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+The Project Gutenberg eBook, Other Worlds, by Garrett P. Serviss
+
+
+This eBook is for the use of anyone anywhere 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
+
+
+
+
+
+Title: Other Worlds
+       Their Nature, Possibilities and Habitability in the Light of the Latest Discoveries
+
+
+Author: Garrett P. Serviss
+
+
+
+Release Date: May 22, 2006  [eBook #18431]
+
+Language: English
+
+Character set encoding: ISO-646-US (US-ASCII)
+
+
+***START OF THE PROJECT GUTENBERG EBOOK OTHER WORLDS***
+
+
+E-text prepared by Suzanne Lybarger, Brian Janes, and the Project
+Gutenberg Online Distributed Proofreading Team (https://www.pgdp.net/)
+
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+
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+
+
+
+
+
+OTHER WORLDS
+
+by
+
+GARRETT P. SERVISS.
+
+
+
+     *     *     *     *     *     *
+
+
+
+OTHER WORLDS.
+
+     Their Nature and Possibilities in the Light of the Latest
+     Discoveries. Illustrated. 12mo. Cloth, $1.20 net; postage
+     additional.
+
+     No science has ever equaled astronomy in its appeal to the
+     imagination, and recently popular interest in the wonders of the
+     starry heavens has been stimulated by surprising discoveries and
+     imaginary discoveries, as well as by a marked tendency of writers
+     of fiction to include other worlds and their possible inhabitants
+     within the field of romance.
+
+     Mr. Serviss's new book on "Other Worlds, their Nature and
+     Possibilities in the Light of the Latest Discoveries," summarizes
+     what is known. With helpful illustrations, the most interesting
+     facts about the planets Venus, Mars, Jupiter, Saturn, etc., as well
+     as about the nearest of all other worlds, the moon, are presented
+     in a popular manner, and always from the point of view of human
+     interest--a point that is too seldom taken by writers on science.
+
+ASTRONOMY WITH AN OPERA-GLASS.
+
+     A Popular Introduction to the Study of the Starry Heavens with the
+     simplest of Optical Instruments. Illustrated. 8vo. Cloth, $1.50.
+
+     "By its aid thousands of people who have resigned themselves to the
+     ignorance in which they were left at school, by our wretched system
+     of teaching by the book only, will thank Mr. Serviss for the
+     suggestions he has so well carried out."--_New York Times._
+
+PLEASURES OF THE TELESCOPE.
+
+     A Descriptive Guide to Amateur Astronomers and All Lovers of the
+     Stars. Illustrated. 8vo. Cloth, $1.50.
+
+     "The volume will be found interesting by those for whom it is
+     written, and will inspire many with a love for the study of
+     astronomy, one of the most far-reaching of the
+     sciences."--_Milwaukee Journal._
+
+D. APPLETON AND COMPANY, NEW YORK.
+
+
+
+     *     *     *     *     *     *
+
+
+
+[Illustration: CHART OF MARS. After Schiaparelli.]
+
+
+
+OTHER WORLDS
+
+Their Nature, Possibilities and Habitability in the Light of the Latest
+Discoveries.
+
+by
+
+GARRETT P. SERVISS
+
+Author of
+"Astronomy with an Opera-glass" and "Pleasures of the Telescope"
+
+With Charts and Illustrations
+
+
+
+
+     "Shall we measure the councils of heaven by the narrow impotence of
+     human faculties, or conceive that silence and solitude reign
+     throughout the mighty empire of nature?"
+
+     --DR. THOMAS CHALMERS.
+
+
+
+
+New York
+D. Appleton and Company
+1901
+Copyright, 1901,
+by D. Appleton and Company.
+
+
+
+
+TO THE MEMORY OF WILLIAM JAY YOUMANS.
+
+
+
+
+PREFACE
+
+
+The point of view of this book is human interest in the other worlds
+around us. It presents the latest discoveries among the planets of the
+solar system, and shows their bearing upon the question of life in those
+planets. It points out the resemblances and the differences between the
+earth and the other worlds that share with it in the light of the sun.
+It shows what we should see and experience if we could visit those
+worlds.
+
+While basing itself upon facts, it does not exclude the discussion of
+interesting probabilities and theories that have commanded wide popular
+attention. It points out, for instance, what is to be thought of the
+idea of interplanetary communication. It indicates what must be the
+outlook of the possible inhabitants of some of the other planets toward
+the earth. As far as may be, it traces the origin and development of the
+other worlds of our system, and presents a graphic picture of their
+present condition as individuals, and of their wonderful contrasts as
+members of a common family.
+
+In short, the aim of the author has been to show how wide, and how rich,
+is the field of interest opened to the human mind by man's discoveries
+concerning worlds, which, though inaccessible to him in a physical
+sense, offer intellectual conquests of the noblest description.
+
+And, finally, in order to assist those who may wish to recognize for
+themselves these other worlds in the sky, this book presents a special
+series of charts to illustrate a method of finding the planets which
+requires no observatory and no instruments, and only such knowledge of
+the starry heavens as anybody can easily acquire.
+
+G.P.S.
+
+
+BOROUGH OF BROOKLYN, NEW YORK CITY,
+_September, 1901._
+
+
+
+
+CONTENTS
+
+
+CHAPTER I
+
+_INTRODUCTORY_                                                1
+
+     Remarkable popular interest in questions concerning
+     other worlds and their inhabitants--Theories of
+     interplanetary communication--The plurality of worlds in
+     literature--Romances of foreign planets--Scientific interest
+     in the subject--Opposing views based on telescopic and
+     spectroscopic revelations--Changes of opinion--Desirability
+     of a popular presentation of the latest facts--The natural
+     tendency to regard other planets as habitable--Some of the
+     conditions and limitations of the problem--The solar system
+     viewed from outer space--The resemblances and contrasts of
+     its various planets--Three planetary groups recognized--The
+     family character of the solar system
+
+
+CHAPTER II
+
+_MERCURY, A WORLD OF TWO FACES AND MANY CONTRASTS_           18
+
+     Grotesqueness of Mercury considered as a world--Its
+     dimensions, mass, and movements--The question of an
+     atmosphere--Mercury's visibility from the earth--Its
+     eccentric orbit, and rapid changes of distance from the
+     sun--Momentous consequences of these peculiarities--A
+     virtual fall of fourteen million miles toward the sun
+     in six weeks--The tremendous heat poured upon Mercury
+     and its great variations--The little planet's singular
+     manner of rotation on its axis--Schiaparelli's astonishing
+     discovery--A day side and a night side--Interesting effects
+     of libration--The heavens as viewed from Mercury--Can it
+     support life?
+
+
+CHAPTER III
+
+_VENUS, THE TWIN OF THE EARTH_                               46
+
+     A planet that matches ours in size--Its beauty in the
+     sky--Remarkable circularity of its orbit--Probable
+     absence of seasons and stable conditions of temperature
+     and weather on Venus--Its dense and abundant atmosphere--Seeing
+     the atmosphere of Venus from the earth--Is the real face of the
+     planet hidden under an atmospheric veil?--Conditions of
+     habitability--All planetary life need not be of the terrestrial
+     type--The limit fixed by destructive temperature--Importance of
+     air and water in the problem--Reasons why Venus may be a
+     more agreeable abode than the earth--Splendor of our globe
+     as seen from Venus--What astronomers on Venus might learn
+     about the earth--A serious question raised--Does Venus, like
+     Mercury, rotate but once in the course of a revolution about
+     the sun?--Reasons for and against that view
+
+
+CHAPTER IV
+
+_MARS, A WORLD MORE ADVANCED THAN OURS_                      85
+
+     Resemblances between Mars and the earth--Its seasons and its
+     white polar caps--Peculiar surface markings--Schiaparelli's
+     discovery of the canals--His description of their appearance
+     and of their duplication--Influence of the seasons on the
+     aspect of the canals--What are the canals?--Mr. Lowell's
+     observations--The theory of irrigation--How the inhabitants
+     of Mars are supposed to have taken advantage of the annual
+     accession of water supplied by the melting of the polar
+     caps--Wonderful details shown in charts of Mars--Curious
+     effects that may follow from the small force of gravity
+     on Mars--Imaginary giants--Reasons for thinking that
+     Mars may be, in an evolutionary sense, older than the
+     earth--Speculations about interplanetary signals from
+     Mars, and their origin--Mars's atmosphere--The question of
+     water--The problem of temperature--Eccentricities of Mars's
+     moons
+
+
+CHAPTER V
+
+_THE ASTEROIDS, A FAMILY OF DWARF WORLDS_                   129
+
+     Only four asteroids large enough to be measured--Remarkable
+     differences in their brightness irrespective of size--Their
+     widely scattered and intermixed orbits--Eccentric orbit of
+     Eros--the nearest celestial body to the earth except the
+     moon--Its existence recorded by photography before it was
+     discovered--Its great and rapid fluctuations in light, and
+     the curious hypotheses based upon them--Is it a fragment of
+     an exploded planet?--The startling theory of Olbers as to
+     the origin of the asteroids revived--Curious results of the
+     slight force of gravity on an asteroid--An imaginary visit
+     to a world only twelve miles in diameter
+
+
+CHAPTER VI
+
+_JUPITER, THE GREATEST OF KNOWN WORLDS_                     160
+
+     Jupiter compared with our globe--His swift rotation on his
+     axis--Remarkable lack of density--The force of gravity on
+     Jupiter--Wonderful clouds--Strange phenomena of the great
+     belts--Brilliant display of colors--The great red spot
+     and the many theories it has given rise to--Curious facts
+     about the varying rates of rotation of the huge planet's
+     surface--The theory of a hidden world in Jupiter--When
+     Jupiter was a companion star to the sun--The miracle of
+     world-making before our eyes--Are Jupiter's satellites
+     habitable?--Magnificent spectacles in the Jovian system
+
+
+CHAPTER VII
+
+_SATURN, A PRODIGY AMONG PLANETS_                           185
+
+     The wonder of the great rings--Saturn's great distance and
+     long year--The least dense of all the planets--It would
+     float in water--What kind of a world is it?--Sir Humphry
+     Davy's imaginary inhabitants of Saturn--Facts about the
+     rings, which are a phenomenon unparalleled in the visible
+     universe--The surprising nature of the rings, as revealed
+     by mathematics and the spectroscope--The question of their
+     origin and ultimate fate--Dr. Dick's idea of their
+     habitability--Swedenborg's curious description of the
+     appearance of the rings from Saturn--Is Saturn a globe of
+     vapor, or of dust?--The nine satellites and "Roche's
+     limit"--The play of spectacular shadows in the Saturnian
+     system--Uranus and Neptune--Is there a yet undiscovered
+     planet greater than Jupiter?
+
+
+CHAPTER VIII
+
+_THE MOON, CHILD OF THE EARTH AND THE SUN_                  212
+
+     The moon a favorite subject for intellectual speculation--Its
+     nearness to the earth graphically illustrated--Ideas of the
+     ancients--Galileo's discoveries--What first raised a serious
+     question as to its habitability--Singularity of the moon's
+     motions--Appearance of its surface to the naked eye and with
+     the telescope--The "seas" and the wonderful mountains and
+     craters--A terrible abyss described--Tycho's mysterious
+     rays--Difference between lunar and terrestrial
+     volcanoes--Mountain-ringed valleys--Gigantic cracks in the
+     lunar globe--Slight force of gravity of the moon and some
+     interesting deductions--The moon a world of giantism--What
+     kind of atmospheric gases can the moon contain--The question
+     of water and of former oceans--The great volcanic cataclysm
+     in the moon's history--Evidence of volcanic and other
+     changes now occurring--Is there vegetation on the
+     moon?--Lunar day and night--The earth as seen from the
+     moon--Discoveries yet to be made
+
+
+CHAPTER IX
+
+_HOW TO FIND THE PLANETS_                                   256
+
+     It is easy to make acquaintance with the planets and to
+     follow them among the stars--The first step a knowledge of
+     the constellations--How this is to be acquired--How to use
+     the Nautical Almanac in connection with the charts in this
+     book--The visibility of Mercury and Venus--The oppositions
+     of Mars, Jupiter, and Saturn
+
+
+INDEX                                                       277
+
+
+
+
+LIST OF ILLUSTRATIONS
+
+
+                                                                   PAGE
+Chart of Mars                                             _Frontispiece_
+
+Diagram showing causes of day and night on portions of Mercury       35
+
+Regions of day and night on Mercury                                  38
+
+Venus's atmosphere seen as a ring of light                           56
+
+View of Jupiter                                            _facing_ 168
+
+Three views of Saturn                                      _facing_ 186
+
+Diagram showing the moon's path through space                       217
+
+The lunar Alps, Apennines, and Caucasus                    _facing_ 222
+
+The moon at first and last quarter                         _facing_ 226
+
+Phases and rotation of the moon                                     250
+
+Charts showing the zodiacal constellations:
+  1. From right ascension 0 hours to 4 hours                        259
+  2.    "           "     4   "   "  8   "                          261
+  3.    "           "     8   "   "  2   "                          263
+  4.    "           "    12   "   " 16   "                          265
+  5.    "           "    16   "   " 20   "                          267
+  6.    "           "    20   "   " 24   "                          269
+
+
+
+
+OTHER WORLDS
+
+
+
+
+CHAPTER I
+
+INTRODUCTORY
+
+
+Other worlds and their inhabitants are remarkably popular subjects of
+speculation at the present time. Every day we hear people asking one
+another if it is true that we shall soon be able to communicate with
+some of the far-off globes, such as Mars, that circle in company with
+our earth about the sun. One of the masters of practical electrical
+science in our time has suggested that the principle of wireless
+telegraphy may be extended to the transmission of messages across space
+from planet to planet. The existence of intelligent inhabitants in some
+of the other planets has become, with many, a matter of conviction, and
+for everybody it presents a question of fascinating interest, which has
+deeply stirred the popular imagination.
+
+The importance of this subject as an intellectual phenomenon of the
+opening century is clearly indicated by the extent to which it has
+entered into recent literature. Poets feel its inspiration, and
+novelists and romancers freely select other planets as the scenes of
+their stories. One tells us of a visit paid by men to the moon, and of
+the wonderful things seen, and adventures had, there. Lucian, it is
+true, did the same thing eighteen hundred years ago, but he had not the
+aid of hints from modern science to guide his speculations and lend
+verisimilitude to his narrative.
+
+Another startles us from our sense of planetary security with a
+realistic account of the invasion of the earth by the terrible sons of
+warlike Mars, seeking to extend their empire by the conquest of foreign
+globes.
+
+Sometimes it is a trip from world to world, a kind of celestial pleasure
+yachting, with depictions of creatures more wonderful than--
+
+    "The anthropophagi and men whose heads
+    Do grow beneath their shoulders"--
+
+that is presented to our imagination; and sometimes we are informed of
+the visions beheld by the temporarily disembodied spirits of trance
+mediums, or other modern thaumaturgists, flitting about among the
+planets.
+
+Then, to vary the theme, we find charming inhabitants of other worlds
+represented as coming down to the earth and sojourning for a time on our
+dull planet, to the delight of susceptible successors of father Adam,
+who become, henceforth, ready to follow their captivating visitors to
+the ends of the universe.
+
+In short, writers of fiction have already established interplanetary
+communication to their entire satisfaction, thus vastly and indefinitely
+enlarging the bounds of romance, and making us so familiar with the
+peculiarities of our remarkable brothers and sisters of Mars, Venus,
+and the moon, that we can not help feeling, notwithstanding the many
+divergences in the descriptions, that we should certainly recognize them
+on sight wherever we might meet them.
+
+But the subject is by no means abandoned to the tellers of tales and the
+dreamers of dreams. Men of science, also, eagerly enter into the
+discussion of the possibilities of other worlds, and become warm over
+it.
+
+Around Mars, in particular, a lively war of opinions rages. Not all
+astronomers have joined in the dispute--some have not imagination
+enough, and some are waiting for more light before choosing sides--but
+those who have entered the arena are divided between two opposed camps.
+One side holds that Mars is not only a world capable of having
+inhabitants, but that it actually has them, and that they have given
+visual proof of their existence and their intelligence through the
+changes they have produced upon its surface. The other side maintains
+that Mars is neither inhabited nor habitable, and that what are taken
+for vast public works and engineering marvels wrought by its
+industrious inhabitants, are nothing but illusions of the telescope, or
+delusions of the observer's mind. Both adduce numerous observations,
+telescopic and spectroscopic, and many arguments, scientific and
+theoretic, to support their respective contentions, but neither side has
+yet been able to convince or silence the other, although both have made
+themselves and their views intensely interesting to the world at large,
+which would very much like to know what the truth really is.
+
+And not only Mars, but Venus--the beauteous twin sister of the earth,
+who, when she glows in the evening sky, makes everybody a lover of the
+stars--and even Mercury, the Moor among the planets, wearing "the
+shadowed livery of the burnished sun," to whom he is "a neighbor and
+near bred," and Jupiter, Saturn, and the moon itself--all these have
+their advocates, who refuse to believe that they are lifeless globes,
+mere reflectors of useless sunshine.
+
+The case of the moon is, in this respect, especially interesting, on
+account of the change that has occurred in the opinions held concerning
+its physical condition. For a very long time our satellite was
+confidently, and almost universally, regarded as an airless, waterless,
+lifeless desert, a completely "dead world," a bare, desiccated skull of
+rock, circling about the living earth.
+
+But within a few years there has been a reaction from this extreme view
+of the lifelessness of the moon. Observers tell us of clouds suddenly
+appearing and then melting to invisibility over volcanic craters; of
+evidences of an atmosphere, rare as compared with ours, yet manifest in
+its effects; of variations of color witnessed in certain places as the
+sunlight drifts over them at changing angles of incidence; of what seem
+to be immense fields of vegetation covering level ground, and of
+appearances indicating the existence of clouds of ice crystals and
+deposits of snow among the mountainous lunar landscapes. Thus, in a
+manner, the moon is rehabilitated, and we are invited to regard its
+silvery beams not as the reflections of the surface of a desert, but as
+sent back to our eyes from the face of a world that yet has some slight
+remnants of life to brighten it.
+
+The suggestion that there is an atmosphere lying close upon the shell of
+the lunar globe, filling the deep cavities that pit its face and
+penetrating to an unknown depth in its interior, recalls a speculation
+of the ingenious and entertaining Fontenelle, in the seventeenth
+century--recently revived and enlarged upon by the author of one of our
+modern romances of adventure in the moon--to the effect that the lunar
+inhabitants dwell beneath the surface of their globe instead of on the
+top of it.
+
+Now, because of this widespread and continually increasing interest in
+the subject of other worlds, and on account of the many curious
+revelations that we owe to modern telescopes and other improved means of
+investigation, it is certainly to be desired that the most important and
+interesting discoveries that have lately been made concerning the
+various globes which together with the earth constitute the sun's
+family, should be assembled in a convenient and popular form--and that
+is the object of this book. Fact is admittedly often stranger and more
+wonderful than fiction, and there are no facts that appeal more
+powerfully to the imagination than do those of astronomy. Technical
+books on astronomy usually either ignore the subject of the habitability
+of the planets, or dismiss it with scarcely any recognition of the
+overpowering human interest that it possesses. Hence, a book written
+specially from the point of view of that subject would appear calculated
+to meet a popular want; and this the more, because, since Mr. Proctor
+wrote his Other Worlds than Ours and M. Flammarion his Pluralite des
+Mondes Habites, many most important and significant discoveries have
+been made that, in several notable instances, have completely altered
+the aspect in which the planets present themselves for our judgment as
+to their conditions of habitability.
+
+No doubt the natural tendency of the mind is to regard all the planets
+as habitable worlds, for there seems to be deeply implanted in human
+nature a consciousness of the universality of life, giving rise to a
+conviction that one world, even in the material sense, is not enough for
+it, but that every planet must belong to its kingdom. We are apt to say
+to ourselves: "The earth is one of a number of planets, all similarly
+circumstanced; the earth is inhabited, why should not the others also be
+inhabited?"
+
+What has been learned of the unity in chemical constitution and
+mechanical operation prevailing throughout the solar system, together
+with the continually accumulating evidence of the common origin of its
+various members, and the identity of the evolutionary processes that
+have brought them into being, all tends to strengthen the _a priori_
+hypothesis that life is a phenomenon general to the entire system, and
+only absent where its essential and fundamental conditions, for special
+and local, and perhaps temporary, reasons, do not exist.
+
+If we look for life in the sun, for instance, while accepting the
+prevalent conception of the sun as a center of intense thermal action,
+we must abandon all our ideas of the physical organization of life
+formed upon what we know of it from experimental evidence. We can not
+imagine any form of life that has ever been presented to our senses as
+existing in the sun.
+
+But this is not generally true of the planets. Life, in our sense of it,
+is a planetary, not a solar, phenomenon, and while we may find reasons
+for believing that on some of the planets the conditions are such that
+creatures organized like ourselves could not survive, yet we can not
+positively say that every form of living organism must necessarily be
+excluded from a world whose environment would be unsuited for us and our
+contemporaries in terrestrial life.
+
+Although our sole knowledge of animated nature is confined to what we
+learn by experience on the earth, yet it is a most entertaining, and by
+no means unedifying, occupation, to seek to apply to the exceedingly
+diversified conditions prevailing in the other planets, as astronomical
+observations reveal them to us, the principles, types, and limitations
+that govern the living creatures of our world, and to judge, as best we
+can, how far those types and limits may be modified or extended so that
+those other planets may reasonably be included among the probable abodes
+of life.
+
+In order to form such judgments each planet must be examined by itself,
+but first it is desirable to glance at the planetary system as a whole.
+To do this we may throw off, in imagination, the dominance of the sun,
+and suppose ourselves to be in the midst of open space, far removed both
+from the sun and the other stars. In this situation it is only by
+chance, or through foreknowledge, that we can distinguish our sun at
+all, for it is lost among the stars; and when we discover it we find
+that it is only one of the smaller and less conspicuous members of the
+sparkling host.
+
+We rapidly approach, and when we have arrived within a distance
+comparable with that of its planets, we see that the sun has increased
+in apparent magnitude, until now it enormously outshines all the other
+stars, and its rays begin to produce the effect of daylight upon the
+orbs that they reach. But we are in no danger of mistaking its apparent
+superiority to its fellow stars for a real one, because we clearly
+perceive that our nearness alone makes it seem so great and
+overpowering.
+
+And now we observe that this star that we have drawn near to has
+attending it a number of minute satellites, faintly shining specks, that
+circle about it as if charmed, like night-wandering insects, by its
+splendor. It is manifest to us at the first glance that without the sun
+these obedient little planets would not exist; it is his attraction that
+binds them together in a system, and his rays that make them visible to
+one another in the abyss of space. Although they vary in relative size,
+yet we observe a striking similarity among them. They are all globular
+bodies, they all turn upon their axes, they all travel about the sun in
+the same direction, and their paths all lie very nearly in one plane.
+Some of them have one or more moons, or satellites, circling about them
+in imitation of their own revolution about the sun. Their family
+relationship to one another and to the sun is so evident that it colors
+our judgment about them as individuals; and when we happen to find, upon
+closer approach, that one of them, the earth, is covered with vegetation
+and water and filled with thousands of species of animated creatures, we
+are disposed to believe, without further examination, that they are all
+alike in this respect, just as they are all alike in receiving light and
+heat from the sun.
+
+This preliminary judgment, arising from the evident unity of the
+planetary system, can only be varied by an examination of its members in
+detail.
+
+One striking fact that commands our attention as soon as we have entered
+the narrow precincts of the solar system is the isolation of the sun and
+its attendants in space. The solar system occupies a disk-shaped, or
+flat circular, expanse, about 5,580,000,000 miles across and relatively
+very thin, the sun being in the center. From the sun to the nearest
+star, or other sun, the distance is approximately five thousand times
+the entire diameter of the solar system. But the vast majority of the
+stars are probably a hundred times yet more remote. In other words, if
+the Solar system be represented by a circular flower-bed ten feet
+across, the nearest star must be placed at a distance of nine and a half
+miles, and the great multitude of the stars at a distance of nine
+hundred miles!
+
+Or, to put it in another way, let us suppose the sun and his planets to
+be represented by a fleet of ships at sea, all included within a space
+about half a mile across; then, in order that there might be no shore
+relatively nearer than the nearest fixed star is to the sun, we should
+have to place our fleet in the middle of the Pacific Ocean, while the
+distance of the main shore of the starry universe would be so immense
+that the whole surface of the earth would be far too small to hold the
+expanse of ocean needed to represent it!
+
+From these general considerations we next proceed to recall some of the
+details of the system of worlds amid which we dwell. Besides the earth,
+the sun has seven other principal planets in attendance. These eight
+planets fall into two classes--the terrestrial planets and the major, or
+jovian, planets. The former class comprises Mercury, Venus, the earth,
+and Mars, and the latter Jupiter, Saturn, Uranus, and Neptune. I have
+named them all in the order of their distance from the sun, beginning
+with the nearest.
+
+The terrestrial planets, taking their class name from _terra_, the
+earth, are relatively close to the sun and comparatively small. The
+major planets--or the jovian planets, if we give them a common title
+based upon the name of their chief, Jupiter or Jove--are relatively
+distant from the sun and are characterized both by great comparative
+size and slight mean density. The terrestrial planets are all included
+within a circle, having the sun for a center, about 140,000,000 miles
+in radius. The space, or gap, between the outermost of them, Mars, and
+the innermost of the jovian planets, Jupiter, is nearly two and a half
+times as broad as the entire radius of the circle within which they are
+included. And not only is the jovian group of planets widely separated
+from the terrestrial group, but the distances between the orbits of its
+four members are likewise very great and progressively increasing.
+Between Jupiter and Saturn is a gap 400,000,000 miles across, and this
+becomes 900,000,000 miles between Saturn and Uranus, and more than
+1,000,000,000 miles between Uranus and Neptune. All of these distances
+are given in round numbers.
+
+Finally, we come to some very extraordinary worlds--if we can call them
+worlds at all--the asteroids. They form a third group, characterized by
+the extreme smallness of its individual members, their astonishing
+number, and the unusual eccentricities and inclinations of their orbits.
+They are situated in the gap between the terrestrial and the jovian
+planets, and about 500 of them have been discovered, while there is
+reason to think that their real number may be many thousands. The
+largest of them is less than 500 miles in diameter, and many of those
+recently discovered may be not more than ten or twenty miles in
+diameter. What marvelous places of abode such little planets would be if
+it were possible to believe them inhabited, we shall see more clearly
+when we come to consider them in their turn. But without regard to the
+question of habitability, the asteroids will be found extremely
+interesting.
+
+In the next chapter we proceed to take up the planets for study as
+individuals, beginning with Mercury, the one nearest the sun.
+
+
+
+
+CHAPTER II
+
+MERCURY, A WORLD OF TWO FACES AND MANY CONTRASTS
+
+
+Mercury, the first of the other worlds that we are going to consider,
+fascinates by its grotesqueness, like a piece of Chinese ivory carving,
+so small is it for its kind and so finished in its eccentric details. In
+a little while we shall see how singular Mercury is in many of the
+particulars of planetary existence, but first of all let us endeavor to
+obtain a clear idea of the actual size and mass of this strange little
+planet. Compared with the earth it is so diminutive that it looks as if
+it had been cut out on the pattern of a satellite rather than that of an
+independent planet. Its diameter, 3,000 miles, only exceeds the moon's
+by less than one half, while both Jupiter and Saturn, among their
+remarkable collections of moons, have each at least one that is
+considerably larger than the planet Mercury. But, insignificant though
+it be in size, it holds the place of honor, nearest to the sun.
+
+It was formerly thought that Mercury possessed a mass greatly in excess
+of that which its size would seem to imply, and some estimates, based
+upon the apparent effect of its attraction on comets, made it equal in
+mean density to lead, or even to the metal mercury. This led to curious
+speculations concerning its probable metallic composition, and the
+possible existence of vast quantities of such heavy elements as gold in
+the frame of the planet. But more recent, and probably more correct,
+computations place Mercury third in the order of density among the
+members of the solar system, the earth ranking as first and Venus as
+second. Mercury's density is now believed to be less than the earth's in
+the ratio of 85 to 100. Accepting this estimate, we find that the force
+of gravity upon the surface of Mercury is only one third as great as
+upon the surface of the earth--i.e., a body weighing 300 pounds on the
+earth would weigh only 100 pounds on Mercury.
+
+This is an important matter, because not only the weight of bodies, but
+the density of the atmosphere and even the nature of its gaseous
+constituents, are affected by the force of gravity, and if we could
+journey from world to world, in our bodily form, it would make a great
+difference to us to find gravity considerably greater or less upon other
+planets than it is upon our own. This alone might suffice to render some
+of the planets impossible places of abode for us, unless a decided
+change were effected in our present physical organization.
+
+One of the first questions that we should ask about a foreign world to
+which we proposed to pay a visit, would relate to its atmosphere. We
+should wish to know in advance if it had air and water, and in what
+proportions and quantities. However its own peculiar inhabitants might
+be supposed able to dispense with these things, to _us_ their presence
+would be essential, and if we did not find them, even a planet that
+blazed with gold and diamonds only waiting to be seized would remain
+perfectly safe from our invasion. Now, in the case of Mercury, some
+doubt on this point exists.
+
+Messrs. Huggins, Vogel, and others have believed that they found
+spectroscopic proof of the existence of both air and the vapor of water
+on Mercury. But the necessary observations are of a very delicate
+nature, and difficult to make, and some astronomers doubt whether we
+possess sufficient proof that Mercury has an atmosphere. At any rate,
+its atmosphere is very rare as compared with the earth's, but we need
+not, on that account, conclude that Mercury is lifeless. Possibly, in
+view of certain other peculiarities soon to be explained, a rare
+atmosphere would be decidedly advantageous.
+
+Being much nearer the sun than the earth is, Mercury can be seen by us
+only in the same quarter of the sky where the sun itself appears. As it
+revolves in its orbit about the sun it is visible, alternately, in the
+evening for a short time after sunset and in the morning for a short
+time before sunrise, but it can never be seen, as the outer planets are
+seen, in the mid-heaven or late at night. When seen low in the twilight,
+at evening or morning, it glows with the brilliance of a bright
+first-magnitude star, and is a beautiful object, though few casual
+watchers of the stars ever catch sight of it. When it is nearest the
+earth and is about to pass between the earth and the sun, it temporarily
+disappears in the glare of the sunlight; and likewise, when it it is
+farthest from the earth and passing around in its orbit on the opposite
+side of the sun, it is concealed by the blinding solar rays.
+Consequently, except with the instruments of an observatory, which are
+able to show it in broad day, Mercury is never visible save during the
+comparatively brief periods of time when it is near its greatest
+apparent distance east or west from the sun.
+
+The nearer a planet is to the sun the more rapidly it is compelled to
+move in its orbit, and Mercury, being the nearest to the sun of all the
+planets, is by far the swiftest footed among them. But its velocity is
+subject to remarkable variation, owing to the peculiar form of the orbit
+in which the planet travels. This is more eccentric than the orbit of
+any other planet, except some of the asteroids. The sun being situated
+in one focus of the elliptical orbit, when Mercury is at perihelion, or
+nearest to the sun, its distance from that body is 28,500,000 miles, but
+when it is at aphelion, or farthest from the sun, its distance is
+43,500,000 miles. The difference is no less than 14,000,000 miles! When
+nearest the sun Mercury darts forward in its orbit at the rate of
+twenty-nine miles in a second, while when farthest from the sun the
+speed is reduced to twenty-three miles.
+
+Now, let us return for a moment to the consideration of the wonderful
+variations in Mercury's distance from the sun, for we shall find that
+their effects are absolutely startling, and that they alone suffice to
+mark a wide difference between Mercury and the earth, considered as the
+abodes of sentient creatures. The total change of distance amounts, as
+already remarked, to 14,000,000 miles, which is almost half the entire
+distance separating the planet from the sun at perihelion. This immense
+variation of distance is emphasized by the rapidity with which it takes
+place. Mercury's periodic time, i.e., the period required for it to make
+a single revolution about the sun--or, in other words, the length of its
+year--is eighty-eight of our days. In just one half of that time, or in
+about six weeks, it passes from aphelion to perihelion; that is to say,
+in six weeks the whole change in its distance from the sun takes place.
