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+The Project Gutenberg eBook of Curiosities of the Sky, by Garrett P. Serviss
+
+This eBook is for the use of anyone anywhere in the United States and
+most other parts of the world at no cost and with almost no restrictions
+whatsoever. You may copy it, give it away or re-use it under the terms
+of the Project Gutenberg License included with this eBook or online at
+www.gutenberg.org. If you are not located in the United States, you
+will have to check the laws of the country where you are located before
+using this eBook.
+
+Title: Curiosities of the Sky
+
+Author: Garrett P. Serviss
+
+Release Date: January 6, 2003 [eBook #6630]
+[Most recently updated: December 27, 2022]
+
+Language: English
+
+Character set encoding: UTF-8
+
+Produced by: William McClain
+
+*** START OF THE PROJECT GUTENBERG EBOOK CURIOSITIES OF THE SKY ***
+
+
+
+
+[Illustration]
+
+
+
+
+Curiosities of the Sky
+
+by Garrett P. Serviss
+
+
+
+
+Contents
+
+ PREFACE
+ CURIOSITIES OF THE SKY
+ I. The Windows of Absolute Night
+ II. Star-Clouds, Star-Clusters, and Star-Streams
+ III. Stellar Migrations
+ IV. The Passing of the Constellations
+ V. Conflagrations in the Heavens
+ VI. Explosive and Whirling Nebulæ
+ VII. The Banners of the Sun
+ VIII. The Zodiacal Light Mystery
+ IX. Marvels of the Aurora
+ X. Strange Adventures of Comets
+ XI. Meteors, Fire-Balls, and Meteorites
+ XII. The Wrecking of the Moon
+ XIII. The Great Mars Problem
+ XIV. The Riddle of the Asteroids
+
+
+
+
+[Illustration: Great spiral nebula in Andromeda]
+
+
+
+
+Illustrations
+
+ Great spiral nebula in Andromeda
+ The Milky Way
+ Star-cluster in Hercules
+ Great southern star-cluster, Omega Centauri
+ The Pleiades
+ The “Great Dipper”
+ Cassiopeia
+ The “Northern Crown”
+ The “Southern Cross”
+ Chart showing location of Tycho’s star, 1572,  and Nova Persei of 1901
+ Nova Persei, with its nebular rings
+ Lord Rosse’s nebula
+ Wonderful spiral in triangulum
+ Spiral in Ursa Major
+ Nebula in Cetus
+ The Orion nebula
+ The Corona
+ A solar “prominence”
+ Auroral beams seen in England
+ Auroral arches of an eliptic form seen in the Arctic regions
+ Auroral curtain seen in Scandinavia
+ Auroral arches seen in Scandinavia
+ Swift’s comet
+ Daniels’ comet
+ Brooks’ comet
+ Curious forms of meteorite trains
+ Section of the atmosphere up to 100 kilometers
+ A meteor photographed in flight
+ Looking across Coon Butte crater from northern rim
+ Trail on south side, Coon Butte crater
+ The craters Clavius, Longomontanus, Tycho, etc.
+ Western part of the Mare Serenitatis
+ Mare Tranquilitatis and surroundings
+ Lunar craters Theophilus and surrounding region
+ Mare Crisium
+ Schiaparelli’s chart of Mars, showing the so-called system of canals
+
+
+
+
+PREFACE
+
+
+What Froude says of history is true also of astronomy: it is the most
+impressive where it transcends explanation. It is not the mathematics
+of astronomy, but the wonder and the mystery that seize upon the
+imagination. The calculation of an eclipse owes all its prestige to the
+sublimity of its data; the operation, in itself, requires no more
+mental effort than the preparation of a railway time-table.
+
+The dominion which astronomy has always held over the minds of men is
+akin to that of poetry; when the former becomes merely instructive and
+the latter purely didactic, both lose their power over the imagination.
+Astronomy is known as the oldest of the sciences, and it will be the
+longest-lived because it will always have arcana that have not been
+penetrated.
+
+Some of the things described in this book are little known to the
+average reader, while others are well known; but all possess the
+fascination of whatever is strange, marvelous, obscure, or
+mysterious—magnified, in this case, by the portentous scale of the
+phenomena.
+
+The idea of the author is to tell about these things in plain language,
+but with as much scientific accuracy as plain language will permit,
+showing the wonder that is in them without getting away from the facts.
+Most of them have hitherto been discussed only in technical form, and
+in treatises that the general public seldom sees and never reads.
+
+Among the topics touched upon are:
+
+The strange unfixedness of the “fixed stars,” the vast migrations of
+the suns and worlds constituting the universe.
+
+The slow passing out of existence of those collocations of stars which
+for thousands of years have formed famous “constellations,” preserving
+the memory of mythological heroes and heroines, and perhaps of
+otherwise unrecorded history.
+
+The tendency of stars to assemble in immense clouds, swarms, and
+clusters.
+
+The existence in some of the richest regions of the universe of
+absolutely black, starless gaps, deeps, or holes, as if one were
+looking out of a window into the murkiest night.
+
+The marvelous phenomena of new, or temporary, stars, which appear as
+suddenly as conflagrations, and often turn into something else as
+eccentric as themselves.
+
+The amazing forms of the “whirlpool,” “spiral,” “pinwheel,” and “lace,”
+or “tress,” nebulæ.
+
+The strange surroundings of the sun, only seen in particular
+circumstances, but evidently playing a constant part in the daily
+phenomena of the solar system.
+
+The mystery of the Zodiacal Light and the Gegenschein.
+
+The extraordinary transformations undergone by comets and their tails.
+
+The prodigies of meteorites and masses of stone and metal fallen from
+the sky.
+
+The cataclysms that have wrecked the moon.
+
+The problem of life and intelligence on the planet Mars.
+
+The problematical origin and fate of the asteroids.
+
+The strange phenomena of the auroral lights.
+
+
+An attempt has been made to develop these topics in an orderly way,
+showing their connection, so that the reader may obtain a broad general
+view of the chief mysteries and problems of astronomy, and an idea of
+the immense field of discovery which still lies, almost unexplored,
+before it.
+
+
+
+
+CURIOSITIES OF THE SKY
+
+
+
+
+I
+The Windows of Absolute Night
+
+
+To most minds mystery is more fascinating than science. But when
+science itself leads straight up to the borders of mystery and there
+comes to a dead stop, saying, “At present I can no longer see my way,”
+the force of the charm is redoubled. On the other hand, the illimitable
+is no less potent in mystery than the invisible, whence the dramatic
+effect of Keats’ “stout Cortez” staring at the boundless Pacific while
+all his men look at each other with a wild surmise, “silent upon a peak
+in Darien.” It is with similar feelings that the astronomer regards
+certain places where from the peaks of the universe his vision seems to
+range out into endless empty space. He sees there the shore of his
+little isthmus, and, beyond, unexplored immensity.
+
+The name, “coal-sacks,” given to these strange voids is hardly
+descriptive. Rather they produce upon the mind the effect of blank
+windows in a lonely house on a pitch-dark night, which, when looked at
+from the brilliant interior, become appalling in their rayless murk.
+Infinity seems to acquire a new meaning in the presence of these black
+openings in the sky, for as one continues to gaze it loses its purely
+metaphysical quality and becomes a kind of entity, like the ocean. The
+observer is conscious that he can actually _see_ the beginning of its
+ebon depths, in which the visible universe appears to float like an
+enchanted island, resplendent within with lights and life and gorgeous
+spectacles, and encircled with screens of crowded stars, but with its
+dazzling vistas ending at the fathomless sea of pure darkness which
+encloses all.
+
+The Galaxy, or Milky Way, surrounds the borders of our island in space
+like a stellar garland, and when openings appear in it they are, by
+contrast, far more impressive than the general darkness of the
+interstellar expanse seen in other directions. Yet even that expanse is
+not everywhere equally dark, for it contains gloomy deeps discernable
+with careful watching. Here, too, contrast plays an important part,
+though less striking than within the galactic region. Some of Sir
+William Herschel’s observations appear to indicate an association
+between these tenebrious spots and neighboring star clouds and nebulæ.
+It is an illuminating bit of astronomical history that when he was
+sweeping the then virgin heavens with his great telescopes he was
+accustomed to say to his sister who, note-book in hand, waited at his
+side to take down his words, fresh with the inspiration of discovery:
+“Prepare to write; the nebulæ are coming; here space is vacant.”
+
+The most famous of the “coal-sacks,” and the first to be brought to
+general attention before astronomers had awakened to the significance
+of such things, lies adjacent to the “Southern Cross,” and is truly an
+amazing phenomenon. It is not alone the conspicuousness of this
+celestial vacancy, opening suddenly in the midst of one of the richest
+parts of the Galaxy, that has given it its fame, but quite as much the
+superstitious awe with which it was regarded by the early explorers of
+the South Seas. To them, as well as to those who listened in rapt
+wonder to their tales, the “Coal-sack” seemed to possess some occult
+connection with the mystic “Cross.” In the eyes of the sailors it was
+not a vacancy so much as a sable reality in the sky, and as,
+shuddering, they stared at it, they piously crossed themselves. It was
+another of the magical wonders of the unknown South, and as such it
+formed the basis of many a “wild surmise” and many a sea-dog’s yarn.
+Scientific investigation has not diminished its prestige, and today no
+traveler in the southern hemisphere is indifferent to its fascinating
+strangeness, while some find it the most impressive spectacle of the
+antarctic heavens.
+
+All around, up to the very edge of the yawning gap, the sheen of the
+Milky Way is surpassingly glorious; but there, as if in obedience to an
+almighty edict, everything vanishes. A single faint star is visible
+within the opening, producing a curious effect upon the sensitive
+spectator, like the sight of a tiny islet in the midst of a black,
+motionless, waveless tarn. The dimensions of the lagoon of darkness,
+which is oval or pear-shaped, are eight degrees by five, so that it
+occupies a space in the sky about one hundred and thirty times greater
+than the area of the full moon. It attracts attention as soon as the
+eye is directed toward the quarter where it exists, and by virtue of
+the rarity of such phenomena it appears a far greater wonder than the
+drifts of stars that are heaped around it. Now that observatories are
+multiplying in the southern hemisphere, the great austral “Coal-sack”
+will, no doubt, receive attention proportioned to its importance as one
+of the most significant features of the sky. Already at the Sydney
+Observatory photographs have shown that the southern portion of this
+Dead Sea of Space is not quite “bottomless,” although its northern part
+defies the longest sounding lines of the astronomer.
+
+There is a similar, but less perfect, “coal-sack” in the northern
+hemisphere, in the constellation of “The Swan,” which, strange to say,
+also contains a well-marked figure of a cross outlined by stars. This
+gap lies near the top of the cross-shaped figure. It is best seen by
+averted vision, which brings out the contrast with the Milky Way, which
+is quite brilliant around it. It does not, however, exercise the same
+weird attraction upon the eye as the southern “Coal-sack,” for instead
+of looking like an absolute void in the sky, it rather appears as if a
+canopy of dark gauze had been drawn over the stars. We shall see the
+possible significance of this appearance later.
+
+
+[Illustration: The Milky Way. Region near M.S. Photographed by
+Professor Barnard]
+
+
+Just above the southern horizon of our northern middle latitudes, in
+summer, where the Milky Way breaks up into vast sheets of nebulous
+luminosity, lying over and between the constellations Scorpio and
+Sagittarius, there is a remarkable assemblage of “coal-sacks,” though
+none is of great size. One of them, near a conspicuous star-cluster in
+Scorpio, M80, is interesting for having been the first of these strange
+objects noted by Herschel. Probably it was its nearness to M80 which
+suggested to his mind the apparent connection of such vacancies with
+star-clusters which we have already mentioned.
+
+But the most marvelous of the “coal-sacks” are those that have been
+found by photography in Sagittarius. One of Barnard’s earliest and most
+excellent photographs includes two of them, both in the star-cluster
+M8. The larger, which is roughly rectangular in outline, contains one
+little star, and its smaller neighbor is lune-shaped—surely a most
+singular form for such an object. Both are associated with curious dark
+lanes running through the clustered stars like trails in the woods.
+Along the borders of these lanes the stars are ranked in parallel rows,
+and what may be called the bottoms of the lanes are not entirely dark,
+but pebbled with faint stellar points. One of them which skirts the two
+dark gaps and traverses the cluster along its greatest diameter is
+edged with lines of stars, recalling the alignment of the trees
+bordering a French highway. This _road of stars_ cannot be less than
+many billions of miles in length!
+
+All about the cluster the bed of the Galaxy is strangely disturbed, and
+in places nearly denuded, as if its contents had been raked away to
+form the immense stack and the smaller accumulations of stars around
+it. The well-known “Trifid Nebula” is also included in the field of the
+photograph, which covers a truly marvelous region, so intricate in its
+mingling of nebulæ, star-clusters, star-swarms, star-streams, and dark
+vacancies that no description can do it justice. Yet, chaotic as it
+appears, there is an unmistakable suggestion of unity about it,
+impressing the beholder with the idea that all the different parts are
+in some way connected, and have not been fortuitously thrown together.
+Miss Agnes M. Clerke made the striking remark that the dusky lanes in
+M8 are exemplified on the largest scale in the great rift dividing the
+Milky Way, from Cygnus in the northern hemisphere all the way to the
+“Cross” in the southern. Similar lanes are found in many other
+clusters, and they are generally associated with flanking rows of
+stars, resembling in their arrangement the thick-set houses and villas
+along the roadways that traverse the approaches to a great city.
+
+But to return to the black gaps. Are they really windows in the
+star-walls of the universe? Some of them look rather as if they had
+been made by a shell fired through a luminous target, allowing the eye
+to range through the hole into the void space beyond. If science is
+discretely silent about these things, what can the more venturesome and
+less responsible imagination suggest? Would a huge “runaway sun,” like
+Arcturus, for instance, make such an opening if it should pass like a
+projectile through the Milky Way? It is at least a stimulating inquiry.
+Being probably many thousands of times more massive than the galactic
+stars, such a stellar missile would not be stopped by them, though its
+direction of flight might be altered. It would drag the small stars
+lying close to its course out of their spheres, but the ultimate
+tendency of its attraction would be to sweep them round in its wake,
+thus producing rather a star-swarm than a vacancy. Those that were very
+close to it might be swept away in its rush and become its satellites,
+careering away with it in its flight into outer space; but those that
+were farther off, and they would, of course, greatly outnumber the
+nearer ones, would tend inward from all sides toward the line of
+flight, as dust and leaves collect behind a speeding motor (though the
+forces operating would be different), and would fill up the hole, if
+hole it were. A swarm thus collected should be rounded in outline and
+bordered with a relatively barren ring from which the stars had been
+“sucked” away. In a general sense the M8 cluster answers to this
+description, but even if we undertook to account for its existence by a
+supposition like the above, the black gaps would remain unexplained,
+unless one could make a further draft on the imagination and suggest
+that the stars had been thrown into a vast eddy, or system of eddies,
+whose vortices appear as dark holes. Only a maelstrom-like motion could
+keep such a funnel open, for without regard to the impulse derived from
+the projectile, the proper motions of the stars themselves would tend
+to fill it. Perhaps some other cause of the whirling motion may be
+found. As we shall see when we come to the spiral nebulæ, gyratory
+movements are exceedingly prevalent throughout the universe, and the
+structure of the Milky Way is everywhere suggestive of them. But this
+is hazardous sport even for the imagination—to play with _suns_ as if
+they were but thistle-down in the wind or corks in a mill-race.
+
+Another question arises: What is the thickness of the hedge of stars
+through which the holes penetrate? Is the depth of the openings
+proportionate to their width? In other words, is the Milky Way round in
+section like a rope, or flat and thin like a ribbon? The answer is not
+obvious, for we have little or no information concerning the relative
+distances of the faint galactic stars. It would be easier, certainly,
+to conceive of openings in a thin belt than in a massive ring, for in
+the first case they would resemble mere rifts and breaks, while in the
+second they would be like wells or bore-holes. Then, too, the fact that
+the Milky Way is not a _continuous_ body but is made up of stars whose
+actual distances apart is great, offers another quandary; persistent
+and sharply bordered apertures in such an assemblage are _a priori_ as
+improbable, if not impossible, as straight, narrow holes running
+through a swarm of bees.
+
+The difficulty of these questions indicates one of the reasons why it
+has been suggested that the seeming gaps, or many of them, are not
+openings at all, but opaque screens cutting off the light from stars
+behind them. That this is quite possible in some cases is shown by
+Barnard’s later photographs, particularly those of the singular region
+around the star Rho Ophiuchi. Here are to be seen somber lanes and
+patches, apparently forming a connected system which covers an immense
+space, and which their discoverer thinks may constitute a “dark
+nebula.” This seems at first a startling suggestion; but, after all,
+why should their not be dark nebulæ as well as visible ones? In truth,
+it has troubled some astronomers to explain the luminosity of the
+bright nebulæ, since it is not to be supposed that matter in so diffuse
+a state can be incandescent through heat, and phosphorescent light is
+in itself a mystery. The supposition is also in accord with what we
+know of the existence of dark solid bodies in space. Many bright stars
+are accompanied by obscure companions, sometimes as massive as
+themselves; the planets are non-luminous; the same is true of meteors
+before they plunge into the atmosphere and become heated by friction;
+and many plausible reasons have been found for believing that space
+contains as many obscure as shining bodies of great size. It is not so
+difficult, after all, then, to believe that there are immense
+collections of shadowy gases and meteoric dust whose presence is only
+manifested when they intercept the light coming from shining bodies
+behind them.
+
+This would account for the apparent extinguishment of light in open
+space, which is indicated by the falling off in relative number of
+telescopic stars below the tenth magnitude. Even as things are, the
+amount of light coming to us from stars too faint to be seen with the
+naked eye is so great that the statement of it generally surprises
+persons who are unfamiliar with the inner facts of astronomy. It has
+been calculated that on a clear night the total starlight from the
+entire celestial sphere amounts to one-sixtieth of the light of the
+full moon; but of this less than one-twenty-fifth is due to stars
+separately distinguished by the eye. If there were no obscuring medium
+in space, it is probable that the amount of starlight would be
+noticeably and perhaps enormously increased.
+
+But while it seems certain that some of the obscure spots in the Milky
+Way are due to the presence of “dark nebulæ,” or concealing veils of
+one kind or another, it is equally certain that there are many which
+are true apertures, however they may have been formed, and by whatever
+forces they may be maintained. These, then, are veritable windows of
+the Galaxy, and when looking out of them one is face to face with the
+great mystery of infinite space. _There_ the known universe visibly
+ends, but manifestly space itself does not end there. It is not within
+the power of thought to conceive an end to space, for the instant we
+think of a terminal point or line the mind leaps forward to the
+_beyond._ There must be space outside as well as inside. Eternity of
+time and infinity of space are ideas that the intellect cannot fully
+grasp, but neither can it grasp the idea of a limitation to either
+space or time. The metaphysical conceptions of hypergeometry, or
+fourth-dimensional space, do not aid us.
+
+Having, then, discovered that the universe is a thing _contained_ in
+something indefinitely greater than itself; having looked out of its
+windows and found only the gloom of starless night outside—what
+conclusions are we to draw concerning the beyond? It _seems_ as empty
+as a vacuum, but is it really so? If it be, then our universe is a
+single atom astray in the infinite; it is the only island in an ocean
+without shores; it is the one oasis in an illimitable desert. Then the
+Milky Way, with its wide-flung garland of stars, is afloat like a tiny
+smoke-wreath amid a horror of immeasurable vacancy, or it is an
+evanescent and solitary ring of sparkling froth cast up for a moment on
+the viewless billows of immensity. From such conclusions the mind
+instinctively shrinks. It prefers to think that there is _something_
+beyond, though we cannot see it. Even the universe could not bear to be
+alone—a Crusoe lost in the Cosmos! As the inhabitants of the most
+elegant château, with its gardens, parks, and crowds of attendants,
+would die of loneliness if they did not know that they have neighbors,
+though not seen, and that a living world of indefinite extent surrounds
+them, so we, when we perceive that the universe has limits, wish to
+feel that it is not solitary; that beyond the hedges and the hills
+there are other centers of life and activity. Could anything be more
+terrible than the thought of an _isolated universe?_ The greater the
+being, the greater the aversion to seclusion. Only the infinite
+satisfies; in that alone the mind finds rest.
+
+We are driven, then, to believe that the universal night which
+envelopes us is not tenantless; that as we stare out of the star-framed
+windows of the Galaxy and see nothing but uniform blackness, the fault
+is with our eyes or is due to an obscuring medium. Since _our_ universe
+is limited in extent, there must be _other_ universes beyond it on all
+sides. Perhaps if we could carry our telescopes to the verge of the
+great “Coal-sack” near the “Cross,” being then on the frontier of our
+starry system, we could discern, sparkling afar off in the vast night,
+some of the outer galaxies. They may be grander than ours, just as many
+of the suns surrounding us are immensely greater than ours. If we could
+take our stand somewhere in the midst of immensity and, with vision of
+infinite reach, look about us, we should perhaps see a countless number
+of stellar systems, amid which ours would be unnoticeable, like a
+single star among the multitude glittering in the terrestial sky on a
+clear night. Some might be in the form of a wreath, like our own; some
+might be globular, like the great star-clusters in Hercules and
+Centaurus; some might be glittering circles, or disks, or rings within
+rings. If we could enter them we should probably find a vast variety of
+composition, including elements unknown to terrestrial chemistry; for
+while the _visible_ universe appears to contain few if any substances
+not existing on the earth or in the sun, we have no warrant to assume
+that others may not exist in infinite space.
+
+And how as to gravitation? We do not _know_ that gravitation acts
+beyond the visible universe, but it is reasonable to suppose that it
+does. At any rate, if we let go _its_ sustaining hand we are lost, and
+can only wander hopelessly in our speculations, like children astray.
+If the empire of gravitation is infinite, then the various outer
+systems must have _some,_ though measuring by our standards an
+imperceptible, attractive influence upon each other, for gravitation
+never lets go its hold, however great the space over which it is
+required to act. Just as the stars about us are all in motion, so the
+starry systems beyond our sight may be in motion, and our system as a
+whole may be moving in concert with them. If this be so, then after
+interminable ages the aspect of the entire system of systems must
+change, its various members assuming new positions with respect to one
+another. In the course of time we may even suppose that our universe
+will approach relatively close to one of the others; and then, if men
+are yet living on the earth, they may glimpse through the openings
+which reveal nothing to us now, the lights of another nearing star
+system, like the signals of a strange squadron, bringing them the
+assurance (which can be but an inference at present) that the ocean of
+space has other argosies venturing on its limitless expanse.
+
+There remains the question of the luminiferous ether by whose agency
+the waves of light are borne through space. The ether is as mysterious
+as gravitation. With regard to ether we only infer its existence from
+the effects which we ascribe to it. Evidently the ether must extend as
+far as the most distant visible stars. But does it continue on
+indefinitely in outer space? If it does, then the invisibility of the
+other systems must be due to their distance diminishing the quantity of
+light that comes from them below the limit of perceptibility, or to the
+interposition of absorbing media; if it does not, then the reason why
+we cannot see them is owing to the absence of a means of conveyance for
+the light waves, as the lack of an interplanetary atmosphere prevents
+us from hearing the thunder of sun-spots. (It is interesting to recall
+that Mr Edison was once credited with the intention to construct a
+gigantic microphone which should render the roar of sun-spots audible
+by transforming the electric vibrations into sound-waves). On this
+supposition each starry system would be enveloped in its own globule of
+ether, and no light could cross from one to another. But the
+probability is that both the ether and gravitation are ubiquitous, and
+that all the stellar systems are immersed in the former like clouds of
+phosphorescent organisms in the sea.
+
+So astronomy carries the mind from height to greater height. Men were
+long in accepting the proofs of the relative insignificance of the
+earth; they were more quickly convinced of the comparative littleness
+of the solar system; and now the evidence assails their reason that
+what they had regarded as _the_ universe is only one mote gleaming in
+the sunbeams of Infinity.
+
+
+
+
+II
+Star-Clouds, Star-Clusters, and Star-Streams
+
+
+In the preceding chapter we have seen something of the strangely
+complicated structure of the Galaxy, or Milky Way. We now proceed to
+study more comprehensively that garlanded “Pathway of the Gods.”
+
+Judged by the eye alone, the Milky Way is one of the most delicately
+beautiful phenomena in the entire realm of nature—a shimmer of silvery
+gauze stretched across the sky; but studied in the light of its
+revelations, it is the most stupendous object presented to human ken.
+Let us consider, first, its appearance to ordinary vision. Its apparent
+position in the sky shifts according to the season. On a serene,
+cloudless summer evening, in the absence of the moon, whose light
+obscures it, one sees the Galaxy spanning the heavens from north to
+southeast of the zenith like a phosphorescent arch. In early spring it
+forms a similar but, upon the whole, less brilliant arch west of the
+zenith. Between spring and summer it lies like a long, faint, twilight
+band along the northern horizon. At the beginning of winter it again
+forms an arch, this time spanning the sky from east to west, a little
+north of the zenith. These are its positions as viewed from the mean
+latitude of the United States. Even the beginner in star-gazing does
+not have to watch it throughout the year in order to be convinced that
+it is, in reality, a great circle, extending entirely around the
+celestial sphere. We appear to be situated near its center, but its
+periphery is evidently far away in the depths of space.
+
+Although to the casual observer it seems but a delicate scarf of light,
+brighter in some places than in others, but hazy and indefinite at the
+best, such is not its appearance to those who study it with care. They
+perceive that it is an organic whole, though marvelously complex in
+detail. The telescope shows that it consists of stars too faint and
+small through excess of distance to be separately visible. Of the
+hundred million suns which some estimates have fixed as the probable
+population of the starry universe, the vast majority (at least thirty
+to one) are included in this strange belt of misty light. But they are
+not uniformly distributed in it; on the contrary, they are arrayed in
+clusters, knots, bunches, clouds, and streams. The appearance is
+somewhat as if the Galaxy consisted of innumerable swarms of
+silver-winged bees, more or less intermixed, some massed together, some
+crossing the paths of others, but all governed by a single purpose
+which leads them to encircle the region of space in which we are
+situated.
+
+From the beginning of the systematic study of the heavens, the fact has
+been recognized that the form of the Milky Way denotes the scheme of
+the sidereal system. At first it was thought that the shape of the
+system was that of a vast round disk, flat like a cheese, and filled
+with stars, our sun and his relatively few neighbors being placed near
+the center. According to this view, the galactic belt was an effect of
+perspective; for when looking in the direction of the plane of the
+disk, the eye ranged through an immense extension of stars which
+blended into a glimmering blur, surrounding us like a ring; while when
+looking out from the sides of the disk we saw but few stars, and in
+those directions the heavens appeared relatively blank. Finally it was
+recognized that this theory did not correspond with the observed
+appearances, and it became evident that the Milky Way was not a mere
+effect of perspective, but an actual band of enormously distant stars,
+forming a circle about the sphere, the central opening of the ring
+(containing many scattered stars) being many times broader than the
+width of the ring itself. Our sun is one of the scattered stars in the
+central opening.
+
+As already remarked, the ring of the Galaxy is very irregular, and in
+places it is partly broken. With its sinuous outline, its pendant
+sprays, its graceful and accordant curves, its bunching of masses, its
+occasional interstices, and the manifest order of a general plan
+governing the jumble of its details, it bears a remarkable resemblance
+to a garland—a fact which appears the more wonderful when we recall its
+composition. That an elm-tree should trace the lines of beauty with its
+leafy and pendulous branches does not surprise us; but we can only gaze
+with growing amazement when we behold _a hundred million suns imitating
+the form of a chaplet!_ And then we have to remember that this form
+furnishes the ground-plan of the universe.
+
+As an indication of the extraordinary speculations to which the mystery
+of the Milky Way has given rise, a theory recently (1909) proposed by
+Prof. George C. Comstock may be mentioned. Starting with the data
+(first) that the number of stars increases as the Milky Way is
+approached, and reaches a maximum in its plane, while on the other hand
+the number of nebulæ is greatest outside the Milky Way and increases
+with distance from it, and (second) that the Milky Way, although a
+complete ring, is broad and diffuse on one side through one-half its
+course—that half alone containing nebulæ—and relatively narrow and well
+defined on the opposite side, the author of this singular speculation
+avers that these facts can best be explained by supposing that the
+invisible universe consists of two interpenetrating parts, one of which
+is a chaos of indefinite extent, strewn with stars and nebulous dust,
+and the other a long, broad but comparatively thin cluster of stars,
+including the sun as one of its central members. This flat star-cluster
+is conceived to be moving edgewise through the chaos, and, according to
+Professor Comstock, it acts after the manner of a snow-plough sweeping
+away the cosmic dust and piling it on either hand above and below the
+plane of the moving cluster. It thus forms a transparent rift, through
+which we see farther and command a view of more stars than through the
+intensified dust-clouds on either hand. This rift is the Milky Way. The
+dust thrown aside toward the poles of the Milky Way is the substance of
+the nebulæ which abound there. Ahead, where the front of the
+star-plough is clearing the way, the chaos is nearer at hand, and
+consequently there the rift subtends a broader angle, and is filled
+with primordial dust, which, having been annexed by the vanguard of the
+star-swarm, forms the nebulæ seen only in that part of the Milky Way.
+But behind, the rift appears narrow because there we look farther away
+between dust-clouds produced ages ago by the front of the plough, and
+no scattered dust remains in that part of the rift.
+
+In quoting an outline of this strikingly original theory the present
+writer should not be understood as assenting to it. That it appears
+bizarre is not, in itself, a reason for rejecting it, when we are
+dealing with so problematical and enigmatical a subject as the Milky
+Way; but the serious objection is that the theory does not sufficiently
+accord with the observed phenomena. There is too much evidence that the
+Milky Way is an organic system, however fantastic its form, to permit
+the belief that it can only be a rift in chaotic clouds. As with every
+organism, we find that its parts are more or less clearly repeated in
+its ensemble. Among all the strange things that the Milky Way contains
+there is nothing so extraordinary as itself. Every astronomer must many
+times have found himself marveling at it in those comparatively rare
+nights when it shows all its beauty and all its strangeness. In its
+great broken rifts, divisions, and spirals are found the gigantic
+prototypes of similar forms in its star-clouds and clusters. As we have
+said, it determines the general shape of the whole sidereal system.
+Some of the brightest stars in the sky appear to hang like jewels
+suspended at the ends of tassels dropped from the Galaxy. Among these
+pendants are the Pleiades and the Hyades. Orion, too, the “Mighty
+Hunter,” is caught in “a loop of light” thrown out from it. The
+majority of the great first-magnitude stars seem related to it, as if
+they formed an inner ring inclined at an angle of some twenty degrees
+to its plane. Many of the long curves that set off from it on both
+sides are accompanied by corresponding curves of lucid stars. In a
+word, it offers every appearance of structural connection with the
+entire starry system. That the universe should have assumed the form of
+a wreath is certainly a matter for astonishment; but it would have been
+still more astonishing if it had been a cube, a rhomboid, or a
+dodecahedron, for then we should have had to suppose that something
+resembling the forces that shape crystals had acted upon the stars, and
+the difficulty of explaining the universe by the laws of gravitation
+would have been increased.
+
+From the Milky Way as a whole we pass to the vast clouds, swarms, and
+clusters of stars of which it is made up. It may be, as some
+astronomers hold, that most of the galactic stars are much smaller than
+the sun, so that their faintness is not due entirely to the effect of
+distance. Still, their intrinsic brilliance attests their solar
+character, and considering their remoteness, which has been estimated
+at not less than ten thousand to twenty thousand light-years (a
+light-year is equal to nearly six thousand thousand million miles)
+their actual masses cannot be extremely small. The minutest of them are
+entitled to be regarded as real suns, and they vary enormously in
+magnitude. The effects of their attractions upon one another can only
+be inferred from their clustering, because their relative movements are
+not apparent on account of the brevity of the observations that we can
+make. But imagine a being for whom a million years would be but as a
+flitting moment; to him the Milky Way would appear in a state of
+ceaseless agitation—swirling with “a fury of whirlpool motion.”
+
+The cloud-like aspect of large parts of the Galaxy must always have
+attracted attention, even from naked-eye observers, but the true
+star-clouds were first satisfactorily represented in Barnard’s
+photographs. The resemblance to actual clouds is often startling. Some
+are close-packed and dense, like cumuli; some are wispy or mottled,
+like cirri. The rifts and modulations, as well as the general outlines,
+are the same as those of clouds of vapor or dust, and one notices also
+the characteristic thinning out at the edges. But we must beware of
+supposing that the component suns are thickly crowded as the particles
+forming an ordinary cloud. They _look,_ indeed, as if they were matted
+together, because of the irradiation of light, but in reality millions
+and billions of miles separate each star from its neighbors.
+Nevertheless they form real assemblages, whose members are far more
+closely related to one another than is our sun to the stars around him,
+and if we were in the Milky Way the aspect of the nocturnal sky would
+be marvelously different from its present appearance.
+
+Stellar clouds are characteristic of the Galaxy and are not found
+beyond its borders, except in the “Magellanic Clouds” of the southern
+hemisphere, which resemble detached portions of the Milky Way. These
+singular objects form as striking a peculiarity of the austral heavens
+as does the great “Coal-sack” described in Chapter 1. But it is their
+isolation that makes them so remarkable, for their composition is
+essentially galactic, and if they were included within its boundaries
+they would not appear more wonderful than many other parts of the Milky
+Way. Placed where they are, they look like masses fallen from the great
+stellar arch. They are full of nebulæ and star-clusters, and show
+striking evidences of spiral movement.
+
+Star-swarms, which are also characteristic features of the Galaxy,
+differ from star-clouds very much in the way that their name would
+imply—_i.e.,_ their component stars are so arranged, even when they are
+countless in number, that the idea of an exceedingly numerous
+assemblage rather than that of a cloud is impressed on the observer’s
+mind. In a star-swarm the separate members are distinguishable because
+they are either larger or nearer than the stars composing a “cloud.” A
+splendid example of a true star-swarm is furnished by Chi Persei, in
+that part of the Milky Way which runs between the constellations
+Perseus and Cassiopeia. This swarm is much coarser than many others,
+and can be seen by the naked eye. In a small telescope it appears
+double, as if the suns composing it had divided into two parties which
+keep on their way side by side, with some commingling of their members
+where the skirts of the two companies come in contact.
+
+Smaller than either star-clouds or star-swarms, and differing from both
+in their organization, are star-clusters. These, unlike the others, are
+found outside as well as inside the Milky Way, although they are more
+numerous inside its boundaries than elsewhere. The term star-cluster is
+sometimes applied, though improperly, to assemblages which are rather
+groups, such, for instance, as the Pleiades. In their most
+characteristic aspect star-clusters are of a globular shape—globes of
+suns! A famous example of a globular star-cluster, but one not included
+in the Milky Way, is the “Great Cluster in Hercules.” This is barely
+visible to the naked eye, but a small telescope shows its character,
+and in a large one it presents a marvelous spectacle. Photographs of
+such clusters are, perhaps, less effective than those of star-clouds,
+because the central condensation of stars in them is so great that
+their light becomes blended in an indistinguishable blur. The beautiful
+effect of the incessant play of infinitesimal rays over the apparently
+compact surface of the cluster, as if it were a globe of the finest
+frosted silver shining in an electric beam, is also lost in a
+photograph. Still, even to the eye looking directly at the cluster
+through a powerful telescope, the central part of the wonderful
+congregation seems almost a solid mass in which the stars are packed
+like the ice crystals in a snowball.
+
+The same question rises to the lips of every observer: How can they
+possibly have been brought into such a situation? The marvel does not
+grow less when we know that, instead of being closely compacted, the
+stars of the cluster are probably separated by millions of miles; for
+we know that their distances apart are slight as compared with their
+remoteness from the Earth. Sir William Herschel estimated their number
+to be about fourteen thousand, but in fact they are uncountable. If we
+could view them from a point just within the edge of the assemblage,
+they would offer the appearance of a hollow hemisphere emblazoned with
+stars of astonishing brilliancy; the near-by ones unparalleled in
+splendor by any celestial object known to us, while the more distant
+ones would resemble ordinary stars. An inhabitant of the cluster would
+not know, except by a process of ratiocination, that he was dwelling in
+a globular assemblage of suns; only from a point far outside would
+their spherical arrangement become evident to the eye. Imagine
+fourteen-thousand fire-balloons with an approach to regularity in a
+spherical space—say, ten miles in diameter; there would be an average
+of less than thirty in every cubic mile, and it would be necessary to
+go to a considerable distance in order to see them as a globular
+aggregation; yet from a point sufficiently far away they would blend
+into a glowing ball.
+
+
+[Illustration: Star-cluster in Hercules
+(Photographed with a two-foot reflector)]
+
+
+Photographs show even better than the best telescopic views that the
+great cluster is surrounded with a multitude of dispersed stars,
+suggestively arrayed in more or less curving lines, which radiate from
+the principle mass, with which their connection is manifest. These
+stars, situated outside the central sphere, look somewhat like vagrant
+bees buzzing round a dense swarm where the queen bee is sitting. Yet
+while there is so much to suggest the operation of central forces,
+bringing and keeping the members of the cluster together, the attentive
+observer is also impressed with the idea that the whole wonderful
+phenomenon may be _the result of explosion._ As soon as this thought
+seizes the mind, confirmation of it seems to be found in the appearance
+of the outlying stars, which could be as readily explained by the
+supposition that they have been blown apart as that they have flocked
+together toward a center. The probable fact that the stars constituting
+the cluster are very much smaller than our sun might be regarded as
+favoring the hypothesis of an explosion. Of their real size we know
+nothing, but, on the basis of an uncertain estimate of their parallax,
+it has been calculated that they may average forty-five thousand miles
+in diameter—something more than half the diameter of the planet
+Jupiter. Assuming the same mean density, fourteen thousand such stars
+might have been formed by the explosion of a body about twice the size
+of the sun. This recalls the theory of Olbers, which has never been
+altogether abandoned or disproved, that the Asteroids were formed by
+the explosion of a planet circulating between the orbits of Mars and
+Jupiter. The Asteroids, whatever their manner of origin, form a ring
+around the sun; but, of course, the explosion of a great independent
+body, not originally revolving about a superior center of gravitational
+force, would not result in the formation of a ring of small bodies, but
+rather of a dispersed mass of them. But back of any speculation of this
+kind lies the problem, at present insoluble: How could the explosion be
+produced? (See the question of explosions in Chapters 6 and 14).
+
+Then, on the other hand, we have the observation of Herschel, since
+abundantly confirmed, that space is unusually vacant in the immediate
+neighborhood of condensed star-clusters and nebulæ, which, as far as it
+goes, might be taken as an indication that the assembled stars had been
+drawn together by their mutual attractions, and that the tendency to
+aggregation is still bringing new members toward the cluster. But in
+that case there must have been an original condensation of stars at
+that point in space. This could probably have been produced by the
+coagulation of a great nebula into stellar nuclei, a process which
+seems now to be taking place in the Orion Nebula.
+
+
+[Illustration: Great southern star-cluster, Omega Centauri]
+
+
+A yet more remarkable globular star-cluster exists in the southern
+hemisphere, Omega Centauri. In this case the central condensation of
+stars presents an almost uniform blaze of light. Like the Hercules
+cluster, that in Centaurus is surrounded with stars scattered over a
+broad field and showing an appearance of radial arrangement. In fact,
+except for its greater richness, Omega Centauri is an exact duplicate
+of its northern rival. Each appears to an imaginative spectator as a
+veritable “city of suns.” Mathematics shrinks from the task of
+disentangling the maze of motions in such an assemblage. It would seem
+that the chance of collisions is not to be neglected, and this idea
+finds a certain degree of confirmation in the appearance of “temporary
+stars” which have more than once blazed out in, or close by, globular
+star-clusters.
+
+This leads up to the notable fact, first established by Professor
+Bailey a few years ago, that such clusters are populous with variable
+stars. Omega Centauri and the Hercules cluster are especially
+remarkable in this respect. The variables found in them are all of
+short period and the changes of light show a noteworthy tendency to
+uniformity. The first thought is that these phenomena must be due to
+collisions among the crowded stars, but, if so, the encounters cannot
+be between the stars themselves, but probably between stars and meteor
+swarms revolving around them. Such periodic collisions might go on for
+ages without the meteors being exhausted by incorporation with the
+stars. This explanation appears all the more probable because one would
+naturally expect that flocks of meteors would abound in a close
+aggregation of stars. It is also consistent with Perrine’s
+discovery—that the globular star clusters are powdered with minute
+stars strewn thickly among the brighter ones.
+
+In speaking of Professor Comstock’s extraordinary theory of the Milky
+Way, the fact was mentioned that, broadly speaking, the nebulæ are less
+numerous in the galactic belt than in the comparatively open spaces on
+either side of it, but that they are, nevertheless, abundant in the
+broader half of the Milky Way which he designates as the front of the
+gigantic “plough” supposed to be forcing its way through the enveloping
+chaos. In and around the Sagittarius region the intermingling of nebulæ
+and galactic star clouds and clusters is particularly remarkable. That
+there is a causal connection no thoughtful person can doubt. We are
+unable to get away from the evidence that a nebula is like a
+seed-ground from which stars spring forth; or we may say that nebulæ
+resemble clouds in whose bosom raindrops are forming. The wonderful
+aspect of the admixtures of nebulæ and star-clusters in Sagittarius has
+been described in Chapter 1. We now come to a still more extraordinary
+phenomenon of this kind—the Pleiades nebulæ.
+
+
+[Illustration: The Pleiades]
+
+
+The group of the Pleiades, although lying outside the main course of
+the Galaxy, is connected with it by a faint loop, and is the scene of
+the most remarkable association of stars and nebulous matter known in
+the visible universe. The naked eye is unaware of the existence of
+nebulæ in the Pleiades, or, at the best, merely suspects that there is
+something of the kind there; and even the most powerful telescopes are
+far from revealing the full wonder of the spectacle; but in photographs
+which have been exposed for many hours consecutively, in order to
+accumulate the impression of the actinic rays, the revelation is
+stunning. The principle stars are seen surrounded by, and, as it were,
+_drowned in,_ dense nebulous clouds of an unparalleled kind. The forms
+assumed by these clouds seem at first sight inexplicable. They look
+like fleeces, or perhaps more like splashes and daubs of luminous paint
+dashed carelessly from a brush. But closer inspection shows that they
+are, to a large extent, _woven_ out of innumerable threads of filmy
+texture, and there are many indications of spiral tendencies. Each of
+the bright stars of the group—Alcyone, Merope, Maia, Electra, Taygeta,
+Atlas—is the focus of a dense fog (totally invisible, remember, alike
+to the naked eye and to the telescope), and these particular stars are
+veiled from sight behind the strange mists. Running in all directions
+across the relatively open spaces are nebulous wisps and streaks of the
+most curious forms. On some of the nebular lines, which are either
+straight throughout, or if they change direction do so at an angle,
+little stars are strung like beads. In one case seven or eight stars
+are thus aligned, and, as if to emphasize their dependence upon the
+chain which connects them, when it makes a slight bend the file of
+stars turns the same way. Many other star rows in the group suggest by
+their arrangement that they, too, were once strung upon similar threads
+which have now disappeared, leaving the stars spaced along their
+ancient tracks. We seem forced to the conclusion that there was a time
+when the Pleiades were embedded in a vast nebula resembling that of
+Orion, and that the cloud has now become so rare by gradual
+condensation into stars that the merest trace of it remains, and this
+would probably have escaped detection but for the remarkable actinic
+power of the radiant matter of which it consists. The richness of many
+of these faint nebulous masses in ultra-violet radiations, which are
+those that specifically affect the photographic plate, is the cause of
+the marvelous revelatory power of celestial photography. So the
+veritable unseen universe, as distinguished from the “unseen universe”
+of metaphysical speculation, is shown to us.
+
+A different kind of association between stars and nebulæ is shown in
+some surprising photographic objects in the constellation Cygnus, where
+long, wispy nebulæ, billions of miles in length, some of them looking
+like tresses streaming in a breeze, lie amid fields of stars which seem
+related to them. But the relation is of a most singular kind, for
+notwithstanding the delicate structure of the long nebulæ they appear
+to act as barriers, causing the stars to heap themselves on one side.
+The stars are two, three, or four times as numerous on one side of the
+nebulæ as on the other. These nebulæ, as far as appearance goes, might
+be likened to rail fences, or thin hedges, against which the wind is
+driving drifts of powdery snow, which, while scattered plentifully all
+around, tends to bank itself on the leeward side of the obstruction.
+The imagination is at a loss to account for these extraordinary
+phenomena; yet there they are, faithfully giving us their images
+whenever the photographic plate is exposed to their radiations.
+
+Thus the more we see of the universe with improved methods of
+observation, and the more we invent aids to human senses, each enabling
+us to penetrate a little deeper into the unseen, the greater becomes
+the mystery. The telescope carried us far, photography is carrying us
+still farther; but what as yet unimagined instrument will take us to
+the bottom, the top, and the end? And then, what hitherto untried power
+of thought will enable us to comprehend the meaning of it all?
+
+
+
+
+III
+Stellar Migrations
+
+
+To the untrained eye the stars and the planets are not distinguishable.
+It is customary to call them all alike “stars.” But since the planets
+more or less rapidly change their places in the sky, in consequence of
+their revolution about the sun, while the stars proper seem to remain
+always in the same relative positions, the latter are spoken of as
+“fixed stars.” In the beginnings of astronomy it was not known that the
+“fixed stars” had any motion independent of their apparent annual
+revolution with the whole sky about the earth as a seeming center. Now,
+however, we know that the term “fixed stars” is paradoxical, for there
+is not a single really fixed object in the whole celestial sphere. The
+apparent fixity in the positions of the stars is due to their immense
+distance, combined with the shortness of the time during which we are
+able to observe them. It is like viewing the plume of smoke issuing
+from a steamer, hull down, at sea: if one does not continue to watch it
+for a long time it appears to be motionless, although in reality it may
+be traveling at great speed across the line of sight. Even the planets
+seem fixed in position if one watches them for a single night only, and
+the more distant ones do not sensibly change their places, except after
+many nights of observation. Neptune, for instance, moves but little
+more than two degrees in the course of an entire year, and in a month
+its change of place is only about one-third of the diameter of the full
+moon.
+
+Yet, fixed as they seem, the stars are actually moving with a speed in
+comparison with which, in some cases, the planets might almost be said
+to stand fast in their tracks. Jupiter’s speed in his orbit is about
+eight miles per second, Neptune’s is less than three and one-half
+miles, and the earth’s is about eighteen and one-half miles; while
+there are “fixed stars” which move two hundred or three hundred miles
+per second. They do not all, however, move with so great a velocity,
+for some appear to travel no faster than the planets. But in all cases,
+notwithstanding their real speed, long-continued and exceedingly
+careful observations are required to demonstrate that they are moving
+at all. No more overwhelming impression of the frightful depths of
+space in which the stars are buried can be obtained than by reflecting
+upon the fact that a star whose actual motion across the line of sight
+amounts to two hundred miles per second does not change its apparent
+place in the sky, in the course of a thousand years, sufficiently to be
+noticed by the casual observer of the heavens!
+
+There is one vast difference between the motions of the stars and those
+of the planets to which attention should be at once called: the
+planets, being under the control of a central force emanating from
+their immediate master, the sun, all move in the same direction and in
+orbits concentric about the sun; the stars, on the other hand, move in
+every conceivable direction and have no apparent center of motion, for
+all efforts to discover such a center have failed. At one time, when
+theology had finally to accept the facts of science, a grandiose
+conception arose in some pious minds, according to which the Throne of
+God was situated at the exact center of His Creation, and, seated
+there, He watched the magnificent spectacle of the starry systems
+obediently revolving around Him. Astronomical discoveries and
+speculations seemed for a time to afford some warrant for this view,
+which was, moreover, an acceptable substitute for the abandoned
+geocentric theory in minds that could only conceive of God as a
+superhuman artificer, constantly admiring his own work. No longer ago
+than the middle of the nineteenth century a German astronomer, Maedler,
+believed that he had actually found the location of the center about
+which the stellar universe revolved. He placed it in the group of the
+Pleiades, and upon his authority an extraordinary imaginative picture
+was sometimes drawn of the star Alcyone, the brightest of the Pleiades,
+as the very seat of the Almighty. This idea even seemed to gain a kind
+of traditional support from the mystic significance, without known
+historical origin, which has for many ages, and among widely separated
+peoples, been attached to the remarkable group of which Alcyone is the
+chief. But since Maedler’s time it has been demonstrated that the
+Pleiades cannot be the center of revolution of the universe, and, as
+already remarked, all attempts to find or fix such a center have proved
+abortive. Yet so powerful was the hold that the theory took upon the
+popular imagination, that even today astronomers are often asked if
+Alcyone is not the probable site of “Jerusalem the Golden.”
+
+If there were a discoverable center of predominant gravitative power,
+to which the motions of all the stars could be referred, those motions
+would appear less mysterious, and we should then be able to conclude
+that the universe was, as a whole, a prototype of the subsidiary
+systems of which it is composed. We should look simply to the law of
+gravitation for an explanation, and, naturally, the center would be
+placed within the opening enclosed by the Milky Way. If it were there
+the Milky Way itself should exhibit signs of revolution about it, like
+a wheel turning upon its hub. No theory of the star motions as a whole
+could stand which failed to take account of the Milky Way as the basis
+of all. But the very form of that divided wreath of stars forbids the
+assumption of its revolution about a center. Even if it could be
+conceived as a wheel having no material center it would not have the
+form which it actually presents. As was shown in Chapter 2, there is
+abundant evidence of motion in the Milky Way; but it is not motion of
+the system as a whole, but motion affecting its separate parts. Instead
+of all moving one way, the galactic stars, as far as their movements
+can be inferred, are governed by local influences and conditions. They
+appear to travel crosswise and in contrary directions, and perhaps they
+eddy around foci where great numbers have assembled; but of a universal
+revolution involving the entire mass we have no evidence.
+
+Most of our knowledge of star motions, called “proper motions,” relates
+to individual stars and to a few groups which happen to be so near that
+the effects of their movements are measurable. In some cases the motion
+is so rapid (not in appearance, but in reality) that the chief
+difficulty is to imagine how it can have been imparted, and what will
+eventually become of the “runaways.” Without a collision, or a series
+of very close approaches to great gravitational centers, a star
+traveling through space at the rate of two hundred or three hundred
+miles per second could not be arrested or turned into an orbit which
+would keep it forever flying within the limits of the visible universe.
+A famous example of these speeding stars is “1830 Groombridge,” a star
+of only the sixth magnitude, and consequently just visible to the naked
+eye, whose motion across the line of sight is so rapid that it moves
+upon the face of the sky a distance equal to the apparent diameter of
+the moon every 280 years. The distance of this star is at least
+200,000,000,000,000 miles, and may be two or three times greater, so
+that its actual speed cannot be less than two hundred, and may be as
+much as four hundred, miles per second. It could be turned into a new
+course by a close approach to a great sun, but it could only be stopped
+by collision, head-on, with a body of enormous mass. Barring such
+accidents it must, as far as we can see, keep on until it has traversed
+our stellar system, whence in may escape and pass out into space
+beyond, to join, perhaps, one of those other universes of which we have
+spoken. Arcturus, one of the greatest suns in the universe, is also a
+runaway, whose speed of flight has been estimated all the way from
+fifty to two hundred miles per second. Arcturus, we have every reason
+to believe, possesses hundreds of times the mass of our sun—think,
+then, of the prodigious momentum that its motion implies! Sirius moves
+more moderately, its motion across the line of sight amounting to only
+ten miles per second, but it is at the same time approaching the sun at
+about the same speed, its actual velocity in space being the resultant
+of the two displacements.
+
+What has been said about the motion of Sirius brings us to another
+aspect of this subject. The fact is, that in every case of stellar
+motion the displacement that we observe represents only a part of the
+actual movement of the star concerned. There are stars whose motion
+carries them straight toward or straight away from the earth, and such
+stars, of course, show no cross motion. But the vast majority are
+traveling in paths inclined from a perpendicular to our line of sight.
+Taken as a whole, the stars may be said to be flying about like the
+molecules in a mass of gas. The discovery of the radial component in
+the movements of the stars is due to the spectroscope. If a star is
+approaching, its spectral lines are shifted toward the violet end of
+the spectrum by an amount depending upon the velocity of approach; if
+it is receding, the lines are correspondingly shifted toward the red
+end. Spectroscopic observation, then, combined with micrometric
+measurements of the cross motion, enables us to detect the real
+movement of the star in space. Sometimes it happens that a star’s
+radial movement is periodically reversed; first it approaches, and then
+it recedes. This indicates that it is revolving around a near-by
+companion, which is often invisible, and superposed upon this motion is
+that of the two stars concerned, which together may be approaching or
+receding or traveling across the line of sight. Thus the complications
+involved in the stellar motions are often exceedingly great and
+puzzling.
+
+Yet another source of complication exists in the movement of our own
+star, the sun. There is no more difficult problem in astronomy than
+that of disentangling the effects of the solar motion from those of the
+motions of the other stars. But the problem, difficult as it is, has
+been solved, and upon its solution depends our knowledge of the speed
+and direction of the movement of the solar system through space, for of
+course the sun carries its planets with it. One element of the solution
+is found in the fact that, as a result of perspective, the stars toward
+which we are going appear to move apart toward all points of the
+compass, while those behind appear to close up together. Then the
+spectroscopic principle already mentioned is invoked for studying the
+shift of the lines, which is toward the violet in the stars ahead of us
+and toward the red in those that we are leaving behind. Of course the
+effects of the independent motions of the stars must be carefully
+excluded. The result of the studies devoted to this subject is to show
+that we are traveling at a speed of twelve to fifteen miles per second
+in a northerly direction, toward the border of the constellations
+Hercules and Lyra. A curious fact is that the more recent estimates
+show that the direction is not very much out of a straight line drawn
+from the sun to the star Vega, one of the most magnificent suns in the
+heavens. But it should not be inferred from this that Vega is drawing
+us on; it is too distant for its gravitation to have such an effect.
+
+Many unaccustomed thoughts are suggested by this mighty voyage of the
+solar system. Whence have we come, and whither do we go? Every year of
+our lives we advance at least 375,000,000 miles. Since the traditional
+time of Adam the sun has led his planets through the wastes of space no
+less than 225,000,000,000 miles, or more than 2400 times the distance
+that separates him from the earth. Go back in imagination to the
+geologic ages, and try to comprehend the distance over which the earth
+has flown. Where was our little planet when it emerged out of the
+clouds of chaos? Where was the sun when his “thunder march” began? What
+strange constellations shone down upon our globe when its masters of
+life were the monstrous beasts of the “Age of Reptiles”? A million
+years is not much of a span of time in geologic reckoning, yet a
+million years ago the earth was farther from its present place in space
+than any of the stars with a measurable parallax are now. It was more
+than seven times as far as Sirius, nearly fourteen times as far as
+Alpha Centauri, three times as far as Vega, and twice as far as
+Arcturus. But some geologists demand two hundred, three hundred, even
+one thousand million years to enable them to account for the
+evolutionary development of the earth and its inhabitants. In a
+thousand million years the earth would have traveled farther than from
+the remotest conceivable depths of the Milky Way!
+
+Other curious reflections arise when we think of the form of the
+earth’s track as it follows the lead of the sun, in a journey which has
+neither known beginning nor conceivable end. There are probably many
+minds which have found a kind of consolation in the thought that every
+year the globe returns to the same place, on the same side of the sun.
+This idea may have an occult connection with our traditional regard for
+anniversaries. When that period of the year returns at which any great
+event in our lives has occurred we have the feeling that the earth, in
+its annual round, has, in a manner, brought us back to the scene of
+that event. We think of the earth’s orbit as a well-worn path which we
+traverse many times in the course of a lifetime. It seems familiar to
+us, and we grow to have a sort of attachment to it. The sun we are
+accustomed to regard as a fixed center in space, like the mill or pump
+around which the harnessed patient mule makes his endless circuits. But
+the real fact is that the earth never returns to the place in space
+where it has once quitted. In consequence of the motion of the sun
+carrying the earth and the other planets along, the track pursued by
+our globe is a vast spiral in space continually developing and never
+returning upon its course. It is probable that the tracks of the sun
+and the others stars are also irregular, and possibly spiral, although,
+as far as can be at present determined, they appear to be practically
+straight. Every star, wherever it may be situated, is attracted by its
+fellow-stars from many sides at once, and although the force is
+minimized by distance, yet in the course of many ages its effects must
+become manifest.
+
+Looked at from another side, is there not something immensely
+stimulating and pleasing to the imagination in the idea of so
+stupendous a journey, which makes all of us the greatest of travelers?
+In the course of a long life a man is transported through space thirty
+thousand million miles; Halley’s Comet does not travel one-quarter as
+far in making one of its immense circuits. And there are adventures on
+this voyage of which we are just beginning to learn to take account.
+Space is full of strange things, and the earth must encounter some of
+them as it advances through the unknown. Many singular speculations
+have been indulged in by astronomers concerning the possible effects
+upon the earth of the varying state of the space that it traverses.
+Even the alternation of hot and glacial periods has sometimes been
+ascribed to this source. When tropical life flourished around the
+poles, as the remains in the rocks assure us, the needed high
+temperature may, it has been thought, have been derived from the
+presence of the earth in a warm region of space. Then, too, there is a
+certain interest for us in the thought of what our familiar planet has
+passed through. We cannot but admire it for its long journeying as we
+admire the traveler who comes to us from remote and unexplored lands,
+or as we gaze with a glow of interest upon the first locomotive that
+has crossed a continent, or a ship that has visited the Arctic or
+Antarctic regions. If we may trust the indications of the present
+course, the earth, piloted by the sun, has come from the Milky Way in
+the far south and may eventually rejoin that mighty band of stars in
+the far north.
+
+While the stars in general appear to travel independently of one
+another, except when they are combined in binary or trinary systems,
+there are notable exceptions to this rule. In some quarters of the sky
+we behold veritable migrations of entire groups of stars whose members
+are too widely separated to show any indications of revolution about a
+common center of gravity. This leads us back again to the wonderful
+group of the Pleiades. All of the principle stars composing that group
+are traveling in virtually parallel lines. Whatever force set them
+going evidently acted upon all alike. This might be explained by the
+assumption that when the original projective force acted upon them they
+were more closely united than they are at present, and that in drifting
+apart they have not lost the impulse of the primal motion. Or it may be
+supposed that they are carried along by some current in space, although
+it would be exceedingly difficult, in the present state of our
+knowledge, to explain the nature of such a current. Yet the theory of a
+current has been proposed. As to an attractive center around which they
+might revolve, none has been found. Another instance of similar
+“star-drift” is furnished by five of the seven stars constituting the
+figure of the “Great Dipper.” In this case the stars concerned are
+separated very widely, the two extreme ones by not less than fifteen
+degrees, so that the idea of a common motion would never have been
+suggested by their aspect in the sky; and the case becomes the more
+remarkable from the fact that among and between them there are other
+stars, some of the same magnitude, which do not share their motion, but
+are traveling in other directions. Still other examples of the same
+phenomenon are found in other parts of the sky. Of course, in the case
+of compact star-clusters, it is assumed that all the members share a
+like motion of translation through space, and the same is probably true
+of dense star-swarms and star-clouds.
+
+The whole question of star-drift has lately assumed a new phase, in
+consequence of the investigations of Kapteyn, Dyson, and Eddington on
+the “systematic motions of the stars.” This research will, it is hoped,
+lead to an understanding of the general law governing the movements of
+the whole body of stars constituting the visible universe. Taking about
+eleven hundred stars whose proper motions have been ascertained with an
+approach to certainty, and which are distributed in all parts of the
+sky, it has been shown that there exists an apparent double drift, in
+two independent streams, moving in different and nearly opposed
+directions. The apex of the motion of what is called “Stream I” is
+situated, according to Professor Kapteyn, in right ascension 85°,
+declination south 11°, which places it just south of the constellation
+Orion; while the apex of “Stream II” is in right ascension 260°,
+declination south 48°, placing it in the constellation Ara, south of
+Scorpio. The two apices differ very nearly 180° in right ascension and
+about 120° in declination. The discovery of these vast star-streams, if
+they really exist, is one of the most extraordinary in modern
+astronomy. It offers the correlation of stellar movements needed as the
+basis of a theory of those movements, but it seems far from revealing a
+physical cause for them. As projected against the celestial sphere the
+stars forming the two opposite streams appear intermingled, some
+obeying one tendency and some the other. As Professor Dyson has said,
+the hypothesis of this double movement is of a revolutionary character,
+and calls for further investigation. Indeed, it seems at first glance
+not less surprising than would be the observation that in a snow-storm
+the flakes over our heads were divided into two parties and driving
+across each other’s course in nearly opposite directions, as if urged
+by interpenetrating winds.
+
+But whatever explanation may eventually be found for the motions of the
+stars, the knowledge of the existence of those motions must always
+afford a new charm to the contemplative observer of the heavens, for
+they impart a sense of life to the starry system that would otherwise
+be lacking. A stagnant universe, with every star fixed immovably in its
+place, would not content the imagination or satisfy our longing for
+ceaseless activity. The majestic grandeur of the evolutions of the
+celestial hosts, the inconceivable vastness of the fields of space in
+which they are executed, the countless numbers, the immeasurable
+distances, the involved convolutions, the flocking and the scattering,
+the interpenetrating marches and countermarches, the strange community
+of impulsion affecting stars that are wide apart in space and causing
+them to traverse the general movement about them like aides and
+despatch-bearers on a battle-field—all these arouse an intensity of
+interest which is heightened by the mystery behind them.
+
+
+
+
+IV
+The Passing of the Constellations
+
+
+From a historical and picturesque point of view, one of the most
+striking results of the motions of the stars described in the last
+chapter is their effect upon the forms of the constellations, which
+have been watched and admired by mankind from a period so early that
+the date of their invention is now unknown. The constellations are
+formed by chance combinations of conspicuous stars, like figures in a
+kaleidoscope, and if our lives were commensurate with the æons of
+cosmic existence we should perceive that the kaleidoscope of the
+heavens was ceaselessly turning and throwing the stars into new
+symmetries. Even if the stars stood fast, the motion of the solar
+system would gradually alter the configurations, as the elements of a
+landscape dissolve and recombine in fresh groupings with the traveler’s
+progress amid them. But with the stars themselves all in motion at
+various speeds and in many directions, the changes occur more rapidly.
+Of course, “rapid” is here understood in a relative sense; the wheel of
+human history to an eye accustomed to the majestic progression of the
+universe would appear to revolve with the velocity of a whirling
+dynamo. Only the deliberation of geological movements can be contrasted
+with the evolution and devolution of the constellations.
+
+And yet this secular fluctuation of the constellation figures is not
+without keen interest for the meditative observer. It is another
+reminder of the swift mutability of terrestial affairs. To the passing
+glance, which is all that we can bestow upon these figures, they appear
+so immutable that they have been called into service to form the most
+lasting records of ancient thought and imagination that we possess. In
+the forms of the constellations, the most beautiful, and, in
+imaginative quality, the finest, mythology that the world has ever
+known has been perpetuated. Yet, in a broad sense, this scroll of human
+thought imprinted on the heavens is as evanescent as the summer clouds.
+Although more enduring than parchment, tombs, pyramids, and temples, it
+is as far as they from truly eternizing the memory of what man has
+fancied and done.
+
+Before studying the effects that the motions of the stars have had and
+will have upon the constellations, it is worth while to consider a
+little further the importance of the stellar pictures as archives of
+history. To emphasize the importance of these effects it is only
+necessary to recall that the constellations register the oldest
+traditions of our race. In the history of primeval religions they are
+the most valuable of documents. Leaving out of account for the moment
+the more familiar mythology of the Greeks, based on something older
+yet, we may refer for illustration to that of the mysterious Maya race
+of America. At Izamal, in Yucatan, says Mr Stansbury Hagar, is a group
+of ruins perched, after the Mexican and Central-American plan, on the
+summits of pyramidal mounds which mark the site of an ancient theogonic
+center of the Mayas. Here the temples all evidently refer to a cult
+based upon the constellations as symbols. The figures and the names, of
+course, were not the same as those that we have derived from our Aryan
+ancestors, but the star groups were the same or nearly so. For
+instance, the loftiest of the temples at Izamal was connected with the
+sign of the constellation known to us as Cancer, marking the place of
+the sun at the summer solstice, at which period the sun was supposed to
+descend at noon like a great bird of fire and consume the offerings
+left upon the altar. Our Scorpio was known to the Mayas as a sign of
+the “Death God.” Our Libra, the “Balance,” with which the idea of a
+divine weighing out of justice has always been connected, seems to be
+identical with the Mayan constellation Teoyaotlatohua, with which was
+associated a temple where dwelt the priests whose special business it
+was to administer justice and to foretell the future by means of
+information obtained from the spirits of the dead. Orion, the “Hunter”
+of our celestial mythology, was among the Mayas a “Warrior,” while
+Sagittarius and others of our constellations were known to them (under
+different names, of course), and all were endowed with a religious
+symbolism. And the same star figures, having the same significance,
+were familiar to the Peruvians, as shown by the temples at Cuzco. Thus
+the imagination of ancient America sought in the constellations symbols
+of the unchanging gods.
+
+But, in fact, there is no nation and no people that has not recognized
+the constellations, and at one period or another in its history
+employed them in some symbolic or representative capacity. As handled
+by the Greeks from prehistoric times, the constellation myths became
+the very soul of poetry. The imagination of that wonderful race
+idealized the principal star groups so effectively that the figures and
+traditions thus attached to them have, for civilized mankind, displaced
+all others, just as Greek art in its highest forms stands without
+parallel and eclipses every rival. The Romans translated no heroes and
+heroines of the mythical period of their history to the sky, and the
+deified Cæsars never entered that lofty company, but the heavens are
+filled with the early myths of the Greeks. Herakles nightly resumes his
+mighty labors in the stars; Zeus, in the form of the white “Bull,”
+Taurus, bears the fair Europa on his back through the celestial waves;
+Andromeda stretches forth her shackled arms in the star-gemmed ether,
+beseeching aid; and Perseus, in a blaze of diamond armor, revives his
+heroic deeds amid sparkling clouds of stellar dust. There, too, sits
+Queen Cassiopeia in her dazzling chair, while the Great King, Cepheus,
+towers gigantic over the pole. Professor Young has significantly
+remarked that a great number of the constellations are connected in
+some way or other with the Argonautic Expedition—that strangely
+fascinating legend of earliest Greek story which has never lost its
+charm for mankind. In view of all this, we may well congratulate
+ourselves that the constellations will outlast our time and the time of
+countless generations to follow us; and yet they are very far from
+being eternal. Let us now study some of the effects of the stellar
+motions upon them.
+
+We begin with the familiar figure of the “Great Dipper.” He who has not
+drunk inspiration from its celestial bowl is not yet admitted to the
+circle of Olympus. This figure is made up of seven conspicuous stars in
+the constellation Ursa Major, the “Greater Bear.” The handle of the
+“Dipper” corresponds to the tail of the imaginary “Bear,” and the bowl
+lies upon his flank. In fact, the figure of a dipper is so evident and
+that of a bear so unevident, that to most persons the “Great Dipper” is
+the only part of the constellation that is recognizable. Of the seven
+stars mentioned, six are of nearly equal brightness, ranking as of the
+second magnitude, while the seventh is of only the third magnitude. The
+difference is very striking, since every increase of one magnitude
+involves an increase of two-and-a-half times in brightness. There
+appears to be little doubt that the faint star, which is situated at
+the junction of the bowl and the handle, is a variable of long period,
+since three hundred years ago it was as bright as its companions. But
+however that may be, its relative faintness at the present time
+interferes but little with the perfection of the “Dipper’s” figure. In
+order the more readily to understand the changes which are taking
+place, it will be well to mention both the names and the Greek letters
+which are attached to the seven stars. Beginning at the star in the
+upper outer edge of the rim of the bowl and running in regular order
+round the bottom and then out to the end of the handle, the names and
+letters are as follows: Dubhe (α), Merak (β), Phaed (γ), Megrez (δ),
+Alioth (ε), Mizar (ζ), and Benetnasch (η). Megrez is the faint star
+already mentioned at the junction of the bowl and handle, and Mizar, in
+the middle of the handle, has a close, naked-eye companion which is
+named Alcor. The Arabs called this singular pair of stars “The Horse
+and Rider.” Merak and Duhbe are called “The Pointers,” because an
+imaginary line drawn northward through them indicates the Pole Star.
+
+
+[Illustration: The “Great Dipper”]
+
+
+Now it has been found that five of these stars—_viz.,_ Merak, Phaed,
+Megrez, Alioth, and Mizar (with its comrade)—are moving with
+practically the same speed in an easterly direction, while the other
+two, Dubhe and Benetnasch, are simultaneously moving westward, the
+motions of Benetnasch being apparently more rapid. The consequence of
+these opposed motions is, of course, that the figure of the “Dipper”
+cannot always have existed and will not continue to exist. In the
+accompanying diagrams it has been thought interesting to show the
+relative positions of these seven stars, as seen from the point which
+the earth now occupies, both in the past and in the future. Arrows
+attached to the stars in the figure representing the present appearance
+of the “Dipper” indicate the directions of the motions and the
+distances over which they will carry the stars in a period of about
+five hundred centuries. The time, no doubt, seems long, but remember
+the vast stretch of ages through which the earth has passed, and then
+reflect that no reason is apparent why our globe should not continue to
+be a scene of animation for ten thousand centuries yet to come. The
+fact that the little star Alcor placed so close to Mizar should
+accompany the latter in its flight is not surprising, but that two of
+the principal stars of the group should be found moving in a direction
+directly opposed to that pursued by the other five is surprising in the
+highest degree; and it recalls the strange theory of a double drift
+affecting all the stars, to which attention was called in the preceding
+chapter. It would appear that Benetnasch and Dubhe belong to one
+“current,” and Merak, Phaed, Megrez, Alioth, and Mizar to the other. As
+far as is known, the motion of the seven stars are not shared by the
+smaller stars scattered about them, but on the theory of currents there
+should be such a community of motion, and further investigation may
+reveal it.
+
+
+[Illustration: Cassiopeia]
+
+
+From the “Great Dipper” we turn to a constellation hardly less
+conspicuous and situated at an equal distance from the pole on the
+other side—Cassiopeia. This famous star-group commemorating the
+romantic Queen of Ethiopia whose vain boasting of her beauty was
+punished by the exposure of her daughter Andromeda to the “Sea
+Monster,” is well-marked by five stars which form an irregular letter
+“W” with its open side toward the pole. Three of these stars are
+usually ranked as of the second magnitude, and two of the third; but to
+ordinary observation they appear of nearly equal brightness, and
+present a very striking picture. They mark out the chair and a part of
+the figure of the beautiful queen. Beginning at the right-hand, or
+western, end of the “W,” their Greek letter designations are: Beta (β),
+Alpha (α), Gamma (γ), Delta (δ), and Epsilon (ε). Four of them, Beta,
+Alpha, Delta, and Epsilon are traveling eastwardly at various speeds,
+while the fifth, Gamma, moves in a westerly direction. The motion of
+Beta is more rapid than that of any of the others. It should be said,
+however, that no little uncertainty attaches to the estimates of the
+rate of motion of stars which are not going very rapidly, and different
+observers often vary considerably in their results.
+
+In the beautiful “Northern Crown,” one of the most perfect and charming
+of all the figures to be found in the stars, the alternate combining
+and scattering effects of the stellar motions are shown by comparing
+the appearance which the constellation must have had five hundred
+centuries ago with that which it has at present and that which it will
+have in the future. The seven principle stars of the asterism, forming
+a surprisingly perfect coronet, have movements in three directions at
+right angles to one another. That in these circumstances they should
+ever have arrived at positions giving them so striking an appearance of
+definite association is certainly surprising; from its aspect one would
+have expected to find a community of movement governing the brilliants
+of the “Crown,” but instead of that we find evidence that they will
+inevitably drift apart and the beautiful figure will dissolve.
+
+A similar fate awaits such asterisms as the “Northern Cross” in Cygnus;
+the “Crow” (Corvus), which stands on the back of the great “Sea
+Serpent,” Hydra, and pecks at his scales; “Job’s Coffin” (Delphinus);
+the “Great Square of Pegasus”; the “Twins” (Gemini); the beautiful
+“Sickle” in Leo; and the exquisite group of the Hyades in Taurus. In
+the case of the Hyades, two controlling movements are manifest: one,
+affecting five of the stars which form the well-known figure of a
+letter “V,” is directed northerly; the other, which controls the
+direction of two stars, has an easterly trend. The chief star of the
+group, Aldebaran, one of the finest of all stars both for its
+brilliance and its color, is the most affected by the easterly motion.
+In time it will drift entirely out of connection with its present
+neighbors. Although the Hyades do not form so compact a group as the
+Pleiades in the same constellation, yet their appearance of
+relationship is sufficient to awaken a feeling of surprise over the
+fact that, as with the stars of the “Dipper,” their association is only
+temporary or apparent.
+
+
+[Illustration: The “Northern Crown”]
+
+
+The great figure of Orion appears to be more lasting, not because its
+stars are physically connected, but because of their great distance,
+which renders their movements too deliberate to be exactly ascertained.
+Two of the greatest of its stars, Betelgeuse and Rigel, possess, as far
+as has been ascertained, no perceptible motion across the line of
+sight, but there is a little movement perceptible in the “Belt.” At the
+present time this consists of an almost perfect straight line, a row of
+second-magnitude stars about equally spaced and of the most striking
+beauty. In the course of time, however, the two right-hand stars,
+Mintaka and Alnilam (how fine are these Arabic star names!) will
+approach each other and form a naked-eye double, but the third, Alnita,
+will drift away eastward, so that the “Belt” will no longer exist.
+
+For one more example, let us go to the southern hemisphere, whose most
+celebrated constellation, the “Southern Cross,” has found a place in
+all modern literatures, although it has no claim to consideration on
+account of association with ancient legends. This most attractive
+asterism, which has never ceased to fascinate the imagination of
+Christendom since it was first devoutly described by the early
+explorers of the South, is but a passing collocation of brilliant
+stars. Yet even in its transfigurations it has been for hundreds of
+centuries, and will continue to be for hundreds of centuries to come, a
+most striking object in the sky. Our figures show its appearance in
+three successive phases: first, as it was fifty thousand years ago
+(viewed from the earth’s present location); second, as it is in our
+day; and, third, as it will be an equal time in the future. The
+nearness of these bright stars to one another—the length of the longer
+beam of the “Cross” is only sixdegrees—makes this group very
+noticeable, whatever the arrangement of its components may be. The
+largest star, at the base of the “Cross,” is of the first magnitude,
+two of the others are of the second magnitude, and the fourth is of the
+third. Other stars, not represented in the figures, increase the effect
+of a celestial blazonry, although they do not help the resemblance to a
+cross.
+
+
+[Illustration: The “Southern Cross”]
+
+
+But since the motion of the solar system itself will, in the course of
+so long a period as fifty thousand years, produce a great change in the
+perspective of the heavens as seen from the earth, by carrying us
+nearly nineteen trillion miles from our present place, why, it may be
+asked, seek to represent future appearances of the constellations which
+we could not hope to see, even if we could survive so long? The answer
+is: Because these things aid the mind to form a picture of the effects
+of the mobility of the starry universe. Only by showing the changes
+from some definite point of view can we arrive at a due comprehension
+of them. The constellations are more or less familiar to everybody, so
+that impending changes of their forms must at once strike the eye and
+the imagination, and make clearer the significance of the movements of
+the stars. If the future history of mankind is to resemble its past and
+if our race is destined to survive yet a million years, then our remote
+descendents will see a “new heavens” if not a “new earth,” and will
+have to invent novel constellations to perpetuate their legends and
+mythologies.
+
+If our knowledge of the relative distances of the stars were more
+complete, it would be an interesting exercise in celestial geometry to
+project the constellations probably visible to the inhabitants of
+worlds revolving around some of the other suns of space. Our sun is too
+insignificant for us to think that he can make a conspicuous appearance
+among them, except, perhaps, in a few cases. As seen, for instance,
+from the nearest known star, Alpha Centauri, the sun would appear of
+the average first magnitude, and consequently from that standpoint he
+might be the gem of some little constellation which had no Sirius, or
+Arcturus, or Vega to eclipse him with its superior splendor. But from
+the distance of the vast majority of the stars the sun would probably
+be invisible to the naked eye, and as seen from nearer systems could
+only rank as a fifth or sixth magnitude star, unnoticed and unknown
+except by the star-charting astronomer.
+
+
+
+
+V
+Conflagrations in the Heavens
+
+
+Suppose it were possible for the world to take fire and burn up—as some
+pessimists think that it will do when the Divine wrath shall have
+sufficiently accumulated against it—nobody out of our own little corner
+of space would ever be aware of the catastrophe! With all their
+telescopes, the astronomers living in the golden light of Arcturus or
+the diamond blaze of Canopus would be unable to detect the least
+glimmer of the conflagration that had destroyed the seat of Adam and
+his descendents, just as now they are totally ignorant of its
+existence.
+
+But at least fifteen times in the course of recorded history men
+looking out from the earth have beheld in the remote depths of space
+great outbursts of fiery light, some of them more splendidly luminous
+than anything else in the firmament except the sun! If _they_ were
+conflagrations, how many million worlds like ours were required to feed
+their blaze?
+
+It is probable that “temporary” or “new” stars, as these wonderful
+apparitions are called, really are conflagrations; not in the sense of
+a bonfire or a burning house or city, but in that of a sudden eruption
+of inconceivable heat and light, such as would result from the
+stripping off the shell of an encrusted sun or the crashing together of
+two mighty orbs flying through space with a hundred times the velocity
+of the swiftest cannon-shot.
+
+Temporary stars are the rarest and most erratic of astronomical
+phenomena. The earliest records relating to them are not very clear,
+and we cannot in every instance be certain that it was one of these
+appearances that the ignorant and superstitious old chroniclers are
+trying to describe. The first temporary star that we are absolutely
+sure of appeared in 1572, and is known as “Tycho’s Star,” because the
+celebrated Danish astronomer (whose remains, with his gold-and-silver
+artificial nose—made necessary by a duel—still intact, were disinterred
+and reburied in 1901) was the first to perceive it in the sky, and the
+most assiduous and successful in his studies of it. As the first fully
+accredited representative of its class, this new star made its entry
+upon the scene with becoming _éclat._ It is characteristic of these
+phenomena that they burst into view with amazing suddenness, and, of
+course, entirely unexpectedly. Tycho’s star appeared in the
+constellation Cassiopeia, near a now well-known and much-watched little
+star named Kappa, on the evening of November 11, 1572. The story has
+often been repeated, but it never loses interest, how Tycho, going home
+that evening, saw people in the street pointing and staring at the sky
+directly over their heads, and following the direction of their hands
+and eyes he was astonished to see, near the zenith, an unknown star of
+surpassing brilliance. It outshone the planet Jupiter, and was
+therefore far brighter than the first magnitude. There was not another
+star in the heavens that could be compared with it in splendor. Tycho
+was not in all respects free from the superstitions of his time—and who
+is?—but he had the true scientific instinct, and immediately he began
+to study the stranger, and to record with the greatest care every
+change in its aspect. First he determined as well as he could with the
+imperfect instruments of his day, many of which he himself had
+invented, the precise location of the phenomena in the sky. Then he
+followed the changes that it underwent. At first it brightened until
+its light equaled or exceeded that of the planet Venus at her
+brightest, a statement which will be appreciated at its full value by
+anyone who has ever watched Venus when she plays her dazzling rôle of
+“Evening Star,” flaring like an arc light in the sunset sky. It even
+became so brilliant as to be visible in full daylight, since, its
+position being circumpolar, it never set in the latitude of Northern
+Europe. Finally it began to fade, turning red as it did so, and in
+March, 1574, it disappeared from Tycho’s searching gaze, and has never
+been seen again from that day to this. None of the astronomers of the
+time could make anything of it. They had not yet as many bases of
+speculation as we possess today.
+
+
+[Illustration: Chart showing location of Tycho’s star, 1572,  and Nova
+Persei of 1901]
+
+
+Tycho’s star has achieved a romantic reputation by being fancifully
+identified with the “Star of Bethlehem,” said to have led the wondering
+Magi from their eastern deserts to the cradle-manger of the Savior in
+Palestine. Many attempts have been made to connect this traditional
+“star” with some known phenomenon of the heavens, and none seems more
+idle than this. Yet it persistently survives, and no astronomer is free
+from eager questions about it addressed by people whose imagination has
+been excited by the legend. It is only necessary to say that the
+supposition of a connection between the phenomenon of the Magi and
+Tycho’s star is without any scientific foundation. It was originally
+based on an unwarranted assumption that the star of Tycho was a
+variable of long period, appearing once every three hundred and fifteen
+years, or thereabout. If that were true there would have been an
+apparition somewhere near the traditional date of the birth of Christ,
+a date which is itself uncertain. But even the data on which the
+assumption was based are inconsistent with the theory. Certain monkish
+records speak of something wonderful appearing in the sky in the years
+1264 and 945, and these were taken to have been outbursts of Tycho’s
+star. Investigation shows that the records more probably refer to
+comets, but even if the objects seen were temporary stars, their dates
+do not suit the hypothesis; from 945 to 1264 there is a gap of 319
+years, and from 1264 to 1572 one of only 308 years; moreover 337 years
+have now (1909) elapsed since Tycho saw the last glimmer of his star.
+Upon a variability so irregular and uncertain as that, even if we felt
+sure that it existed, no conclusion could be found concerning an
+apparition occurring 2000 years ago.
+
+In the year 1600 (the year in which Giordano Bruno was burned at the
+stake for teaching that there is more than one physical world), a
+temporary star of the third magnitude broke out in the constellation
+Cygnus, and curiously enough, considering the rarity of such phenomena,
+only four years later another surprisingly brilliant one appeared in
+the constellation Ophiuchus. This is often called “Kepler’s star,”
+because the great German astronomer devoted to it the same attention
+that Tycho had given to the earlier phenomenon. It, too, like Tycho’s,
+was at first the brightest object in the stellar heavens, although it
+seems never to have quite equaled its famous predecessor in splendor.
+It disappeared after a year, also turning of a red color as it became
+more faint. We shall see the significance of this as we go on. Some of
+Kepler’s contemporaries suggested that the outburst of this star was
+due to a meeting of atoms in space, and idea bearing a striking
+resemblance to the modern theory of “astronomical collisions.”
+
+In 1670, 1848, and 1860 temporary stars made their appearance, but none
+of them was of great brilliance. In 1866 one of the second magnitude
+broke forth in the “Northern Crown” and awoke much interest, because by
+that time the spectroscope had begun to be employed in studying the
+composition of the stars, and Huggins demonstrated that the new star
+consisted largely of incandescent hydrogen. But this star, apparently
+unlike the others mentioned, was not absolutely new. Before its
+outburst it had shown as a star of the ninth magnitude (entirely
+invisible, of course, to the naked eye), and after about six weeks it
+faded to its original condition in which it has ever since remained. In
+1876 a temporary star appeared in the constellation Cygnus, and
+attained at one time the brightness of the second magnitude. Its
+spectrum and its behavior resembled those of its immediate predecessor.
+In 1885, astronomers were surprised to see a sixth-magnitude star
+glimmering in the midst of the hazy cloud of the great Andromeda
+Nebula. It soon absolutely disappeared. Its spectrum was remarkable for
+being “continuous,” like that of the nebula itself. A continuous
+spectrum is supposed to represent a body, or a mass, which is either
+solid or liquid, or composed of gas under great pressure. In January,
+1892, a new star was suddenly seen in the constellation Auriga. It
+never rose much above the fourth magnitude, but it showed a peculiar
+spectrum containing both bright and dark lines of hydrogen.
+
+But a bewildering surprise was now in store; the world was to behold at
+the opening of the twentieth century such a celestial spectacle as had
+not been on view since the times of Tycho and Kepler. Before daylight
+on the morning of February 22, 1901, the Rev. Doctor Anderson, of
+Edinburgh, an amateur astronomer, who had also been the first to see
+the new star in Auriga, beheld a strange object in the constellation
+Perseus not far from the celebrated variable star Algol. He recognized
+its character at once, and immediately telegraphed the news, which
+awoke the startled attention of astronomers all over the world. When
+first seen the new star was no brighter than Algol (less than the
+second magnitude), but within twenty-four hours it was ablaze,
+outshining even the brilliant Capella, and far surpassing the first
+magnitude. At the spot in the sky where it appeared nothing whatever
+was visible on the night before its coming. This is known with
+certainty because a photograph had been made of that very region on
+February 21, and this photograph showed everything down to the twelfth
+magnitude, but not a trace of the stranger which burst into view
+between the 21st and the 22nd like the explosion of a rocket.
+
+Upon one who knew the stars the apparition of this intruder in a
+well-known constellation had the effect of a sudden invasion. The new
+star was not far west of the zenith in the early evening, and in that
+position showed to the best advantage. To see Capella, the hitherto
+unchallenged ruler of that quarter of the sky, abased by comparison
+with this stranger of alien aspect, for there was always an unfamiliar
+look about the “nova,” was decidedly disconcerting. It seemed to
+portend the beginning of a revolution in the heavens. One could
+understand what the effect of such an apparition must have been in the
+superstitious times of Tycho. The star of Tycho had burst forth on the
+northern border of the Milky Way; this one was on its southern border,
+some forty-five degrees farther east.
+
+Astronomers were well-prepared this time for the scientific study of
+the new star, both astronomical photography and spectroscopy having
+been perfected, and the results of their investigations were calculated
+to increase the wonder with which the phenomenon was regarded. The star
+remained at its brightest only a few days; then, like a veritable
+conflagration, it began to languish; and, like the reflection of a
+dying fire, as it sank it began to glow with the red color of embers.
+But its changes were spasmodic; once about every three days it flared
+up only to die away again. During these fluctuations its light varied
+alternately in the ratio of one to six. Finally it took a permanent
+downward course, and after a few months the naked eye could no longer
+perceive it; but it remained visible with telescopes, gradually fading
+until it had sunk to the ninth magnitude. Then another astonishing
+change happened: in August photographs taken at the Yerkes Observatory
+and at Heidelberg showed that the “nova” was _surrounded by a spiral
+nebula!_ The nebula had not been there before, and no one could doubt
+that it represented a phase of the same catastrophe that had produced
+the outburst of the new star. At one time the star seemed virtually to
+have disappeared, as if all its substance had been expanded into the
+nebulous cloud, but always there remained a stellar nucleus about which
+the misty spiral spread wider and ever wider, like a wave expanding
+around a center of disturbance. The nebula too showed a variability of
+brightness, and four condensations which formed in it seemed to have a
+motion of revolution about the star. As time went on the nebula
+continued to expand at a rate which was computed to be not less than
+twenty thousand miles per second! And now the star itself, showing
+indications of having turned into a nebula, behaved in a most erratic
+manner, giving rise to the suspicion that it was about to burst out
+again. But this did not occur, and at length it sunk into a state of
+lethargy from which it has to the present time not recovered. But the
+nebulous spiral has disappeared, and the entire phenomena as it now
+(1909) exists consists of a faint nebulous star of less than the ninth
+magnitude.
+
+The wonderful transformations just described had been forecast in
+advance of the discovery of the nebulous spiral encircling the star by
+the spectroscopic study of the latter. At first there was no suggestion
+of a nebular constitution, but within a month or two characteristic
+nebular lines began to appear, and in less than six months the whole
+spectrum had been transformed to the nebular type. In the mean time the
+shifting of the spectral lines indicated a complication of rapid
+motions in several directions simultaneously. These motions were
+estimated to amount to from one hundred to five hundred miles per
+second.
+
+The human mind is so constituted that it feels forced to seek an
+explanation of so marvelous a phenomenon as this, even in the absence
+of the data needed for a sound conclusion. The most natural hypothesis,
+perhaps, is that of a collision. Such a catastrophe could certainly
+happen. It has been shown, for instance, that in infinity of time the
+earth is sure to be hit by a comet; in the same way it may be asserted
+that, if no time limit is fixed, the sun is certain to run against some
+obstacle in space, either another star, or a dense meteor swarm, or one
+of the dark bodies which there is every reason to believe abound around
+us. The consequences of such a collision are easy to foretell, provided
+that we know the masses and the velocities of the colliding bodies. In
+a preceding chapter we have discussed the motions of the sun and stars,
+and have seen that they are so swift that an encounter between any two
+of them could not but be disastrous. But this is not all; for as soon
+as two stars approached within a few million miles their speed would be
+enormously increased by their reciprocal attractions and, if their
+motion was directed radially with respect to their centers, they would
+come together with a crash that would reduce them both to nebulous
+clouds. It is true that the chances of such a “head-on” collision are
+relatively very small; two stars approaching each other would most
+probably fall into closed orbits around their common center of gravity.
+If there were a collision it would most likely be a grazing one instead
+of a direct front-to-front encounter. But even a close approach,
+without any actual collision, would probably prove disastrous, owing to
+the tidal influence of each of the bodies on the other. Suns, in
+consequence of their enormous masses and dimensions and the
+peculiarities of their constitution, are exceedingly dangerous to one
+another at close quarters. Propinquity awakes in them a mutually
+destructive tendency. Consisting of matter in the gaseous, or perhaps,
+in some cases, liquid, state, their tidal pull upon each other if
+brought close together might burst them asunder, and the photospheric
+envelope being destroyed the internal incandescent mass would gush out,
+bringing fiery death to any planets that were revolving near. Without
+regard to the resulting disturbance of the earth’s orbit, the close
+approach of a great star to the sun would be in the highest degree
+perilous to us. But this is a danger which may properly be regarded as
+indefinitely remote, since, at our present location in space, we are
+certainly far from every star except the sun, and we may feel confident
+that no great invisible body is near, for if there were one we should
+be aware of its presence from the effects of its attraction. As to dark
+nebulæ which may possibly lie in the track that the solar system is
+pursuing at the rate of 375,000,000 miles per year, that is another
+question—and they, too, could be dangerous!
+
+
+[Illustration: Nova Persei, with its nebular rings]
+
+
+This brings us directly back to “Nova Persei,” for among the many
+suggestions offered to explain its outburst, as well as those of other
+temporary stars, one of the most fruitful is that of a collision
+between a star and a vast invisible nebula. Professor Seeliger, of
+Munich, first proposed this theory, but it afterward underwent some
+modifications from others. Stated in a general form, the idea is that a
+huge dark body, perhaps an extinguished sun, encountered in its
+progress through space a widespread flock of small meteors forming a
+dark nebula. As it plunged into the swarm the friction of the
+innumerable collisions with the meteors heated its surface to
+incandescence, and being of vast size it then became visible to us as a
+new star. Meanwhile the motion of the body through the nebula, and its
+rotation upon itself, set up a gyration in the blazing atmosphere
+formed around it by the vaporized meteors; and as this atmosphere
+spread wider, under the laws of gyratory motion a rotation in the
+opposite direction began in the inflamed meteoric cloud outside the
+central part of the vortex. Thus the spectral lines were caused to show
+motion in opposite directions, a part of the incandescent mass
+approaching the earth simultaneously with the retreat of another part.
+So the curious spectroscopic observations before mentioned were
+explained. This theory might also account for the appearance of the
+nebulous spiral first seen some six months after the original outburst.
+The sequent changes in the spectrum of the “nova” are accounted for by
+this theory on the assumption, reasonable enough in itself, that at
+first the invading body would be enveloped in a vaporized atmosphere of
+relatively slight depth, producing by its absorption the fine dark
+lines first observed; but that as time went on and the incessant
+collisions continued, the blazing atmosphere would become very deep and
+extensive, whereupon the appearance of the spectral lines would change,
+and bright lines due to the light of the incandescent meteors
+surrounding the nucleus at a great distance would take the place of the
+original dark ones. The vortex of meteors once formed would protect the
+flying body within from further immediate collisions, the latter now
+occurring mainly among the meteors themselves, and then the central
+blaze would die down, and the original splendor of the phenomenon would
+fade.
+
+But the theories about Nova Persei have been almost as numerous as the
+astronomers who have speculated about it. One of the most startling of
+them assumed that the outburst was caused by the running amuck of a
+dark star which had encountered another star surrounded with planets,
+the renewed outbreaks of light after the principal one had faded being
+due to the successive running down of the unfortunate planets! Yet
+another hypothesis is based on what we have already said of the tidal
+influence that two close approaching suns would have upon each other.
+Supposing two such bodies which had become encrusted, but remained
+incandescent and fluid within, to approach within almost striking
+distance; they would whirl each other about their common center of
+gravity, and at the same time their shells would burst under the tidal
+strain, and their glowing nuclei being disclosed would produce a great
+outburst of light. Applying this theory to a “nova,” like that of 1866
+in the “Northern Crown,” which had been visible as a small star before
+the outbreak, and which afterward resumed its former aspect, we should
+have to assume that a yet shining sun had been approached by a dark
+body whose attraction temporarily burst open its photosphere. It might
+be supposed that in this case the dark body was too far advanced in
+cooling to suffer the same fate from the tidal pull of its victim. But
+a close approach of that kind would be expected to result in the
+formation of a binary system, with orbits of great eccentricity,
+perhaps, and after the lapse of a certain time the outburst should be
+renewed by another approximation of the two bodies. A temporary star of
+that kind would rather be ranked as a variable.
+
+The celebrated French astronomer, Janssen, had a different theory of
+Nova Persei, and of temporary stars in general. According to his idea,
+such phenomena might be the result of chemical changes taking place in
+a sun without interference by, or collision with, another body. Janssen
+was engaged for many years in trying to discover evidence of the
+existence of oxygen in the sun, and he constructed his observatory on
+the summit of Mount Blanc specially to pursue that research. He
+believed that oxygen must surely exist in the sun since we find so many
+other familiar elements included in the constitution of the solar
+globe, and as he was unable to discover satisfactory evidence of its
+presence he assumed that it existed in a form unknown on the earth. If
+it were normally in the sun’s chromosphere, or coronal atmosphere, he
+said, it would combine with the hydrogen which we know is there and
+form an obscuring envelope of water vapor. It exists, then, in a
+special state, uncombined with hydrogen; but let the temperature of the
+sun sink to a critical point and the oxygen will assume its normal
+properties and combine with the hydrogen, producing a mighty outburst
+of light and heat. This, Janssen thought, might explain the phenomena
+of the temporary stars. It would also, he suggested, account for their
+brief career, because the combination of the elements would be quickly
+accomplished, and then the resulting water vapor would form an
+atmosphere cutting off the radiation from the star within.
+
+This theory may be said to have a livelier human interest than some of
+the others, since, according to it, the sun may carry in its very
+constitution a menace to mankind; one does not like to think of it
+being suddenly transformed into a gigantic laboratory for the explosive
+combination of oxygen and hydrogen! But while Janssen’s theory might do
+for some temporary stars, it is inadequate to explain all the phenomena
+of Nova Persei, and particularly the appearance of the great spiral
+nebula that seemed to exhale from the heart of the star. Upon the
+whole, the theory of an encounter between a star and a dark nebula
+seems best to fit the observations. By that hypothesis the expanding
+billow of light surrounding the core of the conflagration is very well
+accounted for, and the spectroscopic peculiarities are also explained.
+
+Dr Gustov Le Bon offers a yet more alarming theory, suggesting that
+temporary stars are the result of _atomic explosion;_ but we shall
+touch upon this more fully in Chapter 14.
+
+Twice in the course of this discussion we have called attention to the
+change of color invariably undergone by temporary stars in the later
+stages of their career. This was conspicuous with Nova Persei which
+glowed more and more redly as it faded, until the nebulous light began
+to overpower that of the stellar nucleus. Nothing could be more
+suggestive of the dying out of a great fire. Moreover, change of color
+from white to red is characteristic of all variable stars of long
+period, such as “Mira” in Cetus. It is also characteristic of stars
+believed to be in the later stages of evolution, and consequently
+approaching extinction, like Antares and Betelgeuse, and still more
+notably certain small stars which “gleam like rubies in the field of
+the telescope.” These last appear to be suns in the closing period of
+existence as self-luminous bodies. Between the white stars, such as
+Sirius and Rigel, and the red stars, such as Aldebaran and Alpha
+Herculis, there is a progressive series of colors from golden yellow
+through orange to deep red. The change is believed to be due to the
+increase of absorbing vapors in the stellar atmosphere as the body
+cools down. In the case of ordinary stars these changes no doubt occupy
+many millions of years, which represent the average duration of solar
+life; but the temporary stars run through similar changes in a few
+months: they resemble ephemeral insects—born in the morning and doomed
+to perish with the going down of the sun.
+
+
+
+
+VI
+Explosive and Whirling Nebulæ
+
+
+One of the most surprising triumphs of celestial photography was
+Professor Keeler’s discovery, in 1899, that the great majority of the
+nebulæ have a distinctly spiral form. This form, previously known in
+Lord Rosse’s great “Whirlpool Nebula,” had been supposed to be
+exceptional; now the photographs, far excelling telescopic views in the
+revelation of nebular forms, showed the spiral to be the typical shape.
+Indeed, it is a question whether all nebulæ are not to some extent
+spiral. The extreme importance of this discovery is shown in the effect
+that it has had upon hitherto prevailing views of solar and planetary
+evolution. For more than three-quarters of a century Laplace’s
+celebrated hypothesis of the manner of origin of the solar system from
+a rotating and contracting nebula surrounding the sun had guided
+speculation on that subject, and had been tentatively extended to cover
+the evolution of systems in general. The apparent forms of some of the
+nebulæ which the telescope had revealed were regarded, and by some are
+still regarded, as giving visual evidence in favor of this theory.
+There is a “ring nebula” in Lyra with a central star, and a “planetary
+nebula” in Gemini bearing no little resemblance to the planet Saturn
+with its rings, both of which appear to be practical realizations of
+Laplace’s idea, and the elliptical rings surrounding the central
+condensation of the Andromeda Nebula may be cited for the same kind of
+proof.
+
+
+[Illustration: Lord Rosse’s nebula]
+
+
+But since Keeler’s discovery there has been a decided turning away of
+speculation another way. The form of the spiral nebulæ seems to be
+entirely inconsistent with the theory of an originally globular or
+disk-shaped nebula condensing around a sun and throwing or leaving off
+rings, to be subsequently shaped into planets. Some astronomers,
+indeed, now reject Laplace’s hypothesis _in toto,_ preferring to think
+that even our solar system originated from a spiral nebula. Since the
+spiral type prevails among the existing nebulæ, we must make any
+mechanical theory of the development of stars and planetary systems
+from them accord with the requirements which that form imposes. A
+glance at the extraordinary variations upon the spiral which Professor
+Keeler’s photographs reveal is sufficient to convince one of the
+difficulty of the task of basing a general theory upon them. In truth,
+it is much easier to criticize Laplace’s hypothesis than to invent a
+satisfactory substitute for it. If the spiral nebulæ seem to oppose it
+there are other nebulæ which appear to support it, and it may be that
+no one fixed theory can account for all the forms of stellar evolution
+in the universe. Our particular planetary system may have originated
+very much as the great French mathematician supposed, while others have
+undergone, or are now undergoing, a different process of development.
+There is always a too strong tendency to regard an important new
+discovery and the theories and speculations based upon it as
+revolutionizing knowledge, and displacing or overthrowing everything
+that went before. Upon the plea that “Laplace only made a guess” more
+recent guesses have been driven to extremes and treated by injudicious
+exponents as “the solid facts at last.”
+
+
+[Illustration: Wonderful spiral in triangulum]
+
+
+Before considering more recent theories than Laplace’s, let us see what
+the nature of the photographic revelations is. The vast celestial
+maelstrom discovered by Lord Rosse in the “Hunting Dogs” may be taken
+as the leading type of the spiral nebulæ, although there are less
+conspicuous objects of the kind which, perhaps, better illustrate some
+of their peculiarities. Lord Rosse’s nebula appears far more wonderful
+in the photographs than in his drawings made with the aid of his giant
+reflecting telescope at Parsonstown, for the photographic plate records
+details that no telescope is capable of showing. Suppose we look at the
+photograph of this object as any person of common sense would look at
+any great and strange natural phenomenon. What is the first thing that
+strikes the mind? It is certainly the appearance of violent whirling
+motion. One would say that the whole glowing mass had been spun about
+with tremendous velocity, or that it had been set rotating so rapidly
+that it had become the victim of “centrifugal force,” one huge fragment
+having broken loose and started to gyrate off into space. Closer
+inspection shows that in addition to the principal focus there are
+various smaller condensations scattered through the mass. These are
+conspicuous in the spirals. Some of them are stellar points, and but
+for the significance of their location we might suppose them to be
+stars which happen to lie in a line between us and the nebula. But when
+we observe how many of them follow most faithfully the curves of the
+spirals we cannot but conclude that they form an essential part of the
+phenomenon; it is not possible to believe that their presence in such
+situations is merely fortuitous. One of the outer spirals has at least
+a dozen of these star-like points strung upon it; some of them sharp,
+small, and distinct, others more blurred and nebulous, suggesting
+different stages of condensation. Even the part which seems to have
+been flung loose from the main mass has, in addition to its central
+condensation, at least one stellar point gleaming in the half-vanished
+spire attached to it. Some of the more distant stars scattered around
+the “whirlpool” look as if they too had been shot out of the mighty
+vortex, afterward condensing into unmistakable solar bodies. There are
+at least two curved rows of minute stars a little beyond the periphery
+of the luminous whirl which clearly follow lines concentric with those
+of the nebulous spirals. Such facts are simply dumbfounding for anyone
+who will bestow sufficient thought upon them, for these are _suns,_
+though they may be small ones; and what a birth is that for a sun!
+
+Look now again at the glowing spirals. We observe that hardly have they
+left the central mass before they begin to coagulate. In some places
+they have a “ropy” aspect; or they are like peascods filled with
+growing seeds, which eventually will become stars. The great focus
+itself shows a similar tendency, especially around its circumference.
+The sense that it imparts of a tremendous shattering force at work is
+overwhelming. There is probably more matter in that whirling and
+bursting nebula than would suffice to make a hundred solar systems! It
+must be confessed at once that there is no confirmation of the
+Laplacean hypothesis here; but what hypothesis will fit the facts?
+There is one which it has been claimed does so, but we shall come to
+that later. In the meanwhile, as a preparation, fix in the memory the
+appearance of that second spiral mass spinning beside its master which
+seems to have spurned it away.
+
+
+[Illustration: Spiral in Ursa Major]
+
+
+For a second example of the spiral nebulæ look at the one in the
+constellation Triangulum. _God, how hath the imagination of puny man
+failed to comprehend Thee!_ Here is creation through destruction with a
+vengeance! The spiral form of the nebula is unmistakable, but it is
+half obliterated amid the turmoil of flying masses hurled away on all
+sides with tornadic fury. The focus itself is splitting asunder under
+the intolerable strain, and in a little while, as time is reckoned in
+the Cosmos, it will be gyrating into stars. And then look at the
+cyclonic rain of already finished stars whirling round the outskirts of
+the storm. Observe how scores of them are yet involved in the fading
+streams of the nebulous spirals; see how they have been thrown into
+vast loops and curves, of a beauty that half redeems the terror of the
+spectacle enclosed within their lines—like iridescent cirri hovering
+about the edges of a hurricane. And so again are suns born!
+
+Let us turn to the exquisite spiral in Ursa Major; how different its
+aspect from that of the other! One would say that if the terrific coil
+in Triangulum has all but destroyed itself in its fury, this one on the
+contrary has just begun its self-demolition. As one gazes one seems to
+see in it the smooth, swift, accelerating motion that precedes
+catastrophe. The central part is still intact, dense, and uniform in
+texture. How graceful are the spirals that smoothly rise from its oval
+rim and, gemmed with little stars, wind off into the darkness until
+they have become as delicate as threads of gossamer! But at bottom the
+story told here is the same—creation by gyration!
+
+Compare with the above the curious mass in Cetus. Here the plane of the
+whirling nebula nearly coincides with our line of sight and we see the
+object at a low angle. It is far advanced and torn to shreds, and if we
+could look at it perpendicularly to its plane it is evident that it
+would closely resemble the spectacle in Triangulum.
+
+Then take the famous Andromeda Nebula (see Frontispiece), which is so
+vast that notwithstanding its immense distance even the naked eye
+perceives it as an enigmatical wisp in the sky. Its image on the
+sensitive plate is the masterpiece of astronomical photography; for
+wild, incomprehensible beauty there is nothing that can be compared
+with it. Here, if anywhere, we look upon the spectacle of creation in
+one of its earliest stages. The Andromeda Nebula is apparently less
+advanced toward transformation into stellar bodies than is that in
+Triangulum. The immense crowd of stars sprinkled over it and its
+neighborhood seem in the main to lie this side of the nebula, and
+consequently to have no connection with it. But incipient stars (in
+some places clusters of them) are seen in the nebulous rings, while one
+or two huge masses seem to give promise of transformation into stellar
+bodies of unusual magnitude. I say “rings” because although the loops
+encompassing the Andromeda Nebula have been called spirals by those who
+wish utterly to demolish Laplace’s hypothesis, yet they are not
+manifestly such, as can be seen on comparing them with the undoubted
+spirals of the Lord Rosse Nebula. They look quite as much like circles
+or ellipses seen at an angle of, say, fifteen or twenty degrees to
+their plane. If they are truly elliptical they accord fairly well with
+Laplace’s idea, except that the scale of magnitude is stupendous, and
+if the Andromeda Nebula is to become a solar system it will surpass
+ours in grandeur beyond all possibility of comparison.
+
+
+[Illustration: Nebula in Cetus]
+
+
+There is one circumstance connected with the spiral nebulæ, and
+conspicuous in the Andromeda Nebula on account of its brightness, which
+makes the question of their origin still more puzzling; they all show
+continuous spectra, which, as we have before remarked, indicate that
+the mass from which the light comes is either solid or liquid, or a gas
+under heavy pressure. Thus nebulæ fall into two classes: the “white”
+nebulæ, giving a continuous spectrum; and the “green” nebulæ whose
+spectra are distinctly gaseous. The Andromeda Nebula is the great
+representative of the former class and the Orion Nebula of the latter.
+The spectrum of the Andromeda Nebula has been interpreted to mean that
+it consists not of luminous gas, but of a flock of stars so distant
+that they are separately indistinguishable even with powerful
+telescopes, just as the component stars of the Milky Way are
+indistinguishable with the naked eye; and upon this has been based the
+suggestion that what we see in Andromeda is an outer universe whose
+stars form a series of elliptical garlands surrounding a central mass
+of amazing richness. But this idea is unacceptable if for no other
+reason than that, as just said, all the spiral nebulæ possess the same
+kind of spectrum, and probably no one would be disposed to regard them
+all as outer universes. As we shall see later, the peculiarity of the
+spectra of the spiral nebulæ is appealed to in support of a modern
+substitute for Laplace’s hypothesis.
+
+Finally, without having by any means exhausted the variety exhibited by
+the spiral nebulæ, let us turn to the great representative of the other
+species, the Orion Nebula. In some ways this is even more marvelous
+than the others. The early drawings with the telescope failed to convey
+an adequate conception either of its sublimity or of its complication
+of structure. It exists in a nebulous region of space, since
+photographs show that nearly the whole constellation is interwoven with
+faintly luminous coils. To behold the entry of the great nebula into
+the field even of a small telescope is a startling experience which
+never loses its novelty. As shown by the photographs, it is an
+inscrutable chaos of perfectly amazing extent, where spiral bands,
+radiating streaks, dense masses, and dark yawning gaps are strangely
+intermingled without apparent order. In one place four conspicuous
+little stars, better seen in a telescope than in the photograph on
+account of the blurring produced by over-exposure, are suggestively
+situated in the midst of a dark opening, and no observer has ever felt
+any doubt that these stars have been formed from the substance of the
+surrounding nebula. There are many other stars scattered over its
+expanse which manifestly owe their origin to the same source. But
+compare the general appearance of this nebula with the others that we
+have studied, and remark the difference. If the unmistakably spiral
+nebulæ resemble bursting fly-wheels or grindstones from whose
+perimeters torrents of sparks are flying, the Orion Nebula rather
+recalls the aspect of a cloud of smoke and fragments produced by the
+explosion of a shell. This idea is enforced by the look of the outer
+portion farthest from the bright half of the nebula, where sharply
+edged clouds with dark spaces behind seem to be billowing away as if
+driven by a wind blowing from the center.
+
+
+[Illustration: The Orion nebula]
+
+
+Next let us consider what scientific speculation has done in the effort
+to explain these mysteries. Laplace’s hypothesis can certainly find no
+standing ground either in the Orion Nebula or in those of a spiral
+configuration, whatever may be its situation with respect to the grand
+Nebula of Andromeda, or the “ring” and “planetary” nebulæ. Some other
+hypothesis more consonant with the appearances must be found. Among the
+many that have been proposed the most elaborate is the “Planetesimal
+Hypothesis” of Professors Chamberlin and Moulton. It is to be remarked
+that it applies to the spiral nebulæ distinctively, and not to an
+apparently chaotic mass of gas like the vast luminous cloud in Orion.
+The gist of the theory is that these curious objects are probably the
+result of close approaches to each other of two independent suns,
+reminding us of what was said on this subject when we were dealing with
+temporary stars. Of the previous history of these appulsing suns the
+theory gives us no account; they are simply supposed to arrive within
+what may be called an effective tide-producing distance, and then the
+drama begins. Some of the probable consequences of such an approach
+have been noticed in Chapter 5; let us now consider them a little more
+in detail.
+
+Tides always go in couples; if there is a tide on one side of a globe
+there will be a corresponding tide on the other side. The cause is to
+be found in the law that the force of gravitation varies inversely as
+the square of the distance; the attraction on the nearest surface of
+the body exercised by another body is greater than on its center, and
+greater yet than on its opposite surface. If two great globes attract
+each other, each tends to draw the other out into an ellipsoidal
+figure; they must be more rigid than steel to resist this—and even then
+they cannot altogether resist. If they are liquid or gaseous they will
+yield readily to the force of distortion, the amount of which will
+depend upon their distance apart, for the nearer they are the greater
+becomes the tidal strain. If they are encrusted without and liquid or
+gaseous in the interior, the internal mass will strive to assume the
+figure demanded by the tidal force, and will, if it can, burst the
+restraining envelope. Now this is virtually the predicament of the body
+we call a sun when in the immediate presence of another body of
+similarly great mass. Such a body is presumably gaseous throughout, the
+component gases being held in a state of rigidity by the compression
+produced by the tremendous gravitational force of their own aggregate
+mass. At the surface such a body is enveloped in a shell of relatively
+cool matter. Now suppose a great attracting body, such as another sun,
+to approach near enough for the difference in its attraction on the two
+opposite sides of the body and on its center to become very great; the
+consequence will be a tidal deformation of the whole body, and it will
+lengthen out along the line of the gravitational pull and draw in at
+the sides, and if its shell offers considerable resistance, but not
+enough to exercise a complete restraint, it will be violently burst
+apart, or blown to atoms, and the internal mass will leap out on the
+two opposite sides in great fiery spouts. In the case of a sun further
+advanced in cooling than ours the interior might be composed of molten
+matter while the exterior crust had become rigid like the shell of an
+egg; then the force of the “tidal explosion” produced by the appulse of
+another sun would be more violent in consequence of the greater
+resistance overcome. Such, then, is the mechanism of the first phase in
+the history of a spiral nebula according to the Planetesimal
+Hypothesis. Two suns, perhaps extinguished ones, have drawn near
+together, and an explosive outburst has occured in one or both. The
+second phase calls for a more agile exercise of the imagination.
+
+To simplify the case, let us suppose that only one of the tugging suns
+is seriously affected by the strain. Its vast wings produced by the
+outburst are twisted into spirals by their rotation and the contending
+attractions exercised upon them, as the two suns, like battleships in
+desperate conflict, curve round each other, concentrating their
+destructive energies. Then immense quantities of débris are scattered
+about in which eddies are created, and finally, as the sun that caused
+the damage goes on its way, leaving its victim to repair its injuries
+as it may, the dispersed matter cools, condenses, and turns into
+streams of solid particles circling in elliptical paths about their
+parent sun. These particles, or fragments, are the “planetesimals” of
+the theory. In consequence of the inevitable intersection of the orbits
+of the planetesimals, nodes are formed where the flying particles meet,
+and at these nodes large masses are gradually accumulated. The larger
+the mass the greater its attraction, and at last the nodal points
+become the nuclei of great aggregations from which planets are shaped.
+
+This, in very brief form, is the Planetesimal Hypothesis which we are
+asked to substitute for that based on Laplace’s suggestion as an
+explanation of the mode of origin of the solar system; and the
+phenomena of the spiral nebulæ are appealed to as offering evident
+support to the new hypothesis. We are reminded that they are elliptical
+in outline, which accords with the hypothesis; that their spectra are
+not gaseous, which shows that they may be composed of solid particles
+like the planetesimals; and that their central masses present an oval
+form, which is what would result from the tidal effects, as just
+described. We also remember that some of them, like the Lord Rosse and
+the Andromeda nebulæ, are visually double, and in these cases we might
+suppose that the two masses represent the tide-burst suns that ventured
+into too close proximity. It may be added that the authors of the
+theory do not insist upon the appulse of two suns as the _only_ way in
+which the planetesimals may have originated, but it is the only
+supposition that has been worked out.
+
+But serious questions remain. It needs, for instance, but a glance at
+the Triangulum monster to convince the observer that it cannot be a
+solar system which is being evolved there, but rather a swarm of stars.
+Many of the detached masses are too vast to admit of the supposition
+that they are to be transformed into planets, in our sense of planets,
+and the distances of the stars which appear to have been originally
+ejected from the focal masses are too great to allow us to liken the
+assemblage that they form to a solar system. Then, too, no nodes such
+as the hypothesis calls for are visible. Moreover, in most of the
+spiral nebulæ the appearances favor the view that the supposititious
+encountering suns have not separated and gone each rejoicing on its
+way, after having inflicted the maximum possible damage on its
+opponent, but that, on the contrary, they remain in close association
+like two wrestlers who cannot escape from each other’s grasp. And this
+is exactly what the law of gravitation demands; stars cannot approach
+one another with impunity, with regard either to their physical make-up
+or their future independence of movement. The theory undertakes to
+avoid this difficulty by assuming that in the case of our system the
+approach of the foreign body to the sun was not a close one—just close
+enough to produce the tidal extrusion of the relatively insignificant
+quantity of matter needed to form the planets. But even then the effect
+of the appulse would be to change the direction of flight, both of the
+sun and of its visitor, and there is no known star in the sky which can
+be selected as the sun’s probable partner in their ancient _pas deux._
+That there are unconquered difficulties in Laplace’s hypothesis no one
+would deny, but in simplicity of conception it is incomparably more
+satisfactory, and with proper modifications could probably be made more
+consonant with existing facts in our solar system than that which is
+offered to replace it. Even as an explanation of the spiral nebulæ, not
+as solar systems in process of formation, but as the birthplaces of
+stellar clusters, the Planetesimal Hypothesis would be open to many
+objections. Granting its assumptions, it has undoubtedly a strong
+mathematical framework, but the trouble is not with the mathematics but
+with the assumptions. Laplace was one of the ablest mathematicians that
+ever lived, but he had never seen a spiral nebula; if he had, he might
+have invented a hypothesis to suit its phenomena. His actual hypothesis
+was intended only for our solar system, and he left it in the form of a
+“note” for the consideration of his successors, with the hope that they
+might be able to discover the full truth, which he confessed was hidden
+from him. It cannot be said that that truth has yet been found, and
+when it is found the chances are that intuition and not logic will have
+led to it.
+
+The spiral nebulæ, then, remain among the greatest riddles of the
+universe, while the gaseous nebulæ, like that of Orion, are no less
+mysterious, although it seems impossible to doubt that both forms give
+birth to stars. It is but natural to look to them for light on the
+question of the origin of our planetary system; but we should not
+forget that the scale of the phenomena in the two cases is vastly
+different, and the forces in operation may be equally different. A hill
+may have been built up by a glacier, while a mountain may be the
+product of volcanic forces or of the upheaval of the strata of the
+planet.
+
+
+
+
+VII
+The Banners of the Sun
+
+
+As all the world knows, the sun, a blinding globe pouring forth an
+inconceivable quantity of light and heat, whose daily passage through
+the sky is caused by the earth’s rotation on its axis, constitutes the
+most important phenomenon of terrestial existence. Viewed with a dark
+glass to take off the glare, or with a telescope, its rim is seen to be
+a sharp and smooth circle, and nothing but dark sky is visible around
+it. Except for the interference of the moon, we should probably never
+have known that there is any more of the sun than our eyes ordinarily
+see.
+
+But when an eclipse of the sun occurs, caused by the interposition of
+the opaque globe of the moon, we see its immediate surroundings, which
+in some respects are more wonderful than the glowing central orb. These
+surroundings, although not in the sense in which we apply the term to
+the gaseous envelope of the earth, may be called the sun’s atmosphere.
+They consist of two very different parts—first, the red “prominences,”
+which resemble tongues of flame ascending thousands of miles above the
+sun’s surface; and, second, the “corona,” which extends to distances of
+millions of miles from the sun, and shines with a soft, glowing light.
+The two combined, when well seen, make a spectacle without parallel
+among the marvels of the sky. Although many attempts have been made to
+render the corona visible when there is no eclipse, all have failed,
+and it is to the moon alone that we owe its revelation. To cover the
+sun’s disk with a circular screen will not answer the purpose because
+of the illumination of the air all about the observer. When the moon
+hides the sun, on the other hand, the sunlight is withdrawn from a
+great cylinder of air extending to the top of the atmosphere and
+spreading many miles around the observer. There is then no glare to
+interfere with the spectacle, and the corona appears in all its
+surprising beauty. The prominences, however, although they were
+discovered during an eclipse, can now, with the aid of the
+spectroscope, be seen at any time. But the prominences are rarely large
+enough to be noticed by the naked eye, while the streamers of the
+corona, stretching far away in space, like ghostly banners blown out
+from the black circle of the obscuring moon, attract every eye, and to
+this weird apparition much of the fear inspired by eclipses has been
+due. But if the corona has been a cause of terror in the past it has
+become a source of growing knowledge in our time.
+
+
+[Illustration: The Corona]
+
+
+The story of the first scientific observation of the corona and the
+prominences is thrillingly interesting, and in fact dramatic. The
+observation was made during the eclipse of 1842, which fortunately was
+visible all over Central and Southern Europe so that scores of
+astronomers saw it. The interest centers in what happened at Pavia in
+Northern Italy, where the English astronomer Francis Baily had set up
+his telescope. The eclipse had begun and Bailey was busy at his
+telescope when, to quote his own words in the account which he wrote
+for the _Memoirs of the Royal Astronomical Society:_
+
+
+I was astounded by a tremendous burst of applause from the streets
+below, and at the same moment was electrified by the sight of one of
+the most brilliant and splendid phenomena that can well be imagined;
+for at that instant the dark body of the moon was suddenly surrounded
+with a corona, or kind of bright glory, similar in shape and magnitude
+to that which painters draw round the heads of saints...
+
+Pavia contains many thousand inhabitants, the major part of whom were
+at this early hour walking about the streets and squares or looking out
+of windows in order to witness this long-talked-of phenomenon; and when
+the total obscuration took place, which was _instantaneous,_ there was
+a universal shout from every observer which “made the welkin ring,” and
+for the moment withdrew my attention from the object with which I was
+immediately occupied. I had, indeed, expected the appearance of a
+luminous circle round the moon during the time of total obscurity; but
+I did not expect, from any of the accounts of preceding eclipses that I
+had read, to witness so magnificent an exhibition as that which took
+place...
+
+Splendid and astonishing, however, as this remarkable phenomenon really
+was, and although it could not fail to call forth the admiration and
+applause of every beholder, yet I must confess that there was at the
+same time something in its singular and wonderful appearance that was
+appalling...
+
+But the most remarkable circumstance attending the phenomenon was the
+appearance of _three large protuberances_ apparently emanating from the
+circumference of the moon, but evidently forming a portion of the
+corona. They had the appearance of mountains of a prodigious elevation;
+their color was red tinged with lilac or purple; perhaps the color of
+the peach-blossom would more nearly represent it. They somewhat
+resembled the tops of the snowy Alpine mountains when colored by the
+rising or the setting sun. They resembled the Alpine mountains in
+another respect, inasmuch as their light was perfectly steady, and had
+none of that flickering or sparkling motion so visible in other parts
+of the corona...
+
+The whole of these protuberances were visible even to the last moment
+of total obscuration, and when the first ray of light was admitted from
+the sun they vanished, with the corona, altogether, and daylight was
+instantly restored.
+
+
+I have quoted nearly all of this remarkable description not alone for
+its intrinsic interest, but because it is the best depiction that can
+be found of the general phenomena of a total solar eclipse. Still, not
+every such eclipse offers an equally magnificent spectacle. The
+eclipses of 1900 and 1905, for instance, which were seen by the writer,
+the first in South Carolina and the second in Spain, fell far short of
+that described by Bailey in splendor and impressiveness. Of course,
+something must be allowed for the effect of surprise; Bailey had not
+expected to see what was so suddenly disclosed to him. But both in 1900
+and 1905 the amount of scattered light in the sky was sufficient in
+itself to make the corona appear faint, and there were no very
+conspicuous prominences visible. Yet on both occasions there was
+manifest among the spectators that mingling of admiration and awe of
+which Bailey speaks. The South Carolinians gave a cheer and the ladies
+waved their handkerchiefs when the corona, ineffably delicate of form
+and texture, _melted_ into sight and then in two minutes melted away
+again. The Spaniards, crowded on the citadel hill of Burgos, with their
+king and his royal retinue in their midst, broke out with a great
+clapping of hands as the awaited spectacle unfolded itself in the sky;
+and on both occasions, before the applause began, after an awed silence
+a low murmur ran through the crowds. At Burgos it is said many made the
+sign of the cross.
