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diff --git a/.gitattributes b/.gitattributes new file mode 100644 index 0000000..d7b82bc --- /dev/null +++ b/.gitattributes @@ -0,0 +1,4 @@ +*.txt text eol=lf +*.htm text eol=lf +*.html text eol=lf +*.md text eol=lf diff --git a/LICENSE.txt b/LICENSE.txt new file mode 100644 index 0000000..6312041 --- /dev/null +++ b/LICENSE.txt @@ -0,0 +1,11 @@ +This eBook, including all associated images, markup, improvements, +metadata, and any other content or labor, has been confirmed to be +in the PUBLIC DOMAIN IN THE UNITED STATES. + +Procedures for determining public domain status are described in +the "Copyright How-To" at https://www.gutenberg.org. + +No investigation has been made concerning possible copyrights in +jurisdictions other than the United States. Anyone seeking to utilize +this eBook outside of the United States should confirm copyright +status under the laws that apply to them. diff --git a/README.md b/README.md new file mode 100644 index 0000000..e7252af --- /dev/null +++ b/README.md @@ -0,0 +1,2 @@ +Project Gutenberg (https://www.gutenberg.org) public repository for +eBook #68394 (https://www.gutenberg.org/ebooks/68394) diff --git a/old/68394-0.txt b/old/68394-0.txt deleted file mode 100644 index ad39e66..0000000 --- a/old/68394-0.txt +++ /dev/null @@ -1,7681 +0,0 @@ -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 *** - -Updated editions will replace the previous one--the old editions will -be renamed. - -Creating the works from print editions not protected by U.S. copyright -law means that no one owns a United States copyright in these works, -so the Foundation (and you!) can copy and distribute it in the -United States without permission and without paying copyright -royalties. Special rules, set forth in the General Terms of Use part -of this license, apply to copying and distributing Project -Gutenberg-tm electronic works to protect the PROJECT GUTENBERG-tm -concept and trademark. 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Trouvelot. - </title> - <style type="text/css"> - -body { - margin-left: 10%; - margin-right: 10%; -} - - h1,h2,h3,h4,h5,h6 { - text-align: center; /* all headings centered */ - clear: both; -} - -p { - margin-top: .51em; - text-align: justify; - margin-bottom: .49em; - text-indent:4%; -} - -.nind {text-indent:0%;} - -hr { - width: 33%; - margin-top: 2em; - margin-bottom: 2em; - margin-left: auto; - margin-right: auto; - clear: both; -} - -table { - margin-left: auto; - margin-right: auto; -} - -/* Notes */ -.footnotes {border: dashed 1px;} - -.footnote {margin-left: 10%; margin-right: 10%; font-size: 0.9em;} - -.footnote .label {position: absolute; right: 84%; text-align: right;} - -.fnanchor { - vertical-align: super; - font-size: .8em; - text-decoration: - none; -} - -/* Images */ -.figcenter { - margin: auto; - text-align: center; -} - -.caption {font-weight: normal; - font-size: 90%; - text-align: right; - padding-bottom: 1em;} - -.caption p -{ - text-align: center; - text-indent: 0; - margin: 0.25em 0; -} - -.caption p.smaller -{ - font-size: smaller; -} - - - </style> - </head> -<body> -<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 -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 <a href="https://www.gutenberg.org">www.gutenberg.org</a>. 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. -</div> - -<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 & 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. -</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—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 & 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—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. -</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—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—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—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.—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.—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—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.—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.—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—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.—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—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—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—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. -</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—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.—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—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.—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.—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—some of them appearing almost -perfectly black. -</p> -<p> -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. -</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—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—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. -</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—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. -</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.—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.—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—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. -</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.—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.—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—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.—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.—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.—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.—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—1 inch.</i> -</p> - -<p><br /></p> - -<h5>PLATE II.—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>—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—1 inch.</i> -</p> - -<p><br /></p> - -<h5>PLATE III.—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—1 inch.</i> -</p> - -<p><br /></p> - -<h5>PLATE IV.—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.—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.—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—1 inch.</i> -</p> - -<p><br /></p> - -<h5>PLATE VII.—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—1 inch.</i> -</p> - -<p><br /></p> - -<h5>PLATE VIII.—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—1 inch.</i> -</p> - -<p><br /></p> - -<h5>PLATE IX.—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—1 inch.</i> -</p> - -<p><br /></p> - -<h5>PLATE X.—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—1 inch.</i> -</p> - -<p><br /></p> - -<h5>PLATE XI.—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.—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.—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.—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.—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> -<div style='text-align:left'> - -<div style='display:block; margin:1em 0'> -Updated editions will replace the previous one—the old editions will -be renamed. -</div> - -<div style='display:block; margin:1em 0'> -Creating the works from print editions not protected by U.S. copyright -law means that no one owns a United States copyright in these works, -so the Foundation (and you!) can copy and distribute it in the United -States without permission and without paying copyright -royalties. 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