+In six weeks Mercury falls 14,000,000 miles--for it _is_ a fall, though
+in a curve instead of a straight line--falls 14,000,000 miles toward the
+sun! And, as it falls, like any other falling body it gains in speed,
+until, having reached the perihelion point, its terrific velocity
+counteracts its approach and it begins to recede. At the end of the next
+six weeks it once more attains its greatest distance, and turns again to
+plunge sunward.
+
+Of course it may be said of every planet having an elliptical orbit
+that between aphelion and perihelion it is falling toward the sun, but
+no other planet than Mercury travels in an orbit sufficiently eccentric,
+and approaches sufficiently near to the sun, to give to the mind so
+vivid an impression of an actual, stupendous fall!
+
+Next let us consider the effects of this rapid fall, or approach, toward
+the sun, which is so foreign to our terrestrial experience, and so
+appalling to the imagination.
+
+First, we must remember that the nearer a planet is to the sun the
+greater is the amount of heat and light that it receives, the variation
+being proportional to the inverse square of the distance. The earth's
+distance from the sun being 93,000,000 miles, while Mercury's is only
+36,000,000, it follows, to begin with, that Mercury gets, on the
+average, more than six and a half times as much heat from the sun as the
+earth does. That alone is enough to make it seem impossible that Mercury
+can be the home of living forms resembling those of the earth, for
+imagine the heat of the sun in the middle of a summer's day increased
+six or seven fold! If there were no mitigating influences, the face of
+the earth would shrivel as in the blast of a furnace, the very stones
+would become incandescent, and the oceans would turn into steam.
+
+Still, notwithstanding the tremendous heat poured upon Mercury as
+compared with that which our planet receives, we can possibly, and for
+the sake of a clearer understanding of the effects of the varying
+distance, which is the object of our present inquiry, find a loophole to
+admit the chance that yet there may be living beings there. We might,
+for instance, suppose that, owing to the rarity of its atmosphere, the
+excessive heat was quickly radiated away, or that there was something in
+the constitution of the atmosphere that greatly modified the effective
+temperature of the sun's rays. But, having satisfied our imagination on
+this point, and placed our supposititious inhabitants in the hot world
+of Mercury, how are we going to meet the conditions imposed by the
+rapid changes of distance--the swift fall of the planet toward the sun,
+followed by the equally swift rush away from it? For change of distance
+implies change of heat and temperature.
+
+It is true that we have a slight effect of this kind on the earth.
+Between midsummer (of the northern hemisphere) and midwinter our planet
+draws 3,000,000 miles nearer the sun, but the change occupies six
+months, and, at the earth's great average distance, the effect of this
+change is too slight to be ordinarily observable, and only the
+astronomer is aware of the consequent increase in the apparent size of
+the sun. It is not to this variation of the sun's distance, but rather
+to the changes of the seasons, depending on the inclination of the
+earth's axis, that we owe the differences of temperature that we
+experience. In other words, the total supply of heat from the sun is not
+far from uniform at all times of the year, and the variations of
+temperature depend upon the distribution of that supply between the
+northern and southern hemispheres, which are alternately inclined
+sunward.
+
+But on Mercury the supply of solar heat is itself variable to an
+enormous extent. In six weeks, as we have seen, Mercury diminishes its
+distance from the sun about one third, which is proportionally ten times
+as great a change of distance as the earth experiences in six months.
+The inhabitants of Mercury in those six pregnant weeks see the sun
+expand in the sky to more than two and a half times its former
+magnitude, while the solar heat poured upon them swiftly augments from
+something more than four and a half times to above eleven times the
+amount received upon the earth! Then, immediately, the retreat of the
+planet begins, the sun visibly shrinks, as a receding balloon becomes
+smaller in the eyes of its watchers, the heat falls off as rapidly as it
+had previously increased, until, the aphelion point being reached, the
+process is again reversed. And thus it goes on unceasingly, the sun
+growing and diminishing in the sky, and the heat increasing and
+decreasing by enormous amounts with astonishing rapidity. It is
+difficult to imagine any way in which atmospheric influences could
+equalize the effects of such violent changes, or any adjustments in the
+physical organization of living beings that could make such changes
+endurable.
+
+But we have only just begun the story of Mercury's peculiarities. We
+come next to an even more remarkable contrast between that planet and
+our own. During the Paris Exposition of 1889 a little company of
+astronomers was assembled at the Juvisy observatory of M. Flammarion,
+near the French capital, listening to one of the most surprising
+disclosures of a secret of nature that any _savant_ ever confided to a
+few trustworthy friends while awaiting a suitable time to make it
+public. It was a secret as full of significance as that which Galileo
+concealed for a time in his celebrated anagram, which, when at length he
+furnished the key, still remained a riddle, for then it read: "The
+Mother of the Loves imitates the Shapes of Cynthia," meaning that the
+planet Venus, when viewed with a telescope, shows phases like those of
+the moon. The secret imparted in confidence to the knot of astronomers
+at Juvisy came from a countryman of Galileo's, Signor G. V.
+Schiaparelli, the Director of the Observatory of Milan, and its purport
+was that the planet Mercury always keeps the same face directed toward
+the sun. Schiaparelli had satisfied himself, by a careful series of
+observations, of the truth of his strange announcement, but before
+giving it to the world he determined to make doubly sure. Early in 1890
+he withdrew the pledge of secrecy from his friends and published his
+discovery.
+
+No one can wonder that the statement was generally received with
+incredulity, for it was in direct contradiction to the conclusions of
+other astronomers, who had long believed that Mercury rotated on its
+axis in a period closely corresponding with that of the earth's
+rotation--that is to say, once every twenty-four hours. Schiaparelli's
+discovery, if it were received as correct, would put Mercury, as a
+planet, in a class by itself, and would distinguish it by a peculiarity
+which had always been recognized as a special feature of the moon, viz.,
+that of rotating on its axis in the same period of time required to
+perform a revolution in its orbit, and, while this seemed natural enough
+for a satellite, almost nobody was prepared for the ascription of such
+eccentric conduct to a planet.
+
+The Italian astronomer based his discovery upon the observation that
+certain markings visible on the disk of Mercury remained in such a
+position with reference to the direction of the sun as to prove that the
+planet's rotation was extremely slow, and he finally satisfied himself
+that there was but one rotation in the course of a revolution about the
+sun. That, of course, means that one side of Mercury always faces toward
+the sun while the opposite side always faces away from it, and neither
+side experiences the alternation of day and night, one having perpetual
+day and the other perpetual night. The older observations, from which
+had been deduced the long accepted opinion that Mercury rotated, like
+the earth, once in about twenty-four hours, had also been made upon the
+markings on the planet's disk, but these are not easily seen, and their
+appearances had evidently been misinterpreted.
+
+The very fact of the difficulty of seeing any details on Mercury tended
+to prevent or delay corroboration of Schiaparelli's discovery. But there
+were two circumstances that contributed to the final acceptance of his
+results. One of these was his well-known experience as an observer and
+the high reputation that he enjoyed among astronomers, and the other was
+the development by Prof. George Darwin of the theory of tidal friction
+in its application to planetary evolution, for this furnished a
+satisfactory explanation of the manner in which a body, situated as near
+the sun as Mercury is, could have its axial rotation gradually reduced
+by the tidal attraction of the sun until it coincided in period with its
+orbital revolution.
+
+Accepting the accuracy of Schiaparelli's discovery, which was
+corroborated in every particular in 1896 by Percival Lowell in a special
+series of observations on Mercury made with his 24-inch telescope at
+Flagstaff, Arizona, and which has also been corroborated by others, we
+see at once how important is its bearing on the habitability of the
+planet. It adds another difficulty to that offered by the remarkable
+changes of distance from the sun, and consequent variations of heat,
+which we have already discussed. In order to bring the situation home to
+our experience, let us, for a moment, imagine the earth fallen into
+Mercury's dilemma. There would then be no succession of day and night,
+such as we at present enjoy, and upon which not alone our comfort but
+perhaps our very existence depends, but, instead, one side of our
+globe--it might be the Asiatic or the American half--would be
+continually in the sunlight, and the other side would lie buried in
+endless night. And this condition, so suggestive of the play of pure
+imagination, this plight of being a two-faced world, like the god
+Janus, one face light and the other face dark, must be the actual state
+of things on Mercury.
+
+There is one interesting qualification. In the case just imagined for
+the earth, supposing it to retain the present inclination of its axis
+while parting with its differential rotation, there would be an
+interchange of day and night once a year in the polar regions. On
+Mercury, whose axis appears to be perpendicular, a similar phenomenon,
+affecting not the polar regions but the eastern and western sides of the
+planet, is produced by the extraordinary eccentricity of its orbit. As
+the planet alternately approaches and recedes from the sun its orbital
+velocity, as we have already remarked, varies between the limits of
+twenty-three and thirty-five miles per second, being most rapid at the
+point nearest the sun. But this variation in the speed of its revolution
+about the sun does not, in any manner, affect the rate of rotation on
+its axis. The latter is perfectly uniform and just fast enough to
+complete one axial turn in the course of a single revolution about the
+sun. The accompanying figure may assist the explanation.
+
+[Illustration: DIAGRAM SHOWING THAT, OWING TO THE ECCENTRICITY OF ITS
+ORBIT, AND ITS VARYING VELOCITY, MERCURY, ALTHOUGH MAKING BUT ONE TURN
+ON ITS AXIS IN THE COURSE OF A REVOLUTION ABOUT THE SUN, NEVERTHELESS
+EXPERIENCES ON PARTS OF ITS SURFACE THE ALTERNATION OF DAY AND NIGHT.]
+
+Let us start with Mercury in perihelion at the point _A_. The little
+cross on the planet stands exactly under the sun and in the center of
+the illuminated hemisphere. The large arrows show the direction in which
+the planet travels in its revolution about the sun, and the small curved
+arrows the direction in which it rotates on its axis. Now, in moving
+along its orbit from _A_ to _B_ the planet, partly because of its
+swifter motion when near the sun, and partly because of the elliptical
+nature of the orbit, traverses a greater angular interval with reference
+to the sun than the cross, moving with the uniform rotation of the
+planet on its axis, is able to traverse in the same time. As drawn in
+the diagram, the cross has moved through exactly ninety degrees, or one
+right angle, while the planet in its orbit has moved through
+considerably more than a right angle. In consequence of this gain of the
+angle of revolution upon the angle of rotation, the cross at _B_ is no
+longer exactly under the sun, nor in the center of the illuminated
+hemisphere. It appears to have shifted its position toward the west,
+while the hemispherical cap of sunshine has slipped eastward over the
+globe of the planet.
+
+In the next following section of the orbit the planet rotates through
+another right angle, but, owing to increased distance from the sun, the
+motion in the orbit now becomes slower until, when the planet arrives at
+aphelion, _C_, the angular difference disappears and the cross is once
+more just under the sun. On returning from aphelion to perihelion the
+same phenomena recur in reverse order and the line between day and night
+on the planet first shifts westward, attaining its limit in that respect
+at _D_, and then, at perihelion, returns to its original position.
+
+Now, if we could stand on the sunward hemisphere of Mercury what, to our
+eyes, would be the effect of this shifting of the sun's position with
+regard to a fixed point on the planet's surface? Manifestly it would
+cause the sun to describe a great arc in the sky, swinging to and fro,
+in an east and west line, like a pendulum bob, the angular extent of the
+swing being a little more than forty-seven degrees, and the time
+required for the sun to pass from its extreme eastern to its extreme
+western position and back again being eighty-eight days. But, owing to
+the eccentricity of the orbit, the sun swings much faster toward the
+east than toward the west, the eastward motion occupying about
+thirty-seven days and the westward motion about fifty-one days.
+
+[Illustration: THE REGIONS OF PERPETUAL DAY, PERPETUAL NIGHT, AND
+ALTERNATE DAY AND NIGHT ON MERCURY. IN THE LEFT-HAND VIEW THE OBSERVER
+LOOKS AT THE PLANET IN THE PLANE OF ITS EQUATOR; IN THE RIGHT-HAND VIEW
+HE LOOKS DOWN ON ITS NORTH POLE.]
+
+Another effect of the libratory motion of the sun as seen from Mercury
+is represented in the next figure, where we have a view of the planet
+showing both the day and the night hemisphere, and where we see that
+between the two there is a region upon which the sun rises and sets once
+every eighty-eight days. There are, in reality, two of these lune-shaped
+regions, one at the east and the other at the west, each between 1,200
+and 1,300 miles broad at the equator. At the sunward edge of these
+regions, once in eighty-eight days, or once in a Mercurial year, the sun
+rises to an elevation of forty-seven degrees, and then descends again
+straight to the horizon from which it rose; at the nightward edge, once
+in eighty-eight days, the sun peeps above the horizon and quickly sinks
+from sight again. The result is that, neglecting the effects of
+atmospheric refraction, which would tend to expand the borders of the
+domain of sunlight, about one quarter of the entire surface of Mercury
+is, with regard to day and night, in a condition resembling that of our
+polar regions, where there is but one day and one night in the course of
+a year--and on Mercury a year is eighty-eight days. One half of the
+remaining three quarters of the planet's surface is bathed in perpetual
+sunshine and the other half is a region of eternal night.
+
+And now again, what of life in such a world as that? On the night side,
+where no sunshine ever penetrates, the temperature must be extremely
+low, hardly greater than the fearful cold of open space, unless
+modifying influences beyond our ken exist. It is certain that if life
+flourishes there, it must be in such forms as can endure continual
+darkness and excessive cold. Some heat would be carried around by
+atmospheric circulation from the sunward side, but not enough, it would
+seem, to keep water from being perpetually frozen, or the ground from
+being baked with unrelaxing frost. It is for the imagination to picture
+underground dwellings, artificial sources of heat, and living forms
+suited to unearthlike environment.
+
+What would be the mental effects of perpetual night upon a race of
+intelligent creatures doomed to that condition? Perhaps not quite so
+grievous as we are apt to think. The constellations in all their
+splendor would circle before their eyes with the revolution of their
+planet about the sun, and with the exception of the sun itself--which
+they could see by making a journey to the opposite hemisphere--all the
+members of the solar system would pass in succession through their
+mid-heaven, and two of them would present themselves with a magnificence
+of planetary display unknown on the earth. Venus, when in opposition
+under the most favorable circumstances, is scarcely more than 24,000,000
+miles from Mercury, and, showing herself at such times with a fully
+illuminated disk--as, owing to her position within the orbit of the
+earth, she never can do when at her least distance from us--she must be
+a phenomenon of unparalleled beauty, at least four times brighter than
+we ever see her, and capable, of course, of casting a strong shadow.
+
+The earth, also, is a splendid star in the midnight sky of Mercury, and
+the moon may be visible to the naked eye as a little attendant circling
+about its brilliant master. The outer planets are slightly less
+conspicuous than they are to us, owing to increase of distance.
+
+The revolution of the heavens as seen from the night side of Mercury is
+quite different in period from that which we are accustomed to, although
+the apparent motion is in the same direction, viz., from east to west.
+The same constellations remain above the horizon for weeks at a time,
+slowly moving westward, with the planets drifting yet more slowly, but
+at different rates, among them; the nearer planets, Venus and the earth,
+showing the most decided tendency to loiter behind the stars.
+
+On the side where eternal sunlight shines the sky of Mercury contains no
+stars. Forever the pitiless blaze of day; forever,
+
+    "All in a hot and copper sky
+    The bloody sun at noon."
+
+As it is difficult to understand how water can exist on the night
+hemisphere, except in the shape of perpetual snow and ice, so it is
+hard to imagine that on the day hemisphere water can ever be
+precipitated from the vaporous form. In truth, there can be very little
+water on Mercury even in the form of vapor, else the spectroscope would
+have given unquestionable evidence of its presence. Those who think that
+Mercury is entirely waterless and almost, if not quite, airless may be
+right. In these respects it would then resemble the moon, and, according
+to some observers, it possesses another characteristic lunar feature in
+the roughening of its surface by what seem to be innumerable volcanic
+craters.
+
+But if we suppose Mercury to possess an atmosphere much rarer than that
+of the earth, we may perceive therein a possible provision against the
+excessive solar heat to which it is subjected, since, as we see on high
+mountains, a light air permits a ready radiation of heat, which does not
+become stored up as in a denser atmosphere.
+
+As the sun pours its heat without cessation upon the day hemisphere the
+warmed air must rise and flow off on all sides into the night
+hemisphere, while cold air rushes in below, to take its place, from the
+region of frost and darkness. The intermediate areas, which see the sun
+part of the time, as explained above, are perhaps the scene of
+contending winds and tempests, where the moisture, if there be any, is
+precipitated, through the rapid cooling of the air, in whelming floods
+and wild snow-storms driven by hurrying blasts from the realm of endless
+night.
+
+Enough seems now to have been said to indicate clearly the hopelessness
+of looking for any analogies between Mercury and the earth which would
+warrant the conclusion that the former planet is capable of supporting
+inhabitants or forms of life resembling those that swarm upon the
+latter. If we would still believe that Mercury is a habitable globe we
+must depend entirely upon the imagination for pictures of creatures able
+to endure its extremes of heat and cold, of light and darkness, of
+instability, swift vicissitude, and violent contrast.
+
+In the next chapter we shall study a more peaceful and even-going world,
+yet one of great brilliancy, which possesses some remarkable
+resemblances to the earth, as well as some surprising divergences from
+it.
+
+
+
+
+CHAPTER III
+
+VENUS, THE TWIN OF THE EARTH
+
+
+We come now to a planet which seems, at the first glance, to afford a
+far more promising outlook than Mercury does for the presence of organic
+life forms bearing some resemblance to those of the earth. One of the
+strongest arguments for regarding Venus as a world much like ours is
+based upon its remarkable similarity to the earth in size and mass,
+because thus we are assured that the force of gravity is practically the
+same upon the two planets, and the force of gravity governs numberless
+physical phenomena of essential importance to both animal and vegetable
+life. The mean diameter of the earth is 7,918 miles; that of Venus is
+7,700 miles. The difference is so slight that if the two planets were
+suspended side by side in the sky, at such a distance that their disks
+resembled that of the full moon, the eye would notice no inequality
+between them.
+
+The mean density of Venus is about nine tenths of that of the earth, and
+the force of gravity upon its surface is in the ratio of about 85 to 100
+as compared to its force on the surface of the earth. A man removed to
+Venus would, consequently, find himself perceptibly lighter than he was
+at home, and would be able to exert his strength with considerably
+greater effect than on his own planet. But the difference would amount
+only to an agreeable variation from accustomed conditions, and would not
+be productive of fundamental changes in the order of nature.
+
+Being, like Mercury, nearer to the sun than the earth is, Venus also is
+visible to us only in the morning or the evening sky. But her distance
+from the sun, slightly exceeding 67,000,000 miles, is nearly double that
+of Mercury, so that, when favorably situated, she becomes a very
+conspicuous object, and, instead of being known almost exclusively by
+astronomers, she is, perhaps, the most popular and most admired of all
+the members of the planetary system, especially when she appears in the
+charming role of the "evening star." As she emerges periodically from
+the blinding glare of the sun's immediate neighborhood and begins to
+soar, bright as an electric balloon, in the twilight, she commands all
+eyes and calls forth exclamations of astonishment and admiration by her
+singular beauty. The intervals between her successive reappearances in
+the evening sky, measured by her synodic period of 584 days, are
+sufficiently long to give an element of surprise and novelty to every
+return of so dazzling a phenomenon.
+
+Even the light of the full moon silvering the tree tops does not
+exercise greater enchantment over the mind of the contemplative
+observer. In either of her roles, as morning or as evening star, Venus
+has no rival. No fixed star can for an instant bear comparison with her.
+What she lacks in vivacity of light--none of the planets twinkles, as do
+all of the true stars--is more than compensated by the imposing size of
+her gleaming disk and the striking beauty of her clear lamplike rays.
+Her color is silvery or golden, according to the state of the
+atmosphere, while the distinction of her appearance in a dark sky is so
+great that no eye can resist its attraction, and I have known an
+unexpected glimpse of Venus to put an end to an animated conversation
+and distract, for a long time, the attention of a party of ladies and
+gentlemen from the social occupation that had brought them together.
+
+As a telescopic object Venus is exceedingly attractive, even when
+considered merely from the point of view of simple beauty. Both Mercury
+and Venus, as they travel about the sun, exhibit phases like those of
+the moon, but Venus, being much larger and much nearer to the earth than
+Mercury, shows her successive phases more effectively, and when she
+shines as a thin crescent in the morning or evening twilight, only a
+very slight magnifying power is required to show the sickle form of her
+disk.
+
+A remarkable difference between Venus and Mercury comes out as soon as
+we examine the shape of the former's orbit. Venus's mean distance from
+the sun is 67,200,000 miles, and her orbit is so nearly a circle, much
+more nearly than that of any other planet, that in the course of a
+revolution her distance from the sun varies less than a million miles.
+The distance of the earth varies 3,000,000 miles, and that of Mercury
+14,000,000. Her period of revolution, or the length of her year, is 225
+of our days. When she comes between the sun and the earth she approaches
+us nearer than any other planet ever gets, except the asteroid Eros, her
+distance at such times being 26,000,000 miles, or about one hundred and
+ten times the distance of the moon.
+
+Being nearer to the sun in the ratio of 67 to 93, Venus receives almost
+twice as much solar light and heat as we get, but less than one third as
+much as Mercury gets. There is reason to believe that her axis, instead
+of being considerably inclined, like that of the earth, is perpendicular
+to the plane of her orbit. Thus Venus introduces to us another novelty
+in the economy of worlds, for with a perpendicular axis of rotation she
+can have no succession of seasons, no winter and summer flitting, one
+upon the other's heels, to and fro between the northern and southern
+hemispheres; but, on the contrary, her climatic conditions must be
+unchangeable, and, on any particular part of her surface, except for
+local causes of variation, the weather remains the same the year around.
+So, as far as temperature is concerned, Venus may have two regions of
+perpetual winter, one around each pole; two belts of perpetual spring in
+the upper middle latitudes, one on each side of the equator; and one
+zone of perpetual summer occupying the equatorial portion of the planet.
+But, of course, these seasonal terms do not strictly apply to Venus, in
+the sense in which we employ them on the earth, for with us spring is
+characterized rather by the change in the quantity of heat and other
+atmospheric conditions that it witnesses than by a certain fixed and
+invariable temperature.
+
+To some minds it may appear very undesirable, from the point of view of
+animate existences, that there should be no alternation of seasons on
+the surface of a planet, but, instead, fixed conditions of climate; yet
+it is not clear that such a state of affairs might not be preferable to
+that with which we are familiar. Even on the earth, we find that
+tropical regions, where the seasonal changes are comparatively moderate,
+present many attractions and advantages in contrast with the violent and
+often destructive vicissitudes of the temperate zones, and nature has
+shown us, within the pale of our own planet, that she is capable of
+bringing forth harvests of fruit and grain without the stimulus of
+alternate frost and sunshine.
+
+Even under the reign of perpetual summer the fields and trees find time
+and opportunity to rest and restore their productive forces.
+
+The circularity of Venus's orbit, and the consequently insignificant
+change in the sun's distance and heating effect, are other elements to
+be considered in estimating the singular constancy in the operation of
+natural agencies upon that interesting planet, which, twin of the earth
+though it be in stature, is evidently not its twin in temperament.
+
+And next as to the all-important question of atmosphere. In what
+precedes, the presence of an atmosphere has been assumed, and,
+fortunately, there is very convincing evidence, both visual and
+spectroscopic, that Venus is well and abundantly supplied with air, by
+which it is not meant that Venus's air is precisely like the mixture of
+oxygen and nitrogen, with a few other gases, which we breathe and call
+by that name. In fact, there are excellent reasons for thinking that the
+atmosphere of Venus differs from the earth's quite as much as some of
+her other characteristics differ from those of our planet. But, however
+it may vary from ours in constitution, the atmosphere of Venus contains
+water vapor, and is exceedingly abundant. Listen to Professor Young:
+
+"Its [Venus's] atmosphere is probably from one and a half to two times
+as extensive and as dense as our own, and the spectroscope shows
+evidence of the presence of water vapor in it."
+
+And Prof. William C. Pickering, basing his statement on the result of
+observations at the mountain observatory of Arequipa, says: "We may feel
+reasonably certain that at the planet's [Venus's] surface the density of
+its atmosphere is many times that of our own."
+
+We do not have to depend upon the spectroscope for evidence that Venus
+has a dense atmosphere, for we can, in a manner, _see_ her atmosphere,
+in consequence of its refractive action upon the sunlight that strikes
+into it near the edge of the planet's globe. This illumination of
+Venus's atmosphere is witnessed both when she is nearly between the sun
+and the earth, and when, being exactly between them, she appears in
+silhouette against the solar disk. During a transit of this kind, in
+1882, many observers, and the present writer was one, saw a bright
+atmospheric bow edging a part of the circumference of Venus when the
+planet was moving upon the face of the sun--a most beautiful and
+impressive spectacle.
+
+Even more curious is an observation made in 1866 by Prof. C.S. Lyman, of
+Yale College, who, when Venus was very near the sun, saw her atmosphere
+_in the form of a luminous ring_. A little fuller explanation of this
+appearance may be of interest.
+
+When approaching inferior conjunction--i.e., passing between the earth
+and sun--Venus appears, with a telescope, in the shape of a very thin
+crescent. Professor Lyman watched this crescent, becoming narrower day
+after day as it approached the sun, and noticed that its extremities
+gradually extended themselves beyond the limits of a semicircle, bending
+to meet one another on the opposite side of the invisible disk of the
+planet, until, at length, they did meet, and he beheld a complete ring
+of silvery light, all that remained visible of the planet Venus! The
+ring was, of course, the illuminated atmosphere of the planet refracting
+the sunlight on all sides around the opaque globe.
+
+In 1874 M. Flammarion witnessed the same phenomenon in similar
+circumstances. One may well envy those who have had the good fortune to
+behold this spectacle--to actually see, as it were, the air that the
+inhabitants of another world are breathing and making resonant with all
+the multitudinous sounds and voices that accompany intelligent life. But
+perhaps some readers will prefer to think that even though an atmosphere
+is there, there is no one to breathe it.
+
+[Illustration: VENUS'S ATMOSPHERE SEEN AS A RING OF LIGHT.]
+
+As the visibility of Venus's atmosphere is unparalleled elsewhere in the
+solar system, it may be worth while to give a graphic illustration of
+it. In the accompanying figure the planet is represented at three
+successive points in its advance toward inferior conjunction. As it
+approaches conjunction it slowly draws nearer the earth, and its
+apparent diameter consequently increases. At _A_ a large part of the
+luminous crescent is composed of the planet's surface reflecting the
+sunshine; at _B_ the ratio of the reflecting surface to the illuminated
+atmosphere has diminished, and the latter has extended, like the curved
+arms of a pair of calipers, far around the unilluminated side of the
+disk; at _C_ the atmosphere is illuminated all around by the sunlight
+coming through it from behind, while the surface of the planet has
+passed entirely out of the light--that is to say, Venus has become an
+invisible globe embraced by a circle of refracted sunshine.
+
+We return to the question of life. With almost twice as much solar heat
+and light as we have, and with a deeper and denser atmosphere than ours,
+it is evident, without seeking other causes of variation, that the
+conditions of life upon Venus are notably different from those with
+which we are acquainted. At first sight it would seem that a dense
+atmosphere, together with a more copious supply of heat, might render
+the surface temperature of Venus unsuitable for organic life as we
+understand it. But so much depends upon the precise composition of the
+atmosphere and upon the relative quantities of its constituents, that it
+will not do to pronounce a positive judgment in such a case, because we
+lack information on too many essential points.
+
+Experiment has shown that the temperature of the air varies with changes
+in the amount of carbonic acid and of water vapor that it contains. It
+has been suggested that in past geologic ages the earth's atmosphere was
+denser and more heavily charged with vapors than it is at present; yet
+even then forms of life suited to their environment existed, and from
+those forms the present inhabitants of our globe have been developed.
+There are several lines of reasoning which may be followed to the
+conclusion that Venus, as a life-bearing world, is younger than the
+earth, and, according to that view, we are at liberty to imagine our
+beautiful sister planet as now passing through some such period in its
+history as that at which the earth had arrived in the age of the
+carboniferous forests, or the age of the gigantic reptiles who ruled
+both land and sea.
+
+But, without making any assumptions as to the phase of evolution which
+life may have attained on Venus, it is also possible to think that the
+planet's thick shell of air, with its abundant vapors, may serve as a
+shield against the excessive solar radiation. Venus is extraordinarily
+brilliant, its reflective power being greatly in excess of Mercury's,
+and it has often been suggested that this may be due to the fact that a
+large share of the sunlight falling upon it is turned back before
+reaching the planet's surface, being reflected both from the atmosphere
+itself and from vast layers of clouds.
+
+Even when viewed with the most powerful telescopes and in the most
+favoring circumstances, the features of Venus's surface are difficult
+to see, and generally extremely difficult. They consist of faint shadowy
+markings, indefinite in outline, and so close to the limit of visibility
+that great uncertainty exists not only as to their shape and their
+precise location upon the planet, but even as to their actual existence.
+No two observers have represented them exactly alike in drawings of the
+planet, and, unfortunately, photography is as yet utterly unable to deal
+with them. Mr. Percival Lowell, in his special studies of Venus in 1896,
+using a 24-inch telescope of great excellence, in the clear and steady
+air of Arizona, found delicate spokelike streaks radiating from a
+rounded spot like a hub, and all of which, in his opinion, were genuine
+and definite markings on the planet's surface. But others, using larger
+telescopes, have failed to perceive the shapes and details depicted by
+Mr. Lowell, and some are disposed to ascribe their appearances to
+Venus's atmosphere. Mr. Lowell himself noticed that the markings seemed
+to have a kind of obscuring veil over them.
+
+In short, all observers of Venus agree in thinking that her atmosphere,
+to a greater or less extent, serves as a mask to conceal her real
+features, and the possibilities of so extensive an atmosphere with
+reference to an adjustment of the peculiar conditions of the planet to
+the requirements of life upon it, are almost unlimited. If we could
+accurately analyze that atmosphere we would have a basis for more exact
+conclusions concerning Venus's habitability.
+
+But the mere existence of the atmosphere is, in itself, a strong
+argument for the habitability of the planet, and as to the temperature,
+we are really not compelled to imagine special adaptations by means of
+which it may be brought into accord with that prevailing upon the earth.
+As long as the temperature does not rise to the _destructive_ point,
+beyond which our experience teaches that no organic life can exist, it
+may very well attain an elevation that would mean extreme discomfort
+from our point of view, without precluding the existence of life even in
+its terrestrial sense.
+
+And would it not be unreasonable to assume that vital phenomena on other
+planets must be subject to exactly the same limitations that we find
+circumscribing them in our world? That kind of assumption has more than
+once led us far astray even in dealing with terrestrial conditions.
+
+It is not so long ago, for instance, since life in the depths of the sea
+was deemed to be demonstrably impossible. The bottom of the ocean, we
+were assured, was a region of eternal darkness and of frightful
+pressure, wherein no living creatures could exist. Yet the first dip of
+the deep-sea trawl brought up animals of marvelous delicacy of
+organization, which, although curiously and wonderfully adapted to live
+in a compressed liquid, collapsed when lifted into a lighter medium, and
+which, despite the assumed perpetual darkness of their profound abode,
+were adorned with variegated colors and furnished with organs of
+phosphorescence whereby they could create for themselves all the light
+they needed.
+
+Even the fixed animals of the sea, growing, like plants, fast to the
+rocks, are frequently vivid with living light, and there is a splendid
+suggestion of nature's powers of adaptation, which may not be entirely
+inapplicable to the problems of life on strange planets, in Alexander
+Agassiz's statement that species of sea animals, living below the depths
+to which sunlight penetrates, "may dwell in total darkness and be
+illuminated at times merely by the movements of abyssal fishes through
+the forests of phosphorescent alcyonarians."
+
+In attempting to judge the habitability of a planet such as Venus we
+must first, as far as possible, generalize the conditions that govern
+life and restrict its boundaries.