+
+It was not long before Bailey’s idea that the prominences were a part
+of the corona was abandoned, and it was perceived that the two
+phenomena were to a great extent independent. At the eclipse of 1868,
+which the astronomers, aroused by the wonderful scene of 1842, and
+eager to test the powers of the newly invented spectroscope, flocked to
+India to witness, Janssen conceived the idea of employing the
+spectroscope to render the prominences visible when there was no
+eclipse. He succeeded the very next day, and these phenomena have been
+studied in that way ever since.
+
+There are recognized two kinds of prominences—the “erruptive” and the
+“quiescent.” The latter, which are cloud-like in form, may be seen
+almost anywhere along the edge of the sun; but the former, which often
+shoot up as if hurled from mighty volcanoes, appear to be associated
+with sun-spots, and appear only above the zones where spots abound.
+Either of them, when seen in projection against the brilliant solar
+disk, appears white, not red, as against a background of sky. The
+quiescent prominences, whose elevation is often from forty thousand to
+sixty thousand miles, consist, as the spectroscope shows, mainly of
+hydrogen and helium. The latter, it will be remembered, is an element
+which was known to be in the sun many years before the discovery that
+it also exists in small quantities on the earth. A fact which may have
+a significance which we cannot at present see is that the emanation
+from radium gradually and spontaneously changes into helium, an
+alchemistical feat of nature that has opened many curious vistas to
+speculative thinkers. The eruptive prominences, which do not spread
+horizontally like the others, but ascend with marvelous velocity to
+elevations of half a million miles or more, are apparently composed
+largely of metallic vapors—_i.e._ metals which are usually solid on the
+earth, but which at solar temperatures are kept in a volatilized state.
+The velocity of their ascent occasionally amounts to three hundred or
+four hundred miles per second. It is known from mathematical
+considerations that the gravitation of the sun would not be able to
+bring back any body that started from its surface with a velocity
+exceeding three hundred and eighty-three miles per second; so it is
+evident that some of the matter hurled forth in eruptive prominences
+may escape from solar control and go speeding out into space, cooling
+and condensing into solid masses. There seems to be no reason why some
+of the projectiles from the sun might not reach the planets. Here,
+then, we have on a relatively small scale, _explosions_ recalling those
+which it has been imagined may be the originating cause of some of the
+sudden phenomena of the stellar heavens.
+
+
+[Illustration: A solar “prominence.” Photographed May 21, 1907]
+
+
+Of the sun-spots it is not our intention here specifically to speak,
+but they evidently have an intimate connection with eruptive
+prominences, as well as some relation, not yet fully understood, with
+the corona. Of the real cause of sun-spots we know virtually nothing,
+but recent studies by Professor Hale and others have revealed a strange
+state of things in the clouds of metallic vapors floating above them
+and their surroundings. Evidences of a cyclonic tendency have been
+found, and Professor Hale has proved that sun-spots are strong magnetic
+fields, and consist of columns of ionized vapors rotating in opposite
+directions in the two hemispheres. A fact which may have the greatest
+significance is that titanium and vanadium have been found both in
+sun-spots and in the remarkable variable Mira Ceti, a star which every
+eleven months, or thereabout, flames up with great brilliancy and then
+sinks back to invisibility with the naked eye. It has been suggested
+that sun-spots are indications of the beginning of a process in the sun
+which will be intensified until it falls into the state of such a star
+as Mira. Stars very far advanced in evolution, without showing
+variability, also exhibit similar spectra; so that there is much reason
+for regarding sunspots as emblems of advancing age.
+
+The association of the corona with sun-spots is less evident than that
+of the eruptive prominences; still such an association exists, for the
+form and extent of the corona vary with the sun-spot period of which we
+shall presently speak. The constitution of the corona remains to be
+discovered. It is evidently in part gaseous, but it also probably
+contains matter in the form of dust and small meteors. It includes one
+substance altogether mysterious—“coronium.” There are reasons for
+thinking that this may be the lightest of all the elements, and
+Professor Young, its discoverer, said that it was “absolutely unique in
+nature; utterly distinct from any other known form of matter,
+terrestial, solar, or cosmical.” The enormous extent of the corona is
+one of its riddles. Since the development of the curious subject of the
+“pressure of light” it has been proposed to account for the
+sustentation of the corona by supposing that it is borne upon the
+billows of light continually poured out from the sun. Experiment has
+proved, what mathematical considerations had previously pointed out as
+probable, that the waves of light exert a pressure or driving force,
+which becomes evident in its effects if the body acted upon is
+sufficiently small. In that case the light pressure will prevail over
+the attraction of gravitation, and propel the attenuated matter away
+from the sun in the teeth of its attraction. The earth itself would be
+driven away if, instead of consisting of a solid globe of immense
+aggregate mass, it were a cloud of microscopic particles. The reason is
+that the pressure varies in proportion to the _surface_ of the body
+acted upon, while the gravitational attraction is proportional to the
+_volume,_ or the total amount of matter in the body. But the surface of
+any body depends upon the _square_ of its diameter, while the volume
+depends upon the _cube_ of the diameter. If, for instance, the diameter
+is represented by 4, the surface will be proportional to 4 × 4, or 16,
+and the volume to 4 × 4 × 4, or 64; but if the diameter is taken as 2,
+the surface will be 2 × 2, or 4, and the volume 2 × 2 × 2, or 8. Now,
+the ratio of 4 to 8 is twice as great as that of 16 to 64. If the
+diameter is still further decreased, the ratio of the surface to the
+volume will proportionally grow larger; in other words, the pressure
+will gain upon the attraction, and whatever their original ratio may
+have been, a time will come, if the diminution of size continues, when
+the pressure will become more effective than the attraction, and the
+body will be driven away. Supposing the particles of the corona to be
+below the critical size for the attraction of a mass like that of the
+sun to control them, they would be driven off into the surrounding
+space and appear around the sun like the clouds of dust around a mill.
+We shall return to this subject in connection with the Zodiacal Light,
+the Aurora, and Comets.
+
+On the other hand, there are parts of the corona which suggest by their
+forms the play of electric or magnetic forces. This is beautifully
+shown in some of the photographs that have been made of the corona
+during recent eclipses. Take, for instance, that of the eclipse of
+1900. The sheaves of light emanating from the poles look precisely like
+the “lines of force” surrounding the poles of a magnet. It will be
+noticed in this photograph that the corona appears to consist of two
+portions: one comprising the polar rays just spoken of, and the other
+consisting of the broader, longer, and less-defined masses of light
+extending out from the equatorial and middle-latitude zones. Yet even
+in this more diffuse part of the phenomenon one can detect the presence
+of submerged curves bearing more or less resemblance to those about the
+poles. Just what part electricity or electro-magnetism plays in the
+mechanism of the solar radiation it is impossible to say, but on the
+assumption that it is a very important part is based the hypothesis
+that there exists a direct solar influence not only upon the magnetism,
+but upon the weather of the earth. This hypothesis has been under
+discussion for half a century, and still we do not know just how much
+truth it represents. It is certain that the outbreak of great
+disturbances on the sun, accompanied by the formation of sun-spots and
+the upshooting of eruptive prominences (phenomena which we should
+naturally expect to be attended by action), have been instantly
+followed by corresponding “magnetic storms” on the earth and brilliant
+displays of the auroral lights. There have been occasions when the
+influence has manifested itself in the most startling ways, a great
+solar outburst being followed by a mysterious gripping of the cable and
+telegraph systems of the world, as if an invisible and irresistible
+hand had seized them. Messages are abruptly cut off, sparks leap from
+the telegraph instruments, and the entire earth seems to have been
+thrown into a magnetic flurry. These occurrences affect the mind with a
+deep impression of the dependence of our planet on the sun, such as we
+do not derive from the more familiar action of the sunlight on the
+growth of plants and other phenomena of life depending on solar
+influences.
+
+Perhaps the theory of solar magnetic influence upon the weather is best
+known in connection with the “sun-spot cycle.” This, at any rate, is,
+as already remarked, closely associated with the corona. Its existence
+was discovered in 1843 by the German astronomer Schwabe. It is a period
+of variable length, averaging about eleven years, during which the
+number of spots visible on the sun first increases to a maximum, then
+diminishes to a minimum, and finally increases again to a maximum. For
+unknown reasons the period is sometimes two or three years longer than
+the average and sometimes as much shorter. Nevertheless, the phenomena
+always recur in the same order. Starting, for instance, with a time
+when the observer can find few or no spots, they gradually increase in
+number and size until a maximum, in both senses, is reached, during
+which the spots are often of enormous size and exceedingly active.
+After two or three years they begin to diminish in number, magnitude,
+and activity until they almost or quite disappear. A strange fact is
+that when a new period opens, the spots appear first in high northern
+and southern latitudes, far from the solar equator, and as the period
+advances they not only increase in number and size, but break out
+nearer and nearer to the equator, the last spots of a vanishing period
+sometimes lingering in the equatorial region after the advance-guard of
+its successor has made its appearance in the high latitudes. Spots are
+never seen on the equator nor near the poles. It was not very long
+after the discovery of the sun-spot cycle that the curious observation
+was made that a striking coincidence existed between the period of the
+sun-spots and another period affecting the general magnetic condition
+of the earth. When a curved line representing the varying number of
+sun-spots was compared with another curve showing the variations in the
+magnetic state of the earth the two were seen to be in almost exact
+accord, a rise in one curve corresponding to a rise in the other, and a
+fall to a fall. Continued observation has proved that this is a real
+coincidence and not an accidental one, so that the connection, although
+as yet unexplained, is accepted as established. But does the influence
+extend further, and directly affect the weather and the seasons as well
+as the magnetic elements of the earth? A final answer to this question
+cannot yet be given, for the evidence is contradictory, and the
+interpretations put upon it depend largely on the predilections of the
+judges.
+
+But, in a broad sense, the sun-spots and the phenomena connected with
+them _must_ have a relation to terrestial meteorology, for they prove
+the sun to be a variable star. Reference was made, a few lines above,
+to the resemblance of the spectra of sun-spots to those of certain
+stars which seem to be failing through age. This in itself is extremely
+suggestive; but if this resemblance had never been discovered, we
+should have been justified in regarding the sun as variable in its
+output of energy; and not only variable, but probably increasingly so.
+The very inequalities in the sun-spot cycle are suspicious. When the
+sun is most spotted its total light may be reduced by one-thousandth
+part, although it is by no means certain that its outgiving of thermal
+radiations is then reduced. A loss of one-thousandth of its luminosity
+would correspond to a decrease of .0025 of a stellar magnitude,
+considering the sun as a star viewed from distant space. So slight a
+change would not be perceptible; but it is not alone sun-spots which
+obscure the solar surface, its entire globe is enveloped with an
+obscuring veil. When studied with a powerful telescope the sun’s
+surface is seen to be thickly mottled with relatively obscure specks,
+so numerous that it has been estimated that they cut off from one-tenth
+to one-twentieth of the light that we should receive from it if the
+whole surface were as brilliant as its brightest parts. The condition
+of other stars warrants the conclusion that this obscuring envelope is
+the product of a process of refrigeration which will gradually make the
+sun more and more variable until its history ends in extinction.
+Looking backward, we see a time when the sun must have been more
+brilliant than it is now. At that time it probably shone with the
+blinding white splendor of such stars as Sirius, Spica, and Vega; now
+it resembles the relatively dull Procyon; in time it will turn ruddy
+and fall into the closing cycle represented by Antares. Considering
+that once it must have been more radiantly powerful than at present,
+one is tempted to wonder if that could have been the time when tropical
+life flourished within the earth’s polar circles, sustained by a
+vivific energy in the sun which it has now lost.
+
+The corona, as we have said, varies with the sun-spot cycle. When the
+spots are abundant and active the corona rises strong above the spotted
+zones, forming immense beams or streamers, which on one occasion, at
+least, had an observed length of _ten million miles._ At the time of a
+spot minimum the corona is less brilliant and has a different outline.
+It is then that the curved polar rays are most conspicuous. Thus the
+vast banners of the sun, shaken out in the eclipse, are signals to tell
+of its varying state, but it will probably be long before we can read
+correctly their messages.
+
+
+
+
+VIII
+The Zodiacal Light Mystery
+
+
+There is a singular phenomenon in the sky—one of the most puzzling of
+all—which has long arrested the attention of astronomers, defying their
+efforts at explanation, but which probably not one in a hundred, and
+possibly not one in a thousand, of the readers of this book has ever
+seen. Yet its name is often spoken, and it is a conspicuous object if
+one knows when and where to look for it, and when well seen it exhibits
+a mystical beauty which at the same time charms and awes the beholder.
+It is called “The Zodiacal Light,” because it lies within the broad
+circle of the Zodiac, marking the sun’s apparent annual path through
+the stars. What it is nobody has yet been able to find out with
+certainty, and books on astronomy usually speak of it with singular
+reserve. But it has given rise to many remarkable theories, and a true
+explanation of it would probably throw light on a great many other
+celestial mysteries. The Milky Way is a more wonderful object to look
+upon, but its nature can be comprehended, while there is a sort of
+uncanniness about the Zodiacal Light which immediately impresses one
+upon seeing it, for its part in the great scheme of extra-terrestrial
+affairs is not evident.
+
+If you are out-of-doors soon after sunset—say, on an evening late in
+the month of February—you may perceive, just after the angry flush of
+the dying winter’s day has faded from the sky, a pale ghostly presence
+rising above the place where the sun went down. The writer remembers
+from boyhood the first time it was pointed out to him and the unearthly
+impression that it made, so that he afterward avoided being out alone
+at night, fearful of seeing the spectral thing again. The phenomenon
+brightens slowly with the fading of the twilight, and soon distinctly
+assumes the shape of an elongated pyramid of pearly light, leaning
+toward the south if the place of observation is in the northern
+hemisphere. It does not impress the observer at all in the same manner
+as the Milky Way; that looks far off and is clearly among the stars,
+but the Zodiacal Light seems closer at hand, as if it were something
+more intimately concerning the earth. To all it immediately suggests a
+connection, also, with the sunken sun. If the night is clear and the
+moon absent (and if you are in the country, for city lights ruin the
+spectacles of the sky), you will be able to watch the apparition for a
+long time. You will observe that the light is brightest near the
+horizon, gradually fading as the pyramidal beam mounts higher, but in
+favorable circumstances it may be traced nearly to the meridian south
+of the zenith, where its apex at last vanishes in the starlight. It
+continues visible during the evenings of March and part of April, after
+which, ordinarily, it is seen no more, or if seen is relatively faint
+and unimpressive. But when autumn comes it appears again, this time not
+like a wraith hovering above the westward tomb of the day-god, but
+rather like a spirit of the morning announcing his reincarnation in the
+east.
+
+The reason why the Zodiacal Light is best seen in our latitudes at the
+periods just mentioned is because at those times the Zodiac is more
+nearly perpendicular to the horizon, first in the west and then in the
+east; and, since the phenomenon is confined within the borders of the
+Zodiac, it cannot be favorably placed for observation when the zodiacal
+plane is but slightly inclined to the horizon. Its faint light requires
+the contrast of a background of dark sky in order to be readily
+perceptible. But within the tropics, where the Zodiac is always at a
+favorable angle, the mysterious light is more constantly visible.
+Nearly all observant travelers in the equatorial regions have taken
+particular note of this phenomenon, for being so much more conspicuous
+there than in the temperate zones it at once catches the eye and holds
+the attention as a novelty. Humboldt mentions it many times in his
+works, for his genius was always attracted by things out of the
+ordinary and difficult of explanation, and he made many careful
+observations on its shape, its brilliancy, and its variations; for
+there can be no doubt that it does vary, and sometimes to an
+astonishing degree. It is said that it once remained practically
+invisible in Europe for several years in succession. During a trip to
+South Africa in 1909 an English astronomer, Mr E. W. Maunder, found a
+remarkable difference between the appearance of the Zodiacal Light on
+his going and coming voyages. In fact, when crossing the equator going
+south he did not see it at all; but on returning he had, on March 6th,
+when one degree south of the equator, a memorable view of it.
+
+It was a bright, clear night, and the Zodiacal Light was
+extraordinarily brilliant—brighter than he had ever seen it before. The
+Milky Way was not to be compared with it. The brightest part extended
+75° from the sun. There was a faint and much narrower extension which
+they could just make out beyond the Pleiades along the ecliptic, but
+the greater part of the Zodiacal Light showed as a broad truncated
+column, and it did not appear nearly as conical as he had before seen
+it.
+
+
+When out of the brief twilight of intertropical lands, where the sun
+drops vertically to the horizon and night rushes on like a wave of
+darkness, the Zodiacal Light shoots to the very zenith, its color is
+described as a golden tint, entirely different from the silvery sheen
+of the Milky Way. If I may venture again to refer to personal
+experiences and impressions, I will recall a view of the Zodiacal Light
+from the summit of the cone of Mt Etna in the autumn of the year 1896
+(more briefly described in _Astronomy with the Naked Eye_). There are
+few lofty mountains so favorably placed as Etna for observations of
+this kind. It was once resorted to by Prof. George E. Hale, in an
+attempt to see the solar corona without an eclipse. Rising directly
+from sea-level to an elevation of nearly eleven thousand feet, the
+observer on its summit at night finds himself, as it were, lost in the
+midst of the sky. But for the black flanks of the great cone on which
+he stands he might fancy himself to be in a balloon. On the occasion to
+which I refer the world beneath was virtually invisible in the moonless
+night. The blaze of the constellations overhead was astonishingly
+brilliant, yet amid all their magnificence my attention was immediately
+drawn to a great tapering light that sprang from the place on the
+horizon where the sun would rise later, and that seemed to be blown out
+over the stars like a long, luminous veil. It was the finest view of
+the Zodiacal light that I had ever enjoyed—thrilling in its
+strangeness—but I was almost disheartened by the indifference of my
+guide, to whom it was only a light and nothing more. If he had no
+science, he had less poetry—rather a remarkable thing, I thought, for a
+child of his clime. The Light appeared to me to be distinctly brighter
+than the visible part of the Milky Way which included the brilliant
+stretches in Auriga and Perseus, and its color, if one may speak of
+color in connection with such an object, seemed richer than that of the
+galactic band; but I did not think of it as yellow, although Humboldt
+has described it as resembling a golden curtain drawn over the stars,
+and Du Chaillu in Equatorial Africa found it of a bright yellow color.
+It may vary in color as in conspicuousness. The fascination of that
+extraordinary sight has never faded from my memory. I turned to regard
+it again and again, although I had never seen the stellar heavens so
+brilliant, and it was one of the last things I looked for when the
+morning glow began softly to mount in the east, and Sicily and the
+Mediterranean slowly emerged from the profound shadow beneath us.
+
+The Zodiacal Light seems never to have attracted from astronomers in
+general the amount of careful attention that it deserves; perhaps
+because so little can really be made of it as far as explanation is
+concerned. I have referred to the restraint that scientific writers
+apparently feel in speaking of it. The grounds for speculation that it
+affords may be too scanty to lead to long discussions, yet it piques
+curiosity, and as we shall see in a moment has finally led to a most
+interesting theory. Once it was the subject of an elaborate series of
+studies which carried the observer all round the world. That was in
+1845—46, during the United States Exploring Expedition that visited the
+then little known Japan. The chaplain of the fleet, the Rev. Mr Jones,
+went out prepared to study the mysterious light in all its phases. He
+saw it from many latitudes on both sides of the equator, and the
+imagination cannot but follow him with keen interest in his
+world-circling tour, keeping his eyes every night fixed upon the
+phantasm overhead, whose position shifted with that of the hidden sun.
+He demonstrated that the flow extends at times completely across the
+celestial dome, although it is relatively faint directly behind the
+earth. On his return the government published a large volume of his
+observations, in which he undertook to show that the phenomenon was due
+to the reflection of sunlight from a ring of meteoric bodies encircling
+the earth. But, after all, this elaborate investigation settled
+nothing.
+
+Prof. E. E. Barnard has more recently devoted much attention to the
+Zodiacal Light, as well as to a strange attendant phenomenon called the
+“Gegenschein,” or Counterglow, because it always appears at that point
+in the sky which is exactly opposite the sun. The Gegenschein is an
+extremely elusive phenomenon, suitable only for eyes that have been
+specially trained to see it. Professor Newcomb has cautiously remarked
+that
+
+it is said that in that point of the heavens directly opposite the sun
+there is an elliptical patch of light... This phenomenon is so
+difficult to account for that its existence is sometimes doubted; yet
+the testimony in its favor is difficult to set aside.
+
+
+It certainly cannot be set aside at all since the observations of
+Barnard. I recall an attempt to see it under his guidance during a
+visit to Mount Hamilton, when he was occupied there with the Lick
+telescope. Of course, both the Gegenschein and the Zodiacal Light are
+too diffuse to be studied with telescopes, which, so to speak, magnify
+them out of existence. They can only be successfully studied with the
+naked eye, since every faintest glimmer that they afford must be
+utilized. This is especially true of the Gegenschein. At Mount
+Hamilton, Mr Barnard pointed out to me its location with reference to
+certain stars, but with all my gazing I could not be sure that I saw
+it. To him, on the contrary, it was obvious; he had studied it for
+months, and was able to indicate its shape, its boundaries, its
+diameter, and the declination of its center with regard to the
+ecliptic. There is not, of course, the shadow of a doubt of the
+existence of the Gegenschein, and yet I question if one person in a
+million has ever seen or ever will see it. The Zodiacal Light, on the
+other hand, is plain enough, provided that the time and the
+circumstances of the observation are properly chosen.
+
+In the attempts to explain the Zodiacal Light, the favorite hypothesis
+has been that it is an appendage of the sun—perhaps simply an extension
+of the corona in the plane of the ecliptic, which is not very far from
+coinciding with that of the sun’s equator. This idea is quite a natural
+one, because of the evident relation of the light to the position of
+the sun. The vast extension of the equatorial wings of the corona in
+1878 gave apparent support to this hypothesis; if the substance of the
+corona could extend ten million miles from the sun, why might it not
+extend even one hundred million, gradually fading out beyond the orbit
+of the earth? A variation of this hypothesis assumes that the
+reflection is due to swarms of meteors circling about the sun, in the
+plane of its equator, all the way from its immediate neighborhood to a
+distance exceeding that of the earth. But in neither form is the
+hypothesis satisfactory; there is nothing in the appearance of the
+corona to indicate that it extends even as far as the planet Mercury,
+while as to meteors, the orbits of the known swarms do not accord with
+the hypothesis, and we have no reason to believe that clouds of others
+exist traveling in the part of space where they would have to be in
+order to answer the requirements of the theory. The extension of the
+corona in 1878 did not resemble in its texture the Zodiacal Light.
+
+Now, it has so often happened in the history of science that an
+important discovery in one branch has thrown unexpected but most
+welcome light upon some pending problem in some other branch, that a
+strong argument might be based upon that fact alone against the too
+exclusive devotion of many investigators to the narrow lines of their
+own particular specialty; and the Zodiacal Light affords a case in
+point, when it is considered in connection with recent discoveries in
+chemistry and physics. From the fact that atoms are compound bodies
+made up of corpuscles at least a thousand times smaller than the
+smallest known atom—a fact which astounded most men of science when it
+was announced a few years ago—a new hypothesis has been developed
+concerning the nature of the Zodiacal Light (as well as other
+astronomical riddles), and this hypothesis comes not from an
+astronomer, but from a chemist and physicist, the Swede, Svante
+Arrhenius. In considering an outline of this new hypothesis we need
+neither accept nor reject it; it is a case rather for suspension of
+judgment.
+
+To begin with, it carries us back to the “pressure of light” mentioned
+in the preceding chapter. The manner in which this pressure is believed
+generally to act was there sufficiently explained, and it only remains
+to see how it is theoretically extended to the particles of matter
+supposed to constitute the Zodiacal Light. We know that corpuscles, or
+“fragments of atoms” negatively electrified, are discharged from hot
+bodies. Streams of these “ions” pour from many flames and from molten
+metals; and the impact of the cathode and ultra-violet rays causes them
+to gush even from cold bodies. In the vast laboratory of the sun it is
+but reasonable to suppose that similar processes are taking place. “As
+a very hot metal emits these corpuscles,” says Prof. J. J. Thomson, “it
+does not seem an improbable hypothesis that they are emitted by that
+very hot body, the sun.” Let it be assumed, then, that the sun does
+emit them; what happens next? Negatively charged corpuscles, it is
+known, serve as nuclei to which particles of matter in the ordinary
+state are attracted, and it is probable that those emitted from the sun
+immediately pick up loads in this manner and so grow in bulk. If they
+grow large enough the gravitation of the sun draws them back, and they
+produce a negative charge in the solar atmosphere. But it is probable
+that many of the particles do not attain the critical size which,
+according to the principles before explained, would enable the
+gravitation of the sun to retain them in opposition to the pressure of
+the waves of light, and with these particles the light pressure is
+dominant. Clouds of them may be supposed to be continually swept away
+from the sun into surrounding space, moving mostly in or near the plane
+of the solar equator, where the greatest activity, as indicated by
+sunspots and related phenomena, is taking place. As they pass outward
+into space many of them encounter the earth. If the earth, like the
+moon, had no atmosphere the particles would impinge directly on its
+surface, giving it a negative electric charge. But the presence of the
+atmosphere changes all that, for the first of the flying particles that
+encounter it impart to it their negative electricity, and then, since
+like electric charges repel like, the storm of particles following will
+be sheered off from the earth, and will stream around it in a maze of
+hyperbolic paths. Those that continue on into space beyond the earth
+may be expected to continue picking up wandering particles of matter
+until their bulk has become so great that the solar attraction prevails
+again over the light pressure acting upon them, and they turn again
+sunward. Passing the earth on their return they will increase the
+amount of dust-clouds careering round it; and these will be further
+increased by the action of the ultra-violet rays of the sunlight
+causing particles to shoot radially away from the earth when the
+negative charge of the upper atmosphere has reached a certain amount,
+which particles, although starting sunward, will be swept back to the
+earth with the oncoming streams. As the final result of all this
+accumulation of flying and gyrating particles in the earth’s
+neighborhood, we are told that the latter must be transformed into the
+semblance of a gigantic solid-headed comet provided with streaming
+tails, the longest of them stretching away from the direction of the
+sun, while another shorter one extends toward the sun. This shorter
+tail is due to the particles that we have just spoken of as being
+driven sunward from the earth by the action of ultra-violet light. No
+doubt this whole subject is too technical for popular statement; but at
+any rate the general reader can understand the picturesque side of the
+theory, for its advocates assure us that if we were on the moon we
+would doubtless be able to see the comet-like tails of the earth, and
+then we could appreciate the part that they play in producing the
+phenomenon of the Zodiacal Light.
+
+That the Light as we see it could be produced by the reflection of
+sunlight from swarms of particles careering round the earth in the
+manner supposed by Arrhenius’ hypothesis is evident enough; and it will
+be observed that the new theory, after all, is only another variant of
+the older one which attributes the Zodiacal Light to an extension of
+the solar corona. But it differs from the older theory in offering an
+explanation of the manner in which the extension is effected, and it
+differentiates between the corona proper and the streams of negative
+particles shot away from the sun. In its details the hypothesis of
+Arrhenius also affords an explanation of many peculiarities of the
+Zodiacal Light, such as that it is confined to the neighborhood of the
+ecliptic, and that it is stronger on the side of the earth which is
+just turning away from a position under the sun than on the other side;
+but it would carry us beyond our limits to go into these particulars.
+The Gegenschein, according to this theory, is a part of the same
+phenomenon as the Zodiacal Light, for by the laws of perspective it is
+evident that the reflection from the streams of particles situated at a
+point directly opposite to the sun would be at a maximum, and this is
+the place which the Gegenschein occupies. Apart from its geometrical
+relations to the position of the sun, the variability of the Zodiacal
+Light appears to affirm its solar dependence, and this too would be
+accounted for by Arrhenius’ hypothesis better than by the old theory of
+coronal extension. The amount of corpuscular discharge from the sun
+must naturally be governed by the state of relative activity or
+inactivity of the latter, and this could not but be reflected in the
+varying splendor of the Zodiacal Light. But much more extended study
+than has yet been given to the subject will be required before we can
+feel that we know with reasonable certainty what this mysterious
+phenomenon really is. By the hypothesis of Arrhenius every planet that
+has an atmosphere must have a Zodiacal Light attending it, but the
+phenomenon is too faint for us to be able to see it in the case, for
+instance, of Venus, whose atmosphere is very abundant. The moon has no
+corresponding “comet’s tail” because, as already explained, of the lack
+of a lunar atmosphere to repel the streams by becoming itself
+electrified; but if there were a lunar Zodiacal Light, no doubt we
+could see it because of the relative nearness of our satellite.
+
+
+
+
+IX
+Marvels of the Aurora
+
+
+One of the most vivid recollections of my early boyhood is that of
+seeing my father return hastily into the house one evening and call out
+to the family: “Come outside and look at the sky!” Ours was a country
+house situated on a commanding site, and as we all emerged from the
+doorway we were dumbfounded to see the heavens filled with pale flames
+which ran licking and quivering over the stars. Instantly there sprang
+into my terrified mind the recollection of an awful description of “the
+Day of Judgment” (the _Dies Iræ_), which I had heard with much
+perturbation of spirit in the Dutch Reformed church from the lips of a
+tall, dark-browed, dreadfully-in-earnest preacher of the old-fashioned
+type. My heart literally sank at sight of the spectacle, for it
+recalled the preacher’s very words; it was just as he had said it would
+be, and it needed the assured bearing of my elders finally to convince
+me that
+
+That Day of Wrath, O dreadful day,
+When Heaven and Earth shall pass away,
+As David and the Sibyl say
+
+
+had not actually come upon us. And even the older members of the
+household were not untouched with misgivings when menacing spots of
+crimson appeared, breaking out now here, now there, in the shuddering
+sky. Toward the north the spectacle was appalling. A huge arch spanned
+an unnaturally dark segment resting on the horizon, and above this arch
+sprang up beams and streamers in a state of incessant agitation,
+sometimes shooting up to the zenith with a velocity that took one’s
+breath, and sometimes suddenly falling into long ranks, and _marching,
+marching, marching,_ like an endless phalanx of fiery specters, and
+moving, as I remember, always from east to west. The absolute silence
+with which these mysterious evolutions were performed and the quavering
+reflections which were thrown upon the ground increased the awfulness
+of the exhibition. Occasionally enormous curtains of lambent flame
+rolled and unrolled with a majestic motion, or were shaken to and fro
+as if by a mighty, noiseless wind. At times, too, a sudden billowing
+rush would be made toward the zenith, and for a minute the sky overhead
+would glow so brightly that the stars seemed to have been consumed. The
+spectacle continued with varying intensity for hours.
+
+
+[Illustration: Auroral beams seen in England]
+
+
+This exhibition occurred in Central New York, a latitude in which the
+Aurora Borealis is seldom seen with so much splendor. I remember
+another similar one seen from the city of New York in November, 1882.
+On this last occasion some observers saw a great upright beam of light
+which majestically moved across the heavens, stalking like an
+apparition in the midst of the auroral pageant, of whose general
+movements it seemed to be independent, maintaining always its upright
+posture, and following a magnetic parallel from east to west. This
+mysterious beam was seen by no less than twenty-six observers in
+different parts of the country, and a comparison of their observations
+led to a curious calculation indicating that the apparition was about
+_one hundred and thirty-three miles tall_ and moved at the speed of ten
+miles per second!
+
+
+[Illustration: Auroral arches of an eliptic form seen in the Arctic
+regions]
+
+
+But, as everybody knows, it is in the Arctic regions that the Aurora,
+or the “Northern Lights,” can best be seen. There, in the long polar
+night, when for months together the sun does not rise, the strange
+coruscations in the sky often afford a kind of spectral daylight in
+unison with the weird scenery of the world of ice. The pages in the
+narratives of Arctic exploration that are devoted to descriptions of
+the wonderful effects of the Northern Lights are second to none that
+man has ever penned in their fascination. The lights, as I have already
+intimated, display astonishing colors, particularly shades of red and
+green, as they flit from place to place in the sky. The discovery that
+the magnetic needle is affected by the Aurora, quivering and darting
+about in a state of extraordinary excitement when the lights are
+playing in the sky, only added to the mystery of the phenomenon until
+its electro-magnetic nature had been established. This became evident
+as soon as it was known that the focus of the displays was the magnetic
+pole; and when the far South was visited the Aurora Australis was
+found, having its center at the South Magnetic Pole. Then, if not
+before, it was clear that the earth was a great globular magnet, having
+its poles of opposite magnetism, and that the auroral lights, whatever
+their precise cause might be, were manifestations of the magnetic
+activity of our planet. After the invention of magnetic telegraphy it
+was found that whenever a great Aurora occurred the telegraph lines
+were interrupted in their operation, and the ocean cables ceased to
+work. Such a phenomenon is called a “magnetic storm.”
+
+The interest excited by the Aurora in scientific circles was greatly
+stimulated when, in the last half of the nineteenth century, it was
+discovered that it is a phenomenon intimately associated with
+disturbances on the sun. The ancient “Zurich Chronicles,” extending
+from the year 1000 to the year 1800, in which both sun-spots visible to
+the naked eye and great displays of the auroral lights were recorded,
+first set Rudolf Wolf on the track of this discovery. The first notable
+proof of the suspected connection was furnished with dramatic emphasis
+by an occurrence which happened on September 1, 1859. Near noon on that
+day two intensely brilliant points suddenly broke out in a group of
+sun-spots which were under observation by Mr R. C. Carrington at his
+observatory at Redhill, England. The points remained visible for not
+more than five minutes, during which interval they moved _thirty-five
+thousand miles_ across the solar disk. Mr R. Hodgson happened to see
+the same phenomenon at his observatory at Highgate, and thus all
+possibility of deception was removed. But neither of the startled
+observers could have anticipated what was to follow, and, indeed, it
+was an occurrence which has never been precisely duplicated. I quote
+the eloquent account given by Miss Clerke in her _History of Astronomy
+During the Nineteenth Century._
+
+This unique phenomenon seemed as if specially designed to accentuate
+the inference of a sympathetic relation between the earth and the sun.
+From August 28 to September 4, 1859, a magnetic storm of unparalleled
+intensity, extent, and duration was in progress over the entire globe.
+Telegraphic communication was everywhere interrupted—except, indeed,
+that it was in some cases found practicable to work the lines _without
+batteries_ by the agency of the earth-currents alone; sparks issued
+from the wires; gorgeous auroras draped the skies in solemn crimson
+over both hemispheres, and even in the tropics; the magnetic needle
+lost all trace of continuity in its movements and darted to and fro as
+if stricken with inexplicable panic. The coincidence was even closer.
+_At the very instant_ of the solar outburst witnessed by Carrington and
+Hodgson the photographic apparatus at Kew registered a marked
+disturbance of all the three magnetic elements; while shortly after the
+ensuing midnight the electric agitation culminated, thrilling the whole
+earth with subtle vibrations, and lighting up the atmosphere from pole
+to pole with coruscating splendors which perhaps dimly recall the times
+when our ancient planet itself shone as a star.
+
+
+If this amazing occurrence stood alone, and as I have already said it
+has never been exactly duplicated, doubt might be felt concerning some
+of the inferences drawn from it; but in varying forms it has been
+repeated many times, so that now hardly anyone questions the reality of
+the assumed connection between solar outbursts and magnetic storms
+accompanied by auroral displays on the earth. It is true that the late
+Lord Kelvin raised difficulties in the way of the hypothesis of a
+direct magnetic action of the sun upon the earth, because it seemed to
+him that an inadmissible quantity of energy was demanded to account for
+such action. But no calculation like that which he made is final, since
+all calculations depend upon the validity of the data; and no authority
+is unshakable in science, because no man can possess omniscience. It
+was Lord Kelvin who, but a few years before the thing was actually
+accomplished, declared that aerial navigation was an impracticable
+dream, and demonstrated its impracticability by calculation. However
+the connection may be brought about, it is as certain as evidence can
+make it that solar outbursts are coincident with terrestial magnetic
+disturbances, and coincident in such a way as to make the inference of
+a causal connection irresistible. The sun is only a little more than a
+hundred times its own diameter away from the earth. Why, then, with the
+subtle connection between them afforded by the ether which conveys to
+us the blinding solar light and the life-sustaining solar heat, should
+it be so difficult to believe that the sun’s enormous electric energies
+find a way to us also? No doubt the impulse coming from the sun acts
+upon the earth after the manner of a touch upon a trigger, releasing
+energies which are already stored up in our planet.
+
+But besides the evidence afforded by such occurrences as have been
+related of an intimate connection between solar outbreaks and
+terrestial magnetic flurries, attended by magnificent auroral displays,
+there is another line of proof pointing in the same direction. Thus, it
+is known that the sun-spot period, as remarked in a preceding chapter,
+coincides in a most remarkable manner with the periodic fluctuations in
+the magnetic state of the earth. This coincidence runs into the most
+astonishing details. For instance, when the sun-spot period shortens,
+the auroral period shortens to precisely the same extent; as the short
+sun-spot periods usually bring the most intense outbreaks of solar
+activity, so the corresponding short auroral periods are attended by
+the most violent magnetic storms; a secular period of about two hundred
+and twenty-two years affecting sun-spots is said to have its auroral
+duplicate; a shorter period of fifty-five and a half years, which some
+observers believe that they have discovered appears also to be common
+to the two phenomena; and yet another “superposed” period of about
+thirty-five years, which some investigators aver exists, affects
+sun-spots and aurora alike. In short, the coincidences are so numerous
+and significant that one would have to throw the doctrine of
+probability to the winds in order to be able to reject the conclusion
+to which they so plainly lead.
+
+
+[Illustration: Auroral curtain seen in Scandinavia]
+
+
+But still the question recurs: How is the influence transmitted? Here
+Arrhenius comes once more with his hypothesis of negative corpuscles,
+or ions, driven away from the sun by light-pressure—a hypothesis which
+seems to explain so many things—and offers it also as an explanation of
+the way in which the sun creates the Aurora. He would give the Aurora
+the same lineage with the Zodiacal Light. To understand the application
+of this theory we must first recall the fact that the earth is a great
+magnet having its two opposite poles of magnetism, one near the Arctic
+and the other near the Antarctic Circle. Like all magnets, the earth is
+surrounded with “lines of force,” which, after the manner of the curved
+rays we saw in the photograph of a solar eclipse, start from a pole,
+rising at first nearly vertically, then bend gradually over, passing
+high above the equator, and finally descending in converging sheaves to
+the opposite pole. Now the axis of the earth is so placed in space that
+it lies at nearly a right angle to the direction of the sun, and as the
+streams of negatively charged particles come pouring on from the sun
+(see the last preceding chapter), they arrive in the greatest numbers
+over the earth’s equatorial regions. There they encounter the lines of
+magnetic force at the place where the latter have their greatest
+elevation above the earth, and where their direction is horizontal to
+the earth’s surface. Obeying a law which has been demonstrated in the
+laboratory, the particles then follow the lines of force toward the
+poles. While they are above the equatorial regions they do not become
+luminescent, because at the great elevation that they there occupy
+there is virtually no atmosphere; but as they pass on toward the north
+and the south they begin to descend with the lines of force, curving
+down to meet at the poles; and, encountering a part of the atmosphere
+comparable in density with what remains in an exhausted Crookes tube,
+they produce a glow of cathode rays. This glow is conceived to
+represent the Aurora, which may consequently be likened to a gigantic
+exhibition of vacuum-tube lights. Anybody who recalls his student days
+in the college laboratory and who has witnessed a display of Northern
+Lights will at once recognize the resemblance between them in colors,
+forms, and behavior. This resemblance had often been noted before
+Arrhenius elaborated his hypothesis.
+
+Without intending to treat his interesting theory as more than a
+possibly correct explanation of the phenomena of the Aurora, we may
+call attention to some apparently confirmatory facts. One of the most
+striking of these relates to a seasonal variation in the average number
+of auroræ. It has been observed that there are more in March and
+September than at any other time of the year, and fewer in June and
+December; moreover (and this is a delicate test as applied to the
+theory), they are slightly rarer in June than in December. Now all
+these facts seem to find a ready explanation in the hypothesis of
+Arrhenius, thus: (1) The particles issuing from the sun are supposed to
+come principally from the regions whose excitement is indicated by the
+presence of sun-spots (which accords with Hale’s observation that
+sun-spots are columns of ionized vapors), and these regions have a
+definite location on either side of the solar equator, seldom
+approaching it nearer than within 5° or 10° north or south, and never
+extending much beyond 35° toward either pole; (2) The equator of the
+sun is inclined about 7° to the plane of the earth’s orbit, from which
+it results that twice in a year—_viz.,_ in June and December—the earth
+is directly over the solar equator, and twice a year—_viz.,_ in March
+and September—when it is farthest north or south of the solar equator,
+it is over the inner edge of the sun-spot belts. Since the corpuscles
+must be supposed to be propelled radially from the sun, few will reach
+the earth when the latter is over the solar equator in June and
+December, but when it is over, or nearly over, the spot belts, in March
+and September, it will be in the line of fire of the more active parts
+of the solar surface, and relatively rich streams of particles will
+reach it. This, as will be seen from what has been said above, is in
+strict accord with the observed variations in the frequency of auroræ.
+Even the fact that somewhat fewer auroræ are seen in June than in
+December also finds its explanation in the known fact that the earth is
+about three million miles nearer the sun in the winter than in the
+summer, and the number of particles reaching it will vary, like the
+intensity of light, inversely as the square of the distance. These
+coincidences are certainly very striking, and they have a cumulative
+force. If we accept the theory, it would appear that we ought to
+congratulate ourselves that the inclination of the sun’s equator is so
+slight, for as things stand the earth is never directly over the most
+active regions of the sun-spots, and consequently never suffers from
+the maximum bombardment of charged particles of which the sun is
+capable. Incessant auroral displays, with their undulating draperies,
+flitting colors, and marching columns might not be objectionable from
+the point of view of picturesqueness, but one magnetic storm of extreme
+intensity following closely upon the heels of another, for months on
+end, crazing the magnetic needle and continually putting the telegraph
+and cable lines out of commission, to say nothing of their effect upon
+“wireless telegraphy”, would hardly add to the charms of terrestrial
+existence.
+
+
+[Illustration: Auroral arches seen in Scandinavia]
+
+
+One or two other curious points in connection with Arrhenius’
+hypothesis may be mentioned. First, the number of auroræ, according to
+his explanation, ought to be greatest in the daytime, when the face of
+the earth on the sunward side is directly exposed to the atomic
+bombardment. Of course visual observation can give us no information
+about this, since the light of the Aurora is never sufficiently intense
+to be visible in the presence of daylight, but the records of the
+magnetic observatories can be, and have been, appealed to for
+information, and they indicate that the facts actually accord with the
+theory. Behind the veil of sunlight in the middle of the afternoon,
+there is good reason to believe, auroral exhibitions often take place
+which would eclipse in magnificence those seen at night if we could
+behold them. Observation shows, too, that auroræ are more frequent
+before than after midnight, which is just what we should expect if they
+originate in the way that Arrhenius supposes. Second, the theory offers
+an explanation of the alleged fact that the formation of clouds in the
+upper air is more frequent in years when auroræ are most abundant,
+because clouds are the result of the condensation of moisture upon
+floating particles in the atmosphere (in an absolutely dustless
+atmosphere there would be no clouds), and it has been proved that
+negative ions like those supposed to come from the sun play a master
+part in the phenomena of cloud formation.
+
+Yet another singular fact, almost mystical in its suggestions, may be
+mentioned. It seems that the dance of the auroral lights occurs most
+frequently during the absence of the moon from the hemisphere in which
+they appear, and that they flee, in greater part, to the opposite
+hemisphere when the moon’s revolution in an orbit considerably inclined
+to the earth’s equator brings her into that where they have been
+performing. Arrhenius himself discovered this curious relation of
+auroral frequency to the position of the moon north or south of the
+equator, and he explains it in this way. The moon, like the earth, is
+exposed to the influx of the ions from the sun; but having no
+atmosphere, or almost none, to interfere with them, they descend
+directly upon her surface and charge her with an electric negative
+potential to a very high degree. In consequence of this she affects the
+electric state of the upper parts of the earth’s atmosphere where they
+lie most directly beneath her, and thus prevents, to a large extent,
+the negative discharges to which the appearance of the Aurora is due.
+And so “the extravagant and erring spirit” of the Aurora avoids the
+moon as Hamlet’s ghost fled at the voice of the cock announcing the
+awakening of the god of day.
+
+There are even other apparent confirmations of the hypothesis, but we
+need not go into them. We shall, however, find one more application of
+it in the next chapter, for it appears to be a kind of cure-all for
+astronomical troubles; at any rate it offers a conceivable solution of
+the question, How does the sun manage to transmit its electric
+influence to the earth? And this solution is so grandiose in
+conception, and so novel in the mental pictures that it offers, that
+its acceptance would not in the least detract from the impression that
+the Aurora makes upon the imagination.
+
+
+
+
+X
+Strange Adventures of Comets
+
+
+The fears and legends of ancient times before Science was born, and the
+superstitions of the Dark Ages, sedulously cultivated for theological
+purposes by monks and priests, have so colored our ideas of the
+influence that comets have had upon the human mind that many readers
+may be surprised to learn that it was the apparition of a wonderful
+comet, that of 1843, which led to the foundation of our greatest
+astronomical institution, the Harvard College Observatory. No doubt the
+comet superstition existed half a century ago, as, indeed, it exists
+yet today, but in this case the marvelous spectacle in the sky proved
+less effective in inspiring terror than in awakening a desire for
+knowledge. Even in the sixteenth century the views that enlightened
+minds took of comets tended powerfully to inspire popular confidence in
+science, and Halley’s prediction, after seeing and studying the motion
+of the comet which appeared in 1682, that it would prove to be a
+regular member of the sun’s family and would be seen returning after a
+period of about seventy-six years, together with the fulfillment of
+that prediction, produced a revulsion from the superstitious notions
+which had so long prevailed.
+
+
+[Illustration: Swift’s comet. Taken at Arequipa, March 30 1892]
+
+
+Then the facts were made plain that comets are subject to the law of
+gravitation equally with the planets; that there are many which
+regularly return to the neighborhood of the sun (perihelion); and that
+these travel in orbits differing from those of the planets only in
+their greater eccentricity, although they have the peculiarity that
+they do not, like the planets, all go round the sun in the same
+direction, and do not keep within the general plane of the planetary
+system, but traverse it sometimes from above and sometimes from below.
+Other comets, including most of the “great” ones, appear to travel in
+parabolic or, in a few cases, hyperbolic orbits, which, not being
+closed curves, never bring them back again. But it is not certain that
+these orbits may not be extremely eccentric ellipses, and that after
+the lapse of hundreds, or thousands, of years the comets that follow
+them may not reappear. The question is an interesting one, because if
+all orbits are really ellipses, then all comets must be permanent
+members of the solar system, while in the contrary case many of them
+are simply visitors, seen once and never to be seen again. The
+hypothesis that comets are originally interlopers might seem to derive
+some support from the fact that the certainly periodic ones are
+associated, in groups, with the great outer planets, whose attraction
+appears to have served as a trap for them by turning them into
+elliptical orbits and thus making them prisoners in the solar system.
+Jupiter, owing to his great mass and his commanding situation in the
+system, is the chief “comet-catcher;” but he catches them not for
+himself, but for the sun. Yet if comets do come originally from without
+the borders of the planetary system, it does not, by any means, follow
+that they were wanderers at large in space before they yielded to the
+overmastering attraction of the sun. Investigation of the known
+cometary orbits, combined with theoretical considerations, has led some
+astronomers to the conclusion that as the sun travels onward through
+space he “picks up _en route_” cometary masses which, without belonging
+strictly to his empire, are borne along in the same vast “cosmical
+current” that carries the solar system.
+
+But while no intelligent person any longer thinks that the appearance
+of a great comet is a token from the heavenly powers of the approaching
+death of a mighty ruler, or the outbreak of a devastating war, or the
+infliction of a terrible plague upon wicked mankind, science itself has
+discovered mysteries about comets which are not less fascinating
+because they are more intellectual than the irrational fancies that
+they have displaced. To bring the subject properly before the mind, let
+us see what the principal phenomena connected with a comet are.
+
+At the present day comets are ordinarily “picked up” with the telescope
+or the photographic plate before any one except their discoverer is
+aware of their existence, and usually they remain so insignificant in
+appearance that only astronomers ever see them. Yet so great is the
+prestige of the word “comet” that the discovery of one of these
+inconspicuous wanderers, and its subsequent movements, become items of
+the day’s news which everybody reads with the feeling, perhaps, that at
+least he knows what is going on in the universe even if he doesn’t
+understand it. But a truly great comet presents quite a different
+proposition. It, too, is apt to be detected coming out of the depths of
+space before the world at large can get a glimpse of it, but as it
+approaches the sun its aspect undergoes a marvelous change. Agitated
+apparently by solar influence, it throws out a long streaming tail of
+nebulous light, directed away from the sun and looking as if blown out
+like a pennon by a powerful wind. Whatever may be the position of the
+comet with regard to the sun, as it circles round him it continually
+keeps its tail on the off side. This, as we shall soon see, is a fact
+of capital importance in relation to the probable nature of comets’
+tails. Almost at the same time that the formation of the tail is
+observed a remarkable change takes place in the comet’s head, which, by
+the way, is invariably and not merely occasionally its most important
+part. On approaching the sun the head usually contracts. Coincidently
+with this contraction a nucleus generally makes its appearance. This is
+a bright, star-like point in the head, and it probably represents the
+totality of solid matter that the comet possesses. But it is regarded
+as extremely unlikely that even the nucleus consists of a uniformly
+solid mass. If it were such, comets would be far more formidable
+visitors when they pass near the planets than they have been found to
+be. The diameter of the nucleus may vary from a few hundred up to
+several thousand miles; the heads, on the average, are from twenty-five
+thousand to one hundred thousand miles in diameter, although a few have
+greatly exceeded these dimensions; that of the comet of 1811, one of
+the most stupendous ever seen, was a million and a quarter miles in
+diameter! As to the tails, not withstanding their enormous length—some
+have been more than a hundred million miles long—there is reason to
+believe that they are of extreme tenuity, “as rare as vacuum.” The
+smallest stars have been seen shining through their most brilliant
+portions with undiminished luster.
+
+After the nucleus has been formed it begins to throw out bright jets
+directed toward the sun. A stream, and sometimes several streams, of
+light also project sunward from the nucleus, occasionally appearing
+like a stunted tail directed oppositely to the real tail. Symmetrical
+envelopes which, seen in section, appear as half circles or parabolas,
+rise sunward from the nucleus, forming a concentric series. The ends of
+these stream backward into the tail, to which they seem to supply
+material. Ordinarily the formation of these ejections and envelopes is
+attended by intense agitation of the nucleus, which twists and turns,
+swinging and gyrating with an appearance of the greatest violence.
+Sometimes the nucleus is seen to break up into several parts. The
+entire heads of some comets have been split asunder in passing close
+around the sun; The comet of 1882 retreated into space after its
+perihelion passage with _five heads_ instead of the one that it had
+originally, and each of these heads had its own tail!
+
+The possession of the spectroscope has enabled astronomers during later
+years to study the chemical composition of comets by analyzing their
+light. At first the only substances thus discovered in them were
+hydro-carbon compounds, due evidently to the gaseous envelopes in which
+some combination of hydrogen with carbon existed. Behind this gaseous
+spectrum was found a faint continuous spectrum ascribed to the nucleus,
+which apparently both reflects the sunlight and gives forth the light
+of a glowing solid or liquid. Subsequently sodium and iron lines were
+found in cometary spectra. The presence of iron would seem to indicate
+that some of these bodies may be much more massive than observations on
+their attractive effects have indicated. In some recent comets, such as
+Morehouse’s, in 1908, several lines have been found, the origin of
+which is unknown.
+
+Without going back of the nineteenth century we may find records of
+some of the most extraordinary comets that man has ever looked upon. In
+1811, still spoken of as “the year of the comet,” because of the
+wonderful vintage ascribed to the skyey visitor, a comet shaped like a
+gigantic sword amazed the whole world, and, as it remained visible for
+seventeen months, was regarded by superstitious persons as a symbol of
+the fearful happenings of Napoleon’s Russian campaign. This comet, the
+extraordinary size of whose head, greatly exceeding that of the sun
+itself, has already been mentioned, was also remarkable for exhibiting
+so great a brilliancy without approaching even to the earth’s distance
+from the sun. But there was once a comet (and only once—in the year
+1729) which never got nearer to the sun than four times the distance of
+the earth and yet appeared as a formidable object in the sky. As
+Professor Young has remarked, “it must have been an enormous comet to
+be visible from such a distance.” And we are to remember that there
+were no great telescopes in the year 1729. That comet affects the
+imagination like a phantom of space peering into the solar system,
+displaying its enormous train afar off (which, if it had approached as
+near as other comets, would probably have become _the_ celestial wonder
+of all human memory), and then turning away and vanishing in the depths
+of immensity.
+
+In 1843 a comet appeared which was so brilliant that it could be seen
+in broad day close beside the sun! This was the first authenticated
+instance of that kind, but the occurrence was to be repeated, as we
+shall see in a moment, less than forty years later.
+
+The splendid comet of 1858, usually called Donati’s, is remembered by
+many persons yet living. It was, perhaps, both as seen by the naked eye
+and with the telescope, the most beautiful comet of which we have any
+record. It too marked a rich vintage year, still remembered in the
+vineyards of France, where there is a popular belief that a great comet
+ripens the grape and imparts to the wine a flavor not attainable by the
+mere skill of the cultivator. There are “comet wines,” carefully
+treasured in certain cellars, and brought forth only when their owner
+wishes to treat his guests to a sip from paradise.
+
+The year 1861 saw another very remarkable comet, of an aspect strangely
+vast and diffuse, which is believed to have swept the earth with its
+immense tail when it passed between us and the sun on the night of June
+30th, an event which produced no other known effect than the appearance
+of an unwonted amount of scattered light in the sky.
+
+The next very notable comet was the “Great Southern Comet” of 1880,
+which was not seen from the northern hemisphere. It mimicked the aspect
+of the famous comet of 1843, and to the great surprise of astronomers
+appeared to be traveling in the same path. This proved to be the rising
+of the curtain for an astronomical sensation unparalleled in its kind;
+for two years later another brilliant comet appeared, first in the
+southern hemisphere, _and it too followed the same track._ The
+startling suggestion was now made that this comet was identical with
+those of 1843 and 1880, its return having been hastened by the
+resistance experienced in passing twice through the coronal envelope,
+and there were some who thought that it would now swing swiftly round
+and then plunge straight into the sun, with consequences that might be
+disastrous to us on account of the “flash of heat” that would be
+produced by the impact. Nervous people were frightened, but observation
+soon proved that the danger was imaginary, for although the comet
+almost grazed the sun, and must have rushed through two or three
+million miles of the coronal region, no retardation of its immense
+velocity was perceptible, and it finally passed away in a damaged
+condition, as before remarked, and has never since appeared.
+
+Then the probable truth was perceived—_viz.,_ that the three comets
+(1843, 1880, and 1882) were not one identical body, but three separate
+ones all traveling in the same orbit. It was found, too, that a comet
+seen in 1668 bore similar insignia of relationship. The natural
+inference was that these four bodies had once formed a single mass
+which had been split apart by the disruptive action of the sun.
+Strength was lent to this hypothesis by the fact that the comet of 1882
+was apparently torn asunder during its perihelion passage, retreating
+into space in a dissevered state. But Prof. George Forbes has a theory
+that the splitting of the original cometary mass was effected by an
+unknown planet, probably greater than Jupiter, situated at a hundred
+times the earth’s distance from the sun, and revolving in a period of a
+thousand years. He supposes that the original comet was not that of
+1668, but one seen in 1556, which has since been “missing,” and that
+its disruption occurred from an encounter with the supposititious
+planet about the year 1700. Truly from every point of view comets are
+the most extraordinary of adventurers!
+
+The comet of 1882 was likewise remarkable for being visible, like its
+predecessor of 1843, in full daylight in close proximity to the sun.
+The story of its detection when almost in contact with the solar disk
+is dramatic. It had been discovered in the southern hemisphere only a
+couple of weeks before its perihelion, which occurred on September
+17th, and on the forenoon of that day it was seen by Doctor Common in
+England, and by Doctor Elkin and Mr Finlay at the Cape of Good Hope,
+almost touching the sun. It looked like a dazzling white bird with
+outspread wings. The southern observers watched it go _right into the
+sun,_ when it instantly disappeared. What had happened was that the
+comet in passing its perihelion point had swung exactly between the
+earth and the sun. On the following morning it was seen from all parts
+of the world close by the sun on the opposite side, and it remained
+thus visible for three days, gradually receding from the solar disk. It
+then became visible for northern observers in the morning sky before
+sunrise, brandishing a portentous sword-shaped tail which, if it had
+been in the evening sky, would have excited the wonder of hundreds of
+millions, but situated where it was, comparatively few ever saw it.
+
+
+[Illustration: Daniels’ comet. August 11, 1907]
+
+
+The application of photography to the study of comets has revealed many
+curious details which might otherwise have escaped detection, or at
+best have remained subject to doubt. It has in particular shown not
+only the precise form of the tails, but the remarkable vicissitudes
+that they undergo. Professor Barnard’s photographs of Brooks’ comet in
+1893 suggested, by the extraordinary changes in the form of the tail
+which they revealed, that the comet was encountering a series of
+obstructions in space which bent and twisted its tail into fantastic
+shapes. The reader will observe the strange form into which the tail
+was thrown on the night of October 21st. A cloud of meteors through
+which the comet was passing might have produced such deformations of
+its tail. In the photograph of Daniels’ comet of 1907, a curious
+striping of the tail will be noticed. The short bright streaks seen in
+the photograph, it may be explained, are the images of stars which are
+drawn out into lines in consequence of the fact that the photographic
+telescope was adjusted to follow the motion of the comet while the
+stars remained at rest.
+
+But the adventures of comets are not confined to possible encounters
+with unknown obstacles. We have referred to the fact that the great
+planets, and especially Jupiter, frequently interfere with the motions
+of comets. This interference is not limited to the original alteration
+of their orbits from possible parabolas to ellipses, but is sometimes
+exercised again and again, turning the bewildered comets into
+elliptical paths of all degrees of eccentricity. A famous example of
+this kind of planetary horse-play is furnished by the story of Lexell’s
+missing comet. This comet was first seen in 1770. Investigation showed
+that it was moving in an orbit which should bring it back to perihelion
+every five and a half years; yet it had never been seen before and,
+although often searched for, has never been seen since. Laplace and
+Leverrier proved mathematically that in 1767 it had approached so close
+to Jupiter as to be involved among the orbits of his satellites. What
+its track had been before is not known, but on that occasion the giant
+planet seized the interloper, threw it into a short elliptic orbit and
+sent it, like an arrested vagrant, to receive sentence at the bar of
+the sun. On this journey it passed within less than 1,500,000 miles of
+the earth. The form of orbit which Jupiter had impressed required, as
+we have said, its return in about five and a half years; but soon after
+1770 it had the misfortune a second time to encounter Jupiter at close
+range, and he, as if dissatisfied with the leniency of the sun, or
+indignant at the stranger’s familiarity, seized the comet and hurled it
+out of the system, or at any rate so far away that it has never since
+been able to rejoin the family circle that basks in the immediate rays
+of the solar hearth. Nor is this the only instance in which Jupiter has
+dealt summarily with small comets that have approached him with too
+little deference.
+
+
+[Illustration: Brooks’ comet. Photographed by Barnard, October 21,
+1893]
+
+
+The function which Jupiter so conspicuously fulfills as master of the
+hounds to the sun is worth considering a little more in detail. To
+change the figure, imagine the sun in its voyage through space to be
+like a majestic battleship surrounded by its scouts. Small vessels (the
+comets, as they are overhauled by the squadron, are taken in charge by
+the scouts, with Jupiter for their chief, and are forced to accompany
+the fleet, but not all are impressed. If a strange comet undertakes to
+run across Jupiter’s bows the latter brings it to, and makes prize of
+it by throwing it into a relatively small ellipse with the sun for its
+focus. Thenceforth, unless, as happened to the unhappy comet of Lexell,
+it encounters Jupiter again in such a way as to be diverted by him into
+a more distant orbit, it can never get away. About thirty comets are
+now known to have thus been captured by the great planet, and they are
+called “Jupiter’s Comet Family.” But, on the other hand, if a wandering
+comet crosses the wake of the chief planetary scout the latter simply
+drives it away by accelerating its motion and compels it to steer off
+into open space. The transformation of comets into meteors will be
+considered in the next chapter, but here, in passing, mention may be
+made of the strange fate of one member of Jupiter’s family, Biela’s
+comet, which, having become over bold in its advances to its captor,
+was, after a few revolutions in is impressed orbit, torn to pieces and
+turned into a flock of meteors.
+
+And now let us return to the mystery of comets’ tails. That we are
+fully justified in speaking of the tails of comets as mysterious is
+proved by the declaration of Sir John Herschel, who averred, in so many
+words, that “there is some profound secret and mystery of nature
+concerned in this phenomenon,” and this profound secret and mystery has
+not yet been altogether cleared up. Nevertheless, the all-explaining
+hypothesis of Arrhenius offers us once more a certain amount of aid.
+Comets’ tails, Arrhenius assures us, are but another result of the
+pressure of light. The reader will recall the applications of this
+theory to the Zodiacal Light and the Aurora. In the form in which we
+now have to deal with it, the supposition is made that as a comet
+approaches the sun eruptions of vapor, due to the solar heat, occur in
+its nucleus. These are naturally most active on the side which is
+directly exposed to the sun, whence the appearance of the immense
+glowing envelopes that surround the nucleus on the sunward side. Among
+the particles of hydro-carbon, and perhaps solid carbon in the state of
+fine dust, which are thus set free there will be many whose size is
+within the critical limit which enables the light-waves from the sun to
+drive them away. Clouds of such particles, then, will stream off behind
+the advancing comet, producing the appearance of a tail. This accounts
+for the fact that the tails of comets are always directed away from the
+sun, and it also explains the varying forms of the tails and the
+extraordinary changes that they undergo. The speed of the particles
+driven before the light-waves must depend upon their size and weight,
+the lightest of a given size traveling the most swiftly. By accretion
+certain particles might grow, thus losing velocity and producing the
+appearance of bunches in the tail, such as have been observed. The
+hypothesis also falls in with the researches of Bredichin, who has
+divided the tails of comets into three principal classes—_viz.:_ (1)
+Those which appear as long, straight rays; (2) Those which have the
+form of curved plumes or scimitars; (3) Those which are short, brushy,
+and curved sharply backward along the comet’s path. In the first type
+he calculates the repulsive force at from twelve to fifteen times the
+force of gravity; in the second at from two to four times; and in the
+third at about one and a half times. The straight tails he ascribes to
+hydrogen because the hydrogen atom is the lightest known; the
+sword-shaped tails to hydro-carbons; and the stumpy tails to vaporized
+iron. It will be seen that, if the force driving off the tails is that
+which Arrhenius assumes it to be, the forms of those appendages would
+accord with those that Bredichin’s theory calls for. At the same time
+we have an explanation of the multiple tails with which some comets
+have adorned themselves. The comet of 1744, for instance, had at one
+time no less than seven tails spread in a wide curved brush behind it.
+Donati’s comet of 1858 also had at least two tails, the principal one
+sword-shaped and the other long, narrow, and as straight as a rule.
+According to Bredichin, the straight tail must have been composed of
+hydrogen, and the other of some form of hydro-carbon whose atoms are
+heavier than those of hydrogen, and, consequently, when swept away by
+the storm of light-waves, followed a curvature depending upon the
+resultant of the forces operating upon them. The seven tails of the
+comet of 1744 presented a kind of diagram graphically exhibiting its
+complex composition, and, if we knew a little more about the
+constituents of a comet, we might be able to say from the amount of
+curvature of the different tails just what were the seven substances of
+which that comet consisted.
+
+If these theories seem to the reader fantastic, at any rate they are no
+more fantastic than the phenomena that they seek to explain.
+
+
+
+
+XI
+Meteors, Fire-Balls, and Meteorites
+
+
+One of the most terrorizing spectacles with which the heavens have ever
+caused the hearts of men to quake occurred on the night of November 13,
+1833. On that night North America, which faced the storm, was under a
+continual rain of fire from about ten o’clock in the evening until
+daybreak.
+
+_The fragments of a comet had struck the earth._
+
+But the meaning of what had happened was not discovered until long
+afterward. To the astronomers who, with astonishment not less than that
+of other people, watched the wonderful scene, it was an unparalleled
+“shower of meteors.” They did not then suspect that those meteors had
+once formed the head of a comet. Light dawned when, a year later, Prof.
+Denison Olmsted, of Yale College, demonstrated that the meteors had all
+moved in parallel orbits around the sun, and that these orbits
+intersected that of the earth at the point where our planet happened to
+be on the memorable night of November 13th. Professor Olmsted even went
+so far as to suggest that the cloud of meteors that had encountered the
+earth might form a diffuse comet; but full recognition of the fact that
+they were cometary débris came later, as the result of further
+investigation. The key to the secret was plainly displayed in the
+spectacle itself, and was noticed without being understood by thousands
+of the terror-stricken beholders. It was _an umbrella of fire_ that had
+opened overhead and covered the heavens; in other words, the meteors
+all radiated from a particular point in the constellation Leo, and,
+being countless as the snowflakes in a winter tempest, they ribbed the
+sky with fiery streaks. Professor Olmsted showed that the radiation of
+the meteors from a fixed point was an effect of perspective, and in
+itself a proof that they were moving in parallel paths when they
+encountered the earth. The fact was noted that there had been a
+similar, but incomparably less brilliant, display of meteors on the
+same day of November, 1832, and it was rightly concluded that these had
+belonged to the same stream, although the true relationship of the
+phenomena was not immediately apprehended. Olmsted ascribed to the
+meteors a revolution about the sun once in every six months, bringing
+them to the intersection of their orbit with that of the earth every
+November 13th; but later investigators found that the real period was
+about thirty-three and one-quarter years, so that the great displays
+were due three times in a century, and their return was confidently
+predicted for the year 1866. The appearance of the meteors in 1832, a
+year before the great display, was ascribed to the great length of the
+stream which they formed in space—so great that they required more than
+two years to cross the earth’s orbit. In 1832 the earth had encountered
+a relatively rare part of the stream, but in 1833, on returning to the
+crossing-place, it found there the richest part of the stream pouring
+across its orbit. This explanation also proved to be correct, and the
+predicted return in 1866 was duly witnessed, although the display was
+much less brilliant than in 1833. It was followed by another in 1867.
+
+
+[Illustration: Curious forms of meteorite trains
+Nos. 1 to 6 show the changes undergone by a train left by a meteorite
+which passed near the “Great Dipper”; 7 shows the changes and drift of
+a train seen in the constellation Virgo; 8 is the singular train of the
+meteorite of February 22, 1909, near the Pole Star. (From _La
+Nature_.)]
+
+
+In the mean time Olmsted’s idea of a cometary relationship of the
+meteors was demonstrated to be correct by the researches of
+Schiaparelli and others, who showed that not only the November meteors,
+but those of August, which are seen more or less abundantly every year,
+traveled in the tracks of well-known comets, and had undoubtedly an
+identical origin with those comets. In other words the comets and the
+meteor-swarms were both remnants of original masses which had probably
+been split up by the action of the sun, or of some planet to which they
+had made close approaches. The annual periodicity of the August meteors
+was ascribed to the fact that the separation had taken place so long
+ago that the meteors had become distributed all around the orbit, in
+consequence of which the earth encountered some of them every year when
+it arrived at the crossing-point. Then Leverrier showed that the
+original comet associated with the November meteors was probably
+brought into the system by the influence of the planet Uranus in the
+year 126 of the Christian era. Afterward Alexander Herschel identified
+the tracks of no less than seventy-six meteor-swarms (most of them
+inconspicuous) with those of comets. The still more recent researches
+of Mr W. F. Denning make it probable that there are no meteors which do
+not belong to a flock or system probably formed by the disintegration
+of a cometary mass; even the apparently sporadic ones which shoot
+across the sky, “lost souls in the night,” being members of flocks
+which have become so widely scattered that the earth sometimes takes
+weeks to pass through the region of space where their paths lie.
+
+The November meteors should have exhibited another pair of spectacles
+in 1899 and 1900, and their failure to do so caused at first much
+disappointment, until it was made plain that a good reason existed for
+their absence. It was found that after their last appearance, in 1867,
+they had been disturbed in their movements by the planets Jupiter and
+Saturn, whose attractions had so shifted the position of their orbit
+that it no longer intersected that of the earth, as it did before.
+Whether another planetary interference will sometime bring the
+principal mass of the November meteors back to the former point of
+intersection with the earth’s orbit is a question for the future to
+decide. It would seem that there may be several parallel streams of the
+November meteors, and that some of them, like those of August, are
+distributed entirely around the orbit, so that every mid-November we
+see a few of them.
+
+We come now to a very remarkable example of the disintegration of a
+comet and the formation of a meteor-stream. In 1826 Biela, of
+Josephstadt, Austria, discovered a comet to which his name was given.
+Calculation showed that it had an orbital period of about six and a
+half years, belonging to Jupiter’s “family.” On one of its returns, in
+1846, it astonished its watchers by suddenly splitting in two. The two
+comets thus formed out of one separated to a distance of about one
+hundred and sixty thousand miles, and then raced side by side,
+sometimes with a curious ligature connecting them, like Siamese twins,
+until they disappeared together in interplanetary space. In 1852 they
+came back, still nearly side by side, but now the distance between them
+had increased to a million and a quarter of miles. After that, at every
+recurrence of their period, astronomers looked for them in vain, until
+1872, when an amazing thing happened. On the night of November 28th,
+when the earth was crossing the plane of the orbit of the missing
+comet, a brilliant shower of meteors burst from the northern sky,
+traveling nearly in the track which the comet should have pursued. The
+astronomers were electrified. Klinkerfues, of Göttingen, telegraphed to
+Pogson, of Madras: _“Biela touched earth; search near Theta Centauri.”_
+Pogson searched in the place indicated and saw a cometary mass
+retreating into the southern heavens, where it was soon swallowed from
+sight!
+
+
+[Illustration: Section of the atmosphere up to 100 kilometers.
+Showing the mean elevation at which meteorites and meteors make their
+appearance. Below are shown the elevation of Mount Everest, the highest
+manned balloon ascent by M. Berson; the height of cirrus clouds; the
+highest free balloon ascent; and the elevation attained by the clouds
+of fire-dust ejected by the Krakatoa eruption in 1883. (From _La
+Nature_)]
+
+
+Since then the Biela meteors have been among the recognized periodic
+spectacles of the sky, and few if any doubt that they represent a
+portion of the missing comet whose disintegration began with the
+separation into two parts in 1846. The comet itself has never since
+been seen. The first display of these meteors, sometimes called the
+“Andromedes,” because they radiate from the constellation Andromeda,
+was remarkable for the great brilliancy of many of the fire-balls that
+shot among the shower of smaller sparks, some of which were described
+as equaling the full moon in size. None of them is known to have
+reached the earth, but during the display of the same meteors in 1885 a
+meteoric mass fell at Mazapil in Northern Mexico (it is now in the
+Museum at Vienna), which many have thought may actually be a piece of
+the original comet of Biela. This brings us to the second branch of our
+subject.
+
+More rare than meteors or falling stars, and more startling, except
+that they never appear in showers, are the huge balls of fire which
+occasionally dart through the sky, lighting up the landscapes beneath
+with their glare, leaving trains of sparks behind them, often producing
+peals of thunder when they explode, and in many cases falling upon the
+earth and burying themselves from a few inches to several feet in the
+soil, from which, more than once, they have been picked up while yet
+hot and fuming. These balls are sometimes called bolides. They are not
+really round in shape, although they often look so while traversing the
+sky, but their forms are fragmentary, and occasionally fantastic. It
+has been supposed that their origin is different from that of the true
+meteors; it has even been conjectured that they may have originated
+from the giant volcanoes of the moon or have been shot out from the sun
+during some of the tremendous explosions that accompany the formation
+of eruptive prominences. By the same reasoning some of them might be
+supposed to have come from some distant star. Others have conjectured
+that they are wanderers in space, of unknown origin, which the earth
+encounters as it journeys on, and Lord Kelvin made a suggestion which
+has become classic because of its imaginative reach—_viz.,_ that the
+first germs of life may have been brought to the earth by one of these
+bodies, “a fragment of an exploded world.”
+
+It is a singular fact that astronomers and scientific men in general
+were among the last to admit the possibility of solid masses falling
+from the sky. The people had believed in the reality of such phenomena
+from the earliest times, but the savants shook their heads and talked
+of superstition. This was the less surprising because no scientifically
+authenticated instance of such an occurrence was known, and the stones
+popularly believed to have fallen from the sky had become the objects
+of worship or superstitious reverence, a fact not calculated to
+recommend them to scientific credence. The celebrated “black stone”
+suspended in the Kaaba at Mecca is one of these reputed gifts from
+heaven; the “Palladium” of ancient Troy was another; and a stone which
+fell near Ensisheim, in Germany, was placed in a church as an object to
+be religiously venerated. Many legends of falling stones existed in
+antiquity, some of them curiously transfigured by the imagination, like
+the “Lion of the Peloponnesus,” which was said to have sprung down from
+the sky upon the Isthmus of Corinth. But near the beginning of the
+nineteenth century, in 1803, a veritable shower of falling stones
+occurred at L’Aigle, in Northern France, and this time astronomers took
+note of the phenomenon and scientifically investigated it. Thousands of
+the strange projectiles came from the sky on this occasion, and were
+scattered over a wide area of country, and some buildings were hit.
+Four years later another shower of stones occurred at Weston, Conn.,
+numbering thousands of individuals. The local alarm created in both
+cases was great, as well it might be, for what could be more
+intimidating than to find the blue vault of heaven suddenly hurling
+solid missiles at the homes of men? After these occurrences it was
+impossible for the most skeptical to doubt any longer, and the regular
+study of “aerolites,” or “meteorites,” began.
+
+One of the first things recognized was the fact that fire-balls are
+solid meteorites in flight, and not gaseous exhalations in the air, as
+some had assumed. They burn in the air during their flight, and
+sometimes, perhaps, are entirely consumed before reaching the ground.
+Their velocity before entering the earth’s atmosphere is equal to that
+of the planets in their orbits—_viz.,_ from twenty to thirty miles per
+second—a fact which proves that the sun is the seat of the central
+force governing them. Their burning in the air is not difficult to
+explain; it is the heat of friction which so quickly brings them to
+incandescence. Calculation shows that a body moving through the air at
+a velocity of about a mile per second will be brought, superficially,
+to the temperature of “red heat” by friction with the atmosphere. If
+its velocity is twenty miles per second the temperature will become
+thousands of degrees. This is the state of affairs with a meteorite
+rushing into the earth’s atmosphere; its surface is liquefied within a
+few seconds after the friction begins to act, and the melted and
+vaporized portion of its mass is swept backward, forming the train of
+sparks that follows every great fire-ball. However, there is one
+phenomenon connected with the trains of meteorites which has never been
+satisfactorily explained: they often persist for long periods of time,
+drifting and turning with the wind, but not ceasing to glow with a
+phosphorescent luminosity. The question is, Whence comes this light? It
+must be light without heat, since the fine dust or vapor of which the
+train can only consist would not retain sufficient heat to render it
+luminous for so long a time. An extremely remarkable incident of this
+kind occurred on February 22, 1909, when an immense fire-ball that
+passed over southern England left a train that remained visible during
+two hours, assuming many curious shapes as it was drifted about by
+currents in the air.
+
+
+[Illustration: A meteor photographed in flight]
+
+
+But notwithstanding the enormous velocity with which meteorites enter
+the air they are soon slowed down to comparatively moderate speed, so
+that when they disappear they are usually traveling not faster than a
+mile a second. The courses of many have been traced by observers
+situated along their track at various points, and thus a knowledge has
+been obtained of their height above the ground during their flight and
+of the length of their visible courses. They generally appear at an
+elevation of eighty or a hundred miles, and are seldom visible after
+having descended to within five miles of the ground, unless the
+observer happens to be near the striking-point, when he may actually
+witness the fall. Frequently they burst while high in the air and their
+fragments are scattered like shrapnel over the surface of the ground,
+sometimes covering an area of several square miles, but of course not
+thickly; different fragments of the same meteorite may reach the ground
+at points several miles apart. The observed length of their courses in
+the atmosphere varies from fifty to five hundred miles. If they
+continued a long time in flight after entering the air, even the
+largest of them would probably be consumed to the last scrap, but their
+fiery career is so short on account of their great speed that the heat
+does not have time to penetrate very deeply, and some that have been
+picked up immediately after their fall have been found cold as ice
+within. Their size after reaching the ground is variable within wide
+limits; some are known which weigh several tons, but the great majority
+weigh only a few pounds and many only a few ounces.
+
+Meteorites are of two kinds: _stony_ meteorites and _iron_ meteorites.
+The former outnumber the latter twenty to one; but many stone
+meteorites contain grains of iron. Nickel is commonly found in iron
+meteorites, so that it might be said that that redoubtable alloy
+nickel-steel is of cosmical invention. Some twenty-five chemical
+elements have been found in meteorites, including carbon and the
+“sun-metal,” helium. The presence of the latter is certainly highly
+suggestive in connection with the question of the origin of meteorites.
+The iron meteorites, besides metallic iron and nickel, of which they
+are almost entirely composed, contain hydrogen, helium, and carbonic
+oxide, and about the only imaginable way in which these gases could
+have become absorbed in the iron would be through the immersion of the
+latter while in a molten or vaporized state in a hot and dense
+atmosphere composed of them, a condition which we know to exist only in
+the envelopes of the sun and the stars.
+
+The existence of carbon in the Canyon Diablo iron meteorites is
+attended by a circumstance of the most singular character—a very “fairy
+tale of science.” In some cases _the carbon has become diamond!_ These
+meteoric diamonds are very small; nevertheless, they are true diamonds,
+resembling in many ways the little black gems produced by Moissan’s
+method with the aid of the electric furnace. The fact that they are
+found embedded in these iron meteorites is another argument in favor of
+the hypothesis of the solar or stellar origin of the latter. To
+appreciate this it is necessary to recall the way in which Moissan made
+his diamonds. It was by a combination of the effects of great heat,
+great pressure, and sudden or rapid superficial cooling on a mass of
+iron containing carbon. When he finally broke open his iron he found it
+a pudding stuffed with miniature black diamonds. When a fragment of the
+Canyon Diablo meteoric iron was polished in Philadelphia over fifteen
+years ago it cut the emery-wheel to pieces, and examination showed that
+the damage had been effected by microscopic diamonds peppered through
+the mass. How were those diamonds formed? If the sun or Sirius was the
+laboratory that prepared them, we can get a glimpse at the process of
+their formation. There is plenty of heat, plenty of pressure, and an
+abundance of vaporized iron in the sun and the stars. When a great
+solar eruption takes place, masses of iron which have absorbed carbon
+may be shot out with a velocity which forbids their return. Plunged
+into the frightful cold of space, their surfaces are quickly cooled, as
+Moissan cooled his prepared iron by throwing it into water, and thus
+the requisite stress is set up within, and, as the iron solidifies, the
+included carbon crystallizes into diamonds. Whether this explanation
+has a germ of truth in it or not, at any rate it is evident that iron
+meteorites were not created in the form in which they come to us; they
+must once have been parts of immeasurably more massive bodies than
+themselves.
+
+The fall of meteorites offers an appreciable, though numerically
+insignificant, peril to the inhabitants of the earth. Historical
+records show perhaps three or four instances of people being killed by
+these bodies. But for the protection afforded by the atmosphere, which
+acts as a very effective shield, the danger would doubtless be very
+much greater. In the absence of an atmosphere not only would more
+meteorites reach the ground, but their striking force would be
+incomparably greater, since, as we have seen, the larger part of their
+original velocity is destroyed by the resistance of the air. A
+meteorite weighing many tons and striking the earth with a velocity of
+twenty or thirty miles per second, would probably cause frightful
+havoc.
+
+
+[Illustration: Looking across Coon Butte crater from northern rim]
+
+
+It is a singular fact that recent investigations seem to have proved
+that an event of this kind actually happened in North America—perhaps
+not longer than a thousand or two thousand years ago. The scene of the
+supposed catastrophe is in northern central Arizona, at Coon Butte,
+where there is a nearly circular crater in the middle of a circular
+elevation or small mountain. The crater is somewhat over four thousand
+feet in diameter, and the surrounding rim, formed of upturned strata
+and ejected rock fragments, rises at its highest point one hundred and
+sixty feet above the plain. The crater is about six hundred feet in
+depth—that is, from the rim to the visible floor or bottom of the
+crater. There is no evidence that volcanic action has ever taken place
+in the immediate neighborhood of Coon Butte. The rock in which the
+crater has been made is composed of horizontal sandstone and limestone
+strata. Between three hundred and four hundred million tons of rock
+fragments have been detached, and a large portion hurled by some cause
+out of the crater. These fragments lie concentrically distributed
+around the crater, and in large measure form the elevation known as
+Coon Butte. The region has been famous for nearly twenty years on
+account of the masses of meteoric iron found scattered about and known
+as the “Canyon Diablo” meteorites. It was one of these masses, which
+consist of nickel-iron containing a small quantity of platinum, and of
+which in all some ten tons have been recovered for sale to the various
+collectors throughout the world, that as before mentioned destroyed the
+grinding-tool at Philadelphia through the cutting power of its embedded
+diamonds. These meteoric irons are scattered about the crater-hill, in
+concentric distribution, to a maximum distance of about five miles.
+When the suggestion was first made in 1896 that a monster meteorite
+might have created by its fall this singular lone crater _in stratified
+rocks,_ it was greeted with incredulous smiles; but since then the
+matter has assumed a different aspect. The Standard Iron Company,
+formed by Messrs. D. M. Barringer, B. C. Tilghman, E. J. Bennitt, and
+S. J. Holsinger, having become, in 1903, the owner of this freak of
+nature, sunk shafts and bored holes to a great depth in the interior of
+the crater, and also trenched the slopes of the mountain, and the
+result of their investigations has proved that the meteoric hypothesis
+of origin is correct. (See the papers published in the _Proceedings of
+the Academy of Natural Sciences of Philadelphia,_ December, 1905,
+wherein it is proved that the United States Geological Survey was wrong
+in believing this crater to have been due to a steam explosion. Since
+that date there has been discovered a great amount of additional
+confirmatory proof). Material of unmistakably meteoric origin was found
+by means of the drills, mixed with crushed rock, to a depth of six
+hundred to seven hundred feet below the floor of the crater, and a
+great deal of it has been found admixed with the ejected rock fragments
+on the outer slopes of the mountain, absolutely proving synchronism
+between the two events, the formation of this great crater and the
+falling of the meteoric iron out of the sky. The drill located in the
+bottom of the crater was sent, in a number of cases, much deeper (over
+one thousand feet) into unaltered horizontal red sandstone strata, but
+no meteoric material was found below this depth (seven hundred feet, or
+between eleven and twelve hundred feet below the level of the
+surrounding plain), which has been assumed as being about the limit of
+penetration. It is not possible to sink a shaft at present, owing to
+the water which has drained into the crater, and which forms, with the
+finely pulverized sandstone, a very troublesome quicksand encountered
+at about two hundred feet below the visible floor of the crater. As
+soon as this water is removed by pumping it will be easy to explore the
+depths of the crater by means of shafts and drifts. The rock strata
+(sandstone and limestone) of which the walls consist present every
+appearance of having been violently upturned by a huge body penetrating
+the earth like a cannon-ball. The general aspect of the crater
+strikingly resembles the impression made by a steel projectile shot
+into an armor-plate. Mr Tilghman has estimated that a meteorite about
+five hundred feet in diameter and moving with a velocity of about five
+miles per second would have made just such a perforation upon striking
+rocks of the character of those found at this place. There was some
+fusion of the colliding masses, and the heat produced some steam from
+the small amount of water in the rocks. As a result there has been
+found at depth a considerable amount of fused quartz (original
+sandstone), and with it innumerable particles or sparks of fused
+nickel-iron (original meteorite). A projectile of that size penetrating
+eleven to twelve hundred feet into the rocky shell of the globe must
+have produced a shock which was perceptible several hundred miles away.
+
+
+[Illustration: Trail on south side, Coon Butte crater]
+
+
+The great velocity ascribed to the supposed meteorite at the moment of
+striking could be accounted for by the fact that it probably plunged
+nearly vertically downward, for it formed a circular crater in the
+rocky crust of the earth. In that case it would have been less retarded
+by the resistance of the atmosphere than are meteorites which enter the
+air at a lower angle and shoot ahead hundreds of miles until friction
+has nearly destroyed their original motion when they drop upon the
+earth. Some meteoric masses of great size, such as Peary’s iron
+meteorite found at Cape York, Greenland, and the almost equally large
+mass discovered at Bacubirito, Mexico, appear to have penetrated but
+slightly on striking the earth. This may be explained by supposing that
+they pursued a long, horizontal course through the air before falling.
+The result would be that, their original velocity having been
+practically destroyed, they would drop to the ground with a velocity
+nearly corresponding to that which gravity would impart within the
+perpendicular distance of their final fall. A
+six-hundred-and-sixty-pound meteorite, which fell at Knyahinya,
+Hungary, striking at an angle of 27° from the vertical, penetrated the
+ground to a depth of eleven feet.
+
+It has been remarked that the Coon Butte meteorite may have fallen not
+longer ago than a few thousand years. This is based upon the fact that
+the geological indications favor the supposition that the event did not
+occur more than five thousand years ago, while on the other hand the
+rings of growth in the cedar-trees growing on the slopes of the crater
+show that they have existed there about seven hundred years. Prof.
+William H. Pickering has recently correlated this with an ancient
+chronicle which states that at Cairo, Egypt, in the year 1029, “many
+stars passed with a great noise.” He remarks that Cairo is about 100°,
+by great circle, from Coon Butte, so that if the meteorite that made
+the crater was a member of a flock of similar bodies which encountered
+the earth moving in parallel lines, some of them might have traversed
+the sky tangent to the earth’s surface at Cairo. That the spectacle
+spoken of in the chronicle was caused by meteorites he deems
+exceedingly probable because of what is said about “a great noise;”
+meteorites are the only celestial phenomena attended with perceptible
+sounds. Professor Pickering conjectures that this supposed flock of
+great meteorites may have formed the nucleus of a comet which struck
+the earth, and he finds confirmation of the idea in the fact that out
+of the ten largest meteorites known, no less than seven were found
+within nine hundred miles of Coon Butte. It would be interesting if we
+could trace back the history of that comet, and find out what malicious
+planet caught it up in its innocent wanderings and hurled it with so
+true an aim at the earth! This remarkable crater is one of the most
+interesting places in the world, for there is absolutely no record of
+such a mass, possibly an iron-headed comet, from outer space having
+come into collision with our earth. The results of the future
+exploration of the depths of the crater will be awaited with much
+interest.
+
+
+
+
+XII
+The Wrecking of the Moon
+
+
+There are sympathetic moods under whose influence one gazes with a
+certain poignant tenderness at the worn face of the moon; that little
+“fossil world” (the child of our mother earth, too) bears such terrible
+scars of its brief convulsive life that a sense of pity is awakened by
+the sight. The moon is the wonder-land of the telescope. Those towering
+mountains, whose “proud aspiring peaks” cast silhouettes of shadow that
+seem drawn with india-ink; those vast plains, enchained with gentle
+winding hills and bordered with giant ranges; those oval “oceans,”
+where one looks expectant for the flash of wind-whipped waves; those
+enchanting “bays” and recesses at the seaward feet of the Alps; those
+broad straits passing between guardian heights incomparably mightier
+than Gibraltar; those locket-like valleys as secluded among their
+mountains as the Vale of Cashmere; those colossal craters that make us
+smile at the pretensions of Vesuvius, Etna, and Cotopaxi; those strange
+white ways which pass with the unconcern of Roman roads across
+mountain, gorge, and valley—all these give the beholder an irresistible
+impression that it is truly a world into which he is looking, a world
+akin to ours, and yet no more like our world than Pompeii is like
+Naples. Its air, its waters, its clouds, its life are gone, and only a
+skeleton remains—a mute but eloquent witness to a cosmical tragedy
+without parallel in the range of human knowledge.
+
+One cannot but regret that the moon, if it ever was the seat of
+intelligent life, has not remained so until our time. Think what the
+consequences would have been if this other world at our very door had
+been found to be both habitable and inhabited! We talk rather airily of
+communicating with Mars by signals; but Mars never approaches nearer
+than 35,000,000 miles, while the moon when nearest is only a little
+more than 220,000 miles away. Given an effective magnifying power of
+five thousand diameters, which will perhaps be possible at the mountain
+observatories as telescopes improve, and we should be able to bring the
+moon within an apparent distance of about forty miles, while the
+corresponding distance for Mars would be more than seven thousand
+miles. But even with existing telescopic powers we can see details on
+the moon no larger than some artificial constructions on the earth. St
+Peter’s at Rome, with the Vatican palace and the great piazza, if
+existing on the moon, would unquestionably be recognizable as something
+else than a freak of nature. Large cities, with their radiating lines
+of communication, would at once betray their real character. Cultivated
+tracts, and the changes produced by the interference of intelligent
+beings, would be clearly recognizable. The electric illumination of a
+large town at night would probably be markedly visible. Gleams of
+reflected sunlight would come to us from the surfaces of the lakes and
+oceans, and a huge “liner” traversing a lunar sea could probably be
+followed by its trail of smoke. As to communications by “wireless”
+signals, which certain enthusiasts have thought of in connection with
+Mars, in the case of the moon they should be a relatively simple
+matter, and the feat might actually be accomplished. Think what a
+literature would grow up about the moon if it were a living world! Its
+very differences from the earth would only accentuate its interest for
+us. Night and day on the moon are each two weeks in length; how
+interesting it would be to watch the manner in which the lunarians
+dealt with such a situation as that. Lunar and terrestrial history
+would keep step with each other, and we should record them both. Truly
+one might well wish to have a neighbor world to study; one would feel
+so much the less alone in space.
+
+It is not impossible that the moon did at one time have inhabitants of
+some kind. But, if so, they vanished with the disappearance of its
+atmosphere and seas, or with the advent of its cataclysmic age. At the
+best, its career as a living world must have been brief. If the water
+and air were gradually absorbed, as some have conjectured, by its
+cooling interior rocks, its surface might, nevertheless, have retained
+them for long ages; but if, as others think, their disappearance was
+due to the escape of their gaseous molecules in consequence of the
+inability of the relatively small lunar gravitation to retain them,
+then the final catastrophe must have been as swift as it was
+inevitable. Accepting Darwin’s hypothesis, that the moon was separated
+from the earth by tidal action while both were yet plastic or nebulous,
+we may reasonably conclude that it began its career with a good supply
+of both water and air, but did not possess sufficient mass to hold them
+permanently. Yet it may have retained them long enough for life to
+develop in many forms upon its surface; in fact, there are so many
+indications that air and water have not always been lacking to the
+lunar world that we are driven to invent theories to explain both their
+former presence and their present absence.
+
+
+[Illustration: The craters Clavius, Longomontanus, Tycho, etc.]
+
+
+But whatever the former condition of the moon may have been, its
+existing appearance gives it a resistless fascination, and it bears so
+clearly the story of a vast catastrophe sculptured on its rocky face
+that the thoughtful observer cannot look upon it without a feeling of
+awe. The gigantic character of the lunar features impresses the
+beholder not less than the universality of the play of destructive
+forces which they attest. Let us make a few comparisons. Take the lunar
+crater called “Tycho”, which is a typical example of its kind. In the
+telescope Tycho appears as a perfect ring surrounding a circular
+depression, in the center of which rises a group of mountains. Its
+superficial resemblance to some terrestrial volcanic craters is very
+striking. Vesuvius, seen from a point vertically above, would no doubt
+look something like that (the resemblance would have been greater when
+the Monte del Cavallo formed a more complete circuit about the crater
+cone). But compare the dimensions. The remains of the outer crater ring
+of Vesuvius are perhaps half a mile in diameter, while the active
+crater itself is only two or three hundred feet across at the most;
+Tycho has a diameter of fifty-four miles! The group of relatively
+insignificant peaks in the center of the crater floor of Tycho is far
+more massive than the entire mountain that we call Vesuvius. The
+largest known volcanic crater on the earth, Aso San, in Japan, has a
+diameter of seven miles; it would take _sixty_ craters like Aso San to
+equal Tycho in area! And Tycho, though one of the most perfect, is by
+no means the largest crater on the moon. Another, called “Theophilus,”
+has a diameter of sixty-four miles, and is eighteen thousand feet deep.
+There are hundreds from ten to forty miles in diameter, and thousands
+from one to ten miles. They are so numerous in many places that they
+break into one another, like the cells of a crushed honeycomb.
+
+The lunar craters differ from those of the earth more fundamentally
+than in the matter of mere size; _they are not situated on the tops of
+mountains._ If they were, and if all the proportions were the same, a
+crater like Tycho might crown a conical peak fifty or one hundred miles
+high! Instead of being cavities in the summits of mountains, the lunar
+craters are rather gigantic sink-holes whose bottoms in many cases lie
+two or three miles below the general surface of the lunar world. Around
+their rims the rocks are piled up to a height of from a few hundred to
+two or three thousand feet, with a comparatively gentle inclination,
+but on the inner side they fall away in gigantic broken precipices
+which make the dizzy cliffs of the Matterhorn seem but “lover’s leaps.”
+Down they drop, ridge below ridge, crag under crag, tottering wall
+beneath wall, until, in a crater named “Newton,” near the south lunar
+pole, they attain a depth where the rays of the sun never reach.
+Nothing more frightful than the spectacle which many of these terrible
+chasms present can be pictured by the imagination. As the lazy lunar
+day slowly advances, the sunshine, unmitigated by clouds or atmospheric
+veil of any kind, creeps across their rims and begins to descend the
+opposite walls. Presently it strikes the ragged crest of a ridge which
+had lain hidden in such darkness as we never know on the earth, and
+runs along it like a line of kindling fire. Rocky pinnacles and needles
+shoot up into the sunlight out of the black depths. Down sinks the line
+of light, mile after mile, and continually new precipices and cliffs
+are brought into view, until at last the vast floor is attained and
+begins to be illuminated. In the meanwhile the sun’s rays, darting
+across the gulf, have touched the summits of the central peaks, twenty
+or thirty miles from the crater’s inmost edge, and they immediately
+kindle and blaze like huge stars amid the darkness. So profound are
+some of these awful craters that days pass before the sun has risen
+high enough above them to chase the last shadows from their depths.
+
+Although several long ranges of mountains resembling those of the earth
+exist on the moon, the great majority of its elevations assume the
+crateriform aspect. Sometimes, instead of a crater, we find an immense
+mountain ring whose form and aspect hardly suggest volcanic action. But
+everywhere the true craters are in evidence, even on the sea-beds,
+although they attain their greatest number and size on those parts of
+the moon—covering sixty per cent of its visible surface—which are
+distinctly mountainous in character and which constitute its most
+brilliant portions. Broadly speaking, the southwestern half of the moon
+is the most mountainous and broken, and the northeastern half the least
+so. Right down through the center, from pole to pole, runs a wonderful
+line of craters and crateriform valleys of a magnitude stupendous even
+for the moon. Another similar line follows the western edge. Three or
+four “seas” are thrust between these mountainous belts. By the effects
+of “libration” parts of the opposite hemisphere of the moon which is
+turned away from the earth are from time to time brought into view, and
+their aspect indicates that that hemisphere resembles in its surface
+features the one which faces the earth. There are many things about the
+craters which seem to give some warrant for the hypothesis which has
+been particularly urged by Mr G. K. Gilbert, that they were formed by
+the impact of meteors; but there are also many things which militate
+against that idea, and, upon the whole, the volcanic theory of their
+origin is to be preferred.
+
+The enormous size of the lunar volcanoes is not so difficult to account
+for when we remember how slight is the force of lunar gravity as
+compared with that of the earth. With equal size and density, bodies on
+the moon weigh only one-sixth as much as on the earth. Impelled by the
+same force, a projectile that would go ten miles on the earth would go
+sixty miles on the moon. A lunar giant thirty-five feet tall would
+weigh no more than an ordinary son of Adam weighs on his greater
+planet. To shoot a body from the earth so that it would not drop back
+again, we should have to start it with a velocity of seven miles per
+second; a mile and a half per second would serve on the moon. It is by
+no means difficult to believe, then, that a lunar volcano might form a
+crater ring eight or ten times broader than the greatest to be found on
+the earth, especially when we reflect that in addition to the
+relatively slight force of gravity, the materials of the lunar crust
+are probably lighter than those of our terrestrial rocks.
+
+
+[Illustration: Western part of the Mare Serenitatis]
+
+
+For similar reasons it seems not impossible that the theory mentioned
+in a former chapter—that some of the meteorites that have fallen upon
+the earth originated from the lunar volcanoes—is well founded. This
+would apply especially to the stony meteorites, for it is hardly to be
+supposed that the moon, at least in its superficial parts, contains
+much iron. It is surely a scene most strange that is thus presented to
+the mind’s eye—that little attendant of the earth’s (the moon has only
+one-fiftieth of the volume, and only one-eightieth of the mass of the
+earth) firing great stones back at its parent planet! And what can have
+been the cause of this furious outbreak of volcanic forces on the moon?
+Evidently it was but a passing stage in its history; it had enjoyed
+more quiet times before. As it cooled down from the plastic state in
+which it parted from the earth, it became incrusted after the normal
+manner of a planet, and then oceans were formed, its atmosphere being
+sufficiently dense to prevent the water from evaporating and the
+would-be oceans from disappearing continually in mist. This, if any,
+must have been the period of life in the lunar world. As we look upon
+the vestiges of that ancient world buried in the wreck that now covers
+so much of its surface, it is difficult to restrain the imagination
+from picturing the scenes which were once presented there; and, in such
+a case, should the imagination be fettered? We give it free rein in
+terrestrial life, and it rewards us with some of our greatest
+intellectual pleasures. The wonderful landscapes of the moon offer it
+an ideal field with just enough half-hidden suggestions of facts to
+stimulate its powers.
+
+The great plains of the _Mare Imbrium_ and the _Mare Serenitatis_ (the
+“Sea of Showers” and the “Sea of Serenity”), bordered in part by lofty
+mountain ranges precisely like terrestrial mountains, scalloped along
+their shores with beautiful bays curving back into the adjoining
+highlands, and united by a great strait passing between the nearly
+abutting ends of the “Lunar Apennines” and the “Lunar Caucasus,” offer
+the elements of a scene of world beauty such as it would be difficult
+to match upon our planet. Look at the finely modulated bottom of the
+ancient sea in Mr Ritchey’s exquisite photograph of the western part of
+the _Mare Serenitatis,_ where one seems to see the play of the watery
+currents heaping the ocean sands in waving lines, making shallows,
+bars, and deeps for the mariner to avoid or seek, and affording a
+playground for the creatures of the main. What geologist would not wish
+to try his hammer on those rocks with their stony pages of fossilized
+history? There is in us an instinct which forbids us to think that
+there was never any life there. If we could visit the moon, there is
+not among us a person so prosaic and unimaginative that he would not,
+the very first thing, begin to search for traces of its inhabitants. We
+would look for them in the deposits on the sea bottoms; we would
+examine the shores wherever the configuration seemed favorable for
+harbors and the sites of maritime cities—forgetting that it may be a
+little ridiculous to ascribe to the ancient lunarians the same ideas
+that have governed the development of our race; we would search through
+the valleys and along the seeming courses of vanished streams; we would
+explore the mountains, not the terrible craters, but the pinnacled
+chains that recall our own Alps and Rockies; seeking everywhere some
+vestige of the transforming presence of intelligent life. Perhaps we
+should find such traces, and perhaps, with all our searching, we should
+find nothing to suggest that life had ever existed amid that universal
+ruin.
+
+
+[Illustration: Mare Tranquilitatis and surroundings]
+
+
+Look again at the border of the “Sea of Serenity”—what a name for such
+a scene!—and observe how it has been rent with almost inconceivable
+violence, the wall of the colossal crater Posidonius dropping
+vertically upon the ancient shore and obliterating it, while its giant
+neighbor, Le Monnier, opens a yawning mouth as if to swallow the sea
+itself. A scene like this makes one question whether, after all, those
+may not be right who have imagined that the so-called sea bottoms are
+really vast plains of frozen lava which gushed up in floods so
+extensive that even the mighty volcanoes were half drowned in the fiery
+sea. This suggestion becomes even stronger when we turn to another of
+the photographs of Mr Ritchey’s wonderful series, showing a part of the
+_Mare Tranquilitatis_ (“Sea of Tranquility”!). Notice how near the
+center of the picture the outline of a huge ring with radiating ridges
+shows through the sea bottom; a fossil volcano submerged in a petrified
+ocean! This is by no means the only instance in which a buried world
+shows itself under the great lunar plains. Yet, as the newer craters in
+the sea itself prove, the volcanic activity survived this other
+catastrophe, or broke out again subsequently, bringing more ruin to
+pile upon ruin.
+
+Yet notwithstanding the evidence which we have just been considering in
+support of the hypothesis that the “seas” are lava floods, Messrs.
+Loewy and Puiseux, the selenographers of the Paris Observatory, are
+convinced that these great plains bear characteristic marks of the
+former presence of immense bodies of water. In that case we should be
+forced to conclude that the later oceans of the moon lay upon vast
+sheets of solidified lava; and thus the catastrophe of the lunar world
+assumes a double aspect, the earliest oceans being swallowed up in
+molten floods issuing from the interior, while the lands were reduced
+to chaos by a universal eruption of tremendous volcanoes; and then a
+period of comparative quiet followed, during which new seas were
+formed, and new life perhaps began to flourish in the lunar world, only
+to end in another cataclysm, which finally put a term to the existence
+of the moon as a life-supporting world.
+
+Suppose we examine two more of Mr Ritchey’s illuminating photographs,
+and, first, the one showing the crater Theophilus and its surroundings.
+We have spoken of Theophilus before, citing the facts that it is
+sixty-four miles in diameter and eighteen thousand feet deep. It will
+be noticed that it has two brother giants—Cyrillus the nearer, and
+Catharina the more distant; but Theophilus is plainly the youngest of
+the trio. Centuries, and perhaps thousands of years, must have elapsed
+between the periods of their upheaval, for the two older craters are
+partly filled with débris, while it is manifest at a glance that when
+the south eastern wall of Theophilus was formed, it broke away and
+destroyed a part of the more ancient ring of Cyrillus. There is no more
+tremendous scene on the moon than this; viewed with a powerful
+telescope, it is absolutely appalling.
+
+
+[Illustration: Lunar craters Theophilus and surrounding region]
+
+
+The next photograph shows, if possible, a still wilder region. It is
+the part of the moon lying between Tycho and the south pole. Tycho is
+seen in the lower left-hand part of the picture. To the right, at the
+edge of the illuminated portion of the moon, are the crater-rings,
+Longomontanus and Wilhelm I, the former being the larger. Between them
+are to be seen the ruins of two or three more ancient craters which,
+together with portions of the walls of Wilhelm I and Longomontanus,
+have been honeycombed with smaller craters. The vast crateriform
+depression above the center of the picture is Clavius, an unrivaled
+wonder of lunar scenery, a hundred and forty-two miles in its greatest
+length, while its whole immense floor has sunk two miles below the
+general surface of the moon outside the ring. The monstrous
+shadow-filled cavity above Clavius toward the right is Blancanus, whose
+aspect here gives a good idea of the appearance of these chasms when
+only their rims are in the sunlight. But observe the indescribable
+savagery of the entire scene. It looks as though the spirit of
+destruction had gone mad in this spot. The mighty craters have broken
+forth one after another, each rending its predecessor; and when their
+work was finished, a minor but yet tremendous outbreak occurred, and
+the face of the moon was gored and punctured with thousands of smaller
+craters. These relatively small craters (small, however, only in a
+lunar sense, for many of them would appear gigantic on the earth)
+recall once more the theory of meteoric impact. It does not seem
+impossible that some of them may have been formed by such an agency.
+
+One would not wish for our planet such a fate as that which has
+overtaken the moon, but we cannot be absolutely sure that something of
+the kind may not be in store for it. We really know nothing of the
+ultimate causes of volcanic activity, and some have suggested that the
+internal energies of the earth may be accumulating instead of dying
+out, and may never yet have exhibited their utmost destructive power.
+Perhaps the best assurance that we can find that the earth will escape
+the catastrophe that has overtaken its satellite is to be found in the
+relatively great force of its gravitation. The moon has been the victim
+of its weakness; given equal forces, and the earth would be the better
+able to withstand them. It is significant, in connection with these
+considerations, that the little planet Mercury, which seems also to
+have parted with its air and water, shows to the telescope some
+indications that it is pitted with craters resembling those that have
+torn to pieces the face of the moon.
+
+Upon the whole, after studying the dreadful lunar landscapes, one
+cannot feel a very enthusiastic sympathy with those who are seeking
+indications of the continued existence of some kind of life on the
+moon; such a world is better without inhabitants. It has met its fate;
+let it go! Fortunately, it is not so near that it cannot hide its scars
+and appear beautiful—except when curiosity impels us to look with the
+penetrating eyes of the astronomer.
+
+
+[Illustration: Mare Crisium]
+
+
+
+
+XIII
+The Great Mars Problem
+
+
+Let any thoughtful person who is acquainted with the general facts of
+astronomy look up at the heavens some night when they appear in their
+greatest splendor, and ask himself what is the strongest impression
+that they make upon his mind. He may not find it easy to frame an
+answer, but when he has succeeded it will probably be to the effect
+that the stars give him an impression of the universality of
+intelligence; they make him feel, as the sun and the moon cannot do,
+that his world is not alone; that all this was not made simply to form
+a gorgeous canopy over the tents of men. If he is of a devout turn of
+mind, he thinks, as he gazes into those fathomless deeps and among
+those bewildering hosts, of the infinite multitude of created beings
+that the Almighty has taken under his care. The narrow ideas of the old
+geocentric theology, which made the earth God’s especial footstool, and
+man his only rational creature, fall away from him like a veil that had
+obscured his vision; they are impossible in the presence of what he
+sees above. Thus the natural tendency, in the light of modern progress,
+is to regard the universe as everywhere filled with life.
+
+But science, which is responsible for this broadening of men’s thoughts
+concerning the universality of life, itself proceeds to set limits. Of
+spiritual existences it pretends to know nothing, but as to physical
+beings, it declares that it can only entertain the supposition of their
+existence where it finds evidence of an environment suited to their
+needs, and such environment may not everywhere exist. Science, though
+repelled by the antiquated theological conception of the supreme
+isolation of man among created beings, regards with complacency the
+probability that there are regions in the universe where no organic
+life exists, stars which shine upon no inhabited worlds, and planets
+which nourish no animate creatures. The astronomical view of the
+universe is that it consists of matter in every stage of evolution:
+some nebulous and chaotic; some just condensing into stars (suns) of
+every magnitude and order; some shaped into finished solar bodies
+surrounded by dependent planets; some forming stars that perhaps have
+no planets, and will have none; some constituting suns that are already
+aging, and will soon lose their radiant energy and disappear; and some
+aggregated into masses that long ago became inert, cold, and rayless,
+and that can only be revivified by means about which we can form
+conjectures, but of which we actually know nothing.
+
+As with the stars, so with the planets, which are the satellites of
+stars. All investigations unite to tell us that the planets are not all
+in the same state of development. As some are large and some small, so
+some are, in an evolutionary sense, young, and some old. As they depend
+upon the suns around which they revolve for their light, heat, and
+other forms of radiant energy, so their condition varies with their
+distance from those suns. Many may never arrive at a state suitable for
+the maintenance of life upon their surfaces; some which are not at
+present in such a state may attain it later; and the forms of life
+themselves may vary with the peculiar environment that different
+planets afford. Thus we see that we are not scientifically justified in
+affirming that life is ubiquitous, although we are thus justified in
+saying that it must be, in a general sense, universal. We might liken
+the universe to a garden known to contain every variety of plant. If on
+entering it we see no flowers, we examine the species before us and
+find that they are not of those which bloom at this particular season,
+or perhaps they are such as never bear flowers. Yet we feel no doubt
+that we shall find flowers somewhere in the garden, because there _are_
+species which bloom at this season, and the garden contains _all_
+varieties.
+
+While it is tacitly assumed that there are planets revolving around
+other stars than the sun, it would be impossible for us to see them
+with any telescope yet invented, and no instrument now in the
+possession of astronomers could assure us of their existence; so the
+only planetary system of which we have visual knowledge is our own.
+Excluding the asteroids, which could not from any point of view be
+considered as habitable, we have in the solar system eight planets of
+various sizes and situated at various distances from the sun. Of these
+eight we know that one, the earth, is inhabited. The question, then,
+arises: Are there any of the others which are inhabited or habitable?
+Since it is our intention to discuss the habitability of only one of
+the seven to which the question applies, the rest may be dismissed in a
+few words. The smallest of them, and the nearest to the sun, is
+Mercury, which is regarded as uninhabitable because it has no
+perceptible supply of water and air, and because, owing to the
+extraordinary eccentricity of its orbit, it is subjected to excessive
+and very rapid alterations in the amount of solar heat and light poured
+upon its surface, such alterations being inconsistent with the
+supposition that it can support living beings. Even its average
+temperature is more than six and a half times that prevailing on the
+earth! Another circumstance which militates against its habitability is
+that, according to the results of the best telescopic studies, it
+always keeps the same face toward the sun, so that one half of the
+planet is perpetually exposed to the fierce solar rays, and the other
+half faces the unmitigated cold of open space. Venus, the next in
+distance from the sun, is almost the exact twin of the earth in size,
+and many arguments may be urged in favor of its habitability, although
+it is suspected of possessing the same peculiarity as Mercury, in
+always keeping the same side sunward. Unfortunately its atmosphere
+appears to be so dense that no permanent markings on its surface are
+certainly visible, and the question of its actual condition must, for
+the present, be left in abeyance. Mars, the first planet more distant
+from the sun than the earth, is the special subject of this chapter,
+and will be described and discussed a few lines further on. Jupiter,
+Saturn, Uranus, and Neptune, the four giant planets, all more distant
+than Mars, and each more distant than the other in the order named, are
+all regarded as uninhabitable because none of them appears to possess
+any degree of solidity. They may have solid or liquid nuclei, but
+exteriorly they seem to be mere balls of cloud. Of course, one can
+imagine what he pleases about the existence of creatures suited to the
+physical constitution of such planets as these, but they must be
+excluded from the category of habitable worlds in the ordinary sense of
+the term. We go back, then, to Mars.
+
+It will be best to begin with a description of the planet. Mars is 4230
+miles in diameter; its surface is not much more than one-quarter as
+extensive as that of the earth (.285). Its mean distance from the sun
+is 141,500,000 miles, 48,500,000 miles greater than that of the earth.
+Since radiant energy varies inversely as the square of distance, Mars
+receives less than half as much solar light and heat as the earth gets.
+Mars’ year (period of revolution round the sun) is 687 days. Its mean
+density is 71 per cent of the earth’s, and the force of gravity on its
+surface is 38 per cent of that on the surface of the earth; _i.e.,_ a
+body weighing one hundred pounds on the earth would, if transported to
+Mars, weigh but thirty-eight pounds. The inclination of its equator to
+the plane of its orbit differs very little from that of the earth’s
+equator, and its axial rotation occupies 24 hours 37 minutes. so that
+the length of day and night, and the extent of the seasonal changes on
+Mars, are almost precisely the same as on the earth. But owing to the
+greater length of its year, the seasons of Mars, while occurring in the
+same order, are almost twice as long as ours. The surface of the planet
+is manifestly solid, like that of our globe, and the telescope reveals
+many permanent markings on it, recalling the appearance of a globe on
+which geographical features have been represented in reddish and dusky
+tints. Around the poles are plainly to be seen rounded white areas,
+which vary in extent with the Martian seasons, nearly vanishing in
+summer and extending widely in winter. The most recent spectroscopic
+determinations indicate that Mars has an atmosphere perhaps as dense as
+that to be found on our loftiest mountain peaks, and there is a
+perceptible amount of watery vapor in this atmosphere. The surface of
+the planet appears to be remarkably level, and it has no mountain
+ranges. No evidences of volcanic action have been discovered on Mars.
+The dusky and reddish areas were regarded by the early observers as
+respectively seas and lands, but at present it is not believed that
+there are any bodies of water on the planet. There has never been much
+doubt expressed that the white areas about the poles represent snow.
+
+It will be seen from this brief description that many remarkable
+resemblances exist between Mars and the earth, and there is nothing
+wonderful in the fact that the question of the habitability of the
+former has become one of extreme and wide-spread interest, giving rise
+to the most diverse views, to many extraordinary speculations, and
+sometimes to regrettably heated controversy. The first champion of the
+habitability of Mars was Sir William Herschel, although even before his
+time the idea had been suggested. He was convinced by the revelations
+of his telescopes, continually increasing in power, that Mars was more
+like the earth than any other planet. He could not resist the testimony
+of the polar snows, whose suggestive conduct was in such striking
+accord with what occurs upon the earth. Gradually, as telescopes
+improved and observers increased in number, the principal features of
+the planet were disclosed and charted, and “areography,” as the
+geography of Mars was called, took its place among the recognized
+branches of astronomical study. But it was not before 1877 that a
+fundamentally new discovery in areography gave a truly sensational turn
+to speculation about life on “the red planet.” In that year Mars made
+one of its nearest approaches to the earth, and was so situated in its
+orbit that it could be observed to great advantage from the northern
+hemisphere of the earth. The celebrated Italian astronomer,
+Schiaparelli, took advantage of this opportunity to make a
+trigonometrical survey of the surface of Mars—as coolly and confidently
+as if he were not taking his sights across a thirty-five-million-mile
+gulf of empty space—and in the course of this survey he was astonished
+to perceive that the reddish areas, then called continents, were
+crossed in many directions by narrow, dusky lines, to which he gave the
+suggestive name of “canals.” Thus a kind of firebrand was cast into the
+field of astronomical speculation, which has ever since produced
+disputes that have sometimes approached the violence of political
+faction. At first the accuracy of Schiaparelli’s observations was
+contested; it required a powerful telescope, and the most excellent
+“seeing,” to render the enigmatical lines visible at all, and many
+searchers were unable to detect them. But Schiaparelli continued his
+studies in the serene sky of Italy, and produced charts of the
+gridironed face of Mars containing so much astonishing detail that one
+had either to reject them _in toto_ or to confess that Schiaparelli was
+right. As subsequent favorable oppositions of Mars occurred, other
+observers began to see the “canals” and to confirm the substantial
+accuracy of the Italian astronomer’s work, and finally few were found
+who would venture to affirm that the “canals” did not exist, whatever
+their meaning might be.
+
+
+[Illustration: Schiaparelli’s chart of Mars, showing the so-called
+system of canals]
+
+
+When Schiaparelli began his observations it was generally believed, as
+we have said, that the dusky areas on Mars were seas, and since
+Schiaparelli thought that the “canals” invariably began and ended at
+the shores of the “seas,” the appropriateness of the title given to the
+lines seemed apparent. Their artificial character was immediately
+assumed by many, because they were too straight and too suggestively
+geometrical in their arrangement to permit the conclusion that they
+were natural watercourses. A most surprising circumstance noted by
+Schiaparelli was that the “canals” made their appearance _after_ the
+melting of the polar snow in the corresponding hemisphere had begun,
+and that they grew darker, longer, and more numerous in proportion as
+the polar liquidation proceeded; another very puzzling observation was
+that many of them became double as the season advanced; close beside an
+already existing “canal,” and in perfect parallelism with it, another
+would gradually make its appearance. That these phenomena actually
+existed and were not illusions was proved by later observations, and
+today they are seen whenever Mars is favorably situated for
+observation.
+
+In the closing decade of the nineteenth century, Mr Percival Lowell
+took up the work where Schiaparelli had virtually dropped it, and soon
+added a great number of “canals” to those previously known, so that in
+his charts the surface of the wonderful little planet appears covered
+as with a spider’s web, the dusky lines criss-crossing in every
+direction, with conspicuous knots wherever a number of them come
+together. Mr Lowell has demonstrated that the areas originally called
+seas, and thus named on the earlier charts, are not bodies of water,
+whatever else they may be. He has also found that the mysterious lines
+do not, as Schiaparelli supposed, begin and end at the edges of the
+dusky regions, but often continue on across them, reaching in some
+cases far up into the polar regions. But Schiaparelli was right in his
+observation that the appearance of the “canals” is synchronous with the
+gradual disappearance of the polar snows, and this fact has become the
+basis of the most extraordinary theory that the subject of life in
+other worlds has ever given birth to.
+
+Now, the effect of such discoveries, as we have related, depends upon
+the type of mind to whose attention they are called. Many are content
+to accept them as strange and inexplicable at present, and to wait for
+further light upon them; others insist upon an immediate inquiry
+concerning their probable nature and meaning. Such an inquiry can only
+be based upon inference proceeding from analogy. Mars, say Mr Lowell
+and those who are of his opinion, is manifestly a solidly incrusted
+planet like the earth; it has an atmosphere, though one of great
+rarity; it has water vapor, as the snows in themselves prove; it has
+the alternation of day and night, and a succession of seasons closely
+resembling those of the earth; its surface is suggestively divided into
+regions of contrasting colors and appearance, and upon that surface we
+see an immense number of lines geometrically arranged, with a system of
+symmetrical intersections where the lines expand into circular and oval
+areas—and all connected with the annual melting of the polar snows in a
+way which irresistibly suggests the interference of intelligence
+directed to a definite end. Why, with so many concurrent circumstances
+to support the hypothesis, should we not regard Mars as an inhabited
+globe?
+
+But the differences between Mars and the earth are in many ways as
+striking as their resemblances. Mars is relatively small; it gets less
+than half as much light and heat as we receive; its atmosphere is so
+rare that it would be distressing to us, even if we could survive in it
+at all; it has no lakes, rivers, or seas; its surface is an endless
+prairie. and its “canals” are phenomena utterly unlike anything on the
+earth. Yet it is precisely upon these divergences between the earth and
+Mars, this repudiation of terrestrial standards, that the theory of
+“life on Mars,” for which Mr Lowell is mainly responsible, is based.
+Because Mars is smaller than the earth, we are told it must necessarily
+be more advanced in planetary evolution, the underlying cause of which
+is the gradual cooling and contraction of the planet’s mass. Mars has
+parted with its internal heat more rapidly than the earth; consequently
+its waters and its atmosphere have been mostly withdrawn by chemical
+combinations, but enough of both yet remain to render life still
+possible on its surface. As the globe of Mars is evolutionally older
+than that of the earth, so its forms of organic life may be
+proportionally further advanced, and its inhabitants may have attained
+a degree of cultivated intelligence much superior to what at present
+exists upon the earth. Understanding the nature and the causes of the
+desiccation of their planet, and possessing engineering science and
+capabilities far in advance of ours, they may be conceived to have
+grappled with the stupendous problem of keeping their world in a
+habitable condition as long as possible. Supposing them to have become
+accustomed to live in their rarefied atmosphere (a thing not
+inconceivable, since men can live for a time at least in air hardly
+less rare), the most pressing problem for them is that of a
+water-supply, without which plant life cannot exist, while animal life
+in turn depends for its existence upon vegetation. The only direction
+in which they can seek water is that of the polar regions, where it is
+alternately condensed into snow and released in the liquid form by the
+effect of the seasonal changes. It is, then, to the annual melting of
+the polar snow-fields that the Martian engineers are supposed to have
+recourse in supplying the needs of their planet, and thus providing the
+means of prolonging their own existence. It is imagined that they have
+for this purpose constructed a stupendous system of irrigation
+extending over the temperate and equatorial regions of the planet. The
+“canals” represent the lines of irrigation, but the narrow streaks that
+we see are not the canals themselves, but the irrigated bands covered
+by them. Their dark hue, and their gradual appearance after the polar
+melting has begun, are due to the growth of vegetation stimulated by
+the water. The rounded areas visible where several “canals” meet and
+cross are called by Mr Lowell “oases.” These are supposed to be the
+principal centers of population and industry. It must be confessed that
+some of them, with their complicated systems of radiating lines, appear
+to answer very well to such a theory. No attempt to explain them by
+analogy with natural phenomena on the earth has proved successful.
