease
+is over seven years; each period of Sun-spot maximum being nearer the
+preceding period of Sun-spot minimum than it is to that next following.
+
+The cause of these fluctuations in the solar energy is at present wholly
+unknown. Some astronomers, however, have attributed it to the influence
+of the planets Venus and Jupiter, the period of revolution of the latter
+planet being not much longer than the Sun-spot period; but this
+supposition lacks confirmation from direct observations, which, so far,
+do not seem to be in favor of the hypothesis. At the present time the
+solar activity is on the increase, and the Sun-spots will probably reach
+their maximum in 1883. The last minimum occurred in 1879, when only
+sixteen small groups of spots were observed during the whole year.
+
+Sun-spots vary in size and appearance; but, unless they are very small,
+in which case they appear as simple black dots, they generally consist
+of two distinct and well-characterized parts, nearly always present.
+There is first, a central part, much darker than the other, and sharply
+divided from it, called the "_Umbra_;" second, a broad, irregular
+radiated fringe of lighter shade, completely surrounding the first, and
+called the "_Penumbra_."
+
+Reduced to its simplest expression, a Sun-spot is a funnel-shaped
+opening through the chromosphere and the photosphere. The inner end of
+the funnel, or opening, gives the form to the umbra, while its sloping
+sides form the penumbra.
+
+The umbra of Sun-spots, whose outlines approximately follow the
+irregularities of the penumbral fringe, has a diameter which generally
+exceeds the width of the penumbral ring. Sometimes it appears uniformly
+black throughout; but it is only so by contrast, as is proved when
+either Mercury or Venus passes near a spot during a transit over the
+Sun's disk. The umbra then appears grayish, when compared with the
+jet-black disk of the planet.
+
+The umbra of spots is rarely so simple as just described; but it is
+frequently occupied, either partly or wholly, by grayish and rosy forms,
+somewhat resembling loosely-entangled muscular fibres. These forms have
+been called the _Gray and Rosy Veils_. Frequently these veils appear as
+if perforated by roundish black holes, improperly called _Nuclei_, which
+permit the sight to penetrate deeper into the interior. To all
+appearance the gray and rosy veils are of the same nature as the
+chromosphere and the faculæ, and are therefore mainly composed of
+hydrogen gas.
+
+Whatever can be known about the interior of the Sun, must be learned
+from the observations of these openings, which are comparatively small.
+But whatever this interior may be, we certainly know that it is not
+homogeneous. Apparently, the Sun is a gigantic bubble, limited by a very
+thin shell. Below this shell exists a large open space filled with
+invisible gases, in which, through the openings constituting the
+Sun-spots, the gray and rosy veils described above are occasionally seen
+floating.
+
+The fringe forming the penumbra of spots is much more complicated than
+the umbra. In its simpler form, it is composed of a multitude of bright,
+independent filaments of different forms and sizes, partly projecting
+one above the other, on the sloping wall of the penumbra, from which
+they seem to proceed. Seen from the Earth, these filaments have somewhat
+the appearance of thatched straw, converging towards the centre of the
+umbra. It is very rare, however, that the convergence of the penumbral
+filaments is regular, and great confusion sometimes arises from the
+entanglement of these filaments. Some of these elements appear straight,
+others are curved or loop-shaped; while still others, much larger and
+brighter than the rest, give a final touch to this chaos of filaments,
+from which results the general thatched and radiating appearance of the
+penumbra.
+
+The extremities of the penumbral filaments, especially of those forming
+the border of the umbra, are usually club-shaped and appear very
+brilliant, as if these elements had been superheated by some forces
+escaping through the opening of the spots.
+
+Besides these characteristics, the Sun-spots have others, which,
+although not always present, properly belong to them. Comparatively few
+spots are so simple as the form just described. Very frequently a spot
+is accompanied by brilliant faculæ, covering part of its umbra and
+penumbra, and appearing to form a part of the spot itself.
+
+When seen projected over Sun-spots, the faculæ appear intensely bright,
+and from these peculiarities they have been called _Luminous Bridges_.
+They are, in fact, bridges, but in most cases they are at considerable
+heights above the spots, kept there by invisible forces. When such spots
+with luminous bridges approach the Sun's limb, it is easy to see, by the
+rapid apparent displacement which they undergo, that they are above the
+general level.
+
+When the spots are closing up, the inverse effect is sometimes observed.
+On several occasions, I have seen huge masses of faculæ advance slowly
+over the penumbra of a spot and fall into the depths of the umbra,
+resembling gigantic cataracts. I have seen narrow branches of faculæ,
+which, after having fallen to great depths in the umbra, floated across
+it and disappeared under the photosphere on the opposite side. I have
+also seen luminous bridges, resembling cables, tightly stretched across
+the spots, slackening slowly, as if loosened at one end, and gently
+curving into the umbra, where they formed immense loops, large enough to
+receive our globe.
+
+It is to be remarked that, in descending under the photospheric shell,
+the bright faculæ and the luminous bridges gradually lose their
+brilliancy. At first they appear grayish, but in descending farther they
+assume more and more the pink color peculiar to the rosy veils. The
+pinkish color acquired by the faculæ when they reach a certain depth
+under the photosphere, is precisely the color of the chromosphere and of
+the solar protuberances, as seen during total eclipses of the Sun--a
+fact which furnishes another proof that the faculæ are of the same
+nature as the protuberances.
+
+I record here an observation which, at first sight, may appear
+paradoxical; but which seems, however, to be of considerable importance,
+as it shows unmistakably that the solar light is mainly, if not
+entirely, generated on its surface, or at least very near to it. On May
+26, 1878, I observed a large group of Sun-spots at a little distance
+from the east limb of the Sun. The spot nearest to the limb was partly
+covered over on its eastern and western sides by bright and massive
+faculæ which concealed about two-thirds of the whole spot, only a
+narrow opening, running from north to south, being left across the
+middle of the spot. Owing to the rotundity of the Sun, the penumbra of
+this spot, although partly covered by the faculæ, could, however, be
+seen on its eastern side, since the sight of the observer could there
+penetrate sidewise under the faculæ. Upon that part of the penumbra
+appeared a strong shadow, representing perfectly the outline of the
+facular mass situated above it. The phenomenon was so apparent that no
+error of observation was possible, and a good drawing of it was secured.
+If this faculæ had been as bright beneath as it was above, it is
+evident that no shadow could have been produced; hence the light of
+these faculæ must have been mainly generated on or very near their
+exterior surfaces. This, with the well-proved fact that the bright
+faculæ lose their light in falling into the interior of the Sun, seems
+to suggest the idea that the bright light emitted by the faculæ, and
+very probably all the solar light, can be generated only on its surface;
+the presence of the coronal atmosphere being perhaps necessary to
+produce it. Several times before this observation, I had suspected that
+some faculæ were casting a shadow, but as this seemed so improbable, my
+attention was not awakened until the phenomenon became so prominent that
+it could not escape notice.
+
+With due attention, some glimpses of the phenomenon can frequently be
+observed through the openings of some of the faculæ projecting over the
+penumbra of Sun-spots. It is very seldom that the structure of the
+penumbra is seen through such openings, which usually appear as dark as
+the umbra of the large spots, although they do not penetrate through the
+photosphere like the latter. It is only when the rents in the faculæ
+are numerous and quite large, that the penumbral structure is recognized
+through them. Since these superficial rents in the faculæ do not extend
+through the photosphere, and appear black, it seems evident that the
+penumbra seen through them cannot be as bright as it is when no faculæ
+are projected upon it, and therefore that the faculæ intercept light
+from the exterior surface, which would otherwise reach the penumbra.
+
+While the matter forming the faculæ sometimes falls into the interior
+of the Sun, the same kind of matter is frequently ejected in enormous
+quantities, and with great force, from the interior, through the visible
+and invisible openings of the photosphere, and form the protuberances
+described in the following section of this manual (Solar protuberances)
+It is not only the incandescent hydrogen gas or the metallic vapors
+which are thus ejected, but also cooler hydrogen gas, which sometimes
+appears as dark clouds on the solar surface. On December 12, 1875, I
+observed such a cloud of hydrogen issuing from the corner of a small
+Sun-spot. It traveled several thousand miles on the solar surface, in a
+north-easterly direction, before it became invisible.
+
+Solar spots are formed in various ways; but, for the most part, the
+apparition of a spot is announced beforehand, by a great commotion of
+the solar surface at the place of its appearance, and by the formation
+of large and bright masses of faculæ, which are usually swollen into
+enormous bubbles by the pressure of the internal gases. These bubbles
+become visible in the spectroscope while they are traversing the solar
+limb, as they are then presented to us sidewise. Under the action of the
+increasing pressure, the base of the faculæ is considerably stretched,
+and, its weakest side finally giving way, the facular mass is torn in
+many places from the solar surface, and is perforated by holes of
+different sizes and forms. The holes thus made along the border of the
+faculæ appear as small black spots, separated more or less by the
+remaining portion of the lacerated faculæ, and they enlarge more and
+more at the expense of the intervening portions, which thus become very
+narrow. This perforated side of the faculæ, offering less resistance,
+is gradually lifted up, as would be the cover of a box, for example,
+while its opposite side remains attached to the surface. The facular
+matter separating the small black holes is greatly stretched during this
+action, and forms long columns and filaments. These appear as luminous
+bridges upon the large and perfectly-formed spot, which is then seen
+under the lifted facular masses. The spots thus made visible are soon
+freed from the facular masses, which are gradually shifted towards the
+opposite side.
+
+In such cases the spots are undoubtedly formed under the faculæ before
+they can be seen. This becomes evident when such spots, not yet cleared
+from the faculæ covering them, are observed near the east limb; since
+in this position the observer can see through the side-openings of the
+faculæ, and sometimes recognize the spots under their cover.
+
+It frequently occurs that the spots thus formed under the faculæ
+continue to be partly covered by the facular clouds, the forces at work
+in them being apparently too feeble to shift them aside. In such cases
+these spots are visible when they are in the vicinity of the limb, where
+they are seen sidewise; but when observed in the east, in being carried
+forward by the solar rotation towards the centre of the disk, they
+gradually diminish in size, and finally become invisible. The
+disappearance of these spots, however, is only apparent, being due to
+the fact that, as they advance towards the centre of the disk, our
+lateral view of them is gradually lost, and they are finally hidden from
+sight by the overhanging faculæ which then serve as a screen between
+the observer and the spot. This class of spots may be called _Lateral
+Spots_, from the fact that they can only be seen laterally, and near the
+Sun's limb.
+
+Solar spots are also formed in various other ways. Some, like those
+represented in Plate I., appearing without being announced by any
+apparent disturbance of the surface, or by the formation of any faculæ,
+form and develop in a very short time. Others, appearing at first as
+very small spots having an umbra and a penumbra, slowly and gradually
+develop into very large spots. This mode of formation, which would seem
+to be the most natural, is, however, quite rare. Spots of this class
+have a duration and permanence not observed in those of any other type.
+These spots of slow and regular development are never accompanied by
+faculæ or luminous bridges, nor have they any gray or rosy veils in
+their interior; a fact which may, perhaps, account for their permanent
+character.
+
+Another class of spots, which is also rare, appear as long and narrow
+crevasses showing the penumbral structure of the ordinary spots; but
+these rarely have any umbra. These long, and sometimes exceedingly
+narrow fissures of the solar envelopes, with their radiated penumbral
+structure, strongly suggest the idea that the photosphere is composed of
+a multitude of filamentary elements having the granulations for summits.
+Such a crevasse is represented on Plate I., and unites the two spots
+which form the group.
+
+The duration of Sun-spots varies greatly. Some last only for a few
+hours; while others continue for weeks and even months at a time, but
+not without undergoing changes.
+
+The modes of disappearance of Sun-spots are as various as those of their
+apparition. The spots rarely close up by a gradual diminution or
+contraction of their umbra and penumbra. This mode of disappearance
+belongs exclusively to the spots deprived of faculæ and veils. One of
+the most common modes of the disappearance of a spot is its invasion by
+large facular masses, which slowly advance upon its penumbra and umbra
+and finally cover it entirely. It is a process precisely the reverse of
+that in which spots are formed by the shifting aside of the faculæ, as
+above described. In other types, the spots close up by the gradual
+enlargement of the luminous bridges traversing them, which are slowly
+transformed into branches of the photosphere, all of the characteristics
+of which they have acquired. In many cases, the spots covered over by
+the faculæ continue to exist for some time, hidden under these masses,
+as is often proved, either by the appearance of small spots on the
+facular mass left at the place they occupied, or even by the
+reappearance of the same spot.
+
+Apart from the general movement of rotation of the solar surface, some
+of the spots seem to be endowed with a proper motion of their own, which
+becomes greater the nearer the spots are to the solar equator. According
+to the observations of Mr. Carrington, the period of rotation of the
+Sun, as deduced from the observations of the solar spots during a period
+of seven years, is 25 days at the equator; while at 50° of heliocentric
+latitude it is 27 days. But the period of rotation, as derived from the
+observations of spots occupying the same latitude, is far from being
+constant, as it varies at different times, with the frequency of the
+spots and with the solar activity, so that at present the law of these
+variations is not well known. From the character of the solar envelope,
+it seems very natural that the rotation should differ in the different
+zones and at different times, since this envelope is not rigid, but very
+movable, and governed by forces which are themselves very variable.
+
+Although it is a general law that the spots near the equator have a more
+rapid motion than those situated in higher latitudes, yet, in many
+cases, the proper motion of the spots is more apparent than real. For
+the most part, the changes of form and the rapid displacements observed
+in some spots are only apparent, and due to the fact that the large
+masses of faculæ which are kept in suspense above them are very
+unstable, and change position with the slightest change in the forces
+holding them in suspension. Since in these cases we view the spots
+through the openings of the faculæ situated above them, the slightest
+motion of these objects produces an apparent motion in the spots,
+although they have remained motionless. Accordingly, it has been
+remarked that of all the spots, those which have the greater proper
+motion are precisely those which have the most faculæ and luminous
+bridges; while the other spots in the same regions, but not attended by
+similar phenomena, are comparatively steady in their movement. These
+last spots are undoubtedly better adapted than any others to exhibit the
+rotation of the Sun; but it is probable that this period of rotation
+will never be known with accuracy, simply because the solar surface is
+unstable, and does not rotate uniformly.
+
+The Sun-spots have a remarkable tendency to form into groups of various
+sizes, but whatever may be the number of spots thus assembled, the group
+is nearly always composed of two principal spots, to which the others
+are only accessories. The tendency of the Sun-spots to assemble in pairs
+is general, and is observed in all latitudes, even among the minute
+temporary groups formed in the polar regions. Whenever several are
+situated quite close together, those belonging to the same group can be
+easily recognized by this character. Whatever may be the position of the
+axis of the two principal spots of a group when it is first formed, this
+axis has a decided tendency to place itself parallel to the solar
+equator, no matter to what latitude the group belongs; and if it is
+disturbed from this position, it soon returns to it when the disturbance
+has ceased.
+
+It is also remarkable that the spots observed at the same time remain in
+nearly the same parallel of latitude for a greater or less period of
+time; but they keep changing their position from year to year, their
+latitude decreasing with the activity of the solar forces.
+
+Among the Sun-spots, those associated with faculæ form the groups which
+attain the largest proportions. When such groups acquire an apparent
+diameter of 1' or more, they are plainly visible to the naked eye, since
+for a spot to be visible to the naked eye on the Sun, it need only
+subtend an angle of 50". I have sometimes seen such groups through a
+smoky atmosphere, when the solar light was so much reduced that the disk
+could be observed directly and without injury to the sight.
+
+The largest spot which ever came under my observation was seen during
+the period from the 13th to the 19th of November, 1870. This spot, which
+was on the northern hemisphere of the Sun, was conspicuous among the
+smaller spots constituting the group to which it belonged, and followed
+them on the east. On November 16th, when it attained its largest size,
+the diameter of its penumbra occupied fully one-fifth of the diameter of
+the Sun; its real diameter being, therefore, not less than 172,000
+miles, or nearly 22 times the diameter of the Earth. As the umbra of
+this spot occupied a little more than one-third of its whole diameter,
+seven globes like our own, placed side by side on a straight line, could
+easily have passed through this immense gap. To fill the area of this
+opening, about 45 such globes would have been needed. This spot was, of
+course, very easily seen with the naked eye, its diameter being almost
+eight times that required for a spot to be visible without a telescope.
+
+Ancient historians often speak of obscurations of the Sun, and it has
+been supposed by some astronomers that this phenomenon might have been
+due in some cases to the apparition of large spots. A few spots on the
+surface of the Sun, like that just described, would sensibly reduce its
+light.
+
+Besides the ordinary Sun-spots already described, others are at times
+observed on the surface of the Sun, which show some of the same
+characteristics, but never attain so large proportions. They always
+appear as if seen through a fog, or veil, between the granulations of
+the solar surface. On account of their vagueness and ill-defined
+contours, I have proposed for these objects the term, "_Veiled Spots_."
+Veiled spots have a shorter duration than the ordinary spots, the
+smaller types sometimes forming and vanishing in a few minutes. Some of
+the larger veiled spots, however, remain visible for several days in
+succession, and show the characteristics of other spots in regard to the
+arrangement of their parts.
+
+The veiled spots have no umbra or penumbra, although they are usually
+accompanied by faculæ resembling those seen near the ordinary spots.
+They are frequently seen in the polar regions, but are there always of
+small size and of short duration. The veiled spots are larger, and more
+apt to arrange themselves into groups, in the regions occupied by the
+ordinary spots, and it is not rare to observe such spots transform
+themselves into ordinary spots, and vice versa. The veiled spots,
+therefore, seem to be ordinary spots filled up, or covered over by the
+granulations and semi-transparent gases composing the chromospheric
+layer. That it is so, becomes more evident, from the fact that large
+Sun-spots in process of diminution are sometimes gradually covered with
+faint and scattered granules which descend in long, narrow filaments,
+and become less and less distinguishable as they attain greater depth.
+This phenomenon, associated with the fact that the luminous bridges seen
+over the Sun-spots which are closing up are sometimes transformed into
+branches which show the characteristic structure of the photosphere,
+goes far to prove that the solar envelopes are mainly composed of an
+innumerable quantity of radial filaments of varying height.
+
+
+[PLATE I.--GROUP OF SUN-SPOTS AND VEILED SPOTS.
+
+Observed on June 17th 1875 at 7 h. 30 m. A.M.]
+
+
+The group of Sun-spots represented in Plate I., was observed and drawn
+on June 17th, 1875, at 7h. 30m. A. M. The first traces of this group
+were seen on June 15th, at noon, and consisted of three small black dots
+disseminated among the granulations. At that time, no disturbance of the
+surface was noticeable, and no faculæ were seen in the vicinity of these
+spots. On June 16th, at 8 o'clock A. M., the three small spots had
+become considerably enlarged, and, as usual, the group consisted of two
+principal spots. Between these two spots all was in motion: the
+granulations, stretched into long, wavy, parallel lines, had somewhat
+the appearance of a liquid in rapid motion. At 1 o'clock, P. M., on the
+same day, the group had considerably enlarged; the faculæ, the
+granulations, and the penumbral filaments being interwoven in an
+indescribable manner. On the morning of the 17th, these spots had
+assumed the complicated form and development represented in the drawing;
+while at the same time two conspicuous veiled spots were seen on the
+left hand, at some distance above the group.
+
+Some luminous bridges are visible upon the left hand spot, traversing
+the penumbra and umbra of this spot in various directions. The umbra of
+one of the spots is occupied, and partly filled with gray and rosy
+veils, similar to those above described, and the granulations of the
+solar surface form a background to the group of spots.
+
+This group of spots was not so remarkable for its size as for its
+complicated structure. The diameter of the group from east to west was
+only 2½ minutes of arc, or about 67,000 miles. The upper part of the
+umbra of the spot situated on the right hand side of the group was
+nearly 7,000 miles in diameter, or less by 1,000 miles than the diameter
+of the Earth. Some of the long filaments composing that part of the
+penumbra, situated on the left hand side of the same spot, were 17,000
+miles in length. One of these fiery elements would be sufficient to
+encircle two-thirds of the circumference of the Earth.
+
+
+
+
+SOLAR PROTUBERANCES
+
+PLATE II
+
+
+The chromosphere forming the outlying envelope of the Sun, is subject,
+as has been shown above, to great disturbances in certain regions,
+causing considerable upheavals of its surface and violent outbursts of
+its gases. From these upheavals and outbursts of the chromosphere result
+certain curious and very interesting forms, which are known under the
+name of "_Solar Protuberances_," "_Prominences_," or "_Flames_."
+
+These singular forms, which could, until recently, be observed only
+during the short duration of the total eclipses of the Sun, can now be
+seen on every clear day with the spectroscope, thanks to Messrs. Janssen
+and Lockyer, to whose researches solar physics is so much indebted.
+
+
+[PLATE II.--SOLAR PROTUBERANCES.
+
+Observed on May 5, 1873 at 9h, 40m. A.M.]
+
+
+The solar protuberances, the Sun-spots, and the faculæ to which they
+are closely related, are confined within the same general regions of the
+Sun, although the protuberances attain higher heliocentric latitudes.
+
+There is certainly a very close relation between the faculæ and the
+solar protuberances, since when a group of the faculæ traverses the
+Sun's limb, protuberances are always seen at the same place. It seems
+very probable that the faculæ and the protuberances are in the main
+identical. The faculæ may be the brighter portion of the protuberances,
+consisting of gases which are still undergoing a high temperature and
+pressure; while the gases which have been relieved from this pressure
+and have lost a considerable amount of their heat, may form that part of
+the protuberances which is only visible on the Sun's limb.
+
+A daily study of the solar protuberances, continued for ten years, has
+shown me that these objects are distributed on two zones which are
+equidistant from the solar equator, and parallel with it. The zone
+arrangement of the protuberances is more easily recognized during the
+years of minimum solar activity, as in these years the zones are very
+narrow and widely separated. During these years the belt of
+protuberances is situated between 40° and 45° of latitude, north and
+south. In years of great solar activity the zones spread considerably on
+either side of these limits, especially towards the equator, which they
+nearly reach, only a narrow belt, usually free from protuberances,
+remaining between them. Towards the poles the zones do not spread so
+much, and there the space free from protuberances is considerably
+greater than it is at the equator.
+
+During years of maximum solar activity, the protuberances, like the
+Sun-spots and the faculæ, are very numerous, very large, and very
+complicated--sometimes occupying a great part of the whole solar limb.
+As many as twenty distinct flames are sometimes observed at one time. In
+years of minimum solar activity, on the contrary, the prominences are
+very few in number, and they are of small size; but, as far as my
+observations go, they are never totally absent.
+
+In general, the solar flames undergo rapid changes, especially those
+which are situated in the vicinity of Sun-spots, although they
+occasionally remain unchanged in appearance and form for several hours
+at a time. The protuberances situated in higher latitudes are less
+liable to great and sudden changes, often retaining the same form for
+several days. The changes observed in the protuberances of the
+equatorial regions are due in part to the comparatively great changes in
+their position with respect to the spectator, which are occasioned by
+the rotation of the Sun. This rotation, of course, has a greater angular
+velocity on the equator than in higher latitudes. In most cases,
+however, the changes of the equatorial protuberances are too great and
+too sudden to be thus explained. They are, in fact, due to the greater
+solar activity developed in the equatorial zones, and wherever spots are
+most numerous.
+
+The solar protuberances appear under various shapes, and are often so
+complicated in appearance that they defy description. Some resemble huge
+clumsy masses having a few perforations on their sides; while others
+form a succession of arches supported by pillars of different styles.
+Others form vertical or inclined columns, often surmounted by cloud-like
+masses, or by various appendages, which sometimes droop gracefully,
+resembling gigantic palm leaves. Some resemble flames driven by the
+wind; others, which are composed of a multitude of long and narrow
+filaments, appear as immense fiery bundles, from which sometimes issue
+long and delicate columns surmounted by torch-like objects of the most
+fantastic pattern. Some others resemble trees, or animal forms, in a
+very striking manner; while still others, apparently detached from the
+solar limb, float above it, forming graceful streamers or clouds of
+various shapes. Some of the protuberances are very massive, while others
+are so thin and transparent as to form a mere veil, through which more
+distant flames can easily be seen.
+
+Notwithstanding this variety of form, two principal classes of solar
+protuberances may be recognized: the cloud-like or quiescent, and the
+eruptive or metallic protuberances.
+
+The first class, which is the most common, comprises all the cloud-like
+protuberances resting upon the chromosphere or floating about it. The
+protuberances of this type often obtain enormous horizontal proportions,
+and it is not rare to see some among them occupying 20° and 30° of the
+solar limb. The height attained by protuberances of this class does not
+correspond in general to their longitudinal extent; although some of
+their branches attain considerable elevations. These prominences very
+seldom have the brilliancy displayed by the other type, and are
+sometimes so faint as to be seen with difficulty. Although it is
+generally stated by observers that some of the protuberances belonging
+to this class are detached from the solar surface, and kept in
+suspension above the surface, like the clouds in our atmosphere, yet it
+seems to me very doubtful whether protuberances are ever disconnected
+from the chromosphere, since, in an experience of ten years, I have
+never been able to satisfy myself that such a thing has occurred. Many
+of them have appeared to me at first sight to be detached from the
+surface, but with a little patience and attention I was always able to
+detect faint traces of filamentary elements connecting them with the
+chromosphere. Quite often I have seen bright protuberances gradually
+lose their light and become invisible, while soon after they had
+regained it, and were as clearly visible as before. Observations of this
+kind seem to show that while the prominences are for the most part
+luminous, there are also a few which are non-luminous and invisible to
+the eye. These dark and invisible forms are most generally found in the
+vicinity of Sun-spots in great activity. When observing such regions
+with the spectroscope, it is not rare to encounter them in the form of
+large dark spots projecting on the solar spectrum near the hydrogen
+lines. On July 28th, 1872, I observed with the spectroscope a dark spot
+of this kind issuing from the vicinity of a large Sun-spot, and
+extending over one-fifth of the diameter of the Sun. This object had
+been independently observed in France a little earlier by M. Chacornac
+with the telescope, in which it appeared as a bluish streak.
+
+The second class of solar protuberances, comprising the eruptive type,
+is the most interesting, inasmuch as it conveys to us a conception of
+the magnitude and violence of the solar forces. The protuberances of
+this class, which are always intensely bright, appear for the most part
+in the immediate vicinity of Sun-spots or faculæ. These protuberances,
+which seem to be due to the outburst of the chromosphere, and to the
+violent ejection of incandescent gases and metallic vapors from the
+interior of the Sun, sometimes attain gigantic proportions and enormous
+heights.
+
+While the spectrum of the protuberances of the cloudy type is simple,
+and usually composed of four hydrogen lines and the yellow line D{3}, that
+of the eruptive class is very complicated, and, besides the hydrogen
+lines and D{3}, it often exhibits the bright lines of sodium, magnesium,
+barium, titanium, and iron, and occasionally, also, a number of other
+bright lines.
+
+The phenomena of a solar outburst are grand and imposing. Suddenly
+immense and acute tongues and jets of flames of a dazzling brilliancy
+rise up from the solar limb and extend in various directions. Some of
+these fiery jets appear perfectly rigid, and remain apparently
+motionless in the midst of the greatest disorder. Immense straps and
+columns form and rise in an instant, bending and waving in all sorts of
+ways and assuming innumerable shapes. Sometimes powerful jets resembling
+molten metal spring up from the Sun, describing graceful parabolas,
+while in their descent they form numerous fiery drops which acquire a
+dazzling brilliancy when they approach the surface.
+
+The upward motion of the protuberances in process of formation is
+sometimes very rapid. Some protuberances have been observed to ascend in
+the solar atmosphere at the rate of from 120 to 497 miles a second.
+Great as this velocity may appear, it is nevertheless insignificant when
+compared with that sometimes attained by protuberances moving in the
+line of sight instead of directly upwards. Movements of this kind are
+indicated by the displacement of the bright or dark lines in the
+spectrum. A remarkable instance of this kind occurred on the 26th of
+June, 1874. On that day I observed a displacement of the hydrogen C line
+corresponding to a velocity of motion of 1,600 miles per second. The
+mass of hydrogen gas in motion producing such a displacement was,
+according to theory, moving towards the Earth at this incredible rate,
+when it instantly vanished from sight as if it had been annihilated, and
+was seen no more.
+
+Until recently the protuberances had not been observed to rise more than
+200,000 miles above the solar surface; but, on October 7th, 1880, a
+flame, which had an elevation of 80,000 miles when I observed it at 8h.
+55m. A. M., had attained the enormous altitude of 350,000 miles when it
+was observed at noon by Professor C. A. Young. If we had such a
+protuberance on the Earth, its summit would be at a height sufficient
+not merely to reach, but to extend 100,000 miles beyond the Moon.
+
+Although the solar protuberances represented in Plate II. have not the
+enormous proportions attained by some of these objects, yet they are as
+characteristic as any of the largest ones, and afford a good
+illustration of the purely eruptive type of protuberances. The height of
+the largest column in the group equals 4' 43", or a little over 126,000
+miles. A large group of Sun-spots was in the vicinity of these
+protuberances when they were observed and delineated.
+
+
+
+
+TOTAL ECLIPSE OF THE SUN
+
+PLATE III
+
+
+A solar eclipse is due to the passage of the Moon directly between the
+observer and the Sun. Such an eclipse can only occur at New Moon, since
+it is only at that time that our satellite passes between us and the
+Sun. The Moon's orbit does not lie precisely in the same plane as the
+orbit of the Earth, but is inclined about five degrees to it, otherwise
+an eclipse of the Sun would occur at every New Moon, and an eclipse of
+the Moon at every Full Moon.
+
+Since the Moon's orbit is inclined to that of the Earth, it must
+necessarily intersect this orbit at two opposite points. These points
+are called the nodes of the Moon's orbit. When our satellite passes
+through either of the nodes when the Moon is new, it appears interposed
+to some extent between the Sun and the Earth, and so produces a solar
+eclipse; while if it passes a node when the Moon is full, it is more or
+less obscured by the Earth's shadow, which then produces an eclipse of
+the Moon. But, on the other hand, when the New Moon and the Full Moon do
+not coincide with the passage of our satellite through the nodes of its
+orbit, no eclipse can occur, since the Moon is not then on a line with
+the Sun and the Earth, but above or below that line.
+
+Owing to the ellipticity of the Moon's orbit, the distance of our
+satellite from the Earth varies considerably during each of its
+revolutions around us, and its apparent diameter is necessarily subject
+to corresponding changes. Sometimes it is greater, sometimes it is less,
+than the apparent diameter of the Sun. If it is greater at the time of a
+solar eclipse, the eclipse will be total to a terrestrial observer
+stationed nearly on the line of the centres of the Sun and Moon, while
+it will be only partial to another observer stationed further from this
+line. But the Moon's distance from the Earth may be so great and its
+apparent diameter consequently so small that even those observers
+nearest the central line of the eclipse see the border of the Sun all
+round the black disk of the Moon; the eclipse is then annular. Even
+during the progress of one and the same eclipse the distance of the Moon
+from the parts of the Earth towards which its shadow is directed may
+vary so much that, while the eclipse is total to some observers, others
+equally near the central line, but stationed at a different place, will
+see it as annular.
+
+The shadow cast by the Moon on the Earth during total eclipses, travels
+along upon the surface of the Earth, in consequence of the daily
+movement of rotation of our globe combined with the movements of the
+Earth and Moon in their orbits. The track of the Moon's shadow over the
+Earth's surface has a general eastward course, so that the more westerly
+observers see it earlier than those east of them. An eclipse may
+continue total at one place for nearly eight minutes, but in ordinary
+cases the total phase is much shorter.
+
+The nodes of the Moon's orbit do not invariably occupy the same
+position, but move nearly uniformly, their position with regard to the
+Sun, Earth, and Moon being at any time approximately what it formerly
+was at a series of times separated by equal intervals from each other.
+Each interval comprises 223 lunations, or 18 years, 11 days, and 7 or 8
+hours. The eclipses which occur within this interval are almost exactly
+repeated during the next similar interval. This period, called the
+"Saros," was well known to the ancients, who were enabled by its means
+to predict eclipses with some certainty.
+
+
+[PLATE III.--TOTAL ECLIPSE OF THE SUN.
+
+Observed July 29, 1878, at Creston, Wyoming Territory]
+
+
+A total eclipse of the Sun is a most beautiful and imposing phenomenon.
+At the predicted time the perfectly round disk of the Sun becomes
+slightly indented at its western limb by the yet invisible Moon. This
+phenomenon is known as the "first contact."
+
+The slight indentation observed gradually increases with the advance of
+the Moon from west to east, the irregularities of the surface of our
+satellite being plainly visible on the border of the dark segment
+advancing on the Sun's disk. With the advance of the Moon on the Sun,
+the light gradually diminishes on the Earth. Every object puts on a dull
+and gloomy appearance, as when night is approaching; while the bright
+sky, losing its light, changes its pure azure for a livid grayish color.
+
+Two or three minutes before totality begins, the solar crescent, reduced
+to minute proportions, gives comparatively so little light that faint
+traces of the Sun's atmosphere appear on the western side behind the
+dark body of the Moon, whose limb then becomes visible outside of the
+Sun. I observed this phenomenon at Creston during the eclipse of 1878.
+From 15 to 20 seconds before totality, the narrow arc of the Sun's disk
+not yet obscured by the Moon seems to break and separate towards the
+extremities of its cusps, which, thus divided, form independent points
+of light, which are called "_Baily's beads_." A moment after, the whole
+solar crescent breaks into numerous beads of light, separated by dark
+intervals, and, suddenly, they all vanish with the last ray of Sunlight,
+and totality has begun with the "second contact." This phenomenon of
+Baily's beads is undoubtedly caused by the irregularities of the Moon's
+border, which, on reaching the solar limb, divide the thin solar
+crescent into as many beads of light and dark intervals as there are
+peaks and ravines seen sidewise on that part of the Moon's limb.
+
+With the disappearance of the last ray of light, the planets and the
+stars of the first and second magnitude seem to light up and become
+visible in the sky. The darkness, which had been gradually creeping in
+with the progress of the eclipse, is then at its maximum. Although
+subject to great variations in different eclipses, the darkness is never
+so great as might be expected from the complete obscuration of our
+luminary, as the part of our atmosphere which is still exposed to the
+direct rays of the Sun, reflects to us some of that light, which thus
+diminishes the darkness resulting from the disappearance of the Sun.
+Usually the darkness is sufficient to prevent the reading of common
+print, and to deceive animals, causing them to act as if night was
+really approaching. During totality the temperature decreases, while the
+humidity of the atmosphere augments.
+
+Simultaneously with the disappearance of Baily's beads, a pale, soft,
+silvery light bursts forth from behind the Moon, as if the Sun, in
+disappearing, had been vaporized and expanded in all directions into
+soft phosphorescent rays and streamers. This pale light is emitted by
+gases constituting the solar atmosphere surrounding the bright nucleus
+now obscured by the dark body of our satellite. This solar atmosphere is
+called _Corona_, from its distant resemblance to the aureola, or glory,
+represented by ancient painters around the heads of saints.
+
+With the bursting forth of the corona, a very thin arc of bright white
+light is seen along the Moon's limb, where the solar crescent has just
+disappeared. This thin arc of light is the reversing layer, which, when
+observed with the spectroscope at that moment, exhibits bright lines
+answering to the dark lines of the ordinary solar spectrum. Immediately
+above this reversing layer, and concentric with it, appears the
+pink-colored chromospheric layer, with its curiously shaped flames and
+protuberances. During totality, the chromosphere and protuberances are
+seen without the aid of the spectroscope, and appear of their natural
+color, which, although somewhat varying in their different parts, is, on
+the whole, pinkish, and similar to that of peach-blossoms; yet it is
+mixed here and there with delicate prismatic hues, among which the pink
+and straw colors predominate.
+
+The color of the corona seems to vary in every eclipse, but as its tints
+are very delicate, it may depend, in a great measure, upon the vision of
+the observer; although there seems to be no doubt that there are real
+variations. At Creston, in 1878, it appeared to both Professor W.
+Harkness and myself of a decided pale greenish hue.
+
+The corona appears under different forms, and has never been observed
+twice alike. Its dimensions are also subject to considerable variations.
+Sometimes it appears regular and very little extended, its distribution
+around the Sun being almost uniform; although in general it spreads a
+little more in the direction of the ecliptic, or of the solar equator.
+At other times it appears much larger and more complicated, and forms
+various wings and appendages, which in some cases, as in 1878, extend to
+immense distances; while delicate rays radiate in straight or curved
+lines from the spaces left in the polar regions between the wings. The
+corona has sometimes appeared as if divided by immense dark gaps,
+apparently free from luminous matter, and strongly resembling the dark
+rifts seen in the tails of comets. This was observed in Spain and Sicily
+during the total eclipse of the Sun in 1870. Different structures,
+forming wisps and streamers of great length, and interlaced in various
+ways, are sometimes present in the corona, while faint but more
+complicated forms, distantly resembling enormous solar protuberances
+with bright nuclei, have also been observed.
+
+As the Moon continues its eastward progress, it gradually covers the
+chromosphere and the solar protuberances on the eastern side of the Sun;
+while, at the same time, the protuberances and the chromosphere on the
+opposite limb gradually appear from under the retreating Moon. Then, the
+thin arc of the reversing layer is visible for an instant, and is
+instantly followed by the appearance of a point of dazzling white light,
+succeeded immediately by the apparition of Daily's beads on each side,
+and totality is over, with this third contact. The corona continues to
+be visible on the eastern side of the Sun for several minutes longer,
+and then rapidly vanishes.
+
+The thin solar crescent increases in breadth as the Moon advances;
+while, at the same time, the darkness and gloom spread over nature
+gradually disappear, and terrestrial objects begin to resume their
+natural appearance. Finally the limb of the Moon separates from that of
+the Sun at the instant of "fourth contact," and the eclipse is over.
+
+The phenomena exhibited by the corona in different eclipses are very
+complex, and, so far, they have not been sufficiently studied to enable
+us to understand the true nature of the solar atmosphere. From the
+spectral analysis of the corona, and the phenomena of polarization, it
+has been learned, at least, that while the matter composing the upper
+part of the solar atmosphere is chiefly composed of an unknown
+substance, producing the green line 1474, its lower part is mainly
+composed of hydrogen gas at different temperatures, a part of which is
+self-luminous, while the other part only reflects the solar light. But
+the proportion of the gaseous particles emitting light, to those simply
+reflecting it, is subject to considerable variations in different
+eclipses. At present it would seem that in years of great solar
+disturbances, the particles emitting light are found in greater quantity
+in the corona than those reflecting it; but further observations will be
+required to confirm these views.
+
+It is very difficult to understand how the corona, which in certain
+eclipses extends only one diameter of the Sun, should, in other cases,
+as in 1878, extend to the enormous distance of twelve times the same
+diameter. Changes of such magnitude in the solar atmosphere, if due to
+the operation of forces with which we are acquainted, cannot yet be
+accounted for by what is known of such forces. Their causes are still as
+mysterious as those concerned in the production of the monstrous tails
+displayed by some comets on their approach to the Sun.
+
+Plate 3, representing the total eclipse of the Sun of July 29th, 1878,
+was drawn from my observations made at Creston, Wyoming Territory, for
+the Naval Observatory. The eclipse is represented as seen in a
+refracting telescope, having an aperture of 6⅓ inches, and as it
+appeared a few seconds before totality was over, and when the
+chromosphere was visible on the western limb of the Sun. The two long
+wings seen on the east and west side of the Sun, appeared considerably
+larger in the sky than they are represented in the picture.
+
+
+
+
+THE AURORA BOREALIS
+
+PLATE IV
+
+
+The name of Polar Auroras is given to certain very remarkable luminous
+meteoric phenomena which appear at intervals above the northern or the
+southern horizons of both hemispheres of the Earth. When the phenomenon
+is produced in our northern sky, it is called "Aurora Borealis," or
+"Northern Lights;" and when it appears in the southern sky, it is called
+"Aurora Australis," or southern aurora.
+
+Marked differences appear in the various auroras observed from our
+northern latitudes. While some simply consist in a pale, faint
+luminosity, hardly distinguishable from twilight, others present the
+most gorgeous and remarkable effects of brightness and colors.
+
+A great aurora is usually indicated in the evening soon after twilight,
+by a peculiar grayish appearance of the northern sky just above the
+horizon. The grayish vapors giving that appearance, continuing to form
+there, soon assume a dark and gloomy aspect, while they gradually take
+the form of a segment of a circle resting on the horizon. At the same
+time that this dark segment is forming, a soft pearly light, which seems
+to issue from its border, spreads up in the sky, where it gradually
+vanishes, being the brightest at its base. This arc of light, gradually
+increasing in extent as well as in brightness, reaches sometimes as far
+as the polar star. On some rare occasions, one or two, and even three,
+concentric arches of bright light form one above the other over the dark
+segment, where they appear as brilliant concentric rainbows. While the
+aurora continues to develop and spread out its immense arc, the border
+of the dark segment loses its regularity and appears indented at several
+places by patches of light, which soon develop into long, narrow,
+diverging rays and streamers of great beauty. For the most part the
+auroral light is either whitish or of a pale, greenish tint; but in some
+cases it exhibits the most beautiful colors, among which the red and
+green predominate. In these cases the rays and streamers, which are
+usually of different colors, produce the most magnificent effects by
+their continual changes and transformations.
+
+The brightness and extent of the auroral rays are likewise subject to
+continual changes. An instant suffices for their development and
+disappearance, which may be succeeded by the sudden appearance of others
+elsewhere, as though the original streamers had been swiftly transported
+to a new place while invisible. It frequently happens that all the
+streamers seem to move sidewise, from west to east, along the arch,
+continuing meanwhile to exhibit their various changes of form and color.
+For a time, these appearances of motion continue to increase, a
+succession of streamers alternately shooting forth and again fading,
+when a sudden lull occurs, during which all motion seems to have ceased.
+The stillness then prevailing is soon succeeded by slight pulsations of
+light, which seem to originate on the border-of the dark segment, and
+are propagated upwards along the streamers, which have now become more
+numerous and active. Slow at first, these pulsations quicken by degrees,
+and after a few minutes the whole northern sky seems to be in rapid
+vibration. The lively upward and downward movement of these streamers
+entitles them to the name of "merry dancers" given them in northern
+countries where they are frequent.
+
+Long waves of light, quickly succeeded by others, are propagated in an
+instant from the horizon to the zenith; these, in their rapid passage,
+cause bends and curves in the streamers, which then, losing their
+original straightness, wave and undulate in graceful folds, resembling
+those of a pennant in a gentle breeze. Although the coruscations add to
+the grandeur of the spectacle, they tend to destroy the diverging
+streamers, which, being disconnected from the dark segment, or torn in
+various ways, are, as it were, bodily carried up towards the zenith.
+
+In this new phase the aurora is transformed into a glorious crown of
+light, called the "Corona." From this corona diverge in all directions
+long streamers of different colors and forms, gracefully undulating in
+numerous folds, like so many banners of light. Some of the largest of
+these streamers appear like fringes composed of short transverse rays of
+different intensity and colors, producing the most fantastic effects,
+when traversed by the pulsations and coruscations which generally run
+across these rays during the great auroral displays.
+
+The aurora has now attained its full development and beauty. It may
+continue in this form for half an hour, but usually the celestial fires
+begin to fade at the end of fifteen or twenty minutes, reviving from
+time to time, but gradually dying out. The northern sky usually appears
+covered by gray and luminous streaks and patches after a great aurora,
+these being occasionally rekindled, but more often they gradually
+disappear, and the sky resumes its usual appearance.
+
+The number of auroras which develop a corona near the zenith is
+comparatively small in our latitudes; but many of them, although not
+exhibited on so grand a scale, are nevertheless very interesting. On
+some very rare occasions the auroral display has been confined almost
+exclusively to the dark segment, which appeared then as if pierced along
+its border by many square openings, like windows, through which appeared
+the bright auroral light.
+
+
+[PLATE IV.--AURORA BOREALIS.
+
+As observed March 1, 1872, at 9h. 25m. P.M.]
+
+
+Among the many auroras which I have had occasion to observe, none are
+more interesting, excepting the type first described, than those which
+form an immense arch of light spanning the heavens from East to West.
+This form of aurora, which is quite rare, I last observed on September
+12th, 1881. All the northern sky was covered with light vapors, when a
+small auroral patch appeared in the East at about 20° above the
+horizon. This patch of light, gradually increasing westward, soon
+reached the zenith, and continued its onward progress until it arrived
+at about 20° above the western horizon, where it stopped. The aurora
+then appeared as a narrow, wavy band of light, crossed by numerous
+parallel rays of different intensity and color. These rays seemed to
+have a rapid motion from West to East along the delicately-fringed
+streamer, which, on the whole, moved southward, while its extremities
+remained undisturbed. Aside from the apparent displacement of the
+fringes, a singular vibrating motion was observed in the auroral band,
+which was traversed by pulsations and long waves of light. The phenomena
+lasted for about twenty minutes, after which the arch was broken in many
+places, and it slowly vanished.
+
+The aurora usually appears in the early part of the evening, and attains
+its full development between ten and eleven o'clock. Although the
+auroral light may have apparently ceased, yet the phenomenon is not at
+an end, as very often a solitary ray is visible from time to time; and
+even towards morning these rays sometimes become quite numerous. On some
+occasions the phenomenon even continues through the following day, and
+is manifested by the radial direction of the cirrus-clouds in the
+heights of our atmosphere. In 1872 I, myself, observed an aurora which
+apparently continued for two or three consecutive days and nights. In
+August, 1859, the northern lights remained visible in the United States
+for a whole week.
+
+The height attained by these meteors is considerable, and it is now
+admitted that they are produced in the rarefied air of the upper regions
+of our atmosphere. From the researches of Professor Elias Loomis on the
+great auroras observed in August and September, 1859, it was ascertained
+that the inferior part of the auroral rays had an altitude of 46 miles,
+while that of their summits was 428 miles. These rays had, therefore, a
+length of 382 miles. From the observation of thirty auroral displays, it
+has been found that the mean height attained by the summit of these
+streamers above the Earth's surface was 450 miles.
+
+But if the auroral streamers are generally manifested at great heights
+in our atmosphere, it would appear from the observations of persons
+living in the regions where the auroras are most frequent, as also from
+those who have been stationed in high northern and southern latitudes,
+that the phenomenon sometimes descends very low. Both Sabine and Parry
+saw the auroral rays projected on a distant mountain; Ross saw them
+almost at sea-level projected on the polar ice; while Wrangel, Franklin,
+and others observed similar phenomena. Dr. Hjaltalin, who has lived in
+latitude 64° 46' north, and has made a particular study of the aurora,
+on one occasion saw the aurora much below the summit of a hill 1,600
+feet high, which was not very far off.
+
+The same aurora is sometimes observed on the same night at places very
+far distant from one another. The great aurora borealis of August 28th,
+1859, for instance, was seen over a space occupying 150° in
+longitude--from California to the Ural Mountains in Russia. It even
+appears now very probable that the phenomenon is universal on our globe,
+and that the northern lights observed in our hemisphere are simultaneous
+with the aurora australis of the southern hemisphere. The aurora of
+September 2d, 1859, was observed all through North and South America,
+the Sandwich Islands, Australia, and Africa; the streamers and
+pulsations of light of the north pole responding to the rays and
+coruscations of the south pole. Of thirty-four auroras observed at
+Hobart Town, in Tasmania, twenty-nine corresponded with aurora borealis
+observed in our hemisphere.
+
+The auroral phenomena, although sometimes visible within the tropics,
+are, however, quite rare in these regions. For the most part they are
+confined within certain zones situated in high latitudes north and
+south. The zone where they are most frequent in our hemisphere forms an
+ellipse, which has the north pole at one of its foci; while the other is
+situated somewhere in North America, in the vicinity of the magnetic
+pole. The central line of the zone upon which the auroras seem to be
+most frequent passes from the northern coast of Alaska through Hudson's
+Bay and Labrador to Iceland, and then follows the northern coast of
+Europe and Asia. The number of auroras diminishes as the observer
+recedes from this zone, and it is only in exceptional cases that they
+are seen near the equator. Near the pole the phenomenon is less frequent
+than it is in the region described. In North America we occupy a
+favorable position for the observation of auroras, as we are nearer the
+magnetic poles than are the Europeans and Asiatics, and we consequently
+have a greater number of auroras in corresponding latitudes.
+
+The position of the dark auroral segment varies with the place occupied
+by the observer, and its centre always corresponds with the magnetic
+meridian. In our Eastern States the auroral segment appears a little to
+the west of the north point; but as the observer proceeds westward it
+gradually approaches this point, and is due north when seen from the
+vicinity of Lake Winnipeg. At Point Barrow, in the extreme north-west of
+the United States, the aurora is observed in the east. In Melville
+Islands, Parry saw it in the south; while in Greenland it is directly in
+the west.
+
+It is stated that auroras are more numerous about the equinoxes than
+they are at any other seasons; and also, when the earth is in perigee,
+than when it is in apogee. An examination which I have made of a
+catalogue by Professor Loomis, comprising 4,137 auroras observed in the
+temperate zone of our hemisphere from 1776 to 1873, sustains this
+statement. During this period, one hundred more auroras were recorded
+during each of the months comprising the equinoxes, than during any
+other months of the year; while eighty more auroras were observed when
+the earth was in perigee, than when it was in apogee. But to establish
+the truth of this assertion on a solid basis, more observations in both
+hemispheres will be required.
+
+The aurora is not simply a terrestrial phenomenon, but is associated in
+some mysterious way with the conditions of the Sun's surface. It is a
+well-known fact that terrestrial magnetism is influenced directly by the
+Sun, which creates the diurnal oscillations of the magnetic needle.
+Between sunrise and two o'clock, the north pole of the needle moves
+towards the west in our northern hemisphere, and in the afternoon and
+evening it moves the other way. These daily oscillations of the needle
+are not uniform in extent; they have a period of regular increase and
+decrease. At a given place the daily oscillations of the magnetic needle
+increase and decrease with regularity during a period which is equal to
+10⅓ years. As this period closely coincides with the Sun-spot period,
+the connection between the variation of the needle and these solar
+disturbances has been recognized.
+
+Auroral phenomena generally accompany the extraordinary perturbations in
+the oscillations of the magnetic needle, which are commonly called
+"magnetic storms," and the greater the auroral displays, the greater are
+the magnetic perturbations. Not only is the needle subject to unusual
+displacements during an aurora, but its movements seem to be
+simultaneous with the pulsations and waving motions of the delicate
+auroral streamers in the sky. When the aurora sends forth a coruscation,
+or a streamer in the sky, the magnetic needle responds to it by a
+vibration. The inference that the auroral phenomena are connected with
+terrestrial magnetism is further supported by the fact that the centre
+of the corona is always situated exactly in the direction of that point
+in the heavens to which the dipping needle is directed.
+
+It has been found that the aurora is a periodical phenomenon, and that
+its period corresponds very closely with those of the magnetic needle
+and Sun-spots. The years which have the most Sun-spots and magnetic
+disturbances have also the most auroras. There is an almost perfect
+similarity between the courses of the three sets of phenomena, from
+which it is concluded that the aurora is connected in some mysterious
+way with the action of the Sun, as well as with the magnetic condition
+of the earth.
+
+A very curious observation, which has been supposed to have some
+connection with this subject, was made on Sept. 1st, 1859, by Mr.
+Carrington and Mr. Hodgson, in England. While these observers, who were
+situated many miles from one another, were both engaged at the same time
+in observing the same Sun-spot, they suddenly saw two luminous spots of
+dazzling brilliancy bursting into sight from the edge of the Sun-spot.
+These objects moved eastward for about five minutes, after which they
+disappeared, having then traveled nearly 34,000 miles. Simultaneously
+with these appearances, a magnetic disturbance was registered at Kew by
+the self-registering magnetic instruments. The very night that followed
+these observations, great magnetic perturbations, accompanied by
+brilliant auroral displays, were observed in Europe. A connection
+between the terrestrial magnetism and the auroral phenomena is further
+proved by the fact that, before the appearance of an aurora, the
+magnetic intensity of our globe considerably increases, but diminishes
+as soon as the first flashes show themselves.
+
+The auroral phenomena are also connected in some way with electricity,
+and generate serious disturbances in the electric currents traversing
+our telegraphic lines, which are thus often rendered useless for the
+transmission of messages during great auroral displays. It sometimes
+happens, however, during such displays, that the telegraphic lines can
+be operated for a long distance, without the assistance of a battery;
+the aurora, or at least its cause, furnishing the necessary electric
+current for the working of the line. During auroras, the telephonic
+lines are also greatly affected, and all kinds of noises and
+crepitations are heard in the instruments.
+
+Two observations of mine, which may have a bearing on the subject,
+present some interest, as they seem to indicate the action of the aurora
+on some of the clouds of our atmosphere. On January 6th, 1872, after I
+had been observing a brilliant aurora for over one hour, an isolated
+black cumulus cloud appeared at a little distance from the western
+extremity of the dark auroral segment. This cloud, probably driven by
+the wind, rapidly advanced eastward, and was soon followed by a
+succession of similar clouds, all starting from the same point. All
+these black clouds apparently followed the same path, which was not a
+straight line, but parallel to and concentric with the border of the
+dark auroral segment. When the first cloud arrived in the vicinity of
+the magnetic meridian passing through the middle of the auroral arc, it
+very rapidly dissolved, and on reaching this meridian became invisible.
+The same phenomenon was observed with the succession of black clouds
+following, each rapidly dissolving as it approached the magnetic
+meridian. This phenomenon of black clouds vanishing like phantoms in
+crossing the magnetic meridian, was observed for nearly an hour. On June
+17th, 1879, I observed a similar phenomenon during a fine auroral
+display. About midway between the horizon and the polar star, but a
+little to the west of the magnetic meridian, there was a large black
+cumulo-stratus cloud which very slowly advanced eastward. As it
+progressed in that direction, its eastern extremity was dissolved in
+traversing the magnetic meridian; while, at the same time, several short
+and quite bright auroral rays issued from its western extremity, which
+in its turn dissolved rapidly, as if burned or melted away in the
+production of the auroral flame.
+
+It seems to be a well observed fact, that during auroras, a strong
+sulphurous odor prevails in high northern latitudes. According to Dr.
+Hjaltalin, during these phenomena, "the ozone of the atmosphere
+increases considerably, and men and animals exposed out of doors emit a
+sulphurous odor when entering a heated room." The Esquimaux and other
+inhabitants of the northern regions assert that great auroras are
+sometimes accompanied by crepitations and crackling noises of various
+sorts. Although these assertions have been denied by several travelers
+who have visited the regions of these phenomena, they are confirmed by
+many competent observers. Dr. Hjaltalin, who has heard these noises
+about six times in a hundred observations, says that they are especially
+audible when the weather is clear and calm; but that when the atmosphere
+is agitated they are not heard. He compares them to the peculiar sound
+produced by a silk cloth when torn asunder, or to the crepitations of
+the electric machine when its motion is accelerated. "When the auroral
+light is much agitated and the streamers show great movements, it is
+then that these noises are heard at different places in the atmosphere."
+
+The spectrum of the auroral light, although it varies with almost every
+aurora, always shows a bright green line on a faint continuous spectrum.
+In addition to this green line I have frequently observed four broad
+diffused bands of greater refrangibility in the spectra of some auroras.
+In two cases, when the auroras appeared red towards the west, the
+spectrum showed a bright red line, in addition to the green line and the
+broad bands described. These facts evidently show that the light of the
+aurora is due to the presence of luminous vapors in our atmosphere; and
+it may reasonably be supposed that these vapors are rendered luminous by
+the passage of electric discharges through them.
+
+
+
+
+THE ZODIACAL LIGHT
+
+PLATE V
+
+
+In our northern latitudes may be seen, on every clear winter and spring
+evening, a column of faint, whitish, nebulous light, rising obliquely
+above the western horizon. A similar phenomenon may also be observed in
+the east, before day-break, on any clear summer or autumn night. To this
+pale, glimmering luminosity the name of "Zodiacal Light" has been given,
+from the fact that it lies in the zodiac along the ecliptic.
+
+In common with all the celestial bodies, the zodiacal light participates
+in the diurnal motion of the sky, and rises and sets with the
+constellations in which it appears. Aside from this apparent motion, it
+is endowed with a motion of its own, accomplished from west to east, in
+a period of a year. In its motion among the stars, the zodiacal light
+always keeps pace with the Sun, and appears as if forming two faint
+luminous wings, resting on opposite sides of this body. In reality it
+extends on each side of the Sun, its axis lying very nearly in the plane
+of the ecliptic.
+
+In our latitudes the phenomena can be observed most advantageously
+towards the equinoxes, in March and September, when twilight is of short
+duration. As we proceed southward it becomes more prominent, and
+gradually increases in size and brightness. It is within the tropical
+regions that the zodiacal light acquires all its splendor: there it is
+visible all the year round, and always appears very nearly perpendicular
+to the horizon, while at the same time its proportions and brilliancy
+are greatly increased.
+
+
+[PLATE V.--THE ZODIACAL LIGHT.
+
+Observed February 20, 1876]
+
+
+The zodiacal light appears under the form of a spear-head, or of a
+narrow cone of light whose base apparently rests on the horizon, while
+its summit rises among the zodiacal constellations. In general
+appearance it somewhat resembles the tail of a large comet whose head is
+below the horizon. The most favorable time to observe this phenomenon in
+the evening, is immediately after the last trace of twilight has
+disappeared; and in the morning, one or two hours before twilight
+appears. When observed with attention, it is seen that the light of the
+zodiacal cone is not uniform, but gradually increases in brightness
+inwardly, especially towards its base, where it sometimes surpasses in
+brilliancy the brightest parts of the Milky-Way. In general, its
+outlines are vague and very difficult to make out, so gradually do they
+blend with the sky. On some favorable occasions, the luminous cone
+appears to be composed of several distinct concentric conical layers,
+having different degrees of brightness, the inner cone being the most
+brilliant of all. There is a remarkable distinction between the evening
+and morning zodiacal light. In our climate, the morning light is pale,
+and never so bright nor so extended as the evening light.
+
+In general, the zodiacal light is whitish and colorless, but in some
+cases it acquires a warm yellowish or reddish tint. These changes of
+color may be accidental and due to atmospheric conditions, and not to
+actual change in the color of the object. Although the zodiacal light is
+quite bright, and produces the impression of having considerable depth,
+yet its transparency is great, since all the stars, except the faint
+ones, can be seen through its substance.
+
+The zodiacal light is subject to considerable variations in brightness,
+and also varies in extent, the apex of its cone varying in distance from
+the Sun's place, from 40 to 90 degrees. These variations cannot be
+attributed to atmospheric causes alone, some of them being due to real
+changes in the zodiacal light itself, whose light and dimensions
+increase or decrease under the action of causes at present unknown. From
+the discussion of a series of observations on the zodiacal light made at
+Paris and Geneva, it appears certain that its light varies from year to
+year, and sometimes even from day to day, independently of atmospheric
+causes. Some of my own observations agree with these results, and one of
+them, at least, seems to indicate changes even more rapid. On December
+18th, 1875, I observed the zodiacal light in a clear sky free from any
+vapors, at six o'clock in the evening. At that time, the point of its
+cone was a little to the north of the ecliptic, at a distance of about
+90 degrees from the Sun's place. Ten minutes later, its summit had sunk
+down 35 degrees, the cone then being reduced to nearly one-half of its
+original dimensions. Ten minutes later, it had risen 25 degrees, and was
+then 80 degrees from the Sun's place, where it remained all the evening.
+On March 22d, 1878, the sky was very clear and the zodiacal light was
+bright when I observed it, at eight o'clock. At that moment the apex of
+the cone of light was a little to the south of the Pleiades, but this
+cone presented an unusual appearance never noticed by me before, its
+northern border appearing much brighter and sharper than usual, while at
+the same time its axis of greatest brightness appeared to be much nearer
+to this northern border than it was to the southern. After a few minutes
+of observation it became evident that the northern border was extending
+itself, as stars which were at some distance from it became gradually
+involved in its light. At the same time that this border spread
+northward, it seemed to diffuse itself, and after a time the cone
+presented its usual appearance, having its southern border brighter and
+better defined than the other. It would have been impossible to
+attribute this sudden change to an atmospheric cause, since only one of
+the borders of the cone participated in it, and since some very faint
+stars near this northern border were not affected in the least while the
+phenomenon occurred. Besides these observations, Cassini, Mairan,
+Humboldt, and many other competent observers have seen pulsations,
+coruscations and bickerings in the light of the cone, which they thought
+could not be attributed to atmospheric causes. It has also been observed
+that at certain periods the zodiacal light has shone with unusual
+intensity for months together.
+
+When this phenomenon is observed from the tropical regions, it is found
+that its axis of symmetry always corresponds with its axis of greatest
+brightness, and that both lie in the plane of the ecliptic, which
+divides its cone into two equal parts. But when the zodiacal light is
+observed in our latitude, the axis of symmetry does not correspond with
+the axis of greatest brightness, and both axes are a little to the north
+of this plane, the axis of symmetry being the farther removed.
+Furthermore, as already stated, the southern border of the cone always
+appears better defined and brighter than the corresponding northern
+margin. It is very probable, if not absolutely certain, that these
+phenomena are exactly reversed when the zodiacal light is observed from
+corresponding latitudes in the southern hemisphere, and that there, its
+axes, both of symmetry and of greatest brightness, appear south of the
+ecliptic, while the northern margin is the brightest. This seems to be
+established by the valuable observations of Rev. George Jones, made on
+board the U. S. steam frigate Mississippi, in California, Japan, and the
+Southern Ocean. "When I was north of the ecliptic," says this observer,
+"the greatest part of the light of the cone appeared to the north of
+this line; when I was to the south of the ecliptic, it appeared to be
+south of it; while when my position was on the ecliptic, or in its
+vicinity, the zodiacal cone was equally divided by this line."
+
+Besides the zodiacal light observed in the East and West, some observers
+have recognized an exceedingly faint, luminous, gauzy band, about 10 or
+12 degrees wide, stretching along the ecliptic from the summit of the
+western to that of the eastern zodiacal cone. This faint narrow belt has
+been called the Zodiacal Band. It has been recognized by Mr. H. C.
+Lewis, who has made a study of this phenomenon, that the zodiacal band
+has its southern margin a little brighter and a little sharper than the
+northern border. This observation is in accordance with similar
+phenomena observed in the zodiacal light, and may have considerable
+importance.
+
+In 1854, Brorsen recognized a faint, roundish, luminous spot in a point
+of the heavens exactly opposite to the place occupied by the Sun, which
+he has called "Gegenschein," or counter-glow. This luminous spot has
+sometimes a small nucleus, which is a little brighter than the rest.
+Night after night this very faint object shifts its position among the
+constellations, keeping always at 180 degrees from the Sun. The position
+of the counter-glow, like that of the zodiacal light and zodiacal band,
+is not precisely on the plane of the ecliptic, but a little to the north
+of this line. It is very probable that near the equator the phenomenon
+would appear different and there would correspond with this plane.
+
+There seems to be some confusion among observers in regard to the
+spectrum of the zodiacal light. Some have seen a bright green line in
+its spectrum, corresponding to that of the aurora borealis; while others
+could only see a faint grayish continuous spectrum, which differs,
+however, from that of a faint solar light, by the fact that it presents
+a well-defined bright zone, gradually blending on each side with the
+fainter light of the continuous spectrum. I have, myself, frequently
+observed the faint continuous spectrum of the zodiacal light, and on one
+occasion recognized the green line of the aurora; but it might have been
+produced by the aurora itself, as yet invisible to the eye, and not by
+the zodiacal light, since, later in the same evening, there was a
+brilliant auroral display. If it were demonstrated that this green line
+exists in the spectrum of the zodiacal light, the fact would have
+importance, as tending to show that the aurora and the zodiacal light
+have a common origin.
+
+Rev. Geo. Jones describes a very curious phenomenon which he observed
+several times a little before the moon rose above the horizon. The
+phenomenon consisted in a short, oblique, luminous cone rising from the
+Moon's place in the direction of the ecliptic. This phenomenon he has
+called the Moon Zodiacal Light. In 1874, I had an opportunity to observe
+a similar phenomenon when the Moon was quite high in the sky. By taking
+the precaution to screen the Moon's disk by the interposition of some
+buildings between it and my eye, I saw two long and narrow cones of
+light parallel to the ecliptic issuing from opposite sides of our
+satellite. The phenomenon could not possibly be attributed to vapors in
+our atmosphere, since the sky was very clear at the moment of the
+observation. Later on, these appendages disappeared with the formation
+of vapors near the Moon, but they reappeared an hour later, when the sky
+had cleared off, and continued visible for twenty minutes longer, and
+then disappeared in a clear sky.
+
+Although the zodiacal light has been studied for over two centuries, no
+wholly satisfactory explanation of the phenomenon has yet been given.
+Now, as in Cassini's time, it is generally considered by astronomers to
+be due to a kind of lens-shaped ring surrounding the Sun, and extending
+a little beyond the Earth's orbit. This ring is supposed to lie in the
+plane of the ecliptic, and to be composed of a multitude of independent
+meteoric particles circulating in closed parallel orbits around the Sun.
+But many difficulties lie in the way of this theory. It seems as
+incompetent to explain the slow and rapid changes in the light of this
+object as it is to explain the contractions and extensions of its cone.
+It fails, moreover, to explain the flickering motions, the coruscations
+observed in its light, or the displacement of its cone and of its axes
+of brightness and symmetry by a mere change in the position of the
+observer. Rev. Geo. Jones, unable to explain by this theory the
+phenomena which came under his observation, has proposed another, which
+supposes the zodiacal light to be produced by a luminous ring
+surrounding the Earth, this ring not extending as far as the orbit of
+the Moon. But this theory also fails in many important points, so that
+at present no satisfactory explanation of the phenomenon can be given.
+
+As the phenomenon is connected in some way with the Sun, and as we have
+many reasons to believe this body to be always more or less electrified,
+it might be supposed that the Sun, acting by induction on our globe,
+develops feeble electric currents in the rarefied gases of the superior
+regions of our atmosphere, and there forms a kind of luminous ridge
+moving with the Sun in a direction contrary to the diurnal motion, and
+so producing the zodiacal light. On this hypothesis, the counter-glow
+would be the result of a smaller cone of light generated by the solar
+induction on the opposite point of the Earth.
+
+Plate 5, which sufficiently explains itself, represents the zodiacal
+light as it appeared in the West on the evening of February 20th, 1876.
+All the stars are placed in their proper position, and their relative
+brightness is approximately shown by corresponding variations in
+size--the usual and almost the only available means of representation.
+Of course, it must be remembered that a star does not, in fact, show any
+disk even in the largest telescopes, where it appears as a mere point of
+light, having more or less brilliancy. The cone of light rises obliquely
+along the ecliptic, and the point forming its summit is found in the
+vicinity of the well-known group of stars, called the Pleiades, in the
+constellation of Taurus, or the Bull.
+
+
+
+
+THE MOON
+
+PLATE VI
+
+
+In its endless journey through space, our globe is not solitary, like
+some of the planets, but is attended by the Moon, our nearest celestial
+neighbor. Although the Moon does not attain to the dignity of a planet,
+and remains a secondary body in the solar system, yet, owing to its
+proximity to our globe, and to the great influence it exerts upon it by
+its powerful attraction, it is to us one of the most important celestial
+bodies.
+
+While the Moon accompanies the Earth around the Sun, it also revolves
+around the Earth at a mean distance of 238,800 miles. For a celestial
+distance this is only a trifling one; the Earth in advancing on its
+orbit travels over such a distance in less than four hours. A cannon
+ball would reach our satellite in nine days; and a telegraphic dispatch
+would be transmitted there in 1½ seconds of time, if a wire could be
+stretched between us and the Moon.
+
+Owing to the ellipticity of the Moon's orbit, its distance from the
+Earth varies considerably, our satellite being sometimes 38,000 miles
+nearer to us than it is at other times. These changes in the distance of
+the Moon occasion corresponding changes from 29' to 33' in its apparent
+diameter. The real diameter of the Moon is 2,160 miles, or a little over
+one-quarter the diameter of our globe; our satellite being 49 times
+smaller than the Earth.
+
+The mean density of the materials composing the Moon is only
+⁶⁄₁₀ that of the materials composing the Earth, and the force of
+gravitation at the surface of our satellite is six times less than it is
+at the surface of our globe. If a person weighing 150 lbs. on our Earth
+could be transported to the Moon, his weight there would be only 25 lbs.
+
+The Moon revolves around the Earth in about 27⅓ days, with a mean
+velocity of one mile per second, the revolution constituting its
+sidereal period. If the Earth were motionless, the lunar month would be
+equal to the sidereal period; but owing to its motion in space, the Sun
+appears to move with the Moon, though more slowly, so that after having
+accomplished one complete revolution, our satellite has yet to advance
+2¼ days before reaching the same apparent position in regard to the
+Earth and the Sun that it had at first. The interval of time comprised
+between two successive New Moons, which is a little over 29½ days,
+constitutes the synodical period of the Moon, or the lunar month.
+
+The Moon is not a self-luminous body, but, like the Earth and the
+planets, it reflects the light which it receives from the Sun, and so
+appears luminous. That such is the case is sufficiently demonstrated by
+the phases exhibited by our satellite in the course of the lunar month.
+Every one is familiar with these phases, which are a consequence of the
+motion of the Moon around the Earth. When our satellite is situated
+between us and the Sun, it is New Moon; since we cannot see its
+illuminated side, which is then turned away from us towards the Sun.
+When, on the contrary, it reaches that point of its orbit which, in
+regard to us, is opposite to the Sun's place, it is Full Moon; since
+from the Earth we can only see the fully illuminated side of our
+satellite. Again, when the Moon arrives at either of the two opposite
+points of its orbit, the direction of which from the Earth is at right
+angles with that of the Sun, it is either the First or the Last Quarter;
+since in these positions we can only see one-half of its illuminated
+disk.
+
+The curve described by the Moon around the Earth lies approximately in a
+plane, this plane being inclined about 5° to the ecliptic. Since our
+satellite, in its motion around us and the Sun, closely follows the
+ecliptic, which is inclined 23½° to the equator, it results that when
+this plane is respectively high or low in the sky, the moon is also high
+or low when crossing the meridian of the observer. In winter that part
+of the ecliptic occupied by the Sun is below the equator, and,
+consequently, the New Moons occurring in that season are low in the sky,
+since at New Moon our satellite must be on the same side of the ecliptic
+with the Sun. But the Full Moons in the same season are necessarily high
+in the sky, since a Full Moon can only occur when our satellite is on
+the opposite side of the ecliptic from the Sun, in which position it is,
+of course, as many degrees above the equator as the Sun is below. The
+Full Moon which happens nearest to the autumnal equinox is commonly
+called the Harvest Moon, from the fact that, after full, its delays in
+rising on successive evenings are very brief and therefore favorable for
+the harvest work in the evening. The same phenomenon occurs in every
+other lunar month, but not sufficiently near the time of Full Moon to be
+noticeable. When, in spring, a day or two after New Moon, our satellite
+begins to show its thin crescent, its position on the ecliptic is north
+as well as east of that occupied by the Sun; hence, its horns are nearly
+upright in direction, and give it a crude resemblance to a tipping bowl,
+from which many people who are unaware of its cause, and that this
+happens every year, draw conclusions as to the amount of rain to be
+expected.
+
+One of the most remarkable features of the Moon's motions is that our
+satellite rotates on its axis in exactly the same period of time
+occupied by its revolution around the Earth, from which it results that
+the Moon always presents to us the same face. To explain this
+peculiarity, astronomers have supposed that the figure of our satellite
+is not perfectly spherical, but elongated, so that the attraction of the
+Earth, acting more powerfully upon its nearest portions, always keeps
+them turned toward us, as if the Moon were united to our globe by a
+string. It is not exactly true, however, that the Moon always presents
+its same side to us, although its period of rotation exactly equals that
+of its revolution; since in consequence of the inclination of its axis
+of rotation to its orbit, combined with the irregularities of its
+orbital motion about us, apparent oscillations in latitude and in
+longitude, called librations, are created, from which it results that
+nearly ⁶⁄₁₀ of the Moon's surface is visible from the Earth at
+one time or another.
+
+The Moon is a familiar object, and every one is aware that our
+satellite, especially when it is fully illuminated, presents a variety
+of bright and dark markings, which, from their distant resemblance to a
+human face, are popularly known as "the man in the moon." A day or two
+after New Moon, when the thin crescent of our satellite is visible above
+the western horizon after sunset, the dark portion of its disk is
+plainly visible, and appears of a pale, ashy gray color, although not
+directly illuminated by the Sun. This phenomenon is due to the
+Earth-shine, or to that portion of solar light which the illuminated
+surface of our globe reflects to the dark side of the Moon, exactly in
+the same manner that the Moon-shine, on our Earth, is due to the solar
+light reflected to our globe by the illuminated Moon.
+
+Seen with a telescope of moderate power, or even with a good
+opera-glass, the Moon presents a peculiar mottled appearance, and has a
+strong resemblance to a globe made of plaster of Paris, on the surface
+of which numerous roundish, saucer-shaped cavities of various sizes are
+scattered at random. This mottled structure is better seen along the
+boundary line called the _terminator_, which divides the illuminated
+from the dark side of the Moon. The line of the terminator always
+appears jagged, and it is very easy to recognize that this irregularity
+is due to the uneven and rugged structure of the surface of our
+satellite.
+
+A glance at the Moon through a larger telescope shows that the bright
+spots recognized with the naked eye belong to very uneven and
+mountainous regions of our satellite, while the dark ones belong to
+comparatively smooth, low surfaces, comparable to those forming the
+great steppes and plains of the Earth. When examined with sufficient
+magnifying power, the white, rugged districts of the Moon appear covered
+over by numerous elevated craggy plateaus, mountain-chains, and deep
+ravines; by steep cliffs and ridges; by peaks of great height and
+cavities of great depth. This rugged formation, which is undoubtedly of
+volcanic origin, gives our satellite a desolate and barren appearance.
+The rugged tract occupies more than one-half of the visible surface of
+the Moon, forming several distinct masses, the principal of which occupy
+the south and south-western part of the disk. That this formation is
+elevated above the general level is proved by the fact that the
+mountains, peaks, and other objects which compose it, all cast a shadow
+opposite to the Sun; and further, that the length of these shadows
+diminishes with the elevation of the Sun above the lunar horizon.
+
+Since Galileo's time the surface of the Moon has been studied by a host
+of astronomers, and accurate maps of its topographical configuration
+have been made, and names given to all features of any prominence. It
+may even be said that in its general features, the visible surface of
+our satellite is now better known to us than is the surface of our own
+Earth.
+
+One of the most striking and common features of the mountainous
+districts of the Moon, is the circular, ring-like disposition of their
+elevated parts, which form numerous crater-like objects of different
+sizes and depths. Many thousands of crater-like objects are visible on
+the Moon through a good telescope, and, considering how numerous the
+small ones are, there is, perhaps, no great exaggeration in fixing their
+number at 50,000, as has been done by some astronomers. These volcanic
+regions of the Moon cannot be compared to anything we know, and far
+surpass in extent those of our globe. The number and size of the craters
+of our most important volcanic regions in Europe, in Asia, in North and
+South America, in Java, in Sumatra, and Borneo, are insignificant when
+compared with those of the Moon. The largest known craters on the Earth
+give only a faint idea of the magnitude of some of the lunar craters.
+The great crater Haleakala, in the Sandwich Islands, probably the
+largest of the terrestrial volcanoes, has a circumference of thirty
+miles, or a diameter of a little less than ten miles. Some of the great
+lunar craters, called walled plains, such as Hipparchus, Ptolemæus,
+etc., have a diameter more than ten times larger than that of Haleakala,
+that of the first being 115 miles and that of the last 100 miles. These
+are, of course, among the largest of the craters of the Moon, although
+there are on our satellite a great number of craters above ten miles in
+diameter.
+
+The crater-forms of the Moon have evidently appeared at different
+periods of time, since small craters are frequently found on the walls
+of larger ones; and, indeed, still smaller craters are not rarely seen
+on the walls of these last. The walls of the lunar craters are usually
+quite elevated above the surrounding surface, some of them attaining
+considerable elevations, especially at some points, which form peaks of
+great height. Newton, the loftiest of all, rises at one point to the
+height of 23,000 feet, while many others range from ten to twenty
+thousand feet in height. Several craters have their floor above the
+general surface--Plato, for instance. Wargentin has its floor nearly on
+a level with the summit of its walls, showing that at some period of its
+history liquid lavas, ejected from within, have filled it to the brim
+and then solidified. The floors of some of the craters are smooth and
+flat, but in general they are occupied by peaks and abrupt mountainous
+masses, which usually form the centre. Many of their outside walls are
+partly or wholly covered by numerous ravines and gullies, winding down
+their steep declivities, branching out and sometimes extending to great
+distances from their base. It would seem that these great volcanic
+mouths have at some time poured out torrents of lavas, which, in their
+descent, carved their passage by the deep gullies now visible.
+Sometimes, also, the crater slopes are strewn with debris, giving them a
+peculiar volcanic appearance.
+
+Notwithstanding their many points of similarity with the volcanoes of
+the Earth, the lunar craters differ from them in many particulars,
+showing that volcanic forces acting on different globes may produce
+widely different results. For example, the floors of terrestrial craters
+are usually situated at considerable elevations above the general
+surface, while those of the lunar craters are generally much depressed,
+the height of their walls being only about one-half the depth of their
+cavities. Again, while on the Earth the mass of the volcanic cones far
+exceeds the capacity of their openings, on the Moon it is not rare to
+see the capacity of the crater cavities exceeding the mass of the
+surrounding walls. On the Earth, the volcanic cones and mouths are
+comparatively regular and smooth, and are generally due to the
+accumulation of the ashes and the debris of all kinds which are ejected
+from the volcanic mouths. On the Moon, very few craters show this
+character, and for the most part their walls have a very different
+structure, being irregular, very rugged, and composed of a succession of
+concentric ridges, rising at many points to great elevations, and
+forming peaks of stupendous height. Again, many of the larger
+terrestrial craters have their interior occupied by a central cone, or
+several such cones, having a volcanic mouth on their summits; on the
+Moon such central cones are very rare. Although many of the large lunar
+craters have their interior occupied by central masses which have been
+often compared to the central cones of our great volcanoes, yet these
+objects have a very different character and origin. For the most part,
+they are mountainous masses of different forms--having very rarely any
+craters on them--and seem to have resulted from the crowding and lifting
+up of the crater floor by the phenomena of subsidence, of which these
+craters show abundant signs. Besides, the terrestrial craters are
+characterized by large and important lava streams, while on the Moon the
+traces of such phenomena are quite rare, and when they are shown, they
+generally differ from those of the Earth by their numerous and
+complicated ramifications, and also by the fact that many of these lava
+streamlets take their origin at a considerable distance from the crater
+slopes, and are grooved and depressed as if the burning liquids which
+are supposed to have produced them had subsequently disappeared, by
+evaporation or otherwise, leaving the furrow empty.
+
+The dark spots of the Moon, when viewed through a telescope, exhibit a
+totally different character, and show that they belong to a different
+formation from that of the brighter portions. These darker tracts do not
+seem to have had a direct volcanic origin like the latter, but rather
+appear to have resulted from the solidification of semi-fluid materials,
+which have overflowed vast areas at different times. The surface of this
+system is comparatively smooth and uniform, only some small craters and
+low ridges being seen upon it. The level and dark appearance of these
+areas led the ancient astronomers to the belief that they were produced
+by a liquid strongly absorbing the rays of light, and were seas like our
+seas. Accordingly, these dark surfaces were called _Maria_, or Seas, a
+name which it is convenient to retain, although it is well known to have
+originated in an error. The so-called seas of the Moon are evidently
+large flat surfaces similar to the deserts, steppes, pampas, and
+prairies of the Earth in general appearance. The great plains of the
+Moon are at a lower level than that of the other formation, and that
+which first attracts the observer's attention is the fact that they are
+surrounded almost on all sides by an irregular line of abrupt cliffs and
+mountain chains, showing phenomena of dislocation. This character of
+dislocation, which is general, and is visible everywhere upon the
+contours of the plains, seems to indicate that phenomena of subsidence,
+either slow or rapid, have occurred on the Moon; while, at the same
+time, the sunken surfaces were overflowed by a semi-fluid liquid, which
+solidified afterwards. The evidences of subsidence and overflowing
+become unmistakable when we observe that, along the borders of the gray
+plains, numerous craters are more or less embedded in the gray
+formation, only parts of the summit of their walls remaining visible, to
+attest that once large craters existed there. The farther from the
+border of the plain the vestiges of these craters are observed, the
+deeper they are embedded in the gray formation. That phenomena of
+subsidence have occurred on a grand scale on the Moon, is further
+indicated by the fact that the singular systems of fractures called
+clefts and rifts generally follow closely the outside border of the gray
+plains, often forming parallel lines of dislocation and fractures. In
+the interior regions of the gray formation, these fractures are
+comparatively rare.
+
+The gray, lava-like formation is obviously of later origin than the
+mountainous system to which belong the embedded craters above described.
+Its comparatively recent origin might also be inferred from the
+smallness of its craters and its low ridges. The few large craters
+observed on this formation evidently belong to the earlier system.
+
+The color of this system of gray plains is far from being uniform. In
+general appearance it is of a bluish gray, but when observed
+attentively, large areas appear tinted with a dusky olive-green, while
+others are slightly tinged with yellow. Some patches appear brownish,
+and even purplish. A remarkable example of the first case is seen on the
+surface, which encloses within a large parallelogram the two conspicuous
+craters, Aristarchus and Herodotus. This surface evidently belongs to a
+different system from that of the Oceanus Procellarum surrounding it,
+as, besides its color, which totally differs from that of the gray
+formation, its surface shows the rugged structure of the volcanic
+formation.
+
+When the Moon is full, some very curious white, luminous streaks are
+seen radiating from different centres, which, for the most part, are
+important craters, occupied by interior mountains. The great crater
+Tycho is the centre of the most imposing of the systems of white
+streaks. Some of the diverging rays of this great centre extend to a
+distance equal to one-quarter of the Moon's circumference, or about
+1,700 miles. The true nature of these luminous streaks is unknown, but
+it seems certain that they have their origin in the crater from which
+they diverge. They do not form any relief on the surface, and are seen
+going up over the mountains and steep walls of the crater, as well as
+down the ravines and on the floors of craters.
+
+The Moon seems to be deprived of an atmosphere; or, if it has any, it
+must be so excessively rare that its density is less than of the density
+of the Earth's atmosphere, since delicate tests afforded by the
+occultation of stars have failed to reveal its presence. Although no
+atmosphere of any consequence exists on the Moon, yet phenomena which I
+have observed seem to indicate the occasional presence there of vapors
+of some sort. On several occasions, I have seen a purplish light over
+some parts of the Moon, which prevented well-known objects being as
+distinctly seen as they were at other times, causing them to appear as
+if seen through a fog. One of the most striking of these observations
+was made on January 4th, 1873, on the crater Kant and its vicinity,
+which then appeared as if seen through luminous purplish vapors. On one
+occasion, the great crater Godin, which was entirely involved in the
+shadow of its western wall, appeared illuminated in its interior by a
+faint purplish light, which enabled me to recognize the structure of
+this interior. The phenomenon could not be attributed in this case to
+reflection, since the Sun, then just rising on the western wall of the
+crater, had not yet grazed the eastern wall, which was invisible. It is
+not impossible that a very rare atmosphere composed of such vapors
+exists in the lower parts of the Moon.
+
+If the Moon has no air, and no liquids of any sort, it seems impossible
+that its surface can maintain any form of life, either vegetable or
+animal, analogous to those on the Earth. In fact, nothing indicating
+life has been detected on the Moon--our satellite looking like a barren,
+lifeless desert. If life is to be found there at all, it must be of a
+very elementary nature. Aside from the want of air and water to sustain
+it, the climatic conditions of our satellite are very unfavorable for
+the development of life. The nights and days of the Moon are each equal
+to nearly fifteen of our days and nights. For fifteen consecutive
+terrestrial days the Sun's light is absent from one hemisphere of the
+Moon; while for the same number of days the Sun pours down on the other
+hemisphere its light and heat, the effects of which are not in any way
+mitigated by an atmosphere. During the long lunar nights the temperature
+must at least fall to that of our polar regions, while during its long
+days it must be far above that of our tropical zone. It has been
+calculated that during the lunar nights the temperature descends to 23°
+below zero, while during the days it rises to 468°, or 256° above the
+boiling point.
+
+It has been a question among astronomers whether changes are still
+taking place at the surface of the Moon. Aside from the fact that
+change, not constancy, is the law of nature, it does not seem doubtful
+that changes occur on the Moon, especially in view of the powerful
+influences of contraction and dilatation to which its materials are
+submitted by its severe alternations of temperature. From the distance
+at which we view our satellite, we cannot expect, of course, to be able
+to see changes, unless they are produced on a large scale. Theoretically
+speaking, the largest telescopes ever constructed ought to show us the
+Moon as it would appear to the naked eye from a distance of 40 miles;
+but in practice it is very different. The difficulty is in the fact
+that, while we magnify the surface of a telescopic image, we are unable
+to increase its light; so that, practically, in magnifying an object, we
+weaken its light proportionally to the magnifying power employed. The
+light of the Moon, especially near the terminator, where we almost
+always make our observations, is not sufficiently bright to bear a very
+high magnifying power, and only moderate ones can be applied to its
+study. What we gain by enlarging an object, we more than lose by the
+weakening of its light. Besides, a high magnifying power, by increasing
+the disturbances generally present in our atmosphere, renders the
+telescopic image unsteady and very indistinct. On the whole, the largest
+telescopes now in existence do not show us our satellite better than if
+we could see it with the naked eye from a distance of 300 miles or more.
+At such a distance only considerable changes would be visible.
+
+Notwithstanding these difficulties, it is believed that changes have
+been detected in Linné, Marius, Messier, and several other craters. An
+observation of mine seems to indicate that changes have recently taken
+place in the great crater Eudoxus. On February 20th, 1877, between 9h.
+30m. and 10h. 30m., I observed a straight, narrow wall crossing this
+crater from east to west, a little to the south of its centre. This wall
+had a considerable elevation, as was proved by the shadow it cast on its
+northern side. Towards its western end this wall appeared as a brilliant
+thread of light on the black shadow cast by the western wall of the
+crater. The first time I had occasion to observe this crater again,
+after this observation, was a year later, on February 17th, 1878; no
+traces of the wall were then detected. Many times since I have tried to
+find this narrow wall again, when the Moon presented the same phase and
+the same illumination, but always with negative results. It seems
+probable that this structure has crumbled down, yet it is very singular
+that so prominent a feature should not have been noticed before.
+
+
+[PLATE VI.--MARE HUMORUM.
+
+From a study made in 1875]
+
+
+The "Mare Humorum," or sea of moisture, as it is called, which is
+represented on Plate VI., is one of the smaller gray lunar plains. Its
+diameter, which is very nearly the same in all directions, is about 270
+miles, the total area of this plain being about 50,000 square miles. It
+is one of the most distinct plains of the Moon, and is easily seen with
+the naked eye on the left-hand side of the disk. The floor of the plain
+is, like that of the other gray plains, traversed by several systems of
+very extended but low hills and ridges, while small craters are
+disseminated upon its surface. The color of this formation is of a dusky
+greenish gray along the border, while in the interior it is of a lighter
+shade, and is of brownish olivaceous tint. This plain, which is
+surrounded by high clefts and rifts, well illustrates the phenomena of
+dislocation and subsidence. The double-ringed crater Vitello, whose
+walls rise from 4,000 to 5,000 feet in height, is seen in the upper
+left-hand corner of the gray plain. Close to Vitello, at the east, is
+the large broken ring-plain Lee, and farther east, and a little below,
+is a similarly broken crater called Doppelmayer. Both of these open
+craters have mountainous masses and peaks on their floor, which is on a
+level with that of the Mare Humorum. A little below, and to the left of
+these objects, is seen a deeply embedded oval crater, whose walls barely
+rise above the level of the plain. On the right-hand side of the great
+plain, is a long _fault_, with a system of fracture running along its
+border. On this right-hand side, may be seen a part of the line of the
+terminator, which separates the light from the darkness. Towards the
+lower right-hand corner, is the great ring-plain Gassendi, 55 miles in
+diameter, with its system of fractures and its central mountains, which
+rise from 3,000 to 4,000 feet above its floor. This crater slopes
+southward towards the plain, showing the subsidence to which it has been
+submitted. While the northern portion of the wall of this crater rises
+to 10,000 feet, that on the plain is only 500 feet high, and is even
+wholly demolished at one place where the floor of the crater is in
+direct communication with the plain. In the lower part of the _mare_,
+and a little to the west of the middle line, is found the crater
+Agatharchides, which shows below its north wall the marks of rills
+impressed by a flood of lava, which once issued from the side of the
+crater. On the left-hand side of the plain, is seen the half-demolished
+crater Hippalus, resembling a large bay, which has its interior strewn
+with peaks and mountains. On this same side can be seen one of the most
+important systems of clefts and fractures visible on the Moon, these
+clefts varying in length from 150 to 200 miles.
+
+
+
+
+ECLIPSES OF THE MOON
+
+PLATE VII
+
+
+Since the Moon is not a self-luminous body, but shines by the light
+which it borrows from the Sun, it follows that when the Sun's light is
+prevented from reaching its surface, our satellite becomes obscured. The
+Earth, like all opaque bodies exposed to sunlight, casts a shadow in
+space, the direction of which is always opposite to the Sun's place. The
+form of the Earth's shadow is that of a long, sharply-pointed cone,
+which has our globe for its base. Its length, varying with the distance
+of the Earth from the Sun, is, on an average, 855,000 miles, or 108
+times the terrestrial diameter. This conical shadow of the Earth,
+divided longitudinally by the plane of the ecliptic, lies half above and
+half below that plane, on which the summit of the shadow describes a
+whole circumference in the course of a year. If the Moon's orbit were
+not inclined to the ecliptic, our satellite would pass at every Full
+Moon directly through the Earth's shadow; but, owing to that
+inclination, it usually passes above or below the shadow. Twice,
+however, during each of its revolutions, it must cross the plane of the
+ecliptic, the points of its orbit where this happens being called nodes.
+Accordingly, if it is near a node at the time of Full Moon, it will
+enter the shadow of the Earth, and become either partly or wholly
+obscured, according to the distance of its centre from the plane of the
+ecliptic. The partial or total obscuration of the Moon's disk thus
+produced constitutes a partial or total eclipse of the Moon. The
+essential conditions for an eclipse of the Moon are, therefore, that our
+satellite must not only be full, but must also be at or very near one of
+its nodes.
+
+Although inferior in importance to the eclipses of the Sun, the eclipses
+of the Moon are, nevertheless, very interesting and remarkable
+phenomena, which never fail to produce a deep impression on the mind of
+the observer, inasmuch as they give him a clear insight into the silent
+motions of the planetary bodies.
+
+At the mean distance of the Moon from the Earth, the diameter of the
+conical shadow cast in space by our globe is more than twice as large as
+that of our satellite. But, besides this pure dark shadow of the Earth,
+its cone is enveloped by a partial shadow called "Penumbra," which is
+produced by the Sun's light being partially, but not wholly, cut off by
+our globe.
+
+While the Moon is passing into the penumbra, a slight reduction of the
+light of that part of the disk which has entered it, is noticeable. As
+the progress of the Moon continues, the reduction becomes more
+remarkable, giving the impression that rare and invisible vapors are
+passing over our satellite. Some time after, a small dark-indentation,
+marking the instant of first contact, appears on the eastern or
+left-hand border of the Moon, which is always the first to encounter the
+Earth's shadow, since our satellite is moving from west to east. The
+dark indentation slowly and gradually enlarges with the onward progress
+of the Moon into the Earth's shadow, while the luminous surface of its
+disk diminishes in the same proportion. The form of the Earth's shadow
+on the Moon's disk clearly indicates the rotundity of our globe by its
+circular outline. Little by little the dark segment covers the Moon's
+disk, and its crescent, at last reduced to a mere thread of light,
+disappears at the moment of the second contact. With this the phase of
+totality begins, our satellite being then completely involved in the
+Earth's shadow.
+
+The Moon remains so eclipsed for a period of time which varies with its
+distance from the Earth, and with the point of its orbit where it
+crosses the conical shadow. When it passes through the middle of this
+shadow, while its distance from our globe is the least, the total phase
+of an eclipse of the Moon may last nearly two hours. The left-hand
+border of our satellite having gone first into the Earth's shadow, is
+also the first to emerge, and, at the moment of doing so, it receives
+the Sun's light, and totality ends with the third contact. The lunar
+crescent gradually increases in breadth after its exit from the shadow,
+and finally the Moon recovers its fully illuminated disk as before, at
+the moment its western border leaves the Earth's shadow. Soon after, it
+passes out of the penumbra, and the eclipse is over. In total eclipses,
+the interval of time from the first to last contact may last 5h. 30m,
+but it is usually shorter.
+
+Soon after the beginning of an eclipse, the dark segment produced by the
+Earth's shadow on the Moon's disk generally appears of a dark grayish
+opaque color, but with the progress of the phenomenon, this dark tint is
+changed into a dull reddish color, which, gradually increasing, attains
+its greatest intensity when the eclipse is total. At that moment the
+color of the Moon is of a dusky, reddish, coppery hue, and the general
+features of the Moon's surface are visible as darker and lighter tints
+of the same color. It sometimes happens, however, that our satellite
+does not exhibit this peculiar coppery tint, but appears either blackish
+or bluish, in which case it is hardly distinguishable from the sky.
+
+It is very rare for the Moon to disappear completely during totality,
+and even when involved in the deepest part of the Earth's shadow, our
+satellite usually remains visible to the naked eye, or, at least, to the
+telescope. This phenomenon is to be attributed to the fact that the
+portion of the solar rays which traverse the lower strata of our
+atmosphere are strongly refracted, and bend inward in such a manner that
+they fall on the Moon, and sufficiently illuminate its surface to make
+it visible. The reddish color observed is caused by the absorption of
+the blue rays of light by the vapors which ordinarily-saturate the lower
+regions of our atmosphere, leaving only red rays to reach the Moon's
+surface. Of course, these phenomena are liable to vary with every
+eclipse, and depend almost exclusively on the meteorological conditions
+of our atmosphere.
+
+In some cases the phase of totality lasts longer than it should,
+according to calculation. This can be attributed to the fact that the
+Earth is enveloped in a dense atmosphere, in which opaque clouds of
+considerable extent are often forming at great elevations. Such strata
+of clouds, in intercepting the Sun's light, would have, of course, the
+effect of increasing the diameter of the Earth's shadow, in a direction
+corresponding to the place they occupy, and, if the Moon were moving in
+this direction, would increase the phase of total obscuration.
+
+The eclipses of the Moon, like those of the Sun, as shown above, have a
+cycle of 18 years, 11 days and 7 hours, and recur after this period of
+time in nearly the same order. They can, therefore, be approximately
+predicted by adding 18y. 11d. 7h. to the date of the eclipses which have
+occurred during the preceding period. During this cycle 70 eclipses will
+occur--41 being eclipses of the Sun and 29 eclipses of the Moon. At no
+time can there ever be more than seven eclipses in a year, and there are
+never less than two. When there are only two eclipses in a year, they
+are both eclipses of the Sun.
+
+Although the number of solar eclipses occurring at some point or other
+of the Earth's surface is greater than that of the eclipses of the Moon,
+yet at any single terrestrial station the eclipses of the Moon are the
+more frequent. While an eclipse of the Sun is only visible on a narrow
+belt, which is but a very small fraction of the hemisphere then
+illuminated by the Sun, an eclipse of the Moon is visible from all the
+points of the Earth which have the Moon above their horizon at the time.
+Furthermore, an eclipse of the Sun is not visible at one time over the
+whole length of its narrow tract, but moves gradually from one end of it
+to the other; while, on the contrary, an eclipse of the Moon begins and
+ends at the very same instant for all places from which it can be seen,
+but, of course, not at the same local time, which varies with the
+longitude of the place.
+
+
+[PLATE VII.--PARTIAL ECLIPSE OF THE MOON.
+
+Observed October 24, 1874]
+
+
+The partial eclipse of the Moon, represented on Plate VII., shows quite
+plainly the configuration of our satellite as seen with the naked eye
+during the eclipse, with its bright and dark spots, and its radiating
+streaks. This eclipse was observed on October 24th, 1874.
+
+
+
+
+THE PLANETS
+
+
+Around the Sun circulate a number of celestial bodies, which are called
+"_Planets_." The planets are opaque bodies, and appear luminous because
+their surfaces reflect the light they receive from the Sun.
+
+The planets are situated at various distances from the Sun, and revolve
+around this body in widely different periods of time, which are,
+however, constant for each planet, so far as ascertained, and doubtless
+are so in the other cases.
+
+The ideal line traced in space by a planet in going around the Sun, is
+called _the orbit_ of the planet; while the period of time employed by a
+planet to travel over its entire orbit and return to its starting point,
+is called _the sidereal revolution_, _or year_ of the planet. The
+dimensions of the orbits of the different planets necessarily vary with
+the distance of these bodies from the Sun, as does also the length of
+their sidereal revolution.
+
+The distance of a planet from the Sun does not remain constant, but is
+subject to variations, which in certain cases are quite large. These
+variations result from the fact that the planetary orbits are not
+perfect circles having the Sun for centre, but curves called
+"_Ellipses_," which have two centres, or foci, one of which is always
+occupied by the Sun. This is in accordance with Kepler's first law.
+
+The ideal point situated midway between the two foci is called _the
+centre of the ellipse_, or _orbit_; while the imaginary straight line
+which passes through both foci and the centre, with its ends at opposite
+points of the ellipse, is called "_the major axis_" of the orbit. It is
+also known as "_the line of the apsides_." The ideal straight line
+which, in passing through the centre of the orbit, cuts the major axis
+at right angles, and is prolonged on either side to opposite points on
+the ellipse, is called "_the minor axis_" of the orbit.
+
+When a planet reaches that extremity of the major axis of its orbit
+which is the nearest to the Sun, it is said to be in its "_perihelion_;"
+while, when it arrives at the other extremity, which is farthest from
+this body, it is said to be in its "_aphelion_." When a planet reaches
+either of the two opposite points of its orbit situated at the
+extremities of its minor axis, it is said to be at its _mean distance_
+from the Sun.
+
+The rapidity with which the planets move on their orbits varies with
+their distance from the Sun; the farther they are from this body, the
+more slowly they move. The rapidity of their motion is greatest when
+they are in perihelion, and least when they are in aphelion, having its
+mean rate when these bodies are crossing either of the extremities of
+the minor axes of their orbits.
+
+The imaginary line which joins the Sun to a planet at any point of its
+orbit, and moves with this planet around the Sun, is called "_the radius
+vector_." According to Kepler's second law, whatever may be the distance
+of a planet from the Sun, the radius vector sweeps over equal areas of
+the plane of the planet's orbit in equal times.
+
+There is a remarkable relation between the distance of the planets from
+the Sun and their period of revolution, in consequence of which the
+squares of their periodic times are respectively equal to the cubes of
+their mean distances from the Sun. From this third law of Kepler, it
+results that the mere knowledge of the mean distance of a planet from
+the Sun enables one to know its period of revolution, and _vice versa_.
+
+The orbit described by the Earth around the Sun in a year, or the
+apparent path of the Sun in the sky, is called "_the ecliptic_." Like
+that of all the planetary orbits, the plane of the ecliptic passes
+through the Sun's centre. The ecliptic has a great importance in
+astronomy, inasmuch as it is the fundamental plane to which the orbits
+and motions of all planets are referred.
+
+The orbits of the larger planets are not quite parallel to the ecliptic,
+but more or less inclined to this plane; although the inclination is
+small, and does not exceed eight degrees. On account of this inclination
+of the orbits, the planets, in accomplishing their revolutions around
+the Sun, are sometimes above and sometimes below the plane of the
+ecliptic. A belt extending 8° on each side of the ecliptic, and,
+therefore, 16° in width, comprises within its limits the orbits of all
+the principal planets. This belt is called "_the Zodiac_."
+
+Since all the planets have the Sun for a common centre, and have their
+orbits inclined to the ecliptic, it follows that each of these orbits
+must necessarily intersect the plane of the ecliptic at two opposite
+points situated at the extremities of a straight line passing through
+the Sun's centre. The two opposite points on a planetary orbit where its
+intersections with the ecliptic occur, are called "_the Nodes_," and the
+imaginary line joining them, which passes through the Sun's centre, is
+called "the line of the nodes." The node situated at the point where a
+planet crosses the ecliptic from the south to the north, is called "_the
+ascending node_" while that situated where the planet crosses from north
+to south, is called "_the descending node_."
+
+The planets circulating around the Sun are eight in number, but, beside
+these, there is a multitude of very small planets, commonly called
+"asteroids," which also revolve around our luminary. The number of
+asteroids at present known surpasses two hundred, and constantly
+increases by new discoveries. In their order of distance from the Sun
+the principal planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn,
+Uranus and Neptune. The orbits of the asteroids are comprised between
+the orbits of Mars and Jupiter.
+
+When the principal planets are considered in regard to their differences
+in size, they are separated into two distinct groups of four planets
+each, viz.: the small planets and the large planets. The orbits of the
+small planets are wholly within the region occupied by the orbits of the
+asteroids, while those of the large planets are wholly without this
+region.
+
+When the planets are considered in regard to their position with
+reference to the Earth, they are called "inferior planets" and "superior
+planets." The inferior planets comprise those whose orbits are within
+the orbit of our globe; while the superior planets are those whose
+orbits lie beyond the orbit of the Earth.
+
+Since the orbits of the inferior planets lie within the orbit of the
+Earth, the angular distances of these bodies from the Sun, as seen from
+the Earth, must always be included within fixed limits; and these
+planets must seem to oscillate from the east to the west, and from the
+west to the east of the Sun during their sidereal revolution. In this
+process of oscillation these planets sometimes pass between the Earth
+and the Sun, and sometimes behind the Sun. When they pass between us and
+the Sun they are said to be in "inferior conjunction," while, when they
+pass behind the Sun, they are said to be in "superior conjunction." When
+such a planet reaches its greatest distance, either east or west, it is
+said to be at its greatest elongation east or west, as the case may be,
+or in quadrature.
+
+The superior planets, whose orbits lie beyond that of the Earth and
+enclose it, present a different appearance. A superior planet never
+passes between the Earth and the Sun, since its orbit lies beyond that
+of our globe, and, therefore, no inferior conjunction of such a planet
+can ever occur. When one of these planets passes beyond the Sun, just
+opposite to the place occupied by the Earth, the planet is said to be in
+"conjunction;" while, when it is on the same side of the Sun with our
+globe, it is said to be in "opposition." While occupying this last
+position, the planet is most advantageously situated for observation,
+since it is then nearer to the Earth. The period comprised between two
+successive conjunctions, or two successive oppositions of a planet, is
+called its "synodical period." This period differs for every planet.
+
+It is supposed that all the planets rotate from west to east, like our
+globe; although no direct evidence of the rotation of Mercury Uranus,
+and Neptune has yet been obtained, it is probable that these planets
+rotate like the others. It results from the rotation of the planets that
+they have their days and nights, like our Earth, but differing in
+duration for every planet.
+
+The axes of rotation of the planets are more or less inclined to their
+respective orbits, and this inclination varies but little in the course
+of time. From the inclination of the axes of rotation of the planets to
+their orbits, it results that these bodies have seasons like those of
+the Earth; but, of course, they differ from our seasons in duration and
+intensity, according to the period of revolution and the inclination of
+the axis of each separate planet.
+
+
+
+
+THE PLANET MARS
+
+PLATE VIII
+
+
+Mars is the fourth of the planets in order of distance from the sun;
+Mercury, Venus and the Earth being respectively the first, second and
+third.
+
+Owing to the great eccentricity of its orbit, the distance of Mars from
+the Sun is subject to considerable variations. When this planet is in
+its aphelion, its distance from the Sun is 152,000,000 miles, but at
+perihelion it is only 126,000,000 miles distant, the planet being
+therefore 26,000,000 miles nearer the Sun at perihelion than at
+aphelion. The mean distance of Mars from the Sun is 139,000,000 miles.
+Light, which travels at the rate of 185,000 miles a second, occupies
+12½ minutes in passing from the Sun to this planet.
+
+While the distance of Mars from the Sun varies considerably, its
+distance from the Earth varies still more. When Mars comes into
+opposition, its distance from our globe is comparatively small,
+especially if the opposition occurs in August, as the two planets are
+then as near together as it is possible for them to be, their distance
+apart being only 33,000,000 miles. But if the opposition occurs in
+February, the distance may be nearly twice as great, or 62,000,000
+miles. On the other hand, when Mars is in conjunction in August, the
+distance between the two planets is the greatest possible, or no less
+than 245,000,000 miles; while, when the conjunction occurs in February,
+it is only 216,000,000 miles. Hence the distance between Mars and the
+Earth varies from .33 to 245 millions of miles; that is, this planet may
+be 212 million miles nearer to us at its nearest oppositions than at its
+most distant conjunctions.
+
+From these varying distances of Mars from the Earth, necessarily result
+great variations in the brightness and apparent size of the planet, as
+seen from our globe. When nearest to us it is a very conspicuous object,
+appearing as a star of the first magnitude, and approaching Jupiter in
+brightness; but when it is farthest it is much reduced, and is hardly
+distinguishable from the stars of the second and even third magnitude.
+In the first position, the apparent diameter of Mars is 26", in the last
+it is reduced to 3" only.
+
+The orbit of Mars has the very small inclination of 1° 51' to the plane
+of the ecliptic. The planet revolves around the Sun in a period of 687
+days, which constitutes its sidereal year, the year of Mars being only
+43 days less than two of our years.
+
+Mars travels along its orbit with a mean velocity of 15 miles per
+second, being about ⁸⁄₁₀ of the velocity of our globe in its
+orbit. The synodical period of Mars is 2 years and 48 days, during which
+the planet passes through all its degrees of brightness.
+
+Mars is a smaller planet than the Earth, its diameter being only 4,200
+miles, and its circumference 13,200 miles. It seems well established
+that it is a little flattened at its poles, but the actual amount of
+this flattening is difficult to obtain. According to Prof. Young, the
+polar compression is ¹⁄₂₁₉.
+
+The surface of this planet is a little over ²⁸⁄₁₀₀ of the
+surface of our globe, and its volume is 6½ times less than that of the
+Earth. Its mass is only about ⅒ while its density is about ¾ that of
+the Earth. The force of gravitation at its surface is nearly ¾ of what
+it is at the surface of our globe.
+
+The planet Mars rotates on an axis inclined 61° 18' to the plane of its
+orbit, so that its equator makes an angle of 28° 42' with the same
+plane. The period of rotation of this planet, which constitutes its
+sidereal day, is 24 h. 37 m. 23 s.
+
+The year of Mars, which is composed of 669⅔ of these Martial days,
+equals 687 of our days, this planet rotating 669⅔ times upon its axis
+during this period. But owing to the movement of Mars around the Sun,
+the number of solar days in the Martial year is only 668⅔, while,
+owing to the same cause, the solar day of Mars is a little longer than
+its sidereal day, and equals 24 h. 39 m. 35 s.
+
+The days and nights on Mars are accordingly nearly of the same length as
+our days and nights, the difference being a little less than
+three-quarters of an hour. But while the days and nights of Mars are
+essentially the same as ours, its seasons are almost twice as long as
+those of the Earth. Their duration for the northern hemisphere,
+expressed in Martial days, is as follows: Spring, 191; Summer, 181;
+Autumn, 149; Winter, 147. While the Spring and Summer of the northern
+hemisphere together last 372 days, the Autumn and Winter of the same
+hemisphere last only 296 days, or 76 days less. Since the summer seasons
+of the northern hemisphere correspond to the winter seasons of the
+southern hemisphere, and vice versa, the northern hemisphere, owing to
+its longer summer, must accumulate a larger quantity of heat than the
+last. But on Mars, as on the Earth, there is a certain law of
+compensation resulting from the eccentricity of the planet's orbit, and
+from the fact that the middle of the summer of the southern hemisphere
+of this planet, coincides with its perihelion. From the greater
+proximity of Mars to the Sun at that time, the southern hemisphere then
+receives more heat in a given time than does the northern hemisphere in
+its summer season. When everything is taken into account, however, it is
+found that the southern hemisphere must have warmer summers and colder
+winters than the northern hemisphere.
+
+Seen with the naked eye, Mars appears as a fiery red star, whose
+intensity of color is surpassed by no other star in the heavens. Seen
+through the telescope, it retains the same red tint, which, however,
+appears less intense, and gradually fades away toward the limb, where it
+is replaced by a white luminous ring.
+
+
+[PLATE VIII.--THE PLANET MARS.
+
+Observed September 3, 1877, at 11h. 55m. P.M.]
+
+
+Mars is a very difficult object to observe, the atmosphere surrounding
+it being sometimes so cloudy and foggy that the sight can hardly
+penetrate through its vapors. When this planet is observed under
+favorable atmospheric conditions, and with sufficient magnifying power,
+its surface, which is of a general reddish tint, is found to be
+diversified by white, gray and dark markings. The dark markings, which
+are the most conspicuous, almost completely surround the planet. They
+are of different forms and sizes, and very irregular, as can be seen on
+Plate VIII., which represents one of the hemispheres of this planet.
+Many of them, especially those situated in the tropical regions of the
+planet, form long narrow bands, whose direction is in the main parallel
+to the Martial equator.
+
+The dusky spots differ very much, both from one another and in their
+several parts, as regards intensity of shade. Some appear almost black,
+while others which appear grayish, are so faint, that they can seldom be
+seen. In the southern hemisphere, the darkest part of the spots is
+generally found along their northern border; especially where there are
+deep indentations.
+
+Some observers have described these spots as being greenish or bluish,
+but I have never been able to see the faintest trace of these colors in
+them, except when they were observed close to the limb, and involved in
+the greenish tinted ring which is always to be seen there. It is
+probably an effect of contrast, since green and red are complementary
+colors, and since this greenish tinge around the limb covers all kinds
+of spots, whether white or dark. When such dark spots, involved in the
+greenish tint, are carried by the rotation towards the centre of the
+disk, they no longer show this greenish color. To me, these spots have
+always appeared dark, and of such tints as would result from a mixture
+of white and black in different proportions; except that on their
+lighter portions they show some of the prevalent reddish tint of the
+Martial surface. It is to be remarked that in moments of superior
+definition of the telescopic image, the intensity of darkness of all the
+spots is considerably increased--some of them appearing almost perfectly
+black.
+
+The markings on the surface of Mars are now tolerably well
+known--especially those of its southern hemisphere, which, owing to the
+greater proximity of the planet to our globe when this hemisphere is
+inclined towards the earth, have been better studied. Those of the
+northern hemisphere are not so well known, since when this hemisphere is
+inclined towards us, the distance of Mars from the Earth is 26,000,000
+miles greater, so that the occasions for observing them are not so
+favorable.
+
+Several charts of Mars are in existence, but as the same nomenclature
+has not been employed in all of them, some confusion has arisen in
+regard to the names given to the most remarkable features of the
+planet's surface. In order to give clearness to the subject, it will be
+necessary here to give a brief description of the principal markings
+represented on Plate VIII. In this the nomenclature will be employed
+which has been adopted by the English observers in the fine chart of Mr.
+Nath. Green. The large dark spot represented on the left-hand side of
+the plate is called De La Rue Ocean. The dark oval spot, isolated in the
+vicinity of the centre of the disk, is called Terby Sea; while the dark,
+irregular form on the right, near the border, represents the western
+extremity of Maraldi Sea.
+
+The dusky spots of Mars seem to be permanent, and to form a part of the
+general surface of the planet. That several among them, at least, are
+permanent, is proved by the fact that they have been observed in the
+same position, and with the same general form, for over two centuries.
+Yet, if we are to depend upon the drawings made fifty years ago by Beer
+and Maedler, it would seem that the permanency of some of them does not
+exist, since a very large spot represented by these astronomers on their
+chart of Mars is not visible now. This object, which, on their map, has
+its middle at 270°, should be precisely under the prominent dark oval
+spot called Terby Sea, seen near the centre of the picture, and would
+extend down almost as far as the northern limb. This can hardly be
+attributed to an error of observation, since these observers were both
+careful, and had great experience in this class of work. It is a very
+singular fact that, at the very same place where Maedler represented the
+spot in question, I found a conspicuous dark mark on December 16, 1881,
+which was certainly not visible in 1877, during one of the most
+favorable oppositions which can ever occur. The object, which is still
+visible (Feb., 1882), consists of an isolated spot situated a little to
+the north of Terby Sea. During the memorable opposition of 1877, I
+investigated thoroughly the markings of Mars, and made over 200 drawings
+of its disk, 32 of which represent the Terby Sea; but this isolated spot
+was not to be seen, unless it be identified with the faint mark,
+represented on the plate, which occupied its place. There cannot be the
+slightest doubt that a change has occurred at that place. Changes in the
+markings have also been suspected on the other hemisphere of the planet.
+
+The well-known fact that the continents, and especially the mountainous
+and denuded districts of our globe, reflect much more light than the
+surfaces covered by water, has led astronomers to suppose that the dark
+spots on Mars are produced by a liquid strongly absorbing the rays of
+light, like the liquids on the surface of the Earth. According to this
+theory the dark spots observed are supposed to be lakes, seas, and
+oceans, similar to our own seas and oceans, while the reddish and
+whitish surfaces separating these dark spots, are supposed to be
+islands, peninsulas, and continents. This supposition seems certainly to
+have a great deal of probability in its favor, although some of the
+lighter markings may have a different origin, and perhaps be due to
+vegetation; but no observer has yet seen in them any of the changes
+which ought to result from change of seasons. Some of the changes in the
+dark spots might also be attributed to the flooding or drying up of
+marshes and low land. The change which I have observed lately might be
+attributed to such a cause, especially as my observation was made
+shortly after the spring equinox of the northern hemisphere of Mars,
+which occurred on December 8th.
+
+Besides the dark spots just described, there are markings of a different
+character and appearance. Among the most conspicuous are two very
+brilliant white oval spots, which always occupy opposite sides of the
+planet. These two bright spots, which correspond very closely with the
+poles of rotation of Mars, have been called "polar spots."
+
+On account of the inclination of the axis of rotation of this planet to
+the ecliptic, it is rare that both of these spots are visible on the
+disk at the same time; and when this occurs, they are seen considerably
+foreshortened, as they are then both on the limb of the planet. Usually
+only one spot is visible, and it appears to its best advantage when the
+region to which it belongs attains its maximum of inclination towards
+the Earth.
+
+The polar spots change considerably in size, as they do also in form.
+Sometimes they occupy nearly one-third of the disk, as is proved by many
+of my observations; while at other times they are so much reduced as to
+be totally invisible. It is to be remarked that the reduction of these
+spots generally corresponds with the summer seasons, and their
+enlargement with the winter seasons of the hemispheres to which they
+respectively belong. From these well-observed facts it would appear that
+a relation exists between the temperature of the two hemispheres of Mars
+and the variations of the white spots observed at its poles. A similar
+relation is known to exist on our globe between the progress of the
+seasons and the melting away and the accumulation of snow in the polar
+regions. Astronomers have been led, accordingly, to attribute the polar
+spots of Mars, with all their variations, to the alternate accumulation
+and melting of snows. On this account, the polar spots of Mars are
+sometimes also called "snow-spots."
+
+Errors have certainly been made by astronomers in some of their
+observations of the so-called polar snow-spots, other objects occupying
+their place having been mistaken for them. A regular series of
+observations on this planet, which I have now continued for seven years,
+has revealed the fact that during the winter seasons of the southern
+hemisphere of Mars, the polar spots are most of the time invisible,
+being covered over by white, opaque, cloud-like forms, strongly
+reflecting light. In 1877, during more than a month, I, myself, mistook
+for the polar spots such a canopy of clouds, which covered at least
+one-fifth of the surface of the whole disk. I only became aware of my
+error when the opaque cloud, beginning to dissolve at the approach of
+the Martial summer, allowed the real polar spot to be seen through its
+vapors, as through a mist at first, and afterwards with great
+distinctness. In this particular case, the snow-spot was considerably
+smaller than the cloudy cap which covered it, and it is to be remarked
+that it was not situated at the centre of this cloudy cap, but was east
+of that centre; a fact which may account for the so-called polar spots
+not being always observed on exactly opposite sides of the disk. From my
+observations of 1877, 1878 and 1880, it appears that at the approach of
+the autumnal equinox of the southern hemisphere of Mars, large, opaque
+masses, like cumulus clouds in form, began to gather in the polar
+regions of that hemisphere, and continued through autumn and winter,
+dissolving only at the approach of spring. These clouds, which varied in
+form and extent, were very unsteady at first, but as the winter drew
+nearer they enlarged and became more permanent, covering large surfaces
+for months at a time.
+
+That the large white spots under consideration are real clouds in the
+atmosphere of Mars, and are not due to a fall of snow, is proved by the
+fact that these spots covered both seas and continents with equal
+facility, even in the equatorial regions of the planet. Snow, of course,
+could not cover the seas of Mars, unless these were all frozen over,
+even in the equatorial zones; therefore, if the dark spots of Mars are
+assumed to be due to water, these large white spots cannot well be
+ascribed to snow.
+
+The real polar spots of Mars seem to be in relief on the surface of the
+planet, since the southern spot often appeared slightly shaded on the
+side opposite to the Sun during my observations in 1877. In certain
+cases, when they are on or very near the limb, they have been observed,
+both by others and by myself, to project from the disk slightly.
+
+The polar spots of Mars are doubtless composed of a material which, like
+our snow or ice, melts under the rays of the Sun; although it seems
+difficult to admit that the Martial snow is identical with our
+terrestrial snow, and that it melts at a like temperature. The south
+polar spot of Mars entirely disappeared from sight in its summer season
+in 1877, although the planet receives less than one-half as much heat as
+we receive from the Sun; yet on our globe the arctic or antarctic ices
+and snow are perpetual--never melting entirely. An important fact
+disclosed by the melting away of the southern polar spot is, that in
+melting it is always surrounded with a very dark surface, which takes
+the place of the melted portion of the spot, as observed by myself in
+1877-78. When the polar spot had entirely disappeared, its place was
+occupied by a very dark spot. Now, if the polar spot is really ice, and
+the dark spots are actual seas, this polar spot must be situated in
+mid-ocean, since, on melting away, it is replaced by a dark spot. If the
+polar spots are composed of a white substance melting under the rays of
+the Sun, as seems altogether probable, its melting point must be above
+that of terrestrial snow.
+
+Many of the dark spots of Mars, and especially those whose northern
+border forms an irregular belt upon the equatorial regions of this
+planet, are bordered on that side by a white luminous belt, following
+all their sinuosities. These white borders are variable. Sometimes they
+are very prominent and intensely bright, especially at some points,
+which occasionally almost equal the polar spots in brilliancy; while at
+other times they are so faint, that they can hardly be distinguished, or
+are even invisible; although the atmosphere is clear and the dark spots
+appear perfectly well defined. While these white borders were invisible,
+I have sometimes watched for several hours at a time to see if I could
+detect any traces of them in places where they usually appear the most
+prominent, but generally without success. On a few occasions, however, I
+had the good fortune to see some of these spots forming gradually in the
+course of one or two hours, at places where nothing of them could be
+seen before.
+
+These whitish fringes forming and vanishing along the coasts of the
+Martial seas have been very little studied by astronomers. From my
+observations made during the last seven years, it appears very probable
+that this belt and its white spots are mainly due to the condensation of
+vapors around, and over high peaks, and extensive mountain chains,
+forming the Martial sea-coasts, as the Andes and Rocky Mountains form
+the sea-coasts of the Pacific Ocean. These high mountains on Mars,
+condensing the vapors into fogs or clouds above them, or at their sides,
+as often happens in our mountainous districts, would certainly suffice
+to produce the phenomena observed. Some of the highest peaks among these
+mountain chains may even have their summits covered with perpetual snow,
+or some substance partaking of the nature of snow. The temporary
+visibility and invisibility of the white spots seen on Mars, as well as
+the rapid transformations they sometimes undergo, may be explained as
+caused by clouds having a high reflective power and a liability to form
+and disappear quickly.
+
+The assumption that these irregular whitish bands and spots are formed
+by the condensation of vapors on mountain chains, and elevated table
+lands, is supported by my observations made in 1877 and 1879. When such
+white spots were traversing the terminator at sunrise, they very often
+projected far into the night side, thus indicating that they were at a
+higher level than that of the general surface. Indentations in the
+terminator, corresponding to large dark spots crossing its line, also
+clearly indicated the depression of the dark spots below the general
+surface. The highest mountainous districts thus observed on Mars, are
+situated between 60° and 70° of south latitude, towards the western
+extremity of Gill Land. The mountain chain, which almost completely
+forms the surface of this land, is so elevated at some points, that they
+not only change the form of the terminator when they are seen upon it,
+but also the limb of the planet, as seen by myself. They then appear so
+brilliant, that the principal summit among them has been mistaken by
+several observers for the polar spot itself, as proved by the wrong
+position assigned to it on their drawings. It seems probable that this
+high peak, which appears always white, is constantly covered with snow,
+or the similar material replacing it on Mars. This high region is
+situated between longitudes 180° and 190°.
+
+The highest mountainous parts belonging to the hemisphere represented on
+Plate VIII., which are nearly always more or less visible as whitish
+spots and bands, form a coast line along the northern (lower) border of
+De La Rue Ocean. This great spot, which is not so simple as it has been
+represented by observers, is in fact divided by two narrow isthmuses,
+one in the north, the other in the east, both joining, in the interior
+of the great ocean, a peninsula heretofore known as Hall Island. Upon
+the south-eastern extremity of this peninsula, a white spot, called
+Dawes Ice Island, was observed in 1865, but it soon disappeared, and was
+after that seen only now and then. It is very probable that this
+so-called Ice Island was due to clouds forming around the summit of some
+high peak of this peninsula.
+
+On the opposite hemisphere to that represented on Plate VIII., the white
+fringes bordering the dark spots are much more conspicuous than they are
+on this side. On the eastern side of a remarkable dark spot called
+Kaiser Sea, they are very bright, and almost always present, although
+they vary considerably, both in brightness and in extent. To the south
+of Kaiser Sea, they are very conspicuous on the eastern border of
+Lockyer Land, forming an elevated and deeply indented coast-line along
+Lambert Sea. There the white spots never disappear entirely, being
+always visible on the north side, where they turn westward along Dawes'
+Ocean--the mountain chain attaining there its greatest altitude. Very
+frequently Lockyer Land, which seems to be a vast plateau, appears
+throughout white and brilliant, this occurring usually towards the
+sunrise or sunset of that region, probably from the condensation of
+vapors and the formation of fogs, but generally this whiteness gradually
+disappears with the progress of the sun above this plateau. Inside of
+the great continents of Mars these temporary white spots are not so
+frequent, but when visible they occupy always the same positions--a fact
+which probably indicates that they occupy the culminating points of
+these continents. One of these temporary white spots inside of the
+continents is represented on Plate VIII., on the left-hand side, below
+De La Rue Ocean, on Maedler Continent.
+
+Although large, opaque, cumulus-shaped, cloud-like forms are seen in the
+polar regions of Mars, such forms are very seldom seen in the tropical
+zones, or, at least, it appears so, from the fact that my observations,
+continued during the last seven years, have disclosed no real opaque
+cloudy forms there. Although the Martial sky is frequently overcast by
+dense vapors or thick fogs in these regions, yet no real opaque clouds
+were ever seen; the most prominent among the dusky spots being faintly
+visible through the vapory veil, when they approached the centre of the
+disk.
+
+Besides these phenomena, which prove that Mars is surrounded by an
+atmosphere having a great deal of similarity to our own, a further proof
+is afforded by the fact that the dark spots, which appear sharply
+defined and black when they are seen near the centre, become less and
+less visible as they advance towards the limb, and are totally invisible
+before they reach it. Moreover, the spectroscope also indicates the
+existence of an atmosphere, and even the presence of watery vapor in it.
+A very curious state of the Martial atmosphere is revealed by my
+observations of 1877-78. During eight consecutive weeks, from December
+12th to February 6th, a whole hemisphere of the planet--precisely that
+represented on Plate VIII.--was completely covered by dense vapors, or a
+thick fog which barely allowed the dark spots to be seen through it,
+even when they were in the centre of the disk. The opposite hemisphere
+of Mars appeared just as clear and calm as possible; there all the spots
+and their minutest details could be seen, and when the planet was
+observed at the proper time, the line separating the foggy from the
+clear side was plainly visible.
+
+The reddish tint observed on the continents of Mars has been supposed by
+some astronomers to be the real color of the atmosphere of this planet.
+But, for many reasons, this explanation is not acceptable. Besides the
+fact that the border of the planet appears white, while it should be
+more red than the other part, owing to the greater depth of atmosphere
+there presented to us, the polar spots, the white bands along the
+sea-coasts, and the cloud-like forms appear perfectly white, not the
+slightest tint of red being visible on them, as would be the case if
+these objects were seen through an atmosphere tinted red. Other
+astronomers have supposed that the vegetation of this planet has a
+reddish color; but this is not supported by observation. It has been
+again supposed, with much more probability, that the surface of Mars is
+composed of an ochreous material which gives the planet its predominant
+ruddy color.
+
+Until lately Mars was supposed to be without a satellite, but in August,
+1877, Professor Hall, of the Washington Observatory, made one of the
+most remarkable discoveries of the time, and found two satellites
+revolving around this planet. These satellites are among the smallest
+known heavenly bodies, their diameter having been estimated at from 6 to
+10 miles for the outer satellite, and from 10 to 40 miles for the inner
+one.
+
+The most extraordinary feature of these bodies is the proximity of the
+inner satellite to the planet, and the consequent rapidity of its
+motion. The distance of the inner satellite from the centre of Mars is
+about 6,000 miles, and from surface to surface it is less than 4,000
+miles, or a little more than the distance from New York to San
+Francisco. The shortest period of revolution of any satellite previously
+known, is that of the inner satellite of Saturn, which is a little more
+than 22½ hours; but the inner satellite of Mars accomplishes its
+revolution in 7h. 38m., or in 17 hours less than the period of rotation
+of the planet upon its axis. The period of revolution of the outer
+satellite is greater, of course, and equals 30h. 7m.
+
+From this rapidity of motion of the inner satellite of Mars, a very
+curious result follows, which at first sight may appear in contradiction
+with the fact that this body has a direct motion, like that of all the
+planets of the solar system, and moves around Mars from west to east.
+While the outer satellite of this planet, in company with all the stars
+and planets, rises in the east and sets in the west, the inner
+satellite, on the contrary, rises in the west and sets in the east.
+Since the period of rotation of Mars is greater than is the period of
+revolution of this satellite, it necessarily follows that this last body
+must constantly be gaining on the rotation, and, consequently, that the
+satellite sets in the east and rises in the west, compassing the whole
+heavens around Mars three times a day, passing through all its phases in
+11 hours, each quarter of this singular Moon lasting less than 3 hours.
+
+It has been shown above that Mars has many points of resemblance to the
+Earth. It has an atmosphere constituted very nearly like ours; it has
+fogs, clouds, rains, snows, and winds. It has water, or at least some
+liquids resembling it; it has rivers, lakes, seas and oceans. It has
+also islands, peninsulas, continents, mountains and valleys. It has two
+Moons, which must create great and rapid tides in the waters of its seas
+and oceans. It has its days and nights, its warm and cold seasons, and
+very likely its vegetation, its prairies and forests, like the Earth. On
+the other hand, its year and seasons are double those of the Earth, and
+its distance from the Sun is greater.
+
+Is this planet, which is certainly constituted very nearly like our
+globe, and seems so nearly fitted for the wants of the human race,
+inhabited by animals and intelligent beings?
+
+To answer this question, either in the negative or in the affirmative,
+would be to step out of the pure province of science, and enter the
+boundless domain of speculation, since no observer has ever seen
+anything indicating that animal life exists on Mars, or on any other
+planet or satellite. So far as observation goes, Mars seems to be a
+planet well suited to sustain animal life, and we may reason from
+analogy that if animal life can exist at all outside of the Earth, Mars
+must have its flora and fauna; it must have its fishes and birds, its
+mammalia and men; although all these living beings must inevitably be
+very different in appearance from their representatives on the Earth, as
+can easily be imagined from the differences existing between the two
+planets. Although all this is possible, and even very probable, yet it
+must be remembered that we have not the slightest evidence that it is
+so; and until we have acquired this evidence, we may only provisionally
+accept this idea as a pleasing hypothesis, which, after all, may be
+wrong and totally unfounded.
+
+
+
+
+THE PLANET JUPITER
+
+PLATE IX
+
+
+Jupiter, the giant of the planetary world, is the fifth in order of
+distance from the Sun, and is next to Mars, our ruddy neighbor. To the
+naked eye, Jupiter appears as a very brilliant star, whose magnitude,
+changing with the distance of this planet from the Earth, sometimes
+approaches that of Venus, our bright morning and evening star.
+
+The mean distance of Jupiter from the Sun is 475,000,000 miles, but
+owing to the eccentricity of its orbit, its distance varies from 452 to
+498 millions of miles. The distance of this planet from the Earth varies
+still more. When nearest to our globe, or in opposition, its distance is
+reduced to 384,000,000 miles, and its apparent diameter increased to
+50"; while when it is farthest, or in conjunction, its distance is
+increased to 567,000,000 miles, and its apparent diameter reduced to
+30"; Jupiter being thus 183,000,000 miles nearer our globe while in
+opposition than when it is in conjunction.
+
+This planet revolves around the Sun in 11 years, 10 months and 17 days,
+or in only 50 days less than 12 terrestrial years. Such is the year of
+this planet. The plane of its orbit is inclined 1° 19' to the ecliptic.
+No planet, except Uranus, has an orbit exhibiting a smaller inclination.
+The planet advances in its orbit at the mean rate of 8 miles a second;
+which is a little less than half the orbital velocity of the Earth.
+
+Jupiter is of enormous proportions. Its equatorial diameter measures
+88,000 miles, and its circumference no less than 276,460 miles, these
+dimensions being 11 times greater than those of the Earth. This planet,
+notwithstanding its huge size, rotates on its axis in not far from 9h.
+55m. 36s., which period constitutes its day. Owing, however, to the
+changeable appearance of its surface, this period cannot be ascertained
+with very great exactitude. In consequence of its rapid rotation, the
+planet is far from spherical, its polar diameter being shorter than the
+equatorial by about ¹⁄₁₆, or 5,500 miles. Its surface is 124
+times the surface of the earth; while its volume is 1,387 times as
+great. If Jupiter occupied the place of our satellite in the sky, it
+would appear 40 times as large as the Moon appears to us, and would
+cover a surface of the heavens 1,600 times that covered by the full
+Moon, and would subtend an angle of 21°. Jupiter's mass does not
+correspond with its great bulk, and is only ¹⁄₁₀₄₇ of the
+mass of the Sun, and 310 times the mass of the earth; its density being
+only ¼ of that of our globe. The force of gravitation at the surface of
+this planet is over 2½ times what it is on the Earth, so that a
+terrestrial object carried to the surface of Jupiter would weigh over
+two and a half times as much as on our globe.
+
+Observed with a telescope, even of moderate aperture, Jupiter, with its
+four attending satellites and its dazzling brilliancy, appears as one of
+the most magnificent objects in the sky. The general appearance of the
+disk is white; but unlike that of Mars, it is brightest towards its
+central parts, and a little darker around the limb, especially on the
+side opposite to the Sun. Although an exterior planet, and so far from
+us, Jupiter shows faint traces of phases when observed near its
+quadratures, but this gibbosity of its disk is very slight, and is
+indicated only by a kind of penumbral shadow on the limb.
+
+When observed with adequate power, the disk of Jupiter is found to be
+highly diversified. The principal features consist of a series of
+alternate light and dark streaks or bands, disposed most of the time
+parallel with the Jovian equator. These bands differ from each other in
+intensity as well as in breadth; those near the equator being usually
+much more prominent than those situated in higher latitudes north and
+south.
+
+The equatorial zone of Jupiter is occupied most of the time by a broad,
+prominent belt 20° or 30° wide, limited on each side by a very dark
+narrow streak. Between these two dark borders, but seldom occupying the
+whole space between them, appears an irregular white belt, apparently
+composed of dense masses of clouds strongly reflecting the Sun's light,
+some of these cloudy masses being very brilliant. The spaces left
+between the cloudy belt and the dark borders, usually exhibit a delicate
+pink or rosy color, which produces a very harmonious effect with the
+varying grayish and bluish shades of some of the belts and streaks seen
+on the disk. Quite often the cloudy belt is broken up, and consists of
+independent cloudy masses, separated by larger or smaller intervals,
+these intervals disclosing the rosy background of this zone.
+
+On each side of the equatorial belt there is usually a broad whitish
+belt, succeeded by a narrow gray band; the space left on each hemisphere
+between these last bands and the limb being usually occupied by two or
+three alternate white and gray bands. A uniform gray segment usually
+forms a sort of polar cap to Jupiter.
+
+When observed under very favorable conditions, all the lighter belts
+appear as if composed of masses of small cloudlets, resembling the white
+opaque clouds seen in our atmosphere. This, as already stated, is
+particularly noticeable on the equatorial belt. It is not unusual, when
+Jupiter is in quadrature, to see some of the most conspicuous white
+spots casting a shadow opposite to the Sun; a fact which sufficiently
+indicates that these spots are at different levels. They probably form
+the summits of vast banks of clouds floating high up in the atmosphere
+of Jupiter.
+
+What we see of Jupiter is chiefly a vaporous, cloudy envelope. If our
+sight penetrates anywhere deeper into the interior, it can only be
+through the narrow fissures of this envelope, which appear as gray or
+dark streaks or spots. That most of the visible surface of Jupiter is
+simply a cloudy covering, is abundantly proved by the proper motion of
+its spots, which sometimes becomes very great.
+
+In periods of calm, very few changes are noticeable in the markings of
+the planet, except, perhaps, some slight modifications of form in the
+cloudy, equatorial belt which, in general, is much more liable to
+changes than the other belts. But the Jovian surface is not always so
+tranquil, great changes being observed during the terrific storms which
+sometimes occur on this mighty planet, when all becomes disorder and
+confusion on its usually calm surface; and nothing on the Earth can give
+us a conception of the velocity with which some of its clouds and spots
+are animated. New belts quickly form, while old ones disappear. The
+usual parallelism of the belts no more exists. Huge, white, cumulus-like
+masses advance and spread out, the rosy equatorial belt enlarges
+sometimes to two or three times its usual size, and occupies two-thirds,
+or more, of the disk, the rosy tint spreading out in a very short time.
+At times very dark bands extending across the disk are transformed into
+knots or dark spots, which encircle the planet with a belt, as it were,
+of jet black beads. Sometimes, also, a secondary but narrower rosy belt
+forms either in the northern or the southern hemisphere, and remains
+visible for a few days or for years at a time.
+
+On May 25, 1876, I witnessed one of the grandest commotions which can be
+conceived as taking place in an atmosphere. All the southern hemisphere
+of Jupiter, from equator to pole, was in rapid motion, the belts and
+spots being transported entirely across the disk, from the eastern to
+the western limb, in one hour's time, during which the equatorial belt
+swelled to twice its original breadth, towards the south.
+
+Now, when one stops for a moment to think what is signified by that
+motion of the dark spots across the little telescopic disk of Jupiter in
+an hour's time, he may arrive at some conception of the magnitude of the
+Jovian storms, compared with those of our globe. The circumference of
+Jupiter's equator, as stated above, is 276,460 miles; half this number,
+or 138,230 miles, represents the length of the equatorial line seen from
+the Earth. Now, after taking into account the rotation of the planet,
+which somewhat diminishes the apparent motion, we arrive at the
+astonishing result that the spots and markings were carried along by
+this Jovian storm, at the enormous rate of 110,584 miles an hour, or
+over 30.7 miles a second. On our globe, a hurricane or tornado, which
+blows at the rate of 100 miles an hour, sweeps everything before it.
+What, then, must be expected from a velocity over 1,105 times as great?
+Enormous as this motion may appear, its occurrence cannot be doubted,
+since it is disclosed by direct observation.
+
+The surface of Jupiter, it would seem, has its periods of calm and
+activity like that of the Sun, although it is not yet known, as it is
+for the latter, that they recur with approximate regularity.
+
+My observations of this planet, which embrace a period of ten years,
+seem to point in that direction, for they show, at least, that Jupiter
+has its years of calm and its years of disturbances. The year 1876 was a
+year of extraordinary disturbance on Jupiter. Changes in the markings
+were going on all the time, and no one form could be recognized the next
+day, or even sometimes the next hour, as shown above. The cloudy
+envelope of the planet was in constant motion, the equatorial belt,
+especially, showing the signs of greatest disturbance, being, for the
+most part, two or three times as wide as in other years. After 1876 the
+calm was very great on the planet, only a slight change now and then
+being noticeable, the same forms being recognized day after day, month
+after month, and even year after year. In one case the same marking has
+been observed for seventeen consecutive months, and in another for
+twenty-eight months. This state of quietude lasted until October, 1880,
+when considerable commotion occurred on the northern hemisphere, where
+large round black spots, somewhat resembling the Sun-spots, formed in
+the cloudy atmosphere, and finally changed, towards the end of December,
+into a narrow pink belt, which still exists.
+
+
+
+[PLATE IX.--THE PLANET JUPITER.
+
+Observed November 1, 1880, at 9h. 30m. P.M.]
+
+
+The most curious marking ever seen on Jupiter is undoubtedly the great
+Red Spot, observed on the southern hemisphere of this planet for the
+last three years. This interesting object, seen first in July, 1878,
+disappeared for a time, reappeared on September 25 of the same year, and
+has remained visible until now. When seen by me in September, it was
+much elongated, and sharply pointed on one side, like a spear-head, but
+it subsequently acquired an irregular form, with short appendages
+protruding from its northern border. At first, the changes were great
+and frequent, but at length it acquired the regular oval form, which,
+with but slight modifications, it has retained until now. During the
+month of November, 1880, I noticed two small black specks upon this Red
+Spot, and they were seen again in January of the succeeding year, by Mr.
+Alvan Clark, Jr. When the spot had attained its oval shape, it appeared
+part of the time surrounded with a white luminous ring of cloudy forms
+which, however, was changing more or less all the time, being sometimes
+invisible. The color of this curious spot is a brilliant rosy red,
+tinged with vermilion, and altogether different in shade from the
+pinkish color of the equatorial belt. The size of the spot varies, but
+of late its changes have been slight. Its longer diameter may be
+estimated at 8,000 miles, and the shorter at 2,200 miles. The Red Spot
+is represented on Plate IX. with its natural color, and as it appears at
+the moments most favorable for observation. In ordinary cases its color
+does not appear so brilliant, but paler.
+
+It is difficult to account for the color of the equatorial belt and that
+of the Red Spot; but it is known, at least, that the material to which
+they are due cannot be situated at the level of the general surface
+visible to us, and especially that of the cloudy forms of the equatorial
+zone. Undoubtedly the red layer lies deeper than the superficial
+envelope of the planet, although it does not seem to be very deeply
+depressed.
+
+Jupiter is attended by four satellites, which revolve around the planet
+at various distances, and shine like stars of the 6th and 7th magnitude.
+It is said that under very favorable circumstances, and in a very clear
+sky, the satellites can be seen with the naked eye, but this requires
+exceptionally keen eyes, since the glare of the planet is so strong as
+to overpower the comparatively faint light of the satellites. However, I
+myself have sometimes seen, without the aid of the telescope, two or
+three of the satellites as a single object, when they were closely
+grouped on the same side of Jupiter.
+
+The four moons of Jupiter are all larger than our Moon, except the
+first, which has about the same diameter. They range in size from 2,300
+to 3,400 miles in diameter, the third being the largest; the
+determination of their diameter is by no means accurate, however, as it
+is difficult to measure such small objects with precision. Their mean
+distance from the centre of Jupiter varies from 267,000 to 1,192,000
+miles, the first satellite, the nearest to the planet, being a little
+farther from Jupiter then our satellite is from us. The four satellites
+revolve around the planet in orbits whose planes have a slight
+inclination to the equator of Jupiter, and consequently to the ecliptic.
+The diameter of the largest satellite is nearly half that of the Earth,
+or 3,436 miles; while its volume is five times that of our Moon. The
+period of revolution of these satellites varies from 1d. 18h. for the
+first, to 16d. 16h. for the last.
+
+Owing to the slight inclination of the plane of their orbits to that of
+the planet, the three first satellites, and generally the fourth, pass
+in front of the disk and also through the shadow of the planet at every
+revolution, and are accordingly eclipsed. Their passages behind
+Jupiter's disk are called occultations; those in front of it, transits.
+The eclipses, the occultations and the transits of the moons of Jupiter
+are interesting and important phenomena; the eclipses being sometimes
+observed for the rough determination of longitudes at sea.
+
+The satellites in transit present some curious phenomena. When they
+enter the disk, they appear intensely luminous upon its grayish border;
+but as they advance, they seem by degrees to lose their brightness,
+until they finally become undistinguishable from the luminous surface of
+Jupiter. It sometimes happens, however, that the first, the third and
+the fourth satellites, after ceasing to appear as bright spots, continue
+to be visible as dark spots upon the bright central portions of the
+planet's disk; but in these cases their disks appear smaller than the
+shadows they cast. Undoubtedly these satellites have extensive
+atmospheres, since they sometimes pass unperceived across the central
+parts of Jupiter, this being probably when their atmospheres are
+condensed into clouds, strongly reflecting light; while when these
+clouds are absent, we can see their actual surface, with traces of the
+dark spots upon them similar to those on Mars.
+
+From the variation in the brightness of these satellites, which is said
+to be always observed in the same part of their orbit, William Herschel
+was led to suppose that these bodies, like our Moon, rotate upon their
+axes in the same period in which they move round the planet, so that
+they always present the same face to Jupiter; but these conclusions have
+been denied. From my observations it is apparent, however, that the
+light reflected by them varies in intensity as well as in color. But
+this is rather to be attributed to the presence of an atmosphere
+surrounding these bodies, which when cloudy reflects more light than
+when clear, with corresponding changes in the color of the light.
+
+The satellites in transit are sometimes preceded or followed, according
+to the position of the Sun, by a round black spot having about the same
+size as the satellite itself. This black spot is the shadow of the
+satellite cast on the vapory envelope of Jupiter, similar to the shadow
+cast by the Moon on the Earth, during eclipses of the Sun; in fact, all
+the Jovian regions traversed by these shadows have the Sun totally
+eclipsed. Sometimes it happens that the shadow appears elliptical. This
+occurs either when it is observed very near the limb, or when entering
+upon a round, cloud-like spot. This effect is attributable to the
+perspective under which the shadow is seen on the spherical globe or
+spot.
+
+The proper motion of the satellites in the Jovian sky is much more rapid
+than that of the Moon in our sky. During one Jovian day of ten hours,
+the first satellite advances 84°; the second, 42°; the third, 20° and
+the fourth, 9°. The first satellite passes from New Moon to its first
+quarter in a little more than a Jovian day, while the fourth occupies
+ten such days in attaining the same phase.
+
+In density, as well as in physical constitution, Jupiter differs widely
+from the interior planets, and especially from the Earth; and, as has
+been shown, it is surrounded by a dense, opaque, cloudy layer, which is
+almost always impenetrable to the sight, and hides from view the
+nucleus, which we may conceive to exist under this vaporous envelope. In
+1876, the year of the great Jovian disturbances, I observed frequently
+in the northern hemisphere of the planet a very curious phenomenon,
+which seems to prove that its cloudy envelope is at times partially
+absent in some places, its vapors being apparently either condensed, or
+transported to other parts of its surface, and that, therefore, a
+considerable part of the real globe of the planet was visible at these
+places. The phenomenon consisted in the deformation of the northern
+limb, which had a much shorter radius on all of this hemisphere situated
+northward of the white belt which adjoins the equatorial zone. The
+deformation of the limb on both sides, where it passed from a longer to
+a shorter radius, was abrupt, and at right angles to the limb, forming
+there a steplike indentation which was very prominent. The polar segment
+having a smaller radius, appeared unusually dark, and was not striped,
+as usual, but uniform in tint throughout. On September 27th, the third
+satellite passed over this dark segment, and emerged from the western
+border, a little below the place where the limb was abruptly deformed,
+as above described. When the satellite had fully emerged from this limb,
+it was apparent that if the portion of the limb having a longer radius
+had been prolonged a little below, and as far as the satellite, it would
+have enclosed it within its border, and thus retarded the time of
+emersion. The depth of deformation of the limb was accordingly greater
+than the diameter of the third satellite, and certainly more than 4,000
+miles. That the phenomenon was real, is proved by the fact that the
+egress of this satellite occurred at least four minutes sooner than the
+time predicted for it in the American Ephemeris. Other observations seem
+to point in the same direction, since some of the satellites which were
+occulted have been seen through the limb of Jupiter by different
+astronomers, as if this limb was sometimes semi-transparent. Another
+observation of mine seems to confirm these conclusions. On April 24th,
+1877, at 15h. 25m. the shadow of the first satellite was projected on
+the dark band forming the northern border of the equatorial belt, the
+shadow being then not far from the east limb. Close to this shadow, and
+on its western side, it was preceded by a secondary shadow, which was
+fainter, but had the same apparent size. This round dark spot was not
+the satellite itself, as I had supposed at first, since this object was
+yet outside of the planet, on the east, and entered upon it only at 16h.
+4m. I watched closely this strange phenomenon, and at 16h. 45m., when
+the shadow had already crossed about ¾ of the disk, it was still
+preceded by the secondary, or mock shadow, as it may be called; the same
+relative distance having been kept all the while between the two
+objects, which had therefore traveled at the same rate. It is obvious
+that this dark spot could not be one of the planet's markings, since the
+shadow of the first satellite moves more quickly on the surface of
+Jupiter than a spot on the same surface travels by the effect of
+rotation, so that in this case the shadow would soon have passed over
+this marking, and left it behind, during the time occupied by the
+observation. From these observations it seems very probable that Jupiter
+has a nucleus, either solid or liquid, which lies several thousand
+mites below the surface of its cloudy envelope. It is also probable that
+the uniformly shaded dark segment seen in 1876, was a portion of the
+surface of this nucleus itself. When the cloudy envelope is
+semi-transparent at the place situated on a line with an occulted
+satellite and the eye of an observer, this satellite may accordingly
+remain visible for a time through the limb, as shown by observation. The
+phenomenon of the mock shadow may also be attributed to a similar cause,
+where semi-transparent vapors receive the shadow of a satellite at their
+surface, while at the same time part of this shadow, passing through the
+semi-transparent vapors, may be seen at the surface of the nucleus, or
+of a layer of opaque clouds situated at some distance below the surface.
+
+Some astronomers are inclined to think that Jupiter is at a high
+temperature, and self-luminous to a certain extent. If this planet is
+self-luminous to any degree, we might expect that some light would be
+thrown upon the satellites when they are crossing the shadow cast into
+space by the planet; but when they cross this shadow they are totally
+invisible in the best telescopes, a proof that they do not receive much
+light from the non-illuminated side of Jupiter. It would, indeed, seem
+probable that some of the intensely white spots occasionally seen on the
+equatorial belt of the planet are self-luminous in a degree, yet not
+enough to render the satellites visible while they are immersed in
+Jupiter's shadow. It does not seem impossible that the planet should
+have the high temperature attributed to it, when we remember the
+terrific storms observed in its atmosphere, which, owing to the great
+distance of Jupiter from the Sun, do not seem to be attributable to this
+body, but rather to some local cause within the envelope of the planet.
+
+Astronomy, which is a science of observation, is naturally silent with
+regard to the inhabitants of Jupiter. If there are any such inhabitants,
+they are confined to the domain of conjecture, under the dense cloudy
+envelope of the planet. The conditions of habitability on Jupiter must
+differ very widely from those of our globe. Comparatively little direct
+light from the Sun reaches the surface of the globe of Jupiter, except
+that which passes through the narrow openings forming the dark clouds.
+All the rest of the planet's surface, being covered perpetually by
+opaque clouds, receives only diffused light. On Jupiter there are
+practically no seasons, since its axis is nearly perpendicular to its
+orbit. The force of gravity on the surface of Jupiter being more than
+double what it is on the Earth, living bodies would there have more than
+double the weight of similar bodies on the Earth. Furthermore, Jupiter
+only receives 0.011 of the light and heat which we receive from the Sun;
+and its year is nearly equal to 12 of our years. If there are living
+beings on Jupiter, they must, then, be entirely different from any known
+to us, and they may have forms never dreamed of in our most fantastic
+conceptions.
+
+The two round black spots represented towards the central parts of Plate
+IX. are the shadows of the first and second satellite; while the two
+round white spots seen on the left of the disk, are the satellites
+themselves, as they appeared at the moment of the observation. The first
+satellite and its shadow are the nearest to the equator; while the
+second satellite and its shadow are higher, the last being projected on
+the Great Red Spot.[2] The row of dark circular spots represented on the
+northern, or lower hemisphere, when they first appeared, had some
+resemblance to Sun-spots without a penumbra, with bright markings around
+them, resembling faculæ. These round spots subsequently enlarged
+considerably, until they united along the entire line, encircling the
+planet, and finally forming a narrow pink belt, which is still visible.
+
+
+[Footnote 2: By an accidental error in enlarging the original drawing,
+the satellites and shadows appear in Plate IX. of double their actual
+size. The error is one easy of mental correction.]
+
+
+
+
+THE PLANET SATURN
+
+PLATE X
+
+
+Saturn, which is next to Jupiter in order of distance from the Sun,
+while not the largest, is certainly the most beautiful and interesting
+of all the planets, with his grand and unique system of rings, and his
+eight satellites, which, like faithful servants, attend the planet's
+interminable journey through space.
+
+Seen with the naked eye, Saturn shines in the night like a star of the
+first magnitude, whose dull, soft whiteness is, however, far from
+attaining the brilliancy of Venus or Jupiter, although it sometimes
+approaches Mars in brightness. Saturn hardly ever exhibits the
+phenomenon of scintillation, or twinkling, a peculiarity which makes it
+easily distinguishable among the stars and planets of the heavens.
+
+The synodical period of Saturn occupies 1 year and 13 days, so that
+every 378 days, on an average, this planet holds the same position in
+the sky relatively to the Sun and the Earth.
+
+The mean distance of Saturn from the Sun is a little over 9½ times that
+of our globe, or 872,000,000 miles. Owing to the orbital eccentricity,
+this distance may increase to 921,000,000 miles, when the planet is in
+aphelion; or decrease to 823,000,000 miles, when it is in perihelion;
+Saturn being therefore 98,000,000 miles nearer to the Sun when in
+perihelion than in aphelion. If gravitation were free to exert its
+influence alone, Saturn would fall into the Sun in 5 years and 2 months.
+
+The distance of Saturn from the Earth varies, according to the position
+of the two planets in their respective orbits. At the time of
+opposition, when the Earth lies between the Sun and Saturn, this
+distance is smallest; while, on the contrary, at the time of
+conjunction, when the Sun lies between the Earth and Saturn, it is
+greatest. Owing, however, to the eccentricity of the orbits of Saturn
+and our globe, and the inclination of their planes to each other, and
+owing also to the variable heliocentric longitude of the perihelion, the
+distance of the two planets from each other at their successive
+conjunctions and oppositions is rendered extremely variable. At present
+it is when the oppositions of Saturn occur in December that this planet
+comes nearest to us; while when the conjunctions take place in June, the
+distance of Saturn from the Earth is the greatest possible. In the
+former case the distance of the planet from our globe is only
+730,000,000 miles; while in the last it is 1,014,000,000 miles, the
+difference between the nearest and farthest points of Saturn's approach
+to us being no less than 284,000,000 miles, or over three times the mean
+distance of the Sun from the Earth.
+
+From the great variations in the distance of Saturn from the Earth,
+necessarily result corresponding changes in the brightness and apparent
+diameter of this body. When it is farthest from us, its angular diameter
+measures but 14"; while, when it is nearest, it measures 20".
+
+The orbit of Saturn is inclined 2°30' to the ecliptic, and its
+eccentricity, which equals 0,056, is over three times that of the
+Earth's orbit.
+
+This planet revolves around the Sun in a period of 29 years and 5½
+months, or 10,759 terrestrial days, which constitutes its sidereal year.
+The extension of the immense curve forming the orbit of this planet, is
+no less than 5,505,000,000 miles, which is traversed by the planet with
+a mean velocity of a little less than 6 miles per second, or three times
+less than the motion of our globe in space.
+
+The real dimensions of the globe of Saturn are not yet known with
+accuracy, and the equatorial diameter has been variously estimated by
+observers, at from 71,000 to 79,000 miles. If we adopt the mean of these
+numbers, 75,000 miles, the circumference of the Saturnian equator would
+measure 235,620 miles, or 9½ times the circumference of our globe; the
+surface of Saturn would be 86 times, and its volume over 810 times that
+of the Earth.
+
+However great the volume of Saturn, its mass is proportionally small,
+being only 90 times greater than that of our globe; the mean density of
+the materials composing this planet being less than that of cork, and
+only 0.68 the density of water. The force of gravitation at the surface
+of Saturn is greater, by a little over ⅑, than it is at the surface of
+the Earth; a body falling in a vacuum at its surface, would travel 17.59
+feet during the first second.
+
+From observations of markings seen on the surface of Saturn, and from
+the study of their apparent displacements on the disk, William Herschel
+found that the planet rotated upon its axis in 10h. 16m. 0.24s. Since
+Herschel's determination, new researches have been made, and lately,
+Professor Hall, noticing a bright spot, followed it for nearly a month,
+observing its transits across the central meridian of the disk. From
+these observations he has obtained for the rotation period 10h. 14m.
+23.8s., a result which agrees very closely with that obtained 82 years
+earlier by Herschel, considering the fact that the markings from which
+the period of rotation is ascertained are not fixed on the planet, but
+are always more or less endowed with proper motion. The velocity of
+rotation at the equator is 21,538 miles per hour, or nearly 6 miles per
+second.
+
+The axis of rotation of Saturn is inclined 64° 18' to the plane of the
+orbit, so that its equator makes an angle of 25° 42' with the same
+plane. The seasons of this planet therefore present greater extremes of
+temperature than those of the Earth, but not quite so great variations
+as the seasons of Mars.
+
+The globe of Saturn is not a perfect sphere, but its figure is that of
+an oblong spheroid, flattened at the poles. The polar compression of
+Saturn is greater than that of any other planet, surpassing even that of
+Jupiter. Though not yet determined with a great degree of accuracy, the
+compression is known to be between ²⁄₁₈ and ⅒ of the equatorial
+diameter; that is, a flattening of about 3,894 miles, at each pole, the
+polar diameter being 7,788 miles shorter than the equatorial.
+
+The internal condition of the planet Saturn, whether solid, liquid or
+gaseous, cannot be discovered from the examination of its surface, as
+its globe is enwrapped in a dense opaque layer of vapors and cloud-like
+forms, through which the sight fails to penetrate. The appearance of
+this vapory envelope is like that of _cumulus_ clouds, and one of its
+characteristics is to arrange itself into alternate bright and dark
+parallel belts, broader than those seen on Jupiter, and also more
+regular and dark. These belts, which are parallel to the equator of the
+planet, vary in curvature with the inclination of its axis of rotation
+to the line of sight.
+
+The belts of Saturn, like those of Jupiter, are not permanent, but keep
+changing more or less rapidly. Sometimes they have been observed to be
+quite numerous; while at other times they are few. Occasionally
+conspicuous white or dark spots are seen on the surface, although the
+phenomenon is quite rare. It is from the observation of such spots that
+Saturn's period of rotation has been determined, as stated above. The
+equatorial zone of Saturn always appears more white and brilliant than
+the other parts, as it also appears more mottled and cloud-like. In late
+years the globe has been characterized, and much adorned, by a pale
+pinkish tint on its equatorial belt, resembling that of Jupiter, but
+somewhat fainter. On either side of the equatorial belt there is a
+narrower band, upon which the mottled appearance is visible. Below
+these, one or two dark belts, separated by narrow white bands, are
+usually seen; but, of late, the bands have been less numerous, being
+replaced in high latitudes by a dark segment, which forms a polar cap to
+Saturn. The globe of Saturn does not anywhere appear perfectly white,
+and when compared with its ring, it looks of a smoky yellowish tint,
+which becomes an ashy gray on its shaded parts. It usually appears
+darker near the limb than in its central portions; although on some
+occasions I have seen portions of the limb appear brighter, as if some
+white spots were traversing it.
+
+Some observers have seen the limb deformed and flattened at different
+places, and W. Herschel even thought such a deformation to be a
+permanent feature of this globe, which he termed diamond-shaped, or
+"square shouldered." But this was evidently an illusion, since the
+planet's limb usually appears perfectly elliptical, although it
+occasionally appears as if flattened at some points, especially where it
+comes in apparent contact with the shadow cast by the globe on the ring,
+as observed by myself many times. But with some attention, it is
+generally found that this deformation is apparent rather than real, and
+is caused by the passage of some large dark spots over the limb, which
+is thus rendered indistinguishable from the dark background upon which
+it is projected.
+
+What distinguishes Saturn from all known planets, or heavenly bodies,
+and makes it unique in our universe, is the marvelous broad flat ring
+which encircles its equator at a considerable distance from it. With a
+low magnifying power this flat ring appears single, but when carefully
+examined with higher powers, it is found to consist of several distinct
+concentric rings and zones, all lying nearly in the same plane with the
+planet's equator.
+
+
+[PLATE X.--THE PLANET SATURN.
+
+Observed on November 30, 1874, at 5h. 30m. P.M.]
+
+
+At first sight only two concentric rings are recognized, the _outer_ and
+the _middle_, or _intermediary_, which are separated by a wide and
+continuous black line, called the _principal division_. This line, and
+indeed all the features of the surface of the rings are better seen, and
+appear more prominent on that part of the ring on either side called the
+_ansa_, or handle. Besides these two conspicuous rings, a third, of very
+dark bluish or purplish color, lies between this middle ring, to which
+it is contiguous, and the planet. This inner ring, which is quite wide,
+is called the _gauze_ or _dusky ring_. Closer examination shows that the
+outer ring is itself divided by a narrow, faint, grayish line called the
+pencil line, which, from its extreme faintness, is only visible on the
+ansæ. Moreover, the middle ring is composed of three concentric zones,
+or belts, which, although not apparently divided by any interval of
+space, are distinguished by the different shadings of the materials
+composing them. The outer zone of this compound middle ring is, by far,
+the brightest of all the system of rings and belts, especially close to
+its external border, where, on favorable occasions, I have seen it
+appear on the ansæ as if mottled over, and covered throughout with
+strongly luminous cloud-like masses. On the ansæ of the double outer
+ring, similar cloudy forms have also been seen at different times. The
+second zone of the middle ring is darker than the first, the innermost
+being darker still. All the characteristic points which have thus been
+described, are shown in Plate X.
+
+Although suspected in 1838, the dusky ring was not recognized before
+1850, when G. P. Bond discovered it with the 15-inch refractor of the
+Cambridge Observatory. It was also independently discovered the same
+year in England by Dawes and Lassell. The dusky ring differs widely in
+appearance and in constitution from the other rings, inasmuch as these
+last are opaque, and either white or grayish, while the former is very
+dark, and yet so transparent that the limb of the planet is plainly seen
+through its substance. On particularly favorable occasions, the
+appearance of this ring resembles that of the fine particles of dust
+floating in a ray of light traversing a dark chamber. Whatever may be
+the material of which this ring is composed, it must be quite rarefied,
+especially towards its inner border, which appears as if composed of
+distinct and minute particles of matter feebly reflecting the solar
+light. That the inner part of the dusky ring is composed of separate
+particles, is proved by the fact that the part of the ring which is seen
+in front of the globe of Saturn has its inner border abruptly deflected
+and curved inward on entering upon the disk, causing it to appear
+considerably narrower than it must be in reality, a peculiarity which is
+shown in the Plate. This phenomenon may be attributed to an effect of
+irradiation, due to the strong light reflected by the central parts of
+the ball, which so reduces the apparent diameter of the individual
+particles that they become invisible to us, especially those near the
+inner border, which are more scattered and less numerous than elsewhere.
+
+The dusky ring, which was described by Bond, Lassell and other
+astronomers as being equally transparent throughout all its width, has
+not been found so by me in later years. The limb of the planet, seen by
+these observers through the whole width of the dusky ring in 1850, could
+not be traced through its outer half by myself in 1872 and 1874, and
+this with the very same instrument used by Bond in his observations of
+1848 and 1850. Moreover, I have plainly seen that its transparency was
+not everywhere equal, but greatest on the inner border, from which it
+gradually decreases, until it becomes opaque, as proved by the gradual
+loss of distinctness of the limb, which vanishes at about the middle of
+the dusky ring. These facts, which have been well ascertained, prove
+that the particles composing this ring are not permanently located, and
+are undergoing changes of relative position. It will be shown that the
+surface of the other rings is also subject to changes, which are
+sometimes very rapid.
+
+The globe of Saturn is not self-luminous, but opaque. It shines by the
+solar light, as is proved by the shadow it casts opposite the Sun upon
+the ring. Although receiving its light from the Sun, Saturn does not
+exhibit any traces of phases, like the other planets nearer to the Sun,
+owing to its great distance from the Earth. When near its quadratures,
+however, the limb opposite to the Sun appears much darker, and shows
+traces of twilight. As far as can be ascertained, the rings, with the
+exception of the inner one, are opaque, as proved by the strong shadow
+which they cast on the globe of Saturn.
+
+The shadows cast by the planet on the ring, and by the ring on the
+planet, are very interesting phenomena, inasmuch as they enable the
+astronomer to recognize the form of the surface which receives them. The
+shadow cast by the ring on the ball is not quite so interesting as the
+other, although it has served to prove that the surface of this globe is
+not smooth, as is likewise suggested by its mottled appearance. I have
+sometimes found, as have also other observers, that the outline of this
+shadow upon the ball was irregular and indented, an observation which
+proves either that the surface of the ball is irregular, or that the
+border of the ring casting the shadow was jagged. The shadow of the
+globe on the rings has much more interest, as it enables us to get at
+some knowledge of the form of the surface of the rings, which otherwise
+is very difficult to discover, owing to the oblique position in which we
+always see them.
+
+In general, the shadow of the ball on the middle ring has its outline
+concave towards the planet; while on the outer ring it is usually
+slanting, and at a greater distance from the limb than on the middle,
+and dusky rings. This form of the shadow evidently proves that the
+middle ring stands at a higher level than the two others, especially
+towards its outer margin. The system seems to increase gradually in
+thickness from the inner border of the dusky ring to the vicinity of the
+outer margin of the middle ring, after which it rapidly diminishes on
+this border, while the surface of the outer ring is almost level.
+
+But this surface is by no means fixed, as its form sometimes changes, as
+proved by my observations and those of others. As may be noticed on
+Plate X., the outline of the shadow of the planet on the rings is
+strongly deviated towards the planet, near the outer margin of the
+middle ring; the notch indicating an abrupt change of level, and a rise
+of the surface at that point. Some observers have endeavored to explain
+these deviations by the phenomena of irradiation, from which it would
+follow that the maximum effect of deviation should be observed where the
+ring is the brightest, which does not accord with observation; as the
+deepest depression in the shadow is not to be found usually at the
+brightest part, which is towards the outer border of the middle ring,
+but occurs near its centre. From these observations it is undoubtedly
+established that the surface of the rings is far from being flat
+throughout, and is, besides, not permanent, but changes, as would, for
+instance, the surface of a large mass of clouds seen from the top of a
+high mountain. In general, the system is thickest not very far from the
+outer border of the intermediary ring.
+
+Some interesting phenomena which I had occasion to observe before and
+after the passage of the Sun through the plane of the rings, on February
+6th, 1878, conclusively show that the surface of this system cannot be
+of a uniform level, but must be thicker towards the outer border of the
+middle ring, thence gradually sloping towards the planet. Many of my
+observations irresistibly lead to this conclusion. As it would, however,
+be out of place to have them recorded here in detail, I will simply give
+one of the most characteristic among them.
+
+From December 18th, 1877, when the Sun was about 41' above the plane of
+the rings, to February 6th, 1878, the day of its passage through their
+plane, the illuminated surface of this system gradually decreased in
+breadth with the lowering of the Sun, until it was lost sight of,
+February 5th, on the eve of the passage of the Sun through their plane.
+The phenomenon in question consisted in the gradual invasion of their
+illuminated surface by what appeared to be a black shadow, apparently
+cast by the front part of the outer portion of the middle ring the
+nearest to the Sun. On January 25th, when the elevation of the Sun above
+the plane of the rings was reduced to 15', the shadow thus cast had
+extended so far on their surface that it reached the shadow cast by the
+globe on the opposite part of the ring in the east, and accordingly the
+remaining portion of the illuminated surface of the eastern ansa then
+appeared entirely disconnected from the ball, by a large dark gap,
+corresponding in breadth to that of the globe's shadow on the rings. On
+February 4th, when the Sun was only 5' above the plane of the rings,
+their illuminated and only visible surface was reduced to a mere thread
+of light, which on the 5th appeared broken into separate points. It is
+evident that the phenomenon was not caused by the obliquity of the ring
+as seen from our globe, since the elevation of the Earth above the plane
+of the rings--which on December 18th was 3° 20'--was still 1° 20' on
+the 4th of February. In ordinary circumstances, when the Sun is a little
+more elevated, and the rings seen at this last angle, they appear quite
+broad and conspicuous, and even the dark open space separating the dusky
+ring from the planet is perfectly visible on the ansæ, where the
+Earth's elevation above their plane is reduced to 40'. It is also
+evident that the phenomenon was not to be attributed to the reduction of
+the light which they received from the Sun, although the illumination in
+February might be expected to be comparatively feeble, since the Sun
+then shone upon the rings so obliquely; yet (on the supposition that
+their surface is flat) they should have been illuminated throughout, and
+if not very brightly, sufficiently so, at least, to make them visible
+and as bright as was the narrow thread of light observed on the 4th of
+February. The phenomenon actually observed may be explained most readily
+by assuming, as other phenomena also indicate, that the surface of the
+ring is not flat, but more elevated towards, or in the vicinity of its
+outer border, from which place it slopes inwardly towards the planet. On
+this assumption, it is evident that the elevated part of the ring the
+nearest to the Sun would cast a shadow, which, with the increasing
+obliquity of the Sun, would gradually cover the whole surface comprised
+within the elevated part, and thus become invisible to us. Several
+observations made by Bond and other observers undoubtedly show the same
+phenomenon, and do not seem to be intelligible on any other supposition.
+From my observations made in 1881 it would appear, however, that the
+opposite surfaces of the rings do not exactly correspond in form, but
+this may not be a permanent feature, as the surface of this system is
+subject to changes, as already shown.
+
+The dimensions of the rings are great, the diameter of the outer one
+being no less than 172,982 miles, the distance from the centre of the
+globe to the outer border of the system being, therefore, 86,491 miles.
+The breadth of the outer ring is 9,941 miles; that of the principal
+division, 2,131 miles; that of the middle ring, 19,902 miles, and that
+of the dusky ring, 8,772 miles. The breadth of all the rings taken
+together is, therefore, 40,746 miles. The interval between the surface
+of Saturn and the inner border of the dusky ring is 7,843 miles.
+
+The thickness of the system of rings has been variously estimated by
+astronomers, on account of the great difficulties attending its
+determination. While Sir John Herschel estimated it at more than 250
+miles, G. P. Bond reduces it to 40 miles. Both of these numbers are
+evidently too small, as so slight a thickness cannot explain the
+observed phenomenon of the shadow cast by a portion of the ring on its
+own surface, when the Sun is very low in its horizon, as shown above.
+
+The plane of the system of rings is inclined 27° to the planet's orbit,
+and is parallel, or at least very nearly so, with the equator of the
+planet, passing, therefore, through its centre, and dividing its globe
+into northern and southern hemispheres. Seen from the Earth, a portion
+of the ring always appears projected in front of the planet, thus
+concealing a small part of its globe, while the opposite portion passes
+behind the globe, which hides it from sight.
+
+As the plane of the ring is not affected by the motion of the planet
+around the Sun, but always remains parallel to itself, it follows that
+as Saturn advances in its orbit the rings must successively present
+themselves to us under various angles of inclination, appearing,
+therefore, more or less elliptical, and presenting two maxima and two
+minima of inclination in the course of one of its revolutions. As the
+revolution of Saturn is accomplished in 29½ years, the maxima and the
+minima must recur every 14 years and 9 months; the maxima being
+separated from the minima by an interval of 7 years and 4½ months.
+
+When Saturn arrives at the two opposite points of its orbit, where the
+major axis of its ring is at right angles to the line joining its centre
+to that of the Sun, the ring, which is then viewed at an inclination of
+27°, the greatest angle at which it can ever be seen, has reached its
+maximum opening, the smaller diameter of its ellipse being then about
+half that of the larger. At this moment the outer ring projects north
+and south beyond the globe, which is then completely enclosed in its
+ellipse. The maximum opening of the northern surface of the ring takes
+place, at present, when Saturn arrives in longitude 262°, in the
+constellation Sagittarius, and that of the southern surface when it
+arrives in longitude 82° in the constellation Taurus. When, on the
+contrary, Saturn reaches the two opposite points of its orbit, where the
+plane of its ring is parallel to the line joining its centre to that of
+the Sun, the opening vanishes, as only the thin edge of the ring is then
+presented to the Sun and receives its light, the rest being in darkness.
+At this moment the ring disappears, except in the largest telescopes,
+where it is seen as an exceedingly thin thread of light; and the
+Saturnian globe, having apparently lost its ring, appears solitary in
+the sky, like the other planets. The disappearance of the ring from this
+cause occurs now when Saturn arrives at 90° from either of the
+positions of maximum inclination, that is, in longitude 352° in the
+constellation Pisces, and in longitude 172° in the constellation Leo.
+
+When the planet is in any other position than one of these last two,
+either the northern or the southern surface of the ring is illuminated
+by the Sun, while the opposite surface is in the night, and does not
+receive any direct sunlight. At the time of the passage of the plane of
+the ring through the Sun's centre, a change takes place in the
+illumination of the ring. If it is the northern surface which has
+received the rays of the Sun during the previous half of the Saturnian
+year, at the moment the plane has passed the centre of the Sun, the
+southern surface, after having been buried in darkness for 14¾ years,
+sees the dawn of its long day of the same length. Such a phenomenon will
+not occur until 1892, when the passage of the Sun from the northern to
+the southern side of the ring will close in twilight the day commenced
+in 1878.
+
+Aside from the periodic disappearance of the ring, resulting from the
+passage of the Sun through its plane, the ring may also disappear from
+other reasons. Just before or just after the time of the passage of the
+Sun through the plane of the ring, the Earth and the Sun may occupy such
+positions, that while the one is north of the plane of the ring, the
+other is south of it, or vice versa, in which event the ring becomes
+invisible, because its dark and non-illuminated surface is presented to
+us. The ring may also become invisible to us when the Earth passes
+through its plane.
+
+Since the distance from Saturn to the Sun is to the distance of the
+Earth from this last body as 9.54 is to 1; and since the circumference
+of a circle increases in the same proportion as its radius, it follows
+that the diameter of the Earth's orbit projected on the orbit of Saturn
+would occupy only part of the latter, or about 12° 2', this being 6°
+1' on either side of the nodes of the rings. To describe such an arc on
+its orbit, it takes Saturn almost 360 days on an average, or almost a
+complete year; the Earth describing therefore almost a whole revolution
+around the Sun during the time it takes Saturn to advance 12° 2' on its
+orbit. Then, when Saturn occupies a position comprised within an arc 6°
+1' from either side of the nodes of its ring, the Earth, by its motion,
+is liable to encounter the plane of the ring, when therefore it will
+only present its thin edge to us, and becomes invisible. At least one
+such encounter is unavoidable within the time during which Saturn
+occupies either of these positions on its orbit; while three frequently
+happen, and two are possible.
+
+The natural impression received by looking at the rings, while seeing
+the ponderous globe of Saturn enclosed in its interior, is that this
+gigantic, but very delicate structure, in order to avoid destruction,
+must be endowed with a swift movement of rotation on an axis
+perpendicular to its plane, and that the centrifugal force thence
+arising counterbalances the powerful attraction of the planet, and thus
+keeps the system in equilibrium.
+
+Theoretically, the rotation of the rings is admitted by every
+astronomer, as being an essential condition to the existence of the
+system, which otherwise, it is thought, would fall upon the planet.
+Although the rotation of the rings seems so probable that it is
+theoretically considered as certain, yet its existence has not been
+satisfactorily demonstrated by direct observation, which alone can
+establish it on a firm basis as a matter of scientific knowledge.
+
+The determination of the period of rotation of the rings, which is
+supposed to be 10h. 32m. 15s., rests only on the observations of W.
+Herschel, made in 1790, from the apparent displacement of irregularities
+on the ring; but his results have been contradicted by other
+observations, and even by those of Herschel himself, made in later
+years.
+
+Although the system of rings is very nearly concentric with the globe of
+Saturn, yet the coincidence is not considered as mathematically exact.
+It seems to have been satisfactorily demonstrated by direct observations
+that the centre of gravity of the system oscillates around that of the
+planet, thus describing a minute orbit. This peculiarity is in
+accordance with theory, which has shown it to be essential to the
+stability of the system.
+
+Besides its system of rings, which makes Saturn the most remarkable
+planet of the solar system, this globe is attended by eight satellites,
+moving in orbits whose planes very nearly coincide with the plane of the
+rings, except that of the most distant one, which has an inclination of
+about 12° 14'. In the order of their distance from the planets, the
+satellites of Saturn are as follows: Mimas, Enceladus, Tethys, Dione,
+Rhea, Titan, Hyperion and Iapetus. The three first satellites are nearer
+to Saturn than the Moon is to the Earth; while Iapetus, the farthest, is
+9½ times the distance of our satellite from us. All the satellites,
+with the exception of the farthest, move more rapidly around Saturn than
+the Moon moves around the Earth; while Iapetus, on the contrary, takes
+almost three times as long to make one revolution.
+
+The period of revolution of the four inner satellites is accomplished in
+less than three days, that of Mimas being only a little more than 22
+hours. From such swiftness of motion, it is easily understood how short
+must be the intervals between the different phases of these satellites.
+Mimas, for instance, passes from New Moon to First Quarter in less than
+6 hours.
+
+The distance of the nearest satellite from the planet's surface is
+84,000 miles, and its distance from the outer ring only 36,000 miles. It
+is difficult to determine the diameter of objects so faint and distant
+as are some of these satellites, but the diameter of Titan, the largest
+of all, is pretty well known, and estimated to be ¹⁄₁₆ the
+diameter of the planet, or more than half the diameter of our globe.
+
+Iapetus is subject to considerable variations in brilliancy, and as the
+maxima and minima always occur when this satellite occupies the same
+parts of its orbit, it was conjectured by W. Herschel that, like our
+Moon, it turns once upon its axis during each of its revolutions about
+the planet. It has been shown by my observations, that Iapetus attains
+its maximum brightness a little before it reaches its greatest western
+elongation, and its minimum on the opposite side.
+
+As the planes of the orbits of the satellites are inclined to the
+planet's orbit, it follows that their transits, occultations and
+eclipses, are only possible when Saturn is near its equinoxes. Passages
+of the satellites and their shadows across the disk, although rare, have
+been observed, and they somewhat resemble the phenomena exhibited by the
+satellites of Jupiter in transit. When the Earth is very near the plane
+of the rings, the satellites, except the farthest, appear to be in a
+straight line nearly coincident with the plane of the rings, and are
+seen occasionally moving along the thin edge of the rings, appearing as
+luminous beads moving on a thread of light.
+
+Owing to the considerable inclination of the axis of rotation of Saturn
+to its orbit, the seasons of this planet must have greater extremes of
+temperature than those of the Earth. As the year of Saturn consists of
+25,217 Saturnian days, each season, on the average, is composed of 6,304
+Saturnian days.
+
+To an observer on Saturn, the immense arches formed by its rings would
+appear as objects of great magnificence, spanning the sky like soft
+colorless rainbows. Moreover, the eight moons, several of which are
+always visible, would be of the highest interest, with their swift
+motions and rapid phases. Mimas, traveling in its orbit at the rate of
+16 of arc per minute of time, moves over a space equal to the apparent
+diameter of our Moon in two minutes, or at the rate of 16° an hour.
+
+Owing to the globular form of Saturn, the rings would be invisible in
+latitudes situated above 65° from its equator, and their apparent form
+and breadth would naturally vary with the latitude. At 63° only a very
+small portion of the outer ring would be visible above the equatorial
+horizon, where it would appear as a small segment of a circle. At 62°
+the principal division would just graze the horizon. At 46° the outer
+portion of the dusky ring would become visible, while at 35° its inner
+edge would appear above the horizon. From 65° of latitude down to the
+equator, the arches of the rings would be seen more and more elevated
+above the equatorial horizon, but at the same time that they are seen
+higher up, their apparent breadth gradually diminishes, owing to the
+effect of foreshortening, and at the equator itself the system would
+only present its thin edge to view.
+
+During the summer seasons of either hemisphere of Saturn, the surface of
+the rings turned towards such hemisphere, being fully illuminated by the
+Sun, is visible from these regions. In the day time its light must be
+feeble and similar to the light reflected by our Moon during sunshine;
+but at night the system would display all its beauty, and the different
+rings, with their divisions and their various reflective powers, must
+present a magnificent sight.
+
+During the nights of the long winter seasons on Saturn, on the contrary,
+the surface of the rings turned towards the hemisphere undergoing
+winter, receives no light from the Sun, and is invisible, or very nearly
+so, except towards morning and evening, when it may be faintly
+illuminated by the secondary light which it receives from the
+illuminated globe of Saturn. Although dark and invisible, the rings may
+make their form apparent at night by the absence of stars from the
+region which they occupy in the sky. Again, in other seasons, the days
+present very curious phenomena. In consequence of the diurnal rotation
+of the planet, the Sun seems to move in circular arcs, which, owing to
+the inclination of Saturn's axis, are more or less elevated above its
+horizon, according to the position of the planet in its orbit. As such
+arcs described by the Sun in the sky of Saturn are liable to encounter
+the rings, the Sun in passing behind them becomes eclipsed. It must be a
+magnificent spectacle to witness the gradual disappearance of the fiery
+globe behind the outer ring, and its early reappearance, but for a
+moment only, through the narrow gap of the principal division; to see it
+vanish again behind the middle ring, to reappear a little later through
+the semi-transparent dusky ring, but very faint and red colored at
+first; and then, gradually brighten up, and finally emerge in all its
+beauty from the inner edge of the dusky ring.
+
+It is in latitude 23° that the rings produce the most prolonged
+eclipses of the Sun. During a period equivalent to ten of our
+terrestrial years, such eclipses continually succeed each other with but
+very short periods of interruption; and even during a long series of
+rotations of Saturn, the Sun remains completely invisible in those
+regions where the apparent arcs which it describes coincide with the
+arcs of the rings. In neighboring latitudes, the eclipses of the Sun,
+although still frequent, would have a shorter and shorter duration as
+the observer should travel north or south. These eclipses of the Sun
+must produce a partial darkness of the regions involved in the shadow of
+the rings, which may be compared to the darkness produced on our globe
+by a total eclipse of the Sun. The frequent recurrence of these
+eclipses, and their comparatively long duration in some regions, must
+still further reduce the duration of the short Saturnian days.
+
+The globe of Saturn, as already shown, casts a shadow on the rings,
+which, according to the position of the planet in its orbit, either
+extends across their whole breadth, or covers only a part of their
+surface. The shadow on the rings rising in the east after sunset,
+ascends to the culminating point of their arcs in the sky, in 2h. 34m.,
+and as rapidly descends on the western horizon, to disappear with
+sunrise. This shadow, when projected on the rings in the sky, must be
+hardly distinguishable from the dark background of the heavens, except
+from the absence of stars in the regions which it occupies. It must
+appear as a large dark gap, separating the rings into two parts, and
+constantly moving from east to west. Possibly the refraction of the
+solar rays, in passing through Saturn's atmosphere, may cast some
+colored light on the rings, similar to that observed on the Moon during
+its eclipses.
+
+An observer on the rings would behold phenomena still more curious, a
+long day of 14¾ years being followed by a long night of 14¾ years. The
+long days of Saturn's rings are, however, diversified by numerous
+eclipses of the Sun, which regularly occur every 10¼ hours; the
+phenomenon being due to the interposition of the globe of Saturn between
+the rings and the Sun. These eclipses produce partial obscurations of
+their surface, lasting from 1½ to 2 hours at a time. Although the
+surface of the rings never receives direct sunlight during their long
+nights, yet they are not plunged all the time in total darkness, as they
+receive some reflected light from that part of the globe of Saturn which
+is illuminated by the Sun. To the supposed observer on the rings, during
+every 10¼ hours, the immense globe would exhibit continually changing
+phases. At first he would see a point of light rapidly ascending from
+the horizon, and appearing under the form of a half crescent of
+considerable radius; 5⅛ hours later, the crescent having gradually
+increased, would appear as a half circle, covering ⅛ of the visible
+heavens, its surface being more than 20,000 times as large as the
+surface of the Moon. Upon this brilliantly illuminated semi-circle would
+be projected the shadows of the rings, appearing as black belts
+separated by a narrow luminous band.
+
+It is very difficult for one to conceive how such a delicate structure,
+as the system of rings appears to be, can keep together in equilibrium
+and avoid destruction from the powerful attraction of the planet on one
+side and the disturbing influence of the satellites on the other. To
+explain it, several hypotheses have been advanced. The rings were first
+supposed to be solid, and upon this supposition Laplace determined the
+necessary conditions for their equilibrium; the most important of which
+require that the cross section of the rings should be an ellipse of
+irregular curvature, and having its major axis directed towards the
+centre of the planet, and also that the system should rotate upon an
+axis perpendicular to the plane of the rings. This theory was superseded
+by another, which supposed the rings to be fluid. This one was soon
+rejected for a third, assuming the system to be composed of vapors or
+gases; and more recently, all these theories were considered untenable,
+and replaced by a fourth, which supposes the system of rings to be made
+up of a congregation of innumerable small, independent bodies, revolving
+around Saturn in concentric zones. Naturally, such a divergence of
+opinion can only result from our comparative ignorance of the subject,
+and sufficiently indicates our inability to explain the phenomena; and
+it must be admitted that, so far, nothing is certainly known about this
+strange system. We shall probably remain in the same uncertainty until
+the rotation of the rings is ascertained by direct observations. It is
+pretty certain, however, that none of these theories account for the
+observed phenomena in their details, although a partial explanation may
+be obtained by borrowing something from each hypothesis.
+
+It has been conjectured, and a theory has been advanced, that the
+breadth of the whole ring system is gradually increasing inwards, and
+that it will come in contact with the planet in about 2,150 years; but
+the question seems to have been settled in the negative by the elaborate
+measurements of the English observers. It is likely that the increase is
+only in the defining power of the instruments.
+
+
+
+
+COMETS
+
+PLATE XI
+
+
+Among the celestial phenomena, none are more interesting than those
+mysterious apparitions from the depths which unexpectedly display their
+strange forms in our familiar constellations, through which they wander
+for a time, until they disappear like phantoms.
+
+A comet, with its luminous diffused head, whence proceeds a long vapory
+appendage gradually fading away in the sky, presents an extraordinary
+aspect, which may well astonish and deeply impress the observer.
+Although these visitors from infinite space do not now inspire dread, as
+in by-gone times, yet, owing to the mystery in which the phenomenon is
+still involved, the apparition of a large comet, even in our days, never
+fails to create a profound sensation, and in some cases that unconscious
+fear which results from the unknown.
+
+The effect of such a spectacle largely depends upon its rarity; but
+since the telescope has been applied to the sounding of the heavens, it
+has been found that the appearance of comets is by no means an unusual
+occurrence. If so few comets, comparatively, are seen, it is because
+most of them are telescopic objects, and are therefore invisible to the
+naked eye. Most of the telescopic comets are not only too faint to be
+perceived by the unaided eye, but are insignificant objects, even when
+observed through the largest telescopes.
+
+It was Kepler's opinion that comets are as numerous in the sky as fishes
+are in the ocean. Undoubtedly the number of these bodies must be great,
+considering that we can only see them when they come into the
+neighborhood of the Earth, and that many even here remain invisible, or
+at least pass unperceived. That many of them have passed unperceived
+heretofore, is proved by the fact that the number of those observed
+becomes greater every year, with the increase of the number of
+instruments used in their search. The number of comets observed with the
+naked eye during historic times is nearly 600, and that of telescopic
+comets, which, of course, all belong to the last few centuries, is more
+than 200, so that we have a total number of about 800 comets of which
+records have been kept. From theoretical considerations, Lambert and
+Arago estimated their entire number at several millions, but such
+speculations have generally no real value, since they cannot be
+established on a firm basis.
+
+Comets remain visible for more or less time, according to their size and
+the nature and position of their orbits, but in general, the large ones
+can be followed with the telescope for several months after they have
+become invisible to the naked eye. The comet of 1861, for example,
+remained telescopically visible for a year, and that of 1811, for 17
+months after disappearing from ordinary sight.
+
+While a comet remains visible, it appears to revolve daily about us like
+the stars in general; but it also moves among the constellations, and
+from this movement its orbit may be computed like that of a planet. From
+the apparent diurnal motion of a comet with the heavens, result the
+changes of position which it seems to undergo in the course of a night.
+The direction of the head and tail of a comet, of course, has only
+changed in regard to the horizon, but not in regard to the sky, in which
+they occupy very nearly the same position throughout a given night, and
+even for many nights in succession.
+
+The movements of the comets in their orbits are, like those of the
+planets, in accordance with Kepler's laws, the Sun occupying one of the
+foci of the orbit they describe; but the orbits of comets differ,
+however, in several points from those of the planets. Their eccentricity
+is always great, being sometimes apparently infinite, in which case the
+orbit is said to be parabolic, or hyperbolic; but the smallness of the
+portion of a cometary orbit which can ordinarily be observed, makes it
+difficult to determine this with certainty. Again, while the planetary
+orbits are usually near the plane of the ecliptic, those of comets
+frequently have great inclinations to that plane, and even when the
+inclination is less than 90°, the comet may have a retrograde movement,
+or, in other words, a movement contrary to the course in which all the
+planets revolve about the Sun.
+
+Notwithstanding these differences between the elements of the orbits of
+the comets and those of the planets, the fact that each has the Sun in
+one focus indicates that the body moving in it is a member of the solar
+system, either for the time, or permanently, according to the nature of
+its orbit.
+
+A distinction may accordingly be made between the comets which are
+permanent members of our solar system and those which are only
+accidental or temporary visitors. Those moving in elliptical orbits
+around the Sun, like the planets, and therefore having a determinate
+period of revolution, from which the time of their successive returns
+may be predicted, are permanent members of our system, and are called
+periodic comets. All comets moving in parabolical or hyperbolical
+curves, are only temporary members of the solar system, being apparently
+strangers who have been diverted from their courses by some disturbing
+influence. No comet is classed as periodical which does not follow a
+perceptibly elliptical orbit. Any comet passing around the Sun at the
+mean distance of the Earth from this body, with a velocity of 26 miles
+per second, will fly off into infinite space, to return to us no more.
+
+The time of revolution of the different periodic comets thus far
+observed varies greatly, as do also the distances to which they recede
+from the Sun at aphelion. Whilst the period of revolution of Encke's
+comet, the shortest thus far known, is only 3½ years, that of the comet
+of 1844, II., is 102,000 years; and whilst the orbit of the first is
+comprised within the orbit of Jupiter, that of the last extends to a
+distance equal to 147 times the distance of Neptune from the Sun. But so
+vast an orbit cannot be accurately determined from the imperfect data at
+our disposal.
+
+The periodic comets are usually divided into two classes. The comets
+whose orbits are within the orbit of Neptune are called interior comets,
+while those whose orbits extend beyond that of Neptune are called
+exterior comets. The known interior periodic comets are twelve in
+number, while, including all the cases in which there is some slight
+evidence of elliptic motion, the number of exterior comets observed is
+six or seven times as great. The periodic comets of short period are
+very interesting objects, inasmuch as by their successive returns they
+afford an opportunity to calculate their motions and to observe the
+physical changes which they undergo in their intervals of absence.
+
+From observation of the periodic comets, it has been learned that the
+same comet never presents twice the same physical appearance at its
+different returns, its size, shape and brilliancy varying so greatly
+that a comet can never be identified by its physical characters alone.
+It is only when its elements have been calculated, and are found to
+agree with those of a cometary orbit previously known, that the two
+comets can be identified one with the other. There are reasons to
+believe that, in general, comets decrease in brightness and size at each
+of their successive returns, and that they are also continually losing
+some of their matter as they traverse their orbits.
+
+When very far away from us, all comets appear nearly alike, consisting
+of a faint nebulosity, of varying dimensions. When a comet first appears
+in the depths of space, and travels towards the Sun, it generally
+resembles a faint, uniformly luminous nebulosity, either circular or
+slightly elongated in form. As it approaches nearer to the Sun, a slight
+condensation of light appears towards its centre, and as it draws still
+nearer, it becomes brighter and brighter, and in condensing forms a kind
+of diffused luminous nucleus. At the same time that the comet acquires
+this concentration of light, the nebulosity gradually becomes elongated
+in the direction of the Sun. These effects generally go on increasing so
+long as the comet is approaching the Sun; the condensation of light
+sometimes forms a bright nucleus, comparable to a very brilliant star,
+while the elongation becomes an immense appendage or tail. When the
+comet has passed its perihelion and recedes from the Sun, the inverse
+phenomena are observed; the comet, decreasing in brightness, gradually
+loses its nucleus and tail, resumes its nebulous aspect, and finally
+vanishes in space, to appear again in due course, if it chance to be a
+periodic comet. While all comets become brighter in approaching the Sun,
+they do not all, however, develop a large tail, some of them showing
+only a slight elongation.
+
+When a comet is first discovered with the telescope at a great distance
+from the Sun, it is difficult to predict whether it will become visible
+to the naked eye, or will remain a telescopic object, as it is only in
+approaching the Sun that these singular bodies acquire their full
+development. Thus, Donati's comet, whose tail became so conspicuous an
+object at its full appearance in 1858, remained two months after its
+discovery by the telescope without any indication of a tail. The comet
+of Halley, which before and after its return in 1759, remained five
+years inside of the orbit of Saturn, showed not the least trace of its
+presence during the greater part of this time. Nothing but calculation
+could then indicate the position in the sky of this invisible object,
+which was so prominent when it approached the Sun.
+
+Another curious phenomenon exhibited by comets, and first noticed by
+Valz, is that in approaching the Sun the nebulosity composing these
+bodies contracts, instead of dilating, as would be naturally supposed
+from the greater amount of solar heat which they must then receive. In
+receding from the Sun, on the contrary, they expand gradually. As comets
+approach the Sun, the tail and nucleus are developed, while the
+nebulosity originally constituting these comets contracts, as if its
+material had been partly consumed in this development. In a certain
+sense it may be said that the comets are partly created by the Sun; in
+more exact terms, the changes of form which they undergo are induced by
+the Sun's action upon them at different distances and under varying
+conditions. Moreover, they are rendered visible by its influence,
+without which they would pass unperceived in our sky. When a comet
+disappears from view, it is not because its apparent diameter is so much
+reduced by the distance that it vanishes, but rather on account of the
+diminution of its light, both that which it receives from the Sun, and
+its own light; these bodies being in some degree self-luminous, as will
+be shown below.
+
+The large comets, such as can be seen with the naked eye, always show
+the following characteristics, on examination with the telescope. A
+condensation of light resembling a diffused star forms the brightest
+part of the comet, this condensation being situated towards the
+extremity the nearest to the Sun. It is this starlike object which is
+called the _nucleus_. The nucleus seems to be entirely enclosed in a
+luminous vapory envelope of the same general texture, called the _coma_.
+This envelope, which is quite variable in brightness and form, is
+brightest next to the nucleus, and gradually fades away as it recedes
+from it. The _nucleus_ and the _coma_, considered as a whole, constitute
+the _head_ of a comet. From the head of a comet proceeds a long trail of
+pale nebulous light, which usually grows wider, but fainter, as it
+recedes from the nucleus, and insensibly vanishes in the sky. This
+delicate appendage, or tail, as it is commonly called, varies very much
+in size and shape, not only in different comets, but in the very same
+comet, at different times. Its direction is generally opposite to that
+of the Sun from the head of the comet.
+
+The nuclei vary very much in brightness, in size and in shape; and while
+in some telescopic comets they are either absent or barely
+distinguishable as a small condensation of light, in bright comets they
+may become plainly visible to the naked eye, and they sometimes even
+surpass in brightness the most brilliant stars of the heavens. But
+whatever may be the size of cometary nuclei, they are subject to sudden
+and rapid changes, and vary from day to day. Sometimes they appear
+exceedingly brilliant and sharply outlined, while at other times they
+are so dim and diffused that they are hardly distinguishable from the
+coma of which they seem then to form a part.
+
+
+[PLATE XI.--THE GREAT COMET OF 1881.
+
+Observed on the night of June 25-26 at 1h. 30m. A.M.]
+
+
+From my observations upon the comets which have appeared since the year
+1873, it is apparent that the changes in the nucleus, coma and tail, are
+due to a solar action, which contracts or expands these objects in such
+a manner that the nuclei become either bright and star-like, or dim and
+diffused, in a very short time. I had excellent opportunity, especially
+in the two large comets of 1881, to observe some of these curious
+changes, a description of which will give an idea of their extent and
+rapidity. On July 2d, 1881, at 9 o'clock, the nucleus of comet 1881,
+III., which is represented on Plate XI., appeared sharply defined,
+bright and considerably flattened crosswise; but half an hour later it
+had considerably enlarged and had become so diffused that it could
+hardly be distinguished from the coma, with which it gradually blended.
+It is perhaps worth mention that, at the time this last observation was
+made, an aurora borealis was visible. This comet 1881, III., underwent
+other very important changes of its nucleus, coma and tail. On June
+25th, the nucleus, which was bright and clearly defined, was ornamented
+with four bright diverging conical wings of light, as shown on Plate XI.
+On the 26th these luminous wings had gone, and the nucleus appeared
+one-third smaller. On the 28th it had enlarged, but on the 29th its
+shape was considerably altered, the nucleus extending in one direction
+to three or four times its diameter on previous nights, and being
+curved, so as to resemble a comma. On the 6th of July the nucleus of
+this comet showed the greatest disturbances. The nucleus, which had
+appeared perfectly round on the evening of the 5th, was found much
+elongated at 10 o'clock on the 6th, forming then a straight, acute, and
+well-defined wedge of light, inclined upwards to the left. The length of
+the nucleus, at this time, was three or four times its ordinary
+diameter. At the same time rapid changes occurred; the strangely shaped
+nucleus soon became unsteady, extending and contracting alternately, and
+varying greatly in brightness. At 10h. 45m., the elongated nucleus, then
+gently curved, took the shape of a succession of luminous knots, which
+at times became so brilliant and distinct that they seemed to be about
+to divide and form separate nuclei; but such a separation did not
+actually occur, at least while I was observing. While these important
+changes were going on in the comet, a bright auroral arch appeared in
+the north, which lasted only a short time. On July 7th, the sky being
+cloudy, no observations were made, but on the 8th I observed the comet
+again. The nucleus had then resumed its circular form, but it was yet
+very unsteady, being sometimes small, bright and sharp, while a few
+seconds later it appeared twice as large, but dim in outlines; and
+sometimes an ill-defined secondary nucleus appeared at its centre. On
+several occasions the nucleus appeared as if it were double, one nucleus
+being apparently projected partly upon the other.
+
+The nuclei of comets are sometimes very small, and in other cases very
+large. Among those which have been measured, the nucleus of the comet of
+1798, I., was only 28 miles in diameter, but that of Donati's comet, in
+1858, was 5,600 miles, and that of the comet of 1845 was 8,000 miles in
+diameter.
+
+The coma of comets is found to be even more variable than the nucleus.
+The changes observed in the coma are generally in close connection with
+those of the nucleus and tail, the same perturbations affecting
+simultaneously the whole comet. While the coma of the comet of 1847 was
+only 18,000 miles in diameter, that of Halley's comet, in 1835, was
+357,000 miles, and that of the comet of 1811 was 1,125,000 miles in
+diameter. In general, as already stated, the coma of a comet decreases
+in size in approaching the Sun. That of Encke's comet, which, on October
+9th, 1838, had a diameter of 281,000 miles, gradually decreased at a
+daily mean rate of 4,088 miles in going towards the Sun; so that, on
+December 17th, when the distance of the comet from the Sun was more than
+four times less than it was on the first date, its diameter was reduced
+to 3,000 miles.
+
+The form of the coma, in that part which is free from the tail, is in
+general a portion of a circle, but is sometimes irregular, with its
+border deformed. Thus, the border of the coma of Halley's comet was
+depressed at one point towards the Sun. I observed a similar phenomenon
+in Coggia's comet, with the great refractor of the Harvard College
+Observatory, on July 13th, 1874, when its border appeared deeply
+depressed on the side nearest to the Sun, as if repelled by this body.
+The coma of comet 1881, III., showed also very singular outlines on the
+nights of the 25th and 26th of June, when its border was so deeply
+depressed that the coma appeared as if it were double. Luminous rays and
+jets often radiate from the nucleus across the coma, and describe
+graceful lateral curves, falling backwards and gradually fading away
+into the tail, of which they then form a part. The rays and jets emitted
+by the nucleus seem at first to obey the solar attraction and travel
+towards the Sun; but they are soon repelled, and move backward towards
+the tail. It is a mystery, as yet unexplained, how these cometary jets,
+which at first seem to obey to the laws of attraction, are compelled to
+retreat apparently by superior opposing forces. Among the forces of
+nature, we know of no other than those of an electrical sort, which
+would act in a similar manner; but this explanation would require us to
+assume some direct electrical communication between the comet and the
+Sun. Considering the distance between the two bodies, and the probable
+absence or great tenuity of the gaseous material in interstellar space,
+such an assumption is a difficult one.
+
+Under the action of the solar forces, the coma also very frequently
+forms itself into concentric luminous arcs, separated by comparatively
+dark intervals. These luminous semi-circles vary in number, but
+sometimes there are as many as four or five at a time. All great comets
+show these concentric curves more or less, but sometimes only a portion
+is visible, the rest of the coma having a different structure. When
+great comets approach near the Sun, their coma is generally composed of
+two distinct parts, an inner and an outer coma, the inner one being due
+to the luminous jets issuing from the nucleus, which, never extending
+very far, form a distinct, bright zone within the fainter exterior coma.
+
+The tails of comets, which are in fact a prolongation of the coma, are
+likewise extremely variable in form. They are sometimes straight like a
+rod; again, are curved like a sabre, or even crooked like an S, as was
+that of the comet of 1769. They are also fan-shaped, pointed, or of the
+same width throughout. Many of these appendages appear longitudinally
+divided through their middle by a narrow, darkish rift, extending from
+the nucleus to the extremity. This peculiarity appears in the comet
+shown on Plate XI. Sometimes the dark rift does not commence near the
+nucleus, but at some distance from it, as I observed in the case of
+comet 1881, III., on June 26th. This dark rift is not a permanent
+feature of a comet's tail, but may be visible one day and not at all the
+next. Comet 1881, III., which had shown a dark rift towards the end of
+June, did not exhibit any such rift during July and August, when, on the
+contrary, its tail appeared brighter in the middle. Coggia's comet,
+which showed so prominent a dark rift in July, 1874, had none on June
+10th. On the contrary, the tail was on that date very bright along its
+middle, as also along each of its edges.
+
+The tail of a comet does not invariably point directly away from the
+Sun, as above mentioned, and sometimes the deviation is considerable;
+for instance, the tail of the comet of 1577 deviated 21° from the point
+opposite to the Sun.
+
+In general, the tail inclines its extremity towards the regions of space
+which it has just left, always presenting its convex border to the
+regions towards which it is moving. It is also a remarkable fact that
+this convex border, moving first in space, always appears brighter and
+sharper than the opposite one, which is often diffused. From these
+peculiarities it would seem that in moving about the Sun the comets
+encounter some resistance to their motion, from the medium through which
+they pass, and that this resistance is sufficient to curve their tails
+away from the course in which they move, and to crowd their particles
+together on the forward side. It is especially when they approach their
+perihelion, and move more rapidly on a curve of a shorter radius, that
+the comets' tails show the greatest curvature, unless their position in
+regard to the observer prevents their being advantageously seen. The
+tail of Donati's comet presented a fair illustration of this
+peculiarity, its curvature having augmented with the velocity of the
+comet's motion about the Sun. But possibly this phenomenon has another
+cause, and may be found rather in the solar repulsion which acts on
+comets and is not instantaneously propagated throughout their mass.
+
+Although, in general, comets have but one tail, it is not very rare to
+see them with multiple tails. The comets of 1807 and 1843 had each a
+double tail; Donati's comet, in 1858, showed several narrow, long
+rectilinear rays, issuing from its abruptly curved tail. The comet of
+1825 had five branches, while that of 1744 exhibited no less than six
+distinct tails diverging from the coma at various angles. In general
+character the multiple and single tails are similar. When a comet has
+two tails, it is not rare for the second to extend in the general
+direction of the Sun, as was the case with the great comet of 1881,
+III., represented on Plate XI. From July 14th to the 21st it exhibited
+quite an extended conical tail, starting obliquely downwards from the
+right side of the coma, and directed towards the Sun. From the 24th of
+July to the 2d of August this secondary tail was exactly opposite in its
+direction from that of the primary tail, and gave to the head a very
+elongated appearance. Comet 1881, IV., also exhibited a secondary
+appendage, not directed towards the Sun, but making an angle of about
+45° with the main tail.
+
+These cometary appendages sometimes attain prodigious dimensions. The
+comets of 1680 and 1769 had tails so extended that, after their heads
+had set under the horizon, the extremities of these immense appendages
+were still seen as far up as the zenith. In a single day the tail of the
+comet of 1843 extended 100°, and it was thrust from the comet "as a
+dart of light" to the enormous distance of 48,500,000 miles, and yet of
+this immense appendage nothing was left on the following day. The tail
+of Donati's comet, in 1858, attained a real length of 42,000,000 miles,
+while that of the great comet of 1843 had the enormous length of
+200,000,000 miles. If this last comet had occupied the position of the
+Sun, which it approached very nearly for a moment, the extremity of its
+tail would have extended 60,000,000 miles beyond the orbit of Mars.
+
+In some cases the tails of comets have been seen undulating and
+vibrating in a manner similar to the undulations and coruscations of
+light characteristic of some auroras. Many observers report having seen
+such phenomena. The comet of 1769 was traversed by luminous waves and
+pulsations, comparable to those seen in the aurora borealis. I myself
+observed these curious undulations in Coggia's comet in 1874, while the
+head of this object was below the horizon. For an hour the undulations
+rapidly succeeded each other, and ran along the whole length of the
+tail.
+
+Some of the brightest comets have shone with such splendor that they
+could be observed easily in full sunshine. Many comets, such as those of
+1577 and 1744, have equaled Sirius and Venus in brilliancy. The great
+comet of 1843, which suddenly appeared in our sky, was so brilliant that
+it was seen by many observers at noon time, within a few degrees from
+the Sun. I remember that I myself saw this remarkable object in the day
+time, with a number of persons, who were gazing at the wonderful
+apparition. So brilliant was this comet, that besides its nucleus and
+head, a portion of its tail was also visible in the day time, provided
+the observer screened his eyes from the full sunlight by standing in the
+shadow of some building.
+
+Of all the bodies revolving around the Sun, none have been known to
+approach so near its surface as did the comet of 1843. When it arrived
+at perihelion, the distance from the centre of its nucleus to the
+surface of the Sun's photosphere was only 96,000 miles, while the
+distance from surface to surface was less than 60,000 miles. This comet,
+then, went through the solar atmosphere, and in traversing it with its
+tremendous velocity of 366 miles per second, may very possibly have
+swept through some solar protuberances, many of which attain much higher
+elevations than that at which the comet passed. The comet of 1680 also
+approached quite near the surface of the Sun, and near enough to
+encounter some of the high solar protuberances, its distance at
+perihelion being about two-thirds of the Moon's distance from the Earth.
+The rapidity of motion of the comet of 1843 was such, when it approached
+the Sun, that it swept through all that part of its orbit which is
+situated north of the plane of the ecliptic in a little more than two
+hours, moving in this short time from one node to the other, or 1800.
+
+But if some comets have a very short perihelion distance, that of others
+is considerable. Such a comet was that of 1729, whose perihelion
+distance was 383,000,000 miles, the perihelion point being situated
+between the orbits of Mars and Jupiter.
+
+While some comets come near enough to the Sun at perihelion to be
+volatilized by its intense heat, others recede so far from it at
+aphelion that they may be said to be frozen. The shortest cometary
+aphelion distance known is that of Encke's comet, whose greatest
+distance from the sun is 388,000,000 miles. But that of the comet of
+1844 is 406,000,000,000 miles from the Sun. The comets of 1863 and 1864
+are so remote in space when they reach their aphelion points that light,
+with its velocity of 185,500 miles a second, would require 171 days in
+the first case, and 230 in the last, to pass from them to the Earth.
+
+The period of revolution of different comets also varies immensely.
+While that of Encke's comet is only 3½ years, that of comet 1864, II.,
+is 280,000 years.
+
+Among the periodic comets of short period, some have exhibited highly
+interesting phenomena. Encke's comet, discovered in 1818, is remarkable
+for the fact that its period of revolution diminishes at each of its
+successive returns, and consequently this comet, with each revolution,
+approaches nearer and nearer to the Sun. The decrease of the period is
+about 2½ hours at each return. Although the decrease is small, if it go
+on in future as it does at present, the inevitable consequence will be
+that this comet will finally fall into the Sun. This curious phenomenon
+of retardation has been attributed by astronomers to the existence of a
+resisting medium filling space, but so rare and ethereal that it does
+not produce any sensible effect on the movements of the planets. But
+some other causes may retard this comet, as similar retardations have
+not been observed in the case of other periodic comets of short period.
+These, however, are not so near to the Sun, and perhaps our luminary may
+be surrounded by matter of extreme tenuity, which does not exist at a
+greater distance from it.
+
+Another of the periodic comets which has exhibited a very remarkable
+phenomenon of transformation is Biela's comet, which divided into two
+distinct parts, moving together in the same direction. When this comet
+was first detected at its return in 1845, it presented nothing unusual,
+but in the early part of 1846 it was noticed by several astronomers to
+be divided into two parts of unequal brightness, forming thus a twin
+comet. At its next return in 1852, the two sister comets were still
+traveling in company, but their distance apart, which in 1846 was
+157,000 miles, had increased to 1,500,000 miles. At the two next returns
+in 1859 and 1865, their position not being very favorably situated for
+observation, the comets were not seen. In 1872 the position should have
+been favorable for observation, and they were consequently searched for,
+but in vain; neither comet was found. An astronomer in the southern
+hemisphere, however, found a comet on the track of Biela's, but
+calculation has shown that the two objects are probably not identical,
+since this comet was two months behind the computed position for
+Biela's. It will be shown in the following chapter that our globe
+probably crossed the orbit of Biela's comet on November 27th, 1872, and
+the phenomena resulting from this passage will be there described.
+
+It is seen from these observations that comets may be lost or dissipated
+in space by causes entirely unknown to us. Biela's comet is not the only
+one which has been thus disintegrated. Ancient historians speak of the
+separation of large comets into two or more parts. In 1661 Hevelius
+observed the apparent division of the comet of that year and its
+reduction to fragments. The return of this comet, calculated for 1790,
+was vainly waited for; the comet was not seen.
+
+Other comets, whose periods of revolution were well known, have
+disappeared, probably never to return. Such is Lexell's comet, whose
+period was 5⁶⁄₁₀ years; also De Vico's comet, both of which are
+now lost. It is supposed that Lexell's comet, which passed twice very
+near the giant planet Jupiter, had its orbit changed from an ellipse to
+a parabola, by the powerful disturbing influence of this planet, and was
+thus lost from our system. Several other comets, in traveling over their
+different orbits, have approached near enough to Saturn, Jupiter and the
+Earth to have their orbits decidedly altered by the powerful attraction
+of these bodies.
+
+But since comets are liable to pass near the planets, and several have
+orbits which approach that of the Earth, it becomes important for us to
+know whether an encounter of such a body with our globe is possible, and
+what would then be the result for us. Although that knowledge would not
+enable us to modify the possibilities of an encounter, yet it is better
+to know the dangers of our navigation through space than to ignore them.
+This question of a collision of the Earth with a comet has been answered
+in different ways, according to the ideas entertained in regard to the
+mass of these bodies. While some have predicted calamities of all kinds,
+such as deluges, conflagrations, or the reduction of the Earth to
+incandescent gases, others have asserted that it would produce no more
+effect than does a fly on encountering a railroad train. In our days
+astronomers entertain very little fears from such an encounter, because
+the probabilities of danger from an occurrence of this sort are very
+slight, the mass of an ordinary comet being so small compared with that
+of our globe. We know with certainty that the Earth has never had an
+encounter with a comet _by which it has been transformed into gases_, at
+least within the several millions of years during which animal and
+vegetable life have left their marks upon the stony pages of its
+history, otherwise these marks would not now be seen. If, then, such an
+accident has not happened during this long period, the chances for its
+occurring must be very small, so small indeed that they might almost be
+left out of the question. It is true that our globe shows signs of great
+perturbations of its surface, but we have not the slightest proofs that
+they resulted from an encounter with a celestial body. It seems very
+probable that our globe passed through the tail of the comet of 1861,
+before it was first seen on June 29th; but nothing unusual was observed,
+except perhaps some phosphorescent light in the atmosphere, which was
+afterwards attributed to this cause.
+
+The density and mass of comets must be comparatively very small. Their
+tails consist of matter of such extreme tenuity that it affects but very
+little the light of the small stars over which they pass. The coma and
+nucleus, however, are not quite so transparent, and may have greater
+masses. On several occasions I have seen the light of stars reduced by
+the interposition of cometary matter, comet 1881, III., presenting
+remarkable cases of this sort. On July 8th, at 10h. 50m., several small
+stars were involved in this comet, one of which passed quite near the
+nucleus through the bright inner coma. At that time the comet was
+greatly disturbed, its nucleus was contracting and enlarging rapidly,
+and becoming bright and again faint in an instant. Every time that the
+nucleus grew larger, the star became invisible, but reappeared the
+moment the nucleus was reduced in size. This phenomenon could not be
+attributed to an atmospheric effect, since, while the nucleus was
+enlarging, a very small inner nucleus was visible within the large
+diffused one, the matter of which had apparently spread over the part of
+the coma in which the star was involved, making it invisible.
+
+That the mass of comets is small, is proved by the fact that they have
+sometimes passed near the planets without disturbing them in any
+sensible manner. Lexell's comet, which in 1770 remained four months very
+near Jupiter, did not affect in the least the orbits, or the motions of
+its satellites. The same comet also came within less than 1,500,000
+miles from the Earth, and on this occasion it was calculated that its
+mass could not have been the ¹⁄₅₀₀₀ part of that of our
+globe, since otherwise the perturbations which it would have caused in
+the elements of the Earth's orbit would have been sensible. There was,
+however, no change. If this comet's mass had been equal to that of our
+globe, the length of our year would have been increased by 2h. 47m. The
+comet of 1837 remained four days within 3,500,000 miles of the Earth,
+with no sensible effect.
+
+It seems quite difficult to admit that the denser part of a comet
+forming the nucleus is solid, as supposed by some physicists, since it
+is so rapidly contracted and dilated by the solar forces, while the
+comet is yet at a too great distance from the Sun to allow these effects
+to be attributed to solar heat alone. This part of a comet, as indeed
+the other parts, seems rather to be in the gaseous than in the solid
+state; the changes observed in the intensity of its light and in its
+structure may be conceived as due to some solar action partaking of the
+nature of electricity.
+
+It has been a question whether comets are self-luminous, or whether they
+simply reflect the solar light. When their light is analyzed by the
+spectroscope, it is found that the nucleus of a comet generally gives a
+continuous spectrum, while the coma and tail give a spectrum consisting
+of several bright diffused bands. The spectrum given by the nucleus is
+rarely bright enough to allow the dark lines of the solar spectrum to be
+discerned upon it; but such lines were reported in the spectrum of comet
+1881, III., a fact proving that this nucleus at least reflected some
+solar light. The nucleus of a comet may be partly self-luminous, and
+either solid, liquid, or composed of incandescent gases submitted to a
+great pressure. As to the coma and tail, they are evidently gaseous, and
+partly, if not entirely, self-luminous, as is proved by the band
+spectrum which they give. The position of these bands, moreover,
+indicates that the luminous gases of which they are composed contain
+carbon. The phenomena of polarization, however, seem to prove that these
+parts of comets also reflect some solar light.
+
+No theory so far proposed, to explain comets and the strange phenomena
+they exhibit, seems to have been successful in its attempts, and the
+mystery in which these bodies have been involved from the beginning of
+their apparition, seems to be now nearly as great as ever. It has been
+supposed that their tails have no real existence, but are due to an
+optical illusion. Prof. Tyndall has endeavored to explain cometary
+phenomena by supposing these bodies to be composed of vapors subject to
+decomposition by the solar radiations, and thus made visible, the head
+and tail being an actinic cloud due to such decompositions. According to
+this view, the tails of comets would not consist of matter projected
+into spacer but simply of matter precipitated by the solar rays in
+traversing the cometary nebulosity. The endeavor has also been made to
+explain the various phenomena presented by comets by an electrical
+action of the Sun on the gases composing these objects. Theories taking
+this as a base seem to us to be more likely to lead to valuable results.
+M. Faye, who has devoted much time and learning to this subject, assumes
+a real repulsive force of the Sun, acting inversely to the square of the
+distance and proportionally to the surface, and not to the mass as
+attraction does. He supposes, however, that this repulsive force is
+generated by the solar heat, and not by electricity. Prof. Wm. Harkness
+says that many circumstances seem to indicate that the comets' tails are
+due, in a great measure, to electrical phenomena.
+
+The fact that the tails of comets are better defined and brighter on the
+forward side, associated with the other fact that they curve the most
+when their motion is most rapid, sufficiently indicates that these
+appendages are material, and that they either encounter some resistance
+from the medium in which they move, or from a solar repulsion. The
+phenomena of condensation and extension, which I have observed in the
+comets of 1874 and 1881, added to the curious behavior exhibited by the
+jets issuing from the nucleus, seem to indicate the action of electrical
+forces rather than of heat. The main difficulty encountered in the
+framing of a theory of comets consists in explaining how so delicate and
+extended objects as their tails seem to be, can be transported and
+whirled around the Sun at their perihelion with such an enormous
+velocity, always keeping opposite to the Sun, and, as expressed by Sir
+John Herschel, "in defiance of the law of gravitation, nay, even of the
+received laws of motion."
+
+To consider the direction of the comets' tails as an indirect effect of
+attraction, seems out of the question; the phenomenon of repulsion so
+plainly exhibited by these objects seems to point to a positive solar
+repulsion, as alone competent to produce these great changes. The
+repulsive action of the Sun on comets' tails might be conceived, for
+instance, as acting in a manner similar to that of a powerful current of
+wind starting from the Sun, and constantly changing in direction, but
+always keeping on a line with the comet. Such a current, acting on a
+comet's tail as if it were a pennant, would drive it behind the nucleus
+just as observed. If it could once be ascertained that the great
+disturbances on comets correspond with the magnetic disturbances on our
+globe and with the display of the auroral light, the electric nature of
+the forces acting so strangely on the comets would be substantially
+demonstrated. I have shown that some of the great disturbances observed
+in the comets of 1874 and 1881 have coincided with auroral displays, and
+it will be shown hereafter that similar displays have also coincided
+with the passage of meteoric showers through our atmosphere. Whether
+these simultaneous phenomena were simple coincidences having no
+connection, or whether they are the result of a common cause, can only
+be ascertained by long continued future observations.
+
+
+
+
+SHOOTING-STARS AND METEORS
+
+PLATE XII
+
+
+While contemplating the heavens on a clear moonless night, we
+occasionally witness the sudden blazing forth of a star-like meteor,
+which glides swiftly and silently across some of the constellations, and
+as suddenly disappears, leaving sometimes along its track a
+phosphorescent trail, which remains visible for a while and gradually
+vanishes. These strange apparitions of the night are called _Falling_ or
+_Shooting-stars_.
+
+There is certainly no clear night throughout the year during which some
+of these meteors do not make their appearance, but their number is quite
+variable. In ordinary nights only four or five will be observed by a
+single person in the course of an hour; but on others they are so
+numerous that it becomes impossible to count them. When the falling
+stars are only a few in number, and appear scattered in the sky, they
+are called _Sporadic Meteors_, and when they appear in great numbers
+they constitute _Meteoric Showers_ or _Swarms_.
+
+Probably there is no celestial phenomenon more impressive than are these
+wonderful pyrotechnic displays, during which the heavens seem to break
+open and give passage to fiery showers, whose luminous drops describe
+fantastic hieroglyphics in the sky. While observing them, one can fully
+realize the terror with which they have sometimes filled beholders, to
+whom it seemed that the stability of the universe had come to an end,
+and that all the stars of the firmament were pouring down upon the Earth
+in deluges of fire.
+
+The ancients have left record of many great meteoric displays, and the
+manner in which they describe them sufficiently indicates the fear
+caused by these mysterious objects. Among the many meteoric showers
+recorded by ancient historians may be mentioned one observed in
+Constantinople, in the month of November, 472, when all the sky appeared
+as if on fire with meteors. In the year 599, meteors were seen on a
+certain night flying in all directions like fiery grasshoppers, and
+giving much alarm to the people. In March, 763, "the stars fell
+suddenly, and in such crowded number that people were much frightened,
+and believed the end of the world had come." On April 10th, 1095, the
+stars fell in such enormous quantity from midnight till morning that
+they were as crowded as are the hail stones during a severe storm.
+
+In modern times the fall of the shooting-stars in great number has been
+frequently recorded. One of the most remarkable meteoric showers of the
+eighteenth century occurred on the night of November 13th, 1799, and was
+observed throughout North and South America and Europe. On this
+memorable night thousands of falling stars were seen traversing the sky
+between midnight and morning. Humboldt and Boupland, then traveling in
+South America, observed the phenomena at Cumana, between two and five
+o'clock in the morning. They saw an innumerable number of shooting-stars
+going from north to south, appearing like brilliant fire-works. Several
+of these meteors left long phosphorescent trails in the sky, and had
+nuclei whose apparent diameter, in some cases, surpassed that of the
+Moon.
+
+The shower of November 13th, 1833, was still more remarkable for the
+great number of meteors which traversed the heavens, and was visible
+over the whole of North and South America. On that occasion the falling
+stars were far too numerous to be counted, and they fell so thickly that
+Prof. Olmsted, of New Haven, who observed them carefully, compared their
+number at the moment of their maximum fall to half that of the flakes of
+snow falling during a heavy storm. This observer estimated at 240,000
+the number of meteors which must have traversed the heavens above the
+horizon during the seven hours while the display was visible.
+
+
+
+[PLATE XII.--THE NOVEMBER METEORS.
+
+As observed between midnight and 5 o'clock A.M. on the night of November
+13-14 1868.]
+
+
+In the years 1866, 1867 and 1868, there were also extraordinary meteoric
+displays on the night of November 13th. It was on the last mentioned
+date that I had the opportunity to observe the remarkable shower of
+shooting-stars of which I have attempted to represent all the
+characteristic points in Plate XII. My observations were begun a little
+after midnight, and continued without interruption till sunrise. Over
+three thousand meteors were observed during this interval of time in the
+part of the sky visible from a northern window of my house. The maximum
+fall occurred between four and five o'clock, when they appeared at a
+mean rate of 15 in a minute.
+
+In general, the falling stars were quite large, many being superior to
+Jupiter in brightness and apparent size, while a few even surpassed
+Venus, and were so brilliant that opaque objects cast a strong shadow
+during their flight. A great many left behind them a luminous train,
+which remained visible for more or less time after the nucleus had
+vanished. In general, these meteors appeared to move either in straight
+or slightly curved orbits; but quite a number among them exhibited very
+extraordinary motions, and followed very complicated paths, some of
+which were quite incomprehensible.
+
+While some moved either in wavy or zig-zag lines, strongly accentuated,
+others, after moving for a time in a straight line, gradually changed
+their course, curving upward or downward, thus moving in a new
+direction. Several among them, which were apparently moving in a
+straight line with great rapidity, suddenly altered their course,
+starting at an abrupt angle in another direction, with no apparent
+slackening in their motion. One of them, which was a very conspicuous
+object, was moving slowly in a straight course, when of a sudden it made
+a sharp turn and continued to travel in a straight line, at an acute
+angle with the first, retreating, and almost going back towards the
+regions from which it originally came. As nearly all the meteors which
+exhibited these extraordinary motions left the trace of their passage in
+the sky by a luminous trail, it was easily ascertained that these
+appearances were not deceptive. On one occasion I noticed that the
+change of direction in the orbit corresponded with the brightening up of
+the meteor thus disturbed in its progress.
+
+Among these meteors, some traveled very slowly, and a few seemed to
+advance as if by jerks, but in general they moved very rapidly. One of
+the meteors thus appearing to move by jerks left a luminous trail, upon
+which the various jerks seemed to be left impressed by a succession of
+bright and faint spaces along the train. Some of the largest meteors
+appeared to rotate upon an axis as they advanced, and most of these
+revolving meteors, as also a great number of the others, seemed to
+explode just before they disappeared, sending bright fiery sparks of
+different colors in all directions, although no sound was at any time
+heard. The largest and most brilliant meteor observed on that night
+appeared at 5h. 30m., a little before sunrise. It was very bright, and
+appeared considerably larger than Venus, having quite a distinct disk.
+This meteor moved very slowly, leaving behind a large phosphorescent
+trail, which seemed to issue from the inside of the nucleus as it
+advanced. For a moment the train increased in size and brightness close
+to the nucleus, which then appeared as an empty transparent sphere,
+sprinkled all over with minute fiery sparks; the nucleus then suddenly
+burst out into luminous particles, which immediately vanished, only the
+luminous trail of considerable dimensions being left.
+
+Many of the trails thus left by the meteors retained their luminosity
+for several minutes, and sometimes for over a quarter of an hour. These
+trails slowly changed their form and position; but it is perhaps
+remarkable that almost all those which I observed on that night assumed
+the same general form--that of an open, irregular ring, or horse-shoe,
+somewhat resembling the letter C. This ring form was subsequently
+transformed into an irregular, roundish cumulus-like cloud. The trail
+left by a very large meteor, which I observed on the evening of
+September 5th, 1880, also exhibited the same general character of
+transformation.
+
+While I was observing a long brilliant trail left by a meteor on the
+night of November 13th, 1868, it was suddenly crossed by another bright
+shooting-star. The latter apparently went through the luminous substance
+forming the trail, which was suddenly altered in form, and considerably
+diminished in brightness simultaneously with this passage, although
+electrical action at some distance might perhaps as well explain the
+sudden change observed.
+
+In the majority of cases the meteors appeared white; but many,
+especially the largest, exhibited a variety of brilliant colors, among
+which the red, blue, green, yellow and purple were the most common. In
+general the trails exhibited about the same color as the nucleus, but
+much fainter, and they were usually pervaded by a greenish tint. In some
+instances the trails were of quite a different color from the nucleus.
+
+The luminous cloud observed at 5h. 30m. on the morning of November 14th,
+1868, after having passed through the series of transformations above
+described, remained visible for a long while after sunrise, appearing
+then as a small cirrus cloud, exactly similar in appearance to the
+hundreds of small cirrus clouds then visible in the sky, which had
+probably the same meteoric origin. For over three hours after sunrise,
+these cirrus clouds remained visible in the sky, moving all together
+with the wind in the high regions of the atmosphere.
+
+Although Plate XII. is intended to represent all the characteristics
+exhibited by the meteors observed on that night, every form represented
+having been obtained by direct observation, yet the number is much
+greater than it was at any single moment during the particular shower of
+1868. As regards number, the intention was to give an idea of a great
+meteoric shower, such as that of 1833, for instance. Although many of
+the falling stars seem to be close to the Earth's surface, yet this is
+only an effect of perspective due to their great distance, very few of
+these meteors ever coming into the lower regions of our atmosphere at
+all.
+
+The phenomena exhibited during other great meteoric showers have been
+similar to those presented by the shower just described, the only
+differences consisting in variations of size and brightness in the
+meteors, and also in the trails, which sometimes are not so numerous as
+they were in 1868.
+
+While some shooting-stars move so rapidly that they can hardly be
+followed in their orbits, others move so slowly that the sight can
+easily follow them, and even remark the peculiarities of their
+movements, some remaining visible for half a minute. Some of the falling
+stars move at the rapid rate of 100 miles a second, but others only 10
+miles a second, and even less. In general, they move about half as fast
+again as the Earth in its orbit. The arcs described by the meteors in
+the sky are variable. While some extend 80° and even 100°, others are
+hardly half a degree in length. While some shooting-stars are so faint
+that they can hardly be seen through the largest telescopes, others are
+so large and brilliant that they can be seen in the day-time. In
+general, a shooting-star of average brightness resembles a star of the
+third or fourth magnitude.
+
+Whatever may be the origin of the shooting-stars, they are, when we see
+them, not in the celestial spaces, like the planets, the comets, or the
+stars, but in our atmosphere, through which they travel as long as they
+remain visible. The height at which they appear and disappear is
+variable, but in general they are about 80 miles above the surface of
+our globe when they are first seen, and at about 55 miles when they
+disappear. In many cases, however, they have been observed at greater
+elevations, as also at smaller. A meteor simultaneously observed at two
+different stations first appeared at the height of 285 miles, and was
+last seen at 192 miles above the Earth's surface; but in rare cases the
+falling stars have been seen below a layer of clouds completely covering
+the sky. I myself saw one such shooting-star a few years since. The fact
+that the meteors are visible at so great elevations, proves that our
+atmosphere extends much farther than was formerly supposed, although at
+these great heights it must be extremely rarefied, and very different
+from what it is in its lower regions.
+
+There is a remarkable difference between the sporadic meteors seen in
+the sky on every night, and the meteoric showers observed only at
+comparatively rare intervals. While the first appear from different
+points in the sky and travel in all directions, being perfectly
+independent, the meteors of a shower all come from the same point of the
+heavens, from which they apparently diverge in all directions. This
+point of divergence of the meteors is called the _radiant point_ of the
+shower. Although the meteors seem to diverge in all directions from the
+radiant point, yet they all move in approximately parallel lines, the
+divergence being an effect of perspective.
+
+Whatever may be the position of the radiant point in the constellations,
+it remains as fixed in the sky as the stars themselves, and participates
+with them in the apparent motion which they undergo by the effect of the
+diurnal motion, and thus rises and sets with the constellation to which
+it belongs. This fact is sufficient to prove that the orbits of these
+meteors are independent of the Earth's motion, and that consequently
+they do not originate in our atmosphere. It has been shown by Encke that
+the radiant point of the meteoric shower of November 13th is precisely
+the point towards which our globe moves in space on November 13th; a
+tangent to the Earth's orbit would pass through this radiant point.
+
+The meteoric showers are particularly remarkable, not merely because of
+the large number of meteors which are visible and the fact that they all
+follow a common orbit, but chiefly because they have a periodic return,
+either after an interval of a year, or after a lapse of several years.
+At the beginning of the present century only two meteoric showers were
+known, those of August 10th and of November 13th, and their periodicity
+had not yet been recognized, although it had begun to be suspected. It
+was only in 1836 that Quetelet and Olbers ventured to predict the
+reappearance of the November meteors in the year 1867. Having made
+further investigations, Prof. Newton, of Yale College, announced their
+return in the year 1866. In both of these years, as also in 1868, the
+meteors were very numerous, and were observed in Europe and in America
+on the night of November 13th. The predictions having thus been
+fulfilled, the periodicity of the meteors was established. Since then,
+other periodic showers have been recognized, although they are much less
+important in regard to number than those of August and November, except
+that of November 27th, which exhibited so brilliant a display in Europe
+in 1872. These successive appearances have established the main fact
+that meteoric showers are more or less visible every year when the Earth
+occupies certain positions in its orbit.
+
+The meteoric shower of the 10th of August has its radiant point situated
+in the vicinity of the variable star Algol, in the constellation
+Perseus, from which its meteors have received the name of Perseids.
+Although varying in splendor, this meteoric swarm never fails to make
+its appearance every year. The Perseids move through our atmosphere at
+the rate of 37 miles per second. The shower usually lasts about six
+hours.
+
+The meteoric shower of November 13th has its radiant point situated in
+the vicinity of the star Gamma, in the constellation Leo, from which its
+meteors have been called Leonids. But while the August meteors recur
+regularly every year, with slight variations, the shower of November
+does not occur with the same regularity. During several years it is
+hardly noticeable, and is even totally absent, while in other years it
+is very remarkable. Every 33 years an extraordinary meteoric shower
+occurs on the 13th of November, and the phenomenon is repeated on the
+two succeeding years at the same date, but with a diminution in its
+splendor at each successive return. The Leonids move in an opposite
+direction to that of the Earth, and travel in our atmosphere with an
+apparent velocity of 45 miles per second, this being about the maximum
+velocity observed in falling stars. But when the motion of our globe is
+taken into account, and a deduction is made of the 18 miles which it
+travels per second, it is found that these meteors move at an actual
+mean rate of 27 miles a second.
+
+In a meteoric shower the stars do not fall uniformly throughout the
+night, there being a time when they appear in greater numbers. Usually
+it is towards morning, between 4 and 6 o'clock, that the maximum occurs.
+The probable cause of this phenomenon will be explained in its place
+hereafter.
+
+The orbits of the meteoric showers are not all approximately in the same
+plane, like those of the planets, but rather resemble those of comets,
+and have all possible inclinations to the ecliptic. Like the comets,
+too, the different meteoric showers have either direct or retrograde
+motion.
+
+The shooting-stars were formerly considered as atmospheric meteors,
+caused by the combustion of inflammable gases generated at the surface
+of the Earth, and transported to the high regions of our atmosphere by
+their low specific gravity. But the considerable height at which they
+usually appear, the great velocity of their motion, the common orbit
+followed by the meteors of the same shower, and the periodicity of their
+recurrence, do not permit us now to entertain these ideas, or to doubt
+their cosmical origin. But what is their nature?