+
+On the earth we find animated existence confined to the surface of the
+crust of the globe, to the lower and denser strata of the atmosphere,
+and to the film of water that constitutes the oceans. It does not exist
+in the heart of the rocks forming the body of the planet nor in the void
+of space surrounding it outside the atmosphere. As the earth condensed
+from the original nebula, and cooled and solidified, a certain quantity
+of matter remained at its surface in the form of free gases and unstable
+compounds, and, within the narrow precincts where these things were,
+lying like a thin shell between the huge inert globe of permanently
+combined elements below, and the equally unchanging realm of the ether
+above, life, a phenomenon depending upon ceaseless changes, combinations
+and recombinations of chemical elements in unstable and temporary union,
+made its appearance, and there only we find it at the present time.
+
+It is because air and water furnish the means for the continual
+transformations by which the bodies of animals and plants are built up
+and afterward disintegrated and dispersed, that we are compelled to
+regard their presence as prerequisites to the existence, on any planet,
+of life in any of the forms in which we are acquainted with it. But if
+we perceive that another world has an atmosphere, and that there is
+water vapor in its atmosphere--both of which conditions are fulfilled by
+Venus--and if we find that that world is bathed in the same sunshine
+that stimulates the living forces of our planet, even though its
+quantity or intensity may be different, then it would seem that we are
+justified in averring that the burden of proof rests upon those who
+would deny the capability of such a world to support inhabitants.
+
+The generally accepted hypothesis of the origin of the solar system
+leads us to believe that Venus has experienced the same process of
+evolution as that which brought the earth into its present condition,
+and we may fairly argue that upon the rocky shell of Venus exists a
+region where chemical combinations and recombinations like those on the
+surface of the earth are taking place. It is surely not essential that
+the life-forming elements should exist in exactly the same states and
+proportions as upon the earth; it is enough if some of them are
+manifestly present. Even on the earth these things have undergone much
+variation in the course of geological history, coincidently with the
+development of various species of life. Just at present the earth
+appears to have reached a stage where everything contributes to the
+maintenance of a very high organization in both the animal and vegetable
+kingdoms.
+
+So each planet that has attained the habitable stage may have a typical
+adjustment of temperature and atmospheric constitution, rendering life
+possible within certain limits peculiar to that planet, and to the
+special conditions prevailing there. Admitting, as there is reason for
+doing, that different planets may be at different stages of development
+in the geological and biological sense, we should, of course, not expect
+to find them inhabited by the same living species. And, since there is
+also reason to believe that no two planets upon arriving at the same
+stage of evolution as globes would possess identical gaseous
+surroundings, there would naturally be differences between their organic
+life forms notwithstanding the similarity of their common phase of
+development in other respects. Thus a departure from the terrestrial
+type in the envelope of gases covering a planet, instead of precluding
+life, would only tend to vary its manifestations.
+
+After all, why should the intensity of the solar radiation upon Venus be
+regarded as inimical to life? The sunbeams awaken life.
+
+It is not impossible that relative nearness to the sun may be an
+advantage to Venus from the biologic point of view. She gets less than
+one third as much heat as Mercury receives on the average, and she gets
+it with almost absolute uniformity. At aphelion Mercury is about two and
+four tenths times hotter than Venus; then it rushes sunward, and within
+forty-four days becomes six times hotter than Venus. In the meantime the
+temperature of the latter, while high as compared with the earth's,
+remains practically unchanged. Not only may Mercury's temperature reach
+the destructive point, and thus be too high for organic life, but
+Mercury gets nothing with either moderation or constancy. It is a world
+both of excessive heat and of violent contrasts of temperature. Venus,
+on the other hand, presents an unparalleled instance of invariableness
+and uniformity. She may well be called the favorite of the sun, and,
+through the advantages of her situation, may be stimulated by him to
+more intense vitality than falls to the lot of the earth.
+
+It is open, at least to the writers of the interplanetary romances now
+so popular, to imagine that on Venus, life, while encompassed with the
+serenity that results from the circular form of her orbit, and the
+unchangeableness of her climates, is richer, warmer, more passionate,
+more exquisite in its forms and more fascinating in its experiences,
+keener of sense, capable of more delicious joys, than is possible to it
+amid the manifold inclemencies of the colder earth.
+
+We have seen that there is excellent authority for saying that Venus's
+atmosphere is from one and a half to two times as dense and as extensive
+as ours. Here is an interesting suggestion of aerial possibilities for
+her inhabitants. If man could but fly, how would he take to himself
+wings and widen his horizons along with the birds! Give him an
+atmosphere the double in density of that which now envelopes him, take
+off a little of his weight, thereby increasing the ratio of his strength
+and activity, put into his nervous system a more puissant stimulus from
+the life-giving sun, and perchance he _would_ fly.
+
+Well, on Venus, apparently, these very conditions actually exist. How,
+then, do intellectual creatures in the world of Venus take wing when
+they choose? Upon what spectacle of fluttering pinions afloat in
+iridescent air, like a Raphael dream of heaven and its angels, might we
+not look down if we could get near enough to our brilliant evening star
+to behold the intimate splendors of its life?
+
+As Venus herself would be the most brilliant member of the celestial
+host to an observer stationed on the night side of Mercury, so the earth
+takes precedence in the midnight sky of Venus. For the inhabitants of
+Venus Mercury is a splendid evening and morning star only, while the
+earth, being an outer planet, is visible at times in that part of the
+sky which is directly opposite to the place of the sun. The light
+reflected from our planet is probably less dazzling than that which
+Venus sends to us, both because, at our greater distance, the sunlight
+is less intense, and because our rarer atmosphere reflects a smaller
+proportion of the rays incident upon it. But the earth is, after all, a
+more brilliant phenomenon seen from Venus than the latter is seen from
+the earth, for the reason that the entire illuminated disk of the earth
+is presented toward our sister planet when the two are at their nearest
+point of approach, whereas, at that time, the larger part of the surface
+of Venus that is turned earthward has no illumination, while the
+illuminated portion is a mere crescent.
+
+Owing, again, to the comparative rarity of the terrestrial atmosphere,
+it is probable that the inhabitants of Venus--assuming their
+existence--enjoy a superb view of the continents, oceans, polar snows,
+and passing clouds that color and variegate the face of the earth. Our
+astronomers can study the full disk of Venus only when she is at her
+greatest distance, and on the opposite side of the sun from us, where
+she is half concealed in the glare. The astronomers of Venus, on the
+other hand, can study the earth under the most favorable conditions of
+observation--that is to say, when it is nearest to them and when, being
+in opposition to the sun, its whole disk is fully illuminated. In fact,
+there is no planet in the entire system which enjoys an outlook toward a
+sister world comparable with that which Venus enjoys with regard to the
+earth. If there be astronomers upon Venus, armed with telescopes, it is
+safe to guess that they possess a knowledge of the surface of the earth
+far exceeding in minuteness and accuracy the knowledge that we possess
+of the features of any heavenly body except the moon. They must long ago
+have been able to form definite conclusions concerning the meteorology
+and the probable habitability of our planet.
+
+It certainly tends to increase our interest in Venus when, granting
+that she is inhabited, we reflect upon the penetrating scrutiny of which
+the earth may be the object whenever Venus--as happens once every 584
+days--passes between us and the sun. The spectacle of our great planet,
+glowing in its fullest splendor in the midnight sky, pied and streaked
+with water, land, cloud, and snow, is one that might well excite among
+the astronomers of another world, so fortunately placed to observe it,
+an interest even greater than that which the recurrence of total solar
+eclipses occasions upon the earth. For the inhabitants of Venus the
+study of the earth must be the most absorbing branch of observational
+astronomy, and the subject, we may imagine, of numberless volumes of
+learned memoirs, far exceeding in the definiteness of their conclusions
+the books that we have written about the physical characteristics of
+other members of the solar system. And, if we are to look for attempts
+on the part of the inhabitants of other worlds to communicate with us by
+signals across the ether, it would certainly seem that Venus is the
+most likely source of such efforts, for from no other planet can those
+features of the earth that give evidence of its habitability be so
+clearly discerned. Of one thing it would seem we may be certain: if
+Venus has intellectual inhabitants they possess far more convincing
+evidence of our existence than we are likely ever to have of theirs.
+
+In referring to the view of the earth from Mercury it was remarked that
+the moon is probably visible to the naked eye. From Venus the moon is
+not only visible, but conspicuous, to the naked eye, circling about the
+earth, and appearing at times to recede from it to a distance of about
+half a degree--equal to the diameter of the full moon as we see it. The
+disk of the earth is not quite four times greater in diameter than that
+of the moon, and nowhere else in the solar system is there an instance
+in which two bodies, no more widely different in size than are the moon
+and the earth, are closely linked together. The moons of the other
+planets that possess satellites are relatively so small that they
+appear in the telescope as mere specks beside their primaries, but the
+moon is so large as compared with the earth that the two must appear, as
+viewed from Venus, like a double planet. To the naked eye they may look
+like a very wide and brilliant double star, probably of contrasted
+colors, the moon being silvery white and the earth, perhaps, now of a
+golden or reddish tinge and now green or blue, according to the part of
+its surface turned toward Venus, and according, also, to the season that
+chances to be reigning over that part.
+
+Such a spectacle could not fail to be of absorbing interest, and we can
+not admit the possibility of intelligent inhabitants on Venus without
+supposing them to watch the motions of the moon and the earth with the
+utmost intentness. The passage of the moon behind and in front of the
+earth, and its eclipses when it goes into the earth's shadow, could be
+seen without the aid of telescopes, while, with such instruments, these
+phenomena would possess the highest scientific interest and importance.
+
+Because the earth has a satellite so easily observable, the astronomers
+of Venus could not remain ignorant of the exact mass of our planet, and
+in that respect they would outstrip us in the race for knowledge, since,
+on account of the lack of a satellite attending Venus, we have been able
+to do no more than make an approximate estimate of her mass.
+
+With telescopes, too, in the case of a solar eclipse occurring at the
+time of the earth's opposition, they could see the black spot formed by
+the shadow of the moon, where the end of its cone moved across the earth
+like the point of an invisible pencil, and could watch it traversing
+continents and oceans, or thrown out in bold contrast upon the white
+background of a great area of clouds. Indeed, the phenomena which our
+globe and its satellite present to Venus must be so varied and wonderful
+that one might well wish to visit that planet merely for the sake of
+beholding them.
+
+Thus far we have found so much of brilliant promise in the earth's twin
+sister that I almost hesitate to approach another phase of the subject
+which may tend to weaken the faith of some readers in the habitability
+of Venus. It may have been observed that heretofore nothing has been
+said as to the planet's rotation period, but, without specifically
+mentioning it, I have tacitly assumed the correctness of the generally
+accepted period of about twenty-four hours, determined by De Vico and
+other observers. This period, closely accordant with the earth's, is, as
+far as it goes, another argument for the habitability of Venus.
+
+But now it must be stated that no less eminent an authority than
+Schiaparelli holds that Venus, as well as Mercury, makes but a single
+turn on its axis in the course of a revolution about the sun, and,
+consequently, is a two-faced world, one side staring eternally at the
+sun and the other side wearing the black mask of endless night.
+
+Schiaparelli made this announcement concerning Venus but a few weeks
+after publishing his discovery of Mercury's peculiar rotation. He
+himself appears to be equally confident in both cases of the
+correctness of his conclusions and the certainty of his observation. As
+with Mercury, several other observers have corroborated him, and
+particularly Percival Lowell in this country. Mr. Lowell, indeed, seems
+unwilling to admit that any doubt can be entertained. Nevertheless, very
+grave doubt is entertained, and that by many, and probably by the
+majority, of the leading professional astronomers and observers. In
+fact, some observers of great ability, equipped with powerful
+instruments, have directly contradicted the results of Schiaparelli and
+his supporters.
+
+The reader may ask: "Why so readily accept Schiaparelli's conclusions
+with regard to Mercury while rejecting them in the case of Venus?"
+
+The reply is twofold. In the first place the markings on Venus, although
+Mr. Lowell sketched them with perfect confidence in 1896, are, by the
+almost unanimous testimony of those who have searched for them with
+telescopes, both large and small, extremely difficult to see,
+indistinct in outline, and perhaps evanescent in character. The sketches
+of no two observers agree, and often they are remarkably unlike. The
+fact has already been mentioned that Mr. Lowell noticed a kind of veil
+partially obscuring the markings, and which he ascribed, no doubt
+correctly, to the planet's atmosphere. But he thinks that,
+notwithstanding the atmospheric veil, the markings noted by him were
+unquestionably permanent features of the planet's real surface.
+Inasmuch, however, as his drawings represent things entirely different
+from what others have seen, there seems to be weight in the suggestion
+that the radiating bands and shadings noticed by him were in some manner
+illusory, and perhaps of atmospheric origin.
+
+If the markings were evidently of a permanent nature and attached to the
+solid shell of the planet, and if they were of sufficient distinctness
+to be seen in substantially the same form by all observers armed with
+competent instruments, then whatever conclusion was drawn from their
+apparent motion as to the period of the planet's rotation would have to
+be accepted. In the case of Mercury the markings, while not easily seen,
+appear to be sufficiently distinct to afford confidence in the result of
+observations based upon them, but Venus's markings have been represented
+in so many different ways that it seems advisable to await more light
+before accepting any extraordinary, and in itself improbable, conclusion
+based upon them.
+
+It should also be added that in 1900 spectroscopic observations by
+Belopolski at Pulkova gave evidence that Venus really rotates rapidly on
+her axis, in a period probably approximating to the twenty-four hours of
+the earth's rotation, thus corroborating the older conclusions.
+
+Belopolski's observation, it may be remarked, was based upon what is
+known as the Doppler principle, which is employed in measuring the
+motion of stars in the line of sight, and in other cases of rapidly
+moving sources of light. According to this principle, when a source of
+light, either original or reflected, is approaching the observer, the
+characteristic lines in its spectrum are shifted toward the blue end,
+and when it is retreating from the observer the lines are shifted toward
+the red end. Now, in the case of a planet rotating rapidly on its axis,
+it is clear that if the observer is situated in, or nearly in, the plane
+of the planet's equator, one edge of its disk will be approaching his
+eye while the opposite edge is retreating, and the lines in the spectrum
+of a beam of light from the advancing edge will be shifted toward the
+blue, while those in the spectrum of the light coming from the
+retreating edge will be shifted toward the red. And, by carefully noting
+the amount of the shifting, the velocity of the planet's rotation can be
+computed. This is what was done by Belopolski in the case of Venus, with
+the result above noted.
+
+Secondly, the theory that Venus rotates but once in the course of a
+revolution finds but slight support from the doctrine of tidal friction,
+as compared with that which it receives when applied to Mercury. The
+effectiveness of the sun's attraction in slowing down the rotation of a
+planet through the braking action of the tides raised in the body of the
+planet while it is yet molten or plastic, varies inversely as the sixth
+power of the planet's distance. For Mercury this effectiveness is nearly
+three hundred times as great as it is for the earth, while for Venus it
+is only seven times as great. While we may admit, then, that Mercury,
+being relatively close to the sun and subject to an enormous braking
+action, lost rotation until--as occurred for a similar reason to the
+moon under the tidal attraction of the earth--it ended by keeping one
+face always toward its master, we are not prepared to make the same
+admission in the case of Venus, where the effective force concerned is
+comparatively so slight.
+
+It should be added, however, that no certain evidence of polar
+compression in the outline of Venus's disk has ever been obtained, and
+this fact would favor the theory of a very slow rotation because a
+plastic globe in swift rotation has its equatorial diameter increased
+and its polar diameter diminished. If Venus were as much flattened at
+the poles as the earth is, it would seem that the fact could not escape
+detection, yet the necessary observations are very difficult, and Venus
+is so brilliant that her light increases the difficulty, while her
+transits across the sun, when she can be seen as a round black disk, are
+very rare phenomena, the latest having occurred in 1874 and 1882, and
+the next not being due until 2004.
+
+Upon the whole, probably the best method of settling the question of
+Venus's rotation is the spectroscopic method, and that, as we saw, has
+already given evidence for the short period.
+
+Even if it were established that Venus keeps always the same face to the
+sun, it might not be necessary to abandon altogether the belief that she
+is habitable, although, of course, the obstacles to that belief would be
+increased. Venus's orbit being so nearly circular, and her orbital
+motion so nearly invariable, she has but a very slight libration with
+reference to the sun, and the east and west lunes on her surface, where
+day and night would alternate once in her year of 225 days, would be so
+narrow as to be practically negligible.
+
+But, owing to her extensive atmosphere, there would be a very broad band
+of twilight on Venus, running entirely around the planet at the inner
+edge of the light hemisphere. What the meteorological conditions within
+this zone would be is purely a matter of conjecture. As in the case of
+Mercury, we should expect an interchange of atmospheric currents between
+the light and dark sides of the planet, the heated air rising under the
+influence of the unsetting sun in one hemisphere, and being replaced by
+an indraught of cold air from the other. The twilight band would
+probably be the scene of atmospheric conflicts and storms, and of
+immense precipitation, if there were oceans on the light hemisphere to
+charge the air with moisture.
+
+It has been suggested that ice and snow might be piled in a vast circle
+of glaciers, belting the planet along the line between perpetual day
+and night, and that where the sunbeams touched these icy deposits near
+the edge of the light hemisphere a marvelous spectacle of prismatic
+hills of crystal would be presented!
+
+It may be remarked that it would be the inhabitants of the dark
+hemisphere who would enjoy the beautiful scene of the earth and the moon
+in opposition.
+
+
+
+
+CHAPTER IV
+
+MARS, A WORLD MORE ADVANCED THAN OURS
+
+
+Mars is the fourth planet in the order of distance from the sun, and the
+outermost member of the terrestrial group. Its mean distance is
+141,500,000 miles, variable, through the eccentricity of its orbit, to
+the extent of about 13,000,000 miles. It will be observed that this is
+only a million miles less than the variation in Mercury's distance from
+the sun, from which, in a previous chapter, were deduced most momentous
+consequences; but, in the case of Mars, the ratio of the variation to
+the mean distance is far smaller than with Mercury, so that the effect
+upon the temperature of the planet is relatively insignificant.
+
+Mars gets a little less than half as much solar light and heat as the
+earth receives, its situation in this respect being just the opposite
+to that of Venus. Its period of orbital revolution, or the length of its
+year, is 687 of our days. The diameter of Mars is 4,200 miles, and its
+density is 73 per cent of the earth's density. Gravity on its surface is
+only 38 per cent of terrestrial gravity--i.e., a one hundred-pound
+weight removed from the earth to Mars would there weigh but thirty-eight
+pounds. Mars evidently has an atmosphere, the details of which we shall
+discuss later.
+
+The poles of the planet are inclined from a perpendicular to the plane
+of its orbit at very nearly the same angle as that of the earth's poles,
+viz., 24 deg. 50 min. Its rotation on its axis is also effected in
+almost the same period as the earth's, viz., 24 hours, 37 minutes.
+
+When in opposition to the sun, Mars may be only about 35,000,000 miles
+from the earth, but its average distance when in that position is more
+than 48,000,000 miles, and may be more than 60,000,000. These
+differences arise from the eccentricities of the orbits of the two
+planets. When on the farther side of the sun--i.e., in conjunction with
+the sun as seen from the earth--Mars's average distance from us is about
+235,000,000 miles. In consequence of these great changes in its
+distance, Mars is sometimes a very conspicuous object in the sky, and at
+other times inconspicuous.
+
+The similarity in the inclination of the axis of the two planets results
+in a close resemblance between the seasons on Mars and on the earth,
+although, owing to the greater length of its year, Mars's seasons are
+much longer than ours. Winter and summer visit in succession its
+northern and southern hemispheres just as occurs on the planet that we
+inhabit, and the torrid, temperate, and frigid zones on its surface have
+nearly the same angular width as on the earth. In this respect Mars is
+the first of the foreign planets we have studied to resemble the earth.
+
+Around each of its poles appears a circular white patch, which visibly
+expands when winter prevails upon it, and rapidly contracts, sometimes
+almost completely disappearing, under a summer sun. From the time of
+Sir William Herschel the almost universal belief among astronomers has
+been that these gleaming polar patches on Mars are composed of snow and
+ice, like the similar glacial caps of the earth, and no one can look at
+them with a telescope and not feel the liveliest interest in the planet
+to which they belong, for they impart to it an appearance of likeness to
+our globe which at first glance is all but irresistible.
+
+To watch one of them apparently melting, becoming perceptibly smaller
+week after week, while the general surface of the corresponding
+hemisphere of the planet deepens in color, and displays a constantly
+increasing wealth of details as summer advances across it, is an
+experience of the most memorable kind, whose effect upon the mind of the
+observer is indescribable.
+
+Early in the history of the telescope it became known that, in addition
+to the polar caps, Mars presented a number of distinct surface features,
+and gradually, as instruments increased in power and observers in
+skill, charts of the planet were produced showing a surface diversified
+somewhat in the manner that characterizes the face of the earth,
+although the permanent forms do not closely resemble those of our
+planet.
+
+Two principal colors exist on the disk of Mars--dark, bluish gray or
+greenish gray, characterizing areas which have generally been regarded
+as seas, and light yellowish red, overspreading broad regions looked
+upon as continents. It was early observed that if the dark regions
+really are seas, the proportion of water to land upon Mars is much
+smaller than upon the earth.
+
+For two especial reasons Mars has generally been regarded as an older or
+more advanced planet than the earth. The first reason is that, accepting
+Laplace's theory of the origin of the planetary system from a series of
+rings left off at the periphery of the contracting solar nebula, Mars
+must have come into existence earlier than the earth, because, being
+more distant from the center of the system, the ring from which it was
+formed would have been separated sooner than the terrestrial ring. The
+second reason is that Mars being smaller and less massive than the earth
+has run through its developments a cooling globe more rapidly. The
+bearing of these things upon the problems of life on Mars will be
+considered hereafter.
+
+And now, once more, Schiaparelli appears as the discoverer of surprising
+facts about one of the most interesting worlds of the solar system.
+During the exceptionally favorable opposition of Mars in 1877, when an
+American astronomer, Asaph Hall, discovered the planet's two minute
+satellites, and again during the opposition of 1879, the Italian
+observer caught sight of an astonishing network of narrow dark lines
+intersecting the so-called continental regions of the planet and
+crossing one another in every direction. Schiaparelli did not see the
+little moons that Hall discovered, and Hall did not perceive the
+enigmatical lines that Schiaparelli detected. Hall had by far the larger
+and more powerful telescope; Schiaparelli had much the more steady and
+favorable atmosphere for astronomical observation. Yet these differences
+in equipment and circumstances do not clearly explain why each observer
+should have seen what the other did not.
+
+There may be a partial explanation in the fact that an observer having
+made a remarkable discovery is naturally inclined to confine his
+attention to it, to the neglect of other things. But it was soon found
+that Schiaparelli's lines--to which he gave the name "canals," merely on
+account of their shape and appearance, and without any intention to
+define their real nature--were excessively difficult telescopic objects.
+Eight or nine years elapsed before any other observer corroborated
+Schiaparelli's observations, and notwithstanding the "sensation" which
+the discovery of the canals produced they were for many years regarded
+by the majority of astronomers as an illusion.
+
+But they were no illusion, and in 1881 Schiaparelli added to the
+astonishment created by his original discovery, and furnished additional
+grounds for skepticism, by announcing that, at certain times, many of
+the canals geminated, or became double! He continued his observations at
+each subsequent opposition, adding to the number of the canals observed,
+and charting them with classical names upon a detailed map of the
+planet's surface.
+
+At length in 1886 Perrotin, at Nice, detected many of Schiaparelli's
+canals, and later they were seen by others. In 1888 Schiaparelli greatly
+extended his observations, and in 1892 and 1894 some of the canals were
+studied with the 36-inch telescope of the Lick Observatory, and in the
+last-named year a very elaborate series of observations upon them was
+made by Percival Lowell and his associates, Prof. William C. Pickering
+and Mr. A.E. Douglass, at Flagstaff, Arizona. Mr. Lowell's charts of the
+planet are the most complete yet produced, containing 184 canals to
+which separate names have been given, besides more than a hundred other
+markings also designated by individual appellations.
+
+It should not be inferred from the fact that Schiaparelli's discovery
+in 1877 excited so much surprise and incredulity that no glimpse of the
+peculiar canal-like markings on Mars had been obtained earlier than
+that. At least as long ago as 1864 Mr. Dawes, in England, had seen and
+sketched half a dozen of the larger canals, or at least the broader
+parts of them, especially where they connect with the dark regions known
+as seas, but Dawes did not see them in their full extent, did not
+recognize their peculiar character, and entirely failed to catch sight
+of the narrower and more numerous ones which constitute the wonderful
+network discovered by the Italian astronomer. Schiaparelli found no less
+than sixty canals during his first series of observations in 1877.
+
+Let us note some of the more striking facts about the canals which
+Schiaparelli has described. We can not do better than quote his own
+words:
+
+"There are on this planet, traversing the continents, long dark lines
+which may be designated as _canals_, although we do not yet know what
+they are. These lines run from one to another of the somber spots that
+are regarded as seas, and form, over the lighter, or continental,
+regions a well-defined network. Their arrangement appears to be
+invariable and permanent; at least, as far as I can judge from four and
+a half years of observation. Nevertheless, their aspect and their degree
+of visibility are not always the same, and depend upon circumstances
+which the present state of our knowledge does not yet permit us to
+explain with certainty. In 1879 a great number were seen which were not
+visible in 1877, and in 1882 all those which had been seen at former
+oppositions were found again, together with new ones. Sometimes these
+canals present themselves in the form of shadowy and vague lines, while
+on other occasions they are clear and precise, like a trace drawn with a
+pen. In general they are traced upon the sphere like the lines of great
+circles; a few show a sensible lateral curvature. They cross one another
+obliquely, or at right angles. They have a breadth of two degrees, or
+120 kilometres [74 miles], and several extend over a length of eighty
+degrees, or 4,800 kilometres [nearly 3,000 miles]. Their tint is very
+nearly the same as that of the seas, usually a little lighter. Every
+canal terminates at both its extremities in a sea, or in another canal;
+there is not a single example of one coming to an end in the midst of
+dry land.
+
+"This is not all. In certain seasons these canals become double. This
+phenomenon seems to appear at a determinate epoch, and to be produced
+simultaneously over the entire surface of the planet's continents. There
+was no indication of it in 1877, during the weeks that preceded and
+followed the summer solstice of that world. A single isolated case
+presented itself in 1879. On the 26th of December, this year--a little
+before the spring equinox, which occurred on Mars on the 21st of
+January, 1880--I noticed the doubling of the Nile [a canal thus named]
+between the Lakes of the Moon and the Ceraunic Gulf. These two regular,
+equal, and parallel lines caused me, I confess, a profound surprise,
+the more so because a few days earlier, on the 23d and the 24th of
+December, I had carefully observed that very region without discovering
+anything of the kind.
+
+"I awaited with curiosity the return of the planet in 1881, to see if an
+analogous phenomenon would present itself in the same place, and I saw
+the same thing reappear on the 11th of January, 1882, one month after
+the spring equinox--which occurred on the 8th of December, 1881. The
+duplication was still more evident at the end of February. On this same
+date, the 11th of January, another duplication had already taken place,
+that of the middle portion of the canal of the Cyclops, adjoining
+Elysium. [Elysium is a part of one of the continental areas.]
+
+"Yet greater was my astonishment when, on the 19th of January, I saw the
+canal Jamuna, which was then in the center of the disk, formed very
+rigidly of two parallel straight lines, crossing the space which
+separates the Niliac Lake from the Gulf of Aurora. At first sight I
+believed it was an illusion, caused by fatigue of the eye and some new
+kind of strabismus, but I had to yield to the evidence. After the 19th
+of January I simply passed from wonder to wonder; successively the
+Orontes, the Euphrates, the Phison, the Ganges, and the larger part of
+the other canals, displayed themselves very clearly and indisputably
+duplicated. There were not less than twenty examples of duplication, of
+which seventeen were observed in the space of a month, from the 19th of
+January to the 19th of February.
+
+"In certain cases it was possible to observe precursory symptoms which
+are not lacking in interest. Thus, on the 13th of January, a light,
+ill-defined shade extended alongside the Ganges; on the 18th and the
+19th one could only distinguish a series of white spots; on the 20th the
+shadow was still indecisive, but on the 21st the duplication was
+perfectly clear, such as I observed it until the 23d of February. The
+duplication of the Euphrates, of the canal of the Titans, and of the
+Pyriphlegethon also began in an uncertain and nebulous form.
+
+"These duplications are not an optical effect depending on increase of
+visual power, as happens in the observation of double stars, and it is
+not the canal itself splitting in two longitudinally. Here is what is
+seen: To the right or left of a pre-existing line, without any change in
+the course and position of that line, one sees another line produce
+itself, equal and parallel to the first, at a distance generally varying
+from six to twelve degrees--i.e., from 350 to 700 kilometres (217 to 434
+miles); even closer ones seem to be produced, but the telescope is not
+powerful enough to distinguish them with certainty. Their tint appears
+to be a quite deep reddish brown. The parallelism is sometimes
+rigorously exact. There is nothing analogous in terrestrial geography.
+Everything indicates that here there is an organization special to the
+planet Mars, probably connected with the course of its seasons."[1]
+
+[Footnote 1: L'Astronomie, vol. i, 1882, pp. 217 _et seq._]
+
+Schiaparelli adds that he took every precaution to avoid the least
+suspicion of illusion. "I am absolutely sure," he says, "of what I have
+observed."
+
+I have quoted his statement, especially about the duplication of the
+canals, at so much length, both on account of its intrinsic interest and
+because it has many times been argued that this particular phenomenon
+must be illusory even though the canals are real.
+
+One of the most significant facts that came out in the early
+observations was the evident connection between the appearance of the
+canals and the seasonal changes on Mars. It was about the time of the
+spring equinox, when the white polar caps had begun to melt, that
+Schiaparelli first noticed the phenomenon of duplication. As the season
+advanced the doubling of the canals increased in frequency and the lines
+became more distinct. In the meantime the polar caps were becoming
+smaller. Broadly speaking, Schiaparelli's observation showed that the
+doubling of the canals occurred principally a little after the spring
+equinox and a little before the autumn equinox; that the phenomenon
+disappeared in large part at the epoch of the winter solstice, and
+disappeared altogether at the epoch of the summer solstice. Moreover, he
+observed that many of the canals, without regard to duplication, were
+invisible at times, and reappeared gradually; faint, scarcely visible
+lines and shadows, deepened and became more distinct until they were
+clearly and sharply defined, and these changes, likewise, were evidently
+seasonal.
+
+The invariable connection of the canals at their terminations with the
+regions called seas, the fact that as the polar caps disappeared the
+sealike expanses surrounding the polar regions deepened in color, and
+other similar considerations soon led to the suggestion that there
+existed on Mars a wonderful system of water circulation, whereby the
+melting of the polar snows, as summer passed alternately from one
+hemisphere to the other, served to reenforce the supply of water in the
+seas, and, through the seas, in the canals traversing the broad
+expanses of dry land that occupy the equatorial regions of the planet.
+The thought naturally occurred that the canals might be of artificial
+origin, and might indicate the existence of a gigantic system of
+irrigation serving to maintain life upon the globe of Mars. The
+geometrical perfection of the lines, their straightness, their absolute
+parallelism when doubled, their remarkable tendency to radiate from
+definite centers, lent strength to the hypothesis of an artificial
+origin. But their enormous size, length, and number tended to stagger
+belief in the ability of the inhabitants of any world to achieve a work
+so stupendous.
+
+After a time a change of view occurred concerning the nature of the
+expanses called seas, and Mr. Lowell, following his observations of
+1894, developed the theory of the water circulation and irrigation of
+Mars in a new form. He and others observed that occasionally canals were
+visible cutting straight across some of the greenish, or bluish-gray,
+areas that had been regarded as seas. This fact suggested that, instead
+of seas, these dark expanses may rather be areas of marshy ground
+covered with vegetation which flourishes and dies away according as the
+supply of water alternately increases and diminishes, while the reddish
+areas known as continents are barren deserts, intersected by canals; and
+as the water released by the melting of the polar snows begins to fill
+the canals, vegetation springs up along their sides and becomes visible
+in the form of long narrow bands.