+
+But a great difficulty yet remains: How to explain the seemingly
+miraculous powers of the supposed engineers? Here recourse is had once
+more to the relative smallness of the planet. We have remarked that the
+force of gravity on Mars is only thirty-eight per cent of that on the
+earth. A steam-shovel driven by a certain horse-power would be nearly
+three times as effective there as here. A man of our stature on Mars
+would find his effective strength increased in the same proportion. But
+just because of the slight force of gravity there, a Martian might
+attain to the traditional stature of Goliath without finding his own
+weight an encumbrance to his activity, while at the same time his huge
+muscles would come into unimpeded play, enabling him single-handed to
+perform labors that would be impossible to a whole gang of terrestrial
+workmen. The effective powers of huge machines would be increased in
+the same way; and to all this must be added the fact that the mean
+density of the materials of which Mars is composed is much less than
+that of the constituents of the earth. Combining all these
+considerations, it becomes much less difficult to conceive that public
+works might be successfully undertaken on Mars which would be
+hopelessly beyond the limits of human accomplishment.
+
+Certain other difficulties have also to be met; as, for instance, the
+relative coldness of the climate of Mars. At its distance it gets
+considerably less than half as much light and heat as we receive. In
+addition to this, the rarity of its atmosphere would naturally be
+expected to decrease the effective temperature at the planet’s surface,
+since an atmosphere acts somewhat like the glass cover of a hot-house
+in retaining the solar heat which has penetrated it. It has been
+calculated that, unless there are mitigating circumstances of which we
+know nothing, the average temperature at the surface of Mars must be
+far below the freezing-point of water. To this it is replied that the
+possible mitigating circumstances spoken of evidently exist in fact,
+because we can _see_ that the watery vapor condenses into snow around
+the poles in winter, but melts again when summer comes. The mitigating
+agent may be supposed to exist in the atmosphere where the presence of
+certain gases would completely alter the temperature gradients.
+
+It might also be objected that it is inconceivable that the Martian
+engineers, however great may be their physical powers, and however
+gigantic the mechanical energies under their control, could force water
+in large quantities from the poles to the equator. This is an
+achievement that measures up to the cosmical standard. It is admitted
+by the champions of the theory that the difficulty is a formidable one;
+but they call attention to the singular fact that on Mars there can be
+found no chains of mountains, and it is even doubtful if ranges of
+hills exist there. The entire surface of the planet appears to be
+almost “as smooth as a billiard ball,” and even the broad regions which
+were once supposed to be seas apparently lie at practically the same
+level as the other parts, since the “canals” in many cases run
+uninterruptedly across them. Lowell’s idea is that these sombre areas
+may be expanses of vegetation covering ground of a more or less marshy
+character, for while the largest of them appear to be permanent, there
+are some which vary coincidently with the variations of the canals.
+
+As to the kind of machinery employed to force the water from the poles,
+it has been conjectured that it may have taken the form of a gigantic
+system of pumps and conduits; and since the Martians are assumed to be
+so far in advance of us in their mastery of scientific principles, the
+hypothesis will at least not be harmed by supposing that they have
+learned to harness forces of nature whose very existence in a
+manageable form is yet unrecognized on the earth. If we wish to let the
+imagination loose, we may conjecture that they have conquered the
+secret of those intra-atomic forces whose resistless energy is
+beginning to become evident to us, but the possibility of whose
+utilization remains a dream, the fulfillment of which nobody dares to
+predict.
+
+Such, in very brief form, is the celebrated theory of Mars as an
+inhabited world. It certainly captivates the imagination, and if we
+believe it to represent the facts, we cannot but watch with the deepest
+sympathy this gallant struggle of an intellectual race to preserve its
+planet from the effects of advancing age and death. We may, indeed,
+wonder whether our own humanity, confronted by such a calamity, could
+be counted on to meet the emergency with equal stoutness of heart and
+inexhaustibleness of resource. Up to the present time we certainly have
+shown no capacity to confront Nature toe to toe, and to seize her by
+the shoulders and turn her round when she refuses to go our way. If we
+could get into wireless telephonic communication with the Martians we
+might learn from their own lips the secret of their more than “Roman
+recovery.”
+
+
+
+
+XIV
+The Riddle of the Asteroids
+
+
+Between the orbits of Mars and Jupiter revolves the most remarkable
+system of little bodies with which we are acquainted—the Asteroids, or
+Minor Planets. Some six hundred are now known, and they may actually
+number thousands. They form virtually a ring about the sun. The most
+striking general fact about them is that they occupy the place in the
+sky which should be occupied, according to Bode’s Law, by a single
+large planet. This fact, as we shall see, has led to the invention of
+one of the most extraordinary theories in astronomy—_viz.,_ that of the
+explosion of a world!
+
+Bode’s Law, so-called, is only an empiric formula, but until the
+discovery of Neptune it accorded so well with the distances of the
+planets that astronomers were disposed to look upon it as really
+representing some underlying principle of planetary distribution. They
+were puzzled by the absence of a planet in the space between Mars and
+Jupiter, where the “law” demanded that there should be one, and an
+association of astronomers was formed to search for it. There was a
+decided sensation when, in 1801, Piazzi, of Palermo, announced that he
+had found a little planet which apparently occupied the place in the
+system which belonged to the missing body. He named it Ceres, and it
+was the first of the Asteroids. The next year Olbers, of Bremen, while
+looking for Ceres with his telescope, stumbled upon another small
+planet which he named Pallas. Immediately he was inspired with the idea
+that these two planets were fragments of a larger one which had
+formerly occupied the vacant place in the planetary ranks, and he
+predicted that others would be found by searching in the neighborhood
+of the intersection of the orbits of the two already discovered. This
+bold prediction was brilliantly fulfilled by the finding of two
+more—Juno in 1804, and Vesta in 1807. Olbers would seem to have been
+led to the invention of his hypothesis of a planetary explosion by the
+faith which astronomers at that time had in Bode’s Law. They appear to
+have thought that several planets revolving in the gap where the “law”
+called for but one could only be accounted for upon the theory that the
+original _one_ had been broken up to form the several. Gravitation
+demanded that the remnants of a planet blown to pieces, no matter how
+their orbits might otherwise differ, should all return at stated
+periods to the point where the explosion had occurred; hence Olbers’
+prediction that any asteroids that might subsequently be discovered
+would be found to have a common point of orbital intersection. And
+curiously enough all of the first asteroids found practically answered
+to this requirement. Olbers’ theory seemed to be established.
+
+After the first four, no more asteroids were found until 1845, when one
+was discovered; then, in 1847, three more were added to the list; and
+after that searchers began to pick them up with such rapidity that by
+the close of the century hundreds were known, and it had become almost
+impossible to keep track of them. The first four are by far the largest
+members of the group, but their actual sizes remained unknown until
+less than twenty years ago. It was long supposed that Vesta was the
+largest, because it shines more brightly than any of the others; but
+finally, in 1895, Barnard, with the Lick telescope, definitely measured
+their diameters, and proved to everybody’s surprise that Ceres is
+really the chief, and Vesta only the third in rank. His measures are as
+follows: Ceres, 477 miles; Pallas, 304 miles; Vesta, 239 miles; and
+Juno, 120 miles. They differ greatly in the reflective power of their
+surfaces, a fact of much significance in connection with the question
+of their origin. Vesta is, surface for surface, rather more than three
+times as brilliant as Ceres, whence the original mistake about its
+magnitude.
+
+Nowadays new asteroids are found frequently by photography, but
+physically they are most insignificant bodies, their average diameter
+probably not exceeding twenty miles, and some are believed not to
+exceed ten. On a planet only ten miles in diameter, assuming the same
+mean density as the earth’s, which is undoubtedly too much, the force
+of gravity would be so slight that an average man would not weigh more
+than three ounces, and could jump off into space whenever he liked.
+
+Although the asteroids all revolve around the sun in the same direction
+as that pursued by the major planets, their orbits are inclined at a
+great variety of angles to the general plane of the planetary system,
+and some of them are very eccentric—almost as much so as the orbits of
+many of the periodic comets. It has even been conjectured that the two
+tiny moons of Mars and the four smaller satellites of Jupiter may be
+asteroids gone astray and captured by those planets. Two of the
+asteroids are exceedingly remarkable for the shapes and positions of
+their orbits; these are Eros, discovered in 1898, and T. G., 1906,
+found eight years later. The latter has a mean distance from the sun
+slightly greater than that of Jupiter, while the mean distance of Eros
+is less than that of Mars. The orbit of Eros is so eccentric that at
+times it approaches within 15,000,000 miles of the earth, nearer than
+any other regular member of the solar system except the moon, thus
+affording an unrivaled means of measuring the solar parallax. But for
+our present purpose the chief interest of Eros lies in its
+extraordinary changes of light.
+
+These changes, although irregular, have been observed and photographed
+many times, and there seems to be no doubt of their reality. Their
+significance consists in their possible connection with the form of the
+little planet, whose diameter is generally estimated at not more than
+twenty miles. Von Oppolzer found, in 1901, that Eros lost three-fourths
+of its brilliancy once in every two hours and thirty-eight minutes.
+Other observers have found slightly different periods of variability,
+but none as long as three hours. The most interesting interpretation
+that has been offered of this phenomenon is that it is due to a great
+irregularity of figure, recalling at once Olbers’ hypothesis. According
+to some, Eros may be double, the two bodies composing it revolving
+around each other at very close quarters; but a more striking, and it
+may be said probable, suggestion is that Eros has a form not unlike
+that of a dumb-bell, or hour-glass, turning rapidly end over end so
+that the area of illuminated surface presented to our eyes continually
+changes, reaching at certain times a minimum when the amount of light
+that it reflects toward the earth is reduced to a quarter of its
+maximum value. Various other bizarre shapes have been ascribed to Eros,
+such, for instance, as that of a flat stone revolving about one of its
+longer axes, so that sometimes we see its face and sometimes its edge.
+
+All of these explanations proceed upon the assumption that Eros cannot
+have a simple globular figure like that of a typical planet, a figure
+which is prescribed by the law of gravitation, but that its shape is
+what may be called accidental; in a word, it is a _fragment,_ for it
+seems impossible to believe that a body formed in interplanetary space,
+either through nebular condensation or through the aggregation of
+particles drawn together by their mutual attractions, should not be
+practically spherical in shape. Nor is Eros the only asteroid that
+gives evidence by variations of brilliancy that there is something
+abnormal in its constitution; several others present the same
+phenomenon in varying degrees. Even Vesta was regarded by Olbers as
+sufficiently variable in its light to warrant the conclusion that it
+was an angular mass instead of a globe. Some of the smaller ones show
+very notable variations, and all in short periods, of three or four
+hours, suggesting that in turning about one of their axes they present
+a surface of variable extent toward the sun and the earth.
+
+The theory which some have preferred—that the variability of light is
+due to the differences of reflective power on different parts of the
+surface—would, if accepted, be hardly less suggestive of the origin of
+these little bodies by the breaking up of a larger one, because the
+most natural explanation of such differences would seem to be that they
+arose from variations in the roughness or smoothness of the reflecting
+surface, which would be characteristic of fragmentary bodies. In the
+case of a large planet alternating expanses of land and water, or of
+vegetation and desert, would produce a notable variation in the amount
+of reflection, but on bodies of the size of the asteroids neither water
+nor vegetation could exist, and an atmosphere would be equally
+impossible.
+
+One of the strongest objections to Olbers’ hypothesis is that only a
+few of the first asteroids discovered travel in orbits which measurably
+satisfy the requirement that they should all intersect at the point
+where the explosion occurred. To this it was at first replied that the
+perturbations of the asteroidal orbits, by the attractions of the major
+planets, would soon displace them in such a manner that they would
+cease to intersect. One of the first investigations undertaken by the
+late Prof. Simon Newcomb was directed to the solution of this question,
+and he arrived at the conclusion that the planetary perturbations could
+not explain the actual situation of the asteroidal orbits. But
+afterward it was pointed out that the difficulty could be avoided by
+supposing that not one but a series of explosions had produced the
+asteroids as they now are. After the primary disruption the fragments
+themselves, according to this suggestion, may have exploded, and then
+the resulting orbits would be as “tangled” as the heart could wish.
+This has so far rehabilitated the explosion theory that it has never
+been entirely abandoned, and the evidence which we have just cited of
+the probably abnormal shapes of Eros and other asteroids has lately
+given it renewed life. It is a subject that needs a thorough
+rediscussion.
+
+We must not fail to mention, however, that there is a rival hypothesis
+which commends itself to many astronomers—_viz.,_ that the asteroids
+were formed out of a relatively scant ring of matter, situated between
+Mars and Jupiter and resembling in composition the immensely more
+massive rings from which, according to Laplace’s hypothesis, the
+planets were born. It is held by the supporters of this theory that the
+attraction of the giant Jupiter was sufficient to prevent the small,
+nebulous ring that gave birth to the asteroids from condensing like the
+others into a single planet.
+
+But if we accept the explosion theory, with its corollary that minor
+explosions followed the principal one, we have still an unanswered
+question before us: What caused the explosions? The idea of _a world
+blowing up_ is too Titanic to be shocking; it rather amuses the
+imagination than seriously impresses it; in a word, it seems
+essentially chimerical. We can by no appeal to experience form a mental
+picture of such an occurrence. Even the moon did not blow up when it
+was wrecked by volcanoes. The explosive nebulæ and new stars are far
+away in space, and suggest no connection with such a catastrophe as the
+bursting of a planet into hundreds of pieces. We cannot conceive of a
+great globe thousands of miles in diameter resembling a pellet of
+gunpowder only awaiting the touch of a match to cause its sudden
+disruption. Somehow the thought of human agency obtrudes itself in
+connection with the word “explosion,” and we smile at the idea that
+giant powder or nitro-glycerine could blow up a planet. Yet it would
+only need _enough_ of them to do it.
+
+After all, we may deceive ourselves in thinking, as we are apt to do,
+that explosive energies lock themselves up only in small masses of
+matter. There are many causes producing explosions in nature, every
+volcanic eruption manifests the activity of some of them. Think of the
+giant power of confined steam; if enough steam could be suddenly
+generated in the center of the earth by a downpour of all the waters of
+the oceans, what might not the consequences be for our globe? In a
+smaller globe, and it has never been estimated that the original
+asteroid was even as large as the moon, such a catastrophe would,
+perhaps, be more easily conceivable; but since we are compelled in this
+case to assume that there was a series of successive explosions, steam
+would hardly answer the purpose; it would be more reasonable to suppose
+that the cause of the explosion was some kind of chemical reaction, or
+something affecting the atoms composing the exploding body. Here Dr
+Gustav Le Bon comes to our aid with a most startling suggestion, based
+on his theory of the dissipation of intra-atomic energy. It will be
+best to quote him at some length from his book on _The Evolution of
+Forces._
+
+“It does not seem at first sight,” says Doctor Le Bon,
+
+
+very comprehensible that worlds which appear more and more stable as
+they cool could become so unstable as to afterward dissociate entirely.
+To explain this phenomenon, we will inquire whether astronomical
+observations do not allow us to witness this dissociation.
+
+We know that the stability of a body in motion, such as a top or a
+bicycle, ceases to be possible when its velocity of rotation descends
+below a certain limit. Once this limit is reached it loses its
+stability and falls to the ground. Prof. J. J. Thomson even interprets
+radio-activity in this manner, and points out that when the speed of
+the elements composing the atoms descends below a certain limit they
+become unstable and tend to lose their equilibria. There would result
+from this a commencement of dissociation, with diminution of their
+potential energy and a corresponding increase of their kinetic energy
+sufficient to launch into space the products of intra-atomic
+disintegration.
+
+It must not be forgotten that the atom being an enormous reservoir of
+energy is by this very fact comparable with explosive bodies. These
+last remain inert so long as their internal equilibria are undisturbed.
+So soon as some cause or other modifies these, they explode and smash
+everything around them after being themselves broken to pieces.
+
+Atoms, therefore, which grow old in consequence of the diminution of a
+part of their intra-atomic energy gradually lose their stability. A
+moment, then, arrives when this stability is so weak that the matter
+disappears by a sort of explosion more or less rapid. The bodies of the
+radium group offer an image of this phenomenon—a rather faint image,
+however, because the atoms of this body have only reached a period of
+instability when the dissociation is rather slow. It probably precedes
+another and more rapid period of dissociation capable of producing
+their final explosion. Bodies such as radium, thorium, etc., represent,
+no doubt, a state of old age at which all bodies must some day arrive,
+and which they already begin to manifest in our universe, since all
+matter is slightly radio-active. It would suffice for the dissociation
+to be fairly general and fairly rapid for an explosion to occur in a
+world where it was manifested.
+
+These theoretical considerations find a solid support in the sudden
+appearances and disappearances of stars. The explosions of a world
+which produce them reveal to us, perhaps, how the universes perish when
+they become old.
+
+As astronomical observations show the relative frequency of these rapid
+destructions, we may ask ourselves whether the end of a universe by a
+sudden explosion after a long period of old age does not represent its
+most general ending.
+
+
+Here, perhaps, it will be well to stop, since, entrancing as the
+subject may be, we know very little about it, and Doctor Le Bon’s
+theory affords a limitless field for the reader’s imagination.
+
+
+
+
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+<div style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Title: Curiosities of the Sky</div>
+<div style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Author: Garrett P. Serviss</div>
+<div style='display:block; margin:1em 0'>Release Date: January 6, 2003 [eBook #6630]<br />
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+<div style='display:block; margin-left:2em; text-indent:-2em'>Produced by: William McClain</div>
+<div style='margin-top:2em; margin-bottom:4em'>*** START OF THE PROJECT GUTENBERG EBOOK CURIOSITIES OF THE SKY ***</div>
+
+<div class="fig" style="width:55%;">
+<img src="images/cover.jpg" style="width:100%;" alt="[Illustration]" />
+</div>
+
+<h1>Curiosities of the Sky</h1>
+
+<h2 class="no-break">by Garrett P. Serviss</h2>
+
+<hr />
+
+<div class="chapter">
+
+<h2>Contents</h2>
+
+<table summary="" style="">
+
+<tr>
+<td> <a href="#pref01">PREFACE</a></td>
+</tr>
+
+<tr>
+<td> <a href="#chap00">CURIOSITIES OF THE SKY</a></td>
+</tr>
+
+<tr>
+<td> <a href="#chap01">I. The Windows of Absolute Night</a></td>
+</tr>
+
+<tr>
+<td> <a href="#chap02">II. Star-Clouds, Star-Clusters, and Star-Streams</a></td>
+</tr>
+
+<tr>
+<td> <a href="#chap03">III. Stellar Migrations</a></td>
+</tr>
+
+<tr>
+<td> <a href="#chap04">IV. The Passing of the Constellations</a></td>
+</tr>
+
+<tr>
+<td> <a href="#chap05">V. Conflagrations in the Heavens</a></td>
+</tr>
+
+<tr>
+<td> <a href="#chap06">VI. Explosive and Whirling Nebulæ</a></td>
+</tr>
+
+<tr>
+<td> <a href="#chap07">VII. The Banners of the Sun</a></td>
+</tr>
+
+<tr>
+<td> <a href="#chap08">VIII. The Zodiacal Light Mystery</a></td>
+</tr>
+
+<tr>
+<td> <a href="#chap09">IX. Marvels of the Aurora</a></td>
+</tr>
+
+<tr>
+<td> <a href="#chap10">X. Strange Adventures of Comets</a></td>
+</tr>
+
+<tr>
+<td> <a href="#chap11">XI. Meteors, Fire-Balls, and Meteorites</a></td>
+</tr>
+
+<tr>
+<td> <a href="#chap12">XII. The Wrecking of the Moon</a></td>
+</tr>
+
+<tr>
+<td> <a href="#chap13">XIII. The Great Mars Problem</a></td>
+</tr>
+
+<tr>
+<td> <a href="#chap14">XIV. The Riddle of the Asteroids</a></td>
+</tr>
+
+</table>
+
+<div class="fig" style="width:100%;">
+<a name="illus01"></a>
+<a href="images/image01.jpg">
+<img src="images/image01.jpg" width="376" height="600" alt="Illustration:" /></a>
+<p class="caption">Great spiral nebula in Andromeda</p>
+</div>
+
+</div><!--end chapter-->
+
+<div class="chapter">
+
+<h2>Illustrations</h2>
+
+<table summary="" style="">
+
+<tr>
+<td> <a href="#illus01">Great spiral nebula in Andromeda</a></td>
+</tr>
+
+<tr>
+<td> <a href="#illus02">The Milky Way</a></td>
+</tr>
+
+<tr>
+<td> <a href="#illus03">Star-cluster in Hercules</a></td>
+</tr>
+
+<tr>
+<td> <a href="#illus04">Great southern star-cluster, Omega Centauri</a></td>
+</tr>
+
+<tr>
+<td> <a href="#illus05">The Pleiades</a></td>
+</tr>
+
+<tr>
+<td> <a href="#illus06">The “Great Dipper”</a></td>
+</tr>
+
+<tr>
+<td> <a href="#illus07">Cassiopeia</a></td>
+</tr>
+
+<tr>
+<td> <a href="#illus08">The “Northern Crown”</a></td>
+</tr>
+
+<tr>
+<td> <a href="#illus09">The “Southern Cross”</a></td>
+</tr>
+
+<tr>
+<td> <a href="#illus10">Chart showing location of Tycho’s star, 1572,  and Nova Persei of 1901</a></td>
+</tr>
+
+<tr>
+<td> <a href="#illus11">Nova Persei, with its nebular rings</a></td>
+</tr>
+
+<tr>
+<td> <a href="#illus12">Lord Rosse’s nebula</a></td>
+</tr>
+
+<tr>
+<td> <a href="#illus13">Wonderful spiral in triangulum</a></td>
+</tr>
+
+<tr>
+<td> <a href="#illus14">Spiral in Ursa Major</a></td>
+</tr>
+
+<tr>
+<td> <a href="#illus15">Nebula in Cetus</a></td>
+</tr>
+
+<tr>
+<td> <a href="#illus16">The Orion nebula</a></td>
+</tr>
+
+<tr>
+<td> <a href="#illus17">The Corona</a></td>
+</tr>
+
+<tr>
+<td> <a href="#illus18">A solar “prominence”</a></td>
+</tr>
+
+<tr>
+<td> <a href="#illus19">Auroral beams seen in England</a></td>
+</tr>
+
+<tr>
+<td> <a href="#illus20">Auroral arches of an eliptic form seen in the Arctic regions</a></td>
+</tr>
+
+<tr>
+<td> <a href="#illus21">Auroral curtain seen in Scandinavia</a></td>
+</tr>
+
+<tr>
+<td> <a href="#illus22">Auroral arches seen in Scandinavia</a></td>
+</tr>
+
+<tr>
+<td> <a href="#illus23">Swift’s comet</a></td>
+</tr>
+
+<tr>
+<td> <a href="#illus24">Daniels’ comet</a></td>
+</tr>
+
+<tr>
+<td> <a href="#illus25">Brooks’ comet</a></td>
+</tr>
+
+<tr>
+<td> <a href="#illus26">Curious forms of meteorite trains</a></td>
+</tr>
+
+<tr>
+<td> <a href="#illus27">Section of the atmosphere up to 100 kilometers</a></td>
+</tr>
+
+<tr>
+<td> <a href="#illus28">A meteor photographed in flight</a></td>
+</tr>
+
+<tr>
+<td> <a href="#illus29">Looking across Coon Butte crater from northern rim</a></td>
+</tr>
+
+<tr>
+<td> <a href="#illus30">Trail on south side, Coon Butte crater</a></td>
+</tr>
+
+<tr>
+<td> <a href="#illus31">The craters Clavius, Longomontanus, Tycho, etc.</a></td>
+</tr>
+
+<tr>
+<td> <a href="#illus32">Western part of the Mare Serenitatis</a></td>
+</tr>
+
+<tr>
+<td> <a href="#illus33">Mare Tranquilitatis and surroundings</a></td>
+</tr>
+
+<tr>
+<td> <a href="#illus34">Lunar craters Theophilus and surrounding region</a></td>
+</tr>
+
+<tr>
+<td> <a href="#illus35">Mare Crisium</a></td>
+</tr>
+
+<tr>
+<td> <a href="#illus36">Schiaparelli’s chart of Mars, showing the so-called system of canals</a></td>
+</tr>
+
+</table>
+
+</div><!--end chapter-->
+
+<div class="chapter">
+
+<h2><a name="pref01"></a>PREFACE</h2>
+
+<p>
+What Froude says of history is true also of astronomy: it is the most
+impressive where it transcends explanation. It is not the mathematics of
+astronomy, but the wonder and the mystery that seize upon the imagination. The
+calculation of an eclipse owes all its prestige to the sublimity of its data;
+the operation, in itself, requires no more mental effort than the preparation
+of a railway time-table.
+</p>
+
+<p>
+The dominion which astronomy has always held over the minds of men is akin to
+that of poetry; when the former becomes merely instructive and the latter
+purely didactic, both lose their power over the imagination. Astronomy is known
+as the oldest of the sciences, and it will be the longest-lived because it will
+always have arcana that have not been penetrated.
+</p>
+
+<p>
+Some of the things described in this book are little known to the average
+reader, while others are well known; but all possess the fascination of
+whatever is strange, marvelous, obscure, or mysterious—magnified, in this case,
+by the portentous scale of the phenomena.
+</p>
+
+<p>
+The idea of the author is to tell about these things in plain language, but
+with as much scientific accuracy as plain language will permit, showing the
+wonder that is in them without getting away from the facts. Most of them have
+hitherto been discussed only in technical form, and in treatises that the
+general public seldom sees and never reads.
+</p>
+
+<p>
+Among the topics touched upon are:
+</p>
+
+<ul> <li>The strange unfixedness of the “fixed stars,” the vast migrations of
+the suns and worlds constituting the universe.</li>
+
+<li>The slow passing out of existence of those collocations of stars which for
+thousands of years have formed famous “constellations,” preserving the memory
+of mythological heroes and heroines, and perhaps of otherwise unrecorded
+history.</li>
+
+<li>The tendency of stars to assemble in immense clouds, swarms, and
+clusters.</li>
+
+<li>The existence in some of the richest regions of the universe of absolutely
+black, starless gaps, deeps, or holes, as if one were looking out of a window
+into the murkiest night.</li>
+
+<li>The marvelous phenomena of new, or temporary, stars, which appear as
+suddenly as conflagrations, and often turn into something else as eccentric as
+themselves.</li>
+
+<li>The amazing forms of the “whirlpool,” “spiral,” “pinwheel,” and “lace,” or
+“tress,” nebulæ.</li>
+
+<li>The strange surroundings of the sun, only seen in particular circumstances,
+but evidently playing a constant part in the daily phenomena of the solar
+system.</li>
+
+<li>The mystery of the Zodiacal Light and the Gegenschein.</li>
+
+<li>The extraordinary transformations undergone by comets and their tails.</li>
+
+<li>The prodigies of meteorites and masses of stone and metal fallen from the
+sky.</li>
+
+<li>The cataclysms that have wrecked the moon.</li>
+
+<li>The problem of life and intelligence on the planet Mars.</li>
+
+<li>The problematical origin and fate of the asteroids.</li>
+
+<li>The strange phenomena of the auroral lights.</li> </ul>
+
+<p>
+An attempt has been made to develop these topics in an orderly way, showing
+their connection, so that the reader may obtain a broad general view of the
+chief mysteries and problems of astronomy, and an idea of the immense field of
+discovery which still lies, almost unexplored, before it.
+</p>
+
+</div><!--end chapter-->
+
+<div class="chapter">
+
+<h2><a name="chap00"></a>CURIOSITIES OF THE SKY</h2>
+
+</div><!--end chapter-->
+
+<div class="chapter">
+
+<h2><a name="chap01"></a>I<br/>
+The Windows of Absolute Night</h2>
+
+<p>
+To most minds mystery is more fascinating than science. But when science itself
+leads straight up to the borders of mystery and there comes to a dead stop,
+saying, “At present I can no longer see my way,” the force of the charm is
+redoubled. On the other hand, the illimitable is no less potent in mystery than
+the invisible, whence the dramatic effect of Keats’ “stout Cortez” staring at
+the boundless Pacific while all his men look at each other with a wild surmise,
+“silent upon a peak in Darien.” It is with similar feelings that the astronomer
+regards certain places where from the peaks of the universe his vision seems to
+range out into endless empty space. He sees there the shore of his little
+isthmus, and, beyond, unexplored immensity.
+</p>
+
+<p>
+The name, “coal-sacks,” given to these strange voids is hardly descriptive.
+Rather they produce upon the mind the effect of blank windows in a lonely house
+on a pitch-dark night, which, when looked at from the brilliant interior,
+become appalling in their rayless murk. Infinity seems to acquire a new meaning
+in the presence of these black openings in the sky, for as one continues to
+gaze it loses its purely metaphysical quality and becomes a kind of entity,
+like the ocean. The observer is conscious that he can actually <i>see</i> the
+beginning of its ebon depths, in which the visible universe appears to float
+like an enchanted island, resplendent within with lights and life and gorgeous
+spectacles, and encircled with screens of crowded stars, but with its dazzling
+vistas ending at the fathomless sea of pure darkness which encloses all.
+</p>
+
+<p>
+The Galaxy, or Milky Way, surrounds the borders of our island in space like a
+stellar garland, and when openings appear in it they are, by contrast, far more
+impressive than the general darkness of the interstellar expanse seen in other
+directions. Yet even that expanse is not everywhere equally dark, for it
+contains gloomy deeps discernable with careful watching. Here, too, contrast
+plays an important part, though less striking than within the galactic region.
+Some of Sir William Herschel’s observations appear to indicate an association
+between these tenebrious spots and neighboring star clouds and nebulæ. It is an
+illuminating bit of astronomical history that when he was sweeping the then
+virgin heavens with his great telescopes he was accustomed to say to his sister
+who, note-book in hand, waited at his side to take down his words, fresh with
+the inspiration of discovery: “Prepare to write; the nebulæ are coming; here
+space is vacant.”
+</p>
+
+<p>
+The most famous of the “coal-sacks,” and the first to be brought to general
+attention before astronomers had awakened to the significance of such things,
+lies adjacent to the “Southern Cross,” and is truly an amazing phenomenon. It
+is not alone the conspicuousness of this celestial vacancy, opening suddenly in
+the midst of one of the richest parts of the Galaxy, that has given it its
+fame, but quite as much the superstitious awe with which it was regarded by the
+early explorers of the South Seas. To them, as well as to those who listened in
+rapt wonder to their tales, the “Coal-sack” seemed to possess some occult
+connection with the mystic “Cross.” In the eyes of the sailors it was not a
+vacancy so much as a sable reality in the sky, and as, shuddering, they stared
+at it, they piously crossed themselves. It was another of the magical wonders
+of the unknown South, and as such it formed the basis of many a “wild surmise”
+and many a sea-dog’s yarn. Scientific investigation has not diminished its
+prestige, and today no traveler in the southern hemisphere is indifferent to
+its fascinating strangeness, while some find it the most impressive spectacle
+of the antarctic heavens.
+</p>
+
+<p>
+All around, up to the very edge of the yawning gap, the sheen of the Milky Way
+is surpassingly glorious; but there, as if in obedience to an almighty edict,
+everything vanishes. A single faint star is visible within the opening,
+producing a curious effect upon the sensitive spectator, like the sight of a
+tiny islet in the midst of a black, motionless, waveless tarn. The dimensions
+of the lagoon of darkness, which is oval or pear-shaped, are eight degrees by
+five, so that it occupies a space in the sky about one hundred and thirty times
+greater than the area of the full moon. It attracts attention as soon as the
+eye is directed toward the quarter where it exists, and by virtue of the rarity
+of such phenomena it appears a far greater wonder than the drifts of stars that
+are heaped around it. Now that observatories are multiplying in the southern
+hemisphere, the great austral “Coal-sack” will, no doubt, receive attention
+proportioned to its importance as one of the most significant features of the
+sky. Already at the Sydney Observatory photographs have shown that the southern
+portion of this Dead Sea of Space is not quite “bottomless,” although its
+northern part defies the longest sounding lines of the astronomer.
+</p>
+
+<p>
+There is a similar, but less perfect, “coal-sack” in the northern hemisphere,
+in the constellation of “The Swan,” which, strange to say, also contains a
+well-marked figure of a cross outlined by stars. This gap lies near the top of
+the cross-shaped figure. It is best seen by averted vision, which brings out
+the contrast with the Milky Way, which is quite brilliant around it. It does
+not, however, exercise the same weird attraction upon the eye as the southern
+“Coal-sack,” for instead of looking like an absolute void in the sky, it rather
+appears as if a canopy of dark gauze had been drawn over the stars. We shall
+see the possible significance of this appearance later.
+</p>
+
+<div class="fig" style="width:100%;">
+<a name="illus02"></a>
+<a href="images/image02.jpg">
+<img src="images/image02.jpg" width="481" height="600" alt="Illustration:" /></a>
+<p class="caption">The Milky Way. Region near M.S. Photographed by Professor
+Barnard</p>
+</div>
+
+<p>
+Just above the southern horizon of our northern middle latitudes, in summer,
+where the Milky Way breaks up into vast sheets of nebulous luminosity, lying
+over and between the constellations Scorpio and Sagittarius, there is a
+remarkable assemblage of “coal-sacks,” though none is of great size. One of
+them, near a conspicuous star-cluster in Scorpio, M80, is interesting for
+having been the first of these strange objects noted by Herschel. Probably it
+was its nearness to M80 which suggested to his mind the apparent connection of
+such vacancies with star-clusters which we have already mentioned.
+</p>
+
+<p>
+But the most marvelous of the “coal-sacks” are those that have been found by
+photography in Sagittarius. One of Barnard’s earliest and most excellent
+photographs includes two of them, both in the star-cluster M8. The larger,
+which is roughly rectangular in outline, contains one little star, and its
+smaller neighbor is lune-shaped—surely a most singular form for such an object.
+Both are associated with curious dark lanes running through the clustered stars
+like trails in the woods. Along the borders of these lanes the stars are ranked
+in parallel rows, and what may be called the bottoms of the lanes are not
+entirely dark, but pebbled with faint stellar points. One of them which skirts
+the two dark gaps and traverses the cluster along its greatest diameter is
+edged with lines of stars, recalling the alignment of the trees bordering a
+French highway. This <i>road of stars</i> cannot be less than many billions of
+miles in length!
+</p>
+
+<p>
+All about the cluster the bed of the Galaxy is strangely disturbed, and in
+places nearly denuded, as if its contents had been raked away to form the
+immense stack and the smaller accumulations of stars around it. The well-known
+“Trifid Nebula” is also included in the field of the photograph, which covers a
+truly marvelous region, so intricate in its mingling of nebulæ, star-clusters,
+star-swarms, star-streams, and dark vacancies that no description can do it
+justice. Yet, chaotic as it appears, there is an unmistakable suggestion of
+unity about it, impressing the beholder with the idea that all the different
+parts are in some way connected, and have not been fortuitously thrown
+together. Miss Agnes M. Clerke made the striking remark that the dusky lanes in
+M8 are exemplified on the largest scale in the great rift dividing the Milky
+Way, from Cygnus in the northern hemisphere all the way to the “Cross” in the
+southern. Similar lanes are found in many other clusters, and they are
+generally associated with flanking rows of stars, resembling in their
+arrangement the thick-set houses and villas along the roadways that traverse
+the approaches to a great city.
+</p>
+
+<p>
+But to return to the black gaps. Are they really windows in the star-walls of
+the universe? Some of them look rather as if they had been made by a shell
+fired through a luminous target, allowing the eye to range through the hole
+into the void space beyond. If science is discretely silent about these things,
+what can the more venturesome and less responsible imagination suggest? Would a
+huge “runaway sun,” like Arcturus, for instance, make such an opening if it
+should pass like a projectile through the Milky Way? It is at least a
+stimulating inquiry. Being probably many thousands of times more massive than
+the galactic stars, such a stellar missile would not be stopped by them, though
+its direction of flight might be altered. It would drag the small stars lying
+close to its course out of their spheres, but the ultimate tendency of its
+attraction would be to sweep them round in its wake, thus producing rather a
+star-swarm than a vacancy. Those that were very close to it might be swept away
+in its rush and become its satellites, careering away with it in its flight
+into outer space; but those that were farther off, and they would, of course,
+greatly outnumber the nearer ones, would tend inward from all sides toward the
+line of flight, as dust and leaves collect behind a speeding motor (though the
+forces operating would be different), and would fill up the hole, if hole it
+were. A swarm thus collected should be rounded in outline and bordered with a
+relatively barren ring from which the stars had been “sucked” away. In a
+general sense the M8 cluster answers to this description, but even if we
+undertook to account for its existence by a supposition like the above, the
+black gaps would remain unexplained, unless one could make a further draft on
+the imagination and suggest that the stars had been thrown into a vast eddy, or
+system of eddies, whose vortices appear as dark holes. Only a maelstrom-like
+motion could keep such a funnel open, for without regard to the impulse derived
+from the projectile, the proper motions of the stars themselves would tend to
+fill it. Perhaps some other cause of the whirling motion may be found. As we
+shall see when we come to the spiral nebulæ, gyratory movements are exceedingly
+prevalent throughout the universe, and the structure of the Milky Way is
+everywhere suggestive of them. But this is hazardous sport even for the
+imagination—to play with <i>suns</i> as if they were but thistle-down in the
+wind or corks in a mill-race.
+</p>
+
+<p>
+Another question arises: What is the thickness of the hedge of stars through
+which the holes penetrate? Is the depth of the openings proportionate to their
+width? In other words, is the Milky Way round in section like a rope, or flat
+and thin like a ribbon? The answer is not obvious, for we have little or no
+information concerning the relative distances of the faint galactic stars. It
+would be easier, certainly, to conceive of openings in a thin belt than in a
+massive ring, for in the first case they would resemble mere rifts and breaks,
+while in the second they would be like wells or bore-holes. Then, too, the fact
+that the Milky Way is not a <i>continuous</i> body but is made up of stars
+whose actual distances apart is great, offers another quandary; persistent and
+sharply bordered apertures in such an assemblage are <i>a priori</i> as
+improbable, if not impossible, as straight, narrow holes running through a
+swarm of bees.
+</p>
+
+<p>
+The difficulty of these questions indicates one of the reasons why it has been
+suggested that the seeming gaps, or many of them, are not openings at all, but
+opaque screens cutting off the light from stars behind them. That this is quite
+possible in some cases is shown by Barnard’s later photographs, particularly
+those of the singular region around the star Rho Ophiuchi. Here are to be seen
+somber lanes and patches, apparently forming a connected system which covers an
+immense space, and which their discoverer thinks may constitute a “dark
+nebula.” This seems at first a startling suggestion; but, after all, why should
+their not be dark nebulæ as well as visible ones? In truth, it has troubled
+some astronomers to explain the luminosity of the bright nebulæ, since it is
+not to be supposed that matter in so diffuse a state can be incandescent
+through heat, and phosphorescent light is in itself a mystery. The supposition
+is also in accord with what we know of the existence of dark solid bodies in
+space. Many bright stars are accompanied by obscure companions, sometimes as
+massive as themselves; the planets are non-luminous; the same is true of
+meteors before they plunge into the atmosphere and become heated by friction;
+and many plausible reasons have been found for believing that space contains as
+many obscure as shining bodies of great size. It is not so difficult, after
+all, then, to believe that there are immense collections of shadowy gases and
+meteoric dust whose presence is only manifested when they intercept the light
+coming from shining bodies behind them.
+</p>
+
+<p>
+This would account for the apparent extinguishment of light in open space,
+which is indicated by the falling off in relative number of telescopic stars
+below the tenth magnitude. Even as things are, the amount of light coming to us
+from stars too faint to be seen with the naked eye is so great that the
+statement of it generally surprises persons who are unfamiliar with the inner
+facts of astronomy. It has been calculated that on a clear night the total
+starlight from the entire celestial sphere amounts to one-sixtieth of the light
+of the full moon; but of this less than one-twenty-fifth is due to stars
+separately distinguished by the eye. If there were no obscuring medium in
+space, it is probable that the amount of starlight would be noticeably and
+perhaps enormously increased.
+</p>
+
+<p>
+But while it seems certain that some of the obscure spots in the Milky Way are
+due to the presence of “dark nebulæ,” or concealing veils of one kind or
+another, it is equally certain that there are many which are true apertures,
+however they may have been formed, and by whatever forces they may be
+maintained. These, then, are veritable windows of the Galaxy, and when looking
+out of them one is face to face with the great mystery of infinite space.
+<i>There</i> the known universe visibly ends, but manifestly space itself does
+not end there. It is not within the power of thought to conceive an end to
+space, for the instant we think of a terminal point or line the mind leaps
+forward to the <i>beyond.</i> There must be space outside as well as inside.
+Eternity of time and infinity of space are ideas that the intellect cannot
+fully grasp, but neither can it grasp the idea of a limitation to either space
+or time. The metaphysical conceptions of hypergeometry, or fourth-dimensional
+space, do not aid us.
+</p>
+
+<p>
+Having, then, discovered that the universe is a thing <i>contained</i> in
+something indefinitely greater than itself; having looked out of its windows
+and found only the gloom of starless night outside—what conclusions are we to
+draw concerning the beyond? It <i>seems</i> as empty as a vacuum, but is it
+really so? If it be, then our universe is a single atom astray in the infinite;
+it is the only island in an ocean without shores; it is the one oasis in an
+illimitable desert. Then the Milky Way, with its wide-flung garland of stars,
+is afloat like a tiny smoke-wreath amid a horror of immeasurable vacancy, or it
+is an evanescent and solitary ring of sparkling froth cast up for a moment on
+the viewless billows of immensity. From such conclusions the mind instinctively
+shrinks. It prefers to think that there is <i>something</i> beyond, though we
+cannot see it. Even the universe could not bear to be alone—a Crusoe lost in
+the Cosmos! As the inhabitants of the most elegant château, with its gardens,
+parks, and crowds of attendants, would die of loneliness if they did not know
+that they have neighbors, though not seen, and that a living world of
+indefinite extent surrounds them, so we, when we perceive that the universe has
+limits, wish to feel that it is not solitary; that beyond the hedges and the
+hills there are other centers of life and activity. Could anything be more
+terrible than the thought of an <i>isolated universe?</i> The greater the
+being, the greater the aversion to seclusion. Only the infinite satisfies; in
+that alone the mind finds rest.
+</p>
+
+<p>
+We are driven, then, to believe that the universal night which envelopes us is
+not tenantless; that as we stare out of the star-framed windows of the Galaxy
+and see nothing but uniform blackness, the fault is with our eyes or is due to
+an obscuring medium. Since <i>our</i> universe is limited in extent, there must
+be <i>other</i> universes beyond it on all sides. Perhaps if we could carry our
+telescopes to the verge of the great “Coal-sack” near the “Cross,” being then
+on the frontier of our starry system, we could discern, sparkling afar off in
+the vast night, some of the outer galaxies. They may be grander than ours, just
+as many of the suns surrounding us are immensely greater than ours. If we could
+take our stand somewhere in the midst of immensity and, with vision of infinite
+reach, look about us, we should perhaps see a countless number of stellar
+systems, amid which ours would be unnoticeable, like a single star among the
+multitude glittering in the terrestial sky on a clear night. Some might be in
+the form of a wreath, like our own; some might be globular, like the great
+star-clusters in Hercules and Centaurus; some might be glittering circles, or
+disks, or rings within rings. If we could enter them we should probably find a
+vast variety of composition, including elements unknown to terrestrial
+chemistry; for while the <i>visible</i> universe appears to contain few if any
+substances not existing on the earth or in the sun, we have no warrant to
+assume that others may not exist in infinite space.
+</p>
+
+<p>
+And how as to gravitation? We do not <i>know</i> that gravitation acts beyond
+the visible universe, but it is reasonable to suppose that it does. At any
+rate, if we let go <i>its</i> sustaining hand we are lost, and can only wander
+hopelessly in our speculations, like children astray. If the empire of
+gravitation is infinite, then the various outer systems must have <i>some,</i>
+though measuring by our standards an imperceptible, attractive influence upon
+each other, for gravitation never lets go its hold, however great the space
+over which it is required to act. Just as the stars about us are all in motion,
+so the starry systems beyond our sight may be in motion, and our system as a
+whole may be moving in concert with them. If this be so, then after
+interminable ages the aspect of the entire system of systems must change, its
+various members assuming new positions with respect to one another. In the
+course of time we may even suppose that our universe will approach relatively
+close to one of the others; and then, if men are yet living on the earth, they
+may glimpse through the openings which reveal nothing to us now, the lights of
+another nearing star system, like the signals of a strange squadron, bringing
+them the assurance (which can be but an inference at present) that the ocean of
+space has other argosies venturing on its limitless expanse.
+</p>
+
+<p>
+There remains the question of the luminiferous ether by whose agency the waves
+of light are borne through space. The ether is as mysterious as gravitation.
+With regard to ether we only infer its existence from the effects which we
+ascribe to it. Evidently the ether must extend as far as the most distant
+visible stars. But does it continue on indefinitely in outer space? If it does,
+then the invisibility of the other systems must be due to their distance
+diminishing the quantity of light that comes from them below the limit of
+perceptibility, or to the interposition of absorbing media; if it does not,
+then the reason why we cannot see them is owing to the absence of a means of
+conveyance for the light waves, as the lack of an interplanetary atmosphere
+prevents us from hearing the thunder of sun-spots. (It is interesting to recall
+that Mr Edison was once credited with the intention to construct a gigantic
+microphone which should render the roar of sun-spots audible by transforming
+the electric vibrations into sound-waves). On this supposition each starry
+system would be enveloped in its own globule of ether, and no light could cross
+from one to another. But the probability is that both the ether and gravitation
+are ubiquitous, and that all the stellar systems are immersed in the former
+like clouds of phosphorescent organisms in the sea.
+</p>
+
+<p>
+So astronomy carries the mind from height to greater height. Men were long in
+accepting the proofs of the relative insignificance of the earth; they were
+more quickly convinced of the comparative littleness of the solar system; and
+now the evidence assails their reason that what they had regarded as <i>the</i>
+universe is only one mote gleaming in the sunbeams of Infinity.
+</p>
+
+</div><!--end chapter-->
+
+<div class="chapter">
+
+<h2><a name="chap02"></a>II<br/>
+Star-Clouds, Star-Clusters, and Star-Streams</h2>
+
+<p>
+In the preceding chapter we have seen something of the strangely complicated
+structure of the Galaxy, or Milky Way. We now proceed to study more
+comprehensively that garlanded “Pathway of the Gods.”
+</p>
+
+<p>
+Judged by the eye alone, the Milky Way is one of the most delicately beautiful
+phenomena in the entire realm of nature—a shimmer of silvery gauze stretched
+across the sky; but studied in the light of its revelations, it is the most
+stupendous object presented to human ken. Let us consider, first, its
+appearance to ordinary vision. Its apparent position in the sky shifts
+according to the season. On a serene, cloudless summer evening, in the absence
+of the moon, whose light obscures it, one sees the Galaxy spanning the heavens
+from north to southeast of the zenith like a phosphorescent arch. In early
+spring it forms a similar but, upon the whole, less brilliant arch west of the
+zenith. Between spring and summer it lies like a long, faint, twilight band
+along the northern horizon. At the beginning of winter it again forms an arch,
+this time spanning the sky from east to west, a little north of the zenith.
+These are its positions as viewed from the mean latitude of the United States.
+Even the beginner in star-gazing does not have to watch it throughout the year
+in order to be convinced that it is, in reality, a great circle, extending
+entirely around the celestial sphere. We appear to be situated near its center,
+but its periphery is evidently far away in the depths of space.
+</p>
+
+<p>
+Although to the casual observer it seems but a delicate scarf of light,
+brighter in some places than in others, but hazy and indefinite at the best,
+such is not its appearance to those who study it with care. They perceive that
+it is an organic whole, though marvelously complex in detail. The telescope
+shows that it consists of stars too faint and small through excess of distance
+to be separately visible. Of the hundred million suns which some estimates have
+fixed as the probable population of the starry universe, the vast majority (at
+least thirty to one) are included in this strange belt of misty light. But they
+are not uniformly distributed in it; on the contrary, they are arrayed in
+clusters, knots, bunches, clouds, and streams. The appearance is somewhat as if
+the Galaxy consisted of innumerable swarms of silver-winged bees, more or less
+intermixed, some massed together, some crossing the paths of others, but all
+governed by a single purpose which leads them to encircle the region of space
+in which we are situated.
+</p>
+
+<p>
+From the beginning of the systematic study of the heavens, the fact has been
+recognized that the form of the Milky Way denotes the scheme of the sidereal
+system. At first it was thought that the shape of the system was that of a vast
+round disk, flat like a cheese, and filled with stars, our sun and his
+relatively few neighbors being placed near the center. According to this view,
+the galactic belt was an effect of perspective; for when looking in the
+direction of the plane of the disk, the eye ranged through an immense extension
+of stars which blended into a glimmering blur, surrounding us like a ring;
+while when looking out from the sides of the disk we saw but few stars, and in
+those directions the heavens appeared relatively blank. Finally it was
+recognized that this theory did not correspond with the observed appearances,
+and it became evident that the Milky Way was not a mere effect of perspective,
+but an actual band of enormously distant stars, forming a circle about the
+sphere, the central opening of the ring (containing many scattered stars) being
+many times broader than the width of the ring itself. Our sun is one of the
+scattered stars in the central opening.
+</p>
+
+<p>
+As already remarked, the ring of the Galaxy is very irregular, and in places it
+is partly broken. With its sinuous outline, its pendant sprays, its graceful
+and accordant curves, its bunching of masses, its occasional interstices, and
+the manifest order of a general plan governing the jumble of its details, it
+bears a remarkable resemblance to a garland—a fact which appears the more
+wonderful when we recall its composition. That an elm-tree should trace the
+lines of beauty with its leafy and pendulous branches does not surprise us; but
+we can only gaze with growing amazement when we behold <i>a hundred million
+suns imitating the form of a chaplet!</i> And then we have to remember that
+this form furnishes the ground-plan of the universe.
+</p>
+
+<p>
+As an indication of the extraordinary speculations to which the mystery of the
+Milky Way has given rise, a theory recently (1909) proposed by Prof. George C.
+Comstock may be mentioned. Starting with the data (first) that the number of
+stars increases as the Milky Way is approached, and reaches a maximum in its
+plane, while on the other hand the number of nebulæ is greatest outside the
+Milky Way and increases with distance from it, and (second) that the Milky Way,
+although a complete ring, is broad and diffuse on one side through one-half its
+course—that half alone containing nebulæ—and relatively narrow and well defined
+on the opposite side, the author of this singular speculation avers that these
+facts can best be explained by supposing that the invisible universe consists
+of two interpenetrating parts, one of which is a chaos of indefinite extent,
+strewn with stars and nebulous dust, and the other a long, broad but
+comparatively thin cluster of stars, including the sun as one of its central
+members. This flat star-cluster is conceived to be moving edgewise through the
+chaos, and, according to Professor Comstock, it acts after the manner of a
+snow-plough sweeping away the cosmic dust and piling it on either hand above
+and below the plane of the moving cluster. It thus forms a transparent rift,
+through which we see farther and command a view of more stars than through the
+intensified dust-clouds on either hand. This rift is the Milky Way. The dust
+thrown aside toward the poles of the Milky Way is the substance of the nebulæ
+which abound there. Ahead, where the front of the star-plough is clearing the
+way, the chaos is nearer at hand, and consequently there the rift subtends a
+broader angle, and is filled with primordial dust, which, having been annexed
+by the vanguard of the star-swarm, forms the nebulæ seen only in that part of
+the Milky Way. But behind, the rift appears narrow because there we look
+farther away between dust-clouds produced ages ago by the front of the plough,
+and no scattered dust remains in that part of the rift.
+</p>
+
+<p>
+In quoting an outline of this strikingly original theory the present writer
+should not be understood as assenting to it. That it appears bizarre is not, in
+itself, a reason for rejecting it, when we are dealing with so problematical
+and enigmatical a subject as the Milky Way; but the serious objection is that
+the theory does not sufficiently accord with the observed phenomena. There is
+too much evidence that the Milky Way is an organic system, however fantastic
+its form, to permit the belief that it can only be a rift in chaotic clouds. As
+with every organism, we find that its parts are more or less clearly repeated
+in its ensemble. Among all the strange things that the Milky Way contains there
+is nothing so extraordinary as itself. Every astronomer must many times have
+found himself marveling at it in those comparatively rare nights when it shows
+all its beauty and all its strangeness. In its great broken rifts, divisions,
+and spirals are found the gigantic prototypes of similar forms in its
+star-clouds and clusters. As we have said, it determines the general shape of
+the whole sidereal system. Some of the brightest stars in the sky appear to
+hang like jewels suspended at the ends of tassels dropped from the Galaxy.
+Among these pendants are the Pleiades and the Hyades. Orion, too, the “Mighty
+Hunter,” is caught in “a loop of light” thrown out from it. The majority of the
+great first-magnitude stars seem related to it, as if they formed an inner ring
+inclined at an angle of some twenty degrees to its plane. Many of the long
+curves that set off from it on both sides are accompanied by corresponding
+curves of lucid stars. In a word, it offers every appearance of structural
+connection with the entire starry system. That the universe should have assumed
+the form of a wreath is certainly a matter for astonishment; but it would have
+been still more astonishing if it had been a cube, a rhomboid, or a
+dodecahedron, for then we should have had to suppose that something resembling
+the forces that shape crystals had acted upon the stars, and the difficulty of
+explaining the universe by the laws of gravitation would have been increased.
+</p>
+
+<p>
+From the Milky Way as a whole we pass to the vast clouds, swarms, and clusters
+of stars of which it is made up. It may be, as some astronomers hold, that most
+of the galactic stars are much smaller than the sun, so that their faintness is
+not due entirely to the effect of distance. Still, their intrinsic brilliance
+attests their solar character, and considering their remoteness, which has been
+estimated at not less than ten thousand to twenty thousand light-years (a
+light-year is equal to nearly six thousand thousand million miles) their actual
+masses cannot be extremely small. The minutest of them are entitled to be
+regarded as real suns, and they vary enormously in magnitude. The effects of
+their attractions upon one another can only be inferred from their clustering,
+because their relative movements are not apparent on account of the brevity of
+the observations that we can make. But imagine a being for whom a million years
+would be but as a flitting moment; to him the Milky Way would appear in a state
+of ceaseless agitation—swirling with “a fury of whirlpool motion.”
+</p>
+
+<p>
+The cloud-like aspect of large parts of the Galaxy must always have attracted
+attention, even from naked-eye observers, but the true star-clouds were first
+satisfactorily represented in Barnard’s photographs. The resemblance to actual
+clouds is often startling. Some are close-packed and dense, like cumuli; some
+are wispy or mottled, like cirri. The rifts and modulations, as well as the
+general outlines, are the same as those of clouds of vapor or dust, and one
+notices also the characteristic thinning out at the edges. But we must beware
+of supposing that the component suns are thickly crowded as the particles
+forming an ordinary cloud. They <i>look,</i> indeed, as if they were matted
+together, because of the irradiation of light, but in reality millions and
+billions of miles separate each star from its neighbors. Nevertheless they form
+real assemblages, whose members are far more closely related to one another
+than is our sun to the stars around him, and if we were in the Milky Way the
+aspect of the nocturnal sky would be marvelously different from its present
+appearance.
+</p>
+
+<p>
+Stellar clouds are characteristic of the Galaxy and are not found beyond its
+borders, except in the “Magellanic Clouds” of the southern hemisphere, which
+resemble detached portions of the Milky Way. These singular objects form as
+striking a peculiarity of the austral heavens as does the great “Coal-sack”
+described in Chapter 1. But it is their isolation that makes them so
+remarkable, for their composition is essentially galactic, and if they were
+included within its boundaries they would not appear more wonderful than many
+other parts of the Milky Way. Placed where they are, they look like masses
+fallen from the great stellar arch. They are full of nebulæ and star-clusters,
+and show striking evidences of spiral movement.
+</p>
+
+<p>
+Star-swarms, which are also characteristic features of the Galaxy, differ from
+star-clouds very much in the way that their name would imply—<i>i.e.,</i> their
+component stars are so arranged, even when they are countless in number, that
+the idea of an exceedingly numerous assemblage rather than that of a cloud is
+impressed on the observer’s mind. In a star-swarm the separate members are
+distinguishable because they are either larger or nearer than the stars
+composing a “cloud.” A splendid example of a true star-swarm is furnished by
+Chi Persei, in that part of the Milky Way which runs between the constellations
+Perseus and Cassiopeia. This swarm is much coarser than many others, and can be
+seen by the naked eye. In a small telescope it appears double, as if the suns
+composing it had divided into two parties which keep on their way side by side,
+with some commingling of their members where the skirts of the two companies
+come in contact.
+</p>
+
+<p>
+Smaller than either star-clouds or star-swarms, and differing from both in
+their organization, are star-clusters. These, unlike the others, are found
+outside as well as inside the Milky Way, although they are more numerous inside
+its boundaries than elsewhere. The term star-cluster is sometimes applied,
+though improperly, to assemblages which are rather groups, such, for instance,
+as the Pleiades. In their most characteristic aspect star-clusters are of a
+globular shape—globes of suns! A famous example of a globular star-cluster, but
+one not included in the Milky Way, is the “Great Cluster in Hercules.” This is
+barely visible to the naked eye, but a small telescope shows its character, and
+in a large one it presents a marvelous spectacle. Photographs of such clusters
+are, perhaps, less effective than those of star-clouds, because the central
+condensation of stars in them is so great that their light becomes blended in
+an indistinguishable blur. The beautiful effect of the incessant play of
+infinitesimal rays over the apparently compact surface of the cluster, as if it
+were a globe of the finest frosted silver shining in an electric beam, is also
+lost in a photograph. Still, even to the eye looking directly at the cluster
+through a powerful telescope, the central part of the wonderful congregation
+seems almost a solid mass in which the stars are packed like the ice crystals
+in a snowball.
+</p>
+
+<p>
+The same question rises to the lips of every observer: How can they possibly
+have been brought into such a situation? The marvel does not grow less when we
+know that, instead of being closely compacted, the stars of the cluster are
+probably separated by millions of miles; for we know that their distances apart
+are slight as compared with their remoteness from the Earth. Sir William
+Herschel estimated their number to be about fourteen thousand, but in fact they
+are uncountable. If we could view them from a point just within the edge of the
+assemblage, they would offer the appearance of a hollow hemisphere emblazoned
+with stars of astonishing brilliancy; the near-by ones unparalleled in splendor
+by any celestial object known to us, while the more distant ones would resemble
+ordinary stars. An inhabitant of the cluster would not know, except by a
+process of ratiocination, that he was dwelling in a globular assemblage of
+suns; only from a point far outside would their spherical arrangement become
+evident to the eye. Imagine fourteen-thousand fire-balloons with an approach to
+regularity in a spherical space—say, ten miles in diameter; there would be an
+average of less than thirty in every cubic mile, and it would be necessary to
+go to a considerable distance in order to see them as a globular aggregation;
+yet from a point sufficiently far away they would blend into a glowing ball.
+</p>
+
+<div class="fig" style="width:100%;">
+<a name="illus03"></a>
+<a href="images/image03.jpg">
+<img src="images/image03.jpg" width="409" height="600" alt="Illustration:" /></a>
+<p class="caption">Star-cluster in Hercules<br/>
+(Photographed with a two-foot reflector)</p>
+</div>
+
+<p>
+Photographs show even better than the best telescopic views that the great
+cluster is surrounded with a multitude of dispersed stars, suggestively arrayed
+in more or less curving lines, which radiate from the principle mass, with
+which their connection is manifest. These stars, situated outside the central
+sphere, look somewhat like vagrant bees buzzing round a dense swarm where the
+queen bee is sitting. Yet while there is so much to suggest the operation of
+central forces, bringing and keeping the members of the cluster together, the
+attentive observer is also impressed with the idea that the whole wonderful
+phenomenon may be <i>the result of explosion.</i> As soon as this thought
+seizes the mind, confirmation of it seems to be found in the appearance of the
+outlying stars, which could be as readily explained by the supposition that
+they have been blown apart as that they have flocked together toward a center.
+The probable fact that the stars constituting the cluster are very much smaller
+than our sun might be regarded as favoring the hypothesis of an explosion. Of
+their real size we know nothing, but, on the basis of an uncertain estimate of
+their parallax, it has been calculated that they may average forty-five
+thousand miles in diameter—something more than half the diameter of the planet
+Jupiter. Assuming the same mean density, fourteen thousand such stars might
+have been formed by the explosion of a body about twice the size of the sun.
+This recalls the theory of Olbers, which has never been altogether abandoned or
+disproved, that the Asteroids were formed by the explosion of a planet
+circulating between the orbits of Mars and Jupiter. The Asteroids, whatever
+their manner of origin, form a ring around the sun; but, of course, the
+explosion of a great independent body, not originally revolving about a
+superior center of gravitational force, would not result in the formation of a
+ring of small bodies, but rather of a dispersed mass of them. But back of any
+speculation of this kind lies the problem, at present insoluble: How could the
+explosion be produced? (See the question of explosions in Chapters 6 and 14).
+</p>
+
+<p>
+Then, on the other hand, we have the observation of Herschel, since abundantly
+confirmed, that space is unusually vacant in the immediate neighborhood of
+condensed star-clusters and nebulæ, which, as far as it goes, might be taken as
+an indication that the assembled stars had been drawn together by their mutual
+attractions, and that the tendency to aggregation is still bringing new members
+toward the cluster. But in that case there must have been an original
+condensation of stars at that point in space. This could probably have been
+produced by the coagulation of a great nebula into stellar nuclei, a process
+which seems now to be taking place in the Orion Nebula.
+</p>
+
+<div class="fig" style="width:100%;">
+<a name="illus04"></a>
+<a href="images/image04.jpg">
+<img src="images/image04.jpg" width="417" height="600" alt="Illustration:" /></a>
+<p class="caption">Great southern star-cluster, Omega Centauri</p>
+</div>
+
+<p>
+A yet more remarkable globular star-cluster exists in the southern hemisphere,
+Omega Centauri. In this case the central condensation of stars presents an
+almost uniform blaze of light. Like the Hercules cluster, that in Centaurus is
+surrounded with stars scattered over a broad field and showing an appearance of
+radial arrangement. In fact, except for its greater richness, Omega Centauri is
+an exact duplicate of its northern rival. Each appears to an imaginative
+spectator as a veritable “city of suns.” Mathematics shrinks from the task of
+disentangling the maze of motions in such an assemblage. It would seem that the
+chance of collisions is not to be neglected, and this idea finds a certain
+degree of confirmation in the appearance of “temporary stars” which have more
+than once blazed out in, or close by, globular star-clusters.
+</p>
+
+<p>
+This leads up to the notable fact, first established by Professor Bailey a few
+years ago, that such clusters are populous with variable stars. Omega Centauri
+and the Hercules cluster are especially remarkable in this respect. The
+variables found in them are all of short period and the changes of light show a
+noteworthy tendency to uniformity. The first thought is that these phenomena
+must be due to collisions among the crowded stars, but, if so, the encounters
+cannot be between the stars themselves, but probably between stars and meteor
+swarms revolving around them. Such periodic collisions might go on for ages
+without the meteors being exhausted by incorporation with the stars. This
+explanation appears all the more probable because one would naturally expect
+that flocks of meteors would abound in a close aggregation of stars. It is also
+consistent with Perrine’s discovery—that the globular star clusters are
+powdered with minute stars strewn thickly among the brighter ones.
+</p>
+
+<p>
+In speaking of Professor Comstock’s extraordinary theory of the Milky Way, the
+fact was mentioned that, broadly speaking, the nebulæ are less numerous in the
+galactic belt than in the comparatively open spaces on either side of it, but
+that they are, nevertheless, abundant in the broader half of the Milky Way
+which he designates as the front of the gigantic “plough” supposed to be
+forcing its way through the enveloping chaos. In and around the Sagittarius
+region the intermingling of nebulæ and galactic star clouds and clusters is
+particularly remarkable. That there is a causal connection no thoughtful person
+can doubt. We are unable to get away from the evidence that a nebula is like a
+seed-ground from which stars spring forth; or we may say that nebulæ resemble
+clouds in whose bosom raindrops are forming. The wonderful aspect of the
+admixtures of nebulæ and star-clusters in Sagittarius has been described in
+Chapter 1. We now come to a still more extraordinary phenomenon of this
+kind—the Pleiades nebulæ.
+</p>
+
+<div class="fig" style="width:100%;">
+<a name="illus05"></a>
+<a href="images/image05.jpg">
+<img src="images/image05.jpg" width="600" height="436" alt="Illustration:" /></a>
+<p class="caption">The Pleiades</p>
+</div>
+
+<p>
+The group of the Pleiades, although lying outside the main course of the
+Galaxy, is connected with it by a faint loop, and is the scene of the most
+remarkable association of stars and nebulous matter known in the visible
+universe. The naked eye is unaware of the existence of nebulæ in the Pleiades,
+or, at the best, merely suspects that there is something of the kind there; and
+even the most powerful telescopes are far from revealing the full wonder of the
+spectacle; but in photographs which have been exposed for many hours
+consecutively, in order to accumulate the impression of the actinic rays, the
+revelation is stunning. The principle stars are seen surrounded by, and, as it
+were, <i>drowned in,</i> dense nebulous clouds of an unparalleled kind. The
+forms assumed by these clouds seem at first sight inexplicable. They look like
+fleeces, or perhaps more like splashes and daubs of luminous paint dashed
+carelessly from a brush. But closer inspection shows that they are, to a large
+extent, <i>woven</i> out of innumerable threads of filmy texture, and there are
+many indications of spiral tendencies. Each of the bright stars of the
+group—Alcyone, Merope, Maia, Electra, Taygeta, Atlas—is the focus of a dense
+fog (totally invisible, remember, alike to the naked eye and to the telescope),
+and these particular stars are veiled from sight behind the strange mists.
+Running in all directions across the relatively open spaces are nebulous wisps
+and streaks of the most curious forms. On some of the nebular lines, which are
+either straight throughout, or if they change direction do so at an angle,
+little stars are strung like beads. In one case seven or eight stars are thus
+aligned, and, as if to emphasize their dependence upon the chain which connects
+them, when it makes a slight bend the file of stars turns the same way. Many
+other star rows in the group suggest by their arrangement that they, too, were
+once strung upon similar threads which have now disappeared, leaving the stars
+spaced along their ancient tracks. We seem forced to the conclusion that there
+was a time when the Pleiades were embedded in a vast nebula resembling that of
+Orion, and that the cloud has now become so rare by gradual condensation into
+stars that the merest trace of it remains, and this would probably have escaped
+detection but for the remarkable actinic power of the radiant matter of which
+it consists. The richness of many of these faint nebulous masses in
+ultra-violet radiations, which are those that specifically affect the
+photographic plate, is the cause of the marvelous revelatory power of celestial
+photography. So the veritable unseen universe, as distinguished from the
+“unseen universe” of metaphysical speculation, is shown to us.
+</p>
+
+<p>
+A different kind of association between stars and nebulæ is shown in some
+surprising photographic objects in the constellation Cygnus, where long, wispy
+nebulæ, billions of miles in length, some of them looking like tresses
+streaming in a breeze, lie amid fields of stars which seem related to them. But
+the relation is of a most singular kind, for notwithstanding the delicate
+structure of the long nebulæ they appear to act as barriers, causing the stars
+to heap themselves on one side. The stars are two, three, or four times as
+numerous on one side of the nebulæ as on the other. These nebulæ, as far as
+appearance goes, might be likened to rail fences, or thin hedges, against which
+the wind is driving drifts of powdery snow, which, while scattered plentifully
+all around, tends to bank itself on the leeward side of the obstruction. The
+imagination is at a loss to account for these extraordinary phenomena; yet
+there they are, faithfully giving us their images whenever the photographic
+plate is exposed to their radiations.
+</p>
+
+<p>
+Thus the more we see of the universe with improved methods of observation, and
+the more we invent aids to human senses, each enabling us to penetrate a little
+deeper into the unseen, the greater becomes the mystery. The telescope carried
+us far, photography is carrying us still farther; but what as yet unimagined
+instrument will take us to the bottom, the top, and the end? And then, what
+hitherto untried power of thought will enable us to comprehend the meaning of
+it all?
+</p>
+
+</div><!--end chapter-->
+
+<div class="chapter">
+
+<h2><a name="chap03"></a>III<br/>
+Stellar Migrations</h2>
+
+<p>
+To the untrained eye the stars and the planets are not distinguishable. It is
+customary to call them all alike “stars.” But since the planets more or less
+rapidly change their places in the sky, in consequence of their revolution
+about the sun, while the stars proper seem to remain always in the same
+relative positions, the latter are spoken of as “fixed stars.” In the
+beginnings of astronomy it was not known that the “fixed stars” had any motion
+independent of their apparent annual revolution with the whole sky about the
+earth as a seeming center. Now, however, we know that the term “fixed stars” is
+paradoxical, for there is not a single really fixed object in the whole
+celestial sphere. The apparent fixity in the positions of the stars is due to
+their immense distance, combined with the shortness of the time during which we
+are able to observe them. It is like viewing the plume of smoke issuing from a
+steamer, hull down, at sea: if one does not continue to watch it for a long
+time it appears to be motionless, although in reality it may be traveling at
+great speed across the line of sight. Even the planets seem fixed in position
+if one watches them for a single night only, and the more distant ones do not
+sensibly change their places, except after many nights of observation. Neptune,
+for instance, moves but little more than two degrees in the course of an entire
+year, and in a month its change of place is only about one-third of the
+diameter of the full moon.
+</p>
+
+<p>
+Yet, fixed as they seem, the stars are actually moving with a speed in
+comparison with which, in some cases, the planets might almost be said to stand
+fast in their tracks. Jupiter’s speed in his orbit is about eight miles per
+second, Neptune’s is less than three and one-half miles, and the earth’s is
+about eighteen and one-half miles; while there are “fixed stars” which move two
+hundred or three hundred miles per second. They do not all, however, move with
+so great a velocity, for some appear to travel no faster than the planets. But
+in all cases, notwithstanding their real speed, long-continued and exceedingly
+careful observations are required to demonstrate that they are moving at all.
+No more overwhelming impression of the frightful depths of space in which the
+stars are buried can be obtained than by reflecting upon the fact that a star
+whose actual motion across the line of sight amounts to two hundred miles per
+second does not change its apparent place in the sky, in the course of a
+thousand years, sufficiently to be noticed by the casual observer of the
+heavens!
+</p>
+
+<p>
+There is one vast difference between the motions of the stars and those of the
+planets to which attention should be at once called: the planets, being under
+the control of a central force emanating from their immediate master, the sun,
+all move in the same direction and in orbits concentric about the sun; the
+stars, on the other hand, move in every conceivable direction and have no
+apparent center of motion, for all efforts to discover such a center have
+failed. At one time, when theology had finally to accept the facts of science,
+a grandiose conception arose in some pious minds, according to which the Throne
+of God was situated at the exact center of His Creation, and, seated there, He
+watched the magnificent spectacle of the starry systems obediently revolving
+around Him. Astronomical discoveries and speculations seemed for a time to
+afford some warrant for this view, which was, moreover, an acceptable
+substitute for the abandoned geocentric theory in minds that could only
+conceive of God as a superhuman artificer, constantly admiring his own work. No
+longer ago than the middle of the nineteenth century a German astronomer,
+Maedler, believed that he had actually found the location of the center about
+which the stellar universe revolved. He placed it in the group of the Pleiades,
+and upon his authority an extraordinary imaginative picture was sometimes drawn
+of the star Alcyone, the brightest of the Pleiades, as the very seat of the
+Almighty. This idea even seemed to gain a kind of traditional support from the
+mystic significance, without known historical origin, which has for many ages,
+and among widely separated peoples, been attached to the remarkable group of
+which Alcyone is the chief. But since Maedler’s time it has been demonstrated
+that the Pleiades cannot be the center of revolution of the universe, and, as
+already remarked, all attempts to find or fix such a center have proved
+abortive. Yet so powerful was the hold that the theory took upon the popular
+imagination, that even today astronomers are often asked if Alcyone is not the
+probable site of “Jerusalem the Golden.”
+</p>
+
+<p>
+If there were a discoverable center of predominant gravitative power, to which
+the motions of all the stars could be referred, those motions would appear less
+mysterious, and we should then be able to conclude that the universe was, as a
+whole, a prototype of the subsidiary systems of which it is composed. We should
+look simply to the law of gravitation for an explanation, and, naturally, the
+center would be placed within the opening enclosed by the Milky Way. If it were
+there the Milky Way itself should exhibit signs of revolution about it, like a
+wheel turning upon its hub. No theory of the star motions as a whole could
+stand which failed to take account of the Milky Way as the basis of all. But
+the very form of that divided wreath of stars forbids the assumption of its
+revolution about a center. Even if it could be conceived as a wheel having no
+material center it would not have the form which it actually presents. As was
+shown in Chapter 2, there is abundant evidence of motion in the Milky Way; but
+it is not motion of the system as a whole, but motion affecting its separate
+parts. Instead of all moving one way, the galactic stars, as far as their
+movements can be inferred, are governed by local influences and conditions.
+They appear to travel crosswise and in contrary directions, and perhaps they
+eddy around foci where great numbers have assembled; but of a universal
+revolution involving the entire mass we have no evidence.
+</p>
+
+<p>
+Most of our knowledge of star motions, called “proper motions,” relates to
+individual stars and to a few groups which happen to be so near that the
+effects of their movements are measurable. In some cases the motion is so rapid
+(not in appearance, but in reality) that the chief difficulty is to imagine how
+it can have been imparted, and what will eventually become of the “runaways.”
+Without a collision, or a series of very close approaches to great
+gravitational centers, a star traveling through space at the rate of two
+hundred or three hundred miles per second could not be arrested or turned into
+an orbit which would keep it forever flying within the limits of the visible
+universe. A famous example of these speeding stars is “1830 Groombridge,” a
+star of only the sixth magnitude, and consequently just visible to the naked
+eye, whose motion across the line of sight is so rapid that it moves upon the
+face of the sky a distance equal to the apparent diameter of the moon every 280
+years. The distance of this star is at least 200,000,000,000,000 miles, and may
+be two or three times greater, so that its actual speed cannot be less than two
+hundred, and may be as much as four hundred, miles per second. It could be
+turned into a new course by a close approach to a great sun, but it could only
+be stopped by collision, head-on, with a body of enormous mass. Barring such
+accidents it must, as far as we can see, keep on until it has traversed our
+stellar system, whence in may escape and pass out into space beyond, to join,
+perhaps, one of those other universes of which we have spoken. Arcturus, one of
+the greatest suns in the universe, is also a runaway, whose speed of flight has
+been estimated all the way from fifty to two hundred miles per second.
+Arcturus, we have every reason to believe, possesses hundreds of times the mass
+of our sun—think, then, of the prodigious momentum that its motion implies!
+Sirius moves more moderately, its motion across the line of sight amounting to
+only ten miles per second, but it is at the same time approaching the sun at
+about the same speed, its actual velocity in space being the resultant of the
+two displacements.
+</p>
+
+<p>
+What has been said about the motion of Sirius brings us to another aspect of
+this subject. The fact is, that in every case of stellar motion the
+displacement that we observe represents only a part of the actual movement of
+the star concerned. There are stars whose motion carries them straight toward
+or straight away from the earth, and such stars, of course, show no cross
+motion. But the vast majority are traveling in paths inclined from a
+perpendicular to our line of sight. Taken as a whole, the stars may be said to
+be flying about like the molecules in a mass of gas. The discovery of the
+radial component in the movements of the stars is due to the spectroscope. If a
+star is approaching, its spectral lines are shifted toward the violet end of
+the spectrum by an amount depending upon the velocity of approach; if it is
+receding, the lines are correspondingly shifted toward the red end.
+Spectroscopic observation, then, combined with micrometric measurements of the
+cross motion, enables us to detect the real movement of the star in space.
+Sometimes it happens that a star’s radial movement is periodically reversed;
+first it approaches, and then it recedes. This indicates that it is revolving
+around a near-by companion, which is often invisible, and superposed upon this
+motion is that of the two stars concerned, which together may be approaching or
+receding or traveling across the line of sight. Thus the complications involved
+in the stellar motions are often exceedingly great and puzzling.
+</p>
+
+<p>
+Yet another source of complication exists in the movement of our own star, the
+sun. There is no more difficult problem in astronomy than that of disentangling
+the effects of the solar motion from those of the motions of the other stars.
+But the problem, difficult as it is, has been solved, and upon its solution
+depends our knowledge of the speed and direction of the movement of the solar
+system through space, for of course the sun carries its planets with it. One
+element of the solution is found in the fact that, as a result of perspective,
+the stars toward which we are going appear to move apart toward all points of
+the compass, while those behind appear to close up together. Then the
+spectroscopic principle already mentioned is invoked for studying the shift of
+the lines, which is toward the violet in the stars ahead of us and toward the
+red in those that we are leaving behind. Of course the effects of the
+independent motions of the stars must be carefully excluded. The result of the
+studies devoted to this subject is to show that we are traveling at a speed of
+twelve to fifteen miles per second in a northerly direction, toward the border
+of the constellations Hercules and Lyra. A curious fact is that the more recent
+estimates show that the direction is not very much out of a straight line drawn
+from the sun to the star Vega, one of the most magnificent suns in the heavens.
+But it should not be inferred from this that Vega is drawing us on; it is too
+distant for its gravitation to have such an effect.
+</p>
+
+<p>
+Many unaccustomed thoughts are suggested by this mighty voyage of the solar
+system. Whence have we come, and whither do we go? Every year of our lives we
+advance at least 375,000,000 miles. Since the traditional time of Adam the sun
+has led his planets through the wastes of space no less than 225,000,000,000
+miles, or more than 2400 times the distance that separates him from the earth.
+Go back in imagination to the geologic ages, and try to comprehend the distance
+over which the earth has flown. Where was our little planet when it emerged out
+of the clouds of chaos? Where was the sun when his “thunder march” began? What
+strange constellations shone down upon our globe when its masters of life were
+the monstrous beasts of the “Age of Reptiles”? A million years is not much of a
+span of time in geologic reckoning, yet a million years ago the earth was
+farther from its present place in space than any of the stars with a measurable
+parallax are now. It was more than seven times as far as Sirius, nearly
+fourteen times as far as Alpha Centauri, three times as far as Vega, and twice
+as far as Arcturus. But some geologists demand two hundred, three hundred, even
+one thousand million years to enable them to account for the evolutionary
+development of the earth and its inhabitants. In a thousand million years the
+earth would have traveled farther than from the remotest conceivable depths of
+the Milky Way!
+</p>
+
+<p>
+Other curious reflections arise when we think of the form of the earth’s track
+as it follows the lead of the sun, in a journey which has neither known
+beginning nor conceivable end. There are probably many minds which have found a
+kind of consolation in the thought that every year the globe returns to the
+same place, on the same side of the sun. This idea may have an occult
+connection with our traditional regard for anniversaries. When that period of
+the year returns at which any great event in our lives has occurred we have the
+feeling that the earth, in its annual round, has, in a manner, brought us back
+to the scene of that event. We think of the earth’s orbit as a well-worn path
+which we traverse many times in the course of a lifetime. It seems familiar to
+us, and we grow to have a sort of attachment to it. The sun we are accustomed
+to regard as a fixed center in space, like the mill or pump around which the
+harnessed patient mule makes his endless circuits. But the real fact is that
+the earth never returns to the place in space where it has once quitted. In
+consequence of the motion of the sun carrying the earth and the other planets
+along, the track pursued by our globe is a vast spiral in space continually
+developing and never returning upon its course. It is probable that the tracks
+of the sun and the others stars are also irregular, and possibly spiral,
+although, as far as can be at present determined, they appear to be practically
+straight. Every star, wherever it may be situated, is attracted by its
+fellow-stars from many sides at once, and although the force is minimized by
+distance, yet in the course of many ages its effects must become manifest.
+</p>
+
+<p>
+Looked at from another side, is there not something immensely stimulating and
+pleasing to the imagination in the idea of so stupendous a journey, which makes
+all of us the greatest of travelers? In the course of a long life a man is
+transported through space thirty thousand million miles; Halley’s Comet does
+not travel one-quarter as far in making one of its immense circuits. And there
+are adventures on this voyage of which we are just beginning to learn to take
+account. Space is full of strange things, and the earth must encounter some of
+them as it advances through the unknown. Many singular speculations have been
+indulged in by astronomers concerning the possible effects upon the earth of
+the varying state of the space that it traverses. Even the alternation of hot
+and glacial periods has sometimes been ascribed to this source. When tropical
+life flourished around the poles, as the remains in the rocks assure us, the
+needed high temperature may, it has been thought, have been derived from the
+presence of the earth in a warm region of space. Then, too, there is a certain
+interest for us in the thought of what our familiar planet has passed through.
+We cannot but admire it for its long journeying as we admire the traveler who
+comes to us from remote and unexplored lands, or as we gaze with a glow of
+interest upon the first locomotive that has crossed a continent, or a ship that
+has visited the Arctic or Antarctic regions. If we may trust the indications of
+the present course, the earth, piloted by the sun, has come from the Milky Way
+in the far south and may eventually rejoin that mighty band of stars in the far
+north.
+</p>
+
+<p>
+While the stars in general appear to travel independently of one another,
+except when they are combined in binary or trinary systems, there are notable
+exceptions to this rule. In some quarters of the sky we behold veritable
+migrations of entire groups of stars whose members are too widely separated to
+show any indications of revolution about a common center of gravity. This leads
+us back again to the wonderful group of the Pleiades. All of the principle
+stars composing that group are traveling in virtually parallel lines. Whatever
+force set them going evidently acted upon all alike. This might be explained by
+the assumption that when the original projective force acted upon them they
+were more closely united than they are at present, and that in drifting apart
+they have not lost the impulse of the primal motion. Or it may be supposed that
+they are carried along by some current in space, although it would be
+exceedingly difficult, in the present state of our knowledge, to explain the
+nature of such a current. Yet the theory of a current has been proposed. As to
+an attractive center around which they might revolve, none has been found.
+Another instance of similar “star-drift” is furnished by five of the seven
+stars constituting the figure of the “Great Dipper.” In this case the stars
+concerned are separated very widely, the two extreme ones by not less than
+fifteen degrees, so that the idea of a common motion would never have been
+suggested by their aspect in the sky; and the case becomes the more remarkable
+from the fact that among and between them there are other stars, some of the
+same magnitude, which do not share their motion, but are traveling in other
+directions. Still other examples of the same phenomenon are found in other
+parts of the sky. Of course, in the case of compact star-clusters, it is
+assumed that all the members share a like motion of translation through space,
+and the same is probably true of dense star-swarms and star-clouds.
+</p>
+
+<p>
+The whole question of star-drift has lately assumed a new phase, in consequence
+of the investigations of Kapteyn, Dyson, and Eddington on the “systematic
+motions of the stars.” This research will, it is hoped, lead to an
+understanding of the general law governing the movements of the whole body of
+stars constituting the visible universe. Taking about eleven hundred stars
+whose proper motions have been ascertained with an approach to certainty, and
+which are distributed in all parts of the sky, it has been shown that there
+exists an apparent double drift, in two independent streams, moving in
+different and nearly opposed directions. The apex of the motion of what is
+called “Stream I” is situated, according to Professor Kapteyn, in right
+ascension 85&deg;, declination south 11&deg;, which places it just south of the
+constellation Orion; while the apex of “Stream II” is in right ascension
+260&deg;, declination south 48&deg;, placing it in the constellation Ara, south
+of Scorpio. The two apices differ very nearly 180&deg; in right ascension and
+about 120&deg; in declination. The discovery of these vast star-streams, if
+they really exist, is one of the most extraordinary in modern astronomy. It
+offers the correlation of stellar movements needed as the basis of a theory of
+those movements, but it seems far from revealing a physical cause for them. As
+projected against the celestial sphere the stars forming the two opposite
+streams appear intermingled, some obeying one tendency and some the other. As
+Professor Dyson has said, the hypothesis of this double movement is of a
+revolutionary character, and calls for further investigation. Indeed, it seems
+at first glance not less surprising than would be the observation that in a
+snow-storm the flakes over our heads were divided into two parties and driving
+across each other’s course in nearly opposite directions, as if urged by
+interpenetrating winds.
+</p>
+
+<p>
+But whatever explanation may eventually be found for the motions of the stars,
+the knowledge of the existence of those motions must always afford a new charm
+to the contemplative observer of the heavens, for they impart a sense of life
+to the starry system that would otherwise be lacking. A stagnant universe, with
+every star fixed immovably in its place, would not content the imagination or
+satisfy our longing for ceaseless activity. The majestic grandeur of the
+evolutions of the celestial hosts, the inconceivable vastness of the fields of
+space in which they are executed, the countless numbers, the immeasurable
+distances, the involved convolutions, the flocking and the scattering, the
+interpenetrating marches and countermarches, the strange community of impulsion
+affecting stars that are wide apart in space and causing them to traverse the
+general movement about them like aides and despatch-bearers on a
+battle-field—all these arouse an intensity of interest which is heightened by
+the mystery behind them.
+</p>
+
+</div><!--end chapter-->
+
+<div class="chapter">
+
+<h2><a name="chap04"></a>IV<br/>
+The Passing of the Constellations</h2>
+
+<p>
+From a historical and picturesque point of view, one of the most striking
+results of the motions of the stars described in the last chapter is their
+effect upon the forms of the constellations, which have been watched and
+admired by mankind from a period so early that the date of their invention is
+now unknown. The constellations are formed by chance combinations of
+conspicuous stars, like figures in a kaleidoscope, and if our lives were
+commensurate with the æons of cosmic existence we should perceive that the
+kaleidoscope of the heavens was ceaselessly turning and throwing the stars into
+new symmetries. Even if the stars stood fast, the motion of the solar system
+would gradually alter the configurations, as the elements of a landscape
+dissolve and recombine in fresh groupings with the traveler’s progress amid
+them. But with the stars themselves all in motion at various speeds and in many
+directions, the changes occur more rapidly. Of course, “rapid” is here
+understood in a relative sense; the wheel of human history to an eye accustomed
+to the majestic progression of the universe would appear to revolve with the
+velocity of a whirling dynamo. Only the deliberation of geological movements
+can be contrasted with the evolution and devolution of the constellations.
+</p>
+
+<p>
+And yet this secular fluctuation of the constellation figures is not without
+keen interest for the meditative observer. It is another reminder of the swift
+mutability of terrestial affairs. To the passing glance, which is all that we
+can bestow upon these figures, they appear so immutable that they have been
+called into service to form the most lasting records of ancient thought and
+imagination that we possess. In the forms of the constellations, the most
+beautiful, and, in imaginative quality, the finest, mythology that the world
+has ever known has been perpetuated. Yet, in a broad sense, this scroll of
+human thought imprinted on the heavens is as evanescent as the summer clouds.
+Although more enduring than parchment, tombs, pyramids, and temples, it is as
+far as they from truly eternizing the memory of what man has fancied and done.
+</p>
+
+<p>
+Before studying the effects that the motions of the stars have had and will
+have upon the constellations, it is worth while to consider a little further
+the importance of the stellar pictures as archives of history. To emphasize the
+importance of these effects it is only necessary to recall that the
+constellations register the oldest traditions of our race. In the history of
+primeval religions they are the most valuable of documents. Leaving out of
+account for the moment the more familiar mythology of the Greeks, based on
+something older yet, we may refer for illustration to that of the mysterious
+Maya race of America. At Izamal, in Yucatan, says Mr Stansbury Hagar, is a
+group of ruins perched, after the Mexican and Central-American plan, on the
+summits of pyramidal mounds which mark the site of an ancient theogonic center
+of the Mayas. Here the temples all evidently refer to a cult based upon the
+constellations as symbols. The figures and the names, of course, were not the
+same as those that we have derived from our Aryan ancestors, but the star
+groups were the same or nearly so. For instance, the loftiest of the temples at
+Izamal was connected with the sign of the constellation known to us as Cancer,
+marking the place of the sun at the summer solstice, at which period the sun
+was supposed to descend at noon like a great bird of fire and consume the
+offerings left upon the altar. Our Scorpio was known to the Mayas as a sign of
+the “Death God.” Our Libra, the “Balance,” with which the idea of a divine
+weighing out of justice has always been connected, seems to be identical with
+the Mayan constellation Teoyaotlatohua, with which was associated a temple
+where dwelt the priests whose special business it was to administer justice and
+to foretell the future by means of information obtained from the spirits of the
+dead. Orion, the “Hunter” of our celestial mythology, was among the Mayas a
+“Warrior,” while Sagittarius and others of our constellations were known to
+them (under different names, of course), and all were endowed with a religious
+symbolism. And the same star figures, having the same significance, were
+familiar to the Peruvians, as shown by the temples at Cuzco. Thus the
+imagination of ancient America sought in the constellations symbols of the
+unchanging gods.
+</p>
+
+<p>
+But, in fact, there is no nation and no people that has not recognized the
+constellations, and at one period or another in its history employed them in
+some symbolic or representative capacity. As handled by the Greeks from
+prehistoric times, the constellation myths became the very soul of poetry. The
+imagination of that wonderful race idealized the principal star groups so
+effectively that the figures and traditions thus attached to them have, for
+civilized mankind, displaced all others, just as Greek art in its highest forms
+stands without parallel and eclipses every rival. The Romans translated no
+heroes and heroines of the mythical period of their history to the sky, and the
+deified Cæsars never entered that lofty company, but the heavens are filled
+with the early myths of the Greeks. Herakles nightly resumes his mighty labors
+in the stars; Zeus, in the form of the white “Bull,” Taurus, bears the fair
+Europa on his back through the celestial waves; Andromeda stretches forth her
+shackled arms in the star-gemmed ether, beseeching aid; and Perseus, in a blaze
+of diamond armor, revives his heroic deeds amid sparkling clouds of stellar
+dust. There, too, sits Queen Cassiopeia in her dazzling chair, while the Great
+King, Cepheus, towers gigantic over the pole. Professor Young has significantly
+remarked that a great number of the constellations are connected in some way or
+other with the Argonautic Expedition—that strangely fascinating legend of
+earliest Greek story which has never lost its charm for mankind. In view of all
+this, we may well congratulate ourselves that the constellations will outlast
+our time and the time of countless generations to follow us; and yet they are
+very far from being eternal. Let us now study some of the effects of the
+stellar motions upon them.
+</p>
+
+<p>
+We begin with the familiar figure of the “Great Dipper.” He who has not drunk
+inspiration from its celestial bowl is not yet admitted to the circle of
+Olympus. This figure is made up of seven conspicuous stars in the constellation
+Ursa Major, the “Greater Bear.” The handle of the “Dipper” corresponds to the
+tail of the imaginary “Bear,” and the bowl lies upon his flank. In fact, the
+figure of a dipper is so evident and that of a bear so unevident, that to most
+persons the “Great Dipper” is the only part of the constellation that is
+recognizable. Of the seven stars mentioned, six are of nearly equal brightness,
+ranking as of the second magnitude, while the seventh is of only the third
+magnitude. The difference is very striking, since every increase of one
+magnitude involves an increase of two-and-a-half times in brightness. There
+appears to be little doubt that the faint star, which is situated at the
+junction of the bowl and the handle, is a variable of long period, since three
+hundred years ago it was as bright as its companions. But however that may be,
+its relative faintness at the present time interferes but little with the
+perfection of the “Dipper’s” figure. In order the more readily to understand
+the changes which are taking place, it will be well to mention both the names
+and the Greek letters which are attached to the seven stars. Beginning at the
+star in the upper outer edge of the rim of the bowl and running in regular
+order round the bottom and then out to the end of the handle, the names and
+letters are as follows: Dubhe (&#945;), Merak (&#946;), Phaed (&#947;), Megrez
+(&#948;), Alioth (&#949;), Mizar (&#950;), and Benetnasch (&#951;). Megrez is
+the faint star already mentioned at the junction of the bowl and handle, and
+Mizar, in the middle of the handle, has a close, naked-eye companion which is
+named Alcor. The Arabs called this singular pair of stars “The Horse and
+Rider.” Merak and Duhbe are called “The Pointers,” because an imaginary line
+drawn northward through them indicates the Pole Star.
+</p>
+
+<div class="fig" style="width:100%;">
+<a name="illus06"></a>
+<a href="images/image06.jpg">
+<img src="images/image06.jpg" width="515" height="600" alt="Illustration:" /></a>
+<p class="caption">The “Great Dipper”</p>
+</div>
+
+<p>
+Now it has been found that five of these stars—<i>viz.,</i> Merak, Phaed,
+Megrez, Alioth, and Mizar (with its comrade)—are moving with practically the
+same speed in an easterly direction, while the other two, Dubhe and Benetnasch,
+are simultaneously moving westward, the motions of Benetnasch being apparently
+more rapid. The consequence of these opposed motions is, of course, that the
+figure of the “Dipper” cannot always have existed and will not continue to
+exist. In the accompanying diagrams it has been thought interesting to show the
+relative positions of these seven stars, as seen from the point which the earth
+now occupies, both in the past and in the future. Arrows attached to the stars
+in the figure representing the present appearance of the “Dipper” indicate the
+directions of the motions and the distances over which they will carry the
+stars in a period of about five hundred centuries. The time, no doubt, seems
+long, but remember the vast stretch of ages through which the earth has passed,
+and then reflect that no reason is apparent why our globe should not continue
+to be a scene of animation for ten thousand centuries yet to come. The fact
+that the little star Alcor placed so close to Mizar should accompany the latter
+in its flight is not surprising, but that two of the principal stars of the
+group should be found moving in a direction directly opposed to that pursued by
+the other five is surprising in the highest degree; and it recalls the strange
+theory of a double drift affecting all the stars, to which attention was called
+in the preceding chapter. It would appear that Benetnasch and Dubhe belong to
+one “current,” and Merak, Phaed, Megrez, Alioth, and Mizar to the other. As far
+as is known, the motion of the seven stars are not shared by the smaller stars
+scattered about them, but on the theory of currents there should be such a
+community of motion, and further investigation may reveal it.
+</p>
+
+<div class="fig" style="width:100%;">
+<a name="illus07"></a>
+<a href="images/image07.jpg">
+<img src="images/image07.jpg" width="519" height="600" alt="Illustration:" /></a>
+<p class="caption">Cassiopeia</p>
+</div>
+
+<p>
+From the “Great Dipper” we turn to a constellation hardly less conspicuous and
+situated at an equal distance from the pole on the other side—Cassiopeia. This
+famous star-group commemorating the romantic Queen of Ethiopia whose vain
+boasting of her beauty was punished by the exposure of her daughter Andromeda
+to the “Sea Monster,” is well-marked by five stars which form an irregular
+letter “W” with its open side toward the pole. Three of these stars are usually
+ranked as of the second magnitude, and two of the third; but to ordinary
+observation they appear of nearly equal brightness, and present a very striking
+picture. They mark out the chair and a part of the figure of the beautiful
+queen. Beginning at the right-hand, or western, end of the “W,” their Greek
+letter designations are: Beta (&#946;), Alpha (&#945;), Gamma (&#947;), Delta
+(&#948;), and Epsilon (&#949;). Four of them, Beta, Alpha, Delta, and Epsilon
+are traveling eastwardly at various speeds, while the fifth, Gamma, moves in a
+westerly direction. The motion of Beta is more rapid than that of any of the
+others. It should be said, however, that no little uncertainty attaches to the
+estimates of the rate of motion of stars which are not going very rapidly, and
+different observers often vary considerably in their results.
+</p>
+
+<p>
+In the beautiful “Northern Crown,” one of the most perfect and charming of all
+the figures to be found in the stars, the alternate combining and scattering
+effects of the stellar motions are shown by comparing the appearance which the
+constellation must have had five hundred centuries ago with that which it has
+at present and that which it will have in the future. The seven principle stars
+of the asterism, forming a surprisingly perfect coronet, have movements in
+three directions at right angles to one another. That in these circumstances
+they should ever have arrived at positions giving them so striking an
+appearance of definite association is certainly surprising; from its aspect one
+would have expected to find a community of movement governing the brilliants of
+the “Crown,” but instead of that we find evidence that they will inevitably
+drift apart and the beautiful figure will dissolve.
+</p>
+
+<p>
+A similar fate awaits such asterisms as the “Northern Cross” in Cygnus; the
+“Crow” (Corvus), which stands on the back of the great “Sea Serpent,” Hydra,
+and pecks at his scales; “Job’s Coffin” (Delphinus); the “Great Square of
+Pegasus”; the “Twins” (Gemini); the beautiful “Sickle” in Leo; and the
+exquisite group of the Hyades in Taurus. In the case of the Hyades, two
+controlling movements are manifest: one, affecting five of the stars which form
+the well-known figure of a letter “V,” is directed northerly; the other, which
+controls the direction of two stars, has an easterly trend. The chief star of
+the group, Aldebaran, one of the finest of all stars both for its brilliance
+and its color, is the most affected by the easterly motion. In time it will
+drift entirely out of connection with its present neighbors. Although the
+Hyades do not form so compact a group as the Pleiades in the same
+constellation, yet their appearance of relationship is sufficient to awaken a
+feeling of surprise over the fact that, as with the stars of the “Dipper,”
+their association is only temporary or apparent.
+</p>
+
+<div class="fig" style="width:100%;">
+<a name="illus08"></a>
+<a href="images/image08.jpg">
+<img src="images/image08.jpg" width="560" height="600" alt="Illustration:" /></a>
+<p class="caption">The “Northern Crown”</p>
+</div>
+
+<p>
+The great figure of Orion appears to be more lasting, not because its stars are
+physically connected, but because of their great distance, which renders their
+movements too deliberate to be exactly ascertained. Two of the greatest of its
+stars, Betelgeuse and Rigel, possess, as far as has been ascertained, no
+perceptible motion across the line of sight, but there is a little movement
+perceptible in the “Belt.” At the present time this consists of an almost
+perfect straight line, a row of second-magnitude stars about equally spaced and
+of the most striking beauty. In the course of time, however, the two right-hand
+stars, Mintaka and Alnilam (how fine are these Arabic star names!) will
+approach each other and form a naked-eye double, but the third, Alnita, will
+drift away eastward, so that the “Belt” will no longer exist.
+</p>
+
+<p>
+For one more example, let us go to the southern hemisphere, whose most
+celebrated constellation, the “Southern Cross,” has found a place in all modern
+literatures, although it has no claim to consideration on account of
+association with ancient legends. This most attractive asterism, which has
+never ceased to fascinate the imagination of Christendom since it was first
+devoutly described by the early explorers of the South, is but a passing
+collocation of brilliant stars. Yet even in its transfigurations it has been
+for hundreds of centuries, and will continue to be for hundreds of centuries to
+come, a most striking object in the sky. Our figures show its appearance in
+three successive phases: first, as it was fifty thousand years ago (viewed from
+the earth’s present location); second, as it is in our day; and, third, as it
+will be an equal time in the future. The nearness of these bright stars to one
+another—the length of the longer beam of the “Cross” is only sixdegrees—makes
+this group very noticeable, whatever the arrangement of its components may be.
+The largest star, at the base of the “Cross,” is of the first magnitude, two of
+the others are of the second magnitude, and the fourth is of the third. Other
+stars, not represented in the figures, increase the effect of a celestial
+blazonry, although they do not help the resemblance to a cross.
+</p>
+
+<div class="fig" style="width:100%;">
+<a name="illus09"></a>
+<a href="images/image09.jpg">
+<img src="images/image09.jpg" width="500" height="444" alt="Illustration:" /></a>
+<p class="caption">The “Southern Cross”</p>
+</div>
+
+<p>
+But since the motion of the solar system itself will, in the course of so long
+a period as fifty thousand years, produce a great change in the perspective of
+the heavens as seen from the earth, by carrying us nearly nineteen trillion
+miles from our present place, why, it may be asked, seek to represent future
+appearances of the constellations which we could not hope to see, even if we
+could survive so long? The answer is: Because these things aid the mind to form
+a picture of the effects of the mobility of the starry universe. Only by
+showing the changes from some definite point of view can we arrive at a due
+comprehension of them. The constellations are more or less familiar to
+everybody, so that impending changes of their forms must at once strike the eye
+and the imagination, and make clearer the significance of the movements of the
+stars. If the future history of mankind is to resemble its past and if our race
+is destined to survive yet a million years, then our remote descendents will
+see a “new heavens” if not a “new earth,” and will have to invent novel
+constellations to perpetuate their legends and mythologies.
+</p>
+
+<p>
+If our knowledge of the relative distances of the stars were more complete, it
+would be an interesting exercise in celestial geometry to project the
+constellations probably visible to the inhabitants of worlds revolving around
+some of the other suns of space. Our sun is too insignificant for us to think
+that he can make a conspicuous appearance among them, except, perhaps, in a few
+cases. As seen, for instance, from the nearest known star, Alpha Centauri, the
+sun would appear of the average first magnitude, and consequently from that
+standpoint he might be the gem of some little constellation which had no
+Sirius, or Arcturus, or Vega to eclipse him with its superior splendor. But
+from the distance of the vast majority of the stars the sun would probably be
+invisible to the naked eye, and as seen from nearer systems could only rank as
+a fifth or sixth magnitude star, unnoticed and unknown except by the
+star-charting astronomer.
+</p>
+
+</div><!--end chapter-->
+
+<div class="chapter">
+
+<h2><a name="chap05"></a>V<br/>
+Conflagrations in the Heavens</h2>
+
+<p>
+Suppose it were possible for the world to take fire and burn up—as some
+pessimists think that it will do when the Divine wrath shall have sufficiently
+accumulated against it—nobody out of our own little corner of space would ever
+be aware of the catastrophe! With all their telescopes, the astronomers living
+in the golden light of Arcturus or the diamond blaze of Canopus would be unable
+to detect the least glimmer of the conflagration that had destroyed the seat of
+Adam and his descendents, just as now they are totally ignorant of its
+existence.
+</p>
+
+<p>
+But at least fifteen times in the course of recorded history men looking out
+from the earth have beheld in the remote depths of space great outbursts of
+fiery light, some of them more splendidly luminous than anything else in the
+firmament except the sun! If <i>they</i> were conflagrations, how many million
+worlds like ours were required to feed their blaze?
+</p>
+
+<p>
+It is probable that “temporary” or “new” stars, as these wonderful apparitions
+are called, really are conflagrations; not in the sense of a bonfire or a
+burning house or city, but in that of a sudden eruption of inconceivable heat
+and light, such as would result from the stripping off the shell of an
+encrusted sun or the crashing together of two mighty orbs flying through space
+with a hundred times the velocity of the swiftest cannon-shot.
+</p>
+
+<p>
+Temporary stars are the rarest and most erratic of astronomical phenomena. The
+earliest records relating to them are not very clear, and we cannot in every
+instance be certain that it was one of these appearances that the ignorant and
+superstitious old chroniclers are trying to describe. The first temporary star
+that we are absolutely sure of appeared in 1572, and is known as “Tycho’s
+Star,” because the celebrated Danish astronomer (whose remains, with his
+gold-and-silver artificial nose—made necessary by a duel—still intact, were
+disinterred and reburied in 1901) was the first to perceive it in the sky, and
+the most assiduous and successful in his studies of it. As the first fully
+accredited representative of its class, this new star made its entry upon the
+scene with becoming <i>éclat.</i> It is characteristic of these phenomena that
+they burst into view with amazing suddenness, and, of course, entirely
+unexpectedly. Tycho’s star appeared in the constellation Cassiopeia, near a now
+well-known and much-watched little star named Kappa, on the evening of November
+11, 1572. The story has often been repeated, but it never loses interest, how
+Tycho, going home that evening, saw people in the street pointing and staring
+at the sky directly over their heads, and following the direction of their
+hands and eyes he was astonished to see, near the zenith, an unknown star of
+surpassing brilliance. It outshone the planet Jupiter, and was therefore far
+brighter than the first magnitude. There was not another star in the heavens
+that could be compared with it in splendor. Tycho was not in all respects free
+from the superstitions of his time—and who is?—but he had the true scientific
+instinct, and immediately he began to study the stranger, and to record with
+the greatest care every change in its aspect. First he determined as well as he
+could with the imperfect instruments of his day, many of which he himself had
+invented, the precise location of the phenomena in the sky. Then he followed
+the changes that it underwent. At first it brightened until its light equaled
+or exceeded that of the planet Venus at her brightest, a statement which will
+be appreciated at its full value by anyone who has ever watched Venus when she
+plays her dazzling rôle of “Evening Star,” flaring like an arc light in the
+sunset sky. It even became so brilliant as to be visible in full daylight,
+since, its position being circumpolar, it never set in the latitude of Northern
+Europe. Finally it began to fade, turning red as it did so, and in March, 1574,
+it disappeared from Tycho’s searching gaze, and has never been seen again from
+that day to this. None of the astronomers of the time could make anything of
+it. They had not yet as many bases of speculation as we possess today.
+</p>
+
+<div class="fig" style="width:100%;">
+<a name="illus10"></a>
+<a href="images/image10.jpg">
+<img src="images/image10.jpg" width="500" height="408" alt="Illustration:" /></a>
+<p class="caption">Chart showing location of Tycho’s star, 1572,  and Nova
+Persei of 1901</p>
+</div>
+
+<p>
+Tycho’s star has achieved a romantic reputation by being fancifully identified
+with the “Star of Bethlehem,” said to have led the wondering Magi from their
+eastern deserts to the cradle-manger of the Savior in Palestine. Many attempts
+have been made to connect this traditional “star” with some known phenomenon of
+the heavens, and none seems more idle than this. Yet it persistently survives,
+and no astronomer is free from eager questions about it addressed by people
+whose imagination has been excited by the legend. It is only necessary to say
+that the supposition of a connection between the phenomenon of the Magi and
+Tycho’s star is without any scientific foundation. It was originally based on
+an unwarranted assumption that the star of Tycho was a variable of long period,
+appearing once every three hundred and fifteen years, or thereabout. If that
+were true there would have been an apparition somewhere near the traditional
+date of the birth of Christ, a date which is itself uncertain. But even the
+data on which the assumption was based are inconsistent with the theory.
+Certain monkish records speak of something wonderful appearing in the sky in
+the years 1264 and 945, and these were taken to have been outbursts of Tycho’s
+star. Investigation shows that the records more probably refer to comets, but
+even if the objects seen were temporary stars, their dates do not suit the
+hypothesis; from 945 to 1264 there is a gap of 319 years, and from 1264 to 1572
+one of only 308 years; moreover 337 years have now (1909) elapsed since Tycho
+saw the last glimmer of his star. Upon a variability so irregular and uncertain
+as that, even if we felt sure that it existed, no conclusion could be found
+concerning an apparition occurring 2000 years ago.
+</p>
+
+<p>
+In the year 1600 (the year in which Giordano Bruno was burned at the stake for
+teaching that there is more than one physical world), a temporary star of the
+third magnitude broke out in the constellation Cygnus, and curiously enough,
+considering the rarity of such phenomena, only four years later another
+surprisingly brilliant one appeared in the constellation Ophiuchus. This is
+often called “Kepler’s star,” because the great German astronomer devoted to it
+the same attention that Tycho had given to the earlier phenomenon. It, too,
+like Tycho’s, was at first the brightest object in the stellar heavens,
+although it seems never to have quite equaled its famous predecessor in
+splendor. It disappeared after a year, also turning of a red color as it became
+more faint. We shall see the significance of this as we go on. Some of Kepler’s
+contemporaries suggested that the outburst of this star was due to a meeting of
+atoms in space, and idea bearing a striking resemblance to the modern theory of
+“astronomical collisions.”
+</p>
+
+<p>
+In 1670, 1848, and 1860 temporary stars made their appearance, but none of them
+was of great brilliance. In 1866 one of the second magnitude broke forth in the
+“Northern Crown” and awoke much interest, because by that time the spectroscope
+had begun to be employed in studying the composition of the stars, and Huggins
+demonstrated that the new star consisted largely of incandescent hydrogen. But
+this star, apparently unlike the others mentioned, was not absolutely new.
+Before its outburst it had shown as a star of the ninth magnitude (entirely
+invisible, of course, to the naked eye), and after about six weeks it faded to
+its original condition in which it has ever since remained. In 1876 a temporary
+star appeared in the constellation Cygnus, and attained at one time the
+brightness of the second magnitude. Its spectrum and its behavior resembled
+those of its immediate predecessor. In 1885, astronomers were surprised to see
+a sixth-magnitude star glimmering in the midst of the hazy cloud of the great
+Andromeda Nebula. It soon absolutely disappeared. Its spectrum was remarkable
+for being “continuous,” like that of the nebula itself. A continuous spectrum
+is supposed to represent a body, or a mass, which is either solid or liquid, or
+composed of gas under great pressure. In January, 1892, a new star was suddenly
+seen in the constellation Auriga. It never rose much above the fourth
+magnitude, but it showed a peculiar spectrum containing both bright and dark
+lines of hydrogen.
+</p>
+
+<p>
+But a bewildering surprise was now in store; the world was to behold at the
+opening of the twentieth century such a celestial spectacle as had not been on
+view since the times of Tycho and Kepler. Before daylight on the morning of
+February 22, 1901, the Rev. Doctor Anderson, of Edinburgh, an amateur
+astronomer, who had also been the first to see the new star in Auriga, beheld a
+strange object in the constellation Perseus not far from the celebrated
+variable star Algol. He recognized its character at once, and immediately
+telegraphed the news, which awoke the startled attention of astronomers all
+over the world. When first seen the new star was no brighter than Algol (less
+than the second magnitude), but within twenty-four hours it was ablaze,
+outshining even the brilliant Capella, and far surpassing the first magnitude.
+At the spot in the sky where it appeared nothing whatever was visible on the
+night before its coming. This is known with certainty because a photograph had
+been made of that very region on February 21, and this photograph showed
+everything down to the twelfth magnitude, but not a trace of the stranger which
+burst into view between the 21st and the 22nd like the explosion of a rocket.
+</p>
+
+<p>
+Upon one who knew the stars the apparition of this intruder in a well-known
+constellation had the effect of a sudden invasion. The new star was not far
+west of the zenith in the early evening, and in that position showed to the
+best advantage. To see Capella, the hitherto unchallenged ruler of that quarter
+of the sky, abased by comparison with this stranger of alien aspect, for there
+was always an unfamiliar look about the “nova,” was decidedly disconcerting. It
+seemed to portend the beginning of a revolution in the heavens. One could
+understand what the effect of such an apparition must have been in the
+superstitious times of Tycho. The star of Tycho had burst forth on the northern
+border of the Milky Way; this one was on its southern border, some forty-five
+degrees farther east.
+</p>
+
+<p>
+Astronomers were well-prepared this time for the scientific study of the new
+star, both astronomical photography and spectroscopy having been perfected, and
+the results of their investigations were calculated to increase the wonder with
+which the phenomenon was regarded. The star remained at its brightest only a
+few days; then, like a veritable conflagration, it began to languish; and, like
+the reflection of a dying fire, as it sank it began to glow with the red color
+of embers. But its changes were spasmodic; once about every three days it
+flared up only to die away again. During these fluctuations its light varied
+alternately in the ratio of one to six. Finally it took a permanent downward
+course, and after a few months the naked eye could no longer perceive it; but
+it remained visible with telescopes, gradually fading until it had sunk to the
+ninth magnitude. Then another astonishing change happened: in August
+photographs taken at the Yerkes Observatory and at Heidelberg showed that the
+“nova” was <i>surrounded by a spiral nebula!</i> The nebula had not been there
+before, and no one could doubt that it represented a phase of the same
+catastrophe that had produced the outburst of the new star. At one time the
+star seemed virtually to have disappeared, as if all its substance had been
+expanded into the nebulous cloud, but always there remained a stellar nucleus
+about which the misty spiral spread wider and ever wider, like a wave expanding
+around a center of disturbance. The nebula too showed a variability of
+brightness, and four condensations which formed in it seemed to have a motion
+of revolution about the star. As time went on the nebula continued to expand at
+a rate which was computed to be not less than twenty thousand miles per second!
+And now the star itself, showing indications of having turned into a nebula,
+behaved in a most erratic manner, giving rise to the suspicion that it was
+about to burst out again. But this did not occur, and at length it sunk into a
+state of lethargy from which it has to the present time not recovered. But the
+nebulous spiral has disappeared, and the entire phenomena as it now (1909)
+exists consists of a faint nebulous star of less than the ninth magnitude.
+</p>
+
+<p>
+The wonderful transformations just described had been forecast in advance of
+the discovery of the nebulous spiral encircling the star by the spectroscopic
+study of the latter. At first there was no suggestion of a nebular
+constitution, but within a month or two characteristic nebular lines began to
+appear, and in less than six months the whole spectrum had been transformed to
+the nebular type. In the mean time the shifting of the spectral lines indicated
+a complication of rapid motions in several directions simultaneously. These
+motions were estimated to amount to from one hundred to five hundred miles per
+second.
+</p>
+
+<p>
+The human mind is so constituted that it feels forced to seek an explanation of
+so marvelous a phenomenon as this, even in the absence of the data needed for a
+sound conclusion. The most natural hypothesis, perhaps, is that of a collision.
+Such a catastrophe could certainly happen. It has been shown, for instance,
+that in infinity of time the earth is sure to be hit by a comet; in the same
+way it may be asserted that, if no time limit is fixed, the sun is certain to
+run against some obstacle in space, either another star, or a dense meteor
+swarm, or one of the dark bodies which there is every reason to believe abound
+around us. The consequences of such a collision are easy to foretell, provided
+that we know the masses and the velocities of the colliding bodies. In a
+preceding chapter we have discussed the motions of the sun and stars, and have
+seen that they are so swift that an encounter between any two of them could not
+but be disastrous. But this is not all; for as soon as two stars approached
+within a few million miles their speed would be enormously increased by their
+reciprocal attractions and, if their motion was directed radially with respect
+to their centers, they would come together with a crash that would reduce them
+both to nebulous clouds. It is true that the chances of such a “head-on”
+collision are relatively very small; two stars approaching each other would
+most probably fall into closed orbits around their common center of gravity. If
+there were a collision it would most likely be a grazing one instead of a
+direct front-to-front encounter. But even a close approach, without any actual
+collision, would probably prove disastrous, owing to the tidal influence of
+each of the bodies on the other. Suns, in consequence of their enormous masses
+and dimensions and the peculiarities of their constitution, are exceedingly
+dangerous to one another at close quarters. Propinquity awakes in them a
+mutually destructive tendency. Consisting of matter in the gaseous, or perhaps,
+in some cases, liquid, state, their tidal pull upon each other if brought close
+together might burst them asunder, and the photospheric envelope being
+destroyed the internal incandescent mass would gush out, bringing fiery death
+to any planets that were revolving near. Without regard to the resulting
+disturbance of the earth’s orbit, the close approach of a great star to the sun
+would be in the highest degree perilous to us. But this is a danger which may
+properly be regarded as indefinitely remote, since, at our present location in
+space, we are certainly far from every star except the sun, and we may feel
+confident that no great invisible body is near, for if there were one we should
+be aware of its presence from the effects of its attraction. As to dark nebulæ
+which may possibly lie in the track that the solar system is pursuing at the
+rate of 375,000,000 miles per year, that is another question—and they, too,
+could be dangerous!
+</p>
+
+<div class="fig" style="width:100%;">
+<a name="illus11"></a>
+<a href="images/image11.jpg">
+<img src="images/image11.jpg" width="600" height="563" alt="Illustration:" /></a>
+<p class="caption">Nova Persei, with its nebular rings</p>
+</div>
+
+<p>
+This brings us directly back to “Nova Persei,” for among the many suggestions
+offered to explain its outburst, as well as those of other temporary stars, one
+of the most fruitful is that of a collision between a star and a vast invisible
+nebula. Professor Seeliger, of Munich, first proposed this theory, but it
+afterward underwent some modifications from others. Stated in a general form,
+the idea is that a huge dark body, perhaps an extinguished sun, encountered in
+its progress through space a widespread flock of small meteors forming a dark
+nebula. As it plunged into the swarm the friction of the innumerable collisions
+with the meteors heated its surface to incandescence, and being of vast size it
+then became visible to us as a new star. Meanwhile the motion of the body
+through the nebula, and its rotation upon itself, set up a gyration in the
+blazing atmosphere formed around it by the vaporized meteors; and as this
+atmosphere spread wider, under the laws of gyratory motion a rotation in the
+opposite direction began in the inflamed meteoric cloud outside the central
+part of the vortex. Thus the spectral lines were caused to show motion in
+opposite directions, a part of the incandescent mass approaching the earth
+simultaneously with the retreat of another part. So the curious spectroscopic
+observations before mentioned were explained. This theory might also account
+for the appearance of the nebulous spiral first seen some six months after the
+original outburst. The sequent changes in the spectrum of the “nova” are
+accounted for by this theory on the assumption, reasonable enough in itself,
+that at first the invading body would be enveloped in a vaporized atmosphere of
+relatively slight depth, producing by its absorption the fine dark lines first
+observed; but that as time went on and the incessant collisions continued, the
+blazing atmosphere would become very deep and extensive, whereupon the
+appearance of the spectral lines would change, and bright lines due to the
+light of the incandescent meteors surrounding the nucleus at a great distance
+would take the place of the original dark ones. The vortex of meteors once
+formed would protect the flying body within from further immediate collisions,
+the latter now occurring mainly among the meteors themselves, and then the
+central blaze would die down, and the original splendor of the phenomenon would
+fade.
+</p>
+
+<p>
+But the theories about Nova Persei have been almost as numerous as the
+astronomers who have speculated about it. One of the most startling of them
+assumed that the outburst was caused by the running amuck of a dark star which
+had encountered another star surrounded with planets, the renewed outbreaks of
+light after the principal one had faded being due to the successive running
+down of the unfortunate planets! Yet another hypothesis is based on what we
+have already said of the tidal influence that two close approaching suns would
+have upon each other. Supposing two such bodies which had become encrusted, but
+remained incandescent and fluid within, to approach within almost striking
+distance; they would whirl each other about their common center of gravity, and
+at the same time their shells would burst under the tidal strain, and their
+glowing nuclei being disclosed would produce a great outburst of light.
+Applying this theory to a “nova,” like that of 1866 in the “Northern Crown,”
+which had been visible as a small star before the outbreak, and which afterward
+resumed its former aspect, we should have to assume that a yet shining sun had
+been approached by a dark body whose attraction temporarily burst open its
+photosphere. It might be supposed that in this case the dark body was too far
+advanced in cooling to suffer the same fate from the tidal pull of its victim.
+But a close approach of that kind would be expected to result in the formation
+of a binary system, with orbits of great eccentricity, perhaps, and after the
+lapse of a certain time the outburst should be renewed by another approximation
+of the two bodies. A temporary star of that kind would rather be ranked as a
+variable.
+</p>
+
+<p>
+The celebrated French astronomer, Janssen, had a different theory of Nova
+Persei, and of temporary stars in general. According to his idea, such
+phenomena might be the result of chemical changes taking place in a sun without
+interference by, or collision with, another body. Janssen was engaged for many
+years in trying to discover evidence of the existence of oxygen in the sun, and
+he constructed his observatory on the summit of Mount Blanc specially to pursue
+that research. He believed that oxygen must surely exist in the sun since we
+find so many other familiar elements included in the constitution of the solar
+globe, and as he was unable to discover satisfactory evidence of its presence
+he assumed that it existed in a form unknown on the earth. If it were normally
+in the sun’s chromosphere, or coronal atmosphere, he said, it would combine
+with the hydrogen which we know is there and form an obscuring envelope of
+water vapor. It exists, then, in a special state, uncombined with hydrogen; but
+let the temperature of the sun sink to a critical point and the oxygen will
+assume its normal properties and combine with the hydrogen, producing a mighty
+outburst of light and heat. This, Janssen thought, might explain the phenomena
+of the temporary stars. It would also, he suggested, account for their brief
+career, because the combination of the elements would be quickly accomplished,
+and then the resulting water vapor would form an atmosphere cutting off the
+radiation from the star within.
+</p>
+
+<p>
+This theory may be said to have a livelier human interest than some of the
+others, since, according to it, the sun may carry in its very constitution a
+menace to mankind; one does not like to think of it being suddenly transformed
+into a gigantic laboratory for the explosive combination of oxygen and
+hydrogen! But while Janssen’s theory might do for some temporary stars, it is
+inadequate to explain all the phenomena of Nova Persei, and particularly the
+appearance of the great spiral nebula that seemed to exhale from the heart of
+the star. Upon the whole, the theory of an encounter between a star and a dark
+nebula seems best to fit the observations. By that hypothesis the expanding
+billow of light surrounding the core of the conflagration is very well
+accounted for, and the spectroscopic peculiarities are also explained.
+</p>
+
+<p>
+Dr Gustov Le Bon offers a yet more alarming theory, suggesting that temporary
+stars are the result of <i>atomic explosion;</i> but we shall touch upon this
+more fully in Chapter 14.
+</p>
+
+<p>
+Twice in the course of this discussion we have called attention to the change
+of color invariably undergone by temporary stars in the later stages of their
+career. This was conspicuous with Nova Persei which glowed more and more redly
+as it faded, until the nebulous light began to overpower that of the stellar
+nucleus. Nothing could be more suggestive of the dying out of a great fire.
+Moreover, change of color from white to red is characteristic of all variable
+stars of long period, such as “Mira” in Cetus. It is also characteristic of
+stars believed to be in the later stages of evolution, and consequently
+approaching extinction, like Antares and Betelgeuse, and still more notably
+certain small stars which “gleam like rubies in the field of the telescope.”