+
+It is now generally admitted that innumerable minute bodies, moving in
+various directions around the Sun, are scattered in the interplanetary
+spaces through which our globe travels. It has been supposed that
+congregations of such minute bodies form elliptical rings, within which
+they are all moving in close parallel orbits around the Sun. On the
+supposition that such rings intersect the orbit of the Earth at the
+proper places, it was practicable to account for the shooting-stars by
+the passage through our atmosphere of the numerous minute cosmical
+bodies composing the rings, and the Leonid and Perseid showers were so
+explained. But when the elements of the orbits of these two last swarms
+came to be better known, and were compared with those of other celestial
+bodies, it was found necessary to alter this theory.
+
+It had for a long while been suspected that some kind of relation
+existed between the shooting-stars and the comets. This idea, vaguely
+formulated by Kepler more than two centuries ago, more clearly expressed
+by Chladni, and still more by Mr. Grey, before the British Association,
+at Liverpool, in 1855, has recently received a brilliant confirmation by
+the researches of Professor Schiaparelli, Director of the Observatory of
+Milan. A thorough investigation of the orbits of the August and November
+meteors led Schiaparelli to the discovery of a remarkable relation
+between meteoric and cometary orbits. By comparing the elements of these
+meteoric orbits with those of comets, he found a very close resemblance
+between the orbit of the August meteors and that of the comet 1862,
+III., and again between the orbit of the November meteors and that of
+Tempel's comet, 1866, I. These resemblances were too striking to be the
+result of mere chance, and demonstrated the identity of these cometary
+orbits with those of the Perseid and Leonid showers. In accordance with
+these new facts, it is now admitted that the meteoric showers result
+from the passage of our globe through swarms of meteoric particles
+following the orbits of comets, which intersect the orbit of the Earth.
+
+Professor Schiaparelli has attempted to show how these meteoric swarms
+were originally scattered along the orbits of comets, by supposing these
+bodies to originate from nebulous masses, which, in entering the sphere
+of attraction of the Sun, are gradually scattered along their orbits,
+and finally form comets followed by long trails of meteoric particles.
+
+It has been shown that in approaching the Sun the comets become
+considerably elongated, their particles being disseminated over immense
+distances by the solar repulsion. It seems probable that, owing to its
+feeble attractive power, the nucleus is incompetent to recall the
+scattered cometary particles and retain them in its grasp when they are
+relieved from the solar repulsion, so that they remain free from the
+nucleus, although they continue to move along its orbit. It is
+supposable that these cometary particles will scatter more and more in
+course of time. Forming at first an elongated meteoric cloud, they will
+finally spread along the whole orbit, and thus form a ring of meteoric
+particles. Since our globe constantly moves in its orbit and daily
+occupies a different position, it follows that at any point where such a
+cometary orbit happens to cross that of the Earth, our globe will
+necessarily encounter the cometary particles as a shower of meteors.
+This encounter will take place at a certain time of the year, either
+yearly, if they form a continuous ring, or after a succession of years,
+if they simply form an elongated cloud. Such meteoric clouds or rings
+would not be visible in ordinary circumstances, even through the largest
+telescopes, except on penetrating the upper regions of our atmosphere,
+when they would appear as showers of falling stars. It is supposed that
+in penetrating our atmosphere, even in its most rarefied regions, these
+meteors are heated by the resistance offered by the air to their motion,
+first becoming luminous and then being finally vaporized and burnt
+before they can reach the surface of the Earth.
+
+The orbit of the comet of 1862, III., which so closely corresponds with
+that of the Perseid meteors, is much more extended than that of Tempel's
+comet corresponding with that of the Leonids. While the first extends
+far beyond the orbit of Neptune, the latter only goes a little beyond
+that of Uranus. The former orbit makes a considerable angle with the
+plane of the Earth's orbit, but the latter is much nearer to parallelism
+with it. The period of revolution of the first is 108 years, and that of
+the last about 33¼ years.
+
+From the fact that the Perseid shower occurs yearly on the 10th of
+August, when the Earth crosses the orbit of the comet of 1862, III., it
+is supposed that the cometary particles producing this shower are
+disseminated along the whole orbit, and form a ring encircling the Sun
+and Earth. To explain the yearly variations in the number of the
+shooting-stars observed, these particles are supposed to be unequally
+distributed over the orbit, being more crowded at one place than they
+are at another. In order to explain the meteoric shower of Leonids,
+which appears in all its splendor every 33 years, and then with
+diminished intensity for two successive years, after which it is without
+importance, it is supposed that the cometary particles of the comet of
+1866, I., have not as yet spread all along the orbit, a sufficient time
+not having been allowed, but form an elongated meteoric cloud, more
+dense in its front than in its rear part. From these considerations it
+has been supposed also that the comet of 1866, I., is of a more recent
+date than that of 1862, III. While Tempel's comet makes its revolution
+around the Sun in about 33 years, this meteoric cloud, which has the
+same period and returns to the same point of its orbit every 33 years,
+encounters our globe for three successive years. The first year we are
+passing through its densest parts, and the two following years in less
+and less crowded parts, from which result the observed phenomena. An
+idea of the extent of this meteoric cloud may be formed from the fact
+that, with its cometary velocity of motion, it takes this cloud three
+years at least to cross the Earth's orbit. From recent researches it
+would appear that the Leonid cloud is not single, but that at least two
+others of smaller importance exist, and have periods of 33¼ years.
+
+Biela's comet, which was divided into two parts in 1846, is another of
+the few comets whose orbit approaches that of the Earth. Possessing this
+knowledge, and knowing then the close connection existing between
+meteors and comets, astronomers supposed that there were sufficient
+reasons to expect a meteoric shower when this comet was passing near the
+Earth. They consequently expected a meteoric display in 1872, when our
+globe was to cross its orbit. Their anticipation was plainly fulfilled,
+and on the night of November 27th, 1872, a splendid meteoric display,
+having its radiant point in the constellation Andromeda, was observed in
+Europe, and also in America, but the meteors seen here were not so
+numerous as in Europe. Other meteoric showers of less importance, such
+as that of April 20th, for instance, have also been identified with
+cometary orbits, so that now no doubt seems to remain as to the identity
+of cometary particles and shooting-stars.
+
+The fact that the maximum number of meteors is always observed in the
+morning hours, supports the hypothesis of the cosmic origin of the
+shooting-stars, since the regions of the Earth where it is morning are
+precisely those fronting the regions towards which our globe is moving
+in space, and accordingly encounter more directly the meteors moving in
+their orbit. The greater abundance of falling stars at that time may
+thus be accounted for.
+
+The number of meteors penetrating our atmosphere must be very great;
+there is not an hour and probably not a minute during which none fall.
+From various considerations, some astronomers have estimated at from
+65,000,000,000 to 146,000,000,000 the total number of shooting-stars
+yearly penetrating in our atmosphere. The actual number is undoubtedly
+great, yet the fact that the meteors are rarely seen through the
+telescope while employed in observing various celestial objects, does
+not indicate that they are so numerous as these figures imply. It is
+only occasionally that one is seen traversing the field of the
+instrument. Even when the sky is observed with a low power eye-piece for
+several hours in succession, many nights may pass without disclosing
+one, although an observer, sweeping the sky more freely with the naked
+eye, may often perceive four or five during an ordinary night.
+
+About the true nature of these bodies nothing is known with certainty.
+From spectrum analysis it seems to be established that most of them
+contain sodium and magnesium, while a few indicate the presence of
+strontium and iron, and in some rare cases there are traces of coal-gas.
+Some of the nuclei give a continuous spectrum, and others a spectrum of
+lines. The trail always gives a spectrum of bright lines which indicates
+its gaseous state. The traces of coal-gas rarely seen in meteors are,
+however, of great importance, as it identifies them more closely with
+the comets, which generally show a similar spectrum. The continuous
+spectra exhibited by some nuclei would indicate that they are
+incandescent and either solid or liquid; but it is difficult to conclude
+from their spectra what is their true nature, since we do not know
+exactly what part the terrestrial atmosphere may play in producing the
+results.
+
+The mass of the shooting-stars is not known with certainty, but the fact
+that during great meteoric showers, none are seen to reach the surface
+of the Earth, all being consumed in a few seconds, sufficiently
+indicates that it must be very small. It has been calculated that those
+equal to Venus in apparent size and brilliancy may weigh several pounds,
+while the faint ones would weigh only a few grains.
+
+If the shooting-stars have even such a mass as that here attributed to
+some of them, the extraordinary motions which I have described above
+seem to be unaccountable. The change of direction of a heavy mass moving
+swiftly cannot be sudden. The semi-circular, the wavy and the angular
+orbits observed could not be described, it would seem, by such a mass
+animated with a great velocity. Although the meteors are said to be
+ignited by the transformation of part of their progressive motion into
+molecular motion, yet it is not observed that the velocity of the
+falling stars diminishes when they are about to disappear. The luminous
+trails they leave in the atmosphere do not appear to be endowed with any
+motion, but remain for a time in their original positions. These facts
+are apparently opposed to the hypothesis that such meteors have any
+appreciable mass. The extraordinary motions exhibited by some meteors
+seem to indicate that some unsuspected force resides in these bodies,
+and causes them to deviate from the laws of ordinary motion.
+
+Although it is very probable that the ordinary shooting-stars have no
+appreciable mass, yet it is known that very heavy meteoric masses
+sometimes fall at the surface of the Earth. Such falls are generally
+preceded by the sudden apparition in the sky of a large, and usually
+very brilliant fire-ball, which traverses the air at a great speed,
+sometimes leaving behind it a luminous trail, after which it explodes
+with a loud sound, and heavy fiery meteoric fragments, diverging in all
+directions, fall at the surface of the Earth. The name of _Aerolites_ or
+_Meteorolites_ is given to these ponderous fragments. As these meteors,
+before they explode and fall to the ground, have many points of
+resemblance with the shooting-stars, they are generally supposed to be
+connected with them, and to have a similar cometary origin. The fact
+that the aerolites differ widely from each other in constitution, and
+are all composed of substances found on the Earth, associated with other
+facts given below, would rather seem to indicate a terrestrial than a
+celestial origin.
+
+If the aerolites belong to the same class of bodies as the falling
+stars, differing from them only in size and mass, it is difficult to see
+why so very few should fall upon the Earth during the great meteoric
+showers, when thousands of shooting-stars traverse our atmosphere. In
+Prof. Kirkwood's "Meteoric Astronomy" are given catalogues of all the
+falls of aerolites and fire-balls which have been observed at the time
+of the periodic meteoric showers of the 10th of August and the 13th of
+November, during a period of 221 years for the Perseids, or August
+showers, and of 318 years for the Leonids, or November showers. During
+221 years, 10 falls of aerolites have been witnessed simultaneously with
+the fall of the Perseids; while during 318 years, only 4 such falls have
+been recorded as having occurred at the time of the Leonid shower. If
+there is any close connection between the shooting-stars and the
+aerolites, we should expect to find a maximum in their fall at the time
+of the great meteoric displays. So far, no maxima or minima have yet
+been discovered in the fall of aerolites; they do not seem, like
+meteoric showers, to be governed by a law of periodicity.
+
+A very remarkable peculiarity of the aerolites is that they seem to have
+a tendency to fall in certain regions. Such are the southern part of
+France, the north of Italy, Hindostan, the central states of North
+America, and Mexico and Brazil. There is a curious contrast existing
+between the quick cometary motion of the aerolites before their
+explosion, and the comparatively slow motion of their fragments as they
+reach the Earth; motion which seems to be no greater than that
+corresponding to their natural fall impeded by the resistance of the
+air. In general, their penetration into the soil upon which they fall
+does not at all correspond to the great velocity with which they move in
+the atmosphere. The fragmentary structures of the aerolites, their
+identity of substance with that of our globe, their great resemblance to
+the volcanic minerals of the Earth, and the fractures and faults which
+some of them exhibit, do not correspond at all with the idea that they
+are cometary particles fallen on the Earth. As far as their structure
+and appearance is concerned, they seem rather to be a volcanic product
+of the interior of the Earth than parts of disintegrated comets. It must
+be admitted that their identity with the shooting-stars is far from
+established, and that they are still involved in mystery.
+
+The so-called meteoric dust gathered at sea and on high mountains may
+have various origins, and may be partly furnished by volcanic dust
+carried to great distances in the atmosphere.
+
+Since millions of shooting-stars penetrate our atmosphere every year and
+remain in it, becoming definitively a part of the Earth, it follows
+that, no matter how small may be the quantity of matter of which they
+are composed, they must gradually increase the volume and mass of our
+globe, although the increase may be exceedingly slow. Supposing every
+one of the shooting-stars penetrating our atmosphere to contain one
+cubic millimeter of matter, it has been calculated that it would take
+nearly 35,000 years to make a deposit one centimeter in thickness all
+over the surface of our globe. Insignificant as this may appear, it is
+probable that the quantity of matter of meteoric origin which is added
+to our globe is much less than has just been supposed.
+
+
+
+
+THE MILKY-WAY OR GALAXY
+
+PLATE XIII
+
+
+During clear nights, when the Moon is below the horizon, the starry
+vault is greatly adorned by an immense belt of soft white light,
+spanning the heavens from one point of the horizon to the opposite
+point, and girdling the celestial sphere in its delicate folds. Every
+one is familiar with this remarkable celestial object, called the
+_Milky-way_ or _Galaxy_.
+
+Seen with the naked eye, the Galaxy appears as an irregular, narrow,
+nebulous belt, apparently composed of cloud-like luminous masses of
+different forms and sizes, separated by comparatively dark intervals.
+These cloud-like masses vary much in luminous intensity, and while some
+among them are very bright and conspicuous, others are so faint that
+they are hard to recognize. In general, the brightest parts of the
+Milky-way are situated along the middle of its belt, while its borders,
+which are usually very faint, gradually vanish in the sky. Some parts of
+the Galaxy, however, show very little of the cloudy structure so
+characteristic of other parts, being almost uniform throughout, except
+towards the borders, which are always fainter. These parts showing
+greater uniformity are also the faintest.
+
+Such is the general appearance of the Milky-way on ordinary nights, but
+on rare occasions, when the atmosphere is particularly pure, it presents
+one of the grandest sights that can be imagined. At such favorable
+moments I have seen the Galaxy gleaming with light, and appearing as if
+composed of star-dust or of precious stones. The strange belt then
+appeared all mottled over and fleecy, its large cloud-like masses being
+subdivided into numerous small, irregular cloudlets of great brilliancy,
+which appeared projected upon a soft luminous background.
+
+The width of the Galaxy is far from being uniform; while in some places
+it is only 4° or 5°, in others it is 15° and even more. In some
+places it appears wavy in outline, at others quite straight; then it
+contracts, to expand a few degrees distant; while at other places it
+sends off branches and loops, varying in form, size and direction, some
+of which are quite prominent, while others are very faint.
+
+Although very irregular in form, the general appearance of the galactic
+belt is that of a regular curve occupying one of the great circles of
+the celestial sphere. The Milky-way completely encircles the heavens,
+but, of course, only one-half is visible at any one moment, since our
+globe prevents the other half from being seen. If, for a moment, we
+imagine ourselves left in space, our globe having vanished from under
+our feet, we should then see the whole Galaxy forming a continuous belt
+in the heavens, at the centre of which we should apparently be situated.
+
+While only one-half of the galactic belt can be seen at once from any
+point on the Earth, yet, according to the position of the observer, a
+larger or smaller portion of the whole can be seen at different times.
+In high northern or southern latitudes but little more than half can be
+seen even by continuous observations; but as we approach the equatorial
+regions, more and more of it becomes visible, until the whole may be
+seen at different hours and seasons. In the latitudes of the northern
+states, about two-thirds of the Galaxy is visible, the rest remaining
+hidden below the horizon; but from the southern states very nearly the
+whole can be seen. The half of the Milky-way visible at any one time
+from any latitude on the Earth never entirely sets below the horizon,
+although in some places it may be so near the horizon as to be rendered
+invisible by vapors. In the latitude of Cambridge, when in its lowest
+position, the summit of its arc is still about 12° or 15° above the
+northern horizon. The great circle of the celestial sphere, occupied by
+the galactic belt, is inclined at an angle of about 63° to the
+celestial equator, and intersects this great circle on one side in the
+constellation Monoceros in 6h. 47m., and on the opposite side in the
+constellations Aquila and Ophiuchus in 18h. 47m. of right ascension; so
+that its northern pole is situated in the constellation Coma Berenices
+in R. A. 12h. 47m., declination N. 27°, and the southern in the
+constellation Cetus in R. A. 0h. 47m., declination S. 27°.
+
+According to the seasons and to the hours of the night at which it is
+observed, the galactic arch presents different inclinations in the sky.
+Owing to its inclination to the equator of the celestial sphere, its
+opposite parts exhibit opposite inclinations when they pass the meridian
+of a place. That part of the Galaxy which is represented on Plate XIII.,
+and which intersects the celestial equator in the constellation Aquila,
+is inclined to the left or towards the east, when it is on the meridian;
+while the opposite part, situated in Monoceros, is inclined to the
+right, or towards the west, when it reaches the meridian. The former
+passes the meridian in the evening in the summer and autumn months; the
+latter, in the winter and spring months.
+
+
+[PLATE XIII.--PART OF THE MILKY WAY.
+
+From a study made during the years 1874, 1875 and 1876]
+
+
+By beginning at its northernmost part, represented at the upper part of
+Plate XIII., situated in "the chair" of the constellation Cassiopeia,
+and descending southwardly, and continuing in the same direction until
+the whole circle is completed, the course of the Milky-way through the
+constellations may be briefly described as follows: From Cassiopeia's
+chair, the Galaxy, forming two streams, descends south, passing partly
+through Lacerta on the left, and Cepheus on the right; at this last
+point it approaches nearest to the polar star. Then it enters Cygnus,
+where it becomes very complicated and bright, and where several large
+cloudy masses are seen terminating its left branch, which passes to the
+right, near the bright star Deneb, the leader of this constellation.
+Below Deneb, the Galaxy is apparently disconnected and separated from
+the northern part by a narrow, irregular dark gap. From this rupture,
+the Milky-way divides into two great streams separated by an irregular
+dark rift. An immense branch extends to the right, which, after having
+formed an important luminous mass between the stars _γ_ and _β_,
+continues its southward progress through parts of Lyra, Vulpecula,
+Hercules, Aquila and Ophiuchus, where it gradually terminates a few
+degrees south of the equator. The main stream on the left, after having
+formed a bright mass around _ε_ Cygni, passes through Vulpecula and
+then Aquila, where it crosses the equinoctial just below the star _η_
+after having involved in its nebulosity the bright star Altair, the
+leader of Aquila. In the southern hemisphere the Galaxy becomes very
+complicated and forms a succession of very bright, irregular masses, the
+upper one being in Scutum Sobieskii, while the others are respectively
+situated in Sagittarius and in Scorpio; the last, just a little above
+our horizon, being always considerably dimmed by vapors. From Scutum
+Sobieskii, the Galaxy expands considerably on the right, and sends a
+branch into Scorpio, in which the fiery red star Antares is somewhat
+involved.
+
+Continuing its course below our horizon, the Milky-way enters Ara and
+Norma, and then, passing partly through Circinus, Centaurus and Musca,
+it reaches the Southern Cross, after having been divided by the large
+dark pear-shaped spot known to navigators as the "Coal-Sack." In Ara and
+Crux the Milky-way attains its maximum of brightness, which there
+surpasses its brightest parts in Cygnus. In Musca, it makes its nearest
+approach to the south pole of the heavens. It then enters Carina and
+Vela, where it spreads out like a fan, and terminates in this last
+constellation, before reaching _λ_, being once more interrupted by a
+dark and very irregular gap, on a line with the two stars _γ_ and _λ_.
+It is noteworthy that this second rupture of continuity of the Galaxy in
+Vela is very nearly opposite, or at about 180° from the break near
+Deneb in Cygnus.
+
+Continuing its course on the other side of the break, the Milky-way
+again spreads out into the shape of a fan, grows narrower in entering
+Puppis, where it is longitudinally divided by darkish channels. It then
+passes above our southern horizon, becoming visible to us, passing
+through part of Canis Major, where its border just grazes the brilliant
+star Sirius. But from Puppis it gradually diminishes in brightness and
+complication, becoming faint and uniform. It enters Monoceros and Orion,
+where it again crosses the equator a little above _δ_, the northernmost
+of the three bright stars in the belt of Orion. Continuing its northward
+course it passes through Gemini, extending as far as Castor and Pollux,
+and then entering Auriga, where it begins to increase in brightness and
+in complication of structure. It passes partly through Camelopardus and
+into Perseus, where an important branch proceeds from its southern
+border.
+
+This branch beginning near the star _θ_, advances towards the
+celebrated variable star Algol, around which it is quite bright and
+complicated. Continuing its course in the same direction, the branch
+rapidly loses its brightness, becoming very faint a little below Algol,
+and passing through _ζ_ Persei, it enters Taurus, leaving the Pleiades
+on its extreme southern margin; and after having passed through _ε_
+where it branches off, it rapidly curves towards the main stream, which
+it joins near _ζ_ Tauri, thus forming an immense loop. The ramification
+projecting near _ε_ Tauri involves in its nebulosity the ruddy star
+Aldebaran and the scattered group of the Hyades. It then advances
+towards the three bright stars _δ_, _ε_ and _ζ_ of the belt of Orion,
+which, together with the sextuple star _θ_ Orionis, are involved in its
+faint nebulosity, and joins the main stream on the equinoctial, having
+thus formed a second loop, whose interior part is comparatively free
+from nebulosity, and contains the fine stars Betelgeuse and Bellatrix.
+
+That portion of the main galactic stream which is comprised between the
+star Deneb in Cygnus, and Capella in Auriga, is divided longitudinally
+by a very irregular, narrow, darkish cleft, comparatively devoid of
+nebulosity, which, however, is interrupted at some points. This dark gap
+sends short branches north and south, the most important of which are
+situated near _ζ_ Cephei and _β_ Cassiopeiæ. Another branch runs from
+_γ_ beyond _ε_ of the constellation last mentioned. The main stream of
+the Galaxy after leaving Perseus, enters Cassiopeia, and sending short
+branches into Andromeda, it completes its immense circle in Cassiopeia's
+chair, where this description was begun.
+
+When examined through the telescope, the appearance of the Milky-way
+completely changes, and its nebulous light is resolved into an immense
+number of stars, too faint to be individually seen with the naked eye.
+When Galileo first directed the telescope to the galactic belt, its
+nebulous, cloud-like masses were at once resolved into stars, even by
+the feeble magnifying power of his instrument. When, much later, Sir
+William Herschel undertook his celebrated star-gaugings of the Galaxy,
+millions of stars blazed out in his powerful telescopes. The stars
+composing this great nebulous belt are so numerous that it is impossible
+to arrive at any definite idea as to their number. From his soundings
+Herschel estimated at 116,000 the number of stars which, on one
+occasion, passed through the field of his telescope in 15 minutes, by
+the simple effect of the diurnal motion of the heavens; and on another
+occasion, a number estimated at 250,000 crossed the field in 41 minutes.
+In a space of 50, comprised between _β_ and _γ_ Cygni, shown on Plate
+XIII., he found no less than 331,000 stars. Prof. Struve has estimated
+at 20,500,000 the number of stars seen in the Milky-way through the
+twenty-foot telescope employed by Herschel in his star-gaugings. Great
+as this number may seem, it is yet far below the truth; as the great
+modern telescopes, according to Professor Newcomb, would very probably
+double the number of stars seen through Herschel's largest telescope,
+and detect from thirty to fifty millions of stars in the Milky-way.
+
+Although the telescope resolves the Galaxy into millions of stars, yet
+the largest instruments fail to penetrate its immense depths. The
+forty-foot telescope of Herschel, and even the giant telescope of Lord
+Rosse, have failed to resolve the Milky-way entirely into stars, the
+most distant ones appearing in them as nebulosities upon which the
+nearer stars are seen projected, the galactic stratum being unfathomable
+by the largest telescopes yet made.
+
+The stars composing the Milky-way are very unevenly distributed, as
+might easily be supposed from the cloud-like appearance of this belt. In
+some regions they are loosely scattered, forming long rows or streams of
+various figures, while in others they congregate into star groups and
+clusters having all imaginable forms, some being compressed into very
+dense globular masses. The intervals left between the clustering masses
+are poorer in stars, and indeed some of them are even totally devoid of
+stars or nebulosity. Such are the great and small "coal-sacks" in the
+southern Galaxy. I have myself detected such a dark space devoid of
+stars and nebulosity in one of the brightest parts of the Milky-way, in
+the constellation Sagittarius, in about 17h. 45m. right ascension, and
+27° 35' south declination. It is a small miniature coal-sack or opening
+in the Galaxy, through which the sight penetrates beyond this great
+assemblage of stars. Close to this, is another narrow opening near a
+small, loose cluster.
+
+Although lacking the optical resources which now enable us to recognize
+the structure of the Milky-way, some of the ancient philosophers had
+succeeded tolerably well in their speculations regarding its nature. It
+was the opinion of Democritus, Pythagoras and Manilius, that the Galaxy
+was nothing else but a vast and confused assemblage of stars, whose
+faint light was the true cause of its milky appearance.
+
+Before the invention of the telescope, no well-founded theory in regard
+to the structure of the Milky-way could, of course, be attempted.
+Although Kepler entertained different ideas in regard to the structure
+of this great belt from those now generally admitted, yet in them may be
+found the starting point of the modern conception of the structure of
+the Galaxy and of the visible universe. In the view of this great mind,
+the Milky-way, with all its stars, formed a vast system, the centre of
+which, and of the universe, was occupied by our Sun. Kepler reasoned
+that the place of the Sun must be near the centre of the galactic belt,
+from the fact this last object appears very nearly as a great circle of
+the celestial sphere, and that its luminous intensity is about the same
+in all its parts.
+
+Half a century later, another attempt to explain the Milky-way was made
+by Wright, of Durham, who rejected the idea of an accidental and
+confused distribution of the stars as inconsistent with the appearance
+of the Galaxy, and regarded them as arranged along a fundamental plane
+corresponding to that of the Milky-way. These ideas which were
+subsequently developed and enlarged by Kant, and then by Lambert,
+constitute what is now known as Kant's theory. According to this theory,
+the stars composing the Galaxy are conceived as being uniformly arranged
+between two flat planes of considerable extension, but which are
+comparatively near together, the Sun occupying a place not very far from
+the centre of this immense starry stratum. As we view this system
+crosswise through its thinnest parts, the stars composing it appear
+scattered and comparatively few in number, but when we view it
+lengthwiser through its most extended parts, they appear condensed and
+extremely numerous, thus giving the impression of a luminous belt
+encircling the heavens. In the conception of Kant, each star was a sun,
+forming the centre of a planetary system. These systems are not
+independent, but are kept together by the bonds of universal
+gravitation. The Galaxy itself is one of these great systems, its
+principal plane being the equivalent of the zodiac in our planetary
+system, while a preponderant body, which might be Sirius, is the
+equivalent of our Sun, and keeps the galactic system together. In the
+universe there are other galaxies, but as they are too distant to be
+resolved into stars, they appear as elliptical nebulæ. Such are, in
+brief, the grand speculations of Kant and Lambert on the Milky-way, and
+the structure of the universe.
+
+Kant's theory rested more on conjectures than on observed facts, and
+needed therefore the sanction of direct observations to be established
+on a firm basis. With this view, Sir William Herschel investigated the
+subject, by a long and laborious series of observations. His plan, which
+was that of "star-gauging," consisted in counting all the stars visible
+in his twenty-foot telescope, comprised in a wide belt cutting the
+Galaxy at right angles, and extending from one of its sides to the
+opposite one, thus embracing 180° of the celestial sphere. In this belt
+he executed 3,400 telescopic star-gaugings of a quarter of a degree
+each, from which he obtained 683 mean gaugings giving the stellar
+density of the corresponding regions.
+
+The general result derived from this immense labor was that the stars
+are fewest in regions the most distant from the galactic belt; while
+from these regions, which correspond to the pole of the Galaxy, they
+gradually increase in number in approaching the Milky-way. The star
+density was found to be extremely variable, and while some of the
+telescopic gaugings detected either no star at all, or only one or two,
+other gaugings gave 500 stars and even more. The average number of stars
+in a field of view of his telescope, obtained for the six zones, each of
+15°, into which Herschel divided up the portion of his observing belt,
+extending from the Galaxy to its pole, is as follows: In the first zone,
+commencing at 90° from the galactic belt and extending towards it, 4
+stars per telescopic field were found; 5 in the second; 8 in the third;
+14 in the fourth; 24 in the fifth and 53 in the sixth, which terminated
+in the Galaxy itself. Very nearly similar results were afterwards found
+by Sir John Herschel, for corresponding regions in the southern
+hemisphere.
+
+From these studies, Herschel concluded that the stellar system is of the
+general form supposed by the Kantian theory, and that its diameter must
+be five times as extended in the direction of the galactic plane, as it
+is in a direction perpendicular to it. To explain the great branch sent
+out by the Galaxy in Cygnus, he supposed a great cleft dividing the
+system edgewise, about half way from its circumference to its centre.
+From suppositions founded on the apparent magnitude and arrangement of
+stars, he estimated that it would take light about 7,000 years to reach
+us from the extremities of the Galaxy, and therefore 14,000 years to
+travel across the system, from one border to the opposite one.
+
+But Herschel's theory concerning the Milky-way rested on the erroneous
+assumption that the stars are uniformly distributed in space, and also
+that his telescopes penetrated through the entire depth of the Galaxy.
+Further study showed him that his telescope of twenty feet, and even his
+great forty-foot telescope, which was estimated to penetrate to a
+distance 2,300 times that of stars of the first magnitude, failed to
+resolve some parts of the Galaxy into stars. Meanwhile, the structure of
+the Milky-way being better known, the irregular condensation of its
+stars became apparent, while the mutual relation existing between binary
+and multiple systems of stars, as also between the stars which form
+clusters, was recognized, as showing evidence of closer association
+between certain groups of stars than between the stars in general.
+Herschel's system, which rested on the assumption of the uniform
+distribution of the stars in space, and on the supposition that the
+telescopes used for his gauges penetrated through the greater depths of
+the Galaxy, being thus found to contradict the facts, was gradually
+abandoned by its author, who adopted another method of estimating the
+relative distances of the stars observed in his gaugings.
+
+This method, founded on photometric principles, consisted in judging the
+penetrating power of his telescope by the brightness of the stars, and
+not, as formerly, by the number which they brought into view. He then
+studied by this new method the structure of the Milky-way and the
+probable distance of the clustering masses of which it is formed,
+concluding that the portion of the Galaxy traversing the constellation
+Orion is the nearest to us. This last result seems indicated by the fact
+that this portion of the Milky-way is the faintest and the most uniform
+of all the galactic belt.
+
+More recently Otto Struve investigated the same subject, and arrived at
+very nearly similar conclusions, which may be briefly stated as follows:
+The galactic system is composed of a countless number of stars,
+spreading out on all sides along a very extended plane. These stars,
+which are very unevenly distributed, show a decided tendency to cluster
+together into individual groups of different sizes and forms, separated
+by comparatively vacant spaces. This layer where the stars congregate in
+such vast numbers may be conceived as a very irregular flat disk,
+sending many branches in various directions, and having a diameter eight
+or ten times its thickness. The size of this starry disk cannot be
+determined, since it is unfathomable in some directions, even when
+examined with the largest telescopes. The Sun, with its attending
+planets, is involved in this immense congregation of suns, of which it
+forms but a small particle, occupying a position at some distance from
+the principal plane of the Galaxy. According to Struve, this distance is
+approximately equal to 208,000 times the radius of the Earth's orbit.
+The Milky-way is mainly composed of star-clusters, two-thirds, perhaps,
+of the whole number visible in the heavens being involved in this great
+belt. In conclusion, our Sun is only one of the individual stars which
+constitute the galactic system, and each of these stars itself is a sun
+similar to our Sun. These individual suns are not independent, but are
+associated in groups varying in number from a few to several thousands,
+the Galaxy itself being nothing but an immense aggregation of such
+clusters, whose whole number of individual suns probably ranges between
+thirty and fifty millions. In this vast system our globe is so
+insignificant that it cannot even be regarded as one of its members.
+According to Dr. Gould, there are reasons to believe that our Sun is a
+member of a small, flattened, bifid cluster, composed of more than 400
+stars, ranging between the first and seventh magnitude, its position in
+this small system being eccentric, but not very far from the galactic
+plane.
+
+The study of the Milky-way, of which Plate XIII. is only a part, was
+undertaken to answer a friendly appeal made by Mr. A. Marth, in the
+Monthly Notices of the Royal Astronomical Society, in 1872. I take
+pleasure in offering him my thanks for the suggestion, and for the
+facility afforded me in this study by his "List of Co-ordinates of Stars
+within and near the Milky-way," which was published with it.
+
+
+
+
+THE STAR-CLUSTERS
+
+PLATE XIV
+
+
+It is a well-known fact that the stars visible to the naked eye are very
+unequally distributed in the heavens, and that while they are loosely
+scattered in some regions, in others they are comparatively numerous,
+sometimes forming groups in which they appear quite close together.
+
+In our northern sky are found a few such agglomerations of stars, which
+are familiar objects to all observers of celestial objects. In the
+constellation Coma Berenices, the stars are small, but quite condensed,
+and form a loosely scattered, faint group. In Taurus, the Hyades and the
+Pleiades, visible during our winter nights, are conspicuous and familiar
+objects which cannot fail to be recognized. In the last group, six stars
+may be easily detected by ordinary eyes on any clear night, but more can
+sometimes be seen; on rare occasions, when the sky was especially
+favorable, I have detected eleven clearly and suspected several others.
+The six stars ordinarily visible, are in order of decreasing brightness,
+as follows: Alcyone, Electra, Atlas, Maia, Taygeta and Merope. Glimpses
+of Celano and Pleione are sometimes obtained.
+
+When the sky is examined with some attention on any clear, moonless
+night, small, hazy, luminous patches, having a cometary aspect, are
+visible here and there to the naked eye. In the constellation Cancer is
+found one of the most conspicuous, called Præsepe, which forms a small
+triangle with the two stars _γ_ and _δ_. In Perseus, and involved in
+the Milky-way, is found another luminous cloud, situated in the
+sword-handle, and almost in a line with the two stars _γ_ and _δ_ of
+Cassiopeia's Chair. In the constellation Hercules, another nebulous mass
+of light, but fainter, is also visible between the stars _η_ and _ζ_
+where it appears as a faint comet, in the depths of space. In Ophiuchus
+and Monoceros are likewise found hazy, luminous patches. In the southern
+sky, several such objects are also visible to the naked eye, being found
+in Sagittarius, in Canis Major and in Puppis; but the most conspicuous
+are those in Centaurus and Toucan. That in Centaurus involves the star
+_ω_ in its pale diffused nebulosity, and that in Toucan is involved in
+the lesser Magellanic cloud.
+
+When the telescope is directed to these nebulous objects, their hazy,
+ill-defined aspect disappears, and they are found to consist of
+individual stars of different magnitudes, which being more or less
+closely grouped together, apparently form a system of their own. These
+groups, which are so well adapted to give us an insight into the
+structure and the vastness of the stellar universe, are called
+_Star-clusters_.
+
+Star-clusters are found of all degrees of aggregation, and while in some
+of them, such as in the Pleiades, in Præsepe and in Perseus, the stars
+are so loosely scattered that an opera glass, and even the naked eye,
+will resolve them; in others, such as in those situated in Hercules,
+Aquarius, Toucan and Centaurus, they are so greatly compressed that even
+in the largest telescopes they appear as a confused mass of blazing
+dust, in which comparatively few individual stars can be distinctly
+recognized. Although only about a dozen Star-clusters can be seen in the
+sky with the naked eye, yet nearly eleven hundred such objects visible
+through the telescope, have been catalogued by astronomers.
+
+The stars composing the different clusters visible in the heavens vary
+greatly in number, and while in some clusters there are only a few, in
+others they are so numerous and crowded that it would be idle to try to
+count them, their number amounting to several thousands. It has been
+calculated by Herschel that some clusters are so closely condensed, that
+in an area not more than ⅒ part of that covered by the Moon, at least
+5,000 stars are agglomerated.
+
+When the group in the Pleiades is seen through the telescope it appears
+more important than it does to the naked eye, and several hundreds of
+stars are found in it. In a study of Tempel's nebula, which is involved
+in the Pleiades, I have mapped out 250 stars, mostly comprised within
+this nebula, with the telescope of 6⅓ inches aperture, which I have
+used for this study.
+
+As a type of a loose, coarse cluster, that in Perseus is one of the
+finest of its class. It appears to the naked eye as a single object, but
+in the telescope it has two centres of condensation, around which
+cluster a great number of bright stars, forming various curves and
+festoons of great beauty. Among its components are found several yellow
+and red stars, which give a most beautiful contrast of colors in this
+gorgeous and sparkling region. In a study which I have made of this twin
+cluster, I have mapped out 664 stars belonging to it, among which are
+two yellow and five red stars.
+
+While some clusters, like those just described, are very easily
+resolvable into stars with the smallest instruments, others yield with
+the greatest difficulty, even to the largest telescopes, in which their
+starry nature is barely suspected. Owing to this peculiarity,
+star-clusters are usually divided into two principal classes. In the
+first class are comprised all the clusters which have been plainly
+resolved into stars, and in the second all those which, although not
+plainly resolvable with the largest instruments now at our disposal,
+show a decided tendency to resolvability, and convey the impression that
+an increase of power in telescopes is the only thing needed to resolve
+them into stars. Of course this classification, which depends on the
+power of telescopes to decide the nature of these objects, is arbitrary,
+and a classification based on spectrum analysis is now substituted for
+it.
+
+The star-clusters are also divided into globular and irregular clusters,
+according to their general form and appearance. The globular clusters,
+which are the most numerous, are usually well-defined objects, more or
+less circular in their general outlines. The rapid increase of
+brightness towards their centres, where the stars composing them are
+greatly condensed, readily conveys the impression that the general form
+of these sparkling masses is globular. The irregular clusters are not so
+rich in stars as the former. Usually their stars are less condensed
+towards the centre, and are, for the most part, so loosely and
+irregularly distributed, that it is impossible to recognize the outlines
+of these clusters or to decide where they terminate. The globular
+clusters are usually quite easily resolvable into stars, either partly
+or wholly, although some among them do not show the least traces of
+resolvability, even in the largest instruments. This may result from
+different causes, and may be attributed either to the minuteness of
+their components or to their great distance from the Earth, many
+star-clusters being at such immense distances that they are beyond our
+means of measurement.
+
+As has been shown in the preceding section, the star-clusters are found
+in great number in the Galaxy; indeed, it is in this region and in its
+vicinity that the greater portion of them are found. In other regions,
+with the exception of the Magellanic clouds, where they are found in
+great number and in every stage of resolution, the clusters are few and
+scattered.
+
+
+[PLATE XIV.--STAR-CLUSTER IN HERCULES.
+
+From a study made in June, 1877]
+
+
+The star-cluster in the constellation Hercules, designated as No. 4,230
+in Sir J. Herschel's catalogue, and which is represented on Plate XIV.,
+is one of the brightest and most condensed in the northern hemisphere,
+although it is not so extended as several others, its angular diameter
+being only 7' or 8'. This object, which was discovered by Halley in
+1714, is one of the most beautiful of its class in the heavens.
+According to Herschel, it is composed of thousands of stars between the
+tenth and fifteenth magnitudes. Undoubtedly the stars composing this
+group are very numerous, although those which can be distinctly seen as
+individual stars, and whose position can be determined, are not so many
+as a superficial look at the object would lead us to suppose. From a
+long study of this cluster, which I have made with instruments of
+various apertures, I have not been able to identify more stars than are
+represented on the plate, although the nebulosity of which this object
+mainly consists, and especially the region situated towards its centre,
+appeared at times granular and blazing with countless points of light,
+too faint and too flickering to be individually recognized. Towards its
+centre there is quite an extended region, whose luminous intensity is
+very great, and which irresistibly conveys the impression of the
+globular structure of this cluster. Besides several outlying appendages,
+formed by its nebulosity, the larger stars recognized in this cluster
+are scattered and distributed in such a way that they form various
+branches, corresponding with those formed by the irresolvable
+nebulosity. At least six or seven of these branches and wings are
+recognized, some of which are curved and bent in various ways, thus
+giving this object a distant resemblance to some crustacean forms.
+Although I have looked for it with care, I have failed to recognize the
+spiral structure attributed to this object by several observers. Among
+the six appendages which I have recognized, some are slightly curved;
+but their curves are sometimes in opposite directions, and two branches
+of the upper portion make so short a bend that they resemble a claw
+rather than a spiral wing. The spectrum of this cluster, like that of
+many objects of its class, is continuous, with the red end deficient.
+
+A little to the north-east of this object is found the cluster No.
+4,294, which, although smaller and less bright than the preceding, is
+still quite interesting. It appears as a distinctly globular cluster
+without wings, and much condensed towards its centre. The stars
+individually recognized in it, although less bright than those of the
+other cluster, are so very curiously distributed in curved lines that
+they give a peculiar appearance to this condensed region.
+
+A little to the north of _γ_ Centauri may be found the great _ω_
+Centauri cluster, No. 3,531, already referred to above. This magnificent
+object, which appears as a blazing globe 20' in diameter, is, according
+to Herschel, the richest in the sky, and is resolved into a countless
+number of stars from the twelfth to the fifteenth magnitude, which are
+greatly compressed towards the centre. The larger stars are so arranged
+as to form a sort of net-work, with two dark spaces in the middle.
+
+The great globular cluster No. 52, involved in the lesser Magellanic
+cloud, in the constellation Toucan, is a beautiful and remarkable
+object. It is composed of three distinct, concentric layers of stars,
+varying in brightness and in degree of condensation in each layer. The
+central mass, which is the largest and most brilliant, is composed of an
+immense number of stars greatly compressed, whose reddish color gives to
+this blazing circle a splendid appearance. Around the sparkling centre
+is a broad circle, composed of less compressed stars, this circle being
+itself involved in another circular layer, where the stars are fainter
+and more scattered and gradually fade away.
+
+Many other great globular clusters are found in various parts of the
+heavens, among which may be mentioned the cluster No. 4,678, in
+Aquarius. This object is composed of several thousand stars of the
+fifteenth magnitude, greatly condensed towards the centre, and, as
+remarked by Sir J. Herschel, since the brightness of this cluster does
+not exceed that of a star of the sixth magnitude, it follows that in
+this case several thousand stars of the fifteenth magnitude equal only a
+star of the sixth magnitude. In the constellation Serpens the globular
+clusters No. 4,083 and No. 4,118 are both conspicuous objects, also No.
+4,687 in Capricornus. In Scutum Sobieskii the cluster No. 4,437 is one
+of the most remarkable of this region. The stars composing it, which are
+quite large and easily made out separately, form various figures, in
+which the square predominates.
+
+Among the loose irregular clusters, some are very remarkable for the
+curious arrangement of their stars. In the constellation Gemini the
+cluster No. 1,360, which is visible to the naked eye, is a magnificent
+object seen through the telescope, in which its sparkling stars form
+curves and festoons of great elegance. The cluster No. 1,467, of the
+same constellation, is remarkable for its triangular form. In the
+constellation Ara the cluster No. 4,233, composed of loosely scattered
+stars, forming various lines and curves, is enclosed on three sides by
+nearly straight single lines of stars. In Scorpio the cluster No. 4,224
+is still more curious, being composed of a continuous ring of loosely
+scattered stars, inside of which is a round, loose cluster, which is
+divided into four parts by a dark cross-shaped gap, in which no stars
+are visible.
+
+Among the 1,034 objects which are now classified as clusters more or
+less resolvable, 565 have been absolutely resolved into stars, and 469
+have been only partly resolved, but are considered as belonging to this
+class of objects. In Sir J. Herschel's catalogue there are 102 clusters
+which are Considered as being globular; among them 30 have been
+positively resolved into stars.
+
+The agglomeration of thousands of stars into a globular cluster cannot
+be conceived, of course, to be simply the result of chance. This
+globular form seems clearly to indicate the existence of some bond of
+union, some general attractive force acting between the different
+members of these systems, which keeps them together, and condenses them
+towards the centre. Herschel regards the loose, irregular clusters as
+systems in a less advanced stage of condensation, but gradually
+concentrating by their mutual attraction into the globular form.
+Although the stars of some globular clusters appear very close together,
+they are not necessarily so, and may be separated by great intervals of
+space. It has been shown that the clusters are agglomeration of suns,
+and that our Sun itself is a member of a cluster composed of several
+hundreds of suns, although, from our point of observation, these do not
+seem very close together. So far as known, the nearest star to us is a
+Centauri, but its distance from the Earth equals 221,000 times the
+distance of the Sun from our globe, a distance which cannot be traversed
+by light in less than three years and five months. It seems very
+probable that if the suns composing the globular clusters appear so near
+together, it is because, in the first place, they are at immense
+distances from us, and in the second, because they appear nearly in a
+line with other suns, which are at a still greater distance from us, and
+on which they accordingly are nearly projected. If one should imagine
+himself placed at the centre of the cluster in Hercules, for instance,
+the stars, which from our Earth seems to be so closely grouped, would
+then quite likely appear very loosely scattered around him in the sky,
+and would resemble the fixed stars as seen from our terrestrial station.
+
+Judging by their loose and irregular distribution, the easily resolvable
+clusters would appear, in general, to be the nearer to us. It is
+probable that the globular clusters do not possess, to a very great
+degree, the regular form which they ordinarily present to us. It seems
+rather more natural to infer that they are irregular, and composed of
+many wings and branches, such as are observed in the cluster in
+Hercules; but as these appendages would necessarily be much poorer in
+stars than the central portions, they would be likely to become
+invisible at a great distance, and therefore the object would appear
+more or less globular; the globular form being simply given by the close
+grouping of the stars in the central portion. It would seem, then, that
+in general, the most loosely scattered and irregular clusters are the
+nearest to us, while the smallest globular clusters and those resolvable
+with most difficulty are the most distant.
+
+In accordance with the theory that the clusters are composed of stars,
+the spectrum of these objects is in general continuous; although, in
+many cases, the red end of the spectrum is either very faint or
+altogether wanting. Many objects presenting in a very high degree the
+principal characteristics exhibited by the true star-clusters, namely, a
+circular or oval mass, whose luminous intensity is greatly condensed
+toward the centre, have not yielded, however, to the resolving power of
+the largest telescopes, although their continuous spectrum is in close
+agreement with their general resemblance to the star-clusters. Although
+such objects may remain irresolvable forever, yet it is highly probable
+that they do not materially differ from the resolvable and partly
+resolvable clusters, except by their enormous distance from us, which
+probably reaches the extreme boundary of our visible universe.
+
+
+
+
+THE NEBULÆ
+
+PLATE XV
+
+
+Besides the foggy, luminous patches which have just been described, a
+few hazy spots of a different kind are also visible to the naked eye on
+any clear, moonless night. These objects mainly differ from the former
+in this particular, that when viewed through the largest telescopes in
+existence they are not resolved into stars, but still retain the same
+cloudy appearance which they present to the unassisted eye. On account
+of the misty and vaporous appearance which they exhibit, these objects
+have been called _Nebulæ_.
+
+Of the 26 nebulous objects visible to the naked eye in the whole
+heavens, 19 belong to the class of star-clusters, and 7 to the class of
+nebulæ. Among the most conspicuous nebulæ visible to the unassisted
+eye, are those in the constellations Argo Navis, Andromeda and Orion.
+
+Besides the seven nebulæ visible to the naked eye, a great number of
+similar objects are visible through the telescope. In Sir John
+Herschel's catalogue of nebulæ and clusters, are found 4,053
+irresolvable nebulæ, and with every increase of the aperture of
+telescopes, new nebulæ, invisible in smaller instruments, are found.
+Notwithstanding their irresolvability it is probable, however, that many
+among them have a stellar structure, which their immense distance
+prevents us from recognizing, and are not therefore true nebulæ. The
+giant telescope of Lord Rosse has shown nebulæ so remote that it has
+been estimated that it takes their light 30 million years to reach the
+Earth.
+
+The nebulæ are very far from being uniformly distributed in space. In
+some regions they are rare, while in others they are numerous and
+crowded together, forming many small, irregular groups, differing in
+size and in richness of aggregation. The grouping of the nebulæ does
+not occur at random in any part of the heavens, as might naturally be
+supposed, but, on the contrary, it is chiefly confined to certain
+regions. Outside of these regions nebulæ are rare and are separated
+from each other by immense intervals; so that these isolated objects
+appear as if they were lost wanderers from the great nebulous systems.
+
+The regions where the nebulæ congregate in great number are very
+extensive, and in a general view there are two vast systems of nebular
+agglomeration, occupying almost opposite points of the heavens, whose
+centres are not very distant from the poles of the Milky-way. In the
+northern hemisphere, the nebulous system is much richer and more
+condensed than in the southern hemisphere. The northern nebulæ are
+principally contained in the constellations Ursa Minor and Major, in
+Draco, Canes Venatici, Bootes, Leo Major and Minor, Coma Berenices, and
+Virgo. In this region, which occupies about ⅛ of the whole surface of
+the heavens, ⅓ of the known nebulæ are assembled. The southern
+nebulæ are more evenly distributed and less numerous, with the
+exception of two comparatively small, but very remarkable centres of
+condensation which, together with many star-clusters, constitute the
+Magellanic clouds.
+
+These two vast nebular groups are by no means regular in outline, and
+send various branches toward each other. They are separated by a wide
+and very irregular belt, comparatively free from nebulæ, which
+encircles the celestial sphere, and whose medial line approximately
+coincides with that of the galactic belt. The Milky-way, so rich in
+star-clusters, is very barren in nebulæ; but it is a very remarkable
+fact, nevertheless, that almost all the brightest, largest, and most
+complicated nebulæ of the heavens are situated either within it, or in
+its immediate vicinity. Such are the great nebulæ in Orion and
+Andromeda; the nebula of _ζ_ Orionis; the Ring nebula in Lyra; the
+bifurcate nebula in Cygnus; the Dumb-bell nebula in Vulpecula; the Fan,
+Horse-shoe, Trifid and Winged nebulæ in Sagittarius; the great nebula
+around _η_ Argus Navis, and the Crab nebula in Taurus.
+
+Aside from the discovery of some of the largest nebulæ by different
+observers, and their subsequent arrangement in catalogues by Lacaille
+and Messier, very little had been done towards the study of these
+objects before 1779, when Sir W. Herschel began to observe them with the
+earnestness of purpose which was one of the distinctive points of the
+character of this great man. He successively published three catalogues
+in 1786, 1789, and 1802, in which the position of 2,500 nebulous objects
+was given. This number was more than doubled before 1864, when Sir John
+Herschel published his catalogue of 5,079 nebulæ and star-clusters. To
+this long list must be added several hundred similar objects, since
+discovered by D'Arrest, Stephan, Gould and others. But, as has been
+shown above, among the so-called nebulæ are many star-clusters which do
+not properly belong to the same class of objects, it being sometimes
+impossible in the present state of our knowledge to know whether a
+nebulous object belongs to one class or to the other.
+
+The nebulæ exhibit a great variety of forms and appearances, and, in
+accordance with their most typical characters, they are usually divided
+into several classes, which are: the Nebulous stars, the Circular, or
+Planetary, the Elliptical, the Annular, the Spiral and Irregular
+nebulæ.
+
+
+[PLATE XV.--THE GREAT NEBULA IN ORION.
+
+From a study made in the years 1875-76]
+
+
+The so-called nebulous stars consist of a faint nebulosity, usually
+circular, surrounding a bright and sharp star, which generally occupies
+its centre. The nebulosity surrounding these stars varies in brightness
+as well as in extent, and while, in general, its light gradually fades
+away, it sometimes terminates quite suddenly. Such nebulosities are
+usually brighter and more condensed towards the central star. The stars
+thus surrounded do not seem, however, to be distinguished from others by
+any additional peculiarity. Some nebulæ of this kind are round, with
+one star in the centre; others are oval and have two stars, one at each
+of their foci. The nebulous star, _τ_ Orionis, represented at the upper
+part of Plate XV., above the great nebula, has a bright star at its
+centre and two smaller ones on the side. The association of double stars
+with nebulæ is very remarkable, and may in some cases indicate a mutual
+relation between them.
+
+The so-called planetary nebulæ derive their name from their likeness to
+the planets, which they resemble in a more or less equable distribution
+of light and in their round or slightly oval form. While some of them
+have edges comparatively sharp and well defined, the outlines of others
+are more hazy and diffused. These nebulæ, which are frequently of a
+bluish tint, are comparatively rare objects, and most of those known
+belong to the southern hemisphere. When seen through large telescopes,
+however, they present a different aspect, and their apparent uniformity
+changes. The largest of these objects, No. 2,343 of the General
+Catalogue, is situated in the Great Bear, close to the star _β_. Its
+apparent diameter is, according to Sir J. Herschel, 2', 40", and "its
+light is equable, except at the edge, where it is a little hazy." In a
+study which I made of this object in 1876, with a refractor 6⅓ inches
+in aperture, I found it decidedly brighter on the preceding side, where
+the brightest part is crescent shaped. In Lord Rosse's telescope its
+disk is transformed into a luminous ring with a fringed border, and two
+small star-like condensations are found within. Another planetary
+nebula, near _χ_ Andromedæ, has also shown an annular structure in
+Rosse's telescope.
+
+The elliptical nebulæ, as their name implies, are elongated, elliptical
+objects; but while some of them are only slightly elongated ovals,
+others form ellipses whose eccentricity is so great that they appear
+almost linear. In all these objects the light is more or less condensed
+towards the centre; but while in some of them the condensation is
+gradual and slight, in others it is so great and sudden that the centre
+of the nebula appears as a large diffused star, somewhat resembling the
+nucleus of a comet. From the general appearance of these objects, it is
+not unlikely that some of them are either flattish, nebulous disks, like
+the planetary nebulæ, or nebulous rings, seen more or less sidewise.
+The condensation of light at their centres does not appear to be
+stellar, but nebulous like the rest, and it is a remarkable fact that
+very few, if any, of these objects are resolvable into stars.
+
+Several elliptical nebulæ are remarkable for having a star at or near
+each of their foci, or at each of their extremities. Such are the
+elliptical nebulæ in Draco, Centaurus, and Sagittarius, Nos. 4,419,
+3,706 and 4,395 of the General Catalogue, the last of which is in the
+vicinity of the triple star _μ_ Sagitarii. Each of these nebulæ has a
+star at each of its foci, while No. 1, in Cetus, has a star at each of
+its extremities.
+
+Among the most remarkable elliptic nebulæ may be mentioned Nos. 1,861
+and 2,373 of Sir J. Herschel's catalogue, both situated in the
+constellation Leo. The first is one of Lord Rosse's spiral nebulæ, and
+the last, which is a very elongated object, is formed of concentric oval
+rings, which are especially visible towards its central part. The
+constellation Draco is particularly remarkable for the number of
+elliptical nebulæ found within its boundaries. Among them are Nos.
+3,939, 4,058, 4,064, 4,087, 4,415, etc., which are quite remarkable
+objects of their class. No. 4,058, of which I have made a study, is
+bright, and has a decided lenticular form with a condensation in the
+centre. Its following edge is better defined than the preceding. In Lord
+Rosse's telescope this object exhibits a narrow, dark, longitudinal, gap
+in its interior.
+
+By far the largest and the finest object of this class is the great
+nebula in Andromeda. Although this object belongs rather to the class of
+irregular nebulæ, yet it is generally considered as an elliptic nebula,
+since its complicated structure, being less prominent, was not
+recognized until 1848, when it was perceived by George P. Bond, Director
+of the Harvard College Observatory. This, the first nebula discovered,
+was found in 1612 by Simon Marius. It is situated in the constellation
+Andromeda, in the vicinity of the star _ν_, and almost in a line with
+the stars _μ_ and _β_ of the same constellation. It is visible to the
+naked eye, and appears as a faint comet-like object. It is represented
+at the upper left hand corner of Plate XIII., on the border of the
+Milky-way, as it appears to the naked eye.
+
+The nebula in Andromeda is one of the brightest in the heavens, and is
+closely attended by two smaller nebulæ. Perhaps it would be rather more
+correct to say that it has three centres of condensation, as the two
+small nebulæ referred to are entirely involved in the same faint and
+extensive nebulosity. Its general form is that of an irregular oval,
+upwards of one degree in breadth and two and a half degrees in length.
+Its brightest and most prominent part, which alone was seen by the
+earlier observers, consists in a very elongated lenticular mass, which
+gradually condenses towards its centre into a blazing, star-like
+nucleus, surrounded by a brilliant nebulous mass. At a little distance
+to the south of this central condensation is found one of the lesser
+centres of condensation noted above, which is globular in appearance,
+with a bright, star-like nucleus like the former. The other centre of
+condensation is found to the north-west of the centre of the principal
+mass, and is quite elongated, with a centre of condensation towards its
+southern extremity, but it is not so bright as the others. Close to the
+western edge of the bright lenticular mass first described, and making a
+very slight angle with its longer axis, are found two narrow and nearly
+rectilinear dark rifts, running almost parallel to each other, and both
+terminating in a slender point in the south. These dark rifts, which are
+almost totally devoid of nebulous matter, are quite rare in nebulæ, and
+afford a good opportunity to watch the changes which this part of the
+nebulæ may undergo.
+
+This nebula has never been positively resolved into stars, although
+Prof. Geo. Bond and others have strongly suspected its resolvability. In
+a study which I have made of it, with the same instrument employed by
+Bond, and also with the great Washington telescope, I detected a decided
+mottled appearance in several places, which might be attributable to a
+beginning of resolvability; but I do not consider this a conclusive
+indication that the nebula is resolvable. The continuous spectrum given
+by this nebula, showing that it is not in the gaseous state which its
+appearance seems to indicate, warrants the conclusion, however, that it
+will ultimately be found to be resolvable. This object, being situated
+on the edge of the Galaxy and involved in its diffused light, has a
+great number of small stars belonging to this belt projected upon it.
+During my observations I have mapped out 1,323 of these stars, none of
+which seems to be in physical connection with the nebula.
+
+Among the circular and elliptical nebulæ a few exhibit a very
+remarkable structure, being apparently perforated, and forming either
+round, slightly oval, or elongated rings of great beauty. These Annular
+nebulæ are among the rarest objects in the heavens. In Scorpio, two
+such nebulæ are found involved in the Milky-way, and also one in
+Cygnus. One of those in Scorpio has two stars involved within its ring,
+at the extremities of its smallest interior diameter. A very elongated
+nebula in the vicinity of the fine triple star _γ_ Andromedæ is also
+annular, and has two stars symmetrically placed at the extremities of
+its greatest interior axis. Another elongated annular nebula is also
+found north of _η_ Pegasi.
+
+The grandest and most remarkable of the annular nebulæ is found in the
+constellation Lyra, about midway between the two stars _β_ and _γ_. It
+is slightly elliptical in form, and according to Prof. E. S. Holden, its
+major axis is 77".3 and the minor 58". From a study and several drawings
+which I have made of this object, with instruments of various apertures,
+I have found it decidedly brighter towards its outer border, at the
+extremities of its minor axis, than at the ends of the major axis. On
+very favorable occasions, some of its brightest parts have appeared
+decidedly, but very faintly mottled, and I have recognized three small
+centres of condensation. Its interior, in which Professor Holden has
+detected a very faint star, is quite strongly nebulous. In Lord Rosse's
+telescope, this nebula is completely surrounded by wisps and appendages
+of all sorts of forms, which I have failed to trace, however, both with
+the refractor of the Harvard College Observatory and with that of the
+Naval Observatory at Washington; Rosse, Secchi and Chacornac, have seen
+this nebula glittering as if it were a "heap of star dust," although its
+spectrum indicates that it is gaseous.
+
+The nebula No. 1,541, in Camelopardus, of which I have also made a study
+and a drawing, is closely allied to the class of annular nebulæ. This
+object, which is quite bright, has a remarkable appearance. It consists
+principally of somewhat more than half of an oval ring, surrounding a
+bright, nebulous mass which condenses around a star; this mass being
+separated from the imperfect ring by a dark interval. Upon the bright
+portion of the ring, and on opposite points, are found two bright stars,
+between which lies the star occupying the central mass. The central mass
+extends at some distance outside of the ring on its open side. Several
+stars are involved in this object.
+
+The Spiral nebulæ are very curious and complicated objects, but they
+are visible only in the largest telescopes. Prominent above all is the
+double spiral nebula No. 3,572, in Canes Venatici, which is not far from
+_η_ Ursa Majoris. In Lord Rosse's telescope, this object presents a
+wonderful spiral disposition, looking somewhat like one of the
+fire-works called pin-wheels, and forming long, curved wisps, diverging
+from two bright centres. The spectrum of this object, however, is not
+that of a gas. In the constellation Virgo, Rosse has detected another
+such nebula. In Cepheus, Triangulum, and Ursa Major, are found other
+spiral nebulæ of smaller size. Lord Rosse has recognized 40 spiral
+nebulæ and suspected a similar structure in 30 others.
+
+The class of the Irregular nebulæ, which will be now considered,
+differs greatly in character from the others, and includes the largest,
+the brightest and the most extraordinary nebulæ in the heavens. The
+nebulæ of this class differ from those belonging to the other classes
+by a want of symmetry in their form and in the distribution of their
+light, as well as by their capricious shapes, and their very complicated
+structure. Another and perhaps the principal difference between them and
+the objects above described, consists in the remarkable fact already
+stated, that they are not, except in rare cases, to be found in the
+regions where the other nebulæ abound. On the contrary, they are found
+in or very near the Milky-way, precisely where the other nebulæ are the
+most rare. This fact, recognized by Sir J. Herschel, led him to consider
+them as "outlying, very distant, and as it were detached fragments of
+the great stratum of the Galaxy." It seems very probable that the reason
+why these objects differ so greatly from the other nebulæ in size,
+brightness and complication of structure, is simply because they are
+much nearer to us than are most of the others. They are perhaps nebulous
+members of our Galaxy. The same remark which has been made of
+star-clusters may be applied to nebulæ. The nearer they are to us, the
+larger, the brighter and the more complicated they will appear, while
+the farther they are removed, the more simple and regular and round they
+will appear, only their brightest and deepest parts being then visible.
+
+The Crab Nebula of Lord Rosse, near _ζ_ Tauri, No. 1,157, is one of the
+interesting objects of this class. It has curious appendages streaming
+off from an oval, luminous mass, which give it a distant resemblance to
+the animal from which it derives its name. The Bifid nebula in Cygnus,
+Nos. 4,400 and 4,616, is another object of this class. It consists of a
+long, narrow, crooked streak, forking out at several places, and passing
+through _χ_ Cygni. Observers, having failed to recognize the connection
+existing between its different centres of brightness, have made distinct
+nebulæ of this extended object.
+
+The Dumb-bell nebula in Vulpecula, No. 4,532, is a bright and curious
+object, with a general resemblance to the instrument from which it
+derives its name. Lord Rosse's telescope has shown many stars in it,
+projected on a nebulous background, and Prof. Bond seems to have thought
+that it showed traces of resolvability, although in the study which I
+made of this nebula with the same instrument used by the latter
+observer, I failed to perceive any such traces. Dr. Huggins finds its
+spectrum gaseous.
+
+The star-cluster, No. 4,400, in Scutum Sobieskii, which is described by
+Sir J. Herschel as a loose cluster of at least 100 stars, I have found
+to be involved in an extensive, although not very bright, nebula, which
+would seem to have escaped his scrutiny. In a study and drawing of this
+nebula made in 1876, its general form is that of an open fan, with the
+exception that the handle is wanting, with deeply indented branches on
+the preceding side, where the brightest stars of the cluster are
+grouped. From its peculiar form, this object might appropriately be
+called the Fan nebula.
+
+The Omega or Horse-shoe nebula, in Sagittarius, No. 4,403, of which I
+have made a study and two drawings, one with a refractor 6⅓ inches in
+aperture, and the other conjointly with Prof. Holden, with the great
+telescope of the Naval Observatory, is a bright and very complicated
+object. Its general appearance in small instruments, with low power, is
+that of a long, narrow pisciform mass of light, from which proceeds on
+the preceding side, the great double loop from which it derives its
+name. But in the great Washington refractor its structure becomes very
+complicated, forming various bright nebulous masses and wisps of great
+extension. Prof. Holden, who has made a careful, comparative study of
+the published drawings of this object, thinks there are reasons to
+believe that its western branch has moved relatively to the stars found
+within its loop. The spectrum of this nebula is gaseous.
+
+The Trifid nebula, No. 4,355, in the same constellation, is also a very
+remarkable object, although it is not so bright as the last. This
+nebula, which I have studied with the refractor of the Cambridge
+Observatory, consists of four principal masses of light, separated by a
+wide and irregular gap branching out in several places. These masses,
+which are brighter along the dark gap, gradually fade away externally. A
+group of stars, two of which are quite bright, is found near the centre
+of the nebula, on the inner edge of the following mass, and close to the
+principal branch of the dark channel. A little to the north, and
+apparently forming a part of this nebula, is a globular-looking nebula,
+having a pale yellow star at its centre. Prof. Holden's studies on this
+nebula show that the triple star, which was centrally situated in the
+dark gap from 1784 to 1833, was found involved in the border of the
+nebulous mass following it, from 1839 to 1877; the change, he thinks,
+is attributable either to the proper motion of the group of stars or to
+that of the nebula itself.
+
+In the same vicinity is found the splendid and very extensive nebula No.
+4,361, in which is involved a loose, but very brilliant star-cluster.
+This nebula and cluster, which I have studied and drawn with a 6⅓ inch
+telescope, is very complicated in structure, and divided by a dark
+irregular gap into three principal masses of light, condensing at one
+point around a star, and at others forming long, bright, gently-curved
+branches, which give to this object a strong resemblance to the wings of
+a bird when extended upwards in the action of flying. From this
+peculiarity this object might appropriately be called the Winged nebula.
+Its spectrum is that of a gas.
+
+The variable star _η_ Argus is completely surrounded by the great
+nebula of the same name, No. 2,197, first delineated by Sir J. Herschel,
+during his residence at the Cape of Good Hope, in 1838. This object,
+which covers more than ⁴⁄₇ of a square degree, is divided into
+three unequal masses, separated by dark oval spots, comparatively free
+from nebulosity, and is suspected to have undergone changes since
+Herschel's time.
+
+In the same field with the double star, _ζ_ Orionis, the most easterly
+of the three bright stars in the belt of Orion, is found another
+irregular nebula of the Trifid type. From the drawings which I have made
+of this object, it appears to be composed of three principal unequal
+masses, separated by a wide, irregular, dark channel, two of the masses
+being quite complicated in structure, and forming curved, nebulous
+streams of considerable length and breadth. This nebula, like the next
+to be described, seems to be connected with the Galaxy by the great
+galactic loop described in another section.
+
+By far the most conspicuous irregular nebula visible from our northern
+States, is the great nebula in Orion, No. 1,179, represented on Plate
+XV. This object, visible to the naked eye, is the brightest and the most
+wonderful nebula in the heavens. It is situated a little to the south of
+the three bright stars in the belt of Orion, and may be readily detected
+surrounding the star _θ_, situated between and in a line with two faint
+stars, the three being in a straight line which points directly towards
+_ε_, the middle star of the three in Orion's belt. The area occupied by
+this nebula is about equal to that occupied by the Moon.
+
+In its brightest parts the nebula in Orion appears as a luminous cloud
+of a pentagonal form, from which issue many luminous appendages of
+various shapes and lengths. This principal mass is divided into
+secondary masses, separated by darkish, irregular intervals. These
+secondary masses in their turn appear mottled and fleecy. Towards the
+lower part of the pentagonal mass is found a roundish dark space,
+comparatively devoid of nebulosity, in which are involved four bright
+stars forming a trapezium, and several fainter ones. The four bright
+stars of the trapezium constitute the quadruple star _θ_ Orionis, from
+which the nebula has received its name. The cloud-like pentagonal form
+is brightest on the north-west of the trapezium, and is surrounded on
+three sides by long, soft, curved wisps, fading insensibly into the
+outer nebulous mass in which they are involved. On the east a broad,
+wavy wing spreads out, and sends an important branch southward.
+South-east of the trapezium are found several curious dark spaces,
+comparatively devoid of nebulosity, especially those on the east, which
+give to this nebula a singular character. Close to the north-eastern
+part of the nebula, or rather in contact with it, is found a small,
+curiously-shaped nebula, condensing around a bright star into a blazing
+nucleus. From this centre it continues northward in a narrow diffused
+stream, which spreads out in passing over the stars _c^1_ and _c^2_; and
+after having sent short branches northward, it curves back to the south
+and joins the main nebula on the west of its starting point, having thus
+formed a great loop which is not shown on the Plate. The nebula also
+forms a loop towards the south, which is partly shown on Plate XV., a
+small branch of which, passing through _τ_ Orionis, the nebulous star
+shown at the top of the Plate, and extending southward, is not here
+represented.
+
+On ordinary nights the nebula in Orion is a splendid object, and
+inspires the observer with amazement; but this is as nothing compared
+with the grand and magnificent sight which it presents during the very
+rare moments when our atmosphere is perfectly clear and steady. I have
+seen this nebula but once under these favorable circumstances, and I was
+surprised by the grandeur of the scene. Then could be detected features
+to be seen at no other time, and its fleecy, floculent, cloud-like
+masses glittered with such intensity that it seemed as if thousands of
+stars were going to blaze out the next moment. Although I observed the
+nebula under such favorable conditions, and with the fifteen-inch
+refractor of the Cambridge Observatory, yet I was disappointed in my
+expectations, and distinguished no new stars or points of light, and
+nothing more than a very bright mass, finely divided into minute blazing
+cloudlets. Although I failed to resolve this nebula into stars, yet Lord
+Rosse, Bond and Secchi thought they had caught glimpses of star dust.
+Its spectrum, however, proves to be mainly that of incandescent gases,
+probably hydrogen and nitrogen. In the curved wisps found in this
+nebula, Lord Rosse and others saw indications of a spiral structure.
+
+Several bright stars are found scattered over this nebula, and besides
+those forming the trapezium, there are three in a row, a little to the
+south-east of that group, which are quite bright and remarkable. Among
+the stars involved in this nebula, few show signs of having a physical
+connection with it, although it seems probable that the group of the
+trapezium is so connected. Some of these stars are variable. The small
+stars represented on this Plate, as on others of the series, are
+somewhat exaggerated in size, as was unavoidable with any process of
+reproduction which could be adopted.
+
+In 1811, W. Herschel was led to suspect that some changes had occurred
+in this nebula, but changes in such complicated and delicate objects are
+not easily ascertained, since, for the most part, we have for comparison
+with our later observations only coarse drawings made by hands unskilled
+in delineation.
+
+Although comparatively rare, double and multiple nebulæ may be found in
+the sky. When this occurs, their constituents most commonly belong to
+the class of spherical nebulæ. Sometimes the components are separated
+and distinct, at other times one of them is projected upon the other,
+either really or by the effect of perspective. Sometimes one is round
+and the other elongated. It is probable that while some of these nebulæ
+are physically associated and form a system, others appear to be so only
+because they happen to be almost in a line with the observer. A double
+nebula in Draco, Nos. 4,127 and 4,128, which I have drawn, is a fair
+type of those which are separated. The first is a globular nebula, and
+the last an oval one, with a star at its centre. The double nebula, Nos.
+858 and 859, in Taurus, which I have also studied, is a type of the
+cases in which one nebula is partly projected on another. In this
+instance both the nebulæ are globular.
+
+The nebulæ in general show very little color in their light, which is
+ordinarily whitish and pale. Some, however, present a decided bluish or
+greenish tint. The great nebula in Orion has a greenish cast, and we
+have seen that some planetary nebulæ are bluish.
+
+It has been a question whether nebulæ are changing. It has already been
+stated that Prof. Holden believes there is ground to suspect that the
+Trifid and Horse-shoe nebulæ have undergone some changes. A nebula near
+_ε_ Tauri has been lost and found again several times. Two other
+nebulæ in the same constellation have presented curious variations.
+One, near a star of the tenth magnitude, exhibited variations of
+brightness like those of the star itself, and for a time disappeared.
+The other, near _ζ_ Tauri, increased in brightness for three months,
+after which it disappeared. In 1859 Tempel discovered a nebula in the
+Pleiades, which has shown some fluctuations. In 1875 I made a long study
+of this object, and drew it carefully a dozen times, but I was not able
+to see any changes in it within the two or three months during which my
+observations were continued. But on Nov. 24, 1876, it was found of a
+different color, being purplish and very faint. On Dec. 23, 1880, it was
+found just as bright and visible as when I drew it in 1875, and on Oct.
+20, 1881, it appeared faint and purplish again, as in 1876. On this last
+night, and on those which followed it, it was impossible for me to trace
+the nebulosity as far as in 1875. I consider this as due to a variation
+in the light of this object, which in 1875 was bright enough to be well
+seen while the Moon after her First Quarter was within ten or fifteen
+degrees from the Pleiades.
+
+From the observations of M. Laugier, it appears that some nebulæ have a
+proper motion, comparable to that of stars. From the displacement of the
+lines of their spectra by their motion in the line of sight, Dr. Huggins
+found that no nebula observed by him has a proper motion surpassing 25
+miles per second. The Ring nebula in Lyra appears to move from us at the
+rate of 3 miles per second, and that in Orion recedes about 17 miles per
+second.
+
+The important question arises, are all the irresolvable nebulæ in the
+heavens to be considered as so many star-clusters, differing only from
+them by the minuteness of their components, or their immense distance
+from us; or are they cosmical clouds, composed of luminous vapors,
+similar to the matter composing the heads and tails of comets?
+Originally, W. Herschel, with many astronomers, thought that all these
+objects were stellar aggregations, too distant to be resolved into
+stars; but he subsequently modified his opinion, and accepted the idea
+that some of them are of a gaseous nature.
+
+No direct proof that the nebulæ are gaseous could be obtained, however,
+before the spectroscope was known. The attempt to analyze the light of
+the nebulæ with this instrument was made in 1864, by Dr. Huggins, who
+directed his spectroscope to the planetary nebula, No. 4,373, in Draco.
+Its spectrum was found to consist of three bright, distinct lines, the
+brightest of which corresponded with the strongest nitrogen line, and
+the feeblest with the hydrogen C line. Besides these lines, it gave also
+a very faint, continuous spectrum, apparently due to a central point of
+condensation. By this observation, the gaseous nature of a nebula was
+for the first time demonstrated. Dr. Huggins thus analyzed 70 nebulæ,
+of which one-third gave a gaseous spectrum, consisting of several bright
+lines, the brightest of which invariably corresponded with the lines of
+nitrogen. The others gave a continuous spectrum, with the red end
+usually deficient. These results indicate that if some of the so-called
+nebulæ are due to an aggregation of stars, either too minute or too
+remote in space to be individually resolved, others are in a gaseous
+state. Yet the faint, continuous spectrum, given by some nebulæ, in
+addition to their gaseous spectra, seems to show that these nebulæ have
+some stars or matter in a different state, either involved in them or
+projected on their surface.
+
+The idea of diffused matter distributed here and there in space, and
+gradually condensing into stars, is by no means new. As early as 1572,
+Tycho Brahé proposed such an hypothesis, to explain the sudden
+apparition of a new star in Cassiopeia, which he considered as formed by
+the recent agglomeration of the "celestial matter" diffused in space.
+Kepler adopted the same idea to explain the new star which appeared in
+Ophiuchus, in 1604. Halley, Lacaille, Mairan and others, entertained the
+same opinion. The hypothesis of a self-luminous, nebulous matter
+diffused in space, and forming here and there immense masses, has been
+proposed from the origin of the telescope, and was adopted by Sir
+William Herschel, who in his grand speculations on the universe
+considered the nebulæ as immense masses of phosphorescent vapors,
+gradually condensing around one or several centres into stars or
+clusters of stars. The evidence afforded by the spectroscope seems to be
+in favor of such an hypothesis, and shows us that gaseous agglomerations
+exist in space.
+
+According to our modern conception, the visible universe is but an
+infinitely small portion of the infinite universe perceived by our mind.
+The great blazing centre around which our little, non-luminous globe
+pursues its endless journey, is only an humble member of a cluster
+comprising four hundred equally powerful suns, as they are believed to
+be, although they appear to us as little twinkling stars. The nearest of
+these stars is 221,000 times as far from the Sun as the Sun is from the
+Earth, and yet this entire cluster is only one among the several hundred
+Star-clusters composing the great galactic nebula in which we are
+involved, comprising thirty or fifty millions of such suns. Among the
+4,000 irresolvable nebulæ in the sky, perhaps over one-half are
+supposed to be galaxies, like our own galaxy, composed of star-clusters,
+and millions of stars. Besides these remote galaxies, vast
+agglomerations of yet uncondensed, nebulous matter exist in space, and
+form the nebulæ proper, in which the genesis of suns is slowly
+elaborated. Although the visible universe is limited by the penetrating
+power of our instruments, yet we see in imagination the infinite
+universe stretching farther and farther; but we know not whether this
+invisible universe is totally devoid of matter, or whether it also is
+filled with millions and millions of suns and galaxies.
+
+
+
+
+APPENDIX
+
+KEY TO THE PLATES
+
+
+PLATE I.--GROUP OF SUN-SPOTS AND VEILED SPOTS.
+
+_Observed June_ 17, 1875, _at 7h. 30m. A. M._
+
+
+The background shows the sun's visible surface, or _photosphere_, as
+seen with a telescope of high power at the most favorable moments,
+composed of innumerable light markings, or granules, separated by a
+network of darker gray. The granules, each some hundreds of miles in
+width, are thought to be the flame-like summits of the radial filaments
+or columns of gas and vapor which compose the photospheric shell. The
+two principal sun-spots of the group here represented show the
+characteristic dark _umbra_ in the centre, overhung by the thatch-like
+_penumbra_, composed of whitish gray filaments. The penumbral filaments
+are not supposed to differ in their nature from those constituting the
+ordinary photosphere, save that they are seen here elongated and
+violently disturbed by the force of gaseous currents. Both spots are
+traversed partly or wholly by bright overlying _faculæ_, or so-called
+_luminous bridges_, depressed portions of which, in the left-hand spot,
+form the _gray and rosy veils_ commonly attendant upon this class of
+spots. In each of these spots, also, the inner ends of projecting
+penumbral filaments have fallen so far within the umbra as to appear
+much darker than the rest. At the right of the upper portion of the
+left-hand spot, is a mass of white facular clouds, honey-combed by dark
+spaces, through which are seen traces of the undeveloped third spot of
+the triple group first observed. If seen upon the sun's limb, this would
+have presented the appearance of a _lateral spot_. Above the right-hand
+spot is a small black "dot," or incipient spot, without distinct
+penumbra. The irregular dark rift below the two large spots and
+connecting them is a spot of the crevasse type, with very slight umbra,
+a still better example of which is seen in a westward
+prolongation of the penumbra of the left-hand spot. In the upper
+left-hand corner of the Plate are seen several small _faculæ_,
+appearing as irregular whitish streaks amongst the granules. In the
+pear-shaped darkening of the solar surface below and at their left, is
+seen a veiled spot, two of which attended this group.
+
+_Approximate scale, 2500 miles--1 inch._
+
+
+PLATE II.--SOLAR PROTUBERANCES.
+
+_Observed May_ 5, 1873, _at 9h. 40m. A. M._
+
+
+A view of an upheaval of the _chromosphere_, or third outlying envelope
+of the sun, as observed with the tele-spectroscope, or telescope with
+spectroscope attached.
+
+The _method of the observation_ requires a word of explanation. Save on
+the rare occasions of a total solar eclipse, no direct telescopic view
+of the solar prominences or flames is possible, owing to the fact that
+the intense white light from the sun's main disk entirely obscures
+the feeble pink light of the chromosphere. A few years ago Messrs.
+Jannsen and Lockyer found that a spectroscope of high dispersive power
+so weakens the spectrum of ordinary sun-light as to show the spectrum of
+bright lines given by the chromosphere, on any clear day. The telescope
+is adjusted so that a portion of the sun's limb, usually near a group of
+active sun-spots, shall be presented before the opened slit of the
+spectroscope. The light of the chromosphere thus admitted along with
+some diffused sun-light from the earth's atmosphere, produces a spectrum
+of intensely bright lines, widely separated, on the fainter background
+of the strongly dispersed spectrum of sun-light. The most prominent of
+these bright lines are those known as the C line (_scarlet_), F line
+(_blue_), which with several others are due to the hydrogen present in
+the chromosphere, the D{3} line (_orange_) ascribed to a little known
+substance called "helium", and occasionally the sodium lines D{1}, D{3},
+(_yellow_). By adjusting the slit upon the scarlet C line, the
+appearances represented in Plate II. were observed as through an
+atmosphere of scarlet light: in the D or F lines identical appearances
+may be seen, but somewhat less clearly defined, as through yellow or
+blue light respectively. Hence the solar times, as here observed with
+the spectroscope in the hydrogen C line, are seen through a portion only
+(the scarlet rays) of the light coming from but one substance (hydrogen)
+of the companion incandescent substances present in the chromosphere.
+The color of the collective chromospheric light is seen directly with
+the telescope during an eclipse (See Plate III.) to be a delicate rosy
+pink.
+
+
+_Description of the Plate._--The black background represents the general
+darkness of the eye-piece to the spectroscope. The broad red stripe
+stretching from top to bottom of the Plate is a portion of the red band
+of the spectrum, magnified about 100 times as compared with the actual
+spectroscopic view. The upper and lower edges of the cross-section of
+dusky red correspond with the edges of the slit, opened widely enough to
+admit a view of the chromospheric crest and of the whole height of the
+protuberances at once. With a narrower opening of the slit this
+background would have been nearly black, its reddish cast increasing
+with the amount of opening and consequent admission of diffused
+sun-light. Rising above the lower edge of the opening is seen a small
+outer segment of the chromosphere, which, as a portion of the sun's
+eastern limb, should be imagined as moving directly towards the
+beholder. The seams and rifts by which its surface is broken, as well as
+the distorted forms of the huge protuberances show the chromosphere to
+be in violent agitation. Some of the most characteristic shapes of the
+_eruptive protuberances_ are presented, as also _cloud-like_ forms
+overtopping the rest. In the immediate foreground the bases of two
+towering columns appear deeply depressed below the general horizon of
+the segment observed, showing an extraordinary velocity of motion of the
+whole uplifted mass toward the observer. The highest of these
+protuberances was 126,000 miles in height at the moment of observation.
+The triple protuberance at the left with two drooping wings and a tall
+swaying spire tipped with a very bright flame, shows by its more
+brilliant color the higher temperature (and possibly compression) to
+which its gases have been subjected. The irregular black bands behind
+this protuberance indicate the presence there of less condensed and
+cooler clouds of the same gases. The dimmer jets of dame rising from the
+chromosphere are either vanishing protuberances, or, as in the case of
+the smallest jet shown at the extreme right of the horizon, are the tops
+of protuberances just coming into view.
+
+_Approximate scale, 6000 miles--1 inch._
+
+
+PLATE III.--TOTAL ECLIPSE OF THE SUN.
+
+_Observed July_ 29, 1878, _at Creston, Wyoming Territory._
+
+
+A telescopic view of the sun's _corona_ or extreme outer atmosphere and
+of the _solar flames_ or _prominences_ during a total eclipse. At the
+moment of observation the dark disk of the moon, while still hiding the
+sun's main body, had passed far enough eastward to allow the rosy pink
+chromospheric prominences to be seen on its western border. On all sides
+of the sun's hidden disk, the _corona_ shows its pale greenish light
+extending in halo-like rays and streamers, and two very remarkable wings
+stretch eastward and westward very nearly in the plane of the ecliptic
+and in the direction of the positions of Mercury and Venus respectively
+at the time of observation. The full extent of these wings could not be
+shown in the Plate without reducing its scale materially, since the
+westerly wing extended no less than twelve times the sun's diameter, and
+the easterly wing nearly as far, or over ten million miles. A circlet of
+bright light immediately bordering the moon's disk is the so-called
+_inner corona_, next to which the wings and streamers arc brightest,
+thence shading off imperceptibly into the twilight sky of the eclipse.
+Other noteworthy peculiarities of the corona, as observed during this
+eclipse, are the varying angles at which the radiating streamers are
+seen to project, the comparatively dark intervals between them, and the
+curved, wisp-like projections seen upon the wings. An especially
+noticeable gap appears where the most westerly of the upward streamers
+abruptly cuts off the view of the long wing. The largest and brightest
+of the curving streamers on the westerly wing coincides with the highest
+flame-like protuberance. To some observers of this eclipse the upward
+and downward streamers seemed pointed at their outer extremities and
+less regular in form.
+
+_Approximate scale, 135,000 miles--1 inch._
+
+
+PLATE IV.--AURORA BOREALIS.
+
+_As observed March_ 1, 1872, _at 9h. 25m. P. M._
+
+
+The view presents the rare spectacle of an aurora spanning the sky from
+east to west in concentric arches. The Polar Star is nearly central in
+the background, the constellation of the Great Bear on the right and
+Cassiopeia's chair on the left. The large star at some distance above
+the horizon on the right is Arcturus. The almost black inner segment of
+the aurora resting upon the horizon, has its summit in the magnetic
+meridian, which was in this case a little west of north, its arc being
+indented by the bases of the ascending streamers. Both streamers and
+arches were, when observed, tremulous with upward pulsations and there
+was also a wave-like movement of the streamers from west to east. The
+prevailing color of this aurora is a pale whitish green and the
+complementary red appears especially at the west end of the auroral
+arch. The summits of the streamers are from four hundred to live hundred
+miles above the earth and the aurora is therefore a phenomenon of the
+terrestrial atmosphere rather than of astronomical observation proper.
+
+
+PLATE V.--THE ZODIACAL LIGHT.
+
+_Observed February_ 20, 1876.
+
+
+An observation of the cone of light whose axis lies along the Zodiac,
+whence it derives its name. It is drawn as seen in the west, with its
+base in the constellation Pisces, and its apex near the familiar group
+of the Pleiades in the constellation Taurus. The first bright star above
+the horizon in the base of the cone is the planet _Venus_ and at some
+distance above is the reddish disk of _Mars_, the two being in rare
+companionship as evening stars. Above the constellation Pisces, two
+bright stars of Aries lie just outside the cone at the right. The
+nearest bright star above these at the right is _Beta_, the leading star
+of the constellation Triangula. Further at the right the three prominent
+stars nearly in a line are, in ascending order, _Delta_, _Beta_ and
+_Gamma_ of the constellation Andromeda. Above these at the left, the
+brightest star of a quadrangular group of four is the remarkable
+variable star _Algol_ (_Beta_) of the constellation Perseus, which
+changes from the second to the fourth magnitude in a period of less than
+three days. At the left and a little above the Pleiades is the ruddy
+star _Aldebaran_, one of the Hyades and chief star in the constellation
+Taurus. These are the principal stars visible in this portion of the sky
+at the time of the observation. Their relative positions are represented
+as seen in the sky and not by the common method of star-atlases, which
+allows for the change from a spherical to a plane surface. Their
+magnitude in the order of brightness is indicated only approximately.
+
+
+PLATE VI.--MARE HUMORUM.
+
+_From a study made in 1875._
+
+
+A view of one of the lunar plains, or so-called _seas_ (_Maria_), with
+an encircling mountainous wall consisting of volcano-like craters in
+various stages of subsidence and dislocation. The sun-light coming from
+the west casts strong shadows from all the elevations eastward, and is
+just rising on the _terminator_, where the rugged structure of the
+Moon's surface is best seen. The lighter portions are the more elevated
+mountainous tracts and crater summits. The detailed description of this
+Plate given in the body of the Manual is repeated here for convenience
+of reference: The Mare Humorum, or sea of moisture, as it is called, is
+one of the smaller gray lunar plains. Its diameter, which is very nearly
+the same in all directions, is about 270 miles, the total area of this
+plain being about 50,000 square miles. It is one of the most distinct
+plains of the Moon, and is easily seen with the naked eye on the
+left-hand side of the disk. The floor of the plain is, like that of the
+other gray plains, traversed by several systems of very extended but low
+hills and ridges, while small craters are disseminated upon its surface.
+The color of this formation is of a dusky greenish gray along the
+border, while in the interior it is of a lighter shade, and is of
+brownish olivaceous tint. This plain, which is surrounded by high clefts
+and rifts, well illustrates the phenomena of dislocation and subsidence.
+The double-ringed crater Vitello, whose walls rise from 4,000 to 5,000
+feet in height, is seen in the upper left-hand corner of the gray plain.
+Close to Vitello at the east is the large broken ring-plain Lee, and
+farther east, and a little below, is a similarly broken crater called
+Doppelmayer. Both of these open craters have mountainous masses and
+peaks on their door, which is on a level with that of the Mare Humorum.
+A little below, and to the left of these objects, is dimly seen a deeply
+imbedded oval crater, whose walls barely rise above the level of the
+plain. On the right-hand side of the great plain is a long _fault_, with
+a system of fracture running along its border. On this right-hand side
+may be seen a part of the line of the terminator, which separates the
+light from the darkness. Towards the lower right-hand corner is the
+great ring-plain Gassendi, 55 miles in diameter, with its system of
+fractures and its central mountains, which rise from 3,000 to 4,000 feet
+above its floor. This crater slopes towards the plain, showing the
+subsidence to which it has been submitted. While the northern portion of
+the wall of this crater rises to 10,000 feet, that on the plain is only
+500 feet high, and is even wholly demolished at one place where the
+floor of the crater is in direct communication with the plain. In the
+lower part of the sea, and a little to the west of the middle line, is
+found the crater Agatharchides, which shows below its north wall the
+marks of rills impressed by a flood of lava, which once issued from the
+side of the crater. On the left-hand side of the plain is seen the
+half-demolished crater Hippalus resembling a large bay, which has its
+interior strewn with peaks and mountains. On this same side can be seen
+one of the most important systems of clefts and fractures visible on the
+Moon, these clefts varying in length from 150 to 200 miles.
+
+_Approximate scale, 15 miles--1 inch._
+
+
+PLATE VII.--PARTIAL ECLIPSE OF THE MOON.
+
+_Observed October_ 24, 1874.
+
+
+A view of the Moon partially obscured by the Earth's shadow, whose
+outline gives ocular proof of the earth's rotundity of form. The
+shadowed part of the Moon's surface is rendered visible by the diffused
+sun-light refracted upon it from the earth's atmosphere. Its reddish
+brown color is due to the absorption, by vapors present near the earth's
+surface, of a considerable part of this dim light. On both the obscured
+and illuminated tracts the configurations of the Moon's surface are seen
+as with the naked eye. The craters appear as distinct patches of lighter
+color, and the noticeably darker areas are the depressed plains or
+_Maria_. The large crater _Tycho_, at the lower part of the disk, is the
+most prominent of these objects, with its extensive system of _radiating
+streaks_. The largest crater above is _Copernicus_, at the left of which
+is _Kepler_ and still above are _Aristarchus_ and _Herodotus_ appearing
+as if blended in one. Above and at the left of the great crater _Tycho_,
+the first dark tract is the _Mare Humorum_ of Plate VI., seen in its
+natural position, with the crater _Gassendi_ at its northern (upper)
+extremity and _Vitello_ on its southern (lower) border. The advancing
+border of the shadow appears, as always, noticeably darker than the
+remainder, an effect probably of contrast. The illuminated segment of
+the Moon's disk has its usual appearance, the lighter portions being the
+more elevated mountainous surfaces, and the dark spaces the floors of
+extensive plains.
+
+_Approximate scale, 140 miles--1 inch._
+
+
+PLATE VIII.--THE PLANET MARS.
+
+_Observed September_ 3, 1877, _at 11h. 55m. P. M._
+
+A view of the southern hemisphere of Mars, when in the most favorable
+position for observation, and when exceptionally free from the clouds,
+which very frequently hide its surface configurations. Since, of all the
+planets, Mars is most like the earth, Plate VIII. may give a fair idea
+of the appearance of our globe to a supposed observer on Mars. The dark
+gray and black markings, are regarded as tracts of water, or of some
+liquid with similar powers of absorbing light; and, for the same reason,
+the lighter portions, of a prevailing reddish tint, are supposed to be
+bodies of land, while the bright white portions are variously due to
+clouds, to polar snow or ice, and the bright rim of white along the
+limb, to the depth of the atmosphere through which the limb is seen. The
+chief permanent features of the planet's surface have been named in
+honor of various astronomers.
+
+The large dark tract on the left is _De La Rue Ocean_, the isolated oval
+spot near the centre is _Terby Sea_, and on the right is the western end
+of _Maraldi Sea_, with strongly indented border. Directly north of
+(below) De La Rue Ocean, is _Maedler Continent_; above it stretches
+_Jacob Land_; and surrounding Terby Sea is _Secchi Continent_. Extending
+into the centre of De La Rue Ocean is a curious double peninsula,
+called, in consequence of the dimness of former observations, _Hall
+Island_. The sharply defined, white-crested northern borders of De La
+Rue Ocean and Maraldi Sea may indicate the existence there of lofty
+coast ranges, more or less constantly covered with opaque clouds
+strongly reflecting light. The white spot in the centre of Maedler
+Continent, of a temporary nature, has a similar explanation. The
+intervals of olivaceous gray on Secchi Continent and elsewhere may
+perhaps be ascribed to the flooding and drying up of marshes and
+lowlands, as these markings have been observed to vary somewhat in
+connection with the change of seasons on Mars. The greenish tints
+observed along the planet's limb, alike on the darker and lighter
+surfaces, are probably due to an optical effect, the green being
+complementary to the prevailing red of the disk. The brilliant oval
+white spot near the southern (upper) pole of the planet is a so-called
+polar spot, in all probability consisting of a material similar to snow
+or ice and here observed in the midst of a dark open sea.
+
+_Approximate scale, 300 miles--1 inch._
+
+
+PLATE IX.--THE PLANET JUPITER.
+
+_Observed November_ 1, 1880, _at 9h. 30m. P. M._
+
+This planet is perpetually wrapped in dense clouds which hide its inner
+globe from view. The drawing shows Jupiter's outer clouded surface with
+its usual series of alternate light and dark belts, the disk as a whole
+appearing brighter in the centre than near the limb. The darker gray and
+black markings indicate in general the lower cloud-levels; that is,
+partial breaks or rifts in the cloudy envelope, whose prevailing depth
+apparently exceeds four thousand miles. While the deepest depression in
+the cloudy envelope is within the limits of the Great Red Spot, the
+vision may not even here penetrate very deeply. Two of Jupiter's four
+moons present bright disks near the planet's western limb, and cast
+their shadows far eastward on the disk, that of the "second satellite"
+falling upon the Red Spot. On the Red Spot are seen in addition two
+small black spots, no explanation of which can yet be offered. The broad
+white ring of clouds bordering the Red Spot appeared in constant motion.
+The central, or equatorial belt, shows brilliant cloudy masses of both
+the _cumulus_ and _stratus_ types, and the underlying gray and black
+cloudy surfaces are pervaded with the pinkish color characteristic of
+this belt. The dark circular spots on the wide white belt next north
+showed in their mode of formation striking resemblances to sun-spots.
+They afterward coalesced into a continuous pink belt. The diffusion of
+pinkish color over the three northernmost dark bands, as here observed,
+is unusual. About either pole is seen the uniform gray segment or polar
+cap. The equatorial diameter is noticeably longer than the polar
+diameter, a consequence of the planet's extraordinary swiftness of
+rotation. To the same cause may also be due chiefly the distribution of
+the cloudy belts parallel to the planet's equator, though the analogy of
+the terrestrial trade-winds fails to explain all the observed phenomena.
+
+_Approximate scale, 5,500 miles--1 inch._
+
+
+PLATE X.--THE PLANET SATURN.
+
+_Observed November_ 30, 1874, _at 5h. 50m. P. M._
+
+
+Saturn is unique amongst the planets in that its globe is encircled by a
+series of concentric rings, which lie in the plane of its equator, and
+consist, according to present theories, of vast throngs of minute bodies
+revolving about the planet, like so many satellites, in closely parallel
+orbits. The globe of Saturn, like that of Jupiter, is surrounded by
+cloudy belts parallel to its equator. The broad equatorial belt, of a
+delicate pinkish tint, is both brighter and more mottled than the
+narrower yellowish white belts, which alternate with darker belts of
+ashy gray on both the north and south sides, but are seen here only on
+the northern side. The disk has an oval shape, owing to the extreme
+polar compression of the globe.
+
+The outer, middle and inner rings, with their various subdivisions, are
+clearly shown in Plate X., and are best seen on the so-called _ansæ_,
+or handles, projecting on either side. The gray _outer ring_ is
+separated by the dusky pencil line into two divisions, both of which
+appear slightly mottled on the ansæ, as if with clouds. The _middle
+ring_ has three sub-divisions which are clearly distinguishable,
+although separated by no dark interval, viz., a brilliant white outer
+zone, distinctly mottled, as seen on the extremities of the ansæ, and
+two interior zones of gradually diminishing brightness. The _gauze_ or
+_dusky ring_ is seen at its full width on the ansæ, but on the
+background of the strongly illuminated globe only its outer and
+presumably denser border is visible. The shadow of the globe on the
+rings is seen on the lower portion of the eastern ansæ. The shadow on
+the dusky ring is with difficulty perceptible; the shadow on the middle
+ring is slightly concave toward the planet, which concavity is abruptly
+increased on the outer zone of this ring; while the shadow on the outer
+ring slants away from the globe. These appearances are fully accounted
+for by supposing a general increase of level from the inner edge of the
+dusky ring to the outer margin of the middle ring, and a uniform lower
+level on the outer ring. Other observers have regarded the deflection of
+the shadow as an effect of irradiation. The inner margin of the double
+outer ring presents on both ansæ a number of slight indentations,
+which, if not actual irregularities in the contour of this ring, may be
+explained as shadows caused by elevations on the outer border of the
+middle ring, or possibly by over-hanging clouds.
+
+_Approximate scale, 6,500 miles--1 inch._
+
+
+PLATE XI.--THE GREAT COMET OF 1881.
+
+_Observed on the night of June_ 25-26, _at 1h. 30m. A. M._
+
+
+A view of the comet 1881, III., drawn as if seen with the naked eye, the
+minute details, however, being reproduced as seen with the telescope.
+The star-like _nucleus_ is attended by four conical wings which cause it
+to appear diamond-shaped. The _coma_ appears double, the brilliant inner
+coma, immediately enveloping the nucleus, being surrounded by a fainter
+exterior coma, which has a noticeable depression corresponding to that
+of the inner edge of the principal coma. The tail is divided lengthwise
+by a dark rift and is brightest on its convex or forward side. An inner
+portion of the tail, brighter than the rest, is more strongly curved, as
+if by solar repulsion. Stars are seen through the brighter parts of the
+tail, as they may be seen even through the coma and nucleus, with little
+diminution of their light.
+
+
+PLATE XII.--THE NOVEMBER METEORS.
+
+_As observed on the night of November_ 13-14, 1868.
+
+
+A partially ideal view of the November Meteors, combining forms observed
+at different times during the night of Nov. 13th, 1868. It is not,
+however, a fanciful view, since a much larger number of meteors were
+observed falling at once during the shower of November, 1833, and at
+other times. The locality of the observation is shown by the Polar Star
+seen near the centre of the Plate, and Cassiopeia's Chair at the left.
+The general direction of the paths of the meteors is from the
+north-east, the _radiant point_ of the shower having been in the
+constellation Leo, beyond and above Ursa Major. While the orbits of the
+meteors are, in general, curved regularly and slightly, several are
+shown with very eccentric paths, among them one which changed its course
+at a sharp angle. In the upper left-hand corner appear two vanishing
+trails of the "ring-form," and several others still further transformed
+into faint luminous patches of cloud. Red, yellow, green, blue and
+purple tints were observed in the meteors and their trails, as
+represented in the Plate.
+
+
+PLATE XIII.--PART OF THE MILKY WAY.
+
+_From a study made during the years_ 1874, 1875 _and_ 1876.
+
+
+The course of the portion of the Galaxy represented in Plate XIII. is as
+follows: From Cassiopeia's chair, three bright stars of which appear at
+the upper edge of the Plate, the Galaxy, forming two streams, descends
+south, passing partly through Lacerta on the left, and Cepheus on the
+right; at this last point it approaches nearest to the Polar Star, which
+is itself outside of the field of view. Then it enters Cygnus, where it
+becomes very complicated and bright, and where several large cloudy
+masses are seen terminating its left branch, which passes to the right,
+near the bright star _Deneb_, the leader of this constellation. Below
+_Deneb_, the Galaxy is apparently disconnected and separated from the
+northern part by a narrow, irregular dark gap. From this rupture, the
+Milky-way divides into two great streams, separated by an irregular dark
+rift. An immense branch extends to the right, which, after having formed
+an important luminous mass between the stars _Gamma_ and _Beta_,
+continues its southward progress through parts of Lyra, Vulpecula,
+Hercules, Aquila and Ophiuchus, where it gradually terminates a few
+degrees south of the equator. The main stream on the left, after having
+formed a bright mass around _Epsilon Cygni_, passes through Vulpecula
+and then Aquila, where it crosses the equinoctial just below the star
+_Eta_, after having involved in its nebulosity the bright star _Altair_,
+the leader of Aquila. In the southern hemisphere the Galaxy becomes very
+complicated and forms a succession of very bright, irregular masses, the
+upper one being in Scutum Sobieskii, while the others are respectively
+situated in Sagittarius and in Scorpio; the last, just a little above
+our horizon, being always considerably dimmed by vapors. From Scutum
+Sobieskii, the Galaxy expands considerably on the right, and sends a
+branch into Scorpio, in which the fiery red star _Antares_ is somewhat
+involved. In the upper left-hand corner of the Plate, at some distance
+from the Milky-way, is seen dimly the Nebula in Andromeda, which becomes
+so magnificent an object to telescopic view.
+
+
+PLATE XIV.--STAR-CLUSTER IN HERCULES.
+
+_From a study made in June_, 1877.
+
+
+In the constellation Hercules, a small nebulous mass is faintly visible
+to the eye, a telescopic view of which is presented in Plate XIV. It is
+one of the most beautiful of the easily resolvable globular clusters.
+The brilliancy of the centre gives the cluster a distinctly globular
+appearance, while the several wings curving in various directions, have
+suggested to some observers an irregularly spiral structure. The large
+stars of the cluster are arranged in several groups which correspond, in
+a general way, with the faintly luminous wings.
+
+
+PLATE XV.--THE GREAT NEBULA IN ORION.
+
+_From a study made in the years_ 1875-76.
+
+
+This nebula, which is one of the most brilliant and wonderful of
+telescopic objects, readily visible to the naked eve as a patch of
+nebulous light immediately surrounding the middle star of the three
+which form the sword of Orion, and a little south of the three
+well-known stars forming the belt. The small stars in this, as in other
+Plates of the series, are somewhat exaggerated in size, as was
+unavoidable with any mode of reproduction that could be employed. The
+bright pentagonal centre of the nebula is traversed by less luminous
+rifts, the several subdivisions thus outlined being irregularly mottled
+as if by bright fleecy clouds. Toward the lower part of this bright
+pentagonal centre is a comparatively dark space containing four bright
+stars which form a trapezium and together constitute the quadruple star
+_Theta Orionis_, which, to the naked eye, appears as the single star in
+the centre of the sword. On three sides of the central mass extend long
+bright wisps, whose curves fail, however, to reveal the spiral structure
+often attributed to this nebula. On the east a broad wing, with
+wave-shaped inner border, stretches southward. East of the trapezium are
+two especially noticeable dark spaces. Close to the main nebula on the
+north-east, a small faint nebula surrounds a bright star, and a branch
+from another faint stream of nebulous matter forming a loop to the
+southward, encloses the nebulous star (_Iota Orionis_) shown at the top
+of the Plate.
+
+*** END OF THE PROJECT GUTENBERG EBOOK 68394 ***
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-<p style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Title: The Trouvelot astronomical drawings manual</p>
-<p style='display:block; margin-top:1em; margin-bottom:0; margin-left:2em; text-indent:-2em'>Author: Étienne Léopold Trouvelot</p>
-<p style='display:block; text-indent:0; margin:1em 0'>Release Date: June 24, 2022 [eBook #68394]</p>
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-<div style='margin-top:2em; margin-bottom:4em'>*** START OF THE PROJECT GUTENBERG EBOOK THE TROUVELOT ASTRONOMICAL DRAWINGS MANUAL ***</div>
-
-
-<div class="figcenter" style="width: 500px;">
-<img src="images/trouvelot_cover.jpg" width="500" alt="" />
-</div>
-
-<p><br /><br /><br /></p>
-
-<h1>THE<br />
-TROUVELOT<br />
-ASTRONOMICAL DRAWINGS<br />
-MANUAL</h1>
-
-<p><br /><br /></p>
-
-<h4>BY</h4>
-
-<h2>E. L. TROUVELOT,</h2>
-
-<p><br /><br /></p>
-
-<h3>FORMERLY CONNECTED WITH THE OBSERVATORY OF HARVARD COLLEGE; FELLOW OF THE<br />
-AMERICAN ACADEMY OF ARTS AND SCIENCES, AND MEMBER OF THE SELENO-GRAPHICAL<br />
-SOCIETY OF GREAT BRITAIN; IN CHARGE OF A<br />
-GOVERNMENT EXPEDITION TO OBSERVE THE<br />
-TOTAL SOLAR ECLIPSE OF 1878.</h3>
-
-<p><br /><br /></p>
-
-<h4>NEW YORK</h4>
-
-<h4>CHARLES SCRIBNER'S SONS</h4>
-
-<h5>1882</h5>
-
-<p><br /><br /><br /></p>
-
-<h4><a id="INTRODUCTION">INTRODUCTION</a></h4>
-
-<p>
-During a study of the heavens, which has now been continued for more
-than fifteen years, I have made a large number of observations
-pertaining to physical astronomy, together with many original drawings
-representing the most interesting celestial objects and phenomena.
-</p>
-<p>
-With a view to making these observations more generally useful, I was
-led, some years ago, to prepare, from this collection of drawings, a
-series of astronomical pictures, which were intended to represent the
-celestial phenomena as they appear to a trained eye and to an
-experienced draughtsman through the great modern telescopes, provided
-with the most delicate instrumental appliances. Over two years were
-spent in the preparation of this series, which consisted of a number of
-large drawings executed in pastel. In 1876, these drawings were
-displayed at the United States Centennial Exhibition at Philadelphia,
-forming a part of the Massachusetts exhibit, in the Department of
-Education and Science.
-</p>
-<p>
-The drawings forming the present series comprise only a part of those
-exhibited at Philadelphia; but, although fewer in number, they are quite
-sufficient to illustrate the principal classes of celestial objects and
-phenomena.
-</p>
-<p>
-While my aim in this work has been to combine scrupulous fidelity and
-accuracy in the details, I have also endeavored to preserve the natural
-elegance and the delicate outlines peculiar to the objects depicted; but
-in this, only a little more than a suggestion is possible, since no
-human skill can reproduce upon paper the majestic beauty and radiance of
-the celestial objects.
-</p>
-<p>
-The plates were prepared under my supervision, from the original pastel
-drawings, and great care has been taken to make the reproduction exact.
-</p>
-<p>
-The instruments employed in the observations, and in the delineation of
-the heavenly bodies represented in the series, have varied in aperture
-from 6 to 26 inches, according to circumstances, and to the nature of
-the object to be studied. The great Washington refractor, kindly placed
-at my disposal by the late Admiral C. H. Davis, has contributed to this
-work, as has also the 26 inch telescope of the University of Virginia,
-while in the hands of its celebrated constructors, Alvan Clark &amp; Sons.
-The spectroscope used was made by Alvan Clark &amp; Sons. Attached to it is
-an excellent diffraction grating, by Mr. L. M. Rutherfurd, to whose
-kindness I am indebted for it.
-</p>
-<p>
-Those unacquainted with the use of optical instruments generally suppose
-that all astronomical drawings are obtained by the photographic process,
-and are, therefore, comparatively easy to procure; but this is not true.
-Although photography renders valuable assistance to the astronomer in
-the case of the Sun and Moon, as proved by the fine photographs of these
-objects taken by M. Janssen and Mr. Rutherfurd; yet, for other subjects,
-its products are in general so blurred and indistinct that no details of
-any great value can be secured. A well-trained eye alone is capable of
-seizing the delicate details of structure and of configuration of the
-heavenly bodies, which are liable to be affected, and even rendered
-invisible, by the slightest changes in our atmosphere.
-</p>
-<p>
-The method employed to secure correctness in the proportions of the
-original drawings is simple, but well adapted to the purpose in view. It
-consists in placing a fine reticule, cut on glass, at the common focus
-of the objective and the eye-piece, so that in viewing an object, its
-telescopic image, appearing projected on the reticule, can be drawn very
-accurately on a sheet of paper ruled with corresponding squares. For a
-series of such reticules I am indebted to the kindness of Professor
-William A. Rogers, of the Harvard College Observatory.
-</p>
-<p>
-The drawings representing telescopic views are inverted, as they appear
-in a refracting telescope&mdash;the South being upward, the North downward,
-the East on the right, and the West on the left. The Comet, the
-Milky-Way, the Eclipse of the Moon, the Aurora Borealis, the Zodiacal
-Light and the Meteors are represented as seen directly in the sky with
-the naked eye. The Comet was, however, drawn with the aid of the
-telescope, without which the delicate structure shown in the drawing
-would not have been visible.
-</p>
-<p>
-The plate representing the November Meteors, or so-called "Leonids," may
-be called an ideal view, since the shooting stars delineated, were not
-observed at the same moment of time, but during the same night. Over
-three thousand Meteors were observed between midnight and five o'clock
-in the morning of the day on which this shower occurred; a dozen being
-sometimes in sight at the same instant. The paths of the Meteors,
-whether curved, wavy, or crooked, and also their delicate colors, are in
-all cases depicted as they were actually observed.
-</p>
-<p>
-In the Manual, I have endeavored to present a general outline of what is
-known, or supposed, on the different subjects and phenomena illustrated
-in the series. The statements made are derived either from the best
-authorities on physical astronomy, or from my original observations,
-which are, for the most part, yet unpublished.
-</p>
-<p>
-The figures in the Manual relating to distance, size, volume, mass,
-etc., are not intended to be strictly exact, being only round numbers,
-which can, therefore, be more easily remembered.
-</p>
-<p>
-It gives me pleasure to acknowledge that the experience acquired in
-making the astronomical drawings published in Volume VIII. of the Annals
-of the Harvard College Observatory, while I was connected with that
-institution, has been of considerable assistance to me in preparing this
-work; although no drawings made while I was so connected have been used
-for this series.
-</p>
-
-<p style="margin-left: 60%;">E. L. TROUVELOT.</p>
-
-<p style="margin-left: 5%;"><i>Cambridge, March, 1882.</i></p>
-
-<p><br /><br /><br /></p>
-
-<h4>CONTENTS</h4>
-
-<p class="nind"><a href="#INTRODUCTION">INTRODUCTION</a><br />
-
-<a href="#LIST_OF_PLATES">LIST OF PLATES</a></p>
-
-<h4><i>THE SUN</i></h4>
-
-<p class="nind"><a href="#SUN">GENERAL REMARKS ON THE SUN</a><br />
-
-<a href="#SUN_SPOTS">SUN-SPOTS AND VEILED SPOTS</a><br />
-
-<a href="#SOLAR_PROTUBERANCES">SOLAR PROTUBERANCES</a><br />
-
-<a href="#TOTAL_ECLIPSE">TOTAL ECLIPSE OF THE SUN</a></p>
-
-<h4><i>THE AURORAL AND ZODIACAL LIGHTS</i></h4>
-
-<p class="nind"><a href="#THE_AURORA_BOREALIS">THE AURORA BOREALIS</a><br />
-
-<a href="#THE_ZODIACAL_LIGHT">THE ZODIACAL LIGHT</a></p>
-
-<h4><i>THE MOON</i></h4>
-
-<p class="nind"><a href="#THE_MOON">THE MOON</a><br />
-
-<a href="#ECLIPSES">ECLIPSES OF THE MOON</a></p>
-
-<h4><i>THE PLANETS</i></h4>
-
-<p class="nind"><a href="#THE_PLANET_MARS">THE PLANET MARS</a><br />
-
-<a href="#THE_PLANET_JUPITER">THE PLANET JUPITER</a><br />
-
-<a href="#THE_PLANET_SATURN">THE PLANET SATURN</a></p>
-
-<h4><i>COMETS AND METEORS</i></h4>
-
-<p class="nind"><a href="#COMETS">COMETS</a><br />
-
-<a href="#SHOOTING_STARS">SHOOTING-STARS AND METEORS</a></p>
-
-<h4><i>THE STELLAR SYSTEMS</i></h4>
-
-<p class="nind"><a href="#THE_MILKY_WAY">THE MILKY-WAY OR GALAXY</a><br />
-
-<a href="#THE_STAR_CLUSTERS">THE STAR-CLUSTERS</a><br />
-
-<a href="#THE_NEBULAE">THE NEBULÆ</a><br />
-
-<a href="#APPENDIX">APPENDIX</a></p>
-
-<p><br /><br /><br /></p>
-
-<h4><a id="LIST_OF_PLATES">LIST OF PLATES</a><a name="FNanchor_1_1" id="FNanchor_1_1"></a><a href="#Footnote_1_1" class="fnanchor">[1]</a></h4>
-
-<h4>PLATE</h4>
-
-<p class="nind"><a href="#figure01">I. GROUP OF SUN-SPOTS AND VEILED SPOTS.</a><br />
-
-<span style="margin-left: 1.5em;"><i>Observed June 17, 1875, at 7 h. 30m. A. M.</i></span><br />
-
-<a href="#figure02">II. SOLAR PROTUBERANCES.</a><br />
-
-<span style="margin-left: 1.5em;"><i>Observed May 5, 1873, at 9h. 40m. A. M.</i></span><br />
-
-<a href="#figure03">III. TOTAL ECLIPSE OF THE SUN.</a><br />
-
-<span style="margin-left: 1.5em;"><i>Observed July 29, 1878, at Creston, Wyoming Territory.</i></span><br />
-
-<a href="#figure04">IV. AURORA BOREALIS.</a><br />
-
-<span style="margin-left: 1.5em;"><i>As observed March 1, 1872, at 9h. 25m. P. M.</i></span><br />
-
-<a href="#figure05">V. THE ZODIACAL LIGHT.</a><br />
-
-<span style="margin-left: 1.5em;"><i>Observed February 20, 1876.</i></span><br />
-
-<a href="#figure06">VI. MARE HUMORUM.</a><br />
-
-<span style="margin-left: 1.5em;"><i>From a study made in 1875.</i></span><br />
-
-<a href="#figure07">VII. PARTIAL ECLIPSE OF THE MOON.</a><br />
-
-<span style="margin-left: 1.5em;"><i>Observed October 24, 1874.</i></span><br />
-
-<a href="#figure08">VIII. THE PLANET MARS.</a><br />
-
-<span style="margin-left: 1.5em;"><i>Observed September 3, 1877, at 11h. 55m. P. M.</i></span><br />
-
-<a href="#figure09">IX. THE PLANET JUPITER.</a><br />
-
-<span style="margin-left: 1.5em;"><i>Observed November 1, 1880, at 9h. 30m. P. M.</i></span><br />
-
-<a href="#figure10">X. THE PLANET SATURN.</a><br />
-
-<span style="margin-left: 1.5em;"><i>Observed November 30, 1874, at 5th. 50m. P. M.</i></span><br />
-
-<a href="#figure11">XI. THE GREAT COMET OF 1881.</a><br />
-
-<span style="margin-left: 1.5em;"><i>Observed on the night of June 25-26, at 1h. 30m. A. M.</i></span><br />
-
-<a href="#figure12">XII. THE NOVEMBER METEORS.</a><br />
-
-<span style="margin-left: 1.5em;"><i>As observed between midnight and 3 o'clock A. M., on the night<br />
-of November 13-14, 1868.</i></span><br />
-
-<a href="#figure13">XIII. PART OF THE MILKY-WAY.</a><br />
-
-<span style="margin-left: 1.5em;"><i>From a study made during the years 1874, 1875 and 1876.</i></span><br />
-
-<a href="#figure14">XIV. STAR-CLUSTER IN HERCULES.</a><br />
-
-<span style="margin-left: 1.5em;"><i>From a study made in June, 1877.</i></span><br />
-
-<a href="#figure15">XV. THE GREAT NEBULA IN ORION.</a><br />
-
-<span style="margin-left: 1.5em;"><i>From a study made in the years 1873-76.</i></span></p>
-
-<p><br /></p>
-
-<div class="footnote">
-
-<p class="nind"><a name="Footnote_1_1" id="Footnote_1_1"></a><a href="#FNanchor_1_1"><span class="label">[1]</span></a>For Key to the Plates, see Appendix.</p></div>
-
-<p><br /></p>
-
-<h5>
-<i>Reproduced from the Original Drawings, by Armstrong &amp; Company,<br />
-Riverside Press, Cambridge, Mass.</i>
-</h5>
-
-<p><br /><br /><br /></p>
-
-<h4><a id="SUN">GENERAL REMARKS ON THE SUN</a></h4>
-
-<p>
-The Sun, the centre of the system which bears its name, is a
-self-luminous sphere, constantly radiating heat and light.
-</p>
-<p>
-Its apparent diameter, as seen at its mean from the Earth, subtends an
-angle of 32', or a little over half a degree. A dime, placed about six
-feet from the eye, would appear of the same proportions, and cover the
-Sun's disk, if projected upon it.
-</p>
-<p>
-That the diameter of the Sun does not appear larger, is due to the great
-distance which separates us from that body. Its distance from the Earth
-is no less than 92,000,000 miles. To bridge this immense gap, would
-require 11,623 globes like the Earth, placed side by side, like beads on
-a string.
-</p>
-<p>
-The Sun is an enormous sphere whose diameter is over 108 times the
-diameter of our globe, or very nearly 860,000 miles. Its radius is
-nearly double the distance from the Earth to the Moon. If we suppose,
-for a moment, the Sun to be hollow, and our globe to be placed at the
-centre of this immense spherical shell, not only could our satellite
-revolve around us at its mean distance of 238,800 miles, as now, but
-another satellite, placed 190,000 miles farther than the Moon, could
-freely revolve likewise, without ever coming in contact with the solar
-envelope.
-</p>
-<p>
-The circumference of this immense sphere measures 2,800,000 miles. While
-a steamer, going at the rate of 300 miles a day, would circumnavigate
-the Earth in 83 days, it would take, at the same rate, nearly 25 years
-to travel around the Sun.
-</p>
-<p>
-The surface of the Sun is nearly 12,000 times the surface of the Earth,
-and its volume is equal to 1,300,000 globes like our own. If all the
-known planets and satellites were united in a single mass, 600 such
-compound masses would be needed to equal the volume of our luminary.
-</p>
-<p>
-Although the density of the Sun is only one-quarter that of the Earth,
-yet the bulk of this body is so enormous that, to counterpoise it, no
-less than 314,760 globes like our Earth would be required.
-</p>
-<p>
-The Sun uniformly revolves around its axis in about 25½ days. Its
-equator is inclined 7° 15' to the plane of the ecliptic, the axis of
-rotation forming, therefore, an angle of 82° 45' with the same plane.
-As the Earth revolves about the Sun in the same direction as that of the
-Sun's rotation, the apparent time of this rotation, as seen by a
-terrestrial spectator, is prolonged from 25½ days to about 27 days and
-7 hours.
-</p>
-<p>
-The rotation of the Sun on its axis, like that of the Earth and the
-other planets, is direct, or accomplished from West to East. To an
-observer on the Earth, looking directly at the Sun, the rotation of this
-body is from left to right, or from East to West.
-</p>
-<p>
-The general appearance of the Sun is that of an intensely luminous disk,
-whose limb, or border, is sharply defined on the heavens. When its
-telescopic image is projected on a screen, or fixed on paper by
-photography, it is noticed that its disk is not uniformly bright
-throughout, but is notably more luminous in its central parts. This
-phenomenon is not accidental, but permanent, and is due in reality to a
-very rare but extensive atmosphere which surrounds the Sun, and absorbs
-the light which that body radiates, proportionally to its thickness,
-which, of course, increases towards the limb, to an observer on the
-Earth.
-</p>
-
-<p><br /><br /></p>
-
-<h4>THE ENVELOPING LAYERS OF THE SUN</h4>
-
-<p>
-The luminous surface of the Sun, or that part visible at all times, and
-which forms its disk, is called the <i>Photosphere</i>, from the property
-it is supposed to possess of generating light. The photosphere does not
-extend to a great depth below the luminous surface, but forms a
-comparatively thin shell, 3,000 or 4,000 miles thick, which is distinct
-from the interior parts, above which it seems to be kept in suspense by
-internal forces. From the observations of some astronomers it would
-appear that the diameter of the photosphere is subject to slight
-variations, and, therefore, that the solar diameter is not a constant
-quantity. From the nature of this envelope, such a result does not seem
-at all impossible, but rather probable.
-</p>
-<p>
-Immediately above the photosphere lies a comparatively thin stratum,
-less than a thousand miles in thickness, called the <i>Reversing Layer</i>.
-This stratum is composed of metallic vapors, which, by absorbing the
-light of particular refrangibilities emanating from the photosphere
-below, produces the dark Fraunhofer lines of the solar spectrum.
-</p>
-<p>
-Above the reversing layer, and resting immediately upon it, is a
-shallow, semi-transparent gaseous layer, which has been called the
-<i>Chromosphere</i>, from the fine tints which it exhibits during total
-eclipses of the Sun, in contrast with the colorless white light radiated
-by the photosphere below. Although visible to a certain extent on the
-disk, the chromosphere is totally invisible on the limb, except with the
-spectroscope, and during eclipses, on account of the nature of its
-light, which is mainly monochromatic, and too feeble, compared with that
-emitted by the photosphere, to be seen.
-</p>
-<p>
-The chromospheric layer, which has a thickness of from 3,000 to 4,000
-miles, is uneven, and is usually upheaved in certain regions, its matter
-being transported to considerable elevations above its general surface,
-apparently by some internal forces. The portions of the chromosphere
-thus lifted up, form curious and complicated figures, which are known
-under the names of <i>Solar Protuberances, or Solar Flames</i>.
-</p>
-<p>
-Above the chromosphere, and rising to an immense but unknown height, is
-the solar atmosphere proper, which is only visible during total eclipses
-of the Sun, and which then surrounds the dark body of the Moon with the
-beautiful rays and glorious nimbus, called the <i>Corona</i>.
-</p>
-<p>
-These four envelopes: the photosphere, the reversing layer, the
-chromosphere, and the corona, constitute the outer portions of our
-luminary.
-</p>
-<p>
-Below the photosphere little can be seen, although it is known, as will
-appear below, that at certain depths cloud-like forms exist, and freely
-float in an interior atmosphere of invisible gases. Beyond this all is
-mystery, and belongs to the domain of hypothesis.
-</p>
-
-<p><br /><br /></p>
-
-<h4>STRUCTURE OF THE PHOTOSPHERE AND CHROMOSPHERE</h4>
-
-<p>
-The apparent uniformity of the solar surface disappears when it is
-examined with a telescope of sufficient aperture and magnifying powers.
-Seen under good atmospheric conditions, the greater part of the solar
-surface appears mottled with an infinite number of small, bright
-granules, irregularly distributed, and separated from each other by a
-gray-tinted background.
-</p>
-<p>
-These objects are known under different names. The terms granules and
-granulations answer very well for the purpose, as they do not imply
-anything positive as to their form and true nature. They have also been
-called <i>Luculœ</i>, <i>Rice Grains</i>, <i>Willow Leaves</i>, etc., by
-different observers.
-</p>
-<p>
-Although having different shapes, the granulations partake more or less
-of the circular or slightly elongated form. Their diameter, which varies
-considerably, has been estimated at from 0".5 to 3", or from 224 to
-1,344 miles. The granulations which attain the largest size appear,
-under good atmospheric conditions, to be composed of several granules,
-closely united and forming an irregular mass, from which short
-appendages protrude in various directions.
-</p>
-<p>
-The number of granulations on the surface of the Sun varies considerably
-under the action of unknown causes. Sometimes they are small and very
-numerous, while at other times they are larger, less numerous, and more
-widely separated. Other things being equal, the granulations are better
-seen in the central regions of the Sun than they are near the limb.
-</p>
-<p>
-Usually the granulations are very unstable; their relative position,
-form, and size undergoing continual changes. Sometimes they are seen to
-congregate or to disperse in an instant, as if acting under the
-influence of attractive and repulsive forces; assembling in groups or
-files, and oftentimes forming capricious figures which are very
-remarkable, but usually of short duration. In an area of great solar
-disturbances, the granulations are often stretched to great distances,
-and form into parallel lines, either straight, wavy, or curved, and they
-have then some resemblance to the flowing of viscous liquids.
-</p>
-<p>
-The granulations are usually terminated either by rounded or sharply
-pointed summits, but they do not all rise to the same height, as can be
-ascertained with the spectroscope when they are seen sidewise on the
-limb. In the regions where they are most abundant, they usually attain
-greater elevations, and when observed on the limb with the spectroscope,
-they appear as slender acute flames.
-</p>
-<p>
-The granulations terminated by sharply-pointed crests, although observed
-in all latitudes, seem to be characteristic of certain regions. A daily
-study of the chromosphere, extending over a period of ten years, has
-shown me that the polar regions are rarely ever free from these objects,
-which are less frequent in other parts of the Sun. In the polar regions
-they are sometimes so abundant that they completely form the solar limb.
-These forms of granulation are comparatively rare in the equatorial
-zones, and when seen there, they never have the permanency which they
-exhibit in the polar regions. When observed in the equatorial regions,
-they usually appear in small groups, in the vicinity of sun spots, or
-they are at least enclosed in areas of disturbances where such spots are
-in process of formation. In these regions they often attain greater
-elevations than those seen in high latitudes.
-</p>
-<p>
-As we are certain that in the equatorial zones these slender flames (<i>i.
-e.</i>, granulations) are a sure sign of local disturbance, it may be
-reasonably supposed that the same kind of energy producing them nearly
-always prevails in the polar regions, although it is there much weaker,
-and never reaches beyond certain narrow limits.
-</p>
-<p>
-Studied with the spectroscope, the granulations are found to be composed
-in the main of incandescent hydrogen gas, and of an unknown substance
-provisionally called "helium." Among the most brilliant of them are
-found traces of incandescent metallic vapors, belonging to various
-substances found on our globe.
-</p>
-<p>
-The chromosphere is not fixed, but varies considerably in thickness in
-its different parts, from day to day. Its thickness is usually greater
-in the polar regions, where it sometimes exceeds 6,700 miles. In the
-equatorial regions the chromosphere very rarely attains this height, and
-when it does, the rising is local and occupies only a small area. In
-these regions it is sometimes so shallow that its depth is only a few
-seconds, and is then quite difficult to measure. These numbers give, of
-course, the extreme limits of the variations of the chromosphere; but,
-nearly always, it is more shallow in the equatorial regions; and, as far
-as my observations go, the difference in thickness between the polar and
-equatorial zones is greater in years of calm than it is in years of
-great solar activity. But ten years of observation are not sufficient to
-warrant any definite conclusions on this subject.
-</p>
-<p>
-There is undoubtedly some relation between the greater thickness of the
-chromosphere in the polar regions, and the abundance and permanence of
-the sharply-pointed granulations observed in the same regions. This
-becomes more evident when we know that the appearance of
-similarly-pointed flames in the equatorial zones is always accompanied
-with a local thickening of the chromosphere. The thickening in the polar
-regions may be only apparent, and not due to a greater accumulation of
-chromospheric gases there; but may be caused by some kind of repulsive
-action or polarity, which lifts up and extends the summit of the
-granulations in a manner similar to the well-known mode of electric
-repulsion and polarity.
-</p>
-<p>
-As it seems very probable that the heat and light emanating from the Sun
-are mainly generated at the base of the granulations, in the filamentary
-elements composing the chromosphere and photosphere, it would follow
-that, as the size and number of these objects constantly vary, the
-amount of heat and light emitted by the Sun should also vary in the same
-proportion.
-</p>
-<p>
-The granulations of the solar surface are represented on Plate I., and
-form the general background to the group of Sun-spots forming the
-picture.
-</p>
-
-<p><br /><br /></p>
-
-<h4>THE FACULÆ</h4>
-
-<p>
-Although the solar surface is mainly covered with the luminous
-granulations and the grayish background above described, it is very rare
-that its appearance is so simple and uniform as already represented. For
-the most part, on the contrary, it is diversified by larger, brighter,
-and more complicated forms, which are especially visible towards the
-border of the Sun. Owing to their extraordinary brilliancy, these
-objects have been called <i>Faculœ</i> (torches).
-</p>
-<p>
-Although the faculæ are very seldom seen well beyond 50 heliocentric
-degrees from the limb, yet they exist, and are as numerous in the
-central parts of the disk as they are towards the border; since they
-form a part of the solar surface, and participate in its movement of
-rotation. Their appearance near the limb has been attributed to the
-effect of absorption produced by the solar atmosphere on the
-light from the photosphere; but this explanation seems inadequate,
-and does not solve the problem. The well-known fact that the solar
-protuberances&mdash;which are in a great measure identical with the
-faculæ&mdash;are much brighter at the base than they are at the summit,
-perhaps gives a clue to the explanation of the phenomenon; especially
-since we know that, in general, the summit of the protuberances is
-considerably broader than their base. When these objects are observed in
-the vicinity of the limb, they present their brightest parts to the
-observer, since, in this position, they are seen more or less sidewise;
-and, therefore, they appear bright and distinct. But as the faculæ
-recede from the limb, their sides, being seen under a constantly
-decreasing angle, appear more and more foreshortened; and, therefore,
-these objects grow less bright and less distinct, until they finally
-become invisible, when their bases are covered over by the broad, dusky
-summit generally terminating the protuberances.
-</p>
-<p>
-The faculæ appear as bright and luminous masses or streaks on the
-granular surface of the Sun, but they differ considerably in form and
-size. Two types at least are distinguishable. In their simplest form
-they appear either as isolated white spots, or as groups of such spots
-covering large surfaces, and somewhat resembling large flakes of snow.
-In their most characteristic types they appear as intensely luminous,
-heavy masses, from which, in most cases, issue intricate ramifications,
-sometimes extending to great distances. Generally, the ramifications
-issuing from the masses of faculæ have their largest branches directed
-in the main towards the eastern limb of the Sun. Some of these branches
-have gigantic proportions. Occasionally they extend over 60° and even
-80° of the solar surface, and, therefore, attain a length of from
-450,000 to 600,000 miles.
-</p>
-<p>
-Although the faculæ may be said to be seen everywhere on the surface of
-the Sun, there is a vast difference in different regions, with regard to
-their size, number, and brilliancy. They are largest, most abundant, and
-brightest on two intermediate zones parallel to the solar equator, and
-extending 35° or 40° to the north and to the south of this line. The
-breadth of these zones varies considerably with the activity of the
-solar forces. When they are most active, the faculæ spread on either
-side, but especially towards the equator, where they sometimes nearly
-meet those of the other zone. In years of little solar activity the
-belts formed by the faculæ are very narrow&mdash;the elements composing
-them being very few and small, although they never entirely disappear.
-</p>
-<p>
-The faculæ are very unstable, and are constantly changing: those of the
-small types sometimes form and vanish in a few minutes. When an area of
-disturbance of the solar surface is observed for some time, all seems in
-confusion; the movements of the granulations become unusually violent;
-they congregate in all sorts of ways, and thus frequently form temporary
-faculæ. Action of this kind is, for the most part, peculiar to the
-polar regions of the Sun.
-</p>
-<p>
-The larger faculæ have undoubtedly another origin, as they seem to be
-mainly formed by the ejection of incandescent gases and metallic vapors
-from the interior of the photosphere. In their process of development
-some of the heavy masses of faculæ are swollen up to great heights,
-being torn in all sorts of ways, showing large rents and fissures
-through which the sight can penetrate.
-</p>
-<p>
-Very few faculæ are represented in Plate I.; several streaks are shown
-at the upper left-hand corner, some appearing as whitish ramifications
-among the granulations representing the general solar surface.
-</p>
-
-<p><br /><br /><br /></p>
-
-<h4><a id="SUN_SPOTS">SUN-SPOTS AND VEILED SPOTS</a>
-<br /><br />
-PLATE I</h4>
-
-<p>
-Besides the brilliant faculæ already described, much more conspicuous
-markings, though of a totally different character, are very frequently
-observed on the Sun. On account of their darkish appearance, which is in
-strong contrast with the white envelope of our luminary, these markings
-were called <i>Maculæ</i>, or Sun-spots, by their earlier observers.
-</p>
-<p>
-The Sun-spots are not equally distributed on the solar surface; but like
-the faculæ, to which they are closely related, they occupy two
-zones&mdash;one on each side of the equator. These zones are comprised
-between 10° and 35° of north latitude, and 10° and 35° of south
-latitude. Between these two zones is a belt 20° in width, where the
-Sun-spots are rarely seen.
-</p>
-<p>
-Above the latitudes 35° north and south, the Sun-spots are rare, and it
-is only occasionally, and during years of great solar activity, that
-they appear in these regions; in only a few cases have spots of
-considerable size been seen there. A few observers, however, have seen
-spots as far as 40° and 50 from the equator; and La Hire even observed
-one in 70° of north latitude; but these cases are exceedingly rare. It
-is not uncommon, however, to see very small spots, or groups of such
-spots, within 8° or 10° from the poles.
-</p>
-<p>
-The activity of the Sun is subject to considerable fluctuation, and
-accordingly the Sun-spots vary in size and number in different years.
-During some years they are large, complicated, and very numerous; while
-in others they are small and scarce, and are sometimes totally absent
-for weeks and months together. The fluctuations in the frequency of
-Sun-spots are supposed to be periodical in their character, although
-their periods do not always appear to recur at exactly regular
-intervals. Sometimes the period is found to be only nine years, while at
-other times it extends to twelve years. The period generally adopted now
-is 11⅒ years, nearly; but further investigations are needed to
-understand the true nature of the phenomenon.
-</p>
-<p>
-The number of Sun-spots does not symmetrically augment and diminish, but
-the increase is more rapid than the diminution.
-</p>
-<p>
-The period of increase is only about four years, while that of decrease
-is over seven years; each period of Sun-spot maximum being nearer the
-preceding period of Sun-spot minimum than it is to that next following.
-</p>
-<p>
-The cause of these fluctuations in the solar energy is at present wholly
-unknown. Some astronomers, however, have attributed it to the influence
-of the planets Venus and Jupiter, the period of revolution of the latter
-planet being not much longer than the Sun-spot period; but this
-supposition lacks confirmation from direct observations, which, so far,
-do not seem to be in favor of the hypothesis. At the present time the
-solar activity is on the increase, and the Sun-spots will probably reach
-their maximum in 1883. The last minimum occurred in 1879, when only
-sixteen small groups of spots were observed during the whole year.
-</p>
-<p>
-Sun-spots vary in size and appearance; but, unless they are very small,
-in which case they appear as simple black dots, they generally consist
-of two distinct and well-characterized parts, nearly always present.
-There is first, a central part, much darker than the other, and sharply
-divided from it, called the "<i>Umbra</i>;" second, a broad, irregular
-radiated fringe of lighter shade, completely surrounding the first, and
-called the "<i>Penumbra</i>."
-</p>
-<p>
-Reduced to its simplest expression, a Sun-spot is a funnel-shaped
-opening through the chromosphere and the photosphere. The inner end of
-the funnel, or opening, gives the form to the umbra, while its sloping
-sides form the penumbra.
-</p>
-<p>
-The umbra of Sun-spots, whose outlines approximately follow the
-irregularities of the penumbral fringe, has a diameter which generally
-exceeds the width of the penumbral ring. Sometimes it appears uniformly
-black throughout; but it is only so by contrast, as is proved when
-either Mercury or Venus passes near a spot during a transit over the
-Sun's disk. The umbra then appears grayish, when compared with the
-jet-black disk of the planet.
-</p>
-<p>
-The umbra of spots is rarely so simple as just described; but it is
-frequently occupied, either partly or wholly, by grayish and rosy forms,
-somewhat resembling loosely-entangled muscular fibres. These forms have
-been called the <i>Gray and Rosy Veils</i>. Frequently these veils appear
-as if perforated by roundish black holes, improperly called <i>Nuclei</i>,
-which permit the sight to penetrate deeper into the interior. To all
-appearance the gray and rosy veils are of the same nature as the
-chromosphere and the faculæ, and are therefore mainly composed of
-hydrogen gas.
-</p>
-<p>
-Whatever can be known about the interior of the Sun, must be learned
-from the observations of these openings, which are comparatively small.
-But whatever this interior may be, we certainly know that it is not
-homogeneous. Apparently, the Sun is a gigantic bubble, limited by a very
-thin shell. Below this shell exists a large open space filled with
-invisible gases, in which, through the openings constituting the
-Sun-spots, the gray and rosy veils described above are occasionally seen
-floating.
-</p>
-<p>
-The fringe forming the penumbra of spots is much more complicated than
-the umbra. In its simpler form, it is composed of a multitude of bright,
-independent filaments of different forms and sizes, partly projecting
-one above the other, on the sloping wall of the penumbra, from which
-they seem to proceed. Seen from the Earth, these filaments have somewhat
-the appearance of thatched straw, converging towards the centre of the
-umbra. It is very rare, however, that the convergence of the penumbral
-filaments is regular, and great confusion sometimes arises from the
-entanglement of these filaments. Some of these elements appear straight,
-others are curved or loop-shaped; while still others, much larger and
-brighter than the rest, give a final touch to this chaos of filaments,
-from which results the general thatched and radiating appearance of the
-penumbra.
-</p>
-<p>
-The extremities of the penumbral filaments, especially of those forming
-the border of the umbra, are usually club-shaped and appear very
-brilliant, as if these elements had been superheated by some forces
-escaping through the opening of the spots.
-</p>
-<p>
-Besides these characteristics, the Sun-spots have others, which,
-although not always present, properly belong to them. Comparatively few
-spots are so simple as the form just described. Very frequently a spot
-is accompanied by brilliant faculæ, covering part of its umbra and
-penumbra, and appearing to form a part of the spot itself.
-</p>
-<p>
-When seen projected over Sun-spots, the faculæ appear intensely bright,
-and from these peculiarities they have been called <i>Luminous Bridges</i>.
-They are, in fact, bridges, but in most cases they are at considerable
-heights above the spots, kept there by invisible forces. When such spots
-with luminous bridges approach the Sun's limb, it is easy to see, by the
-rapid apparent displacement which they undergo, that they are above the
-general level.
-</p>
-<p>
-When the spots are closing up, the inverse effect is sometimes observed.
-On several occasions, I have seen huge masses of faculæ advance slowly
-over the penumbra of a spot and fall into the depths of the umbra,
-resembling gigantic cataracts. I have seen narrow branches of faculæ,
-which, after having fallen to great depths in the umbra, floated across
-it and disappeared under the photosphere on the opposite side. I have
-also seen luminous bridges, resembling cables, tightly stretched across
-the spots, slackening slowly, as if loosened at one end, and gently
-curving into the umbra, where they formed immense loops, large enough to
-receive our globe.
-</p>
-<p>
-It is to be remarked that, in descending under the photospheric shell,
-the bright faculæ and the luminous bridges gradually lose their
-brilliancy. At first they appear grayish, but in descending farther they
-assume more and more the pink color peculiar to the rosy veils. The
-pinkish color acquired by the faculæ when they reach a certain depth
-under the photosphere, is precisely the color of the chromosphere and of
-the solar protuberances, as seen during total eclipses of the Sun&mdash;a
-fact which furnishes another proof that the faculæ are of the same
-nature as the protuberances.
-</p>
-<p>
-I record here an observation which, at first sight, may appear
-paradoxical; but which seems, however, to be of considerable importance,
-as it shows unmistakably that the solar light is mainly, if not
-entirely, generated on its surface, or at least very near to it. On May
-26, 1878, I observed a large group of Sun-spots at a little distance
-from the east limb of the Sun. The spot nearest to the limb was partly
-covered over on its eastern and western sides by bright and massive
-faculæ which concealed about two-thirds of the whole spot, only a
-narrow opening, running from north to south, being left across the
-middle of the spot. Owing to the rotundity of the Sun, the penumbra of
-this spot, although partly covered by the faculæ, could, however, be
-seen on its eastern side, since the sight of the observer could there
-penetrate sidewise under the faculæ. Upon that part of the penumbra
-appeared a strong shadow, representing perfectly the outline of the
-facular mass situated above it. The phenomenon was so apparent that no
-error of observation was possible, and a good drawing of it was secured.
-If this faculæ had been as bright beneath as it was above, it is
-evident that no shadow could have been produced; hence the light of
-these faculæ must have been mainly generated on or very near their
-exterior surfaces. This, with the well-proved fact that the bright
-faculæ lose their light in falling into the interior of the Sun, seems
-to suggest the idea that the bright light emitted by the faculæ, and
-very probably all the solar light, can be generated only on its surface;
-the presence of the coronal atmosphere being perhaps necessary to
-produce it. Several times before this observation, I had suspected that
-some faculæ were casting a shadow, but as this seemed so improbable, my
-attention was not awakened until the phenomenon became so prominent that
-it could not escape notice.
-</p>
-<p>
-With due attention, some glimpses of the phenomenon can frequently be
-observed through the openings of some of the faculæ projecting over the
-penumbra of Sun-spots. It is very seldom that the structure of the
-penumbra is seen through such openings, which usually appear as dark as
-the umbra of the large spots, although they do not penetrate through the
-photosphere like the latter. It is only when the rents in the faculæ
-are numerous and quite large, that the penumbral structure is recognized
-through them. Since these superficial rents in the faculæ do not extend
-through the photosphere, and appear black, it seems evident that the
-penumbra seen through them cannot be as bright as it is when no faculæ
-are projected upon it, and therefore that the faculæ intercept light
-from the exterior surface, which would otherwise reach the penumbra.
-</p>
-<p>
-While the matter forming the faculæ sometimes falls into the interior
-of the Sun, the same kind of matter is frequently ejected in enormous
-quantities, and with great force, from the interior, through the visible
-and invisible openings of the photosphere, and form the protuberances
-described in the following section of this manual (Solar protuberances)
-It is not only the incandescent hydrogen gas or the metallic vapors
-which are thus ejected, but also cooler hydrogen gas, which sometimes
-appears as dark clouds on the solar surface. On December 12, 1875, I
-observed such a cloud of hydrogen issuing from the corner of a small
-Sun-spot. It traveled several thousand miles on the solar surface, in a
-north-easterly direction, before it became invisible.
-</p>
-<p>
-Solar spots are formed in various ways; but, for the most part, the
-apparition of a spot is announced beforehand, by a great commotion of
-the solar surface at the place of its appearance, and by the formation
-of large and bright masses of faculæ, which are usually swollen into
-enormous bubbles by the pressure of the internal gases. These bubbles
-become visible in the spectroscope while they are traversing the solar
-limb, as they are then presented to us sidewise. Under the action of the
-increasing pressure, the base of the faculæ is considerably stretched,
-and, its weakest side finally giving way, the facular mass is torn in
-many places from the solar surface, and is perforated by holes of
-different sizes and forms. The holes thus made along the border of the
-faculæ appear as small black spots, separated more or less by the
-remaining portion of the lacerated faculæ, and they enlarge more and
-more at the expense of the intervening portions, which thus become very
-narrow. This perforated side of the faculæ, offering less resistance,
-is gradually lifted up, as would be the cover of a box, for example,
-while its opposite side remains attached to the surface. The facular
-matter separating the small black holes is greatly stretched during this
-action, and forms long columns and filaments. These appear as luminous
-bridges upon the large and perfectly-formed spot, which is then seen
-under the lifted facular masses. The spots thus made visible are soon
-freed from the facular masses, which are gradually shifted towards the
-opposite side.
-</p>
-<p>
-In such cases the spots are undoubtedly formed under the faculæ before
-they can be seen. This becomes evident when such spots, not yet cleared
-from the faculæ covering them, are observed near the east limb; since
-in this position the observer can see through the side-openings of the
-faculæ, and sometimes recognize the spots under their cover.
-</p>
-<p>
-It frequently occurs that the spots thus formed under the faculæ
-continue to be partly covered by the facular clouds, the forces at work
-in them being apparently too feeble to shift them aside. In such cases
-these spots are visible when they are in the vicinity of the limb, where
-they are seen sidewise; but when observed in the east, in being carried
-forward by the solar rotation towards the centre of the disk, they
-gradually diminish in size, and finally become invisible. The
-disappearance of these spots, however, is only apparent, being due to
-the fact that, as they advance towards the centre of the disk, our
-lateral view of them is gradually lost, and they are finally hidden from
-sight by the overhanging faculæ which then serve as a screen between
-the observer and the spot. This class of spots may be called <i>Lateral
-Spots</i>, from the fact that they can only be seen laterally, and near the
-Sun's limb.
-</p>
-<p>
-Solar spots are also formed in various other ways. Some, like those
-represented in Plate I., appearing without being announced by any
-apparent disturbance of the surface, or by the formation of any faculæ,
-form and develop in a very short time. Others, appearing at first as
-very small spots having an umbra and a penumbra, slowly and gradually
-develop into very large spots. This mode of formation, which would seem
-to be the most natural, is, however, quite rare. Spots of this class
-have a duration and permanence not observed in those of any other type.
-These spots of slow and regular development are never accompanied by
-faculæ or luminous bridges, nor have they any gray or rosy veils in
-their interior; a fact which may, perhaps, account for their permanent
-character.
-</p>
-<p>
-Another class of spots, which is also rare, appear as long and narrow
-crevasses showing the penumbral structure of the ordinary spots; but
-these rarely have any umbra. These long, and sometimes exceedingly
-narrow fissures of the solar envelopes, with their radiated penumbral
-structure, strongly suggest the idea that the photosphere is composed of
-a multitude of filamentary elements having the granulations for summits.
-Such a crevasse is represented on Plate I., and unites the two spots
-which form the group.
-</p>
-<p>
-The duration of Sun-spots varies greatly. Some last only for a few
-hours; while others continue for weeks and even months at a time, but
-not without undergoing changes.
-</p>
-<p>
-The modes of disappearance of Sun-spots are as various as those of their
-apparition. The spots rarely close up by a gradual diminution or
-contraction of their umbra and penumbra. This mode of disappearance
-belongs exclusively to the spots deprived of faculæ and veils. One of
-the most common modes of the disappearance of a spot is its invasion by
-large facular masses, which slowly advance upon its penumbra and umbra
-and finally cover it entirely. It is a process precisely the reverse of
-that in which spots are formed by the shifting aside of the faculæ, as
-above described. In other types, the spots close up by the gradual
-enlargement of the luminous bridges traversing them, which are slowly
-transformed into branches of the photosphere, all of the characteristics
-of which they have acquired. In many cases, the spots covered over by
-the faculæ continue to exist for some time, hidden under these masses,
-as is often proved, either by the appearance of small spots on the
-facular mass left at the place they occupied, or even by the
-reappearance of the same spot.
-</p>
-<p>
-Apart from the general movement of rotation of the solar surface, some
-of the spots seem to be endowed with a proper motion of their own, which
-becomes greater the nearer the spots are to the solar equator. According
-to the observations of Mr. Carrington, the period of rotation of the
-Sun, as deduced from the observations of the solar spots during a period
-of seven years, is 25 days at the equator; while at 50° of heliocentric
-latitude it is 27 days. But the period of rotation, as derived from the
-observations of spots occupying the same latitude, is far from being
-constant, as it varies at different times, with the frequency of the
-spots and with the solar activity, so that at present the law of these
-variations is not well known. From the character of the solar envelope,
-it seems very natural that the rotation should differ in the different
-zones and at different times, since this envelope is not rigid, but very
-movable, and governed by forces which are themselves very variable.
-</p>
-<p>
-Although it is a general law that the spots near the equator have a more
-rapid motion than those situated in higher latitudes, yet, in many
-cases, the proper motion of the spots is more apparent than real. For
-the most part, the changes of form and the rapid displacements observed
-in some spots are only apparent, and due to the fact that the large
-masses of faculæ which are kept in suspense above them are very
-unstable, and change position with the slightest change in the forces
-holding them in suspension. Since in these cases we view the spots
-through the openings of the faculæ situated above them, the slightest
-motion of these objects produces an apparent motion in the spots,
-although they have remained motionless. Accordingly, it has been
-remarked that of all the spots, those which have the greater proper
-motion are precisely those which have the most faculæ and luminous
-bridges; while the other spots in the same regions, but not attended by
-similar phenomena, are comparatively steady in their movement. These
-last spots are undoubtedly better adapted than any others to exhibit the
-rotation of the Sun; but it is probable that this period of rotation
-will never be known with accuracy, simply because the solar surface is
-unstable, and does not rotate uniformly.
-</p>
-<p>
-The Sun-spots have a remarkable tendency to form into groups of various
-sizes, but whatever may be the number of spots thus assembled, the group
-is nearly always composed of two principal spots, to which the others
-are only accessories. The tendency of the Sun-spots to assemble in pairs
-is general, and is observed in all latitudes, even among the minute
-temporary groups formed in the polar regions. Whenever several are
-situated quite close together, those belonging to the same group can be
-easily recognized by this character. Whatever may be the position of the
-axis of the two principal spots of a group when it is first formed, this
-axis has a decided tendency to place itself parallel to the solar
-equator, no matter to what latitude the group belongs; and if it is
-disturbed from this position, it soon returns to it when the disturbance
-has ceased.
-</p>
-<p>
-It is also remarkable that the spots observed at the same time remain in
-nearly the same parallel of latitude for a greater or less period of
-time; but they keep changing their position from year to year, their
-latitude decreasing with the activity of the solar forces.
-</p>
-<p>
-Among the Sun-spots, those associated with faculæ form the groups which
-attain the largest proportions. When such groups acquire an apparent
-diameter of 1' or more, they are plainly visible to the naked eye, since
-for a spot to be visible to the naked eye on the Sun, it need only
-subtend an angle of 50". I have sometimes seen such groups through a
-smoky atmosphere, when the solar light was so much reduced that the disk
-could be observed directly and without injury to the sight.
-</p>
-<p>
-The largest spot which ever came under my observation was seen during
-the period from the 13th to the 19th of November, 1870. This spot, which
-was on the northern hemisphere of the Sun, was conspicuous among the
-smaller spots constituting the group to which it belonged, and followed
-them on the east. On November 16th, when it attained its largest size,
-the diameter of its penumbra occupied fully one-fifth of the diameter of
-the Sun; its real diameter being, therefore, not less than 172,000
-miles, or nearly 22 times the diameter of the Earth. As the umbra of
-this spot occupied a little more than one-third of its whole diameter,
-seven globes like our own, placed side by side on a straight line, could
-easily have passed through this immense gap. To fill the area of this
-opening, about 45 such globes would have been needed. This spot was, of
-course, very easily seen with the naked eye, its diameter being almost
-eight times that required for a spot to be visible without a telescope.
-</p>
-<p>
-Ancient historians often speak of obscurations of the Sun, and it has
-been supposed by some astronomers that this phenomenon might have been
-due in some cases to the apparition of large spots. A few spots on the
-surface of the Sun, like that just described, would sensibly reduce its
-light.
-</p>
-<p>
-Besides the ordinary Sun-spots already described, others are at times
-observed on the surface of the Sun, which show some of the same
-characteristics, but never attain so large proportions. They always
-appear as if seen through a fog, or veil, between the granulations of
-the solar surface. On account of their vagueness and ill-defined contours,
-I have proposed for these objects the term, "<i>Veiled Spots</i>."
-Veiled spots have a shorter duration than the ordinary spots, the
-smaller types sometimes forming and vanishing in a few minutes. Some of
-the larger veiled spots, however, remain visible for several days in
-succession, and show the characteristics of other spots in regard to the
-arrangement of their parts.
-</p>
-<p>
-The veiled spots have no umbra or penumbra, although they are usually
-accompanied by faculæ resembling those seen near the ordinary spots.
-They are frequently seen in the polar regions, but are there always of
-small size and of short duration. The veiled spots are larger, and more
-apt to arrange themselves into groups, in the regions occupied by the
-ordinary spots, and it is not rare to observe such spots transform
-themselves into ordinary spots, and vice versa. The veiled spots,
-therefore, seem to be ordinary spots filled up, or covered over by the
-granulations and semi-transparent gases composing the chromospheric
-layer. That it is so, becomes more evident, from the fact that large
-Sun-spots in process of diminution are sometimes gradually covered with
-faint and scattered granules which descend in long, narrow filaments,
-and become less and less distinguishable as they attain greater depth.
-This phenomenon, associated with the fact that the luminous bridges seen
-over the Sun-spots which are closing up are sometimes transformed into
-branches which show the characteristic structure of the photosphere,
-goes far to prove that the solar envelopes are mainly composed of an
-innumerable quantity of radial filaments of varying height.
-</p>
-
-<p><br /></p>
-
-<div class="figcenter" style="width: 400px;">
-<a id="figure01"></a>
-<br />
-<img src="images/figure01.jpg" width="400" alt="" />
-<div class="caption">
-<p>PLATE I.&mdash;GROUP OF SUN-SPOTS AND VEILED SPOTS.</p>
-<p class="smaller">Observed on June 17th 1875 at 7 h. 30 m. A.M.</p>
-</div></div>
-
-<p><br /></p>
-
-<p>
-The group of Sun-spots represented in Plate I., was observed and drawn
-on June 17th, 1875, at 7h. 30m. A. M. The first traces of this group
-were seen on June 15th, at noon, and consisted of three small black dots
-disseminated among the granulations. At that time, no disturbance of the
-surface was noticeable, and no faculæ were seen in the vicinity of these
-spots. On June 16th, at 8 o'clock A. M., the three small spots had
-become considerably enlarged, and, as usual, the group consisted of two
-principal spots. Between these two spots all was in motion: the
-granulations, stretched into long, wavy, parallel lines, had somewhat
-the appearance of a liquid in rapid motion. At 1 o'clock, P. M., on the
-same day, the group had considerably enlarged; the faculæ, the
-granulations, and the penumbral filaments being interwoven in an
-indescribable manner. On the morning of the 17th, these spots had
-assumed the complicated form and development represented in the drawing;
-while at the same time two conspicuous veiled spots were seen on the
-left hand, at some distance above the group.
-</p>
-<p>
-Some luminous bridges are visible upon the left hand spot, traversing
-the penumbra and umbra of this spot in various directions. The umbra of
-one of the spots is occupied, and partly filled with gray and rosy
-veils, similar to those above described, and the granulations of the
-solar surface form a background to the group of spots.
-</p>
-<p>
-This group of spots was not so remarkable for its size as for its
-complicated structure. The diameter of the group from east to west was
-only 2½ minutes of arc, or about 67,000 miles. The upper part of the
-umbra of the spot situated on the right hand side of the group was
-nearly 7,000 miles in diameter, or less by 1,000 miles than the diameter
-of the Earth. Some of the long filaments composing that part of the
-penumbra, situated on the left hand side of the same spot, were 17,000
-miles in length. One of these fiery elements would be sufficient to
-encircle two-thirds of the circumference of the Earth.
-</p>
-
-<p><br /><br /><br /></p>
-
-<h4><a id="SOLAR_PROTUBERANCES">SOLAR PROTUBERANCES</a>
-<br /><br />
-PLATE II</h4>
-
-<p>
-The chromosphere forming the outlying envelope of the Sun, is subject,
-as has been shown above, to great disturbances in certain regions,
-causing considerable upheavals of its surface and violent outbursts of
-its gases. From these upheavals and outbursts of the chromosphere result
-certain curious and very interesting forms, which are known under the
-name of "<i>Solar Protuberances</i>," "<i>Prominences</i>," or
-"<i>Flames</i>."
-</p>
-<p>
-These singular forms, which could, until recently, be observed only
-during the short duration of the total eclipses of the Sun, can now be
-seen on every clear day with the spectroscope, thanks to Messrs. Janssen
-and Lockyer, to whose researches solar physics is so much indebted.
-</p>
-
-<p><br /></p>
-
-<div class="figcenter" style="width: 400px;">
-<a id="figure02"></a>
-<br />
-<img src="images/figure02.jpg" width="400" alt="" />
-<div class="caption">
-<p>PLATE II.&mdash;SOLAR PROTUBERANCES.</p>
-<p class="smaller">Observed on May 5, 1873 at 9h, 40m. A.M.</p>
-</div></div>
-
-<p><br /></p>
-
-<p>
-The solar protuberances, the Sun-spots, and the faculæ to which they
-are closely related, are confined within the same general regions of the
-Sun, although the protuberances attain higher heliocentric latitudes.
-</p>
-<p>
-There is certainly a very close relation between the faculæ and the
-solar protuberances, since when a group of the faculæ traverses the
-Sun's limb, protuberances are always seen at the same place. It seems
-very probable that the faculæ and the protuberances are in the main
-identical. The faculæ may be the brighter portion of the protuberances,
-consisting of gases which are still undergoing a high temperature and
-pressure; while the gases which have been relieved from this pressure
-and have lost a considerable amount of their heat, may form that part of
-the protuberances which is only visible on the Sun's limb.
-</p>
-<p>
-A daily study of the solar protuberances, continued for ten years, has
-shown me that these objects are distributed on two zones which are
-equidistant from the solar equator, and parallel with it. The zone
-arrangement of the protuberances is more easily recognized during the
-years of minimum solar activity, as in these years the zones are very
-narrow and widely separated. During these years the belt of
-protuberances is situated between 40° and 45° of latitude, north and
-south. In years of great solar activity the zones spread considerably on
-either side of these limits, especially towards the equator, which they
-nearly reach, only a narrow belt, usually free from protuberances,
-remaining between them. Towards the poles the zones do not spread so
-much, and there the space free from protuberances is considerably
-greater than it is at the equator.
-</p>
-<p>
-During years of maximum solar activity, the protuberances, like the
-Sun-spots and the faculæ, are very numerous, very large, and very
-complicated&mdash;sometimes occupying a great part of the whole solar limb.
-As many as twenty distinct flames are sometimes observed at one time. In
-years of minimum solar activity, on the contrary, the prominences are
-very few in number, and they are of small size; but, as far as my
-observations go, they are never totally absent.
-</p>
-<p>
-In general, the solar flames undergo rapid changes, especially those
-which are situated in the vicinity of Sun-spots, although they
-occasionally remain unchanged in appearance and form for several hours
-at a time. The protuberances situated in higher latitudes are less
-liable to great and sudden changes, often retaining the same form for
-several days. The changes observed in the protuberances of the
-equatorial regions are due in part to the comparatively great changes in
-their position with respect to the spectator, which are occasioned by
-the rotation of the Sun. This rotation, of course, has a greater angular
-velocity on the equator than in higher latitudes. In most cases,
-however, the changes of the equatorial protuberances are too great and
-too sudden to be thus explained. They are, in fact, due to the greater
-solar activity developed in the equatorial zones, and wherever spots are
-most numerous.
-</p>
-<p>
-The solar protuberances appear under various shapes, and are often so
-complicated in appearance that they defy description. Some resemble huge
-clumsy masses having a few perforations on their sides; while others
-form a succession of arches supported by pillars of different styles.
-Others form vertical or inclined columns, often surmounted by cloud-like
-masses, or by various appendages, which sometimes droop gracefully,
-resembling gigantic palm leaves. Some resemble flames driven by the
-wind; others, which are composed of a multitude of long and narrow
-filaments, appear as immense fiery bundles, from which sometimes issue
-long and delicate columns surmounted by torch-like objects of the most
-fantastic pattern. Some others resemble trees, or animal forms, in a
-very striking manner; while still others, apparently detached from the
-solar limb, float above it, forming graceful streamers or clouds of
-various shapes. Some of the protuberances are very massive, while others
-are so thin and transparent as to form a mere veil, through which more
-distant flames can easily be seen.
-</p>
-<p>
-Notwithstanding this variety of form, two principal classes of solar
-protuberances may be recognized: the cloud-like or quiescent, and the
-eruptive or metallic protuberances.
-</p>
-<p>
-The first class, which is the most common, comprises all the cloud-like
-protuberances resting upon the chromosphere or floating about it. The
-protuberances of this type often obtain enormous horizontal proportions,
-and it is not rare to see some among them occupying 20° and 30° of the
-solar limb. The height attained by protuberances of this class does not
-correspond in general to their longitudinal extent; although some of
-their branches attain considerable elevations. These prominences very
-seldom have the brilliancy displayed by the other type, and are
-sometimes so faint as to be seen with difficulty. Although it is
-generally stated by observers that some of the protuberances belonging
-to this class are detached from the solar surface, and kept in
-suspension above the surface, like the clouds in our atmosphere, yet it
-seems to me very doubtful whether protuberances are ever disconnected
-from the chromosphere, since, in an experience of ten years, I have
-never been able to satisfy myself that such a thing has occurred. Many
-of them have appeared to me at first sight to be detached from the
-surface, but with a little patience and attention I was always able to
-detect faint traces of filamentary elements connecting them with the
-chromosphere. Quite often I have seen bright protuberances gradually
-lose their light and become invisible, while soon after they had
-regained it, and were as clearly visible as before. Observations of this
-kind seem to show that while the prominences are for the most part
-luminous, there are also a few which are non-luminous and invisible to
-the eye. These dark and invisible forms are most generally found in the
-vicinity of Sun-spots in great activity. When observing such regions
-with the spectroscope, it is not rare to encounter them in the form of
-large dark spots projecting on the solar spectrum near the hydrogen
-lines. On July 28th, 1872, I observed with the spectroscope a dark spot
-of this kind issuing from the vicinity of a large Sun-spot, and
-extending over one-fifth of the diameter of the Sun. This object had
-been independently observed in France a little earlier by M. Chacornac
-with the telescope, in which it appeared as a bluish streak.
-</p>
-<p>
-The second class of solar protuberances, comprising the eruptive type,
-is the most interesting, inasmuch as it conveys to us a conception of
-the magnitude and violence of the solar forces. The protuberances of
-this class, which are always intensely bright, appear for the most part
-in the immediate vicinity of Sun-spots or faculæ. These protuberances,
-which seem to be due to the outburst of the chromosphere, and to the
-violent ejection of incandescent gases and metallic vapors from the
-interior of the Sun, sometimes attain gigantic proportions and enormous
-heights.
-</p>
-<p>
-While the spectrum of the protuberances of the cloudy type is simple,
-and usually composed of four hydrogen lines and the yellow line
-D<sub>3</sub>, that of the eruptive class is very complicated, and,
-besides the hydrogen lines and D<sub>3</sub>, it often exhibits the
-bright lines of sodium, magnesium, barium, titanium, and iron, and
-occasionally, also, a number of other bright lines.
-</p>
-<p>
-The phenomena of a solar outburst are grand and imposing. Suddenly
-immense and acute tongues and jets of flames of a dazzling brilliancy
-rise up from the solar limb and extend in various directions. Some of
-these fiery jets appear perfectly rigid, and remain apparently
-motionless in the midst of the greatest disorder. Immense straps and
-columns form and rise in an instant, bending and waving in all sorts of
-ways and assuming innumerable shapes. Sometimes powerful jets resembling
-molten metal spring up from the Sun, describing graceful parabolas,
-while in their descent they form numerous fiery drops which acquire a
-dazzling brilliancy when they approach the surface.
-</p>
-<p>
-The upward motion of the protuberances in process of formation is
-sometimes very rapid. Some protuberances have been observed to ascend in
-the solar atmosphere at the rate of from 120 to 497 miles a second.
-Great as this velocity may appear, it is nevertheless insignificant when
-compared with that sometimes attained by protuberances moving in the
-line of sight instead of directly upwards. Movements of this kind are
-indicated by the displacement of the bright or dark lines in the
-spectrum. A remarkable instance of this kind occurred on the 26th of
-June, 1874. On that day I observed a displacement of the hydrogen C line
-corresponding to a velocity of motion of 1,600 miles per second. The
-mass of hydrogen gas in motion producing such a displacement was,
-according to theory, moving towards the Earth at this incredible rate,
-when it instantly vanished from sight as if it had been annihilated, and
-was seen no more.
-</p>
-<p>
-Until recently the protuberances had not been observed to rise more than
-200,000 miles above the solar surface; but, on October 7th, 1880, a
-flame, which had an elevation of 80,000 miles when I observed it at 8h.
-55m. A. M., had attained the enormous altitude of 350,000 miles when it
-was observed at noon by Professor C. A. Young. If we had such a
-protuberance on the Earth, its summit would be at a height sufficient
-not merely to reach, but to extend 100,000 miles beyond the Moon.
-</p>
-<p>
-Although the solar protuberances represented in Plate II. have not the
-enormous proportions attained by some of these objects, yet they are as
-characteristic as any of the largest ones, and afford a good
-illustration of the purely eruptive type of protuberances. The height of
-the largest column in the group equals 4' 43", or a little over 126,000
-miles. A large group of Sun-spots was in the vicinity of these
-protuberances when they were observed and delineated.
-</p>
-
-<p><br /><br /><br /></p>
-
-<h4><a id="TOTAL_ECLIPSE">TOTAL ECLIPSE OF THE SUN</a>
-<br /><br />
-PLATE III</h4>
-
-<p>
-A solar eclipse is due to the passage of the Moon directly between the
-observer and the Sun. Such an eclipse can only occur at New Moon, since
-it is only at that time that our satellite passes between us and the
-Sun. The Moon's orbit does not lie precisely in the same plane as the
-orbit of the Earth, but is inclined about five degrees to it, otherwise
-an eclipse of the Sun would occur at every New Moon, and an eclipse of
-the Moon at every Full Moon.
-</p>
-<p>
-Since the Moon's orbit is inclined to that of the Earth, it must
-necessarily intersect this orbit at two opposite points. These points
-are called the nodes of the Moon's orbit. When our satellite passes
-through either of the nodes when the Moon is new, it appears interposed
-to some extent between the Sun and the Earth, and so produces a solar
-eclipse; while if it passes a node when the Moon is full, it is more or
-less obscured by the Earth's shadow, which then produces an eclipse of
-the Moon. But, on the other hand, when the New Moon and the Full Moon do
-not coincide with the passage of our satellite through the nodes of its
-orbit, no eclipse can occur, since the Moon is not then on a line with
-the Sun and the Earth, but above or below that line.
-</p>
-<p>
-Owing to the ellipticity of the Moon's orbit, the distance of our
-satellite from the Earth varies considerably during each of its
-revolutions around us, and its apparent diameter is necessarily subject
-to corresponding changes. Sometimes it is greater, sometimes it is less,
-than the apparent diameter of the Sun. If it is greater at the time of a
-solar eclipse, the eclipse will be total to a terrestrial observer
-stationed nearly on the line of the centres of the Sun and Moon, while
-it will be only partial to another observer stationed further from this
-line. But the Moon's distance from the Earth may be so great and its
-apparent diameter consequently so small that even those observers
-nearest the central line of the eclipse see the border of the Sun all
-round the black disk of the Moon; the eclipse is then annular. Even
-during the progress of one and the same eclipse the distance of the Moon
-from the parts of the Earth towards which its shadow is directed may
-vary so much that, while the eclipse is total to some observers, others
-equally near the central line, but stationed at a different place, will
-see it as annular.
-</p>
-<p>
-The shadow cast by the Moon on the Earth during total eclipses, travels
-along upon the surface of the Earth, in consequence of the daily
-movement of rotation of our globe combined with the movements of the
-Earth and Moon in their orbits. The track of the Moon's shadow over the
-Earth's surface has a general eastward course, so that the more westerly
-observers see it earlier than those east of them. An eclipse may
-continue total at one place for nearly eight minutes, but in ordinary
-cases the total phase is much shorter.
-</p>
-<p>
-The nodes of the Moon's orbit do not invariably occupy the same
-position, but move nearly uniformly, their position with regard to the
-Sun, Earth, and Moon being at any time approximately what it formerly
-was at a series of times separated by equal intervals from each other.
-Each interval comprises 223 lunations, or 18 years, 11 days, and 7 or 8
-hours. The eclipses which occur within this interval are almost exactly
-repeated during the next similar interval. This period, called the
-"Saros," was well known to the ancients, who were enabled by its means
-to predict eclipses with some certainty.
-</p>
-
-<p><br /></p>
-
-<div class="figcenter" style="width: 400px;">
-<a id="figure03"></a>
-<br />
-<img src="images/figure03.jpg" width="400" alt="" />
-<div class="caption">
-<p>PLATE III.&mdash;TOTAL ECLIPSE OF THE SUN.</p>
-<p class="smaller">Observed July 29, 1878, at Creston, Wyoming Territory</p>
-</div></div>
-
-<p><br /></p>
-
-<p>
-A total eclipse of the Sun is a most beautiful and imposing phenomenon.
-At the predicted time the perfectly round disk of the Sun becomes
-slightly indented at its western limb by the yet invisible Moon. This
-phenomenon is known as the "first contact."
-</p>
-<p>
-The slight indentation observed gradually increases with the advance of
-the Moon from west to east, the irregularities of the surface of our
-satellite being plainly visible on the border of the dark segment
-advancing on the Sun's disk. With the advance of the Moon on the Sun,
-the light gradually diminishes on the Earth. Every object puts on a dull
-and gloomy appearance, as when night is approaching; while the bright
-sky, losing its light, changes its pure azure for a livid grayish color.
-</p>
-<p>
-Two or three minutes before totality begins, the solar crescent, reduced
-to minute proportions, gives comparatively so little light that faint
-traces of the Sun's atmosphere appear on the western side behind the
-dark body of the Moon, whose limb then becomes visible outside of the
-Sun. I observed this phenomenon at Creston during the eclipse of 1878.
-From 15 to 20 seconds before totality, the narrow arc of the Sun's disk
-not yet obscured by the Moon seems to break and separate towards the
-extremities of its cusps, which, thus divided, form independent points of
-light, which are called "<i>Baily's beads</i>." A moment after, the whole
-solar crescent breaks into numerous beads of light, separated by dark
-intervals, and, suddenly, they all vanish with the last ray of Sunlight,
-and totality has begun with the "second contact." This phenomenon of
-Baily's beads is undoubtedly caused by the irregularities of the Moon's
-border, which, on reaching the solar limb, divide the thin solar
-crescent into as many beads of light and dark intervals as there are
-peaks and ravines seen sidewise on that part of the Moon's limb.
-</p>
-<p>
-With the disappearance of the last ray of light, the planets and the
-stars of the first and second magnitude seem to light up and become
-visible in the sky. The darkness, which had been gradually creeping in
-with the progress of the eclipse, is then at its maximum. Although
-subject to great variations in different eclipses, the darkness is never
-so great as might be expected from the complete obscuration of our
-luminary, as the part of our atmosphere which is still exposed to the
-direct rays of the Sun, reflects to us some of that light, which thus
-diminishes the darkness resulting from the disappearance of the Sun.
-Usually the darkness is sufficient to prevent the reading of common
-print, and to deceive animals, causing them to act as if night was
-really approaching. During totality the temperature decreases, while the
-humidity of the atmosphere augments.
-</p>
-<p>
-Simultaneously with the disappearance of Baily's beads, a pale, soft,
-silvery light bursts forth from behind the Moon, as if the Sun, in
-disappearing, had been vaporized and expanded in all directions into
-soft phosphorescent rays and streamers. This pale light is emitted by
-gases constituting the solar atmosphere surrounding the bright nucleus
-now obscured by the dark body of our satellite. This solar atmosphere is
-called <i>Corona</i>, from its distant resemblance to the aureola, or
-glory, represented by ancient painters around the heads of saints.
-</p>
-<p>
-With the bursting forth of the corona, a very thin arc of bright white
-light is seen along the Moon's limb, where the solar crescent has just
-disappeared. This thin arc of light is the reversing layer, which, when
-observed with the spectroscope at that moment, exhibits bright lines
-answering to the dark lines of the ordinary solar spectrum. Immediately
-above this reversing layer, and concentric with it, appears the
-pink-colored chromospheric layer, with its curiously shaped flames and
-protuberances. During totality, the chromosphere and protuberances are
-seen without the aid of the spectroscope, and appear of their natural
-color, which, although somewhat varying in their different parts, is, on
-the whole, pinkish, and similar to that of peach-blossoms; yet it is
-mixed here and there with delicate prismatic hues, among which the pink
-and straw colors predominate.
-</p>
-<p>
-The color of the corona seems to vary in every eclipse, but as its tints
-are very delicate, it may depend, in a great measure, upon the vision of
-the observer; although there seems to be no doubt that there are real
-variations. At Creston, in 1878, it appeared to both Professor W.
-Harkness and myself of a decided pale greenish hue.
-</p>
-<p>
-The corona appears under different forms, and has never been observed
-twice alike. Its dimensions are also subject to considerable variations.
-Sometimes it appears regular and very little extended, its distribution
-around the Sun being almost uniform; although in general it spreads a
-little more in the direction of the ecliptic, or of the solar equator.
-At other times it appears much larger and more complicated, and forms
-various wings and appendages, which in some cases, as in 1878, extend to
-immense distances; while delicate rays radiate in straight or curved
-lines from the spaces left in the polar regions between the wings. The
-corona has sometimes appeared as if divided by immense dark gaps,
-apparently free from luminous matter, and strongly resembling the dark
-rifts seen in the tails of comets. This was observed in Spain and Sicily
-during the total eclipse of the Sun in 1870. Different structures,
-forming wisps and streamers of great length, and interlaced in various
-ways, are sometimes present in the corona, while faint but more
-complicated forms, distantly resembling enormous solar protuberances
-with bright nuclei, have also been observed.
-</p>
-<p>
-As the Moon continues its eastward progress, it gradually covers the
-chromosphere and the solar protuberances on the eastern side of the Sun;
-while, at the same time, the protuberances and the chromosphere on the
-opposite limb gradually appear from under the retreating Moon. Then, the
-thin arc of the reversing layer is visible for an instant, and is
-instantly followed by the appearance of a point of dazzling white light,
-succeeded immediately by the apparition of Daily's beads on each side,
-and totality is over, with this third contact. The corona continues to
-be visible on the eastern side of the Sun for several minutes longer,
-and then rapidly vanishes.
-</p>
-<p>
-The thin solar crescent increases in breadth as the Moon advances;
-while, at the same time, the darkness and gloom spread over nature
-gradually disappear, and terrestrial objects begin to resume their
-natural appearance. Finally the limb of the Moon separates from that of
-the Sun at the instant of "fourth contact," and the eclipse is over.
-</p>
-<p>
-The phenomena exhibited by the corona in different eclipses are very
-complex, and, so far, they have not been sufficiently studied to enable
-us to understand the true nature of the solar atmosphere. From the
-spectral analysis of the corona, and the phenomena of polarization, it
-has been learned, at least, that while the matter composing the upper
-part of the solar atmosphere is chiefly composed of an unknown
-substance, producing the green line 1474, its lower part is mainly
-composed of hydrogen gas at different temperatures, a part of which is
-self-luminous, while the other part only reflects the solar light. But
-the proportion of the gaseous particles emitting light, to those simply
-reflecting it, is subject to considerable variations in different
-eclipses. At present it would seem that in years of great solar
-disturbances, the particles emitting light are found in greater quantity
-in the corona than those reflecting it; but further observations will be
-required to confirm these views.
-</p>
-<p>
-It is very difficult to understand how the corona, which in certain
-eclipses extends only one diameter of the Sun, should, in other cases,
-as in 1878, extend to the enormous distance of twelve times the same
-diameter. Changes of such magnitude in the solar atmosphere, if due to
-the operation of forces with which we are acquainted, cannot yet be
-accounted for by what is known of such forces. Their causes are still as
-mysterious as those concerned in the production of the monstrous tails
-displayed by some comets on their approach to the Sun.
-</p>
-<p>
-Plate 3, representing the total eclipse of the Sun of July 29th, 1878,
-was drawn from my observations made at Creston, Wyoming Territory, for
-the Naval Observatory. The eclipse is represented as seen in a
-refracting telescope, having an aperture of 6⅓ inches, and as it
-appeared a few seconds before totality was over, and when the
-chromosphere was visible on the western limb of the Sun. The two long
-wings seen on the east and west side of the Sun, appeared considerably
-larger in the sky than they are represented in the picture.
-</p>
-
-<p><br /><br /><br /></p>
-
-<h4><a id="THE_AURORA_BOREALIS">THE AURORA BOREALIS</a>
-<br /><br />
-PLATE IV</h4>
-
-<p>
-The name of Polar Auroras is given to certain very remarkable luminous
-meteoric phenomena which appear at intervals above the northern or the
-southern horizons of both hemispheres of the Earth. When the phenomenon
-is produced in our northern sky, it is called "Aurora Borealis," or
-"Northern Lights;" and when it appears in the southern sky, it is called
-"Aurora Australis," or southern aurora.
-</p>
-<p>
-Marked differences appear in the various auroras observed from our
-northern latitudes. While some simply consist in a pale, faint
-luminosity, hardly distinguishable from twilight, others present the
-most gorgeous and remarkable effects of brightness and colors.
-</p>
-<p>
-A great aurora is usually indicated in the evening soon after twilight,
-by a peculiar grayish appearance of the northern sky just above the
-horizon. The grayish vapors giving that appearance, continuing to form
-there, soon assume a dark and gloomy aspect, while they gradually take
-the form of a segment of a circle resting on the horizon. At the same
-time that this dark segment is forming, a soft pearly light, which seems
-to issue from its border, spreads up in the sky, where it gradually
-vanishes, being the brightest at its base. This arc of light, gradually
-increasing in extent as well as in brightness, reaches sometimes as far
-as the polar star. On some rare occasions, one or two, and even three,
-concentric arches of bright light form one above the other over the dark
-segment, where they appear as brilliant concentric rainbows. While the
-aurora continues to develop and spread out its immense arc, the border
-of the dark segment loses its regularity and appears indented at several
-places by patches of light, which soon develop into long, narrow,
-diverging rays and streamers of great beauty. For the most part the
-auroral light is either whitish or of a pale, greenish tint; but in some
-cases it exhibits the most beautiful colors, among which the red and
-green predominate. In these cases the rays and streamers, which are
-usually of different colors, produce the most magnificent effects by
-their continual changes and transformations.
-</p>
-<p>
-The brightness and extent of the auroral rays are likewise subject to
-continual changes. An instant suffices for their development and
-disappearance, which may be succeeded by the sudden appearance of others
-elsewhere, as though the original streamers had been swiftly transported
-to a new place while invisible. It frequently happens that all the
-streamers seem to move sidewise, from west to east, along the arch,
-continuing meanwhile to exhibit their various changes of form and color.
-For a time, these appearances of motion continue to increase, a
-succession of streamers alternately shooting forth and again fading,
-when a sudden lull occurs, during which all motion seems to have ceased.
-The stillness then prevailing is soon succeeded by slight pulsations of
-light, which seem to originate on the border-of the dark segment, and
-are propagated upwards along the streamers, which have now become more
-numerous and active. Slow at first, these pulsations quicken by degrees,
-and after a few minutes the whole northern sky seems to be in rapid
-vibration. The lively upward and downward movement of these streamers
-entitles them to the name of "merry dancers" given them in northern
-countries where they are frequent.
-</p>
-<p>
-Long waves of light, quickly succeeded by others, are propagated in an
-instant from the horizon to the zenith; these, in their rapid passage,
-cause bends and curves in the streamers, which then, losing their
-original straightness, wave and undulate in graceful folds, resembling
-those of a pennant in a gentle breeze. Although the coruscations add to
-the grandeur of the spectacle, they tend to destroy the diverging
-streamers, which, being disconnected from the dark segment, or torn in
-various ways, are, as it were, bodily carried up towards the zenith.
-</p>
-<p>
-In this new phase the aurora is transformed into a glorious crown of
-light, called the "Corona." From this corona diverge in all directions
-long streamers of different colors and forms, gracefully undulating in
-numerous folds, like so many banners of light. Some of the largest of
-these streamers appear like fringes composed of short transverse rays of
-different intensity and colors, producing the most fantastic effects,
-when traversed by the pulsations and coruscations which generally run
-across these rays during the great auroral displays.
-</p>
-<p>
-The aurora has now attained its full development and beauty. It may
-continue in this form for half an hour, but usually the celestial fires
-begin to fade at the end of fifteen or twenty minutes, reviving from
-time to time, but gradually dying out. The northern sky usually appears
-covered by gray and luminous streaks and patches after a great aurora,
-these being occasionally rekindled, but more often they gradually
-disappear, and the sky resumes its usual appearance.
-</p>
-<p>
-The number of auroras which develop a corona near the zenith is
-comparatively small in our latitudes; but many of them, although not
-exhibited on so grand a scale, are nevertheless very interesting. On
-some very rare occasions the auroral display has been confined almost
-exclusively to the dark segment, which appeared then as if pierced along
-its border by many square openings, like windows, through which appeared
-the bright auroral light.
-</p>
-
-<p><br /></p>
-
-<div class="figcenter" style="width: 400px;">
-<a id="figure04"></a>
-<br />
-<img src="images/figure04.jpg" width="400" alt="" />
-<div class="caption">
-<p>PLATE IV.&mdash;AURORA BOREALIS.</p>
-<p class="smaller">As observed March 1, 1872, at 9h. 25m. P.M.</p>
-</div></div>
-
-<p><br /></p>
-
-<p>
-Among the many auroras which I have had occasion to observe, none are
-more interesting, excepting the type first described, than those which
-form an immense arch of light spanning the heavens from East to West.
-This form of aurora, which is quite rare, I last observed on September
-12th, 1881. All the northern sky was covered with light vapors, when a
-small auroral patch appeared in the East at about 20° above the
-horizon. This patch of light, gradually increasing westward, soon
-reached the zenith, and continued its onward progress until it arrived
-at about 20° above the western horizon, where it stopped. The aurora
-then appeared as a narrow, wavy band of light, crossed by numerous
-parallel rays of different intensity and color. These rays seemed to
-have a rapid motion from West to East along the delicately-fringed
-streamer, which, on the whole, moved southward, while its extremities
-remained undisturbed. Aside from the apparent displacement of the
-fringes, a singular vibrating motion was observed in the auroral band,
-which was traversed by pulsations and long waves of light. The phenomena
-lasted for about twenty minutes, after which the arch was broken in many
-places, and it slowly vanished.
-</p>
-<p>
-The aurora usually appears in the early part of the evening, and attains
-its full development between ten and eleven o'clock. Although the
-auroral light may have apparently ceased, yet the phenomenon is not at
-an end, as very often a solitary ray is visible from time to time; and
-even towards morning these rays sometimes become quite numerous. On some
-occasions the phenomenon even continues through the following day, and
-is manifested by the radial direction of the cirrus-clouds in the
-heights of our atmosphere. In 1872 I, myself, observed an aurora which
-apparently continued for two or three consecutive days and nights. In
-August, 1859, the northern lights remained visible in the United States
-for a whole week.
-</p>
-<p>
-The height attained by these meteors is considerable, and it is now
-admitted that they are produced in the rarefied air of the upper regions
-of our atmosphere. From the researches of Professor Elias Loomis on the
-great auroras observed in August and September, 1859, it was ascertained
-that the inferior part of the auroral rays had an altitude of 46 miles,
-while that of their summits was 428 miles. These rays had, therefore, a
-length of 382 miles. From the observation of thirty auroral displays, it
-has been found that the mean height attained by the summit of these
-streamers above the Earth's surface was 450 miles.
-</p>
-<p>
-But if the auroral streamers are generally manifested at great heights
-in our atmosphere, it would appear from the observations of persons
-living in the regions where the auroras are most frequent, as also from
-those who have been stationed in high northern and southern latitudes,
-that the phenomenon sometimes descends very low. Both Sabine and Parry
-saw the auroral rays projected on a distant mountain; Ross saw them
-almost at sea-level projected on the polar ice; while Wrangel, Franklin,
-and others observed similar phenomena. Dr. Hjaltalin, who has lived in
-latitude 64° 46' north, and has made a particular study of the aurora,
-on one occasion saw the aurora much below the summit of a hill 1,600
-feet high, which was not very far off.
-</p>
-<p>
-The same aurora is sometimes observed on the same night at places very
-far distant from one another. The great aurora borealis of August 28th,
-1859, for instance, was seen over a space occupying 150° in
-longitude&mdash;from California to the Ural Mountains in Russia. It even
-appears now very probable that the phenomenon is universal on our globe,
-and that the northern lights observed in our hemisphere are simultaneous
-with the aurora australis of the southern hemisphere. The aurora of
-September 2d, 1859, was observed all through North and South America,
-the Sandwich Islands, Australia, and Africa; the streamers and
-pulsations of light of the north pole responding to the rays and
-coruscations of the south pole. Of thirty-four auroras observed at
-Hobart Town, in Tasmania, twenty-nine corresponded with aurora borealis
-observed in our hemisphere.
-</p>
-<p>
-The auroral phenomena, although sometimes visible within the tropics,
-are, however, quite rare in these regions. For the most part they are
-confined within certain zones situated in high latitudes north and
-south. The zone where they are most frequent in our hemisphere forms an
-ellipse, which has the north pole at one of its foci; while the other is
-situated somewhere in North America, in the vicinity of the magnetic
-pole. The central line of the zone upon which the auroras seem to be
-most frequent passes from the northern coast of Alaska through Hudson's
-Bay and Labrador to Iceland, and then follows the northern coast of
-Europe and Asia. The number of auroras diminishes as the observer
-recedes from this zone, and it is only in exceptional cases that they
-are seen near the equator. Near the pole the phenomenon is less frequent
-than it is in the region described. In North America we occupy a
-favorable position for the observation of auroras, as we are nearer the
-magnetic poles than are the Europeans and Asiatics, and we consequently
-have a greater number of auroras in corresponding latitudes.
-</p>
-<p>
-The position of the dark auroral segment varies with the place occupied
-by the observer, and its centre always corresponds with the magnetic
-meridian. In our Eastern States the auroral segment appears a little to
-the west of the north point; but as the observer proceeds westward it
-gradually approaches this point, and is due north when seen from the
-vicinity of Lake Winnipeg. At Point Barrow, in the extreme north-west of
-the United States, the aurora is observed in the east. In Melville
-Islands, Parry saw it in the south; while in Greenland it is directly in
-the west.
-</p>
-<p>
-It is stated that auroras are more numerous about the equinoxes than
-they are at any other seasons; and also, when the earth is in perigee,
-than when it is in apogee. An examination which I have made of a
-catalogue by Professor Loomis, comprising 4,137 auroras observed in the
-temperate zone of our hemisphere from 1776 to 1873, sustains this
-statement. During this period, one hundred more auroras were recorded
-during each of the months comprising the equinoxes, than during any
-other months of the year; while eighty more auroras were observed when
-the earth was in perigee, than when it was in apogee. But to establish
-the truth of this assertion on a solid basis, more observations in both
-hemispheres will be required.
-</p>
-<p>
-The aurora is not simply a terrestrial phenomenon, but is associated in
-some mysterious way with the conditions of the Sun's surface. It is a
-well-known fact that terrestrial magnetism is influenced directly by the
-Sun, which creates the diurnal oscillations of the magnetic needle.
-Between sunrise and two o'clock, the north pole of the needle moves
-towards the west in our northern hemisphere, and in the afternoon and
-evening it moves the other way. These daily oscillations of the needle
-are not uniform in extent; they have a period of regular increase and
-decrease. At a given place the daily oscillations of the magnetic needle
-increase and decrease with regularity during a period which is equal to
-10⅓ years. As this period closely coincides with the Sun-spot period,
-the connection between the variation of the needle and these solar
-disturbances has been recognized.
-</p>
-<p>
-Auroral phenomena generally accompany the extraordinary perturbations in
-the oscillations of the magnetic needle, which are commonly called
-"magnetic storms," and the greater the auroral displays, the greater are
-the magnetic perturbations. Not only is the needle subject to unusual
-displacements during an aurora, but its movements seem to be
-simultaneous with the pulsations and waving motions of the delicate
-auroral streamers in the sky. When the aurora sends forth a coruscation,
-or a streamer in the sky, the magnetic needle responds to it by a
-vibration. The inference that the auroral phenomena are connected with
-terrestrial magnetism is further supported by the fact that the centre
-of the corona is always situated exactly in the direction of that point
-in the heavens to which the dipping needle is directed.
-</p>
-<p>
-It has been found that the aurora is a periodical phenomenon, and that
-its period corresponds very closely with those of the magnetic needle
-and Sun-spots. The years which have the most Sun-spots and magnetic
-disturbances have also the most auroras. There is an almost perfect
-similarity between the courses of the three sets of phenomena, from
-which it is concluded that the aurora is connected in some mysterious
-way with the action of the Sun, as well as with the magnetic condition
-of the earth.
-</p>
-<p>
-A very curious observation, which has been supposed to have some
-connection with this subject, was made on Sept. 1st, 1859, by Mr.
-Carrington and Mr. Hodgson, in England. While these observers, who were
-situated many miles from one another, were both engaged at the same time
-in observing the same Sun-spot, they suddenly saw two luminous spots of
-dazzling brilliancy bursting into sight from the edge of the Sun-spot.
-These objects moved eastward for about five minutes, after which they
-disappeared, having then traveled nearly 34,000 miles. Simultaneously
-with these appearances, a magnetic disturbance was registered at Kew by
-the self-registering magnetic instruments. The very night that followed
-these observations, great magnetic perturbations, accompanied by
-brilliant auroral displays, were observed in Europe. A connection
-between the terrestrial magnetism and the auroral phenomena is further
-proved by the fact that, before the appearance of an aurora, the
-magnetic intensity of our globe considerably increases, but diminishes
-as soon as the first flashes show themselves.
-</p>
-<p>
-The auroral phenomena are also connected in some way with electricity,
-and generate serious disturbances in the electric currents traversing
-our telegraphic lines, which are thus often rendered useless for the
-transmission of messages during great auroral displays. It sometimes
-happens, however, during such displays, that the telegraphic lines can
-be operated for a long distance, without the assistance of a battery;
-the aurora, or at least its cause, furnishing the necessary electric
-current for the working of the line. During auroras, the telephonic
-lines are also greatly affected, and all kinds of noises and
-crepitations are heard in the instruments.
-</p>
-<p>
-Two observations of mine, which may have a bearing on the subject,
-present some interest, as they seem to indicate the action of the aurora
-on some of the clouds of our atmosphere. On January 6th, 1872, after I
-had been observing a brilliant aurora for over one hour, an isolated
-black cumulus cloud appeared at a little distance from the western
-extremity of the dark auroral segment. This cloud, probably driven by
-the wind, rapidly advanced eastward, and was soon followed by a
-succession of similar clouds, all starting from the same point. All
-these black clouds apparently followed the same path, which was not a
-straight line, but parallel to and concentric with the border of the
-dark auroral segment. When the first cloud arrived in the vicinity of
-the magnetic meridian passing through the middle of the auroral arc, it
-very rapidly dissolved, and on reaching this meridian became invisible.
-The same phenomenon was observed with the succession of black clouds
-following, each rapidly dissolving as it approached the magnetic
-meridian. This phenomenon of black clouds vanishing like phantoms in
-crossing the magnetic meridian, was observed for nearly an hour. On June
-17th, 1879, I observed a similar phenomenon during a fine auroral
-display. About midway between the horizon and the polar star, but a
-little to the west of the magnetic meridian, there was a large black
-cumulo-stratus cloud which very slowly advanced eastward. As it
-progressed in that direction, its eastern extremity was dissolved in
-traversing the magnetic meridian; while, at the same time, several short
-and quite bright auroral rays issued from its western extremity, which
-in its turn dissolved rapidly, as if burned or melted away in the
-production of the auroral flame.
-</p>
-<p>
-It seems to be a well observed fact, that during auroras, a strong
-sulphurous odor prevails in high northern latitudes. According to Dr.
-Hjaltalin, during these phenomena, "the ozone of the atmosphere
-increases considerably, and men and animals exposed out of doors emit a
-sulphurous odor when entering a heated room." The Esquimaux and other
-inhabitants of the northern regions assert that great auroras are
-sometimes accompanied by crepitations and crackling noises of various
-sorts. Although these assertions have been denied by several travelers
-who have visited the regions of these phenomena, they are confirmed by
-many competent observers. Dr. Hjaltalin, who has heard these noises
-about six times in a hundred observations, says that they are especially
-audible when the weather is clear and calm; but that when the atmosphere
-is agitated they are not heard. He compares them to the peculiar sound
-produced by a silk cloth when torn asunder, or to the crepitations of
-the electric machine when its motion is accelerated. "When the auroral
-light is much agitated and the streamers show great movements, it is
-then that these noises are heard at different places in the atmosphere."
-</p>
-<p>
-The spectrum of the auroral light, although it varies with almost every
-aurora, always shows a bright green line on a faint continuous spectrum.
-In addition to this green line I have frequently observed four broad
-diffused bands of greater refrangibility in the spectra of some auroras.
-In two cases, when the auroras appeared red towards the west, the
-spectrum showed a bright red line, in addition to the green line and the
-broad bands described. These facts evidently show that the light of the
-aurora is due to the presence of luminous vapors in our atmosphere; and
-it may reasonably be supposed that these vapors are rendered luminous by
-the passage of electric discharges through them.
-</p>
-
-<p><br /><br /><br /></p>
-
-<h4><a id="THE_ZODIACAL_LIGHT">THE ZODIACAL LIGHT</a>
-<br /><br />
-PLATE V</h4>
-
-<p>
-In our northern latitudes may be seen, on every clear winter and spring
-evening, a column of faint, whitish, nebulous light, rising obliquely
-above the western horizon. A similar phenomenon may also be observed in
-the east, before day-break, on any clear summer or autumn night. To this
-pale, glimmering luminosity the name of "Zodiacal Light" has been given,
-from the fact that it lies in the zodiac along the ecliptic.
-</p>
-<p>
-In common with all the celestial bodies, the zodiacal light participates
-in the diurnal motion of the sky, and rises and sets with the
-constellations in which it appears. Aside from this apparent motion, it
-is endowed with a motion of its own, accomplished from west to east, in
-a period of a year. In its motion among the stars, the zodiacal light
-always keeps pace with the Sun, and appears as if forming two faint
-luminous wings, resting on opposite sides of this body. In reality it
-extends on each side of the Sun, its axis lying very nearly in the plane
-of the ecliptic.
-</p>
-<p>
-In our latitudes the phenomena can be observed most advantageously
-towards the equinoxes, in March and September, when twilight is of short
-duration. As we proceed southward it becomes more prominent, and
-gradually increases in size and brightness. It is within the tropical
-regions that the zodiacal light acquires all its splendor: there it is
-visible all the year round, and always appears very nearly perpendicular
-to the horizon, while at the same time its proportions and brilliancy
-are greatly increased.
-</p>
-
-<p><br /></p>
-
-<div class="figcenter" style="width: 400px;">
-<a id="figure05"></a>
-<br />
-<img src="images/figure05.jpg" width="400" alt="" />
-<div class="caption">
-<p>PLATE V.&mdash;THE ZODIACAL LIGHT.</p>
-<p class="smaller">Observed February 20, 1876</p>
-</div></div>
-
-<p><br /></p>
-
-<p>
-The zodiacal light appears under the form of a spear-head, or of a
-narrow cone of light whose base apparently rests on the horizon, while
-its summit rises among the zodiacal constellations. In general
-appearance it somewhat resembles the tail of a large comet whose head is
-below the horizon. The most favorable time to observe this phenomenon in
-the evening, is immediately after the last trace of twilight has
-disappeared; and in the morning, one or two hours before twilight
-appears. When observed with attention, it is seen that the light of the
-zodiacal cone is not uniform, but gradually increases in brightness
-inwardly, especially towards its base, where it sometimes surpasses in
-brilliancy the brightest parts of the Milky-Way. In general, its
-outlines are vague and very difficult to make out, so gradually do they
-blend with the sky. On some favorable occasions, the luminous cone
-appears to be composed of several distinct concentric conical layers,
-having different degrees of brightness, the inner cone being the most
-brilliant of all. There is a remarkable distinction between the evening
-and morning zodiacal light. In our climate, the morning light is pale,
-and never so bright nor so extended as the evening light.
-</p>
-<p>
-In general, the zodiacal light is whitish and colorless, but in some
-cases it acquires a warm yellowish or reddish tint. These changes of
-color may be accidental and due to atmospheric conditions, and not to
-actual change in the color of the object. Although the zodiacal light is
-quite bright, and produces the impression of having considerable depth,
-yet its transparency is great, since all the stars, except the faint
-ones, can be seen through its substance.
-</p>
-<p>
-The zodiacal light is subject to considerable variations in brightness,
-and also varies in extent, the apex of its cone varying in distance from
-the Sun's place, from 40 to 90 degrees. These variations cannot be
-attributed to atmospheric causes alone, some of them being due to real
-changes in the zodiacal light itself, whose light and dimensions
-increase or decrease under the action of causes at present unknown. From
-the discussion of a series of observations on the zodiacal light made at
-Paris and Geneva, it appears certain that its light varies from year to
-year, and sometimes even from day to day, independently of atmospheric
-causes. Some of my own observations agree with these results, and one of
-them, at least, seems to indicate changes even more rapid. On December
-18th, 1875, I observed the zodiacal light in a clear sky free from any
-vapors, at six o'clock in the evening. At that time, the point of its
-cone was a little to the north of the ecliptic, at a distance of about
-90 degrees from the Sun's place. Ten minutes later, its summit had sunk
-down 35 degrees, the cone then being reduced to nearly one-half of its
-original dimensions. Ten minutes later, it had risen 25 degrees, and was
-then 80 degrees from the Sun's place, where it remained all the evening.
-On March 22d, 1878, the sky was very clear and the zodiacal light was
-bright when I observed it, at eight o'clock. At that moment the apex of
-the cone of light was a little to the south of the Pleiades, but this
-cone presented an unusual appearance never noticed by me before, its
-northern border appearing much brighter and sharper than usual, while at
-the same time its axis of greatest brightness appeared to be much nearer
-to this northern border than it was to the southern. After a few minutes
-of observation it became evident that the northern border was extending
-itself, as stars which were at some distance from it became gradually
-involved in its light. At the same time that this border spread
-northward, it seemed to diffuse itself, and after a time the cone
-presented its usual appearance, having its southern border brighter and
-better defined than the other. It would have been impossible to
-attribute this sudden change to an atmospheric cause, since only one of
-the borders of the cone participated in it, and since some very faint
-stars near this northern border were not affected in the least while the
-phenomenon occurred. Besides these observations, Cassini, Mairan,
-Humboldt, and many other competent observers have seen pulsations,
-coruscations and bickerings in the light of the cone, which they thought
-could not be attributed to atmospheric causes. It has also been observed
-that at certain periods the zodiacal light has shone with unusual
-intensity for months together.
-</p>
-<p>
-When this phenomenon is observed from the tropical regions, it is found
-that its axis of symmetry always corresponds with its axis of greatest
-brightness, and that both lie in the plane of the ecliptic, which
-divides its cone into two equal parts. But when the zodiacal light is
-observed in our latitude, the axis of symmetry does not correspond with
-the axis of greatest brightness, and both axes are a little to the north
-of this plane, the axis of symmetry being the farther removed.
-Furthermore, as already stated, the southern border of the cone always
-appears better defined and brighter than the corresponding northern
-margin. It is very probable, if not absolutely certain, that these
-phenomena are exactly reversed when the zodiacal light is observed from
-corresponding latitudes in the southern hemisphere, and that there, its
-axes, both of symmetry and of greatest brightness, appear south of the
-ecliptic, while the northern margin is the brightest. This seems to be
-established by the valuable observations of Rev. George Jones, made on
-board the U. S. steam frigate Mississippi, in California, Japan, and the
-Southern Ocean. "When I was north of the ecliptic," says this observer,
-"the greatest part of the light of the cone appeared to the north of
-this line; when I was to the south of the ecliptic, it appeared to be
-south of it; while when my position was on the ecliptic, or in its
-vicinity, the zodiacal cone was equally divided by this line."
-</p>
-<p>
-Besides the zodiacal light observed in the East and West, some observers
-have recognized an exceedingly faint, luminous, gauzy band, about 10 or
-12 degrees wide, stretching along the ecliptic from the summit of the
-western to that of the eastern zodiacal cone. This faint narrow belt has
-been called the Zodiacal Band. It has been recognized by Mr. H. C.
-Lewis, who has made a study of this phenomenon, that the zodiacal band
-has its southern margin a little brighter and a little sharper than the
-northern border. This observation is in accordance with similar
-phenomena observed in the zodiacal light, and may have considerable
-importance.
-</p>
-<p>
-In 1854, Brorsen recognized a faint, roundish, luminous spot in a point
-of the heavens exactly opposite to the place occupied by the Sun, which
-he has called "Gegenschein," or counter-glow. This luminous spot has
-sometimes a small nucleus, which is a little brighter than the rest.
-Night after night this very faint object shifts its position among the
-constellations, keeping always at 180 degrees from the Sun. The position
-of the counter-glow, like that of the zodiacal light and zodiacal band,
-is not precisely on the plane of the ecliptic, but a little to the north
-of this line. It is very probable that near the equator the phenomenon
-would appear different and there would correspond with this plane.
-</p>
-<p>
-There seems to be some confusion among observers in regard to the
-spectrum of the zodiacal light. Some have seen a bright green line in
-its spectrum, corresponding to that of the aurora borealis; while others
-could only see a faint grayish continuous spectrum, which differs,
-however, from that of a faint solar light, by the fact that it presents
-a well-defined bright zone, gradually blending on each side with the
-fainter light of the continuous spectrum. I have, myself, frequently
-observed the faint continuous spectrum of the zodiacal light, and on one
-occasion recognized the green line of the aurora; but it might have been
-produced by the aurora itself, as yet invisible to the eye, and not by
-the zodiacal light, since, later in the same evening, there was a
-brilliant auroral display. If it were demonstrated that this green line
-exists in the spectrum of the zodiacal light, the fact would have
-importance, as tending to show that the aurora and the zodiacal light
-have a common origin.
-</p>
-<p>
-Rev. Geo. Jones describes a very curious phenomenon which he observed
-several times a little before the moon rose above the horizon. The
-phenomenon consisted in a short, oblique, luminous cone rising from the
-Moon's place in the direction of the ecliptic. This phenomenon he has
-called the Moon Zodiacal Light. In 1874, I had an opportunity to observe
-a similar phenomenon when the Moon was quite high in the sky. By taking
-the precaution to screen the Moon's disk by the interposition of some
-buildings between it and my eye, I saw two long and narrow cones of
-light parallel to the ecliptic issuing from opposite sides of our
-satellite. The phenomenon could not possibly be attributed to vapors in
-our atmosphere, since the sky was very clear at the moment of the
-observation. Later on, these appendages disappeared with the formation
-of vapors near the Moon, but they reappeared an hour later, when the sky
-had cleared off, and continued visible for twenty minutes longer, and
-then disappeared in a clear sky.
-</p>
-<p>
-Although the zodiacal light has been studied for over two centuries, no
-wholly satisfactory explanation of the phenomenon has yet been given.
-Now, as in Cassini's time, it is generally considered by astronomers to
-be due to a kind of lens-shaped ring surrounding the Sun, and extending
-a little beyond the Earth's orbit. This ring is supposed to lie in the
-plane of the ecliptic, and to be composed of a multitude of independent
-meteoric particles circulating in closed parallel orbits around the Sun.
-But many difficulties lie in the way of this theory. It seems as
-incompetent to explain the slow and rapid changes in the light of this
-object as it is to explain the contractions and extensions of its cone.
-It fails, moreover, to explain the flickering motions, the coruscations
-observed in its light, or the displacement of its cone and of its axes
-of brightness and symmetry by a mere change in the position of the
-observer. Rev. Geo. Jones, unable to explain by this theory the
-phenomena which came under his observation, has proposed another, which
-supposes the zodiacal light to be produced by a luminous ring
-surrounding the Earth, this ring not extending as far as the orbit of
-the Moon. But this theory also fails in many important points, so that
-at present no satisfactory explanation of the phenomenon can be given.
-</p>
-<p>
-As the phenomenon is connected in some way with the Sun, and as we have
-many reasons to believe this body to be always more or less electrified,
-it might be supposed that the Sun, acting by induction on our globe,
-develops feeble electric currents in the rarefied gases of the superior
-regions of our atmosphere, and there forms a kind of luminous ridge
-moving with the Sun in a direction contrary to the diurnal motion, and
-so producing the zodiacal light. On this hypothesis, the counter-glow
-would be the result of a smaller cone of light generated by the solar
-induction on the opposite point of the Earth.
-</p>
-<p>
-Plate 5, which sufficiently explains itself, represents the zodiacal
-light as it appeared in the West on the evening of February 20th, 1876.
-All the stars are placed in their proper position, and their relative
-brightness is approximately shown by corresponding variations in
-size&mdash;the usual and almost the only available means of representation.
-Of course, it must be remembered that a star does not, in fact, show any
-disk even in the largest telescopes, where it appears as a mere point of
-light, having more or less brilliancy. The cone of light rises obliquely
-along the ecliptic, and the point forming its summit is found in the
-vicinity of the well-known group of stars, called the Pleiades, in the
-constellation of Taurus, or the Bull.
-</p>
-
-<p><br /><br /><br /></p>
-
-<h4><a id="THE_MOON">THE MOON</a>
-<br /><br />
-PLATE VI</h4>
-
-<p>
-In its endless journey through space, our globe is not solitary, like
-some of the planets, but is attended by the Moon, our nearest celestial
-neighbor. Although the Moon does not attain to the dignity of a planet,
-and remains a secondary body in the solar system, yet, owing to its
-proximity to our globe, and to the great influence it exerts upon it by
-its powerful attraction, it is to us one of the most important celestial
-bodies.
-</p>
-<p>
-While the Moon accompanies the Earth around the Sun, it also revolves
-around the Earth at a mean distance of 238,800 miles. For a celestial
-distance this is only a trifling one; the Earth in advancing on its
-orbit travels over such a distance in less than four hours. A cannon
-ball would reach our satellite in nine days; and a telegraphic dispatch
-would be transmitted there in 1½ seconds of time, if a wire could be
-stretched between us and the Moon.
-</p>
-<p>
-Owing to the ellipticity of the Moon's orbit, its distance from the
-Earth varies considerably, our satellite being sometimes 38,000 miles
-nearer to us than it is at other times. These changes in the distance of
-the Moon occasion corresponding changes from 29' to 33' in its apparent
-diameter. The real diameter of the Moon is 2,160 miles, or a little over
-one-quarter the diameter of our globe; our satellite being 49 times
-smaller than the Earth.
-</p>
-<p>
-The mean density of the materials composing the Moon is only
-⁶⁄₁₀ that of the materials composing the Earth, and the force of
-gravitation at the surface of our satellite is six times less than it is
-at the surface of our globe. If a person weighing 150 lbs. on our Earth
-could be transported to the Moon, his weight there would be only 25 lbs.
-</p>
-<p>
-The Moon revolves around the Earth in about 27⅓ days, with a mean
-velocity of one mile per second, the revolution constituting its
-sidereal period. If the Earth were motionless, the lunar month would be
-equal to the sidereal period; but owing to its motion in space, the Sun
-appears to move with the Moon, though more slowly, so that after having
-accomplished one complete revolution, our satellite has yet to advance
-2¼ days before reaching the same apparent position in regard to the
-Earth and the Sun that it had at first. The interval of time comprised
-between two successive New Moons, which is a little over 29½ days,
-constitutes the synodical period of the Moon, or the lunar month.
-</p>
-<p>
-The Moon is not a self-luminous body, but, like the Earth and the
-planets, it reflects the light which it receives from the Sun, and so
-appears luminous. That such is the case is sufficiently demonstrated by
-the phases exhibited by our satellite in the course of the lunar month.
-Every one is familiar with these phases, which are a consequence of the
-motion of the Moon around the Earth. When our satellite is situated
-between us and the Sun, it is New Moon; since we cannot see its
-illuminated side, which is then turned away from us towards the Sun.
-When, on the contrary, it reaches that point of its orbit which, in
-regard to us, is opposite to the Sun's place, it is Full Moon; since
-from the Earth we can only see the fully illuminated side of our
-satellite. Again, when the Moon arrives at either of the two opposite
-points of its orbit, the direction of which from the Earth is at right
-angles with that of the Sun, it is either the First or the Last Quarter;
-since in these positions we can only see one-half of its illuminated
-disk.
-</p>
-<p>
-The curve described by the Moon around the Earth lies approximately in a
-plane, this plane being inclined about 5° to the ecliptic. Since our
-satellite, in its motion around us and the Sun, closely follows the
-ecliptic, which is inclined 23½° to the equator, it results that when
-this plane is respectively high or low in the sky, the moon is also high
-or low when crossing the meridian of the observer. In winter that part
-of the ecliptic occupied by the Sun is below the equator, and,
-consequently, the New Moons occurring in that season are low in the sky,
-since at New Moon our satellite must be on the same side of the ecliptic
-with the Sun. But the Full Moons in the same season are necessarily high
-in the sky, since a Full Moon can only occur when our satellite is on
-the opposite side of the ecliptic from the Sun, in which position it is,
-of course, as many degrees above the equator as the Sun is below. The
-Full Moon which happens nearest to the autumnal equinox is commonly
-called the Harvest Moon, from the fact that, after full, its delays in
-rising on successive evenings are very brief and therefore favorable for
-the harvest work in the evening. The same phenomenon occurs in every
-other lunar month, but not sufficiently near the time of Full Moon to be
-noticeable. When, in spring, a day or two after New Moon, our satellite
-begins to show its thin crescent, its position on the ecliptic is north
-as well as east of that occupied by the Sun; hence, its horns are nearly
-upright in direction, and give it a crude resemblance to a tipping bowl,
-from which many people who are unaware of its cause, and that this
-happens every year, draw conclusions as to the amount of rain to be
-expected.
-</p>
-<p>
-One of the most remarkable features of the Moon's motions is that our
-satellite rotates on its axis in exactly the same period of time
-occupied by its revolution around the Earth, from which it results that
-the Moon always presents to us the same face. To explain this
-peculiarity, astronomers have supposed that the figure of our satellite
-is not perfectly spherical, but elongated, so that the attraction of the
-Earth, acting more powerfully upon its nearest portions, always keeps
-them turned toward us, as if the Moon were united to our globe by a
-string. It is not exactly true, however, that the Moon always presents
-its same side to us, although its period of rotation exactly equals that
-of its revolution; since in consequence of the inclination of its axis
-of rotation to its orbit, combined with the irregularities of its
-orbital motion about us, apparent oscillations in latitude and in
-longitude, called librations, are created, from which it results that
-nearly ⁶⁄₁₀ of the Moon's surface is visible from the Earth at
-one time or another.
-</p>
-<p>
-The Moon is a familiar object, and every one is aware that our
-satellite, especially when it is fully illuminated, presents a variety
-of bright and dark markings, which, from their distant resemblance to a
-human face, are popularly known as "the man in the moon." A day or two
-after New Moon, when the thin crescent of our satellite is visible above
-the western horizon after sunset, the dark portion of its disk is
-plainly visible, and appears of a pale, ashy gray color, although not
-directly illuminated by the Sun. This phenomenon is due to the
-Earth-shine, or to that portion of solar light which the illuminated
-surface of our globe reflects to the dark side of the Moon, exactly in
-the same manner that the Moon-shine, on our Earth, is due to the solar
-light reflected to our globe by the illuminated Moon.
-</p>
-<p>
-Seen with a telescope of moderate power, or even with a good
-opera-glass, the Moon presents a peculiar mottled appearance, and has a
-strong resemblance to a globe made of plaster of Paris, on the surface
-of which numerous roundish, saucer-shaped cavities of various sizes are
-scattered at random. This mottled structure is better seen along the
-boundary line called the <i>terminator</i>, which divides the illuminated
-from the dark side of the Moon. The line of the terminator always
-appears jagged, and it is very easy to recognize that this irregularity
-is due to the uneven and rugged structure of the surface of our
-satellite.
-</p>
-<p>
-A glance at the Moon through a larger telescope shows that the bright
-spots recognized with the naked eye belong to very uneven and
-mountainous regions of our satellite, while the dark ones belong to
-comparatively smooth, low surfaces, comparable to those forming the
-great steppes and plains of the Earth. When examined with sufficient
-magnifying power, the white, rugged districts of the Moon appear covered
-over by numerous elevated craggy plateaus, mountain-chains, and deep
-ravines; by steep cliffs and ridges; by peaks of great height and
-cavities of great depth. This rugged formation, which is undoubtedly of
-volcanic origin, gives our satellite a desolate and barren appearance.
-The rugged tract occupies more than one-half of the visible surface of
-the Moon, forming several distinct masses, the principal of which occupy
-the south and south-western part of the disk. That this formation is
-elevated above the general level is proved by the fact that the
-mountains, peaks, and other objects which compose it, all cast a shadow
-opposite to the Sun; and further, that the length of these shadows
-diminishes with the elevation of the Sun above the lunar horizon.
-</p>
-<p>
-Since Galileo's time the surface of the Moon has been studied by a host
-of astronomers, and accurate maps of its topographical configuration
-have been made, and names given to all features of any prominence. It
-may even be said that in its general features, the visible surface of
-our satellite is now better known to us than is the surface of our own
-Earth.
-</p>
-<p>
-One of the most striking and common features of the mountainous
-districts of the Moon, is the circular, ring-like disposition of their
-elevated parts, which form numerous crater-like objects of different
-sizes and depths. Many thousands of crater-like objects are visible on
-the Moon through a good telescope, and, considering how numerous the
-small ones are, there is, perhaps, no great exaggeration in fixing their
-number at 50,000, as has been done by some astronomers. These volcanic
-regions of the Moon cannot be compared to anything we know, and far
-surpass in extent those of our globe. The number and size of the craters
-of our most important volcanic regions in Europe, in Asia, in North and
-South America, in Java, in Sumatra, and Borneo, are insignificant when
-compared with those of the Moon. The largest known craters on the Earth
-give only a faint idea of the magnitude of some of the lunar craters.
-The great crater Haleakala, in the Sandwich Islands, probably the
-largest of the terrestrial volcanoes, has a circumference of thirty
-miles, or a diameter of a little less than ten miles. Some of the great
-lunar craters, called walled plains, such as Hipparchus, Ptolemæus,
-etc., have a diameter more than ten times larger than that of Haleakala,
-that of the first being 115 miles and that of the last 100 miles. These
-are, of course, among the largest of the craters of the Moon, although
-there are on our satellite a great number of craters above ten miles in
-diameter.
-</p>
-<p>
-The crater-forms of the Moon have evidently appeared at different
-periods of time, since small craters are frequently found on the walls
-of larger ones; and, indeed, still smaller craters are not rarely seen
-on the walls of these last. The walls of the lunar craters are usually
-quite elevated above the surrounding surface, some of them attaining
-considerable elevations, especially at some points, which form peaks of
-great height. Newton, the loftiest of all, rises at one point to the
-height of 23,000 feet, while many others range from ten to twenty
-thousand feet in height. Several craters have their floor above the
-general surface&mdash;Plato, for instance. Wargentin has its floor nearly
-on a level with the summit of its walls, showing that at some period of its
-history liquid lavas, ejected from within, have filled it to the brim
-and then solidified. The floors of some of the craters are smooth and
-flat, but in general they are occupied by peaks and abrupt mountainous
-masses, which usually form the centre. Many of their outside walls are
-partly or wholly covered by numerous ravines and gullies, winding down
-their steep declivities, branching out and sometimes extending to great
-distances from their base. It would seem that these great volcanic
-mouths have at some time poured out torrents of lavas, which, in their
-descent, carved their passage by the deep gullies now visible.
-Sometimes, also, the crater slopes are strewn with debris, giving them a
-peculiar volcanic appearance.
-</p>
-<p>
-Notwithstanding their many points of similarity with the volcanoes of
-the Earth, the lunar craters differ from them in many particulars,
-showing that volcanic forces acting on different globes may produce
-widely different results. For example, the floors of terrestrial craters
-are usually situated at considerable elevations above the general
-surface, while those of the lunar craters are generally much depressed,
-the height of their walls being only about one-half the depth of their
-cavities. Again, while on the Earth the mass of the volcanic cones far
-exceeds the capacity of their openings, on the Moon it is not rare to
-see the capacity of the crater cavities exceeding the mass of the
-surrounding walls. On the Earth, the volcanic cones and mouths are
-comparatively regular and smooth, and are generally due to the
-accumulation of the ashes and the debris of all kinds which are ejected
-from the volcanic mouths. On the Moon, very few craters show this
-character, and for the most part their walls have a very different
-structure, being irregular, very rugged, and composed of a succession of
-concentric ridges, rising at many points to great elevations, and
-forming peaks of stupendous height. Again, many of the larger
-terrestrial craters have their interior occupied by a central cone, or
-several such cones, having a volcanic mouth on their summits; on the
-Moon such central cones are very rare. Although many of the large lunar
-craters have their interior occupied by central masses which have been
-often compared to the central cones of our great volcanoes, yet these
-objects have a very different character and origin. For the most part,
-they are mountainous masses of different forms&mdash;having very rarely any
-craters on them&mdash;and seem to have resulted from the crowding and
-lifting up of the crater floor by the phenomena of subsidence, of which
-these craters show abundant signs. Besides, the terrestrial craters are
-characterized by large and important lava streams, while on the Moon the
-traces of such phenomena are quite rare, and when they are shown, they
-generally differ from those of the Earth by their numerous and
-complicated ramifications, and also by the fact that many of these lava
-streamlets take their origin at a considerable distance from the crater
-slopes, and are grooved and depressed as if the burning liquids which
-are supposed to have produced them had subsequently disappeared, by
-evaporation or otherwise, leaving the furrow empty.
-</p>
-<p>
-The dark spots of the Moon, when viewed through a telescope, exhibit a
-totally different character, and show that they belong to a different
-formation from that of the brighter portions. These darker tracts do not
-seem to have had a direct volcanic origin like the latter, but rather
-appear to have resulted from the solidification of semi-fluid materials,
-which have overflowed vast areas at different times. The surface of this
-system is comparatively smooth and uniform, only some small craters and
-low ridges being seen upon it. The level and dark appearance of these
-areas led the ancient astronomers to the belief that they were produced
-by a liquid strongly absorbing the rays of light, and were seas like our
-seas. Accordingly, these dark surfaces were called <i>Maria</i>, or Seas, a
-name which it is convenient to retain, although it is well known to have
-originated in an error. The so-called seas of the Moon are evidently
-large flat surfaces similar to the deserts, steppes, pampas, and
-prairies of the Earth in general appearance. The great plains of the
-Moon are at a lower level than that of the other formation, and that
-which first attracts the observer's attention is the fact that they are
-surrounded almost on all sides by an irregular line of abrupt cliffs and
-mountain chains, showing phenomena of dislocation. This character of
-dislocation, which is general, and is visible everywhere upon the
-contours of the plains, seems to indicate that phenomena of subsidence,
-either slow or rapid, have occurred on the Moon; while, at the same
-time, the sunken surfaces were overflowed by a semi-fluid liquid, which
-solidified afterwards. The evidences of subsidence and overflowing
-become unmistakable when we observe that, along the borders of the gray
-plains, numerous craters are more or less embedded in the gray
-formation, only parts of the summit of their walls remaining visible, to
-attest that once large craters existed there. The farther from the
-border of the plain the vestiges of these craters are observed, the
-deeper they are embedded in the gray formation. That phenomena of
-subsidence have occurred on a grand scale on the Moon, is further
-indicated by the fact that the singular systems of fractures called
-clefts and rifts generally follow closely the outside border of the gray
-plains, often forming parallel lines of dislocation and fractures. In
-the interior regions of the gray formation, these fractures are
-comparatively rare.
-</p>
-<p>
-The gray, lava-like formation is obviously of later origin than the
-mountainous system to which belong the embedded craters above described.
-Its comparatively recent origin might also be inferred from the
-smallness of its craters and its low ridges. The few large craters
-observed on this formation evidently belong to the earlier system.
-</p>
-<p>
-The color of this system of gray plains is far from being uniform. In
-general appearance it is of a bluish gray, but when observed
-attentively, large areas appear tinted with a dusky olive-green, while
-others are slightly tinged with yellow. Some patches appear brownish,
-and even purplish. A remarkable example of the first case is seen on the
-surface, which encloses within a large parallelogram the two conspicuous
-craters, Aristarchus and Herodotus. This surface evidently belongs to a
-different system from that of the Oceanus Procellarum surrounding it,
-as, besides its color, which totally differs from that of the gray
-formation, its surface shows the rugged structure of the volcanic
-formation.
-</p>
-<p>
-When the Moon is full, some very curious white, luminous streaks are
-seen radiating from different centres, which, for the most part, are
-important craters, occupied by interior mountains. The great crater
-Tycho is the centre of the most imposing of the systems of white
-streaks. Some of the diverging rays of this great centre extend to a
-distance equal to one-quarter of the Moon's circumference, or about
-1,700 miles. The true nature of these luminous streaks is unknown, but
-it seems certain that they have their origin in the crater from which
-they diverge. They do not form any relief on the surface, and are seen
-going up over the mountains and steep walls of the crater, as well as
-down the ravines and on the floors of craters.
-</p>
-<p>
-The Moon seems to be deprived of an atmosphere; or, if it has any, it
-must be so excessively rare that its density is less than of the density
-of the Earth's atmosphere, since delicate tests afforded by the
-occultation of stars have failed to reveal its presence. Although no
-atmosphere of any consequence exists on the Moon, yet phenomena which I
-have observed seem to indicate the occasional presence there of vapors
-of some sort. On several occasions, I have seen a purplish light over
-some parts of the Moon, which prevented well-known objects being as
-distinctly seen as they were at other times, causing them to appear as
-if seen through a fog. One of the most striking of these observations
-was made on January 4th, 1873, on the crater Kant and its vicinity,
-which then appeared as if seen through luminous purplish vapors. On one
-occasion, the great crater Godin, which was entirely involved in the
-shadow of its western wall, appeared illuminated in its interior by a
-faint purplish light, which enabled me to recognize the structure of
-this interior. The phenomenon could not be attributed in this case to
-reflection, since the Sun, then just rising on the western wall of the
-crater, had not yet grazed the eastern wall, which was invisible. It is
-not impossible that a very rare atmosphere composed of such vapors
-exists in the lower parts of the Moon.
-</p>
-<p>
-If the Moon has no air, and no liquids of any sort, it seems impossible
-that its surface can maintain any form of life, either vegetable or
-animal, analogous to those on the Earth. In fact, nothing indicating life
-has been detected on the Moon&mdash;our satellite looking like a barren,
-lifeless desert. If life is to be found there at all, it must be of a
-very elementary nature. Aside from the want of air and water to sustain
-it, the climatic conditions of our satellite are very unfavorable for
-the development of life. The nights and days of the Moon are each equal
-to nearly fifteen of our days and nights. For fifteen consecutive
-terrestrial days the Sun's light is absent from one hemisphere of the
-Moon; while for the same number of days the Sun pours down on the other
-hemisphere its light and heat, the effects of which are not in any way
-mitigated by an atmosphere. During the long lunar nights the temperature
-must at least fall to that of our polar regions, while during its long
-days it must be far above that of our tropical zone. It has been
-calculated that during the lunar nights the temperature descends to 23°
-below zero, while during the days it rises to 468°, or 256° above the
-boiling point.
-</p>
-<p>
-It has been a question among astronomers whether changes are still
-taking place at the surface of the Moon. Aside from the fact that
-change, not constancy, is the law of nature, it does not seem doubtful
-that changes occur on the Moon, especially in view of the powerful
-influences of contraction and dilatation to which its materials are
-submitted by its severe alternations of temperature. From the distance
-at which we view our satellite, we cannot expect, of course, to be able
-to see changes, unless they are produced on a large scale. Theoretically
-speaking, the largest telescopes ever constructed ought to show us the
-Moon as it would appear to the naked eye from a distance of 40 miles;
-but in practice it is very different. The difficulty is in the fact
-that, while we magnify the surface of a telescopic image, we are unable
-to increase its light; so that, practically, in magnifying an object, we
-weaken its light proportionally to the magnifying power employed. The
-light of the Moon, especially near the terminator, where we almost
-always make our observations, is not sufficiently bright to bear a very
-high magnifying power, and only moderate ones can be applied to its
-study. What we gain by enlarging an object, we more than lose by the
-weakening of its light. Besides, a high magnifying power, by increasing
-the disturbances generally present in our atmosphere, renders the
-telescopic image unsteady and very indistinct. On the whole, the largest
-telescopes now in existence do not show us our satellite better than if
-we could see it with the naked eye from a distance of 300 miles or more.
-At such a distance only considerable changes would be visible.
-</p>
-<p>
-Notwithstanding these difficulties, it is believed that changes have
-been detected in Linné, Marius, Messier, and several other craters. An
-observation of mine seems to indicate that changes have recently taken
-place in the great crater Eudoxus. On February 20th, 1877, between 9h.
-30m. and 10h. 30m., I observed a straight, narrow wall crossing this
-crater from east to west, a little to the south of its centre. This wall
-had a considerable elevation, as was proved by the shadow it cast on its
-northern side. Towards its western end this wall appeared as a brilliant
-thread of light on the black shadow cast by the western wall of the
-crater. The first time I had occasion to observe this crater again,
-after this observation, was a year later, on February 17th, 1878; no
-traces of the wall were then detected. Many times since I have tried to
-find this narrow wall again, when the Moon presented the same phase and
-the same illumination, but always with negative results. It seems
-probable that this structure has crumbled down, yet it is very singular
-that so prominent a feature should not have been noticed before.
-</p>
-
-<p><br /></p>
-
-<div class="figcenter" style="width: 400px;">
-<a id="figure06"></a>
-<br />
-<img src="images/figure06.jpg" width="400" alt="" />
-<div class="caption">
-<p>PLATE VI.&mdash;MARE HUMORUM.</p>
-<p class="smaller">From a study made in 1875</p>
-</div></div>
-
-<p><br /></p>
-
-<p>
-The "Mare Humorum," or sea of moisture, as it is called, which is
-represented on Plate VI., is one of the smaller gray lunar plains. Its
-diameter, which is very nearly the same in all directions, is about 270
-miles, the total area of this plain being about 50,000 square miles. It
-is one of the most distinct plains of the Moon, and is easily seen with
-the naked eye on the left-hand side of the disk. The floor of the plain
-is, like that of the other gray plains, traversed by several systems of
-very extended but low hills and ridges, while small craters are
-disseminated upon its surface. The color of this formation is of a dusky
-greenish gray along the border, while in the interior it is of a lighter
-shade, and is of brownish olivaceous tint. This plain, which is
-surrounded by high clefts and rifts, well illustrates the phenomena of
-dislocation and subsidence. The double-ringed crater Vitello, whose
-walls rise from 4,000 to 5,000 feet in height, is seen in the upper
-left-hand corner of the gray plain. Close to Vitello, at the east, is
-the large broken ring-plain Lee, and farther east, and a little below,
-is a similarly broken crater called Doppelmayer. Both of these open
-craters have mountainous masses and peaks on their floor, which is on a
-level with that of the Mare Humorum. A little below, and to the left of
-these objects, is seen a deeply embedded oval crater, whose walls barely
-rise above the level of the plain. On the right-hand side of the great
-plain, is a long <i>fault</i>, with a system of fracture running along its
-border. On this right-hand side, may be seen a part of the line of the
-terminator, which separates the light from the darkness. Towards the
-lower right-hand corner, is the great ring-plain Gassendi, 55 miles in
-diameter, with its system of fractures and its central mountains, which
-rise from 3,000 to 4,000 feet above its floor. This crater slopes
-southward towards the plain, showing the subsidence to which it has been
-submitted. While the northern portion of the wall of this crater rises
-to 10,000 feet, that on the plain is only 500 feet high, and is even
-wholly demolished at one place where the floor of the crater is in
-direct communication with the plain. In the lower part of the <i>mare</i>,
-and a little to the west of the middle line, is found the crater
-Agatharchides, which shows below its north wall the marks of rills
-impressed by a flood of lava, which once issued from the side of the
-crater. On the left-hand side of the plain, is seen the half-demolished
-crater Hippalus, resembling a large bay, which has its interior strewn
-with peaks and mountains. On this same side can be seen one of the most
-important systems of clefts and fractures visible on the Moon, these
-clefts varying in length from 150 to 200 miles.
-</p>
-
-<p><br /><br /><br /></p>
-
-<h4><a id="ECLIPSES">ECLIPSES OF THE MOON</a>
-<br /><br />
-PLATE VII</h4>
-
-<p>
-Since the Moon is not a self-luminous body, but shines by the light
-which it borrows from the Sun, it follows that when the Sun's light is
-prevented from reaching its surface, our satellite becomes obscured. The
-Earth, like all opaque bodies exposed to sunlight, casts a shadow in
-space, the direction of which is always opposite to the Sun's place. The
-form of the Earth's shadow is that of a long, sharply-pointed cone,
-which has our globe for its base. Its length, varying with the distance
-of the Earth from the Sun, is, on an average, 855,000 miles, or 108
-times the terrestrial diameter. This conical shadow of the Earth,
-divided longitudinally by the plane of the ecliptic, lies half above and
-half below that plane, on which the summit of the shadow describes a
-whole circumference in the course of a year. If the Moon's orbit were
-not inclined to the ecliptic, our satellite would pass at every Full
-Moon directly through the Earth's shadow; but, owing to that
-inclination, it usually passes above or below the shadow. Twice,
-however, during each of its revolutions, it must cross the plane of the
-ecliptic, the points of its orbit where this happens being called nodes.
-Accordingly, if it is near a node at the time of Full Moon, it will
-enter the shadow of the Earth, and become either partly or wholly
-obscured, according to the distance of its centre from the plane of the
-ecliptic. The partial or total obscuration of the Moon's disk thus
-produced constitutes a partial or total eclipse of the Moon. The
-essential conditions for an eclipse of the Moon are, therefore, that our
-satellite must not only be full, but must also be at or very near one of
-its nodes.
-</p>
-<p>
-Although inferior in importance to the eclipses of the Sun, the eclipses
-of the Moon are, nevertheless, very interesting and remarkable
-phenomena, which never fail to produce a deep impression on the mind of
-the observer, inasmuch as they give him a clear insight into the silent
-motions of the planetary bodies.
-</p>
-<p>
-At the mean distance of the Moon from the Earth, the diameter of the
-conical shadow cast in space by our globe is more than twice as large as
-that of our satellite. But, besides this pure dark shadow of the Earth,
-its cone is enveloped by a partial shadow called "Penumbra," which is
-produced by the Sun's light being partially, but not wholly, cut off by
-our globe.