+
+According to this theory, the phenomena called canals are simply lines
+of vegetation, the real canals being individually too small to be
+detected. It may be supposed that from a central supply canal irrigation
+ditches are extended for a distance of twenty or thirty miles on each
+side, thus producing a strip of fertile soil from forty to sixty miles
+wide, and hundreds, or in some cases two or three thousands, of miles in
+length.
+
+The water supply being limited, the inhabitants can not undertake to
+irrigate the entire surface of the thirsty land, and convenience of
+circulation induces them to extend the irrigated areas in the form of
+long lines. The surface of Mars, according to Lowell's observation, is
+remarkably flat and level, so that no serious obstacle exists to the
+extension of the canal system in straight bands as undeviating as arcs
+of great circles.
+
+Wherever two or more canals meet, or cross, a rounded dark spot from a
+hundred miles, or less, to three hundred miles in diameter, is seen. An
+astonishing number of these appear on Mr. Lowell's charts. Occasionally,
+as occurs at the singular spot named Lacus Solis, several canals
+converging from all points of the compass meet at a central point like
+the spokes of a wheel; in other cases, as, for instance, that of the
+long canal named Eumenides, with its continuation Orcus, a single
+conspicuous line is seen threading a large number of round dark spots,
+which present the appearance of a row of beads upon a string. These
+circular spots, which some have regarded as lakes, Mr. Lowell believes
+are rather oases in the great deserts, and granting the correctness of
+his theory of the canals the aptness of this designation is apparent.[2]
+
+[Footnote 2: The reader can find many of these "canals" and "oases," as
+well as some of the other regions on Mars that have received names, in
+the frontispiece.]
+
+Wherever several canals, that is to say, several bands of vegetation or
+bands of life, meet, it is reasonable to assume that an irrigated and
+habitable area of considerable extent will be developed, and in such
+places the imagination may picture the location of the chief centers of
+population, perhaps in the form of large cities, or perhaps in groups of
+smaller towns and villages. The so-called Lacus Solis is one of these
+localities.
+
+So, likewise, it seems but natural that along the course of a broad,
+well-irrigated band a number of expansions should occur, driving back
+the bounds of the desert, forming rounded areas of vegetation, and thus
+affording a footing for population. Wherever two bands cross such areas
+would be sure to exist, and in almost every instance of crossing the
+telescope actually shows them.
+
+As to the gemination or duplication of many of the lines which, at the
+beginning of the season, appear single, it may be suggested that, in the
+course of the development of the vast irrigation system of the planet
+parallel bands of cultivation have been established, one receiving its
+water supply from the canals of the other, and consequently lagging a
+little behind in visibility as the water slowly percolates through the
+soil and awakens the vegetation. Or else, the character of the
+vegetation itself may differ as between two such parallel bands, one
+being supplied with plants that spring up and mature quickly when the
+soil about their roots is moistened, while the plants in the twin band
+respond more slowly to stimulation.
+
+Objection has been made to the theory of the artificial origin of the
+canals of Mars on the ground, already mentioned, that the work required
+to construct them would be beyond the capacity of any race of creatures
+resembling man. The reply that has been made to this is twofold. In the
+first place, it should be remembered that the theory, as Mr. Lowell
+presents it, does not assert that the visible lines are the actual
+canals, but only that they are strips of territory intersected, like
+Holland or the center of the plain of Lombardy, by innumerable
+irrigation canals and ditches. To construct such works is clearly not an
+impossible undertaking, although it does imply great industry and
+concentration of effort.
+
+In the second place, since the force of gravity on Mars is in the ratio
+of only 38 to 100 compared with the earth's, it is evident that the
+diminished weight of all bodies to be handled would give the inhabitants
+of Mars an advantage over those of the earth in the performance of
+manual labor, provided that they possess physical strength and activity
+as great as ours. But, in consequence of this very fact of the slighter
+force of gravity, a man upon Mars could attain a much greater size, and
+consequently much greater muscular strength, than his fellows upon the
+earth possess without being oppressed by his own weight. In other words,
+as far as the force of gravity may be considered as the decisive factor,
+Mars could be inhabited by giants fifteen feet tall, who would be
+relatively just as active, and just as little impeded in their movements
+by the weight of their bodies, as a six-footer is upon the earth. But
+they would possess far more physical strength than we do, while, in
+doing work, they would have much lighter materials to deal with.
+
+Whether the theory that the canals of Mars really are canals is true or
+not, at any rate there can now be no doubt as to the existence of the
+strange lines which bear that designation. The suggestion has been
+offered that their builders may no longer be in existence, Mars having
+already passed the point in its history where life must cease upon its
+surface. This brings us to consider again the statement, made near the
+beginning of this chapter, that Mars is, perhaps, at a more advanced
+stage of development than the earth. If we accept this view, then,
+provided there was originally some resemblance between Mars's life forms
+and those of the earth, the inhabitants of that planet would, at every
+step, probably be in front of their terrestrial rivals, so that at the
+present time they should stand well in advance. Mr. Lowell has, perhaps,
+put this view of the relative advancement in evolution of Mars and its
+inhabitants as picturesquely as anybody.
+
+"In Mars," he says, "we have before us the spectacle of a world
+relatively well on in years, a world much older than the earth. To so
+much about his age Mars bears witness on his face. He shows unmistakable
+signs of being old. Advancing planetary years have left their mark
+legible there. His continents are all smoothed down; his oceans have all
+dried up.... Mars being thus old himself, we know that evolution on his
+surface must be similarly advanced. This only informs us of its
+condition relative to the planet's capabilities. Of its actual state our
+data are not definite enough to furnish much deduction. But from the
+fact that our own development has been comparatively a recent thing, and
+that a long time would be needed to bring even Mars to his present
+geological condition, we may judge any life he may support to be not
+only relatively, but really older than our own. From the little we can
+see such appears to be the case. The evidence of handicraft, if such it
+be, points to a highly intelligent mind behind it. Irrigation,
+unscientifically conducted, would not give us such truly wonderful
+mathematical fitness in the several parts to the whole as we there
+behold.... Quite possibly such Martian folk are possessed of inventions
+of which we have not dreamed, and with them electrophones and
+kinetoscopes are things of a bygone past, preserved with veneration in
+museums as relics of the clumsy contrivances of the simple childhood of
+the race. Certainly what we see hints at the existence of beings who are
+in advance of, not behind us, in the journey of life."[3]
+
+[Footnote 3: Mars, by Percival Lowell, p. 207 _et seq._]
+
+Granted the existence of such a race as is thus described, and to them
+it might not seem a too appalling enterprise, when their planet had
+become decrepit, with its atmosphere thinned out and its supply of water
+depleted, to grapple with the destroying hand of nature and to prolong
+the career of their world by feats of chemistry and engineering as yet
+beyond the compass of human knowledge.
+
+It is confidence, bred from considerations like these, in the superhuman
+powers of the supposed inhabitants of Mars that has led to the popular
+idea that they are trying to communicate by signals with the earth.
+Certain enigmatical spots of light, seen at the edge of the illuminated
+disk of Mars, and projecting into the unilluminated part--for Mars,
+although an outer planet, shows at particular times a gibbous phase
+resembling that of the moon just before or just after the period of full
+moon--have been interpreted by some, but without any scientific
+evidence, as of artificial origin.
+
+Upon the assumption that these bright points, and others occasionally
+seen elsewhere on the planet's disk, are intended by the Martians for
+signals to the earth, entertaining calculations have been made as to the
+quantity of light that would be required in the form of a "flash signal"
+to be visible across the distance separating the two planets. The
+results of the calculations have hardly been encouraging to possible
+investors in interplanetary telegraphy, since it appears that
+heliographic mirrors with reflecting surfaces measured by square miles,
+instead of square inches, would be required to send a visible beam from
+the earth to Mars or _vice versa_.
+
+The projections of light on Mars can be explained much more simply and
+reasonably. Various suggestions have been made about them; among others,
+that they are masses of cloud reflecting the sunshine; that they are
+areas of snow; and that they are the summits of mountains crowned with
+ice and encircled with clouds. In fact, a huge mountain mass lying on
+the terminator, or the line between day and night, would produce the
+effect of a tongue of light projecting into the darkness without
+assuming that it was snow-covered or capped with clouds, as any one may
+convince himself by studying the moon with a telescope when the
+terminator lies across some of its most mountainous regions. To be sure,
+there is reason to think that the surface of Mars is remarkably flat;
+yet even so the planet may have some mountains, and on a globe the
+greater part of whose shell is smooth any projections would be
+conspicuous, particularly where the sunlight fell at a low angle across
+them.
+
+Another form in which the suggestion of interplanetary communication has
+been urged is plainly an outgrowth of the invention and surprising
+developments of wireless telegraphy. The human mind is so constituted
+that whenever it obtains any new glimpse into the arcana of nature it
+immediately imagines an indefinite and all but unlimited extension of
+its view in that direction. So to many it has not appeared unreasonable
+to assume that, since it is possible to transmit electric impulses for
+considerable distances over the earth's surface by the simple
+propagation of a series of waves, or undulations, without connecting
+wires, it may also be possible to send such impulses through the ether
+from planet to planet.
+
+The fact that the electric undulations employed in wireless telegraphy
+pass between stations connected by the crust of the earth itself, and
+immersed in a common atmospheric envelope, is not deemed by the
+supporters of the theory in question as a very serious objection, for,
+they contend, electric waves are a phenomenon of the ether, which
+extends throughout space, and, given sufficient energy, such waves could
+cross the gap between world and world.
+
+But nobody has shown how much energy would be needed for such a purpose,
+and much less has anybody indicated a way in which the required energy
+could be artificially developed, or cunningly filched from the stores of
+nature. It is, then, purely an assumption, an interesting figment of
+the mind, that certain curious disturbances in the electrical state of
+the air and the earth, affecting delicate electric instruments,
+possessing a marked periodicity in brief intervals of time, and not yet
+otherwise accounted for, are due to the throbbing, in the all-enveloping
+ether, of impulses transmitted from instruments controlled by the
+_savants_ of Mars, whose insatiable thirst for knowledge, and presumably
+burning desire to learn whether there is not within reach some more
+fortunate world than their half-dried-up globe, has led them into a
+desperate attempt to "call up" the earth on their interplanetary
+telephone, with the hope that we are wise and skilful enough to
+understand and answer them.
+
+In what language they intend to converse no one has yet undertaken to
+tell, but the suggestion has sapiently been made that, mathematical
+facts being invariable, the eternal equality of two plus two with four
+might serve as a basis of understanding, and that a statement of that
+truth sent by electric taps across the ocean of ether would be a
+convincing assurance that the inhabitants of the planet from which the
+message came at least enjoyed the advantages of a common-school
+education.
+
+But, while speculation upon this subject rests on unverified, and at
+present unverifiable, assumptions, of course everybody would rejoice if
+such a thing were possible, for consider what zest and charm would be
+added to human life if messages, even of the simplest description, could
+be sent to and received from intelligent beings inhabiting other
+planets! It is because of this hold that it possesses upon the
+imagination, and the pleasing pictures that it conjures up, that the
+idea of interplanetary communication, once broached, has become so
+popular a topic, even though everybody sees that it should not be taken
+too seriously.
+
+The subject of the atmosphere of Mars can not be dismissed without
+further consideration than we have yet given it, because those who think
+the planet uninhabitable base their opinion largely upon the assumed
+absence of sufficient air to support life. It was long ago recognized
+that, other things being equal, a planet of small mass must possess a
+less dense atmosphere than one of large mass. Assuming that each planet
+originally drew from a common stock, and that the amount and density of
+its atmosphere is measured by its force of gravity, it can be shown that
+Mars should have an atmosphere less than one fifth as dense as the
+earth's.
+
+Dr. Johnstone Stoney has attacked the problem of planetary atmospheres
+in another way. Knowing the force of gravity on a planet, it is easy to
+calculate the velocity with which a body, or a particle, would have to
+start radially from the planet in order to escape from its gravitational
+control. For the earth this critical velocity is about seven miles per
+second; for Mars about three miles per second. Estimating the velocity
+of the molecules of the various atmospheric gases, according to the
+kinetic theory, Dr. Stoney finds that some of the smaller planets, and
+the moon, are gravitationally incapable of retaining all of these gases
+in the form of an atmosphere. Among the atmospheric constituents that,
+according to this view, Mars would be unable permanently to retain is
+water vapor. Indeed, he supposes that even the earth is slowly losing
+its water by evaporation into space, and on Mars, owing to the slight
+force of gravity there, this process would go on much more rapidly, so
+that, in this way, we have a means of accounting for the apparent drying
+up of that planet, while we may be led to anticipate that at some time
+in the remote future the earth also will begin to suffer from lack of
+water, and that eventually the chasms of the sea will yawn empty and
+desolate under a cloudless sky.
+
+But it is not certain that the original supply of atmospheric elements
+was in every case proportional to the respective force of gravity of a
+planet. The fact that Venus appears to have an atmosphere more extensive
+and denser than the earth's, although its force of gravity is a little
+less than that of our globe, indicates at once a variation as between
+these two planets in the amount of atmospheric material at their
+disposal. This may be a detail depending upon differences in the mode,
+or in the stage, of their evolution. Thus, after all, Dr. Stoney's
+theory may be substantially correct and yet Mars may retain sufficient
+water to form clouds, to be precipitated in snow, and to fill its canals
+after each annual melting of the polar caps, because the original supply
+was abundant, and its escape is a gradual process, only to be completed
+by age-long steps.
+
+Even though the evidence of the spectroscope, as far as it goes, seems
+to lend support to the theory that there is no water vapor in the
+atmosphere of Mars, we can not disregard the visual evidence that,
+nevertheless, water vapor exists there.
+
+What are the polar caps if they are not snow? Frozen carbon dioxide, it
+has been suggested; but this is hardly satisfactory, for it offers no
+explanation of the fact that when the polar caps diminish, and in
+proportion as they diminish, the "seas" and the canals darken and
+expand, whereas a reasonable explanation of the correlation of these
+phenomena is offered if we accept the view that the polar caps consist
+of snow.
+
+Then there are many observations on record indicating the existence of
+clouds in Mars's atmosphere. Sometimes a considerable area of its
+surface has been observed to be temporarily obscured, not by dense
+masses of cloud such as accompany the progress of great cyclonic storms
+across the continents and oceans of the earth, but by comparatively thin
+veils of vapor such as would be expected to form in an atmosphere so
+comparatively rare as that of Mars. And these clouds, in some instances
+at least, appear, like the cirrus streaks and dapples in our own air, to
+float at a great elevation. Mr. Douglass, one of Mr. Lowell's associates
+in the observations of 1894 at Flagstaff, Arizona, observed what he
+believed to be a cloud over the unilluminated part of Mars's disk,
+which, by micrometric measurement and estimate, was drifting at an
+elevation of about fifteen miles above the surface of the planet. This
+was seen on two successive days, November 25th and November 26th, and it
+underwent curious fluctuations in visibility, besides moving in a
+northerly direction at the rate of some thirteen miles an hour. But,
+upon the whole, as Mr. Lowell remarks, the atmosphere of Mars is
+remarkably free of clouds.
+
+The reader will remember that Mars gets a little less than half as much
+heat from the sun as the earth gets. This fact also has been used as an
+argument against the habitability of the planet. In truth, those who
+think that life in the solar system is confined to the earth alone
+insist upon an almost exact reproduction of terrestrial conditions as a
+_sine qua non_ to the habitability of any other planet. Venus, they
+think, is too hot, and Mars too cold, as if life were rather a happy
+accident than the result of the operation of general laws applicable
+under a wide variety of conditions. All that we are really justified in
+asserting is that Venus may be too hot and Mars too cold for _us_. Of
+course, if we adopt the opinion held by some that the temperature on
+Mars is constantly so low that water would remain perpetually frozen, it
+does throw the question of the kind of life that could be maintained
+there into the realm of pure conjecture.
+
+The argument in favor of an extremely low temperature on Mars is based
+on the law of the diminution of radiant energy inversely as the square
+of the distance, together with the assumption that no qualifying
+circumstances, or no modification of that law, can enter into the
+problem. According to this view, it could be shown that the temperature
+on Mars never rises above -200 deg. F. But it is a view that seems to be
+directly opposed to the evidence of the telescope, for all who have
+studied Mars under favorable conditions of observation have been
+impressed by the rapid and extensive changes that the appearance of its
+surface undergoes coincidently with the variation of the planet's
+seasons. It has its winter aspect and its summer aspect, perfectly
+distinct and recognizable, in each hemisphere by turns, and whether the
+polar caps be snow or carbon dioxide, at any rate they melt and
+disappear under a high sun, thus proving that an accumulation of heat
+takes place.
+
+Professor Young says: "As to the temperature of Mars we have no certain
+knowledge. On the one hand, we know that on account of the planet's
+distance from the sun the intensity of solar radiation upon its surface
+must be less than here in the ratio of 1 to (1.524)^2--i.e., only about
+43 per cent as great as with us; its 'solar constant' must be less than
+13 calories against our 30. Then, too, the low density of its
+atmosphere, probably less at the planet's surface than on the tops of
+our highest mountains, would naturally assist to keep down the
+temperature to a point far below the freezing-point of water. But, on
+the other hand, things certainly _look_ as if the polar caps were really
+masses of _snow_ and _ice_ deposited from vapor in the planet's
+atmosphere, and as if these actually melted during the Martian summer,
+sending floods of water through the channels provided for them, and
+causing the growth of vegetation along their banks. We are driven,
+therefore, to suppose either that the planet has sources of heat
+internal or external which are not yet explained, or else, as long ago
+suggested, that the polar 'snow' may possibly be composed of something
+else than frozen _water_."[4]
+
+[Footnote 4: General Astronomy, by Charles A. Young. Revised edition,
+1898, p. 363.]
+
+Even while granting the worst that can be said for the low temperature
+of Mars, the persistent believer in its habitability could take refuge
+in the results of recent experiments which have proved that bacterial
+life is able to resist the utmost degree of cold that can be applied,
+microscopic organisms perfectly retaining their vitality--or at least
+their power to resume it--when subjected to the fearfully low
+temperature of liquid air. But then he would be open to the reply that
+the organisms thus treated are in a torpid condition and deprived of all
+activity until revived by the application of heat; and the picture of a
+world in a state of perpetual sleep is not particularly attractive,
+unless the fortunate prince who is destined to awake the slumbering
+beauty can also be introduced into the romance.[5]
+
+[Footnote 5: Many of the present difficulties about temperatures on the
+various planets would be beautifully disposed of if we could accept the
+theory urged by Mr. Cope Whitehouse, to the effect that the sun is not
+really a hot body at all, and that what we call solar light and heat are
+only local manifestations produced in our atmosphere by the
+transformation of some other form of energy transmitted from the sun;
+very much as the electric impulses carried by a wire from the
+transmitting to the receiving station on a telephone line are translated
+by the receiver into waves of sound. According to this theory, which is
+here mentioned only as an ingenuity and because something of the kind so
+frequently turns up in one form or another in popular semi-scientific
+literature, the amount of heat and light on a planet would depend mainly
+upon local causes.]
+
+To an extent which most of us, perhaps, do not fully appreciate, we are
+indebted for many of the pleasures and conveniences and some of the
+necessities of life on our planet to its faithful attendant, the moon.
+Neither Mercury nor Venus has a moon, but Mars has two moons. This
+statement, standing alone, might lead to the conclusion that, as far as
+the advantages a satellite can afford to the inhabitants of its master
+planet are concerned, the people of Mars are doubly fortunate. So they
+would be, perhaps, if Mars's moons were bodies comparable in size with
+our moon, but in fact they are hardly more than a pair of very
+entertaining astronomical toys. The larger of the two, Phobos, is
+believed to be about seven miles in diameter; the smaller, Deimos, only
+five or six miles. Their dimensions thus resemble those of the more
+minute of the asteroids, and the suggestion has even been made that they
+may be captured asteroids which have fallen under the gravitational
+control of Mars.
+
+The diameters just mentioned are Professor Pickering's estimates, based
+on the amount of light the little satellites reflect, for they are much
+too small to present measurable disks. Deimos is 14,600 miles from the
+center of Mars and 12,500 miles from its surface. Phobos is 5,800 miles
+from the center of the planet and only 3,700 from the surface. Deimos
+completes a revolution about the planet in thirty hours and eighteen
+minutes, and Phobos in the astonishingly short period--although, of
+course, it is in strict accord with the law of gravitation and in that
+sense not astonishing--of seven hours and thirty-nine minutes.
+
+Since Mars takes twenty-four hours and thirty-seven minutes for one
+rotation on its axis, it is evident that Phobos goes round the planet
+three times in the course of a single Martian day and night, rising,
+contrary to the general motion of the heavens, in the west, running in a
+few hours through all the phases that our moon exhibits in the course of
+a month, and setting, where the sun and all the stars rise, in the east.
+Deimos, on the other hand, has a period of revolution five or six hours
+longer than that of the planet's axial rotation, so that it rises, like
+the other heavenly bodies, in the east; but, because its motion is so
+nearly equal, in angular velocity, to that of Mars's rotation, it shifts
+very slowly through the sky toward the west, and for two or three
+successive days and nights it remains above the horizon, the sun
+overtaking and passing it again and again, while, in the meantime, its
+protean face swiftly changes from full circle to half-moon, from
+half-moon to crescent, from crescent back to half, and from half to
+full, and so on without ceasing.
+
+And during this time Phobos is rushing through the sky in the opposite
+direction, as if in defiance of the fundamental law of celestial
+revolution, making a complete circuit three times every twenty-four
+hours, and changing the shape of its disk four times as rapidly as
+Deimos does! Truly, if we were suddenly transported to Mars, we might
+well believe that we had arrived in the mother world of lunatics, and
+that its two moons were bewitched. Yet it must not be supposed that all
+the peculiarities just mentioned would be clearly seen from the surface
+of Mars by eyes like ours. The phases of Phobos would probably be
+discernible to the naked eye, but those of Deimos would require a
+telescope in order to be seen, for, notwithstanding their nearness to
+the planet, Mars's moons are inconspicuous phenomena even to the
+Martians themselves. Professor Young's estimate is that Phobos may shed
+upon Mars one-sixtieth and Deimos one-twelve-hundredth as much reflected
+moonlight as our moon sends to the earth. Accordingly, a "moonlit night"
+on Mars can have no such charm as we associate with the phrase. But it
+is surely a tribute to the power and perfection of our telescopes that
+we have been able to discover the existence of objects so minute and
+inconspicuous, situated at a distance of many millions of miles, and
+half concealed by the glaring light of the planet close around which
+they revolve.
+
+If Mars's moons were as massive as our moon is they would raise
+tremendous tides upon Mars, and would affect the circulation of water in
+the canals, but, in fact, their tidal effects are even more
+insignificant than their light-giving powers. But for astronomers on
+Mars they would be objects of absorbing interest.
+
+Upon quitting Mars we pass to the second distinctive planetary group of
+the solar system, that of the asteroids.
+
+
+
+
+CHAPTER V
+
+THE ASTEROIDS, A FAMILY OF DWARF WORLDS
+
+
+Beyond Mars, in the broad gap separating the terrestrial from the Jovian
+planets, are the asteroids, of which nearly five hundred have been
+discovered and designated by individual names or numbers. But any
+statement concerning the known number of asteroids can remain valid for
+but a short time, because new ones are continually found, especially by
+the aid of photography. Very few of the asteroids are of measurable
+size. Among these are the four that were the first to be
+discovered--Ceres, Pallas, Juno, and Vesta. Their diameters, according
+to the measurements of Prof. E.E. Barnard, of the Yerkes Observatory,
+are as follows: Ceres, 477 miles; Pallas, 304 miles; Juno, 120 miles;
+Vesta, 239 miles.
+
+It is only necessary to mention these diameters in order to indicate how
+wide is the difference between the asteroids and such planets as the
+earth, Venus, or Mars. The entire surface of the largest asteroid,
+Ceres, does not equal the republic of Mexico in area. But Ceres itself
+is gigantic in comparison with the vast majority of the asteroids, many
+of which, it is believed, do not exceed twenty miles in diameter, while
+there may be hundreds or thousands of others still smaller--ten miles,
+five miles, or perhaps only a few rods, in diameter!
+
+Curiously enough, the asteroid which appears brightest, and which it
+would naturally be inferred is the largest, really stands third in the
+order of measured size. This is Vesta, whose diameter, according to
+Barnard, is only 239 miles. It is estimated that the surface of Vesta
+possesses about four times greater light-reflecting power than the
+surface of Ceres. Some observations have also shown a variation in the
+intensity of the light from Vesta, a most interesting fact, which
+becomes still more significant when considered in connection with the
+great variability of another most extraordinary member of the asteroidal
+family, Eros, which is to be described presently.
+
+The orbits of the asteroids are scattered over a zone about 200,000,000
+miles broad. The mean distance from the sun of the nearest asteroid,
+Eros, is 135,000,000 miles, and that of the most distant, Thule,
+400,000,000 miles. Wide gaps exist in the asteroidal zone where few or
+no members of the group are to be found, and Prof. Daniel Kirkwood long
+ago demonstrated the influence of Jupiter in producing these gaps.
+Almost no asteroids, as he showed, revolve at such a distance from the
+sun that their periods of revolution are exactly commensurable with that
+of Jupiter. Originally there may have been many thus situated, but the
+attraction of the great planet has, in the course of time, swept those
+zones clean.
+
+Many of the asteroids have very eccentric orbits, and their orbits are
+curiously intermixed, varying widely among themselves, both in
+ellipticity and in inclination to the common plane of the solar system.
+
+Considered with reference to the shape and position of its orbit, the
+most unique of these little worlds is Eros, which was discovered in 1898
+by De Witt, at Berlin, and which, on account of its occasional near
+approach to the earth, has lately been utilized in a fresh attempt to
+obtain a closer approximation to the true distance of the sun from the
+earth. The mean distance of Eros from the sun is 135,000,000 miles, its
+greatest distance is 166,000,000 miles, and its least distance
+105,000,000 miles. It will thus be seen that, although all the other
+asteroids are situated beyond Mars, Eros, at its mean distance, is
+nearer to the sun than Mars is. When in aphelion, or at its greatest
+distance, Eros is outside of the orbit of Mars, but when in perihelion
+it is so much inside of Mars's orbit that it comes surprisingly near the
+earth.
+
+Indeed, there are times when Eros is nearer to the earth than any other
+celestial body ever gets except the moon--and, it might be added,
+except meteors and, by chance, a comet, or a comet's tail. Its least
+possible distance from the earth is less than 14,000,000 miles, and it
+was nearly as close as that, without anybody knowing or suspecting the
+fact, in 1894, four years in advance of its discovery. Yet the fact,
+strange as the statement may seem, had been recorded without being
+recognized. After De Witt's discovery of Eros in 1898, at a time when it
+was by no means as near the earth as it had been some years before,
+Prof. E.C. Pickering ascertained that it had several times imprinted its
+image on the photographic plates of the Harvard Observatory, with which
+pictures of the sky are systematically taken, but had remained
+unnoticed, or had been taken for an ordinary star among the thousands of
+star images surrounding it. From these telltale plates it was
+ascertained that in 1894 it had been in perihelion very near the earth,
+and had shone with the brilliance of a seventh-magnitude star.
+
+It will, unfortunately, be a long time before Eros comes quite as near
+us as it did on that occasion, when we failed to see it, for its close
+approaches to the earth are not frequent. Prof. Solon I. Bailey selects
+the oppositions of Eros in 1931 and 1938 as probably the most favorable
+that will occur during the first half of the twentieth century.
+
+We turn to the extraordinary fluctuations in the light of Eros, and the
+equally extraordinary conclusions drawn from them. While the little
+asteroid, whose diameter is estimated to be in the neighborhood of
+twenty or twenty-five miles, was being assiduously watched and
+photographed during its opposition in the winter of 1900-1901, several
+observers discovered that its light was variable to the extent of more
+than a whole magnitude; some said as much as two magnitudes. When it is
+remembered that an increase of one stellar magnitude means an accession
+of light in the ratio of 2.5 to 1, and an increase of two magnitudes an
+accession of 6.25 to 1, the significance of such variations as Eros
+exhibited becomes immediately apparent. The shortness of the period
+within which the cycle of changes occurred, about two hours and a half,
+made the variation more noticeable, and at the same time suggested a
+ready explanation, viz., that the asteroid was rapidly turning on its
+axis, a thing, in itself, quite in accordance with the behavior of other
+celestial bodies and naturally to be expected.
+
+But careful observation showed that there were marked irregularities in
+the light fluctuations, indicating that Eros either had a very strange
+distribution of light and dark areas covering its surface, or that
+instead of being a globular body it was of some extremely irregular
+shape, so that as it rotated it presented successively larger and
+smaller reflecting surfaces toward the sun and the earth. One
+interesting suggestion was that the little planet is in reality double,
+the two components revolving around their common center of gravity, like
+a close binary star, and mutually eclipsing one another. But this theory
+seems hardly competent to explain the very great fluctuation in light,
+and a better one, probably, is that suggested by Prof. E.C. Pickering,
+that Eros is shaped something like a dumb-bell.
+
+We can picture such a mass, in imagination, tumbling end over end in its
+orbit so as to present at one moment the broad sides of both bells,
+together with their connecting neck, toward the sun, and, at the same
+time, toward the observer on the earth, and, at another moment, only the
+end of one of the bells, the other bell and the neck being concealed in
+shadow. In this way the successive gain and loss of sixfold in the
+amount of light might be accounted for. Owing to the great distance the
+real form of the asteroid is imperceptible even with powerful
+telescopes, but the effect of a change in the amount of reflecting
+surface presented produces, necessarily, an alternate waxing and waning
+of the light. As far as the fluctuations are concerned, they might also
+be explained by supposing that the shape of the asteroid is that of a
+flat disk, rotating about one of its larger diameters so as to present,
+alternately, its edge and its broadside to the sun. And, perhaps, in
+order completely to account for all the observed eccentricities of the
+light of Eros, the irregularity of form may have to be supplemented by
+certain assumptions as to the varying reflective capacity of different
+parts of the misshapen mass.
+
+The invaluable Harvard photographs show that long before Eros was
+recognized as an asteroid its light variations had been automatically
+registered on the plates. Some of the plates, Prof. E.C. Pickering says,
+had had an exposure of an hour or more, and, owing to its motion, Eros
+had formed a trail on each of these plates, which in some cases showed
+distinct variations in brightness. Differences in the amount of
+variation at different times will largely depend upon the position of
+the earth with respect to the axis of rotation.
+
+Another interesting deduction may be made from the changes that the
+light of Eros undergoes. We have already remarked that one of the larger
+asteroids, and the one which appears to the eye as the most brilliant
+of all, Vesta, has been suspected of variability, but not so extensive
+as that of Eros. Olbers, at the beginning of the last century, was of
+the opinion that Vesta's variations were due to its being not a globe
+but an angular mass. So he was led by a similar phenomenon to precisely
+the same opinion about Vesta that has lately been put forth concerning
+Eros. The importance of this coincidence is that it tends to revive a
+remarkable theory of the origin of the asteroids which has long been in
+abeyance, and, in the minds of many, perhaps discredited.
+
+This theory, which is due to Olbers, begins with the startling
+assumption that a planet, perhaps as large as Mars, formerly revolving
+in an orbit situated between the orbits of Mars and Jupiter, was
+destroyed by an explosion! Although, at first glance, such a catastrophe
+may appear too wildly improbable for belief, yet it was not the
+improbability of a world's blowing up that led to a temporary
+abandonment of Olbers's bold theory. The great French mathematician
+Lagrange investigated the explosive force "which would be necessary to
+detach a fragment of matter from a planet revolving at a given distance
+from the sun," and published the results in the Connaissance des Temps
+for 1814.