+These last appear to be suns in the closing period of existence as
+self-luminous bodies. Between the white stars, such as Sirius and Rigel, and
+the red stars, such as Aldebaran and Alpha Herculis, there is a progressive
+series of colors from golden yellow through orange to deep red. The change is
+believed to be due to the increase of absorbing vapors in the stellar
+atmosphere as the body cools down. In the case of ordinary stars these changes
+no doubt occupy many millions of years, which represent the average duration of
+solar life; but the temporary stars run through similar changes in a few
+months: they resemble ephemeral insects—born in the morning and doomed to
+perish with the going down of the sun.
+</p>
+
+</div><!--end chapter-->
+
+<div class="chapter">
+
+<h2><a name="chap06"></a>VI<br/>
+Explosive and Whirling Nebulæ</h2>
+
+<p>
+One of the most surprising triumphs of celestial photography was Professor
+Keeler’s discovery, in 1899, that the great majority of the nebulæ have a
+distinctly spiral form. This form, previously known in Lord Rosse’s great
+“Whirlpool Nebula,” had been supposed to be exceptional; now the photographs,
+far excelling telescopic views in the revelation of nebular forms, showed the
+spiral to be the typical shape. Indeed, it is a question whether all nebulæ are
+not to some extent spiral. The extreme importance of this discovery is shown in
+the effect that it has had upon hitherto prevailing views of solar and
+planetary evolution. For more than three-quarters of a century Laplace’s
+celebrated hypothesis of the manner of origin of the solar system from a
+rotating and contracting nebula surrounding the sun had guided speculation on
+that subject, and had been tentatively extended to cover the evolution of
+systems in general. The apparent forms of some of the nebulæ which the
+telescope had revealed were regarded, and by some are still regarded, as giving
+visual evidence in favor of this theory. There is a “ring nebula” in Lyra with
+a central star, and a “planetary nebula” in Gemini bearing no little
+resemblance to the planet Saturn with its rings, both of which appear to be
+practical realizations of Laplace’s idea, and the elliptical rings surrounding
+the central condensation of the Andromeda Nebula may be cited for the same kind
+of proof.
+</p>
+
+<div class="fig" style="width:100%;">
+<a name="illus12"></a>
+<a href="images/image12.jpg">
+<img src="images/image12.jpg" width="425" height="600" alt="Illustration:" /></a>
+<p class="caption">Lord Rosse’s nebula</p>
+</div>
+
+<p>
+But since Keeler’s discovery there has been a decided turning away of
+speculation another way. The form of the spiral nebulæ seems to be entirely
+inconsistent with the theory of an originally globular or disk-shaped nebula
+condensing around a sun and throwing or leaving off rings, to be subsequently
+shaped into planets. Some astronomers, indeed, now reject Laplace’s hypothesis
+<i>in toto,</i> preferring to think that even our solar system originated from
+a spiral nebula. Since the spiral type prevails among the existing nebulæ, we
+must make any mechanical theory of the development of stars and planetary
+systems from them accord with the requirements which that form imposes. A
+glance at the extraordinary variations upon the spiral which Professor Keeler’s
+photographs reveal is sufficient to convince one of the difficulty of the task
+of basing a general theory upon them. In truth, it is much easier to criticize
+Laplace’s hypothesis than to invent a satisfactory substitute for it. If the
+spiral nebulæ seem to oppose it there are other nebulæ which appear to support
+it, and it may be that no one fixed theory can account for all the forms of
+stellar evolution in the universe. Our particular planetary system may have
+originated very much as the great French mathematician supposed, while others
+have undergone, or are now undergoing, a different process of development.
+There is always a too strong tendency to regard an important new discovery and
+the theories and speculations based upon it as revolutionizing knowledge, and
+displacing or overthrowing everything that went before. Upon the plea that
+“Laplace only made a guess” more recent guesses have been driven to extremes
+and treated by injudicious exponents as “the solid facts at last.”
+</p>
+
+<div class="fig" style="width:100%;">
+<a name="illus13"></a>
+<a href="images/image13.jpg">
+<img src="images/image13.jpg" width="482" height="600" alt="Illustration:" /></a>
+<p class="caption">Wonderful spiral in triangulum</p>
+</div>
+
+<p>
+Before considering more recent theories than Laplace’s, let us see what the
+nature of the photographic revelations is. The vast celestial maelstrom
+discovered by Lord Rosse in the “Hunting Dogs” may be taken as the leading type
+of the spiral nebulæ, although there are less conspicuous objects of the kind
+which, perhaps, better illustrate some of their peculiarities. Lord Rosse’s
+nebula appears far more wonderful in the photographs than in his drawings made
+with the aid of his giant reflecting telescope at Parsonstown, for the
+photographic plate records details that no telescope is capable of showing.
+Suppose we look at the photograph of this object as any person of common sense
+would look at any great and strange natural phenomenon. What is the first thing
+that strikes the mind? It is certainly the appearance of violent whirling
+motion. One would say that the whole glowing mass had been spun about with
+tremendous velocity, or that it had been set rotating so rapidly that it had
+become the victim of “centrifugal force,” one huge fragment having broken loose
+and started to gyrate off into space. Closer inspection shows that in addition
+to the principal focus there are various smaller condensations scattered
+through the mass. These are conspicuous in the spirals. Some of them are
+stellar points, and but for the significance of their location we might suppose
+them to be stars which happen to lie in a line between us and the nebula. But
+when we observe how many of them follow most faithfully the curves of the
+spirals we cannot but conclude that they form an essential part of the
+phenomenon; it is not possible to believe that their presence in such
+situations is merely fortuitous. One of the outer spirals has at least a dozen
+of these star-like points strung upon it; some of them sharp, small, and
+distinct, others more blurred and nebulous, suggesting different stages of
+condensation. Even the part which seems to have been flung loose from the main
+mass has, in addition to its central condensation, at least one stellar point
+gleaming in the half-vanished spire attached to it. Some of the more distant
+stars scattered around the “whirlpool” look as if they too had been shot out of
+the mighty vortex, afterward condensing into unmistakable solar bodies. There
+are at least two curved rows of minute stars a little beyond the periphery of
+the luminous whirl which clearly follow lines concentric with those of the
+nebulous spirals. Such facts are simply dumbfounding for anyone who will bestow
+sufficient thought upon them, for these are <i>suns,</i> though they may be
+small ones; and what a birth is that for a sun!
+</p>
+
+<p>
+Look now again at the glowing spirals. We observe that hardly have they left
+the central mass before they begin to coagulate. In some places they have a
+“ropy” aspect; or they are like peascods filled with growing seeds, which
+eventually will become stars. The great focus itself shows a similar tendency,
+especially around its circumference. The sense that it imparts of a tremendous
+shattering force at work is overwhelming. There is probably more matter in that
+whirling and bursting nebula than would suffice to make a hundred solar
+systems! It must be confessed at once that there is no confirmation of the
+Laplacean hypothesis here; but what hypothesis will fit the facts? There is one
+which it has been claimed does so, but we shall come to that later. In the
+meanwhile, as a preparation, fix in the memory the appearance of that second
+spiral mass spinning beside its master which seems to have spurned it away.
+</p>
+
+<div class="fig" style="width:100%;">
+<a name="illus14"></a>
+<a href="images/image14.jpg">
+<img src="images/image14.jpg" width="407" height="600" alt="Illustration:" /></a>
+<p class="caption">Spiral in Ursa Major</p>
+</div>
+
+<p>
+For a second example of the spiral nebulæ look at the one in the constellation
+Triangulum. <i>God, how hath the imagination of puny man failed to comprehend
+Thee!</i> Here is creation through destruction with a vengeance! The spiral
+form of the nebula is unmistakable, but it is half obliterated amid the turmoil
+of flying masses hurled away on all sides with tornadic fury. The focus itself
+is splitting asunder under the intolerable strain, and in a little while, as
+time is reckoned in the Cosmos, it will be gyrating into stars. And then look
+at the cyclonic rain of already finished stars whirling round the outskirts of
+the storm. Observe how scores of them are yet involved in the fading streams of
+the nebulous spirals; see how they have been thrown into vast loops and curves,
+of a beauty that half redeems the terror of the spectacle enclosed within their
+lines—like iridescent cirri hovering about the edges of a hurricane. And so
+again are suns born!
+</p>
+
+<p>
+Let us turn to the exquisite spiral in Ursa Major; how different its aspect
+from that of the other! One would say that if the terrific coil in Triangulum
+has all but destroyed itself in its fury, this one on the contrary has just
+begun its self-demolition. As one gazes one seems to see in it the smooth,
+swift, accelerating motion that precedes catastrophe. The central part is still
+intact, dense, and uniform in texture. How graceful are the spirals that
+smoothly rise from its oval rim and, gemmed with little stars, wind off into
+the darkness until they have become as delicate as threads of gossamer! But at
+bottom the story told here is the same—creation by gyration!
+</p>
+
+<p>
+Compare with the above the curious mass in Cetus. Here the plane of the
+whirling nebula nearly coincides with our line of sight and we see the object
+at a low angle. It is far advanced and torn to shreds, and if we could look at
+it perpendicularly to its plane it is evident that it would closely resemble
+the spectacle in Triangulum.
+</p>
+
+<p>
+Then take the famous Andromeda Nebula (see Frontispiece), which is so vast that
+notwithstanding its immense distance even the naked eye perceives it as an
+enigmatical wisp in the sky. Its image on the sensitive plate is the
+masterpiece of astronomical photography; for wild, incomprehensible beauty
+there is nothing that can be compared with it. Here, if anywhere, we look upon
+the spectacle of creation in one of its earliest stages. The Andromeda Nebula
+is apparently less advanced toward transformation into stellar bodies than is
+that in Triangulum. The immense crowd of stars sprinkled over it and its
+neighborhood seem in the main to lie this side of the nebula, and consequently
+to have no connection with it. But incipient stars (in some places clusters of
+them) are seen in the nebulous rings, while one or two huge masses seem to give
+promise of transformation into stellar bodies of unusual magnitude. I say
+“rings” because although the loops encompassing the Andromeda Nebula have been
+called spirals by those who wish utterly to demolish Laplace’s hypothesis, yet
+they are not manifestly such, as can be seen on comparing them with the
+undoubted spirals of the Lord Rosse Nebula. They look quite as much like
+circles or ellipses seen at an angle of, say, fifteen or twenty degrees to
+their plane. If they are truly elliptical they accord fairly well with
+Laplace’s idea, except that the scale of magnitude is stupendous, and if the
+Andromeda Nebula is to become a solar system it will surpass ours in grandeur
+beyond all possibility of comparison.
+</p>
+
+<div class="fig" style="width:100%;">
+<a name="illus15"></a>
+<a href="images/image15.jpg">
+<img src="images/image15.jpg" width="457" height="600" alt="Illustration:" /></a>
+<p class="caption">Nebula in Cetus</p>
+</div>
+
+<p>
+There is one circumstance connected with the spiral nebulæ, and conspicuous in
+the Andromeda Nebula on account of its brightness, which makes the question of
+their origin still more puzzling; they all show continuous spectra, which, as
+we have before remarked, indicate that the mass from which the light comes is
+either solid or liquid, or a gas under heavy pressure. Thus nebulæ fall into
+two classes: the “white” nebulæ, giving a continuous spectrum; and the “green”
+nebulæ whose spectra are distinctly gaseous. The Andromeda Nebula is the great
+representative of the former class and the Orion Nebula of the latter. The
+spectrum of the Andromeda Nebula has been interpreted to mean that it consists
+not of luminous gas, but of a flock of stars so distant that they are
+separately indistinguishable even with powerful telescopes, just as the
+component stars of the Milky Way are indistinguishable with the naked eye; and
+upon this has been based the suggestion that what we see in Andromeda is an
+outer universe whose stars form a series of elliptical garlands surrounding a
+central mass of amazing richness. But this idea is unacceptable if for no other
+reason than that, as just said, all the spiral nebulæ possess the same kind of
+spectrum, and probably no one would be disposed to regard them all as outer
+universes. As we shall see later, the peculiarity of the spectra of the spiral
+nebulæ is appealed to in support of a modern substitute for Laplace’s
+hypothesis.
+</p>
+
+<p>
+Finally, without having by any means exhausted the variety exhibited by the
+spiral nebulæ, let us turn to the great representative of the other species,
+the Orion Nebula. In some ways this is even more marvelous than the others. The
+early drawings with the telescope failed to convey an adequate conception
+either of its sublimity or of its complication of structure. It exists in a
+nebulous region of space, since photographs show that nearly the whole
+constellation is interwoven with faintly luminous coils. To behold the entry of
+the great nebula into the field even of a small telescope is a startling
+experience which never loses its novelty. As shown by the photographs, it is an
+inscrutable chaos of perfectly amazing extent, where spiral bands, radiating
+streaks, dense masses, and dark yawning gaps are strangely intermingled without
+apparent order. In one place four conspicuous little stars, better seen in a
+telescope than in the photograph on account of the blurring produced by
+over-exposure, are suggestively situated in the midst of a dark opening, and no
+observer has ever felt any doubt that these stars have been formed from the
+substance of the surrounding nebula. There are many other stars scattered over
+its expanse which manifestly owe their origin to the same source. But compare
+the general appearance of this nebula with the others that we have studied, and
+remark the difference. If the unmistakably spiral nebulæ resemble bursting
+fly-wheels or grindstones from whose perimeters torrents of sparks are flying,
+the Orion Nebula rather recalls the aspect of a cloud of smoke and fragments
+produced by the explosion of a shell. This idea is enforced by the look of the
+outer portion farthest from the bright half of the nebula, where sharply edged
+clouds with dark spaces behind seem to be billowing away as if driven by a wind
+blowing from the center.
+</p>
+
+<div class="fig" style="width:100%;">
+<a name="illus16"></a>
+<a href="images/image16.jpg">
+<img src="images/image16.jpg" width="600" height="595" alt="Illustration:" /></a>
+<p class="caption">The Orion nebula</p>
+</div>
+
+<p>
+Next let us consider what scientific speculation has done in the effort to
+explain these mysteries. Laplace’s hypothesis can certainly find no standing
+ground either in the Orion Nebula or in those of a spiral configuration,
+whatever may be its situation with respect to the grand Nebula of Andromeda, or
+the “ring” and “planetary” nebulæ. Some other hypothesis more consonant with
+the appearances must be found. Among the many that have been proposed the most
+elaborate is the “Planetesimal Hypothesis” of Professors Chamberlin and
+Moulton. It is to be remarked that it applies to the spiral nebulæ
+distinctively, and not to an apparently chaotic mass of gas like the vast
+luminous cloud in Orion. The gist of the theory is that these curious objects
+are probably the result of close approaches to each other of two independent
+suns, reminding us of what was said on this subject when we were dealing with
+temporary stars. Of the previous history of these appulsing suns the theory
+gives us no account; they are simply supposed to arrive within what may be
+called an effective tide-producing distance, and then the drama begins. Some of
+the probable consequences of such an approach have been noticed in Chapter 5;
+let us now consider them a little more in detail.
+</p>
+
+<p>
+Tides always go in couples; if there is a tide on one side of a globe there
+will be a corresponding tide on the other side. The cause is to be found in the
+law that the force of gravitation varies inversely as the square of the
+distance; the attraction on the nearest surface of the body exercised by
+another body is greater than on its center, and greater yet than on its
+opposite surface. If two great globes attract each other, each tends to draw
+the other out into an ellipsoidal figure; they must be more rigid than steel to
+resist this—and even then they cannot altogether resist. If they are liquid or
+gaseous they will yield readily to the force of distortion, the amount of which
+will depend upon their distance apart, for the nearer they are the greater
+becomes the tidal strain. If they are encrusted without and liquid or gaseous
+in the interior, the internal mass will strive to assume the figure demanded by
+the tidal force, and will, if it can, burst the restraining envelope. Now this
+is virtually the predicament of the body we call a sun when in the immediate
+presence of another body of similarly great mass. Such a body is presumably
+gaseous throughout, the component gases being held in a state of rigidity by
+the compression produced by the tremendous gravitational force of their own
+aggregate mass. At the surface such a body is enveloped in a shell of
+relatively cool matter. Now suppose a great attracting body, such as another
+sun, to approach near enough for the difference in its attraction on the two
+opposite sides of the body and on its center to become very great; the
+consequence will be a tidal deformation of the whole body, and it will lengthen
+out along the line of the gravitational pull and draw in at the sides, and if
+its shell offers considerable resistance, but not enough to exercise a complete
+restraint, it will be violently burst apart, or blown to atoms, and the
+internal mass will leap out on the two opposite sides in great fiery spouts. In
+the case of a sun further advanced in cooling than ours the interior might be
+composed of molten matter while the exterior crust had become rigid like the
+shell of an egg; then the force of the “tidal explosion” produced by the
+appulse of another sun would be more violent in consequence of the greater
+resistance overcome. Such, then, is the mechanism of the first phase in the
+history of a spiral nebula according to the Planetesimal Hypothesis. Two suns,
+perhaps extinguished ones, have drawn near together, and an explosive outburst
+has occured in one or both. The second phase calls for a more agile exercise of
+the imagination.
+</p>
+
+<p>
+To simplify the case, let us suppose that only one of the tugging suns is
+seriously affected by the strain. Its vast wings produced by the outburst are
+twisted into spirals by their rotation and the contending attractions exercised
+upon them, as the two suns, like battleships in desperate conflict, curve round
+each other, concentrating their destructive energies. Then immense quantities
+of débris are scattered about in which eddies are created, and finally, as the
+sun that caused the damage goes on its way, leaving its victim to repair its
+injuries as it may, the dispersed matter cools, condenses, and turns into
+streams of solid particles circling in elliptical paths about their parent sun.
+These particles, or fragments, are the “planetesimals” of the theory. In
+consequence of the inevitable intersection of the orbits of the planetesimals,
+nodes are formed where the flying particles meet, and at these nodes large
+masses are gradually accumulated. The larger the mass the greater its
+attraction, and at last the nodal points become the nuclei of great
+aggregations from which planets are shaped.
+</p>
+
+<p>
+This, in very brief form, is the Planetesimal Hypothesis which we are asked to
+substitute for that based on Laplace’s suggestion as an explanation of the mode
+of origin of the solar system; and the phenomena of the spiral nebulæ are
+appealed to as offering evident support to the new hypothesis. We are reminded
+that they are elliptical in outline, which accords with the hypothesis; that
+their spectra are not gaseous, which shows that they may be composed of solid
+particles like the planetesimals; and that their central masses present an oval
+form, which is what would result from the tidal effects, as just described. We
+also remember that some of them, like the Lord Rosse and the Andromeda nebulæ,
+are visually double, and in these cases we might suppose that the two masses
+represent the tide-burst suns that ventured into too close proximity. It may be
+added that the authors of the theory do not insist upon the appulse of two suns
+as the <i>only</i> way in which the planetesimals may have originated, but it
+is the only supposition that has been worked out.
+</p>
+
+<p>
+But serious questions remain. It needs, for instance, but a glance at the
+Triangulum monster to convince the observer that it cannot be a solar system
+which is being evolved there, but rather a swarm of stars. Many of the detached
+masses are too vast to admit of the supposition that they are to be transformed
+into planets, in our sense of planets, and the distances of the stars which
+appear to have been originally ejected from the focal masses are too great to
+allow us to liken the assemblage that they form to a solar system. Then, too,
+no nodes such as the hypothesis calls for are visible. Moreover, in most of the
+spiral nebulæ the appearances favor the view that the supposititious
+encountering suns have not separated and gone each rejoicing on its way, after
+having inflicted the maximum possible damage on its opponent, but that, on the
+contrary, they remain in close association like two wrestlers who cannot escape
+from each other’s grasp. And this is exactly what the law of gravitation
+demands; stars cannot approach one another with impunity, with regard either to
+their physical make-up or their future independence of movement. The theory
+undertakes to avoid this difficulty by assuming that in the case of our system
+the approach of the foreign body to the sun was not a close one—just close
+enough to produce the tidal extrusion of the relatively insignificant quantity
+of matter needed to form the planets. But even then the effect of the appulse
+would be to change the direction of flight, both of the sun and of its visitor,
+and there is no known star in the sky which can be selected as the sun’s
+probable partner in their ancient <i>pas deux.</i> That there are unconquered
+difficulties in Laplace’s hypothesis no one would deny, but in simplicity of
+conception it is incomparably more satisfactory, and with proper modifications
+could probably be made more consonant with existing facts in our solar system
+than that which is offered to replace it. Even as an explanation of the spiral
+nebulæ, not as solar systems in process of formation, but as the birthplaces of
+stellar clusters, the Planetesimal Hypothesis would be open to many objections.
+Granting its assumptions, it has undoubtedly a strong mathematical framework,
+but the trouble is not with the mathematics but with the assumptions. Laplace
+was one of the ablest mathematicians that ever lived, but he had never seen a
+spiral nebula; if he had, he might have invented a hypothesis to suit its
+phenomena. His actual hypothesis was intended only for our solar system, and he
+left it in the form of a “note” for the consideration of his successors, with
+the hope that they might be able to discover the full truth, which he confessed
+was hidden from him. It cannot be said that that truth has yet been found, and
+when it is found the chances are that intuition and not logic will have led to
+it.
+</p>
+
+<p>
+The spiral nebulæ, then, remain among the greatest riddles of the universe,
+while the gaseous nebulæ, like that of Orion, are no less mysterious, although
+it seems impossible to doubt that both forms give birth to stars. It is but
+natural to look to them for light on the question of the origin of our
+planetary system; but we should not forget that the scale of the phenomena in
+the two cases is vastly different, and the forces in operation may be equally
+different. A hill may have been built up by a glacier, while a mountain may be
+the product of volcanic forces or of the upheaval of the strata of the planet.
+</p>
+
+</div><!--end chapter-->
+
+<div class="chapter">
+
+<h2><a name="chap07"></a>VII<br/>
+The Banners of the Sun</h2>
+
+<p>
+As all the world knows, the sun, a blinding globe pouring forth an
+inconceivable quantity of light and heat, whose daily passage through the sky
+is caused by the earth’s rotation on its axis, constitutes the most important
+phenomenon of terrestial existence. Viewed with a dark glass to take off the
+glare, or with a telescope, its rim is seen to be a sharp and smooth circle,
+and nothing but dark sky is visible around it. Except for the interference of
+the moon, we should probably never have known that there is any more of the sun
+than our eyes ordinarily see.
+</p>
+
+<p>
+But when an eclipse of the sun occurs, caused by the interposition of the
+opaque globe of the moon, we see its immediate surroundings, which in some
+respects are more wonderful than the glowing central orb. These surroundings,
+although not in the sense in which we apply the term to the gaseous envelope of
+the earth, may be called the sun’s atmosphere. They consist of two very
+different parts—first, the red “prominences,” which resemble tongues of flame
+ascending thousands of miles above the sun’s surface; and, second, the
+“corona,” which extends to distances of millions of miles from the sun, and
+shines with a soft, glowing light. The two combined, when well seen, make a
+spectacle without parallel among the marvels of the sky. Although many attempts
+have been made to render the corona visible when there is no eclipse, all have
+failed, and it is to the moon alone that we owe its revelation. To cover the
+sun’s disk with a circular screen will not answer the purpose because of the
+illumination of the air all about the observer. When the moon hides the sun, on
+the other hand, the sunlight is withdrawn from a great cylinder of air
+extending to the top of the atmosphere and spreading many miles around the
+observer. There is then no glare to interfere with the spectacle, and the
+corona appears in all its surprising beauty. The prominences, however, although
+they were discovered during an eclipse, can now, with the aid of the
+spectroscope, be seen at any time. But the prominences are rarely large enough
+to be noticed by the naked eye, while the streamers of the corona, stretching
+far away in space, like ghostly banners blown out from the black circle of the
+obscuring moon, attract every eye, and to this weird apparition much of the
+fear inspired by eclipses has been due. But if the corona has been a cause of
+terror in the past it has become a source of growing knowledge in our time.
+</p>
+
+<div class="fig" style="width:100%;">
+<a name="illus17"></a>
+<a href="images/image17.jpg">
+<img src="images/image17.jpg" width="600" height="454" alt="Illustration:" /></a>
+<p class="caption">The Corona</p>
+</div>
+
+<p>
+The story of the first scientific observation of the corona and the prominences
+is thrillingly interesting, and in fact dramatic. The observation was made
+during the eclipse of 1842, which fortunately was visible all over Central and
+Southern Europe so that scores of astronomers saw it. The interest centers in
+what happened at Pavia in Northern Italy, where the English astronomer Francis
+Baily had set up his telescope. The eclipse had begun and Bailey was busy at
+his telescope when, to quote his own words in the account which he wrote for
+the <i>Memoirs of the Royal Astronomical Society:</i>
+</p>
+
+<div class="letter">
+<p>
+I was astounded by a tremendous burst of applause from the streets below, and
+at the same moment was electrified by the sight of one of the most brilliant
+and splendid phenomena that can well be imagined; for at that instant the dark
+body of the moon was suddenly surrounded with a corona, or kind of bright
+glory, similar in shape and magnitude to that which painters draw round the
+heads of saints...
+</p>
+
+<p>
+Pavia contains many thousand inhabitants, the major part of whom were at this
+early hour walking about the streets and squares or looking out of windows in
+order to witness this long-talked-of phenomenon; and when the total obscuration
+took place, which was <i>instantaneous,</i> there was a universal shout from
+every observer which “made the welkin ring,” and for the moment withdrew my
+attention from the object with which I was immediately occupied. I had, indeed,
+expected the appearance of a luminous circle round the moon during the time of
+total obscurity; but I did not expect, from any of the accounts of preceding
+eclipses that I had read, to witness so magnificent an exhibition as that which
+took place...
+</p>
+
+<p>
+Splendid and astonishing, however, as this remarkable phenomenon really was,
+and although it could not fail to call forth the admiration and applause of
+every beholder, yet I must confess that there was at the same time something in
+its singular and wonderful appearance that was appalling...
+</p>
+
+<p>
+But the most remarkable circumstance attending the phenomenon was the
+appearance of <i>three large protuberances</i> apparently emanating from the
+circumference of the moon, but evidently forming a portion of the corona. They
+had the appearance of mountains of a prodigious elevation; their color was red
+tinged with lilac or purple; perhaps the color of the peach-blossom would more
+nearly represent it. They somewhat resembled the tops of the snowy Alpine
+mountains when colored by the rising or the setting sun. They resembled the
+Alpine mountains in another respect, inasmuch as their light was perfectly
+steady, and had none of that flickering or sparkling motion so visible in other
+parts of the corona...
+</p>
+
+<p>
+The whole of these protuberances were visible even to the last moment of total
+obscuration, and when the first ray of light was admitted from the sun they
+vanished, with the corona, altogether, and daylight was instantly restored.
+</p>
+</div>
+
+<p>
+I have quoted nearly all of this remarkable description not alone for its
+intrinsic interest, but because it is the best depiction that can be found of
+the general phenomena of a total solar eclipse. Still, not every such eclipse
+offers an equally magnificent spectacle. The eclipses of 1900 and 1905, for
+instance, which were seen by the writer, the first in South Carolina and the
+second in Spain, fell far short of that described by Bailey in splendor and
+impressiveness. Of course, something must be allowed for the effect of
+surprise; Bailey had not expected to see what was so suddenly disclosed to him.
+But both in 1900 and 1905 the amount of scattered light in the sky was
+sufficient in itself to make the corona appear faint, and there were no very
+conspicuous prominences visible. Yet on both occasions there was manifest among
+the spectators that mingling of admiration and awe of which Bailey speaks. The
+South Carolinians gave a cheer and the ladies waved their handkerchiefs when
+the corona, ineffably delicate of form and texture, <i>melted</i> into sight
+and then in two minutes melted away again. The Spaniards, crowded on the
+citadel hill of Burgos, with their king and his royal retinue in their midst,
+broke out with a great clapping of hands as the awaited spectacle unfolded
+itself in the sky; and on both occasions, before the applause began, after an
+awed silence a low murmur ran through the crowds. At Burgos it is said many
+made the sign of the cross.
+</p>
+
+<p>
+It was not long before Bailey’s idea that the prominences were a part of the
+corona was abandoned, and it was perceived that the two phenomena were to a
+great extent independent. At the eclipse of 1868, which the astronomers,
+aroused by the wonderful scene of 1842, and eager to test the powers of the
+newly invented spectroscope, flocked to India to witness, Janssen conceived the
+idea of employing the spectroscope to render the prominences visible when there
+was no eclipse. He succeeded the very next day, and these phenomena have been
+studied in that way ever since.
+</p>
+
+<p>
+There are recognized two kinds of prominences—the “erruptive” and the
+“quiescent.” The latter, which are cloud-like in form, may be seen almost
+anywhere along the edge of the sun; but the former, which often shoot up as if
+hurled from mighty volcanoes, appear to be associated with sun-spots, and
+appear only above the zones where spots abound. Either of them, when seen in
+projection against the brilliant solar disk, appears white, not red, as against
+a background of sky. The quiescent prominences, whose elevation is often from
+forty thousand to sixty thousand miles, consist, as the spectroscope shows,
+mainly of hydrogen and helium. The latter, it will be remembered, is an element
+which was known to be in the sun many years before the discovery that it also
+exists in small quantities on the earth. A fact which may have a significance
+which we cannot at present see is that the emanation from radium gradually and
+spontaneously changes into helium, an alchemistical feat of nature that has
+opened many curious vistas to speculative thinkers. The eruptive prominences,
+which do not spread horizontally like the others, but ascend with marvelous
+velocity to elevations of half a million miles or more, are apparently composed
+largely of metallic vapors—<i>i.e.</i> metals which are usually solid on the
+earth, but which at solar temperatures are kept in a volatilized state. The
+velocity of their ascent occasionally amounts to three hundred or four hundred
+miles per second. It is known from mathematical considerations that the
+gravitation of the sun would not be able to bring back any body that started
+from its surface with a velocity exceeding three hundred and eighty-three miles
+per second; so it is evident that some of the matter hurled forth in eruptive
+prominences may escape from solar control and go speeding out into space,
+cooling and condensing into solid masses. There seems to be no reason why some
+of the projectiles from the sun might not reach the planets. Here, then, we
+have on a relatively small scale, <i>explosions</i> recalling those which it
+has been imagined may be the originating cause of some of the sudden phenomena
+of the stellar heavens.
+</p>
+
+<div class="fig" style="width:100%;">
+<a name="illus18"></a>
+<a href="images/image18.jpg">
+<img src="images/image18.jpg" width="478" height="600" alt="Illustration:" /></a>
+<p class="caption">A solar “prominence.” Photographed May 21, 1907</p>
+</div>
+
+<p>
+Of the sun-spots it is not our intention here specifically to speak, but they
+evidently have an intimate connection with eruptive prominences, as well as
+some relation, not yet fully understood, with the corona. Of the real cause of
+sun-spots we know virtually nothing, but recent studies by Professor Hale and
+others have revealed a strange state of things in the clouds of metallic vapors
+floating above them and their surroundings. Evidences of a cyclonic tendency
+have been found, and Professor Hale has proved that sun-spots are strong
+magnetic fields, and consist of columns of ionized vapors rotating in opposite
+directions in the two hemispheres. A fact which may have the greatest
+significance is that titanium and vanadium have been found both in sun-spots
+and in the remarkable variable Mira Ceti, a star which every eleven months, or
+thereabout, flames up with great brilliancy and then sinks back to invisibility
+with the naked eye. It has been suggested that sun-spots are indications of the
+beginning of a process in the sun which will be intensified until it falls into
+the state of such a star as Mira. Stars very far advanced in evolution, without
+showing variability, also exhibit similar spectra; so that there is much reason
+for regarding sunspots as emblems of advancing age.
+</p>
+
+<p>
+The association of the corona with sun-spots is less evident than that of the
+eruptive prominences; still such an association exists, for the form and extent
+of the corona vary with the sun-spot period of which we shall presently speak.
+The constitution of the corona remains to be discovered. It is evidently in
+part gaseous, but it also probably contains matter in the form of dust and
+small meteors. It includes one substance altogether mysterious—“coronium.”
+There are reasons for thinking that this may be the lightest of all the
+elements, and Professor Young, its discoverer, said that it was “absolutely
+unique in nature; utterly distinct from any other known form of matter,
+terrestial, solar, or cosmical.” The enormous extent of the corona is one of
+its riddles. Since the development of the curious subject of the “pressure of
+light” it has been proposed to account for the sustentation of the corona by
+supposing that it is borne upon the billows of light continually poured out
+from the sun. Experiment has proved, what mathematical considerations had
+previously pointed out as probable, that the waves of light exert a pressure or
+driving force, which becomes evident in its effects if the body acted upon is
+sufficiently small. In that case the light pressure will prevail over the
+attraction of gravitation, and propel the attenuated matter away from the sun
+in the teeth of its attraction. The earth itself would be driven away if,
+instead of consisting of a solid globe of immense aggregate mass, it were a
+cloud of microscopic particles. The reason is that the pressure varies in
+proportion to the <i>surface</i> of the body acted upon, while the
+gravitational attraction is proportional to the <i>volume,</i> or the total
+amount of matter in the body. But the surface of any body depends upon the
+<i>square</i> of its diameter, while the volume depends upon the <i>cube</i> of
+the diameter. If, for instance, the diameter is represented by 4, the surface
+will be proportional to 4 &times; 4, or 16, and the volume to 4 &times; 4
+&times; 4, or 64; but if the diameter is taken as 2, the surface will be 2
+&times; 2, or 4, and the volume 2 &times; 2 &times; 2, or 8. Now, the ratio of
+4 to 8 is twice as great as that of 16 to 64. If the diameter is still further
+decreased, the ratio of the surface to the volume will proportionally grow
+larger; in other words, the pressure will gain upon the attraction, and
+whatever their original ratio may have been, a time will come, if the
+diminution of size continues, when the pressure will become more effective than
+the attraction, and the body will be driven away. Supposing the particles of
+the corona to be below the critical size for the attraction of a mass like that
+of the sun to control them, they would be driven off into the surrounding space
+and appear around the sun like the clouds of dust around a mill. We shall
+return to this subject in connection with the Zodiacal Light, the Aurora, and
+Comets.
+</p>
+
+<p>
+On the other hand, there are parts of the corona which suggest by their forms
+the play of electric or magnetic forces. This is beautifully shown in some of
+the photographs that have been made of the corona during recent eclipses. Take,
+for instance, that of the eclipse of 1900. The sheaves of light emanating from
+the poles look precisely like the “lines of force” surrounding the poles of a
+magnet. It will be noticed in this photograph that the corona appears to
+consist of two portions: one comprising the polar rays just spoken of, and the
+other consisting of the broader, longer, and less-defined masses of light
+extending out from the equatorial and middle-latitude zones. Yet even in this
+more diffuse part of the phenomenon one can detect the presence of submerged
+curves bearing more or less resemblance to those about the poles. Just what
+part electricity or electro-magnetism plays in the mechanism of the solar
+radiation it is impossible to say, but on the assumption that it is a very
+important part is based the hypothesis that there exists a direct solar
+influence not only upon the magnetism, but upon the weather of the earth. This
+hypothesis has been under discussion for half a century, and still we do not
+know just how much truth it represents. It is certain that the outbreak of
+great disturbances on the sun, accompanied by the formation of sun-spots and
+the upshooting of eruptive prominences (phenomena which we should naturally
+expect to be attended by action), have been instantly followed by corresponding
+“magnetic storms” on the earth and brilliant displays of the auroral lights.
+There have been occasions when the influence has manifested itself in the most
+startling ways, a great solar outburst being followed by a mysterious gripping
+of the cable and telegraph systems of the world, as if an invisible and
+irresistible hand had seized them. Messages are abruptly cut off, sparks leap
+from the telegraph instruments, and the entire earth seems to have been thrown
+into a magnetic flurry. These occurrences affect the mind with a deep
+impression of the dependence of our planet on the sun, such as we do not derive
+from the more familiar action of the sunlight on the growth of plants and other
+phenomena of life depending on solar influences.
+</p>
+
+<p>
+Perhaps the theory of solar magnetic influence upon the weather is best known
+in connection with the “sun-spot cycle.” This, at any rate, is, as already
+remarked, closely associated with the corona. Its existence was discovered in
+1843 by the German astronomer Schwabe. It is a period of variable length,
+averaging about eleven years, during which the number of spots visible on the
+sun first increases to a maximum, then diminishes to a minimum, and finally
+increases again to a maximum. For unknown reasons the period is sometimes two
+or three years longer than the average and sometimes as much shorter.
+Nevertheless, the phenomena always recur in the same order. Starting, for
+instance, with a time when the observer can find few or no spots, they
+gradually increase in number and size until a maximum, in both senses, is
+reached, during which the spots are often of enormous size and exceedingly
+active. After two or three years they begin to diminish in number, magnitude,
+and activity until they almost or quite disappear. A strange fact is that when
+a new period opens, the spots appear first in high northern and southern
+latitudes, far from the solar equator, and as the period advances they not only
+increase in number and size, but break out nearer and nearer to the equator,
+the last spots of a vanishing period sometimes lingering in the equatorial
+region after the advance-guard of its successor has made its appearance in the
+high latitudes. Spots are never seen on the equator nor near the poles. It was
+not very long after the discovery of the sun-spot cycle that the curious
+observation was made that a striking coincidence existed between the period of
+the sun-spots and another period affecting the general magnetic condition of
+the earth. When a curved line representing the varying number of sun-spots was
+compared with another curve showing the variations in the magnetic state of the
+earth the two were seen to be in almost exact accord, a rise in one curve
+corresponding to a rise in the other, and a fall to a fall. Continued
+observation has proved that this is a real coincidence and not an accidental
+one, so that the connection, although as yet unexplained, is accepted as
+established. But does the influence extend further, and directly affect the
+weather and the seasons as well as the magnetic elements of the earth? A final
+answer to this question cannot yet be given, for the evidence is contradictory,
+and the interpretations put upon it depend largely on the predilections of the
+judges.
+</p>
+
+<p>
+But, in a broad sense, the sun-spots and the phenomena connected with them
+<i>must</i> have a relation to terrestial meteorology, for they prove the sun
+to be a variable star. Reference was made, a few lines above, to the
+resemblance of the spectra of sun-spots to those of certain stars which seem to
+be failing through age. This in itself is extremely suggestive; but if this
+resemblance had never been discovered, we should have been justified in
+regarding the sun as variable in its output of energy; and not only variable,
+but probably increasingly so. The very inequalities in the sun-spot cycle are
+suspicious. When the sun is most spotted its total light may be reduced by
+one-thousandth part, although it is by no means certain that its outgiving of
+thermal radiations is then reduced. A loss of one-thousandth of its luminosity
+would correspond to a decrease of .0025 of a stellar magnitude, considering the
+sun as a star viewed from distant space. So slight a change would not be
+perceptible; but it is not alone sun-spots which obscure the solar surface, its
+entire globe is enveloped with an obscuring veil. When studied with a powerful
+telescope the sun’s surface is seen to be thickly mottled with relatively
+obscure specks, so numerous that it has been estimated that they cut off from
+one-tenth to one-twentieth of the light that we should receive from it if the
+whole surface were as brilliant as its brightest parts. The condition of other
+stars warrants the conclusion that this obscuring envelope is the product of a
+process of refrigeration which will gradually make the sun more and more
+variable until its history ends in extinction. Looking backward, we see a time
+when the sun must have been more brilliant than it is now. At that time it
+probably shone with the blinding white splendor of such stars as Sirius, Spica,
+and Vega; now it resembles the relatively dull Procyon; in time it will turn
+ruddy and fall into the closing cycle represented by Antares. Considering that
+once it must have been more radiantly powerful than at present, one is tempted
+to wonder if that could have been the time when tropical life flourished within
+the earth’s polar circles, sustained by a vivific energy in the sun which it
+has now lost.
+</p>
+
+<p>
+The corona, as we have said, varies with the sun-spot cycle. When the spots are
+abundant and active the corona rises strong above the spotted zones, forming
+immense beams or streamers, which on one occasion, at least, had an observed
+length of <i>ten million miles.</i> At the time of a spot minimum the corona is
+less brilliant and has a different outline. It is then that the curved polar
+rays are most conspicuous. Thus the vast banners of the sun, shaken out in the
+eclipse, are signals to tell of its varying state, but it will probably be long
+before we can read correctly their messages.
+</p>
+
+</div><!--end chapter-->
+
+<div class="chapter">
+
+<h2><a name="chap08"></a>VIII<br/>
+The Zodiacal Light Mystery</h2>
+
+<p>
+There is a singular phenomenon in the sky—one of the most puzzling of all—which
+has long arrested the attention of astronomers, defying their efforts at
+explanation, but which probably not one in a hundred, and possibly not one in a
+thousand, of the readers of this book has ever seen. Yet its name is often
+spoken, and it is a conspicuous object if one knows when and where to look for
+it, and when well seen it exhibits a mystical beauty which at the same time
+charms and awes the beholder. It is called “The Zodiacal Light,” because it
+lies within the broad circle of the Zodiac, marking the sun’s apparent annual
+path through the stars. What it is nobody has yet been able to find out with
+certainty, and books on astronomy usually speak of it with singular reserve.
+But it has given rise to many remarkable theories, and a true explanation of it
+would probably throw light on a great many other celestial mysteries. The Milky
+Way is a more wonderful object to look upon, but its nature can be
+comprehended, while there is a sort of uncanniness about the Zodiacal Light
+which immediately impresses one upon seeing it, for its part in the great
+scheme of extra-terrestrial affairs is not evident.
+</p>
+
+<p>
+If you are out-of-doors soon after sunset—say, on an evening late in the month
+of February—you may perceive, just after the angry flush of the dying winter’s
+day has faded from the sky, a pale ghostly presence rising above the place
+where the sun went down. The writer remembers from boyhood the first time it
+was pointed out to him and the unearthly impression that it made, so that he
+afterward avoided being out alone at night, fearful of seeing the spectral
+thing again. The phenomenon brightens slowly with the fading of the twilight,
+and soon distinctly assumes the shape of an elongated pyramid of pearly light,
+leaning toward the south if the place of observation is in the northern
+hemisphere. It does not impress the observer at all in the same manner as the
+Milky Way; that looks far off and is clearly among the stars, but the Zodiacal
+Light seems closer at hand, as if it were something more intimately concerning
+the earth. To all it immediately suggests a connection, also, with the sunken
+sun. If the night is clear and the moon absent (and if you are in the country,
+for city lights ruin the spectacles of the sky), you will be able to watch the
+apparition for a long time. You will observe that the light is brightest near
+the horizon, gradually fading as the pyramidal beam mounts higher, but in
+favorable circumstances it may be traced nearly to the meridian south of the
+zenith, where its apex at last vanishes in the starlight. It continues visible
+during the evenings of March and part of April, after which, ordinarily, it is
+seen no more, or if seen is relatively faint and unimpressive. But when autumn
+comes it appears again, this time not like a wraith hovering above the westward
+tomb of the day-god, but rather like a spirit of the morning announcing his
+reincarnation in the east.
+</p>
+
+<p>
+The reason why the Zodiacal Light is best seen in our latitudes at the periods
+just mentioned is because at those times the Zodiac is more nearly
+perpendicular to the horizon, first in the west and then in the east; and,
+since the phenomenon is confined within the borders of the Zodiac, it cannot be
+favorably placed for observation when the zodiacal plane is but slightly
+inclined to the horizon. Its faint light requires the contrast of a background
+of dark sky in order to be readily perceptible. But within the tropics, where
+the Zodiac is always at a favorable angle, the mysterious light is more
+constantly visible. Nearly all observant travelers in the equatorial regions
+have taken particular note of this phenomenon, for being so much more
+conspicuous there than in the temperate zones it at once catches the eye and
+holds the attention as a novelty. Humboldt mentions it many times in his works,
+for his genius was always attracted by things out of the ordinary and difficult
+of explanation, and he made many careful observations on its shape, its
+brilliancy, and its variations; for there can be no doubt that it does vary,
+and sometimes to an astonishing degree. It is said that it once remained
+practically invisible in Europe for several years in succession. During a trip
+to South Africa in 1909 an English astronomer, Mr E. W. Maunder, found a
+remarkable difference between the appearance of the Zodiacal Light on his going
+and coming voyages. In fact, when crossing the equator going south he did not
+see it at all; but on returning he had, on March 6th, when one degree south of
+the equator, a memorable view of it.
+</p>
+
+<p class="letter">
+It was a bright, clear night, and the Zodiacal Light was extraordinarily
+brilliant—brighter than he had ever seen it before. The Milky Way was not to be
+compared with it. The brightest part extended 75&deg; from the sun. There was a
+faint and much narrower extension which they could just make out beyond the
+Pleiades along the ecliptic, but the greater part of the Zodiacal Light showed
+as a broad truncated column, and it did not appear nearly as conical as he had
+before seen it.
+</p>
+
+<p>
+When out of the brief twilight of intertropical lands, where the sun drops
+vertically to the horizon and night rushes on like a wave of darkness, the
+Zodiacal Light shoots to the very zenith, its color is described as a golden
+tint, entirely different from the silvery sheen of the Milky Way. If I may
+venture again to refer to personal experiences and impressions, I will recall a
+view of the Zodiacal Light from the summit of the cone of Mt Etna in the autumn
+of the year 1896 (more briefly described in <i>Astronomy with the Naked
+Eye</i>). There are few lofty mountains so favorably placed as Etna for
+observations of this kind. It was once resorted to by Prof. George E. Hale, in
+an attempt to see the solar corona without an eclipse. Rising directly from
+sea-level to an elevation of nearly eleven thousand feet, the observer on its
+summit at night finds himself, as it were, lost in the midst of the sky. But
+for the black flanks of the great cone on which he stands he might fancy
+himself to be in a balloon. On the occasion to which I refer the world beneath
+was virtually invisible in the moonless night. The blaze of the constellations
+overhead was astonishingly brilliant, yet amid all their magnificence my
+attention was immediately drawn to a great tapering light that sprang from the
+place on the horizon where the sun would rise later, and that seemed to be
+blown out over the stars like a long, luminous veil. It was the finest view of
+the Zodiacal light that I had ever enjoyed—thrilling in its strangeness—but I
+was almost disheartened by the indifference of my guide, to whom it was only a
+light and nothing more. If he had no science, he had less poetry—rather a
+remarkable thing, I thought, for a child of his clime. The Light appeared to me
+to be distinctly brighter than the visible part of the Milky Way which included
+the brilliant stretches in Auriga and Perseus, and its color, if one may speak
+of color in connection with such an object, seemed richer than that of the
+galactic band; but I did not think of it as yellow, although Humboldt has
+described it as resembling a golden curtain drawn over the stars, and Du
+Chaillu in Equatorial Africa found it of a bright yellow color. It may vary in
+color as in conspicuousness. The fascination of that extraordinary sight has
+never faded from my memory. I turned to regard it again and again, although I
+had never seen the stellar heavens so brilliant, and it was one of the last
+things I looked for when the morning glow began softly to mount in the east,
+and Sicily and the Mediterranean slowly emerged from the profound shadow
+beneath us.
+</p>
+
+<p>
+The Zodiacal Light seems never to have attracted from astronomers in general
+the amount of careful attention that it deserves; perhaps because so little can
+really be made of it as far as explanation is concerned. I have referred to the
+restraint that scientific writers apparently feel in speaking of it. The
+grounds for speculation that it affords may be too scanty to lead to long
+discussions, yet it piques curiosity, and as we shall see in a moment has
+finally led to a most interesting theory. Once it was the subject of an
+elaborate series of studies which carried the observer all round the world.
+That was in 1845—46, during the United States Exploring Expedition that visited
+the then little known Japan. The chaplain of the fleet, the Rev. Mr Jones, went
+out prepared to study the mysterious light in all its phases. He saw it from
+many latitudes on both sides of the equator, and the imagination cannot but
+follow him with keen interest in his world-circling tour, keeping his eyes
+every night fixed upon the phantasm overhead, whose position shifted with that
+of the hidden sun. He demonstrated that the flow extends at times completely
+across the celestial dome, although it is relatively faint directly behind the
+earth. On his return the government published a large volume of his
+observations, in which he undertook to show that the phenomenon was due to the
+reflection of sunlight from a ring of meteoric bodies encircling the earth.
+But, after all, this elaborate investigation settled nothing.
+</p>
+
+<p>
+Prof. E. E. Barnard has more recently devoted much attention to the Zodiacal
+Light, as well as to a strange attendant phenomenon called the “Gegenschein,”
+or Counterglow, because it always appears at that point in the sky which is
+exactly opposite the sun. The Gegenschein is an extremely elusive phenomenon,
+suitable only for eyes that have been specially trained to see it. Professor
+Newcomb has cautiously remarked that
+</p>
+
+<p class="letter">
+it is said that in that point of the heavens directly opposite the sun there is
+an elliptical patch of light... This phenomenon is so difficult to account for
+that its existence is sometimes doubted; yet the testimony in its favor is
+difficult to set aside.
+</p>
+
+<p>
+It certainly cannot be set aside at all since the observations of Barnard. I
+recall an attempt to see it under his guidance during a visit to Mount
+Hamilton, when he was occupied there with the Lick telescope. Of course, both
+the Gegenschein and the Zodiacal Light are too diffuse to be studied with
+telescopes, which, so to speak, magnify them out of existence. They can only be
+successfully studied with the naked eye, since every faintest glimmer that they
+afford must be utilized. This is especially true of the Gegenschein. At Mount
+Hamilton, Mr Barnard pointed out to me its location with reference to certain
+stars, but with all my gazing I could not be sure that I saw it. To him, on the
+contrary, it was obvious; he had studied it for months, and was able to
+indicate its shape, its boundaries, its diameter, and the declination of its
+center with regard to the ecliptic. There is not, of course, the shadow of a
+doubt of the existence of the Gegenschein, and yet I question if one person in
+a million has ever seen or ever will see it. The Zodiacal Light, on the other
+hand, is plain enough, provided that the time and the circumstances of the
+observation are properly chosen.
+</p>
+
+<p>
+In the attempts to explain the Zodiacal Light, the favorite hypothesis has been
+that it is an appendage of the sun—perhaps simply an extension of the corona in
+the plane of the ecliptic, which is not very far from coinciding with that of
+the sun’s equator. This idea is quite a natural one, because of the evident
+relation of the light to the position of the sun. The vast extension of the
+equatorial wings of the corona in 1878 gave apparent support to this
+hypothesis; if the substance of the corona could extend ten million miles from
+the sun, why might it not extend even one hundred million, gradually fading out
+beyond the orbit of the earth? A variation of this hypothesis assumes that the
+reflection is due to swarms of meteors circling about the sun, in the plane of
+its equator, all the way from its immediate neighborhood to a distance
+exceeding that of the earth. But in neither form is the hypothesis
+satisfactory; there is nothing in the appearance of the corona to indicate that
+it extends even as far as the planet Mercury, while as to meteors, the orbits
+of the known swarms do not accord with the hypothesis, and we have no reason to
+believe that clouds of others exist traveling in the part of space where they
+would have to be in order to answer the requirements of the theory. The
+extension of the corona in 1878 did not resemble in its texture the Zodiacal
+Light.
+</p>
+
+<p>
+Now, it has so often happened in the history of science that an important
+discovery in one branch has thrown unexpected but most welcome light upon some
+pending problem in some other branch, that a strong argument might be based
+upon that fact alone against the too exclusive devotion of many investigators
+to the narrow lines of their own particular specialty; and the Zodiacal Light
+affords a case in point, when it is considered in connection with recent
+discoveries in chemistry and physics. From the fact that atoms are compound
+bodies made up of corpuscles at least a thousand times smaller than the
+smallest known atom—a fact which astounded most men of science when it was
+announced a few years ago—a new hypothesis has been developed concerning the
+nature of the Zodiacal Light (as well as other astronomical riddles), and this
+hypothesis comes not from an astronomer, but from a chemist and physicist, the
+Swede, Svante Arrhenius. In considering an outline of this new hypothesis we
+need neither accept nor reject it; it is a case rather for suspension of
+judgment.
+</p>
+
+<p>
+To begin with, it carries us back to the “pressure of light” mentioned in the
+preceding chapter. The manner in which this pressure is believed generally to
+act was there sufficiently explained, and it only remains to see how it is
+theoretically extended to the particles of matter supposed to constitute the
+Zodiacal Light. We know that corpuscles, or “fragments of atoms” negatively
+electrified, are discharged from hot bodies. Streams of these “ions” pour from
+many flames and from molten metals; and the impact of the cathode and
+ultra-violet rays causes them to gush even from cold bodies. In the vast
+laboratory of the sun it is but reasonable to suppose that similar processes
+are taking place. “As a very hot metal emits these corpuscles,” says Prof. J.
+J. Thomson, “it does not seem an improbable hypothesis that they are emitted by
+that very hot body, the sun.” Let it be assumed, then, that the sun does emit
+them; what happens next? Negatively charged corpuscles, it is known, serve as
+nuclei to which particles of matter in the ordinary state are attracted, and it
+is probable that those emitted from the sun immediately pick up loads in this
+manner and so grow in bulk. If they grow large enough the gravitation of the
+sun draws them back, and they produce a negative charge in the solar
+atmosphere. But it is probable that many of the particles do not attain the
+critical size which, according to the principles before explained, would enable
+the gravitation of the sun to retain them in opposition to the pressure of the
+waves of light, and with these particles the light pressure is dominant. Clouds
+of them may be supposed to be continually swept away from the sun into
+surrounding space, moving mostly in or near the plane of the solar equator,
+where the greatest activity, as indicated by sunspots and related phenomena, is
+taking place. As they pass outward into space many of them encounter the earth.
+If the earth, like the moon, had no atmosphere the particles would impinge
+directly on its surface, giving it a negative electric charge. But the presence
+of the atmosphere changes all that, for the first of the flying particles that
+encounter it impart to it their negative electricity, and then, since like
+electric charges repel like, the storm of particles following will be sheered
+off from the earth, and will stream around it in a maze of hyperbolic paths.
+Those that continue on into space beyond the earth may be expected to continue
+picking up wandering particles of matter until their bulk has become so great
+that the solar attraction prevails again over the light pressure acting upon
+them, and they turn again sunward. Passing the earth on their return they will
+increase the amount of dust-clouds careering round it; and these will be
+further increased by the action of the ultra-violet rays of the sunlight
+causing particles to shoot radially away from the earth when the negative
+charge of the upper atmosphere has reached a certain amount, which particles,
+although starting sunward, will be swept back to the earth with the oncoming
+streams. As the final result of all this accumulation of flying and gyrating
+particles in the earth’s neighborhood, we are told that the latter must be
+transformed into the semblance of a gigantic solid-headed comet provided with
+streaming tails, the longest of them stretching away from the direction of the
+sun, while another shorter one extends toward the sun. This shorter tail is due
+to the particles that we have just spoken of as being driven sunward from the
+earth by the action of ultra-violet light. No doubt this whole subject is too
+technical for popular statement; but at any rate the general reader can
+understand the picturesque side of the theory, for its advocates assure us that
+if we were on the moon we would doubtless be able to see the comet-like tails
+of the earth, and then we could appreciate the part that they play in producing
+the phenomenon of the Zodiacal Light.
+</p>
+
+<p>
+That the Light as we see it could be produced by the reflection of sunlight
+from swarms of particles careering round the earth in the manner supposed by
+Arrhenius’ hypothesis is evident enough; and it will be observed that the new
+theory, after all, is only another variant of the older one which attributes
+the Zodiacal Light to an extension of the solar corona. But it differs from the
+older theory in offering an explanation of the manner in which the extension is
+effected, and it differentiates between the corona proper and the streams of
+negative particles shot away from the sun. In its details the hypothesis of
+Arrhenius also affords an explanation of many peculiarities of the Zodiacal
+Light, such as that it is confined to the neighborhood of the ecliptic, and
+that it is stronger on the side of the earth which is just turning away from a
+position under the sun than on the other side; but it would carry us beyond our
+limits to go into these particulars. The Gegenschein, according to this theory,
+is a part of the same phenomenon as the Zodiacal Light, for by the laws of
+perspective it is evident that the reflection from the streams of particles
+situated at a point directly opposite to the sun would be at a maximum, and
+this is the place which the Gegenschein occupies. Apart from its geometrical
+relations to the position of the sun, the variability of the Zodiacal Light
+appears to affirm its solar dependence, and this too would be accounted for by
+Arrhenius’ hypothesis better than by the old theory of coronal extension. The
+amount of corpuscular discharge from the sun must naturally be governed by the
+state of relative activity or inactivity of the latter, and this could not but
+be reflected in the varying splendor of the Zodiacal Light. But much more
+extended study than has yet been given to the subject will be required before
+we can feel that we know with reasonable certainty what this mysterious
+phenomenon really is. By the hypothesis of Arrhenius every planet that has an
+atmosphere must have a Zodiacal Light attending it, but the phenomenon is too
+faint for us to be able to see it in the case, for instance, of Venus, whose
+atmosphere is very abundant. The moon has no corresponding “comet’s tail”
+because, as already explained, of the lack of a lunar atmosphere to repel the
+streams by becoming itself electrified; but if there were a lunar Zodiacal
+Light, no doubt we could see it because of the relative nearness of our
+satellite.
+</p>
+
+</div><!--end chapter-->
+
+<div class="chapter">
+
+<h2><a name="chap09"></a>IX<br/>
+Marvels of the Aurora</h2>
+
+<p>
+One of the most vivid recollections of my early boyhood is that of seeing my
+father return hastily into the house one evening and call out to the family:
+“Come outside and look at the sky!” Ours was a country house situated on a
+commanding site, and as we all emerged from the doorway we were dumbfounded to
+see the heavens filled with pale flames which ran licking and quivering over
+the stars. Instantly there sprang into my terrified mind the recollection of an
+awful description of “the Day of Judgment” (the <i>Dies Iræ</i>), which I had
+heard with much perturbation of spirit in the Dutch Reformed church from the
+lips of a tall, dark-browed, dreadfully-in-earnest preacher of the
+old-fashioned type. My heart literally sank at sight of the spectacle, for it
+recalled the preacher’s very words; it was just as he had said it would be, and
+it needed the assured bearing of my elders finally to convince me that
+</p>
+
+<p class="poem">
+That Day of Wrath, O dreadful day,<br/>
+When Heaven and Earth shall pass away,<br/>
+As David and the Sibyl say
+</p>
+
+<p class="noindent">
+had not actually come upon us. And even the older members of the household were
+not untouched with misgivings when menacing spots of crimson appeared, breaking
+out now here, now there, in the shuddering sky. Toward the north the spectacle
+was appalling. A huge arch spanned an unnaturally dark segment resting on the
+horizon, and above this arch sprang up beams and streamers in a state of
+incessant agitation, sometimes shooting up to the zenith with a velocity that
+took one’s breath, and sometimes suddenly falling into long ranks, and
+<i>marching, marching, marching,</i> like an endless phalanx of fiery specters,
+and moving, as I remember, always from east to west. The absolute silence with
+which these mysterious evolutions were performed and the quavering reflections
+which were thrown upon the ground increased the awfulness of the exhibition.
+Occasionally enormous curtains of lambent flame rolled and unrolled with a
+majestic motion, or were shaken to and fro as if by a mighty, noiseless wind.
+At times, too, a sudden billowing rush would be made toward the zenith, and for
+a minute the sky overhead would glow so brightly that the stars seemed to have
+been consumed. The spectacle continued with varying intensity for hours.
+</p>
+
+<div class="fig" style="width:100%;">
+<a name="illus19"></a>
+<a href="images/image19.jpg">
+<img src="images/image19.jpg" width="600" height="459" alt="Illustration:" /></a>
+<p class="caption">Auroral beams seen in England</p>
+</div>
+
+<p>
+This exhibition occurred in Central New York, a latitude in which the Aurora
+Borealis is seldom seen with so much splendor. I remember another similar one
+seen from the city of New York in November, 1882. On this last occasion some
+observers saw a great upright beam of light which majestically moved across the
+heavens, stalking like an apparition in the midst of the auroral pageant, of
+whose general movements it seemed to be independent, maintaining always its
+upright posture, and following a magnetic parallel from east to west. This
+mysterious beam was seen by no less than twenty-six observers in different
+parts of the country, and a comparison of their observations led to a curious
+calculation indicating that the apparition was about <i>one hundred and
+thirty-three miles tall</i> and moved at the speed of ten miles per second!
+</p>
+
+<div class="fig" style="width:100%;">
+<a name="illus20"></a>
+<a href="images/image20.jpg">
+<img src="images/image20.jpg" width="600" height="403" alt="Illustration:" /></a>
+<p class="caption">Auroral arches of an eliptic form seen in the Arctic regions</p>
+</div>
+
+<p>
+But, as everybody knows, it is in the Arctic regions that the Aurora, or the
+“Northern Lights,” can best be seen. There, in the long polar night, when for
+months together the sun does not rise, the strange coruscations in the sky
+often afford a kind of spectral daylight in unison with the weird scenery of
+the world of ice. The pages in the narratives of Arctic exploration that are
+devoted to descriptions of the wonderful effects of the Northern Lights are
+second to none that man has ever penned in their fascination. The lights, as I
+have already intimated, display astonishing colors, particularly shades of red
+and green, as they flit from place to place in the sky. The discovery that the
+magnetic needle is affected by the Aurora, quivering and darting about in a
+state of extraordinary excitement when the lights are playing in the sky, only
+added to the mystery of the phenomenon until its electro-magnetic nature had
+been established. This became evident as soon as it was known that the focus of
+the displays was the magnetic pole; and when the far South was visited the
+Aurora Australis was found, having its center at the South Magnetic Pole. Then,
+if not before, it was clear that the earth was a great globular magnet, having
+its poles of opposite magnetism, and that the auroral lights, whatever their
+precise cause might be, were manifestations of the magnetic activity of our
+planet. After the invention of magnetic telegraphy it was found that whenever a
+great Aurora occurred the telegraph lines were interrupted in their operation,
+and the ocean cables ceased to work. Such a phenomenon is called a “magnetic
+storm.”
+</p>
+
+<p>
+The interest excited by the Aurora in scientific circles was greatly stimulated
+when, in the last half of the nineteenth century, it was discovered that it is
+a phenomenon intimately associated with disturbances on the sun. The ancient
+“Zurich Chronicles,” extending from the year 1000 to the year 1800, in which
+both sun-spots visible to the naked eye and great displays of the auroral
+lights were recorded, first set Rudolf Wolf on the track of this discovery. The
+first notable proof of the suspected connection was furnished with dramatic
+emphasis by an occurrence which happened on September 1, 1859. Near noon on
+that day two intensely brilliant points suddenly broke out in a group of
+sun-spots which were under observation by Mr R. C. Carrington at his
+observatory at Redhill, England. The points remained visible for not more than
+five minutes, during which interval they moved <i>thirty-five thousand
+miles</i> across the solar disk. Mr R. Hodgson happened to see the same
+phenomenon at his observatory at Highgate, and thus all possibility of
+deception was removed. But neither of the startled observers could have
+anticipated what was to follow, and, indeed, it was an occurrence which has
+never been precisely duplicated. I quote the eloquent account given by Miss
+Clerke in her <i>History of Astronomy During the Nineteenth Century.</i>
+</p>
+
+<p class="letter">
+This unique phenomenon seemed as if specially designed to accentuate the
+inference of a sympathetic relation between the earth and the sun. From August
+28 to September 4, 1859, a magnetic storm of unparalleled intensity, extent,
+and duration was in progress over the entire globe. Telegraphic communication
+was everywhere interrupted—except, indeed, that it was in some cases found
+practicable to work the lines <i>without batteries</i> by the agency of the
+earth-currents alone; sparks issued from the wires; gorgeous auroras draped the
+skies in solemn crimson over both hemispheres, and even in the tropics; the
+magnetic needle lost all trace of continuity in its movements and darted to and
+fro as if stricken with inexplicable panic. The coincidence was even closer.
+<i>At the very instant</i> of the solar outburst witnessed by Carrington and
+Hodgson the photographic apparatus at Kew registered a marked disturbance of
+all the three magnetic elements; while shortly after the ensuing midnight the
+electric agitation culminated, thrilling the whole earth with subtle
+vibrations, and lighting up the atmosphere from pole to pole with coruscating
+splendors which perhaps dimly recall the times when our ancient planet itself
+shone as a star.
+</p>
+
+<p>
+If this amazing occurrence stood alone, and as I have already said it has never
+been exactly duplicated, doubt might be felt concerning some of the inferences
+drawn from it; but in varying forms it has been repeated many times, so that
+now hardly anyone questions the reality of the assumed connection between solar
+outbursts and magnetic storms accompanied by auroral displays on the earth. It
+is true that the late Lord Kelvin raised difficulties in the way of the
+hypothesis of a direct magnetic action of the sun upon the earth, because it
+seemed to him that an inadmissible quantity of energy was demanded to account
+for such action. But no calculation like that which he made is final, since all
+calculations depend upon the validity of the data; and no authority is
+unshakable in science, because no man can possess omniscience. It was Lord
+Kelvin who, but a few years before the thing was actually accomplished,
+declared that aerial navigation was an impracticable dream, and demonstrated
+its impracticability by calculation. However the connection may be brought
+about, it is as certain as evidence can make it that solar outbursts are
+coincident with terrestial magnetic disturbances, and coincident in such a way
+as to make the inference of a causal connection irresistible. The sun is only a
+little more than a hundred times its own diameter away from the earth. Why,
+then, with the subtle connection between them afforded by the ether which
+conveys to us the blinding solar light and the life-sustaining solar heat,
+should it be so difficult to believe that the sun’s enormous electric energies
+find a way to us also? No doubt the impulse coming from the sun acts upon the
+earth after the manner of a touch upon a trigger, releasing energies which are
+already stored up in our planet.
+</p>
+
+<p>
+But besides the evidence afforded by such occurrences as have been related of
+an intimate connection between solar outbreaks and terrestial magnetic
+flurries, attended by magnificent auroral displays, there is another line of
+proof pointing in the same direction. Thus, it is known that the sun-spot
+period, as remarked in a preceding chapter, coincides in a most remarkable
+manner with the periodic fluctuations in the magnetic state of the earth. This
+coincidence runs into the most astonishing details. For instance, when the
+sun-spot period shortens, the auroral period shortens to precisely the same
+extent; as the short sun-spot periods usually bring the most intense outbreaks
+of solar activity, so the corresponding short auroral periods are attended by
+the most violent magnetic storms; a secular period of about two hundred and
+twenty-two years affecting sun-spots is said to have its auroral duplicate; a
+shorter period of fifty-five and a half years, which some observers believe
+that they have discovered appears also to be common to the two phenomena; and
+yet another “superposed” period of about thirty-five years, which some
+investigators aver exists, affects sun-spots and aurora alike. In short, the
+coincidences are so numerous and significant that one would have to throw the
+doctrine of probability to the winds in order to be able to reject the
+conclusion to which they so plainly lead.
+</p>
+
+<div class="fig" style="width:100%;">
+<a name="illus21"></a>
+<a href="images/image21.jpg">
+<img src="images/image21.jpg" width="600" height="395" alt="Illustration:" /></a>
+<p class="caption">Auroral curtain seen in Scandinavia</p>
+</div>
+
+<p>
+But still the question recurs: How is the influence transmitted? Here Arrhenius
+comes once more with his hypothesis of negative corpuscles, or ions, driven
+away from the sun by light-pressure—a hypothesis which seems to explain so many
+things—and offers it also as an explanation of the way in which the sun creates
+the Aurora. He would give the Aurora the same lineage with the Zodiacal Light.
+To understand the application of this theory we must first recall the fact that
+the earth is a great magnet having its two opposite poles of magnetism, one
+near the Arctic and the other near the Antarctic Circle. Like all magnets, the
+earth is surrounded with “lines of force,” which, after the manner of the
+curved rays we saw in the photograph of a solar eclipse, start from a pole,
+rising at first nearly vertically, then bend gradually over, passing high above
+the equator, and finally descending in converging sheaves to the opposite pole.
+Now the axis of the earth is so placed in space that it lies at nearly a right
+angle to the direction of the sun, and as the streams of negatively charged
+particles come pouring on from the sun (see the last preceding chapter), they
+arrive in the greatest numbers over the earth’s equatorial regions. There they
+encounter the lines of magnetic force at the place where the latter have their
+greatest elevation above the earth, and where their direction is horizontal to
+the earth’s surface. Obeying a law which has been demonstrated in the
+laboratory, the particles then follow the lines of force toward the poles.
+While they are above the equatorial regions they do not become luminescent,
+because at the great elevation that they there occupy there is virtually no
+atmosphere; but as they pass on toward the north and the south they begin to
+descend with the lines of force, curving down to meet at the poles; and,
+encountering a part of the atmosphere comparable in density with what remains
+in an exhausted Crookes tube, they produce a glow of cathode rays. This glow is
+conceived to represent the Aurora, which may consequently be likened to a
+gigantic exhibition of vacuum-tube lights. Anybody who recalls his student days
+in the college laboratory and who has witnessed a display of Northern Lights
+will at once recognize the resemblance between them in colors, forms, and
+behavior. This resemblance had often been noted before Arrhenius elaborated his
+hypothesis.
+</p>
+
+<p>
+Without intending to treat his interesting theory as more than a possibly
+correct explanation of the phenomena of the Aurora, we may call attention to
+some apparently confirmatory facts. One of the most striking of these relates
+to a seasonal variation in the average number of auroræ. It has been observed
+that there are more in March and September than at any other time of the year,
+and fewer in June and December; moreover (and this is a delicate test as
+applied to the theory), they are slightly rarer in June than in December. Now
+all these facts seem to find a ready explanation in the hypothesis of
+Arrhenius, thus: (1) The particles issuing from the sun are supposed to come
+principally from the regions whose excitement is indicated by the presence of
+sun-spots (which accords with Hale’s observation that sun-spots are columns of
+ionized vapors), and these regions have a definite location on either side of
+the solar equator, seldom approaching it nearer than within 5&deg; or 10&deg;
+north or south, and never extending much beyond 35&deg; toward either pole; (2)
+The equator of the sun is inclined about 7&deg; to the plane of the earth’s
+orbit, from which it results that twice in a year—<i>viz.,</i> in June and
+December—the earth is directly over the solar equator, and twice a
+year—<i>viz.,</i> in March and September—when it is farthest north or south of
+the solar equator, it is over the inner edge of the sun-spot belts. Since the
+corpuscles must be supposed to be propelled radially from the sun, few will
+reach the earth when the latter is over the solar equator in June and December,
+but when it is over, or nearly over, the spot belts, in March and September, it
+will be in the line of fire of the more active parts of the solar surface, and
+relatively rich streams of particles will reach it. This, as will be seen from
+what has been said above, is in strict accord with the observed variations in
+the frequency of auroræ. Even the fact that somewhat fewer auroræ are seen in
+June than in December also finds its explanation in the known fact that the
+earth is about three million miles nearer the sun in the winter than in the
+summer, and the number of particles reaching it will vary, like the intensity
+of light, inversely as the square of the distance. These coincidences are
+certainly very striking, and they have a cumulative force. If we accept the
+theory, it would appear that we ought to congratulate ourselves that the
+inclination of the sun’s equator is so slight, for as things stand the earth is
+never directly over the most active regions of the sun-spots, and consequently
+never suffers from the maximum bombardment of charged particles of which the
+sun is capable. Incessant auroral displays, with their undulating draperies,
+flitting colors, and marching columns might not be objectionable from the point
+of view of picturesqueness, but one magnetic storm of extreme intensity
+following closely upon the heels of another, for months on end, crazing the
+magnetic needle and continually putting the telegraph and cable lines out of
+commission, to say nothing of their effect upon “wireless telegraphy”, would
+hardly add to the charms of terrestrial existence.
+</p>
+
+<div class="fig" style="width:100%;">
+<a name="illus22"></a>
+<a href="images/image22.jpg">
+<img src="images/image22.jpg" width="600" height="462" alt="Illustration:" /></a>
+<p class="caption">Auroral arches seen in Scandinavia</p>
+</div>
+
+<p>
+One or two other curious points in connection with Arrhenius’ hypothesis may be
+mentioned. First, the number of auroræ, according to his explanation, ought to
+be greatest in the daytime, when the face of the earth on the sunward side is
+directly exposed to the atomic bombardment. Of course visual observation can
+give us no information about this, since the light of the Aurora is never
+sufficiently intense to be visible in the presence of daylight, but the records
+of the magnetic observatories can be, and have been, appealed to for
+information, and they indicate that the facts actually accord with the theory.
+Behind the veil of sunlight in the middle of the afternoon, there is good
+reason to believe, auroral exhibitions often take place which would eclipse in
+magnificence those seen at night if we could behold them. Observation shows,
+too, that auroræ are more frequent before than after midnight, which is just
+what we should expect if they originate in the way that Arrhenius supposes.
+Second, the theory offers an explanation of the alleged fact that the formation
+of clouds in the upper air is more frequent in years when auroræ are most
+abundant, because clouds are the result of the condensation of moisture upon
+floating particles in the atmosphere (in an absolutely dustless atmosphere
+there would be no clouds), and it has been proved that negative ions like those
+supposed to come from the sun play a master part in the phenomena of cloud
+formation.
+</p>
+
+<p>
+Yet another singular fact, almost mystical in its suggestions, may be
+mentioned. It seems that the dance of the auroral lights occurs most frequently
+during the absence of the moon from the hemisphere in which they appear, and
+that they flee, in greater part, to the opposite hemisphere when the moon’s
+revolution in an orbit considerably inclined to the earth’s equator brings her
+into that where they have been performing. Arrhenius himself discovered this
+curious relation of auroral frequency to the position of the moon north or
+south of the equator, and he explains it in this way. The moon, like the earth,
+is exposed to the influx of the ions from the sun; but having no atmosphere, or
+almost none, to interfere with them, they descend directly upon her surface and
+charge her with an electric negative potential to a very high degree. In
+consequence of this she affects the electric state of the upper parts of the
+earth’s atmosphere where they lie most directly beneath her, and thus prevents,
+to a large extent, the negative discharges to which the appearance of the
+Aurora is due. And so “the extravagant and erring spirit” of the Aurora avoids
+the moon as Hamlet’s ghost fled at the voice of the cock announcing the
+awakening of the god of day.
+</p>
+
+<p>
+There are even other apparent confirmations of the hypothesis, but we need not
+go into them. We shall, however, find one more application of it in the next
+chapter, for it appears to be a kind of cure-all for astronomical troubles; at
+any rate it offers a conceivable solution of the question, How does the sun
+manage to transmit its electric influence to the earth? And this solution is so
+grandiose in conception, and so novel in the mental pictures that it offers,
+that its acceptance would not in the least detract from the impression that the
+Aurora makes upon the imagination.
+</p>
+
+</div><!--end chapter-->
+
+<div class="chapter">
+
+<h2><a name="chap10"></a>X<br/>
+Strange Adventures of Comets</h2>
+
+<p>
+The fears and legends of ancient times before Science was born, and the
+superstitions of the Dark Ages, sedulously cultivated for theological purposes
+by monks and priests, have so colored our ideas of the influence that comets
+have had upon the human mind that many readers may be surprised to learn that
+it was the apparition of a wonderful comet, that of 1843, which led to the
+foundation of our greatest astronomical institution, the Harvard College
+Observatory. No doubt the comet superstition existed half a century ago, as,
+indeed, it exists yet today, but in this case the marvelous spectacle in the
+sky proved less effective in inspiring terror than in awakening a desire for
+knowledge. Even in the sixteenth century the views that enlightened minds took
+of comets tended powerfully to inspire popular confidence in science, and
+Halley’s prediction, after seeing and studying the motion of the comet which
+appeared in 1682, that it would prove to be a regular member of the sun’s
+family and would be seen returning after a period of about seventy-six years,
+together with the fulfillment of that prediction, produced a revulsion from the
+superstitious notions which had so long prevailed.
+</p>
+
+<div class="fig" style="width:100%;">
+<a name="illus23"></a>
+<a href="images/image23.jpg">
+<img src="images/image23.jpg" width="508" height="600" alt="Illustration:" /></a>
+<p class="caption">Swift’s comet. Taken at Arequipa, March 30 1892</p>
+</div>
+
+<p>
+Then the facts were made plain that comets are subject to the law of
+gravitation equally with the planets; that there are many which regularly
+return to the neighborhood of the sun (perihelion); and that these travel in
+orbits differing from those of the planets only in their greater eccentricity,
+although they have the peculiarity that they do not, like the planets, all go
+round the sun in the same direction, and do not keep within the general plane
+of the planetary system, but traverse it sometimes from above and sometimes
+from below. Other comets, including most of the “great” ones, appear to travel
+in parabolic or, in a few cases, hyperbolic orbits, which, not being closed
+curves, never bring them back again. But it is not certain that these orbits
+may not be extremely eccentric ellipses, and that after the lapse of hundreds,
+or thousands, of years the comets that follow them may not reappear. The
+question is an interesting one, because if all orbits are really ellipses, then
+all comets must be permanent members of the solar system, while in the contrary
+case many of them are simply visitors, seen once and never to be seen again.
+The hypothesis that comets are originally interlopers might seem to derive some
+support from the fact that the certainly periodic ones are associated, in
+groups, with the great outer planets, whose attraction appears to have served
+as a trap for them by turning them into elliptical orbits and thus making them
+prisoners in the solar system. Jupiter, owing to his great mass and his
+commanding situation in the system, is the chief “comet-catcher;” but he
+catches them not for himself, but for the sun. Yet if comets do come originally
+from without the borders of the planetary system, it does not, by any means,
+follow that they were wanderers at large in space before they yielded to the
+overmastering attraction of the sun. Investigation of the known cometary
+orbits, combined with theoretical considerations, has led some astronomers to
+the conclusion that as the sun travels onward through space he “picks up <i>en
+route</i>” cometary masses which, without belonging strictly to his empire, are
+borne along in the same vast “cosmical current” that carries the solar system.
+</p>
+
+<p>
+But while no intelligent person any longer thinks that the appearance of a
+great comet is a token from the heavenly powers of the approaching death of a
+mighty ruler, or the outbreak of a devastating war, or the infliction of a
+terrible plague upon wicked mankind, science itself has discovered mysteries
+about comets which are not less fascinating because they are more intellectual
+than the irrational fancies that they have displaced. To bring the subject
+properly before the mind, let us see what the principal phenomena connected
+with a comet are.
+</p>
+
+<p>
+At the present day comets are ordinarily “picked up” with the telescope or the
+photographic plate before any one except their discoverer is aware of their
+existence, and usually they remain so insignificant in appearance that only
+astronomers ever see them. Yet so great is the prestige of the word “comet”
+that the discovery of one of these inconspicuous wanderers, and its subsequent
+movements, become items of the day’s news which everybody reads with the
+feeling, perhaps, that at least he knows what is going on in the universe even
+if he doesn’t understand it. But a truly great comet presents quite a different
+proposition. It, too, is apt to be detected coming out of the depths of space
+before the world at large can get a glimpse of it, but as it approaches the sun
+its aspect undergoes a marvelous change. Agitated apparently by solar
+influence, it throws out a long streaming tail of nebulous light, directed away
+from the sun and looking as if blown out like a pennon by a powerful wind.
+Whatever may be the position of the comet with regard to the sun, as it circles
+round him it continually keeps its tail on the off side. This, as we shall soon
+see, is a fact of capital importance in relation to the probable nature of
+comets’ tails. Almost at the same time that the formation of the tail is
+observed a remarkable change takes place in the comet’s head, which, by the
+way, is invariably and not merely occasionally its most important part. On
+approaching the sun the head usually contracts. Coincidently with this
+contraction a nucleus generally makes its appearance. This is a bright,
+star-like point in the head, and it probably represents the totality of solid
+matter that the comet possesses. But it is regarded as extremely unlikely that
+even the nucleus consists of a uniformly solid mass. If it were such, comets
+would be far more formidable visitors when they pass near the planets than they
+have been found to be. The diameter of the nucleus may vary from a few hundred
+up to several thousand miles; the heads, on the average, are from twenty-five
+thousand to one hundred thousand miles in diameter, although a few have greatly
+exceeded these dimensions; that of the comet of 1811, one of the most
+stupendous ever seen, was a million and a quarter miles in diameter! As to the
+tails, not withstanding their enormous length—some have been more than a
+hundred million miles long—there is reason to believe that they are of extreme
+tenuity, “as rare as vacuum.” The smallest stars have been seen shining through
+their most brilliant portions with undiminished luster.
+</p>
+
+<p>
+After the nucleus has been formed it begins to throw out bright jets directed
+toward the sun. A stream, and sometimes several streams, of light also project
+sunward from the nucleus, occasionally appearing like a stunted tail directed
+oppositely to the real tail. Symmetrical envelopes which, seen in section,
+appear as half circles or parabolas, rise sunward from the nucleus, forming a
+concentric series. The ends of these stream backward into the tail, to which
+they seem to supply material. Ordinarily the formation of these ejections and
+envelopes is attended by intense agitation of the nucleus, which twists and
+turns, swinging and gyrating with an appearance of the greatest violence.
+Sometimes the nucleus is seen to break up into several parts. The entire heads
+of some comets have been split asunder in passing close around the sun; The
+comet of 1882 retreated into space after its perihelion passage with <i>five
+heads</i> instead of the one that it had originally, and each of these heads
+had its own tail!
+</p>
+
+<p>
+The possession of the spectroscope has enabled astronomers during later years
+to study the chemical composition of comets by analyzing their light. At first
+the only substances thus discovered in them were hydro-carbon compounds, due
+evidently to the gaseous envelopes in which some combination of hydrogen with
+carbon existed. Behind this gaseous spectrum was found a faint continuous
+spectrum ascribed to the nucleus, which apparently both reflects the sunlight
+and gives forth the light of a glowing solid or liquid. Subsequently sodium and
+iron lines were found in cometary spectra. The presence of iron would seem to
+indicate that some of these bodies may be much more massive than observations
+on their attractive effects have indicated. In some recent comets, such as
+Morehouse’s, in 1908, several lines have been found, the origin of which is
+unknown.
+</p>
+
+<p>
+Without going back of the nineteenth century we may find records of some of the
+most extraordinary comets that man has ever looked upon. In 1811, still spoken
+of as “the year of the comet,” because of the wonderful vintage ascribed to the
+skyey visitor, a comet shaped like a gigantic sword amazed the whole world,
+and, as it remained visible for seventeen months, was regarded by superstitious
+persons as a symbol of the fearful happenings of Napoleon’s Russian campaign.
+This comet, the extraordinary size of whose head, greatly exceeding that of the
+sun itself, has already been mentioned, was also remarkable for exhibiting so
+great a brilliancy without approaching even to the earth’s distance from the
+sun. But there was once a comet (and only once—in the year 1729) which never
+got nearer to the sun than four times the distance of the earth and yet
+appeared as a formidable object in the sky. As Professor Young has remarked,
+“it must have been an enormous comet to be visible from such a distance.” And
+we are to remember that there were no great telescopes in the year 1729. That
+comet affects the imagination like a phantom of space peering into the solar
+system, displaying its enormous train afar off (which, if it had approached as
+near as other comets, would probably have become <i>the</i> celestial wonder of
+all human memory), and then turning away and vanishing in the depths of
+immensity.
+</p>
+
+<p>
+In 1843 a comet appeared which was so brilliant that it could be seen in broad
+day close beside the sun! This was the first authenticated instance of that
+kind, but the occurrence was to be repeated, as we shall see in a moment, less
+than forty years later.
+</p>
+
+<p>
+The splendid comet of 1858, usually called Donati’s, is remembered by many
+persons yet living. It was, perhaps, both as seen by the naked eye and with the
+telescope, the most beautiful comet of which we have any record. It too marked
+a rich vintage year, still remembered in the vineyards of France, where there
+is a popular belief that a great comet ripens the grape and imparts to the wine
+a flavor not attainable by the mere skill of the cultivator. There are “comet
+wines,” carefully treasured in certain cellars, and brought forth only when
+their owner wishes to treat his guests to a sip from paradise.
+</p>
+
+<p>
+The year 1861 saw another very remarkable comet, of an aspect strangely vast
+and diffuse, which is believed to have swept the earth with its immense tail
+when it passed between us and the sun on the night of June 30th, an event which
+produced no other known effect than the appearance of an unwonted amount of
+scattered light in the sky.
+</p>
+
+<p>
+The next very notable comet was the “Great Southern Comet” of 1880, which was
+not seen from the northern hemisphere. It mimicked the aspect of the famous
+comet of 1843, and to the great surprise of astronomers appeared to be
+traveling in the same path. This proved to be the rising of the curtain for an
+astronomical sensation unparalleled in its kind; for two years later another
+brilliant comet appeared, first in the southern hemisphere, <i>and it too
+followed the same track.</i> The startling suggestion was now made that this
+comet was identical with those of 1843 and 1880, its return having been
+hastened by the resistance experienced in passing twice through the coronal
+envelope, and there were some who thought that it would now swing swiftly round
+and then plunge straight into the sun, with consequences that might be
+disastrous to us on account of the “flash of heat” that would be produced by
+the impact. Nervous people were frightened, but observation soon proved that
+the danger was imaginary, for although the comet almost grazed the sun, and
+must have rushed through two or three million miles of the coronal region, no
+retardation of its immense velocity was perceptible, and it finally passed away
+in a damaged condition, as before remarked, and has never since appeared.
+</p>
+
+<p>
+Then the probable truth was perceived—<i>viz.,</i> that the three comets (1843,
+1880, and 1882) were not one identical body, but three separate ones all
+traveling in the same orbit. It was found, too, that a comet seen in 1668 bore
+similar insignia of relationship. The natural inference was that these four
+bodies had once formed a single mass which had been split apart by the
+disruptive action of the sun. Strength was lent to this hypothesis by the fact
+that the comet of 1882 was apparently torn asunder during its perihelion
+passage, retreating into space in a dissevered state. But Prof. George Forbes
+has a theory that the splitting of the original cometary mass was effected by
+an unknown planet, probably greater than Jupiter, situated at a hundred times
+the earth’s distance from the sun, and revolving in a period of a thousand
+years. He supposes that the original comet was not that of 1668, but one seen
+in 1556, which has since been “missing,” and that its disruption occurred from
+an encounter with the supposititious planet about the year 1700. Truly from
+every point of view comets are the most extraordinary of adventurers!
+</p>
+
+<p>
+The comet of 1882 was likewise remarkable for being visible, like its
+predecessor of 1843, in full daylight in close proximity to the sun. The story
+of its detection when almost in contact with the solar disk is dramatic. It had
+been discovered in the southern hemisphere only a couple of weeks before its
+perihelion, which occurred on September 17th, and on the forenoon of that day
+it was seen by Doctor Common in England, and by Doctor Elkin and Mr Finlay at
+the Cape of Good Hope, almost touching the sun. It looked like a dazzling white
+bird with outspread wings. The southern observers watched it go <i>right into
+the sun,</i> when it instantly disappeared. What had happened was that the
+comet in passing its perihelion point had swung exactly between the earth and
+the sun. On the following morning it was seen from all parts of the world close
+by the sun on the opposite side, and it remained thus visible for three days,
+gradually receding from the solar disk. It then became visible for northern
+observers in the morning sky before sunrise, brandishing a portentous
+sword-shaped tail which, if it had been in the evening sky, would have excited
+the wonder of hundreds of millions, but situated where it was, comparatively
+few ever saw it.
+</p>
+
+<div class="fig" style="width:100%;">
+<a name="illus24"></a>
+<a href="images/image24.jpg">
+<img src="images/image24.jpg" width="285" height="650" alt="Illustration:" /></a>
+<p class="caption">Daniels’ comet. August 11, 1907</p>
+</div>
+
+<p>
+The application of photography to the study of comets has revealed many curious
+details which might otherwise have escaped detection, or at best have remained
+subject to doubt. It has in particular shown not only the precise form of the
+tails, but the remarkable vicissitudes that they undergo. Professor Barnard’s
+photographs of Brooks’ comet in 1893 suggested, by the extraordinary changes in
+the form of the tail which they revealed, that the comet was encountering a
+series of obstructions in space which bent and twisted its tail into fantastic
+shapes. The reader will observe the strange form into which the tail was thrown
+on the night of October 21st. A cloud of meteors through which the comet was
+passing might have produced such deformations of its tail. In the photograph of
+Daniels’ comet of 1907, a curious striping of the tail will be noticed. The
+short bright streaks seen in the photograph, it may be explained, are the
+images of stars which are drawn out into lines in consequence of the fact that
+the photographic telescope was adjusted to follow the motion of the comet while
+the stars remained at rest.
+</p>
+
+<p>
+But the adventures of comets are not confined to possible encounters with
+unknown obstacles. We have referred to the fact that the great planets, and
+especially Jupiter, frequently interfere with the motions of comets. This
+interference is not limited to the original alteration of their orbits from
+possible parabolas to ellipses, but is sometimes exercised again and again,
+turning the bewildered comets into elliptical paths of all degrees of
+eccentricity. A famous example of this kind of planetary horse-play is
+furnished by the story of Lexell’s missing comet. This comet was first seen in
+1770. Investigation showed that it was moving in an orbit which should bring it
+back to perihelion every five and a half years; yet it had never been seen
+before and, although often searched for, has never been seen since. Laplace and
+Leverrier proved mathematically that in 1767 it had approached so close to
+Jupiter as to be involved among the orbits of his satellites. What its track
+had been before is not known, but on that occasion the giant planet seized the
+interloper, threw it into a short elliptic orbit and sent it, like an arrested
+vagrant, to receive sentence at the bar of the sun. On this journey it passed
+within less than 1,500,000 miles of the earth. The form of orbit which Jupiter
+had impressed required, as we have said, its return in about five and a half
+years; but soon after 1770 it had the misfortune a second time to encounter
+Jupiter at close range, and he, as if dissatisfied with the leniency of the
+sun, or indignant at the stranger’s familiarity, seized the comet and hurled it
+out of the system, or at any rate so far away that it has never since been able
+to rejoin the family circle that basks in the immediate rays of the solar
+hearth. Nor is this the only instance in which Jupiter has dealt summarily with
+small comets that have approached him with too little deference.
+</p>
+
+<div class="fig" style="width:100%;">
+<a name="illus25"></a>
+<a href="images/image25.jpg">
+<img src="images/image25.jpg" width="600" height="463" alt="Illustration:" /></a>
+<p class="caption">Brooks’ comet. Photographed by Barnard, October 21, 1893</p>
+</div>
+
+<p>
+The function which Jupiter so conspicuously fulfills as master of the hounds to
+the sun is worth considering a little more in detail. To change the figure,
+imagine the sun in its voyage through space to be like a majestic battleship
+surrounded by its scouts. Small vessels (the comets, as they are overhauled by
+the squadron, are taken in charge by the scouts, with Jupiter for their chief,
+and are forced to accompany the fleet, but not all are impressed. If a strange
+comet undertakes to run across Jupiter’s bows the latter brings it to, and
+makes prize of it by throwing it into a relatively small ellipse with the sun
+for its focus. Thenceforth, unless, as happened to the unhappy comet of Lexell,
+it encounters Jupiter again in such a way as to be diverted by him into a more
+distant orbit, it can never get away. About thirty comets are now known to have
+thus been captured by the great planet, and they are called “Jupiter’s Comet
+Family.” But, on the other hand, if a wandering comet crosses the wake of the
+chief planetary scout the latter simply drives it away by accelerating its
+motion and compels it to steer off into open space. The transformation of
+comets into meteors will be considered in the next chapter, but here, in
+passing, mention may be made of the strange fate of one member of Jupiter’s
+family, Biela’s comet, which, having become over bold in its advances to its
+captor, was, after a few revolutions in is impressed orbit, torn to pieces and
+turned into a flock of meteors.
+</p>
+
+<p>
+And now let us return to the mystery of comets’ tails. That we are fully
+justified in speaking of the tails of comets as mysterious is proved by the
+declaration of Sir John Herschel, who averred, in so many words, that “there is
+some profound secret and mystery of nature concerned in this phenomenon,” and
+this profound secret and mystery has not yet been altogether cleared up.
+Nevertheless, the all-explaining hypothesis of Arrhenius offers us once more a
+certain amount of aid. Comets’ tails, Arrhenius assures us, are but another
+result of the pressure of light. The reader will recall the applications of
+this theory to the Zodiacal Light and the Aurora. In the form in which we now
+have to deal with it, the supposition is made that as a comet approaches the
+sun eruptions of vapor, due to the solar heat, occur in its nucleus. These are
+naturally most active on the side which is directly exposed to the sun, whence
+the appearance of the immense glowing envelopes that surround the nucleus on
+the sunward side. Among the particles of hydro-carbon, and perhaps solid carbon
+in the state of fine dust, which are thus set free there will be many whose
+size is within the critical limit which enables the light-waves from the sun to
+drive them away. Clouds of such particles, then, will stream off behind the
+advancing comet, producing the appearance of a tail. This accounts for the fact
+that the tails of comets are always directed away from the sun, and it also
+explains the varying forms of the tails and the extraordinary changes that they
+undergo. The speed of the particles driven before the light-waves must depend
+upon their size and weight, the lightest of a given size traveling the most
+swiftly. By accretion certain particles might grow, thus losing velocity and
+producing the appearance of bunches in the tail, such as have been observed.
+The hypothesis also falls in with the researches of Bredichin, who has divided
+the tails of comets into three principal classes—<i>viz.:</i> (1) Those which
+appear as long, straight rays; (2) Those which have the form of curved plumes
+or scimitars; (3) Those which are short, brushy, and curved sharply backward
+along the comet’s path. In the first type he calculates the repulsive force at
+from twelve to fifteen times the force of gravity; in the second at from two to
+four times; and in the third at about one and a half times. The straight tails
+he ascribes to hydrogen because the hydrogen atom is the lightest known; the
+sword-shaped tails to hydro-carbons; and the stumpy tails to vaporized iron. It
+will be seen that, if the force driving off the tails is that which Arrhenius
+assumes it to be, the forms of those appendages would accord with those that
+Bredichin’s theory calls for. At the same time we have an explanation of the
+multiple tails with which some comets have adorned themselves. The comet of
+1744, for instance, had at one time no less than seven tails spread in a wide
+curved brush behind it. Donati’s comet of 1858 also had at least two tails, the
+principal one sword-shaped and the other long, narrow, and as straight as a
+rule. According to Bredichin, the straight tail must have been composed of
+hydrogen, and the other of some form of hydro-carbon whose atoms are heavier
+than those of hydrogen, and, consequently, when swept away by the storm of
+light-waves, followed a curvature depending upon the resultant of the forces
+operating upon them. The seven tails of the comet of 1744 presented a kind of
+diagram graphically exhibiting its complex composition, and, if we knew a
+little more about the constituents of a comet, we might be able to say from the
+amount of curvature of the different tails just what were the seven substances
+of which that comet consisted.
+</p>
+
+<p>
+If these theories seem to the reader fantastic, at any rate they are no more
+fantastic than the phenomena that they seek to explain.
+</p>
+
+</div><!--end chapter-->
+
+<div class="chapter">
+
+<h2><a name="chap11"></a>XI<br/>
+Meteors, Fire-Balls, and Meteorites</h2>
+
+<p>
+One of the most terrorizing spectacles with which the heavens have ever caused
+the hearts of men to quake occurred on the night of November 13, 1833. On that
+night North America, which faced the storm, was under a continual rain of fire
+from about ten o’clock in the evening until daybreak.
+</p>
+
+<p>
+<i>The fragments of a comet had struck the earth.</i>
+</p>
+
+<p>
+But the meaning of what had happened was not discovered until long afterward.
+To the astronomers who, with astonishment not less than that of other people,
+watched the wonderful scene, it was an unparalleled “shower of meteors.” They
+did not then suspect that those meteors had once formed the head of a comet.
+Light dawned when, a year later, Prof. Denison Olmsted, of Yale College,
+demonstrated that the meteors had all moved in parallel orbits around the sun,
+and that these orbits intersected that of the earth at the point where our
+planet happened to be on the memorable night of November 13th. Professor
+Olmsted even went so far as to suggest that the cloud of meteors that had
+encountered the earth might form a diffuse comet; but full recognition of the
+fact that they were cometary débris came later, as the result of further
+investigation. The key to the secret was plainly displayed in the spectacle
+itself, and was noticed without being understood by thousands of the
+terror-stricken beholders. It was <i>an umbrella of fire</i> that had opened
+overhead and covered the heavens; in other words, the meteors all radiated from
+a particular point in the constellation Leo, and, being countless as the
+snowflakes in a winter tempest, they ribbed the sky with fiery streaks.
+Professor Olmsted showed that the radiation of the meteors from a fixed point
+was an effect of perspective, and in itself a proof that they were moving in
+parallel paths when they encountered the earth. The fact was noted that there
+had been a similar, but incomparably less brilliant, display of meteors on the
+same day of November, 1832, and it was rightly concluded that these had
+belonged to the same stream, although the true relationship of the phenomena
+was not immediately apprehended. Olmsted ascribed to the meteors a revolution
+about the sun once in every six months, bringing them to the intersection of
+their orbit with that of the earth every November 13th; but later investigators
+found that the real period was about thirty-three and one-quarter years, so
+that the great displays were due three times in a century, and their return was
+confidently predicted for the year 1866. The appearance of the meteors in 1832,
+a year before the great display, was ascribed to the great length of the stream
+which they formed in space—so great that they required more than two years to
+cross the earth’s orbit. In 1832 the earth had encountered a relatively rare
+part of the stream, but in 1833, on returning to the crossing-place, it found
+there the richest part of the stream pouring across its orbit. This explanation
+also proved to be correct, and the predicted return in 1866 was duly witnessed,
+although the display was much less brilliant than in 1833. It was followed by
+another in 1867.
+</p>
+
+<div class="fig" style="width:100%;">
+<a name="illus26"></a>
+<a href="images/image26.jpg">
+<img src="images/image26.jpg" width="600" height="459" alt="Illustration:" /></a>
+<p class="caption">Curious forms of meteorite trains<br/>
+Nos. 1 to 6 show the changes undergone by a train left by a meteorite which passed near the “Great Dipper”;
+7 shows the changes and drift of a train seen in the constellation Virgo; 8 is the singular train
+of the meteorite of February 22, 1909, near the Pole Star. (From <i>La Nature</i>.)</p>
+</div>
+
+<p>
+In the mean time Olmsted’s idea of a cometary relationship of the meteors was
+demonstrated to be correct by the researches of Schiaparelli and others, who
+showed that not only the November meteors, but those of August, which are seen
+more or less abundantly every year, traveled in the tracks of well-known
+comets, and had undoubtedly an identical origin with those comets. In other
+words the comets and the meteor-swarms were both remnants of original masses
+which had probably been split up by the action of the sun, or of some planet to
+which they had made close approaches. The annual periodicity of the August
+meteors was ascribed to the fact that the separation had taken place so long
+ago that the meteors had become distributed all around the orbit, in
+consequence of which the earth encountered some of them every year when it
+arrived at the crossing-point. Then Leverrier showed that the original comet
+associated with the November meteors was probably brought into the system by
+the influence of the planet Uranus in the year 126 of the Christian era.
+Afterward Alexander Herschel identified the tracks of no less than seventy-six
+meteor-swarms (most of them inconspicuous) with those of comets. The still more
+recent researches of Mr W. F. Denning make it probable that there are no
+meteors which do not belong to a flock or system probably formed by the
+disintegration of a cometary mass; even the apparently sporadic ones which
+shoot across the sky, “lost souls in the night,” being members of flocks which
+have become so widely scattered that the earth sometimes takes weeks to pass
+through the region of space where their paths lie.
+</p>
+
+<p>
+The November meteors should have exhibited another pair of spectacles in 1899
+and 1900, and their failure to do so caused at first much disappointment, until
+it was made plain that a good reason existed for their absence. It was found
+that after their last appearance, in 1867, they had been disturbed in their
+movements by the planets Jupiter and Saturn, whose attractions had so shifted
+the position of their orbit that it no longer intersected that of the earth, as
+it did before. Whether another planetary interference will sometime bring the
+principal mass of the November meteors back to the former point of intersection
+with the earth’s orbit is a question for the future to decide. It would seem
+that there may be several parallel streams of the November meteors, and that
+some of them, like those of August, are distributed entirely around the orbit,
+so that every mid-November we see a few of them.
+</p>
+
+<p>
+We come now to a very remarkable example of the disintegration of a comet and
+the formation of a meteor-stream. In 1826 Biela, of Josephstadt, Austria,
+discovered a comet to which his name was given. Calculation showed that it had
+an orbital period of about six and a half years, belonging to Jupiter’s
+“family.” On one of its returns, in 1846, it astonished its watchers by
+suddenly splitting in two. The two comets thus formed out of one separated to a
+distance of about one hundred and sixty thousand miles, and then raced side by
+side, sometimes with a curious ligature connecting them, like Siamese twins,
+until they disappeared together in interplanetary space. In 1852 they came
+back, still nearly side by side, but now the distance between them had
+increased to a million and a quarter of miles. After that, at every recurrence
+of their period, astronomers looked for them in vain, until 1872, when an
+amazing thing happened. On the night of November 28th, when the earth was
+crossing the plane of the orbit of the missing comet, a brilliant shower of
+meteors burst from the northern sky, traveling nearly in the track which the
+comet should have pursued. The astronomers were electrified. Klinkerfues, of
+Göttingen, telegraphed to Pogson, of Madras: <i>“Biela touched earth; search
+near Theta Centauri.”</i> Pogson searched in the place indicated and saw a
+cometary mass retreating into the southern heavens, where it was soon swallowed
+from sight!
+</p>
+
+<div class="fig" style="width:100%;">
+<a name="illus27"></a>
+<a href="images/image27.jpg">
+<img src="images/image27.jpg" width="151" height="700" alt="Illustration:" /></a>
+<p class="caption">Section of the atmosphere up to 100 kilometers.<br/>
+Showing the mean elevation at which meteorites and meteors make their
+appearance. Below are shown the elevation of Mount Everest, the highest manned
+balloon ascent by M. Berson; the height of cirrus clouds; the highest free
+balloon ascent; and the elevation attained by the clouds of fire-dust ejected
+by the Krakatoa eruption in 1883. (From <i>La Nature</i>)
+</p>
+</div>
+
+<p>
+Since then the Biela meteors have been among the recognized periodic spectacles
+of the sky, and few if any doubt that they represent a portion of the missing
+comet whose disintegration began with the separation into two parts in 1846.
+The comet itself has never since been seen. The first display of these meteors,
+sometimes called the “Andromedes,” because they radiate from the constellation
+Andromeda, was remarkable for the great brilliancy of many of the fire-balls
+that shot among the shower of smaller sparks, some of which were described as
+equaling the full moon in size. None of them is known to have reached the
+earth, but during the display of the same meteors in 1885 a meteoric mass fell
+at Mazapil in Northern Mexico (it is now in the Museum at Vienna), which many
+have thought may actually be a piece of the original comet of Biela. This
+brings us to the second branch of our subject.
+</p>
+
+<p>
+More rare than meteors or falling stars, and more startling, except that they
+never appear in showers, are the huge balls of fire which occasionally dart
+through the sky, lighting up the landscapes beneath with their glare, leaving
+trains of sparks behind them, often producing peals of thunder when they
+explode, and in many cases falling upon the earth and burying themselves from a
+few inches to several feet in the soil, from which, more than once, they have
+been picked up while yet hot and fuming. These balls are sometimes called
+bolides. They are not really round in shape, although they often look so while
+traversing the sky, but their forms are fragmentary, and occasionally
+fantastic. It has been supposed that their origin is different from that of the
+true meteors; it has even been conjectured that they may have originated from
+the giant volcanoes of the moon or have been shot out from the sun during some
+of the tremendous explosions that accompany the formation of eruptive
+prominences. By the same reasoning some of them might be supposed to have come
+from some distant star. Others have conjectured that they are wanderers in
+space, of unknown origin, which the earth encounters as it journeys on, and
+Lord Kelvin made a suggestion which has become classic because of its
+imaginative reach—<i>viz.,</i> that the first germs of life may have been
+brought to the earth by one of these bodies, “a fragment of an exploded world.”
+</p>
+
+<p>
+It is a singular fact that astronomers and scientific men in general were among
+the last to admit the possibility of solid masses falling from the sky. The
+people had believed in the reality of such phenomena from the earliest times,
+but the savants shook their heads and talked of superstition. This was the less
+surprising because no scientifically authenticated instance of such an
+occurrence was known, and the stones popularly believed to have fallen from the
+sky had become the objects of worship or superstitious reverence, a fact not
+calculated to recommend them to scientific credence. The celebrated “black
+stone” suspended in the Kaaba at Mecca is one of these reputed gifts from
+heaven; the “Palladium” of ancient Troy was another; and a stone which fell
+near Ensisheim, in Germany, was placed in a church as an object to be
+religiously venerated. Many legends of falling stones existed in antiquity,
+some of them curiously transfigured by the imagination, like the “Lion of the
+Peloponnesus,” which was said to have sprung down from the sky upon the Isthmus
+of Corinth. But near the beginning of the nineteenth century, in 1803, a
+veritable shower of falling stones occurred at L’Aigle, in Northern France, and
+this time astronomers took note of the phenomenon and scientifically
+investigated it. Thousands of the strange projectiles came from the sky on this
+occasion, and were scattered over a wide area of country, and some buildings
+were hit. Four years later another shower of stones occurred at Weston, Conn.,
+numbering thousands of individuals. The local alarm created in both cases was
+great, as well it might be, for what could be more intimidating than to find
+the blue vault of heaven suddenly hurling solid missiles at the homes of men?
+After these occurrences it was impossible for the most skeptical to doubt any
+longer, and the regular study of “aerolites,” or “meteorites,” began.
+</p>
+
+<p>
+One of the first things recognized was the fact that fire-balls are solid
+meteorites in flight, and not gaseous exhalations in the air, as some had
+assumed. They burn in the air during their flight, and sometimes, perhaps, are
+entirely consumed before reaching the ground. Their velocity before entering
+the earth’s atmosphere is equal to that of the planets in their
+orbits—<i>viz.,</i> from twenty to thirty miles per second—a fact which proves
+that the sun is the seat of the central force governing them. Their burning in
+the air is not difficult to explain; it is the heat of friction which so
+quickly brings them to incandescence. Calculation shows that a body moving
+through the air at a velocity of about a mile per second will be brought,
+superficially, to the temperature of “red heat” by friction with the
+atmosphere. If its velocity is twenty miles per second the temperature will
+become thousands of degrees. This is the state of affairs with a meteorite
+rushing into the earth’s atmosphere; its surface is liquefied within a few
+seconds after the friction begins to act, and the melted and vaporized portion
+of its mass is swept backward, forming the train of sparks that follows every
+great fire-ball. However, there is one phenomenon connected with the trains of
+meteorites which has never been satisfactorily explained: they often persist
+for long periods of time, drifting and turning with the wind, but not ceasing
+to glow with a phosphorescent luminosity. The question is, Whence comes this
+light? It must be light without heat, since the fine dust or vapor of which the
+train can only consist would not retain sufficient heat to render it luminous
+for so long a time. An extremely remarkable incident of this kind occurred on
+February 22, 1909, when an immense fire-ball that passed over southern England
+left a train that remained visible during two hours, assuming many curious
+shapes as it was drifted about by currents in the air.
+</p>
+
+<div class="fig" style="width:100%;">
+<a name="illus28"></a>
+<a href="images/image28.jpg">
+<img src="images/image28.jpg" width="474" height="600" alt="Illustration:" /></a>
+<p class="caption">A meteor photographed in flight</p>
+</div>
+
+<p>
+But notwithstanding the enormous velocity with which meteorites enter the air
+they are soon slowed down to comparatively moderate speed, so that when they
+disappear they are usually traveling not faster than a mile a second. The
+courses of many have been traced by observers situated along their track at
+various points, and thus a knowledge has been obtained of their height above
+the ground during their flight and of the length of their visible courses. They
+generally appear at an elevation of eighty or a hundred miles, and are seldom
+visible after having descended to within five miles of the ground, unless the
+observer happens to be near the striking-point, when he may actually witness
+the fall. Frequently they burst while high in the air and their fragments are
+scattered like shrapnel over the surface of the ground, sometimes covering an
+area of several square miles, but of course not thickly; different fragments of
+the same meteorite may reach the ground at points several miles apart. The
+observed length of their courses in the atmosphere varies from fifty to five
+hundred miles. If they continued a long time in flight after entering the air,
+even the largest of them would probably be consumed to the last scrap, but
+their fiery career is so short on account of their great speed that the heat
+does not have time to penetrate very deeply, and some that have been picked up
+immediately after their fall have been found cold as ice within. Their size
+after reaching the ground is variable within wide limits; some are known which
+weigh several tons, but the great majority weigh only a few pounds and many
+only a few ounces.
+</p>
+
+<p>
+Meteorites are of two kinds: <i>stony</i> meteorites and <i>iron</i>
+meteorites. The former outnumber the latter twenty to one; but many stone
+meteorites contain grains of iron. Nickel is commonly found in iron meteorites,
+so that it might be said that that redoubtable alloy nickel-steel is of
+cosmical invention. Some twenty-five chemical elements have been found in
+meteorites, including carbon and the “sun-metal,” helium. The presence of the
+latter is certainly highly suggestive in connection with the question of the
+origin of meteorites. The iron meteorites, besides metallic iron and nickel, of
+which they are almost entirely composed, contain hydrogen, helium, and carbonic
+oxide, and about the only imaginable way in which these gases could have become
+absorbed in the iron would be through the immersion of the latter while in a
+molten or vaporized state in a hot and dense atmosphere composed of them, a
+condition which we know to exist only in the envelopes of the sun and the
+stars.
+</p>
+
+<p>
+The existence of carbon in the Canyon Diablo iron meteorites is attended by a
+circumstance of the most singular character—a very “fairy tale of science.” In
+some cases <i>the carbon has become diamond!</i> These meteoric diamonds are
+very small; nevertheless, they are true diamonds, resembling in many ways the
+little black gems produced by Moissan’s method with the aid of the electric
+furnace. The fact that they are found embedded in these iron meteorites is
+another argument in favor of the hypothesis of the solar or stellar origin of
+the latter. To appreciate this it is necessary to recall the way in which
+Moissan made his diamonds. It was by a combination of the effects of great
+heat, great pressure, and sudden or rapid superficial cooling on a mass of iron
+containing carbon. When he finally broke open his iron he found it a pudding
+stuffed with miniature black diamonds. When a fragment of the Canyon Diablo
+meteoric iron was polished in Philadelphia over fifteen years ago it cut the
+emery-wheel to pieces, and examination showed that the damage had been effected
+by microscopic diamonds peppered through the mass. How were those diamonds
+formed? If the sun or Sirius was the laboratory that prepared them, we can get
+a glimpse at the process of their formation. There is plenty of heat, plenty of
+pressure, and an abundance of vaporized iron in the sun and the stars. When a
+great solar eruption takes place, masses of iron which have absorbed carbon may
+be shot out with a velocity which forbids their return. Plunged into the
+frightful cold of space, their surfaces are quickly cooled, as Moissan cooled
+his prepared iron by throwing it into water, and thus the requisite stress is
+set up within, and, as the iron solidifies, the included carbon crystallizes
+into diamonds. Whether this explanation has a germ of truth in it or not, at
+any rate it is evident that iron meteorites were not created in the form in
+which they come to us; they must once have been parts of immeasurably more
+massive bodies than themselves.
+</p>
+
+<p>
+The fall of meteorites offers an appreciable, though numerically insignificant,
+peril to the inhabitants of the earth. Historical records show perhaps three or
+four instances of people being killed by these bodies. But for the protection
+afforded by the atmosphere, which acts as a very effective shield, the danger
+would doubtless be very much greater. In the absence of an atmosphere not only
+would more meteorites reach the ground, but their striking force would be
+incomparably greater, since, as we have seen, the larger part of their original
+velocity is destroyed by the resistance of the air. A meteorite weighing many
+tons and striking the earth with a velocity of twenty or thirty miles per
+second, would probably cause frightful havoc.
+</p>
+
+<div class="fig" style="width:100%;">
+<a name="illus29"></a>
+<a href="images/image29.jpg">
+<img src="images/image29.jpg" width="600" height="406" alt="Illustration:" /></a>
+<p class="caption">Looking across Coon Butte crater from northern rim</p>
+</div>
+
+<p>
+It is a singular fact that recent investigations seem to have proved that an
+event of this kind actually happened in North America—perhaps not longer than a
+thousand or two thousand years ago. The scene of the supposed catastrophe is in
+northern central Arizona, at Coon Butte, where there is a nearly circular
+crater in the middle of a circular elevation or small mountain. The crater is
+somewhat over four thousand feet in diameter, and the surrounding rim, formed
+of upturned strata and ejected rock fragments, rises at its highest point one
+hundred and sixty feet above the plain. The crater is about six hundred feet in
+depth—that is, from the rim to the visible floor or bottom of the crater. There
+is no evidence that volcanic action has ever taken place in the immediate
+neighborhood of Coon Butte. The rock in which the crater has been made is
+composed of horizontal sandstone and limestone strata. Between three hundred
+and four hundred million tons of rock fragments have been detached, and a large
+portion hurled by some cause out of the crater. These fragments lie
+concentrically distributed around the crater, and in large measure form the
+elevation known as Coon Butte. The region has been famous for nearly twenty
+years on account of the masses of meteoric iron found scattered about and known
+as the “Canyon Diablo” meteorites. It was one of these masses, which consist of
+nickel-iron containing a small quantity of platinum, and of which in all some
+ten tons have been recovered for sale to the various collectors throughout the
+world, that as before mentioned destroyed the grinding-tool at Philadelphia
+through the cutting power of its embedded diamonds. These meteoric irons are
+scattered about the crater-hill, in concentric distribution, to a maximum
+distance of about five miles. When the suggestion was first made in 1896 that a
+monster meteorite might have created by its fall this singular lone crater
+<i>in stratified rocks,</i> it was greeted with incredulous smiles; but since
+then the matter has assumed a different aspect. The Standard Iron Company,
+formed by Messrs. D. M. Barringer, B. C. Tilghman, E. J. Bennitt, and S. J.
+Holsinger, having become, in 1903, the owner of this freak of nature, sunk
+shafts and bored holes to a great depth in the interior of the crater, and also
+trenched the slopes of the mountain, and the result of their investigations has
+proved that the meteoric hypothesis of origin is correct. (See the papers
+published in the <i>Proceedings of the Academy of Natural Sciences of
+Philadelphia,</i> December, 1905, wherein it is proved that the United States
+Geological Survey was wrong in believing this crater to have been due to a
+steam explosion. Since that date there has been discovered a great amount of
+additional confirmatory proof). Material of unmistakably meteoric origin was
+found by means of the drills, mixed with crushed rock, to a depth of six
+hundred to seven hundred feet below the floor of the crater, and a great deal
+of it has been found admixed with the ejected rock fragments on the outer
+slopes of the mountain, absolutely proving synchronism between the two events,
+the formation of this great crater and the falling of the meteoric iron out of
+the sky. The drill located in the bottom of the crater was sent, in a number of
+cases, much deeper (over one thousand feet) into unaltered horizontal red
+sandstone strata, but no meteoric material was found below this depth (seven
+hundred feet, or between eleven and twelve hundred feet below the level of the
+surrounding plain), which has been assumed as being about the limit of
+penetration. It is not possible to sink a shaft at present, owing to the water
+which has drained into the crater, and which forms, with the finely pulverized
+sandstone, a very troublesome quicksand encountered at about two hundred feet
+below the visible floor of the crater. As soon as this water is removed by
+pumping it will be easy to explore the depths of the crater by means of shafts
+and drifts. The rock strata (sandstone and limestone) of which the walls
+consist present every appearance of having been violently upturned by a huge
+body penetrating the earth like a cannon-ball. The general aspect of the crater
+strikingly resembles the impression made by a steel projectile shot into an
+armor-plate. Mr Tilghman has estimated that a meteorite about five hundred feet
+in diameter and moving with a velocity of about five miles per second would
+have made just such a perforation upon striking rocks of the character of those
+found at this place. There was some fusion of the colliding masses, and the
+heat produced some steam from the small amount of water in the rocks. As a
+result there has been found at depth a considerable amount of fused quartz
+(original sandstone), and with it innumerable particles or sparks of fused
+nickel-iron (original meteorite). A projectile of that size penetrating eleven
+to twelve hundred feet into the rocky shell of the globe must have produced a
+shock which was perceptible several hundred miles away.
+</p>
+
+<div class="fig" style="width:100%;">
+<a name="illus30"></a>
+<a href="images/image30.jpg">
+<img src="images/image30.jpg" width="600" height="405" alt="Illustration:" /></a>
+<p class="caption">Trail on south side, Coon Butte crater</p>
+</div>
+
+<p>
+The great velocity ascribed to the supposed meteorite at the moment of striking
+could be accounted for by the fact that it probably plunged nearly vertically
+downward, for it formed a circular crater in the rocky crust of the earth. In
+that case it would have been less retarded by the resistance of the atmosphere
+than are meteorites which enter the air at a lower angle and shoot ahead
+hundreds of miles until friction has nearly destroyed their original motion
+when they drop upon the earth. Some meteoric masses of great size, such as
+Peary’s iron meteorite found at Cape York, Greenland, and the almost equally
+large mass discovered at Bacubirito, Mexico, appear to have penetrated but
+slightly on striking the earth. This may be explained by supposing that they
+pursued a long, horizontal course through the air before falling. The result
+would be that, their original velocity having been practically destroyed, they
+would drop to the ground with a velocity nearly corresponding to that which
+gravity would impart within the perpendicular distance of their final fall. A
+six-hundred-and-sixty-pound meteorite, which fell at Knyahinya, Hungary,
+striking at an angle of 27&deg; from the vertical, penetrated the ground to a
+depth of eleven feet.
+</p>
+
+<p>
+It has been remarked that the Coon Butte meteorite may have fallen not longer
+ago than a few thousand years. This is based upon the fact that the geological
+indications favor the supposition that the event did not occur more than five
+thousand years ago, while on the other hand the rings of growth in the
+cedar-trees growing on the slopes of the crater show that they have existed
+there about seven hundred years. Prof. William H. Pickering has recently
+correlated this with an ancient chronicle which states that at Cairo, Egypt, in
+the year 1029, “many stars passed with a great noise.” He remarks that Cairo is
+about 100&deg;, by great circle, from Coon Butte, so that if the meteorite that
+made the crater was a member of a flock of similar bodies which encountered the
+earth moving in parallel lines, some of them might have traversed the sky
+tangent to the earth’s surface at Cairo. That the spectacle spoken of in the
+chronicle was caused by meteorites he deems exceedingly probable because of
+what is said about “a great noise;” meteorites are the only celestial phenomena
+attended with perceptible sounds. Professor Pickering conjectures that this
+supposed flock of great meteorites may have formed the nucleus of a comet which
+struck the earth, and he finds confirmation of the idea in the fact that out of
+the ten largest meteorites known, no less than seven were found within nine
+hundred miles of Coon Butte. It would be interesting if we could trace back the
+history of that comet, and find out what malicious planet caught it up in its
+innocent wanderings and hurled it with so true an aim at the earth! This
+remarkable crater is one of the most interesting places in the world, for there
+is absolutely no record of such a mass, possibly an iron-headed comet, from
+outer space having come into collision with our earth. The results of the
+future exploration of the depths of the crater will be awaited with much
+interest.
+</p>
+
+</div><!--end chapter-->
+
+<div class="chapter">
+
+<h2><a name="chap12"></a>XII<br/>
+The Wrecking of the Moon</h2>
+
+<p>
+There are sympathetic moods under whose influence one gazes with a certain
+poignant tenderness at the worn face of the moon; that little “fossil world”
+(the child of our mother earth, too) bears such terrible scars of its brief
+convulsive life that a sense of pity is awakened by the sight. The moon is the
+wonder-land of the telescope. Those towering mountains, whose “proud aspiring
+peaks” cast silhouettes of shadow that seem drawn with india-ink; those vast
+plains, enchained with gentle winding hills and bordered with giant ranges;
+those oval “oceans,” where one looks expectant for the flash of wind-whipped
+waves; those enchanting “bays” and recesses at the seaward feet of the Alps;
+those broad straits passing between guardian heights incomparably mightier than
+Gibraltar; those locket-like valleys as secluded among their mountains as the
+Vale of Cashmere; those colossal craters that make us smile at the pretensions
+of Vesuvius, Etna, and Cotopaxi; those strange white ways which pass with the
+unconcern of Roman roads across mountain, gorge, and valley—all these give the
+beholder an irresistible impression that it is truly a world into which he is
+looking, a world akin to ours, and yet no more like our world than Pompeii is
+like Naples. Its air, its waters, its clouds, its life are gone, and only a
+skeleton remains—a mute but eloquent witness to a cosmical tragedy without
+parallel in the range of human knowledge.