-</p>
-<p>
-While the Moon is passing into the penumbra, a slight reduction of the
-light of that part of the disk which has entered it, is noticeable. As
-the progress of the Moon continues, the reduction becomes more
-remarkable, giving the impression that rare and invisible vapors are
-passing over our satellite. Some time after, a small dark-indentation,
-marking the instant of first contact, appears on the eastern or
-left-hand border of the Moon, which is always the first to encounter the
-Earth's shadow, since our satellite is moving from west to east. The
-dark indentation slowly and gradually enlarges with the onward progress
-of the Moon into the Earth's shadow, while the luminous surface of its
-disk diminishes in the same proportion. The form of the Earth's shadow
-on the Moon's disk clearly indicates the rotundity of our globe by its
-circular outline. Little by little the dark segment covers the Moon's
-disk, and its crescent, at last reduced to a mere thread of light,
-disappears at the moment of the second contact. With this the phase of
-totality begins, our satellite being then completely involved in the
-Earth's shadow.
-</p>
-<p>
-The Moon remains so eclipsed for a period of time which varies with its
-distance from the Earth, and with the point of its orbit where it
-crosses the conical shadow. When it passes through the middle of this
-shadow, while its distance from our globe is the least, the total phase
-of an eclipse of the Moon may last nearly two hours. The left-hand
-border of our satellite having gone first into the Earth's shadow, is
-also the first to emerge, and, at the moment of doing so, it receives
-the Sun's light, and totality ends with the third contact. The lunar
-crescent gradually increases in breadth after its exit from the shadow,
-and finally the Moon recovers its fully illuminated disk as before, at
-the moment its western border leaves the Earth's shadow. Soon after, it
-passes out of the penumbra, and the eclipse is over. In total eclipses,
-the interval of time from the first to last contact may last 5h. 30m,
-but it is usually shorter.
-</p>
-<p>
-Soon after the beginning of an eclipse, the dark segment produced by the
-Earth's shadow on the Moon's disk generally appears of a dark grayish
-opaque color, but with the progress of the phenomenon, this dark tint is
-changed into a dull reddish color, which, gradually increasing, attains
-its greatest intensity when the eclipse is total. At that moment the
-color of the Moon is of a dusky, reddish, coppery hue, and the general
-features of the Moon's surface are visible as darker and lighter tints
-of the same color. It sometimes happens, however, that our satellite
-does not exhibit this peculiar coppery tint, but appears either blackish
-or bluish, in which case it is hardly distinguishable from the sky.
-</p>
-<p>
-It is very rare for the Moon to disappear completely during totality,
-and even when involved in the deepest part of the Earth's shadow, our
-satellite usually remains visible to the naked eye, or, at least, to the
-telescope. This phenomenon is to be attributed to the fact that the
-portion of the solar rays which traverse the lower strata of our
-atmosphere are strongly refracted, and bend inward in such a manner that
-they fall on the Moon, and sufficiently illuminate its surface to make
-it visible. The reddish color observed is caused by the absorption of
-the blue rays of light by the vapors which ordinarily-saturate the lower
-regions of our atmosphere, leaving only red rays to reach the Moon's
-surface. Of course, these phenomena are liable to vary with every
-eclipse, and depend almost exclusively on the meteorological conditions
-of our atmosphere.
-</p>
-<p>
-In some cases the phase of totality lasts longer than it should,
-according to calculation. This can be attributed to the fact that the
-Earth is enveloped in a dense atmosphere, in which opaque clouds of
-considerable extent are often forming at great elevations. Such strata
-of clouds, in intercepting the Sun's light, would have, of course, the
-effect of increasing the diameter of the Earth's shadow, in a direction
-corresponding to the place they occupy, and, if the Moon were moving in
-this direction, would increase the phase of total obscuration.
-</p>
-<p>
-The eclipses of the Moon, like those of the Sun, as shown above, have a
-cycle of 18 years, 11 days and 7 hours, and recur after this period of
-time in nearly the same order. They can, therefore, be approximately
-predicted by adding 18y. 11d. 7h. to the date of the eclipses which have
-occurred during the preceding period. During this cycle 70 eclipses will
-occur&mdash;41 being eclipses of the Sun and 29 eclipses of the Moon. At no
-time can there ever be more than seven eclipses in a year, and there are
-never less than two. When there are only two eclipses in a year, they
-are both eclipses of the Sun.
-</p>
-<p>
-Although the number of solar eclipses occurring at some point or other
-of the Earth's surface is greater than that of the eclipses of the Moon,
-yet at any single terrestrial station the eclipses of the Moon are the
-more frequent. While an eclipse of the Sun is only visible on a narrow
-belt, which is but a very small fraction of the hemisphere then
-illuminated by the Sun, an eclipse of the Moon is visible from all the
-points of the Earth which have the Moon above their horizon at the time.
-Furthermore, an eclipse of the Sun is not visible at one time over the
-whole length of its narrow tract, but moves gradually from one end of it
-to the other; while, on the contrary, an eclipse of the Moon begins and
-ends at the very same instant for all places from which it can be seen,
-but, of course, not at the same local time, which varies with the
-longitude of the place.
-</p>
-
-<p><br /></p>
-
-<div class="figcenter" style="width: 400px;">
-<a id="figure07"></a>
-<br />
-<img src="images/figure07.jpg" width="400" alt="" />
-<div class="caption">
-<p>PLATE VII.&mdash;PARTIAL ECLIPSE OF THE MOON.</p>
-<p class="smaller">Observed October 24, 1874</p>
-</div></div>
-
-<p><br /></p>
-
-<p>
-The partial eclipse of the Moon, represented on Plate VII., shows quite
-plainly the configuration of our satellite as seen with the naked eye
-during the eclipse, with its bright and dark spots, and its radiating
-streaks. This eclipse was observed on October 24th, 1874.
-</p>
-
-<p><br /><br /><br /></p>
-
-<h4>THE PLANETS</h4>
-
-<p>
-Around the Sun circulate a number of celestial bodies, which are called
-"<i>Planets</i>." The planets are opaque bodies, and appear luminous
-because their surfaces reflect the light they receive from the Sun.
-</p>
-<p>
-The planets are situated at various distances from the Sun, and revolve
-around this body in widely different periods of time, which are,
-however, constant for each planet, so far as ascertained, and doubtless
-are so in the other cases.
-</p>
-<p>
-The ideal line traced in space by a planet in going around the Sun, is
-called <i>the orbit</i> of the planet; while the period of time employed by
-a planet to travel over its entire orbit and return to its starting point,
-is called <i>the sidereal revolution</i>, <i>or year</i> of the planet. The
-dimensions of the orbits of the different planets necessarily vary with
-the distance of these bodies from the Sun, as does also the length of
-their sidereal revolution.
-</p>
-<p>
-The distance of a planet from the Sun does not remain constant, but is
-subject to variations, which in certain cases are quite large. These
-variations result from the fact that the planetary orbits are not
-perfect circles having the Sun for centre, but curves called
-"<i>Ellipses</i>," which have two centres, or foci, one of which is always
-occupied by the Sun. This is in accordance with Kepler's first law.
-</p>
-<p>
-The ideal point situated midway between the two foci is called <i>the
-centre of the ellipse</i>, or <i>orbit</i>; while the imaginary straight
-line which passes through both foci and the centre, with its ends at
-opposite points of the ellipse, is called "<i>the major axis</i>" of the
-orbit. It is also known as "<i>the line of the apsides</i>." The ideal
-straight line which, in passing through the centre of the orbit, cuts
-the major axis at right angles, and is prolonged on either side to
-opposite points on the ellipse, is called "<i>the minor axis</i>" of the
-orbit.
-</p>
-<p>
-When a planet reaches that extremity of the major axis of its orbit
-which is the nearest to the Sun, it is said to be in its
-"<i>perihelion</i>;" while, when it arrives at the other extremity,
-which is farthest from this body, it is said to be in its
-"<i>aphelion</i>." When a planet reaches either of the two opposite
-points of its orbit situated at the extremities of its minor axis, it is
-said to be at its <i>mean distance</i> from the Sun.
-</p>
-<p>
-The rapidity with which the planets move on their orbits varies with
-their distance from the Sun; the farther they are from this body, the
-more slowly they move. The rapidity of their motion is greatest when
-they are in perihelion, and least when they are in aphelion, having its
-mean rate when these bodies are crossing either of the extremities of
-the minor axes of their orbits.
-</p>
-<p>
-The imaginary line which joins the Sun to a planet at any point of its
-orbit, and moves with this planet around the Sun, is called "<i>the radius
-vector</i>." According to Kepler's second law, whatever may be the distance
-of a planet from the Sun, the radius vector sweeps over equal areas of
-the plane of the planet's orbit in equal times.
-</p>
-<p>
-There is a remarkable relation between the distance of the planets from
-the Sun and their period of revolution, in consequence of which the
-squares of their periodic times are respectively equal to the cubes of
-their mean distances from the Sun. From this third law of Kepler, it
-results that the mere knowledge of the mean distance of a planet from
-the Sun enables one to know its period of revolution, and <i>vice
-versa</i>.
-</p>
-<p>
-The orbit described by the Earth around the Sun in a year, or the
-apparent path of the Sun in the sky, is called "<i>the ecliptic</i>." Like
-that of all the planetary orbits, the plane of the ecliptic passes
-through the Sun's centre. The ecliptic has a great importance in
-astronomy, inasmuch as it is the fundamental plane to which the orbits
-and motions of all planets are referred.
-</p>
-<p>
-The orbits of the larger planets are not quite parallel to the ecliptic,
-but more or less inclined to this plane; although the inclination is
-small, and does not exceed eight degrees. On account of this inclination
-of the orbits, the planets, in accomplishing their revolutions around
-the Sun, are sometimes above and sometimes below the plane of the
-ecliptic. A belt extending 8° on each side of the ecliptic, and,
-therefore, 16° in width, comprises within its limits the orbits of all
-the principal planets. This belt is called "<i>the Zodiac</i>."
-</p>
-<p>
-Since all the planets have the Sun for a common centre, and have their
-orbits inclined to the ecliptic, it follows that each of these orbits
-must necessarily intersect the plane of the ecliptic at two opposite
-points situated at the extremities of a straight line passing through
-the Sun's centre. The two opposite points on a planetary orbit where its
-intersections with the ecliptic occur, are called "<i>the Nodes</i>," and
-the imaginary line joining them, which passes through the Sun's centre, is
-called "the line of the nodes." The node situated at the point where a
-planet crosses the ecliptic from the south to the north, is called "<i>the
-ascending node</i>" while that situated where the planet crosses from north
-to south, is called "<i>the descending node</i>."
-</p>
-<p>
-The planets circulating around the Sun are eight in number, but, beside
-these, there is a multitude of very small planets, commonly called
-"asteroids," which also revolve around our luminary. The number of
-asteroids at present known surpasses two hundred, and constantly
-increases by new discoveries. In their order of distance from the Sun
-the principal planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn,
-Uranus and Neptune. The orbits of the asteroids are comprised between
-the orbits of Mars and Jupiter.
-</p>
-<p>
-When the principal planets are considered in regard to their differences
-in size, they are separated into two distinct groups of four planets
-each, viz.: the small planets and the large planets. The orbits of the
-small planets are wholly within the region occupied by the orbits of the
-asteroids, while those of the large planets are wholly without this
-region.
-</p>
-<p>
-When the planets are considered in regard to their position with
-reference to the Earth, they are called "inferior planets" and "superior
-planets." The inferior planets comprise those whose orbits are within
-the orbit of our globe; while the superior planets are those whose
-orbits lie beyond the orbit of the Earth.
-</p>
-<p>
-Since the orbits of the inferior planets lie within the orbit of the
-Earth, the angular distances of these bodies from the Sun, as seen from
-the Earth, must always be included within fixed limits; and these
-planets must seem to oscillate from the east to the west, and from the
-west to the east of the Sun during their sidereal revolution. In this
-process of oscillation these planets sometimes pass between the Earth
-and the Sun, and sometimes behind the Sun. When they pass between us and
-the Sun they are said to be in "inferior conjunction," while, when they
-pass behind the Sun, they are said to be in "superior conjunction." When
-such a planet reaches its greatest distance, either east or west, it is
-said to be at its greatest elongation east or west, as the case may be,
-or in quadrature.
-</p>
-<p>
-The superior planets, whose orbits lie beyond that of the Earth and
-enclose it, present a different appearance. A superior planet never
-passes between the Earth and the Sun, since its orbit lies beyond that
-of our globe, and, therefore, no inferior conjunction of such a planet
-can ever occur. When one of these planets passes beyond the Sun, just
-opposite to the place occupied by the Earth, the planet is said to be in
-"conjunction;" while, when it is on the same side of the Sun with our
-globe, it is said to be in "opposition." While occupying this last
-position, the planet is most advantageously situated for observation,
-since it is then nearer to the Earth. The period comprised between two
-successive conjunctions, or two successive oppositions of a planet, is
-called its "synodical period." This period differs for every planet.
-</p>
-<p>
-It is supposed that all the planets rotate from west to east, like our
-globe; although no direct evidence of the rotation of Mercury Uranus,
-and Neptune has yet been obtained, it is probable that these planets
-rotate like the others. It results from the rotation of the planets that
-they have their days and nights, like our Earth, but differing in
-duration for every planet.
-</p>
-<p>
-The axes of rotation of the planets are more or less inclined to their
-respective orbits, and this inclination varies but little in the course
-of time. From the inclination of the axes of rotation of the planets to
-their orbits, it results that these bodies have seasons like those of
-the Earth; but, of course, they differ from our seasons in duration and
-intensity, according to the period of revolution and the inclination of
-the axis of each separate planet.
-</p>
-
-<p><br /><br /><br /></p>
-
-<h4><a id="THE_PLANET_MARS">THE PLANET MARS</a>
-<br /><br />
-PLATE VIII</h4>
-
-<p>
-Mars is the fourth of the planets in order of distance from the sun;
-Mercury, Venus and the Earth being respectively the first, second and
-third.
-</p>
-<p>
-Owing to the great eccentricity of its orbit, the distance of Mars from
-the Sun is subject to considerable variations. When this planet is in
-its aphelion, its distance from the Sun is 152,000,000 miles, but at
-perihelion it is only 126,000,000 miles distant, the planet being
-therefore 26,000,000 miles nearer the Sun at perihelion than at
-aphelion. The mean distance of Mars from the Sun is 139,000,000 miles.
-Light, which travels at the rate of 185,000 miles a second, occupies
-12½ minutes in passing from the Sun to this planet.
-</p>
-<p>
-While the distance of Mars from the Sun varies considerably, its
-distance from the Earth varies still more. When Mars comes into
-opposition, its distance from our globe is comparatively small,
-especially if the opposition occurs in August, as the two planets are
-then as near together as it is possible for them to be, their distance
-apart being only 33,000,000 miles. But if the opposition occurs in
-February, the distance may be nearly twice as great, or 62,000,000
-miles. On the other hand, when Mars is in conjunction in August, the
-distance between the two planets is the greatest possible, or no less
-than 245,000,000 miles; while, when the conjunction occurs in February,
-it is only 216,000,000 miles. Hence the distance between Mars and the
-Earth varies from .33 to 245 millions of miles; that is, this planet may
-be 212 million miles nearer to us at its nearest oppositions than at its
-most distant conjunctions.
-</p>
-<p>
-From these varying distances of Mars from the Earth, necessarily result
-great variations in the brightness and apparent size of the planet, as
-seen from our globe. When nearest to us it is a very conspicuous object,
-appearing as a star of the first magnitude, and approaching Jupiter in
-brightness; but when it is farthest it is much reduced, and is hardly
-distinguishable from the stars of the second and even third magnitude.
-In the first position, the apparent diameter of Mars is 26", in the last
-it is reduced to 3" only.
-</p>
-<p>
-The orbit of Mars has the very small inclination of 1° 51' to the plane
-of the ecliptic. The planet revolves around the Sun in a period of 687
-days, which constitutes its sidereal year, the year of Mars being only
-43 days less than two of our years.
-</p>
-<p>
-Mars travels along its orbit with a mean velocity of 15 miles per
-second, being about ⁸⁄₁₀ of the velocity of our globe in its
-orbit. The synodical period of Mars is 2 years and 48 days, during which
-the planet passes through all its degrees of brightness.
-</p>
-<p>
-Mars is a smaller planet than the Earth, its diameter being only 4,200
-miles, and its circumference 13,200 miles. It seems well established
-that it is a little flattened at its poles, but the actual amount of
-this flattening is difficult to obtain. According to Prof. Young, the
-polar compression is ¹⁄₂₁₉.
-</p>
-<p>
-The surface of this planet is a little over ²⁸⁄₁₀₀ of the
-surface of our globe, and its volume is 6½ times less than that of the
-Earth. Its mass is only about ⅒ while its density is about ¾ that of
-the Earth. The force of gravitation at its surface is nearly ¾ of what
-it is at the surface of our globe.
-</p>
-<p>
-The planet Mars rotates on an axis inclined 61° 18' to the plane of its
-orbit, so that its equator makes an angle of 28° 42' with the same
-plane. The period of rotation of this planet, which constitutes its
-sidereal day, is 24 h. 37 m. 23 s.
-</p>
-<p>
-The year of Mars, which is composed of 669⅔ of these Martial days,
-equals 687 of our days, this planet rotating 669⅔ times upon its axis
-during this period. But owing to the movement of Mars around the Sun,
-the number of solar days in the Martial year is only 668⅔, while,
-owing to the same cause, the solar day of Mars is a little longer than
-its sidereal day, and equals 24 h. 39 m. 35 s.
-</p>
-<p>
-The days and nights on Mars are accordingly nearly of the same length as
-our days and nights, the difference being a little less than
-three-quarters of an hour. But while the days and nights of Mars are
-essentially the same as ours, its seasons are almost twice as long as
-those of the Earth. Their duration for the northern hemisphere,
-expressed in Martial days, is as follows: Spring, 191; Summer, 181;
-Autumn, 149; Winter, 147. While the Spring and Summer of the northern
-hemisphere together last 372 days, the Autumn and Winter of the same
-hemisphere last only 296 days, or 76 days less. Since the summer seasons
-of the northern hemisphere correspond to the winter seasons of the
-southern hemisphere, and vice versa, the northern hemisphere, owing to
-its longer summer, must accumulate a larger quantity of heat than the
-last. But on Mars, as on the Earth, there is a certain law of
-compensation resulting from the eccentricity of the planet's orbit, and
-from the fact that the middle of the summer of the southern hemisphere
-of this planet, coincides with its perihelion. From the greater
-proximity of Mars to the Sun at that time, the southern hemisphere then
-receives more heat in a given time than does the northern hemisphere in
-its summer season. When everything is taken into account, however, it is
-found that the southern hemisphere must have warmer summers and colder
-winters than the northern hemisphere.
-</p>
-<p>
-Seen with the naked eye, Mars appears as a fiery red star, whose
-intensity of color is surpassed by no other star in the heavens. Seen
-through the telescope, it retains the same red tint, which, however,
-appears less intense, and gradually fades away toward the limb, where it
-is replaced by a white luminous ring.
-</p>
-
-<p><br /></p>
-
-<div class="figcenter" style="width: 400px;">
-<a id="figure08"></a>
-<br />
-<img src="images/figure08.jpg" width="400" alt="" />
-<div class="caption">
-<p>PLATE VIII.&mdash;THE PLANET MARS.</p>
-<p class="smaller">Observed September 3, 1877, at 11h. 55m. P.M.</p>
-</div></div>
-
-<p><br /></p>
-
-<p>
-Mars is a very difficult object to observe, the atmosphere surrounding
-it being sometimes so cloudy and foggy that the sight can hardly
-penetrate through its vapors. When this planet is observed under
-favorable atmospheric conditions, and with sufficient magnifying power,
-its surface, which is of a general reddish tint, is found to be
-diversified by white, gray and dark markings. The dark markings, which
-are the most conspicuous, almost completely surround the planet. They
-are of different forms and sizes, and very irregular, as can be seen on
-Plate VIII., which represents one of the hemispheres of this planet.
-Many of them, especially those situated in the tropical regions of the
-planet, form long narrow bands, whose direction is in the main parallel
-to the Martial equator.
-</p>
-<p>
-The dusky spots differ very much, both from one another and in their
-several parts, as regards intensity of shade. Some appear almost black,
-while others which appear grayish, are so faint, that they can seldom be
-seen. In the southern hemisphere, the darkest part of the spots is
-generally found along their northern border; especially where there are
-deep indentations.
-</p>
-<p>
-Some observers have described these spots as being greenish or bluish,
-but I have never been able to see the faintest trace of these colors in
-them, except when they were observed close to the limb, and involved in
-the greenish tinted ring which is always to be seen there. It is
-probably an effect of contrast, since green and red are complementary
-colors, and since this greenish tinge around the limb covers all kinds
-of spots, whether white or dark. When such dark spots, involved in the
-greenish tint, are carried by the rotation towards the centre of the
-disk, they no longer show this greenish color. To me, these spots have
-always appeared dark, and of such tints as would result from a mixture
-of white and black in different proportions; except that on their
-lighter portions they show some of the prevalent reddish tint of the
-Martial surface. It is to be remarked that in moments of superior
-definition of the telescopic image, the intensity of darkness of all the
-spots is considerably increased&mdash;some of them appearing almost
-perfectly black.
-</p>
-<p>
-The markings on the surface of Mars are now tolerably well
-known&mdash;especially those of its southern hemisphere, which, owing to
-the greater proximity of the planet to our globe when this hemisphere is
-inclined towards the earth, have been better studied. Those of the
-northern hemisphere are not so well known, since when this hemisphere is
-inclined towards us, the distance of Mars from the Earth is 26,000,000
-miles greater, so that the occasions for observing them are not so
-favorable.
-</p>
-<p>
-Several charts of Mars are in existence, but as the same nomenclature
-has not been employed in all of them, some confusion has arisen in
-regard to the names given to the most remarkable features of the
-planet's surface. In order to give clearness to the subject, it will be
-necessary here to give a brief description of the principal markings
-represented on Plate VIII. In this the nomenclature will be employed
-which has been adopted by the English observers in the fine chart of Mr.
-Nath. Green. The large dark spot represented on the left-hand side of
-the plate is called De La Rue Ocean. The dark oval spot, isolated in the
-vicinity of the centre of the disk, is called Terby Sea; while the dark,
-irregular form on the right, near the border, represents the western
-extremity of Maraldi Sea.
-</p>
-<p>
-The dusky spots of Mars seem to be permanent, and to form a part of the
-general surface of the planet. That several among them, at least, are
-permanent, is proved by the fact that they have been observed in the
-same position, and with the same general form, for over two centuries.
-Yet, if we are to depend upon the drawings made fifty years ago by Beer
-and Maedler, it would seem that the permanency of some of them does not
-exist, since a very large spot represented by these astronomers on their
-chart of Mars is not visible now. This object, which, on their map, has
-its middle at 270°, should be precisely under the prominent dark oval
-spot called Terby Sea, seen near the centre of the picture, and would
-extend down almost as far as the northern limb. This can hardly be
-attributed to an error of observation, since these observers were both
-careful, and had great experience in this class of work. It is a very
-singular fact that, at the very same place where Maedler represented the
-spot in question, I found a conspicuous dark mark on December 16, 1881,
-which was certainly not visible in 1877, during one of the most
-favorable oppositions which can ever occur. The object, which is still
-visible (Feb., 1882), consists of an isolated spot situated a little to
-the north of Terby Sea. During the memorable opposition of 1877, I
-investigated thoroughly the markings of Mars, and made over 200 drawings
-of its disk, 32 of which represent the Terby Sea; but this isolated spot
-was not to be seen, unless it be identified with the faint mark,
-represented on the plate, which occupied its place. There cannot be the
-slightest doubt that a change has occurred at that place. Changes in the
-markings have also been suspected on the other hemisphere of the planet.
-</p>
-<p>
-The well-known fact that the continents, and especially the mountainous
-and denuded districts of our globe, reflect much more light than the
-surfaces covered by water, has led astronomers to suppose that the dark
-spots on Mars are produced by a liquid strongly absorbing the rays of
-light, like the liquids on the surface of the Earth. According to this
-theory the dark spots observed are supposed to be lakes, seas, and
-oceans, similar to our own seas and oceans, while the reddish and
-whitish surfaces separating these dark spots, are supposed to be
-islands, peninsulas, and continents. This supposition seems certainly to
-have a great deal of probability in its favor, although some of the
-lighter markings may have a different origin, and perhaps be due to
-vegetation; but no observer has yet seen in them any of the changes
-which ought to result from change of seasons. Some of the changes in the
-dark spots might also be attributed to the flooding or drying up of
-marshes and low land. The change which I have observed lately might be
-attributed to such a cause, especially as my observation was made
-shortly after the spring equinox of the northern hemisphere of Mars,
-which occurred on December 8th.
-</p>
-<p>
-Besides the dark spots just described, there are markings of a different
-character and appearance. Among the most conspicuous are two very
-brilliant white oval spots, which always occupy opposite sides of the
-planet. These two bright spots, which correspond very closely with the
-poles of rotation of Mars, have been called "polar spots."
-</p>
-<p>
-On account of the inclination of the axis of rotation of this planet to
-the ecliptic, it is rare that both of these spots are visible on the
-disk at the same time; and when this occurs, they are seen considerably
-foreshortened, as they are then both on the limb of the planet. Usually
-only one spot is visible, and it appears to its best advantage when the
-region to which it belongs attains its maximum of inclination towards
-the Earth.
-</p>
-<p>
-The polar spots change considerably in size, as they do also in form.
-Sometimes they occupy nearly one-third of the disk, as is proved by many
-of my observations; while at other times they are so much reduced as to
-be totally invisible. It is to be remarked that the reduction of these
-spots generally corresponds with the summer seasons, and their
-enlargement with the winter seasons of the hemispheres to which they
-respectively belong. From these well-observed facts it would appear that
-a relation exists between the temperature of the two hemispheres of Mars
-and the variations of the white spots observed at its poles. A similar
-relation is known to exist on our globe between the progress of the
-seasons and the melting away and the accumulation of snow in the polar
-regions. Astronomers have been led, accordingly, to attribute the polar
-spots of Mars, with all their variations, to the alternate accumulation
-and melting of snows. On this account, the polar spots of Mars are
-sometimes also called "snow-spots."
-</p>
-<p>
-Errors have certainly been made by astronomers in some of their
-observations of the so-called polar snow-spots, other objects occupying
-their place having been mistaken for them. A regular series of
-observations on this planet, which I have now continued for seven years,
-has revealed the fact that during the winter seasons of the southern
-hemisphere of Mars, the polar spots are most of the time invisible,
-being covered over by white, opaque, cloud-like forms, strongly
-reflecting light. In 1877, during more than a month, I, myself, mistook
-for the polar spots such a canopy of clouds, which covered at least
-one-fifth of the surface of the whole disk. I only became aware of my
-error when the opaque cloud, beginning to dissolve at the approach of
-the Martial summer, allowed the real polar spot to be seen through its
-vapors, as through a mist at first, and afterwards with great
-distinctness. In this particular case, the snow-spot was considerably
-smaller than the cloudy cap which covered it, and it is to be remarked
-that it was not situated at the centre of this cloudy cap, but was east
-of that centre; a fact which may account for the so-called polar spots
-not being always observed on exactly opposite sides of the disk. From my
-observations of 1877, 1878 and 1880, it appears that at the approach of
-the autumnal equinox of the southern hemisphere of Mars, large, opaque
-masses, like cumulus clouds in form, began to gather in the polar
-regions of that hemisphere, and continued through autumn and winter,
-dissolving only at the approach of spring. These clouds, which varied in
-form and extent, were very unsteady at first, but as the winter drew
-nearer they enlarged and became more permanent, covering large surfaces
-for months at a time.
-</p>
-<p>
-That the large white spots under consideration are real clouds in the
-atmosphere of Mars, and are not due to a fall of snow, is proved by the
-fact that these spots covered both seas and continents with equal
-facility, even in the equatorial regions of the planet. Snow, of course,
-could not cover the seas of Mars, unless these were all frozen over,
-even in the equatorial zones; therefore, if the dark spots of Mars are
-assumed to be due to water, these large white spots cannot well be
-ascribed to snow.
-</p>
-<p>
-The real polar spots of Mars seem to be in relief on the surface of the
-planet, since the southern spot often appeared slightly shaded on the
-side opposite to the Sun during my observations in 1877. In certain
-cases, when they are on or very near the limb, they have been observed,
-both by others and by myself, to project from the disk slightly.
-</p>
-<p>
-The polar spots of Mars are doubtless composed of a material which, like
-our snow or ice, melts under the rays of the Sun; although it seems
-difficult to admit that the Martial snow is identical with our
-terrestrial snow, and that it melts at a like temperature. The south
-polar spot of Mars entirely disappeared from sight in its summer season
-in 1877, although the planet receives less than one-half as much heat as
-we receive from the Sun; yet on our globe the arctic or antarctic ices
-and snow are perpetual&mdash;never melting entirely. An important fact
-disclosed by the melting away of the southern polar spot is, that in
-melting it is always surrounded with a very dark surface, which takes
-the place of the melted portion of the spot, as observed by myself in
-1877-78. When the polar spot had entirely disappeared, its place was
-occupied by a very dark spot. Now, if the polar spot is really ice, and
-the dark spots are actual seas, this polar spot must be situated in
-mid-ocean, since, on melting away, it is replaced by a dark spot. If the
-polar spots are composed of a white substance melting under the rays of
-the Sun, as seems altogether probable, its melting point must be above
-that of terrestrial snow.
-</p>
-<p>
-Many of the dark spots of Mars, and especially those whose northern
-border forms an irregular belt upon the equatorial regions of this
-planet, are bordered on that side by a white luminous belt, following
-all their sinuosities. These white borders are variable. Sometimes they
-are very prominent and intensely bright, especially at some points,
-which occasionally almost equal the polar spots in brilliancy; while at
-other times they are so faint, that they can hardly be distinguished, or
-are even invisible; although the atmosphere is clear and the dark spots
-appear perfectly well defined. While these white borders were invisible,
-I have sometimes watched for several hours at a time to see if I could
-detect any traces of them in places where they usually appear the most
-prominent, but generally without success. On a few occasions, however, I
-had the good fortune to see some of these spots forming gradually in the
-course of one or two hours, at places where nothing of them could be
-seen before.
-</p>
-<p>
-These whitish fringes forming and vanishing along the coasts of the
-Martial seas have been very little studied by astronomers. From my
-observations made during the last seven years, it appears very probable
-that this belt and its white spots are mainly due to the condensation of
-vapors around, and over high peaks, and extensive mountain chains,
-forming the Martial sea-coasts, as the Andes and Rocky Mountains form
-the sea-coasts of the Pacific Ocean. These high mountains on Mars,
-condensing the vapors into fogs or clouds above them, or at their sides,
-as often happens in our mountainous districts, would certainly suffice
-to produce the phenomena observed. Some of the highest peaks among these
-mountain chains may even have their summits covered with perpetual snow,
-or some substance partaking of the nature of snow. The temporary
-visibility and invisibility of the white spots seen on Mars, as well as
-the rapid transformations they sometimes undergo, may be explained as
-caused by clouds having a high reflective power and a liability to form
-and disappear quickly.
-</p>
-<p>
-The assumption that these irregular whitish bands and spots are formed
-by the condensation of vapors on mountain chains, and elevated table
-lands, is supported by my observations made in 1877 and 1879. When such
-white spots were traversing the terminator at sunrise, they very often
-projected far into the night side, thus indicating that they were at a
-higher level than that of the general surface. Indentations in the
-terminator, corresponding to large dark spots crossing its line, also
-clearly indicated the depression of the dark spots below the general
-surface. The highest mountainous districts thus observed on Mars, are
-situated between 60° and 70° of south latitude, towards the western
-extremity of Gill Land. The mountain chain, which almost completely
-forms the surface of this land, is so elevated at some points, that they
-not only change the form of the terminator when they are seen upon it,
-but also the limb of the planet, as seen by myself. They then appear so
-brilliant, that the principal summit among them has been mistaken by
-several observers for the polar spot itself, as proved by the wrong
-position assigned to it on their drawings. It seems probable that this
-high peak, which appears always white, is constantly covered with snow,
-or the similar material replacing it on Mars. This high region is
-situated between longitudes 18o° and 190°.
-</p>
-<p>
-The highest mountainous parts belonging to the hemisphere represented on
-Plate VIII., which are nearly always more or less visible as whitish
-spots and bands, form a coast line along the northern (lower) border of
-De La Rue Ocean. This great spot, which is not so simple as it has been
-represented by observers, is in fact divided by two narrow isthmuses,
-one in the north, the other in the east, both joining, in the interior
-of the great ocean, a peninsula heretofore known as Hall Island. Upon
-the south-eastern extremity of this peninsula, a white spot, called
-Dawes Ice Island, was observed in 1865, but it soon disappeared, and was
-after that seen only now and then. It is very probable that this
-so-called Ice Island was due to clouds forming around the summit of some
-high peak of this peninsula.
-</p>
-<p>
-On the opposite hemisphere to that represented on Plate VIII., the white
-fringes bordering the dark spots are much more conspicuous than they are
-on this side. On the eastern side of a remarkable dark spot called
-Kaiser Sea, they are very bright, and almost always present, although
-they vary considerably, both in brightness and in extent. To the south
-of Kaiser Sea, they are very conspicuous on the eastern border of
-Lockyer Land, forming an elevated and deeply indented coast-line along
-Lambert Sea. There the white spots never disappear entirely, being
-always visible on the north side, where they turn westward along Dawes'
-Ocean&mdash;the mountain chain attaining there its greatest altitude. Very
-frequently Lockyer Land, which seems to be a vast plateau, appears
-throughout white and brilliant, this occurring usually towards the
-sunrise or sunset of that region, probably from the condensation of
-vapors and the formation of fogs, but generally this whiteness gradually
-disappears with the progress of the sun above this plateau. Inside of
-the great continents of Mars these temporary white spots are not so
-frequent, but when visible they occupy always the same positions&mdash;a
-fact which probably indicates that they occupy the culminating points of
-these continents. One of these temporary white spots inside of the
-continents is represented on Plate VIII., on the left-hand side, below
-De La Rue Ocean, on Maedler Continent.
-</p>
-<p>
-Although large, opaque, cumulus-shaped, cloud-like forms are seen in the
-polar regions of Mars, such forms are very seldom seen in the tropical
-zones, or, at least, it appears so, from the fact that my observations,
-continued during the last seven years, have disclosed no real opaque
-cloudy forms there. Although the Martial sky is frequently overcast by
-dense vapors or thick fogs in these regions, yet no real opaque clouds
-were ever seen; the most prominent among the dusky spots being faintly
-visible through the vapory veil, when they approached the centre of the
-disk.
-</p>
-<p>
-Besides these phenomena, which prove that Mars is surrounded by an
-atmosphere having a great deal of similarity to our own, a further proof
-is afforded by the fact that the dark spots, which appear sharply
-defined and black when they are seen near the centre, become less and
-less visible as they advance towards the limb, and are totally invisible
-before they reach it. Moreover, the spectroscope also indicates the
-existence of an atmosphere, and even the presence of watery vapor in it.
-A very curious state of the Martial atmosphere is revealed by my
-observations of 1877-78. During eight consecutive weeks, from December
-12th to February 6th, a whole hemisphere of the planet&mdash;precisely that
-represented on Plate VIII.&mdash;was completely covered by dense vapors, or
-a thick fog which barely allowed the dark spots to be seen through it,
-even when they were in the centre of the disk. The opposite hemisphere
-of Mars appeared just as clear and calm as possible; there all the spots
-and their minutest details could be seen, and when the planet was
-observed at the proper time, the line separating the foggy from the
-clear side was plainly visible.
-</p>
-<p>
-The reddish tint observed on the continents of Mars has been supposed by
-some astronomers to be the real color of the atmosphere of this planet.
-But, for many reasons, this explanation is not acceptable. Besides the
-fact that the border of the planet appears white, while it should be
-more red than the other part, owing to the greater depth of atmosphere
-there presented to us, the polar spots, the white bands along the
-sea-coasts, and the cloud-like forms appear perfectly white, not the
-slightest tint of red being visible on them, as would be the case if
-these objects were seen through an atmosphere tinted red. Other
-astronomers have supposed that the vegetation of this planet has a
-reddish color; but this is not supported by observation. It has been
-again supposed, with much more probability, that the surface of Mars is
-composed of an ochreous material which gives the planet its predominant
-ruddy color.
-</p>
-<p>
-Until lately Mars was supposed to be without a satellite, but in August,
-1877, Professor Hall, of the Washington Observatory, made one of the
-most remarkable discoveries of the time, and found two satellites
-revolving around this planet. These satellites are among the smallest
-known heavenly bodies, their diameter having been estimated at from 6 to
-10 miles for the outer satellite, and from 10 to 40 miles for the inner
-one.
-</p>
-<p>
-The most extraordinary feature of these bodies is the proximity of the
-inner satellite to the planet, and the consequent rapidity of its
-motion. The distance of the inner satellite from the centre of Mars is
-about 6,000 miles, and from surface to surface it is less than 4,000
-miles, or a little more than the distance from New York to San
-Francisco. The shortest period of revolution of any satellite previously
-known, is that of the inner satellite of Saturn, which is a little more
-than 22½ hours; but the inner satellite of Mars accomplishes its
-revolution in 7h. 38m., or in 17 hours less than the period of rotation
-of the planet upon its axis. The period of revolution of the outer
-satellite is greater, of course, and equals 30h. 7m.
-</p>
-<p>
-From this rapidity of motion of the inner satellite of Mars, a very
-curious result follows, which at first sight may appear in contradiction
-with the fact that this body has a direct motion, like that of all the
-planets of the solar system, and moves around Mars from west to east.
-While the outer satellite of this planet, in company with all the stars
-and planets, rises in the east and sets in the west, the inner
-satellite, on the contrary, rises in the west and sets in the east.
-Since the period of rotation of Mars is greater than is the period of
-revolution of this satellite, it necessarily follows that this last body
-must constantly be gaining on the rotation, and, consequently, that the
-satellite sets in the east and rises in the west, compassing the whole
-heavens around Mars three times a day, passing through all its phases in
-11 hours, each quarter of this singular Moon lasting less than 3 hours.
-</p>
-<p>
-It has been shown above that Mars has many points of resemblance to the
-Earth. It has an atmosphere constituted very nearly like ours; it has
-fogs, clouds, rains, snows, and winds. It has water, or at least some
-liquids resembling it; it has rivers, lakes, seas and oceans. It has
-also islands, peninsulas, continents, mountains and valleys. It has two
-Moons, which must create great and rapid tides in the waters of its seas
-and oceans. It has its days and nights, its warm and cold seasons, and
-very likely its vegetation, its prairies and forests, like the Earth. On
-the other hand, its year and seasons are double those of the Earth, and
-its distance from the Sun is greater.
-</p>
-<p>
-Is this planet, which is certainly constituted very nearly like our
-globe, and seems so nearly fitted for the wants of the human race,
-inhabited by animals and intelligent beings?
-</p>
-<p>
-To answer this question, either in the negative or in the affirmative,
-would be to step out of the pure province of science, and enter the
-boundless domain of speculation, since no observer has ever seen
-anything indicating that animal life exists on Mars, or on any other
-planet or satellite. So far as observation goes, Mars seems to be a
-planet well suited to sustain animal life, and we may reason from
-analogy that if animal life can exist at all outside of the Earth, Mars
-must have its flora and fauna; it must have its fishes and birds, its
-mammalia and men; although all these living beings must inevitably be
-very different in appearance from their representatives on the Earth, as
-can easily be imagined from the differences existing between the two
-planets. Although all this is possible, and even very probable, yet it
-must be remembered that we have not the slightest evidence that it is
-so; and until we have acquired this evidence, we may only provisionally
-accept this idea as a pleasing hypothesis, which, after all, may be
-wrong and totally unfounded.
-</p>
-
-<p><br /><br /><br /></p>
-
-<h4><a id="THE_PLANET_JUPITER">THE PLANET JUPITER</a>
-<br /><br />
-PLATE IX</h4>
-
-<p>
-Jupiter, the giant of the planetary world, is the fifth in order of
-distance from the Sun, and is next to Mars, our ruddy neighbor. To the
-naked eye, Jupiter appears as a very brilliant star, whose magnitude,
-changing with the distance of this planet from the Earth, sometimes
-approaches that of Venus, our bright morning and evening star.
-</p>
-<p>
-The mean distance of Jupiter from the Sun is 475,000,000 miles, but
-owing to the eccentricity of its orbit, its distance varies from 452 to
-498 millions of miles. The distance of this planet from the Earth varies
-still more. When nearest to our globe, or in opposition, its distance is
-reduced to 384,000,000 miles, and its apparent diameter increased to
-50"; while when it is farthest, or in conjunction, its distance is
-increased to 567,000,000 miles, and its apparent diameter reduced to
-30"; Jupiter being thus 183,000,000 miles nearer our globe while in
-opposition than when it is in conjunction.
-</p>
-<p>
-This planet revolves around the Sun in 11 years, 10 months and 17 days,
-or in only 50 days less than 12 terrestrial years. Such is the year of
-this planet. The plane of its orbit is inclined 1° 19' to the ecliptic.
-No planet, except Uranus, has an orbit exhibiting a smaller inclination.
-The planet advances in its orbit at the mean rate of 8 miles a second;
-which is a little less than half the orbital velocity of the Earth.
-</p>
-<p>
-Jupiter is of enormous proportions. Its equatorial diameter measures
-88,000 miles, and its circumference no less than 276,460 miles, these
-dimensions being 11 times greater than those of the Earth. This planet,
-notwithstanding its huge size, rotates on its axis in not far from 9h.
-55m. 36s., which period constitutes its day. Owing, however, to the
-changeable appearance of its surface, this period cannot be ascertained
-with very great exactitude. In consequence of its rapid rotation, the
-planet is far from spherical, its polar diameter being shorter than the
-equatorial by about ¹⁄₁₆, or 5,500 miles. Its surface is 124
-times the surface of the earth; while its volume is 1,387 times as
-great. If Jupiter occupied the place of our satellite in the sky, it
-would appear 40 times as large as the Moon appears to us, and would
-cover a surface of the heavens 1,600 times that covered by the full
-Moon, and would subtend an angle of 21°. Jupiter's mass does not
-correspond with its great bulk, and is only ¹⁄₁₀₄₇ of the
-mass of the Sun, and 310 times the mass of the earth; its density being
-only ¼ of that of our globe. The force of gravitation at the surface of
-this planet is over 2½ times what it is on the Earth, so that a
-terrestrial object carried to the surface of Jupiter would weigh over
-two and a half times as much as on our globe.
-</p>
-<p>
-Observed with a telescope, even of moderate aperture, Jupiter, with its
-four attending satellites and its dazzling brilliancy, appears as one of
-the most magnificent objects in the sky. The general appearance of the
-disk is white; but unlike that of Mars, it is brightest towards its
-central parts, and a little darker around the limb, especially on the
-side opposite to the Sun. Although an exterior planet, and so far from
-us, Jupiter shows faint traces of phases when observed near its
-quadratures, but this gibbosity of its disk is very slight, and is
-indicated only by a kind of penumbral shadow on the limb.
-</p>
-<p>
-When observed with adequate power, the disk of Jupiter is found to be
-highly diversified. The principal features consist of a series of
-alternate light and dark streaks or bands, disposed most of the time
-parallel with the Jovian equator. These bands differ from each other in
-intensity as well as in breadth; those near the equator being usually
-much more prominent than those situated in higher latitudes north and
-south.
-</p>
-<p>
-The equatorial zone of Jupiter is occupied most of the time by a broad,
-prominent belt 20° or 30° wide, limited on each side by a very dark
-narrow streak. Between these two dark borders, but seldom occupying the
-whole space between them, appears an irregular white belt, apparently
-composed of dense masses of clouds strongly reflecting the Sun's light,
-some of these cloudy masses being very brilliant. The spaces left
-between the cloudy belt and the dark borders, usually exhibit a delicate
-pink or rosy color, which produces a very harmonious effect with the
-varying grayish and bluish shades of some of the belts and streaks seen
-on the disk. Quite often the cloudy belt is broken up, and consists of
-independent cloudy masses, separated by larger or smaller intervals,
-these intervals disclosing the rosy background of this zone.
-</p>
-<p>
-On each side of the equatorial belt there is usually a broad whitish
-belt, succeeded by a narrow gray band; the space left on each hemisphere
-between these last bands and the limb being usually occupied by two or
-three alternate white and gray bands. A uniform gray segment usually
-forms a sort of polar cap to Jupiter.
-</p>
-<p>
-When observed under very favorable conditions, all the lighter belts
-appear as if composed of masses of small cloudlets, resembling the white
-opaque clouds seen in our atmosphere. This, as already stated, is
-particularly noticeable on the equatorial belt. It is not unusual, when
-Jupiter is in quadrature, to see some of the most conspicuous white
-spots casting a shadow opposite to the Sun; a fact which sufficiently
-indicates that these spots are at different levels. They probably form
-the summits of vast banks of clouds floating high up in the atmosphere
-of Jupiter.
-</p>
-<p>
-What we see of Jupiter is chiefly a vaporous, cloudy envelope. If our
-sight penetrates anywhere deeper into the interior, it can only be
-through the narrow fissures of this envelope, which appear as gray or
-dark streaks or spots. That most of the visible surface of Jupiter is
-simply a cloudy covering, is abundantly proved by the proper motion of
-its spots, which sometimes becomes very great.
-</p>
-<p>
-In periods of calm, very few changes are noticeable in the markings of
-the planet, except, perhaps, some slight modifications of form in the
-cloudy, equatorial belt which, in general, is much more liable to
-changes than the other belts. But the Jovian surface is not always so
-tranquil, great changes being observed during the terrific storms which
-sometimes occur on this mighty planet, when all becomes disorder and
-confusion on its usually calm surface; and nothing on the Earth can give
-us a conception of the velocity with which some of its clouds and spots
-are animated. New belts quickly form, while old ones disappear. The
-usual parallelism of the belts no more exists. Huge, white, cumulus-like
-masses advance and spread out, the rosy equatorial belt enlarges
-sometimes to two or three times its usual size, and occupies two-thirds,
-or more, of the disk, the rosy tint spreading out in a very short time.
-At times very dark bands extending across the disk are transformed into
-knots or dark spots, which encircle the planet with a belt, as it were,
-of jet black beads. Sometimes, also, a secondary but narrower rosy belt
-forms either in the northern or the southern hemisphere, and remains
-visible for a few days or for years at a time.
-</p>
-<p>
-On May 25, 1876, I witnessed one of the grandest commotions which can be
-conceived as taking place in an atmosphere. All the southern hemisphere
-of Jupiter, from equator to pole, was in rapid motion, the belts and
-spots being transported entirely across the disk, from the eastern to
-the western limb, in one hour's time, during which the equatorial belt
-swelled to twice its original breadth, towards the south.
-</p>
-<p>
-Now, when one stops for a moment to think what is signified by that
-motion of the dark spots across the little telescopic disk of Jupiter in
-an hour's time, he may arrive at some conception of the magnitude of the
-Jovian storms, compared with those of our globe. The circumference of
-Jupiter's equator, as stated above, is 276,460 miles; half this number,
-or 138,230 miles, represents the length of the equatorial line seen from
-the Earth. Now, after taking into account the rotation of the planet,
-which somewhat diminishes the apparent motion, we arrive at the
-astonishing result that the spots and markings were carried along by
-this Jovian storm, at the enormous rate of 110,584 miles an hour, or
-over 30.7 miles a second. On our globe, a hurricane or tornado, which
-blows at the rate of 100 miles an hour, sweeps everything before it.
-What, then, must be expected from a velocity over 1,105 times as great?
-Enormous as this motion may appear, its occurrence cannot be doubted,
-since it is disclosed by direct observation.
-</p>
-<p>
-The surface of Jupiter, it would seem, has its periods of calm and
-activity like that of the Sun, although it is not yet known, as it is
-for the latter, that they recur with approximate regularity.
-</p>
-<p>
-My observations of this planet, which embrace a period of ten years,
-seem to point in that direction, for they show, at least, that Jupiter
-has its years of calm and its years of disturbances. The year 1876 was a
-year of extraordinary disturbance on Jupiter. Changes in the markings
-were going on all the time, and no one form could be recognized the next
-day, or even sometimes the next hour, as shown above. The cloudy
-envelope of the planet was in constant motion, the equatorial belt,
-especially, showing the signs of greatest disturbance, being, for the
-most part, two or three times as wide as in other years. After 1876 the
-calm was very great on the planet, only a slight change now and then
-being noticeable, the same forms being recognized day after day, month
-after month, and even year after year. In one case the same marking has
-been observed for seventeen consecutive months, and in another for
-twenty-eight months. This state of quietude lasted until October, 1880,
-when considerable commotion occurred on the northern hemisphere, where
-large round black spots, somewhat resembling the Sun-spots, formed in
-the cloudy atmosphere, and finally changed, towards the end of December,
-into a narrow pink belt, which still exists.
-</p>
-
-<p><br /></p>
-
-<div class="figcenter" style="width: 400px;">
-<a id="figure09"></a>
-<br />
-<img src="images/figure09.jpg" width="400" alt="" />
-<div class="caption">
-<p>PLATE IX.&mdash;THE PLANET JUPITER.</p>
-<p class="smaller">Observed November 1, 1880, at 9h. 30m. P.M.</p>
-</div></div>
-
-<p><br /></p>
-
-<p>
-The most curious marking ever seen on Jupiter is undoubtedly the great
-Red Spot, observed on the southern hemisphere of this planet for the
-last three years. This interesting object, seen first in July, 1878,
-disappeared for a time, reappeared on September 25 of the same year, and
-has remained visible until now. When seen by me in September, it was
-much elongated, and sharply pointed on one side, like a spear-head, but
-it subsequently acquired an irregular form, with short appendages
-protruding from its northern border. At first, the changes were great
-and frequent, but at length it acquired the regular oval form, which,
-with but slight modifications, it has retained until now. During the
-month of November, 1880, I noticed two small black specks upon this Red
-Spot, and they were seen again in January of the succeeding year, by Mr.
-Alvan Clark, Jr. When the spot had attained its oval shape, it appeared
-part of the time surrounded with a white luminous ring of cloudy forms
-which, however, was changing more or less all the time, being sometimes
-invisible. The color of this curious spot is a brilliant rosy red,
-tinged with vermilion, and altogether different in shade from the
-pinkish color of the equatorial belt. The size of the spot varies, but
-of late its changes have been slight. Its longer diameter may be
-estimated at 8,000 miles, and the shorter at 2,200 miles. The Red Spot
-is represented on Plate IX. with its natural color, and as it appears at
-the moments most favorable for observation. In ordinary cases its color
-does not appear so brilliant, but paler.
-</p>
-<p>
-It is difficult to account for the color of the equatorial belt and that
-of the Red Spot; but it is known, at least, that the material to which
-they are due cannot be situated at the level of the general surface
-visible to us, and especially that of the cloudy forms of the equatorial
-zone. Undoubtedly the red layer lies deeper than the superficial
-envelope of the planet, although it does not seem to be very deeply
-depressed.
-</p>
-<p>
-Jupiter is attended by four satellites, which revolve around the planet
-at various distances, and shine like stars of the 6th and 7th magnitude.
-It is said that under very favorable circumstances, and in a very clear
-sky, the satellites can be seen with the naked eye, but this requires
-exceptionally keen eyes, since the glare of the planet is so strong as
-to overpower the comparatively faint light of the satellites. However, I
-myself have sometimes seen, without the aid of the telescope, two or
-three of the satellites as a single object, when they were closely
-grouped on the same side of Jupiter.
-</p>
-<p>
-The four moons of Jupiter are all larger than our Moon, except the
-first, which has about the same diameter. They range in size from 2,300
-to 3,400 miles in diameter, the third being the largest; the
-determination of their diameter is by no means accurate, however, as it
-is difficult to measure such small objects with precision. Their mean
-distance from the centre of Jupiter varies from 267,000 to 1,192,000
-miles, the first satellite, the nearest to the planet, being a little
-farther from Jupiter then our satellite is from us. The four satellites
-revolve around the planet in orbits whose planes have a slight
-inclination to the equator of Jupiter, and consequently to the ecliptic.
-The diameter of the largest satellite is nearly half that of the Earth,
-or 3,436 miles; while its volume is five times that of our Moon. The
-period of revolution of these satellites varies from 1d. 18h. for the
-first, to 16d. 16h. for the last.
-</p>
-<p>
-Owing to the slight inclination of the plane of their orbits to that of
-the planet, the three first satellites, and generally the fourth, pass
-in front of the disk and also through the shadow of the planet at every
-revolution, and are accordingly eclipsed. Their passages behind
-Jupiter's disk are called occultations; those in front of it, transits.
-The eclipses, the occultations and the transits of the moons of Jupiter
-are interesting and important phenomena; the eclipses being sometimes
-observed for the rough determination of longitudes at sea.
-</p>
-<p>
-The satellites in transit present some curious phenomena. When they
-enter the disk, they appear intensely luminous upon its grayish border;
-but as they advance, they seem by degrees to lose their brightness,
-until they finally become undistinguishable from the luminous surface of
-Jupiter. It sometimes happens, however, that the first, the third and
-the fourth satellites, after ceasing to appear as bright spots, continue
-to be visible as dark spots upon the bright central portions of the
-planet's disk; but in these cases their disks appear smaller than the
-shadows they cast. Undoubtedly these satellites have extensive
-atmospheres, since they sometimes pass unperceived across the central
-parts of Jupiter, this being probably when their atmospheres are
-condensed into clouds, strongly reflecting light; while when these
-clouds are absent, we can see their actual surface, with traces of the
-dark spots upon them similar to those on Mars.
-</p>
-<p>
-From the variation in the brightness of these satellites, which is said
-to be always observed in the same part of their orbit, William Herschel
-was led to suppose that these bodies, like our Moon, rotate upon their
-axes in the same period in which they move round the planet, so that
-they always present the same face to Jupiter; but these conclusions have
-been denied. From my observations it is apparent, however, that the
-light reflected by them varies in intensity as well as in color. But
-this is rather to be attributed to the presence of an atmosphere
-surrounding these bodies, which when cloudy reflects more light than
-when clear, with corresponding changes in the color of the light.
-</p>
-<p>
-The satellites in transit are sometimes preceded or followed, according
-to the position of the Sun, by a round black spot having about the same
-size as the satellite itself. This black spot is the shadow of the
-satellite cast on the vapory envelope of Jupiter, similar to the shadow
-cast by the Moon on the Earth, during eclipses of the Sun; in fact, all
-the Jovian regions traversed by these shadows have the Sun totally
-eclipsed. Sometimes it happens that the shadow appears elliptical. This
-occurs either when it is observed very near the limb, or when entering
-upon a round, cloud-like spot. This effect is attributable to the
-perspective under which the shadow is seen on the spherical globe or
-spot.
-</p>
-<p>
-The proper motion of the satellites in the Jovian sky is much more rapid
-than that of the Moon in our sky. During one Jovian day of ten hours,
-the first satellite advances 84°; the second, 42°; the third, 20° and
-the fourth, 9°. The first satellite passes from New Moon to its first
-quarter in a little more than a Jovian day, while the fourth occupies
-ten such days in attaining the same phase.
-</p>
-<p>
-In density, as well as in physical constitution, Jupiter differs widely
-from the interior planets, and especially from the Earth; and, as has
-been shown, it is surrounded by a dense, opaque, cloudy layer, which is
-almost always impenetrable to the sight, and hides from view the
-nucleus, which we may conceive to exist under this vaporous envelope. In
-1876, the year of the great Jovian disturbances, I observed frequently
-in the northern hemisphere of the planet a very curious phenomenon,
-which seems to prove that its cloudy envelope is at times partially
-absent in some places, its vapors being apparently either condensed, or
-transported to other parts of its surface, and that, therefore, a
-considerable part of the real globe of the planet was visible at these
-places. The phenomenon consisted in the deformation of the northern
-limb, which had a much shorter radius on all of this hemisphere situated
-northward of the white belt which adjoins the equatorial zone. The
-deformation of the limb on both sides, where it passed from a longer to
-a shorter radius, was abrupt, and at right angles to the limb, forming
-there a steplike indentation which was very prominent. The polar segment
-having a smaller radius, appeared unusually dark, and was not striped,
-as usual, but uniform in tint throughout. On September 27th, the third
-satellite passed over this dark segment, and emerged from the western
-border, a little below the place where the limb was abruptly deformed,
-as above described. When the satellite had fully emerged from this limb,
-it was apparent that if the portion of the limb having a longer radius
-had been prolonged a little below, and as far as the satellite, it would
-have enclosed it within its border, and thus retarded the time of
-emersion. The depth of deformation of the limb was accordingly greater
-than the diameter of the third satellite, and certainly more than 4,000
-miles. That the phenomenon was real, is proved by the fact that the
-egress of this satellite occurred at least four minutes sooner than the
-time predicted for it in the American Ephemeris. Other observations seem
-to point in the same direction, since some of the satellites which were
-occulted have been seen through the limb of Jupiter by different
-astronomers, as if this limb was sometimes semi-transparent. Another
-observation of mine seems to confirm these conclusions. On April 24th,
-1877, at 15h. 25m. the shadow of the first satellite was projected on
-the dark band forming the northern border of the equatorial belt, the
-shadow being then not far from the east limb. Close to this shadow, and
-on its western side, it was preceded by a secondary shadow, which was
-fainter, but had the same apparent size. This round dark spot was not
-the satellite itself, as I had supposed at first, since this object was
-yet outside of the planet, on the east, and entered upon it only at 16h.
-4m. I watched closely this strange phenomenon, and at 16h. 45m., when
-the shadow had already crossed about ¾ of the disk, it was still
-preceded by the secondary, or mock shadow, as it may be called; the same
-relative distance having been kept all the while between the two
-objects, which had therefore traveled at the same rate. It is obvious
-that this dark spot could not be one of the planet's markings, since the
-shadow of the first satellite moves more quickly on the surface of
-Jupiter than a spot on the same surface travels by the effect of
-rotation, so that in this case the shadow would soon have passed over
-this marking, and left it behind, during the time occupied by the
-observation. From these observations it seems very probable that Jupiter
-has a nucleus, either solid or liquid, which lies several thousand
-mites below the surface of its cloudy envelope. It is also probable that
-the uniformly shaded dark segment seen in 1876, was a portion of the
-surface of this nucleus itself. When the cloudy envelope is
-semi-transparent at the place situated on a line with an occulted
-satellite and the eye of an observer, this satellite may accordingly
-remain visible for a time through the limb, as shown by observation. The
-phenomenon of the mock shadow may also be attributed to a similar cause,
-where semi-transparent vapors receive the shadow of a satellite at their
-surface, while at the same time part of this shadow, passing through the
-semi-transparent vapors, may be seen at the surface of the nucleus, or
-of a layer of opaque clouds situated at some distance below the surface.
-</p>
-<p>
-Some astronomers are inclined to think that Jupiter is at a high
-temperature, and self-luminous to a certain extent. If this planet is
-self-luminous to any degree, we might expect that some light would be
-thrown upon the satellites when they are crossing the shadow cast into
-space by the planet; but when they cross this shadow they are totally
-invisible in the best telescopes, a proof that they do not receive much
-light from the non-illuminated side of Jupiter. It would, indeed, seem
-probable that some of the intensely white spots occasionally seen on the
-equatorial belt of the planet are self-luminous in a degree, yet not
-enough to render the satellites visible while they are immersed in
-Jupiter's shadow. It does not seem impossible that the planet should
-have the high temperature attributed to it, when we remember the
-terrific storms observed in its atmosphere, which, owing to the great
-distance of Jupiter from the Sun, do not seem to be attributable to this
-body, but rather to some local cause within the envelope of the planet.
-</p>
-<p>
-Astronomy, which is a science of observation, is naturally silent with
-regard to the inhabitants of Jupiter. If there are any such inhabitants,
-they are confined to the domain of conjecture, under the dense cloudy
-envelope of the planet. The conditions of habitability on Jupiter must
-differ very widely from those of our globe. Comparatively little direct
-light from the Sun reaches the surface of the globe of Jupiter, except
-that which passes through the narrow openings forming the dark clouds.
-All the rest of the planet's surface, being covered perpetually by
-opaque clouds, receives only diffused light. On Jupiter there are
-practically no seasons, since its axis is nearly perpendicular to its
-orbit. The force of gravity on the surface of Jupiter being more than
-double what it is on the Earth, living bodies would there have more than
-double the weight of similar bodies on the Earth. Furthermore, Jupiter
-only receives 0.011 of the light and heat which we receive from the Sun;
-and its year is nearly equal to 12 of our years. If there are living
-beings on Jupiter, they must, then, be entirely different from any known
-to us, and they may have forms never dreamed of in our most fantastic
-conceptions.
-</p>
-<p>
-The two round black spots represented towards the central parts of Plate
-IX. are the shadows of the first and second satellite; while the two
-round white spots seen on the left of the disk, are the satellites
-themselves, as they appeared at the moment of the observation. The first
-satellite and its shadow are the nearest to the equator; while the
-second satellite and its shadow are higher, the last being projected on
-the Great Red Spot.<a name="FNanchor_2_1" id="FNanchor_2_1"></a><a href="#Footnote_2_1" class="fnanchor">[2]</a> The row of dark circular spots represented on the
-northern, or lower hemisphere, when they first appeared, had some
-resemblance to Sun-spots without a penumbra, with bright markings around
-them, resembling faculæ. These round spots subsequently enlarged
-considerably, until they united along the entire line, encircling the
-planet, and finally forming a narrow pink belt, which is still visible.
-</p>
-
-<p><br /></p>
-
-<div class="footnote">
-
-<p class="nind"><a name="Footnote_2_1" id="Footnote_2_1"></a><a href="#FNanchor_2_1"><span class="label">[2]</span></a>By an accidental error in enlarging the original drawing,
-the satellites and shadows appear in Plate IX. of double their actual
-size. The error is one easy of mental correction.</p></div>
-
-<p><br /><br /><br /></p>
-
-<h4><a id="THE_PLANET_SATURN">THE PLANET SATURN</a>
-<br /><br />
-PLATE X</h4>
-
-<p>
-Saturn, which is next to Jupiter in order of distance from the Sun,
-while not the largest, is certainly the most beautiful and interesting
-of all the planets, with his grand and unique system of rings, and his
-eight satellites, which, like faithful servants, attend the planet's
-interminable journey through space.
-</p>
-<p>
-Seen with the naked eye, Saturn shines in the night like a star of the
-first magnitude, whose dull, soft whiteness is, however, far from
-attaining the brilliancy of Venus or Jupiter, although it sometimes
-approaches Mars in brightness. Saturn hardly ever exhibits the
-phenomenon of scintillation, or twinkling, a peculiarity which makes it
-easily distinguishable among the stars and planets of the heavens.
-</p>
-<p>
-The synodical period of Saturn occupies 1 year and 13 days, so that
-every 378 days, on an average, this planet holds the same position in
-the sky relatively to the Sun and the Earth.
-</p>
-<p>
-The mean distance of Saturn from the Sun is a little over 9½ times that
-of our globe, or 872,000,000 miles. Owing to the orbital eccentricity,
-this distance may increase to 921,000,000 miles, when the planet is in
-aphelion; or decrease to 823,000,000 miles, when it is in perihelion;
-Saturn being therefore 98,000,000 miles nearer to the Sun when in
-perihelion than in aphelion. If gravitation were free to exert its
-influence alone, Saturn would fall into the Sun in 5 years and 2 months.
-</p>
-<p>
-The distance of Saturn from the Earth varies, according to the position
-of the two planets in their respective orbits. At the time of
-opposition, when the Earth lies between the Sun and Saturn, this
-distance is smallest; while, on the contrary, at the time of
-conjunction, when the Sun lies between the Earth and Saturn, it is
-greatest. Owing, however, to the eccentricity of the orbits of Saturn
-and our globe, and the inclination of their planes to each other, and
-owing also to the variable heliocentric longitude of the perihelion, the
-distance of the two planets from each other at their successive
-conjunctions and oppositions is rendered extremely variable. At present
-it is when the oppositions of Saturn occur in December that this planet
-comes nearest to us; while when the conjunctions take place in June, the
-distance of Saturn from the Earth is the greatest possible. In the
-former case the distance of the planet from our globe is only
-730,000,000 miles; while in the last it is 1,014,000,000 miles, the
-difference between the nearest and farthest points of Saturn's approach
-to us being no less than 284,000,000 miles, or over three times the mean
-distance of the Sun from the Earth.
-</p>
-<p>
-From the great variations in the distance of Saturn from the Earth,
-necessarily result corresponding changes in the brightness and apparent
-diameter of this body. When it is farthest from us, its angular diameter
-measures but 14"; while, when it is nearest, it measures 20".
-</p>
-<p>
-The orbit of Saturn is inclined 2°30' to the ecliptic, and its
-eccentricity, which equals 0,056, is over three times that of the
-Earth's orbit.
-</p>
-<p>
-This planet revolves around the Sun in a period of 29 years and 5½
-months, or 10,759 terrestrial days, which constitutes its sidereal year.
-The extension of the immense curve forming the orbit of this planet, is
-no less than 5,505,000,000 miles, which is traversed by the planet with
-a mean velocity of a little less than 6 miles per second, or three times
-less than the motion of our globe in space.
-</p>
-<p>
-The real dimensions of the globe of Saturn are not yet known with
-accuracy, and the equatorial diameter has been variously estimated by
-observers, at from 71,000 to 79,000 miles. If we adopt the mean of these
-numbers, 75,000 miles, the circumference of the Saturnian equator would
-measure 235,620 miles, or 9½ times the circumference of our globe; the
-surface of Saturn would be 86 times, and its volume over 810 times that
-of the Earth.
-</p>
-<p>
-However great the volume of Saturn, its mass is proportionally small,
-being only 90 times greater than that of our globe; the mean density of
-the materials composing this planet being less than that of cork, and
-only 0.68 the density of water. The force of gravitation at the surface
-of Saturn is greater, by a little over ⅑, than it is at the surface of
-the Earth; a body falling in a vacuum at its surface, would travel 17.59
-feet during the first second.
-</p>
-<p>
-From observations of markings seen on the surface of Saturn, and from
-the study of their apparent displacements on the disk, William Herschel
-found that the planet rotated upon its axis in 10h. 16m. 0.24s. Since
-Herschel's determination, new researches have been made, and lately,
-Professor Hall, noticing a bright spot, followed it for nearly a month,
-observing its transits across the central meridian of the disk. From
-these observations he has obtained for the rotation period 10h. 14m.
-23.8s., a result which agrees very closely with that obtained 82 years
-earlier by Herschel, considering the fact that the markings from which
-the period of rotation is ascertained are not fixed on the planet, but
-are always more or less endowed with proper motion. The velocity of
-rotation at the equator is 21,538 miles per hour, or nearly 6 miles per
-second.
-</p>
-<p>
-The axis of rotation of Saturn is inclined 64° 18' to the plane of the
-orbit, so that its equator makes an angle of 25° 42' with the same
-plane. The seasons of this planet therefore present greater extremes of
-temperature than those of the Earth, but not quite so great variations
-as the seasons of Mars.
-</p>
-<p>
-The globe of Saturn is not a perfect sphere, but its figure is that of
-an oblong spheroid, flattened at the poles. The polar compression of
-Saturn is greater than that of any other planet, surpassing even that of
-Jupiter. Though not yet determined with a great degree of accuracy, the
-compression is known to be between ²⁄₁₈ and ⅒ of the equatorial
-diameter; that is, a flattening of about 3,894 miles, at each pole, the
-polar diameter being 7,788 miles shorter than the equatorial.
-</p>
-<p>
-The internal condition of the planet Saturn, whether solid, liquid or
-gaseous, cannot be discovered from the examination of its surface, as
-its globe is enwrapped in a dense opaque layer of vapors and cloud-like
-forms, through which the sight fails to penetrate. The appearance of
-this vapory envelope is like that of <i>cumulus</i> clouds, and one of its
-characteristics is to arrange itself into alternate bright and dark
-parallel belts, broader than those seen on Jupiter, and also more
-regular and dark. These belts, which are parallel to the equator of the
-planet, vary in curvature with the inclination of its axis of rotation
-to the line of sight.
-</p>
-<p>
-The belts of Saturn, like those of Jupiter, are not permanent, but keep
-changing more or less rapidly. Sometimes they have been observed to be
-quite numerous; while at other times they are few. Occasionally
-conspicuous white or dark spots are seen on the surface, although the
-phenomenon is quite rare. It is from the observation of such spots that
-Saturn's period of rotation has been determined, as stated above. The
-equatorial zone of Saturn always appears more white and brilliant than
-the other parts, as it also appears more mottled and cloud-like. In late
-years the globe has been characterized, and much adorned, by a pale
-pinkish tint on its equatorial belt, resembling that of Jupiter, but
-somewhat fainter. On either side of the equatorial belt there is a
-narrower band, upon which the mottled appearance is visible. Below
-these, one or two dark belts, separated by narrow white bands, are
-usually seen; but, of late, the bands have been less numerous, being
-replaced in high latitudes by a dark segment, which forms a polar cap to
-Saturn. The globe of Saturn does not anywhere appear perfectly white,
-and when compared with its ring, it looks of a smoky yellowish tint,
-which becomes an ashy gray on its shaded parts. It usually appears
-darker near the limb than in its central portions; although on some
-occasions I have seen portions of the limb appear brighter, as if some
-white spots were traversing it.
-</p>
-<p>
-Some observers have seen the limb deformed and flattened at different
-places, and W. Herschel even thought such a deformation to be a
-permanent feature of this globe, which he termed diamond-shaped, or
-"square shouldered." But this was evidently an illusion, since the
-planet's limb usually appears perfectly elliptical, although it
-occasionally appears as if flattened at some points, especially where it
-comes in apparent contact with the shadow cast by the globe on the ring,
-as observed by myself many times. But with some attention, it is
-generally found that this deformation is apparent rather than real, and
-is caused by the passage of some large dark spots over the limb, which
-is thus rendered indistinguishable from the dark background upon which
-it is projected.
-</p>
-<p>
-What distinguishes Saturn from all known planets, or heavenly bodies,
-and makes it unique in our universe, is the marvelous broad flat ring
-which encircles its equator at a considerable distance from it. With a
-low magnifying power this flat ring appears single, but when carefully
-examined with higher powers, it is found to consist of several distinct
-concentric rings and zones, all lying nearly in the same plane with the
-planet's equator.
-</p>
-
-<p><br /></p>
-
-<div class="figcenter" style="width: 400px;">
-<a id="figure10"></a>
-<br />
-<img src="images/figure10.jpg" width="400" alt="" />
-<div class="caption">
-<p>PLATE X.&mdash;THE PLANET SATURN.</p>
-<p class="smaller">Observed on November 30, 1874, at 5h. 30m. P.M.</p>
-</div></div>
-
-<p><br /></p>
-
-<p>
-At first sight only two concentric rings are recognized, the
-<i>outer</i> and the <i>middle</i>, or <i>intermediary</i>, which are
-separated by a wide and continuous black line, called the <i>principal
-division</i>. This line, and indeed all the features of the surface of
-the rings are better seen, and appear more prominent on that part of the
-ring on either side called the <i>ansa</i>, or handle. Besides these two
-conspicuous rings, a third, of very dark bluish or purplish color, lies
-between this middle ring, to which it is contiguous, and the planet.
-This inner ring, which is quite wide, is called the <i>gauze</i> or
-<i>dusky ring</i>. Closer examination shows that the outer ring is
-itself divided by a narrow, faint, grayish line called the pencil line,
-which, from its extreme faintness, is only visible on the ansæ.
-Moreover, the middle ring is composed of three concentric zones, or
-belts, which, although not apparently divided by any interval of space,
-are distinguished by the different shadings of the materials composing
-them. The outer zone of this compound middle ring is, by far, the
-brightest of all the system of rings and belts, especially close to its
-external border, where, on favorable occasions, I have seen it appear on
-the ansæ as if mottled over, and covered throughout with strongly
-luminous cloud-like masses. On the ansæ of the double outer ring,
-similar cloudy forms have also been seen at different times. The second
-zone of the middle ring is darker than the first, the innermost being
-darker still. All the characteristic points which have thus been
-described, are shown in Plate X.
-</p>
-<p>
-Although suspected in 1838, the dusky ring was not recognized before
-1850, when G. P. Bond discovered it with the 15-inch refractor of the
-Cambridge Observatory. It was also independently discovered the same
-year in England by Dawes and Lassell. The dusky ring differs widely in
-appearance and in constitution from the other rings, inasmuch as these
-last are opaque, and either white or grayish, while the former is very
-dark, and yet so transparent that the limb of the planet is plainly seen
-through its substance. On particularly favorable occasions, the
-appearance of this ring resembles that of the fine particles of dust
-floating in a ray of light traversing a dark chamber. Whatever may be
-the material of which this ring is composed, it must be quite rarefied,
-especially towards its inner border, which appears as if composed of
-distinct and minute particles of matter feebly reflecting the solar
-light. That the inner part of the dusky ring is composed of separate
-particles, is proved by the fact that the part of the ring which is seen
-in front of the globe of Saturn has its inner border abruptly deflected
-and curved inward on entering upon the disk, causing it to appear
-considerably narrower than it must be in reality, a peculiarity which is
-shown in the Plate. This phenomenon may be attributed to an effect of
-irradiation, due to the strong light reflected by the central parts of
-the ball, which so reduces the apparent diameter of the individual
-particles that they become invisible to us, especially those near the
-inner border, which are more scattered and less numerous than elsewhere.
-</p>
-<p>
-The dusky ring, which was described by Bond, Lassell and other
-astronomers as being equally transparent throughout all its width, has
-not been found so by me in later years. The limb of the planet, seen by
-these observers through the whole width of the dusky ring in 1850, could
-not be traced through its outer half by myself in 1872 and 1874, and
-this with the very same instrument used by Bond in his observations of
-1848 and 1850. Moreover, I have plainly seen that its transparency was
-not everywhere equal, but greatest on the inner border, from which it
-gradually decreases, until it becomes opaque, as proved by the gradual
-loss of distinctness of the limb, which vanishes at about the middle of
-the dusky ring. These facts, which have been well ascertained, prove
-that the particles composing this ring are not permanently located, and
-are undergoing changes of relative position. It will be shown that the
-surface of the other rings is also subject to changes, which are
-sometimes very rapid.
-</p>
-<p>
-The globe of Saturn is not self-luminous, but opaque. It shines by the
-solar light, as is proved by the shadow it casts opposite the Sun upon
-the ring. Although receiving its light from the Sun, Saturn does not
-exhibit any traces of phases, like the other planets nearer to the Sun,
-owing to its great distance from the Earth. When near its quadratures,
-however, the limb opposite to the Sun appears much darker, and shows
-traces of twilight. As far as can be ascertained, the rings, with the
-exception of the inner one, are opaque, as proved by the strong shadow
-which they cast on the globe of Saturn.
-</p>
-<p>
-The shadows cast by the planet on the ring, and by the ring on the
-planet, are very interesting phenomena, inasmuch as they enable the
-astronomer to recognize the form of the surface which receives them. The
-shadow cast by the ring on the ball is not quite so interesting as the
-other, although it has served to prove that the surface of this globe is
-not smooth, as is likewise suggested by its mottled appearance. I have
-sometimes found, as have also other observers, that the outline of this
-shadow upon the ball was irregular and indented, an observation which
-proves either that the surface of the ball is irregular, or that the
-border of the ring casting the shadow was jagged. The shadow of the
-globe on the rings has much more interest, as it enables us to get at
-some knowledge of the form of the surface of the rings, which otherwise
-is very difficult to discover, owing to the oblique position in which we
-always see them.
-</p>
-<p>
-In general, the shadow of the ball on the middle ring has its outline
-concave towards the planet; while on the outer ring it is usually
-slanting, and at a greater distance from the limb than on the middle,
-and dusky rings. This form of the shadow evidently proves that the
-middle ring stands at a higher level than the two others, especially
-towards its outer margin. The system seems to increase gradually in
-thickness from the inner border of the dusky ring to the vicinity of the
-outer margin of the middle ring, after which it rapidly diminishes on
-this border, while the surface of the outer ring is almost level.
-</p>
-<p>
-But this surface is by no means fixed, as its form sometimes changes, as
-proved by my observations and those of others. As may be noticed on
-Plate X., the outline of the shadow of the planet on the rings is
-strongly deviated towards the planet, near the outer margin of the
-middle ring; the notch indicating an abrupt change of level, and a rise
-of the surface at that point. Some observers have endeavored to explain
-these deviations by the phenomena of irradiation, from which it would
-follow that the maximum effect of deviation should be observed where the
-ring is the brightest, which does not accord with observation; as the
-deepest depression in the shadow is not to be found usually at the
-brightest part, which is towards the outer border of the middle ring,
-but occurs near its centre. From these observations it is undoubtedly
-established that the surface of the rings is far from being flat
-throughout, and is, besides, not permanent, but changes, as would, for
-instance, the surface of a large mass of clouds seen from the top of a
-high mountain. In general, the system is thickest not very far from the
-outer border of the intermediary ring.
-</p>
-<p>
-Some interesting phenomena which I had occasion to observe before and
-after the passage of the Sun through the plane of the rings, on February
-6th, 1878, conclusively show that the surface of this system cannot be
-of a uniform level, but must be thicker towards the outer border of the
-middle ring, thence gradually sloping towards the planet. Many of my
-observations irresistibly lead to this conclusion. As it would, however,
-be out of place to have them recorded here in detail, I will simply give
-one of the most characteristic among them.
-</p>
-<p>
-From December 18th, 1877, when the Sun was about 41' above the plane of
-the rings, to February 6th, 1878, the day of its passage through their
-plane, the illuminated surface of this system gradually decreased in
-breadth with the lowering of the Sun, until it was lost sight of,
-February 5th, on the eve of the passage of the Sun through their plane.
-The phenomenon in question consisted in the gradual invasion of their
-illuminated surface by what appeared to be a black shadow, apparently
-cast by the front part of the outer portion of the middle ring the
-nearest to the Sun. On January 25th, when the elevation of the Sun above
-the plane of the rings was reduced to 15', the shadow thus cast had
-extended so far on their surface that it reached the shadow cast by the
-globe on the opposite part of the ring in the east, and accordingly the
-remaining portion of the illuminated surface of the eastern ansa then
-appeared entirely disconnected from the ball, by a large dark gap,
-corresponding in breadth to that of the globe's shadow on the rings. On
-February 4th, when the Sun was only 5' above the plane of the rings,
-their illuminated and only visible surface was reduced to a mere thread
-of light, which on the 5th appeared broken into separate points. It is
-evident that the phenomenon was not caused by the obliquity of the ring
-as seen from our globe, since the elevation of the Earth above the plane
-of the rings&mdash;which on December 18th was 3° 20'&mdash;was still 1° 20'
-on the 4th of February. In ordinary circumstances, when the Sun is a little
-more elevated, and the rings seen at this last angle, they appear quite
-broad and conspicuous, and even the dark open space separating the dusky
-ring from the planet is perfectly visible on the ansæ, where the
-Earth's elevation above their plane is reduced to 40'. It is also
-evident that the phenomenon was not to be attributed to the reduction of
-the light which they received from the Sun, although the illumination in
-February might be expected to be comparatively feeble, since the Sun
-then shone upon the rings so obliquely; yet (on the supposition that
-their surface is flat) they should have been illuminated throughout, and
-if not very brightly, sufficiently so, at least, to make them visible
-and as bright as was the narrow thread of light observed on the 4th of
-February. The phenomenon actually observed may be explained most readily
-by assuming, as other phenomena also indicate, that the surface of the
-ring is not flat, but more elevated towards, or in the vicinity of its
-outer border, from which place it slopes inwardly towards the planet. On
-this assumption, it is evident that the elevated part of the ring the
-nearest to the Sun would cast a shadow, which, with the increasing
-obliquity of the Sun, would gradually cover the whole surface comprised
-within the elevated part, and thus become invisible to us. Several
-observations made by Bond and other observers undoubtedly show the same
-phenomenon, and do not seem to be intelligible on any other supposition.
-From my observations made in 1881 it would appear, however, that the
-opposite surfaces of the rings do not exactly correspond in form, but
-this may not be a permanent feature, as the surface of this system is
-subject to changes, as already shown.
-</p>
-<p>
-The dimensions of the rings are great, the diameter of the outer one
-being no less than 172,982 miles, the distance from the centre of the
-globe to the outer border of the system being, therefore, 86,491 miles.
-The breadth of the outer ring is 9,941 miles; that of the principal
-division, 2,131 miles; that of the middle ring, 19,902 miles, and that
-of the dusky ring, 8,772 miles. The breadth of all the rings taken
-together is, therefore, 40,746 miles. The interval between the surface
-of Saturn and the inner border of the dusky ring is 7,843 miles.
-</p>
-<p>
-The thickness of the system of rings has been variously estimated by
-astronomers, on account of the great difficulties attending its
-determination. While Sir John Herschel estimated it at more than 250
-miles, G. P. Bond reduces it to 40 miles. Both of these numbers are
-evidently too small, as so slight a thickness cannot explain the
-observed phenomenon of the shadow cast by a portion of the ring on its
-own surface, when the Sun is very low in its horizon, as shown above.
-</p>
-<p>
-The plane of the system of rings is inclined 27° to the planet's orbit,
-and is parallel, or at least very nearly so, with the equator of the
-planet, passing, therefore, through its centre, and dividing its globe
-into northern and southern hemispheres. Seen from the Earth, a portion
-of the ring always appears projected in front of the planet, thus
-concealing a small part of its globe, while the opposite portion passes
-behind the globe, which hides it from sight.
-</p>
-<p>
-As the plane of the ring is not affected by the motion of the planet
-around the Sun, but always remains parallel to itself, it follows that
-as Saturn advances in its orbit the rings must successively present
-themselves to us under various angles of inclination, appearing,
-therefore, more or less elliptical, and presenting two maxima and two
-minima of inclination in the course of one of its revolutions. As the
-revolution of Saturn is accomplished in 29½ years, the maxima and the
-minima must recur every 14 years and 9 months; the maxima being
-separated from the minima by an interval of 7 years and 4½ months.
-</p>
-<p>
-When Saturn arrives at the two opposite points of its orbit, where the
-major axis of its ring is at right angles to the line joining its centre
-to that of the Sun, the ring, which is then viewed at an inclination of
-27°, the greatest angle at which it can ever be seen, has reached its
-maximum opening, the smaller diameter of its ellipse being then about
-half that of the larger. At this moment the outer ring projects north
-and south beyond the globe, which is then completely enclosed in its
-ellipse. The maximum opening of the northern surface of the ring takes
-place, at present, when Saturn arrives in longitude 262°, in the
-constellation Sagittarius, and that of the southern surface when it
-arrives in longitude 82° in the constellation Taurus. When, on the
-contrary, Saturn reaches the two opposite points of its orbit, where the
-plane of its ring is parallel to the line joining its centre to that of
-the Sun, the opening vanishes, as only the thin edge of the ring is then
-presented to the Sun and receives its light, the rest being in darkness.
-At this moment the ring disappears, except in the largest telescopes,
-where it is seen as an exceedingly thin thread of light; and the
-Saturnian globe, having apparently lost its ring, appears solitary in
-the sky, like the other planets. The disappearance of the ring from this
-cause occurs now when Saturn arrives at 90° from either of the
-positions of maximum inclination, that is, in longitude 352° in the
-constellation Pisces, and in longitude 172° in the constellation Leo.
-</p>
-<p>
-When the planet is in any other position than one of these last two,
-either the northern or the southern surface of the ring is illuminated
-by the Sun, while the opposite surface is in the night, and does not
-receive any direct sunlight. At the time of the passage of the plane of
-the ring through the Sun's centre, a change takes place in the
-illumination of the ring. If it is the northern surface which has
-received the rays of the Sun during the previous half of the Saturnian
-year, at the moment the plane has passed the centre of the Sun, the
-southern surface, after having been buried in darkness for 14¾ years,
-sees the dawn of its long day of the same length. Such a phenomenon will
-not occur until 1892, when the passage of the Sun from the northern to
-the southern side of the ring will close in twilight the day commenced
-in 1878.
-</p>
-<p>
-Aside from the periodic disappearance of the ring, resulting from the
-passage of the Sun through its plane, the ring may also disappear from
-other reasons. Just before or just after the time of the passage of the
-Sun through the plane of the ring, the Earth and the Sun may occupy such
-positions, that while the one is north of the plane of the ring, the
-other is south of it, or vice versa, in which event the ring becomes
-invisible, because its dark and non-illuminated surface is presented to
-us. The ring may also become invisible to us when the Earth passes
-through its plane.
-</p>
-<p>
-Since the distance from Saturn to the Sun is to the distance of the
-Earth from this last body as 9.54 is to 1; and since the circumference
-of a circle increases in the same proportion as its radius, it follows
-that the diameter of the Earth's orbit projected on the orbit of Saturn
-would occupy only part of the latter, or about 12° 2', this being 6°
-1' on either side of the nodes of the rings. To describe such an arc on
-its orbit, it takes Saturn almost 360 days on an average, or almost a
-complete year; the Earth describing therefore almost a whole revolution
-around the Sun during the time it takes Saturn to advance 12° 2' on its
-orbit. Then, when Saturn occupies a position comprised within an arc 6°
-1' from either side of the nodes of its ring, the Earth, by its motion,
-is liable to encounter the plane of the ring, when therefore it will
-only present its thin edge to us, and becomes invisible. At least one
-such encounter is unavoidable within the time during which Saturn
-occupies either of these positions on its orbit; while three frequently
-happen, and two are possible.
-</p>
-<p>
-The natural impression received by looking at the rings, while seeing
-the ponderous globe of Saturn enclosed in its interior, is that this
-gigantic, but very delicate structure, in order to avoid destruction,
-must be endowed with a swift movement of rotation on an axis
-perpendicular to its plane, and that the centrifugal force thence
-arising counterbalances the powerful attraction of the planet, and thus
-keeps the system in equilibrium.
-</p>
-<p>
-Theoretically, the rotation of the rings is admitted by every
-astronomer, as being an essential condition to the existence of the
-system, which otherwise, it is thought, would fall upon the planet.
-Although the rotation of the rings seems so probable that it is
-theoretically considered as certain, yet its existence has not been
-satisfactorily demonstrated by direct observation, which alone can
-establish it on a firm basis as a matter of scientific knowledge.
-</p>
-<p>
-The determination of the period of rotation of the rings, which is
-supposed to be 10h. 32m. 15s., rests only on the observations of W.
-Herschel, made in 1790, from the apparent displacement of irregularities
-on the ring; but his results have been contradicted by other
-observations, and even by those of Herschel himself, made in later
-years.
-</p>
-<p>
-Although the system of rings is very nearly concentric with the globe of
-Saturn, yet the coincidence is not considered as mathematically exact.
-It seems to have been satisfactorily demonstrated by direct observations
-that the centre of gravity of the system oscillates around that of the
-planet, thus describing a minute orbit. This peculiarity is in
-accordance with theory, which has shown it to be essential to the
-stability of the system.
-</p>
-<p>
-Besides its system of rings, which makes Saturn the most remarkable
-planet of the solar system, this globe is attended by eight satellites,
-moving in orbits whose planes very nearly coincide with the plane of the
-rings, except that of the most distant one, which has an inclination of
-about 12° 14'. In the order of their distance from the planets, the
-satellites of Saturn are as follows: Mimas, Enceladus, Tethys, Dione,
-Rhea, Titan, Hyperion and Iapetus. The three first satellites are nearer
-to Saturn than the Moon is to the Earth; while Iapetus, the farthest, is
-9½ times the distance of our satellite from us. All the satellites,
-with the exception of the farthest, move more rapidly around Saturn than
-the Moon moves around the Earth; while Iapetus, on the contrary, takes
-almost three times as long to make one revolution.
-</p>
-<p>
-The period of revolution of the four inner satellites is accomplished in
-less than three days, that of Mimas being only a little more than 22
-hours. From such swiftness of motion, it is easily understood how short
-must be the intervals between the different phases of these satellites.
-Mimas, for instance, passes from New Moon to First Quarter in less than
-6 hours.
-</p>
-<p>
-The distance of the nearest satellite from the planet's surface is
-84,000 miles, and its distance from the outer ring only 36,000 miles. It
-is difficult to determine the diameter of objects so faint and distant
-as are some of these satellites, but the diameter of Titan, the largest
-of all, is pretty well known, and estimated to be ¹⁄₁₆ the
-diameter of the planet, or more than half the diameter of our globe.
-</p>
-<p>
-Iapetus is subject to considerable variations in brilliancy, and as the
-maxima and minima always occur when this satellite occupies the same
-parts of its orbit, it was conjectured by W. Herschel that, like our
-Moon, it turns once upon its axis during each of its revolutions about
-the planet. It has been shown by my observations, that Iapetus attains
-its maximum brightness a little before it reaches its greatest western
-elongation, and its minimum on the opposite side.
-</p>
-<p>
-As the planes of the orbits of the satellites are inclined to the
-planet's orbit, it follows that their transits, occultations and
-eclipses, are only possible when Saturn is near its equinoxes. Passages
-of the satellites and their shadows across the disk, although rare, have
-been observed, and they somewhat resemble the phenomena exhibited by the
-satellites of Jupiter in transit. When the Earth is very near the plane
-of the rings, the satellites, except the farthest, appear to be in a
-straight line nearly coincident with the plane of the rings, and are
-seen occasionally moving along the thin edge of the rings, appearing as
-luminous beads moving on a thread of light.
-</p>
-<p>
-Owing to the considerable inclination of the axis of rotation of Saturn
-to its orbit, the seasons of this planet must have greater extremes of
-temperature than those of the Earth. As the year of Saturn consists of
-25,217 Saturnian days, each season, on the average, is composed of 6,304
-Saturnian days.
-</p>
-<p>
-To an observer on Saturn, the immense arches formed by its rings would
-appear as objects of great magnificence, spanning the sky like soft
-colorless rainbows. Moreover, the eight moons, several of which are
-always visible, would be of the highest interest, with their swift
-motions and rapid phases. Mimas, traveling in its orbit at the rate of
-16 of arc per minute of time, moves over a space equal to the apparent
-diameter of our Moon in two minutes, or at the rate of 16° an hour.
-</p>
-<p>
-Owing to the globular form of Saturn, the rings would be invisible in
-latitudes situated above 65° from its equator, and their apparent form
-and breadth would naturally vary with the latitude. At 63° only a very
-small portion of the outer ring would be visible above the equatorial
-horizon, where it would appear as a small segment of a circle. At 62°
-the principal division would just graze the horizon. At 46° the outer
-portion of the dusky ring would become visible, while at 35° its inner
-edge would appear above the horizon. From 65° of latitude down to the
-equator, the arches of the rings would be seen more and more elevated
-above the equatorial horizon, but at the same time that they are seen
-higher up, their apparent breadth gradually diminishes, owing to the
-effect of foreshortening, and at the equator itself the system would
-only present its thin edge to view.
-</p>
-<p>
-During the summer seasons of either hemisphere of Saturn, the surface of
-the rings turned towards such hemisphere, being fully illuminated by the
-Sun, is visible from these regions. In the day time its light must be
-feeble and similar to the light reflected by our Moon during sunshine;
-but at night the system would display all its beauty, and the different
-rings, with their divisions and their various reflective powers, must
-present a magnificent sight.
-</p>
-<p>
-During the nights of the long winter seasons on Saturn, on the contrary,
-the surface of the rings turned towards the hemisphere undergoing
-winter, receives no light from the Sun, and is invisible, or very nearly
-so, except towards morning and evening, when it may be faintly
-illuminated by the secondary light which it receives from the
-illuminated globe of Saturn. Although dark and invisible, the rings may
-make their form apparent at night by the absence of stars from the
-region which they occupy in the sky. Again, in other seasons, the days
-present very curious phenomena. In consequence of the diurnal rotation
-of the planet, the Sun seems to move in circular arcs, which, owing to
-the inclination of Saturn's axis, are more or less elevated above its
-horizon, according to the position of the planet in its orbit. As such
-arcs described by the Sun in the sky of Saturn are liable to encounter
-the rings, the Sun in passing behind them becomes eclipsed. It must be a
-magnificent spectacle to witness the gradual disappearance of the fiery
-globe behind the outer ring, and its early reappearance, but for a
-moment only, through the narrow gap of the principal division; to see it
-vanish again behind the middle ring, to reappear a little later through
-the semi-transparent dusky ring, but very faint and red colored at
-first; and then, gradually brighten up, and finally emerge in all its
-beauty from the inner edge of the dusky ring.
-</p>
-<p>
-It is in latitude 23° that the rings produce the most prolonged
-eclipses of the Sun. During a period equivalent to ten of our
-terrestrial years, such eclipses continually succeed each other with but
-very short periods of interruption; and even during a long series of
-rotations of Saturn, the Sun remains completely invisible in those
-regions where the apparent arcs which it describes coincide with the
-arcs of the rings. In neighboring latitudes, the eclipses of the Sun,
-although still frequent, would have a shorter and shorter duration as
-the observer should travel north or south. These eclipses of the Sun
-must produce a partial darkness of the regions involved in the shadow of
-the rings, which may be compared to the darkness produced on our globe
-by a total eclipse of the Sun. The frequent recurrence of these
-eclipses, and their comparatively long duration in some regions, must
-still further reduce the duration of the short Saturnian days.
-</p>
-<p>
-The globe of Saturn, as already shown, casts a shadow on the rings,
-which, according to the position of the planet in its orbit, either
-extends across their whole breadth, or covers only a part of their
-surface. The shadow on the rings rising in the east after sunset,
-ascends to the culminating point of their arcs in the sky, in 2h. 34m.,
-and as rapidly descends on the western horizon, to disappear with
-sunrise. This shadow, when projected on the rings in the sky, must be
-hardly distinguishable from the dark background of the heavens, except
-from the absence of stars in the regions which it occupies. It must
-appear as a large dark gap, separating the rings into two parts, and
-constantly moving from east to west. Possibly the refraction of the
-solar rays, in passing through Saturn's atmosphere, may cast some
-colored light on the rings, similar to that observed on the Moon during
-its eclipses.
-</p>
-<p>
-An observer on the rings would behold phenomena still more curious, a
-long day of 14¾ years being followed by a long night of 14¾ years. The
-long days of Saturn's rings are, however, diversified by numerous
-eclipses of the Sun, which regularly occur every 10¼ hours; the
-phenomenon being due to the interposition of the globe of Saturn between
-the rings and the Sun. These eclipses produce partial obscurations of
-their surface, lasting from 1½ to 2 hours at a time. Although the
-surface of the rings never receives direct sunlight during their long
-nights, yet they are not plunged all the time in total darkness, as they
-receive some reflected light from that part of the globe of Saturn which
-is illuminated by the Sun. To the supposed observer on the rings, during
-every 10¼ hours, the immense globe would exhibit continually changing
-phases. At first he would see a point of light rapidly ascending from
-the horizon, and appearing under the form of a half crescent of
-considerable radius; 5⅛ hours later, the crescent having gradually
-increased, would appear as a half circle, covering ⅛ of the visible
-heavens, its surface being more than 20,000 times as large as the
-surface of the Moon. Upon this brilliantly illuminated semi-circle would
-be projected the shadows of the rings, appearing as black belts
-separated by a narrow luminous band.
-</p>
-<p>
-It is very difficult for one to conceive how such a delicate structure,
-as the system of rings appears to be, can keep together in equilibrium
-and avoid destruction from the powerful attraction of the planet on one
-side and the disturbing influence of the satellites on the other. To
-explain it, several hypotheses have been advanced. The rings were first
-supposed to be solid, and upon this supposition Laplace determined the
-necessary conditions for their equilibrium; the most important of which
-require that the cross section of the rings should be an ellipse of
-irregular curvature, and having its major axis directed towards the
-centre of the planet, and also that the system should rotate upon an
-axis perpendicular to the plane of the rings. This theory was superseded
-by another, which supposed the rings to be fluid. This one was soon
-rejected for a third, assuming the system to be composed of vapors or
-gases; and more recently, all these theories were considered untenable,
-and replaced by a fourth, which supposes the system of rings to be made
-up of a congregation of innumerable small, independent bodies, revolving
-around Saturn in concentric zones. Naturally, such a divergence of
-opinion can only result from our comparative ignorance of the subject,
-and sufficiently indicates our inability to explain the phenomena; and
-it must be admitted that, so far, nothing is certainly known about this
-strange system. We shall probably remain in the same uncertainty until
-the rotation of the rings is ascertained by direct observations. It is
-pretty certain, however, that none of these theories account for the
-observed phenomena in their details, although a partial explanation may
-be obtained by borrowing something from each hypothesis.
-</p>
-<p>
-It has been conjectured, and a theory has been advanced, that the
-breadth of the whole ring system is gradually increasing inwards, and
-that it will come in contact with the planet in about 2,150 years; but
-the question seems to have been settled in the negative by the elaborate
-measurements of the English observers. It is likely that the increase is
-only in the defining power of the instruments.
-</p>
-
-<p><br /><br /><br /></p>
-
-<h4><a id="COMETS">COMETS</a>
-<br /><br />
-PLATE XI</h4>
-
-<p>
-Among the celestial phenomena, none are more interesting than those
-mysterious apparitions from the depths which unexpectedly display their
-strange forms in our familiar constellations, through which they wander
-for a time, until they disappear like phantoms.
-</p>
-<p>
-A comet, with its luminous diffused head, whence proceeds a long vapory
-appendage gradually fading away in the sky, presents an extraordinary
-aspect, which may well astonish and deeply impress the observer.
-Although these visitors from infinite space do not now inspire dread, as
-in by-gone times, yet, owing to the mystery in which the phenomenon is
-still involved, the apparition of a large comet, even in our days, never
-fails to create a profound sensation, and in some cases that unconscious
-fear which results from the unknown.
-</p>
-<p>
-The effect of such a spectacle largely depends upon its rarity; but
-since the telescope has been applied to the sounding of the heavens, it
-has been found that the appearance of comets is by no means an unusual
-occurrence. If so few comets, comparatively, are seen, it is because
-most of them are telescopic objects, and are therefore invisible to the
-naked eye. Most of the telescopic comets are not only too faint to be
-perceived by the unaided eye, but are insignificant objects, even when
-observed through the largest telescopes.
-</p>
-<p>
-It was Kepler's opinion that comets are as numerous in the sky as fishes
-are in the ocean. Undoubtedly the number of these bodies must be great,
-considering that we can only see them when they come into the
-neighborhood of the Earth, and that many even here remain invisible, or
-at least pass unperceived. That many of them have passed unperceived
-heretofore, is proved by the fact that the number of those observed
-becomes greater every year, with the increase of the number of
-instruments used in their search. The number of comets observed with the
-naked eye during historic times is nearly 600, and that of telescopic
-comets, which, of course, all belong to the last few centuries, is more
-than 200, so that we have a total number of about 800 comets of which
-records have been kept. From theoretical considerations, Lambert and
-Arago estimated their entire number at several millions, but such
-speculations have generally no real value, since they cannot be
-established on a firm basis.
-</p>
-<p>
-Comets remain visible for more or less time, according to their size and
-the nature and position of their orbits, but in general, the large ones
-can be followed with the telescope for several months after they have
-become invisible to the naked eye. The comet of 1861, for example,
-remained telescopically visible for a year, and that of 1811, for 17
-months after disappearing from ordinary sight.
-</p>
-<p>
-While a comet remains visible, it appears to revolve daily about us like
-the stars in general; but it also moves among the constellations, and
-from this movement its orbit may be computed like that of a planet. From
-the apparent diurnal motion of a comet with the heavens, result the
-changes of position which it seems to undergo in the course of a night.
-The direction of the head and tail of a comet, of course, has only
-changed in regard to the horizon, but not in regard to the sky, in which
-they occupy very nearly the same position throughout a given night, and
-even for many nights in succession.
-</p>
-<p>
-The movements of the comets in their orbits are, like those of the
-planets, in accordance with Kepler's laws, the Sun occupying one of the
-foci of the orbit they describe; but the orbits of comets differ,
-however, in several points from those of the planets. Their eccentricity
-is always great, being sometimes apparently infinite, in which case the
-orbit is said to be parabolic, or hyperbolic; but the smallness of the
-portion of a cometary orbit which can ordinarily be observed, makes it
-difficult to determine this with certainty. Again, while the planetary
-orbits are usually near the plane of the ecliptic, those of comets
-frequently have great inclinations to that plane, and even when the
-inclination is less than 90°, the comet may have a retrograde movement,
-or, in other words, a movement contrary to the course in which all the
-planets revolve about the Sun.
-</p>
-<p>
-Notwithstanding these differences between the elements of the orbits of
-the comets and those of the planets, the fact that each has the Sun in
-one focus indicates that the body moving in it is a member of the solar
-system, either for the time, or permanently, according to the nature of
-its orbit.
-</p>
-<p>
-A distinction may accordingly be made between the comets which are
-permanent members of our solar system and those which are only
-accidental or temporary visitors. Those moving in elliptical orbits
-around the Sun, like the planets, and therefore having a determinate
-period of revolution, from which the time of their successive returns
-may be predicted, are permanent members of our system, and are called
-periodic comets. All comets moving in parabolical or hyperbolical
-curves, are only temporary members of the solar system, being apparently
-strangers who have been diverted from their courses by some disturbing
-influence. No comet is classed as periodical which does not follow a
-perceptibly elliptical orbit. Any comet passing around the Sun at the
-mean distance of the Earth from this body, with a velocity of 26 miles
-per second, will fly off into infinite space, to return to us no more.
-</p>
-<p>
-The time of revolution of the different periodic comets thus far
-observed varies greatly, as do also the distances to which they recede
-from the Sun at aphelion. Whilst the period of revolution of Encke's
-comet, the shortest thus far known, is only 3½ years, that of the comet
-of 1844, II., is 102,000 years; and whilst the orbit of the first is
-comprised within the orbit of Jupiter, that of the last extends to a
-distance equal to 147 times the distance of Neptune from the Sun. But so
-vast an orbit cannot be accurately determined from the imperfect data at
-our disposal.
-</p>
-<p>
-The periodic comets are usually divided into two classes. The comets
-whose orbits are within the orbit of Neptune are called interior comets,
-while those whose orbits extend beyond that of Neptune are called
-exterior comets. The known interior periodic comets are twelve in
-number, while, including all the cases in which there is some slight
-evidence of elliptic motion, the number of exterior comets observed is
-six or seven times as great. The periodic comets of short period are
-very interesting objects, inasmuch as by their successive returns they
-afford an opportunity to calculate their motions and to observe the
-physical changes which they undergo in their intervals of absence.
-</p>
-<p>
-From observation of the periodic comets, it has been learned that the
-same comet never presents twice the same physical appearance at its
-different returns, its size, shape and brilliancy varying so greatly
-that a comet can never be identified by its physical characters alone.
-It is only when its elements have been calculated, and are found to
-agree with those of a cometary orbit previously known, that the two
-comets can be identified one with the other. There are reasons to
-believe that, in general, comets decrease in brightness and size at each
-of their successive returns, and that they are also continually losing
-some of their matter as they traverse their orbits.
-</p>
-<p>
-When very far away from us, all comets appear nearly alike, consisting
-of a faint nebulosity, of varying dimensions. When a comet first appears
-in the depths of space, and travels towards the Sun, it generally
-resembles a faint, uniformly luminous nebulosity, either circular or
-slightly elongated in form. As it approaches nearer to the Sun, a slight
-condensation of light appears towards its centre, and as it draws still
-nearer, it becomes brighter and brighter, and in condensing forms a kind
-of diffused luminous nucleus. At the same time that the comet acquires
-this concentration of light, the nebulosity gradually becomes elongated
-in the direction of the Sun. These effects generally go on increasing so
-long as the comet is approaching the Sun; the condensation of light
-sometimes forms a bright nucleus, comparable to a very brilliant star,
-while the elongation becomes an immense appendage or tail. When the
-comet has passed its perihelion and recedes from the Sun, the inverse
-phenomena are observed; the comet, decreasing in brightness, gradually
-loses its nucleus and tail, resumes its nebulous aspect, and finally
-vanishes in space, to appear again in due course, if it chance to be a
-periodic comet. While all comets become brighter in approaching the Sun,
-they do not all, however, develop a large tail, some of them showing
-only a slight elongation.
-</p>
-<p>
-When a comet is first discovered with the telescope at a great distance
-from the Sun, it is difficult to predict whether it will become visible
-to the naked eye, or will remain a telescopic object, as it is only in
-approaching the Sun that these singular bodies acquire their full
-development. Thus, Donati's comet, whose tail became so conspicuous an
-object at its full appearance in 1858, remained two months after its
-discovery by the telescope without any indication of a tail. The comet
-of Halley, which before and after its return in 1759, remained five
-years inside of the orbit of Saturn, showed not the least trace of its
-presence during the greater part of this time. Nothing but calculation
-could then indicate the position in the sky of this invisible object,
-which was so prominent when it approached the Sun.
-</p>
-<p>
-Another curious phenomenon exhibited by comets, and first noticed by
-Valz, is that in approaching the Sun the nebulosity composing these
-bodies contracts, instead of dilating, as would be naturally supposed
-from the greater amount of solar heat which they must then receive. In
-receding from the Sun, on the contrary, they expand gradually. As comets
-approach the Sun, the tail and nucleus are developed, while the
-nebulosity originally constituting these comets contracts, as if its
-material had been partly consumed in this development. In a certain
-sense it may be said that the comets are partly created by the Sun; in
-more exact terms, the changes of form which they undergo are induced by
-the Sun's action upon them at different distances and under varying
-conditions. Moreover, they are rendered visible by its influence,
-without which they would pass unperceived in our sky. When a comet
-disappears from view, it is not because its apparent diameter is so much
-reduced by the distance that it vanishes, but rather on account of the
-diminution of its light, both that which it receives from the Sun, and
-its own light; these bodies being in some degree self-luminous, as will
-be shown below.
-</p>
-<p>
-The large comets, such as can be seen with the naked eye, always show
-the following characteristics, on examination with the telescope. A
-condensation of light resembling a diffused star forms the brightest
-part of the comet, this condensation being situated towards the
-extremity the nearest to the Sun. It is this starlike object which is
-called the <i>nucleus</i>. The nucleus seems to be entirely enclosed in
-a luminous vapory envelope of the same general texture, called the
-<i>coma</i>. This envelope, which is quite variable in brightness and
-form, is brightest next to the nucleus, and gradually fades away as it
-recedes from it. The <i>nucleus</i> and the <i>coma</i>, considered as a
-whole, constitute the <i>head</i> of a comet. From the head of a comet
-proceeds a long trail of pale nebulous light, which usually grows wider,
-but fainter, as it recedes from the nucleus, and insensibly vanishes in
-the sky. This delicate appendage, or tail, as it is commonly called,
-varies very much in size and shape, not only in different comets, but in
-the very same comet, at different times. Its direction is generally
-opposite to that of the Sun from the head of the comet.
-</p>
-<p>
-The nuclei vary very much in brightness, in size and in shape; and while
-in some telescopic comets they are either absent or barely
-distinguishable as a small condensation of light, in bright comets they
-may become plainly visible to the naked eye, and they sometimes even
-surpass in brightness the most brilliant stars of the heavens. But
-whatever may be the size of cometary nuclei, they are subject to sudden
-and rapid changes, and vary from day to day. Sometimes they appear
-exceedingly brilliant and sharply outlined, while at other times they
-are so dim and diffused that they are hardly distinguishable from the
-coma of which they seem then to form a part.
-</p>
-
-<p><br /></p>
-
-<div class="figcenter" style="width: 400px;">
-<a id="figure11"></a>
-<br />
-<img src="images/figure11.jpg" width="400" alt="" />
-<div class="caption">
-<p>PLATE XI.&mdash;THE GREAT COMET OF 1881.</p>
-<p class="smaller">Observed on the night of June 25-26 at 1h. 30m. A.M.</p>
-</div></div>
-
-<p><br /></p>
-
-<p>
-From my observations upon the comets which have appeared since the year
-1873, it is apparent that the changes in the nucleus, coma and tail, are
-due to a solar action, which contracts or expands these objects in such
-a manner that the nuclei become either bright and star-like, or dim and
-diffused, in a very short time. I had excellent opportunity, especially
-in the two large comets of 1881, to observe some of these curious
-changes, a description of which will give an idea of their extent and
-rapidity. On July 2d, 1881, at 9 o'clock, the nucleus of comet 1881,
-III., which is represented on Plate XI., appeared sharply defined,
-bright and considerably flattened crosswise; but half an hour later it
-had considerably enlarged and had become so diffused that it could
-hardly be distinguished from the coma, with which it gradually blended.
-It is perhaps worth mention that, at the time this last observation was
-made, an aurora borealis was visible. This comet 1881, III., underwent
-other very important changes of its nucleus, coma and tail. On June
-25th, the nucleus, which was bright and clearly defined, was ornamented
-with four bright diverging conical wings of light, as shown on Plate XI.
-On the 26th these luminous wings had gone, and the nucleus appeared
-one-third smaller. On the 28th it had enlarged, but on the 29th its
-shape was considerably altered, the nucleus extending in one direction
-to three or four times its diameter on previous nights, and being
-curved, so as to resemble a comma. On the 6th of July the nucleus of
-this comet showed the greatest disturbances. The nucleus, which had
-appeared perfectly round on the evening of the 5th, was found much
-elongated at 10 o'clock on the 6th, forming then a straight, acute, and
-well-defined wedge of light, inclined upwards to the left. The length of
-the nucleus, at this time, was three or four times its ordinary
-diameter. At the same time rapid changes occurred; the strangely shaped
-nucleus soon became unsteady, extending and contracting alternately, and
-varying greatly in brightness. At 10h. 45m., the elongated nucleus, then
-gently curved, took the shape of a succession of luminous knots, which
-at times became so brilliant and distinct that they seemed to be about
-to divide and form separate nuclei; but such a separation did not
-actually occur, at least while I was observing. While these important
-changes were going on in the comet, a bright auroral arch appeared in
-the north, which lasted only a short time. On July 7th, the sky being
-cloudy, no observations were made, but on the 8th I observed the comet
-again. The nucleus had then resumed its circular form, but it was yet
-very unsteady, being sometimes small, bright and sharp, while a few
-seconds later it appeared twice as large, but dim in outlines; and
-sometimes an ill-defined secondary nucleus appeared at its centre. On
-several occasions the nucleus appeared as if it were double, one nucleus
-being apparently projected partly upon the other.
-</p>
-<p>
-The nuclei of comets are sometimes very small, and in other cases very
-large. Among those which have been measured, the nucleus of the comet of
-1798, I., was only 28 miles in diameter, but that of Donati's comet, in
-1858, was 5,600 miles, and that of the comet of 1845 was 8,000 miles in
-diameter.
-</p>
-<p>
-The coma of comets is found to be even more variable than the nucleus.
-The changes observed in the coma are generally in close connection with
-those of the nucleus and tail, the same perturbations affecting
-simultaneously the whole comet. While the coma of the comet of 1847 was
-only 18,000 miles in diameter, that of Halley's comet, in 1835, was
-357,000 miles, and that of the comet of 1811 was 1,125,000 miles in
-diameter. In general, as already stated, the coma of a comet decreases
-in size in approaching the Sun. That of Encke's comet, which, on October
-9th, 1838, had a diameter of 281,000 miles, gradually decreased at a
-daily mean rate of 4,088 miles in going towards the Sun; so that, on
-December 17th, when the distance of the comet from the Sun was more than
-four times less than it was on the first date, its diameter was reduced
-to 3,000 miles.
-</p>
-<p>
-The form of the coma, in that part which is free from the tail, is in
-general a portion of a circle, but is sometimes irregular, with its
-border deformed. Thus, the border of the coma of Halley's comet was
-depressed at one point towards the Sun. I observed a similar phenomenon
-in Coggia's comet, with the great refractor of the Harvard College
-Observatory, on July 13th, 1874, when its border appeared deeply
-depressed on the side nearest to the Sun, as if repelled by this body.
-The coma of comet 1881, III., showed also very singular outlines on the
-nights of the 25th and 26th of June, when its border was so deeply
-depressed that the coma appeared as if it were double. Luminous rays and
-jets often radiate from the nucleus across the coma, and describe
-graceful lateral curves, falling backwards and gradually fading away
-into the tail, of which they then form a part. The rays and jets emitted
-by the nucleus seem at first to obey the solar attraction and travel
-towards the Sun; but they are soon repelled, and move backward towards
-the tail. It is a mystery, as yet unexplained, how these cometary jets,
-which at first seem to obey to the laws of attraction, are compelled to
-retreat apparently by superior opposing forces. Among the forces of
-nature, we know of no other than those of an electrical sort, which
-would act in a similar manner; but this explanation would require us to
-assume some direct electrical communication between the comet and the
-Sun. Considering the distance between the two bodies, and the probable
-absence or great tenuity of the gaseous material in interstellar space,
-such an assumption is a difficult one.
-</p>
-<p>
-Under the action of the solar forces, the coma also very frequently
-forms itself into concentric luminous arcs, separated by comparatively
-dark intervals. These luminous semi-circles vary in number, but
-sometimes there are as many as four or five at a time. All great comets
-show these concentric curves more or less, but sometimes only a portion
-is visible, the rest of the coma having a different structure. When
-great comets approach near the Sun, their coma is generally composed of
-two distinct parts, an inner and an outer coma, the inner one being due
-to the luminous jets issuing from the nucleus, which, never extending
-very far, form a distinct, bright zone within the fainter exterior coma.
-</p>
-<p>
-The tails of comets, which are in fact a prolongation of the coma, are
-likewise extremely variable in form. They are sometimes straight like a
-rod; again, are curved like a sabre, or even crooked like an S, as was
-that of the comet of 1769. They are also fan-shaped, pointed, or of the
-same width throughout. Many of these appendages appear longitudinally
-divided through their middle by a narrow, darkish rift, extending from
-the nucleus to the extremity. This peculiarity appears in the comet
-shown on Plate XI. Sometimes the dark rift does not commence near the
-nucleus, but at some distance from it, as I observed in the case of
-comet 1881, III., on June 26th. This dark rift is not a permanent
-feature of a comet's tail, but may be visible one day and not at all the
-next. Comet 1881, III., which had shown a dark rift towards the end of
-June, did not exhibit any such rift during July and August, when, on the
-contrary, its tail appeared brighter in the middle. Coggia's comet,
-which showed so prominent a dark rift in July, 1874, had none on June
-10th. On the contrary, the tail was on that date very bright along its
-middle, as also along each of its edges.
-</p>
-<p>
-The tail of a comet does not invariably point directly away from the
-Sun, as above mentioned, and sometimes the deviation is considerable;
-for instance, the tail of the comet of 1577 deviated 21° from the point
-opposite to the Sun.
-</p>
-<p>
-In general, the tail inclines its extremity towards the regions of space
-which it has just left, always presenting its convex border to the
-regions towards which it is moving. It is also a remarkable fact that
-this convex border, moving first in space, always appears brighter and
-sharper than the opposite one, which is often diffused. From these
-peculiarities it would seem that in moving about the Sun the comets
-encounter some resistance to their motion, from the medium through which
-they pass, and that this resistance is sufficient to curve their tails
-away from the course in which they move, and to crowd their particles
-together on the forward side. It is especially when they approach their
-perihelion, and move more rapidly on a curve of a shorter radius, that
-the comets' tails show the greatest curvature, unless their position in
-regard to the observer prevents their being advantageously seen. The
-tail of Donati's comet presented a fair illustration of this
-peculiarity, its curvature having augmented with the velocity of the
-comet's motion about the Sun. But possibly this phenomenon has another
-cause, and may be found rather in the solar repulsion which acts on
-comets and is not instantaneously propagated throughout their mass.
-</p>
-<p>
-Although, in general, comets have but one tail, it is not very rare to
-see them with multiple tails. The comets of 1807 and 1843 had each a
-double tail; Donati's comet, in 1858, showed several narrow, long
-rectilinear rays, issuing from its abruptly curved tail. The comet of
-1825 had five branches, while that of 1744 exhibited no less than six
-distinct tails diverging from the coma at various angles. In general
-character the multiple and single tails are similar. When a comet has
-two tails, it is not rare for the second to extend in the general
-direction of the Sun, as was the case with the great comet of 1881,
-III., represented on Plate XI. From July 14th to the 21st it exhibited
-quite an extended conical tail, starting obliquely downwards from the
-right side of the coma, and directed towards the Sun. From the 24th of
-July to the 2d of August this secondary tail was exactly opposite in its
-direction from that of the primary tail, and gave to the head a very
-elongated appearance. Comet 1881, IV., also exhibited a secondary
-appendage, not directed towards the Sun, but making an angle of about
-45° with the main tail.
-</p>
-<p>
-These cometary appendages sometimes attain prodigious dimensions. The
-comets of 1680 and 1769 had tails so extended that, after their heads
-had set under the horizon, the extremities of these immense appendages
-were still seen as far up as the zenith. In a single day the tail of the
-comet of 1843 extended 100°, and it was thrust from the comet "as a
-dart of light" to the enormous distance of 48,500,000 miles, and yet of
-this immense appendage nothing was left on the following day. The tail
-of Donati's comet, in 1858, attained a real length of 42,000,000 miles,
-while that of the great comet of 1843 had the enormous length of
-200,000,000 miles. If this last comet had occupied the position of the
-Sun, which it approached very nearly for a moment, the extremity of its
-tail would have extended 60,000,000 miles beyond the orbit of Mars.
-</p>
-<p>
-In some cases the tails of comets have been seen undulating and
-vibrating in a manner similar to the undulations and coruscations of
-light characteristic of some auroras. Many observers report having seen
-such phenomena. The comet of 1769 was traversed by luminous waves and
-pulsations, comparable to those seen in the aurora borealis. I myself
-observed these curious undulations in Coggia's comet in 1874, while the
-head of this object was below the horizon. For an hour the undulations
-rapidly succeeded each other, and ran along the whole length of the
-tail.
-</p>
-<p>
-Some of the brightest comets have shone with such splendor that they
-could be observed easily in full sunshine. Many comets, such as those of
-1577 and 1744, have equaled Sirius and Venus in brilliancy. The great
-comet of 1843, which suddenly appeared in our sky, was so brilliant that
-it was seen by many observers at noon time, within a few degrees from
-the Sun. I remember that I myself saw this remarkable object in the day
-time, with a number of persons, who were gazing at the wonderful
-apparition. So brilliant was this comet, that besides its nucleus and
-head, a portion of its tail was also visible in the day time, provided
-the observer screened his eyes from the full sunlight by standing in the
-shadow of some building.
-</p>
-<p>
-Of all the bodies revolving around the Sun, none have been known to
-approach so near its surface as did the comet of 1843. When it arrived
-at perihelion, the distance from the centre of its nucleus to the
-surface of the Sun's photosphere was only 96,000 miles, while the
-distance from surface to surface was less than 60,000 miles. This comet,
-then, went through the solar atmosphere, and in traversing it with its
-tremendous velocity of 366 miles per second, may very possibly have
-swept through some solar protuberances, many of which attain much higher
-elevations than that at which the comet passed. The comet of 1680 also
-approached quite near the surface of the Sun, and near enough to
-encounter some of the high solar protuberances, its distance at
-perihelion being about two-thirds of the Moon's distance from the Earth.
-The rapidity of motion of the comet of 1843 was such, when it approached
-the Sun, that it swept through all that part of its orbit which is
-situated north of the plane of the ecliptic in a little more than two
-hours, moving in this short time from one node to the other, or 1800.
-</p>
-<p>
-But if some comets have a very short perihelion distance, that of others
-is considerable. Such a comet was that of 1729, whose perihelion
-distance was 383,000,000 miles, the perihelion point being situated
-between the orbits of Mars and Jupiter.
-</p>
-<p>
-While some comets come near enough to the Sun at perihelion to be
-volatilized by its intense heat, others recede so far from it at
-aphelion that they may be said to be frozen. The shortest cometary
-aphelion distance known is that of Encke's comet, whose greatest
-distance from the sun is 388,000,000 miles. But that of the comet of
-1844 is 406,000,000,000 miles from the Sun. The comets of 1863 and 1864
-are so remote in space when they reach their aphelion points that light,
-with its velocity of 185,500 miles a second, would require 171 days in
-the first case, and 230 in the last, to pass from them to the Earth.
-</p>
-<p>
-The period of revolution of different comets also varies immensely.
-While that of Encke's comet is only 3½ years, that of comet 1864, II.,
-is 280,000 years.
-</p>
-<p>
-Among the periodic comets of short period, some have exhibited highly
-interesting phenomena. Encke's comet, discovered in 1818, is remarkable
-for the fact that its period of revolution diminishes at each of its
-successive returns, and consequently this comet, with each revolution,
-approaches nearer and nearer to the Sun. The decrease of the period is
-about 2½ hours at each return. Although the decrease is small, if it go
-on in future as it does at present, the inevitable consequence will be
-that this comet will finally fall into the Sun. This curious phenomenon
-of retardation has been attributed by astronomers to the existence of a
-resisting medium filling space, but so rare and ethereal that it does
-not produce any sensible effect on the movements of the planets. But
-some other causes may retard this comet, as similar retardations have
-not been observed in the case of other periodic comets of short period.
-These, however, are not so near to the Sun, and perhaps our luminary may
-be surrounded by matter of extreme tenuity, which does not exist at a
-greater distance from it.
-</p>
-<p>
-Another of the periodic comets which has exhibited a very remarkable
-phenomenon of transformation is Biela's comet, which divided into two
-distinct parts, moving together in the same direction. When this comet
-was first detected at its return in 1845, it presented nothing unusual,
-but in the early part of 1846 it was noticed by several astronomers to
-be divided into two parts of unequal brightness, forming thus a twin
-comet. At its next return in 1852, the two sister comets were still
-traveling in company, but their distance apart, which in 1846 was
-157,000 miles, had increased to 1,500,000 miles. At the two next returns
-in 1859 and 1865, their position not being very favorably situated for
-observation, the comets were not seen. In 1872 the position should have
-been favorable for observation, and they were consequently searched for,
-but in vain; neither comet was found. An astronomer in the southern
-hemisphere, however, found a comet on the track of Biela's, but
-calculation has shown that the two objects are probably not identical,
-since this comet was two months behind the computed position for
-Biela's. It will be shown in the following chapter that our globe
-probably crossed the orbit of Biela's comet on November 27th, 1872, and
-the phenomena resulting from this passage will be there described.
-</p>
-<p>
-It is seen from these observations that comets may be lost or dissipated
-in space by causes entirely unknown to us. Biela's comet is not the only
-one which has been thus disintegrated. Ancient historians speak of the
-separation of large comets into two or more parts. In 1661 Hevelius
-observed the apparent division of the comet of that year and its
-reduction to fragments. The return of this comet, calculated for 1790,
-was vainly waited for; the comet was not seen.
-</p>
-<p>
-Other comets, whose periods of revolution were well known, have
-disappeared, probably never to return. Such is Lexell's comet, whose
-period was 5⁶⁄₁₀ years; also De Vico's comet, both of which are
-now lost. It is supposed that Lexell's comet, which passed twice very
-near the giant planet Jupiter, had its orbit changed from an ellipse to
-a parabola, by the powerful disturbing influence of this planet, and was
-thus lost from our system. Several other comets, in traveling over their
-different orbits, have approached near enough to Saturn, Jupiter and the
-Earth to have their orbits decidedly altered by the powerful attraction
-of these bodies.
-</p>
-<p>
-But since comets are liable to pass near the planets, and several have
-orbits which approach that of the Earth, it becomes important for us to
-know whether an encounter of such a body with our globe is possible, and
-what would then be the result for us. Although that knowledge would not
-enable us to modify the possibilities of an encounter, yet it is better
-to know the dangers of our navigation through space than to ignore them.
-This question of a collision of the Earth with a comet has been answered
-in different ways, according to the ideas entertained in regard to the
-mass of these bodies. While some have predicted calamities of all kinds,
-such as deluges, conflagrations, or the reduction of the Earth to
-incandescent gases, others have asserted that it would produce no more
-effect than does a fly on encountering a railroad train. In our days
-astronomers entertain very little fears from such an encounter, because
-the probabilities of danger from an occurrence of this sort are very
-slight, the mass of an ordinary comet being so small compared with that
-of our globe. We know with certainty that the Earth has never had an
-encounter with a comet <i>by which it has been transformed into gases</i>,
-at least within the several millions of years during which animal and
-vegetable life have left their marks upon the stony pages of its
-history, otherwise these marks would not now be seen. If, then, such an
-accident has not happened during this long period, the chances for its
-occurring must be very small, so small indeed that they might almost be
-left out of the question. It is true that our globe shows signs of great
-perturbations of its surface, but we have not the slightest proofs that
-they resulted from an encounter with a celestial body. It seems very
-probable that our globe passed through the tail of the comet of 1861,
-before it was first seen on June 29th; but nothing unusual was observed,
-except perhaps some phosphorescent light in the atmosphere, which was
-afterwards attributed to this cause.
-</p>
-<p>
-The density and mass of comets must be comparatively very small. Their
-tails consist of matter of such extreme tenuity that it affects but very
-little the light of the small stars over which they pass. The coma and
-nucleus, however, are not quite so transparent, and may have greater
-masses. On several occasions I have seen the light of stars reduced by
-the interposition of cometary matter, comet 1881, III., presenting
-remarkable cases of this sort. On July 8th, at 10h. 50m., several small
-stars were involved in this comet, one of which passed quite near the
-nucleus through the bright inner coma. At that time the comet was
-greatly disturbed, its nucleus was contracting and enlarging rapidly,
-and becoming bright and again faint in an instant. Every time that the
-nucleus grew larger, the star became invisible, but reappeared the
-moment the nucleus was reduced in size. This phenomenon could not be
-attributed to an atmospheric effect, since, while the nucleus was
-enlarging, a very small inner nucleus was visible within the large
-diffused one, the matter of which had apparently spread over the part of
-the coma in which the star was involved, making it invisible.
-</p>
-<p>
-That the mass of comets is small, is proved by the fact that they have
-sometimes passed near the planets without disturbing them in any
-sensible manner. Lexell's comet, which in 1770 remained four months very
-near Jupiter, did not affect in the least the orbits, or the motions of
-its satellites. The same comet also came within less than 1,500,000
-miles from the Earth, and on this occasion it was calculated that its
-mass could not have been the ¹⁄₅₀₀₀ part of that of our
-globe, since otherwise the perturbations which it would have caused in
-the elements of the Earth's orbit would have been sensible. There was,
-however, no change. If this comet's mass had been equal to that of our
-globe, the length of our year would have been increased by 2h. 47m. The
-comet of 1837 remained four days within 3,500,000 miles of the Earth,
-with no sensible effect.
-</p>
-<p>
-It seems quite difficult to admit that the denser part of a comet
-forming the nucleus is solid, as supposed by some physicists, since it
-is so rapidly contracted and dilated by the solar forces, while the
-comet is yet at a too great distance from the Sun to allow these effects
-to be attributed to solar heat alone. This part of a comet, as indeed
-the other parts, seems rather to be in the gaseous than in the solid
-state; the changes observed in the intensity of its light and in its
-structure may be conceived as due to some solar action partaking of the
-nature of electricity.
-</p>
-<p>
-It has been a question whether comets are self-luminous, or whether they
-simply reflect the solar light. When their light is analyzed by the
-spectroscope, it is found that the nucleus of a comet generally gives a
-continuous spectrum, while the coma and tail give a spectrum consisting
-of several bright diffused bands. The spectrum given by the nucleus is
-rarely bright enough to allow the dark lines of the solar spectrum to be
-discerned upon it; but such lines were reported in the spectrum of comet
-1881, III., a fact proving that this nucleus at least reflected some
-solar light. The nucleus of a comet may be partly self-luminous, and
-either solid, liquid, or composed of incandescent gases submitted to a
-great pressure. As to the coma and tail, they are evidently gaseous, and
-partly, if not entirely, self-luminous, as is proved by the band
-spectrum which they give. The position of these bands, moreover,
-indicates that the luminous gases of which they are composed contain
-carbon. The phenomena of polarization, however, seem to prove that these
-parts of comets also reflect some solar light.
-</p>
-<p>
-No theory so far proposed, to explain comets and the strange phenomena
-they exhibit, seems to have been successful in its attempts, and the
-mystery in which these bodies have been involved from the beginning of
-their apparition, seems to be now nearly as great as ever. It has been
-supposed that their tails have no real existence, but are due to an
-optical illusion. Prof. Tyndall has endeavored to explain cometary
-phenomena by supposing these bodies to be composed of vapors subject to
-decomposition by the solar radiations, and thus made visible, the head
-and tail being an actinic cloud due to such decompositions. According to
-this view, the tails of comets would not consist of matter projected
-into spacer but simply of matter precipitated by the solar rays in
-traversing the cometary nebulosity. The endeavor has also been made to
-explain the various phenomena presented by comets by an electrical
-action of the Sun on the gases composing these objects. Theories taking
-this as a base seem to us to be more likely to lead to valuable results.
-M. Faye, who has devoted much time and learning to this subject, assumes
-a real repulsive force of the Sun, acting inversely to the square of the
-distance and proportionally to the surface, and not to the mass as
-attraction does. He supposes, however, that this repulsive force is
-generated by the solar heat, and not by electricity. Prof. Wm. Harkness
-says that many circumstances seem to indicate that the comets' tails are
-due, in a great measure, to electrical phenomena.
-</p>
-<p>
-The fact that the tails of comets are better defined and brighter on the
-forward side, associated with the other fact that they curve the most
-when their motion is most rapid, sufficiently indicates that these
-appendages are material, and that they either encounter some resistance
-from the medium in which they move, or from a solar repulsion. The
-phenomena of condensation and extension, which I have observed in the
-comets of 1874 and 1881, added to the curious behavior exhibited by the
-jets issuing from the nucleus, seem to indicate the action of electrical
-forces rather than of heat. The main difficulty encountered in the
-framing of a theory of comets consists in explaining how so delicate and
-extended objects as their tails seem to be, can be transported and
-whirled around the Sun at their perihelion with such an enormous
-velocity, always keeping opposite to the Sun, and, as expressed by Sir
-John Herschel, "in defiance of the law of gravitation, nay, even of the
-received laws of motion."
-</p>
-<p>
-To consider the direction of the comets' tails as an indirect effect of
-attraction, seems out of the question; the phenomenon of repulsion so
-plainly exhibited by these objects seems to point to a positive solar
-repulsion, as alone competent to produce these great changes. The
-repulsive action of the Sun on comets' tails might be conceived, for
-instance, as acting in a manner similar to that of a powerful current of
-wind starting from the Sun, and constantly changing in direction, but
-always keeping on a line with the comet. Such a current, acting on a
-comet's tail as if it were a pennant, would drive it behind the nucleus
-just as observed. If it could once be ascertained that the great
-disturbances on comets correspond with the magnetic disturbances on our
-globe and with the display of the auroral light, the electric nature of
-the forces acting so strangely on the comets would be substantially
-demonstrated. I have shown that some of the great disturbances observed
-in the comets of 1874 and 1881 have coincided with auroral displays, and
-it will be shown hereafter that similar displays have also coincided
-with the passage of meteoric showers through our atmosphere. Whether
-these simultaneous phenomena were simple coincidences having no
-connection, or whether they are the result of a common cause, can only
-be ascertained by long continued future observations.
-</p>
-
-<p><br /><br /><br /></p>
-
-<h4><a id="SHOOTING_STARS">SHOOTING-STARS AND METEORS</a>
-<br /><br />
-PLATE XII</h4>
-
-<p>
-While contemplating the heavens on a clear moonless night, we
-occasionally witness the sudden blazing forth of a star-like meteor,
-which glides swiftly and silently across some of the constellations, and
-as suddenly disappears, leaving sometimes along its track a
-phosphorescent trail, which remains visible for a while and gradually
-vanishes. These strange apparitions of the night are called <i>Falling</i>
-or <i>Shooting-stars</i>.
-</p>
-<p>
-There is certainly no clear night throughout the year during which some
-of these meteors do not make their appearance, but their number is quite
-variable. In ordinary nights only four or five will be observed by a
-single person in the course of an hour; but on others they are so
-numerous that it becomes impossible to count them. When the falling
-stars are only a few in number, and appear scattered in the sky, they
-are called <i>Sporadic Meteors</i>, and when they appear in great numbers
-they constitute <i>Meteoric Showers</i> or <i>Swarms</i>.
-</p>
-<p>
-Probably there is no celestial phenomenon more impressive than are these
-wonderful pyrotechnic displays, during which the heavens seem to break
-open and give passage to fiery showers, whose luminous drops describe
-fantastic hieroglyphics in the sky. While observing them, one can fully
-realize the terror with which they have sometimes filled beholders, to
-whom it seemed that the stability of the universe had come to an end,
-and that all the stars of the firmament were pouring down upon the Earth
-in deluges of fire.
-</p>
-<p>
-The ancients have left record of many great meteoric displays, and the
-manner in which they describe them sufficiently indicates the fear
-caused by these mysterious objects. Among the many meteoric showers
-recorded by ancient historians may be mentioned one observed in
-Constantinople, in the month of November, 472, when all the sky appeared
-as if on fire with meteors. In the year 599, meteors were seen on a
-certain night flying in all directions like fiery grasshoppers, and
-giving much alarm to the people. In March, 763, "the stars fell
-suddenly, and in such crowded number that people were much frightened,
-and believed the end of the world had come." On April 10th, 1095, the
-stars fell in such enormous quantity from midnight till morning that
-they were as crowded as are the hail stones during a severe storm.
-</p>
-<p>
-In modern times the fall of the shooting-stars in great number has been
-frequently recorded. One of the most remarkable meteoric showers of the
-eighteenth century occurred on the night of November 13th, 1799, and was
-observed throughout North and South America and Europe. On this
-memorable night thousands of falling stars were seen traversing the sky
-between midnight and morning. Humboldt and Boupland, then traveling in
-South America, observed the phenomena at Cumana, between two and five
-o'clock in the morning. They saw an innumerable number of shooting-stars
-going from north to south, appearing like brilliant fire-works. Several
-of these meteors left long phosphorescent trails in the sky, and had
-nuclei whose apparent diameter, in some cases, surpassed that of the
-Moon.
-</p>
-<p>
-The shower of November 13th, 1833, was still more remarkable for the
-great number of meteors which traversed the heavens, and was visible
-over the whole of North and South America. On that occasion the falling
-stars were far too numerous to be counted, and they fell so thickly that
-Prof. Olmsted, of New Haven, who observed them carefully, compared their
-number at the moment of their maximum fall to half that of the flakes of
-snow falling during a heavy storm. This observer estimated at 240,000
-the number of meteors which must have traversed the heavens above the
-horizon during the seven hours while the display was visible.
-</p>
-
-<p><br /></p>
-
-<div class="figcenter" style="width: 400px;">
-<a id="figure12"></a>
-<br />
-<img src="images/figure12.jpg" width="400" alt="" />
-<div class="caption">
-<p>PLATE XII.&mdash;THE NOVEMBER METEORS.</p>
-<p class="smaller">As observed between midnight and 5 o'clock A.M. on the night
-of November 13-14 1868.</p>
-</div></div>
-
-<p><br /></p>
-
-<p>
-In the years 1866, 1867 and 1868, there were also extraordinary meteoric
-displays on the night of November 13th. It was on the last mentioned
-date that I had the opportunity to observe the remarkable shower of
-shooting-stars of which I have attempted to represent all the
-characteristic points in Plate XII. My observations were begun a little
-after midnight, and continued without interruption till sunrise. Over
-three thousand meteors were observed during this interval of time in the
-part of the sky visible from a northern window of my house. The maximum
-fall occurred between four and five o'clock, when they appeared at a
-mean rate of 15 in a minute.
-</p>
-<p>
-In general, the falling stars were quite large, many being superior to
-Jupiter in brightness and apparent size, while a few even surpassed
-Venus, and were so brilliant that opaque objects cast a strong shadow
-during their flight. A great many left behind them a luminous train,
-which remained visible for more or less time after the nucleus had
-vanished. In general, these meteors appeared to move either in straight
-or slightly curved orbits; but quite a number among them exhibited very
-extraordinary motions, and followed very complicated paths, some of
-which were quite incomprehensible.
-</p>
-<p>
-While some moved either in wavy or zig-zag lines, strongly accentuated,
-others, after moving for a time in a straight line, gradually changed
-their course, curving upward or downward, thus moving in a new
-direction. Several among them, which were apparently moving in a
-straight line with great rapidity, suddenly altered their course,
-starting at an abrupt angle in another direction, with no apparent
-slackening in their motion. One of them, which was a very conspicuous
-object, was moving slowly in a straight course, when of a sudden it made
-a sharp turn and continued to travel in a straight line, at an acute
-angle with the first, retreating, and almost going back towards the
-regions from which it originally came. As nearly all the meteors which
-exhibited these extraordinary motions left the trace of their passage in
-the sky by a luminous trail, it was easily ascertained that these
-appearances were not deceptive. On one occasion I noticed that the
-change of direction in the orbit corresponded with the brightening up of
-the meteor thus disturbed in its progress.
-</p>
-<p>
-Among these meteors, some traveled very slowly, and a few seemed to
-advance as if by jerks, but in general they moved very rapidly. One of
-the meteors thus appearing to move by jerks left a luminous trail, upon
-which the various jerks seemed to be left impressed by a succession of
-bright and faint spaces along the train. Some of the largest meteors
-appeared to rotate upon an axis as they advanced, and most of these
-revolving meteors, as also a great number of the others, seemed to
-explode just before they disappeared, sending bright fiery sparks of
-different colors in all directions, although no sound was at any time
-heard. The largest and most brilliant meteor observed on that night
-appeared at 5h. 30m., a little before sunrise. It was very bright, and
-appeared considerably larger than Venus, having quite a distinct disk.
-This meteor moved very slowly, leaving behind a large phosphorescent
-trail, which seemed to issue from the inside of the nucleus as it
-advanced. For a moment the train increased in size and brightness close
-to the nucleus, which then appeared as an empty transparent sphere,
-sprinkled all over with minute fiery sparks; the nucleus then suddenly
-burst out into luminous particles, which immediately vanished, only the
-luminous trail of considerable dimensions being left.
-</p>
-<p>
-Many of the trails thus left by the meteors retained their luminosity
-for several minutes, and sometimes for over a quarter of an hour. These
-trails slowly changed their form and position; but it is perhaps
-remarkable that almost all those which I observed on that night assumed
-the same general form&mdash;that of an open, irregular ring, or horse-shoe,
-somewhat resembling the letter C. This ring form was subsequently
-transformed into an irregular, roundish cumulus-like cloud. The trail
-left by a very large meteor, which I observed on the evening of
-September 5th, 1880, also exhibited the same general character of
-transformation.
-</p>
-<p>
-While I was observing a long brilliant trail left by a meteor on the
-night of November 13th, 1868, it was suddenly crossed by another bright
-shooting-star. The latter apparently went through the luminous substance
-forming the trail, which was suddenly altered in form, and considerably
-diminished in brightness simultaneously with this passage, although
-electrical action at some distance might perhaps as well explain the
-sudden change observed.
-</p>
-<p>
-In the majority of cases the meteors appeared white; but many,
-especially the largest, exhibited a variety of brilliant colors, among
-which the red, blue, green, yellow and purple were the most common. In
-general the trails exhibited about the same color as the nucleus, but
-much fainter, and they were usually pervaded by a greenish tint. In some
-instances the trails were of quite a different color from the nucleus.
-</p>
-<p>
-The luminous cloud observed at 5h. 30m. on the morning of November 14th,
-1868, after having passed through the series of transformations above
-described, remained visible for a long while after sunrise, appearing
-then as a small cirrus cloud, exactly similar in appearance to the
-hundreds of small cirrus clouds then visible in the sky, which had
-probably the same meteoric origin. For over three hours after sunrise,
-these cirrus clouds remained visible in the sky, moving all together
-with the wind in the high regions of the atmosphere.
-</p>
-<p>
-Although Plate XII. is intended to represent all the characteristics
-exhibited by the meteors observed on that night, every form represented
-having been obtained by direct observation, yet the number is much
-greater than it was at any single moment during the particular shower of
-1868. As regards number, the intention was to give an idea of a great
-meteoric shower, such as that of 1833, for instance. Although many of
-the falling stars seem to be close to the Earth's surface, yet this is
-only an effect of perspective due to their great distance, very few of
-these meteors ever coming into the lower regions of our atmosphere at
-all.
-</p>
-<p>
-The phenomena exhibited during other great meteoric showers have been
-similar to those presented by the shower just described, the only
-differences consisting in variations of size and brightness in the
-meteors, and also in the trails, which sometimes are not so numerous as
-they were in 1868.
-</p>
-<p>
-While some shooting-stars move so rapidly that they can hardly be
-followed in their orbits, others move so slowly that the sight can
-easily follow them, and even remark the peculiarities of their
-movements, some remaining visible for half a minute. Some of the falling
-stars move at the rapid rate of 100 miles a second, but others only 10
-miles a second, and even less. In general, they move about half as fast
-again as the Earth in its orbit. The arcs described by the meteors in
-the sky are variable. While some extend 8o° and even 100°, others are
-hardly half a degree in length. While some shooting-stars are so faint
-that they can hardly be seen through the largest telescopes, others are
-so large and brilliant that they can be seen in the day-time. In
-general, a shooting-star of average brightness resembles a star of the
-third or fourth magnitude.
-</p>
-<p>
-Whatever may be the origin of the shooting-stars, they are, when we see
-them, not in the celestial spaces, like the planets, the comets, or the
-stars, but in our atmosphere, through which they travel as long as they
-remain visible. The height at which they appear and disappear is
-variable, but in general they are about 80 miles above the surface of
-our globe when they are first seen, and at about 55 miles when they
-disappear. In many cases, however, they have been observed at greater
-elevations, as also at smaller. A meteor simultaneously observed at two
-different stations first appeared at the height of 285 miles, and was
-last seen at 192 miles above the Earth's surface; but in rare cases the
-falling stars have been seen below a layer of clouds completely covering
-the sky. I myself saw one such shooting-star a few years since. The fact
-that the meteors are visible at so great elevations, proves that our
-atmosphere extends much farther than was formerly supposed, although at
-these great heights it must be extremely rarefied, and very different
-from what it is in its lower regions.
-</p>
-<p>
-There is a remarkable difference between the sporadic meteors seen in
-the sky on every night, and the meteoric showers observed only at
-comparatively rare intervals. While the first appear from different
-points in the sky and travel in all directions, being perfectly
-independent, the meteors of a shower all come from the same point of the
-heavens, from which they apparently diverge in all directions. This
-point of divergence of the meteors is called the <i>radiant point</i> of
-the shower. Although the meteors seem to diverge in all directions from the
-radiant point, yet they all move in approximately parallel lines, the
-divergence being an effect of perspective.
-</p>
-<p>
-Whatever may be the position of the radiant point in the constellations,
-it remains as fixed in the sky as the stars themselves, and participates
-with them in the apparent motion which they undergo by the effect of the
-diurnal motion, and thus rises and sets with the constellation to which
-it belongs. This fact is sufficient to prove that the orbits of these
-meteors are independent of the Earth's motion, and that consequently
-they do not originate in our atmosphere. It has been shown by Encke that
-the radiant point of the meteoric shower of November 13th is precisely
-the point towards which our globe moves in space on November 13th; a
-tangent to the Earth's orbit would pass through this radiant point.
-</p>
-<p>
-The meteoric showers are particularly remarkable, not merely because of
-the large number of meteors which are visible and the fact that they all
-follow a common orbit, but chiefly because they have a periodic return,
-either after an interval of a year, or after a lapse of several years.
-At the beginning of the present century only two meteoric showers were
-known, those of August 10th and of November 13th, and their periodicity
-had not yet been recognized, although it had begun to be suspected. It
-was only in 1836 that Quetelet and Olbers ventured to predict the
-reappearance of the November meteors in the year 1867. Having made
-further investigations, Prof. Newton, of Yale College, announced their
-return in the year 1866. In both of these years, as also in 1868, the
-meteors were very numerous, and were observed in Europe and in America
-on the night of November 13th. The predictions having thus been
-fulfilled, the periodicity of the meteors was established. Since then,
-other periodic showers have been recognized, although they are much less
-important in regard to number than those of August and November, except
-that of November 27th, which exhibited so brilliant a display in Europe
-in 1872. These successive appearances have established the main fact
-that meteoric showers are more or less visible every year when the Earth
-occupies certain positions in its orbit.
-</p>
-<p>
-The meteoric shower of the 10th of August has its radiant point situated
-in the vicinity of the variable star Algol, in the constellation
-Perseus, from which its meteors have received the name of Perseids.
-Although varying in splendor, this meteoric swarm never fails to make
-its appearance every year. The Perseids move through our atmosphere at
-the rate of 37 miles per second. The shower usually lasts about six
-hours.
-</p>
-<p>
-The meteoric shower of November 13th has its radiant point situated in
-the vicinity of the star Gamma, in the constellation Leo, from which its
-meteors have been called Leonids. But while the August meteors recur
-regularly every year, with slight variations, the shower of November
-does not occur with the same regularity. During several years it is
-hardly noticeable, and is even totally absent, while in other years it
-is very remarkable. Every 33 years an extraordinary meteoric shower
-occurs on the 13th of November, and the phenomenon is repeated on the
-two succeeding years at the same date, but with a diminution in its
-splendor at each successive return. The Leonids move in an opposite
-direction to that of the Earth, and travel in our atmosphere with an
-apparent velocity of 45 miles per second, this being about the maximum
-velocity observed in falling stars. But when the motion of our globe is
-taken into account, and a deduction is made of the 18 miles which it
-travels per second, it is found that these meteors move at an actual
-mean rate of 27 miles a second.
-</p>
-<p>
-In a meteoric shower the stars do not fall uniformly throughout the
-night, there being a time when they appear in greater numbers. Usually
-it is towards morning, between 4 and 6 o'clock, that the maximum occurs.
-The probable cause of this phenomenon will be explained in its place
-hereafter.
-</p>
-<p>
-The orbits of the meteoric showers are not all approximately in the same
-plane, like those of the planets, but rather resemble those of comets,
-and have all possible inclinations to the ecliptic. Like the comets,
-too, the different meteoric showers have either direct or retrograde
-motion.
-</p>
-<p>
-The shooting-stars were formerly considered as atmospheric meteors,
-caused by the combustion of inflammable gases generated at the surface
-of the Earth, and transported to the high regions of our atmosphere by
-their low specific gravity. But the considerable height at which they
-usually appear, the great velocity of their motion, the common orbit
-followed by the meteors of the same shower, and the periodicity of their
-recurrence, do not permit us now to entertain these ideas, or to doubt
-their cosmical origin. But what is their nature?
-</p>
-<p>
-It is now generally admitted that innumerable minute bodies, moving in
-various directions around the Sun, are scattered in the interplanetary
-spaces through which our globe travels. It has been supposed that
-congregations of such minute bodies form elliptical rings, within which
-they are all moving in close parallel orbits around the Sun. On the
-supposition that such rings intersect the orbit of the Earth at the
-proper places, it was practicable to account for the shooting-stars by
-the passage through our atmosphere of the numerous minute cosmical
-bodies composing the rings, and the Leonid and Perseid showers were so
-explained. But when the elements of the orbits of these two last swarms
-came to be better known, and were compared with those of other celestial
-bodies, it was found necessary to alter this theory.
-</p>
-<p>
-It had for a long while been suspected that some kind of relation
-existed between the shooting-stars and the comets. This idea, vaguely
-formulated by Kepler more than two centuries ago, more clearly expressed
-by Chladni, and still more by Mr. Grey, before the British Association,
-at Liverpool, in 1855, has recently received a brilliant confirmation by
-the researches of Professor Schiaparelli, Director of the Observatory of
-Milan. A thorough investigation of the orbits of the August and November
-meteors led Schiaparelli to the discovery of a remarkable relation
-between meteoric and cometary orbits. By comparing the elements of these
-meteoric orbits with those of comets, he found a very close resemblance
-between the orbit of the August meteors and that of the comet 1862,
-III., and again between the orbit of the November meteors and that of
-Tempel's comet, 1866, I. These resemblances were too striking to be the
-result of mere chance, and demonstrated the identity of these cometary
-orbits with those of the Perseid and Leonid showers. In accordance with
-these new facts, it is now admitted that the meteoric showers result
-from the passage of our globe through swarms of meteoric particles
-following the orbits of comets, which intersect the orbit of the Earth.
-</p>
-<p>
-Professor Schiaparelli has attempted to show how these meteoric swarms
-were originally scattered along the orbits of comets, by supposing these
-bodies to originate from nebulous masses, which, in entering the sphere
-of attraction of the Sun, are gradually scattered along their orbits,
-and finally form comets followed by long trails of meteoric particles.
-</p>
-<p>
-It has been shown that in approaching the Sun the comets become
-considerably elongated, their particles being disseminated over immense
-distances by the solar repulsion. It seems probable that, owing to its
-feeble attractive power, the nucleus is incompetent to recall the
-scattered cometary particles and retain them in its grasp when they are
-relieved from the solar repulsion, so that they remain free from the
-nucleus, although they continue to move along its orbit. It is
-supposable that these cometary particles will scatter more and more in
-course of time. Forming at first an elongated meteoric cloud, they will
-finally spread along the whole orbit, and thus form a ring of meteoric
-particles. Since our globe constantly moves in its orbit and daily
-occupies a different position, it follows that at any point where such a
-cometary orbit happens to cross that of the Earth, our globe will
-necessarily encounter the cometary particles as a shower of meteors.
-This encounter will take place at a certain time of the year, either
-yearly, if they form a continuous ring, or after a succession of years,
-if they simply form an elongated cloud. Such meteoric clouds or rings
-would not be visible in ordinary circumstances, even through the largest
-telescopes, except on penetrating the upper regions of our atmosphere,
-when they would appear as showers of falling stars. It is supposed that
-in penetrating our atmosphere, even in its most rarefied regions, these
-meteors are heated by the resistance offered by the air to their motion,
-first becoming luminous and then being finally vaporized and burnt
-before they can reach the surface of the Earth.
-</p>
-<p>
-The orbit of the comet of 1862, III., which so closely corresponds with
-that of the Perseid meteors, is much more extended than that of Tempel's
-comet corresponding with that of the Leonids. While the first extends
-far beyond the orbit of Neptune, the latter only goes a little beyond
-that of Uranus. The former orbit makes a considerable angle with the
-plane of the Earth's orbit, but the latter is much nearer to parallelism
-with it. The period of revolution of the first is 108 years, and that of
-the last about 33¼ years.
-</p>
-<p>
-From the fact that the Perseid shower occurs yearly on the 10th of
-August, when the Earth crosses the orbit of the comet of 1862, III., it
-is supposed that the cometary particles producing this shower are
-disseminated along the whole orbit, and form a ring encircling the Sun
-and Earth. To explain the yearly variations in the number of the
-shooting-stars observed, these particles are supposed to be unequally
-distributed over the orbit, being more crowded at one place than they
-are at another. In order to explain the meteoric shower of Leonids,
-which appears in all its splendor every 33 years, and then with
-diminished intensity for two successive years, after which it is without
-importance, it is supposed that the cometary particles of the comet of
-1866, I., have not as yet spread all along the orbit, a sufficient time
-not having been allowed, but form an elongated meteoric cloud, more
-dense in its front than in its rear part. From these considerations it
-has been supposed also that the comet of 1866, I., is of a more recent
-date than that of 1862, III. While Tempel's comet makes its revolution
-around the Sun in about 33 years, this meteoric cloud, which has the
-same period and returns to the same point of its orbit every 33 years,
-encounters our globe for three successive years. The first year we are
-passing through its densest parts, and the two following years in less
-and less crowded parts, from which result the observed phenomena. An
-idea of the extent of this meteoric cloud may be formed from the fact
-that, with its cometary velocity of motion, it takes this cloud three
-years at least to cross the Earth's orbit. From recent researches it
-would appear that the Leonid cloud is not single, but that at least two
-others of smaller importance exist, and have periods of 33¼ years.
-</p>
-<p>
-Biela's comet, which was divided into two parts in 1846, is another of
-the few comets whose orbit approaches that of the Earth. Possessing this
-knowledge, and knowing then the close connection existing between
-meteors and comets, astronomers supposed that there were sufficient
-reasons to expect a meteoric shower when this comet was passing near the
-Earth. They consequently expected a meteoric display in 1872, when our
-globe was to cross its orbit. Their anticipation was plainly fulfilled,
-and on the night of November 27th, 1872, a splendid meteoric display,
-having its radiant point in the constellation Andromeda, was observed in
-Europe, and also in America, but the meteors seen here were not so
-numerous as in Europe. Other meteoric showers of less importance, such
-as that of April 20th, for instance, have also been identified with
-cometary orbits, so that now no doubt seems to remain as to the identity
-of cometary particles and shooting-stars.
-</p>
-<p>
-The fact that the maximum number of meteors is always observed in the
-morning hours, supports the hypothesis of the cosmic origin of the
-shooting-stars, since the regions of the Earth where it is morning are
-precisely those fronting the regions towards which our globe is moving
-in space, and accordingly encounter more directly the meteors moving in
-their orbit. The greater abundance of falling stars at that time may
-thus be accounted for.
-</p>
-<p>
-The number of meteors penetrating our atmosphere must be very great;
-there is not an hour and probably not a minute during which none fall.
-From various considerations, some astronomers have estimated at from
-65,000,000,000 to 146,000,000,000 the total number of shooting-stars
-yearly penetrating in our atmosphere. The actual number is undoubtedly
-great, yet the fact that the meteors are rarely seen through the
-telescope while employed in observing various celestial objects, does
-not indicate that they are so numerous as these figures imply. It is
-only occasionally that one is seen traversing the field of the
-instrument. Even when the sky is observed with a low power eye-piece for
-several hours in succession, many nights may pass without disclosing
-one, although an observer, sweeping the sky more freely with the naked
-eye, may often perceive four or five during an ordinary night.
-</p>
-<p>
-About the true nature of these bodies nothing is known with certainty.
-From spectrum analysis it seems to be established that most of them
-contain sodium and magnesium, while a few indicate the presence of
-strontium and iron, and in some rare cases there are traces of coal-gas.
-Some of the nuclei give a continuous spectrum, and others a spectrum of
-lines. The trail always gives a spectrum of bright lines which indicates
-its gaseous state. The traces of coal-gas rarely seen in meteors are,
-however, of great importance, as it identifies them more closely with
-the comets, which generally show a similar spectrum. The continuous
-spectra exhibited by some nuclei would indicate that they are
-incandescent and either solid or liquid; but it is difficult to conclude
-from their spectra what is their true nature, since we do not know
-exactly what part the terrestrial atmosphere may play in producing the
-results.
-</p>
-<p>
-The mass of the shooting-stars is not known with certainty, but the fact
-that during great meteoric showers, none are seen to reach the surface
-of the Earth, all being consumed in a few seconds, sufficiently
-indicates that it must be very small. It has been calculated that those
-equal to Venus in apparent size and brilliancy may weigh several pounds,
-while the faint ones would weigh only a few grains.
-</p>
-<p>
-If the shooting-stars have even such a mass as that here attributed to
-some of them, the extraordinary motions which I have described above
-seem to be unaccountable. The change of direction of a heavy mass moving
-swiftly cannot be sudden. The semi-circular, the wavy and the angular
-orbits observed could not be described, it would seem, by such a mass
-animated with a great velocity. Although the meteors are said to be
-ignited by the transformation of part of their progressive motion into
-molecular motion, yet it is not observed that the velocity of the
-falling stars diminishes when they are about to disappear. The luminous
-trails they leave in the atmosphere do not appear to be endowed with any
-motion, but remain for a time in their original positions. These facts
-are apparently opposed to the hypothesis that such meteors have any
-appreciable mass. The extraordinary motions exhibited by some meteors
-seem to indicate that some unsuspected force resides in these bodies,
-and causes them to deviate from the laws of ordinary motion.
-</p>
-<p>
-Although it is very probable that the ordinary shooting-stars have no
-appreciable mass, yet it is known that very heavy meteoric masses
-sometimes fall at the surface of the Earth. Such falls are generally
-preceded by the sudden apparition in the sky of a large, and usually
-very brilliant fire-ball, which traverses the air at a great speed,
-sometimes leaving behind it a luminous trail, after which it explodes
-with a loud sound, and heavy fiery meteoric fragments, diverging in all
-directions, fall at the surface of the Earth. The name of
-<i>Aerolites</i> or <i>Meteorolites</i> is given to these ponderous
-fragments. As these meteors, before they explode and fall to the ground,
-have many points of resemblance with the shooting-stars, they are
-generally supposed to be connected with them, and to have a similar
-cometary origin. The fact that the aerolites differ widely from each
-other in constitution, and are all composed of substances found on the
-Earth, associated with other facts given below, would rather seem to
-indicate a terrestrial than a celestial origin.
-</p>
-<p>
-If the aerolites belong to the same class of bodies as the falling
-stars, differing from them only in size and mass, it is difficult to see
-why so very few should fall upon the Earth during the great meteoric
-showers, when thousands of shooting-stars traverse our atmosphere. In
-Prof. Kirkwood's "Meteoric Astronomy" are given catalogues of all the
-falls of aerolites and fire-balls which have been observed at the time
-of the periodic meteoric showers of the 10th of August and the 13th of
-November, during a period of 221 years for the Perseids, or August
-showers, and of 318 years for the Leonids, or November showers. During
-221 years, 10 falls of aerolites have been witnessed simultaneously with
-the fall of the Perseids; while during 318 years, only 4 such falls have
-been recorded as having occurred at the time of the Leonid shower. If
-there is any close connection between the shooting-stars and the
-aerolites, we should expect to find a maximum in their fall at the time
-of the great meteoric displays. So far, no maxima or minima have yet
-been discovered in the fall of aerolites; they do not seem, like
-meteoric showers, to be governed by a law of periodicity.
-</p>
-<p>
-A very remarkable peculiarity of the aerolites is that they seem to have
-a tendency to fall in certain regions. Such are the southern part of
-France, the north of Italy, Hindostan, the central states of North
-America, and Mexico and Brazil. There is a curious contrast existing
-between the quick cometary motion of the aerolites before their
-explosion, and the comparatively slow motion of their fragments as they
-reach the Earth; motion which seems to be no greater than that
-corresponding to their natural fall impeded by the resistance of the
-air. In general, their penetration into the soil upon which they fall
-does not at all correspond to the great velocity with which they move in
-the atmosphere. The fragmentary structures of the aerolites, their
-identity of substance with that of our globe, their great resemblance to
-the volcanic minerals of the Earth, and the fractures and faults which
-some of them exhibit, do not correspond at all with the idea that they
-are cometary particles fallen on the Earth. As far as their structure
-and appearance is concerned, they seem rather to be a volcanic product
-of the interior of the Earth than parts of disintegrated comets. It must
-be admitted that their identity with the shooting-stars is far from
-established, and that they are still involved in mystery.
-</p>
-<p>
-The so-called meteoric dust gathered at sea and on high mountains may
-have various origins, and may be partly furnished by volcanic dust
-carried to great distances in the atmosphere.
-</p>
-<p>
-Since millions of shooting-stars penetrate our atmosphere every year and
-remain in it, becoming definitively a part of the Earth, it follows
-that, no matter how small may be the quantity of matter of which they
-are composed, they must gradually increase the volume and mass of our
-globe, although the increase may be exceedingly slow. Supposing every
-one of the shooting-stars penetrating our atmosphere to contain one
-cubic millimeter of matter, it has been calculated that it would take
-nearly 35,000 years to make a deposit one centimeter in thickness all
-over the surface of our globe. Insignificant as this may appear, it is
-probable that the quantity of matter of meteoric origin which is added
-to our globe is much less than has just been supposed.
-</p>
-
-<p><br /><br /><br /></p>
-
-<h4><a id="THE_MILKY_WAY">THE MILKY-WAY OR GALAXY</a>
-<br /><br />
-PLATE XIII</h4>
-
-<p>
-During clear nights, when the Moon is below the horizon, the starry
-vault is greatly adorned by an immense belt of soft white light,
-spanning the heavens from one point of the horizon to the opposite
-point, and girdling the celestial sphere in its delicate folds. Every
-one is familiar with this remarkable celestial object, called the
-<i>Milky-way</i> or <i>Galaxy</i>.
-</p>
-<p>
-Seen with the naked eye, the Galaxy appears as an irregular, narrow,
-nebulous belt, apparently composed of cloud-like luminous masses of
-different forms and sizes, separated by comparatively dark intervals.
-These cloud-like masses vary much in luminous intensity, and while some
-among them are very bright and conspicuous, others are so faint that
-they are hard to recognize. In general, the brightest parts of the
-Milky-way are situated along the middle of its belt, while its borders,
-which are usually very faint, gradually vanish in the sky. Some parts of
-the Galaxy, however, show very little of the cloudy structure so
-characteristic of other parts, being almost uniform throughout, except
-towards the borders, which are always fainter. These parts showing
-greater uniformity are also the faintest.
-</p>
-<p>
-Such is the general appearance of the Milky-way on ordinary nights, but
-on rare occasions, when the atmosphere is particularly pure, it presents
-one of the grandest sights that can be imagined. At such favorable
-moments I have seen the Galaxy gleaming with light, and appearing as if
-composed of star-dust or of precious stones. The strange belt then
-appeared all mottled over and fleecy, its large cloud-like masses being
-subdivided into numerous small, irregular cloudlets of great brilliancy,
-which appeared projected upon a soft luminous background.
-</p>
-<p>
-The width of the Galaxy is far from being uniform; while in some places
-it is only 4° or 5°, in others it is 15° and even more. In some
-places it appears wavy in outline, at others quite straight; then it
-contracts, to expand a few degrees distant; while at other places it
-sends off branches and loops, varying in form, size and direction, some
-of which are quite prominent, while others are very faint.
-</p>
-<p>
-Although very irregular in form, the general appearance of the galactic
-belt is that of a regular curve occupying one of the great circles of
-the celestial sphere. The Milky-way completely encircles the heavens,
-but, of course, only one-half is visible at any one moment, since our
-globe prevents the other half from being seen. If, for a moment, we
-imagine ourselves left in space, our globe having vanished from under
-our feet, we should then see the whole Galaxy forming a continuous belt
-in the heavens, at the centre of which we should apparently be situated.
-</p>
-<p>
-While only one-half of the galactic belt can be seen at once from any
-point on the Earth, yet, according to the position of the observer, a
-larger or smaller portion of the whole can be seen at different times.
-In high northern or southern latitudes but little more than half can be
-seen even by continuous observations; but as we approach the equatorial
-regions, more and more of it becomes visible, until the whole may be
-seen at different hours and seasons. In the latitudes of the northern
-states, about two-thirds of the Galaxy is visible, the rest remaining
-hidden below the horizon; but from the southern states very nearly the
-whole can be seen. The half of the Milky-way visible at any one time
-from any latitude on the Earth never entirely sets below the horizon,
-although in some places it may be so near the horizon as to be rendered
-invisible by vapors. In the latitude of Cambridge, when in its lowest
-position, the summit of its arc is still about 12° or 15° above the
-northern horizon. The great circle of the celestial sphere, occupied by
-the galactic belt, is inclined at an angle of about 63° to the
-celestial equator, and intersects this great circle on one side in the
-constellation Monoceros in 6h. 47m., and on the opposite side in the
-constellations Aquila and Ophiuchus in 18h. 47m. of right ascension; so
-that its northern pole is situated in the constellation Coma Berenices
-in R. A. 12h. 47m., declination N. 27°, and the southern in the
-constellation Cetus in R. A. 0h. 47m., declination S. 27°.
-</p>
-<p>
-According to the seasons and to the hours of the night at which it is
-observed, the galactic arch presents different inclinations in the sky.
-Owing to its inclination to the equator of the celestial sphere, its
-opposite parts exhibit opposite inclinations when they pass the meridian
-of a place. That part of the Galaxy which is represented on Plate XIII.,
-and which intersects the celestial equator in the constellation Aquila,
-is inclined to the left or towards the east, when it is on the meridian;
-while the opposite part, situated in Monoceros, is inclined to the
-right, or towards the west, when it reaches the meridian. The former
-passes the meridian in the evening in the summer and autumn months; the
-latter, in the winter and spring months.
-</p>
-
-<p><br /></p>
-
-<div class="figcenter" style="width: 400px;">
-<a id="figure13"></a>
-<br />
-<img src="images/figure13.jpg" width="400" alt="" />
-<div class="caption">
-<p>PLATE XIII.&mdash;PART OF THE MILKY WAY.</p>
-<p class="smaller">From a study made during the years 1874, 1875 and 1876</p>
-</div></div>
-
-<p><br /></p>
-
-<p>
-By beginning at its northernmost part, represented at the upper part of
-Plate XIII., situated in "the chair" of the constellation Cassiopeia,
-and descending southwardly, and continuing in the same direction until
-the whole circle is completed, the course of the Milky-way through the
-constellations may be briefly described as follows: From Cassiopeia's
-chair, the Galaxy, forming two streams, descends south, passing partly
-through Lacerta on the left, and Cepheus on the right; at this last
-point it approaches nearest to the polar star. Then it enters Cygnus,
-where it becomes very complicated and bright, and where several large
-cloudy masses are seen terminating its left branch, which passes to the
-right, near the bright star Deneb, the leader of this constellation.
-Below Deneb, the Galaxy is apparently disconnected and separated from
-the northern part by a narrow, irregular dark gap. From this rupture,
-the Milky-way divides into two great streams separated by an irregular
-dark rift. An immense branch extends to the right, which, after having
-formed an important luminous mass between the stars <i>γ</i> and <i>β</i>,
-continues its southward progress through parts of Lyra, Vulpecula,
-Hercules, Aquila and Ophiuchus, where it gradually terminates a few
-degrees south of the equator. The main stream on the left, after having
-formed a bright mass around <i>ε</i> Cygni, passes through Vulpecula and
-then Aquila, where it crosses the equinoctial just below the star <i>η</i>
-after having involved in its nebulosity the bright star Altair, the
-leader of Aquila. In the southern hemisphere the Galaxy becomes very
-complicated and forms a succession of very bright, irregular masses, the
-upper one being in Scutum Sobieskii, while the others are respectively
-situated in Sagittarius and in Scorpio; the last, just a little above
-our horizon, being always considerably dimmed by vapors. From Scutum
-Sobieskii, the Galaxy expands considerably on the right, and sends a
-branch into Scorpio, in which the fiery red star Antares is somewhat
-involved.
-</p>
-<p>
-Continuing its course below our horizon, the Milky-way enters Ara and
-Norma, and then, passing partly through Circinus, Centaurus and Musca,
-it reaches the Southern Cross, after having been divided by the large
-dark pear-shaped spot known to navigators as the "Coal-Sack." In Ara and
-Crux the Milky-way attains its maximum of brightness, which there
-surpasses its brightest parts in Cygnus. In Musca, it makes its nearest
-approach to the south pole of the heavens. It then enters Carina and
-Vela, where it spreads out like a fan, and terminates in this last
-constellation, before reaching <i>λ</i>, being once more interrupted by a
-dark and very irregular gap, on a line with the two stars <i>γ</i> and
-<i>λ</i>. It is noteworthy that this second rupture of continuity of the
-Galaxy in Vela is very nearly opposite, or at about 18o° from the break
-near Deneb in Cygnus.
-</p>
-<p>
-Continuing its course on the other side of the break, the Milky-way
-again spreads out into the shape of a fan, grows narrower in entering
-Puppis, where it is longitudinally divided by darkish channels. It then
-passes above our southern horizon, becoming visible to us, passing
-through part of Canis Major, where its border just grazes the brilliant
-star Sirius. But from Puppis it gradually diminishes in brightness and
-complication, becoming faint and uniform. It enters Monoceros and Orion,
-where it again crosses the equator a little above <i>δ</i>, the
-northernmost of the three bright stars in the belt of Orion. Continuing
-its northward course it passes through Gemini, extending as far as
-Castor and Pollux, and then entering Auriga, where it begins to increase
-in brightness and in complication of structure. It passes partly through
-Camelopardus and into Perseus, where an important branch proceeds from
-its southern border.
-</p>
-<p>
-This branch beginning near the star <i>θ</i>, advances towards the
-celebrated variable star Algol, around which it is quite bright and
-complicated. Continuing its course in the same direction, the branch
-rapidly loses its brightness, becoming very faint a little below Algol,
-and passing through <i>ζ</i> Persei, it enters Taurus, leaving the
-Pleiades on its extreme southern margin; and after having passed through
-<i>ε</i> where it branches off, it rapidly curves towards the main
-stream, which it joins near <i>ζ</i> Tauri, thus forming an immense
-loop. The ramification projecting near <i>ε</i> Tauri involves in its
-nebulosity the ruddy star Aldebaran and the scattered group of the
-Hyades. It then advances towards the three bright stars <i>δ</i>,
-<i>ε</i> and <i>ζ</i> of the belt of Orion, which, together with the
-sextuple star <i>θ</i> Orionis, are involved in its faint nebulosity,
-and joins the main stream on the equinoctial, having thus formed a
-second loop, whose interior part is comparatively free from nebulosity,
-and contains the fine stars Betelgeuse and Bellatrix.
-</p>
-<p>
-That portion of the main galactic stream which is comprised between the
-star Deneb in Cygnus, and Capella in Auriga, is divided longitudinally
-by a very irregular, narrow, darkish cleft, comparatively devoid of
-nebulosity, which, however, is interrupted at some points. This dark gap
-sends short branches north and south, the most important of which are
-situated near <i>ζ</i> Cephei and <i>β</i> Cassiopeiæ. Another branch
-runs from <i>γ</i> beyond <i>ε</i> of the constellation last
-mentioned. The main stream of the Galaxy after leaving Perseus, enters
-Cassiopeia, and sending short branches into Andromeda, it completes its
-immense circle in Cassiopeia's chair, where this description was begun.
-</p>
-<p>
-When examined through the telescope, the appearance of the Milky-way
-completely changes, and its nebulous light is resolved into an immense
-number of stars, too faint to be individually seen with the naked eye.
-When Galileo first directed the telescope to the galactic belt, its
-nebulous, cloud-like masses were at once resolved into stars, even by
-the feeble magnifying power of his instrument. When, much later, Sir
-William Herschel undertook his celebrated star-gaugings of the Galaxy,
-millions of stars blazed out in his powerful telescopes. The stars
-composing this great nebulous belt are so numerous that it is impossible
-to arrive at any definite idea as to their number. From his soundings
-Herschel estimated at 116,000 the number of stars which, on one
-occasion, passed through the field of his telescope in 15 minutes, by
-the simple effect of the diurnal motion of the heavens; and on another
-occasion, a number estimated at 250,000 crossed the field in 41 minutes.
-In a space of 50, comprised between <i>β</i> and <i>γ</i> Cygni, shown
-on Plate XIII., he found no less than 331,000 stars. Prof. Struve has
-estimated at 20,500,000 the number of stars seen in the Milky-way
-through the twenty-foot telescope employed by Herschel in his
-star-gaugings. Great as this number may seem, it is yet far below the
-truth; as the great modern telescopes, according to Professor Newcomb,
-would very probably double the number of stars seen through Herschel's
-largest telescope, and detect from thirty to fifty millions of stars in
-the Milky-way.
-</p>
-<p>
-Although the telescope resolves the Galaxy into millions of stars, yet
-the largest instruments fail to penetrate its immense depths. The
-forty-foot telescope of Herschel, and even the giant telescope of Lord
-Rosse, have failed to resolve the Milky-way entirely into stars, the
-most distant ones appearing in them as nebulosities upon which the
-nearer stars are seen projected, the galactic stratum being unfathomable
-by the largest telescopes yet made.
-</p>
-<p>
-The stars composing the Milky-way are very unevenly distributed, as
-might easily be supposed from the cloud-like appearance of this belt. In
-some regions they are loosely scattered, forming long rows or streams of
-various figures, while in others they congregate into star groups and
-clusters having all imaginable forms, some being compressed into very
-dense globular masses. The intervals left between the clustering masses
-are poorer in stars, and indeed some of them are even totally devoid of
-stars or nebulosity. Such are the great and small "coal-sacks" in the
-southern Galaxy. I have myself detected such a dark space devoid of
-stars and nebulosity in one of the brightest parts of the Milky-way, in
-the constellation Sagittarius, in about 17h. 45m. right ascension, and
-27° 35' south declination. It is a small miniature coal-sack or opening
-in the Galaxy, through which the sight penetrates beyond this great
-assemblage of stars. Close to this, is another narrow opening near a
-small, loose cluster.
-</p>
-<p>
-Although lacking the optical resources which now enable us to recognize
-the structure of the Milky-way, some of the ancient philosophers had
-succeeded tolerably well in their speculations regarding its nature. It
-was the opinion of Democritus, Pythagoras and Manilius, that the Galaxy
-was nothing else but a vast and confused assemblage of stars, whose
-faint light was the true cause of its milky appearance.
-</p>
-<p>
-Before the invention of the telescope, no well-founded theory in regard
-to the structure of the Milky-way could, of course, be attempted.
-Although Kepler entertained different ideas in regard to the structure
-of this great belt from those now generally admitted, yet in them may be
-found the starting point of the modern conception of the structure of
-the Galaxy and of the visible universe. In the view of this great mind,
-the Milky-way, with all its stars, formed a vast system, the centre of
-which, and of the universe, was occupied by our Sun. Kepler reasoned
-that the place of the Sun must be near the centre of the galactic belt,
-from the fact this last object appears very nearly as a great circle of
-the celestial sphere, and that its luminous intensity is about the same
-in all its parts.
-</p>
-<p>
-Half a century later, another attempt to explain the Milky-way was made
-by Wright, of Durham, who rejected the idea of an accidental and
-confused distribution of the stars as inconsistent with the appearance
-of the Galaxy, and regarded them as arranged along a fundamental plane
-corresponding to that of the Milky-way. These ideas which were
-subsequently developed and enlarged by Kant, and then by Lambert,
-constitute what is now known as Kant's theory. According to this theory,
-the stars composing the Galaxy are conceived as being uniformly arranged
-between two flat planes of considerable extension, but which are
-comparatively near together, the Sun occupying a place not very far from
-the centre of this immense starry stratum. As we view this system
-crosswise through its thinnest parts, the stars composing it appear
-scattered and comparatively few in number, but when we view it
-lengthwiser through its most extended parts, they appear condensed and
-extremely numerous, thus giving the impression of a luminous belt
-encircling the heavens. In the conception of Kant, each star was a sun,
-forming the centre of a planetary system. These systems are not
-independent, but are kept together by the bonds of universal
-gravitation. The Galaxy itself is one of these great systems, its
-principal plane being the equivalent of the zodiac in our planetary
-system, while a preponderant body, which might be Sirius, is the
-equivalent of our Sun, and keeps the galactic system together. In the
-universe there are other galaxies, but as they are too distant to be
-resolved into stars, they appear as elliptical nebulæ. Such are, in
-brief, the grand speculations of Kant and Lambert on the Milky-way, and
-the structure of the universe.
-</p>
-<p>
-Kant's theory rested more on conjectures than on observed facts, and
-needed therefore the sanction of direct observations to be established
-on a firm basis. With this view, Sir William Herschel investigated the
-subject, by a long and laborious series of observations. His plan, which
-was that of "star-gauging," consisted in counting all the stars visible
-in his twenty-foot telescope, comprised in a wide belt cutting the
-Galaxy at right angles, and extending from one of its sides to the
-opposite one, thus embracing 180° of the celestial sphere. In this belt
-he executed 3,400 telescopic star-gaugings of a quarter of a degree
-each, from which he obtained 683 mean gaugings giving the stellar
-density of the corresponding regions.
-</p>
-<p>
-The general result derived from this immense labor was that the stars
-are fewest in regions the most distant from the galactic belt; while
-from these regions, which correspond to the pole of the Galaxy, they
-gradually increase in number in approaching the Milky-way. The star
-density was found to be extremely variable, and while some of the
-telescopic gaugings detected either no star at all, or only one or two,
-other gaugings gave 500 stars and even more. The average number of stars
-in a field of view of his telescope, obtained for the six zones, each of
-15°, into which Herschel divided up the portion of his observing belt,
-extending from the Galaxy to its pole, is as follows: In the first zone,
-commencing at 90° from the galactic belt and extending towards it, 4
-stars per telescopic field were found; 5 in the second; 8 in the third;
-14 in the fourth; 24 in the fifth and 53 in the sixth, which terminated
-in the Galaxy itself. Very nearly similar results were afterwards found
-by Sir John Herschel, for corresponding regions in the southern
-hemisphere.
-</p>
-<p>
-From these studies, Herschel concluded that the stellar system is of the
-general form supposed by the Kantian theory, and that its diameter must
-be five times as extended in the direction of the galactic plane, as it
-is in a direction perpendicular to it. To explain the great branch sent
-out by the Galaxy in Cygnus, he supposed a great cleft dividing the
-system edgewise, about half way from its circumference to its centre.
-From suppositions founded on the apparent magnitude and arrangement of
-stars, he estimated that it would take light about 7,000 years to reach
-us from the extremities of the Galaxy, and therefore 14,000 years to
-travel across the system, from one border to the opposite one.
-</p>
-<p>
-But Herschel's theory concerning the Milky-way rested on the erroneous
-assumption that the stars are uniformly distributed in space, and also
-that his telescopes penetrated through the entire depth of the Galaxy.
-Further study showed him that his telescope of twenty feet, and even his
-great forty-foot telescope, which was estimated to penetrate to a
-distance 2,300 times that of stars of the first magnitude, failed to
-resolve some parts of the Galaxy into stars. Meanwhile, the structure of
-the Milky-way being better known, the irregular condensation of its
-stars became apparent, while the mutual relation existing between binary
-and multiple systems of stars, as also between the stars which form
-clusters, was recognized, as showing evidence of closer association
-between certain groups of stars than between the stars in general.
-Herschel's system, which rested on the assumption of the uniform
-distribution of the stars in space, and on the supposition that the
-telescopes used for his gauges penetrated through the greater depths of
-the Galaxy, being thus found to contradict the facts, was gradually
-abandoned by its author, who adopted another method of estimating the
-relative distances of the stars observed in his gaugings.
-</p>
-<p>
-This method, founded on photometric principles, consisted in judging the
-penetrating power of his telescope by the brightness of the stars, and
-not, as formerly, by the number which they brought into view. He then
-studied by this new method the structure of the Milky-way and the
-probable distance of the clustering masses of which it is formed,
-concluding that the portion of the Galaxy traversing the constellation
-Orion is the nearest to us. This last result seems indicated by the fact
-that this portion of the Milky-way is the faintest and the most uniform
-of all the galactic belt.
-</p>
-<p>
-More recently Otto Struve investigated the same subject, and arrived at
-very nearly similar conclusions, which may be briefly stated as follows:
-The galactic system is composed of a countless number of stars,
-spreading out on all sides along a very extended plane. These stars,
-which are very unevenly distributed, show a decided tendency to cluster
-together into individual groups of different sizes and forms, separated
-by comparatively vacant spaces. This layer where the stars congregate in
-such vast numbers may be conceived as a very irregular flat disk,
-sending many branches in various directions, and having a diameter eight
-or ten times its thickness. The size of this starry disk cannot be
-determined, since it is unfathomable in some directions, even when
-examined with the largest telescopes. The Sun, with its attending
-planets, is involved in this immense congregation of suns, of which it
-forms but a small particle, occupying a position at some distance from
-the principal plane of the Galaxy. According to Struve, this distance is
-approximately equal to 208,000 times the radius of the Earth's orbit.
-The Milky-way is mainly composed of star-clusters, two-thirds, perhaps,
-of the whole number visible in the heavens being involved in this great
-belt. In conclusion, our Sun is only one of the individual stars which
-constitute the galactic system, and each of these stars itself is a sun
-similar to our Sun. These individual suns are not independent, but are
-associated in groups varying in number from a few to several thousands,
-the Galaxy itself being nothing but an immense aggregation of such
-clusters, whose whole number of individual suns probably ranges between
-thirty and fifty millions. In this vast system our globe is so
-insignificant that it cannot even be regarded as one of its members.
-According to Dr. Gould, there are reasons to believe that our Sun is a
-member of a small, flattened, bifid cluster, composed of more than 400
-stars, ranging between the first and seventh magnitude, its position in
-this small system being eccentric, but not very far from the galactic
-plane.
-</p>
-<p>
-The study of the Milky-way, of which Plate XIII. is only a part, was
-undertaken to answer a friendly appeal made by Mr. A. Marth, in the
-Monthly Notices of the Royal Astronomical Society, in 1872. I take
-pleasure in offering him my thanks for the suggestion, and for the
-facility afforded me in this study by his "List of Co-ordinates of Stars
-within and near the Milky-way," which was published with it.
-</p>
-
-<p><br /><br /><br /></p>
-
-<h4><a id="THE_STAR_CLUSTERS">THE STAR-CLUSTERS</a>
-<br /><br />
-PLATE XIV</h4>
-
-<p>
-It is a well-known fact that the stars visible to the naked eye are very
-unequally distributed in the heavens, and that while they are loosely
-scattered in some regions, in others they are comparatively numerous,
-sometimes forming groups in which they appear quite close together.
-</p>
-<p>
-In our northern sky are found a few such agglomerations of stars, which
-are familiar objects to all observers of celestial objects. In the
-constellation Coma Berenices, the stars are small, but quite condensed,
-and form a loosely scattered, faint group. In Taurus, the Hyades and the
-Pleiades, visible during our winter nights, are conspicuous and familiar
-objects which cannot fail to be recognized. In the last group, six stars
-may be easily detected by ordinary eyes on any clear night, but more can
-sometimes be seen; on rare occasions, when the sky was especially
-favorable, I have detected eleven clearly and suspected several others.
-The six stars ordinarily visible, are in order of decreasing brightness,
-as follows: Alcyone, Electra, Atlas, Maia, Taygeta and Merope. Glimpses
-of Celano and Pleione are sometimes obtained.
-</p>
-<p>
-When the sky is examined with some attention on any clear, moonless
-night, small, hazy, luminous patches, having a cometary aspect, are
-visible here and there to the naked eye. In the constellation Cancer is
-found one of the most conspicuous, called Præsepe, which forms a small
-triangle with the two stars <i>γ</i> and <i>δ</i>. In Perseus, and
-involved in the Milky-way, is found another luminous cloud, situated in
-the sword-handle, and almost in a line with the two stars <i>γ</i> and
-<i>δ</i> of Cassiopeia's Chair. In the constellation Hercules, another
-nebulous mass of light, but fainter, is also visible between the stars
-<i>η</i> and <i>ζ</i> where it appears as a faint comet, in the depths
-of space. In Ophiuchus and Monoceros are likewise found hazy, luminous
-patches. In the southern sky, several such objects are also visible to
-the naked eye, being found in Sagittarius, in Canis Major and in Puppis;
-but the most conspicuous are those in Centaurus and Toucan. That in
-Centaurus involves the star <i>ω</i> in its pale diffused nebulosity,
-and that in Toucan is involved in the lesser Magellanic cloud.
-</p>
-<p>
-When the telescope is directed to these nebulous objects, their hazy,
-ill-defined aspect disappears, and they are found to consist of
-individual stars of different magnitudes, which being more or less
-closely grouped together, apparently form a system of their own. These
-groups, which are so well adapted to give us an insight into the
-structure and the vastness of the stellar universe, are called
-<i>Star-clusters</i>.
-</p>
-<p>
-Star-clusters are found of all degrees of aggregation, and while in some
-of them, such as in the Pleiades, in Præsepe and in Perseus, the stars
-are so loosely scattered that an opera glass, and even the naked eye,
-will resolve them; in others, such as in those situated in Hercules,
-Aquarius, Toucan and Centaurus, they are so greatly compressed that even
-in the largest telescopes they appear as a confused mass of blazing
-dust, in which comparatively few individual stars can be distinctly
-recognized. Although only about a dozen Star-clusters can be seen in the
-sky with the naked eye, yet nearly eleven hundred such objects visible
-through the telescope, have been catalogued by astronomers.
-</p>
-<p>
-The stars composing the different clusters visible in the heavens vary
-greatly in number, and while in some clusters there are only a few, in
-others they are so numerous and crowded that it would be idle to try to
-count them, their number amounting to several thousands. It has been
-calculated by Herschel that some clusters are so closely condensed, that
-in an area not more than ⅒ part of that covered by the Moon, at least
-5,000 stars are agglomerated.
-</p>
-<p>
-When the group in the Pleiades is seen through the telescope it appears
-more important than it does to the naked eye, and several hundreds of
-stars are found in it. In a study of Tempel's nebula, which is involved
-in the Pleiades, I have mapped out 250 stars, mostly comprised within
-this nebula, with the telescope of 6⅓ inches aperture, which I have
-used for this study.
-</p>
-<p>
-As a type of a loose, coarse cluster, that in Perseus is one of the
-finest of its class. It appears to the naked eye as a single object, but
-in the telescope it has two centres of condensation, around which
-cluster a great number of bright stars, forming various curves and
-festoons of great beauty. Among its components are found several yellow
-and red stars, which give a most beautiful contrast of colors in this
-gorgeous and sparkling region. In a study which I have made of this twin
-cluster, I have mapped out 664 stars belonging to it, among which are
-two yellow and five red stars.
-</p>
-<p>
-While some clusters, like those just described, are very easily
-resolvable into stars with the smallest instruments, others yield with
-the greatest difficulty, even to the largest telescopes, in which their
-starry nature is barely suspected. Owing to this peculiarity,
-star-clusters are usually divided into two principal classes. In the
-first class are comprised all the clusters which have been plainly
-resolved into stars, and in the second all those which, although not
-plainly resolvable with the largest instruments now at our disposal,
-show a decided tendency to resolvability, and convey the impression that
-an increase of power in telescopes is the only thing needed to resolve
-them into stars. Of course this classification, which depends on the
-power of telescopes to decide the nature of these objects, is arbitrary,
-and a classification based on spectrum analysis is now substituted for
-it.
-</p>
-<p>
-The star-clusters are also divided into globular and irregular clusters,
-according to their general form and appearance. The globular clusters,
-which are the most numerous, are usually well-defined objects, more or
-less circular in their general outlines. The rapid increase of
-brightness towards their centres, where the stars composing them are
-greatly condensed, readily conveys the impression that the general form
-of these sparkling masses is globular. The irregular clusters are not so
-rich in stars as the former. Usually their stars are less condensed
-towards the centre, and are, for the most part, so loosely and
-irregularly distributed, that it is impossible to recognize the outlines
-of these clusters or to decide where they terminate. The globular
-clusters are usually quite easily resolvable into stars, either partly
-or wholly, although some among them do not show the least traces of
-resolvability, even in the largest instruments. This may result from
-different causes, and may be attributed either to the minuteness of
-their components or to their great distance from the Earth, many
-star-clusters being at such immense distances that they are beyond our
-means of measurement.
-</p>
-<p>
-As has been shown in the preceding section, the star-clusters are found
-in great number in the Galaxy; indeed, it is in this region and in its
-vicinity that the greater portion of them are found. In other regions,
-with the exception of the Magellanic clouds, where they are found in
-great number and in every stage of resolution, the clusters are few and
-scattered.
-</p>
-
-<p><br /></p>
-
-<div class="figcenter" style="width: 400px;">
-<a id="figure14"></a>
-<br />
-<img src="images/figure14.jpg" width="400" alt="" />
-<div class="caption">
-<p>PLATE XIV.&mdash;STAR-CLUSTER IN HERCULES.</p>
-<p class="smaller">From a study made in June, 1877</p>
-</div></div>
-
-<p><br /></p>
-
-<p>
-The star-cluster in the constellation Hercules, designated as No. 4,230
-in Sir J. Herschel's catalogue, and which is represented on Plate XIV.,
-is one of the brightest and most condensed in the northern hemisphere,
-although it is not so extended as several others, its angular diameter
-being only 7' or 8'. This object, which was discovered by Halley in
-1714, is one of the most beautiful of its class in the heavens.
-According to Herschel, it is composed of thousands of stars between the
-tenth and fifteenth magnitudes. Undoubtedly the stars composing this
-group are very numerous, although those which can be distinctly seen as
-individual stars, and whose position can be determined, are not so many
-as a superficial look at the object would lead us to suppose. From a
-long study of this cluster, which I have made with instruments of
-various apertures, I have not been able to identify more stars than are
-represented on the plate, although the nebulosity of which this object
-mainly consists, and especially the region situated towards its centre,
-appeared at times granular and blazing with countless points of light,
-too faint and too flickering to be individually recognized. Towards its
-centre there is quite an extended region, whose luminous intensity is
-very great, and which irresistibly conveys the impression of the
-globular structure of this cluster. Besides several outlying appendages,
-formed by its nebulosity, the larger stars recognized in this cluster
-are scattered and distributed in such a way that they form various
-branches, corresponding with those formed by the irresolvable
-nebulosity. At least six or seven of these branches and wings are
-recognized, some of which are curved and bent in various ways, thus
-giving this object a distant resemblance to some crustacean forms.
-Although I have looked for it with care, I have failed to recognize the
-spiral structure attributed to this object by several observers. Among
-the six appendages which I have recognized, some are slightly curved;
-but their curves are sometimes in opposite directions, and two branches
-of the upper portion make so short a bend that they resemble a claw
-rather than a spiral wing. The spectrum of this cluster, like that of
-many objects of its class, is continuous, with the red end deficient.
-</p>
-<p>
-A little to the north-east of this object is found the cluster No.
-4,294, which, although smaller and less bright than the preceding, is
-still quite interesting. It appears as a distinctly globular cluster
-without wings, and much condensed towards its centre. The stars
-individually recognized in it, although less bright than those of the
-other cluster, are so very curiously distributed in curved lines that
-they give a peculiar appearance to this condensed region.
-</p>
-<p>
-A little to the north of <i>γ</i> Centauri may be found the great <i>ω</i>
-Centauri cluster, No. 3,531, already referred to above. This magnificent
-object, which appears as a blazing globe 20' in diameter, is, according
-to Herschel, the richest in the sky, and is resolved into a countless
-number of stars from the twelfth to the fifteenth magnitude, which are
-greatly compressed towards the centre. The larger stars are so arranged
-as to form a sort of net-work, with two dark spaces in the middle.
-</p>
-<p>
-The great globular cluster No. 52, involved in the lesser Magellanic
-cloud, in the constellation Toucan, is a beautiful and remarkable
-object. It is composed of three distinct, concentric layers of stars,
-varying in brightness and in degree of condensation in each layer. The
-central mass, which is the largest and most brilliant, is composed of an
-immense number of stars greatly compressed, whose reddish color gives to
-this blazing circle a splendid appearance. Around the sparkling centre
-is a broad circle, composed of less compressed stars, this circle being
-itself involved in another circular layer, where the stars are fainter
-and more scattered and gradually fade away.
-</p>
-<p>
-Many other great globular clusters are found in various parts of the
-heavens, among which may be mentioned the cluster No. 4,678, in
-Aquarius. This object is composed of several thousand stars of the
-fifteenth magnitude, greatly condensed towards the centre, and, as
-remarked by Sir J. Herschel, since the brightness of this cluster does
-not exceed that of a star of the sixth magnitude, it follows that in
-this case several thousand stars of the fifteenth magnitude equal only a
-star of the sixth magnitude. In the constellation Serpens the globular
-clusters No. 4,083 and No. 4,118 are both conspicuous objects, also No.
-4,687 in Capricornus. In Scutum Sobieskii the cluster No. 4,437 is one
-of the most remarkable of this region. The stars composing it, which are
-quite large and easily made out separately, form various figures, in
-which the square predominates.
-</p>
-<p>
-Among the loose irregular clusters, some are very remarkable for the
-curious arrangement of their stars. In the constellation Gemini the
-cluster No. 1,360, which is visible to the naked eye, is a magnificent
-object seen through the telescope, in which its sparkling stars form
-curves and festoons of great elegance. The cluster No. 1,467, of the
-same constellation, is remarkable for its triangular form. In the
-constellation Ara the cluster No. 4,233, composed of loosely scattered
-stars, forming various lines and curves, is enclosed on three sides by
-nearly straight single lines of stars. In Scorpio the cluster No. 4,224
-is still more curious, being composed of a continuous ring of loosely
-scattered stars, inside of which is a round, loose cluster, which is
-divided into four parts by a dark cross-shaped gap, in which no stars
-are visible.
-</p>
-<p>
-Among the 1,034 objects which are now classified as clusters more or
-less resolvable, 565 have been absolutely resolved into stars, and 469
-have been only partly resolved, but are considered as belonging to this
-class of objects. In Sir J. Herschel's catalogue there are 102 clusters
-which are Considered as being globular; among them 30 have been
-positively resolved into stars.
-</p>
-<p>
-The agglomeration of thousands of stars into a globular cluster cannot
-be conceived, of course, to be simply the result of chance. This
-globular form seems clearly to indicate the existence of some bond of
-union, some general attractive force acting between the different
-members of these systems, which keeps them together, and condenses them
-towards the centre. Herschel regards the loose, irregular clusters as
-systems in a less advanced stage of condensation, but gradually
-concentrating by their mutual attraction into the globular form.
-Although the stars of some globular clusters appear very close together,
-they are not necessarily so, and may be separated by great intervals of
-space. It has been shown that the clusters are agglomeration of suns,
-and that our Sun itself is a member of a cluster composed of several
-hundreds of suns, although, from our point of observation, these do not
-seem very close together. So far as known, the nearest star to us is a
-Centauri, but its distance from the Earth equals 221,000 times the
-distance of the Sun from our globe, a distance which cannot be traversed
-by light in less than three years and five months. It seems very
-probable that if the suns composing the globular clusters appear so near
-together, it is because, in the first place, they are at immense
-distances from us, and in the second, because they appear nearly in a
-line with other suns, which are at a still greater distance from us, and
-on which they accordingly are nearly projected. If one should imagine
-himself placed at the centre of the cluster in Hercules, for instance,
-the stars, which from our Earth seems to be so closely grouped, would
-then quite likely appear very loosely scattered around him in the sky,
-and would resemble the fixed stars as seen from our terrestrial station.
-</p>
-<p>
-Judging by their loose and irregular distribution, the easily resolvable
-clusters would appear, in general, to be the nearer to us. It is
-probable that the globular clusters do not possess, to a very great
-degree, the regular form which they ordinarily present to us. It seems
-rather more natural to infer that they are irregular, and composed of
-many wings and branches, such as are observed in the cluster in
-Hercules; but as these appendages would necessarily be much poorer in
-stars than the central portions, they would be likely to become
-invisible at a great distance, and therefore the object would appear
-more or less globular; the globular form being simply given by the close
-grouping of the stars in the central portion. It would seem, then, that
-in general, the most loosely scattered and irregular clusters are the
-nearest to us, while the smallest globular clusters and those resolvable
-with most difficulty are the most distant.
-</p>
-<p>
-In accordance with the theory that the clusters are composed of stars,
-the spectrum of these objects is in general continuous; although, in
-many cases, the red end of the spectrum is either very faint or
-altogether wanting. Many objects presenting in a very high degree the
-principal characteristics exhibited by the true star-clusters, namely, a
-circular or oval mass, whose luminous intensity is greatly condensed
-toward the centre, have not yielded, however, to the resolving power of
-the largest telescopes, although their continuous spectrum is in close
-agreement with their general resemblance to the star-clusters. Although
-such objects may remain irresolvable forever, yet it is highly probable
-that they do not materially differ from the resolvable and partly
-resolvable clusters, except by their enormous distance from us, which
-probably reaches the extreme boundary of our visible universe.
-</p>
-
-<p><br /><br /><br /></p>
-
-<h4><a id="THE_NEBULAE">THE NEBULÆ</a>
-<br /><br />
-PLATE XV</h4>
-
-<p>
-Besides the foggy, luminous patches which have just been described, a
-few hazy spots of a different kind are also visible to the naked eye on
-any clear, moonless night. These objects mainly differ from the former
-in this particular, that when viewed through the largest telescopes in
-existence they are not resolved into stars, but still retain the same
-cloudy appearance which they present to the unassisted eye. On account
-of the misty and vaporous appearance which they exhibit, these objects
-have been called <i>Nebulæ</i>.
-</p>
-<p>
-Of the 26 nebulous objects visible to the naked eye in the whole
-heavens, 19 belong to the class of star-clusters, and 7 to the class of
-nebulæ. Among the most conspicuous nebulæ visible to the unassisted
-eye, are those in the constellations Argo Navis, Andromeda and Orion.
-</p>
-<p>
-Besides the seven nebulæ visible to the naked eye, a great number of
-similar objects are visible through the telescope. In Sir John
-Herschel's catalogue of nebulæ and clusters, are found 4,053
-irresolvable nebulæ, and with every increase of the aperture of
-telescopes, new nebulæ, invisible in smaller instruments, are found.
-Notwithstanding their irresolvability it is probable, however, that many
-among them have a stellar structure, which their immense distance
-prevents us from recognizing, and are not therefore true nebulæ. The
-giant telescope of Lord Rosse has shown nebulæ so remote that it has
-been estimated that it takes their light 30 million years to reach the
-Earth.
-</p>
-<p>
-The nebulæ are very far from being uniformly distributed in space. In
-some regions they are rare, while in others they are numerous and
-crowded together, forming many small, irregular groups, differing in
-size and in richness of aggregation. The grouping of the nebulæ does
-not occur at random in any part of the heavens, as might naturally be
-supposed, but, on the contrary, it is chiefly confined to certain
-regions. Outside of these regions nebulæ are rare and are separated
-from each other by immense intervals; so that these isolated objects
-appear as if they were lost wanderers from the great nebulous systems.
-</p>
-<p>
-The regions where the nebulæ congregate in great number are very
-extensive, and in a general view there are two vast systems of nebular
-agglomeration, occupying almost opposite points of the heavens, whose
-centres are not very distant from the poles of the Milky-way. In the
-northern hemisphere, the nebulous system is much richer and more
-condensed than in the southern hemisphere. The northern nebulæ are
-principally contained in the constellations Ursa Minor and Major, in
-Draco, Canes Venatici, Bootes, Leo Major and Minor, Coma Berenices, and
-Virgo. In this region, which occupies about ⅛ of the whole surface of
-the heavens, ⅓ of the known nebulæ are assembled. The southern
-nebulæ are more evenly distributed and less numerous, with the
-exception of two comparatively small, but very remarkable centres of
-condensation which, together with many star-clusters, constitute the
-Magellanic clouds.
-</p>
-<p>
-These two vast nebular groups are by no means regular in outline, and
-send various branches toward each other. They are separated by a wide
-and very irregular belt, comparatively free from nebulæ, which
-encircles the celestial sphere, and whose medial line approximately
-coincides with that of the galactic belt. The Milky-way, so rich in
-star-clusters, is very barren in nebulæ; but it is a very remarkable
-fact, nevertheless, that almost all the brightest, largest, and most
-complicated nebulæ of the heavens are situated either within it, or in
-its immediate vicinity. Such are the great nebulæ in Orion and
-Andromeda; the nebula of <i>ζ</i> Orionis; the Ring nebula in Lyra; the
-bifurcate nebula in Cygnus; the Dumb-bell nebula in Vulpecula; the Fan,
-Horse-shoe, Trifid and Winged nebulæ in Sagittarius; the great nebula
-around <i>η</i> Argus Navis, and the Crab nebula in Taurus.
-</p>
-<p>
-Aside from the discovery of some of the largest nebulæ by different
-observers, and their subsequent arrangement in catalogues by Lacaille
-and Messier, very little had been done towards the study of these
-objects before 1779, when Sir W. Herschel began to observe them with the
-earnestness of purpose which was one of the distinctive points of the
-character of this great man. He successively published three catalogues
-in 1786, 1789, and 1802, in which the position of 2,500 nebulous objects
-was given. This number was more than doubled before 1864, when Sir John
-Herschel published his catalogue of 5,079 nebulæ and star-clusters. To
-this long list must be added several hundred similar objects, since
-discovered by D'Arrest, Stephan, Gould and others. But, as has been
-shown above, among the so-called nebulæ are many star-clusters which do
-not properly belong to the same class of objects, it being sometimes
-impossible in the present state of our knowledge to know whether a
-nebulous object belongs to one class or to the other.
-</p>
-<p>
-The nebulæ exhibit a great variety of forms and appearances, and, in
-accordance with their most typical characters, they are usually divided
-into several classes, which are: the Nebulous stars, the Circular, or
-Planetary, the Elliptical, the Annular, the Spiral and Irregular
-nebulæ.
-</p>
-
-<p><br /></p>
-
-<div class="figcenter" style="width: 400px;">
-<a id="figure15"></a>
-<br />
-<img src="images/figure15.jpg" width="400" alt="" />
-<div class="caption">
-<p>PLATE XV.&mdash;THE GREAT NEBULA IN ORION.</p>
-<p class="smaller">From a study made in the years 1875-76</p>
-</div></div>
-
-<p><br /></p>
-
-<p>
-The so-called nebulous stars consist of a faint nebulosity, usually
-circular, surrounding a bright and sharp star, which generally occupies
-its centre. The nebulosity surrounding these stars varies in brightness
-as well as in extent, and while, in general, its light gradually fades
-away, it sometimes terminates quite suddenly. Such nebulosities are
-usually brighter and more condensed towards the central star. The stars
-thus surrounded do not seem, however, to be distinguished from others by
-any additional peculiarity. Some nebulæ of this kind are round, with
-one star in the centre; others are oval and have two stars, one at each of
-their foci. The nebulous star, <i>τ</i> Orionis, represented at the upper
-part of Plate XV., above the great nebula, has a bright star at its
-centre and two smaller ones on the side. The association of double stars
-with nebulæ is very remarkable, and may in some cases indicate a mutual
-relation between them.
-</p>
-<p>
-The so-called planetary nebulæ derive their name from their likeness to
-the planets, which they resemble in a more or less equable distribution
-of light and in their round or slightly oval form. While some of them
-have edges comparatively sharp and well defined, the outlines of others
-are more hazy and diffused. These nebulæ, which are frequently of a
-bluish tint, are comparatively rare objects, and most of those known
-belong to the southern hemisphere. When seen through large telescopes,
-however, they present a different aspect, and their apparent uniformity
-changes. The largest of these objects, No. 2,343 of the General
-Catalogue, is situated in the Great Bear, close to the star <i>β</i>. Its
-apparent diameter is, according to Sir J. Herschel, 2', 40", and "its
-light is equable, except at the edge, where it is a little hazy." In a
-study which I made of this object in 1876, with a refractor 6⅓ inches
-in aperture, I found it decidedly brighter on the preceding side, where
-the brightest part is crescent shaped. In Lord Rosse's telescope its
-disk is transformed into a luminous ring with a fringed border, and two
-small star-like condensations are found within. Another planetary
-nebula, near <i>χ</i> Andromedæ, has also shown an annular structure in
-Rosse's telescope.
-</p>
-<p>
-The elliptical nebulæ, as their name implies, are elongated, elliptical
-objects; but while some of them are only slightly elongated ovals,
-others form ellipses whose eccentricity is so great that they appear
-almost linear. In all these objects the light is more or less condensed
-towards the centre; but while in some of them the condensation is
-gradual and slight, in others it is so great and sudden that the centre
-of the nebula appears as a large diffused star, somewhat resembling the
-nucleus of a comet. From the general appearance of these objects, it is
-not unlikely that some of them are either flattish, nebulous disks, like
-the planetary nebulæ, or nebulous rings, seen more or less sidewise.
-The condensation of light at their centres does not appear to be
-stellar, but nebulous like the rest, and it is a remarkable fact that
-very few, if any, of these objects are resolvable into stars.
-</p>
-<p>
-Several elliptical nebulæ are remarkable for having a star at or near
-each of their foci, or at each of their extremities. Such are the
-elliptical nebulæ in Draco, Centaurus, and Sagittarius, Nos. 4,419,
-3,706 and 4,395 of the General Catalogue, the last of which is in the
-vicinity of the triple star <i>μ</i> Sagitarii. Each of these nebulæ has a
-star at each of its foci, while No. 1, in Cetus, has a star at each of
-its extremities.
-</p>
-<p>
-Among the most remarkable elliptic nebulæ may be mentioned Nos. 1,861
-and 2,373 of Sir J. Herschel's catalogue, both situated in the
-constellation Leo. The first is one of Lord Rosse's spiral nebulæ, and
-the last, which is a very elongated object, is formed of concentric oval
-rings, which are especially visible towards its central part. The
-constellation Draco is particularly remarkable for the number of
-elliptical nebulæ found within its boundaries. Among them are Nos.
-3,939, 4,058, 4,064, 4,087, 4,415, etc., which are quite remarkable
-objects of their class. No. 4,058, of which I have made a study, is
-bright, and has a decided lenticular form with a condensation in the
-centre. Its following edge is better defined than the preceding. In Lord
-Rosse's telescope this object exhibits a narrow, dark, longitudinal, gap
-in its interior.
-</p>
-<p>
-By far the largest and the finest object of this class is the great
-nebula in Andromeda. Although this object belongs rather to the class of
-irregular nebulæ, yet it is generally considered as an elliptic nebula,
-since its complicated structure, being less prominent, was not
-recognized until 1848, when it was perceived by George P. Bond, Director
-of the Harvard College Observatory. This, the first nebula discovered,
-was found in 1612 by Simon Marius. It is situated in the constellation
-Andromeda, in the vicinity of the star <i>ν</i>, and almost in a line with
-the stars <i>μ</i> and <i>β</i> of the same constellation. It is visible to
-the naked eye, and appears as a faint comet-like object. It is represented
-at the upper left hand corner of Plate XIII., on the border of the
-Milky-way, as it appears to the naked eye.
-</p>
-<p>
-The nebula in Andromeda is one of the brightest in the heavens, and is
-closely attended by two smaller nebulæ. Perhaps it would be rather more
-correct to say that it has three centres of condensation, as the two
-small nebulæ referred to are entirely involved in the same faint and
-extensive nebulosity. Its general form is that of an irregular oval,
-upwards of one degree in breadth and two and a half degrees in length.
-Its brightest and most prominent part, which alone was seen by the
-earlier observers, consists in a very elongated lenticular mass, which
-gradually condenses towards its centre into a blazing, star-like
-nucleus, surrounded by a brilliant nebulous mass. At a little distance
-to the south of this central condensation is found one of the lesser
-centres of condensation noted above, which is globular in appearance,
-with a bright, star-like nucleus like the former. The other centre of
-condensation is found to the north-west of the centre of the principal
-mass, and is quite elongated, with a centre of condensation towards its
-southern extremity, but it is not so bright as the others. Close to the
-western edge of the bright lenticular mass first described, and making a
-very slight angle with its longer axis, are found two narrow and nearly
-rectilinear dark rifts, running almost parallel to each other, and both
-terminating in a slender point in the south. These dark rifts, which are
-almost totally devoid of nebulous matter, are quite rare in nebulæ, and
-afford a good opportunity to watch the changes which this part of the
-nebulæ may undergo.
-</p>
-<p>
-This nebula has never been positively resolved into stars, although
-Prof. Geo. Bond and others have strongly suspected its resolvability. In
-a study which I have made of it, with the same instrument employed by
-Bond, and also with the great Washington telescope, I detected a decided
-mottled appearance in several places, which might be attributable to a
-beginning of resolvability; but I do not consider this a conclusive
-indication that the nebula is resolvable. The continuous spectrum given
-by this nebula, showing that it is not in the gaseous state which its
-appearance seems to indicate, warrants the conclusion, however, that it
-will ultimately be found to be resolvable. This object, being situated
-on the edge of the Galaxy and involved in its diffused light, has a
-great number of small stars belonging to this belt projected upon it.
-During my observations I have mapped out 1,323 of these stars, none of
-which seems to be in physical connection with the nebula.
-</p>
-<p>
-Among the circular and elliptical nebulæ a few exhibit a very
-remarkable structure, being apparently perforated, and forming either
-round, slightly oval, or elongated rings of great beauty. These Annular
-nebulæ are among the rarest objects in the heavens. In Scorpio, two
-such nebulæ are found involved in the Milky-way, and also one in
-Cygnus. One of those in Scorpio has two stars involved within its ring,
-at the extremities of its smallest interior diameter. A very elongated
-nebula in the vicinity of the fine triple star <i>γ</i> Andromedæ is also
-annular, and has two stars symmetrically placed at the extremities of
-its greatest interior axis. Another elongated annular nebula is also
-found north of <i>η</i> Pegasi.
-</p>
-<p>
-The grandest and most remarkable of the annular nebulæ is found in the
-constellation Lyra, about midway between the two stars <i>β</i> and
-<i>γ</i>. It is slightly elliptical in form, and according to Prof. E.
-S. Holden, its major axis is 77".3 and the minor 58". From a study and
-several drawings which I have made of this object, with instruments of
-various apertures, I have found it decidedly brighter towards its outer
-border, at the extremities of its minor axis, than at the ends of the
-major axis. On very favorable occasions, some of its brightest parts
-have appeared decidedly, but very faintly mottled, and I have recognized
-three small centres of condensation. Its interior, in which Professor
-Holden has detected a very faint star, is quite strongly nebulous. In
-Lord Rosse's telescope, this nebula is completely surrounded by wisps
-and appendages of all sorts of forms, which I have failed to trace,
-however, both with the refractor of the Harvard College Observatory and
-with that of the Naval Observatory at Washington; Rosse, Secchi and
-Chacornac, have seen this nebula glittering as if it were a "heap of
-star dust," although its spectrum indicates that it is gaseous.
-</p>
-<p>
-The nebula No. 1,541, in Camelopardus, of which I have also made a study
-and a drawing, is closely allied to the class of annular nebulæ. This
-object, which is quite bright, has a remarkable appearance. It consists
-principally of somewhat more than half of an oval ring, surrounding a
-bright, nebulous mass which condenses around a star; this mass being
-separated from the imperfect ring by a dark interval. Upon the bright
-portion of the ring, and on opposite points, are found two bright stars,
-between which lies the star occupying the central mass. The central mass
-extends at some distance outside of the ring on its open side. Several
-stars are involved in this object.
-</p>
-<p>
-The Spiral nebulæ are very curious and complicated objects, but they
-are visible only in the largest telescopes. Prominent above all is the
-double spiral nebula No. 3,572, in Canes Venatici, which is not far from
-<i>η</i> Ursa Majoris. In Lord Rosse's telescope, this object presents a
-wonderful spiral disposition, looking somewhat like one of the
-fire-works called pin-wheels, and forming long, curved wisps, diverging
-from two bright centres. The spectrum of this object, however, is not
-that of a gas. In the constellation Virgo, Rosse has detected another
-such nebula. In Cepheus, Triangulum, and Ursa Major, are found other
-spiral nebulæ of smaller size. Lord Rosse has recognized 40 spiral
-nebulæ and suspected a similar structure in 30 others.
-</p>
-<p>
-The class of the Irregular nebulæ, which will be now considered,
-differs greatly in character from the others, and includes the largest,
-the brightest and the most extraordinary nebulæ in the heavens. The
-nebulæ of this class differ from those belonging to the other classes
-by a want of symmetry in their form and in the distribution of their
-light, as well as by their capricious shapes, and their very complicated
-structure. Another and perhaps the principal difference between them and
-the objects above described, consists in the remarkable fact already
-stated, that they are not, except in rare cases, to be found in the
-regions where the other nebulæ abound. On the contrary, they are found
-in or very near the Milky-way, precisely where the other nebulæ are the
-most rare. This fact, recognized by Sir J. Herschel, led him to consider
-them as "outlying, very distant, and as it were detached fragments of
-the great stratum of the Galaxy." It seems very probable that the reason
-why these objects differ so greatly from the other nebulæ in size,
-brightness and complication of structure, is simply because they are
-much nearer to us than are most of the others. They are perhaps nebulous
-members of our Galaxy. The same remark which has been made of
-star-clusters may be applied to nebulæ. The nearer they are to us, the
-larger, the brighter and the more complicated they will appear, while
-the farther they are removed, the more simple and regular and round they
-will appear, only their brightest and deepest parts being then visible.
-</p>
-<p>
-The Crab Nebula of Lord Rosse, near <i>ζ</i> Tauri, No. 1,157, is one of
-the interesting objects of this class. It has curious appendages streaming
-off from an oval, luminous mass, which give it a distant resemblance to
-the animal from which it derives its name. The Bifid nebula in Cygnus,
-Nos. 4,400 and 4,616, is another object of this class. It consists of a
-long, narrow, crooked streak, forking out at several places, and passing
-through <i>χ</i> Cygni. Observers, having failed to recognize the
-connection existing between its different centres of brightness, have made
-distinct nebulæ of this extended object.
-</p>
-<p>
-The Dumb-bell nebula in Vulpecula, No. 4,532, is a bright and curious
-object, with a general resemblance to the instrument from which it
-derives its name. Lord Rosse's telescope has shown many stars in it,
-projected on a nebulous background, and Prof. Bond seems to have thought
-that it showed traces of resolvability, although in the study which I
-made of this nebula with the same instrument used by the latter
-observer, I failed to perceive any such traces. Dr. Huggins finds its
-spectrum gaseous.
-</p>
-<p>
-The star-cluster, No. 4,400, in Scutum Sobieskii, which is described by
-Sir J. Herschel as a loose cluster of at least 100 stars, I have found
-to be involved in an extensive, although not very bright, nebula, which
-would seem to have escaped his scrutiny. In a study and drawing of this
-nebula made in 1876, its general form is that of an open fan, with the
-exception that the handle is wanting, with deeply indented branches on
-the preceding side, where the brightest stars of the cluster are
-grouped. From its peculiar form, this object might appropriately be
-called the Fan nebula.
-</p>
-<p>
-The Omega or Horse-shoe nebula, in Sagittarius, No. 4,403, of which I
-have made a study and two drawings, one with a refractor 6⅓ inches in
-aperture, and the other conjointly with Prof. Holden, with the great
-telescope of the Naval Observatory, is a bright and very complicated
-object. Its general appearance in small instruments, with low power, is
-that of a long, narrow pisciform mass of light, from which proceeds on
-the preceding side, the great double loop from which it derives its
-name. But in the great Washington refractor its structure becomes very
-complicated, forming various bright nebulous masses and wisps of great
-extension. Prof. Holden, who has made a careful, comparative study of
-the published drawings of this object, thinks there are reasons to
-believe that its western branch has moved relatively to the stars found
-within its loop. The spectrum of this nebula is gaseous.
-</p>
-<p>
-The Trifid nebula, No. 4,355, in the same constellation, is also a very
-remarkable object, although it is not so bright as the last. This
-nebula, which I have studied with the refractor of the Cambridge
-Observatory, consists of four principal masses of light, separated by a
-wide and irregular gap branching out in several places. These masses,
-which are brighter along the dark gap, gradually fade away externally. A
-group of stars, two of which are quite bright, is found near the centre
-of the nebula, on the inner edge of the following mass, and close to the
-principal branch of the dark channel. A little to the north, and
-apparently forming a part of this nebula, is a globular-looking nebula,
-having a pale yellow star at its centre. Prof. Holden's studies on this
-nebula show that the triple star, which was centrally situated in the
-dark gap from 1784 to 1833, was found involved in the border of the
-nebulous mass following it, from 1839 to 1877; the change, he thinks,
-is attributable either to the proper motion of the group of stars or to
-that of the nebula itself.
-</p>
-<p>
-In the same vicinity is found the splendid and very extensive nebula No.
-4,361, in which is involved a loose, but very brilliant star-cluster.
-This nebula and cluster, which I have studied and drawn with a 6⅓ inch
-telescope, is very complicated in structure, and divided by a dark
-irregular gap into three principal masses of light, condensing at one
-point around a star, and at others forming long, bright, gently-curved
-branches, which give to this object a strong resemblance to the wings of
-a bird when extended upwards in the action of flying. From this
-peculiarity this object might appropriately be called the Winged nebula.
-Its spectrum is that of a gas.
-</p>
-<p>
-The variable star <i>η</i> Argus is completely surrounded by the great
-nebula of the same name, No. 2,197, first delineated by Sir J. Herschel,
-during his residence at the Cape of Good Hope, in 1838. This object,
-which covers more than ⁴⁄₇ of a square degree, is divided into
-three unequal masses, separated by dark oval spots, comparatively free
-from nebulosity, and is suspected to have undergone changes since
-Herschel's time.
-</p>
-<p>
-In the same field with the double star, <i>ζ</i> Orionis, the most easterly
-of the three bright stars in the belt of Orion, is found another
-irregular nebula of the Trifid type. From the drawings which I have made
-of this object, it appears to be composed of three principal unequal
-masses, separated by a wide, irregular, dark channel, two of the masses
-being quite complicated in structure, and forming curved, nebulous
-streams of considerable length and breadth. This nebula, like the next
-to be described, seems to be connected with the Galaxy by the great
-galactic loop described in another section.
-</p>
-<p>
-By far the most conspicuous irregular nebula visible from our northern
-States, is the great nebula in Orion, No. 1,179, represented on Plate
-XV. This object, visible to the naked eye, is the brightest and the most
-wonderful nebula in the heavens. It is situated a little to the south of
-the three bright stars in the belt of Orion, and may be readily detected
-surrounding the star <i>θ</i>, situated between and in a line with two
-faint stars, the three being in a straight line which points directly
-towards <i>ε</i>, the middle star of the three in Orion's belt. The
-area occupied by this nebula is about equal to that occupied by the
-Moon.
-</p>
-<p>
-In its brightest parts the nebula in Orion appears as a luminous cloud
-of a pentagonal form, from which issue many luminous appendages of
-various shapes and lengths. This principal mass is divided into
-secondary masses, separated by darkish, irregular intervals. These
-secondary masses in their turn appear mottled and fleecy. Towards the
-lower part of the pentagonal mass is found a roundish dark space,
-comparatively devoid of nebulosity, in which are involved four bright
-stars forming a trapezium, and several fainter ones. The four bright
-stars of the trapezium constitute the quadruple star <i>θ</i> Orionis,
-from which the nebula has received its name. The cloud-like pentagonal
-form is brightest on the north-west of the trapezium, and is surrounded
-on three sides by long, soft, curved wisps, fading insensibly into the
-outer nebulous mass in which they are involved. On the east a broad,
-wavy wing spreads out, and sends an important branch southward.
-South-east of the trapezium are found several curious dark spaces,
-comparatively devoid of nebulosity, especially those on the east, which
-give to this nebula a singular character. Close to the north-eastern
-part of the nebula, or rather in contact with it, is found a small,
-curiously-shaped nebula, condensing around a bright star into a blazing
-nucleus. From this centre it continues northward in a narrow diffused
-stream, which spreads out in passing over the stars <i>c<sup>1</sup></i>
-and <i>c<sup>2</sup></i>; and after having sent short branches
-northward, it curves back to the south and joins the main nebula on the
-west of its starting point, having thus formed a great loop which is not
-shown on the Plate. The nebula also forms a loop towards the south,
-which is partly shown on Plate XV., a small branch of which, passing
-through <i>τ</i> Orionis, the nebulous star shown at the top of the
-Plate, and extending southward, is not here represented.
-</p>
-<p>
-On ordinary nights the nebula in Orion is a splendid object, and
-inspires the observer with amazement; but this is as nothing compared
-with the grand and magnificent sight which it presents during the very
-rare moments when our atmosphere is perfectly clear and steady. I have
-seen this nebula but once under these favorable circumstances, and I was
-surprised by the grandeur of the scene. Then could be detected features
-to be seen at no other time, and its fleecy, floculent, cloud-like
-masses glittered with such intensity that it seemed as if thousands of
-stars were going to blaze out the next moment. Although I observed the
-nebula under such favorable conditions, and with the fifteen-inch
-refractor of the Cambridge Observatory, yet I was disappointed in my
-expectations, and distinguished no new stars or points of light, and
-nothing more than a very bright mass, finely divided into minute blazing
-cloudlets. Although I failed to resolve this nebula into stars, yet Lord
-Rosse, Bond and Secchi thought they had caught glimpses of star dust.
-Its spectrum, however, proves to be mainly that of incandescent gases,
-probably hydrogen and nitrogen. In the curved wisps found in this
-nebula, Lord Rosse and others saw indications of a spiral structure.
-</p>
-<p>
-Several bright stars are found scattered over this nebula, and besides
-those forming the trapezium, there are three in a row, a little to the
-south-east of that group, which are quite bright and remarkable. Among
-the stars involved in this nebula, few show signs of having a physical
-connection with it, although it seems probable that the group of the
-trapezium is so connected. Some of these stars are variable. The small
-stars represented on this Plate, as on others of the series, are
-somewhat exaggerated in size, as was unavoidable with any process of
-reproduction which could be adopted.
-</p>
-<p>
-In 1811, W. Herschel was led to suspect that some changes had occurred
-in this nebula, but changes in such complicated and delicate objects are
-not easily ascertained, since, for the most part, we have for comparison
-with our later observations only coarse drawings made by hands unskilled
-in delineation.
-</p>
-<p>
-Although comparatively rare, double and multiple nebulæ may be found in
-the sky. When this occurs, their constituents most commonly belong to
-the class of spherical nebulæ. Sometimes the components are separated
-and distinct, at other times one of them is projected upon the other,
-either really or by the effect of perspective. Sometimes one is round
-and the other elongated. It is probable that while some of these nebulæ
-are physically associated and form a system, others appear to be so only
-because they happen to be almost in a line with the observer. A double
-nebula in Draco, Nos. 4,127 and 4,128, which I have drawn, is a fair
-type of those which are separated. The first is a globular nebula, and
-the last an oval one, with a star at its centre. The double nebula, Nos.
-858 and 859, in Taurus, which I have also studied, is a type of the
-cases in which one nebula is partly projected on another. In this
-instance both the nebulæ are globular.
-</p>
-<p>
-The nebulæ in general show very little color in their light, which is
-ordinarily whitish and pale. Some, however, present a decided bluish or
-greenish tint. The great nebula in Orion has a greenish cast, and we
-have seen that some planetary nebulæ are bluish.
-</p>
-<p>
-It has been a question whether nebulæ are changing. It has already been
-stated that Prof. Holden believes there is ground to suspect that the
-Trifid and Horse-shoe nebulæ have undergone some changes. A nebula near
-<i>ε</i> Tauri has been lost and found again several times. Two other
-nebulæ in the same constellation have presented curious variations.
-One, near a star of the tenth magnitude, exhibited variations of
-brightness like those of the star itself, and for a time disappeared.
-The other, near <i>ζ</i> Tauri, increased in brightness for three months,
-after which it disappeared. In 1859 Tempel discovered a nebula in the
-Pleiades, which has shown some fluctuations. In 1875 I made a long study
-of this object, and drew it carefully a dozen times, but I was not able
-to see any changes in it within the two or three months during which my
-observations were continued. But on Nov. 24, 1876, it was found of a
-different color, being purplish and very faint. On Dec. 23, 1880, it was
-found just as bright and visible as when I drew it in 1875, and on Oct.
-20, 1881, it appeared faint and purplish again, as in 1876. On this last
-night, and on those which followed it, it was impossible for me to trace
-the nebulosity as far as in 1875. I consider this as due to a variation
-in the light of this object, which in 1875 was bright enough to be well
-seen while the Moon after her First Quarter was within ten or fifteen
-degrees from the Pleiades.
-</p>
-<p>
-From the observations of M. Laugier, it appears that some nebulæ have a
-proper motion, comparable to that of stars. From the displacement of the
-lines of their spectra by their motion in the line of sight, Dr. Huggins
-found that no nebula observed by him has a proper motion surpassing 25
-miles per second. The Ring nebula in Lyra appears to move from us at the
-rate of 3 miles per second, and that in Orion recedes about 17 miles per
-second.
-</p>
-<p>
-The important question arises, are all the irresolvable nebulæ in the
-heavens to be considered as so many star-clusters, differing only from
-them by the minuteness of their components, or their immense distance
-from us; or are they cosmical clouds, composed of luminous vapors,
-similar to the matter composing the heads and tails of comets?
-Originally, W. Herschel, with many astronomers, thought that all these
-objects were stellar aggregations, too distant to be resolved into
-stars; but he subsequently modified his opinion, and accepted the idea
-that some of them are of a gaseous nature.
-</p>
-<p>
-No direct proof that the nebulæ are gaseous could be obtained, however,
-before the spectroscope was known. The attempt to analyze the light of
-the nebulæ with this instrument was made in 1864, by Dr. Huggins, who
-directed his spectroscope to the planetary nebula, No. 4,373, in Draco.
-Its spectrum was found to consist of three bright, distinct lines, the
-brightest of which corresponded with the strongest nitrogen line, and
-the feeblest with the hydrogen C line. Besides these lines, it gave also
-a very faint, continuous spectrum, apparently due to a central point of
-condensation. By this observation, the gaseous nature of a nebula was
-for the first time demonstrated. Dr. Huggins thus analyzed 70 nebulæ,
-of which one-third gave a gaseous spectrum, consisting of several bright
-lines, the brightest of which invariably corresponded with the lines of
-nitrogen. The others gave a continuous spectrum, with the red end
-usually deficient. These results indicate that if some of the so-called
-nebulæ are due to an aggregation of stars, either too minute or too
-remote in space to be individually resolved, others are in a gaseous
-state. Yet the faint, continuous spectrum, given by some nebulæ, in
-addition to their gaseous spectra, seems to show that these nebulæ have
-some stars or matter in a different state, either involved in them or
-projected on their surface.
-</p>
-<p>
-The idea of diffused matter distributed here and there in space, and
-gradually condensing into stars, is by no means new. As early as 1572,
-Tycho Brahé proposed such an hypothesis, to explain the sudden
-apparition of a new star in Cassiopeia, which he considered as formed by
-the recent agglomeration of the "celestial matter" diffused in space.
-Kepler adopted the same idea to explain the new star which appeared in
-Ophiuchus, in 1604. Halley, Lacaille, Mairan and others, entertained the
-same opinion. The hypothesis of a self-luminous, nebulous matter
-diffused in space, and forming here and there immense masses, has been
-proposed from the origin of the telescope, and was adopted by Sir
-William Herschel, who in his grand speculations on the universe
-considered the nebulæ as immense masses of phosphorescent vapors,
-gradually condensing around one or several centres into stars or
-clusters of stars. The evidence afforded by the spectroscope seems to be
-in favor of such an hypothesis, and shows us that gaseous agglomerations
-exist in space.
-</p>
-<p>
-According to our modern conception, the visible universe is but an
-infinitely small portion of the infinite universe perceived by our mind.
-The great blazing centre around which our little, non-luminous globe
-pursues its endless journey, is only an humble member of a cluster
-comprising four hundred equally powerful suns, as they are believed to
-be, although they appear to us as little twinkling stars. The nearest of
-these stars is 221,000 times as far from the Sun as the Sun is from the
-Earth, and yet this entire cluster is only one among the several hundred
-Star-clusters composing the great galactic nebula in which we are
-involved, comprising thirty or fifty millions of such suns. Among the
-4,000 irresolvable nebulæ in the sky, perhaps over one-half are
-supposed to be galaxies, like our own galaxy, composed of star-clusters,
-and millions of stars. Besides these remote galaxies, vast
-agglomerations of yet uncondensed, nebulous matter exist in space, and
-form the nebulæ proper, in which the genesis of suns is slowly
-elaborated. Although the visible universe is limited by the penetrating
-power of our instruments, yet we see in imagination the infinite
-universe stretching farther and farther; but we know not whether this
-invisible universe is totally devoid of matter, or whether it also is
-filled with millions and millions of suns and galaxies.
-</p>
-
-<p><br /><br /><br /></p>
-
-<h4><a id="APPENDIX">APPENDIX</a></h4>
-
-<h4>KEY TO THE PLATES</h4>
-
-<p><br /></p>
-
-<h5>PLATE I.&mdash;GROUP OF SUN-SPOTS AND VEILED SPOTS.
-<br />
-<i>Observed June</i> 17, 1875, <i>at 7h. 30m. A. M.</i></h5>
-
-<p>
-The background shows the sun's visible surface, or <i>photosphere</i>,
-as seen with a telescope of high power at the most favorable moments,
-composed of innumerable light markings, or granules, separated by a
-network of darker gray. The granules, each some hundreds of miles in
-width, are thought to be the flame-like summits of the radial filaments
-or columns of gas and vapor which compose the photospheric shell. The
-two principal sun-spots of the group here represented show the
-characteristic dark <i>umbra</i> in the centre, overhung by the
-thatch-like <i>penumbra</i>, composed of whitish gray filaments. The
-penumbral filaments are not supposed to differ in their nature from
-those constituting the ordinary photosphere, save that they are seen
-here elongated and violently disturbed by the force of gaseous currents.
-Both spots are traversed partly or wholly by bright overlying
-<i>faculæ</i>, or so-called <i>luminous bridges</i>, depressed portions
-of which, in the left-hand spot, form the <i>gray and rosy veils</i>
-commonly attendant upon this class of spots. In each of these spots,
-also, the inner ends of projecting penumbral filaments have fallen so
-far within the umbra as to appear much darker than the rest. At the
-right of the upper portion of the left-hand spot, is a mass of white
-facular clouds, honey-combed by dark spaces, through which are seen
-traces of the undeveloped third spot of the triple group first observed.
-If seen upon the sun's limb, this would have presented the appearance of
-a <i>lateral spot</i>. Above the right-hand spot is a small black "dot,"
-or incipient spot, without distinct penumbra. The irregular dark rift
-below the two large spots and connecting them is a spot of the crevasse
-type, with very slight umbra, a still better example of which is seen in
-a westward prolongation of the penumbra of the left-hand spot. In the
-upper left-hand corner of the Plate are seen several small
-<i>faculæ</i>, appearing as irregular whitish streaks amongst the
-granules. In the pear-shaped darkening of the solar surface below and at
-their left, is seen a veiled spot, two of which attended this group.
-</p>
-<p>
-<i>Approximate scale, 2500 miles&mdash;1 inch.</i>
-</p>
-
-<p><br /></p>
-
-<h5>PLATE II.&mdash;SOLAR PROTUBERANCES.
-<br />
-<i>Observed May</i> 5, 1873, <i>at 9h. 40m. A. M.</i></h5>
-
-<p>
-A view of an upheaval of the <i>chromosphere</i>, or third outlying
-envelope of the sun, as observed with the tele-spectroscope, or
-telescope with spectroscope attached.
-</p>
-<p>
-The <i>method of the observation</i> requires a word of explanation.
-Save on the rare occasions of a total solar eclipse, no direct
-telescopic view of the solar prominences or flames is possible, owing to
-the fact that the intense white light from the sun's main disk entirely
-obscures the feeble pink light of the chromosphere. A few years ago
-Messrs. Jannsen and Lockyer found that a spectroscope of high dispersive
-power so weakens the spectrum of ordinary sun-light as to show the
-spectrum of bright lines given by the chromosphere, on any clear day.