+
+"Applying his results to the earth, Lagrange found that if the velocity
+of the detached fragment exceeded that of a cannon ball in the
+proportion of 121 to 1 the fragment would become a comet with a direct
+motion; but if the velocity rose in the proportion of 156 to 1 the
+motion of the comet would be retrograde. If the velocity was less than
+in either of these cases the fragment would revolve as a planet in an
+elliptic orbit. For any other planet besides the earth the velocity of
+explosion corresponding to the different cases would vary in the inverse
+ratio of the square root of the mean distance. It would therefore
+manifestly be less as the planet was more distant from the sun. In the
+case of each of the four smaller planets (only the four asteroids,
+Ceres, Pallas, Juno, and Vesta, were known at that time), the velocity
+of explosion indicated by their observed motion would be less than
+twenty times the velocity of a cannon ball."[6]
+
+[Footnote 6: Grant's History of Physical Astronomy, p. 241.]
+
+Instead, then, of being discredited by its assumption of so strange a
+catastrophe, Olbers's theory fell into desuetude because of its apparent
+failure to account for the position of the orbits of many of the
+asteroids after a large number of those bodies had been discovered. He
+calculated that the orbits of all the fragments of his exploded planet
+would have nearly equal mean distances, and a common point of
+intersection in the heavens, through which every fragment of the
+original mass would necessarily pass in each revolution. At first the
+orbits of the asteroids discovered seemed to answer to these conditions,
+and Olbers was even able to use his theory as a means of predicting the
+position of yet undetected asteroids. Only Ceres and Pallas had been
+discovered when he put forth his theory, but when Juno and Vesta were
+found they fell in with his predictions so well that the theory was
+generally regarded as being virtually established; while the
+fluctuations in the light of Vesta, as we have before remarked, led
+Olbers to assert that that body was of a fragmental shape, thus strongly
+supporting his explosion hypothesis.
+
+Afterward, when the orbits of many asteroids had been investigated, the
+soundness of Olbers's theory began to be questioned. The fact that the
+orbits did not all intersect at a common point could easily be disposed
+of, as Professor Newcomb has pointed out, by simply placing the date of
+the explosion sufficiently far back, say millions of years ago, for the
+secular changes produced by the attraction of the larger planets would
+effectively mix up the orbits. But when the actual effects of these
+secular changes were calculated for particular asteroids the result
+seemed to show that "the orbits could never have intersected unless some
+of them have in the meantime been altered by the attraction of the
+small planets on each other. Such an action is not impossible, but it is
+impossible to determine it, owing to the great number of these bodies
+and our ignorance of their masses."[7]
+
+[Footnote 7: Popular Astronomy, by Simon Newcomb, p. 335.]
+
+Yet the theory has never been entirely thrown out, and now that the
+discovery of the light fluctuations of Eros lends support to Olbers's
+assertion of the irregular shape of some of the asteroids, it is very
+interesting to recall what so high an authority as Professor Young said
+on the subject before the discovery of Eros:
+
+"It is true, as has often been urged, that this theory in its original
+form, as presented by Olbers, can not be correct. No _single_ explosion
+of a planet could give rise to the present assemblage of orbits, nor is
+it possible that even the perturbations of Jupiter could have converted
+a set of orbits originally all crossing at one point (the point of
+explosion) into the present tangle. The smaller orbits are so small
+that, however turned about, they lie wholly inside the larger and can
+not be made to intersect them. If, however, we admit a _series_ of
+explosions, this difficulty is removed; and if we grant an explosion at
+all, there seems to be nothing improbable in the hypothesis that the
+fragments formed by the bursting of the parent mass would carry away
+within themselves the same forces and reactions which caused the
+original bursting, so that they themselves would be likely enough to
+explode at some time in their later history."[8]
+
+[Footnote 8: General Astronomy, by Charles A. Young. Revised edition,
+1898, p. 372.]
+
+The rival theory of the origin of the asteroids is that which assumes
+that the planetary ring originally left off from the contracting solar
+nebula between the orbits of Mars and Jupiter was so violently perturbed
+by the attraction of the latter planet that, instead of being shaped
+into a single globe, it was broken up into many fragments. Either
+hypothesis presents an attractive picture; but that which presupposes
+the bursting asunder of a large planet, which might at least have borne
+the germs of life, and the subsequent shattering of its parts into
+smaller fragments, like the secondary explosions of the pieces of a
+pyrotechnic bomb, certainly is by far the more impressive in its appeal
+to the imagination, and would seem to offer excellent material for some
+of the extra-terrestrial romances now so popular. It is a startling
+thought that a world can possibly carry within itself, like a dynamite
+cartridge, the means of its own disruption; but the idea does not appear
+so extremely improbable when we recall the evidence of collisions or
+explosions, happening on a tremendous scale, in the case of new or
+temporary stars.[9]
+
+[Footnote 9: "Since the discovery of Eros, the extraordinary position of
+its orbit has led to the suggestion that possibly Mars itself, instead
+of being regarded as primarily a major planet, belonging to the
+terrestrial group, ought rather to be considered as the greatest of the
+asteroids, and a part of the original body from which the asteroidal
+system was formed."--J. Bauschinger, Astronomische Nachrichten, No.
+3542.]
+
+Coming to the question of life upon the asteroids, it seems clear that
+they must be excluded from the list of habitable worlds, whatever we
+may choose to think of the possible habitability of the original planet
+through whose destruction they may have come into existence. The largest
+of them possesses a force of gravity far too slight to enable it to
+retain any of the gases or vapors that are recognized as constituting an
+atmosphere. But they afford a captivating field for speculation, which
+need not be altogether avoided, for it offers some graphic illustrations
+of the law of gravitation. A few years ago I wrote, for the
+entertainment of an audience which preferred to meet science attired in
+a garb woven largely from the strands of fancy, an account of some of
+the peculiarities of such minute globes as the asteroids, which I
+reproduce here because it gives, perhaps, a livelier picture of those
+little bodies, from the point of view of ordinary human interest, than
+could be presented in any other way.
+
+
+A WAIF OF SPACE
+
+One night as I was waiting, watch in hand, for an occultation, and
+striving hard to keep awake, for it had been a hot and exhausting
+summer's day, while my wife--we were then in our honeymoon--sat
+sympathetically by my side, I suddenly found myself withdrawn from the
+telescope, and standing in a place that appeared entirely strange. It
+was a very smooth bit of ground, and, to my surprise, there was no
+horizon in sight; that is to say, the surface of the ground disappeared
+on all sides at a short distance off, and beyond nothing but sky was
+visible. I thought I must be on the top of a stupendous mountain, and
+yet I was puzzled to understand how the face of the earth could be so
+far withdrawn. Presently I became aware that there was some one by me
+whom I could not see.
+
+"You are not on a mountain," my companion said, and as he spoke a cold
+shiver ran along my back-bone; "you are on an asteroid, one of those
+miniature planets, as you astronomers call them, and of which you have
+discovered several hundred revolving between the orbits of Mars and
+Jupiter. This is the little globe that you have glimpsed occasionally
+with your telescope, and that you, or some of your fellows, have been
+kind enough to name Menippe."
+
+Then I perceived that my companion, whose address had hardly been
+reassuring, was a gigantic inhabitant of the little planet, towering up
+to a height of three quarters of a mile. For a moment I was highly
+amused, standing by his foot, which swelled up like a hill, and
+straining my neck backward to get a look up along the precipice of his
+leg, which, curiously enough, I observed was clothed in rough homespun,
+the woolly knots of the cloth appearing of tremendous size, while it
+bagged at the knee like any terrestrial trousers' leg. His great head
+and face I could see far above me, as it were, in the clouds. Yet I was
+not at all astonished.
+
+"This is all right," I said to myself. "Of course on Menippe the people
+must be as large as this, for the little planet is only a dozen miles
+in diameter, and the force of gravity is consequently so small that a
+man without loss of activity, or inconvenience, can grow three quarters
+of a mile tall."
+
+Suddenly an idea occurred to me. "Just to think what a jump I can make!
+Why, only the other day I was figuring it out that a man could easily
+jump a thousand feet high from the surface of Menippe, and now here I
+actually am on Menippe. I'll jump."
+
+The sensation of that glorious rise skyward was delightful beyond
+expression. My legs seemed to have become as powerful as the engines of
+a transatlantic liner, and with one spring I rose smoothly and swiftly,
+and as straight as an arrow, surmounting the giant's foot, passing his
+knee and attaining nearly to the level of his hip. Then I felt that the
+momentum of my leap was exhausted, and despite my efforts I slowly
+turned head downward, glancing in affright at the ground a quarter of a
+mile below me, on which I expected to be dashed to pieces. But a
+moment's thought convinced me that I should get no hurt, for with so
+slight a force of gravity it would be more like floating than falling.
+Just then the Menippean caught me with his monstrous hand and lifted me
+to the level of his face.
+
+"I should like to know," I said, "how you manage to live up here; you
+are so large and your planet is so little."
+
+"Now, you are altogether too inquisitive," replied the giant. "You go!"
+
+He stooped down, placed me on the toe of his boot, and drew back his
+foot to kick me off.
+
+It flashed into my mind that my situation had now become very serious. I
+knew well what the effects of the small attractive force of these
+diminutive planets must be, for I had often amused myself with
+calculations about them. In this moment of peril I did not forget my
+mathematics. It was clear that if the giant propelled me with sufficient
+velocity I should be shot into space, never to return. How great would
+that velocity have to be? My mind worked like lightning on this problem.
+The diameter of Menippe I knew did not exceed twelve miles. Its mean
+density, as near as I could judge, was about the same as that of the
+earth. Its attraction must therefore be as its radius, or nearly 660
+times less than that of the earth. A well-known formula enables us to
+compute the velocity a body would acquire in falling from an infinite
+distance to the earth or any other planet whose size and force of
+gravity are known. The same formula, taken in the opposite sense, of
+course, shows how fast a body must start from a planet in order that it
+may be freed from its control. The formula is V = square root of (2
+gr.), in which "g" is the acceleration of gravity, equal for the earth
+to 32 feet in a second, and "r" is the radius of the attracting body. On
+Menippe I knew "g" must equal about one twentieth of a foot, and "r"
+31,680 feet. Like a flash I applied the formula while the giant's
+muscles were yet tightening for the kick: 31,680 x 1/20 x 2 = 3,168, the
+square root of which is a fraction more than 56. Fifty-six feet in a
+second, then, was the critical velocity with which I must be kicked off
+in order that I might never return. I perceived at once that the giant
+would be able to accomplish it. I turned and shouted up at him:
+
+"Hold on, I have something to say to you!"
+
+I dimly saw his mountainous face puckered into mighty wrinkles, out of
+which his eyes glared fiercely, and the next moment I was sailing into
+space. I could no more have kept a balance than the earth can stand
+still upon its axis. I had become a small planet myself, and, like all
+planets, I rotated. Yet the motion did not dizzy me, and soon I became
+intensely interested in the panorama of creation that was spread around
+me. For some time, whenever my face was turned toward the little globe
+of Menippe, I saw the giant, partly in profile against the sky, with his
+back bent and his hands upon his knees, watching me with an occasional
+approving nod of his big head. He looked so funny standing there on his
+little seven-by-nine world, like a clown on a performing ball, that,
+despite my terrible situation, I shook my sides with laughter. There was
+no echo in the profundity of empty space.
+
+Soon Menippe dwindled to a point, and I saw her inhospitable inhabitant
+no more. Then I watched the sun and the blazing firmament around, for
+there was at the same time broad day and midnight for me. The sunlight,
+being no longer diffused by an atmosphere, did not conceal the face of
+the sky, and I could see the stars shining close to the orb of day. I
+recognized the various planets much more easily than I had been
+accustomed to do, and, with a twinge at my heart, saw the earth
+traveling along in its distant orbit, splendid in the sunshine. I
+thought of my wife sitting alone by the telescope in the darkness and
+silence, wondering what had become of me. I asked myself, "How in the
+world can I ever get back there again?" Then I smiled to think of the
+ridiculous figure I cut, out here in space, exposed to the eyes of the
+universe, a rotating, gyrating, circumambulating astronomer, an
+animated teetotum lost in the sky. I saw no reason to hope that I should
+not go on thus forever, revolving around the sun until my bones,
+whitening among the stars, might be revealed to the superlative powers
+of some future telescope, and become a subject of absorbing interest,
+the topic of many a learned paper for the astronomers of a future age.
+Afterward I was comforted by the reflection that in airless space,
+although I might die and my body become desiccated, yet there could be
+no real decay; even my garments would probably last forever. The
+_savants_, after all, should never speculate on my bones.
+
+I saw the ruddy disk of Mars, and the glinting of his icy poles, as the
+beautiful planet rolled far below me. "If I could only get there," I
+thought, "I should know what those canals of Schiaparelli are, and even
+if I could never return to the earth, I should doubtless meet with a
+warm welcome among the Martians. What a lion I should be!" I looked
+longingly at the distant planet, the outlines of whose continents and
+seas appeared most enticing, but when I tried to propel myself in that
+direction I only kicked against nothingness. I groaned in desperation.
+
+Suddenly something darted by me flying sunward; then another and
+another. In a minute I was surrounded by strange projectiles. Every
+instant I expected to be dashed in pieces by them. They sped with the
+velocity of lightning. Hundreds, thousands of them were all about me. My
+chance of not being hit was not one in a million, and yet I escaped. The
+sweat of terror was upon me, but I did not lose my head. "A comet has
+met me," I said. "These missiles are the meteoric stones of which it is
+composed." And now I noticed that as they rushed along collisions took
+place, and flashes of electricity darted from one to another. A pale
+luminosity dimmed the stars. I did not doubt that, as seen from the
+earth, the comet was already flinging the splendors of its train upon
+the bosom of the night.
+
+While I was wondering at my immunity amid such a rain of
+death-threatening bolts, I became aware that their velocity was sensibly
+diminishing. This fact I explained by supposing that I was drawn along
+with them. Notwithstanding the absence of any collision with my body,
+the overpowering attraction of the whole mass of meteors was overcoming
+my tangential force and bearing me in their direction. At first I
+rejoiced at this circumstance, for at any rate the comet would save me
+from the dreadful fate of becoming an asteroid. A little further
+reflection, however, showed me that I had gone from the frying-pan into
+the fire. The direction of my expulsion from Menippe had been such that
+I had fallen into an orbit that would have carried me around the sun
+without passing very close to the solar body. Now, being swept along by
+the comet, whose perihelion probably lay in the immediate neighborhood
+of the sun, I saw no way of escape from the frightful fate of being
+broiled alive. Even where I was, the untempered rays of the sun scorched
+me, and I knew that within two or three hundred thousand miles of the
+solar surface the heat must be sufficient to melt the hardest rocks. I
+was aware that experiments with burning-glasses had sufficiently
+demonstrated that fact.
+
+But perforce I resigned myself to my fate. At any rate it would the
+sooner be all over. In fact, I almost forgot my awful situation in the
+interest awakened by the phenomena of the comet. I was in the midst of
+its very head. I was one of its component particles. I was a meteor
+among a million millions of others. If I could only get back to the
+earth, what news could I not carry to Signor Schiaparelli and Mr.
+Lockyer and Dr. Bredichin about the composition of comets! But, alas!
+the world could never know what I now saw. Nobody on yonder gleaming
+earth, watching the magnificent advance of this "specter of the skies,"
+would ever dream that there was a lost astronomer in its blazing head. I
+should be burned and rent to pieces amid the terrors of its perihelion
+passage, and my fragments would be strewn along the comet's orbit, to
+become, in course of time, particles in a swarm of aerolites. Perchance,
+through the effects of some unforeseen perturbation, the earth might
+encounter that swarm. Thus only could I ever return to the bosom of my
+mother planet. I took a positive pleasure in imagining that one of my
+calcined bones might eventually flash for a moment, a falling star, in
+the atmosphere of the earth, leaving its atoms to slowly settle through
+the air, until finally they rested in the soil from which they had
+sprung.
+
+From such reflections I was aroused by the approach of the crisis. The
+head of the comet had become an exceedingly uncomfortable place. The
+collisions among the meteors were constantly increasing in number and
+violence. How I escaped destruction I could not comprehend, but in fact
+I was unconscious of danger from that source. I had become in spirit an
+actual component of the clashing, roaring mass. Tremendous sparks of
+electricity, veritable lightning strokes, darted about me in every
+direction, but I bore a charmed life. As the comet drew in nearer to
+the sun, under the terrible stress of the solar attraction, the meteors
+seemed to crowd closer, crashing and grinding together, while the whole
+mass swayed and shrieked with the uproar of a million tormented devils.
+The heat had become terrific. I saw stone and iron melted like snow and
+dissipated in steam. Stupendous jets of white-hot vapor shot upward,
+and, driven off by the electrical repulsion of the sun, streamed
+backward into the tail.
+
+Suddenly I myself became sensible of the awful heat. It seemed without
+warning to have penetrated my vitals. With a yell I jerked my feet from
+a boiling rock and flung my arms despairingly over my head.
+
+"You had better be careful," said my wife, "or you'll knock over the
+telescope."
+
+I rubbed my eyes, shook myself, and rose.
+
+"I must have been dreaming," I said.
+
+"I should think it was a very lively dream," she replied.
+
+I responded after the manner of a young man newly wed.
+
+At this moment the occultation began.
+
+
+
+
+CHAPTER VI
+
+JUPITER, THE GREATEST OF KNOWN WORLDS
+
+
+When we are thinking of worlds, and trying to exalt the imagination with
+them, it is well to turn to Jupiter, for there is a planet worth
+pondering upon! A world thirteen hundred times as voluminous as the
+earth is a phenomenon calculated to make us feel somewhat as the
+inhabitant of a rural village does when his amazed vision ranges across
+the million roofs of a metropolis. Jupiter is the first of the outer and
+greater planets, the major, or Jovian, group. His mean diameter is
+86,500 miles, and his average girth more than 270,000 miles. An
+inhabitant of Jupiter, in making a trip around his planet, along any
+great circle of the sphere, would have to travel more than 30,000 miles
+farther than the distance between the earth and the moon. The polar
+compression of Jupiter, owing to his rapid rotation, amounts in the
+aggregate to more than 5,000 miles, the equatorial diameter being 88,200
+miles and the polar diameter 83,000 miles.
+
+Jupiter's mean distance from the sun is 483,000,000 miles, and the
+eccentricity of his orbit is sufficient to make this distance variable
+to the extent of 21,000,000 miles; but, in view of his great average
+distance, the consequent variation in the amount of solar light and heat
+received by the planet is not of serious importance.
+
+When he is in opposition to the sun as seen from the earth Jupiter's
+mean distance from us is about 390,000,000 miles. His year, or period of
+revolution about the sun, is somewhat less than twelve of our years
+(11.86 years). His axis is very nearly upright to the plane of his
+orbit, so that, as upon Venus, there is practically no variation of
+seasons. Gigantic though he is in dimensions, Jupiter is the swiftest of
+all the planets in axial rotation. While the earth requires twenty-four
+hours to make a complete turn, Jupiter takes less than ten hours (nine
+hours fifty-five minutes), and a point on his equator moves, in
+consequence of axial rotation, between 27,000 and 28,000 miles in an
+hour.
+
+The density of the mighty planet is slight, only about one quarter of
+the mean density of the earth and virtually the same as that of the sun.
+This fact at once calls attention to a contrast between Jupiter and our
+globe that is even more significant than their immense difference in
+size. The force of gravity upon Jupiter's surface is more than two and a
+half times greater than upon the earth's surface (more accurately 2.65
+times), so that a hundred-pound weight removed from the planet on which
+we live to Jupiter would there weigh 265 pounds, and an average man,
+similarly transported, would be oppressed with a weight of at least 400
+pounds. But, as a result of the rapid rotation of the great planet, and
+the ellipticity of its figure, the unfortunate visitor could find a
+perceptible relief from his troublesome weight by seeking the planet's
+equator, where the centrifugal tendency would remove about twenty pounds
+from every one hundred as compared with his weight at the poles.
+
+If we could go to the moon, or to Mercury, Venus, or Mars, we may be
+certain that upon reaching any of those globes we should find ourselves
+upon a solid surface, probably composed of rock not unlike the rocky
+crust of the earth; but with Jupiter the case would evidently be very
+different. As already remarked, the mean density of that planet is only
+one quarter of the earth's density, or only one third greater than the
+density of water. Consequently the visitor, in attempting to set foot
+upon Jupiter, might find no solid supporting surface, but would be in a
+situation as embarrassing as that of Milton's Satan when he undertook to
+cross the domain of Chaos:
+
+    "Fluttering his pinions vain, plumb down he drops,
+    Ten thousand fathom deep, and to this hour
+    Down had been falling had not, by ill chance,
+    The strong rebuff of some tumultuous cloud.
+    Instinct with fire and niter, hurried him
+    As many miles aloft; that fury stayed,
+    Quenched in a boggy Syrtis, neither sea
+    Nor good dry land, nigh foundered, as he fares,
+    Treading the crude consistence, half on foot,
+    Half flying."
+
+The probability that nothing resembling a solid crust, nor, perhaps,
+even a liquid shell, would be found at the visible surface of Jupiter,
+is increased by considering that the surface density must be much less
+than the mean density of the planet taken as a whole, and since the
+latter but little exceeds the density of water, it is likely that at the
+surface everything is in a state resembling that of cloud or smoke. Our
+imaginary visitor upon reaching Jupiter would, under the influence of
+the planet's strong force of gravity, drop out of sight, with the speed
+of a shot, swallowed up in the vast atmosphere of probably hot, and
+perhaps partially incandescent, gases. When he had sunk--supposing his
+identity could be preserved--to a depth of thousands of miles he might
+not yet have found any solid part of the planet; and, perchance, there
+is no solid nucleus even at the very center.
+
+The cloudy aspect of Jupiter immediately strikes the telescopic
+observer. The huge planet is filled with color, and with the animation
+of constant movement, but there is no appearance of markings, like those
+on Mars, recalling the look of the earth. There are no white polar caps,
+and no shadings that suggest the outlines of continents and oceans. What
+every observer, even with the smallest telescope, perceives at once is a
+pair of strongly defined dark belts, one on either side of, and both
+parallel to, the planet's equator. These belts are dark compared with
+the equatorial band between them and with the general surface of the
+planet toward the north and the south, but they are not of a gray or
+neutral shade. On the contrary, they show decided, and, at times,
+brilliant colors, usually of a reddish tone. More delicate tints,
+sometimes a fine pink, salmon, or even light green, are occasionally to
+be seen about the equatorial zone, and the borders of the belts, while
+near the poles the surface is shadowed with bluish gray, imperceptibly
+deepening from the lighter hues of the equator.
+
+All this variety of tone and color makes of a telescopic view of Jupiter
+a picture that will not quickly fade from the memory; while if an
+instrument of considerable power is used, so that the wonderful details
+of the belts, with their scalloped edges, their diagonal filaments,
+their many divisions, and their curious light and dark spots, are made
+plain, the observer is deeply impressed with the strangeness of the
+spectacle, and the more so as he reflects upon the enormous real
+magnitude of that which is spread before his eye. The whole earth
+flattened out would be but a small blotch on that gigantic disk!
+
+Then, the visible rotation of the great Jovian globe, whose effects
+become evident to a practised eye after but a few minutes' watching,
+heightens the impression. And the presence of the four satellites, whose
+motions in their orbits are also evident, through the change in their
+positions, during the course of a single not prolonged observation,
+adds its influence to the effectiveness of the scene. Indeed, color and
+motion are so conspicuous in the immense spectacle presented by Jupiter
+that they impart to it a powerful suggestion of life, which the mind
+does not readily divest itself of when compelled to face the evidence
+that Jupiter is as widely different from the earth, and as diametrically
+opposed to lifelike conditions, as we comprehend them, as a planet
+possibly could be.
+
+The great belts lie in latitudes about corresponding to those in which
+the trade-winds blow upon the earth, and it has often been suggested
+that their existence indicates a similarity between the atmospheric
+circulation of Jupiter and that of the world in which we live. No doubt
+there are times when the earth, seen with a telescope from a distant
+planet, would present a belted appearance somewhat resembling that of
+Jupiter, but there would almost certainly be no similar display of
+colors in the clouds, and the latter would exhibit no such persistence
+in general form and position as characterizes those of Jupiter. Our
+clouds are formed by the action of the sun, producing evaporation of
+water; on Jupiter, whose mean distance from the sun is more than five
+times as great as ours, the intensity of the solar rays is reduced to
+less than one twenty-fifth part of their intensity on the earth, so that
+the evaporation can not be equally active there, and the tendency to
+form aerial currents and great systems of winds must be proportionally
+slight. In brief, the clouds of Jupiter are probably of an entirely
+different origin from that of terrestrial clouds, and rather resemble
+the chaotic masses of vapor that enveloped the earth when it was still
+in a seminebulous condition, and before its crust had formed.
+
+Although the strongest features of the disk of Jupiter are the great
+cloud belts, and the white or colored spots in the equatorial zone, yet
+the telescope shows many markings north and south of the belts,
+including a number of narrower and fainter belts, and small light or
+dark spots. None of them is absolutely fixed in position with
+reference to others. In other words, all of the spots, belts, and
+markings shift their places to a perceptible extent, the changes being
+generally very slow and regular, but occasionally quite rapid. The main
+belts never entirely disappear, and never depart very far from their
+mean positions with respect to the equator, but the smaller belts toward
+the north and south are more or less evanescent. Round or oblong spots,
+as distinguished from belts, are still more variable and transient. The
+main belts themselves show great internal commotion, frequently
+splitting up, through a considerable part of their length, and sometimes
+apparently throwing out projections into the lighter equatorial zone,
+which occasionally resemble bridges, diagonally spanning the broad space
+between the belts.
+
+[Illustration: JUPITER AS SEEN AT THE LICK OBSERVATORY IN 1889. THE
+GREAT RED SPOT IS VISIBLE, TOGETHER WITH THE INDENTATION IN THE SOUTH
+BELT.]
+
+Perhaps the most puzzling phenomenon that has ever made its appearance
+on Jupiter is the celebrated "great red spot," which was first noticed
+in 1878, although it has since been shown to be probably identical with
+a similar spot seen in 1869, and possibly with one noticed in 1857.
+This spot, soon after its discovery in 1878, became a clearly defined
+red oval, lying near the southern edge of the south belt in latitude
+about 30 deg. Its length was nearly one third of the diameter of the disk
+and its width almost one quarter as great as its length. Translated into
+terrestrial measure, it was about 30,000 miles long and 7,000 miles
+broad.
+
+In 1879 it seemed to deepen in color until it became a truly wonderful
+object, its redness of hue irresistibly suggesting the idea that it was
+something hot and glowing. During the following years it underwent
+various changes of appearance, now fading almost to invisibility and now
+brightening again, but without ever completely vanishing, and it is
+still (1901) faintly visible.
+
+Nobody has yet suggested an altogether probable and acceptable theory as
+to its nature. Some have said that it might be a part of the red-hot
+crust of the planet elevated above the level of the clouds; others that
+its appearance might be due to the clearing off of the clouds above a
+heated region of the globe beneath, rendering the latter visible through
+the opening; others that it was perhaps a mass of smoke and vapor
+ejected from a gigantic volcano, or from the vents covering a broad area
+of volcanic action; others that it might be a vast incandescent slag
+floating upon the molten globe of the planet and visible through, or
+above, the enveloping clouds; and others have thought that it could be
+nothing but a cloud among clouds, differing, for unknown reasons, in
+composition and cohesion from its surroundings. All of these hypotheses
+except the last imply the existence, just beneath the visible cloud
+shell, of a more or less stable and continuous surface, either solid or
+liquid.
+
+When the red spot began to lose distinctness a kind of veil seemed to be
+drawn over it, as if light clouds, floating at a superior elevation, had
+drifted across it. At times it has been reduced in this manner to a
+faint oval ring, the rim remaining visible after the central part has
+faded from sight.
+
+One of the most remarkable phenomena connected with the mysterious spot
+is a great bend, or scallop, in the southern edge of the south belt
+adjacent to the spot. This looks as if it were produced by the spot, or
+by the same cause to which the spot owes its existence. If the spot were
+an immense mountainous elevation, and the belt a current of liquid, or
+of clouds, flowing past its base, one would expect to see some such bend
+in the stream. The visual evidence that the belt is driven, or forced,
+away from the neighborhood of the spot seems complete. The appearance of
+repulsion between them is very striking, and even when the spot fades
+nearly to invisibility the curve remains equally distinct, so that in
+using a telescope too small to reveal the spot itself one may discover
+its location by observing the bow in the south belt. The suggestion of a
+resemblance to the flowing of a stream past the foot of an elevated
+promontory, or mountain, is strengthened by the fact, which was
+observed early in the history of the spot, that markings involved in the
+south belt have a quicker rate of rotation about the planet's axis than
+that of the red spot, so that such markings, first seen in the rear of
+the red spot, gradually overtake and pass it, and eventually leave it
+behind, as boats in a river drift past a rock lying in the midst of the
+current.
+
+This leads us to another significant fact concerning the peculiar
+condition of Jupiter's surface. Not only does the south belt move
+perceptibly faster than the red spot, but, generally speaking, the
+various markings on the surface of the planet move at different rates
+according as they are nearer to or farther from the equator. Between the
+equator and latitude 30 deg. or 40 deg. there is a difference of six
+minutes in the rotation period--i.e., the equatorial parts turn round
+the axis so much faster than the parts north and south of them, that in
+one rotation they gain six minutes of time. In other words, the clouds
+over Jupiter's equator flow past those in the middle latitudes with a
+relative velocity of 270 miles per hour. But there are no sharp lines
+of separation between the different velocities; on the contrary, the
+swiftness of rotation gradually diminishes from the equator toward the
+poles, as it manifestly could not do if the visible surface of Jupiter
+were solid.
+
+In this respect Jupiter resembles the sun, whose surface also has
+different rates of rotation diminishing from the equator. Measured by
+the motion of spots on or near the equator, Jupiter's rotation period is
+about nine hours fifty minutes; measured by the motion of spots in the
+middle latitudes, it is about nine hours fifty-six minutes. The red spot
+completes a rotation in a little less than nine hours and fifty-six
+minutes, but its period can not be positively given for the singular
+reason that it is variable. The variation amounts to only a few seconds
+in the course of several years, but it is nevertheless certain. The
+phenomenon of variable motion is not, however, peculiar to the red spot.
+Mr. W.F. Denning, who has studied Jupiter for a quarter of a century,
+says:
+
+"It is well known that in different latitudes of Jupiter there are
+currents, forming the belts and zones, moving at various rates of speed.
+In many instances the velocity changes from year to year. And it is a
+singular circumstance that in the same current a uniform motion is not
+maintained in all parts of the circumference. Certain spots move faster
+than others, so that if we would obtain a fair value for the rotation
+period of any current it is not sufficient to derive it from one marking
+alone; we must follow a number of objects distributed in different
+longitudes along the current and deduce a mean from the whole."[10]
+
+[Footnote 10: The Observatory, No. 286, December, 1899.]
+
+Nor is this all. Observation indicates that if we could look at a
+vertical section of Jupiter's atmosphere we should behold an equally
+remarkable contrast and conflict of motions. There is evidence that some
+of the visible spots, or clouds, lie at a greater elevation than others,
+and it has been observed that the deeper ones move more rapidly. This
+fact has led some observers to conclude that the deep-lying spots may be
+a part of the actual surface of the planet. But if we could think that
+there is any solid nucleus, or core, in the body of Jupiter, it would
+seem, on account of the slight mean density of the planet, that it can
+not lie so near the visible surface, but must be at a depth of
+thousands, perhaps tens of thousands, of miles. Since the telescope is
+unable to penetrate the cloudy envelope we can only guess at the actual
+constitution of the interior of Jupiter's globe. In a spirit of mere
+speculative curiosity it has been suggested that deep under the clouds
+of the great planet there may be a comparatively small solid globe, even
+a habitable world, closed round by a firmament all its own, whose vault,
+raised 30,000 or 40,000 miles above the surface of the imprisoned
+planet, appears only an unbroken dome, too distant to reveal its real
+nature to watchers below, except, perhaps, under telescopic scrutiny;
+enclosing, as in a shell, a transparent atmosphere, and deriving its
+illumination partly from the sunlight that may filter through, but
+mainly from some luminous source within.