+</p>
+
+<p>
+One cannot but regret that the moon, if it ever was the seat of intelligent
+life, has not remained so until our time. Think what the consequences would
+have been if this other world at our very door had been found to be both
+habitable and inhabited! We talk rather airily of communicating with Mars by
+signals; but Mars never approaches nearer than 35,000,000 miles, while the moon
+when nearest is only a little more than 220,000 miles away. Given an effective
+magnifying power of five thousand diameters, which will perhaps be possible at
+the mountain observatories as telescopes improve, and we should be able to
+bring the moon within an apparent distance of about forty miles, while the
+corresponding distance for Mars would be more than seven thousand miles. But
+even with existing telescopic powers we can see details on the moon no larger
+than some artificial constructions on the earth. St Peter’s at Rome, with the
+Vatican palace and the great piazza, if existing on the moon, would
+unquestionably be recognizable as something else than a freak of nature. Large
+cities, with their radiating lines of communication, would at once betray their
+real character. Cultivated tracts, and the changes produced by the interference
+of intelligent beings, would be clearly recognizable. The electric illumination
+of a large town at night would probably be markedly visible. Gleams of
+reflected sunlight would come to us from the surfaces of the lakes and oceans,
+and a huge “liner” traversing a lunar sea could probably be followed by its
+trail of smoke. As to communications by “wireless” signals, which certain
+enthusiasts have thought of in connection with Mars, in the case of the moon
+they should be a relatively simple matter, and the feat might actually be
+accomplished. Think what a literature would grow up about the moon if it were a
+living world! Its very differences from the earth would only accentuate its
+interest for us. Night and day on the moon are each two weeks in length; how
+interesting it would be to watch the manner in which the lunarians dealt with
+such a situation as that. Lunar and terrestrial history would keep step with
+each other, and we should record them both. Truly one might well wish to have a
+neighbor world to study; one would feel so much the less alone in space.
+</p>
+
+<p>
+It is not impossible that the moon did at one time have inhabitants of some
+kind. But, if so, they vanished with the disappearance of its atmosphere and
+seas, or with the advent of its cataclysmic age. At the best, its career as a
+living world must have been brief. If the water and air were gradually
+absorbed, as some have conjectured, by its cooling interior rocks, its surface
+might, nevertheless, have retained them for long ages; but if, as others think,
+their disappearance was due to the escape of their gaseous molecules in
+consequence of the inability of the relatively small lunar gravitation to
+retain them, then the final catastrophe must have been as swift as it was
+inevitable. Accepting Darwin’s hypothesis, that the moon was separated from the
+earth by tidal action while both were yet plastic or nebulous, we may
+reasonably conclude that it began its career with a good supply of both water
+and air, but did not possess sufficient mass to hold them permanently. Yet it
+may have retained them long enough for life to develop in many forms upon its
+surface; in fact, there are so many indications that air and water have not
+always been lacking to the lunar world that we are driven to invent theories to
+explain both their former presence and their present absence.
+</p>
+
+<div class="fig" style="width:100%;">
+<a name="illus31"></a>
+<a href="images/image31.jpg">
+<img src="images/image31.jpg" width="405" height="600" alt="Illustration:" /></a>
+<p class="caption">The craters Clavius, Longomontanus, Tycho, etc.</p>
+</div>
+
+<p>
+But whatever the former condition of the moon may have been, its existing
+appearance gives it a resistless fascination, and it bears so clearly the story
+of a vast catastrophe sculptured on its rocky face that the thoughtful observer
+cannot look upon it without a feeling of awe. The gigantic character of the
+lunar features impresses the beholder not less than the universality of the
+play of destructive forces which they attest. Let us make a few comparisons.
+Take the lunar crater called “Tycho”, which is a typical example of its kind.
+In the telescope Tycho appears as a perfect ring surrounding a circular
+depression, in the center of which rises a group of mountains. Its superficial
+resemblance to some terrestrial volcanic craters is very striking. Vesuvius,
+seen from a point vertically above, would no doubt look something like that
+(the resemblance would have been greater when the Monte del Cavallo formed a
+more complete circuit about the crater cone). But compare the dimensions. The
+remains of the outer crater ring of Vesuvius are perhaps half a mile in
+diameter, while the active crater itself is only two or three hundred feet
+across at the most; Tycho has a diameter of fifty-four miles! The group of
+relatively insignificant peaks in the center of the crater floor of Tycho is
+far more massive than the entire mountain that we call Vesuvius. The largest
+known volcanic crater on the earth, Aso San, in Japan, has a diameter of seven
+miles; it would take <i>sixty</i> craters like Aso San to equal Tycho in area!
+And Tycho, though one of the most perfect, is by no means the largest crater on
+the moon. Another, called “Theophilus,” has a diameter of sixty-four miles, and
+is eighteen thousand feet deep. There are hundreds from ten to forty miles in
+diameter, and thousands from one to ten miles. They are so numerous in many
+places that they break into one another, like the cells of a crushed honeycomb.
+</p>
+
+<p>
+The lunar craters differ from those of the earth more fundamentally than in the
+matter of mere size; <i>they are not situated on the tops of mountains.</i> If
+they were, and if all the proportions were the same, a crater like Tycho might
+crown a conical peak fifty or one hundred miles high! Instead of being cavities
+in the summits of mountains, the lunar craters are rather gigantic sink-holes
+whose bottoms in many cases lie two or three miles below the general surface of
+the lunar world. Around their rims the rocks are piled up to a height of from a
+few hundred to two or three thousand feet, with a comparatively gentle
+inclination, but on the inner side they fall away in gigantic broken precipices
+which make the dizzy cliffs of the Matterhorn seem but “lover’s leaps.” Down
+they drop, ridge below ridge, crag under crag, tottering wall beneath wall,
+until, in a crater named “Newton,” near the south lunar pole, they attain a
+depth where the rays of the sun never reach. Nothing more frightful than the
+spectacle which many of these terrible chasms present can be pictured by the
+imagination. As the lazy lunar day slowly advances, the sunshine, unmitigated
+by clouds or atmospheric veil of any kind, creeps across their rims and begins
+to descend the opposite walls. Presently it strikes the ragged crest of a ridge
+which had lain hidden in such darkness as we never know on the earth, and runs
+along it like a line of kindling fire. Rocky pinnacles and needles shoot up
+into the sunlight out of the black depths. Down sinks the line of light, mile
+after mile, and continually new precipices and cliffs are brought into view,
+until at last the vast floor is attained and begins to be illuminated. In the
+meanwhile the sun’s rays, darting across the gulf, have touched the summits of
+the central peaks, twenty or thirty miles from the crater’s inmost edge, and
+they immediately kindle and blaze like huge stars amid the darkness. So
+profound are some of these awful craters that days pass before the sun has
+risen high enough above them to chase the last shadows from their depths.
+</p>
+
+<p>
+Although several long ranges of mountains resembling those of the earth exist
+on the moon, the great majority of its elevations assume the crateriform
+aspect. Sometimes, instead of a crater, we find an immense mountain ring whose
+form and aspect hardly suggest volcanic action. But everywhere the true craters
+are in evidence, even on the sea-beds, although they attain their greatest
+number and size on those parts of the moon—covering sixty per cent of its
+visible surface—which are distinctly mountainous in character and which
+constitute its most brilliant portions. Broadly speaking, the southwestern half
+of the moon is the most mountainous and broken, and the northeastern half the
+least so. Right down through the center, from pole to pole, runs a wonderful
+line of craters and crateriform valleys of a magnitude stupendous even for the
+moon. Another similar line follows the western edge. Three or four “seas” are
+thrust between these mountainous belts. By the effects of “libration” parts of
+the opposite hemisphere of the moon which is turned away from the earth are
+from time to time brought into view, and their aspect indicates that that
+hemisphere resembles in its surface features the one which faces the earth.
+There are many things about the craters which seem to give some warrant for the
+hypothesis which has been particularly urged by Mr G. K. Gilbert, that they
+were formed by the impact of meteors; but there are also many things which
+militate against that idea, and, upon the whole, the volcanic theory of their
+origin is to be preferred.
+</p>
+
+<p>
+The enormous size of the lunar volcanoes is not so difficult to account for
+when we remember how slight is the force of lunar gravity as compared with that
+of the earth. With equal size and density, bodies on the moon weigh only
+one-sixth as much as on the earth. Impelled by the same force, a projectile
+that would go ten miles on the earth would go sixty miles on the moon. A lunar
+giant thirty-five feet tall would weigh no more than an ordinary son of Adam
+weighs on his greater planet. To shoot a body from the earth so that it would
+not drop back again, we should have to start it with a velocity of seven miles
+per second; a mile and a half per second would serve on the moon. It is by no
+means difficult to believe, then, that a lunar volcano might form a crater ring
+eight or ten times broader than the greatest to be found on the earth,
+especially when we reflect that in addition to the relatively slight force of
+gravity, the materials of the lunar crust are probably lighter than those of
+our terrestrial rocks.
+</p>
+
+<div class="fig" style="width:100%;">
+<a name="illus32"></a>
+<a href="images/image32.jpg">
+<img src="images/image32.jpg" width="409" height="600" alt="Illustration:" /></a>
+<p class="caption">Western part of the Mare Serenitatis</p>
+</div>
+
+<p>
+For similar reasons it seems not impossible that the theory mentioned in a
+former chapter—that some of the meteorites that have fallen upon the earth
+originated from the lunar volcanoes—is well founded. This would apply
+especially to the stony meteorites, for it is hardly to be supposed that the
+moon, at least in its superficial parts, contains much iron. It is surely a
+scene most strange that is thus presented to the mind’s eye—that little
+attendant of the earth’s (the moon has only one-fiftieth of the volume, and
+only one-eightieth of the mass of the earth) firing great stones back at its
+parent planet! And what can have been the cause of this furious outbreak of
+volcanic forces on the moon? Evidently it was but a passing stage in its
+history; it had enjoyed more quiet times before. As it cooled down from the
+plastic state in which it parted from the earth, it became incrusted after the
+normal manner of a planet, and then oceans were formed, its atmosphere being
+sufficiently dense to prevent the water from evaporating and the would-be
+oceans from disappearing continually in mist. This, if any, must have been the
+period of life in the lunar world. As we look upon the vestiges of that ancient
+world buried in the wreck that now covers so much of its surface, it is
+difficult to restrain the imagination from picturing the scenes which were once
+presented there; and, in such a case, should the imagination be fettered? We
+give it free rein in terrestrial life, and it rewards us with some of our
+greatest intellectual pleasures. The wonderful landscapes of the moon offer it
+an ideal field with just enough half-hidden suggestions of facts to stimulate
+its powers.
+</p>
+
+<p>
+The great plains of the <i>Mare Imbrium</i> and the <i>Mare Serenitatis</i>
+(the “Sea of Showers” and the “Sea of Serenity”), bordered in part by lofty
+mountain ranges precisely like terrestrial mountains, scalloped along their
+shores with beautiful bays curving back into the adjoining highlands, and
+united by a great strait passing between the nearly abutting ends of the “Lunar
+Apennines” and the “Lunar Caucasus,” offer the elements of a scene of world
+beauty such as it would be difficult to match upon our planet. Look at the
+finely modulated bottom of the ancient sea in Mr Ritchey’s exquisite photograph
+of the western part of the <i>Mare Serenitatis,</i> where one seems to see the
+play of the watery currents heaping the ocean sands in waving lines, making
+shallows, bars, and deeps for the mariner to avoid or seek, and affording a
+playground for the creatures of the main. What geologist would not wish to try
+his hammer on those rocks with their stony pages of fossilized history? There
+is in us an instinct which forbids us to think that there was never any life
+there. If we could visit the moon, there is not among us a person so prosaic
+and unimaginative that he would not, the very first thing, begin to search for
+traces of its inhabitants. We would look for them in the deposits on the sea
+bottoms; we would examine the shores wherever the configuration seemed
+favorable for harbors and the sites of maritime cities—forgetting that it may
+be a little ridiculous to ascribe to the ancient lunarians the same ideas that
+have governed the development of our race; we would search through the valleys
+and along the seeming courses of vanished streams; we would explore the
+mountains, not the terrible craters, but the pinnacled chains that recall our
+own Alps and Rockies; seeking everywhere some vestige of the transforming
+presence of intelligent life. Perhaps we should find such traces, and perhaps,
+with all our searching, we should find nothing to suggest that life had ever
+existed amid that universal ruin.
+</p>
+
+<div class="fig" style="width:100%;">
+<a name="illus33"></a>
+<a href="images/image33.jpg">
+<img src="images/image33.jpg" width="421" height="600" alt="Illustration:" /></a>
+<p class="caption">Mare Tranquilitatis and surroundings</p>
+</div>
+
+<p>
+Look again at the border of the “Sea of Serenity”—what a name for such a
+scene!—and observe how it has been rent with almost inconceivable violence, the
+wall of the colossal crater Posidonius dropping vertically upon the ancient
+shore and obliterating it, while its giant neighbor, Le Monnier, opens a
+yawning mouth as if to swallow the sea itself. A scene like this makes one
+question whether, after all, those may not be right who have imagined that the
+so-called sea bottoms are really vast plains of frozen lava which gushed up in
+floods so extensive that even the mighty volcanoes were half drowned in the
+fiery sea. This suggestion becomes even stronger when we turn to another of the
+photographs of Mr Ritchey’s wonderful series, showing a part of the <i>Mare
+Tranquilitatis</i> (“Sea of Tranquility”!). Notice how near the center of the
+picture the outline of a huge ring with radiating ridges shows through the sea
+bottom; a fossil volcano submerged in a petrified ocean! This is by no means
+the only instance in which a buried world shows itself under the great lunar
+plains. Yet, as the newer craters in the sea itself prove, the volcanic
+activity survived this other catastrophe, or broke out again subsequently,
+bringing more ruin to pile upon ruin.
+</p>
+
+<p>
+Yet notwithstanding the evidence which we have just been considering in support
+of the hypothesis that the “seas” are lava floods, Messrs. Loewy and Puiseux,
+the selenographers of the Paris Observatory, are convinced that these great
+plains bear characteristic marks of the former presence of immense bodies of
+water. In that case we should be forced to conclude that the later oceans of
+the moon lay upon vast sheets of solidified lava; and thus the catastrophe of
+the lunar world assumes a double aspect, the earliest oceans being swallowed up
+in molten floods issuing from the interior, while the lands were reduced to
+chaos by a universal eruption of tremendous volcanoes; and then a period of
+comparative quiet followed, during which new seas were formed, and new life
+perhaps began to flourish in the lunar world, only to end in another cataclysm,
+which finally put a term to the existence of the moon as a life-supporting
+world.
+</p>
+
+<p>
+Suppose we examine two more of Mr Ritchey’s illuminating photographs, and,
+first, the one showing the crater Theophilus and its surroundings. We have
+spoken of Theophilus before, citing the facts that it is sixty-four miles in
+diameter and eighteen thousand feet deep. It will be noticed that it has two
+brother giants—Cyrillus the nearer, and Catharina the more distant; but
+Theophilus is plainly the youngest of the trio. Centuries, and perhaps
+thousands of years, must have elapsed between the periods of their upheaval,
+for the two older craters are partly filled with débris, while it is manifest
+at a glance that when the south eastern wall of Theophilus was formed, it broke
+away and destroyed a part of the more ancient ring of Cyrillus. There is no
+more tremendous scene on the moon than this; viewed with a powerful telescope,
+it is absolutely appalling.
+</p>
+
+<div class="fig" style="width:100%;">
+<a name="illus34"></a>
+<a href="images/image34.jpg">
+<img src="images/image34.jpg" width="425" height="600" alt="Illustration:" /></a>
+<p class="caption">Lunar craters Theophilus and surrounding region</p>
+</div>
+
+<p>
+The next photograph shows, if possible, a still wilder region. It is the part
+of the moon lying between Tycho and the south pole. Tycho is seen in the lower
+left-hand part of the picture. To the right, at the edge of the illuminated
+portion of the moon, are the crater-rings, Longomontanus and Wilhelm I, the
+former being the larger. Between them are to be seen the ruins of two or three
+more ancient craters which, together with portions of the walls of Wilhelm I
+and Longomontanus, have been honeycombed with smaller craters. The vast
+crateriform depression above the center of the picture is Clavius, an unrivaled
+wonder of lunar scenery, a hundred and forty-two miles in its greatest length,
+while its whole immense floor has sunk two miles below the general surface of
+the moon outside the ring. The monstrous shadow-filled cavity above Clavius
+toward the right is Blancanus, whose aspect here gives a good idea of the
+appearance of these chasms when only their rims are in the sunlight. But
+observe the indescribable savagery of the entire scene. It looks as though the
+spirit of destruction had gone mad in this spot. The mighty craters have broken
+forth one after another, each rending its predecessor; and when their work was
+finished, a minor but yet tremendous outbreak occurred, and the face of the
+moon was gored and punctured with thousands of smaller craters. These
+relatively small craters (small, however, only in a lunar sense, for many of
+them would appear gigantic on the earth) recall once more the theory of
+meteoric impact. It does not seem impossible that some of them may have been
+formed by such an agency.
+</p>
+
+<p>
+One would not wish for our planet such a fate as that which has overtaken the
+moon, but we cannot be absolutely sure that something of the kind may not be in
+store for it. We really know nothing of the ultimate causes of volcanic
+activity, and some have suggested that the internal energies of the earth may
+be accumulating instead of dying out, and may never yet have exhibited their
+utmost destructive power. Perhaps the best assurance that we can find that the
+earth will escape the catastrophe that has overtaken its satellite is to be
+found in the relatively great force of its gravitation. The moon has been the
+victim of its weakness; given equal forces, and the earth would be the better
+able to withstand them. It is significant, in connection with these
+considerations, that the little planet Mercury, which seems also to have parted
+with its air and water, shows to the telescope some indications that it is
+pitted with craters resembling those that have torn to pieces the face of the
+moon.
+</p>
+
+<p>
+Upon the whole, after studying the dreadful lunar landscapes, one cannot feel a
+very enthusiastic sympathy with those who are seeking indications of the
+continued existence of some kind of life on the moon; such a world is better
+without inhabitants. It has met its fate; let it go! Fortunately, it is not so
+near that it cannot hide its scars and appear beautiful—except when curiosity
+impels us to look with the penetrating eyes of the astronomer.
+</p>
+
+<div class="fig" style="width:100%;">
+<a name="illus35"></a>
+<a href="images/image35.jpg">
+<img src="images/image35.jpg" width="484" height="600" alt="Illustration:" /></a>
+<p class="caption">Mare Crisium</p>
+</div>
+
+</div><!--end chapter-->
+
+<div class="chapter">
+
+<h2><a name="chap13"></a>XIII<br/>
+The Great Mars Problem</h2>
+
+<p>
+Let any thoughtful person who is acquainted with the general facts of astronomy
+look up at the heavens some night when they appear in their greatest splendor,
+and ask himself what is the strongest impression that they make upon his mind.
+He may not find it easy to frame an answer, but when he has succeeded it will
+probably be to the effect that the stars give him an impression of the
+universality of intelligence; they make him feel, as the sun and the moon
+cannot do, that his world is not alone; that all this was not made simply to
+form a gorgeous canopy over the tents of men. If he is of a devout turn of
+mind, he thinks, as he gazes into those fathomless deeps and among those
+bewildering hosts, of the infinite multitude of created beings that the
+Almighty has taken under his care. The narrow ideas of the old geocentric
+theology, which made the earth God’s especial footstool, and man his only
+rational creature, fall away from him like a veil that had obscured his vision;
+they are impossible in the presence of what he sees above. Thus the natural
+tendency, in the light of modern progress, is to regard the universe as
+everywhere filled with life.
+</p>
+
+<p>
+But science, which is responsible for this broadening of men’s thoughts
+concerning the universality of life, itself proceeds to set limits. Of
+spiritual existences it pretends to know nothing, but as to physical beings, it
+declares that it can only entertain the supposition of their existence where it
+finds evidence of an environment suited to their needs, and such environment
+may not everywhere exist. Science, though repelled by the antiquated
+theological conception of the supreme isolation of man among created beings,
+regards with complacency the probability that there are regions in the universe
+where no organic life exists, stars which shine upon no inhabited worlds, and
+planets which nourish no animate creatures. The astronomical view of the
+universe is that it consists of matter in every stage of evolution: some
+nebulous and chaotic; some just condensing into stars (suns) of every magnitude
+and order; some shaped into finished solar bodies surrounded by dependent
+planets; some forming stars that perhaps have no planets, and will have none;
+some constituting suns that are already aging, and will soon lose their radiant
+energy and disappear; and some aggregated into masses that long ago became
+inert, cold, and rayless, and that can only be revivified by means about which
+we can form conjectures, but of which we actually know nothing.
+</p>
+
+<p>
+As with the stars, so with the planets, which are the satellites of stars. All
+investigations unite to tell us that the planets are not all in the same state
+of development. As some are large and some small, so some are, in an
+evolutionary sense, young, and some old. As they depend upon the suns around
+which they revolve for their light, heat, and other forms of radiant energy, so
+their condition varies with their distance from those suns. Many may never
+arrive at a state suitable for the maintenance of life upon their surfaces;
+some which are not at present in such a state may attain it later; and the
+forms of life themselves may vary with the peculiar environment that different
+planets afford. Thus we see that we are not scientifically justified in
+affirming that life is ubiquitous, although we are thus justified in saying
+that it must be, in a general sense, universal. We might liken the universe to
+a garden known to contain every variety of plant. If on entering it we see no
+flowers, we examine the species before us and find that they are not of those
+which bloom at this particular season, or perhaps they are such as never bear
+flowers. Yet we feel no doubt that we shall find flowers somewhere in the
+garden, because there <i>are</i> species which bloom at this season, and the
+garden contains <i>all</i> varieties.
+</p>
+
+<p>
+While it is tacitly assumed that there are planets revolving around other stars
+than the sun, it would be impossible for us to see them with any telescope yet
+invented, and no instrument now in the possession of astronomers could assure
+us of their existence; so the only planetary system of which we have visual
+knowledge is our own. Excluding the asteroids, which could not from any point
+of view be considered as habitable, we have in the solar system eight planets
+of various sizes and situated at various distances from the sun. Of these eight
+we know that one, the earth, is inhabited. The question, then, arises: Are
+there any of the others which are inhabited or habitable? Since it is our
+intention to discuss the habitability of only one of the seven to which the
+question applies, the rest may be dismissed in a few words. The smallest of
+them, and the nearest to the sun, is Mercury, which is regarded as
+uninhabitable because it has no perceptible supply of water and air, and
+because, owing to the extraordinary eccentricity of its orbit, it is subjected
+to excessive and very rapid alterations in the amount of solar heat and light
+poured upon its surface, such alterations being inconsistent with the
+supposition that it can support living beings. Even its average temperature is
+more than six and a half times that prevailing on the earth! Another
+circumstance which militates against its habitability is that, according to the
+results of the best telescopic studies, it always keeps the same face toward
+the sun, so that one half of the planet is perpetually exposed to the fierce
+solar rays, and the other half faces the unmitigated cold of open space. Venus,
+the next in distance from the sun, is almost the exact twin of the earth in
+size, and many arguments may be urged in favor of its habitability, although it
+is suspected of possessing the same peculiarity as Mercury, in always keeping
+the same side sunward. Unfortunately its atmosphere appears to be so dense that
+no permanent markings on its surface are certainly visible, and the question of
+its actual condition must, for the present, be left in abeyance. Mars, the
+first planet more distant from the sun than the earth, is the special subject
+of this chapter, and will be described and discussed a few lines further on.
+Jupiter, Saturn, Uranus, and Neptune, the four giant planets, all more distant
+than Mars, and each more distant than the other in the order named, are all
+regarded as uninhabitable because none of them appears to possess any degree of
+solidity. They may have solid or liquid nuclei, but exteriorly they seem to be
+mere balls of cloud. Of course, one can imagine what he pleases about the
+existence of creatures suited to the physical constitution of such planets as
+these, but they must be excluded from the category of habitable worlds in the
+ordinary sense of the term. We go back, then, to Mars.
+</p>
+
+<p>
+It will be best to begin with a description of the planet. Mars is 4230 miles
+in diameter; its surface is not much more than one-quarter as extensive as that
+of the earth (.285). Its mean distance from the sun is 141,500,000 miles,
+48,500,000 miles greater than that of the earth. Since radiant energy varies
+inversely as the square of distance, Mars receives less than half as much solar
+light and heat as the earth gets. Mars’ year (period of revolution round the
+sun) is 687 days. Its mean density is 71 per cent of the earth’s, and the force
+of gravity on its surface is 38 per cent of that on the surface of the earth;
+<i>i.e.,</i> a body weighing one hundred pounds on the earth would, if
+transported to Mars, weigh but thirty-eight pounds. The inclination of its
+equator to the plane of its orbit differs very little from that of the earth’s
+equator, and its axial rotation occupies 24 hours 37 minutes. so that the
+length of day and night, and the extent of the seasonal changes on Mars, are
+almost precisely the same as on the earth. But owing to the greater length of
+its year, the seasons of Mars, while occurring in the same order, are almost
+twice as long as ours. The surface of the planet is manifestly solid, like that
+of our globe, and the telescope reveals many permanent markings on it,
+recalling the appearance of a globe on which geographical features have been
+represented in reddish and dusky tints. Around the poles are plainly to be seen
+rounded white areas, which vary in extent with the Martian seasons, nearly
+vanishing in summer and extending widely in winter. The most recent
+spectroscopic determinations indicate that Mars has an atmosphere perhaps as
+dense as that to be found on our loftiest mountain peaks, and there is a
+perceptible amount of watery vapor in this atmosphere. The surface of the
+planet appears to be remarkably level, and it has no mountain ranges. No
+evidences of volcanic action have been discovered on Mars. The dusky and
+reddish areas were regarded by the early observers as respectively seas and
+lands, but at present it is not believed that there are any bodies of water on
+the planet. There has never been much doubt expressed that the white areas
+about the poles represent snow.
+</p>
+
+<p>
+It will be seen from this brief description that many remarkable resemblances
+exist between Mars and the earth, and there is nothing wonderful in the fact
+that the question of the habitability of the former has become one of extreme
+and wide-spread interest, giving rise to the most diverse views, to many
+extraordinary speculations, and sometimes to regrettably heated controversy.
+The first champion of the habitability of Mars was Sir William Herschel,
+although even before his time the idea had been suggested. He was convinced by
+the revelations of his telescopes, continually increasing in power, that Mars
+was more like the earth than any other planet. He could not resist the
+testimony of the polar snows, whose suggestive conduct was in such striking
+accord with what occurs upon the earth. Gradually, as telescopes improved and
+observers increased in number, the principal features of the planet were
+disclosed and charted, and “areography,” as the geography of Mars was called,
+took its place among the recognized branches of astronomical study. But it was
+not before 1877 that a fundamentally new discovery in areography gave a truly
+sensational turn to speculation about life on “the red planet.” In that year
+Mars made one of its nearest approaches to the earth, and was so situated in
+its orbit that it could be observed to great advantage from the northern
+hemisphere of the earth. The celebrated Italian astronomer, Schiaparelli, took
+advantage of this opportunity to make a trigonometrical survey of the surface
+of Mars—as coolly and confidently as if he were not taking his sights across a
+thirty-five-million-mile gulf of empty space—and in the course of this survey
+he was astonished to perceive that the reddish areas, then called continents,
+were crossed in many directions by narrow, dusky lines, to which he gave the
+suggestive name of “canals.” Thus a kind of firebrand was cast into the field
+of astronomical speculation, which has ever since produced disputes that have
+sometimes approached the violence of political faction. At first the accuracy
+of Schiaparelli’s observations was contested; it required a powerful telescope,
+and the most excellent “seeing,” to render the enigmatical lines visible at
+all, and many searchers were unable to detect them. But Schiaparelli continued
+his studies in the serene sky of Italy, and produced charts of the gridironed
+face of Mars containing so much astonishing detail that one had either to
+reject them <i>in toto</i> or to confess that Schiaparelli was right. As
+subsequent favorable oppositions of Mars occurred, other observers began to see
+the “canals” and to confirm the substantial accuracy of the Italian
+astronomer’s work, and finally few were found who would venture to affirm that
+the “canals” did not exist, whatever their meaning might be.
+</p>
+
+<div class="fig" style="width:100%;">
+<a name="illus36"></a>
+<a href="images/image36.jpg">
+<img src="images/image36.jpg" width="750" height="419" alt="Illustration:" /></a>
+<p class="caption">Schiaparelli’s chart of Mars, showing the so-called system
+of canals</p>
+</div>
+
+<p>
+When Schiaparelli began his observations it was generally believed, as we have
+said, that the dusky areas on Mars were seas, and since Schiaparelli thought
+that the “canals” invariably began and ended at the shores of the “seas,” the
+appropriateness of the title given to the lines seemed apparent. Their
+artificial character was immediately assumed by many, because they were too
+straight and too suggestively geometrical in their arrangement to permit the
+conclusion that they were natural watercourses. A most surprising circumstance
+noted by Schiaparelli was that the “canals” made their appearance <i>after</i>
+the melting of the polar snow in the corresponding hemisphere had begun, and
+that they grew darker, longer, and more numerous in proportion as the polar
+liquidation proceeded; another very puzzling observation was that many of them
+became double as the season advanced; close beside an already existing “canal,”
+and in perfect parallelism with it, another would gradually make its
+appearance. That these phenomena actually existed and were not illusions was
+proved by later observations, and today they are seen whenever Mars is
+favorably situated for observation.
+</p>
+
+<p>
+In the closing decade of the nineteenth century, Mr Percival Lowell took up the
+work where Schiaparelli had virtually dropped it, and soon added a great number
+of “canals” to those previously known, so that in his charts the surface of the
+wonderful little planet appears covered as with a spider’s web, the dusky lines
+criss-crossing in every direction, with conspicuous knots wherever a number of
+them come together. Mr Lowell has demonstrated that the areas originally called
+seas, and thus named on the earlier charts, are not bodies of water, whatever
+else they may be. He has also found that the mysterious lines do not, as
+Schiaparelli supposed, begin and end at the edges of the dusky regions, but
+often continue on across them, reaching in some cases far up into the polar
+regions. But Schiaparelli was right in his observation that the appearance of
+the “canals” is synchronous with the gradual disappearance of the polar snows,
+and this fact has become the basis of the most extraordinary theory that the
+subject of life in other worlds has ever given birth to.
+</p>
+
+<p>
+Now, the effect of such discoveries, as we have related, depends upon the type
+of mind to whose attention they are called. Many are content to accept them as
+strange and inexplicable at present, and to wait for further light upon them;
+others insist upon an immediate inquiry concerning their probable nature and
+meaning. Such an inquiry can only be based upon inference proceeding from
+analogy. Mars, say Mr Lowell and those who are of his opinion, is manifestly a
+solidly incrusted planet like the earth; it has an atmosphere, though one of
+great rarity; it has water vapor, as the snows in themselves prove; it has the
+alternation of day and night, and a succession of seasons closely resembling
+those of the earth; its surface is suggestively divided into regions of
+contrasting colors and appearance, and upon that surface we see an immense
+number of lines geometrically arranged, with a system of symmetrical
+intersections where the lines expand into circular and oval areas—and all
+connected with the annual melting of the polar snows in a way which
+irresistibly suggests the interference of intelligence directed to a definite
+end. Why, with so many concurrent circumstances to support the hypothesis,
+should we not regard Mars as an inhabited globe?
+</p>
+
+<p>
+But the differences between Mars and the earth are in many ways as striking as
+their resemblances. Mars is relatively small; it gets less than half as much
+light and heat as we receive; its atmosphere is so rare that it would be
+distressing to us, even if we could survive in it at all; it has no lakes,
+rivers, or seas; its surface is an endless prairie. and its “canals” are
+phenomena utterly unlike anything on the earth. Yet it is precisely upon these
+divergences between the earth and Mars, this repudiation of terrestrial
+standards, that the theory of “life on Mars,” for which Mr Lowell is mainly
+responsible, is based. Because Mars is smaller than the earth, we are told it
+must necessarily be more advanced in planetary evolution, the underlying cause
+of which is the gradual cooling and contraction of the planet’s mass. Mars has
+parted with its internal heat more rapidly than the earth; consequently its
+waters and its atmosphere have been mostly withdrawn by chemical combinations,
+but enough of both yet remain to render life still possible on its surface. As
+the globe of Mars is evolutionally older than that of the earth, so its forms
+of organic life may be proportionally further advanced, and its inhabitants may
+have attained a degree of cultivated intelligence much superior to what at
+present exists upon the earth. Understanding the nature and the causes of the
+desiccation of their planet, and possessing engineering science and
+capabilities far in advance of ours, they may be conceived to have grappled
+with the stupendous problem of keeping their world in a habitable condition as
+long as possible. Supposing them to have become accustomed to live in their
+rarefied atmosphere (a thing not inconceivable, since men can live for a time
+at least in air hardly less rare), the most pressing problem for them is that
+of a water-supply, without which plant life cannot exist, while animal life in
+turn depends for its existence upon vegetation. The only direction in which
+they can seek water is that of the polar regions, where it is alternately
+condensed into snow and released in the liquid form by the effect of the
+seasonal changes. It is, then, to the annual melting of the polar snow-fields
+that the Martian engineers are supposed to have recourse in supplying the needs
+of their planet, and thus providing the means of prolonging their own
+existence. It is imagined that they have for this purpose constructed a
+stupendous system of irrigation extending over the temperate and equatorial
+regions of the planet. The “canals” represent the lines of irrigation, but the
+narrow streaks that we see are not the canals themselves, but the irrigated
+bands covered by them. Their dark hue, and their gradual appearance after the
+polar melting has begun, are due to the growth of vegetation stimulated by the
+water. The rounded areas visible where several “canals” meet and cross are
+called by Mr Lowell “oases.” These are supposed to be the principal centers of
+population and industry. It must be confessed that some of them, with their
+complicated systems of radiating lines, appear to answer very well to such a
+theory. No attempt to explain them by analogy with natural phenomena on the
+earth has proved successful.
+</p>
+
+<p>
+But a great difficulty yet remains: How to explain the seemingly miraculous
+powers of the supposed engineers? Here recourse is had once more to the
+relative smallness of the planet. We have remarked that the force of gravity on
+Mars is only thirty-eight per cent of that on the earth. A steam-shovel driven
+by a certain horse-power would be nearly three times as effective there as
+here. A man of our stature on Mars would find his effective strength increased
+in the same proportion. But just because of the slight force of gravity there,
+a Martian might attain to the traditional stature of Goliath without finding
+his own weight an encumbrance to his activity, while at the same time his huge
+muscles would come into unimpeded play, enabling him single-handed to perform
+labors that would be impossible to a whole gang of terrestrial workmen. The
+effective powers of huge machines would be increased in the same way; and to
+all this must be added the fact that the mean density of the materials of which
+Mars is composed is much less than that of the constituents of the earth.
+Combining all these considerations, it becomes much less difficult to conceive
+that public works might be successfully undertaken on Mars which would be
+hopelessly beyond the limits of human accomplishment.
+</p>
+
+<p>
+Certain other difficulties have also to be met; as, for instance, the relative
+coldness of the climate of Mars. At its distance it gets considerably less than
+half as much light and heat as we receive. In addition to this, the rarity of
+its atmosphere would naturally be expected to decrease the effective
+temperature at the planet’s surface, since an atmosphere acts somewhat like the
+glass cover of a hot-house in retaining the solar heat which has penetrated it.
+It has been calculated that, unless there are mitigating circumstances of which
+we know nothing, the average temperature at the surface of Mars must be far
+below the freezing-point of water. To this it is replied that the possible
+mitigating circumstances spoken of evidently exist in fact, because we can
+<i>see</i> that the watery vapor condenses into snow around the poles in
+winter, but melts again when summer comes. The mitigating agent may be supposed
+to exist in the atmosphere where the presence of certain gases would completely
+alter the temperature gradients.
+</p>
+
+<p>
+It might also be objected that it is inconceivable that the Martian engineers,
+however great may be their physical powers, and however gigantic the mechanical
+energies under their control, could force water in large quantities from the
+poles to the equator. This is an achievement that measures up to the cosmical
+standard. It is admitted by the champions of the theory that the difficulty is
+a formidable one; but they call attention to the singular fact that on Mars
+there can be found no chains of mountains, and it is even doubtful if ranges of
+hills exist there. The entire surface of the planet appears to be almost “as
+smooth as a billiard ball,” and even the broad regions which were once supposed
+to be seas apparently lie at practically the same level as the other parts,
+since the “canals” in many cases run uninterruptedly across them. Lowell’s idea
+is that these sombre areas may be expanses of vegetation covering ground of a
+more or less marshy character, for while the largest of them appear to be
+permanent, there are some which vary coincidently with the variations of the
+canals.
+</p>
+
+<p>
+As to the kind of machinery employed to force the water from the poles, it has
+been conjectured that it may have taken the form of a gigantic system of pumps
+and conduits; and since the Martians are assumed to be so far in advance of us
+in their mastery of scientific principles, the hypothesis will at least not be
+harmed by supposing that they have learned to harness forces of nature whose
+very existence in a manageable form is yet unrecognized on the earth. If we
+wish to let the imagination loose, we may conjecture that they have conquered
+the secret of those intra-atomic forces whose resistless energy is beginning to
+become evident to us, but the possibility of whose utilization remains a dream,
+the fulfillment of which nobody dares to predict.
+</p>
+
+<p>
+Such, in very brief form, is the celebrated theory of Mars as an inhabited
+world. It certainly captivates the imagination, and if we believe it to
+represent the facts, we cannot but watch with the deepest sympathy this gallant
+struggle of an intellectual race to preserve its planet from the effects of
+advancing age and death. We may, indeed, wonder whether our own humanity,
+confronted by such a calamity, could be counted on to meet the emergency with
+equal stoutness of heart and inexhaustibleness of resource. Up to the present
+time we certainly have shown no capacity to confront Nature toe to toe, and to
+seize her by the shoulders and turn her round when she refuses to go our way.
+If we could get into wireless telephonic communication with the Martians we
+might learn from their own lips the secret of their more than “Roman recovery.”
+</p>
+
+</div><!--end chapter-->
+
+<div class="chapter">
+
+<h2><a name="chap14"></a>XIV<br/>
+The Riddle of the Asteroids</h2>
+
+<p>
+Between the orbits of Mars and Jupiter revolves the most remarkable system of
+little bodies with which we are acquainted—the Asteroids, or Minor Planets.
+Some six hundred are now known, and they may actually number thousands. They
+form virtually a ring about the sun. The most striking general fact about them
+is that they occupy the place in the sky which should be occupied, according to
+Bode’s Law, by a single large planet. This fact, as we shall see, has led to
+the invention of one of the most extraordinary theories in
+astronomy—<i>viz.,</i> that of the explosion of a world!
+</p>
+
+<p>
+Bode’s Law, so-called, is only an empiric formula, but until the discovery of
+Neptune it accorded so well with the distances of the planets that astronomers
+were disposed to look upon it as really representing some underlying principle
+of planetary distribution. They were puzzled by the absence of a planet in the
+space between Mars and Jupiter, where the “law” demanded that there should be
+one, and an association of astronomers was formed to search for it. There was a
+decided sensation when, in 1801, Piazzi, of Palermo, announced that he had
+found a little planet which apparently occupied the place in the system which
+belonged to the missing body. He named it Ceres, and it was the first of the
+Asteroids. The next year Olbers, of Bremen, while looking for Ceres with his
+telescope, stumbled upon another small planet which he named Pallas.
+Immediately he was inspired with the idea that these two planets were fragments
+of a larger one which had formerly occupied the vacant place in the planetary
+ranks, and he predicted that others would be found by searching in the
+neighborhood of the intersection of the orbits of the two already discovered.
+This bold prediction was brilliantly fulfilled by the finding of two more—Juno
+in 1804, and Vesta in 1807. Olbers would seem to have been led to the invention
+of his hypothesis of a planetary explosion by the faith which astronomers at
+that time had in Bode’s Law. They appear to have thought that several planets
+revolving in the gap where the “law” called for but one could only be accounted
+for upon the theory that the original <i>one</i> had been broken up to form the
+several. Gravitation demanded that the remnants of a planet blown to pieces, no
+matter how their orbits might otherwise differ, should all return at stated
+periods to the point where the explosion had occurred; hence Olbers’ prediction
+that any asteroids that might subsequently be discovered would be found to have
+a common point of orbital intersection. And curiously enough all of the first
+asteroids found practically answered to this requirement. Olbers’ theory seemed
+to be established.
+</p>
+
+<p>
+After the first four, no more asteroids were found until 1845, when one was
+discovered; then, in 1847, three more were added to the list; and after that
+searchers began to pick them up with such rapidity that by the close of the
+century hundreds were known, and it had become almost impossible to keep track
+of them. The first four are by far the largest members of the group, but their
+actual sizes remained unknown until less than twenty years ago. It was long
+supposed that Vesta was the largest, because it shines more brightly than any
+of the others; but finally, in 1895, Barnard, with the Lick telescope,
+definitely measured their diameters, and proved to everybody’s surprise that
+Ceres is really the chief, and Vesta only the third in rank. His measures are
+as follows: Ceres, 477 miles; Pallas, 304 miles; Vesta, 239 miles; and Juno,
+120 miles. They differ greatly in the reflective power of their surfaces, a
+fact of much significance in connection with the question of their origin.
+Vesta is, surface for surface, rather more than three times as brilliant as
+Ceres, whence the original mistake about its magnitude.
+</p>
+
+<p>
+Nowadays new asteroids are found frequently by photography, but physically they
+are most insignificant bodies, their average diameter probably not exceeding
+twenty miles, and some are believed not to exceed ten. On a planet only ten
+miles in diameter, assuming the same mean density as the earth’s, which is
+undoubtedly too much, the force of gravity would be so slight that an average
+man would not weigh more than three ounces, and could jump off into space
+whenever he liked.
+</p>
+
+<p>
+Although the asteroids all revolve around the sun in the same direction as that
+pursued by the major planets, their orbits are inclined at a great variety of
+angles to the general plane of the planetary system, and some of them are very
+eccentric—almost as much so as the orbits of many of the periodic comets. It
+has even been conjectured that the two tiny moons of Mars and the four smaller
+satellites of Jupiter may be asteroids gone astray and captured by those
+planets. Two of the asteroids are exceedingly remarkable for the shapes and
+positions of their orbits; these are Eros, discovered in 1898, and T. G., 1906,
+found eight years later. The latter has a mean distance from the sun slightly
+greater than that of Jupiter, while the mean distance of Eros is less than that
+of Mars. The orbit of Eros is so eccentric that at times it approaches within
+15,000,000 miles of the earth, nearer than any other regular member of the
+solar system except the moon, thus affording an unrivaled means of measuring
+the solar parallax. But for our present purpose the chief interest of Eros lies
+in its extraordinary changes of light.
+</p>
+
+<p>
+These changes, although irregular, have been observed and photographed many
+times, and there seems to be no doubt of their reality. Their significance
+consists in their possible connection with the form of the little planet, whose
+diameter is generally estimated at not more than twenty miles. Von Oppolzer
+found, in 1901, that Eros lost three-fourths of its brilliancy once in every
+two hours and thirty-eight minutes. Other observers have found slightly
+different periods of variability, but none as long as three hours. The most
+interesting interpretation that has been offered of this phenomenon is that it
+is due to a great irregularity of figure, recalling at once Olbers’ hypothesis.
+According to some, Eros may be double, the two bodies composing it revolving
+around each other at very close quarters; but a more striking, and it may be
+said probable, suggestion is that Eros has a form not unlike that of a
+dumb-bell, or hour-glass, turning rapidly end over end so that the area of
+illuminated surface presented to our eyes continually changes, reaching at
+certain times a minimum when the amount of light that it reflects toward the
+earth is reduced to a quarter of its maximum value. Various other bizarre
+shapes have been ascribed to Eros, such, for instance, as that of a flat stone
+revolving about one of its longer axes, so that sometimes we see its face and
+sometimes its edge.
+</p>
+
+<p>
+All of these explanations proceed upon the assumption that Eros cannot have a
+simple globular figure like that of a typical planet, a figure which is
+prescribed by the law of gravitation, but that its shape is what may be called
+accidental; in a word, it is a <i>fragment,</i> for it seems impossible to
+believe that a body formed in interplanetary space, either through nebular
+condensation or through the aggregation of particles drawn together by their
+mutual attractions, should not be practically spherical in shape. Nor is Eros
+the only asteroid that gives evidence by variations of brilliancy that there is
+something abnormal in its constitution; several others present the same
+phenomenon in varying degrees. Even Vesta was regarded by Olbers as
+sufficiently variable in its light to warrant the conclusion that it was an
+angular mass instead of a globe. Some of the smaller ones show very notable
+variations, and all in short periods, of three or four hours, suggesting that
+in turning about one of their axes they present a surface of variable extent
+toward the sun and the earth.
+</p>
+
+<p>
+The theory which some have preferred—that the variability of light is due to
+the differences of reflective power on different parts of the surface—would, if
+accepted, be hardly less suggestive of the origin of these little bodies by the
+breaking up of a larger one, because the most natural explanation of such
+differences would seem to be that they arose from variations in the roughness
+or smoothness of the reflecting surface, which would be characteristic of
+fragmentary bodies. In the case of a large planet alternating expanses of land
+and water, or of vegetation and desert, would produce a notable variation in
+the amount of reflection, but on bodies of the size of the asteroids neither
+water nor vegetation could exist, and an atmosphere would be equally
+impossible.
+</p>
+
+<p>
+One of the strongest objections to Olbers’ hypothesis is that only a few of the
+first asteroids discovered travel in orbits which measurably satisfy the
+requirement that they should all intersect at the point where the explosion
+occurred. To this it was at first replied that the perturbations of the
+asteroidal orbits, by the attractions of the major planets, would soon displace
+them in such a manner that they would cease to intersect. One of the first
+investigations undertaken by the late Prof. Simon Newcomb was directed to the
+solution of this question, and he arrived at the conclusion that the planetary
+perturbations could not explain the actual situation of the asteroidal orbits.
+But afterward it was pointed out that the difficulty could be avoided by
+supposing that not one but a series of explosions had produced the asteroids as
+they now are. After the primary disruption the fragments themselves, according
+to this suggestion, may have exploded, and then the resulting orbits would be
+as “tangled” as the heart could wish. This has so far rehabilitated the
+explosion theory that it has never been entirely abandoned, and the evidence
+which we have just cited of the probably abnormal shapes of Eros and other
+asteroids has lately given it renewed life. It is a subject that needs a
+thorough rediscussion.
+</p>
+
+<p>
+We must not fail to mention, however, that there is a rival hypothesis which
+commends itself to many astronomers—<i>viz.,</i> that the asteroids were formed
+out of a relatively scant ring of matter, situated between Mars and Jupiter and
+resembling in composition the immensely more massive rings from which,
+according to Laplace’s hypothesis, the planets were born. It is held by the
+supporters of this theory that the attraction of the giant Jupiter was
+sufficient to prevent the small, nebulous ring that gave birth to the asteroids
+from condensing like the others into a single planet.
+</p>
+
+<p>
+But if we accept the explosion theory, with its corollary that minor explosions
+followed the principal one, we have still an unanswered question before us:
+What caused the explosions? The idea of <i>a world blowing up</i> is too
+Titanic to be shocking; it rather amuses the imagination than seriously
+impresses it; in a word, it seems essentially chimerical. We can by no appeal
+to experience form a mental picture of such an occurrence. Even the moon did
+not blow up when it was wrecked by volcanoes. The explosive nebulæ and new
+stars are far away in space, and suggest no connection with such a catastrophe
+as the bursting of a planet into hundreds of pieces. We cannot conceive of a
+great globe thousands of miles in diameter resembling a pellet of gunpowder
+only awaiting the touch of a match to cause its sudden disruption. Somehow the
+thought of human agency obtrudes itself in connection with the word
+“explosion,” and we smile at the idea that giant powder or nitro-glycerine
+could blow up a planet. Yet it would only need <i>enough</i> of them to do it.
+</p>
+
+<p>
+After all, we may deceive ourselves in thinking, as we are apt to do, that
+explosive energies lock themselves up only in small masses of matter. There are
+many causes producing explosions in nature, every volcanic eruption manifests
+the activity of some of them. Think of the giant power of confined steam; if
+enough steam could be suddenly generated in the center of the earth by a
+downpour of all the waters of the oceans, what might not the consequences be
+for our globe? In a smaller globe, and it has never been estimated that the
+original asteroid was even as large as the moon, such a catastrophe would,
+perhaps, be more easily conceivable; but since we are compelled in this case to
+assume that there was a series of successive explosions, steam would hardly
+answer the purpose; it would be more reasonable to suppose that the cause of
+the explosion was some kind of chemical reaction, or something affecting the
+atoms composing the exploding body. Here Dr Gustav Le Bon comes to our aid with
+a most startling suggestion, based on his theory of the dissipation of
+intra-atomic energy. It will be best to quote him at some length from his book
+on <i>The Evolution of Forces.</i>
+</p>
+
+<p>
+“It does not seem at first sight,” says Doctor Le Bon,
+</p>
+
+<div class="letter">
+<p>
+very comprehensible that worlds which appear more and more stable as they cool
+could become so unstable as to afterward dissociate entirely. To explain this
+phenomenon, we will inquire whether astronomical observations do not allow us
+to witness this dissociation.
+</p>
+
+<p>
+We know that the stability of a body in motion, such as a top or a bicycle,
+ceases to be possible when its velocity of rotation descends below a certain
+limit. Once this limit is reached it loses its stability and falls to the
+ground. Prof. J. J. Thomson even interprets radio-activity in this manner, and
+points out that when the speed of the elements composing the atoms descends
+below a certain limit they become unstable and tend to lose their equilibria.
+There would result from this a commencement of dissociation, with diminution of
+their potential energy and a corresponding increase of their kinetic energy
+sufficient to launch into space the products of intra-atomic disintegration.
+</p>
+
+<p>
+It must not be forgotten that the atom being an enormous reservoir of energy is
+by this very fact comparable with explosive bodies. These last remain inert so
+long as their internal equilibria are undisturbed. So soon as some cause or
+other modifies these, they explode and smash everything around them after being
+themselves broken to pieces.
+</p>
+
+<p>
+Atoms, therefore, which grow old in consequence of the diminution of a part of
+their intra-atomic energy gradually lose their stability. A moment, then,
+arrives when this stability is so weak that the matter disappears by a sort of
+explosion more or less rapid. The bodies of the radium group offer an image of
+this phenomenon—a rather faint image, however, because the atoms of this body
+have only reached a period of instability when the dissociation is rather slow.
+It probably precedes another and more rapid period of dissociation capable of
+producing their final explosion. Bodies such as radium, thorium, etc.,
+represent, no doubt, a state of old age at which all bodies must some day
+arrive, and which they already begin to manifest in our universe, since all
+matter is slightly radio-active. It would suffice for the dissociation to be
+fairly general and fairly rapid for an explosion to occur in a world where it
+was manifested.
+</p>
+
+<p>
+These theoretical considerations find a solid support in the sudden appearances
+and disappearances of stars. The explosions of a world which produce them
+reveal to us, perhaps, how the universes perish when they become old.
+</p>
+
+<p>
+As astronomical observations show the relative frequency of these rapid
+destructions, we may ask ourselves whether the end of a universe by a sudden
+explosion after a long period of old age does not represent its most general
+ending.
+</p>
+</div>
+
+<p>
+Here, perhaps, it will be well to stop, since, entrancing as the subject may
+be, we know very little about it, and Doctor Le Bon’s theory affords a
+limitless field for the reader’s imagination.
+</p>
+
+</div><!--end chapter-->
+
+<div style='display:block; margin-top:4em'>*** END OF THE PROJECT GUTENBERG EBOOK CURIOSITIES OF THE SKY ***</div>
+<div style='text-align:left'>
+
+<div style='display:block; margin:1em 0'>
+Updated editions will replace the previous one&#8212;the old editions will
+be renamed.
+</div>
+
+<div style='display:block; margin:1em 0'>
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+Project Gutenberg (https://www.gutenberg.org) public repository for
+eBook #6630 (https://www.gutenberg.org/ebooks/6630)
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+The Project Gutenberg EBook of Curiosities of the Sky, by Garrett 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: Curiosities of the Sky
+
+Author: Garrett Serviss
+
+Posting Date: August 25, 2012 [EBook #6630]
+Release Date: October, 2004
+First Posted: January 6, 2003
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK CURIOSITIES OF THE SKY ***
+
+
+
+
+Produced by William McClain
+
+
+
+
+
+
+
+
+
+
+                         Curiosities of the Sky
+
+by Garrett Serviss
+
+Curiosities of the Sky was first published in 1909 and the text is in
+the public domain. The transcription was done by William McClain
+(info@sattre-press.com), 2002.
+
+A printed version of this book is available from Sattre Press
+(http://csky.sattre-press.com). It includes extensive annotations, a
+new introduction and all the original photographs and diagrams.
+_________________________________________________________________
+
+Preface
+
+What Froude says of history is true also of astronomy: it is the most
+impressive where it transcends explanation. It is not the mathematics
+of astronomy, but the wonder and the mystery that seize upon the
+imagination. The calculation of an eclipse owes all its prestige to
+the sublimity of its data; the operation, in itself, requires no more
+mental effort than the preparation of a railway time-table.
+
+The dominion which astronomy has always held over the minds of men is
+akin to that of poetry; when the former becomes merely instructive and
+the latter purely didactic, both lose their power over the
+imagination. Astronomy is known as the oldest of the sciences, and it
+will be the longest-lived because it will always have arcana that have
+not been penetrated.
+
+Some of the things described in this book are little known to the
+average reader, while others are well known; but all possess the
+fascination of whatever is strange, marvelous, obscure, or mysterious
+-- magnified, in this case, by the portentous scale of the phenomena.
+
+The idea of the author is to tell about these things in plain
+language, but with as much scientific accuracy as plain language will
+permit, showing the wonder that is in them without getting away from
+the facts. Most of them have hitherto been discussed only in technical
+form, and in treatises that the general public seldom sees and never
+reads.
+
+Among the topics touched upon are:
+  * The strange unfixedness of the ``fixed stars,'' the vast
+    migrations of the suns and worlds constituting the universe.
+  * The slow passing out of existence of those collocations of stars
+    which for thousands of years have formed famous
+    ``constellations,'' preserving the memory of mythological heroes
+    and heroines, and perhaps of otherwise unrecorded history.
+  * The tendency of stars to assemble in immense clouds, swarms, and
+    clusters.
+  * The existence in some of the richest regions of the universe of
+    absolutely black, starless gaps, deeps, or holes, as if one were
+    looking out of a window into the murkiest night.
+  * The marvelous phenomena of new, or temporary, stars, which appear
+    as suddenly as conflagrations, and often turn into something else
+    as eccentric as themselves.
+  * The amazing forms of the ``whirlpool,'' ``spiral,'' ``pinwheel,''
+    and ``lace,'' or ``tress,'' nebul�.
+  * The strange surroundings of the sun, only seen in particular
+    circumstances, but evidently playing a constant part in the daily
+    phenomena of the solar system.
+  * The mystery of the Zodiacal Light and the Gegenschein.
+  * The extraordinary transformations undergone by comets and their
+    tails.
+  * The prodigies of meteorites and masses of stone and metal fallen
+    from the sky.
+  * The cataclysms that have wrecked the moon.
+  * The problem of life and intelligence on the planet Mars.
+  * The problematical origin and fate of the asteroids.
+  * The strange phenomena of the auroral lights.
+
+An attempt has been made to develop these topics in an orderly way,
+showing their connection, so that the reader may obtain a broad
+general view of the chief mysteries and problems of astronomy, and an
+idea of the immense field of discovery which still lies, almost
+unexplored, before it.
+
+The Windows of Absolute Night
+
+To most minds mystery is more fascinating than science. But when
+science itself leads straight up to the borders of mystery and there
+comes to a dead stop, saying, ``At present I can no longer see my
+way,'' the force of the charm is redoubled. On the other hand, the
+illimitable is no less potent in mystery than the invisible, whence
+the dramatic effect of Keats' ``stout Cortez'' staring at the
+boundless Pacific while all his men look at each other with a wild
+surmise, ``silent upon a peak in Darien.'' It is with similar feelings
+that the astronomer regards certain places where from the peaks of the
+universe his vision seems to range out into endless empty space. He
+sees there the shore of his little isthmus, and, beyond, unexplored
+immensity.
+
+The name, ``coal-sacks,'' given to these strange voids is hardly
+descriptive. Rather they produce upon the mind the effect of blank
+windows in a lonely house on a pitch-dark night, which, when looked at
+from the brilliant interior, become appalling in their rayless murk.
+Infinity seems to acquire a new meaning in the presence of these black
+openings in the sky, for as one continues to gaze it loses its purely
+metaphysical quality and becomes a kind of entity, like the ocean. The
+observer is conscious that he can actually see the beginning of its
+ebon depths, in which the visible universe appears to float like an
+enchanted island, resplendent within with lights and life and gorgeous
+spectacles, and encircled with screens of crowded stars, but with its
+dazzling vistas ending at the fathomless sea of pure darkness which
+encloses all.
+
+The Galaxy, or Milky Way, surrounds the borders of our island in space
+like a stellar garland, and when openings appear in it they are, by
+contrast, far more impressive than the general darkness of the
+interstellar expanse seen in other directions. Yet even that expanse
+is not everywhere equally dark, for it contains gloomy deeps
+discernable with careful watching. Here, too, contrast plays an
+important part, though less striking than within the galactic region.
+Some of Sir William Herschel's observations appear to indicate an
+association between these tenebrious spots and neighboring star clouds
+and nebul�. It is an illuminating bit of astronomical history that
+when he was sweeping the then virgin heavens with his great telescopes
+he was accustomed to say to his sister who, note-book in hand, waited
+at his side to take down his words, fresh with the inspiration of
+discovery: ``Prepare to write; the nebul� are coming; here space is
+vacant.''
+
+The most famous of the ``coal-sacks,'' and the first to be brought to
+general attention before astronomers had awakened to the significance
+of such things, lies adjacent to the ``Southern Cross,'' and is truly
+an amazing phenomenon. It is not alone the conspicuousness of this
+celestial vacancy, opening suddenly in the midst of one of the richest
+parts of the Galaxy, that has given it its fame, but quite as much the
+superstitious awe with which it was regarded by the early explorers of
+the South Seas. To them, as well as to those who listened in rapt
+wonder to their tales, the ``Coal-sack'' seemed to possess some occult
+connection with the mystic ``Cross.'' In the eyes of the sailors it
+was not a vacancy so much as a sable reality in the sky, and as,
+shuddering, they stared at it, they piously crossed themselves. It was
+another of the magical wonders of the unknown South, and as such it
+formed the basis of many a ``wild surmise'' and many a sea-dog's yarn.
+Scientific investigation has not diminished its prestige, and today no
+traveler in the southern hemisphere is indifferent to its fascinating
+strangeness, while some find it the most impressive spectacle of the
+antarctic heavens.
+
+All around, up to the very edge of the yawning gap, the sheen of the
+Milky Way is surpassingly glorious; but there, as if in obedience to
+an almighty edict, everything vanishes. A single faint star is visible
+within the opening, producing a curious effect upon the sensitive
+spectator, like the sight of a tiny islet in the midst of a black,
+motionless, waveless tarn. The dimensions of the lagoon of darkness,
+which is oval or pear-shaped, are eight degrees by five, so that it
+occupies a space in the sky about one hundred and thirty times greater
+than the area of the full moon. It attracts attention as soon as the
+eye is directed toward the quarter where it exists, and by virtue of
+the rarity of such phenomena it appears a far greater wonder than the
+drifts of stars that are heaped around it. Now that observatories are
+multiplying in the southern hemisphere, the great austral
+``Coal-sack'' will, no doubt, receive attention proportioned to its
+importance as one of the most significant features of the sky. Already
+at the Sydney Observatory photographs have shown that the southern
+portion of this Dead Sea of Space is not quite ``bottomless,''
+although its northern part defies the longest sounding lines of the
+astronomer.
+
+There is a similar, but less perfect, ``coal-sack'' in the northern
+hemisphere, in the constellation of ``The Swan,'' which, strange to
+say, also contains a well-marked figure of a cross outlined by stars.
+This gap lies near the top of the cross-shaped figure. It is best seen
+by averted vision, which brings out the contrast with the Milky Way,
+which is quite brilliant around it. It does not, however, exercise the
+same weird attraction upon the eye as the southern ``Coal-sack,'' for
+instead of looking like an absolute void in the sky, it rather appears
+as if a canopy of dark gauze had been drawn over the stars. We shall
+see the possible significance of this appearance later.
+
+Just above the southern horizon of our northern middle latitudes, in
+summer, where the Milky Way breaks up into vast sheets of nebulous
+luminosity, lying over and between the constellations Scorpio and
+Sagittarius, there is a remarkable assemblage of ``coal-sacks,''
+though none is of great size. One of them, near a conspicuous
+star-cluster in Scorpio, M80, is interesting for having been the first
+of these strange objects noted by Herschel. Probably it was its
+nearness to M80 which suggested to his mind the apparent connection of
+such vacancies with star-clusters which we have already mentioned.
+
+But the most marvelous of the ``coal-sacks'' are those that have been
+found by photography in Sagittarius. One of Barnard's earliest and
+most excellent photographs includes two of them, both in the
+star-cluster M8. The larger, which is roughly rectangular in outline,
+contains one little star, and its smaller neighbor is lune-shaped --
+surely a most singular form for such an object. Both are associated
+with curious dark lanes running through the clustered stars like
+trails in the woods. Along the borders of these lanes the stars are
+ranked in parallel rows, and what may be called the bottoms of the
+lanes are not entirely dark, but pebbled with faint stellar points.
+One of them which skirts the two dark gaps and traverses the cluster
+along its greatest diameter is edged with lines of stars, recalling
+the alignment of the trees bordering a French highway. This road of
+stars cannot be less than many billions of miles in length!
+
+All about the cluster the bed of the Galaxy is strangely disturbed,
+and in places nearly denuded, as if its contents had been raked away
+to form the immense stack and the smaller accumulations of stars
+around it. The well-known ``Trifid Nebula'' is also included in the
+field of the photograph, which covers a truly marvelous region, so
+intricate in its mingling of nebul�, star-clusters, star-swarms,
+star-streams, and dark vacancies that no description can do it
+justice. Yet, chaotic as it appears, there is an unmistakable
+suggestion of unity about it, impressing the beholder with the idea
+that all the different parts are in some way connected, and have not
+been fortuitously thrown together. Miss Agnes M. Clerke made the
+striking remark that the dusky lanes in M8 are exemplified on the
+largest scale in the great rift dividing the Milky Way, from Cygnus in
+the northern hemisphere all the way to the ``Cross'' in the southern.
+Similar lanes are found in many other clusters, and they are generally
+associated with flanking rows of stars, resembling in their
+arrangement the thick-set houses and villas along the roadways that
+traverse the approaches to a great city.
+
+But to return to the black gaps. Are they really windows in the
+star-walls of the universe? Some of them look rather as if they had
+been made by a shell fired through a luminous target, allowing the eye
+to range through the hole into the void space beyond. If science is
+discretely silent about these things, what can the more venturesome
+and less responsible imagination suggest? Would a huge ``runaway
+sun,'' like Arcturus, for instance, make such an opening if it should
+pass like a projectile through the Milky Way? It is at least a
+stimulating inquiry. Being probably many thousands of times more
+massive than the galactic stars, such a stellar missile would not be
+stopped by them, though its direction of flight might be altered. It
+would drag the small stars lying close to its course out of their
+spheres, but the ultimate tendency of its attraction would be to sweep
+them round in its wake, thus producing rather a star-swarm than a
+vacancy. Those that were very close to it might be swept away in its
+rush and become its satellites, careering away with it in its flight
+into outer space; but those that were farther off, and they would, of
+course, greatly outnumber the nearer ones, would tend inward from all
+sides toward the line of flight, as dust and leaves collect behind a
+speeding motor (though the forces operating would be different), and
+would fill up the hole, if hole it were. A swarm thus collected should
+be rounded in outline and bordered with a relatively barren ring from
+which the stars had been ``sucked'' away. In a general sense the M8
+cluster answers to this description, but even if we undertook to
+account for its existence by a supposition like the above, the black
+gaps would remain unexplained, unless one could make a further draft
+on the imagination and suggest that the stars had been thrown into a
+vast eddy, or system of eddies, whose vortices appear as dark holes.
+Only a maelstrom-like motion could keep such a funnel open, for
+without regard to the impulse derived from the projectile, the proper
+motions of the stars themselves would tend to fill it. Perhaps some
+other cause of the whirling motion may be found. As we shall see when
+we come to the spiral nebul�, gyratory movements are exceedingly
+prevalent throughout the universe, and the structure of the Milky Way
+is everywhere suggestive of them. But this is hazardous sport even for
+the imagination -- to play with suns as if they were but thistle-down
+in the wind or corks in a mill-race.
+
+Another question arises: What is the thickness of the hedge of stars
+through which the holes penetrate? Is the depth of the openings
+proportionate to their width? In other words, is the Milky Way round
+in section like a rope, or flat and thin like a ribbon? The answer is
+not obvious, for we have little or no information concerning the
+relative distances of the faint galactic stars. It would be easier,
+certainly, to conceive of openings in a thin belt than in a massive
+ring, for in the first case they would resemble mere rifts and breaks,
+while in the second they would be like wells or bore-holes. Then, too,
+the fact that the Milky Way is not a continuous body but is made up of
+stars whose actual distances apart is great, offers another quandary;
+persistent and sharply bordered apertures in such an assemblage are a
+priori as improbable, if not impossible, as straight, narrow holes
+running through a swarm of bees.
+
+The difficulty of these questions indicates one of the reasons why it
+has been suggested that the seeming gaps, or many of them, are not
+openings at all, but opaque screens cutting off the light from stars
+behind them. That this is quite possible in some cases is shown by
+Barnard's later photographs, particularly those of the singular region
+around the star Rho Ophiuchi. Here are to be seen somber lanes and
+patches, apparently forming a connected system which covers an immense
+space, and which their discoverer thinks may constitute a ``dark
+nebula.'' This seems at first a startling suggestion; but, after all,
+why should their not be dark nebul� as well as visible ones? In truth,
+it has troubled some astronomers to explain the luminosity of the
+bright nebul�, since it is not to be supposed that matter in so
+diffuse a state can be incandescent through heat, and phosphorescent
+light is in itself a mystery. The supposition is also in accord with
+what we know of the existence of dark solid bodies in space. Many
+bright stars are accompanied by obscure companions, sometimes as
+massive as themselves; the planets are non-luminous; the same is true
+of meteors before they plunge into the atmosphere and become heated by
+friction; and many plausible reasons have been found for believing
+that space contains as many obscure as shining bodies of great size.
+It is not so difficult, after all, then, to believe that there are
+immense collections of shadowy gases and meteoric dust whose presence
+is only manifested when they intercept the light coming from shining
+bodies behind them.
+
+This would account for the apparent extinguishment of light in open
+space, which is indicated by the falling off in relative number of
+telescopic stars below the tenth magnitude. Even as things are, the
+amount of light coming to us from stars too faint to be seen with the
+naked eye is so great that the statement of it generally surprises
+persons who are unfamiliar with the inner facts of astronomy. It has
+been calculated that on a clear night the total starlight from the
+entire celestial sphere amounts to one-sixtieth of the light of the
+full moon; but of this less than one-twenty-fifth is due to stars
+separately distinguished by the eye. If there were no obscuring medium
+in space, it is probable that the amount of starlight would be
+noticeably and perhaps enormously increased.
+
+But while it seems certain that some of the obscure spots in the Milky
+Way are due to the presence of ``dark nebul�,'' or concealing veils of
+one kind or another, it is equally certain that there are many which
+are true apertures, however they may have been formed, and by whatever
+forces they may be maintained. These, then, are veritable windows of
+the Galaxy, and when looking out of them one is face to face with the
+great mystery of infinite space. There the known universe visibly
+ends, but manifestly space itself does not end there. It is not within
+the power of thought to conceive an end to space, for the instant we
+think of a terminal point or line the mind leaps forward to the
+beyond. There must be space outside as well as inside. Eternity of
+time and infinity of space are ideas that the intellect cannot fully
+grasp, but neither can it grasp the idea of a limitation to either
+space or time. The metaphysical conceptions of hypergeometry, or
+fourth-dimensional space, do not aid us.
+
+Having, then, discovered that the universe is a thing contained in
+something indefinitely greater than itself; having looked out of its
+windows and found only the gloom of starless night outside -- what
+conclusions are we to draw concerning the beyond? It seems as empty as
+a vacuum, but is it really so? If it be, then our universe is a single
+atom astray in the infinite; it is the only island in an ocean without
+shores; it is the one oasis in an illimitable desert. Then the Milky
+Way, with its wide-flung garland of stars, is afloat like a tiny
+smoke-wreath amid a horror of immeasurable vacancy, or it is an
+evanescent and solitary ring of sparkling froth cast up for a moment
+on the viewless billows of immensity. From such conclusions the mind
+instinctively shrinks. It prefers to think that there is something
+beyond, though we cannot see it. Even the universe could not bear to
+be alone -- a Crusoe lost in the Cosmos! As the inhabitants of the
+most elegant ch�teau, with its gardens, parks, and crowds of
+attendants, would die of loneliness if they did not know that they
+have neighbors, though not seen, and that a living world of indefinite
+extent surrounds them, so we, when we perceive that the universe has
+limits, wish to feel that it is not solitary; that beyond the hedges
+and the hills there are other centers of life and activity. Could
+anything be more terrible than the thought of an isolated universe?
+The greater the being, the greater the aversion to seclusion. Only the
+infinite satisfies; in that alone the mind finds rest.
+
+We are driven, then, to believe that the universal night which
+envelopes us is not tenantless; that as we stare out of the
+star-framed windows of the Galaxy and see nothing but uniform
+blackness, the fault is with our eyes or is due to an obscuring
+medium. Since our universe is limited in extent, there must be other
+universes beyond it on all sides. Perhaps if we could carry our
+telescopes to the verge of the great ``Coal-sack'' near the ``Cross,''
+being then on the frontier of our starry system, we could discern,
+sparkling afar off in the vast night, some of the outer galaxies. They
+may be grander than ours, just as many of the suns surrounding us are
+immensely greater than ours. If we could take our stand somewhere in
+the midst of immensity and, with vision of infinite reach, look about
+us, we should perhaps see a countless number of stellar systems, amid
+which ours would be unnoticeable, like a single star among the
+multitude glittering in the terrestial sky on a clear night. Some
+might be in the form of a wreath, like our own; some might be
+globular, like the great star-clusters in Hercules and Centaurus; some
+might be glittering circles, or disks, or rings within rings. If we
+could enter them we should probably find a vast variety of
+composition, including elements unknown to terrestrial chemistry; for
+while the visible universe appears to contain few if any substances
+not existing on the earth or in the sun, we have no warrant to assume
+that others may not exist in infinite space.
+
+And how as to gravitation? We do not know that gravitation acts beyond
+the visible universe, but it is reasonable to suppose that it does. At
+any rate, if we let go its sustaining hand we are lost, and can only
+wander hopelessly in our speculations, like children astray. If the
+empire of gravitation is infinite, then the various outer systems must
+have some, though measuring by our standards an imperceptible,
+attractive influence upon each other, for gravitation never lets go
+its hold, however great the space over which it is required to act.
+Just as the stars about us are all in motion, so the starry systems
+beyond our sight may be in motion, and our system as a whole may be
+moving in concert with them. If this be so, then after interminable
+ages the aspect of the entire system of systems must change, its
+various members assuming new positions with respect to one another. In
+the course of time we may even suppose that our universe will approach
+relatively close to one of the others; and then, if men are yet living
+on the earth, they may glimpse through the openings which reveal
+nothing to us now, the lights of another nearing star system, like the
+signals of a strange squadron, bringing them the assurance (which can
+be but an inference at present) that the ocean of space has other
+argosies venturing on its limitless expanse.
+
+There remains the question of the luminiferous ether by whose agency
+the waves of light are borne through space. The ether is as mysterious
+as gravitation. With regard to ether we only infer its existence from
+the effects which we ascribe to it. Evidently the ether must extend as
+far as the most distant visible stars. But does it continue on
+indefinitely in outer space? If it does, then the invisibility of the
+other systems must be due to their distance diminishing the quantity
+of light that comes from them below the limit of perceptibility, or to
+the interposition of absorbing media; if it does not, then the reason
+why we cannot see them is owing to the absence of a means of
+conveyance for the light waves, as the lack of an interplanetary
+atmosphere prevents us from hearing the thunder of sun-spots. (It is
+interesting to recall that Mr Edison was once credited with the
+intention to construct a gigantic microphone which should render the
+roar of sun-spots audible by transforming the electric vibrations into
+sound-waves). On this supposition each starry system would be
+enveloped in its own globule of ether, and no light could cross from
+one to another. But the probability is that both the ether and
+gravitation are ubiquitous, and that all the stellar systems are
+immersed in the former like clouds of phosphorescent organisms in the
+sea.
+
+So astronomy carries the mind from height to greater height. Men were
+long in accepting the proofs of the relative insignificance of the
+earth; they were more quickly convinced of the comparative littleness
+of the solar system; and now the evidence assails their reason that
+what they had regarded as the universe is only one mote gleaming in
+the sunbeams of Infinity.
+
+Star-Clouds, Star-Clusters, and Star-Streams
+
+In the preceding chapter we have seen something of the strangely
+complicated structure of the Galaxy, or Milky Way. We now proceed to
+study more comprehensively that garlanded ``Pathway of the Gods.''
+
+Judged by the eye alone, the Milky Way is one of the most delicately
+beautiful phenomena in the entire realm of nature -- a shimmer of
+silvery gauze stretched across the sky; but studied in the light of
+its revelations, it is the most stupendous object presented to human
+ken. Let us consider, first, its appearance to ordinary vision. Its
+apparent position in the sky shifts according to the season. On a
+serene, cloudless summer evening, in the absence of the moon, whose
+light obscures it, one sees the Galaxy spanning the heavens from north
+to southeast of the zenith like a phosphorescent arch. In early spring
+it forms a similar but, upon the whole, less brilliant arch west of
+the zenith. Between spring and summer it lies like a long, faint,
+twilight band along the northern horizon. At the beginning of winter
+it again forms an arch, this time spanning the sky from east to west,
+a little north of the zenith. These are its positions as viewed from
+the mean latitude of the United States. Even the beginner in
+star-gazing does not have to watch it throughout the year in order to
+be convinced that it is, in reality, a great circle, extending
+entirely around the celestial sphere. We appear to be situated near
+its center, but its periphery is evidently far away in the depths of
+space.
+
+Although to the casual observer it seems but a delicate scarf of
+light, brighter in some places than in others, but hazy and indefinite
+at the best, such is not its appearance to those who study it with
+care. They perceive that it is an organic whole, though marvelously
+complex in detail. The telescope shows that it consists of stars too
+faint and small through excess of distance to be separately visible.
+Of the hundred million suns which some estimates have fixed as the
+probable population of the starry universe, the vast majority (at
+least thirty to one) are included in this strange belt of misty light.
+But they are not uniformly distributed in it; on the contrary, they
+are arrayed in clusters, knots, bunches, clouds, and streams. The
+appearance is somewhat as if the Galaxy consisted of innumerable
+swarms of silver-winged bees, more or less intermixed, some massed
+together, some crossing the paths of others, but all governed by a
+single purpose which leads them to encircle the region of space in
+which we are situated.
+
+From the beginning of the systematic study of the heavens, the fact
+has been recognized that the form of the Milky Way denotes the scheme
+of the sidereal system. At first it was thought that the shape of the
+system was that of a vast round disk, flat like a cheese, and filled
+with stars, our sun and his relatively few neighbors being placed near
+the center. According to this view, the galactic belt was an effect of
+perspective; for when looking in the direction of the plane of the
+disk, the eye ranged through an immense extension of stars which
+blended into a glimmering blur, surrounding us like a ring; while when
+looking out from the sides of the disk we saw but few stars, and in
+those directions the heavens appeared relatively blank. Finally it was
+recognized that this theory did not correspond with the observed
+appearances, and it became evident that the Milky Way was not a mere
+effect of perspective, but an actual band of enormously distant stars,
+forming a circle about the sphere, the central opening of the ring
+(containing many scattered stars) being many times broader than the
+width of the ring itself. Our sun is one of the scattered stars in the
+central opening.
+
+As already remarked, the ring of the Galaxy is very irregular, and in
+places it is partly broken. With its sinuous outline, its pendant
+sprays, its graceful and accordant curves, its bunching of masses, its
+occasional interstices, and the manifest order of a general plan
+governing the jumble of its details, it bears a remarkable resemblance
+to a garland -- a fact which appears the more wonderful when we recall
+its composition. That an elm-tree should trace the lines of beauty
+with its leafy and pendulous branches does not surprise us; but we can
+only gaze with growing amazement when we behold a hundred million suns
+imitating the form of a chaplet! And then we have to remember that
+this form furnishes the ground-plan of the universe.
+
+As an indication of the extraordinary speculations to which the
+mystery of the Milky Way has given rise, a theory recently (1909)
+proposed by Prof. George C. Comstock may be mentioned. Starting with
+the data (first) that the number of stars increases as the Milky Way
+is approached, and reaches a maximum in its plane, while on the other
+hand the number of nebul� is greatest outside the Milky Way and
+increases with distance from it, and (second) that the Milky Way,
+although a complete ring, is broad and diffuse on one side through
+one-half its course -- that half alone containing nebul� -- and
+relatively narrow and well defined on the opposite side, the author of
+this singular speculation avers that these facts can best be explained
+by supposing that the invisible universe consists of two
+interpenetrating parts, one of which is a chaos of indefinite extent,
+strewn with stars and nebulous dust, and the other a long, broad but
+comparatively thin cluster of stars, including the sun as one of its
+central members. This flat star-cluster is conceived to be moving
+edgewise through the chaos, and, according to Professor Comstock, it
+acts after the manner of a snow-plough sweeping away the cosmic dust
+and piling it on either hand above and below the plane of the moving
+cluster. It thus forms a transparent rift, through which we see
+farther and command a view of more stars than through the intensified
+dust-clouds on either hand. This rift is the Milky Way. The dust
+thrown aside toward the poles of the Milky Way is the substance of the
+nebul� which abound there. Ahead, where the front of the star-plough
+is clearing the way, the chaos is nearer at hand, and consequently
+there the rift subtends a broader angle, and is filled with primordial
+dust, which, having been annexed by the vanguard of the star-swarm,
+forms the nebul� seen only in that part of the Milky Way. But behind,
+the rift appears narrow because there we look farther away between
+dust-clouds produced ages ago by the front of the plough, and no
+scattered dust remains in that part of the rift.
+
+In quoting an outline of this strikingly original theory the present
+writer should not be understood as assenting to it. That it appears
+bizarre is not, in itself, a reason for rejecting it, when we are
+dealing with so problematical and enigmatical a subject as the Milky
+Way; but the serious objection is that the theory does not
+sufficiently accord with the observed phenomena. There is too much
+evidence that the Milky Way is an organic system, however fantastic
+its form, to permit the belief that it can only be a rift in chaotic
+clouds. As with every organism, we find that its parts are more or
+less clearly repeated in its ensemble. Among all the strange things
+that the Milky Way contains there is nothing so extraordinary as
+itself. Every astronomer must many times have found himself marveling
+at it in those comparatively rare nights when it shows all its beauty
+and all its strangeness. In its great broken rifts, divisions, and
+spirals are found the gigantic prototypes of similar forms in its
+star-clouds and clusters. As we have said, it determines the general
+shape of the whole sidereal system. Some of the brightest stars in the
+sky appear to hang like jewels suspended at the ends of tassels
+dropped from the Galaxy. Among these pendants are the Pleiades and the
+Hyades. Orion, too, the ``Mighty Hunter,'' is caught in ``a loop of
+light'' thrown out from it. The majority of the great first-magnitude
+stars seem related to it, as if they formed an inner ring inclined at
+an angle of some twenty degrees to its plane. Many of the long curves
+that set off from it on both sides are accompanied by corresponding
+curves of lucid stars. In a word, it offers every appearance of
+structural connection with the entire starry system. That the universe
+should have assumed the form of a wreath is certainly a matter for
+astonishment; but it would have been still more astonishing if it had
+been a cube, a rhomboid, or a dodecahedron, for then we should have
+had to suppose that something resembling the forces that shape
+crystals had acted upon the stars, and the difficulty of explaining
+the universe by the laws of gravitation would have been increased.
+
+From the Milky Way as a whole we pass to the vast clouds, swarms, and
+clusters of stars of which it is made up. It may be, as some
+astronomers hold, that most of the galactic stars are much smaller
+than the sun, so that their faintness is not due entirely to the
+effect of distance. Still, their intrinsic brilliance attests their
+solar character, and considering their remoteness, which has been
+estimated at not less than ten thousand to twenty thousand light-years
+(a light-year is equal to nearly six thousand thousand million miles)
+their actual masses cannot be extremely small. The minutest of them
+are entitled to be regarded as real suns, and they vary enormously in
+magnitude. The effects of their attractions upon one another can only
+be inferred from their clustering, because their relative movements
+are not apparent on account of the brevity of the observations that we
+can make. But imagine a being for whom a million years would be but as
+a flitting moment; to him the Milky Way would appear in a state of
+ceaseless agitation -- swirling with ``a fury of whirlpool motion.''
+
+The cloud-like aspect of large parts of the Galaxy must always have
+attracted attention, even from naked-eye observers, but the true
+star-clouds were first satisfactorily represented in Barnard's
+photographs. The resemblance to actual clouds is often startling. Some
+are close-packed and dense, like cumuli; some are wispy or mottled,
+like cirri. The rifts and modulations, as well as the general
+outlines, are the same as those of clouds of vapor or dust, and one
+notices also the characteristic thinning out at the edges. But we must
+beware of supposing that the component suns are thickly crowded as the
+particles forming an ordinary cloud. They look, indeed, as if they
+were matted together, because of the irradiation of light, but in
+reality millions and billions of miles separate each star from its
+neighbors. Nevertheless they form real assemblages, whose members are
+far more closely related to one another than is our sun to the stars
+around him, and if we were in the Milky Way the aspect of the
+nocturnal sky would be marvelously different from its present
+appearance.
+
+Stellar clouds are characteristic of the Galaxy and are not found
+beyond its borders, except in the ``Magellanic Clouds'' of the
+southern hemisphere, which resemble detached portions of the Milky
+Way. These singular objects form as striking a peculiarity of the
+austral heavens as does the great ``Coal-sack'' described in Chapter
+1. But it is their isolation that makes them so remarkable, for their
+composition is essentially galactic, and if they were included within
+its boundaries they would not appear more wonderful than many other
+parts of the Milky Way. Placed where they are, they look like masses
+fallen from the great stellar arch. They are full of nebul� and
+star-clusters, and show striking evidences of spiral movement.
+
+Star-swarms, which are also characteristic features of the Galaxy,
+differ from star-clouds very much in the way that their name would
+imply -- i.e., their component stars are so arranged, even when they
+are countless in number, that the idea of an exceedingly numerous
+assemblage rather than that of a cloud is impressed on the observer's
+mind. In a star-swarm the separate members are distinguishable because
+they are either larger or nearer than the stars composing a ``cloud.''
+A splendid example of a true star-swarm is furnished by Chi Persei, in
+that part of the Milky Way which runs between the constellations
+Perseus and Cassiopeia. This swarm is much coarser than many others,
+and can be seen by the naked eye. In a small telescope it appears
+double, as if the suns composing it had divided into two parties which
+keep on their way side by side, with some commingling of their members
+where the skirts of the two companies come in contact.
+
+Smaller than either star-clouds or star-swarms, and differing from
+both in their organization, are star-clusters. These, unlike the
+others, are found outside as well as inside the Milky Way, although
+they are more numerous inside its boundaries than elsewhere. The term
+star-cluster is sometimes applied, though improperly, to assemblages
+which are rather groups, such, for instance, as the Pleiades. In their
+most characteristic aspect star-clusters are of a globular shape --
+globes of suns! A famous example of a globular star-cluster, but one
+not included in the Milky Way, is the ``Great Cluster in Hercules.''
+This is barely visible to the naked eye, but a small telescope shows
+its character, and in a large one it presents a marvelous spectacle.
+Photographs of such clusters are, perhaps, less effective than those
+of star-clouds, because the central condensation of stars in them is
+so great that their light becomes blended in an indistinguishable
+blur. The beautiful effect of the incessant play of infinitesimal rays
+over the apparently compact surface of the cluster, as if it were a
+globe of the finest frosted silver shining in an electric beam, is
+also lost in a photograph. Still, even to the eye looking directly at
+the cluster through a powerful telescope, the central part of the
+wonderful congregation seems almost a solid mass in which the stars
+are packed like the ice crystals in a snowball.
+
+The same question rises to the lips of every observer: How can they
+possibly have been brought into such a situation? The marvel does not
+grow less when we know that, instead of being closely compacted, the
+stars of the cluster are probably separated by millions of miles; for
+we know that their distances apart are slight as compared with their
+remoteness from the Earth. Sir William Herschel estimated their number
+to be about fourteen thousand, but in fact they are uncountable. If we
+could view them from a point just within the edge of the assemblage,
+they would offer the appearance of a hollow hemisphere emblazoned with
+stars of astonishing brilliancy; the near-by ones unparalleled in
+splendor by any celestial object known to us, while the more distant
+ones would resemble ordinary stars. An inhabitant of the cluster would
+not know, except by a process of ratiocination, that he was dwelling
+in a globular assemblage of suns; only from a point far outside would
+their spherical arrangement become evident to the eye. Imagine
+fourteen-thousand fire-balloons with an approach to regularity in a
+spherical space -- say, ten miles in diameter; there would be an
+average of less than thirty in every cubic mile, and it would be
+necessary to go to a considerable distance in order to see them as a
+globular aggregation; yet from a point sufficiently far away they
+would blend into a glowing ball.
+
+Photographs show even better than the best telescopic views that the
+great cluster is surrounded with a multitude of dispersed stars,
+suggestively arrayed in more or less curving lines, which radiate from
+the principle mass, with which their connection is manifest. These
+stars, situated outside the central sphere, look somewhat like vagrant
+bees buzzing round a dense swarm where the queen bee is sitting. Yet
+while there is so much to suggest the operation of central forces,
+bringing and keeping the members of the cluster together, the
+attentive observer is also impressed with the idea that the whole
+wonderful phenomenon may be the result of explosion. As soon as this
+thought seizes the mind, confirmation of it seems to be found in the
+appearance of the outlying stars, which could be as readily explained
+by the supposition that they have been blown apart as that they have
+flocked together toward a center. The probable fact that the stars
+constituting the cluster are very much smaller than our sun might be
+regarded as favoring the hypothesis of an explosion. Of their real
+size we know nothing, but, on the basis of an uncertain estimate of
+their parallax, it has been calculated that they may average
+forty-five thousand miles in diameter -- something more than half the
+diameter of the planet Jupiter. Assuming the same mean density,
+fourteen thousand such stars might have been formed by the explosion
+of a body about twice the size of the sun. This recalls the theory of
+Olbers, which has never been altogether abandoned or disproved, that
+the Asteroids were formed by the explosion of a planet circulating
+between the orbits of Mars and Jupiter. The Asteroids, whatever their
+manner of origin, form a ring around the sun; but, of course, the
+explosion of a great independent body, not originally revolving about
+a superior center of gravitational force, would not result in the
+formation of a ring of small bodies, but rather of a dispersed mass of
+them. But back of any speculation of this kind lies the problem, at
+present insoluble: How could the explosion be produced? (See the
+question of explosions in Chapters 6 and 14).
+
+Then, on the other hand, we have the observation of Herschel, since
+abundantly confirmed, that space is unusually vacant in the immediate
+neighborhood of condensed star-clusters and nebul�, which, as far as
+it goes, might be taken as an indication that the assembled stars had
+been drawn together by their mutual attractions, and that the tendency
+to aggregation is still bringing new members toward the cluster. But
+in that case there must have been an original condensation of stars at
+that point in space. This could probably have been produced by the
+coagulation of a great nebula into stellar nuclei, a process which
+seems now to be taking place in the Orion Nebula.
+
+A yet more remarkable globular star-cluster exists in the southern
+hemisphere, Omega Centauri. In this case the central condensation of
+stars presents an almost uniform blaze of light. Like the Hercules
+cluster, that in Centaurus is surrounded with stars scattered over a
+broad field and showing an appearance of radial arrangement. In fact,
+except for its greater richness, Omega Centauri is an exact duplicate
+of its northern rival. Each appears to an imaginative spectator as a
+veritable ``city of suns.'' Mathematics shrinks from the task of
+disentangling the maze of motions in such an assemblage. It would seem
+that the chance of collisions is not to be neglected, and this idea
+finds a certain degree of confirmation in the appearance of
+``temporary stars'' which have more than once blazed out in, or close
+by, globular star-clusters.
+
+This leads up to the notable fact, first established by Professor
+Bailey a few years ago, that such clusters are populous with variable
+stars. Omega Centauri and the Hercules cluster are especially
+remarkable in this respect. The variables found in them are all of
+short period and the changes of light show a noteworthy tendency to
+uniformity. The first thought is that these phenomena must be due to
+collisions among the crowded stars, but, if so, the encounters cannot
+be between the stars themselves, but probably between stars and meteor
+swarms revolving around them. Such periodic collisions might go on for
+ages without the meteors being exhausted by incorporation with the
+stars. This explanation appears all the more probable because one
+would naturally expect that flocks of meteors would abound in a close
+aggregation of stars. It is also consistent with Perrine's discovery
+-- that the globular star clusters are powdered with minute stars
+strewn thickly among the brighter ones.
+
+In speaking of Professor Comstock's extraordinary theory of the Milky
+Way, the fact was mentioned that, broadly speaking, the nebul� are
+less numerous in the galactic belt than in the comparatively open
+spaces on either side of it, but that they are, nevertheless, abundant
+in the broader half of the Milky Way which he designates as the front
+of the gigantic ``plough'' supposed to be forcing its way through the
+enveloping chaos. In and around the Sagittarius region the
+intermingling of nebul� and galactic star clouds and clusters is
+particularly remarkable. That there is a causal connection no
+thoughtful person can doubt. We are unable to get away from the
+evidence that a nebula is like a seed-ground from which stars spring
+forth; or we may say that nebul� resemble clouds in whose bosom
+raindrops are forming. The wonderful aspect of the admixtures of
+nebul� and star-clusters in Sagittarius has been described in Chapter
+1. We now come to a still more extraordinary phenomenon of this kind
+-- the Pleiades nebul�.
+
+The group of the Pleiades, although lying outside the main course of
+the Galaxy, is connected with it by a faint loop, and is the scene of
+the most remarkable association of stars and nebulous matter known in
+the visible universe. The naked eye is unaware of the existence of
+nebul� in the Pleiades, or, at the best, merely suspects that there is
+something of the kind there; and even the most powerful telescopes are
+far from revealing the full wonder of the spectacle; but in
+photographs which have been exposed for many hours consecutively, in
+order to accumulate the impression of the actinic rays, the revelation
+is stunning. The principle stars are seen surrounded by, and, as it
+were, drowned in, dense nebulous clouds of an unparalleled kind. The
+forms assumed by these clouds seem at first sight inexplicable. They
+look like fleeces, or perhaps more like splashes and daubs of luminous
+paint dashed carelessly from a brush. But closer inspection shows that
+they are, to a large extent, woven out of innumerable threads of filmy
+texture, and there are many indications of spiral tendencies. Each of
+the bright stars of the group -- Alcyone, Merope, Maia, Electra,
+Taygeta, Atlas -- is the focus of a dense fog (totally invisible,
+remember, alike to the naked eye and to the telescope), and these
+particular stars are veiled from sight behind the strange mists.
+Running in all directions across the relatively open spaces are
+nebulous wisps and streaks of the most curious forms. On some of the
+nebular lines, which are either straight throughout, or if they change
+direction do so at an angle, little stars are strung like beads. In
+one case seven or eight stars are thus aligned, and, as if to
+emphasize their dependence upon the chain which connects them, when it
+makes a slight bend the file of stars turns the same way. Many other
+star rows in the group suggest by their arrangement that they, too,
+were once strung upon similar threads which have now disappeared,
+leaving the stars spaced along their ancient tracks. We seem forced to
+the conclusion that there was a time when the Pleiades were embedded
+in a vast nebula resembling that of Orion, and that the cloud has now
+become so rare by gradual condensation into stars that the merest
+trace of it remains, and this would probably have escaped detection
+but for the remarkable actinic power of the radiant matter of which it
+consists. The richness of many of these faint nebulous masses in
+ultra-violet radiations, which are those that specifically affect the
+photographic plate, is the cause of the marvelous revelatory power of
+celestial photography. So the veritable unseen universe, as
+distinguished from the ``unseen universe'' of metaphysical
+speculation, is shown to us.
+
+A different kind of association between stars and nebul� is shown in
+some surprising photographic objects in the constellation Cygnus,
+where long, wispy nebul�, billions of miles in length, some of them
+looking like tresses streaming in a breeze, lie amid fields of stars
+which seem related to them. But the relation is of a most singular
+kind, for notwithstanding the delicate structure of the long nebul�
+they appear to act as barriers, causing the stars to heap themselves
+on one side. The stars are two, three, or four times as numerous on
+one side of the nebul� as on the other. These nebul�, as far as
+appearance goes, might be likened to rail fences, or thin hedges,
+against which the wind is driving drifts of powdery snow, which, while
+scattered plentifully all around, tends to bank itself on the leeward
+side of the obstruction. The imagination is at a loss to account for
+these extraordinary phenomena; yet there they are, faithfully giving
+us their images whenever the photographic plate is exposed to their
+radiations.
+
+Thus the more we see of the universe with improved methods of
+observation, and the more we invent aids to human senses, each
+enabling us to penetrate a little deeper into the unseen, the greater
+becomes the mystery. The telescope carried us far, photography is
+carrying us still farther; but what as yet unimagined instrument will
+take us to the bottom, the top, and the end? And then, what hitherto
+untried power of thought will enable us to comprehend the meaning of
+it all?
+
+Stellar Migrations
+
+To the untrained eye the stars and the planets are not
+distinguishable. It is customary to call them all alike ``stars.'' But
+since the planets more or less rapidly change their places in the sky,
+in consequence of their revolution about the sun, while the stars
+proper seem to remain always in the same relative positions, the
+latter are spoken of as ``fixed stars.'' In the beginnings of
+astronomy it was not known that the ``fixed stars'' had any motion
+independent of their apparent annual revolution with the whole sky
+about the earth as a seeming center. Now, however, we know that the
+term ``fixed stars'' is paradoxical, for there is not a single really
+fixed object in the whole celestial sphere. The apparent fixity in the
+positions of the stars is due to their immense distance, combined with
+the shortness of the time during which we are able to observe them. It
+is like viewing the plume of smoke issuing from a steamer, hull down,
+at sea: if one does not continue to watch it for a long time it
+appears to be motionless, although in reality it may be traveling at
+great speed across the line of sight. Even the planets seem fixed in
+position if one watches them for a single night only, and the more
+distant ones do not sensibly change their places, except after many
+nights of observation. Neptune, for instance, moves but little more
+than two degrees in the course of an entire year, and in a month its
+change of place is only about one-third of the diameter of the full
+moon.
+
+Yet, fixed as they seem, the stars are actually moving with a speed in
+comparison with which, in some cases, the planets might almost be said
+to stand fast in their tracks. Jupiter's speed in his orbit is about
+eight miles per second, Neptune's is less than three and one-half
+miles, and the earth's is about eighteen and one-half miles; while
+there are ``fixed stars'' which move two hundred or three hundred
+miles per second. They do not all, however, move with so great a
+velocity, for some appear to travel no faster than the planets. But in
+all cases, notwithstanding their real speed, long-continued and
+exceedingly careful observations are required to demonstrate that they
+are moving at all. No more overwhelming impression of the frightful
+depths of space in which the stars are buried can be obtained than by
+reflecting upon the fact that a star whose actual motion across the
+line of sight amounts to two hundred miles per second does not change
+its apparent place in the sky, in the course of a thousand years,
+sufficiently to be noticed by the casual observer of the heavens!
+
+There is one vast difference between the motions of the stars and
+those of the planets to which attention should be at once called: the
+planets, being under the control of a central force emanating from
+their immediate master, the sun, all move in the same direction and in
+orbits concentric about the sun; the stars, on the other hand, move in
+every conceivable direction and have no apparent center of motion, for
+all efforts to discover such a center have failed. At one time, when
+theology had finally to accept the facts of science, a grandiose
+conception arose in some pious minds, according to which the Throne of
+God was situated at the exact center of His Creation, and, seated
+there, He watched the magnificent spectacle of the starry systems
+obediently revolving around Him. Astronomical discoveries and
+speculations seemed for a time to afford some warrant for this view,
+which was, moreover, an acceptable substitute for the abandoned
+geocentric theory in minds that could only conceive of God as a
+superhuman artificer, constantly admiring his own work. No longer ago
+than the middle of the nineteenth century a German astronomer,
+Maedler, believed that he had actually found the location of the
+center about which the stellar universe revolved. He placed it in the
+group of the Pleiades, and upon his authority an extraordinary
+imaginative picture was sometimes drawn of the star Alcyone, the
+brightest of the Pleiades, as the very seat of the Almighty. This idea
+even seemed to gain a kind of traditional support from the mystic
+significance, without known historical origin, which has for many
+ages, and among widely separated peoples, been attached to the
+remarkable group of which Alcyone is the chief. But since Maedler's
+time it has been demonstrated that the Pleiades cannot be the center
+of revolution of the universe, and, as already remarked, all attempts
+to find or fix such a center have proved abortive. Yet so powerful was
+the hold that the theory took upon the popular imagination, that even
+today astronomers are often asked if Alcyone is not the probable site
+of ``Jerusalem the Golden.''
+
+If there were a discoverable center of predominant gravitative power,
+to which the motions of all the stars could be referred, those motions
+would appear less mysterious, and we should then be able to conclude
+that the universe was, as a whole, a prototype of the subsidiary
+systems of which it is composed. We should look simply to the law of
+gravitation for an explanation, and, naturally, the center would be
+placed within the opening enclosed by the Milky Way. If it were there
+the Milky Way itself should exhibit signs of revolution about it, like
+a wheel turning upon its hub. No theory of the star motions as a whole
+could stand which failed to take account of the Milky Way as the basis
+of all. But the very form of that divided wreath of stars forbids the
+assumption of its revolution about a center. Even if it could be
+conceived as a wheel having no material center it would not have the
+form which it actually presents. As was shown in Chapter 2, there is
+abundant evidence of motion in the Milky Way; but it is not motion of
+the system as a whole, but motion affecting its separate parts.
+Instead of all moving one way, the galactic stars, as far as their
+movements can be inferred, are governed by local influences and
+conditions. They appear to travel crosswise and in contrary
+directions, and perhaps they eddy around foci where great numbers have
+assembled; but of a universal revolution involving the entire mass we
+have no evidence.
+
+Most of our knowledge of star motions, called ``proper motions,''
+relates to individual stars and to a few groups which happen to be so
+near that the effects of their movements are measurable. In some cases
+the motion is so rapid (not in appearance, but in reality) that the
+chief difficulty is to imagine how it can have been imparted, and what
+will eventually become of the ``runaways.'' Without a collision, or a
+series of very close approaches to great gravitational centers, a star
+traveling through space at the rate of two hundred or three hundred
+miles per second could not be arrested or turned into an orbit which
+would keep it forever flying within the limits of the visible
+universe. A famous example of these speeding stars is ``1830
+Groombridge,'' a star of only the sixth magnitude, and consequently
+just visible to the naked eye, whose motion across the line of sight
+is so rapid that it moves upon the face of the sky a distance equal to
+the apparent diameter of the moon every 280 years. The distance of
+this star is at least 200,000,000,000,000 miles, and may be two or
+three times greater, so that its actual speed cannot be less than two
+hundred, and may be as much as four hundred, miles per second. It
+could be turned into a new course by a close approach to a great sun,
+but it could only be stopped by collision, head-on, with a body of
+enormous mass. Barring such accidents it must, as far as we can see,
+keep on until it has traversed our stellar system, whence in may
+escape and pass out into space beyond, to join, perhaps, one of those
+other universes of which we have spoken. Arcturus, one of the greatest
+suns in the universe, is also a runaway, whose speed of flight has
+been estimated all the way from fifty to two hundred miles per second.
+Arcturus, we have every reason to believe, possesses hundreds of times
+the mass of our sun -- think, then, of the prodigious momentum that
+its motion implies! Sirius moves more moderately, its motion across
+the line of sight amounting to only ten miles per second, but it is at
+the same time approaching the sun at about the same speed, its actual
+velocity in space being the resultant of the two displacements.
+
+What has been said about the motion of Sirius brings us to another
+aspect of this subject. The fact is, that in every case of stellar
+motion the displacement that we observe represents only a part of the
+actual movement of the star concerned. There are stars whose motion
+carries them straight toward or straight away from the earth, and such
+stars, of course, show no cross motion. But the vast majority are
+traveling in paths inclined from a perpendicular to our line of sight.
+Taken as a whole, the stars may be said to be flying about like the
+molecules in a mass of gas. The discovery of the radial component in
+the movements of the stars is due to the spectroscope. If a star is
+approaching, its spectral lines are shifted toward the violet end of
+the spectrum by an amount depending upon the velocity of approach; if
+it is receding, the lines are correspondingly shifted toward the red
+end. Spectroscopic observation, then, combined with micrometric
+measurements of the cross motion, enables us to detect the real
+movement of the star in space. Sometimes it happens that a star's
+radial movement is periodically reversed; first it approaches, and
+then it recedes. This indicates that it is revolving around a near-by
+companion, which is often invisible, and superposed upon this motion
+is that of the two stars concerned, which together may be approaching
+or receding or traveling across the line of sight. Thus the
+complications involved in the stellar motions are often exceedingly
+great and puzzling.
+
+Yet another source of complication exists in the movement of our own
+star, the sun. There is no more difficult problem in astronomy than
+that of disentangling the effects of the solar motion from those of
+the motions of the other stars. But the problem, difficult as it is,
+has been solved, and upon its solution depends our knowledge of the
+speed and direction of the movement of the solar system through space,
+for of course the sun carries its planets with it. One element of the
+solution is found in the fact that, as a result of perspective, the
+stars toward which we are going appear to move apart toward all points
+of the compass, while those behind appear to close up together. Then
+the spectroscopic principle already mentioned is invoked for studying
+the shift of the lines, which is toward the violet in the stars ahead
+of us and toward the red in those that we are leaving behind. Of
+course the effects of the independent motions of the stars must be
+carefully excluded. The result of the studies devoted to this subject
+is to show that we are traveling at a speed of twelve to fifteen miles
+per second in a northerly direction, toward the border of the
+constellations Hercules and Lyra. A curious fact is that the more
+recent estimates show that the direction is not very much out of a
+straight line drawn from the sun to the star Vega, one of the most
+magnificent suns in the heavens. But it should not be inferred from
+this that Vega is drawing us on; it is too distant for its gravitation
+to have such an effect.
+
+Many unaccustomed thoughts are suggested by this mighty voyage of the
+solar system. Whence have we come, and whither do we go? Every year of
+our lives we advance at least 375,000,000 miles. Since the traditional
+time of Adam the sun has led his planets through the wastes of space
+no less than 225,000,000,000 miles, or more than 2400 times the
+distance that separates him from the earth. Go back in imagination to
+the geologic ages, and try to comprehend the distance over which the
+earth has flown. Where was our little planet when it emerged out of
+the clouds of chaos? Where was the sun when his ``thunder march''
+began? What strange constellations shone down upon our globe when its
+masters of life were the monstrous beasts of the ``Age of Reptiles''?
+A million years is not much of a span of time in geologic reckoning,
+yet a million years ago the earth was farther from its present place
+in space than any of the stars with a measurable parallax are now. It
+was more than seven times as far as Sirius, nearly fourteen times as
+far as Alpha Centauri, three times as far as Vega, and twice as far as
+Arcturus. But some geologists demand two hundred, three hundred, even
+one thousand million years to enable them to account for the
+evolutionary development of the earth and its inhabitants. In a
+thousand million years the earth would have traveled farther than from
+the remotest conceivable depths of the Milky Way!
+
+Other curious reflections arise when we think of the form of the
+earth's track as it follows the lead of the sun, in a journey which
+has neither known beginning nor conceivable end. There are probably
+many minds which have found a kind of consolation in the thought that
+every year the globe returns to the same place, on the same side of
+the sun. This idea may have an occult connection with our traditional
+regard for anniversaries. When that period of the year returns at
+which any great event in our lives has occurred we have the feeling
+that the earth, in its annual round, has, in a manner, brought us back
+to the scene of that event. We think of the earth's orbit as a
+well-worn path which we traverse many times in the course of a
+lifetime. It seems familiar to us, and we grow to have a sort of
+attachment to it. The sun we are accustomed to regard as a fixed
+center in space, like the mill or pump around which the harnessed
+patient mule makes his endless circuits. But the real fact is that the
+earth never returns to the place in space where it has once quitted.
+In consequence of the motion of the sun carrying the earth and the
+other planets along, the track pursued by our globe is a vast spiral
+in space continually developing and never returning upon its course.
+It is probable that the tracks of the sun and the others stars are
+also irregular, and possibly spiral, although, as far as can be at
+present determined, they appear to be practically straight. Every
+star, wherever it may be situated, is attracted by its fellow-stars
+from many sides at once, and although the force is minimized by
+distance, yet in the course of many ages its effects must become
+manifest.
+
+Looked at from another side, is there not something immensely
+stimulating and pleasing to the imagination in the idea of so
+stupendous a journey, which makes all of us the greatest of travelers?
+In the course of a long life a man is transported through space thirty
+thousand million miles; Halley's Comet does not travel one-quarter as
+far in making one of its immense circuits. And there are adventures on
+this voyage of which we are just beginning to learn to take account.
+Space is full of strange things, and the earth must encounter some of
+them as it advances through the unknown. Many singular speculations
+have been indulged in by astronomers concerning the possible effects
+upon the earth of the varying state of the space that it traverses.
+Even the alternation of hot and glacial periods has sometimes been
+ascribed to this source. When tropical life flourished around the
+poles, as the remains in the rocks assure us, the needed high
+temperature may, it has been thought, have been derived from the
+presence of the earth in a warm region of space. Then, too, there is a
+certain interest for us in the thought of what our familiar planet has
+passed through. We cannot but admire it for its long journeying as we
+admire the traveler who comes to us from remote and unexplored lands,
+or as we gaze with a glow of interest upon the first locomotive that
+has crossed a continent, or a ship that has visited the Arctic or
+Antarctic regions. If we may trust the indications of the present
+course, the earth, piloted by the sun, has come from the Milky Way in
+the far south and may eventually rejoin that mighty band of stars in
+the far north.
+
+While the stars in general appear to travel independently of one
+another, except when they are combined in binary or trinary systems,
+there are notable exceptions to this rule. In some quarters of the sky
+we behold veritable migrations of entire groups of stars whose members
+are too widely separated to show any indications of revolution about a
+common center of gravity. This leads us back again to the wonderful
+group of the Pleiades. All of the principle stars composing that group
+are traveling in virtually parallel lines. Whatever force set them
+going evidently acted upon all alike. This might be explained by the
+assumption that when the original projective force acted upon them
+they were more closely united than they are at present, and that in
+drifting apart they have not lost the impulse of the primal motion. Or
+it may be supposed that they are carried along by some current in
+space, although it would be exceedingly difficult, in the present
+state of our knowledge, to explain the nature of such a current. Yet
+the theory of a current has been proposed. As to an attractive center
+around which they might revolve, none has been found. Another instance
+of similar ``star-drift'' is furnished by five of the seven stars
+constituting the figure of the ``Great Dipper.'' In this case the
+stars concerned are separated very widely, the two extreme ones by not
+less than fifteen degrees, so that the idea of a common motion would
+never have been suggested by their aspect in the sky; and the case
+becomes the more remarkable from the fact that among and between them
+there are other stars, some of the same magnitude, which do not share
+their motion, but are traveling in other directions. Still other
+examples of the same phenomenon are found in other parts of the sky.
+Of course, in the case of compact star-clusters, it is assumed that
+all the members share a like motion of translation through space, and
+the same is probably true of dense star-swarms and star-clouds.
+
+The whole question of star-drift has lately assumed a new phase, in
+consequence of the investigations of Kapteyn, Dyson, and Eddington on
+the ``systematic motions of the stars.'' This research will, it is
+hoped, lead to an understanding of the general law governing the
+movements of the whole body of stars constituting the visible
+universe. Taking about eleven hundred stars whose proper motions have
+been ascertained with an approach to certainty, and which are
+distributed in all parts of the sky, it has been shown that there
+exists an apparent double drift, in two independent streams, moving in
+different and nearly opposed directions. The apex of the motion of
+what is called ``Stream I'' is situated, according to Professor
+Kapteyn, in right ascension 85�, declination south 11�, which places
+it just south of the constellation Orion; while the apex of ``Stream
+II'' is in right ascension 260�, declination south 48�, placing it in
+the constellation Ara, south of Scorpio. The two apices differ very
+nearly 180� in right ascension and about 120� in declination. The
+discovery of these vast star-streams, if they really exist, is one of
+the most extraordinary in modern astronomy. It offers the correlation
+of stellar movements needed as the basis of a theory of those
+movements, but it seems far from revealing a physical cause for them.
+As projected against the celestial sphere the stars forming the two
+opposite streams appear intermingled, some obeying one tendency and
+some the other. As Professor Dyson has said, the hypothesis of this
+double movement is of a revolutionary character, and calls for further
+investigation. Indeed, it seems at first glance not less surprising
+than would be the observation that in a snow-storm the flakes over our
+heads were divided into two parties and driving across each other's
+course in nearly opposite directions, as if urged by interpenetrating
+winds.
+
+But whatever explanation may eventually be found for the motions of
+the stars, the knowledge of the existence of those motions must always
+afford a new charm to the contemplative observer of the heavens, for
+they impart a sense of life to the starry system that would otherwise
+be lacking. A stagnant universe, with every star fixed immovably in
+its place, would not content the imagination or satisfy our longing
+for ceaseless activity. The majestic grandeur of the evolutions of the
+celestial hosts, the inconceivable vastness of the fields of space in
+which they are executed, the countless numbers, the immeasurable
+distances, the involved convolutions, the flocking and the scattering,
+the interpenetrating marches and countermarches, the strange community
+of impulsion affecting stars that are wide apart in space and causing
+them to traverse the general movement about them like aides and
+despatch-bearers on a battle-field -- all these arouse an intensity of
+interest which is heightened by the mystery behind them.
+
+The Passing of the Constellations
+
+From a historical and picturesque point of view, one of the most
+striking results of the motions of the stars described in the last
+chapter is their effect upon the forms of the constellations, which
+have been watched and admired by mankind from a period so early that
+the date of their invention is now unknown. The constellations are
+formed by chance combinations of conspicuous stars, like figures in a
+kaleidoscope, and if our lives were commensurate with the �ons of
+cosmic existence we should perceive that the kaleidoscope of the
+heavens was ceaselessly turning and throwing the stars into new
+symmetries. Even if the stars stood fast, the motion of the solar
+system would gradually alter the configurations, as the elements of a
+landscape dissolve and recombine in fresh groupings with the
+traveler's progress amid them. But with the stars themselves all in
+motion at various speeds and in many directions, the changes occur
+more rapidly. Of course, ``rapid'' is here understood in a relative
+sense; the wheel of human history to an eye accustomed to the majestic
+progression of the universe would appear to revolve with the velocity
+of a whirling dynamo. Only the deliberation of geological movements
+can be contrasted with the evolution and devolution of the
+constellations.
+
+And yet this secular fluctuation of the constellation figures is not
+without keen interest for the meditative observer. It is another
+reminder of the swift mutability of terrestial affairs. To the passing
+glance, which is all that we can bestow upon these figures, they
+appear so immutable that they have been called into service to form
+the most lasting records of ancient thought and imagination that we
+possess. In the forms of the constellations, the most beautiful, and,
+in imaginative quality, the finest, mythology that the world has ever
+known has been perpetuated. Yet, in a broad sense, this scroll of
+human thought imprinted on the heavens is as evanescent as the summer
+clouds. Although more enduring than parchment, tombs, pyramids, and
+temples, it is as far as they from truly eternizing the memory of what
+man has fancied and done.
+
+Before studying the effects that the motions of the stars have had and
+will have upon the constellations, it is worth while to consider a
+little further the importance of the stellar pictures as archives of
+history. To emphasize the importance of these effects it is only
+necessary to recall that the constellations register the oldest
+traditions of our race. In the history of primeval religions they are
+the most valuable of documents. Leaving out of account for the moment
+the more familiar mythology of the Greeks, based on something older
+yet, we may refer for illustration to that of the mysterious Maya race
+of America. At Izamal, in Yucatan, says Mr Stansbury Hagar, is a group
+of ruins perched, after the Mexican and Central-American plan, on the
+summits of pyramidal mounds which mark the site of an ancient
+theogonic center of the Mayas. Here the temples all evidently refer to
+a cult based upon the constellations as symbols. The figures and the
+names, of course, were not the same as those that we have derived from
+our Aryan ancestors, but the star groups were the same or nearly so.
+For instance, the loftiest of the temples at Izamal was connected with
+the sign of the constellation known to us as Cancer, marking the place
+of the sun at the summer solstice, at which period the sun was
+supposed to descend at noon like a great bird of fire and consume the
+offerings left upon the altar. Our Scorpio was known to the Mayas as a
+sign of the ``Death God.'' Our Libra, the ``Balance,'' with which the
+idea of a divine weighing out of justice has always been connected,
+seems to be identical with the Mayan constellation Teoyaotlatohua,
+with which was associated a temple where dwelt the priests whose
+special business it was to administer justice and to foretell the
+future by means of information obtained from the spirits of the dead.
+Orion, the ``Hunter'' of our celestial mythology, was among the Mayas
+a ``Warrior,'' while Sagittarius and others of our constellations were
+known to them (under different names, of course), and all were endowed
+with a religious symbolism. And the same star figures, having the same
+significance, were familiar to the Peruvians, as shown by the temples
+at Cuzco. Thus the imagination of ancient America sought in the
+constellations symbols of the unchanging gods.
+
+But, in fact, there is no nation and no people that has not recognized
+the constellations, and at one period or another in its history
+employed them in some symbolic or representative capacity. As handled
+by the Greeks from prehistoric times, the constellation myths became
+the very soul of poetry. The imagination of that wonderful race
+idealized the principal star groups so effectively that the figures
+and traditions thus attached to them have, for civilized mankind,
+displaced all others, just as Greek art in its highest forms stands
+without parallel and eclipses every rival. The Romans translated no
+heroes and heroines of the mythical period of their history to the
+sky, and the deified C�sars never entered that lofty company, but the
+heavens are filled with the early myths of the Greeks. Herakles
+nightly resumes his mighty labors in the stars; Zeus, in the form of
+the white ``Bull,'' Taurus, bears the fair Europa on his back through
+the celestial waves; Andromeda stretches forth her shackled arms in
+the star-gemmed ether, beseeching aid; and Perseus, in a blaze of
+diamond armor, revives his heroic deeds amid sparkling clouds of
+stellar dust. There, too, sits Queen Cassiopeia in her dazzling chair,
+while the Great King, Cepheus, towers gigantic over the pole.
+Professor Young has significantly remarked that a great number of the
+constellations are connected in some way or other with the Argonautic
+Expedition -- that strangely fascinating legend of earliest Greek
+story which has never lost its charm for mankind. In view of all this,
+we may well congratulate ourselves that the constellations will
+outlast our time and the time of countless generations to follow us;
+and yet they are very far from being eternal. Let us now study some of
+the effects of the stellar motions upon them.
+
+We begin with the familiar figure of the ``Great Dipper.'' He who has
+not drunk inspiration from its celestial bowl is not yet admitted to
+the circle of Olympus. This figure is made up of seven conspicuous
+stars in the constellation Ursa Major, the ``Greater Bear.'' The
+handle of the ``Dipper'' corresponds to the tail of the imaginary
+``Bear,'' and the bowl lies upon his flank. In fact, the figure of a
+dipper is so evident and that of a bear so unevident, that to most
+persons the ``Great Dipper'' is the only part of the constellation
+that is recognizable. Of the seven stars mentioned, six are of nearly
+equal brightness, ranking as of the second magnitude, while the
+seventh is of only the third magnitude. The difference is very
+striking, since every increase of one magnitude involves an increase
+of two-and-a-half times in brightness. There appears to be little
+doubt that the faint star, which is situated at the junction of the
+bowl and the handle, is a variable of long period, since three hundred
+years ago it was as bright as its companions. But however that may be,
+its relative faintness at the present time interferes but little with
+the perfection of the ``Dipper's'' figure. In order the more readily
+to understand the changes which are taking place, it will be well to
+mention both the names and the Greek letters which are attached to the
+seven stars. Beginning at the star in the upper outer edge of the rim
+of the bowl and running in regular order round the bottom and then out
+to the end of the handle, the names and letters are as follows: Dubhe
+({\alpha}), Merak ({\beta}), Phaed ({\gamma}), Megrez ({\delta}),
+Alioth ({\epsilon}), Mizar ({\zeta}), and Benetnasch ({\eta}). Megrez
+is the faint star already mentioned at the junction of the bowl and
+handle, and Mizar, in the middle of the handle, has a close, naked-eye
+companion which is named Alcor. The Arabs called this singular pair of
+stars ``The Horse and Rider.'' Merak and Duhbe are called ``The
+Pointers,'' because an imaginary line drawn northward through them
+indicates the Pole Star.
+
+Now it has been found that five of these stars -- viz., Merak, Phaed,
+Megrez, Alioth, and Mizar (with its comrade) -- are moving with
+practically the same speed in an easterly direction, while the other
+two, Dubhe and Benetnasch, are simultaneously moving westward, the
+motions of Benetnasch being apparently more rapid. The consequence of
+these opposed motions is, of course, that the figure of the ``Dipper''
+cannot always have existed and will not continue to exist. In the
+accompanying diagrams it has been thought interesting to show the
+relative positions of these seven stars, as seen from the point which
+the earth now occupies, both in the past and in the future. Arrows
+attached to the stars in the figure representing the present
+appearance of the ``Dipper'' indicate the directions of the motions
+and the distances over which they will carry the stars in a period of
+about five hundred centuries. The time, no doubt, seems long, but
+remember the vast stretch of ages through which the earth has passed,
+and then reflect that no reason is apparent why our globe should not
+continue to be a scene of animation for ten thousand centuries yet to
+come. The fact that the little star Alcor placed so close to Mizar
+should accompany the latter in its flight is not surprising, but that
+two of the principal stars of the group should be found moving in a
+direction directly opposed to that pursued by the other five is
+surprising in the highest degree; and it recalls the strange theory of
+a double drift affecting all the stars, to which attention was called
+in the preceding chapter. It would appear that Benetnasch and Dubhe
+belong to one ``current,'' and Merak, Phaed, Megrez, Alioth, and Mizar
+to the other. As far as is known, the motion of the seven stars are
+not shared by the smaller stars scattered about them, but on the
+theory of currents there should be such a community of motion, and
+further investigation may reveal it.
+
+From the ``Great Dipper'' we turn to a constellation hardly less
+conspicuous and situated at an equal distance from the pole on the
+other side -- Cassiopeia. This famous star-group commemorating the
+romantic Queen of Ethiopia whose vain boasting of her beauty was
+punished by the exposure of her daughter Andromeda to the ``Sea
+Monster,'' is well-marked by five stars which form an irregular letter
+``W'' with its open side toward the pole. Three of these stars are
+usually ranked as of the second magnitude, and two of the third; but
+to ordinary observation they appear of nearly equal brightness, and
+present a very striking picture. They mark out the chair and a part of
+the figure of the beautiful queen. Beginning at the right-hand, or
+western, end of the ``W,'' their Greek letter designations are: Beta
+({\beta}), Alpha ({\alpha}), Gamma ({\gamma}), Delta ({\delta}), and
+Epsilon ({\epsilon}). Four of them, Beta, Alpha, Delta, and Epsilon
+are traveling eastwardly at various speeds, while the fifth, Gamma,
+moves in a westerly direction. The motion of Beta is more rapid than
+that of any of the others. It should be said, however, that no little
+uncertainty attaches to the estimates of the rate of motion of stars
+which are not going very rapidly, and different observers often vary
+considerably in their results.
+
+In the beautiful ``Northern Crown,'' one of the most perfect and
+charming of all the figures to be found in the stars, the alternate
+combining and scattering effects of the stellar motions are shown by
+comparing the appearance which the constellation must have had five
+hundred centuries ago with that which it has at present and that which
+it will have in the future. The seven principle stars of the asterism,
+forming a surprisingly perfect coronet, have movements in three
+directions at right angles to one another. That in these circumstances
+they should ever have arrived at positions giving them so striking an
+appearance of definite association is certainly surprising; from its
+aspect one would have expected to find a community of movement
+governing the brilliants of the ``Crown,'' but instead of that we find
+evidence that they will inevitably drift apart and the beautiful
+figure will dissolve.
+
+A similar fate awaits such asterisms as the ``Northern Cross'' in
+Cygnus; the ``Crow'' (Corvus), which stands on the back of the great
+``Sea Serpent,'' Hydra, and pecks at his scales; ``Job's Coffin''
+(Delphinus); the ``Great Square of Pegasus''; the ``Twins'' (Gemini);
+the beautiful ``Sickle'' in Leo; and the exquisite group of the Hyades
+in Taurus. In the case of the Hyades, two controlling movements are
+manifest: one, affecting five of the stars which form the well-known
+figure of a letter ``V,'' is directed northerly; the other, which
+controls the direction of two stars, has an easterly trend. The chief
+star of the group, Aldebaran, one of the finest of all stars both for
+its brilliance and its color, is the most affected by the easterly
+motion. In time it will drift entirely out of connection with its
+present neighbors. Although the Hyades do not form so compact a group
+as the Pleiades in the same constellation, yet their appearance of
+relationship is sufficient to awaken a feeling of surprise over the
+fact that, as with the stars of the ``Dipper,'' their association is
+only temporary or apparent.