-The telescope is adjusted so that a portion of the sun's limb, usually
-near a group of active sun-spots, shall be presented before the opened
-slit of the spectroscope. The light of the chromosphere thus admitted
-along with some diffused sun-light from the earth's atmosphere, produces
-a spectrum of intensely bright lines, widely separated, on the fainter
-background of the strongly dispersed spectrum of sun-light. The most
-prominent of these bright lines are those known as the C line
-(<i>scarlet</i>), F line (<i>blue</i>), which with several others are
-due to the hydrogen present in the chromosphere, the D<sub>3</sub> line
-(<i>orange</i>) ascribed to a little known substance called "helium",
-and occasionally the sodium lines D<sub>1</sub>, D<sub>3</sub>,
-(<i>yellow</i>). By adjusting the slit upon the scarlet C line, the
-appearances represented in Plate II. were observed as through an
-atmosphere of scarlet light: in the D or F lines identical appearances
-may be seen, but somewhat less clearly defined, as through yellow or
-blue light respectively. Hence the solar times, as here observed with
-the spectroscope in the hydrogen C line, are seen through a portion only
-(the scarlet rays) of the light coming from but one substance (hydrogen)
-of the companion incandescent substances present in the chromosphere.
-The color of the collective chromospheric light is seen directly with
-the telescope during an eclipse (See Plate III.) to be a delicate rosy
-pink.
-</p>
-
-<p><br /></p>
-
-<p>
-<i>Description of the Plate.</i>&mdash;The black background represents
-the general darkness of the eye-piece to the spectroscope. The broad red
-stripe stretching from top to bottom of the Plate is a portion of the
-red band of the spectrum, magnified about 100 times as compared with the
-actual spectroscopic view. The upper and lower edges of the
-cross-section of dusky red correspond with the edges of the slit, opened
-widely enough to admit a view of the chromospheric crest and of the
-whole height of the protuberances at once. With a narrower opening of
-the slit this background would have been nearly black, its reddish cast
-increasing with the amount of opening and consequent admission of
-diffused sun-light. Rising above the lower edge of the opening is seen a
-small outer segment of the chromosphere, which, as a portion of the
-sun's eastern limb, should be imagined as moving directly towards the
-beholder. The seams and rifts by which its surface is broken, as well as
-the distorted forms of the huge protuberances show the chromosphere to
-be in violent agitation. Some of the most characteristic shapes of the
-<i>eruptive protuberances</i> are presented, as also <i>cloud-like</i>
-forms overtopping the rest. In the immediate foreground the bases of two
-towering columns appear deeply depressed below the general horizon of
-the segment observed, showing an extraordinary velocity of motion of the
-whole uplifted mass toward the observer. The highest of these
-protuberances was 126,000 miles in height at the moment of observation.
-The triple protuberance at the left with two drooping wings and a tall
-swaying spire tipped with a very bright flame, shows by its more
-brilliant color the higher temperature (and possibly compression) to
-which its gases have been subjected. The irregular black bands behind
-this protuberance indicate the presence there of less condensed and
-cooler clouds of the same gases. The dimmer jets of dame rising from the
-chromosphere are either vanishing protuberances, or, as in the case of
-the smallest jet shown at the extreme right of the horizon, are the tops
-of protuberances just coming into view.
-</p>
-<p>
-<i>Approximate scale, 6000 miles&mdash;1 inch.</i>
-</p>
-
-<p><br /></p>
-
-<h5>PLATE III.&mdash;TOTAL ECLIPSE OF THE SUN.
-<br />
-<i>Observed July</i> 29, 1878, <i>at Creston, Wyoming Territory.</i></h5>
-
-<p>
-A telescopic view of the sun's <i>corona</i> or extreme outer atmosphere
-and of the <i>solar flames</i> or <i>prominences</i> during a total
-eclipse. At the moment of observation the dark disk of the moon, while
-still hiding the sun's main body, had passed far enough eastward to
-allow the rosy pink chromospheric prominences to be seen on its western
-border. On all sides of the sun's hidden disk, the <i>corona</i> shows
-its pale greenish light extending in halo-like rays and streamers, and
-two very remarkable wings stretch eastward and westward very nearly in
-the plane of the ecliptic and in the direction of the positions of
-Mercury and Venus respectively at the time of observation. The full
-extent of these wings could not be shown in the Plate without reducing
-its scale materially, since the westerly wing extended no less than
-twelve times the sun's diameter, and the easterly wing nearly as far, or
-over ten million miles. A circlet of bright light immediately bordering
-the moon's disk is the so-called <i>inner corona</i>, next to which the
-wings and streamers arc brightest, thence shading off imperceptibly into
-the twilight sky of the eclipse. Other noteworthy peculiarities of the
-corona, as observed during this eclipse, are the varying angles at which
-the radiating streamers are seen to project, the comparatively dark
-intervals between them, and the curved, wisp-like projections seen upon
-the wings. An especially noticeable gap appears where the most westerly
-of the upward streamers abruptly cuts off the view of the long wing. The
-largest and brightest of the curving streamers on the westerly wing
-coincides with the highest flame-like protuberance. To some observers of
-this eclipse the upward and downward streamers seemed pointed at their
-outer extremities and less regular in form.
-</p>
-<p>
-<i>Approximate scale, 135,000 miles&mdash;1 inch.</i>
-</p>
-
-<p><br /></p>
-
-<h5>PLATE IV.&mdash;AURORA BOREALIS.
-<br />
-<i>As observed March</i> 1, 1872, <i>at 9h. 25m. P. M.</i></h5>
-
-<p>
-The view presents the rare spectacle of an aurora spanning the sky from
-east to west in concentric arches. The Polar Star is nearly central in
-the background, the constellation of the Great Bear on the right and
-Cassiopeia's chair on the left. The large star at some distance above
-the horizon on the right is Arcturus. The almost black inner segment of
-the aurora resting upon the horizon, has its summit in the magnetic
-meridian, which was in this case a little west of north, its arc being
-indented by the bases of the ascending streamers. Both streamers and
-arches were, when observed, tremulous with upward pulsations and there
-was also a wave-like movement of the streamers from west to east. The
-prevailing color of this aurora is a pale whitish green and the
-complementary red appears especially at the west end of the auroral
-arch. The summits of the streamers are from four hundred to live hundred
-miles above the earth and the aurora is therefore a phenomenon of the
-terrestrial atmosphere rather than of astronomical observation proper.
-</p>
-
-<p><br /></p>
-
-<h5>PLATE V.&mdash;THE ZODIACAL LIGHT.
-<br />
-<i>Observed February</i> 20, 1876.</h5>
-
-<p>
-An observation of the cone of light whose axis lies along the Zodiac,
-whence it derives its name. It is drawn as seen in the west, with its
-base in the constellation Pisces, and its apex near the familiar group
-of the Pleiades in the constellation Taurus. The first bright star above
-the horizon in the base of the cone is the planet <i>Venus</i> and at
-some distance above is the reddish disk of <i>Mars</i>, the two being in
-rare companionship as evening stars. Above the constellation Pisces, two
-bright stars of Aries lie just outside the cone at the right. The
-nearest bright star above these at the right is <i>Beta</i>, the leading
-star of the constellation Triangula. Further at the right the three
-prominent stars nearly in a line are, in ascending order, <i>Delta</i>,
-<i>Beta</i> and <i>Gamma</i> of the constellation Andromeda. Above these
-at the left, the brightest star of a quadrangular group of four is the
-remarkable variable star <i>Algol</i> (<i>Beta</i>) of the constellation
-Perseus, which changes from the second to the fourth magnitude in a
-period of less than three days. At the left and a little above the
-Pleiades is the ruddy star <i>Aldebaran</i>, one of the Hyades and chief
-star in the constellation Taurus. These are the principal stars visible
-in this portion of the sky at the time of the observation. Their
-relative positions are represented as seen in the sky and not by the
-common method of star-atlases, which allows for the change from a
-spherical to a plane surface. Their magnitude in the order of brightness
-is indicated only approximately.
-</p>
-
-<p><br /></p>
-
-<h5>PLATE VI.&mdash;MARE HUMORUM.
-<br />
-<i>From a study made in 1875.</i></h5>
-
-<p>
-A view of one of the lunar plains, or so-called <i>seas</i>
-(<i>Maria</i>), with an encircling mountainous wall consisting of
-volcano-like craters in various stages of subsidence and dislocation.
-The sun-light coming from the west casts strong shadows from all the
-elevations eastward, and is just rising on the <i>terminator</i>, where
-the rugged structure of the Moon's surface is best seen. The lighter
-portions are the more elevated mountainous tracts and crater summits.
-The detailed description of this Plate given in the body of the Manual
-is repeated here for convenience of reference: The Mare Humorum, or sea
-of moisture, as it is called, is one of the smaller gray lunar plains.
-Its diameter, which is very nearly the same in all directions, is about
-270 miles, the total area of this plain being about 50,000 square miles.
-It is one of the most distinct plains of the Moon, and is easily seen
-with the naked eye on the left-hand side of the disk. The floor of the
-plain is, like that of the other gray plains, traversed by several
-systems of very extended but low hills and ridges, while small craters
-are disseminated upon its surface. The color of this formation is of a
-dusky greenish gray along the border, while in the interior it is of a
-lighter shade, and is of brownish olivaceous tint. This plain, which is
-surrounded by high clefts and rifts, well illustrates the phenomena of
-dislocation and subsidence. The double-ringed crater Vitello, whose
-walls rise from 4,000 to 5,000 feet in height, is seen in the upper
-left-hand corner of the gray plain. Close to Vitello at the east is the
-large broken ring-plain Lee, and farther east, and a little below, is a
-similarly broken crater called Doppelmayer. Both of these open craters
-have mountainous masses and peaks on their door, which is on a level
-with that of the Mare Humorum. A little below, and to the left of these
-objects, is dimly seen a deeply imbedded oval crater, whose walls barely
-rise above the level of the plain. On the right-hand side of the great
-plain is a long <i>fault</i>, with a system of fracture running along
-its border. On this right-hand side may be seen a part of the line of
-the terminator, which separates the light from the darkness. Towards the
-lower right-hand corner is the great ring-plain Gassendi, 55 miles in
-diameter, with its system of fractures and its central mountains, which
-rise from 3,000 to 4,000 feet above its floor. This crater slopes
-towards the plain, showing the subsidence to which it has been
-submitted. While the northern portion of the wall of this crater rises
-to 10,000 feet, that on the plain is only 500 feet high, and is even
-wholly demolished at one place where the floor of the crater is in
-direct communication with the plain. In the lower part of the sea, and a
-little to the west of the middle line, is found the crater
-Agatharchides, which shows below its north wall the marks of rills
-impressed by a flood of lava, which once issued from the side of the
-crater. On the left-hand side of the plain is seen the half-demolished
-crater Hippalus resembling a large bay, which has its interior strewn
-with peaks and mountains. On this same side can be seen one of the most
-important systems of clefts and fractures visible on the Moon, these
-clefts varying in length from 150 to 200 miles.
-</p>
-<p>
-<i>Approximate scale, 15 miles&mdash;1 inch.</i>
-</p>
-
-<p><br /></p>
-
-<h5>PLATE VII.&mdash;PARTIAL ECLIPSE OF THE MOON.
-<br />
-<i>Observed October</i> 24, 1874.</h5>
-
-<p>
-A view of the Moon partially obscured by the Earth's shadow, whose
-outline gives ocular proof of the earth's rotundity of form. The
-shadowed part of the Moon's surface is rendered visible by the diffused
-sun-light refracted upon it from the earth's atmosphere. Its reddish
-brown color is due to the absorption, by vapors present near the earth's
-surface, of a considerable part of this dim light. On both the obscured
-and illuminated tracts the configurations of the Moon's surface are seen
-as with the naked eye. The craters appear as distinct patches of lighter
-color, and the noticeably darker areas are the depressed plains or
-<i>Maria</i>. The large crater <i>Tycho</i>, at the lower part of the
-disk, is the most prominent of these objects, with its extensive system
-of <i>radiating streaks</i>. The largest crater above is
-<i>Copernicus</i>, at the left of which is <i>Kepler</i> and still above
-are <i>Aristarchus</i> and <i>Herodotus</i> appearing as if blended in
-one. Above and at the left of the great crater <i>Tycho</i>, the first
-dark tract is the <i>Mare Humorum</i> of Plate VI., seen in its natural
-position, with the crater <i>Gassendi</i> at its northern (upper)
-extremity and <i>Vitello</i> on its southern (lower) border. The
-advancing border of the shadow appears, as always, noticeably darker
-than the remainder, an effect probably of contrast. The illuminated
-segment of the Moon's disk has its usual appearance, the lighter
-portions being the more elevated mountainous surfaces, and the dark
-spaces the floors of extensive plains.
-</p>
-<p>
-<i>Approximate scale, 140 miles&mdash;1 inch.</i>
-</p>
-
-<p><br /></p>
-
-<h5>PLATE VIII.&mdash;THE PLANET MARS.
-<br />
-<i>Observed September</i> 3, 1877, <i>at 11h. 55m. P. M.</i></h5>
-
-<p>
-A view of the southern hemisphere of Mars, when in the most favorable
-position for observation, and when exceptionally free from the clouds,
-which very frequently hide its surface configurations. Since, of all the
-planets, Mars is most like the earth, Plate VIII. may give a fair idea
-of the appearance of our globe to a supposed observer on Mars. The dark
-gray and black markings, are regarded as tracts of water, or of some
-liquid with similar powers of absorbing light; and, for the same reason,
-the lighter portions, of a prevailing reddish tint, are supposed to be
-bodies of land, while the bright white portions are variously due to
-clouds, to polar snow or ice, and the bright rim of white along the
-limb, to the depth of the atmosphere through which the limb is seen. The
-chief permanent features of the planet's surface have been named in
-honor of various astronomers.
-</p>
-<p>
-The large dark tract on the left is <i>De La Rue Ocean</i>, the isolated
-oval spot near the centre is <i>Terby Sea</i>, and on the right is the
-western end of <i>Maraldi Sea</i>, with strongly indented border.
-Directly north of (below) De La Rue Ocean, is <i>Maedler Continent</i>;
-above it stretches <i>Jacob Land</i>; and surrounding Terby Sea is
-<i>Secchi Continent</i>. Extending into the centre of De La Rue Ocean is
-a curious double peninsula, called, in consequence of the dimness of
-former observations, <i>Hall Island</i>. The sharply defined,
-white-crested northern borders of De La Rue Ocean and Maraldi Sea may
-indicate the existence there of lofty coast ranges, more or less
-constantly covered with opaque clouds strongly reflecting light. The
-white spot in the centre of Maedler Continent, of a temporary nature,
-has a similar explanation. The intervals of olivaceous gray on Secchi
-Continent and elsewhere may perhaps be ascribed to the flooding and
-drying up of marshes and lowlands, as these markings have been observed
-to vary somewhat in connection with the change of seasons on Mars. The
-greenish tints observed along the planet's limb, alike on the darker and
-lighter surfaces, are probably due to an optical effect, the green being
-complementary to the prevailing red of the disk. The brilliant oval
-white spot near the southern (upper) pole of the planet is a so-called
-polar spot, in all probability consisting of a material similar to snow
-or ice and here observed in the midst of a dark open sea.
-</p>
-<p>
-<i>Approximate scale, 300 miles&mdash;1 inch.</i>
-</p>
-
-<p><br /></p>
-
-<h5>PLATE IX.&mdash;THE PLANET JUPITER.
-<br />
-<i>Observed November</i> 1, 1880, <i>at 9h. 30m. P. M.</i></h5>
-
-<p>
-This planet is perpetually wrapped in dense clouds which hide its inner
-globe from view. The drawing shows Jupiter's outer clouded surface with
-its usual series of alternate light and dark belts, the disk as a whole
-appearing brighter in the centre than near the limb. The darker gray and
-black markings indicate in general the lower cloud-levels; that is,
-partial breaks or rifts in the cloudy envelope, whose prevailing depth
-apparently exceeds four thousand miles. While the deepest depression in
-the cloudy envelope is within the limits of the Great Red Spot, the
-vision may not even here penetrate very deeply. Two of Jupiter's four
-moons present bright disks near the planet's western limb, and cast
-their shadows far eastward on the disk, that of the "second satellite"
-falling upon the Red Spot. On the Red Spot are seen in addition two
-small black spots, no explanation of which can yet be offered. The broad
-white ring of clouds bordering the Red Spot appeared in constant motion.
-The central, or equatorial belt, shows brilliant cloudy masses of both
-the <i>cumulus</i> and <i>stratus</i> types, and the underlying gray and
-black cloudy surfaces are pervaded with the pinkish color characteristic of
-this belt. The dark circular spots on the wide white belt next north
-showed in their mode of formation striking resemblances to sun-spots.
-They afterward coalesced into a continuous pink belt. The diffusion of
-pinkish color over the three northernmost dark bands, as here observed,
-is unusual. About either pole is seen the uniform gray segment or polar
-cap. The equatorial diameter is noticeably longer than the polar
-diameter, a consequence of the planet's extraordinary swiftness of
-rotation. To the same cause may also be due chiefly the distribution of
-the cloudy belts parallel to the planet's equator, though the analogy of
-the terrestrial trade-winds fails to explain all the observed phenomena.
-</p>
-<p>
-<i>Approximate scale, 5,500 miles&mdash;1 inch.</i>
-</p>
-
-<p><br /></p>
-
-<h5>PLATE X.&mdash;THE PLANET SATURN.
-<br />
-<i>Observed November</i> 30, 1874, <i>at 5h. 50m. P. M.</i></h5>
-
-<p>
-Saturn is unique amongst the planets in that its globe is encircled by a
-series of concentric rings, which lie in the plane of its equator, and
-consist, according to present theories, of vast throngs of minute bodies
-revolving about the planet, like so many satellites, in closely parallel
-orbits. The globe of Saturn, like that of Jupiter, is surrounded by
-cloudy belts parallel to its equator. The broad equatorial belt, of a
-delicate pinkish tint, is both brighter and more mottled than the
-narrower yellowish white belts, which alternate with darker belts of
-ashy gray on both the north and south sides, but are seen here only on
-the northern side. The disk has an oval shape, owing to the extreme
-polar compression of the globe.
-</p>
-<p>
-The outer, middle and inner rings, with their various subdivisions, are
-clearly shown in Plate X., and are best seen on the so-called <i>ansæ</i>,
-or handles, projecting on either side. The gray <i>outer ring</i> is
-separated by the dusky pencil line into two divisions, both of which
-appear slightly mottled on the ansæ, as if with clouds. The <i>middle
-ring</i> has three sub-divisions which are clearly distinguishable,
-although separated by no dark interval, viz., a brilliant white outer
-zone, distinctly mottled, as seen on the extremities of the ansæ, and
-two interior zones of gradually diminishing brightness. The <i>gauze</i> or
-<i>dusky ring</i> is seen at its full width on the ansæ, but on the
-background of the strongly illuminated globe only its outer and
-presumably denser border is visible. The shadow of the globe on the
-rings is seen on the lower portion of the eastern ansæ. The shadow on
-the dusky ring is with difficulty perceptible; the shadow on the middle
-ring is slightly concave toward the planet, which concavity is abruptly
-increased on the outer zone of this ring; while the shadow on the outer
-ring slants away from the globe. These appearances are fully accounted
-for by supposing a general increase of level from the inner edge of the
-dusky ring to the outer margin of the middle ring, and a uniform lower
-level on the outer ring. Other observers have regarded the deflection of
-the shadow as an effect of irradiation. The inner margin of the double
-outer ring presents on both ansæ a number of slight indentations,
-which, if not actual irregularities in the contour of this ring, may be
-explained as shadows caused by elevations on the outer border of the
-middle ring, or possibly by over-hanging clouds.
-</p>
-<p>
-<i>Approximate scale, 6,500 miles&mdash;1 inch.</i>
-</p>
-
-<p><br /></p>
-
-<h5>PLATE XI.&mdash;THE GREAT COMET OF 1881.
-<br />
-<i>Observed on the night of June</i> 25-26, <i>at 1h. 30m. A. M.</i></h5>
-
-<p>
-A view of the comet 1881, III., drawn as if seen with the naked eye, the
-minute details, however, being reproduced as seen with the telescope.
-The star-like <i>nucleus</i> is attended by four conical wings which
-cause it to appear diamond-shaped. The <i>coma</i> appears double, the
-brilliant inner coma, immediately enveloping the nucleus, being
-surrounded by a fainter exterior coma, which has a noticeable depression
-corresponding to that of the inner edge of the principal coma. The tail
-is divided lengthwise by a dark rift and is brightest on its convex or
-forward side. An inner portion of the tail, brighter than the rest, is
-more strongly curved, as if by solar repulsion. Stars are seen through
-the brighter parts of the tail, as they may be seen even through the
-coma and nucleus, with little diminution of their light.
-</p>
-
-<p><br /></p>
-
-<h5>PLATE XII.&mdash;THE NOVEMBER METEORS.
-<br />
-<i>As observed on the night of November</i> 13-14, 1868.</h5>
-
-<p>
-A partially ideal view of the November Meteors, combining forms observed
-at different times during the night of Nov. 13th, 1868. It is not,
-however, a fanciful view, since a much larger number of meteors were
-observed falling at once during the shower of November, 1833, and at
-other times. The locality of the observation is shown by the Polar Star
-seen near the centre of the Plate, and Cassiopeia's Chair at the left.
-The general direction of the paths of the meteors is from the
-north-east, the <i>radiant point</i> of the shower having been in the
-constellation Leo, beyond and above Ursa Major. While the orbits of the
-meteors are, in general, curved regularly and slightly, several are
-shown with very eccentric paths, among them one which changed its course
-at a sharp angle. In the upper left-hand corner appear two vanishing
-trails of the "ring-form," and several others still further transformed
-into faint luminous patches of cloud. Red, yellow, green, blue and
-purple tints were observed in the meteors and their trails, as
-represented in the Plate.
-</p>
-
-<p><br /></p>
-
-<h5>PLATE XIII.&mdash;PART OF THE MILKY WAY.
-<br />
-<i>From a study made during the years</i> 1874, 1875 <i>and</i> 1876.</h5>
-
-<p>
-The course of the portion of the Galaxy represented in Plate XIII. is as
-follows: From Cassiopeia's chair, three bright stars of which appear at
-the upper edge of the Plate, the Galaxy, forming two streams, descends
-south, passing partly through Lacerta on the left, and Cepheus on the
-right; at this last point it approaches nearest to the Polar Star, which
-is itself outside of the field of view. Then it enters Cygnus, where it
-becomes very complicated and bright, and where several large cloudy
-masses are seen terminating its left branch, which passes to the right,
-near the bright star <i>Deneb</i>, the leader of this constellation.
-Below <i>Deneb</i>, the Galaxy is apparently disconnected and separated
-from the northern part by a narrow, irregular dark gap. From this
-rupture, the Milky-way divides into two great streams, separated by an
-irregular dark rift. An immense branch extends to the right, which,
-after having formed an important luminous mass between the stars
-<i>Gamma</i> and <i>Beta</i>, continues its southward progress through
-parts of Lyra, Vulpecula, Hercules, Aquila and Ophiuchus, where it
-gradually terminates a few degrees south of the equator. The main stream
-on the left, after having formed a bright mass around <i>Epsilon
-Cygni</i>, passes through Vulpecula and then Aquila, where it crosses
-the equinoctial just below the star <i>Eta</i>, after having involved in
-its nebulosity the bright star <i>Altair</i>, the leader of Aquila. In
-the southern hemisphere the Galaxy becomes very complicated and forms a
-succession of very bright, irregular masses, the upper one being in
-Scutum Sobieskii, while the others are respectively situated in
-Sagittarius and in Scorpio; the last, just a little above our horizon,
-being always considerably dimmed by vapors. From Scutum Sobieskii, the
-Galaxy expands considerably on the right, and sends a branch into
-Scorpio, in which the fiery red star <i>Antares</i> is somewhat
-involved. In the upper left-hand corner of the Plate, at some distance
-from the Milky-way, is seen dimly the Nebula in Andromeda, which becomes
-so magnificent an object to telescopic view.
-</p>
-
-<p><br /></p>
-
-<h5>PLATE XIV.&mdash;STAR-CLUSTER IN HERCULES.
-<br />
-<i>From a study made in June</i>, 1877.</h5>
-
-<p>
-In the constellation Hercules, a small nebulous mass is faintly visible
-to the eye, a telescopic view of which is presented in Plate XIV. It is
-one of the most beautiful of the easily resolvable globular clusters.
-The brilliancy of the centre gives the cluster a distinctly globular
-appearance, while the several wings curving in various directions, have
-suggested to some observers an irregularly spiral structure. The large
-stars of the cluster are arranged in several groups which correspond, in
-a general way, with the faintly luminous wings.
-</p>
-
-<p><br /></p>
-
-<h5>PLATE XV.&mdash;THE GREAT NEBULA IN ORION.
-<br />
-<i>From a study made in the years</i> 1875-76.</h5>
-
-<p>
-This nebula, which is one of the most brilliant and wonderful of
-telescopic objects, readily visible to the naked eve as a patch of
-nebulous light immediately surrounding the middle star of the three
-which form the sword of Orion, and a little south of the three
-well-known stars forming the belt. The small stars in this, as in other
-Plates of the series, are somewhat exaggerated in size, as was
-unavoidable with any mode of reproduction that could be employed. The
-bright pentagonal centre of the nebula is traversed by less luminous
-rifts, the several subdivisions thus outlined being irregularly mottled
-as if by bright fleecy clouds. Toward the lower part of this bright
-pentagonal centre is a comparatively dark space containing four bright
-stars which form a trapezium and together constitute the quadruple star
-<i>Theta Orionis</i>, which, to the naked eye, appears as the single star
-in the centre of the sword. On three sides of the central mass extend long
-bright wisps, whose curves fail, however, to reveal the spiral structure
-often attributed to this nebula. On the east a broad wing, with
-wave-shaped inner border, stretches southward. East of the trapezium are
-two especially noticeable dark spaces. Close to the main nebula on the
-north-east, a small faint nebula surrounds a bright star, and a branch
-from another faint stream of nebulous matter forming a loop to the
-southward, encloses the nebulous star (<i>Iota Orionis</i>) shown at the
-top of the Plate.
-</p>
-
-<p><br /><br /><br /></p>
-
-<div style='display:block; margin-top:4em'>*** END OF THE PROJECT GUTENBERG EBOOK THE TROUVELOT ASTRONOMICAL DRAWINGS MANUAL ***</div>
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+<div>*** START OF THE PROJECT GUTENBERG EBOOK 68394 ***</div>
+
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/trouvelot_cover.jpg" width="500" alt="" />
+</div>
+
+<p><br /><br /><br /></p>
+
+<h1>THE<br />
+TROUVELOT<br />
+ASTRONOMICAL DRAWINGS<br />
+MANUAL</h1>
+
+<p><br /><br /></p>
+
+<h4>BY</h4>
+
+<h2>E. L. TROUVELOT,</h2>
+
+<p><br /><br /></p>
+
+<h3>FORMERLY CONNECTED WITH THE OBSERVATORY OF HARVARD COLLEGE; FELLOW OF THE<br />
+AMERICAN ACADEMY OF ARTS AND SCIENCES, AND MEMBER OF THE SELENO-GRAPHICAL<br />
+SOCIETY OF GREAT BRITAIN; IN CHARGE OF A<br />
+GOVERNMENT EXPEDITION TO OBSERVE THE<br />
+TOTAL SOLAR ECLIPSE OF 1878.</h3>
+
+<p><br /><br /></p>
+
+<h4>NEW YORK</h4>
+
+<h4>CHARLES SCRIBNER'S SONS</h4>
+
+<h5>1882</h5>
+
+<p><br /><br /><br /></p>
+
+<h4><a id="INTRODUCTION">INTRODUCTION</a></h4>
+
+<p>
+During a study of the heavens, which has now been continued for more
+than fifteen years, I have made a large number of observations
+pertaining to physical astronomy, together with many original drawings
+representing the most interesting celestial objects and phenomena.
+</p>
+<p>
+With a view to making these observations more generally useful, I was
+led, some years ago, to prepare, from this collection of drawings, a
+series of astronomical pictures, which were intended to represent the
+celestial phenomena as they appear to a trained eye and to an
+experienced draughtsman through the great modern telescopes, provided
+with the most delicate instrumental appliances. Over two years were
+spent in the preparation of this series, which consisted of a number of
+large drawings executed in pastel. In 1876, these drawings were
+displayed at the United States Centennial Exhibition at Philadelphia,
+forming a part of the Massachusetts exhibit, in the Department of
+Education and Science.
+</p>
+<p>
+The drawings forming the present series comprise only a part of those
+exhibited at Philadelphia; but, although fewer in number, they are quite
+sufficient to illustrate the principal classes of celestial objects and
+phenomena.
+</p>
+<p>
+While my aim in this work has been to combine scrupulous fidelity and
+accuracy in the details, I have also endeavored to preserve the natural
+elegance and the delicate outlines peculiar to the objects depicted; but
+in this, only a little more than a suggestion is possible, since no
+human skill can reproduce upon paper the majestic beauty and radiance of
+the celestial objects.
+</p>
+<p>
+The plates were prepared under my supervision, from the original pastel
+drawings, and great care has been taken to make the reproduction exact.
+</p>
+<p>
+The instruments employed in the observations, and in the delineation of
+the heavenly bodies represented in the series, have varied in aperture
+from 6 to 26 inches, according to circumstances, and to the nature of
+the object to be studied. The great Washington refractor, kindly placed
+at my disposal by the late Admiral C. H. Davis, has contributed to this
+work, as has also the 26 inch telescope of the University of Virginia,
+while in the hands of its celebrated constructors, Alvan Clark &amp; Sons.
+The spectroscope used was made by Alvan Clark &amp; Sons. Attached to it is
+an excellent diffraction grating, by Mr. L. M. Rutherfurd, to whose
+kindness I am indebted for it.
+</p>
+<p>
+Those unacquainted with the use of optical instruments generally suppose
+that all astronomical drawings are obtained by the photographic process,
+and are, therefore, comparatively easy to procure; but this is not true.
+Although photography renders valuable assistance to the astronomer in
+the case of the Sun and Moon, as proved by the fine photographs of these
+objects taken by M. Janssen and Mr. Rutherfurd; yet, for other subjects,
+its products are in general so blurred and indistinct that no details of
+any great value can be secured. A well-trained eye alone is capable of
+seizing the delicate details of structure and of configuration of the
+heavenly bodies, which are liable to be affected, and even rendered
+invisible, by the slightest changes in our atmosphere.
+</p>
+<p>
+The method employed to secure correctness in the proportions of the
+original drawings is simple, but well adapted to the purpose in view. It
+consists in placing a fine reticule, cut on glass, at the common focus
+of the objective and the eye-piece, so that in viewing an object, its
+telescopic image, appearing projected on the reticule, can be drawn very
+accurately on a sheet of paper ruled with corresponding squares. For a
+series of such reticules I am indebted to the kindness of Professor
+William A. Rogers, of the Harvard College Observatory.
+</p>
+<p>
+The drawings representing telescopic views are inverted, as they appear
+in a refracting telescope&mdash;the South being upward, the North downward,
+the East on the right, and the West on the left. The Comet, the
+Milky-Way, the Eclipse of the Moon, the Aurora Borealis, the Zodiacal
+Light and the Meteors are represented as seen directly in the sky with
+the naked eye. The Comet was, however, drawn with the aid of the
+telescope, without which the delicate structure shown in the drawing
+would not have been visible.
+</p>
+<p>
+The plate representing the November Meteors, or so-called "Leonids," may
+be called an ideal view, since the shooting stars delineated, were not
+observed at the same moment of time, but during the same night. Over
+three thousand Meteors were observed between midnight and five o'clock
+in the morning of the day on which this shower occurred; a dozen being
+sometimes in sight at the same instant. The paths of the Meteors,
+whether curved, wavy, or crooked, and also their delicate colors, are in
+all cases depicted as they were actually observed.
+</p>
+<p>
+In the Manual, I have endeavored to present a general outline of what is
+known, or supposed, on the different subjects and phenomena illustrated
+in the series. The statements made are derived either from the best
+authorities on physical astronomy, or from my original observations,
+which are, for the most part, yet unpublished.
+</p>
+<p>
+The figures in the Manual relating to distance, size, volume, mass,
+etc., are not intended to be strictly exact, being only round numbers,
+which can, therefore, be more easily remembered.
+</p>
+<p>
+It gives me pleasure to acknowledge that the experience acquired in
+making the astronomical drawings published in Volume VIII. of the Annals
+of the Harvard College Observatory, while I was connected with that
+institution, has been of considerable assistance to me in preparing this
+work; although no drawings made while I was so connected have been used
+for this series.
+</p>
+
+<p style="margin-left: 60%;">E. L. TROUVELOT.</p>
+
+<p style="margin-left: 5%;"><i>Cambridge, March, 1882.</i></p>
+
+<p><br /><br /><br /></p>
+
+<h4>CONTENTS</h4>
+
+<p class="nind"><a href="#INTRODUCTION">INTRODUCTION</a><br />
+
+<a href="#LIST_OF_PLATES">LIST OF PLATES</a></p>
+
+<h4><i>THE SUN</i></h4>
+
+<p class="nind"><a href="#SUN">GENERAL REMARKS ON THE SUN</a><br />
+
+<a href="#SUN_SPOTS">SUN-SPOTS AND VEILED SPOTS</a><br />
+
+<a href="#SOLAR_PROTUBERANCES">SOLAR PROTUBERANCES</a><br />
+
+<a href="#TOTAL_ECLIPSE">TOTAL ECLIPSE OF THE SUN</a></p>
+
+<h4><i>THE AURORAL AND ZODIACAL LIGHTS</i></h4>
+
+<p class="nind"><a href="#THE_AURORA_BOREALIS">THE AURORA BOREALIS</a><br />
+
+<a href="#THE_ZODIACAL_LIGHT">THE ZODIACAL LIGHT</a></p>
+
+<h4><i>THE MOON</i></h4>
+
+<p class="nind"><a href="#THE_MOON">THE MOON</a><br />
+
+<a href="#ECLIPSES">ECLIPSES OF THE MOON</a></p>
+
+<h4><i>THE PLANETS</i></h4>
+
+<p class="nind"><a href="#THE_PLANET_MARS">THE PLANET MARS</a><br />
+
+<a href="#THE_PLANET_JUPITER">THE PLANET JUPITER</a><br />
+
+<a href="#THE_PLANET_SATURN">THE PLANET SATURN</a></p>
+
+<h4><i>COMETS AND METEORS</i></h4>
+
+<p class="nind"><a href="#COMETS">COMETS</a><br />
+
+<a href="#SHOOTING_STARS">SHOOTING-STARS AND METEORS</a></p>
+
+<h4><i>THE STELLAR SYSTEMS</i></h4>
+
+<p class="nind"><a href="#THE_MILKY_WAY">THE MILKY-WAY OR GALAXY</a><br />
+
+<a href="#THE_STAR_CLUSTERS">THE STAR-CLUSTERS</a><br />
+
+<a href="#THE_NEBULAE">THE NEBULÆ</a><br />
+
+<a href="#APPENDIX">APPENDIX</a></p>
+
+<p><br /><br /><br /></p>
+
+<h4><a id="LIST_OF_PLATES">LIST OF PLATES</a><a name="FNanchor_1_1" id="FNanchor_1_1"></a><a href="#Footnote_1_1" class="fnanchor">[1]</a></h4>
+
+<h4>PLATE</h4>
+
+<p class="nind"><a href="#figure01">I. GROUP OF SUN-SPOTS AND VEILED SPOTS.</a><br />
+
+<span style="margin-left: 1.5em;"><i>Observed June 17, 1875, at 7 h. 30m. A. M.</i></span><br />
+
+<a href="#figure02">II. SOLAR PROTUBERANCES.</a><br />
+
+<span style="margin-left: 1.5em;"><i>Observed May 5, 1873, at 9h. 40m. A. M.</i></span><br />
+
+<a href="#figure03">III. TOTAL ECLIPSE OF THE SUN.</a><br />
+
+<span style="margin-left: 1.5em;"><i>Observed July 29, 1878, at Creston, Wyoming Territory.</i></span><br />
+
+<a href="#figure04">IV. AURORA BOREALIS.</a><br />
+
+<span style="margin-left: 1.5em;"><i>As observed March 1, 1872, at 9h. 25m. P. M.</i></span><br />
+
+<a href="#figure05">V. THE ZODIACAL LIGHT.</a><br />
+
+<span style="margin-left: 1.5em;"><i>Observed February 20, 1876.</i></span><br />
+
+<a href="#figure06">VI. MARE HUMORUM.</a><br />
+
+<span style="margin-left: 1.5em;"><i>From a study made in 1875.</i></span><br />
+
+<a href="#figure07">VII. PARTIAL ECLIPSE OF THE MOON.</a><br />
+
+<span style="margin-left: 1.5em;"><i>Observed October 24, 1874.</i></span><br />
+
+<a href="#figure08">VIII. THE PLANET MARS.</a><br />
+
+<span style="margin-left: 1.5em;"><i>Observed September 3, 1877, at 11h. 55m. P. M.</i></span><br />
+
+<a href="#figure09">IX. THE PLANET JUPITER.</a><br />
+
+<span style="margin-left: 1.5em;"><i>Observed November 1, 1880, at 9h. 30m. P. M.</i></span><br />
+
+<a href="#figure10">X. THE PLANET SATURN.</a><br />
+
+<span style="margin-left: 1.5em;"><i>Observed November 30, 1874, at 5th. 50m. P. M.</i></span><br />
+
+<a href="#figure11">XI. THE GREAT COMET OF 1881.</a><br />
+
+<span style="margin-left: 1.5em;"><i>Observed on the night of June 25-26, at 1h. 30m. A. M.</i></span><br />
+
+<a href="#figure12">XII. THE NOVEMBER METEORS.</a><br />
+
+<span style="margin-left: 1.5em;"><i>As observed between midnight and 3 o'clock A. M., on the night<br />
+of November 13-14, 1868.</i></span><br />
+
+<a href="#figure13">XIII. PART OF THE MILKY-WAY.</a><br />
+
+<span style="margin-left: 1.5em;"><i>From a study made during the years 1874, 1875 and 1876.</i></span><br />
+
+<a href="#figure14">XIV. STAR-CLUSTER IN HERCULES.</a><br />
+
+<span style="margin-left: 1.5em;"><i>From a study made in June, 1877.</i></span><br />
+
+<a href="#figure15">XV. THE GREAT NEBULA IN ORION.</a><br />
+
+<span style="margin-left: 1.5em;"><i>From a study made in the years 1873-76.</i></span></p>
+
+<p><br /></p>
+
+<div class="footnote">
+
+<p class="nind"><a name="Footnote_1_1" id="Footnote_1_1"></a><a href="#FNanchor_1_1"><span class="label">[1]</span></a>For Key to the Plates, see Appendix.</p></div>
+
+<p><br /></p>
+
+<h5>
+<i>Reproduced from the Original Drawings, by Armstrong &amp; Company,<br />
+Riverside Press, Cambridge, Mass.</i>
+</h5>
+
+<p><br /><br /><br /></p>
+
+<h4><a id="SUN">GENERAL REMARKS ON THE SUN</a></h4>
+
+<p>
+The Sun, the centre of the system which bears its name, is a
+self-luminous sphere, constantly radiating heat and light.
+</p>
+<p>
+Its apparent diameter, as seen at its mean from the Earth, subtends an
+angle of 32', or a little over half a degree. A dime, placed about six
+feet from the eye, would appear of the same proportions, and cover the
+Sun's disk, if projected upon it.
+</p>
+<p>
+That the diameter of the Sun does not appear larger, is due to the great
+distance which separates us from that body. Its distance from the Earth
+is no less than 92,000,000 miles. To bridge this immense gap, would
+require 11,623 globes like the Earth, placed side by side, like beads on
+a string.
+</p>
+<p>
+The Sun is an enormous sphere whose diameter is over 108 times the
+diameter of our globe, or very nearly 860,000 miles. Its radius is
+nearly double the distance from the Earth to the Moon. If we suppose,
+for a moment, the Sun to be hollow, and our globe to be placed at the
+centre of this immense spherical shell, not only could our satellite
+revolve around us at its mean distance of 238,800 miles, as now, but
+another satellite, placed 190,000 miles farther than the Moon, could
+freely revolve likewise, without ever coming in contact with the solar
+envelope.
+</p>
+<p>
+The circumference of this immense sphere measures 2,800,000 miles. While
+a steamer, going at the rate of 300 miles a day, would circumnavigate
+the Earth in 83 days, it would take, at the same rate, nearly 25 years
+to travel around the Sun.
+</p>
+<p>
+The surface of the Sun is nearly 12,000 times the surface of the Earth,
+and its volume is equal to 1,300,000 globes like our own. If all the
+known planets and satellites were united in a single mass, 600 such
+compound masses would be needed to equal the volume of our luminary.
+</p>
+<p>
+Although the density of the Sun is only one-quarter that of the Earth,
+yet the bulk of this body is so enormous that, to counterpoise it, no
+less than 314,760 globes like our Earth would be required.
+</p>
+<p>
+The Sun uniformly revolves around its axis in about 25½ days. Its
+equator is inclined 7° 15' to the plane of the ecliptic, the axis of
+rotation forming, therefore, an angle of 82° 45' with the same plane.
+As the Earth revolves about the Sun in the same direction as that of the
+Sun's rotation, the apparent time of this rotation, as seen by a
+terrestrial spectator, is prolonged from 25½ days to about 27 days and
+7 hours.
+</p>
+<p>
+The rotation of the Sun on its axis, like that of the Earth and the
+other planets, is direct, or accomplished from West to East. To an
+observer on the Earth, looking directly at the Sun, the rotation of this
+body is from left to right, or from East to West.
+</p>
+<p>
+The general appearance of the Sun is that of an intensely luminous disk,
+whose limb, or border, is sharply defined on the heavens. When its
+telescopic image is projected on a screen, or fixed on paper by
+photography, it is noticed that its disk is not uniformly bright
+throughout, but is notably more luminous in its central parts. This
+phenomenon is not accidental, but permanent, and is due in reality to a
+very rare but extensive atmosphere which surrounds the Sun, and absorbs
+the light which that body radiates, proportionally to its thickness,
+which, of course, increases towards the limb, to an observer on the
+Earth.
+</p>
+
+<p><br /><br /></p>
+
+<h4>THE ENVELOPING LAYERS OF THE SUN</h4>
+
+<p>
+The luminous surface of the Sun, or that part visible at all times, and
+which forms its disk, is called the <i>Photosphere</i>, from the property
+it is supposed to possess of generating light. The photosphere does not
+extend to a great depth below the luminous surface, but forms a
+comparatively thin shell, 3,000 or 4,000 miles thick, which is distinct
+from the interior parts, above which it seems to be kept in suspense by
+internal forces. From the observations of some astronomers it would
+appear that the diameter of the photosphere is subject to slight
+variations, and, therefore, that the solar diameter is not a constant
+quantity. From the nature of this envelope, such a result does not seem
+at all impossible, but rather probable.
+</p>
+<p>
+Immediately above the photosphere lies a comparatively thin stratum,
+less than a thousand miles in thickness, called the <i>Reversing Layer</i>.
+This stratum is composed of metallic vapors, which, by absorbing the
+light of particular refrangibilities emanating from the photosphere
+below, produces the dark Fraunhofer lines of the solar spectrum.
+</p>
+<p>
+Above the reversing layer, and resting immediately upon it, is a
+shallow, semi-transparent gaseous layer, which has been called the
+<i>Chromosphere</i>, from the fine tints which it exhibits during total
+eclipses of the Sun, in contrast with the colorless white light radiated
+by the photosphere below. Although visible to a certain extent on the
+disk, the chromosphere is totally invisible on the limb, except with the
+spectroscope, and during eclipses, on account of the nature of its
+light, which is mainly monochromatic, and too feeble, compared with that
+emitted by the photosphere, to be seen.
+</p>
+<p>
+The chromospheric layer, which has a thickness of from 3,000 to 4,000
+miles, is uneven, and is usually upheaved in certain regions, its matter
+being transported to considerable elevations above its general surface,
+apparently by some internal forces. The portions of the chromosphere
+thus lifted up, form curious and complicated figures, which are known
+under the names of <i>Solar Protuberances, or Solar Flames</i>.
+</p>
+<p>
+Above the chromosphere, and rising to an immense but unknown height, is
+the solar atmosphere proper, which is only visible during total eclipses
+of the Sun, and which then surrounds the dark body of the Moon with the
+beautiful rays and glorious nimbus, called the <i>Corona</i>.
+</p>
+<p>
+These four envelopes: the photosphere, the reversing layer, the
+chromosphere, and the corona, constitute the outer portions of our
+luminary.
+</p>
+<p>
+Below the photosphere little can be seen, although it is known, as will
+appear below, that at certain depths cloud-like forms exist, and freely
+float in an interior atmosphere of invisible gases. Beyond this all is
+mystery, and belongs to the domain of hypothesis.
+</p>
+
+<p><br /><br /></p>
+
+<h4>STRUCTURE OF THE PHOTOSPHERE AND CHROMOSPHERE</h4>
+
+<p>
+The apparent uniformity of the solar surface disappears when it is
+examined with a telescope of sufficient aperture and magnifying powers.
+Seen under good atmospheric conditions, the greater part of the solar
+surface appears mottled with an infinite number of small, bright
+granules, irregularly distributed, and separated from each other by a
+gray-tinted background.
+</p>
+<p>
+These objects are known under different names. The terms granules and
+granulations answer very well for the purpose, as they do not imply
+anything positive as to their form and true nature. They have also been
+called <i>Luculœ</i>, <i>Rice Grains</i>, <i>Willow Leaves</i>, etc., by
+different observers.
+</p>
+<p>
+Although having different shapes, the granulations partake more or less
+of the circular or slightly elongated form. Their diameter, which varies
+considerably, has been estimated at from 0".5 to 3", or from 224 to
+1,344 miles. The granulations which attain the largest size appear,
+under good atmospheric conditions, to be composed of several granules,
+closely united and forming an irregular mass, from which short
+appendages protrude in various directions.
+</p>
+<p>
+The number of granulations on the surface of the Sun varies considerably
+under the action of unknown causes. Sometimes they are small and very
+numerous, while at other times they are larger, less numerous, and more
+widely separated. Other things being equal, the granulations are better
+seen in the central regions of the Sun than they are near the limb.
+</p>
+<p>
+Usually the granulations are very unstable; their relative position,
+form, and size undergoing continual changes. Sometimes they are seen to
+congregate or to disperse in an instant, as if acting under the
+influence of attractive and repulsive forces; assembling in groups or
+files, and oftentimes forming capricious figures which are very
+remarkable, but usually of short duration. In an area of great solar
+disturbances, the granulations are often stretched to great distances,
+and form into parallel lines, either straight, wavy, or curved, and they
+have then some resemblance to the flowing of viscous liquids.
+</p>
+<p>
+The granulations are usually terminated either by rounded or sharply
+pointed summits, but they do not all rise to the same height, as can be
+ascertained with the spectroscope when they are seen sidewise on the
+limb. In the regions where they are most abundant, they usually attain
+greater elevations, and when observed on the limb with the spectroscope,
+they appear as slender acute flames.
+</p>
+<p>
+The granulations terminated by sharply-pointed crests, although observed
+in all latitudes, seem to be characteristic of certain regions. A daily
+study of the chromosphere, extending over a period of ten years, has
+shown me that the polar regions are rarely ever free from these objects,
+which are less frequent in other parts of the Sun. In the polar regions
+they are sometimes so abundant that they completely form the solar limb.
+These forms of granulation are comparatively rare in the equatorial
+zones, and when seen there, they never have the permanency which they
+exhibit in the polar regions. When observed in the equatorial regions,
+they usually appear in small groups, in the vicinity of sun spots, or
+they are at least enclosed in areas of disturbances where such spots are
+in process of formation. In these regions they often attain greater
+elevations than those seen in high latitudes.
+</p>
+<p>
+As we are certain that in the equatorial zones these slender flames (<i>i.
+e.</i>, granulations) are a sure sign of local disturbance, it may be
+reasonably supposed that the same kind of energy producing them nearly
+always prevails in the polar regions, although it is there much weaker,
+and never reaches beyond certain narrow limits.
+</p>
+<p>
+Studied with the spectroscope, the granulations are found to be composed
+in the main of incandescent hydrogen gas, and of an unknown substance
+provisionally called "helium." Among the most brilliant of them are
+found traces of incandescent metallic vapors, belonging to various
+substances found on our globe.
+</p>
+<p>
+The chromosphere is not fixed, but varies considerably in thickness in
+its different parts, from day to day. Its thickness is usually greater
+in the polar regions, where it sometimes exceeds 6,700 miles. In the
+equatorial regions the chromosphere very rarely attains this height, and
+when it does, the rising is local and occupies only a small area. In
+these regions it is sometimes so shallow that its depth is only a few
+seconds, and is then quite difficult to measure. These numbers give, of
+course, the extreme limits of the variations of the chromosphere; but,
+nearly always, it is more shallow in the equatorial regions; and, as far
+as my observations go, the difference in thickness between the polar and
+equatorial zones is greater in years of calm than it is in years of
+great solar activity. But ten years of observation are not sufficient to
+warrant any definite conclusions on this subject.
+</p>
+<p>
+There is undoubtedly some relation between the greater thickness of the
+chromosphere in the polar regions, and the abundance and permanence of
+the sharply-pointed granulations observed in the same regions. This
+becomes more evident when we know that the appearance of
+similarly-pointed flames in the equatorial zones is always accompanied
+with a local thickening of the chromosphere. The thickening in the polar
+regions may be only apparent, and not due to a greater accumulation of
+chromospheric gases there; but may be caused by some kind of repulsive
+action or polarity, which lifts up and extends the summit of the
+granulations in a manner similar to the well-known mode of electric
+repulsion and polarity.
+</p>
+<p>
+As it seems very probable that the heat and light emanating from the Sun
+are mainly generated at the base of the granulations, in the filamentary
+elements composing the chromosphere and photosphere, it would follow
+that, as the size and number of these objects constantly vary, the
+amount of heat and light emitted by the Sun should also vary in the same
+proportion.
+</p>
+<p>
+The granulations of the solar surface are represented on Plate I., and
+form the general background to the group of Sun-spots forming the
+picture.
+</p>
+
+<p><br /><br /></p>
+
+<h4>THE FACULÆ</h4>
+
+<p>
+Although the solar surface is mainly covered with the luminous
+granulations and the grayish background above described, it is very rare
+that its appearance is so simple and uniform as already represented. For
+the most part, on the contrary, it is diversified by larger, brighter,
+and more complicated forms, which are especially visible towards the
+border of the Sun. Owing to their extraordinary brilliancy, these
+objects have been called <i>Faculœ</i> (torches).
+</p>
+<p>
+Although the faculæ are very seldom seen well beyond 50 heliocentric
+degrees from the limb, yet they exist, and are as numerous in the
+central parts of the disk as they are towards the border; since they
+form a part of the solar surface, and participate in its movement of
+rotation. Their appearance near the limb has been attributed to the
+effect of absorption produced by the solar atmosphere on the
+light from the photosphere; but this explanation seems inadequate,
+and does not solve the problem. The well-known fact that the solar
+protuberances&mdash;which are in a great measure identical with the
+faculæ&mdash;are much brighter at the base than they are at the summit,
+perhaps gives a clue to the explanation of the phenomenon; especially
+since we know that, in general, the summit of the protuberances is
+considerably broader than their base. When these objects are observed in
+the vicinity of the limb, they present their brightest parts to the
+observer, since, in this position, they are seen more or less sidewise;
+and, therefore, they appear bright and distinct. But as the faculæ
+recede from the limb, their sides, being seen under a constantly
+decreasing angle, appear more and more foreshortened; and, therefore,
+these objects grow less bright and less distinct, until they finally
+become invisible, when their bases are covered over by the broad, dusky
+summit generally terminating the protuberances.
+</p>
+<p>
+The faculæ appear as bright and luminous masses or streaks on the
+granular surface of the Sun, but they differ considerably in form and
+size. Two types at least are distinguishable. In their simplest form
+they appear either as isolated white spots, or as groups of such spots
+covering large surfaces, and somewhat resembling large flakes of snow.
+In their most characteristic types they appear as intensely luminous,
+heavy masses, from which, in most cases, issue intricate ramifications,
+sometimes extending to great distances. Generally, the ramifications
+issuing from the masses of faculæ have their largest branches directed
+in the main towards the eastern limb of the Sun. Some of these branches
+have gigantic proportions. Occasionally they extend over 60° and even
+80° of the solar surface, and, therefore, attain a length of from
+450,000 to 600,000 miles.
+</p>
+<p>
+Although the faculæ may be said to be seen everywhere on the surface of
+the Sun, there is a vast difference in different regions, with regard to
+their size, number, and brilliancy. They are largest, most abundant, and
+brightest on two intermediate zones parallel to the solar equator, and
+extending 35° or 40° to the north and to the south of this line. The
+breadth of these zones varies considerably with the activity of the
+solar forces. When they are most active, the faculæ spread on either
+side, but especially towards the equator, where they sometimes nearly
+meet those of the other zone. In years of little solar activity the
+belts formed by the faculæ are very narrow&mdash;the elements composing
+them being very few and small, although they never entirely disappear.
+</p>
+<p>
+The faculæ are very unstable, and are constantly changing: those of the
+small types sometimes form and vanish in a few minutes. When an area of
+disturbance of the solar surface is observed for some time, all seems in
+confusion; the movements of the granulations become unusually violent;
+they congregate in all sorts of ways, and thus frequently form temporary
+faculæ. Action of this kind is, for the most part, peculiar to the
+polar regions of the Sun.
+</p>
+<p>
+The larger faculæ have undoubtedly another origin, as they seem to be
+mainly formed by the ejection of incandescent gases and metallic vapors
+from the interior of the photosphere. In their process of development
+some of the heavy masses of faculæ are swollen up to great heights,
+being torn in all sorts of ways, showing large rents and fissures
+through which the sight can penetrate.
+</p>
+<p>
+Very few faculæ are represented in Plate I.; several streaks are shown
+at the upper left-hand corner, some appearing as whitish ramifications
+among the granulations representing the general solar surface.
+</p>
+
+<p><br /><br /><br /></p>
+
+<h4><a id="SUN_SPOTS">SUN-SPOTS AND VEILED SPOTS</a>
+<br /><br />
+PLATE I</h4>
+
+<p>
+Besides the brilliant faculæ already described, much more conspicuous
+markings, though of a totally different character, are very frequently
+observed on the Sun. On account of their darkish appearance, which is in
+strong contrast with the white envelope of our luminary, these markings
+were called <i>Maculæ</i>, or Sun-spots, by their earlier observers.
+</p>
+<p>
+The Sun-spots are not equally distributed on the solar surface; but like
+the faculæ, to which they are closely related, they occupy two
+zones&mdash;one on each side of the equator. These zones are comprised
+between 10° and 35° of north latitude, and 10° and 35° of south
+latitude. Between these two zones is a belt 20° in width, where the
+Sun-spots are rarely seen.
+</p>
+<p>
+Above the latitudes 35° north and south, the Sun-spots are rare, and it
+is only occasionally, and during years of great solar activity, that
+they appear in these regions; in only a few cases have spots of
+considerable size been seen there. A few observers, however, have seen
+spots as far as 40° and 50 from the equator; and La Hire even observed
+one in 70° of north latitude; but these cases are exceedingly rare. It
+is not uncommon, however, to see very small spots, or groups of such
+spots, within 8° or 10° from the poles.
+</p>
+<p>
+The activity of the Sun is subject to considerable fluctuation, and
+accordingly the Sun-spots vary in size and number in different years.
+During some years they are large, complicated, and very numerous; while
+in others they are small and scarce, and are sometimes totally absent
+for weeks and months together. The fluctuations in the frequency of
+Sun-spots are supposed to be periodical in their character, although
+their periods do not always appear to recur at exactly regular
+intervals. Sometimes the period is found to be only nine years, while at
+other times it extends to twelve years. The period generally adopted now
+is 11⅒ years, nearly; but further investigations are needed to
+understand the true nature of the phenomenon.
+</p>
+<p>
+The number of Sun-spots does not symmetrically augment and diminish, but
+the increase is more rapid than the diminution.
+</p>
+<p>
+The period of increase is only about four years, while that of decrease
+is over seven years; each period of Sun-spot maximum being nearer the
+preceding period of Sun-spot minimum than it is to that next following.
+</p>
+<p>
+The cause of these fluctuations in the solar energy is at present wholly
+unknown. Some astronomers, however, have attributed it to the influence
+of the planets Venus and Jupiter, the period of revolution of the latter
+planet being not much longer than the Sun-spot period; but this
+supposition lacks confirmation from direct observations, which, so far,
+do not seem to be in favor of the hypothesis. At the present time the
+solar activity is on the increase, and the Sun-spots will probably reach
+their maximum in 1883. The last minimum occurred in 1879, when only
+sixteen small groups of spots were observed during the whole year.
+</p>
+<p>
+Sun-spots vary in size and appearance; but, unless they are very small,
+in which case they appear as simple black dots, they generally consist
+of two distinct and well-characterized parts, nearly always present.
+There is first, a central part, much darker than the other, and sharply
+divided from it, called the "<i>Umbra</i>;" second, a broad, irregular
+radiated fringe of lighter shade, completely surrounding the first, and
+called the "<i>Penumbra</i>."
+</p>
+<p>
+Reduced to its simplest expression, a Sun-spot is a funnel-shaped
+opening through the chromosphere and the photosphere. The inner end of
+the funnel, or opening, gives the form to the umbra, while its sloping
+sides form the penumbra.
+</p>
+<p>
+The umbra of Sun-spots, whose outlines approximately follow the
+irregularities of the penumbral fringe, has a diameter which generally
+exceeds the width of the penumbral ring. Sometimes it appears uniformly
+black throughout; but it is only so by contrast, as is proved when
+either Mercury or Venus passes near a spot during a transit over the
+Sun's disk. The umbra then appears grayish, when compared with the
+jet-black disk of the planet.
+</p>
+<p>
+The umbra of spots is rarely so simple as just described; but it is
+frequently occupied, either partly or wholly, by grayish and rosy forms,
+somewhat resembling loosely-entangled muscular fibres. These forms have
+been called the <i>Gray and Rosy Veils</i>. Frequently these veils appear
+as if perforated by roundish black holes, improperly called <i>Nuclei</i>,
+which permit the sight to penetrate deeper into the interior. To all
+appearance the gray and rosy veils are of the same nature as the
+chromosphere and the faculæ, and are therefore mainly composed of
+hydrogen gas.
+</p>
+<p>
+Whatever can be known about the interior of the Sun, must be learned
+from the observations of these openings, which are comparatively small.
+But whatever this interior may be, we certainly know that it is not
+homogeneous. Apparently, the Sun is a gigantic bubble, limited by a very
+thin shell. Below this shell exists a large open space filled with
+invisible gases, in which, through the openings constituting the
+Sun-spots, the gray and rosy veils described above are occasionally seen
+floating.
+</p>
+<p>
+The fringe forming the penumbra of spots is much more complicated than
+the umbra. In its simpler form, it is composed of a multitude of bright,
+independent filaments of different forms and sizes, partly projecting
+one above the other, on the sloping wall of the penumbra, from which
+they seem to proceed. Seen from the Earth, these filaments have somewhat
+the appearance of thatched straw, converging towards the centre of the
+umbra. It is very rare, however, that the convergence of the penumbral
+filaments is regular, and great confusion sometimes arises from the
+entanglement of these filaments. Some of these elements appear straight,
+others are curved or loop-shaped; while still others, much larger and
+brighter than the rest, give a final touch to this chaos of filaments,
+from which results the general thatched and radiating appearance of the
+penumbra.
+</p>
+<p>
+The extremities of the penumbral filaments, especially of those forming
+the border of the umbra, are usually club-shaped and appear very
+brilliant, as if these elements had been superheated by some forces
+escaping through the opening of the spots.
+</p>
+<p>
+Besides these characteristics, the Sun-spots have others, which,
+although not always present, properly belong to them. Comparatively few
+spots are so simple as the form just described. Very frequently a spot
+is accompanied by brilliant faculæ, covering part of its umbra and
+penumbra, and appearing to form a part of the spot itself.
+</p>
+<p>
+When seen projected over Sun-spots, the faculæ appear intensely bright,
+and from these peculiarities they have been called <i>Luminous Bridges</i>.
+They are, in fact, bridges, but in most cases they are at considerable
+heights above the spots, kept there by invisible forces. When such spots
+with luminous bridges approach the Sun's limb, it is easy to see, by the
+rapid apparent displacement which they undergo, that they are above the
+general level.
+</p>
+<p>
+When the spots are closing up, the inverse effect is sometimes observed.
+On several occasions, I have seen huge masses of faculæ advance slowly
+over the penumbra of a spot and fall into the depths of the umbra,
+resembling gigantic cataracts. I have seen narrow branches of faculæ,
+which, after having fallen to great depths in the umbra, floated across
+it and disappeared under the photosphere on the opposite side. I have
+also seen luminous bridges, resembling cables, tightly stretched across
+the spots, slackening slowly, as if loosened at one end, and gently
+curving into the umbra, where they formed immense loops, large enough to
+receive our globe.
+</p>
+<p>
+It is to be remarked that, in descending under the photospheric shell,
+the bright faculæ and the luminous bridges gradually lose their
+brilliancy. At first they appear grayish, but in descending farther they
+assume more and more the pink color peculiar to the rosy veils. The
+pinkish color acquired by the faculæ when they reach a certain depth
+under the photosphere, is precisely the color of the chromosphere and of
+the solar protuberances, as seen during total eclipses of the Sun&mdash;a
+fact which furnishes another proof that the faculæ are of the same
+nature as the protuberances.
+</p>
+<p>
+I record here an observation which, at first sight, may appear
+paradoxical; but which seems, however, to be of considerable importance,
+as it shows unmistakably that the solar light is mainly, if not
+entirely, generated on its surface, or at least very near to it. On May
+26, 1878, I observed a large group of Sun-spots at a little distance
+from the east limb of the Sun. The spot nearest to the limb was partly
+covered over on its eastern and western sides by bright and massive
+faculæ which concealed about two-thirds of the whole spot, only a
+narrow opening, running from north to south, being left across the
+middle of the spot. Owing to the rotundity of the Sun, the penumbra of
+this spot, although partly covered by the faculæ, could, however, be
+seen on its eastern side, since the sight of the observer could there
+penetrate sidewise under the faculæ. Upon that part of the penumbra
+appeared a strong shadow, representing perfectly the outline of the
+facular mass situated above it. The phenomenon was so apparent that no
+error of observation was possible, and a good drawing of it was secured.
+If this faculæ had been as bright beneath as it was above, it is
+evident that no shadow could have been produced; hence the light of
+these faculæ must have been mainly generated on or very near their
+exterior surfaces. This, with the well-proved fact that the bright
+faculæ lose their light in falling into the interior of the Sun, seems
+to suggest the idea that the bright light emitted by the faculæ, and
+very probably all the solar light, can be generated only on its surface;
+the presence of the coronal atmosphere being perhaps necessary to
+produce it. Several times before this observation, I had suspected that
+some faculæ were casting a shadow, but as this seemed so improbable, my
+attention was not awakened until the phenomenon became so prominent that
+it could not escape notice.
+</p>
+<p>
+With due attention, some glimpses of the phenomenon can frequently be
+observed through the openings of some of the faculæ projecting over the
+penumbra of Sun-spots. It is very seldom that the structure of the
+penumbra is seen through such openings, which usually appear as dark as
+the umbra of the large spots, although they do not penetrate through the
+photosphere like the latter. It is only when the rents in the faculæ
+are numerous and quite large, that the penumbral structure is recognized
+through them. Since these superficial rents in the faculæ do not extend
+through the photosphere, and appear black, it seems evident that the
+penumbra seen through them cannot be as bright as it is when no faculæ
+are projected upon it, and therefore that the faculæ intercept light
+from the exterior surface, which would otherwise reach the penumbra.
+</p>
+<p>
+While the matter forming the faculæ sometimes falls into the interior
+of the Sun, the same kind of matter is frequently ejected in enormous
+quantities, and with great force, from the interior, through the visible
+and invisible openings of the photosphere, and form the protuberances
+described in the following section of this manual (Solar protuberances)
+It is not only the incandescent hydrogen gas or the metallic vapors
+which are thus ejected, but also cooler hydrogen gas, which sometimes
+appears as dark clouds on the solar surface. On December 12, 1875, I
+observed such a cloud of hydrogen issuing from the corner of a small
+Sun-spot. It traveled several thousand miles on the solar surface, in a
+north-easterly direction, before it became invisible.
+</p>
+<p>
+Solar spots are formed in various ways; but, for the most part, the
+apparition of a spot is announced beforehand, by a great commotion of
+the solar surface at the place of its appearance, and by the formation
+of large and bright masses of faculæ, which are usually swollen into
+enormous bubbles by the pressure of the internal gases. These bubbles
+become visible in the spectroscope while they are traversing the solar
+limb, as they are then presented to us sidewise. Under the action of the
+increasing pressure, the base of the faculæ is considerably stretched,
+and, its weakest side finally giving way, the facular mass is torn in
+many places from the solar surface, and is perforated by holes of
+different sizes and forms. The holes thus made along the border of the
+faculæ appear as small black spots, separated more or less by the
+remaining portion of the lacerated faculæ, and they enlarge more and
+more at the expense of the intervening portions, which thus become very
+narrow. This perforated side of the faculæ, offering less resistance,
+is gradually lifted up, as would be the cover of a box, for example,
+while its opposite side remains attached to the surface. The facular
+matter separating the small black holes is greatly stretched during this
+action, and forms long columns and filaments. These appear as luminous
+bridges upon the large and perfectly-formed spot, which is then seen
+under the lifted facular masses. The spots thus made visible are soon
+freed from the facular masses, which are gradually shifted towards the
+opposite side.
+</p>
+<p>
+In such cases the spots are undoubtedly formed under the faculæ before
+they can be seen. This becomes evident when such spots, not yet cleared
+from the faculæ covering them, are observed near the east limb; since
+in this position the observer can see through the side-openings of the
+faculæ, and sometimes recognize the spots under their cover.
+</p>
+<p>
+It frequently occurs that the spots thus formed under the faculæ
+continue to be partly covered by the facular clouds, the forces at work
+in them being apparently too feeble to shift them aside. In such cases
+these spots are visible when they are in the vicinity of the limb, where
+they are seen sidewise; but when observed in the east, in being carried
+forward by the solar rotation towards the centre of the disk, they
+gradually diminish in size, and finally become invisible. The
+disappearance of these spots, however, is only apparent, being due to
+the fact that, as they advance towards the centre of the disk, our
+lateral view of them is gradually lost, and they are finally hidden from
+sight by the overhanging faculæ which then serve as a screen between
+the observer and the spot. This class of spots may be called <i>Lateral
+Spots</i>, from the fact that they can only be seen laterally, and near the
+Sun's limb.
+</p>
+<p>
+Solar spots are also formed in various other ways. Some, like those
+represented in Plate I., appearing without being announced by any
+apparent disturbance of the surface, or by the formation of any faculæ,
+form and develop in a very short time. Others, appearing at first as
+very small spots having an umbra and a penumbra, slowly and gradually
+develop into very large spots. This mode of formation, which would seem
+to be the most natural, is, however, quite rare. Spots of this class
+have a duration and permanence not observed in those of any other type.
+These spots of slow and regular development are never accompanied by
+faculæ or luminous bridges, nor have they any gray or rosy veils in
+their interior; a fact which may, perhaps, account for their permanent
+character.
+</p>
+<p>
+Another class of spots, which is also rare, appear as long and narrow
+crevasses showing the penumbral structure of the ordinary spots; but
+these rarely have any umbra. These long, and sometimes exceedingly
+narrow fissures of the solar envelopes, with their radiated penumbral
+structure, strongly suggest the idea that the photosphere is composed of
+a multitude of filamentary elements having the granulations for summits.
+Such a crevasse is represented on Plate I., and unites the two spots
+which form the group.
+</p>
+<p>
+The duration of Sun-spots varies greatly. Some last only for a few
+hours; while others continue for weeks and even months at a time, but
+not without undergoing changes.
+</p>
+<p>
+The modes of disappearance of Sun-spots are as various as those of their
+apparition. The spots rarely close up by a gradual diminution or
+contraction of their umbra and penumbra. This mode of disappearance
+belongs exclusively to the spots deprived of faculæ and veils. One of
+the most common modes of the disappearance of a spot is its invasion by
+large facular masses, which slowly advance upon its penumbra and umbra
+and finally cover it entirely. It is a process precisely the reverse of
+that in which spots are formed by the shifting aside of the faculæ, as
+above described. In other types, the spots close up by the gradual
+enlargement of the luminous bridges traversing them, which are slowly
+transformed into branches of the photosphere, all of the characteristics
+of which they have acquired. In many cases, the spots covered over by
+the faculæ continue to exist for some time, hidden under these masses,
+as is often proved, either by the appearance of small spots on the
+facular mass left at the place they occupied, or even by the
+reappearance of the same spot.
+</p>
+<p>
+Apart from the general movement of rotation of the solar surface, some
+of the spots seem to be endowed with a proper motion of their own, which
+becomes greater the nearer the spots are to the solar equator. According
+to the observations of Mr. Carrington, the period of rotation of the
+Sun, as deduced from the observations of the solar spots during a period
+of seven years, is 25 days at the equator; while at 50° of heliocentric
+latitude it is 27 days. But the period of rotation, as derived from the
+observations of spots occupying the same latitude, is far from being
+constant, as it varies at different times, with the frequency of the
+spots and with the solar activity, so that at present the law of these
+variations is not well known. From the character of the solar envelope,
+it seems very natural that the rotation should differ in the different
+zones and at different times, since this envelope is not rigid, but very
+movable, and governed by forces which are themselves very variable.
+</p>
+<p>
+Although it is a general law that the spots near the equator have a more
+rapid motion than those situated in higher latitudes, yet, in many
+cases, the proper motion of the spots is more apparent than real. For
+the most part, the changes of form and the rapid displacements observed
+in some spots are only apparent, and due to the fact that the large
+masses of faculæ which are kept in suspense above them are very
+unstable, and change position with the slightest change in the forces
+holding them in suspension. Since in these cases we view the spots
+through the openings of the faculæ situated above them, the slightest
+motion of these objects produces an apparent motion in the spots,
+although they have remained motionless. Accordingly, it has been
+remarked that of all the spots, those which have the greater proper
+motion are precisely those which have the most faculæ and luminous
+bridges; while the other spots in the same regions, but not attended by
+similar phenomena, are comparatively steady in their movement. These
+last spots are undoubtedly better adapted than any others to exhibit the
+rotation of the Sun; but it is probable that this period of rotation
+will never be known with accuracy, simply because the solar surface is
+unstable, and does not rotate uniformly.
+</p>
+<p>
+The Sun-spots have a remarkable tendency to form into groups of various
+sizes, but whatever may be the number of spots thus assembled, the group
+is nearly always composed of two principal spots, to which the others
+are only accessories. The tendency of the Sun-spots to assemble in pairs
+is general, and is observed in all latitudes, even among the minute
+temporary groups formed in the polar regions. Whenever several are
+situated quite close together, those belonging to the same group can be
+easily recognized by this character. Whatever may be the position of the
+axis of the two principal spots of a group when it is first formed, this
+axis has a decided tendency to place itself parallel to the solar
+equator, no matter to what latitude the group belongs; and if it is
+disturbed from this position, it soon returns to it when the disturbance
+has ceased.
+</p>
+<p>
+It is also remarkable that the spots observed at the same time remain in
+nearly the same parallel of latitude for a greater or less period of
+time; but they keep changing their position from year to year, their
+latitude decreasing with the activity of the solar forces.
+</p>
+<p>
+Among the Sun-spots, those associated with faculæ form the groups which
+attain the largest proportions. When such groups acquire an apparent
+diameter of 1' or more, they are plainly visible to the naked eye, since
+for a spot to be visible to the naked eye on the Sun, it need only
+subtend an angle of 50". I have sometimes seen such groups through a
+smoky atmosphere, when the solar light was so much reduced that the disk
+could be observed directly and without injury to the sight.
+</p>
+<p>
+The largest spot which ever came under my observation was seen during
+the period from the 13th to the 19th of November, 1870. This spot, which
+was on the northern hemisphere of the Sun, was conspicuous among the
+smaller spots constituting the group to which it belonged, and followed
+them on the east. On November 16th, when it attained its largest size,
+the diameter of its penumbra occupied fully one-fifth of the diameter of
+the Sun; its real diameter being, therefore, not less than 172,000
+miles, or nearly 22 times the diameter of the Earth. As the umbra of
+this spot occupied a little more than one-third of its whole diameter,
+seven globes like our own, placed side by side on a straight line, could
+easily have passed through this immense gap. To fill the area of this
+opening, about 45 such globes would have been needed. This spot was, of
+course, very easily seen with the naked eye, its diameter being almost
+eight times that required for a spot to be visible without a telescope.
+</p>
+<p>
+Ancient historians often speak of obscurations of the Sun, and it has
+been supposed by some astronomers that this phenomenon might have been
+due in some cases to the apparition of large spots. A few spots on the
+surface of the Sun, like that just described, would sensibly reduce its
+light.
+</p>
+<p>
+Besides the ordinary Sun-spots already described, others are at times
+observed on the surface of the Sun, which show some of the same
+characteristics, but never attain so large proportions. They always
+appear as if seen through a fog, or veil, between the granulations of
+the solar surface. On account of their vagueness and ill-defined contours,
+I have proposed for these objects the term, "<i>Veiled Spots</i>."
+Veiled spots have a shorter duration than the ordinary spots, the
+smaller types sometimes forming and vanishing in a few minutes. Some of
+the larger veiled spots, however, remain visible for several days in
+succession, and show the characteristics of other spots in regard to the
+arrangement of their parts.
+</p>
+<p>
+The veiled spots have no umbra or penumbra, although they are usually
+accompanied by faculæ resembling those seen near the ordinary spots.
+They are frequently seen in the polar regions, but are there always of
+small size and of short duration. The veiled spots are larger, and more
+apt to arrange themselves into groups, in the regions occupied by the
+ordinary spots, and it is not rare to observe such spots transform
+themselves into ordinary spots, and vice versa. The veiled spots,
+therefore, seem to be ordinary spots filled up, or covered over by the
+granulations and semi-transparent gases composing the chromospheric
+layer. That it is so, becomes more evident, from the fact that large
+Sun-spots in process of diminution are sometimes gradually covered with
+faint and scattered granules which descend in long, narrow filaments,
+and become less and less distinguishable as they attain greater depth.
+This phenomenon, associated with the fact that the luminous bridges seen
+over the Sun-spots which are closing up are sometimes transformed into
+branches which show the characteristic structure of the photosphere,
+goes far to prove that the solar envelopes are mainly composed of an
+innumerable quantity of radial filaments of varying height.
+</p>
+
+<p><br /></p>
+
+<div class="figcenter" style="width: 400px;">
+<a id="figure01"></a>
+<br />
+<img src="images/figure01.jpg" width="400" alt="" />
+<div class="caption">
+<p>PLATE I.&mdash;GROUP OF SUN-SPOTS AND VEILED SPOTS.</p>
+<p class="smaller">Observed on June 17th 1875 at 7 h. 30 m. A.M.</p>
+</div></div>
+
+<p><br /></p>
+
+<p>
+The group of Sun-spots represented in Plate I., was observed and drawn
+on June 17th, 1875, at 7h. 30m. A. M. The first traces of this group
+were seen on June 15th, at noon, and consisted of three small black dots
+disseminated among the granulations. At that time, no disturbance of the
+surface was noticeable, and no faculæ were seen in the vicinity of these
+spots. On June 16th, at 8 o'clock A. M., the three small spots had
+become considerably enlarged, and, as usual, the group consisted of two
+principal spots. Between these two spots all was in motion: the
+granulations, stretched into long, wavy, parallel lines, had somewhat
+the appearance of a liquid in rapid motion. At 1 o'clock, P. M., on the
+same day, the group had considerably enlarged; the faculæ, the
+granulations, and the penumbral filaments being interwoven in an
+indescribable manner. On the morning of the 17th, these spots had
+assumed the complicated form and development represented in the drawing;
+while at the same time two conspicuous veiled spots were seen on the
+left hand, at some distance above the group.
+</p>
+<p>
+Some luminous bridges are visible upon the left hand spot, traversing
+the penumbra and umbra of this spot in various directions. The umbra of
+one of the spots is occupied, and partly filled with gray and rosy
+veils, similar to those above described, and the granulations of the
+solar surface form a background to the group of spots.
+</p>
+<p>
+This group of spots was not so remarkable for its size as for its
+complicated structure. The diameter of the group from east to west was
+only 2½ minutes of arc, or about 67,000 miles. The upper part of the
+umbra of the spot situated on the right hand side of the group was
+nearly 7,000 miles in diameter, or less by 1,000 miles than the diameter
+of the Earth. Some of the long filaments composing that part of the
+penumbra, situated on the left hand side of the same spot, were 17,000
+miles in length. One of these fiery elements would be sufficient to
+encircle two-thirds of the circumference of the Earth.
+</p>
+
+<p><br /><br /><br /></p>
+
+<h4><a id="SOLAR_PROTUBERANCES">SOLAR PROTUBERANCES</a>
+<br /><br />
+PLATE II</h4>
+
+<p>
+The chromosphere forming the outlying envelope of the Sun, is subject,
+as has been shown above, to great disturbances in certain regions,
+causing considerable upheavals of its surface and violent outbursts of
+its gases. From these upheavals and outbursts of the chromosphere result
+certain curious and very interesting forms, which are known under the
+name of "<i>Solar Protuberances</i>," "<i>Prominences</i>," or
+"<i>Flames</i>."
+</p>
+<p>
+These singular forms, which could, until recently, be observed only
+during the short duration of the total eclipses of the Sun, can now be
+seen on every clear day with the spectroscope, thanks to Messrs. Janssen
+and Lockyer, to whose researches solar physics is so much indebted.
+</p>
+
+<p><br /></p>
+
+<div class="figcenter" style="width: 400px;">
+<a id="figure02"></a>
+<br />
+<img src="images/figure02.jpg" width="400" alt="" />
+<div class="caption">
+<p>PLATE II.&mdash;SOLAR PROTUBERANCES.</p>
+<p class="smaller">Observed on May 5, 1873 at 9h, 40m. A.M.</p>
+</div></div>
+
+<p><br /></p>
+
+<p>
+The solar protuberances, the Sun-spots, and the faculæ to which they
+are closely related, are confined within the same general regions of the
+Sun, although the protuberances attain higher heliocentric latitudes.
+</p>
+<p>
+There is certainly a very close relation between the faculæ and the
+solar protuberances, since when a group of the faculæ traverses the
+Sun's limb, protuberances are always seen at the same place. It seems
+very probable that the faculæ and the protuberances are in the main
+identical. The faculæ may be the brighter portion of the protuberances,
+consisting of gases which are still undergoing a high temperature and
+pressure; while the gases which have been relieved from this pressure
+and have lost a considerable amount of their heat, may form that part of
+the protuberances which is only visible on the Sun's limb.
+</p>
+<p>
+A daily study of the solar protuberances, continued for ten years, has
+shown me that these objects are distributed on two zones which are
+equidistant from the solar equator, and parallel with it. The zone
+arrangement of the protuberances is more easily recognized during the
+years of minimum solar activity, as in these years the zones are very
+narrow and widely separated. During these years the belt of
+protuberances is situated between 40° and 45° of latitude, north and
+south. In years of great solar activity the zones spread considerably on
+either side of these limits, especially towards the equator, which they
+nearly reach, only a narrow belt, usually free from protuberances,
+remaining between them. Towards the poles the zones do not spread so
+much, and there the space free from protuberances is considerably
+greater than it is at the equator.
+</p>
+<p>
+During years of maximum solar activity, the protuberances, like the
+Sun-spots and the faculæ, are very numerous, very large, and very
+complicated&mdash;sometimes occupying a great part of the whole solar limb.
+As many as twenty distinct flames are sometimes observed at one time. In
+years of minimum solar activity, on the contrary, the prominences are
+very few in number, and they are of small size; but, as far as my
+observations go, they are never totally absent.
+</p>
+<p>
+In general, the solar flames undergo rapid changes, especially those
+which are situated in the vicinity of Sun-spots, although they
+occasionally remain unchanged in appearance and form for several hours
+at a time. The protuberances situated in higher latitudes are less
+liable to great and sudden changes, often retaining the same form for
+several days. The changes observed in the protuberances of the
+equatorial regions are due in part to the comparatively great changes in
+their position with respect to the spectator, which are occasioned by
+the rotation of the Sun. This rotation, of course, has a greater angular
+velocity on the equator than in higher latitudes. In most cases,
+however, the changes of the equatorial protuberances are too great and
+too sudden to be thus explained. They are, in fact, due to the greater
+solar activity developed in the equatorial zones, and wherever spots are
+most numerous.
+</p>
+<p>
+The solar protuberances appear under various shapes, and are often so
+complicated in appearance that they defy description. Some resemble huge
+clumsy masses having a few perforations on their sides; while others
+form a succession of arches supported by pillars of different styles.
+Others form vertical or inclined columns, often surmounted by cloud-like
+masses, or by various appendages, which sometimes droop gracefully,
+resembling gigantic palm leaves. Some resemble flames driven by the
+wind; others, which are composed of a multitude of long and narrow
+filaments, appear as immense fiery bundles, from which sometimes issue
+long and delicate columns surmounted by torch-like objects of the most
+fantastic pattern. Some others resemble trees, or animal forms, in a
+very striking manner; while still others, apparently detached from the
+solar limb, float above it, forming graceful streamers or clouds of
+various shapes. Some of the protuberances are very massive, while others
+are so thin and transparent as to form a mere veil, through which more
+distant flames can easily be seen.
+</p>
+<p>
+Notwithstanding this variety of form, two principal classes of solar
+protuberances may be recognized: the cloud-like or quiescent, and the
+eruptive or metallic protuberances.
+</p>
+<p>
+The first class, which is the most common, comprises all the cloud-like
+protuberances resting upon the chromosphere or floating about it. The
+protuberances of this type often obtain enormous horizontal proportions,
+and it is not rare to see some among them occupying 20° and 30° of the
+solar limb. The height attained by protuberances of this class does not
+correspond in general to their longitudinal extent; although some of
+their branches attain considerable elevations. These prominences very
+seldom have the brilliancy displayed by the other type, and are
+sometimes so faint as to be seen with difficulty. Although it is
+generally stated by observers that some of the protuberances belonging
+to this class are detached from the solar surface, and kept in
+suspension above the surface, like the clouds in our atmosphere, yet it
+seems to me very doubtful whether protuberances are ever disconnected
+from the chromosphere, since, in an experience of ten years, I have
+never been able to satisfy myself that such a thing has occurred. Many
+of them have appeared to me at first sight to be detached from the
+surface, but with a little patience and attention I was always able to
+detect faint traces of filamentary elements connecting them with the
+chromosphere. Quite often I have seen bright protuberances gradually
+lose their light and become invisible, while soon after they had
+regained it, and were as clearly visible as before. Observations of this
+kind seem to show that while the prominences are for the most part
+luminous, there are also a few which are non-luminous and invisible to
+the eye. These dark and invisible forms are most generally found in the
+vicinity of Sun-spots in great activity. When observing such regions
+with the spectroscope, it is not rare to encounter them in the form of
+large dark spots projecting on the solar spectrum near the hydrogen
+lines. On July 28th, 1872, I observed with the spectroscope a dark spot
+of this kind issuing from the vicinity of a large Sun-spot, and
+extending over one-fifth of the diameter of the Sun. This object had
+been independently observed in France a little earlier by M. Chacornac
+with the telescope, in which it appeared as a bluish streak.
+</p>
+<p>
+The second class of solar protuberances, comprising the eruptive type,
+is the most interesting, inasmuch as it conveys to us a conception of
+the magnitude and violence of the solar forces. The protuberances of
+this class, which are always intensely bright, appear for the most part
+in the immediate vicinity of Sun-spots or faculæ. These protuberances,
+which seem to be due to the outburst of the chromosphere, and to the
+violent ejection of incandescent gases and metallic vapors from the
+interior of the Sun, sometimes attain gigantic proportions and enormous
+heights.
+</p>
+<p>
+While the spectrum of the protuberances of the cloudy type is simple,
+and usually composed of four hydrogen lines and the yellow line
+D<sub>3</sub>, that of the eruptive class is very complicated, and,
+besides the hydrogen lines and D<sub>3</sub>, it often exhibits the
+bright lines of sodium, magnesium, barium, titanium, and iron, and
+occasionally, also, a number of other bright lines.
+</p>
+<p>
+The phenomena of a solar outburst are grand and imposing. Suddenly
+immense and acute tongues and jets of flames of a dazzling brilliancy
+rise up from the solar limb and extend in various directions. Some of
+these fiery jets appear perfectly rigid, and remain apparently
+motionless in the midst of the greatest disorder. Immense straps and
+columns form and rise in an instant, bending and waving in all sorts of
+ways and assuming innumerable shapes. Sometimes powerful jets resembling
+molten metal spring up from the Sun, describing graceful parabolas,
+while in their descent they form numerous fiery drops which acquire a
+dazzling brilliancy when they approach the surface.
+</p>
+<p>
+The upward motion of the protuberances in process of formation is
+sometimes very rapid. Some protuberances have been observed to ascend in
+the solar atmosphere at the rate of from 120 to 497 miles a second.
+Great as this velocity may appear, it is nevertheless insignificant when
+compared with that sometimes attained by protuberances moving in the
+line of sight instead of directly upwards. Movements of this kind are
+indicated by the displacement of the bright or dark lines in the
+spectrum. A remarkable instance of this kind occurred on the 26th of
+June, 1874. On that day I observed a displacement of the hydrogen C line
+corresponding to a velocity of motion of 1,600 miles per second. The
+mass of hydrogen gas in motion producing such a displacement was,
+according to theory, moving towards the Earth at this incredible rate,
+when it instantly vanished from sight as if it had been annihilated, and
+was seen no more.
+</p>
+<p>
+Until recently the protuberances had not been observed to rise more than
+200,000 miles above the solar surface; but, on October 7th, 1880, a
+flame, which had an elevation of 80,000 miles when I observed it at 8h.
+55m. A. M., had attained the enormous altitude of 350,000 miles when it
+was observed at noon by Professor C. A. Young. If we had such a
+protuberance on the Earth, its summit would be at a height sufficient
+not merely to reach, but to extend 100,000 miles beyond the Moon.
+</p>
+<p>
+Although the solar protuberances represented in Plate II. have not the
+enormous proportions attained by some of these objects, yet they are as
+characteristic as any of the largest ones, and afford a good
+illustration of the purely eruptive type of protuberances. The height of
+the largest column in the group equals 4' 43", or a little over 126,000
+miles. A large group of Sun-spots was in the vicinity of these
+protuberances when they were observed and delineated.
+</p>
+
+<p><br /><br /><br /></p>
+
+<h4><a id="TOTAL_ECLIPSE">TOTAL ECLIPSE OF THE SUN</a>
+<br /><br />
+PLATE III</h4>
+
+<p>
+A solar eclipse is due to the passage of the Moon directly between the
+observer and the Sun. Such an eclipse can only occur at New Moon, since
+it is only at that time that our satellite passes between us and the
+Sun. The Moon's orbit does not lie precisely in the same plane as the
+orbit of the Earth, but is inclined about five degrees to it, otherwise
+an eclipse of the Sun would occur at every New Moon, and an eclipse of
+the Moon at every Full Moon.
+</p>
+<p>
+Since the Moon's orbit is inclined to that of the Earth, it must
+necessarily intersect this orbit at two opposite points. These points
+are called the nodes of the Moon's orbit. When our satellite passes
+through either of the nodes when the Moon is new, it appears interposed
+to some extent between the Sun and the Earth, and so produces a solar
+eclipse; while if it passes a node when the Moon is full, it is more or
+less obscured by the Earth's shadow, which then produces an eclipse of
+the Moon. But, on the other hand, when the New Moon and the Full Moon do
+not coincide with the passage of our satellite through the nodes of its
+orbit, no eclipse can occur, since the Moon is not then on a line with
+the Sun and the Earth, but above or below that line.
+</p>
+<p>
+Owing to the ellipticity of the Moon's orbit, the distance of our
+satellite from the Earth varies considerably during each of its
+revolutions around us, and its apparent diameter is necessarily subject
+to corresponding changes. Sometimes it is greater, sometimes it is less,
+than the apparent diameter of the Sun. If it is greater at the time of a
+solar eclipse, the eclipse will be total to a terrestrial observer
+stationed nearly on the line of the centres of the Sun and Moon, while
+it will be only partial to another observer stationed further from this
+line. But the Moon's distance from the Earth may be so great and its
+apparent diameter consequently so small that even those observers
+nearest the central line of the eclipse see the border of the Sun all
+round the black disk of the Moon; the eclipse is then annular. Even
+during the progress of one and the same eclipse the distance of the Moon
+from the parts of the Earth towards which its shadow is directed may
+vary so much that, while the eclipse is total to some observers, others
+equally near the central line, but stationed at a different place, will
+see it as annular.
+</p>
+<p>
+The shadow cast by the Moon on the Earth during total eclipses, travels
+along upon the surface of the Earth, in consequence of the daily
+movement of rotation of our globe combined with the movements of the
+Earth and Moon in their orbits. The track of the Moon's shadow over the
+Earth's surface has a general eastward course, so that the more westerly
+observers see it earlier than those east of them. An eclipse may
+continue total at one place for nearly eight minutes, but in ordinary
+cases the total phase is much shorter.
+</p>
+<p>
+The nodes of the Moon's orbit do not invariably occupy the same
+position, but move nearly uniformly, their position with regard to the
+Sun, Earth, and Moon being at any time approximately what it formerly
+was at a series of times separated by equal intervals from each other.
+Each interval comprises 223 lunations, or 18 years, 11 days, and 7 or 8
+hours. The eclipses which occur within this interval are almost exactly
+repeated during the next similar interval. This period, called the
+"Saros," was well known to the ancients, who were enabled by its means
+to predict eclipses with some certainty.
+</p>
+
+<p><br /></p>
+
+<div class="figcenter" style="width: 400px;">
+<a id="figure03"></a>
+<br />
+<img src="images/figure03.jpg" width="400" alt="" />
+<div class="caption">
+<p>PLATE III.&mdash;TOTAL ECLIPSE OF THE SUN.</p>
+<p class="smaller">Observed July 29, 1878, at Creston, Wyoming Territory</p>
+</div></div>
+
+<p><br /></p>
+
+<p>
+A total eclipse of the Sun is a most beautiful and imposing phenomenon.
+At the predicted time the perfectly round disk of the Sun becomes
+slightly indented at its western limb by the yet invisible Moon. This
+phenomenon is known as the "first contact."
+</p>
+<p>
+The slight indentation observed gradually increases with the advance of
+the Moon from west to east, the irregularities of the surface of our
+satellite being plainly visible on the border of the dark segment
+advancing on the Sun's disk. With the advance of the Moon on the Sun,
+the light gradually diminishes on the Earth. Every object puts on a dull
+and gloomy appearance, as when night is approaching; while the bright
+sky, losing its light, changes its pure azure for a livid grayish color.
+</p>
+<p>
+Two or three minutes before totality begins, the solar crescent, reduced
+to minute proportions, gives comparatively so little light that faint
+traces of the Sun's atmosphere appear on the western side behind the
+dark body of the Moon, whose limb then becomes visible outside of the
+Sun. I observed this phenomenon at Creston during the eclipse of 1878.
+From 15 to 20 seconds before totality, the narrow arc of the Sun's disk
+not yet obscured by the Moon seems to break and separate towards the
+extremities of its cusps, which, thus divided, form independent points of
+light, which are called "<i>Baily's beads</i>." A moment after, the whole
+solar crescent breaks into numerous beads of light, separated by dark
+intervals, and, suddenly, they all vanish with the last ray of Sunlight,
+and totality has begun with the "second contact." This phenomenon of
+Baily's beads is undoubtedly caused by the irregularities of the Moon's
+border, which, on reaching the solar limb, divide the thin solar
+crescent into as many beads of light and dark intervals as there are
+peaks and ravines seen sidewise on that part of the Moon's limb.
+</p>
+<p>
+With the disappearance of the last ray of light, the planets and the
+stars of the first and second magnitude seem to light up and become
+visible in the sky. The darkness, which had been gradually creeping in
+with the progress of the eclipse, is then at its maximum. Although
+subject to great variations in different eclipses, the darkness is never
+so great as might be expected from the complete obscuration of our
+luminary, as the part of our atmosphere which is still exposed to the
+direct rays of the Sun, reflects to us some of that light, which thus
+diminishes the darkness resulting from the disappearance of the Sun.
+Usually the darkness is sufficient to prevent the reading of common
+print, and to deceive animals, causing them to act as if night was
+really approaching. During totality the temperature decreases, while the
+humidity of the atmosphere augments.
+</p>
+<p>
+Simultaneously with the disappearance of Baily's beads, a pale, soft,
+silvery light bursts forth from behind the Moon, as if the Sun, in
+disappearing, had been vaporized and expanded in all directions into
+soft phosphorescent rays and streamers. This pale light is emitted by
+gases constituting the solar atmosphere surrounding the bright nucleus
+now obscured by the dark body of our satellite. This solar atmosphere is
+called <i>Corona</i>, from its distant resemblance to the aureola, or
+glory, represented by ancient painters around the heads of saints.
+</p>
+<p>
+With the bursting forth of the corona, a very thin arc of bright white
+light is seen along the Moon's limb, where the solar crescent has just
+disappeared. This thin arc of light is the reversing layer, which, when
+observed with the spectroscope at that moment, exhibits bright lines
+answering to the dark lines of the ordinary solar spectrum. Immediately
+above this reversing layer, and concentric with it, appears the
+pink-colored chromospheric layer, with its curiously shaped flames and
+protuberances. During totality, the chromosphere and protuberances are
+seen without the aid of the spectroscope, and appear of their natural
+color, which, although somewhat varying in their different parts, is, on
+the whole, pinkish, and similar to that of peach-blossoms; yet it is
+mixed here and there with delicate prismatic hues, among which the pink
+and straw colors predominate.
+</p>
+<p>
+The color of the corona seems to vary in every eclipse, but as its tints
+are very delicate, it may depend, in a great measure, upon the vision of
+the observer; although there seems to be no doubt that there are real
+variations. At Creston, in 1878, it appeared to both Professor W.
+Harkness and myself of a decided pale greenish hue.
+</p>
+<p>
+The corona appears under different forms, and has never been observed
+twice alike. Its dimensions are also subject to considerable variations.
+Sometimes it appears regular and very little extended, its distribution
+around the Sun being almost uniform; although in general it spreads a
+little more in the direction of the ecliptic, or of the solar equator.
+At other times it appears much larger and more complicated, and forms
+various wings and appendages, which in some cases, as in 1878, extend to
+immense distances; while delicate rays radiate in straight or curved
+lines from the spaces left in the polar regions between the wings. The
+corona has sometimes appeared as if divided by immense dark gaps,
+apparently free from luminous matter, and strongly resembling the dark
+rifts seen in the tails of comets. This was observed in Spain and Sicily
+during the total eclipse of the Sun in 1870. Different structures,
+forming wisps and streamers of great length, and interlaced in various
+ways, are sometimes present in the corona, while faint but more
+complicated forms, distantly resembling enormous solar protuberances
+with bright nuclei, have also been observed.
+</p>
+<p>
+As the Moon continues its eastward progress, it gradually covers the
+chromosphere and the solar protuberances on the eastern side of the Sun;
+while, at the same time, the protuberances and the chromosphere on the
+opposite limb gradually appear from under the retreating Moon. Then, the
+thin arc of the reversing layer is visible for an instant, and is
+instantly followed by the appearance of a point of dazzling white light,
+succeeded immediately by the apparition of Daily's beads on each side,
+and totality is over, with this third contact. The corona continues to
+be visible on the eastern side of the Sun for several minutes longer,
+and then rapidly vanishes.
+</p>
+<p>
+The thin solar crescent increases in breadth as the Moon advances;
+while, at the same time, the darkness and gloom spread over nature
+gradually disappear, and terrestrial objects begin to resume their
+natural appearance. Finally the limb of the Moon separates from that of
+the Sun at the instant of "fourth contact," and the eclipse is over.
+</p>
+<p>
+The phenomena exhibited by the corona in different eclipses are very
+complex, and, so far, they have not been sufficiently studied to enable
+us to understand the true nature of the solar atmosphere. From the
+spectral analysis of the corona, and the phenomena of polarization, it
+has been learned, at least, that while the matter composing the upper
+part of the solar atmosphere is chiefly composed of an unknown
+substance, producing the green line 1474, its lower part is mainly
+composed of hydrogen gas at different temperatures, a part of which is
+self-luminous, while the other part only reflects the solar light. But
+the proportion of the gaseous particles emitting light, to those simply
+reflecting it, is subject to considerable variations in different
+eclipses. At present it would seem that in years of great solar
+disturbances, the particles emitting light are found in greater quantity
+in the corona than those reflecting it; but further observations will be
+required to confirm these views.
+</p>
+<p>
+It is very difficult to understand how the corona, which in certain
+eclipses extends only one diameter of the Sun, should, in other cases,
+as in 1878, extend to the enormous distance of twelve times the same
+diameter. Changes of such magnitude in the solar atmosphere, if due to
+the operation of forces with which we are acquainted, cannot yet be
+accounted for by what is known of such forces. Their causes are still as
+mysterious as those concerned in the production of the monstrous tails
+displayed by some comets on their approach to the Sun.
+</p>
+<p>
+Plate 3, representing the total eclipse of the Sun of July 29th, 1878,
+was drawn from my observations made at Creston, Wyoming Territory, for
+the Naval Observatory. The eclipse is represented as seen in a
+refracting telescope, having an aperture of 6⅓ inches, and as it
+appeared a few seconds before totality was over, and when the
+chromosphere was visible on the western limb of the Sun. The two long
+wings seen on the east and west side of the Sun, appeared considerably
+larger in the sky than they are represented in the picture.
+</p>
+
+<p><br /><br /><br /></p>
+
+<h4><a id="THE_AURORA_BOREALIS">THE AURORA BOREALIS</a>
+<br /><br />
+PLATE IV</h4>
+
+<p>
+The name of Polar Auroras is given to certain very remarkable luminous
+meteoric phenomena which appear at intervals above the northern or the
+southern horizons of both hemispheres of the Earth. When the phenomenon
+is produced in our northern sky, it is called "Aurora Borealis," or
+"Northern Lights;" and when it appears in the southern sky, it is called
+"Aurora Australis," or southern aurora.
+</p>
+<p>
+Marked differences appear in the various auroras observed from our
+northern latitudes. While some simply consist in a pale, faint
+luminosity, hardly distinguishable from twilight, others present the
+most gorgeous and remarkable effects of brightness and colors.
+</p>
+<p>
+A great aurora is usually indicated in the evening soon after twilight,
+by a peculiar grayish appearance of the northern sky just above the
+horizon. The grayish vapors giving that appearance, continuing to form
+there, soon assume a dark and gloomy aspect, while they gradually take
+the form of a segment of a circle resting on the horizon. At the same
+time that this dark segment is forming, a soft pearly light, which seems
+to issue from its border, spreads up in the sky, where it gradually
+vanishes, being the brightest at its base. This arc of light, gradually
+increasing in extent as well as in brightness, reaches sometimes as far
+as the polar star. On some rare occasions, one or two, and even three,
+concentric arches of bright light form one above the other over the dark
+segment, where they appear as brilliant concentric rainbows. While the
+aurora continues to develop and spread out its immense arc, the border
+of the dark segment loses its regularity and appears indented at several
+places by patches of light, which soon develop into long, narrow,
+diverging rays and streamers of great beauty. For the most part the
+auroral light is either whitish or of a pale, greenish tint; but in some
+cases it exhibits the most beautiful colors, among which the red and
+green predominate. In these cases the rays and streamers, which are
+usually of different colors, produce the most magnificent effects by
+their continual changes and transformations.
+</p>
+<p>
+The brightness and extent of the auroral rays are likewise subject to
+continual changes. An instant suffices for their development and
+disappearance, which may be succeeded by the sudden appearance of others
+elsewhere, as though the original streamers had been swiftly transported
+to a new place while invisible. It frequently happens that all the
+streamers seem to move sidewise, from west to east, along the arch,
+continuing meanwhile to exhibit their various changes of form and color.
+For a time, these appearances of motion continue to increase, a
+succession of streamers alternately shooting forth and again fading,
+when a sudden lull occurs, during which all motion seems to have ceased.
+The stillness then prevailing is soon succeeded by slight pulsations of
+light, which seem to originate on the border-of the dark segment, and
+are propagated upwards along the streamers, which have now become more
+numerous and active. Slow at first, these pulsations quicken by degrees,
+and after a few minutes the whole northern sky seems to be in rapid
+vibration. The lively upward and downward movement of these streamers
+entitles them to the name of "merry dancers" given them in northern
+countries where they are frequent.
+</p>
+<p>
+Long waves of light, quickly succeeded by others, are propagated in an
+instant from the horizon to the zenith; these, in their rapid passage,
+cause bends and curves in the streamers, which then, losing their
+original straightness, wave and undulate in graceful folds, resembling
+those of a pennant in a gentle breeze. Although the coruscations add to
+the grandeur of the spectacle, they tend to destroy the diverging
+streamers, which, being disconnected from the dark segment, or torn in
+various ways, are, as it were, bodily carried up towards the zenith.
+</p>
+<p>
+In this new phase the aurora is transformed into a glorious crown of
+light, called the "Corona." From this corona diverge in all directions
+long streamers of different colors and forms, gracefully undulating in
+numerous folds, like so many banners of light. Some of the largest of
+these streamers appear like fringes composed of short transverse rays of
+different intensity and colors, producing the most fantastic effects,
+when traversed by the pulsations and coruscations which generally run
+across these rays during the great auroral displays.
+</p>
+<p>
+The aurora has now attained its full development and beauty. It may
+continue in this form for half an hour, but usually the celestial fires
+begin to fade at the end of fifteen or twenty minutes, reviving from
+time to time, but gradually dying out. The northern sky usually appears
+covered by gray and luminous streaks and patches after a great aurora,
+these being occasionally rekindled, but more often they gradually
+disappear, and the sky resumes its usual appearance.
+</p>
+<p>
+The number of auroras which develop a corona near the zenith is
+comparatively small in our latitudes; but many of them, although not
+exhibited on so grand a scale, are nevertheless very interesting. On
+some very rare occasions the auroral display has been confined almost
+exclusively to the dark segment, which appeared then as if pierced along
+its border by many square openings, like windows, through which appeared
+the bright auroral light.
+</p>
+
+<p><br /></p>
+
+<div class="figcenter" style="width: 400px;">
+<a id="figure04"></a>
+<br />
+<img src="images/figure04.jpg" width="400" alt="" />
+<div class="caption">
+<p>PLATE IV.&mdash;AURORA BOREALIS.</p>
+<p class="smaller">As observed March 1, 1872, at 9h. 25m. P.M.</p>
+</div></div>
+
+<p><br /></p>
+
+<p>
+Among the many auroras which I have had occasion to observe, none are
+more interesting, excepting the type first described, than those which
+form an immense arch of light spanning the heavens from East to West.
+This form of aurora, which is quite rare, I last observed on September
+12th, 1881. All the northern sky was covered with light vapors, when a
+small auroral patch appeared in the East at about 20° above the
+horizon. This patch of light, gradually increasing westward, soon
+reached the zenith, and continued its onward progress until it arrived
+at about 20° above the western horizon, where it stopped. The aurora
+then appeared as a narrow, wavy band of light, crossed by numerous
+parallel rays of different intensity and color. These rays seemed to
+have a rapid motion from West to East along the delicately-fringed
+streamer, which, on the whole, moved southward, while its extremities
+remained undisturbed. Aside from the apparent displacement of the
+fringes, a singular vibrating motion was observed in the auroral band,
+which was traversed by pulsations and long waves of light. The phenomena
+lasted for about twenty minutes, after which the arch was broken in many
+places, and it slowly vanished.
+</p>
+<p>
+The aurora usually appears in the early part of the evening, and attains
+its full development between ten and eleven o'clock. Although the
+auroral light may have apparently ceased, yet the phenomenon is not at
+an end, as very often a solitary ray is visible from time to time; and
+even towards morning these rays sometimes become quite numerous. On some
+occasions the phenomenon even continues through the following day, and
+is manifested by the radial direction of the cirrus-clouds in the
+heights of our atmosphere. In 1872 I, myself, observed an aurora which
+apparently continued for two or three consecutive days and nights. In
+August, 1859, the northern lights remained visible in the United States
+for a whole week.
+</p>
+<p>
+The height attained by these meteors is considerable, and it is now
+admitted that they are produced in the rarefied air of the upper regions
+of our atmosphere. From the researches of Professor Elias Loomis on the
+great auroras observed in August and September, 1859, it was ascertained
+that the inferior part of the auroral rays had an altitude of 46 miles,
+while that of their summits was 428 miles. These rays had, therefore, a
+length of 382 miles. From the observation of thirty auroral displays, it
+has been found that the mean height attained by the summit of these
+streamers above the Earth's surface was 450 miles.
+</p>
+<p>
+But if the auroral streamers are generally manifested at great heights
+in our atmosphere, it would appear from the observations of persons
+living in the regions where the auroras are most frequent, as also from
+those who have been stationed in high northern and southern latitudes,
+that the phenomenon sometimes descends very low. Both Sabine and Parry
+saw the auroral rays projected on a distant mountain; Ross saw them
+almost at sea-level projected on the polar ice; while Wrangel, Franklin,
+and others observed similar phenomena. Dr. Hjaltalin, who has lived in
+latitude 64° 46' north, and has made a particular study of the aurora,
+on one occasion saw the aurora much below the summit of a hill 1,600
+feet high, which was not very far off.
+</p>
+<p>
+The same aurora is sometimes observed on the same night at places very
+far distant from one another. The great aurora borealis of August 28th,
+1859, for instance, was seen over a space occupying 150° in
+longitude&mdash;from California to the Ural Mountains in Russia. It even
+appears now very probable that the phenomenon is universal on our globe,
+and that the northern lights observed in our hemisphere are simultaneous
+with the aurora australis of the southern hemisphere. The aurora of
+September 2d, 1859, was observed all through North and South America,
+the Sandwich Islands, Australia, and Africa; the streamers and
+pulsations of light of the north pole responding to the rays and
+coruscations of the south pole. Of thirty-four auroras observed at
+Hobart Town, in Tasmania, twenty-nine corresponded with aurora borealis
+observed in our hemisphere.
+</p>
+<p>
+The auroral phenomena, although sometimes visible within the tropics,
+are, however, quite rare in these regions. For the most part they are
+confined within certain zones situated in high latitudes north and
+south. The zone where they are most frequent in our hemisphere forms an
+ellipse, which has the north pole at one of its foci; while the other is
+situated somewhere in North America, in the vicinity of the magnetic
+pole. The central line of the zone upon which the auroras seem to be
+most frequent passes from the northern coast of Alaska through Hudson's
+Bay and Labrador to Iceland, and then follows the northern coast of
+Europe and Asia. The number of auroras diminishes as the observer
+recedes from this zone, and it is only in exceptional cases that they
+are seen near the equator. Near the pole the phenomenon is less frequent
+than it is in the region described. In North America we occupy a
+favorable position for the observation of auroras, as we are nearer the
+magnetic poles than are the Europeans and Asiatics, and we consequently
+have a greater number of auroras in corresponding latitudes.
+</p>
+<p>
+The position of the dark auroral segment varies with the place occupied
+by the observer, and its centre always corresponds with the magnetic
+meridian. In our Eastern States the auroral segment appears a little to
+the west of the north point; but as the observer proceeds westward it
+gradually approaches this point, and is due north when seen from the
+vicinity of Lake Winnipeg. At Point Barrow, in the extreme north-west of
+the United States, the aurora is observed in the east. In Melville
+Islands, Parry saw it in the south; while in Greenland it is directly in
+the west.
+</p>
+<p>
+It is stated that auroras are more numerous about the equinoxes than
+they are at any other seasons; and also, when the earth is in perigee,
+than when it is in apogee. An examination which I have made of a
+catalogue by Professor Loomis, comprising 4,137 auroras observed in the
+temperate zone of our hemisphere from 1776 to 1873, sustains this
+statement. During this period, one hundred more auroras were recorded
+during each of the months comprising the equinoxes, than during any
+other months of the year; while eighty more auroras were observed when
+the earth was in perigee, than when it was in apogee. But to establish
+the truth of this assertion on a solid basis, more observations in both
+hemispheres will be required.
+</p>
+<p>
+The aurora is not simply a terrestrial phenomenon, but is associated in
+some mysterious way with the conditions of the Sun's surface. It is a
+well-known fact that terrestrial magnetism is influenced directly by the
+Sun, which creates the diurnal oscillations of the magnetic needle.
+Between sunrise and two o'clock, the north pole of the needle moves
+towards the west in our northern hemisphere, and in the afternoon and
+evening it moves the other way. These daily oscillations of the needle
+are not uniform in extent; they have a period of regular increase and
+decrease. At a given place the daily oscillations of the magnetic needle
+increase and decrease with regularity during a period which is equal to
+10⅓ years. As this period closely coincides with the Sun-spot period,
+the connection between the variation of the needle and these solar
+disturbances has been recognized.
+</p>
+<p>
+Auroral phenomena generally accompany the extraordinary perturbations in
+the oscillations of the magnetic needle, which are commonly called
+"magnetic storms," and the greater the auroral displays, the greater are
+the magnetic perturbations. Not only is the needle subject to unusual
+displacements during an aurora, but its movements seem to be
+simultaneous with the pulsations and waving motions of the delicate
+auroral streamers in the sky. When the aurora sends forth a coruscation,
+or a streamer in the sky, the magnetic needle responds to it by a
+vibration. The inference that the auroral phenomena are connected with
+terrestrial magnetism is further supported by the fact that the centre
+of the corona is always situated exactly in the direction of that point
+in the heavens to which the dipping needle is directed.
+</p>
+<p>
+It has been found that the aurora is a periodical phenomenon, and that
+its period corresponds very closely with those of the magnetic needle
+and Sun-spots. The years which have the most Sun-spots and magnetic
+disturbances have also the most auroras. There is an almost perfect
+similarity between the courses of the three sets of phenomena, from
+which it is concluded that the aurora is connected in some mysterious
+way with the action of the Sun, as well as with the magnetic condition
+of the earth.
+</p>
+<p>
+A very curious observation, which has been supposed to have some
+connection with this subject, was made on Sept. 1st, 1859, by Mr.
+Carrington and Mr. Hodgson, in England. While these observers, who were
+situated many miles from one another, were both engaged at the same time
+in observing the same Sun-spot, they suddenly saw two luminous spots of
+dazzling brilliancy bursting into sight from the edge of the Sun-spot.
+These objects moved eastward for about five minutes, after which they
+disappeared, having then traveled nearly 34,000 miles. Simultaneously
+with these appearances, a magnetic disturbance was registered at Kew by
+the self-registering magnetic instruments. The very night that followed
+these observations, great magnetic perturbations, accompanied by
+brilliant auroral displays, were observed in Europe. A connection
+between the terrestrial magnetism and the auroral phenomena is further
+proved by the fact that, before the appearance of an aurora, the
+magnetic intensity of our globe considerably increases, but diminishes
+as soon as the first flashes show themselves.
+</p>
+<p>
+The auroral phenomena are also connected in some way with electricity,
+and generate serious disturbances in the electric currents traversing
+our telegraphic lines, which are thus often rendered useless for the
+transmission of messages during great auroral displays. It sometimes
+happens, however, during such displays, that the telegraphic lines can
+be operated for a long distance, without the assistance of a battery;
+the aurora, or at least its cause, furnishing the necessary electric
+current for the working of the line. During auroras, the telephonic
+lines are also greatly affected, and all kinds of noises and
+crepitations are heard in the instruments.
+</p>
+<p>
+Two observations of mine, which may have a bearing on the subject,
+present some interest, as they seem to indicate the action of the aurora
+on some of the clouds of our atmosphere. On January 6th, 1872, after I
+had been observing a brilliant aurora for over one hour, an isolated
+black cumulus cloud appeared at a little distance from the western
+extremity of the dark auroral segment. This cloud, probably driven by
+the wind, rapidly advanced eastward, and was soon followed by a
+succession of similar clouds, all starting from the same point. All
+these black clouds apparently followed the same path, which was not a
+straight line, but parallel to and concentric with the border of the
+dark auroral segment. When the first cloud arrived in the vicinity of
+the magnetic meridian passing through the middle of the auroral arc, it
+very rapidly dissolved, and on reaching this meridian became invisible.
+The same phenomenon was observed with the succession of black clouds
+following, each rapidly dissolving as it approached the magnetic
+meridian. This phenomenon of black clouds vanishing like phantoms in
+crossing the magnetic meridian, was observed for nearly an hour. On June
+17th, 1879, I observed a similar phenomenon during a fine auroral
+display. About midway between the horizon and the polar star, but a
+little to the west of the magnetic meridian, there was a large black
+cumulo-stratus cloud which very slowly advanced eastward. As it
+progressed in that direction, its eastern extremity was dissolved in
+traversing the magnetic meridian; while, at the same time, several short
+and quite bright auroral rays issued from its western extremity, which
+in its turn dissolved rapidly, as if burned or melted away in the
+production of the auroral flame.
+</p>
+<p>
+It seems to be a well observed fact, that during auroras, a strong
+sulphurous odor prevails in high northern latitudes. According to Dr.
+Hjaltalin, during these phenomena, "the ozone of the atmosphere
+increases considerably, and men and animals exposed out of doors emit a
+sulphurous odor when entering a heated room." The Esquimaux and other
+inhabitants of the northern regions assert that great auroras are
+sometimes accompanied by crepitations and crackling noises of various
+sorts. Although these assertions have been denied by several travelers
+who have visited the regions of these phenomena, they are confirmed by
+many competent observers. Dr. Hjaltalin, who has heard these noises
+about six times in a hundred observations, says that they are especially
+audible when the weather is clear and calm; but that when the atmosphere
+is agitated they are not heard. He compares them to the peculiar sound
+produced by a silk cloth when torn asunder, or to the crepitations of
+the electric machine when its motion is accelerated. "When the auroral
+light is much agitated and the streamers show great movements, it is
+then that these noises are heard at different places in the atmosphere."
+</p>
+<p>
+The spectrum of the auroral light, although it varies with almost every
+aurora, always shows a bright green line on a faint continuous spectrum.
+In addition to this green line I have frequently observed four broad
+diffused bands of greater refrangibility in the spectra of some auroras.
+In two cases, when the auroras appeared red towards the west, the
+spectrum showed a bright red line, in addition to the green line and the
+broad bands described. These facts evidently show that the light of the
+aurora is due to the presence of luminous vapors in our atmosphere; and
+it may reasonably be supposed that these vapors are rendered luminous by
+the passage of electric discharges through them.
+</p>
+
+<p><br /><br /><br /></p>
+
+<h4><a id="THE_ZODIACAL_LIGHT">THE ZODIACAL LIGHT</a>
+<br /><br />
+PLATE V</h4>
+
+<p>
+In our northern latitudes may be seen, on every clear winter and spring
+evening, a column of faint, whitish, nebulous light, rising obliquely
+above the western horizon. A similar phenomenon may also be observed in
+the east, before day-break, on any clear summer or autumn night. To this
+pale, glimmering luminosity the name of "Zodiacal Light" has been given,
+from the fact that it lies in the zodiac along the ecliptic.
+</p>
+<p>
+In common with all the celestial bodies, the zodiacal light participates
+in the diurnal motion of the sky, and rises and sets with the
+constellations in which it appears. Aside from this apparent motion, it
+is endowed with a motion of its own, accomplished from west to east, in
+a period of a year. In its motion among the stars, the zodiacal light
+always keeps pace with the Sun, and appears as if forming two faint
+luminous wings, resting on opposite sides of this body. In reality it
+extends on each side of the Sun, its axis lying very nearly in the plane
+of the ecliptic.
+</p>
+<p>
+In our latitudes the phenomena can be observed most advantageously
+towards the equinoxes, in March and September, when twilight is of short
+duration. As we proceed southward it becomes more prominent, and
+gradually increases in size and brightness. It is within the tropical
+regions that the zodiacal light acquires all its splendor: there it is
+visible all the year round, and always appears very nearly perpendicular
+to the horizon, while at the same time its proportions and brilliancy
+are greatly increased.
+</p>
+
+<p><br /></p>
+
+<div class="figcenter" style="width: 400px;">
+<a id="figure05"></a>
+<br />
+<img src="images/figure05.jpg" width="400" alt="" />
+<div class="caption">
+<p>PLATE V.&mdash;THE ZODIACAL LIGHT.</p>
+<p class="smaller">Observed February 20, 1876</p>
+</div></div>
+
+<p><br /></p>
+
+<p>
+The zodiacal light appears under the form of a spear-head, or of a
+narrow cone of light whose base apparently rests on the horizon, while
+its summit rises among the zodiacal constellations. In general
+appearance it somewhat resembles the tail of a large comet whose head is
+below the horizon. The most favorable time to observe this phenomenon in
+the evening, is immediately after the last trace of twilight has
+disappeared; and in the morning, one or two hours before twilight
+appears. When observed with attention, it is seen that the light of the
+zodiacal cone is not uniform, but gradually increases in brightness
+inwardly, especially towards its base, where it sometimes surpasses in
+brilliancy the brightest parts of the Milky-Way. In general, its
+outlines are vague and very difficult to make out, so gradually do they
+blend with the sky. On some favorable occasions, the luminous cone
+appears to be composed of several distinct concentric conical layers,
+having different degrees of brightness, the inner cone being the most
+brilliant of all. There is a remarkable distinction between the evening
+and morning zodiacal light. In our climate, the morning light is pale,
+and never so bright nor so extended as the evening light.
+</p>
+<p>
+In general, the zodiacal light is whitish and colorless, but in some
+cases it acquires a warm yellowish or reddish tint. These changes of
+color may be accidental and due to atmospheric conditions, and not to
+actual change in the color of the object. Although the zodiacal light is
+quite bright, and produces the impression of having considerable depth,
+yet its transparency is great, since all the stars, except the faint
+ones, can be seen through its substance.
+</p>
+<p>
+The zodiacal light is subject to considerable variations in brightness,
+and also varies in extent, the apex of its cone varying in distance from
+the Sun's place, from 40 to 90 degrees. These variations cannot be
+attributed to atmospheric causes alone, some of them being due to real
+changes in the zodiacal light itself, whose light and dimensions
+increase or decrease under the action of causes at present unknown. From
+the discussion of a series of observations on the zodiacal light made at
+Paris and Geneva, it appears certain that its light varies from year to
+year, and sometimes even from day to day, independently of atmospheric
+causes. Some of my own observations agree with these results, and one of
+them, at least, seems to indicate changes even more rapid. On December
+18th, 1875, I observed the zodiacal light in a clear sky free from any
+vapors, at six o'clock in the evening. At that time, the point of its
+cone was a little to the north of the ecliptic, at a distance of about
+90 degrees from the Sun's place. Ten minutes later, its summit had sunk
+down 35 degrees, the cone then being reduced to nearly one-half of its
+original dimensions. Ten minutes later, it had risen 25 degrees, and was
+then 80 degrees from the Sun's place, where it remained all the evening.
+On March 22d, 1878, the sky was very clear and the zodiacal light was
+bright when I observed it, at eight o'clock. At that moment the apex of
+the cone of light was a little to the south of the Pleiades, but this
+cone presented an unusual appearance never noticed by me before, its
+northern border appearing much brighter and sharper than usual, while at
+the same time its axis of greatest brightness appeared to be much nearer
+to this northern border than it was to the southern. After a few minutes
+of observation it became evident that the northern border was extending
+itself, as stars which were at some distance from it became gradually
+involved in its light. At the same time that this border spread
+northward, it seemed to diffuse itself, and after a time the cone
+presented its usual appearance, having its southern border brighter and
+better defined than the other. It would have been impossible to
+attribute this sudden change to an atmospheric cause, since only one of
+the borders of the cone participated in it, and since some very faint
+stars near this northern border were not affected in the least while the
+phenomenon occurred. Besides these observations, Cassini, Mairan,
+Humboldt, and many other competent observers have seen pulsations,
+coruscations and bickerings in the light of the cone, which they thought
+could not be attributed to atmospheric causes. It has also been observed
+that at certain periods the zodiacal light has shone with unusual
+intensity for months together.
+</p>
+<p>
+When this phenomenon is observed from the tropical regions, it is found
+that its axis of symmetry always corresponds with its axis of greatest
+brightness, and that both lie in the plane of the ecliptic, which
+divides its cone into two equal parts. But when the zodiacal light is
+observed in our latitude, the axis of symmetry does not correspond with
+the axis of greatest brightness, and both axes are a little to the north
+of this plane, the axis of symmetry being the farther removed.
+Furthermore, as already stated, the southern border of the cone always
+appears better defined and brighter than the corresponding northern
+margin. It is very probable, if not absolutely certain, that these
+phenomena are exactly reversed when the zodiacal light is observed from
+corresponding latitudes in the southern hemisphere, and that there, its
+axes, both of symmetry and of greatest brightness, appear south of the
+ecliptic, while the northern margin is the brightest. This seems to be
+established by the valuable observations of Rev. George Jones, made on
+board the U. S. steam frigate Mississippi, in California, Japan, and the
+Southern Ocean. "When I was north of the ecliptic," says this observer,
+"the greatest part of the light of the cone appeared to the north of
+this line; when I was to the south of the ecliptic, it appeared to be
+south of it; while when my position was on the ecliptic, or in its
+vicinity, the zodiacal cone was equally divided by this line."
+</p>
+<p>
+Besides the zodiacal light observed in the East and West, some observers
+have recognized an exceedingly faint, luminous, gauzy band, about 10 or
+12 degrees wide, stretching along the ecliptic from the summit of the
+western to that of the eastern zodiacal cone. This faint narrow belt has
+been called the Zodiacal Band. It has been recognized by Mr. H. C.
+Lewis, who has made a study of this phenomenon, that the zodiacal band
+has its southern margin a little brighter and a little sharper than the
+northern border. This observation is in accordance with similar
+phenomena observed in the zodiacal light, and may have considerable
+importance.
+</p>
+<p>
+In 1854, Brorsen recognized a faint, roundish, luminous spot in a point
+of the heavens exactly opposite to the place occupied by the Sun, which
+he has called "Gegenschein," or counter-glow. This luminous spot has
+sometimes a small nucleus, which is a little brighter than the rest.
+Night after night this very faint object shifts its position among the
+constellations, keeping always at 180 degrees from the Sun. The position
+of the counter-glow, like that of the zodiacal light and zodiacal band,
+is not precisely on the plane of the ecliptic, but a little to the north
+of this line. It is very probable that near the equator the phenomenon
+would appear different and there would correspond with this plane.
+</p>
+<p>
+There seems to be some confusion among observers in regard to the
+spectrum of the zodiacal light. Some have seen a bright green line in
+its spectrum, corresponding to that of the aurora borealis; while others
+could only see a faint grayish continuous spectrum, which differs,
+however, from that of a faint solar light, by the fact that it presents
+a well-defined bright zone, gradually blending on each side with the
+fainter light of the continuous spectrum. I have, myself, frequently
+observed the faint continuous spectrum of the zodiacal light, and on one
+occasion recognized the green line of the aurora; but it might have been
+produced by the aurora itself, as yet invisible to the eye, and not by
+the zodiacal light, since, later in the same evening, there was a
+brilliant auroral display. If it were demonstrated that this green line
+exists in the spectrum of the zodiacal light, the fact would have
+importance, as tending to show that the aurora and the zodiacal light
+have a common origin.
+</p>
+<p>
+Rev. Geo. Jones describes a very curious phenomenon which he observed
+several times a little before the moon rose above the horizon. The
+phenomenon consisted in a short, oblique, luminous cone rising from the
+Moon's place in the direction of the ecliptic. This phenomenon he has
+called the Moon Zodiacal Light. In 1874, I had an opportunity to observe
+a similar phenomenon when the Moon was quite high in the sky. By taking
+the precaution to screen the Moon's disk by the interposition of some
+buildings between it and my eye, I saw two long and narrow cones of
+light parallel to the ecliptic issuing from opposite sides of our
+satellite. The phenomenon could not possibly be attributed to vapors in
+our atmosphere, since the sky was very clear at the moment of the
+observation. Later on, these appendages disappeared with the formation
+of vapors near the Moon, but they reappeared an hour later, when the sky
+had cleared off, and continued visible for twenty minutes longer, and
+then disappeared in a clear sky.
+</p>
+<p>
+Although the zodiacal light has been studied for over two centuries, no
+wholly satisfactory explanation of the phenomenon has yet been given.
+Now, as in Cassini's time, it is generally considered by astronomers to
+be due to a kind of lens-shaped ring surrounding the Sun, and extending
+a little beyond the Earth's orbit. This ring is supposed to lie in the
+plane of the ecliptic, and to be composed of a multitude of independent
+meteoric particles circulating in closed parallel orbits around the Sun.
+But many difficulties lie in the way of this theory. It seems as
+incompetent to explain the slow and rapid changes in the light of this
+object as it is to explain the contractions and extensions of its cone.
+It fails, moreover, to explain the flickering motions, the coruscations
+observed in its light, or the displacement of its cone and of its axes
+of brightness and symmetry by a mere change in the position of the
+observer. Rev. Geo. Jones, unable to explain by this theory the
+phenomena which came under his observation, has proposed another, which
+supposes the zodiacal light to be produced by a luminous ring
+surrounding the Earth, this ring not extending as far as the orbit of
+the Moon. But this theory also fails in many important points, so that
+at present no satisfactory explanation of the phenomenon can be given.
+</p>
+<p>
+As the phenomenon is connected in some way with the Sun, and as we have
+many reasons to believe this body to be always more or less electrified,
+it might be supposed that the Sun, acting by induction on our globe,
+develops feeble electric currents in the rarefied gases of the superior
+regions of our atmosphere, and there forms a kind of luminous ridge
+moving with the Sun in a direction contrary to the diurnal motion, and
+so producing the zodiacal light. On this hypothesis, the counter-glow
+would be the result of a smaller cone of light generated by the solar
+induction on the opposite point of the Earth.
+</p>
+<p>
+Plate 5, which sufficiently explains itself, represents the zodiacal
+light as it appeared in the West on the evening of February 20th, 1876.
+All the stars are placed in their proper position, and their relative
+brightness is approximately shown by corresponding variations in
+size&mdash;the usual and almost the only available means of representation.
+Of course, it must be remembered that a star does not, in fact, show any
+disk even in the largest telescopes, where it appears as a mere point of
+light, having more or less brilliancy. The cone of light rises obliquely
+along the ecliptic, and the point forming its summit is found in the
+vicinity of the well-known group of stars, called the Pleiades, in the
+constellation of Taurus, or the Bull.
+</p>
+
+<p><br /><br /><br /></p>
+
+<h4><a id="THE_MOON">THE MOON</a>
+<br /><br />
+PLATE VI</h4>
+
+<p>
+In its endless journey through space, our globe is not solitary, like
+some of the planets, but is attended by the Moon, our nearest celestial
+neighbor. Although the Moon does not attain to the dignity of a planet,
+and remains a secondary body in the solar system, yet, owing to its
+proximity to our globe, and to the great influence it exerts upon it by
+its powerful attraction, it is to us one of the most important celestial
+bodies.
+</p>
+<p>
+While the Moon accompanies the Earth around the Sun, it also revolves
+around the Earth at a mean distance of 238,800 miles. For a celestial
+distance this is only a trifling one; the Earth in advancing on its
+orbit travels over such a distance in less than four hours. A cannon
+ball would reach our satellite in nine days; and a telegraphic dispatch
+would be transmitted there in 1½ seconds of time, if a wire could be
+stretched between us and the Moon.
+</p>
+<p>
+Owing to the ellipticity of the Moon's orbit, its distance from the
+Earth varies considerably, our satellite being sometimes 38,000 miles
+nearer to us than it is at other times. These changes in the distance of
+the Moon occasion corresponding changes from 29' to 33' in its apparent
+diameter. The real diameter of the Moon is 2,160 miles, or a little over
+one-quarter the diameter of our globe; our satellite being 49 times
+smaller than the Earth.
+</p>
+<p>
+The mean density of the materials composing the Moon is only
+⁶⁄₁₀ that of the materials composing the Earth, and the force of
+gravitation at the surface of our satellite is six times less than it is
+at the surface of our globe. If a person weighing 150 lbs. on our Earth
+could be transported to the Moon, his weight there would be only 25 lbs.
+</p>
+<p>
+The Moon revolves around the Earth in about 27⅓ days, with a mean
+velocity of one mile per second, the revolution constituting its
+sidereal period. If the Earth were motionless, the lunar month would be
+equal to the sidereal period; but owing to its motion in space, the Sun
+appears to move with the Moon, though more slowly, so that after having
+accomplished one complete revolution, our satellite has yet to advance
+2¼ days before reaching the same apparent position in regard to the
+Earth and the Sun that it had at first. The interval of time comprised
+between two successive New Moons, which is a little over 29½ days,
+constitutes the synodical period of the Moon, or the lunar month.
+</p>
+<p>
+The Moon is not a self-luminous body, but, like the Earth and the
+planets, it reflects the light which it receives from the Sun, and so
+appears luminous. That such is the case is sufficiently demonstrated by
+the phases exhibited by our satellite in the course of the lunar month.
+Every one is familiar with these phases, which are a consequence of the
+motion of the Moon around the Earth. When our satellite is situated
+between us and the Sun, it is New Moon; since we cannot see its
+illuminated side, which is then turned away from us towards the Sun.
+When, on the contrary, it reaches that point of its orbit which, in
+regard to us, is opposite to the Sun's place, it is Full Moon; since
+from the Earth we can only see the fully illuminated side of our
+satellite. Again, when the Moon arrives at either of the two opposite
+points of its orbit, the direction of which from the Earth is at right
+angles with that of the Sun, it is either the First or the Last Quarter;
+since in these positions we can only see one-half of its illuminated
+disk.
+</p>
+<p>
+The curve described by the Moon around the Earth lies approximately in a
+plane, this plane being inclined about 5° to the ecliptic. Since our
+satellite, in its motion around us and the Sun, closely follows the
+ecliptic, which is inclined 23½° to the equator, it results that when
+this plane is respectively high or low in the sky, the moon is also high
+or low when crossing the meridian of the observer. In winter that part
+of the ecliptic occupied by the Sun is below the equator, and,
+consequently, the New Moons occurring in that season are low in the sky,
+since at New Moon our satellite must be on the same side of the ecliptic
+with the Sun. But the Full Moons in the same season are necessarily high
+in the sky, since a Full Moon can only occur when our satellite is on
+the opposite side of the ecliptic from the Sun, in which position it is,
+of course, as many degrees above the equator as the Sun is below. The
+Full Moon which happens nearest to the autumnal equinox is commonly
+called the Harvest Moon, from the fact that, after full, its delays in
+rising on successive evenings are very brief and therefore favorable for
+the harvest work in the evening. The same phenomenon occurs in every
+other lunar month, but not sufficiently near the time of Full Moon to be
+noticeable. When, in spring, a day or two after New Moon, our satellite
+begins to show its thin crescent, its position on the ecliptic is north
+as well as east of that occupied by the Sun; hence, its horns are nearly
+upright in direction, and give it a crude resemblance to a tipping bowl,
+from which many people who are unaware of its cause, and that this
+happens every year, draw conclusions as to the amount of rain to be
+expected.
+</p>
+<p>
+One of the most remarkable features of the Moon's motions is that our
+satellite rotates on its axis in exactly the same period of time
+occupied by its revolution around the Earth, from which it results that
+the Moon always presents to us the same face. To explain this
+peculiarity, astronomers have supposed that the figure of our satellite
+is not perfectly spherical, but elongated, so that the attraction of the
+Earth, acting more powerfully upon its nearest portions, always keeps
+them turned toward us, as if the Moon were united to our globe by a
+string. It is not exactly true, however, that the Moon always presents
+its same side to us, although its period of rotation exactly equals that
+of its revolution; since in consequence of the inclination of its axis
+of rotation to its orbit, combined with the irregularities of its
+orbital motion about us, apparent oscillations in latitude and in
+longitude, called librations, are created, from which it results that
+nearly ⁶⁄₁₀ of the Moon's surface is visible from the Earth at
+one time or another.
+</p>
+<p>
+The Moon is a familiar object, and every one is aware that our
+satellite, especially when it is fully illuminated, presents a variety
+of bright and dark markings, which, from their distant resemblance to a
+human face, are popularly known as "the man in the moon." A day or two
+after New Moon, when the thin crescent of our satellite is visible above
+the western horizon after sunset, the dark portion of its disk is
+plainly visible, and appears of a pale, ashy gray color, although not
+directly illuminated by the Sun. This phenomenon is due to the
+Earth-shine, or to that portion of solar light which the illuminated
+surface of our globe reflects to the dark side of the Moon, exactly in
+the same manner that the Moon-shine, on our Earth, is due to the solar
+light reflected to our globe by the illuminated Moon.
+</p>
+<p>
+Seen with a telescope of moderate power, or even with a good
+opera-glass, the Moon presents a peculiar mottled appearance, and has a
+strong resemblance to a globe made of plaster of Paris, on the surface
+of which numerous roundish, saucer-shaped cavities of various sizes are
+scattered at random. This mottled structure is better seen along the
+boundary line called the <i>terminator</i>, which divides the illuminated
+from the dark side of the Moon. The line of the terminator always
+appears jagged, and it is very easy to recognize that this irregularity
+is due to the uneven and rugged structure of the surface of our
+satellite.
+</p>
+<p>
+A glance at the Moon through a larger telescope shows that the bright
+spots recognized with the naked eye belong to very uneven and
+mountainous regions of our satellite, while the dark ones belong to
+comparatively smooth, low surfaces, comparable to those forming the
+great steppes and plains of the Earth. When examined with sufficient
+magnifying power, the white, rugged districts of the Moon appear covered
+over by numerous elevated craggy plateaus, mountain-chains, and deep
+ravines; by steep cliffs and ridges; by peaks of great height and
+cavities of great depth. This rugged formation, which is undoubtedly of
+volcanic origin, gives our satellite a desolate and barren appearance.
+The rugged tract occupies more than one-half of the visible surface of
+the Moon, forming several distinct masses, the principal of which occupy
+the south and south-western part of the disk. That this formation is
+elevated above the general level is proved by the fact that the
+mountains, peaks, and other objects which compose it, all cast a shadow
+opposite to the Sun; and further, that the length of these shadows
+diminishes with the elevation of the Sun above the lunar horizon.
+</p>
+<p>
+Since Galileo's time the surface of the Moon has been studied by a host
+of astronomers, and accurate maps of its topographical configuration
+have been made, and names given to all features of any prominence. It
+may even be said that in its general features, the visible surface of
+our satellite is now better known to us than is the surface of our own
+Earth.
+</p>
+<p>
+One of the most striking and common features of the mountainous
+districts of the Moon, is the circular, ring-like disposition of their
+elevated parts, which form numerous crater-like objects of different
+sizes and depths. Many thousands of crater-like objects are visible on
+the Moon through a good telescope, and, considering how numerous the
+small ones are, there is, perhaps, no great exaggeration in fixing their
+number at 50,000, as has been done by some astronomers. These volcanic
+regions of the Moon cannot be compared to anything we know, and far
+surpass in extent those of our globe. The number and size of the craters
+of our most important volcanic regions in Europe, in Asia, in North and
+South America, in Java, in Sumatra, and Borneo, are insignificant when
+compared with those of the Moon. The largest known craters on the Earth
+give only a faint idea of the magnitude of some of the lunar craters.
+The great crater Haleakala, in the Sandwich Islands, probably the
+largest of the terrestrial volcanoes, has a circumference of thirty
+miles, or a diameter of a little less than ten miles. Some of the great
+lunar craters, called walled plains, such as Hipparchus, Ptolemæus,
+etc., have a diameter more than ten times larger than that of Haleakala,
+that of the first being 115 miles and that of the last 100 miles. These
+are, of course, among the largest of the craters of the Moon, although
+there are on our satellite a great number of craters above ten miles in
+diameter.
+</p>
+<p>
+The crater-forms of the Moon have evidently appeared at different
+periods of time, since small craters are frequently found on the walls
+of larger ones; and, indeed, still smaller craters are not rarely seen
+on the walls of these last. The walls of the lunar craters are usually
+quite elevated above the surrounding surface, some of them attaining
+considerable elevations, especially at some points, which form peaks of
+great height. Newton, the loftiest of all, rises at one point to the
+height of 23,000 feet, while many others range from ten to twenty
+thousand feet in height. Several craters have their floor above the
+general surface&mdash;Plato, for instance. Wargentin has its floor nearly
+on a level with the summit of its walls, showing that at some period of its
+history liquid lavas, ejected from within, have filled it to the brim
+and then solidified. The floors of some of the craters are smooth and
+flat, but in general they are occupied by peaks and abrupt mountainous
+masses, which usually form the centre. Many of their outside walls are
+partly or wholly covered by numerous ravines and gullies, winding down
+their steep declivities, branching out and sometimes extending to great
+distances from their base. It would seem that these great volcanic
+mouths have at some time poured out torrents of lavas, which, in their
+descent, carved their passage by the deep gullies now visible.
+Sometimes, also, the crater slopes are strewn with debris, giving them a
+peculiar volcanic appearance.
+</p>
+<p>
+Notwithstanding their many points of similarity with the volcanoes of
+the Earth, the lunar craters differ from them in many particulars,
+showing that volcanic forces acting on different globes may produce
+widely different results. For example, the floors of terrestrial craters
+are usually situated at considerable elevations above the general
+surface, while those of the lunar craters are generally much depressed,
+the height of their walls being only about one-half the depth of their
+cavities. Again, while on the Earth the mass of the volcanic cones far
+exceeds the capacity of their openings, on the Moon it is not rare to
+see the capacity of the crater cavities exceeding the mass of the
+surrounding walls. On the Earth, the volcanic cones and mouths are
+comparatively regular and smooth, and are generally due to the
+accumulation of the ashes and the debris of all kinds which are ejected
+from the volcanic mouths. On the Moon, very few craters show this
+character, and for the most part their walls have a very different
+structure, being irregular, very rugged, and composed of a succession of
+concentric ridges, rising at many points to great elevations, and
+forming peaks of stupendous height. Again, many of the larger
+terrestrial craters have their interior occupied by a central cone, or
+several such cones, having a volcanic mouth on their summits; on the
+Moon such central cones are very rare. Although many of the large lunar
+craters have their interior occupied by central masses which have been
+often compared to the central cones of our great volcanoes, yet these
+objects have a very different character and origin. For the most part,
+they are mountainous masses of different forms&mdash;having very rarely any
+craters on them&mdash;and seem to have resulted from the crowding and
+lifting up of the crater floor by the phenomena of subsidence, of which
+these craters show abundant signs. Besides, the terrestrial craters are
+characterized by large and important lava streams, while on the Moon the
+traces of such phenomena are quite rare, and when they are shown, they
+generally differ from those of the Earth by their numerous and
+complicated ramifications, and also by the fact that many of these lava
+streamlets take their origin at a considerable distance from the crater
+slopes, and are grooved and depressed as if the burning liquids which
+are supposed to have produced them had subsequently disappeared, by
+evaporation or otherwise, leaving the furrow empty.
+</p>
+<p>
+The dark spots of the Moon, when viewed through a telescope, exhibit a
+totally different character, and show that they belong to a different
+formation from that of the brighter portions. These darker tracts do not
+seem to have had a direct volcanic origin like the latter, but rather
+appear to have resulted from the solidification of semi-fluid materials,
+which have overflowed vast areas at different times. The surface of this
+system is comparatively smooth and uniform, only some small craters and
+low ridges being seen upon it. The level and dark appearance of these
+areas led the ancient astronomers to the belief that they were produced
+by a liquid strongly absorbing the rays of light, and were seas like our
+seas. Accordingly, these dark surfaces were called <i>Maria</i>, or Seas, a
+name which it is convenient to retain, although it is well known to have
+originated in an error. The so-called seas of the Moon are evidently
+large flat surfaces similar to the deserts, steppes, pampas, and
+prairies of the Earth in general appearance. The great plains of the
+Moon are at a lower level than that of the other formation, and that
+which first attracts the observer's attention is the fact that they are
+surrounded almost on all sides by an irregular line of abrupt cliffs and
+mountain chains, showing phenomena of dislocation. This character of
+dislocation, which is general, and is visible everywhere upon the
+contours of the plains, seems to indicate that phenomena of subsidence,
+either slow or rapid, have occurred on the Moon; while, at the same
+time, the sunken surfaces were overflowed by a semi-fluid liquid, which
+solidified afterwards. The evidences of subsidence and overflowing
+become unmistakable when we observe that, along the borders of the gray
+plains, numerous craters are more or less embedded in the gray
+formation, only parts of the summit of their walls remaining visible, to
+attest that once large craters existed there. The farther from the
+border of the plain the vestiges of these craters are observed, the
+deeper they are embedded in the gray formation. That phenomena of
+subsidence have occurred on a grand scale on the Moon, is further
+indicated by the fact that the singular systems of fractures called
+clefts and rifts generally follow closely the outside border of the gray
+plains, often forming parallel lines of dislocation and fractures. In
+the interior regions of the gray formation, these fractures are
+comparatively rare.
+</p>
+<p>
+The gray, lava-like formation is obviously of later origin than the
+mountainous system to which belong the embedded craters above described.
+Its comparatively recent origin might also be inferred from the
+smallness of its craters and its low ridges. The few large craters
+observed on this formation evidently belong to the earlier system.
+</p>
+<p>
+The color of this system of gray plains is far from being uniform. In
+general appearance it is of a bluish gray, but when observed
+attentively, large areas appear tinted with a dusky olive-green, while
+others are slightly tinged with yellow. Some patches appear brownish,
+and even purplish. A remarkable example of the first case is seen on the
+surface, which encloses within a large parallelogram the two conspicuous
+craters, Aristarchus and Herodotus. This surface evidently belongs to a
+different system from that of the Oceanus Procellarum surrounding it,
+as, besides its color, which totally differs from that of the gray
+formation, its surface shows the rugged structure of the volcanic
+formation.
+</p>
+<p>
+When the Moon is full, some very curious white, luminous streaks are
+seen radiating from different centres, which, for the most part, are
+important craters, occupied by interior mountains. The great crater
+Tycho is the centre of the most imposing of the systems of white
+streaks. Some of the diverging rays of this great centre extend to a
+distance equal to one-quarter of the Moon's circumference, or about
+1,700 miles. The true nature of these luminous streaks is unknown, but
+it seems certain that they have their origin in the crater from which
+they diverge. They do not form any relief on the surface, and are seen
+going up over the mountains and steep walls of the crater, as well as
+down the ravines and on the floors of craters.
+</p>
+<p>
+The Moon seems to be deprived of an atmosphere; or, if it has any, it
+must be so excessively rare that its density is less than of the density
+of the Earth's atmosphere, since delicate tests afforded by the
+occultation of stars have failed to reveal its presence. Although no
+atmosphere of any consequence exists on the Moon, yet phenomena which I
+have observed seem to indicate the occasional presence there of vapors
+of some sort. On several occasions, I have seen a purplish light over
+some parts of the Moon, which prevented well-known objects being as
+distinctly seen as they were at other times, causing them to appear as
+if seen through a fog. One of the most striking of these observations
+was made on January 4th, 1873, on the crater Kant and its vicinity,
+which then appeared as if seen through luminous purplish vapors. On one
+occasion, the great crater Godin, which was entirely involved in the
+shadow of its western wall, appeared illuminated in its interior by a
+faint purplish light, which enabled me to recognize the structure of
+this interior. The phenomenon could not be attributed in this case to
+reflection, since the Sun, then just rising on the western wall of the
+crater, had not yet grazed the eastern wall, which was invisible. It is
+not impossible that a very rare atmosphere composed of such vapors
+exists in the lower parts of the Moon.
+</p>
+<p>
+If the Moon has no air, and no liquids of any sort, it seems impossible
+that its surface can maintain any form of life, either vegetable or
+animal, analogous to those on the Earth. In fact, nothing indicating life
+has been detected on the Moon&mdash;our satellite looking like a barren,
+lifeless desert. If life is to be found there at all, it must be of a
+very elementary nature. Aside from the want of air and water to sustain
+it, the climatic conditions of our satellite are very unfavorable for
+the development of life. The nights and days of the Moon are each equal
+to nearly fifteen of our days and nights. For fifteen consecutive
+terrestrial days the Sun's light is absent from one hemisphere of the
+Moon; while for the same number of days the Sun pours down on the other
+hemisphere its light and heat, the effects of which are not in any way
+mitigated by an atmosphere. During the long lunar nights the temperature
+must at least fall to that of our polar regions, while during its long
+days it must be far above that of our tropical zone. It has been
+calculated that during the lunar nights the temperature descends to 23°
+below zero, while during the days it rises to 468°, or 256° above the
+boiling point.
+</p>
+<p>
+It has been a question among astronomers whether changes are still
+taking place at the surface of the Moon. Aside from the fact that
+change, not constancy, is the law of nature, it does not seem doubtful
+that changes occur on the Moon, especially in view of the powerful
+influences of contraction and dilatation to which its materials are
+submitted by its severe alternations of temperature. From the distance
+at which we view our satellite, we cannot expect, of course, to be able
+to see changes, unless they are produced on a large scale. Theoretically
+speaking, the largest telescopes ever constructed ought to show us the
+Moon as it would appear to the naked eye from a distance of 40 miles;
+but in practice it is very different. The difficulty is in the fact
+that, while we magnify the surface of a telescopic image, we are unable
+to increase its light; so that, practically, in magnifying an object, we
+weaken its light proportionally to the magnifying power employed. The
+light of the Moon, especially near the terminator, where we almost
+always make our observations, is not sufficiently bright to bear a very
+high magnifying power, and only moderate ones can be applied to its
+study. What we gain by enlarging an object, we more than lose by the
+weakening of its light. Besides, a high magnifying power, by increasing
+the disturbances generally present in our atmosphere, renders the
+telescopic image unsteady and very indistinct. On the whole, the largest
+telescopes now in existence do not show us our satellite better than if
+we could see it with the naked eye from a distance of 300 miles or more.
+At such a distance only considerable changes would be visible.
+</p>
+<p>
+Notwithstanding these difficulties, it is believed that changes have
+been detected in Linné, Marius, Messier, and several other craters. An
+observation of mine seems to indicate that changes have recently taken
+place in the great crater Eudoxus. On February 20th, 1877, between 9h.
+30m. and 10h. 30m., I observed a straight, narrow wall crossing this
+crater from east to west, a little to the south of its centre. This wall
+had a considerable elevation, as was proved by the shadow it cast on its
+northern side. Towards its western end this wall appeared as a brilliant
+thread of light on the black shadow cast by the western wall of the
+crater. The first time I had occasion to observe this crater again,
+after this observation, was a year later, on February 17th, 1878; no
+traces of the wall were then detected. Many times since I have tried to
+find this narrow wall again, when the Moon presented the same phase and
+the same illumination, but always with negative results. It seems
+probable that this structure has crumbled down, yet it is very singular
+that so prominent a feature should not have been noticed before.
+</p>
+
+<p><br /></p>
+
+<div class="figcenter" style="width: 400px;">
+<a id="figure06"></a>
+<br />
+<img src="images/figure06.jpg" width="400" alt="" />
+<div class="caption">
+<p>PLATE VI.&mdash;MARE HUMORUM.</p>
+<p class="smaller">From a study made in 1875</p>
+</div></div>
+
+<p><br /></p>
+
+<p>
+The "Mare Humorum," or sea of moisture, as it is called, which is
+represented on Plate VI., is one of the smaller gray lunar plains. Its
+diameter, which is very nearly the same in all directions, is about 270
+miles, the total area of this plain being about 50,000 square miles. It
+is one of the most distinct plains of the Moon, and is easily seen with
+the naked eye on the left-hand side of the disk. The floor of the plain
+is, like that of the other gray plains, traversed by several systems of
+very extended but low hills and ridges, while small craters are
+disseminated upon its surface. The color of this formation is of a dusky
+greenish gray along the border, while in the interior it is of a lighter
+shade, and is of brownish olivaceous tint. This plain, which is
+surrounded by high clefts and rifts, well illustrates the phenomena of
+dislocation and subsidence. The double-ringed crater Vitello, whose
+walls rise from 4,000 to 5,000 feet in height, is seen in the upper
+left-hand corner of the gray plain. Close to Vitello, at the east, is
+the large broken ring-plain Lee, and farther east, and a little below,
+is a similarly broken crater called Doppelmayer. Both of these open
+craters have mountainous masses and peaks on their floor, which is on a
+level with that of the Mare Humorum. A little below, and to the left of
+these objects, is seen a deeply embedded oval crater, whose walls barely
+rise above the level of the plain. On the right-hand side of the great
+plain, is a long <i>fault</i>, with a system of fracture running along its
+border. On this right-hand side, may be seen a part of the line of the
+terminator, which separates the light from the darkness. Towards the
+lower right-hand corner, is the great ring-plain Gassendi, 55 miles in
+diameter, with its system of fractures and its central mountains, which
+rise from 3,000 to 4,000 feet above its floor. This crater slopes
+southward towards the plain, showing the subsidence to which it has been
+submitted. While the northern portion of the wall of this crater rises
+to 10,000 feet, that on the plain is only 500 feet high, and is even
+wholly demolished at one place where the floor of the crater is in
+direct communication with the plain. In the lower part of the <i>mare</i>,
+and a little to the west of the middle line, is found the crater
+Agatharchides, which shows below its north wall the marks of rills
+impressed by a flood of lava, which once issued from the side of the
+crater. On the left-hand side of the plain, is seen the half-demolished
+crater Hippalus, resembling a large bay, which has its interior strewn
+with peaks and mountains. On this same side can be seen one of the most
+important systems of clefts and fractures visible on the Moon, these
+clefts varying in length from 150 to 200 miles.
+</p>
+
+<p><br /><br /><br /></p>
+
+<h4><a id="ECLIPSES">ECLIPSES OF THE MOON</a>
+<br /><br />
+PLATE VII</h4>
+
+<p>
+Since the Moon is not a self-luminous body, but shines by the light
+which it borrows from the Sun, it follows that when the Sun's light is
+prevented from reaching its surface, our satellite becomes obscured. The
+Earth, like all opaque bodies exposed to sunlight, casts a shadow in
+space, the direction of which is always opposite to the Sun's place. The
+form of the Earth's shadow is that of a long, sharply-pointed cone,
+which has our globe for its base. Its length, varying with the distance
+of the Earth from the Sun, is, on an average, 855,000 miles, or 108
+times the terrestrial diameter. This conical shadow of the Earth,
+divided longitudinally by the plane of the ecliptic, lies half above and
+half below that plane, on which the summit of the shadow describes a
+whole circumference in the course of a year. If the Moon's orbit were
+not inclined to the ecliptic, our satellite would pass at every Full
+Moon directly through the Earth's shadow; but, owing to that
+inclination, it usually passes above or below the shadow. Twice,
+however, during each of its revolutions, it must cross the plane of the
+ecliptic, the points of its orbit where this happens being called nodes.
+Accordingly, if it is near a node at the time of Full Moon, it will
+enter the shadow of the Earth, and become either partly or wholly
+obscured, according to the distance of its centre from the plane of the
+ecliptic. The partial or total obscuration of the Moon's disk thus
+produced constitutes a partial or total eclipse of the Moon. The
+essential conditions for an eclipse of the Moon are, therefore, that our
+satellite must not only be full, but must also be at or very near one of
+its nodes.
+</p>
+<p>
+Although inferior in importance to the eclipses of the Sun, the eclipses
+of the Moon are, nevertheless, very interesting and remarkable
+phenomena, which never fail to produce a deep impression on the mind of
+the observer, inasmuch as they give him a clear insight into the silent
+motions of the planetary bodies.
+</p>
+<p>
+At the mean distance of the Moon from the Earth, the diameter of the
+conical shadow cast in space by our globe is more than twice as large as
+that of our satellite. But, besides this pure dark shadow of the Earth,
+its cone is enveloped by a partial shadow called "Penumbra," which is
+produced by the Sun's light being partially, but not wholly, cut off by
+our globe.
+</p>
+<p>
+While the Moon is passing into the penumbra, a slight reduction of the
+light of that part of the disk which has entered it, is noticeable. As
+the progress of the Moon continues, the reduction becomes more
+remarkable, giving the impression that rare and invisible vapors are
+passing over our satellite. Some time after, a small dark-indentation,
+marking the instant of first contact, appears on the eastern or
+left-hand border of the Moon, which is always the first to encounter the
+Earth's shadow, since our satellite is moving from west to east. The
+dark indentation slowly and gradually enlarges with the onward progress
+of the Moon into the Earth's shadow, while the luminous surface of its
+disk diminishes in the same proportion. The form of the Earth's shadow
+on the Moon's disk clearly indicates the rotundity of our globe by its
+circular outline. Little by little the dark segment covers the Moon's
+disk, and its crescent, at last reduced to a mere thread of light,
+disappears at the moment of the second contact. With this the phase of
+totality begins, our satellite being then completely involved in the
+Earth's shadow.
+</p>
+<p>
+The Moon remains so eclipsed for a period of time which varies with its
+distance from the Earth, and with the point of its orbit where it
+crosses the conical shadow. When it passes through the middle of this
+shadow, while its distance from our globe is the least, the total phase
+of an eclipse of the Moon may last nearly two hours. The left-hand
+border of our satellite having gone first into the Earth's shadow, is
+also the first to emerge, and, at the moment of doing so, it receives
+the Sun's light, and totality ends with the third contact. The lunar
+crescent gradually increases in breadth after its exit from the shadow,
+and finally the Moon recovers its fully illuminated disk as before, at
+the moment its western border leaves the Earth's shadow. Soon after, it
+passes out of the penumbra, and the eclipse is over. In total eclipses,
+the interval of time from the first to last contact may last 5h. 30m,
+but it is usually shorter.
+</p>
+<p>
+Soon after the beginning of an eclipse, the dark segment produced by the
+Earth's shadow on the Moon's disk generally appears of a dark grayish
+opaque color, but with the progress of the phenomenon, this dark tint is
+changed into a dull reddish color, which, gradually increasing, attains
+its greatest intensity when the eclipse is total. At that moment the
+color of the Moon is of a dusky, reddish, coppery hue, and the general
+features of the Moon's surface are visible as darker and lighter tints
+of the same color. It sometimes happens, however, that our satellite
+does not exhibit this peculiar coppery tint, but appears either blackish
+or bluish, in which case it is hardly distinguishable from the sky.
+</p>
+<p>
+It is very rare for the Moon to disappear completely during totality,
+and even when involved in the deepest part of the Earth's shadow, our
+satellite usually remains visible to the naked eye, or, at least, to the
+telescope. This phenomenon is to be attributed to the fact that the
+portion of the solar rays which traverse the lower strata of our
+atmosphere are strongly refracted, and bend inward in such a manner that
+they fall on the Moon, and sufficiently illuminate its surface to make
+it visible. The reddish color observed is caused by the absorption of
+the blue rays of light by the vapors which ordinarily-saturate the lower
+regions of our atmosphere, leaving only red rays to reach the Moon's
+surface. Of course, these phenomena are liable to vary with every
+eclipse, and depend almost exclusively on the meteorological conditions
+of our atmosphere.
+</p>
+<p>
+In some cases the phase of totality lasts longer than it should,
+according to calculation. This can be attributed to the fact that the
+Earth is enveloped in a dense atmosphere, in which opaque clouds of
+considerable extent are often forming at great elevations. Such strata
+of clouds, in intercepting the Sun's light, would have, of course, the
+effect of increasing the diameter of the Earth's shadow, in a direction
+corresponding to the place they occupy, and, if the Moon were moving in
+this direction, would increase the phase of total obscuration.
+</p>
+<p>
+The eclipses of the Moon, like those of the Sun, as shown above, have a
+cycle of 18 years, 11 days and 7 hours, and recur after this period of
+time in nearly the same order. They can, therefore, be approximately
+predicted by adding 18y. 11d. 7h. to the date of the eclipses which have
+occurred during the preceding period. During this cycle 70 eclipses will
+occur&mdash;41 being eclipses of the Sun and 29 eclipses of the Moon. At no
+time can there ever be more than seven eclipses in a year, and there are
+never less than two. When there are only two eclipses in a year, they
+are both eclipses of the Sun.
+</p>
+<p>
+Although the number of solar eclipses occurring at some point or other
+of the Earth's surface is greater than that of the eclipses of the Moon,
+yet at any single terrestrial station the eclipses of the Moon are the
+more frequent. While an eclipse of the Sun is only visible on a narrow
+belt, which is but a very small fraction of the hemisphere then
+illuminated by the Sun, an eclipse of the Moon is visible from all the
+points of the Earth which have the Moon above their horizon at the time.
+Furthermore, an eclipse of the Sun is not visible at one time over the
+whole length of its narrow tract, but moves gradually from one end of it
+to the other; while, on the contrary, an eclipse of the Moon begins and
+ends at the very same instant for all places from which it can be seen,
+but, of course, not at the same local time, which varies with the
+longitude of the place.
+</p>
+
+<p><br /></p>
+
+<div class="figcenter" style="width: 400px;">
+<a id="figure07"></a>
+<br />
+<img src="images/figure07.jpg" width="400" alt="" />
+<div class="caption">
+<p>PLATE VII.&mdash;PARTIAL ECLIPSE OF THE MOON.</p>
+<p class="smaller">Observed October 24, 1874</p>
+</div></div>
+
+<p><br /></p>
+
+<p>
+The partial eclipse of the Moon, represented on Plate VII., shows quite
+plainly the configuration of our satellite as seen with the naked eye
+during the eclipse, with its bright and dark spots, and its radiating
+streaks. This eclipse was observed on October 24th, 1874.
+</p>
+
+<p><br /><br /><br /></p>
+
+<h4>THE PLANETS</h4>
+
+<p>
+Around the Sun circulate a number of celestial bodies, which are called
+"<i>Planets</i>." The planets are opaque bodies, and appear luminous
+because their surfaces reflect the light they receive from the Sun.
+</p>
+<p>
+The planets are situated at various distances from the Sun, and revolve
+around this body in widely different periods of time, which are,
+however, constant for each planet, so far as ascertained, and doubtless
+are so in the other cases.
+</p>
+<p>
+The ideal line traced in space by a planet in going around the Sun, is
+called <i>the orbit</i> of the planet; while the period of time employed by
+a planet to travel over its entire orbit and return to its starting point,
+is called <i>the sidereal revolution</i>, <i>or year</i> of the planet. The
+dimensions of the orbits of the different planets necessarily vary with
+the distance of these bodies from the Sun, as does also the length of
+their sidereal revolution.
+</p>
+<p>
+The distance of a planet from the Sun does not remain constant, but is
+subject to variations, which in certain cases are quite large. These
+variations result from the fact that the planetary orbits are not
+perfect circles having the Sun for centre, but curves called
+"<i>Ellipses</i>," which have two centres, or foci, one of which is always
+occupied by the Sun. This is in accordance with Kepler's first law.
+</p>
+<p>
+The ideal point situated midway between the two foci is called <i>the
+centre of the ellipse</i>, or <i>orbit</i>; while the imaginary straight
+line which passes through both foci and the centre, with its ends at
+opposite points of the ellipse, is called "<i>the major axis</i>" of the
+orbit. It is also known as "<i>the line of the apsides</i>." The ideal
+straight line which, in passing through the centre of the orbit, cuts
+the major axis at right angles, and is prolonged on either side to
+opposite points on the ellipse, is called "<i>the minor axis</i>" of the
+orbit.
+</p>
+<p>
+When a planet reaches that extremity of the major axis of its orbit
+which is the nearest to the Sun, it is said to be in its
+"<i>perihelion</i>;" while, when it arrives at the other extremity,
+which is farthest from this body, it is said to be in its
+"<i>aphelion</i>." When a planet reaches either of the two opposite
+points of its orbit situated at the extremities of its minor axis, it is
+said to be at its <i>mean distance</i> from the Sun.
+</p>
+<p>
+The rapidity with which the planets move on their orbits varies with
+their distance from the Sun; the farther they are from this body, the
+more slowly they move. The rapidity of their motion is greatest when
+they are in perihelion, and least when they are in aphelion, having its
+mean rate when these bodies are crossing either of the extremities of
+the minor axes of their orbits.
+</p>
+<p>
+The imaginary line which joins the Sun to a planet at any point of its
+orbit, and moves with this planet around the Sun, is called "<i>the radius
+vector</i>." According to Kepler's second law, whatever may be the distance
+of a planet from the Sun, the radius vector sweeps over equal areas of
+the plane of the planet's orbit in equal times.
+</p>
+<p>
+There is a remarkable relation between the distance of the planets from
+the Sun and their period of revolution, in consequence of which the
+squares of their periodic times are respectively equal to the cubes of
+their mean distances from the Sun. From this third law of Kepler, it
+results that the mere knowledge of the mean distance of a planet from
+the Sun enables one to know its period of revolution, and <i>vice
+versa</i>.
+</p>
+<p>
+The orbit described by the Earth around the Sun in a year, or the
+apparent path of the Sun in the sky, is called "<i>the ecliptic</i>." Like
+that of all the planetary orbits, the plane of the ecliptic passes
+through the Sun's centre. The ecliptic has a great importance in
+astronomy, inasmuch as it is the fundamental plane to which the orbits
+and motions of all planets are referred.
+</p>
+<p>
+The orbits of the larger planets are not quite parallel to the ecliptic,
+but more or less inclined to this plane; although the inclination is
+small, and does not exceed eight degrees. On account of this inclination
+of the orbits, the planets, in accomplishing their revolutions around
+the Sun, are sometimes above and sometimes below the plane of the
+ecliptic. A belt extending 8° on each side of the ecliptic, and,
+therefore, 16° in width, comprises within its limits the orbits of all
+the principal planets. This belt is called "<i>the Zodiac</i>."
+</p>
+<p>
+Since all the planets have the Sun for a common centre, and have their
+orbits inclined to the ecliptic, it follows that each of these orbits
+must necessarily intersect the plane of the ecliptic at two opposite
+points situated at the extremities of a straight line passing through
+the Sun's centre. The two opposite points on a planetary orbit where its
+intersections with the ecliptic occur, are called "<i>the Nodes</i>," and
+the imaginary line joining them, which passes through the Sun's centre, is
+called "the line of the nodes." The node situated at the point where a
+planet crosses the ecliptic from the south to the north, is called "<i>the
+ascending node</i>" while that situated where the planet crosses from north
+to south, is called "<i>the descending node</i>."
+</p>
+<p>
+The planets circulating around the Sun are eight in number, but, beside
+these, there is a multitude of very small planets, commonly called
+"asteroids," which also revolve around our luminary. The number of
+asteroids at present known surpasses two hundred, and constantly
+increases by new discoveries. In their order of distance from the Sun
+the principal planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn,
+Uranus and Neptune. The orbits of the asteroids are comprised between
+the orbits of Mars and Jupiter.
+</p>
+<p>
+When the principal planets are considered in regard to their differences
+in size, they are separated into two distinct groups of four planets
+each, viz.: the small planets and the large planets. The orbits of the
+small planets are wholly within the region occupied by the orbits of the
+asteroids, while those of the large planets are wholly without this
+region.
+</p>
+<p>
+When the planets are considered in regard to their position with
+reference to the Earth, they are called "inferior planets" and "superior
+planets." The inferior planets comprise those whose orbits are within
+the orbit of our globe; while the superior planets are those whose
+orbits lie beyond the orbit of the Earth.
+</p>
+<p>
+Since the orbits of the inferior planets lie within the orbit of the
+Earth, the angular distances of these bodies from the Sun, as seen from
+the Earth, must always be included within fixed limits; and these
+planets must seem to oscillate from the east to the west, and from the
+west to the east of the Sun during their sidereal revolution. In this
+process of oscillation these planets sometimes pass between the Earth
+and the Sun, and sometimes behind the Sun. When they pass between us and
+the Sun they are said to be in "inferior conjunction," while, when they
+pass behind the Sun, they are said to be in "superior conjunction." When
+such a planet reaches its greatest distance, either east or west, it is
+said to be at its greatest elongation east or west, as the case may be,
+or in quadrature.
+</p>
+<p>
+The superior planets, whose orbits lie beyond that of the Earth and
+enclose it, present a different appearance. A superior planet never
+passes between the Earth and the Sun, since its orbit lies beyond that
+of our globe, and, therefore, no inferior conjunction of such a planet
+can ever occur. When one of these planets passes beyond the Sun, just
+opposite to the place occupied by the Earth, the planet is said to be in
+"conjunction;" while, when it is on the same side of the Sun with our
+globe, it is said to be in "opposition." While occupying this last
+position, the planet is most advantageously situated for observation,
+since it is then nearer to the Earth. The period comprised between two
+successive conjunctions, or two successive oppositions of a planet, is
+called its "synodical period." This period differs for every planet.
+</p>
+<p>
+It is supposed that all the planets rotate from west to east, like our
+globe; although no direct evidence of the rotation of Mercury Uranus,
+and Neptune has yet been obtained, it is probable that these planets
+rotate like the others. It results from the rotation of the planets that
+they have their days and nights, like our Earth, but differing in
+duration for every planet.
+</p>
+<p>
+The axes of rotation of the planets are more or less inclined to their
+respective orbits, and this inclination varies but little in the course
+of time. From the inclination of the axes of rotation of the planets to
+their orbits, it results that these bodies have seasons like those of
+the Earth; but, of course, they differ from our seasons in duration and
+intensity, according to the period of revolution and the inclination of
+the axis of each separate planet.
+</p>
+
+<p><br /><br /><br /></p>
+
+<h4><a id="THE_PLANET_MARS">THE PLANET MARS</a>
+<br /><br />
+PLATE VIII</h4>
+
+<p>
+Mars is the fourth of the planets in order of distance from the sun;
+Mercury, Venus and the Earth being respectively the first, second and
+third.
+</p>
+<p>
+Owing to the great eccentricity of its orbit, the distance of Mars from
+the Sun is subject to considerable variations. When this planet is in
+its aphelion, its distance from the Sun is 152,000,000 miles, but at
+perihelion it is only 126,000,000 miles distant, the planet being
+therefore 26,000,000 miles nearer the Sun at perihelion than at
+aphelion. The mean distance of Mars from the Sun is 139,000,000 miles.
+Light, which travels at the rate of 185,000 miles a second, occupies
+12½ minutes in passing from the Sun to this planet.
+</p>
+<p>
+While the distance of Mars from the Sun varies considerably, its
+distance from the Earth varies still more. When Mars comes into
+opposition, its distance from our globe is comparatively small,
+especially if the opposition occurs in August, as the two planets are
+then as near together as it is possible for them to be, their distance
+apart being only 33,000,000 miles. But if the opposition occurs in
+February, the distance may be nearly twice as great, or 62,000,000
+miles. On the other hand, when Mars is in conjunction in August, the
+distance between the two planets is the greatest possible, or no less
+than 245,000,000 miles; while, when the conjunction occurs in February,
+it is only 216,000,000 miles. Hence the distance between Mars and the
+Earth varies from .33 to 245 millions of miles; that is, this planet may
+be 212 million miles nearer to us at its nearest oppositions than at its
+most distant conjunctions.
+</p>
+<p>
+From these varying distances of Mars from the Earth, necessarily result
+great variations in the brightness and apparent size of the planet, as
+seen from our globe. When nearest to us it is a very conspicuous object,
+appearing as a star of the first magnitude, and approaching Jupiter in
+brightness; but when it is farthest it is much reduced, and is hardly
+distinguishable from the stars of the second and even third magnitude.
+In the first position, the apparent diameter of Mars is 26", in the last
+it is reduced to 3" only.
+</p>
+<p>
+The orbit of Mars has the very small inclination of 1° 51' to the plane
+of the ecliptic. The planet revolves around the Sun in a period of 687
+days, which constitutes its sidereal year, the year of Mars being only
+43 days less than two of our years.
+</p>
+<p>
+Mars travels along its orbit with a mean velocity of 15 miles per
+second, being about ⁸⁄₁₀ of the velocity of our globe in its
+orbit. The synodical period of Mars is 2 years and 48 days, during which
+the planet passes through all its degrees of brightness.
+</p>
+<p>
+Mars is a smaller planet than the Earth, its diameter being only 4,200
+miles, and its circumference 13,200 miles. It seems well established
+that it is a little flattened at its poles, but the actual amount of
+this flattening is difficult to obtain. According to Prof. Young, the
+polar compression is ¹⁄₂₁₉.
+</p>
+<p>
+The surface of this planet is a little over ²⁸⁄₁₀₀ of the
+surface of our globe, and its volume is 6½ times less than that of the
+Earth. Its mass is only about ⅒ while its density is about ¾ that of
+the Earth. The force of gravitation at its surface is nearly ¾ of what
+it is at the surface of our globe.
+</p>
+<p>
+The planet Mars rotates on an axis inclined 61° 18' to the plane of its
+orbit, so that its equator makes an angle of 28° 42' with the same
+plane. The period of rotation of this planet, which constitutes its
+sidereal day, is 24 h. 37 m. 23 s.
+</p>
+<p>
+The year of Mars, which is composed of 669⅔ of these Martial days,
+equals 687 of our days, this planet rotating 669⅔ times upon its axis
+during this period. But owing to the movement of Mars around the Sun,
+the number of solar days in the Martial year is only 668⅔, while,
+owing to the same cause, the solar day of Mars is a little longer than
+its sidereal day, and equals 24 h. 39 m. 35 s.
+</p>
+<p>
+The days and nights on Mars are accordingly nearly of the same length as
+our days and nights, the difference being a little less than
+three-quarters of an hour. But while the days and nights of Mars are
+essentially the same as ours, its seasons are almost twice as long as
+those of the Earth. Their duration for the northern hemisphere,
+expressed in Martial days, is as follows: Spring, 191; Summer, 181;
+Autumn, 149; Winter, 147. While the Spring and Summer of the northern
+hemisphere together last 372 days, the Autumn and Winter of the same
+hemisphere last only 296 days, or 76 days less. Since the summer seasons
+of the northern hemisphere correspond to the winter seasons of the
+southern hemisphere, and vice versa, the northern hemisphere, owing to
+its longer summer, must accumulate a larger quantity of heat than the
+last. But on Mars, as on the Earth, there is a certain law of
+compensation resulting from the eccentricity of the planet's orbit, and
+from the fact that the middle of the summer of the southern hemisphere
+of this planet, coincides with its perihelion. From the greater
+proximity of Mars to the Sun at that time, the southern hemisphere then
+receives more heat in a given time than does the northern hemisphere in
+its summer season. When everything is taken into account, however, it is
+found that the southern hemisphere must have warmer summers and colder
+winters than the northern hemisphere.
+</p>
+<p>
+Seen with the naked eye, Mars appears as a fiery red star, whose
+intensity of color is surpassed by no other star in the heavens. Seen
+through the telescope, it retains the same red tint, which, however,
+appears less intense, and gradually fades away toward the limb, where it
+is replaced by a white luminous ring.
+</p>
+
+<p><br /></p>
+
+<div class="figcenter" style="width: 400px;">
+<a id="figure08"></a>
+<br />
+<img src="images/figure08.jpg" width="400" alt="" />
+<div class="caption">
+<p>PLATE VIII.&mdash;THE PLANET MARS.</p>
+<p class="smaller">Observed September 3, 1877, at 11h. 55m. P.M.</p>
+</div></div>
+
+<p><br /></p>
+
+<p>
+Mars is a very difficult object to observe, the atmosphere surrounding
+it being sometimes so cloudy and foggy that the sight can hardly
+penetrate through its vapors. When this planet is observed under
+favorable atmospheric conditions, and with sufficient magnifying power,
+its surface, which is of a general reddish tint, is found to be
+diversified by white, gray and dark markings. The dark markings, which
+are the most conspicuous, almost completely surround the planet. They
+are of different forms and sizes, and very irregular, as can be seen on
+Plate VIII., which represents one of the hemispheres of this planet.
+Many of them, especially those situated in the tropical regions of the
+planet, form long narrow bands, whose direction is in the main parallel
+to the Martial equator.
+</p>
+<p>
+The dusky spots differ very much, both from one another and in their
+several parts, as regards intensity of shade. Some appear almost black,
+while others which appear grayish, are so faint, that they can seldom be
+seen. In the southern hemisphere, the darkest part of the spots is
+generally found along their northern border; especially where there are
+deep indentations.
+</p>
+<p>
+Some observers have described these spots as being greenish or bluish,
+but I have never been able to see the faintest trace of these colors in
+them, except when they were observed close to the limb, and involved in
+the greenish tinted ring which is always to be seen there. It is
+probably an effect of contrast, since green and red are complementary
+colors, and since this greenish tinge around the limb covers all kinds
+of spots, whether white or dark. When such dark spots, involved in the
+greenish tint, are carried by the rotation towards the centre of the
+disk, they no longer show this greenish color. To me, these spots have
+always appeared dark, and of such tints as would result from a mixture
+of white and black in different proportions; except that on their
+lighter portions they show some of the prevalent reddish tint of the
+Martial surface. It is to be remarked that in moments of superior
+definition of the telescopic image, the intensity of darkness of all the
+spots is considerably increased&mdash;some of them appearing almost
+perfectly black.
+</p>
+<p>
+The markings on the surface of Mars are now tolerably well
+known&mdash;especially those of its southern hemisphere, which, owing to
+the greater proximity of the planet to our globe when this hemisphere is
+inclined towards the earth, have been better studied. Those of the
+northern hemisphere are not so well known, since when this hemisphere is
+inclined towards us, the distance of Mars from the Earth is 26,000,000
+miles greater, so that the occasions for observing them are not so
+favorable.
+</p>
+<p>
+Several charts of Mars are in existence, but as the same nomenclature
+has not been employed in all of them, some confusion has arisen in
+regard to the names given to the most remarkable features of the
+planet's surface. In order to give clearness to the subject, it will be
+necessary here to give a brief description of the principal markings
+represented on Plate VIII. In this the nomenclature will be employed
+which has been adopted by the English observers in the fine chart of Mr.
+Nath. Green. The large dark spot represented on the left-hand side of
+the plate is called De La Rue Ocean. The dark oval spot, isolated in the
+vicinity of the centre of the disk, is called Terby Sea; while the dark,
+irregular form on the right, near the border, represents the western
+extremity of Maraldi Sea.
+</p>
+<p>
+The dusky spots of Mars seem to be permanent, and to form a part of the
+general surface of the planet. That several among them, at least, are
+permanent, is proved by the fact that they have been observed in the
+same position, and with the same general form, for over two centuries.
+Yet, if we are to depend upon the drawings made fifty years ago by Beer
+and Maedler, it would seem that the permanency of some of them does not
+exist, since a very large spot represented by these astronomers on their
+chart of Mars is not visible now. This object, which, on their map, has
+its middle at 270°, should be precisely under the prominent dark oval
+spot called Terby Sea, seen near the centre of the picture, and would
+extend down almost as far as the northern limb. This can hardly be
+attributed to an error of observation, since these observers were both
+careful, and had great experience in this class of work. It is a very
+singular fact that, at the very same place where Maedler represented the
+spot in question, I found a conspicuous dark mark on December 16, 1881,
+which was certainly not visible in 1877, during one of the most
+favorable oppositions which can ever occur. The object, which is still
+visible (Feb., 1882), consists of an isolated spot situated a little to
+the north of Terby Sea. During the memorable opposition of 1877, I
+investigated thoroughly the markings of Mars, and made over 200 drawings
+of its disk, 32 of which represent the Terby Sea; but this isolated spot
+was not to be seen, unless it be identified with the faint mark,
+represented on the plate, which occupied its place. There cannot be the
+slightest doubt that a change has occurred at that place. Changes in the
+markings have also been suspected on the other hemisphere of the planet.
+</p>
+<p>
+The well-known fact that the continents, and especially the mountainous
+and denuded districts of our globe, reflect much more light than the
+surfaces covered by water, has led astronomers to suppose that the dark
+spots on Mars are produced by a liquid strongly absorbing the rays of
+light, like the liquids on the surface of the Earth. According to this
+theory the dark spots observed are supposed to be lakes, seas, and
+oceans, similar to our own seas and oceans, while the reddish and
+whitish surfaces separating these dark spots, are supposed to be
+islands, peninsulas, and continents. This supposition seems certainly to
+have a great deal of probability in its favor, although some of the
+lighter markings may have a different origin, and perhaps be due to
+vegetation; but no observer has yet seen in them any of the changes
+which ought to result from change of seasons. Some of the changes in the
+dark spots might also be attributed to the flooding or drying up of
+marshes and low land. The change which I have observed lately might be
+attributed to such a cause, especially as my observation was made
+shortly after the spring equinox of the northern hemisphere of Mars,
+which occurred on December 8th.
+</p>
+<p>
+Besides the dark spots just described, there are markings of a different
+character and appearance. Among the most conspicuous are two very
+brilliant white oval spots, which always occupy opposite sides of the
+planet. These two bright spots, which correspond very closely with the
+poles of rotation of Mars, have been called "polar spots."
+</p>
+<p>
+On account of the inclination of the axis of rotation of this planet to
+the ecliptic, it is rare that both of these spots are visible on the
+disk at the same time; and when this occurs, they are seen considerably
+foreshortened, as they are then both on the limb of the planet. Usually
+only one spot is visible, and it appears to its best advantage when the
+region to which it belongs attains its maximum of inclination towards
+the Earth.
+</p>
+<p>
+The polar spots change considerably in size, as they do also in form.
+Sometimes they occupy nearly one-third of the disk, as is proved by many
+of my observations; while at other times they are so much reduced as to
+be totally invisible. It is to be remarked that the reduction of these
+spots generally corresponds with the summer seasons, and their
+enlargement with the winter seasons of the hemispheres to which they
+respectively belong. From these well-observed facts it would appear that
+a relation exists between the temperature of the two hemispheres of Mars
+and the variations of the white spots observed at its poles. A similar
+relation is known to exist on our globe between the progress of the
+seasons and the melting away and the accumulation of snow in the polar
+regions. Astronomers have been led, accordingly, to attribute the polar
+spots of Mars, with all their variations, to the alternate accumulation
+and melting of snows. On this account, the polar spots of Mars are
+sometimes also called "snow-spots."
+</p>
+<p>
+Errors have certainly been made by astronomers in some of their
+observations of the so-called polar snow-spots, other objects occupying
+their place having been mistaken for them. A regular series of
+observations on this planet, which I have now continued for seven years,
+has revealed the fact that during the winter seasons of the southern
+hemisphere of Mars, the polar spots are most of the time invisible,
+being covered over by white, opaque, cloud-like forms, strongly
+reflecting light. In 1877, during more than a month, I, myself, mistook
+for the polar spots such a canopy of clouds, which covered at least
+one-fifth of the surface of the whole disk. I only became aware of my
+error when the opaque cloud, beginning to dissolve at the approach of
+the Martial summer, allowed the real polar spot to be seen through its
+vapors, as through a mist at first, and afterwards with great
+distinctness. In this particular case, the snow-spot was considerably
+smaller than the cloudy cap which covered it, and it is to be remarked
+that it was not situated at the centre of this cloudy cap, but was east
+of that centre; a fact which may account for the so-called polar spots
+not being always observed on exactly opposite sides of the disk. From my
+observations of 1877, 1878 and 1880, it appears that at the approach of
+the autumnal equinox of the southern hemisphere of Mars, large, opaque
+masses, like cumulus clouds in form, began to gather in the polar
+regions of that hemisphere, and continued through autumn and winter,
+dissolving only at the approach of spring. These clouds, which varied in
+form and extent, were very unsteady at first, but as the winter drew
+nearer they enlarged and became more permanent, covering large surfaces
+for months at a time.
+</p>
+<p>
+That the large white spots under consideration are real clouds in the
+atmosphere of Mars, and are not due to a fall of snow, is proved by the
+fact that these spots covered both seas and continents with equal
+facility, even in the equatorial regions of the planet. Snow, of course,
+could not cover the seas of Mars, unless these were all frozen over,
+even in the equatorial zones; therefore, if the dark spots of Mars are
+assumed to be due to water, these large white spots cannot well be
+ascribed to snow.
+</p>
+<p>
+The real polar spots of Mars seem to be in relief on the surface of the
+planet, since the southern spot often appeared slightly shaded on the
+side opposite to the Sun during my observations in 1877. In certain
+cases, when they are on or very near the limb, they have been observed,
+both by others and by myself, to project from the disk slightly.
+</p>
+<p>
+The polar spots of Mars are doubtless composed of a material which, like
+our snow or ice, melts under the rays of the Sun; although it seems
+difficult to admit that the Martial snow is identical with our
+terrestrial snow, and that it melts at a like temperature. The south
+polar spot of Mars entirely disappeared from sight in its summer season
+in 1877, although the planet receives less than one-half as much heat as
+we receive from the Sun; yet on our globe the arctic or antarctic ices
+and snow are perpetual&mdash;never melting entirely. An important fact
+disclosed by the melting away of the southern polar spot is, that in
+melting it is always surrounded with a very dark surface, which takes
+the place of the melted portion of the spot, as observed by myself in
+1877-78. When the polar spot had entirely disappeared, its place was
+occupied by a very dark spot. Now, if the polar spot is really ice, and
+the dark spots are actual seas, this polar spot must be situated in
+mid-ocean, since, on melting away, it is replaced by a dark spot. If the
+polar spots are composed of a white substance melting under the rays of
+the Sun, as seems altogether probable, its melting point must be above
+that of terrestrial snow.
+</p>
+<p>
+Many of the dark spots of Mars, and especially those whose northern
+border forms an irregular belt upon the equatorial regions of this
+planet, are bordered on that side by a white luminous belt, following
+all their sinuosities. These white borders are variable. Sometimes they
+are very prominent and intensely bright, especially at some points,
+which occasionally almost equal the polar spots in brilliancy; while at
+other times they are so faint, that they can hardly be distinguished, or
+are even invisible; although the atmosphere is clear and the dark spots
+appear perfectly well defined. While these white borders were invisible,
+I have sometimes watched for several hours at a time to see if I could
+detect any traces of them in places where they usually appear the most
+prominent, but generally without success. On a few occasions, however, I
+had the good fortune to see some of these spots forming gradually in the
+course of one or two hours, at places where nothing of them could be
+seen before.
+</p>
+<p>
+These whitish fringes forming and vanishing along the coasts of the
+Martial seas have been very little studied by astronomers. From my
+observations made during the last seven years, it appears very probable
+that this belt and its white spots are mainly due to the condensation of
+vapors around, and over high peaks, and extensive mountain chains,
+forming the Martial sea-coasts, as the Andes and Rocky Mountains form
+the sea-coasts of the Pacific Ocean. These high mountains on Mars,
+condensing the vapors into fogs or clouds above them, or at their sides,
+as often happens in our mountainous districts, would certainly suffice
+to produce the phenomena observed. Some of the highest peaks among these
+mountain chains may even have their summits covered with perpetual snow,
+or some substance partaking of the nature of snow. The temporary
+visibility and invisibility of the white spots seen on Mars, as well as
+the rapid transformations they sometimes undergo, may be explained as
+caused by clouds having a high reflective power and a liability to form
+and disappear quickly.
+</p>
+<p>
+The assumption that these irregular whitish bands and spots are formed
+by the condensation of vapors on mountain chains, and elevated table
+lands, is supported by my observations made in 1877 and 1879. When such
+white spots were traversing the terminator at sunrise, they very often
+projected far into the night side, thus indicating that they were at a
+higher level than that of the general surface. Indentations in the
+terminator, corresponding to large dark spots crossing its line, also
+clearly indicated the depression of the dark spots below the general
+surface. The highest mountainous districts thus observed on Mars, are
+situated between 60° and 70° of south latitude, towards the western
+extremity of Gill Land. The mountain chain, which almost completely
+forms the surface of this land, is so elevated at some points, that they
+not only change the form of the terminator when they are seen upon it,
+but also the limb of the planet, as seen by myself. They then appear so
+brilliant, that the principal summit among them has been mistaken by
+several observers for the polar spot itself, as proved by the wrong
+position assigned to it on their drawings. It seems probable that this
+high peak, which appears always white, is constantly covered with snow,
+or the similar material replacing it on Mars. This high region is
+situated between longitudes 18o° and 190°.
+</p>
+<p>
+The highest mountainous parts belonging to the hemisphere represented on
+Plate VIII., which are nearly always more or less visible as whitish
+spots and bands, form a coast line along the northern (lower) border of
+De La Rue Ocean. This great spot, which is not so simple as it has been
+represented by observers, is in fact divided by two narrow isthmuses,
+one in the north, the other in the east, both joining, in the interior
+of the great ocean, a peninsula heretofore known as Hall Island. Upon
+the south-eastern extremity of this peninsula, a white spot, called
+Dawes Ice Island, was observed in 1865, but it soon disappeared, and was
+after that seen only now and then. It is very probable that this
+so-called Ice Island was due to clouds forming around the summit of some
+high peak of this peninsula.
+</p>
+<p>
+On the opposite hemisphere to that represented on Plate VIII., the white
+fringes bordering the dark spots are much more conspicuous than they are
+on this side. On the eastern side of a remarkable dark spot called
+Kaiser Sea, they are very bright, and almost always present, although
+they vary considerably, both in brightness and in extent. To the south
+of Kaiser Sea, they are very conspicuous on the eastern border of
+Lockyer Land, forming an elevated and deeply indented coast-line along
+Lambert Sea. There the white spots never disappear entirely, being
+always visible on the north side, where they turn westward along Dawes'
+Ocean&mdash;the mountain chain attaining there its greatest altitude. Very
+frequently Lockyer Land, which seems to be a vast plateau, appears
+throughout white and brilliant, this occurring usually towards the
+sunrise or sunset of that region, probably from the condensation of
+vapors and the formation of fogs, but generally this whiteness gradually
+disappears with the progress of the sun above this plateau. Inside of
+the great continents of Mars these temporary white spots are not so
+frequent, but when visible they occupy always the same positions&mdash;a
+fact which probably indicates that they occupy the culminating points of
+these continents. One of these temporary white spots inside of the
+continents is represented on Plate VIII., on the left-hand side, below
+De La Rue Ocean, on Maedler Continent.
+</p>
+<p>
+Although large, opaque, cumulus-shaped, cloud-like forms are seen in the
+polar regions of Mars, such forms are very seldom seen in the tropical
+zones, or, at least, it appears so, from the fact that my observations,
+continued during the last seven years, have disclosed no real opaque
+cloudy forms there. Although the Martial sky is frequently overcast by
+dense vapors or thick fogs in these regions, yet no real opaque clouds
+were ever seen; the most prominent among the dusky spots being faintly
+visible through the vapory veil, when they approached the centre of the
+disk.
+</p>
+<p>
+Besides these phenomena, which prove that Mars is surrounded by an
+atmosphere having a great deal of similarity to our own, a further proof
+is afforded by the fact that the dark spots, which appear sharply
+defined and black when they are seen near the centre, become less and
+less visible as they advance towards the limb, and are totally invisible
+before they reach it. Moreover, the spectroscope also indicates the
+existence of an atmosphere, and even the presence of watery vapor in it.
+A very curious state of the Martial atmosphere is revealed by my
+observations of 1877-78. During eight consecutive weeks, from December
+12th to February 6th, a whole hemisphere of the planet&mdash;precisely that
+represented on Plate VIII.&mdash;was completely covered by dense vapors, or
+a thick fog which barely allowed the dark spots to be seen through it,
+even when they were in the centre of the disk. The opposite hemisphere
+of Mars appeared just as clear and calm as possible; there all the spots
+and their minutest details could be seen, and when the planet was
+observed at the proper time, the line separating the foggy from the
+clear side was plainly visible.
+</p>
+<p>
+The reddish tint observed on the continents of Mars has been supposed by
+some astronomers to be the real color of the atmosphere of this planet.
+But, for many reasons, this explanation is not acceptable. Besides the
+fact that the border of the planet appears white, while it should be
+more red than the other part, owing to the greater depth of atmosphere
+there presented to us, the polar spots, the white bands along the
+sea-coasts, and the cloud-like forms appear perfectly white, not the
+slightest tint of red being visible on them, as would be the case if
+these objects were seen through an atmosphere tinted red. Other
+astronomers have supposed that the vegetation of this planet has a
+reddish color; but this is not supported by observation. It has been
+again supposed, with much more probability, that the surface of Mars is
+composed of an ochreous material which gives the planet its predominant
+ruddy color.
+</p>
+<p>
+Until lately Mars was supposed to be without a satellite, but in August,
+1877, Professor Hall, of the Washington Observatory, made one of the
+most remarkable discoveries of the time, and found two satellites
+revolving around this planet. These satellites are among the smallest
+known heavenly bodies, their diameter having been estimated at from 6 to
+10 miles for the outer satellite, and from 10 to 40 miles for the inner
+one.
+</p>
+<p>
+The most extraordinary feature of these bodies is the proximity of the
+inner satellite to the planet, and the consequent rapidity of its
+motion. The distance of the inner satellite from the centre of Mars is
+about 6,000 miles, and from surface to surface it is less than 4,000
+miles, or a little more than the distance from New York to San
+Francisco. The shortest period of revolution of any satellite previously
+known, is that of the inner satellite of Saturn, which is a little more
+than 22½ hours; but the inner satellite of Mars accomplishes its
+revolution in 7h. 38m., or in 17 hours less than the period of rotation
+of the planet upon its axis. The period of revolution of the outer
+satellite is greater, of course, and equals 30h. 7m.
+</p>
+<p>
+From this rapidity of motion of the inner satellite of Mars, a very
+curious result follows, which at first sight may appear in contradiction
+with the fact that this body has a direct motion, like that of all the
+planets of the solar system, and moves around Mars from west to east.
+While the outer satellite of this planet, in company with all the stars
+and planets, rises in the east and sets in the west, the inner
+satellite, on the contrary, rises in the west and sets in the east.
+Since the period of rotation of Mars is greater than is the period of
+revolution of this satellite, it necessarily follows that this last body
+must constantly be gaining on the rotation, and, consequently, that the
+satellite sets in the east and rises in the west, compassing the whole
+heavens around Mars three times a day, passing through all its phases in
+11 hours, each quarter of this singular Moon lasting less than 3 hours.
+</p>
+<p>
+It has been shown above that Mars has many points of resemblance to the
+Earth. It has an atmosphere constituted very nearly like ours; it has
+fogs, clouds, rains, snows, and winds. It has water, or at least some
+liquids resembling it; it has rivers, lakes, seas and oceans. It has
+also islands, peninsulas, continents, mountains and valleys. It has two
+Moons, which must create great and rapid tides in the waters of its seas
+and oceans. It has its days and nights, its warm and cold seasons, and
+very likely its vegetation, its prairies and forests, like the Earth. On
+the other hand, its year and seasons are double those of the Earth, and
+its distance from the Sun is greater.
+</p>
+<p>
+Is this planet, which is certainly constituted very nearly like our
+globe, and seems so nearly fitted for the wants of the human race,
+inhabited by animals and intelligent beings?
+</p>
+<p>
+To answer this question, either in the negative or in the affirmative,
+would be to step out of the pure province of science, and enter the
+boundless domain of speculation, since no observer has ever seen
+anything indicating that animal life exists on Mars, or on any other
+planet or satellite. So far as observation goes, Mars seems to be a
+planet well suited to sustain animal life, and we may reason from
+analogy that if animal life can exist at all outside of the Earth, Mars
+must have its flora and fauna; it must have its fishes and birds, its
+mammalia and men; although all these living beings must inevitably be
+very different in appearance from their representatives on the Earth, as
+can easily be imagined from the differences existing between the two
+planets. Although all this is possible, and even very probable, yet it
+must be remembered that we have not the slightest evidence that it is
+so; and until we have acquired this evidence, we may only provisionally
+accept this idea as a pleasing hypothesis, which, after all, may be
+wrong and totally unfounded.
+</p>
+
+<p><br /><br /><br /></p>
+
+<h4><a id="THE_PLANET_JUPITER">THE PLANET JUPITER</a>
+<br /><br />
+PLATE IX</h4>
+
+<p>
+Jupiter, the giant of the planetary world, is the fifth in order of
+distance from the Sun, and is next to Mars, our ruddy neighbor. To the
+naked eye, Jupiter appears as a very brilliant star, whose magnitude,
+changing with the distance of this planet from the Earth, sometimes
+approaches that of Venus, our bright morning and evening star.
+</p>
+<p>
+The mean distance of Jupiter from the Sun is 475,000,000 miles, but
+owing to the eccentricity of its orbit, its distance varies from 452 to
+498 millions of miles. The distance of this planet from the Earth varies
+still more. When nearest to our globe, or in opposition, its distance is
+reduced to 384,000,000 miles, and its apparent diameter increased to
+50"; while when it is farthest, or in conjunction, its distance is
+increased to 567,000,000 miles, and its apparent diameter reduced to
+30"; Jupiter being thus 183,000,000 miles nearer our globe while in
+opposition than when it is in conjunction.
+</p>
+<p>
+This planet revolves around the Sun in 11 years, 10 months and 17 days,
+or in only 50 days less than 12 terrestrial years. Such is the year of
+this planet. The plane of its orbit is inclined 1° 19' to the ecliptic.
+No planet, except Uranus, has an orbit exhibiting a smaller inclination.
+The planet advances in its orbit at the mean rate of 8 miles a second;
+which is a little less than half the orbital velocity of the Earth.
+</p>
+<p>
+Jupiter is of enormous proportions. Its equatorial diameter measures
+88,000 miles, and its circumference no less than 276,460 miles, these
+dimensions being 11 times greater than those of the Earth. This planet,
+notwithstanding its huge size, rotates on its axis in not far from 9h.
+55m. 36s., which period constitutes its day. Owing, however, to the
+changeable appearance of its surface, this period cannot be ascertained
+with very great exactitude. In consequence of its rapid rotation, the
+planet is far from spherical, its polar diameter being shorter than the
+equatorial by about ¹⁄₁₆, or 5,500 miles. Its surface is 124
+times the surface of the earth; while its volume is 1,387 times as
+great. If Jupiter occupied the place of our satellite in the sky, it
+would appear 40 times as large as the Moon appears to us, and would
+cover a surface of the heavens 1,600 times that covered by the full
+Moon, and would subtend an angle of 21°. Jupiter's mass does not
+correspond with its great bulk, and is only ¹⁄₁₀₄₇ of the
+mass of the Sun, and 310 times the mass of the earth; its density being
+only ¼ of that of our globe. The force of gravitation at the surface of
+this planet is over 2½ times what it is on the Earth, so that a
+terrestrial object carried to the surface of Jupiter would weigh over
+two and a half times as much as on our globe.
+</p>
+<p>
+Observed with a telescope, even of moderate aperture, Jupiter, with its
+four attending satellites and its dazzling brilliancy, appears as one of
+the most magnificent objects in the sky. The general appearance of the
+disk is white; but unlike that of Mars, it is brightest towards its
+central parts, and a little darker around the limb, especially on the
+side opposite to the Sun. Although an exterior planet, and so far from
+us, Jupiter shows faint traces of phases when observed near its
+quadratures, but this gibbosity of its disk is very slight, and is
+indicated only by a kind of penumbral shadow on the limb.
+</p>
+<p>
+When observed with adequate power, the disk of Jupiter is found to be
+highly diversified. The principal features consist of a series of
+alternate light and dark streaks or bands, disposed most of the time
+parallel with the Jovian equator. These bands differ from each other in
+intensity as well as in breadth; those near the equator being usually
+much more prominent than those situated in higher latitudes north and
+south.
+</p>
+<p>
+The equatorial zone of Jupiter is occupied most of the time by a broad,
+prominent belt 20° or 30° wide, limited on each side by a very dark
+narrow streak. Between these two dark borders, but seldom occupying the
+whole space between them, appears an irregular white belt, apparently
+composed of dense masses of clouds strongly reflecting the Sun's light,
+some of these cloudy masses being very brilliant. The spaces left
+between the cloudy belt and the dark borders, usually exhibit a delicate
+pink or rosy color, which produces a very harmonious effect with the
+varying grayish and bluish shades of some of the belts and streaks seen
+on the disk. Quite often the cloudy belt is broken up, and consists of
+independent cloudy masses, separated by larger or smaller intervals,
+these intervals disclosing the rosy background of this zone.
+</p>
+<p>
+On each side of the equatorial belt there is usually a broad whitish
+belt, succeeded by a narrow gray band; the space left on each hemisphere
+between these last bands and the limb being usually occupied by two or
+three alternate white and gray bands. A uniform gray segment usually
+forms a sort of polar cap to Jupiter.
+</p>
+<p>
+When observed under very favorable conditions, all the lighter belts
+appear as if composed of masses of small cloudlets, resembling the white
+opaque clouds seen in our atmosphere. This, as already stated, is
+particularly noticeable on the equatorial belt. It is not unusual, when
+Jupiter is in quadrature, to see some of the most conspicuous white
+spots casting a shadow opposite to the Sun; a fact which sufficiently
+indicates that these spots are at different levels. They probably form
+the summits of vast banks of clouds floating high up in the atmosphere
+of Jupiter.
+</p>
+<p>
+What we see of Jupiter is chiefly a vaporous, cloudy envelope. If our
+sight penetrates anywhere deeper into the interior, it can only be
+through the narrow fissures of this envelope, which appear as gray or
+dark streaks or spots. That most of the visible surface of Jupiter is
+simply a cloudy covering, is abundantly proved by the proper motion of
+its spots, which sometimes becomes very great.
+</p>
+<p>
+In periods of calm, very few changes are noticeable in the markings of
+the planet, except, perhaps, some slight modifications of form in the
+cloudy, equatorial belt which, in general, is much more liable to
+changes than the other belts. But the Jovian surface is not always so
+tranquil, great changes being observed during the terrific storms which
+sometimes occur on this mighty planet, when all becomes disorder and
+confusion on its usually calm surface; and nothing on the Earth can give
+us a conception of the velocity with which some of its clouds and spots
+are animated. New belts quickly form, while old ones disappear. The
+usual parallelism of the belts no more exists. Huge, white, cumulus-like
+masses advance and spread out, the rosy equatorial belt enlarges
+sometimes to two or three times its usual size, and occupies two-thirds,
+or more, of the disk, the rosy tint spreading out in a very short time.
+At times very dark bands extending across the disk are transformed into
+knots or dark spots, which encircle the planet with a belt, as it were,
+of jet black beads. Sometimes, also, a secondary but narrower rosy belt
+forms either in the northern or the southern hemisphere, and remains
+visible for a few days or for years at a time.
+</p>
+<p>
+On May 25, 1876, I witnessed one of the grandest commotions which can be
+conceived as taking place in an atmosphere. All the southern hemisphere
+of Jupiter, from equator to pole, was in rapid motion, the belts and
+spots being transported entirely across the disk, from the eastern to
+the western limb, in one hour's time, during which the equatorial belt
+swelled to twice its original breadth, towards the south.
+</p>
+<p>
+Now, when one stops for a moment to think what is signified by that
+motion of the dark spots across the little telescopic disk of Jupiter in
+an hour's time, he may arrive at some conception of the magnitude of the
+Jovian storms, compared with those of our globe. The circumference of
+Jupiter's equator, as stated above, is 276,460 miles; half this number,
+or 138,230 miles, represents the length of the equatorial line seen from
+the Earth. Now, after taking into account the rotation of the planet,
+which somewhat diminishes the apparent motion, we arrive at the
+astonishing result that the spots and markings were carried along by
+this Jovian storm, at the enormous rate of 110,584 miles an hour, or
+over 30.7 miles a second. On our globe, a hurricane or tornado, which
+blows at the rate of 100 miles an hour, sweeps everything before it.
+What, then, must be expected from a velocity over 1,105 times as great?
+Enormous as this motion may appear, its occurrence cannot be doubted,
+since it is disclosed by direct observation.
+</p>
+<p>
+The surface of Jupiter, it would seem, has its periods of calm and
+activity like that of the Sun, although it is not yet known, as it is
+for the latter, that they recur with approximate regularity.
+</p>
+<p>
+My observations of this planet, which embrace a period of ten years,
+seem to point in that direction, for they show, at least, that Jupiter
+has its years of calm and its years of disturbances. The year 1876 was a
+year of extraordinary disturbance on Jupiter. Changes in the markings
+were going on all the time, and no one form could be recognized the next
+day, or even sometimes the next hour, as shown above. The cloudy
+envelope of the planet was in constant motion, the equatorial belt,
+especially, showing the signs of greatest disturbance, being, for the
+most part, two or three times as wide as in other years. After 1876 the
+calm was very great on the planet, only a slight change now and then
+being noticeable, the same forms being recognized day after day, month
+after month, and even year after year. In one case the same marking has
+been observed for seventeen consecutive months, and in another for
+twenty-eight months. This state of quietude lasted until October, 1880,
+when considerable commotion occurred on the northern hemisphere, where
+large round black spots, somewhat resembling the Sun-spots, formed in
+the cloudy atmosphere, and finally changed, towards the end of December,
+into a narrow pink belt, which still exists.
+</p>
+
+<p><br /></p>
+
+<div class="figcenter" style="width: 400px;">
+<a id="figure09"></a>
+<br />
+<img src="images/figure09.jpg" width="400" alt="" />
+<div class="caption">
+<p>PLATE IX.&mdash;THE PLANET JUPITER.</p>
+<p class="smaller">Observed November 1, 1880, at 9h. 30m. P.M.</p>
+</div></div>
+
+<p><br /></p>
+
+<p>
+The most curious marking ever seen on Jupiter is undoubtedly the great
+Red Spot, observed on the southern hemisphere of this planet for the
+last three years. This interesting object, seen first in July, 1878,
+disappeared for a time, reappeared on September 25 of the same year, and
+has remained visible until now. When seen by me in September, it was
+much elongated, and sharply pointed on one side, like a spear-head, but
+it subsequently acquired an irregular form, with short appendages
+protruding from its northern border. At first, the changes were great
+and frequent, but at length it acquired the regular oval form, which,
+with but slight modifications, it has retained until now. During the
+month of November, 1880, I noticed two small black specks upon this Red
+Spot, and they were seen again in January of the succeeding year, by Mr.
+Alvan Clark, Jr. When the spot had attained its oval shape, it appeared
+part of the time surrounded with a white luminous ring of cloudy forms
+which, however, was changing more or less all the time, being sometimes
+invisible. The color of this curious spot is a brilliant rosy red,
+tinged with vermilion, and altogether different in shade from the
+pinkish color of the equatorial belt. The size of the spot varies, but
+of late its changes have been slight. Its longer diameter may be
+estimated at 8,000 miles, and the shorter at 2,200 miles. The Red Spot
+is represented on Plate IX. with its natural color, and as it appears at
+the moments most favorable for observation. In ordinary cases its color
+does not appear so brilliant, but paler.
+</p>
+<p>
+It is difficult to account for the color of the equatorial belt and that
+of the Red Spot; but it is known, at least, that the material to which
+they are due cannot be situated at the level of the general surface
+visible to us, and especially that of the cloudy forms of the equatorial
+zone. Undoubtedly the red layer lies deeper than the superficial
+envelope of the planet, although it does not seem to be very deeply
+depressed.
+</p>
+<p>
+Jupiter is attended by four satellites, which revolve around the planet
+at various distances, and shine like stars of the 6th and 7th magnitude.
+It is said that under very favorable circumstances, and in a very clear
+sky, the satellites can be seen with the naked eye, but this requires
+exceptionally keen eyes, since the glare of the planet is so strong as
+to overpower the comparatively faint light of the satellites. However, I
+myself have sometimes seen, without the aid of the telescope, two or
+three of the satellites as a single object, when they were closely
+grouped on the same side of Jupiter.
+</p>
+<p>
+The four moons of Jupiter are all larger than our Moon, except the
+first, which has about the same diameter. They range in size from 2,300
+to 3,400 miles in diameter, the third being the largest; the
+determination of their diameter is by no means accurate, however, as it
+is difficult to measure such small objects with precision. Their mean
+distance from the centre of Jupiter varies from 267,000 to 1,192,000
+miles, the first satellite, the nearest to the planet, being a little
+farther from Jupiter then our satellite is from us. The four satellites
+revolve around the planet in orbits whose planes have a slight
+inclination to the equator of Jupiter, and consequently to the ecliptic.
+The diameter of the largest satellite is nearly half that of the Earth,
+or 3,436 miles; while its volume is five times that of our Moon. The
+period of revolution of these satellites varies from 1d. 18h. for the
+first, to 16d. 16h. for the last.
+</p>
+<p>
+Owing to the slight inclination of the plane of their orbits to that of
+the planet, the three first satellites, and generally the fourth, pass
+in front of the disk and also through the shadow of the planet at every
+revolution, and are accordingly eclipsed. Their passages behind
+Jupiter's disk are called occultations; those in front of it, transits.
+The eclipses, the occultations and the transits of the moons of Jupiter
+are interesting and important phenomena; the eclipses being sometimes
+observed for the rough determination of longitudes at sea.
+</p>
+<p>
+The satellites in transit present some curious phenomena. When they
+enter the disk, they appear intensely luminous upon its grayish border;
+but as they advance, they seem by degrees to lose their brightness,
+until they finally become undistinguishable from the luminous surface of
+Jupiter. It sometimes happens, however, that the first, the third and
+the fourth satellites, after ceasing to appear as bright spots, continue
+to be visible as dark spots upon the bright central portions of the
+planet's disk; but in these cases their disks appear smaller than the
+shadows they cast. Undoubtedly these satellites have extensive
+atmospheres, since they sometimes pass unperceived across the central
+parts of Jupiter, this being probably when their atmospheres are
+condensed into clouds, strongly reflecting light; while when these
+clouds are absent, we can see their actual surface, with traces of the
+dark spots upon them similar to those on Mars.
+</p>
+<p>
+From the variation in the brightness of these satellites, which is said
+to be always observed in the same part of their orbit, William Herschel
+was led to suppose that these bodies, like our Moon, rotate upon their
+axes in the same period in which they move round the planet, so that
+they always present the same face to Jupiter; but these conclusions have
+been denied. From my observations it is apparent, however, that the
+light reflected by them varies in intensity as well as in color. But
+this is rather to be attributed to the presence of an atmosphere
+surrounding these bodies, which when cloudy reflects more light than
+when clear, with corresponding changes in the color of the light.
+</p>
+<p>
+The satellites in transit are sometimes preceded or followed, according
+to the position of the Sun, by a round black spot having about the same
+size as the satellite itself. This black spot is the shadow of the
+satellite cast on the vapory envelope of Jupiter, similar to the shadow
+cast by the Moon on the Earth, during eclipses of the Sun; in fact, all
+the Jovian regions traversed by these shadows have the Sun totally
+eclipsed. Sometimes it happens that the shadow appears elliptical. This
+occurs either when it is observed very near the limb, or when entering
+upon a round, cloud-like spot. This effect is attributable to the
+perspective under which the shadow is seen on the spherical globe or
+spot.
+</p>
+<p>
+The proper motion of the satellites in the Jovian sky is much more rapid
+than that of the Moon in our sky. During one Jovian day of ten hours,
+the first satellite advances 84°; the second, 42°; the third, 20° and
+the fourth, 9°. The first satellite passes from New Moon to its first
+quarter in a little more than a Jovian day, while the fourth occupies
+ten such days in attaining the same phase.
+</p>
+<p>
+In density, as well as in physical constitution, Jupiter differs widely
+from the interior planets, and especially from the Earth; and, as has
+been shown, it is surrounded by a dense, opaque, cloudy layer, which is
+almost always impenetrable to the sight, and hides from view the
+nucleus, which we may conceive to exist under this vaporous envelope. In
+1876, the year of the great Jovian disturbances, I observed frequently
+in the northern hemisphere of the planet a very curious phenomenon,
+which seems to prove that its cloudy envelope is at times partially
+absent in some places, its vapors being apparently either condensed, or
+transported to other parts of its surface, and that, therefore, a
+considerable part of the real globe of the planet was visible at these
+places. The phenomenon consisted in the deformation of the northern
+limb, which had a much shorter radius on all of this hemisphere situated
+northward of the white belt which adjoins the equatorial zone. The
+deformation of the limb on both sides, where it passed from a longer to
+a shorter radius, was abrupt, and at right angles to the limb, forming
+there a steplike indentation which was very prominent. The polar segment
+having a smaller radius, appeared unusually dark, and was not striped,
+as usual, but uniform in tint throughout. On September 27th, the third
+satellite passed over this dark segment, and emerged from the western
+border, a little below the place where the limb was abruptly deformed,
+as above described. When the satellite had fully emerged from this limb,
+it was apparent that if the portion of the limb having a longer radius
+had been prolonged a little below, and as far as the satellite, it would
+have enclosed it within its border, and thus retarded the time of
+emersion. The depth of deformation of the limb was accordingly greater
+than the diameter of the third satellite, and certainly more than 4,000
+miles. That the phenomenon was real, is proved by the fact that the
+egress of this satellite occurred at least four minutes sooner than the
+time predicted for it in the American Ephemeris. Other observations seem
+to point in the same direction, since some of the satellites which were
+occulted have been seen through the limb of Jupiter by different
+astronomers, as if this limb was sometimes semi-transparent. Another
+observation of mine seems to confirm these conclusions. On April 24th,
+1877, at 15h. 25m. the shadow of the first satellite was projected on
+the dark band forming the northern border of the equatorial belt, the
+shadow being then not far from the east limb. Close to this shadow, and
+on its western side, it was preceded by a secondary shadow, which was
+fainter, but had the same apparent size. This round dark spot was not
+the satellite itself, as I had supposed at first, since this object was
+yet outside of the planet, on the east, and entered upon it only at 16h.
+4m. I watched closely this strange phenomenon, and at 16h. 45m., when
+the shadow had already crossed about ¾ of the disk, it was still
+preceded by the secondary, or mock shadow, as it may be called; the same
+relative distance having been kept all the while between the two
+objects, which had therefore traveled at the same rate. It is obvious
+that this dark spot could not be one of the planet's markings, since the
+shadow of the first satellite moves more quickly on the surface of
+Jupiter than a spot on the same surface travels by the effect of
+rotation, so that in this case the shadow would soon have passed over
+this marking, and left it behind, during the time occupied by the
+observation. From these observations it seems very probable that Jupiter
+has a nucleus, either solid or liquid, which lies several thousand
+mites below the surface of its cloudy envelope. It is also probable that
+the uniformly shaded dark segment seen in 1876, was a portion of the
+surface of this nucleus itself. When the cloudy envelope is
+semi-transparent at the place situated on a line with an occulted
+satellite and the eye of an observer, this satellite may accordingly
+remain visible for a time through the limb, as shown by observation. The
+phenomenon of the mock shadow may also be attributed to a similar cause,
+where semi-transparent vapors receive the shadow of a satellite at their
+surface, while at the same time part of this shadow, passing through the
+semi-transparent vapors, may be seen at the surface of the nucleus, or
+of a layer of opaque clouds situated at some distance below the surface.
+</p>
+<p>
+Some astronomers are inclined to think that Jupiter is at a high
+temperature, and self-luminous to a certain extent. If this planet is
+self-luminous to any degree, we might expect that some light would be
+thrown upon the satellites when they are crossing the shadow cast into
+space by the planet; but when they cross this shadow they are totally
+invisible in the best telescopes, a proof that they do not receive much
+light from the non-illuminated side of Jupiter. It would, indeed, seem
+probable that some of the intensely white spots occasionally seen on the
+equatorial belt of the planet are self-luminous in a degree, yet not
+enough to render the satellites visible while they are immersed in
+Jupiter's shadow. It does not seem impossible that the planet should
+have the high temperature attributed to it, when we remember the
+terrific storms observed in its atmosphere, which, owing to the great
+distance of Jupiter from the Sun, do not seem to be attributable to this
+body, but rather to some local cause within the envelope of the planet.
+</p>
+<p>
+Astronomy, which is a science of observation, is naturally silent with
+regard to the inhabitants of Jupiter. If there are any such inhabitants,
+they are confined to the domain of conjecture, under the dense cloudy
+envelope of the planet. The conditions of habitability on Jupiter must
+differ very widely from those of our globe. Comparatively little direct
+light from the Sun reaches the surface of the globe of Jupiter, except
+that which passes through the narrow openings forming the dark clouds.
+All the rest of the planet's surface, being covered perpetually by
+opaque clouds, receives only diffused light. On Jupiter there are
+practically no seasons, since its axis is nearly perpendicular to its
+orbit. The force of gravity on the surface of Jupiter being more than
+double what it is on the Earth, living bodies would there have more than
+double the weight of similar bodies on the Earth. Furthermore, Jupiter
+only receives 0.011 of the light and heat which we receive from the Sun;
+and its year is nearly equal to 12 of our years. If there are living
+beings on Jupiter, they must, then, be entirely different from any known
+to us, and they may have forms never dreamed of in our most fantastic
+conceptions.
+</p>
+<p>
+The two round black spots represented towards the central parts of Plate
+IX. are the shadows of the first and second satellite; while the two
+round white spots seen on the left of the disk, are the satellites
+themselves, as they appeared at the moment of the observation. The first
+satellite and its shadow are the nearest to the equator; while the
+second satellite and its shadow are higher, the last being projected on
+the Great Red Spot.<a name="FNanchor_2_1" id="FNanchor_2_1"></a><a href="#Footnote_2_1" class="fnanchor">[2]</a> The row of dark circular spots represented on the
+northern, or lower hemisphere, when they first appeared, had some
+resemblance to Sun-spots without a penumbra, with bright markings around
+them, resembling faculæ. These round spots subsequently enlarged
+considerably, until they united along the entire line, encircling the
+planet, and finally forming a narrow pink belt, which is still visible.
+</p>
+
+<p><br /></p>
+
+<div class="footnote">
+
+<p class="nind"><a name="Footnote_2_1" id="Footnote_2_1"></a><a href="#FNanchor_2_1"><span class="label">[2]</span></a>By an accidental error in enlarging the original drawing,
+the satellites and shadows appear in Plate IX. of double their actual
+size. The error is one easy of mental correction.</p></div>
+
+<p><br /><br /><br /></p>
+
+<h4><a id="THE_PLANET_SATURN">THE PLANET SATURN</a>
+<br /><br />
+PLATE X</h4>
+
+<p>
+Saturn, which is next to Jupiter in order of distance from the Sun,
+while not the largest, is certainly the most beautiful and interesting
+of all the planets, with his grand and unique system of rings, and his
+eight satellites, which, like faithful servants, attend the planet's
+interminable journey through space.
+</p>
+<p>
+Seen with the naked eye, Saturn shines in the night like a star of the
+first magnitude, whose dull, soft whiteness is, however, far from
+attaining the brilliancy of Venus or Jupiter, although it sometimes
+approaches Mars in brightness. Saturn hardly ever exhibits the
+phenomenon of scintillation, or twinkling, a peculiarity which makes it
+easily distinguishable among the stars and planets of the heavens.
+</p>
+<p>
+The synodical period of Saturn occupies 1 year and 13 days, so that
+every 378 days, on an average, this planet holds the same position in
+the sky relatively to the Sun and the Earth.
+</p>
+<p>
+The mean distance of Saturn from the Sun is a little over 9½ times that
+of our globe, or 872,000,000 miles. Owing to the orbital eccentricity,
+this distance may increase to 921,000,000 miles, when the planet is in
+aphelion; or decrease to 823,000,000 miles, when it is in perihelion;
+Saturn being therefore 98,000,000 miles nearer to the Sun when in
+perihelion than in aphelion. If gravitation were free to exert its
+influence alone, Saturn would fall into the Sun in 5 years and 2 months.
+</p>
+<p>
+The distance of Saturn from the Earth varies, according to the position
+of the two planets in their respective orbits. At the time of
+opposition, when the Earth lies between the Sun and Saturn, this
+distance is smallest; while, on the contrary, at the time of
+conjunction, when the Sun lies between the Earth and Saturn, it is
+greatest. Owing, however, to the eccentricity of the orbits of Saturn
+and our globe, and the inclination of their planes to each other, and
+owing also to the variable heliocentric longitude of the perihelion, the
+distance of the two planets from each other at their successive
+conjunctions and oppositions is rendered extremely variable. At present
+it is when the oppositions of Saturn occur in December that this planet
+comes nearest to us; while when the conjunctions take place in June, the
+distance of Saturn from the Earth is the greatest possible. In the
+former case the distance of the planet from our globe is only
+730,000,000 miles; while in the last it is 1,014,000,000 miles, the
+difference between the nearest and farthest points of Saturn's approach
+to us being no less than 284,000,000 miles, or over three times the mean
+distance of the Sun from the Earth.
+</p>
+<p>
+From the great variations in the distance of Saturn from the Earth,
+necessarily result corresponding changes in the brightness and apparent
+diameter of this body. When it is farthest from us, its angular diameter
+measures but 14"; while, when it is nearest, it measures 20".
+</p>
+<p>
+The orbit of Saturn is inclined 2°30' to the ecliptic, and its
+eccentricity, which equals 0,056, is over three times that of the
+Earth's orbit.
+</p>
+<p>
+This planet revolves around the Sun in a period of 29 years and 5½
+months, or 10,759 terrestrial days, which constitutes its sidereal year.
+The extension of the immense curve forming the orbit of this planet, is
+no less than 5,505,000,000 miles, which is traversed by the planet with
+a mean velocity of a little less than 6 miles per second, or three times
+less than the motion of our globe in space.
+</p>
+<p>
+The real dimensions of the globe of Saturn are not yet known with
+accuracy, and the equatorial diameter has been variously estimated by
+observers, at from 71,000 to 79,000 miles. If we adopt the mean of these
+numbers, 75,000 miles, the circumference of the Saturnian equator would
+measure 235,620 miles, or 9½ times the circumference of our globe; the
+surface of Saturn would be 86 times, and its volume over 810 times that
+of the Earth.
+</p>
+<p>
+However great the volume of Saturn, its mass is proportionally small,
+being only 90 times greater than that of our globe; the mean density of
+the materials composing this planet being less than that of cork, and
+only 0.68 the density of water. The force of gravitation at the surface
+of Saturn is greater, by a little over ⅑, than it is at the surface of
+the Earth; a body falling in a vacuum at its surface, would travel 17.59
+feet during the first second.
+</p>
+<p>
+From observations of markings seen on the surface of Saturn, and from
+the study of their apparent displacements on the disk, William Herschel
+found that the planet rotated upon its axis in 10h. 16m. 0.24s. Since
+Herschel's determination, new researches have been made, and lately,
+Professor Hall, noticing a bright spot, followed it for nearly a month,
+observing its transits across the central meridian of the disk. From
+these observations he has obtained for the rotation period 10h. 14m.
+23.8s., a result which agrees very closely with that obtained 82 years
+earlier by Herschel, considering the fact that the markings from which
+the period of rotation is ascertained are not fixed on the planet, but
+are always more or less endowed with proper motion. The velocity of
+rotation at the equator is 21,538 miles per hour, or nearly 6 miles per
+second.
+</p>
+<p>
+The axis of rotation of Saturn is inclined 64° 18' to the plane of the
+orbit, so that its equator makes an angle of 25° 42' with the same
+plane. The seasons of this planet therefore present greater extremes of
+temperature than those of the Earth, but not quite so great variations
+as the seasons of Mars.
+</p>
+<p>
+The globe of Saturn is not a perfect sphere, but its figure is that of
+an oblong spheroid, flattened at the poles. The polar compression of
+Saturn is greater than that of any other planet, surpassing even that of
+Jupiter. Though not yet determined with a great degree of accuracy, the
+compression is known to be between ²⁄₁₈ and ⅒ of the equatorial
+diameter; that is, a flattening of about 3,894 miles, at each pole, the
+polar diameter being 7,788 miles shorter than the equatorial.
+</p>
+<p>
+The internal condition of the planet Saturn, whether solid, liquid or
+gaseous, cannot be discovered from the examination of its surface, as
+its globe is enwrapped in a dense opaque layer of vapors and cloud-like
+forms, through which the sight fails to penetrate. The appearance of
+this vapory envelope is like that of <i>cumulus</i> clouds, and one of its
+characteristics is to arrange itself into alternate bright and dark
+parallel belts, broader than those seen on Jupiter, and also more
+regular and dark. These belts, which are parallel to the equator of the
+planet, vary in curvature with the inclination of its axis of rotation
+to the line of sight.
+</p>
+<p>
+The belts of Saturn, like those of Jupiter, are not permanent, but keep
+changing more or less rapidly. Sometimes they have been observed to be
+quite numerous; while at other times they are few. Occasionally
+conspicuous white or dark spots are seen on the surface, although the
+phenomenon is quite rare. It is from the observation of such spots that
+Saturn's period of rotation has been determined, as stated above. The
+equatorial zone of Saturn always appears more white and brilliant than
+the other parts, as it also appears more mottled and cloud-like. In late
+years the globe has been characterized, and much adorned, by a pale
+pinkish tint on its equatorial belt, resembling that of Jupiter, but
+somewhat fainter. On either side of the equatorial belt there is a
+narrower band, upon which the mottled appearance is visible. Below
+these, one or two dark belts, separated by narrow white bands, are
+usually seen; but, of late, the bands have been less numerous, being
+replaced in high latitudes by a dark segment, which forms a polar cap to
+Saturn. The globe of Saturn does not anywhere appear perfectly white,
+and when compared with its ring, it looks of a smoky yellowish tint,
+which becomes an ashy gray on its shaded parts. It usually appears
+darker near the limb than in its central portions; although on some
+occasions I have seen portions of the limb appear brighter, as if some
+white spots were traversing it.
+</p>
+<p>
+Some observers have seen the limb deformed and flattened at different
+places, and W. Herschel even thought such a deformation to be a
+permanent feature of this globe, which he termed diamond-shaped, or
+"square shouldered." But this was evidently an illusion, since the
+planet's limb usually appears perfectly elliptical, although it
+occasionally appears as if flattened at some points, especially where it
+comes in apparent contact with the shadow cast by the globe on the ring,
+as observed by myself many times. But with some attention, it is
+generally found that this deformation is apparent rather than real, and
+is caused by the passage of some large dark spots over the limb, which
+is thus rendered indistinguishable from the dark background upon which
+it is projected.
+</p>
+<p>
+What distinguishes Saturn from all known planets, or heavenly bodies,
+and makes it unique in our universe, is the marvelous broad flat ring
+which encircles its equator at a considerable distance from it. With a
+low magnifying power this flat ring appears single, but when carefully
+examined with higher powers, it is found to consist of several distinct
+concentric rings and zones, all lying nearly in the same plane with the
+planet's equator.
+</p>
+
+<p><br /></p>
+
+<div class="figcenter" style="width: 400px;">
+<a id="figure10"></a>
+<br />
+<img src="images/figure10.jpg" width="400" alt="" />
+<div class="caption">
+<p>PLATE X.&mdash;THE PLANET SATURN.</p>
+<p class="smaller">Observed on November 30, 1874, at 5h. 30m. P.M.</p>
+</div></div>
+
+<p><br /></p>
+
+<p>
+At first sight only two concentric rings are recognized, the
+<i>outer</i> and the <i>middle</i>, or <i>intermediary</i>, which are
+separated by a wide and continuous black line, called the <i>principal
+division</i>. This line, and indeed all the features of the surface of
+the rings are better seen, and appear more prominent on that part of the
+ring on either side called the <i>ansa</i>, or handle. Besides these two
+conspicuous rings, a third, of very dark bluish or purplish color, lies
+between this middle ring, to which it is contiguous, and the planet.
+This inner ring, which is quite wide, is called the <i>gauze</i> or
+<i>dusky ring</i>. Closer examination shows that the outer ring is
+itself divided by a narrow, faint, grayish line called the pencil line,
+which, from its extreme faintness, is only visible on the ansæ.
+Moreover, the middle ring is composed of three concentric zones, or
+belts, which, although not apparently divided by any interval of space,
+are distinguished by the different shadings of the materials composing
+them. The outer zone of this compound middle ring is, by far, the
+brightest of all the system of rings and belts, especially close to its
+external border, where, on favorable occasions, I have seen it appear on
+the ansæ as if mottled over, and covered throughout with strongly
+luminous cloud-like masses. On the ansæ of the double outer ring,
+similar cloudy forms have also been seen at different times. The second
+zone of the middle ring is darker than the first, the innermost being
+darker still. All the characteristic points which have thus been
+described, are shown in Plate X.
+</p>
+<p>
+Although suspected in 1838, the dusky ring was not recognized before
+1850, when G. P. Bond discovered it with the 15-inch refractor of the
+Cambridge Observatory. It was also independently discovered the same
+year in England by Dawes and Lassell. The dusky ring differs widely in
+appearance and in constitution from the other rings, inasmuch as these
+last are opaque, and either white or grayish, while the former is very
+dark, and yet so transparent that the limb of the planet is plainly seen
+through its substance. On particularly favorable occasions, the
+appearance of this ring resembles that of the fine particles of dust
+floating in a ray of light traversing a dark chamber. Whatever may be
+the material of which this ring is composed, it must be quite rarefied,
+especially towards its inner border, which appears as if composed of
+distinct and minute particles of matter feebly reflecting the solar
+light. That the inner part of the dusky ring is composed of separate
+particles, is proved by the fact that the part of the ring which is seen
+in front of the globe of Saturn has its inner border abruptly deflected
+and curved inward on entering upon the disk, causing it to appear
+considerably narrower than it must be in reality, a peculiarity which is
+shown in the Plate. This phenomenon may be attributed to an effect of
+irradiation, due to the strong light reflected by the central parts of
+the ball, which so reduces the apparent diameter of the individual
+particles that they become invisible to us, especially those near the
+inner border, which are more scattered and less numerous than elsewhere.
+</p>
+<p>
+The dusky ring, which was described by Bond, Lassell and other
+astronomers as being equally transparent throughout all its width, has
+not been found so by me in later years. The limb of the planet, seen by
+these observers through the whole width of the dusky ring in 1850, could
+not be traced through its outer half by myself in 1872 and 1874, and
+this with the very same instrument used by Bond in his observations of
+1848 and 1850. Moreover, I have plainly seen that its transparency was
+not everywhere equal, but greatest on the inner border, from which it
+gradually decreases, until it becomes opaque, as proved by the gradual
+loss of distinctness of the limb, which vanishes at about the middle of
+the dusky ring. These facts, which have been well ascertained, prove
+that the particles composing this ring are not permanently located, and
+are undergoing changes of relative position. It will be shown that the
+surface of the other rings is also subject to changes, which are
+sometimes very rapid.
+</p>
+<p>
+The globe of Saturn is not self-luminous, but opaque. It shines by the
+solar light, as is proved by the shadow it casts opposite the Sun upon
+the ring. Although receiving its light from the Sun, Saturn does not
+exhibit any traces of phases, like the other planets nearer to the Sun,
+owing to its great distance from the Earth. When near its quadratures,
+however, the limb opposite to the Sun appears much darker, and shows
+traces of twilight. As far as can be ascertained, the rings, with the
+exception of the inner one, are opaque, as proved by the strong shadow
+which they cast on the globe of Saturn.
+</p>
+<p>
+The shadows cast by the planet on the ring, and by the ring on the
+planet, are very interesting phenomena, inasmuch as they enable the
+astronomer to recognize the form of the surface which receives them. The
+shadow cast by the ring on the ball is not quite so interesting as the
+other, although it has served to prove that the surface of this globe is
+not smooth, as is likewise suggested by its mottled appearance. I have
+sometimes found, as have also other observers, that the outline of this
+shadow upon the ball was irregular and indented, an observation which
+proves either that the surface of the ball is irregular, or that the
+border of the ring casting the shadow was jagged. The shadow of the
+globe on the rings has much more interest, as it enables us to get at
+some knowledge of the form of the surface of the rings, which otherwise
+is very difficult to discover, owing to the oblique position in which we
+always see them.
+</p>
+<p>
+In general, the shadow of the ball on the middle ring has its outline
+concave towards the planet; while on the outer ring it is usually
+slanting, and at a greater distance from the limb than on the middle,
+and dusky rings. This form of the shadow evidently proves that the
+middle ring stands at a higher level than the two others, especially
+towards its outer margin. The system seems to increase gradually in
+thickness from the inner border of the dusky ring to the vicinity of the
+outer margin of the middle ring, after which it rapidly diminishes on
+this border, while the surface of the outer ring is almost level.
+</p>
+<p>
+But this surface is by no means fixed, as its form sometimes changes, as
+proved by my observations and those of others. As may be noticed on
+Plate X., the outline of the shadow of the planet on the rings is
+strongly deviated towards the planet, near the outer margin of the
+middle ring; the notch indicating an abrupt change of level, and a rise
+of the surface at that point. Some observers have endeavored to explain
+these deviations by the phenomena of irradiation, from which it would
+follow that the maximum effect of deviation should be observed where the
+ring is the brightest, which does not accord with observation; as the
+deepest depression in the shadow is not to be found usually at the
+brightest part, which is towards the outer border of the middle ring,
+but occurs near its centre. From these observations it is undoubtedly
+established that the surface of the rings is far from being flat
+throughout, and is, besides, not permanent, but changes, as would, for
+instance, the surface of a large mass of clouds seen from the top of a
+high mountain. In general, the system is thickest not very far from the
+outer border of the intermediary ring.
+</p>
+<p>
+Some interesting phenomena which I had occasion to observe before and
+after the passage of the Sun through the plane of the rings, on February
+6th, 1878, conclusively show that the surface of this system cannot be
+of a uniform level, but must be thicker towards the outer border of the
+middle ring, thence gradually sloping towards the planet. Many of my
+observations irresistibly lead to this conclusion. As it would, however,
+be out of place to have them recorded here in detail, I will simply give
+one of the most characteristic among them.
+</p>
+<p>
+From December 18th, 1877, when the Sun was about 41' above the plane of
+the rings, to February 6th, 1878, the day of its passage through their
+plane, the illuminated surface of this system gradually decreased in
+breadth with the lowering of the Sun, until it was lost sight of,
+February 5th, on the eve of the passage of the Sun through their plane.