+
+But is not Jupiter almost equally fascinating to the imagination, if we
+dismiss all attempts to picture a humanly impossible world shut up
+within it, and turn rather to consider what its future may be, guided by
+the not unreasonable hypothesis that, because of its immense size and
+mass, it is still in a chaotic condition? Mention has been made of the
+resemblance of Jupiter to the sun by virtue of their similar manner of
+rotation. This is not the only reason for looking upon Jupiter as being,
+in some respects, almost as much a solar as a planetary body. Its
+exceptional brightness rather favors the view that a small part of the
+light by which it shines comes from its own incandescence. In size and
+mass it is half-way between the earth and the sun. Jupiter is eleven
+times greater than the earth in diameter and thirteen hundred times
+greater in volume; the sun is ten times greater than Jupiter in diameter
+and a thousand times greater in volume. The mean density of Jupiter, as
+we have seen, is almost exactly the same as the sun's.
+
+Now, the history of the solar system, according to the nebular
+hypothesis, is a history of cooling and condensation. The sun, a
+thousand times larger than Jupiter, has not yet sufficiently cooled and
+contracted to become incrusted, except with a shell of incandescent
+metallic clouds; Jupiter, a thousand times smaller than the sun, has
+cooled and contracted until it is but slightly, if at all, incandescent
+at its surface, while its thickening shell, although still composed of
+vapor and smoke, and still probably hot, has grown so dense that it
+entirely cuts off the luminous radiation from within; the earth, to
+carry the comparison one step further, being more than a thousand times
+smaller than Jupiter, has progressed so far in the process of cooling
+that its original shell of vapor has given place to one of solid rock.
+
+A sudden outburst of light from Jupiter, such as occurs occasionally in
+a star that is losing its radiance through the condensation of
+absorbing vapors around it, would furnish strong corroboration of the
+theory that Jupiter is really an extinguished sun which is now on the
+way to become a planet in the terrestrial sense.
+
+Not very long ago, as time is reckoned in astronomy, our sun, viewed
+from the distance of the nearer fixed stars, may have appeared as a
+binary star, the brighter component of the pair being the sun itself and
+the fainter one the body now called the planet Jupiter. Supposing the
+latter to have had the same intrinsic brilliance, surface for surface,
+as the sun, it would have radiated one hundred times less light than the
+sun. A difference of one hundredfold between the light of two stars
+means that they are six magnitudes apart; or, in other words, from a
+point in space where the sun appeared as bright as what we call a
+first-magnitude star, its companion, Jupiter, would have shone as a
+sixth-magnitude star. Many stars have companions proportionally much
+fainter than that. The companion of Sirius, for instance, is at least
+ten thousand times less bright than its great comrade.
+
+Looking at Jupiter in this way, it interests us not as the probable
+abode of intelligent life, but as a world in the making, a world,
+moreover, which, when it is completed--if it ever shall be after the
+terrestrial pattern--will dwarf our globe into insignificance. That
+stupendous miracle of world-making which is dimly painted in the grand
+figures employed by the writers of Genesis, and the composers of other
+cosmogonic legends, is here actually going on before our eyes. The
+telescope shows us in the cloudy face of Jupiter the moving of the
+spirit upon the face of the great deep. What the final result will be we
+can not tell, but clearly the end of the grand processes there in
+operation has not yet been reached.
+
+The interesting suggestion was made and urged by Mr. Proctor that if
+Jupiter itself is in no condition at present to bear life, its
+satellites may be, in that respect, more happily circumstanced. It can
+not be said that very much has been learned about the satellites of
+Jupiter since Proctor's day, and his suggestion is no less and no more
+probable now than it was when first offered.
+
+There has been cumulative evidence that Jupiter's satellites obey the
+same law that governs the rotation of our moon, viz., that which compels
+them always to keep the same face turned toward their primary, and this
+would clearly affect, although it might not preclude, their
+habitability. With the exception of the minute fifth satellite
+discovered by Barnard in 1892, they are all of sufficient size to retain
+at least some traces of an atmosphere. In fact, one of them is larger
+than the planet Mars, and another is of nearly the same size as that
+planet, while the smallest of the four principal ones is about equal to
+our moon. Under the powerful attraction of Jupiter they travel rapidly,
+and viewed from the surface of that planet they would offer a wonderful
+spectacle.
+
+They are continually causing solar eclipses and themselves undergoing
+eclipse in Jupiter's shadow, and their swiftly changing aspects and
+groupings would be watched by an astronomer on Jupiter with undying
+interest.
+
+But far more wonderful would be the spectacle presented by Jupiter to
+inhabitants dwelling on his moons. From the nearer moon, in particular,
+which is situated less than 220,000 miles from Jupiter's surface, the
+great planet would be an overwhelming phenomenon in the sky.
+
+Its immense disk, hanging overhead, would cover a circle of the
+firmament twenty degrees in diameter, or, in round numbers, forty times
+the diameter of the full moon as seen from the earth! It would shed a
+great amount of light and heat, and thus would more or less effectively
+supply the deficit of solar radiation, for we must remember that Jupiter
+and his satellites receive from the sun less than one twenty-fifth as
+much light and heat as the earth receives.
+
+The maze of contending motions, the rapid flow and eddying of cloud
+belts, the outburst of strange fiery spots, the display of rich, varied,
+and constantly changing colors, which astonish and delight the
+telescopic observer on the earth, would be exhibited to the naked eye of
+an inhabitant of Jupiter's nearest moon far more clearly than the
+greatest telescope is able to reveal them to us.
+
+Here, again, the mind is carried back to long past ages in the history
+of the planet on which we dwell. It is believed by some that our moon
+may have contained inhabitants when the earth was still hot and glowing,
+as Jupiter appears to be now, and that, as the earth cooled and became
+habitable, the moon gradually parted with its atmosphere and water so
+that its living races perished almost coincidently with the beginning of
+life on the earth. If we accept this view and apply it to the case of
+Jupiter we may conclude that when that enormous globe has cooled and
+settled down to a possibly habitable condition, its four attendant moons
+will suffer the fate that overtook the earth's satellite, and in their
+turn become barren and death-stricken, while the great orb that once
+nurtured them with its light and heat receives the Promethean fire and
+begins to bloom with life.
+
+
+
+
+CHAPTER VII
+
+SATURN, A PRODIGY AMONG PLANETS
+
+
+One of the first things that persons unaccustomed to astronomical
+observations ask to see when they have an opportunity to look through a
+telescope is the planet Saturn. Many telescopic views in the heavens
+disappoint the beginner, but that of Saturn does not. Even though the
+planet may not look as large as he expects to see it from what he has
+been told of the magnifying power employed, the untrained observer is
+sure to be greatly impressed by the wonderful rings, suspended around it
+as if by a miracle. No previous inspection of pictures of these rings
+can rob them of their effect upon the eye and the mind. They are
+overwhelming in their inimitable singularity, and they leave every
+spectator truly amazed. Sir John Herschel has remarked that they have
+the appearance of an "elaborately artificial mechanism." They have even
+been regarded as habitable bodies! What we are to think of that
+proposition we shall see when we come to consider their composition and
+probable origin. In the meantime let us recall the main facts of
+Saturn's dimensions and situation in the solar system.
+
+Saturn is the second of the major, or Jovian, group of planets, and is
+situated at a mean distance from the sun of 886,000,000 miles. We need
+not consider the eccentricity of its orbit, which, although relatively
+not very great, produces a variation of 50,000,000 miles in its distance
+from the sun, because, at its immense mean distance, this change would
+not be of much importance with regard to the planet's habitability or
+non-habitability. Under the most favorable conditions Saturn can never
+be nearer than 744,000,000 miles to the earth, or eight times the sun's
+distance from us. It receives from the sun about one ninetieth of the
+light and heat that we get.
+
+[Illustration: SATURN IN ITS THREE PRINCIPAL PHASES AS SEEN FROM THE
+EARTH. From a drawing by Bond.]
+
+Saturn takes twenty-nine and a half years to complete a journey about
+the sun. Like Jupiter, it rotates very rapidly on its axis, the period
+being ten hours and fourteen minutes. Its axis of rotation is inclined
+not far from the same angle as that of the earth's axis (26 deg.
+49 min.), so that its seasons should resemble ours, although their
+alternations are extremely slow in consequence of the enormous length
+of Saturn's year.
+
+Not including the rings in the calculation, Saturn exceeds the earth in
+size 760 times. The addition of the rings would not, however, greatly
+alter the result of the comparison, because, although the total surface
+of the rings, counting both faces, exceeds the earth's surface about 160
+times, their volume, owing to their surprising thinness, is only about
+six times the volume of the earth, and their mass, in consequence of
+their slight density, is very much less than the earth's, perhaps,
+indeed, inappreciable in comparison.
+
+Saturn's mean diameter is 73,000 miles, and its polar compression is
+even greater than that of Jupiter, a difference of 7,000 miles--almost
+comparable with the entire diameter of the earth--existing between its
+equatorial and its polar diameter, the former being 75,000 and the
+latter 68,000 miles.
+
+We found the density of Jupiter astonishingly slight, but that of Saturn
+is slighter still. Jupiter would sink if thrown into water, but Saturn
+would actually float, if not "like a cork," yet quite as buoyantly as
+many kinds of wood, for its mean density is only three quarters that of
+water, or one eighth of the earth's. In fact, there is no known planet
+whose density is so slight as Saturn's. Thus it happens that,
+notwithstanding its vast size and mass, the force of gravity upon Saturn
+is nearly the same as upon our globe. Upon visiting Venus we should find
+ourselves weighing a little less than at home, and upon visiting Saturn
+a little more, but in neither case would the difference be very
+important. If the relative weight of bodies on the surfaces of planets
+formed the sole test of their habitability, Venus and Saturn would both
+rank with the earth as suitable abodes for men.
+
+But the exceedingly slight density of Saturn seems to be most reasonably
+accounted for on the supposition that, like Jupiter, it is in a vaporous
+condition, still very hot within--although but slightly, if at all,
+incandescent at the surface--and, therefore, unsuited to contain life.
+It is hardly worth while to speculate about any solid nucleus within,
+because, even if such a thing were possible, or probable, it must lie
+forever hidden from our eyes. But if we accept the theory that Saturn is
+in an early formative stage, and that, millions of years hence, it may
+become an incrusted and habitable globe, we shall, at least, follow the
+analogy of what we believe to have been the history of the earth, except
+that Saturn's immense distance from the sun will always prevent it from
+receiving an amount of solar radiation consistent with our ideas of what
+is required by a living world. Of course, since one can imagine what he
+chooses, it is possible to suppose inhabitants suited to existence in a
+world composed only of whirling clouds, and a poet with the imagination
+of a Milton might give us very imposing and stirring images of such
+creatures and their chaotic surroundings, but fancies like these can
+have no basis in human experience, and consequently can make no claim
+upon scientific recognition.
+
+Or, as an alternative, it might be assumed that Saturn is composed of
+lighter elements and materials than those which constitute the earth and
+the other solid planets in the more immediate neighborhood of the sun.
+But such an assumption would put us entirely at sea as regards the forms
+of organic life that could exist upon a planet of that description, and,
+like Sir Humphry Davy in the Vision, that occupies the first chapter of
+his quaintly charming Consolations in Travel, or, the Last Days of a
+Philosopher, we should be thrown entirely upon the resources of the
+imagination in representing to ourselves the nature and appearance of
+its inhabitants. Yet minds of unquestioned power and sincerity have in
+all ages found pleasure and even profit in such exercises, and with
+every fresh discovery arises a new flight of fancies like butterflies
+from a roadside pool. As affording a glimpse into the mind of a
+remarkable man, as well as a proof of the fascination of such subjects,
+it will be interesting to quote from the book just mentioned Davy's
+description of his imaginary inhabitants of Saturn:
+
+"I saw below me a surface infinitely diversified, something like that of
+an immense glacier covered with large columnar masses, which appeared as
+if formed of glass, and from which were suspended rounded forms of
+various sizes which, if they had not been transparent, I might have
+supposed to be fruit. From what appeared to me to be analogous to
+bright-blue ice, streams of the richest tint of rose color or purple
+burst forth and flowed into basins, forming lakes or seas of the same
+color. Looking through the atmosphere toward the heavens, I saw
+brilliant opaque clouds, of an azure color, that reflected the light of
+the sun, which had to my eyes an entirely new aspect and appeared
+smaller, as if seen through a dense blue mist.
+
+"I saw moving on the surface below me immense masses, the forms of which
+I find it impossible to describe. They had systems for locomotion
+similar to those of the morse, or sea-horse, but I saw, with great
+surprise, that they moved from place to place by six extremely thin
+membranes, which they used as wings. Their colors were varied and
+beautiful, but principally azure and rose color. I saw numerous
+convolutions of tubes, more analogous to the trunk of the elephant than
+to anything else I can imagine, occupying what I supposed to be the
+upper parts of the body. It was with a species of terror that I saw one
+of them mounting upward, apparently flying toward those opaque clouds
+which I have before mentioned.
+
+"'I know what your feelings are,' said the Genius; 'you want analogies,
+and all the elements of knowledge to comprehend the scene before you.
+You are in the same state in which a fly would be whose microscopic eye
+was changed for one similar to that of man, and you are wholly unable to
+associate what you now see with your former knowledge. But those beings
+who are before you, and who appear to you almost as imperfect in their
+functions as the zoophytes of the polar sea, to which they are not
+unlike in their apparent organization to your eyes, have a sphere of
+sensibility and intellectual enjoyment far superior to that of the
+inhabitants of your earth. Each of those tubes, which appears like the
+trunk of an elephant, is an organ of peculiar motion or sensation. They
+have many modes of perception of which you are wholly ignorant, at the
+same time that their sphere of vision is infinitely more extended than
+yours, and their organs of touch far more perfect and exquisite.'"
+
+After descanting upon the advantages of Saturn's position for surveying
+some of the phenomena of the solar system and of outer space, and the
+consequent immense advances that the Saturnians have made in
+astronomical knowledge, the Genius continues:
+
+"'If I were to show you the different parts of the surface of this
+planet you would see the marvelous results of the powers possessed by
+these highly intellectual beings, and of the wonderful manner in which
+they have applied and modified matter. Those columnar masses, which seem
+to you as if rising out of a mass of ice below, are results of art, and
+processes are going on within them connected with the formation and
+perfection of their food. The brilliant-colored fluids are the results
+of such operations as on the earth would be performed in your
+laboratories, or more properly in your refined culinary apparatus, for
+they are connected with their system of nourishment. Those opaque azure
+clouds, to which you saw a few minutes ago one of those beings directing
+his course, are works of art, and places in which they move through
+different regions of their atmosphere, and command the temperature and
+the quantity of light most fitted for their philosophical researches,
+or most convenient for the purposes of life.'"[11]
+
+[Footnote 11: Davy, of course, was aware that, owing to increase of
+distance, the sun would appear to an inhabitant of Saturn with a disk
+only one ninetieth as great in area as that which it presents to our
+eyes.]
+
+But, while Saturn does not appear, with our present knowledge, to hold
+out any encouragement to those who would regard it as the abode of
+living creatures capable of being described in any terms except those of
+pure imagination, yet it is so unique a curiosity among the heavenly
+bodies that one returns again and again to the contemplation of its
+strange details. Saturn has nine moons, but some of them are relatively
+small bodies--the ninth, discovered photographically by Professor
+Pickering in 1899, being especially minute--and others are situated at
+great distances from the planet, and for these reasons, together with
+the fact that the sunlight is so feeble upon them that, surface for
+surface, they have only one ninetieth as much illumination as our moon
+receives, they can not make a very brilliant display in the Saturnian
+sky. To astronomers on Saturn they would, of course, be intensely
+interesting because of their perturbations and particularly the effect
+of their attraction on the rings.
+
+This brings us again to the consideration of those marvelous appendages,
+and to the statement of facts about them which we have not yet recalled.
+
+If the reader will take a ball three inches in diameter to represent the
+globe of Saturn, and, out of the center of a circular piece of
+writing-paper seven inches in diameter, will cut a round hole three and
+three quarter inches across, and will then place the ball in the middle
+of the hole in the paper, he will have a very fair representation of the
+relative proportions of Saturn and its rings. To represent the main gap
+or division in the rings he might draw, a little more than three eighths
+of an inch from the outer edge of the paper disk, a pencil line about a
+sixteenth of an inch broad.
+
+Perhaps the most striking fact that becomes conspicuous in making such a
+model of the Saturnian system is the exceeding thinness of the rings as
+compared with their enormous extent. They are about 170,000 miles across
+from outer edge to outer edge, and about 38,000 miles broad from outer
+edge to inner edge--including the gauze ring presently to be
+mentioned--yet their thickness probably does not surpass one hundred
+miles! In fact, the sheet of paper in our imaginary model is several
+times too thick to represent the true relative thickness of Saturn's
+rings.
+
+Several narrow gaps in the rings have been detected from time to time,
+but there is only one such gap that is always clearly to be seen, the
+one already mentioned, situated about 10,000 miles from the outer edge
+and about 1,600 miles in width. Inside of this gap the broadest and
+brightest ring appears, having a width of about 16,500 miles. For some
+reason this great ring is most brilliant near the gap, and its
+brightness gradually falls off toward its inner side. At a distance of
+something less than 20,000 miles from the planet--or perhaps it would
+be more correct to say above the planet, for the rings hang directly
+over Saturn's equator--the broad, bright ring merges into a mysterious
+gauzelike object, also in the form of a ring, which extends to within
+9,000 or 10,000 miles of the planet's surface, and therefore itself has
+a width of say 10,000 miles.
+
+In consequence of the thinness of the rings they completely disappear
+from the range of vision of small telescopes when, as occurs once in
+every fifteen years, they are seen exactly edgewise from the earth. In a
+telescope powerful enough to reveal them when in that situation they
+resemble a thin, glowing needle run through the ball of the planet. The
+rings will be in this position in 1907, and again in 1922.
+
+The opacity of the rings is proved by the shadow which they cast upon
+the ball of the planet. This is particularly manifest at the time when
+they are edgewise to the earth, for the sun being situated slightly
+above or below the plane of the rings then throws their shadow across
+Saturn close to its equator. When they are canted at a considerable
+angle to our line of sight their shadow is seen on the planet, bordering
+their outer edge where they cross the ball.
+
+The gauze ring, the detection of which as a faintly luminous phenomenon
+requires a powerful telescope, can be seen with slighter telescopic
+power in the form of a light shade projected against the planet at the
+inner edge of the broad bright ring. The explanation of the existence of
+this peculiar object depends upon the nature of the entire system,
+which, instead of being, as the earliest observers thought it, a solid
+ring or series of concentric rings, is composed of innumerable small
+bodies, like meteorites, perhaps, in size, circulating independently but
+in comparatively close juxtaposition to one another about Saturn, and
+presenting to our eyes, because of their great number and of our
+enormous distance, the appearance of solid, uniform rings. So a flock of
+ducks may look from afar like a continuous black line or band, although
+if we were near them we should perceive that a considerable space
+separates each individual from his neighbors.
+
+The fact that this is the constitution of Saturn's rings can be
+confidently stated because it has been mathematically proved that they
+could not exist if they were either solid or liquid bodies in a
+continuous form, and because the late Prof. James E. Keeler demonstrated
+with the spectroscope, by means of the Doppler principle, already
+explained in the chapter on Venus, that the rings circulate about the
+planet with varying velocities according to their distance from Saturn's
+center, exactly as independent satellites would do.
+
+It might be said, then, that Saturn, instead of having nine satellites
+only, has untold millions of them, traveling in orbits so closely
+contiguous that they form the appearance of a vast ring.
+
+As to their origin, it may be supposed that they are a relic of a ring
+of matter left in suspension during the contraction of the globe of
+Saturn from a nebulous mass, just as the rings from which the various
+planets are supposed to have been formed were left off during the
+contraction of the main body of the original solar nebula. Other similar
+rings originally surrounding Saturn may have become satellites, but the
+matter composing the existing rings is so close to the planet that it
+falls within the critical distance known as "Roche's limit," within
+which, owing to the tidal effect of the planet's attraction, no body so
+large as a true satellite could exist, and accordingly in the process of
+formation of the Saturnian system this matter, instead of being
+aggregated into a single satellite, has remained spread out in the form
+of a ring, although its substance long ago passed from the vaporous and
+liquid to the solid form. We have spoken of the rings as being composed
+of meteorites, but perhaps their component particles may be so small as
+to answer more closely to the definition of dust. In these rings of
+dust, or meteorites, disturbances are produced by the attraction of the
+planet and that of the outer satellites, and it is yet a question
+whether they are a stable and permanent feature of Saturn, or will, in
+the course of time, be destroyed.[12]
+
+[Footnote 12: For further details about Saturn's rings, see The Tides,
+by G.H. Darwin, chap. xx.]
+
+It has been thought that the gauze ring is variable in brightness. This
+would tend to show that it is composed of bodies which have been drawn
+in toward the planet from the principal mass of the rings, and these
+bodies may end their career by falling upon the planet. This process,
+indefinitely continued, would result in the total disappearance of the
+rings--Saturn would finally swallow them, as the old god from whom the
+planet gets its name is fabled to have swallowed his children.
+
+Near the beginning of this chapter reference was made to the fact that
+Saturn's rings have been regarded as habitable bodies. That, of course,
+was before the discovery that they were not solid. Knowing what we now
+know about them, even Dr. Thomas Dick, the great Scotch popularizer of
+astronomy in the first half of the nineteenth century, would have been
+compelled to abandon his theory that Saturn's rings were crowded with
+inhabitants. At the rate of 280 to the square mile he reckoned that they
+could easily contain 8,078,102,266,080 people.
+
+He even seems to have regarded their edges--in his time their actual
+thinness was already well known--as useful ground for the support of
+living creatures, for he carefully calculated the aggregate area of
+these edges and found that it considerably exceeded the area of the
+entire surface of the earth. Indeed, Dr. Dick found room for more
+inhabitants on Saturn's rings than on Saturn itself, for, excluding the
+gauze ring, undiscovered in his day, the two surfaces of the rings are
+greater in area than the surface of the globe of the planet. He did not
+attack the problem of the weight of bodies on worlds in the form of
+broad, flat, thin, surfaces like Saturn's rings, or indulge in any
+reflections on the interrelations of the inhabitants of the opposite
+sides, although he described the wonderful appearance of Saturn and
+other celestial objects as viewed from the rings.
+
+But all these speculations fall to the ground in face of the simple fact
+that if we could reach Saturn's rings we should find nothing to stand
+upon, except a cloud of swiftly flying dust or a swarm of meteors,
+swayed by contending attractions. And, indeed, it is likely that upon
+arriving in the immediate neighborhood of the rings they would virtually
+disappear! Seen close at hand their component particles might be so
+widely separated that all appearance of connection between them would
+vanish, and it has been estimated that from Saturn's surface the rings,
+instead of presenting a gorgeous arch spanning the heavens, may be
+visible only as a faintly gleaming band, like the Milky Way or the
+zodiacal light. In this respect the mystic Swedenborg appears to have
+had a clearer conception of the true nature of Saturn's rings than did
+Dr. Dick, for in his book on The Earths in the Universe he says--using
+the word "belt" to describe the phenomenon of the rings:
+
+"Being questioned concerning that great belt which appears from our
+earth to rise above the horizon of that planet, and to vary its
+situations, they [the inhabitants of Saturn] said that it does not
+appear to them as a belt, but only as somewhat whitish, like snow in the
+heaven, in various directions."
+
+In view of such observations as that of Prof. E.E. Barnard, in 1892,
+showing that a satellite passing through the shadow of Saturn's rings
+does not entirely disappear--a fact which proves that the rings are
+partially transparent to the sunlight--one might be tempted to ask
+whether Saturn itself, considering its astonishing lack of density, is
+not composed, at least in its outer parts, of separate particles of
+matter revolving independently about their center of attraction, and
+presenting the appearance of a smooth, uniform shell reflecting the
+light of the sun. In other words, may not Saturn be, exteriorly, a globe
+of dust instead of a globe of vapor? Certainly the rings, incoherent and
+translucent though they be, reflect the sunlight to our eyes, at least
+from the brighter part of their surface, with a brilliance comparable
+with that of the globe of the planet itself.
+
+As bearing on the question of the interior condition of Saturn and
+Jupiter, it should, perhaps, be said that mathematical considerations,
+based on the figures of equilibrium of rotating liquid masses, lead to
+the conclusion that those planets are comparatively very dense within.
+Professor Darwin puts the statement very strongly, as follows: "In this
+way it is known with certainty that the central portions of the planets
+Jupiter and Saturn are much denser, compared to their superficial
+portions, than is the case with the earth."[13]
+
+[Footnote 13: The Tides, by G.H. Darwin, p. 333.]
+
+The globe and rings of Saturn witness an imposing spectacle of gigantic
+moving shadows. The great ball stretches its vast shade across the full
+width of the rings at times, and the rings, as we have seen, throw their
+shadow in a belt, whose position slowly changes, across the ball,
+sweeping from the equator, now toward one pole and now toward the
+other. The sun shines alternately on each side of the rings for a space
+of nearly fifteen years--a day fifteen years long! And then, when that
+face of the ring is turned away from the sun, there ensues a night of
+fifteen years' duration also.
+
+Whatever appearance the rings may present from the equator and the
+middle latitudes on Saturn, from the polar regions they would be totally
+invisible. As one passed toward the north, or the south, pole he would
+see the upper part of the arch of the rings gradually sink toward the
+horizon until at length, somewhere in the neighborhood of the polar
+circle, it would finally disappear, hidden by the round shoulder of the
+great globe.
+
+
+URANUS, NEPTUNE, AND THE SUSPECTED ULTRANEPTUNIAN PLANET
+
+What has been said of Jupiter and Saturn applies also to the remaining
+members of the Jovian group of planets, Uranus and Neptune, viz., that
+their density is so small that it seems probable that they can not, at
+the present time, be in a habitable planetary condition. All four of
+these outer, larger planets have, in comparatively recent times, been
+solar orbs, small companions of the sun. The density of Uranus is about
+one fifth greater than that of water, and slightly greater than that of
+Neptune. Uranus is 32,000 miles in diameter, and Neptune 35,000 miles.
+Curiously enough, the force of gravity upon each of these two large
+planets is a little less than upon the earth. This arises from the fact
+that in reckoning gravity on the surface of a planet not only the mass
+of the planet, but its diameter or radius, must be considered. Gravity
+varies directly as the mass, but inversely as the square of the radius,
+and for this reason a large planet of small density may exercise a less
+force of gravity at its surface than does a small planet of great
+density.
+
+The mean distance of Uranus from the sun is about 1,780,000,000 miles,
+and its period of revolution is eighty-four years; Neptune's mean
+distance is about 2,800,000,000 miles, and its period of revolution is
+about 164 years.
+
+Uranus has four satellites, and Neptune one. The remarkable thing about
+these satellites is that they revolve _backward_, or contrary to the
+direction in which all the other satellites belonging to the solar
+system revolve, and in which all the other planets rotate on their axis.
+In the case of Uranus, the plane in which the satellites revolve is not
+far from a position at right angles to the plane of the ecliptic; but in
+the case of Neptune, the plane of revolution of the satellites is tipped
+much farther backward. Since in every other case the satellites of a
+planet are situated nearly in the plane of the planet's equator, it may
+be assumed that the same rule holds with Uranus and Neptune; and, that
+being so, we must conclude that those planets rotate backward on their
+axes. This has an important bearing on the nebular hypothesis of the
+origin of the solar system, and at one time was thought to furnish a
+convincing argument against that hypothesis; but it has been shown that
+by a modification of Laplace's theory the peculiar behavior of Uranus
+and Neptune can be reconciled with it.
+
+Very little is known of the surfaces of Uranus and Neptune. Indications
+of the existence of belts resembling those of Jupiter have been found in
+the case of both planets. There are similar belts on Saturn, and as they
+seem to be characteristic of large, rapidly rotating bodies of small
+density, it was to be expected that they would be found on Uranus and
+Neptune.
+
+The very interesting opinion is entertained by some astronomers that
+there is at least one other great planet beyond Neptune. The orbits of
+certain comets are relied upon as furnishing evidence of the existence
+of such a body. Prof. George Forbes has estimated that this, as yet
+undiscovered, planet may be even greater than Jupiter in mass, and may
+be situated at a distance from the sun one hundred times as great as the
+earth's, where it revolves in an orbit a single circuit of which
+requires a thousand years.
+
+Whether this planet, with a year a thousand of our years in length, will
+ever be seen with a telescope, or whether its existence will ever, in
+some other manner, be fully demonstrated, can not yet be told. It will
+be remembered that Neptune was discovered by means of computations based
+upon its disturbing attraction on Uranus before it had ever been
+recognized with the telescope. But when the astronomers in the
+observatories were told by their mathematical brethren where to look
+they found the planet within half an hour after the search began. So it
+is possible the suspected great planet beyond Neptune may be within the
+range of telescopic vision, but may not be detected until elaborate
+calculations have deduced its place in the heavens. As a populous city
+is said to furnish the best hiding-place for a man who would escape the
+attention of his fellow beings, so the star-sprinkled sky is able to
+conceal among its multitudes worlds both great and small until the most
+painstaking detective methods bring them to recognition.
+
+
+
+
+CHAPTER VIII
+
+THE MOON, CHILD OF THE EARTH AND THE SUN
+
+
+Very naturally the moon has always been a great favorite with those who,
+either in a scientific or in a literary spirit, have speculated about
+the plurality of inhabited worlds. The reasons for the preference
+accorded to the moon in this regard are evident. Unless a comet should
+brush us--as a comet is suspected of having done already--no celestial
+body, of any pretensions to size, can ever approach as near to the earth
+as the moon is, at least while the solar system continues to obey the
+organic laws that now control it. It is only a step from the earth to
+the moon. What are 240,000 miles in comparison with the distances of the
+stars, or even with the distances of the planets? Jupiter, driving
+between the earth and the moon, would occupy more than one third of the
+intervening space with the chariot of his mighty globe; Saturn, with
+broad wings outspread, would span more than two thirds of the distance;
+and the sun, so far from being able to get through at all, would overlap
+the way more than 300,000 miles on each side.
+
+In consequence, of course, of its nearness, the moon is the only member
+of the planetary system whose principal features are visible to the
+naked eye. In truth, the naked eye perceives the larger configurations
+of the lunar surface more clearly than the most powerful telescope shows
+the details on the disk of Mars. Long before the time of Galileo and the
+invention of the telescope, men had noticed that the face of the moon
+bears a resemblance to the appearance that the earth would present if
+viewed from afar off. In remote antiquity there were philosophers who
+thought that the moon was an inhabited world, and very early the
+romancers took up the theme. Lucian, the Voltaire of the second century
+of our era, mercilessly scourged the pretenders of the earth from an
+imaginary point of vantage on the moon, which enabled him to peer down
+into their secrets. Lucian's description of the appearance of the earth
+from the moon shows how clearly defined in his day had become the
+conception of our globe as only an atom in space.
+
+"Especially did it occur to me to laugh at the men who were quarreling
+about the boundaries of their land, and at those who were proud because
+they cultivated the Sikyonian plain, or owned that part of Marathon
+around Oenoe, or held possession of a thousand acres at Acharnae. Of
+the whole of Greece, as it then appeared to me from above, being about
+the size of four fingers, I think Attica was in proportion a mere speck.
+So that I wondered on what condition it was left to these rich men to be
+proud."[14]
+
+[Footnote 14: Ikaromenippus; or, Above the Clouds. Prof. D.C. Brown's
+translation.]
+
+Such scenes as Lucian beheld, in imagination, upon the earth while
+looking from the moon, many would fain behold, with telescopic aid,
+upon the moon while looking from the earth. Galileo believed that the
+details of the lunar surface revealed by his telescope closely resembled
+in their nature the features of the earth's surface, and for a long
+time, as the telescope continued to be improved, observers were
+impressed with the belief that the moon possessed not only mountains and
+plains, but seas and oceans also.
+
+It was the discovery that the moon has no perceptible atmosphere that
+first seriously undermined the theory of its habitability. Yet, as was
+remarked in the introductory chapter, there has of late been some change
+of view concerning a lunar atmosphere; but the change has been not so
+much in the ascertained facts as in the way of looking at those facts.
+
+But before we discuss this matter, it will be well to state what is
+known beyond peradventure about the moon.