+
+The great figure of Orion appears to be more lasting, not because its
+stars are physically connected, but because of their great distance,
+which renders their movements too deliberate to be exactly
+ascertained. Two of the greatest of its stars, Betelgeuse and Rigel,
+possess, as far as has been ascertained, no perceptible motion across
+the line of sight, but there is a little movement perceptible in the
+``Belt.'' At the present time this consists of an almost perfect
+straight line, a row of second-magnitude stars about equally spaced
+and of the most striking beauty. In the course of time, however, the
+two right-hand stars, Mintaka and Alnilam (how fine are these Arabic
+star names!) will approach each other and form a naked-eye double, but
+the third, Alnita, will drift away eastward, so that the ``Belt'' will
+no longer exist.
+
+For one more example, let us go to the southern hemisphere, whose most
+celebrated constellation, the ``Southern Cross,'' has found a place in
+all modern literatures, although it has no claim to consideration on
+account of association with ancient legends. This most attractive
+asterism, which has never ceased to fascinate the imagination of
+Christendom since it was first devoutly described by the early
+explorers of the South, is but a passing collocation of brilliant
+stars. Yet even in its transfigurations it has been for hundreds of
+centuries, and will continue to be for hundreds of centuries to come,
+a most striking object in the sky. Our figures show its appearance in
+three successive phases: first, as it was fifty thousand years ago
+(viewed from the earth's present location); second, as it is in our
+day; and, third, as it will be an equal time in the future. The
+nearness of these bright stars to one another -- the length of the
+longer beam of the ``Cross'' is only six degrees -- makes this group
+very noticeable, whatever the arrangement of its components may be.
+The largest star, at the base of the ``Cross,'' is of the first
+magnitude, two of the others are of the second magnitude, and the
+fourth is of the third. Other stars, not represented in the figures,
+increase the effect of a celestial blazonry, although they do not help
+the resemblance to a cross.
+
+But since the motion of the solar system itself will, in the course of
+so long a period as fifty thousand years, produce a great change in
+the perspective of the heavens as seen from the earth, by carrying us
+nearly nineteen trillion miles from our present place, why, it may be
+asked, seek to represent future appearances of the constellations
+which we could not hope to see, even if we could survive so long? The
+answer is: Because these things aid the mind to form a picture of the
+effects of the mobility of the starry universe. Only by showing the
+changes from some definite point of view can we arrive at a due
+comprehension of them. The constellations are more or less familiar to
+everybody, so that impending changes of their forms must at once
+strike the eye and the imagination, and make clearer the significance
+of the movements of the stars. If the future history of mankind is to
+resemble its past and if our race is destined to survive yet a million
+years, then our remote descendents will see a ``new heavens'' if not a
+``new earth,'' and will have to invent novel constellations to
+perpetuate their legends and mythologies.
+
+If our knowledge of the relative distances of the stars were more
+complete, it would be an interesting exercise in celestial geometry to
+project the constellations probably visible to the inhabitants of
+worlds revolving around some of the other suns of space. Our sun is
+too insignificant for us to think that he can make a conspicuous
+appearance among them, except, perhaps, in a few cases. As seen, for
+instance, from the nearest known star, Alpha Centauri, the sun would
+appear of the average first magnitude, and consequently from that
+standpoint he might be the gem of some little constellation which had
+no Sirius, or Arcturus, or Vega to eclipse him with its superior
+splendor. But from the distance of the vast majority of the stars the
+sun would probably be invisible to the naked eye, and as seen from
+nearer systems could only rank as a fifth or sixth magnitude star,
+unnoticed and unknown except by the star-charting astronomer.
+
+Conflagrations in the Heavens
+
+Suppose it were possible for the world to take fire and burn up -- as
+some pessimists think that it will do when the Divine wrath shall have
+sufficiently accumulated against it -- nobody out of our own little
+corner of space would ever be aware of the catastrophe! With all their
+telescopes, the astronomers living in the golden light of Arcturus or
+the diamond blaze of Canopus would be unable to detect the least
+glimmer of the conflagration that had destroyed the seat of Adam and
+his descendents, just as now they are totally ignorant of its
+existence.
+
+But at least fifteen times in the course of recorded history men
+looking out from the earth have beheld in the remote depths of space
+great outbursts of fiery light, some of them more splendidly luminous
+than anything else in the firmament except the sun! If they were
+conflagrations, how many million worlds like ours were required to
+feed their blaze?
+
+It is probable that ``temporary'' or ``new'' stars, as these wonderful
+apparitions are called, really are conflagrations; not in the sense of
+a bonfire or a burning house or city, but in that of a sudden eruption
+of inconceivable heat and light, such as would result from the
+stripping off the shell of an encrusted sun or the crashing together
+of two mighty orbs flying through space with a hundred times the
+velocity of the swiftest cannon-shot.
+
+Temporary stars are the rarest and most erratic of astronomical
+phenomena. The earliest records relating to them are not very clear,
+and we cannot in every instance be certain that it was one of these
+appearances that the ignorant and superstitious old chroniclers are
+trying to describe. The first temporary star that we are absolutely
+sure of appeared in 1572, and is known as ``Tycho's Star,'' because
+the celebrated Danish astronomer (whose remains, with his
+gold-and-silver artificial nose -- made necessary by a duel -- still
+intact, were disinterred and reburied in 1901) was the first to
+perceive it in the sky, and the most assiduous and successful in his
+studies of it. As the first fully accredited representative of its
+class, this new star made its entry upon the scene with becoming
+�clat. It is characteristic of these phenomena that they burst into
+view with amazing suddenness, and, of course, entirely unexpectedly.
+Tycho's star appeared in the constellation Cassiopeia, near a now
+well-known and much-watched little star named Kappa, on the evening of
+November 11, 1572. The story has often been repeated, but it never
+loses interest, how Tycho, going home that evening, saw people in the
+street pointing and staring at the sky directly over their heads, and
+following the direction of their hands and eyes he was astonished to
+see, near the zenith, an unknown star of surpassing brilliance. It
+outshone the planet Jupiter, and was therefore far brighter than the
+first magnitude. There was not another star in the heavens that could
+be compared with it in splendor. Tycho was not in all respects free
+from the superstitions of his time -- and who is? -- but he had the
+true scientific instinct, and immediately he began to study the
+stranger, and to record with the greatest care every change in its
+aspect. First he determined as well as he could with the imperfect
+instruments of his day, many of which he himself had invented, the
+precise location of the phenomena in the sky. Then he followed the
+changes that it underwent. At first it brightened until its light
+equaled or exceeded that of the planet Venus at her brightest, a
+statement which will be appreciated at its full value by anyone who
+has ever watched Venus when she plays her dazzling r�le of ``Evening
+Star,'' flaring like an arc light in the sunset sky. It even became so
+brilliant as to be visible in full daylight, since, its position being
+circumpolar, it never set in the latitude of Northern Europe. Finally
+it began to fade, turning red as it did so, and in March, 1574, it
+disappeared from Tycho's searching gaze, and has never been seen again
+from that day to this. None of the astronomers of the time could make
+anything of it. They had not yet as many bases of speculation as we
+possess today.
+
+Tycho's star has achieved a romantic reputation by being fancifully
+identified with the ``Star of Bethlehem,'' said to have led the
+wondering Magi from their eastern deserts to the cradle-manger of the
+Savior in Palestine. Many attempts have been made to connect this
+traditional ``star'' with some known phenomenon of the heavens, and
+none seems more idle than this. Yet it persistently survives, and no
+astronomer is free from eager questions about it addressed by people
+whose imagination has been excited by the legend. It is only necessary
+to say that the supposition of a connection between the phenomenon of
+the Magi and Tycho's star is without any scientific foundation. It was
+originally based on an unwarranted assumption that the star of Tycho
+was a variable of long period, appearing once every three hundred and
+fifteen years, or thereabout. If that were true there would have been
+an apparition somewhere near the traditional date of the birth of
+Christ, a date which is itself uncertain. But even the data on which
+the assumption was based are inconsistent with the theory. Certain
+monkish records speak of something wonderful appearing in the sky in
+the years 1264 and 945, and these were taken to have been outbursts of
+Tycho's star. Investigation shows that the records more probably refer
+to comets, but even if the objects seen were temporary stars, their
+dates do not suit the hypothesis; from 945 to 1264 there is a gap of
+319 years, and from 1264 to 1572 one of only 308 years; moreover 337
+years have now (1909) elapsed since Tycho saw the last glimmer of his
+star. Upon a variability so irregular and uncertain as that, even if
+we felt sure that it existed, no conclusion could be found concerning
+an apparition occurring 2000 years ago.
+
+In the year 1600 (the year in which Giordano Bruno was burned at the
+stake for teaching that there is more than one physical world), a
+temporary star of the third magnitude broke out in the constellation
+Cygnus, and curiously enough, considering the rarity of such
+phenomena, only four years later another surprisingly brilliant one
+appeared in the constellation Ophiuchus. This is often called
+``Kepler's star,'' because the great German astronomer devoted to it
+the same attention that Tycho had given to the earlier phenomenon. It,
+too, like Tycho's, was at first the brightest object in the stellar
+heavens, although it seems never to have quite equaled its famous
+predecessor in splendor. It disappeared after a year, also turning of
+a red color as it became more faint. We shall see the significance of
+this as we go on. Some of Kepler's contemporaries suggested that the
+outburst of this star was due to a meeting of atoms in space, and idea
+bearing a striking resemblance to the modern theory of ``astronomical
+collisions.''
+
+In 1670, 1848, and 1860 temporary stars made their appearance, but
+none of them was of great brilliance. In 1866 one of the second
+magnitude broke forth in the ``Northern Crown'' and awoke much
+interest, because by that time the spectroscope had begun to be
+employed in studying the composition of the stars, and Huggins
+demonstrated that the new star consisted largely of incandescent
+hydrogen. But this star, apparently unlike the others mentioned, was
+not absolutely new. Before its outburst it had shown as a star of the
+ninth magnitude (entirely invisible, of course, to the naked eye), and
+after about six weeks it faded to its original condition in which it
+has ever since remained. In 1876 a temporary star appeared in the
+constellation Cygnus, and attained at one time the brightness of the
+second magnitude. Its spectrum and its behavior resembled those of its
+immediate predecessor. In 1885, astronomers were surprised to see a
+sixth-magnitude star glimmering in the midst of the hazy cloud of the
+great Andromeda Nebula. It soon absolutely disappeared. Its spectrum
+was remarkable for being ``continuous,'' like that of the nebula
+itself. A continuous spectrum is supposed to represent a body, or a
+mass, which is either solid or liquid, or composed of gas under great
+pressure. In January, 1892, a new star was suddenly seen in the
+constellation Auriga. It never rose much above the fourth magnitude,
+but it showed a peculiar spectrum containing both bright and dark
+lines of hydrogen.
+
+But a bewildering surprise was now in store; the world was to behold
+at the opening of the twentieth century such a celestial spectacle as
+had not been on view since the times of Tycho and Kepler. Before
+daylight on the morning of February 22, 1901, the Rev. Doctor
+Anderson, of Edinburgh, an amateur astronomer, who had also been the
+first to see the new star in Auriga, beheld a strange object in the
+constellation Perseus not far from the celebrated variable star Algol.
+He recognized its character at once, and immediately telegraphed the
+news, which awoke the startled attention of astronomers all over the
+world. When first seen the new star was no brighter than Algol (less
+than the second magnitude), but within twenty-four hours it was
+ablaze, outshining even the brilliant Capella, and far surpassing the
+first magnitude. At the spot in the sky where it appeared nothing
+whatever was visible on the night before its coming. This is known
+with certainty because a photograph had been made of that very region
+on February 21, and this photograph showed everything down to the
+twelfth magnitude, but not a trace of the stranger which burst into
+view between the 21st and the 22nd like the explosion of a rocket.
+
+Upon one who knew the stars the apparition of this intruder in a
+well-known constellation had the effect of a sudden invasion. The new
+star was not far west of the zenith in the early evening, and in that
+position showed to the best advantage. To see Capella, the hitherto
+unchallenged ruler of that quarter of the sky, abased by comparison
+with this stranger of alien aspect, for there was always an unfamiliar
+look about the ``nova,'' was decidedly disconcerting. It seemed to
+portend the beginning of a revolution in the heavens. One could
+understand what the effect of such an apparition must have been in the
+superstitious times of Tycho. The star of Tycho had burst forth on the
+northern border of the Milky Way; this one was on its southern border,
+some forty-five degrees farther east.
+
+Astronomers were well-prepared this time for the scientific study of
+the new star, both astronomical photography and spectroscopy having
+been perfected, and the results of their investigations were
+calculated to increase the wonder with which the phenomenon was
+regarded. The star remained at its brightest only a few days; then,
+like a veritable conflagration, it began to languish; and, like the
+reflection of a dying fire, as it sank it began to glow with the red
+color of embers. But its changes were spasmodic; once about every
+three days it flared up only to die away again. During these
+fluctuations its light varied alternately in the ratio of one to six.
+Finally it took a permanent downward course, and after a few months
+the naked eye could no longer perceive it; but it remained visible
+with telescopes, gradually fading until it had sunk to the ninth
+magnitude. Then another astonishing change happened: in August
+photographs taken at the Yerkes Observatory and at Heidelberg showed
+that the ``nova'' was surrounded by a spiral nebula! The nebula had
+not been there before, and no one could doubt that it represented a
+phase of the same catastrophe that had produced the outburst of the
+new star. At one time the star seemed virtually to have disappeared,
+as if all its substance had been expanded into the nebulous cloud, but
+always there remained a stellar nucleus about which the misty spiral
+spread wider and ever wider, like a wave expanding around a center of
+disturbance. The nebula too showed a variability of brightness, and
+four condensations which formed in it seemed to have a motion of
+revolution about the star. As time went on the nebula continued to
+expand at a rate which was computed to be not less than twenty
+thousand miles per second! And now the star itself, showing
+indications of having turned into a nebula, behaved in a most erratic
+manner, giving rise to the suspicion that it was about to burst out
+again. But this did not occur, and at length it sunk into a state of
+lethargy from which it has to the present time not recovered. But the
+nebulous spiral has disappeared, and the entire phenomena as it now
+(1909) exists consists of a faint nebulous star of less than the ninth
+magnitude.
+
+The wonderful transformations just described had been forecast in
+advance of the discovery of the nebulous spiral encircling the star by
+the spectroscopic study of the latter. At first there was no
+suggestion of a nebular constitution, but within a month or two
+characteristic nebular lines began to appear, and in less than six
+months the whole spectrum had been transformed to the nebular type. In
+the mean time the shifting of the spectral lines indicated a
+complication of rapid motions in several directions simultaneously.
+These motions were estimated to amount to from one hundred to five
+hundred miles per second.
+
+The human mind is so constituted that it feels forced to seek an
+explanation of so marvelous a phenomenon as this, even in the absence
+of the data needed for a sound conclusion. The most natural
+hypothesis, perhaps, is that of a collision. Such a catastrophe could
+certainly happen. It has been shown, for instance, that in infinity of
+time the earth is sure to be hit by a comet; in the same way it may be
+asserted that, if no time limit is fixed, the sun is certain to run
+against some obstacle in space, either another star, or a dense meteor
+swarm, or one of the dark bodies which there is every reason to
+believe abound around us. The consequences of such a collision are
+easy to foretell, provided that we know the masses and the velocities
+of the colliding bodies. In a preceding chapter we have discussed the
+motions of the sun and stars, and have seen that they are so swift
+that an encounter between any two of them could not but be disastrous.
+But this is not all; for as soon as two stars approached within a few
+million miles their speed would be enormously increased by their
+reciprocal attractions and, if their motion was directed radially with
+respect to their centers, they would come together with a crash that
+would reduce them both to nebulous clouds. It is true that the chances
+of such a ``head-on'' collision are relatively very small; two stars
+approaching each other would most probably fall into closed orbits
+around their common center of gravity. If there were a collision it
+would most likely be a grazing one instead of a direct front-to-front
+encounter. But even a close approach, without any actual collision,
+would probably prove disastrous, owing to the tidal influence of each
+of the bodies on the other. Suns, in consequence of their enormous
+masses and dimensions and the peculiarities of their constitution, are
+exceedingly dangerous to one another at close quarters. Propinquity
+awakes in them a mutually destructive tendency. Consisting of matter
+in the gaseous, or perhaps, in some cases, liquid, state, their tidal
+pull upon each other if brought close together might burst them
+asunder, and the photospheric envelope being destroyed the internal
+incandescent mass would gush out, bringing fiery death to any planets
+that were revolving near. Without regard to the resulting disturbance
+of the earth's orbit, the close approach of a great star to the sun
+would be in the highest degree perilous to us. But this is a danger
+which may properly be regarded as indefinitely remote, since, at our
+present location in space, we are certainly far from every star except
+the sun, and we may feel confident that no great invisible body is
+near, for if there were one we should be aware of its presence from
+the effects of its attraction. As to dark nebul� which may possibly
+lie in the track that the solar system is pursuing at the rate of
+375,000,000 miles per year, that is another question -- and they, too,
+could be dangerous!
+
+This brings us directly back to ``Nova Persei,'' for among the many
+suggestions offered to explain its outburst, as well as those of other
+temporary stars, one of the most fruitful is that of a collision
+between a star and a vast invisible nebula. Professor Seeliger, of
+Munich, first proposed this theory, but it afterward underwent some
+modifications from others. Stated in a general form, the idea is that
+a huge dark body, perhaps an extinguished sun, encountered in its
+progress through space a widespread flock of small meteors forming a
+dark nebula. As it plunged into the swarm the friction of the
+innumerable collisions with the meteors heated its surface to
+incandescence, and being of vast size it then became visible to us as
+a new star. Meanwhile the motion of the body through the nebula, and
+its rotation upon itself, set up a gyration in the blazing atmosphere
+formed around it by the vaporized meteors; and as this atmosphere
+spread wider, under the laws of gyratory motion a rotation in the
+opposite direction began in the inflamed meteoric cloud outside the
+central part of the vortex. Thus the spectral lines were caused to
+show motion in opposite directions, a part of the incandescent mass
+approaching the earth simultaneously with the retreat of another part.
+So the curious spectroscopic observations before mentioned were
+explained. This theory might also account for the appearance of the
+nebulous spiral first seen some six months after the original
+outburst. The sequent changes in the spectrum of the ``nova'' are
+accounted for by this theory on the assumption, reasonable enough in
+itself, that at first the invading body would be enveloped in a
+vaporized atmosphere of relatively slight depth, producing by its
+absorption the fine dark lines first observed; but that as time went
+on and the incessant collisions continued, the blazing atmosphere
+would become very deep and extensive, whereupon the appearance of the
+spectral lines would change, and bright lines due to the light of the
+incandescent meteors surrounding the nucleus at a great distance would
+take the place of the original dark ones. The vortex of meteors once
+formed would protect the flying body within from further immediate
+collisions, the latter now occurring mainly among the meteors
+themselves, and then the central blaze would die down, and the
+original splendor of the phenomenon would fade.
+
+But the theories about Nova Persei have been almost as numerous as the
+astronomers who have speculated about it. One of the most startling of
+them assumed that the outburst was caused by the running amuck of a
+dark star which had encountered another star surrounded with planets,
+the renewed outbreaks of light after the principal one had faded being
+due to the successive running down of the unfortunate planets! Yet
+another hypothesis is based on what we have already said of the tidal
+influence that two close approaching suns would have upon each other.
+Supposing two such bodies which had become encrusted, but remained
+incandescent and fluid within, to approach within almost striking
+distance; they would whirl each other about their common center of
+gravity, and at the same time their shells would burst under the tidal
+strain, and their glowing nuclei being disclosed would produce a great
+outburst of light. Applying this theory to a ``nova,'' like that of
+1866 in the ``Northern Crown,'' which had been visible as a small star
+before the outbreak, and which afterward resumed its former aspect, we
+should have to assume that a yet shining sun had been approached by a
+dark body whose attraction temporarily burst open its photosphere. It
+might be supposed that in this case the dark body was too far advanced
+in cooling to suffer the same fate from the tidal pull of its victim.
+But a close approach of that kind would be expected to result in the
+formation of a binary system, with orbits of great eccentricity,
+perhaps, and after the lapse of a certain time the outburst should be
+renewed by another approximation of the two bodies. A temporary star
+of that kind would rather be ranked as a variable.
+
+The celebrated French astronomer, Janssen, had a different theory of
+Nova Persei, and of temporary stars in general. According to his idea,
+such phenomena might be the result of chemical changes taking place in
+a sun without interference by, or collision with, another body.
+Janssen was engaged for many years in trying to discover evidence of
+the existence of oxygen in the sun, and he constructed his observatory
+on the summit of Mount Blanc specially to pursue that research. He
+believed that oxygen must surely exist in the sun since we find so
+many other familiar elements included in the constitution of the solar
+globe, and as he was unable to discover satisfactory evidence of its
+presence he assumed that it existed in a form unknown on the earth. If
+it were normally in the sun's chromosphere, or coronal atmosphere, he
+said, it would combine with the hydrogen which we know is there and
+form an obscuring envelope of water vapor. It exists, then, in a
+special state, uncombined with hydrogen; but let the temperature of
+the sun sink to a critical point and the oxygen will assume its normal
+properties and combine with the hydrogen, producing a mighty outburst
+of light and heat. This, Janssen thought, might explain the phenomena
+of the temporary stars. It would also, he suggested, account for their
+brief career, because the combination of the elements would be quickly
+accomplished, and then the resulting water vapor would form an
+atmosphere cutting off the radiation from the star within.
+
+This theory may be said to have a livelier human interest than some of
+the others, since, according to it, the sun may carry in its very
+constitution a menace to mankind; one does not like to think of it
+being suddenly transformed into a gigantic laboratory for the
+explosive combination of oxygen and hydrogen! But while Janssen's
+theory might do for some temporary stars, it is inadequate to explain
+all the phenomena of Nova Persei, and particularly the appearance of
+the great spiral nebula that seemed to exhale from the heart of the
+star. Upon the whole, the theory of an encounter between a star and a
+dark nebula seems best to fit the observations. By that hypothesis the
+expanding billow of light surrounding the core of the conflagration is
+very well accounted for, and the spectroscopic peculiarities are also
+explained.
+
+Dr Gustov Le Bon offers a yet more alarming theory, suggesting that
+temporary stars are the result of atomic explosion; but we shall touch
+upon this more fully in Chapter 14.
+
+Twice in the course of this discussion we have called attention to the
+change of color invariably undergone by temporary stars in the later
+stages of their career. This was conspicuous with Nova Persei which
+glowed more and more redly as it faded, until the nebulous light began
+to overpower that of the stellar nucleus. Nothing could be more
+suggestive of the dying out of a great fire. Moreover, change of color
+from white to red is characteristic of all variable stars of long
+period, such as ``Mira'' in Cetus. It is also characteristic of stars
+believed to be in the later stages of evolution, and consequently
+approaching extinction, like Antares and Betelgeuse, and still more
+notably certain small stars which ``gleam like rubies in the field of
+the telescope.'' These last appear to be suns in the closing period of
+existence as self-luminous bodies. Between the white stars, such as
+Sirius and Rigel, and the red stars, such as Aldebaran and Alpha
+Herculis, there is a progressive series of colors from golden yellow
+through orange to deep red. The change is believed to be due to the
+increase of absorbing vapors in the stellar atmosphere as the body
+cools down. In the case of ordinary stars these changes no doubt
+occupy many millions of years, which represent the average duration of
+solar life; but the temporary stars run through similar changes in a
+few months: they resemble ephemeral insects -- born in the morning and
+doomed to perish with the going down of the sun.
+
+Explosive and Whirling Nebul�
+
+One of the most surprising triumphs of celestial photography was
+Professor Keeler's discovery, in 1899, that the great majority of the
+nebul� have a distinctly spiral form. This form, previously known in
+Lord Rosse's great ``Whirlpool Nebula,'' had been supposed to be
+exceptional; now the photographs, far excelling telescopic views in
+the revelation of nebular forms, showed the spiral to be the typical
+shape. Indeed, it is a question whether all nebul� are not to some
+extent spiral. The extreme importance of this discovery is shown in
+the effect that it has had upon hitherto prevailing views of solar and
+planetary evolution. For more than three-quarters of a century
+Laplace's celebrated hypothesis of the manner of origin of the solar
+system from a rotating and contracting nebula surrounding the sun had
+guided speculation on that subject, and had been tentatively extended
+to cover the evolution of systems in general. The apparent forms of
+some of the nebul� which the telescope had revealed were regarded, and
+by some are still regarded, as giving visual evidence in favor of this
+theory. There is a ``ring nebula'' in Lyra with a central star, and a
+``planetary nebula'' in Gemini bearing no little resemblance to the
+planet Saturn with its rings, both of which appear to be practical
+realizations of Laplace's idea, and the elliptical rings surrounding
+the central condensation of the Andromeda Nebula may be cited for the
+same kind of proof.
+
+But since Keeler's discovery there has been a decided turning away of
+speculation another way. The form of the spiral nebul� seems to be
+entirely inconsistent with the theory of an originally globular or
+disk-shaped nebula condensing around a sun and throwing or leaving off
+rings, to be subsequently shaped into planets. Some astronomers,
+indeed, now reject Laplace's hypothesis in toto, preferring to think
+that even our solar system originated from a spiral nebula. Since the
+spiral type prevails among the existing nebul�, we must make any
+mechanical theory of the development of stars and planetary systems
+from them accord with the requirements which that form imposes. A
+glance at the extraordinary variations upon the spiral which Professor
+Keeler's photographs reveal is sufficient to convince one of the
+difficulty of the task of basing a general theory upon them. In truth,
+it is much easier to criticize Laplace's hypothesis than to invent a
+satisfactory substitute for it. If the spiral nebul� seem to oppose it
+there are other nebul� which appear to support it, and it may be that
+no one fixed theory can account for all the forms of stellar evolution
+in the universe. Our particular planetary system may have originated
+very much as the great French mathematician supposed, while others
+have undergone, or are now undergoing, a different process of
+development. There is always a too strong tendency to regard an
+important new discovery and the theories and speculations based upon
+it as revolutionizing knowledge, and displacing or overthrowing
+everything that went before. Upon the plea that ``Laplace only made a
+guess'' more recent guesses have been driven to extremes and treated
+by injudicious exponents as ``the solid facts at last.''
+
+Before considering more recent theories than Laplace's, let us see
+what the nature of the photographic revelations is. The vast celestial
+maelstrom discovered by Lord Rosse in the ``Hunting Dogs'' may be
+taken as the leading type of the spiral nebul�, although there are
+less conspicuous objects of the kind which, perhaps, better illustrate
+some of their peculiarities. Lord Rosse's nebula appears far more
+wonderful in the photographs than in his drawings made with the aid of
+his giant reflecting telescope at Parsonstown, for the photographic
+plate records details that no telescope is capable of showing. Suppose
+we look at the photograph of this object as any person of common sense
+would look at any great and strange natural phenomenon. What is the
+first thing that strikes the mind? It is certainly the appearance of
+violent whirling motion. One would say that the whole glowing mass had
+been spun about with tremendous velocity, or that it had been set
+rotating so rapidly that it had become the victim of ``centrifugal
+force,'' one huge fragment having broken loose and started to gyrate
+off into space. Closer inspection shows that in addition to the
+principal focus there are various smaller condensations scattered
+through the mass. These are conspicuous in the spirals. Some of them
+are stellar points, and but for the significance of their location we
+might suppose them to be stars which happen to lie in a line between
+us and the nebula. But when we observe how many of them follow most
+faithfully the curves of the spirals we cannot but conclude that they
+form an essential part of the phenomenon; it is not possible to
+believe that their presence in such situations is merely fortuitous.
+One of the outer spirals has at least a dozen of these star-like
+points strung upon it; some of them sharp, small, and distinct, others
+more blurred and nebulous, suggesting different stages of
+condensation. Even the part which seems to have been flung loose from
+the main mass has, in addition to its central condensation, at least
+one stellar point gleaming in the half-vanished spire attached to it.
+Some of the more distant stars scattered around the ``whirlpool'' look
+as if they too had been shot out of the mighty vortex, afterward
+condensing into unmistakable solar bodies. There are at least two
+curved rows of minute stars a little beyond the periphery of the
+luminous whirl which clearly follow lines concentric with those of the
+nebulous spirals. Such facts are simply dumbfounding for anyone who
+will bestow sufficient thought upon them, for these are suns, though
+they may be small ones; and what a birth is that for a sun!
+
+Look now again at the glowing spirals. We observe that hardly have
+they left the central mass before they begin to coagulate. In some
+places they have a ``ropy'' aspect; or they are like peascods filled
+with growing seeds, which eventually will become stars. The great
+focus itself shows a similar tendency, especially around its
+circumference. The sense that it imparts of a tremendous shattering
+force at work is overwhelming. There is probably more matter in that
+whirling and bursting nebula than would suffice to make a hundred
+solar systems! It must be confessed at once that there is no
+confirmation of the Laplacean hypothesis here; but what hypothesis
+will fit the facts? There is one which it has been claimed does so,
+but we shall come to that later. In the meanwhile, as a preparation,
+fix in the memory the appearance of that second spiral mass spinning
+beside its master which seems to have spurned it away.
+
+For a second example of the spiral nebul� look at the one in the
+constellation Triangulum. God, how hath the imagination of puny man
+failed to comprehend Thee! Here is creation through destruction with a
+vengeance! The spiral form of the nebula is unmistakable, but it is
+half obliterated amid the turmoil of flying masses hurled away on all
+sides with tornadic fury. The focus itself is splitting asunder under
+the intolerable strain, and in a little while, as time is reckoned in
+the Cosmos, it will be gyrating into stars. And then look at the
+cyclonic rain of already finished stars whirling round the outskirts
+of the storm. Observe how scores of them are yet involved in the
+fading streams of the nebulous spirals; see how they have been thrown
+into vast loops and curves, of a beauty that half redeems the terror
+of the spectacle enclosed within their lines -- like iridescent cirri
+hovering about the edges of a hurricane. And so again are suns born!
+
+Let us turn to the exquisite spiral in Ursa Major; how different its
+aspect from that of the other! One would say that if the terrific coil
+in Triangulum has all but destroyed itself in its fury, this one on
+the contrary has just begun its self-demolition. As one gazes one
+seems to see in it the smooth, swift, accelerating motion that
+precedes catastrophe. The central part is still intact, dense, and
+uniform in texture. How graceful are the spirals that smoothly rise
+from its oval rim and, gemmed with little stars, wind off into the
+darkness until they have become as delicate as threads of gossamer!
+But at bottom the story told here is the same -- creation by gyration!
+
+Compare with the above the curious mass in Cetus. Here the plane of
+the whirling nebula nearly coincides with our line of sight and we see
+the object at a low angle. It is far advanced and torn to shreds, and
+if we could look at it perpendicularly to its plane it is evident that
+it would closely resemble the spectacle in Triangulum.
+
+Then take the famous Andromeda Nebula (see Frontispiece), which is so
+vast that notwithstanding its immense distance even the naked eye
+perceives it as an enigmatical wisp in the sky. Its image on the
+sensitive plate is the masterpiece of astronomical photography; for
+wild, incomprehensible beauty there is nothing that can be compared
+with it. Here, if anywhere, we look upon the spectacle of creation in
+one of its earliest stages. The Andromeda Nebula is apparently less
+advanced toward transformation into stellar bodies than is that in
+Triangulum. The immense crowd of stars sprinkled over it and its
+neighborhood seem in the main to lie this side of the nebula, and
+consequently to have no connection with it. But incipient stars (in
+some places clusters of them) are seen in the nebulous rings, while
+one or two huge masses seem to give promise of transformation into
+stellar bodies of unusual magnitude. I say ``rings'' because although
+the loops encompassing the Andromeda Nebula have been called spirals
+by those who wish utterly to demolish Laplace's hypothesis, yet they
+are not manifestly such, as can be seen on comparing them with the
+undoubted spirals of the Lord Rosse Nebula. They look quite as much
+like circles or ellipses seen at an angle of, say, fifteen or twenty
+degrees to their plane. If they are truly elliptical they accord
+fairly well with Laplace's idea, except that the scale of magnitude is
+stupendous, and if the Andromeda Nebula is to become a solar system it
+will surpass ours in grandeur beyond all possibility of comparison.
+
+There is one circumstance connected with the spiral nebul�, and
+conspicuous in the Andromeda Nebula on account of its brightness,
+which makes the question of their origin still more puzzling; they all
+show continuous spectra, which, as we have before remarked, indicate
+that the mass from which the light comes is either solid or liquid, or
+a gas under heavy pressure. Thus nebul� fall into two classes: the
+``white'' nebul�, giving a continuous spectrum; and the ``green''
+nebul� whose spectra are distinctly gaseous. The Andromeda Nebula is
+the great representative of the former class and the Orion Nebula of
+the latter. The spectrum of the Andromeda Nebula has been interpreted
+to mean that it consists not of luminous gas, but of a flock of stars
+so distant that they are separately indistinguishable even with
+powerful telescopes, just as the component stars of the Milky Way are
+indistinguishable with the naked eye; and upon this has been based the
+suggestion that what we see in Andromeda is an outer universe whose
+stars form a series of elliptical garlands surrounding a central mass
+of amazing richness. But this idea is unacceptable if for no other
+reason than that, as just said, all the spiral nebul� possess the same
+kind of spectrum, and probably no one would be disposed to regard them
+all as outer universes. As we shall see later, the peculiarity of the
+spectra of the spiral nebul� is appealed to in support of a modern
+substitute for Laplace's hypothesis.
+
+Finally, without having by any means exhausted the variety exhibited
+by the spiral nebul�, let us turn to the great representative of the
+other species, the Orion Nebula. In some ways this is even more
+marvelous than the others. The early drawings with the telescope
+failed to convey an adequate conception either of its sublimity or of
+its complication of structure. It exists in a nebulous region of
+space, since photographs show that nearly the whole constellation is
+interwoven with faintly luminous coils. To behold the entry of the
+great nebula into the field even of a small telescope is a startling
+experience which never loses its novelty. As shown by the photographs,
+it is an inscrutable chaos of perfectly amazing extent, where spiral
+bands, radiating streaks, dense masses, and dark yawning gaps are
+strangely intermingled without apparent order. In one place four
+conspicuous little stars, better seen in a telescope than in the
+photograph on account of the blurring produced by over-exposure, are
+suggestively situated in the midst of a dark opening, and no observer
+has ever felt any doubt that these stars have been formed from the
+substance of the surrounding nebula. There are many other stars
+scattered over its expanse which manifestly owe their origin to the
+same source. But compare the general appearance of this nebula with
+the others that we have studied, and remark the difference. If the
+unmistakably spiral nebul� resemble bursting fly-wheels or grindstones
+from whose perimeters torrents of sparks are flying, the Orion Nebula
+rather recalls the aspect of a cloud of smoke and fragments produced
+by the explosion of a shell. This idea is enforced by the look of the
+outer portion farthest from the bright half of the nebula, where
+sharply edged clouds with dark spaces behind seem to be billowing away
+as if driven by a wind blowing from the center.
+
+Next let us consider what scientific speculation has done in the
+effort to explain these mysteries. Laplace's hypothesis can certainly
+find no standing ground either in the Orion Nebula or in those of a
+spiral configuration, whatever may be its situation with respect to
+the grand Nebula of Andromeda, or the ``ring'' and ``planetary''
+nebul�. Some other hypothesis more consonant with the appearances must
+be found. Among the many that have been proposed the most elaborate is
+the ``Planetesimal Hypothesis'' of Professors Chamberlin and Moulton.
+It is to be remarked that it applies to the spiral nebul�
+distinctively, and not to an apparently chaotic mass of gas like the
+vast luminous cloud in Orion. The gist of the theory is that these
+curious objects are probably the result of close approaches to each
+other of two independent suns, reminding us of what was said on this
+subject when we were dealing with temporary stars. Of the previous
+history of these appulsing suns the theory gives us no account; they
+are simply supposed to arrive within what may be called an effective
+tide-producing distance, and then the drama begins. Some of the
+probable consequences of such an approach have been noticed in Chapter
+5; let us now consider them a little more in detail.
+
+Tides always go in couples; if there is a tide on one side of a globe
+there will be a corresponding tide on the other side. The cause is to
+be found in the law that the force of gravitation varies inversely as
+the square of the distance; the attraction on the nearest surface of
+the body exercised by another body is greater than on its center, and
+greater yet than on its opposite surface. If two great globes attract
+each other, each tends to draw the other out into an ellipsoidal
+figure; they must be more rigid than steel to resist this -- and even
+then they cannot altogether resist. If they are liquid or gaseous they
+will yield readily to the force of distortion, the amount of which
+will depend upon their distance apart, for the nearer they are the
+greater becomes the tidal strain. If they are encrusted without and
+liquid or gaseous in the interior, the internal mass will strive to
+assume the figure demanded by the tidal force, and will, if it can,
+burst the restraining envelope. Now this is virtually the predicament
+of the body we call a sun when in the immediate presence of another
+body of similarly great mass. Such a body is presumably gaseous
+throughout, the component gases being held in a state of rigidity by
+the compression produced by the tremendous gravitational force of
+their own aggregate mass. At the surface such a body is enveloped in a
+shell of relatively cool matter. Now suppose a great attracting body,
+such as another sun, to approach near enough for the difference in its
+attraction on the two opposite sides of the body and on its center to
+become very great; the consequence will be a tidal deformation of the
+whole body, and it will lengthen out along the line of the
+gravitational pull and draw in at the sides, and if its shell offers
+considerable resistance, but not enough to exercise a complete
+restraint, it will be violently burst apart, or blown to atoms, and
+the internal mass will leap out on the two opposite sides in great
+fiery spouts. In the case of a sun further advanced in cooling than
+ours the interior might be composed of molten matter while the
+exterior crust had become rigid like the shell of an egg; then the
+force of the ``tidal explosion'' produced by the appulse of another
+sun would be more violent in consequence of the greater resistance
+overcome. Such, then, is the mechanism of the first phase in the
+history of a spiral nebula according to the Planetesimal Hypothesis.
+Two suns, perhaps extinguished ones, have drawn near together, and an
+explosive outburst has occured in one or both. The second phase calls
+for a more agile exercise of the imagination.
+
+To simplify the case, let us suppose that only one of the tugging suns
+is seriously affected by the strain. Its vast wings produced by the
+outburst are twisted into spirals by their rotation and the contending
+attractions exercised upon them, as the two suns, like battleships in
+desperate conflict, curve round each other, concentrating their
+destructive energies. Then immense quantities of d�bris are scattered
+about in which eddies are created, and finally, as the sun that caused
+the damage goes on its way, leaving its victim to repair its injuries
+as it may, the dispersed matter cools, condenses, and turns into
+streams of solid particles circling in elliptical paths about their
+parent sun. These particles, or fragments, are the ``planetesimals''
+of the theory. In consequence of the inevitable intersection of the
+orbits of the planetesimals, nodes are formed where the flying
+particles meet, and at these nodes large masses are gradually
+accumulated. The larger the mass the greater its attraction, and at
+last the nodal points become the nuclei of great aggregations from
+which planets are shaped.
+
+This, in very brief form, is the Planetesimal Hypothesis which we are
+asked to substitute for that based on Laplace's suggestion as an
+explanation of the mode of origin of the solar system; and the
+phenomena of the spiral nebul� are appealed to as offering evident
+support to the new hypothesis. We are reminded that they are
+elliptical in outline, which accords with the hypothesis; that their
+spectra are not gaseous, which shows that they may be composed of
+solid particles like the planetesimals; and that their central masses
+present an oval form, which is what would result from the tidal
+effects, as just described. We also remember that some of them, like
+the Lord Rosse and the Andromeda nebul�, are visually double, and in
+these cases we might suppose that the two masses represent the
+tide-burst suns that ventured into too close proximity. It may be
+added that the authors of the theory do not insist upon the appulse of
+two suns as the only way in which the planetesimals may have
+originated, but it is the only supposition that has been worked out.
+
+But serious questions remain. It needs, for instance, but a glance at
+the Triangulum monster to convince the observer that it cannot be a
+solar system which is being evolved there, but rather a swarm of
+stars. Many of the detached masses are too vast to admit of the
+supposition that they are to be transformed into planets, in our sense
+of planets, and the distances of the stars which appear to have been
+originally ejected from the focal masses are too great to allow us to
+liken the assemblage that they form to a solar system. Then, too, no
+nodes such as the hypothesis calls for are visible. Moreover, in most
+of the spiral nebul� the appearances favor the view that the
+supposititious encountering suns have not separated and gone each
+rejoicing on its way, after having inflicted the maximum possible
+damage on its opponent, but that, on the contrary, they remain in
+close association like two wrestlers who cannot escape from each
+other's grasp. And this is exactly what the law of gravitation
+demands; stars cannot approach one another with impunity, with regard
+either to their physical make-up or their future independence of
+movement. The theory undertakes to avoid this difficulty by assuming
+that in the case of our system the approach of the foreign body to the
+sun was not a close one -- just close enough to produce the tidal
+extrusion of the relatively insignificant quantity of matter needed to
+form the planets. But even then the effect of the appulse would be to
+change the direction of flight, both of the sun and of its visitor,
+and there is no known star in the sky which can be selected as the
+sun's probable partner in their ancient pas deux. That there are
+unconquered difficulties in Laplace's hypothesis no one would deny,
+but in simplicity of conception it is incomparably more satisfactory,
+and with proper modifications could probably be made more consonant
+with existing facts in our solar system than that which is offered to
+replace it. Even as an explanation of the spiral nebul�, not as solar
+systems in process of formation, but as the birthplaces of stellar
+clusters, the Planetesimal Hypothesis would be open to many
+objections. Granting its assumptions, it has undoubtedly a strong
+mathematical framework, but the trouble is not with the mathematics
+but with the assumptions. Laplace was one of the ablest mathematicians
+that ever lived, but he had never seen a spiral nebula; if he had, he
+might have invented a hypothesis to suit its phenomena. His actual
+hypothesis was intended only for our solar system, and he left it in
+the form of a ``note'' for the consideration of his successors, with
+the hope that they might be able to discover the full truth, which he
+confessed was hidden from him. It cannot be said that that truth has
+yet been found, and when it is found the chances are that intuition
+and not logic will have led to it.
+
+The spiral nebul�, then, remain among the greatest riddles of the
+universe, while the gaseous nebul�, like that of Orion, are no less
+mysterious, although it seems impossible to doubt that both forms give
+birth to stars. It is but natural to look to them for light on the
+question of the origin of our planetary system; but we should not
+forget that the scale of the phenomena in the two cases is vastly
+different, and the forces in operation may be equally different. A
+hill may have been built up by a glacier, while a mountain may be the
+product of volcanic forces or of the upheaval of the strata of the
+planet.
+
+The Banners of the Sun
+
+As all the world knows, the sun, a blinding globe pouring forth an
+inconceivable quantity of light and heat, whose daily passage through
+the sky is caused by the earth's rotation on its axis, constitutes the
+most important phenomenon of terrestial existence. Viewed with a dark
+glass to take off the glare, or with a telescope, its rim is seen to
+be a sharp and smooth circle, and nothing but dark sky is visible
+around it. Except for the interference of the moon, we should probably
+never have known that there is any more of the sun than our eyes
+ordinarily see.
+
+But when an eclipse of the sun occurs, caused by the interposition of
+the opaque globe of the moon, we see its immediate surroundings, which
+in some respects are more wonderful than the glowing central orb.
+These surroundings, although not in the sense in which we apply the
+term to the gaseous envelope of the earth, may be called the sun's
+atmosphere. They consist of two very different parts -- first, the red
+``prominences,'' which resemble tongues of flame ascending thousands
+of miles above the sun's surface; and, second, the ``corona,'' which
+extends to distances of millions of miles from the sun, and shines
+with a soft, glowing light. The two combined, when well seen, make a
+spectacle without parallel among the marvels of the sky. Although many
+attempts have been made to render the corona visible when there is no
+eclipse, all have failed, and it is to the moon alone that we owe its
+revelation. To cover the sun's disk with a circular screen will not
+answer the purpose because of the illumination of the air all about
+the observer. When the moon hides the sun, on the other hand, the
+sunlight is withdrawn from a great cylinder of air extending to the
+top of the atmosphere and spreading many miles around the observer.
+There is then no glare to interfere with the spectacle, and the corona
+appears in all its surprising beauty. The prominences, however,
+although they were discovered during an eclipse, can now, with the aid
+of the spectroscope, be seen at any time. But the prominences are
+rarely large enough to be noticed by the naked eye, while the
+streamers of the corona, stretching far away in space, like ghostly
+banners blown out from the black circle of the obscuring moon, attract
+every eye, and to this weird apparition much of the fear inspired by
+eclipses has been due. But if the corona has been a cause of terror in
+the past it has become a source of growing knowledge in our time.
+
+The story of the first scientific observation of the corona and the
+prominences is thrillingly interesting, and in fact dramatic. The
+observation was made during the eclipse of 1842, which fortunately was
+visible all over Central and Southern Europe so that scores of
+astronomers saw it. The interest centers in what happened at Pavia in
+Northern Italy, where the English astronomer Francis Baily had set up
+his telescope. The eclipse had begun and Bailey was busy at his
+telescope when, to quote his own words in the account which he wrote
+for the Memoirs of the Royal Astronomical Society:
+
+  I was astounded by a tremendous burst of applause from the streets
+  below, and at the same moment was electrified by the sight of one
+  of the most brilliant and splendid phenomena that can well be
+  imagined; for at that instant the dark body of the moon was
+  suddenly surrounded with a corona, or kind of bright glory, similar
+  in shape and magnitude to that which painters draw round the heads
+  of saints...
+
+  Pavia contains many thousand inhabitants, the major part of whom
+  were at this early hour walking about the streets and squares or
+  looking out of windows in order to witness this long-talked-of
+  phenomenon; and when the total obscuration took place, which was
+  instantaneous, there was a universal shout from every observer
+  which ``made the welkin ring,'' and for the moment withdrew my
+  attention from the object with which I was immediately occupied. I
+  had, indeed, expected the appearance of a luminous circle round the
+  moon during the time of total obscurity; but I did not expect, from
+  any of the accounts of preceding eclipses that I had read, to
+  witness so magnificent an exhibition as that which took place...
+
+  Splendid and astonishing, however, as this remarkable phenomenon
+  really was, and although it could not fail to call forth the
+  admiration and applause of every beholder, yet I must confess that
+  there was at the same time something in its singular and wonderful
+  appearance that was appalling...
+
+  But the most remarkable circumstance attending the phenomenon was
+  the appearance of three large protuberances apparently emanating
+  from the circumference of the moon, but evidently forming a portion
+  of the corona. They had the appearance of mountains of a prodigious
+  elevation; their color was red tinged with lilac or purple; perhaps
+  the color of the peach-blossom would more nearly represent it. They
+  somewhat resembled the tops of the snowy Alpine mountains when
+  colored by the rising or the setting sun. They resembled the Alpine
+  mountains in another respect, inasmuch as their light was perfectly
+  steady, and had none of that flickering or sparkling motion so
+  visible in other parts of the corona...
+
+  The whole of these protuberances were visible even to the last
+  moment of total obscuration, and when the first ray of light was
+  admitted from the sun they vanished, with the corona, altogether,
+  and daylight was instantly restored.
+
+I have quoted nearly all of this remarkable description not alone for
+its intrinsic interest, but because it is the best depiction that can
+be found of the general phenomena of a total solar eclipse. Still, not
+every such eclipse offers an equally magnificent spectacle. The
+eclipses of 1900 and 1905, for instance, which were seen by the
+writer, the first in South Carolina and the second in Spain, fell far
+short of that described by Bailey in splendor and impressiveness. Of
+course, something must be allowed for the effect of surprise; Bailey
+had not expected to see what was so suddenly disclosed to him. But
+both in 1900 and 1905 the amount of scattered light in the sky was
+sufficient in itself to make the corona appear faint, and there were
+no very conspicuous prominences visible. Yet on both occasions there
+was manifest among the spectators that mingling of admiration and awe
+of which Bailey speaks. The South Carolinians gave a cheer and the
+ladies waved their handkerchiefs when the corona, ineffably delicate
+of form and texture, melted into sight and then in two minutes melted
+away again. The Spaniards, crowded on the citadel hill of Burgos, with
+their king and his royal retinue in their midst, broke out with a
+great clapping of hands as the awaited spectacle unfolded itself in
+the sky; and on both occasions, before the applause began, after an
+awed silence a low murmur ran through the crowds. At Burgos it is said
+many made the sign of the cross.
+
+It was not long before Bailey's idea that the prominences were a part
+of the corona was abandoned, and it was perceived that the two
+phenomena were to a great extent independent. At the eclipse of 1868,
+which the astronomers, aroused by the wonderful scene of 1842, and
+eager to test the powers of the newly invented spectroscope, flocked
+to India to witness, Janssen conceived the idea of employing the
+spectroscope to render the prominences visible when there was no
+eclipse. He succeeded the very next day, and these phenomena have been
+studied in that way ever since.
+
+There are recognized two kinds of prominences -- the ``erruptive'' and
+the ``quiescent.'' The latter, which are cloud-like in form, may be
+seen almost anywhere along the edge of the sun; but the former, which
+often shoot up as if hurled from mighty volcanoes, appear to be
+associated with sun-spots, and appear only above the zones where spots
+abound. Either of them, when seen in projection against the brilliant
+solar disk, appears white, not red, as against a background of sky.
+The quiescent prominences, whose elevation is often from forty
+thousand to sixty thousand miles, consist, as the spectroscope shows,
+mainly of hydrogen and helium. The latter, it will be remembered, is
+an element which was known to be in the sun many years before the
+discovery that it also exists in small quantities on the earth. A fact
+which may have a significance which we cannot at present see is that
+the emanation from radium gradually and spontaneously changes into
+helium, an alchemistical feat of nature that has opened many curious
+vistas to speculative thinkers. The eruptive prominences, which do not
+spread horizontally like the others, but ascend with marvelous
+velocity to elevations of half a million miles or more, are apparently
+composed largely of metallic vapors -- i.e. metals which are usually
+solid on the earth, but which at solar temperatures are kept in a
+volatilized state. The velocity of their ascent occasionally amounts
+to three hundred or four hundred miles per second. It is known from
+mathematical considerations that the gravitation of the sun would not
+be able to bring back any body that started from its surface with a
+velocity exceeding three hundred and eighty-three miles per second; so
+it is evident that some of the matter hurled forth in eruptive
+prominences may escape from solar control and go speeding out into
+space, cooling and condensing into solid masses. There seems to be no
+reason why some of the projectiles from the sun might not reach the
+planets. Here, then, we have on a relatively small scale, explosions
+recalling those which it has been imagined may be the originating
+cause of some of the sudden phenomena of the stellar heavens.
+
+Of the sun-spots it is not our intention here specifically to speak,
+but they evidently have an intimate connection with eruptive
+prominences, as well as some relation, not yet fully understood, with
+the corona. Of the real cause of sun-spots we know virtually nothing,
+but recent studies by Professor Hale and others have revealed a
+strange state of things in the clouds of metallic vapors floating
+above them and their surroundings. Evidences of a cyclonic tendency
+have been found, and Professor Hale has proved that sun-spots are
+strong magnetic fields, and consist of columns of ionized vapors
+rotating in opposite directions in the two hemispheres. A fact which
+may have the greatest significance is that titanium and vanadium have
+been found both in sun-spots and in the remarkable variable Mira Ceti,
+a star which every eleven months, or thereabout, flames up with great
+brilliancy and then sinks back to invisibility with the naked eye. It
+has been suggested that sun-spots are indications of the beginning of
+a process in the sun which will be intensified until it falls into the
+state of such a star as Mira. Stars very far advanced in evolution,
+without showing variability, also exhibit similar spectra; so that
+there is much reason for regarding sunspots as emblems of advancing
+age.
+
+The association of the corona with sun-spots is less evident than that
+of the eruptive prominences; still such an association exists, for the
+form and extent of the corona vary with the sun-spot period of which
+we shall presently speak. The constitution of the corona remains to be
+discovered. It is evidently in part gaseous, but it also probably
+contains matter in the form of dust and small meteors. It includes one
+substance altogether mysterious -- ``coronium.'' There are reasons for
+thinking that this may be the lightest of all the elements, and
+Professor Young, its discoverer, said that it was ``absolutely unique
+in nature; utterly distinct from any other known form of matter,
+terrestial, solar, or cosmical.'' The enormous extent of the corona is
+one of its riddles. Since the development of the curious subject of
+the ``pressure of light'' it has been proposed to account for the
+sustentation of the corona by supposing that it is borne upon the
+billows of light continually poured out from the sun. Experiment has
+proved, what mathematical considerations had previously pointed out as
+probable, that the waves of light exert a pressure or driving force,
+which becomes evident in its effects if the body acted upon is
+sufficiently small. In that case the light pressure will prevail over
+the attraction of gravitation, and propel the attenuated matter away
+from the sun in the teeth of its attraction. The earth itself would be
+driven away if, instead of consisting of a solid globe of immense
+aggregate mass, it were a cloud of microscopic particles. The reason
+is that the pressure varies in proportion to the surface of the body
+acted upon, while the gravitational attraction is proportional to the
+volume, or the total amount of matter in the body. But the surface of
+any body depends upon the square of its diameter, while the volume
+depends upon the cube of the diameter. If, for instance, the diameter
+is represented by 4, the surface will be proportional to 4 � 4, or 16,
+and the volume to 4 � 4 � 4, or 64; but if the diameter is taken as 2,
+the surface will be 2 � 2, or 4, and the volume 2 � 2 � 2, or 8. Now,
+the ratio of 4 to 8 is twice as great as that of 16 to 64. If the
+diameter is still further decreased, the ratio of the surface to the
+volume will proportionally grow larger; in other words, the pressure
+will gain upon the attraction, and whatever their original ratio may
+have been, a time will come, if the diminution of size continues, when
+the pressure will become more effective than the attraction, and the
+body will be driven away. Supposing the particles of the corona to be
+below the critical size for the attraction of a mass like that of the
+sun to control them, they would be driven off into the surrounding
+space and appear around the sun like the clouds of dust around a mill.
+We shall return to this subject in connection with the Zodiacal Light,
+the Aurora, and Comets.
+
+On the other hand, there are parts of the corona which suggest by
+their forms the play of electric or magnetic forces. This is
+beautifully shown in some of the photographs that have been made of
+the corona during recent eclipses. Take, for instance, that of the
+eclipse of 1900. The sheaves of light emanating from the poles look
+precisely like the ``lines of force'' surrounding the poles of a
+magnet. It will be noticed in this photograph that the corona appears
+to consist of two portions: one comprising the polar rays just spoken
+of, and the other consisting of the broader, longer, and less-defined
+masses of light extending out from the equatorial and middle-latitude
+zones. Yet even in this more diffuse part of the phenomenon one can
+detect the presence of submerged curves bearing more or less
+resemblance to those about the poles. Just what part electricity or
+electro-magnetism plays in the mechanism of the solar radiation it is
+impossible to say, but on the assumption that it is a very important
+part is based the hypothesis that there exists a direct solar
+influence not only upon the magnetism, but upon the weather of the
+earth. This hypothesis has been under discussion for half a century,
+and still we do not know just how much truth it represents. It is
+certain that the outbreak of great disturbances on the sun,
+accompanied by the formation of sun-spots and the upshooting of
+eruptive prominences (phenomena which we should naturally expect to be
+attended by action), have been instantly followed by corresponding
+``magnetic storms'' on the earth and brilliant displays of the auroral
+lights. There have been occasions when the influence has manifested
+itself in the most startling ways, a great solar outburst being
+followed by a mysterious gripping of the cable and telegraph systems
+of the world, as if an invisible and irresistible hand had seized
+them. Messages are abruptly cut off, sparks leap from the telegraph
+instruments, and the entire earth seems to have been thrown into a
+magnetic flurry. These occurrences affect the mind with a deep
+impression of the dependence of our planet on the sun, such as we do
+not derive from the more familiar action of the sunlight on the growth
+of plants and other phenomena of life depending on solar influences.
+
+Perhaps the theory of solar magnetic influence upon the weather is
+best known in connection with the ``sun-spot cycle.'' This, at any
+rate, is, as already remarked, closely associated with the corona. Its
+existence was discovered in 1843 by the German astronomer Schwabe. It
+is a period of variable length, averaging about eleven years, during
+which the number of spots visible on the sun first increases to a
+maximum, then diminishes to a minimum, and finally increases again to
+a maximum. For unknown reasons the period is sometimes two or three
+years longer than the average and sometimes as much shorter.
+Nevertheless, the phenomena always recur in the same order. Starting,
+for instance, with a time when the observer can find few or no spots,
+they gradually increase in number and size until a maximum, in both
+senses, is reached, during which the spots are often of enormous size
+and exceedingly active. After two or three years they begin to
+diminish in number, magnitude, and activity until they almost or quite
+disappear. A strange fact is that when a new period opens, the spots
+appear first in high northern and southern latitudes, far from the
+solar equator, and as the period advances they not only increase in
+number and size, but break out nearer and nearer to the equator, the
+last spots of a vanishing period sometimes lingering in the equatorial
+region after the advance-guard of its successor has made its
+appearance in the high latitudes. Spots are never seen on the equator
+nor near the poles. It was not very long after the discovery of the
+sun-spot cycle that the curious observation was made that a striking
+coincidence existed between the period of the sun-spots and another
+period affecting the general magnetic condition of the earth. When a
+curved line representing the varying number of sun-spots was compared
+with another curve showing the variations in the magnetic state of the
+earth the two were seen to be in almost exact accord, a rise in one
+curve corresponding to a rise in the other, and a fall to a fall.
+Continued observation has proved that this is a real coincidence and
+not an accidental one, so that the connection, although as yet
+unexplained, is accepted as established. But does the influence extend
+further, and directly affect the weather and the seasons as well as
+the magnetic elements of the earth? A final answer to this question
+cannot yet be given, for the evidence is contradictory, and the
+interpretations put upon it depend largely on the predilections of the
+judges.
+
+But, in a broad sense, the sun-spots and the phenomena connected with
+them must have a relation to terrestial meteorology, for they prove
+the sun to be a variable star. Reference was made, a few lines above,
+to the resemblance of the spectra of sun-spots to those of certain
+stars which seem to be failing through age. This in itself is
+extremely suggestive; but if this resemblance had never been
+discovered, we should have been justified in regarding the sun as
+variable in its output of energy; and not only variable, but probably
+increasingly so. The very inequalities in the sun-spot cycle are
+suspicious. When the sun is most spotted its total light may be
+reduced by one-thousandth part, although it is by no means certain
+that its outgiving of thermal radiations is then reduced. A loss of
+one-thousandth of its luminosity would correspond to a decrease of
+.0025 of a stellar magnitude, considering the sun as a star viewed
+from distant space. So slight a change would not be perceptible; but
+it is not alone sun-spots which obscure the solar surface, its entire
+globe is enveloped with an obscuring veil. When studied with a
+powerful telescope the sun's surface is seen to be thickly mottled
+with relatively obscure specks, so numerous that it has been estimated
+that they cut off from one-tenth to one-twentieth of the light that we
+should receive from it if the whole surface were as brilliant as its
+brightest parts. The condition of other stars warrants the conclusion
+that this obscuring envelope is the product of a process of
+refrigeration which will gradually make the sun more and more variable
+until its history ends in extinction. Looking backward, we see a time
+when the sun must have been more brilliant than it is now. At that
+time it probably shone with the blinding white splendor of such stars
+as Sirius, Spica, and Vega; now it resembles the relatively dull
+Procyon; in time it will turn ruddy and fall into the closing cycle
+represented by Antares. Considering that once it must have been more
+radiantly powerful than at present, one is tempted to wonder if that
+could have been the time when tropical life flourished within the
+earth's polar circles, sustained by a vivific energy in the sun which
+it has now lost.
+
+The corona, as we have said, varies with the sun-spot cycle. When the
+spots are abundant and active the corona rises strong above the
+spotted zones, forming immense beams or streamers, which on one
+occasion, at least, had an observed length of ten million miles. At
+the time of a spot minimum the corona is less brilliant and has a
+different outline. It is then that the curved polar rays are most
+conspicuous. Thus the vast banners of the sun, shaken out in the
+eclipse, are signals to tell of its varying state, but it will
+probably be long before we can read correctly their messages.
+
+The Zodiacal Light Mystery
+
+There is a singular phenomenon in the sky -- one of the most puzzling
+of all -- which has long arrested the attention of astronomers,
+defying their efforts at explanation, but which probably not one in a
+hundred, and possibly not one in a thousand, of the readers of this
+book has ever seen. Yet its name is often spoken, and it is a
+conspicuous object if one knows when and where to look for it, and
+when well seen it exhibits a mystical beauty which at the same time
+charms and awes the beholder. It is called ``The Zodiacal Light,''
+because it lies within the broad circle of the Zodiac, marking the
+sun's apparent annual path through the stars. What it is nobody has
+yet been able to find out with certainty, and books on astronomy
+usually speak of it with singular reserve. But it has given rise to
+many remarkable theories, and a true explanation of it would probably
+throw light on a great many other celestial mysteries. The Milky Way
+is a more wonderful object to look upon, but its nature can be
+comprehended, while there is a sort of uncanniness about the Zodiacal
+Light which immediately impresses one upon seeing it, for its part in
+the great scheme of extra-terrestrial affairs is not evident.
+
+If you are out-of-doors soon after sunset -- say, on an evening late
+in the month of February -- you may perceive, just after the angry
+flush of the dying winter's day has faded from the sky, a pale ghostly
+presence rising above the place where the sun went down. The writer
+remembers from boyhood the first time it was pointed out to him and
+the unearthly impression that it made, so that he afterward avoided
+being out alone at night, fearful of seeing the spectral thing again.
+The phenomenon brightens slowly with the fading of the twilight, and
+soon distinctly assumes the shape of an elongated pyramid of pearly
+light, leaning toward the south if the place of observation is in the
+northern hemisphere. It does not impress the observer at all in the
+same manner as the Milky Way; that looks far off and is clearly among
+the stars, but the Zodiacal Light seems closer at hand, as if it were
+something more intimately concerning the earth. To all it immediately
+suggests a connection, also, with the sunken sun. If the night is
+clear and the moon absent (and if you are in the country, for city
+lights ruin the spectacles of the sky), you will be able to watch the
+apparition for a long time. You will observe that the light is
+brightest near the horizon, gradually fading as the pyramidal beam
+mounts higher, but in favorable circumstances it may be traced nearly
+to the meridian south of the zenith, where its apex at last vanishes
+in the starlight. It continues visible during the evenings of March
+and part of April, after which, ordinarily, it is seen no more, or if
+seen is relatively faint and unimpressive. But when autumn comes it
+appears again, this time not like a wraith hovering above the westward
+tomb of the day-god, but rather like a spirit of the morning
+announcing his reincarnation in the east.
+
+The reason why the Zodiacal Light is best seen in our latitudes at the
+periods just mentioned is because at those times the Zodiac is more
+nearly perpendicular to the horizon, first in the west and then in the
+east; and, since the phenomenon is confined within the borders of the
+Zodiac, it cannot be favorably placed for observation when the
+zodiacal plane is but slightly inclined to the horizon. Its faint
+light requires the contrast of a background of dark sky in order to be
+readily perceptible. But within the tropics, where the Zodiac is
+always at a favorable angle, the mysterious light is more constantly
+visible. Nearly all observant travelers in the equatorial regions have
+taken particular note of this phenomenon, for being so much more
+conspicuous there than in the temperate zones it at once catches the
+eye and holds the attention as a novelty. Humboldt mentions it many
+times in his works, for his genius was always attracted by things out
+of the ordinary and difficult of explanation, and he made many careful
+observations on its shape, its brilliancy, and its variations; for
+there can be no doubt that it does vary, and sometimes to an
+astonishing degree. It is said that it once remained practically
+invisible in Europe for several years in succession. During a trip to
+South Africa in 1909 an English astronomer, Mr E. W. Maunder, found a
+remarkable difference between the appearance of the Zodiacal Light on
+his going and coming voyages. In fact, when crossing the equator going
+south he did not see it at all; but on returning he had, on March 6th,
+when one degree south of the equator, a memorable view of it.
+
+  It was a bright, clear night, and the Zodiacal Light was
+  extraordinarily brilliant -- brighter than he had ever seen it
+  before. The Milky Way was not to be compared with it. The brightest
+  part extended 75� from the sun. There was a faint and much narrower
+  extension which they could just make out beyond the Pleiades along
+  the ecliptic, but the greater part of the Zodiacal Light showed as
+  a broad truncated column, and it did not appear nearly as conical
+  as he had before seen it.
+
+When out of the brief twilight of intertropical lands, where the sun
+drops vertically to the horizon and night rushes on like a wave of
+darkness, the Zodiacal Light shoots to the very zenith, its color is
+described as a golden tint, entirely different from the silvery sheen
+of the Milky Way. If I may venture again to refer to personal
+experiences and impressions, I will recall a view of the Zodiacal
+Light from the summit of the cone of Mt Etna in the autumn of the year
+1896 (more briefly described in Astronomy with the Naked Eye). There
+are few lofty mountains so favorably placed as Etna for observations
+of this kind. It was once resorted to by Prof. George E. Hale, in an
+attempt to see the solar corona without an eclipse. Rising directly
+from sea-level to an elevation of nearly eleven thousand feet, the
+observer on its summit at night finds himself, as it were, lost in the
+midst of the sky. But for the black flanks of the great cone on which
+he stands he might fancy himself to be in a balloon. On the occasion
+to which I refer the world beneath was virtually invisible in the
+moonless night. The blaze of the constellations overhead was
+astonishingly brilliant, yet amid all their magnificence my attention
+was immediately drawn to a great tapering light that sprang from the
+place on the horizon where the sun would rise later, and that seemed
+to be blown out over the stars like a long, luminous veil. It was the
+finest view of the Zodiacal light that I had ever enjoyed -- thrilling
+in its strangeness -- but I was almost disheartened by the
+indifference of my guide, to whom it was only a light and nothing
+more. If he had no science, he had less poetry -- rather a remarkable
+thing, I thought, for a child of his clime. The Light appeared to me
+to be distinctly brighter than the visible part of the Milky Way which
+included the brilliant stretches in Auriga and Perseus, and its color,
+if one may speak of color in connection with such an object, seemed
+richer than that of the galactic band; but I did not think of it as
+yellow, although Humboldt has described it as resembling a golden
+curtain drawn over the stars, and Du Chaillu in Equatorial Africa
+found it of a bright yellow color. It may vary in color as in
+conspicuousness. The fascination of that extraordinary sight has never
+faded from my memory. I turned to regard it again and again, although
+I had never seen the stellar heavens so brilliant, and it was one of
+the last things I looked for when the morning glow began softly to
+mount in the east, and Sicily and the Mediterranean slowly emerged
+from the profound shadow beneath us.
+
+The Zodiacal Light seems never to have attracted from astronomers in
+general the amount of careful attention that it deserves; perhaps
+because so little can really be made of it as far as explanation is
+concerned. I have referred to the restraint that scientific writers
+apparently feel in speaking of it. The grounds for speculation that it
+affords may be too scanty to lead to long discussions, yet it piques
+curiosity, and as we shall see in a moment has finally led to a most
+interesting theory. Once it was the subject of an elaborate series of
+studies which carried the observer all round the world. That was in
+1845--46, during the United States Exploring Expedition that visited
+the then little known Japan. The chaplain of the fleet, the Rev. Mr
+Jones, went out prepared to study the mysterious light in all its
+phases. He saw it from many latitudes on both sides of the equator,
+and the imagination cannot but follow him with keen interest in his
+world-circling tour, keeping his eyes every night fixed upon the
+phantasm overhead, whose position shifted with that of the hidden sun.
+He demonstrated that the flow extends at times completely across the
+celestial dome, although it is relatively faint directly behind the
+earth. On his return the government published a large volume of his
+observations, in which he undertook to show that the phenomenon was
+due to the reflection of sunlight from a ring of meteoric bodies
+encircling the earth. But, after all, this elaborate investigation
+settled nothing.
+
+Prof. E. E. Barnard has more recently devoted much attention to the
+Zodiacal Light, as well as to a strange attendant phenomenon called
+the ``Gegenschein,'' or Counterglow, because it always appears at that
+point in the sky which is exactly opposite the sun. The Gegenschein is
+an extremely elusive phenomenon, suitable only for eyes that have been
+specially trained to see it. Professor Newcomb has cautiously remarked
+that
+
+  it is said that in that point of the heavens directly opposite the
+  sun there is an elliptical patch of light... This phenomenon is so
+  difficult to account for that its existence is sometimes doubted;
+  yet the testimony in its favor is difficult to set aside.
+
+It certainly cannot be set aside at all since the observations of
+Barnard. I recall an attempt to see it under his guidance during a
+visit to Mount Hamilton, when he was occupied there with the Lick
+telescope. Of course, both the Gegenschein and the Zodiacal Light are
+too diffuse to be studied with telescopes, which, so to speak, magnify
+them out of existence. They can only be successfully studied with the
+naked eye, since every faintest glimmer that they afford must be
+utilized. This is especially true of the Gegenschein. At Mount
+Hamilton, Mr Barnard pointed out to me its location with reference to
+certain stars, but with all my gazing I could not be sure that I saw
+it. To him, on the contrary, it was obvious; he had studied it for
+months, and was able to indicate its shape, its boundaries, its
+diameter, and the declination of its center with regard to the
+ecliptic. There is not, of course, the shadow of a doubt of the
+existence of the Gegenschein, and yet I question if one person in a
+million has ever seen or ever will see it. The Zodiacal Light, on the
+other hand, is plain enough, provided that the time and the
+circumstances of the observation are properly chosen.
+
+In the attempts to explain the Zodiacal Light, the favorite hypothesis
+has been that it is an appendage of the sun -- perhaps simply an
+extension of the corona in the plane of the ecliptic, which is not
+very far from coinciding with that of the sun's equator. This idea is
+quite a natural one, because of the evident relation of the light to
+the position of the sun. The vast extension of the equatorial wings of
+the corona in 1878 gave apparent support to this hypothesis; if the
+substance of the corona could extend ten million miles from the sun,
+why might it not extend even one hundred million, gradually fading out
+beyond the orbit of the earth? A variation of this hypothesis assumes
+that the reflection is due to swarms of meteors circling about the
+sun, in the plane of its equator, all the way from its immediate
+neighborhood to a distance exceeding that of the earth. But in neither
+form is the hypothesis satisfactory; there is nothing in the
+appearance of the corona to indicate that it extends even as far as
+the planet Mercury, while as to meteors, the orbits of the known
+swarms do not accord with the hypothesis, and we have no reason to
+believe that clouds of others exist traveling in the part of space
+where they would have to be in order to answer the requirements of the
+theory. The extension of the corona in 1878 did not resemble in its
+texture the Zodiacal Light.
+
+Now, it has so often happened in the history of science that an
+important discovery in one branch has thrown unexpected but most
+welcome light upon some pending problem in some other branch, that a
+strong argument might be based upon that fact alone against the too
+exclusive devotion of many investigators to the narrow lines of their
+own particular specialty; and the Zodiacal Light affords a case in
+point, when it is considered in connection with recent discoveries in
+chemistry and physics. From the fact that atoms are compound bodies
+made up of corpuscles at least a thousand times smaller than the
+smallest known atom -- a fact which astounded most men of science when
+it was announced a few years ago -- a new hypothesis has been
+developed concerning the nature of the Zodiacal Light (as well as
+other astronomical riddles), and this hypothesis comes not from an
+astronomer, but from a chemist and physicist, the Swede, Svante
+Arrhenius. In considering an outline of this new hypothesis we need
+neither accept nor reject it; it is a case rather for suspension of
+judgment.
+
+To begin with, it carries us back to the ``pressure of light''
+mentioned in the preceding chapter. The manner in which this pressure
+is believed generally to act was there sufficiently explained, and it
+only remains to see how it is theoretically extended to the particles
+of matter supposed to constitute the Zodiacal Light. We know that
+corpuscles, or ``fragments of atoms'' negatively electrified, are
+discharged from hot bodies. Streams of these ``ions'' pour from many
+flames and from molten metals; and the impact of the cathode and
+ultra-violet rays causes them to gush even from cold bodies. In the
+vast laboratory of the sun it is but reasonable to suppose that
+similar processes are taking place. ``As a very hot metal emits these
+corpuscles,'' says Prof. J. J. Thomson, ``it does not seem an
+improbable hypothesis that they are emitted by that very hot body, the
+sun.'' Let it be assumed, then, that the sun does emit them; what
+happens next? Negatively charged corpuscles, it is known, serve as
+nuclei to which particles of matter in the ordinary state are
+attracted, and it is probable that those emitted from the sun
+immediately pick up loads in this manner and so grow in bulk. If they
+grow large enough the gravitation of the sun draws them back, and they
+produce a negative charge in the solar atmosphere. But it is probable
+that many of the particles do not attain the critical size which,
+according to the principles before explained, would enable the
+gravitation of the sun to retain them in opposition to the pressure of
+the waves of light, and with these particles the light pressure is
+dominant. Clouds of them may be supposed to be continually swept away
+from the sun into surrounding space, moving mostly in or near the
+plane of the solar equator, where the greatest activity, as indicated
+by sunspots and related phenomena, is taking place. As they pass
+outward into space many of them encounter the earth. If the earth,
+like the moon, had no atmosphere the particles would impinge directly
+on its surface, giving it a negative electric charge. But the presence
+of the atmosphere changes all that, for the first of the flying
+particles that encounter it impart to it their negative electricity,
+and then, since like electric charges repel like, the storm of
+particles following will be sheered off from the earth, and will
+stream around it in a maze of hyperbolic paths. Those that continue on
+into space beyond the earth may be expected to continue picking up
+wandering particles of matter until their bulk has become so great
+that the solar attraction prevails again over the light pressure
+acting upon them, and they turn again sunward. Passing the earth on
+their return they will increase the amount of dust-clouds careering
+round it; and these will be further increased by the action of the
+ultra-violet rays of the sunlight causing particles to shoot radially
+away from the earth when the negative charge of the upper atmosphere
+has reached a certain amount, which particles, although starting
+sunward, will be swept back to the earth with the oncoming streams. As
+the final result of all this accumulation of flying and gyrating
+particles in the earth's neighborhood, we are told that the latter
+must be transformed into the semblance of a gigantic solid-headed
+comet provided with streaming tails, the longest of them stretching
+away from the direction of the sun, while another shorter one extends
+toward the sun. This shorter tail is due to the particles that we have
+just spoken of as being driven sunward from the earth by the action of
+ultra-violet light. No doubt this whole subject is too technical for
+popular statement; but at any rate the general reader can understand
+the picturesque side of the theory, for its advocates assure us that
+if we were on the moon we would doubtless be able to see the
+comet-like tails of the earth, and then we could appreciate the part
+that they play in producing the phenomenon of the Zodiacal Light.
+
+That the Light as we see it could be produced by the reflection of
+sunlight from swarms of particles careering round the earth in the
+manner supposed by Arrhenius' hypothesis is evident enough; and it
+will be observed that the new theory, after all, is only another
+variant of the older one which attributes the Zodiacal Light to an
+extension of the solar corona. But it differs from the older theory in
+offering an explanation of the manner in which the extension is
+effected, and it differentiates between the corona proper and the
+streams of negative particles shot away from the sun. In its details
+the hypothesis of Arrhenius also affords an explanation of many
+peculiarities of the Zodiacal Light, such as that it is confined to
+the neighborhood of the ecliptic, and that it is stronger on the side
+of the earth which is just turning away from a position under the sun
+than on the other side; but it would carry us beyond our limits to go
+into these particulars. The Gegenschein, according to this theory, is
+a part of the same phenomenon as the Zodiacal Light, for by the laws
+of perspective it is evident that the reflection from the streams of
+particles situated at a point directly opposite to the sun would be at
+a maximum, and this is the place which the Gegenschein occupies. Apart
+from its geometrical relations to the position of the sun, the
+variability of the Zodiacal Light appears to affirm its solar
+dependence, and this too would be accounted for by Arrhenius'
+hypothesis better than by the old theory of coronal extension. The
+amount of corpuscular discharge from the sun must naturally be
+governed by the state of relative activity or inactivity of the
+latter, and this could not but be reflected in the varying splendor of
+the Zodiacal Light. But much more extended study than has yet been
+given to the subject will be required before we can feel that we know
+with reasonable certainty what this mysterious phenomenon really is.
+By the hypothesis of Arrhenius every planet that has an atmosphere
+must have a Zodiacal Light attending it, but the phenomenon is too
+faint for us to be able to see it in the case, for instance, of Venus,
+whose atmosphere is very abundant. The moon has no corresponding
+``comet's tail'' because, as already explained, of the lack of a lunar
+atmosphere to repel the streams by becoming itself electrified; but if
+there were a lunar Zodiacal Light, no doubt we could see it because of
+the relative nearness of our satellite.
+
+Marvels of the Aurora
+
+One of the most vivid recollections of my early boyhood is that of
+seeing my father return hastily into the house one evening and call
+out to the family: ``Come outside and look at the sky!'' Ours was a
+country house situated on a commanding site, and as we all emerged
+from the doorway we were dumbfounded to see the heavens filled with
+pale flames which ran licking and quivering over the stars. Instantly
+there sprang into my terrified mind the recollection of an awful
+description of ``the Day of Judgment'' (the Dies Ir�), which I had
+heard with much perturbation of spirit in the Dutch Reformed church
+from the lips of a tall, dark-browed, dreadfully-in-earnest preacher
+of the old-fashioned type. My heart literally sank at sight of the
+spectacle, for it recalled the preacher's very words; it was just as
+he had said it would be, and it needed the assured bearing of my
+elders finally to convince me that
+
+  That Day of Wrath, O dreadful day,
+  When Heaven and Earth shall pass away,
+  As David and the Sibyl say
+
+had not actually come upon us. And even the older members of the
+household were not untouched with misgivings when menacing spots of
+crimson appeared, breaking out now here, now there, in the shuddering
+sky. Toward the north the spectacle was appalling. A huge arch spanned
+an unnaturally dark segment resting on the horizon, and above this
+arch sprang up beams and streamers in a state of incessant agitation,
+sometimes shooting up to the zenith with a velocity that took one's
+breath, and sometimes suddenly falling into long ranks, and marching,
+marching, marching, like an endless phalanx of fiery specters, and
+moving, as I remember, always from east to west. The absolute silence
+with which these mysterious evolutions were performed and the
+quavering reflections which were thrown upon the ground increased the
+awfulness of the exhibition. Occasionally enormous curtains of lambent
+flame rolled and unrolled with a majestic motion, or were shaken to
+and fro as if by a mighty, noiseless wind. At times, too, a sudden
+billowing rush would be made toward the zenith, and for a minute the
+sky overhead would glow so brightly that the stars seemed to have been
+consumed. The spectacle continued with varying intensity for hours.
+
+This exhibition occurred in Central New York, a latitude in which the
+Aurora Borealis is seldom seen with so much splendor. I remember
+another similar one seen from the city of New York in November, 1882.
+On this last occasion some observers saw a great upright beam of light
+which majestically moved across the heavens, stalking like an
+apparition in the midst of the auroral pageant, of whose general
+movements it seemed to be independent, maintaining always its upright
+posture, and following a magnetic parallel from east to west. This
+mysterious beam was seen by no less than twenty-six observers in
+different parts of the country, and a comparison of their observations
+led to a curious calculation indicating that the apparition was about
+one hundred and thirty-three miles tall and moved at the speed of ten
+miles per second!
+
+But, as everybody knows, it is in the Arctic regions that the Aurora,
+or the ``Northern Lights,'' can best be seen. There, in the long polar
+night, when for months together the sun does not rise, the strange
+coruscations in the sky often afford a kind of spectral daylight in
+unison with the weird scenery of the world of ice. The pages in the
+narratives of Arctic exploration that are devoted to descriptions of
+the wonderful effects of the Northern Lights are second to none that
+man has ever penned in their fascination. The lights, as I have
+already intimated, display astonishing colors, particularly shades of
+red and green, as they flit from place to place in the sky. The
+discovery that the magnetic needle is affected by the Aurora,
+quivering and darting about in a state of extraordinary excitement
+when the lights are playing in the sky, only added to the mystery of
+the phenomenon until its electro-magnetic nature had been established.
+This became evident as soon as it was known that the focus of the
+displays was the magnetic pole; and when the far South was visited the
+Aurora Australis was found, having its center at the South Magnetic
+Pole. Then, if not before, it was clear that the earth was a great
+globular magnet, having its poles of opposite magnetism, and that the
+auroral lights, whatever their precise cause might be, were
+manifestations of the magnetic activity of our planet. After the
+invention of magnetic telegraphy it was found that whenever a great
+Aurora occurred the telegraph lines were interrupted in their
+operation, and the ocean cables ceased to work. Such a phenomenon is
+called a ``magnetic storm.''
+
+The interest excited by the Aurora in scientific circles was greatly
+stimulated when, in the last half of the nineteenth century, it was
+discovered that it is a phenomenon intimately associated with
+disturbances on the sun. The ancient ``Zurich Chronicles,'' extending
+from the year 1000 to the year 1800, in which both sun-spots visible
+to the naked eye and great displays of the auroral lights were
+recorded, first set Rudolf Wolf on the track of this discovery. The
+first notable proof of the suspected connection was furnished with
+dramatic emphasis by an occurrence which happened on September 1,
+1859. Near noon on that day two intensely brilliant points suddenly
+broke out in a group of sun-spots which were under observation by Mr
+R. C. Carrington at his observatory at Redhill, England. The points
+remained visible for not more than five minutes, during which interval
+they moved thirty-five thousand miles across the solar disk. Mr R.
+Hodgson happened to see the same phenomenon at his observatory at
+Highgate, and thus all possibility of deception was removed. But
+neither of the startled observers could have anticipated what was to
+follow, and, indeed, it was an occurrence which has never been
+precisely duplicated. I quote the eloquent account given by Miss
+Clerke in her History of Astronomy During the Nineteenth Century.
+
+  This unique phenomenon seemed as if specially designed to
+  accentuate the inference of a sympathetic relation between the
+  earth and the sun. From August 28 to September 4, 1859, a magnetic
+  storm of unparalleled intensity, extent, and duration was in
+  progress over the entire globe. Telegraphic communication was
+  everywhere interrupted -- except, indeed, that it was in some cases
+  found practicable to work the lines without batteries by the agency
+  of the earth-currents alone; sparks issued from the wires; gorgeous
+  auroras draped the skies in solemn crimson over both hemispheres,
+  and even in the tropics; the magnetic needle lost all trace of
+  continuity in its movements and darted to and fro as if stricken
+  with inexplicable panic. The coincidence was even closer. At the
+  very instant of the solar outburst witnessed by Carrington and
+  Hodgson the photographic apparatus at Kew registered a marked
+  disturbance of all the three magnetic elements; while shortly after
+  the ensuing midnight the electric agitation culminated, thrilling
+  the whole earth with subtle vibrations, and lighting up the
+  atmosphere from pole to pole with coruscating splendors which
+  perhaps dimly recall the times when our ancient planet itself shone
+  as a star.
+
+If this amazing occurrence stood alone, and as I have already said it
+has never been exactly duplicated, doubt might be felt concerning some
+of the inferences drawn from it; but in varying forms it has been
+repeated many times, so that now hardly anyone questions the reality
+of the assumed connection between solar outbursts and magnetic storms
+accompanied by auroral displays on the earth. It is true that the late
+Lord Kelvin raised difficulties in the way of the hypothesis of a
+direct magnetic action of the sun upon the earth, because it seemed to
+him that an inadmissible quantity of energy was demanded to account
+for such action. But no calculation like that which he made is final,
+since all calculations depend upon the validity of the data; and no
+authority is unshakable in science, because no man can possess
+omniscience. It was Lord Kelvin who, but a few years before the thing
+was actually accomplished, declared that aerial navigation was an
+impracticable dream, and demonstrated its impracticability by
+calculation. However the connection may be brought about, it is as
+certain as evidence can make it that solar outbursts are coincident
+with terrestial magnetic disturbances, and coincident in such a way as
+to make the inference of a causal connection irresistible. The sun is
+only a little more than a hundred times its own diameter away from the
+earth. Why, then, with the subtle connection between them afforded by
+the ether which conveys to us the blinding solar light and the
+life-sustaining solar heat, should it be so difficult to believe that
+the sun's enormous electric energies find a way to us also? No doubt
+the impulse coming from the sun acts upon the earth after the manner
+of a touch upon a trigger, releasing energies which are already stored
+up in our planet.
+
+But besides the evidence afforded by such occurrences as have been
+related of an intimate connection between solar outbreaks and
+terrestial magnetic flurries, attended by magnificent auroral
+displays, there is another line of proof pointing in the same
+direction. Thus, it is known that the sun-spot period, as remarked in
+a preceding chapter, coincides in a most remarkable manner with the
+periodic fluctuations in the magnetic state of the earth. This
+coincidence runs into the most astonishing details. For instance, when
+the sun-spot period shortens, the auroral period shortens to precisely
+the same extent; as the short sun-spot periods usually bring the most
+intense outbreaks of solar activity, so the corresponding short
+auroral periods are attended by the most violent magnetic storms; a
+secular period of about two hundred and twenty-two years affecting
+sun-spots is said to have its auroral duplicate; a shorter period of
+fifty-five and a half years, which some observers believe that they
+have discovered appears also to be common to the two phenomena; and
+yet another ``superposed'' period of about thirty-five years, which
+some investigators aver exists, affects sun-spots and aurora alike. In
+short, the coincidences are so numerous and significant that one would
+have to throw the doctrine of probability to the winds in order to be
+able to reject the conclusion to which they so plainly lead.
+
+But still the question recurs: How is the influence transmitted? Here
+Arrhenius comes once more with his hypothesis of negative corpuscles,
+or ions, driven away from the sun by light-pressure -- a hypothesis
+which seems to explain so many things -- and offers it also as an
+explanation of the way in which the sun creates the Aurora. He would
+give the Aurora the same lineage with the Zodiacal Light. To
+understand the application of this theory we must first recall the
+fact that the earth is a great magnet having its two opposite poles of
+magnetism, one near the Arctic and the other near the Antarctic
+Circle. Like all magnets, the earth is surrounded with ``lines of
+force,'' which, after the manner of the curved rays we saw in the
+photograph of a solar eclipse, start from a pole, rising at first
+nearly vertically, then bend gradually over, passing high above the
+equator, and finally descending in converging sheaves to the opposite
+pole. Now the axis of the earth is so placed in space that it lies at
+nearly a right angle to the direction of the sun, and as the streams
+of negatively charged particles come pouring on from the sun (see the
+last preceding chapter), they arrive in the greatest numbers over the
+earth's equatorial regions. There they encounter the lines of magnetic
+force at the place where the latter have their greatest elevation
+above the earth, and where their direction is horizontal to the
+earth's surface. Obeying a law which has been demonstrated in the
+laboratory, the particles then follow the lines of force toward the
+poles. While they are above the equatorial regions they do not become
+luminescent, because at the great elevation that they there occupy
+there is virtually no atmosphere; but as they pass on toward the north
+and the south they begin to descend with the lines of force, curving
+down to meet at the poles; and, encountering a part of the atmosphere
+comparable in density with what remains in an exhausted Crookes tube,
+they produce a glow of cathode rays. This glow is conceived to
+represent the Aurora, which may consequently be likened to a gigantic
+exhibition of vacuum-tube lights. Anybody who recalls his student days
+in the college laboratory and who has witnessed a display of Northern
+Lights will at once recognize the resemblance between them in colors,
+forms, and behavior. This resemblance had often been noted before
+Arrhenius elaborated his hypothesis.
+
+Without intending to treat his interesting theory as more than a
+possibly correct explanation of the phenomena of the Aurora, we may
+call attention to some apparently confirmatory facts. One of the most
+striking of these relates to a seasonal variation in the average
+number of auror�. It has been observed that there are more in March
+and September than at any other time of the year, and fewer in June
+and December; moreover (and this is a delicate test as applied to the
+theory), they are slightly rarer in June than in December. Now all
+these facts seem to find a ready explanation in the hypothesis of
+Arrhenius, thus: (1) The particles issuing from the sun are supposed
+to come principally from the regions whose excitement is indicated by
+the presence of sun-spots (which accords with Hale's observation that
+sun-spots are columns of ionized vapors), and these regions have a
+definite location on either side of the solar equator, seldom
+approaching it nearer than within 5� or 10� north or south, and never
+extending much beyond 35� toward either pole; (2) The equator of the
+sun is inclined about 7� to the plane of the earth's orbit, from which
+it results that twice in a year -- viz., in June and December -- the
+earth is directly over the solar equator, and twice a year -- viz., in
+March and September -- when it is farthest north or south of the solar
+equator, it is over the inner edge of the sun-spot belts. Since the
+corpuscles must be supposed to be propelled radially from the sun, few
+will reach the earth when the latter is over the solar equator in June
+and December, but when it is over, or nearly over, the spot belts, in
+March and September, it will be in the line of fire of the more active
+parts of the solar surface, and relatively rich streams of particles
+will reach it. This, as will be seen from what has been said above, is
+in strict accord with the observed variations in the frequency of
+auror�. Even the fact that somewhat fewer auror� are seen in June than
+in December also finds its explanation in the known fact that the
+earth is about three million miles nearer the sun in the winter than
+in the summer, and the number of particles reaching it will vary, like
+the intensity of light, inversely as the square of the distance. These
+coincidences are certainly very striking, and they have a cumulative
+force. If we accept the theory, it would appear that we ought to
+congratulate ourselves that the inclination of the sun's equator is so
+slight, for as things stand the earth is never directly over the most
+active regions of the sun-spots, and consequently never suffers from
+the maximum bombardment of charged particles of which the sun is
+capable. Incessant auroral displays, with their undulating draperies,
+flitting colors, and marching columns might not be objectionable from
+the point of view of picturesqueness, but one magnetic storm of
+extreme intensity following closely upon the heels of another, for
+months on end, crazing the magnetic needle and continually putting the
+telegraph and cable lines out of commission, to say nothing of their
+effect upon ``wireless telegraphy'', would hardly add to the charms of
+terrestrial existence.
+
+One or two other curious points in connection with Arrhenius'
+hypothesis may be mentioned. First, the number of auror�, according to
+his explanation, ought to be greatest in the daytime, when the face of
+the earth on the sunward side is directly exposed to the atomic
+bombardment. Of course visual observation can give us no information
+about this, since the light of the Aurora is never sufficiently
+intense to be visible in the presence of daylight, but the records of
+the magnetic observatories can be, and have been, appealed to for
+information, and they indicate that the facts actually accord with the
+theory. Behind the veil of sunlight in the middle of the afternoon,
+there is good reason to believe, auroral exhibitions often take place
+which would eclipse in magnificence those seen at night if we could
+behold them. Observation shows, too, that auror� are more frequent
+before than after midnight, which is just what we should expect if
+they originate in the way that Arrhenius supposes. Second, the theory
+offers an explanation of the alleged fact that the formation of clouds
+in the upper air is more frequent in years when auror� are most
+abundant, because clouds are the result of the condensation of
+moisture upon floating particles in the atmosphere (in an absolutely
+dustless atmosphere there would be no clouds), and it has been proved
+that negative ions like those supposed to come from the sun play a
+master part in the phenomena of cloud formation.
+
+Yet another singular fact, almost mystical in its suggestions, may be
+mentioned. It seems that the dance of the auroral lights occurs most
+frequently during the absence of the moon from the hemisphere in which
+they appear, and that they flee, in greater part, to the opposite
+hemisphere when the moon's revolution in an orbit considerably
+inclined to the earth's equator brings her into that where they have
+been performing. Arrhenius himself discovered this curious relation of
+auroral frequency to the position of the moon north or south of the
+equator, and he explains it in this way. The moon, like the earth, is
+exposed to the influx of the ions from the sun; but having no
+atmosphere, or almost none, to interfere with them, they descend
+directly upon her surface and charge her with an electric negative
+potential to a very high degree. In consequence of this she affects
+the electric state of the upper parts of the earth's atmosphere where
+they lie most directly beneath her, and thus prevents, to a large
+extent, the negative discharges to which the appearance of the Aurora
+is due. And so ``the extravagant and erring spirit'' of the Aurora
+avoids the moon as Hamlet's ghost fled at the voice of the cock
+announcing the awakening of the god of day.
+
+There are even other apparent confirmations of the hypothesis, but we
+need not go into them. We shall, however, find one more application of
+it in the next chapter, for it appears to be a kind of cure-all for
+astronomical troubles; at any rate it offers a conceivable solution of
+the question, How does the sun manage to transmit its electric
+influence to the earth? And this solution is so grandiose in
+conception, and so novel in the mental pictures that it offers, that
+its acceptance would not in the least detract from the impression that
+the Aurora makes upon the imagination.
+
+Strange Adventures of Comets
+
+The fears and legends of ancient times before Science was born, and
+the superstitions of the Dark Ages, sedulously cultivated for
+theological purposes by monks and priests, have so colored our ideas
+of the influence that comets have had upon the human mind that many
+readers may be surprised to learn that it was the apparition of a
+wonderful comet, that of 1843, which led to the foundation of our
+greatest astronomical institution, the Harvard College Observatory. No
+doubt the comet superstition existed half a century ago, as, indeed,
+it exists yet today, but in this case the marvelous spectacle in the
+sky proved less effective in inspiring terror than in awakening a
+desire for knowledge. Even in the sixteenth century the views that
+enlightened minds took of comets tended powerfully to inspire popular
+confidence in science, and Halley's prediction, after seeing and
+studying the motion of the comet which appeared in 1682, that it would
+prove to be a regular member of the sun's family and would be seen
+returning after a period of about seventy-six years, together with the
+fulfillment of that prediction, produced a revulsion from the
+superstitious notions which had so long prevailed.
+
+Then the facts were made plain that comets are subject to the law of
+gravitation equally with the planets; that there are many which
+regularly return to the neighborhood of the sun (perihelion); and that
+these travel in orbits differing from those of the planets only in
+their greater eccentricity, although they have the peculiarity that
+they do not, like the planets, all go round the sun in the same
+direction, and do not keep within the general plane of the planetary
+system, but traverse it sometimes from above and sometimes from below.
+Other comets, including most of the ``great'' ones, appear to travel
+in parabolic or, in a few cases, hyperbolic orbits, which, not being
+closed curves, never bring them back again. But it is not certain that
+these orbits may not be extremely eccentric ellipses, and that after
+the lapse of hundreds, or thousands, of years the comets that follow
+them may not reappear. The question is an interesting one, because if
+all orbits are really ellipses, then all comets must be permanent
+members of the solar system, while in the contrary case many of them
+are simply visitors, seen once and never to be seen again. The
+hypothesis that comets are originally interlopers might seem to derive
+some support from the fact that the certainly periodic ones are
+associated, in groups, with the great outer planets, whose attraction
+appears to have served as a trap for them by turning them into
+elliptical orbits and thus making them prisoners in the solar system.
+Jupiter, owing to his great mass and his commanding situation in the
+system, is the chief ``comet-catcher;'' but he catches them not for
+himself, but for the sun. Yet if comets do come originally from
+without the borders of the planetary system, it does not, by any
+means, follow that they were wanderers at large in space before they
+yielded to the overmastering attraction of the sun. Investigation of
+the known cometary orbits, combined with theoretical considerations,
+has led some astronomers to the conclusion that as the sun travels
+onward through space he ``picks up en route'' cometary masses which,
+without belonging strictly to his empire, are borne along in the same
+vast ``cosmical current'' that carries the solar system.
+
+But while no intelligent person any longer thinks that the appearance
+of a great comet is a token from the heavenly powers of the
+approaching death of a mighty ruler, or the outbreak of a devastating
+war, or the infliction of a terrible plague upon wicked mankind,
+science itself has discovered mysteries about comets which are not
+less fascinating because they are more intellectual than the
+irrational fancies that they have displaced. To bring the subject
+properly before the mind, let us see what the principal phenomena
+connected with a comet are.
+
+At the present day comets are ordinarily ``picked up'' with the
+telescope or the photographic plate before any one except their
+discoverer is aware of their existence, and usually they remain so
+insignificant in appearance that only astronomers ever see them. Yet
+so great is the prestige of the word ``comet'' that the discovery of
+one of these inconspicuous wanderers, and its subsequent movements,
+become items of the day's news which everybody reads with the feeling,
+perhaps, that at least he knows what is going on in the universe even
+if he doesn't understand it. But a truly great comet presents quite a
+different proposition. It, too, is apt to be detected coming out of
+the depths of space before the world at large can get a glimpse of it,
+but as it approaches the sun its aspect undergoes a marvelous change.
+Agitated apparently by solar influence, it throws out a long streaming
+tail of nebulous light, directed away from the sun and looking as if
+blown out like a pennon by a powerful wind. Whatever may be the
+position of the comet with regard to the sun, as it circles round him
+it continually keeps its tail on the off side. This, as we shall soon
+see, is a fact of capital importance in relation to the probable
+nature of comets' tails. Almost at the same time that the formation of
+the tail is observed a remarkable change takes place in the comet's
+head, which, by the way, is invariably and not merely occasionally its
+most important part. On approaching the sun the head usually
+contracts. Coincidently with this contraction a nucleus generally
+makes its appearance. This is a bright, star-like point in the head,
+and it probably represents the totality of solid matter that the comet
+possesses. But it is regarded as extremely unlikely that even the
+nucleus consists of a uniformly solid mass. If it were such, comets
+would be far more formidable visitors when they pass near the planets
+than they have been found to be. The diameter of the nucleus may vary
+from a few hundred up to several thousand miles; the heads, on the
+average, are from twenty-five thousand to one hundred thousand miles
+in diameter, although a few have greatly exceeded these dimensions;
+that of the comet of 1811, one of the most stupendous ever seen, was a
+million and a quarter miles in diameter! As to the tails, not
+withstanding their enormous length -- some have been more than a
+hundred million miles long -- there is reason to believe that they are
+of extreme tenuity, ``as rare as vacuum.'' The smallest stars have
+been seen shining through their most brilliant portions with
+undiminished luster.
+
+After the nucleus has been formed it begins to throw out bright jets
+directed toward the sun. A stream, and sometimes several streams, of
+light also project sunward from the nucleus, occasionally appearing
+like a stunted tail directed oppositely to the real tail. Symmetrical
+envelopes which, seen in section, appear as half circles or parabolas,
+rise sunward from the nucleus, forming a concentric series. The ends
+of these stream backward into the tail, to which they seem to supply
+material. Ordinarily the formation of these ejections and envelopes is
+attended by intense agitation of the nucleus, which twists and turns,
+swinging and gyrating with an appearance of the greatest violence.
+Sometimes the nucleus is seen to break up into several parts. The
+entire heads of some comets have been split asunder in passing close
+around the sun; The comet of 1882 retreated into space after its
+perihelion passage with five heads instead of the one that it had
+originally, and each of these heads had its own tail!
+
+The possession of the spectroscope has enabled astronomers during
+later years to study the chemical composition of comets by analyzing
+their light. At first the only substances thus discovered in them were
+hydro-carbon compounds, due evidently to the gaseous envelopes in
+which some combination of hydrogen with carbon existed. Behind this
+gaseous spectrum was found a faint continuous spectrum ascribed to the
+nucleus, which apparently both reflects the sunlight and gives forth
+the light of a glowing solid or liquid. Subsequently sodium and iron
+lines were found in cometary spectra. The presence of iron would seem
+to indicate that some of these bodies may be much more massive than
+observations on their attractive effects have indicated. In some
+recent comets, such as Morehouse's, in 1908, several lines have been
+found, the origin of which is unknown.
+
+Without going back of the nineteenth century we may find records of
+some of the most extraordinary comets that man has ever looked upon.
+In 1811, still spoken of as ``the year of the comet,'' because of the
+wonderful vintage ascribed to the skyey visitor, a comet shaped like a
+gigantic sword amazed the whole world, and, as it remained visible for
+seventeen months, was regarded by superstitious persons as a symbol of
+the fearful happenings of Napoleon's Russian campaign. This comet, the
+extraordinary size of whose head, greatly exceeding that of the sun
+itself, has already been mentioned, was also remarkable for exhibiting
+so great a brilliancy without approaching even to the earth's distance
+from the sun. But there was once a comet (and only once -- in the year
+1729) which never got nearer to the sun than four times the distance
+of the earth and yet appeared as a formidable object in the sky. As
+Professor Young has remarked, ``it must have been an enormous comet to
+be visible from such a distance.'' And we are to remember that there
+were no great telescopes in the year 1729. That comet affects the
+imagination like a phantom of space peering into the solar system,
+displaying its enormous train afar off (which, if it had approached as
+near as other comets, would probably have become the celestial wonder
+of all human memory), and then turning away and vanishing in the
+depths of immensity.
+
+In 1843 a comet appeared which was so brilliant that it could be seen
+in broad day close beside the sun! This was the first authenticated
+instance of that kind, but the occurrence was to be repeated, as we
+shall see in a moment, less than forty years later.
+
+The splendid comet of 1858, usually called Donati's, is remembered by
+many persons yet living. It was, perhaps, both as seen by the naked
+eye and with the telescope, the most beautiful comet of which we have
+any record. It too marked a rich vintage year, still remembered in the
+vineyards of France, where there is a popular belief that a great
+comet ripens the grape and imparts to the wine a flavor not attainable
+by the mere skill of the cultivator. There are ``comet wines,''
+carefully treasured in certain cellars, and brought forth only when
+their owner wishes to treat his guests to a sip from paradise.
+
+The year 1861 saw another very remarkable comet, of an aspect
+strangely vast and diffuse, which is believed to have swept the earth
+with its immense tail when it passed between us and the sun on the
+night of June 30th, an event which produced no other known effect than
+the appearance of an unwonted amount of scattered light in the sky.
+
+The next very notable comet was the ``Great Southern Comet'' of 1880,
+which was not seen from the northern hemisphere. It mimicked the
+aspect of the famous comet of 1843, and to the great surprise of
+astronomers appeared to be traveling in the same path. This proved to
+be the rising of the curtain for an astronomical sensation
+unparalleled in its kind; for two years later another brilliant comet
+appeared, first in the southern hemisphere, and it too followed the
+same track. The startling suggestion was now made that this comet was
+identical with those of 1843 and 1880, its return having been hastened
+by the resistance experienced in passing twice through the coronal
+envelope, and there were some who thought that it would now swing
+swiftly round and then plunge straight into the sun, with consequences
+that might be disastrous to us on account of the ``flash of heat''
+that would be produced by the impact. Nervous people were frightened,
+but observation soon proved that the danger was imaginary, for
+although the comet almost grazed the sun, and must have rushed through
+two or three million miles of the coronal region, no retardation of
+its immense velocity was perceptible, and it finally passed away in a
+damaged condition, as before remarked, and has never since appeared.
+
+Then the probable truth was perceived -- viz., that the three comets
+(1843, 1880, and 1882) were not one identical body, but three separate
+ones all traveling in the same orbit. It was found, too, that a comet
+seen in 1668 bore similar insignia of relationship. The natural
+inference was that these four bodies had once formed a single mass
+which had been split apart by the disruptive action of the sun.
+Strength was lent to this hypothesis by the fact that the comet of
+1882 was apparently torn asunder during its perihelion passage,
+retreating into space in a dissevered state. But Prof. George Forbes
+has a theory that the splitting of the original cometary mass was
+effected by an unknown planet, probably greater than Jupiter, situated
+at a hundred times the earth's distance from the sun, and revolving in
+a period of a thousand years. He supposes that the original comet was
+not that of 1668, but one seen in 1556, which has since been
+``missing,'' and that its disruption occurred from an encounter with
+the supposititious planet about the year 1700. Truly from every point
+of view comets are the most extraordinary of adventurers!
+
+The comet of 1882 was likewise remarkable for being visible, like its
+predecessor of 1843, in full daylight in close proximity to the sun.
+The story of its detection when almost in contact with the solar disk
+is dramatic. It had been discovered in the southern hemisphere only a
+couple of weeks before its perihelion, which occurred on September
+17th, and on the forenoon of that day it was seen by Doctor Common in
+England, and by Doctor Elkin and Mr Finlay at the Cape of Good Hope,
+almost touching the sun. It looked like a dazzling white bird with
+outspread wings. The southern observers watched it go right into the
+sun, when it instantly disappeared. What had happened was that the
+comet in passing its perihelion point had swung exactly between the
+earth and the sun. On the following morning it was seen from all parts
+of the world close by the sun on the opposite side, and it remained
+thus visible for three days, gradually receding from the solar disk.
+It then became visible for northern observers in the morning sky
+before sunrise, brandishing a portentous sword-shaped tail which, if
+it had been in the evening sky, would have excited the wonder of
+hundreds of millions, but situated where it was, comparatively few
+ever saw it.
+
+The application of photography to the study of comets has revealed
+many curious details which might otherwise have escaped detection, or
+at best have remained subject to doubt. It has in particular shown not
+only the precise form of the tails, but the remarkable vicissitudes
+that they undergo. Professor Barnard's photographs of Brooks' comet in
+1893 suggested, by the extraordinary changes in the form of the tail
+which they revealed, that the comet was encountering a series of
+obstructions in space which bent and twisted its tail into fantastic
+shapes. The reader will observe the strange form into which the tail
+was thrown on the night of October 21st. A cloud of meteors through
+which the comet was passing might have produced such deformations of
+its tail. In the photograph of Daniels' comet of 1907, a curious
+striping of the tail will be noticed. The short bright streaks seen in
+the photograph, it may be explained, are the images of stars which are
+drawn out into lines in consequence of the fact that the photographic
+telescope was adjusted to follow the motion of the comet while the
+stars remained at rest.
+
+But the adventures of comets are not confined to possible encounters
+with unknown obstacles. We have referred to the fact that the great
+planets, and especially Jupiter, frequently interfere with the motions
+of comets. This interference is not limited to the original alteration
+of their orbits from possible parabolas to ellipses, but is sometimes
+exercised again and again, turning the bewildered comets into
+elliptical paths of all degrees of eccentricity. A famous example of
+this kind of planetary horse-play is furnished by the story of
+Lexell's missing comet. This comet was first seen in 1770.
+Investigation showed that it was moving in an orbit which should bring
+it back to perihelion every five and a half years; yet it had never
+been seen before and, although often searched for, has never been seen
+since. Laplace and Leverrier proved mathematically that in 1767 it had
+approached so close to Jupiter as to be involved among the orbits of
+his satellites. What its track had been before is not known, but on
+that occasion the giant planet seized the interloper, threw it into a
+short elliptic orbit and sent it, like an arrested vagrant, to receive
+sentence at the bar of the sun. On this journey it passed within less
+than 1,500,000 miles of the earth. The form of orbit which Jupiter had
+impressed required, as we have said, its return in about five and a
+half years; but soon after 1770 it had the misfortune a second time to
+encounter Jupiter at close range, and he, as if dissatisfied with the
+leniency of the sun, or indignant at the stranger's familiarity,
+seized the comet and hurled it out of the system, or at any rate so
+far away that it has never since been able to rejoin the family circle
+that basks in the immediate rays of the solar hearth. Nor is this the
+only instance in which Jupiter has dealt summarily with small comets
+that have approached him with too little deference.
+
+The function which Jupiter so conspicuously fulfills as master of the
+hounds to the sun is worth considering a little more in detail. To
+change the figure, imagine the sun in its voyage through space to be
+like a majestic battleship surrounded by its scouts. Small vessels
+(the comets, as they are overhauled by the squadron, are taken in
+charge by the scouts, with Jupiter for their chief, and are forced to
+accompany the fleet, but not all are impressed. If a strange comet
+undertakes to run across Jupiter's bows the latter brings it to, and
+makes prize of it by throwing it into a relatively small ellipse with
+the sun for its focus. Thenceforth, unless, as happened to the unhappy
+comet of Lexell, it encounters Jupiter again in such a way as to be
+diverted by him into a more distant orbit, it can never get away.
+About thirty comets are now known to have thus been captured by the
+great planet, and they are called ``Jupiter's Comet Family.'' But, on
+the other hand, if a wandering comet crosses the wake of the chief
+planetary scout the latter simply drives it away by accelerating its
+motion and compels it to steer off into open space. The transformation
+of comets into meteors will be considered in the next chapter, but
+here, in passing, mention may be made of the strange fate of one
+member of Jupiter's family, Biela's comet, which, having become over
+bold in its advances to its captor, was, after a few revolutions in is
+impressed orbit, torn to pieces and turned into a flock of meteors.
+
+And now let us return to the mystery of comets' tails. That we are
+fully justified in speaking of the tails of comets as mysterious is
+proved by the declaration of Sir John Herschel, who averred, in so
+many words, that ``there is some profound secret and mystery of nature
+concerned in this phenomenon,'' and this profound secret and mystery
+has not yet been altogether cleared up. Nevertheless, the
+all-explaining hypothesis of Arrhenius offers us once more a certain
+amount of aid. Comets' tails, Arrhenius assures us, are but another
+result of the pressure of light. The reader will recall the
+applications of this theory to the Zodiacal Light and the Aurora. In
+the form in which we now have to deal with it, the supposition is made
+that as a comet approaches the sun eruptions of vapor, due to the
+solar heat, occur in its nucleus. These are naturally most active on
+the side which is directly exposed to the sun, whence the appearance
+of the immense glowing envelopes that surround the nucleus on the
+sunward side. Among the particles of hydro-carbon, and perhaps solid
+carbon in the state of fine dust, which are thus set free there will
+be many whose size is within the critical limit which enables the
+light-waves from the sun to drive them away. Clouds of such particles,
+then, will stream off behind the advancing comet, producing the
+appearance of a tail. This accounts for the fact that the tails of
+comets are always directed away from the sun, and it also explains the
+varying forms of the tails and the extraordinary changes that they
+undergo. The speed of the particles driven before the light-waves must
+depend upon their size and weight, the lightest of a given size
+traveling the most swiftly. By accretion certain particles might grow,
+thus losing velocity and producing the appearance of bunches in the
+tail, such as have been observed. The hypothesis also falls in with
+the researches of Bredichin, who has divided the tails of comets into
+three principal classes -- viz.: (1) Those which appear as long,
+straight rays; (2) Those which have the form of curved plumes or
+scimitars; (3) Those which are short, brushy, and curved sharply
+backward along the comet's path. In the first type he calculates the
+repulsive force at from twelve to fifteen times the force of gravity;
+in the second at from two to four times; and in the third at about one
+and a half times. The straight tails he ascribes to hydrogen because
+the hydrogen atom is the lightest known; the sword-shaped tails to
+hydro-carbons; and the stumpy tails to vaporized iron. It will be seen
+that, if the force driving off the tails is that which Arrhenius
+assumes it to be, the forms of those appendages would accord with
+those that Bredichin's theory calls for. At the same time we have an
+explanation of the multiple tails with which some comets have adorned
+themselves. The comet of 1744, for instance, had at one time no less
+than seven tails spread in a wide curved brush behind it. Donati's
+comet of 1858 also had at least two tails, the principal one
+sword-shaped and the other long, narrow, and as straight as a rule.
+According to Bredichin, the straight tail must have been composed of
+hydrogen, and the other of some form of hydro-carbon whose atoms are
+heavier than those of hydrogen, and, consequently, when swept away by
+the storm of light-waves, followed a curvature depending upon the
+resultant of the forces operating upon them. The seven tails of the
+comet of 1744 presented a kind of diagram graphically exhibiting its
+complex composition, and, if we knew a little more about the
+constituents of a comet, we might be able to say from the amount of
+curvature of the different tails just what were the seven substances
+of which that comet consisted.
+
+If these theories seem to the reader fantastic, at any rate they are
+no more fantastic than the phenomena that they seek to explain.
+
+Meteors, Fire-Balls, and Meteorites
+
+One of the most terrorizing spectacles with which the heavens have
+ever caused the hearts of men to quake occurred on the night of
+November 13, 1833. On that night North America, which faced the storm,
+was under a continual rain of fire from about ten o'clock in the
+evening until daybreak.
+
+The fragments of a comet had struck the earth.
+
+But the meaning of what had happened was not discovered until long
+afterward. To the astronomers who, with astonishment not less than
+that of other people, watched the wonderful scene, it was an
+unparalleled ``shower of meteors.'' They did not then suspect that
+those meteors had once formed the head of a comet. Light dawned when,
+a year later, Prof. Denison Olmsted, of Yale College, demonstrated
+that the meteors had all moved in parallel orbits around the sun, and
+that these orbits intersected that of the earth at the point where our
+planet happened to be on the memorable night of November 13th.
+Professor Olmsted even went so far as to suggest that the cloud of
+meteors that had encountered the earth might form a diffuse comet; but
+full recognition of the fact that they were cometary d�bris came
+later, as the result of further investigation. The key to the secret
+was plainly displayed in the spectacle itself, and was noticed without
+being understood by thousands of the terror-stricken beholders. It was
+an umbrella of fire that had opened overhead and covered the heavens;
+in other words, the meteors all radiated from a particular point in
+the constellation Leo, and, being countless as the snowflakes in a
+winter tempest, they ribbed the sky with fiery streaks. Professor
+Olmsted showed that the radiation of the meteors from a fixed point
+was an effect of perspective, and in itself a proof that they were
+moving in parallel paths when they encountered the earth. The fact was
+noted that there had been a similar, but incomparably less brilliant,
+display of meteors on the same day of November, 1832, and it was
+rightly concluded that these had belonged to the same stream, although
+the true relationship of the phenomena was not immediately
+apprehended. Olmsted ascribed to the meteors a revolution about the
+sun once in every six months, bringing them to the intersection of
+their orbit with that of the earth every November 13th; but later
+investigators found that the real period was about thirty-three and
+one-quarter years, so that the great displays were due three times in
+a century, and their return was confidently predicted for the year
+1866. The appearance of the meteors in 1832, a year before the great
+display, was ascribed to the great length of the stream which they
+formed in space -- so great that they required more than two years to
+cross the earth's orbit. In 1832 the earth had encountered a
+relatively rare part of the stream, but in 1833, on returning to the
+crossing-place, it found there the richest part of the stream pouring
+across its orbit. This explanation also proved to be correct, and the
+predicted return in 1866 was duly witnessed, although the display was
+much less brilliant than in 1833. It was followed by another in 1867.
+
+In the mean time Olmsted's idea of a cometary relationship of the
+meteors was demonstrated to be correct by the researches of
+Schiaparelli and others, who showed that not only the November
+meteors, but those of August, which are seen more or less abundantly
+every year, traveled in the tracks of well-known comets, and had
+undoubtedly an identical origin with those comets. In other words the
+comets and the meteor-swarms were both remnants of original masses
+which had probably been split up by the action of the sun, or of some
+planet to which they had made close approaches. The annual periodicity
+of the August meteors was ascribed to the fact that the separation had
+taken place so long ago that the meteors had become distributed all
+around the orbit, in consequence of which the earth encountered some
+of them every year when it arrived at the crossing-point. Then
+Leverrier showed that the original comet associated with the November
+meteors was probably brought into the system by the influence of the
+planet Uranus in the year 126 of the Christian era. Afterward
+Alexander Herschel identified the tracks of no less than seventy-six
+meteor-swarms (most of them inconspicuous) with those of comets. The
+still more recent researches of Mr W. F. Denning make it probable that
+there are no meteors which do not belong to a flock or system probably
+formed by the disintegration of a cometary mass; even the apparently
+sporadic ones which shoot across the sky, ``lost souls in the night,''
+being members of flocks which have become so widely scattered that the
+earth sometimes takes weeks to pass through the region of space where
+their paths lie.
+
+The November meteors should have exhibited another pair of spectacles
+in 1899 and 1900, and their failure to do so caused at first much
+disappointment, until it was made plain that a good reason existed for
+their absence. It was found that after their last appearance, in 1867,
+they had been disturbed in their movements by the planets Jupiter and
+Saturn, whose attractions had so shifted the position of their orbit
+that it no longer intersected that of the earth, as it did before.
+Whether another planetary interference will sometime bring the
+principal mass of the November meteors back to the former point of
+intersection with the earth's orbit is a question for the future to
+decide. It would seem that there may be several parallel streams of
+the November meteors, and that some of them, like those of August, are
+distributed entirely around the orbit, so that every mid-November we
+see a few of them.
+
+We come now to a very remarkable example of the disintegration of a
+comet and the formation of a meteor-stream. In 1826 Biela, of
+Josephstadt, Austria, discovered a comet to which his name was given.
+Calculation showed that it had an orbital period of about six and a
+half years, belonging to Jupiter's ``family.'' On one of its returns,
+in 1846, it astonished its watchers by suddenly splitting in two. The
+two comets thus formed out of one separated to a distance of about one
+hundred and sixty thousand miles, and then raced side by side,
+sometimes with a curious ligature connecting them, like Siamese twins,
+until they disappeared together in interplanetary space. In 1852 they
+came back, still nearly side by side, but now the distance between
+them had increased to a million and a quarter of miles. After that, at
+every recurrence of their period, astronomers looked for them in vain,
+until 1872, when an amazing thing happened. On the night of November
+28th, when the earth was crossing the plane of the orbit of the
+missing comet, a brilliant shower of meteors burst from the northern
+sky, traveling nearly in the track which the comet should have
+pursued. The astronomers were electrified. Klinkerfues, of G�ttingen,
+telegraphed to Pogson, of Madras: ``Biela touched earth; search near
+Theta Centauri.'' Pogson searched in the place indicated and saw a
+cometary mass retreating into the southern heavens, where it was soon
+swallowed from sight!
+
+Since then the Biela meteors have been among the recognized periodic
+spectacles of the sky, and few if any doubt that they represent a
+portion of the missing comet whose disintegration began with the
+separation into two parts in 1846. The comet itself has never since
+been seen. The first display of these meteors, sometimes called the
+``Andromedes,'' because they radiate from the constellation Andromeda,
+was remarkable for the great brilliancy of many of the fire-balls that
+shot among the shower of smaller sparks, some of which were described
+as equaling the full moon in size. None of them is known to have
+reached the earth, but during the display of the same meteors in 1885
+a meteoric mass fell at Mazapil in Northern Mexico (it is now in the
+Museum at Vienna), which many have thought may actually be a piece of
+the original comet of Biela. This brings us to the second branch of
+our subject.
+
+More rare than meteors or falling stars, and more startling, except
+that they never appear in showers, are the huge balls of fire which
+occasionally dart through the sky, lighting up the landscapes beneath
+with their glare, leaving trains of sparks behind them, often
+producing peals of thunder when they explode, and in many cases
+falling upon the earth and burying themselves from a few inches to
+several feet in the soil, from which, more than once, they have been
+picked up while yet hot and fuming. These balls are sometimes called
+bolides. They are not really round in shape, although they often look
+so while traversing the sky, but their forms are fragmentary, and
+occasionally fantastic. It has been supposed that their origin is
+different from that of the true meteors; it has even been conjectured
+that they may have originated from the giant volcanoes of the moon or
+have been shot out from the sun during some of the tremendous
+explosions that accompany the formation of eruptive prominences. By
+the same reasoning some of them might be supposed to have come from
+some distant star. Others have conjectured that they are wanderers in
+space, of unknown origin, which the earth encounters as it journeys
+on, and Lord Kelvin made a suggestion which has become classic because
+of its imaginative reach -- viz., that the first germs of life may
+have been brought to the earth by one of these bodies, ``a fragment of
+an exploded world.''
+
+It is a singular fact that astronomers and scientific men in general
+were among the last to admit the possibility of solid masses falling
+from the sky. The people had believed in the reality of such phenomena
+from the earliest times, but the savants shook their heads and talked
+of superstition. This was the less surprising because no
+scientifically authenticated instance of such an occurrence was known,
+and the stones popularly believed to have fallen from the sky had
+become the objects of worship or superstitious reverence, a fact not
+calculated to recommend them to scientific credence. The celebrated
+``black stone'' suspended in the Kaaba at Mecca is one of these
+reputed gifts from heaven; the ``Palladium'' of ancient Troy was
+another; and a stone which fell near Ensisheim, in Germany, was placed
+in a church as an object to be religiously venerated. Many legends of
+falling stones existed in antiquity, some of them curiously
+transfigured by the imagination, like the ``Lion of the
+Peloponnesus,'' which was said to have sprung down from the sky upon
+the Isthmus of Corinth. But near the beginning of the nineteenth
+century, in 1803, a veritable shower of falling stones occurred at
+L'Aigle, in Northern France, and this time astronomers took note of
+the phenomenon and scientifically investigated it. Thousands of the
+strange projectiles came from the sky on this occasion, and were
+scattered over a wide area of country, and some buildings were hit.
+Four years later another shower of stones occurred at Weston, Conn.,
+numbering thousands of individuals. The local alarm created in both
+cases was great, as well it might be, for what could be more
+intimidating than to find the blue vault of heaven suddenly hurling
+solid missiles at the homes of men? After these occurrences it was
+impossible for the most skeptical to doubt any longer, and the regular
+study of ``aerolites,'' or ``meteorites,'' began.
+
+One of the first things recognized was the fact that fire-balls are
+solid meteorites in flight, and not gaseous exhalations in the air, as
+some had assumed. They burn in the air during their flight, and
+sometimes, perhaps, are entirely consumed before reaching the ground.