+The phenomenon in question consisted in the gradual invasion of their
+illuminated surface by what appeared to be a black shadow, apparently
+cast by the front part of the outer portion of the middle ring the
+nearest to the Sun. On January 25th, when the elevation of the Sun above
+the plane of the rings was reduced to 15', the shadow thus cast had
+extended so far on their surface that it reached the shadow cast by the
+globe on the opposite part of the ring in the east, and accordingly the
+remaining portion of the illuminated surface of the eastern ansa then
+appeared entirely disconnected from the ball, by a large dark gap,
+corresponding in breadth to that of the globe's shadow on the rings. On
+February 4th, when the Sun was only 5' above the plane of the rings,
+their illuminated and only visible surface was reduced to a mere thread
+of light, which on the 5th appeared broken into separate points. It is
+evident that the phenomenon was not caused by the obliquity of the ring
+as seen from our globe, since the elevation of the Earth above the plane
+of the rings&mdash;which on December 18th was 3° 20'&mdash;was still 1° 20'
+on the 4th of February. In ordinary circumstances, when the Sun is a little
+more elevated, and the rings seen at this last angle, they appear quite
+broad and conspicuous, and even the dark open space separating the dusky
+ring from the planet is perfectly visible on the ansæ, where the
+Earth's elevation above their plane is reduced to 40'. It is also
+evident that the phenomenon was not to be attributed to the reduction of
+the light which they received from the Sun, although the illumination in
+February might be expected to be comparatively feeble, since the Sun
+then shone upon the rings so obliquely; yet (on the supposition that
+their surface is flat) they should have been illuminated throughout, and
+if not very brightly, sufficiently so, at least, to make them visible
+and as bright as was the narrow thread of light observed on the 4th of
+February. The phenomenon actually observed may be explained most readily
+by assuming, as other phenomena also indicate, that the surface of the
+ring is not flat, but more elevated towards, or in the vicinity of its
+outer border, from which place it slopes inwardly towards the planet. On
+this assumption, it is evident that the elevated part of the ring the
+nearest to the Sun would cast a shadow, which, with the increasing
+obliquity of the Sun, would gradually cover the whole surface comprised
+within the elevated part, and thus become invisible to us. Several
+observations made by Bond and other observers undoubtedly show the same
+phenomenon, and do not seem to be intelligible on any other supposition.
+From my observations made in 1881 it would appear, however, that the
+opposite surfaces of the rings do not exactly correspond in form, but
+this may not be a permanent feature, as the surface of this system is
+subject to changes, as already shown.
+</p>
+<p>
+The dimensions of the rings are great, the diameter of the outer one
+being no less than 172,982 miles, the distance from the centre of the
+globe to the outer border of the system being, therefore, 86,491 miles.
+The breadth of the outer ring is 9,941 miles; that of the principal
+division, 2,131 miles; that of the middle ring, 19,902 miles, and that
+of the dusky ring, 8,772 miles. The breadth of all the rings taken
+together is, therefore, 40,746 miles. The interval between the surface
+of Saturn and the inner border of the dusky ring is 7,843 miles.
+</p>
+<p>
+The thickness of the system of rings has been variously estimated by
+astronomers, on account of the great difficulties attending its
+determination. While Sir John Herschel estimated it at more than 250
+miles, G. P. Bond reduces it to 40 miles. Both of these numbers are
+evidently too small, as so slight a thickness cannot explain the
+observed phenomenon of the shadow cast by a portion of the ring on its
+own surface, when the Sun is very low in its horizon, as shown above.
+</p>
+<p>
+The plane of the system of rings is inclined 27° to the planet's orbit,
+and is parallel, or at least very nearly so, with the equator of the
+planet, passing, therefore, through its centre, and dividing its globe
+into northern and southern hemispheres. Seen from the Earth, a portion
+of the ring always appears projected in front of the planet, thus
+concealing a small part of its globe, while the opposite portion passes
+behind the globe, which hides it from sight.
+</p>
+<p>
+As the plane of the ring is not affected by the motion of the planet
+around the Sun, but always remains parallel to itself, it follows that
+as Saturn advances in its orbit the rings must successively present
+themselves to us under various angles of inclination, appearing,
+therefore, more or less elliptical, and presenting two maxima and two
+minima of inclination in the course of one of its revolutions. As the
+revolution of Saturn is accomplished in 29½ years, the maxima and the
+minima must recur every 14 years and 9 months; the maxima being
+separated from the minima by an interval of 7 years and 4½ months.
+</p>
+<p>
+When Saturn arrives at the two opposite points of its orbit, where the
+major axis of its ring is at right angles to the line joining its centre
+to that of the Sun, the ring, which is then viewed at an inclination of
+27°, the greatest angle at which it can ever be seen, has reached its
+maximum opening, the smaller diameter of its ellipse being then about
+half that of the larger. At this moment the outer ring projects north
+and south beyond the globe, which is then completely enclosed in its
+ellipse. The maximum opening of the northern surface of the ring takes
+place, at present, when Saturn arrives in longitude 262°, in the
+constellation Sagittarius, and that of the southern surface when it
+arrives in longitude 82° in the constellation Taurus. When, on the
+contrary, Saturn reaches the two opposite points of its orbit, where the
+plane of its ring is parallel to the line joining its centre to that of
+the Sun, the opening vanishes, as only the thin edge of the ring is then
+presented to the Sun and receives its light, the rest being in darkness.
+At this moment the ring disappears, except in the largest telescopes,
+where it is seen as an exceedingly thin thread of light; and the
+Saturnian globe, having apparently lost its ring, appears solitary in
+the sky, like the other planets. The disappearance of the ring from this
+cause occurs now when Saturn arrives at 90° from either of the
+positions of maximum inclination, that is, in longitude 352° in the
+constellation Pisces, and in longitude 172° in the constellation Leo.
+</p>
+<p>
+When the planet is in any other position than one of these last two,
+either the northern or the southern surface of the ring is illuminated
+by the Sun, while the opposite surface is in the night, and does not
+receive any direct sunlight. At the time of the passage of the plane of
+the ring through the Sun's centre, a change takes place in the
+illumination of the ring. If it is the northern surface which has
+received the rays of the Sun during the previous half of the Saturnian
+year, at the moment the plane has passed the centre of the Sun, the
+southern surface, after having been buried in darkness for 14¾ years,
+sees the dawn of its long day of the same length. Such a phenomenon will
+not occur until 1892, when the passage of the Sun from the northern to
+the southern side of the ring will close in twilight the day commenced
+in 1878.
+</p>
+<p>
+Aside from the periodic disappearance of the ring, resulting from the
+passage of the Sun through its plane, the ring may also disappear from
+other reasons. Just before or just after the time of the passage of the
+Sun through the plane of the ring, the Earth and the Sun may occupy such
+positions, that while the one is north of the plane of the ring, the
+other is south of it, or vice versa, in which event the ring becomes
+invisible, because its dark and non-illuminated surface is presented to
+us. The ring may also become invisible to us when the Earth passes
+through its plane.
+</p>
+<p>
+Since the distance from Saturn to the Sun is to the distance of the
+Earth from this last body as 9.54 is to 1; and since the circumference
+of a circle increases in the same proportion as its radius, it follows
+that the diameter of the Earth's orbit projected on the orbit of Saturn
+would occupy only part of the latter, or about 12° 2', this being 6°
+1' on either side of the nodes of the rings. To describe such an arc on
+its orbit, it takes Saturn almost 360 days on an average, or almost a
+complete year; the Earth describing therefore almost a whole revolution
+around the Sun during the time it takes Saturn to advance 12° 2' on its
+orbit. Then, when Saturn occupies a position comprised within an arc 6°
+1' from either side of the nodes of its ring, the Earth, by its motion,
+is liable to encounter the plane of the ring, when therefore it will
+only present its thin edge to us, and becomes invisible. At least one
+such encounter is unavoidable within the time during which Saturn
+occupies either of these positions on its orbit; while three frequently
+happen, and two are possible.
+</p>
+<p>
+The natural impression received by looking at the rings, while seeing
+the ponderous globe of Saturn enclosed in its interior, is that this
+gigantic, but very delicate structure, in order to avoid destruction,
+must be endowed with a swift movement of rotation on an axis
+perpendicular to its plane, and that the centrifugal force thence
+arising counterbalances the powerful attraction of the planet, and thus
+keeps the system in equilibrium.
+</p>
+<p>
+Theoretically, the rotation of the rings is admitted by every
+astronomer, as being an essential condition to the existence of the
+system, which otherwise, it is thought, would fall upon the planet.
+Although the rotation of the rings seems so probable that it is
+theoretically considered as certain, yet its existence has not been
+satisfactorily demonstrated by direct observation, which alone can
+establish it on a firm basis as a matter of scientific knowledge.
+</p>
+<p>
+The determination of the period of rotation of the rings, which is
+supposed to be 10h. 32m. 15s., rests only on the observations of W.
+Herschel, made in 1790, from the apparent displacement of irregularities
+on the ring; but his results have been contradicted by other
+observations, and even by those of Herschel himself, made in later
+years.
+</p>
+<p>
+Although the system of rings is very nearly concentric with the globe of
+Saturn, yet the coincidence is not considered as mathematically exact.
+It seems to have been satisfactorily demonstrated by direct observations
+that the centre of gravity of the system oscillates around that of the
+planet, thus describing a minute orbit. This peculiarity is in
+accordance with theory, which has shown it to be essential to the
+stability of the system.
+</p>
+<p>
+Besides its system of rings, which makes Saturn the most remarkable
+planet of the solar system, this globe is attended by eight satellites,
+moving in orbits whose planes very nearly coincide with the plane of the
+rings, except that of the most distant one, which has an inclination of
+about 12° 14'. In the order of their distance from the planets, the
+satellites of Saturn are as follows: Mimas, Enceladus, Tethys, Dione,
+Rhea, Titan, Hyperion and Iapetus. The three first satellites are nearer
+to Saturn than the Moon is to the Earth; while Iapetus, the farthest, is
+9½ times the distance of our satellite from us. All the satellites,
+with the exception of the farthest, move more rapidly around Saturn than
+the Moon moves around the Earth; while Iapetus, on the contrary, takes
+almost three times as long to make one revolution.
+</p>
+<p>
+The period of revolution of the four inner satellites is accomplished in
+less than three days, that of Mimas being only a little more than 22
+hours. From such swiftness of motion, it is easily understood how short
+must be the intervals between the different phases of these satellites.
+Mimas, for instance, passes from New Moon to First Quarter in less than
+6 hours.
+</p>
+<p>
+The distance of the nearest satellite from the planet's surface is
+84,000 miles, and its distance from the outer ring only 36,000 miles. It
+is difficult to determine the diameter of objects so faint and distant
+as are some of these satellites, but the diameter of Titan, the largest
+of all, is pretty well known, and estimated to be ¹⁄₁₆ the
+diameter of the planet, or more than half the diameter of our globe.
+</p>
+<p>
+Iapetus is subject to considerable variations in brilliancy, and as the
+maxima and minima always occur when this satellite occupies the same
+parts of its orbit, it was conjectured by W. Herschel that, like our
+Moon, it turns once upon its axis during each of its revolutions about
+the planet. It has been shown by my observations, that Iapetus attains
+its maximum brightness a little before it reaches its greatest western
+elongation, and its minimum on the opposite side.
+</p>
+<p>
+As the planes of the orbits of the satellites are inclined to the
+planet's orbit, it follows that their transits, occultations and
+eclipses, are only possible when Saturn is near its equinoxes. Passages
+of the satellites and their shadows across the disk, although rare, have
+been observed, and they somewhat resemble the phenomena exhibited by the
+satellites of Jupiter in transit. When the Earth is very near the plane
+of the rings, the satellites, except the farthest, appear to be in a
+straight line nearly coincident with the plane of the rings, and are
+seen occasionally moving along the thin edge of the rings, appearing as
+luminous beads moving on a thread of light.
+</p>
+<p>
+Owing to the considerable inclination of the axis of rotation of Saturn
+to its orbit, the seasons of this planet must have greater extremes of
+temperature than those of the Earth. As the year of Saturn consists of
+25,217 Saturnian days, each season, on the average, is composed of 6,304
+Saturnian days.
+</p>
+<p>
+To an observer on Saturn, the immense arches formed by its rings would
+appear as objects of great magnificence, spanning the sky like soft
+colorless rainbows. Moreover, the eight moons, several of which are
+always visible, would be of the highest interest, with their swift
+motions and rapid phases. Mimas, traveling in its orbit at the rate of
+16 of arc per minute of time, moves over a space equal to the apparent
+diameter of our Moon in two minutes, or at the rate of 16° an hour.
+</p>
+<p>
+Owing to the globular form of Saturn, the rings would be invisible in
+latitudes situated above 65° from its equator, and their apparent form
+and breadth would naturally vary with the latitude. At 63° only a very
+small portion of the outer ring would be visible above the equatorial
+horizon, where it would appear as a small segment of a circle. At 62°
+the principal division would just graze the horizon. At 46° the outer
+portion of the dusky ring would become visible, while at 35° its inner
+edge would appear above the horizon. From 65° of latitude down to the
+equator, the arches of the rings would be seen more and more elevated
+above the equatorial horizon, but at the same time that they are seen
+higher up, their apparent breadth gradually diminishes, owing to the
+effect of foreshortening, and at the equator itself the system would
+only present its thin edge to view.
+</p>
+<p>
+During the summer seasons of either hemisphere of Saturn, the surface of
+the rings turned towards such hemisphere, being fully illuminated by the
+Sun, is visible from these regions. In the day time its light must be
+feeble and similar to the light reflected by our Moon during sunshine;
+but at night the system would display all its beauty, and the different
+rings, with their divisions and their various reflective powers, must
+present a magnificent sight.
+</p>
+<p>
+During the nights of the long winter seasons on Saturn, on the contrary,
+the surface of the rings turned towards the hemisphere undergoing
+winter, receives no light from the Sun, and is invisible, or very nearly
+so, except towards morning and evening, when it may be faintly
+illuminated by the secondary light which it receives from the
+illuminated globe of Saturn. Although dark and invisible, the rings may
+make their form apparent at night by the absence of stars from the
+region which they occupy in the sky. Again, in other seasons, the days
+present very curious phenomena. In consequence of the diurnal rotation
+of the planet, the Sun seems to move in circular arcs, which, owing to
+the inclination of Saturn's axis, are more or less elevated above its
+horizon, according to the position of the planet in its orbit. As such
+arcs described by the Sun in the sky of Saturn are liable to encounter
+the rings, the Sun in passing behind them becomes eclipsed. It must be a
+magnificent spectacle to witness the gradual disappearance of the fiery
+globe behind the outer ring, and its early reappearance, but for a
+moment only, through the narrow gap of the principal division; to see it
+vanish again behind the middle ring, to reappear a little later through
+the semi-transparent dusky ring, but very faint and red colored at
+first; and then, gradually brighten up, and finally emerge in all its
+beauty from the inner edge of the dusky ring.
+</p>
+<p>
+It is in latitude 23° that the rings produce the most prolonged
+eclipses of the Sun. During a period equivalent to ten of our
+terrestrial years, such eclipses continually succeed each other with but
+very short periods of interruption; and even during a long series of
+rotations of Saturn, the Sun remains completely invisible in those
+regions where the apparent arcs which it describes coincide with the
+arcs of the rings. In neighboring latitudes, the eclipses of the Sun,
+although still frequent, would have a shorter and shorter duration as
+the observer should travel north or south. These eclipses of the Sun
+must produce a partial darkness of the regions involved in the shadow of
+the rings, which may be compared to the darkness produced on our globe
+by a total eclipse of the Sun. The frequent recurrence of these
+eclipses, and their comparatively long duration in some regions, must
+still further reduce the duration of the short Saturnian days.
+</p>
+<p>
+The globe of Saturn, as already shown, casts a shadow on the rings,
+which, according to the position of the planet in its orbit, either
+extends across their whole breadth, or covers only a part of their
+surface. The shadow on the rings rising in the east after sunset,
+ascends to the culminating point of their arcs in the sky, in 2h. 34m.,
+and as rapidly descends on the western horizon, to disappear with
+sunrise. This shadow, when projected on the rings in the sky, must be
+hardly distinguishable from the dark background of the heavens, except
+from the absence of stars in the regions which it occupies. It must
+appear as a large dark gap, separating the rings into two parts, and
+constantly moving from east to west. Possibly the refraction of the
+solar rays, in passing through Saturn's atmosphere, may cast some
+colored light on the rings, similar to that observed on the Moon during
+its eclipses.
+</p>
+<p>
+An observer on the rings would behold phenomena still more curious, a
+long day of 14¾ years being followed by a long night of 14¾ years. The
+long days of Saturn's rings are, however, diversified by numerous
+eclipses of the Sun, which regularly occur every 10¼ hours; the
+phenomenon being due to the interposition of the globe of Saturn between
+the rings and the Sun. These eclipses produce partial obscurations of
+their surface, lasting from 1½ to 2 hours at a time. Although the
+surface of the rings never receives direct sunlight during their long
+nights, yet they are not plunged all the time in total darkness, as they
+receive some reflected light from that part of the globe of Saturn which
+is illuminated by the Sun. To the supposed observer on the rings, during
+every 10¼ hours, the immense globe would exhibit continually changing
+phases. At first he would see a point of light rapidly ascending from
+the horizon, and appearing under the form of a half crescent of
+considerable radius; 5⅛ hours later, the crescent having gradually
+increased, would appear as a half circle, covering ⅛ of the visible
+heavens, its surface being more than 20,000 times as large as the
+surface of the Moon. Upon this brilliantly illuminated semi-circle would
+be projected the shadows of the rings, appearing as black belts
+separated by a narrow luminous band.
+</p>
+<p>
+It is very difficult for one to conceive how such a delicate structure,
+as the system of rings appears to be, can keep together in equilibrium
+and avoid destruction from the powerful attraction of the planet on one
+side and the disturbing influence of the satellites on the other. To
+explain it, several hypotheses have been advanced. The rings were first
+supposed to be solid, and upon this supposition Laplace determined the
+necessary conditions for their equilibrium; the most important of which
+require that the cross section of the rings should be an ellipse of
+irregular curvature, and having its major axis directed towards the
+centre of the planet, and also that the system should rotate upon an
+axis perpendicular to the plane of the rings. This theory was superseded
+by another, which supposed the rings to be fluid. This one was soon
+rejected for a third, assuming the system to be composed of vapors or
+gases; and more recently, all these theories were considered untenable,
+and replaced by a fourth, which supposes the system of rings to be made
+up of a congregation of innumerable small, independent bodies, revolving
+around Saturn in concentric zones. Naturally, such a divergence of
+opinion can only result from our comparative ignorance of the subject,
+and sufficiently indicates our inability to explain the phenomena; and
+it must be admitted that, so far, nothing is certainly known about this
+strange system. We shall probably remain in the same uncertainty until
+the rotation of the rings is ascertained by direct observations. It is
+pretty certain, however, that none of these theories account for the
+observed phenomena in their details, although a partial explanation may
+be obtained by borrowing something from each hypothesis.
+</p>
+<p>
+It has been conjectured, and a theory has been advanced, that the
+breadth of the whole ring system is gradually increasing inwards, and
+that it will come in contact with the planet in about 2,150 years; but
+the question seems to have been settled in the negative by the elaborate
+measurements of the English observers. It is likely that the increase is
+only in the defining power of the instruments.
+</p>
+
+<p><br /><br /><br /></p>
+
+<h4><a id="COMETS">COMETS</a>
+<br /><br />
+PLATE XI</h4>
+
+<p>
+Among the celestial phenomena, none are more interesting than those
+mysterious apparitions from the depths which unexpectedly display their
+strange forms in our familiar constellations, through which they wander
+for a time, until they disappear like phantoms.
+</p>
+<p>
+A comet, with its luminous diffused head, whence proceeds a long vapory
+appendage gradually fading away in the sky, presents an extraordinary
+aspect, which may well astonish and deeply impress the observer.
+Although these visitors from infinite space do not now inspire dread, as
+in by-gone times, yet, owing to the mystery in which the phenomenon is
+still involved, the apparition of a large comet, even in our days, never
+fails to create a profound sensation, and in some cases that unconscious
+fear which results from the unknown.
+</p>
+<p>
+The effect of such a spectacle largely depends upon its rarity; but
+since the telescope has been applied to the sounding of the heavens, it
+has been found that the appearance of comets is by no means an unusual
+occurrence. If so few comets, comparatively, are seen, it is because
+most of them are telescopic objects, and are therefore invisible to the
+naked eye. Most of the telescopic comets are not only too faint to be
+perceived by the unaided eye, but are insignificant objects, even when
+observed through the largest telescopes.
+</p>
+<p>
+It was Kepler's opinion that comets are as numerous in the sky as fishes
+are in the ocean. Undoubtedly the number of these bodies must be great,
+considering that we can only see them when they come into the
+neighborhood of the Earth, and that many even here remain invisible, or
+at least pass unperceived. That many of them have passed unperceived
+heretofore, is proved by the fact that the number of those observed
+becomes greater every year, with the increase of the number of
+instruments used in their search. The number of comets observed with the
+naked eye during historic times is nearly 600, and that of telescopic
+comets, which, of course, all belong to the last few centuries, is more
+than 200, so that we have a total number of about 800 comets of which
+records have been kept. From theoretical considerations, Lambert and
+Arago estimated their entire number at several millions, but such
+speculations have generally no real value, since they cannot be
+established on a firm basis.
+</p>
+<p>
+Comets remain visible for more or less time, according to their size and
+the nature and position of their orbits, but in general, the large ones
+can be followed with the telescope for several months after they have
+become invisible to the naked eye. The comet of 1861, for example,
+remained telescopically visible for a year, and that of 1811, for 17
+months after disappearing from ordinary sight.
+</p>
+<p>
+While a comet remains visible, it appears to revolve daily about us like
+the stars in general; but it also moves among the constellations, and
+from this movement its orbit may be computed like that of a planet. From
+the apparent diurnal motion of a comet with the heavens, result the
+changes of position which it seems to undergo in the course of a night.
+The direction of the head and tail of a comet, of course, has only
+changed in regard to the horizon, but not in regard to the sky, in which
+they occupy very nearly the same position throughout a given night, and
+even for many nights in succession.
+</p>
+<p>
+The movements of the comets in their orbits are, like those of the
+planets, in accordance with Kepler's laws, the Sun occupying one of the
+foci of the orbit they describe; but the orbits of comets differ,
+however, in several points from those of the planets. Their eccentricity
+is always great, being sometimes apparently infinite, in which case the
+orbit is said to be parabolic, or hyperbolic; but the smallness of the
+portion of a cometary orbit which can ordinarily be observed, makes it
+difficult to determine this with certainty. Again, while the planetary
+orbits are usually near the plane of the ecliptic, those of comets
+frequently have great inclinations to that plane, and even when the
+inclination is less than 90°, the comet may have a retrograde movement,
+or, in other words, a movement contrary to the course in which all the
+planets revolve about the Sun.
+</p>
+<p>
+Notwithstanding these differences between the elements of the orbits of
+the comets and those of the planets, the fact that each has the Sun in
+one focus indicates that the body moving in it is a member of the solar
+system, either for the time, or permanently, according to the nature of
+its orbit.
+</p>
+<p>
+A distinction may accordingly be made between the comets which are
+permanent members of our solar system and those which are only
+accidental or temporary visitors. Those moving in elliptical orbits
+around the Sun, like the planets, and therefore having a determinate
+period of revolution, from which the time of their successive returns
+may be predicted, are permanent members of our system, and are called
+periodic comets. All comets moving in parabolical or hyperbolical
+curves, are only temporary members of the solar system, being apparently
+strangers who have been diverted from their courses by some disturbing
+influence. No comet is classed as periodical which does not follow a
+perceptibly elliptical orbit. Any comet passing around the Sun at the
+mean distance of the Earth from this body, with a velocity of 26 miles
+per second, will fly off into infinite space, to return to us no more.
+</p>
+<p>
+The time of revolution of the different periodic comets thus far
+observed varies greatly, as do also the distances to which they recede
+from the Sun at aphelion. Whilst the period of revolution of Encke's
+comet, the shortest thus far known, is only 3½ years, that of the comet
+of 1844, II., is 102,000 years; and whilst the orbit of the first is
+comprised within the orbit of Jupiter, that of the last extends to a
+distance equal to 147 times the distance of Neptune from the Sun. But so
+vast an orbit cannot be accurately determined from the imperfect data at
+our disposal.
+</p>
+<p>
+The periodic comets are usually divided into two classes. The comets
+whose orbits are within the orbit of Neptune are called interior comets,
+while those whose orbits extend beyond that of Neptune are called
+exterior comets. The known interior periodic comets are twelve in
+number, while, including all the cases in which there is some slight
+evidence of elliptic motion, the number of exterior comets observed is
+six or seven times as great. The periodic comets of short period are
+very interesting objects, inasmuch as by their successive returns they
+afford an opportunity to calculate their motions and to observe the
+physical changes which they undergo in their intervals of absence.
+</p>
+<p>
+From observation of the periodic comets, it has been learned that the
+same comet never presents twice the same physical appearance at its
+different returns, its size, shape and brilliancy varying so greatly
+that a comet can never be identified by its physical characters alone.
+It is only when its elements have been calculated, and are found to
+agree with those of a cometary orbit previously known, that the two
+comets can be identified one with the other. There are reasons to
+believe that, in general, comets decrease in brightness and size at each
+of their successive returns, and that they are also continually losing
+some of their matter as they traverse their orbits.
+</p>
+<p>
+When very far away from us, all comets appear nearly alike, consisting
+of a faint nebulosity, of varying dimensions. When a comet first appears
+in the depths of space, and travels towards the Sun, it generally
+resembles a faint, uniformly luminous nebulosity, either circular or
+slightly elongated in form. As it approaches nearer to the Sun, a slight
+condensation of light appears towards its centre, and as it draws still
+nearer, it becomes brighter and brighter, and in condensing forms a kind
+of diffused luminous nucleus. At the same time that the comet acquires
+this concentration of light, the nebulosity gradually becomes elongated
+in the direction of the Sun. These effects generally go on increasing so
+long as the comet is approaching the Sun; the condensation of light
+sometimes forms a bright nucleus, comparable to a very brilliant star,
+while the elongation becomes an immense appendage or tail. When the
+comet has passed its perihelion and recedes from the Sun, the inverse
+phenomena are observed; the comet, decreasing in brightness, gradually
+loses its nucleus and tail, resumes its nebulous aspect, and finally
+vanishes in space, to appear again in due course, if it chance to be a
+periodic comet. While all comets become brighter in approaching the Sun,
+they do not all, however, develop a large tail, some of them showing
+only a slight elongation.
+</p>
+<p>
+When a comet is first discovered with the telescope at a great distance
+from the Sun, it is difficult to predict whether it will become visible
+to the naked eye, or will remain a telescopic object, as it is only in
+approaching the Sun that these singular bodies acquire their full
+development. Thus, Donati's comet, whose tail became so conspicuous an
+object at its full appearance in 1858, remained two months after its
+discovery by the telescope without any indication of a tail. The comet
+of Halley, which before and after its return in 1759, remained five
+years inside of the orbit of Saturn, showed not the least trace of its
+presence during the greater part of this time. Nothing but calculation
+could then indicate the position in the sky of this invisible object,
+which was so prominent when it approached the Sun.
+</p>
+<p>
+Another curious phenomenon exhibited by comets, and first noticed by
+Valz, is that in approaching the Sun the nebulosity composing these
+bodies contracts, instead of dilating, as would be naturally supposed
+from the greater amount of solar heat which they must then receive. In
+receding from the Sun, on the contrary, they expand gradually. As comets
+approach the Sun, the tail and nucleus are developed, while the
+nebulosity originally constituting these comets contracts, as if its
+material had been partly consumed in this development. In a certain
+sense it may be said that the comets are partly created by the Sun; in
+more exact terms, the changes of form which they undergo are induced by
+the Sun's action upon them at different distances and under varying
+conditions. Moreover, they are rendered visible by its influence,
+without which they would pass unperceived in our sky. When a comet
+disappears from view, it is not because its apparent diameter is so much
+reduced by the distance that it vanishes, but rather on account of the
+diminution of its light, both that which it receives from the Sun, and
+its own light; these bodies being in some degree self-luminous, as will
+be shown below.
+</p>
+<p>
+The large comets, such as can be seen with the naked eye, always show
+the following characteristics, on examination with the telescope. A
+condensation of light resembling a diffused star forms the brightest
+part of the comet, this condensation being situated towards the
+extremity the nearest to the Sun. It is this starlike object which is
+called the <i>nucleus</i>. The nucleus seems to be entirely enclosed in
+a luminous vapory envelope of the same general texture, called the
+<i>coma</i>. This envelope, which is quite variable in brightness and
+form, is brightest next to the nucleus, and gradually fades away as it
+recedes from it. The <i>nucleus</i> and the <i>coma</i>, considered as a
+whole, constitute the <i>head</i> of a comet. From the head of a comet
+proceeds a long trail of pale nebulous light, which usually grows wider,
+but fainter, as it recedes from the nucleus, and insensibly vanishes in
+the sky. This delicate appendage, or tail, as it is commonly called,
+varies very much in size and shape, not only in different comets, but in
+the very same comet, at different times. Its direction is generally
+opposite to that of the Sun from the head of the comet.
+</p>
+<p>
+The nuclei vary very much in brightness, in size and in shape; and while
+in some telescopic comets they are either absent or barely
+distinguishable as a small condensation of light, in bright comets they
+may become plainly visible to the naked eye, and they sometimes even
+surpass in brightness the most brilliant stars of the heavens. But
+whatever may be the size of cometary nuclei, they are subject to sudden
+and rapid changes, and vary from day to day. Sometimes they appear
+exceedingly brilliant and sharply outlined, while at other times they
+are so dim and diffused that they are hardly distinguishable from the
+coma of which they seem then to form a part.
+</p>
+
+<p><br /></p>
+
+<div class="figcenter" style="width: 400px;">
+<a id="figure11"></a>
+<br />
+<img src="images/figure11.jpg" width="400" alt="" />
+<div class="caption">
+<p>PLATE XI.&mdash;THE GREAT COMET OF 1881.</p>
+<p class="smaller">Observed on the night of June 25-26 at 1h. 30m. A.M.</p>
+</div></div>
+
+<p><br /></p>
+
+<p>
+From my observations upon the comets which have appeared since the year
+1873, it is apparent that the changes in the nucleus, coma and tail, are
+due to a solar action, which contracts or expands these objects in such
+a manner that the nuclei become either bright and star-like, or dim and
+diffused, in a very short time. I had excellent opportunity, especially
+in the two large comets of 1881, to observe some of these curious
+changes, a description of which will give an idea of their extent and
+rapidity. On July 2d, 1881, at 9 o'clock, the nucleus of comet 1881,
+III., which is represented on Plate XI., appeared sharply defined,
+bright and considerably flattened crosswise; but half an hour later it
+had considerably enlarged and had become so diffused that it could
+hardly be distinguished from the coma, with which it gradually blended.
+It is perhaps worth mention that, at the time this last observation was
+made, an aurora borealis was visible. This comet 1881, III., underwent
+other very important changes of its nucleus, coma and tail. On June
+25th, the nucleus, which was bright and clearly defined, was ornamented
+with four bright diverging conical wings of light, as shown on Plate XI.
+On the 26th these luminous wings had gone, and the nucleus appeared
+one-third smaller. On the 28th it had enlarged, but on the 29th its
+shape was considerably altered, the nucleus extending in one direction
+to three or four times its diameter on previous nights, and being
+curved, so as to resemble a comma. On the 6th of July the nucleus of
+this comet showed the greatest disturbances. The nucleus, which had
+appeared perfectly round on the evening of the 5th, was found much
+elongated at 10 o'clock on the 6th, forming then a straight, acute, and
+well-defined wedge of light, inclined upwards to the left. The length of
+the nucleus, at this time, was three or four times its ordinary
+diameter. At the same time rapid changes occurred; the strangely shaped
+nucleus soon became unsteady, extending and contracting alternately, and
+varying greatly in brightness. At 10h. 45m., the elongated nucleus, then
+gently curved, took the shape of a succession of luminous knots, which
+at times became so brilliant and distinct that they seemed to be about
+to divide and form separate nuclei; but such a separation did not
+actually occur, at least while I was observing. While these important
+changes were going on in the comet, a bright auroral arch appeared in
+the north, which lasted only a short time. On July 7th, the sky being
+cloudy, no observations were made, but on the 8th I observed the comet
+again. The nucleus had then resumed its circular form, but it was yet
+very unsteady, being sometimes small, bright and sharp, while a few
+seconds later it appeared twice as large, but dim in outlines; and
+sometimes an ill-defined secondary nucleus appeared at its centre. On
+several occasions the nucleus appeared as if it were double, one nucleus
+being apparently projected partly upon the other.
+</p>
+<p>
+The nuclei of comets are sometimes very small, and in other cases very
+large. Among those which have been measured, the nucleus of the comet of
+1798, I., was only 28 miles in diameter, but that of Donati's comet, in
+1858, was 5,600 miles, and that of the comet of 1845 was 8,000 miles in
+diameter.
+</p>
+<p>
+The coma of comets is found to be even more variable than the nucleus.
+The changes observed in the coma are generally in close connection with
+those of the nucleus and tail, the same perturbations affecting
+simultaneously the whole comet. While the coma of the comet of 1847 was
+only 18,000 miles in diameter, that of Halley's comet, in 1835, was
+357,000 miles, and that of the comet of 1811 was 1,125,000 miles in
+diameter. In general, as already stated, the coma of a comet decreases
+in size in approaching the Sun. That of Encke's comet, which, on October
+9th, 1838, had a diameter of 281,000 miles, gradually decreased at a
+daily mean rate of 4,088 miles in going towards the Sun; so that, on
+December 17th, when the distance of the comet from the Sun was more than
+four times less than it was on the first date, its diameter was reduced
+to 3,000 miles.
+</p>
+<p>
+The form of the coma, in that part which is free from the tail, is in
+general a portion of a circle, but is sometimes irregular, with its
+border deformed. Thus, the border of the coma of Halley's comet was
+depressed at one point towards the Sun. I observed a similar phenomenon
+in Coggia's comet, with the great refractor of the Harvard College
+Observatory, on July 13th, 1874, when its border appeared deeply
+depressed on the side nearest to the Sun, as if repelled by this body.
+The coma of comet 1881, III., showed also very singular outlines on the
+nights of the 25th and 26th of June, when its border was so deeply
+depressed that the coma appeared as if it were double. Luminous rays and
+jets often radiate from the nucleus across the coma, and describe
+graceful lateral curves, falling backwards and gradually fading away
+into the tail, of which they then form a part. The rays and jets emitted
+by the nucleus seem at first to obey the solar attraction and travel
+towards the Sun; but they are soon repelled, and move backward towards
+the tail. It is a mystery, as yet unexplained, how these cometary jets,
+which at first seem to obey to the laws of attraction, are compelled to
+retreat apparently by superior opposing forces. Among the forces of
+nature, we know of no other than those of an electrical sort, which
+would act in a similar manner; but this explanation would require us to
+assume some direct electrical communication between the comet and the
+Sun. Considering the distance between the two bodies, and the probable
+absence or great tenuity of the gaseous material in interstellar space,
+such an assumption is a difficult one.
+</p>
+<p>
+Under the action of the solar forces, the coma also very frequently
+forms itself into concentric luminous arcs, separated by comparatively
+dark intervals. These luminous semi-circles vary in number, but
+sometimes there are as many as four or five at a time. All great comets
+show these concentric curves more or less, but sometimes only a portion
+is visible, the rest of the coma having a different structure. When
+great comets approach near the Sun, their coma is generally composed of
+two distinct parts, an inner and an outer coma, the inner one being due
+to the luminous jets issuing from the nucleus, which, never extending
+very far, form a distinct, bright zone within the fainter exterior coma.
+</p>
+<p>
+The tails of comets, which are in fact a prolongation of the coma, are
+likewise extremely variable in form. They are sometimes straight like a
+rod; again, are curved like a sabre, or even crooked like an S, as was
+that of the comet of 1769. They are also fan-shaped, pointed, or of the
+same width throughout. Many of these appendages appear longitudinally
+divided through their middle by a narrow, darkish rift, extending from
+the nucleus to the extremity. This peculiarity appears in the comet
+shown on Plate XI. Sometimes the dark rift does not commence near the
+nucleus, but at some distance from it, as I observed in the case of
+comet 1881, III., on June 26th. This dark rift is not a permanent
+feature of a comet's tail, but may be visible one day and not at all the
+next. Comet 1881, III., which had shown a dark rift towards the end of
+June, did not exhibit any such rift during July and August, when, on the
+contrary, its tail appeared brighter in the middle. Coggia's comet,
+which showed so prominent a dark rift in July, 1874, had none on June
+10th. On the contrary, the tail was on that date very bright along its
+middle, as also along each of its edges.
+</p>
+<p>
+The tail of a comet does not invariably point directly away from the
+Sun, as above mentioned, and sometimes the deviation is considerable;
+for instance, the tail of the comet of 1577 deviated 21° from the point
+opposite to the Sun.
+</p>
+<p>
+In general, the tail inclines its extremity towards the regions of space
+which it has just left, always presenting its convex border to the
+regions towards which it is moving. It is also a remarkable fact that
+this convex border, moving first in space, always appears brighter and
+sharper than the opposite one, which is often diffused. From these
+peculiarities it would seem that in moving about the Sun the comets
+encounter some resistance to their motion, from the medium through which
+they pass, and that this resistance is sufficient to curve their tails
+away from the course in which they move, and to crowd their particles
+together on the forward side. It is especially when they approach their
+perihelion, and move more rapidly on a curve of a shorter radius, that
+the comets' tails show the greatest curvature, unless their position in
+regard to the observer prevents their being advantageously seen. The
+tail of Donati's comet presented a fair illustration of this
+peculiarity, its curvature having augmented with the velocity of the
+comet's motion about the Sun. But possibly this phenomenon has another
+cause, and may be found rather in the solar repulsion which acts on
+comets and is not instantaneously propagated throughout their mass.
+</p>
+<p>
+Although, in general, comets have but one tail, it is not very rare to
+see them with multiple tails. The comets of 1807 and 1843 had each a
+double tail; Donati's comet, in 1858, showed several narrow, long
+rectilinear rays, issuing from its abruptly curved tail. The comet of
+1825 had five branches, while that of 1744 exhibited no less than six
+distinct tails diverging from the coma at various angles. In general
+character the multiple and single tails are similar. When a comet has
+two tails, it is not rare for the second to extend in the general
+direction of the Sun, as was the case with the great comet of 1881,
+III., represented on Plate XI. From July 14th to the 21st it exhibited
+quite an extended conical tail, starting obliquely downwards from the
+right side of the coma, and directed towards the Sun. From the 24th of
+July to the 2d of August this secondary tail was exactly opposite in its
+direction from that of the primary tail, and gave to the head a very
+elongated appearance. Comet 1881, IV., also exhibited a secondary
+appendage, not directed towards the Sun, but making an angle of about
+45° with the main tail.
+</p>
+<p>
+These cometary appendages sometimes attain prodigious dimensions. The
+comets of 1680 and 1769 had tails so extended that, after their heads
+had set under the horizon, the extremities of these immense appendages
+were still seen as far up as the zenith. In a single day the tail of the
+comet of 1843 extended 100°, and it was thrust from the comet "as a
+dart of light" to the enormous distance of 48,500,000 miles, and yet of
+this immense appendage nothing was left on the following day. The tail
+of Donati's comet, in 1858, attained a real length of 42,000,000 miles,
+while that of the great comet of 1843 had the enormous length of
+200,000,000 miles. If this last comet had occupied the position of the
+Sun, which it approached very nearly for a moment, the extremity of its
+tail would have extended 60,000,000 miles beyond the orbit of Mars.
+</p>
+<p>
+In some cases the tails of comets have been seen undulating and
+vibrating in a manner similar to the undulations and coruscations of
+light characteristic of some auroras. Many observers report having seen
+such phenomena. The comet of 1769 was traversed by luminous waves and
+pulsations, comparable to those seen in the aurora borealis. I myself
+observed these curious undulations in Coggia's comet in 1874, while the
+head of this object was below the horizon. For an hour the undulations
+rapidly succeeded each other, and ran along the whole length of the
+tail.
+</p>
+<p>
+Some of the brightest comets have shone with such splendor that they
+could be observed easily in full sunshine. Many comets, such as those of
+1577 and 1744, have equaled Sirius and Venus in brilliancy. The great
+comet of 1843, which suddenly appeared in our sky, was so brilliant that
+it was seen by many observers at noon time, within a few degrees from
+the Sun. I remember that I myself saw this remarkable object in the day
+time, with a number of persons, who were gazing at the wonderful
+apparition. So brilliant was this comet, that besides its nucleus and
+head, a portion of its tail was also visible in the day time, provided
+the observer screened his eyes from the full sunlight by standing in the
+shadow of some building.
+</p>
+<p>
+Of all the bodies revolving around the Sun, none have been known to
+approach so near its surface as did the comet of 1843. When it arrived
+at perihelion, the distance from the centre of its nucleus to the
+surface of the Sun's photosphere was only 96,000 miles, while the
+distance from surface to surface was less than 60,000 miles. This comet,
+then, went through the solar atmosphere, and in traversing it with its
+tremendous velocity of 366 miles per second, may very possibly have
+swept through some solar protuberances, many of which attain much higher
+elevations than that at which the comet passed. The comet of 1680 also
+approached quite near the surface of the Sun, and near enough to
+encounter some of the high solar protuberances, its distance at
+perihelion being about two-thirds of the Moon's distance from the Earth.
+The rapidity of motion of the comet of 1843 was such, when it approached
+the Sun, that it swept through all that part of its orbit which is
+situated north of the plane of the ecliptic in a little more than two
+hours, moving in this short time from one node to the other, or 1800.
+</p>
+<p>
+But if some comets have a very short perihelion distance, that of others
+is considerable. Such a comet was that of 1729, whose perihelion
+distance was 383,000,000 miles, the perihelion point being situated
+between the orbits of Mars and Jupiter.
+</p>
+<p>
+While some comets come near enough to the Sun at perihelion to be
+volatilized by its intense heat, others recede so far from it at
+aphelion that they may be said to be frozen. The shortest cometary
+aphelion distance known is that of Encke's comet, whose greatest
+distance from the sun is 388,000,000 miles. But that of the comet of
+1844 is 406,000,000,000 miles from the Sun. The comets of 1863 and 1864
+are so remote in space when they reach their aphelion points that light,
+with its velocity of 185,500 miles a second, would require 171 days in
+the first case, and 230 in the last, to pass from them to the Earth.
+</p>
+<p>
+The period of revolution of different comets also varies immensely.
+While that of Encke's comet is only 3½ years, that of comet 1864, II.,
+is 280,000 years.
+</p>
+<p>
+Among the periodic comets of short period, some have exhibited highly
+interesting phenomena. Encke's comet, discovered in 1818, is remarkable
+for the fact that its period of revolution diminishes at each of its
+successive returns, and consequently this comet, with each revolution,
+approaches nearer and nearer to the Sun. The decrease of the period is
+about 2½ hours at each return. Although the decrease is small, if it go
+on in future as it does at present, the inevitable consequence will be
+that this comet will finally fall into the Sun. This curious phenomenon
+of retardation has been attributed by astronomers to the existence of a
+resisting medium filling space, but so rare and ethereal that it does
+not produce any sensible effect on the movements of the planets. But
+some other causes may retard this comet, as similar retardations have
+not been observed in the case of other periodic comets of short period.
+These, however, are not so near to the Sun, and perhaps our luminary may
+be surrounded by matter of extreme tenuity, which does not exist at a
+greater distance from it.
+</p>
+<p>
+Another of the periodic comets which has exhibited a very remarkable
+phenomenon of transformation is Biela's comet, which divided into two
+distinct parts, moving together in the same direction. When this comet
+was first detected at its return in 1845, it presented nothing unusual,
+but in the early part of 1846 it was noticed by several astronomers to
+be divided into two parts of unequal brightness, forming thus a twin
+comet. At its next return in 1852, the two sister comets were still
+traveling in company, but their distance apart, which in 1846 was
+157,000 miles, had increased to 1,500,000 miles. At the two next returns
+in 1859 and 1865, their position not being very favorably situated for
+observation, the comets were not seen. In 1872 the position should have
+been favorable for observation, and they were consequently searched for,
+but in vain; neither comet was found. An astronomer in the southern
+hemisphere, however, found a comet on the track of Biela's, but
+calculation has shown that the two objects are probably not identical,
+since this comet was two months behind the computed position for
+Biela's. It will be shown in the following chapter that our globe
+probably crossed the orbit of Biela's comet on November 27th, 1872, and
+the phenomena resulting from this passage will be there described.
+</p>
+<p>
+It is seen from these observations that comets may be lost or dissipated
+in space by causes entirely unknown to us. Biela's comet is not the only
+one which has been thus disintegrated. Ancient historians speak of the
+separation of large comets into two or more parts. In 1661 Hevelius
+observed the apparent division of the comet of that year and its
+reduction to fragments. The return of this comet, calculated for 1790,
+was vainly waited for; the comet was not seen.
+</p>
+<p>
+Other comets, whose periods of revolution were well known, have
+disappeared, probably never to return. Such is Lexell's comet, whose
+period was 5⁶⁄₁₀ years; also De Vico's comet, both of which are
+now lost. It is supposed that Lexell's comet, which passed twice very
+near the giant planet Jupiter, had its orbit changed from an ellipse to
+a parabola, by the powerful disturbing influence of this planet, and was
+thus lost from our system. Several other comets, in traveling over their
+different orbits, have approached near enough to Saturn, Jupiter and the
+Earth to have their orbits decidedly altered by the powerful attraction
+of these bodies.
+</p>
+<p>
+But since comets are liable to pass near the planets, and several have
+orbits which approach that of the Earth, it becomes important for us to
+know whether an encounter of such a body with our globe is possible, and
+what would then be the result for us. Although that knowledge would not
+enable us to modify the possibilities of an encounter, yet it is better
+to know the dangers of our navigation through space than to ignore them.
+This question of a collision of the Earth with a comet has been answered
+in different ways, according to the ideas entertained in regard to the
+mass of these bodies. While some have predicted calamities of all kinds,
+such as deluges, conflagrations, or the reduction of the Earth to
+incandescent gases, others have asserted that it would produce no more
+effect than does a fly on encountering a railroad train. In our days
+astronomers entertain very little fears from such an encounter, because
+the probabilities of danger from an occurrence of this sort are very
+slight, the mass of an ordinary comet being so small compared with that
+of our globe. We know with certainty that the Earth has never had an
+encounter with a comet <i>by which it has been transformed into gases</i>,
+at least within the several millions of years during which animal and
+vegetable life have left their marks upon the stony pages of its
+history, otherwise these marks would not now be seen. If, then, such an
+accident has not happened during this long period, the chances for its
+occurring must be very small, so small indeed that they might almost be
+left out of the question. It is true that our globe shows signs of great
+perturbations of its surface, but we have not the slightest proofs that
+they resulted from an encounter with a celestial body. It seems very
+probable that our globe passed through the tail of the comet of 1861,
+before it was first seen on June 29th; but nothing unusual was observed,
+except perhaps some phosphorescent light in the atmosphere, which was
+afterwards attributed to this cause.
+</p>
+<p>
+The density and mass of comets must be comparatively very small. Their
+tails consist of matter of such extreme tenuity that it affects but very
+little the light of the small stars over which they pass. The coma and
+nucleus, however, are not quite so transparent, and may have greater
+masses. On several occasions I have seen the light of stars reduced by
+the interposition of cometary matter, comet 1881, III., presenting
+remarkable cases of this sort. On July 8th, at 10h. 50m., several small
+stars were involved in this comet, one of which passed quite near the
+nucleus through the bright inner coma. At that time the comet was
+greatly disturbed, its nucleus was contracting and enlarging rapidly,
+and becoming bright and again faint in an instant. Every time that the
+nucleus grew larger, the star became invisible, but reappeared the
+moment the nucleus was reduced in size. This phenomenon could not be
+attributed to an atmospheric effect, since, while the nucleus was
+enlarging, a very small inner nucleus was visible within the large
+diffused one, the matter of which had apparently spread over the part of
+the coma in which the star was involved, making it invisible.
+</p>
+<p>
+That the mass of comets is small, is proved by the fact that they have
+sometimes passed near the planets without disturbing them in any
+sensible manner. Lexell's comet, which in 1770 remained four months very
+near Jupiter, did not affect in the least the orbits, or the motions of
+its satellites. The same comet also came within less than 1,500,000
+miles from the Earth, and on this occasion it was calculated that its
+mass could not have been the ¹⁄₅₀₀₀ part of that of our
+globe, since otherwise the perturbations which it would have caused in
+the elements of the Earth's orbit would have been sensible. There was,
+however, no change. If this comet's mass had been equal to that of our
+globe, the length of our year would have been increased by 2h. 47m. The
+comet of 1837 remained four days within 3,500,000 miles of the Earth,
+with no sensible effect.
+</p>
+<p>
+It seems quite difficult to admit that the denser part of a comet
+forming the nucleus is solid, as supposed by some physicists, since it
+is so rapidly contracted and dilated by the solar forces, while the
+comet is yet at a too great distance from the Sun to allow these effects
+to be attributed to solar heat alone. This part of a comet, as indeed
+the other parts, seems rather to be in the gaseous than in the solid
+state; the changes observed in the intensity of its light and in its
+structure may be conceived as due to some solar action partaking of the
+nature of electricity.
+</p>
+<p>
+It has been a question whether comets are self-luminous, or whether they
+simply reflect the solar light. When their light is analyzed by the
+spectroscope, it is found that the nucleus of a comet generally gives a
+continuous spectrum, while the coma and tail give a spectrum consisting
+of several bright diffused bands. The spectrum given by the nucleus is
+rarely bright enough to allow the dark lines of the solar spectrum to be
+discerned upon it; but such lines were reported in the spectrum of comet
+1881, III., a fact proving that this nucleus at least reflected some
+solar light. The nucleus of a comet may be partly self-luminous, and
+either solid, liquid, or composed of incandescent gases submitted to a
+great pressure. As to the coma and tail, they are evidently gaseous, and
+partly, if not entirely, self-luminous, as is proved by the band
+spectrum which they give. The position of these bands, moreover,
+indicates that the luminous gases of which they are composed contain
+carbon. The phenomena of polarization, however, seem to prove that these
+parts of comets also reflect some solar light.
+</p>
+<p>
+No theory so far proposed, to explain comets and the strange phenomena
+they exhibit, seems to have been successful in its attempts, and the
+mystery in which these bodies have been involved from the beginning of
+their apparition, seems to be now nearly as great as ever. It has been
+supposed that their tails have no real existence, but are due to an
+optical illusion. Prof. Tyndall has endeavored to explain cometary
+phenomena by supposing these bodies to be composed of vapors subject to
+decomposition by the solar radiations, and thus made visible, the head
+and tail being an actinic cloud due to such decompositions. According to
+this view, the tails of comets would not consist of matter projected
+into spacer but simply of matter precipitated by the solar rays in
+traversing the cometary nebulosity. The endeavor has also been made to
+explain the various phenomena presented by comets by an electrical
+action of the Sun on the gases composing these objects. Theories taking
+this as a base seem to us to be more likely to lead to valuable results.
+M. Faye, who has devoted much time and learning to this subject, assumes
+a real repulsive force of the Sun, acting inversely to the square of the
+distance and proportionally to the surface, and not to the mass as
+attraction does. He supposes, however, that this repulsive force is
+generated by the solar heat, and not by electricity. Prof. Wm. Harkness
+says that many circumstances seem to indicate that the comets' tails are
+due, in a great measure, to electrical phenomena.
+</p>
+<p>
+The fact that the tails of comets are better defined and brighter on the
+forward side, associated with the other fact that they curve the most
+when their motion is most rapid, sufficiently indicates that these
+appendages are material, and that they either encounter some resistance
+from the medium in which they move, or from a solar repulsion. The
+phenomena of condensation and extension, which I have observed in the
+comets of 1874 and 1881, added to the curious behavior exhibited by the
+jets issuing from the nucleus, seem to indicate the action of electrical
+forces rather than of heat. The main difficulty encountered in the
+framing of a theory of comets consists in explaining how so delicate and
+extended objects as their tails seem to be, can be transported and
+whirled around the Sun at their perihelion with such an enormous
+velocity, always keeping opposite to the Sun, and, as expressed by Sir
+John Herschel, "in defiance of the law of gravitation, nay, even of the
+received laws of motion."
+</p>
+<p>
+To consider the direction of the comets' tails as an indirect effect of
+attraction, seems out of the question; the phenomenon of repulsion so
+plainly exhibited by these objects seems to point to a positive solar
+repulsion, as alone competent to produce these great changes. The
+repulsive action of the Sun on comets' tails might be conceived, for
+instance, as acting in a manner similar to that of a powerful current of
+wind starting from the Sun, and constantly changing in direction, but
+always keeping on a line with the comet. Such a current, acting on a
+comet's tail as if it were a pennant, would drive it behind the nucleus
+just as observed. If it could once be ascertained that the great
+disturbances on comets correspond with the magnetic disturbances on our
+globe and with the display of the auroral light, the electric nature of
+the forces acting so strangely on the comets would be substantially
+demonstrated. I have shown that some of the great disturbances observed
+in the comets of 1874 and 1881 have coincided with auroral displays, and
+it will be shown hereafter that similar displays have also coincided
+with the passage of meteoric showers through our atmosphere. Whether
+these simultaneous phenomena were simple coincidences having no
+connection, or whether they are the result of a common cause, can only
+be ascertained by long continued future observations.
+</p>
+
+<p><br /><br /><br /></p>
+
+<h4><a id="SHOOTING_STARS">SHOOTING-STARS AND METEORS</a>
+<br /><br />
+PLATE XII</h4>
+
+<p>
+While contemplating the heavens on a clear moonless night, we
+occasionally witness the sudden blazing forth of a star-like meteor,
+which glides swiftly and silently across some of the constellations, and
+as suddenly disappears, leaving sometimes along its track a
+phosphorescent trail, which remains visible for a while and gradually
+vanishes. These strange apparitions of the night are called <i>Falling</i>
+or <i>Shooting-stars</i>.
+</p>
+<p>
+There is certainly no clear night throughout the year during which some
+of these meteors do not make their appearance, but their number is quite
+variable. In ordinary nights only four or five will be observed by a
+single person in the course of an hour; but on others they are so
+numerous that it becomes impossible to count them. When the falling
+stars are only a few in number, and appear scattered in the sky, they
+are called <i>Sporadic Meteors</i>, and when they appear in great numbers
+they constitute <i>Meteoric Showers</i> or <i>Swarms</i>.
+</p>
+<p>
+Probably there is no celestial phenomenon more impressive than are these
+wonderful pyrotechnic displays, during which the heavens seem to break
+open and give passage to fiery showers, whose luminous drops describe
+fantastic hieroglyphics in the sky. While observing them, one can fully
+realize the terror with which they have sometimes filled beholders, to
+whom it seemed that the stability of the universe had come to an end,
+and that all the stars of the firmament were pouring down upon the Earth
+in deluges of fire.
+</p>
+<p>
+The ancients have left record of many great meteoric displays, and the
+manner in which they describe them sufficiently indicates the fear
+caused by these mysterious objects. Among the many meteoric showers
+recorded by ancient historians may be mentioned one observed in
+Constantinople, in the month of November, 472, when all the sky appeared
+as if on fire with meteors. In the year 599, meteors were seen on a
+certain night flying in all directions like fiery grasshoppers, and
+giving much alarm to the people. In March, 763, "the stars fell
+suddenly, and in such crowded number that people were much frightened,
+and believed the end of the world had come." On April 10th, 1095, the
+stars fell in such enormous quantity from midnight till morning that
+they were as crowded as are the hail stones during a severe storm.
+</p>
+<p>
+In modern times the fall of the shooting-stars in great number has been
+frequently recorded. One of the most remarkable meteoric showers of the
+eighteenth century occurred on the night of November 13th, 1799, and was
+observed throughout North and South America and Europe. On this
+memorable night thousands of falling stars were seen traversing the sky
+between midnight and morning. Humboldt and Boupland, then traveling in
+South America, observed the phenomena at Cumana, between two and five
+o'clock in the morning. They saw an innumerable number of shooting-stars
+going from north to south, appearing like brilliant fire-works. Several
+of these meteors left long phosphorescent trails in the sky, and had
+nuclei whose apparent diameter, in some cases, surpassed that of the
+Moon.
+</p>
+<p>
+The shower of November 13th, 1833, was still more remarkable for the
+great number of meteors which traversed the heavens, and was visible
+over the whole of North and South America. On that occasion the falling
+stars were far too numerous to be counted, and they fell so thickly that
+Prof. Olmsted, of New Haven, who observed them carefully, compared their
+number at the moment of their maximum fall to half that of the flakes of
+snow falling during a heavy storm. This observer estimated at 240,000
+the number of meteors which must have traversed the heavens above the
+horizon during the seven hours while the display was visible.
+</p>
+
+<p><br /></p>
+
+<div class="figcenter" style="width: 400px;">
+<a id="figure12"></a>
+<br />
+<img src="images/figure12.jpg" width="400" alt="" />
+<div class="caption">
+<p>PLATE XII.&mdash;THE NOVEMBER METEORS.</p>
+<p class="smaller">As observed between midnight and 5 o'clock A.M. on the night
+of November 13-14 1868.</p>
+</div></div>
+
+<p><br /></p>
+
+<p>
+In the years 1866, 1867 and 1868, there were also extraordinary meteoric
+displays on the night of November 13th. It was on the last mentioned
+date that I had the opportunity to observe the remarkable shower of
+shooting-stars of which I have attempted to represent all the
+characteristic points in Plate XII. My observations were begun a little
+after midnight, and continued without interruption till sunrise. Over
+three thousand meteors were observed during this interval of time in the
+part of the sky visible from a northern window of my house. The maximum
+fall occurred between four and five o'clock, when they appeared at a
+mean rate of 15 in a minute.
+</p>
+<p>
+In general, the falling stars were quite large, many being superior to
+Jupiter in brightness and apparent size, while a few even surpassed
+Venus, and were so brilliant that opaque objects cast a strong shadow
+during their flight. A great many left behind them a luminous train,
+which remained visible for more or less time after the nucleus had
+vanished. In general, these meteors appeared to move either in straight
+or slightly curved orbits; but quite a number among them exhibited very
+extraordinary motions, and followed very complicated paths, some of
+which were quite incomprehensible.
+</p>
+<p>
+While some moved either in wavy or zig-zag lines, strongly accentuated,
+others, after moving for a time in a straight line, gradually changed
+their course, curving upward or downward, thus moving in a new
+direction. Several among them, which were apparently moving in a
+straight line with great rapidity, suddenly altered their course,
+starting at an abrupt angle in another direction, with no apparent
+slackening in their motion. One of them, which was a very conspicuous
+object, was moving slowly in a straight course, when of a sudden it made
+a sharp turn and continued to travel in a straight line, at an acute
+angle with the first, retreating, and almost going back towards the
+regions from which it originally came. As nearly all the meteors which
+exhibited these extraordinary motions left the trace of their passage in
+the sky by a luminous trail, it was easily ascertained that these
+appearances were not deceptive. On one occasion I noticed that the
+change of direction in the orbit corresponded with the brightening up of
+the meteor thus disturbed in its progress.
+</p>
+<p>
+Among these meteors, some traveled very slowly, and a few seemed to
+advance as if by jerks, but in general they moved very rapidly. One of
+the meteors thus appearing to move by jerks left a luminous trail, upon
+which the various jerks seemed to be left impressed by a succession of
+bright and faint spaces along the train. Some of the largest meteors
+appeared to rotate upon an axis as they advanced, and most of these
+revolving meteors, as also a great number of the others, seemed to
+explode just before they disappeared, sending bright fiery sparks of
+different colors in all directions, although no sound was at any time
+heard. The largest and most brilliant meteor observed on that night
+appeared at 5h. 30m., a little before sunrise. It was very bright, and
+appeared considerably larger than Venus, having quite a distinct disk.
+This meteor moved very slowly, leaving behind a large phosphorescent
+trail, which seemed to issue from the inside of the nucleus as it
+advanced. For a moment the train increased in size and brightness close
+to the nucleus, which then appeared as an empty transparent sphere,
+sprinkled all over with minute fiery sparks; the nucleus then suddenly
+burst out into luminous particles, which immediately vanished, only the
+luminous trail of considerable dimensions being left.
+</p>
+<p>
+Many of the trails thus left by the meteors retained their luminosity
+for several minutes, and sometimes for over a quarter of an hour. These
+trails slowly changed their form and position; but it is perhaps
+remarkable that almost all those which I observed on that night assumed
+the same general form&mdash;that of an open, irregular ring, or horse-shoe,
+somewhat resembling the letter C. This ring form was subsequently
+transformed into an irregular, roundish cumulus-like cloud. The trail
+left by a very large meteor, which I observed on the evening of
+September 5th, 1880, also exhibited the same general character of
+transformation.
+</p>
+<p>
+While I was observing a long brilliant trail left by a meteor on the
+night of November 13th, 1868, it was suddenly crossed by another bright
+shooting-star. The latter apparently went through the luminous substance
+forming the trail, which was suddenly altered in form, and considerably
+diminished in brightness simultaneously with this passage, although
+electrical action at some distance might perhaps as well explain the
+sudden change observed.
+</p>
+<p>
+In the majority of cases the meteors appeared white; but many,
+especially the largest, exhibited a variety of brilliant colors, among
+which the red, blue, green, yellow and purple were the most common. In
+general the trails exhibited about the same color as the nucleus, but
+much fainter, and they were usually pervaded by a greenish tint. In some
+instances the trails were of quite a different color from the nucleus.
+</p>
+<p>
+The luminous cloud observed at 5h. 30m. on the morning of November 14th,
+1868, after having passed through the series of transformations above
+described, remained visible for a long while after sunrise, appearing
+then as a small cirrus cloud, exactly similar in appearance to the
+hundreds of small cirrus clouds then visible in the sky, which had
+probably the same meteoric origin. For over three hours after sunrise,
+these cirrus clouds remained visible in the sky, moving all together
+with the wind in the high regions of the atmosphere.
+</p>
+<p>
+Although Plate XII. is intended to represent all the characteristics
+exhibited by the meteors observed on that night, every form represented
+having been obtained by direct observation, yet the number is much
+greater than it was at any single moment during the particular shower of
+1868. As regards number, the intention was to give an idea of a great
+meteoric shower, such as that of 1833, for instance. Although many of
+the falling stars seem to be close to the Earth's surface, yet this is
+only an effect of perspective due to their great distance, very few of
+these meteors ever coming into the lower regions of our atmosphere at
+all.
+</p>
+<p>
+The phenomena exhibited during other great meteoric showers have been
+similar to those presented by the shower just described, the only
+differences consisting in variations of size and brightness in the
+meteors, and also in the trails, which sometimes are not so numerous as
+they were in 1868.
+</p>
+<p>
+While some shooting-stars move so rapidly that they can hardly be
+followed in their orbits, others move so slowly that the sight can
+easily follow them, and even remark the peculiarities of their
+movements, some remaining visible for half a minute. Some of the falling
+stars move at the rapid rate of 100 miles a second, but others only 10
+miles a second, and even less. In general, they move about half as fast
+again as the Earth in its orbit. The arcs described by the meteors in
+the sky are variable. While some extend 8o° and even 100°, others are
+hardly half a degree in length. While some shooting-stars are so faint
+that they can hardly be seen through the largest telescopes, others are
+so large and brilliant that they can be seen in the day-time. In
+general, a shooting-star of average brightness resembles a star of the
+third or fourth magnitude.
+</p>
+<p>
+Whatever may be the origin of the shooting-stars, they are, when we see
+them, not in the celestial spaces, like the planets, the comets, or the
+stars, but in our atmosphere, through which they travel as long as they
+remain visible. The height at which they appear and disappear is
+variable, but in general they are about 80 miles above the surface of
+our globe when they are first seen, and at about 55 miles when they
+disappear. In many cases, however, they have been observed at greater
+elevations, as also at smaller. A meteor simultaneously observed at two
+different stations first appeared at the height of 285 miles, and was
+last seen at 192 miles above the Earth's surface; but in rare cases the
+falling stars have been seen below a layer of clouds completely covering
+the sky. I myself saw one such shooting-star a few years since. The fact
+that the meteors are visible at so great elevations, proves that our
+atmosphere extends much farther than was formerly supposed, although at
+these great heights it must be extremely rarefied, and very different
+from what it is in its lower regions.
+</p>
+<p>
+There is a remarkable difference between the sporadic meteors seen in
+the sky on every night, and the meteoric showers observed only at
+comparatively rare intervals. While the first appear from different
+points in the sky and travel in all directions, being perfectly
+independent, the meteors of a shower all come from the same point of the
+heavens, from which they apparently diverge in all directions. This
+point of divergence of the meteors is called the <i>radiant point</i> of
+the shower. Although the meteors seem to diverge in all directions from the
+radiant point, yet they all move in approximately parallel lines, the
+divergence being an effect of perspective.
+</p>
+<p>
+Whatever may be the position of the radiant point in the constellations,
+it remains as fixed in the sky as the stars themselves, and participates
+with them in the apparent motion which they undergo by the effect of the
+diurnal motion, and thus rises and sets with the constellation to which
+it belongs. This fact is sufficient to prove that the orbits of these
+meteors are independent of the Earth's motion, and that consequently
+they do not originate in our atmosphere. It has been shown by Encke that
+the radiant point of the meteoric shower of November 13th is precisely
+the point towards which our globe moves in space on November 13th; a
+tangent to the Earth's orbit would pass through this radiant point.
+</p>
+<p>
+The meteoric showers are particularly remarkable, not merely because of
+the large number of meteors which are visible and the fact that they all
+follow a common orbit, but chiefly because they have a periodic return,
+either after an interval of a year, or after a lapse of several years.
+At the beginning of the present century only two meteoric showers were
+known, those of August 10th and of November 13th, and their periodicity
+had not yet been recognized, although it had begun to be suspected. It
+was only in 1836 that Quetelet and Olbers ventured to predict the
+reappearance of the November meteors in the year 1867. Having made
+further investigations, Prof. Newton, of Yale College, announced their
+return in the year 1866. In both of these years, as also in 1868, the
+meteors were very numerous, and were observed in Europe and in America
+on the night of November 13th. The predictions having thus been
+fulfilled, the periodicity of the meteors was established. Since then,
+other periodic showers have been recognized, although they are much less
+important in regard to number than those of August and November, except
+that of November 27th, which exhibited so brilliant a display in Europe
+in 1872. These successive appearances have established the main fact
+that meteoric showers are more or less visible every year when the Earth
+occupies certain positions in its orbit.
+</p>
+<p>
+The meteoric shower of the 10th of August has its radiant point situated
+in the vicinity of the variable star Algol, in the constellation
+Perseus, from which its meteors have received the name of Perseids.
+Although varying in splendor, this meteoric swarm never fails to make
+its appearance every year. The Perseids move through our atmosphere at
+the rate of 37 miles per second. The shower usually lasts about six
+hours.
+</p>
+<p>
+The meteoric shower of November 13th has its radiant point situated in
+the vicinity of the star Gamma, in the constellation Leo, from which its
+meteors have been called Leonids. But while the August meteors recur
+regularly every year, with slight variations, the shower of November
+does not occur with the same regularity. During several years it is
+hardly noticeable, and is even totally absent, while in other years it
+is very remarkable. Every 33 years an extraordinary meteoric shower
+occurs on the 13th of November, and the phenomenon is repeated on the
+two succeeding years at the same date, but with a diminution in its
+splendor at each successive return. The Leonids move in an opposite
+direction to that of the Earth, and travel in our atmosphere with an
+apparent velocity of 45 miles per second, this being about the maximum
+velocity observed in falling stars. But when the motion of our globe is
+taken into account, and a deduction is made of the 18 miles which it
+travels per second, it is found that these meteors move at an actual
+mean rate of 27 miles a second.
+</p>
+<p>
+In a meteoric shower the stars do not fall uniformly throughout the
+night, there being a time when they appear in greater numbers. Usually
+it is towards morning, between 4 and 6 o'clock, that the maximum occurs.
+The probable cause of this phenomenon will be explained in its place
+hereafter.
+</p>
+<p>
+The orbits of the meteoric showers are not all approximately in the same
+plane, like those of the planets, but rather resemble those of comets,
+and have all possible inclinations to the ecliptic. Like the comets,
+too, the different meteoric showers have either direct or retrograde
+motion.
+</p>
+<p>
+The shooting-stars were formerly considered as atmospheric meteors,
+caused by the combustion of inflammable gases generated at the surface
+of the Earth, and transported to the high regions of our atmosphere by
+their low specific gravity. But the considerable height at which they
+usually appear, the great velocity of their motion, the common orbit
+followed by the meteors of the same shower, and the periodicity of their
+recurrence, do not permit us now to entertain these ideas, or to doubt
+their cosmical origin. But what is their nature?
+</p>
+<p>
+It is now generally admitted that innumerable minute bodies, moving in
+various directions around the Sun, are scattered in the interplanetary
+spaces through which our globe travels. It has been supposed that
+congregations of such minute bodies form elliptical rings, within which
+they are all moving in close parallel orbits around the Sun. On the
+supposition that such rings intersect the orbit of the Earth at the
+proper places, it was practicable to account for the shooting-stars by
+the passage through our atmosphere of the numerous minute cosmical
+bodies composing the rings, and the Leonid and Perseid showers were so
+explained. But when the elements of the orbits of these two last swarms
+came to be better known, and were compared with those of other celestial
+bodies, it was found necessary to alter this theory.
+</p>
+<p>
+It had for a long while been suspected that some kind of relation
+existed between the shooting-stars and the comets. This idea, vaguely
+formulated by Kepler more than two centuries ago, more clearly expressed
+by Chladni, and still more by Mr. Grey, before the British Association,
+at Liverpool, in 1855, has recently received a brilliant confirmation by
+the researches of Professor Schiaparelli, Director of the Observatory of
+Milan. A thorough investigation of the orbits of the August and November
+meteors led Schiaparelli to the discovery of a remarkable relation
+between meteoric and cometary orbits. By comparing the elements of these
+meteoric orbits with those of comets, he found a very close resemblance
+between the orbit of the August meteors and that of the comet 1862,
+III., and again between the orbit of the November meteors and that of
+Tempel's comet, 1866, I. These resemblances were too striking to be the
+result of mere chance, and demonstrated the identity of these cometary
+orbits with those of the Perseid and Leonid showers. In accordance with
+these new facts, it is now admitted that the meteoric showers result
+from the passage of our globe through swarms of meteoric particles
+following the orbits of comets, which intersect the orbit of the Earth.
+</p>
+<p>
+Professor Schiaparelli has attempted to show how these meteoric swarms
+were originally scattered along the orbits of comets, by supposing these
+bodies to originate from nebulous masses, which, in entering the sphere
+of attraction of the Sun, are gradually scattered along their orbits,
+and finally form comets followed by long trails of meteoric particles.
+</p>
+<p>
+It has been shown that in approaching the Sun the comets become
+considerably elongated, their particles being disseminated over immense
+distances by the solar repulsion. It seems probable that, owing to its
+feeble attractive power, the nucleus is incompetent to recall the
+scattered cometary particles and retain them in its grasp when they are
+relieved from the solar repulsion, so that they remain free from the
+nucleus, although they continue to move along its orbit. It is
+supposable that these cometary particles will scatter more and more in
+course of time. Forming at first an elongated meteoric cloud, they will
+finally spread along the whole orbit, and thus form a ring of meteoric
+particles. Since our globe constantly moves in its orbit and daily
+occupies a different position, it follows that at any point where such a
+cometary orbit happens to cross that of the Earth, our globe will
+necessarily encounter the cometary particles as a shower of meteors.
+This encounter will take place at a certain time of the year, either
+yearly, if they form a continuous ring, or after a succession of years,
+if they simply form an elongated cloud. Such meteoric clouds or rings
+would not be visible in ordinary circumstances, even through the largest
+telescopes, except on penetrating the upper regions of our atmosphere,
+when they would appear as showers of falling stars. It is supposed that
+in penetrating our atmosphere, even in its most rarefied regions, these
+meteors are heated by the resistance offered by the air to their motion,
+first becoming luminous and then being finally vaporized and burnt
+before they can reach the surface of the Earth.
+</p>
+<p>
+The orbit of the comet of 1862, III., which so closely corresponds with
+that of the Perseid meteors, is much more extended than that of Tempel's
+comet corresponding with that of the Leonids. While the first extends
+far beyond the orbit of Neptune, the latter only goes a little beyond
+that of Uranus. The former orbit makes a considerable angle with the
+plane of the Earth's orbit, but the latter is much nearer to parallelism
+with it. The period of revolution of the first is 108 years, and that of
+the last about 33¼ years.
+</p>
+<p>
+From the fact that the Perseid shower occurs yearly on the 10th of
+August, when the Earth crosses the orbit of the comet of 1862, III., it
+is supposed that the cometary particles producing this shower are
+disseminated along the whole orbit, and form a ring encircling the Sun
+and Earth. To explain the yearly variations in the number of the
+shooting-stars observed, these particles are supposed to be unequally
+distributed over the orbit, being more crowded at one place than they
+are at another. In order to explain the meteoric shower of Leonids,
+which appears in all its splendor every 33 years, and then with
+diminished intensity for two successive years, after which it is without
+importance, it is supposed that the cometary particles of the comet of
+1866, I., have not as yet spread all along the orbit, a sufficient time
+not having been allowed, but form an elongated meteoric cloud, more
+dense in its front than in its rear part. From these considerations it
+has been supposed also that the comet of 1866, I., is of a more recent
+date than that of 1862, III. While Tempel's comet makes its revolution
+around the Sun in about 33 years, this meteoric cloud, which has the
+same period and returns to the same point of its orbit every 33 years,
+encounters our globe for three successive years. The first year we are
+passing through its densest parts, and the two following years in less
+and less crowded parts, from which result the observed phenomena. An
+idea of the extent of this meteoric cloud may be formed from the fact
+that, with its cometary velocity of motion, it takes this cloud three
+years at least to cross the Earth's orbit. From recent researches it
+would appear that the Leonid cloud is not single, but that at least two
+others of smaller importance exist, and have periods of 33¼ years.
+</p>
+<p>
+Biela's comet, which was divided into two parts in 1846, is another of
+the few comets whose orbit approaches that of the Earth. Possessing this
+knowledge, and knowing then the close connection existing between
+meteors and comets, astronomers supposed that there were sufficient
+reasons to expect a meteoric shower when this comet was passing near the
+Earth. They consequently expected a meteoric display in 1872, when our
+globe was to cross its orbit. Their anticipation was plainly fulfilled,
+and on the night of November 27th, 1872, a splendid meteoric display,
+having its radiant point in the constellation Andromeda, was observed in
+Europe, and also in America, but the meteors seen here were not so
+numerous as in Europe. Other meteoric showers of less importance, such
+as that of April 20th, for instance, have also been identified with
+cometary orbits, so that now no doubt seems to remain as to the identity
+of cometary particles and shooting-stars.
+</p>
+<p>
+The fact that the maximum number of meteors is always observed in the
+morning hours, supports the hypothesis of the cosmic origin of the
+shooting-stars, since the regions of the Earth where it is morning are
+precisely those fronting the regions towards which our globe is moving
+in space, and accordingly encounter more directly the meteors moving in
+their orbit. The greater abundance of falling stars at that time may
+thus be accounted for.
+</p>
+<p>
+The number of meteors penetrating our atmosphere must be very great;
+there is not an hour and probably not a minute during which none fall.
+From various considerations, some astronomers have estimated at from
+65,000,000,000 to 146,000,000,000 the total number of shooting-stars
+yearly penetrating in our atmosphere. The actual number is undoubtedly
+great, yet the fact that the meteors are rarely seen through the
+telescope while employed in observing various celestial objects, does
+not indicate that they are so numerous as these figures imply. It is
+only occasionally that one is seen traversing the field of the
+instrument. Even when the sky is observed with a low power eye-piece for
+several hours in succession, many nights may pass without disclosing
+one, although an observer, sweeping the sky more freely with the naked
+eye, may often perceive four or five during an ordinary night.
+</p>
+<p>
+About the true nature of these bodies nothing is known with certainty.
+From spectrum analysis it seems to be established that most of them
+contain sodium and magnesium, while a few indicate the presence of
+strontium and iron, and in some rare cases there are traces of coal-gas.
+Some of the nuclei give a continuous spectrum, and others a spectrum of
+lines. The trail always gives a spectrum of bright lines which indicates
+its gaseous state. The traces of coal-gas rarely seen in meteors are,
+however, of great importance, as it identifies them more closely with
+the comets, which generally show a similar spectrum. The continuous
+spectra exhibited by some nuclei would indicate that they are
+incandescent and either solid or liquid; but it is difficult to conclude
+from their spectra what is their true nature, since we do not know
+exactly what part the terrestrial atmosphere may play in producing the
+results.
+</p>
+<p>
+The mass of the shooting-stars is not known with certainty, but the fact
+that during great meteoric showers, none are seen to reach the surface
+of the Earth, all being consumed in a few seconds, sufficiently
+indicates that it must be very small. It has been calculated that those
+equal to Venus in apparent size and brilliancy may weigh several pounds,
+while the faint ones would weigh only a few grains.
+</p>
+<p>
+If the shooting-stars have even such a mass as that here attributed to
+some of them, the extraordinary motions which I have described above
+seem to be unaccountable. The change of direction of a heavy mass moving
+swiftly cannot be sudden. The semi-circular, the wavy and the angular
+orbits observed could not be described, it would seem, by such a mass
+animated with a great velocity. Although the meteors are said to be
+ignited by the transformation of part of their progressive motion into
+molecular motion, yet it is not observed that the velocity of the
+falling stars diminishes when they are about to disappear. The luminous
+trails they leave in the atmosphere do not appear to be endowed with any
+motion, but remain for a time in their original positions. These facts
+are apparently opposed to the hypothesis that such meteors have any
+appreciable mass. The extraordinary motions exhibited by some meteors
+seem to indicate that some unsuspected force resides in these bodies,
+and causes them to deviate from the laws of ordinary motion.
+</p>
+<p>
+Although it is very probable that the ordinary shooting-stars have no
+appreciable mass, yet it is known that very heavy meteoric masses
+sometimes fall at the surface of the Earth. Such falls are generally
+preceded by the sudden apparition in the sky of a large, and usually
+very brilliant fire-ball, which traverses the air at a great speed,
+sometimes leaving behind it a luminous trail, after which it explodes
+with a loud sound, and heavy fiery meteoric fragments, diverging in all
+directions, fall at the surface of the Earth. The name of
+<i>Aerolites</i> or <i>Meteorolites</i> is given to these ponderous
+fragments. As these meteors, before they explode and fall to the ground,
+have many points of resemblance with the shooting-stars, they are
+generally supposed to be connected with them, and to have a similar
+cometary origin. The fact that the aerolites differ widely from each
+other in constitution, and are all composed of substances found on the
+Earth, associated with other facts given below, would rather seem to
+indicate a terrestrial than a celestial origin.
+</p>
+<p>
+If the aerolites belong to the same class of bodies as the falling
+stars, differing from them only in size and mass, it is difficult to see
+why so very few should fall upon the Earth during the great meteoric
+showers, when thousands of shooting-stars traverse our atmosphere. In
+Prof. Kirkwood's "Meteoric Astronomy" are given catalogues of all the
+falls of aerolites and fire-balls which have been observed at the time
+of the periodic meteoric showers of the 10th of August and the 13th of
+November, during a period of 221 years for the Perseids, or August
+showers, and of 318 years for the Leonids, or November showers. During
+221 years, 10 falls of aerolites have been witnessed simultaneously with
+the fall of the Perseids; while during 318 years, only 4 such falls have
+been recorded as having occurred at the time of the Leonid shower. If
+there is any close connection between the shooting-stars and the
+aerolites, we should expect to find a maximum in their fall at the time
+of the great meteoric displays. So far, no maxima or minima have yet
+been discovered in the fall of aerolites; they do not seem, like
+meteoric showers, to be governed by a law of periodicity.
+</p>
+<p>
+A very remarkable peculiarity of the aerolites is that they seem to have
+a tendency to fall in certain regions. Such are the southern part of
+France, the north of Italy, Hindostan, the central states of North
+America, and Mexico and Brazil. There is a curious contrast existing
+between the quick cometary motion of the aerolites before their
+explosion, and the comparatively slow motion of their fragments as they
+reach the Earth; motion which seems to be no greater than that
+corresponding to their natural fall impeded by the resistance of the
+air. In general, their penetration into the soil upon which they fall
+does not at all correspond to the great velocity with which they move in
+the atmosphere. The fragmentary structures of the aerolites, their
+identity of substance with that of our globe, their great resemblance to
+the volcanic minerals of the Earth, and the fractures and faults which
+some of them exhibit, do not correspond at all with the idea that they
+are cometary particles fallen on the Earth. As far as their structure
+and appearance is concerned, they seem rather to be a volcanic product
+of the interior of the Earth than parts of disintegrated comets. It must
+be admitted that their identity with the shooting-stars is far from
+established, and that they are still involved in mystery.
+</p>
+<p>
+The so-called meteoric dust gathered at sea and on high mountains may
+have various origins, and may be partly furnished by volcanic dust
+carried to great distances in the atmosphere.
+</p>
+<p>
+Since millions of shooting-stars penetrate our atmosphere every year and
+remain in it, becoming definitively a part of the Earth, it follows
+that, no matter how small may be the quantity of matter of which they
+are composed, they must gradually increase the volume and mass of our
+globe, although the increase may be exceedingly slow. Supposing every
+one of the shooting-stars penetrating our atmosphere to contain one
+cubic millimeter of matter, it has been calculated that it would take
+nearly 35,000 years to make a deposit one centimeter in thickness all
+over the surface of our globe. Insignificant as this may appear, it is
+probable that the quantity of matter of meteoric origin which is added
+to our globe is much less than has just been supposed.
+</p>
+
+<p><br /><br /><br /></p>
+
+<h4><a id="THE_MILKY_WAY">THE MILKY-WAY OR GALAXY</a>
+<br /><br />
+PLATE XIII</h4>
+
+<p>
+During clear nights, when the Moon is below the horizon, the starry
+vault is greatly adorned by an immense belt of soft white light,
+spanning the heavens from one point of the horizon to the opposite
+point, and girdling the celestial sphere in its delicate folds. Every
+one is familiar with this remarkable celestial object, called the
+<i>Milky-way</i> or <i>Galaxy</i>.
+</p>
+<p>
+Seen with the naked eye, the Galaxy appears as an irregular, narrow,
+nebulous belt, apparently composed of cloud-like luminous masses of
+different forms and sizes, separated by comparatively dark intervals.
+These cloud-like masses vary much in luminous intensity, and while some
+among them are very bright and conspicuous, others are so faint that
+they are hard to recognize. In general, the brightest parts of the
+Milky-way are situated along the middle of its belt, while its borders,
+which are usually very faint, gradually vanish in the sky. Some parts of
+the Galaxy, however, show very little of the cloudy structure so
+characteristic of other parts, being almost uniform throughout, except
+towards the borders, which are always fainter. These parts showing
+greater uniformity are also the faintest.
+</p>
+<p>
+Such is the general appearance of the Milky-way on ordinary nights, but
+on rare occasions, when the atmosphere is particularly pure, it presents
+one of the grandest sights that can be imagined. At such favorable
+moments I have seen the Galaxy gleaming with light, and appearing as if
+composed of star-dust or of precious stones. The strange belt then
+appeared all mottled over and fleecy, its large cloud-like masses being
+subdivided into numerous small, irregular cloudlets of great brilliancy,
+which appeared projected upon a soft luminous background.
+</p>
+<p>
+The width of the Galaxy is far from being uniform; while in some places
+it is only 4° or 5°, in others it is 15° and even more. In some
+places it appears wavy in outline, at others quite straight; then it
+contracts, to expand a few degrees distant; while at other places it
+sends off branches and loops, varying in form, size and direction, some
+of which are quite prominent, while others are very faint.
+</p>
+<p>
+Although very irregular in form, the general appearance of the galactic
+belt is that of a regular curve occupying one of the great circles of
+the celestial sphere. The Milky-way completely encircles the heavens,
+but, of course, only one-half is visible at any one moment, since our
+globe prevents the other half from being seen. If, for a moment, we
+imagine ourselves left in space, our globe having vanished from under
+our feet, we should then see the whole Galaxy forming a continuous belt
+in the heavens, at the centre of which we should apparently be situated.
+</p>
+<p>
+While only one-half of the galactic belt can be seen at once from any
+point on the Earth, yet, according to the position of the observer, a
+larger or smaller portion of the whole can be seen at different times.
+In high northern or southern latitudes but little more than half can be
+seen even by continuous observations; but as we approach the equatorial
+regions, more and more of it becomes visible, until the whole may be
+seen at different hours and seasons. In the latitudes of the northern
+states, about two-thirds of the Galaxy is visible, the rest remaining
+hidden below the horizon; but from the southern states very nearly the
+whole can be seen. The half of the Milky-way visible at any one time
+from any latitude on the Earth never entirely sets below the horizon,
+although in some places it may be so near the horizon as to be rendered
+invisible by vapors. In the latitude of Cambridge, when in its lowest
+position, the summit of its arc is still about 12° or 15° above the
+northern horizon. The great circle of the celestial sphere, occupied by
+the galactic belt, is inclined at an angle of about 63° to the
+celestial equator, and intersects this great circle on one side in the
+constellation Monoceros in 6h. 47m., and on the opposite side in the
+constellations Aquila and Ophiuchus in 18h. 47m. of right ascension; so
+that its northern pole is situated in the constellation Coma Berenices
+in R. A. 12h. 47m., declination N. 27°, and the southern in the
+constellation Cetus in R. A. 0h. 47m., declination S. 27°.
+</p>
+<p>
+According to the seasons and to the hours of the night at which it is
+observed, the galactic arch presents different inclinations in the sky.
+Owing to its inclination to the equator of the celestial sphere, its
+opposite parts exhibit opposite inclinations when they pass the meridian
+of a place. That part of the Galaxy which is represented on Plate XIII.,
+and which intersects the celestial equator in the constellation Aquila,
+is inclined to the left or towards the east, when it is on the meridian;
+while the opposite part, situated in Monoceros, is inclined to the
+right, or towards the west, when it reaches the meridian. The former
+passes the meridian in the evening in the summer and autumn months; the
+latter, in the winter and spring months.
+</p>
+
+<p><br /></p>
+
+<div class="figcenter" style="width: 400px;">
+<a id="figure13"></a>
+<br />
+<img src="images/figure13.jpg" width="400" alt="" />
+<div class="caption">
+<p>PLATE XIII.&mdash;PART OF THE MILKY WAY.</p>
+<p class="smaller">From a study made during the years 1874, 1875 and 1876</p>
+</div></div>
+
+<p><br /></p>
+
+<p>
+By beginning at its northernmost part, represented at the upper part of
+Plate XIII., situated in "the chair" of the constellation Cassiopeia,
+and descending southwardly, and continuing in the same direction until
+the whole circle is completed, the course of the Milky-way through the
+constellations may be briefly described as follows: From Cassiopeia's
+chair, the Galaxy, forming two streams, descends south, passing partly
+through Lacerta on the left, and Cepheus on the right; at this last
+point it approaches nearest to the polar star. Then it enters Cygnus,
+where it becomes very complicated and bright, and where several large
+cloudy masses are seen terminating its left branch, which passes to the
+right, near the bright star Deneb, the leader of this constellation.
+Below Deneb, the Galaxy is apparently disconnected and separated from
+the northern part by a narrow, irregular dark gap. From this rupture,
+the Milky-way divides into two great streams separated by an irregular
+dark rift. An immense branch extends to the right, which, after having
+formed an important luminous mass between the stars <i>γ</i> and <i>β</i>,
+continues its southward progress through parts of Lyra, Vulpecula,
+Hercules, Aquila and Ophiuchus, where it gradually terminates a few
+degrees south of the equator. The main stream on the left, after having
+formed a bright mass around <i>ε</i> Cygni, passes through Vulpecula and
+then Aquila, where it crosses the equinoctial just below the star <i>η</i>
+after having involved in its nebulosity the bright star Altair, the
+leader of Aquila. In the southern hemisphere the Galaxy becomes very
+complicated and forms a succession of very bright, irregular masses, the
+upper one being in Scutum Sobieskii, while the others are respectively
+situated in Sagittarius and in Scorpio; the last, just a little above
+our horizon, being always considerably dimmed by vapors. From Scutum
+Sobieskii, the Galaxy expands considerably on the right, and sends a
+branch into Scorpio, in which the fiery red star Antares is somewhat
+involved.
+</p>
+<p>
+Continuing its course below our horizon, the Milky-way enters Ara and
+Norma, and then, passing partly through Circinus, Centaurus and Musca,
+it reaches the Southern Cross, after having been divided by the large
+dark pear-shaped spot known to navigators as the "Coal-Sack." In Ara and
+Crux the Milky-way attains its maximum of brightness, which there
+surpasses its brightest parts in Cygnus. In Musca, it makes its nearest
+approach to the south pole of the heavens. It then enters Carina and
+Vela, where it spreads out like a fan, and terminates in this last
+constellation, before reaching <i>λ</i>, being once more interrupted by a
+dark and very irregular gap, on a line with the two stars <i>γ</i> and
+<i>λ</i>. It is noteworthy that this second rupture of continuity of the
+Galaxy in Vela is very nearly opposite, or at about 18o° from the break
+near Deneb in Cygnus.
+</p>
+<p>
+Continuing its course on the other side of the break, the Milky-way
+again spreads out into the shape of a fan, grows narrower in entering
+Puppis, where it is longitudinally divided by darkish channels. It then
+passes above our southern horizon, becoming visible to us, passing
+through part of Canis Major, where its border just grazes the brilliant
+star Sirius. But from Puppis it gradually diminishes in brightness and
+complication, becoming faint and uniform. It enters Monoceros and Orion,
+where it again crosses the equator a little above <i>δ</i>, the
+northernmost of the three bright stars in the belt of Orion. Continuing
+its northward course it passes through Gemini, extending as far as
+Castor and Pollux, and then entering Auriga, where it begins to increase
+in brightness and in complication of structure. It passes partly through
+Camelopardus and into Perseus, where an important branch proceeds from
+its southern border.
+</p>
+<p>
+This branch beginning near the star <i>θ</i>, advances towards the
+celebrated variable star Algol, around which it is quite bright and
+complicated. Continuing its course in the same direction, the branch
+rapidly loses its brightness, becoming very faint a little below Algol,
+and passing through <i>ζ</i> Persei, it enters Taurus, leaving the
+Pleiades on its extreme southern margin; and after having passed through
+<i>ε</i> where it branches off, it rapidly curves towards the main
+stream, which it joins near <i>ζ</i> Tauri, thus forming an immense
+loop. The ramification projecting near <i>ε</i> Tauri involves in its
+nebulosity the ruddy star Aldebaran and the scattered group of the
+Hyades. It then advances towards the three bright stars <i>δ</i>,
+<i>ε</i> and <i>ζ</i> of the belt of Orion, which, together with the
+sextuple star <i>θ</i> Orionis, are involved in its faint nebulosity,
+and joins the main stream on the equinoctial, having thus formed a
+second loop, whose interior part is comparatively free from nebulosity,
+and contains the fine stars Betelgeuse and Bellatrix.
+</p>
+<p>
+That portion of the main galactic stream which is comprised between the
+star Deneb in Cygnus, and Capella in Auriga, is divided longitudinally
+by a very irregular, narrow, darkish cleft, comparatively devoid of
+nebulosity, which, however, is interrupted at some points. This dark gap
+sends short branches north and south, the most important of which are
+situated near <i>ζ</i> Cephei and <i>β</i> Cassiopeiæ. Another branch
+runs from <i>γ</i> beyond <i>ε</i> of the constellation last
+mentioned. The main stream of the Galaxy after leaving Perseus, enters
+Cassiopeia, and sending short branches into Andromeda, it completes its
+immense circle in Cassiopeia's chair, where this description was begun.
+</p>
+<p>
+When examined through the telescope, the appearance of the Milky-way
+completely changes, and its nebulous light is resolved into an immense
+number of stars, too faint to be individually seen with the naked eye.
+When Galileo first directed the telescope to the galactic belt, its
+nebulous, cloud-like masses were at once resolved into stars, even by
+the feeble magnifying power of his instrument. When, much later, Sir
+William Herschel undertook his celebrated star-gaugings of the Galaxy,
+millions of stars blazed out in his powerful telescopes. The stars
+composing this great nebulous belt are so numerous that it is impossible
+to arrive at any definite idea as to their number. From his soundings
+Herschel estimated at 116,000 the number of stars which, on one
+occasion, passed through the field of his telescope in 15 minutes, by
+the simple effect of the diurnal motion of the heavens; and on another
+occasion, a number estimated at 250,000 crossed the field in 41 minutes.
+In a space of 50, comprised between <i>β</i> and <i>γ</i> Cygni, shown
+on Plate XIII., he found no less than 331,000 stars. Prof. Struve has
+estimated at 20,500,000 the number of stars seen in the Milky-way
+through the twenty-foot telescope employed by Herschel in his
+star-gaugings. Great as this number may seem, it is yet far below the
+truth; as the great modern telescopes, according to Professor Newcomb,
+would very probably double the number of stars seen through Herschel's
+largest telescope, and detect from thirty to fifty millions of stars in
+the Milky-way.
+</p>
+<p>
+Although the telescope resolves the Galaxy into millions of stars, yet
+the largest instruments fail to penetrate its immense depths. The
+forty-foot telescope of Herschel, and even the giant telescope of Lord
+Rosse, have failed to resolve the Milky-way entirely into stars, the
+most distant ones appearing in them as nebulosities upon which the
+nearer stars are seen projected, the galactic stratum being unfathomable
+by the largest telescopes yet made.
+</p>
+<p>
+The stars composing the Milky-way are very unevenly distributed, as
+might easily be supposed from the cloud-like appearance of this belt. In
+some regions they are loosely scattered, forming long rows or streams of
+various figures, while in others they congregate into star groups and
+clusters having all imaginable forms, some being compressed into very
+dense globular masses. The intervals left between the clustering masses
+are poorer in stars, and indeed some of them are even totally devoid of
+stars or nebulosity. Such are the great and small "coal-sacks" in the
+southern Galaxy. I have myself detected such a dark space devoid of
+stars and nebulosity in one of the brightest parts of the Milky-way, in
+the constellation Sagittarius, in about 17h. 45m. right ascension, and
+27° 35' south declination. It is a small miniature coal-sack or opening
+in the Galaxy, through which the sight penetrates beyond this great
+assemblage of stars. Close to this, is another narrow opening near a
+small, loose cluster.
+</p>
+<p>
+Although lacking the optical resources which now enable us to recognize
+the structure of the Milky-way, some of the ancient philosophers had
+succeeded tolerably well in their speculations regarding its nature. It
+was the opinion of Democritus, Pythagoras and Manilius, that the Galaxy
+was nothing else but a vast and confused assemblage of stars, whose
+faint light was the true cause of its milky appearance.
+</p>
+<p>
+Before the invention of the telescope, no well-founded theory in regard
+to the structure of the Milky-way could, of course, be attempted.
+Although Kepler entertained different ideas in regard to the structure
+of this great belt from those now generally admitted, yet in them may be
+found the starting point of the modern conception of the structure of
+the Galaxy and of the visible universe. In the view of this great mind,
+the Milky-way, with all its stars, formed a vast system, the centre of
+which, and of the universe, was occupied by our Sun. Kepler reasoned
+that the place of the Sun must be near the centre of the galactic belt,
+from the fact this last object appears very nearly as a great circle of
+the celestial sphere, and that its luminous intensity is about the same
+in all its parts.
+</p>
+<p>
+Half a century later, another attempt to explain the Milky-way was made
+by Wright, of Durham, who rejected the idea of an accidental and
+confused distribution of the stars as inconsistent with the appearance
+of the Galaxy, and regarded them as arranged along a fundamental plane
+corresponding to that of the Milky-way. These ideas which were
+subsequently developed and enlarged by Kant, and then by Lambert,
+constitute what is now known as Kant's theory. According to this theory,
+the stars composing the Galaxy are conceived as being uniformly arranged
+between two flat planes of considerable extension, but which are
+comparatively near together, the Sun occupying a place not very far from
+the centre of this immense starry stratum. As we view this system
+crosswise through its thinnest parts, the stars composing it appear
+scattered and comparatively few in number, but when we view it
+lengthwiser through its most extended parts, they appear condensed and
+extremely numerous, thus giving the impression of a luminous belt
+encircling the heavens. In the conception of Kant, each star was a sun,
+forming the centre of a planetary system. These systems are not
+independent, but are kept together by the bonds of universal
+gravitation. The Galaxy itself is one of these great systems, its
+principal plane being the equivalent of the zodiac in our planetary
+system, while a preponderant body, which might be Sirius, is the
+equivalent of our Sun, and keeps the galactic system together. In the
+universe there are other galaxies, but as they are too distant to be
+resolved into stars, they appear as elliptical nebulæ. Such are, in
+brief, the grand speculations of Kant and Lambert on the Milky-way, and
+the structure of the universe.
+</p>
+<p>
+Kant's theory rested more on conjectures than on observed facts, and
+needed therefore the sanction of direct observations to be established
+on a firm basis. With this view, Sir William Herschel investigated the
+subject, by a long and laborious series of observations. His plan, which
+was that of "star-gauging," consisted in counting all the stars visible
+in his twenty-foot telescope, comprised in a wide belt cutting the
+Galaxy at right angles, and extending from one of its sides to the
+opposite one, thus embracing 180° of the celestial sphere. In this belt
+he executed 3,400 telescopic star-gaugings of a quarter of a degree
+each, from which he obtained 683 mean gaugings giving the stellar
+density of the corresponding regions.
+</p>
+<p>
+The general result derived from this immense labor was that the stars
+are fewest in regions the most distant from the galactic belt; while
+from these regions, which correspond to the pole of the Galaxy, they
+gradually increase in number in approaching the Milky-way. The star
+density was found to be extremely variable, and while some of the
+telescopic gaugings detected either no star at all, or only one or two,
+other gaugings gave 500 stars and even more. The average number of stars
+in a field of view of his telescope, obtained for the six zones, each of
+15°, into which Herschel divided up the portion of his observing belt,
+extending from the Galaxy to its pole, is as follows: In the first zone,
+commencing at 90° from the galactic belt and extending towards it, 4
+stars per telescopic field were found; 5 in the second; 8 in the third;
+14 in the fourth; 24 in the fifth and 53 in the sixth, which terminated
+in the Galaxy itself. Very nearly similar results were afterwards found
+by Sir John Herschel, for corresponding regions in the southern
+hemisphere.
+</p>
+<p>
+From these studies, Herschel concluded that the stellar system is of the
+general form supposed by the Kantian theory, and that its diameter must
+be five times as extended in the direction of the galactic plane, as it
+is in a direction perpendicular to it. To explain the great branch sent
+out by the Galaxy in Cygnus, he supposed a great cleft dividing the
+system edgewise, about half way from its circumference to its centre.
+From suppositions founded on the apparent magnitude and arrangement of
+stars, he estimated that it would take light about 7,000 years to reach
+us from the extremities of the Galaxy, and therefore 14,000 years to
+travel across the system, from one border to the opposite one.
+</p>
+<p>
+But Herschel's theory concerning the Milky-way rested on the erroneous
+assumption that the stars are uniformly distributed in space, and also
+that his telescopes penetrated through the entire depth of the Galaxy.
+Further study showed him that his telescope of twenty feet, and even his
+great forty-foot telescope, which was estimated to penetrate to a
+distance 2,300 times that of stars of the first magnitude, failed to
+resolve some parts of the Galaxy into stars. Meanwhile, the structure of
+the Milky-way being better known, the irregular condensation of its
+stars became apparent, while the mutual relation existing between binary
+and multiple systems of stars, as also between the stars which form
+clusters, was recognized, as showing evidence of closer association
+between certain groups of stars than between the stars in general.
+Herschel's system, which rested on the assumption of the uniform
+distribution of the stars in space, and on the supposition that the
+telescopes used for his gauges penetrated through the greater depths of
+the Galaxy, being thus found to contradict the facts, was gradually
+abandoned by its author, who adopted another method of estimating the
+relative distances of the stars observed in his gaugings.
+</p>
+<p>
+This method, founded on photometric principles, consisted in judging the
+penetrating power of his telescope by the brightness of the stars, and
+not, as formerly, by the number which they brought into view. He then
+studied by this new method the structure of the Milky-way and the
+probable distance of the clustering masses of which it is formed,
+concluding that the portion of the Galaxy traversing the constellation
+Orion is the nearest to us. This last result seems indicated by the fact
+that this portion of the Milky-way is the faintest and the most uniform
+of all the galactic belt.
+</p>
+<p>
+More recently Otto Struve investigated the same subject, and arrived at
+very nearly similar conclusions, which may be briefly stated as follows:
+The galactic system is composed of a countless number of stars,
+spreading out on all sides along a very extended plane. These stars,
+which are very unevenly distributed, show a decided tendency to cluster
+together into individual groups of different sizes and forms, separated
+by comparatively vacant spaces. This layer where the stars congregate in
+such vast numbers may be conceived as a very irregular flat disk,
+sending many branches in various directions, and having a diameter eight
+or ten times its thickness. The size of this starry disk cannot be
+determined, since it is unfathomable in some directions, even when
+examined with the largest telescopes. The Sun, with its attending
+planets, is involved in this immense congregation of suns, of which it
+forms but a small particle, occupying a position at some distance from
+the principal plane of the Galaxy. According to Struve, this distance is
+approximately equal to 208,000 times the radius of the Earth's orbit.
+The Milky-way is mainly composed of star-clusters, two-thirds, perhaps,
+of the whole number visible in the heavens being involved in this great
+belt. In conclusion, our Sun is only one of the individual stars which
+constitute the galactic system, and each of these stars itself is a sun
+similar to our Sun. These individual suns are not independent, but are
+associated in groups varying in number from a few to several thousands,
+the Galaxy itself being nothing but an immense aggregation of such
+clusters, whose whole number of individual suns probably ranges between
+thirty and fifty millions. In this vast system our globe is so
+insignificant that it cannot even be regarded as one of its members.
+According to Dr. Gould, there are reasons to believe that our Sun is a
+member of a small, flattened, bifid cluster, composed of more than 400
+stars, ranging between the first and seventh magnitude, its position in
+this small system being eccentric, but not very far from the galactic
+plane.
+</p>
+<p>
+The study of the Milky-way, of which Plate XIII. is only a part, was
+undertaken to answer a friendly appeal made by Mr. A. Marth, in the
+Monthly Notices of the Royal Astronomical Society, in 1872. I take
+pleasure in offering him my thanks for the suggestion, and for the
+facility afforded me in this study by his "List of Co-ordinates of Stars
+within and near the Milky-way," which was published with it.
+</p>
+
+<p><br /><br /><br /></p>
+
+<h4><a id="THE_STAR_CLUSTERS">THE STAR-CLUSTERS</a>
+<br /><br />
+PLATE XIV</h4>
+
+<p>
+It is a well-known fact that the stars visible to the naked eye are very
+unequally distributed in the heavens, and that while they are loosely
+scattered in some regions, in others they are comparatively numerous,
+sometimes forming groups in which they appear quite close together.
+</p>
+<p>
+In our northern sky are found a few such agglomerations of stars, which
+are familiar objects to all observers of celestial objects. In the
+constellation Coma Berenices, the stars are small, but quite condensed,
+and form a loosely scattered, faint group. In Taurus, the Hyades and the
+Pleiades, visible during our winter nights, are conspicuous and familiar
+objects which cannot fail to be recognized. In the last group, six stars
+may be easily detected by ordinary eyes on any clear night, but more can
+sometimes be seen; on rare occasions, when the sky was especially
+favorable, I have detected eleven clearly and suspected several others.
+The six stars ordinarily visible, are in order of decreasing brightness,
+as follows: Alcyone, Electra, Atlas, Maia, Taygeta and Merope. Glimpses
+of Celano and Pleione are sometimes obtained.
+</p>
+<p>
+When the sky is examined with some attention on any clear, moonless
+night, small, hazy, luminous patches, having a cometary aspect, are
+visible here and there to the naked eye. In the constellation Cancer is
+found one of the most conspicuous, called Præsepe, which forms a small
+triangle with the two stars <i>γ</i> and <i>δ</i>. In Perseus, and
+involved in the Milky-way, is found another luminous cloud, situated in
+the sword-handle, and almost in a line with the two stars <i>γ</i> and
+<i>δ</i> of Cassiopeia's Chair. In the constellation Hercules, another
+nebulous mass of light, but fainter, is also visible between the stars
+<i>η</i> and <i>ζ</i> where it appears as a faint comet, in the depths
+of space. In Ophiuchus and Monoceros are likewise found hazy, luminous
+patches. In the southern sky, several such objects are also visible to
+the naked eye, being found in Sagittarius, in Canis Major and in Puppis;
+but the most conspicuous are those in Centaurus and Toucan. That in
+Centaurus involves the star <i>ω</i> in its pale diffused nebulosity,
+and that in Toucan is involved in the lesser Magellanic cloud.
+</p>
+<p>
+When the telescope is directed to these nebulous objects, their hazy,
+ill-defined aspect disappears, and they are found to consist of
+individual stars of different magnitudes, which being more or less
+closely grouped together, apparently form a system of their own. These
+groups, which are so well adapted to give us an insight into the
+structure and the vastness of the stellar universe, are called
+<i>Star-clusters</i>.
+</p>
+<p>
+Star-clusters are found of all degrees of aggregation, and while in some
+of them, such as in the Pleiades, in Præsepe and in Perseus, the stars
+are so loosely scattered that an opera glass, and even the naked eye,
+will resolve them; in others, such as in those situated in Hercules,
+Aquarius, Toucan and Centaurus, they are so greatly compressed that even
+in the largest telescopes they appear as a confused mass of blazing
+dust, in which comparatively few individual stars can be distinctly
+recognized. Although only about a dozen Star-clusters can be seen in the
+sky with the naked eye, yet nearly eleven hundred such objects visible
+through the telescope, have been catalogued by astronomers.
+</p>
+<p>
+The stars composing the different clusters visible in the heavens vary
+greatly in number, and while in some clusters there are only a few, in
+others they are so numerous and crowded that it would be idle to try to
+count them, their number amounting to several thousands. It has been
+calculated by Herschel that some clusters are so closely condensed, that
+in an area not more than ⅒ part of that covered by the Moon, at least
+5,000 stars are agglomerated.
+</p>
+<p>
+When the group in the Pleiades is seen through the telescope it appears
+more important than it does to the naked eye, and several hundreds of
+stars are found in it. In a study of Tempel's nebula, which is involved
+in the Pleiades, I have mapped out 250 stars, mostly comprised within
+this nebula, with the telescope of 6⅓ inches aperture, which I have
+used for this study.
+</p>
+<p>
+As a type of a loose, coarse cluster, that in Perseus is one of the
+finest of its class. It appears to the naked eye as a single object, but
+in the telescope it has two centres of condensation, around which
+cluster a great number of bright stars, forming various curves and
+festoons of great beauty. Among its components are found several yellow
+and red stars, which give a most beautiful contrast of colors in this
+gorgeous and sparkling region. In a study which I have made of this twin
+cluster, I have mapped out 664 stars belonging to it, among which are
+two yellow and five red stars.
+</p>
+<p>
+While some clusters, like those just described, are very easily
+resolvable into stars with the smallest instruments, others yield with
+the greatest difficulty, even to the largest telescopes, in which their
+starry nature is barely suspected. Owing to this peculiarity,
+star-clusters are usually divided into two principal classes. In the
+first class are comprised all the clusters which have been plainly
+resolved into stars, and in the second all those which, although not
+plainly resolvable with the largest instruments now at our disposal,
+show a decided tendency to resolvability, and convey the impression that
+an increase of power in telescopes is the only thing needed to resolve
+them into stars. Of course this classification, which depends on the
+power of telescopes to decide the nature of these objects, is arbitrary,
+and a classification based on spectrum analysis is now substituted for
+it.
+</p>
+<p>
+The star-clusters are also divided into globular and irregular clusters,
+according to their general form and appearance. The globular clusters,
+which are the most numerous, are usually well-defined objects, more or
+less circular in their general outlines. The rapid increase of
+brightness towards their centres, where the stars composing them are
+greatly condensed, readily conveys the impression that the general form
+of these sparkling masses is globular. The irregular clusters are not so
+rich in stars as the former. Usually their stars are less condensed
+towards the centre, and are, for the most part, so loosely and
+irregularly distributed, that it is impossible to recognize the outlines
+of these clusters or to decide where they terminate. The globular
+clusters are usually quite easily resolvable into stars, either partly
+or wholly, although some among them do not show the least traces of
+resolvability, even in the largest instruments. This may result from
+different causes, and may be attributed either to the minuteness of
+their components or to their great distance from the Earth, many
+star-clusters being at such immense distances that they are beyond our
+means of measurement.
+</p>
+<p>
+As has been shown in the preceding section, the star-clusters are found
+in great number in the Galaxy; indeed, it is in this region and in its
+vicinity that the greater portion of them are found. In other regions,
+with the exception of the Magellanic clouds, where they are found in
+great number and in every stage of resolution, the clusters are few and
+scattered.
+</p>
+
+<p><br /></p>
+
+<div class="figcenter" style="width: 400px;">
+<a id="figure14"></a>
+<br />
+<img src="images/figure14.jpg" width="400" alt="" />
+<div class="caption">
+<p>PLATE XIV.&mdash;STAR-CLUSTER IN HERCULES.</p>
+<p class="smaller">From a study made in June, 1877</p>
+</div></div>
+
+<p><br /></p>
+
+<p>
+The star-cluster in the constellation Hercules, designated as No. 4,230
+in Sir J. Herschel's catalogue, and which is represented on Plate XIV.,
+is one of the brightest and most condensed in the northern hemisphere,
+although it is not so extended as several others, its angular diameter
+being only 7' or 8'. This object, which was discovered by Halley in
+1714, is one of the most beautiful of its class in the heavens.
+According to Herschel, it is composed of thousands of stars between the
+tenth and fifteenth magnitudes. Undoubtedly the stars composing this
+group are very numerous, although those which can be distinctly seen as
+individual stars, and whose position can be determined, are not so many
+as a superficial look at the object would lead us to suppose. From a
+long study of this cluster, which I have made with instruments of
+various apertures, I have not been able to identify more stars than are
+represented on the plate, although the nebulosity of which this object
+mainly consists, and especially the region situated towards its centre,
+appeared at times granular and blazing with countless points of light,
+too faint and too flickering to be individually recognized. Towards its
+centre there is quite an extended region, whose luminous intensity is
+very great, and which irresistibly conveys the impression of the
+globular structure of this cluster. Besides several outlying appendages,
+formed by its nebulosity, the larger stars recognized in this cluster
+are scattered and distributed in such a way that they form various
+branches, corresponding with those formed by the irresolvable
+nebulosity. At least six or seven of these branches and wings are
+recognized, some of which are curved and bent in various ways, thus
+giving this object a distant resemblance to some crustacean forms.
+Although I have looked for it with care, I have failed to recognize the
+spiral structure attributed to this object by several observers. Among
+the six appendages which I have recognized, some are slightly curved;
+but their curves are sometimes in opposite directions, and two branches
+of the upper portion make so short a bend that they resemble a claw
+rather than a spiral wing. The spectrum of this cluster, like that of
+many objects of its class, is continuous, with the red end deficient.
+</p>
+<p>
+A little to the north-east of this object is found the cluster No.
+4,294, which, although smaller and less bright than the preceding, is
+still quite interesting. It appears as a distinctly globular cluster
+without wings, and much condensed towards its centre. The stars
+individually recognized in it, although less bright than those of the
+other cluster, are so very curiously distributed in curved lines that
+they give a peculiar appearance to this condensed region.
+</p>
+<p>
+A little to the north of <i>γ</i> Centauri may be found the great <i>ω</i>
+Centauri cluster, No. 3,531, already referred to above. This magnificent
+object, which appears as a blazing globe 20' in diameter, is, according
+to Herschel, the richest in the sky, and is resolved into a countless
+number of stars from the twelfth to the fifteenth magnitude, which are
+greatly compressed towards the centre. The larger stars are so arranged
+as to form a sort of net-work, with two dark spaces in the middle.
+</p>
+<p>
+The great globular cluster No. 52, involved in the lesser Magellanic
+cloud, in the constellation Toucan, is a beautiful and remarkable
+object. It is composed of three distinct, concentric layers of stars,
+varying in brightness and in degree of condensation in each layer. The
+central mass, which is the largest and most brilliant, is composed of an
+immense number of stars greatly compressed, whose reddish color gives to
+this blazing circle a splendid appearance. Around the sparkling centre
+is a broad circle, composed of less compressed stars, this circle being
+itself involved in another circular layer, where the stars are fainter
+and more scattered and gradually fade away.
+</p>
+<p>
+Many other great globular clusters are found in various parts of the
+heavens, among which may be mentioned the cluster No. 4,678, in
+Aquarius. This object is composed of several thousand stars of the
+fifteenth magnitude, greatly condensed towards the centre, and, as
+remarked by Sir J. Herschel, since the brightness of this cluster does
+not exceed that of a star of the sixth magnitude, it follows that in
+this case several thousand stars of the fifteenth magnitude equal only a
+star of the sixth magnitude. In the constellation Serpens the globular
+clusters No. 4,083 and No. 4,118 are both conspicuous objects, also No.
+4,687 in Capricornus. In Scutum Sobieskii the cluster No. 4,437 is one
+of the most remarkable of this region. The stars composing it, which are
+quite large and easily made out separately, form various figures, in
+which the square predominates.
+</p>
+<p>
+Among the loose irregular clusters, some are very remarkable for the
+curious arrangement of their stars. In the constellation Gemini the
+cluster No. 1,360, which is visible to the naked eye, is a magnificent
+object seen through the telescope, in which its sparkling stars form
+curves and festoons of great elegance. The cluster No. 1,467, of the
+same constellation, is remarkable for its triangular form. In the
+constellation Ara the cluster No. 4,233, composed of loosely scattered
+stars, forming various lines and curves, is enclosed on three sides by
+nearly straight single lines of stars. In Scorpio the cluster No. 4,224
+is still more curious, being composed of a continuous ring of loosely
+scattered stars, inside of which is a round, loose cluster, which is
+divided into four parts by a dark cross-shaped gap, in which no stars
+are visible.
+</p>
+<p>
+Among the 1,034 objects which are now classified as clusters more or
+less resolvable, 565 have been absolutely resolved into stars, and 469
+have been only partly resolved, but are considered as belonging to this
+class of objects. In Sir J. Herschel's catalogue there are 102 clusters
+which are Considered as being globular; among them 30 have been
+positively resolved into stars.
+</p>
+<p>
+The agglomeration of thousands of stars into a globular cluster cannot
+be conceived, of course, to be simply the result of chance. This
+globular form seems clearly to indicate the existence of some bond of
+union, some general attractive force acting between the different
+members of these systems, which keeps them together, and condenses them
+towards the centre. Herschel regards the loose, irregular clusters as
+systems in a less advanced stage of condensation, but gradually
+concentrating by their mutual attraction into the globular form.
+Although the stars of some globular clusters appear very close together,
+they are not necessarily so, and may be separated by great intervals of
+space. It has been shown that the clusters are agglomeration of suns,
+and that our Sun itself is a member of a cluster composed of several
+hundreds of suns, although, from our point of observation, these do not
+seem very close together. So far as known, the nearest star to us is a
+Centauri, but its distance from the Earth equals 221,000 times the
+distance of the Sun from our globe, a distance which cannot be traversed
+by light in less than three years and five months. It seems very
+probable that if the suns composing the globular clusters appear so near
+together, it is because, in the first place, they are at immense
+distances from us, and in the second, because they appear nearly in a
+line with other suns, which are at a still greater distance from us, and
+on which they accordingly are nearly projected. If one should imagine
+himself placed at the centre of the cluster in Hercules, for instance,
+the stars, which from our Earth seems to be so closely grouped, would
+then quite likely appear very loosely scattered around him in the sky,
+and would resemble the fixed stars as seen from our terrestrial station.
+</p>
+<p>
+Judging by their loose and irregular distribution, the easily resolvable
+clusters would appear, in general, to be the nearer to us. It is
+probable that the globular clusters do not possess, to a very great
+degree, the regular form which they ordinarily present to us. It seems
+rather more natural to infer that they are irregular, and composed of
+many wings and branches, such as are observed in the cluster in
+Hercules; but as these appendages would necessarily be much poorer in
+stars than the central portions, they would be likely to become
+invisible at a great distance, and therefore the object would appear
+more or less globular; the globular form being simply given by the close
+grouping of the stars in the central portion. It would seem, then, that
+in general, the most loosely scattered and irregular clusters are the
+nearest to us, while the smallest globular clusters and those resolvable
+with most difficulty are the most distant.
+</p>
+<p>
+In accordance with the theory that the clusters are composed of stars,
+the spectrum of these objects is in general continuous; although, in
+many cases, the red end of the spectrum is either very faint or
+altogether wanting. Many objects presenting in a very high degree the
+principal characteristics exhibited by the true star-clusters, namely, a
+circular or oval mass, whose luminous intensity is greatly condensed
+toward the centre, have not yielded, however, to the resolving power of
+the largest telescopes, although their continuous spectrum is in close
+agreement with their general resemblance to the star-clusters. Although
+such objects may remain irresolvable forever, yet it is highly probable
+that they do not materially differ from the resolvable and partly
+resolvable clusters, except by their enormous distance from us, which
+probably reaches the extreme boundary of our visible universe.
+</p>
+
+<p><br /><br /><br /></p>
+
+<h4><a id="THE_NEBULAE">THE NEBULÆ</a>
+<br /><br />
+PLATE XV</h4>
+
+<p>
+Besides the foggy, luminous patches which have just been described, a
+few hazy spots of a different kind are also visible to the naked eye on
+any clear, moonless night. These objects mainly differ from the former
+in this particular, that when viewed through the largest telescopes in
+existence they are not resolved into stars, but still retain the same
+cloudy appearance which they present to the unassisted eye. On account
+of the misty and vaporous appearance which they exhibit, these objects
+have been called <i>Nebulæ</i>.
+</p>
+<p>
+Of the 26 nebulous objects visible to the naked eye in the whole
+heavens, 19 belong to the class of star-clusters, and 7 to the class of
+nebulæ. Among the most conspicuous nebulæ visible to the unassisted
+eye, are those in the constellations Argo Navis, Andromeda and Orion.
+</p>
+<p>
+Besides the seven nebulæ visible to the naked eye, a great number of
+similar objects are visible through the telescope. In Sir John
+Herschel's catalogue of nebulæ and clusters, are found 4,053
+irresolvable nebulæ, and with every increase of the aperture of
+telescopes, new nebulæ, invisible in smaller instruments, are found.
+Notwithstanding their irresolvability it is probable, however, that many
+among them have a stellar structure, which their immense distance
+prevents us from recognizing, and are not therefore true nebulæ. The
+giant telescope of Lord Rosse has shown nebulæ so remote that it has
+been estimated that it takes their light 30 million years to reach the
+Earth.
+</p>
+<p>
+The nebulæ are very far from being uniformly distributed in space. In
+some regions they are rare, while in others they are numerous and
+crowded together, forming many small, irregular groups, differing in
+size and in richness of aggregation. The grouping of the nebulæ does
+not occur at random in any part of the heavens, as might naturally be
+supposed, but, on the contrary, it is chiefly confined to certain
+regions. Outside of these regions nebulæ are rare and are separated
+from each other by immense intervals; so that these isolated objects
+appear as if they were lost wanderers from the great nebulous systems.
+</p>
+<p>
+The regions where the nebulæ congregate in great number are very
+extensive, and in a general view there are two vast systems of nebular
+agglomeration, occupying almost opposite points of the heavens, whose
+centres are not very distant from the poles of the Milky-way. In the
+northern hemisphere, the nebulous system is much richer and more
+condensed than in the southern hemisphere. The northern nebulæ are
+principally contained in the constellations Ursa Minor and Major, in
+Draco, Canes Venatici, Bootes, Leo Major and Minor, Coma Berenices, and
+Virgo. In this region, which occupies about ⅛ of the whole surface of
+the heavens, ⅓ of the known nebulæ are assembled. The southern
+nebulæ are more evenly distributed and less numerous, with the
+exception of two comparatively small, but very remarkable centres of
+condensation which, together with many star-clusters, constitute the
+Magellanic clouds.
+</p>
+<p>
+These two vast nebular groups are by no means regular in outline, and
+send various branches toward each other. They are separated by a wide
+and very irregular belt, comparatively free from nebulæ, which
+encircles the celestial sphere, and whose medial line approximately
+coincides with that of the galactic belt. The Milky-way, so rich in
+star-clusters, is very barren in nebulæ; but it is a very remarkable
+fact, nevertheless, that almost all the brightest, largest, and most
+complicated nebulæ of the heavens are situated either within it, or in
+its immediate vicinity. Such are the great nebulæ in Orion and
+Andromeda; the nebula of <i>ζ</i> Orionis; the Ring nebula in Lyra; the
+bifurcate nebula in Cygnus; the Dumb-bell nebula in Vulpecula; the Fan,
+Horse-shoe, Trifid and Winged nebulæ in Sagittarius; the great nebula
+around <i>η</i> Argus Navis, and the Crab nebula in Taurus.
+</p>
+<p>
+Aside from the discovery of some of the largest nebulæ by different
+observers, and their subsequent arrangement in catalogues by Lacaille
+and Messier, very little had been done towards the study of these
+objects before 1779, when Sir W. Herschel began to observe them with the
+earnestness of purpose which was one of the distinctive points of the
+character of this great man. He successively published three catalogues
+in 1786, 1789, and 1802, in which the position of 2,500 nebulous objects
+was given. This number was more than doubled before 1864, when Sir John
+Herschel published his catalogue of 5,079 nebulæ and star-clusters. To
+this long list must be added several hundred similar objects, since
+discovered by D'Arrest, Stephan, Gould and others. But, as has been
+shown above, among the so-called nebulæ are many star-clusters which do
+not properly belong to the same class of objects, it being sometimes
+impossible in the present state of our knowledge to know whether a
+nebulous object belongs to one class or to the other.
+</p>
+<p>
+The nebulæ exhibit a great variety of forms and appearances, and, in
+accordance with their most typical characters, they are usually divided
+into several classes, which are: the Nebulous stars, the Circular, or
+Planetary, the Elliptical, the Annular, the Spiral and Irregular
+nebulæ.
+</p>
+
+<p><br /></p>
+
+<div class="figcenter" style="width: 400px;">
+<a id="figure15"></a>
+<br />
+<img src="images/figure15.jpg" width="400" alt="" />
+<div class="caption">
+<p>PLATE XV.&mdash;THE GREAT NEBULA IN ORION.</p>
+<p class="smaller">From a study made in the years 1875-76</p>
+</div></div>
+
+<p><br /></p>
+
+<p>
+The so-called nebulous stars consist of a faint nebulosity, usually
+circular, surrounding a bright and sharp star, which generally occupies
+its centre. The nebulosity surrounding these stars varies in brightness
+as well as in extent, and while, in general, its light gradually fades
+away, it sometimes terminates quite suddenly. Such nebulosities are
+usually brighter and more condensed towards the central star. The stars
+thus surrounded do not seem, however, to be distinguished from others by
+any additional peculiarity. Some nebulæ of this kind are round, with
+one star in the centre; others are oval and have two stars, one at each of
+their foci. The nebulous star, <i>τ</i> Orionis, represented at the upper
+part of Plate XV., above the great nebula, has a bright star at its
+centre and two smaller ones on the side. The association of double stars
+with nebulæ is very remarkable, and may in some cases indicate a mutual
+relation between them.
+</p>
+<p>
+The so-called planetary nebulæ derive their name from their likeness to
+the planets, which they resemble in a more or less equable distribution
+of light and in their round or slightly oval form. While some of them
+have edges comparatively sharp and well defined, the outlines of others
+are more hazy and diffused. These nebulæ, which are frequently of a
+bluish tint, are comparatively rare objects, and most of those known
+belong to the southern hemisphere. When seen through large telescopes,
+however, they present a different aspect, and their apparent uniformity
+changes. The largest of these objects, No. 2,343 of the General
+Catalogue, is situated in the Great Bear, close to the star <i>β</i>. Its
+apparent diameter is, according to Sir J. Herschel, 2', 40", and "its
+light is equable, except at the edge, where it is a little hazy." In a
+study which I made of this object in 1876, with a refractor 6⅓ inches
+in aperture, I found it decidedly brighter on the preceding side, where
+the brightest part is crescent shaped. In Lord Rosse's telescope its
+disk is transformed into a luminous ring with a fringed border, and two
+small star-like condensations are found within. Another planetary
+nebula, near <i>χ</i> Andromedæ, has also shown an annular structure in
+Rosse's telescope.
+</p>
+<p>
+The elliptical nebulæ, as their name implies, are elongated, elliptical
+objects; but while some of them are only slightly elongated ovals,
+others form ellipses whose eccentricity is so great that they appear
+almost linear. In all these objects the light is more or less condensed
+towards the centre; but while in some of them the condensation is
+gradual and slight, in others it is so great and sudden that the centre
+of the nebula appears as a large diffused star, somewhat resembling the
+nucleus of a comet. From the general appearance of these objects, it is
+not unlikely that some of them are either flattish, nebulous disks, like
+the planetary nebulæ, or nebulous rings, seen more or less sidewise.
+The condensation of light at their centres does not appear to be
+stellar, but nebulous like the rest, and it is a remarkable fact that
+very few, if any, of these objects are resolvable into stars.
+</p>
+<p>
+Several elliptical nebulæ are remarkable for having a star at or near
+each of their foci, or at each of their extremities. Such are the
+elliptical nebulæ in Draco, Centaurus, and Sagittarius, Nos. 4,419,
+3,706 and 4,395 of the General Catalogue, the last of which is in the
+vicinity of the triple star <i>μ</i> Sagitarii. Each of these nebulæ has a
+star at each of its foci, while No. 1, in Cetus, has a star at each of
+its extremities.
+</p>
+<p>
+Among the most remarkable elliptic nebulæ may be mentioned Nos. 1,861
+and 2,373 of Sir J. Herschel's catalogue, both situated in the
+constellation Leo. The first is one of Lord Rosse's spiral nebulæ, and
+the last, which is a very elongated object, is formed of concentric oval
+rings, which are especially visible towards its central part. The
+constellation Draco is particularly remarkable for the number of
+elliptical nebulæ found within its boundaries. Among them are Nos.
+3,939, 4,058, 4,064, 4,087, 4,415, etc., which are quite remarkable
+objects of their class. No. 4,058, of which I have made a study, is
+bright, and has a decided lenticular form with a condensation in the
+centre. Its following edge is better defined than the preceding. In Lord
+Rosse's telescope this object exhibits a narrow, dark, longitudinal, gap
+in its interior.
+</p>
+<p>
+By far the largest and the finest object of this class is the great
+nebula in Andromeda. Although this object belongs rather to the class of
+irregular nebulæ, yet it is generally considered as an elliptic nebula,
+since its complicated structure, being less prominent, was not
+recognized until 1848, when it was perceived by George P. Bond, Director
+of the Harvard College Observatory. This, the first nebula discovered,
+was found in 1612 by Simon Marius. It is situated in the constellation
+Andromeda, in the vicinity of the star <i>ν</i>, and almost in a line with
+the stars <i>μ</i> and <i>β</i> of the same constellation. It is visible to
+the naked eye, and appears as a faint comet-like object. It is represented
+at the upper left hand corner of Plate XIII., on the border of the
+Milky-way, as it appears to the naked eye.
+</p>
+<p>
+The nebula in Andromeda is one of the brightest in the heavens, and is
+closely attended by two smaller nebulæ. Perhaps it would be rather more
+correct to say that it has three centres of condensation, as the two
+small nebulæ referred to are entirely involved in the same faint and
+extensive nebulosity. Its general form is that of an irregular oval,
+upwards of one degree in breadth and two and a half degrees in length.
+Its brightest and most prominent part, which alone was seen by the
+earlier observers, consists in a very elongated lenticular mass, which
+gradually condenses towards its centre into a blazing, star-like
+nucleus, surrounded by a brilliant nebulous mass. At a little distance
+to the south of this central condensation is found one of the lesser
+centres of condensation noted above, which is globular in appearance,
+with a bright, star-like nucleus like the former. The other centre of
+condensation is found to the north-west of the centre of the principal
+mass, and is quite elongated, with a centre of condensation towards its
+southern extremity, but it is not so bright as the others. Close to the
+western edge of the bright lenticular mass first described, and making a
+very slight angle with its longer axis, are found two narrow and nearly
+rectilinear dark rifts, running almost parallel to each other, and both
+terminating in a slender point in the south. These dark rifts, which are
+almost totally devoid of nebulous matter, are quite rare in nebulæ, and
+afford a good opportunity to watch the changes which this part of the
+nebulæ may undergo.
+</p>
+<p>
+This nebula has never been positively resolved into stars, although
+Prof. Geo. Bond and others have strongly suspected its resolvability. In
+a study which I have made of it, with the same instrument employed by
+Bond, and also with the great Washington telescope, I detected a decided
+mottled appearance in several places, which might be attributable to a
+beginning of resolvability; but I do not consider this a conclusive
+indication that the nebula is resolvable. The continuous spectrum given
+by this nebula, showing that it is not in the gaseous state which its
+appearance seems to indicate, warrants the conclusion, however, that it
+will ultimately be found to be resolvable. This object, being situated
+on the edge of the Galaxy and involved in its diffused light, has a
+great number of small stars belonging to this belt projected upon it.
+During my observations I have mapped out 1,323 of these stars, none of
+which seems to be in physical connection with the nebula.
+</p>
+<p>
+Among the circular and elliptical nebulæ a few exhibit a very
+remarkable structure, being apparently perforated, and forming either
+round, slightly oval, or elongated rings of great beauty. These Annular
+nebulæ are among the rarest objects in the heavens. In Scorpio, two
+such nebulæ are found involved in the Milky-way, and also one in
+Cygnus. One of those in Scorpio has two stars involved within its ring,
+at the extremities of its smallest interior diameter. A very elongated
+nebula in the vicinity of the fine triple star <i>γ</i> Andromedæ is also
+annular, and has two stars symmetrically placed at the extremities of
+its greatest interior axis. Another elongated annular nebula is also
+found north of <i>η</i> Pegasi.
+</p>
+<p>
+The grandest and most remarkable of the annular nebulæ is found in the
+constellation Lyra, about midway between the two stars <i>β</i> and
+<i>γ</i>. It is slightly elliptical in form, and according to Prof. E.
+S. Holden, its major axis is 77".3 and the minor 58". From a study and
+several drawings which I have made of this object, with instruments of
+various apertures, I have found it decidedly brighter towards its outer
+border, at the extremities of its minor axis, than at the ends of the
+major axis. On very favorable occasions, some of its brightest parts
+have appeared decidedly, but very faintly mottled, and I have recognized
+three small centres of condensation. Its interior, in which Professor
+Holden has detected a very faint star, is quite strongly nebulous. In
+Lord Rosse's telescope, this nebula is completely surrounded by wisps
+and appendages of all sorts of forms, which I have failed to trace,
+however, both with the refractor of the Harvard College Observatory and
+with that of the Naval Observatory at Washington; Rosse, Secchi and
+Chacornac, have seen this nebula glittering as if it were a "heap of
+star dust," although its spectrum indicates that it is gaseous.
+</p>
+<p>
+The nebula No. 1,541, in Camelopardus, of which I have also made a study
+and a drawing, is closely allied to the class of annular nebulæ. This
+object, which is quite bright, has a remarkable appearance. It consists
+principally of somewhat more than half of an oval ring, surrounding a
+bright, nebulous mass which condenses around a star; this mass being
+separated from the imperfect ring by a dark interval. Upon the bright
+portion of the ring, and on opposite points, are found two bright stars,
+between which lies the star occupying the central mass. The central mass
+extends at some distance outside of the ring on its open side. Several
+stars are involved in this object.
+</p>
+<p>
+The Spiral nebulæ are very curious and complicated objects, but they
+are visible only in the largest telescopes. Prominent above all is the
+double spiral nebula No. 3,572, in Canes Venatici, which is not far from
+<i>η</i> Ursa Majoris. In Lord Rosse's telescope, this object presents a
+wonderful spiral disposition, looking somewhat like one of the
+fire-works called pin-wheels, and forming long, curved wisps, diverging
+from two bright centres. The spectrum of this object, however, is not
+that of a gas. In the constellation Virgo, Rosse has detected another
+such nebula. In Cepheus, Triangulum, and Ursa Major, are found other
+spiral nebulæ of smaller size. Lord Rosse has recognized 40 spiral
+nebulæ and suspected a similar structure in 30 others.
+</p>
+<p>
+The class of the Irregular nebulæ, which will be now considered,
+differs greatly in character from the others, and includes the largest,
+the brightest and the most extraordinary nebulæ in the heavens. The
+nebulæ of this class differ from those belonging to the other classes
+by a want of symmetry in their form and in the distribution of their
+light, as well as by their capricious shapes, and their very complicated
+structure. Another and perhaps the principal difference between them and
+the objects above described, consists in the remarkable fact already
+stated, that they are not, except in rare cases, to be found in the
+regions where the other nebulæ abound. On the contrary, they are found
+in or very near the Milky-way, precisely where the other nebulæ are the
+most rare. This fact, recognized by Sir J. Herschel, led him to consider
+them as "outlying, very distant, and as it were detached fragments of
+the great stratum of the Galaxy." It seems very probable that the reason
+why these objects differ so greatly from the other nebulæ in size,
+brightness and complication of structure, is simply because they are
+much nearer to us than are most of the others. They are perhaps nebulous
+members of our Galaxy. The same remark which has been made of
+star-clusters may be applied to nebulæ. The nearer they are to us, the
+larger, the brighter and the more complicated they will appear, while
+the farther they are removed, the more simple and regular and round they
+will appear, only their brightest and deepest parts being then visible.
+</p>
+<p>
+The Crab Nebula of Lord Rosse, near <i>ζ</i> Tauri, No. 1,157, is one of
+the interesting objects of this class. It has curious appendages streaming
+off from an oval, luminous mass, which give it a distant resemblance to
+the animal from which it derives its name. The Bifid nebula in Cygnus,
+Nos. 4,400 and 4,616, is another object of this class. It consists of a
+long, narrow, crooked streak, forking out at several places, and passing
+through <i>χ</i> Cygni. Observers, having failed to recognize the
+connection existing between its different centres of brightness, have made
+distinct nebulæ of this extended object.
+</p>
+<p>
+The Dumb-bell nebula in Vulpecula, No. 4,532, is a bright and curious
+object, with a general resemblance to the instrument from which it
+derives its name. Lord Rosse's telescope has shown many stars in it,
+projected on a nebulous background, and Prof. Bond seems to have thought
+that it showed traces of resolvability, although in the study which I
+made of this nebula with the same instrument used by the latter
+observer, I failed to perceive any such traces. Dr. Huggins finds its
+spectrum gaseous.
+</p>
+<p>
+The star-cluster, No. 4,400, in Scutum Sobieskii, which is described by
+Sir J. Herschel as a loose cluster of at least 100 stars, I have found
+to be involved in an extensive, although not very bright, nebula, which
+would seem to have escaped his scrutiny. In a study and drawing of this
+nebula made in 1876, its general form is that of an open fan, with the
+exception that the handle is wanting, with deeply indented branches on
+the preceding side, where the brightest stars of the cluster are
+grouped. From its peculiar form, this object might appropriately be
+called the Fan nebula.
+</p>
+<p>
+The Omega or Horse-shoe nebula, in Sagittarius, No. 4,403, of which I
+have made a study and two drawings, one with a refractor 6⅓ inches in
+aperture, and the other conjointly with Prof. Holden, with the great
+telescope of the Naval Observatory, is a bright and very complicated
+object. Its general appearance in small instruments, with low power, is
+that of a long, narrow pisciform mass of light, from which proceeds on
+the preceding side, the great double loop from which it derives its
+name. But in the great Washington refractor its structure becomes very
+complicated, forming various bright nebulous masses and wisps of great
+extension. Prof. Holden, who has made a careful, comparative study of
+the published drawings of this object, thinks there are reasons to
+believe that its western branch has moved relatively to the stars found
+within its loop. The spectrum of this nebula is gaseous.
+</p>
+<p>
+The Trifid nebula, No. 4,355, in the same constellation, is also a very
+remarkable object, although it is not so bright as the last. This
+nebula, which I have studied with the refractor of the Cambridge
+Observatory, consists of four principal masses of light, separated by a
+wide and irregular gap branching out in several places. These masses,
+which are brighter along the dark gap, gradually fade away externally. A
+group of stars, two of which are quite bright, is found near the centre
+of the nebula, on the inner edge of the following mass, and close to the
+principal branch of the dark channel. A little to the north, and
+apparently forming a part of this nebula, is a globular-looking nebula,
+having a pale yellow star at its centre. Prof. Holden's studies on this
+nebula show that the triple star, which was centrally situated in the
+dark gap from 1784 to 1833, was found involved in the border of the
+nebulous mass following it, from 1839 to 1877; the change, he thinks,
+is attributable either to the proper motion of the group of stars or to
+that of the nebula itself.
+</p>
+<p>
+In the same vicinity is found the splendid and very extensive nebula No.
+4,361, in which is involved a loose, but very brilliant star-cluster.
+This nebula and cluster, which I have studied and drawn with a 6⅓ inch
+telescope, is very complicated in structure, and divided by a dark
+irregular gap into three principal masses of light, condensing at one
+point around a star, and at others forming long, bright, gently-curved
+branches, which give to this object a strong resemblance to the wings of
+a bird when extended upwards in the action of flying. From this
+peculiarity this object might appropriately be called the Winged nebula.
+Its spectrum is that of a gas.
+</p>
+<p>
+The variable star <i>η</i> Argus is completely surrounded by the great
+nebula of the same name, No. 2,197, first delineated by Sir J. Herschel,
+during his residence at the Cape of Good Hope, in 1838. This object,
+which covers more than ⁴⁄₇ of a square degree, is divided into
+three unequal masses, separated by dark oval spots, comparatively free
+from nebulosity, and is suspected to have undergone changes since
+Herschel's time.
+</p>
+<p>
+In the same field with the double star, <i>ζ</i> Orionis, the most easterly
+of the three bright stars in the belt of Orion, is found another
+irregular nebula of the Trifid type. From the drawings which I have made
+of this object, it appears to be composed of three principal unequal
+masses, separated by a wide, irregular, dark channel, two of the masses
+being quite complicated in structure, and forming curved, nebulous
+streams of considerable length and breadth. This nebula, like the next
+to be described, seems to be connected with the Galaxy by the great
+galactic loop described in another section.
+</p>
+<p>
+By far the most conspicuous irregular nebula visible from our northern
+States, is the great nebula in Orion, No. 1,179, represented on Plate
+XV. This object, visible to the naked eye, is the brightest and the most
+wonderful nebula in the heavens. It is situated a little to the south of
+the three bright stars in the belt of Orion, and may be readily detected
+surrounding the star <i>θ</i>, situated between and in a line with two
+faint stars, the three being in a straight line which points directly
+towards <i>ε</i>, the middle star of the three in Orion's belt. The
+area occupied by this nebula is about equal to that occupied by the
+Moon.
+</p>
+<p>
+In its brightest parts the nebula in Orion appears as a luminous cloud
+of a pentagonal form, from which issue many luminous appendages of
+various shapes and lengths. This principal mass is divided into
+secondary masses, separated by darkish, irregular intervals. These
+secondary masses in their turn appear mottled and fleecy. Towards the
+lower part of the pentagonal mass is found a roundish dark space,
+comparatively devoid of nebulosity, in which are involved four bright
+stars forming a trapezium, and several fainter ones. The four bright
+stars of the trapezium constitute the quadruple star <i>θ</i> Orionis,
+from which the nebula has received its name. The cloud-like pentagonal
+form is brightest on the north-west of the trapezium, and is surrounded
+on three sides by long, soft, curved wisps, fading insensibly into the
+outer nebulous mass in which they are involved. On the east a broad,
+wavy wing spreads out, and sends an important branch southward.
+South-east of the trapezium are found several curious dark spaces,
+comparatively devoid of nebulosity, especially those on the east, which
+give to this nebula a singular character. Close to the north-eastern
+part of the nebula, or rather in contact with it, is found a small,
+curiously-shaped nebula, condensing around a bright star into a blazing
+nucleus. From this centre it continues northward in a narrow diffused
+stream, which spreads out in passing over the stars <i>c<sup>1</sup></i>
+and <i>c<sup>2</sup></i>; and after having sent short branches
+northward, it curves back to the south and joins the main nebula on the
+west of its starting point, having thus formed a great loop which is not
+shown on the Plate. The nebula also forms a loop towards the south,
+which is partly shown on Plate XV., a small branch of which, passing
+through <i>τ</i> Orionis, the nebulous star shown at the top of the
+Plate, and extending southward, is not here represented.
+</p>
+<p>
+On ordinary nights the nebula in Orion is a splendid object, and
+inspires the observer with amazement; but this is as nothing compared
+with the grand and magnificent sight which it presents during the very
+rare moments when our atmosphere is perfectly clear and steady. I have
+seen this nebula but once under these favorable circumstances, and I was
+surprised by the grandeur of the scene. Then could be detected features
+to be seen at no other time, and its fleecy, floculent, cloud-like
+masses glittered with such intensity that it seemed as if thousands of
+stars were going to blaze out the next moment. Although I observed the
+nebula under such favorable conditions, and with the fifteen-inch
+refractor of the Cambridge Observatory, yet I was disappointed in my
+expectations, and distinguished no new stars or points of light, and
+nothing more than a very bright mass, finely divided into minute blazing
+cloudlets. Although I failed to resolve this nebula into stars, yet Lord
+Rosse, Bond and Secchi thought they had caught glimpses of star dust.
+Its spectrum, however, proves to be mainly that of incandescent gases,
+probably hydrogen and nitrogen. In the curved wisps found in this
+nebula, Lord Rosse and others saw indications of a spiral structure.
+</p>
+<p>
+Several bright stars are found scattered over this nebula, and besides
+those forming the trapezium, there are three in a row, a little to the
+south-east of that group, which are quite bright and remarkable. Among
+the stars involved in this nebula, few show signs of having a physical
+connection with it, although it seems probable that the group of the
+trapezium is so connected. Some of these stars are variable. The small
+stars represented on this Plate, as on others of the series, are
+somewhat exaggerated in size, as was unavoidable with any process of
+reproduction which could be adopted.
+</p>
+<p>
+In 1811, W. Herschel was led to suspect that some changes had occurred
+in this nebula, but changes in such complicated and delicate objects are
+not easily ascertained, since, for the most part, we have for comparison
+with our later observations only coarse drawings made by hands unskilled
+in delineation.
+</p>
+<p>
+Although comparatively rare, double and multiple nebulæ may be found in
+the sky. When this occurs, their constituents most commonly belong to
+the class of spherical nebulæ. Sometimes the components are separated
+and distinct, at other times one of them is projected upon the other,
+either really or by the effect of perspective. Sometimes one is round
+and the other elongated. It is probable that while some of these nebulæ
+are physically associated and form a system, others appear to be so only
+because they happen to be almost in a line with the observer. A double
+nebula in Draco, Nos. 4,127 and 4,128, which I have drawn, is a fair
+type of those which are separated. The first is a globular nebula, and
+the last an oval one, with a star at its centre. The double nebula, Nos.
+858 and 859, in Taurus, which I have also studied, is a type of the
+cases in which one nebula is partly projected on another. In this
+instance both the nebulæ are globular.
+</p>
+<p>
+The nebulæ in general show very little color in their light, which is
+ordinarily whitish and pale. Some, however, present a decided bluish or
+greenish tint. The great nebula in Orion has a greenish cast, and we
+have seen that some planetary nebulæ are bluish.
+</p>
+<p>
+It has been a question whether nebulæ are changing. It has already been
+stated that Prof. Holden believes there is ground to suspect that the
+Trifid and Horse-shoe nebulæ have undergone some changes. A nebula near
+<i>ε</i> Tauri has been lost and found again several times. Two other
+nebulæ in the same constellation have presented curious variations.
+One, near a star of the tenth magnitude, exhibited variations of
+brightness like those of the star itself, and for a time disappeared.
+The other, near <i>ζ</i> Tauri, increased in brightness for three months,
+after which it disappeared. In 1859 Tempel discovered a nebula in the
+Pleiades, which has shown some fluctuations. In 1875 I made a long study
+of this object, and drew it carefully a dozen times, but I was not able
+to see any changes in it within the two or three months during which my
+observations were continued. But on Nov. 24, 1876, it was found of a
+different color, being purplish and very faint. On Dec. 23, 1880, it was
+found just as bright and visible as when I drew it in 1875, and on Oct.
+20, 1881, it appeared faint and purplish again, as in 1876. On this last
+night, and on those which followed it, it was impossible for me to trace
+the nebulosity as far as in 1875. I consider this as due to a variation
+in the light of this object, which in 1875 was bright enough to be well
+seen while the Moon after her First Quarter was within ten or fifteen
+degrees from the Pleiades.
+</p>
+<p>
+From the observations of M. Laugier, it appears that some nebulæ have a
+proper motion, comparable to that of stars. From the displacement of the
+lines of their spectra by their motion in the line of sight, Dr. Huggins
+found that no nebula observed by him has a proper motion surpassing 25
+miles per second. The Ring nebula in Lyra appears to move from us at the
+rate of 3 miles per second, and that in Orion recedes about 17 miles per
+second.
+</p>
+<p>
+The important question arises, are all the irresolvable nebulæ in the
+heavens to be considered as so many star-clusters, differing only from
+them by the minuteness of their components, or their immense distance
+from us; or are they cosmical clouds, composed of luminous vapors,
+similar to the matter composing the heads and tails of comets?
+Originally, W. Herschel, with many astronomers, thought that all these
+objects were stellar aggregations, too distant to be resolved into
+stars; but he subsequently modified his opinion, and accepted the idea
+that some of them are of a gaseous nature.
+</p>
+<p>
+No direct proof that the nebulæ are gaseous could be obtained, however,
+before the spectroscope was known. The attempt to analyze the light of
+the nebulæ with this instrument was made in 1864, by Dr. Huggins, who
+directed his spectroscope to the planetary nebula, No. 4,373, in Draco.
+Its spectrum was found to consist of three bright, distinct lines, the
+brightest of which corresponded with the strongest nitrogen line, and
+the feeblest with the hydrogen C line. Besides these lines, it gave also
+a very faint, continuous spectrum, apparently due to a central point of
+condensation. By this observation, the gaseous nature of a nebula was
+for the first time demonstrated. Dr. Huggins thus analyzed 70 nebulæ,
+of which one-third gave a gaseous spectrum, consisting of several bright
+lines, the brightest of which invariably corresponded with the lines of
+nitrogen. The others gave a continuous spectrum, with the red end
+usually deficient. These results indicate that if some of the so-called
+nebulæ are due to an aggregation of stars, either too minute or too
+remote in space to be individually resolved, others are in a gaseous
+state. Yet the faint, continuous spectrum, given by some nebulæ, in
+addition to their gaseous spectra, seems to show that these nebulæ have
+some stars or matter in a different state, either involved in them or
+projected on their surface.
+</p>
+<p>
+The idea of diffused matter distributed here and there in space, and
+gradually condensing into stars, is by no means new. As early as 1572,
+Tycho Brahé proposed such an hypothesis, to explain the sudden
+apparition of a new star in Cassiopeia, which he considered as formed by
+the recent agglomeration of the "celestial matter" diffused in space.
+Kepler adopted the same idea to explain the new star which appeared in
+Ophiuchus, in 1604. Halley, Lacaille, Mairan and others, entertained the
+same opinion. The hypothesis of a self-luminous, nebulous matter
+diffused in space, and forming here and there immense masses, has been
+proposed from the origin of the telescope, and was adopted by Sir
+William Herschel, who in his grand speculations on the universe
+considered the nebulæ as immense masses of phosphorescent vapors,
+gradually condensing around one or several centres into stars or
+clusters of stars. The evidence afforded by the spectroscope seems to be
+in favor of such an hypothesis, and shows us that gaseous agglomerations
+exist in space.
+</p>
+<p>
+According to our modern conception, the visible universe is but an
+infinitely small portion of the infinite universe perceived by our mind.
+The great blazing centre around which our little, non-luminous globe
+pursues its endless journey, is only an humble member of a cluster
+comprising four hundred equally powerful suns, as they are believed to
+be, although they appear to us as little twinkling stars. The nearest of
+these stars is 221,000 times as far from the Sun as the Sun is from the
+Earth, and yet this entire cluster is only one among the several hundred
+Star-clusters composing the great galactic nebula in which we are
+involved, comprising thirty or fifty millions of such suns. Among the
+4,000 irresolvable nebulæ in the sky, perhaps over one-half are
+supposed to be galaxies, like our own galaxy, composed of star-clusters,
+and millions of stars. Besides these remote galaxies, vast
+agglomerations of yet uncondensed, nebulous matter exist in space, and
+form the nebulæ proper, in which the genesis of suns is slowly
+elaborated. Although the visible universe is limited by the penetrating
+power of our instruments, yet we see in imagination the infinite
+universe stretching farther and farther; but we know not whether this
+invisible universe is totally devoid of matter, or whether it also is
+filled with millions and millions of suns and galaxies.
+</p>
+
+<p><br /><br /><br /></p>
+
+<h4><a id="APPENDIX">APPENDIX</a></h4>
+
+<h4>KEY TO THE PLATES</h4>
+
+<p><br /></p>
+
+<h5>PLATE I.&mdash;GROUP OF SUN-SPOTS AND VEILED SPOTS.
+<br />
+<i>Observed June</i> 17, 1875, <i>at 7h. 30m. A. M.</i></h5>
+
+<p>
+The background shows the sun's visible surface, or <i>photosphere</i>,
+as seen with a telescope of high power at the most favorable moments,
+composed of innumerable light markings, or granules, separated by a
+network of darker gray. The granules, each some hundreds of miles in
+width, are thought to be the flame-like summits of the radial filaments
+or columns of gas and vapor which compose the photospheric shell. The
+two principal sun-spots of the group here represented show the
+characteristic dark <i>umbra</i> in the centre, overhung by the
+thatch-like <i>penumbra</i>, composed of whitish gray filaments. The
+penumbral filaments are not supposed to differ in their nature from
+those constituting the ordinary photosphere, save that they are seen
+here elongated and violently disturbed by the force of gaseous currents.
+Both spots are traversed partly or wholly by bright overlying
+<i>faculæ</i>, or so-called <i>luminous bridges</i>, depressed portions
+of which, in the left-hand spot, form the <i>gray and rosy veils</i>
+commonly attendant upon this class of spots. In each of these spots,
+also, the inner ends of projecting penumbral filaments have fallen so
+far within the umbra as to appear much darker than the rest. At the
+right of the upper portion of the left-hand spot, is a mass of white
+facular clouds, honey-combed by dark spaces, through which are seen
+traces of the undeveloped third spot of the triple group first observed.
+If seen upon the sun's limb, this would have presented the appearance of
+a <i>lateral spot</i>. Above the right-hand spot is a small black "dot,"
+or incipient spot, without distinct penumbra. The irregular dark rift
+below the two large spots and connecting them is a spot of the crevasse
+type, with very slight umbra, a still better example of which is seen in
+a westward prolongation of the penumbra of the left-hand spot. In the
+upper left-hand corner of the Plate are seen several small
+<i>faculæ</i>, appearing as irregular whitish streaks amongst the
+granules. In the pear-shaped darkening of the solar surface below and at
+their left, is seen a veiled spot, two of which attended this group.
+</p>
+<p>
+<i>Approximate scale, 2500 miles&mdash;1 inch.</i>
+</p>
+
+<p><br /></p>
+
+<h5>PLATE II.&mdash;SOLAR PROTUBERANCES.
+<br />
+<i>Observed May</i> 5, 1873, <i>at 9h. 40m. A. M.</i></h5>
+
+<p>
+A view of an upheaval of the <i>chromosphere</i>, or third outlying
+envelope of the sun, as observed with the tele-spectroscope, or
+telescope with spectroscope attached.
+</p>
+<p>
+The <i>method of the observation</i> requires a word of explanation.
+Save on the rare occasions of a total solar eclipse, no direct
+telescopic view of the solar prominences or flames is possible, owing to
+the fact that the intense white light from the sun's main disk entirely
+obscures the feeble pink light of the chromosphere. A few years ago
+Messrs. Jannsen and Lockyer found that a spectroscope of high dispersive
+power so weakens the spectrum of ordinary sun-light as to show the
+spectrum of bright lines given by the chromosphere, on any clear day.
+The telescope is adjusted so that a portion of the sun's limb, usually
+near a group of active sun-spots, shall be presented before the opened
+slit of the spectroscope. The light of the chromosphere thus admitted
+along with some diffused sun-light from the earth's atmosphere, produces
+a spectrum of intensely bright lines, widely separated, on the fainter
+background of the strongly dispersed spectrum of sun-light. The most
+prominent of these bright lines are those known as the C line
+(<i>scarlet</i>), F line (<i>blue</i>), which with several others are
+due to the hydrogen present in the chromosphere, the D<sub>3</sub> line
+(<i>orange</i>) ascribed to a little known substance called "helium",
+and occasionally the sodium lines D<sub>1</sub>, D<sub>3</sub>,
+(<i>yellow</i>). By adjusting the slit upon the scarlet C line, the
+appearances represented in Plate II. were observed as through an
+atmosphere of scarlet light: in the D or F lines identical appearances
+may be seen, but somewhat less clearly defined, as through yellow or
+blue light respectively. Hence the solar times, as here observed with
+the spectroscope in the hydrogen C line, are seen through a portion only
+(the scarlet rays) of the light coming from but one substance (hydrogen)
+of the companion incandescent substances present in the chromosphere.
+The color of the collective chromospheric light is seen directly with
+the telescope during an eclipse (See Plate III.) to be a delicate rosy
+pink.
+</p>
+
+<p><br /></p>
+
+<p>
+<i>Description of the Plate.</i>&mdash;The black background represents
+the general darkness of the eye-piece to the spectroscope. The broad red
+stripe stretching from top to bottom of the Plate is a portion of the
+red band of the spectrum, magnified about 100 times as compared with the
+actual spectroscopic view. The upper and lower edges of the
+cross-section of dusky red correspond with the edges of the slit, opened
+widely enough to admit a view of the chromospheric crest and of the
+whole height of the protuberances at once. With a narrower opening of
+the slit this background would have been nearly black, its reddish cast
+increasing with the amount of opening and consequent admission of
+diffused sun-light. Rising above the lower edge of the opening is seen a
+small outer segment of the chromosphere, which, as a portion of the
+sun's eastern limb, should be imagined as moving directly towards the
+beholder. The seams and rifts by which its surface is broken, as well as
+the distorted forms of the huge protuberances show the chromosphere to
+be in violent agitation. Some of the most characteristic shapes of the
+<i>eruptive protuberances</i> are presented, as also <i>cloud-like</i>
+forms overtopping the rest. In the immediate foreground the bases of two
+towering columns appear deeply depressed below the general horizon of
+the segment observed, showing an extraordinary velocity of motion of the
+whole uplifted mass toward the observer. The highest of these
+protuberances was 126,000 miles in height at the moment of observation.
+The triple protuberance at the left with two drooping wings and a tall
+swaying spire tipped with a very bright flame, shows by its more
+brilliant color the higher temperature (and possibly compression) to
+which its gases have been subjected. The irregular black bands behind
+this protuberance indicate the presence there of less condensed and
+cooler clouds of the same gases. The dimmer jets of dame rising from the
+chromosphere are either vanishing protuberances, or, as in the case of
+the smallest jet shown at the extreme right of the horizon, are the tops
+of protuberances just coming into view.
+</p>
+<p>
+<i>Approximate scale, 6000 miles&mdash;1 inch.</i>
+</p>
+
+<p><br /></p>
+
+<h5>PLATE III.&mdash;TOTAL ECLIPSE OF THE SUN.
+<br />
+<i>Observed July</i> 29, 1878, <i>at Creston, Wyoming Territory.</i></h5>
+
+<p>
+A telescopic view of the sun's <i>corona</i> or extreme outer atmosphere
+and of the <i>solar flames</i> or <i>prominences</i> during a total
+eclipse. At the moment of observation the dark disk of the moon, while
+still hiding the sun's main body, had passed far enough eastward to
+allow the rosy pink chromospheric prominences to be seen on its western
+border. On all sides of the sun's hidden disk, the <i>corona</i> shows
+its pale greenish light extending in halo-like rays and streamers, and
+two very remarkable wings stretch eastward and westward very nearly in
+the plane of the ecliptic and in the direction of the positions of
+Mercury and Venus respectively at the time of observation. The full
+extent of these wings could not be shown in the Plate without reducing
+its scale materially, since the westerly wing extended no less than
+twelve times the sun's diameter, and the easterly wing nearly as far, or
+over ten million miles. A circlet of bright light immediately bordering
+the moon's disk is the so-called <i>inner corona</i>, next to which the
+wings and streamers arc brightest, thence shading off imperceptibly into
+the twilight sky of the eclipse. Other noteworthy peculiarities of the
+corona, as observed during this eclipse, are the varying angles at which
+the radiating streamers are seen to project, the comparatively dark
+intervals between them, and the curved, wisp-like projections seen upon
+the wings. An especially noticeable gap appears where the most westerly
+of the upward streamers abruptly cuts off the view of the long wing. The
+largest and brightest of the curving streamers on the westerly wing
+coincides with the highest flame-like protuberance. To some observers of
+this eclipse the upward and downward streamers seemed pointed at their
+outer extremities and less regular in form.
+</p>
+<p>
+<i>Approximate scale, 135,000 miles&mdash;1 inch.</i>
+</p>
+
+<p><br /></p>
+
+<h5>PLATE IV.&mdash;AURORA BOREALIS.
+<br />
+<i>As observed March</i> 1, 1872, <i>at 9h. 25m. P. M.</i></h5>
+
+<p>
+The view presents the rare spectacle of an aurora spanning the sky from
+east to west in concentric arches. The Polar Star is nearly central in
+the background, the constellation of the Great Bear on the right and
+Cassiopeia's chair on the left. The large star at some distance above
+the horizon on the right is Arcturus. The almost black inner segment of
+the aurora resting upon the horizon, has its summit in the magnetic
+meridian, which was in this case a little west of north, its arc being
+indented by the bases of the ascending streamers. Both streamers and
+arches were, when observed, tremulous with upward pulsations and there
+was also a wave-like movement of the streamers from west to east. The
+prevailing color of this aurora is a pale whitish green and the
+complementary red appears especially at the west end of the auroral
+arch. The summits of the streamers are from four hundred to live hundred
+miles above the earth and the aurora is therefore a phenomenon of the
+terrestrial atmosphere rather than of astronomical observation proper.
+</p>
+
+<p><br /></p>
+
+<h5>PLATE V.&mdash;THE ZODIACAL LIGHT.
+<br />
+<i>Observed February</i> 20, 1876.</h5>
+
+<p>
+An observation of the cone of light whose axis lies along the Zodiac,
+whence it derives its name. It is drawn as seen in the west, with its
+base in the constellation Pisces, and its apex near the familiar group
+of the Pleiades in the constellation Taurus. The first bright star above
+the horizon in the base of the cone is the planet <i>Venus</i> and at
+some distance above is the reddish disk of <i>Mars</i>, the two being in
+rare companionship as evening stars. Above the constellation Pisces, two
+bright stars of Aries lie just outside the cone at the right. The
+nearest bright star above these at the right is <i>Beta</i>, the leading
+star of the constellation Triangula. Further at the right the three
+prominent stars nearly in a line are, in ascending order, <i>Delta</i>,
+<i>Beta</i> and <i>Gamma</i> of the constellation Andromeda. Above these
+at the left, the brightest star of a quadrangular group of four is the
+remarkable variable star <i>Algol</i> (<i>Beta</i>) of the constellation
+Perseus, which changes from the second to the fourth magnitude in a
+period of less than three days. At the left and a little above the
+Pleiades is the ruddy star <i>Aldebaran</i>, one of the Hyades and chief
+star in the constellation Taurus. These are the principal stars visible
+in this portion of the sky at the time of the observation. Their
+relative positions are represented as seen in the sky and not by the
+common method of star-atlases, which allows for the change from a
+spherical to a plane surface. Their magnitude in the order of brightness
+is indicated only approximately.
+</p>
+
+<p><br /></p>
+
+<h5>PLATE VI.&mdash;MARE HUMORUM.
+<br />
+<i>From a study made in 1875.</i></h5>
+
+<p>
+A view of one of the lunar plains, or so-called <i>seas</i>
+(<i>Maria</i>), with an encircling mountainous wall consisting of
+volcano-like craters in various stages of subsidence and dislocation.
+The sun-light coming from the west casts strong shadows from all the
+elevations eastward, and is just rising on the <i>terminator</i>, where
+the rugged structure of the Moon's surface is best seen. The lighter
+portions are the more elevated mountainous tracts and crater summits.
+The detailed description of this Plate given in the body of the Manual
+is repeated here for convenience of reference: The Mare Humorum, or sea
+of moisture, as it is called, is one of the smaller gray lunar plains.
+Its diameter, which is very nearly the same in all directions, is about
+270 miles, the total area of this plain being about 50,000 square miles.
+It is one of the most distinct plains of the Moon, and is easily seen
+with the naked eye on the left-hand side of the disk. The floor of the
+plain is, like that of the other gray plains, traversed by several
+systems of very extended but low hills and ridges, while small craters
+are disseminated upon its surface. The color of this formation is of a
+dusky greenish gray along the border, while in the interior it is of a
+lighter shade, and is of brownish olivaceous tint. This plain, which is
+surrounded by high clefts and rifts, well illustrates the phenomena of
+dislocation and subsidence. The double-ringed crater Vitello, whose
+walls rise from 4,000 to 5,000 feet in height, is seen in the upper
+left-hand corner of the gray plain. Close to Vitello at the east is the
+large broken ring-plain Lee, and farther east, and a little below, is a
+similarly broken crater called Doppelmayer. Both of these open craters
+have mountainous masses and peaks on their door, which is on a level
+with that of the Mare Humorum. A little below, and to the left of these
+objects, is dimly seen a deeply imbedded oval crater, whose walls barely
+rise above the level of the plain. On the right-hand side of the great
+plain is a long <i>fault</i>, with a system of fracture running along
+its border. On this right-hand side may be seen a part of the line of
+the terminator, which separates the light from the darkness. Towards the
+lower right-hand corner is the great ring-plain Gassendi, 55 miles in
+diameter, with its system of fractures and its central mountains, which
+rise from 3,000 to 4,000 feet above its floor. This crater slopes
+towards the plain, showing the subsidence to which it has been
+submitted. While the northern portion of the wall of this crater rises
+to 10,000 feet, that on the plain is only 500 feet high, and is even
+wholly demolished at one place where the floor of the crater is in
+direct communication with the plain. In the lower part of the sea, and a
+little to the west of the middle line, is found the crater
+Agatharchides, which shows below its north wall the marks of rills
+impressed by a flood of lava, which once issued from the side of the
+crater. On the left-hand side of the plain is seen the half-demolished
+crater Hippalus resembling a large bay, which has its interior strewn
+with peaks and mountains. On this same side can be seen one of the most
+important systems of clefts and fractures visible on the Moon, these
+clefts varying in length from 150 to 200 miles.
+</p>
+<p>
+<i>Approximate scale, 15 miles&mdash;1 inch.</i>
+</p>
+
+<p><br /></p>
+
+<h5>PLATE VII.&mdash;PARTIAL ECLIPSE OF THE MOON.
+<br />
+<i>Observed October</i> 24, 1874.</h5>
+
+<p>
+A view of the Moon partially obscured by the Earth's shadow, whose
+outline gives ocular proof of the earth's rotundity of form. The
+shadowed part of the Moon's surface is rendered visible by the diffused
+sun-light refracted upon it from the earth's atmosphere. Its reddish
+brown color is due to the absorption, by vapors present near the earth's
+surface, of a considerable part of this dim light. On both the obscured
+and illuminated tracts the configurations of the Moon's surface are seen
+as with the naked eye. The craters appear as distinct patches of lighter
+color, and the noticeably darker areas are the depressed plains or
+<i>Maria</i>. The large crater <i>Tycho</i>, at the lower part of the
+disk, is the most prominent of these objects, with its extensive system
+of <i>radiating streaks</i>. The largest crater above is
+<i>Copernicus</i>, at the left of which is <i>Kepler</i> and still above
+are <i>Aristarchus</i> and <i>Herodotus</i> appearing as if blended in
+one. Above and at the left of the great crater <i>Tycho</i>, the first
+dark tract is the <i>Mare Humorum</i> of Plate VI., seen in its natural
+position, with the crater <i>Gassendi</i> at its northern (upper)
+extremity and <i>Vitello</i> on its southern (lower) border. The
+advancing border of the shadow appears, as always, noticeably darker
+than the remainder, an effect probably of contrast. The illuminated
+segment of the Moon's disk has its usual appearance, the lighter
+portions being the more elevated mountainous surfaces, and the dark
+spaces the floors of extensive plains.
+</p>
+<p>
+<i>Approximate scale, 140 miles&mdash;1 inch.</i>
+</p>
+
+<p><br /></p>
+
+<h5>PLATE VIII.&mdash;THE PLANET MARS.
+<br />
+<i>Observed September</i> 3, 1877, <i>at 11h. 55m. P. M.</i></h5>
+
+<p>
+A view of the southern hemisphere of Mars, when in the most favorable
+position for observation, and when exceptionally free from the clouds,
+which very frequently hide its surface configurations. Since, of all the
+planets, Mars is most like the earth, Plate VIII. may give a fair idea
+of the appearance of our globe to a supposed observer on Mars. The dark
+gray and black markings, are regarded as tracts of water, or of some
+liquid with similar powers of absorbing light; and, for the same reason,
+the lighter portions, of a prevailing reddish tint, are supposed to be
+bodies of land, while the bright white portions are variously due to
+clouds, to polar snow or ice, and the bright rim of white along the
+limb, to the depth of the atmosphere through which the limb is seen. The
+chief permanent features of the planet's surface have been named in
+honor of various astronomers.
+</p>
+<p>
+The large dark tract on the left is <i>De La Rue Ocean</i>, the isolated
+oval spot near the centre is <i>Terby Sea</i>, and on the right is the
+western end of <i>Maraldi Sea</i>, with strongly indented border.
+Directly north of (below) De La Rue Ocean, is <i>Maedler Continent</i>;
+above it stretches <i>Jacob Land</i>; and surrounding Terby Sea is
+<i>Secchi Continent</i>. Extending into the centre of De La Rue Ocean is
+a curious double peninsula, called, in consequence of the dimness of
+former observations, <i>Hall Island</i>. The sharply defined,
+white-crested northern borders of De La Rue Ocean and Maraldi Sea may
+indicate the existence there of lofty coast ranges, more or less
+constantly covered with opaque clouds strongly reflecting light. The
+white spot in the centre of Maedler Continent, of a temporary nature,
+has a similar explanation. The intervals of olivaceous gray on Secchi
+Continent and elsewhere may perhaps be ascribed to the flooding and
+drying up of marshes and lowlands, as these markings have been observed
+to vary somewhat in connection with the change of seasons on Mars. The
+greenish tints observed along the planet's limb, alike on the darker and
+lighter surfaces, are probably due to an optical effect, the green being
+complementary to the prevailing red of the disk. The brilliant oval
+white spot near the southern (upper) pole of the planet is a so-called
+polar spot, in all probability consisting of a material similar to snow
+or ice and here observed in the midst of a dark open sea.
+</p>
+<p>
+<i>Approximate scale, 300 miles&mdash;1 inch.</i>
+</p>
+
+<p><br /></p>
+
+<h5>PLATE IX.&mdash;THE PLANET JUPITER.
+<br />
+<i>Observed November</i> 1, 1880, <i>at 9h. 30m. P. M.</i></h5>
+
+<p>
+This planet is perpetually wrapped in dense clouds which hide its inner
+globe from view. The drawing shows Jupiter's outer clouded surface with
+its usual series of alternate light and dark belts, the disk as a whole
+appearing brighter in the centre than near the limb. The darker gray and
+black markings indicate in general the lower cloud-levels; that is,
+partial breaks or rifts in the cloudy envelope, whose prevailing depth
+apparently exceeds four thousand miles. While the deepest depression in
+the cloudy envelope is within the limits of the Great Red Spot, the
+vision may not even here penetrate very deeply. Two of Jupiter's four
+moons present bright disks near the planet's western limb, and cast
+their shadows far eastward on the disk, that of the "second satellite"
+falling upon the Red Spot. On the Red Spot are seen in addition two
+small black spots, no explanation of which can yet be offered. The broad
+white ring of clouds bordering the Red Spot appeared in constant motion.
+The central, or equatorial belt, shows brilliant cloudy masses of both
+the <i>cumulus</i> and <i>stratus</i> types, and the underlying gray and
+black cloudy surfaces are pervaded with the pinkish color characteristic of
+this belt. The dark circular spots on the wide white belt next north
+showed in their mode of formation striking resemblances to sun-spots.
+They afterward coalesced into a continuous pink belt. The diffusion of
+pinkish color over the three northernmost dark bands, as here observed,
+is unusual. About either pole is seen the uniform gray segment or polar
+cap. The equatorial diameter is noticeably longer than the polar
+diameter, a consequence of the planet's extraordinary swiftness of
+rotation. To the same cause may also be due chiefly the distribution of
+the cloudy belts parallel to the planet's equator, though the analogy of
+the terrestrial trade-winds fails to explain all the observed phenomena.
+</p>
+<p>
+<i>Approximate scale, 5,500 miles&mdash;1 inch.</i>
+</p>
+
+<p><br /></p>
+
+<h5>PLATE X.&mdash;THE PLANET SATURN.
+<br />
+<i>Observed November</i> 30, 1874, <i>at 5h. 50m. P. M.</i></h5>
+
+<p>
+Saturn is unique amongst the planets in that its globe is encircled by a
+series of concentric rings, which lie in the plane of its equator, and
+consist, according to present theories, of vast throngs of minute bodies
+revolving about the planet, like so many satellites, in closely parallel
+orbits. The globe of Saturn, like that of Jupiter, is surrounded by
+cloudy belts parallel to its equator. The broad equatorial belt, of a
+delicate pinkish tint, is both brighter and more mottled than the
+narrower yellowish white belts, which alternate with darker belts of
+ashy gray on both the north and south sides, but are seen here only on
+the northern side. The disk has an oval shape, owing to the extreme
+polar compression of the globe.
+</p>
+<p>
+The outer, middle and inner rings, with their various subdivisions, are
+clearly shown in Plate X., and are best seen on the so-called <i>ansæ</i>,
+or handles, projecting on either side. The gray <i>outer ring</i> is
+separated by the dusky pencil line into two divisions, both of which
+appear slightly mottled on the ansæ, as if with clouds. The <i>middle
+ring</i> has three sub-divisions which are clearly distinguishable,
+although separated by no dark interval, viz., a brilliant white outer
+zone, distinctly mottled, as seen on the extremities of the ansæ, and
+two interior zones of gradually diminishing brightness. The <i>gauze</i> or
+<i>dusky ring</i> is seen at its full width on the ansæ, but on the
+background of the strongly illuminated globe only its outer and
+presumably denser border is visible. The shadow of the globe on the
+rings is seen on the lower portion of the eastern ansæ. The shadow on
+the dusky ring is with difficulty perceptible; the shadow on the middle
+ring is slightly concave toward the planet, which concavity is abruptly
+increased on the outer zone of this ring; while the shadow on the outer
+ring slants away from the globe. These appearances are fully accounted
+for by supposing a general increase of level from the inner edge of the
+dusky ring to the outer margin of the middle ring, and a uniform lower
+level on the outer ring. Other observers have regarded the deflection of
+the shadow as an effect of irradiation. The inner margin of the double
+outer ring presents on both ansæ a number of slight indentations,
+which, if not actual irregularities in the contour of this ring, may be
+explained as shadows caused by elevations on the outer border of the
+middle ring, or possibly by over-hanging clouds.
+</p>
+<p>
+<i>Approximate scale, 6,500 miles&mdash;1 inch.</i>
+</p>
+
+<p><br /></p>
+
+<h5>PLATE XI.&mdash;THE GREAT COMET OF 1881.
+<br />
+<i>Observed on the night of June</i> 25-26, <i>at 1h. 30m. A. M.</i></h5>
+
+<p>
+A view of the comet 1881, III., drawn as if seen with the naked eye, the
+minute details, however, being reproduced as seen with the telescope.
+The star-like <i>nucleus</i> is attended by four conical wings which
+cause it to appear diamond-shaped. The <i>coma</i> appears double, the
+brilliant inner coma, immediately enveloping the nucleus, being
+surrounded by a fainter exterior coma, which has a noticeable depression
+corresponding to that of the inner edge of the principal coma. The tail
+is divided lengthwise by a dark rift and is brightest on its convex or
+forward side. An inner portion of the tail, brighter than the rest, is
+more strongly curved, as if by solar repulsion. Stars are seen through
+the brighter parts of the tail, as they may be seen even through the
+coma and nucleus, with little diminution of their light.
+</p>
+
+<p><br /></p>
+
+<h5>PLATE XII.&mdash;THE NOVEMBER METEORS.
+<br />
+<i>As observed on the night of November</i> 13-14, 1868.</h5>
+
+<p>
+A partially ideal view of the November Meteors, combining forms observed
+at different times during the night of Nov. 13th, 1868. It is not,
+however, a fanciful view, since a much larger number of meteors were
+observed falling at once during the shower of November, 1833, and at
+other times. The locality of the observation is shown by the Polar Star
+seen near the centre of the Plate, and Cassiopeia's Chair at the left.
+The general direction of the paths of the meteors is from the
+north-east, the <i>radiant point</i> of the shower having been in the
+constellation Leo, beyond and above Ursa Major. While the orbits of the
+meteors are, in general, curved regularly and slightly, several are
+shown with very eccentric paths, among them one which changed its course
+at a sharp angle. In the upper left-hand corner appear two vanishing
+trails of the "ring-form," and several others still further transformed
+into faint luminous patches of cloud. Red, yellow, green, blue and
+purple tints were observed in the meteors and their trails, as
+represented in the Plate.
+</p>
+
+<p><br /></p>
+
+<h5>PLATE XIII.&mdash;PART OF THE MILKY WAY.
+<br />
+<i>From a study made during the years</i> 1874, 1875 <i>and</i> 1876.</h5>
+
+<p>
+The course of the portion of the Galaxy represented in Plate XIII. is as
+follows: From Cassiopeia's chair, three bright stars of which appear at
+the upper edge of the Plate, the Galaxy, forming two streams, descends
+south, passing partly through Lacerta on the left, and Cepheus on the
+right; at this last point it approaches nearest to the Polar Star, which
+is itself outside of the field of view. Then it enters Cygnus, where it
+becomes very complicated and bright, and where several large cloudy
+masses are seen terminating its left branch, which passes to the right,
+near the bright star <i>Deneb</i>, the leader of this constellation.
+Below <i>Deneb</i>, the Galaxy is apparently disconnected and separated
+from the northern part by a narrow, irregular dark gap. From this
+rupture, the Milky-way divides into two great streams, separated by an
+irregular dark rift. An immense branch extends to the right, which,
+after having formed an important luminous mass between the stars
+<i>Gamma</i> and <i>Beta</i>, continues its southward progress through
+parts of Lyra, Vulpecula, Hercules, Aquila and Ophiuchus, where it
+gradually terminates a few degrees south of the equator. The main stream
+on the left, after having formed a bright mass around <i>Epsilon
+Cygni</i>, passes through Vulpecula and then Aquila, where it crosses
+the equinoctial just below the star <i>Eta</i>, after having involved in
+its nebulosity the bright star <i>Altair</i>, the leader of Aquila. In
+the southern hemisphere the Galaxy becomes very complicated and forms a
+succession of very bright, irregular masses, the upper one being in
+Scutum Sobieskii, while the others are respectively situated in
+Sagittarius and in Scorpio; the last, just a little above our horizon,
+being always considerably dimmed by vapors. From Scutum Sobieskii, the
+Galaxy expands considerably on the right, and sends a branch into
+Scorpio, in which the fiery red star <i>Antares</i> is somewhat
+involved. In the upper left-hand corner of the Plate, at some distance
+from the Milky-way, is seen dimly the Nebula in Andromeda, which becomes
+so magnificent an object to telescopic view.
+</p>
+
+<p><br /></p>
+
+<h5>PLATE XIV.&mdash;STAR-CLUSTER IN HERCULES.
+<br />
+<i>From a study made in June</i>, 1877.</h5>
+
+<p>
+In the constellation Hercules, a small nebulous mass is faintly visible
+to the eye, a telescopic view of which is presented in Plate XIV. It is
+one of the most beautiful of the easily resolvable globular clusters.
+The brilliancy of the centre gives the cluster a distinctly globular
+appearance, while the several wings curving in various directions, have
+suggested to some observers an irregularly spiral structure. The large
+stars of the cluster are arranged in several groups which correspond, in
+a general way, with the faintly luminous wings.
+</p>
+
+<p><br /></p>
+
+<h5>PLATE XV.&mdash;THE GREAT NEBULA IN ORION.
+<br />
+<i>From a study made in the years</i> 1875-76.</h5>
+
+<p>
+This nebula, which is one of the most brilliant and wonderful of
+telescopic objects, readily visible to the naked eve as a patch of
+nebulous light immediately surrounding the middle star of the three
+which form the sword of Orion, and a little south of the three
+well-known stars forming the belt. The small stars in this, as in other
+Plates of the series, are somewhat exaggerated in size, as was
+unavoidable with any mode of reproduction that could be employed. The
+bright pentagonal centre of the nebula is traversed by less luminous
+rifts, the several subdivisions thus outlined being irregularly mottled
+as if by bright fleecy clouds. Toward the lower part of this bright
+pentagonal centre is a comparatively dark space containing four bright
+stars which form a trapezium and together constitute the quadruple star
+<i>Theta Orionis</i>, which, to the naked eye, appears as the single star
+in the centre of the sword. On three sides of the central mass extend long
+bright wisps, whose curves fail, however, to reveal the spiral structure
+often attributed to this nebula. On the east a broad wing, with
+wave-shaped inner border, stretches southward. East of the trapezium are
+two especially noticeable dark spaces. Close to the main nebula on the
+north-east, a small faint nebula surrounds a bright star, and a branch
+from another faint stream of nebulous matter forming a loop to the
+southward, encloses the nebulous star (<i>Iota Orionis</i>) shown at the
+top of the Plate.
+</p>
+
+<p><br /><br /><br /></p>
+
+<div>*** END OF THE PROJECT GUTENBERG EBOOK 68394 ***</div>
+</body>
+
+</html>
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+The Project Gutenberg eBook of The Trouvelot astronomical drawings
+manual, by Étienne Léopold Trouvelot
+
+This eBook is for the use of anyone anywhere in the United States and
+most other parts of the world at no cost and with almost no restrictions
+whatsoever. You may copy it, give it away or re-use it under the terms
+of the Project Gutenberg License included with this eBook or online at
+www.gutenberg.org. If you are not located in the United States, you
+will have to check the laws of the country where you are located before
+using this eBook.
+
+Title: The Trouvelot astronomical drawings manual
+
+Author: Étienne Léopold Trouvelot
+
+Release Date: June 24, 2022 [eBook #68394]
+
+Language: English
+
+Produced by: Laura Natal Rodrigues (Images generously made available by
+             Hathi Trust Digital Library and Wikipedia Commons.)
+
+*** START OF THE PROJECT GUTENBERG EBOOK THE TROUVELOT ASTRONOMICAL
+DRAWINGS MANUAL ***
+
+
+THE
+TROUVELOT
+ASTRONOMICAL DRAWINGS
+MANUAL
+
+
+
+
+BY
+
+E. L. TROUVELOT,
+
+
+FORMERLY CONNECTED WITH THE OBSERVATORY OF HARVARD COLLEGE; FELLOW OF THE
+AMERICAN ACADEMY OF ARTS AND SCIENCES, AND MEMBER OF THE SELENO-GRAPHICAL
+SOCIETY OF GREAT BRITAIN; IN CHARGE OF A
+GOVERNMENT EXPEDITION TO OBSERVE THE
+TOTAL SOLAR ECLIPSE OF 1878.
+
+
+
+
+NEW YORK
+
+CHARLES SCRIBNER'S SONS
+
+1882
+
+
+
+
+INTRODUCTION
+
+
+During a study of the heavens, which has now been continued for more
+than fifteen years, I have made a large number of observations
+pertaining to physical astronomy, together with many original drawings
+representing the most interesting celestial objects and phenomena.
+
+With a view to making these observations more generally useful, I was
+led, some years ago, to prepare, from this collection of drawings, a
+series of astronomical pictures, which were intended to represent the
+celestial phenomena as they appear to a trained eye and to an
+experienced draughtsman through the great modern telescopes, provided
+with the most delicate instrumental appliances. Over two years were
+spent in the preparation of this series, which consisted of a number of
+large drawings executed in pastel. In 1876, these drawings were
+displayed at the United States Centennial Exhibition at Philadelphia,
+forming a part of the Massachusetts exhibit, in the Department of
+Education and Science.
+
+The drawings forming the present series comprise only a part of those
+exhibited at Philadelphia; but, although fewer in number, they are quite
+sufficient to illustrate the principal classes of celestial objects and
+phenomena.
+
+While my aim in this work has been to combine scrupulous fidelity and
+accuracy in the details, I have also endeavored to preserve the natural
+elegance and the delicate outlines peculiar to the objects depicted; but
+in this, only a little more than a suggestion is possible, since no
+human skill can reproduce upon paper the majestic beauty and radiance of
+the celestial objects.
+
+The plates were prepared under my supervision, from the original pastel
+drawings, and great care has been taken to make the reproduction exact.
+
+The instruments employed in the observations, and in the delineation of
+the heavenly bodies represented in the series, have varied in aperture
+from 6 to 26 inches, according to circumstances, and to the nature of
+the object to be studied. The great Washington refractor, kindly placed
+at my disposal by the late Admiral C. H. Davis, has contributed to this
+work, as has also the 26 inch telescope of the University of Virginia,
+while in the hands of its celebrated constructors, Alvan Clark & Sons.
+The spectroscope used was made by Alvan Clark & Sons. Attached to it is
+an excellent diffraction grating, by Mr. L. M. Rutherfurd, to whose
+kindness I am indebted for it.
+
+Those unacquainted with the use of optical instruments generally suppose
+that all astronomical drawings are obtained by the photographic process,
+and are, therefore, comparatively easy to procure; but this is not true.
+Although photography renders valuable assistance to the astronomer in
+the case of the Sun and Moon, as proved by the fine photographs of these
+objects taken by M. Janssen and Mr. Rutherfurd; yet, for other subjects,
+its products are in general so blurred and indistinct that no details of
+any great value can be secured. A well-trained eye alone is capable of
+seizing the delicate details of structure and of configuration of the
+heavenly bodies, which are liable to be affected, and even rendered
+invisible, by the slightest changes in our atmosphere.
+
+The method employed to secure correctness in the proportions of the
+original drawings is simple, but well adapted to the purpose in view. It
+consists in placing a fine reticule, cut on glass, at the common focus
+of the objective and the eye-piece, so that in viewing an object, its
+telescopic image, appearing projected on the reticule, can be drawn very
+accurately on a sheet of paper ruled with corresponding squares. For a
+series of such reticules I am indebted to the kindness of Professor
+William A. Rogers, of the Harvard College Observatory.
+
+The drawings representing telescopic views are inverted, as they appear
+in a refracting telescope--the South being upward, the North downward,
+the East on the right, and the West on the left. The Comet, the
+Milky-Way, the Eclipse of the Moon, the Aurora Borealis, the Zodiacal
+Light and the Meteors are represented as seen directly in the sky with
+the naked eye. The Comet was, however, drawn with the aid of the
+telescope, without which the delicate structure shown in the drawing
+would not have been visible.
+
+The plate representing the November Meteors, or so-called "Leonids," may
+be called an ideal view, since the shooting stars delineated, were not
+observed at the same moment of time, but during the same night. Over
+three thousand Meteors were observed between midnight and five o'clock
+in the morning of the day on which this shower occurred; a dozen being
+sometimes in sight at the same instant. The paths of the Meteors,
+whether curved, wavy, or crooked, and also their delicate colors, are in
+all cases depicted as they were actually observed.
+
+In the Manual, I have endeavored to present a general outline of what is
+known, or supposed, on the different subjects and phenomena illustrated
+in the series. The statements made are derived either from the best
+authorities on physical astronomy, or from my original observations,
+which are, for the most part, yet unpublished.
+
+The figures in the Manual relating to distance, size, volume, mass,
+etc., are not intended to be strictly exact, being only round numbers,
+which can, therefore, be more easily remembered.
+
+It gives me pleasure to acknowledge that the experience acquired in
+making the astronomical drawings published in Volume VIII. of the Annals
+of the Harvard College Observatory, while I was connected with that
+institution, has been of considerable assistance to me in preparing this
+work; although no drawings made while I was so connected have been used
+for this series.
+
+
+                             E. L. TROUVELOT.
+
+_Cambridge, March, 1882._
+
+
+
+
+CONTENTS
+
+
+INTRODUCTION
+
+LIST OF PLATES
+
+_THE SUN_
+
+GENERAL REMARKS ON THE SUN
+
+SUN-SPOTS AND VEILED SPOTS
+
+SOLAR PROTUBERANCES
+
+TOTAL ECLIPSE OF THE SUN
+
+_THE AURORAL AND ZODIACAL LIGHTS_
+
+THE AURORA BOREALIS
+
+THE ZODIACAL LIGHT
+
+_THE MOON_
+
+THE MOON
+
+ECLIPSES OF THE MOON
+
+_THE PLANETS_
+
+THE PLANETS
+
+THE PLANET MARS
+
+THE PLANET JUPITER
+
+THE PLANET SATURN
+
+_COMETS AND METEORS_
+
+COMETS
+
+SHOOTING-STARS AND METEORS
+
+_THE STELLAR SYSTEMS_
+
+THE MILKY-WAY OR GALAXY
+
+THE STAR-CLUSTERS
+
+THE NEBULÆ
+
+APPENDIX
+
+
+
+
+LIST OF PLATES[1]
+
+PLATE
+
+I. GROUP OF SUN-SPOTS AND VEILED SPOTS.
+
+_Observed June 17, 1875, at 7 h. 30m. A. M._
+
+II. SOLAR PROTUBERANCES.
+
+_Observed May 5, 1873, at 9h. 40m. A. M._
+
+III. TOTAL ECLIPSE OF THE SUN.
+
+_Observed July 29, 1878, at Creston, Wyoming Territory._
+
+IV. AURORA BOREALIS.
+
+_As observed March 1, 1872, at 9h. 25m. P. M._
+
+V. THE ZODIACAL LIGHT.
+
+_Observed February 20, 1876._
+
+VI. MARE HUMORUM.
+
+_From a study made in 1875._
+
+VII. PARTIAL ECLIPSE OF THE MOON.
+
+_Observed October 24, 1874._
+
+VIII. THE PLANET MARS.
+
+_Observed September 3, 1877, at 11h. 55m. P. M._
+
+IX. THE PLANET JUPITER.
+
+_Observed November 1, 1880, at 9h. 30m. P. M._
+
+X. THE PLANET SATURN.
+
+_Observed November 30, 1874, at 5th. 50m. P. M._
+
+XI. THE GREAT COMET OF 1881.
+
+_Observed on the night of June 25-26, at 1h. 30m. A. M._
+
+XII. THE NOVEMBER METEORS.
+
+_As observed between midnight and 3 o'clock A. M., on the night
+of November 13-14, 1868._
+
+XIII. PART OF THE MILKY-WAY.
+
+_From a study made during the years 1874, 1875 and 1876._
+
+XIV. STAR-CLUSTER IN HERCULES.
+
+_From a study made in June, 1877._
+
+XV. THE GREAT NEBULA IN ORION.
+
+_From a study made in the years 1873-76._
+
+
+[Footnote 1: For Key to the Plates, see Appendix.]
+
+
+_Reproduced from the Original Drawings, by Armstrong & Company,
+Riverside Press, Cambridge, Mass._
+
+
+
+
+GENERAL REMARKS ON THE SUN
+
+
+The Sun, the centre of the system which bears its name, is a
+self-luminous sphere, constantly radiating heat and light.
+
+Its apparent diameter, as seen at its mean from the Earth, subtends an
+angle of 32', or a little over half a degree. A dime, placed about six
+feet from the eye, would appear of the same proportions, and cover the
+Sun's disk, if projected upon it.
+
+That the diameter of the Sun does not appear larger, is due to the great
+distance which separates us from that body. Its distance from the Earth
+is no less than 92,000,000 miles. To bridge this immense gap, would
+require 11,623 globes like the Earth, placed side by side, like beads on
+a string.
+
+The Sun is an enormous sphere whose diameter is over 108 times the
+diameter of our globe, or very nearly 860,000 miles. Its radius is
+nearly double the distance from the Earth to the Moon. If we suppose,
+for a moment, the Sun to be hollow, and our globe to be placed at the
+centre of this immense spherical shell, not only could our satellite
+revolve around us at its mean distance of 238,800 miles, as now, but
+another satellite, placed 190,000 miles farther than the Moon, could
+freely revolve likewise, without ever coming in contact with the solar
+envelope.
+
+The circumference of this immense sphere measures 2,800,000 miles. While
+a steamer, going at the rate of 300 miles a day, would circumnavigate
+the Earth in 83 days, it would take, at the same rate, nearly 25 years
+to travel around the Sun.
+
+The surface of the Sun is nearly 12,000 times the surface of the Earth,
+and its volume is equal to 1,300,000 globes like our own. If all the
+known planets and satellites were united in a single mass, 600 such
+compound masses would be needed to equal the volume of our luminary.
+
+Although the density of the Sun is only one-quarter that of the Earth,
+yet the bulk of this body is so enormous that, to counterpoise it, no
+less than 314,760 globes like our Earth would be required.
+
+The Sun uniformly revolves around its axis in about 25½ days. Its
+equator is inclined 7° 15' to the plane of the ecliptic, the axis of
+rotation forming, therefore, an angle of 82° 45' with the same plane.
+As the Earth revolves about the Sun in the same direction as that of the
+Sun's rotation, the apparent time of this rotation, as seen by a
+terrestrial spectator, is prolonged from 25½ days to about 27 days and
+7 hours.
+
+The rotation of the Sun on its axis, like that of the Earth and the
+other planets, is direct, or accomplished from West to East. To an
+observer on the Earth, looking directly at the Sun, the rotation of this
+body is from left to right, or from East to West.
+
+The general appearance of the Sun is that of an intensely luminous disk,
+whose limb, or border, is sharply defined on the heavens. When its
+telescopic image is projected on a screen, or fixed on paper by
+photography, it is noticed that its disk is not uniformly bright
+throughout, but is notably more luminous in its central parts. This
+phenomenon is not accidental, but permanent, and is due in reality to a
+very rare but extensive atmosphere which surrounds the Sun, and absorbs
+the light which that body radiates, proportionally to its thickness,
+which, of course, increases towards the limb, to an observer on the
+Earth.
+
+
+THE ENVELOPING LAYERS OF THE SUN
+
+
+The luminous surface of the Sun, or that part visible at all times, and
+which forms its disk, is called the _Photosphere_, from the property it
+is supposed to possess of generating light. The photosphere does not
+extend to a great depth below the luminous surface, but forms a
+comparatively thin shell, 3,000 or 4,000 miles thick, which is distinct
+from the interior parts, above which it seems to be kept in suspense by
+internal forces. From the observations of some astronomers it would
+appear that the diameter of the photosphere is subject to slight
+variations, and, therefore, that the solar diameter is not a constant
+quantity. From the nature of this envelope, such a result does not seem
+at all impossible, but rather probable.
+
+Immediately above the photosphere lies a comparatively thin stratum,
+less than a thousand miles in thickness, called the _Reversing Layer_.
+This stratum is composed of metallic vapors, which, by absorbing the
+light of particular refrangibilities emanating from the photosphere
+below, produces the dark Fraunhofer lines of the solar spectrum.
+
+Above the reversing layer, and resting immediately upon it, is a
+shallow, semi-transparent gaseous layer, which has been called the
+_Chromosphere_, from the fine tints which it exhibits during total
+eclipses of the Sun, in contrast with the colorless white light radiated
+by the photosphere below. Although visible to a certain extent on the
+disk, the chromosphere is totally invisible on the limb, except with the
+spectroscope, and during eclipses, on account of the nature of its
+light, which is mainly monochromatic, and too feeble, compared with that
+emitted by the photosphere, to be seen.
+
+The chromospheric layer, which has a thickness of from 3,000 to 4,000
+miles, is uneven, and is usually upheaved in certain regions, its matter
+being transported to considerable elevations above its general surface,
+apparently by some internal forces. The portions of the chromosphere
+thus lifted up, form curious and complicated figures, which are known
+under the names of _Solar Protuberances, or Solar Flames_.
+
+Above the chromosphere, and rising to an immense but unknown height, is
+the solar atmosphere proper, which is only visible during total eclipses
+of the Sun, and which then surrounds the dark body of the Moon with the
+beautiful rays and glorious nimbus, called the _Corona_.
+
+These four envelopes: the photosphere, the reversing layer, the
+chromosphere, and the corona, constitute the outer portions of our
+luminary.
+
+Below the photosphere little can be seen, although it is known, as will
+appear below, that at certain depths cloud-like forms exist, and freely
+float in an interior atmosphere of invisible gases. Beyond this all is
+mystery, and belongs to the domain of hypothesis.
+
+
+STRUCTURE OF THE PHOTOSPHERE AND CHROMOSPHERE
+
+
+The apparent uniformity of the solar surface disappears when it is
+examined with a telescope of sufficient aperture and magnifying powers.
+Seen under good atmospheric conditions, the greater part of the solar
+surface appears mottled with an infinite number of small, bright
+granules, irregularly distributed, and separated from each other by a
+gray-tinted background.
+
+These objects are known under different names. The terms granules and
+granulations answer very well for the purpose, as they do not imply
+anything positive as to their form and true nature. They have also been
+called _Luculœ_, _Rice Grains_, _Willow Leaves_, etc., by different
+observers.
+
+Although having different shapes, the granulations partake more or less
+of the circular or slightly elongated form. Their diameter, which varies
+considerably, has been estimated at from 0".5 to 3", or from 224 to
+1,344 miles. The granulations which attain the largest size appear,
+under good atmospheric conditions, to be composed of several granules,
+closely united and forming an irregular mass, from which short
+appendages protrude in various directions.
+
+The number of granulations on the surface of the Sun varies considerably
+under the action of unknown causes. Sometimes they are small and very
+numerous, while at other times they are larger, less numerous, and more
+widely separated. Other things being equal, the granulations are better
+seen in the central regions of the Sun than they are near the limb.
+
+Usually the granulations are very unstable; their relative position,
+form, and size undergoing continual changes. Sometimes they are seen to
+congregate or to disperse in an instant, as if acting under the
+influence of attractive and repulsive forces; assembling in groups or
+files, and oftentimes forming capricious figures which are very
+remarkable, but usually of short duration. In an area of great solar
+disturbances, the granulations are often stretched to great distances,
+and form into parallel lines, either straight, wavy, or curved, and they
+have then some resemblance to the flowing of viscous liquids.
+
+The granulations are usually terminated either by rounded or sharply
+pointed summits, but they do not all rise to the same height, as can be
+ascertained with the spectroscope when they are seen sidewise on the
+limb. In the regions where they are most abundant, they usually attain
+greater elevations, and when observed on the limb with the spectroscope,
+they appear as slender acute flames.
+
+The granulations terminated by sharply-pointed crests, although observed
+in all latitudes, seem to be characteristic of certain regions. A daily
+study of the chromosphere, extending over a period of ten years, has
+shown me that the polar regions are rarely ever free from these objects,
+which are less frequent in other parts of the Sun. In the polar regions
+they are sometimes so abundant that they completely form the solar limb.
+These forms of granulation are comparatively rare in the equatorial
+zones, and when seen there, they never have the permanency which they
+exhibit in the polar regions. When observed in the equatorial regions,
+they usually appear in small groups, in the vicinity of sun spots, or
+they are at least enclosed in areas of disturbances where such spots are
+in process of formation. In these regions they often attain greater
+elevations than those seen in high latitudes.
+
+As we are certain that in the equatorial zones these slender flames (_i.
+e._, granulations) are a sure sign of local disturbance, it may be
+reasonably supposed that the same kind of energy producing them nearly
+always prevails in the polar regions, although it is there much weaker,
+and never reaches beyond certain narrow limits.
+
+Studied with the spectroscope, the granulations are found to be composed
+in the main of incandescent hydrogen gas, and of an unknown substance
+provisionally called "helium." Among the most brilliant of them are
+found traces of incandescent metallic vapors, belonging to various
+substances found on our globe.
+
+The chromosphere is not fixed, but varies considerably in thickness in
+its different parts, from day to day. Its thickness is usually greater
+in the polar regions, where it sometimes exceeds 6,700 miles. In the
+equatorial regions the chromosphere very rarely attains this height, and
+when it does, the rising is local and occupies only a small area. In
+these regions it is sometimes so shallow that its depth is only a few
+seconds, and is then quite difficult to measure. These numbers give, of
+course, the extreme limits of the variations of the chromosphere; but,
+nearly always, it is more shallow in the equatorial regions; and, as far
+as my observations go, the difference in thickness between the polar and
+equatorial zones is greater in years of calm than it is in years of
+great solar activity. But ten years of observation are not sufficient to
+warrant any definite conclusions on this subject.
+
+There is undoubtedly some relation between the greater thickness of the
+chromosphere in the polar regions, and the abundance and permanence of
+the sharply-pointed granulations observed in the same regions. This
+becomes more evident when we know that the appearance of
+similarly-pointed flames in the equatorial zones is always accompanied
+with a local thickening of the chromosphere. The thickening in the polar
+regions may be only apparent, and not due to a greater accumulation of
+chromospheric gases there; but may be caused by some kind of repulsive
+action or polarity, which lifts up and extends the summit of the
+granulations in a manner similar to the well-known mode of electric
+repulsion and polarity.
+
+As it seems very probable that the heat and light emanating from the Sun
+are mainly generated at the base of the granulations, in the filamentary
+elements composing the chromosphere and photosphere, it would follow
+that, as the size and number of these objects constantly vary, the
+amount of heat and light emitted by the Sun should also vary in the same
+proportion.
+
+The granulations of the solar surface are represented on Plate I., and
+form the general background to the group of Sun-spots forming the
+picture.
+
+
+THE FACULÆ
+
+
+Although the solar surface is mainly covered with the luminous
+granulations and the grayish background above described, it is very rare
+that its appearance is so simple and uniform as already represented. For
+the most part, on the contrary, it is diversified by larger, brighter,
+and more complicated forms, which are especially visible towards the
+border of the Sun. Owing to their extraordinary brilliancy, these
+objects have been called _Faculœ_ (torches).
+
+Although the faculæ are very seldom seen well beyond 50 heliocentric
+degrees from the limb, yet they exist, and are as numerous in the
+central parts of the disk as they are towards the border; since they
+form a part of the solar surface, and participate in its movement of
+rotation. Their appearance near the limb has been attributed to the
+effect of absorption produced by the solar atmosphere on the
+light from the photosphere; but this explanation seems inadequate,
+and does not solve the problem. The well-known fact that the solar
+protuberances--which are in a great measure identical with the
+faculæ--are much brighter at the base than they are at the summit,
+perhaps gives a clue to the explanation of the phenomenon; especially
+since we know that, in general, the summit of the protuberances is
+considerably broader than their base. When these objects are observed in
+the vicinity of the limb, they present their brightest parts to the
+observer, since, in this position, they are seen more or less sidewise;
+and, therefore, they appear bright and distinct. But as the faculæ
+recede from the limb, their sides, being seen under a constantly
+decreasing angle, appear more and more foreshortened; and, therefore,
+these objects grow less bright and less distinct, until they finally
+become invisible, when their bases are covered over by the broad, dusky
+summit generally terminating the protuberances.
+
+The faculæ appear as bright and luminous masses or streaks on the
+granular surface of the Sun, but they differ considerably in form and
+size. Two types at least are distinguishable. In their simplest form
+they appear either as isolated white spots, or as groups of such spots
+covering large surfaces, and somewhat resembling large flakes of snow.
+In their most characteristic types they appear as intensely luminous,
+heavy masses, from which, in most cases, issue intricate ramifications,
+sometimes extending to great distances. Generally, the ramifications
+issuing from the masses of faculæ have their largest branches directed
+in the main towards the eastern limb of the Sun. Some of these branches
+have gigantic proportions. Occasionally they extend over 60° and even
+80° of the solar surface, and, therefore, attain a length of from
+450,000 to 600,000 miles.
+
+Although the faculæ may be said to be seen everywhere on the surface of
+the Sun, there is a vast difference in different regions, with regard to
+their size, number, and brilliancy. They are largest, most abundant, and
+brightest on two intermediate zones parallel to the solar equator, and
+extending 35° or 40° to the north and to the south of this line. The
+breadth of these zones varies considerably with the activity of the
+solar forces. When they are most active, the faculæ spread on either
+side, but especially towards the equator, where they sometimes nearly
+meet those of the other zone. In years of little solar activity the
+belts formed by the faculæ are very narrow--the elements composing them
+being very few and small, although they never entirely disappear.
+
+The faculæ are very unstable, and are constantly changing: those of the
+small types sometimes form and vanish in a few minutes. When an area of
+disturbance of the solar surface is observed for some time, all seems in
+confusion; the movements of the granulations become unusually violent;
+they congregate in all sorts of ways, and thus frequently form temporary
+faculæ. Action of this kind is, for the most part, peculiar to the
+polar regions of the Sun.
+
+The larger faculæ have undoubtedly another origin, as they seem to be
+mainly formed by the ejection of incandescent gases and metallic vapors
+from the interior of the photosphere. In their process of development
+some of the heavy masses of faculæ are swollen up to great heights,
+being torn in all sorts of ways, showing large rents and fissures
+through which the sight can penetrate.
+
+Very few faculæ are represented in Plate I.; several streaks are shown
+at the upper left-hand corner, some appearing as whitish ramifications
+among the granulations representing the general solar surface.
+
+
+
+
+SUN-SPOTS AND VEILED SPOTS
+
+PLATE I
+
+
+Besides the brilliant faculæ already described, much more conspicuous
+markings, though of a totally different character, are very frequently
+observed on the Sun. On account of their darkish appearance, which is in
+strong contrast with the white envelope of our luminary, these markings
+were called _Maculæ_, or Sun-spots, by their earlier observers.
+
+The Sun-spots are not equally distributed on the solar surface; but like
+the faculæ, to which they are closely related, they occupy two
+zones--one on each side of the equator. These zones are comprised
+between 10° and 35° of north latitude, and 10° and 35° of south
+latitude. Between these two zones is a belt 20° in width, where the
+Sun-spots are rarely seen.
+
+Above the latitudes 35° north and south, the Sun-spots are rare, and it
+is only occasionally, and during years of great solar activity, that
+they appear in these regions; in only a few cases have spots of
+considerable size been seen there. A few observers, however, have seen
+spots as far as 40° and 50 from the equator; and La Hire even observed
+one in 70° of north latitude; but these cases are exceedingly rare. It
+is not uncommon, however, to see very small spots, or groups of such
+spots, within 8° or 10° from the poles.
+
+The activity of the Sun is subject to considerable fluctuation, and
+accordingly the Sun-spots vary in size and number in different years.
+During some years they are large, complicated, and very numerous; while
+in others they are small and scarce, and are sometimes totally absent
+for weeks and months together. The fluctuations in the frequency of
+Sun-spots are supposed to be periodical in their character, although
+their periods do not always appear to recur at exactly regular
+intervals. Sometimes the period is found to be only nine years, while at
+other times it extends to twelve years. The period generally adopted now
+is 11⅒ years, nearly; but further investigations are needed to
+understand the true nature of the phenomenon.
+
+The number of Sun-spots does not symmetrically augment and diminish, but
+the increase is more rapid than the diminution.
+
+The period of increase is only about four years, while that of decrease
+is over seven years; each period of Sun-spot maximum being nearer the
+preceding period of Sun-spot minimum than it is to that next following.
+
+The cause of these fluctuations in the solar energy is at present wholly
+unknown. Some astronomers, however, have attributed it to the influence
+of the planets Venus and Jupiter, the period of revolution of the latter
+planet being not much longer than the Sun-spot period; but this
+supposition lacks confirmation from direct observations, which, so far,
+do not seem to be in favor of the hypothesis. At the present time the
+solar activity is on the increase, and the Sun-spots will probably reach
+their maximum in 1883. The last minimum occurred in 1879, when only
+sixteen small groups of spots were observed during the whole year.
+
+Sun-spots vary in size and appearance; but, unless they are very small,
+in which case they appear as simple black dots, they generally consist
+of two distinct and well-characterized parts, nearly always present.
+There is first, a central part, much darker than the other, and sharply
+divided from it, called the "_Umbra_;" second, a broad, irregular
+radiated fringe of lighter shade, completely surrounding the first, and
+called the "_Penumbra_."
+
+Reduced to its simplest expression, a Sun-spot is a funnel-shaped
+opening through the chromosphere and the photosphere. The inner end of
+the funnel, or opening, gives the form to the umbra, while its sloping
+sides form the penumbra.
+
+The umbra of Sun-spots, whose outlines approximately follow the
+irregularities of the penumbral fringe, has a diameter which generally
+exceeds the width of the penumbral ring. Sometimes it appears uniformly
+black throughout; but it is only so by contrast, as is proved when
+either Mercury or Venus passes near a spot during a transit over the
+Sun's disk. The umbra then appears grayish, when compared with the
+jet-black disk of the planet.
+
+The umbra of spots is rarely so simple as just described; but it is
+frequently occupied, either partly or wholly, by grayish and rosy forms,
+somewhat resembling loosely-entangled muscular fibres. These forms have
+been called the _Gray and Rosy Veils_. Frequently these veils appear as
+if perforated by roundish black holes, improperly called _Nuclei_, which
+permit the sight to penetrate deeper into the interior. To all
+appearance the gray and rosy veils are of the same nature as the
+chromosphere and the faculæ, and are therefore mainly composed of
+hydrogen gas.
+
+Whatever can be known about the interior of the Sun, must be learned
+from the observations of these openings, which are comparatively small.
+But whatever this interior may be, we certainly know that it is not
+homogeneous. Apparently, the Sun is a gigantic bubble, limited by a very
+thin shell. Below this shell exists a large open space filled with
+invisible gases, in which, through the openings constituting the
+Sun-spots, the gray and rosy veils described above are occasionally seen
+floating.
+
+The fringe forming the penumbra of spots is much more complicated than
+the umbra. In its simpler form, it is composed of a multitude of bright,
+independent filaments of different forms and sizes, partly projecting
+one above the other, on the sloping wall of the penumbra, from which
+they seem to proceed. Seen from the Earth, these filaments have somewhat
+the appearance of thatched straw, converging towards the centre of the
+umbra. It is very rare, however, that the convergence of the penumbral
+filaments is regular, and great confusion sometimes arises from the
+entanglement of these filaments. Some of these elements appear straight,
+others are curved or loop-shaped; while still others, much larger and
+brighter than the rest, give a final touch to this chaos of filaments,
+from which results the general thatched and radiating appearance of the
+penumbra.
+
+The extremities of the penumbral filaments, especially of those forming
+the border of the umbra, are usually club-shaped and appear very
+brilliant, as if these elements had been superheated by some forces
+escaping through the opening of the spots.
+
+Besides these characteristics, the Sun-spots have others, which,
+although not always present, properly belong to them. Comparatively few
+spots are so simple as the form just described. Very frequently a spot
+is accompanied by brilliant faculæ, covering part of its umbra and
+penumbra, and appearing to form a part of the spot itself.
+
+When seen projected over Sun-spots, the faculæ appear intensely bright,
+and from these peculiarities they have been called _Luminous Bridges_.
+They are, in fact, bridges, but in most cases they are at considerable
+heights above the spots, kept there by invisible forces. When such spots
+with luminous bridges approach the Sun's limb, it is easy to see, by the
+rapid apparent displacement which they undergo, that they are above the
+general level.
+
+When the spots are closing up, the inverse effect is sometimes observed.
+On several occasions, I have seen huge masses of faculæ advance slowly
+over the penumbra of a spot and fall into the depths of the umbra,
+resembling gigantic cataracts. I have seen narrow branches of faculæ,
+which, after having fallen to great depths in the umbra, floated across
+it and disappeared under the photosphere on the opposite side. I have
+also seen luminous bridges, resembling cables, tightly stretched across
+the spots, slackening slowly, as if loosened at one end, and gently
+curving into the umbra, where they formed immense loops, large enough to
+receive our globe.
+
+It is to be remarked that, in descending under the photospheric shell,
+the bright faculæ and the luminous bridges gradually lose their
+brilliancy. At first they appear grayish, but in descending farther they
+assume more and more the pink color peculiar to the rosy veils. The
+pinkish color acquired by the faculæ when they reach a certain depth
+under the photosphere, is precisely the color of the chromosphere and of
+the solar protuberances, as seen during total eclipses of the Sun--a
+fact which furnishes another proof that the faculæ are of the same
+nature as the protuberances.
+
+I record here an observation which, at first sight, may appear
+paradoxical; but which seems, however, to be of considerable importance,
+as it shows unmistakably that the solar light is mainly, if not
+entirely, generated on its surface, or at least very near to it. On May
+26, 1878, I observed a large group of Sun-spots at a little distance
+from the east limb of the Sun. The spot nearest to the limb was partly
+covered over on its eastern and western sides by bright and massive
+faculæ which concealed about two-thirds of the whole spot, only a
+narrow opening, running from north to south, being left across the
+middle of the spot. Owing to the rotundity of the Sun, the penumbra of
+this spot, although partly covered by the faculæ, could, however, be
+seen on its eastern side, since the sight of the observer could there
+penetrate sidewise under the faculæ. Upon that part of the penumbra
+appeared a strong shadow, representing perfectly the outline of the
+facular mass situated above it. The phenomenon was so apparent that no
+error of observation was possible, and a good drawing of it was secured.
+If this faculæ had been as bright beneath as it was above, it is
+evident that no shadow could have been produced; hence the light of
+these faculæ must have been mainly generated on or very near their
+exterior surfaces. This, with the well-proved fact that the bright
+faculæ lose their light in falling into the interior of the Sun, seems
+to suggest the idea that the bright light emitted by the faculæ, and
+very probably all the solar light, can be generated only on its surface;
+the presence of the coronal atmosphere being perhaps necessary to
+produce it. Several times before this observation, I had suspected that
+some faculæ were casting a shadow, but as this seemed so improbable, my
+attention was not awakened until the phenomenon became so prominent that
+it could not escape notice.
+
+With due attention, some glimpses of the phenomenon can frequently be
+observed through the openings of some of the faculæ projecting over the
+penumbra of Sun-spots. It is very seldom that the structure of the
+penumbra is seen through such openings, which usually appear as dark as
+the umbra of the large spots, although they do not penetrate through the
+photosphere like the latter. It is only when the rents in the faculæ
+are numerous and quite large, that the penumbral structure is recognized
+through them. Since these superficial rents in the faculæ do not extend
+through the photosphere, and appear black, it seems evident that the
+penumbra seen through them cannot be as bright as it is when no faculæ
+are projected upon it, and therefore that the faculæ intercept light
+from the exterior surface, which would otherwise reach the penumbra.
+
+While the matter forming the faculæ sometimes falls into the interior
+of the Sun, the same kind of matter is frequently ejected in enormous
+quantities, and with great force, from the interior, through the visible
+and invisible openings of the photosphere, and form the protuberances
+described in the following section of this manual (Solar protuberances)
+It is not only the incandescent hydrogen gas or the metallic vapors
+which are thus ejected, but also cooler hydrogen gas, which sometimes
+appears as dark clouds on the solar surface. On December 12, 1875, I
+observed such a cloud of hydrogen issuing from the corner of a small
+Sun-spot. It traveled several thousand miles on the solar surface, in a
+north-easterly direction, before it became invisible.
+
+Solar spots are formed in various ways; but, for the most part, the
+apparition of a spot is announced beforehand, by a great commotion of
+the solar surface at the place of its appearance, and by the formation
+of large and bright masses of faculæ, which are usually swollen into
+enormous bubbles by the pressure of the internal gases. These bubbles
+become visible in the spectroscope while they are traversing the solar
+limb, as they are then presented to us sidewise. Under the action of the
+increasing pressure, the base of the faculæ is considerably stretched,
+and, its weakest side finally giving way, the facular mass is torn in
+many places from the solar surface, and is perforated by holes of
+different sizes and forms. The holes thus made along the border of the
+faculæ appear as small black spots, separated more or less by the
+remaining portion of the lacerated faculæ, and they enlarge more and
+more at the expense of the intervening portions, which thus become very
+narrow. This perforated side of the faculæ, offering less resistance,
+is gradually lifted up, as would be the cover of a box, for example,
+while its opposite side remains attached to the surface. The facular
+matter separating the small black holes is greatly stretched during this
+action, and forms long columns and filaments. These appear as luminous
+bridges upon the large and perfectly-formed spot, which is then seen
+under the lifted facular masses. The spots thus made visible are soon
+freed from the facular masses, which are gradually shifted towards the
+opposite side.
+
+In such cases the spots are undoubtedly formed under the faculæ before
+they can be seen. This becomes evident when such spots, not yet cleared
+from the faculæ covering them, are observed near the east limb; since
+in this position the observer can see through the side-openings of the
+faculæ, and sometimes recognize the spots under their cover.
+
+It frequently occurs that the spots thus formed under the faculæ
+continue to be partly covered by the facular clouds, the forces at work
+in them being apparently too feeble to shift them aside. In such cases
+these spots are visible when they are in the vicinity of the limb, where
+they are seen sidewise; but when observed in the east, in being carried
+forward by the solar rotation towards the centre of the disk, they
+gradually diminish in size, and finally become invisible. The
+disappearance of these spots, however, is only apparent, being due to
+the fact that, as they advance towards the centre of the disk, our
+lateral view of them is gradually lost, and they are finally hidden from
+sight by the overhanging faculæ which then serve as a screen between
+the observer and the spot. This class of spots may be called _Lateral
+Spots_, from the fact that they can only be seen laterally, and near the
+Sun's limb.
+
+Solar spots are also formed in various other ways. Some, like those
+represented in Plate I., appearing without being announced by any
+apparent disturbance of the surface, or by the formation of any faculæ,
+form and develop in a very short time. Others, appearing at first as
+very small spots having an umbra and a penumbra, slowly and gradually
+develop into very large spots. This mode of formation, which would seem
+to be the most natural, is, however, quite rare. Spots of this class
+have a duration and permanence not observed in those of any other type.
+These spots of slow and regular development are never accompanied by
+faculæ or luminous bridges, nor have they any gray or rosy veils in
+their interior; a fact which may, perhaps, account for their permanent
+character.
+
+Another class of spots, which is also rare, appear as long and narrow
+crevasses showing the penumbral structure of the ordinary spots; but
+these rarely have any umbra. These long, and sometimes exceedingly
+narrow fissures of the solar envelopes, with their radiated penumbral
+structure, strongly suggest the idea that the photosphere is composed of
+a multitude of filamentary elements having the granulations for summits.
+Such a crevasse is represented on Plate I., and unites the two spots
+which form the group.
+
+The duration of Sun-spots varies greatly. Some last only for a few
+hours; while others continue for weeks and even months at a time, but
+not without undergoing changes.
+
+The modes of disappearance of Sun-spots are as various as those of their
+apparition. The spots rarely close up by a gradual diminution or
+contraction of their umbra and penumbra. This mode of disappearance
+belongs exclusively to the spots deprived of faculæ and veils. One of
+the most common modes of the disappearance of a spot is its invasion by
+large facular masses, which slowly advance upon its penumbra and umbra
+and finally cover it entirely. It is a process precisely the reverse of
+that in which spots are formed by the shifting aside of the faculæ, as
+above described. In other types, the spots close up by the gradual
+enlargement of the luminous bridges traversing them, which are slowly
+transformed into branches of the photosphere, all of the characteristics
+of which they have acquired. In many cases, the spots covered over by
+the faculæ continue to exist for some time, hidden under these masses,
+as is often proved, either by the appearance of small spots on the
+facular mass left at the place they occupied, or even by the
+reappearance of the same spot.
+
+Apart from the general movement of rotation of the solar surface, some
+of the spots seem to be endowed with a proper motion of their own, which
+becomes greater the nearer the spots are to the solar equator. According
+to the observations of Mr. Carrington, the period of rotation of the
+Sun, as deduced from the observations of the solar spots during a period
+of seven years, is 25 days at the equator; while at 50° of heliocentric
+latitude it is 27 days. But the period of rotation, as derived from the
+observations of spots occupying the same latitude, is far from being
+constant, as it varies at different times, with the frequency of the
+spots and with the solar activity, so that at present the law of these
+variations is not well known. From the character of the solar envelope,
+it seems very natural that the rotation should differ in the different
+zones and at different times, since this envelope is not rigid, but very
+movable, and governed by forces which are themselves very variable.
+
+Although it is a general law that the spots near the equator have a more
+rapid motion than those situated in higher latitudes, yet, in many
+cases, the proper motion of the spots is more apparent than real. For
+the most part, the changes of form and the rapid displacements observed
+in some spots are only apparent, and due to the fact that the large
+masses of faculæ which are kept in suspense above them are very
+unstable, and change position with the slightest change in the forces
+holding them in suspension. Since in these cases we view the spots
+through the openings of the faculæ situated above them, the slightest
+motion of these objects produces an apparent motion in the spots,
+although they have remained motionless. Accordingly, it has been
+remarked that of all the spots, those which have the greater proper
+motion are precisely those which have the most faculæ and luminous
+bridges; while the other spots in the same regions, but not attended by
+similar phenomena, are comparatively steady in their movement. These
+last spots are undoubtedly better adapted than any others to exhibit the
+rotation of the Sun; but it is probable that this period of rotation
+will never be known with accuracy, simply because the solar surface is
+unstable, and does not rotate uniformly.
+
+The Sun-spots have a remarkable tendency to form into groups of various
+sizes, but whatever may be the number of spots thus assembled, the group
+is nearly always composed of two principal spots, to which the others
+are only accessories. The tendency of the Sun-spots to assemble in pairs
+is general, and is observed in all latitudes, even among the minute
+temporary groups formed in the polar regions. Whenever several are
+situated quite close together, those belonging to the same group can be
+easily recognized by this character. Whatever may be the position of the
+axis of the two principal spots of a group when it is first formed, this
+axis has a decided tendency to place itself parallel to the solar
+equator, no matter to what latitude the group belongs; and if it is
+disturbed from this position, it soon returns to it when the disturbance
+has ceased.
+
+It is also remarkable that the spots observed at the same time remain in
+nearly the same parallel of latitude for a greater or less period of
+time; but they keep changing their position from year to year, their
+latitude decreasing with the activity of the solar forces.
+
+Among the Sun-spots, those associated with faculæ form the groups which
+attain the largest proportions. When such groups acquire an apparent
+diameter of 1' or more, they are plainly visible to the naked eye, since
+for a spot to be visible to the naked eye on the Sun, it need only
+subtend an angle of 50". I have sometimes seen such groups through a
+smoky atmosphere, when the solar light was so much reduced that the disk
+could be observed directly and without injury to the sight.
+
+The largest spot which ever came under my observation was seen during
+the period from the 13th to the 19th of November, 1870. This spot, which
+was on the northern hemisphere of the Sun, was conspicuous among the
+smaller spots constituting the group to which it belonged, and followed
+them on the east. On November 16th, when it attained its largest size,
+the diameter of its penumbra occupied fully one-fifth of the diameter of
+the Sun; its real diameter being, therefore, not less than 172,000
+miles, or nearly 22 times the diameter of the Earth. As the umbra of
+this spot occupied a little more than one-third of its whole diameter,
+seven globes like our own, placed side by side on a straight line, could
+easily have passed through this immense gap. To fill the area of this
+opening, about 45 such globes would have been needed. This spot was, of
+course, very easily seen with the naked eye, its diameter being almost
+eight times that required for a spot to be visible without a telescope.
+
+Ancient historians often speak of obscurations of the Sun, and it has
+been supposed by some astronomers that this phenomenon might have been
+due in some cases to the apparition of large spots. A few spots on the
+surface of the Sun, like that just described, would sensibly reduce its
+light.
+
+Besides the ordinary Sun-spots already described, others are at times
+observed on the surface of the Sun, which show some of the same
+characteristics, but never attain so large proportions. They always
+appear as if seen through a fog, or veil, between the granulations of
+the solar surface. On account of their vagueness and ill-defined
+contours, I have proposed for these objects the term, "_Veiled Spots_."
+Veiled spots have a shorter duration than the ordinary spots, the
+smaller types sometimes forming and vanishing in a few minutes. Some of
+the larger veiled spots, however, remain visible for several days in
+succession, and show the characteristics of other spots in regard to the
+arrangement of their parts.
+
+The veiled spots have no umbra or penumbra, although they are usually
+accompanied by faculæ resembling those seen near the ordinary spots.
+They are frequently seen in the polar regions, but are there always of
+small size and of short duration. The veiled spots are larger, and more
+apt to arrange themselves into groups, in the regions occupied by the
+ordinary spots, and it is not rare to observe such spots transform
+themselves into ordinary spots, and vice versa. The veiled spots,
+therefore, seem to be ordinary spots filled up, or covered over by the
+granulations and semi-transparent gases composing the chromospheric
+layer. That it is so, becomes more evident, from the fact that large
+Sun-spots in process of diminution are sometimes gradually covered with
+faint and scattered granules which descend in long, narrow filaments,
+and become less and less distinguishable as they attain greater depth.
+This phenomenon, associated with the fact that the luminous bridges seen
+over the Sun-spots which are closing up are sometimes transformed into
+branches which show the characteristic structure of the photosphere,
+goes far to prove that the solar envelopes are mainly composed of an
+innumerable quantity of radial filaments of varying height.
+
+
+[PLATE I.--GROUP OF SUN-SPOTS AND VEILED SPOTS.
+
+Observed on June 17th 1875 at 7 h. 30 m. A.M.]
+
+
+The group of Sun-spots represented in Plate I., was observed and drawn
+on June 17th, 1875, at 7h. 30m. A. M. The first traces of this group
+were seen on June 15th, at noon, and consisted of three small black dots
+disseminated among the granulations. At that time, no disturbance of the
+surface was noticeable, and no faculæ were seen in the vicinity of these
+spots. On June 16th, at 8 o'clock A. M., the three small spots had
+become considerably enlarged, and, as usual, the group consisted of two
+principal spots. Between these two spots all was in motion: the
+granulations, stretched into long, wavy, parallel lines, had somewhat
+the appearance of a liquid in rapid motion. At 1 o'clock, P. M., on the
+same day, the group had considerably enlarged; the faculæ, the
+granulations, and the penumbral filaments being interwoven in an
+indescribable manner. On the morning of the 17th, these spots had
+assumed the complicated form and development represented in the drawing;
+while at the same time two conspicuous veiled spots were seen on the
+left hand, at some distance above the group.
+
+Some luminous bridges are visible upon the left hand spot, traversing
+the penumbra and umbra of this spot in various directions. The umbra of
+one of the spots is occupied, and partly filled with gray and rosy
+veils, similar to those above described, and the granulations of the
+solar surface form a background to the group of spots.
+
+This group of spots was not so remarkable for its size as for its
+complicated structure. The diameter of the group from east to west was
+only 2½ minutes of arc, or about 67,000 miles. The upper part of the
+umbra of the spot situated on the right hand side of the group was
+nearly 7,000 miles in diameter, or less by 1,000 miles than the diameter
+of the Earth. Some of the long filaments composing that part of the
+penumbra, situated on the left hand side of the same spot, were 17,000
+miles in length. One of these fiery elements would be sufficient to
+encircle two-thirds of the circumference of the Earth.
+
+
+
+
+SOLAR PROTUBERANCES
+
+PLATE II
+
+
+The chromosphere forming the outlying envelope of the Sun, is subject,
+as has been shown above, to great disturbances in certain regions,
+causing considerable upheavals of its surface and violent outbursts of
+its gases. From these upheavals and outbursts of the chromosphere result
+certain curious and very interesting forms, which are known under the
+name of "_Solar Protuberances_," "_Prominences_," or "_Flames_."
+
+These singular forms, which could, until recently, be observed only
+during the short duration of the total eclipses of the Sun, can now be
+seen on every clear day with the spectroscope, thanks to Messrs. Janssen
+and Lockyer, to whose researches solar physics is so much indebted.
+
+
+[PLATE II.--SOLAR PROTUBERANCES.
+
+Observed on May 5, 1873 at 9h, 40m. A.M.]
+
+
+The solar protuberances, the Sun-spots, and the faculæ to which they
+are closely related, are confined within the same general regions of the
+Sun, although the protuberances attain higher heliocentric latitudes.
+
+There is certainly a very close relation between the faculæ and the
+solar protuberances, since when a group of the faculæ traverses the
+Sun's limb, protuberances are always seen at the same place. It seems
+very probable that the faculæ and the protuberances are in the main
+identical. The faculæ may be the brighter portion of the protuberances,
+consisting of gases which are still undergoing a high temperature and
+pressure; while the gases which have been relieved from this pressure
+and have lost a considerable amount of their heat, may form that part of
+the protuberances which is only visible on the Sun's limb.
+
+A daily study of the solar protuberances, continued for ten years, has
+shown me that these objects are distributed on two zones which are
+equidistant from the solar equator, and parallel with it. The zone
+arrangement of the protuberances is more easily recognized during the
+years of minimum solar activity, as in these years the zones are very
+narrow and widely separated. During these years the belt of
+protuberances is situated between 40° and 45° of latitude, north and
+south. In years of great solar activity the zones spread considerably on
+either side of these limits, especially towards the equator, which they
+nearly reach, only a narrow belt, usually free from protuberances,
+remaining between them. Towards the poles the zones do not spread so
+much, and there the space free from protuberances is considerably
+greater than it is at the equator.
+
+During years of maximum solar activity, the protuberances, like the
+Sun-spots and the faculæ, are very numerous, very large, and very
+complicated--sometimes occupying a great part of the whole solar limb.
+As many as twenty distinct flames are sometimes observed at one time. In
+years of minimum solar activity, on the contrary, the prominences are
+very few in number, and they are of small size; but, as far as my
+observations go, they are never totally absent.
+
+In general, the solar flames undergo rapid changes, especially those
+which are situated in the vicinity of Sun-spots, although they
+occasionally remain unchanged in appearance and form for several hours
+at a time. The protuberances situated in higher latitudes are less
+liable to great and sudden changes, often retaining the same form for
+several days. The changes observed in the protuberances of the
+equatorial regions are due in part to the comparatively great changes in
+their position with respect to the spectator, which are occasioned by
+the rotation of the Sun. This rotation, of course, has a greater angular
+velocity on the equator than in higher latitudes. In most cases,
+however, the changes of the equatorial protuberances are too great and
+too sudden to be thus explained. They are, in fact, due to the greater
+solar activity developed in the equatorial zones, and wherever spots are
+most numerous.
+
+The solar protuberances appear under various shapes, and are often so
+complicated in appearance that they defy description. Some resemble huge
+clumsy masses having a few perforations on their sides; while others
+form a succession of arches supported by pillars of different styles.
+Others form vertical or inclined columns, often surmounted by cloud-like
+masses, or by various appendages, which sometimes droop gracefully,
+resembling gigantic palm leaves. Some resemble flames driven by the
+wind; others, which are composed of a multitude of long and narrow
+filaments, appear as immense fiery bundles, from which sometimes issue
+long and delicate columns surmounted by torch-like objects of the most
+fantastic pattern. Some others resemble trees, or animal forms, in a
+very striking manner; while still others, apparently detached from the
+solar limb, float above it, forming graceful streamers or clouds of
+various shapes. Some of the protuberances are very massive, while others
+are so thin and transparent as to form a mere veil, through which more
+distant flames can easily be seen.
+
+Notwithstanding this variety of form, two principal classes of solar
+protuberances may be recognized: the cloud-like or quiescent, and the
+eruptive or metallic protuberances.
+
+The first class, which is the most common, comprises all the cloud-like
+protuberances resting upon the chromosphere or floating about it. The
+protuberances of this type often obtain enormous horizontal proportions,
+and it is not rare to see some among them occupying 20° and 30° of the
+solar limb. The height attained by protuberances of this class does not
+correspond in general to their longitudinal extent; although some of
+their branches attain considerable elevations. These prominences very
+seldom have the brilliancy displayed by the other type, and are
+sometimes so faint as to be seen with difficulty. Although it is
+generally stated by observers that some of the protuberances belonging
+to this class are detached from the solar surface, and kept in
+suspension above the surface, like the clouds in our atmosphere, yet it
+seems to me very doubtful whether protuberances are ever disconnected
+from the chromosphere, since, in an experience of ten years, I have
+never been able to satisfy myself that such a thing has occurred. Many
+of them have appeared to me at first sight to be detached from the
+surface, but with a little patience and attention I was always able to
+detect faint traces of filamentary elements connecting them with the
+chromosphere. Quite often I have seen bright protuberances gradually
+lose their light and become invisible, while soon after they had
+regained it, and were as clearly visible as before. Observations of this
+kind seem to show that while the prominences are for the most part
+luminous, there are also a few which are non-luminous and invisible to
+the eye. These dark and invisible forms are most generally found in the
+vicinity of Sun-spots in great activity. When observing such regions
+with the spectroscope, it is not rare to encounter them in the form of
+large dark spots projecting on the solar spectrum near the hydrogen
+lines. On July 28th, 1872, I observed with the spectroscope a dark spot
+of this kind issuing from the vicinity of a large Sun-spot, and
+extending over one-fifth of the diameter of the Sun. This object had
+been independently observed in France a little earlier by M. Chacornac
+with the telescope, in which it appeared as a bluish streak.
+
+The second class of solar protuberances, comprising the eruptive type,
+is the most interesting, inasmuch as it conveys to us a conception of
+the magnitude and violence of the solar forces. The protuberances of
+this class, which are always intensely bright, appear for the most part
+in the immediate vicinity of Sun-spots or faculæ. These protuberances,
+which seem to be due to the outburst of the chromosphere, and to the
+violent ejection of incandescent gases and metallic vapors from the
+interior of the Sun, sometimes attain gigantic proportions and enormous
+heights.
+
+While the spectrum of the protuberances of the cloudy type is simple,
+and usually composed of four hydrogen lines and the yellow line D{3}, that
+of the eruptive class is very complicated, and, besides the hydrogen
+lines and D{3}, it often exhibits the bright lines of sodium, magnesium,
+barium, titanium, and iron, and occasionally, also, a number of other
+bright lines.