+
+Its mean distance from the earth is usually called, for the sake of a
+round number, 240,000 miles, but more accurately stated it is 238,840
+miles. This is variable to the extent of more than 31,000 miles, on
+account of the eccentricity of its orbit, and the eccentricity itself is
+variable, in consequence of the perturbing attractions of the earth and
+the sun, so that the distance of the moon from the earth is continually
+changing. It may be as far away as 253,000 miles and as near as 221,600
+miles.
+
+Although the orbit of the moon is generally represented, for
+convenience, as an ellipse about the earth, it is, in reality, a varying
+curve, having the sun for its real focus, and always concave toward the
+latter. This is a fact that can be more readily explained with the aid
+of a diagram.
+
+[Illustration: THE MOON'S PATH WITH RESPECT TO THE SUN AND THE EARTH.]
+
+In the accompanying cut, when the earth is at _A_ the moon is between it
+and the sun, in the phase called new moon. At this point the earth's
+orbit about the sun is more curved than the moon's, and the earth is
+moving relatively faster than the moon, so that when it arrives at _B_
+it is ahead of the moon, and we see the latter to the right of the
+earth, in the phase called first quarter. The earth being at this time
+ahead of the moon, the effect of its attraction, combined with that of
+the sun, tends to hasten the moon onward in its orbit about the sun, and
+the moon begins to travel more swiftly, until it overtakes the earth at
+_C_, and appears on the side opposite the sun, in the phase called full
+moon. At this point the moon's orbit about the sun has a shorter radius
+of curvature than the earth's. In traveling from _C_ to _D_ the moon
+still moves more rapidly than the earth, and, having passed it, appears
+at _D_ to the left of the earth, in the phase called third quarter. Now,
+the earth being behind the moon, the effect of its attraction combined
+with the sun's tends to retard the moon in its orbit about the sun,
+with the result that the moon moves again less rapidly than the earth,
+and the latter overtakes it, so that, upon reaching _E_, the two are
+once more in the same relative positions that they occupied at _A_, and
+it is again new moon. Thus it will be seen that, although the real orbit
+of the moon has the sun for its center of revolution, nevertheless, in
+consequence of the attraction of the earth, combined in varying
+directions with that of the sun, the moon, once every month, makes a
+complete circuit of our globe.
+
+The above explanation should not be taken for a mathematical
+demonstration of the moon's motion, but simply for a graphical
+illustration of how the moon appears to revolve about the earth while
+really obeying the sun's attraction as completely as the earth does.
+
+There is no other planet that has a moon relatively as large as ours.
+The moon's diameter is 2,163 miles. Its volume, compared with the
+earth's, is in the ratio of 1 to 49, and its density is about six
+tenths of the earth's. This makes its mass to that of our globe about as
+1 to 81. In other words, it would take eighty-one moons to
+counterbalance the earth. Before speaking of the force of gravity on the
+moon we will examine the character of the lunar surface.
+
+To the naked eye the moon's face appears variegated with dusky patches,
+while a few points of superior brilliance shine amid the brighter
+portions, especially in the southern and eastern quarters, where immense
+craters like Tycho and Copernicus are visible to a keen eye, gleaming
+like polished buttons. With a telescope, even of moderate power, the
+surface of the moon presents a scene of astonishing complexity, in which
+strangeness, beauty, and grandeur are all combined. The half of the moon
+turned earthward contains an area of 7,300,000 square miles, a little
+greater than the area of South America and a little less than that of
+North America. Of these 7,300,000 square miles, about 2,900,000 square
+miles are occupied by the gray, or dusky, expanses, called in lunar
+geography, or selenography, _maria_--i.e., "seas." Whatever they may
+once have been, they are not now seas, but dry plains, bordered in many
+places by precipitous cliffs and mountains, varied in level by low
+ridges and regions of depression, intersected occasionally by immense
+cracks, having the width and depth of our mightiest river canons, and
+sprinkled with bright points and crater pits. The remaining 4,400,000
+square miles are mainly occupied by mountains of the most extraordinary
+character. Owing partly to roughness of the surface and partly to more
+brilliant reflective power, the mountainous regions of the moon appear
+bright in comparison with the dull-colored plains.
+
+Some of the lunar mountains lie in long, massive chains, with towering
+peaks, profound gorges, narrow valleys, vast amphitheaters, and beetling
+precipices. Looking at them with a powerful telescope, the observer
+might well fancy himself to be gazing down from an immense height into
+the heart of the untraveled Himalayas. But these, imposing though they
+are, do not constitute the most wonderful feature of the mountain
+scenery of the moon.
+
+Appearing sometimes on the shores of the "seas," sometimes in the midst
+of broad plains, sometimes along the course of mountain chains, and
+sometimes in magnificent rows, following for hundreds of miles the
+meridians of the lunar globe, are tremendous, mountain-walled, circular
+chasms, called craters. Frequently they have in the middle of their
+depressed interior floors a peak, or a cluster of peaks. Their inner and
+outer walls are seamed with ridges, and what look like gigantic streams
+of frozen lava surround them. The resemblance that they bear to the
+craters of volcanoes is, at first sight, so striking that probably
+nobody would ever have thought of questioning the truth of the statement
+that they are such craters but for their incredible magnitude. Many of
+them exceed fifty miles in diameter, and some of them sink two, three,
+four, and more miles below the loftiest points upon their walls! There
+is a chasm, 140 miles long and 70 broad, named Newton, situated about
+200 miles from the south pole of the moon, whose floor lies 24,000 feet
+below the summit of a peak that towers just above it on the east! This
+abyss is so profound that the shadows of its enclosing precipices never
+entirely quit it, and the larger part of its bottom is buried in endless
+night.
+
+One can not but shudder at the thought of standing on the broken walls
+of Newton, and gazing down into a cavity of such stupendous depth that
+if Chimborazo were thrown into it, the head of the mighty Andean peak
+would be thousands of feet beneath the observer.
+
+A different example of the crater mountains of the moon is the
+celebrated Tycho, situated in latitude about 43 deg. south, corresponding
+with the latitude of southern New Zealand on the earth. Tycho is nearly
+circular and a little more than 54 miles across. The highest point on
+its wall is about 17,000 feet above the interior. In the middle of its
+floor is a mountain 5,000 or 6,000 feet high. Tycho is especially
+remarkable for the vast system of whitish streaks, or rays, which
+starting from its outer walls, spread in all directions over the face of
+the moon, many of them, running, without deviation, hundreds of miles
+across mountains, craters, and plains. These rays are among the greatest
+of lunar mysteries, and we shall have more to say of them.
+
+[Illustration: THE LUNAR ALPS, APENNINES, AND CAUCASUS.
+Photographed with the Lick Telescope.]
+
+Copernicus, a crater mountain situated about 10 deg. north of the equator,
+in the eastern hemisphere of the moon, is another wonderful object, 56
+miles in diameter, a polygon appearing, when not intently studied, as a
+circle, 11,000 or 12,000 feet deep, and having a group of relatively low
+peaks in the center of its floor. Around Copernicus an extensive area of
+the moon's surface is whitened with something resembling the rays of
+Tycho, but more irregular in appearance. Copernicus lies within the edge
+of the great plain named the _Oceanus Procellarum_, or "Ocean of
+Storms," and farther east, in the midst of the "ocean," is a smaller
+crater mountain, named Kepler, which is also enveloped by a whitish
+area, covering the lunar surface as if it were the result of extensive
+outflows of light-colored lava.
+
+In one important particular the crater mountains of the moon differ from
+terrestrial volcanoes. This difference is clearly described by Nasmyth
+and Carpenter in their book on The Moon:
+
+"While the terrestrial crater is generally a hollow on a mountain top,
+with its flat bottom high above the level of the surrounding country,
+those upon the moon have their lowest points depressed more or less
+deeply below the general surface of the moon, the external height being
+frequently only a half or one third of the internal depth."
+
+It has been suggested that these gigantic rings are only "basal wrecks"
+of volcanic mountains, whose conical summits have been blown away,
+leaving vast crateriform hollows where the mighty peaks once stood; but
+the better opinion seems to be that which assumes that the rings were
+formed by volcanic action very much as we now see them. If such a crater
+as Copernicus or the still larger one named Theophilus, which is
+situated in the western hemisphere of the moon, on the shore of the "Sea
+of Nectar," ever had a conical mountain rising from its rim, the height
+attained by the peak, if the average slope were about 30 deg., would have
+been truly stupendous--fifteen or eighteen miles!
+
+There is a kind of ring mountains, found in many places on the moon,
+whose forms and surroundings do not, as the craters heretofore described
+do, suggest at first sight a volcanic origin. These are rather level
+plains of an oval or circular outline, enclosed by a wall of mountains.
+The finest example is, perhaps, the dark-gray Plato, situated in 50 deg. of
+north latitude, near an immense mountain uplift named the Lunar Alps,
+and on the northern shore of the _Mare Imbrium_, or "Sea of Showers."
+Plato appears as an oval plain, very smooth and level, about 60 miles in
+length, and completely surrounded by mountains, quite precipitous on the
+inner side, and rising in their highest peaks to an elevation of 6,000
+to 7,000 feet. Enclosed plains, bearing more or less resemblance to
+Plato--sometimes smooth within, and sometimes broken with small peaks
+and craters or hilly ridges--are to be found scattered over almost all
+parts of the moon. If our satellite was ever an inhabited world like the
+earth, while its surface was in its present condition, these valleys
+must have presented an extraordinary spectacle. It has been thought that
+they may once have been filled with water, forming lakes that recall the
+curious Crater Lake of Oregon.
+
+[Illustration: THE MOON AT FIRST AND LAST QUARTER (WESTERN AND EASTERN
+HEMISPHERES). Photographed with the Lick Telescope.]
+
+It is not my intention to give a complete description of the various
+lunar features, and I mention but one other--the "clefts" or "rills,"
+which are to be seen running across the surface like cracks. One of the
+most remarkable of these is found in the _Oceanus Procellarum_, near the
+crater-mountain Aristarchus, which is famed for the intense brilliance
+of its central peak, whose reflective power is so great that it was once
+supposed to be aflame with volcanic fire. The cleft, or crack, in
+question is very erratic in its course, and many miles in length, and
+it terminates in a ringed plain named Herodotus not far east of
+Aristarchus, breaking through the wall of the plain and entering the
+interior. Many other similar chasms or canons exist on the moon, some
+crossing plains, some cleaving mountain walls, and some forming a
+network of intersecting clefts. Mr. Thomas Gwyn Elger has this to say on
+the subject of the lunar clefts:
+
+"If, as seems most probable, these gigantic cracks are due to
+contractions of the moon's surface, it is not impossible, in spite of
+the assertions of the text-books to the effect that our satellite is now
+a 'changeless world,' that emanations may proceed from these fissures,
+even if, under the monthly alternations of extreme temperatures, surface
+changes do not now occasionally take place from this cause also. Should
+this be so, the appearance of new rills and the extension and
+modification of those already existing may reasonably be looked for."
+
+Mr. Elger then proceeds to describe his discovery in 1883, in the
+ring-plain Mersenius, of a cleft never noticed before, and which seems
+to have been of recent formation.[15]
+
+[Footnote 15: The Moon, a Full Description and Map of its Principal
+Features, by Thomas Gwyn Elger, 1895.
+
+Those who desire to read detailed descriptions of lunar scenery may
+consult, in addition to Mr. Elger's book, the following: The Moon,
+considered as a Planet, a World, and a Satellite, by James Nasmyth and
+James Carpenter, 1874; The Moon, and the Condition and Configurations of
+its Surface, by Edmund Neison, 1876. See also Annals of Harvard College
+Observatory, vol. xxxii, part ii, 1900, for observations made by Prof.
+William H. Pickering at the Arequipa Observatory.]
+
+We now return to the question of the force of lunar gravity. This we
+find to be only one sixth as great as gravity on the surface of the
+earth. It is by far the smallest force of gravity that we have found
+anywhere except on the asteroids. Employing the same method of
+comparison that was made in the case of Mars, we compute that a man on
+the moon could attain a height of thirty-six feet without being
+relatively more unwieldy than a six-foot descendant of Adam is on the
+earth.
+
+Whether this furnishes a sound reason for assuming that the lunar
+inhabitants, if any exist or have ever existed, should be preposterous
+giants is questionable; yet such an assumption receives a certain degree
+of support from the observed fact that the natural features of the moon
+are framed on an exaggerated scale as compared with the earth's. We have
+just observed that the moon is characterized by vast mountain rings,
+attaining in many cases a diameter exceeding fifty miles. If these are
+volcanic craters, it is evident, at a glance, that the mightiest
+volcanoes of the earth fall into insignificance beside them. Now, the
+slight force of gravity on the moon has been appealed to as a reason why
+volcanic explosions on the lunar globe should produce incomparably
+greater effects than upon the earth, where the ejected materials are so
+much heavier. The same force that would throw a volcanic bomb a mile
+high on the earth could throw it six miles high on the moon. The giant
+cannon that we have placed in one of our coast forts, which is said to
+be able to hurl a projectile to a distance of fifteen miles, could send
+the same projectile ninety miles on the moon. An athlete who can clear a
+horizontal bar at a height of six feet on the earth could clear the same
+bar at a height of thirty-six feet on the moon. In other words, he could
+jump over a house, unless, indeed, the lunarians really are giants, and
+live in houses proportioned to their own dimensions and to the size of
+their mountains. In that case, our athlete would have to content himself
+with jumping over a lunarian, whose head he could just clear--with the
+hat off.
+
+These things are not only amusing, but important. There can be no
+question that the force of gravity on the moon actually is as slight as
+it has just been described. So, even without calling in imaginary
+inhabitants to lend it interest, the comparative inability of the moon
+to arrest bodies in motion becomes a fact of much significance. It has
+led to the theory that meteorites may have originally been shot out of
+the moon's great volcanoes, when those volcanoes were active, and may
+have circulated about the sun until various perturbations have brought
+them down upon the earth. A body shot radially from the surface of the
+moon would need to have a velocity of only about a mile and a half in a
+second in order to escape from the moon's control, and we can believe
+that a lunar volcano when in action could have imparted such a velocity,
+all the more readily because with modern gunpowders we have been able to
+give to projectiles a speed one half as great as that needed for
+liberation from lunar gravity.
+
+Another consequence of the small gravitative power of the moon bears
+upon the all-important question of atmosphere. According to the theory
+of Dr. Johnstone Stoney, heretofore referred to, oxygen, nitrogen, and
+water vapor would all gradually escape from the moon, if originally
+placed upon it, because, by the kinetic theory, the maximum velocities
+of their molecules are greater than a mile and a half per second. The
+escape would not occur instantly, nor all at once, for it would be only
+the molecules at the upper surface of the atmosphere which were moving
+with their greatest velocity, and in a direction radial to the center of
+the moon, that would get away; but in the course of time this gradual
+leakage would result in the escape of all of those gases.[16]
+
+[Footnote 16: The discovery of free hydrogen in the earth's atmosphere,
+by Professor Dewar, 1901, bears upon the theory of the escape of gases
+from a planet, and may modify the view above expressed. Since hydrogen
+is theoretically incapable of being permanently retained in the free
+state by the earth, its presence in the atmosphere indicates either that
+there is an influx from space or that it emanates from the earth's
+crust. In a similar way it may be assumed that atmospheric gases can be
+given off from the crust of the moon, thus, to a greater or less extent,
+supplying the place of the molecules that escape.]
+
+After it had been found that, to ordinary tests, the moon offered no
+evidence of the possession of an atmosphere, and before Dr. Stoney's
+theory was broached, it was supposed by many that the moon had lost its
+original supply of air by absorption into its interior. The oxygen was
+supposed to have entered into combination with the cooling rocks and
+minerals, thus being withdrawn from the atmosphere, and the nitrogen was
+imagined to have disappeared also within the lunar crust. For it seems
+to have always been tacitly assumed that the phenomenon to be accounted
+for was not so much the _absence_ of a lunar atmosphere as its
+_disappearance_. But disappearance, of course, implies previous
+existence. In like manner it has always been a commonly accepted view
+that the moon probably once had enough water to form lakes and seas.
+
+These, it has been calculated, could have been absorbed into the lunar
+globe as it cooled off. But Johnstone Stoney's theory offers another
+method by which they could have escaped, through evaporation and the
+gradual flight of the molecules into open space. Possibly both methods
+have been in operation, a portion of the constituents of the former
+atmosphere and oceans having entered into chemical combinations in the
+lunar crust, and the remainder having vanished in consequence of the
+lack of sufficient gravitative force to retain them.
+
+But why, it may be asked, should it be assumed that the moon ever had
+things which it does not now possess? Perhaps no entirely satisfactory
+reply can be made. Some observers have believed that they detected
+unmistakable indications of alluvial deposits on lunar plains, and of
+the existence of beaches on the shores of the "seas." Messrs. Loewy and
+Puiseux, of the Paris Observatory, whose photographs of the moon are
+perhaps the finest yet made, say on this subject:
+
+"There exists, from the point of view of relief, a general similarity
+between the 'seas' of the moon and the plateaux which are covered to-day
+by terrestrial oceans. In these convex surfaces are more frequent than
+concave basins, thrown back usually toward the verge of the depressed
+space. In the same way the 'seas' of the moon present, generally at the
+edges, rather pronounced depressions. In one case, as in the other, we
+observe normal deformations of a shrinking globe shielded from the
+erosive action of rain, which tends, on the contrary, in all the
+abundantly watered parts of the earth to make the concave surfaces
+predominate. The explanation of this structure, such as is admitted at
+present by geologists, seems to us equally valid for the moon."[17]
+
+[Footnote 17: Comptes Rendus, June 26, July 3, 1899.]
+
+It might be urged that there is evidence of former volcanic activity on
+the moon of such a nature that explosions of steam must have played a
+part in the phenomena, and if there was steam, of course there was
+water.
+
+But perhaps the most convincing argument tending to show that the moon
+once had a supply of water, of which some remnant may yet remain below
+the surface of the lunar globe, is based upon the probable similarity in
+composition of the earth and the moon. This similarity results almost
+equally whether we regard the moon as having originated in a ring of
+matter left off from the contracting mass that became the earth, or
+whether we accept the suggestion of Prof. G.H. Darwin, that the moon is
+the veritable offspring of the earth, brought into being by the
+assistance of the tidal influence of the sun. The latter hypothesis is
+the more picturesque of the two, and, at present, is probably the more
+generally favored. It depends upon the theory of tidal friction, which
+was referred to in Chapter III, as offering an explanation of the manner
+in which the rotation of the planet Mercury has been slowed down until
+its rotary period coincides with that of its revolution.
+
+The gist of the hypothesis in question is that at a very early period in
+its history, when the earth was probably yet in a fluid condition, it
+rotated with extreme rapidity on its axis, and was, at the same time,
+greatly agitated by the tidal attraction of the sun, and finally huge
+masses were detached from the earth which, ultimately uniting, became
+the moon.[18]
+
+[Footnote 18: The Tides, by G.H. Darwin, chapter xvi.]
+
+Born in this manner from the very substance of the earth, the moon would
+necessarily be composed, in the main, of the same elements as the globe
+on which we dwell, and is it conceivable that it should not have carried
+with it both air and water, or the gases from which they were to be
+formed? If the moon ever had enough of these prime requisites to enable
+it to support forms of life comparable with those of the earth, the
+disappearance of that life must have been a direct consequence of the
+gradual vanishing of the lunar air and water. The secular drying up of
+the oceans and wasting away of the atmosphere on our little neighbor
+world involved a vast, all-embracing tragedy, some of the earlier scenes
+of which, if theories be correct, are now reenacted on the
+half-desiccated planet Mars--a planet, by the way, which in size, mass,
+and ability to retain vital gases stands about half-way between the
+earth and the moon.
+
+One of the most interesting facts about the moon is that its surface
+affords evidence of a cataclysm which has wiped out many, and perhaps
+nearly all, of the records of its earlier history, that were once
+written upon its face. Even on the earth there have been geological
+catastrophes destroying or burying the accumulated results of ages of
+undisturbed progress, but on the moon these effects have been
+transcendent. The story of the tremendous disaster that overtook the
+moon is partly written in its giant volcanoes. Although it may be true,
+as some maintain, that there is yet volcanic action going on upon the
+lunar surface, it is evident that such action must be insignificant in
+comparison with that which took place ages ago.
+
+There is a spot in the western hemisphere of the moon, on the border of
+a placid bay or "sea," that I can never look at without a feeling of awe
+and almost of shrinking. There, within a space about 250 miles in length
+by 100 in width, is an exhibition of the most terrifying effects of
+volcanic energy that the eye of man can anywhere behold. Three immense
+craters--Theophilus, 64 miles across and 3-1/2 miles deep; Cyrillus, 60
+miles across and 15,000 feet deep; and Catharina, 70 miles across and
+from 8,000 to 16,000 feet deep--form an interlinked chain of mountain
+rings, ridges, precipices, chasms, and bottomless pits that take away
+one's breath.
+
+But when the first impression of astonishment and dismay produced by
+this overwhelming spectacle has somewhat abated, the thoughtful observer
+will note that here the moon is telling him a part of her wonderful
+story, depicted in characters so plain that he needs no instruction in
+order to decipher their meaning. He will observe that this ruin was not
+all wrought at once or simultaneously. Theophilus, the crater-mountain
+at the northwestern end of the chain, whose bottom lies deepest of all,
+is the youngest of these giants, though the most imposing. For a
+distance of forty miles the lofty wall of Theophilus has piled itself
+upon the ruins of the wall of Cyrillus, and the circumference of the
+circle of its tremendous crater has been forcibly thrust within the
+original rim of the more ancient crater, which was thus rudely compelled
+to make room for its more vigorous rival and successor.
+
+The observer will also notice that Catharina, the huge pit at the
+southeastern end of the chain, bears evidence of yet greater age. Its
+original walls, fragments of which still stand in broken grandeur,
+towering to a height of 16,000 feet, have, throughout the greater part
+of their circuit, been riddled by the outbreak of smaller craters, and
+torn asunder and thrown down on all sides.
+
+In the vast enclosure that was originally the floor of the
+crater-mountain Catharina, several crater rings, only a third, a
+quarter, or a fifth as great in diameter, have broken forth, and these
+in turn have been partially destroyed, while in the interior of the
+oldest of them yet smaller craters, a nest of them, mere Etnas,
+Cotopaxis, and Kilaueas in magnitude, simple pinheads on the moon, have
+opened their tiny jaws in weak and ineffective expression of the waning
+energies of a still later epoch, which followed the truly heroic age of
+lunar vulcanicity.
+
+This is only one example among hundreds, scattered all over the moon,
+which show how the surface of our satellite has suffered upheaval after
+upheaval. It is possible that some of the small craters, not included
+within the walls of the greater ones, may represent an early stage in
+the era of volcanic activity that wrecked the moon, but where larger and
+smaller are grouped together a certain progression can be seen, tending
+finally to extinction. The internal energies reached a maximum and then
+fell off in strength until they died out completely.
+
+It can hardly be supposed that the life-bearing phase of lunar
+history--if there ever was one--could survive the outbreak of the
+volcanic cataclysm. North America, or Europe, if subjected to such an
+experience as the continental areas of the moon have passed through,
+would be, in proportion, worse wrecked than the most fearfully battered
+steel victim of a modern sea fight, and one can readily understand that,
+in such circumstances, those now beautiful and populous continents would
+exhibit, from a distance, scarcely any token of their present
+topographical features, to say nothing of any relics of their occupation
+by living creatures.
+
+There are other interesting glimpses to be had of an older world in the
+moon than that whose scarred face is now beautified for us by distance.
+Not far from Theophilus and the other great crater-mountains just
+described, at the upper, or southern, end of the level expanse called
+the "Sea of Nectar," is a broad, semicircular bay whose shores are
+formed by the walls of a partially destroyed crater named Fracastorius.
+It is evident that this bay, and the larger part of the "Sea of Nectar,"
+have been created by an outwelling of liquid lavas, which formed a
+smooth floor over a portion of the pre-existing surface of the moon, and
+broke down and submerged a large part of the mountain ring of
+Fracastorius, leaving the more ancient walls standing at the southern
+end, while, outlined by depressions and corrugations in the rocky
+blanket, are certain half-defined forms belonging to the buried world
+beneath.
+
+Near Copernicus, some years ago, as Dr. Edward S. Holden pointed out,
+photographs made with the great Lick telescope, then under his
+direction, showed, in skeleton outline, a huge ring buried beneath some
+vast outflow of molten matter and undiscerned by telescopic observers.
+And Mr. Elger, who was a most industrious observer and careful
+interpreter of lunar scenery, speaks of "the undoubted existence of the
+relics of an earlier lunar world beneath the smooth superficies of the
+_maria_."
+
+Although, as already remarked, it seems necessary to assume that any
+life existing in the moon prior to its great volcanic outburst must have
+ceased at that time, yet the possibility may be admitted that life could
+reappear upon the moon after its surface had again become quiet and
+comparatively undisturbed. Germs of the earlier life might have
+survived, despite the terrible nature of the catastrophe. But the
+conditions on the moon at present are such that even the most confident
+advocates of the view that the lunar world is not entirely dead do not
+venture to assume that anything beyond the lowest and simplest organic
+forms--mainly, if not wholly, in the shape of vegetation--can exist
+there. The impression that even such life is possible rests upon the
+accumulating evidence of the existence of a lunar atmosphere, and of
+visible changes, some apparently of a volcanic character and some not,
+on the moon's surface.
+
+Prof. William H. Pickering, who is, perhaps, more familiar with the
+telescopic and photographic aspects of the moon than any other American
+astronomer, has recorded numberless instances of change in minute
+details of the lunar landscapes. He regards some of his observations
+made at Arequipa as "pointing very strongly to the existence of
+vegetation upon the surface of the moon in large quantities at the
+present time." The mountain-ringed valley of Plato is one of the places
+in the lunar world where the visible changes have been most frequently
+observed, and more than one student of the moon has reached the
+conclusion that something very like the appearances that vegetation
+would produce is to be seen in that valley.
+
+Professor Pickering has thoroughly discussed the observations relating
+to a celebrated crater named Linne in the _Mare Serenitatis_, and after
+reading his description of its changes of appearance one can hardly
+reject his conclusion that Linne is an active volcanic vent, but
+variable in its manifestations. This is only one of a number of similar
+instances among the smaller craters of the moon. The giant ones are
+evidently entirely extinct, but some of the minor vents give occasional
+signs of activity. Nor should it be assumed that these relatively slight
+manifestations of volcanic action are really insignificant. As Professor
+Pickering shows, they may be regarded as comparable with the greatest
+volcanic phenomena now witnessed on the earth, and, speaking again of
+Plato, he says of its evidences of volcanic action:
+
+"It is, I believe, more active than any area of similar size upon the
+earth. There seems to be no evidences of lava, but the white streaks
+indicate apparently something analogous to snow or clouds. There must be
+a certain escape of gases, presumably steam and carbonic acid, the
+former of which, probably, aids in the production of the white
+markings."[19]
+
+[Footnote 19: Annals of Harvard College Observatory, vol. xxxii, part
+ii, 1900.]
+
+To Professor Pickering we owe the suggestion that the wonderful rays
+emanating from Tycho consist of some whitish substance blown by the
+wind, not from Tycho itself, but from lines of little volcanic vents or
+craters lying along the course of the rays. This substance may be
+volcanic powder or snow, in the form of minute ice crystals. Mr. Elger
+remarks of this theory that the "confused network of streaks" around
+Copernicus seems to respond to it more happily than the rays of Tycho
+do, because of the lack of definiteness of direction so manifest in the
+case of the rays.
+
+As an encouragement to amateur observers who may be disposed to find out
+for themselves whether or not changes now take place in the moon, the
+following sentence from the introduction to Professor Pickering's
+chapter on Plato in the Harvard Observatory Annals, volume xxxii, will
+prove useful and interesting:
+
+"In reviewing the history of selenography, one must be impressed by the
+singular fact that, while most of the astronomers who have made a
+special study of the moon, such as Schroeter, Maedler, Schmidt, Webb,
+Neison, and Elger, have all believed that its surface was still subject
+to changes readily visible from the earth, the great majority of
+astronomers who have paid little attention to the subject have quite as
+strenuously denied the existence of such changes."
+
+In regard to the lunar atmosphere, it may be said, in a word, that even
+those who advocate the existence of vegetation and of clouds of dust or
+ice crystals on the moon do not predicate any greater amount, or greater
+density, of atmosphere than do those who consider the moon to be wholly
+dead and inert. Professor Pickering himself showed, from his
+observations, that the horizontal refraction of the lunar atmosphere,
+instead of being less than 2 sec., as formerly stated, was less than
+0.4 sec. Yet he found visual evidence that on the sunlit side of the
+moon this rare atmosphere was filled to a height of four miles with some
+absorbing medium which was absent on the dark side, and which was
+apparently an emanation from the lunar crust, occurring after sunrise.
+And Messrs. Loewy and Puiseux, of the Paris Observatory, say, after
+showing reasons for thinking that the great volcanic eruptions belong to
+a recent period in the history of the moon, that "the diffusion of
+cinders to great distances infers a gaseous envelope of a certain
+density.... The resistance of the atmosphere must have been sufficient
+to retard the fall of this dust [the reference is to the white trails,
+like those from Tycho], during its transport over a distance of more
+than 1,000 kilometers [620 miles]."[20]
+
+[Footnote 20: Comptes Rendus, June 23, July 3, 1899.]
+
+We come now to a brief consideration of certain peculiarities in the
+motions of the moon, and in the phenomena of day and night on its
+surface. The moon keeps the same side forever turned toward the earth,
+behaving, in this respect, as Mercury does with regard to the sun. The
+consequence is that the lunar globe makes but one rotation on its axis
+in the course of a month, or in the course of one revolution about the
+earth. Some of the results of this practical identity of the periods of
+rotation and revolution are illustrated in the diagram on page 250. The
+moon really undergoes considerable libration, recalling the libration of
+Mercury, which was explained in the chapter on that planet, and in
+consequence we are able to see a little way round into the opposite
+lunar hemisphere, now on this side and now on the other, but in the
+diagram this libration has been neglected. If it had been represented we
+should have found that, instead of only one half, about three fifths of
+the total superficies of the moon are visible from the earth at one time
+or another.
+
+[Illustration: PHASES AND ROTATION OF THE MOON.]
+
+Perhaps it should be remarked that in drawing the moon's orbit about the
+earth as a center we offer no contradiction to what was shown earlier
+in this chapter. The moon does travel around the earth, and its orbit
+about our globe may, for our present purpose, be treated independently
+of its motion about the sun. Let the central globe, then, represent the
+earth, and let the sun be supposed to shine from the left-hand side of
+the diagram. A little cross is erected at a fixed spot on the globe of
+the moon.
+
+At _A_ the moon is between the earth and the sun, or in the phase of new
+moon. The lunar hemisphere facing the earth is now buried in night,
+except so far as the light reflected from the earth illuminates it, and
+this illumination, it is interesting to remember, is about fourteen
+times as great--reckoned by the relative areas of the reflecting
+surfaces--as that which the full moon sends to the earth. An inhabitant
+of the moon, standing beside the cross, sees the earth in the form of a
+huge full moon directly above his head, but, as far as the sun is
+concerned, it is midnight for him.
+
+In the course of about seven days the moon travels to _B_. In the
+meantime it has turned one quarter of the way around its axis, and the
+spot marked by the cross is still directly under the earth. For the
+lunar inhabitant standing on that spot the sun is now on the point of
+rising, and he sees the earth no longer in the shape of a full moon, but
+in that of a half-moon. The lunar globe itself appears, at the same
+time from the earth, as a half-moon, being in the position or phase that
+we call first quarter.
+
+Seven more days elapse, and the moon arrives at _C_, opposite to the
+position of the sun, and with the earth between it and the solar orb. It
+is now high noon for our lunarian standing beside the cross, while the
+earth over his head appears, if he sees it at all, only as a black disk
+close to the sun, or--as would sometimes be the case--covering the sun,
+and encircled with a beautiful ring of light produced by the refraction
+of its atmosphere. (Recall the similar phenomenon in the case of Venus.)
+The moon seen from the earth is now in the phase called full moon.