+Their velocity before entering the earth's atmosphere is equal to that
+of the planets in their orbits -- viz., from twenty to thirty miles
+per second -- a fact which proves that the sun is the seat of the
+central force governing them. Their burning in the air is not
+difficult to explain; it is the heat of friction which so quickly
+brings them to incandescence. Calculation shows that a body moving
+through the air at a velocity of about a mile per second will be
+brought, superficially, to the temperature of ``red heat'' by friction
+with the atmosphere. If its velocity is twenty miles per second the
+temperature will become thousands of degrees. This is the state of
+affairs with a meteorite rushing into the earth's atmosphere; its
+surface is liquefied within a few seconds after the friction begins to
+act, and the melted and vaporized portion of its mass is swept
+backward, forming the train of sparks that follows every great
+fire-ball. However, there is one phenomenon connected with the trains
+of meteorites which has never been satisfactorily explained: they
+often persist for long periods of time, drifting and turning with the
+wind, but not ceasing to glow with a phosphorescent luminosity. The
+question is, Whence comes this light? It must be light without heat,
+since the fine dust or vapor of which the train can only consist would
+not retain sufficient heat to render it luminous for so long a time.
+An extremely remarkable incident of this kind occurred on February 22,
+1909, when an immense fire-ball that passed over southern England left
+a train that remained visible during two hours, assuming many curious
+shapes as it was drifted about by currents in the air.
+
+But notwithstanding the enormous velocity with which meteorites enter
+the air they are soon slowed down to comparatively moderate speed, so
+that when they disappear they are usually traveling not faster than a
+mile a second. The courses of many have been traced by observers
+situated along their track at various points, and thus a knowledge has
+been obtained of their height above the ground during their flight and
+of the length of their visible courses. They generally appear at an
+elevation of eighty or a hundred miles, and are seldom visible after
+having descended to within five miles of the ground, unless the
+observer happens to be near the striking-point, when he may actually
+witness the fall. Frequently they burst while high in the air and
+their fragments are scattered like shrapnel over the surface of the
+ground, sometimes covering an area of several square miles, but of
+course not thickly; different fragments of the same meteorite may
+reach the ground at points several miles apart. The observed length of
+their courses in the atmosphere varies from fifty to five hundred
+miles. If they continued a long time in flight after entering the air,
+even the largest of them would probably be consumed to the last scrap,
+but their fiery career is so short on account of their great speed
+that the heat does not have time to penetrate very deeply, and some
+that have been picked up immediately after their fall have been found
+cold as ice within. Their size after reaching the ground is variable
+within wide limits; some are known which weigh several tons, but the
+great majority weigh only a few pounds and many only a few ounces.
+
+Meteorites are of two kinds: stony meteorites and iron meteorites. The
+former outnumber the latter twenty to one; but many stone meteorites
+contain grains of iron. Nickel is commonly found in iron meteorites,
+so that it might be said that that redoubtable alloy nickel-steel is
+of cosmical invention. Some twenty-five chemical elements have been
+found in meteorites, including carbon and the ``sun-metal,'' helium.
+The presence of the latter is certainly highly suggestive in
+connection with the question of the origin of meteorites. The iron
+meteorites, besides metallic iron and nickel, of which they are almost
+entirely composed, contain hydrogen, helium, and carbonic oxide, and
+about the only imaginable way in which these gases could have become
+absorbed in the iron would be through the immersion of the latter
+while in a molten or vaporized state in a hot and dense atmosphere
+composed of them, a condition which we know to exist only in the
+envelopes of the sun and the stars.
+
+The existence of carbon in the Canyon Diablo iron meteorites is
+attended by a circumstance of the most singular character -- a very
+``fairy tale of science.'' In some cases the carbon has become
+diamond! These meteoric diamonds are very small; nevertheless, they
+are true diamonds, resembling in many ways the little black gems
+produced by Moissan's method with the aid of the electric furnace. The
+fact that they are found embedded in these iron meteorites is another
+argument in favor of the hypothesis of the solar or stellar origin of
+the latter. To appreciate this it is necessary to recall the way in
+which Moissan made his diamonds. It was by a combination of the
+effects of great heat, great pressure, and sudden or rapid superficial
+cooling on a mass of iron containing carbon. When he finally broke
+open his iron he found it a pudding stuffed with miniature black
+diamonds. When a fragment of the Canyon Diablo meteoric iron was
+polished in Philadelphia over fifteen years ago it cut the emery-wheel
+to pieces, and examination showed that the damage had been effected by
+microscopic diamonds peppered through the mass. How were those
+diamonds formed? If the sun or Sirius was the laboratory that prepared
+them, we can get a glimpse at the process of their formation. There is
+plenty of heat, plenty of pressure, and an abundance of vaporized iron
+in the sun and the stars. When a great solar eruption takes place,
+masses of iron which have absorbed carbon may be shot out with a
+velocity which forbids their return. Plunged into the frightful cold
+of space, their surfaces are quickly cooled, as Moissan cooled his
+prepared iron by throwing it into water, and thus the requisite stress
+is set up within, and, as the iron solidifies, the included carbon
+crystallizes into diamonds. Whether this explanation has a germ of
+truth in it or not, at any rate it is evident that iron meteorites
+were not created in the form in which they come to us; they must once
+have been parts of immeasurably more massive bodies than themselves.
+
+The fall of meteorites offers an appreciable, though numerically
+insignificant, peril to the inhabitants of the earth. Historical
+records show perhaps three or four instances of people being killed by
+these bodies. But for the protection afforded by the atmosphere, which
+acts as a very effective shield, the danger would doubtless be very
+much greater. In the absence of an atmosphere not only would more
+meteorites reach the ground, but their striking force would be
+incomparably greater, since, as we have seen, the larger part of their
+original velocity is destroyed by the resistance of the air. A
+meteorite weighing many tons and striking the earth with a velocity of
+twenty or thirty miles per second, would probably cause frightful
+havoc.
+
+It is a singular fact that recent investigations seem to have proved
+that an event of this kind actually happened in North America --
+perhaps not longer than a thousand or two thousand years ago. The
+scene of the supposed catastrophe is in northern central Arizona, at
+Coon Butte, where there is a nearly circular crater in the middle of a
+circular elevation or small mountain. The crater is somewhat over four
+thousand feet in diameter, and the surrounding rim, formed of upturned
+strata and ejected rock fragments, rises at its highest point one
+hundred and sixty feet above the plain. The crater is about six
+hundred feet in depth -- that is, from the rim to the visible floor or
+bottom of the crater. There is no evidence that volcanic action has
+ever taken place in the immediate neighborhood of Coon Butte. The rock
+in which the crater has been made is composed of horizontal sandstone
+and limestone strata. Between three hundred and four hundred million
+tons of rock fragments have been detached, and a large portion hurled
+by some cause out of the crater. These fragments lie concentrically
+distributed around the crater, and in large measure form the elevation
+known as Coon Butte. The region has been famous for nearly twenty
+years on account of the masses of meteoric iron found scattered about
+and known as the ``Canyon Diablo'' meteorites. It was one of these
+masses, which consist of nickel-iron containing a small quantity of
+platinum, and of which in all some ten tons have been recovered for
+sale to the various collectors throughout the world, that as before
+mentioned destroyed the grinding-tool at Philadelphia through the
+cutting power of its embedded diamonds. These meteoric irons are
+scattered about the crater-hill, in concentric distribution, to a
+maximum distance of about five miles. When the suggestion was first
+made in 1896 that a monster meteorite might have created by its fall
+this singular lone crater in stratified rocks, it was greeted with
+incredulous smiles; but since then the matter has assumed a different
+aspect. The Standard Iron Company, formed by Messrs. D. M. Barringer,
+B. C. Tilghman, E. J. Bennitt, and S. J. Holsinger, having become, in
+1903, the owner of this freak of nature, sunk shafts and bored holes
+to a great depth in the interior of the crater, and also trenched the
+slopes of the mountain, and the result of their investigations has
+proved that the meteoric hypothesis of origin is correct. (See the
+papers published in the Proceedings of the Academy of Natural Sciences
+of Philadelphia, December, 1905, wherein it is proved that the United
+States Geological Survey was wrong in believing this crater to have
+been due to a steam explosion. Since that date there has been
+discovered a great amount of additional confirmatory proof). Material
+of unmistakably meteoric origin was found by means of the drills,
+mixed with crushed rock, to a depth of six hundred to seven hundred
+feet below the floor of the crater, and a great deal of it has been
+found admixed with the ejected rock fragments on the outer slopes of
+the mountain, absolutely proving synchronism between the two events,
+the formation of this great crater and the falling of the meteoric
+iron out of the sky. The drill located in the bottom of the crater was
+sent, in a number of cases, much deeper (over one thousand feet) into
+unaltered horizontal red sandstone strata, but no meteoric material
+was found below this depth (seven hundred feet, or between eleven and
+twelve hundred feet below the level of the surrounding plain), which
+has been assumed as being about the limit of penetration. It is not
+possible to sink a shaft at present, owing to the water which has
+drained into the crater, and which forms, with the finely pulverized
+sandstone, a very troublesome quicksand encountered at about two
+hundred feet below the visible floor of the crater. As soon as this
+water is removed by pumping it will be easy to explore the depths of
+the crater by means of shafts and drifts. The rock strata (sandstone
+and limestone) of which the walls consist present every appearance of
+having been violently upturned by a huge body penetrating the earth
+like a cannon-ball. The general aspect of the crater strikingly
+resembles the impression made by a steel projectile shot into an
+armor-plate. Mr Tilghman has estimated that a meteorite about five
+hundred feet in diameter and moving with a velocity of about five
+miles per second would have made just such a perforation upon striking
+rocks of the character of those found at this place. There was some
+fusion of the colliding masses, and the heat produced some steam from
+the small amount of water in the rocks. As a result there has been
+found at depth a considerable amount of fused quartz (original
+sandstone), and with it innumerable particles or sparks of fused
+nickel-iron (original meteorite). A projectile of that size
+penetrating eleven to twelve hundred feet into the rocky shell of the
+globe must have produced a shock which was perceptible several hundred
+miles away.
+
+The great velocity ascribed to the supposed meteorite at the moment of
+striking could be accounted for by the fact that it probably plunged
+nearly vertically downward, for it formed a circular crater in the
+rocky crust of the earth. In that case it would have been less
+retarded by the resistance of the atmosphere than are meteorites which
+enter the air at a lower angle and shoot ahead hundreds of miles until
+friction has nearly destroyed their original motion when they drop
+upon the earth. Some meteoric masses of great size, such as Peary's
+iron meteorite found at Cape York, Greenland, and the almost equally
+large mass discovered at Bacubirito, Mexico, appear to have penetrated
+but slightly on striking the earth. This may be explained by supposing
+that they pursued a long, horizontal course through the air before
+falling. The result would be that, their original velocity having been
+practically destroyed, they would drop to the ground with a velocity
+nearly corresponding to that which gravity would impart within the
+perpendicular distance of their final fall. A
+six-hundred-and-sixty-pound meteorite, which fell at Knyahinya,
+Hungary, striking at an angle of 27� from the vertical, penetrated the
+ground to a depth of eleven feet.
+
+It has been remarked that the Coon Butte meteorite may have fallen not
+longer ago than a few thousand years. This is based upon the fact that
+the geological indications favor the supposition that the event did
+not occur more than five thousand years ago, while on the other hand
+the rings of growth in the cedar-trees growing on the slopes of the
+crater show that they have existed there about seven hundred years.
+Prof. William H. Pickering has recently correlated this with an
+ancient chronicle which states that at Cairo, Egypt, in the year 1029,
+``many stars passed with a great noise.'' He remarks that Cairo is
+about 100�, by great circle, from Coon Butte, so that if the meteorite
+that made the crater was a member of a flock of similar bodies which
+encountered the earth moving in parallel lines, some of them might
+have traversed the sky tangent to the earth's surface at Cairo. That
+the spectacle spoken of in the chronicle was caused by meteorites he
+deems exceedingly probable because of what is said about ``a great
+noise;'' meteorites are the only celestial phenomena attended with
+perceptible sounds. Professor Pickering conjectures that this supposed
+flock of great meteorites may have formed the nucleus of a comet which
+struck the earth, and he finds confirmation of the idea in the fact
+that out of the ten largest meteorites known, no less than seven were
+found within nine hundred miles of Coon Butte. It would be interesting
+if we could trace back the history of that comet, and find out what
+malicious planet caught it up in its innocent wanderings and hurled it
+with so true an aim at the earth! This remarkable crater is one of the
+most interesting places in the world, for there is absolutely no
+record of such a mass, possibly an iron-headed comet, from outer space
+having come into collision with our earth. The results of the future
+exploration of the depths of the crater will be awaited with much
+interest.
+
+The Wrecking of the Moon
+
+There are sympathetic moods under whose influence one gazes with a
+certain poignant tenderness at the worn face of the moon; that little
+``fossil world'' (the child of our mother earth, too) bears such
+terrible scars of its brief convulsive life that a sense of pity is
+awakened by the sight. The moon is the wonder-land of the telescope.
+Those towering mountains, whose ``proud aspiring peaks'' cast
+silhouettes of shadow that seem drawn with india-ink; those vast
+plains, enchained with gentle winding hills and bordered with giant
+ranges; those oval ``oceans,'' where one looks expectant for the flash
+of wind-whipped waves; those enchanting ``bays'' and recesses at the
+seaward feet of the Alps; those broad straits passing between guardian
+heights incomparably mightier than Gibraltar; those locket-like
+valleys as secluded among their mountains as the Vale of Cashmere;
+those colossal craters that make us smile at the pretensions of
+Vesuvius, Etna, and Cotopaxi; those strange white ways which pass with
+the unconcern of Roman roads across mountain, gorge, and valley -- all
+these give the beholder an irresistible impression that it is truly a
+world into which he is looking, a world akin to ours, and yet no more
+like our world than Pompeii is like Naples. Its air, its waters, its
+clouds, its life are gone, and only a skeleton remains -- a mute but
+eloquent witness to a cosmical tragedy without parallel in the range
+of human knowledge.
+
+One cannot but regret that the moon, if it ever was the seat of
+intelligent life, has not remained so until our time. Think what the
+consequences would have been if this other world at our very door had
+been found to be both habitable and inhabited! We talk rather airily
+of communicating with Mars by signals; but Mars never approaches
+nearer than 35,000,000 miles, while the moon when nearest is only a
+little more than 220,000 miles away. Given an effective magnifying
+power of five thousand diameters, which will perhaps be possible at
+the mountain observatories as telescopes improve, and we should be
+able to bring the moon within an apparent distance of about forty
+miles, while the corresponding distance for Mars would be more than
+seven thousand miles. But even with existing telescopic powers we can
+see details on the moon no larger than some artificial constructions
+on the earth. St Peter's at Rome, with the Vatican palace and the
+great piazza, if existing on the moon, would unquestionably be
+recognizable as something else than a freak of nature. Large cities,
+with their radiating lines of communication, would at once betray
+their real character. Cultivated tracts, and the changes produced by
+the interference of intelligent beings, would be clearly recognizable.
+The electric illumination of a large town at night would probably be
+markedly visible. Gleams of reflected sunlight would come to us from
+the surfaces of the lakes and oceans, and a huge ``liner'' traversing
+a lunar sea could probably be followed by its trail of smoke. As to
+communications by ``wireless'' signals, which certain enthusiasts have
+thought of in connection with Mars, in the case of the moon they
+should be a relatively simple matter, and the feat might actually be
+accomplished. Think what a literature would grow up about the moon if
+it were a living world! Its very differences from the earth would only
+accentuate its interest for us. Night and day on the moon are each two
+weeks in length; how interesting it would be to watch the manner in
+which the lunarians dealt with such a situation as that. Lunar and
+terrestrial history would keep step with each other, and we should
+record them both. Truly one might well wish to have a neighbor world
+to study; one would feel so much the less alone in space.
+
+It is not impossible that the moon did at one time have inhabitants of
+some kind. But, if so, they vanished with the disappearance of its
+atmosphere and seas, or with the advent of its cataclysmic age. At the
+best, its career as a living world must have been brief. If the water
+and air were gradually absorbed, as some have conjectured, by its
+cooling interior rocks, its surface might, nevertheless, have retained
+them for long ages; but if, as others think, their disappearance was
+due to the escape of their gaseous molecules in consequence of the
+inability of the relatively small lunar gravitation to retain them,
+then the final catastrophe must have been as swift as it was
+inevitable. Accepting Darwin's hypothesis, that the moon was separated
+from the earth by tidal action while both were yet plastic or
+nebulous, we may reasonably conclude that it began its career with a
+good supply of both water and air, but did not possess sufficient mass
+to hold them permanently. Yet it may have retained them long enough
+for life to develop in many forms upon its surface; in fact, there are
+so many indications that air and water have not always been lacking to
+the lunar world that we are driven to invent theories to explain both
+their former presence and their present absence.
+
+But whatever the former condition of the moon may have been, its
+existing appearance gives it a resistless fascination, and it bears so
+clearly the story of a vast catastrophe sculptured on its rocky face
+that the thoughtful observer cannot look upon it without a feeling of
+awe. The gigantic character of the lunar features impresses the
+beholder not less than the universality of the play of destructive
+forces which they attest. Let us make a few comparisons. Take the
+lunar crater called ``Tycho'', which is a typical example of its kind.
+In the telescope Tycho appears as a perfect ring surrounding a
+circular depression, in the center of which rises a group of
+mountains. Its superficial resemblance to some terrestrial volcanic
+craters is very striking. Vesuvius, seen from a point vertically
+above, would no doubt look something like that (the resemblance would
+have been greater when the Monte del Cavallo formed a more complete
+circuit about the crater cone). But compare the dimensions. The
+remains of the outer crater ring of Vesuvius are perhaps half a mile
+in diameter, while the active crater itself is only two or three
+hundred feet across at the most; Tycho has a diameter of fifty-four
+miles! The group of relatively insignificant peaks in the center of
+the crater floor of Tycho is far more massive than the entire mountain
+that we call Vesuvius. The largest known volcanic crater on the earth,
+Aso San, in Japan, has a diameter of seven miles; it would take sixty
+craters like Aso San to equal Tycho in area! And Tycho, though one of
+the most perfect, is by no means the largest crater on the moon.
+Another, called ``Theophilus,'' has a diameter of sixty-four miles,
+and is eighteen thousand feet deep. There are hundreds from ten to
+forty miles in diameter, and thousands from one to ten miles. They are
+so numerous in many places that they break into one another, like the
+cells of a crushed honeycomb.
+
+The lunar craters differ from those of the earth more fundamentally
+than in the matter of mere size; they are not situated on the tops of
+mountains. If they were, and if all the proportions were the same, a
+crater like Tycho might crown a conical peak fifty or one hundred
+miles high! Instead of being cavities in the summits of mountains, the
+lunar craters are rather gigantic sink-holes whose bottoms in many
+cases lie two or three miles below the general surface of the lunar
+world. Around their rims the rocks are piled up to a height of from a
+few hundred to two or three thousand feet, with a comparatively gentle
+inclination, but on the inner side they fall away in gigantic broken
+precipices which make the dizzy cliffs of the Matterhorn seem but
+``lover's leaps.'' Down they drop, ridge below ridge, crag under crag,
+tottering wall beneath wall, until, in a crater named ``Newton,'' near
+the south lunar pole, they attain a depth where the rays of the sun
+never reach. Nothing more frightful than the spectacle which many of
+these terrible chasms present can be pictured by the imagination. As
+the lazy lunar day slowly advances, the sunshine, unmitigated by
+clouds or atmospheric veil of any kind, creeps across their rims and
+begins to descend the opposite walls. Presently it strikes the ragged
+crest of a ridge which had lain hidden in such darkness as we never
+know on the earth, and runs along it like a line of kindling fire.
+Rocky pinnacles and needles shoot up into the sunlight out of the
+black depths. Down sinks the line of light, mile after mile, and
+continually new precipices and cliffs are brought into view, until at
+last the vast floor is attained and begins to be illuminated. In the
+meanwhile the sun's rays, darting across the gulf, have touched the
+summits of the central peaks, twenty or thirty miles from the crater's
+inmost edge, and they immediately kindle and blaze like huge stars
+amid the darkness. So profound are some of these awful craters that
+days pass before the sun has risen high enough above them to chase the
+last shadows from their depths.
+
+Although several long ranges of mountains resembling those of the
+earth exist on the moon, the great majority of its elevations assume
+the crateriform aspect. Sometimes, instead of a crater, we find an
+immense mountain ring whose form and aspect hardly suggest volcanic
+action. But everywhere the true craters are in evidence, even on the
+sea-beds, although they attain their greatest number and size on those
+parts of the moon -- covering sixty per cent of its visible surface --
+which are distinctly mountainous in character and which constitute its
+most brilliant portions. Broadly speaking, the southwestern half of
+the moon is the most mountainous and broken, and the northeastern half
+the least so. Right down through the center, from pole to pole, runs a
+wonderful line of craters and crateriform valleys of a magnitude
+stupendous even for the moon. Another similar line follows the western
+edge. Three or four ``seas'' are thrust between these mountainous
+belts. By the effects of ``libration'' parts of the opposite
+hemisphere of the moon which is turned away from the earth are from
+time to time brought into view, and their aspect indicates that that
+hemisphere resembles in its surface features the one which faces the
+earth. There are many things about the craters which seem to give some
+warrant for the hypothesis which has been particularly urged by Mr G.
+K. Gilbert, that they were formed by the impact of meteors; but there
+are also many things which militate against that idea, and, upon the
+whole, the volcanic theory of their origin is to be preferred.
+
+The enormous size of the lunar volcanoes is not so difficult to
+account for when we remember how slight is the force of lunar gravity
+as compared with that of the earth. With equal size and density,
+bodies on the moon weigh only one-sixth as much as on the earth.
+Impelled by the same force, a projectile that would go ten miles on
+the earth would go sixty miles on the moon. A lunar giant thirty-five
+feet tall would weigh no more than an ordinary son of Adam weighs on
+his greater planet. To shoot a body from the earth so that it would
+not drop back again, we should have to start it with a velocity of
+seven miles per second; a mile and a half per second would serve on
+the moon. It is by no means difficult to believe, then, that a lunar
+volcano might form a crater ring eight or ten times broader than the
+greatest to be found on the earth, especially when we reflect that in
+addition to the relatively slight force of gravity, the materials of
+the lunar crust are probably lighter than those of our terrestrial
+rocks.
+
+For similar reasons it seems not impossible that the theory mentioned
+in a former chapter -- that some of the meteorites that have fallen
+upon the earth originated from the lunar volcanoes -- is well founded.
+This would apply especially to the stony meteorites, for it is hardly
+to be supposed that the moon, at least in its superficial parts,
+contains much iron. It is surely a scene most strange that is thus
+presented to the mind's eye -- that little attendant of the earth's
+(the moon has only one-fiftieth of the volume, and only one-eightieth
+of the mass of the earth) firing great stones back at its parent
+planet! And what can have been the cause of this furious outbreak of
+volcanic forces on the moon? Evidently it was but a passing stage in
+its history; it had enjoyed more quiet times before. As it cooled down
+from the plastic state in which it parted from the earth, it became
+incrusted after the normal manner of a planet, and then oceans were
+formed, its atmosphere being sufficiently dense to prevent the water
+from evaporating and the would-be oceans from disappearing continually
+in mist. This, if any, must have been the period of life in the lunar
+world. As we look upon the vestiges of that ancient world buried in
+the wreck that now covers so much of its surface, it is difficult to
+restrain the imagination from picturing the scenes which were once
+presented there; and, in such a case, should the imagination be
+fettered? We give it free rein in terrestrial life, and it rewards us
+with some of our greatest intellectual pleasures. The wonderful
+landscapes of the moon offer it an ideal field with just enough
+half-hidden suggestions of facts to stimulate its powers.
+
+The great plains of the Mare Imbrium and the Mare Serenitatis (the
+``Sea of Showers'' and the ``Sea of Serenity''), bordered in part by
+lofty mountain ranges precisely like terrestrial mountains, scalloped
+along their shores with beautiful bays curving back into the adjoining
+highlands, and united by a great strait passing between the nearly
+abutting ends of the ``Lunar Apennines'' and the ``Lunar Caucasus,''
+offer the elements of a scene of world beauty such as it would be
+difficult to match upon our planet. Look at the finely modulated
+bottom of the ancient sea in Mr Ritchey's exquisite photograph of the
+western part of the Mare Serenitatis, where one seems to see the play
+of the watery currents heaping the ocean sands in waving lines, making
+shallows, bars, and deeps for the mariner to avoid or seek, and
+affording a playground for the creatures of the main. What geologist
+would not wish to try his hammer on those rocks with their stony pages
+of fossilized history? There is in us an instinct which forbids us to
+think that there was never any life there. If we could visit the moon,
+there is not among us a person so prosaic and unimaginative that he
+would not, the very first thing, begin to search for traces of its
+inhabitants. We would look for them in the deposits on the sea
+bottoms; we would examine the shores wherever the configuration seemed
+favorable for harbors and the sites of maritime cities -- forgetting
+that it may be a little ridiculous to ascribe to the ancient lunarians
+the same ideas that have governed the development of our race; we
+would search through the valleys and along the seeming courses of
+vanished streams; we would explore the mountains, not the terrible
+craters, but the pinnacled chains that recall our own Alps and
+Rockies; seeking everywhere some vestige of the transforming presence
+of intelligent life. Perhaps we should find such traces, and perhaps,
+with all our searching, we should find nothing to suggest that life
+had ever existed amid that universal ruin.
+
+Look again at the border of the ``Sea of Serenity'' -- what a name for
+such a scene! -- and observe how it has been rent with almost
+inconceivable violence, the wall of the colossal crater Posidonius
+dropping vertically upon the ancient shore and obliterating it, while
+its giant neighbor, Le Monnier, opens a yawning mouth as if to swallow
+the sea itself. A scene like this makes one question whether, after
+all, those may not be right who have imagined that the so-called sea
+bottoms are really vast plains of frozen lava which gushed up in
+floods so extensive that even the mighty volcanoes were half drowned
+in the fiery sea. This suggestion becomes even stronger when we turn
+to another of the photographs of Mr Ritchey's wonderful series,
+showing a part of the Mare Tranquilitatis (``Sea of Tranquility''!).
+Notice how near the center of the picture the outline of a huge ring
+with radiating ridges shows through the sea bottom; a fossil volcano
+submerged in a petrified ocean! This is by no means the only instance
+in which a buried world shows itself under the great lunar plains.
+Yet, as the newer craters in the sea itself prove, the volcanic
+activity survived this other catastrophe, or broke out again
+subsequently, bringing more ruin to pile upon ruin.
+
+Yet notwithstanding the evidence which we have just been considering
+in support of the hypothesis that the ``seas'' are lava floods,
+Messrs. Loewy and Puiseux, the selenographers of the Paris
+Observatory, are convinced that these great plains bear characteristic
+marks of the former presence of immense bodies of water. In that case
+we should be forced to conclude that the later oceans of the moon lay
+upon vast sheets of solidified lava; and thus the catastrophe of the
+lunar world assumes a double aspect, the earliest oceans being
+swallowed up in molten floods issuing from the interior, while the
+lands were reduced to chaos by a universal eruption of tremendous
+volcanoes; and then a period of comparative quiet followed, during
+which new seas were formed, and new life perhaps began to flourish in
+the lunar world, only to end in another cataclysm, which finally put a
+term to the existence of the moon as a life-supporting world.
+
+Suppose we examine two more of Mr Ritchey's illuminating photographs,
+and, first, the one showing the crater Theophilus and its
+surroundings. We have spoken of Theophilus before, citing the facts
+that it is sixty-four miles in diameter and eighteen thousand feet
+deep. It will be noticed that it has two brother giants -- Cyrillus
+the nearer, and Catharina the more distant; but Theophilus is plainly
+the youngest of the trio. Centuries, and perhaps thousands of years,
+must have elapsed between the periods of their upheaval, for the two
+older craters are partly filled with d�bris, while it is manifest at a
+glance that when the south eastern wall of Theophilus was formed, it
+broke away and destroyed a part of the more ancient ring of Cyrillus.
+There is no more tremendous scene on the moon than this; viewed with a
+powerful telescope, it is absolutely appalling.
+
+The next photograph shows, if possible, a still wilder region. It is
+the part of the moon lying between Tycho and the south pole. Tycho is
+seen in the lower left-hand part of the picture. To the right, at the
+edge of the illuminated portion of the moon, are the crater-rings,
+Longomontanus and Wilhelm I, the former being the larger. Between them
+are to be seen the ruins of two or three more ancient craters which,
+together with portions of the walls of Wilhelm I and Longomontanus,
+have been honeycombed with smaller craters. The vast crateriform
+depression above the center of the picture is Clavius, an unrivaled
+wonder of lunar scenery, a hundred and forty-two miles in its greatest
+length, while its whole immense floor has sunk two miles below the
+general surface of the moon outside the ring. The monstrous
+shadow-filled cavity above Clavius toward the right is Blancanus,
+whose aspect here gives a good idea of the appearance of these chasms
+when only their rims are in the sunlight. But observe the
+indescribable savagery of the entire scene. It looks as though the
+spirit of destruction had gone mad in this spot. The mighty craters
+have broken forth one after another, each rending its predecessor; and
+when their work was finished, a minor but yet tremendous outbreak
+occurred, and the face of the moon was gored and punctured with
+thousands of smaller craters. These relatively small craters (small,
+however, only in a lunar sense, for many of them would appear gigantic
+on the earth) recall once more the theory of meteoric impact. It does
+not seem impossible that some of them may have been formed by such an
+agency.
+
+One would not wish for our planet such a fate as that which has
+overtaken the moon, but we cannot be absolutely sure that something of
+the kind may not be in store for it. We really know nothing of the
+ultimate causes of volcanic activity, and some have suggested that the
+internal energies of the earth may be accumulating instead of dying
+out, and may never yet have exhibited their utmost destructive power.
+Perhaps the best assurance that we can find that the earth will escape
+the catastrophe that has overtaken its satellite is to be found in the
+relatively great force of its gravitation. The moon has been the
+victim of its weakness; given equal forces, and the earth would be the
+better able to withstand them. It is significant, in connection with
+these considerations, that the little planet Mercury, which seems also
+to have parted with its air and water, shows to the telescope some
+indications that it is pitted with craters resembling those that have
+torn to pieces the face of the moon.
+
+Upon the whole, after studying the dreadful lunar landscapes, one
+cannot feel a very enthusiastic sympathy with those who are seeking
+indications of the continued existence of some kind of life on the
+moon; such a world is better without inhabitants. It has met its fate;
+let it go! Fortunately, it is not so near that it cannot hide its
+scars and appear beautiful -- except when curiosity impels us to look
+with the penetrating eyes of the astronomer.
+
+The Great Mars Problem
+
+Let any thoughtful person who is acquainted with the general facts of
+astronomy look up at the heavens some night when they appear in their
+greatest splendor, and ask himself what is the strongest impression
+that they make upon his mind. He may not find it easy to frame an
+answer, but when he has succeeded it will probably be to the effect
+that the stars give him an impression of the universality of
+intelligence; they make him feel, as the sun and the moon cannot do,
+that his world is not alone; that all this was not made simply to form
+a gorgeous canopy over the tents of men. If he is of a devout turn of
+mind, he thinks, as he gazes into those fathomless deeps and among
+those bewildering hosts, of the infinite multitude of created beings
+that the Almighty has taken under his care. The narrow ideas of the
+old geocentric theology, which made the earth God's especial
+footstool, and man his only rational creature, fall away from him like
+a veil that had obscured his vision; they are impossible in the
+presence of what he sees above. Thus the natural tendency, in the
+light of modern progress, is to regard the universe as everywhere
+filled with life.
+
+But science, which is responsible for this broadening of men's
+thoughts concerning the universality of life, itself proceeds to set
+limits. Of spiritual existences it pretends to know nothing, but as to
+physical beings, it declares that it can only entertain the
+supposition of their existence where it finds evidence of an
+environment suited to their needs, and such environment may not
+everywhere exist. Science, though repelled by the antiquated
+theological conception of the supreme isolation of man among created
+beings, regards with complacency the probability that there are
+regions in the universe where no organic life exists, stars which
+shine upon no inhabited worlds, and planets which nourish no animate
+creatures. The astronomical view of the universe is that it consists
+of matter in every stage of evolution: some nebulous and chaotic; some
+just condensing into stars (suns) of every magnitude and order; some
+shaped into finished solar bodies surrounded by dependent planets;
+some forming stars that perhaps have no planets, and will have none;
+some constituting suns that are already aging, and will soon lose
+their radiant energy and disappear; and some aggregated into masses
+that long ago became inert, cold, and rayless, and that can only be
+revivified by means about which we can form conjectures, but of which
+we actually know nothing.
+
+As with the stars, so with the planets, which are the satellites of
+stars. All investigations unite to tell us that the planets are not
+all in the same state of development. As some are large and some
+small, so some are, in an evolutionary sense, young, and some old. As
+they depend upon the suns around which they revolve for their light,
+heat, and other forms of radiant energy, so their condition varies
+with their distance from those suns. Many may never arrive at a state
+suitable for the maintenance of life upon their surfaces; some which
+are not at present in such a state may attain it later; and the forms
+of life themselves may vary with the peculiar environment that
+different planets afford. Thus we see that we are not scientifically
+justified in affirming that life is ubiquitous, although we are thus
+justified in saying that it must be, in a general sense, universal. We
+might liken the universe to a garden known to contain every variety of
+plant. If on entering it we see no flowers, we examine the species
+before us and find that they are not of those which bloom at this
+particular season, or perhaps they are such as never bear flowers. Yet
+we feel no doubt that we shall find flowers somewhere in the garden,
+because there are species which bloom at this season, and the garden
+contains all varieties.
+
+While it is tacitly assumed that there are planets revolving around
+other stars than the sun, it would be impossible for us to see them
+with any telescope yet invented, and no instrument now in the
+possession of astronomers could assure us of their existence; so the
+only planetary system of which we have visual knowledge is our own.
+Excluding the asteroids, which could not from any point of view be
+considered as habitable, we have in the solar system eight planets of
+various sizes and situated at various distances from the sun. Of these
+eight we know that one, the earth, is inhabited. The question, then,
+arises: Are there any of the others which are inhabited or habitable?
+Since it is our intention to discuss the habitability of only one of
+the seven to which the question applies, the rest may be dismissed in
+a few words. The smallest of them, and the nearest to the sun, is
+Mercury, which is regarded as uninhabitable because it has no
+perceptible supply of water and air, and because, owing to the
+extraordinary eccentricity of its orbit, it is subjected to excessive
+and very rapid alterations in the amount of solar heat and light
+poured upon its surface, such alterations being inconsistent with the
+supposition that it can support living beings. Even its average
+temperature is more than six and a half times that prevailing on the
+earth! Another circumstance which militates against its habitability
+is that, according to the results of the best telescopic studies, it
+always keeps the same face toward the sun, so that one half of the
+planet is perpetually exposed to the fierce solar rays, and the other
+half faces the unmitigated cold of open space. Venus, the next in
+distance from the sun, is almost the exact twin of the earth in size,
+and many arguments may be urged in favor of its habitability, although
+it is suspected of possessing the same peculiarity as Mercury, in
+always keeping the same side sunward. Unfortunately its atmosphere
+appears to be so dense that no permanent markings on its surface are
+certainly visible, and the question of its actual condition must, for
+the present, be left in abeyance. Mars, the first planet more distant
+from the sun than the earth, is the special subject of this chapter,
+and will be described and discussed a few lines further on. Jupiter,
+Saturn, Uranus, and Neptune, the four giant planets, all more distant
+than Mars, and each more distant than the other in the order named,
+are all regarded as uninhabitable because none of them appears to
+possess any degree of solidity. They may have solid or liquid nuclei,
+but exteriorly they seem to be mere balls of cloud. Of course, one can
+imagine what he pleases about the existence of creatures suited to the
+physical constitution of such planets as these, but they must be
+excluded from the category of habitable worlds in the ordinary sense
+of the term. We go back, then, to Mars.
+
+It will be best to begin with a description of the planet. Mars is
+4230 miles in diameter; its surface is not much more than one-quarter
+as extensive as that of the earth (.285). Its mean distance from the
+sun is 141,500,000 miles, 48,500,000 miles greater than that of the
+earth. Since radiant energy varies inversely as the square of
+distance, Mars receives less than half as much solar light and heat as
+the earth gets. Mars' year (period of revolution round the sun) is 687
+days. Its mean density is 71 per cent of the earth's, and the force of
+gravity on its surface is 38 per cent of that on the surface of the
+earth; i.e., a body weighing one hundred pounds on the earth would, if
+transported to Mars, weigh but thirty-eight pounds. The inclination of
+its equator to the plane of its orbit differs very little from that of
+the earth's equator, and its axial rotation occupies 24 hours 37
+minutes. so that the length of day and night, and the extent of the
+seasonal changes on Mars, are almost precisely the same as on the
+earth. But owing to the greater length of its year, the seasons of
+Mars, while occurring in the same order, are almost twice as long as
+ours. The surface of the planet is manifestly solid, like that of our
+globe, and the telescope reveals many permanent markings on it,
+recalling the appearance of a globe on which geographical features
+have been represented in reddish and dusky tints. Around the poles are
+plainly to be seen rounded white areas, which vary in extent with the
+Martian seasons, nearly vanishing in summer and extending widely in
+winter. The most recent spectroscopic determinations indicate that
+Mars has an atmosphere perhaps as dense as that to be found on our
+loftiest mountain peaks, and there is a perceptible amount of watery
+vapor in this atmosphere. The surface of the planet appears to be
+remarkably level, and it has no mountain ranges. No evidences of
+volcanic action have been discovered on Mars. The dusky and reddish
+areas were regarded by the early observers as respectively seas and
+lands, but at present it is not believed that there are any bodies of
+water on the planet. There has never been much doubt expressed that
+the white areas about the poles represent snow.
+
+It will be seen from this brief description that many remarkable
+resemblances exist between Mars and the earth, and there is nothing
+wonderful in the fact that the question of the habitability of the
+former has become one of extreme and wide-spread interest, giving rise
+to the most diverse views, to many extraordinary speculations, and
+sometimes to regrettably heated controversy. The first champion of the
+habitability of Mars was Sir William Herschel, although even before
+his time the idea had been suggested. He was convinced by the
+revelations of his telescopes, continually increasing in power, that
+Mars was more like the earth than any other planet. He could not
+resist the testimony of the polar snows, whose suggestive conduct was
+in such striking accord with what occurs upon the earth. Gradually, as
+telescopes improved and observers increased in number, the principal
+features of the planet were disclosed and charted, and ``areography,''
+as the geography of Mars was called, took its place among the
+recognized branches of astronomical study. But it was not before 1877
+that a fundamentally new discovery in areography gave a truly
+sensational turn to speculation about life on ``the red planet.'' In
+that year Mars made one of its nearest approaches to the earth, and
+was so situated in its orbit that it could be observed to great
+advantage from the northern hemisphere of the earth. The celebrated
+Italian astronomer, Schiaparelli, took advantage of this opportunity
+to make a trigonometrical survey of the surface of Mars -- as coolly
+and confidently as if he were not taking his sights across a
+thirty-five-million-mile gulf of empty space -- and in the course of
+this survey he was astonished to perceive that the reddish areas, then
+called continents, were crossed in many directions by narrow, dusky
+lines, to which he gave the suggestive name of ``canals.'' Thus a kind
+of firebrand was cast into the field of astronomical speculation,
+which has ever since produced disputes that have sometimes approached
+the violence of political faction. At first the accuracy of
+Schiaparelli's observations was contested; it required a powerful
+telescope, and the most excellent ``seeing,'' to render the
+enigmatical lines visible at all, and many searchers were unable to
+detect them. But Schiaparelli continued his studies in the serene sky
+of Italy, and produced charts of the gridironed face of Mars
+containing so much astonishing detail that one had either to reject
+them in toto or to confess that Schiaparelli was right. As subsequent
+favorable oppositions of Mars occurred, other observers began to see
+the ``canals'' and to confirm the substantial accuracy of the Italian
+astronomer's work, and finally few were found who would venture to
+affirm that the ``canals'' did not exist, whatever their meaning might
+be.
+
+When Schiaparelli began his observations it was generally believed, as
+we have said, that the dusky areas on Mars were seas, and since
+Schiaparelli thought that the ``canals'' invariably began and ended at
+the shores of the ``seas,'' the appropriateness of the title given to
+the lines seemed apparent. Their artificial character was immediately
+assumed by many, because they were too straight and too suggestively
+geometrical in their arrangement to permit the conclusion that they
+were natural watercourses. A most surprising circumstance noted by
+Schiaparelli was that the ``canals'' made their appearance after the
+melting of the polar snow in the corresponding hemisphere had begun,
+and that they grew darker, longer, and more numerous in proportion as
+the polar liquidation proceeded; another very puzzling observation was
+that many of them became double as the season advanced; close beside
+an already existing ``canal,'' and in perfect parallelism with it,
+another would gradually make its appearance. That these phenomena
+actually existed and were not illusions was proved by later
+observations, and today they are seen whenever Mars is favorably
+situated for observation.
+
+In the closing decade of the nineteenth century, Mr Percival Lowell
+took up the work where Schiaparelli had virtually dropped it, and soon
+added a great number of ``canals'' to those previously known, so that
+in his charts the surface of the wonderful little planet appears
+covered as with a spider's web, the dusky lines criss-crossing in
+every direction, with conspicuous knots wherever a number of them come
+together. Mr Lowell has demonstrated that the areas originally called
+seas, and thus named on the earlier charts, are not bodies of water,
+whatever else they may be. He has also found that the mysterious lines
+do not, as Schiaparelli supposed, begin and end at the edges of the
+dusky regions, but often continue on across them, reaching in some
+cases far up into the polar regions. But Schiaparelli was right in his
+observation that the appearance of the ``canals'' is synchronous with
+the gradual disappearance of the polar snows, and this fact has become
+the basis of the most extraordinary theory that the subject of life in
+other worlds has ever given birth to.
+
+Now, the effect of such discoveries, as we have related, depends upon
+the type of mind to whose attention they are called. Many are content
+to accept them as strange and inexplicable at present, and to wait for
+further light upon them; others insist upon an immediate inquiry
+concerning their probable nature and meaning. Such an inquiry can only
+be based upon inference proceeding from analogy. Mars, say Mr Lowell
+and those who are of his opinion, is manifestly a solidly incrusted
+planet like the earth; it has an atmosphere, though one of great
+rarity; it has water vapor, as the snows in themselves prove; it has
+the alternation of day and night, and a succession of seasons closely
+resembling those of the earth; its surface is suggestively divided
+into regions of contrasting colors and appearance, and upon that
+surface we see an immense number of lines geometrically arranged, with
+a system of symmetrical intersections where the lines expand into
+circular and oval areas -- and all connected with the annual melting
+of the polar snows in a way which irresistibly suggests the
+interference of intelligence directed to a definite end. Why, with so
+many concurrent circumstances to support the hypothesis, should we not
+regard Mars as an inhabited globe?
+
+But the differences between Mars and the earth are in many ways as
+striking as their resemblances. Mars is relatively small; it gets less
+than half as much light and heat as we receive; its atmosphere is so
+rare that it would be distressing to us, even if we could survive in
+it at all; it has no lakes, rivers, or seas; its surface is an endless
+prairie. and its ``canals'' are phenomena utterly unlike anything on
+the earth. Yet it is precisely upon these divergences between the
+earth and Mars, this repudiation of terrestrial standards, that the
+theory of ``life on Mars,'' for which Mr Lowell is mainly responsible,
+is based. Because Mars is smaller than the earth, we are told it must
+necessarily be more advanced in planetary evolution, the underlying
+cause of which is the gradual cooling and contraction of the planet's
+mass. Mars has parted with its internal heat more rapidly than the
+earth; consequently its waters and its atmosphere have been mostly
+withdrawn by chemical combinations, but enough of both yet remain to
+render life still possible on its surface. As the globe of Mars is
+evolutionally older than that of the earth, so its forms of organic
+life may be proportionally further advanced, and its inhabitants may
+have attained a degree of cultivated intelligence much superior to
+what at present exists upon the earth. Understanding the nature and
+the causes of the desiccation of their planet, and possessing
+engineering science and capabilities far in advance of ours, they may
+be conceived to have grappled with the stupendous problem of keeping
+their world in a habitable condition as long as possible. Supposing
+them to have become accustomed to live in their rarefied atmosphere (a
+thing not inconceivable, since men can live for a time at least in air
+hardly less rare), the most pressing problem for them is that of a
+water-supply, without which plant life cannot exist, while animal life
+in turn depends for its existence upon vegetation. The only direction
+in which they can seek water is that of the polar regions, where it is
+alternately condensed into snow and released in the liquid form by the
+effect of the seasonal changes. It is, then, to the annual melting of
+the polar snow-fields that the Martian engineers are supposed to have
+recourse in supplying the needs of their planet, and thus providing
+the means of prolonging their own existence. It is imagined that they
+have for this purpose constructed a stupendous system of irrigation
+extending over the temperate and equatorial regions of the planet. The
+``canals'' represent the lines of irrigation, but the narrow streaks
+that we see are not the canals themselves, but the irrigated bands
+covered by them. Their dark hue, and their gradual appearance after
+the polar melting has begun, are due to the growth of vegetation
+stimulated by the water. The rounded areas visible where several
+``canals'' meet and cross are called by Mr Lowell ``oases.'' These are
+supposed to be the principal centers of population and industry. It
+must be confessed that some of them, with their complicated systems of
+radiating lines, appear to answer very well to such a theory. No
+attempt to explain them by analogy with natural phenomena on the earth
+has proved successful.
+
+But a great difficulty yet remains: How to explain the seemingly
+miraculous powers of the supposed engineers? Here recourse is had once
+more to the relative smallness of the planet. We have remarked that
+the force of gravity on Mars is only thirty-eight per cent of that on
+the earth. A steam-shovel driven by a certain horse-power would be
+nearly three times as effective there as here. A man of our stature on
+Mars would find his effective strength increased in the same
+proportion. But just because of the slight force of gravity there, a
+Martian might attain to the traditional stature of Goliath without
+finding his own weight an encumbrance to his activity, while at the
+same time his huge muscles would come into unimpeded play, enabling
+him single-handed to perform labors that would be impossible to a
+whole gang of terrestrial workmen. The effective powers of huge
+machines would be increased in the same way; and to all this must be
+added the fact that the mean density of the materials of which Mars is
+composed is much less than that of the constituents of the earth.
+Combining all these considerations, it becomes much less difficult to
+conceive that public works might be successfully undertaken on Mars
+which would be hopelessly beyond the limits of human accomplishment.
+
+Certain other difficulties have also to be met; as, for instance, the
+relative coldness of the climate of Mars. At its distance it gets
+considerably less than half as much light and heat as we receive. In
+addition to this, the rarity of its atmosphere would naturally be
+expected to decrease the effective temperature at the planet's
+surface, since an atmosphere acts somewhat like the glass cover of a
+hot-house in retaining the solar heat which has penetrated it. It has
+been calculated that, unless there are mitigating circumstances of
+which we know nothing, the average temperature at the surface of Mars
+must be far below the freezing-point of water. To this it is replied
+that the possible mitigating circumstances spoken of evidently exist
+in fact, because we can see that the watery vapor condenses into snow
+around the poles in winter, but melts again when summer comes. The
+mitigating agent may be supposed to exist in the atmosphere where the
+presence of certain gases would completely alter the temperature
+gradients.
+
+It might also be objected that it is inconceivable that the Martian
+engineers, however great may be their physical powers, and however
+gigantic the mechanical energies under their control, could force
+water in large quantities from the poles to the equator. This is an
+achievement that measures up to the cosmical standard. It is admitted
+by the champions of the theory that the difficulty is a formidable
+one; but they call attention to the singular fact that on Mars there
+can be found no chains of mountains, and it is even doubtful if ranges
+of hills exist there. The entire surface of the planet appears to be
+almost ``as smooth as a billiard ball,'' and even the broad regions
+which were once supposed to be seas apparently lie at practically the
+same level as the other parts, since the ``canals'' in many cases run
+uninterruptedly across them. Lowell's idea is that these sombre areas
+may be expanses of vegetation covering ground of a more or less marshy
+character, for while the largest of them appear to be permanent, there
+are some which vary coincidently with the variations of the canals.
+
+As to the kind of machinery employed to force the water from the
+poles, it has been conjectured that it may have taken the form of a
+gigantic system of pumps and conduits; and since the Martians are
+assumed to be so far in advance of us in their mastery of scientific
+principles, the hypothesis will at least not be harmed by supposing
+that they have learned to harness forces of nature whose very
+existence in a manageable form is yet unrecognized on the earth. If we
+wish to let the imagination loose, we may conjecture that they have
+conquered the secret of those intra-atomic forces whose resistless
+energy is beginning to become evident to us, but the possibility of
+whose utilization remains a dream, the fulfillment of which nobody
+dares to predict.
+
+Such, in very brief form, is the celebrated theory of Mars as an
+inhabited world. It certainly captivates the imagination, and if we
+believe it to represent the facts, we cannot but watch with the
+deepest sympathy this gallant struggle of an intellectual race to
+preserve its planet from the effects of advancing age and death. We
+may, indeed, wonder whether our own humanity, confronted by such a
+calamity, could be counted on to meet the emergency with equal
+stoutness of heart and inexhaustibleness of resource. Up to the
+present time we certainly have shown no capacity to confront Nature
+toe to toe, and to seize her by the shoulders and turn her round when
+she refuses to go our way. If we could get into wireless telephonic
+communication with the Martians we might learn from their own lips the
+secret of their more than ``Roman recovery.''
+
+The Riddle of the Asteroids
+
+Between the orbits of Mars and Jupiter revolves the most remarkable
+system of little bodies with which we are acquainted -- the Asteroids,
+or Minor Planets. Some six hundred are now known, and they may
+actually number thousands. They form virtually a ring about the sun.
+The most striking general fact about them is that they occupy the
+place in the sky which should be occupied, according to Bode's Law, by
+a single large planet. This fact, as we shall see, has led to the
+invention of one of the most extraordinary theories in astronomy --
+viz., that of the explosion of a world!
+
+Bode's Law, so-called, is only an empiric formula, but until the
+discovery of Neptune it accorded so well with the distances of the
+planets that astronomers were disposed to look upon it as really
+representing some underlying principle of planetary distribution. They
+were puzzled by the absence of a planet in the space between Mars and
+Jupiter, where the ``law'' demanded that there should be one, and an
+association of astronomers was formed to search for it. There was a
+decided sensation when, in 1801, Piazzi, of Palermo, announced that he
+had found a little planet which apparently occupied the place in the
+system which belonged to the missing body. He named it Ceres, and it
+was the first of the Asteroids. The next year Olbers, of Bremen, while
+looking for Ceres with his telescope, stumbled upon another small
+planet which he named Pallas. Immediately he was inspired with the
+idea that these two planets were fragments of a larger one which had
+formerly occupied the vacant place in the planetary ranks, and he
+predicted that others would be found by searching in the neighborhood
+of the intersection of the orbits of the two already discovered. This
+bold prediction was brilliantly fulfilled by the finding of two more
+-- Juno in 1804, and Vesta in 1807. Olbers would seem to have been led
+to the invention of his hypothesis of a planetary explosion by the
+faith which astronomers at that time had in Bode's Law. They appear to
+have thought that several planets revolving in the gap where the
+``law'' called for but one could only be accounted for upon the theory
+that the original one had been broken up to form the several.
+Gravitation demanded that the remnants of a planet blown to pieces, no
+matter how their orbits might otherwise differ, should all return at
+stated periods to the point where the explosion had occurred; hence
+Olbers' prediction that any asteroids that might subsequently be
+discovered would be found to have a common point of orbital
+intersection. And curiously enough all of the first asteroids found
+practically answered to this requirement. Olbers' theory seemed to be
+established.
+
+After the first four, no more asteroids were found until 1845, when
+one was discovered; then, in 1847, three more were added to the list;
+and after that searchers began to pick them up with such rapidity that
+by the close of the century hundreds were known, and it had become
+almost impossible to keep track of them. The first four are by far the
+largest members of the group, but their actual sizes remained unknown
+until less than twenty years ago. It was long supposed that Vesta was
+the largest, because it shines more brightly than any of the others;
+but finally, in 1895, Barnard, with the Lick telescope, definitely
+measured their diameters, and proved to everybody's surprise that
+Ceres is really the chief, and Vesta only the third in rank. His
+measures are as follows: Ceres, 477 miles; Pallas, 304 miles; Vesta,
+239 miles; and Juno, 120 miles. They differ greatly in the reflective
+power of their surfaces, a fact of much significance in connection
+with the question of their origin. Vesta is, surface for surface,
+rather more than three times as brilliant as Ceres, whence the
+original mistake about its magnitude.
+
+Nowadays new asteroids are found frequently by photography, but
+physically they are most insignificant bodies, their average diameter
+probably not exceeding twenty miles, and some are believed not to
+exceed ten. On a planet only ten miles in diameter, assuming the same
+mean density as the earth's, which is undoubtedly too much, the force
+of gravity would be so slight that an average man would not weigh more
+than three ounces, and could jump off into space whenever he liked.
+
+Although the asteroids all revolve around the sun in the same
+direction as that pursued by the major planets, their orbits are
+inclined at a great variety of angles to the general plane of the
+planetary system, and some of them are very eccentric -- almost as
+much so as the orbits of many of the periodic comets. It has even been
+conjectured that the two tiny moons of Mars and the four smaller
+satellites of Jupiter may be asteroids gone astray and captured by
+those planets. Two of the asteroids are exceedingly remarkable for the
+shapes and positions of their orbits; these are Eros, discovered in
+1898, and T. G., 1906, found eight years later. The latter has a mean
+distance from the sun slightly greater than that of Jupiter, while the
+mean distance of Eros is less than that of Mars. The orbit of Eros is
+so eccentric that at times it approaches within 15,000,000 miles of
+the earth, nearer than any other regular member of the solar system
+except the moon, thus affording an unrivaled means of measuring the
+solar parallax. But for our present purpose the chief interest of Eros
+lies in its extraordinary changes of light.
+
+These changes, although irregular, have been observed and photographed
+many times, and there seems to be no doubt of their reality. Their
+significance consists in their possible connection with the form of
+the little planet, whose diameter is generally estimated at not more
+than twenty miles. Von Oppolzer found, in 1901, that Eros lost
+three-fourths of its brilliancy once in every two hours and
+thirty-eight minutes. Other observers have found slightly different
+periods of variability, but none as long as three hours. The most
+interesting interpretation that has been offered of this phenomenon is
+that it is due to a great irregularity of figure, recalling at once
+Olbers' hypothesis. According to some, Eros may be double, the two
+bodies composing it revolving around each other at very close
+quarters; but a more striking, and it may be said probable, suggestion
+is that Eros has a form not unlike that of a dumb-bell, or hour-glass,
+turning rapidly end over end so that the area of illuminated surface
+presented to our eyes continually changes, reaching at certain times a
+minimum when the amount of light that it reflects toward the earth is
+reduced to a quarter of its maximum value. Various other bizarre
+shapes have been ascribed to Eros, such, for instance, as that of a
+flat stone revolving about one of its longer axes, so that sometimes
+we see its face and sometimes its edge.
+
+All of these explanations proceed upon the assumption that Eros cannot
+have a simple globular figure like that of a typical planet, a figure
+which is prescribed by the law of gravitation, but that its shape is
+what may be called accidental; in a word, it is a fragment, for it
+seems impossible to believe that a body formed in interplanetary
+space, either through nebular condensation or through the aggregation
+of particles drawn together by their mutual attractions, should not be
+practically spherical in shape. Nor is Eros the only asteroid that
+gives evidence by variations of brilliancy that there is something
+abnormal in its constitution; several others present the same
+phenomenon in varying degrees. Even Vesta was regarded by Olbers as
+sufficiently variable in its light to warrant the conclusion that it
+was an angular mass instead of a globe. Some of the smaller ones show
+very notable variations, and all in short periods, of three or four
+hours, suggesting that in turning about one of their axes they present
+a surface of variable extent toward the sun and the earth.
+
+The theory which some have preferred -- that the variability of light
+is due to the differences of reflective power on different parts of
+the surface -- would, if accepted, be hardly less suggestive of the
+origin of these little bodies by the breaking up of a larger one,
+because the most natural explanation of such differences would seem to
+be that they arose from variations in the roughness or smoothness of
+the reflecting surface, which would be characteristic of fragmentary
+bodies. In the case of a large planet alternating expanses of land and
+water, or of vegetation and desert, would produce a notable variation
+in the amount of reflection, but on bodies of the size of the
+asteroids neither water nor vegetation could exist, and an atmosphere
+would be equally impossible.
+
+One of the strongest objections to Olbers' hypothesis is that only a
+few of the first asteroids discovered travel in orbits which
+measurably satisfy the requirement that they should all intersect at
+the point where the explosion occurred. To this it was at first
+replied that the perturbations of the asteroidal orbits, by the
+attractions of the major planets, would soon displace them in such a
+manner that they would cease to intersect. One of the first
+investigations undertaken by the late Prof. Simon Newcomb was directed
+to the solution of this question, and he arrived at the conclusion
+that the planetary perturbations could not explain the actual
+situation of the asteroidal orbits. But afterward it was pointed out
+that the difficulty could be avoided by supposing that not one but a
+series of explosions had produced the asteroids as they now are. After
+the primary disruption the fragments themselves, according to this
+suggestion, may have exploded, and then the resulting orbits would be
+as ``tangled'' as the heart could wish. This has so far rehabilitated
+the explosion theory that it has never been entirely abandoned, and
+the evidence which we have just cited of the probably abnormal shapes
+of Eros and other asteroids has lately given it renewed life. It is a
+subject that needs a thorough rediscussion.
+
+We must not fail to mention, however, that there is a rival hypothesis
+which commends itself to many astronomers -- viz., that the asteroids
+were formed out of a relatively scant ring of matter, situated between
+Mars and Jupiter and resembling in composition the immensely more
+massive rings from which, according to Laplace's hypothesis, the
+planets were born. It is held by the supporters of this theory that
+the attraction of the giant Jupiter was sufficient to prevent the
+small, nebulous ring that gave birth to the asteroids from condensing
+like the others into a single planet.
+
+But if we accept the explosion theory, with its corollary that minor
+explosions followed the principal one, we have still an unanswered
+question before us: What caused the explosions? The idea of a world
+blowing up is too Titanic to be shocking; it rather amuses the
+imagination than seriously impresses it; in a word, it seems
+essentially chimerical. We can by no appeal to experience form a
+mental picture of such an occurrence. Even the moon did not blow up
+when it was wrecked by volcanoes. The explosive nebul� and new stars
+are far away in space, and suggest no connection with such a
+catastrophe as the bursting of a planet into hundreds of pieces. We
+cannot conceive of a great globe thousands of miles in diameter
+resembling a pellet of gunpowder only awaiting the touch of a match to
+cause its sudden disruption. Somehow the thought of human agency
+obtrudes itself in connection with the word ``explosion,'' and we
+smile at the idea that giant powder or nitro-glycerine could blow up a
+planet. Yet it would only need enough of them to do it.
+
+After all, we may deceive ourselves in thinking, as we are apt to do,
+that explosive energies lock themselves up only in small masses of
+matter. There are many causes producing explosions in nature, every
+volcanic eruption manifests the activity of some of them. Think of the
+giant power of confined steam; if enough steam could be suddenly
+generated in the center of the earth by a downpour of all the waters
+of the oceans, what might not the consequences be for our globe? In a
+smaller globe, and it has never been estimated that the original
+asteroid was even as large as the moon, such a catastrophe would,
+perhaps, be more easily conceivable; but since we are compelled in
+this case to assume that there was a series of successive explosions,
+steam would hardly answer the purpose; it would be more reasonable to
+suppose that the cause of the explosion was some kind of chemical
+reaction, or something affecting the atoms composing the exploding
+body. Here Dr Gustav Le Bon comes to our aid with a most startling
+suggestion, based on his theory of the dissipation of intra-atomic
+energy. It will be best to quote him at some length from his book on
+The Evolution of Forces.
+
+``It does not seem at first sight,'' says Doctor Le Bon,
+
+  very comprehensible that worlds which appear more and more stable
+  as they cool could become so unstable as to afterward dissociate
+  entirely. To explain this phenomenon, we will inquire whether
+  astronomical observations do not allow us to witness this
+  dissociation.
+
+  We know that the stability of a body in motion, such as a top or a
+  bicycle, ceases to be possible when its velocity of rotation
+  descends below a certain limit. Once this limit is reached it loses
+  its stability and falls to the ground. Prof. J. J. Thomson even
+  interprets radio-activity in this manner, and points out that when
+  the speed of the elements composing the atoms descends below a
+  certain limit they become unstable and tend to lose their
+  equilibria. There would result from this a commencement of
+  dissociation, with diminution of their potential energy and a
+  corresponding increase of their kinetic energy sufficient to launch
+  into space the products of intra-atomic disintegration.
+
+  It must not be forgotten that the atom being an enormous reservoir
+  of energy is by this very fact comparable with explosive bodies.
+  These last remain inert so long as their internal equilibria are
+  undisturbed. So soon as some cause or other modifies these, they
+  explode and smash everything around them after being themselves
+  broken to pieces.
+
+  Atoms, therefore, which grow old in consequence of the diminution
+  of a part of their intra-atomic energy gradually lose their
+  stability. A moment, then, arrives when this stability is so weak
+  that the matter disappears by a sort of explosion more or less
+  rapid. The bodies of the radium group offer an image of this
+  phenomenon -- a rather faint image, however, because the atoms of
+  this body have only reached a period of instability when the
+  dissociation is rather slow. It probably precedes another and more
+  rapid period of dissociation capable of producing their final
+  explosion. Bodies such as radium, thorium, etc., represent, no
+  doubt, a state of old age at which all bodies must some day arrive,
+  and which they already begin to manifest in our universe, since all
+  matter is slightly radio-active. It would suffice for the
+  dissociation to be fairly general and fairly rapid for an explosion
+  to occur in a world where it was manifested.
+
+  These theoretical considerations find a solid support in the sudden
+  appearances and disappearances of stars. The explosions of a world
+  which produce them reveal to us, perhaps, how the universes perish
+  when they become old.
+
+  As astronomical observations show the relative frequency of these
+  rapid destructions, we may ask ourselves whether the end of a
+  universe by a sudden explosion after a long period of old age does
+  not represent its most general ending.
+
+Here, perhaps, it will be well to stop, since, entrancing as the
+subject may be, we know very little about it, and Doctor Le Bon's
+theory affords a limitless field for the reader's imagination.
+  _________________________________________________________________
+
+A printed version of this book is available from Sattre Press
+(http://csky.sattre-press.com). It includes extensive annotations, a
+new introduction and all the original photographs and diagrams.
+
+
+
+
+
+
+
+
+
+
+End of Project Gutenberg's Curiosities of the Sky, by Garrett Serviss
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+The Project Gutenberg EBook of Curiosities of the Sky, by Garrett 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: Curiosities of the Sky
+
+Author: Garrett Serviss
+
+Posting Date: August 25, 2012 [EBook #6630]
+Release Date: October, 2004
+First Posted: January 6, 2003
+
+Language: English
+
+Character set encoding: ASCII
+
+*** START OF THIS PROJECT GUTENBERG EBOOK CURIOSITIES OF THE SKY ***
+
+
+
+
+Produced by William McClain
+
+
+
+
+
+
+
+
+
+
+                         Curiosities of the Sky
+
+by Garrett Serviss
+
+Curiosities of the Sky was first published in 1909 and the text is in
+the public domain. The transcription was done by William McClain
+(info@sattre-press.com), 2002.
+
+A printed version of this book is available from Sattre Press
+(http://csky.sattre-press.com). It includes extensive annotations, a
+new introduction and all the original photographs and diagrams.
+_________________________________________________________________
+
+Preface
+
+What Froude says of history is true also of astronomy: it is the most
+impressive where it transcends explanation. It is not the mathematics
+of astronomy, but the wonder and the mystery that seize upon the
+imagination. The calculation of an eclipse owes all its prestige to
+the sublimity of its data; the operation, in itself, requires no more
+mental effort than the preparation of a railway time-table.
+
+The dominion which astronomy has always held over the minds of men is
+akin to that of poetry; when the former becomes merely instructive and
+the latter purely didactic, both lose their power over the
+imagination. Astronomy is known as the oldest of the sciences, and it
+will be the longest-lived because it will always have arcana that have
+not been penetrated.
+
+Some of the things described in this book are little known to the
+average reader, while others are well known; but all possess the
+fascination of whatever is strange, marvelous, obscure, or mysterious
+-- magnified, in this case, by the portentous scale of the phenomena.
+
+The idea of the author is to tell about these things in plain
+language, but with as much scientific accuracy as plain language will
+permit, showing the wonder that is in them without getting away from
+the facts. Most of them have hitherto been discussed only in technical
+form, and in treatises that the general public seldom sees and never
+reads.
+
+Among the topics touched upon are:
+  * The strange unfixedness of the ``fixed stars,'' the vast
+    migrations of the suns and worlds constituting the universe.
+  * The slow passing out of existence of those collocations of stars
+    which for thousands of years have formed famous
+    ``constellations,'' preserving the memory of mythological heroes
+    and heroines, and perhaps of otherwise unrecorded history.
+  * The tendency of stars to assemble in immense clouds, swarms, and
+    clusters.
+  * The existence in some of the richest regions of the universe of
+    absolutely black, starless gaps, deeps, or holes, as if one were
+    looking out of a window into the murkiest night.
+  * The marvelous phenomena of new, or temporary, stars, which appear
+    as suddenly as conflagrations, and often turn into something else
+    as eccentric as themselves.
+  * The amazing forms of the ``whirlpool,'' ``spiral,'' ``pinwheel,''
+    and ``lace,'' or ``tress,'' nebulae.
+  * The strange surroundings of the sun, only seen in particular
+    circumstances, but evidently playing a constant part in the daily
+    phenomena of the solar system.
+  * The mystery of the Zodiacal Light and the Gegenschein.
+  * The extraordinary transformations undergone by comets and their
+    tails.
+  * The prodigies of meteorites and masses of stone and metal fallen
+    from the sky.
+  * The cataclysms that have wrecked the moon.
+  * The problem of life and intelligence on the planet Mars.
+  * The problematical origin and fate of the asteroids.
+  * The strange phenomena of the auroral lights.
+
+An attempt has been made to develop these topics in an orderly way,
+showing their connection, so that the reader may obtain a broad
+general view of the chief mysteries and problems of astronomy, and an
+idea of the immense field of discovery which still lies, almost
+unexplored, before it.
+
+The Windows of Absolute Night
+
+To most minds mystery is more fascinating than science. But when
+science itself leads straight up to the borders of mystery and there
+comes to a dead stop, saying, ``At present I can no longer see my
+way,'' the force of the charm is redoubled. On the other hand, the
+illimitable is no less potent in mystery than the invisible, whence
+the dramatic effect of Keats' ``stout Cortez'' staring at the
+boundless Pacific while all his men look at each other with a wild
+surmise, ``silent upon a peak in Darien.'' It is with similar feelings
+that the astronomer regards certain places where from the peaks of the
+universe his vision seems to range out into endless empty space. He
+sees there the shore of his little isthmus, and, beyond, unexplored
+immensity.
+
+The name, ``coal-sacks,'' given to these strange voids is hardly
+descriptive. Rather they produce upon the mind the effect of blank
+windows in a lonely house on a pitch-dark night, which, when looked at
+from the brilliant interior, become appalling in their rayless murk.
+Infinity seems to acquire a new meaning in the presence of these black
+openings in the sky, for as one continues to gaze it loses its purely
+metaphysical quality and becomes a kind of entity, like the ocean. The
+observer is conscious that he can actually see the beginning of its
+ebon depths, in which the visible universe appears to float like an
+enchanted island, resplendent within with lights and life and gorgeous
+spectacles, and encircled with screens of crowded stars, but with its
+dazzling vistas ending at the fathomless sea of pure darkness which
+encloses all.
+
+The Galaxy, or Milky Way, surrounds the borders of our island in space
+like a stellar garland, and when openings appear in it they are, by
+contrast, far more impressive than the general darkness of the
+interstellar expanse seen in other directions. Yet even that expanse
+is not everywhere equally dark, for it contains gloomy deeps
+discernable with careful watching. Here, too, contrast plays an
+important part, though less striking than within the galactic region.
+Some of Sir William Herschel's observations appear to indicate an
+association between these tenebrious spots and neighboring star clouds
+and nebulae. It is an illuminating bit of astronomical history that
+when he was sweeping the then virgin heavens with his great telescopes
+he was accustomed to say to his sister who, note-book in hand, waited
+at his side to take down his words, fresh with the inspiration of
+discovery: ``Prepare to write; the nebulae are coming; here space is
+vacant.''
+
+The most famous of the ``coal-sacks,'' and the first to be brought to
+general attention before astronomers had awakened to the significance
+of such things, lies adjacent to the ``Southern Cross,'' and is truly
+an amazing phenomenon. It is not alone the conspicuousness of this
+celestial vacancy, opening suddenly in the midst of one of the richest
+parts of the Galaxy, that has given it its fame, but quite as much the
+superstitious awe with which it was regarded by the early explorers of
+the South Seas. To them, as well as to those who listened in rapt
+wonder to their tales, the ``Coal-sack'' seemed to possess some occult
+connection with the mystic ``Cross.'' In the eyes of the sailors it
+was not a vacancy so much as a sable reality in the sky, and as,
+shuddering, they stared at it, they piously crossed themselves. It was
+another of the magical wonders of the unknown South, and as such it
+formed the basis of many a ``wild surmise'' and many a sea-dog's yarn.
+Scientific investigation has not diminished its prestige, and today no
+traveler in the southern hemisphere is indifferent to its fascinating
+strangeness, while some find it the most impressive spectacle of the
+antarctic heavens.
+
+All around, up to the very edge of the yawning gap, the sheen of the
+Milky Way is surpassingly glorious; but there, as if in obedience to
+an almighty edict, everything vanishes. A single faint star is visible
+within the opening, producing a curious effect upon the sensitive
+spectator, like the sight of a tiny islet in the midst of a black,
+motionless, waveless tarn. The dimensions of the lagoon of darkness,
+which is oval or pear-shaped, are eight degrees by five, so that it
+occupies a space in the sky about one hundred and thirty times greater
+than the area of the full moon. It attracts attention as soon as the
+eye is directed toward the quarter where it exists, and by virtue of
+the rarity of such phenomena it appears a far greater wonder than the
+drifts of stars that are heaped around it. Now that observatories are
+multiplying in the southern hemisphere, the great austral
+``Coal-sack'' will, no doubt, receive attention proportioned to its
+importance as one of the most significant features of the sky. Already
+at the Sydney Observatory photographs have shown that the southern
+portion of this Dead Sea of Space is not quite ``bottomless,''
+although its northern part defies the longest sounding lines of the
+astronomer.
+
+There is a similar, but less perfect, ``coal-sack'' in the northern
+hemisphere, in the constellation of ``The Swan,'' which, strange to
+say, also contains a well-marked figure of a cross outlined by stars.
+This gap lies near the top of the cross-shaped figure. It is best seen
+by averted vision, which brings out the contrast with the Milky Way,
+which is quite brilliant around it. It does not, however, exercise the
+same weird attraction upon the eye as the southern ``Coal-sack,'' for
+instead of looking like an absolute void in the sky, it rather appears
+as if a canopy of dark gauze had been drawn over the stars. We shall
+see the possible significance of this appearance later.
+
+Just above the southern horizon of our northern middle latitudes, in
+summer, where the Milky Way breaks up into vast sheets of nebulous
+luminosity, lying over and between the constellations Scorpio and
+Sagittarius, there is a remarkable assemblage of ``coal-sacks,''
+though none is of great size. One of them, near a conspicuous
+star-cluster in Scorpio, M80, is interesting for having been the first
+of these strange objects noted by Herschel. Probably it was its
+nearness to M80 which suggested to his mind the apparent connection of
+such vacancies with star-clusters which we have already mentioned.
+
+But the most marvelous of the ``coal-sacks'' are those that have been
+found by photography in Sagittarius. One of Barnard's earliest and
+most excellent photographs includes two of them, both in the
+star-cluster M8. The larger, which is roughly rectangular in outline,
+contains one little star, and its smaller neighbor is lune-shaped --
+surely a most singular form for such an object. Both are associated
+with curious dark lanes running through the clustered stars like
+trails in the woods. Along the borders of these lanes the stars are
+ranked in parallel rows, and what may be called the bottoms of the
+lanes are not entirely dark, but pebbled with faint stellar points.
+One of them which skirts the two dark gaps and traverses the cluster
+along its greatest diameter is edged with lines of stars, recalling
+the alignment of the trees bordering a French highway. This road of
+stars cannot be less than many billions of miles in length!
+
+All about the cluster the bed of the Galaxy is strangely disturbed,
+and in places nearly denuded, as if its contents had been raked away
+to form the immense stack and the smaller accumulations of stars
+around it. The well-known ``Trifid Nebula'' is also included in the
+field of the photograph, which covers a truly marvelous region, so
+intricate in its mingling of nebulae, star-clusters, star-swarms,
+star-streams, and dark vacancies that no description can do it
+justice. Yet, chaotic as it appears, there is an unmistakable
+suggestion of unity about it, impressing the beholder with the idea
+that all the different parts are in some way connected, and have not
+been fortuitously thrown together. Miss Agnes M. Clerke made the
+striking remark that the dusky lanes in M8 are exemplified on the
+largest scale in the great rift dividing the Milky Way, from Cygnus in
+the northern hemisphere all the way to the ``Cross'' in the southern.
+Similar lanes are found in many other clusters, and they are generally
+associated with flanking rows of stars, resembling in their
+arrangement the thick-set houses and villas along the roadways that
+traverse the approaches to a great city.
+
+But to return to the black gaps. Are they really windows in the
+star-walls of the universe? Some of them look rather as if they had
+been made by a shell fired through a luminous target, allowing the eye
+to range through the hole into the void space beyond. If science is
+discretely silent about these things, what can the more venturesome
+and less responsible imagination suggest? Would a huge ``runaway
+sun,'' like Arcturus, for instance, make such an opening if it should
+pass like a projectile through the Milky Way? It is at least a
+stimulating inquiry. Being probably many thousands of times more
+massive than the galactic stars, such a stellar missile would not be
+stopped by them, though its direction of flight might be altered. It
+would drag the small stars lying close to its course out of their
+spheres, but the ultimate tendency of its attraction would be to sweep
+them round in its wake, thus producing rather a star-swarm than a
+vacancy. Those that were very close to it might be swept away in its
+rush and become its satellites, careering away with it in its flight
+into outer space; but those that were farther off, and they would, of
+course, greatly outnumber the nearer ones, would tend inward from all
+sides toward the line of flight, as dust and leaves collect behind a
+speeding motor (though the forces operating would be different), and
+would fill up the hole, if hole it were. A swarm thus collected should
+be rounded in outline and bordered with a relatively barren ring from
+which the stars had been ``sucked'' away. In a general sense the M8
+cluster answers to this description, but even if we undertook to
+account for its existence by a supposition like the above, the black
+gaps would remain unexplained, unless one could make a further draft
+on the imagination and suggest that the stars had been thrown into a
+vast eddy, or system of eddies, whose vortices appear as dark holes.
+Only a maelstrom-like motion could keep such a funnel open, for
+without regard to the impulse derived from the projectile, the proper
+motions of the stars themselves would tend to fill it. Perhaps some
+other cause of the whirling motion may be found. As we shall see when
+we come to the spiral nebulae, gyratory movements are exceedingly
+prevalent throughout the universe, and the structure of the Milky Way
+is everywhere suggestive of them. But this is hazardous sport even for
+the imagination -- to play with suns as if they were but thistle-down
+in the wind or corks in a mill-race.
+
+Another question arises: What is the thickness of the hedge of stars
+through which the holes penetrate? Is the depth of the openings
+proportionate to their width? In other words, is the Milky Way round
+in section like a rope, or flat and thin like a ribbon? The answer is
+not obvious, for we have little or no information concerning the
+relative distances of the faint galactic stars. It would be easier,
+certainly, to conceive of openings in a thin belt than in a massive
+ring, for in the first case they would resemble mere rifts and breaks,
+while in the second they would be like wells or bore-holes. Then, too,
+the fact that the Milky Way is not a continuous body but is made up of
+stars whose actual distances apart is great, offers another quandary;
+persistent and sharply bordered apertures in such an assemblage are a
+priori as improbable, if not impossible, as straight, narrow holes
+running through a swarm of bees.
+
+The difficulty of these questions indicates one of the reasons why it
+has been suggested that the seeming gaps, or many of them, are not
+openings at all, but opaque screens cutting off the light from stars
+behind them. That this is quite possible in some cases is shown by
+Barnard's later photographs, particularly those of the singular region
+around the star Rho Ophiuchi. Here are to be seen somber lanes and
+patches, apparently forming a connected system which covers an immense
+space, and which their discoverer thinks may constitute a ``dark
+nebula.'' This seems at first a startling suggestion; but, after all,
+why should their not be dark nebulae as well as visible ones? In truth,
+it has troubled some astronomers to explain the luminosity of the
+bright nebulae, since it is not to be supposed that matter in so
+diffuse a state can be incandescent through heat, and phosphorescent
+light is in itself a mystery. The supposition is also in accord with
+what we know of the existence of dark solid bodies in space. Many
+bright stars are accompanied by obscure companions, sometimes as
+massive as themselves; the planets are non-luminous; the same is true
+of meteors before they plunge into the atmosphere and become heated by
+friction; and many plausible reasons have been found for believing
+that space contains as many obscure as shining bodies of great size.
+It is not so difficult, after all, then, to believe that there are
+immense collections of shadowy gases and meteoric dust whose presence
+is only manifested when they intercept the light coming from shining
+bodies behind them.
+
+This would account for the apparent extinguishment of light in open
+space, which is indicated by the falling off in relative number of
+telescopic stars below the tenth magnitude. Even as things are, the
+amount of light coming to us from stars too faint to be seen with the
+naked eye is so great that the statement of it generally surprises
+persons who are unfamiliar with the inner facts of astronomy. It has
+been calculated that on a clear night the total starlight from the
+entire celestial sphere amounts to one-sixtieth of the light of the
+full moon; but of this less than one-twenty-fifth is due to stars
+separately distinguished by the eye. If there were no obscuring medium
+in space, it is probable that the amount of starlight would be
+noticeably and perhaps enormously increased.
+
+But while it seems certain that some of the obscure spots in the Milky
+Way are due to the presence of ``dark nebulae,'' or concealing veils of
+one kind or another, it is equally certain that there are many which
+are true apertures, however they may have been formed, and by whatever
+forces they may be maintained. These, then, are veritable windows of
+the Galaxy, and when looking out of them one is face to face with the
+great mystery of infinite space. There the known universe visibly
+ends, but manifestly space itself does not end there. It is not within
+the power of thought to conceive an end to space, for the instant we
+think of a terminal point or line the mind leaps forward to the
+beyond. There must be space outside as well as inside. Eternity of
+time and infinity of space are ideas that the intellect cannot fully
+grasp, but neither can it grasp the idea of a limitation to either
+space or time. The metaphysical conceptions of hypergeometry, or
+fourth-dimensional space, do not aid us.
+
+Having, then, discovered that the universe is a thing contained in
+something indefinitely greater than itself; having looked out of its
+windows and found only the gloom of starless night outside -- what
+conclusions are we to draw concerning the beyond? It seems as empty as
+a vacuum, but is it really so? If it be, then our universe is a single
+atom astray in the infinite; it is the only island in an ocean without
+shores; it is the one oasis in an illimitable desert. Then the Milky
+Way, with its wide-flung garland of stars, is afloat like a tiny
+smoke-wreath amid a horror of immeasurable vacancy, or it is an
+evanescent and solitary ring of sparkling froth cast up for a moment
+on the viewless billows of immensity. From such conclusions the mind
+instinctively shrinks. It prefers to think that there is something
+beyond, though we cannot see it. Even the universe could not bear to
+be alone -- a Crusoe lost in the Cosmos! As the inhabitants of the
+most elegant chateau, with its gardens, parks, and crowds of
+attendants, would die of loneliness if they did not know that they
+have neighbors, though not seen, and that a living world of indefinite
+extent surrounds them, so we, when we perceive that the universe has
+limits, wish to feel that it is not solitary; that beyond the hedges
+and the hills there are other centers of life and activity. Could
+anything be more terrible than the thought of an isolated universe?
+The greater the being, the greater the aversion to seclusion. Only the
+infinite satisfies; in that alone the mind finds rest.
+
+We are driven, then, to believe that the universal night which
+envelopes us is not tenantless; that as we stare out of the
+star-framed windows of the Galaxy and see nothing but uniform
+blackness, the fault is with our eyes or is due to an obscuring
+medium. Since our universe is limited in extent, there must be other
+universes beyond it on all sides. Perhaps if we could carry our
+telescopes to the verge of the great ``Coal-sack'' near the ``Cross,''
+being then on the frontier of our starry system, we could discern,
+sparkling afar off in the vast night, some of the outer galaxies. They
+may be grander than ours, just as many of the suns surrounding us are
+immensely greater than ours. If we could take our stand somewhere in
+the midst of immensity and, with vision of infinite reach, look about
+us, we should perhaps see a countless number of stellar systems, amid
+which ours would be unnoticeable, like a single star among the
+multitude glittering in the terrestial sky on a clear night. Some
+might be in the form of a wreath, like our own; some might be
+globular, like the great star-clusters in Hercules and Centaurus; some
+might be glittering circles, or disks, or rings within rings. If we
+could enter them we should probably find a vast variety of
+composition, including elements unknown to terrestrial chemistry; for
+while the visible universe appears to contain few if any substances
+not existing on the earth or in the sun, we have no warrant to assume
+that others may not exist in infinite space.
+
+And how as to gravitation? We do not know that gravitation acts beyond
+the visible universe, but it is reasonable to suppose that it does. At
+any rate, if we let go its sustaining hand we are lost, and can only
+wander hopelessly in our speculations, like children astray. If the
+empire of gravitation is infinite, then the various outer systems must
+have some, though measuring by our standards an imperceptible,
+attractive influence upon each other, for gravitation never lets go
+its hold, however great the space over which it is required to act.
+Just as the stars about us are all in motion, so the starry systems
+beyond our sight may be in motion, and our system as a whole may be
+moving in concert with them. If this be so, then after interminable
+ages the aspect of the entire system of systems must change, its
+various members assuming new positions with respect to one another. In
+the course of time we may even suppose that our universe will approach
+relatively close to one of the others; and then, if men are yet living
+on the earth, they may glimpse through the openings which reveal
+nothing to us now, the lights of another nearing star system, like the
+signals of a strange squadron, bringing them the assurance (which can
+be but an inference at present) that the ocean of space has other
+argosies venturing on its limitless expanse.
+
+There remains the question of the luminiferous ether by whose agency
+the waves of light are borne through space. The ether is as mysterious
+as gravitation. With regard to ether we only infer its existence from
+the effects which we ascribe to it. Evidently the ether must extend as
+far as the most distant visible stars. But does it continue on
+indefinitely in outer space? If it does, then the invisibility of the
+other systems must be due to their distance diminishing the quantity
+of light that comes from them below the limit of perceptibility, or to
+the interposition of absorbing media; if it does not, then the reason
+why we cannot see them is owing to the absence of a means of
+conveyance for the light waves, as the lack of an interplanetary
+atmosphere prevents us from hearing the thunder of sun-spots. (It is
+interesting to recall that Mr Edison was once credited with the
+intention to construct a gigantic microphone which should render the
+roar of sun-spots audible by transforming the electric vibrations into
+sound-waves). On this supposition each starry system would be
+enveloped in its own globule of ether, and no light could cross from
+one to another. But the probability is that both the ether and
+gravitation are ubiquitous, and that all the stellar systems are
+immersed in the former like clouds of phosphorescent organisms in the
+sea.
+
+So astronomy carries the mind from height to greater height. Men were
+long in accepting the proofs of the relative insignificance of the
+earth; they were more quickly convinced of the comparative littleness
+of the solar system; and now the evidence assails their reason that
+what they had regarded as the universe is only one mote gleaming in
+the sunbeams of Infinity.
+
+Star-Clouds, Star-Clusters, and Star-Streams
+
+In the preceding chapter we have seen something of the strangely
+complicated structure of the Galaxy, or Milky Way. We now proceed to
+study more comprehensively that garlanded ``Pathway of the Gods.''
+
+Judged by the eye alone, the Milky Way is one of the most delicately
+beautiful phenomena in the entire realm of nature -- a shimmer of
+silvery gauze stretched across the sky; but studied in the light of
+its revelations, it is the most stupendous object presented to human
+ken. Let us consider, first, its appearance to ordinary vision. Its
+apparent position in the sky shifts according to the season. On a
+serene, cloudless summer evening, in the absence of the moon, whose
+light obscures it, one sees the Galaxy spanning the heavens from north
+to southeast of the zenith like a phosphorescent arch. In early spring
+it forms a similar but, upon the whole, less brilliant arch west of
+the zenith. Between spring and summer it lies like a long, faint,
+twilight band along the northern horizon. At the beginning of winter
+it again forms an arch, this time spanning the sky from east to west,
+a little north of the zenith. These are its positions as viewed from
+the mean latitude of the United States. Even the beginner in
+star-gazing does not have to watch it throughout the year in order to
+be convinced that it is, in reality, a great circle, extending
+entirely around the celestial sphere. We appear to be situated near
+its center, but its periphery is evidently far away in the depths of
+space.
+
+Although to the casual observer it seems but a delicate scarf of
+light, brighter in some places than in others, but hazy and indefinite
+at the best, such is not its appearance to those who study it with
+care. They perceive that it is an organic whole, though marvelously
+complex in detail. The telescope shows that it consists of stars too
+faint and small through excess of distance to be separately visible.
+Of the hundred million suns which some estimates have fixed as the
+probable population of the starry universe, the vast majority (at
+least thirty to one) are included in this strange belt of misty light.
+But they are not uniformly distributed in it; on the contrary, they
+are arrayed in clusters, knots, bunches, clouds, and streams. The
+appearance is somewhat as if the Galaxy consisted of innumerable
+swarms of silver-winged bees, more or less intermixed, some massed
+together, some crossing the paths of others, but all governed by a
+single purpose which leads them to encircle the region of space in
+which we are situated.
+
+From the beginning of the systematic study of the heavens, the fact
+has been recognized that the form of the Milky Way denotes the scheme
+of the sidereal system. At first it was thought that the shape of the
+system was that of a vast round disk, flat like a cheese, and filled
+with stars, our sun and his relatively few neighbors being placed near
+the center. According to this view, the galactic belt was an effect of
+perspective; for when looking in the direction of the plane of the
+disk, the eye ranged through an immense extension of stars which
+blended into a glimmering blur, surrounding us like a ring; while when
+looking out from the sides of the disk we saw but few stars, and in
+those directions the heavens appeared relatively blank. Finally it was
+recognized that this theory did not correspond with the observed
+appearances, and it became evident that the Milky Way was not a mere
+effect of perspective, but an actual band of enormously distant stars,
+forming a circle about the sphere, the central opening of the ring
+(containing many scattered stars) being many times broader than the
+width of the ring itself. Our sun is one of the scattered stars in the
+central opening.
+
+As already remarked, the ring of the Galaxy is very irregular, and in
+places it is partly broken. With its sinuous outline, its pendant
+sprays, its graceful and accordant curves, its bunching of masses, its
+occasional interstices, and the manifest order of a general plan
+governing the jumble of its details, it bears a remarkable resemblance
+to a garland -- a fact which appears the more wonderful when we recall
+its composition. That an elm-tree should trace the lines of beauty
+with its leafy and pendulous branches does not surprise us; but we can
+only gaze with growing amazement when we behold a hundred million suns
+imitating the form of a chaplet! And then we have to remember that
+this form furnishes the ground-plan of the universe.
+
+As an indication of the extraordinary speculations to which the
+mystery of the Milky Way has given rise, a theory recently (1909)
+proposed by Prof. George C. Comstock may be mentioned. Starting with
+the data (first) that the number of stars increases as the Milky Way
+is approached, and reaches a maximum in its plane, while on the other
+hand the number of nebulae is greatest outside the Milky Way and
+increases with distance from it, and (second) that the Milky Way,
+although a complete ring, is broad and diffuse on one side through
+one-half its course -- that half alone containing nebulae -- and
+relatively narrow and well defined on the opposite side, the author of
+this singular speculation avers that these facts can best be explained
+by supposing that the invisible universe consists of two
+interpenetrating parts, one of which is a chaos of indefinite extent,
+strewn with stars and nebulous dust, and the other a long, broad but
+comparatively thin cluster of stars, including the sun as one of its
+central members. This flat star-cluster is conceived to be moving
+edgewise through the chaos, and, according to Professor Comstock, it
+acts after the manner of a snow-plough sweeping away the cosmic dust
+and piling it on either hand above and below the plane of the moving
+cluster. It thus forms a transparent rift, through which we see
+farther and command a view of more stars than through the intensified
+dust-clouds on either hand. This rift is the Milky Way. The dust
+thrown aside toward the poles of the Milky Way is the substance of the
+nebulae which abound there. Ahead, where the front of the star-plough
+is clearing the way, the chaos is nearer at hand, and consequently
+there the rift subtends a broader angle, and is filled with primordial
+dust, which, having been annexed by the vanguard of the star-swarm,
+forms the nebulae seen only in that part of the Milky Way. But behind,
+the rift appears narrow because there we look farther away between
+dust-clouds produced ages ago by the front of the plough, and no
+scattered dust remains in that part of the rift.
+
+In quoting an outline of this strikingly original theory the present
+writer should not be understood as assenting to it. That it appears
+bizarre is not, in itself, a reason for rejecting it, when we are
+dealing with so problematical and enigmatical a subject as the Milky
+Way; but the serious objection is that the theory does not
+sufficiently accord with the observed phenomena. There is too much
+evidence that the Milky Way is an organic system, however fantastic
+its form, to permit the belief that it can only be a rift in chaotic
+clouds. As with every organism, we find that its parts are more or
+less clearly repeated in its ensemble. Among all the strange things
+that the Milky Way contains there is nothing so extraordinary as
+itself. Every astronomer must many times have found himself marveling
+at it in those comparatively rare nights when it shows all its beauty
+and all its strangeness. In its great broken rifts, divisions, and
+spirals are found the gigantic prototypes of similar forms in its
+star-clouds and clusters. As we have said, it determines the general
+shape of the whole sidereal system. Some of the brightest stars in the
+sky appear to hang like jewels suspended at the ends of tassels
+dropped from the Galaxy. Among these pendants are the Pleiades and the
+Hyades. Orion, too, the ``Mighty Hunter,'' is caught in ``a loop of
+light'' thrown out from it. The majority of the great first-magnitude
+stars seem related to it, as if they formed an inner ring inclined at
+an angle of some twenty degrees to its plane. Many of the long curves
+that set off from it on both sides are accompanied by corresponding
+curves of lucid stars. In a word, it offers every appearance of
+structural connection with the entire starry system. That the universe
+should have assumed the form of a wreath is certainly a matter for
+astonishment; but it would have been still more astonishing if it had
+been a cube, a rhomboid, or a dodecahedron, for then we should have
+had to suppose that something resembling the forces that shape
+crystals had acted upon the stars, and the difficulty of explaining
+the universe by the laws of gravitation would have been increased.
+
+From the Milky Way as a whole we pass to the vast clouds, swarms, and
+clusters of stars of which it is made up. It may be, as some
+astronomers hold, that most of the galactic stars are much smaller
+than the sun, so that their faintness is not due entirely to the
+effect of distance. Still, their intrinsic brilliance attests their
+solar character, and considering their remoteness, which has been
+estimated at not less than ten thousand to twenty thousand light-years
+(a light-year is equal to nearly six thousand thousand million miles)
+their actual masses cannot be extremely small. The minutest of them
+are entitled to be regarded as real suns, and they vary enormously in
+magnitude. The effects of their attractions upon one another can only
+be inferred from their clustering, because their relative movements
+are not apparent on account of the brevity of the observations that we
+can make. But imagine a being for whom a million years would be but as
+a flitting moment; to him the Milky Way would appear in a state of
+ceaseless agitation -- swirling with ``a fury of whirlpool motion.''
+
+The cloud-like aspect of large parts of the Galaxy must always have
+attracted attention, even from naked-eye observers, but the true
+star-clouds were first satisfactorily represented in Barnard's
+photographs. The resemblance to actual clouds is often startling. Some
+are close-packed and dense, like cumuli; some are wispy or mottled,
+like cirri. The rifts and modulations, as well as the general
+outlines, are the same as those of clouds of vapor or dust, and one
+notices also the characteristic thinning out at the edges. But we must
+beware of supposing that the component suns are thickly crowded as the
+particles forming an ordinary cloud. They look, indeed, as if they
+were matted together, because of the irradiation of light, but in
+reality millions and billions of miles separate each star from its
+neighbors. Nevertheless they form real assemblages, whose members are
+far more closely related to one another than is our sun to the stars
+around him, and if we were in the Milky Way the aspect of the
+nocturnal sky would be marvelously different from its present
+appearance.
+
+Stellar clouds are characteristic of the Galaxy and are not found
+beyond its borders, except in the ``Magellanic Clouds'' of the
+southern hemisphere, which resemble detached portions of the Milky
+Way. These singular objects form as striking a peculiarity of the
+austral heavens as does the great ``Coal-sack'' described in Chapter
+1. But it is their isolation that makes them so remarkable, for their
+composition is essentially galactic, and if they were included within
+its boundaries they would not appear more wonderful than many other
+parts of the Milky Way. Placed where they are, they look like masses
+fallen from the great stellar arch. They are full of nebulae and
+star-clusters, and show striking evidences of spiral movement.
+
+Star-swarms, which are also characteristic features of the Galaxy,
+differ from star-clouds very much in the way that their name would
+imply -- i.e., their component stars are so arranged, even when they
+are countless in number, that the idea of an exceedingly numerous
+assemblage rather than that of a cloud is impressed on the observer's
+mind. In a star-swarm the separate members are distinguishable because
+they are either larger or nearer than the stars composing a ``cloud.''
+A splendid example of a true star-swarm is furnished by Chi Persei, in
+that part of the Milky Way which runs between the constellations
+Perseus and Cassiopeia. This swarm is much coarser than many others,
+and can be seen by the naked eye. In a small telescope it appears
+double, as if the suns composing it had divided into two parties which
+keep on their way side by side, with some commingling of their members
+where the skirts of the two companies come in contact.
+
+Smaller than either star-clouds or star-swarms, and differing from
+both in their organization, are star-clusters. These, unlike the
+others, are found outside as well as inside the Milky Way, although
+they are more numerous inside its boundaries than elsewhere. The term
+star-cluster is sometimes applied, though improperly, to assemblages
+which are rather groups, such, for instance, as the Pleiades. In their
+most characteristic aspect star-clusters are of a globular shape --
+globes of suns! A famous example of a globular star-cluster, but one
+not included in the Milky Way, is the ``Great Cluster in Hercules.''
+This is barely visible to the naked eye, but a small telescope shows
+its character, and in a large one it presents a marvelous spectacle.
+Photographs of such clusters are, perhaps, less effective than those
+of star-clouds, because the central condensation of stars in them is
+so great that their light becomes blended in an indistinguishable
+blur. The beautiful effect of the incessant play of infinitesimal rays
+over the apparently compact surface of the cluster, as if it were a
+globe of the finest frosted silver shining in an electric beam, is
+also lost in a photograph. Still, even to the eye looking directly at
+the cluster through a powerful telescope, the central part of the
+wonderful congregation seems almost a solid mass in which the stars
+are packed like the ice crystals in a snowball.
+
+The same question rises to the lips of every observer: How can they
+possibly have been brought into such a situation? The marvel does not
+grow less when we know that, instead of being closely compacted, the
+stars of the cluster are probably separated by millions of miles; for
+we know that their distances apart are slight as compared with their
+remoteness from the Earth. Sir William Herschel estimated their number
+to be about fourteen thousand, but in fact they are uncountable. If we
+could view them from a point just within the edge of the assemblage,
+they would offer the appearance of a hollow hemisphere emblazoned with
+stars of astonishing brilliancy; the near-by ones unparalleled in
+splendor by any celestial object known to us, while the more distant
+ones would resemble ordinary stars. An inhabitant of the cluster would
+not know, except by a process of ratiocination, that he was dwelling
+in a globular assemblage of suns; only from a point far outside would
+their spherical arrangement become evident to the eye. Imagine
+fourteen-thousand fire-balloons with an approach to regularity in a
+spherical space -- say, ten miles in diameter; there would be an
+average of less than thirty in every cubic mile, and it would be
+necessary to go to a considerable distance in order to see them as a
+globular aggregation; yet from a point sufficiently far away they
+would blend into a glowing ball.
+
+Photographs show even better than the best telescopic views that the
+great cluster is surrounded with a multitude of dispersed stars,
+suggestively arrayed in more or less curving lines, which radiate from
+the principle mass, with which their connection is manifest. These
+stars, situated outside the central sphere, look somewhat like vagrant
+bees buzzing round a dense swarm where the queen bee is sitting. Yet
+while there is so much to suggest the operation of central forces,
+bringing and keeping the members of the cluster together, the
+attentive observer is also impressed with the idea that the whole
+wonderful phenomenon may be the result of explosion. As soon as this
+thought seizes the mind, confirmation of it seems to be found in the
+appearance of the outlying stars, which could be as readily explained
+by the supposition that they have been blown apart as that they have
+flocked together toward a center. The probable fact that the stars
+constituting the cluster are very much smaller than our sun might be
+regarded as favoring the hypothesis of an explosion. Of their real
+size we know nothing, but, on the basis of an uncertain estimate of
+their parallax, it has been calculated that they may average
+forty-five thousand miles in diameter -- something more than half the
+diameter of the planet Jupiter. Assuming the same mean density,
+fourteen thousand such stars might have been formed by the explosion
+of a body about twice the size of the sun. This recalls the theory of
+Olbers, which has never been altogether abandoned or disproved, that
+the Asteroids were formed by the explosion of a planet circulating
+between the orbits of Mars and Jupiter. The Asteroids, whatever their
+manner of origin, form a ring around the sun; but, of course, the
+explosion of a great independent body, not originally revolving about
+a superior center of gravitational force, would not result in the
+formation of a ring of small bodies, but rather of a dispersed mass of
+them. But back of any speculation of this kind lies the problem, at
+present insoluble: How could the explosion be produced? (See the
+question of explosions in Chapters 6 and 14).
+
+Then, on the other hand, we have the observation of Herschel, since
+abundantly confirmed, that space is unusually vacant in the immediate
+neighborhood of condensed star-clusters and nebulae, which, as far as
+it goes, might be taken as an indication that the assembled stars had
+been drawn together by their mutual attractions, and that the tendency
+to aggregation is still bringing new members toward the cluster. But
+in that case there must have been an original condensation of stars at
+that point in space. This could probably have been produced by the
+coagulation of a great nebula into stellar nuclei, a process which
+seems now to be taking place in the Orion Nebula.
+
+A yet more remarkable globular star-cluster exists in the southern
+hemisphere, Omega Centauri. In this case the central condensation of
+stars presents an almost uniform blaze of light. Like the Hercules
+cluster, that in Centaurus is surrounded with stars scattered over a
+broad field and showing an appearance of radial arrangement. In fact,
+except for its greater richness, Omega Centauri is an exact duplicate
+of its northern rival. Each appears to an imaginative spectator as a
+veritable ``city of suns.'' Mathematics shrinks from the task of
+disentangling the maze of motions in such an assemblage. It would seem
+that the chance of collisions is not to be neglected, and this idea
+finds a certain degree of confirmation in the appearance of
+``temporary stars'' which have more than once blazed out in, or close
+by, globular star-clusters.
+
+This leads up to the notable fact, first established by Professor
+Bailey a few years ago, that such clusters are populous with variable
+stars. Omega Centauri and the Hercules cluster are especially
+remarkable in this respect. The variables found in them are all of
+short period and the changes of light show a noteworthy tendency to
+uniformity. The first thought is that these phenomena must be due to
+collisions among the crowded stars, but, if so, the encounters cannot
+be between the stars themselves, but probably between stars and meteor
+swarms revolving around them. Such periodic collisions might go on for
+ages without the meteors being exhausted by incorporation with the
+stars. This explanation appears all the more probable because one
+would naturally expect that flocks of meteors would abound in a close
+aggregation of stars. It is also consistent with Perrine's discovery
+-- that the globular star clusters are powdered with minute stars
+strewn thickly among the brighter ones.
+
+In speaking of Professor Comstock's extraordinary theory of the Milky
+Way, the fact was mentioned that, broadly speaking, the nebulae are
+less numerous in the galactic belt than in the comparatively open
+spaces on either side of it, but that they are, nevertheless, abundant
+in the broader half of the Milky Way which he designates as the front
+of the gigantic ``plough'' supposed to be forcing its way through the
+enveloping chaos. In and around the Sagittarius region the
+intermingling of nebulae and galactic star clouds and clusters is
+particularly remarkable. That there is a causal connection no
+thoughtful person can doubt. We are unable to get away from the
+evidence that a nebula is like a seed-ground from which stars spring
+forth; or we may say that nebulae resemble clouds in whose bosom
+raindrops are forming. The wonderful aspect of the admixtures of
+nebulae and star-clusters in Sagittarius has been described in Chapter
+1. We now come to a still more extraordinary phenomenon of this kind
+-- the Pleiades nebulae.
+
+The group of the Pleiades, although lying outside the main course of
+the Galaxy, is connected with it by a faint loop, and is the scene of
+the most remarkable association of stars and nebulous matter known in
+the visible universe. The naked eye is unaware of the existence of
+nebulae in the Pleiades, or, at the best, merely suspects that there is
+something of the kind there; and even the most powerful telescopes are
+far from revealing the full wonder of the spectacle; but in
+photographs which have been exposed for many hours consecutively, in
+order to accumulate the impression of the actinic rays, the revelation
+is stunning. The principle stars are seen surrounded by, and, as it
+were, drowned in, dense nebulous clouds of an unparalleled kind. The
+forms assumed by these clouds seem at first sight inexplicable. They
+look like fleeces, or perhaps more like splashes and daubs of luminous
+paint dashed carelessly from a brush. But closer inspection shows that
+they are, to a large extent, woven out of innumerable threads of filmy
+texture, and there are many indications of spiral tendencies. Each of
+the bright stars of the group -- Alcyone, Merope, Maia, Electra,
+Taygeta, Atlas -- is the focus of a dense fog (totally invisible,
+remember, alike to the naked eye and to the telescope), and these
+particular stars are veiled from sight behind the strange mists.
+Running in all directions across the relatively open spaces are
+nebulous wisps and streaks of the most curious forms. On some of the
+nebular lines, which are either straight throughout, or if they change
+direction do so at an angle, little stars are strung like beads. In
+one case seven or eight stars are thus aligned, and, as if to
+emphasize their dependence upon the chain which connects them, when it
+makes a slight bend the file of stars turns the same way. Many other
+star rows in the group suggest by their arrangement that they, too,
+were once strung upon similar threads which have now disappeared,
+leaving the stars spaced along their ancient tracks. We seem forced to
+the conclusion that there was a time when the Pleiades were embedded
+in a vast nebula resembling that of Orion, and that the cloud has now
+become so rare by gradual condensation into stars that the merest
+trace of it remains, and this would probably have escaped detection
+but for the remarkable actinic power of the radiant matter of which it
+consists. The richness of many of these faint nebulous masses in
+ultra-violet radiations, which are those that specifically affect the
+photographic plate, is the cause of the marvelous revelatory power of
+celestial photography. So the veritable unseen universe, as
+distinguished from the ``unseen universe'' of metaphysical
+speculation, is shown to us.
+
+A different kind of association between stars and nebulae is shown in
+some surprising photographic objects in the constellation Cygnus,
+where long, wispy nebulae, billions of miles in length, some of them
+looking like tresses streaming in a breeze, lie amid fields of stars
+which seem related to them. But the relation is of a most singular
+kind, for notwithstanding the delicate structure of the long nebulae
+they appear to act as barriers, causing the stars to heap themselves
+on one side. The stars are two, three, or four times as numerous on
+one side of the nebulae as on the other. These nebulae, as far as
+appearance goes, might be likened to rail fences, or thin hedges,
+against which the wind is driving drifts of powdery snow, which, while
+scattered plentifully all around, tends to bank itself on the leeward
+side of the obstruction. The imagination is at a loss to account for
+these extraordinary phenomena; yet there they are, faithfully giving
+us their images whenever the photographic plate is exposed to their
+radiations.
+
+Thus the more we see of the universe with improved methods of
+observation, and the more we invent aids to human senses, each
+enabling us to penetrate a little deeper into the unseen, the greater
+becomes the mystery. The telescope carried us far, photography is
+carrying us still farther; but what as yet unimagined instrument will
+take us to the bottom, the top, and the end? And then, what hitherto
+untried power of thought will enable us to comprehend the meaning of
+it all?
+
+Stellar Migrations
+
+To the untrained eye the stars and the planets are not
+distinguishable. It is customary to call them all alike ``stars.'' But
+since the planets more or less rapidly change their places in the sky,
+in consequence of their revolution about the sun, while the stars
+proper seem to remain always in the same relative positions, the
+latter are spoken of as ``fixed stars.'' In the beginnings of
+astronomy it was not known that the ``fixed stars'' had any motion
+independent of their apparent annual revolution with the whole sky
+about the earth as a seeming center. Now, however, we know that the
+term ``fixed stars'' is paradoxical, for there is not a single really
+fixed object in the whole celestial sphere. The apparent fixity in the
+positions of the stars is due to their immense distance, combined with
+the shortness of the time during which we are able to observe them. It
+is like viewing the plume of smoke issuing from a steamer, hull down,
+at sea: if one does not continue to watch it for a long time it
+appears to be motionless, although in reality it may be traveling at
+great speed across the line of sight. Even the planets seem fixed in
+position if one watches them for a single night only, and the more
+distant ones do not sensibly change their places, except after many
+nights of observation. Neptune, for instance, moves but little more
+than two degrees in the course of an entire year, and in a month its
+change of place is only about one-third of the diameter of the full
+moon.
+
+Yet, fixed as they seem, the stars are actually moving with a speed in
+comparison with which, in some cases, the planets might almost be said
+to stand fast in their tracks. Jupiter's speed in his orbit is about
+eight miles per second, Neptune's is less than three and one-half
+miles, and the earth's is about eighteen and one-half miles; while
+there are ``fixed stars'' which move two hundred or three hundred
+miles per second. They do not all, however, move with so great a
+velocity, for some appear to travel no faster than the planets. But in
+all cases, notwithstanding their real speed, long-continued and
+exceedingly careful observations are required to demonstrate that they
+are moving at all. No more overwhelming impression of the frightful
+depths of space in which the stars are buried can be obtained than by
+reflecting upon the fact that a star whose actual motion across the
+line of sight amounts to two hundred miles per second does not change
+its apparent place in the sky, in the course of a thousand years,
+sufficiently to be noticed by the casual observer of the heavens!
+
+There is one vast difference between the motions of the stars and
+those of the planets to which attention should be at once called: the
+planets, being under the control of a central force emanating from
+their immediate master, the sun, all move in the same direction and in
+orbits concentric about the sun; the stars, on the other hand, move in
+every conceivable direction and have no apparent center of motion, for
+all efforts to discover such a center have failed. At one time, when
+theology had finally to accept the facts of science, a grandiose
+conception arose in some pious minds, according to which the Throne of
+God was situated at the exact center of His Creation, and, seated
+there, He watched the magnificent spectacle of the starry systems
+obediently revolving around Him. Astronomical discoveries and
+speculations seemed for a time to afford some warrant for this view,
+which was, moreover, an acceptable substitute for the abandoned
+geocentric theory in minds that could only conceive of God as a
+superhuman artificer, constantly admiring his own work. No longer ago
+than the middle of the nineteenth century a German astronomer,
+Maedler, believed that he had actually found the location of the
+center about which the stellar universe revolved. He placed it in the
+group of the Pleiades, and upon his authority an extraordinary
+imaginative picture was sometimes drawn of the star Alcyone, the
+brightest of the Pleiades, as the very seat of the Almighty. This idea
+even seemed to gain a kind of traditional support from the mystic
+significance, without known historical origin, which has for many
+ages, and among widely separated peoples, been attached to the
+remarkable group of which Alcyone is the chief. But since Maedler's
+time it has been demonstrated that the Pleiades cannot be the center
+of revolution of the universe, and, as already remarked, all attempts
+to find or fix such a center have proved abortive. Yet so powerful was
+the hold that the theory took upon the popular imagination, that even
+today astronomers are often asked if Alcyone is not the probable site
+of ``Jerusalem the Golden.''
+
+If there were a discoverable center of predominant gravitative power,
+to which the motions of all the stars could be referred, those motions
+would appear less mysterious, and we should then be able to conclude
+that the universe was, as a whole, a prototype of the subsidiary
+systems of which it is composed. We should look simply to the law of
+gravitation for an explanation, and, naturally, the center would be
+placed within the opening enclosed by the Milky Way. If it were there
+the Milky Way itself should exhibit signs of revolution about it, like
+a wheel turning upon its hub. No theory of the star motions as a whole
+could stand which failed to take account of the Milky Way as the basis
+of all. But the very form of that divided wreath of stars forbids the
+assumption of its revolution about a center. Even if it could be
+conceived as a wheel having no material center it would not have the
+form which it actually presents. As was shown in Chapter 2, there is
+abundant evidence of motion in the Milky Way; but it is not motion of
+the system as a whole, but motion affecting its separate parts.
+Instead of all moving one way, the galactic stars, as far as their
+movements can be inferred, are governed by local influences and
+conditions. They appear to travel crosswise and in contrary
+directions, and perhaps they eddy around foci where great numbers have
+assembled; but of a universal revolution involving the entire mass we
+have no evidence.
+
+Most of our knowledge of star motions, called ``proper motions,''
+relates to individual stars and to a few groups which happen to be so
+near that the effects of their movements are measurable. In some cases
+the motion is so rapid (not in appearance, but in reality) that the
+chief difficulty is to imagine how it can have been imparted, and what
+will eventually become of the ``runaways.'' Without a collision, or a
+series of very close approaches to great gravitational centers, a star
+traveling through space at the rate of two hundred or three hundred
+miles per second could not be arrested or turned into an orbit which
+would keep it forever flying within the limits of the visible
+universe. A famous example of these speeding stars is ``1830
+Groombridge,'' a star of only the sixth magnitude, and consequently
+just visible to the naked eye, whose motion across the line of sight
+is so rapid that it moves upon the face of the sky a distance equal to
+the apparent diameter of the moon every 280 years. The distance of
+this star is at least 200,000,000,000,000 miles, and may be two or
+three times greater, so that its actual speed cannot be less than two
+hundred, and may be as much as four hundred, miles per second. It
+could be turned into a new course by a close approach to a great sun,
+but it could only be stopped by collision, head-on, with a body of
+enormous mass. Barring such accidents it must, as far as we can see,
+keep on until it has traversed our stellar system, whence in may
+escape and pass out into space beyond, to join, perhaps, one of those
+other universes of which we have spoken. Arcturus, one of the greatest
+suns in the universe, is also a runaway, whose speed of flight has
+been estimated all the way from fifty to two hundred miles per second.
+Arcturus, we have every reason to believe, possesses hundreds of times
+the mass of our sun -- think, then, of the prodigious momentum that
+its motion implies! Sirius moves more moderately, its motion across
+the line of sight amounting to only ten miles per second, but it is at
+the same time approaching the sun at about the same speed, its actual
+velocity in space being the resultant of the two displacements.
+
+What has been said about the motion of Sirius brings us to another
+aspect of this subject. The fact is, that in every case of stellar
+motion the displacement that we observe represents only a part of the
+actual movement of the star concerned. There are stars whose motion
+carries them straight toward or straight away from the earth, and such
+stars, of course, show no cross motion. But the vast majority are
+traveling in paths inclined from a perpendicular to our line of sight.
+Taken as a whole, the stars may be said to be flying about like the
+molecules in a mass of gas. The discovery of the radial component in
+the movements of the stars is due to the spectroscope. If a star is
+approaching, its spectral lines are shifted toward the violet end of
+the spectrum by an amount depending upon the velocity of approach; if
+it is receding, the lines are correspondingly shifted toward the red
+end. Spectroscopic observation, then, combined with micrometric
+measurements of the cross motion, enables us to detect the real
+movement of the star in space. Sometimes it happens that a star's
+radial movement is periodically reversed; first it approaches, and
+then it recedes. This indicates that it is revolving around a near-by
+companion, which is often invisible, and superposed upon this motion
+is that of the two stars concerned, which together may be approaching
+or receding or traveling across the line of sight. Thus the
+complications involved in the stellar motions are often exceedingly
+great and puzzling.
+
+Yet another source of complication exists in the movement of our own
+star, the sun. There is no more difficult problem in astronomy than
+that of disentangling the effects of the solar motion from those of
+the motions of the other stars. But the problem, difficult as it is,
+has been solved, and upon its solution depends our knowledge of the
+speed and direction of the movement of the solar system through space,
+for of course the sun carries its planets with it. One element of the
+solution is found in the fact that, as a result of perspective, the
+stars toward which we are going appear to move apart toward all points
+of the compass, while those behind appear to close up together. Then
+the spectroscopic principle already mentioned is invoked for studying
+the shift of the lines, which is toward the violet in the stars ahead
+of us and toward the red in those that we are leaving behind. Of
+course the effects of the independent motions of the stars must be
+carefully excluded. The result of the studies devoted to this subject
+is to show that we are traveling at a speed of twelve to fifteen miles
+per second in a northerly direction, toward the border of the
+constellations Hercules and Lyra. A curious fact is that the more
+recent estimates show that the direction is not very much out of a
+straight line drawn from the sun to the star Vega, one of the most
+magnificent suns in the heavens. But it should not be inferred from
+this that Vega is drawing us on; it is too distant for its gravitation
+to have such an effect.
+
+Many unaccustomed thoughts are suggested by this mighty voyage of the
+solar system. Whence have we come, and whither do we go? Every year of
+our lives we advance at least 375,000,000 miles. Since the traditional
+time of Adam the sun has led his planets through the wastes of space
+no less than 225,000,000,000 miles, or more than 2400 times the
+distance that separates him from the earth. Go back in imagination to
+the geologic ages, and try to comprehend the distance over which the
+earth has flown. Where was our little planet when it emerged out of
+the clouds of chaos? Where was the sun when his ``thunder march''
+began? What strange constellations shone down upon our globe when its
+masters of life were the monstrous beasts of the ``Age of Reptiles''?
+A million years is not much of a span of time in geologic reckoning,
+yet a million years ago the earth was farther from its present place
+in space than any of the stars with a measurable parallax are now. It
+was more than seven times as far as Sirius, nearly fourteen times as
+far as Alpha Centauri, three times as far as Vega, and twice as far as
+Arcturus. But some geologists demand two hundred, three hundred, even
+one thousand million years to enable them to account for the
+evolutionary development of the earth and its inhabitants. In a
+thousand million years the earth would have traveled farther than from
+the remotest conceivable depths of the Milky Way!
+
+Other curious reflections arise when we think of the form of the
+earth's track as it follows the lead of the sun, in a journey which
+has neither known beginning nor conceivable end. There are probably
+many minds which have found a kind of consolation in the thought that
+every year the globe returns to the same place, on the same side of
+the sun. This idea may have an occult connection with our traditional
+regard for anniversaries. When that period of the year returns at
+which any great event in our lives has occurred we have the feeling
+that the earth, in its annual round, has, in a manner, brought us back
+to the scene of that event. We think of the earth's orbit as a
+well-worn path which we traverse many times in the course of a
+lifetime. It seems familiar to us, and we grow to have a sort of
+attachment to it. The sun we are accustomed to regard as a fixed
+center in space, like the mill or pump around which the harnessed
+patient mule makes his endless circuits. But the real fact is that the
+earth never returns to the place in space where it has once quitted.
+In consequence of the motion of the sun carrying the earth and the
+other planets along, the track pursued by our globe is a vast spiral
+in space continually developing and never returning upon its course.
+It is probable that the tracks of the sun and the others stars are
+also irregular, and possibly spiral, although, as far as can be at
+present determined, they appear to be practically straight. Every
+star, wherever it may be situated, is attracted by its fellow-stars
+from many sides at once, and although the force is minimized by
+distance, yet in the course of many ages its effects must become
+manifest.
+
+Looked at from another side, is there not something immensely
+stimulating and pleasing to the imagination in the idea of so
+stupendous a journey, which makes all of us the greatest of travelers?
+In the course of a long life a man is transported through space thirty
+thousand million miles; Halley's Comet does not travel one-quarter as
+far in making one of its immense circuits. And there are adventures on
+this voyage of which we are just beginning to learn to take account.
+Space is full of strange things, and the earth must encounter some of
+them as it advances through the unknown. Many singular speculations
+have been indulged in by astronomers concerning the possible effects
+upon the earth of the varying state of the space that it traverses.
+Even the alternation of hot and glacial periods has sometimes been
+ascribed to this source. When tropical life flourished around the
+poles, as the remains in the rocks assure us, the needed high
+temperature may, it has been thought, have been derived from the
+presence of the earth in a warm region of space. Then, too, there is a
+certain interest for us in the thought of what our familiar planet has
+passed through. We cannot but admire it for its long journeying as we
+admire the traveler who comes to us from remote and unexplored lands,
+or as we gaze with a glow of interest upon the first locomotive that
+has crossed a continent, or a ship that has visited the Arctic or
+Antarctic regions. If we may trust the indications of the present
+course, the earth, piloted by the sun, has come from the Milky Way in
+the far south and may eventually rejoin that mighty band of stars in
+the far north.
+
+While the stars in general appear to travel independently of one
+another, except when they are combined in binary or trinary systems,
+there are notable exceptions to this rule. In some quarters of the sky
+we behold veritable migrations of entire groups of stars whose members
+are too widely separated to show any indications of revolution about a
+common center of gravity. This leads us back again to the wonderful
+group of the Pleiades. All of the principle stars composing that group
+are traveling in virtually parallel lines. Whatever force set them
+going evidently acted upon all alike. This might be explained by the
+assumption that when the original projective force acted upon them
+they were more closely united than they are at present, and that in
+drifting apart they have not lost the impulse of the primal motion. Or
+it may be supposed that they are carried along by some current in
+space, although it would be exceedingly difficult, in the present
+state of our knowledge, to explain the nature of such a current. Yet
+the theory of a current has been proposed. As to an attractive center
+around which they might revolve, none has been found. Another instance
+of similar ``star-drift'' is furnished by five of the seven stars
+constituting the figure of the ``Great Dipper.'' In this case the
+stars concerned are separated very widely, the two extreme ones by not
+less than fifteen degrees, so that the idea of a common motion would
+never have been suggested by their aspect in the sky; and the case
+becomes the more remarkable from the fact that among and between them
+there are other stars, some of the same magnitude, which do not share
+their motion, but are traveling in other directions. Still other
+examples of the same phenomenon are found in other parts of the sky.
+Of course, in the case of compact star-clusters, it is assumed that
+all the members share a like motion of translation through space, and
+the same is probably true of dense star-swarms and star-clouds.
+
+The whole question of star-drift has lately assumed a new phase, in
+consequence of the investigations of Kapteyn, Dyson, and Eddington on
+the ``systematic motions of the stars.'' This research will, it is
+hoped, lead to an understanding of the general law governing the
+movements of the whole body of stars constituting the visible
+universe. Taking about eleven hundred stars whose proper motions have
+been ascertained with an approach to certainty, and which are
+distributed in all parts of the sky, it has been shown that there
+exists an apparent double drift, in two independent streams, moving in
+different and nearly opposed directions. The apex of the motion of
+what is called ``Stream I'' is situated, according to Professor
+Kapteyn, in right ascension 85 deg., declination south 11 deg., which places
+it just south of the constellation Orion; while the apex of ``Stream
+II'' is in right ascension 260 deg., declination south 48 deg., placing it in
+the constellation Ara, south of Scorpio. The two apices differ very
+nearly 180 deg. in right ascension and about 120 deg. in declination. The
+discovery of these vast star-streams, if they really exist, is one of
+the most extraordinary in modern astronomy. It offers the correlation
+of stellar movements needed as the basis of a theory of those
+movements, but it seems far from revealing a physical cause for them.
+As projected against the celestial sphere the stars forming the two
+opposite streams appear intermingled, some obeying one tendency and
+some the other. As Professor Dyson has said, the hypothesis of this
+double movement is of a revolutionary character, and calls for further
+investigation. Indeed, it seems at first glance not less surprising
+than would be the observation that in a snow-storm the flakes over our
+heads were divided into two parties and driving across each other's
+course in nearly opposite directions, as if urged by interpenetrating
+winds.