+
+The phenomena of a solar outburst are grand and imposing. Suddenly
+immense and acute tongues and jets of flames of a dazzling brilliancy
+rise up from the solar limb and extend in various directions. Some of
+these fiery jets appear perfectly rigid, and remain apparently
+motionless in the midst of the greatest disorder. Immense straps and
+columns form and rise in an instant, bending and waving in all sorts of
+ways and assuming innumerable shapes. Sometimes powerful jets resembling
+molten metal spring up from the Sun, describing graceful parabolas,
+while in their descent they form numerous fiery drops which acquire a
+dazzling brilliancy when they approach the surface.
+
+The upward motion of the protuberances in process of formation is
+sometimes very rapid. Some protuberances have been observed to ascend in
+the solar atmosphere at the rate of from 120 to 497 miles a second.
+Great as this velocity may appear, it is nevertheless insignificant when
+compared with that sometimes attained by protuberances moving in the
+line of sight instead of directly upwards. Movements of this kind are
+indicated by the displacement of the bright or dark lines in the
+spectrum. A remarkable instance of this kind occurred on the 26th of
+June, 1874. On that day I observed a displacement of the hydrogen C line
+corresponding to a velocity of motion of 1,600 miles per second. The
+mass of hydrogen gas in motion producing such a displacement was,
+according to theory, moving towards the Earth at this incredible rate,
+when it instantly vanished from sight as if it had been annihilated, and
+was seen no more.
+
+Until recently the protuberances had not been observed to rise more than
+200,000 miles above the solar surface; but, on October 7th, 1880, a
+flame, which had an elevation of 80,000 miles when I observed it at 8h.
+55m. A. M., had attained the enormous altitude of 350,000 miles when it
+was observed at noon by Professor C. A. Young. If we had such a
+protuberance on the Earth, its summit would be at a height sufficient
+not merely to reach, but to extend 100,000 miles beyond the Moon.
+
+Although the solar protuberances represented in Plate II. have not the
+enormous proportions attained by some of these objects, yet they are as
+characteristic as any of the largest ones, and afford a good
+illustration of the purely eruptive type of protuberances. The height of
+the largest column in the group equals 4' 43", or a little over 126,000
+miles. A large group of Sun-spots was in the vicinity of these
+protuberances when they were observed and delineated.
+
+
+
+
+TOTAL ECLIPSE OF THE SUN
+
+PLATE III
+
+
+A solar eclipse is due to the passage of the Moon directly between the
+observer and the Sun. Such an eclipse can only occur at New Moon, since
+it is only at that time that our satellite passes between us and the
+Sun. The Moon's orbit does not lie precisely in the same plane as the
+orbit of the Earth, but is inclined about five degrees to it, otherwise
+an eclipse of the Sun would occur at every New Moon, and an eclipse of
+the Moon at every Full Moon.
+
+Since the Moon's orbit is inclined to that of the Earth, it must
+necessarily intersect this orbit at two opposite points. These points
+are called the nodes of the Moon's orbit. When our satellite passes
+through either of the nodes when the Moon is new, it appears interposed
+to some extent between the Sun and the Earth, and so produces a solar
+eclipse; while if it passes a node when the Moon is full, it is more or
+less obscured by the Earth's shadow, which then produces an eclipse of
+the Moon. But, on the other hand, when the New Moon and the Full Moon do
+not coincide with the passage of our satellite through the nodes of its
+orbit, no eclipse can occur, since the Moon is not then on a line with
+the Sun and the Earth, but above or below that line.
+
+Owing to the ellipticity of the Moon's orbit, the distance of our
+satellite from the Earth varies considerably during each of its
+revolutions around us, and its apparent diameter is necessarily subject
+to corresponding changes. Sometimes it is greater, sometimes it is less,
+than the apparent diameter of the Sun. If it is greater at the time of a
+solar eclipse, the eclipse will be total to a terrestrial observer
+stationed nearly on the line of the centres of the Sun and Moon, while
+it will be only partial to another observer stationed further from this
+line. But the Moon's distance from the Earth may be so great and its
+apparent diameter consequently so small that even those observers
+nearest the central line of the eclipse see the border of the Sun all
+round the black disk of the Moon; the eclipse is then annular. Even
+during the progress of one and the same eclipse the distance of the Moon
+from the parts of the Earth towards which its shadow is directed may
+vary so much that, while the eclipse is total to some observers, others
+equally near the central line, but stationed at a different place, will
+see it as annular.
+
+The shadow cast by the Moon on the Earth during total eclipses, travels
+along upon the surface of the Earth, in consequence of the daily
+movement of rotation of our globe combined with the movements of the
+Earth and Moon in their orbits. The track of the Moon's shadow over the
+Earth's surface has a general eastward course, so that the more westerly
+observers see it earlier than those east of them. An eclipse may
+continue total at one place for nearly eight minutes, but in ordinary
+cases the total phase is much shorter.
+
+The nodes of the Moon's orbit do not invariably occupy the same
+position, but move nearly uniformly, their position with regard to the
+Sun, Earth, and Moon being at any time approximately what it formerly
+was at a series of times separated by equal intervals from each other.
+Each interval comprises 223 lunations, or 18 years, 11 days, and 7 or 8
+hours. The eclipses which occur within this interval are almost exactly
+repeated during the next similar interval. This period, called the
+"Saros," was well known to the ancients, who were enabled by its means
+to predict eclipses with some certainty.
+
+
+[PLATE III.--TOTAL ECLIPSE OF THE SUN.
+
+Observed July 29, 1878, at Creston, Wyoming Territory]
+
+
+A total eclipse of the Sun is a most beautiful and imposing phenomenon.
+At the predicted time the perfectly round disk of the Sun becomes
+slightly indented at its western limb by the yet invisible Moon. This
+phenomenon is known as the "first contact."
+
+The slight indentation observed gradually increases with the advance of
+the Moon from west to east, the irregularities of the surface of our
+satellite being plainly visible on the border of the dark segment
+advancing on the Sun's disk. With the advance of the Moon on the Sun,
+the light gradually diminishes on the Earth. Every object puts on a dull
+and gloomy appearance, as when night is approaching; while the bright
+sky, losing its light, changes its pure azure for a livid grayish color.
+
+Two or three minutes before totality begins, the solar crescent, reduced
+to minute proportions, gives comparatively so little light that faint
+traces of the Sun's atmosphere appear on the western side behind the
+dark body of the Moon, whose limb then becomes visible outside of the
+Sun. I observed this phenomenon at Creston during the eclipse of 1878.
+From 15 to 20 seconds before totality, the narrow arc of the Sun's disk
+not yet obscured by the Moon seems to break and separate towards the
+extremities of its cusps, which, thus divided, form independent points
+of light, which are called "_Baily's beads_." A moment after, the whole
+solar crescent breaks into numerous beads of light, separated by dark
+intervals, and, suddenly, they all vanish with the last ray of Sunlight,
+and totality has begun with the "second contact." This phenomenon of
+Baily's beads is undoubtedly caused by the irregularities of the Moon's
+border, which, on reaching the solar limb, divide the thin solar
+crescent into as many beads of light and dark intervals as there are
+peaks and ravines seen sidewise on that part of the Moon's limb.
+
+With the disappearance of the last ray of light, the planets and the
+stars of the first and second magnitude seem to light up and become
+visible in the sky. The darkness, which had been gradually creeping in
+with the progress of the eclipse, is then at its maximum. Although
+subject to great variations in different eclipses, the darkness is never
+so great as might be expected from the complete obscuration of our
+luminary, as the part of our atmosphere which is still exposed to the
+direct rays of the Sun, reflects to us some of that light, which thus
+diminishes the darkness resulting from the disappearance of the Sun.
+Usually the darkness is sufficient to prevent the reading of common
+print, and to deceive animals, causing them to act as if night was
+really approaching. During totality the temperature decreases, while the
+humidity of the atmosphere augments.
+
+Simultaneously with the disappearance of Baily's beads, a pale, soft,
+silvery light bursts forth from behind the Moon, as if the Sun, in
+disappearing, had been vaporized and expanded in all directions into
+soft phosphorescent rays and streamers. This pale light is emitted by
+gases constituting the solar atmosphere surrounding the bright nucleus
+now obscured by the dark body of our satellite. This solar atmosphere is
+called _Corona_, from its distant resemblance to the aureola, or glory,
+represented by ancient painters around the heads of saints.
+
+With the bursting forth of the corona, a very thin arc of bright white
+light is seen along the Moon's limb, where the solar crescent has just
+disappeared. This thin arc of light is the reversing layer, which, when
+observed with the spectroscope at that moment, exhibits bright lines
+answering to the dark lines of the ordinary solar spectrum. Immediately
+above this reversing layer, and concentric with it, appears the
+pink-colored chromospheric layer, with its curiously shaped flames and
+protuberances. During totality, the chromosphere and protuberances are
+seen without the aid of the spectroscope, and appear of their natural
+color, which, although somewhat varying in their different parts, is, on
+the whole, pinkish, and similar to that of peach-blossoms; yet it is
+mixed here and there with delicate prismatic hues, among which the pink
+and straw colors predominate.
+
+The color of the corona seems to vary in every eclipse, but as its tints
+are very delicate, it may depend, in a great measure, upon the vision of
+the observer; although there seems to be no doubt that there are real
+variations. At Creston, in 1878, it appeared to both Professor W.
+Harkness and myself of a decided pale greenish hue.
+
+The corona appears under different forms, and has never been observed
+twice alike. Its dimensions are also subject to considerable variations.
+Sometimes it appears regular and very little extended, its distribution
+around the Sun being almost uniform; although in general it spreads a
+little more in the direction of the ecliptic, or of the solar equator.
+At other times it appears much larger and more complicated, and forms
+various wings and appendages, which in some cases, as in 1878, extend to
+immense distances; while delicate rays radiate in straight or curved
+lines from the spaces left in the polar regions between the wings. The
+corona has sometimes appeared as if divided by immense dark gaps,
+apparently free from luminous matter, and strongly resembling the dark
+rifts seen in the tails of comets. This was observed in Spain and Sicily
+during the total eclipse of the Sun in 1870. Different structures,
+forming wisps and streamers of great length, and interlaced in various
+ways, are sometimes present in the corona, while faint but more
+complicated forms, distantly resembling enormous solar protuberances
+with bright nuclei, have also been observed.
+
+As the Moon continues its eastward progress, it gradually covers the
+chromosphere and the solar protuberances on the eastern side of the Sun;
+while, at the same time, the protuberances and the chromosphere on the
+opposite limb gradually appear from under the retreating Moon. Then, the
+thin arc of the reversing layer is visible for an instant, and is
+instantly followed by the appearance of a point of dazzling white light,
+succeeded immediately by the apparition of Daily's beads on each side,
+and totality is over, with this third contact. The corona continues to
+be visible on the eastern side of the Sun for several minutes longer,
+and then rapidly vanishes.
+
+The thin solar crescent increases in breadth as the Moon advances;
+while, at the same time, the darkness and gloom spread over nature
+gradually disappear, and terrestrial objects begin to resume their
+natural appearance. Finally the limb of the Moon separates from that of
+the Sun at the instant of "fourth contact," and the eclipse is over.
+
+The phenomena exhibited by the corona in different eclipses are very
+complex, and, so far, they have not been sufficiently studied to enable
+us to understand the true nature of the solar atmosphere. From the
+spectral analysis of the corona, and the phenomena of polarization, it
+has been learned, at least, that while the matter composing the upper
+part of the solar atmosphere is chiefly composed of an unknown
+substance, producing the green line 1474, its lower part is mainly
+composed of hydrogen gas at different temperatures, a part of which is
+self-luminous, while the other part only reflects the solar light. But
+the proportion of the gaseous particles emitting light, to those simply
+reflecting it, is subject to considerable variations in different
+eclipses. At present it would seem that in years of great solar
+disturbances, the particles emitting light are found in greater quantity
+in the corona than those reflecting it; but further observations will be
+required to confirm these views.
+
+It is very difficult to understand how the corona, which in certain
+eclipses extends only one diameter of the Sun, should, in other cases,
+as in 1878, extend to the enormous distance of twelve times the same
+diameter. Changes of such magnitude in the solar atmosphere, if due to
+the operation of forces with which we are acquainted, cannot yet be
+accounted for by what is known of such forces. Their causes are still as
+mysterious as those concerned in the production of the monstrous tails
+displayed by some comets on their approach to the Sun.
+
+Plate 3, representing the total eclipse of the Sun of July 29th, 1878,
+was drawn from my observations made at Creston, Wyoming Territory, for
+the Naval Observatory. The eclipse is represented as seen in a
+refracting telescope, having an aperture of 6⅓ inches, and as it
+appeared a few seconds before totality was over, and when the
+chromosphere was visible on the western limb of the Sun. The two long
+wings seen on the east and west side of the Sun, appeared considerably
+larger in the sky than they are represented in the picture.
+
+
+
+
+THE AURORA BOREALIS
+
+PLATE IV
+
+
+The name of Polar Auroras is given to certain very remarkable luminous
+meteoric phenomena which appear at intervals above the northern or the
+southern horizons of both hemispheres of the Earth. When the phenomenon
+is produced in our northern sky, it is called "Aurora Borealis," or
+"Northern Lights;" and when it appears in the southern sky, it is called
+"Aurora Australis," or southern aurora.
+
+Marked differences appear in the various auroras observed from our
+northern latitudes. While some simply consist in a pale, faint
+luminosity, hardly distinguishable from twilight, others present the
+most gorgeous and remarkable effects of brightness and colors.
+
+A great aurora is usually indicated in the evening soon after twilight,
+by a peculiar grayish appearance of the northern sky just above the
+horizon. The grayish vapors giving that appearance, continuing to form
+there, soon assume a dark and gloomy aspect, while they gradually take
+the form of a segment of a circle resting on the horizon. At the same
+time that this dark segment is forming, a soft pearly light, which seems
+to issue from its border, spreads up in the sky, where it gradually
+vanishes, being the brightest at its base. This arc of light, gradually
+increasing in extent as well as in brightness, reaches sometimes as far
+as the polar star. On some rare occasions, one or two, and even three,
+concentric arches of bright light form one above the other over the dark
+segment, where they appear as brilliant concentric rainbows. While the
+aurora continues to develop and spread out its immense arc, the border
+of the dark segment loses its regularity and appears indented at several
+places by patches of light, which soon develop into long, narrow,
+diverging rays and streamers of great beauty. For the most part the
+auroral light is either whitish or of a pale, greenish tint; but in some
+cases it exhibits the most beautiful colors, among which the red and
+green predominate. In these cases the rays and streamers, which are
+usually of different colors, produce the most magnificent effects by
+their continual changes and transformations.
+
+The brightness and extent of the auroral rays are likewise subject to
+continual changes. An instant suffices for their development and
+disappearance, which may be succeeded by the sudden appearance of others
+elsewhere, as though the original streamers had been swiftly transported
+to a new place while invisible. It frequently happens that all the
+streamers seem to move sidewise, from west to east, along the arch,
+continuing meanwhile to exhibit their various changes of form and color.
+For a time, these appearances of motion continue to increase, a
+succession of streamers alternately shooting forth and again fading,
+when a sudden lull occurs, during which all motion seems to have ceased.
+The stillness then prevailing is soon succeeded by slight pulsations of
+light, which seem to originate on the border-of the dark segment, and
+are propagated upwards along the streamers, which have now become more
+numerous and active. Slow at first, these pulsations quicken by degrees,
+and after a few minutes the whole northern sky seems to be in rapid
+vibration. The lively upward and downward movement of these streamers
+entitles them to the name of "merry dancers" given them in northern
+countries where they are frequent.
+
+Long waves of light, quickly succeeded by others, are propagated in an
+instant from the horizon to the zenith; these, in their rapid passage,
+cause bends and curves in the streamers, which then, losing their
+original straightness, wave and undulate in graceful folds, resembling
+those of a pennant in a gentle breeze. Although the coruscations add to
+the grandeur of the spectacle, they tend to destroy the diverging
+streamers, which, being disconnected from the dark segment, or torn in
+various ways, are, as it were, bodily carried up towards the zenith.
+
+In this new phase the aurora is transformed into a glorious crown of
+light, called the "Corona." From this corona diverge in all directions
+long streamers of different colors and forms, gracefully undulating in
+numerous folds, like so many banners of light. Some of the largest of
+these streamers appear like fringes composed of short transverse rays of
+different intensity and colors, producing the most fantastic effects,
+when traversed by the pulsations and coruscations which generally run
+across these rays during the great auroral displays.
+
+The aurora has now attained its full development and beauty. It may
+continue in this form for half an hour, but usually the celestial fires
+begin to fade at the end of fifteen or twenty minutes, reviving from
+time to time, but gradually dying out. The northern sky usually appears
+covered by gray and luminous streaks and patches after a great aurora,
+these being occasionally rekindled, but more often they gradually
+disappear, and the sky resumes its usual appearance.
+
+The number of auroras which develop a corona near the zenith is
+comparatively small in our latitudes; but many of them, although not
+exhibited on so grand a scale, are nevertheless very interesting. On
+some very rare occasions the auroral display has been confined almost
+exclusively to the dark segment, which appeared then as if pierced along
+its border by many square openings, like windows, through which appeared
+the bright auroral light.
+
+
+[PLATE IV.--AURORA BOREALIS.
+
+As observed March 1, 1872, at 9h. 25m. P.M.]
+
+
+Among the many auroras which I have had occasion to observe, none are
+more interesting, excepting the type first described, than those which
+form an immense arch of light spanning the heavens from East to West.
+This form of aurora, which is quite rare, I last observed on September
+12th, 1881. All the northern sky was covered with light vapors, when a
+small auroral patch appeared in the East at about 20° above the
+horizon. This patch of light, gradually increasing westward, soon
+reached the zenith, and continued its onward progress until it arrived
+at about 20° above the western horizon, where it stopped. The aurora
+then appeared as a narrow, wavy band of light, crossed by numerous
+parallel rays of different intensity and color. These rays seemed to
+have a rapid motion from West to East along the delicately-fringed
+streamer, which, on the whole, moved southward, while its extremities
+remained undisturbed. Aside from the apparent displacement of the
+fringes, a singular vibrating motion was observed in the auroral band,
+which was traversed by pulsations and long waves of light. The phenomena
+lasted for about twenty minutes, after which the arch was broken in many
+places, and it slowly vanished.
+
+The aurora usually appears in the early part of the evening, and attains
+its full development between ten and eleven o'clock. Although the
+auroral light may have apparently ceased, yet the phenomenon is not at
+an end, as very often a solitary ray is visible from time to time; and
+even towards morning these rays sometimes become quite numerous. On some
+occasions the phenomenon even continues through the following day, and
+is manifested by the radial direction of the cirrus-clouds in the
+heights of our atmosphere. In 1872 I, myself, observed an aurora which
+apparently continued for two or three consecutive days and nights. In
+August, 1859, the northern lights remained visible in the United States
+for a whole week.
+
+The height attained by these meteors is considerable, and it is now
+admitted that they are produced in the rarefied air of the upper regions
+of our atmosphere. From the researches of Professor Elias Loomis on the
+great auroras observed in August and September, 1859, it was ascertained
+that the inferior part of the auroral rays had an altitude of 46 miles,
+while that of their summits was 428 miles. These rays had, therefore, a
+length of 382 miles. From the observation of thirty auroral displays, it
+has been found that the mean height attained by the summit of these
+streamers above the Earth's surface was 450 miles.
+
+But if the auroral streamers are generally manifested at great heights
+in our atmosphere, it would appear from the observations of persons
+living in the regions where the auroras are most frequent, as also from
+those who have been stationed in high northern and southern latitudes,
+that the phenomenon sometimes descends very low. Both Sabine and Parry
+saw the auroral rays projected on a distant mountain; Ross saw them
+almost at sea-level projected on the polar ice; while Wrangel, Franklin,
+and others observed similar phenomena. Dr. Hjaltalin, who has lived in
+latitude 64° 46' north, and has made a particular study of the aurora,
+on one occasion saw the aurora much below the summit of a hill 1,600
+feet high, which was not very far off.
+
+The same aurora is sometimes observed on the same night at places very
+far distant from one another. The great aurora borealis of August 28th,
+1859, for instance, was seen over a space occupying 150° in
+longitude--from California to the Ural Mountains in Russia. It even
+appears now very probable that the phenomenon is universal on our globe,
+and that the northern lights observed in our hemisphere are simultaneous
+with the aurora australis of the southern hemisphere. The aurora of
+September 2d, 1859, was observed all through North and South America,
+the Sandwich Islands, Australia, and Africa; the streamers and
+pulsations of light of the north pole responding to the rays and
+coruscations of the south pole. Of thirty-four auroras observed at
+Hobart Town, in Tasmania, twenty-nine corresponded with aurora borealis
+observed in our hemisphere.
+
+The auroral phenomena, although sometimes visible within the tropics,
+are, however, quite rare in these regions. For the most part they are
+confined within certain zones situated in high latitudes north and
+south. The zone where they are most frequent in our hemisphere forms an
+ellipse, which has the north pole at one of its foci; while the other is
+situated somewhere in North America, in the vicinity of the magnetic
+pole. The central line of the zone upon which the auroras seem to be
+most frequent passes from the northern coast of Alaska through Hudson's
+Bay and Labrador to Iceland, and then follows the northern coast of
+Europe and Asia. The number of auroras diminishes as the observer
+recedes from this zone, and it is only in exceptional cases that they
+are seen near the equator. Near the pole the phenomenon is less frequent
+than it is in the region described. In North America we occupy a
+favorable position for the observation of auroras, as we are nearer the
+magnetic poles than are the Europeans and Asiatics, and we consequently
+have a greater number of auroras in corresponding latitudes.
+
+The position of the dark auroral segment varies with the place occupied
+by the observer, and its centre always corresponds with the magnetic
+meridian. In our Eastern States the auroral segment appears a little to
+the west of the north point; but as the observer proceeds westward it
+gradually approaches this point, and is due north when seen from the
+vicinity of Lake Winnipeg. At Point Barrow, in the extreme north-west of
+the United States, the aurora is observed in the east. In Melville
+Islands, Parry saw it in the south; while in Greenland it is directly in
+the west.
+
+It is stated that auroras are more numerous about the equinoxes than
+they are at any other seasons; and also, when the earth is in perigee,
+than when it is in apogee. An examination which I have made of a
+catalogue by Professor Loomis, comprising 4,137 auroras observed in the
+temperate zone of our hemisphere from 1776 to 1873, sustains this
+statement. During this period, one hundred more auroras were recorded
+during each of the months comprising the equinoxes, than during any
+other months of the year; while eighty more auroras were observed when
+the earth was in perigee, than when it was in apogee. But to establish
+the truth of this assertion on a solid basis, more observations in both
+hemispheres will be required.
+
+The aurora is not simply a terrestrial phenomenon, but is associated in
+some mysterious way with the conditions of the Sun's surface. It is a
+well-known fact that terrestrial magnetism is influenced directly by the
+Sun, which creates the diurnal oscillations of the magnetic needle.
+Between sunrise and two o'clock, the north pole of the needle moves
+towards the west in our northern hemisphere, and in the afternoon and
+evening it moves the other way. These daily oscillations of the needle
+are not uniform in extent; they have a period of regular increase and
+decrease. At a given place the daily oscillations of the magnetic needle
+increase and decrease with regularity during a period which is equal to
+10⅓ years. As this period closely coincides with the Sun-spot period,
+the connection between the variation of the needle and these solar
+disturbances has been recognized.
+
+Auroral phenomena generally accompany the extraordinary perturbations in
+the oscillations of the magnetic needle, which are commonly called
+"magnetic storms," and the greater the auroral displays, the greater are
+the magnetic perturbations. Not only is the needle subject to unusual
+displacements during an aurora, but its movements seem to be
+simultaneous with the pulsations and waving motions of the delicate
+auroral streamers in the sky. When the aurora sends forth a coruscation,
+or a streamer in the sky, the magnetic needle responds to it by a
+vibration. The inference that the auroral phenomena are connected with
+terrestrial magnetism is further supported by the fact that the centre
+of the corona is always situated exactly in the direction of that point
+in the heavens to which the dipping needle is directed.
+
+It has been found that the aurora is a periodical phenomenon, and that
+its period corresponds very closely with those of the magnetic needle
+and Sun-spots. The years which have the most Sun-spots and magnetic
+disturbances have also the most auroras. There is an almost perfect
+similarity between the courses of the three sets of phenomena, from
+which it is concluded that the aurora is connected in some mysterious
+way with the action of the Sun, as well as with the magnetic condition
+of the earth.
+
+A very curious observation, which has been supposed to have some
+connection with this subject, was made on Sept. 1st, 1859, by Mr.
+Carrington and Mr. Hodgson, in England. While these observers, who were
+situated many miles from one another, were both engaged at the same time
+in observing the same Sun-spot, they suddenly saw two luminous spots of
+dazzling brilliancy bursting into sight from the edge of the Sun-spot.
+These objects moved eastward for about five minutes, after which they
+disappeared, having then traveled nearly 34,000 miles. Simultaneously
+with these appearances, a magnetic disturbance was registered at Kew by
+the self-registering magnetic instruments. The very night that followed
+these observations, great magnetic perturbations, accompanied by
+brilliant auroral displays, were observed in Europe. A connection
+between the terrestrial magnetism and the auroral phenomena is further
+proved by the fact that, before the appearance of an aurora, the
+magnetic intensity of our globe considerably increases, but diminishes
+as soon as the first flashes show themselves.
+
+The auroral phenomena are also connected in some way with electricity,
+and generate serious disturbances in the electric currents traversing
+our telegraphic lines, which are thus often rendered useless for the
+transmission of messages during great auroral displays. It sometimes
+happens, however, during such displays, that the telegraphic lines can
+be operated for a long distance, without the assistance of a battery;
+the aurora, or at least its cause, furnishing the necessary electric
+current for the working of the line. During auroras, the telephonic
+lines are also greatly affected, and all kinds of noises and
+crepitations are heard in the instruments.
+
+Two observations of mine, which may have a bearing on the subject,
+present some interest, as they seem to indicate the action of the aurora
+on some of the clouds of our atmosphere. On January 6th, 1872, after I
+had been observing a brilliant aurora for over one hour, an isolated
+black cumulus cloud appeared at a little distance from the western
+extremity of the dark auroral segment. This cloud, probably driven by
+the wind, rapidly advanced eastward, and was soon followed by a
+succession of similar clouds, all starting from the same point. All
+these black clouds apparently followed the same path, which was not a
+straight line, but parallel to and concentric with the border of the
+dark auroral segment. When the first cloud arrived in the vicinity of
+the magnetic meridian passing through the middle of the auroral arc, it
+very rapidly dissolved, and on reaching this meridian became invisible.
+The same phenomenon was observed with the succession of black clouds
+following, each rapidly dissolving as it approached the magnetic
+meridian. This phenomenon of black clouds vanishing like phantoms in
+crossing the magnetic meridian, was observed for nearly an hour. On June
+17th, 1879, I observed a similar phenomenon during a fine auroral
+display. About midway between the horizon and the polar star, but a
+little to the west of the magnetic meridian, there was a large black
+cumulo-stratus cloud which very slowly advanced eastward. As it
+progressed in that direction, its eastern extremity was dissolved in
+traversing the magnetic meridian; while, at the same time, several short
+and quite bright auroral rays issued from its western extremity, which
+in its turn dissolved rapidly, as if burned or melted away in the
+production of the auroral flame.
+
+It seems to be a well observed fact, that during auroras, a strong
+sulphurous odor prevails in high northern latitudes. According to Dr.
+Hjaltalin, during these phenomena, "the ozone of the atmosphere
+increases considerably, and men and animals exposed out of doors emit a
+sulphurous odor when entering a heated room." The Esquimaux and other
+inhabitants of the northern regions assert that great auroras are
+sometimes accompanied by crepitations and crackling noises of various
+sorts. Although these assertions have been denied by several travelers
+who have visited the regions of these phenomena, they are confirmed by
+many competent observers. Dr. Hjaltalin, who has heard these noises
+about six times in a hundred observations, says that they are especially
+audible when the weather is clear and calm; but that when the atmosphere
+is agitated they are not heard. He compares them to the peculiar sound
+produced by a silk cloth when torn asunder, or to the crepitations of
+the electric machine when its motion is accelerated. "When the auroral
+light is much agitated and the streamers show great movements, it is
+then that these noises are heard at different places in the atmosphere."
+
+The spectrum of the auroral light, although it varies with almost every
+aurora, always shows a bright green line on a faint continuous spectrum.
+In addition to this green line I have frequently observed four broad
+diffused bands of greater refrangibility in the spectra of some auroras.
+In two cases, when the auroras appeared red towards the west, the
+spectrum showed a bright red line, in addition to the green line and the
+broad bands described. These facts evidently show that the light of the
+aurora is due to the presence of luminous vapors in our atmosphere; and
+it may reasonably be supposed that these vapors are rendered luminous by
+the passage of electric discharges through them.
+
+
+
+
+THE ZODIACAL LIGHT
+
+PLATE V
+
+
+In our northern latitudes may be seen, on every clear winter and spring
+evening, a column of faint, whitish, nebulous light, rising obliquely
+above the western horizon. A similar phenomenon may also be observed in
+the east, before day-break, on any clear summer or autumn night. To this
+pale, glimmering luminosity the name of "Zodiacal Light" has been given,
+from the fact that it lies in the zodiac along the ecliptic.
+
+In common with all the celestial bodies, the zodiacal light participates
+in the diurnal motion of the sky, and rises and sets with the
+constellations in which it appears. Aside from this apparent motion, it
+is endowed with a motion of its own, accomplished from west to east, in
+a period of a year. In its motion among the stars, the zodiacal light
+always keeps pace with the Sun, and appears as if forming two faint
+luminous wings, resting on opposite sides of this body. In reality it
+extends on each side of the Sun, its axis lying very nearly in the plane
+of the ecliptic.
+
+In our latitudes the phenomena can be observed most advantageously
+towards the equinoxes, in March and September, when twilight is of short
+duration. As we proceed southward it becomes more prominent, and
+gradually increases in size and brightness. It is within the tropical
+regions that the zodiacal light acquires all its splendor: there it is
+visible all the year round, and always appears very nearly perpendicular
+to the horizon, while at the same time its proportions and brilliancy
+are greatly increased.
+
+
+[PLATE V.--THE ZODIACAL LIGHT.
+
+Observed February 20, 1876]
+
+
+The zodiacal light appears under the form of a spear-head, or of a
+narrow cone of light whose base apparently rests on the horizon, while
+its summit rises among the zodiacal constellations. In general
+appearance it somewhat resembles the tail of a large comet whose head is
+below the horizon. The most favorable time to observe this phenomenon in
+the evening, is immediately after the last trace of twilight has
+disappeared; and in the morning, one or two hours before twilight
+appears. When observed with attention, it is seen that the light of the
+zodiacal cone is not uniform, but gradually increases in brightness
+inwardly, especially towards its base, where it sometimes surpasses in
+brilliancy the brightest parts of the Milky-Way. In general, its
+outlines are vague and very difficult to make out, so gradually do they
+blend with the sky. On some favorable occasions, the luminous cone
+appears to be composed of several distinct concentric conical layers,
+having different degrees of brightness, the inner cone being the most
+brilliant of all. There is a remarkable distinction between the evening
+and morning zodiacal light. In our climate, the morning light is pale,
+and never so bright nor so extended as the evening light.
+
+In general, the zodiacal light is whitish and colorless, but in some
+cases it acquires a warm yellowish or reddish tint. These changes of
+color may be accidental and due to atmospheric conditions, and not to
+actual change in the color of the object. Although the zodiacal light is
+quite bright, and produces the impression of having considerable depth,
+yet its transparency is great, since all the stars, except the faint
+ones, can be seen through its substance.
+
+The zodiacal light is subject to considerable variations in brightness,
+and also varies in extent, the apex of its cone varying in distance from
+the Sun's place, from 40 to 90 degrees. These variations cannot be
+attributed to atmospheric causes alone, some of them being due to real
+changes in the zodiacal light itself, whose light and dimensions
+increase or decrease under the action of causes at present unknown. From
+the discussion of a series of observations on the zodiacal light made at
+Paris and Geneva, it appears certain that its light varies from year to
+year, and sometimes even from day to day, independently of atmospheric
+causes. Some of my own observations agree with these results, and one of
+them, at least, seems to indicate changes even more rapid. On December
+18th, 1875, I observed the zodiacal light in a clear sky free from any
+vapors, at six o'clock in the evening. At that time, the point of its
+cone was a little to the north of the ecliptic, at a distance of about
+90 degrees from the Sun's place. Ten minutes later, its summit had sunk
+down 35 degrees, the cone then being reduced to nearly one-half of its
+original dimensions. Ten minutes later, it had risen 25 degrees, and was
+then 80 degrees from the Sun's place, where it remained all the evening.
+On March 22d, 1878, the sky was very clear and the zodiacal light was
+bright when I observed it, at eight o'clock. At that moment the apex of
+the cone of light was a little to the south of the Pleiades, but this
+cone presented an unusual appearance never noticed by me before, its
+northern border appearing much brighter and sharper than usual, while at
+the same time its axis of greatest brightness appeared to be much nearer
+to this northern border than it was to the southern. After a few minutes
+of observation it became evident that the northern border was extending
+itself, as stars which were at some distance from it became gradually
+involved in its light. At the same time that this border spread
+northward, it seemed to diffuse itself, and after a time the cone
+presented its usual appearance, having its southern border brighter and
+better defined than the other. It would have been impossible to
+attribute this sudden change to an atmospheric cause, since only one of
+the borders of the cone participated in it, and since some very faint
+stars near this northern border were not affected in the least while the
+phenomenon occurred. Besides these observations, Cassini, Mairan,
+Humboldt, and many other competent observers have seen pulsations,
+coruscations and bickerings in the light of the cone, which they thought
+could not be attributed to atmospheric causes. It has also been observed
+that at certain periods the zodiacal light has shone with unusual
+intensity for months together.
+
+When this phenomenon is observed from the tropical regions, it is found
+that its axis of symmetry always corresponds with its axis of greatest
+brightness, and that both lie in the plane of the ecliptic, which
+divides its cone into two equal parts. But when the zodiacal light is
+observed in our latitude, the axis of symmetry does not correspond with
+the axis of greatest brightness, and both axes are a little to the north
+of this plane, the axis of symmetry being the farther removed.
+Furthermore, as already stated, the southern border of the cone always
+appears better defined and brighter than the corresponding northern
+margin. It is very probable, if not absolutely certain, that these
+phenomena are exactly reversed when the zodiacal light is observed from
+corresponding latitudes in the southern hemisphere, and that there, its
+axes, both of symmetry and of greatest brightness, appear south of the
+ecliptic, while the northern margin is the brightest. This seems to be
+established by the valuable observations of Rev. George Jones, made on
+board the U. S. steam frigate Mississippi, in California, Japan, and the
+Southern Ocean. "When I was north of the ecliptic," says this observer,
+"the greatest part of the light of the cone appeared to the north of
+this line; when I was to the south of the ecliptic, it appeared to be
+south of it; while when my position was on the ecliptic, or in its
+vicinity, the zodiacal cone was equally divided by this line."
+
+Besides the zodiacal light observed in the East and West, some observers
+have recognized an exceedingly faint, luminous, gauzy band, about 10 or
+12 degrees wide, stretching along the ecliptic from the summit of the
+western to that of the eastern zodiacal cone. This faint narrow belt has
+been called the Zodiacal Band. It has been recognized by Mr. H. C.
+Lewis, who has made a study of this phenomenon, that the zodiacal band
+has its southern margin a little brighter and a little sharper than the
+northern border. This observation is in accordance with similar
+phenomena observed in the zodiacal light, and may have considerable
+importance.
+
+In 1854, Brorsen recognized a faint, roundish, luminous spot in a point
+of the heavens exactly opposite to the place occupied by the Sun, which
+he has called "Gegenschein," or counter-glow. This luminous spot has
+sometimes a small nucleus, which is a little brighter than the rest.
+Night after night this very faint object shifts its position among the
+constellations, keeping always at 180 degrees from the Sun. The position
+of the counter-glow, like that of the zodiacal light and zodiacal band,
+is not precisely on the plane of the ecliptic, but a little to the north
+of this line. It is very probable that near the equator the phenomenon
+would appear different and there would correspond with this plane.
+
+There seems to be some confusion among observers in regard to the
+spectrum of the zodiacal light. Some have seen a bright green line in
+its spectrum, corresponding to that of the aurora borealis; while others
+could only see a faint grayish continuous spectrum, which differs,
+however, from that of a faint solar light, by the fact that it presents
+a well-defined bright zone, gradually blending on each side with the
+fainter light of the continuous spectrum. I have, myself, frequently
+observed the faint continuous spectrum of the zodiacal light, and on one
+occasion recognized the green line of the aurora; but it might have been
+produced by the aurora itself, as yet invisible to the eye, and not by
+the zodiacal light, since, later in the same evening, there was a
+brilliant auroral display. If it were demonstrated that this green line
+exists in the spectrum of the zodiacal light, the fact would have
+importance, as tending to show that the aurora and the zodiacal light
+have a common origin.
+
+Rev. Geo. Jones describes a very curious phenomenon which he observed
+several times a little before the moon rose above the horizon. The
+phenomenon consisted in a short, oblique, luminous cone rising from the
+Moon's place in the direction of the ecliptic. This phenomenon he has
+called the Moon Zodiacal Light. In 1874, I had an opportunity to observe
+a similar phenomenon when the Moon was quite high in the sky. By taking
+the precaution to screen the Moon's disk by the interposition of some
+buildings between it and my eye, I saw two long and narrow cones of
+light parallel to the ecliptic issuing from opposite sides of our
+satellite. The phenomenon could not possibly be attributed to vapors in
+our atmosphere, since the sky was very clear at the moment of the
+observation. Later on, these appendages disappeared with the formation
+of vapors near the Moon, but they reappeared an hour later, when the sky
+had cleared off, and continued visible for twenty minutes longer, and
+then disappeared in a clear sky.
+
+Although the zodiacal light has been studied for over two centuries, no
+wholly satisfactory explanation of the phenomenon has yet been given.
+Now, as in Cassini's time, it is generally considered by astronomers to
+be due to a kind of lens-shaped ring surrounding the Sun, and extending
+a little beyond the Earth's orbit. This ring is supposed to lie in the
+plane of the ecliptic, and to be composed of a multitude of independent
+meteoric particles circulating in closed parallel orbits around the Sun.
+But many difficulties lie in the way of this theory. It seems as
+incompetent to explain the slow and rapid changes in the light of this
+object as it is to explain the contractions and extensions of its cone.
+It fails, moreover, to explain the flickering motions, the coruscations
+observed in its light, or the displacement of its cone and of its axes
+of brightness and symmetry by a mere change in the position of the
+observer. Rev. Geo. Jones, unable to explain by this theory the
+phenomena which came under his observation, has proposed another, which
+supposes the zodiacal light to be produced by a luminous ring
+surrounding the Earth, this ring not extending as far as the orbit of
+the Moon. But this theory also fails in many important points, so that
+at present no satisfactory explanation of the phenomenon can be given.
+
+As the phenomenon is connected in some way with the Sun, and as we have
+many reasons to believe this body to be always more or less electrified,
+it might be supposed that the Sun, acting by induction on our globe,
+develops feeble electric currents in the rarefied gases of the superior
+regions of our atmosphere, and there forms a kind of luminous ridge
+moving with the Sun in a direction contrary to the diurnal motion, and
+so producing the zodiacal light. On this hypothesis, the counter-glow
+would be the result of a smaller cone of light generated by the solar
+induction on the opposite point of the Earth.
+
+Plate 5, which sufficiently explains itself, represents the zodiacal
+light as it appeared in the West on the evening of February 20th, 1876.
+All the stars are placed in their proper position, and their relative
+brightness is approximately shown by corresponding variations in
+size--the usual and almost the only available means of representation.
+Of course, it must be remembered that a star does not, in fact, show any
+disk even in the largest telescopes, where it appears as a mere point of
+light, having more or less brilliancy. The cone of light rises obliquely
+along the ecliptic, and the point forming its summit is found in the
+vicinity of the well-known group of stars, called the Pleiades, in the
+constellation of Taurus, or the Bull.
+
+
+
+
+THE MOON
+
+PLATE VI
+
+
+In its endless journey through space, our globe is not solitary, like
+some of the planets, but is attended by the Moon, our nearest celestial
+neighbor. Although the Moon does not attain to the dignity of a planet,
+and remains a secondary body in the solar system, yet, owing to its
+proximity to our globe, and to the great influence it exerts upon it by
+its powerful attraction, it is to us one of the most important celestial
+bodies.
+
+While the Moon accompanies the Earth around the Sun, it also revolves
+around the Earth at a mean distance of 238,800 miles. For a celestial
+distance this is only a trifling one; the Earth in advancing on its
+orbit travels over such a distance in less than four hours. A cannon
+ball would reach our satellite in nine days; and a telegraphic dispatch
+would be transmitted there in 1½ seconds of time, if a wire could be
+stretched between us and the Moon.
+
+Owing to the ellipticity of the Moon's orbit, its distance from the
+Earth varies considerably, our satellite being sometimes 38,000 miles
+nearer to us than it is at other times. These changes in the distance of
+the Moon occasion corresponding changes from 29' to 33' in its apparent
+diameter. The real diameter of the Moon is 2,160 miles, or a little over
+one-quarter the diameter of our globe; our satellite being 49 times
+smaller than the Earth.
+
+The mean density of the materials composing the Moon is only
+⁶⁄₁₀ that of the materials composing the Earth, and the force of
+gravitation at the surface of our satellite is six times less than it is
+at the surface of our globe. If a person weighing 150 lbs. on our Earth
+could be transported to the Moon, his weight there would be only 25 lbs.
+
+The Moon revolves around the Earth in about 27⅓ days, with a mean
+velocity of one mile per second, the revolution constituting its
+sidereal period. If the Earth were motionless, the lunar month would be
+equal to the sidereal period; but owing to its motion in space, the Sun
+appears to move with the Moon, though more slowly, so that after having
+accomplished one complete revolution, our satellite has yet to advance
+2¼ days before reaching the same apparent position in regard to the
+Earth and the Sun that it had at first. The interval of time comprised
+between two successive New Moons, which is a little over 29½ days,
+constitutes the synodical period of the Moon, or the lunar month.
+
+The Moon is not a self-luminous body, but, like the Earth and the
+planets, it reflects the light which it receives from the Sun, and so
+appears luminous. That such is the case is sufficiently demonstrated by
+the phases exhibited by our satellite in the course of the lunar month.
+Every one is familiar with these phases, which are a consequence of the
+motion of the Moon around the Earth. When our satellite is situated
+between us and the Sun, it is New Moon; since we cannot see its
+illuminated side, which is then turned away from us towards the Sun.
+When, on the contrary, it reaches that point of its orbit which, in
+regard to us, is opposite to the Sun's place, it is Full Moon; since
+from the Earth we can only see the fully illuminated side of our
+satellite. Again, when the Moon arrives at either of the two opposite
+points of its orbit, the direction of which from the Earth is at right
+angles with that of the Sun, it is either the First or the Last Quarter;
+since in these positions we can only see one-half of its illuminated
+disk.
+
+The curve described by the Moon around the Earth lies approximately in a
+plane, this plane being inclined about 5° to the ecliptic. Since our
+satellite, in its motion around us and the Sun, closely follows the
+ecliptic, which is inclined 23½° to the equator, it results that when
+this plane is respectively high or low in the sky, the moon is also high
+or low when crossing the meridian of the observer. In winter that part
+of the ecliptic occupied by the Sun is below the equator, and,
+consequently, the New Moons occurring in that season are low in the sky,
+since at New Moon our satellite must be on the same side of the ecliptic
+with the Sun. But the Full Moons in the same season are necessarily high
+in the sky, since a Full Moon can only occur when our satellite is on
+the opposite side of the ecliptic from the Sun, in which position it is,
+of course, as many degrees above the equator as the Sun is below. The
+Full Moon which happens nearest to the autumnal equinox is commonly
+called the Harvest Moon, from the fact that, after full, its delays in
+rising on successive evenings are very brief and therefore favorable for
+the harvest work in the evening. The same phenomenon occurs in every
+other lunar month, but not sufficiently near the time of Full Moon to be
+noticeable. When, in spring, a day or two after New Moon, our satellite
+begins to show its thin crescent, its position on the ecliptic is north
+as well as east of that occupied by the Sun; hence, its horns are nearly
+upright in direction, and give it a crude resemblance to a tipping bowl,
+from which many people who are unaware of its cause, and that this
+happens every year, draw conclusions as to the amount of rain to be
+expected.
+
+One of the most remarkable features of the Moon's motions is that our
+satellite rotates on its axis in exactly the same period of time
+occupied by its revolution around the Earth, from which it results that
+the Moon always presents to us the same face. To explain this
+peculiarity, astronomers have supposed that the figure of our satellite
+is not perfectly spherical, but elongated, so that the attraction of the
+Earth, acting more powerfully upon its nearest portions, always keeps
+them turned toward us, as if the Moon were united to our globe by a
+string. It is not exactly true, however, that the Moon always presents
+its same side to us, although its period of rotation exactly equals that
+of its revolution; since in consequence of the inclination of its axis
+of rotation to its orbit, combined with the irregularities of its
+orbital motion about us, apparent oscillations in latitude and in
+longitude, called librations, are created, from which it results that
+nearly ⁶⁄₁₀ of the Moon's surface is visible from the Earth at
+one time or another.
+
+The Moon is a familiar object, and every one is aware that our
+satellite, especially when it is fully illuminated, presents a variety
+of bright and dark markings, which, from their distant resemblance to a
+human face, are popularly known as "the man in the moon." A day or two
+after New Moon, when the thin crescent of our satellite is visible above
+the western horizon after sunset, the dark portion of its disk is
+plainly visible, and appears of a pale, ashy gray color, although not
+directly illuminated by the Sun. This phenomenon is due to the
+Earth-shine, or to that portion of solar light which the illuminated
+surface of our globe reflects to the dark side of the Moon, exactly in
+the same manner that the Moon-shine, on our Earth, is due to the solar
+light reflected to our globe by the illuminated Moon.
+
+Seen with a telescope of moderate power, or even with a good
+opera-glass, the Moon presents a peculiar mottled appearance, and has a
+strong resemblance to a globe made of plaster of Paris, on the surface
+of which numerous roundish, saucer-shaped cavities of various sizes are
+scattered at random. This mottled structure is better seen along the
+boundary line called the _terminator_, which divides the illuminated
+from the dark side of the Moon. The line of the terminator always
+appears jagged, and it is very easy to recognize that this irregularity
+is due to the uneven and rugged structure of the surface of our
+satellite.
+
+A glance at the Moon through a larger telescope shows that the bright
+spots recognized with the naked eye belong to very uneven and
+mountainous regions of our satellite, while the dark ones belong to
+comparatively smooth, low surfaces, comparable to those forming the
+great steppes and plains of the Earth. When examined with sufficient
+magnifying power, the white, rugged districts of the Moon appear covered
+over by numerous elevated craggy plateaus, mountain-chains, and deep
+ravines; by steep cliffs and ridges; by peaks of great height and
+cavities of great depth. This rugged formation, which is undoubtedly of
+volcanic origin, gives our satellite a desolate and barren appearance.
+The rugged tract occupies more than one-half of the visible surface of
+the Moon, forming several distinct masses, the principal of which occupy
+the south and south-western part of the disk. That this formation is
+elevated above the general level is proved by the fact that the
+mountains, peaks, and other objects which compose it, all cast a shadow
+opposite to the Sun; and further, that the length of these shadows
+diminishes with the elevation of the Sun above the lunar horizon.
+
+Since Galileo's time the surface of the Moon has been studied by a host
+of astronomers, and accurate maps of its topographical configuration
+have been made, and names given to all features of any prominence. It
+may even be said that in its general features, the visible surface of
+our satellite is now better known to us than is the surface of our own
+Earth.
+
+One of the most striking and common features of the mountainous
+districts of the Moon, is the circular, ring-like disposition of their
+elevated parts, which form numerous crater-like objects of different
+sizes and depths. Many thousands of crater-like objects are visible on
+the Moon through a good telescope, and, considering how numerous the
+small ones are, there is, perhaps, no great exaggeration in fixing their
+number at 50,000, as has been done by some astronomers. These volcanic
+regions of the Moon cannot be compared to anything we know, and far
+surpass in extent those of our globe. The number and size of the craters
+of our most important volcanic regions in Europe, in Asia, in North and
+South America, in Java, in Sumatra, and Borneo, are insignificant when
+compared with those of the Moon. The largest known craters on the Earth
+give only a faint idea of the magnitude of some of the lunar craters.
+The great crater Haleakala, in the Sandwich Islands, probably the
+largest of the terrestrial volcanoes, has a circumference of thirty
+miles, or a diameter of a little less than ten miles. Some of the great
+lunar craters, called walled plains, such as Hipparchus, Ptolemæus,
+etc., have a diameter more than ten times larger than that of Haleakala,
+that of the first being 115 miles and that of the last 100 miles. These
+are, of course, among the largest of the craters of the Moon, although
+there are on our satellite a great number of craters above ten miles in
+diameter.
+
+The crater-forms of the Moon have evidently appeared at different
+periods of time, since small craters are frequently found on the walls
+of larger ones; and, indeed, still smaller craters are not rarely seen
+on the walls of these last. The walls of the lunar craters are usually
+quite elevated above the surrounding surface, some of them attaining
+considerable elevations, especially at some points, which form peaks of
+great height. Newton, the loftiest of all, rises at one point to the
+height of 23,000 feet, while many others range from ten to twenty
+thousand feet in height. Several craters have their floor above the
+general surface--Plato, for instance. Wargentin has its floor nearly on
+a level with the summit of its walls, showing that at some period of its
+history liquid lavas, ejected from within, have filled it to the brim
+and then solidified. The floors of some of the craters are smooth and
+flat, but in general they are occupied by peaks and abrupt mountainous
+masses, which usually form the centre. Many of their outside walls are
+partly or wholly covered by numerous ravines and gullies, winding down
+their steep declivities, branching out and sometimes extending to great
+distances from their base. It would seem that these great volcanic
+mouths have at some time poured out torrents of lavas, which, in their
+descent, carved their passage by the deep gullies now visible.
+Sometimes, also, the crater slopes are strewn with debris, giving them a
+peculiar volcanic appearance.
+
+Notwithstanding their many points of similarity with the volcanoes of
+the Earth, the lunar craters differ from them in many particulars,
+showing that volcanic forces acting on different globes may produce
+widely different results. For example, the floors of terrestrial craters
+are usually situated at considerable elevations above the general
+surface, while those of the lunar craters are generally much depressed,
+the height of their walls being only about one-half the depth of their
+cavities. Again, while on the Earth the mass of the volcanic cones far
+exceeds the capacity of their openings, on the Moon it is not rare to
+see the capacity of the crater cavities exceeding the mass of the
+surrounding walls. On the Earth, the volcanic cones and mouths are
+comparatively regular and smooth, and are generally due to the
+accumulation of the ashes and the debris of all kinds which are ejected
+from the volcanic mouths. On the Moon, very few craters show this
+character, and for the most part their walls have a very different
+structure, being irregular, very rugged, and composed of a succession of
+concentric ridges, rising at many points to great elevations, and
+forming peaks of stupendous height. Again, many of the larger
+terrestrial craters have their interior occupied by a central cone, or
+several such cones, having a volcanic mouth on their summits; on the
+Moon such central cones are very rare. Although many of the large lunar
+craters have their interior occupied by central masses which have been
+often compared to the central cones of our great volcanoes, yet these
+objects have a very different character and origin. For the most part,
+they are mountainous masses of different forms--having very rarely any
+craters on them--and seem to have resulted from the crowding and lifting
+up of the crater floor by the phenomena of subsidence, of which these
+craters show abundant signs. Besides, the terrestrial craters are
+characterized by large and important lava streams, while on the Moon the
+traces of such phenomena are quite rare, and when they are shown, they
+generally differ from those of the Earth by their numerous and
+complicated ramifications, and also by the fact that many of these lava
+streamlets take their origin at a considerable distance from the crater
+slopes, and are grooved and depressed as if the burning liquids which
+are supposed to have produced them had subsequently disappeared, by
+evaporation or otherwise, leaving the furrow empty.
+
+The dark spots of the Moon, when viewed through a telescope, exhibit a
+totally different character, and show that they belong to a different
+formation from that of the brighter portions. These darker tracts do not
+seem to have had a direct volcanic origin like the latter, but rather
+appear to have resulted from the solidification of semi-fluid materials,
+which have overflowed vast areas at different times. The surface of this
+system is comparatively smooth and uniform, only some small craters and
+low ridges being seen upon it. The level and dark appearance of these
+areas led the ancient astronomers to the belief that they were produced
+by a liquid strongly absorbing the rays of light, and were seas like our
+seas. Accordingly, these dark surfaces were called _Maria_, or Seas, a
+name which it is convenient to retain, although it is well known to have
+originated in an error. The so-called seas of the Moon are evidently
+large flat surfaces similar to the deserts, steppes, pampas, and
+prairies of the Earth in general appearance. The great plains of the
+Moon are at a lower level than that of the other formation, and that
+which first attracts the observer's attention is the fact that they are
+surrounded almost on all sides by an irregular line of abrupt cliffs and
+mountain chains, showing phenomena of dislocation. This character of
+dislocation, which is general, and is visible everywhere upon the
+contours of the plains, seems to indicate that phenomena of subsidence,
+either slow or rapid, have occurred on the Moon; while, at the same
+time, the sunken surfaces were overflowed by a semi-fluid liquid, which
+solidified afterwards. The evidences of subsidence and overflowing
+become unmistakable when we observe that, along the borders of the gray
+plains, numerous craters are more or less embedded in the gray
+formation, only parts of the summit of their walls remaining visible, to
+attest that once large craters existed there. The farther from the
+border of the plain the vestiges of these craters are observed, the
+deeper they are embedded in the gray formation. That phenomena of
+subsidence have occurred on a grand scale on the Moon, is further
+indicated by the fact that the singular systems of fractures called
+clefts and rifts generally follow closely the outside border of the gray
+plains, often forming parallel lines of dislocation and fractures. In
+the interior regions of the gray formation, these fractures are
+comparatively rare.
+
+The gray, lava-like formation is obviously of later origin than the
+mountainous system to which belong the embedded craters above described.
+Its comparatively recent origin might also be inferred from the
+smallness of its craters and its low ridges. The few large craters
+observed on this formation evidently belong to the earlier system.
+
+The color of this system of gray plains is far from being uniform. In
+general appearance it is of a bluish gray, but when observed
+attentively, large areas appear tinted with a dusky olive-green, while
+others are slightly tinged with yellow. Some patches appear brownish,
+and even purplish. A remarkable example of the first case is seen on the
+surface, which encloses within a large parallelogram the two conspicuous
+craters, Aristarchus and Herodotus. This surface evidently belongs to a
+different system from that of the Oceanus Procellarum surrounding it,
+as, besides its color, which totally differs from that of the gray
+formation, its surface shows the rugged structure of the volcanic
+formation.
+
+When the Moon is full, some very curious white, luminous streaks are
+seen radiating from different centres, which, for the most part, are
+important craters, occupied by interior mountains. The great crater
+Tycho is the centre of the most imposing of the systems of white
+streaks. Some of the diverging rays of this great centre extend to a
+distance equal to one-quarter of the Moon's circumference, or about
+1,700 miles. The true nature of these luminous streaks is unknown, but
+it seems certain that they have their origin in the crater from which
+they diverge. They do not form any relief on the surface, and are seen
+going up over the mountains and steep walls of the crater, as well as
+down the ravines and on the floors of craters.
+
+The Moon seems to be deprived of an atmosphere; or, if it has any, it
+must be so excessively rare that its density is less than of the density
+of the Earth's atmosphere, since delicate tests afforded by the
+occultation of stars have failed to reveal its presence. Although no
+atmosphere of any consequence exists on the Moon, yet phenomena which I
+have observed seem to indicate the occasional presence there of vapors
+of some sort. On several occasions, I have seen a purplish light over
+some parts of the Moon, which prevented well-known objects being as
+distinctly seen as they were at other times, causing them to appear as
+if seen through a fog. One of the most striking of these observations
+was made on January 4th, 1873, on the crater Kant and its vicinity,
+which then appeared as if seen through luminous purplish vapors. On one
+occasion, the great crater Godin, which was entirely involved in the
+shadow of its western wall, appeared illuminated in its interior by a
+faint purplish light, which enabled me to recognize the structure of
+this interior. The phenomenon could not be attributed in this case to
+reflection, since the Sun, then just rising on the western wall of the
+crater, had not yet grazed the eastern wall, which was invisible. It is
+not impossible that a very rare atmosphere composed of such vapors
+exists in the lower parts of the Moon.
+
+If the Moon has no air, and no liquids of any sort, it seems impossible
+that its surface can maintain any form of life, either vegetable or
+animal, analogous to those on the Earth. In fact, nothing indicating
+life has been detected on the Moon--our satellite looking like a barren,
+lifeless desert. If life is to be found there at all, it must be of a
+very elementary nature. Aside from the want of air and water to sustain
+it, the climatic conditions of our satellite are very unfavorable for
+the development of life. The nights and days of the Moon are each equal
+to nearly fifteen of our days and nights. For fifteen consecutive
+terrestrial days the Sun's light is absent from one hemisphere of the
+Moon; while for the same number of days the Sun pours down on the other
+hemisphere its light and heat, the effects of which are not in any way
+mitigated by an atmosphere. During the long lunar nights the temperature
+must at least fall to that of our polar regions, while during its long
+days it must be far above that of our tropical zone. It has been
+calculated that during the lunar nights the temperature descends to 23°
+below zero, while during the days it rises to 468°, or 256° above the
+boiling point.
+
+It has been a question among astronomers whether changes are still
+taking place at the surface of the Moon. Aside from the fact that
+change, not constancy, is the law of nature, it does not seem doubtful
+that changes occur on the Moon, especially in view of the powerful
+influences of contraction and dilatation to which its materials are
+submitted by its severe alternations of temperature. From the distance
+at which we view our satellite, we cannot expect, of course, to be able
+to see changes, unless they are produced on a large scale. Theoretically
+speaking, the largest telescopes ever constructed ought to show us the
+Moon as it would appear to the naked eye from a distance of 40 miles;
+but in practice it is very different. The difficulty is in the fact
+that, while we magnify the surface of a telescopic image, we are unable
+to increase its light; so that, practically, in magnifying an object, we
+weaken its light proportionally to the magnifying power employed. The
+light of the Moon, especially near the terminator, where we almost
+always make our observations, is not sufficiently bright to bear a very
+high magnifying power, and only moderate ones can be applied to its
+study. What we gain by enlarging an object, we more than lose by the
+weakening of its light. Besides, a high magnifying power, by increasing
+the disturbances generally present in our atmosphere, renders the
+telescopic image unsteady and very indistinct. On the whole, the largest
+telescopes now in existence do not show us our satellite better than if
+we could see it with the naked eye from a distance of 300 miles or more.
+At such a distance only considerable changes would be visible.
+
+Notwithstanding these difficulties, it is believed that changes have
+been detected in Linné, Marius, Messier, and several other craters. An
+observation of mine seems to indicate that changes have recently taken
+place in the great crater Eudoxus. On February 20th, 1877, between 9h.
+30m. and 10h. 30m., I observed a straight, narrow wall crossing this
+crater from east to west, a little to the south of its centre. This wall
+had a considerable elevation, as was proved by the shadow it cast on its
+northern side. Towards its western end this wall appeared as a brilliant
+thread of light on the black shadow cast by the western wall of the
+crater. The first time I had occasion to observe this crater again,
+after this observation, was a year later, on February 17th, 1878; no
+traces of the wall were then detected. Many times since I have tried to
+find this narrow wall again, when the Moon presented the same phase and
+the same illumination, but always with negative results. It seems
+probable that this structure has crumbled down, yet it is very singular
+that so prominent a feature should not have been noticed before.
+
+
+[PLATE VI.--MARE HUMORUM.
+
+From a study made in 1875]
+
+
+The "Mare Humorum," or sea of moisture, as it is called, which is
+represented on Plate VI., is one of the smaller gray lunar plains. Its
+diameter, which is very nearly the same in all directions, is about 270
+miles, the total area of this plain being about 50,000 square miles. It
+is one of the most distinct plains of the Moon, and is easily seen with
+the naked eye on the left-hand side of the disk. The floor of the plain
+is, like that of the other gray plains, traversed by several systems of
+very extended but low hills and ridges, while small craters are
+disseminated upon its surface. The color of this formation is of a dusky
+greenish gray along the border, while in the interior it is of a lighter
+shade, and is of brownish olivaceous tint. This plain, which is
+surrounded by high clefts and rifts, well illustrates the phenomena of
+dislocation and subsidence. The double-ringed crater Vitello, whose
+walls rise from 4,000 to 5,000 feet in height, is seen in the upper
+left-hand corner of the gray plain. Close to Vitello, at the east, is
+the large broken ring-plain Lee, and farther east, and a little below,
+is a similarly broken crater called Doppelmayer. Both of these open
+craters have mountainous masses and peaks on their floor, which is on a
+level with that of the Mare Humorum. A little below, and to the left of
+these objects, is seen a deeply embedded oval crater, whose walls barely
+rise above the level of the plain. On the right-hand side of the great
+plain, is a long _fault_, with a system of fracture running along its
+border. On this right-hand side, may be seen a part of the line of the
+terminator, which separates the light from the darkness. Towards the
+lower right-hand corner, is the great ring-plain Gassendi, 55 miles in
+diameter, with its system of fractures and its central mountains, which
+rise from 3,000 to 4,000 feet above its floor. This crater slopes
+southward towards the plain, showing the subsidence to which it has been
+submitted. While the northern portion of the wall of this crater rises
+to 10,000 feet, that on the plain is only 500 feet high, and is even
+wholly demolished at one place where the floor of the crater is in
+direct communication with the plain. In the lower part of the _mare_,
+and a little to the west of the middle line, is found the crater
+Agatharchides, which shows below its north wall the marks of rills
+impressed by a flood of lava, which once issued from the side of the
+crater. On the left-hand side of the plain, is seen the half-demolished
+crater Hippalus, resembling a large bay, which has its interior strewn
+with peaks and mountains. On this same side can be seen one of the most
+important systems of clefts and fractures visible on the Moon, these
+clefts varying in length from 150 to 200 miles.
+
+
+
+
+ECLIPSES OF THE MOON
+
+PLATE VII
+
+
+Since the Moon is not a self-luminous body, but shines by the light
+which it borrows from the Sun, it follows that when the Sun's light is
+prevented from reaching its surface, our satellite becomes obscured. The
+Earth, like all opaque bodies exposed to sunlight, casts a shadow in
+space, the direction of which is always opposite to the Sun's place. The
+form of the Earth's shadow is that of a long, sharply-pointed cone,
+which has our globe for its base. Its length, varying with the distance
+of the Earth from the Sun, is, on an average, 855,000 miles, or 108
+times the terrestrial diameter. This conical shadow of the Earth,
+divided longitudinally by the plane of the ecliptic, lies half above and
+half below that plane, on which the summit of the shadow describes a
+whole circumference in the course of a year. If the Moon's orbit were
+not inclined to the ecliptic, our satellite would pass at every Full
+Moon directly through the Earth's shadow; but, owing to that
+inclination, it usually passes above or below the shadow. Twice,
+however, during each of its revolutions, it must cross the plane of the
+ecliptic, the points of its orbit where this happens being called nodes.
+Accordingly, if it is near a node at the time of Full Moon, it will
+enter the shadow of the Earth, and become either partly or wholly
+obscured, according to the distance of its centre from the plane of the
+ecliptic. The partial or total obscuration of the Moon's disk thus
+produced constitutes a partial or total eclipse of the Moon. The
+essential conditions for an eclipse of the Moon are, therefore, that our
+satellite must not only be full, but must also be at or very near one of
+its nodes.
+
+Although inferior in importance to the eclipses of the Sun, the eclipses
+of the Moon are, nevertheless, very interesting and remarkable
+phenomena, which never fail to produce a deep impression on the mind of
+the observer, inasmuch as they give him a clear insight into the silent
+motions of the planetary bodies.
+
+At the mean distance of the Moon from the Earth, the diameter of the
+conical shadow cast in space by our globe is more than twice as large as
+that of our satellite. But, besides this pure dark shadow of the Earth,
+its cone is enveloped by a partial shadow called "Penumbra," which is
+produced by the Sun's light being partially, but not wholly, cut off by
+our globe.
+
+While the Moon is passing into the penumbra, a slight reduction of the
+light of that part of the disk which has entered it, is noticeable. As
+the progress of the Moon continues, the reduction becomes more
+remarkable, giving the impression that rare and invisible vapors are
+passing over our satellite. Some time after, a small dark-indentation,
+marking the instant of first contact, appears on the eastern or
+left-hand border of the Moon, which is always the first to encounter the
+Earth's shadow, since our satellite is moving from west to east. The
+dark indentation slowly and gradually enlarges with the onward progress
+of the Moon into the Earth's shadow, while the luminous surface of its
+disk diminishes in the same proportion. The form of the Earth's shadow
+on the Moon's disk clearly indicates the rotundity of our globe by its
+circular outline. Little by little the dark segment covers the Moon's
+disk, and its crescent, at last reduced to a mere thread of light,
+disappears at the moment of the second contact. With this the phase of
+totality begins, our satellite being then completely involved in the
+Earth's shadow.
+
+The Moon remains so eclipsed for a period of time which varies with its
+distance from the Earth, and with the point of its orbit where it
+crosses the conical shadow. When it passes through the middle of this
+shadow, while its distance from our globe is the least, the total phase
+of an eclipse of the Moon may last nearly two hours. The left-hand
+border of our satellite having gone first into the Earth's shadow, is
+also the first to emerge, and, at the moment of doing so, it receives
+the Sun's light, and totality ends with the third contact. The lunar
+crescent gradually increases in breadth after its exit from the shadow,
+and finally the Moon recovers its fully illuminated disk as before, at
+the moment its western border leaves the Earth's shadow. Soon after, it
+passes out of the penumbra, and the eclipse is over. In total eclipses,
+the interval of time from the first to last contact may last 5h. 30m,
+but it is usually shorter.
+
+Soon after the beginning of an eclipse, the dark segment produced by the
+Earth's shadow on the Moon's disk generally appears of a dark grayish
+opaque color, but with the progress of the phenomenon, this dark tint is
+changed into a dull reddish color, which, gradually increasing, attains
+its greatest intensity when the eclipse is total. At that moment the
+color of the Moon is of a dusky, reddish, coppery hue, and the general
+features of the Moon's surface are visible as darker and lighter tints
+of the same color. It sometimes happens, however, that our satellite
+does not exhibit this peculiar coppery tint, but appears either blackish
+or bluish, in which case it is hardly distinguishable from the sky.
+
+It is very rare for the Moon to disappear completely during totality,
+and even when involved in the deepest part of the Earth's shadow, our
+satellite usually remains visible to the naked eye, or, at least, to the
+telescope. This phenomenon is to be attributed to the fact that the
+portion of the solar rays which traverse the lower strata of our
+atmosphere are strongly refracted, and bend inward in such a manner that
+they fall on the Moon, and sufficiently illuminate its surface to make
+it visible. The reddish color observed is caused by the absorption of
+the blue rays of light by the vapors which ordinarily-saturate the lower
+regions of our atmosphere, leaving only red rays to reach the Moon's
+surface. Of course, these phenomena are liable to vary with every
+eclipse, and depend almost exclusively on the meteorological conditions
+of our atmosphere.
+
+In some cases the phase of totality lasts longer than it should,
+according to calculation. This can be attributed to the fact that the
+Earth is enveloped in a dense atmosphere, in which opaque clouds of
+considerable extent are often forming at great elevations. Such strata
+of clouds, in intercepting the Sun's light, would have, of course, the
+effect of increasing the diameter of the Earth's shadow, in a direction
+corresponding to the place they occupy, and, if the Moon were moving in
+this direction, would increase the phase of total obscuration.
+
+The eclipses of the Moon, like those of the Sun, as shown above, have a
+cycle of 18 years, 11 days and 7 hours, and recur after this period of
+time in nearly the same order. They can, therefore, be approximately
+predicted by adding 18y. 11d. 7h. to the date of the eclipses which have
+occurred during the preceding period. During this cycle 70 eclipses will
+occur--41 being eclipses of the Sun and 29 eclipses of the Moon. At no
+time can there ever be more than seven eclipses in a year, and there are
+never less than two. When there are only two eclipses in a year, they
+are both eclipses of the Sun.
+
+Although the number of solar eclipses occurring at some point or other
+of the Earth's surface is greater than that of the eclipses of the Moon,
+yet at any single terrestrial station the eclipses of the Moon are the
+more frequent. While an eclipse of the Sun is only visible on a narrow
+belt, which is but a very small fraction of the hemisphere then
+illuminated by the Sun, an eclipse of the Moon is visible from all the
+points of the Earth which have the Moon above their horizon at the time.
+Furthermore, an eclipse of the Sun is not visible at one time over the
+whole length of its narrow tract, but moves gradually from one end of it
+to the other; while, on the contrary, an eclipse of the Moon begins and
+ends at the very same instant for all places from which it can be seen,
+but, of course, not at the same local time, which varies with the
+longitude of the place.
+
+
+[PLATE VII.--PARTIAL ECLIPSE OF THE MOON.
+
+Observed October 24, 1874]
+
+
+The partial eclipse of the Moon, represented on Plate VII., shows quite
+plainly the configuration of our satellite as seen with the naked eye
+during the eclipse, with its bright and dark spots, and its radiating
+streaks. This eclipse was observed on October 24th, 1874.
+
+
+
+
+THE PLANETS
+
+
+Around the Sun circulate a number of celestial bodies, which are called
+"_Planets_." The planets are opaque bodies, and appear luminous because
+their surfaces reflect the light they receive from the Sun.
+
+The planets are situated at various distances from the Sun, and revolve
+around this body in widely different periods of time, which are,
+however, constant for each planet, so far as ascertained, and doubtless
+are so in the other cases.
+
+The ideal line traced in space by a planet in going around the Sun, is
+called _the orbit_ of the planet; while the period of time employed by a
+planet to travel over its entire orbit and return to its starting point,
+is called _the sidereal revolution_, _or year_ of the planet. The
+dimensions of the orbits of the different planets necessarily vary with
+the distance of these bodies from the Sun, as does also the length of
+their sidereal revolution.
+
+The distance of a planet from the Sun does not remain constant, but is
+subject to variations, which in certain cases are quite large. These
+variations result from the fact that the planetary orbits are not
+perfect circles having the Sun for centre, but curves called
+"_Ellipses_," which have two centres, or foci, one of which is always
+occupied by the Sun. This is in accordance with Kepler's first law.
+
+The ideal point situated midway between the two foci is called _the
+centre of the ellipse_, or _orbit_; while the imaginary straight line
+which passes through both foci and the centre, with its ends at opposite
+points of the ellipse, is called "_the major axis_" of the orbit. It is
+also known as "_the line of the apsides_." The ideal straight line
+which, in passing through the centre of the orbit, cuts the major axis
+at right angles, and is prolonged on either side to opposite points on
+the ellipse, is called "_the minor axis_" of the orbit.
+
+When a planet reaches that extremity of the major axis of its orbit
+which is the nearest to the Sun, it is said to be in its "_perihelion_;"
+while, when it arrives at the other extremity, which is farthest from
+this body, it is said to be in its "_aphelion_." When a planet reaches
+either of the two opposite points of its orbit situated at the
+extremities of its minor axis, it is said to be at its _mean distance_
+from the Sun.
+
+The rapidity with which the planets move on their orbits varies with
+their distance from the Sun; the farther they are from this body, the
+more slowly they move. The rapidity of their motion is greatest when
+they are in perihelion, and least when they are in aphelion, having its
+mean rate when these bodies are crossing either of the extremities of
+the minor axes of their orbits.
+
+The imaginary line which joins the Sun to a planet at any point of its
+orbit, and moves with this planet around the Sun, is called "_the radius
+vector_." According to Kepler's second law, whatever may be the distance
+of a planet from the Sun, the radius vector sweeps over equal areas of
+the plane of the planet's orbit in equal times.
+
+There is a remarkable relation between the distance of the planets from
+the Sun and their period of revolution, in consequence of which the
+squares of their periodic times are respectively equal to the cubes of
+their mean distances from the Sun. From this third law of Kepler, it
+results that the mere knowledge of the mean distance of a planet from
+the Sun enables one to know its period of revolution, and _vice versa_.
+
+The orbit described by the Earth around the Sun in a year, or the
+apparent path of the Sun in the sky, is called "_the ecliptic_." Like
+that of all the planetary orbits, the plane of the ecliptic passes
+through the Sun's centre. The ecliptic has a great importance in
+astronomy, inasmuch as it is the fundamental plane to which the orbits
+and motions of all planets are referred.
+
+The orbits of the larger planets are not quite parallel to the ecliptic,
+but more or less inclined to this plane; although the inclination is
+small, and does not exceed eight degrees. On account of this inclination
+of the orbits, the planets, in accomplishing their revolutions around
+the Sun, are sometimes above and sometimes below the plane of the
+ecliptic. A belt extending 8° on each side of the ecliptic, and,
+therefore, 16° in width, comprises within its limits the orbits of all
+the principal planets. This belt is called "_the Zodiac_."
+
+Since all the planets have the Sun for a common centre, and have their
+orbits inclined to the ecliptic, it follows that each of these orbits
+must necessarily intersect the plane of the ecliptic at two opposite
+points situated at the extremities of a straight line passing through
+the Sun's centre. The two opposite points on a planetary orbit where its
+intersections with the ecliptic occur, are called "_the Nodes_," and the
+imaginary line joining them, which passes through the Sun's centre, is
+called "the line of the nodes." The node situated at the point where a
+planet crosses the ecliptic from the south to the north, is called "_the
+ascending node_" while that situated where the planet crosses from north
+to south, is called "_the descending node_."
+
+The planets circulating around the Sun are eight in number, but, beside
+these, there is a multitude of very small planets, commonly called
+"asteroids," which also revolve around our luminary. The number of
+asteroids at present known surpasses two hundred, and constantly
+increases by new discoveries. In their order of distance from the Sun
+the principal planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn,
+Uranus and Neptune. The orbits of the asteroids are comprised between
+the orbits of Mars and Jupiter.
+
+When the principal planets are considered in regard to their differences
+in size, they are separated into two distinct groups of four planets
+each, viz.: the small planets and the large planets. The orbits of the
+small planets are wholly within the region occupied by the orbits of the
+asteroids, while those of the large planets are wholly without this
+region.
+
+When the planets are considered in regard to their position with
+reference to the Earth, they are called "inferior planets" and "superior
+planets." The inferior planets comprise those whose orbits are within
+the orbit of our globe; while the superior planets are those whose
+orbits lie beyond the orbit of the Earth.
+
+Since the orbits of the inferior planets lie within the orbit of the
+Earth, the angular distances of these bodies from the Sun, as seen from
+the Earth, must always be included within fixed limits; and these
+planets must seem to oscillate from the east to the west, and from the
+west to the east of the Sun during their sidereal revolution. In this
+process of oscillation these planets sometimes pass between the Earth
+and the Sun, and sometimes behind the Sun. When they pass between us and
+the Sun they are said to be in "inferior conjunction," while, when they
+pass behind the Sun, they are said to be in "superior conjunction." When
+such a planet reaches its greatest distance, either east or west, it is
+said to be at its greatest elongation east or west, as the case may be,
+or in quadrature.
+
+The superior planets, whose orbits lie beyond that of the Earth and
+enclose it, present a different appearance. A superior planet never
+passes between the Earth and the Sun, since its orbit lies beyond that
+of our globe, and, therefore, no inferior conjunction of such a planet
+can ever occur. When one of these planets passes beyond the Sun, just
+opposite to the place occupied by the Earth, the planet is said to be in
+"conjunction;" while, when it is on the same side of the Sun with our
+globe, it is said to be in "opposition." While occupying this last
+position, the planet is most advantageously situated for observation,
+since it is then nearer to the Earth. The period comprised between two
+successive conjunctions, or two successive oppositions of a planet, is
+called its "synodical period." This period differs for every planet.
+
+It is supposed that all the planets rotate from west to east, like our
+globe; although no direct evidence of the rotation of Mercury Uranus,
+and Neptune has yet been obtained, it is probable that these planets
+rotate like the others. It results from the rotation of the planets that
+they have their days and nights, like our Earth, but differing in
+duration for every planet.
+
+The axes of rotation of the planets are more or less inclined to their
+respective orbits, and this inclination varies but little in the course
+of time. From the inclination of the axes of rotation of the planets to
+their orbits, it results that these bodies have seasons like those of
+the Earth; but, of course, they differ from our seasons in duration and
+intensity, according to the period of revolution and the inclination of
+the axis of each separate planet.
+
+
+
+
+THE PLANET MARS
+
+PLATE VIII
+
+
+Mars is the fourth of the planets in order of distance from the sun;
+Mercury, Venus and the Earth being respectively the first, second and
+third.
+
+Owing to the great eccentricity of its orbit, the distance of Mars from
+the Sun is subject to considerable variations. When this planet is in
+its aphelion, its distance from the Sun is 152,000,000 miles, but at
+perihelion it is only 126,000,000 miles distant, the planet being
+therefore 26,000,000 miles nearer the Sun at perihelion than at
+aphelion. The mean distance of Mars from the Sun is 139,000,000 miles.
+Light, which travels at the rate of 185,000 miles a second, occupies
+12½ minutes in passing from the Sun to this planet.
+
+While the distance of Mars from the Sun varies considerably, its
+distance from the Earth varies still more. When Mars comes into
+opposition, its distance from our globe is comparatively small,
+especially if the opposition occurs in August, as the two planets are
+then as near together as it is possible for them to be, their distance
+apart being only 33,000,000 miles. But if the opposition occurs in
+February, the distance may be nearly twice as great, or 62,000,000
+miles. On the other hand, when Mars is in conjunction in August, the
+distance between the two planets is the greatest possible, or no less
+than 245,000,000 miles; while, when the conjunction occurs in February,
+it is only 216,000,000 miles. Hence the distance between Mars and the
+Earth varies from .33 to 245 millions of miles; that is, this planet may
+be 212 million miles nearer to us at its nearest oppositions than at its
+most distant conjunctions.
+
+From these varying distances of Mars from the Earth, necessarily result
+great variations in the brightness and apparent size of the planet, as
+seen from our globe. When nearest to us it is a very conspicuous object,
+appearing as a star of the first magnitude, and approaching Jupiter in
+brightness; but when it is farthest it is much reduced, and is hardly
+distinguishable from the stars of the second and even third magnitude.
+In the first position, the apparent diameter of Mars is 26", in the last
+it is reduced to 3" only.
+
+The orbit of Mars has the very small inclination of 1° 51' to the plane
+of the ecliptic. The planet revolves around the Sun in a period of 687
+days, which constitutes its sidereal year, the year of Mars being only
+43 days less than two of our years.
+
+Mars travels along its orbit with a mean velocity of 15 miles per
+second, being about ⁸⁄₁₀ of the velocity of our globe in its
+orbit. The synodical period of Mars is 2 years and 48 days, during which
+the planet passes through all its degrees of brightness.
+
+Mars is a smaller planet than the Earth, its diameter being only 4,200
+miles, and its circumference 13,200 miles. It seems well established
+that it is a little flattened at its poles, but the actual amount of
+this flattening is difficult to obtain. According to Prof. Young, the
+polar compression is ¹⁄₂₁₉.
+
+The surface of this planet is a little over ²⁸⁄₁₀₀ of the
+surface of our globe, and its volume is 6½ times less than that of the
+Earth. Its mass is only about ⅒ while its density is about ¾ that of
+the Earth. The force of gravitation at its surface is nearly ¾ of what
+it is at the surface of our globe.
+
+The planet Mars rotates on an axis inclined 61° 18' to the plane of its
+orbit, so that its equator makes an angle of 28° 42' with the same
+plane. The period of rotation of this planet, which constitutes its
+sidereal day, is 24 h. 37 m. 23 s.
+
+The year of Mars, which is composed of 669⅔ of these Martial days,
+equals 687 of our days, this planet rotating 669⅔ times upon its axis
+during this period. But owing to the movement of Mars around the Sun,
+the number of solar days in the Martial year is only 668⅔, while,
+owing to the same cause, the solar day of Mars is a little longer than
+its sidereal day, and equals 24 h. 39 m. 35 s.
+
+The days and nights on Mars are accordingly nearly of the same length as
+our days and nights, the difference being a little less than
+three-quarters of an hour. But while the days and nights of Mars are
+essentially the same as ours, its seasons are almost twice as long as
+those of the Earth. Their duration for the northern hemisphere,
+expressed in Martial days, is as follows: Spring, 191; Summer, 181;
+Autumn, 149; Winter, 147. While the Spring and Summer of the northern
+hemisphere together last 372 days, the Autumn and Winter of the same
+hemisphere last only 296 days, or 76 days less. Since the summer seasons
+of the northern hemisphere correspond to the winter seasons of the
+southern hemisphere, and vice versa, the northern hemisphere, owing to
+its longer summer, must accumulate a larger quantity of heat than the
+last. But on Mars, as on the Earth, there is a certain law of
+compensation resulting from the eccentricity of the planet's orbit, and
+from the fact that the middle of the summer of the southern hemisphere
+of this planet, coincides with its perihelion. From the greater
+proximity of Mars to the Sun at that time, the southern hemisphere then
+receives more heat in a given time than does the northern hemisphere in
+its summer season. When everything is taken into account, however, it is
+found that the southern hemisphere must have warmer summers and colder
+winters than the northern hemisphere.
+
+Seen with the naked eye, Mars appears as a fiery red star, whose
+intensity of color is surpassed by no other star in the heavens. Seen
+through the telescope, it retains the same red tint, which, however,
+appears less intense, and gradually fades away toward the limb, where it
+is replaced by a white luminous ring.
+
+
+[PLATE VIII.--THE PLANET MARS.
+
+Observed September 3, 1877, at 11h. 55m. P.M.]
+
+
+Mars is a very difficult object to observe, the atmosphere surrounding
+it being sometimes so cloudy and foggy that the sight can hardly
+penetrate through its vapors. When this planet is observed under
+favorable atmospheric conditions, and with sufficient magnifying power,
+its surface, which is of a general reddish tint, is found to be
+diversified by white, gray and dark markings. The dark markings, which
+are the most conspicuous, almost completely surround the planet. They
+are of different forms and sizes, and very irregular, as can be seen on
+Plate VIII., which represents one of the hemispheres of this planet.
+Many of them, especially those situated in the tropical regions of the
+planet, form long narrow bands, whose direction is in the main parallel
+to the Martial equator.
+
+The dusky spots differ very much, both from one another and in their
+several parts, as regards intensity of shade. Some appear almost black,
+while others which appear grayish, are so faint, that they can seldom be
+seen. In the southern hemisphere, the darkest part of the spots is
+generally found along their northern border; especially where there are
+deep indentations.
+
+Some observers have described these spots as being greenish or bluish,
+but I have never been able to see the faintest trace of these colors in
+them, except when they were observed close to the limb, and involved in
+the greenish tinted ring which is always to be seen there. It is
+probably an effect of contrast, since green and red are complementary
+colors, and since this greenish tinge around the limb covers all kinds
+of spots, whether white or dark. When such dark spots, involved in the
+greenish tint, are carried by the rotation towards the centre of the
+disk, they no longer show this greenish color. To me, these spots have
+always appeared dark, and of such tints as would result from a mixture
+of white and black in different proportions; except that on their
+lighter portions they show some of the prevalent reddish tint of the
+Martial surface. It is to be remarked that in moments of superior
+definition of the telescopic image, the intensity of darkness of all the
+spots is considerably increased--some of them appearing almost perfectly
+black.
+
+The markings on the surface of Mars are now tolerably well
+known--especially those of its southern hemisphere, which, owing to the
+greater proximity of the planet to our globe when this hemisphere is
+inclined towards the earth, have been better studied. Those of the
+northern hemisphere are not so well known, since when this hemisphere is
+inclined towards us, the distance of Mars from the Earth is 26,000,000
+miles greater, so that the occasions for observing them are not so
+favorable.
+
+Several charts of Mars are in existence, but as the same nomenclature
+has not been employed in all of them, some confusion has arisen in
+regard to the names given to the most remarkable features of the
+planet's surface. In order to give clearness to the subject, it will be
+necessary here to give a brief description of the principal markings
+represented on Plate VIII. In this the nomenclature will be employed
+which has been adopted by the English observers in the fine chart of Mr.
+Nath. Green. The large dark spot represented on the left-hand side of
+the plate is called De La Rue Ocean. The dark oval spot, isolated in the
+vicinity of the centre of the disk, is called Terby Sea; while the dark,
+irregular form on the right, near the border, represents the western
+extremity of Maraldi Sea.
+
+The dusky spots of Mars seem to be permanent, and to form a part of the
+general surface of the planet. That several among them, at least, are
+permanent, is proved by the fact that they have been observed in the
+same position, and with the same general form, for over two centuries.
+Yet, if we are to depend upon the drawings made fifty years ago by Beer
+and Maedler, it would seem that the permanency of some of them does not
+exist, since a very large spot represented by these astronomers on their
+chart of Mars is not visible now. This object, which, on their map, has
+its middle at 270°, should be precisely under the prominent dark oval
+spot called Terby Sea, seen near the centre of the picture, and would
+extend down almost as far as the northern limb. This can hardly be
+attributed to an error of observation, since these observers were both
+careful, and had great experience in this class of work. It is a very
+singular fact that, at the very same place where Maedler represented the
+spot in question, I found a conspicuous dark mark on December 16, 1881,
+which was certainly not visible in 1877, during one of the most
+favorable oppositions which can ever occur. The object, which is still
+visible (Feb., 1882), consists of an isolated spot situated a little to
+the north of Terby Sea. During the memorable opposition of 1877, I
+investigated thoroughly the markings of Mars, and made over 200 drawings
+of its disk, 32 of which represent the Terby Sea; but this isolated spot
+was not to be seen, unless it be identified with the faint mark,
+represented on the plate, which occupied its place. There cannot be the
+slightest doubt that a change has occurred at that place. Changes in the
+markings have also been suspected on the other hemisphere of the planet.
+
+The well-known fact that the continents, and especially the mountainous
+and denuded districts of our globe, reflect much more light than the
+surfaces covered by water, has led astronomers to suppose that the dark
+spots on Mars are produced by a liquid strongly absorbing the rays of
+light, like the liquids on the surface of the Earth. According to this
+theory the dark spots observed are supposed to be lakes, seas, and
+oceans, similar to our own seas and oceans, while the reddish and
+whitish surfaces separating these dark spots, are supposed to be
+islands, peninsulas, and continents. This supposition seems certainly to
+have a great deal of probability in its favor, although some of the
+lighter markings may have a different origin, and perhaps be due to
+vegetation; but no observer has yet seen in them any of the changes
+which ought to result from change of seasons. Some of the changes in the
+dark spots might also be attributed to the flooding or drying up of
+marshes and low land. The change which I have observed lately might be
+attributed to such a cause, especially as my observation was made
+shortly after the spring equinox of the northern hemisphere of Mars,
+which occurred on December 8th.
+
+Besides the dark spots just described, there are markings of a different
+character and appearance. Among the most conspicuous are two very
+brilliant white oval spots, which always occupy opposite sides of the
+planet. These two bright spots, which correspond very closely with the
+poles of rotation of Mars, have been called "polar spots."
+
+On account of the inclination of the axis of rotation of this planet to
+the ecliptic, it is rare that both of these spots are visible on the
+disk at the same time; and when this occurs, they are seen considerably
+foreshortened, as they are then both on the limb of the planet. Usually
+only one spot is visible, and it appears to its best advantage when the
+region to which it belongs attains its maximum of inclination towards
+the Earth.
+
+The polar spots change considerably in size, as they do also in form.
+Sometimes they occupy nearly one-third of the disk, as is proved by many
+of my observations; while at other times they are so much reduced as to
+be totally invisible. It is to be remarked that the reduction of these
+spots generally corresponds with the summer seasons, and their
+enlargement with the winter seasons of the hemispheres to which they
+respectively belong. From these well-observed facts it would appear that
+a relation exists between the temperature of the two hemispheres of Mars
+and the variations of the white spots observed at its poles. A similar
+relation is known to exist on our globe between the progress of the
+seasons and the melting away and the accumulation of snow in the polar
+regions. Astronomers have been led, accordingly, to attribute the polar
+spots of Mars, with all their variations, to the alternate accumulation
+and melting of snows. On this account, the polar spots of Mars are
+sometimes also called "snow-spots."
+
+Errors have certainly been made by astronomers in some of their
+observations of the so-called polar snow-spots, other objects occupying
+their place having been mistaken for them. A regular series of
+observations on this planet, which I have now continued for seven years,
+has revealed the fact that during the winter seasons of the southern
+hemisphere of Mars, the polar spots are most of the time invisible,
+being covered over by white, opaque, cloud-like forms, strongly
+reflecting light. In 1877, during more than a month, I, myself, mistook
+for the polar spots such a canopy of clouds, which covered at least
+one-fifth of the surface of the whole disk. I only became aware of my
+error when the opaque cloud, beginning to dissolve at the approach of
+the Martial summer, allowed the real polar spot to be seen through its
+vapors, as through a mist at first, and afterwards with great
+distinctness. In this particular case, the snow-spot was considerably
+smaller than the cloudy cap which covered it, and it is to be remarked
+that it was not situated at the centre of this cloudy cap, but was east
+of that centre; a fact which may account for the so-called polar spots
+not being always observed on exactly opposite sides of the disk. From my
+observations of 1877, 1878 and 1880, it appears that at the approach of
+the autumnal equinox of the southern hemisphere of Mars, large, opaque
+masses, like cumulus clouds in form, began to gather in the polar
+regions of that hemisphere, and continued through autumn and winter,
+dissolving only at the approach of spring. These clouds, which varied in
+form and extent, were very unsteady at first, but as the winter drew
+nearer they enlarged and became more permanent, covering large surfaces
+for months at a time.
+
+That the large white spots under consideration are real clouds in the
+atmosphere of Mars, and are not due to a fall of snow, is proved by the
+fact that these spots covered both seas and continents with equal
+facility, even in the equatorial regions of the planet. Snow, of course,
+could not cover the seas of Mars, unless these were all frozen over,
+even in the equatorial zones; therefore, if the dark spots of Mars are
+assumed to be due to water, these large white spots cannot well be
+ascribed to snow.
+
+The real polar spots of Mars seem to be in relief on the surface of the
+planet, since the southern spot often appeared slightly shaded on the
+side opposite to the Sun during my observations in 1877. In certain
+cases, when they are on or very near the limb, they have been observed,
+both by others and by myself, to project from the disk slightly.
+
+The polar spots of Mars are doubtless composed of a material which, like
+our snow or ice, melts under the rays of the Sun; although it seems
+difficult to admit that the Martial snow is identical with our
+terrestrial snow, and that it melts at a like temperature. The south
+polar spot of Mars entirely disappeared from sight in its summer season
+in 1877, although the planet receives less than one-half as much heat as
+we receive from the Sun; yet on our globe the arctic or antarctic ices
+and snow are perpetual--never melting entirely. An important fact
+disclosed by the melting away of the southern polar spot is, that in
+melting it is always surrounded with a very dark surface, which takes
+the place of the melted portion of the spot, as observed by myself in
+1877-78. When the polar spot had entirely disappeared, its place was
+occupied by a very dark spot. Now, if the polar spot is really ice, and
+the dark spots are actual seas, this polar spot must be situated in
+mid-ocean, since, on melting away, it is replaced by a dark spot. If the
+polar spots are composed of a white substance melting under the rays of
+the Sun, as seems altogether probable, its melting point must be above
+that of terrestrial snow.
+
+Many of the dark spots of Mars, and especially those whose northern
+border forms an irregular belt upon the equatorial regions of this
+planet, are bordered on that side by a white luminous belt, following
+all their sinuosities. These white borders are variable. Sometimes they
+are very prominent and intensely bright, especially at some points,
+which occasionally almost equal the polar spots in brilliancy; while at
+other times they are so faint, that they can hardly be distinguished, or
+are even invisible; although the atmosphere is clear and the dark spots
+appear perfectly well defined. While these white borders were invisible,
+I have sometimes watched for several hours at a time to see if I could
+detect any traces of them in places where they usually appear the most
+prominent, but generally without success. On a few occasions, however, I
+had the good fortune to see some of these spots forming gradually in the
+course of one or two hours, at places where nothing of them could be
+seen before.
+
+These whitish fringes forming and vanishing along the coasts of the
+Martial seas have been very little studied by astronomers. From my
+observations made during the last seven years, it appears very probable
+that this belt and its white spots are mainly due to the condensation of
+vapors around, and over high peaks, and extensive mountain chains,
+forming the Martial sea-coasts, as the Andes and Rocky Mountains form
+the sea-coasts of the Pacific Ocean. These high mountains on Mars,
+condensing the vapors into fogs or clouds above them, or at their sides,
+as often happens in our mountainous districts, would certainly suffice
+to produce the phenomena observed. Some of the highest peaks among these
+mountain chains may even have their summits covered with perpetual snow,
+or some substance partaking of the nature of snow. The temporary
+visibility and invisibility of the white spots seen on Mars, as well as
+the rapid transformations they sometimes undergo, may be explained as
+caused by clouds having a high reflective power and a liability to form
+and disappear quickly.
+
+The assumption that these irregular whitish bands and spots are formed
+by the condensation of vapors on mountain chains, and elevated table
+lands, is supported by my observations made in 1877 and 1879. When such
+white spots were traversing the terminator at sunrise, they very often
+projected far into the night side, thus indicating that they were at a
+higher level than that of the general surface. Indentations in the
+terminator, corresponding to large dark spots crossing its line, also
+clearly indicated the depression of the dark spots below the general
+surface. The highest mountainous districts thus observed on Mars, are
+situated between 60° and 70° of south latitude, towards the western
+extremity of Gill Land. The mountain chain, which almost completely
+forms the surface of this land, is so elevated at some points, that they
+not only change the form of the terminator when they are seen upon it,
+but also the limb of the planet, as seen by myself. They then appear so
+brilliant, that the principal summit among them has been mistaken by
+several observers for the polar spot itself, as proved by the wrong
+position assigned to it on their drawings. It seems probable that this
+high peak, which appears always white, is constantly covered with snow,
+or the similar material replacing it on Mars. This high region is
+situated between longitudes 180° and 190°.
+
+The highest mountainous parts belonging to the hemisphere represented on
+Plate VIII., which are nearly always more or less visible as whitish
+spots and bands, form a coast line along the northern (lower) border of
+De La Rue Ocean. This great spot, which is not so simple as it has been
+represented by observers, is in fact divided by two narrow isthmuses,
+one in the north, the other in the east, both joining, in the interior
+of the great ocean, a peninsula heretofore known as Hall Island. Upon
+the south-eastern extremity of this peninsula, a white spot, called
+Dawes Ice Island, was observed in 1865, but it soon disappeared, and was
+after that seen only now and then. It is very probable that this
+so-called Ice Island was due to clouds forming around the summit of some
+high peak of this peninsula.
+
+On the opposite hemisphere to that represented on Plate VIII., the white
+fringes bordering the dark spots are much more conspicuous than they are
+on this side. On the eastern side of a remarkable dark spot called
+Kaiser Sea, they are very bright, and almost always present, although
+they vary considerably, both in brightness and in extent. To the south
+of Kaiser Sea, they are very conspicuous on the eastern border of
+Lockyer Land, forming an elevated and deeply indented coast-line along
+Lambert Sea. There the white spots never disappear entirely, being
+always visible on the north side, where they turn westward along Dawes'
+Ocean--the mountain chain attaining there its greatest altitude. Very
+frequently Lockyer Land, which seems to be a vast plateau, appears
+throughout white and brilliant, this occurring usually towards the
+sunrise or sunset of that region, probably from the condensation of
+vapors and the formation of fogs, but generally this whiteness gradually
+disappears with the progress of the sun above this plateau. Inside of
+the great continents of Mars these temporary white spots are not so
+frequent, but when visible they occupy always the same positions--a fact
+which probably indicates that they occupy the culminating points of
+these continents. One of these temporary white spots inside of the
+continents is represented on Plate VIII., on the left-hand side, below
+De La Rue Ocean, on Maedler Continent.
+
+Although large, opaque, cumulus-shaped, cloud-like forms are seen in the
+polar regions of Mars, such forms are very seldom seen in the tropical
+zones, or, at least, it appears so, from the fact that my observations,
+continued during the last seven years, have disclosed no real opaque
+cloudy forms there. Although the Martial sky is frequently overcast by
+dense vapors or thick fogs in these regions, yet no real opaque clouds
+were ever seen; the most prominent among the dusky spots being faintly
+visible through the vapory veil, when they approached the centre of the
+disk.
+
+Besides these phenomena, which prove that Mars is surrounded by an
+atmosphere having a great deal of similarity to our own, a further proof
+is afforded by the fact that the dark spots, which appear sharply
+defined and black when they are seen near the centre, become less and
+less visible as they advance towards the limb, and are totally invisible
+before they reach it. Moreover, the spectroscope also indicates the
+existence of an atmosphere, and even the presence of watery vapor in it.
+A very curious state of the Martial atmosphere is revealed by my
+observations of 1877-78. During eight consecutive weeks, from December
+12th to February 6th, a whole hemisphere of the planet--precisely that
+represented on Plate VIII.--was completely covered by dense vapors, or a
+thick fog which barely allowed the dark spots to be seen through it,
+even when they were in the centre of the disk. The opposite hemisphere
+of Mars appeared just as clear and calm as possible; there all the spots
+and their minutest details could be seen, and when the planet was
+observed at the proper time, the line separating the foggy from the
+clear side was plainly visible.
+
+The reddish tint observed on the continents of Mars has been supposed by
+some astronomers to be the real color of the atmosphere of this planet.
+But, for many reasons, this explanation is not acceptable. Besides the
+fact that the border of the planet appears white, while it should be
+more red than the other part, owing to the greater depth of atmosphere
+there presented to us, the polar spots, the white bands along the
+sea-coasts, and the cloud-like forms appear perfectly white, not the
+slightest tint of red being visible on them, as would be the case if
+these objects were seen through an atmosphere tinted red. Other
+astronomers have supposed that the vegetation of this planet has a
+reddish color; but this is not supported by observation. It has been
+again supposed, with much more probability, that the surface of Mars is
+composed of an ochreous material which gives the planet its predominant
+ruddy color.
+
+Until lately Mars was supposed to be without a satellite, but in August,
+1877, Professor Hall, of the Washington Observatory, made one of the
+most remarkable discoveries of the time, and found two satellites
+revolving around this planet. These satellites are among the smallest
+known heavenly bodies, their diameter having been estimated at from 6 to
+10 miles for the outer satellite, and from 10 to 40 miles for the inner
+one.
+
+The most extraordinary feature of these bodies is the proximity of the
+inner satellite to the planet, and the consequent rapidity of its
+motion. The distance of the inner satellite from the centre of Mars is
+about 6,000 miles, and from surface to surface it is less than 4,000
+miles, or a little more than the distance from New York to San
+Francisco. The shortest period of revolution of any satellite previously
+known, is that of the inner satellite of Saturn, which is a little more
+than 22½ hours; but the inner satellite of Mars accomplishes its
+revolution in 7h. 38m., or in 17 hours less than the period of rotation
+of the planet upon its axis. The period of revolution of the outer
+satellite is greater, of course, and equals 30h. 7m.
+
+From this rapidity of motion of the inner satellite of Mars, a very
+curious result follows, which at first sight may appear in contradiction
+with the fact that this body has a direct motion, like that of all the
+planets of the solar system, and moves around Mars from west to east.
+While the outer satellite of this planet, in company with all the stars
+and planets, rises in the east and sets in the west, the inner
+satellite, on the contrary, rises in the west and sets in the east.
+Since the period of rotation of Mars is greater than is the period of
+revolution of this satellite, it necessarily follows that this last body
+must constantly be gaining on the rotation, and, consequently, that the
+satellite sets in the east and rises in the west, compassing the whole
+heavens around Mars three times a day, passing through all its phases in
+11 hours, each quarter of this singular Moon lasting less than 3 hours.
+
+It has been shown above that Mars has many points of resemblance to the
+Earth. It has an atmosphere constituted very nearly like ours; it has
+fogs, clouds, rains, snows, and winds. It has water, or at least some
+liquids resembling it; it has rivers, lakes, seas and oceans. It has
+also islands, peninsulas, continents, mountains and valleys. It has two
+Moons, which must create great and rapid tides in the waters of its seas
+and oceans. It has its days and nights, its warm and cold seasons, and
+very likely its vegetation, its prairies and forests, like the Earth. On
+the other hand, its year and seasons are double those of the Earth, and
+its distance from the Sun is greater.
+
+Is this planet, which is certainly constituted very nearly like our
+globe, and seems so nearly fitted for the wants of the human race,
+inhabited by animals and intelligent beings?
+
+To answer this question, either in the negative or in the affirmative,
+would be to step out of the pure province of science, and enter the
+boundless domain of speculation, since no observer has ever seen
+anything indicating that animal life exists on Mars, or on any other
+planet or satellite. So far as observation goes, Mars seems to be a
+planet well suited to sustain animal life, and we may reason from
+analogy that if animal life can exist at all outside of the Earth, Mars
+must have its flora and fauna; it must have its fishes and birds, its
+mammalia and men; although all these living beings must inevitably be
+very different in appearance from their representatives on the Earth, as
+can easily be imagined from the differences existing between the two
+planets. Although all this is possible, and even very probable, yet it
+must be remembered that we have not the slightest evidence that it is
+so; and until we have acquired this evidence, we may only provisionally
+accept this idea as a pleasing hypothesis, which, after all, may be
+wrong and totally unfounded.
+
+
+
+
+THE PLANET JUPITER
+
+PLATE IX
+
+
+Jupiter, the giant of the planetary world, is the fifth in order of
+distance from the Sun, and is next to Mars, our ruddy neighbor. To the
+naked eye, Jupiter appears as a very brilliant star, whose magnitude,
+changing with the distance of this planet from the Earth, sometimes
+approaches that of Venus, our bright morning and evening star.
+
+The mean distance of Jupiter from the Sun is 475,000,000 miles, but
+owing to the eccentricity of its orbit, its distance varies from 452 to
+498 millions of miles. The distance of this planet from the Earth varies
+still more. When nearest to our globe, or in opposition, its distance is
+reduced to 384,000,000 miles, and its apparent diameter increased to
+50"; while when it is farthest, or in conjunction, its distance is
+increased to 567,000,000 miles, and its apparent diameter reduced to
+30"; Jupiter being thus 183,000,000 miles nearer our globe while in
+opposition than when it is in conjunction.
+
+This planet revolves around the Sun in 11 years, 10 months and 17 days,
+or in only 50 days less than 12 terrestrial years. Such is the year of
+this planet. The plane of its orbit is inclined 1° 19' to the ecliptic.
+No planet, except Uranus, has an orbit exhibiting a smaller inclination.
+The planet advances in its orbit at the mean rate of 8 miles a second;
+which is a little less than half the orbital velocity of the Earth.
+
+Jupiter is of enormous proportions. Its equatorial diameter measures
+88,000 miles, and its circumference no less than 276,460 miles, these
+dimensions being 11 times greater than those of the Earth. This planet,
+notwithstanding its huge size, rotates on its axis in not far from 9h.
+55m. 36s., which period constitutes its day. Owing, however, to the
+changeable appearance of its surface, this period cannot be ascertained
+with very great exactitude. In consequence of its rapid rotation, the
+planet is far from spherical, its polar diameter being shorter than the
+equatorial by about ¹⁄₁₆, or 5,500 miles. Its surface is 124
+times the surface of the earth; while its volume is 1,387 times as
+great. If Jupiter occupied the place of our satellite in the sky, it
+would appear 40 times as large as the Moon appears to us, and would
+cover a surface of the heavens 1,600 times that covered by the full
+Moon, and would subtend an angle of 21°. Jupiter's mass does not
+correspond with its great bulk, and is only ¹⁄₁₀₄₇ of the
+mass of the Sun, and 310 times the mass of the earth; its density being
+only ¼ of that of our globe. The force of gravitation at the surface of
+this planet is over 2½ times what it is on the Earth, so that a
+terrestrial object carried to the surface of Jupiter would weigh over
+two and a half times as much as on our globe.
+
+Observed with a telescope, even of moderate aperture, Jupiter, with its
+four attending satellites and its dazzling brilliancy, appears as one of
+the most magnificent objects in the sky. The general appearance of the
+disk is white; but unlike that of Mars, it is brightest towards its
+central parts, and a little darker around the limb, especially on the
+side opposite to the Sun. Although an exterior planet, and so far from
+us, Jupiter shows faint traces of phases when observed near its
+quadratures, but this gibbosity of its disk is very slight, and is
+indicated only by a kind of penumbral shadow on the limb.
+
+When observed with adequate power, the disk of Jupiter is found to be
+highly diversified. The principal features consist of a series of
+alternate light and dark streaks or bands, disposed most of the time
+parallel with the Jovian equator. These bands differ from each other in
+intensity as well as in breadth; those near the equator being usually
+much more prominent than those situated in higher latitudes north and
+south.
+
+The equatorial zone of Jupiter is occupied most of the time by a broad,
+prominent belt 20° or 30° wide, limited on each side by a very dark
+narrow streak. Between these two dark borders, but seldom occupying the
+whole space between them, appears an irregular white belt, apparently
+composed of dense masses of clouds strongly reflecting the Sun's light,
+some of these cloudy masses being very brilliant. The spaces left
+between the cloudy belt and the dark borders, usually exhibit a delicate
+pink or rosy color, which produces a very harmonious effect with the
+varying grayish and bluish shades of some of the belts and streaks seen
+on the disk. Quite often the cloudy belt is broken up, and consists of
+independent cloudy masses, separated by larger or smaller intervals,
+these intervals disclosing the rosy background of this zone.
+
+On each side of the equatorial belt there is usually a broad whitish
+belt, succeeded by a narrow gray band; the space left on each hemisphere
+between these last bands and the limb being usually occupied by two or
+three alternate white and gray bands. A uniform gray segment usually
+forms a sort of polar cap to Jupiter.
+
+When observed under very favorable conditions, all the lighter belts
+appear as if composed of masses of small cloudlets, resembling the white
+opaque clouds seen in our atmosphere. This, as already stated, is
+particularly noticeable on the equatorial belt. It is not unusual, when
+Jupiter is in quadrature, to see some of the most conspicuous white
+spots casting a shadow opposite to the Sun; a fact which sufficiently
+indicates that these spots are at different levels. They probably form
+the summits of vast banks of clouds floating high up in the atmosphere
+of Jupiter.
+
+What we see of Jupiter is chiefly a vaporous, cloudy envelope. If our
+sight penetrates anywhere deeper into the interior, it can only be
+through the narrow fissures of this envelope, which appear as gray or
+dark streaks or spots. That most of the visible surface of Jupiter is
+simply a cloudy covering, is abundantly proved by the proper motion of
+its spots, which sometimes becomes very great.
+
+In periods of calm, very few changes are noticeable in the markings of
+the planet, except, perhaps, some slight modifications of form in the
+cloudy, equatorial belt which, in general, is much more liable to
+changes than the other belts. But the Jovian surface is not always so
+tranquil, great changes being observed during the terrific storms which
+sometimes occur on this mighty planet, when all becomes disorder and
+confusion on its usually calm surface; and nothing on the Earth can give
+us a conception of the velocity with which some of its clouds and spots
+are animated. New belts quickly form, while old ones disappear. The
+usual parallelism of the belts no more exists. Huge, white, cumulus-like
+masses advance and spread out, the rosy equatorial belt enlarges
+sometimes to two or three times its usual size, and occupies two-thirds,
+or more, of the disk, the rosy tint spreading out in a very short time.
+At times very dark bands extending across the disk are transformed into
+knots or dark spots, which encircle the planet with a belt, as it were,
+of jet black beads. Sometimes, also, a secondary but narrower rosy belt
+forms either in the northern or the southern hemisphere, and remains
+visible for a few days or for years at a time.
+
+On May 25, 1876, I witnessed one of the grandest commotions which can be
+conceived as taking place in an atmosphere. All the southern hemisphere
+of Jupiter, from equator to pole, was in rapid motion, the belts and
+spots being transported entirely across the disk, from the eastern to
+the western limb, in one hour's time, during which the equatorial belt
+swelled to twice its original breadth, towards the south.
+
+Now, when one stops for a moment to think what is signified by that
+motion of the dark spots across the little telescopic disk of Jupiter in
+an hour's time, he may arrive at some conception of the magnitude of the
+Jovian storms, compared with those of our globe. The circumference of
+Jupiter's equator, as stated above, is 276,460 miles; half this number,
+or 138,230 miles, represents the length of the equatorial line seen from
+the Earth. Now, after taking into account the rotation of the planet,
+which somewhat diminishes the apparent motion, we arrive at the
+astonishing result that the spots and markings were carried along by
+this Jovian storm, at the enormous rate of 110,584 miles an hour, or
+over 30.7 miles a second. On our globe, a hurricane or tornado, which
+blows at the rate of 100 miles an hour, sweeps everything before it.
+What, then, must be expected from a velocity over 1,105 times as great?
+Enormous as this motion may appear, its occurrence cannot be doubted,
+since it is disclosed by direct observation.
+
+The surface of Jupiter, it would seem, has its periods of calm and
+activity like that of the Sun, although it is not yet known, as it is
+for the latter, that they recur with approximate regularity.
+
+My observations of this planet, which embrace a period of ten years,
+seem to point in that direction, for they show, at least, that Jupiter
+has its years of calm and its years of disturbances. The year 1876 was a
+year of extraordinary disturbance on Jupiter. Changes in the markings
+were going on all the time, and no one form could be recognized the next
+day, or even sometimes the next hour, as shown above. The cloudy
+envelope of the planet was in constant motion, the equatorial belt,
+especially, showing the signs of greatest disturbance, being, for the
+most part, two or three times as wide as in other years. After 1876 the
+calm was very great on the planet, only a slight change now and then
+being noticeable, the same forms being recognized day after day, month
+after month, and even year after year. In one case the same marking has
+been observed for seventeen consecutive months, and in another for
+twenty-eight months. This state of quietude lasted until October, 1880,
+when considerable commotion occurred on the northern hemisphere, where
+large round black spots, somewhat resembling the Sun-spots, formed in
+the cloudy atmosphere, and finally changed, towards the end of December,
+into a narrow pink belt, which still exists.
+
+
+
+[PLATE IX.--THE PLANET JUPITER.
+
+Observed November 1, 1880, at 9h. 30m. P.M.]
+
+
+The most curious marking ever seen on Jupiter is undoubtedly the great
+Red Spot, observed on the southern hemisphere of this planet for the
+last three years. This interesting object, seen first in July, 1878,
+disappeared for a time, reappeared on September 25 of the same year, and
+has remained visible until now. When seen by me in September, it was
+much elongated, and sharply pointed on one side, like a spear-head, but
+it subsequently acquired an irregular form, with short appendages
+protruding from its northern border. At first, the changes were great
+and frequent, but at length it acquired the regular oval form, which,
+with but slight modifications, it has retained until now. During the
+month of November, 1880, I noticed two small black specks upon this Red
+Spot, and they were seen again in January of the succeeding year, by Mr.
+Alvan Clark, Jr. When the spot had attained its oval shape, it appeared
+part of the time surrounded with a white luminous ring of cloudy forms
+which, however, was changing more or less all the time, being sometimes
+invisible. The color of this curious spot is a brilliant rosy red,
+tinged with vermilion, and altogether different in shade from the
+pinkish color of the equatorial belt. The size of the spot varies, but
+of late its changes have been slight. Its longer diameter may be
+estimated at 8,000 miles, and the shorter at 2,200 miles. The Red Spot
+is represented on Plate IX. with its natural color, and as it appears at
+the moments most favorable for observation. In ordinary cases its color
+does not appear so brilliant, but paler.
+
+It is difficult to account for the color of the equatorial belt and that
+of the Red Spot; but it is known, at least, that the material to which
+they are due cannot be situated at the level of the general surface
+visible to us, and especially that of the cloudy forms of the equatorial
+zone. Undoubtedly the red layer lies deeper than the superficial
+envelope of the planet, although it does not seem to be very deeply
+depressed.
+
+Jupiter is attended by four satellites, which revolve around the planet
+at various distances, and shine like stars of the 6th and 7th magnitude.
+It is said that under very favorable circumstances, and in a very clear
+sky, the satellites can be seen with the naked eye, but this requires
+exceptionally keen eyes, since the glare of the planet is so strong as
+to overpower the comparatively faint light of the satellites. However, I
+myself have sometimes seen, without the aid of the telescope, two or
+three of the satellites as a single object, when they were closely
+grouped on the same side of Jupiter.
+
+The four moons of Jupiter are all larger than our Moon, except the
+first, which has about the same diameter. They range in size from 2,300
+to 3,400 miles in diameter, the third being the largest; the
+determination of their diameter is by no means accurate, however, as it
+is difficult to measure such small objects with precision. Their mean
+distance from the centre of Jupiter varies from 267,000 to 1,192,000
+miles, the first satellite, the nearest to the planet, being a little
+farther from Jupiter then our satellite is from us. The four satellites
+revolve around the planet in orbits whose planes have a slight
+inclination to the equator of Jupiter, and consequently to the ecliptic.
+The diameter of the largest satellite is nearly half that of the Earth,
+or 3,436 miles; while its volume is five times that of our Moon. The
+period of revolution of these satellites varies from 1d. 18h. for the
+first, to 16d. 16h. for the last.
+
+Owing to the slight inclination of the plane of their orbits to that of
+the planet, the three first satellites, and generally the fourth, pass
+in front of the disk and also through the shadow of the planet at every
+revolution, and are accordingly eclipsed. Their passages behind
+Jupiter's disk are called occultations; those in front of it, transits.
+The eclipses, the occultations and the transits of the moons of Jupiter
+are interesting and important phenomena; the eclipses being sometimes
+observed for the rough determination of longitudes at sea.
+
+The satellites in transit present some curious phenomena. When they
+enter the disk, they appear intensely luminous upon its grayish border;
+but as they advance, they seem by degrees to lose their brightness,
+until they finally become undistinguishable from the luminous surface of
+Jupiter. It sometimes happens, however, that the first, the third and
+the fourth satellites, after ceasing to appear as bright spots, continue
+to be visible as dark spots upon the bright central portions of the
+planet's disk; but in these cases their disks appear smaller than the
+shadows they cast. Undoubtedly these satellites have extensive
+atmospheres, since they sometimes pass unperceived across the central
+parts of Jupiter, this being probably when their atmospheres are
+condensed into clouds, strongly reflecting light; while when these
+clouds are absent, we can see their actual surface, with traces of the
+dark spots upon them similar to those on Mars.
+
+From the variation in the brightness of these satellites, which is said
+to be always observed in the same part of their orbit, William Herschel
+was led to suppose that these bodies, like our Moon, rotate upon their
+axes in the same period in which they move round the planet, so that
+they always present the same face to Jupiter; but these conclusions have
+been denied. From my observations it is apparent, however, that the
+light reflected by them varies in intensity as well as in color. But
+this is rather to be attributed to the presence of an atmosphere
+surrounding these bodies, which when cloudy reflects more light than
+when clear, with corresponding changes in the color of the light.
+
+The satellites in transit are sometimes preceded or followed, according
+to the position of the Sun, by a round black spot having about the same
+size as the satellite itself. This black spot is the shadow of the
+satellite cast on the vapory envelope of Jupiter, similar to the shadow
+cast by the Moon on the Earth, during eclipses of the Sun; in fact, all
+the Jovian regions traversed by these shadows have the Sun totally
+eclipsed. Sometimes it happens that the shadow appears elliptical. This
+occurs either when it is observed very near the limb, or when entering
+upon a round, cloud-like spot. This effect is attributable to the
+perspective under which the shadow is seen on the spherical globe or
+spot.
+
+The proper motion of the satellites in the Jovian sky is much more rapid
+than that of the Moon in our sky. During one Jovian day of ten hours,
+the first satellite advances 84°; the second, 42°; the third, 20° and
+the fourth, 9°. The first satellite passes from New Moon to its first
+quarter in a little more than a Jovian day, while the fourth occupies
+ten such days in attaining the same phase.
+
+In density, as well as in physical constitution, Jupiter differs widely
+from the interior planets, and especially from the Earth; and, as has
+been shown, it is surrounded by a dense, opaque, cloudy layer, which is
+almost always impenetrable to the sight, and hides from view the
+nucleus, which we may conceive to exist under this vaporous envelope. In
+1876, the year of the great Jovian disturbances, I observed frequently
+in the northern hemisphere of the planet a very curious phenomenon,
+which seems to prove that its cloudy envelope is at times partially
+absent in some places, its vapors being apparently either condensed, or
+transported to other parts of its surface, and that, therefore, a
+considerable part of the real globe of the planet was visible at these
+places. The phenomenon consisted in the deformation of the northern
+limb, which had a much shorter radius on all of this hemisphere situated
+northward of the white belt which adjoins the equatorial zone. The
+deformation of the limb on both sides, where it passed from a longer to
+a shorter radius, was abrupt, and at right angles to the limb, forming
+there a steplike indentation which was very prominent. The polar segment
+having a smaller radius, appeared unusually dark, and was not striped,
+as usual, but uniform in tint throughout. On September 27th, the third
+satellite passed over this dark segment, and emerged from the western
+border, a little below the place where the limb was abruptly deformed,
+as above described. When the satellite had fully emerged from this limb,
+it was apparent that if the portion of the limb having a longer radius
+had been prolonged a little below, and as far as the satellite, it would
+have enclosed it within its border, and thus retarded the time of
+emersion. The depth of deformation of the limb was accordingly greater
+than the diameter of the third satellite, and certainly more than 4,000
+miles. That the phenomenon was real, is proved by the fact that the
+egress of this satellite occurred at least four minutes sooner than the
+time predicted for it in the American Ephemeris. Other observations seem
+to point in the same direction, since some of the satellites which were
+occulted have been seen through the limb of Jupiter by different
+astronomers, as if this limb was sometimes semi-transparent. Another
+observation of mine seems to confirm these conclusions. On April 24th,
+1877, at 15h. 25m. the shadow of the first satellite was projected on
+the dark band forming the northern border of the equatorial belt, the
+shadow being then not far from the east limb. Close to this shadow, and
+on its western side, it was preceded by a secondary shadow, which was
+fainter, but had the same apparent size. This round dark spot was not
+the satellite itself, as I had supposed at first, since this object was
+yet outside of the planet, on the east, and entered upon it only at 16h.
+4m. I watched closely this strange phenomenon, and at 16h. 45m., when
+the shadow had already crossed about ¾ of the disk, it was still
+preceded by the secondary, or mock shadow, as it may be called; the same
+relative distance having been kept all the while between the two
+objects, which had therefore traveled at the same rate. It is obvious
+that this dark spot could not be one of the planet's markings, since the
+shadow of the first satellite moves more quickly on the surface of
+Jupiter than a spot on the same surface travels by the effect of
+rotation, so that in this case the shadow would soon have passed over
+this marking, and left it behind, during the time occupied by the
+observation. From these observations it seems very probable that Jupiter
+has a nucleus, either solid or liquid, which lies several thousand
+mites below the surface of its cloudy envelope. It is also probable that
+the uniformly shaded dark segment seen in 1876, was a portion of the
+surface of this nucleus itself. When the cloudy envelope is
+semi-transparent at the place situated on a line with an occulted
+satellite and the eye of an observer, this satellite may accordingly
+remain visible for a time through the limb, as shown by observation. The
+phenomenon of the mock shadow may also be attributed to a similar cause,
+where semi-transparent vapors receive the shadow of a satellite at their
+surface, while at the same time part of this shadow, passing through the
+semi-transparent vapors, may be seen at the surface of the nucleus, or
+of a layer of opaque clouds situated at some distance below the surface.
+
+Some astronomers are inclined to think that Jupiter is at a high
+temperature, and self-luminous to a certain extent. If this planet is
+self-luminous to any degree, we might expect that some light would be
+thrown upon the satellites when they are crossing the shadow cast into
+space by the planet; but when they cross this shadow they are totally
+invisible in the best telescopes, a proof that they do not receive much
+light from the non-illuminated side of Jupiter. It would, indeed, seem
+probable that some of the intensely white spots occasionally seen on the
+equatorial belt of the planet are self-luminous in a degree, yet not
+enough to render the satellites visible while they are immersed in
+Jupiter's shadow. It does not seem impossible that the planet should
+have the high temperature attributed to it, when we remember the
+terrific storms observed in its atmosphere, which, owing to the great
+distance of Jupiter from the Sun, do not seem to be attributable to this
+body, but rather to some local cause within the envelope of the planet.
+
+Astronomy, which is a science of observation, is naturally silent with
+regard to the inhabitants of Jupiter. If there are any such inhabitants,
+they are confined to the domain of conjecture, under the dense cloudy
+envelope of the planet. The conditions of habitability on Jupiter must
+differ very widely from those of our globe. Comparatively little direct
+light from the Sun reaches the surface of the globe of Jupiter, except
+that which passes through the narrow openings forming the dark clouds.
+All the rest of the planet's surface, being covered perpetually by
+opaque clouds, receives only diffused light. On Jupiter there are
+practically no seasons, since its axis is nearly perpendicular to its
+orbit. The force of gravity on the surface of Jupiter being more than
+double what it is on the Earth, living bodies would there have more than
+double the weight of similar bodies on the Earth. Furthermore, Jupiter
+only receives 0.011 of the light and heat which we receive from the Sun;
+and its year is nearly equal to 12 of our years. If there are living
+beings on Jupiter, they must, then, be entirely different from any known
+to us, and they may have forms never dreamed of in our most fantastic
+conceptions.
+
+The two round black spots represented towards the central parts of Plate
+IX. are the shadows of the first and second satellite; while the two
+round white spots seen on the left of the disk, are the satellites
+themselves, as they appeared at the moment of the observation. The first
+satellite and its shadow are the nearest to the equator; while the
+second satellite and its shadow are higher, the last being projected on
+the Great Red Spot.[2] The row of dark circular spots represented on the
+northern, or lower hemisphere, when they first appeared, had some
+resemblance to Sun-spots without a penumbra, with bright markings around
+them, resembling faculæ. These round spots subsequently enlarged
+considerably, until they united along the entire line, encircling the
+planet, and finally forming a narrow pink belt, which is still visible.
+
+
+[Footnote 2: By an accidental error in enlarging the original drawing,
+the satellites and shadows appear in Plate IX. of double their actual
+size. The error is one easy of mental correction.]
+
+
+
+
+THE PLANET SATURN
+
+PLATE X
+
+
+Saturn, which is next to Jupiter in order of distance from the Sun,
+while not the largest, is certainly the most beautiful and interesting
+of all the planets, with his grand and unique system of rings, and his
+eight satellites, which, like faithful servants, attend the planet's
+interminable journey through space.
+
+Seen with the naked eye, Saturn shines in the night like a star of the
+first magnitude, whose dull, soft whiteness is, however, far from
+attaining the brilliancy of Venus or Jupiter, although it sometimes
+approaches Mars in brightness. Saturn hardly ever exhibits the
+phenomenon of scintillation, or twinkling, a peculiarity which makes it
+easily distinguishable among the stars and planets of the heavens.
+
+The synodical period of Saturn occupies 1 year and 13 days, so that
+every 378 days, on an average, this planet holds the same position in
+the sky relatively to the Sun and the Earth.
+
+The mean distance of Saturn from the Sun is a little over 9½ times that
+of our globe, or 872,000,000 miles. Owing to the orbital eccentricity,
+this distance may increase to 921,000,000 miles, when the planet is in
+aphelion; or decrease to 823,000,000 miles, when it is in perihelion;
+Saturn being therefore 98,000,000 miles nearer to the Sun when in
+perihelion than in aphelion. If gravitation were free to exert its
+influence alone, Saturn would fall into the Sun in 5 years and 2 months.
+
+The distance of Saturn from the Earth varies, according to the position
+of the two planets in their respective orbits. At the time of
+opposition, when the Earth lies between the Sun and Saturn, this
+distance is smallest; while, on the contrary, at the time of
+conjunction, when the Sun lies between the Earth and Saturn, it is
+greatest. Owing, however, to the eccentricity of the orbits of Saturn
+and our globe, and the inclination of their planes to each other, and
+owing also to the variable heliocentric longitude of the perihelion, the
+distance of the two planets from each other at their successive
+conjunctions and oppositions is rendered extremely variable. At present
+it is when the oppositions of Saturn occur in December that this planet
+comes nearest to us; while when the conjunctions take place in June, the
+distance of Saturn from the Earth is the greatest possible. In the
+former case the distance of the planet from our globe is only
+730,000,000 miles; while in the last it is 1,014,000,000 miles, the
+difference between the nearest and farthest points of Saturn's approach
+to us being no less than 284,000,000 miles, or over three times the mean
+distance of the Sun from the Earth.
+
+From the great variations in the distance of Saturn from the Earth,
+necessarily result corresponding changes in the brightness and apparent
+diameter of this body. When it is farthest from us, its angular diameter
+measures but 14"; while, when it is nearest, it measures 20".
+
+The orbit of Saturn is inclined 2°30' to the ecliptic, and its
+eccentricity, which equals 0,056, is over three times that of the
+Earth's orbit.
+
+This planet revolves around the Sun in a period of 29 years and 5½
+months, or 10,759 terrestrial days, which constitutes its sidereal year.
+The extension of the immense curve forming the orbit of this planet, is
+no less than 5,505,000,000 miles, which is traversed by the planet with
+a mean velocity of a little less than 6 miles per second, or three times
+less than the motion of our globe in space.
+
+The real dimensions of the globe of Saturn are not yet known with
+accuracy, and the equatorial diameter has been variously estimated by
+observers, at from 71,000 to 79,000 miles. If we adopt the mean of these
+numbers, 75,000 miles, the circumference of the Saturnian equator would
+measure 235,620 miles, or 9½ times the circumference of our globe; the
+surface of Saturn would be 86 times, and its volume over 810 times that
+of the Earth.
+
+However great the volume of Saturn, its mass is proportionally small,
+being only 90 times greater than that of our globe; the mean density of
+the materials composing this planet being less than that of cork, and
+only 0.68 the density of water. The force of gravitation at the surface
+of Saturn is greater, by a little over ⅑, than it is at the surface of
+the Earth; a body falling in a vacuum at its surface, would travel 17.59
+feet during the first second.
+
+From observations of markings seen on the surface of Saturn, and from
+the study of their apparent displacements on the disk, William Herschel
+found that the planet rotated upon its axis in 10h. 16m. 0.24s. Since
+Herschel's determination, new researches have been made, and lately,
+Professor Hall, noticing a bright spot, followed it for nearly a month,
+observing its transits across the central meridian of the disk. From
+these observations he has obtained for the rotation period 10h. 14m.
+23.8s., a result which agrees very closely with that obtained 82 years
+earlier by Herschel, considering the fact that the markings from which
+the period of rotation is ascertained are not fixed on the planet, but
+are always more or less endowed with proper motion. The velocity of
+rotation at the equator is 21,538 miles per hour, or nearly 6 miles per
+second.
+
+The axis of rotation of Saturn is inclined 64° 18' to the plane of the
+orbit, so that its equator makes an angle of 25° 42' with the same
+plane. The seasons of this planet therefore present greater extremes of
+temperature than those of the Earth, but not quite so great variations
+as the seasons of Mars.
+
+The globe of Saturn is not a perfect sphere, but its figure is that of
+an oblong spheroid, flattened at the poles. The polar compression of
+Saturn is greater than that of any other planet, surpassing even that of
+Jupiter. Though not yet determined with a great degree of accuracy, the
+compression is known to be between ²⁄₁₈ and ⅒ of the equatorial
+diameter; that is, a flattening of about 3,894 miles, at each pole, the
+polar diameter being 7,788 miles shorter than the equatorial.
+
+The internal condition of the planet Saturn, whether solid, liquid or
+gaseous, cannot be discovered from the examination of its surface, as
+its globe is enwrapped in a dense opaque layer of vapors and cloud-like
+forms, through which the sight fails to penetrate. The appearance of
+this vapory envelope is like that of _cumulus_ clouds, and one of its
+characteristics is to arrange itself into alternate bright and dark
+parallel belts, broader than those seen on Jupiter, and also more
+regular and dark. These belts, which are parallel to the equator of the
+planet, vary in curvature with the inclination of its axis of rotation
+to the line of sight.
+
+The belts of Saturn, like those of Jupiter, are not permanent, but keep
+changing more or less rapidly. Sometimes they have been observed to be
+quite numerous; while at other times they are few. Occasionally
+conspicuous white or dark spots are seen on the surface, although the
+phenomenon is quite rare. It is from the observation of such spots that
+Saturn's period of rotation has been determined, as stated above. The
+equatorial zone of Saturn always appears more white and brilliant than
+the other parts, as it also appears more mottled and cloud-like. In late
+years the globe has been characterized, and much adorned, by a pale
+pinkish tint on its equatorial belt, resembling that of Jupiter, but
+somewhat fainter. On either side of the equatorial belt there is a
+narrower band, upon which the mottled appearance is visible. Below
+these, one or two dark belts, separated by narrow white bands, are
+usually seen; but, of late, the bands have been less numerous, being
+replaced in high latitudes by a dark segment, which forms a polar cap to
+Saturn. The globe of Saturn does not anywhere appear perfectly white,
+and when compared with its ring, it looks of a smoky yellowish tint,
+which becomes an ashy gray on its shaded parts. It usually appears
+darker near the limb than in its central portions; although on some
+occasions I have seen portions of the limb appear brighter, as if some
+white spots were traversing it.
+
+Some observers have seen the limb deformed and flattened at different
+places, and W. Herschel even thought such a deformation to be a
+permanent feature of this globe, which he termed diamond-shaped, or
+"square shouldered." But this was evidently an illusion, since the
+planet's limb usually appears perfectly elliptical, although it
+occasionally appears as if flattened at some points, especially where it
+comes in apparent contact with the shadow cast by the globe on the ring,
+as observed by myself many times. But with some attention, it is
+generally found that this deformation is apparent rather than real, and
+is caused by the passage of some large dark spots over the limb, which
+is thus rendered indistinguishable from the dark background upon which
+it is projected.
+
+What distinguishes Saturn from all known planets, or heavenly bodies,
+and makes it unique in our universe, is the marvelous broad flat ring
+which encircles its equator at a considerable distance from it. With a
+low magnifying power this flat ring appears single, but when carefully
+examined with higher powers, it is found to consist of several distinct
+concentric rings and zones, all lying nearly in the same plane with the
+planet's equator.
+
+
+[PLATE X.--THE PLANET SATURN.
+
+Observed on November 30, 1874, at 5h. 30m. P.M.]
+
+
+At first sight only two concentric rings are recognized, the _outer_ and
+the _middle_, or _intermediary_, which are separated by a wide and
+continuous black line, called the _principal division_. This line, and
+indeed all the features of the surface of the rings are better seen, and
+appear more prominent on that part of the ring on either side called the
+_ansa_, or handle. Besides these two conspicuous rings, a third, of very
+dark bluish or purplish color, lies between this middle ring, to which
+it is contiguous, and the planet. This inner ring, which is quite wide,
+is called the _gauze_ or _dusky ring_. Closer examination shows that the
+outer ring is itself divided by a narrow, faint, grayish line called the
+pencil line, which, from its extreme faintness, is only visible on the
+ansæ. Moreover, the middle ring is composed of three concentric zones,
+or belts, which, although not apparently divided by any interval of
+space, are distinguished by the different shadings of the materials
+composing them. The outer zone of this compound middle ring is, by far,
+the brightest of all the system of rings and belts, especially close to
+its external border, where, on favorable occasions, I have seen it
+appear on the ansæ as if mottled over, and covered throughout with
+strongly luminous cloud-like masses. On the ansæ of the double outer
+ring, similar cloudy forms have also been seen at different times. The
+second zone of the middle ring is darker than the first, the innermost
+being darker still. All the characteristic points which have thus been
+described, are shown in Plate X.
+
+Although suspected in 1838, the dusky ring was not recognized before
+1850, when G. P. Bond discovered it with the 15-inch refractor of the
+Cambridge Observatory. It was also independently discovered the same
+year in England by Dawes and Lassell. The dusky ring differs widely in
+appearance and in constitution from the other rings, inasmuch as these
+last are opaque, and either white or grayish, while the former is very
+dark, and yet so transparent that the limb of the planet is plainly seen
+through its substance. On particularly favorable occasions, the
+appearance of this ring resembles that of the fine particles of dust
+floating in a ray of light traversing a dark chamber. Whatever may be
+the material of which this ring is composed, it must be quite rarefied,
+especially towards its inner border, which appears as if composed of
+distinct and minute particles of matter feebly reflecting the solar
+light. That the inner part of the dusky ring is composed of separate
+particles, is proved by the fact that the part of the ring which is seen
+in front of the globe of Saturn has its inner border abruptly deflected
+and curved inward on entering upon the disk, causing it to appear
+considerably narrower than it must be in reality, a peculiarity which is
+shown in the Plate. This phenomenon may be attributed to an effect of
+irradiation, due to the strong light reflected by the central parts of
+the ball, which so reduces the apparent diameter of the individual
+particles that they become invisible to us, especially those near the
+inner border, which are more scattered and less numerous than elsewhere.
+
+The dusky ring, which was described by Bond, Lassell and other
+astronomers as being equally transparent throughout all its width, has
+not been found so by me in later years. The limb of the planet, seen by
+these observers through the whole width of the dusky ring in 1850, could
+not be traced through its outer half by myself in 1872 and 1874, and
+this with the very same instrument used by Bond in his observations of
+1848 and 1850. Moreover, I have plainly seen that its transparency was
+not everywhere equal, but greatest on the inner border, from which it
+gradually decreases, until it becomes opaque, as proved by the gradual
+loss of distinctness of the limb, which vanishes at about the middle of
+the dusky ring. These facts, which have been well ascertained, prove
+that the particles composing this ring are not permanently located, and
+are undergoing changes of relative position. It will be shown that the
+surface of the other rings is also subject to changes, which are
+sometimes very rapid.
+
+The globe of Saturn is not self-luminous, but opaque. It shines by the
+solar light, as is proved by the shadow it casts opposite the Sun upon
+the ring. Although receiving its light from the Sun, Saturn does not
+exhibit any traces of phases, like the other planets nearer to the Sun,
+owing to its great distance from the Earth. When near its quadratures,
+however, the limb opposite to the Sun appears much darker, and shows
+traces of twilight. As far as can be ascertained, the rings, with the
+exception of the inner one, are opaque, as proved by the strong shadow
+which they cast on the globe of Saturn.
+
+The shadows cast by the planet on the ring, and by the ring on the
+planet, are very interesting phenomena, inasmuch as they enable the
+astronomer to recognize the form of the surface which receives them. The
+shadow cast by the ring on the ball is not quite so interesting as the
+other, although it has served to prove that the surface of this globe is
+not smooth, as is likewise suggested by its mottled appearance. I have
+sometimes found, as have also other observers, that the outline of this
+shadow upon the ball was irregular and indented, an observation which
+proves either that the surface of the ball is irregular, or that the
+border of the ring casting the shadow was jagged. The shadow of the
+globe on the rings has much more interest, as it enables us to get at
+some knowledge of the form of the surface of the rings, which otherwise
+is very difficult to discover, owing to the oblique position in which we
+always see them.
+
+In general, the shadow of the ball on the middle ring has its outline
+concave towards the planet; while on the outer ring it is usually
+slanting, and at a greater distance from the limb than on the middle,
+and dusky rings. This form of the shadow evidently proves that the
+middle ring stands at a higher level than the two others, especially
+towards its outer margin. The system seems to increase gradually in
+thickness from the inner border of the dusky ring to the vicinity of the
+outer margin of the middle ring, after which it rapidly diminishes on
+this border, while the surface of the outer ring is almost level.
+
+But this surface is by no means fixed, as its form sometimes changes, as
+proved by my observations and those of others. As may be noticed on
+Plate X., the outline of the shadow of the planet on the rings is
+strongly deviated towards the planet, near the outer margin of the
+middle ring; the notch indicating an abrupt change of level, and a rise
+of the surface at that point. Some observers have endeavored to explain
+these deviations by the phenomena of irradiation, from which it would
+follow that the maximum effect of deviation should be observed where the
+ring is the brightest, which does not accord with observation; as the
+deepest depression in the shadow is not to be found usually at the
+brightest part, which is towards the outer border of the middle ring,
+but occurs near its centre. From these observations it is undoubtedly
+established that the surface of the rings is far from being flat
+throughout, and is, besides, not permanent, but changes, as would, for
+instance, the surface of a large mass of clouds seen from the top of a
+high mountain. In general, the system is thickest not very far from the
+outer border of the intermediary ring.
+
+Some interesting phenomena which I had occasion to observe before and
+after the passage of the Sun through the plane of the rings, on February
+6th, 1878, conclusively show that the surface of this system cannot be
+of a uniform level, but must be thicker towards the outer border of the
+middle ring, thence gradually sloping towards the planet. Many of my
+observations irresistibly lead to this conclusion. As it would, however,
+be out of place to have them recorded here in detail, I will simply give
+one of the most characteristic among them.
+
+From December 18th, 1877, when the Sun was about 41' above the plane of
+the rings, to February 6th, 1878, the day of its passage through their
+plane, the illuminated surface of this system gradually decreased in
+breadth with the lowering of the Sun, until it was lost sight of,
+February 5th, on the eve of the passage of the Sun through their plane.
+The phenomenon in question consisted in the gradual invasion of their
+illuminated surface by what appeared to be a black shadow, apparently
+cast by the front part of the outer portion of the middle ring the
+nearest to the Sun. On January 25th, when the elevation of the Sun above
+the plane of the rings was reduced to 15', the shadow thus cast had
+extended so far on their surface that it reached the shadow cast by the
+globe on the opposite part of the ring in the east, and accordingly the
+remaining portion of the illuminated surface of the eastern ansa then
+appeared entirely disconnected from the ball, by a large dark gap,
+corresponding in breadth to that of the globe's shadow on the rings. On
+February 4th, when the Sun was only 5' above the plane of the rings,
+their illuminated and only visible surface was reduced to a mere thread
+of light, which on the 5th appeared broken into separate points. It is
+evident that the phenomenon was not caused by the obliquity of the ring
+as seen from our globe, since the elevation of the Earth above the plane
+of the rings--which on December 18th was 3° 20'--was still 1° 20' on
+the 4th of February. In ordinary circumstances, when the Sun is a little
+more elevated, and the rings seen at this last angle, they appear quite
+broad and conspicuous, and even the dark open space separating the dusky
+ring from the planet is perfectly visible on the ansæ, where the
+Earth's elevation above their plane is reduced to 40'. It is also
+evident that the phenomenon was not to be attributed to the reduction of
+the light which they received from the Sun, although the illumination in
+February might be expected to be comparatively feeble, since the Sun
+then shone upon the rings so obliquely; yet (on the supposition that
+their surface is flat) they should have been illuminated throughout, and
+if not very brightly, sufficiently so, at least, to make them visible
+and as bright as was the narrow thread of light observed on the 4th of
+February. The phenomenon actually observed may be explained most readily
+by assuming, as other phenomena also indicate, that the surface of the
+ring is not flat, but more elevated towards, or in the vicinity of its
+outer border, from which place it slopes inwardly towards the planet. On
+this assumption, it is evident that the elevated part of the ring the
+nearest to the Sun would cast a shadow, which, with the increasing
+obliquity of the Sun, would gradually cover the whole surface comprised
+within the elevated part, and thus become invisible to us. Several
+observations made by Bond and other observers undoubtedly show the same
+phenomenon, and do not seem to be intelligible on any other supposition.
+From my observations made in 1881 it would appear, however, that the
+opposite surfaces of the rings do not exactly correspond in form, but
+this may not be a permanent feature, as the surface of this system is
+subject to changes, as already shown.
+
+The dimensions of the rings are great, the diameter of the outer one
+being no less than 172,982 miles, the distance from the centre of the
+globe to the outer border of the system being, therefore, 86,491 miles.
+The breadth of the outer ring is 9,941 miles; that of the principal
+division, 2,131 miles; that of the middle ring, 19,902 miles, and that
+of the dusky ring, 8,772 miles. The breadth of all the rings taken
+together is, therefore, 40,746 miles. The interval between the surface
+of Saturn and the inner border of the dusky ring is 7,843 miles.
+
+The thickness of the system of rings has been variously estimated by
+astronomers, on account of the great difficulties attending its
+determination. While Sir John Herschel estimated it at more than 250
+miles, G. P. Bond reduces it to 40 miles. Both of these numbers are
+evidently too small, as so slight a thickness cannot explain the
+observed phenomenon of the shadow cast by a portion of the ring on its
+own surface, when the Sun is very low in its horizon, as shown above.
+
+The plane of the system of rings is inclined 27° to the planet's orbit,
+and is parallel, or at least very nearly so, with the equator of the
+planet, passing, therefore, through its centre, and dividing its globe
+into northern and southern hemispheres. Seen from the Earth, a portion
+of the ring always appears projected in front of the planet, thus
+concealing a small part of its globe, while the opposite portion passes
+behind the globe, which hides it from sight.
+
+As the plane of the ring is not affected by the motion of the planet
+around the Sun, but always remains parallel to itself, it follows that
+as Saturn advances in its orbit the rings must successively present
+themselves to us under various angles of inclination, appearing,
+therefore, more or less elliptical, and presenting two maxima and two
+minima of inclination in the course of one of its revolutions. As the
+revolution of Saturn is accomplished in 29½ years, the maxima and the
+minima must recur every 14 years and 9 months; the maxima being
+separated from the minima by an interval of 7 years and 4½ months.
+
+When Saturn arrives at the two opposite points of its orbit, where the
+major axis of its ring is at right angles to the line joining its centre
+to that of the Sun, the ring, which is then viewed at an inclination of
+27°, the greatest angle at which it can ever be seen, has reached its
+maximum opening, the smaller diameter of its ellipse being then about
+half that of the larger. At this moment the outer ring projects north
+and south beyond the globe, which is then completely enclosed in its
+ellipse. The maximum opening of the northern surface of the ring takes
+place, at present, when Saturn arrives in longitude 262°, in the
+constellation Sagittarius, and that of the southern surface when it
+arrives in longitude 82° in the constellation Taurus. When, on the
+contrary, Saturn reaches the two opposite points of its orbit, where the
+plane of its ring is parallel to the line joining its centre to that of
+the Sun, the opening vanishes, as only the thin edge of the ring is then
+presented to the Sun and receives its light, the rest being in darkness.
+At this moment the ring disappears, except in the largest telescopes,
+where it is seen as an exceedingly thin thread of light; and the
+Saturnian globe, having apparently lost its ring, appears solitary in
+the sky, like the other planets. The disappearance of the ring from this
+cause occurs now when Saturn arrives at 90° from either of the
+positions of maximum inclination, that is, in longitude 352° in the
+constellation Pisces, and in longitude 172° in the constellation Leo.
+
+When the planet is in any other position than one of these last two,
+either the northern or the southern surface of the ring is illuminated
+by the Sun, while the opposite surface is in the night, and does not
+receive any direct sunlight. At the time of the passage of the plane of
+the ring through the Sun's centre, a change takes place in the
+illumination of the ring. If it is the northern surface which has
+received the rays of the Sun during the previous half of the Saturnian
+year, at the moment the plane has passed the centre of the Sun, the
+southern surface, after having been buried in darkness for 14¾ years,
+sees the dawn of its long day of the same length. Such a phenomenon will
+not occur until 1892, when the passage of the Sun from the northern to
+the southern side of the ring will close in twilight the day commenced
+in 1878.
+
+Aside from the periodic disappearance of the ring, resulting from the
+passage of the Sun through its plane, the ring may also disappear from
+other reasons. Just before or just after the time of the passage of the
+Sun through the plane of the ring, the Earth and the Sun may occupy such
+positions, that while the one is north of the plane of the ring, the
+other is south of it, or vice versa, in which event the ring becomes
+invisible, because its dark and non-illuminated surface is presented to
+us. The ring may also become invisible to us when the Earth passes
+through its plane.
+
+Since the distance from Saturn to the Sun is to the distance of the
+Earth from this last body as 9.54 is to 1; and since the circumference
+of a circle increases in the same proportion as its radius, it follows
+that the diameter of the Earth's orbit projected on the orbit of Saturn
+would occupy only part of the latter, or about 12° 2', this being 6°
+1' on either side of the nodes of the rings. To describe such an arc on
+its orbit, it takes Saturn almost 360 days on an average, or almost a
+complete year; the Earth describing therefore almost a whole revolution
+around the Sun during the time it takes Saturn to advance 12° 2' on its
+orbit. Then, when Saturn occupies a position comprised within an arc 6°
+1' from either side of the nodes of its ring, the Earth, by its motion,
+is liable to encounter the plane of the ring, when therefore it will
+only present its thin edge to us, and becomes invisible. At least one
+such encounter is unavoidable within the time during which Saturn
+occupies either of these positions on its orbit; while three frequently
+happen, and two are possible.
+
+The natural impression received by looking at the rings, while seeing
+the ponderous globe of Saturn enclosed in its interior, is that this
+gigantic, but very delicate structure, in order to avoid destruction,
+must be endowed with a swift movement of rotation on an axis
+perpendicular to its plane, and that the centrifugal force thence
+arising counterbalances the powerful attraction of the planet, and thus
+keeps the system in equilibrium.
+
+Theoretically, the rotation of the rings is admitted by every
+astronomer, as being an essential condition to the existence of the
+system, which otherwise, it is thought, would fall upon the planet.
+Although the rotation of the rings seems so probable that it is
+theoretically considered as certain, yet its existence has not been
+satisfactorily demonstrated by direct observation, which alone can
+establish it on a firm basis as a matter of scientific knowledge.
+
+The determination of the period of rotation of the rings, which is
+supposed to be 10h. 32m. 15s., rests only on the observations of W.
+Herschel, made in 1790, from the apparent displacement of irregularities
+on the ring; but his results have been contradicted by other
+observations, and even by those of Herschel himself, made in later
+years.
+
+Although the system of rings is very nearly concentric with the globe of
+Saturn, yet the coincidence is not considered as mathematically exact.
+It seems to have been satisfactorily demonstrated by direct observations
+that the centre of gravity of the system oscillates around that of the
+planet, thus describing a minute orbit. This peculiarity is in
+accordance with theory, which has shown it to be essential to the
+stability of the system.
+
+Besides its system of rings, which makes Saturn the most remarkable
+planet of the solar system, this globe is attended by eight satellites,
+moving in orbits whose planes very nearly coincide with the plane of the
+rings, except that of the most distant one, which has an inclination of
+about 12° 14'. In the order of their distance from the planets, the
+satellites of Saturn are as follows: Mimas, Enceladus, Tethys, Dione,
+Rhea, Titan, Hyperion and Iapetus. The three first satellites are nearer
+to Saturn than the Moon is to the Earth; while Iapetus, the farthest, is
+9½ times the distance of our satellite from us. All the satellites,
+with the exception of the farthest, move more rapidly around Saturn than
+the Moon moves around the Earth; while Iapetus, on the contrary, takes
+almost three times as long to make one revolution.
+
+The period of revolution of the four inner satellites is accomplished in
+less than three days, that of Mimas being only a little more than 22
+hours. From such swiftness of motion, it is easily understood how short
+must be the intervals between the different phases of these satellites.
+Mimas, for instance, passes from New Moon to First Quarter in less than
+6 hours.
+
+The distance of the nearest satellite from the planet's surface is
+84,000 miles, and its distance from the outer ring only 36,000 miles. It
+is difficult to determine the diameter of objects so faint and distant
+as are some of these satellites, but the diameter of Titan, the largest
+of all, is pretty well known, and estimated to be ¹⁄₁₆ the
+diameter of the planet, or more than half the diameter of our globe.
+
+Iapetus is subject to considerable variations in brilliancy, and as the
+maxima and minima always occur when this satellite occupies the same
+parts of its orbit, it was conjectured by W. Herschel that, like our
+Moon, it turns once upon its axis during each of its revolutions about
+the planet. It has been shown by my observations, that Iapetus attains
+its maximum brightness a little before it reaches its greatest western
+elongation, and its minimum on the opposite side.
+
+As the planes of the orbits of the satellites are inclined to the
+planet's orbit, it follows that their transits, occultations and
+eclipses, are only possible when Saturn is near its equinoxes. Passages
+of the satellites and their shadows across the disk, although rare, have
+been observed, and they somewhat resemble the phenomena exhibited by the
+satellites of Jupiter in transit. When the Earth is very near the plane
+of the rings, the satellites, except the farthest, appear to be in a
+straight line nearly coincident with the plane of the rings, and are
+seen occasionally moving along the thin edge of the rings, appearing as
+luminous beads moving on a thread of light.
+
+Owing to the considerable inclination of the axis of rotation of Saturn
+to its orbit, the seasons of this planet must have greater extremes of
+temperature than those of the Earth. As the year of Saturn consists of
+25,217 Saturnian days, each season, on the average, is composed of 6,304
+Saturnian days.
+
+To an observer on Saturn, the immense arches formed by its rings would
+appear as objects of great magnificence, spanning the sky like soft
+colorless rainbows. Moreover, the eight moons, several of which are
+always visible, would be of the highest interest, with their swift
+motions and rapid phases. Mimas, traveling in its orbit at the rate of
+16 of arc per minute of time, moves over a space equal to the apparent
+diameter of our Moon in two minutes, or at the rate of 16° an hour.
+
+Owing to the globular form of Saturn, the rings would be invisible in
+latitudes situated above 65° from its equator, and their apparent form
+and breadth would naturally vary with the latitude. At 63° only a very
+small portion of the outer ring would be visible above the equatorial
+horizon, where it would appear as a small segment of a circle. At 62°
+the principal division would just graze the horizon. At 46° the outer
+portion of the dusky ring would become visible, while at 35° its inner
+edge would appear above the horizon. From 65° of latitude down to the
+equator, the arches of the rings would be seen more and more elevated
+above the equatorial horizon, but at the same time that they are seen
+higher up, their apparent breadth gradually diminishes, owing to the
+effect of foreshortening, and at the equator itself the system would
+only present its thin edge to view.
+
+During the summer seasons of either hemisphere of Saturn, the surface of
+the rings turned towards such hemisphere, being fully illuminated by the
+Sun, is visible from these regions. In the day time its light must be
+feeble and similar to the light reflected by our Moon during sunshine;
+but at night the system would display all its beauty, and the different
+rings, with their divisions and their various reflective powers, must
+present a magnificent sight.
+
+During the nights of the long winter seasons on Saturn, on the contrary,
+the surface of the rings turned towards the hemisphere undergoing
+winter, receives no light from the Sun, and is invisible, or very nearly
+so, except towards morning and evening, when it may be faintly
+illuminated by the secondary light which it receives from the
+illuminated globe of Saturn. Although dark and invisible, the rings may
+make their form apparent at night by the absence of stars from the
+region which they occupy in the sky. Again, in other seasons, the days
+present very curious phenomena. In consequence of the diurnal rotation
+of the planet, the Sun seems to move in circular arcs, which, owing to
+the inclination of Saturn's axis, are more or less elevated above its
+horizon, according to the position of the planet in its orbit. As such
+arcs described by the Sun in the sky of Saturn are liable to encounter
+the rings, the Sun in passing behind them becomes eclipsed. It must be a
+magnificent spectacle to witness the gradual disappearance of the fiery
+globe behind the outer ring, and its early reappearance, but for a
+moment only, through the narrow gap of the principal division; to see it
+vanish again behind the middle ring, to reappear a little later through
+the semi-transparent dusky ring, but very faint and red colored at
+first; and then, gradually brighten up, and finally emerge in all its
+beauty from the inner edge of the dusky ring.
+
+It is in latitude 23° that the rings produce the most prolonged
+eclipses of the Sun. During a period equivalent to ten of our
+terrestrial years, such eclipses continually succeed each other with but
+very short periods of interruption; and even during a long series of
+rotations of Saturn, the Sun remains completely invisible in those
+regions where the apparent arcs which it describes coincide with the
+arcs of the rings. In neighboring latitudes, the eclipses of the Sun,
+although still frequent, would have a shorter and shorter duration as
+the observer should travel north or south. These eclipses of the Sun
+must produce a partial darkness of the regions involved in the shadow of
+the rings, which may be compared to the darkness produced on our globe
+by a total eclipse of the Sun. The frequent recurrence of these
+eclipses, and their comparatively long duration in some regions, must
+still further reduce the duration of the short Saturnian days.
+
+The globe of Saturn, as already shown, casts a shadow on the rings,
+which, according to the position of the planet in its orbit, either
+extends across their whole breadth, or covers only a part of their
+surface. The shadow on the rings rising in the east after sunset,
+ascends to the culminating point of their arcs in the sky, in 2h. 34m.,
+and as rapidly descends on the western horizon, to disappear with
+sunrise. This shadow, when projected on the rings in the sky, must be
+hardly distinguishable from the dark background of the heavens, except
+from the absence of stars in the regions which it occupies. It must
+appear as a large dark gap, separating the rings into two parts, and
+constantly moving from east to west. Possibly the refraction of the
+solar rays, in passing through Saturn's atmosphere, may cast some
+colored light on the rings, similar to that observed on the Moon during
+its eclipses.
+
+An observer on the rings would behold phenomena still more curious, a
+long day of 14¾ years being followed by a long night of 14¾ years. The
+long days of Saturn's rings are, however, diversified by numerous
+eclipses of the Sun, which regularly occur every 10¼ hours; the
+phenomenon being due to the interposition of the globe of Saturn between
+the rings and the Sun. These eclipses produce partial obscurations of
+their surface, lasting from 1½ to 2 hours at a time. Although the
+surface of the rings never receives direct sunlight during their long
+nights, yet they are not plunged all the time in total darkness, as they
+receive some reflected light from that part of the globe of Saturn which
+is illuminated by the Sun. To the supposed observer on the rings, during
+every 10¼ hours, the immense globe would exhibit continually changing
+phases. At first he would see a point of light rapidly ascending from
+the horizon, and appearing under the form of a half crescent of
+considerable radius; 5⅛ hours later, the crescent having gradually
+increased, would appear as a half circle, covering ⅛ of the visible
+heavens, its surface being more than 20,000 times as large as the
+surface of the Moon. Upon this brilliantly illuminated semi-circle would
+be projected the shadows of the rings, appearing as black belts
+separated by a narrow luminous band.
+
+It is very difficult for one to conceive how such a delicate structure,
+as the system of rings appears to be, can keep together in equilibrium
+and avoid destruction from the powerful attraction of the planet on one
+side and the disturbing influence of the satellites on the other. To
+explain it, several hypotheses have been advanced. The rings were first
+supposed to be solid, and upon this supposition Laplace determined the
+necessary conditions for their equilibrium; the most important of which
+require that the cross section of the rings should be an ellipse of
+irregular curvature, and having its major axis directed towards the
+centre of the planet, and also that the system should rotate upon an
+axis perpendicular to the plane of the rings. This theory was superseded
+by another, which supposed the rings to be fluid. This one was soon
+rejected for a third, assuming the system to be composed of vapors or
+gases; and more recently, all these theories were considered untenable,
+and replaced by a fourth, which supposes the system of rings to be made
+up of a congregation of innumerable small, independent bodies, revolving
+around Saturn in concentric zones. Naturally, such a divergence of
+opinion can only result from our comparative ignorance of the subject,
+and sufficiently indicates our inability to explain the phenomena; and
+it must be admitted that, so far, nothing is certainly known about this
+strange system. We shall probably remain in the same uncertainty until
+the rotation of the rings is ascertained by direct observations. It is
+pretty certain, however, that none of these theories account for the
+observed phenomena in their details, although a partial explanation may
+be obtained by borrowing something from each hypothesis.
+
+It has been conjectured, and a theory has been advanced, that the
+breadth of the whole ring system is gradually increasing inwards, and
+that it will come in contact with the planet in about 2,150 years; but
+the question seems to have been settled in the negative by the elaborate
+measurements of the English observers. It is likely that the increase is
+only in the defining power of the instruments.
+
+
+
+
+COMETS
+
+PLATE XI
+
+
+Among the celestial phenomena, none are more interesting than those
+mysterious apparitions from the depths which unexpectedly display their
+strange forms in our familiar constellations, through which they wander
+for a time, until they disappear like phantoms.
+
+A comet, with its luminous diffused head, whence proceeds a long vapory
+appendage gradually fading away in the sky, presents an extraordinary
+aspect, which may well astonish and deeply impress the observer.
+Although these visitors from infinite space do not now inspire dread, as
+in by-gone times, yet, owing to the mystery in which the phenomenon is
+still involved, the apparition of a large comet, even in our days, never
+fails to create a profound sensation, and in some cases that unconscious
+fear which results from the unknown.
+
+The effect of such a spectacle largely depends upon its rarity; but
+since the telescope has been applied to the sounding of the heavens, it
+has been found that the appearance of comets is by no means an unusual
+occurrence. If so few comets, comparatively, are seen, it is because
+most of them are telescopic objects, and are therefore invisible to the
+naked eye. Most of the telescopic comets are not only too faint to be
+perceived by the unaided eye, but are insignificant objects, even when
+observed through the largest telescopes.
+
+It was Kepler's opinion that comets are as numerous in the sky as fishes
+are in the ocean. Undoubtedly the number of these bodies must be great,
+considering that we can only see them when they come into the
+neighborhood of the Earth, and that many even here remain invisible, or
+at least pass unperceived. That many of them have passed unperceived
+heretofore, is proved by the fact that the number of those observed
+becomes greater every year, with the increase of the number of
+instruments used in their search. The number of comets observed with the
+naked eye during historic times is nearly 600, and that of telescopic
+comets, which, of course, all belong to the last few centuries, is more
+than 200, so that we have a total number of about 800 comets of which
+records have been kept. From theoretical considerations, Lambert and
+Arago estimated their entire number at several millions, but such
+speculations have generally no real value, since they cannot be
+established on a firm basis.
+
+Comets remain visible for more or less time, according to their size and
+the nature and position of their orbits, but in general, the large ones
+can be followed with the telescope for several months after they have
+become invisible to the naked eye. The comet of 1861, for example,
+remained telescopically visible for a year, and that of 1811, for 17
+months after disappearing from ordinary sight.
+
+While a comet remains visible, it appears to revolve daily about us like
+the stars in general; but it also moves among the constellations, and
+from this movement its orbit may be computed like that of a planet. From
+the apparent diurnal motion of a comet with the heavens, result the
+changes of position which it seems to undergo in the course of a night.
+The direction of the head and tail of a comet, of course, has only
+changed in regard to the horizon, but not in regard to the sky, in which
+they occupy very nearly the same position throughout a given night, and
+even for many nights in succession.
+
+The movements of the comets in their orbits are, like those of the
+planets, in accordance with Kepler's laws, the Sun occupying one of the
+foci of the orbit they describe; but the orbits of comets differ,
+however, in several points from those of the planets. Their eccentricity
+is always great, being sometimes apparently infinite, in which case the
+orbit is said to be parabolic, or hyperbolic; but the smallness of the
+portion of a cometary orbit which can ordinarily be observed, makes it
+difficult to determine this with certainty. Again, while the planetary
+orbits are usually near the plane of the ecliptic, those of comets
+frequently have great inclinations to that plane, and even when the
+inclination is less than 90°, the comet may have a retrograde movement,
+or, in other words, a movement contrary to the course in which all the
+planets revolve about the Sun.
+
+Notwithstanding these differences between the elements of the orbits of
+the comets and those of the planets, the fact that each has the Sun in
+one focus indicates that the body moving in it is a member of the solar
+system, either for the time, or permanently, according to the nature of
+its orbit.
+
+A distinction may accordingly be made between the comets which are
+permanent members of our solar system and those which are only
+accidental or temporary visitors. Those moving in elliptical orbits
+around the Sun, like the planets, and therefore having a determinate
+period of revolution, from which the time of their successive returns
+may be predicted, are permanent members of our system, and are called
+periodic comets. All comets moving in parabolical or hyperbolical
+curves, are only temporary members of the solar system, being apparently
+strangers who have been diverted from their courses by some disturbing
+influence. No comet is classed as periodical which does not follow a
+perceptibly elliptical orbit. Any comet passing around the Sun at the
+mean distance of the Earth from this body, with a velocity of 26 miles
+per second, will fly off into infinite space, to return to us no more.
+
+The time of revolution of the different periodic comets thus far
+observed varies greatly, as do also the distances to which they recede
+from the Sun at aphelion. Whilst the period of revolution of Encke's
+comet, the shortest thus far known, is only 3½ years, that of the comet
+of 1844, II., is 102,000 years; and whilst the orbit of the first is
+comprised within the orbit of Jupiter, that of the last extends to a
+distance equal to 147 times the distance of Neptune from the Sun. But so
+vast an orbit cannot be accurately determined from the imperfect data at
+our disposal.
+
+The periodic comets are usually divided into two classes. The comets
+whose orbits are within the orbit of Neptune are called interior comets,
+while those whose orbits extend beyond that of Neptune are called
+exterior comets. The known interior periodic comets are twelve in
+number, while, including all the cases in which there is some slight
+evidence of elliptic motion, the number of exterior comets observed is
+six or seven times as great. The periodic comets of short period are
+very interesting objects, inasmuch as by their successive returns they
+afford an opportunity to calculate their motions and to observe the
+physical changes which they undergo in their intervals of absence.
+
+From observation of the periodic comets, it has been learned that the
+same comet never presents twice the same physical appearance at its
+different returns, its size, shape and brilliancy varying so greatly
+that a comet can never be identified by its physical characters alone.
+It is only when its elements have been calculated, and are found to
+agree with those of a cometary orbit previously known, that the two
+comets can be identified one with the other. There are reasons to
+believe that, in general, comets decrease in brightness and size at each
+of their successive returns, and that they are also continually losing
+some of their matter as they traverse their orbits.
+
+When very far away from us, all comets appear nearly alike, consisting
+of a faint nebulosity, of varying dimensions. When a comet first appears
+in the depths of space, and travels towards the Sun, it generally
+resembles a faint, uniformly luminous nebulosity, either circular or
+slightly elongated in form. As it approaches nearer to the Sun, a slight
+condensation of light appears towards its centre, and as it draws still
+nearer, it becomes brighter and brighter, and in condensing forms a kind
+of diffused luminous nucleus. At the same time that the comet acquires
+this concentration of light, the nebulosity gradually becomes elongated
+in the direction of the Sun. These effects generally go on increasing so
+long as the comet is approaching the Sun; the condensation of light
+sometimes forms a bright nucleus, comparable to a very brilliant star,
+while the elongation becomes an immense appendage or tail. When the
+comet has passed its perihelion and recedes from the Sun, the inverse
+phenomena are observed; the comet, decreasing in brightness, gradually
+loses its nucleus and tail, resumes its nebulous aspect, and finally
+vanishes in space, to appear again in due course, if it chance to be a
+periodic comet. While all comets become brighter in approaching the Sun,
+they do not all, however, develop a large tail, some of them showing
+only a slight elongation.
+
+When a comet is first discovered with the telescope at a great distance
+from the Sun, it is difficult to predict whether it will become visible
+to the naked eye, or will remain a telescopic object, as it is only in
+approaching the Sun that these singular bodies acquire their full
+development. Thus, Donati's comet, whose tail became so conspicuous an
+object at its full appearance in 1858, remained two months after its
+discovery by the telescope without any indication of a tail. The comet
+of Halley, which before and after its return in 1759, remained five
+years inside of the orbit of Saturn, showed not the least trace of its
+presence during the greater part of this time. Nothing but calculation
+could then indicate the position in the sky of this invisible object,
+which was so prominent when it approached the Sun.
+
+Another curious phenomenon exhibited by comets, and first noticed by
+Valz, is that in approaching the Sun the nebulosity composing these
+bodies contracts, instead of dilating, as would be naturally supposed
+from the greater amount of solar heat which they must then receive. In
+receding from the Sun, on the contrary, they expand gradually. As comets
+approach the Sun, the tail and nucleus are developed, while the
+nebulosity originally constituting these comets contracts, as if its
+material had been partly consumed in this development. In a certain
+sense it may be said that the comets are partly created by the Sun; in
+more exact terms, the changes of form which they undergo are induced by
+the Sun's action upon them at different distances and under varying
+conditions. Moreover, they are rendered visible by its influence,
+without which they would pass unperceived in our sky. When a comet
+disappears from view, it is not because its apparent diameter is so much
+reduced by the distance that it vanishes, but rather on account of the
+diminution of its light, both that which it receives from the Sun, and
+its own light; these bodies being in some degree self-luminous, as will
+be shown below.
+
+The large comets, such as can be seen with the naked eye, always show
+the following characteristics, on examination with the telescope. A
+condensation of light resembling a diffused star forms the brightest
+part of the comet, this condensation being situated towards the
+extremity the nearest to the Sun. It is this starlike object which is
+called the _nucleus_. The nucleus seems to be entirely enclosed in a
+luminous vapory envelope of the same general texture, called the _coma_.
+This envelope, which is quite variable in brightness and form, is
+brightest next to the nucleus, and gradually fades away as it recedes
+from it. The _nucleus_ and the _coma_, considered as a whole, constitute
+the _head_ of a comet. From the head of a comet proceeds a long trail of
+pale nebulous light, which usually grows wider, but fainter, as it
+recedes from the nucleus, and insensibly vanishes in the sky. This
+delicate appendage, or tail, as it is commonly called, varies very much
+in size and shape, not only in different comets, but in the very same
+comet, at different times. Its direction is generally opposite to that
+of the Sun from the head of the comet.
+
+The nuclei vary very much in brightness, in size and in shape; and while
+in some telescopic comets they are either absent or barely
+distinguishable as a small condensation of light, in bright comets they
+may become plainly visible to the naked eye, and they sometimes even
+surpass in brightness the most brilliant stars of the heavens. But
+whatever may be the size of cometary nuclei, they are subject to sudden
+and rapid changes, and vary from day to day. Sometimes they appear
+exceedingly brilliant and sharply outlined, while at other times they
+are so dim and diffused that they are hardly distinguishable from the
+coma of which they seem then to form a part.
+
+
+[PLATE XI.--THE GREAT COMET OF 1881.
+
+Observed on the night of June 25-26 at 1h. 30m. A.M.]
+
+
+From my observations upon the comets which have appeared since the year
+1873, it is apparent that the changes in the nucleus, coma and tail, are
+due to a solar action, which contracts or expands these objects in such
+a manner that the nuclei become either bright and star-like, or dim and
+diffused, in a very short time. I had excellent opportunity, especially
+in the two large comets of 1881, to observe some of these curious
+changes, a description of which will give an idea of their extent and
+rapidity. On July 2d, 1881, at 9 o'clock, the nucleus of comet 1881,
+III., which is represented on Plate XI., appeared sharply defined,
+bright and considerably flattened crosswise; but half an hour later it
+had considerably enlarged and had become so diffused that it could
+hardly be distinguished from the coma, with which it gradually blended.
+It is perhaps worth mention that, at the time this last observation was
+made, an aurora borealis was visible. This comet 1881, III., underwent
+other very important changes of its nucleus, coma and tail. On June
+25th, the nucleus, which was bright and clearly defined, was ornamented
+with four bright diverging conical wings of light, as shown on Plate XI.
+On the 26th these luminous wings had gone, and the nucleus appeared
+one-third smaller. On the 28th it had enlarged, but on the 29th its
+shape was considerably altered, the nucleus extending in one direction
+to three or four times its diameter on previous nights, and being
+curved, so as to resemble a comma. On the 6th of July the nucleus of
+this comet showed the greatest disturbances. The nucleus, which had
+appeared perfectly round on the evening of the 5th, was found much
+elongated at 10 o'clock on the 6th, forming then a straight, acute, and
+well-defined wedge of light, inclined upwards to the left. The length of
+the nucleus, at this time, was three or four times its ordinary
+diameter. At the same time rapid changes occurred; the strangely shaped
+nucleus soon became unsteady, extending and contracting alternately, and
+varying greatly in brightness. At 10h. 45m., the elongated nucleus, then
+gently curved, took the shape of a succession of luminous knots, which
+at times became so brilliant and distinct that they seemed to be about
+to divide and form separate nuclei; but such a separation did not
+actually occur, at least while I was observing. While these important
+changes were going on in the comet, a bright auroral arch appeared in
+the north, which lasted only a short time. On July 7th, the sky being
+cloudy, no observations were made, but on the 8th I observed the comet
+again. The nucleus had then resumed its circular form, but it was yet
+very unsteady, being sometimes small, bright and sharp, while a few
+seconds later it appeared twice as large, but dim in outlines; and
+sometimes an ill-defined secondary nucleus appeared at its centre. On
+several occasions the nucleus appeared as if it were double, one nucleus
+being apparently projected partly upon the other.
+
+The nuclei of comets are sometimes very small, and in other cases very
+large. Among those which have been measured, the nucleus of the comet of
+1798, I., was only 28 miles in diameter, but that of Donati's comet, in
+1858, was 5,600 miles, and that of the comet of 1845 was 8,000 miles in
+diameter.
+
+The coma of comets is found to be even more variable than the nucleus.
+The changes observed in the coma are generally in close connection with
+those of the nucleus and tail, the same perturbations affecting
+simultaneously the whole comet. While the coma of the comet of 1847 was
+only 18,000 miles in diameter, that of Halley's comet, in 1835, was
+357,000 miles, and that of the comet of 1811 was 1,125,000 miles in
+diameter. In general, as already stated, the coma of a comet decreases
+in size in approaching the Sun. That of Encke's comet, which, on October
+9th, 1838, had a diameter of 281,000 miles, gradually decreased at a
+daily mean rate of 4,088 miles in going towards the Sun; so that, on
+December 17th, when the distance of the comet from the Sun was more than
+four times less than it was on the first date, its diameter was reduced
+to 3,000 miles.
+
+The form of the coma, in that part which is free from the tail, is in
+general a portion of a circle, but is sometimes irregular, with its
+border deformed. Thus, the border of the coma of Halley's comet was
+depressed at one point towards the Sun. I observed a similar phenomenon
+in Coggia's comet, with the great refractor of the Harvard College
+Observatory, on July 13th, 1874, when its border appeared deeply
+depressed on the side nearest to the Sun, as if repelled by this body.
+The coma of comet 1881, III., showed also very singular outlines on the
+nights of the 25th and 26th of June, when its border was so deeply
+depressed that the coma appeared as if it were double. Luminous rays and
+jets often radiate from the nucleus across the coma, and describe
+graceful lateral curves, falling backwards and gradually fading away
+into the tail, of which they then form a part. The rays and jets emitted
+by the nucleus seem at first to obey the solar attraction and travel
+towards the Sun; but they are soon repelled, and move backward towards
+the tail. It is a mystery, as yet unexplained, how these cometary jets,
+which at first seem to obey to the laws of attraction, are compelled to
+retreat apparently by superior opposing forces. Among the forces of
+nature, we know of no other than those of an electrical sort, which
+would act in a similar manner; but this explanation would require us to
+assume some direct electrical communication between the comet and the
+Sun. Considering the distance between the two bodies, and the probable
+absence or great tenuity of the gaseous material in interstellar space,
+such an assumption is a difficult one.
+
+Under the action of the solar forces, the coma also very frequently
+forms itself into concentric luminous arcs, separated by comparatively
+dark intervals. These luminous semi-circles vary in number, but
+sometimes there are as many as four or five at a time. All great comets
+show these concentric curves more or less, but sometimes only a portion
+is visible, the rest of the coma having a different structure. When
+great comets approach near the Sun, their coma is generally composed of
+two distinct parts, an inner and an outer coma, the inner one being due
+to the luminous jets issuing from the nucleus, which, never extending
+very far, form a distinct, bright zone within the fainter exterior coma.
+
+The tails of comets, which are in fact a prolongation of the coma, are
+likewise extremely variable in form. They are sometimes straight like a
+rod; again, are curved like a sabre, or even crooked like an S, as was
+that of the comet of 1769. They are also fan-shaped, pointed, or of the
+same width throughout. Many of these appendages appear longitudinally
+divided through their middle by a narrow, darkish rift, extending from
+the nucleus to the extremity. This peculiarity appears in the comet
+shown on Plate XI. Sometimes the dark rift does not commence near the
+nucleus, but at some distance from it, as I observed in the case of
+comet 1881, III., on June 26th. This dark rift is not a permanent
+feature of a comet's tail, but may be visible one day and not at all the
+next. Comet 1881, III., which had shown a dark rift towards the end of
+June, did not exhibit any such rift during July and August, when, on the
+contrary, its tail appeared brighter in the middle. Coggia's comet,
+which showed so prominent a dark rift in July, 1874, had none on June
+10th. On the contrary, the tail was on that date very bright along its
+middle, as also along each of its edges.
+
+The tail of a comet does not invariably point directly away from the
+Sun, as above mentioned, and sometimes the deviation is considerable;
+for instance, the tail of the comet of 1577 deviated 21° from the point
+opposite to the Sun.
+
+In general, the tail inclines its extremity towards the regions of space
+which it has just left, always presenting its convex border to the
+regions towards which it is moving. It is also a remarkable fact that
+this convex border, moving first in space, always appears brighter and
+sharper than the opposite one, which is often diffused. From these
+peculiarities it would seem that in moving about the Sun the comets
+encounter some resistance to their motion, from the medium through which
+they pass, and that this resistance is sufficient to curve their tails
+away from the course in which they move, and to crowd their particles
+together on the forward side. It is especially when they approach their
+perihelion, and move more rapidly on a curve of a shorter radius, that
+the comets' tails show the greatest curvature, unless their position in
+regard to the observer prevents their being advantageously seen. The
+tail of Donati's comet presented a fair illustration of this
+peculiarity, its curvature having augmented with the velocity of the
+comet's motion about the Sun. But possibly this phenomenon has another
+cause, and may be found rather in the solar repulsion which acts on
+comets and is not instantaneously propagated throughout their mass.
+
+Although, in general, comets have but one tail, it is not very rare to
+see them with multiple tails. The comets of 1807 and 1843 had each a
+double tail; Donati's comet, in 1858, showed several narrow, long
+rectilinear rays, issuing from its abruptly curved tail. The comet of
+1825 had five branches, while that of 1744 exhibited no less than six
+distinct tails diverging from the coma at various angles. In general
+character the multiple and single tails are similar. When a comet has
+two tails, it is not rare for the second to extend in the general
+direction of the Sun, as was the case with the great comet of 1881,
+III., represented on Plate XI. From July 14th to the 21st it exhibited
+quite an extended conical tail, starting obliquely downwards from the
+right side of the coma, and directed towards the Sun. From the 24th of
+July to the 2d of August this secondary tail was exactly opposite in its
+direction from that of the primary tail, and gave to the head a very
+elongated appearance. Comet 1881, IV., also exhibited a secondary
+appendage, not directed towards the Sun, but making an angle of about
+45° with the main tail.
+
+These cometary appendages sometimes attain prodigious dimensions. The
+comets of 1680 and 1769 had tails so extended that, after their heads
+had set under the horizon, the extremities of these immense appendages
+were still seen as far up as the zenith. In a single day the tail of the
+comet of 1843 extended 100°, and it was thrust from the comet "as a
+dart of light" to the enormous distance of 48,500,000 miles, and yet of
+this immense appendage nothing was left on the following day. The tail
+of Donati's comet, in 1858, attained a real length of 42,000,000 miles,
+while that of the great comet of 1843 had the enormous length of
+200,000,000 miles. If this last comet had occupied the position of the
+Sun, which it approached very nearly for a moment, the extremity of its
+tail would have extended 60,000,000 miles beyond the orbit of Mars.
+
+In some cases the tails of comets have been seen undulating and
+vibrating in a manner similar to the undulations and coruscations of
+light characteristic of some auroras. Many observers report having seen
+such phenomena. The comet of 1769 was traversed by luminous waves and
+pulsations, comparable to those seen in the aurora borealis. I myself
+observed these curious undulations in Coggia's comet in 1874, while the
+head of this object was below the horizon. For an hour the undulations
+rapidly succeeded each other, and ran along the whole length of the
+tail.
+
+Some of the brightest comets have shone with such splendor that they
+could be observed easily in full sunshine. Many comets, such as those of
+1577 and 1744, have equaled Sirius and Venus in brilliancy. The great
+comet of 1843, which suddenly appeared in our sky, was so brilliant that
+it was seen by many observers at noon time, within a few degrees from
+the Sun. I remember that I myself saw this remarkable object in the day
+time, with a number of persons, who were gazing at the wonderful
+apparition. So brilliant was this comet, that besides its nucleus and
+head, a portion of its tail was also visible in the day time, provided
+the observer screened his eyes from the full sunlight by standing in the
+shadow of some building.
+
+Of all the bodies revolving around the Sun, none have been known to
+approach so near its surface as did the comet of 1843. When it arrived
+at perihelion, the distance from the centre of its nucleus to the
+surface of the Sun's photosphere was only 96,000 miles, while the
+distance from surface to surface was less than 60,000 miles. This comet,
+then, went through the solar atmosphere, and in traversing it with its
+tremendous velocity of 366 miles per second, may very possibly have
+swept through some solar protuberances, many of which attain much higher
+elevations than that at which the comet passed. The comet of 1680 also
+approached quite near the surface of the Sun, and near enough to
+encounter some of the high solar protuberances, its distance at
+perihelion being about two-thirds of the Moon's distance from the Earth.
+The rapidity of motion of the comet of 1843 was such, when it approached
+the Sun, that it swept through all that part of its orbit which is
+situated north of the plane of the ecliptic in a little more than two
+hours, moving in this short time from one node to the other, or 1800.
+
+But if some comets have a very short perihelion distance, that of others
+is considerable. Such a comet was that of 1729, whose perihelion
+distance was 383,000,000 miles, the perihelion point being situated
+between the orbits of Mars and Jupiter.
+
+While some comets come near enough to the Sun at perihelion to be
+volatilized by its intense heat, others recede so far from it at
+aphelion that they may be said to be frozen. The shortest cometary
+aphelion distance known is that of Encke's comet, whose greatest
+distance from the sun is 388,000,000 miles. But that of the comet of
+1844 is 406,000,000,000 miles from the Sun. The comets of 1863 and 1864
+are so remote in space when they reach their aphelion points that light,
+with its velocity of 185,500 miles a second, would require 171 days in
+the first case, and 230 in the last, to pass from them to the Earth.
+
+The period of revolution of different comets also varies immensely.
+While that of Encke's comet is only 3½ years, that of comet 1864, II.,
+is 280,000 years.
+
+Among the periodic comets of short period, some have exhibited highly
+interesting phenomena. Encke's comet, discovered in 1818, is remarkable
+for the fact that its period of revolution diminishes at each of its
+successive returns, and consequently this comet, with each revolution,
+approaches nearer and nearer to the Sun. The decrease of the period is
+about 2½ hours at each return. Although the decrease is small, if it go
+on in future as it does at present, the inevitable consequence will be
+that this comet will finally fall into the Sun. This curious phenomenon
+of retardation has been attributed by astronomers to the existence of a
+resisting medium filling space, but so rare and ethereal that it does
+not produce any sensible effect on the movements of the planets. But
+some other causes may retard this comet, as similar retardations have
+not been observed in the case of other periodic comets of short period.
+These, however, are not so near to the Sun, and perhaps our luminary may
+be surrounded by matter of extreme tenuity, which does not exist at a
+greater distance from it.
+
+Another of the periodic comets which has exhibited a very remarkable
+phenomenon of transformation is Biela's comet, which divided into two
+distinct parts, moving together in the same direction. When this comet
+was first detected at its return in 1845, it presented nothing unusual,
+but in the early part of 1846 it was noticed by several astronomers to
+be divided into two parts of unequal brightness, forming thus a twin
+comet. At its next return in 1852, the two sister comets were still
+traveling in company, but their distance apart, which in 1846 was
+157,000 miles, had increased to 1,500,000 miles. At the two next returns
+in 1859 and 1865, their position not being very favorably situated for
+observation, the comets were not seen. In 1872 the position should have
+been favorable for observation, and they were consequently searched for,
+but in vain; neither comet was found. An astronomer in the southern
+hemisphere, however, found a comet on the track of Biela's, but
+calculation has shown that the two objects are probably not identical,
+since this comet was two months behind the computed position for
+Biela's. It will be shown in the following chapter that our globe
+probably crossed the orbit of Biela's comet on November 27th, 1872, and
+the phenomena resulting from this passage will be there described.
+
+It is seen from these observations that comets may be lost or dissipated
+in space by causes entirely unknown to us. Biela's comet is not the only
+one which has been thus disintegrated. Ancient historians speak of the
+separation of large comets into two or more parts. In 1661 Hevelius
+observed the apparent division of the comet of that year and its
+reduction to fragments. The return of this comet, calculated for 1790,
+was vainly waited for; the comet was not seen.
+
+Other comets, whose periods of revolution were well known, have
+disappeared, probably never to return. Such is Lexell's comet, whose
+period was 5⁶⁄₁₀ years; also De Vico's comet, both of which are
+now lost. It is supposed that Lexell's comet, which passed twice very
+near the giant planet Jupiter, had its orbit changed from an ellipse to
+a parabola, by the powerful disturbing influence of this planet, and was
+thus lost from our system. Several other comets, in traveling over their
+different orbits, have approached near enough to Saturn, Jupiter and the
+Earth to have their orbits decidedly altered by the powerful attraction
+of these bodies.
+
+But since comets are liable to pass near the planets, and several have
+orbits which approach that of the Earth, it becomes important for us to
+know whether an encounter of such a body with our globe is possible, and
+what would then be the result for us. Although that knowledge would not
+enable us to modify the possibilities of an encounter, yet it is better
+to know the dangers of our navigation through space than to ignore them.
+This question of a collision of the Earth with a comet has been answered
+in different ways, according to the ideas entertained in regard to the
+mass of these bodies. While some have predicted calamities of all kinds,
+such as deluges, conflagrations, or the reduction of the Earth to
+incandescent gases, others have asserted that it would produce no more
+effect than does a fly on encountering a railroad train. In our days
+astronomers entertain very little fears from such an encounter, because
+the probabilities of danger from an occurrence of this sort are very
+slight, the mass of an ordinary comet being so small compared with that
+of our globe. We know with certainty that the Earth has never had an
+encounter with a comet _by which it has been transformed into gases_, at
+least within the several millions of years during which animal and
+vegetable life have left their marks upon the stony pages of its
+history, otherwise these marks would not now be seen. If, then, such an
+accident has not happened during this long period, the chances for its
+occurring must be very small, so small indeed that they might almost be
+left out of the question. It is true that our globe shows signs of great
+perturbations of its surface, but we have not the slightest proofs that
+they resulted from an encounter with a celestial body. It seems very
+probable that our globe passed through the tail of the comet of 1861,
+before it was first seen on June 29th; but nothing unusual was observed,
+except perhaps some phosphorescent light in the atmosphere, which was
+afterwards attributed to this cause.
+
+The density and mass of comets must be comparatively very small. Their
+tails consist of matter of such extreme tenuity that it affects but very
+little the light of the small stars over which they pass. The coma and
+nucleus, however, are not quite so transparent, and may have greater
+masses. On several occasions I have seen the light of stars reduced by
+the interposition of cometary matter, comet 1881, III., presenting
+remarkable cases of this sort. On July 8th, at 10h. 50m., several small
+stars were involved in this comet, one of which passed quite near the
+nucleus through the bright inner coma. At that time the comet was
+greatly disturbed, its nucleus was contracting and enlarging rapidly,
+and becoming bright and again faint in an instant. Every time that the
+nucleus grew larger, the star became invisible, but reappeared the
+moment the nucleus was reduced in size. This phenomenon could not be
+attributed to an atmospheric effect, since, while the nucleus was
+enlarging, a very small inner nucleus was visible within the large
+diffused one, the matter of which had apparently spread over the part of
+the coma in which the star was involved, making it invisible.
+
+That the mass of comets is small, is proved by the fact that they have
+sometimes passed near the planets without disturbing them in any
+sensible manner. Lexell's comet, which in 1770 remained four months very
+near Jupiter, did not affect in the least the orbits, or the motions of
+its satellites. The same comet also came within less than 1,500,000
+miles from the Earth, and on this occasion it was calculated that its
+mass could not have been the ¹⁄₅₀₀₀ part of that of our
+globe, since otherwise the perturbations which it would have caused in
+the elements of the Earth's orbit would have been sensible. There was,
+however, no change. If this comet's mass had been equal to that of our
+globe, the length of our year would have been increased by 2h. 47m. The
+comet of 1837 remained four days within 3,500,000 miles of the Earth,
+with no sensible effect.
+
+It seems quite difficult to admit that the denser part of a comet
+forming the nucleus is solid, as supposed by some physicists, since it
+is so rapidly contracted and dilated by the solar forces, while the
+comet is yet at a too great distance from the Sun to allow these effects
+to be attributed to solar heat alone. This part of a comet, as indeed
+the other parts, seems rather to be in the gaseous than in the solid
+state; the changes observed in the intensity of its light and in its
+structure may be conceived as due to some solar action partaking of the
+nature of electricity.
+
+It has been a question whether comets are self-luminous, or whether they
+simply reflect the solar light. When their light is analyzed by the
+spectroscope, it is found that the nucleus of a comet generally gives a
+continuous spectrum, while the coma and tail give a spectrum consisting
+of several bright diffused bands. The spectrum given by the nucleus is
+rarely bright enough to allow the dark lines of the solar spectrum to be
+discerned upon it; but such lines were reported in the spectrum of comet
+1881, III., a fact proving that this nucleus at least reflected some
+solar light. The nucleus of a comet may be partly self-luminous, and
+either solid, liquid, or composed of incandescent gases submitted to a
+great pressure. As to the coma and tail, they are evidently gaseous, and
+partly, if not entirely, self-luminous, as is proved by the band
+spectrum which they give. The position of these bands, moreover,
+indicates that the luminous gases of which they are composed contain
+carbon. The phenomena of polarization, however, seem to prove that these
+parts of comets also reflect some solar light.
+
+No theory so far proposed, to explain comets and the strange phenomena
+they exhibit, seems to have been successful in its attempts, and the
+mystery in which these bodies have been involved from the beginning of
+their apparition, seems to be now nearly as great as ever. It has been
+supposed that their tails have no real existence, but are due to an
+optical illusion. Prof. Tyndall has endeavored to explain cometary
+phenomena by supposing these bodies to be composed of vapors subject to
+decomposition by the solar radiations, and thus made visible, the head
+and tail being an actinic cloud due to such decompositions. According to
+this view, the tails of comets would not consist of matter projected
+into spacer but simply of matter precipitated by the solar rays in
+traversing the cometary nebulosity. The endeavor has also been made to
+explain the various phenomena presented by comets by an electrical
+action of the Sun on the gases composing these objects. Theories taking
+this as a base seem to us to be more likely to lead to valuable results.
+M. Faye, who has devoted much time and learning to this subject, assumes
+a real repulsive force of the Sun, acting inversely to the square of the
+distance and proportionally to the surface, and not to the mass as
+attraction does. He supposes, however, that this repulsive force is
+generated by the solar heat, and not by electricity. Prof. Wm. Harkness
+says that many circumstances seem to indicate that the comets' tails are
+due, in a great measure, to electrical phenomena.
+
+The fact that the tails of comets are better defined and brighter on the
+forward side, associated with the other fact that they curve the most
+when their motion is most rapid, sufficiently indicates that these
+appendages are material, and that they either encounter some resistance
+from the medium in which they move, or from a solar repulsion. The
+phenomena of condensation and extension, which I have observed in the
+comets of 1874 and 1881, added to the curious behavior exhibited by the
+jets issuing from the nucleus, seem to indicate the action of electrical
+forces rather than of heat. The main difficulty encountered in the
+framing of a theory of comets consists in explaining how so delicate and
+extended objects as their tails seem to be, can be transported and
+whirled around the Sun at their perihelion with such an enormous
+velocity, always keeping opposite to the Sun, and, as expressed by Sir
+John Herschel, "in defiance of the law of gravitation, nay, even of the
+received laws of motion."
+
+To consider the direction of the comets' tails as an indirect effect of
+attraction, seems out of the question; the phenomenon of repulsion so
+plainly exhibited by these objects seems to point to a positive solar
+repulsion, as alone competent to produce these great changes. The
+repulsive action of the Sun on comets' tails might be conceived, for
+instance, as acting in a manner similar to that of a powerful current of
+wind starting from the Sun, and constantly changing in direction, but
+always keeping on a line with the comet. Such a current, acting on a
+comet's tail as if it were a pennant, would drive it behind the nucleus
+just as observed. If it could once be ascertained that the great
+disturbances on comets correspond with the magnetic disturbances on our
+globe and with the display of the auroral light, the electric nature of
+the forces acting so strangely on the comets would be substantially
+demonstrated. I have shown that some of the great disturbances observed
+in the comets of 1874 and 1881 have coincided with auroral displays, and
+it will be shown hereafter that similar displays have also coincided
+with the passage of meteoric showers through our atmosphere. Whether
+these simultaneous phenomena were simple coincidences having no
+connection, or whether they are the result of a common cause, can only
+be ascertained by long continued future observations.
+
+
+
+
+SHOOTING-STARS AND METEORS
+
+PLATE XII
+
+
+While contemplating the heavens on a clear moonless night, we
+occasionally witness the sudden blazing forth of a star-like meteor,
+which glides swiftly and silently across some of the constellations, and
+as suddenly disappears, leaving sometimes along its track a
+phosphorescent trail, which remains visible for a while and gradually
+vanishes. These strange apparitions of the night are called _Falling_ or
+_Shooting-stars_.
+
+There is certainly no clear night throughout the year during which some
+of these meteors do not make their appearance, but their number is quite
+variable. In ordinary nights only four or five will be observed by a
+single person in the course of an hour; but on others they are so
+numerous that it becomes impossible to count them. When the falling
+stars are only a few in number, and appear scattered in the sky, they
+are called _Sporadic Meteors_, and when they appear in great numbers
+they constitute _Meteoric Showers_ or _Swarms_.
+
+Probably there is no celestial phenomenon more impressive than are these
+wonderful pyrotechnic displays, during which the heavens seem to break
+open and give passage to fiery showers, whose luminous drops describe
+fantastic hieroglyphics in the sky. While observing them, one can fully
+realize the terror with which they have sometimes filled beholders, to
+whom it seemed that the stability of the universe had come to an end,
+and that all the stars of the firmament were pouring down upon the Earth
+in deluges of fire.
+
+The ancients have left record of many great meteoric displays, and the
+manner in which they describe them sufficiently indicates the fear
+caused by these mysterious objects. Among the many meteoric showers
+recorded by ancient historians may be mentioned one observed in
+Constantinople, in the month of November, 472, when all the sky appeared
+as if on fire with meteors. In the year 599, meteors were seen on a
+certain night flying in all directions like fiery grasshoppers, and
+giving much alarm to the people. In March, 763, "the stars fell
+suddenly, and in such crowded number that people were much frightened,
+and believed the end of the world had come." On April 10th, 1095, the
+stars fell in such enormous quantity from midnight till morning that
+they were as crowded as are the hail stones during a severe storm.
+
+In modern times the fall of the shooting-stars in great number has been
+frequently recorded. One of the most remarkable meteoric showers of the
+eighteenth century occurred on the night of November 13th, 1799, and was
+observed throughout North and South America and Europe. On this
+memorable night thousands of falling stars were seen traversing the sky
+between midnight and morning. Humboldt and Boupland, then traveling in
+South America, observed the phenomena at Cumana, between two and five
+o'clock in the morning. They saw an innumerable number of shooting-stars
+going from north to south, appearing like brilliant fire-works. Several
+of these meteors left long phosphorescent trails in the sky, and had
+nuclei whose apparent diameter, in some cases, surpassed that of the
+Moon.
+
+The shower of November 13th, 1833, was still more remarkable for the
+great number of meteors which traversed the heavens, and was visible
+over the whole of North and South America. On that occasion the falling
+stars were far too numerous to be counted, and they fell so thickly that
+Prof. Olmsted, of New Haven, who observed them carefully, compared their
+number at the moment of their maximum fall to half that of the flakes of
+snow falling during a heavy storm. This observer estimated at 240,000
+the number of meteors which must have traversed the heavens above the
+horizon during the seven hours while the display was visible.
+
+
+
+[PLATE XII.--THE NOVEMBER METEORS.
+
+As observed between midnight and 5 o'clock A.M. on the night of November
+13-14 1868.]
+
+
+In the years 1866, 1867 and 1868, there were also extraordinary meteoric
+displays on the night of November 13th. It was on the last mentioned
+date that I had the opportunity to observe the remarkable shower of
+shooting-stars of which I have attempted to represent all the
+characteristic points in Plate XII. My observations were begun a little
+after midnight, and continued without interruption till sunrise. Over
+three thousand meteors were observed during this interval of time in the
+part of the sky visible from a northern window of my house. The maximum
+fall occurred between four and five o'clock, when they appeared at a
+mean rate of 15 in a minute.
+
+In general, the falling stars were quite large, many being superior to
+Jupiter in brightness and apparent size, while a few even surpassed
+Venus, and were so brilliant that opaque objects cast a strong shadow
+during their flight. A great many left behind them a luminous train,
+which remained visible for more or less time after the nucleus had
+vanished. In general, these meteors appeared to move either in straight
+or slightly curved orbits; but quite a number among them exhibited very
+extraordinary motions, and followed very complicated paths, some of
+which were quite incomprehensible.
+
+While some moved either in wavy or zig-zag lines, strongly accentuated,
+others, after moving for a time in a straight line, gradually changed
+their course, curving upward or downward, thus moving in a new
+direction. Several among them, which were apparently moving in a
+straight line with great rapidity, suddenly altered their course,
+starting at an abrupt angle in another direction, with no apparent
+slackening in their motion. One of them, which was a very conspicuous
+object, was moving slowly in a straight course, when of a sudden it made
+a sharp turn and continued to travel in a straight line, at an acute
+angle with the first, retreating, and almost going back towards the
+regions from which it originally came. As nearly all the meteors which
+exhibited these extraordinary motions left the trace of their passage in
+the sky by a luminous trail, it was easily ascertained that these
+appearances were not deceptive. On one occasion I noticed that the
+change of direction in the orbit corresponded with the brightening up of
+the meteor thus disturbed in its progress.
+
+Among these meteors, some traveled very slowly, and a few seemed to
+advance as if by jerks, but in general they moved very rapidly. One of
+the meteors thus appearing to move by jerks left a luminous trail, upon
+which the various jerks seemed to be left impressed by a succession of
+bright and faint spaces along the train. Some of the largest meteors
+appeared to rotate upon an axis as they advanced, and most of these
+revolving meteors, as also a great number of the others, seemed to
+explode just before they disappeared, sending bright fiery sparks of
+different colors in all directions, although no sound was at any time
+heard. The largest and most brilliant meteor observed on that night
+appeared at 5h. 30m., a little before sunrise. It was very bright, and
+appeared considerably larger than Venus, having quite a distinct disk.
+This meteor moved very slowly, leaving behind a large phosphorescent
+trail, which seemed to issue from the inside of the nucleus as it
+advanced. For a moment the train increased in size and brightness close
+to the nucleus, which then appeared as an empty transparent sphere,
+sprinkled all over with minute fiery sparks; the nucleus then suddenly
+burst out into luminous particles, which immediately vanished, only the
+luminous trail of considerable dimensions being left.
+
+Many of the trails thus left by the meteors retained their luminosity
+for several minutes, and sometimes for over a quarter of an hour. These
+trails slowly changed their form and position; but it is perhaps
+remarkable that almost all those which I observed on that night assumed
+the same general form--that of an open, irregular ring, or horse-shoe,
+somewhat resembling the letter C. This ring form was subsequently
+transformed into an irregular, roundish cumulus-like cloud. The trail
+left by a very large meteor, which I observed on the evening of
+September 5th, 1880, also exhibited the same general character of
+transformation.
+
+While I was observing a long brilliant trail left by a meteor on the
+night of November 13th, 1868, it was suddenly crossed by another bright
+shooting-star. The latter apparently went through the luminous substance
+forming the trail, which was suddenly altered in form, and considerably
+diminished in brightness simultaneously with this passage, although
+electrical action at some distance might perhaps as well explain the
+sudden change observed.
+
+In the majority of cases the meteors appeared white; but many,
+especially the largest, exhibited a variety of brilliant colors, among
+which the red, blue, green, yellow and purple were the most common. In
+general the trails exhibited about the same color as the nucleus, but
+much fainter, and they were usually pervaded by a greenish tint. In some
+instances the trails were of quite a different color from the nucleus.
+
+The luminous cloud observed at 5h. 30m. on the morning of November 14th,
+1868, after having passed through the series of transformations above
+described, remained visible for a long while after sunrise, appearing
+then as a small cirrus cloud, exactly similar in appearance to the
+hundreds of small cirrus clouds then visible in the sky, which had
+probably the same meteoric origin. For over three hours after sunrise,
+these cirrus clouds remained visible in the sky, moving all together
+with the wind in the high regions of the atmosphere.
+
+Although Plate XII. is intended to represent all the characteristics
+exhibited by the meteors observed on that night, every form represented
+having been obtained by direct observation, yet the number is much
+greater than it was at any single moment during the particular shower of
+1868. As regards number, the intention was to give an idea of a great
+meteoric shower, such as that of 1833, for instance. Although many of
+the falling stars seem to be close to the Earth's surface, yet this is
+only an effect of perspective due to their great distance, very few of
+these meteors ever coming into the lower regions of our atmosphere at
+all.
+
+The phenomena exhibited during other great meteoric showers have been
+similar to those presented by the shower just described, the only
+differences consisting in variations of size and brightness in the
+meteors, and also in the trails, which sometimes are not so numerous as
+they were in 1868.
+
+While some shooting-stars move so rapidly that they can hardly be
+followed in their orbits, others move so slowly that the sight can
+easily follow them, and even remark the peculiarities of their
+movements, some remaining visible for half a minute. Some of the falling
+stars move at the rapid rate of 100 miles a second, but others only 10
+miles a second, and even less. In general, they move about half as fast
+again as the Earth in its orbit. The arcs described by the meteors in
+the sky are variable. While some extend 80° and even 100°, others are
+hardly half a degree in length. While some shooting-stars are so faint
+that they can hardly be seen through the largest telescopes, others are
+so large and brilliant that they can be seen in the day-time. In
+general, a shooting-star of average brightness resembles a star of the
+third or fourth magnitude.
+
+Whatever may be the origin of the shooting-stars, they are, when we see
+them, not in the celestial spaces, like the planets, the comets, or the
+stars, but in our atmosphere, through which they travel as long as they
+remain visible. The height at which they appear and disappear is
+variable, but in general they are about 80 miles above the surface of
+our globe when they are first seen, and at about 55 miles when they
+disappear. In many cases, however, they have been observed at greater
+elevations, as also at smaller. A meteor simultaneously observed at two
+different stations first appeared at the height of 285 miles, and was
+last seen at 192 miles above the Earth's surface; but in rare cases the
+falling stars have been seen below a layer of clouds completely covering
+the sky. I myself saw one such shooting-star a few years since. The fact
+that the meteors are visible at so great elevations, proves that our
+atmosphere extends much farther than was formerly supposed, although at
+these great heights it must be extremely rarefied, and very different
+from what it is in its lower regions.
+
+There is a remarkable difference between the sporadic meteors seen in
+the sky on every night, and the meteoric showers observed only at
+comparatively rare intervals. While the first appear from different
+points in the sky and travel in all directions, being perfectly
+independent, the meteors of a shower all come from the same point of the
+heavens, from which they apparently diverge in all directions. This
+point of divergence of the meteors is called the _radiant point_ of the
+shower. Although the meteors seem to diverge in all directions from the
+radiant point, yet they all move in approximately parallel lines, the
+divergence being an effect of perspective.
+
+Whatever may be the position of the radiant point in the constellations,
+it remains as fixed in the sky as the stars themselves, and participates
+with them in the apparent motion which they undergo by the effect of the
+diurnal motion, and thus rises and sets with the constellation to which
+it belongs. This fact is sufficient to prove that the orbits of these
+meteors are independent of the Earth's motion, and that consequently
+they do not originate in our atmosphere. It has been shown by Encke that
+the radiant point of the meteoric shower of November 13th is precisely
+the point towards which our globe moves in space on November 13th; a
+tangent to the Earth's orbit would pass through this radiant point.
+
+The meteoric showers are particularly remarkable, not merely because of
+the large number of meteors which are visible and the fact that they all
+follow a common orbit, but chiefly because they have a periodic return,
+either after an interval of a year, or after a lapse of several years.
+At the beginning of the present century only two meteoric showers were
+known, those of August 10th and of November 13th, and their periodicity
+had not yet been recognized, although it had begun to be suspected. It
+was only in 1836 that Quetelet and Olbers ventured to predict the
+reappearance of the November meteors in the year 1867. Having made
+further investigations, Prof. Newton, of Yale College, announced their
+return in the year 1866. In both of these years, as also in 1868, the
+meteors were very numerous, and were observed in Europe and in America
+on the night of November 13th. The predictions having thus been
+fulfilled, the periodicity of the meteors was established. Since then,
+other periodic showers have been recognized, although they are much less
+important in regard to number than those of August and November, except
+that of November 27th, which exhibited so brilliant a display in Europe
+in 1872. These successive appearances have established the main fact
+that meteoric showers are more or less visible every year when the Earth
+occupies certain positions in its orbit.
+
+The meteoric shower of the 10th of August has its radiant point situated
+in the vicinity of the variable star Algol, in the constellation
+Perseus, from which its meteors have received the name of Perseids.
+Although varying in splendor, this meteoric swarm never fails to make
+its appearance every year. The Perseids move through our atmosphere at
+the rate of 37 miles per second. The shower usually lasts about six
+hours.
+
+The meteoric shower of November 13th has its radiant point situated in
+the vicinity of the star Gamma, in the constellation Leo, from which its
+meteors have been called Leonids. But while the August meteors recur
+regularly every year, with slight variations, the shower of November
+does not occur with the same regularity. During several years it is
+hardly noticeable, and is even totally absent, while in other years it
+is very remarkable. Every 33 years an extraordinary meteoric shower
+occurs on the 13th of November, and the phenomenon is repeated on the
+two succeeding years at the same date, but with a diminution in its
+splendor at each successive return. The Leonids move in an opposite
+direction to that of the Earth, and travel in our atmosphere with an
+apparent velocity of 45 miles per second, this being about the maximum
+velocity observed in falling stars. But when the motion of our globe is
+taken into account, and a deduction is made of the 18 miles which it
+travels per second, it is found that these meteors move at an actual
+mean rate of 27 miles a second.
+
+In a meteoric shower the stars do not fall uniformly throughout the
+night, there being a time when they appear in greater numbers. Usually
+it is towards morning, between 4 and 6 o'clock, that the maximum occurs.
+The probable cause of this phenomenon will be explained in its place
+hereafter.
+
+The orbits of the meteoric showers are not all approximately in the same
+plane, like those of the planets, but rather resemble those of comets,
+and have all possible inclinations to the ecliptic. Like the comets,
+too, the different meteoric showers have either direct or retrograde
+motion.
+
+The shooting-stars were formerly considered as atmospheric meteors,
+caused by the combustion of inflammable gases generated at the surface
+of the Earth, and transported to the high regions of our atmosphere by
+their low specific gravity. But the considerable height at which they
+usually appear, the great velocity of their motion, the common orbit
+followed by the meteors of the same shower, and the periodicity of their
+recurrence, do not permit us now to entertain these ideas, or to doubt
+their cosmical origin. But what is their nature?
+
+It is now generally admitted that innumerable minute bodies, moving in
+various directions around the Sun, are scattered in the interplanetary
+spaces through which our globe travels. It has been supposed that
+congregations of such minute bodies form elliptical rings, within which
+they are all moving in close parallel orbits around the Sun. On the
+supposition that such rings intersect the orbit of the Earth at the
+proper places, it was practicable to account for the shooting-stars by
+the passage through our atmosphere of the numerous minute cosmical
+bodies composing the rings, and the Leonid and Perseid showers were so
+explained. But when the elements of the orbits of these two last swarms
+came to be better known, and were compared with those of other celestial
+bodies, it was found necessary to alter this theory.
+
+It had for a long while been suspected that some kind of relation
+existed between the shooting-stars and the comets. This idea, vaguely
+formulated by Kepler more than two centuries ago, more clearly expressed
+by Chladni, and still more by Mr. Grey, before the British Association,
+at Liverpool, in 1855, has recently received a brilliant confirmation by
+the researches of Professor Schiaparelli, Director of the Observatory of
+Milan. A thorough investigation of the orbits of the August and November
+meteors led Schiaparelli to the discovery of a remarkable relation
+between meteoric and cometary orbits. By comparing the elements of these
+meteoric orbits with those of comets, he found a very close resemblance
+between the orbit of the August meteors and that of the comet 1862,
+III., and again between the orbit of the November meteors and that of
+Tempel's comet, 1866, I. These resemblances were too striking to be the
+result of mere chance, and demonstrated the identity of these cometary
+orbits with those of the Perseid and Leonid showers. In accordance with
+these new facts, it is now admitted that the meteoric showers result
+from the passage of our globe through swarms of meteoric particles
+following the orbits of comets, which intersect the orbit of the Earth.
+
+Professor Schiaparelli has attempted to show how these meteoric swarms
+were originally scattered along the orbits of comets, by supposing these
+bodies to originate from nebulous masses, which, in entering the sphere
+of attraction of the Sun, are gradually scattered along their orbits,
+and finally form comets followed by long trails of meteoric particles.
+
+It has been shown that in approaching the Sun the comets become
+considerably elongated, their particles being disseminated over immense
+distances by the solar repulsion. It seems probable that, owing to its
+feeble attractive power, the nucleus is incompetent to recall the
+scattered cometary particles and retain them in its grasp when they are
+relieved from the solar repulsion, so that they remain free from the
+nucleus, although they continue to move along its orbit. It is
+supposable that these cometary particles will scatter more and more in
+course of time. Forming at first an elongated meteoric cloud, they will
+finally spread along the whole orbit, and thus form a ring of meteoric
+particles. Since our globe constantly moves in its orbit and daily
+occupies a different position, it follows that at any point where such a
+cometary orbit happens to cross that of the Earth, our globe will
+necessarily encounter the cometary particles as a shower of meteors.
+This encounter will take place at a certain time of the year, either
+yearly, if they form a continuous ring, or after a succession of years,
+if they simply form an elongated cloud. Such meteoric clouds or rings
+would not be visible in ordinary circumstances, even through the largest
+telescopes, except on penetrating the upper regions of our atmosphere,
+when they would appear as showers of falling stars. It is supposed that
+in penetrating our atmosphere, even in its most rarefied regions, these
+meteors are heated by the resistance offered by the air to their motion,
+first becoming luminous and then being finally vaporized and burnt
+before they can reach the surface of the Earth.
+
+The orbit of the comet of 1862, III., which so closely corresponds with
+that of the Perseid meteors, is much more extended than that of Tempel's
+comet corresponding with that of the Leonids. While the first extends
+far beyond the orbit of Neptune, the latter only goes a little beyond
+that of Uranus. The former orbit makes a considerable angle with the
+plane of the Earth's orbit, but the latter is much nearer to parallelism
+with it. The period of revolution of the first is 108 years, and that of
+the last about 33¼ years.
+
+From the fact that the Perseid shower occurs yearly on the 10th of
+August, when the Earth crosses the orbit of the comet of 1862, III., it
+is supposed that the cometary particles producing this shower are
+disseminated along the whole orbit, and form a ring encircling the Sun
+and Earth. To explain the yearly variations in the number of the
+shooting-stars observed, these particles are supposed to be unequally
+distributed over the orbit, being more crowded at one place than they
+are at another. In order to explain the meteoric shower of Leonids,
+which appears in all its splendor every 33 years, and then with
+diminished intensity for two successive years, after which it is without
+importance, it is supposed that the cometary particles of the comet of
+1866, I., have not as yet spread all along the orbit, a sufficient time
+not having been allowed, but form an elongated meteoric cloud, more
+dense in its front than in its rear part. From these considerations it
+has been supposed also that the comet of 1866, I., is of a more recent
+date than that of 1862, III. While Tempel's comet makes its revolution
+around the Sun in about 33 years, this meteoric cloud, which has the
+same period and returns to the same point of its orbit every 33 years,
+encounters our globe for three successive years. The first year we are
+passing through its densest parts, and the two following years in less
+and less crowded parts, from which result the observed phenomena. An
+idea of the extent of this meteoric cloud may be formed from the fact
+that, with its cometary velocity of motion, it takes this cloud three
+years at least to cross the Earth's orbit. From recent researches it
+would appear that the Leonid cloud is not single, but that at least two
+others of smaller importance exist, and have periods of 33¼ years.
+
+Biela's comet, which was divided into two parts in 1846, is another of
+the few comets whose orbit approaches that of the Earth. Possessing this
+knowledge, and knowing then the close connection existing between
+meteors and comets, astronomers supposed that there were sufficient
+reasons to expect a meteoric shower when this comet was passing near the
+Earth. They consequently expected a meteoric display in 1872, when our
+globe was to cross its orbit. Their anticipation was plainly fulfilled,
+and on the night of November 27th, 1872, a splendid meteoric display,
+having its radiant point in the constellation Andromeda, was observed in
+Europe, and also in America, but the meteors seen here were not so
+numerous as in Europe. Other meteoric showers of less importance, such
+as that of April 20th, for instance, have also been identified with
+cometary orbits, so that now no doubt seems to remain as to the identity
+of cometary particles and shooting-stars.
+
+The fact that the maximum number of meteors is always observed in the
+morning hours, supports the hypothesis of the cosmic origin of the
+shooting-stars, since the regions of the Earth where it is morning are
+precisely those fronting the regions towards which our globe is moving
+in space, and accordingly encounter more directly the meteors moving in
+their orbit. The greater abundance of falling stars at that time may
+thus be accounted for.
+
+The number of meteors penetrating our atmosphere must be very great;
+there is not an hour and probably not a minute during which none fall.
+From various considerations, some astronomers have estimated at from
+65,000,000,000 to 146,000,000,000 the total number of shooting-stars
+yearly penetrating in our atmosphere. The actual number is undoubtedly
+great, yet the fact that the meteors are rarely seen through the
+telescope while employed in observing various celestial objects, does
+not indicate that they are so numerous as these figures imply. It is
+only occasionally that one is seen traversing the field of the
+instrument. Even when the sky is observed with a low power eye-piece for
+several hours in succession, many nights may pass without disclosing
+one, although an observer, sweeping the sky more freely with the naked
+eye, may often perceive four or five during an ordinary night.
+
+About the true nature of these bodies nothing is known with certainty.
+From spectrum analysis it seems to be established that most of them
+contain sodium and magnesium, while a few indicate the presence of
+strontium and iron, and in some rare cases there are traces of coal-gas.
+Some of the nuclei give a continuous spectrum, and others a spectrum of
+lines. The trail always gives a spectrum of bright lines which indicates
+its gaseous state. The traces of coal-gas rarely seen in meteors are,
+however, of great importance, as it identifies them more closely with
+the comets, which generally show a similar spectrum. The continuous
+spectra exhibited by some nuclei would indicate that they are
+incandescent and either solid or liquid; but it is difficult to conclude
+from their spectra what is their true nature, since we do not know
+exactly what part the terrestrial atmosphere may play in producing the
+results.
+
+The mass of the shooting-stars is not known with certainty, but the fact
+that during great meteoric showers, none are seen to reach the surface
+of the Earth, all being consumed in a few seconds, sufficiently
+indicates that it must be very small. It has been calculated that those
+equal to Venus in apparent size and brilliancy may weigh several pounds,
+while the faint ones would weigh only a few grains.
+
+If the shooting-stars have even such a mass as that here attributed to
+some of them, the extraordinary motions which I have described above
+seem to be unaccountable. The change of direction of a heavy mass moving
+swiftly cannot be sudden. The semi-circular, the wavy and the angular
+orbits observed could not be described, it would seem, by such a mass
+animated with a great velocity. Although the meteors are said to be
+ignited by the transformation of part of their progressive motion into
+molecular motion, yet it is not observed that the velocity of the
+falling stars diminishes when they are about to disappear. The luminous
+trails they leave in the atmosphere do not appear to be endowed with any
+motion, but remain for a time in their original positions. These facts
+are apparently opposed to the hypothesis that such meteors have any
+appreciable mass. The extraordinary motions exhibited by some meteors
+seem to indicate that some unsuspected force resides in these bodies,
+and causes them to deviate from the laws of ordinary motion.
+
+Although it is very probable that the ordinary shooting-stars have no
+appreciable mass, yet it is known that very heavy meteoric masses
+sometimes fall at the surface of the Earth. Such falls are generally
+preceded by the sudden apparition in the sky of a large, and usually
+very brilliant fire-ball, which traverses the air at a great speed,
+sometimes leaving behind it a luminous trail, after which it explodes
+with a loud sound, and heavy fiery meteoric fragments, diverging in all
+directions, fall at the surface of the Earth. The name of _Aerolites_ or
+_Meteorolites_ is given to these ponderous fragments. As these meteors,
+before they explode and fall to the ground, have many points of
+resemblance with the shooting-stars, they are generally supposed to be
+connected with them, and to have a similar cometary origin. The fact
+that the aerolites differ widely from each other in constitution, and
+are all composed of substances found on the Earth, associated with other
+facts given below, would rather seem to indicate a terrestrial than a
+celestial origin.
+
+If the aerolites belong to the same class of bodies as the falling
+stars, differing from them only in size and mass, it is difficult to see
+why so very few should fall upon the Earth during the great meteoric
+showers, when thousands of shooting-stars traverse our atmosphere. In
+Prof. Kirkwood's "Meteoric Astronomy" are given catalogues of all the
+falls of aerolites and fire-balls which have been observed at the time
+of the periodic meteoric showers of the 10th of August and the 13th of
+November, during a period of 221 years for the Perseids, or August
+showers, and of 318 years for the Leonids, or November showers. During
+221 years, 10 falls of aerolites have been witnessed simultaneously with
+the fall of the Perseids; while during 318 years, only 4 such falls have
+been recorded as having occurred at the time of the Leonid shower. If
+there is any close connection between the shooting-stars and the
+aerolites, we should expect to find a maximum in their fall at the time
+of the great meteoric displays. So far, no maxima or minima have yet
+been discovered in the fall of aerolites; they do not seem, like
+meteoric showers, to be governed by a law of periodicity.
+
+A very remarkable peculiarity of the aerolites is that they seem to have
+a tendency to fall in certain regions. Such are the southern part of
+France, the north of Italy, Hindostan, the central states of North
+America, and Mexico and Brazil. There is a curious contrast existing
+between the quick cometary motion of the aerolites before their
+explosion, and the comparatively slow motion of their fragments as they
+reach the Earth; motion which seems to be no greater than that
+corresponding to their natural fall impeded by the resistance of the
+air. In general, their penetration into the soil upon which they fall
+does not at all correspond to the great velocity with which they move in
+the atmosphere. The fragmentary structures of the aerolites, their
+identity of substance with that of our globe, their great resemblance to
+the volcanic minerals of the Earth, and the fractures and faults which
+some of them exhibit, do not correspond at all with the idea that they
+are cometary particles fallen on the Earth. As far as their structure
+and appearance is concerned, they seem rather to be a volcanic product
+of the interior of the Earth than parts of disintegrated comets. It must
+be admitted that their identity with the shooting-stars is far from
+established, and that they are still involved in mystery.
+
+The so-called meteoric dust gathered at sea and on high mountains may
+have various origins, and may be partly furnished by volcanic dust
+carried to great distances in the atmosphere.
+
+Since millions of shooting-stars penetrate our atmosphere every year and
+remain in it, becoming definitively a part of the Earth, it follows
+that, no matter how small may be the quantity of matter of which they
+are composed, they must gradually increase the volume and mass of our
+globe, although the increase may be exceedingly slow. Supposing every
+one of the shooting-stars penetrating our atmosphere to contain one
+cubic millimeter of matter, it has been calculated that it would take
+nearly 35,000 years to make a deposit one centimeter in thickness all
+over the surface of our globe. Insignificant as this may appear, it is
+probable that the quantity of matter of meteoric origin which is added
+to our globe is much less than has just been supposed.
+
+
+
+
+THE MILKY-WAY OR GALAXY
+
+PLATE XIII
+
+
+During clear nights, when the Moon is below the horizon, the starry
+vault is greatly adorned by an immense belt of soft white light,
+spanning the heavens from one point of the horizon to the opposite
+point, and girdling the celestial sphere in its delicate folds. Every
+one is familiar with this remarkable celestial object, called the
+_Milky-way_ or _Galaxy_.
+
+Seen with the naked eye, the Galaxy appears as an irregular, narrow,
+nebulous belt, apparently composed of cloud-like luminous masses of
+different forms and sizes, separated by comparatively dark intervals.
+These cloud-like masses vary much in luminous intensity, and while some
+among them are very bright and conspicuous, others are so faint that
+they are hard to recognize. In general, the brightest parts of the
+Milky-way are situated along the middle of its belt, while its borders,
+which are usually very faint, gradually vanish in the sky. Some parts of
+the Galaxy, however, show very little of the cloudy structure so
+characteristic of other parts, being almost uniform throughout, except
+towards the borders, which are always fainter. These parts showing
+greater uniformity are also the faintest.
+
+Such is the general appearance of the Milky-way on ordinary nights, but
+on rare occasions, when the atmosphere is particularly pure, it presents
+one of the grandest sights that can be imagined. At such favorable
+moments I have seen the Galaxy gleaming with light, and appearing as if
+composed of star-dust or of precious stones. The strange belt then
+appeared all mottled over and fleecy, its large cloud-like masses being
+subdivided into numerous small, irregular cloudlets of great brilliancy,
+which appeared projected upon a soft luminous background.
+
+The width of the Galaxy is far from being uniform; while in some places
+it is only 4° or 5°, in others it is 15° and even more. In some
+places it appears wavy in outline, at others quite straight; then it
+contracts, to expand a few degrees distant; while at other places it
+sends off branches and loops, varying in form, size and direction, some
+of which are quite prominent, while others are very faint.
+
+Although very irregular in form, the general appearance of the galactic
+belt is that of a regular curve occupying one of the great circles of
+the celestial sphere. The Milky-way completely encircles the heavens,
+but, of course, only one-half is visible at any one moment, since our
+globe prevents the other half from being seen. If, for a moment, we
+imagine ourselves left in space, our globe having vanished from under
+our feet, we should then see the whole Galaxy forming a continuous belt
+in the heavens, at the centre of which we should apparently be situated.
+
+While only one-half of the galactic belt can be seen at once from any
+point on the Earth, yet, according to the position of the observer, a
+larger or smaller portion of the whole can be seen at different times.
+In high northern or southern latitudes but little more than half can be
+seen even by continuous observations; but as we approach the equatorial
+regions, more and more of it becomes visible, until the whole may be
+seen at different hours and seasons. In the latitudes of the northern
+states, about two-thirds of the Galaxy is visible, the rest remaining
+hidden below the horizon; but from the southern states very nearly the
+whole can be seen. The half of the Milky-way visible at any one time
+from any latitude on the Earth never entirely sets below the horizon,
+although in some places it may be so near the horizon as to be rendered
+invisible by vapors. In the latitude of Cambridge, when in its lowest
+position, the summit of its arc is still about 12° or 15° above the
+northern horizon. The great circle of the celestial sphere, occupied by
+the galactic belt, is inclined at an angle of about 63° to the
+celestial equator, and intersects this great circle on one side in the
+constellation Monoceros in 6h. 47m., and on the opposite side in the
+constellations Aquila and Ophiuchus in 18h. 47m. of right ascension; so
+that its northern pole is situated in the constellation Coma Berenices
+in R. A. 12h. 47m., declination N. 27°, and the southern in the
+constellation Cetus in R. A. 0h. 47m., declination S. 27°.
+
+According to the seasons and to the hours of the night at which it is
+observed, the galactic arch presents different inclinations in the sky.
+Owing to its inclination to the equator of the celestial sphere, its
+opposite parts exhibit opposite inclinations when they pass the meridian
+of a place. That part of the Galaxy which is represented on Plate XIII.,
+and which intersects the celestial equator in the constellation Aquila,
+is inclined to the left or towards the east, when it is on the meridian;
+while the opposite part, situated in Monoceros, is inclined to the
+right, or towards the west, when it reaches the meridian. The former
+passes the meridian in the evening in the summer and autumn months; the
+latter, in the winter and spring months.
+
+
+[PLATE XIII.--PART OF THE MILKY WAY.
+
+From a study made during the years 1874, 1875 and 1876]
+
+
+By beginning at its northernmost part, represented at the upper part of
+Plate XIII., situated in "the chair" of the constellation Cassiopeia,
+and descending southwardly, and continuing in the same direction until
+the whole circle is completed, the course of the Milky-way through the
+constellations may be briefly described as follows: From Cassiopeia's
+chair, the Galaxy, forming two streams, descends south, passing partly
+through Lacerta on the left, and Cepheus on the right; at this last
+point it approaches nearest to the polar star. Then it enters Cygnus,
+where it becomes very complicated and bright, and where several large
+cloudy masses are seen terminating its left branch, which passes to the
+right, near the bright star Deneb, the leader of this constellation.
+Below Deneb, the Galaxy is apparently disconnected and separated from
+the northern part by a narrow, irregular dark gap. From this rupture,
+the Milky-way divides into two great streams separated by an irregular
+dark rift. An immense branch extends to the right, which, after having
+formed an important luminous mass between the stars _γ_ and _β_,
+continues its southward progress through parts of Lyra, Vulpecula,
+Hercules, Aquila and Ophiuchus, where it gradually terminates a few
+degrees south of the equator. The main stream on the left, after having
+formed a bright mass around _ε_ Cygni, passes through Vulpecula and
+then Aquila, where it crosses the equinoctial just below the star _η_
+after having involved in its nebulosity the bright star Altair, the
+leader of Aquila. In the southern hemisphere the Galaxy becomes very
+complicated and forms a succession of very bright, irregular masses, the
+upper one being in Scutum Sobieskii, while the others are respectively
+situated in Sagittarius and in Scorpio; the last, just a little above
+our horizon, being always considerably dimmed by vapors. From Scutum
+Sobieskii, the Galaxy expands considerably on the right, and sends a
+branch into Scorpio, in which the fiery red star Antares is somewhat
+involved.
+
+Continuing its course below our horizon, the Milky-way enters Ara and
+Norma, and then, passing partly through Circinus, Centaurus and Musca,
+it reaches the Southern Cross, after having been divided by the large
+dark pear-shaped spot known to navigators as the "Coal-Sack." In Ara and
+Crux the Milky-way attains its maximum of brightness, which there
+surpasses its brightest parts in Cygnus. In Musca, it makes its nearest
+approach to the south pole of the heavens. It then enters Carina and
+Vela, where it spreads out like a fan, and terminates in this last
+constellation, before reaching _λ_, being once more interrupted by a
+dark and very irregular gap, on a line with the two stars _γ_ and _λ_.
+It is noteworthy that this second rupture of continuity of the Galaxy in
+Vela is very nearly opposite, or at about 180° from the break near
+Deneb in Cygnus.
+
+Continuing its course on the other side of the break, the Milky-way
+again spreads out into the shape of a fan, grows narrower in entering
+Puppis, where it is longitudinally divided by darkish channels. It then
+passes above our southern horizon, becoming visible to us, passing
+through part of Canis Major, where its border just grazes the brilliant
+star Sirius. But from Puppis it gradually diminishes in brightness and
+complication, becoming faint and uniform. It enters Monoceros and Orion,
+where it again crosses the equator a little above _δ_, the northernmost
+of the three bright stars in the belt of Orion. Continuing its northward
+course it passes through Gemini, extending as far as Castor and Pollux,
+and then entering Auriga, where it begins to increase in brightness and
+in complication of structure. It passes partly through Camelopardus and
+into Perseus, where an important branch proceeds from its southern
+border.
+
+This branch beginning near the star _θ_, advances towards the
+celebrated variable star Algol, around which it is quite bright and
+complicated. Continuing its course in the same direction, the branch
+rapidly loses its brightness, becoming very faint a little below Algol,
+and passing through _ζ_ Persei, it enters Taurus, leaving the Pleiades
+on its extreme southern margin; and after having passed through _ε_
+where it branches off, it rapidly curves towards the main stream, which
+it joins near _ζ_ Tauri, thus forming an immense loop. The ramification
+projecting near _ε_ Tauri involves in its nebulosity the ruddy star
+Aldebaran and the scattered group of the Hyades. It then advances
+towards the three bright stars _δ_, _ε_ and _ζ_ of the belt of Orion,
+which, together with the sextuple star _θ_ Orionis, are involved in its
+faint nebulosity, and joins the main stream on the equinoctial, having
+thus formed a second loop, whose interior part is comparatively free
+from nebulosity, and contains the fine stars Betelgeuse and Bellatrix.
+
+That portion of the main galactic stream which is comprised between the
+star Deneb in Cygnus, and Capella in Auriga, is divided longitudinally
+by a very irregular, narrow, darkish cleft, comparatively devoid of
+nebulosity, which, however, is interrupted at some points. This dark gap
+sends short branches north and south, the most important of which are
+situated near _ζ_ Cephei and _β_ Cassiopeiæ. Another branch runs from
+_γ_ beyond _ε_ of the constellation last mentioned. The main stream of
+the Galaxy after leaving Perseus, enters Cassiopeia, and sending short
+branches into Andromeda, it completes its immense circle in Cassiopeia's
+chair, where this description was begun.
+
+When examined through the telescope, the appearance of the Milky-way
+completely changes, and its nebulous light is resolved into an immense
+number of stars, too faint to be individually seen with the naked eye.
+When Galileo first directed the telescope to the galactic belt, its
+nebulous, cloud-like masses were at once resolved into stars, even by
+the feeble magnifying power of his instrument. When, much later, Sir
+William Herschel undertook his celebrated star-gaugings of the Galaxy,
+millions of stars blazed out in his powerful telescopes. The stars
+composing this great nebulous belt are so numerous that it is impossible
+to arrive at any definite idea as to their number. From his soundings
+Herschel estimated at 116,000 the number of stars which, on one
+occasion, passed through the field of his telescope in 15 minutes, by
+the simple effect of the diurnal motion of the heavens; and on another
+occasion, a number estimated at 250,000 crossed the field in 41 minutes.
+In a space of 50, comprised between _β_ and _γ_ Cygni, shown on Plate
+XIII., he found no less than 331,000 stars. Prof. Struve has estimated
+at 20,500,000 the number of stars seen in the Milky-way through the
+twenty-foot telescope employed by Herschel in his star-gaugings. Great
+as this number may seem, it is yet far below the truth; as the great
+modern telescopes, according to Professor Newcomb, would very probably
+double the number of stars seen through Herschel's largest telescope,
+and detect from thirty to fifty millions of stars in the Milky-way.
+
+Although the telescope resolves the Galaxy into millions of stars, yet
+the largest instruments fail to penetrate its immense depths. The
+forty-foot telescope of Herschel, and even the giant telescope of Lord
+Rosse, have failed to resolve the Milky-way entirely into stars, the
+most distant ones appearing in them as nebulosities upon which the
+nearer stars are seen projected, the galactic stratum being unfathomable
+by the largest telescopes yet made.
+
+The stars composing the Milky-way are very unevenly distributed, as
+might easily be supposed from the cloud-like appearance of this belt. In
+some regions they are loosely scattered, forming long rows or streams of
+various figures, while in others they congregate into star groups and
+clusters having all imaginable forms, some being compressed into very
+dense globular masses. The intervals left between the clustering masses
+are poorer in stars, and indeed some of them are even totally devoid of
+stars or nebulosity. Such are the great and small "coal-sacks" in the
+southern Galaxy. I have myself detected such a dark space devoid of
+stars and nebulosity in one of the brightest parts of the Milky-way, in
+the constellation Sagittarius, in about 17h. 45m. right ascension, and
+27° 35' south declination. It is a small miniature coal-sack or opening
+in the Galaxy, through which the sight penetrates beyond this great
+assemblage of stars. Close to this, is another narrow opening near a
+small, loose cluster.
+
+Although lacking the optical resources which now enable us to recognize
+the structure of the Milky-way, some of the ancient philosophers had
+succeeded tolerably well in their speculations regarding its nature. It
+was the opinion of Democritus, Pythagoras and Manilius, that the Galaxy
+was nothing else but a vast and confused assemblage of stars, whose
+faint light was the true cause of its milky appearance.
+
+Before the invention of the telescope, no well-founded theory in regard
+to the structure of the Milky-way could, of course, be attempted.
+Although Kepler entertained different ideas in regard to the structure
+of this great belt from those now generally admitted, yet in them may be
+found the starting point of the modern conception of the structure of
+the Galaxy and of the visible universe. In the view of this great mind,
+the Milky-way, with all its stars, formed a vast system, the centre of
+which, and of the universe, was occupied by our Sun. Kepler reasoned
+that the place of the Sun must be near the centre of the galactic belt,
+from the fact this last object appears very nearly as a great circle of
+the celestial sphere, and that its luminous intensity is about the same
+in all its parts.
+
+Half a century later, another attempt to explain the Milky-way was made
+by Wright, of Durham, who rejected the idea of an accidental and
+confused distribution of the stars as inconsistent with the appearance
+of the Galaxy, and regarded them as arranged along a fundamental plane
+corresponding to that of the Milky-way. These ideas which were
+subsequently developed and enlarged by Kant, and then by Lambert,
+constitute what is now known as Kant's theory. According to this theory,
+the stars composing the Galaxy are conceived as being uniformly arranged
+between two flat planes of considerable extension, but which are
+comparatively near together, the Sun occupying a place not very far from
+the centre of this immense starry stratum. As we view this system
+crosswise through its thinnest parts, the stars composing it appear
+scattered and comparatively few in number, but when we view it
+lengthwiser through its most extended parts, they appear condensed and
+extremely numerous, thus giving the impression of a luminous belt
+encircling the heavens. In the conception of Kant, each star was a sun,
+forming the centre of a planetary system. These systems are not
+independent, but are kept together by the bonds of universal
+gravitation. The Galaxy itself is one of these great systems, its
+principal plane being the equivalent of the zodiac in our planetary
+system, while a preponderant body, which might be Sirius, is the
+equivalent of our Sun, and keeps the galactic system together. In the
+universe there are other galaxies, but as they are too distant to be
+resolved into stars, they appear as elliptical nebulæ. Such are, in
+brief, the grand speculations of Kant and Lambert on the Milky-way, and
+the structure of the universe.