+
+Another lapse of seven days, and the moon is at _D_, in the phase called
+third quarter, while the earth, viewed from the cross on the moon, which
+is still pointed directly at it, appears again in the shape of a huge
+half-moon.
+
+During the next seven days the moon returns to its original position at
+_A_, and becomes once more new moon, with "full earth" shining upon it.
+
+Now it is evident that in consequence of the peculiar law of the moon's
+rotation its days and nights are each about two of our weeks, or
+fourteen days, in length. That hemisphere of the moon which is in the
+full sunlight at _A_, for instance, is buried in the middle of night at
+_C_. The result is different than in the case of Mercury, because the
+body toward which the moon always keeps the same face directed is not
+the luminous sun, but the non-luminous earth.
+
+It is believed that the moon acquired this manner of rotation in
+consequence of the tidal friction exercised upon it by the earth. The
+tidal attraction of the earth exceeds that of the sun upon the moon
+because the earth is so much nearer than the sun is, and tidal
+attraction varies inversely as the cube of the distance. In fact, the
+braking effect of tidal friction varies inversely as the sixth power of
+the distance, so that the ability of the earth to stop the rotation of
+the moon on its axis is immensely greater than that of the sun. This
+power was effectively applied while the moon was yet a molten mass, so
+that it is probable that the moon has rotated just as it does now for
+millions of years.
+
+As was remarked a little while ago, the moon traveling in an elliptical
+orbit about the earth has a libratory movement which, if represented in
+our picture, would cause the cross to swing now a little one way and now
+a little the other, and thus produce an apparent pendulum motion of the
+earth in the sky, similar to that of the sun as seen from Mercury. But
+it is not necessary to go into the details of this phenomenon. The
+reader, if he chooses, can deduce them for himself.
+
+But we may inquire a little into the effects of the long days and nights
+of the moon. In consequence of the extreme rarity of the lunar
+atmosphere, it is believed that the heat of the sun falling upon it
+during a day two weeks in length, is radiated away so rapidly that the
+surface of the lunar rocks never rises above the freezing temperature
+of water. On the night side, with no warm atmospheric blanket such as
+the earth enjoys, the temperature may fall far toward absolute zero, the
+most merciful figure that has been suggested for it being 200 deg. below
+the zero of our ordinary thermometers! But there is much uncertainty
+about the actual temperature on the moon, and different experiments, in
+the attempt to make a direct measurement of it, have yielded discordant
+results. At one time, for instance, Lord Rosse believed he had
+demonstrated that at lunar noon the temperature of the rocks rose above
+the boiling-point of water. But afterward he changed his mind and
+favored the theory of a low temperature.
+
+In this and in other respects much remains to be discovered concerning
+our interesting satellite, and there is plenty of room, and an abundance
+of original occupation, for new observers of the lunar world.
+
+
+
+
+CHAPTER IX
+
+HOW TO FIND THE PLANETS
+
+
+There is no reason why everybody should not know the principal planets
+at sight nearly as well as everybody knows the moon. It only requires a
+little intelligent application to become acquainted with the other
+worlds that have been discussed in the foregoing chapters, and to be
+able to follow their courses through the sky and recognize them wherever
+they appear. No telescope, or any other instrument whatever, is required
+for the purpose. There is but one preliminary requirement, just as every
+branch of human knowledge presupposes its A B C. This is an acquaintance
+with the constellations and the principal stars--not a difficult thing
+to obtain.
+
+Almost everybody knows the "Great Dipper" from childhood's days,
+except, perhaps, those who have had the misfortune to spend their youth
+under the glare of city lights. Some know Orion when he shines
+gloriously in the winter heavens. Many are able to point out the north
+star, or pole star, as everybody should be able to do. All this forms a
+good beginning, and may serve as the basis for the rapid acquirement of
+a general knowledge of the geography of the heavens.
+
+If you are fortunate enough to number an astronomer among your
+acquaintance--an amateur will do as well as a professor--you may, with
+his aid, make a short cut to a knowledge of the stars. Otherwise you
+must depend upon books and charts. My Astronomy with an Opera-Glass was
+prepared for this very purpose. For simply learning the constellations
+and the chief stars you need no opera-glass or other instrument. With
+the aid of the charts, familiarize yourself with the appearance of the
+constellations by noticing the characteristic arrangements of their
+chief stars. You need pay no attention to any except the bright stars,
+and those that are conspicuous enough to thrust themselves upon your
+attention.
+
+Learn by observation at what seasons particular constellations are on,
+or near, the meridian--i.e., the north and south line through the middle
+of the heavens. Make yourself especially familiar with the so-called
+zodiacal constellations, which are, in their order, running around the
+heavens from west to east: Aries, Taurus, Gemini, Cancer, Leo, Virgo,
+Libra, Scorpio, Sagittarius, Capricornus, Aquarius, and Pisces. The
+importance of these particular constellations arises from the fact that
+it is across them that the tracks of the planets lie, and when you are
+familiar with the fixed stars belonging to them you will be able
+immediately to recognize a stranger appearing among them, and will
+correctly conclude that it is one of the planets.[21] How to tell
+which planet it may be, it is the object of this chapter to show you. As
+an indispensable aid--unless you happen already to possess a complete
+star atlas on a larger scale--I have drawn the six charts of the
+zodiacal constellations and their neighbors that are included in this
+chapter.
+
+[Footnote 21: In our latitudes, planets are never seen in the northern
+quarter of the sky. When on the meridian, they are always somewhere
+between the zenith and the southern horizon.]
+
+[Illustration: CHART NO. 1.--FROM RIGHT ASCENSION 0 HOURS TO 4 HOURS;
+DECLINATION 30 deg. NORTH TO 10 deg. SOUTH.]
+
+Having learned to recognize the constellations and their chief stars on
+sight, one other step, an extremely easy one, remains to be taken before
+beginning your search for the planets--buy the American Ephemeris and
+Nautical Almanac for the current year. It is published under the
+direction of the United States Naval Observatory at Washington, and can
+be purchased for one dollar.
+
+This book, which may appear to you rather bulky and formidable for an
+almanac, contains hundreds of pages and scores of tables to which you
+need pay no attention. They are for navigators and astronomers, and are
+much more innocent than they look. The plain citizen, seeking only an
+introduction to the planets, can return their stare and pass by,
+without feeling in the least humiliated.
+
+[Illustration: CHART NO. 2.--FROM RIGHT ASCENSION 4 HOURS TO 8 HOURS;
+DECLINATION 30 deg. NORTH TO 10 deg. SOUTH.]
+
+In the front part of the book, after the long calendar, and the tables
+relating to the sun and the moon, will be found about thirty pages of
+tables headed, in large black letters, with the names of the
+planets--Mercury, Venus, Mars, Jupiter, Saturn, etc. Two months are
+represented on each page, and opposite the number of each successive day
+of the month the position of the planet is given in hours, minutes, and
+seconds of right ascension, and degrees, minutes, and seconds of north
+and south declination, the sign + meaning north, and the sign - south.
+Do not trouble yourself with the seconds in either column, and take the
+minutes only when the number is large. The hours of right ascension and
+the degrees of declination are the main things to be noticed.
+
+Right ascension, by the way, expresses the distance of a celestial body,
+such as a star or a planet, east of the vernal equinox, or the first
+point of Aries, which is an arbitrary point on the equator of the
+heavens, which serves, like the meridian of Greenwich on the earth, as
+a starting-place for reckoning longitude. The entire circuit of the
+heavens along the equator is divided into twenty-four hours of right
+ascension, each hour covering 15 deg. of space. If a planet then is in
+right ascension (usually printed for short R.A.) 0 h. 0 m. 0 s., it is on
+the meridian of the vernal equinox, or the celestial Greenwich; if it is
+in R.A. 1 h., it will be found 15 deg. east of the vernal equinox, and
+so on.
+
+[Illustration: CHART NO. 3.--FROM RIGHT ASCENSION 8 HOURS TO 12 HOURS;
+DECLINATION 30 deg. NORTH TO 10 deg. SOUTH.]
+
+Declination (printed D. or Dec.) expresses the distance of a celestial
+body north or south of the equator of the heavens.
+
+With these explanations we may proceed to find a planet by the aid of
+the Nautical Almanac and our charts. I take, for example, the ephemeris
+for the year 1901, and I look under the heading "Jupiter" on page 239,
+for the month of July. Opposite the 15th day of the month I find the
+right ascension to be 18 h. 27 m., neglecting the seconds. Now 27
+minutes are so near to half an hour that, for our purposes, we may say
+Jupiter is in R.A. 18 h. 30 m. I set this down on a slip of paper, and
+then examine the declination column, where I find that on July 15
+Jupiter is in south declination (the sign - meaning south, as before
+explained) 23 deg. 17 min. 52 sec., which is almost 23 deg. 18 min.,
+and, for our purposes, we may call this 23 deg. 20 min., which is what
+I set down on my slip.
+
+[Illustration: CHART NO. 4.--FROM RIGHT ASCENSION 12 HOURS TO 16 HOURS;
+DECLINATION 10 deg. NORTH TO 30 deg. SOUTH.]
+
+Next, I turn to Chart No. 5, in this chapter, where I find the meridian
+line of R.A. 18 h. running through the center of the chart. I know that
+Jupiter is to be looked for about 30 m. east, or to the left, of that
+line. At the bottom and top of the chart, every twenty minutes of R.A.
+is indicated, so that it is easy, with the eye, or with the aid of a
+ruler, to place the vertical line at some point of which Jupiter is to
+be found.
+
+[Illustration: CHART NO. 5.--FROM RIGHT ASCENSION 16 HOURS TO 20 HOURS;
+DECLINATION 10 deg. NORTH TO 30 deg. SOUTH.]
+
+Then I consult my note of the declination of the planet. It is south
+23 deg. 20 min. On the vertical borders of the chart I find the figures of
+the declination, and I observe that 0 deg. Dec., which represents the
+equator of the heavens, is near the top of the chart, while each parallel
+horizontal line across the chart indicates 10 deg. north or south of its
+next neighbor. Next to the bottom of the chart I find the parallel of
+20 deg., and I see that every five degrees is indicated by the figures at
+the sides. By the eye, or with the aid of a ruler, I easily estimate
+where the horizontal line of 23 deg. would fall, and since 20 min. is the
+third of a degree I perceive that it is, for the rough purpose of merely
+finding a conspicuous planet, negligible, although it, too, can be
+included in the estimate, if thought desirable.
+
+Having already found the vertical line on which Jupiter is placed and
+having now found the horizontal line also, I have simply to regard their
+crossing point, which will be the situation of the planet among the
+stars. I note that it is in the constellation Sagittarius in a certain
+position with reference to a familiar group of stars in that
+constellation, and when I look at the heavens, there, in the place thus
+indicated, Jupiter stands revealed.
+
+[Illustration: CHART NO. 6.--FROM RIGHT ASCENSION 20 HOURS TO 24 HOURS
+(0 II.); DECLINATION 10 deg. NORTH TO 30 deg. SOUTH.]
+
+The reader will readily perceive that, in a precisely similar manner,
+any planet can be located, at any time of the year, and at any point in
+its course about the heavens. But it may turn out that the place
+occupied by the planet is too near the sun to render it easily, or at
+all, visible. Such a case can be recognized, either from a general
+knowledge of the location of the constellations at various seasons, or
+with the aid of the Nautical Almanac, where at the beginning of each set
+of monthly tables in the calendar the sun's right ascension and
+declination will be found. In locating the sun, if you find that its
+right ascension differs by less than an hour, one way or the other, from
+that of the planet sought, it is useless to look for the latter. If the
+planet is situated west of the sun--to the right on the chart--then it
+is to be looked for in the east before sunrise. But if it is east of the
+sun--to the left on the chart--then you must seek it in the west after
+sunset.
+
+For instance, I look for the planet Mercury on October 12, 1901. I find
+its R.A. to be 14 h. 40 m. and its Dec. 18 deg. 36 min. Looking at the
+sun's place for October 12th, I find it to be R.A. 13 h. 8 m. and Dec.
+7 deg. 14 min. Placing them both on Chart No. 4, I discover that Mercury
+is well to the east, or left hand of the sun, and will consequently be
+visible in the western sky after sundown.
+
+Additional guidance will be found by noting the following facts about
+the charts:
+
+The meridian (the north and south line) runs through the middle of Chart
+No. 1 between 11 and 12 o'clock P.M. on November 1st, between 9 and 10
+o'clock P.M. on December 1st, and between 7 and 8 o'clock P.M. on
+January 1st.
+
+The meridian runs through the middle of Chart No. 2 between 11 and 12
+o'clock P.M. on January 1st, between 9 and 10 o'clock P.M. on February
+1st, and between 7 and 8 o'clock P.M. on March 1st.
+
+The meridian runs through the middle of Chart No. 3 between 11 and 12
+o'clock P.M. on March 1st, between 9 and 10 o'clock P.M. on April 1st,
+and between 7 and 8 o'clock P.M. on May 1st.
+
+The meridian runs through the middle of Chart No. 4 between 11 and 12
+o'clock P.M. on May 1st, between 9 and 10 o'clock P.M. on June 1st, and
+between 7 and 8 o'clock P.M. on July 1st.
+
+The meridian runs through the middle of Chart No. 5 between 11 and 12
+o'clock P.M. on July 1st, between 9 and 10 o'clock P.M. on August 1st,
+and between 7 and 8 o'clock P.M. on September 1st.
+
+The meridian runs through the middle of Chart No. 6 between 11 and 12
+o'clock P.M. on September 1st, between 9 and 10 o'clock P.M. on October
+1st, and between 7 and 8 o'clock P.M. on November 1st.
+
+Note well, also, these particulars about the charts: Chart No. 1
+includes the first four hours of right ascension, from 0 h. to 4 h.
+inclusive; Chart No. 2 includes 4 h. to 8 h.; Chart No. 3, 8 h. to 12
+h.; Chart No. 4, 12 h. to 16 h.; Chart No. 5, 16 h. to 20 h.; and Chart
+No. 6, 20 h. to 24 h., which completes the circuit. In the first three
+charts the line of 0 deg., or the equator, is found near the bottom, and in
+the last three near the top. This is a matter of convenience in
+arrangement, based upon the fact that the ecliptic, which, and not the
+equator, marks the center of the zodiac, indicates the position of the
+tracks of the planets among the stars; and the ecliptic, being inclined
+23 deg. to the plane of the equator, lies half to the north and half to the
+south of the latter.
+
+Those who, after all, may not care to consult the ephemeris in order to
+find the planets, may be able to locate them, simply from a knowledge of
+their situation among the constellations. Some ordinary almanacs tell in
+what constellations the principal planets are to be found at various
+times of the year. Having once found them in this way, it is
+comparatively easy to keep track of them thereafter through a general
+knowledge of their movements. Jupiter, for instance, requiring a period
+of nearly twelve years to make a single journey around the sun, moves
+about 30 deg. eastward among the stars every year. The zodiacal
+constellations are roughly about 30 deg. in length, and as Jupiter was in
+Sagittarius in 1901, he will be in Capricornus in 1902. Saturn,
+requiring nearly thirty years for a revolution around the sun, moves
+only between 12 deg. and 13 deg. eastward every year, and, being in
+conjunction with Jupiter in Sagittarius in 1901, does not get beyond the
+border of that constellation in 1902.
+
+Jupiter having been in opposition to the sun June 30, 1901, will be
+similarly placed early in August, 1902, the time from one opposition of
+Jupiter to the next being 399 days.
+
+Saturn passes from one opposition to the next in 378 days, so that
+having been in that position July 5, 1901, it reaches it again about
+July 18, 1902.
+
+Mars requires about 687 days to complete a revolution, and comes into
+conjunction with the earth, or opposition to the sun--the best position
+for observation--on the average once every 780 days. Mars was in
+opposition near the end of February, 1901, and some of its future
+oppositions will be in March, 1903; May, 1905; July, 1907; and
+September, 1909. The oppositions of 1907 and 1909 will be unusually
+favorable ones, for they will occur when the planet is comparatively
+near the earth. When a planet is in opposition to the sun it is on the
+meridian, the north and south line, at midnight.
+
+Mercury and Venus being nearer the sun than the earth is, can never be
+seen very far from the place of the sun itself. Venus recedes much
+farther from the solar orb than Mercury does, but both are visible only
+in the sunset or the sunrise sky. All almanacs tell at what times these
+planets play their respective roles as morning or as evening stars. In
+the case of Mercury about 116 days on the average elapse between its
+reappearances; in the case of Venus, about 584 days. The latter, for
+instance, having become an evening star at the end of April, 1901, will
+become an evening star again in December, 1902.
+
+With the aid of the Nautical Almanac and the charts the amateur will
+find no difficulty, after a little practise, in keeping track of any of
+the planets.
+
+In the back part of the Nautical Almanac will be found two pages headed
+"Phenomena: Planetary Configurations." With the aid of these the student
+can determine the position of the planets with respect to the sun and
+the moon, and with respect to one another. The meaning of the various
+symbols used in the tables will be found explained on a page facing the
+calendar at the beginning of the book. From these tables, among other
+things, the times of greatest elongation from the sun of the planets
+Mercury and Venus can be found.
+
+It may be added that only bright stars, and stars easily seen, are
+included in the charts, and there will be no danger of mistaking any of
+these stars for a planet, if the observer first carefully learns to
+recognize their configurations. Neither Mars, Jupiter, nor Saturn ever
+appears as faint as any of the stars, except those of the first
+magnitude, included in the charts. Uranus and Neptune being invisible to
+the naked eye--Uranus can occasionally be just glimpsed by a keen
+eye--are too faint to be found without the aid of more effective
+appliances.
+
+
+
+
+INDEX
+
+
+Agassiz, Alexander, on deep-sea animals, 63.
+
+Asteroids, the, 16, 129.
+  brightness of, 130.
+  imaginary adventures on, 146.
+  life on, 144.
+  number of, known, 129.
+  orbits of, 132.
+  origin of, 138, 143.
+  size of, 129.
+
+Aristarchus, lunar crater, 226.
+
+Atmosphere, importance of, 20.
+
+
+Bailey, Solon I., on oppositions of Eros, 134.
+
+Barnard, E.E., discovers fifth satellite of Jupiter, 181.
+  measures asteroids, 129.
+  on Saturn's rings, 205.
+
+Belopolski, on rotation of Venus, 79.
+
+
+Ceres, an asteroid, 129, 130.
+
+Clefts in the moon, 226.
+
+Copernicus, lunar crater, 223, 242.
+
+
+Darwin, George H., on Jupiter and Saturn, 206.
+  on origin of moon, 235.
+  theory of tidal friction, 32.
+
+Davy, Sir Humphry, on Saturn, 190.
+
+Dawes sees canals on Mars, 93.
+
+Deimos, satellite of Mars, 125.
+
+Denning, W.F., description of Jupiter, 175.
+
+De Vico on rotation of Venus, 76.
+
+Dewar, James, discovers free hydrogen in air, 232.
+
+De Witt discovers Eros, 133.
+
+Dick, Thomas, on Saturn, 201.
+
+Douglass, A.E., sees Mars's canals, 92.
+  sees clouds in Mars, 119.
+
+Doppler's principle, 79, 200.
+
+
+Earth and moon's orbit, 217.
+  birth of moon from, 236.
+  change of distance from sun, 27.
+  less advanced than Mars, 89.
+  older than Venus, 58.
+  seen from Mercury, 41.
+  seen from Venus, 69-71, 75.
+  seen from moon, 214.
+
+Earth, similarity to Venus, 46.
+  supposed signals to and from Mars, 110.
+
+Elger, T.G., on cracks in moon, 227.
+  on Tycho's rays, 246.
+
+Ephemeris, how to use, 260, 264.
+
+Eros, an asteroid, 131-134, 136, 137.
+
+
+Flammarion, C., observes Venus's atmosphere, 56.
+  on plurality of worlds, 8.
+
+Forbes, Prof. George, on ultra-Neptunian planet, 210.
+
+
+Galileo on lunar world, 215.
+
+Gravity, as affecting life on planets, 20, 46.
+
+
+Hall, Asaph, discovers Mars's moons, 90.
+
+Herodotus, lunar crater, 227.
+
+Herschel, Sir John, on Saturn, 185.
+
+Holden, E.S., on photograph of lunar crater, 242.
+
+Huggins on Mercury's atmosphere, 21.
+
+
+Inhabitants of foreign planets, 1, 4, 5.
+
+Interplanetary communication, 1, 3, 72, 110, 112.
+
+
+Juno, an asteroid, 129.
+
+Jupiter, cloudy aspect of, 165.
+  density of, 162.
+  distance of, 161.
+  equatorial belts on, 165.
+  future of, 180.
+  gravity on, 162.
+  great red spot on, 169.
+  markings outside the belts, 168.
+  and the nebular theory, 178.
+  once a companion star, 179.
+  polar compression of, 161.
+  possibly yet incandescent, 177.
+  question of a denser core, 176.
+  resemblance of, to sun, 174.
+  rotation of, 161, 173.
+  satellites of, 166, 181.
+  seen from satellites, 182.
+  size of, 160.
+  solar light and heat on, 182.
+  south belt of, 172.
+  surface conditions of, 163.
+  theories about the red spot, 170.
+  trade-winds and the belts of, 167.
+  various rates of rotation of, 173.
+  visibility of rotation of, 166.
+
+
+Keeler, J.E., on Saturn's rings, 200.
+
+Kepler, lunar crater, 223.
+
+Kinetic theory of gases, 116.
+
+Kirkwood, Daniel, on asteroids, 131.
+
+
+Lagrange on Olbers's theory, 139.
+
+Lick Observatory and Mars's canals, 92.
+
+Life, a planetary phenomenon, 10.
+  in sea depths, 62.
+  on planets, 62, 63.
+  prime requisites of, 64.
+  resisting extreme cold, 123.
+  universality of, 9.
+
+Loewy and Puiseux, on lunar atmosphere, 248.
+  on lunar "seas," 234.
+
+Lowell, Percival, description of Mars, 108.
+  on markings of Venus, 60.
+  on Mercury's rotation, 33.
+  on rotation of Venus, 77.
+  sees Mars's canals, 92.
+  theory of Martian canals, 101.
+
+Lucian, on appearance of earth from moon, 213.
+
+Lyman, C.S., observes Venus's atmosphere, 55.
+
+
+Mars, age of, 89.
+  atmosphere of, 86, 115, 117.
+  bands of life on, 104.
+  canals on, 90.
+    described by Schiaparelli, 93.
+    gemination of, 91, 105.
+    have builders of, disappeared? 107.
+    and irrigation, 101.
+    and lines of vegetation, 102.
+    and seasonal changes, 99.
+    and water circulation, 100.
+  carbon dioxide on, 118.
+  circular spots or "oases" on, 103.
+  colors of, 89.
+  dimensions of, 86.
+  distance of, 85, 86.
+  enigmatical lights on, 111.
+  gravity on, 86.
+  inclination of axis, 86.
+  length of year, 86.
+  Lowell's theory of, 101.
+  light and heat on, 85.
+  moonlight on, 128.
+  orbit of, 85.
+  polar caps of, 87, 118.
+  possible size of inhabitants, 106.
+  satellites of, 90, 124, 126.
+  seasons on, 87.
+  supposed signals from, 110, 112.
+  temperature of, 120, 122.
+  water vapor on, 117.
+
+Mercury, atmosphere of, 21, 28, 43, 44.
+  day and night on, 34, 38, 40.
+  dimensions, 18.
+  earth seen from, 41.
+  habitability of, 33, 40, 44.
+  heavens seen from, 41, 42.
+  heat and light on, 25, 28.
+  holds place of honor, 19.
+  length of year, 24.
+  mass of, 19.
+  moon visible from, 41.
+  resemblances to moon, 43.
+  rotation of, 30.
+  shape of orbit, 23.
+  sun as seen from, 37.
+  velocity in orbit, 23.
+  Venus seen from, 41.
+  virtual fall toward sun, 24.
+  visibility of, 21.
+  water on, 43.
+
+Moon, the area of surface, 219.
+  atmosphere of, 7, 215, 231, 247, 248.
+  clouds on, 6, 245.
+  constitution of, 236.
+  craters, 221.
+  day and night on, 254.
+  distance of, 212, 215.
+  density of, 219.
+  former cataclysm on, 237.
+  former life on, 241, 243.
+  giantism on, 228, 229.
+  gravity on, 219, 228, 229.
+  libration of, 249.
+  meteorites and, 230.
+  mountains on, 220.
+  the older world in, 242.
+  origin of, 235.
+  phases and motions of, 250.
+  rotation of, 249.
+  seas of, 234.
+  size of, 218.
+  snow on, 246.
+  speculation about, 212.
+  temperature of, 255.
+  vegetation on, 6, 244, 247.
+  visibility of features of, 213.
+
+
+Nasmyth and Carpenter on lunar craters, 224.
+
+Neptune, description of, 208-210.
+
+Newcomb, Simon, on Olbers's theory, 141.
+
+Newton, lunar crater, 222.
+
+
+Olbers's theory of planetary explosion, 138.
+  on Vesta's light, 138.
+
+
+Pallas, an asteroid, 129.
+
+Perrotin sees canals on Mars, 92.
+
+Phobos, satellite of Mars, 125.
+
+Pickering, E.C., discovers ninth moon of Saturn, 195.
+  finds Eros on Harvard plates, 133.
+  on shape of Eros, 136.
+  on light of Eros, 137.
+
+Pickering, W.H., on lunar atmosphere, 247.
+  observes changes in moon, 244.
+  sees Mars's canals, 92.
+  theory of Tycho's rays, 246.
+  on Venus's atmosphere, 54.
+
+Planets, classification of, 15.
+  how to find, 256, 273.
+  resemblances among, 12.
+
+Plato, lunar ring plain, 225.
+
+Plurality of worlds in literature, 2.
+  subject ignored, 8.
+
+Proctor, R.A., on Jupiter's moons, 180.
+  on other worlds, 8.
+
+Roche's limit, 201.
+
+Rosse, Lord, on temperature of moon, 255.
+
+
+Saturn, age of, 189.
+  composition of, 190.
+  density of, 188.
+  distance of, 186.
+  the gauze ring, 199-202.
+  gravity on, 188.
+  inclination of axis, 187.
+  interior of, 206.
+  length of year, 186.
+  popular telescopic object, 185.
+  rings of, 185, 196.
+    gaps in, 197.
+    origin of, 200.
+    periodic disappearance of, 198.
+    seen from planet, 207.
+    shadow of, 198.
+  rotation of, 187.
+  satellites of, 195.
+  size of, 187.
+
+Schiaparelli discovers canals on Mars, 90.
+  describes Martian canals, 93.
+  discovers Mercury's rotation, 30, 32.
+  on rotation of Venus, 76.
+
+Solar system, shape and size of, 14.
+  unity of, 9.
+  viewed from space, 11.
+
+Stoney, Johnstone, on atmospheres of planets, 116.
+  on escape of gases from moon, 231.
+
+Sun, the, isolation in space, 13.
+  no life on, 10.
+  resemblances with Jupiter, 174.
+
+Swedenborg, on Saturn's rings, 204.
+
+
+Tidal friction, 80, 81, 236, 253.
+
+Tycho, lunar crater, 222.
+
+
+Ultra-Neptunian planet, 210.
+
+Uranus, description of, 208-210.
+
+
+Venus, age of, 58.
+  atmosphere of, 53, 55, 59, 61, 68.
+  absence of seasons on, 51.
+  density of, 47.
+  distance of, 47, 50.
+  gravity on, 46, 47.
+  inclination of axis, 50.
+  life on, 57, 58, 61, 65, 67, 68, 82, 117.
+  light and heat on, 50-57.
+  orbit of, 50.
+  phases of, 49.
+  resemblances of, to earth, 46.
+  rotation of, 76, 79, 80.
+  size of, 46.
+  twilight on, 83.
+  visibility of, 47.
+
+Vesta, an asteroid, 129, 130, 138.
+
+Vogel on Mercury's atmosphere, 21.
+
+
+Wireless telegraphy, 1, 112.
+
+
+Young, C.A., on Olbers's theory of asteroids, 142.
+  on temperature of Mars, 122.
+  on Venus's atmosphere, 53.
+
+
+Zodiac, the, 258.
+
+
+THE END
+
+
+
+
+A NEW BOOK BY PROF. GROOS.
+
+The Play of Man.
+
+By KARL GROOS, Professor of Philosophy in the University of Basel, and
+author of "The Play of Animals." Translated, with the author's
+cooperation, by Elizabeth L. Baldwin, and edited, with a Preface and
+Appendix, by Prof. J. Mark Baldwin, of Princeton University. 12mo.
+Cloth, $1.50 net; postage, 12 cents additional.
+
+     The results of Professor Groos's original and acute investigations
+     are of peculiar value to those who are interested in psychology and
+     sociology, and they are of great importance to educators. He
+     presents the anthropological aspects of the subject treated in his
+     psychological study of the Play of Animals, which has already
+     become a classic. Professor Groos, who agrees with the followers of
+     Weismann, develops the great importance of the child's play as
+     tending to strengthen his inheritance in the acquisition of
+     adaptations to his environment. The influence of play on character,
+     and its relation to education, are suggestively indicated. The
+     playful manifestations affecting the child himself and those
+     affecting his relations to others have been carefully classified,
+     and the reader is led from the simpler exercises of the sensory
+     apparatus through a variety of divisions to inner imitations and
+     social play. The biological, aesthetic, ethical, and pedagogical
+     standpoints receive much attention from the investigator. While
+     this book is an illuminating contribution to scientific literature,
+     it is of eminently practical value. Its illustrations and lessons
+     will be studied and applied by educators, and the importance of
+     this original presentation of a most fertile subject will be
+     appreciated by parents as well as by those who are interested as
+     general students of sociological and psychological themes.
+
+D. APPLETON AND COMPANY, NEW YORK.
+
+
+
+
+D. APPLETON AND COMPANY'S PUBLICATIONS.
+
+RICHARD A. PROCTOR'S WORKS.
+
+
+_OTHER WORLDS THAN OURS: The Plurality of Worlds, Studied under the
+Light of Recent Scientific Researches._ With Illustrations, some
+colored. 12mo. Cloth, $1.75.
+
+     CONTENTS.--Introduction.--What the Earth teaches us.--What we learn
+     from the Sun.--The Inferior Planets.--Mars, the Miniature of our
+     Earth.--Jupiter, the Giant of the Solar System.--Saturn, the Ringed
+     World.--Uranus and Neptune, the Arctic Planets.--The Moon and other
+     Satellites.--Meteors and Comets: their Office in the Solar
+     System.--Other Suns than Ours.--Of Minor Stars, and of the
+     Distribution of Stars in Space.--The Nebulae: are they External
+     Galaxies?--Supervision and Control.
+
+
+_OUR PLACE AMONG INFINITIES._ A Series of Essays contrasting our Little
+Abode in Space and Time with the Infinities around us. To which are
+added Essays on the Jewish Sabbath and Astrology. 12mo. Cloth, $1.75.
+
+     CONTENTS.--Past and Future of the Earth.--Seeming Wastes in
+     Nature.--New Theory of Life in other Worlds.--A Missing Comet.--The
+     Lost Comet and its Meteor Train.--Jupiter.--Saturn and its
+     System.--A Giant Sun.--The Star Depths.--Star Gauging.--Saturn and
+     the Sabbath of the Jews.--Thoughts on Astrology.
+
+
+_THE EXPANSE OF HEAVEN._ A Series of Essays on the Wonders of the
+Firmament. 12mo. Cloth, $2.00.
+
+     CONTENTS.--A Dream that was not all a Dream.--The Sun.--The Queen
+     of Night.--The Evening Star.--The Ruddy Planet.--Life in the Ruddy
+     Planet.--The Prince of Planets.--Jupiter's Family of Moons.--The
+     Ring-Girdled Planet.--Newton and the Law of the Universe.--The
+     Discovery of Two Giant Planets.--The Lost Comet.--Visitants from
+     the Star Depths.--Whence come the Comets?--The Comet Families of
+     the Giant Planets.--The Earth's Journey through Showers.--How the
+     Planets Grew.--Our Daily Light.--The Flight of Light.--A Cluster of
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