+
+Kant's theory rested more on conjectures than on observed facts, and
+needed therefore the sanction of direct observations to be established
+on a firm basis. With this view, Sir William Herschel investigated the
+subject, by a long and laborious series of observations. His plan, which
+was that of "star-gauging," consisted in counting all the stars visible
+in his twenty-foot telescope, comprised in a wide belt cutting the
+Galaxy at right angles, and extending from one of its sides to the
+opposite one, thus embracing 180° of the celestial sphere. In this belt
+he executed 3,400 telescopic star-gaugings of a quarter of a degree
+each, from which he obtained 683 mean gaugings giving the stellar
+density of the corresponding regions.
+
+The general result derived from this immense labor was that the stars
+are fewest in regions the most distant from the galactic belt; while
+from these regions, which correspond to the pole of the Galaxy, they
+gradually increase in number in approaching the Milky-way. The star
+density was found to be extremely variable, and while some of the
+telescopic gaugings detected either no star at all, or only one or two,
+other gaugings gave 500 stars and even more. The average number of stars
+in a field of view of his telescope, obtained for the six zones, each of
+15°, into which Herschel divided up the portion of his observing belt,
+extending from the Galaxy to its pole, is as follows: In the first zone,
+commencing at 90° from the galactic belt and extending towards it, 4
+stars per telescopic field were found; 5 in the second; 8 in the third;
+14 in the fourth; 24 in the fifth and 53 in the sixth, which terminated
+in the Galaxy itself. Very nearly similar results were afterwards found
+by Sir John Herschel, for corresponding regions in the southern
+hemisphere.
+
+From these studies, Herschel concluded that the stellar system is of the
+general form supposed by the Kantian theory, and that its diameter must
+be five times as extended in the direction of the galactic plane, as it
+is in a direction perpendicular to it. To explain the great branch sent
+out by the Galaxy in Cygnus, he supposed a great cleft dividing the
+system edgewise, about half way from its circumference to its centre.
+From suppositions founded on the apparent magnitude and arrangement of
+stars, he estimated that it would take light about 7,000 years to reach
+us from the extremities of the Galaxy, and therefore 14,000 years to
+travel across the system, from one border to the opposite one.
+
+But Herschel's theory concerning the Milky-way rested on the erroneous
+assumption that the stars are uniformly distributed in space, and also
+that his telescopes penetrated through the entire depth of the Galaxy.
+Further study showed him that his telescope of twenty feet, and even his
+great forty-foot telescope, which was estimated to penetrate to a
+distance 2,300 times that of stars of the first magnitude, failed to
+resolve some parts of the Galaxy into stars. Meanwhile, the structure of
+the Milky-way being better known, the irregular condensation of its
+stars became apparent, while the mutual relation existing between binary
+and multiple systems of stars, as also between the stars which form
+clusters, was recognized, as showing evidence of closer association
+between certain groups of stars than between the stars in general.
+Herschel's system, which rested on the assumption of the uniform
+distribution of the stars in space, and on the supposition that the
+telescopes used for his gauges penetrated through the greater depths of
+the Galaxy, being thus found to contradict the facts, was gradually
+abandoned by its author, who adopted another method of estimating the
+relative distances of the stars observed in his gaugings.
+
+This method, founded on photometric principles, consisted in judging the
+penetrating power of his telescope by the brightness of the stars, and
+not, as formerly, by the number which they brought into view. He then
+studied by this new method the structure of the Milky-way and the
+probable distance of the clustering masses of which it is formed,
+concluding that the portion of the Galaxy traversing the constellation
+Orion is the nearest to us. This last result seems indicated by the fact
+that this portion of the Milky-way is the faintest and the most uniform
+of all the galactic belt.
+
+More recently Otto Struve investigated the same subject, and arrived at
+very nearly similar conclusions, which may be briefly stated as follows:
+The galactic system is composed of a countless number of stars,
+spreading out on all sides along a very extended plane. These stars,
+which are very unevenly distributed, show a decided tendency to cluster
+together into individual groups of different sizes and forms, separated
+by comparatively vacant spaces. This layer where the stars congregate in
+such vast numbers may be conceived as a very irregular flat disk,
+sending many branches in various directions, and having a diameter eight
+or ten times its thickness. The size of this starry disk cannot be
+determined, since it is unfathomable in some directions, even when
+examined with the largest telescopes. The Sun, with its attending
+planets, is involved in this immense congregation of suns, of which it
+forms but a small particle, occupying a position at some distance from
+the principal plane of the Galaxy. According to Struve, this distance is
+approximately equal to 208,000 times the radius of the Earth's orbit.
+The Milky-way is mainly composed of star-clusters, two-thirds, perhaps,
+of the whole number visible in the heavens being involved in this great
+belt. In conclusion, our Sun is only one of the individual stars which
+constitute the galactic system, and each of these stars itself is a sun
+similar to our Sun. These individual suns are not independent, but are
+associated in groups varying in number from a few to several thousands,
+the Galaxy itself being nothing but an immense aggregation of such
+clusters, whose whole number of individual suns probably ranges between
+thirty and fifty millions. In this vast system our globe is so
+insignificant that it cannot even be regarded as one of its members.
+According to Dr. Gould, there are reasons to believe that our Sun is a
+member of a small, flattened, bifid cluster, composed of more than 400
+stars, ranging between the first and seventh magnitude, its position in
+this small system being eccentric, but not very far from the galactic
+plane.
+
+The study of the Milky-way, of which Plate XIII. is only a part, was
+undertaken to answer a friendly appeal made by Mr. A. Marth, in the
+Monthly Notices of the Royal Astronomical Society, in 1872. I take
+pleasure in offering him my thanks for the suggestion, and for the
+facility afforded me in this study by his "List of Co-ordinates of Stars
+within and near the Milky-way," which was published with it.
+
+
+
+
+THE STAR-CLUSTERS
+
+PLATE XIV
+
+
+It is a well-known fact that the stars visible to the naked eye are very
+unequally distributed in the heavens, and that while they are loosely
+scattered in some regions, in others they are comparatively numerous,
+sometimes forming groups in which they appear quite close together.
+
+In our northern sky are found a few such agglomerations of stars, which
+are familiar objects to all observers of celestial objects. In the
+constellation Coma Berenices, the stars are small, but quite condensed,
+and form a loosely scattered, faint group. In Taurus, the Hyades and the
+Pleiades, visible during our winter nights, are conspicuous and familiar
+objects which cannot fail to be recognized. In the last group, six stars
+may be easily detected by ordinary eyes on any clear night, but more can
+sometimes be seen; on rare occasions, when the sky was especially
+favorable, I have detected eleven clearly and suspected several others.
+The six stars ordinarily visible, are in order of decreasing brightness,
+as follows: Alcyone, Electra, Atlas, Maia, Taygeta and Merope. Glimpses
+of Celano and Pleione are sometimes obtained.
+
+When the sky is examined with some attention on any clear, moonless
+night, small, hazy, luminous patches, having a cometary aspect, are
+visible here and there to the naked eye. In the constellation Cancer is
+found one of the most conspicuous, called Præsepe, which forms a small
+triangle with the two stars _γ_ and _δ_. In Perseus, and involved in
+the Milky-way, is found another luminous cloud, situated in the
+sword-handle, and almost in a line with the two stars _γ_ and _δ_ of
+Cassiopeia's Chair. In the constellation Hercules, another nebulous mass
+of light, but fainter, is also visible between the stars _η_ and _ζ_
+where it appears as a faint comet, in the depths of space. In Ophiuchus
+and Monoceros are likewise found hazy, luminous patches. In the southern
+sky, several such objects are also visible to the naked eye, being found
+in Sagittarius, in Canis Major and in Puppis; but the most conspicuous
+are those in Centaurus and Toucan. That in Centaurus involves the star
+_ω_ in its pale diffused nebulosity, and that in Toucan is involved in
+the lesser Magellanic cloud.
+
+When the telescope is directed to these nebulous objects, their hazy,
+ill-defined aspect disappears, and they are found to consist of
+individual stars of different magnitudes, which being more or less
+closely grouped together, apparently form a system of their own. These
+groups, which are so well adapted to give us an insight into the
+structure and the vastness of the stellar universe, are called
+_Star-clusters_.
+
+Star-clusters are found of all degrees of aggregation, and while in some
+of them, such as in the Pleiades, in Præsepe and in Perseus, the stars
+are so loosely scattered that an opera glass, and even the naked eye,
+will resolve them; in others, such as in those situated in Hercules,
+Aquarius, Toucan and Centaurus, they are so greatly compressed that even
+in the largest telescopes they appear as a confused mass of blazing
+dust, in which comparatively few individual stars can be distinctly
+recognized. Although only about a dozen Star-clusters can be seen in the
+sky with the naked eye, yet nearly eleven hundred such objects visible
+through the telescope, have been catalogued by astronomers.
+
+The stars composing the different clusters visible in the heavens vary
+greatly in number, and while in some clusters there are only a few, in
+others they are so numerous and crowded that it would be idle to try to
+count them, their number amounting to several thousands. It has been
+calculated by Herschel that some clusters are so closely condensed, that
+in an area not more than ⅒ part of that covered by the Moon, at least
+5,000 stars are agglomerated.
+
+When the group in the Pleiades is seen through the telescope it appears
+more important than it does to the naked eye, and several hundreds of
+stars are found in it. In a study of Tempel's nebula, which is involved
+in the Pleiades, I have mapped out 250 stars, mostly comprised within
+this nebula, with the telescope of 6⅓ inches aperture, which I have
+used for this study.
+
+As a type of a loose, coarse cluster, that in Perseus is one of the
+finest of its class. It appears to the naked eye as a single object, but
+in the telescope it has two centres of condensation, around which
+cluster a great number of bright stars, forming various curves and
+festoons of great beauty. Among its components are found several yellow
+and red stars, which give a most beautiful contrast of colors in this
+gorgeous and sparkling region. In a study which I have made of this twin
+cluster, I have mapped out 664 stars belonging to it, among which are
+two yellow and five red stars.
+
+While some clusters, like those just described, are very easily
+resolvable into stars with the smallest instruments, others yield with
+the greatest difficulty, even to the largest telescopes, in which their
+starry nature is barely suspected. Owing to this peculiarity,
+star-clusters are usually divided into two principal classes. In the
+first class are comprised all the clusters which have been plainly
+resolved into stars, and in the second all those which, although not
+plainly resolvable with the largest instruments now at our disposal,
+show a decided tendency to resolvability, and convey the impression that
+an increase of power in telescopes is the only thing needed to resolve
+them into stars. Of course this classification, which depends on the
+power of telescopes to decide the nature of these objects, is arbitrary,
+and a classification based on spectrum analysis is now substituted for
+it.
+
+The star-clusters are also divided into globular and irregular clusters,
+according to their general form and appearance. The globular clusters,
+which are the most numerous, are usually well-defined objects, more or
+less circular in their general outlines. The rapid increase of
+brightness towards their centres, where the stars composing them are
+greatly condensed, readily conveys the impression that the general form
+of these sparkling masses is globular. The irregular clusters are not so
+rich in stars as the former. Usually their stars are less condensed
+towards the centre, and are, for the most part, so loosely and
+irregularly distributed, that it is impossible to recognize the outlines
+of these clusters or to decide where they terminate. The globular
+clusters are usually quite easily resolvable into stars, either partly
+or wholly, although some among them do not show the least traces of
+resolvability, even in the largest instruments. This may result from
+different causes, and may be attributed either to the minuteness of
+their components or to their great distance from the Earth, many
+star-clusters being at such immense distances that they are beyond our
+means of measurement.
+
+As has been shown in the preceding section, the star-clusters are found
+in great number in the Galaxy; indeed, it is in this region and in its
+vicinity that the greater portion of them are found. In other regions,
+with the exception of the Magellanic clouds, where they are found in
+great number and in every stage of resolution, the clusters are few and
+scattered.
+
+
+[PLATE XIV.--STAR-CLUSTER IN HERCULES.
+
+From a study made in June, 1877]
+
+
+The star-cluster in the constellation Hercules, designated as No. 4,230
+in Sir J. Herschel's catalogue, and which is represented on Plate XIV.,
+is one of the brightest and most condensed in the northern hemisphere,
+although it is not so extended as several others, its angular diameter
+being only 7' or 8'. This object, which was discovered by Halley in
+1714, is one of the most beautiful of its class in the heavens.
+According to Herschel, it is composed of thousands of stars between the
+tenth and fifteenth magnitudes. Undoubtedly the stars composing this
+group are very numerous, although those which can be distinctly seen as
+individual stars, and whose position can be determined, are not so many
+as a superficial look at the object would lead us to suppose. From a
+long study of this cluster, which I have made with instruments of
+various apertures, I have not been able to identify more stars than are
+represented on the plate, although the nebulosity of which this object
+mainly consists, and especially the region situated towards its centre,
+appeared at times granular and blazing with countless points of light,
+too faint and too flickering to be individually recognized. Towards its
+centre there is quite an extended region, whose luminous intensity is
+very great, and which irresistibly conveys the impression of the
+globular structure of this cluster. Besides several outlying appendages,
+formed by its nebulosity, the larger stars recognized in this cluster
+are scattered and distributed in such a way that they form various
+branches, corresponding with those formed by the irresolvable
+nebulosity. At least six or seven of these branches and wings are
+recognized, some of which are curved and bent in various ways, thus
+giving this object a distant resemblance to some crustacean forms.
+Although I have looked for it with care, I have failed to recognize the
+spiral structure attributed to this object by several observers. Among
+the six appendages which I have recognized, some are slightly curved;
+but their curves are sometimes in opposite directions, and two branches
+of the upper portion make so short a bend that they resemble a claw
+rather than a spiral wing. The spectrum of this cluster, like that of
+many objects of its class, is continuous, with the red end deficient.
+
+A little to the north-east of this object is found the cluster No.
+4,294, which, although smaller and less bright than the preceding, is
+still quite interesting. It appears as a distinctly globular cluster
+without wings, and much condensed towards its centre. The stars
+individually recognized in it, although less bright than those of the
+other cluster, are so very curiously distributed in curved lines that
+they give a peculiar appearance to this condensed region.
+
+A little to the north of _γ_ Centauri may be found the great _ω_
+Centauri cluster, No. 3,531, already referred to above. This magnificent
+object, which appears as a blazing globe 20' in diameter, is, according
+to Herschel, the richest in the sky, and is resolved into a countless
+number of stars from the twelfth to the fifteenth magnitude, which are
+greatly compressed towards the centre. The larger stars are so arranged
+as to form a sort of net-work, with two dark spaces in the middle.
+
+The great globular cluster No. 52, involved in the lesser Magellanic
+cloud, in the constellation Toucan, is a beautiful and remarkable
+object. It is composed of three distinct, concentric layers of stars,
+varying in brightness and in degree of condensation in each layer. The
+central mass, which is the largest and most brilliant, is composed of an
+immense number of stars greatly compressed, whose reddish color gives to
+this blazing circle a splendid appearance. Around the sparkling centre
+is a broad circle, composed of less compressed stars, this circle being
+itself involved in another circular layer, where the stars are fainter
+and more scattered and gradually fade away.
+
+Many other great globular clusters are found in various parts of the
+heavens, among which may be mentioned the cluster No. 4,678, in
+Aquarius. This object is composed of several thousand stars of the
+fifteenth magnitude, greatly condensed towards the centre, and, as
+remarked by Sir J. Herschel, since the brightness of this cluster does
+not exceed that of a star of the sixth magnitude, it follows that in
+this case several thousand stars of the fifteenth magnitude equal only a
+star of the sixth magnitude. In the constellation Serpens the globular
+clusters No. 4,083 and No. 4,118 are both conspicuous objects, also No.
+4,687 in Capricornus. In Scutum Sobieskii the cluster No. 4,437 is one
+of the most remarkable of this region. The stars composing it, which are
+quite large and easily made out separately, form various figures, in
+which the square predominates.
+
+Among the loose irregular clusters, some are very remarkable for the
+curious arrangement of their stars. In the constellation Gemini the
+cluster No. 1,360, which is visible to the naked eye, is a magnificent
+object seen through the telescope, in which its sparkling stars form
+curves and festoons of great elegance. The cluster No. 1,467, of the
+same constellation, is remarkable for its triangular form. In the
+constellation Ara the cluster No. 4,233, composed of loosely scattered
+stars, forming various lines and curves, is enclosed on three sides by
+nearly straight single lines of stars. In Scorpio the cluster No. 4,224
+is still more curious, being composed of a continuous ring of loosely
+scattered stars, inside of which is a round, loose cluster, which is
+divided into four parts by a dark cross-shaped gap, in which no stars
+are visible.
+
+Among the 1,034 objects which are now classified as clusters more or
+less resolvable, 565 have been absolutely resolved into stars, and 469
+have been only partly resolved, but are considered as belonging to this
+class of objects. In Sir J. Herschel's catalogue there are 102 clusters
+which are Considered as being globular; among them 30 have been
+positively resolved into stars.
+
+The agglomeration of thousands of stars into a globular cluster cannot
+be conceived, of course, to be simply the result of chance. This
+globular form seems clearly to indicate the existence of some bond of
+union, some general attractive force acting between the different
+members of these systems, which keeps them together, and condenses them
+towards the centre. Herschel regards the loose, irregular clusters as
+systems in a less advanced stage of condensation, but gradually
+concentrating by their mutual attraction into the globular form.
+Although the stars of some globular clusters appear very close together,
+they are not necessarily so, and may be separated by great intervals of
+space. It has been shown that the clusters are agglomeration of suns,
+and that our Sun itself is a member of a cluster composed of several
+hundreds of suns, although, from our point of observation, these do not
+seem very close together. So far as known, the nearest star to us is a
+Centauri, but its distance from the Earth equals 221,000 times the
+distance of the Sun from our globe, a distance which cannot be traversed
+by light in less than three years and five months. It seems very
+probable that if the suns composing the globular clusters appear so near
+together, it is because, in the first place, they are at immense
+distances from us, and in the second, because they appear nearly in a
+line with other suns, which are at a still greater distance from us, and
+on which they accordingly are nearly projected. If one should imagine
+himself placed at the centre of the cluster in Hercules, for instance,
+the stars, which from our Earth seems to be so closely grouped, would
+then quite likely appear very loosely scattered around him in the sky,
+and would resemble the fixed stars as seen from our terrestrial station.
+
+Judging by their loose and irregular distribution, the easily resolvable
+clusters would appear, in general, to be the nearer to us. It is
+probable that the globular clusters do not possess, to a very great
+degree, the regular form which they ordinarily present to us. It seems
+rather more natural to infer that they are irregular, and composed of
+many wings and branches, such as are observed in the cluster in
+Hercules; but as these appendages would necessarily be much poorer in
+stars than the central portions, they would be likely to become
+invisible at a great distance, and therefore the object would appear
+more or less globular; the globular form being simply given by the close
+grouping of the stars in the central portion. It would seem, then, that
+in general, the most loosely scattered and irregular clusters are the
+nearest to us, while the smallest globular clusters and those resolvable
+with most difficulty are the most distant.
+
+In accordance with the theory that the clusters are composed of stars,
+the spectrum of these objects is in general continuous; although, in
+many cases, the red end of the spectrum is either very faint or
+altogether wanting. Many objects presenting in a very high degree the
+principal characteristics exhibited by the true star-clusters, namely, a
+circular or oval mass, whose luminous intensity is greatly condensed
+toward the centre, have not yielded, however, to the resolving power of
+the largest telescopes, although their continuous spectrum is in close
+agreement with their general resemblance to the star-clusters. Although
+such objects may remain irresolvable forever, yet it is highly probable
+that they do not materially differ from the resolvable and partly
+resolvable clusters, except by their enormous distance from us, which
+probably reaches the extreme boundary of our visible universe.
+
+
+
+
+THE NEBULÆ
+
+PLATE XV
+
+
+Besides the foggy, luminous patches which have just been described, a
+few hazy spots of a different kind are also visible to the naked eye on
+any clear, moonless night. These objects mainly differ from the former
+in this particular, that when viewed through the largest telescopes in
+existence they are not resolved into stars, but still retain the same
+cloudy appearance which they present to the unassisted eye. On account
+of the misty and vaporous appearance which they exhibit, these objects
+have been called _Nebulæ_.
+
+Of the 26 nebulous objects visible to the naked eye in the whole
+heavens, 19 belong to the class of star-clusters, and 7 to the class of
+nebulæ. Among the most conspicuous nebulæ visible to the unassisted
+eye, are those in the constellations Argo Navis, Andromeda and Orion.
+
+Besides the seven nebulæ visible to the naked eye, a great number of
+similar objects are visible through the telescope. In Sir John
+Herschel's catalogue of nebulæ and clusters, are found 4,053
+irresolvable nebulæ, and with every increase of the aperture of
+telescopes, new nebulæ, invisible in smaller instruments, are found.
+Notwithstanding their irresolvability it is probable, however, that many
+among them have a stellar structure, which their immense distance
+prevents us from recognizing, and are not therefore true nebulæ. The
+giant telescope of Lord Rosse has shown nebulæ so remote that it has
+been estimated that it takes their light 30 million years to reach the
+Earth.
+
+The nebulæ are very far from being uniformly distributed in space. In
+some regions they are rare, while in others they are numerous and
+crowded together, forming many small, irregular groups, differing in
+size and in richness of aggregation. The grouping of the nebulæ does
+not occur at random in any part of the heavens, as might naturally be
+supposed, but, on the contrary, it is chiefly confined to certain
+regions. Outside of these regions nebulæ are rare and are separated
+from each other by immense intervals; so that these isolated objects
+appear as if they were lost wanderers from the great nebulous systems.
+
+The regions where the nebulæ congregate in great number are very
+extensive, and in a general view there are two vast systems of nebular
+agglomeration, occupying almost opposite points of the heavens, whose
+centres are not very distant from the poles of the Milky-way. In the
+northern hemisphere, the nebulous system is much richer and more
+condensed than in the southern hemisphere. The northern nebulæ are
+principally contained in the constellations Ursa Minor and Major, in
+Draco, Canes Venatici, Bootes, Leo Major and Minor, Coma Berenices, and
+Virgo. In this region, which occupies about ⅛ of the whole surface of
+the heavens, ⅓ of the known nebulæ are assembled. The southern
+nebulæ are more evenly distributed and less numerous, with the
+exception of two comparatively small, but very remarkable centres of
+condensation which, together with many star-clusters, constitute the
+Magellanic clouds.
+
+These two vast nebular groups are by no means regular in outline, and
+send various branches toward each other. They are separated by a wide
+and very irregular belt, comparatively free from nebulæ, which
+encircles the celestial sphere, and whose medial line approximately
+coincides with that of the galactic belt. The Milky-way, so rich in
+star-clusters, is very barren in nebulæ; but it is a very remarkable
+fact, nevertheless, that almost all the brightest, largest, and most
+complicated nebulæ of the heavens are situated either within it, or in
+its immediate vicinity. Such are the great nebulæ in Orion and
+Andromeda; the nebula of _ζ_ Orionis; the Ring nebula in Lyra; the
+bifurcate nebula in Cygnus; the Dumb-bell nebula in Vulpecula; the Fan,
+Horse-shoe, Trifid and Winged nebulæ in Sagittarius; the great nebula
+around _η_ Argus Navis, and the Crab nebula in Taurus.
+
+Aside from the discovery of some of the largest nebulæ by different
+observers, and their subsequent arrangement in catalogues by Lacaille
+and Messier, very little had been done towards the study of these
+objects before 1779, when Sir W. Herschel began to observe them with the
+earnestness of purpose which was one of the distinctive points of the
+character of this great man. He successively published three catalogues
+in 1786, 1789, and 1802, in which the position of 2,500 nebulous objects
+was given. This number was more than doubled before 1864, when Sir John
+Herschel published his catalogue of 5,079 nebulæ and star-clusters. To
+this long list must be added several hundred similar objects, since
+discovered by D'Arrest, Stephan, Gould and others. But, as has been
+shown above, among the so-called nebulæ are many star-clusters which do
+not properly belong to the same class of objects, it being sometimes
+impossible in the present state of our knowledge to know whether a
+nebulous object belongs to one class or to the other.
+
+The nebulæ exhibit a great variety of forms and appearances, and, in
+accordance with their most typical characters, they are usually divided
+into several classes, which are: the Nebulous stars, the Circular, or
+Planetary, the Elliptical, the Annular, the Spiral and Irregular
+nebulæ.
+
+
+[PLATE XV.--THE GREAT NEBULA IN ORION.
+
+From a study made in the years 1875-76]
+
+
+The so-called nebulous stars consist of a faint nebulosity, usually
+circular, surrounding a bright and sharp star, which generally occupies
+its centre. The nebulosity surrounding these stars varies in brightness
+as well as in extent, and while, in general, its light gradually fades
+away, it sometimes terminates quite suddenly. Such nebulosities are
+usually brighter and more condensed towards the central star. The stars
+thus surrounded do not seem, however, to be distinguished from others by
+any additional peculiarity. Some nebulæ of this kind are round, with
+one star in the centre; others are oval and have two stars, one at each
+of their foci. The nebulous star, _τ_ Orionis, represented at the upper
+part of Plate XV., above the great nebula, has a bright star at its
+centre and two smaller ones on the side. The association of double stars
+with nebulæ is very remarkable, and may in some cases indicate a mutual
+relation between them.
+
+The so-called planetary nebulæ derive their name from their likeness to
+the planets, which they resemble in a more or less equable distribution
+of light and in their round or slightly oval form. While some of them
+have edges comparatively sharp and well defined, the outlines of others
+are more hazy and diffused. These nebulæ, which are frequently of a
+bluish tint, are comparatively rare objects, and most of those known
+belong to the southern hemisphere. When seen through large telescopes,
+however, they present a different aspect, and their apparent uniformity
+changes. The largest of these objects, No. 2,343 of the General
+Catalogue, is situated in the Great Bear, close to the star _β_. Its
+apparent diameter is, according to Sir J. Herschel, 2', 40", and "its
+light is equable, except at the edge, where it is a little hazy." In a
+study which I made of this object in 1876, with a refractor 6⅓ inches
+in aperture, I found it decidedly brighter on the preceding side, where
+the brightest part is crescent shaped. In Lord Rosse's telescope its
+disk is transformed into a luminous ring with a fringed border, and two
+small star-like condensations are found within. Another planetary
+nebula, near _χ_ Andromedæ, has also shown an annular structure in
+Rosse's telescope.
+
+The elliptical nebulæ, as their name implies, are elongated, elliptical
+objects; but while some of them are only slightly elongated ovals,
+others form ellipses whose eccentricity is so great that they appear
+almost linear. In all these objects the light is more or less condensed
+towards the centre; but while in some of them the condensation is
+gradual and slight, in others it is so great and sudden that the centre
+of the nebula appears as a large diffused star, somewhat resembling the
+nucleus of a comet. From the general appearance of these objects, it is
+not unlikely that some of them are either flattish, nebulous disks, like
+the planetary nebulæ, or nebulous rings, seen more or less sidewise.
+The condensation of light at their centres does not appear to be
+stellar, but nebulous like the rest, and it is a remarkable fact that
+very few, if any, of these objects are resolvable into stars.
+
+Several elliptical nebulæ are remarkable for having a star at or near
+each of their foci, or at each of their extremities. Such are the
+elliptical nebulæ in Draco, Centaurus, and Sagittarius, Nos. 4,419,
+3,706 and 4,395 of the General Catalogue, the last of which is in the
+vicinity of the triple star _μ_ Sagitarii. Each of these nebulæ has a
+star at each of its foci, while No. 1, in Cetus, has a star at each of
+its extremities.
+
+Among the most remarkable elliptic nebulæ may be mentioned Nos. 1,861
+and 2,373 of Sir J. Herschel's catalogue, both situated in the
+constellation Leo. The first is one of Lord Rosse's spiral nebulæ, and
+the last, which is a very elongated object, is formed of concentric oval
+rings, which are especially visible towards its central part. The
+constellation Draco is particularly remarkable for the number of
+elliptical nebulæ found within its boundaries. Among them are Nos.
+3,939, 4,058, 4,064, 4,087, 4,415, etc., which are quite remarkable
+objects of their class. No. 4,058, of which I have made a study, is
+bright, and has a decided lenticular form with a condensation in the
+centre. Its following edge is better defined than the preceding. In Lord
+Rosse's telescope this object exhibits a narrow, dark, longitudinal, gap
+in its interior.
+
+By far the largest and the finest object of this class is the great
+nebula in Andromeda. Although this object belongs rather to the class of
+irregular nebulæ, yet it is generally considered as an elliptic nebula,
+since its complicated structure, being less prominent, was not
+recognized until 1848, when it was perceived by George P. Bond, Director
+of the Harvard College Observatory. This, the first nebula discovered,
+was found in 1612 by Simon Marius. It is situated in the constellation
+Andromeda, in the vicinity of the star _ν_, and almost in a line with
+the stars _μ_ and _β_ of the same constellation. It is visible to the
+naked eye, and appears as a faint comet-like object. It is represented
+at the upper left hand corner of Plate XIII., on the border of the
+Milky-way, as it appears to the naked eye.
+
+The nebula in Andromeda is one of the brightest in the heavens, and is
+closely attended by two smaller nebulæ. Perhaps it would be rather more
+correct to say that it has three centres of condensation, as the two
+small nebulæ referred to are entirely involved in the same faint and
+extensive nebulosity. Its general form is that of an irregular oval,
+upwards of one degree in breadth and two and a half degrees in length.
+Its brightest and most prominent part, which alone was seen by the
+earlier observers, consists in a very elongated lenticular mass, which
+gradually condenses towards its centre into a blazing, star-like
+nucleus, surrounded by a brilliant nebulous mass. At a little distance
+to the south of this central condensation is found one of the lesser
+centres of condensation noted above, which is globular in appearance,
+with a bright, star-like nucleus like the former. The other centre of
+condensation is found to the north-west of the centre of the principal
+mass, and is quite elongated, with a centre of condensation towards its
+southern extremity, but it is not so bright as the others. Close to the
+western edge of the bright lenticular mass first described, and making a
+very slight angle with its longer axis, are found two narrow and nearly
+rectilinear dark rifts, running almost parallel to each other, and both
+terminating in a slender point in the south. These dark rifts, which are
+almost totally devoid of nebulous matter, are quite rare in nebulæ, and
+afford a good opportunity to watch the changes which this part of the
+nebulæ may undergo.
+
+This nebula has never been positively resolved into stars, although
+Prof. Geo. Bond and others have strongly suspected its resolvability. In
+a study which I have made of it, with the same instrument employed by
+Bond, and also with the great Washington telescope, I detected a decided
+mottled appearance in several places, which might be attributable to a
+beginning of resolvability; but I do not consider this a conclusive
+indication that the nebula is resolvable. The continuous spectrum given
+by this nebula, showing that it is not in the gaseous state which its
+appearance seems to indicate, warrants the conclusion, however, that it
+will ultimately be found to be resolvable. This object, being situated
+on the edge of the Galaxy and involved in its diffused light, has a
+great number of small stars belonging to this belt projected upon it.
+During my observations I have mapped out 1,323 of these stars, none of
+which seems to be in physical connection with the nebula.
+
+Among the circular and elliptical nebulæ a few exhibit a very
+remarkable structure, being apparently perforated, and forming either
+round, slightly oval, or elongated rings of great beauty. These Annular
+nebulæ are among the rarest objects in the heavens. In Scorpio, two
+such nebulæ are found involved in the Milky-way, and also one in
+Cygnus. One of those in Scorpio has two stars involved within its ring,
+at the extremities of its smallest interior diameter. A very elongated
+nebula in the vicinity of the fine triple star _γ_ Andromedæ is also
+annular, and has two stars symmetrically placed at the extremities of
+its greatest interior axis. Another elongated annular nebula is also
+found north of _η_ Pegasi.
+
+The grandest and most remarkable of the annular nebulæ is found in the
+constellation Lyra, about midway between the two stars _β_ and _γ_. It
+is slightly elliptical in form, and according to Prof. E. S. Holden, its
+major axis is 77".3 and the minor 58". From a study and several drawings
+which I have made of this object, with instruments of various apertures,
+I have found it decidedly brighter towards its outer border, at the
+extremities of its minor axis, than at the ends of the major axis. On
+very favorable occasions, some of its brightest parts have appeared
+decidedly, but very faintly mottled, and I have recognized three small
+centres of condensation. Its interior, in which Professor Holden has
+detected a very faint star, is quite strongly nebulous. In Lord Rosse's
+telescope, this nebula is completely surrounded by wisps and appendages
+of all sorts of forms, which I have failed to trace, however, both with
+the refractor of the Harvard College Observatory and with that of the
+Naval Observatory at Washington; Rosse, Secchi and Chacornac, have seen
+this nebula glittering as if it were a "heap of star dust," although its
+spectrum indicates that it is gaseous.
+
+The nebula No. 1,541, in Camelopardus, of which I have also made a study
+and a drawing, is closely allied to the class of annular nebulæ. This
+object, which is quite bright, has a remarkable appearance. It consists
+principally of somewhat more than half of an oval ring, surrounding a
+bright, nebulous mass which condenses around a star; this mass being
+separated from the imperfect ring by a dark interval. Upon the bright
+portion of the ring, and on opposite points, are found two bright stars,
+between which lies the star occupying the central mass. The central mass
+extends at some distance outside of the ring on its open side. Several
+stars are involved in this object.
+
+The Spiral nebulæ are very curious and complicated objects, but they
+are visible only in the largest telescopes. Prominent above all is the
+double spiral nebula No. 3,572, in Canes Venatici, which is not far from
+_η_ Ursa Majoris. In Lord Rosse's telescope, this object presents a
+wonderful spiral disposition, looking somewhat like one of the
+fire-works called pin-wheels, and forming long, curved wisps, diverging
+from two bright centres. The spectrum of this object, however, is not
+that of a gas. In the constellation Virgo, Rosse has detected another
+such nebula. In Cepheus, Triangulum, and Ursa Major, are found other
+spiral nebulæ of smaller size. Lord Rosse has recognized 40 spiral
+nebulæ and suspected a similar structure in 30 others.
+
+The class of the Irregular nebulæ, which will be now considered,
+differs greatly in character from the others, and includes the largest,
+the brightest and the most extraordinary nebulæ in the heavens. The
+nebulæ of this class differ from those belonging to the other classes
+by a want of symmetry in their form and in the distribution of their
+light, as well as by their capricious shapes, and their very complicated
+structure. Another and perhaps the principal difference between them and
+the objects above described, consists in the remarkable fact already
+stated, that they are not, except in rare cases, to be found in the
+regions where the other nebulæ abound. On the contrary, they are found
+in or very near the Milky-way, precisely where the other nebulæ are the
+most rare. This fact, recognized by Sir J. Herschel, led him to consider
+them as "outlying, very distant, and as it were detached fragments of
+the great stratum of the Galaxy." It seems very probable that the reason
+why these objects differ so greatly from the other nebulæ in size,
+brightness and complication of structure, is simply because they are
+much nearer to us than are most of the others. They are perhaps nebulous
+members of our Galaxy. The same remark which has been made of
+star-clusters may be applied to nebulæ. The nearer they are to us, the
+larger, the brighter and the more complicated they will appear, while
+the farther they are removed, the more simple and regular and round they
+will appear, only their brightest and deepest parts being then visible.
+
+The Crab Nebula of Lord Rosse, near _ζ_ Tauri, No. 1,157, is one of the
+interesting objects of this class. It has curious appendages streaming
+off from an oval, luminous mass, which give it a distant resemblance to
+the animal from which it derives its name. The Bifid nebula in Cygnus,
+Nos. 4,400 and 4,616, is another object of this class. It consists of a
+long, narrow, crooked streak, forking out at several places, and passing
+through _χ_ Cygni. Observers, having failed to recognize the connection
+existing between its different centres of brightness, have made distinct
+nebulæ of this extended object.
+
+The Dumb-bell nebula in Vulpecula, No. 4,532, is a bright and curious
+object, with a general resemblance to the instrument from which it
+derives its name. Lord Rosse's telescope has shown many stars in it,
+projected on a nebulous background, and Prof. Bond seems to have thought
+that it showed traces of resolvability, although in the study which I
+made of this nebula with the same instrument used by the latter
+observer, I failed to perceive any such traces. Dr. Huggins finds its
+spectrum gaseous.
+
+The star-cluster, No. 4,400, in Scutum Sobieskii, which is described by
+Sir J. Herschel as a loose cluster of at least 100 stars, I have found
+to be involved in an extensive, although not very bright, nebula, which
+would seem to have escaped his scrutiny. In a study and drawing of this
+nebula made in 1876, its general form is that of an open fan, with the
+exception that the handle is wanting, with deeply indented branches on
+the preceding side, where the brightest stars of the cluster are
+grouped. From its peculiar form, this object might appropriately be
+called the Fan nebula.
+
+The Omega or Horse-shoe nebula, in Sagittarius, No. 4,403, of which I
+have made a study and two drawings, one with a refractor 6⅓ inches in
+aperture, and the other conjointly with Prof. Holden, with the great
+telescope of the Naval Observatory, is a bright and very complicated
+object. Its general appearance in small instruments, with low power, is
+that of a long, narrow pisciform mass of light, from which proceeds on
+the preceding side, the great double loop from which it derives its
+name. But in the great Washington refractor its structure becomes very
+complicated, forming various bright nebulous masses and wisps of great
+extension. Prof. Holden, who has made a careful, comparative study of
+the published drawings of this object, thinks there are reasons to
+believe that its western branch has moved relatively to the stars found
+within its loop. The spectrum of this nebula is gaseous.
+
+The Trifid nebula, No. 4,355, in the same constellation, is also a very
+remarkable object, although it is not so bright as the last. This
+nebula, which I have studied with the refractor of the Cambridge
+Observatory, consists of four principal masses of light, separated by a
+wide and irregular gap branching out in several places. These masses,
+which are brighter along the dark gap, gradually fade away externally. A
+group of stars, two of which are quite bright, is found near the centre
+of the nebula, on the inner edge of the following mass, and close to the
+principal branch of the dark channel. A little to the north, and
+apparently forming a part of this nebula, is a globular-looking nebula,
+having a pale yellow star at its centre. Prof. Holden's studies on this
+nebula show that the triple star, which was centrally situated in the
+dark gap from 1784 to 1833, was found involved in the border of the
+nebulous mass following it, from 1839 to 1877; the change, he thinks,
+is attributable either to the proper motion of the group of stars or to
+that of the nebula itself.
+
+In the same vicinity is found the splendid and very extensive nebula No.
+4,361, in which is involved a loose, but very brilliant star-cluster.
+This nebula and cluster, which I have studied and drawn with a 6⅓ inch
+telescope, is very complicated in structure, and divided by a dark
+irregular gap into three principal masses of light, condensing at one
+point around a star, and at others forming long, bright, gently-curved
+branches, which give to this object a strong resemblance to the wings of
+a bird when extended upwards in the action of flying. From this
+peculiarity this object might appropriately be called the Winged nebula.
+Its spectrum is that of a gas.
+
+The variable star _η_ Argus is completely surrounded by the great
+nebula of the same name, No. 2,197, first delineated by Sir J. Herschel,
+during his residence at the Cape of Good Hope, in 1838. This object,
+which covers more than ⁴⁄₇ of a square degree, is divided into
+three unequal masses, separated by dark oval spots, comparatively free
+from nebulosity, and is suspected to have undergone changes since
+Herschel's time.
+
+In the same field with the double star, _ζ_ Orionis, the most easterly
+of the three bright stars in the belt of Orion, is found another
+irregular nebula of the Trifid type. From the drawings which I have made
+of this object, it appears to be composed of three principal unequal
+masses, separated by a wide, irregular, dark channel, two of the masses
+being quite complicated in structure, and forming curved, nebulous
+streams of considerable length and breadth. This nebula, like the next
+to be described, seems to be connected with the Galaxy by the great
+galactic loop described in another section.
+
+By far the most conspicuous irregular nebula visible from our northern
+States, is the great nebula in Orion, No. 1,179, represented on Plate
+XV. This object, visible to the naked eye, is the brightest and the most
+wonderful nebula in the heavens. It is situated a little to the south of
+the three bright stars in the belt of Orion, and may be readily detected
+surrounding the star _θ_, situated between and in a line with two faint
+stars, the three being in a straight line which points directly towards
+_ε_, the middle star of the three in Orion's belt. The area occupied by
+this nebula is about equal to that occupied by the Moon.
+
+In its brightest parts the nebula in Orion appears as a luminous cloud
+of a pentagonal form, from which issue many luminous appendages of
+various shapes and lengths. This principal mass is divided into
+secondary masses, separated by darkish, irregular intervals. These
+secondary masses in their turn appear mottled and fleecy. Towards the
+lower part of the pentagonal mass is found a roundish dark space,
+comparatively devoid of nebulosity, in which are involved four bright
+stars forming a trapezium, and several fainter ones. The four bright
+stars of the trapezium constitute the quadruple star _θ_ Orionis, from
+which the nebula has received its name. The cloud-like pentagonal form
+is brightest on the north-west of the trapezium, and is surrounded on
+three sides by long, soft, curved wisps, fading insensibly into the
+outer nebulous mass in which they are involved. On the east a broad,
+wavy wing spreads out, and sends an important branch southward.
+South-east of the trapezium are found several curious dark spaces,
+comparatively devoid of nebulosity, especially those on the east, which
+give to this nebula a singular character. Close to the north-eastern
+part of the nebula, or rather in contact with it, is found a small,
+curiously-shaped nebula, condensing around a bright star into a blazing
+nucleus. From this centre it continues northward in a narrow diffused
+stream, which spreads out in passing over the stars _c^1_ and _c^2_; and
+after having sent short branches northward, it curves back to the south
+and joins the main nebula on the west of its starting point, having thus
+formed a great loop which is not shown on the Plate. The nebula also
+forms a loop towards the south, which is partly shown on Plate XV., a
+small branch of which, passing through _τ_ Orionis, the nebulous star
+shown at the top of the Plate, and extending southward, is not here
+represented.
+
+On ordinary nights the nebula in Orion is a splendid object, and
+inspires the observer with amazement; but this is as nothing compared
+with the grand and magnificent sight which it presents during the very
+rare moments when our atmosphere is perfectly clear and steady. I have
+seen this nebula but once under these favorable circumstances, and I was
+surprised by the grandeur of the scene. Then could be detected features
+to be seen at no other time, and its fleecy, floculent, cloud-like
+masses glittered with such intensity that it seemed as if thousands of
+stars were going to blaze out the next moment. Although I observed the
+nebula under such favorable conditions, and with the fifteen-inch
+refractor of the Cambridge Observatory, yet I was disappointed in my
+expectations, and distinguished no new stars or points of light, and
+nothing more than a very bright mass, finely divided into minute blazing
+cloudlets. Although I failed to resolve this nebula into stars, yet Lord
+Rosse, Bond and Secchi thought they had caught glimpses of star dust.
+Its spectrum, however, proves to be mainly that of incandescent gases,
+probably hydrogen and nitrogen. In the curved wisps found in this
+nebula, Lord Rosse and others saw indications of a spiral structure.
+
+Several bright stars are found scattered over this nebula, and besides
+those forming the trapezium, there are three in a row, a little to the
+south-east of that group, which are quite bright and remarkable. Among
+the stars involved in this nebula, few show signs of having a physical
+connection with it, although it seems probable that the group of the
+trapezium is so connected. Some of these stars are variable. The small
+stars represented on this Plate, as on others of the series, are
+somewhat exaggerated in size, as was unavoidable with any process of
+reproduction which could be adopted.
+
+In 1811, W. Herschel was led to suspect that some changes had occurred
+in this nebula, but changes in such complicated and delicate objects are
+not easily ascertained, since, for the most part, we have for comparison
+with our later observations only coarse drawings made by hands unskilled
+in delineation.
+
+Although comparatively rare, double and multiple nebulæ may be found in
+the sky. When this occurs, their constituents most commonly belong to
+the class of spherical nebulæ. Sometimes the components are separated
+and distinct, at other times one of them is projected upon the other,
+either really or by the effect of perspective. Sometimes one is round
+and the other elongated. It is probable that while some of these nebulæ
+are physically associated and form a system, others appear to be so only
+because they happen to be almost in a line with the observer. A double
+nebula in Draco, Nos. 4,127 and 4,128, which I have drawn, is a fair
+type of those which are separated. The first is a globular nebula, and
+the last an oval one, with a star at its centre. The double nebula, Nos.
+858 and 859, in Taurus, which I have also studied, is a type of the
+cases in which one nebula is partly projected on another. In this
+instance both the nebulæ are globular.
+
+The nebulæ in general show very little color in their light, which is
+ordinarily whitish and pale. Some, however, present a decided bluish or
+greenish tint. The great nebula in Orion has a greenish cast, and we
+have seen that some planetary nebulæ are bluish.
+
+It has been a question whether nebulæ are changing. It has already been
+stated that Prof. Holden believes there is ground to suspect that the
+Trifid and Horse-shoe nebulæ have undergone some changes. A nebula near
+_ε_ Tauri has been lost and found again several times. Two other
+nebulæ in the same constellation have presented curious variations.
+One, near a star of the tenth magnitude, exhibited variations of
+brightness like those of the star itself, and for a time disappeared.
+The other, near _ζ_ Tauri, increased in brightness for three months,
+after which it disappeared. In 1859 Tempel discovered a nebula in the
+Pleiades, which has shown some fluctuations. In 1875 I made a long study
+of this object, and drew it carefully a dozen times, but I was not able
+to see any changes in it within the two or three months during which my
+observations were continued. But on Nov. 24, 1876, it was found of a
+different color, being purplish and very faint. On Dec. 23, 1880, it was
+found just as bright and visible as when I drew it in 1875, and on Oct.
+20, 1881, it appeared faint and purplish again, as in 1876. On this last
+night, and on those which followed it, it was impossible for me to trace
+the nebulosity as far as in 1875. I consider this as due to a variation
+in the light of this object, which in 1875 was bright enough to be well
+seen while the Moon after her First Quarter was within ten or fifteen
+degrees from the Pleiades.
+
+From the observations of M. Laugier, it appears that some nebulæ have a
+proper motion, comparable to that of stars. From the displacement of the
+lines of their spectra by their motion in the line of sight, Dr. Huggins
+found that no nebula observed by him has a proper motion surpassing 25
+miles per second. The Ring nebula in Lyra appears to move from us at the
+rate of 3 miles per second, and that in Orion recedes about 17 miles per
+second.
+
+The important question arises, are all the irresolvable nebulæ in the
+heavens to be considered as so many star-clusters, differing only from
+them by the minuteness of their components, or their immense distance
+from us; or are they cosmical clouds, composed of luminous vapors,
+similar to the matter composing the heads and tails of comets?
+Originally, W. Herschel, with many astronomers, thought that all these
+objects were stellar aggregations, too distant to be resolved into
+stars; but he subsequently modified his opinion, and accepted the idea
+that some of them are of a gaseous nature.
+
+No direct proof that the nebulæ are gaseous could be obtained, however,
+before the spectroscope was known. The attempt to analyze the light of
+the nebulæ with this instrument was made in 1864, by Dr. Huggins, who
+directed his spectroscope to the planetary nebula, No. 4,373, in Draco.
+Its spectrum was found to consist of three bright, distinct lines, the
+brightest of which corresponded with the strongest nitrogen line, and
+the feeblest with the hydrogen C line. Besides these lines, it gave also
+a very faint, continuous spectrum, apparently due to a central point of
+condensation. By this observation, the gaseous nature of a nebula was
+for the first time demonstrated. Dr. Huggins thus analyzed 70 nebulæ,
+of which one-third gave a gaseous spectrum, consisting of several bright
+lines, the brightest of which invariably corresponded with the lines of
+nitrogen. The others gave a continuous spectrum, with the red end
+usually deficient. These results indicate that if some of the so-called
+nebulæ are due to an aggregation of stars, either too minute or too
+remote in space to be individually resolved, others are in a gaseous
+state. Yet the faint, continuous spectrum, given by some nebulæ, in
+addition to their gaseous spectra, seems to show that these nebulæ have
+some stars or matter in a different state, either involved in them or
+projected on their surface.
+
+The idea of diffused matter distributed here and there in space, and
+gradually condensing into stars, is by no means new. As early as 1572,
+Tycho Brahé proposed such an hypothesis, to explain the sudden
+apparition of a new star in Cassiopeia, which he considered as formed by
+the recent agglomeration of the "celestial matter" diffused in space.
+Kepler adopted the same idea to explain the new star which appeared in
+Ophiuchus, in 1604. Halley, Lacaille, Mairan and others, entertained the
+same opinion. The hypothesis of a self-luminous, nebulous matter
+diffused in space, and forming here and there immense masses, has been
+proposed from the origin of the telescope, and was adopted by Sir
+William Herschel, who in his grand speculations on the universe
+considered the nebulæ as immense masses of phosphorescent vapors,
+gradually condensing around one or several centres into stars or
+clusters of stars. The evidence afforded by the spectroscope seems to be
+in favor of such an hypothesis, and shows us that gaseous agglomerations
+exist in space.
+
+According to our modern conception, the visible universe is but an
+infinitely small portion of the infinite universe perceived by our mind.
+The great blazing centre around which our little, non-luminous globe
+pursues its endless journey, is only an humble member of a cluster
+comprising four hundred equally powerful suns, as they are believed to
+be, although they appear to us as little twinkling stars. The nearest of
+these stars is 221,000 times as far from the Sun as the Sun is from the
+Earth, and yet this entire cluster is only one among the several hundred
+Star-clusters composing the great galactic nebula in which we are
+involved, comprising thirty or fifty millions of such suns. Among the
+4,000 irresolvable nebulæ in the sky, perhaps over one-half are
+supposed to be galaxies, like our own galaxy, composed of star-clusters,
+and millions of stars. Besides these remote galaxies, vast
+agglomerations of yet uncondensed, nebulous matter exist in space, and
+form the nebulæ proper, in which the genesis of suns is slowly
+elaborated. Although the visible universe is limited by the penetrating
+power of our instruments, yet we see in imagination the infinite
+universe stretching farther and farther; but we know not whether this
+invisible universe is totally devoid of matter, or whether it also is
+filled with millions and millions of suns and galaxies.
+
+
+
+
+APPENDIX
+
+KEY TO THE PLATES
+
+
+PLATE I.--GROUP OF SUN-SPOTS AND VEILED SPOTS.
+
+_Observed June_ 17, 1875, _at 7h. 30m. A. M._
+
+
+The background shows the sun's visible surface, or _photosphere_, as
+seen with a telescope of high power at the most favorable moments,
+composed of innumerable light markings, or granules, separated by a
+network of darker gray. The granules, each some hundreds of miles in
+width, are thought to be the flame-like summits of the radial filaments
+or columns of gas and vapor which compose the photospheric shell. The
+two principal sun-spots of the group here represented show the
+characteristic dark _umbra_ in the centre, overhung by the thatch-like
+_penumbra_, composed of whitish gray filaments. The penumbral filaments
+are not supposed to differ in their nature from those constituting the
+ordinary photosphere, save that they are seen here elongated and
+violently disturbed by the force of gaseous currents. Both spots are
+traversed partly or wholly by bright overlying _faculæ_, or so-called
+_luminous bridges_, depressed portions of which, in the left-hand spot,
+form the _gray and rosy veils_ commonly attendant upon this class of
+spots. In each of these spots, also, the inner ends of projecting
+penumbral filaments have fallen so far within the umbra as to appear
+much darker than the rest. At the right of the upper portion of the
+left-hand spot, is a mass of white facular clouds, honey-combed by dark
+spaces, through which are seen traces of the undeveloped third spot of
+the triple group first observed. If seen upon the sun's limb, this would
+have presented the appearance of a _lateral spot_. Above the right-hand
+spot is a small black "dot," or incipient spot, without distinct
+penumbra. The irregular dark rift below the two large spots and
+connecting them is a spot of the crevasse type, with very slight umbra,
+a still better example of which is seen in a westward
+prolongation of the penumbra of the left-hand spot. In the upper
+left-hand corner of the Plate are seen several small _faculæ_,
+appearing as irregular whitish streaks amongst the granules. In the
+pear-shaped darkening of the solar surface below and at their left, is
+seen a veiled spot, two of which attended this group.
+
+_Approximate scale, 2500 miles--1 inch._
+
+
+PLATE II.--SOLAR PROTUBERANCES.
+
+_Observed May_ 5, 1873, _at 9h. 40m. A. M._
+
+
+A view of an upheaval of the _chromosphere_, or third outlying envelope
+of the sun, as observed with the tele-spectroscope, or telescope with
+spectroscope attached.
+
+The _method of the observation_ requires a word of explanation. Save on
+the rare occasions of a total solar eclipse, no direct telescopic view
+of the solar prominences or flames is possible, owing to the fact that
+the intense white light from the sun's main disk entirely obscures
+the feeble pink light of the chromosphere. A few years ago Messrs.
+Jannsen and Lockyer found that a spectroscope of high dispersive power
+so weakens the spectrum of ordinary sun-light as to show the spectrum of
+bright lines given by the chromosphere, on any clear day. The telescope
+is adjusted so that a portion of the sun's limb, usually near a group of
+active sun-spots, shall be presented before the opened slit of the
+spectroscope. The light of the chromosphere thus admitted along with
+some diffused sun-light from the earth's atmosphere, produces a spectrum
+of intensely bright lines, widely separated, on the fainter background
+of the strongly dispersed spectrum of sun-light. The most prominent of
+these bright lines are those known as the C line (_scarlet_), F line
+(_blue_), which with several others are due to the hydrogen present in
+the chromosphere, the D{3} line (_orange_) ascribed to a little known
+substance called "helium", and occasionally the sodium lines D{1}, D{3},
+(_yellow_). By adjusting the slit upon the scarlet C line, the
+appearances represented in Plate II. were observed as through an
+atmosphere of scarlet light: in the D or F lines identical appearances
+may be seen, but somewhat less clearly defined, as through yellow or
+blue light respectively. Hence the solar times, as here observed with
+the spectroscope in the hydrogen C line, are seen through a portion only
+(the scarlet rays) of the light coming from but one substance (hydrogen)
+of the companion incandescent substances present in the chromosphere.
+The color of the collective chromospheric light is seen directly with
+the telescope during an eclipse (See Plate III.) to be a delicate rosy
+pink.
+
+
+_Description of the Plate._--The black background represents the general
+darkness of the eye-piece to the spectroscope. The broad red stripe
+stretching from top to bottom of the Plate is a portion of the red band
+of the spectrum, magnified about 100 times as compared with the actual
+spectroscopic view. The upper and lower edges of the cross-section of
+dusky red correspond with the edges of the slit, opened widely enough to
+admit a view of the chromospheric crest and of the whole height of the
+protuberances at once. With a narrower opening of the slit this
+background would have been nearly black, its reddish cast increasing
+with the amount of opening and consequent admission of diffused
+sun-light. Rising above the lower edge of the opening is seen a small
+outer segment of the chromosphere, which, as a portion of the sun's
+eastern limb, should be imagined as moving directly towards the
+beholder. The seams and rifts by which its surface is broken, as well as
+the distorted forms of the huge protuberances show the chromosphere to
+be in violent agitation. Some of the most characteristic shapes of the
+_eruptive protuberances_ are presented, as also _cloud-like_ forms
+overtopping the rest. In the immediate foreground the bases of two
+towering columns appear deeply depressed below the general horizon of
+the segment observed, showing an extraordinary velocity of motion of the
+whole uplifted mass toward the observer. The highest of these
+protuberances was 126,000 miles in height at the moment of observation.
+The triple protuberance at the left with two drooping wings and a tall
+swaying spire tipped with a very bright flame, shows by its more
+brilliant color the higher temperature (and possibly compression) to
+which its gases have been subjected. The irregular black bands behind
+this protuberance indicate the presence there of less condensed and
+cooler clouds of the same gases. The dimmer jets of dame rising from the
+chromosphere are either vanishing protuberances, or, as in the case of
+the smallest jet shown at the extreme right of the horizon, are the tops
+of protuberances just coming into view.
+
+_Approximate scale, 6000 miles--1 inch._
+
+
+PLATE III.--TOTAL ECLIPSE OF THE SUN.
+
+_Observed July_ 29, 1878, _at Creston, Wyoming Territory._
+
+
+A telescopic view of the sun's _corona_ or extreme outer atmosphere and
+of the _solar flames_ or _prominences_ during a total eclipse. At the
+moment of observation the dark disk of the moon, while still hiding the
+sun's main body, had passed far enough eastward to allow the rosy pink
+chromospheric prominences to be seen on its western border. On all sides
+of the sun's hidden disk, the _corona_ shows its pale greenish light
+extending in halo-like rays and streamers, and two very remarkable wings
+stretch eastward and westward very nearly in the plane of the ecliptic
+and in the direction of the positions of Mercury and Venus respectively
+at the time of observation. The full extent of these wings could not be
+shown in the Plate without reducing its scale materially, since the
+westerly wing extended no less than twelve times the sun's diameter, and
+the easterly wing nearly as far, or over ten million miles. A circlet of
+bright light immediately bordering the moon's disk is the so-called
+_inner corona_, next to which the wings and streamers arc brightest,
+thence shading off imperceptibly into the twilight sky of the eclipse.
+Other noteworthy peculiarities of the corona, as observed during this
+eclipse, are the varying angles at which the radiating streamers are
+seen to project, the comparatively dark intervals between them, and the
+curved, wisp-like projections seen upon the wings. An especially
+noticeable gap appears where the most westerly of the upward streamers
+abruptly cuts off the view of the long wing. The largest and brightest
+of the curving streamers on the westerly wing coincides with the highest
+flame-like protuberance. To some observers of this eclipse the upward
+and downward streamers seemed pointed at their outer extremities and
+less regular in form.
+
+_Approximate scale, 135,000 miles--1 inch._
+
+
+PLATE IV.--AURORA BOREALIS.
+
+_As observed March_ 1, 1872, _at 9h. 25m. P. M._
+
+
+The view presents the rare spectacle of an aurora spanning the sky from
+east to west in concentric arches. The Polar Star is nearly central in
+the background, the constellation of the Great Bear on the right and
+Cassiopeia's chair on the left. The large star at some distance above
+the horizon on the right is Arcturus. The almost black inner segment of
+the aurora resting upon the horizon, has its summit in the magnetic
+meridian, which was in this case a little west of north, its arc being
+indented by the bases of the ascending streamers. Both streamers and
+arches were, when observed, tremulous with upward pulsations and there
+was also a wave-like movement of the streamers from west to east. The
+prevailing color of this aurora is a pale whitish green and the
+complementary red appears especially at the west end of the auroral
+arch. The summits of the streamers are from four hundred to live hundred
+miles above the earth and the aurora is therefore a phenomenon of the
+terrestrial atmosphere rather than of astronomical observation proper.
+
+
+PLATE V.--THE ZODIACAL LIGHT.
+
+_Observed February_ 20, 1876.
+
+
+An observation of the cone of light whose axis lies along the Zodiac,
+whence it derives its name. It is drawn as seen in the west, with its
+base in the constellation Pisces, and its apex near the familiar group
+of the Pleiades in the constellation Taurus. The first bright star above
+the horizon in the base of the cone is the planet _Venus_ and at some
+distance above is the reddish disk of _Mars_, the two being in rare
+companionship as evening stars. Above the constellation Pisces, two
+bright stars of Aries lie just outside the cone at the right. The
+nearest bright star above these at the right is _Beta_, the leading star
+of the constellation Triangula. Further at the right the three prominent
+stars nearly in a line are, in ascending order, _Delta_, _Beta_ and
+_Gamma_ of the constellation Andromeda. Above these at the left, the
+brightest star of a quadrangular group of four is the remarkable
+variable star _Algol_ (_Beta_) of the constellation Perseus, which
+changes from the second to the fourth magnitude in a period of less than
+three days. At the left and a little above the Pleiades is the ruddy
+star _Aldebaran_, one of the Hyades and chief star in the constellation
+Taurus. These are the principal stars visible in this portion of the sky
+at the time of the observation. Their relative positions are represented
+as seen in the sky and not by the common method of star-atlases, which
+allows for the change from a spherical to a plane surface. Their
+magnitude in the order of brightness is indicated only approximately.
+
+
+PLATE VI.--MARE HUMORUM.
+
+_From a study made in 1875._
+
+
+A view of one of the lunar plains, or so-called _seas_ (_Maria_), with
+an encircling mountainous wall consisting of volcano-like craters in
+various stages of subsidence and dislocation. The sun-light coming from
+the west casts strong shadows from all the elevations eastward, and is
+just rising on the _terminator_, where the rugged structure of the
+Moon's surface is best seen. The lighter portions are the more elevated
+mountainous tracts and crater summits. The detailed description of this
+Plate given in the body of the Manual is repeated here for convenience
+of reference: The Mare Humorum, or sea of moisture, as it is called, is
+one of the smaller gray lunar plains. Its diameter, which is very nearly
+the same in all directions, is about 270 miles, the total area of this
+plain being about 50,000 square miles. It is one of the most distinct
+plains of the Moon, and is easily seen with the naked eye on the
+left-hand side of the disk. The floor of the plain is, like that of the
+other gray plains, traversed by several systems of very extended but low
+hills and ridges, while small craters are disseminated upon its surface.
+The color of this formation is of a dusky greenish gray along the
+border, while in the interior it is of a lighter shade, and is of
+brownish olivaceous tint. This plain, which is surrounded by high clefts
+and rifts, well illustrates the phenomena of dislocation and subsidence.
+The double-ringed crater Vitello, whose walls rise from 4,000 to 5,000
+feet in height, is seen in the upper left-hand corner of the gray plain.
+Close to Vitello at the east is the large broken ring-plain Lee, and
+farther east, and a little below, is a similarly broken crater called
+Doppelmayer. Both of these open craters have mountainous masses and
+peaks on their door, which is on a level with that of the Mare Humorum.
+A little below, and to the left of these objects, is dimly seen a deeply
+imbedded oval crater, whose walls barely rise above the level of the
+plain. On the right-hand side of the great plain is a long _fault_, with
+a system of fracture running along its border. On this right-hand side
+may be seen a part of the line of the terminator, which separates the
+light from the darkness. Towards the lower right-hand corner is the
+great ring-plain Gassendi, 55 miles in diameter, with its system of
+fractures and its central mountains, which rise from 3,000 to 4,000 feet
+above its floor. This crater slopes towards the plain, showing the
+subsidence to which it has been submitted. While the northern portion of
+the wall of this crater rises to 10,000 feet, that on the plain is only
+500 feet high, and is even wholly demolished at one place where the
+floor of the crater is in direct communication with the plain. In the
+lower part of the sea, and a little to the west of the middle line, is
+found the crater Agatharchides, which shows below its north wall the
+marks of rills impressed by a flood of lava, which once issued from the
+side of the crater. On the left-hand side of the plain is seen the
+half-demolished crater Hippalus resembling a large bay, which has its
+interior strewn with peaks and mountains. On this same side can be seen
+one of the most important systems of clefts and fractures visible on the
+Moon, these clefts varying in length from 150 to 200 miles.
+
+_Approximate scale, 15 miles--1 inch._
+
+
+PLATE VII.--PARTIAL ECLIPSE OF THE MOON.
+
+_Observed October_ 24, 1874.
+
+
+A view of the Moon partially obscured by the Earth's shadow, whose
+outline gives ocular proof of the earth's rotundity of form. The
+shadowed part of the Moon's surface is rendered visible by the diffused
+sun-light refracted upon it from the earth's atmosphere. Its reddish
+brown color is due to the absorption, by vapors present near the earth's
+surface, of a considerable part of this dim light. On both the obscured
+and illuminated tracts the configurations of the Moon's surface are seen
+as with the naked eye. The craters appear as distinct patches of lighter
+color, and the noticeably darker areas are the depressed plains or
+_Maria_. The large crater _Tycho_, at the lower part of the disk, is the
+most prominent of these objects, with its extensive system of _radiating
+streaks_. The largest crater above is _Copernicus_, at the left of which
+is _Kepler_ and still above are _Aristarchus_ and _Herodotus_ appearing
+as if blended in one. Above and at the left of the great crater _Tycho_,
+the first dark tract is the _Mare Humorum_ of Plate VI., seen in its
+natural position, with the crater _Gassendi_ at its northern (upper)
+extremity and _Vitello_ on its southern (lower) border. The advancing
+border of the shadow appears, as always, noticeably darker than the
+remainder, an effect probably of contrast. The illuminated segment of
+the Moon's disk has its usual appearance, the lighter portions being the
+more elevated mountainous surfaces, and the dark spaces the floors of
+extensive plains.
+
+_Approximate scale, 140 miles--1 inch._
+
+
+PLATE VIII.--THE PLANET MARS.
+
+_Observed September_ 3, 1877, _at 11h. 55m. P. M._
+
+A view of the southern hemisphere of Mars, when in the most favorable
+position for observation, and when exceptionally free from the clouds,
+which very frequently hide its surface configurations. Since, of all the
+planets, Mars is most like the earth, Plate VIII. may give a fair idea
+of the appearance of our globe to a supposed observer on Mars. The dark
+gray and black markings, are regarded as tracts of water, or of some
+liquid with similar powers of absorbing light; and, for the same reason,
+the lighter portions, of a prevailing reddish tint, are supposed to be
+bodies of land, while the bright white portions are variously due to
+clouds, to polar snow or ice, and the bright rim of white along the
+limb, to the depth of the atmosphere through which the limb is seen. The
+chief permanent features of the planet's surface have been named in
+honor of various astronomers.
+
+The large dark tract on the left is _De La Rue Ocean_, the isolated oval
+spot near the centre is _Terby Sea_, and on the right is the western end
+of _Maraldi Sea_, with strongly indented border. Directly north of
+(below) De La Rue Ocean, is _Maedler Continent_; above it stretches
+_Jacob Land_; and surrounding Terby Sea is _Secchi Continent_. Extending
+into the centre of De La Rue Ocean is a curious double peninsula,
+called, in consequence of the dimness of former observations, _Hall
+Island_. The sharply defined, white-crested northern borders of De La
+Rue Ocean and Maraldi Sea may indicate the existence there of lofty
+coast ranges, more or less constantly covered with opaque clouds
+strongly reflecting light. The white spot in the centre of Maedler
+Continent, of a temporary nature, has a similar explanation. The
+intervals of olivaceous gray on Secchi Continent and elsewhere may
+perhaps be ascribed to the flooding and drying up of marshes and
+lowlands, as these markings have been observed to vary somewhat in
+connection with the change of seasons on Mars. The greenish tints
+observed along the planet's limb, alike on the darker and lighter
+surfaces, are probably due to an optical effect, the green being
+complementary to the prevailing red of the disk. The brilliant oval
+white spot near the southern (upper) pole of the planet is a so-called
+polar spot, in all probability consisting of a material similar to snow
+or ice and here observed in the midst of a dark open sea.
+
+_Approximate scale, 300 miles--1 inch._
+
+
+PLATE IX.--THE PLANET JUPITER.
+
+_Observed November_ 1, 1880, _at 9h. 30m. P. M._
+
+This planet is perpetually wrapped in dense clouds which hide its inner
+globe from view. The drawing shows Jupiter's outer clouded surface with
+its usual series of alternate light and dark belts, the disk as a whole
+appearing brighter in the centre than near the limb. The darker gray and
+black markings indicate in general the lower cloud-levels; that is,
+partial breaks or rifts in the cloudy envelope, whose prevailing depth
+apparently exceeds four thousand miles. While the deepest depression in
+the cloudy envelope is within the limits of the Great Red Spot, the
+vision may not even here penetrate very deeply. Two of Jupiter's four
+moons present bright disks near the planet's western limb, and cast
+their shadows far eastward on the disk, that of the "second satellite"
+falling upon the Red Spot. On the Red Spot are seen in addition two
+small black spots, no explanation of which can yet be offered. The broad
+white ring of clouds bordering the Red Spot appeared in constant motion.
+The central, or equatorial belt, shows brilliant cloudy masses of both
+the _cumulus_ and _stratus_ types, and the underlying gray and black
+cloudy surfaces are pervaded with the pinkish color characteristic of
+this belt. The dark circular spots on the wide white belt next north
+showed in their mode of formation striking resemblances to sun-spots.
+They afterward coalesced into a continuous pink belt. The diffusion of
+pinkish color over the three northernmost dark bands, as here observed,
+is unusual. About either pole is seen the uniform gray segment or polar
+cap. The equatorial diameter is noticeably longer than the polar
+diameter, a consequence of the planet's extraordinary swiftness of
+rotation. To the same cause may also be due chiefly the distribution of
+the cloudy belts parallel to the planet's equator, though the analogy of
+the terrestrial trade-winds fails to explain all the observed phenomena.
+
+_Approximate scale, 5,500 miles--1 inch._
+
+
+PLATE X.--THE PLANET SATURN.
+
+_Observed November_ 30, 1874, _at 5h. 50m. P. M._
+
+
+Saturn is unique amongst the planets in that its globe is encircled by a
+series of concentric rings, which lie in the plane of its equator, and
+consist, according to present theories, of vast throngs of minute bodies
+revolving about the planet, like so many satellites, in closely parallel
+orbits. The globe of Saturn, like that of Jupiter, is surrounded by
+cloudy belts parallel to its equator. The broad equatorial belt, of a
+delicate pinkish tint, is both brighter and more mottled than the
+narrower yellowish white belts, which alternate with darker belts of
+ashy gray on both the north and south sides, but are seen here only on
+the northern side. The disk has an oval shape, owing to the extreme
+polar compression of the globe.
+
+The outer, middle and inner rings, with their various subdivisions, are
+clearly shown in Plate X., and are best seen on the so-called _ansæ_,
+or handles, projecting on either side. The gray _outer ring_ is
+separated by the dusky pencil line into two divisions, both of which
+appear slightly mottled on the ansæ, as if with clouds. The _middle
+ring_ has three sub-divisions which are clearly distinguishable,
+although separated by no dark interval, viz., a brilliant white outer
+zone, distinctly mottled, as seen on the extremities of the ansæ, and
+two interior zones of gradually diminishing brightness. The _gauze_ or
+_dusky ring_ is seen at its full width on the ansæ, but on the
+background of the strongly illuminated globe only its outer and
+presumably denser border is visible. The shadow of the globe on the
+rings is seen on the lower portion of the eastern ansæ. The shadow on
+the dusky ring is with difficulty perceptible; the shadow on the middle
+ring is slightly concave toward the planet, which concavity is abruptly
+increased on the outer zone of this ring; while the shadow on the outer
+ring slants away from the globe. These appearances are fully accounted
+for by supposing a general increase of level from the inner edge of the
+dusky ring to the outer margin of the middle ring, and a uniform lower
+level on the outer ring. Other observers have regarded the deflection of
+the shadow as an effect of irradiation. The inner margin of the double
+outer ring presents on both ansæ a number of slight indentations,
+which, if not actual irregularities in the contour of this ring, may be
+explained as shadows caused by elevations on the outer border of the
+middle ring, or possibly by over-hanging clouds.
+
+_Approximate scale, 6,500 miles--1 inch._
+
+
+PLATE XI.--THE GREAT COMET OF 1881.
+
+_Observed on the night of June_ 25-26, _at 1h. 30m. A. M._
+
+
+A view of the comet 1881, III., drawn as if seen with the naked eye, the
+minute details, however, being reproduced as seen with the telescope.
+The star-like _nucleus_ is attended by four conical wings which cause it
+to appear diamond-shaped. The _coma_ appears double, the brilliant inner
+coma, immediately enveloping the nucleus, being surrounded by a fainter
+exterior coma, which has a noticeable depression corresponding to that
+of the inner edge of the principal coma. The tail is divided lengthwise
+by a dark rift and is brightest on its convex or forward side. An inner
+portion of the tail, brighter than the rest, is more strongly curved, as
+if by solar repulsion. Stars are seen through the brighter parts of the
+tail, as they may be seen even through the coma and nucleus, with little
+diminution of their light.
+
+
+PLATE XII.--THE NOVEMBER METEORS.
+
+_As observed on the night of November_ 13-14, 1868.
+
+
+A partially ideal view of the November Meteors, combining forms observed
+at different times during the night of Nov. 13th, 1868. It is not,
+however, a fanciful view, since a much larger number of meteors were
+observed falling at once during the shower of November, 1833, and at
+other times. The locality of the observation is shown by the Polar Star
+seen near the centre of the Plate, and Cassiopeia's Chair at the left.
+The general direction of the paths of the meteors is from the
+north-east, the _radiant point_ of the shower having been in the
+constellation Leo, beyond and above Ursa Major. While the orbits of the
+meteors are, in general, curved regularly and slightly, several are
+shown with very eccentric paths, among them one which changed its course
+at a sharp angle. In the upper left-hand corner appear two vanishing
+trails of the "ring-form," and several others still further transformed
+into faint luminous patches of cloud. Red, yellow, green, blue and
+purple tints were observed in the meteors and their trails, as
+represented in the Plate.
+
+
+PLATE XIII.--PART OF THE MILKY WAY.
+
+_From a study made during the years_ 1874, 1875 _and_ 1876.
+
+
+The course of the portion of the Galaxy represented in Plate XIII. is as
+follows: From Cassiopeia's chair, three bright stars of which appear at
+the upper edge of the Plate, the Galaxy, forming two streams, descends
+south, passing partly through Lacerta on the left, and Cepheus on the
+right; at this last point it approaches nearest to the Polar Star, which
+is itself outside of the field of view. Then it enters Cygnus, where it
+becomes very complicated and bright, and where several large cloudy
+masses are seen terminating its left branch, which passes to the right,
+near the bright star _Deneb_, the leader of this constellation. Below
+_Deneb_, the Galaxy is apparently disconnected and separated from the
+northern part by a narrow, irregular dark gap. From this rupture, the
+Milky-way divides into two great streams, separated by an irregular dark
+rift. An immense branch extends to the right, which, after having formed
+an important luminous mass between the stars _Gamma_ and _Beta_,
+continues its southward progress through parts of Lyra, Vulpecula,
+Hercules, Aquila and Ophiuchus, where it gradually terminates a few
+degrees south of the equator. The main stream on the left, after having
+formed a bright mass around _Epsilon Cygni_, passes through Vulpecula
+and then Aquila, where it crosses the equinoctial just below the star
+_Eta_, after having involved in its nebulosity the bright star _Altair_,
+the leader of Aquila. In the southern hemisphere the Galaxy becomes very
+complicated and forms a succession of very bright, irregular masses, the
+upper one being in Scutum Sobieskii, while the others are respectively
+situated in Sagittarius and in Scorpio; the last, just a little above
+our horizon, being always considerably dimmed by vapors. From Scutum
+Sobieskii, the Galaxy expands considerably on the right, and sends a
+branch into Scorpio, in which the fiery red star _Antares_ is somewhat
+involved. In the upper left-hand corner of the Plate, at some distance
+from the Milky-way, is seen dimly the Nebula in Andromeda, which becomes
+so magnificent an object to telescopic view.
+
+
+PLATE XIV.--STAR-CLUSTER IN HERCULES.
+
+_From a study made in June_, 1877.
+
+
+In the constellation Hercules, a small nebulous mass is faintly visible
+to the eye, a telescopic view of which is presented in Plate XIV. It is
+one of the most beautiful of the easily resolvable globular clusters.
+The brilliancy of the centre gives the cluster a distinctly globular
+appearance, while the several wings curving in various directions, have
+suggested to some observers an irregularly spiral structure. The large
+stars of the cluster are arranged in several groups which correspond, in
+a general way, with the faintly luminous wings.
+
+
+PLATE XV.--THE GREAT NEBULA IN ORION.
+
+_From a study made in the years_ 1875-76.
+
+
+This nebula, which is one of the most brilliant and wonderful of
+telescopic objects, readily visible to the naked eve as a patch of
+nebulous light immediately surrounding the middle star of the three
+which form the sword of Orion, and a little south of the three
+well-known stars forming the belt. The small stars in this, as in other
+Plates of the series, are somewhat exaggerated in size, as was
+unavoidable with any mode of reproduction that could be employed. The
+bright pentagonal centre of the nebula is traversed by less luminous
+rifts, the several subdivisions thus outlined being irregularly mottled
+as if by bright fleecy clouds. Toward the lower part of this bright
+pentagonal centre is a comparatively dark space containing four bright
+stars which form a trapezium and together constitute the quadruple star
+_Theta Orionis_, which, to the naked eye, appears as the single star in
+the centre of the sword. On three sides of the central mass extend long
+bright wisps, whose curves fail, however, to reveal the spiral structure
+often attributed to this nebula. On the east a broad wing, with
+wave-shaped inner border, stretches southward. East of the trapezium are
+two especially noticeable dark spaces. Close to the main nebula on the
+north-east, a small faint nebula surrounds a bright star, and a branch
+from another faint stream of nebulous matter forming a loop to the
+southward, encloses the nebulous star (_Iota Orionis_) shown at the top
+of the Plate.
+
+*** END OF THE PROJECT GUTENBERG EBOOK THE TROUVELOT ASTRONOMICAL
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+      manual by Étienne L. Trouvelot.
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+<p style='text-align:center; font-size:1.2em; font-weight:bold'>The Project Gutenberg eBook of The Trouvelot astronomical drawings manual, by Étienne Léopold Trouvelot</p>
+<div style='display:block; margin:1em 0'>
+This eBook is for the use of anyone anywhere in the United States and
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+
+<p style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Title: The Trouvelot astronomical drawings manual</p>
+<p style='display:block; margin-top:1em; margin-bottom:0; margin-left:2em; text-indent:-2em'>Author: Étienne Léopold Trouvelot</p>
+<p style='display:block; text-indent:0; margin:1em 0'>Release Date: June 24, 2022 [eBook #68394]</p>
+<p style='display:block; text-indent:0; margin:1em 0'>Language: English</p>
+  <p style='display:block; margin-top:1em; margin-bottom:0; margin-left:2em; text-indent:-2em; text-align:left'>Produced by: Laura Natal Rodrigues (Images generously made available by Hathi Trust Digital Library and Wikipedia Commons.)</p>
+<div style='margin-top:2em; margin-bottom:4em'>*** START OF THE PROJECT GUTENBERG EBOOK THE TROUVELOT ASTRONOMICAL DRAWINGS MANUAL ***</div>
+
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/trouvelot_cover.jpg" width="500" alt="" />
+</div>
+
+<p><br /><br /><br /></p>
+
+<h1>THE<br />
+TROUVELOT<br />
+ASTRONOMICAL DRAWINGS<br />
+MANUAL</h1>
+
+<p><br /><br /></p>
+
+<h4>BY</h4>
+
+<h2>E. L. TROUVELOT,</h2>
+
+<p><br /><br /></p>
+
+<h3>FORMERLY CONNECTED WITH THE OBSERVATORY OF HARVARD COLLEGE; FELLOW OF THE<br />
+AMERICAN ACADEMY OF ARTS AND SCIENCES, AND MEMBER OF THE SELENO-GRAPHICAL<br />
+SOCIETY OF GREAT BRITAIN; IN CHARGE OF A<br />
+GOVERNMENT EXPEDITION TO OBSERVE THE<br />
+TOTAL SOLAR ECLIPSE OF 1878.</h3>
+
+<p><br /><br /></p>
+
+<h4>NEW YORK</h4>
+
+<h4>CHARLES SCRIBNER'S SONS</h4>
+
+<h5>1882</h5>
+
+<p><br /><br /><br /></p>
+
+<h4><a id="INTRODUCTION">INTRODUCTION</a></h4>
+
+<p>
+During a study of the heavens, which has now been continued for more
+than fifteen years, I have made a large number of observations
+pertaining to physical astronomy, together with many original drawings
+representing the most interesting celestial objects and phenomena.
+</p>
+<p>
+With a view to making these observations more generally useful, I was
+led, some years ago, to prepare, from this collection of drawings, a
+series of astronomical pictures, which were intended to represent the
+celestial phenomena as they appear to a trained eye and to an
+experienced draughtsman through the great modern telescopes, provided
+with the most delicate instrumental appliances. Over two years were
+spent in the preparation of this series, which consisted of a number of
+large drawings executed in pastel. In 1876, these drawings were
+displayed at the United States Centennial Exhibition at Philadelphia,
+forming a part of the Massachusetts exhibit, in the Department of
+Education and Science.
+</p>
+<p>
+The drawings forming the present series comprise only a part of those
+exhibited at Philadelphia; but, although fewer in number, they are quite
+sufficient to illustrate the principal classes of celestial objects and
+phenomena.
+</p>
+<p>
+While my aim in this work has been to combine scrupulous fidelity and
+accuracy in the details, I have also endeavored to preserve the natural
+elegance and the delicate outlines peculiar to the objects depicted; but
+in this, only a little more than a suggestion is possible, since no
+human skill can reproduce upon paper the majestic beauty and radiance of
+the celestial objects.
+</p>
+<p>
+The plates were prepared under my supervision, from the original pastel
+drawings, and great care has been taken to make the reproduction exact.
+</p>
+<p>
+The instruments employed in the observations, and in the delineation of
+the heavenly bodies represented in the series, have varied in aperture
+from 6 to 26 inches, according to circumstances, and to the nature of
+the object to be studied. The great Washington refractor, kindly placed
+at my disposal by the late Admiral C. H. Davis, has contributed to this
+work, as has also the 26 inch telescope of the University of Virginia,
+while in the hands of its celebrated constructors, Alvan Clark &amp; Sons.
+The spectroscope used was made by Alvan Clark &amp; Sons. Attached to it is
+an excellent diffraction grating, by Mr. L. M. Rutherfurd, to whose
+kindness I am indebted for it.
+</p>
+<p>
+Those unacquainted with the use of optical instruments generally suppose
+that all astronomical drawings are obtained by the photographic process,
+and are, therefore, comparatively easy to procure; but this is not true.
+Although photography renders valuable assistance to the astronomer in
+the case of the Sun and Moon, as proved by the fine photographs of these
+objects taken by M. Janssen and Mr. Rutherfurd; yet, for other subjects,
+its products are in general so blurred and indistinct that no details of
+any great value can be secured. A well-trained eye alone is capable of
+seizing the delicate details of structure and of configuration of the
+heavenly bodies, which are liable to be affected, and even rendered
+invisible, by the slightest changes in our atmosphere.
+</p>
+<p>
+The method employed to secure correctness in the proportions of the
+original drawings is simple, but well adapted to the purpose in view. It
+consists in placing a fine reticule, cut on glass, at the common focus
+of the objective and the eye-piece, so that in viewing an object, its
+telescopic image, appearing projected on the reticule, can be drawn very
+accurately on a sheet of paper ruled with corresponding squares. For a
+series of such reticules I am indebted to the kindness of Professor
+William A. Rogers, of the Harvard College Observatory.
+</p>
+<p>
+The drawings representing telescopic views are inverted, as they appear
+in a refracting telescope&mdash;the South being upward, the North downward,
+the East on the right, and the West on the left. The Comet, the
+Milky-Way, the Eclipse of the Moon, the Aurora Borealis, the Zodiacal
+Light and the Meteors are represented as seen directly in the sky with
+the naked eye. The Comet was, however, drawn with the aid of the
+telescope, without which the delicate structure shown in the drawing
+would not have been visible.
+</p>
+<p>
+The plate representing the November Meteors, or so-called "Leonids," may
+be called an ideal view, since the shooting stars delineated, were not
+observed at the same moment of time, but during the same night. Over
+three thousand Meteors were observed between midnight and five o'clock
+in the morning of the day on which this shower occurred; a dozen being
+sometimes in sight at the same instant. The paths of the Meteors,
+whether curved, wavy, or crooked, and also their delicate colors, are in
+all cases depicted as they were actually observed.
+</p>
+<p>
+In the Manual, I have endeavored to present a general outline of what is
+known, or supposed, on the different subjects and phenomena illustrated
+in the series. The statements made are derived either from the best
+authorities on physical astronomy, or from my original observations,
+which are, for the most part, yet unpublished.
+</p>
+<p>
+The figures in the Manual relating to distance, size, volume, mass,
+etc., are not intended to be strictly exact, being only round numbers,
+which can, therefore, be more easily remembered.
+</p>
+<p>
+It gives me pleasure to acknowledge that the experience acquired in
+making the astronomical drawings published in Volume VIII. of the Annals
+of the Harvard College Observatory, while I was connected with that
+institution, has been of considerable assistance to me in preparing this
+work; although no drawings made while I was so connected have been used
+for this series.
+</p>
+
+<p style="margin-left: 60%;">E. L. TROUVELOT.</p>
+
+<p style="margin-left: 5%;"><i>Cambridge, March, 1882.</i></p>
+
+<p><br /><br /><br /></p>
+
+<h4>CONTENTS</h4>
+
+<p class="nind"><a href="#INTRODUCTION">INTRODUCTION</a><br />
+
+<a href="#LIST_OF_PLATES">LIST OF PLATES</a></p>
+
+<h4><i>THE SUN</i></h4>
+
+<p class="nind"><a href="#SUN">GENERAL REMARKS ON THE SUN</a><br />
+
+<a href="#SUN_SPOTS">SUN-SPOTS AND VEILED SPOTS</a><br />
+
+<a href="#SOLAR_PROTUBERANCES">SOLAR PROTUBERANCES</a><br />
+
+<a href="#TOTAL_ECLIPSE">TOTAL ECLIPSE OF THE SUN</a></p>
+
+<h4><i>THE AURORAL AND ZODIACAL LIGHTS</i></h4>
+
+<p class="nind"><a href="#THE_AURORA_BOREALIS">THE AURORA BOREALIS</a><br />
+
+<a href="#THE_ZODIACAL_LIGHT">THE ZODIACAL LIGHT</a></p>
+
+<h4><i>THE MOON</i></h4>
+
+<p class="nind"><a href="#THE_MOON">THE MOON</a><br />
+
+<a href="#ECLIPSES">ECLIPSES OF THE MOON</a></p>
+
+<h4><i>THE PLANETS</i></h4>
+
+<p class="nind"><a href="#THE_PLANET_MARS">THE PLANET MARS</a><br />
+
+<a href="#THE_PLANET_JUPITER">THE PLANET JUPITER</a><br />
+
+<a href="#THE_PLANET_SATURN">THE PLANET SATURN</a></p>
+
+<h4><i>COMETS AND METEORS</i></h4>
+
+<p class="nind"><a href="#COMETS">COMETS</a><br />
+
+<a href="#SHOOTING_STARS">SHOOTING-STARS AND METEORS</a></p>
+
+<h4><i>THE STELLAR SYSTEMS</i></h4>
+
+<p class="nind"><a href="#THE_MILKY_WAY">THE MILKY-WAY OR GALAXY</a><br />
+
+<a href="#THE_STAR_CLUSTERS">THE STAR-CLUSTERS</a><br />
+
+<a href="#THE_NEBULAE">THE NEBULÆ</a><br />
+
+<a href="#APPENDIX">APPENDIX</a></p>
+
+<p><br /><br /><br /></p>
+
+<h4><a id="LIST_OF_PLATES">LIST OF PLATES</a><a name="FNanchor_1_1" id="FNanchor_1_1"></a><a href="#Footnote_1_1" class="fnanchor">[1]</a></h4>
+
+<h4>PLATE</h4>
+
+<p class="nind"><a href="#figure01">I. GROUP OF SUN-SPOTS AND VEILED SPOTS.</a><br />
+
+<span style="margin-left: 1.5em;"><i>Observed June 17, 1875, at 7 h. 30m. A. M.</i></span><br />
+
+<a href="#figure02">II. SOLAR PROTUBERANCES.</a><br />
+
+<span style="margin-left: 1.5em;"><i>Observed May 5, 1873, at 9h. 40m. A. M.</i></span><br />
+
+<a href="#figure03">III. TOTAL ECLIPSE OF THE SUN.</a><br />
+
+<span style="margin-left: 1.5em;"><i>Observed July 29, 1878, at Creston, Wyoming Territory.</i></span><br />
+
+<a href="#figure04">IV. AURORA BOREALIS.</a><br />
+
+<span style="margin-left: 1.5em;"><i>As observed March 1, 1872, at 9h. 25m. P. M.</i></span><br />
+
+<a href="#figure05">V. THE ZODIACAL LIGHT.</a><br />
+
+<span style="margin-left: 1.5em;"><i>Observed February 20, 1876.</i></span><br />
+
+<a href="#figure06">VI. MARE HUMORUM.</a><br />
+
+<span style="margin-left: 1.5em;"><i>From a study made in 1875.</i></span><br />
+
+<a href="#figure07">VII. PARTIAL ECLIPSE OF THE MOON.</a><br />
+
+<span style="margin-left: 1.5em;"><i>Observed October 24, 1874.</i></span><br />
+
+<a href="#figure08">VIII. THE PLANET MARS.</a><br />
+
+<span style="margin-left: 1.5em;"><i>Observed September 3, 1877, at 11h. 55m. P. M.</i></span><br />
+
+<a href="#figure09">IX. THE PLANET JUPITER.</a><br />
+
+<span style="margin-left: 1.5em;"><i>Observed November 1, 1880, at 9h. 30m. P. M.</i></span><br />
+
+<a href="#figure10">X. THE PLANET SATURN.</a><br />
+
+<span style="margin-left: 1.5em;"><i>Observed November 30, 1874, at 5th. 50m. P. M.</i></span><br />
+
+<a href="#figure11">XI. THE GREAT COMET OF 1881.</a><br />
+
+<span style="margin-left: 1.5em;"><i>Observed on the night of June 25-26, at 1h. 30m. A. M.</i></span><br />
+
+<a href="#figure12">XII. THE NOVEMBER METEORS.</a><br />
+
+<span style="margin-left: 1.5em;"><i>As observed between midnight and 3 o'clock A. M., on the night<br />
+of November 13-14, 1868.</i></span><br />
+
+<a href="#figure13">XIII. PART OF THE MILKY-WAY.</a><br />
+
+<span style="margin-left: 1.5em;"><i>From a study made during the years 1874, 1875 and 1876.</i></span><br />
+
+<a href="#figure14">XIV. STAR-CLUSTER IN HERCULES.</a><br />
+
+<span style="margin-left: 1.5em;"><i>From a study made in June, 1877.</i></span><br />
+
+<a href="#figure15">XV. THE GREAT NEBULA IN ORION.</a><br />
+
+<span style="margin-left: 1.5em;"><i>From a study made in the years 1873-76.</i></span></p>
+
+<p><br /></p>
+
+<div class="footnote">
+
+<p class="nind"><a name="Footnote_1_1" id="Footnote_1_1"></a><a href="#FNanchor_1_1"><span class="label">[1]</span></a>For Key to the Plates, see Appendix.</p></div>
+
+<p><br /></p>
+
+<h5>
+<i>Reproduced from the Original Drawings, by Armstrong &amp; Company,<br />
+Riverside Press, Cambridge, Mass.</i>
+</h5>
+
+<p><br /><br /><br /></p>
+
+<h4><a id="SUN">GENERAL REMARKS ON THE SUN</a></h4>
+
+<p>
+The Sun, the centre of the system which bears its name, is a
+self-luminous sphere, constantly radiating heat and light.
+</p>
+<p>
+Its apparent diameter, as seen at its mean from the Earth, subtends an
+angle of 32', or a little over half a degree. A dime, placed about six
+feet from the eye, would appear of the same proportions, and cover the
+Sun's disk, if projected upon it.
+</p>
+<p>
+That the diameter of the Sun does not appear larger, is due to the great
+distance which separates us from that body. Its distance from the Earth
+is no less than 92,000,000 miles. To bridge this immense gap, would
+require 11,623 globes like the Earth, placed side by side, like beads on
+a string.
+</p>
+<p>
+The Sun is an enormous sphere whose diameter is over 108 times the
+diameter of our globe, or very nearly 860,000 miles. Its radius is
+nearly double the distance from the Earth to the Moon. If we suppose,
+for a moment, the Sun to be hollow, and our globe to be placed at the
+centre of this immense spherical shell, not only could our satellite
+revolve around us at its mean distance of 238,800 miles, as now, but
+another satellite, placed 190,000 miles farther than the Moon, could
+freely revolve likewise, without ever coming in contact with the solar
+envelope.
+</p>
+<p>
+The circumference of this immense sphere measures 2,800,000 miles. While
+a steamer, going at the rate of 300 miles a day, would circumnavigate
+the Earth in 83 days, it would take, at the same rate, nearly 25 years
+to travel around the Sun.
+</p>
+<p>
+The surface of the Sun is nearly 12,000 times the surface of the Earth,
+and its volume is equal to 1,300,000 globes like our own. If all the
+known planets and satellites were united in a single mass, 600 such
+compound masses would be needed to equal the volume of our luminary.
+</p>
+<p>
+Although the density of the Sun is only one-quarter that of the Earth,
+yet the bulk of this body is so enormous that, to counterpoise it, no
+less than 314,760 globes like our Earth would be required.
+</p>
+<p>
+The Sun uniformly revolves around its axis in about 25½ days. Its
+equator is inclined 7° 15' to the plane of the ecliptic, the axis of
+rotation forming, therefore, an angle of 82° 45' with the same plane.
+As the Earth revolves about the Sun in the same direction as that of the
+Sun's rotation, the apparent time of this rotation, as seen by a
+terrestrial spectator, is prolonged from 25½ days to about 27 days and
+7 hours.
+</p>
+<p>
+The rotation of the Sun on its axis, like that of the Earth and the
+other planets, is direct, or accomplished from West to East. To an
+observer on the Earth, looking directly at the Sun, the rotation of this
+body is from left to right, or from East to West.
+</p>
+<p>
+The general appearance of the Sun is that of an intensely luminous disk,
+whose limb, or border, is sharply defined on the heavens. When its
+telescopic image is projected on a screen, or fixed on paper by
+photography, it is noticed that its disk is not uniformly bright
+throughout, but is notably more luminous in its central parts. This
+phenomenon is not accidental, but permanent, and is due in reality to a
+very rare but extensive atmosphere which surrounds the Sun, and absorbs
+the light which that body radiates, proportionally to its thickness,
+which, of course, increases towards the limb, to an observer on the
+Earth.
+</p>
+
+<p><br /><br /></p>
+
+<h4>THE ENVELOPING LAYERS OF THE SUN</h4>
+
+<p>
+The luminous surface of the Sun, or that part visible at all times, and
+which forms its disk, is called the <i>Photosphere</i>, from the property
+it is supposed to possess of generating light. The photosphere does not
+extend to a great depth below the luminous surface, but forms a
+comparatively thin shell, 3,000 or 4,000 miles thick, which is distinct
+from the interior parts, above which it seems to be kept in suspense by
+internal forces. From the observations of some astronomers it would
+appear that the diameter of the photosphere is subject to slight
+variations, and, therefore, that the solar diameter is not a constant
+quantity. From the nature of this envelope, such a result does not seem
+at all impossible, but rather probable.
+</p>
+<p>
+Immediately above the photosphere lies a comparatively thin stratum,
+less than a thousand miles in thickness, called the <i>Reversing Layer</i>.
+This stratum is composed of metallic vapors, which, by absorbing the
+light of particular refrangibilities emanating from the photosphere
+below, produces the dark Fraunhofer lines of the solar spectrum.
+</p>
+<p>
+Above the reversing layer, and resting immediately upon it, is a
+shallow, semi-transparent gaseous layer, which has been called the
+<i>Chromosphere</i>, from the fine tints which it exhibits during total
+eclipses of the Sun, in contrast with the colorless white light radiated
+by the photosphere below. Although visible to a certain extent on the
+disk, the chromosphere is totally invisible on the limb, except with the
+spectroscope, and during eclipses, on account of the nature of its
+light, which is mainly monochromatic, and too feeble, compared with that
+emitted by the photosphere, to be seen.
+</p>
+<p>
+The chromospheric layer, which has a thickness of from 3,000 to 4,000
+miles, is uneven, and is usually upheaved in certain regions, its matter
+being transported to considerable elevations above its general surface,
+apparently by some internal forces. The portions of the chromosphere
+thus lifted up, form curious and complicated figures, which are known
+under the names of <i>Solar Protuberances, or Solar Flames</i>.
+</p>
+<p>
+Above the chromosphere, and rising to an immense but unknown height, is
+the solar atmosphere proper, which is only visible during total eclipses
+of the Sun, and which then surrounds the dark body of the Moon with the
+beautiful rays and glorious nimbus, called the <i>Corona</i>.
+</p>
+<p>
+These four envelopes: the photosphere, the reversing layer, the
+chromosphere, and the corona, constitute the outer portions of our
+luminary.
+</p>
+<p>
+Below the photosphere little can be seen, although it is known, as will
+appear below, that at certain depths cloud-like forms exist, and freely
+float in an interior atmosphere of invisible gases. Beyond this all is
+mystery, and belongs to the domain of hypothesis.
+</p>
+
+<p><br /><br /></p>
+
+<h4>STRUCTURE OF THE PHOTOSPHERE AND CHROMOSPHERE</h4>
+
+<p>
+The apparent uniformity of the solar surface disappears when it is
+examined with a telescope of sufficient aperture and magnifying powers.
+Seen under good atmospheric conditions, the greater part of the solar
+surface appears mottled with an infinite number of small, bright
+granules, irregularly distributed, and separated from each other by a
+gray-tinted background.
+</p>
+<p>
+These objects are known under different names. The terms granules and
+granulations answer very well for the purpose, as they do not imply
+anything positive as to their form and true nature. They have also been
+called <i>Luculœ</i>, <i>Rice Grains</i>, <i>Willow Leaves</i>, etc., by
+different observers.
+</p>
+<p>
+Although having different shapes, the granulations partake more or less
+of the circular or slightly elongated form. Their diameter, which varies
+considerably, has been estimated at from 0".5 to 3", or from 224 to
+1,344 miles. The granulations which attain the largest size appear,
+under good atmospheric conditions, to be composed of several granules,
+closely united and forming an irregular mass, from which short
+appendages protrude in various directions.
+</p>
+<p>
+The number of granulations on the surface of the Sun varies considerably
+under the action of unknown causes. Sometimes they are small and very
+numerous, while at other times they are larger, less numerous, and more
+widely separated. Other things being equal, the granulations are better
+seen in the central regions of the Sun than they are near the limb.
+</p>
+<p>
+Usually the granulations are very unstable; their relative position,
+form, and size undergoing continual changes. Sometimes they are seen to
+congregate or to disperse in an instant, as if acting under the
+influence of attractive and repulsive forces; assembling in groups or
+files, and oftentimes forming capricious figures which are very
+remarkable, but usually of short duration. In an area of great solar
+disturbances, the granulations are often stretched to great distances,
+and form into parallel lines, either straight, wavy, or curved, and they
+have then some resemblance to the flowing of viscous liquids.
+</p>
+<p>
+The granulations are usually terminated either by rounded or sharply
+pointed summits, but they do not all rise to the same height, as can be
+ascertained with the spectroscope when they are seen sidewise on the
+limb. In the regions where they are most abundant, they usually attain
+greater elevations, and when observed on the limb with the spectroscope,
+they appear as slender acute flames.
+</p>
+<p>
+The granulations terminated by sharply-pointed crests, although observed
+in all latitudes, seem to be characteristic of certain regions. A daily
+study of the chromosphere, extending over a period of ten years, has
+shown me that the polar regions are rarely ever free from these objects,
+which are less frequent in other parts of the Sun. In the polar regions
+they are sometimes so abundant that they completely form the solar limb.
+These forms of granulation are comparatively rare in the equatorial
+zones, and when seen there, they never have the permanency which they
+exhibit in the polar regions. When observed in the equatorial regions,
+they usually appear in small groups, in the vicinity of sun spots, or
+they are at least enclosed in areas of disturbances where such spots are
+in process of formation. In these regions they often attain greater
+elevations than those seen in high latitudes.
+</p>
+<p>
+As we are certain that in the equatorial zones these slender flames (<i>i.
+e.</i>, granulations) are a sure sign of local disturbance, it may be
+reasonably supposed that the same kind of energy producing them nearly
+always prevails in the polar regions, although it is there much weaker,
+and never reaches beyond certain narrow limits.
+</p>
+<p>
+Studied with the spectroscope, the granulations are found to be composed
+in the main of incandescent hydrogen gas, and of an unknown substance
+provisionally called "helium." Among the most brilliant of them are
+found traces of incandescent metallic vapors, belonging to various
+substances found on our globe.
+</p>
+<p>
+The chromosphere is not fixed, but varies considerably in thickness in
+its different parts, from day to day. Its thickness is usually greater
+in the polar regions, where it sometimes exceeds 6,700 miles. In the
+equatorial regions the chromosphere very rarely attains this height, and
+when it does, the rising is local and occupies only a small area. In
+these regions it is sometimes so shallow that its depth is only a few
+seconds, and is then quite difficult to measure. These numbers give, of
+course, the extreme limits of the variations of the chromosphere; but,
+nearly always, it is more shallow in the equatorial regions; and, as far
+as my observations go, the difference in thickness between the polar and
+equatorial zones is greater in years of calm than it is in years of
+great solar activity. But ten years of observation are not sufficient to
+warrant any definite conclusions on this subject.
+</p>
+<p>
+There is undoubtedly some relation between the greater thickness of the
+chromosphere in the polar regions, and the abundance and permanence of
+the sharply-pointed granulations observed in the same regions. This
+becomes more evident when we know that the appearance of
+similarly-pointed flames in the equatorial zones is always accompanied
+with a local thickening of the chromosphere. The thickening in the polar
+regions may be only apparent, and not due to a greater accumulation of
+chromospheric gases there; but may be caused by some kind of repulsive
+action or polarity, which lifts up and extends the summit of the
+granulations in a manner similar to the well-known mode of electric
+repulsion and polarity.
+</p>
+<p>
+As it seems very probable that the heat and light emanating from the Sun
+are mainly generated at the base of the granulations, in the filamentary
+elements composing the chromosphere and photosphere, it would follow
+that, as the size and number of these objects constantly vary, the
+amount of heat and light emitted by the Sun should also vary in the same
+proportion.
+</p>
+<p>
+The granulations of the solar surface are represented on Plate I., and
+form the general background to the group of Sun-spots forming the
+picture.
+</p>
+
+<p><br /><br /></p>
+
+<h4>THE FACULÆ</h4>
+
+<p>
+Although the solar surface is mainly covered with the luminous
+granulations and the grayish background above described, it is very rare
+that its appearance is so simple and uniform as already represented. For
+the most part, on the contrary, it is diversified by larger, brighter,
+and more complicated forms, which are especially visible towards the
+border of the Sun. Owing to their extraordinary brilliancy, these
+objects have been called <i>Faculœ</i> (torches).
+</p>
+<p>
+Although the faculæ are very seldom seen well beyond 50 heliocentric
+degrees from the limb, yet they exist, and are as numerous in the
+central parts of the disk as they are towards the border; since they
+form a part of the solar surface, and participate in its movement of
+rotation. Their appearance near the limb has been attributed to the
+effect of absorption produced by the solar atmosphere on the
+light from the photosphere; but this explanation seems inadequate,
+and does not solve the problem. The well-known fact that the solar
+protuberances&mdash;which are in a great measure identical with the
+faculæ&mdash;are much brighter at the base than they are at the summit,
+perhaps gives a clue to the explanation of the phenomenon; especially
+since we know that, in general, the summit of the protuberances is
+considerably broader than their base. When these objects are observed in
+the vicinity of the limb, they present their brightest parts to the
+observer, since, in this position, they are seen more or less sidewise;
+and, therefore, they appear bright and distinct. But as the faculæ
+recede from the limb, their sides, being seen under a constantly
+decreasing angle, appear more and more foreshortened; and, therefore,
+these objects grow less bright and less distinct, until they finally
+become invisible, when their bases are covered over by the broad, dusky
+summit generally terminating the protuberances.
+</p>
+<p>
+The faculæ appear as bright and luminous masses or streaks on the
+granular surface of the Sun, but they differ considerably in form and
+size. Two types at least are distinguishable. In their simplest form
+they appear either as isolated white spots, or as groups of such spots
+covering large surfaces, and somewhat resembling large flakes of snow.
+In their most characteristic types they appear as intensely luminous,
+heavy masses, from which, in most cases, issue intricate ramifications,
+sometimes extending to great distances. Generally, the ramifications
+issuing from the masses of faculæ have their largest branches directed
+in the main towards the eastern limb of the Sun. Some of these branches
+have gigantic proportions. Occasionally they extend over 60° and even
+80° of the solar surface, and, therefore, attain a length of from
+450,000 to 600,000 miles.
+</p>
+<p>
+Although the faculæ may be said to be seen everywhere on the surface of
+the Sun, there is a vast difference in different regions, with regard to
+their size, number, and brilliancy. They are largest, most abundant, and
+brightest on two intermediate zones parallel to the solar equator, and
+extending 35° or 40° to the north and to the south of this line. The
+breadth of these zones varies considerably with the activity of the
+solar forces. When they are most active, the faculæ spread on either
+side, but especially towards the equator, where they sometimes nearly
+meet those of the other zone. In years of little solar activity the
+belts formed by the faculæ are very narrow&mdash;the elements composing
+them being very few and small, although they never entirely disappear.
+</p>
+<p>
+The faculæ are very unstable, and are constantly changing: those of the
+small types sometimes form and vanish in a few minutes. When an area of
+disturbance of the solar surface is observed for some time, all seems in
+confusion; the movements of the granulations become unusually violent;
+they congregate in all sorts of ways, and thus frequently form temporary
+faculæ. Action of this kind is, for the most part, peculiar to the
+polar regions of the Sun.
+</p>
+<p>
+The larger faculæ have undoubtedly another origin, as they seem to be
+mainly formed by the ejection of incandescent gases and metallic vapors
+from the interior of the photosphere. In their process of development
+some of the heavy masses of faculæ are swollen up to great heights,
+being torn in all sorts of ways, showing large rents and fissures
+through which the sight can penetrate.
+</p>
+<p>
+Very few faculæ are represented in Plate I.; several streaks are shown
+at the upper left-hand corner, some appearing as whitish ramifications
+among the granulations representing the general solar surface.
+</p>
+
+<p><br /><br /><br /></p>
+
+<h4><a id="SUN_SPOTS">SUN-SPOTS AND VEILED SPOTS</a>
+<br /><br />
+PLATE I</h4>
+
+<p>
+Besides the brilliant faculæ already described, much more conspicuous
+markings, though of a totally different character, are very frequently
+observed on the Sun. On account of their darkish appearance, which is in
+strong contrast with the white envelope of our luminary, these markings
+were called <i>Maculæ</i>, or Sun-spots, by their earlier observers.
+</p>
+<p>
+The Sun-spots are not equally distributed on the solar surface; but like
+the faculæ, to which they are closely related, they occupy two
+zones&mdash;one on each side of the equator. These zones are comprised
+between 10° and 35° of north latitude, and 10° and 35° of south
+latitude. Between these two zones is a belt 20° in width, where the
+Sun-spots are rarely seen.
+</p>
+<p>
+Above the latitudes 35° north and south, the Sun-spots are rare, and it
+is only occasionally, and during years of great solar activity, that
+they appear in these regions; in only a few cases have spots of
+considerable size been seen there. A few observers, however, have seen
+spots as far as 40° and 50 from the equator; and La Hire even observed
+one in 70° of north latitude; but these cases are exceedingly rare. It
+is not uncommon, however, to see very small spots, or groups of such
+spots, within 8° or 10° from the poles.
+</p>
+<p>
+The activity of the Sun is subject to considerable fluctuation, and
+accordingly the Sun-spots vary in size and number in different years.
+During some years they are large, complicated, and very numerous; while
+in others they are small and scarce, and are sometimes totally absent
+for weeks and months together. The fluctuations in the frequency of
+Sun-spots are supposed to be periodical in their character, although
+their periods do not always appear to recur at exactly regular
+intervals. Sometimes the period is found to be only nine years, while at
+other times it extends to twelve years. The period generally adopted now
+is 11⅒ years, nearly; but further investigations are needed to
+understand the true nature of the phenomenon.
+</p>
+<p>
+The number of Sun-spots does not symmetrically augment and diminish, but
+the increase is more rapid than the diminution.
+</p>
+<p>
+The period of increase is only about four years, while that of decrease
+is over seven years; each period of Sun-spot maximum being nearer the
+preceding period of Sun-spot minimum than it is to that next following.
+</p>
+<p>
+The cause of these fluctuations in the solar energy is at present wholly
+unknown. Some astronomers, however, have attributed it to the influence
+of the planets Venus and Jupiter, the period of revolution of the latter
+planet being not much longer than the Sun-spot period; but this
+supposition lacks confirmation from direct observations, which, so far,
+do not seem to be in favor of the hypothesis. At the present time the
+solar activity is on the increase, and the Sun-spots will probably reach
+their maximum in 1883. The last minimum occurred in 1879, when only
+sixteen small groups of spots were observed during the whole year.
+</p>
+<p>
+Sun-spots vary in size and appearance; but, unless they are very small,
+in which case they appear as simple black dots, they generally consist
+of two distinct and well-characterized parts, nearly always present.
+There is first, a central part, much darker than the other, and sharply
+divided from it, called the "<i>Umbra</i>;" second, a broad, irregular
+radiated fringe of lighter shade, completely surrounding the first, and
+called the "<i>Penumbra</i>."
+</p>
+<p>
+Reduced to its simplest expression, a Sun-spot is a funnel-shaped
+opening through the chromosphere and the photosphere. The inner end of
+the funnel, or opening, gives the form to the umbra, while its sloping
+sides form the penumbra.
+</p>
+<p>
+The umbra of Sun-spots, whose outlines approximately follow the
+irregularities of the penumbral fringe, has a diameter which generally
+exceeds the width of the penumbral ring. Sometimes it appears uniformly
+black throughout; but it is only so by contrast, as is proved when
+either Mercury or Venus passes near a spot during a transit over the
+Sun's disk. The umbra then appears grayish, when compared with the
+jet-black disk of the planet.
+</p>
+<p>
+The umbra of spots is rarely so simple as just described; but it is
+frequently occupied, either partly or wholly, by grayish and rosy forms,
+somewhat resembling loosely-entangled muscular fibres. These forms have
+been called the <i>Gray and Rosy Veils</i>. Frequently these veils appear
+as if perforated by roundish black holes, improperly called <i>Nuclei</i>,
+which permit the sight to penetrate deeper into the interior. To all
+appearance the gray and rosy veils are of the same nature as the
+chromosphere and the faculæ, and are therefore mainly composed of
+hydrogen gas.
+</p>
+<p>
+Whatever can be known about the interior of the Sun, must be learned
+from the observations of these openings, which are comparatively small.
+But whatever this interior may be, we certainly know that it is not
+homogeneous. Apparently, the Sun is a gigantic bubble, limited by a very
+thin shell. Below this shell exists a large open space filled with
+invisible gases, in which, through the openings constituting the
+Sun-spots, the gray and rosy veils described above are occasionally seen
+floating.
+</p>
+<p>
+The fringe forming the penumbra of spots is much more complicated than
+the umbra. In its simpler form, it is composed of a multitude of bright,
+independent filaments of different forms and sizes, partly projecting
+one above the other, on the sloping wall of the penumbra, from which
+they seem to proceed. Seen from the Earth, these filaments have somewhat
+the appearance of thatched straw, converging towards the centre of the
+umbra. It is very rare, however, that the convergence of the penumbral
+filaments is regular, and great confusion sometimes arises from the
+entanglement of these filaments. Some of these elements appear straight,
+others are curved or loop-shaped; while still others, much larger and
+brighter than the rest, give a final touch to this chaos of filaments,
+from which results the general thatched and radiating appearance of the
+penumbra.
+</p>
+<p>
+The extremities of the penumbral filaments, especially of those forming
+the border of the umbra, are usually club-shaped and appear very
+brilliant, as if these elements had been superheated by some forces
+escaping through the opening of the spots.
+</p>
+<p>
+Besides these characteristics, the Sun-spots have others, which,
+although not always present, properly belong to them. Comparatively few
+spots are so simple as the form just described. Very frequently a spot
+is accompanied by brilliant faculæ, covering part of its umbra and
+penumbra, and appearing to form a part of the spot itself.
+</p>
+<p>
+When seen projected over Sun-spots, the faculæ appear intensely bright,
+and from these peculiarities they have been called <i>Luminous Bridges</i>.
+They are, in fact, bridges, but in most cases they are at considerable
+heights above the spots, kept there by invisible forces. When such spots
+with luminous bridges approach the Sun's limb, it is easy to see, by the
+rapid apparent displacement which they undergo, that they are above the
+general level.
+</p>
+<p>
+When the spots are closing up, the inverse effect is sometimes observed.
+On several occasions, I have seen huge masses of faculæ advance slowly
+over the penumbra of a spot and fall into the depths of the umbra,
+resembling gigantic cataracts. I have seen narrow branches of faculæ,
+which, after having fallen to great depths in the umbra, floated across
+it and disappeared under the photosphere on the opposite side. I have
+also seen luminous bridges, resembling cables, tightly stretched across
+the spots, slackening slowly, as if loosened at one end, and gently
+curving into the umbra, where they formed immense loops, large enough to
+receive our globe.
+</p>
+<p>
+It is to be remarked that, in descending under the photospheric shell,
+the bright faculæ and the luminous bridges gradually lose their
+brilliancy. At first they appear grayish, but in descending farther they
+assume more and more the pink color peculiar to the rosy veils. The
+pinkish color acquired by the faculæ when they reach a certain depth
+under the photosphere, is precisely the color of the chromosphere and of
+the solar protuberances, as seen during total eclipses of the Sun&mdash;a
+fact which furnishes another proof that the faculæ are of the same
+nature as the protuberances.
+</p>
+<p>
+I record here an observation which, at first sight, may appear
+paradoxical; but which seems, however, to be of considerable importance,
+as it shows unmistakably that the solar light is mainly, if not
+entirely, generated on its surface, or at least very near to it. On May
+26, 1878, I observed a large group of Sun-spots at a little distance
+from the east limb of the Sun. The spot nearest to the limb was partly
+covered over on its eastern and western sides by bright and massive
+faculæ which concealed about two-thirds of the whole spot, only a
+narrow opening, running from north to south, being left across the
+middle of the spot. Owing to the rotundity of the Sun, the penumbra of
+this spot, although partly covered by the faculæ, could, however, be
+seen on its eastern side, since the sight of the observer could there
+penetrate sidewise under the faculæ. Upon that part of the penumbra
+appeared a strong shadow, representing perfectly the outline of the
+facular mass situated above it. The phenomenon was so apparent that no
+error of observation was possible, and a good drawing of it was secured.
+If this faculæ had been as bright beneath as it was above, it is
+evident that no shadow could have been produced; hence the light of
+these faculæ must have been mainly generated on or very near their
+exterior surfaces. This, with the well-proved fact that the bright
+faculæ lose their light in falling into the interior of the Sun, seems
+to suggest the idea that the bright light emitted by the faculæ, and
+very probably all the solar light, can be generated only on its surface;
+the presence of the coronal atmosphere being perhaps necessary to
+produce it. Several times before this observation, I had suspected that
+some faculæ were casting a shadow, but as this seemed so improbable, my
+attention was not awakened until the phenomenon became so prominent that
+it could not escape notice.
+</p>
+<p>
+With due attention, some glimpses of the phenomenon can frequently be
+observed through the openings of some of the faculæ projecting over the
+penumbra of Sun-spots. It is very seldom that the structure of the
+penumbra is seen through such openings, which usually appear as dark as
+the umbra of the large spots, although they do not penetrate through the
+photosphere like the latter. It is only when the rents in the faculæ
+are numerous and quite large, that the penumbral structure is recognized
+through them. Since these superficial rents in the faculæ do not extend
+through the photosphere, and appear black, it seems evident that the
+penumbra seen through them cannot be as bright as it is when no faculæ
+are projected upon it, and therefore that the faculæ intercept light
+from the exterior surface, which would otherwise reach the penumbra.
+</p>
+<p>
+While the matter forming the faculæ sometimes falls into the interior
+of the Sun, the same kind of matter is frequently ejected in enormous
+quantities, and with great force, from the interior, through the visible
+and invisible openings of the photosphere, and form the protuberances
+described in the following section of this manual (Solar protuberances)
+It is not only the incandescent hydrogen gas or the metallic vapors
+which are thus ejected, but also cooler hydrogen gas, which sometimes
+appears as dark clouds on the solar surface. On December 12, 1875, I
+observed such a cloud of hydrogen issuing from the corner of a small
+Sun-spot. It traveled several thousand miles on the solar surface, in a
+north-easterly direction, before it became invisible.
+</p>
+<p>
+Solar spots are formed in various ways; but, for the most part, the
+apparition of a spot is announced beforehand, by a great commotion of
+the solar surface at the place of its appearance, and by the formation
+of large and bright masses of faculæ, which are usually swollen into
+enormous bubbles by the pressure of the internal gases. These bubbles
+become visible in the spectroscope while they are traversing the solar
+limb, as they are then presented to us sidewise. Under the action of the
+increasing pressure, the base of the faculæ is considerably stretched,
+and, its weakest side finally giving way, the facular mass is torn in
+many places from the solar surface, and is perforated by holes of
+different sizes and forms. The holes thus made along the border of the
+faculæ appear as small black spots, separated more or less by the
+remaining portion of the lacerated faculæ, and they enlarge more and
+more at the expense of the intervening portions, which thus become very
+narrow. This perforated side of the faculæ, offering less resistance,
+is gradually lifted up, as would be the cover of a box, for example,
+while its opposite side remains attached to the surface. The facular
+matter separating the small black holes is greatly stretched during this
+action, and forms long columns and filaments. These appear as luminous
+bridges upon the large and perfectly-formed spot, which is then seen
+under the lifted facular masses. The spots thus made visible are soon
+freed from the facular masses, which are gradually shifted towards the
+opposite side.
+</p>
+<p>
+In such cases the spots are undoubtedly formed under the faculæ before
+they can be seen. This becomes evident when such spots, not yet cleared
+from the faculæ covering them, are observed near the east limb; since
+in this position the observer can see through the side-openings of the
+faculæ, and sometimes recognize the spots under their cover.
+</p>
+<p>
+It frequently occurs that the spots thus formed under the faculæ
+continue to be partly covered by the facular clouds, the forces at work
+in them being apparently too feeble to shift them aside. In such cases
+these spots are visible when they are in the vicinity of the limb, where
+they are seen sidewise; but when observed in the east, in being carried
+forward by the solar rotation towards the centre of the disk, they
+gradually diminish in size, and finally become invisible. The
+disappearance of these spots, however, is only apparent, being due to
+the fact that, as they advance towards the centre of the disk, our
+lateral view of them is gradually lost, and they are finally hidden from
+sight by the overhanging faculæ which then serve as a screen between
+the observer and the spot. This class of spots may be called <i>Lateral
+Spots</i>, from the fact that they can only be seen laterally, and near the
+Sun's limb.
+</p>
+<p>
+Solar spots are also formed in various other ways. Some, like those
+represented in Plate I., appearing without being announced by any
+apparent disturbance of the surface, or by the formation of any faculæ,
+form and develop in a very short time. Others, appearing at first as
+very small spots having an umbra and a penumbra, slowly and gradually
+develop into very large spots. This mode of formation, which would seem
+to be the most natural, is, however, quite rare. Spots of this class
+have a duration and permanence not observed in those of any other type.
+These spots of slow and regular development are never accompanied by
+faculæ or luminous bridges, nor have they any gray or rosy veils in
+their interior; a fact which may, perhaps, account for their permanent
+character.
+</p>
+<p>
+Another class of spots, which is also rare, appear as long and narrow
+crevasses showing the penumbral structure of the ordinary spots; but
+these rarely have any umbra. These long, and sometimes exceedingly
+narrow fissures of the solar envelopes, with their radiated penumbral
+structure, strongly suggest the idea that the photosphere is composed of
+a multitude of filamentary elements having the granulations for summits.
+Such a crevasse is represented on Plate I., and unites the two spots
+which form the group.
+</p>
+<p>
+The duration of Sun-spots varies greatly. Some last only for a few
+hours; while others continue for weeks and even months at a time, but
+not without undergoing changes.
+</p>
+<p>
+The modes of disappearance of Sun-spots are as various as those of their
+apparition. The spots rarely close up by a gradual diminution or
+contraction of their umbra and penumbra. This mode of disappearance
+belongs exclusively to the spots deprived of faculæ and veils. One of
+the most common modes of the disappearance of a spot is its invasion by
+large facular masses, which slowly advance upon its penumbra and umbra
+and finally cover it entirely. It is a process precisely the reverse of
+that in which spots are formed by the shifting aside of the faculæ, as
+above described. In other types, the spots close up by the gradual
+enlargement of the luminous bridges traversing them, which are slowly
+transformed into branches of the photosphere, all of the characteristics
+of which they have acquired. In many cases, the spots covered over by
+the faculæ continue to exist for some time, hidden under these masses,
+as is often proved, either by the appearance of small spots on the
+facular mass left at the place they occupied, or even by the
+reappearance of the same spot.
+</p>
+<p>
+Apart from the general movement of rotation of the solar surface, some
+of the spots seem to be endowed with a proper motion of their own, which
+becomes greater the nearer the spots are to the solar equator. According
+to the observations of Mr. Carrington, the period of rotation of the
+Sun, as deduced from the observations of the solar spots during a period
+of seven years, is 25 days at the equator; while at 50° of heliocentric
+latitude it is 27 days. But the period of rotation, as derived from the
+observations of spots occupying the same latitude, is far from being
+constant, as it varies at different times, with the frequency of the
+spots and with the solar activity, so that at present the law of these
+variations is not well known. From the character of the solar envelope,
+it seems very natural that the rotation should differ in the different
+zones and at different times, since this envelope is not rigid, but very
+movable, and governed by forces which are themselves very variable.
+</p>
+<p>
+Although it is a general law that the spots near the equator have a more
+rapid motion than those situated in higher latitudes, yet, in many
+cases, the proper motion of the spots is more apparent than real. For
+the most part, the changes of form and the rapid displacements observed
+in some spots are only apparent, and due to the fact that the large
+masses of faculæ which are kept in suspense above them are very
+unstable, and change position with the slightest change in the forces
+holding them in suspension. Since in these cases we view the spots
+through the openings of the faculæ situated above them, the slightest
+motion of these objects produces an apparent motion in the spots,
+although they have remained motionless. Accordingly, it has been
+remarked that of all the spots, those which have the greater proper
+motion are precisely those which have the most faculæ and luminous
+bridges; while the other spots in the same regions, but not attended by
+similar phenomena, are comparatively steady in their movement. These
+last spots are undoubtedly better adapted than any others to exhibit the
+rotation of the Sun; but it is probable that this period of rotation
+will never be known with accuracy, simply because the solar surface is
+unstable, and does not rotate uniformly.
+</p>
+<p>
+The Sun-spots have a remarkable tendency to form into groups of various
+sizes, but whatever may be the number of spots thus assembled, the group
+is nearly always composed of two principal spots, to which the others
+are only accessories. The tendency of the Sun-spots to assemble in pairs
+is general, and is observed in all latitudes, even among the minute
+temporary groups formed in the polar regions. Whenever several are
+situated quite close together, those belonging to the same group can be
+easily recognized by this character. Whatever may be the position of the
+axis of the two principal spots of a group when it is first formed, this
+axis has a decided tendency to place itself parallel to the solar
+equator, no matter to what latitude the group belongs; and if it is
+disturbed from this position, it soon returns to it when the disturbance
+has ceased.
+</p>
+<p>
+It is also remarkable that the spots observed at the same time remain in
+nearly the same parallel of latitude for a greater or less period of
+time; but they keep changing their position from year to year, their
+latitude decreasing with the activity of the solar forces.
+</p>
+<p>
+Among the Sun-spots, those associated with faculæ form the groups which
+attain the largest proportions. When such groups acquire an apparent
+diameter of 1' or more, they are plainly visible to the naked eye, since
+for a spot to be visible to the naked eye on the Sun, it need only
+subtend an angle of 50". I have sometimes seen such groups through a
+smoky atmosphere, when the solar light was so much reduced that the disk
+could be observed directly and without injury to the sight.
+</p>
+<p>
+The largest spot which ever came under my observation was seen during
+the period from the 13th to the 19th of November, 1870. This spot, which
+was on the northern hemisphere of the Sun, was conspicuous among the
+smaller spots constituting the group to which it belonged, and followed
+them on the east. On November 16th, when it attained its largest size,
+the diameter of its penumbra occupied fully one-fifth of the diameter of
+the Sun; its real diameter being, therefore, not less than 172,000
+miles, or nearly 22 times the diameter of the Earth. As the umbra of
+this spot occupied a little more than one-third of its whole diameter,
+seven globes like our own, placed side by side on a straight line, could
+easily have passed through this immense gap. To fill the area of this
+opening, about 45 such globes would have been needed. This spot was, of
+course, very easily seen with the naked eye, its diameter being almost
+eight times that required for a spot to be visible without a telescope.
+</p>
+<p>
+Ancient historians often speak of obscurations of the Sun, and it has
+been supposed by some astronomers that this phenomenon might have been
+due in some cases to the apparition of large spots. A few spots on the
+surface of the Sun, like that just described, would sensibly reduce its
+light.
+</p>
+<p>
+Besides the ordinary Sun-spots already described, others are at times
+observed on the surface of the Sun, which show some of the same
+characteristics, but never attain so large proportions. They always
+appear as if seen through a fog, or veil, between the granulations of
+the solar surface. On account of their vagueness and ill-defined contours,
+I have proposed for these objects the term, "<i>Veiled Spots</i>."
+Veiled spots have a shorter duration than the ordinary spots, the
+smaller types sometimes forming and vanishing in a few minutes. Some of
+the larger veiled spots, however, remain visible for several days in
+succession, and show the characteristics of other spots in regard to the
+arrangement of their parts.
+</p>
+<p>
+The veiled spots have no umbra or penumbra, although they are usually
+accompanied by faculæ resembling those seen near the ordinary spots.
+They are frequently seen in the polar regions, but are there always of
+small size and of short duration. The veiled spots are larger, and more
+apt to arrange themselves into groups, in the regions occupied by the
+ordinary spots, and it is not rare to observe such spots transform
+themselves into ordinary spots, and vice versa. The veiled spots,
+therefore, seem to be ordinary spots filled up, or covered over by the
+granulations and semi-transparent gases composing the chromospheric
+layer. That it is so, becomes more evident, from the fact that large
+Sun-spots in process of diminution are sometimes gradually covered with
+faint and scattered granules which descend in long, narrow filaments,
+and become less and less distinguishable as they attain greater depth.
+This phenomenon, associated with the fact that the luminous bridges seen
+over the Sun-spots which are closing up are sometimes transformed into
+branches which show the characteristic structure of the photosphere,
+goes far to prove that the solar envelopes are mainly composed of an
+innumerable quantity of radial filaments of varying height.
+</p>
+
+<p><br /></p>
+
+<div class="figcenter" style="width: 400px;">
+<a id="figure01"></a>
+<br />
+<img src="images/figure01.jpg" width="400" alt="" />
+<div class="caption">
+<p>PLATE I.&mdash;GROUP OF SUN-SPOTS AND VEILED SPOTS.</p>
+<p class="smaller">Observed on June 17th 1875 at 7 h. 30 m. A.M.</p>
+</div></div>
+
+<p><br /></p>
+
+<p>
+The group of Sun-spots represented in Plate I., was observed and drawn
+on June 17th, 1875, at 7h. 30m. A. M. The first traces of this group
+were seen on June 15th, at noon, and consisted of three small black dots
+disseminated among the granulations. At that time, no disturbance of the
+surface was noticeable, and no faculæ were seen in the vicinity of these
+spots. On June 16th, at 8 o'clock A. M., the three small spots had
+become considerably enlarged, and, as usual, the group consisted of two
+principal spots. Between these two spots all was in motion: the
+granulations, stretched into long, wavy, parallel lines, had somewhat
+the appearance of a liquid in rapid motion. At 1 o'clock, P. M., on the
+same day, the group had considerably enlarged; the faculæ, the
+granulations, and the penumbral filaments being interwoven in an
+indescribable manner. On the morning of the 17th, these spots had
+assumed the complicated form and development represented in the drawing;
+while at the same time two conspicuous veiled spots were seen on the
+left hand, at some distance above the group.
+</p>
+<p>
+Some luminous bridges are visible upon the left hand spot, traversing
+the penumbra and umbra of this spot in various directions. The umbra of
+one of the spots is occupied, and partly filled with gray and rosy
+veils, similar to those above described, and the granulations of the
+solar surface form a background to the group of spots.
+</p>
+<p>
+This group of spots was not so remarkable for its size as for its
+complicated structure. The diameter of the group from east to west was
+only 2½ minutes of arc, or about 67,000 miles. The upper part of the
+umbra of the spot situated on the right hand side of the group was
+nearly 7,000 miles in diameter, or less by 1,000 miles than the diameter
+of the Earth. Some of the long filaments composing that part of the
+penumbra, situated on the left hand side of the same spot, were 17,000
+miles in length. One of these fiery elements would be sufficient to
+encircle two-thirds of the circumference of the Earth.
+</p>
+
+<p><br /><br /><br /></p>
+
+<h4><a id="SOLAR_PROTUBERANCES">SOLAR PROTUBERANCES</a>
+<br /><br />
+PLATE II</h4>
+
+<p>
+The chromosphere forming the outlying envelope of the Sun, is subject,
+as has been shown above, to great disturbances in certain regions,
+causing considerable upheavals of its surface and violent outbursts of
+its gases. From these upheavals and outbursts of the chromosphere result
+certain curious and very interesting forms, which are known under the
+name of "<i>Solar Protuberances</i>," "<i>Prominences</i>," or
+"<i>Flames</i>."
+</p>
+<p>
+These singular forms, which could, until recently, be observed only
+during the short duration of the total eclipses of the Sun, can now be
+seen on every clear day with the spectroscope, thanks to Messrs. Janssen
+and Lockyer, to whose researches solar physics is so much indebted.
+</p>
+
+<p><br /></p>
+
+<div class="figcenter" style="width: 400px;">
+<a id="figure02"></a>
+<br />
+<img src="images/figure02.jpg" width="400" alt="" />
+<div class="caption">
+<p>PLATE II.&mdash;SOLAR PROTUBERANCES.</p>
+<p class="smaller">Observed on May 5, 1873 at 9h, 40m. A.M.</p>
+</div></div>
+
+<p><br /></p>
+
+<p>
+The solar protuberances, the Sun-spots, and the faculæ to which they
+are closely related, are confined within the same general regions of the
+Sun, although the protuberances attain higher heliocentric latitudes.
+</p>
+<p>
+There is certainly a very close relation between the faculæ and the
+solar protuberances, since when a group of the faculæ traverses the
+Sun's limb, protuberances are always seen at the same place. It seems
+very probable that the faculæ and the protuberances are in the main
+identical. The faculæ may be the brighter portion of the protuberances,
+consisting of gases which are still undergoing a high temperature and
+pressure; while the gases which have been relieved from this pressure
+and have lost a considerable amount of their heat, may form that part of
+the protuberances which is only visible on the Sun's limb.
+</p>
+<p>
+A daily study of the solar protuberances, continued for ten years, has
+shown me that these objects are distributed on two zones which are
+equidistant from the solar equator, and parallel with it. The zone
+arrangement of the protuberances is more easily recognized during the
+years of minimum solar activity, as in these years the zones are very
+narrow and widely separated. During these years the belt of
+protuberances is situated between 40° and 45° of latitude, north and
+south. In years of great solar activity the zones spread considerably on
+either side of these limits, especially towards the equator, which they
+nearly reach, only a narrow belt, usually free from protuberances,
+remaining between them. Towards the poles the zones do not spread so
+much, and there the space free from protuberances is considerably
+greater than it is at the equator.
+</p>
+<p>
+During years of maximum solar activity, the protuberances, like the
+Sun-spots and the faculæ, are very numerous, very large, and very
+complicated&mdash;sometimes occupying a great part of the whole solar limb.
+As many as twenty distinct flames are sometimes observed at one time. In
+years of minimum solar activity, on the contrary, the prominences are
+very few in number, and they are of small size; but, as far as my
+observations go, they are never totally absent.
+</p>
+<p>
+In general, the solar flames undergo rapid changes, especially those
+which are situated in the vicinity of Sun-spots, although they
+occasionally remain unchanged in appearance and form for several hours
+at a time. The protuberances situated in higher latitudes are less
+liable to great and sudden changes, often retaining the same form for
+several days. The changes observed in the protuberances of the
+equatorial regions are due in part to the comparatively great changes in
+their position with respect to the spectator, which are occasioned by
+the rotation of the Sun. This rotation, of course, has a greater angular
+velocity on the equator than in higher latitudes. In most cases,
+however, the changes of the equatorial protuberances are too great and
+too sudden to be thus explained. They are, in fact, due to the greater
+solar activity developed in the equatorial zones, and wherever spots are
+most numerous.
+</p>
+<p>
+The solar protuberances appear under various shapes, and are often so
+complicated in appearance that they defy description. Some resemble huge
+clumsy masses having a few perforations on their sides; while others
+form a succession of arches supported by pillars of different styles.
+Others form vertical or inclined columns, often surmounted by cloud-like
+masses, or by various appendages, which sometimes droop gracefully,
+resembling gigantic palm leaves. Some resemble flames driven by the
+wind; others, which are composed of a multitude of long and narrow
+filaments, appear as immense fiery bundles, from which sometimes issue
+long and delicate columns surmounted by torch-like objects of the most
+fantastic pattern. Some others resemble trees, or animal forms, in a
+very striking manner; while still others, apparently detached from the
+solar limb, float above it, forming graceful streamers or clouds of
+various shapes. Some of the protuberances are very massive, while others
+are so thin and transparent as to form a mere veil, through which more
+distant flames can easily be seen.
+</p>
+<p>
+Notwithstanding this variety of form, two principal classes of solar
+protuberances may be recognized: the cloud-like or quiescent, and the
+eruptive or metallic protuberances.
+</p>
+<p>
+The first class, which is the most common, comprises all the cloud-like
+protuberances resting upon the chromosphere or floating about it. The
+protuberances of this type often obtain enormous horizontal proportions,
+and it is not rare to see some among them occupying 20° and 30° of the
+solar limb. The height attained by protuberances of this class does not
+correspond in general to their longitudinal extent; although some of
+their branches attain considerable elevations. These prominences very
+seldom have the brilliancy displayed by the other type, and are
+sometimes so faint as to be seen with difficulty. Although it is
+generally stated by observers that some of the protuberances belonging
+to this class are detached from the solar surface, and kept in
+suspension above the surface, like the clouds in our atmosphere, yet it
+seems to me very doubtful whether protuberances are ever disconnected
+from the chromosphere, since, in an experience of ten years, I have
+never been able to satisfy myself that such a thing has occurred. Many
+of them have appeared to me at first sight to be detached from the
+surface, but with a little patience and attention I was always able to
+detect faint traces of filamentary elements connecting them with the
+chromosphere. Quite often I have seen bright protuberances gradually
+lose their light and become invisible, while soon after they had
+regained it, and were as clearly visible as before. Observations of this
+kind seem to show that while the prominences are for the most part
+luminous, there are also a few which are non-luminous and invisible to
+the eye. These dark and invisible forms are most generally found in the
+vicinity of Sun-spots in great activity. When observing such regions
+with the spectroscope, it is not rare to encounter them in the form of
+large dark spots projecting on the solar spectrum near the hydrogen
+lines. On July 28th, 1872, I observed with the spectroscope a dark spot
+of this kind issuing from the vicinity of a large Sun-spot, and
+extending over one-fifth of the diameter of the Sun. This object had
+been independently observed in France a little earlier by M. Chacornac
+with the telescope, in which it appeared as a bluish streak.
+</p>
+<p>
+The second class of solar protuberances, comprising the eruptive type,
+is the most interesting, inasmuch as it conveys to us a conception of
+the magnitude and violence of the solar forces. The protuberances of
+this class, which are always intensely bright, appear for the most part
+in the immediate vicinity of Sun-spots or faculæ. These protuberances,
+which seem to be due to the outburst of the chromosphere, and to the
+violent ejection of incandescent gases and metallic vapors from the
+interior of the Sun, sometimes attain gigantic proportions and enormous
+heights.
+</p>
+<p>
+While the spectrum of the protuberances of the cloudy type is simple,
+and usually composed of four hydrogen lines and the yellow line
+D<sub>3</sub>, that of the eruptive class is very complicated, and,
+besides the hydrogen lines and D<sub>3</sub>, it often exhibits the
+bright lines of sodium, magnesium, barium, titanium, and iron, and
+occasionally, also, a number of other bright lines.
+</p>
+<p>
+The phenomena of a solar outburst are grand and imposing. Suddenly
+immense and acute tongues and jets of flames of a dazzling brilliancy
+rise up from the solar limb and extend in various directions. Some of
+these fiery jets appear perfectly rigid, and remain apparently
+motionless in the midst of the greatest disorder. Immense straps and
+columns form and rise in an instant, bending and waving in all sorts of
+ways and assuming innumerable shapes. Sometimes powerful jets resembling
+molten metal spring up from the Sun, describing graceful parabolas,
+while in their descent they form numerous fiery drops which acquire a
+dazzling brilliancy when they approach the surface.
+</p>
+<p>
+The upward motion of the protuberances in process of formation is
+sometimes very rapid. Some protuberances have been observed to ascend in
+the solar atmosphere at the rate of from 120 to 497 miles a second.
+Great as this velocity may appear, it is nevertheless insignificant when
+compared with that sometimes attained by protuberances moving in the
+line of sight instead of directly upwards. Movements of this kind are
+indicated by the displacement of the bright or dark lines in the
+spectrum. A remarkable instance of this kind occurred on the 26th of
+June, 1874. On that day I observed a displacement of the hydrogen C line
+corresponding to a velocity of motion of 1,600 miles per second. The
+mass of hydrogen gas in motion producing such a displacement was,
+according to theory, moving towards the Earth at this incredible rate,
+when it instantly vanished from sight as if it had been annihilated, and
+was seen no more.
+</p>
+<p>
+Until recently the protuberances had not been observed to rise more than
+200,000 miles above the solar surface; but, on October 7th, 1880, a
+flame, which had an elevation of 80,000 miles when I observed it at 8h.
+55m. A. M., had attained the enormous altitude of 350,000 miles when it
+was observed at noon by Professor C. A. Young. If we had such a
+protuberance on the Earth, its summit would be at a height sufficient
+not merely to reach, but to extend 100,000 miles beyond the Moon.
+</p>
+<p>
+Although the solar protuberances represented in Plate II. have not the
+enormous proportions attained by some of these objects, yet they are as
+characteristic as any of the largest ones, and afford a good
+illustration of the purely eruptive type of protuberances. The height of
+the largest column in the group equals 4' 43", or a little over 126,000
+miles. A large group of Sun-spots was in the vicinity of these
+protuberances when they were observed and delineated.
+</p>
+
+<p><br /><br /><br /></p>
+
+<h4><a id="TOTAL_ECLIPSE">TOTAL ECLIPSE OF THE SUN</a>
+<br /><br />
+PLATE III</h4>
+
+<p>
+A solar eclipse is due to the passage of the Moon directly between the
+observer and the Sun. Such an eclipse can only occur at New Moon, since
+it is only at that time that our satellite passes between us and the
+Sun. The Moon's orbit does not lie precisely in the same plane as the
+orbit of the Earth, but is inclined about five degrees to it, otherwise
+an eclipse of the Sun would occur at every New Moon, and an eclipse of
+the Moon at every Full Moon.
+</p>
+<p>
+Since the Moon's orbit is inclined to that of the Earth, it must
+necessarily intersect this orbit at two opposite points. These points
+are called the nodes of the Moon's orbit. When our satellite passes
+through either of the nodes when the Moon is new, it appears interposed
+to some extent between the Sun and the Earth, and so produces a solar
+eclipse; while if it passes a node when the Moon is full, it is more or
+less obscured by the Earth's shadow, which then produces an eclipse of
+the Moon. But, on the other hand, when the New Moon and the Full Moon do
+not coincide with the passage of our satellite through the nodes of its
+orbit, no eclipse can occur, since the Moon is not then on a line with
+the Sun and the Earth, but above or below that line.
+</p>
+<p>
+Owing to the ellipticity of the Moon's orbit, the distance of our
+satellite from the Earth varies considerably during each of its
+revolutions around us, and its apparent diameter is necessarily subject
+to corresponding changes. Sometimes it is greater, sometimes it is less,
+than the apparent diameter of the Sun. If it is greater at the time of a
+solar eclipse, the eclipse will be total to a terrestrial observer
+stationed nearly on the line of the centres of the Sun and Moon, while
+it will be only partial to another observer stationed further from this
+line. But the Moon's distance from the Earth may be so great and its
+apparent diameter consequently so small that even those observers
+nearest the central line of the eclipse see the border of the Sun all
+round the black disk of the Moon; the eclipse is then annular. Even
+during the progress of one and the same eclipse the distance of the Moon
+from the parts of the Earth towards which its shadow is directed may
+vary so much that, while the eclipse is total to some observers, others
+equally near the central line, but stationed at a different place, will
+see it as annular.
+</p>
+<p>
+The shadow cast by the Moon on the Earth during total eclipses, travels
+along upon the surface of the Earth, in consequence of the daily
+movement of rotation of our globe combined with the movements of the
+Earth and Moon in their orbits. The track of the Moon's shadow over the
+Earth's surface has a general eastward course, so that the more westerly
+observers see it earlier than those east of them. An eclipse may
+continue total at one place for nearly eight minutes, but in ordinary
+cases the total phase is much shorter.
+</p>
+<p>
+The nodes of the Moon's orbit do not invariably occupy the same
+position, but move nearly uniformly, their position with regard to the
+Sun, Earth, and Moon being at any time approximately what it formerly
+was at a series of times separated by equal intervals from each other.
+Each interval comprises 223 lunations, or 18 years, 11 days, and 7 or 8
+hours. The eclipses which occur within this interval are almost exactly
+repeated during the next similar interval. This period, called the
+"Saros," was well known to the ancients, who were enabled by its means
+to predict eclipses with some certainty.
+</p>
+
+<p><br /></p>
+
+<div class="figcenter" style="width: 400px;">
+<a id="figure03"></a>
+<br />
+<img src="images/figure03.jpg" width="400" alt="" />
+<div class="caption">
+<p>PLATE III.&mdash;TOTAL ECLIPSE OF THE SUN.</p>
+<p class="smaller">Observed July 29, 1878, at Creston, Wyoming Territory</p>
+</div></div>
+
+<p><br /></p>
+
+<p>
+A total eclipse of the Sun is a most beautiful and imposing phenomenon.
+At the predicted time the perfectly round disk of the Sun becomes
+slightly indented at its western limb by the yet invisible Moon. This
+phenomenon is known as the "first contact."
+</p>
+<p>
+The slight indentation observed gradually increases with the advance of
+the Moon from west to east, the irregularities of the surface of our
+satellite being plainly visible on the border of the dark segment
+advancing on the Sun's disk. With the advance of the Moon on the Sun,
+the light gradually diminishes on the Earth. Every object puts on a dull
+and gloomy appearance, as when night is approaching; while the bright
+sky, losing its light, changes its pure azure for a livid grayish color.
+</p>
+<p>
+Two or three minutes before totality begins, the solar crescent, reduced
+to minute proportions, gives comparatively so little light that faint
+traces of the Sun's atmosphere appear on the western side behind the
+dark body of the Moon, whose limb then becomes visible outside of the
+Sun. I observed this phenomenon at Creston during the eclipse of 1878.
+From 15 to 20 seconds before totality, the narrow arc of the Sun's disk
+not yet obscured by the Moon seems to break and separate towards the
+extremities of its cusps, which, thus divided, form independent points of
+light, which are called "<i>Baily's beads</i>." A moment after, the whole
+solar crescent breaks into numerous beads of light, separated by dark
+intervals, and, suddenly, they all vanish with the last ray of Sunlight,
+and totality has begun with the "second contact." This phenomenon of
+Baily's beads is undoubtedly caused by the irregularities of the Moon's
+border, which, on reaching the solar limb, divide the thin solar
+crescent into as many beads of light and dark intervals as there are
+peaks and ravines seen sidewise on that part of the Moon's limb.
+</p>
+<p>
+With the disappearance of the last ray of light, the planets and the
+stars of the first and second magnitude seem to light up and become
+visible in the sky. The darkness, which had been gradually creeping in
+with the progress of the eclipse, is then at its maximum. Although
+subject to great variations in different eclipses, the darkness is never
+so great as might be expected from the complete obscuration of our
+luminary, as the part of our atmosphere which is still exposed to the
+direct rays of the Sun, reflects to us some of that light, which thus
+diminishes the darkness resulting from the disappearance of the Sun.
+Usually the darkness is sufficient to prevent the reading of common
+print, and to deceive animals, causing them to act as if night was
+really approaching. During totality the temperature decreases, while the
+humidity of the atmosphere augments.
+</p>
+<p>
+Simultaneously with the disappearance of Baily's beads, a pale, soft,
+silvery light bursts forth from behind the Moon, as if the Sun, in
+disappearing, had been vaporized and expanded in all directions into
+soft phosphorescent rays and streamers. This pale light is emitted by
+gases constituting the solar atmosphere surrounding the bright nucleus
+now obscured by the dark body of our satellite. This solar atmosphere is
+called <i>Corona</i>, from its distant resemblance to the aureola, or
+glory, represented by ancient painters around the heads of saints.
+</p>
+<p>
+With the bursting forth of the corona, a very thin arc of bright white
+light is seen along the Moon's limb, where the solar crescent has just
+disappeared. This thin arc of light is the reversing layer, which, when
+observed with the spectroscope at that moment, exhibits bright lines
+answering to the dark lines of the ordinary solar spectrum. Immediately
+above this reversing layer, and concentric with it, appears the
+pink-colored chromospheric layer, with its curiously shaped flames and
+protuberances. During totality, the chromosphere and protuberances are
+seen without the aid of the spectroscope, and appear of their natural
+color, which, although somewhat varying in their different parts, is, on
+the whole, pinkish, and similar to that of peach-blossoms; yet it is
+mixed here and there with delicate prismatic hues, among which the pink
+and straw colors predominate.
+</p>
+<p>
+The color of the corona seems to vary in every eclipse, but as its tints
+are very delicate, it may depend, in a great measure, upon the vision of
+the observer; although there seems to be no doubt that there are real
+variations. At Creston, in 1878, it appeared to both Professor W.
+Harkness and myself of a decided pale greenish hue.
+</p>
+<p>
+The corona appears under different forms, and has never been observed
+twice alike. Its dimensions are also subject to considerable variations.
+Sometimes it appears regular and very little extended, its distribution
+around the Sun being almost uniform; although in general it spreads a
+little more in the direction of the ecliptic, or of the solar equator.
+At other times it appears much larger and more complicated, and forms
+various wings and appendages, which in some cases, as in 1878, extend to
+immense distances; while delicate rays radiate in straight or curved
+lines from the spaces left in the polar regions between the wings. The
+corona has sometimes appeared as if divided by immense dark gaps,
+apparently free from luminous matter, and strongly resembling the dark
+rifts seen in the tails of comets. This was observed in Spain and Sicily
+during the total eclipse of the Sun in 1870. Different structures,
+forming wisps and streamers of great length, and interlaced in various
+ways, are sometimes present in the corona, while faint but more
+complicated forms, distantly resembling enormous solar protuberances
+with bright nuclei, have also been observed.
+</p>
+<p>
+As the Moon continues its eastward progress, it gradually covers the
+chromosphere and the solar protuberances on the eastern side of the Sun;
+while, at the same time, the protuberances and the chromosphere on the
+opposite limb gradually appear from under the retreating Moon. Then, the
+thin arc of the reversing layer is visible for an instant, and is
+instantly followed by the appearance of a point of dazzling white light,
+succeeded immediately by the apparition of Daily's beads on each side,
+and totality is over, with this third contact. The corona continues to
+be visible on the eastern side of the Sun for several minutes longer,
+and then rapidly vanishes.
+</p>
+<p>
+The thin solar crescent increases in breadth as the Moon advances;
+while, at the same time, the darkness and gloom spread over nature
+gradually disappear, and terrestrial objects begin to resume their
+natural appearance. Finally the limb of the Moon separates from that of
+the Sun at the instant of "fourth contact," and the eclipse is over.
+</p>
+<p>
+The phenomena exhibited by the corona in different eclipses are very
+complex, and, so far, they have not been sufficiently studied to enable
+us to understand the true nature of the solar atmosphere. From the
+spectral analysis of the corona, and the phenomena of polarization, it
+has been learned, at least, that while the matter composing the upper
+part of the solar atmosphere is chiefly composed of an unknown
+substance, producing the green line 1474, its lower part is mainly
+composed of hydrogen gas at different temperatures, a part of which is
+self-luminous, while the other part only reflects the solar light. But
+the proportion of the gaseous particles emitting light, to those simply
+reflecting it, is subject to considerable variations in different
+eclipses. At present it would seem that in years of great solar
+disturbances, the particles emitting light are found in greater quantity
+in the corona than those reflecting it; but further observations will be
+required to confirm these views.
+</p>
+<p>
+It is very difficult to understand how the corona, which in certain
+eclipses extends only one diameter of the Sun, should, in other cases,
+as in 1878, extend to the enormous distance of twelve times the same
+diameter. Changes of such magnitude in the solar atmosphere, if due to
+the operation of forces with which we are acquainted, cannot yet be
+accounted for by what is known of such forces. Their causes are still as
+mysterious as those concerned in the production of the monstrous tails
+displayed by some comets on their approach to the Sun.
+</p>
+<p>
+Plate 3, representing the total eclipse of the Sun of July 29th, 1878,
+was drawn from my observations made at Creston, Wyoming Territory, for
+the Naval Observatory. The eclipse is represented as seen in a
+refracting telescope, having an aperture of 6⅓ inches, and as it
+appeared a few seconds before totality was over, and when the
+chromosphere was visible on the western limb of the Sun. The two long
+wings seen on the east and west side of the Sun, appeared considerably
+larger in the sky than they are represented in the picture.
+</p>
+
+<p><br /><br /><br /></p>
+
+<h4><a id="THE_AURORA_BOREALIS">THE AURORA BOREALIS</a>
+<br /><br />
+PLATE IV</h4>
+
+<p>
+The name of Polar Auroras is given to certain very remarkable luminous
+meteoric phenomena which appear at intervals above the northern or the
+southern horizons of both hemispheres of the Earth. When the phenomenon
+is produced in our northern sky, it is called "Aurora Borealis," or
+"Northern Lights;" and when it appears in the southern sky, it is called
+"Aurora Australis," or southern aurora.
+</p>
+<p>
+Marked differences appear in the various auroras observed from our
+northern latitudes. While some simply consist in a pale, faint
+luminosity, hardly distinguishable from twilight, others present the
+most gorgeous and remarkable effects of brightness and colors.
+</p>
+<p>
+A great aurora is usually indicated in the evening soon after twilight,
+by a peculiar grayish appearance of the northern sky just above the
+horizon. The grayish vapors giving that appearance, continuing to form
+there, soon assume a dark and gloomy aspect, while they gradually take
+the form of a segment of a circle resting on the horizon. At the same
+time that this dark segment is forming, a soft pearly light, which seems
+to issue from its border, spreads up in the sky, where it gradually
+vanishes, being the brightest at its base. This arc of light, gradually
+increasing in extent as well as in brightness, reaches sometimes as far
+as the polar star. On some rare occasions, one or two, and even three,
+concentric arches of bright light form one above the other over the dark
+segment, where they appear as brilliant concentric rainbows. While the
+aurora continues to develop and spread out its immense arc, the border
+of the dark segment loses its regularity and appears indented at several
+places by patches of light, which soon develop into long, narrow,
+diverging rays and streamers of great beauty. For the most part the
+auroral light is either whitish or of a pale, greenish tint; but in some
+cases it exhibits the most beautiful colors, among which the red and
+green predominate. In these cases the rays and streamers, which are
+usually of different colors, produce the most magnificent effects by
+their continual changes and transformations.
+</p>
+<p>
+The brightness and extent of the auroral rays are likewise subject to
+continual changes. An instant suffices for their development and
+disappearance, which may be succeeded by the sudden appearance of others
+elsewhere, as though the original streamers had been swiftly transported
+to a new place while invisible. It frequently happens that all the
+streamers seem to move sidewise, from west to east, along the arch,
+continuing meanwhile to exhibit their various changes of form and color.
+For a time, these appearances of motion continue to increase, a
+succession of streamers alternately shooting forth and again fading,
+when a sudden lull occurs, during which all motion seems to have ceased.
+The stillness then prevailing is soon succeeded by slight pulsations of
+light, which seem to originate on the border-of the dark segment, and
+are propagated upwards along the streamers, which have now become more
+numerous and active. Slow at first, these pulsations quicken by degrees,
+and after a few minutes the whole northern sky seems to be in rapid
+vibration. The lively upward and downward movement of these streamers
+entitles them to the name of "merry dancers" given them in northern
+countries where they are frequent.
+</p>
+<p>
+Long waves of light, quickly succeeded by others, are propagated in an
+instant from the horizon to the zenith; these, in their rapid passage,
+cause bends and curves in the streamers, which then, losing their
+original straightness, wave and undulate in graceful folds, resembling
+those of a pennant in a gentle breeze. Although the coruscations add to
+the grandeur of the spectacle, they tend to destroy the diverging
+streamers, which, being disconnected from the dark segment, or torn in
+various ways, are, as it were, bodily carried up towards the zenith.
+</p>
+<p>
+In this new phase the aurora is transformed into a glorious crown of
+light, called the "Corona." From this corona diverge in all directions
+long streamers of different colors and forms, gracefully undulating in
+numerous folds, like so many banners of light. Some of the largest of
+these streamers appear like fringes composed of short transverse rays of
+different intensity and colors, producing the most fantastic effects,
+when traversed by the pulsations and coruscations which generally run
+across these rays during the great auroral displays.
+</p>
+<p>
+The aurora has now attained its full development and beauty. It may
+continue in this form for half an hour, but usually the celestial fires
+begin to fade at the end of fifteen or twenty minutes, reviving from
+time to time, but gradually dying out. The northern sky usually appears
+covered by gray and luminous streaks and patches after a great aurora,
+these being occasionally rekindled, but more often they gradually
+disappear, and the sky resumes its usual appearance.
+</p>
+<p>
+The number of auroras which develop a corona near the zenith is
+comparatively small in our latitudes; but many of them, although not
+exhibited on so grand a scale, are nevertheless very interesting. On
+some very rare occasions the auroral display has been confined almost
+exclusively to the dark segment, which appeared then as if pierced along
+its border by many square openings, like windows, through which appeared
+the bright auroral light.
+</p>
+
+<p><br /></p>
+
+<div class="figcenter" style="width: 400px;">
+<a id="figure04"></a>
+<br />
+<img src="images/figure04.jpg" width="400" alt="" />
+<div class="caption">
+<p>PLATE IV.&mdash;AURORA BOREALIS.</p>
+<p class="smaller">As observed March 1, 1872, at 9h. 25m. P.M.</p>
+</div></div>
+
+<p><br /></p>
+
+<p>
+Among the many auroras which I have had occasion to observe, none are
+more interesting, excepting the type first described, than those which
+form an immense arch of light spanning the heavens from East to West.
+This form of aurora, which is quite rare, I last observed on September
+12th, 1881. All the northern sky was covered with light vapors, when a
+small auroral patch appeared in the East at about 20° above the
+horizon. This patch of light, gradually increasing westward, soon
+reached the zenith, and continued its onward progress until it arrived
+at about 20° above the western horizon, where it stopped. The aurora
+then appeared as a narrow, wavy band of light, crossed by numerous
+parallel rays of different intensity and color. These rays seemed to
+have a rapid motion from West to East along the delicately-fringed
+streamer, which, on the whole, moved southward, while its extremities
+remained undisturbed. Aside from the apparent displacement of the
+fringes, a singular vibrating motion was observed in the auroral band,
+which was traversed by pulsations and long waves of light. The phenomena
+lasted for about twenty minutes, after which the arch was broken in many
+places, and it slowly vanished.
+</p>
+<p>
+The aurora usually appears in the early part of the evening, and attains
+its full development between ten and eleven o'clock. Although the
+auroral light may have apparently ceased, yet the phenomenon is not at
+an end, as very often a solitary ray is visible from time to time; and
+even towards morning these rays sometimes become quite numerous. On some
+occasions the phenomenon even continues through the following day, and
+is manifested by the radial direction of the cirrus-clouds in the
+heights of our atmosphere. In 1872 I, myself, observed an aurora which
+apparently continued for two or three consecutive days and nights. In
+August, 1859, the northern lights remained visible in the United States
+for a whole week.
+</p>
+<p>
+The height attained by these meteors is considerable, and it is now
+admitted that they are produced in the rarefied air of the upper regions
+of our atmosphere. From the researches of Professor Elias Loomis on the
+great auroras observed in August and September, 1859, it was ascertained
+that the inferior part of the auroral rays had an altitude of 46 miles,
+while that of their summits was 428 miles. These rays had, therefore, a
+length of 382 miles. From the observation of thirty auroral displays, it
+has been found that the mean height attained by the summit of these
+streamers above the Earth's surface was 450 miles.
+</p>
+<p>
+But if the auroral streamers are generally manifested at great heights
+in our atmosphere, it would appear from the observations of persons
+living in the regions where the auroras are most frequent, as also from
+those who have been stationed in high northern and southern latitudes,
+that the phenomenon sometimes descends very low. Both Sabine and Parry
+saw the auroral rays projected on a distant mountain; Ross saw them
+almost at sea-level projected on the polar ice; while Wrangel, Franklin,
+and others observed similar phenomena. Dr. Hjaltalin, who has lived in
+latitude 64° 46' north, and has made a particular study of the aurora,
+on one occasion saw the aurora much below the summit of a hill 1,600
+feet high, which was not very far off.
+</p>
+<p>
+The same aurora is sometimes observed on the same night at places very
+far distant from one another. The great aurora borealis of August 28th,
+1859, for instance, was seen over a space occupying 150° in
+longitude&mdash;from California to the Ural Mountains in Russia. It even
+appears now very probable that the phenomenon is universal on our globe,
+and that the northern lights observed in our hemisphere are simultaneous
+with the aurora australis of the southern hemisphere. The aurora of
+September 2d, 1859, was observed all through North and South America,
+the Sandwich Islands, Australia, and Africa; the streamers and
+pulsations of light of the north pole responding to the rays and
+coruscations of the south pole. Of thirty-four auroras observed at
+Hobart Town, in Tasmania, twenty-nine corresponded with aurora borealis
+observed in our hemisphere.
+</p>
+<p>
+The auroral phenomena, although sometimes visible within the tropics,
+are, however, quite rare in these regions. For the most part they are
+confined within certain zones situated in high latitudes north and
+south. The zone where they are most frequent in our hemisphere forms an
+ellipse, which has the north pole at one of its foci; while the other is
+situated somewhere in North America, in the vicinity of the magnetic
+pole. The central line of the zone upon which the auroras seem to be
+most frequent passes from the northern coast of Alaska through Hudson's
+Bay and Labrador to Iceland, and then follows the northern coast of
+Europe and Asia. The number of auroras diminishes as the observer
+recedes from this zone, and it is only in exceptional cases that they
+are seen near the equator. Near the pole the phenomenon is less frequent
+than it is in the region described. In North America we occupy a
+favorable position for the observation of auroras, as we are nearer the
+magnetic poles than are the Europeans and Asiatics, and we consequently
+have a greater number of auroras in corresponding latitudes.
+</p>
+<p>
+The position of the dark auroral segment varies with the place occupied
+by the observer, and its centre always corresponds with the magnetic
+meridian. In our Eastern States the auroral segment appears a little to
+the west of the north point; but as the observer proceeds westward it
+gradually approaches this point, and is due north when seen from the
+vicinity of Lake Winnipeg. At Point Barrow, in the extreme north-west of
+the United States, the aurora is observed in the east. In Melville
+Islands, Parry saw it in the south; while in Greenland it is directly in
+the west.
+</p>
+<p>
+It is stated that auroras are more numerous about the equinoxes than
+they are at any other seasons; and also, when the earth is in perigee,
+than when it is in apogee. An examination which I have made of a
+catalogue by Professor Loomis, comprising 4,137 auroras observed in the
+temperate zone of our hemisphere from 1776 to 1873, sustains this
+statement. During this period, one hundred more auroras were recorded
+during each of the months comprising the equinoxes, than during any
+other months of the year; while eighty more auroras were observed when
+the earth was in perigee, than when it was in apogee. But to establish
+the truth of this assertion on a solid basis, more observations in both
+hemispheres will be required.
+</p>
+<p>
+The aurora is not simply a terrestrial phenomenon, but is associated in
+some mysterious way with the conditions of the Sun's surface. It is a
+well-known fact that terrestrial magnetism is influenced directly by the
+Sun, which creates the diurnal oscillations of the magnetic needle.
+Between sunrise and two o'clock, the north pole of the needle moves
+towards the west in our northern hemisphere, and in the afternoon and
+evening it moves the other way. These daily oscillations of the needle
+are not uniform in extent; they have a period of regular increase and
+decrease. At a given place the daily oscillations of the magnetic needle
+increase and decrease with regularity during a period which is equal to
+10⅓ years. As this period closely coincides with the Sun-spot period,
+the connection between the variation of the needle and these solar
+disturbances has been recognized.
+</p>
+<p>
+Auroral phenomena generally accompany the extraordinary perturbations in
+the oscillations of the magnetic needle, which are commonly called
+"magnetic storms," and the greater the auroral displays, the greater are
+the magnetic perturbations. Not only is the needle subject to unusual
+displacements during an aurora, but its movements seem to be
+simultaneous with the pulsations and waving motions of the delicate
+auroral streamers in the sky. When the aurora sends forth a coruscation,
+or a streamer in the sky, the magnetic needle responds to it by a
+vibration. The inference that the auroral phenomena are connected with
+terrestrial magnetism is further supported by the fact that the centre
+of the corona is always situated exactly in the direction of that point
+in the heavens to which the dipping needle is directed.
+</p>
+<p>
+It has been found that the aurora is a periodical phenomenon, and that
+its period corresponds very closely with those of the magnetic needle
+and Sun-spots. The years which have the most Sun-spots and magnetic
+disturbances have also the most auroras. There is an almost perfect
+similarity between the courses of the three sets of phenomena, from
+which it is concluded that the aurora is connected in some mysterious
+way with the action of the Sun, as well as with the magnetic condition
+of the earth.
+</p>
+<p>
+A very curious observation, which has been supposed to have some
+connection with this subject, was made on Sept. 1st, 1859, by Mr.
+Carrington and Mr. Hodgson, in England. While these observers, who were
+situated many miles from one another, were both engaged at the same time
+in observing the same Sun-spot, they suddenly saw two luminous spots of
+dazzling brilliancy bursting into sight from the edge of the Sun-spot.
+These objects moved eastward for about five minutes, after which they
+disappeared, having then traveled nearly 34,000 miles. Simultaneously
+with these appearances, a magnetic disturbance was registered at Kew by
+the self-registering magnetic instruments. The very night that followed
+these observations, great magnetic perturbations, accompanied by
+brilliant auroral displays, were observed in Europe. A connection
+between the terrestrial magnetism and the auroral phenomena is further
+proved by the fact that, before the appearance of an aurora, the
+magnetic intensity of our globe considerably increases, but diminishes
+as soon as the first flashes show themselves.
+</p>
+<p>
+The auroral phenomena are also connected in some way with electricity,
+and generate serious disturbances in the electric currents traversing
+our telegraphic lines, which are thus often rendered useless for the
+transmission of messages during great auroral displays. It sometimes
+happens, however, during such displays, that the telegraphic lines can
+be operated for a long distance, without the assistance of a battery;
+the aurora, or at least its cause, furnishing the necessary electric
+current for the working of the line. During auroras, the telephonic
+lines are also greatly affected, and all kinds of noises and
+crepitations are heard in the instruments.
+</p>
+<p>
+Two observations of mine, which may have a bearing on the subject,
+present some interest, as they seem to indicate the action of the aurora
+on some of the clouds of our atmosphere. On January 6th, 1872, after I
+had been observing a brilliant aurora for over one hour, an isolated
+black cumulus cloud appeared at a little distance from the western
+extremity of the dark auroral segment. This cloud, probably driven by
+the wind, rapidly advanced eastward, and was soon followed by a
+succession of similar clouds, all starting from the same point. All
+these black clouds apparently followed the same path, which was not a
+straight line, but parallel to and concentric with the border of the
+dark auroral segment. When the first cloud arrived in the vicinity of
+the magnetic meridian passing through the middle of the auroral arc, it
+very rapidly dissolved, and on reaching this meridian became invisible.
+The same phenomenon was observed with the succession of black clouds
+following, each rapidly dissolving as it approached the magnetic
+meridian. This phenomenon of black clouds vanishing like phantoms in
+crossing the magnetic meridian, was observed for nearly an hour. On June
+17th, 1879, I observed a similar phenomenon during a fine auroral
+display. About midway between the horizon and the polar star, but a
+little to the west of the magnetic meridian, there was a large black
+cumulo-stratus cloud which very slowly advanced eastward. As it
+progressed in that direction, its eastern extremity was dissolved in
+traversing the magnetic meridian; while, at the same time, several short
+and quite bright auroral rays issued from its western extremity, which
+in its turn dissolved rapidly, as if burned or melted away in the
+production of the auroral flame.
+</p>
+<p>
+It seems to be a well observed fact, that during auroras, a strong
+sulphurous odor prevails in high northern latitudes. According to Dr.
+Hjaltalin, during these phenomena, "the ozone of the atmosphere
+increases considerably, and men and animals exposed out of doors emit a
+sulphurous odor when entering a heated room." The Esquimaux and other
+inhabitants of the northern regions assert that great auroras are
+sometimes accompanied by crepitations and crackling noises of various
+sorts. Although these assertions have been denied by several travelers
+who have visited the regions of these phenomena, they are confirmed by
+many competent observers. Dr. Hjaltalin, who has heard these noises
+about six times in a hundred observations, says that they are especially
+audible when the weather is clear and calm; but that when the atmosphere
+is agitated they are not heard. He compares them to the peculiar sound
+produced by a silk cloth when torn asunder, or to the crepitations of
+the electric machine when its motion is accelerated. "When the auroral
+light is much agitated and the streamers show great movements, it is
+then that these noises are heard at different places in the atmosphere."
+</p>
+<p>
+The spectrum of the auroral light, although it varies with almost every
+aurora, always shows a bright green line on a faint continuous spectrum.
+In addition to this green line I have frequently observed four broad
+diffused bands of greater refrangibility in the spectra of some auroras.
+In two cases, when the auroras appeared red towards the west, the
+spectrum showed a bright red line, in addition to the green line and the
+broad bands described. These facts evidently show that the light of the
+aurora is due to the presence of luminous vapors in our atmosphere; and
+it may reasonably be supposed that these vapors are rendered luminous by
+the passage of electric discharges through them.
+</p>
+
+<p><br /><br /><br /></p>
+
+<h4><a id="THE_ZODIACAL_LIGHT">THE ZODIACAL LIGHT</a>
+<br /><br />
+PLATE V</h4>
+
+<p>
+In our northern latitudes may be seen, on every clear winter and spring
+evening, a column of faint, whitish, nebulous light, rising obliquely
+above the western horizon. A similar phenomenon may also be observed in
+the east, before day-break, on any clear summer or autumn night. To this
+pale, glimmering luminosity the name of "Zodiacal Light" has been given,
+from the fact that it lies in the zodiac along the ecliptic.
+</p>
+<p>
+In common with all the celestial bodies, the zodiacal light participates
+in the diurnal motion of the sky, and rises and sets with the
+constellations in which it appears. Aside from this apparent motion, it
+is endowed with a motion of its own, accomplished from west to east, in
+a period of a year. In its motion among the stars, the zodiacal light
+always keeps pace with the Sun, and appears as if forming two faint
+luminous wings, resting on opposite sides of this body. In reality it
+extends on each side of the Sun, its axis lying very nearly in the plane
+of the ecliptic.
+</p>
+<p>
+In our latitudes the phenomena can be observed most advantageously
+towards the equinoxes, in March and September, when twilight is of short
+duration. As we proceed southward it becomes more prominent, and
+gradually increases in size and brightness. It is within the tropical
+regions that the zodiacal light acquires all its splendor: there it is
+visible all the year round, and always appears very nearly perpendicular
+to the horizon, while at the same time its proportions and brilliancy
+are greatly increased.
+</p>
+
+<p><br /></p>
+
+<div class="figcenter" style="width: 400px;">
+<a id="figure05"></a>
+<br />
+<img src="images/figure05.jpg" width="400" alt="" />
+<div class="caption">
+<p>PLATE V.&mdash;THE ZODIACAL LIGHT.</p>
+<p class="smaller">Observed February 20, 1876</p>
+</div></div>
+
+<p><br /></p>
+
+<p>
+The zodiacal light appears under the form of a spear-head, or of a
+narrow cone of light whose base apparently rests on the horizon, while
+its summit rises among the zodiacal constellations. In general
+appearance it somewhat resembles the tail of a large comet whose head is
+below the horizon. The most favorable time to observe this phenomenon in
+the evening, is immediately after the last trace of twilight has
+disappeared; and in the morning, one or two hours before twilight
+appears. When observed with attention, it is seen that the light of the
+zodiacal cone is not uniform, but gradually increases in brightness
+inwardly, especially towards its base, where it sometimes surpasses in
+brilliancy the brightest parts of the Milky-Way. In general, its
+outlines are vague and very difficult to make out, so gradually do they
+blend with the sky. On some favorable occasions, the luminous cone
+appears to be composed of several distinct concentric conical layers,
+having different degrees of brightness, the inner cone being the most
+brilliant of all. There is a remarkable distinction between the evening
+and morning zodiacal light. In our climate, the morning light is pale,
+and never so bright nor so extended as the evening light.
+</p>
+<p>
+In general, the zodiacal light is whitish and colorless, but in some
+cases it acquires a warm yellowish or reddish tint. These changes of
+color may be accidental and due to atmospheric conditions, and not to
+actual change in the color of the object. Although the zodiacal light is
+quite bright, and produces the impression of having considerable depth,
+yet its transparency is great, since all the stars, except the faint
+ones, can be seen through its substance.
+</p>
+<p>
+The zodiacal light is subject to considerable variations in brightness,
+and also varies in extent, the apex of its cone varying in distance from
+the Sun's place, from 40 to 90 degrees. These variations cannot be
+attributed to atmospheric causes alone, some of them being due to real
+changes in the zodiacal light itself, whose light and dimensions
+increase or decrease under the action of causes at present unknown. From
+the discussion of a series of observations on the zodiacal light made at
+Paris and Geneva, it appears certain that its light varies from year to
+year, and sometimes even from day to day, independently of atmospheric
+causes. Some of my own observations agree with these results, and one of
+them, at least, seems to indicate changes even more rapid. On December
+18th, 1875, I observed the zodiacal light in a clear sky free from any
+vapors, at six o'clock in the evening. At that time, the point of its
+cone was a little to the north of the ecliptic, at a distance of about
+90 degrees from the Sun's place. Ten minutes later, its summit had sunk
+down 35 degrees, the cone then being reduced to nearly one-half of its
+original dimensions. Ten minutes later, it had risen 25 degrees, and was
+then 80 degrees from the Sun's place, where it remained all the evening.
+On March 22d, 1878, the sky was very clear and the zodiacal light was
+bright when I observed it, at eight o'clock. At that moment the apex of
+the cone of light was a little to the south of the Pleiades, but this
+cone presented an unusual appearance never noticed by me before, its
+northern border appearing much brighter and sharper than usual, while at
+the same time its axis of greatest brightness appeared to be much nearer
+to this northern border than it was to the southern. After a few minutes
+of observation it became evident that the northern border was extending
+itself, as stars which were at some distance from it became gradually
+involved in its light. At the same time that this border spread
+northward, it seemed to diffuse itself, and after a time the cone
+presented its usual appearance, having its southern border brighter and
+better defined than the other. It would have been impossible to
+attribute this sudden change to an atmospheric cause, since only one of
+the borders of the cone participated in it, and since some very faint
+stars near this northern border were not affected in the least while the
+phenomenon occurred. Besides these observations, Cassini, Mairan,
+Humboldt, and many other competent observers have seen pulsations,
+coruscations and bickerings in the light of the cone, which they thought
+could not be attributed to atmospheric causes. It has also been observed
+that at certain periods the zodiacal light has shone with unusual
+intensity for months together.
+</p>
+<p>
+When this phenomenon is observed from the tropical regions, it is found
+that its axis of symmetry always corresponds with its axis of greatest
+brightness, and that both lie in the plane of the ecliptic, which
+divides its cone into two equal parts. But when the zodiacal light is
+observed in our latitude, the axis of symmetry does not correspond with
+the axis of greatest brightness, and both axes are a little to the north
+of this plane, the axis of symmetry being the farther removed.
+Furthermore, as already stated, the southern border of the cone always
+appears better defined and brighter than the corresponding northern
+margin. It is very probable, if not absolutely certain, that these
+phenomena are exactly reversed when the zodiacal light is observed from
+corresponding latitudes in the southern hemisphere, and that there, its
+axes, both of symmetry and of greatest brightness, appear south of the
+ecliptic, while the northern margin is the brightest. This seems to be
+established by the valuable observations of Rev. George Jones, made on
+board the U. S. steam frigate Mississippi, in California, Japan, and the
+Southern Ocean. "When I was north of the ecliptic," says this observer,
+"the greatest part of the light of the cone appeared to the north of
+this line; when I was to the south of the ecliptic, it appeared to be
+south of it; while when my position was on the ecliptic, or in its
+vicinity, the zodiacal cone was equally divided by this line."
+</p>
+<p>
+Besides the zodiacal light observed in the East and West, some observers
+have recognized an exceedingly faint, luminous, gauzy band, about 10 or
+12 degrees wide, stretching along the ecliptic from the summit of the
+western to that of the eastern zodiacal cone. This faint narrow belt has
+been called the Zodiacal Band. It has been recognized by Mr. H. C.
+Lewis, who has made a study of this phenomenon, that the zodiacal band
+has its southern margin a little brighter and a little sharper than the
+northern border. This observation is in accordance with similar
+phenomena observed in the zodiacal light, and may have considerable
+importance.
+</p>
+<p>
+In 1854, Brorsen recognized a faint, roundish, luminous spot in a point
+of the heavens exactly opposite to the place occupied by the Sun, which
+he has called "Gegenschein," or counter-glow. This luminous spot has
+sometimes a small nucleus, which is a little brighter than the rest.
+Night after night this very faint object shifts its position among the
+constellations, keeping always at 180 degrees from the Sun. The position
+of the counter-glow, like that of the zodiacal light and zodiacal band,
+is not precisely on the plane of the ecliptic, but a little to the north
+of this line. It is very probable that near the equator the phenomenon
+would appear different and there would correspond with this plane.
+</p>
+<p>
+There seems to be some confusion among observers in regard to the
+spectrum of the zodiacal light. Some have seen a bright green line in
+its spectrum, corresponding to that of the aurora borealis; while others
+could only see a faint grayish continuous spectrum, which differs,
+however, from that of a faint solar light, by the fact that it presents
+a well-defined bright zone, gradually blending on each side with the
+fainter light of the continuous spectrum. I have, myself, frequently
+observed the faint continuous spectrum of the zodiacal light, and on one
+occasion recognized the green line of the aurora; but it might have been
+produced by the aurora itself, as yet invisible to the eye, and not by
+the zodiacal light, since, later in the same evening, there was a
+brilliant auroral display. If it were demonstrated that this green line
+exists in the spectrum of the zodiacal light, the fact would have
+importance, as tending to show that the aurora and the zodiacal light
+have a common origin.
+</p>
+<p>
+Rev. Geo. Jones describes a very curious phenomenon which he observed
+several times a little before the moon rose above the horizon. The
+phenomenon consisted in a short, oblique, luminous cone rising from the
+Moon's place in the direction of the ecliptic. This phenomenon he has
+called the Moon Zodiacal Light. In 1874, I had an opportunity to observe
+a similar phenomenon when the Moon was quite high in the sky. By taking
+the precaution to screen the Moon's disk by the interposition of some
+buildings between it and my eye, I saw two long and narrow cones of
+light parallel to the ecliptic issuing from opposite sides of our
+satellite. The phenomenon could not possibly be attributed to vapors in
+our atmosphere, since the sky was very clear at the moment of the
+observation. Later on, these appendages disappeared with the formation
+of vapors near the Moon, but they reappeared an hour later, when the sky
+had cleared off, and continued visible for twenty minutes longer, and
+then disappeared in a clear sky.
+</p>
+<p>
+Although the zodiacal light has been studied for over two centuries, no
+wholly satisfactory explanation of the phenomenon has yet been given.
+Now, as in Cassini's time, it is generally considered by astronomers to
+be due to a kind of lens-shaped ring surrounding the Sun, and extending
+a little beyond the Earth's orbit. This ring is supposed to lie in the
+plane of the ecliptic, and to be composed of a multitude of independent
+meteoric particles circulating in closed parallel orbits around the Sun.
+But many difficulties lie in the way of this theory. It seems as
+incompetent to explain the slow and rapid changes in the light of this
+object as it is to explain the contractions and extensions of its cone.
+It fails, moreover, to explain the flickering motions, the coruscations
+observed in its light, or the displacement of its cone and of its axes
+of brightness and symmetry by a mere change in the position of the
+observer. Rev. Geo. Jones, unable to explain by this theory the
+phenomena which came under his observation, has proposed another, which
+supposes the zodiacal light to be produced by a luminous ring
+surrounding the Earth, this ring not extending as far as the orbit of
+the Moon. But this theory also fails in many important points, so that
+at present no satisfactory explanation of the phenomenon can be given.
+</p>
+<p>
+As the phenomenon is connected in some way with the Sun, and as we have
+many reasons to believe this body to be always more or less electrified,
+it might be supposed that the Sun, acting by induction on our globe,
+develops feeble electric currents in the rarefied gases of the superior
+regions of our atmosphere, and there forms a kind of luminous ridge
+moving with the Sun in a direction contrary to the diurnal motion, and
+so producing the zodiacal light. On this hypothesis, the counter-glow
+would be the result of a smaller cone of light generated by the solar
+induction on the opposite point of the Earth.
+</p>
+<p>
+Plate 5, which sufficiently explains itself, represents the zodiacal
+light as it appeared in the West on the evening of February 20th, 1876.
+All the stars are placed in their proper position, and their relative
+brightness is approximately shown by corresponding variations in
+size&mdash;the usual and almost the only available means of representation.
+Of course, it must be remembered that a star does not, in fact, show any
+disk even in the largest telescopes, where it appears as a mere point of
+light, having more or less brilliancy. The cone of light rises obliquely
+along the ecliptic, and the point forming its summit is found in the
+vicinity of the well-known group of stars, called the Pleiades, in the
+constellation of Taurus, or the Bull.
+</p>
+
+<p><br /><br /><br /></p>
+
+<h4><a id="THE_MOON">THE MOON</a>
+<br /><br />
+PLATE VI</h4>
+
+<p>
+In its endless journey through space, our globe is not solitary, like
+some of the planets, but is attended by the Moon, our nearest celestial
+neighbor. Although the Moon does not attain to the dignity of a planet,
+and remains a secondary body in the solar system, yet, owing to its
+proximity to our globe, and to the great influence it exerts upon it by
+its powerful attraction, it is to us one of the most important celestial
+bodies.
+</p>
+<p>
+While the Moon accompanies the Earth around the Sun, it also revolves
+around the Earth at a mean distance of 238,800 miles. For a celestial
+distance this is only a trifling one; the Earth in advancing on its
+orbit travels over such a distance in less than four hours. A cannon
+ball would reach our satellite in nine days; and a telegraphic dispatch
+would be transmitted there in 1½ seconds of time, if a wire could be
+stretched between us and the Moon.
+</p>
+<p>
+Owing to the ellipticity of the Moon's orbit, its distance from the
+Earth varies considerably, our satellite being sometimes 38,000 miles
+nearer to us than it is at other times. These changes in the distance of
+the Moon occasion corresponding changes from 29' to 33' in its apparent
+diameter. The real diameter of the Moon is 2,160 miles, or a little over
+one-quarter the diameter of our globe; our satellite being 49 times
+smaller than the Earth.
+</p>
+<p>
+The mean density of the materials composing the Moon is only
+⁶⁄₁₀ that of the materials composing the Earth, and the force of
+gravitation at the surface of our satellite is six times less than it is
+at the surface of our globe. If a person weighing 150 lbs. on our Earth
+could be transported to the Moon, his weight there would be only 25 lbs.
+</p>
+<p>
+The Moon revolves around the Earth in about 27⅓ days, with a mean
+velocity of one mile per second, the revolution constituting its
+sidereal period. If the Earth were motionless, the lunar month would be
+equal to the sidereal period; but owing to its motion in space, the Sun
+appears to move with the Moon, though more slowly, so that after having
+accomplished one complete revolution, our satellite has yet to advance
+2¼ days before reaching the same apparent position in regard to the
+Earth and the Sun that it had at first. The interval of time comprised
+between two successive New Moons, which is a little over 29½ days,
+constitutes the synodical period of the Moon, or the lunar month.
+</p>
+<p>
+The Moon is not a self-luminous body, but, like the Earth and the
+planets, it reflects the light which it receives from the Sun, and so
+appears luminous. That such is the case is sufficiently demonstrated by
+the phases exhibited by our satellite in the course of the lunar month.
+Every one is familiar with these phases, which are a consequence of the
+motion of the Moon around the Earth. When our satellite is situated
+between us and the Sun, it is New Moon; since we cannot see its
+illuminated side, which is then turned away from us towards the Sun.
+When, on the contrary, it reaches that point of its orbit which, in
+regard to us, is opposite to the Sun's place, it is Full Moon; since
+from the Earth we can only see the fully illuminated side of our
+satellite. Again, when the Moon arrives at either of the two opposite
+points of its orbit, the direction of which from the Earth is at right
+angles with that of the Sun, it is either the First or the Last Quarter;
+since in these positions we can only see one-half of its illuminated
+disk.
+</p>
+<p>
+The curve described by the Moon around the Earth lies approximately in a
+plane, this plane being inclined about 5° to the ecliptic. Since our
+satellite, in its motion around us and the Sun, closely follows the
+ecliptic, which is inclined 23½° to the equator, it results that when
+this plane is respectively high or low in the sky, the moon is also high
+or low when crossing the meridian of the observer. In winter that part
+of the ecliptic occupied by the Sun is below the equator, and,
+consequently, the New Moons occurring in that season are low in the sky,
+since at New Moon our satellite must be on the same side of the ecliptic
+with the Sun. But the Full Moons in the same season are necessarily high
+in the sky, since a Full Moon can only occur when our satellite is on
+the opposite side of the ecliptic from the Sun, in which position it is,
+of course, as many degrees above the equator as the Sun is below. The
+Full Moon which happens nearest to the autumnal equinox is commonly
+called the Harvest Moon, from the fact that, after full, its delays in
+rising on successive evenings are very brief and therefore favorable for
+the harvest work in the evening. The same phenomenon occurs in every
+other lunar month, but not sufficiently near the time of Full Moon to be
+noticeable. When, in spring, a day or two after New Moon, our satellite
+begins to show its thin crescent, its position on the ecliptic is north
+as well as east of that occupied by the Sun; hence, its horns are nearly
+upright in direction, and give it a crude resemblance to a tipping bowl,
+from which many people who are unaware of its cause, and that this
+happens every year, draw conclusions as to the amount of rain to be
+expected.
+</p>
+<p>
+One of the most remarkable features of the Moon's motions is that our
+satellite rotates on its axis in exactly the same period of time
+occupied by its revolution around the Earth, from which it results that
+the Moon always presents to us the same face. To explain this
+peculiarity, astronomers have supposed that the figure of our satellite
+is not perfectly spherical, but elongated, so that the attraction of the
+Earth, acting more powerfully upon its nearest portions, always keeps
+them turned toward us, as if the Moon were united to our globe by a
+string. It is not exactly true, however, that the Moon always presents
+its same side to us, although its period of rotation exactly equals that
+of its revolution; since in consequence of the inclination of its axis
+of rotation to its orbit, combined with the irregularities of its
+orbital motion about us, apparent oscillations in latitude and in
+longitude, called librations, are created, from which it results that
+nearly ⁶⁄₁₀ of the Moon's surface is visible from the Earth at
+one time or another.
+</p>
+<p>
+The Moon is a familiar object, and every one is aware that our
+satellite, especially when it is fully illuminated, presents a variety
+of bright and dark markings, which, from their distant resemblance to a
+human face, are popularly known as "the man in the moon." A day or two
+after New Moon, when the thin crescent of our satellite is visible above
+the western horizon after sunset, the dark portion of its disk is
+plainly visible, and appears of a pale, ashy gray color, although not
+directly illuminated by the Sun. This phenomenon is due to the
+Earth-shine, or to that portion of solar light which the illuminated
+surface of our globe reflects to the dark side of the Moon, exactly in
+the same manner that the Moon-shine, on our Earth, is due to the solar
+light reflected to our globe by the illuminated Moon.
+</p>
+<p>
+Seen with a telescope of moderate power, or even with a good
+opera-glass, the Moon presents a peculiar mottled appearance, and has a
+strong resemblance to a globe made of plaster of Paris, on the surface
+of which numerous roundish, saucer-shaped cavities of various sizes are
+scattered at random. This mottled structure is better seen along the
+boundary line called the <i>terminator</i>, which divides the illuminated
+from the dark side of the Moon. The line of the terminator always
+appears jagged, and it is very easy to recognize that this irregularity
+is due to the uneven and rugged structure of the surface of our
+satellite.
+</p>
+<p>
+A glance at the Moon through a larger telescope shows that the bright
+spots recognized with the naked eye belong to very uneven and
+mountainous regions of our satellite, while the dark ones belong to
+comparatively smooth, low surfaces, comparable to those forming the
+great steppes and plains of the Earth. When examined with sufficient
+magnifying power, the white, rugged districts of the Moon appear covered
+over by numerous elevated craggy plateaus, mountain-chains, and deep
+ravines; by steep cliffs and ridges; by peaks of great height and
+cavities of great depth. This rugged formation, which is undoubtedly of
+volcanic origin, gives our satellite a desolate and barren appearance.
+The rugged tract occupies more than one-half of the visible surface of
+the Moon, forming several distinct masses, the principal of which occupy
+the south and south-western part of the disk. That this formation is
+elevated above the general level is proved by the fact that the
+mountains, peaks, and other objects which compose it, all cast a shadow
+opposite to the Sun; and further, that the length of these shadows
+diminishes with the elevation of the Sun above the lunar horizon.
+</p>
+<p>
+Since Galileo's time the surface of the Moon has been studied by a host
+of astronomers, and accurate maps of its topographical configuration
+have been made, and names given to all features of any prominence. It
+may even be said that in its general features, the visible surface of
+our satellite is now better known to us than is the surface of our own
+Earth.
+</p>
+<p>
+One of the most striking and common features of the mountainous
+districts of the Moon, is the circular, ring-like disposition of their
+elevated parts, which form numerous crater-like objects of different
+sizes and depths. Many thousands of crater-like objects are visible on
+the Moon through a good telescope, and, considering how numerous the
+small ones are, there is, perhaps, no great exaggeration in fixing their
+number at 50,000, as has been done by some astronomers. These volcanic
+regions of the Moon cannot be compared to anything we know, and far
+surpass in extent those of our globe. The number and size of the craters
+of our most important volcanic regions in Europe, in Asia, in North and
+South America, in Java, in Sumatra, and Borneo, are insignificant when
+compared with those of the Moon. The largest known craters on the Earth
+give only a faint idea of the magnitude of some of the lunar craters.
+The great crater Haleakala, in the Sandwich Islands, probably the
+largest of the terrestrial volcanoes, has a circumference of thirty
+miles, or a diameter of a little less than ten miles. Some of the great
+lunar craters, called walled plains, such as Hipparchus, Ptolemæus,
+etc., have a diameter more than ten times larger than that of Haleakala,
+that of the first being 115 miles and that of the last 100 miles. These
+are, of course, among the largest of the craters of the Moon, although
+there are on our satellite a great number of craters above ten miles in
+diameter.
+</p>
+<p>
+The crater-forms of the Moon have evidently appeared at different
+periods of time, since small craters are frequently found on the walls
+of larger ones; and, indeed, still smaller craters are not rarely seen
+on the walls of these last. The walls of the lunar craters are usually
+quite elevated above the surrounding surface, some of them attaining
+considerable elevations, especially at some points, which form peaks of
+great height. Newton, the loftiest of all, rises at one point to the
+height of 23,000 feet, while many others range from ten to twenty
+thousand feet in height. Several craters have their floor above the
+general surface&mdash;Plato, for instance. Wargentin has its floor nearly
+on a level with the summit of its walls, showing that at some period of its
+history liquid lavas, ejected from within, have filled it to the brim
+and then solidified. The floors of some of the craters are smooth and
+flat, but in general they are occupied by peaks and abrupt mountainous
+masses, which usually form the centre. Many of their outside walls are
+partly or wholly covered by numerous ravines and gullies, winding down
+their steep declivities, branching out and sometimes extending to great
+distances from their base. It would seem that these great volcanic
+mouths have at some time poured out torrents of lavas, which, in their
+descent, carved their passage by the deep gullies now visible.
+Sometimes, also, the crater slopes are strewn with debris, giving them a
+peculiar volcanic appearance.
+</p>
+<p>
+Notwithstanding their many points of similarity with the volcanoes of
+the Earth, the lunar craters differ from them in many particulars,
+showing that volcanic forces acting on different globes may produce
+widely different results. For example, the floors of terrestrial craters
+are usually situated at considerable elevations above the general
+surface, while those of the lunar craters are generally much depressed,
+the height of their walls being only about one-half the depth of their
+cavities. Again, while on the Earth the mass of the volcanic cones far
+exceeds the capacity of their openings, on the Moon it is not rare to
+see the capacity of the crater cavities exceeding the mass of the
+surrounding walls. On the Earth, the volcanic cones and mouths are
+comparatively regular and smooth, and are generally due to the
+accumulation of the ashes and the debris of all kinds which are ejected
+from the volcanic mouths. On the Moon, very few craters show this
+character, and for the most part their walls have a very different
+structure, being irregular, very rugged, and composed of a succession of
+concentric ridges, rising at many points to great elevations, and
+forming peaks of stupendous height. Again, many of the larger
+terrestrial craters have their interior occupied by a central cone, or
+several such cones, having a volcanic mouth on their summits; on the
+Moon such central cones are very rare. Although many of the large lunar
+craters have their interior occupied by central masses which have been
+often compared to the central cones of our great volcanoes, yet these
+objects have a very different character and origin. For the most part,
+they are mountainous masses of different forms&mdash;having very rarely any
+craters on them&mdash;and seem to have resulted from the crowding and
+lifting up of the crater floor by the phenomena of subsidence, of which
+these craters show abundant signs. Besides, the terrestrial craters are
+characterized by large and important lava streams, while on the Moon the
+traces of such phenomena are quite rare, and when they are shown, they
+generally differ from those of the Earth by their numerous and
+complicated ramifications, and also by the fact that many of these lava
+streamlets take their origin at a considerable distance from the crater
+slopes, and are grooved and depressed as if the burning liquids which
+are supposed to have produced them had subsequently disappeared, by
+evaporation or otherwise, leaving the furrow empty.
+</p>
+<p>
+The dark spots of the Moon, when viewed through a telescope, exhibit a
+totally different character, and show that they belong to a different
+formation from that of the brighter portions. These darker tracts do not
+seem to have had a direct volcanic origin like the latter, but rather
+appear to have resulted from the solidification of semi-fluid materials,
+which have overflowed vast areas at different times. The surface of this
+system is comparatively smooth and uniform, only some small craters and
+low ridges being seen upon it. The level and dark appearance of these
+areas led the ancient astronomers to the belief that they were produced
+by a liquid strongly absorbing the rays of light, and were seas like our
+seas. Accordingly, these dark surfaces were called <i>Maria</i>, or Seas, a
+name which it is convenient to retain, although it is well known to have
+originated in an error. The so-called seas of the Moon are evidently
+large flat surfaces similar to the deserts, steppes, pampas, and
+prairies of the Earth in general appearance. The great plains of the
+Moon are at a lower level than that of the other formation, and that
+which first attracts the observer's attention is the fact that they are
+surrounded almost on all sides by an irregular line of abrupt cliffs and
+mountain chains, showing phenomena of dislocation. This character of
+dislocation, which is general, and is visible everywhere upon the
+contours of the plains, seems to indicate that phenomena of subsidence,
+either slow or rapid, have occurred on the Moon; while, at the same
+time, the sunken surfaces were overflowed by a semi-fluid liquid, which
+solidified afterwards. The evidences of subsidence and overflowing
+become unmistakable when we observe that, along the borders of the gray
+plains, numerous craters are more or less embedded in the gray
+formation, only parts of the summit of their walls remaining visible, to
+attest that once large craters existed there. The farther from the
+border of the plain the vestiges of these craters are observed, the
+deeper they are embedded in the gray formation. That phenomena of
+subsidence have occurred on a grand scale on the Moon, is further
+indicated by the fact that the singular systems of fractures called
+clefts and rifts generally follow closely the outside border of the gray
+plains, often forming parallel lines of dislocation and fractures. In
+the interior regions of the gray formation, these fractures are
+comparatively rare.
+</p>
+<p>
+The gray, lava-like formation is obviously of later origin than the
+mountainous system to which belong the embedded craters above described.
+Its comparatively recent origin might also be inferred from the
+smallness of its craters and its low ridges. The few large craters
+observed on this formation evidently belong to the earlier system.
+</p>
+<p>
+The color of this system of gray plains is far from being uniform. In
+general appearance it is of a bluish gray, but when observed
+attentively, large areas appear tinted with a dusky olive-green, while
+others are slightly tinged with yellow. Some patches appear brownish,
+and even purplish. A remarkable example of the first case is seen on the
+surface, which encloses within a large parallelogram the two conspicuous
+craters, Aristarchus and Herodotus. This surface evidently belongs to a
+different system from that of the Oceanus Procellarum surrounding it,
+as, besides its color, which totally differs from that of the gray
+formation, its surface shows the rugged structure of the volcanic
+formation.
+</p>
+<p>
+When the Moon is full, some very curious white, luminous streaks are
+seen radiating from different centres, which, for the most part, are
+important craters, occupied by interior mountains. The great crater
+Tycho is the centre of the most imposing of the systems of white
+streaks. Some of the diverging rays of this great centre extend to a
+distance equal to one-quarter of the Moon's circumference, or about
+1,700 miles. The true nature of these luminous streaks is unknown, but
+it seems certain that they have their origin in the crater from which
+they diverge. They do not form any relief on the surface, and are seen
+going up over the mountains and steep walls of the crater, as well as
+down the ravines and on the floors of craters.
+</p>
+<p>
+The Moon seems to be deprived of an atmosphere; or, if it has any, it
+must be so excessively rare that its density is less than of the density
+of the Earth's atmosphere, since delicate tests afforded by the
+occultation of stars have failed to reveal its presence. Although no
+atmosphere of any consequence exists on the Moon, yet phenomena which I
+have observed seem to indicate the occasional presence there of vapors
+of some sort. On several occasions, I have seen a purplish light over
+some parts of the Moon, which prevented well-known objects being as
+distinctly seen as they were at other times, causing them to appear as
+if seen through a fog. One of the most striking of these observations
+was made on January 4th, 1873, on the crater Kant and its vicinity,
+which then appeared as if seen through luminous purplish vapors. On one
+occasion, the great crater Godin, which was entirely involved in the
+shadow of its western wall, appeared illuminated in its interior by a
+faint purplish light, which enabled me to recognize the structure of
+this interior. The phenomenon could not be attributed in this case to
+reflection, since the Sun, then just rising on the western wall of the
+crater, had not yet grazed the eastern wall, which was invisible. It is
+not impossible that a very rare atmosphere composed of such vapors
+exists in the lower parts of the Moon.
+</p>
+<p>
+If the Moon has no air, and no liquids of any sort, it seems impossible
+that its surface can maintain any form of life, either vegetable or
+animal, analogous to those on the Earth. In fact, nothing indicating life
+has been detected on the Moon&mdash;our satellite looking like a barren,
+lifeless desert. If life is to be found there at all, it must be of a
+very elementary nature. Aside from the want of air and water to sustain
+it, the climatic conditions of our satellite are very unfavorable for
+the development of life. The nights and days of the Moon are each equal
+to nearly fifteen of our days and nights. For fifteen consecutive
+terrestrial days the Sun's light is absent from one hemisphere of the
+Moon; while for the same number of days the Sun pours down on the other
+hemisphere its light and heat, the effects of which are not in any way
+mitigated by an atmosphere. During the long lunar nights the temperature
+must at least fall to that of our polar regions, while during its long
+days it must be far above that of our tropical zone. It has been
+calculated that during the lunar nights the temperature descends to 23°
+below zero, while during the days it rises to 468°, or 256° above the
+boiling point.
+</p>
+<p>
+It has been a question among astronomers whether changes are still
+taking place at the surface of the Moon. Aside from the fact that
+change, not constancy, is the law of nature, it does not seem doubtful
+that changes occur on the Moon, especially in view of the powerful
+influences of contraction and dilatation to which its materials are
+submitted by its severe alternations of temperature. From the distance
+at which we view our satellite, we cannot expect, of course, to be able
+to see changes, unless they are produced on a large scale. Theoretically
+speaking, the largest telescopes ever constructed ought to show us the
+Moon as it would appear to the naked eye from a distance of 40 miles;
+but in practice it is very different. The difficulty is in the fact
+that, while we magnify the surface of a telescopic image, we are unable
+to increase its light; so that, practically, in magnifying an object, we
+weaken its light proportionally to the magnifying power employed. The
+light of the Moon, especially near the terminator, where we almost
+always make our observations, is not sufficiently bright to bear a very
+high magnifying power, and only moderate ones can be applied to its
+study. What we gain by enlarging an object, we more than lose by the
+weakening of its light. Besides, a high magnifying power, by increasing
+the disturbances generally present in our atmosphere, renders the
+telescopic image unsteady and very indistinct. On the whole, the largest
+telescopes now in existence do not show us our satellite better than if
+we could see it with the naked eye from a distance of 300 miles or more.
+At such a distance only considerable changes would be visible.
+</p>
+<p>
+Notwithstanding these difficulties, it is believed that changes have
+been detected in Linné, Marius, Messier, and several other craters. An
+observation of mine seems to indicate that changes have recently taken
+place in the great crater Eudoxus. On February 20th, 1877, between 9h.
+30m. and 10h. 30m., I observed a straight, narrow wall crossing this
+crater from east to west, a little to the south of its centre. This wall
+had a considerable elevation, as was proved by the shadow it cast on its
+northern side. Towards its western end this wall appeared as a brilliant
+thread of light on the black shadow cast by the western wall of the
+crater. The first time I had occasion to observe this crater again,
+after this observation, was a year later, on February 17th, 1878; no
+traces of the wall were then detected. Many times since I have tried to
+find this narrow wall again, when the Moon presented the same phase and
+the same illumination, but always with negative results. It seems
+probable that this structure has crumbled down, yet it is very singular
+that so prominent a feature should not have been noticed before.
+</p>
+
+<p><br /></p>
+
+<div class="figcenter" style="width: 400px;">
+<a id="figure06"></a>
+<br />
+<img src="images/figure06.jpg" width="400" alt="" />
+<div class="caption">
+<p>PLATE VI.&mdash;MARE HUMORUM.</p>
+<p class="smaller">From a study made in 1875</p>
+</div></div>
+
+<p><br /></p>
+
+<p>
+The "Mare Humorum," or sea of moisture, as it is called, which is
+represented on Plate VI., is one of the smaller gray lunar plains. Its
+diameter, which is very nearly the same in all directions, is about 270
+miles, the total area of this plain being about 50,000 square miles. It
+is one of the most distinct plains of the Moon, and is easily seen with
+the naked eye on the left-hand side of the disk. The floor of the plain
+is, like that of the other gray plains, traversed by several systems of
+very extended but low hills and ridges, while small craters are
+disseminated upon its surface. The color of this formation is of a dusky
+greenish gray along the border, while in the interior it is of a lighter
+shade, and is of brownish olivaceous tint. This plain, which is
+surrounded by high clefts and rifts, well illustrates the phenomena of
+dislocation and subsidence. The double-ringed crater Vitello, whose
+walls rise from 4,000 to 5,000 feet in height, is seen in the upper
+left-hand corner of the gray plain. Close to Vitello, at the east, is
+the large broken ring-plain Lee, and farther east, and a little below,
+is a similarly broken crater called Doppelmayer. Both of these open
+craters have mountainous masses and peaks on their floor, which is on a
+level with that of the Mare Humorum. A little below, and to the left of
+these objects, is seen a deeply embedded oval crater, whose walls barely
+rise above the level of the plain. On the right-hand side of the great
+plain, is a long <i>fault</i>, with a system of fracture running along its
+border. On this right-hand side, may be seen a part of the line of the
+terminator, which separates the light from the darkness. Towards the
+lower right-hand corner, is the great ring-plain Gassendi, 55 miles in
+diameter, with its system of fractures and its central mountains, which
+rise from 3,000 to 4,000 feet above its floor. This crater slopes
+southward towards the plain, showing the subsidence to which it has been
+submitted. While the northern portion of the wall of this crater rises
+to 10,000 feet, that on the plain is only 500 feet high, and is even
+wholly demolished at one place where the floor of the crater is in
+direct communication with the plain. In the lower part of the <i>mare</i>,
+and a little to the west of the middle line, is found the crater
+Agatharchides, which shows below its north wall the marks of rills
+impressed by a flood of lava, which once issued from the side of the
+crater. On the left-hand side of the plain, is seen the half-demolished
+crater Hippalus, resembling a large bay, which has its interior strewn
+with peaks and mountains. On this same side can be seen one of the most
+important systems of clefts and fractures visible on the Moon, these
+clefts varying in length from 150 to 200 miles.
+</p>
+
+<p><br /><br /><br /></p>
+
+<h4><a id="ECLIPSES">ECLIPSES OF THE MOON</a>
+<br /><br />
+PLATE VII</h4>
+
+<p>
+Since the Moon is not a self-luminous body, but shines by the light
+which it borrows from the Sun, it follows that when the Sun's light is
+prevented from reaching its surface, our satellite becomes obscured. The
+Earth, like all opaque bodies exposed to sunlight, casts a shadow in
+space, the direction of which is always opposite to the Sun's place. The
+form of the Earth's shadow is that of a long, sharply-pointed cone,
+which has our globe for its base. Its length, varying with the distance
+of the Earth from the Sun, is, on an average, 855,000 miles, or 108
+times the terrestrial diameter. This conical shadow of the Earth,
+divided longitudinally by the plane of the ecliptic, lies half above and
+half below that plane, on which the summit of the shadow describes a
+whole circumference in the course of a year. If the Moon's orbit were
+not inclined to the ecliptic, our satellite would pass at every Full
+Moon directly through the Earth's shadow; but, owing to that
+inclination, it usually passes above or below the shadow. Twice,
+however, during each of its revolutions, it must cross the plane of the
+ecliptic, the points of its orbit where this happens being called nodes.
+Accordingly, if it is near a node at the time of Full Moon, it will
+enter the shadow of the Earth, and become either partly or wholly
+obscured, according to the distance of its centre from the plane of the
+ecliptic. The partial or total obscuration of the Moon's disk thus
+produced constitutes a partial or total eclipse of the Moon. The
+essential conditions for an eclipse of the Moon are, therefore, that our
+satellite must not only be full, but must also be at or very near one of
+its nodes.
+</p>
+<p>
+Although inferior in importance to the eclipses of the Sun, the eclipses
+of the Moon are, nevertheless, very interesting and remarkable
+phenomena, which never fail to produce a deep impression on the mind of
+the observer, inasmuch as they give him a clear insight into the silent
+motions of the planetary bodies.
+</p>
+<p>
+At the mean distance of the Moon from the Earth, the diameter of the
+conical shadow cast in space by our globe is more than twice as large as
+that of our satellite. But, besides this pure dark shadow of the Earth,
+its cone is enveloped by a partial shadow called "Penumbra," which is
+produced by the Sun's light being partially, but not wholly, cut off by
+our globe.
+</p>
+<p>
+While the Moon is passing into the penumbra, a slight reduction of the
+light of that part of the disk which has entered it, is noticeable. As
+the progress of the Moon continues, the reduction becomes more
+remarkable, giving the impression that rare and invisible vapors are
+passing over our satellite. Some time after, a small dark-indentation,
+marking the instant of first contact, appears on the eastern or
+left-hand border of the Moon, which is always the first to encounter the
+Earth's shadow, since our satellite is moving from west to east. The
+dark indentation slowly and gradually enlarges with the onward progress
+of the Moon into the Earth's shadow, while the luminous surface of its
+disk diminishes in the same proportion. The form of the Earth's shadow
+on the Moon's disk clearly indicates the rotundity of our globe by its
+circular outline. Little by little the dark segment covers the Moon's
+disk, and its crescent, at last reduced to a mere thread of light,
+disappears at the moment of the second contact. With this the phase of
+totality begins, our satellite being then completely involved in the
+Earth's shadow.
+</p>
+<p>
+The Moon remains so eclipsed for a period of time which varies with its
+distance from the Earth, and with the point of its orbit where it
+crosses the conical shadow. When it passes through the middle of this
+shadow, while its distance from our globe is the least, the total phase
+of an eclipse of the Moon may last nearly two hours. The left-hand
+border of our satellite having gone first into the Earth's shadow, is
+also the first to emerge, and, at the moment of doing so, it receives
+the Sun's light, and totality ends with the third contact. The lunar
+crescent gradually increases in breadth after its exit from the shadow,
+and finally the Moon recovers its fully illuminated disk as before, at
+the moment its western border leaves the Earth's shadow. Soon after, it
+passes out of the penumbra, and the eclipse is over. In total eclipses,
+the interval of time from the first to last contact may last 5h. 30m,
+but it is usually shorter.
+</p>
+<p>
+Soon after the beginning of an eclipse, the dark segment produced by the
+Earth's shadow on the Moon's disk generally appears of a dark grayish
+opaque color, but with the progress of the phenomenon, this dark tint is
+changed into a dull reddish color, which, gradually increasing, attains
+its greatest intensity when the eclipse is total. At that moment the
+color of the Moon is of a dusky, reddish, coppery hue, and the general
+features of the Moon's surface are visible as darker and lighter tints
+of the same color. It sometimes happens, however, that our satellite
+does not exhibit this peculiar coppery tint, but appears either blackish
+or bluish, in which case it is hardly distinguishable from the sky.
+</p>
+<p>
+It is very rare for the Moon to disappear completely during totality,
+and even when involved in the deepest part of the Earth's shadow, our
+satellite usually remains visible to the naked eye, or, at least, to the
+telescope. This phenomenon is to be attributed to the fact that the
+portion of the solar rays which traverse the lower strata of our
+atmosphere are strongly refracted, and bend inward in such a manner that
+they fall on the Moon, and sufficiently illuminate its surface to make
+it visible. The reddish color observed is caused by the absorption of
+the blue rays of light by the vapors which ordinarily-saturate the lower
+regions of our atmosphere, leaving only red rays to reach the Moon's
+surface. Of course, these phenomena are liable to vary with every
+eclipse, and depend almost exclusively on the meteorological conditions
+of our atmosphere.
+</p>
+<p>
+In some cases the phase of totality lasts longer than it should,
+according to calculation. This can be attributed to the fact that the
+Earth is enveloped in a dense atmosphere, in which opaque clouds of
+considerable extent are often forming at great elevations. Such strata
+of clouds, in intercepting the Sun's light, would have, of course, the
+effect of increasing the diameter of the Earth's shadow, in a direction
+corresponding to the place they occupy, and, if the Moon were moving in
+this direction, would increase the phase of total obscuration.
+</p>
+<p>
+The eclipses of the Moon, like those of the Sun, as shown above, have a
+cycle of 18 years, 11 days and 7 hours, and recur after this period of
+time in nearly the same order. They can, therefore, be approximately
+predicted by adding 18y. 11d. 7h. to the date of the eclipses which have
+occurred during the preceding period. During this cycle 70 eclipses will
+occur&mdash;41 being eclipses of the Sun and 29 eclipses of the Moon. At no
+time can there ever be more than seven eclipses in a year, and there are
+never less than two. When there are only two eclipses in a year, they
+are both eclipses of the Sun.
+</p>
+<p>
+Although the number of solar eclipses occurring at some point or other
+of the Earth's surface is greater than that of the eclipses of the Moon,
+yet at any single terrestrial station the eclipses of the Moon are the
+more frequent. While an eclipse of the Sun is only visible on a narrow
+belt, which is but a very small fraction of the hemisphere then
+illuminated by the Sun, an eclipse of the Moon is visible from all the
+points of the Earth which have the Moon above their horizon at the time.
+Furthermore, an eclipse of the Sun is not visible at one time over the
+whole length of its narrow tract, but moves gradually from one end of it
+to the other; while, on the contrary, an eclipse of the Moon begins and
+ends at the very same instant for all places from which it can be seen,
+but, of course, not at the same local time, which varies with the
+longitude of the place.
+</p>
+
+<p><br /></p>
+
+<div class="figcenter" style="width: 400px;">
+<a id="figure07"></a>
+<br />
+<img src="images/figure07.jpg" width="400" alt="" />
+<div class="caption">
+<p>PLATE VII.&mdash;PARTIAL ECLIPSE OF THE MOON.</p>
+<p class="smaller">Observed October 24, 1874</p>
+</div></div>
+
+<p><br /></p>
+
+<p>
+The partial eclipse of the Moon, represented on Plate VII., shows quite
+plainly the configuration of our satellite as seen with the naked eye
+during the eclipse, with its bright and dark spots, and its radiating
+streaks. This eclipse was observed on October 24th, 1874.
+</p>
+
+<p><br /><br /><br /></p>
+
+<h4>THE PLANETS</h4>
+
+<p>
+Around the Sun circulate a number of celestial bodies, which are called
+"<i>Planets</i>." The planets are opaque bodies, and appear luminous
+because their surfaces reflect the light they receive from the Sun.
+</p>
+<p>
+The planets are situated at various distances from the Sun, and revolve
+around this body in widely different periods of time, which are,
+however, constant for each planet, so far as ascertained, and doubtless
+are so in the other cases.
+</p>
+<p>
+The ideal line traced in space by a planet in going around the Sun, is
+called <i>the orbit</i> of the planet; while the period of time employed by
+a planet to travel over its entire orbit and return to its starting point,
+is called <i>the sidereal revolution</i>, <i>or year</i> of the planet. The
+dimensions of the orbits of the different planets necessarily vary with
+the distance of these bodies from the Sun, as does also the length of
+their sidereal revolution.
+</p>
+<p>
+The distance of a planet from the Sun does not remain constant, but is
+subject to variations, which in certain cases are quite large. These
+variations result from the fact that the planetary orbits are not
+perfect circles having the Sun for centre, but curves called
+"<i>Ellipses</i>," which have two centres, or foci, one of which is always
+occupied by the Sun. This is in accordance with Kepler's first law.
+</p>
+<p>
+The ideal point situated midway between the two foci is called <i>the
+centre of the ellipse</i>, or <i>orbit</i>; while the imaginary straight
+line which passes through both foci and the centre, with its ends at
+opposite points of the ellipse, is called "<i>the major axis</i>" of the
+orbit. It is also known as "<i>the line of the apsides</i>." The ideal
+straight line which, in passing through the centre of the orbit, cuts
+the major axis at right angles, and is prolonged on either side to
+opposite points on the ellipse, is called "<i>the minor axis</i>" of the
+orbit.
+</p>
+<p>
+When a planet reaches that extremity of the major axis of its orbit
+which is the nearest to the Sun, it is said to be in its
+"<i>perihelion</i>;" while, when it arrives at the other extremity,
+which is farthest from this body, it is said to be in its
+"<i>aphelion</i>." When a planet reaches either of the two opposite
+points of its orbit situated at the extremities of its minor axis, it is
+said to be at its <i>mean distance</i> from the Sun.
+</p>
+<p>
+The rapidity with which the planets move on their orbits varies with
+their distance from the Sun; the farther they are from this body, the
+more slowly they move. The rapidity of their motion is greatest when
+they are in perihelion, and least when they are in aphelion, having its
+mean rate when these bodies are crossing either of the extremities of
+the minor axes of their orbits.
+</p>
+<p>
+The imaginary line which joins the Sun to a planet at any point of its
+orbit, and moves with this planet around the Sun, is called "<i>the radius
+vector</i>." According to Kepler's second law, whatever may be the distance
+of a planet from the Sun, the radius vector sweeps over equal areas of
+the plane of the planet's orbit in equal times.
+</p>
+<p>
+There is a remarkable relation between the distance of the planets from
+the Sun and their period of revolution, in consequence of which the
+squares of their periodic times are respectively equal to the cubes of
+their mean distances from the Sun. From this third law of Kepler, it
+results that the mere knowledge of the mean distance of a planet from
+the Sun enables one to know its period of revolution, and <i>vice
+versa</i>.
+</p>
+<p>
+The orbit described by the Earth around the Sun in a year, or the
+apparent path of the Sun in the sky, is called "<i>the ecliptic</i>." Like
+that of all the planetary orbits, the plane of the ecliptic passes
+through the Sun's centre. The ecliptic has a great importance in
+astronomy, inasmuch as it is the fundamental plane to which the orbits
+and motions of all planets are referred.
+</p>
+<p>
+The orbits of the larger planets are not quite parallel to the ecliptic,
+but more or less inclined to this plane; although the inclination is
+small, and does not exceed eight degrees. On account of this inclination
+of the orbits, the planets, in accomplishing their revolutions around
+the Sun, are sometimes above and sometimes below the plane of the
+ecliptic. A belt extending 8° on each side of the ecliptic, and,
+therefore, 16° in width, comprises within its limits the orbits of all
+the principal planets. This belt is called "<i>the Zodiac</i>."
+</p>
+<p>
+Since all the planets have the Sun for a common centre, and have their
+orbits inclined to the ecliptic, it follows that each of these orbits
+must necessarily intersect the plane of the ecliptic at two opposite
+points situated at the extremities of a straight line passing through
+the Sun's centre. The two opposite points on a planetary orbit where its
+intersections with the ecliptic occur, are called "<i>the Nodes</i>," and
+the imaginary line joining them, which passes through the Sun's centre, is
+called "the line of the nodes." The node situated at the point where a
+planet crosses the ecliptic from the south to the north, is called "<i>the
+ascending node</i>" while that situated where the planet crosses from north
+to south, is called "<i>the descending node</i>."
+</p>
+<p>
+The planets circulating around the Sun are eight in number, but, beside
+these, there is a multitude of very small planets, commonly called
+"asteroids," which also revolve around our luminary. The number of
+asteroids at present known surpasses two hundred, and constantly
+increases by new discoveries. In their order of distance from the Sun
+the principal planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn,
+Uranus and Neptune. The orbits of the asteroids are comprised between
+the orbits of Mars and Jupiter.
+</p>
+<p>
+When the principal planets are considered in regard to their differences
+in size, they are separated into two distinct groups of four planets
+each, viz.: the small planets and the large planets. The orbits of the
+small planets are wholly within the region occupied by the orbits of the
+asteroids, while those of the large planets are wholly without this
+region.
+</p>
+<p>
+When the planets are considered in regard to their position with
+reference to the Earth, they are called "inferior planets" and "superior
+planets." The inferior planets comprise those whose orbits are within
+the orbit of our globe; while the superior planets are those whose
+orbits lie beyond the orbit of the Earth.
+</p>
+<p>
+Since the orbits of the inferior planets lie within the orbit of the
+Earth, the angular distances of these bodies from the Sun, as seen from
+the Earth, must always be included within fixed limits; and these
+planets must seem to oscillate from the east to the west, and from the
+west to the east of the Sun during their sidereal revolution. In this
+process of oscillation these planets sometimes pass between the Earth
+and the Sun, and sometimes behind the Sun. When they pass between us and
+the Sun they are said to be in "inferior conjunction," while, when they
+pass behind the Sun, they are said to be in "superior conjunction." When
+such a planet reaches its greatest distance, either east or west, it is
+said to be at its greatest elongation east or west, as the case may be,
+or in quadrature.
+</p>
+<p>
+The superior planets, whose orbits lie beyond that of the Earth and
+enclose it, present a different appearance. A superior planet never
+passes between the Earth and the Sun, since its orbit lies beyond that
+of our globe, and, therefore, no inferior conjunction of such a planet
+can ever occur. When one of these planets passes beyond the Sun, just
+opposite to the place occupied by the Earth, the planet is said to be in
+"conjunction;" while, when it is on the same side of the Sun with our
+globe, it is said to be in "opposition." While occupying this last
+position, the planet is most advantageously situated for observation,
+since it is then nearer to the Earth. The period comprised between two
+successive conjunctions, or two successive oppositions of a planet, is
+called its "synodical period." This period differs for every planet.
+</p>
+<p>
+It is supposed that all the planets rotate from west to east, like our
+globe; although no direct evidence of the rotation of Mercury Uranus,
+and Neptune has yet been obtained, it is probable that these planets
+rotate like the others. It results from the rotation of the planets that
+they have their days and nights, like our Earth, but differing in
+duration for every planet.
+</p>
+<p>
+The axes of rotation of the planets are more or less inclined to their
+respective orbits, and this inclination varies but little in the course
+of time. From the inclination of the axes of rotation of the planets to
+their orbits, it results that these bodies have seasons like those of
+the Earth; but, of course, they differ from our seasons in duration and
+intensity, according to the period of revolution and the inclination of
+the axis of each separate planet.
+</p>
+
+<p><br /><br /><br /></p>
+
+<h4><a id="THE_PLANET_MARS">THE PLANET MARS</a>
+<br /><br />
+PLATE VIII</h4>
+
+<p>
+Mars is the fourth of the planets in order of distance from the sun;
+Mercury, Venus and the Earth being respectively the first, second and
+third.
+</p>
+<p>
+Owing to the great eccentricity of its orbit, the distance of Mars from
+the Sun is subject to considerable variations. When this planet is in
+its aphelion, its distance from the Sun is 152,000,000 miles, but at
+perihelion it is only 126,000,000 miles distant, the planet being
+therefore 26,000,000 miles nearer the Sun at perihelion than at
+aphelion. The mean distance of Mars from the Sun is 139,000,000 miles.
+Light, which travels at the rate of 185,000 miles a second, occupies
+12½ minutes in passing from the Sun to this planet.
+</p>
+<p>
+While the distance of Mars from the Sun varies considerably, its
+distance from the Earth varies still more. When Mars comes into
+opposition, its distance from our globe is comparatively small,
+especially if the opposition occurs in August, as the two planets are
+then as near together as it is possible for them to be, their distance
+apart being only 33,000,000 miles. But if the opposition occurs in
+February, the distance may be nearly twice as great, or 62,000,000
+miles. On the other hand, when Mars is in conjunction in August, the
+distance between the two planets is the greatest possible, or no less
+than 245,000,000 miles; while, when the conjunction occurs in February,
+it is only 216,000,000 miles. Hence the distance between Mars and the
+Earth varies from .33 to 245 millions of miles; that is, this planet may
+be 212 million miles nearer to us at its nearest oppositions than at its
+most distant conjunctions.
+</p>
+<p>
+From these varying distances of Mars from the Earth, necessarily result
+great variations in the brightness and apparent size of the planet, as
+seen from our globe. When nearest to us it is a very conspicuous object,
+appearing as a star of the first magnitude, and approaching Jupiter in
+brightness; but when it is farthest it is much reduced, and is hardly
+distinguishable from the stars of the second and even third magnitude.
+In the first position, the apparent diameter of Mars is 26", in the last
+it is reduced to 3" only.
+</p>
+<p>
+The orbit of Mars has the very small inclination of 1° 51' to the plane
+of the ecliptic. The planet revolves around the Sun in a period of 687
+days, which constitutes its sidereal year, the year of Mars being only
+43 days less than two of our years.
+</p>
+<p>
+Mars travels along its orbit with a mean velocity of 15 miles per
+second, being about ⁸⁄₁₀ of the velocity of our globe in its
+orbit. The synodical period of Mars is 2 years and 48 days, during which
+the planet passes through all its degrees of brightness.
+</p>
+<p>
+Mars is a smaller planet than the Earth, its diameter being only 4,200
+miles, and its circumference 13,200 miles. It seems well established
+that it is a little flattened at its poles, but the actual amount of
+this flattening is difficult to obtain. According to Prof. Young, the
+polar compression is ¹⁄₂₁₉.
+</p>
+<p>
+The surface of this planet is a little over ²⁸⁄₁₀₀ of the
+surface of our globe, and its volume is 6½ times less than that of the
+Earth. Its mass is only about ⅒ while its density is about ¾ that of
+the Earth. The force of gravitation at its surface is nearly ¾ of what
+it is at the surface of our globe.
+</p>
+<p>
+The planet Mars rotates on an axis inclined 61° 18' to the plane of its
+orbit, so that its equator makes an angle of 28° 42' with the same
+plane. The period of rotation of this planet, which constitutes its
+sidereal day, is 24 h. 37 m. 23 s.
+</p>
+<p>
+The year of Mars, which is composed of 669⅔ of these Martial days,
+equals 687 of our days, this planet rotating 669⅔ times upon its axis
+during this period. But owing to the movement of Mars around the Sun,
+the number of solar days in the Martial year is only 668⅔, while,
+owing to the same cause, the solar day of Mars is a little longer than
+its sidereal day, and equals 24 h. 39 m. 35 s.
+</p>
+<p>
+The days and nights on Mars are accordingly nearly of the same length as
+our days and nights, the difference being a little less than
+three-quarters of an hour. But while the days and nights of Mars are
+essentially the same as ours, its seasons are almost twice as long as
+those of the Earth. Their duration for the northern hemisphere,
+expressed in Martial days, is as follows: Spring, 191; Summer, 181;
+Autumn, 149; Winter, 147. While the Spring and Summer of the northern
+hemisphere together last 372 days, the Autumn and Winter of the same
+hemisphere last only 296 days, or 76 days less. Since the summer seasons
+of the northern hemisphere correspond to the winter seasons of the
+southern hemisphere, and vice versa, the northern hemisphere, owing to
+its longer summer, must accumulate a larger quantity of heat than the
+last. But on Mars, as on the Earth, there is a certain law of
+compensation resulting from the eccentricity of the planet's orbit, and
+from the fact that the middle of the summer of the southern hemisphere
+of this planet, coincides with its perihelion. From the greater
+proximity of Mars to the Sun at that time, the southern hemisphere then
+receives more heat in a given time than does the northern hemisphere in
+its summer season. When everything is taken into account, however, it is
+found that the southern hemisphere must have warmer summers and colder
+winters than the northern hemisphere.
+</p>
+<p>
+Seen with the naked eye, Mars appears as a fiery red star, whose
+intensity of color is surpassed by no other star in the heavens. Seen
+through the telescope, it retains the same red tint, which, however,
+appears less intense, and gradually fades away toward the limb, where it
+is replaced by a white luminous ring.
+</p>
+
+<p><br /></p>
+
+<div class="figcenter" style="width: 400px;">
+<a id="figure08"></a>
+<br />
+<img src="images/figure08.jpg" width="400" alt="" />
+<div class="caption">
+<p>PLATE VIII.&mdash;THE PLANET MARS.</p>
+<p class="smaller">Observed September 3, 1877, at 11h. 55m. P.M.</p>
+</div></div>
+
+<p><br /></p>
+
+<p>
+Mars is a very difficult object to observe, the atmosphere surrounding
+it being sometimes so cloudy and foggy that the sight can hardly
+penetrate through its vapors. When this planet is observed under
+favorable atmospheric conditions, and with sufficient magnifying power,
+its surface, which is of a general reddish tint, is found to be
+diversified by white, gray and dark markings. The dark markings, which
+are the most conspicuous, almost completely surround the planet. They
+are of different forms and sizes, and very irregular, as can be seen on
+Plate VIII., which represents one of the hemispheres of this planet.
+Many of them, especially those situated in the tropical regions of the
+planet, form long narrow bands, whose direction is in the main parallel
+to the Martial equator.
+</p>
+<p>
+The dusky spots differ very much, both from one another and in their
+several parts, as regards intensity of shade. Some appear almost black,
+while others which appear grayish, are so faint, that they can seldom be
+seen. In the southern hemisphere, the darkest part of the spots is
+generally found along their northern border; especially where there are
+deep indentations.
+</p>
+<p>
+Some observers have described these spots as being greenish or bluish,
+but I have never been able to see the faintest trace of these colors in
+them, except when they were observed close to the limb, and involved in
+the greenish tinted ring which is always to be seen there. It is
+probably an effect of contrast, since green and red are complementary
+colors, and since this greenish tinge around the limb covers all kinds
+of spots, whether white or dark. When such dark spots, involved in the
+greenish tint, are carried by the rotation towards the centre of the
+disk, they no longer show this greenish color. To me, these spots have
+always appeared dark, and of such tints as would result from a mixture
+of white and black in different proportions; except that on their
+lighter portions they show some of the prevalent reddish tint of the
+Martial surface. It is to be remarked that in moments of superior
+definition of the telescopic image, the intensity of darkness of all the
+spots is considerably increased&mdash;some of them appearing almost
+perfectly black.
+</p>
+<p>
+The markings on the surface of Mars are now tolerably well
+known&mdash;especially those of its southern hemisphere, which, owing to
+the greater proximity of the planet to our globe when this hemisphere is
+inclined towards the earth, have been better studied. Those of the
+northern hemisphere are not so well known, since when this hemisphere is
+inclined towards us, the distance of Mars from the Earth is 26,000,000
+miles greater, so that the occasions for observing them are not so
+favorable.
+</p>
+<p>
+Several charts of Mars are in existence, but as the same nomenclature
+has not been employed in all of them, some confusion has arisen in
+regard to the names given to the most remarkable features of the
+planet's surface. In order to give clearness to the subject, it will be
+necessary here to give a brief description of the principal markings
+represented on Plate VIII. In this the nomenclature will be employed
+which has been adopted by the English observers in the fine chart of Mr.
+Nath. Green. The large dark spot represented on the left-hand side of
+the plate is called De La Rue Ocean. The dark oval spot, isolated in the
+vicinity of the centre of the disk, is called Terby Sea; while the dark,
+irregular form on the right, near the border, represents the western
+extremity of Maraldi Sea.
+</p>
+<p>
+The dusky spots of Mars seem to be permanent, and to form a part of the
+general surface of the planet. That several among them, at least, are
+permanent, is proved by the fact that they have been observed in the
+same position, and with the same general form, for over two centuries.
+Yet, if we are to depend upon the drawings made fifty years ago by Beer
+and Maedler, it would seem that the permanency of some of them does not
+exist, since a very large spot represented by these astronomers on their
+chart of Mars is not visible now. This object, which, on their map, has
+its middle at 270°, should be precisely under the prominent dark oval
+spot called Terby Sea, seen near the centre of the picture, and would
+extend down almost as far as the northern limb. This can hardly be
+attributed to an error of observation, since these observers were both
+careful, and had great experience in this class of work. It is a very
+singular fact that, at the very same place where Maedler represented the
+spot in question, I found a conspicuous dark mark on December 16, 1881,
+which was certainly not visible in 1877, during one of the most
+favorable oppositions which can ever occur. The object, which is still
+visible (Feb., 1882), consists of an isolated spot situated a little to
+the north of Terby Sea. During the memorable opposition of 1877, I
+investigated thoroughly the markings of Mars, and made over 200 drawings
+of its disk, 32 of which represent the Terby Sea; but this isolated spot
+was not to be seen, unless it be identified with the faint mark,
+represented on the plate, which occupied its place. There cannot be the
+slightest doubt that a change has occurred at that place. Changes in the
+markings have also been suspected on the other hemisphere of the planet.
+</p>
+<p>
+The well-known fact that the continents, and especially the mountainous
+and denuded districts of our globe, reflect much more light than the
+surfaces covered by water, has led astronomers to suppose that the dark
+spots on Mars are produced by a liquid strongly absorbing the rays of
+light, like the liquids on the surface of the Earth. According to this
+theory the dark spots observed are supposed to be lakes, seas, and
+oceans, similar to our own seas and oceans, while the reddish and
+whitish surfaces separating these dark spots, are supposed to be
+islands, peninsulas, and continents. This supposition seems certainly to
+have a great deal of probability in its favor, although some of the
+lighter markings may have a different origin, and perhaps be due to
+vegetation; but no observer has yet seen in them any of the changes
+which ought to result from change of seasons. Some of the changes in the
+dark spots might also be attributed to the flooding or drying up of
+marshes and low land. The change which I have observed lately might be
+attributed to such a cause, especially as my observation was made
+shortly after the spring equinox of the northern hemisphere of Mars,
+which occurred on December 8th.
+</p>
+<p>
+Besides the dark spots just described, there are markings of a different
+character and appearance. Among the most conspicuous are two very
+brilliant white oval spots, which always occupy opposite sides of the
+planet. These two bright spots, which correspond very closely with the
+poles of rotation of Mars, have been called "polar spots."
+</p>
+<p>
+On account of the inclination of the axis of rotation of this planet to
+the ecliptic, it is rare that both of these spots are visible on the
+disk at the same time; and when this occurs, they are seen considerably
+foreshortened, as they are then both on the limb of the planet. Usually
+only one spot is visible, and it appears to its best advantage when the
+region to which it belongs attains its maximum of inclination towards
+the Earth.
+</p>
+<p>
+The polar spots change considerably in size, as they do also in form.
+Sometimes they occupy nearly one-third of the disk, as is proved by many
+of my observations; while at other times they are so much reduced as to
+be totally invisible. It is to be remarked that the reduction of these
+spots generally corresponds with the summer seasons, and their
+enlargement with the winter seasons of the hemispheres to which they
+respectively belong. From these well-observed facts it would appear that
+a relation exists between the temperature of the two hemispheres of Mars
+and the variations of the white spots observed at its poles. A similar
+relation is known to exist on our globe between the progress of the
+seasons and the melting away and the accumulation of snow in the polar
+regions. Astronomers have been led, accordingly, to attribute the polar
+spots of Mars, with all their variations, to the alternate accumulation
+and melting of snows. On this account, the polar spots of Mars are
+sometimes also called "snow-spots."
+</p>
+<p>
+Errors have certainly been made by astronomers in some of their
+observations of the so-called polar snow-spots, other objects occupying
+their place having been mistaken for them. A regular series of
+observations on this planet, which I have now continued for seven years,
+has revealed the fact that during the winter seasons of the southern
+hemisphere of Mars, the polar spots are most of the time invisible,
+being covered over by white, opaque, cloud-like forms, strongly
+reflecting light. In 1877, during more than a month, I, myself, mistook
+for the polar spots such a canopy of clouds, which covered at least
+one-fifth of the surface of the whole disk. I only became aware of my
+error when the opaque cloud, beginning to dissolve at the approach of
+the Martial summer, allowed the real polar spot to be seen through its
+vapors, as through a mist at first, and afterwards with great
+distinctness. In this particular case, the snow-spot was considerably
+smaller than the cloudy cap which covered it, and it is to be remarked
+that it was not situated at the centre of this cloudy cap, but was east
+of that centre; a fact which may account for the so-called polar spots
+not being always observed on exactly opposite sides of the disk. From my
+observations of 1877, 1878 and 1880, it appears that at the approach of
+the autumnal equinox of the southern hemisphere of Mars, large, opaque
+masses, like cumulus clouds in form, began to gather in the polar
+regions of that hemisphere, and continued through autumn and winter,
+dissolving only at the approach of spring. These clouds, which varied in
+form and extent, were very unsteady at first, but as the winter drew
+nearer they enlarged and became more permanent, covering large surfaces
+for months at a time.
+</p>
+<p>
+That the large white spots under consideration are real clouds in the
+atmosphere of Mars, and are not due to a fall of snow, is proved by the
+fact that these spots covered both seas and continents with equal
+facility, even in the equatorial regions of the planet. Snow, of course,
+could not cover the seas of Mars, unless these were all frozen over,
+even in the equatorial zones; therefore, if the dark spots of Mars are
+assumed to be due to water, these large white spots cannot well be
+ascribed to snow.
+</p>
+<p>
+The real polar spots of Mars seem to be in relief on the surface of the
+planet, since the southern spot often appeared slightly shaded on the
+side opposite to the Sun during my observations in 1877. In certain
+cases, when they are on or very near the limb, they have been observed,
+both by others and by myself, to project from the disk slightly.
+</p>
+<p>
+The polar spots of Mars are doubtless composed of a material which, like
+our snow or ice, melts under the rays of the Sun; although it seems
+difficult to admit that the Martial snow is identical with our
+terrestrial snow, and that it melts at a like temperature. The south
+polar spot of Mars entirely disappeared from sight in its summer season
+in 1877, although the planet receives less than one-half as much heat as
+we receive from the Sun; yet on our globe the arctic or antarctic ices
+and snow are perpetual&mdash;never melting entirely. An important fact
+disclosed by the melting away of the southern polar spot is, that in
+melting it is always surrounded with a very dark surface, which takes
+the place of the melted portion of the spot, as observed by myself in
+1877-78. When the polar spot had entirely disappeared, its place was
+occupied by a very dark spot. Now, if the polar spot is really ice, and
+the dark spots are actual seas, this polar spot must be situated in
+mid-ocean, since, on melting away, it is replaced by a dark spot. If the
+polar spots are composed of a white substance melting under the rays of
+the Sun, as seems altogether probable, its melting point must be above
+that of terrestrial snow.
+</p>
+<p>
+Many of the dark spots of Mars, and especially those whose northern
+border forms an irregular belt upon the equatorial regions of this
+planet, are bordered on that side by a white luminous belt, following
+all their sinuosities. These white borders are variable. Sometimes they
+are very prominent and intensely bright, especially at some points,
+which occasionally almost equal the polar spots in brilliancy; while at
+other times they are so faint, that they can hardly be distinguished, or
+are even invisible; although the atmosphere is clear and the dark spots
+appear perfectly well defined. While these white borders were invisible,
+I have sometimes watched for several hours at a time to see if I could
+detect any traces of them in places where they usually appear the most
+prominent, but generally without success. On a few occasions, however, I
+had the good fortune to see some of these spots forming gradually in the
+course of one or two hours, at places where nothing of them could be
+seen before.
+</p>
+<p>
+These whitish fringes forming and vanishing along the coasts of the
+Martial seas have been very little studied by astronomers. From my
+observations made during the last seven years, it appears very probable
+that this belt and its white spots are mainly due to the condensation of
+vapors around, and over high peaks, and extensive mountain chains,
+forming the Martial sea-coasts, as the Andes and Rocky Mountains form
+the sea-coasts of the Pacific Ocean. These high mountains on Mars,
+condensing the vapors into fogs or clouds above them, or at their sides,
+as often happens in our mountainous districts, would certainly suffice
+to produce the phenomena observed. Some of the highest peaks among these
+mountain chains may even have their summits covered with perpetual snow,
+or some substance partaking of the nature of snow. The temporary
+visibility and invisibility of the white spots seen on Mars, as well as
+the rapid transformations they sometimes undergo, may be explained as
+caused by clouds having a high reflective power and a liability to form
+and disappear quickly.
+</p>
+<p>
+The assumption that these irregular whitish bands and spots are formed
+by the condensation of vapors on mountain chains, and elevated table
+lands, is supported by my observations made in 1877 and 1879. When such
+white spots were traversing the terminator at sunrise, they very often
+projected far into the night side, thus indicating that they were at a
+higher level than that of the general surface. Indentations in the
+terminator, corresponding to large dark spots crossing its line, also
+clearly indicated the depression of the dark spots below the general
+surface. The highest mountainous districts thus observed on Mars, are
+situated between 60° and 70° of south latitude, towards the western
+extremity of Gill Land. The mountain chain, which almost completely
+forms the surface of this land, is so elevated at some points, that they
+not only change the form of the terminator when they are seen upon it,
+but also the limb of the planet, as seen by myself. They then appear so
+brilliant, that the principal summit among them has been mistaken by
+several observers for the polar spot itself, as proved by the wrong
+position assigned to it on their drawings. It seems probable that this
+high peak, which appears always white, is constantly covered with snow,
+or the similar material replacing it on Mars. This high region is
+situated between longitudes 18o° and 190°.
+</p>
+<p>
+The highest mountainous parts belonging to the hemisphere represented on
+Plate VIII., which are nearly always more or less visible as whitish
+spots and bands, form a coast line along the northern (lower) border of
+De La Rue Ocean. This great spot, which is not so simple as it has been
+represented by observers, is in fact divided by two narrow isthmuses,
+one in the north, the other in the east, both joining, in the interior
+of the great ocean, a peninsula heretofore known as Hall Island. Upon
+the south-eastern extremity of this peninsula, a white spot, called
+Dawes Ice Island, was observed in 1865, but it soon disappeared, and was
+after that seen only now and then. It is very probable that this
+so-called Ice Island was due to clouds forming around the summit of some
+high peak of this peninsula.
+</p>
+<p>
+On the opposite hemisphere to that represented on Plate VIII., the white
+fringes bordering the dark spots are much more conspicuous than they are
+on this side. On the eastern side of a remarkable dark spot called
+Kaiser Sea, they are very bright, and almost always present, although
+they vary considerably, both in brightness and in extent. To the south
+of Kaiser Sea, they are very conspicuous on the eastern border of
+Lockyer Land, forming an elevated and deeply indented coast-line along
+Lambert Sea. There the white spots never disappear entirely, being
+always visible on the north side, where they turn westward along Dawes'
+Ocean&mdash;the mountain chain attaining there its greatest altitude. Very
+frequently Lockyer Land, which seems to be a vast plateau, appears
+throughout white and brilliant, this occurring usually towards the
+sunrise or sunset of that region, probably from the condensation of
+vapors and the formation of fogs, but generally this whiteness gradually
+disappears with the progress of the sun above this plateau. Inside of
+the great continents of Mars these temporary white spots are not so
+frequent, but when visible they occupy always the same positions&mdash;a
+fact which probably indicates that they occupy the culminating points of
+these continents. One of these temporary white spots inside of the
+continents is represented on Plate VIII., on the left-hand side, below
+De La Rue Ocean, on Maedler Continent.
+</p>
+<p>
+Although large, opaque, cumulus-shaped, cloud-like forms are seen in the
+polar regions of Mars, such forms are very seldom seen in the tropical
+zones, or, at least, it appears so, from the fact that my observations,
+continued during the last seven years, have disclosed no real opaque
+cloudy forms there. Although the Martial sky is frequently overcast by
+dense vapors or thick fogs in these regions, yet no real opaque clouds
+were ever seen; the most prominent among the dusky spots being faintly
+visible through the vapory veil, when they approached the centre of the
+disk.
+</p>
+<p>
+Besides these phenomena, which prove that Mars is surrounded by an
+atmosphere having a great deal of similarity to our own, a further proof
+is afforded by the fact that the dark spots, which appear sharply
+defined and black when they are seen near the centre, become less and
+less visible as they advance towards the limb, and are totally invisible
+before they reach it. Moreover, the spectroscope also indicates the
+existence of an atmosphere, and even the presence of watery vapor in it.
+A very curious state of the Martial atmosphere is revealed by my
+observations of 1877-78. During eight consecutive weeks, from December
+12th to February 6th, a whole hemisphere of the planet&mdash;precisely that
+represented on Plate VIII.&mdash;was completely covered by dense vapors, or
+a thick fog which barely allowed the dark spots to be seen through it,
+even when they were in the centre of the disk. The opposite hemisphere
+of Mars appeared just as clear and calm as possible; there all the spots
+and their minutest details could be seen, and when the planet was
+observed at the proper time, the line separating the foggy from the
+clear side was plainly visible.
+</p>
+<p>
+The reddish tint observed on the continents of Mars has been supposed by
+some astronomers to be the real color of the atmosphere of this planet.
+But, for many reasons, this explanation is not acceptable. Besides the
+fact that the border of the planet appears white, while it should be
+more red than the other part, owing to the greater depth of atmosphere
+there presented to us, the polar spots, the white bands along the
+sea-coasts, and the cloud-like forms appear perfectly white, not the
+slightest tint of red being visible on them, as would be the case if
+these objects were seen through an atmosphere tinted red. Other
+astronomers have supposed that the vegetation of this planet has a
+reddish color; but this is not supported by observation. It has been
+again supposed, with much more probability, that the surface of Mars is
+composed of an ochreous material which gives the planet its predominant
+ruddy color.
+</p>
+<p>
+Until lately Mars was supposed to be without a satellite, but in August,
+1877, Professor Hall, of the Washington Observatory, made one of the
+most remarkable discoveries of the time, and found two satellites
+revolving around this planet. These satellites are among the smallest
+known heavenly bodies, their diameter having been estimated at from 6 to
+10 miles for the outer satellite, and from 10 to 40 miles for the inner
+one.
+</p>
+<p>
+The most extraordinary feature of these bodies is the proximity of the
+inner satellite to the planet, and the consequent rapidity of its
+motion. The distance of the inner satellite from the centre of Mars is
+about 6,000 miles, and from surface to surface it is less than 4,000
+miles, or a little more than the distance from New York to San
+Francisco. The shortest period of revolution of any satellite previously
+known, is that of the inner satellite of Saturn, which is a little more
+than 22½ hours; but the inner satellite of Mars accomplishes its
+revolution in 7h. 38m., or in 17 hours less than the period of rotation
+of the planet upon its axis. The period of revolution of the outer
+satellite is greater, of course, and equals 30h. 7m.
+</p>
+<p>
+From this rapidity of motion of the inner satellite of Mars, a very
+curious result follows, which at first sight may appear in contradiction
+with the fact that this body has a direct motion, like that of all the
+planets of the solar system, and moves around Mars from west to east.
+While the outer satellite of this planet, in company with all the stars
+and planets, rises in the east and sets in the west, the inner
+satellite, on the contrary, rises in the west and sets in the east.
+Since the period of rotation of Mars is greater than is the period of
+revolution of this satellite, it necessarily follows that this last body
+must constantly be gaining on the rotation, and, consequently, that the
+satellite sets in the east and rises in the west, compassing the whole
+heavens around Mars three times a day, passing through all its phases in
+11 hours, each quarter of this singular Moon lasting less than 3 hours.
+</p>
+<p>
+It has been shown above that Mars has many points of resemblance to the
+Earth. It has an atmosphere constituted very nearly like ours; it has
+fogs, clouds, rains, snows, and winds. It has water, or at least some
+liquids resembling it; it has rivers, lakes, seas and oceans. It has
+also islands, peninsulas, continents, mountains and valleys. It has two
+Moons, which must create great and rapid tides in the waters of its seas
+and oceans. It has its days and nights, its warm and cold seasons, and
+very likely its vegetation, its prairies and forests, like the Earth. On
+the other hand, its year and seasons are double those of the Earth, and
+its distance from the Sun is greater.
+</p>
+<p>
+Is this planet, which is certainly constituted very nearly like our
+globe, and seems so nearly fitted for the wants of the human race,
+inhabited by animals and intelligent beings?
+</p>
+<p>
+To answer this question, either in the negative or in the affirmative,
+would be to step out of the pure province of science, and enter the
+boundless domain of speculation, since no observer has ever seen
+anything indicating that animal life exists on Mars, or on any other
+planet or satellite. So far as observation goes, Mars seems to be a
+planet well suited to sustain animal life, and we may reason from
+analogy that if animal life can exist at all outside of the Earth, Mars
+must have its flora and fauna; it must have its fishes and birds, its
+mammalia and men; although all these living beings must inevitably be
+very different in appearance from their representatives on the Earth, as
+can easily be imagined from the differences existing between the two
+planets. Although all this is possible, and even very probable, yet it
+must be remembered that we have not the slightest evidence that it is
+so; and until we have acquired this evidence, we may only provisionally
+accept this idea as a pleasing hypothesis, which, after all, may be
+wrong and totally unfounded.
+</p>
+
+<p><br /><br /><br /></p>
+
+<h4><a id="THE_PLANET_JUPITER">THE PLANET JUPITER</a>
+<br /><br />
+PLATE IX</h4>
+
+<p>
+Jupiter, the giant of the planetary world, is the fifth in order of
+distance from the Sun, and is next to Mars, our ruddy neighbor. To the
+naked eye, Jupiter appears as a very brilliant star, whose magnitude,
+changing with the distance of this planet from the Earth, sometimes
+approaches that of Venus, our bright morning and evening star.
+</p>
+<p>
+The mean distance of Jupiter from the Sun is 475,000,000 miles, but
+owing to the eccentricity of its orbit, its distance varies from 452 to
+498 millions of miles. The distance of this planet from the Earth varies
+still more. When nearest to our globe, or in opposition, its distance is
+reduced to 384,000,000 miles, and its apparent diameter increased to
+50"; while when it is farthest, or in conjunction, its distance is
+increased to 567,000,000 miles, and its apparent diameter reduced to
+30"; Jupiter being thus 183,000,000 miles nearer our globe while in
+opposition than when it is in conjunction.
+</p>
+<p>
+This planet revolves around the Sun in 11 years, 10 months and 17 days,
+or in only 50 days less than 12 terrestrial years. Such is the year of
+this planet. The plane of its orbit is inclined 1° 19' to the ecliptic.
+No planet, except Uranus, has an orbit exhibiting a smaller inclination.
+The planet advances in its orbit at the mean rate of 8 miles a second;
+which is a little less than half the orbital velocity of the Earth.
+</p>
+<p>
+Jupiter is of enormous proportions. Its equatorial diameter measures
+88,000 miles, and its circumference no less than 276,460 miles, these
+dimensions being 11 times greater than those of the Earth. This planet,
+notwithstanding its huge size, rotates on its axis in not far from 9h.
+55m. 36s., which period constitutes its day. Owing, however, to the
+changeable appearance of its surface, this period cannot be ascertained
+with very great exactitude. In consequence of its rapid rotation, the
+planet is far from spherical, its polar diameter being shorter than the
+equatorial by about ¹⁄₁₆, or 5,500 miles. Its surface is 124
+times the surface of the earth; while its volume is 1,387 times as
+great. If Jupiter occupied the place of our satellite in the sky, it
+would appear 40 times as large as the Moon appears to us, and would
+cover a surface of the heavens 1,600 times that covered by the full
+Moon, and would subtend an angle of 21°. Jupiter's mass does not
+correspond with its great bulk, and is only ¹⁄₁₀₄₇ of the
+mass of the Sun, and 310 times the mass of the earth; its density being
+only ¼ of that of our globe. The force of gravitation at the surface of
+this planet is over 2½ times what it is on the Earth, so that a
+terrestrial object carried to the surface of Jupiter would weigh over
+two and a half times as much as on our globe.
+</p>
+<p>
+Observed with a telescope, even of moderate aperture, Jupiter, with its
+four attending satellites and its dazzling brilliancy, appears as one of
+the most magnificent objects in the sky. The general appearance of the
+disk is white; but unlike that of Mars, it is brightest towards its
+central parts, and a little darker around the limb, especially on the
+side opposite to the Sun. Although an exterior planet, and so far from
+us, Jupiter shows faint traces of phases when observed near its
+quadratures, but this gibbosity of its disk is very slight, and is
+indicated only by a kind of penumbral shadow on the limb.
+</p>
+<p>
+When observed with adequate power, the disk of Jupiter is found to be
+highly diversified. The principal features consist of a series of
+alternate light and dark streaks or bands, disposed most of the time
+parallel with the Jovian equator. These bands differ from each other in
+intensity as well as in breadth; those near the equator being usually
+much more prominent than those situated in higher latitudes north and
+south.
+</p>
+<p>
+The equatorial zone of Jupiter is occupied most of the time by a broad,
+prominent belt 20° or 30° wide, limited on each side by a very dark
+narrow streak. Between these two dark borders, but seldom occupying the
+whole space between them, appears an irregular white belt, apparently
+composed of dense masses of clouds strongly reflecting the Sun's light,
+some of these cloudy masses being very brilliant. The spaces left
+between the cloudy belt and the dark borders, usually exhibit a delicate
+pink or rosy color, which produces a very harmonious effect with the
+varying grayish and bluish shades of some of the belts and streaks seen
+on the disk. Quite often the cloudy belt is broken up, and consists of
+independent cloudy masses, separated by larger or smaller intervals,
+these intervals disclosing the rosy background of this zone.
+</p>
+<p>
+On each side of the equatorial belt there is usually a broad whitish
+belt, succeeded by a narrow gray band; the space left on each hemisphere
+between these last bands and the limb being usually occupied by two or
+three alternate white and gray bands. A uniform gray segment usually
+forms a sort of polar cap to Jupiter.
+</p>
+<p>
+When observed under very favorable conditions, all the lighter belts
+appear as if composed of masses of small cloudlets, resembling the white
+opaque clouds seen in our atmosphere. This, as already stated, is
+particularly noticeable on the equatorial belt. It is not unusual, when
+Jupiter is in quadrature, to see some of the most conspicuous white
+spots casting a shadow opposite to the Sun; a fact which sufficiently
+indicates that these spots are at different levels. They probably form
+the summits of vast banks of clouds floating high up in the atmosphere
+of Jupiter.
+</p>
+<p>
+What we see of Jupiter is chiefly a vaporous, cloudy envelope. If our
+sight penetrates anywhere deeper into the interior, it can only be
+through the narrow fissures of this envelope, which appear as gray or
+dark streaks or spots. That most of the visible surface of Jupiter is
+simply a cloudy covering, is abundantly proved by the proper motion of
+its spots, which sometimes becomes very great.
+</p>
+<p>
+In periods of calm, very few changes are noticeable in the markings of
+the planet, except, perhaps, some slight modifications of form in the
+cloudy, equatorial belt which, in general, is much more liable to
+changes than the other belts. But the Jovian surface is not always so
+tranquil, great changes being observed during the terrific storms which
+sometimes occur on this mighty planet, when all becomes disorder and
+confusion on its usually calm surface; and nothing on the Earth can give
+us a conception of the velocity with which some of its clouds and spots
+are animated. New belts quickly form, while old ones disappear. The
+usual parallelism of the belts no more exists. Huge, white, cumulus-like
+masses advance and spread out, the rosy equatorial belt enlarges
+sometimes to two or three times its usual size, and occupies two-thirds,
+or more, of the disk, the rosy tint spreading out in a very short time.
+At times very dark bands extending across the disk are transformed into
+knots or dark spots, which encircle the planet with a belt, as it were,
+of jet black beads. Sometimes, also, a secondary but narrower rosy belt
+forms either in the northern or the southern hemisphere, and remains
+visible for a few days or for years at a time.
+</p>
+<p>
+On May 25, 1876, I witnessed one of the grandest commotions which can be
+conceived as taking place in an atmosphere. All the southern hemisphere
+of Jupiter, from equator to pole, was in rapid motion, the belts and
+spots being transported entirely across the disk, from the eastern to
+the western limb, in one hour's time, during which the equatorial belt
+swelled to twice its original breadth, towards the south.
+</p>
+<p>
+Now, when one stops for a moment to think what is signified by that
+motion of the dark spots across the little telescopic disk of Jupiter in
+an hour's time, he may arrive at some conception of the magnitude of the
+Jovian storms, compared with those of our globe. The circumference of
+Jupiter's equator, as stated above, is 276,460 miles; half this number,
+or 138,230 miles, represents the length of the equatorial line seen from
+the Earth. Now, after taking into account the rotation of the planet,
+which somewhat diminishes the apparent motion, we arrive at the
+astonishing result that the spots and markings were carried along by
+this Jovian storm, at the enormous rate of 110,584 miles an hour, or
+over 30.7 miles a second. On our globe, a hurricane or tornado, which
+blows at the rate of 100 miles an hour, sweeps everything before it.
+What, then, must be expected from a velocity over 1,105 times as great?
+Enormous as this motion may appear, its occurrence cannot be doubted,
+since it is disclosed by direct observation.
+</p>
+<p>
+The surface of Jupiter, it would seem, has its periods of calm and
+activity like that of the Sun, although it is not yet known, as it is
+for the latter, that they recur with approximate regularity.
+</p>
+<p>
+My observations of this planet, which embrace a period of ten years,
+seem to point in that direction, for they show, at least, that Jupiter
+has its years of calm and its years of disturbances. The year 1876 was a
+year of extraordinary disturbance on Jupiter. Changes in the markings
+were going on all the time, and no one form could be recognized the next
+day, or even sometimes the next hour, as shown above. The cloudy
+envelope of the planet was in constant motion, the equatorial belt,
+especially, showing the signs of greatest disturbance, being, for the
+most part, two or three times as wide as in other years. After 1876 the
+calm was very great on the planet, only a slight change now and then
+being noticeable, the same forms being recognized day after day, month
+after month, and even year after year. In one case the same marking has
+been observed for seventeen consecutive months, and in another for
+twenty-eight months. This state of quietude lasted until October, 1880,
+when considerable commotion occurred on the northern hemisphere, where
+large round black spots, somewhat resembling the Sun-spots, formed in
+the cloudy atmosphere, and finally changed, towards the end of December,
+into a narrow pink belt, which still exists.
+</p>
+
+<p><br /></p>
+
+<div class="figcenter" style="width: 400px;">
+<a id="figure09"></a>
+<br />
+<img src="images/figure09.jpg" width="400" alt="" />
+<div class="caption">
+<p>PLATE IX.&mdash;THE PLANET JUPITER.</p>
+<p class="smaller">Observed November 1, 1880, at 9h. 30m. P.M.</p>
+</div></div>
+
+<p><br /></p>
+
+<p>
+The most curious marking ever seen on Jupiter is undoubtedly the great
+Red Spot, observed on the southern hemisphere of this planet for the
+last three years. This interesting object, seen first in July, 1878,
+disappeared for a time, reappeared on September 25 of the same year, and
+has remained visible until now. When seen by me in September, it was
+much elongated, and sharply pointed on one side, like a spear-head, but
+it subsequently acquired an irregular form, with short appendages
+protruding from its northern border. At first, the changes were great
+and frequent, but at length it acquired the regular oval form, which,
+with but slight modifications, it has retained until now. During the
+month of November, 1880, I noticed two small black specks upon this Red
+Spot, and they were seen again in January of the succeeding year, by Mr.
+Alvan Clark, Jr. When the spot had attained its oval shape, it appeared
+part of the time surrounded with a white luminous ring of cloudy forms
+which, however, was changing more or less all the time, being sometimes
+invisible. The color of this curious spot is a brilliant rosy red,
+tinged with vermilion, and altogether different in shade from the
+pinkish color of the equatorial belt. The size of the spot varies, but
+of late its changes have been slight. Its longer diameter may be
+estimated at 8,000 miles, and the shorter at 2,200 miles. The Red Spot
+is represented on Plate IX. with its natural color, and as it appears at
+the moments most favorable for observation. In ordinary cases its color
+does not appear so brilliant, but paler.
+</p>
+<p>
+It is difficult to account for the color of the equatorial belt and that
+of the Red Spot; but it is known, at least, that the material to which
+they are due cannot be situated at the level of the general surface
+visible to us, and especially that of the cloudy forms of the equatorial
+zone. Undoubtedly the red layer lies deeper than the superficial
+envelope of the planet, although it does not seem to be very deeply
+depressed.
+</p>
+<p>
+Jupiter is attended by four satellites, which revolve around the planet
+at various distances, and shine like stars of the 6th and 7th magnitude.
+It is said that under very favorable circumstances, and in a very clear
+sky, the satellites can be seen with the naked eye, but this requires
+exceptionally keen eyes, since the glare of the planet is so strong as
+to overpower the comparatively faint light of the satellites. However, I
+myself have sometimes seen, without the aid of the telescope, two or
+three of the satellites as a single object, when they were closely
+grouped on the same side of Jupiter.
+</p>
+<p>
+The four moons of Jupiter are all larger than our Moon, except the
+first, which has about the same diameter. They range in size from 2,300
+to 3,400 miles in diameter, the third being the largest; the
+determination of their diameter is by no means accurate, however, as it
+is difficult to measure such small objects with precision. Their mean
+distance from the centre of Jupiter varies from 267,000 to 1,192,000
+miles, the first satellite, the nearest to the planet, being a little
+farther from Jupiter then our satellite is from us. The four satellites
+revolve around the planet in orbits whose planes have a slight
+inclination to the equator of Jupiter, and consequently to the ecliptic.
+The diameter of the largest satellite is nearly half that of the Earth,
+or 3,436 miles; while its volume is five times that of our Moon. The
+period of revolution of these satellites varies from 1d. 18h. for the
+first, to 16d. 16h. for the last.
+</p>
+<p>
+Owing to the slight inclination of the plane of their orbits to that of
+the planet, the three first satellites, and generally the fourth, pass
+in front of the disk and also through the shadow of the planet at every
+revolution, and are accordingly eclipsed. Their passages behind
+Jupiter's disk are called occultations; those in front of it, transits.
+The eclipses, the occultations and the transits of the moons of Jupiter
+are interesting and important phenomena; the eclipses being sometimes
+observed for the rough determination of longitudes at sea.
+</p>
+<p>
+The satellites in transit present some curious phenomena. When they
+enter the disk, they appear intensely luminous upon its grayish border;
+but as they advance, they seem by degrees to lose their brightness,
+until they finally become undistinguishable from the luminous surface of
+Jupiter. It sometimes happens, however, that the first, the third and
+the fourth satellites, after ceasing to appear as bright spots, continue
+to be visible as dark spots upon the bright central portions of the
+planet's disk; but in these cases their disks appear smaller than the
+shadows they cast. Undoubtedly these satellites have extensive
+atmospheres, since they sometimes pass unperceived across the central
+parts of Jupiter, this being probably when their atmospheres are
+condensed into clouds, strongly reflecting light; while when these
+clouds are absent, we can see their actual surface, with traces of the
+dark spots upon them similar to those on Mars.
+</p>
+<p>
+From the variation in the brightness of these satellites, which is said
+to be always observed in the same part of their orbit, William Herschel
+was led to suppose that these bodies, like our Moon, rotate upon their
+axes in the same period in which they move round the planet, so that
+they always present the same face to Jupiter; but these conclusions have
+been denied. From my observations it is apparent, however, that the
+light reflected by them varies in intensity as well as in color. But
+this is rather to be attributed to the presence of an atmosphere
+surrounding these bodies, which when cloudy reflects more light than
+when clear, with corresponding changes in the color of the light.
+</p>
+<p>
+The satellites in transit are sometimes preceded or followed, according
+to the position of the Sun, by a round black spot having about the same
+size as the satellite itself. This black spot is the shadow of the
+satellite cast on the vapory envelope of Jupiter, similar to the shadow
+cast by the Moon on the Earth, during eclipses of the Sun; in fact, all
+the Jovian regions traversed by these shadows have the Sun totally
+eclipsed. Sometimes it happens that the shadow appears elliptical. This
+occurs either when it is observed very near the limb, or when entering
+upon a round, cloud-like spot. This effect is attributable to the
+perspective under which the shadow is seen on the spherical globe or
+spot.
+</p>
+<p>
+The proper motion of the satellites in the Jovian sky is much more rapid
+than that of the Moon in our sky. During one Jovian day of ten hours,
+the first satellite advances 84°; the second, 42°; the third, 20° and
+the fourth, 9°. The first satellite passes from New Moon to its first
+quarter in a little more than a Jovian day, while the fourth occupies
+ten such days in attaining the same phase.
+</p>
+<p>
+In density, as well as in physical constitution, Jupiter differs widely
+from the interior planets, and especially from the Earth; and, as has
+been shown, it is surrounded by a dense, opaque, cloudy layer, which is
+almost always impenetrable to the sight, and hides from view the
+nucleus, which we may conceive to exist under this vaporous envelope. In
+1876, the year of the great Jovian disturbances, I observed frequently
+in the northern hemisphere of the planet a very curious phenomenon,
+which seems to prove that its cloudy envelope is at times partially
+absent in some places, its vapors being apparently either condensed, or
+transported to other parts of its surface, and that, therefore, a
+considerable part of the real globe of the planet was visible at these
+places. The phenomenon consisted in the deformation of the northern
+limb, which had a much shorter radius on all of this hemisphere situated
+northward of the white belt which adjoins the equatorial zone. The
+deformation of the limb on both sides, where it passed from a longer to
+a shorter radius, was abrupt, and at right angles to the limb, forming
+there a steplike indentation which was very prominent. The polar segment
+having a smaller radius, appeared unusually dark, and was not striped,
+as usual, but uniform in tint throughout. On September 27th, the third
+satellite passed over this dark segment, and emerged from the western
+border, a little below the place where the limb was abruptly deformed,
+as above described. When the satellite had fully emerged from this limb,
+it was apparent that if the portion of the limb having a longer radius
+had been prolonged a little below, and as far as the satellite, it would
+have enclosed it within its border, and thus retarded the time of
+emersion. The depth of deformation of the limb was accordingly greater
+than the diameter of the third satellite, and certainly more than 4,000
+miles. That the phenomenon was real, is proved by the fact that the
+egress of this satellite occurred at least four minutes sooner than the
+time predicted for it in the American Ephemeris. Other observations seem
+to point in the same direction, since some of the satellites which were
+occulted have been seen through the limb of Jupiter by different
+astronomers, as if this limb was sometimes semi-transparent. Another
+observation of mine seems to confirm these conclusions. On April 24th,
+1877, at 15h. 25m. the shadow of the first satellite was projected on
+the dark band forming the northern border of the equatorial belt, the
+shadow being then not far from the east limb. Close to this shadow, and
+on its western side, it was preceded by a secondary shadow, which was
+fainter, but had the same apparent size. This round dark spot was not
+the satellite itself, as I had supposed at first, since this object was
+yet outside of the planet, on the east, and entered upon it only at 16h.
+4m. I watched closely this strange phenomenon, and at 16h. 45m., when
+the shadow had already crossed about ¾ of the disk, it was still
+preceded by the secondary, or mock shadow, as it may be called; the same
+relative distance having been kept all the while between the two
+objects, which had therefore traveled at the same rate. It is obvious
+that this dark spot could not be one of the planet's markings, since the
+shadow of the first satellite moves more quickly on the surface of
+Jupiter than a spot on the same surface travels by the effect of
+rotation, so that in this case the shadow would soon have passed over
+this marking, and left it behind, during the time occupied by the
+observation. From these observations it seems very probable that Jupiter
+has a nucleus, either solid or liquid, which lies several thousand
+mites below the surface of its cloudy envelope. It is also probable that
+the uniformly shaded dark segment seen in 1876, was a portion of the
+surface of this nucleus itself. When the cloudy envelope is
+semi-transparent at the place situated on a line with an occulted
+satellite and the eye of an observer, this satellite may accordingly
+remain visible for a time through the limb, as shown by observation. The
+phenomenon of the mock shadow may also be attributed to a similar cause,
+where semi-transparent vapors receive the shadow of a satellite at their
+surface, while at the same time part of this shadow, passing through the
+semi-transparent vapors, may be seen at the surface of the nucleus, or
+of a layer of opaque clouds situated at some distance below the surface.
+</p>
+<p>
+Some astronomers are inclined to think that Jupiter is at a high
+temperature, and self-luminous to a certain extent. If this planet is
+self-luminous to any degree, we might expect that some light would be
+thrown upon the satellites when they are crossing the shadow cast into
+space by the planet; but when they cross this shadow they are totally
+invisible in the best telescopes, a proof that they do not receive much
+light from the non-illuminated side of Jupiter. It would, indeed, seem
+probable that some of the intensely white spots occasionally seen on the
+equatorial belt of the planet are self-luminous in a degree, yet not
+enough to render the satellites visible while they are immersed in
+Jupiter's shadow. It does not seem impossible that the planet should
+have the high temperature attributed to it, when we remember the
+terrific storms observed in its atmosphere, which, owing to the great
+distance of Jupiter from the Sun, do not seem to be attributable to this
+body, but rather to some local cause within the envelope of the planet.
+</p>
+<p>
+Astronomy, which is a science of observation, is naturally silent with
+regard to the inhabitants of Jupiter. If there are any such inhabitants,
+they are confined to the domain of conjecture, under the dense cloudy
+envelope of the planet. The conditions of habitability on Jupiter must
+differ very widely from those of our globe. Comparatively little direct
+light from the Sun reaches the surface of the globe of Jupiter, except
+that which passes through the narrow openings forming the dark clouds.
+All the rest of the planet's surface, being covered perpetually by
+opaque clouds, receives only diffused light. On Jupiter there are
+practically no seasons, since its axis is nearly perpendicular to its
+orbit. The force of gravity on the surface of Jupiter being more than
+double what it is on the Earth, living bodies would there have more than
+double the weight of similar bodies on the Earth. Furthermore, Jupiter
+only receives 0.011 of the light and heat which we receive from the Sun;
+and its year is nearly equal to 12 of our years. If there are living
+beings on Jupiter, they must, then, be entirely different from any known
+to us, and they may have forms never dreamed of in our most fantastic
+conceptions.
+</p>
+<p>
+The two round black spots represented towards the central parts of Plate
+IX. are the shadows of the first and second satellite; while the two
+round white spots seen on the left of the disk, are the satellites
+themselves, as they appeared at the moment of the observation. The first
+satellite and its shadow are the nearest to the equator; while the
+second satellite and its shadow are higher, the last being projected on
+the Great Red Spot.<a name="FNanchor_2_1" id="FNanchor_2_1"></a><a href="#Footnote_2_1" class="fnanchor">[2]</a> The row of dark circular spots represented on the
+northern, or lower hemisphere, when they first appeared, had some
+resemblance to Sun-spots without a penumbra, with bright markings around
+them, resembling faculæ. These round spots subsequently enlarged
+considerably, until they united along the entire line, encircling the
+planet, and finally forming a narrow pink belt, which is still visible.
+</p>
+
+<p><br /></p>
+
+<div class="footnote">
+
+<p class="nind"><a name="Footnote_2_1" id="Footnote_2_1"></a><a href="#FNanchor_2_1"><span class="label">[2]</span></a>By an accidental error in enlarging the original drawing,
+the satellites and shadows appear in Plate IX. of double their actual
+size. The error is one easy of mental correction.</p></div>
+
+<p><br /><br /><br /></p>
+
+<h4><a id="THE_PLANET_SATURN">THE PLANET SATURN</a>
+<br /><br />
+PLATE X</h4>
+
+<p>
+Saturn, which is next to Jupiter in order of distance from the Sun,
+while not the largest, is certainly the most beautiful and interesting
+of all the planets, with his grand and unique system of rings, and his
+eight satellites, which, like faithful servants, attend the planet's
+interminable journey through space.
+</p>
+<p>
+Seen with the naked eye, Saturn shines in the night like a star of the
+first magnitude, whose dull, soft whiteness is, however, far from
+attaining the brilliancy of Venus or Jupiter, although it sometimes
+approaches Mars in brightness. Saturn hardly ever exhibits the
+phenomenon of scintillation, or twinkling, a peculiarity which makes it
+easily distinguishable among the stars and planets of the heavens.
+</p>
+<p>
+The synodical period of Saturn occupies 1 year and 13 days, so that
+every 378 days, on an average, this planet holds the same position in
+the sky relatively to the Sun and the Earth.
+</p>
+<p>
+The mean distance of Saturn from the Sun is a little over 9½ times that
+of our globe, or 872,000,000 miles. Owing to the orbital eccentricity,
+this distance may increase to 921,000,000 miles, when the planet is in
+aphelion; or decrease to 823,000,000 miles, when it is in perihelion;
+Saturn being therefore 98,000,000 miles nearer to the Sun when in
+perihelion than in aphelion. If gravitation were free to exert its
+influence alone, Saturn would fall into the Sun in 5 years and 2 months.
+</p>
+<p>
+The distance of Saturn from the Earth varies, according to the position
+of the two planets in their respective orbits. At the time of
+opposition, when the Earth lies between the Sun and Saturn, this
+distance is smallest; while, on the contrary, at the time of
+conjunction, when the Sun lies between the Earth and Saturn, it is
+greatest. Owing, however, to the eccentricity of the orbits of Saturn
+and our globe, and the inclination of their planes to each other, and
+owing also to the variable heliocentric longitude of the perihelion, the
+distance of the two planets from each other at their successive
+conjunctions and oppositions is rendered extremely variable. At present
+it is when the oppositions of Saturn occur in December that this planet
+comes nearest to us; while when the conjunctions take place in June, the
+distance of Saturn from the Earth is the greatest possible. In the
+former case the distance of the planet from our globe is only
+730,000,000 miles; while in the last it is 1,014,000,000 miles, the
+difference between the nearest and farthest points of Saturn's approach
+to us being no less than 284,000,000 miles, or over three times the mean
+distance of the Sun from the Earth.
+</p>
+<p>
+From the great variations in the distance of Saturn from the Earth,
+necessarily result corresponding changes in the brightness and apparent
+diameter of this body. When it is farthest from us, its angular diameter
+measures but 14"; while, when it is nearest, it measures 20".
+</p>
+<p>
+The orbit of Saturn is inclined 2°30' to the ecliptic, and its
+eccentricity, which equals 0,056, is over three times that of the
+Earth's orbit.
+</p>
+<p>
+This planet revolves around the Sun in a period of 29 years and 5½
+months, or 10,759 terrestrial days, which constitutes its sidereal year.
+The extension of the immense curve forming the orbit of this planet, is
+no less than 5,505,000,000 miles, which is traversed by the planet with
+a mean velocity of a little less than 6 miles per second, or three times
+less than the motion of our globe in space.
+</p>
+<p>
+The real dimensions of the globe of Saturn are not yet known with
+accuracy, and the equatorial diameter has been variously estimated by
+observers, at from 71,000 to 79,000 miles. If we adopt the mean of these
+numbers, 75,000 miles, the circumference of the Saturnian equator would
+measure 235,620 miles, or 9½ times the circumference of our globe; the
+surface of Saturn would be 86 times, and its volume over 810 times that
+of the Earth.
+</p>
+<p>
+However great the volume of Saturn, its mass is proportionally small,
+being only 90 times greater than that of our globe; the mean density of
+the materials composing this planet being less than that of cork, and
+only 0.68 the density of water. The force of gravitation at the surface
+of Saturn is greater, by a little over ⅑, than it is at the surface of
+the Earth; a body falling in a vacuum at its surface, would travel 17.59
+feet during the first second.
+</p>
+<p>
+From observations of markings seen on the surface of Saturn, and from
+the study of their apparent displacements on the disk, William Herschel
+found that the planet rotated upon its axis in 10h. 16m. 0.24s. Since
+Herschel's determination, new researches have been made, and lately,
+Professor Hall, noticing a bright spot, followed it for nearly a month,
+observing its transits across the central meridian of the disk. From
+these observations he has obtained for the rotation period 10h. 14m.
+23.8s., a result which agrees very closely with that obtained 82 years
+earlier by Herschel, considering the fact that the markings from which
+the period of rotation is ascertained are not fixed on the planet, but
+are always more or less endowed with proper motion. The velocity of
+rotation at the equator is 21,538 miles per hour, or nearly 6 miles per
+second.
+</p>
+<p>
+The axis of rotation of Saturn is inclined 64° 18' to the plane of the
+orbit, so that its equator makes an angle of 25° 42' with the same
+plane. The seasons of this planet therefore present greater extremes of
+temperature than those of the Earth, but not quite so great variations
+as the seasons of Mars.
+</p>
+<p>
+The globe of Saturn is not a perfect sphere, but its figure is that of
+an oblong spheroid, flattened at the poles. The polar compression of
+Saturn is greater than that of any other planet, surpassing even that of
+Jupiter. Though not yet determined with a great degree of accuracy, the
+compression is known to be between ²⁄₁₈ and ⅒ of the equatorial
+diameter; that is, a flattening of about 3,894 miles, at each pole, the
+polar diameter being 7,788 miles shorter than the equatorial.
+</p>
+<p>
+The internal condition of the planet Saturn, whether solid, liquid or
+gaseous, cannot be discovered from the examination of its surface, as
+its globe is enwrapped in a dense opaque layer of vapors and cloud-like
+forms, through which the sight fails to penetrate. The appearance of
+this vapory envelope is like that of <i>cumulus</i> clouds, and one of its
+characteristics is to arrange itself into alternate bright and dark
+parallel belts, broader than those seen on Jupiter, and also more
+regular and dark. These belts, which are parallel to the equator of the
+planet, vary in curvature with the inclination of its axis of rotation
+to the line of sight.
+</p>
+<p>
+The belts of Saturn, like those of Jupiter, are not permanent, but keep
+changing more or less rapidly. Sometimes they have been observed to be
+quite numerous; while at other times they are few. Occasionally
+conspicuous white or dark spots are seen on the surface, although the
+phenomenon is quite rare. It is from the observation of such spots that
+Saturn's period of rotation has been determined, as stated above. The
+equatorial zone of Saturn always appears more white and brilliant than
+the other parts, as it also appears more mottled and cloud-like. In late
+years the globe has been characterized, and much adorned, by a pale
+pinkish tint on its equatorial belt, resembling that of Jupiter, but
+somewhat fainter. On either side of the equatorial belt there is a
+narrower band, upon which the mottled appearance is visible. Below
+these, one or two dark belts, separated by narrow white bands, are
+usually seen; but, of late, the bands have been less numerous, being
+replaced in high latitudes by a dark segment, which forms a polar cap to
+Saturn. The globe of Saturn does not anywhere appear perfectly white,
+and when compared with its ring, it looks of a smoky yellowish tint,
+which becomes an ashy gray on its shaded parts. It usually appears
+darker near the limb than in its central portions; although on some
+occasions I have seen portions of the limb appear brighter, as if some
+white spots were traversing it.
+</p>
+<p>
+Some observers have seen the limb deformed and flattened at different
+places, and W. Herschel even thought such a deformation to be a
+permanent feature of this globe, which he termed diamond-shaped, or
+"square shouldered." But this was evidently an illusion, since the
+planet's limb usually appears perfectly elliptical, although it
+occasionally appears as if flattened at some points, especially where it
+comes in apparent contact with the shadow cast by the globe on the ring,
+as observed by myself many times. But with some attention, it is
+generally found that this deformation is apparent rather than real, and
+is caused by the passage of some large dark spots over the limb, which
+is thus rendered indistinguishable from the dark background upon which
+it is projected.
+</p>
+<p>
+What distinguishes Saturn from all known planets, or heavenly bodies,
+and makes it unique in our universe, is the marvelous broad flat ring
+which encircles its equator at a considerable distance from it. With a
+low magnifying power this flat ring appears single, but when carefully
+examined with higher powers, it is found to consist of several distinct
+concentric rings and zones, all lying nearly in the same plane with the
+planet's equator.
+</p>
+
+<p><br /></p>
+
+<div class="figcenter" style="width: 400px;">
+<a id="figure10"></a>
+<br />
+<img src="images/figure10.jpg" width="400" alt="" />
+<div class="caption">
+<p>PLATE X.&mdash;THE PLANET SATURN.</p>
+<p class="smaller">Observed on November 30, 1874, at 5h. 30m. P.M.</p>
+</div></div>
+
+<p><br /></p>
+
+<p>
+At first sight only two concentric rings are recognized, the
+<i>outer</i> and the <i>middle</i>, or <i>intermediary</i>, which are
+separated by a wide and continuous black line, called the <i>principal
+division</i>. This line, and indeed all the features of the surface of
+the rings are better seen, and appear more prominent on that part of the
+ring on either side called the <i>ansa</i>, or handle. Besides these two
+conspicuous rings, a third, of very dark bluish or purplish color, lies
+between this middle ring, to which it is contiguous, and the planet.
+This inner ring, which is quite wide, is called the <i>gauze</i> or
+<i>dusky ring</i>. Closer examination shows that the outer ring is
+itself divided by a narrow, faint, grayish line called the pencil line,
+which, from its extreme faintness, is only visible on the ansæ.
+Moreover, the middle ring is composed of three concentric zones, or
+belts, which, although not apparently divided by any interval of space,
+are distinguished by the different shadings of the materials composing
+them. The outer zone of this compound middle ring is, by far, the
+brightest of all the system of rings and belts, especially close to its
+external border, where, on favorable occasions, I have seen it appear on
+the ansæ as if mottled over, and covered throughout with strongly
+luminous cloud-like masses. On the ansæ of the double outer ring,
+similar cloudy forms have also been seen at different times. The second
+zone of the middle ring is darker than the first, the innermost being
+darker still. All the characteristic points which have thus been
+described, are shown in Plate X.
+</p>
+<p>
+Although suspected in 1838, the dusky ring was not recognized before
+1850, when G. P. Bond discovered it with the 15-inch refractor of the
+Cambridge Observatory. It was also independently discovered the same
+year in England by Dawes and Lassell. The dusky ring differs widely in
+appearance and in constitution from the other rings, inasmuch as these
+last are opaque, and either white or grayish, while the former is very
+dark, and yet so transparent that the limb of the planet is plainly seen
+through its substance. On particularly favorable occasions, the
+appearance of this ring resembles that of the fine particles of dust
+floating in a ray of light traversing a dark chamber. Whatever may be
+the material of which this ring is composed, it must be quite rarefied,
+especially towards its inner border, which appears as if composed of
+distinct and minute particles of matter feebly reflecting the solar
+light. That the inner part of the dusky ring is composed of separate
+particles, is proved by the fact that the part of the ring which is seen
+in front of the globe of Saturn has its inner border abruptly deflected
+and curved inward on entering upon the disk, causing it to appear
+considerably narrower than it must be in reality, a peculiarity which is
+shown in the Plate. This phenomenon may be attributed to an effect of
+irradiation, due to the strong light reflected by the central parts of
+the ball, which so reduces the apparent diameter of the individual
+particles that they become invisible to us, especially those near the
+inner border, which are more scattered and less numerous than elsewhere.
+</p>
+<p>
+The dusky ring, which was described by Bond, Lassell and other
+astronomers as being equally transparent throughout all its width, has
+not been found so by me in later years. The limb of the planet, seen by
+these observers through the whole width of the dusky ring in 1850, could
+not be traced through its outer half by myself in 1872 and 1874, and
+this with the very same instrument used by Bond in his observations of
+1848 and 1850. Moreover, I have plainly seen that its transparency was
+not everywhere equal, but greatest on the inner border, from which it
+gradually decreases, until it becomes opaque, as proved by the gradual
+loss of distinctness of the limb, which vanishes at about the middle of
+the dusky ring. These facts, which have been well ascertained, prove
+that the particles composing this ring are not permanently located, and
+are undergoing changes of relative position. It will be shown that the
+surface of the other rings is also subject to changes, which are
+sometimes very rapid.
+</p>
+<p>
+The globe of Saturn is not self-luminous, but opaque. It shines by the
+solar light, as is proved by the shadow it casts opposite the Sun upon
+the ring. Although receiving its light from the Sun, Saturn does not
+exhibit any traces of phases, like the other planets nearer to the Sun,
+owing to its great distance from the Earth. When near its quadratures,
+however, the limb opposite to the Sun appears much darker, and shows
+traces of twilight. As far as can be ascertained, the rings, with the
+exception of the inner one, are opaque, as proved by the strong shadow
+which they cast on the globe of Saturn.
+</p>
+<p>
+The shadows cast by the planet on the ring, and by the ring on the
+planet, are very interesting phenomena, inasmuch as they enable the
+astronomer to recognize the form of the surface which receives them. The
+shadow cast by the ring on the ball is not quite so interesting as the
+other, although it has served to prove that the surface of this globe is
+not smooth, as is likewise suggested by its mottled appearance. I have
+sometimes found, as have also other observers, that the outline of this
+shadow upon the ball was irregular and indented, an observation which
+proves either that the surface of the ball is irregular, or that the
+border of the ring casting the shadow was jagged. The shadow of the
+globe on the rings has much more interest, as it enables us to get at
+some knowledge of the form of the surface of the rings, which otherwise
+is very difficult to discover, owing to the oblique position in which we
+always see them.
+</p>
+<p>
+In general, the shadow of the ball on the middle ring has its outline
+concave towards the planet; while on the outer ring it is usually
+slanting, and at a greater distance from the limb than on the middle,
+and dusky rings. This form of the shadow evidently proves that the
+middle ring stands at a higher level than the two others, especially
+towards its outer margin. The system seems to increase gradually in
+thickness from the inner border of the dusky ring to the vicinity of the
+outer margin of the middle ring, after which it rapidly diminishes on
+this border, while the surface of the outer ring is almost level.
+</p>
+<p>
+But this surface is by no means fixed, as its form sometimes changes, as
+proved by my observations and those of others. As may be noticed on
+Plate X., the outline of the shadow of the planet on the rings is
+strongly deviated towards the planet, near the outer margin of the
+middle ring; the notch indicating an abrupt change of level, and a rise
+of the surface at that point. Some observers have endeavored to explain
+these deviations by the phenomena of irradiation, from which it would
+follow that the maximum effect of deviation should be observed where the
+ring is the brightest, which does not accord with observation; as the
+deepest depression in the shadow is not to be found usually at the
+brightest part, which is towards the outer border of the middle ring,
+but occurs near its centre. From these observations it is undoubtedly
+established that the surface of the rings is far from being flat
+throughout, and is, besides, not permanent, but changes, as would, for
+instance, the surface of a large mass of clouds seen from the top of a
+high mountain. In general, the system is thickest not very far from the
+outer border of the intermediary ring.
+</p>
+<p>
+Some interesting phenomena which I had occasion to observe before and
+after the passage of the Sun through the plane of the rings, on February
+6th, 1878, conclusively show that the surface of this system cannot be
+of a uniform level, but must be thicker towards the outer border of the
+middle ring, thence gradually sloping towards the planet. Many of my
+observations irresistibly lead to this conclusion. As it would, however,
+be out of place to have them recorded here in detail, I will simply give
+one of the most characteristic among them.
+</p>
+<p>
+From December 18th, 1877, when the Sun was about 41' above the plane of
+the rings, to February 6th, 1878, the day of its passage through their
+plane, the illuminated surface of this system gradually decreased in
+breadth with the lowering of the Sun, until it was lost sight of,
+February 5th, on the eve of the passage of the Sun through their plane.
+The phenomenon in question consisted in the gradual invasion of their
+illuminated surface by what appeared to be a black shadow, apparently
+cast by the front part of the outer portion of the middle ring the
+nearest to the Sun. On January 25th, when the elevation of the Sun above
+the plane of the rings was reduced to 15', the shadow thus cast had
+extended so far on their surface that it reached the shadow cast by the
+globe on the opposite part of the ring in the east, and accordingly the
+remaining portion of the illuminated surface of the eastern ansa then
+appeared entirely disconnected from the ball, by a large dark gap,
+corresponding in breadth to that of the globe's shadow on the rings. On
+February 4th, when the Sun was only 5' above the plane of the rings,
+their illuminated and only visible surface was reduced to a mere thread
+of light, which on the 5th appeared broken into separate points. It is
+evident that the phenomenon was not caused by the obliquity of the ring
+as seen from our globe, since the elevation of the Earth above the plane
+of the rings&mdash;which on December 18th was 3° 20'&mdash;was still 1° 20'
+on the 4th of February. In ordinary circumstances, when the Sun is a little
+more elevated, and the rings seen at this last angle, they appear quite
+broad and conspicuous, and even the dark open space separating the dusky
+ring from the planet is perfectly visible on the ansæ, where the
+Earth's elevation above their plane is reduced to 40'. It is also
+evident that the phenomenon was not to be attributed to the reduction of
+the light which they received from the Sun, although the illumination in
+February might be expected to be comparatively feeble, since the Sun
+then shone upon the rings so obliquely; yet (on the supposition that
+their surface is flat) they should have been illuminated throughout, and
+if not very brightly, sufficiently so, at least, to make them visible
+and as bright as was the narrow thread of light observed on the 4th of
+February. The phenomenon actually observed may be explained most readily
+by assuming, as other phenomena also indicate, that the surface of the
+ring is not flat, but more elevated towards, or in the vicinity of its
+outer border, from which place it slopes inwardly towards the planet. On
+this assumption, it is evident that the elevated part of the ring the
+nearest to the Sun would cast a shadow, which, with the increasing
+obliquity of the Sun, would gradually cover the whole surface comprised
+within the elevated part, and thus become invisible to us. Several
+observations made by Bond and other observers undoubtedly show the same
+phenomenon, and do not seem to be intelligible on any other supposition.
+From my observations made in 1881 it would appear, however, that the
+opposite surfaces of the rings do not exactly correspond in form, but
+this may not be a permanent feature, as the surface of this system is
+subject to changes, as already shown.
+</p>
+<p>
+The dimensions of the rings are great, the diameter of the outer one
+being no less than 172,982 miles, the distance from the centre of the
+globe to the outer border of the system being, therefore, 86,491 miles.
+The breadth of the outer ring is 9,941 miles; that of the principal
+division, 2,131 miles; that of the middle ring, 19,902 miles, and that
+of the dusky ring, 8,772 miles. The breadth of all the rings taken
+together is, therefore, 40,746 miles. The interval between the surface
+of Saturn and the inner border of the dusky ring is 7,843 miles.
+</p>
+<p>
+The thickness of the system of rings has been variously estimated by
+astronomers, on account of the great difficulties attending its
+determination. While Sir John Herschel estimated it at more than 250
+miles, G. P. Bond reduces it to 40 miles. Both of these numbers are
+evidently too small, as so slight a thickness cannot explain the
+observed phenomenon of the shadow cast by a portion of the ring on its
+own surface, when the Sun is very low in its horizon, as shown above.
+</p>
+<p>
+The plane of the system of rings is inclined 27° to the planet's orbit,
+and is parallel, or at least very nearly so, with the equator of the
+planet, passing, therefore, through its centre, and dividing its globe
+into northern and southern hemispheres. Seen from the Earth, a portion
+of the ring always appears projected in front of the planet, thus
+concealing a small part of its globe, while the opposite portion passes
+behind the globe, which hides it from sight.
+</p>
+<p>
+As the plane of the ring is not affected by the motion of the planet
+around the Sun, but always remains parallel to itself, it follows that
+as Saturn advances in its orbit the rings must successively present
+themselves to us under various angles of inclination, appearing,
+therefore, more or less elliptical, and presenting two maxima and two
+minima of inclination in the course of one of its revolutions. As the
+revolution of Saturn is accomplished in 29½ years, the maxima and the
+minima must recur every 14 years and 9 months; the maxima being
+separated from the minima by an interval of 7 years and 4½ months.
+</p>
+<p>
+When Saturn arrives at the two opposite points of its orbit, where the
+major axis of its ring is at right angles to the line joining its centre
+to that of the Sun, the ring, which is then viewed at an inclination of
+27°, the greatest angle at which it can ever be seen, has reached its
+maximum opening, the smaller diameter of its ellipse being then about
+half that of the larger. At this moment the outer ring projects north
+and south beyond the globe, which is then completely enclosed in its
+ellipse. The maximum opening of the northern surface of the ring takes
+place, at present, when Saturn arrives in longitude 262°, in the
+constellation Sagittarius, and that of the southern surface when it
+arrives in longitude 82° in the constellation Taurus. When, on the
+contrary, Saturn reaches the two opposite points of its orbit, where the
+plane of its ring is parallel to the line joining its centre to that of
+the Sun, the opening vanishes, as only the thin edge of the ring is then
+presented to the Sun and receives its light, the rest being in darkness.
+At this moment the ring disappears, except in the largest telescopes,
+where it is seen as an exceedingly thin thread of light; and the
+Saturnian globe, having apparently lost its ring, appears solitary in
+the sky, like the other planets. The disappearance of the ring from this
+cause occurs now when Saturn arrives at 90° from either of the
+positions of maximum inclination, that is, in longitude 352° in the
+constellation Pisces, and in longitude 172° in the constellation Leo.
+</p>
+<p>
+When the planet is in any other position than one of these last two,
+either the northern or the southern surface of the ring is illuminated
+by the Sun, while the opposite surface is in the night, and does not
+receive any direct sunlight. At the time of the passage of the plane of
+the ring through the Sun's centre, a change takes place in the
+illumination of the ring. If it is the northern surface which has
+received the rays of the Sun during the previous half of the Saturnian
+year, at the moment the plane has passed the centre of the Sun, the
+southern surface, after having been buried in darkness for 14¾ years,
+sees the dawn of its long day of the same length. Such a phenomenon will
+not occur until 1892, when the passage of the Sun from the northern to
+the southern side of the ring will close in twilight the day commenced
+in 1878.
+</p>
+<p>
+Aside from the periodic disappearance of the ring, resulting from the
+passage of the Sun through its plane, the ring may also disappear from
+other reasons. Just before or just after the time of the passage of the
+Sun through the plane of the ring, the Earth and the Sun may occupy such
+positions, that while the one is north of the plane of the ring, the
+other is south of it, or vice versa, in which event the ring becomes
+invisible, because its dark and non-illuminated surface is presented to
+us. The ring may also become invisible to us when the Earth passes
+through its plane.
+</p>
+<p>
+Since the distance from Saturn to the Sun is to the distance of the
+Earth from this last body as 9.54 is to 1; and since the circumference
+of a circle increases in the same proportion as its radius, it follows
+that the diameter of the Earth's orbit projected on the orbit of Saturn
+would occupy only part of the latter, or about 12° 2', this being 6°
+1' on either side of the nodes of the rings. To describe such an arc on
+its orbit, it takes Saturn almost 360 days on an average, or almost a
+complete year; the Earth describing therefore almost a whole revolution
+around the Sun during the time it takes Saturn to advance 12° 2' on its
+orbit. Then, when Saturn occupies a position comprised within an arc 6°
+1' from either side of the nodes of its ring, the Earth, by its motion,
+is liable to encounter the plane of the ring, when therefore it will
+only present its thin edge to us, and becomes invisible. At least one
+such encounter is unavoidable within the time during which Saturn
+occupies either of these positions on its orbit; while three frequently
+happen, and two are possible.
+</p>
+<p>
+The natural impression received by looking at the rings, while seeing
+the ponderous globe of Saturn enclosed in its interior, is that this
+gigantic, but very delicate structure, in order to avoid destruction,
+must be endowed with a swift movement of rotation on an axis
+perpendicular to its plane, and that the centrifugal force thence
+arising counterbalances the powerful attraction of the planet, and thus
+keeps the system in equilibrium.
+</p>
+<p>
+Theoretically, the rotation of the rings is admitted by every
+astronomer, as being an essential condition to the existence of the
+system, which otherwise, it is thought, would fall upon the planet.
+Although the rotation of the rings seems so probable that it is
+theoretically considered as certain, yet its existence has not been
+satisfactorily demonstrated by direct observation, which alone can
+establish it on a firm basis as a matter of scientific knowledge.
+</p>
+<p>
+The determination of the period of rotation of the rings, which is
+supposed to be 10h. 32m. 15s., rests only on the observations of W.
+Herschel, made in 1790, from the apparent displacement of irregularities
+on the ring; but his results have been contradicted by other
+observations, and even by those of Herschel himself, made in later
+years.
+</p>
+<p>
+Although the system of rings is very nearly concentric with the globe of
+Saturn, yet the coincidence is not considered as mathematically exact.
+It seems to have been satisfactorily demonstrated by direct observations
+that the centre of gravity of the system oscillates around that of the
+planet, thus describing a minute orbit. This peculiarity is in
+accordance with theory, which has shown it to be essential to the
+stability of the system.
+</p>
+<p>
+Besides its system of rings, which makes Saturn the most remarkable
+planet of the solar system, this globe is attended by eight satellites,
+moving in orbits whose planes very nearly coincide with the plane of the
+rings, except that of the most distant one, which has an inclination of
+about 12° 14'. In the order of their distance from the planets, the
+satellites of Saturn are as follows: Mimas, Enceladus, Tethys, Dione,
+Rhea, Titan, Hyperion and Iapetus. The three first satellites are nearer
+to Saturn than the Moon is to the Earth; while Iapetus, the farthest, is
+9½ times the distance of our satellite from us. All the satellites,
+with the exception of the farthest, move more rapidly around Saturn than
+the Moon moves around the Earth; while Iapetus, on the contrary, takes
+almost three times as long to make one revolution.
+</p>
+<p>
+The period of revolution of the four inner satellites is accomplished in
+less than three days, that of Mimas being only a little more than 22
+hours. From such swiftness of motion, it is easily understood how short
+must be the intervals between the different phases of these satellites.
+Mimas, for instance, passes from New Moon to First Quarter in less than
+6 hours.
+</p>
+<p>
+The distance of the nearest satellite from the planet's surface is
+84,000 miles, and its distance from the outer ring only 36,000 miles. It
+is difficult to determine the diameter of objects so faint and distant
+as are some of these satellites, but the diameter of Titan, the largest
+of all, is pretty well known, and estimated to be ¹⁄₁₆ the
+diameter of the planet, or more than half the diameter of our globe.
+</p>
+<p>
+Iapetus is subject to considerable variations in brilliancy, and as the
+maxima and minima always occur when this satellite occupies the same
+parts of its orbit, it was conjectured by W. Herschel that, like our
+Moon, it turns once upon its axis during each of its revolutions about
+the planet. It has been shown by my observations, that Iapetus attains
+its maximum brightness a little before it reaches its greatest western
+elongation, and its minimum on the opposite side.
+</p>
+<p>
+As the planes of the orbits of the satellites are inclined to the
+planet's orbit, it follows that their transits, occultations and
+eclipses, are only possible when Saturn is near its equinoxes. Passages
+of the satellites and their shadows across the disk, although rare, have
+been observed, and they somewhat resemble the phenomena exhibited by the
+satellites of Jupiter in transit. When the Earth is very near the plane
+of the rings, the satellites, except the farthest, appear to be in a
+straight line nearly coincident with the plane of the rings, and are
+seen occasionally moving along the thin edge of the rings, appearing as
+luminous beads moving on a thread of light.
+</p>
+<p>
+Owing to the considerable inclination of the axis of rotation of Saturn
+to its orbit, the seasons of this planet must have greater extremes of
+temperature than those of the Earth. As the year of Saturn consists of
+25,217 Saturnian days, each season, on the average, is composed of 6,304
+Saturnian days.
+</p>
+<p>
+To an observer on Saturn, the immense arches formed by its rings would
+appear as objects of great magnificence, spanning the sky like soft
+colorless rainbows. Moreover, the eight moons, several of which are
+always visible, would be of the highest interest, with their swift
+motions and rapid phases. Mimas, traveling in its orbit at the rate of
+16 of arc per minute of time, moves over a space equal to the apparent
+diameter of our Moon in two minutes, or at the rate of 16° an hour.
+</p>
+<p>
+Owing to the globular form of Saturn, the rings would be invisible in
+latitudes situated above 65° from its equator, and their apparent form
+and breadth would naturally vary with the latitude. At 63° only a very
+small portion of the outer ring would be visible above the equatorial
+horizon, where it would appear as a small segment of a circle. At 62°
+the principal division would just graze the horizon. At 46° the outer
+portion of the dusky ring would become visible, while at 35° its inner
+edge would appear above the horizon. From 65° of latitude down to the
+equator, the arches of the rings would be seen more and more elevated
+above the equatorial horizon, but at the same time that they are seen
+higher up, their apparent breadth gradually diminishes, owing to the
+effect of foreshortening, and at the equator itself the system would
+only present its thin edge to view.
+</p>
+<p>
+During the summer seasons of either hemisphere of Saturn, the surface of
+the rings turned towards such hemisphere, being fully illuminated by the
+Sun, is visible from these regions. In the day time its light must be
+feeble and similar to the light reflected by our Moon during sunshine;
+but at night the system would display all its beauty, and the different
+rings, with their divisions and their various reflective powers, must
+present a magnificent sight.
+</p>
+<p>
+During the nights of the long winter seasons on Saturn, on the contrary,
+the surface of the rings turned towards the hemisphere undergoing
+winter, receives no light from the Sun, and is invisible, or very nearly
+so, except towards morning and evening, when it may be faintly
+illuminated by the secondary light which it receives from the
+illuminated globe of Saturn. Although dark and invisible, the rings may
+make their form apparent at night by the absence of stars from the
+region which they occupy in the sky. Again, in other seasons, the days
+present very curious phenomena. In consequence of the diurnal rotation
+of the planet, the Sun seems to move in circular arcs, which, owing to
+the inclination of Saturn's axis, are more or less elevated above its
+horizon, according to the position of the planet in its orbit. As such
+arcs described by the Sun in the sky of Saturn are liable to encounter
+the rings, the Sun in passing behind them becomes eclipsed. It must be a
+magnificent spectacle to witness the gradual disappearance of the fiery
+globe behind the outer ring, and its early reappearance, but for a
+moment only, through the narrow gap of the principal division; to see it
+vanish again behind the middle ring, to reappear a little later through
+the semi-transparent dusky ring, but very faint and red colored at
+first; and then, gradually brighten up, and finally emerge in all its
+beauty from the inner edge of the dusky ring.
+</p>
+<p>
+It is in latitude 23° that the rings produce the most prolonged
+eclipses of the Sun. During a period equivalent to ten of our
+terrestrial years, such eclipses continually succeed each other with but
+very short periods of interruption; and even during a long series of
+rotations of Saturn, the Sun remains completely invisible in those
+regions where the apparent arcs which it describes coincide with the
+arcs of the rings. In neighboring latitudes, the eclipses of the Sun,
+although still frequent, would have a shorter and shorter duration as
+the observer should travel north or south. These eclipses of the Sun
+must produce a partial darkness of the regions involved in the shadow of
+the rings, which may be compared to the darkness produced on our globe
+by a total eclipse of the Sun. The frequent recurrence of these
+eclipses, and their comparatively long duration in some regions, must
+still further reduce the duration of the short Saturnian days.
+</p>
+<p>
+The globe of Saturn, as already shown, casts a shadow on the rings,
+which, according to the position of the planet in its orbit, either
+extends across their whole breadth, or covers only a part of their
+surface. The shadow on the rings rising in the east after sunset,
+ascends to the culminating point of their arcs in the sky, in 2h. 34m.,
+and as rapidly descends on the western horizon, to disappear with
+sunrise. This shadow, when projected on the rings in the sky, must be
+hardly distinguishable from the dark background of the heavens, except
+from the absence of stars in the regions which it occupies. It must
+appear as a large dark gap, separating the rings into two parts, and
+constantly moving from east to west. Possibly the refraction of the
+solar rays, in passing through Saturn's atmosphere, may cast some
+colored light on the rings, similar to that observed on the Moon during
+its eclipses.
+</p>
+<p>
+An observer on the rings would behold phenomena still more curious, a
+long day of 14¾ years being followed by a long night of 14¾ years. The
+long days of Saturn's rings are, however, diversified by numerous
+eclipses of the Sun, which regularly occur every 10¼ hours; the
+phenomenon being due to the interposition of the globe of Saturn between
+the rings and the Sun. These eclipses produce partial obscurations of
+their surface, lasting from 1½ to 2 hours at a time. Although the
+surface of the rings never receives direct sunlight during their long
+nights, yet they are not plunged all the time in total darkness, as they
+receive some reflected light from that part of the globe of Saturn which
+is illuminated by the Sun. To the supposed observer on the rings, during
+every 10¼ hours, the immense globe would exhibit continually changing
+phases. At first he would see a point of light rapidly ascending from
+the horizon, and appearing under the form of a half crescent of
+considerable radius; 5⅛ hours later, the crescent having gradually
+increased, would appear as a half circle, covering ⅛ of the visible
+heavens, its surface being more than 20,000 times as large as the
+surface of the Moon. Upon this brilliantly illuminated semi-circle would
+be projected the shadows of the rings, appearing as black belts
+separated by a narrow luminous band.
+</p>
+<p>
+It is very difficult for one to conceive how such a delicate structure,
+as the system of rings appears to be, can keep together in equilibrium
+and avoid destruction from the powerful attraction of the planet on one
+side and the disturbing influence of the satellites on the other. To
+explain it, several hypotheses have been advanced. The rings were first
+supposed to be solid, and upon this supposition Laplace determined the
+necessary conditions for their equilibrium; the most important of which
+require that the cross section of the rings should be an ellipse of
+irregular curvature, and having its major axis directed towards the
+centre of the planet, and also that the system should rotate upon an
+axis perpendicular to the plane of the rings. This theory was superseded
+by another, which supposed the rings to be fluid. This one was soon
+rejected for a third, assuming the system to be composed of vapors or
+gases; and more recently, all these theories were considered untenable,
+and replaced by a fourth, which supposes the system of rings to be made
+up of a congregation of innumerable small, independent bodies, revolving
+around Saturn in concentric zones. Naturally, such a divergence of
+opinion can only result from our comparative ignorance of the subject,
+and sufficiently indicates our inability to explain the phenomena; and
+it must be admitted that, so far, nothing is certainly known about this
+strange system. We shall probably remain in the same uncertainty until
+the rotation of the rings is ascertained by direct observations. It is
+pretty certain, however, that none of these theories account for the
+observed phenomena in their details, although a partial explanation may
+be obtained by borrowing something from each hypothesis.
+</p>
+<p>
+It has been conjectured, and a theory has been advanced, that the
+breadth of the whole ring system is gradually increasing inwards, and
+that it will come in contact with the planet in about 2,150 years; but
+the question seems to have been settled in the negative by the elaborate
+measurements of the English observers. It is likely that the increase is
+only in the defining power of the instruments.
+</p>
+
+<p><br /><br /><br /></p>
+
+<h4><a id="COMETS">COMETS</a>
+<br /><br />
+PLATE XI</h4>
+
+<p>
+Among the celestial phenomena, none are more interesting than those
+mysterious apparitions from the depths which unexpectedly display their
+strange forms in our familiar constellations, through which they wander
+for a time, until they disappear like phantoms.
+</p>
+<p>
+A comet, with its luminous diffused head, whence proceeds a long vapory
+appendage gradually fading away in the sky, presents an extraordinary
+aspect, which may well astonish and deeply impress the observer.
+Although these visitors from infinite space do not now inspire dread, as
+in by-gone times, yet, owing to the mystery in which the phenomenon is
+still involved, the apparition of a large comet, even in our days, never
+fails to create a profound sensation, and in some cases that unconscious
+fear which results from the unknown.
+</p>
+<p>
+The effect of such a spectacle largely depends upon its rarity; but
+since the telescope has been applied to the sounding of the heavens, it
+has been found that the appearance of comets is by no means an unusual
+occurrence. If so few comets, comparatively, are seen, it is because
+most of them are telescopic objects, and are therefore invisible to the
+naked eye. Most of the telescopic comets are not only too faint to be
+perceived by the unaided eye, but are insignificant objects, even when
+observed through the largest telescopes.
+</p>
+<p>
+It was Kepler's opinion that comets are as numerous in the sky as fishes
+are in the ocean. Undoubtedly the number of these bodies must be great,
+considering that we can only see them when they come into the
+neighborhood of the Earth, and that many even here remain invisible, or
+at least pass unperceived. That many of them have passed unperceived
+heretofore, is proved by the fact that the number of those observed
+becomes greater every year, with the increase of the number of
+instruments used in their search. The number of comets observed with the
+naked eye during historic times is nearly 600, and that of telescopic
+comets, which, of course, all belong to the last few centuries, is more
+than 200, so that we have a total number of about 800 comets of which
+records have been kept. From theoretical considerations, Lambert and
+Arago estimated their entire number at several millions, but such
+speculations have generally no real value, since they cannot be
+established on a firm basis.
+</p>
+<p>
+Comets remain visible for more or less time, according to their size and
+the nature and position of their orbits, but in general, the large ones
+can be followed with the telescope for several months after they have
+become invisible to the naked eye. The comet of 1861, for example,
+remained telescopically visible for a year, and that of 1811, for 17
+months after disappearing from ordinary sight.
+</p>
+<p>
+While a comet remains visible, it appears to revolve daily about us like
+the stars in general; but it also moves among the constellations, and
+from this movement its orbit may be computed like that of a planet. From
+the apparent diurnal motion of a comet with the heavens, result the
+changes of position which it seems to undergo in the course of a night.
+The direction of the head and tail of a comet, of course, has only
+changed in regard to the horizon, but not in regard to the sky, in which
+they occupy very nearly the same position throughout a given night, and
+even for many nights in succession.
+</p>
+<p>
+The movements of the comets in their orbits are, like those of the
+planets, in accordance with Kepler's laws, the Sun occupying one of the
+foci of the orbit they describe; but the orbits of comets differ,
+however, in several points from those of the planets. Their eccentricity
+is always great, being sometimes apparently infinite, in which case the
+orbit is said to be parabolic, or hyperbolic; but the smallness of the
+portion of a cometary orbit which can ordinarily be observed, makes it
+difficult to determine this with certainty. Again, while the planetary
+orbits are usually near the plane of the ecliptic, those of comets
+frequently have great inclinations to that plane, and even when the
+inclination is less than 90°, the comet may have a retrograde movement,
+or, in other words, a movement contrary to the course in which all the
+planets revolve about the Sun.
+</p>
+<p>
+Notwithstanding these differences between the elements of the orbits of
+the comets and those of the planets, the fact that each has the Sun in
+one focus indicates that the body moving in it is a member of the solar
+system, either for the time, or permanently, according to the nature of
+its orbit.
+</p>
+<p>
+A distinction may accordingly be made between the comets which are
+permanent members of our solar system and those which are only
+accidental or temporary visitors. Those moving in elliptical orbits
+around the Sun, like the planets, and therefore having a determinate
+period of revolution, from which the time of their successive returns
+may be predicted, are permanent members of our system, and are called
+periodic comets. All comets moving in parabolical or hyperbolical
+curves, are only temporary members of the solar system, being apparently
+strangers who have been diverted from their courses by some disturbing
+influence. No comet is classed as periodical which does not follow a
+perceptibly elliptical orbit. Any comet passing around the Sun at the
+mean distance of the Earth from this body, with a velocity of 26 miles
+per second, will fly off into infinite space, to return to us no more.
+</p>
+<p>
+The time of revolution of the different periodic comets thus far
+observed varies greatly, as do also the distances to which they recede
+from the Sun at aphelion. Whilst the period of revolution of Encke's
+comet, the shortest thus far known, is only 3½ years, that of the comet
+of 1844, II., is 102,000 years; and whilst the orbit of the first is
+comprised within the orbit of Jupiter, that of the last extends to a
+distance equal to 147 times the distance of Neptune from the Sun. But so
+vast an orbit cannot be accurately determined from the imperfect data at
+our disposal.
+</p>
+<p>
+The periodic comets are usually divided into two classes. The comets
+whose orbits are within the orbit of Neptune are called interior comets,
+while those whose orbits extend beyond that of Neptune are called
+exterior comets. The known interior periodic comets are twelve in
+number, while, including all the cases in which there is some slight
+evidence of elliptic motion, the number of exterior comets observed is
+six or seven times as great. The periodic comets of short period are
+very interesting objects, inasmuch as by their successive returns they
+afford an opportunity to calculate their motions and to observe the
+physical changes which they undergo in their intervals of absence.
+</p>
+<p>
+From observation of the periodic comets, it has been learned that the
+same comet never presents twice the same physical appearance at its
+different returns, its size, shape and brilliancy varying so greatly
+that a comet can never be identified by its physical characters alone.
+It is only when its elements have been calculated, and are found to
+agree with those of a cometary orbit previously known, that the two
+comets can be identified one with the other. There are reasons to
+believe that, in general, comets decrease in brightness and size at each
+of their successive returns, and that they are also continually losing
+some of their matter as they traverse their orbits.
+</p>
+<p>
+When very far away from us, all comets appear nearly alike, consisting
+of a faint nebulosity, of varying dimensions. When a comet first appears
+in the depths of space, and travels towards the Sun, it generally
+resembles a faint, uniformly luminous nebulosity, either circular or
+slightly elongated in form. As it approaches nearer to the Sun, a slight
+condensation of light appears towards its centre, and as it draws still
+nearer, it becomes brighter and brighter, and in condensing forms a kind
+of diffused luminous nucleus. At the same time that the comet acquires
+this concentration of light, the nebulosity gradually becomes elongated
+in the direction of the Sun. These effects generally go on increasing so
+long as the comet is approaching the Sun; the condensation of light
+sometimes forms a bright nucleus, comparable to a very brilliant star,
+while the elongation becomes an immense appendage or tail. When the
+comet has passed its perihelion and recedes from the Sun, the inverse
+phenomena are observed; the comet, decreasing in brightness, gradually
+loses its nucleus and tail, resumes its nebulous aspect, and finally
+vanishes in space, to appear again in due course, if it chance to be a
+periodic comet. While all comets become brighter in approaching the Sun,
+they do not all, however, develop a large tail, some of them showing
+only a slight elongation.
+</p>
+<p>
+When a comet is first discovered with the telescope at a great distance
+from the Sun, it is difficult to predict whether it will become visible
+to the naked eye, or will remain a telescopic object, as it is only in
+approaching the Sun that these singular bodies acquire their full
+development. Thus, Donati's comet, whose tail became so conspicuous an
+object at its full appearance in 1858, remained two months after its
+discovery by the telescope without any indication of a tail. The comet
+of Halley, which before and after its return in 1759, remained five
+years inside of the orbit of Saturn, showed not the least trace of its
+presence during the greater part of this time. Nothing but calculation
+could then indicate the position in the sky of this invisible object,
+which was so prominent when it approached the Sun.
+</p>
+<p>
+Another curious phenomenon exhibited by comets, and first noticed by
+Valz, is that in approaching the Sun the nebulosity composing these
+bodies contracts, instead of dilating, as would be naturally supposed
+from the greater amount of solar heat which they must then receive. In
+receding from the Sun, on the contrary, they expand gradually. As comets
+approach the Sun, the tail and nucleus are developed, while the
+nebulosity originally constituting these comets contracts, as if its
+material had been partly consumed in this development. In a certain
+sense it may be said that the comets are partly created by the Sun; in
+more exact terms, the changes of form which they undergo are induced by
+the Sun's action upon them at different distances and under varying
+conditions. Moreover, they are rendered visible by its influence,
+without which they would pass unperceived in our sky. When a comet
+disappears from view, it is not because its apparent diameter is so much
+reduced by the distance that it vanishes, but rather on account of the
+diminution of its light, both that which it receives from the Sun, and
+its own light; these bodies being in some degree self-luminous, as will
+be shown below.
+</p>
+<p>
+The large comets, such as can be seen with the naked eye, always show
+the following characteristics, on examination with the telescope. A
+condensation of light resembling a diffused star forms the brightest
+part of the comet, this condensation being situated towards the
+extremity the nearest to the Sun. It is this starlike object which is
+called the <i>nucleus</i>. The nucleus seems to be entirely enclosed in
+a luminous vapory envelope of the same general texture, called the
+<i>coma</i>. This envelope, which is quite variable in brightness and
+form, is brightest next to the nucleus, and gradually fades away as it
+recedes from it. The <i>nucleus</i> and the <i>coma</i>, considered as a
+whole, constitute the <i>head</i> of a comet. From the head of a comet
+proceeds a long trail of pale nebulous light, which usually grows wider,
+but fainter, as it recedes from the nucleus, and insensibly vanishes in
+the sky. This delicate appendage, or tail, as it is commonly called,
+varies very much in size and shape, not only in different comets, but in
+the very same comet, at different times. Its direction is generally
+opposite to that of the Sun from the head of the comet.
+</p>
+<p>
+The nuclei vary very much in brightness, in size and in shape; and while
+in some telescopic comets they are either absent or barely
+distinguishable as a small condensation of light, in bright comets they
+may become plainly visible to the naked eye, and they sometimes even
+surpass in brightness the most brilliant stars of the heavens. But
+whatever may be the size of cometary nuclei, they are subject to sudden
+and rapid changes, and vary from day to day. Sometimes they appear
+exceedingly brilliant and sharply outlined, while at other times they
+are so dim and diffused that they are hardly distinguishable from the
+coma of which they seem then to form a part.
+</p>
+
+<p><br /></p>
+
+<div class="figcenter" style="width: 400px;">
+<a id="figure11"></a>
+<br />
+<img src="images/figure11.jpg" width="400" alt="" />
+<div class="caption">
+<p>PLATE XI.&mdash;THE GREAT COMET OF 1881.</p>
+<p class="smaller">Observed on the night of June 25-26 at 1h. 30m. A.M.</p>
+</div></div>
+
+<p><br /></p>
+
+<p>
+From my observations upon the comets which have appeared since the year
+1873, it is apparent that the changes in the nucleus, coma and tail, are
+due to a solar action, which contracts or expands these objects in such
+a manner that the nuclei become either bright and star-like, or dim and
+diffused, in a very short time. I had excellent opportunity, especially
+in the two large comets of 1881, to observe some of these curious
+changes, a description of which will give an idea of their extent and
+rapidity. On July 2d, 1881, at 9 o'clock, the nucleus of comet 1881,
+III., which is represented on Plate XI., appeared sharply defined,
+bright and considerably flattened crosswise; but half an hour later it
+had considerably enlarged and had become so diffused that it could
+hardly be distinguished from the coma, with which it gradually blended.
+It is perhaps worth mention that, at the time this last observation was
+made, an aurora borealis was visible. This comet 1881, III., underwent
+other very important changes of its nucleus, coma and tail. On June
+25th, the nucleus, which was bright and clearly defined, was ornamented
+with four bright diverging conical wings of light, as shown on Plate XI.
+On the 26th these luminous wings had gone, and the nucleus appeared
+one-third smaller. On the 28th it had enlarged, but on the 29th its
+shape was considerably altered, the nucleus extending in one direction
+to three or four times its diameter on previous nights, and being
+curved, so as to resemble a comma. On the 6th of July the nucleus of
+this comet showed the greatest disturbances. The nucleus, which had
+appeared perfectly round on the evening of the 5th, was found much
+elongated at 10 o'clock on the 6th, forming then a straight, acute, and
+well-defined wedge of light, inclined upwards to the left. The length of
+the nucleus, at this time, was three or four times its ordinary
+diameter. At the same time rapid changes occurred; the strangely shaped
+nucleus soon became unsteady, extending and contracting alternately, and
+varying greatly in brightness. At 10h. 45m., the elongated nucleus, then
+gently curved, took the shape of a succession of luminous knots, which
+at times became so brilliant and distinct that they seemed to be about
+to divide and form separate nuclei; but such a separation did not
+actually occur, at least while I was observing. While these important
+changes were going on in the comet, a bright auroral arch appeared in
+the north, which lasted only a short time. On July 7th, the sky being
+cloudy, no observations were made, but on the 8th I observed the comet
+again. The nucleus had then resumed its circular form, but it was yet
+very unsteady, being sometimes small, bright and sharp, while a few
+seconds later it appeared twice as large, but dim in outlines; and
+sometimes an ill-defined secondary nucleus appeared at its centre. On
+several occasions the nucleus appeared as if it were double, one nucleus
+being apparently projected partly upon the other.
+</p>
+<p>
+The nuclei of comets are sometimes very small, and in other cases very
+large. Among those which have been measured, the nucleus of the comet of
+1798, I., was only 28 miles in diameter, but that of Donati's comet, in
+1858, was 5,600 miles, and that of the comet of 1845 was 8,000 miles in
+diameter.
+</p>
+<p>
+The coma of comets is found to be even more variable than the nucleus.
+The changes observed in the coma are generally in close connection with
+those of the nucleus and tail, the same perturbations affecting
+simultaneously the whole comet. While the coma of the comet of 1847 was
+only 18,000 miles in diameter, that of Halley's comet, in 1835, was
+357,000 miles, and that of the comet of 1811 was 1,125,000 miles in
+diameter. In general, as already stated, the coma of a comet decreases
+in size in approaching the Sun. That of Encke's comet, which, on October
+9th, 1838, had a diameter of 281,000 miles, gradually decreased at a
+daily mean rate of 4,088 miles in going towards the Sun; so that, on
+December 17th, when the distance of the comet from the Sun was more than
+four times less than it was on the first date, its diameter was reduced
+to 3,000 miles.
+</p>
+<p>
+The form of the coma, in that part which is free from the tail, is in
+general a portion of a circle, but is sometimes irregular, with its
+border deformed. Thus, the border of the coma of Halley's comet was
+depressed at one point towards the Sun. I observed a similar phenomenon
+in Coggia's comet, with the great refractor of the Harvard College
+Observatory, on July 13th, 1874, when its border appeared deeply
+depressed on the side nearest to the Sun, as if repelled by this body.
+The coma of comet 1881, III., showed also very singular outlines on the
+nights of the 25th and 26th of June, when its border was so deeply
+depressed that the coma appeared as if it were double. Luminous rays and
+jets often radiate from the nucleus across the coma, and describe
+graceful lateral curves, falling backwards and gradually fading away
+into the tail, of which they then form a part. The rays and jets emitted
+by the nucleus seem at first to obey the solar attraction and travel
+towards the Sun; but they are soon repelled, and move backward towards
+the tail. It is a mystery, as yet unexplained, how these cometary jets,
+which at first seem to obey to the laws of attraction, are compelled to
+retreat apparently by superior opposing forces. Among the forces of
+nature, we know of no other than those of an electrical sort, which
+would act in a similar manner; but this explanation would require us to
+assume some direct electrical communication between the comet and the
+Sun. Considering the distance between the two bodies, and the probable
+absence or great tenuity of the gaseous material in interstellar space,
+such an assumption is a difficult one.
+</p>
+<p>
+Under the action of the solar forces, the coma also very frequently
+forms itself into concentric luminous arcs, separated by comparatively
+dark intervals. These luminous semi-circles vary in number, but
+sometimes there are as many as four or five at a time. All great comets
+show these concentric curves more or less, but sometimes only a portion
+is visible, the rest of the coma having a different structure. When
+great comets approach near the Sun, their coma is generally composed of
+two distinct parts, an inner and an outer coma, the inner one being due
+to the luminous jets issuing from the nucleus, which, never extending
+very far, form a distinct, bright zone within the fainter exterior coma.
+</p>
+<p>
+The tails of comets, which are in fact a prolongation of the coma, are
+likewise extremely variable in form. They are sometimes straight like a
+rod; again, are curved like a sabre, or even crooked like an S, as was
+that of the comet of 1769. They are also fan-shaped, pointed, or of the
+same width throughout. Many of these appendages appear longitudinally
+divided through their middle by a narrow, darkish rift, extending from
+the nucleus to the extremity. This peculiarity appears in the comet
+shown on Plate XI. Sometimes the dark rift does not commence near the
+nucleus, but at some distance from it, as I observed in the case of
+comet 1881, III., on June 26th. This dark rift is not a permanent
+feature of a comet's tail, but may be visible one day and not at all the
+next. Comet 1881, III., which had shown a dark rift towards the end of
+June, did not exhibit any such rift during July and August, when, on the
+contrary, its tail appeared brighter in the middle. Coggia's comet,
+which showed so prominent a dark rift in July, 1874, had none on June
+10th. On the contrary, the tail was on that date very bright along its
+middle, as also along each of its edges.
+</p>
+<p>
+The tail of a comet does not invariably point directly away from the
+Sun, as above mentioned, and sometimes the deviation is considerable;
+for instance, the tail of the comet of 1577 deviated 21° from the point
+opposite to the Sun.
+</p>
+<p>
+In general, the tail inclines its extremity towards the regions of space
+which it has just left, always presenting its convex border to the
+regions towards which it is moving. It is also a remarkable fact that
+this convex border, moving first in space, always appears brighter and
+sharper than the opposite one, which is often diffused. From these
+peculiarities it would seem that in moving about the Sun the comets
+encounter some resistance to their motion, from the medium through which
+they pass, and that this resistance is sufficient to curve their tails
+away from the course in which they move, and to crowd their particles
+together on the forward side. It is especially when they approach their
+perihelion, and move more rapidly on a curve of a shorter radius, that
+the comets' tails show the greatest curvature, unless their position in
+regard to the observer prevents their being advantageously seen. The
+tail of Donati's comet presented a fair illustration of this
+peculiarity, its curvature having augmented with the velocity of the
+comet's motion about the Sun. But possibly this phenomenon has another
+cause, and may be found rather in the solar repulsion which acts on
+comets and is not instantaneously propagated throughout their mass.
+</p>
+<p>
+Although, in general, comets have but one tail, it is not very rare to
+see them with multiple tails. The comets of 1807 and 1843 had each a
+double tail; Donati's comet, in 1858, showed several narrow, long
+rectilinear rays, issuing from its abruptly curved tail. The comet of
+1825 had five branches, while that of 1744 exhibited no less than six
+distinct tails diverging from the coma at various angles. In general
+character the multiple and single tails are similar. When a comet has
+two tails, it is not rare for the second to extend in the general
+direction of the Sun, as was the case with the great comet of 1881,
+III., represented on Plate XI. From July 14th to the 21st it exhibited
+quite an extended conical tail, starting obliquely downwards from the
+right side of the coma, and directed towards the Sun. From the 24th of
+July to the 2d of August this secondary tail was exactly opposite in its
+direction from that of the primary tail, and gave to the head a very
+elongated appearance. Comet 1881, IV., also exhibited a secondary
+appendage, not directed towards the Sun, but making an angle of about
+45° with the main tail.
+</p>
+<p>
+These cometary appendages sometimes attain prodigious dimensions. The
+comets of 1680 and 1769 had tails so extended that, after their heads
+had set under the horizon, the extremities of these immense appendages
+were still seen as far up as the zenith. In a single day the tail of the
+comet of 1843 extended 100°, and it was thrust from the comet "as a
+dart of light" to the enormous distance of 48,500,000 miles, and yet of
+this immense appendage nothing was left on the following day. The tail
+of Donati's comet, in 1858, attained a real length of 42,000,000 miles,
+while that of the great comet of 1843 had the enormous length of
+200,000,000 miles. If this last comet had occupied the position of the
+Sun, which it approached very nearly for a moment, the extremity of its
+tail would have extended 60,000,000 miles beyond the orbit of Mars.
+</p>
+<p>
+In some cases the tails of comets have been seen undulating and
+vibrating in a manner similar to the undulations and coruscations of
+light characteristic of some auroras. Many observers report having seen
+such phenomena. The comet of 1769 was traversed by luminous waves and
+pulsations, comparable to those seen in the aurora borealis. I myself
+observed these curious undulations in Coggia's comet in 1874, while the
+head of this object was below the horizon. For an hour the undulations
+rapidly succeeded each other, and ran along the whole length of the
+tail.
+</p>
+<p>
+Some of the brightest comets have shone with such splendor that they
+could be observed easily in full sunshine. Many comets, such as those of
+1577 and 1744, have equaled Sirius and Venus in brilliancy. The great
+comet of 1843, which suddenly appeared in our sky, was so brilliant that
+it was seen by many observers at noon time, within a few degrees from
+the Sun. I remember that I myself saw this remarkable object in the day
+time, with a number of persons, who were gazing at the wonderful
+apparition. So brilliant was this comet, that besides its nucleus and
+head, a portion of its tail was also visible in the day time, provided
+the observer screened his eyes from the full sunlight by standing in the
+shadow of some building.
+</p>
+<p>
+Of all the bodies revolving around the Sun, none have been known to
+approach so near its surface as did the comet of 1843. When it arrived
+at perihelion, the distance from the centre of its nucleus to the
+surface of the Sun's photosphere was only 96,000 miles, while the
+distance from surface to surface was less than 60,000 miles. This comet,
+then, went through the solar atmosphere, and in traversing it with its
+tremendous velocity of 366 miles per second, may very possibly have
+swept through some solar protuberances, many of which attain much higher
+elevations than that at which the comet passed. The comet of 1680 also
+approached quite near the surface of the Sun, and near enough to
+encounter some of the high solar protuberances, its distance at
+perihelion being about two-thirds of the Moon's distance from the Earth.
+The rapidity of motion of the comet of 1843 was such, when it approached
+the Sun, that it swept through all that part of its orbit which is
+situated north of the plane of the ecliptic in a little more than two
+hours, moving in this short time from one node to the other, or 1800.
+</p>
+<p>
+But if some comets have a very short perihelion distance, that of others
+is considerable. Such a comet was that of 1729, whose perihelion
+distance was 383,000,000 miles, the perihelion point being situated
+between the orbits of Mars and Jupiter.
+</p>
+<p>
+While some comets come near enough to the Sun at perihelion to be
+volatilized by its intense heat, others recede so far from it at
+aphelion that they may be said to be frozen. The shortest cometary
+aphelion distance known is that of Encke's comet, whose greatest
+distance from the sun is 388,000,000 miles. But that of the comet of
+1844 is 406,000,000,000 miles from the Sun. The comets of 1863 and 1864
+are so remote in space when they reach their aphelion points that light,
+with its velocity of 185,500 miles a second, would require 171 days in
+the first case, and 230 in the last, to pass from them to the Earth.
+</p>
+<p>
+The period of revolution of different comets also varies immensely.
+While that of Encke's comet is only 3½ years, that of comet 1864, II.,
+is 280,000 years.
+</p>
+<p>
+Among the periodic comets of short period, some have exhibited highly
+interesting phenomena. Encke's comet, discovered in 1818, is remarkable
+for the fact that its period of revolution diminishes at each of its
+successive returns, and consequently this comet, with each revolution,
+approaches nearer and nearer to the Sun. The decrease of the period is
+about 2½ hours at each return. Although the decrease is small, if it go
+on in future as it does at present, the inevitable consequence will be
+that this comet will finally fall into the Sun. This curious phenomenon
+of retardation has been attributed by astronomers to the existence of a
+resisting medium filling space, but so rare and ethereal that it does
+not produce any sensible effect on the movements of the planets. But
+some other causes may retard this comet, as similar retardations have
+not been observed in the case of other periodic comets of short period.
+These, however, are not so near to the Sun, and perhaps our luminary may
+be surrounded by matter of extreme tenuity, which does not exist at a
+greater distance from it.
+</p>
+<p>
+Another of the periodic comets which has exhibited a very remarkable
+phenomenon of transformation is Biela's comet, which divided into two
+distinct parts, moving together in the same direction. When this comet
+was first detected at its return in 1845, it presented nothing unusual,
+but in the early part of 1846 it was noticed by several astronomers to
+be divided into two parts of unequal brightness, forming thus a twin
+comet. At its next return in 1852, the two sister comets were still
+traveling in company, but their distance apart, which in 1846 was
+157,000 miles, had increased to 1,500,000 miles. At the two next returns
+in 1859 and 1865, their position not being very favorably situated for
+observation, the comets were not seen. In 1872 the position should have
+been favorable for observation, and they were consequently searched for,
+but in vain; neither comet was found. An astronomer in the southern
+hemisphere, however, found a comet on the track of Biela's, but
+calculation has shown that the two objects are probably not identical,
+since this comet was two months behind the computed position for
+Biela's. It will be shown in the following chapter that our globe
+probably crossed the orbit of Biela's comet on November 27th, 1872, and
+the phenomena resulting from this passage will be there described.
+</p>
+<p>
+It is seen from these observations that comets may be lost or dissipated
+in space by causes entirely unknown to us. Biela's comet is not the only
+one which has been thus disintegrated. Ancient historians speak of the
+separation of large comets into two or more parts. In 1661 Hevelius
+observed the apparent division of the comet of that year and its
+reduction to fragments. The return of this comet, calculated for 1790,
+was vainly waited for; the comet was not seen.
+</p>
+<p>
+Other comets, whose periods of revolution were well known, have
+disappeared, probably never to return. Such is Lexell's comet, whose
+period was 5⁶⁄₁₀ years; also De Vico's comet, both of which are
+now lost. It is supposed that Lexell's comet, which passed twice very
+near the giant planet Jupiter, had its orbit changed from an ellipse to
+a parabola, by the powerful disturbing influence of this planet, and was
+thus lost from our system. Several other comets, in traveling over their
+different orbits, have approached near enough to Saturn, Jupiter and the
+Earth to have their orbits decidedly altered by the powerful attraction
+of these bodies.
+</p>
+<p>
+But since comets are liable to pass near the planets, and several have
+orbits which approach that of the Earth, it becomes important for us to
+know whether an encounter of such a body with our globe is possible, and
+what would then be the result for us. Although that knowledge would not
+enable us to modify the possibilities of an encounter, yet it is better
+to know the dangers of our navigation through space than to ignore them.
+This question of a collision of the Earth with a comet has been answered
+in different ways, according to the ideas entertained in regard to the
+mass of these bodies. While some have predicted calamities of all kinds,
+such as deluges, conflagrations, or the reduction of the Earth to
+incandescent gases, others have asserted that it would produce no more
+effect than does a fly on encountering a railroad train. In our days
+astronomers entertain very little fears from such an encounter, because
+the probabilities of danger from an occurrence of this sort are very
+slight, the mass of an ordinary comet being so small compared with that
+of our globe. We know with certainty that the Earth has never had an
+encounter with a comet <i>by which it has been transformed into gases</i>,
+at least within the several millions of years during which animal and
+vegetable life have left their marks upon the stony pages of its
+history, otherwise these marks would not now be seen. If, then, such an
+accident has not happened during this long period, the chances for its
+occurring must be very small, so small indeed that they might almost be
+left out of the question. It is true that our globe shows signs of great
+perturbations of its surface, but we have not the slightest proofs that
+they resulted from an encounter with a celestial body. It seems very
+probable that our globe passed through the tail of the comet of 1861,
+before it was first seen on June 29th; but nothing unusual was observed,
+except perhaps some phosphorescent light in the atmosphere, which was
+afterwards attributed to this cause.
+</p>
+<p>
+The density and mass of comets must be comparatively very small. Their
+tails consist of matter of such extreme tenuity that it affects but very
+little the light of the small stars over which they pass. The coma and
+nucleus, however, are not quite so transparent, and may have greater
+masses. On several occasions I have seen the light of stars reduced by
+the interposition of cometary matter, comet 1881, III., presenting
+remarkable cases of this sort. On July 8th, at 10h. 50m., several small
+stars were involved in this comet, one of which passed quite near the
+nucleus through the bright inner coma. At that time the comet was
+greatly disturbed, its nucleus was contracting and enlarging rapidly,
+and becoming bright and again faint in an instant. Every time that the
+nucleus grew larger, the star became invisible, but reappeared the
+moment the nucleus was reduced in size. This phenomenon could not be
+attributed to an atmospheric effect, since, while the nucleus was
+enlarging, a very small inner nucleus was visible within the large
+diffused one, the matter of which had apparently spread over the part of
+the coma in which the star was involved, making it invisible.
+</p>
+<p>
+That the mass of comets is small, is proved by the fact that they have
+sometimes passed near the planets without disturbing them in any
+sensible manner. Lexell's comet, which in 1770 remained four months very
+near Jupiter, did not affect in the least the orbits, or the motions of
+its satellites. The same comet also came within less than 1,500,000
+miles from the Earth, and on this occasion it was calculated that its
+mass could not have been the ¹⁄₅₀₀₀ part of that of our
+globe, since otherwise the perturbations which it would have caused in
+the elements of the Earth's orbit would have been sensible. There was,
+however, no change. If this comet's mass had been equal to that of our
+globe, the length of our year would have been increased by 2h. 47m. The
+comet of 1837 remained four days within 3,500,000 miles of the Earth,
+with no sensible effect.
+</p>
+<p>
+It seems quite difficult to admit that the denser part of a comet
+forming the nucleus is solid, as supposed by some physicists, since it
+is so rapidly contracted and dilated by the solar forces, while the
+comet is yet at a too great distance from the Sun to allow these effects
+to be attributed to solar heat alone. This part of a comet, as indeed
+the other parts, seems rather to be in the gaseous than in the solid
+state; the changes observed in the intensity of its light and in its
+structure may be conceived as due to some solar action partaking of the
+nature of electricity.
+</p>
+<p>
+It has been a question whether comets are self-luminous, or whether they
+simply reflect the solar light. When their light is analyzed by the
+spectroscope, it is found that the nucleus of a comet generally gives a
+continuous spectrum, while the coma and tail give a spectrum consisting
+of several bright diffused bands. The spectrum given by the nucleus is
+rarely bright enough to allow the dark lines of the solar spectrum to be
+discerned upon it; but such lines were reported in the spectrum of comet
+1881, III., a fact proving that this nucleus at least reflected some
+solar light. The nucleus of a comet may be partly self-luminous, and
+either solid, liquid, or composed of incandescent gases submitted to a
+great pressure. As to the coma and tail, they are evidently gaseous, and
+partly, if not entirely, self-luminous, as is proved by the band
+spectrum which they give. The position of these bands, moreover,
+indicates that the luminous gases of which they are composed contain
+carbon. The phenomena of polarization, however, seem to prove that these
+parts of comets also reflect some solar light.
+</p>
+<p>
+No theory so far proposed, to explain comets and the strange phenomena
+they exhibit, seems to have been successful in its attempts, and the
+mystery in which these bodies have been involved from the beginning of
+their apparition, seems to be now nearly as great as ever. It has been
+supposed that their tails have no real existence, but are due to an
+optical illusion. Prof. Tyndall has endeavored to explain cometary
+phenomena by supposing these bodies to be composed of vapors subject to
+decomposition by the solar radiations, and thus made visible, the head
+and tail being an actinic cloud due to such decompositions. According to
+this view, the tails of comets would not consist of matter projected
+into spacer but simply of matter precipitated by the solar rays in
+traversing the cometary nebulosity. The endeavor has also been made to
+explain the various phenomena presented by comets by an electrical
+action of the Sun on the gases composing these objects. Theories taking
+this as a base seem to us to be more likely to lead to valuable results.
+M. Faye, who has devoted much time and learning to this subject, assumes
+a real repulsive force of the Sun, acting inversely to the square of the
+distance and proportionally to the surface, and not to the mass as
+attraction does. He supposes, however, that this repulsive force is
+generated by the solar heat, and not by electricity. Prof. Wm. Harkness
+says that many circumstances seem to indicate that the comets' tails are
+due, in a great measure, to electrical phenomena.
+</p>
+<p>
+The fact that the tails of comets are better defined and brighter on the
+forward side, associated with the other fact that they curve the most
+when their motion is most rapid, sufficiently indicates that these
+appendages are material, and that they either encounter some resistance
+from the medium in which they move, or from a solar repulsion. The
+phenomena of condensation and extension, which I have observed in the
+comets of 1874 and 1881, added to the curious behavior exhibited by the
+jets issuing from the nucleus, seem to indicate the action of electrical
+forces rather than of heat. The main difficulty encountered in the
+framing of a theory of comets consists in explaining how so delicate and
+extended objects as their tails seem to be, can be transported and
+whirled around the Sun at their perihelion with such an enormous
+velocity, always keeping opposite to the Sun, and, as expressed by Sir
+John Herschel, "in defiance of the law of gravitation, nay, even of the
+received laws of motion."
+</p>
+<p>
+To consider the direction of the comets' tails as an indirect effect of
+attraction, seems out of the question; the phenomenon of repulsion so
+plainly exhibited by these objects seems to point to a positive solar
+repulsion, as alone competent to produce these great changes. The
+repulsive action of the Sun on comets' tails might be conceived, for
+instance, as acting in a manner similar to that of a powerful current of
+wind starting from the Sun, and constantly changing in direction, but
+always keeping on a line with the comet. Such a current, acting on a
+comet's tail as if it were a pennant, would drive it behind the nucleus
+just as observed. If it could once be ascertained that the great
+disturbances on comets correspond with the magnetic disturbances on our
+globe and with the display of the auroral light, the electric nature of
+the forces acting so strangely on the comets would be substantially
+demonstrated. I have shown that some of the great disturbances observed
+in the comets of 1874 and 1881 have coincided with auroral displays, and
+it will be shown hereafter that similar displays have also coincided
+with the passage of meteoric showers through our atmosphere. Whether
+these simultaneous phenomena were simple coincidences having no
+connection, or whether they are the result of a common cause, can only
+be ascertained by long continued future observations.
+</p>
+
+<p><br /><br /><br /></p>
+
+<h4><a id="SHOOTING_STARS">SHOOTING-STARS AND METEORS</a>
+<br /><br />
+PLATE XII</h4>
+
+<p>
+While contemplating the heavens on a clear moonless night, we
+occasionally witness the sudden blazing forth of a star-like meteor,
+which glides swiftly and silently across some of the constellations, and
+as suddenly disappears, leaving sometimes along its track a
+phosphorescent trail, which remains visible for a while and gradually
+vanishes. These strange apparitions of the night are called <i>Falling</i>
+or <i>Shooting-stars</i>.
+</p>
+<p>
+There is certainly no clear night throughout the year during which some
+of these meteors do not make their appearance, but their number is quite
+variable. In ordinary nights only four or five will be observed by a
+single person in the course of an hour; but on others they are so
+numerous that it becomes impossible to count them. When the falling
+stars are only a few in number, and appear scattered in the sky, they
+are called <i>Sporadic Meteors</i>, and when they appear in great numbers
+they constitute <i>Meteoric Showers</i> or <i>Swarms</i>.
+</p>
+<p>
+Probably there is no celestial phenomenon more impressive than are these
+wonderful pyrotechnic displays, during which the heavens seem to break
+open and give passage to fiery showers, whose luminous drops describe
+fantastic hieroglyphics in the sky. While observing them, one can fully
+realize the terror with which they have sometimes filled beholders, to
+whom it seemed that the stability of the universe had come to an end,
+and that all the stars of the firmament were pouring down upon the Earth
+in deluges of fire.
+</p>
+<p>
+The ancients have left record of many great meteoric displays, and the
+manner in which they describe them sufficiently indicates the fear
+caused by these mysterious objects. Among the many meteoric showers
+recorded by ancient historians may be mentioned one observed in
+Constantinople, in the month of November, 472, when all the sky appeared
+as if on fire with meteors. In the year 599, meteors were seen on a
+certain night flying in all directions like fiery grasshoppers, and
+giving much alarm to the people. In March, 763, "the stars fell
+suddenly, and in such crowded number that people were much frightened,
+and believed the end of the world had come." On April 10th, 1095, the
+stars fell in such enormous quantity from midnight till morning that
+they were as crowded as are the hail stones during a severe storm.
+</p>
+<p>
+In modern times the fall of the shooting-stars in great number has been
+frequently recorded. One of the most remarkable meteoric showers of the
+eighteenth century occurred on the night of November 13th, 1799, and was
+observed throughout North and South America and Europe. On this
+memorable night thousands of falling stars were seen traversing the sky
+between midnight and morning. Humboldt and Boupland, then traveling in
+South America, observed the phenomena at Cumana, between two and five
+o'clock in the morning. They saw an innumerable number of shooting-stars
+going from north to south, appearing like brilliant fire-works. Several
+of these meteors left long phosphorescent trails in the sky, and had
+nuclei whose apparent diameter, in some cases, surpassed that of the
+Moon.
+</p>
+<p>
+The shower of November 13th, 1833, was still more remarkable for the
+great number of meteors which traversed the heavens, and was visible
+over the whole of North and South America. On that occasion the falling
+stars were far too numerous to be counted, and they fell so thickly that
+Prof. Olmsted, of New Haven, who observed them carefully, compared their
+number at the moment of their maximum fall to half that of the flakes of
+snow falling during a heavy storm. This observer estimated at 240,000
+the number of meteors which must have traversed the heavens above the
+horizon during the seven hours while the display was visible.
+</p>
+
+<p><br /></p>
+
+<div class="figcenter" style="width: 400px;">
+<a id="figure12"></a>
+<br />
+<img src="images/figure12.jpg" width="400" alt="" />
+<div class="caption">
+<p>PLATE XII.&mdash;THE NOVEMBER METEORS.</p>
+<p class="smaller">As observed between midnight and 5 o'clock A.M. on the night
+of November 13-14 1868.</p>
+</div></div>
+
+<p><br /></p>
+
+<p>
+In the years 1866, 1867 and 1868, there were also extraordinary meteoric
+displays on the night of November 13th. It was on the last mentioned
+date that I had the opportunity to observe the remarkable shower of
+shooting-stars of which I have attempted to represent all the
+characteristic points in Plate XII. My observations were begun a little
+after midnight, and continued without interruption till sunrise. Over
+three thousand meteors were observed during this interval of time in the
+part of the sky visible from a northern window of my house. The maximum
+fall occurred between four and five o'clock, when they appeared at a
+mean rate of 15 in a minute.
+</p>
+<p>
+In general, the falling stars were quite large, many being superior to
+Jupiter in brightness and apparent size, while a few even surpassed
+Venus, and were so brilliant that opaque objects cast a strong shadow
+during their flight. A great many left behind them a luminous train,
+which remained visible for more or less time after the nucleus had
+vanished. In general, these meteors appeared to move either in straight
+or slightly curved orbits; but quite a number among them exhibited very
+extraordinary motions, and followed very complicated paths, some of
+which were quite incomprehensible.
+</p>
+<p>
+While some moved either in wavy or zig-zag lines, strongly accentuated,
+others, after moving for a time in a straight line, gradually changed
+their course, curving upward or downward, thus moving in a new
+direction. Several among them, which were apparently moving in a
+straight line with great rapidity, suddenly altered their course,
+starting at an abrupt angle in another direction, with no apparent
+slackening in their motion. One of them, which was a very conspicuous
+object, was moving slowly in a straight course, when of a sudden it made
+a sharp turn and continued to travel in a straight line, at an acute
+angle with the first, retreating, and almost going back towards the
+regions from which it originally came. As nearly all the meteors which
+exhibited these extraordinary motions left the trace of their passage in
+the sky by a luminous trail, it was easily ascertained that these
+appearances were not deceptive. On one occasion I noticed that the
+change of direction in the orbit corresponded with the brightening up of
+the meteor thus disturbed in its progress.
+</p>
+<p>
+Among these meteors, some traveled very slowly, and a few seemed to
+advance as if by jerks, but in general they moved very rapidly. One of
+the meteors thus appearing to move by jerks left a luminous trail, upon
+which the various jerks seemed to be left impressed by a succession of
+bright and faint spaces along the train. Some of the largest meteors
+appeared to rotate upon an axis as they advanced, and most of these
+revolving meteors, as also a great number of the others, seemed to
+explode just before they disappeared, sending bright fiery sparks of
+different colors in all directions, although no sound was at any time
+heard. The largest and most brilliant meteor observed on that night
+appeared at 5h. 30m., a little before sunrise. It was very bright, and
+appeared considerably larger than Venus, having quite a distinct disk.
+This meteor moved very slowly, leaving behind a large phosphorescent
+trail, which seemed to issue from the inside of the nucleus as it
+advanced. For a moment the train increased in size and brightness close
+to the nucleus, which then appeared as an empty transparent sphere,
+sprinkled all over with minute fiery sparks; the nucleus then suddenly
+burst out into luminous particles, which immediately vanished, only the
+luminous trail of considerable dimensions being left.
+</p>
+<p>
+Many of the trails thus left by the meteors retained their luminosity
+for several minutes, and sometimes for over a quarter of an hour. These
+trails slowly changed their form and position; but it is perhaps
+remarkable that almost all those which I observed on that night assumed
+the same general form&mdash;that of an open, irregular ring, or horse-shoe,
+somewhat resembling the letter C. This ring form was subsequently
+transformed into an irregular, roundish cumulus-like cloud. The trail
+left by a very large meteor, which I observed on the evening of
+September 5th, 1880, also exhibited the same general character of
+transformation.
+</p>
+<p>
+While I was observing a long brilliant trail left by a meteor on the
+night of November 13th, 1868, it was suddenly crossed by another bright
+shooting-star. The latter apparently went through the luminous substance
+forming the trail, which was suddenly altered in form, and considerably
+diminished in brightness simultaneously with this passage, although
+electrical action at some distance might perhaps as well explain the
+sudden change observed.
+</p>
+<p>
+In the majority of cases the meteors appeared white; but many,
+especially the largest, exhibited a variety of brilliant colors, among
+which the red, blue, green, yellow and purple were the most common. In
+general the trails exhibited about the same color as the nucleus, but
+much fainter, and they were usually pervaded by a greenish tint. In some
+instances the trails were of quite a different color from the nucleus.
+</p>
+<p>
+The luminous cloud observed at 5h. 30m. on the morning of November 14th,
+1868, after having passed through the series of transformations above
+described, remained visible for a long while after sunrise, appearing
+then as a small cirrus cloud, exactly similar in appearance to the
+hundreds of small cirrus clouds then visible in the sky, which had
+probably the same meteoric origin. For over three hours after sunrise,
+these cirrus clouds remained visible in the sky, moving all together
+with the wind in the high regions of the atmosphere.
+</p>
+<p>
+Although Plate XII. is intended to represent all the characteristics
+exhibited by the meteors observed on that night, every form represented
+having been obtained by direct observation, yet the number is much
+greater than it was at any single moment during the particular shower of
+1868. As regards number, the intention was to give an idea of a great
+meteoric shower, such as that of 1833, for instance. Although many of
+the falling stars seem to be close to the Earth's surface, yet this is
+only an effect of perspective due to their great distance, very few of
+these meteors ever coming into the lower regions of our atmosphere at
+all.
+</p>
+<p>
+The phenomena exhibited during other great meteoric showers have been
+similar to those presented by the shower just described, the only
+differences consisting in variations of size and brightness in the
+meteors, and also in the trails, which sometimes are not so numerous as
+they were in 1868.
+</p>
+<p>
+While some shooting-stars move so rapidly that they can hardly be
+followed in their orbits, others move so slowly that the sight can
+easily follow them, and even remark the peculiarities of their
+movements, some remaining visible for half a minute. Some of the falling
+stars move at the rapid rate of 100 miles a second, but others only 10
+miles a second, and even less. In general, they move about half as fast
+again as the Earth in its orbit. The arcs described by the meteors in
+the sky are variable. While some extend 8o° and even 100°, others are
+hardly half a degree in length. While some shooting-stars are so faint
+that they can hardly be seen through the largest telescopes, others are
+so large and brilliant that they can be seen in the day-time. In
+general, a shooting-star of average brightness resembles a star of the
+third or fourth magnitude.
+</p>
+<p>
+Whatever may be the origin of the shooting-stars, they are, when we see
+them, not in the celestial spaces, like the planets, the comets, or the
+stars, but in our atmosphere, through which they travel as long as they
+remain visible. The height at which they appear and disappear is
+variable, but in general they are about 80 miles above the surface of
+our globe when they are first seen, and at about 55 miles when they
+disappear. In many cases, however, they have been observed at greater
+elevations, as also at smaller. A meteor simultaneously observed at two
+different stations first appeared at the height of 285 miles, and was
+last seen at 192 miles above the Earth's surface; but in rare cases the
+falling stars have been seen below a layer of clouds completely covering
+the sky. I myself saw one such shooting-star a few years since. The fact
+that the meteors are visible at so great elevations, proves that our
+atmosphere extends much farther than was formerly supposed, although at
+these great heights it must be extremely rarefied, and very different
+from what it is in its lower regions.
+</p>
+<p>
+There is a remarkable difference between the sporadic meteors seen in
+the sky on every night, and the meteoric showers observed only at
+comparatively rare intervals. While the first appear from different
+points in the sky and travel in all directions, being perfectly
+independent, the meteors of a shower all come from the same point of the
+heavens, from which they apparently diverge in all directions. This
+point of divergence of the meteors is called the <i>radiant point</i> of
+the shower. Although the meteors seem to diverge in all directions from the
+radiant point, yet they all move in approximately parallel lines, the
+divergence being an effect of perspective.
+</p>
+<p>
+Whatever may be the position of the radiant point in the constellations,
+it remains as fixed in the sky as the stars themselves, and participates
+with them in the apparent motion which they undergo by the effect of the
+diurnal motion, and thus rises and sets with the constellation to which
+it belongs. This fact is sufficient to prove that the orbits of these
+meteors are independent of the Earth's motion, and that consequently
+they do not originate in our atmosphere. It has been shown by Encke that
+the radiant point of the meteoric shower of November 13th is precisely
+the point towards which our globe moves in space on November 13th; a
+tangent to the Earth's orbit would pass through this radiant point.
+</p>
+<p>
+The meteoric showers are particularly remarkable, not merely because of
+the large number of meteors which are visible and the fact that they all
+follow a common orbit, but chiefly because they have a periodic return,
+either after an interval of a year, or after a lapse of several years.
+At the beginning of the present century only two meteoric showers were
+known, those of August 10th and of November 13th, and their periodicity
+had not yet been recognized, although it had begun to be suspected. It
+was only in 1836 that Quetelet and Olbers ventured to predict the
+reappearance of the November meteors in the year 1867. Having made
+further investigations, Prof. Newton, of Yale College, announced their
+return in the year 1866. In both of these years, as also in 1868, the
+meteors were very numerous, and were observed in Europe and in America
+on the night of November 13th. The predictions having thus been
+fulfilled, the periodicity of the meteors was established. Since then,
+other periodic showers have been recognized, although they are much less
+important in regard to number than those of August and November, except
+that of November 27th, which exhibited so brilliant a display in Europe
+in 1872. These successive appearances have established the main fact
+that meteoric showers are more or less visible every year when the Earth
+occupies certain positions in its orbit.
+</p>
+<p>
+The meteoric shower of the 10th of August has its radiant point situated
+in the vicinity of the variable star Algol, in the constellation
+Perseus, from which its meteors have received the name of Perseids.
+Although varying in splendor, this meteoric swarm never fails to make
+its appearance every year. The Perseids move through our atmosphere at
+the rate of 37 miles per second. The shower usually lasts about six
+hours.
+</p>
+<p>
+The meteoric shower of November 13th has its radiant point situated in
+the vicinity of the star Gamma, in the constellation Leo, from which its
+meteors have been called Leonids. But while the August meteors recur
+regularly every year, with slight variations, the shower of November
+does not occur with the same regularity. During several years it is
+hardly noticeable, and is even totally absent, while in other years it
+is very remarkable. Every 33 years an extraordinary meteoric shower
+occurs on the 13th of November, and the phenomenon is repeated on the
+two succeeding years at the same date, but with a diminution in its
+splendor at each successive return. The Leonids move in an opposite
+direction to that of the Earth, and travel in our atmosphere with an
+apparent velocity of 45 miles per second, this being about the maximum
+velocity observed in falling stars. But when the motion of our globe is
+taken into account, and a deduction is made of the 18 miles which it
+travels per second, it is found that these meteors move at an actual
+mean rate of 27 miles a second.
+</p>
+<p>
+In a meteoric shower the stars do not fall uniformly throughout the
+night, there being a time when they appear in greater numbers. Usually
+it is towards morning, between 4 and 6 o'clock, that the maximum occurs.
+The probable cause of this phenomenon will be explained in its place
+hereafter.
+</p>
+<p>
+The orbits of the meteoric showers are not all approximately in the same
+plane, like those of the planets, but rather resemble those of comets,
+and have all possible inclinations to the ecliptic. Like the comets,
+too, the different meteoric showers have either direct or retrograde
+motion.
+</p>
+<p>
+The shooting-stars were formerly considered as atmospheric meteors,
+caused by the combustion of inflammable gases generated at the surface
+of the Earth, and transported to the high regions of our atmosphere by
+their low specific gravity. But the considerable height at which they
+usually appear, the great velocity of their motion, the common orbit
+followed by the meteors of the same shower, and the periodicity of their
+recurrence, do not permit us now to entertain these ideas, or to doubt
+their cosmical origin. But what is their nature?
+</p>
+<p>
+It is now generally admitted that innumerable minute bodies, moving in
+various directions around the Sun, are scattered in the interplanetary
+spaces through which our globe travels. It has been supposed that
+congregations of such minute bodies form elliptical rings, within which
+they are all moving in close parallel orbits around the Sun. On the
+supposition that such rings intersect the orbit of the Earth at the
+proper places, it was practicable to account for the shooting-stars by
+the passage through our atmosphere of the numerous minute cosmical
+bodies composing the rings, and the Leonid and Perseid showers were so
+explained. But when the elements of the orbits of these two last swarms
+came to be better known, and were compared with those of other celestial
+bodies, it was found necessary to alter this theory.
+</p>
+<p>
+It had for a long while been suspected that some kind of relation
+existed between the shooting-stars and the comets. This idea, vaguely
+formulated by Kepler more than two centuries ago, more clearly expressed
+by Chladni, and still more by Mr. Grey, before the British Association,
+at Liverpool, in 1855, has recently received a brilliant confirmation by
+the researches of Professor Schiaparelli, Director of the Observatory of
+Milan. A thorough investigation of the orbits of the August and November
+meteors led Schiaparelli to the discovery of a remarkable relation
+between meteoric and cometary orbits. By comparing the elements of these
+meteoric orbits with those of comets, he found a very close resemblance
+between the orbit of the August meteors and that of the comet 1862,
+III., and again between the orbit of the November meteors and that of
+Tempel's comet, 1866, I. These resemblances were too striking to be the
+result of mere chance, and demonstrated the identity of these cometary
+orbits with those of the Perseid and Leonid showers. In accordance with
+these new facts, it is now admitted that the meteoric showers result
+from the passage of our globe through swarms of meteoric particles
+following the orbits of comets, which intersect the orbit of the Earth.
+</p>
+<p>
+Professor Schiaparelli has attempted to show how these meteoric swarms
+were originally scattered along the orbits of comets, by supposing these
+bodies to originate from nebulous masses, which, in entering the sphere
+of attraction of the Sun, are gradually scattered along their orbits,
+and finally form comets followed by long trails of meteoric particles.
+</p>
+<p>
+It has been shown that in approaching the Sun the comets become
+considerably elongated, their particles being disseminated over immense
+distances by the solar repulsion. It seems probable that, owing to its
+feeble attractive power, the nucleus is incompetent to recall the
+scattered cometary particles and retain them in its grasp when they are
+relieved from the solar repulsion, so that they remain free from the
+nucleus, although they continue to move along its orbit. It is
+supposable that these cometary particles will scatter more and more in
+course of time. Forming at first an elongated meteoric cloud, they will
+finally spread along the whole orbit, and thus form a ring of meteoric
+particles. Since our globe constantly moves in its orbit and daily
+occupies a different position, it follows that at any point where such a
+cometary orbit happens to cross that of the Earth, our globe will
+necessarily encounter the cometary particles as a shower of meteors.
+This encounter will take place at a certain time of the year, either
+yearly, if they form a continuous ring, or after a succession of years,
+if they simply form an elongated cloud. Such meteoric clouds or rings
+would not be visible in ordinary circumstances, even through the largest
+telescopes, except on penetrating the upper regions of our atmosphere,
+when they would appear as showers of falling stars. It is supposed that
+in penetrating our atmosphere, even in its most rarefied regions, these
+meteors are heated by the resistance offered by the air to their motion,
+first becoming luminous and then being finally vaporized and burnt
+before they can reach the surface of the Earth.
+</p>
+<p>
+The orbit of the comet of 1862, III., which so closely corresponds with
+that of the Perseid meteors, is much more extended than that of Tempel's
+comet corresponding with that of the Leonids. While the first extends
+far beyond the orbit of Neptune, the latter only goes a little beyond
+that of Uranus. The former orbit makes a considerable angle with the
+plane of the Earth's orbit, but the latter is much nearer to parallelism
+with it. The period of revolution of the first is 108 years, and that of
+the last about 33¼ years.
+</p>
+<p>
+From the fact that the Perseid shower occurs yearly on the 10th of
+August, when the Earth crosses the orbit of the comet of 1862, III., it
+is supposed that the cometary particles producing this shower are
+disseminated along the whole orbit, and form a ring encircling the Sun
+and Earth. To explain the yearly variations in the number of the
+shooting-stars observed, these particles are supposed to be unequally
+distributed over the orbit, being more crowded at one place than they
+are at another. In order to explain the meteoric shower of Leonids,
+which appears in all its splendor every 33 years, and then with
+diminished intensity for two successive years, after which it is without
+importance, it is supposed that the cometary particles of the comet of
+1866, I., have not as yet spread all along the orbit, a sufficient time
+not having been allowed, but form an elongated meteoric cloud, more
+dense in its front than in its rear part. From these considerations it
+has been supposed also that the comet of 1866, I., is of a more recent
+date than that of 1862, III. While Tempel's comet makes its revolution
+around the Sun in about 33 years, this meteoric cloud, which has the
+same period and returns to the same point of its orbit every 33 years,
+encounters our globe for three successive years. The first year we are
+passing through its densest parts, and the two following years in less
+and less crowded parts, from which result the observed phenomena. An
+idea of the extent of this meteoric cloud may be formed from the fact
+that, with its cometary velocity of motion, it takes this cloud three
+years at least to cross the Earth's orbit. From recent researches it
+would appear that the Leonid cloud is not single, but that at least two
+others of smaller importance exist, and have periods of 33¼ years.
+</p>
+<p>
+Biela's comet, which was divided into two parts in 1846, is another of
+the few comets whose orbit approaches that of the Earth. Possessing this
+knowledge, and knowing then the close connection existing between
+meteors and comets, astronomers supposed that there were sufficient
+reasons to expect a meteoric shower when this comet was passing near the
+Earth. They consequently expected a meteoric display in 1872, when our
+globe was to cross its orbit. Their anticipation was plainly fulfilled,
+and on the night of November 27th, 1872, a splendid meteoric display,
+having its radiant point in the constellation Andromeda, was observed in
+Europe, and also in America, but the meteors seen here were not so
+numerous as in Europe. Other meteoric showers of less importance, such
+as that of April 20th, for instance, have also been identified with
+cometary orbits, so that now no doubt seems to remain as to the identity
+of cometary particles and shooting-stars.
+</p>
+<p>
+The fact that the maximum number of meteors is always observed in the
+morning hours, supports the hypothesis of the cosmic origin of the
+shooting-stars, since the regions of the Earth where it is morning are
+precisely those fronting the regions towards which our globe is moving
+in space, and accordingly encounter more directly the meteors moving in
+their orbit. The greater abundance of falling stars at that time may
+thus be accounted for.
+</p>
+<p>
+The number of meteors penetrating our atmosphere must be very great;
+there is not an hour and probably not a minute during which none fall.
+From various considerations, some astronomers have estimated at from
+65,000,000,000 to 146,000,000,000 the total number of shooting-stars
+yearly penetrating in our atmosphere. The actual number is undoubtedly
+great, yet the fact that the meteors are rarely seen through the
+telescope while employed in observing various celestial objects, does
+not indicate that they are so numerous as these figures imply. It is
+only occasionally that one is seen traversing the field of the
+instrument. Even when the sky is observed with a low power eye-piece for
+several hours in succession, many nights may pass without disclosing
+one, although an observer, sweeping the sky more freely with the naked
+eye, may often perceive four or five during an ordinary night.
+</p>
+<p>
+About the true nature of these bodies nothing is known with certainty.
+From spectrum analysis it seems to be established that most of them
+contain sodium and magnesium, while a few indicate the presence of
+strontium and iron, and in some rare cases there are traces of coal-gas.
+Some of the nuclei give a continuous spectrum, and others a spectrum of
+lines. The trail always gives a spectrum of bright lines which indicates
+its gaseous state. The traces of coal-gas rarely seen in meteors are,
+however, of great importance, as it identifies them more closely with
+the comets, which generally show a similar spectrum. The continuous
+spectra exhibited by some nuclei would indicate that they are
+incandescent and either solid or liquid; but it is difficult to conclude
+from their spectra what is their true nature, since we do not know
+exactly what part the terrestrial atmosphere may play in producing the
+results.
+</p>
+<p>
+The mass of the shooting-stars is not known with certainty, but the fact
+that during great meteoric showers, none are seen to reach the surface
+of the Earth, all being consumed in a few seconds, sufficiently
+indicates that it must be very small. It has been calculated that those
+equal to Venus in apparent size and brilliancy may weigh several pounds,
+while the faint ones would weigh only a few grains.
+</p>
+<p>
+If the shooting-stars have even such a mass as that here attributed to
+some of them, the extraordinary motions which I have described above
+seem to be unaccountable. The change of direction of a heavy mass moving
+swiftly cannot be sudden. The semi-circular, the wavy and the angular
+orbits observed could not be described, it would seem, by such a mass
+animated with a great velocity. Although the meteors are said to be
+ignited by the transformation of part of their progressive motion into
+molecular motion, yet it is not observed that the velocity of the
+falling stars diminishes when they are about to disappear. The luminous
+trails they leave in the atmosphere do not appear to be endowed with any
+motion, but remain for a time in their original positions. These facts
+are apparently opposed to the hypothesis that such meteors have any
+appreciable mass. The extraordinary motions exhibited by some meteors
+seem to indicate that some unsuspected force resides in these bodies,
+and causes them to deviate from the laws of ordinary motion.
+</p>
+<p>
+Although it is very probable that the ordinary shooting-stars have no
+appreciable mass, yet it is known that very heavy meteoric masses
+sometimes fall at the surface of the Earth. Such falls are generally
+preceded by the sudden apparition in the sky of a large, and usually
+very brilliant fire-ball, which traverses the air at a great speed,
+sometimes leaving behind it a luminous trail, after which it explodes
+with a loud sound, and heavy fiery meteoric fragments, diverging in all
+directions, fall at the surface of the Earth. The name of
+<i>Aerolites</i> or <i>Meteorolites</i> is given to these ponderous
+fragments. As these meteors, before they explode and fall to the ground,
+have many points of resemblance with the shooting-stars, they are
+generally supposed to be connected with them, and to have a similar
+cometary origin. The fact that the aerolites differ widely from each
+other in constitution, and are all composed of substances found on the
+Earth, associated with other facts given below, would rather seem to
+indicate a terrestrial than a celestial origin.
+</p>
+<p>
+If the aerolites belong to the same class of bodies as the falling
+stars, differing from them only in size and mass, it is difficult to see
+why so very few should fall upon the Earth during the great meteoric
+showers, when thousands of shooting-stars traverse our atmosphere. In
+Prof. Kirkwood's "Meteoric Astronomy" are given catalogues of all the
+falls of aerolites and fire-balls which have been observed at the time
+of the periodic meteoric showers of the 10th of August and the 13th of
+November, during a period of 221 years for the Perseids, or August
+showers, and of 318 years for the Leonids, or November showers. During
+221 years, 10 falls of aerolites have been witnessed simultaneously with
+the fall of the Perseids; while during 318 years, only 4 such falls have
+been recorded as having occurred at the time of the Leonid shower. If
+there is any close connection between the shooting-stars and the
+aerolites, we should expect to find a maximum in their fall at the time
+of the great meteoric displays. So far, no maxima or minima have yet
+been discovered in the fall of aerolites; they do not seem, like
+meteoric showers, to be governed by a law of periodicity.
+</p>
+<p>
+A very remarkable peculiarity of the aerolites is that they seem to have
+a tendency to fall in certain regions. Such are the southern part of
+France, the north of Italy, Hindostan, the central states of North
+America, and Mexico and Brazil. There is a curious contrast existing
+between the quick cometary motion of the aerolites before their
+explosion, and the comparatively slow motion of their fragments as they
+reach the Earth; motion which seems to be no greater than that
+corresponding to their natural fall impeded by the resistance of the
+air. In general, their penetration into the soil upon which they fall
+does not at all correspond to the great velocity with which they move in
+the atmosphere. The fragmentary structures of the aerolites, their
+identity of substance with that of our globe, their great resemblance to
+the volcanic minerals of the Earth, and the fractures and faults which
+some of them exhibit, do not correspond at all with the idea that they
+are cometary particles fallen on the Earth. As far as their structure
+and appearance is concerned, they seem rather to be a volcanic product
+of the interior of the Earth than parts of disintegrated comets. It must
+be admitted that their identity with the shooting-stars is far from
+established, and that they are still involved in mystery.
+</p>
+<p>
+The so-called meteoric dust gathered at sea and on high mountains may
+have various origins, and may be partly furnished by volcanic dust
+carried to great distances in the atmosphere.
+</p>
+<p>
+Since millions of shooting-stars penetrate our atmosphere every year and
+remain in it, becoming definitively a part of the Earth, it follows
+that, no matter how small may be the quantity of matter of which they
+are composed, they must gradually increase the volume and mass of our
+globe, although the increase may be exceedingly slow. Supposing every
+one of the shooting-stars penetrating our atmosphere to contain one
+cubic millimeter of matter, it has been calculated that it would take
+nearly 35,000 years to make a deposit one centimeter in thickness all
+over the surface of our globe. Insignificant as this may appear, it is
+probable that the quantity of matter of meteoric origin which is added
+to our globe is much less than has just been supposed.
+</p>
+
+<p><br /><br /><br /></p>
+
+<h4><a id="THE_MILKY_WAY">THE MILKY-WAY OR GALAXY</a>
+<br /><br />
+PLATE XIII</h4>
+
+<p>
+During clear nights, when the Moon is below the horizon, the starry
+vault is greatly adorned by an immense belt of soft white light,
+spanning the heavens from one point of the horizon to the opposite
+point, and girdling the celestial sphere in its delicate folds. Every
+one is familiar with this remarkable celestial object, called the
+<i>Milky-way</i> or <i>Galaxy</i>.
+</p>
+<p>
+Seen with the naked eye, the Galaxy appears as an irregular, narrow,
+nebulous belt, apparently composed of cloud-like luminous masses of
+different forms and sizes, separated by comparatively dark intervals.
+These cloud-like masses vary much in luminous intensity, and while some
+among them are very bright and conspicuous, others are so faint that
+they are hard to recognize. In general, the brightest parts of the
+Milky-way are situated along the middle of its belt, while its borders,
+which are usually very faint, gradually vanish in the sky. Some parts of
+the Galaxy, however, show very little of the cloudy structure so
+characteristic of other parts, being almost uniform throughout, except
+towards the borders, which are always fainter. These parts showing
+greater uniformity are also the faintest.
+</p>
+<p>
+Such is the general appearance of the Milky-way on ordinary nights, but
+on rare occasions, when the atmosphere is particularly pure, it presents
+one of the grandest sights that can be imagined. At such favorable
+moments I have seen the Galaxy gleaming with light, and appearing as if
+composed of star-dust or of precious stones. The strange belt then
+appeared all mottled over and fleecy, its large cloud-like masses being
+subdivided into numerous small, irregular cloudlets of great brilliancy,
+which appeared projected upon a soft luminous background.
+</p>
+<p>
+The width of the Galaxy is far from being uniform; while in some places
+it is only 4° or 5°, in others it is 15° and even more. In some
+places it appears wavy in outline, at others quite straight; then it
+contracts, to expand a few degrees distant; while at other places it
+sends off branches and loops, varying in form, size and direction, some
+of which are quite prominent, while others are very faint.
+</p>
+<p>
+Although very irregular in form, the general appearance of the galactic
+belt is that of a regular curve occupying one of the great circles of
+the celestial sphere. The Milky-way completely encircles the heavens,
+but, of course, only one-half is visible at any one moment, since our
+globe prevents the other half from being seen. If, for a moment, we
+imagine ourselves left in space, our globe having vanished from under
+our feet, we should then see the whole Galaxy forming a continuous belt
+in the heavens, at the centre of which we should apparently be situated.
+</p>
+<p>
+While only one-half of the galactic belt can be seen at once from any
+point on the Earth, yet, according to the position of the observer, a
+larger or smaller portion of the whole can be seen at different times.
+In high northern or southern latitudes but little more than half can be
+seen even by continuous observations; but as we approach the equatorial
+regions, more and more of it becomes visible, until the whole may be
+seen at different hours and seasons. In the latitudes of the northern
+states, about two-thirds of the Galaxy is visible, the rest remaining
+hidden below the horizon; but from the southern states very nearly the
+whole can be seen. The half of the Milky-way visible at any one time
+from any latitude on the Earth never entirely sets below the horizon,
+although in some places it may be so near the horizon as to be rendered
+invisible by vapors. In the latitude of Cambridge, when in its lowest
+position, the summit of its arc is still about 12° or 15° above the
+northern horizon. The great circle of the celestial sphere, occupied by
+the galactic belt, is inclined at an angle of about 63° to the
+celestial equator, and intersects this great circle on one side in the
+constellation Monoceros in 6h. 47m., and on the opposite side in the
+constellations Aquila and Ophiuchus in 18h. 47m. of right ascension; so
+that its northern pole is situated in the constellation Coma Berenices
+in R. A. 12h. 47m., declination N. 27°, and the southern in the
+constellation Cetus in R. A. 0h. 47m., declination S. 27°.
+</p>
+<p>
+According to the seasons and to the hours of the night at which it is
+observed, the galactic arch presents different inclinations in the sky.
+Owing to its inclination to the equator of the celestial sphere, its
+opposite parts exhibit opposite inclinations when they pass the meridian
+of a place. That part of the Galaxy which is represented on Plate XIII.,
+and which intersects the celestial equator in the constellation Aquila,
+is inclined to the left or towards the east, when it is on the meridian;
+while the opposite part, situated in Monoceros, is inclined to the
+right, or towards the west, when it reaches the meridian. The former
+passes the meridian in the evening in the summer and autumn months; the
+latter, in the winter and spring months.
+</p>
+
+<p><br /></p>
+
+<div class="figcenter" style="width: 400px;">
+<a id="figure13"></a>
+<br />
+<img src="images/figure13.jpg" width="400" alt="" />
+<div class="caption">
+<p>PLATE XIII.&mdash;PART OF THE MILKY WAY.</p>
+<p class="smaller">From a study made during the years 1874, 1875 and 1876</p>
+</div></div>
+
+<p><br /></p>
+
+<p>
+By beginning at its northernmost part, represented at the upper part of
+Plate XIII., situated in "the chair" of the constellation Cassiopeia,
+and descending southwardly, and continuing in the same direction until
+the whole circle is completed, the course of the Milky-way through the
+constellations may be briefly described as follows: From Cassiopeia's
+chair, the Galaxy, forming two streams, descends south, passing partly
+through Lacerta on the left, and Cepheus on the right; at this last
+point it approaches nearest to the polar star. Then it enters Cygnus,
+where it becomes very complicated and bright, and where several large
+cloudy masses are seen terminating its left branch, which passes to the
+right, near the bright star Deneb, the leader of this constellation.
+Below Deneb, the Galaxy is apparently disconnected and separated from
+the northern part by a narrow, irregular dark gap. From this rupture,
+the Milky-way divides into two great streams separated by an irregular
+dark rift. An immense branch extends to the right, which, after having
+formed an important luminous mass between the stars <i>γ</i> and <i>β</i>,
+continues its southward progress through parts of Lyra, Vulpecula,
+Hercules, Aquila and Ophiuchus, where it gradually terminates a few
+degrees south of the equator. The main stream on the left, after having
+formed a bright mass around <i>ε</i> Cygni, passes through Vulpecula and
+then Aquila, where it crosses the equinoctial just below the star <i>η</i>
+after having involved in its nebulosity the bright star Altair, the
+leader of Aquila. In the southern hemisphere the Galaxy becomes very
+complicated and forms a succession of very bright, irregular masses, the
+upper one being in Scutum Sobieskii, while the others are respectively
+situated in Sagittarius and in Scorpio; the last, just a little above
+our horizon, being always considerably dimmed by vapors. From Scutum
+Sobieskii, the Galaxy expands considerably on the right, and sends a
+branch into Scorpio, in which the fiery red star Antares is somewhat
+involved.
+</p>
+<p>
+Continuing its course below our horizon, the Milky-way enters Ara and
+Norma, and then, passing partly through Circinus, Centaurus and Musca,
+it reaches the Southern Cross, after having been divided by the large
+dark pear-shaped spot known to navigators as the "Coal-Sack." In Ara and
+Crux the Milky-way attains its maximum of brightness, which there
+surpasses its brightest parts in Cygnus. In Musca, it makes its nearest
+approach to the south pole of the heavens. It then enters Carina and
+Vela, where it spreads out like a fan, and terminates in this last
+constellation, before reaching <i>λ</i>, being once more interrupted by a
+dark and very irregular gap, on a line with the two stars <i>γ</i> and
+<i>λ</i>. It is noteworthy that this second rupture of continuity of the
+Galaxy in Vela is very nearly opposite, or at about 18o° from the break
+near Deneb in Cygnus.
+</p>
+<p>
+Continuing its course on the other side of the break, the Milky-way
+again spreads out into the shape of a fan, grows narrower in entering
+Puppis, where it is longitudinally divided by darkish channels. It then
+passes above our southern horizon, becoming visible to us, passing
+through part of Canis Major, where its border just grazes the brilliant
+star Sirius. But from Puppis it gradually diminishes in brightness and
+complication, becoming faint and uniform. It enters Monoceros and Orion,
+where it again crosses the equator a little above <i>δ</i>, the
+northernmost of the three bright stars in the belt of Orion. Continuing
+its northward course it passes through Gemini, extending as far as
+Castor and Pollux, and then entering Auriga, where it begins to increase
+in brightness and in complication of structure. It passes partly through
+Camelopardus and into Perseus, where an important branch proceeds from
+its southern border.
+</p>
+<p>
+This branch beginning near the star <i>θ</i>, advances towards the
+celebrated variable star Algol, around which it is quite bright and
+complicated. Continuing its course in the same direction, the branch
+rapidly loses its brightness, becoming very faint a little below Algol,
+and passing through <i>ζ</i> Persei, it enters Taurus, leaving the
+Pleiades on its extreme southern margin; and after having passed through
+<i>ε</i> where it branches off, it rapidly curves towards the main
+stream, which it joins near <i>ζ</i> Tauri, thus forming an immense
+loop. The ramification projecting near <i>ε</i> Tauri involves in its
+nebulosity the ruddy star Aldebaran and the scattered group of the
+Hyades. It then advances towards the three bright stars <i>δ</i>,
+<i>ε</i> and <i>ζ</i> of the belt of Orion, which, together with the
+sextuple star <i>θ</i> Orionis, are involved in its faint nebulosity,
+and joins the main stream on the equinoctial, having thus formed a
+second loop, whose interior part is comparatively free from nebulosity,
+and contains the fine stars Betelgeuse and Bellatrix.
+</p>
+<p>
+That portion of the main galactic stream which is comprised between the
+star Deneb in Cygnus, and Capella in Auriga, is divided longitudinally
+by a very irregular, narrow, darkish cleft, comparatively devoid of
+nebulosity, which, however, is interrupted at some points. This dark gap
+sends short branches north and south, the most important of which are
+situated near <i>ζ</i> Cephei and <i>β</i> Cassiopeiæ. Another branch
+runs from <i>γ</i> beyond <i>ε</i> of the constellation last
+mentioned. The main stream of the Galaxy after leaving Perseus, enters
+Cassiopeia, and sending short branches into Andromeda, it completes its
+immense circle in Cassiopeia's chair, where this description was begun.
+</p>
+<p>
+When examined through the telescope, the appearance of the Milky-way
+completely changes, and its nebulous light is resolved into an immense
+number of stars, too faint to be individually seen with the naked eye.
+When Galileo first directed the telescope to the galactic belt, its
+nebulous, cloud-like masses were at once resolved into stars, even by
+the feeble magnifying power of his instrument. When, much later, Sir
+William Herschel undertook his celebrated star-gaugings of the Galaxy,
+millions of stars blazed out in his powerful telescopes. The stars
+composing this great nebulous belt are so numerous that it is impossible
+to arrive at any definite idea as to their number. From his soundings
+Herschel estimated at 116,000 the number of stars which, on one
+occasion, passed through the field of his telescope in 15 minutes, by
+the simple effect of the diurnal motion of the heavens; and on another
+occasion, a number estimated at 250,000 crossed the field in 41 minutes.
+In a space of 50, comprised between <i>β</i> and <i>γ</i> Cygni, shown
+on Plate XIII., he found no less than 331,000 stars. Prof. Struve has
+estimated at 20,500,000 the number of stars seen in the Milky-way
+through the twenty-foot telescope employed by Herschel in his
+star-gaugings. Great as this number may seem, it is yet far below the
+truth; as the great modern telescopes, according to Professor Newcomb,
+would very probably double the number of stars seen through Herschel's
+largest telescope, and detect from thirty to fifty millions of stars in
+the Milky-way.
+</p>
+<p>
+Although the telescope resolves the Galaxy into millions of stars, yet
+the largest instruments fail to penetrate its immense depths. The
+forty-foot telescope of Herschel, and even the giant telescope of Lord
+Rosse, have failed to resolve the Milky-way entirely into stars, the
+most distant ones appearing in them as nebulosities upon which the
+nearer stars are seen projected, the galactic stratum being unfathomable
+by the largest telescopes yet made.
+</p>
+<p>
+The stars composing the Milky-way are very unevenly distributed, as
+might easily be supposed from the cloud-like appearance of this belt. In
+some regions they are loosely scattered, forming long rows or streams of
+various figures, while in others they congregate into star groups and
+clusters having all imaginable forms, some being compressed into very
+dense globular masses. The intervals left between the clustering masses
+are poorer in stars, and indeed some of them are even totally devoid of
+stars or nebulosity. Such are the great and small "coal-sacks" in the
+southern Galaxy. I have myself detected such a dark space devoid of
+stars and nebulosity in one of the brightest parts of the Milky-way, in
+the constellation Sagittarius, in about 17h. 45m. right ascension, and
+27° 35' south declination. It is a small miniature coal-sack or opening
+in the Galaxy, through which the sight penetrates beyond this great
+assemblage of stars. Close to this, is another narrow opening near a
+small, loose cluster.
+</p>
+<p>
+Although lacking the optical resources which now enable us to recognize
+the structure of the Milky-way, some of the ancient philosophers had
+succeeded tolerably well in their speculations regarding its nature. It
+was the opinion of Democritus, Pythagoras and Manilius, that the Galaxy
+was nothing else but a vast and confused assemblage of stars, whose
+faint light was the true cause of its milky appearance.
+</p>
+<p>
+Before the invention of the telescope, no well-founded theory in regard
+to the structure of the Milky-way could, of course, be attempted.
+Although Kepler entertained different ideas in regard to the structure
+of this great belt from those now generally admitted, yet in them may be
+found the starting point of the modern conception of the structure of
+the Galaxy and of the visible universe. In the view of this great mind,
+the Milky-way, with all its stars, formed a vast system, the centre of
+which, and of the universe, was occupied by our Sun. Kepler reasoned
+that the place of the Sun must be near the centre of the galactic belt,
+from the fact this last object appears very nearly as a great circle of
+the celestial sphere, and that its luminous intensity is about the same
+in all its parts.
+</p>
+<p>
+Half a century later, another attempt to explain the Milky-way was made
+by Wright, of Durham, who rejected the idea of an accidental and
+confused distribution of the stars as inconsistent with the appearance
+of the Galaxy, and regarded them as arranged along a fundamental plane
+corresponding to that of the Milky-way. These ideas which were
+subsequently developed and enlarged by Kant, and then by Lambert,
+constitute what is now known as Kant's theory. According to this theory,
+the stars composing the Galaxy are conceived as being uniformly arranged
+between two flat planes of considerable extension, but which are
+comparatively near together, the Sun occupying a place not very far from
+the centre of this immense starry stratum. As we view this system
+crosswise through its thinnest parts, the stars composing it appear
+scattered and comparatively few in number, but when we view it
+lengthwiser through its most extended parts, they appear condensed and
+extremely numerous, thus giving the impression of a luminous belt
+encircling the heavens. In the conception of Kant, each star was a sun,
+forming the centre of a planetary system. These systems are not
+independent, but are kept together by the bonds of universal
+gravitation. The Galaxy itself is one of these great systems, its
+principal plane being the equivalent of the zodiac in our planetary
+system, while a preponderant body, which might be Sirius, is the
+equivalent of our Sun, and keeps the galactic system together. In the
+universe there are other galaxies, but as they are too distant to be
+resolved into stars, they appear as elliptical nebulæ. Such are, in
+brief, the grand speculations of Kant and Lambert on the Milky-way, and
+the structure of the universe.
+</p>
+<p>
+Kant's theory rested more on conjectures than on observed facts, and
+needed therefore the sanction of direct observations to be established
+on a firm basis. With this view, Sir William Herschel investigated the
+subject, by a long and laborious series of observations. His plan, which
+was that of "star-gauging," consisted in counting all the stars visible
+in his twenty-foot telescope, comprised in a wide belt cutting the
+Galaxy at right angles, and extending from one of its sides to the
+opposite one, thus embracing 180° of the celestial sphere. In this belt
+he executed 3,400 telescopic star-gaugings of a quarter of a degree
+each, from which he obtained 683 mean gaugings giving the stellar
+density of the corresponding regions.
+</p>
+<p>
+The general result derived from this immense labor was that the stars
+are fewest in regions the most distant from the galactic belt; while
+from these regions, which correspond to the pole of the Galaxy, they
+gradually increase in number in approaching the Milky-way. The star
+density was found to be extremely variable, and while some of the
+telescopic gaugings detected either no star at all, or only one or two,
+other gaugings gave 500 stars and even more. The average number of stars
+in a field of view of his telescope, obtained for the six zones, each of
+15°, into which Herschel divided up the portion of his observing belt,
+extending from the Galaxy to its pole, is as follows: In the first zone,
+commencing at 90° from the galactic belt and extending towards it, 4
+stars per telescopic field were found; 5 in the second; 8 in the third;
+14 in the fourth; 24 in the fifth and 53 in the sixth, which terminated
+in the Galaxy itself. Very nearly similar results were afterwards found
+by Sir John Herschel, for corresponding regions in the southern
+hemisphere.
+</p>
+<p>
+From these studies, Herschel concluded that the stellar system is of the
+general form supposed by the Kantian theory, and that its diameter must
+be five times as extended in the direction of the galactic plane, as it
+is in a direction perpendicular to it. To explain the great branch sent
+out by the Galaxy in Cygnus, he supposed a great cleft dividing the
+system edgewise, about half way from its circumference to its centre.
+From suppositions founded on the apparent magnitude and arrangement of
+stars, he estimated that it would take light about 7,000 years to reach
+us from the extremities of the Galaxy, and therefore 14,000 years to
+travel across the system, from one border to the opposite one.
+</p>
+<p>
+But Herschel's theory concerning the Milky-way rested on the erroneous
+assumption that the stars are uniformly distributed in space, and also
+that his telescopes penetrated through the entire depth of the Galaxy.
+Further study showed him that his telescope of twenty feet, and even his
+great forty-foot telescope, which was estimated to penetrate to a
+distance 2,300 times that of stars of the first magnitude, failed to
+resolve some parts of the Galaxy into stars. Meanwhile, the structure of
+the Milky-way being better known, the irregular condensation of its
+stars became apparent, while the mutual relation existing between binary
+and multiple systems of stars, as also between the stars which form
+clusters, was recognized, as showing evidence of closer association
+between certain groups of stars than between the stars in general.
+Herschel's system, which rested on the assumption of the uniform
+distribution of the stars in space, and on the supposition that the
+telescopes used for his gauges penetrated through the greater depths of
+the Galaxy, being thus found to contradict the facts, was gradually
+abandoned by its author, who adopted another method of estimating the
+relative distances of the stars observed in his gaugings.
+</p>
+<p>
+This method, founded on photometric principles, consisted in judging the
+penetrating power of his telescope by the brightness of the stars, and
+not, as formerly, by the number which they brought into view. He then
+studied by this new method the structure of the Milky-way and the
+probable distance of the clustering masses of which it is formed,
+concluding that the portion of the Galaxy traversing the constellation
+Orion is the nearest to us. This last result seems indicated by the fact
+that this portion of the Milky-way is the faintest and the most uniform
+of all the galactic belt.
+</p>
+<p>
+More recently Otto Struve investigated the same subject, and arrived at
+very nearly similar conclusions, which may be briefly stated as follows:
+The galactic system is composed of a countless number of stars,
+spreading out on all sides along a very extended plane. These stars,
+which are very unevenly distributed, show a decided tendency to cluster
+together into individual groups of different sizes and forms, separated
+by comparatively vacant spaces. This layer where the stars congregate in
+such vast numbers may be conceived as a very irregular flat disk,
+sending many branches in various directions, and having a diameter eight
+or ten times its thickness. The size of this starry disk cannot be
+determined, since it is unfathomable in some directions, even when
+examined with the largest telescopes. The Sun, with its attending
+planets, is involved in this immense congregation of suns, of which it
+forms but a small particle, occupying a position at some distance from
+the principal plane of the Galaxy. According to Struve, this distance is
+approximately equal to 208,000 times the radius of the Earth's orbit.
+The Milky-way is mainly composed of star-clusters, two-thirds, perhaps,
+of the whole number visible in the heavens being involved in this great
+belt. In conclusion, our Sun is only one of the individual stars which
+constitute the galactic system, and each of these stars itself is a sun
+similar to our Sun. These individual suns are not independent, but are
+associated in groups varying in number from a few to several thousands,
+the Galaxy itself being nothing but an immense aggregation of such
+clusters, whose whole number of individual suns probably ranges between
+thirty and fifty millions. In this vast system our globe is so
+insignificant that it cannot even be regarded as one of its members.
+According to Dr. Gould, there are reasons to believe that our Sun is a
+member of a small, flattened, bifid cluster, composed of more than 400
+stars, ranging between the first and seventh magnitude, its position in
+this small system being eccentric, but not very far from the galactic
+plane.
+</p>
+<p>
+The study of the Milky-way, of which Plate XIII. is only a part, was
+undertaken to answer a friendly appeal made by Mr. A. Marth, in the
+Monthly Notices of the Royal Astronomical Society, in 1872. I take
+pleasure in offering him my thanks for the suggestion, and for the
+facility afforded me in this study by his "List of Co-ordinates of Stars
+within and near the Milky-way," which was published with it.
+</p>
+
+<p><br /><br /><br /></p>
+
+<h4><a id="THE_STAR_CLUSTERS">THE STAR-CLUSTERS</a>
+<br /><br />
+PLATE XIV</h4>
+
+<p>
+It is a well-known fact that the stars visible to the naked eye are very
+unequally distributed in the heavens, and that while they are loosely
+scattered in some regions, in others they are comparatively numerous,
+sometimes forming groups in which they appear quite close together.
+</p>
+<p>
+In our northern sky are found a few such agglomerations of stars, which
+are familiar objects to all observers of celestial objects. In the
+constellation Coma Berenices, the stars are small, but quite condensed,
+and form a loosely scattered, faint group. In Taurus, the Hyades and the
+Pleiades, visible during our winter nights, are conspicuous and familiar
+objects which cannot fail to be recognized. In the last group, six stars
+may be easily detected by ordinary eyes on any clear night, but more can
+sometimes be seen; on rare occasions, when the sky was especially
+favorable, I have detected eleven clearly and suspected several others.
+The six stars ordinarily visible, are in order of decreasing brightness,
+as follows: Alcyone, Electra, Atlas, Maia, Taygeta and Merope. Glimpses
+of Celano and Pleione are sometimes obtained.
+</p>
+<p>
+When the sky is examined with some attention on any clear, moonless
+night, small, hazy, luminous patches, having a cometary aspect, are
+visible here and there to the naked eye. In the constellation Cancer is
+found one of the most conspicuous, called Præsepe, which forms a small
+triangle with the two stars <i>γ</i> and <i>δ</i>. In Perseus, and
+involved in the Milky-way, is found another luminous cloud, situated in
+the sword-handle, and almost in a line with the two stars <i>γ</i> and
+<i>δ</i> of Cassiopeia's Chair. In the constellation Hercules, another
+nebulous mass of light, but fainter, is also visible between the stars
+<i>η</i> and <i>ζ</i> where it appears as a faint comet, in the depths
+of space. In Ophiuchus and Monoceros are likewise found hazy, luminous
+patches. In the southern sky, several such objects are also visible to
+the naked eye, being found in Sagittarius, in Canis Major and in Puppis;
+but the most conspicuous are those in Centaurus and Toucan. That in
+Centaurus involves the star <i>ω</i> in its pale diffused nebulosity,
+and that in Toucan is involved in the lesser Magellanic cloud.
+</p>
+<p>
+When the telescope is directed to these nebulous objects, their hazy,
+ill-defined aspect disappears, and they are found to consist of
+individual stars of different magnitudes, which being more or less
+closely grouped together, apparently form a system of their own. These
+groups, which are so well adapted to give us an insight into the
+structure and the vastness of the stellar universe, are called
+<i>Star-clusters</i>.
+</p>
+<p>
+Star-clusters are found of all degrees of aggregation, and while in some
+of them, such as in the Pleiades, in Præsepe and in Perseus, the stars
+are so loosely scattered that an opera glass, and even the naked eye,
+will resolve them; in others, such as in those situated in Hercules,
+Aquarius, Toucan and Centaurus, they are so greatly compressed that even
+in the largest telescopes they appear as a confused mass of blazing
+dust, in which comparatively few individual stars can be distinctly
+recognized. Although only about a dozen Star-clusters can be seen in the
+sky with the naked eye, yet nearly eleven hundred such objects visible
+through the telescope, have been catalogued by astronomers.
+</p>
+<p>
+The stars composing the different clusters visible in the heavens vary
+greatly in number, and while in some clusters there are only a few, in
+others they are so numerous and crowded that it would be idle to try to
+count them, their number amounting to several thousands. It has been
+calculated by Herschel that some clusters are so closely condensed, that
+in an area not more than ⅒ part of that covered by the Moon, at least
+5,000 stars are agglomerated.
+</p>
+<p>
+When the group in the Pleiades is seen through the telescope it appears
+more important than it does to the naked eye, and several hundreds of
+stars are found in it. In a study of Tempel's nebula, which is involved
+in the Pleiades, I have mapped out 250 stars, mostly comprised within
+this nebula, with the telescope of 6⅓ inches aperture, which I have
+used for this study.
+</p>
+<p>
+As a type of a loose, coarse cluster, that in Perseus is one of the
+finest of its class. It appears to the naked eye as a single object, but
+in the telescope it has two centres of condensation, around which
+cluster a great number of bright stars, forming various curves and
+festoons of great beauty. Among its components are found several yellow
+and red stars, which give a most beautiful contrast of colors in this
+gorgeous and sparkling region. In a study which I have made of this twin
+cluster, I have mapped out 664 stars belonging to it, among which are
+two yellow and five red stars.
+</p>
+<p>
+While some clusters, like those just described, are very easily
+resolvable into stars with the smallest instruments, others yield with
+the greatest difficulty, even to the largest telescopes, in which their
+starry nature is barely suspected. Owing to this peculiarity,
+star-clusters are usually divided into two principal classes. In the
+first class are comprised all the clusters which have been plainly
+resolved into stars, and in the second all those which, although not
+plainly resolvable with the largest instruments now at our disposal,
+show a decided tendency to resolvability, and convey the impression that
+an increase of power in telescopes is the only thing needed to resolve
+them into stars. Of course this classification, which depends on the
+power of telescopes to decide the nature of these objects, is arbitrary,
+and a classification based on spectrum analysis is now substituted for
+it.
+</p>
+<p>
+The star-clusters are also divided into globular and irregular clusters,
+according to their general form and appearance. The globular clusters,
+which are the most numerous, are usually well-defined objects, more or
+less circular in their general outlines. The rapid increase of
+brightness towards their centres, where the stars composing them are
+greatly condensed, readily conveys the impression that the general form
+of these sparkling masses is globular. The irregular clusters are not so
+rich in stars as the former. Usually their stars are less condensed
+towards the centre, and are, for the most part, so loosely and
+irregularly distributed, that it is impossible to recognize the outlines
+of these clusters or to decide where they terminate. The globular
+clusters are usually quite easily resolvable into stars, either partly
+or wholly, although some among them do not show the least traces of
+resolvability, even in the largest instruments. This may result from
+different causes, and may be attributed either to the minuteness of
+their components or to their great distance from the Earth, many
+star-clusters being at such immense distances that they are beyond our
+means of measurement.
+</p>
+<p>
+As has been shown in the preceding section, the star-clusters are found
+in great number in the Galaxy; indeed, it is in this region and in its
+vicinity that the greater portion of them are found. In other regions,
+with the exception of the Magellanic clouds, where they are found in
+great number and in every stage of resolution, the clusters are few and
+scattered.
+</p>
+
+<p><br /></p>
+
+<div class="figcenter" style="width: 400px;">
+<a id="figure14"></a>
+<br />
+<img src="images/figure14.jpg" width="400" alt="" />
+<div class="caption">
+<p>PLATE XIV.&mdash;STAR-CLUSTER IN HERCULES.</p>
+<p class="smaller">From a study made in June, 1877</p>
+</div></div>
+
+<p><br /></p>
+
+<p>
+The star-cluster in the constellation Hercules, designated as No. 4,230
+in Sir J. Herschel's catalogue, and which is represented on Plate XIV.,
+is one of the brightest and most condensed in the northern hemisphere,
+although it is not so extended as several others, its angular diameter
+being only 7' or 8'. This object, which was discovered by Halley in
+1714, is one of the most beautiful of its class in the heavens.
+According to Herschel, it is composed of thousands of stars between the
+tenth and fifteenth magnitudes. Undoubtedly the stars composing this
+group are very numerous, although those which can be distinctly seen as
+individual stars, and whose position can be determined, are not so many
+as a superficial look at the object would lead us to suppose. From a
+long study of this cluster, which I have made with instruments of
+various apertures, I have not been able to identify more stars than are
+represented on the plate, although the nebulosity of which this object
+mainly consists, and especially the region situated towards its centre,
+appeared at times granular and blazing with countless points of light,
+too faint and too flickering to be individually recognized. Towards its
+centre there is quite an extended region, whose luminous intensity is
+very great, and which irresistibly conveys the impression of the
+globular structure of this cluster. Besides several outlying appendages,
+formed by its nebulosity, the larger stars recognized in this cluster
+are scattered and distributed in such a way that they form various
+branches, corresponding with those formed by the irresolvable
+nebulosity. At least six or seven of these branches and wings are
+recognized, some of which are curved and bent in various ways, thus
+giving this object a distant resemblance to some crustacean forms.
+Although I have looked for it with care, I have failed to recognize the
+spiral structure attributed to this object by several observers. Among
+the six appendages which I have recognized, some are slightly curved;
+but their curves are sometimes in opposite directions, and two branches
+of the upper portion make so short a bend that they resemble a claw
+rather than a spiral wing. The spectrum of this cluster, like that of
+many objects of its class, is continuous, with the red end deficient.
+</p>
+<p>
+A little to the north-east of this object is found the cluster No.
+4,294, which, although smaller and less bright than the preceding, is
+still quite interesting. It appears as a distinctly globular cluster
+without wings, and much condensed towards its centre. The stars
+individually recognized in it, although less bright than those of the
+other cluster, are so very curiously distributed in curved lines that
+they give a peculiar appearance to this condensed region.
+</p>
+<p>
+A little to the north of <i>γ</i> Centauri may be found the great <i>ω</i>
+Centauri cluster, No. 3,531, already referred to above. This magnificent
+object, which appears as a blazing globe 20' in diameter, is, according
+to Herschel, the richest in the sky, and is resolved into a countless
+number of stars from the twelfth to the fifteenth magnitude, which are
+greatly compressed towards the centre. The larger stars are so arranged
+as to form a sort of net-work, with two dark spaces in the middle.
+</p>
+<p>
+The great globular cluster No. 52, involved in the lesser Magellanic
+cloud, in the constellation Toucan, is a beautiful and remarkable
+object. It is composed of three distinct, concentric layers of stars,
+varying in brightness and in degree of condensation in each layer. The
+central mass, which is the largest and most brilliant, is composed of an
+immense number of stars greatly compressed, whose reddish color gives to
+this blazing circle a splendid appearance. Around the sparkling centre
+is a broad circle, composed of less compressed stars, this circle being
+itself involved in another circular layer, where the stars are fainter
+and more scattered and gradually fade away.
+</p>
+<p>
+Many other great globular clusters are found in various parts of the
+heavens, among which may be mentioned the cluster No. 4,678, in
+Aquarius. This object is composed of several thousand stars of the
+fifteenth magnitude, greatly condensed towards the centre, and, as
+remarked by Sir J. Herschel, since the brightness of this cluster does
+not exceed that of a star of the sixth magnitude, it follows that in
+this case several thousand stars of the fifteenth magnitude equal only a
+star of the sixth magnitude. In the constellation Serpens the globular
+clusters No. 4,083 and No. 4,118 are both conspicuous objects, also No.
+4,687 in Capricornus. In Scutum Sobieskii the cluster No. 4,437 is one
+of the most remarkable of this region. The stars composing it, which are
+quite large and easily made out separately, form various figures, in
+which the square predominates.
+</p>
+<p>
+Among the loose irregular clusters, some are very remarkable for the
+curious arrangement of their stars. In the constellation Gemini the
+cluster No. 1,360, which is visible to the naked eye, is a magnificent
+object seen through the telescope, in which its sparkling stars form
+curves and festoons of great elegance. The cluster No. 1,467, of the
+same constellation, is remarkable for its triangular form. In the
+constellation Ara the cluster No. 4,233, composed of loosely scattered
+stars, forming various lines and curves, is enclosed on three sides by
+nearly straight single lines of stars. In Scorpio the cluster No. 4,224
+is still more curious, being composed of a continuous ring of loosely
+scattered stars, inside of which is a round, loose cluster, which is
+divided into four parts by a dark cross-shaped gap, in which no stars
+are visible.
+</p>
+<p>
+Among the 1,034 objects which are now classified as clusters more or
+less resolvable, 565 have been absolutely resolved into stars, and 469
+have been only partly resolved, but are considered as belonging to this
+class of objects. In Sir J. Herschel's catalogue there are 102 clusters
+which are Considered as being globular; among them 30 have been
+positively resolved into stars.
+</p>
+<p>
+The agglomeration of thousands of stars into a globular cluster cannot
+be conceived, of course, to be simply the result of chance. This
+globular form seems clearly to indicate the existence of some bond of
+union, some general attractive force acting between the different
+members of these systems, which keeps them together, and condenses them
+towards the centre. Herschel regards the loose, irregular clusters as
+systems in a less advanced stage of condensation, but gradually
+concentrating by their mutual attraction into the globular form.
+Although the stars of some globular clusters appear very close together,
+they are not necessarily so, and may be separated by great intervals of
+space. It has been shown that the clusters are agglomeration of suns,
+and that our Sun itself is a member of a cluster composed of several
+hundreds of suns, although, from our point of observation, these do not
+seem very close together. So far as known, the nearest star to us is a
+Centauri, but its distance from the Earth equals 221,000 times the
+distance of the Sun from our globe, a distance which cannot be traversed
+by light in less than three years and five months. It seems very
+probable that if the suns composing the globular clusters appear so near
+together, it is because, in the first place, they are at immense
+distances from us, and in the second, because they appear nearly in a
+line with other suns, which are at a still greater distance from us, and
+on which they accordingly are nearly projected. If one should imagine
+himself placed at the centre of the cluster in Hercules, for instance,
+the stars, which from our Earth seems to be so closely grouped, would
+then quite likely appear very loosely scattered around him in the sky,
+and would resemble the fixed stars as seen from our terrestrial station.
+</p>
+<p>
+Judging by their loose and irregular distribution, the easily resolvable
+clusters would appear, in general, to be the nearer to us. It is
+probable that the globular clusters do not possess, to a very great
+degree, the regular form which they ordinarily present to us. It seems
+rather more natural to infer that they are irregular, and composed of
+many wings and branches, such as are observed in the cluster in
+Hercules; but as these appendages would necessarily be much poorer in
+stars than the central portions, they would be likely to become
+invisible at a great distance, and therefore the object would appear
+more or less globular; the globular form being simply given by the close
+grouping of the stars in the central portion. It would seem, then, that
+in general, the most loosely scattered and irregular clusters are the
+nearest to us, while the smallest globular clusters and those resolvable
+with most difficulty are the most distant.
+</p>
+<p>
+In accordance with the theory that the clusters are composed of stars,
+the spectrum of these objects is in general continuous; although, in
+many cases, the red end of the spectrum is either very faint or
+altogether wanting. Many objects presenting in a very high degree the
+principal characteristics exhibited by the true star-clusters, namely, a
+circular or oval mass, whose luminous intensity is greatly condensed
+toward the centre, have not yielded, however, to the resolving power of
+the largest telescopes, although their continuous spectrum is in close
+agreement with their general resemblance to the star-clusters. Although
+such objects may remain irresolvable forever, yet it is highly probable
+that they do not materially differ from the resolvable and partly
+resolvable clusters, except by their enormous distance from us, which
+probably reaches the extreme boundary of our visible universe.
+</p>
+
+<p><br /><br /><br /></p>
+
+<h4><a id="THE_NEBULAE">THE NEBULÆ</a>
+<br /><br />
+PLATE XV</h4>
+
+<p>
+Besides the foggy, luminous patches which have just been described, a
+few hazy spots of a different kind are also visible to the naked eye on
+any clear, moonless night. These objects mainly differ from the former
+in this particular, that when viewed through the largest telescopes in
+existence they are not resolved into stars, but still retain the same
+cloudy appearance which they present to the unassisted eye. On account
+of the misty and vaporous appearance which they exhibit, these objects
+have been called <i>Nebulæ</i>.
+</p>
+<p>
+Of the 26 nebulous objects visible to the naked eye in the whole
+heavens, 19 belong to the class of star-clusters, and 7 to the class of
+nebulæ. Among the most conspicuous nebulæ visible to the unassisted
+eye, are those in the constellations Argo Navis, Andromeda and Orion.
+</p>
+<p>
+Besides the seven nebulæ visible to the naked eye, a great number of
+similar objects are visible through the telescope. In Sir John
+Herschel's catalogue of nebulæ and clusters, are found 4,053
+irresolvable nebulæ, and with every increase of the aperture of
+telescopes, new nebulæ, invisible in smaller instruments, are found.
+Notwithstanding their irresolvability it is probable, however, that many
+among them have a stellar structure, which their immense distance
+prevents us from recognizing, and are not therefore true nebulæ. The
+giant telescope of Lord Rosse has shown nebulæ so remote that it has
+been estimated that it takes their light 30 million years to reach the
+Earth.
+</p>
+<p>
+The nebulæ are very far from being uniformly distributed in space. In
+some regions they are rare, while in others they are numerous and
+crowded together, forming many small, irregular groups, differing in
+size and in richness of aggregation. The grouping of the nebulæ does
+not occur at random in any part of the heavens, as might naturally be
+supposed, but, on the contrary, it is chiefly confined to certain
+regions. Outside of these regions nebulæ are rare and are separated
+from each other by immense intervals; so that these isolated objects
+appear as if they were lost wanderers from the great nebulous systems.
+</p>
+<p>
+The regions where the nebulæ congregate in great number are very
+extensive, and in a general view there are two vast systems of nebular
+agglomeration, occupying almost opposite points of the heavens, whose
+centres are not very distant from the poles of the Milky-way. In the
+northern hemisphere, the nebulous system is much richer and more
+condensed than in the southern hemisphere. The northern nebulæ are
+principally contained in the constellations Ursa Minor and Major, in
+Draco, Canes Venatici, Bootes, Leo Major and Minor, Coma Berenices, and
+Virgo. In this region, which occupies about ⅛ of the whole surface of
+the heavens, ⅓ of the known nebulæ are assembled. The southern
+nebulæ are more evenly distributed and less numerous, with the
+exception of two comparatively small, but very remarkable centres of
+condensation which, together with many star-clusters, constitute the
+Magellanic clouds.
+</p>
+<p>
+These two vast nebular groups are by no means regular in outline, and
+send various branches toward each other. They are separated by a wide
+and very irregular belt, comparatively free from nebulæ, which
+encircles the celestial sphere, and whose medial line approximately
+coincides with that of the galactic belt. The Milky-way, so rich in
+star-clusters, is very barren in nebulæ; but it is a very remarkable
+fact, nevertheless, that almost all the brightest, largest, and most
+complicated nebulæ of the heavens are situated either within it, or in
+its immediate vicinity. Such are the great nebulæ in Orion and
+Andromeda; the nebula of <i>ζ</i> Orionis; the Ring nebula in Lyra; the
+bifurcate nebula in Cygnus; the Dumb-bell nebula in Vulpecula; the Fan,
+Horse-shoe, Trifid and Winged nebulæ in Sagittarius; the great nebula
+around <i>η</i> Argus Navis, and the Crab nebula in Taurus.
+</p>
+<p>
+Aside from the discovery of some of the largest nebulæ by different
+observers, and their subsequent arrangement in catalogues by Lacaille
+and Messier, very little had been done towards the study of these
+objects before 1779, when Sir W. Herschel began to observe them with the
+earnestness of purpose which was one of the distinctive points of the
+character of this great man. He successively published three catalogues
+in 1786, 1789, and 1802, in which the position of 2,500 nebulous objects
+was given. This number was more than doubled before 1864, when Sir John
+Herschel published his catalogue of 5,079 nebulæ and star-clusters. To
+this long list must be added several hundred similar objects, since
+discovered by D'Arrest, Stephan, Gould and others. But, as has been
+shown above, among the so-called nebulæ are many star-clusters which do
+not properly belong to the same class of objects, it being sometimes
+impossible in the present state of our knowledge to know whether a
+nebulous object belongs to one class or to the other.
+</p>
+<p>
+The nebulæ exhibit a great variety of forms and appearances, and, in
+accordance with their most typical characters, they are usually divided
+into several classes, which are: the Nebulous stars, the Circular, or
+Planetary, the Elliptical, the Annular, the Spiral and Irregular
+nebulæ.
+</p>
+
+<p><br /></p>
+
+<div class="figcenter" style="width: 400px;">
+<a id="figure15"></a>
+<br />
+<img src="images/figure15.jpg" width="400" alt="" />
+<div class="caption">
+<p>PLATE XV.&mdash;THE GREAT NEBULA IN ORION.</p>
+<p class="smaller">From a study made in the years 1875-76</p>
+</div></div>
+
+<p><br /></p>
+
+<p>
+The so-called nebulous stars consist of a faint nebulosity, usually
+circular, surrounding a bright and sharp star, which generally occupies
+its centre. The nebulosity surrounding these stars varies in brightness
+as well as in extent, and while, in general, its light gradually fades
+away, it sometimes terminates quite suddenly. Such nebulosities are
+usually brighter and more condensed towards the central star. The stars
+thus surrounded do not seem, however, to be distinguished from others by
+any additional peculiarity. Some nebulæ of this kind are round, with
+one star in the centre; others are oval and have two stars, one at each of
+their foci. The nebulous star, <i>τ</i> Orionis, represented at the upper
+part of Plate XV., above the great nebula, has a bright star at its
+centre and two smaller ones on the side. The association of double stars
+with nebulæ is very remarkable, and may in some cases indicate a mutual
+relation between them.
+</p>
+<p>
+The so-called planetary nebulæ derive their name from their likeness to
+the planets, which they resemble in a more or less equable distribution
+of light and in their round or slightly oval form. While some of them
+have edges comparatively sharp and well defined, the outlines of others
+are more hazy and diffused. These nebulæ, which are frequently of a
+bluish tint, are comparatively rare objects, and most of those known
+belong to the southern hemisphere. When seen through large telescopes,
+however, they present a different aspect, and their apparent uniformity
+changes. The largest of these objects, No. 2,343 of the General
+Catalogue, is situated in the Great Bear, close to the star <i>β</i>. Its
+apparent diameter is, according to Sir J. Herschel, 2', 40", and "its
+light is equable, except at the edge, where it is a little hazy." In a
+study which I made of this object in 1876, with a refractor 6⅓ inches
+in aperture, I found it decidedly brighter on the preceding side, where
+the brightest part is crescent shaped. In Lord Rosse's telescope its
+disk is transformed into a luminous ring with a fringed border, and two
+small star-like condensations are found within. Another planetary
+nebula, near <i>χ</i> Andromedæ, has also shown an annular structure in
+Rosse's telescope.
+</p>
+<p>
+The elliptical nebulæ, as their name implies, are elongated, elliptical
+objects; but while some of them are only slightly elongated ovals,
+others form ellipses whose eccentricity is so great that they appear
+almost linear. In all these objects the light is more or less condensed
+towards the centre; but while in some of them the condensation is
+gradual and slight, in others it is so great and sudden that the centre
+of the nebula appears as a large diffused star, somewhat resembling the
+nucleus of a comet. From the general appearance of these objects, it is
+not unlikely that some of them are either flattish, nebulous disks, like
+the planetary nebulæ, or nebulous rings, seen more or less sidewise.
+The condensation of light at their centres does not appear to be
+stellar, but nebulous like the rest, and it is a remarkable fact that
+very few, if any, of these objects are resolvable into stars.
+</p>
+<p>
+Several elliptical nebulæ are remarkable for having a star at or near
+each of their foci, or at each of their extremities. Such are the
+elliptical nebulæ in Draco, Centaurus, and Sagittarius, Nos. 4,419,
+3,706 and 4,395 of the General Catalogue, the last of which is in the
+vicinity of the triple star <i>μ</i> Sagitarii. Each of these nebulæ has a
+star at each of its foci, while No. 1, in Cetus, has a star at each of
+its extremities.
+</p>
+<p>
+Among the most remarkable elliptic nebulæ may be mentioned Nos. 1,861
+and 2,373 of Sir J. Herschel's catalogue, both situated in the
+constellation Leo. The first is one of Lord Rosse's spiral nebulæ, and
+the last, which is a very elongated object, is formed of concentric oval
+rings, which are especially visible towards its central part. The
+constellation Draco is particularly remarkable for the number of
+elliptical nebulæ found within its boundaries. Among them are Nos.
+3,939, 4,058, 4,064, 4,087, 4,415, etc., which are quite remarkable
+objects of their class. No. 4,058, of which I have made a study, is
+bright, and has a decided lenticular form with a condensation in the
+centre. Its following edge is better defined than the preceding. In Lord
+Rosse's telescope this object exhibits a narrow, dark, longitudinal, gap
+in its interior.
+</p>
+<p>
+By far the largest and the finest object of this class is the great
+nebula in Andromeda. Although this object belongs rather to the class of
+irregular nebulæ, yet it is generally considered as an elliptic nebula,
+since its complicated structure, being less prominent, was not
+recognized until 1848, when it was perceived by George P. Bond, Director
+of the Harvard College Observatory. This, the first nebula discovered,
+was found in 1612 by Simon Marius. It is situated in the constellation
+Andromeda, in the vicinity of the star <i>ν</i>, and almost in a line with
+the stars <i>μ</i> and <i>β</i> of the same constellation. It is visible to
+the naked eye, and appears as a faint comet-like object. It is represented
+at the upper left hand corner of Plate XIII., on the border of the
+Milky-way, as it appears to the naked eye.
+</p>
+<p>
+The nebula in Andromeda is one of the brightest in the heavens, and is
+closely attended by two smaller nebulæ. Perhaps it would be rather more
+correct to say that it has three centres of condensation, as the two
+small nebulæ referred to are entirely involved in the same faint and
+extensive nebulosity. Its general form is that of an irregular oval,
+upwards of one degree in breadth and two and a half degrees in length.
+Its brightest and most prominent part, which alone was seen by the
+earlier observers, consists in a very elongated lenticular mass, which
+gradually condenses towards its centre into a blazing, star-like
+nucleus, surrounded by a brilliant nebulous mass. At a little distance
+to the south of this central condensation is found one of the lesser
+centres of condensation noted above, which is globular in appearance,
+with a bright, star-like nucleus like the former. The other centre of
+condensation is found to the north-west of the centre of the principal
+mass, and is quite elongated, with a centre of condensation towards its
+southern extremity, but it is not so bright as the others. Close to the
+western edge of the bright lenticular mass first described, and making a
+very slight angle with its longer axis, are found two narrow and nearly
+rectilinear dark rifts, running almost parallel to each other, and both
+terminating in a slender point in the south. These dark rifts, which are
+almost totally devoid of nebulous matter, are quite rare in nebulæ, and
+afford a good opportunity to watch the changes which this part of the
+nebulæ may undergo.
+</p>
+<p>
+This nebula has never been positively resolved into stars, although
+Prof. Geo. Bond and others have strongly suspected its resolvability. In
+a study which I have made of it, with the same instrument employed by
+Bond, and also with the great Washington telescope, I detected a decided
+mottled appearance in several places, which might be attributable to a
+beginning of resolvability; but I do not consider this a conclusive
+indication that the nebula is resolvable. The continuous spectrum given
+by this nebula, showing that it is not in the gaseous state which its
+appearance seems to indicate, warrants the conclusion, however, that it
+will ultimately be found to be resolvable. This object, being situated
+on the edge of the Galaxy and involved in its diffused light, has a
+great number of small stars belonging to this belt projected upon it.
+During my observations I have mapped out 1,323 of these stars, none of
+which seems to be in physical connection with the nebula.
+</p>
+<p>
+Among the circular and elliptical nebulæ a few exhibit a very
+remarkable structure, being apparently perforated, and forming either
+round, slightly oval, or elongated rings of great beauty. These Annular
+nebulæ are among the rarest objects in the heavens. In Scorpio, two
+such nebulæ are found involved in the Milky-way, and also one in
+Cygnus. One of those in Scorpio has two stars involved within its ring,
+at the extremities of its smallest interior diameter. A very elongated
+nebula in the vicinity of the fine triple star <i>γ</i> Andromedæ is also
+annular, and has two stars symmetrically placed at the extremities of
+its greatest interior axis. Another elongated annular nebula is also
+found north of <i>η</i> Pegasi.
+</p>
+<p>
+The grandest and most remarkable of the annular nebulæ is found in the
+constellation Lyra, about midway between the two stars <i>β</i> and
+<i>γ</i>. It is slightly elliptical in form, and according to Prof. E.
+S. Holden, its major axis is 77".3 and the minor 58". From a study and
+several drawings which I have made of this object, with instruments of
+various apertures, I have found it decidedly brighter towards its outer
+border, at the extremities of its minor axis, than at the ends of the
+major axis. On very favorable occasions, some of its brightest parts
+have appeared decidedly, but very faintly mottled, and I have recognized
+three small centres of condensation. Its interior, in which Professor
+Holden has detected a very faint star, is quite strongly nebulous. In
+Lord Rosse's telescope, this nebula is completely surrounded by wisps
+and appendages of all sorts of forms, which I have failed to trace,
+however, both with the refractor of the Harvard College Observatory and
+with that of the Naval Observatory at Washington; Rosse, Secchi and
+Chacornac, have seen this nebula glittering as if it were a "heap of
+star dust," although its spectrum indicates that it is gaseous.
+</p>
+<p>
+The nebula No. 1,541, in Camelopardus, of which I have also made a study
+and a drawing, is closely allied to the class of annular nebulæ. This
+object, which is quite bright, has a remarkable appearance. It consists
+principally of somewhat more than half of an oval ring, surrounding a
+bright, nebulous mass which condenses around a star; this mass being
+separated from the imperfect ring by a dark interval. Upon the bright
+portion of the ring, and on opposite points, are found two bright stars,
+between which lies the star occupying the central mass. The central mass
+extends at some distance outside of the ring on its open side. Several
+stars are involved in this object.
+</p>
+<p>
+The Spiral nebulæ are very curious and complicated objects, but they
+are visible only in the largest telescopes. Prominent above all is the
+double spiral nebula No. 3,572, in Canes Venatici, which is not far from
+<i>η</i> Ursa Majoris. In Lord Rosse's telescope, this object presents a
+wonderful spiral disposition, looking somewhat like one of the
+fire-works called pin-wheels, and forming long, curved wisps, diverging
+from two bright centres. The spectrum of this object, however, is not
+that of a gas. In the constellation Virgo, Rosse has detected another
+such nebula. In Cepheus, Triangulum, and Ursa Major, are found other
+spiral nebulæ of smaller size. Lord Rosse has recognized 40 spiral
+nebulæ and suspected a similar structure in 30 others.
+</p>
+<p>
+The class of the Irregular nebulæ, which will be now considered,
+differs greatly in character from the others, and includes the largest,
+the brightest and the most extraordinary nebulæ in the heavens. The
+nebulæ of this class differ from those belonging to the other classes
+by a want of symmetry in their form and in the distribution of their
+light, as well as by their capricious shapes, and their very complicated
+structure. Another and perhaps the principal difference between them and
+the objects above described, consists in the remarkable fact already
+stated, that they are not, except in rare cases, to be found in the
+regions where the other nebulæ abound. On the contrary, they are found
+in or very near the Milky-way, precisely where the other nebulæ are the
+most rare. This fact, recognized by Sir J. Herschel, led him to consider
+them as "outlying, very distant, and as it were detached fragments of
+the great stratum of the Galaxy." It seems very probable that the reason
+why these objects differ so greatly from the other nebulæ in size,
+brightness and complication of structure, is simply because they are
+much nearer to us than are most of the others. They are perhaps nebulous
+members of our Galaxy. The same remark which has been made of
+star-clusters may be applied to nebulæ. The nearer they are to us, the
+larger, the brighter and the more complicated they will appear, while
+the farther they are removed, the more simple and regular and round they
+will appear, only their brightest and deepest parts being then visible.
+</p>
+<p>
+The Crab Nebula of Lord Rosse, near <i>ζ</i> Tauri, No. 1,157, is one of
+the interesting objects of this class. It has curious appendages streaming
+off from an oval, luminous mass, which give it a distant resemblance to
+the animal from which it derives its name. The Bifid nebula in Cygnus,
+Nos. 4,400 and 4,616, is another object of this class. It consists of a
+long, narrow, crooked streak, forking out at several places, and passing
+through <i>χ</i> Cygni. Observers, having failed to recognize the
+connection existing between its different centres of brightness, have made
+distinct nebulæ of this extended object.
+</p>
+<p>
+The Dumb-bell nebula in Vulpecula, No. 4,532, is a bright and curious
+object, with a general resemblance to the instrument from which it
+derives its name. Lord Rosse's telescope has shown many stars in it,
+projected on a nebulous background, and Prof. Bond seems to have thought
+that it showed traces of resolvability, although in the study which I
+made of this nebula with the same instrument used by the latter
+observer, I failed to perceive any such traces. Dr. Huggins finds its
+spectrum gaseous.
+</p>
+<p>
+The star-cluster, No. 4,400, in Scutum Sobieskii, which is described by
+Sir J. Herschel as a loose cluster of at least 100 stars, I have found
+to be involved in an extensive, although not very bright, nebula, which
+would seem to have escaped his scrutiny. In a study and drawing of this
+nebula made in 1876, its general form is that of an open fan, with the
+exception that the handle is wanting, with deeply indented branches on
+the preceding side, where the brightest stars of the cluster are
+grouped. From its peculiar form, this object might appropriately be
+called the Fan nebula.
+</p>
+<p>
+The Omega or Horse-shoe nebula, in Sagittarius, No. 4,403, of which I
+have made a study and two drawings, one with a refractor 6⅓ inches in
+aperture, and the other conjointly with Prof. Holden, with the great
+telescope of the Naval Observatory, is a bright and very complicated
+object. Its general appearance in small instruments, with low power, is
+that of a long, narrow pisciform mass of light, from which proceeds on
+the preceding side, the great double loop from which it derives its
+name. But in the great Washington refractor its structure becomes very
+complicated, forming various bright nebulous masses and wisps of great
+extension. Prof. Holden, who has made a careful, comparative study of
+the published drawings of this object, thinks there are reasons to
+believe that its western branch has moved relatively to the stars found
+within its loop. The spectrum of this nebula is gaseous.
+</p>
+<p>
+The Trifid nebula, No. 4,355, in the same constellation, is also a very
+remarkable object, although it is not so bright as the last. This
+nebula, which I have studied with the refractor of the Cambridge
+Observatory, consists of four principal masses of light, separated by a
+wide and irregular gap branching out in several places. These masses,
+which are brighter along the dark gap, gradually fade away externally. A
+group of stars, two of which are quite bright, is found near the centre
+of the nebula, on the inner edge of the following mass, and close to the
+principal branch of the dark channel. A little to the north, and
+apparently forming a part of this nebula, is a globular-looking nebula,
+having a pale yellow star at its centre. Prof. Holden's studies on this
+nebula show that the triple star, which was centrally situated in the
+dark gap from 1784 to 1833, was found involved in the border of the
+nebulous mass following it, from 1839 to 1877; the change, he thinks,
+is attributable either to the proper motion of the group of stars or to
+that of the nebula itself.
+</p>
+<p>
+In the same vicinity is found the splendid and very extensive nebula No.
+4,361, in which is involved a loose, but very brilliant star-cluster.
+This nebula and cluster, which I have studied and drawn with a 6⅓ inch
+telescope, is very complicated in structure, and divided by a dark
+irregular gap into three principal masses of light, condensing at one
+point around a star, and at others forming long, bright, gently-curved
+branches, which give to this object a strong resemblance to the wings of
+a bird when extended upwards in the action of flying. From this
+peculiarity this object might appropriately be called the Winged nebula.
+Its spectrum is that of a gas.
+</p>
+<p>
+The variable star <i>η</i> Argus is completely surrounded by the great
+nebula of the same name, No. 2,197, first delineated by Sir J. Herschel,
+during his residence at the Cape of Good Hope, in 1838. This object,
+which covers more than ⁴⁄₇ of a square degree, is divided into
+three unequal masses, separated by dark oval spots, comparatively free
+from nebulosity, and is suspected to have undergone changes since
+Herschel's time.
+</p>
+<p>
+In the same field with the double star, <i>ζ</i> Orionis, the most easterly
+of the three bright stars in the belt of Orion, is found another
+irregular nebula of the Trifid type. From the drawings which I have made
+of this object, it appears to be composed of three principal unequal
+masses, separated by a wide, irregular, dark channel, two of the masses
+being quite complicated in structure, and forming curved, nebulous
+streams of considerable length and breadth. This nebula, like the next
+to be described, seems to be connected with the Galaxy by the great
+galactic loop described in another section.
+</p>
+<p>
+By far the most conspicuous irregular nebula visible from our northern
+States, is the great nebula in Orion, No. 1,179, represented on Plate
+XV. This object, visible to the naked eye, is the brightest and the most
+wonderful nebula in the heavens. It is situated a little to the south of
+the three bright stars in the belt of Orion, and may be readily detected
+surrounding the star <i>θ</i>, situated between and in a line with two
+faint stars, the three being in a straight line which points directly
+towards <i>ε</i>, the middle star of the three in Orion's belt. The
+area occupied by this nebula is about equal to that occupied by the
+Moon.
+</p>
+<p>
+In its brightest parts the nebula in Orion appears as a luminous cloud
+of a pentagonal form, from which issue many luminous appendages of
+various shapes and lengths. This principal mass is divided into
+secondary masses, separated by darkish, irregular intervals. These
+secondary masses in their turn appear mottled and fleecy. Towards the
+lower part of the pentagonal mass is found a roundish dark space,
+comparatively devoid of nebulosity, in which are involved four bright
+stars forming a trapezium, and several fainter ones. The four bright
+stars of the trapezium constitute the quadruple star <i>θ</i> Orionis,
+from which the nebula has received its name. The cloud-like pentagonal
+form is brightest on the north-west of the trapezium, and is surrounded
+on three sides by long, soft, curved wisps, fading insensibly into the
+outer nebulous mass in which they are involved. On the east a broad,
+wavy wing spreads out, and sends an important branch southward.
+South-east of the trapezium are found several curious dark spaces,
+comparatively devoid of nebulosity, especially those on the east, which
+give to this nebula a singular character. Close to the north-eastern
+part of the nebula, or rather in contact with it, is found a small,
+curiously-shaped nebula, condensing around a bright star into a blazing
+nucleus. From this centre it continues northward in a narrow diffused
+stream, which spreads out in passing over the stars <i>c<sup>1</sup></i>
+and <i>c<sup>2</sup></i>; and after having sent short branches
+northward, it curves back to the south and joins the main nebula on the
+west of its starting point, having thus formed a great loop which is not
+shown on the Plate. The nebula also forms a loop towards the south,
+which is partly shown on Plate XV., a small branch of which, passing
+through <i>τ</i> Orionis, the nebulous star shown at the top of the
+Plate, and extending southward, is not here represented.
+</p>
+<p>
+On ordinary nights the nebula in Orion is a splendid object, and
+inspires the observer with amazement; but this is as nothing compared
+with the grand and magnificent sight which it presents during the very
+rare moments when our atmosphere is perfectly clear and steady. I have
+seen this nebula but once under these favorable circumstances, and I was
+surprised by the grandeur of the scene. Then could be detected features
+to be seen at no other time, and its fleecy, floculent, cloud-like
+masses glittered with such intensity that it seemed as if thousands of
+stars were going to blaze out the next moment. Although I observed the
+nebula under such favorable conditions, and with the fifteen-inch
+refractor of the Cambridge Observatory, yet I was disappointed in my
+expectations, and distinguished no new stars or points of light, and
+nothing more than a very bright mass, finely divided into minute blazing
+cloudlets. Although I failed to resolve this nebula into stars, yet Lord
+Rosse, Bond and Secchi thought they had caught glimpses of star dust.
+Its spectrum, however, proves to be mainly that of incandescent gases,
+probably hydrogen and nitrogen. In the curved wisps found in this
+nebula, Lord Rosse and others saw indications of a spiral structure.
+</p>
+<p>
+Several bright stars are found scattered over this nebula, and besides
+those forming the trapezium, there are three in a row, a little to the
+south-east of that group, which are quite bright and remarkable. Among
+the stars involved in this nebula, few show signs of having a physical
+connection with it, although it seems probable that the group of the
+trapezium is so connected. Some of these stars are variable. The small
+stars represented on this Plate, as on others of the series, are
+somewhat exaggerated in size, as was unavoidable with any process of
+reproduction which could be adopted.
+</p>
+<p>
+In 1811, W. Herschel was led to suspect that some changes had occurred
+in this nebula, but changes in such complicated and delicate objects are
+not easily ascertained, since, for the most part, we have for comparison
+with our later observations only coarse drawings made by hands unskilled
+in delineation.
+</p>
+<p>
+Although comparatively rare, double and multiple nebulæ may be found in
+the sky. When this occurs, their constituents most commonly belong to
+the class of spherical nebulæ. Sometimes the components are separated
+and distinct, at other times one of them is projected upon the other,
+either really or by the effect of perspective. Sometimes one is round
+and the other elongated. It is probable that while some of these nebulæ
+are physically associated and form a system, others appear to be so only
+because they happen to be almost in a line with the observer. A double
+nebula in Draco, Nos. 4,127 and 4,128, which I have drawn, is a fair
+type of those which are separated. The first is a globular nebula, and
+the last an oval one, with a star at its centre. The double nebula, Nos.
+858 and 859, in Taurus, which I have also studied, is a type of the
+cases in which one nebula is partly projected on another. In this
+instance both the nebulæ are globular.
+</p>
+<p>
+The nebulæ in general show very little color in their light, which is
+ordinarily whitish and pale. Some, however, present a decided bluish or
+greenish tint. The great nebula in Orion has a greenish cast, and we
+have seen that some planetary nebulæ are bluish.
+</p>
+<p>
+It has been a question whether nebulæ are changing. It has already been
+stated that Prof. Holden believes there is ground to suspect that the
+Trifid and Horse-shoe nebulæ have undergone some changes. A nebula near
+<i>ε</i> Tauri has been lost and found again several times. Two other
+nebulæ in the same constellation have presented curious variations.
+One, near a star of the tenth magnitude, exhibited variations of
+brightness like those of the star itself, and for a time disappeared.
+The other, near <i>ζ</i> Tauri, increased in brightness for three months,
+after which it disappeared. In 1859 Tempel discovered a nebula in the
+Pleiades, which has shown some fluctuations. In 1875 I made a long study
+of this object, and drew it carefully a dozen times, but I was not able
+to see any changes in it within the two or three months during which my
+observations were continued. But on Nov. 24, 1876, it was found of a
+different color, being purplish and very faint. On Dec. 23, 1880, it was
+found just as bright and visible as when I drew it in 1875, and on Oct.
+20, 1881, it appeared faint and purplish again, as in 1876. On this last
+night, and on those which followed it, it was impossible for me to trace
+the nebulosity as far as in 1875. I consider this as due to a variation
+in the light of this object, which in 1875 was bright enough to be well
+seen while the Moon after her First Quarter was within ten or fifteen
+degrees from the Pleiades.
+</p>
+<p>
+From the observations of M. Laugier, it appears that some nebulæ have a
+proper motion, comparable to that of stars. From the displacement of the
+lines of their spectra by their motion in the line of sight, Dr. Huggins
+found that no nebula observed by him has a proper motion surpassing 25
+miles per second. The Ring nebula in Lyra appears to move from us at the
+rate of 3 miles per second, and that in Orion recedes about 17 miles per
+second.
+</p>
+<p>
+The important question arises, are all the irresolvable nebulæ in the
+heavens to be considered as so many star-clusters, differing only from
+them by the minuteness of their components, or their immense distance
+from us; or are they cosmical clouds, composed of luminous vapors,
+similar to the matter composing the heads and tails of comets?
+Originally, W. Herschel, with many astronomers, thought that all these
+objects were stellar aggregations, too distant to be resolved into
+stars; but he subsequently modified his opinion, and accepted the idea
+that some of them are of a gaseous nature.
+</p>
+<p>
+No direct proof that the nebulæ are gaseous could be obtained, however,
+before the spectroscope was known. The attempt to analyze the light of
+the nebulæ with this instrument was made in 1864, by Dr. Huggins, who
+directed his spectroscope to the planetary nebula, No. 4,373, in Draco.
+Its spectrum was found to consist of three bright, distinct lines, the
+brightest of which corresponded with the strongest nitrogen line, and
+the feeblest with the hydrogen C line. Besides these lines, it gave also
+a very faint, continuous spectrum, apparently due to a central point of
+condensation. By this observation, the gaseous nature of a nebula was
+for the first time demonstrated. Dr. Huggins thus analyzed 70 nebulæ,
+of which one-third gave a gaseous spectrum, consisting of several bright
+lines, the brightest of which invariably corresponded with the lines of
+nitrogen. The others gave a continuous spectrum, with the red end
+usually deficient. These results indicate that if some of the so-called
+nebulæ are due to an aggregation of stars, either too minute or too
+remote in space to be individually resolved, others are in a gaseous
+state. Yet the faint, continuous spectrum, given by some nebulæ, in
+addition to their gaseous spectra, seems to show that these nebulæ have
+some stars or matter in a different state, either involved in them or
+projected on their surface.
+</p>
+<p>
+The idea of diffused matter distributed here and there in space, and
+gradually condensing into stars, is by no means new. As early as 1572,
+Tycho Brahé proposed such an hypothesis, to explain the sudden
+apparition of a new star in Cassiopeia, which he considered as formed by
+the recent agglomeration of the "celestial matter" diffused in space.
+Kepler adopted the same idea to explain the new star which appeared in
+Ophiuchus, in 1604. Halley, Lacaille, Mairan and others, entertained the
+same opinion. The hypothesis of a self-luminous, nebulous matter
+diffused in space, and forming here and there immense masses, has been
+proposed from the origin of the telescope, and was adopted by Sir
+William Herschel, who in his grand speculations on the universe
+considered the nebulæ as immense masses of phosphorescent vapors,
+gradually condensing around one or several centres into stars or
+clusters of stars. The evidence afforded by the spectroscope seems to be
+in favor of such an hypothesis, and shows us that gaseous agglomerations
+exist in space.
+</p>
+<p>
+According to our modern conception, the visible universe is but an
+infinitely small portion of the infinite universe perceived by our mind.
+The great blazing centre around which our little, non-luminous globe
+pursues its endless journey, is only an humble member of a cluster
+comprising four hundred equally powerful suns, as they are believed to
+be, although they appear to us as little twinkling stars. The nearest of
+these stars is 221,000 times as far from the Sun as the Sun is from the
+Earth, and yet this entire cluster is only one among the several hundred
+Star-clusters composing the great galactic nebula in which we are
+involved, comprising thirty or fifty millions of such suns. Among the
+4,000 irresolvable nebulæ in the sky, perhaps over one-half are
+supposed to be galaxies, like our own galaxy, composed of star-clusters,
+and millions of stars. Besides these remote galaxies, vast
+agglomerations of yet uncondensed, nebulous matter exist in space, and
+form the nebulæ proper, in which the genesis of suns is slowly
+elaborated. Although the visible universe is limited by the penetrating
+power of our instruments, yet we see in imagination the infinite
+universe stretching farther and farther; but we know not whether this
+invisible universe is totally devoid of matter, or whether it also is
+filled with millions and millions of suns and galaxies.
+</p>
+
+<p><br /><br /><br /></p>
+
+<h4><a id="APPENDIX">APPENDIX</a></h4>
+
+<h4>KEY TO THE PLATES</h4>
+
+<p><br /></p>
+
+<h5>PLATE I.&mdash;GROUP OF SUN-SPOTS AND VEILED SPOTS.
+<br />
+<i>Observed June</i> 17, 1875, <i>at 7h. 30m. A. M.</i></h5>
+
+<p>
+The background shows the sun's visible surface, or <i>photosphere</i>,
+as seen with a telescope of high power at the most favorable moments,
+composed of innumerable light markings, or granules, separated by a
+network of darker gray. The granules, each some hundreds of miles in
+width, are thought to be the flame-like summits of the radial filaments
+or columns of gas and vapor which compose the photospheric shell. The
+two principal sun-spots of the group here represented show the
+characteristic dark <i>umbra</i> in the centre, overhung by the
+thatch-like <i>penumbra</i>, composed of whitish gray filaments. The
+penumbral filaments are not supposed to differ in their nature from
+those constituting the ordinary photosphere, save that they are seen
+here elongated and violently disturbed by the force of gaseous currents.
+Both spots are traversed partly or wholly by bright overlying
+<i>faculæ</i>, or so-called <i>luminous bridges</i>, depressed portions
+of which, in the left-hand spot, form the <i>gray and rosy veils</i>
+commonly attendant upon this class of spots. In each of these spots,
+also, the inner ends of projecting penumbral filaments have fallen so
+far within the umbra as to appear much darker than the rest. At the
+right of the upper portion of the left-hand spot, is a mass of white
+facular clouds, honey-combed by dark spaces, through which are seen
+traces of the undeveloped third spot of the triple group first observed.
+If seen upon the sun's limb, this would have presented the appearance of
+a <i>lateral spot</i>. Above the right-hand spot is a small black "dot,"
+or incipient spot, without distinct penumbra. The irregular dark rift
+below the two large spots and connecting them is a spot of the crevasse
+type, with very slight umbra, a still better example of which is seen in
+a westward prolongation of the penumbra of the left-hand spot. In the
+upper left-hand corner of the Plate are seen several small
+<i>faculæ</i>, appearing as irregular whitish streaks amongst the
+granules. In the pear-shaped darkening of the solar surface below and at
+their left, is seen a veiled spot, two of which attended this group.
+</p>
+<p>
+<i>Approximate scale, 2500 miles&mdash;1 inch.</i>
+</p>
+
+<p><br /></p>
+
+<h5>PLATE II.&mdash;SOLAR PROTUBERANCES.
+<br />
+<i>Observed May</i> 5, 1873, <i>at 9h. 40m. A. M.</i></h5>
+
+<p>
+A view of an upheaval of the <i>chromosphere</i>, or third outlying
+envelope of the sun, as observed with the tele-spectroscope, or
+telescope with spectroscope attached.
+</p>
+<p>
+The <i>method of the observation</i> requires a word of explanation.
+Save on the rare occasions of a total solar eclipse, no direct
+telescopic view of the solar prominences or flames is possible, owing to
+the fact that the intense white light from the sun's main disk entirely
+obscures the feeble pink light of the chromosphere. A few years ago
+Messrs. Jannsen and Lockyer found that a spectroscope of high dispersive
+power so weakens the spectrum of ordinary sun-light as to show the
+spectrum of bright lines given by the chromosphere, on any clear day.
+The telescope is adjusted so that a portion of the sun's limb, usually
+near a group of active sun-spots, shall be presented before the opened
+slit of the spectroscope. The light of the chromosphere thus admitted
+along with some diffused sun-light from the earth's atmosphere, produces
+a spectrum of intensely bright lines, widely separated, on the fainter
+background of the strongly dispersed spectrum of sun-light. The most
+prominent of these bright lines are those known as the C line
+(<i>scarlet</i>), F line (<i>blue</i>), which with several others are
+due to the hydrogen present in the chromosphere, the D<sub>3</sub> line
+(<i>orange</i>) ascribed to a little known substance called "helium",
+and occasionally the sodium lines D<sub>1</sub>, D<sub>3</sub>,
+(<i>yellow</i>). By adjusting the slit upon the scarlet C line, the
+appearances represented in Plate II. were observed as through an
+atmosphere of scarlet light: in the D or F lines identical appearances
+may be seen, but somewhat less clearly defined, as through yellow or
+blue light respectively. Hence the solar times, as here observed with
+the spectroscope in the hydrogen C line, are seen through a portion only
+(the scarlet rays) of the light coming from but one substance (hydrogen)
+of the companion incandescent substances present in the chromosphere.
+The color of the collective chromospheric light is seen directly with
+the telescope during an eclipse (See Plate III.) to be a delicate rosy
+pink.
+</p>
+
+<p><br /></p>
+
+<p>
+<i>Description of the Plate.</i>&mdash;The black background represents
+the general darkness of the eye-piece to the spectroscope. The broad red
+stripe stretching from top to bottom of the Plate is a portion of the
+red band of the spectrum, magnified about 100 times as compared with the
+actual spectroscopic view. The upper and lower edges of the
+cross-section of dusky red correspond with the edges of the slit, opened
+widely enough to admit a view of the chromospheric crest and of the
+whole height of the protuberances at once. With a narrower opening of
+the slit this background would have been nearly black, its reddish cast
+increasing with the amount of opening and consequent admission of
+diffused sun-light. Rising above the lower edge of the opening is seen a
+small outer segment of the chromosphere, which, as a portion of the
+sun's eastern limb, should be imagined as moving directly towards the
+beholder. The seams and rifts by which its surface is broken, as well as
+the distorted forms of the huge protuberances show the chromosphere to
+be in violent agitation. Some of the most characteristic shapes of the
+<i>eruptive protuberances</i> are presented, as also <i>cloud-like</i>
+forms overtopping the rest. In the immediate foreground the bases of two
+towering columns appear deeply depressed below the general horizon of
+the segment observed, showing an extraordinary velocity of motion of the
+whole uplifted mass toward the observer. The highest of these
+protuberances was 126,000 miles in height at the moment of observation.
+The triple protuberance at the left with two drooping wings and a tall
+swaying spire tipped with a very bright flame, shows by its more
+brilliant color the higher temperature (and possibly compression) to
+which its gases have been subjected. The irregular black bands behind
+this protuberance indicate the presence there of less condensed and
+cooler clouds of the same gases. The dimmer jets of dame rising from the
+chromosphere are either vanishing protuberances, or, as in the case of
+the smallest jet shown at the extreme right of the horizon, are the tops
+of protuberances just coming into view.
+</p>
+<p>
+<i>Approximate scale, 6000 miles&mdash;1 inch.</i>
+</p>
+
+<p><br /></p>
+
+<h5>PLATE III.&mdash;TOTAL ECLIPSE OF THE SUN.
+<br />
+<i>Observed July</i> 29, 1878, <i>at Creston, Wyoming Territory.</i></h5>
+
+<p>
+A telescopic view of the sun's <i>corona</i> or extreme outer atmosphere
+and of the <i>solar flames</i> or <i>prominences</i> during a total
+eclipse. At the moment of observation the dark disk of the moon, while
+still hiding the sun's main body, had passed far enough eastward to
+allow the rosy pink chromospheric prominences to be seen on its western
+border. On all sides of the sun's hidden disk, the <i>corona</i> shows
+its pale greenish light extending in halo-like rays and streamers, and
+two very remarkable wings stretch eastward and westward very nearly in
+the plane of the ecliptic and in the direction of the positions of
+Mercury and Venus respectively at the time of observation. The full
+extent of these wings could not be shown in the Plate without reducing
+its scale materially, since the westerly wing extended no less than
+twelve times the sun's diameter, and the easterly wing nearly as far, or
+over ten million miles. A circlet of bright light immediately bordering
+the moon's disk is the so-called <i>inner corona</i>, next to which the
+wings and streamers arc brightest, thence shading off imperceptibly into
+the twilight sky of the eclipse. Other noteworthy peculiarities of the
+corona, as observed during this eclipse, are the varying angles at which
+the radiating streamers are seen to project, the comparatively dark
+intervals between them, and the curved, wisp-like projections seen upon
+the wings. An especially noticeable gap appears where the most westerly
+of the upward streamers abruptly cuts off the view of the long wing. The
+largest and brightest of the curving streamers on the westerly wing
+coincides with the highest flame-like protuberance. To some observers of
+this eclipse the upward and downward streamers seemed pointed at their
+outer extremities and less regular in form.
+</p>
+<p>
+<i>Approximate scale, 135,000 miles&mdash;1 inch.</i>
+</p>
+
+<p><br /></p>
+
+<h5>PLATE IV.&mdash;AURORA BOREALIS.
+<br />
+<i>As observed March</i> 1, 1872, <i>at 9h. 25m. P. M.</i></h5>
+
+<p>
+The view presents the rare spectacle of an aurora spanning the sky from
+east to west in concentric arches. The Polar Star is nearly central in
+the background, the constellation of the Great Bear on the right and
+Cassiopeia's chair on the left. The large star at some distance above
+the horizon on the right is Arcturus. The almost black inner segment of
+the aurora resting upon the horizon, has its summit in the magnetic
+meridian, which was in this case a little west of north, its arc being
+indented by the bases of the ascending streamers. Both streamers and
+arches were, when observed, tremulous with upward pulsations and there
+was also a wave-like movement of the streamers from west to east. The
+prevailing color of this aurora is a pale whitish green and the
+complementary red appears especially at the west end of the auroral
+arch. The summits of the streamers are from four hundred to live hundred
+miles above the earth and the aurora is therefore a phenomenon of the
+terrestrial atmosphere rather than of astronomical observation proper.
+</p>
+
+<p><br /></p>
+
+<h5>PLATE V.&mdash;THE ZODIACAL LIGHT.
+<br />
+<i>Observed February</i> 20, 1876.</h5>
+
+<p>
+An observation of the cone of light whose axis lies along the Zodiac,
+whence it derives its name. It is drawn as seen in the west, with its
+base in the constellation Pisces, and its apex near the familiar group
+of the Pleiades in the constellation Taurus. The first bright star above
+the horizon in the base of the cone is the planet <i>Venus</i> and at
+some distance above is the reddish disk of <i>Mars</i>, the two being in
+rare companionship as evening stars. Above the constellation Pisces, two
+bright stars of Aries lie just outside the cone at the right. The
+nearest bright star above these at the right is <i>Beta</i>, the leading
+star of the constellation Triangula. Further at the right the three
+prominent stars nearly in a line are, in ascending order, <i>Delta</i>,
+<i>Beta</i> and <i>Gamma</i> of the constellation Andromeda. Above these
+at the left, the brightest star of a quadrangular group of four is the
+remarkable variable star <i>Algol</i> (<i>Beta</i>) of the constellation
+Perseus, which changes from the second to the fourth magnitude in a
+period of less than three days. At the left and a little above the
+Pleiades is the ruddy star <i>Aldebaran</i>, one of the Hyades and chief
+star in the constellation Taurus. These are the principal stars visible
+in this portion of the sky at the time of the observation. Their
+relative positions are represented as seen in the sky and not by the
+common method of star-atlases, which allows for the change from a
+spherical to a plane surface. Their magnitude in the order of brightness
+is indicated only approximately.
+</p>
+
+<p><br /></p>
+
+<h5>PLATE VI.&mdash;MARE HUMORUM.
+<br />
+<i>From a study made in 1875.</i></h5>
+
+<p>
+A view of one of the lunar plains, or so-called <i>seas</i>
+(<i>Maria</i>), with an encircling mountainous wall consisting of
+volcano-like craters in various stages of subsidence and dislocation.
+The sun-light coming from the west casts strong shadows from all the
+elevations eastward, and is just rising on the <i>terminator</i>, where
+the rugged structure of the Moon's surface is best seen. The lighter
+portions are the more elevated mountainous tracts and crater summits.
+The detailed description of this Plate given in the body of the Manual
+is repeated here for convenience of reference: The Mare Humorum, or sea
+of moisture, as it is called, is one of the smaller gray lunar plains.
+Its diameter, which is very nearly the same in all directions, is about
+270 miles, the total area of this plain being about 50,000 square miles.
+It is one of the most distinct plains of the Moon, and is easily seen
+with the naked eye on the left-hand side of the disk. The floor of the
+plain is, like that of the other gray plains, traversed by several
+systems of very extended but low hills and ridges, while small craters
+are disseminated upon its surface. The color of this formation is of a
+dusky greenish gray along the border, while in the interior it is of a
+lighter shade, and is of brownish olivaceous tint. This plain, which is
+surrounded by high clefts and rifts, well illustrates the phenomena of
+dislocation and subsidence. The double-ringed crater Vitello, whose
+walls rise from 4,000 to 5,000 feet in height, is seen in the upper
+left-hand corner of the gray plain. Close to Vitello at the east is the
+large broken ring-plain Lee, and farther east, and a little below, is a
+similarly broken crater called Doppelmayer. Both of these open craters
+have mountainous masses and peaks on their door, which is on a level
+with that of the Mare Humorum. A little below, and to the left of these
+objects, is dimly seen a deeply imbedded oval crater, whose walls barely
+rise above the level of the plain. On the right-hand side of the great
+plain is a long <i>fault</i>, with a system of fracture running along
+its border. On this right-hand side may be seen a part of the line of
+the terminator, which separates the light from the darkness. Towards the
+lower right-hand corner is the great ring-plain Gassendi, 55 miles in
+diameter, with its system of fractures and its central mountains, which
+rise from 3,000 to 4,000 feet above its floor. This crater slopes
+towards the plain, showing the subsidence to which it has been
+submitted. While the northern portion of the wall of this crater rises
+to 10,000 feet, that on the plain is only 500 feet high, and is even
+wholly demolished at one place where the floor of the crater is in
+direct communication with the plain. In the lower part of the sea, and a
+little to the west of the middle line, is found the crater
+Agatharchides, which shows below its north wall the marks of rills
+impressed by a flood of lava, which once issued from the side of the
+crater. On the left-hand side of the plain is seen the half-demolished
+crater Hippalus resembling a large bay, which has its interior strewn
+with peaks and mountains. On this same side can be seen one of the most
+important systems of clefts and fractures visible on the Moon, these
+clefts varying in length from 150 to 200 miles.
+</p>
+<p>
+<i>Approximate scale, 15 miles&mdash;1 inch.</i>
+</p>
+
+<p><br /></p>
+
+<h5>PLATE VII.&mdash;PARTIAL ECLIPSE OF THE MOON.
+<br />
+<i>Observed October</i> 24, 1874.</h5>
+
+<p>
+A view of the Moon partially obscured by the Earth's shadow, whose
+outline gives ocular proof of the earth's rotundity of form. The
+shadowed part of the Moon's surface is rendered visible by the diffused
+sun-light refracted upon it from the earth's atmosphere. Its reddish
+brown color is due to the absorption, by vapors present near the earth's
+surface, of a considerable part of this dim light. On both the obscured
+and illuminated tracts the configurations of the Moon's surface are seen
+as with the naked eye. The craters appear as distinct patches of lighter
+color, and the noticeably darker areas are the depressed plains or
+<i>Maria</i>. The large crater <i>Tycho</i>, at the lower part of the
+disk, is the most prominent of these objects, with its extensive system
+of <i>radiating streaks</i>. The largest crater above is
+<i>Copernicus</i>, at the left of which is <i>Kepler</i> and still above
+are <i>Aristarchus</i> and <i>Herodotus</i> appearing as if blended in
+one. Above and at the left of the great crater <i>Tycho</i>, the first
+dark tract is the <i>Mare Humorum</i> of Plate VI., seen in its natural
+position, with the crater <i>Gassendi</i> at its northern (upper)
+extremity and <i>Vitello</i> on its southern (lower) border. The
+advancing border of the shadow appears, as always, noticeably darker
+than the remainder, an effect probably of contrast. The illuminated
+segment of the Moon's disk has its usual appearance, the lighter
+portions being the more elevated mountainous surfaces, and the dark
+spaces the floors of extensive plains.
+</p>
+<p>
+<i>Approximate scale, 140 miles&mdash;1 inch.</i>
+</p>
+
+<p><br /></p>
+
+<h5>PLATE VIII.&mdash;THE PLANET MARS.
+<br />
+<i>Observed September</i> 3, 1877, <i>at 11h. 55m. P. M.</i></h5>
+
+<p>
+A view of the southern hemisphere of Mars, when in the most favorable
+position for observation, and when exceptionally free from the clouds,
+which very frequently hide its surface configurations. Since, of all the
+planets, Mars is most like the earth, Plate VIII. may give a fair idea
+of the appearance of our globe to a supposed observer on Mars. The dark
+gray and black markings, are regarded as tracts of water, or of some
+liquid with similar powers of absorbing light; and, for the same reason,
+the lighter portions, of a prevailing reddish tint, are supposed to be
+bodies of land, while the bright white portions are variously due to
+clouds, to polar snow or ice, and the bright rim of white along the
+limb, to the depth of the atmosphere through which the limb is seen. The
+chief permanent features of the planet's surface have been named in
+honor of various astronomers.
+</p>
+<p>
+The large dark tract on the left is <i>De La Rue Ocean</i>, the isolated
+oval spot near the centre is <i>Terby Sea</i>, and on the right is the
+western end of <i>Maraldi Sea</i>, with strongly indented border.
+Directly north of (below) De La Rue Ocean, is <i>Maedler Continent</i>;
+above it stretches <i>Jacob Land</i>; and surrounding Terby Sea is
+<i>Secchi Continent</i>. Extending into the centre of De La Rue Ocean is
+a curious double peninsula, called, in consequence of the dimness of
+former observations, <i>Hall Island</i>. The sharply defined,
+white-crested northern borders of De La Rue Ocean and Maraldi Sea may
+indicate the existence there of lofty coast ranges, more or less
+constantly covered with opaque clouds strongly reflecting light. The
+white spot in the centre of Maedler Continent, of a temporary nature,
+has a similar explanation. The intervals of olivaceous gray on Secchi
+Continent and elsewhere may perhaps be ascribed to the flooding and
+drying up of marshes and lowlands, as these markings have been observed
+to vary somewhat in connection with the change of seasons on Mars. The
+greenish tints observed along the planet's limb, alike on the darker and
+lighter surfaces, are probably due to an optical effect, the green being
+complementary to the prevailing red of the disk. The brilliant oval
+white spot near the southern (upper) pole of the planet is a so-called
+polar spot, in all probability consisting of a material similar to snow
+or ice and here observed in the midst of a dark open sea.
+</p>
+<p>
+<i>Approximate scale, 300 miles&mdash;1 inch.</i>
+</p>
+
+<p><br /></p>
+
+<h5>PLATE IX.&mdash;THE PLANET JUPITER.
+<br />
+<i>Observed November</i> 1, 1880, <i>at 9h. 30m. P. M.</i></h5>
+
+<p>
+This planet is perpetually wrapped in dense clouds which hide its inner
+globe from view. The drawing shows Jupiter's outer clouded surface with
+its usual series of alternate light and dark belts, the disk as a whole
+appearing brighter in the centre than near the limb. The darker gray and
+black markings indicate in general the lower cloud-levels; that is,
+partial breaks or rifts in the cloudy envelope, whose prevailing depth
+apparently exceeds four thousand miles. While the deepest depression in
+the cloudy envelope is within the limits of the Great Red Spot, the
+vision may not even here penetrate very deeply. Two of Jupiter's four
+moons present bright disks near the planet's western limb, and cast
+their shadows far eastward on the disk, that of the "second satellite"
+falling upon the Red Spot. On the Red Spot are seen in addition two
+small black spots, no explanation of which can yet be offered. The broad
+white ring of clouds bordering the Red Spot appeared in constant motion.
+The central, or equatorial belt, shows brilliant cloudy masses of both
+the <i>cumulus</i> and <i>stratus</i> types, and the underlying gray and
+black cloudy surfaces are pervaded with the pinkish color characteristic of
+this belt. The dark circular spots on the wide white belt next north
+showed in their mode of formation striking resemblances to sun-spots.
+They afterward coalesced into a continuous pink belt. The diffusion of
+pinkish color over the three northernmost dark bands, as here observed,
+is unusual. About either pole is seen the uniform gray segment or polar
+cap. The equatorial diameter is noticeably longer than the polar
+diameter, a consequence of the planet's extraordinary swiftness of
+rotation. To the same cause may also be due chiefly the distribution of
+the cloudy belts parallel to the planet's equator, though the analogy of
+the terrestrial trade-winds fails to explain all the observed phenomena.
+</p>
+<p>
+<i>Approximate scale, 5,500 miles&mdash;1 inch.</i>
+</p>
+
+<p><br /></p>
+
+<h5>PLATE X.&mdash;THE PLANET SATURN.
+<br />
+<i>Observed November</i> 30, 1874, <i>at 5h. 50m. P. M.</i></h5>
+
+<p>
+Saturn is unique amongst the planets in that its globe is encircled by a
+series of concentric rings, which lie in the plane of its equator, and
+consist, according to present theories, of vast throngs of minute bodies
+revolving about the planet, like so many satellites, in closely parallel
+orbits. The globe of Saturn, like that of Jupiter, is surrounded by
+cloudy belts parallel to its equator. The broad equatorial belt, of a
+delicate pinkish tint, is both brighter and more mottled than the
+narrower yellowish white belts, which alternate with darker belts of
+ashy gray on both the north and south sides, but are seen here only on
+the northern side. The disk has an oval shape, owing to the extreme
+polar compression of the globe.
+</p>
+<p>
+The outer, middle and inner rings, with their various subdivisions, are
+clearly shown in Plate X., and are best seen on the so-called <i>ansæ</i>,
+or handles, projecting on either side. The gray <i>outer ring</i> is
+separated by the dusky pencil line into two divisions, both of which
+appear slightly mottled on the ansæ, as if with clouds. The <i>middle
+ring</i> has three sub-divisions which are clearly distinguishable,
+although separated by no dark interval, viz., a brilliant white outer
+zone, distinctly mottled, as seen on the extremities of the ansæ, and
+two interior zones of gradually diminishing brightness. The <i>gauze</i> or
+<i>dusky ring</i> is seen at its full width on the ansæ, but on the
+background of the strongly illuminated globe only its outer and
+presumably denser border is visible. The shadow of the globe on the
+rings is seen on the lower portion of the eastern ansæ. The shadow on
+the dusky ring is with difficulty perceptible; the shadow on the middle
+ring is slightly concave toward the planet, which concavity is abruptly
+increased on the outer zone of this ring; while the shadow on the outer
+ring slants away from the globe. These appearances are fully accounted
+for by supposing a general increase of level from the inner edge of the
+dusky ring to the outer margin of the middle ring, and a uniform lower
+level on the outer ring. Other observers have regarded the deflection of
+the shadow as an effect of irradiation. The inner margin of the double
+outer ring presents on both ansæ a number of slight indentations,
+which, if not actual irregularities in the contour of this ring, may be
+explained as shadows caused by elevations on the outer border of the
+middle ring, or possibly by over-hanging clouds.
+</p>
+<p>
+<i>Approximate scale, 6,500 miles&mdash;1 inch.</i>
+</p>
+
+<p><br /></p>
+
+<h5>PLATE XI.&mdash;THE GREAT COMET OF 1881.
+<br />
+<i>Observed on the night of June</i> 25-26, <i>at 1h. 30m. A. M.</i></h5>
+
+<p>
+A view of the comet 1881, III., drawn as if seen with the naked eye, the
+minute details, however, being reproduced as seen with the telescope.
+The star-like <i>nucleus</i> is attended by four conical wings which
+cause it to appear diamond-shaped. The <i>coma</i> appears double, the
+brilliant inner coma, immediately enveloping the nucleus, being
+surrounded by a fainter exterior coma, which has a noticeable depression
+corresponding to that of the inner edge of the principal coma. The tail
+is divided lengthwise by a dark rift and is brightest on its convex or
+forward side. An inner portion of the tail, brighter than the rest, is
+more strongly curved, as if by solar repulsion. Stars are seen through
+the brighter parts of the tail, as they may be seen even through the
+coma and nucleus, with little diminution of their light.
+</p>
+
+<p><br /></p>
+
+<h5>PLATE XII.&mdash;THE NOVEMBER METEORS.
+<br />
+<i>As observed on the night of November</i> 13-14, 1868.</h5>
+
+<p>
+A partially ideal view of the November Meteors, combining forms observed
+at different times during the night of Nov. 13th, 1868. It is not,
+however, a fanciful view, since a much larger number of meteors were
+observed falling at once during the shower of November, 1833, and at
+other times. The locality of the observation is shown by the Polar Star
+seen near the centre of the Plate, and Cassiopeia's Chair at the left.
+The general direction of the paths of the meteors is from the
+north-east, the <i>radiant point</i> of the shower having been in the
+constellation Leo, beyond and above Ursa Major. While the orbits of the
+meteors are, in general, curved regularly and slightly, several are
+shown with very eccentric paths, among them one which changed its course
+at a sharp angle. In the upper left-hand corner appear two vanishing
+trails of the "ring-form," and several others still further transformed
+into faint luminous patches of cloud. Red, yellow, green, blue and
+purple tints were observed in the meteors and their trails, as
+represented in the Plate.
+</p>
+
+<p><br /></p>
+
+<h5>PLATE XIII.&mdash;PART OF THE MILKY WAY.
+<br />
+<i>From a study made during the years</i> 1874, 1875 <i>and</i> 1876.</h5>
+
+<p>
+The course of the portion of the Galaxy represented in Plate XIII. is as
+follows: From Cassiopeia's chair, three bright stars of which appear at
+the upper edge of the Plate, the Galaxy, forming two streams, descends
+south, passing partly through Lacerta on the left, and Cepheus on the
+right; at this last point it approaches nearest to the Polar Star, which
+is itself outside of the field of view. Then it enters Cygnus, where it
+becomes very complicated and bright, and where several large cloudy
+masses are seen terminating its left branch, which passes to the right,
+near the bright star <i>Deneb</i>, the leader of this constellation.
+Below <i>Deneb</i>, the Galaxy is apparently disconnected and separated
+from the northern part by a narrow, irregular dark gap. From this
+rupture, the Milky-way divides into two great streams, separated by an
+irregular dark rift. An immense branch extends to the right, which,
+after having formed an important luminous mass between the stars
+<i>Gamma</i> and <i>Beta</i>, continues its southward progress through
+parts of Lyra, Vulpecula, Hercules, Aquila and Ophiuchus, where it
+gradually terminates a few degrees south of the equator. The main stream
+on the left, after having formed a bright mass around <i>Epsilon
+Cygni</i>, passes through Vulpecula and then Aquila, where it crosses
+the equinoctial just below the star <i>Eta</i>, after having involved in
+its nebulosity the bright star <i>Altair</i>, the leader of Aquila. In
+the southern hemisphere the Galaxy becomes very complicated and forms a
+succession of very bright, irregular masses, the upper one being in
+Scutum Sobieskii, while the others are respectively situated in
+Sagittarius and in Scorpio; the last, just a little above our horizon,
+being always considerably dimmed by vapors. From Scutum Sobieskii, the
+Galaxy expands considerably on the right, and sends a branch into
+Scorpio, in which the fiery red star <i>Antares</i> is somewhat
+involved. In the upper left-hand corner of the Plate, at some distance
+from the Milky-way, is seen dimly the Nebula in Andromeda, which becomes
+so magnificent an object to telescopic view.
+</p>
+
+<p><br /></p>
+
+<h5>PLATE XIV.&mdash;STAR-CLUSTER IN HERCULES.
+<br />
+<i>From a study made in June</i>, 1877.</h5>
+
+<p>
+In the constellation Hercules, a small nebulous mass is faintly visible
+to the eye, a telescopic view of which is presented in Plate XIV. It is
+one of the most beautiful of the easily resolvable globular clusters.
+The brilliancy of the centre gives the cluster a distinctly globular
+appearance, while the several wings curving in various directions, have
+suggested to some observers an irregularly spiral structure. The large
+stars of the cluster are arranged in several groups which correspond, in
+a general way, with the faintly luminous wings.
+</p>
+
+<p><br /></p>
+
+<h5>PLATE XV.&mdash;THE GREAT NEBULA IN ORION.
+<br />
+<i>From a study made in the years</i> 1875-76.</h5>
+
+<p>
+This nebula, which is one of the most brilliant and wonderful of
+telescopic objects, readily visible to the naked eve as a patch of
+nebulous light immediately surrounding the middle star of the three
+which form the sword of Orion, and a little south of the three
+well-known stars forming the belt. The small stars in this, as in other
+Plates of the series, are somewhat exaggerated in size, as was
+unavoidable with any mode of reproduction that could be employed. The
+bright pentagonal centre of the nebula is traversed by less luminous
+rifts, the several subdivisions thus outlined being irregularly mottled
+as if by bright fleecy clouds. Toward the lower part of this bright
+pentagonal centre is a comparatively dark space containing four bright
+stars which form a trapezium and together constitute the quadruple star
+<i>Theta Orionis</i>, which, to the naked eye, appears as the single star
+in the centre of the sword. On three sides of the central mass extend long
+bright wisps, whose curves fail, however, to reveal the spiral structure
+often attributed to this nebula. On the east a broad wing, with
+wave-shaped inner border, stretches southward. East of the trapezium are
+two especially noticeable dark spaces. Close to the main nebula on the
+north-east, a small faint nebula surrounds a bright star, and a branch
+from another faint stream of nebulous matter forming a loop to the
+southward, encloses the nebulous star (<i>Iota Orionis</i>) shown at the
+top of the Plate.
+</p>
+
+<p><br /><br /><br /></p>
+
+<div style='display:block; margin-top:4em'>*** END OF THE PROJECT GUTENBERG EBOOK THE TROUVELOT ASTRONOMICAL DRAWINGS MANUAL ***</div>
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author	nfenwick <nfenwick@pglaf.org>	2025-01-21 19:06:50 -0800
committer	nfenwick <nfenwick@pglaf.org>	2025-01-21 19:06:50 -0800
commit	bdfd67aca015a3f49eb2f6eb3b73f776850541c1 (patch)
tree	0a02bcd51c29637dfed14b512d50a3d24505c480
parent	ad9ea4cff9fb37672c0cccd57fa6ba171b906bb2 (diff)