+
+But whatever explanation may eventually be found for the motions of
+the stars, the knowledge of the existence of those motions must always
+afford a new charm to the contemplative observer of the heavens, for
+they impart a sense of life to the starry system that would otherwise
+be lacking. A stagnant universe, with every star fixed immovably in
+its place, would not content the imagination or satisfy our longing
+for ceaseless activity. The majestic grandeur of the evolutions of the
+celestial hosts, the inconceivable vastness of the fields of space in
+which they are executed, the countless numbers, the immeasurable
+distances, the involved convolutions, the flocking and the scattering,
+the interpenetrating marches and countermarches, the strange community
+of impulsion affecting stars that are wide apart in space and causing
+them to traverse the general movement about them like aides and
+despatch-bearers on a battle-field -- all these arouse an intensity of
+interest which is heightened by the mystery behind them.
+
+The Passing of the Constellations
+
+From a historical and picturesque point of view, one of the most
+striking results of the motions of the stars described in the last
+chapter is their effect upon the forms of the constellations, which
+have been watched and admired by mankind from a period so early that
+the date of their invention is now unknown. The constellations are
+formed by chance combinations of conspicuous stars, like figures in a
+kaleidoscope, and if our lives were commensurate with the aeons of
+cosmic existence we should perceive that the kaleidoscope of the
+heavens was ceaselessly turning and throwing the stars into new
+symmetries. Even if the stars stood fast, the motion of the solar
+system would gradually alter the configurations, as the elements of a
+landscape dissolve and recombine in fresh groupings with the
+traveler's progress amid them. But with the stars themselves all in
+motion at various speeds and in many directions, the changes occur
+more rapidly. Of course, ``rapid'' is here understood in a relative
+sense; the wheel of human history to an eye accustomed to the majestic
+progression of the universe would appear to revolve with the velocity
+of a whirling dynamo. Only the deliberation of geological movements
+can be contrasted with the evolution and devolution of the
+constellations.
+
+And yet this secular fluctuation of the constellation figures is not
+without keen interest for the meditative observer. It is another
+reminder of the swift mutability of terrestial affairs. To the passing
+glance, which is all that we can bestow upon these figures, they
+appear so immutable that they have been called into service to form
+the most lasting records of ancient thought and imagination that we
+possess. In the forms of the constellations, the most beautiful, and,
+in imaginative quality, the finest, mythology that the world has ever
+known has been perpetuated. Yet, in a broad sense, this scroll of
+human thought imprinted on the heavens is as evanescent as the summer
+clouds. Although more enduring than parchment, tombs, pyramids, and
+temples, it is as far as they from truly eternizing the memory of what
+man has fancied and done.
+
+Before studying the effects that the motions of the stars have had and
+will have upon the constellations, it is worth while to consider a
+little further the importance of the stellar pictures as archives of
+history. To emphasize the importance of these effects it is only
+necessary to recall that the constellations register the oldest
+traditions of our race. In the history of primeval religions they are
+the most valuable of documents. Leaving out of account for the moment
+the more familiar mythology of the Greeks, based on something older
+yet, we may refer for illustration to that of the mysterious Maya race
+of America. At Izamal, in Yucatan, says Mr Stansbury Hagar, is a group
+of ruins perched, after the Mexican and Central-American plan, on the
+summits of pyramidal mounds which mark the site of an ancient
+theogonic center of the Mayas. Here the temples all evidently refer to
+a cult based upon the constellations as symbols. The figures and the
+names, of course, were not the same as those that we have derived from
+our Aryan ancestors, but the star groups were the same or nearly so.
+For instance, the loftiest of the temples at Izamal was connected with
+the sign of the constellation known to us as Cancer, marking the place
+of the sun at the summer solstice, at which period the sun was
+supposed to descend at noon like a great bird of fire and consume the
+offerings left upon the altar. Our Scorpio was known to the Mayas as a
+sign of the ``Death God.'' Our Libra, the ``Balance,'' with which the
+idea of a divine weighing out of justice has always been connected,
+seems to be identical with the Mayan constellation Teoyaotlatohua,
+with which was associated a temple where dwelt the priests whose
+special business it was to administer justice and to foretell the
+future by means of information obtained from the spirits of the dead.
+Orion, the ``Hunter'' of our celestial mythology, was among the Mayas
+a ``Warrior,'' while Sagittarius and others of our constellations were
+known to them (under different names, of course), and all were endowed
+with a religious symbolism. And the same star figures, having the same
+significance, were familiar to the Peruvians, as shown by the temples
+at Cuzco. Thus the imagination of ancient America sought in the
+constellations symbols of the unchanging gods.
+
+But, in fact, there is no nation and no people that has not recognized
+the constellations, and at one period or another in its history
+employed them in some symbolic or representative capacity. As handled
+by the Greeks from prehistoric times, the constellation myths became
+the very soul of poetry. The imagination of that wonderful race
+idealized the principal star groups so effectively that the figures
+and traditions thus attached to them have, for civilized mankind,
+displaced all others, just as Greek art in its highest forms stands
+without parallel and eclipses every rival. The Romans translated no
+heroes and heroines of the mythical period of their history to the
+sky, and the deified Caesars never entered that lofty company, but the
+heavens are filled with the early myths of the Greeks. Herakles
+nightly resumes his mighty labors in the stars; Zeus, in the form of
+the white ``Bull,'' Taurus, bears the fair Europa on his back through
+the celestial waves; Andromeda stretches forth her shackled arms in
+the star-gemmed ether, beseeching aid; and Perseus, in a blaze of
+diamond armor, revives his heroic deeds amid sparkling clouds of
+stellar dust. There, too, sits Queen Cassiopeia in her dazzling chair,
+while the Great King, Cepheus, towers gigantic over the pole.
+Professor Young has significantly remarked that a great number of the
+constellations are connected in some way or other with the Argonautic
+Expedition -- that strangely fascinating legend of earliest Greek
+story which has never lost its charm for mankind. In view of all this,
+we may well congratulate ourselves that the constellations will
+outlast our time and the time of countless generations to follow us;
+and yet they are very far from being eternal. Let us now study some of
+the effects of the stellar motions upon them.
+
+We begin with the familiar figure of the ``Great Dipper.'' He who has
+not drunk inspiration from its celestial bowl is not yet admitted to
+the circle of Olympus. This figure is made up of seven conspicuous
+stars in the constellation Ursa Major, the ``Greater Bear.'' The
+handle of the ``Dipper'' corresponds to the tail of the imaginary
+``Bear,'' and the bowl lies upon his flank. In fact, the figure of a
+dipper is so evident and that of a bear so unevident, that to most
+persons the ``Great Dipper'' is the only part of the constellation
+that is recognizable. Of the seven stars mentioned, six are of nearly
+equal brightness, ranking as of the second magnitude, while the
+seventh is of only the third magnitude. The difference is very
+striking, since every increase of one magnitude involves an increase
+of two-and-a-half times in brightness. There appears to be little
+doubt that the faint star, which is situated at the junction of the
+bowl and the handle, is a variable of long period, since three hundred
+years ago it was as bright as its companions. But however that may be,
+its relative faintness at the present time interferes but little with
+the perfection of the ``Dipper's'' figure. In order the more readily
+to understand the changes which are taking place, it will be well to
+mention both the names and the Greek letters which are attached to the
+seven stars. Beginning at the star in the upper outer edge of the rim
+of the bowl and running in regular order round the bottom and then out
+to the end of the handle, the names and letters are as follows: Dubhe
+({\alpha}), Merak ({\beta}), Phaed ({\gamma}), Megrez ({\delta}),
+Alioth ({\epsilon}), Mizar ({\zeta}), and Benetnasch ({\eta}). Megrez
+is the faint star already mentioned at the junction of the bowl and
+handle, and Mizar, in the middle of the handle, has a close, naked-eye
+companion which is named Alcor. The Arabs called this singular pair of
+stars ``The Horse and Rider.'' Merak and Duhbe are called ``The
+Pointers,'' because an imaginary line drawn northward through them
+indicates the Pole Star.
+
+Now it has been found that five of these stars -- viz., Merak, Phaed,
+Megrez, Alioth, and Mizar (with its comrade) -- are moving with
+practically the same speed in an easterly direction, while the other
+two, Dubhe and Benetnasch, are simultaneously moving westward, the
+motions of Benetnasch being apparently more rapid. The consequence of
+these opposed motions is, of course, that the figure of the ``Dipper''
+cannot always have existed and will not continue to exist. In the
+accompanying diagrams it has been thought interesting to show the
+relative positions of these seven stars, as seen from the point which
+the earth now occupies, both in the past and in the future. Arrows
+attached to the stars in the figure representing the present
+appearance of the ``Dipper'' indicate the directions of the motions
+and the distances over which they will carry the stars in a period of
+about five hundred centuries. The time, no doubt, seems long, but
+remember the vast stretch of ages through which the earth has passed,
+and then reflect that no reason is apparent why our globe should not
+continue to be a scene of animation for ten thousand centuries yet to
+come. The fact that the little star Alcor placed so close to Mizar
+should accompany the latter in its flight is not surprising, but that
+two of the principal stars of the group should be found moving in a
+direction directly opposed to that pursued by the other five is
+surprising in the highest degree; and it recalls the strange theory of
+a double drift affecting all the stars, to which attention was called
+in the preceding chapter. It would appear that Benetnasch and Dubhe
+belong to one ``current,'' and Merak, Phaed, Megrez, Alioth, and Mizar
+to the other. As far as is known, the motion of the seven stars are
+not shared by the smaller stars scattered about them, but on the
+theory of currents there should be such a community of motion, and
+further investigation may reveal it.
+
+From the ``Great Dipper'' we turn to a constellation hardly less
+conspicuous and situated at an equal distance from the pole on the
+other side -- Cassiopeia. This famous star-group commemorating the
+romantic Queen of Ethiopia whose vain boasting of her beauty was
+punished by the exposure of her daughter Andromeda to the ``Sea
+Monster,'' is well-marked by five stars which form an irregular letter
+``W'' with its open side toward the pole. Three of these stars are
+usually ranked as of the second magnitude, and two of the third; but
+to ordinary observation they appear of nearly equal brightness, and
+present a very striking picture. They mark out the chair and a part of
+the figure of the beautiful queen. Beginning at the right-hand, or
+western, end of the ``W,'' their Greek letter designations are: Beta
+({\beta}), Alpha ({\alpha}), Gamma ({\gamma}), Delta ({\delta}), and
+Epsilon ({\epsilon}). Four of them, Beta, Alpha, Delta, and Epsilon
+are traveling eastwardly at various speeds, while the fifth, Gamma,
+moves in a westerly direction. The motion of Beta is more rapid than
+that of any of the others. It should be said, however, that no little
+uncertainty attaches to the estimates of the rate of motion of stars
+which are not going very rapidly, and different observers often vary
+considerably in their results.
+
+In the beautiful ``Northern Crown,'' one of the most perfect and
+charming of all the figures to be found in the stars, the alternate
+combining and scattering effects of the stellar motions are shown by
+comparing the appearance which the constellation must have had five
+hundred centuries ago with that which it has at present and that which
+it will have in the future. The seven principle stars of the asterism,
+forming a surprisingly perfect coronet, have movements in three
+directions at right angles to one another. That in these circumstances
+they should ever have arrived at positions giving them so striking an
+appearance of definite association is certainly surprising; from its
+aspect one would have expected to find a community of movement
+governing the brilliants of the ``Crown,'' but instead of that we find
+evidence that they will inevitably drift apart and the beautiful
+figure will dissolve.
+
+A similar fate awaits such asterisms as the ``Northern Cross'' in
+Cygnus; the ``Crow'' (Corvus), which stands on the back of the great
+``Sea Serpent,'' Hydra, and pecks at his scales; ``Job's Coffin''
+(Delphinus); the ``Great Square of Pegasus''; the ``Twins'' (Gemini);
+the beautiful ``Sickle'' in Leo; and the exquisite group of the Hyades
+in Taurus. In the case of the Hyades, two controlling movements are
+manifest: one, affecting five of the stars which form the well-known
+figure of a letter ``V,'' is directed northerly; the other, which
+controls the direction of two stars, has an easterly trend. The chief
+star of the group, Aldebaran, one of the finest of all stars both for
+its brilliance and its color, is the most affected by the easterly
+motion. In time it will drift entirely out of connection with its
+present neighbors. Although the Hyades do not form so compact a group
+as the Pleiades in the same constellation, yet their appearance of
+relationship is sufficient to awaken a feeling of surprise over the
+fact that, as with the stars of the ``Dipper,'' their association is
+only temporary or apparent.
+
+The great figure of Orion appears to be more lasting, not because its
+stars are physically connected, but because of their great distance,
+which renders their movements too deliberate to be exactly
+ascertained. Two of the greatest of its stars, Betelgeuse and Rigel,
+possess, as far as has been ascertained, no perceptible motion across
+the line of sight, but there is a little movement perceptible in the
+``Belt.'' At the present time this consists of an almost perfect
+straight line, a row of second-magnitude stars about equally spaced
+and of the most striking beauty. In the course of time, however, the
+two right-hand stars, Mintaka and Alnilam (how fine are these Arabic
+star names!) will approach each other and form a naked-eye double, but
+the third, Alnita, will drift away eastward, so that the ``Belt'' will
+no longer exist.
+
+For one more example, let us go to the southern hemisphere, whose most
+celebrated constellation, the ``Southern Cross,'' has found a place in
+all modern literatures, although it has no claim to consideration on
+account of association with ancient legends. This most attractive
+asterism, which has never ceased to fascinate the imagination of
+Christendom since it was first devoutly described by the early
+explorers of the South, is but a passing collocation of brilliant
+stars. Yet even in its transfigurations it has been for hundreds of
+centuries, and will continue to be for hundreds of centuries to come,
+a most striking object in the sky. Our figures show its appearance in
+three successive phases: first, as it was fifty thousand years ago
+(viewed from the earth's present location); second, as it is in our
+day; and, third, as it will be an equal time in the future. The
+nearness of these bright stars to one another -- the length of the
+longer beam of the ``Cross'' is only six degrees -- makes this group
+very noticeable, whatever the arrangement of its components may be.
+The largest star, at the base of the ``Cross,'' is of the first
+magnitude, two of the others are of the second magnitude, and the
+fourth is of the third. Other stars, not represented in the figures,
+increase the effect of a celestial blazonry, although they do not help
+the resemblance to a cross.
+
+But since the motion of the solar system itself will, in the course of
+so long a period as fifty thousand years, produce a great change in
+the perspective of the heavens as seen from the earth, by carrying us
+nearly nineteen trillion miles from our present place, why, it may be
+asked, seek to represent future appearances of the constellations
+which we could not hope to see, even if we could survive so long? The
+answer is: Because these things aid the mind to form a picture of the
+effects of the mobility of the starry universe. Only by showing the
+changes from some definite point of view can we arrive at a due
+comprehension of them. The constellations are more or less familiar to
+everybody, so that impending changes of their forms must at once
+strike the eye and the imagination, and make clearer the significance
+of the movements of the stars. If the future history of mankind is to
+resemble its past and if our race is destined to survive yet a million
+years, then our remote descendents will see a ``new heavens'' if not a
+``new earth,'' and will have to invent novel constellations to
+perpetuate their legends and mythologies.
+
+If our knowledge of the relative distances of the stars were more
+complete, it would be an interesting exercise in celestial geometry to
+project the constellations probably visible to the inhabitants of
+worlds revolving around some of the other suns of space. Our sun is
+too insignificant for us to think that he can make a conspicuous
+appearance among them, except, perhaps, in a few cases. As seen, for
+instance, from the nearest known star, Alpha Centauri, the sun would
+appear of the average first magnitude, and consequently from that
+standpoint he might be the gem of some little constellation which had
+no Sirius, or Arcturus, or Vega to eclipse him with its superior
+splendor. But from the distance of the vast majority of the stars the
+sun would probably be invisible to the naked eye, and as seen from
+nearer systems could only rank as a fifth or sixth magnitude star,
+unnoticed and unknown except by the star-charting astronomer.
+
+Conflagrations in the Heavens
+
+Suppose it were possible for the world to take fire and burn up -- as
+some pessimists think that it will do when the Divine wrath shall have
+sufficiently accumulated against it -- nobody out of our own little
+corner of space would ever be aware of the catastrophe! With all their
+telescopes, the astronomers living in the golden light of Arcturus or
+the diamond blaze of Canopus would be unable to detect the least
+glimmer of the conflagration that had destroyed the seat of Adam and
+his descendents, just as now they are totally ignorant of its
+existence.
+
+But at least fifteen times in the course of recorded history men
+looking out from the earth have beheld in the remote depths of space
+great outbursts of fiery light, some of them more splendidly luminous
+than anything else in the firmament except the sun! If they were
+conflagrations, how many million worlds like ours were required to
+feed their blaze?
+
+It is probable that ``temporary'' or ``new'' stars, as these wonderful
+apparitions are called, really are conflagrations; not in the sense of
+a bonfire or a burning house or city, but in that of a sudden eruption
+of inconceivable heat and light, such as would result from the
+stripping off the shell of an encrusted sun or the crashing together
+of two mighty orbs flying through space with a hundred times the
+velocity of the swiftest cannon-shot.
+
+Temporary stars are the rarest and most erratic of astronomical
+phenomena. The earliest records relating to them are not very clear,
+and we cannot in every instance be certain that it was one of these
+appearances that the ignorant and superstitious old chroniclers are
+trying to describe. The first temporary star that we are absolutely
+sure of appeared in 1572, and is known as ``Tycho's Star,'' because
+the celebrated Danish astronomer (whose remains, with his
+gold-and-silver artificial nose -- made necessary by a duel -- still
+intact, were disinterred and reburied in 1901) was the first to
+perceive it in the sky, and the most assiduous and successful in his
+studies of it. As the first fully accredited representative of its
+class, this new star made its entry upon the scene with becoming
+eclat. It is characteristic of these phenomena that they burst into
+view with amazing suddenness, and, of course, entirely unexpectedly.
+Tycho's star appeared in the constellation Cassiopeia, near a now
+well-known and much-watched little star named Kappa, on the evening of
+November 11, 1572. The story has often been repeated, but it never
+loses interest, how Tycho, going home that evening, saw people in the
+street pointing and staring at the sky directly over their heads, and
+following the direction of their hands and eyes he was astonished to
+see, near the zenith, an unknown star of surpassing brilliance. It
+outshone the planet Jupiter, and was therefore far brighter than the
+first magnitude. There was not another star in the heavens that could
+be compared with it in splendor. Tycho was not in all respects free
+from the superstitions of his time -- and who is? -- but he had the
+true scientific instinct, and immediately he began to study the
+stranger, and to record with the greatest care every change in its
+aspect. First he determined as well as he could with the imperfect
+instruments of his day, many of which he himself had invented, the
+precise location of the phenomena in the sky. Then he followed the
+changes that it underwent. At first it brightened until its light
+equaled or exceeded that of the planet Venus at her brightest, a
+statement which will be appreciated at its full value by anyone who
+has ever watched Venus when she plays her dazzling role of ``Evening
+Star,'' flaring like an arc light in the sunset sky. It even became so
+brilliant as to be visible in full daylight, since, its position being
+circumpolar, it never set in the latitude of Northern Europe. Finally
+it began to fade, turning red as it did so, and in March, 1574, it
+disappeared from Tycho's searching gaze, and has never been seen again
+from that day to this. None of the astronomers of the time could make
+anything of it. They had not yet as many bases of speculation as we
+possess today.
+
+Tycho's star has achieved a romantic reputation by being fancifully
+identified with the ``Star of Bethlehem,'' said to have led the
+wondering Magi from their eastern deserts to the cradle-manger of the
+Savior in Palestine. Many attempts have been made to connect this
+traditional ``star'' with some known phenomenon of the heavens, and
+none seems more idle than this. Yet it persistently survives, and no
+astronomer is free from eager questions about it addressed by people
+whose imagination has been excited by the legend. It is only necessary
+to say that the supposition of a connection between the phenomenon of
+the Magi and Tycho's star is without any scientific foundation. It was
+originally based on an unwarranted assumption that the star of Tycho
+was a variable of long period, appearing once every three hundred and
+fifteen years, or thereabout. If that were true there would have been
+an apparition somewhere near the traditional date of the birth of
+Christ, a date which is itself uncertain. But even the data on which
+the assumption was based are inconsistent with the theory. Certain
+monkish records speak of something wonderful appearing in the sky in
+the years 1264 and 945, and these were taken to have been outbursts of
+Tycho's star. Investigation shows that the records more probably refer
+to comets, but even if the objects seen were temporary stars, their
+dates do not suit the hypothesis; from 945 to 1264 there is a gap of
+319 years, and from 1264 to 1572 one of only 308 years; moreover 337
+years have now (1909) elapsed since Tycho saw the last glimmer of his
+star. Upon a variability so irregular and uncertain as that, even if
+we felt sure that it existed, no conclusion could be found concerning
+an apparition occurring 2000 years ago.
+
+In the year 1600 (the year in which Giordano Bruno was burned at the
+stake for teaching that there is more than one physical world), a
+temporary star of the third magnitude broke out in the constellation
+Cygnus, and curiously enough, considering the rarity of such
+phenomena, only four years later another surprisingly brilliant one
+appeared in the constellation Ophiuchus. This is often called
+``Kepler's star,'' because the great German astronomer devoted to it
+the same attention that Tycho had given to the earlier phenomenon. It,
+too, like Tycho's, was at first the brightest object in the stellar
+heavens, although it seems never to have quite equaled its famous
+predecessor in splendor. It disappeared after a year, also turning of
+a red color as it became more faint. We shall see the significance of
+this as we go on. Some of Kepler's contemporaries suggested that the
+outburst of this star was due to a meeting of atoms in space, and idea
+bearing a striking resemblance to the modern theory of ``astronomical
+collisions.''
+
+In 1670, 1848, and 1860 temporary stars made their appearance, but
+none of them was of great brilliance. In 1866 one of the second
+magnitude broke forth in the ``Northern Crown'' and awoke much
+interest, because by that time the spectroscope had begun to be
+employed in studying the composition of the stars, and Huggins
+demonstrated that the new star consisted largely of incandescent
+hydrogen. But this star, apparently unlike the others mentioned, was
+not absolutely new. Before its outburst it had shown as a star of the
+ninth magnitude (entirely invisible, of course, to the naked eye), and
+after about six weeks it faded to its original condition in which it
+has ever since remained. In 1876 a temporary star appeared in the
+constellation Cygnus, and attained at one time the brightness of the
+second magnitude. Its spectrum and its behavior resembled those of its
+immediate predecessor. In 1885, astronomers were surprised to see a
+sixth-magnitude star glimmering in the midst of the hazy cloud of the
+great Andromeda Nebula. It soon absolutely disappeared. Its spectrum
+was remarkable for being ``continuous,'' like that of the nebula
+itself. A continuous spectrum is supposed to represent a body, or a
+mass, which is either solid or liquid, or composed of gas under great
+pressure. In January, 1892, a new star was suddenly seen in the
+constellation Auriga. It never rose much above the fourth magnitude,
+but it showed a peculiar spectrum containing both bright and dark
+lines of hydrogen.
+
+But a bewildering surprise was now in store; the world was to behold
+at the opening of the twentieth century such a celestial spectacle as
+had not been on view since the times of Tycho and Kepler. Before
+daylight on the morning of February 22, 1901, the Rev. Doctor
+Anderson, of Edinburgh, an amateur astronomer, who had also been the
+first to see the new star in Auriga, beheld a strange object in the
+constellation Perseus not far from the celebrated variable star Algol.
+He recognized its character at once, and immediately telegraphed the
+news, which awoke the startled attention of astronomers all over the
+world. When first seen the new star was no brighter than Algol (less
+than the second magnitude), but within twenty-four hours it was
+ablaze, outshining even the brilliant Capella, and far surpassing the
+first magnitude. At the spot in the sky where it appeared nothing
+whatever was visible on the night before its coming. This is known
+with certainty because a photograph had been made of that very region
+on February 21, and this photograph showed everything down to the
+twelfth magnitude, but not a trace of the stranger which burst into
+view between the 21st and the 22nd like the explosion of a rocket.
+
+Upon one who knew the stars the apparition of this intruder in a
+well-known constellation had the effect of a sudden invasion. The new
+star was not far west of the zenith in the early evening, and in that
+position showed to the best advantage. To see Capella, the hitherto
+unchallenged ruler of that quarter of the sky, abased by comparison
+with this stranger of alien aspect, for there was always an unfamiliar
+look about the ``nova,'' was decidedly disconcerting. It seemed to
+portend the beginning of a revolution in the heavens. One could
+understand what the effect of such an apparition must have been in the
+superstitious times of Tycho. The star of Tycho had burst forth on the
+northern border of the Milky Way; this one was on its southern border,
+some forty-five degrees farther east.
+
+Astronomers were well-prepared this time for the scientific study of
+the new star, both astronomical photography and spectroscopy having
+been perfected, and the results of their investigations were
+calculated to increase the wonder with which the phenomenon was
+regarded. The star remained at its brightest only a few days; then,
+like a veritable conflagration, it began to languish; and, like the
+reflection of a dying fire, as it sank it began to glow with the red
+color of embers. But its changes were spasmodic; once about every
+three days it flared up only to die away again. During these
+fluctuations its light varied alternately in the ratio of one to six.
+Finally it took a permanent downward course, and after a few months
+the naked eye could no longer perceive it; but it remained visible
+with telescopes, gradually fading until it had sunk to the ninth
+magnitude. Then another astonishing change happened: in August
+photographs taken at the Yerkes Observatory and at Heidelberg showed
+that the ``nova'' was surrounded by a spiral nebula! The nebula had
+not been there before, and no one could doubt that it represented a
+phase of the same catastrophe that had produced the outburst of the
+new star. At one time the star seemed virtually to have disappeared,
+as if all its substance had been expanded into the nebulous cloud, but
+always there remained a stellar nucleus about which the misty spiral
+spread wider and ever wider, like a wave expanding around a center of
+disturbance. The nebula too showed a variability of brightness, and
+four condensations which formed in it seemed to have a motion of
+revolution about the star. As time went on the nebula continued to
+expand at a rate which was computed to be not less than twenty
+thousand miles per second! And now the star itself, showing
+indications of having turned into a nebula, behaved in a most erratic
+manner, giving rise to the suspicion that it was about to burst out
+again. But this did not occur, and at length it sunk into a state of
+lethargy from which it has to the present time not recovered. But the
+nebulous spiral has disappeared, and the entire phenomena as it now
+(1909) exists consists of a faint nebulous star of less than the ninth
+magnitude.
+
+The wonderful transformations just described had been forecast in
+advance of the discovery of the nebulous spiral encircling the star by
+the spectroscopic study of the latter. At first there was no
+suggestion of a nebular constitution, but within a month or two
+characteristic nebular lines began to appear, and in less than six
+months the whole spectrum had been transformed to the nebular type. In
+the mean time the shifting of the spectral lines indicated a
+complication of rapid motions in several directions simultaneously.
+These motions were estimated to amount to from one hundred to five
+hundred miles per second.
+
+The human mind is so constituted that it feels forced to seek an
+explanation of so marvelous a phenomenon as this, even in the absence
+of the data needed for a sound conclusion. The most natural
+hypothesis, perhaps, is that of a collision. Such a catastrophe could
+certainly happen. It has been shown, for instance, that in infinity of
+time the earth is sure to be hit by a comet; in the same way it may be
+asserted that, if no time limit is fixed, the sun is certain to run
+against some obstacle in space, either another star, or a dense meteor
+swarm, or one of the dark bodies which there is every reason to
+believe abound around us. The consequences of such a collision are
+easy to foretell, provided that we know the masses and the velocities
+of the colliding bodies. In a preceding chapter we have discussed the
+motions of the sun and stars, and have seen that they are so swift
+that an encounter between any two of them could not but be disastrous.
+But this is not all; for as soon as two stars approached within a few
+million miles their speed would be enormously increased by their
+reciprocal attractions and, if their motion was directed radially with
+respect to their centers, they would come together with a crash that
+would reduce them both to nebulous clouds. It is true that the chances
+of such a ``head-on'' collision are relatively very small; two stars
+approaching each other would most probably fall into closed orbits
+around their common center of gravity. If there were a collision it
+would most likely be a grazing one instead of a direct front-to-front
+encounter. But even a close approach, without any actual collision,
+would probably prove disastrous, owing to the tidal influence of each
+of the bodies on the other. Suns, in consequence of their enormous
+masses and dimensions and the peculiarities of their constitution, are
+exceedingly dangerous to one another at close quarters. Propinquity
+awakes in them a mutually destructive tendency. Consisting of matter
+in the gaseous, or perhaps, in some cases, liquid, state, their tidal
+pull upon each other if brought close together might burst them
+asunder, and the photospheric envelope being destroyed the internal
+incandescent mass would gush out, bringing fiery death to any planets
+that were revolving near. Without regard to the resulting disturbance
+of the earth's orbit, the close approach of a great star to the sun
+would be in the highest degree perilous to us. But this is a danger
+which may properly be regarded as indefinitely remote, since, at our
+present location in space, we are certainly far from every star except
+the sun, and we may feel confident that no great invisible body is
+near, for if there were one we should be aware of its presence from
+the effects of its attraction. As to dark nebulae which may possibly
+lie in the track that the solar system is pursuing at the rate of
+375,000,000 miles per year, that is another question -- and they, too,
+could be dangerous!
+
+This brings us directly back to ``Nova Persei,'' for among the many
+suggestions offered to explain its outburst, as well as those of other
+temporary stars, one of the most fruitful is that of a collision
+between a star and a vast invisible nebula. Professor Seeliger, of
+Munich, first proposed this theory, but it afterward underwent some
+modifications from others. Stated in a general form, the idea is that
+a huge dark body, perhaps an extinguished sun, encountered in its
+progress through space a widespread flock of small meteors forming a
+dark nebula. As it plunged into the swarm the friction of the
+innumerable collisions with the meteors heated its surface to
+incandescence, and being of vast size it then became visible to us as
+a new star. Meanwhile the motion of the body through the nebula, and
+its rotation upon itself, set up a gyration in the blazing atmosphere
+formed around it by the vaporized meteors; and as this atmosphere
+spread wider, under the laws of gyratory motion a rotation in the
+opposite direction began in the inflamed meteoric cloud outside the
+central part of the vortex. Thus the spectral lines were caused to
+show motion in opposite directions, a part of the incandescent mass
+approaching the earth simultaneously with the retreat of another part.
+So the curious spectroscopic observations before mentioned were
+explained. This theory might also account for the appearance of the
+nebulous spiral first seen some six months after the original
+outburst. The sequent changes in the spectrum of the ``nova'' are
+accounted for by this theory on the assumption, reasonable enough in
+itself, that at first the invading body would be enveloped in a
+vaporized atmosphere of relatively slight depth, producing by its
+absorption the fine dark lines first observed; but that as time went
+on and the incessant collisions continued, the blazing atmosphere
+would become very deep and extensive, whereupon the appearance of the
+spectral lines would change, and bright lines due to the light of the
+incandescent meteors surrounding the nucleus at a great distance would
+take the place of the original dark ones. The vortex of meteors once
+formed would protect the flying body within from further immediate
+collisions, the latter now occurring mainly among the meteors
+themselves, and then the central blaze would die down, and the
+original splendor of the phenomenon would fade.
+
+But the theories about Nova Persei have been almost as numerous as the
+astronomers who have speculated about it. One of the most startling of
+them assumed that the outburst was caused by the running amuck of a
+dark star which had encountered another star surrounded with planets,
+the renewed outbreaks of light after the principal one had faded being
+due to the successive running down of the unfortunate planets! Yet
+another hypothesis is based on what we have already said of the tidal
+influence that two close approaching suns would have upon each other.
+Supposing two such bodies which had become encrusted, but remained
+incandescent and fluid within, to approach within almost striking
+distance; they would whirl each other about their common center of
+gravity, and at the same time their shells would burst under the tidal
+strain, and their glowing nuclei being disclosed would produce a great
+outburst of light. Applying this theory to a ``nova,'' like that of
+1866 in the ``Northern Crown,'' which had been visible as a small star
+before the outbreak, and which afterward resumed its former aspect, we
+should have to assume that a yet shining sun had been approached by a
+dark body whose attraction temporarily burst open its photosphere. It
+might be supposed that in this case the dark body was too far advanced
+in cooling to suffer the same fate from the tidal pull of its victim.
+But a close approach of that kind would be expected to result in the
+formation of a binary system, with orbits of great eccentricity,
+perhaps, and after the lapse of a certain time the outburst should be
+renewed by another approximation of the two bodies. A temporary star
+of that kind would rather be ranked as a variable.
+
+The celebrated French astronomer, Janssen, had a different theory of
+Nova Persei, and of temporary stars in general. According to his idea,
+such phenomena might be the result of chemical changes taking place in
+a sun without interference by, or collision with, another body.
+Janssen was engaged for many years in trying to discover evidence of
+the existence of oxygen in the sun, and he constructed his observatory
+on the summit of Mount Blanc specially to pursue that research. He
+believed that oxygen must surely exist in the sun since we find so
+many other familiar elements included in the constitution of the solar
+globe, and as he was unable to discover satisfactory evidence of its
+presence he assumed that it existed in a form unknown on the earth. If
+it were normally in the sun's chromosphere, or coronal atmosphere, he
+said, it would combine with the hydrogen which we know is there and
+form an obscuring envelope of water vapor. It exists, then, in a
+special state, uncombined with hydrogen; but let the temperature of
+the sun sink to a critical point and the oxygen will assume its normal
+properties and combine with the hydrogen, producing a mighty outburst
+of light and heat. This, Janssen thought, might explain the phenomena
+of the temporary stars. It would also, he suggested, account for their
+brief career, because the combination of the elements would be quickly
+accomplished, and then the resulting water vapor would form an
+atmosphere cutting off the radiation from the star within.
+
+This theory may be said to have a livelier human interest than some of
+the others, since, according to it, the sun may carry in its very
+constitution a menace to mankind; one does not like to think of it
+being suddenly transformed into a gigantic laboratory for the
+explosive combination of oxygen and hydrogen! But while Janssen's
+theory might do for some temporary stars, it is inadequate to explain
+all the phenomena of Nova Persei, and particularly the appearance of
+the great spiral nebula that seemed to exhale from the heart of the
+star. Upon the whole, the theory of an encounter between a star and a
+dark nebula seems best to fit the observations. By that hypothesis the
+expanding billow of light surrounding the core of the conflagration is
+very well accounted for, and the spectroscopic peculiarities are also
+explained.
+
+Dr Gustov Le Bon offers a yet more alarming theory, suggesting that
+temporary stars are the result of atomic explosion; but we shall touch
+upon this more fully in Chapter 14.
+
+Twice in the course of this discussion we have called attention to the
+change of color invariably undergone by temporary stars in the later
+stages of their career. This was conspicuous with Nova Persei which
+glowed more and more redly as it faded, until the nebulous light began
+to overpower that of the stellar nucleus. Nothing could be more
+suggestive of the dying out of a great fire. Moreover, change of color
+from white to red is characteristic of all variable stars of long
+period, such as ``Mira'' in Cetus. It is also characteristic of stars
+believed to be in the later stages of evolution, and consequently
+approaching extinction, like Antares and Betelgeuse, and still more
+notably certain small stars which ``gleam like rubies in the field of
+the telescope.'' These last appear to be suns in the closing period of
+existence as self-luminous bodies. Between the white stars, such as
+Sirius and Rigel, and the red stars, such as Aldebaran and Alpha
+Herculis, there is a progressive series of colors from golden yellow
+through orange to deep red. The change is believed to be due to the
+increase of absorbing vapors in the stellar atmosphere as the body
+cools down. In the case of ordinary stars these changes no doubt
+occupy many millions of years, which represent the average duration of
+solar life; but the temporary stars run through similar changes in a
+few months: they resemble ephemeral insects -- born in the morning and
+doomed to perish with the going down of the sun.
+
+Explosive and Whirling Nebulae
+
+One of the most surprising triumphs of celestial photography was
+Professor Keeler's discovery, in 1899, that the great majority of the
+nebulae have a distinctly spiral form. This form, previously known in
+Lord Rosse's great ``Whirlpool Nebula,'' had been supposed to be
+exceptional; now the photographs, far excelling telescopic views in
+the revelation of nebular forms, showed the spiral to be the typical
+shape. Indeed, it is a question whether all nebulae are not to some
+extent spiral. The extreme importance of this discovery is shown in
+the effect that it has had upon hitherto prevailing views of solar and
+planetary evolution. For more than three-quarters of a century
+Laplace's celebrated hypothesis of the manner of origin of the solar
+system from a rotating and contracting nebula surrounding the sun had
+guided speculation on that subject, and had been tentatively extended
+to cover the evolution of systems in general. The apparent forms of
+some of the nebulae which the telescope had revealed were regarded, and
+by some are still regarded, as giving visual evidence in favor of this
+theory. There is a ``ring nebula'' in Lyra with a central star, and a
+``planetary nebula'' in Gemini bearing no little resemblance to the
+planet Saturn with its rings, both of which appear to be practical
+realizations of Laplace's idea, and the elliptical rings surrounding
+the central condensation of the Andromeda Nebula may be cited for the
+same kind of proof.
+
+But since Keeler's discovery there has been a decided turning away of
+speculation another way. The form of the spiral nebulae seems to be
+entirely inconsistent with the theory of an originally globular or
+disk-shaped nebula condensing around a sun and throwing or leaving off
+rings, to be subsequently shaped into planets. Some astronomers,
+indeed, now reject Laplace's hypothesis in toto, preferring to think
+that even our solar system originated from a spiral nebula. Since the
+spiral type prevails among the existing nebulae, we must make any
+mechanical theory of the development of stars and planetary systems
+from them accord with the requirements which that form imposes. A
+glance at the extraordinary variations upon the spiral which Professor
+Keeler's photographs reveal is sufficient to convince one of the
+difficulty of the task of basing a general theory upon them. In truth,
+it is much easier to criticize Laplace's hypothesis than to invent a
+satisfactory substitute for it. If the spiral nebulae seem to oppose it
+there are other nebulae which appear to support it, and it may be that
+no one fixed theory can account for all the forms of stellar evolution
+in the universe. Our particular planetary system may have originated
+very much as the great French mathematician supposed, while others
+have undergone, or are now undergoing, a different process of
+development. There is always a too strong tendency to regard an
+important new discovery and the theories and speculations based upon
+it as revolutionizing knowledge, and displacing or overthrowing
+everything that went before. Upon the plea that ``Laplace only made a
+guess'' more recent guesses have been driven to extremes and treated
+by injudicious exponents as ``the solid facts at last.''
+
+Before considering more recent theories than Laplace's, let us see
+what the nature of the photographic revelations is. The vast celestial
+maelstrom discovered by Lord Rosse in the ``Hunting Dogs'' may be
+taken as the leading type of the spiral nebulae, although there are
+less conspicuous objects of the kind which, perhaps, better illustrate
+some of their peculiarities. Lord Rosse's nebula appears far more
+wonderful in the photographs than in his drawings made with the aid of
+his giant reflecting telescope at Parsonstown, for the photographic
+plate records details that no telescope is capable of showing. Suppose
+we look at the photograph of this object as any person of common sense
+would look at any great and strange natural phenomenon. What is the
+first thing that strikes the mind? It is certainly the appearance of
+violent whirling motion. One would say that the whole glowing mass had
+been spun about with tremendous velocity, or that it had been set
+rotating so rapidly that it had become the victim of ``centrifugal
+force,'' one huge fragment having broken loose and started to gyrate
+off into space. Closer inspection shows that in addition to the
+principal focus there are various smaller condensations scattered
+through the mass. These are conspicuous in the spirals. Some of them
+are stellar points, and but for the significance of their location we
+might suppose them to be stars which happen to lie in a line between
+us and the nebula. But when we observe how many of them follow most
+faithfully the curves of the spirals we cannot but conclude that they
+form an essential part of the phenomenon; it is not possible to
+believe that their presence in such situations is merely fortuitous.
+One of the outer spirals has at least a dozen of these star-like
+points strung upon it; some of them sharp, small, and distinct, others
+more blurred and nebulous, suggesting different stages of
+condensation. Even the part which seems to have been flung loose from
+the main mass has, in addition to its central condensation, at least
+one stellar point gleaming in the half-vanished spire attached to it.
+Some of the more distant stars scattered around the ``whirlpool'' look
+as if they too had been shot out of the mighty vortex, afterward
+condensing into unmistakable solar bodies. There are at least two
+curved rows of minute stars a little beyond the periphery of the
+luminous whirl which clearly follow lines concentric with those of the
+nebulous spirals. Such facts are simply dumbfounding for anyone who
+will bestow sufficient thought upon them, for these are suns, though
+they may be small ones; and what a birth is that for a sun!
+
+Look now again at the glowing spirals. We observe that hardly have
+they left the central mass before they begin to coagulate. In some
+places they have a ``ropy'' aspect; or they are like peascods filled
+with growing seeds, which eventually will become stars. The great
+focus itself shows a similar tendency, especially around its
+circumference. The sense that it imparts of a tremendous shattering
+force at work is overwhelming. There is probably more matter in that
+whirling and bursting nebula than would suffice to make a hundred
+solar systems! It must be confessed at once that there is no
+confirmation of the Laplacean hypothesis here; but what hypothesis
+will fit the facts? There is one which it has been claimed does so,
+but we shall come to that later. In the meanwhile, as a preparation,
+fix in the memory the appearance of that second spiral mass spinning
+beside its master which seems to have spurned it away.
+
+For a second example of the spiral nebulae look at the one in the
+constellation Triangulum. God, how hath the imagination of puny man
+failed to comprehend Thee! Here is creation through destruction with a
+vengeance! The spiral form of the nebula is unmistakable, but it is
+half obliterated amid the turmoil of flying masses hurled away on all
+sides with tornadic fury. The focus itself is splitting asunder under
+the intolerable strain, and in a little while, as time is reckoned in
+the Cosmos, it will be gyrating into stars. And then look at the
+cyclonic rain of already finished stars whirling round the outskirts
+of the storm. Observe how scores of them are yet involved in the
+fading streams of the nebulous spirals; see how they have been thrown
+into vast loops and curves, of a beauty that half redeems the terror
+of the spectacle enclosed within their lines -- like iridescent cirri
+hovering about the edges of a hurricane. And so again are suns born!
+
+Let us turn to the exquisite spiral in Ursa Major; how different its
+aspect from that of the other! One would say that if the terrific coil
+in Triangulum has all but destroyed itself in its fury, this one on
+the contrary has just begun its self-demolition. As one gazes one
+seems to see in it the smooth, swift, accelerating motion that
+precedes catastrophe. The central part is still intact, dense, and
+uniform in texture. How graceful are the spirals that smoothly rise
+from its oval rim and, gemmed with little stars, wind off into the
+darkness until they have become as delicate as threads of gossamer!
+But at bottom the story told here is the same -- creation by gyration!
+
+Compare with the above the curious mass in Cetus. Here the plane of
+the whirling nebula nearly coincides with our line of sight and we see
+the object at a low angle. It is far advanced and torn to shreds, and
+if we could look at it perpendicularly to its plane it is evident that
+it would closely resemble the spectacle in Triangulum.
+
+Then take the famous Andromeda Nebula (see Frontispiece), which is so
+vast that notwithstanding its immense distance even the naked eye
+perceives it as an enigmatical wisp in the sky. Its image on the
+sensitive plate is the masterpiece of astronomical photography; for
+wild, incomprehensible beauty there is nothing that can be compared
+with it. Here, if anywhere, we look upon the spectacle of creation in
+one of its earliest stages. The Andromeda Nebula is apparently less
+advanced toward transformation into stellar bodies than is that in
+Triangulum. The immense crowd of stars sprinkled over it and its
+neighborhood seem in the main to lie this side of the nebula, and
+consequently to have no connection with it. But incipient stars (in
+some places clusters of them) are seen in the nebulous rings, while
+one or two huge masses seem to give promise of transformation into
+stellar bodies of unusual magnitude. I say ``rings'' because although
+the loops encompassing the Andromeda Nebula have been called spirals
+by those who wish utterly to demolish Laplace's hypothesis, yet they
+are not manifestly such, as can be seen on comparing them with the
+undoubted spirals of the Lord Rosse Nebula. They look quite as much
+like circles or ellipses seen at an angle of, say, fifteen or twenty
+degrees to their plane. If they are truly elliptical they accord
+fairly well with Laplace's idea, except that the scale of magnitude is
+stupendous, and if the Andromeda Nebula is to become a solar system it
+will surpass ours in grandeur beyond all possibility of comparison.
+
+There is one circumstance connected with the spiral nebulae, and
+conspicuous in the Andromeda Nebula on account of its brightness,
+which makes the question of their origin still more puzzling; they all
+show continuous spectra, which, as we have before remarked, indicate
+that the mass from which the light comes is either solid or liquid, or
+a gas under heavy pressure. Thus nebulae fall into two classes: the
+``white'' nebulae, giving a continuous spectrum; and the ``green''
+nebulae whose spectra are distinctly gaseous. The Andromeda Nebula is
+the great representative of the former class and the Orion Nebula of
+the latter. The spectrum of the Andromeda Nebula has been interpreted
+to mean that it consists not of luminous gas, but of a flock of stars
+so distant that they are separately indistinguishable even with
+powerful telescopes, just as the component stars of the Milky Way are
+indistinguishable with the naked eye; and upon this has been based the
+suggestion that what we see in Andromeda is an outer universe whose
+stars form a series of elliptical garlands surrounding a central mass
+of amazing richness. But this idea is unacceptable if for no other
+reason than that, as just said, all the spiral nebulae possess the same
+kind of spectrum, and probably no one would be disposed to regard them
+all as outer universes. As we shall see later, the peculiarity of the
+spectra of the spiral nebulae is appealed to in support of a modern
+substitute for Laplace's hypothesis.
+
+Finally, without having by any means exhausted the variety exhibited
+by the spiral nebulae, let us turn to the great representative of the
+other species, the Orion Nebula. In some ways this is even more
+marvelous than the others. The early drawings with the telescope
+failed to convey an adequate conception either of its sublimity or of
+its complication of structure. It exists in a nebulous region of
+space, since photographs show that nearly the whole constellation is
+interwoven with faintly luminous coils. To behold the entry of the
+great nebula into the field even of a small telescope is a startling
+experience which never loses its novelty. As shown by the photographs,
+it is an inscrutable chaos of perfectly amazing extent, where spiral
+bands, radiating streaks, dense masses, and dark yawning gaps are
+strangely intermingled without apparent order. In one place four
+conspicuous little stars, better seen in a telescope than in the
+photograph on account of the blurring produced by over-exposure, are
+suggestively situated in the midst of a dark opening, and no observer
+has ever felt any doubt that these stars have been formed from the
+substance of the surrounding nebula. There are many other stars
+scattered over its expanse which manifestly owe their origin to the
+same source. But compare the general appearance of this nebula with
+the others that we have studied, and remark the difference. If the
+unmistakably spiral nebulae resemble bursting fly-wheels or grindstones
+from whose perimeters torrents of sparks are flying, the Orion Nebula
+rather recalls the aspect of a cloud of smoke and fragments produced
+by the explosion of a shell. This idea is enforced by the look of the
+outer portion farthest from the bright half of the nebula, where
+sharply edged clouds with dark spaces behind seem to be billowing away
+as if driven by a wind blowing from the center.
+
+Next let us consider what scientific speculation has done in the
+effort to explain these mysteries. Laplace's hypothesis can certainly
+find no standing ground either in the Orion Nebula or in those of a
+spiral configuration, whatever may be its situation with respect to
+the grand Nebula of Andromeda, or the ``ring'' and ``planetary''
+nebulae. Some other hypothesis more consonant with the appearances must
+be found. Among the many that have been proposed the most elaborate is
+the ``Planetesimal Hypothesis'' of Professors Chamberlin and Moulton.
+It is to be remarked that it applies to the spiral nebulae
+distinctively, and not to an apparently chaotic mass of gas like the
+vast luminous cloud in Orion. The gist of the theory is that these
+curious objects are probably the result of close approaches to each
+other of two independent suns, reminding us of what was said on this
+subject when we were dealing with temporary stars. Of the previous
+history of these appulsing suns the theory gives us no account; they
+are simply supposed to arrive within what may be called an effective
+tide-producing distance, and then the drama begins. Some of the
+probable consequences of such an approach have been noticed in Chapter
+5; let us now consider them a little more in detail.
+
+Tides always go in couples; if there is a tide on one side of a globe
+there will be a corresponding tide on the other side. The cause is to
+be found in the law that the force of gravitation varies inversely as
+the square of the distance; the attraction on the nearest surface of
+the body exercised by another body is greater than on its center, and
+greater yet than on its opposite surface. If two great globes attract
+each other, each tends to draw the other out into an ellipsoidal
+figure; they must be more rigid than steel to resist this -- and even
+then they cannot altogether resist. If they are liquid or gaseous they
+will yield readily to the force of distortion, the amount of which
+will depend upon their distance apart, for the nearer they are the
+greater becomes the tidal strain. If they are encrusted without and
+liquid or gaseous in the interior, the internal mass will strive to
+assume the figure demanded by the tidal force, and will, if it can,
+burst the restraining envelope. Now this is virtually the predicament
+of the body we call a sun when in the immediate presence of another
+body of similarly great mass. Such a body is presumably gaseous
+throughout, the component gases being held in a state of rigidity by
+the compression produced by the tremendous gravitational force of
+their own aggregate mass. At the surface such a body is enveloped in a
+shell of relatively cool matter. Now suppose a great attracting body,
+such as another sun, to approach near enough for the difference in its
+attraction on the two opposite sides of the body and on its center to
+become very great; the consequence will be a tidal deformation of the
+whole body, and it will lengthen out along the line of the
+gravitational pull and draw in at the sides, and if its shell offers
+considerable resistance, but not enough to exercise a complete
+restraint, it will be violently burst apart, or blown to atoms, and
+the internal mass will leap out on the two opposite sides in great
+fiery spouts. In the case of a sun further advanced in cooling than
+ours the interior might be composed of molten matter while the
+exterior crust had become rigid like the shell of an egg; then the
+force of the ``tidal explosion'' produced by the appulse of another
+sun would be more violent in consequence of the greater resistance
+overcome. Such, then, is the mechanism of the first phase in the
+history of a spiral nebula according to the Planetesimal Hypothesis.
+Two suns, perhaps extinguished ones, have drawn near together, and an
+explosive outburst has occured in one or both. The second phase calls
+for a more agile exercise of the imagination.
+
+To simplify the case, let us suppose that only one of the tugging suns
+is seriously affected by the strain. Its vast wings produced by the
+outburst are twisted into spirals by their rotation and the contending
+attractions exercised upon them, as the two suns, like battleships in
+desperate conflict, curve round each other, concentrating their
+destructive energies. Then immense quantities of debris are scattered
+about in which eddies are created, and finally, as the sun that caused
+the damage goes on its way, leaving its victim to repair its injuries
+as it may, the dispersed matter cools, condenses, and turns into
+streams of solid particles circling in elliptical paths about their
+parent sun. These particles, or fragments, are the ``planetesimals''
+of the theory. In consequence of the inevitable intersection of the
+orbits of the planetesimals, nodes are formed where the flying
+particles meet, and at these nodes large masses are gradually
+accumulated. The larger the mass the greater its attraction, and at
+last the nodal points become the nuclei of great aggregations from
+which planets are shaped.
+
+This, in very brief form, is the Planetesimal Hypothesis which we are
+asked to substitute for that based on Laplace's suggestion as an
+explanation of the mode of origin of the solar system; and the
+phenomena of the spiral nebulae are appealed to as offering evident
+support to the new hypothesis. We are reminded that they are
+elliptical in outline, which accords with the hypothesis; that their
+spectra are not gaseous, which shows that they may be composed of
+solid particles like the planetesimals; and that their central masses
+present an oval form, which is what would result from the tidal
+effects, as just described. We also remember that some of them, like
+the Lord Rosse and the Andromeda nebulae, are visually double, and in
+these cases we might suppose that the two masses represent the
+tide-burst suns that ventured into too close proximity. It may be
+added that the authors of the theory do not insist upon the appulse of
+two suns as the only way in which the planetesimals may have
+originated, but it is the only supposition that has been worked out.
+
+But serious questions remain. It needs, for instance, but a glance at
+the Triangulum monster to convince the observer that it cannot be a
+solar system which is being evolved there, but rather a swarm of
+stars. Many of the detached masses are too vast to admit of the
+supposition that they are to be transformed into planets, in our sense
+of planets, and the distances of the stars which appear to have been
+originally ejected from the focal masses are too great to allow us to
+liken the assemblage that they form to a solar system. Then, too, no
+nodes such as the hypothesis calls for are visible. Moreover, in most
+of the spiral nebulae the appearances favor the view that the
+supposititious encountering suns have not separated and gone each
+rejoicing on its way, after having inflicted the maximum possible
+damage on its opponent, but that, on the contrary, they remain in
+close association like two wrestlers who cannot escape from each
+other's grasp. And this is exactly what the law of gravitation
+demands; stars cannot approach one another with impunity, with regard
+either to their physical make-up or their future independence of
+movement. The theory undertakes to avoid this difficulty by assuming
+that in the case of our system the approach of the foreign body to the
+sun was not a close one -- just close enough to produce the tidal
+extrusion of the relatively insignificant quantity of matter needed to
+form the planets. But even then the effect of the appulse would be to
+change the direction of flight, both of the sun and of its visitor,
+and there is no known star in the sky which can be selected as the
+sun's probable partner in their ancient pas deux. That there are
+unconquered difficulties in Laplace's hypothesis no one would deny,
+but in simplicity of conception it is incomparably more satisfactory,
+and with proper modifications could probably be made more consonant
+with existing facts in our solar system than that which is offered to
+replace it. Even as an explanation of the spiral nebulae, not as solar
+systems in process of formation, but as the birthplaces of stellar
+clusters, the Planetesimal Hypothesis would be open to many
+objections. Granting its assumptions, it has undoubtedly a strong
+mathematical framework, but the trouble is not with the mathematics
+but with the assumptions. Laplace was one of the ablest mathematicians
+that ever lived, but he had never seen a spiral nebula; if he had, he
+might have invented a hypothesis to suit its phenomena. His actual
+hypothesis was intended only for our solar system, and he left it in
+the form of a ``note'' for the consideration of his successors, with
+the hope that they might be able to discover the full truth, which he
+confessed was hidden from him. It cannot be said that that truth has
+yet been found, and when it is found the chances are that intuition
+and not logic will have led to it.
+
+The spiral nebulae, then, remain among the greatest riddles of the
+universe, while the gaseous nebulae, like that of Orion, are no less
+mysterious, although it seems impossible to doubt that both forms give
+birth to stars. It is but natural to look to them for light on the
+question of the origin of our planetary system; but we should not
+forget that the scale of the phenomena in the two cases is vastly
+different, and the forces in operation may be equally different. A
+hill may have been built up by a glacier, while a mountain may be the
+product of volcanic forces or of the upheaval of the strata of the
+planet.
+
+The Banners of the Sun
+
+As all the world knows, the sun, a blinding globe pouring forth an
+inconceivable quantity of light and heat, whose daily passage through
+the sky is caused by the earth's rotation on its axis, constitutes the
+most important phenomenon of terrestial existence. Viewed with a dark
+glass to take off the glare, or with a telescope, its rim is seen to
+be a sharp and smooth circle, and nothing but dark sky is visible
+around it. Except for the interference of the moon, we should probably
+never have known that there is any more of the sun than our eyes
+ordinarily see.
+
+But when an eclipse of the sun occurs, caused by the interposition of
+the opaque globe of the moon, we see its immediate surroundings, which
+in some respects are more wonderful than the glowing central orb.
+These surroundings, although not in the sense in which we apply the
+term to the gaseous envelope of the earth, may be called the sun's
+atmosphere. They consist of two very different parts -- first, the red
+``prominences,'' which resemble tongues of flame ascending thousands
+of miles above the sun's surface; and, second, the ``corona,'' which
+extends to distances of millions of miles from the sun, and shines
+with a soft, glowing light. The two combined, when well seen, make a
+spectacle without parallel among the marvels of the sky. Although many
+attempts have been made to render the corona visible when there is no
+eclipse, all have failed, and it is to the moon alone that we owe its
+revelation. To cover the sun's disk with a circular screen will not
+answer the purpose because of the illumination of the air all about
+the observer. When the moon hides the sun, on the other hand, the
+sunlight is withdrawn from a great cylinder of air extending to the
+top of the atmosphere and spreading many miles around the observer.
+There is then no glare to interfere with the spectacle, and the corona
+appears in all its surprising beauty. The prominences, however,
+although they were discovered during an eclipse, can now, with the aid
+of the spectroscope, be seen at any time. But the prominences are
+rarely large enough to be noticed by the naked eye, while the
+streamers of the corona, stretching far away in space, like ghostly
+banners blown out from the black circle of the obscuring moon, attract
+every eye, and to this weird apparition much of the fear inspired by
+eclipses has been due. But if the corona has been a cause of terror in
+the past it has become a source of growing knowledge in our time.
+
+The story of the first scientific observation of the corona and the
+prominences is thrillingly interesting, and in fact dramatic. The
+observation was made during the eclipse of 1842, which fortunately was
+visible all over Central and Southern Europe so that scores of
+astronomers saw it. The interest centers in what happened at Pavia in
+Northern Italy, where the English astronomer Francis Baily had set up
+his telescope. The eclipse had begun and Bailey was busy at his
+telescope when, to quote his own words in the account which he wrote
+for the Memoirs of the Royal Astronomical Society:
+
+  I was astounded by a tremendous burst of applause from the streets
+  below, and at the same moment was electrified by the sight of one
+  of the most brilliant and splendid phenomena that can well be
+  imagined; for at that instant the dark body of the moon was
+  suddenly surrounded with a corona, or kind of bright glory, similar
+  in shape and magnitude to that which painters draw round the heads
+  of saints...
+
+  Pavia contains many thousand inhabitants, the major part of whom
+  were at this early hour walking about the streets and squares or
+  looking out of windows in order to witness this long-talked-of
+  phenomenon; and when the total obscuration took place, which was
+  instantaneous, there was a universal shout from every observer
+  which ``made the welkin ring,'' and for the moment withdrew my
+  attention from the object with which I was immediately occupied. I
+  had, indeed, expected the appearance of a luminous circle round the
+  moon during the time of total obscurity; but I did not expect, from
+  any of the accounts of preceding eclipses that I had read, to
+  witness so magnificent an exhibition as that which took place...
+
+  Splendid and astonishing, however, as this remarkable phenomenon
+  really was, and although it could not fail to call forth the
+  admiration and applause of every beholder, yet I must confess that
+  there was at the same time something in its singular and wonderful
+  appearance that was appalling...
+
+  But the most remarkable circumstance attending the phenomenon was
+  the appearance of three large protuberances apparently emanating
+  from the circumference of the moon, but evidently forming a portion
+  of the corona. They had the appearance of mountains of a prodigious
+  elevation; their color was red tinged with lilac or purple; perhaps
+  the color of the peach-blossom would more nearly represent it. They
+  somewhat resembled the tops of the snowy Alpine mountains when
+  colored by the rising or the setting sun. They resembled the Alpine
+  mountains in another respect, inasmuch as their light was perfectly
+  steady, and had none of that flickering or sparkling motion so
+  visible in other parts of the corona...
+
+  The whole of these protuberances were visible even to the last
+  moment of total obscuration, and when the first ray of light was
+  admitted from the sun they vanished, with the corona, altogether,
+  and daylight was instantly restored.
+
+I have quoted nearly all of this remarkable description not alone for
+its intrinsic interest, but because it is the best depiction that can
+be found of the general phenomena of a total solar eclipse. Still, not
+every such eclipse offers an equally magnificent spectacle. The
+eclipses of 1900 and 1905, for instance, which were seen by the
+writer, the first in South Carolina and the second in Spain, fell far
+short of that described by Bailey in splendor and impressiveness. Of
+course, something must be allowed for the effect of surprise; Bailey
+had not expected to see what was so suddenly disclosed to him. But
+both in 1900 and 1905 the amount of scattered light in the sky was
+sufficient in itself to make the corona appear faint, and there were
+no very conspicuous prominences visible. Yet on both occasions there
+was manifest among the spectators that mingling of admiration and awe
+of which Bailey speaks. The South Carolinians gave a cheer and the
+ladies waved their handkerchiefs when the corona, ineffably delicate
+of form and texture, melted into sight and then in two minutes melted
+away again. The Spaniards, crowded on the citadel hill of Burgos, with
+their king and his royal retinue in their midst, broke out with a
+great clapping of hands as the awaited spectacle unfolded itself in
+the sky; and on both occasions, before the applause began, after an
+awed silence a low murmur ran through the crowds. At Burgos it is said
+many made the sign of the cross.
+
+It was not long before Bailey's idea that the prominences were a part
+of the corona was abandoned, and it was perceived that the two
+phenomena were to a great extent independent. At the eclipse of 1868,
+which the astronomers, aroused by the wonderful scene of 1842, and
+eager to test the powers of the newly invented spectroscope, flocked
+to India to witness, Janssen conceived the idea of employing the
+spectroscope to render the prominences visible when there was no
+eclipse. He succeeded the very next day, and these phenomena have been
+studied in that way ever since.
+
+There are recognized two kinds of prominences -- the ``erruptive'' and
+the ``quiescent.'' The latter, which are cloud-like in form, may be
+seen almost anywhere along the edge of the sun; but the former, which
+often shoot up as if hurled from mighty volcanoes, appear to be
+associated with sun-spots, and appear only above the zones where spots
+abound. Either of them, when seen in projection against the brilliant
+solar disk, appears white, not red, as against a background of sky.
+The quiescent prominences, whose elevation is often from forty
+thousand to sixty thousand miles, consist, as the spectroscope shows,
+mainly of hydrogen and helium. The latter, it will be remembered, is
+an element which was known to be in the sun many years before the
+discovery that it also exists in small quantities on the earth. A fact
+which may have a significance which we cannot at present see is that
+the emanation from radium gradually and spontaneously changes into
+helium, an alchemistical feat of nature that has opened many curious
+vistas to speculative thinkers. The eruptive prominences, which do not
+spread horizontally like the others, but ascend with marvelous
+velocity to elevations of half a million miles or more, are apparently
+composed largely of metallic vapors -- i.e. metals which are usually
+solid on the earth, but which at solar temperatures are kept in a
+volatilized state. The velocity of their ascent occasionally amounts
+to three hundred or four hundred miles per second. It is known from
+mathematical considerations that the gravitation of the sun would not
+be able to bring back any body that started from its surface with a
+velocity exceeding three hundred and eighty-three miles per second; so
+it is evident that some of the matter hurled forth in eruptive
+prominences may escape from solar control and go speeding out into
+space, cooling and condensing into solid masses. There seems to be no
+reason why some of the projectiles from the sun might not reach the
+planets. Here, then, we have on a relatively small scale, explosions
+recalling those which it has been imagined may be the originating
+cause of some of the sudden phenomena of the stellar heavens.
+
+Of the sun-spots it is not our intention here specifically to speak,
+but they evidently have an intimate connection with eruptive
+prominences, as well as some relation, not yet fully understood, with
+the corona. Of the real cause of sun-spots we know virtually nothing,
+but recent studies by Professor Hale and others have revealed a
+strange state of things in the clouds of metallic vapors floating
+above them and their surroundings. Evidences of a cyclonic tendency
+have been found, and Professor Hale has proved that sun-spots are
+strong magnetic fields, and consist of columns of ionized vapors
+rotating in opposite directions in the two hemispheres. A fact which
+may have the greatest significance is that titanium and vanadium have
+been found both in sun-spots and in the remarkable variable Mira Ceti,
+a star which every eleven months, or thereabout, flames up with great
+brilliancy and then sinks back to invisibility with the naked eye. It
+has been suggested that sun-spots are indications of the beginning of
+a process in the sun which will be intensified until it falls into the
+state of such a star as Mira. Stars very far advanced in evolution,
+without showing variability, also exhibit similar spectra; so that
+there is much reason for regarding sunspots as emblems of advancing
+age.
+
+The association of the corona with sun-spots is less evident than that
+of the eruptive prominences; still such an association exists, for the
+form and extent of the corona vary with the sun-spot period of which
+we shall presently speak. The constitution of the corona remains to be
+discovered. It is evidently in part gaseous, but it also probably
+contains matter in the form of dust and small meteors. It includes one
+substance altogether mysterious -- ``coronium.'' There are reasons for
+thinking that this may be the lightest of all the elements, and
+Professor Young, its discoverer, said that it was ``absolutely unique
+in nature; utterly distinct from any other known form of matter,
+terrestial, solar, or cosmical.'' The enormous extent of the corona is
+one of its riddles. Since the development of the curious subject of
+the ``pressure of light'' it has been proposed to account for the
+sustentation of the corona by supposing that it is borne upon the
+billows of light continually poured out from the sun. Experiment has
+proved, what mathematical considerations had previously pointed out as
+probable, that the waves of light exert a pressure or driving force,
+which becomes evident in its effects if the body acted upon is
+sufficiently small. In that case the light pressure will prevail over
+the attraction of gravitation, and propel the attenuated matter away
+from the sun in the teeth of its attraction. The earth itself would be
+driven away if, instead of consisting of a solid globe of immense
+aggregate mass, it were a cloud of microscopic particles. The reason
+is that the pressure varies in proportion to the surface of the body
+acted upon, while the gravitational attraction is proportional to the
+volume, or the total amount of matter in the body. But the surface of
+any body depends upon the square of its diameter, while the volume
+depends upon the cube of the diameter. If, for instance, the diameter
+is represented by 4, the surface will be proportional to 4 x 4, or 16,
+and the volume to 4 x 4 x 4, or 64; but if the diameter is taken as 2,
+the surface will be 2 x 2, or 4, and the volume 2 x 2 x 2, or 8. Now,
+the ratio of 4 to 8 is twice as great as that of 16 to 64. If the
+diameter is still further decreased, the ratio of the surface to the
+volume will proportionally grow larger; in other words, the pressure
+will gain upon the attraction, and whatever their original ratio may
+have been, a time will come, if the diminution of size continues, when
+the pressure will become more effective than the attraction, and the
+body will be driven away. Supposing the particles of the corona to be
+below the critical size for the attraction of a mass like that of the
+sun to control them, they would be driven off into the surrounding
+space and appear around the sun like the clouds of dust around a mill.
+We shall return to this subject in connection with the Zodiacal Light,
+the Aurora, and Comets.
+
+On the other hand, there are parts of the corona which suggest by
+their forms the play of electric or magnetic forces. This is
+beautifully shown in some of the photographs that have been made of
+the corona during recent eclipses. Take, for instance, that of the
+eclipse of 1900. The sheaves of light emanating from the poles look
+precisely like the ``lines of force'' surrounding the poles of a
+magnet. It will be noticed in this photograph that the corona appears
+to consist of two portions: one comprising the polar rays just spoken
+of, and the other consisting of the broader, longer, and less-defined
+masses of light extending out from the equatorial and middle-latitude
+zones. Yet even in this more diffuse part of the phenomenon one can
+detect the presence of submerged curves bearing more or less
+resemblance to those about the poles. Just what part electricity or
+electro-magnetism plays in the mechanism of the solar radiation it is
+impossible to say, but on the assumption that it is a very important
+part is based the hypothesis that there exists a direct solar
+influence not only upon the magnetism, but upon the weather of the
+earth. This hypothesis has been under discussion for half a century,
+and still we do not know just how much truth it represents. It is
+certain that the outbreak of great disturbances on the sun,
+accompanied by the formation of sun-spots and the upshooting of
+eruptive prominences (phenomena which we should naturally expect to be
+attended by action), have been instantly followed by corresponding
+``magnetic storms'' on the earth and brilliant displays of the auroral
+lights. There have been occasions when the influence has manifested
+itself in the most startling ways, a great solar outburst being
+followed by a mysterious gripping of the cable and telegraph systems
+of the world, as if an invisible and irresistible hand had seized
+them. Messages are abruptly cut off, sparks leap from the telegraph
+instruments, and the entire earth seems to have been thrown into a
+magnetic flurry. These occurrences affect the mind with a deep
+impression of the dependence of our planet on the sun, such as we do
+not derive from the more familiar action of the sunlight on the growth
+of plants and other phenomena of life depending on solar influences.
+
+Perhaps the theory of solar magnetic influence upon the weather is
+best known in connection with the ``sun-spot cycle.'' This, at any
+rate, is, as already remarked, closely associated with the corona. Its
+existence was discovered in 1843 by the German astronomer Schwabe. It
+is a period of variable length, averaging about eleven years, during
+which the number of spots visible on the sun first increases to a
+maximum, then diminishes to a minimum, and finally increases again to
+a maximum. For unknown reasons the period is sometimes two or three
+years longer than the average and sometimes as much shorter.
+Nevertheless, the phenomena always recur in the same order. Starting,
+for instance, with a time when the observer can find few or no spots,
+they gradually increase in number and size until a maximum, in both
+senses, is reached, during which the spots are often of enormous size
+and exceedingly active. After two or three years they begin to
+diminish in number, magnitude, and activity until they almost or quite
+disappear. A strange fact is that when a new period opens, the spots
+appear first in high northern and southern latitudes, far from the
+solar equator, and as the period advances they not only increase in
+number and size, but break out nearer and nearer to the equator, the
+last spots of a vanishing period sometimes lingering in the equatorial
+region after the advance-guard of its successor has made its
+appearance in the high latitudes. Spots are never seen on the equator
+nor near the poles. It was not very long after the discovery of the
+sun-spot cycle that the curious observation was made that a striking
+coincidence existed between the period of the sun-spots and another
+period affecting the general magnetic condition of the earth. When a
+curved line representing the varying number of sun-spots was compared
+with another curve showing the variations in the magnetic state of the
+earth the two were seen to be in almost exact accord, a rise in one
+curve corresponding to a rise in the other, and a fall to a fall.
+Continued observation has proved that this is a real coincidence and
+not an accidental one, so that the connection, although as yet
+unexplained, is accepted as established. But does the influence extend
+further, and directly affect the weather and the seasons as well as
+the magnetic elements of the earth? A final answer to this question
+cannot yet be given, for the evidence is contradictory, and the
+interpretations put upon it depend largely on the predilections of the
+judges.
+
+But, in a broad sense, the sun-spots and the phenomena connected with
+them must have a relation to terrestial meteorology, for they prove
+the sun to be a variable star. Reference was made, a few lines above,
+to the resemblance of the spectra of sun-spots to those of certain
+stars which seem to be failing through age. This in itself is
+extremely suggestive; but if this resemblance had never been
+discovered, we should have been justified in regarding the sun as
+variable in its output of energy; and not only variable, but probably
+increasingly so. The very inequalities in the sun-spot cycle are
+suspicious. When the sun is most spotted its total light may be
+reduced by one-thousandth part, although it is by no means certain
+that its outgiving of thermal radiations is then reduced. A loss of
+one-thousandth of its luminosity would correspond to a decrease of
+.0025 of a stellar magnitude, considering the sun as a star viewed
+from distant space. So slight a change would not be perceptible; but
+it is not alone sun-spots which obscure the solar surface, its entire
+globe is enveloped with an obscuring veil. When studied with a
+powerful telescope the sun's surface is seen to be thickly mottled
+with relatively obscure specks, so numerous that it has been estimated
+that they cut off from one-tenth to one-twentieth of the light that we
+should receive from it if the whole surface were as brilliant as its
+brightest parts. The condition of other stars warrants the conclusion
+that this obscuring envelope is the product of a process of
+refrigeration which will gradually make the sun more and more variable
+until its history ends in extinction. Looking backward, we see a time
+when the sun must have been more brilliant than it is now. At that
+time it probably shone with the blinding white splendor of such stars
+as Sirius, Spica, and Vega; now it resembles the relatively dull
+Procyon; in time it will turn ruddy and fall into the closing cycle
+represented by Antares. Considering that once it must have been more
+radiantly powerful than at present, one is tempted to wonder if that
+could have been the time when tropical life flourished within the
+earth's polar circles, sustained by a vivific energy in the sun which
+it has now lost.
+
+The corona, as we have said, varies with the sun-spot cycle. When the
+spots are abundant and active the corona rises strong above the
+spotted zones, forming immense beams or streamers, which on one
+occasion, at least, had an observed length of ten million miles. At
+the time of a spot minimum the corona is less brilliant and has a
+different outline. It is then that the curved polar rays are most
+conspicuous. Thus the vast banners of the sun, shaken out in the
+eclipse, are signals to tell of its varying state, but it will
+probably be long before we can read correctly their messages.
+
+The Zodiacal Light Mystery
+
+There is a singular phenomenon in the sky -- one of the most puzzling
+of all -- which has long arrested the attention of astronomers,
+defying their efforts at explanation, but which probably not one in a
+hundred, and possibly not one in a thousand, of the readers of this
+book has ever seen. Yet its name is often spoken, and it is a
+conspicuous object if one knows when and where to look for it, and
+when well seen it exhibits a mystical beauty which at the same time
+charms and awes the beholder. It is called ``The Zodiacal Light,''
+because it lies within the broad circle of the Zodiac, marking the
+sun's apparent annual path through the stars. What it is nobody has
+yet been able to find out with certainty, and books on astronomy
+usually speak of it with singular reserve. But it has given rise to
+many remarkable theories, and a true explanation of it would probably
+throw light on a great many other celestial mysteries. The Milky Way
+is a more wonderful object to look upon, but its nature can be
+comprehended, while there is a sort of uncanniness about the Zodiacal
+Light which immediately impresses one upon seeing it, for its part in
+the great scheme of extra-terrestrial affairs is not evident.
+
+If you are out-of-doors soon after sunset -- say, on an evening late
+in the month of February -- you may perceive, just after the angry
+flush of the dying winter's day has faded from the sky, a pale ghostly
+presence rising above the place where the sun went down. The writer
+remembers from boyhood the first time it was pointed out to him and
+the unearthly impression that it made, so that he afterward avoided
+being out alone at night, fearful of seeing the spectral thing again.
+The phenomenon brightens slowly with the fading of the twilight, and
+soon distinctly assumes the shape of an elongated pyramid of pearly
+light, leaning toward the south if the place of observation is in the
+northern hemisphere. It does not impress the observer at all in the
+same manner as the Milky Way; that looks far off and is clearly among
+the stars, but the Zodiacal Light seems closer at hand, as if it were
+something more intimately concerning the earth. To all it immediately
+suggests a connection, also, with the sunken sun. If the night is
+clear and the moon absent (and if you are in the country, for city
+lights ruin the spectacles of the sky), you will be able to watch the
+apparition for a long time. You will observe that the light is
+brightest near the horizon, gradually fading as the pyramidal beam
+mounts higher, but in favorable circumstances it may be traced nearly
+to the meridian south of the zenith, where its apex at last vanishes
+in the starlight. It continues visible during the evenings of March
+and part of April, after which, ordinarily, it is seen no more, or if
+seen is relatively faint and unimpressive. But when autumn comes it
+appears again, this time not like a wraith hovering above the westward
+tomb of the day-god, but rather like a spirit of the morning
+announcing his reincarnation in the east.
+
+The reason why the Zodiacal Light is best seen in our latitudes at the
+periods just mentioned is because at those times the Zodiac is more
+nearly perpendicular to the horizon, first in the west and then in the
+east; and, since the phenomenon is confined within the borders of the
+Zodiac, it cannot be favorably placed for observation when the
+zodiacal plane is but slightly inclined to the horizon. Its faint
+light requires the contrast of a background of dark sky in order to be
+readily perceptible. But within the tropics, where the Zodiac is
+always at a favorable angle, the mysterious light is more constantly
+visible. Nearly all observant travelers in the equatorial regions have
+taken particular note of this phenomenon, for being so much more
+conspicuous there than in the temperate zones it at once catches the
+eye and holds the attention as a novelty. Humboldt mentions it many
+times in his works, for his genius was always attracted by things out
+of the ordinary and difficult of explanation, and he made many careful
+observations on its shape, its brilliancy, and its variations; for
+there can be no doubt that it does vary, and sometimes to an
+astonishing degree. It is said that it once remained practically
+invisible in Europe for several years in succession. During a trip to
+South Africa in 1909 an English astronomer, Mr E. W. Maunder, found a
+remarkable difference between the appearance of the Zodiacal Light on
+his going and coming voyages. In fact, when crossing the equator going
+south he did not see it at all; but on returning he had, on March 6th,
+when one degree south of the equator, a memorable view of it.
+
+  It was a bright, clear night, and the Zodiacal Light was
+  extraordinarily brilliant -- brighter than he had ever seen it
+  before. The Milky Way was not to be compared with it. The brightest
+  part extended 75 deg. from the sun. There was a faint and much narrower
+  extension which they could just make out beyond the Pleiades along
+  the ecliptic, but the greater part of the Zodiacal Light showed as
+  a broad truncated column, and it did not appear nearly as conical
+  as he had before seen it.
+
+When out of the brief twilight of intertropical lands, where the sun
+drops vertically to the horizon and night rushes on like a wave of
+darkness, the Zodiacal Light shoots to the very zenith, its color is
+described as a golden tint, entirely different from the silvery sheen
+of the Milky Way. If I may venture again to refer to personal
+experiences and impressions, I will recall a view of the Zodiacal
+Light from the summit of the cone of Mt Etna in the autumn of the year
+1896 (more briefly described in Astronomy with the Naked Eye). There
+are few lofty mountains so favorably placed as Etna for observations
+of this kind. It was once resorted to by Prof. George E. Hale, in an
+attempt to see the solar corona without an eclipse. Rising directly
+from sea-level to an elevation of nearly eleven thousand feet, the
+observer on its summit at night finds himself, as it were, lost in the
+midst of the sky. But for the black flanks of the great cone on which
+he stands he might fancy himself to be in a balloon. On the occasion
+to which I refer the world beneath was virtually invisible in the
+moonless night. The blaze of the constellations overhead was
+astonishingly brilliant, yet amid all their magnificence my attention
+was immediately drawn to a great tapering light that sprang from the
+place on the horizon where the sun would rise later, and that seemed
+to be blown out over the stars like a long, luminous veil. It was the
+finest view of the Zodiacal light that I had ever enjoyed -- thrilling
+in its strangeness -- but I was almost disheartened by the
+indifference of my guide, to whom it was only a light and nothing
+more. If he had no science, he had less poetry -- rather a remarkable
+thing, I thought, for a child of his clime. The Light appeared to me
+to be distinctly brighter than the visible part of the Milky Way which
+included the brilliant stretches in Auriga and Perseus, and its color,
+if one may speak of color in connection with such an object, seemed
+richer than that of the galactic band; but I did not think of it as
+yellow, although Humboldt has described it as resembling a golden
+curtain drawn over the stars, and Du Chaillu in Equatorial Africa
+found it of a bright yellow color. It may vary in color as in
+conspicuousness. The fascination of that extraordinary sight has never
+faded from my memory. I turned to regard it again and again, although
+I had never seen the stellar heavens so brilliant, and it was one of
+the last things I looked for when the morning glow began softly to
+mount in the east, and Sicily and the Mediterranean slowly emerged
+from the profound shadow beneath us.
+
+The Zodiacal Light seems never to have attracted from astronomers in
+general the amount of careful attention that it deserves; perhaps
+because so little can really be made of it as far as explanation is
+concerned. I have referred to the restraint that scientific writers
+apparently feel in speaking of it. The grounds for speculation that it
+affords may be too scanty to lead to long discussions, yet it piques
+curiosity, and as we shall see in a moment has finally led to a most
+interesting theory. Once it was the subject of an elaborate series of
+studies which carried the observer all round the world. That was in
+1845--46, during the United States Exploring Expedition that visited
+the then little known Japan. The chaplain of the fleet, the Rev. Mr
+Jones, went out prepared to study the mysterious light in all its
+phases. He saw it from many latitudes on both sides of the equator,
+and the imagination cannot but follow him with keen interest in his
+world-circling tour, keeping his eyes every night fixed upon the
+phantasm overhead, whose position shifted with that of the hidden sun.
+He demonstrated that the flow extends at times completely across the
+celestial dome, although it is relatively faint directly behind the
+earth. On his return the government published a large volume of his
+observations, in which he undertook to show that the phenomenon was
+due to the reflection of sunlight from a ring of meteoric bodies
+encircling the earth. But, after all, this elaborate investigation
+settled nothing.
+
+Prof. E. E. Barnard has more recently devoted much attention to the
+Zodiacal Light, as well as to a strange attendant phenomenon called
+the ``Gegenschein,'' or Counterglow, because it always appears at that
+point in the sky which is exactly opposite the sun. The Gegenschein is
+an extremely elusive phenomenon, suitable only for eyes that have been
+specially trained to see it. Professor Newcomb has cautiously remarked
+that
+
+  it is said that in that point of the heavens directly opposite the
+  sun there is an elliptical patch of light... This phenomenon is so
+  difficult to account for that its existence is sometimes doubted;
+  yet the testimony in its favor is difficult to set aside.
+
+It certainly cannot be set aside at all since the observations of
+Barnard. I recall an attempt to see it under his guidance during a
+visit to Mount Hamilton, when he was occupied there with the Lick
+telescope. Of course, both the Gegenschein and the Zodiacal Light are
+too diffuse to be studied with telescopes, which, so to speak, magnify
+them out of existence. They can only be successfully studied with the
+naked eye, since every faintest glimmer that they afford must be
+utilized. This is especially true of the Gegenschein. At Mount
+Hamilton, Mr Barnard pointed out to me its location with reference to
+certain stars, but with all my gazing I could not be sure that I saw
+it. To him, on the contrary, it was obvious; he had studied it for
+months, and was able to indicate its shape, its boundaries, its
+diameter, and the declination of its center with regard to the
+ecliptic. There is not, of course, the shadow of a doubt of the
+existence of the Gegenschein, and yet I question if one person in a
+million has ever seen or ever will see it. The Zodiacal Light, on the
+other hand, is plain enough, provided that the time and the
+circumstances of the observation are properly chosen.
+
+In the attempts to explain the Zodiacal Light, the favorite hypothesis
+has been that it is an appendage of the sun -- perhaps simply an
+extension of the corona in the plane of the ecliptic, which is not
+very far from coinciding with that of the sun's equator. This idea is
+quite a natural one, because of the evident relation of the light to
+the position of the sun. The vast extension of the equatorial wings of
+the corona in 1878 gave apparent support to this hypothesis; if the
+substance of the corona could extend ten million miles from the sun,
+why might it not extend even one hundred million, gradually fading out
+beyond the orbit of the earth? A variation of this hypothesis assumes
+that the reflection is due to swarms of meteors circling about the
+sun, in the plane of its equator, all the way from its immediate
+neighborhood to a distance exceeding that of the earth. But in neither
+form is the hypothesis satisfactory; there is nothing in the
+appearance of the corona to indicate that it extends even as far as
+the planet Mercury, while as to meteors, the orbits of the known
+swarms do not accord with the hypothesis, and we have no reason to
+believe that clouds of others exist traveling in the part of space
+where they would have to be in order to answer the requirements of the
+theory. The extension of the corona in 1878 did not resemble in its
+texture the Zodiacal Light.
+
+Now, it has so often happened in the history of science that an
+important discovery in one branch has thrown unexpected but most
+welcome light upon some pending problem in some other branch, that a
+strong argument might be based upon that fact alone against the too
+exclusive devotion of many investigators to the narrow lines of their
+own particular specialty; and the Zodiacal Light affords a case in
+point, when it is considered in connection with recent discoveries in
+chemistry and physics. From the fact that atoms are compound bodies
+made up of corpuscles at least a thousand times smaller than the
+smallest known atom -- a fact which astounded most men of science when
+it was announced a few years ago -- a new hypothesis has been
+developed concerning the nature of the Zodiacal Light (as well as
+other astronomical riddles), and this hypothesis comes not from an
+astronomer, but from a chemist and physicist, the Swede, Svante
+Arrhenius. In considering an outline of this new hypothesis we need
+neither accept nor reject it; it is a case rather for suspension of
+judgment.
+
+To begin with, it carries us back to the ``pressure of light''
+mentioned in the preceding chapter. The manner in which this pressure
+is believed generally to act was there sufficiently explained, and it
+only remains to see how it is theoretically extended to the particles
+of matter supposed to constitute the Zodiacal Light. We know that
+corpuscles, or ``fragments of atoms'' negatively electrified, are
+discharged from hot bodies. Streams of these ``ions'' pour from many
+flames and from molten metals; and the impact of the cathode and
+ultra-violet rays causes them to gush even from cold bodies. In the
+vast laboratory of the sun it is but reasonable to suppose that
+similar processes are taking place. ``As a very hot metal emits these
+corpuscles,'' says Prof. J. J. Thomson, ``it does not seem an
+improbable hypothesis that they are emitted by that very hot body, the
+sun.'' Let it be assumed, then, that the sun does emit them; what
+happens next? Negatively charged corpuscles, it is known, serve as
+nuclei to which particles of matter in the ordinary state are
+attracted, and it is probable that those emitted from the sun
+immediately pick up loads in this manner and so grow in bulk. If they
+grow large enough the gravitation of the sun draws them back, and they
+produce a negative charge in the solar atmosphere. But it is probable
+that many of the particles do not attain the critical size which,
+according to the principles before explained, would enable the
+gravitation of the sun to retain them in opposition to the pressure of
+the waves of light, and with these particles the light pressure is
+dominant. Clouds of them may be supposed to be continually swept away
+from the sun into surrounding space, moving mostly in or near the
+plane of the solar equator, where the greatest activity, as indicated
+by sunspots and related phenomena, is taking place. As they pass
+outward into space many of them encounter the earth. If the earth,
+like the moon, had no atmosphere the particles would impinge directly
+on its surface, giving it a negative electric charge. But the presence
+of the atmosphere changes all that, for the first of the flying
+particles that encounter it impart to it their negative electricity,
+and then, since like electric charges repel like, the storm of
+particles following will be sheered off from the earth, and will
+stream around it in a maze of hyperbolic paths. Those that continue on
+into space beyond the earth may be expected to continue picking up
+wandering particles of matter until their bulk has become so great
+that the solar attraction prevails again over the light pressure
+acting upon them, and they turn again sunward. Passing the earth on
+their return they will increase the amount of dust-clouds careering
+round it; and these will be further increased by the action of the
+ultra-violet rays of the sunlight causing particles to shoot radially
+away from the earth when the negative charge of the upper atmosphere
+has reached a certain amount, which particles, although starting
+sunward, will be swept back to the earth with the oncoming streams. As
+the final result of all this accumulation of flying and gyrating
+particles in the earth's neighborhood, we are told that the latter
+must be transformed into the semblance of a gigantic solid-headed
+comet provided with streaming tails, the longest of them stretching
+away from the direction of the sun, while another shorter one extends
+toward the sun. This shorter tail is due to the particles that we have
+just spoken of as being driven sunward from the earth by the action of
+ultra-violet light. No doubt this whole subject is too technical for
+popular statement; but at any rate the general reader can understand
+the picturesque side of the theory, for its advocates assure us that
+if we were on the moon we would doubtless be able to see the
+comet-like tails of the earth, and then we could appreciate the part
+that they play in producing the phenomenon of the Zodiacal Light.
+
+That the Light as we see it could be produced by the reflection of
+sunlight from swarms of particles careering round the earth in the
+manner supposed by Arrhenius' hypothesis is evident enough; and it
+will be observed that the new theory, after all, is only another
+variant of the older one which attributes the Zodiacal Light to an
+extension of the solar corona. But it differs from the older theory in
+offering an explanation of the manner in which the extension is
+effected, and it differentiates between the corona proper and the
+streams of negative particles shot away from the sun. In its details
+the hypothesis of Arrhenius also affords an explanation of many
+peculiarities of the Zodiacal Light, such as that it is confined to
+the neighborhood of the ecliptic, and that it is stronger on the side
+of the earth which is just turning away from a position under the sun
+than on the other side; but it would carry us beyond our limits to go
+into these particulars. The Gegenschein, according to this theory, is
+a part of the same phenomenon as the Zodiacal Light, for by the laws
+of perspective it is evident that the reflection from the streams of
+particles situated at a point directly opposite to the sun would be at
+a maximum, and this is the place which the Gegenschein occupies. Apart
+from its geometrical relations to the position of the sun, the
+variability of the Zodiacal Light appears to affirm its solar
+dependence, and this too would be accounted for by Arrhenius'
+hypothesis better than by the old theory of coronal extension. The
+amount of corpuscular discharge from the sun must naturally be
+governed by the state of relative activity or inactivity of the
+latter, and this could not but be reflected in the varying splendor of
+the Zodiacal Light. But much more extended study than has yet been
+given to the subject will be required before we can feel that we know
+with reasonable certainty what this mysterious phenomenon really is.
+By the hypothesis of Arrhenius every planet that has an atmosphere
+must have a Zodiacal Light attending it, but the phenomenon is too
+faint for us to be able to see it in the case, for instance, of Venus,
+whose atmosphere is very abundant. The moon has no corresponding
+``comet's tail'' because, as already explained, of the lack of a lunar
+atmosphere to repel the streams by becoming itself electrified; but if
+there were a lunar Zodiacal Light, no doubt we could see it because of
+the relative nearness of our satellite.
+
+Marvels of the Aurora
+
+One of the most vivid recollections of my early boyhood is that of
+seeing my father return hastily into the house one evening and call
+out to the family: ``Come outside and look at the sky!'' Ours was a
+country house situated on a commanding site, and as we all emerged
+from the doorway we were dumbfounded to see the heavens filled with
+pale flames which ran licking and quivering over the stars. Instantly
+there sprang into my terrified mind the recollection of an awful
+description of ``the Day of Judgment'' (the Dies Irae), which I had
+heard with much perturbation of spirit in the Dutch Reformed church
+from the lips of a tall, dark-browed, dreadfully-in-earnest preacher
+of the old-fashioned type. My heart literally sank at sight of the
+spectacle, for it recalled the preacher's very words; it was just as
+he had said it would be, and it needed the assured bearing of my
+elders finally to convince me that
+
+  That Day of Wrath, O dreadful day,
+  When Heaven and Earth shall pass away,
+  As David and the Sibyl say
+
+had not actually come upon us. And even the older members of the
+household were not untouched with misgivings when menacing spots of
+crimson appeared, breaking out now here, now there, in the shuddering
+sky. Toward the north the spectacle was appalling. A huge arch spanned
+an unnaturally dark segment resting on the horizon, and above this
+arch sprang up beams and streamers in a state of incessant agitation,
+sometimes shooting up to the zenith with a velocity that took one's
+breath, and sometimes suddenly falling into long ranks, and marching,
+marching, marching, like an endless phalanx of fiery specters, and
+moving, as I remember, always from east to west. The absolute silence
+with which these mysterious evolutions were performed and the
+quavering reflections which were thrown upon the ground increased the
+awfulness of the exhibition. Occasionally enormous curtains of lambent
+flame rolled and unrolled with a majestic motion, or were shaken to
+and fro as if by a mighty, noiseless wind. At times, too, a sudden
+billowing rush would be made toward the zenith, and for a minute the
+sky overhead would glow so brightly that the stars seemed to have been
+consumed. The spectacle continued with varying intensity for hours.
+
+This exhibition occurred in Central New York, a latitude in which the
+Aurora Borealis is seldom seen with so much splendor. I remember
+another similar one seen from the city of New York in November, 1882.
+On this last occasion some observers saw a great upright beam of light
+which majestically moved across the heavens, stalking like an
+apparition in the midst of the auroral pageant, of whose general
+movements it seemed to be independent, maintaining always its upright
+posture, and following a magnetic parallel from east to west. This
+mysterious beam was seen by no less than twenty-six observers in
+different parts of the country, and a comparison of their observations
+led to a curious calculation indicating that the apparition was about
+one hundred and thirty-three miles tall and moved at the speed of ten
+miles per second!
+
+But, as everybody knows, it is in the Arctic regions that the Aurora,
+or the ``Northern Lights,'' can best be seen. There, in the long polar
+night, when for months together the sun does not rise, the strange
+coruscations in the sky often afford a kind of spectral daylight in
+unison with the weird scenery of the world of ice. The pages in the
+narratives of Arctic exploration that are devoted to descriptions of
+the wonderful effects of the Northern Lights are second to none that
+man has ever penned in their fascination. The lights, as I have
+already intimated, display astonishing colors, particularly shades of
+red and green, as they flit from place to place in the sky. The
+discovery that the magnetic needle is affected by the Aurora,
+quivering and darting about in a state of extraordinary excitement
+when the lights are playing in the sky, only added to the mystery of
+the phenomenon until its electro-magnetic nature had been established.
+This became evident as soon as it was known that the focus of the
+displays was the magnetic pole; and when the far South was visited the
+Aurora Australis was found, having its center at the South Magnetic
+Pole. Then, if not before, it was clear that the earth was a great
+globular magnet, having its poles of opposite magnetism, and that the
+auroral lights, whatever their precise cause might be, were
+manifestations of the magnetic activity of our planet. After the
+invention of magnetic telegraphy it was found that whenever a great
+Aurora occurred the telegraph lines were interrupted in their
+operation, and the ocean cables ceased to work. Such a phenomenon is
+called a ``magnetic storm.''
+
+The interest excited by the Aurora in scientific circles was greatly
+stimulated when, in the last half of the nineteenth century, it was
+discovered that it is a phenomenon intimately associated with
+disturbances on the sun. The ancient ``Zurich Chronicles,'' extending
+from the year 1000 to the year 1800, in which both sun-spots visible
+to the naked eye and great displays of the auroral lights were
+recorded, first set Rudolf Wolf on the track of this discovery. The
+first notable proof of the suspected connection was furnished with
+dramatic emphasis by an occurrence which happened on September 1,
+1859. Near noon on that day two intensely brilliant points suddenly
+broke out in a group of sun-spots which were under observation by Mr
+R. C. Carrington at his observatory at Redhill, England. The points
+remained visible for not more than five minutes, during which interval
+they moved thirty-five thousand miles across the solar disk. Mr R.
+Hodgson happened to see the same phenomenon at his observatory at
+Highgate, and thus all possibility of deception was removed. But
+neither of the startled observers could have anticipated what was to
+follow, and, indeed, it was an occurrence which has never been
+precisely duplicated. I quote the eloquent account given by Miss
+Clerke in her History of Astronomy During the Nineteenth Century.
+
+  This unique phenomenon seemed as if specially designed to
+  accentuate the inference of a sympathetic relation between the
+  earth and the sun. From August 28 to September 4, 1859, a magnetic
+  storm of unparalleled intensity, extent, and duration was in
+  progress over the entire globe. Telegraphic communication was
+  everywhere interrupted -- except, indeed, that it was in some cases
+  found practicable to work the lines without batteries by the agency
+  of the earth-currents alone; sparks issued from the wires; gorgeous
+  auroras draped the skies in solemn crimson over both hemispheres,
+  and even in the tropics; the magnetic needle lost all trace of
+  continuity in its movements and darted to and fro as if stricken
+  with inexplicable panic. The coincidence was even closer. At the
+  very instant of the solar outburst witnessed by Carrington and
+  Hodgson the photographic apparatus at Kew registered a marked
+  disturbance of all the three magnetic elements; while shortly after
+  the ensuing midnight the electric agitation culminated, thrilling
+  the whole earth with subtle vibrations, and lighting up the
+  atmosphere from pole to pole with coruscating splendors which
+  perhaps dimly recall the times when our ancient planet itself shone
+  as a star.
+
+If this amazing occurrence stood alone, and as I have already said it
+has never been exactly duplicated, doubt might be felt concerning some
+of the inferences drawn from it; but in varying forms it has been
+repeated many times, so that now hardly anyone questions the reality
+of the assumed connection between solar outbursts and magnetic storms
+accompanied by auroral displays on the earth. It is true that the late
+Lord Kelvin raised difficulties in the way of the hypothesis of a
+direct magnetic action of the sun upon the earth, because it seemed to
+him that an inadmissible quantity of energy was demanded to account
+for such action. But no calculation like that which he made is final,
+since all calculations depend upon the validity of the data; and no
+authority is unshakable in science, because no man can possess
+omniscience. It was Lord Kelvin who, but a few years before the thing
+was actually accomplished, declared that aerial navigation was an
+impracticable dream, and demonstrated its impracticability by
+calculation. However the connection may be brought about, it is as
+certain as evidence can make it that solar outbursts are coincident
+with terrestial magnetic disturbances, and coincident in such a way as
+to make the inference of a causal connection irresistible. The sun is
+only a little more than a hundred times its own diameter away from the
+earth. Why, then, with the subtle connection between them afforded by
+the ether which conveys to us the blinding solar light and the
+life-sustaining solar heat, should it be so difficult to believe that
+the sun's enormous electric energies find a way to us also? No doubt
+the impulse coming from the sun acts upon the earth after the manner
+of a touch upon a trigger, releasing energies which are already stored
+up in our planet.
+
+But besides the evidence afforded by such occurrences as have been
+related of an intimate connection between solar outbreaks and
+terrestial magnetic flurries, attended by magnificent auroral
+displays, there is another line of proof pointing in the same
+direction. Thus, it is known that the sun-spot period, as remarked in
+a preceding chapter, coincides in a most remarkable manner with the
+periodic fluctuations in the magnetic state of the earth. This
+coincidence runs into the most astonishing details. For instance, when
+the sun-spot period shortens, the auroral period shortens to precisely
+the same extent; as the short sun-spot periods usually bring the most
+intense outbreaks of solar activity, so the corresponding short
+auroral periods are attended by the most violent magnetic storms; a
+secular period of about two hundred and twenty-two years affecting
+sun-spots is said to have its auroral duplicate; a shorter period of
+fifty-five and a half years, which some observers believe that they
+have discovered appears also to be common to the two phenomena; and
+yet another ``superposed'' period of about thirty-five years, which
+some investigators aver exists, affects sun-spots and aurora alike. In
+short, the coincidences are so numerous and significant that one would
+have to throw the doctrine of probability to the winds in order to be
+able to reject the conclusion to which they so plainly lead.
+
+But still the question recurs: How is the influence transmitted? Here
+Arrhenius comes once more with his hypothesis of negative corpuscles,
+or ions, driven away from the sun by light-pressure -- a hypothesis
+which seems to explain so many things -- and offers it also as an
+explanation of the way in which the sun creates the Aurora. He would
+give the Aurora the same lineage with the Zodiacal Light. To
+understand the application of this theory we must first recall the
+fact that the earth is a great magnet having its two opposite poles of
+magnetism, one near the Arctic and the other near the Antarctic
+Circle. Like all magnets, the earth is surrounded with ``lines of
+force,'' which, after the manner of the curved rays we saw in the
+photograph of a solar eclipse, start from a pole, rising at first
+nearly vertically, then bend gradually over, passing high above the
+equator, and finally descending in converging sheaves to the opposite
+pole. Now the axis of the earth is so placed in space that it lies at
+nearly a right angle to the direction of the sun, and as the streams
+of negatively charged particles come pouring on from the sun (see the
+last preceding chapter), they arrive in the greatest numbers over the
+earth's equatorial regions. There they encounter the lines of magnetic
+force at the place where the latter have their greatest elevation
+above the earth, and where their direction is horizontal to the
+earth's surface. Obeying a law which has been demonstrated in the
+laboratory, the particles then follow the lines of force toward the
+poles. While they are above the equatorial regions they do not become
+luminescent, because at the great elevation that they there occupy
+there is virtually no atmosphere; but as they pass on toward the north
+and the south they begin to descend with the lines of force, curving
+down to meet at the poles; and, encountering a part of the atmosphere
+comparable in density with what remains in an exhausted Crookes tube,
+they produce a glow of cathode rays. This glow is conceived to
+represent the Aurora, which may consequently be likened to a gigantic
+exhibition of vacuum-tube lights. Anybody who recalls his student days
+in the college laboratory and who has witnessed a display of Northern
+Lights will at once recognize the resemblance between them in colors,
+forms, and behavior. This resemblance had often been noted before
+Arrhenius elaborated his hypothesis.
+
+Without intending to treat his interesting theory as more than a
+possibly correct explanation of the phenomena of the Aurora, we may
+call attention to some apparently confirmatory facts. One of the most
+striking of these relates to a seasonal variation in the average
+number of aurorae. It has been observed that there are more in March
+and September than at any other time of the year, and fewer in June
+and December; moreover (and this is a delicate test as applied to the
+theory), they are slightly rarer in June than in December. Now all
+these facts seem to find a ready explanation in the hypothesis of
+Arrhenius, thus: (1) The particles issuing from the sun are supposed
+to come principally from the regions whose excitement is indicated by
+the presence of sun-spots (which accords with Hale's observation that
+sun-spots are columns of ionized vapors), and these regions have a
+definite location on either side of the solar equator, seldom
+approaching it nearer than within 5 deg. or 10 deg. north or south, and never
+extending much beyond 35 deg. toward either pole; (2) The equator of the
+sun is inclined about 7 deg. to the plane of the earth's orbit, from which
+it results that twice in a year -- viz., in June and December -- the
+earth is directly over the solar equator, and twice a year -- viz., in
+March and September -- when it is farthest north or south of the solar
+equator, it is over the inner edge of the sun-spot belts. Since the
+corpuscles must be supposed to be propelled radially from the sun, few
+will reach the earth when the latter is over the solar equator in June
+and December, but when it is over, or nearly over, the spot belts, in
+March and September, it will be in the line of fire of the more active
+parts of the solar surface, and relatively rich streams of particles
+will reach it. This, as will be seen from what has been said above, is
+in strict accord with the observed variations in the frequency of
+aurorae. Even the fact that somewhat fewer aurorae are seen in June than
+in December also finds its explanation in the known fact that the
+earth is about three million miles nearer the sun in the winter than
+in the summer, and the number of particles reaching it will vary, like
+the intensity of light, inversely as the square of the distance. These
+coincidences are certainly very striking, and they have a cumulative
+force. If we accept the theory, it would appear that we ought to
+congratulate ourselves that the inclination of the sun's equator is so
+slight, for as things stand the earth is never directly over the most
+active regions of the sun-spots, and consequently never suffers from
+the maximum bombardment of charged particles of which the sun is
+capable. Incessant auroral displays, with their undulating draperies,
+flitting colors, and marching columns might not be objectionable from
+the point of view of picturesqueness, but one magnetic storm of
+extreme intensity following closely upon the heels of another, for
+months on end, crazing the magnetic needle and continually putting the
+telegraph and cable lines out of commission, to say nothing of their
+effect upon ``wireless telegraphy'', would hardly add to the charms of
+terrestrial existence.
+
+One or two other curious points in connection with Arrhenius'
+hypothesis may be mentioned. First, the number of aurorae, according to
+his explanation, ought to be greatest in the daytime, when the face of
+the earth on the sunward side is directly exposed to the atomic
+bombardment. Of course visual observation can give us no information
+about this, since the light of the Aurora is never sufficiently
+intense to be visible in the presence of daylight, but the records of
+the magnetic observatories can be, and have been, appealed to for
+information, and they indicate that the facts actually accord with the
+theory. Behind the veil of sunlight in the middle of the afternoon,
+there is good reason to believe, auroral exhibitions often take place
+which would eclipse in magnificence those seen at night if we could
+behold them. Observation shows, too, that aurorae are more frequent
+before than after midnight, which is just what we should expect if
+they originate in the way that Arrhenius supposes. Second, the theory
+offers an explanation of the alleged fact that the formation of clouds
+in the upper air is more frequent in years when aurorae are most
+abundant, because clouds are the result of the condensation of
+moisture upon floating particles in the atmosphere (in an absolutely
+dustless atmosphere there would be no clouds), and it has been proved
+that negative ions like those supposed to come from the sun play a
+master part in the phenomena of cloud formation.
+
+Yet another singular fact, almost mystical in its suggestions, may be
+mentioned. It seems that the dance of the auroral lights occurs most
+frequently during the absence of the moon from the hemisphere in which
+they appear, and that they flee, in greater part, to the opposite
+hemisphere when the moon's revolution in an orbit considerably
+inclined to the earth's equator brings her into that where they have
+been performing. Arrhenius himself discovered this curious relation of
+auroral frequency to the position of the moon north or south of the
+equator, and he explains it in this way. The moon, like the earth, is
+exposed to the influx of the ions from the sun; but having no
+atmosphere, or almost none, to interfere with them, they descend
+directly upon her surface and charge her with an electric negative
+potential to a very high degree. In consequence of this she affects
+the electric state of the upper parts of the earth's atmosphere where
+they lie most directly beneath her, and thus prevents, to a large
+extent, the negative discharges to which the appearance of the Aurora
+is due. And so ``the extravagant and erring spirit'' of the Aurora
+avoids the moon as Hamlet's ghost fled at the voice of the cock
+announcing the awakening of the god of day.
+
+There are even other apparent confirmations of the hypothesis, but we
+need not go into them. We shall, however, find one more application of
+it in the next chapter, for it appears to be a kind of cure-all for
+astronomical troubles; at any rate it offers a conceivable solution of
+the question, How does the sun manage to transmit its electric
+influence to the earth? And this solution is so grandiose in
+conception, and so novel in the mental pictures that it offers, that
+its acceptance would not in the least detract from the impression that
+the Aurora makes upon the imagination.
+
+Strange Adventures of Comets
+
+The fears and legends of ancient times before Science was born, and
+the superstitions of the Dark Ages, sedulously cultivated for
+theological purposes by monks and priests, have so colored our ideas
+of the influence that comets have had upon the human mind that many
+readers may be surprised to learn that it was the apparition of a
+wonderful comet, that of 1843, which led to the foundation of our
+greatest astronomical institution, the Harvard College Observatory. No
+doubt the comet superstition existed half a century ago, as, indeed,
+it exists yet today, but in this case the marvelous spectacle in the
+sky proved less effective in inspiring terror than in awakening a
+desire for knowledge. Even in the sixteenth century the views that
+enlightened minds took of comets tended powerfully to inspire popular
+confidence in science, and Halley's prediction, after seeing and
+studying the motion of the comet which appeared in 1682, that it would
+prove to be a regular member of the sun's family and would be seen
+returning after a period of about seventy-six years, together with the
+fulfillment of that prediction, produced a revulsion from the
+superstitious notions which had so long prevailed.
+
+Then the facts were made plain that comets are subject to the law of
+gravitation equally with the planets; that there are many which
+regularly return to the neighborhood of the sun (perihelion); and that
+these travel in orbits differing from those of the planets only in
+their greater eccentricity, although they have the peculiarity that
+they do not, like the planets, all go round the sun in the same
+direction, and do not keep within the general plane of the planetary
+system, but traverse it sometimes from above and sometimes from below.
+Other comets, including most of the ``great'' ones, appear to travel
+in parabolic or, in a few cases, hyperbolic orbits, which, not being
+closed curves, never bring them back again. But it is not certain that
+these orbits may not be extremely eccentric ellipses, and that after
+the lapse of hundreds, or thousands, of years the comets that follow
+them may not reappear. The question is an interesting one, because if
+all orbits are really ellipses, then all comets must be permanent
+members of the solar system, while in the contrary case many of them
+are simply visitors, seen once and never to be seen again. The
+hypothesis that comets are originally interlopers might seem to derive
+some support from the fact that the certainly periodic ones are
+associated, in groups, with the great outer planets, whose attraction
+appears to have served as a trap for them by turning them into
+elliptical orbits and thus making them prisoners in the solar system.
+Jupiter, owing to his great mass and his commanding situation in the
+system, is the chief ``comet-catcher;'' but he catches them not for
+himself, but for the sun. Yet if comets do come originally from
+without the borders of the planetary system, it does not, by any
+means, follow that they were wanderers at large in space before they
+yielded to the overmastering attraction of the sun. Investigation of
+the known cometary orbits, combined with theoretical considerations,
+has led some astronomers to the conclusion that as the sun travels
+onward through space he ``picks up en route'' cometary masses which,
+without belonging strictly to his empire, are borne along in the same
+vast ``cosmical current'' that carries the solar system.
+
+But while no intelligent person any longer thinks that the appearance
+of a great comet is a token from the heavenly powers of the
+approaching death of a mighty ruler, or the outbreak of a devastating
+war, or the infliction of a terrible plague upon wicked mankind,
+science itself has discovered mysteries about comets which are not
+less fascinating because they are more intellectual than the
+irrational fancies that they have displaced. To bring the subject
+properly before the mind, let us see what the principal phenomena
+connected with a comet are.
+
+At the present day comets are ordinarily ``picked up'' with the
+telescope or the photographic plate before any one except their
+discoverer is aware of their existence, and usually they remain so
+insignificant in appearance that only astronomers ever see them. Yet
+so great is the prestige of the word ``comet'' that the discovery of
+one of these inconspicuous wanderers, and its subsequent movements,
+become items of the day's news which everybody reads with the feeling,
+perhaps, that at least he knows what is going on in the universe even
+if he doesn't understand it. But a truly great comet presents quite a
+different proposition. It, too, is apt to be detected coming out of
+the depths of space before the world at large can get a glimpse of it,
+but as it approaches the sun its aspect undergoes a marvelous change.
+Agitated apparently by solar influence, it throws out a long streaming
+tail of nebulous light, directed away from the sun and looking as if
+blown out like a pennon by a powerful wind. Whatever may be the
+position of the comet with regard to the sun, as it circles round him
+it continually keeps its tail on the off side. This, as we shall soon
+see, is a fact of capital importance in relation to the probable
+nature of comets' tails. Almost at the same time that the formation of
+the tail is observed a remarkable change takes place in the comet's
+head, which, by the way, is invariably and not merely occasionally its
+most important part. On approaching the sun the head usually
+contracts. Coincidently with this contraction a nucleus generally
+makes its appearance. This is a bright, star-like point in the head,
+and it probably represents the totality of solid matter that the comet
+possesses. But it is regarded as extremely unlikely that even the
+nucleus consists of a uniformly solid mass. If it were such, comets
+would be far more formidable visitors when they pass near the planets
+than they have been found to be. The diameter of the nucleus may vary
+from a few hundred up to several thousand miles; the heads, on the
+average, are from twenty-five thousand to one hundred thousand miles
+in diameter, although a few have greatly exceeded these dimensions;
+that of the comet of 1811, one of the most stupendous ever seen, was a
+million and a quarter miles in diameter! As to the tails, not
+withstanding their enormous length -- some have been more than a
+hundred million miles long -- there is reason to believe that they are
+of extreme tenuity, ``as rare as vacuum.'' The smallest stars have
+been seen shining through their most brilliant portions with
+undiminished luster.
+
+After the nucleus has been formed it begins to throw out bright jets
+directed toward the sun. A stream, and sometimes several streams, of
+light also project sunward from the nucleus, occasionally appearing
+like a stunted tail directed oppositely to the real tail. Symmetrical
+envelopes which, seen in section, appear as half circles or parabolas,
+rise sunward from the nucleus, forming a concentric series. The ends
+of these stream backward into the tail, to which they seem to supply
+material. Ordinarily the formation of these ejections and envelopes is
+attended by intense agitation of the nucleus, which twists and turns,
+swinging and gyrating with an appearance of the greatest violence.
+Sometimes the nucleus is seen to break up into several parts. The
+entire heads of some comets have been split asunder in passing close
+around the sun; The comet of 1882 retreated into space after its
+perihelion passage with five heads instead of the one that it had
+originally, and each of these heads had its own tail!
+
+The possession of the spectroscope has enabled astronomers during
+later years to study the chemical composition of comets by analyzing
+their light. At first the only substances thus discovered in them were
+hydro-carbon compounds, due evidently to the gaseous envelopes in
+which some combination of hydrogen with carbon existed. Behind this
+gaseous spectrum was found a faint continuous spectrum ascribed to the
+nucleus, which apparently both reflects the sunlight and gives forth
+the light of a glowing solid or liquid. Subsequently sodium and iron
+lines were found in cometary spectra. The presence of iron would seem
+to indicate that some of these bodies may be much more massive than
+observations on their attractive effects have indicated. In some
+recent comets, such as Morehouse's, in 1908, several lines have been
+found, the origin of which is unknown.
+
+Without going back of the nineteenth century we may find records of
+some of the most extraordinary comets that man has ever looked upon.
+In 1811, still spoken of as ``the year of the comet,'' because of the
+wonderful vintage ascribed to the skyey visitor, a comet shaped like a
+gigantic sword amazed the whole world, and, as it remained visible for
+seventeen months, was regarded by superstitious persons as a symbol of
+the fearful happenings of Napoleon's Russian campaign. This comet, the
+extraordinary size of whose head, greatly exceeding that of the sun
+itself, has already been mentioned, was also remarkable for exhibiting
+so great a brilliancy without approaching even to the earth's distance
+from the sun. But there was once a comet (and only once -- in the year
+1729) which never got nearer to the sun than four times the distance
+of the earth and yet appeared as a formidable object in the sky. As
+Professor Young has remarked, ``it must have been an enormous comet to
+be visible from such a distance.'' And we are to remember that there
+were no great telescopes in the year 1729. That comet affects the
+imagination like a phantom of space peering into the solar system,
+displaying its enormous train afar off (which, if it had approached as
+near as other comets, would probably have become the celestial wonder
+of all human memory), and then turning away and vanishing in the
+depths of immensity.
+
+In 1843 a comet appeared which was so brilliant that it could be seen
+in broad day close beside the sun! This was the first authenticated
+instance of that kind, but the occurrence was to be repeated, as we
+shall see in a moment, less than forty years later.
+
+The splendid comet of 1858, usually called Donati's, is remembered by
+many persons yet living. It was, perhaps, both as seen by the naked
+eye and with the telescope, the most beautiful comet of which we have
+any record. It too marked a rich vintage year, still remembered in the
+vineyards of France, where there is a popular belief that a great
+comet ripens the grape and imparts to the wine a flavor not attainable
+by the mere skill of the cultivator. There are ``comet wines,''
+carefully treasured in certain cellars, and brought forth only when
+their owner wishes to treat his guests to a sip from paradise.
+
+The year 1861 saw another very remarkable comet, of an aspect
+strangely vast and diffuse, which is believed to have swept the earth
+with its immense tail when it passed between us and the sun on the
+night of June 30th, an event which produced no other known effect than
+the appearance of an unwonted amount of scattered light in the sky.
+
+The next very notable comet was the ``Great Southern Comet'' of 1880,
+which was not seen from the northern hemisphere. It mimicked the
+aspect of the famous comet of 1843, and to the great surprise of
+astronomers appeared to be traveling in the same path. This proved to
+be the rising of the curtain for an astronomical sensation
+unparalleled in its kind; for two years later another brilliant comet
+appeared, first in the southern hemisphere, and it too followed the
+same track. The startling suggestion was now made that this comet was
+identical with those of 1843 and 1880, its return having been hastened
+by the resistance experienced in passing twice through the coronal
+envelope, and there were some who thought that it would now swing
+swiftly round and then plunge straight into the sun, with consequences
+that might be disastrous to us on account of the ``flash of heat''
+that would be produced by the impact. Nervous people were frightened,
+but observation soon proved that the danger was imaginary, for
+although the comet almost grazed the sun, and must have rushed through
+two or three million miles of the coronal region, no retardation of
+its immense velocity was perceptible, and it finally passed away in a
+damaged condition, as before remarked, and has never since appeared.
+
+Then the probable truth was perceived -- viz., that the three comets
+(1843, 1880, and 1882) were not one identical body, but three separate
+ones all traveling in the same orbit. It was found, too, that a comet
+seen in 1668 bore similar insignia of relationship. The natural
+inference was that these four bodies had once formed a single mass
+which had been split apart by the disruptive action of the sun.
+Strength was lent to this hypothesis by the fact that the comet of
+1882 was apparently torn asunder during its perihelion passage,
+retreating into space in a dissevered state. But Prof. George Forbes
+has a theory that the splitting of the original cometary mass was
+effected by an unknown planet, probably greater than Jupiter, situated
+at a hundred times the earth's distance from the sun, and revolving in
+a period of a thousand years. He supposes that the original comet was
+not that of 1668, but one seen in 1556, which has since been
+``missing,'' and that its disruption occurred from an encounter with
+the supposititious planet about the year 1700. Truly from every point
+of view comets are the most extraordinary of adventurers!
+
+The comet of 1882 was likewise remarkable for being visible, like its
+predecessor of 1843, in full daylight in close proximity to the sun.
+The story of its detection when almost in contact with the solar disk
+is dramatic. It had been discovered in the southern hemisphere only a
+couple of weeks before its perihelion, which occurred on September
+17th, and on the forenoon of that day it was seen by Doctor Common in
+England, and by Doctor Elkin and Mr Finlay at the Cape of Good Hope,
+almost touching the sun. It looked like a dazzling white bird with
+outspread wings. The southern observers watched it go right into the
+sun, when it instantly disappeared. What had happened was that the
+comet in passing its perihelion point had swung exactly between the
+earth and the sun. On the following morning it was seen from all parts
+of the world close by the sun on the opposite side, and it remained
+thus visible for three days, gradually receding from the solar disk.
+It then became visible for northern observers in the morning sky
+before sunrise, brandishing a portentous sword-shaped tail which, if
+it had been in the evening sky, would have excited the wonder of
+hundreds of millions, but situated where it was, comparatively few
+ever saw it.
+
+The application of photography to the study of comets has revealed
+many curious details which might otherwise have escaped detection, or
+at best have remained subject to doubt. It has in particular shown not
+only the precise form of the tails, but the remarkable vicissitudes
+that they undergo. Professor Barnard's photographs of Brooks' comet in
+1893 suggested, by the extraordinary changes in the form of the tail
+which they revealed, that the comet was encountering a series of
+obstructions in space which bent and twisted its tail into fantastic
+shapes. The reader will observe the strange form into which the tail
+was thrown on the night of October 21st. A cloud of meteors through
+which the comet was passing might have produced such deformations of
+its tail. In the photograph of Daniels' comet of 1907, a curious
+striping of the tail will be noticed. The short bright streaks seen in
+the photograph, it may be explained, are the images of stars which are
+drawn out into lines in consequence of the fact that the photographic
+telescope was adjusted to follow the motion of the comet while the
+stars remained at rest.
+
+But the adventures of comets are not confined to possible encounters
+with unknown obstacles. We have referred to the fact that the great
+planets, and especially Jupiter, frequently interfere with the motions
+of comets. This interference is not limited to the original alteration
+of their orbits from possible parabolas to ellipses, but is sometimes
+exercised again and again, turning the bewildered comets into
+elliptical paths of all degrees of eccentricity. A famous example of
+this kind of planetary horse-play is furnished by the story of
+Lexell's missing comet. This comet was first seen in 1770.
+Investigation showed that it was moving in an orbit which should bring
+it back to perihelion every five and a half years; yet it had never
+been seen before and, although often searched for, has never been seen
+since. Laplace and Leverrier proved mathematically that in 1767 it had
+approached so close to Jupiter as to be involved among the orbits of
+his satellites. What its track had been before is not known, but on
+that occasion the giant planet seized the interloper, threw it into a
+short elliptic orbit and sent it, like an arrested vagrant, to receive
+sentence at the bar of the sun. On this journey it passed within less
+than 1,500,000 miles of the earth. The form of orbit which Jupiter had
+impressed required, as we have said, its return in about five and a
+half years; but soon after 1770 it had the misfortune a second time to
+encounter Jupiter at close range, and he, as if dissatisfied with the
+leniency of the sun, or indignant at the stranger's familiarity,
+seized the comet and hurled it out of the system, or at any rate so
+far away that it has never since been able to rejoin the family circle
+that basks in the immediate rays of the solar hearth. Nor is this the
+only instance in which Jupiter has dealt summarily with small comets
+that have approached him with too little deference.
+
+The function which Jupiter so conspicuously fulfills as master of the
+hounds to the sun is worth considering a little more in detail. To
+change the figure, imagine the sun in its voyage through space to be
+like a majestic battleship surrounded by its scouts. Small vessels
+(the comets, as they are overhauled by the squadron, are taken in
+charge by the scouts, with Jupiter for their chief, and are forced to
+accompany the fleet, but not all are impressed. If a strange comet
+undertakes to run across Jupiter's bows the latter brings it to, and
+makes prize of it by throwing it into a relatively small ellipse with
+the sun for its focus. Thenceforth, unless, as happened to the unhappy
+comet of Lexell, it encounters Jupiter again in such a way as to be
+diverted by him into a more distant orbit, it can never get away.
+About thirty comets are now known to have thus been captured by the
+great planet, and they are called ``Jupiter's Comet Family.'' But, on
+the other hand, if a wandering comet crosses the wake of the chief
+planetary scout the latter simply drives it away by accelerating its
+motion and compels it to steer off into open space. The transformation
+of comets into meteors will be considered in the next chapter, but
+here, in passing, mention may be made of the strange fate of one
+member of Jupiter's family, Biela's comet, which, having become over
+bold in its advances to its captor, was, after a few revolutions in is
+impressed orbit, torn to pieces and turned into a flock of meteors.
+
+And now let us return to the mystery of comets' tails. That we are
+fully justified in speaking of the tails of comets as mysterious is
+proved by the declaration of Sir John Herschel, who averred, in so
+many words, that ``there is some profound secret and mystery of nature
+concerned in this phenomenon,'' and this profound secret and mystery
+has not yet been altogether cleared up. Nevertheless, the
+all-explaining hypothesis of Arrhenius offers us once more a certain
+amount of aid. Comets' tails, Arrhenius assures us, are but another
+result of the pressure of light. The reader will recall the
+applications of this theory to the Zodiacal Light and the Aurora. In
+the form in which we now have to deal with it, the supposition is made
+that as a comet approaches the sun eruptions of vapor, due to the
+solar heat, occur in its nucleus. These are naturally most active on
+the side which is directly exposed to the sun, whence the appearance
+of the immense glowing envelopes that surround the nucleus on the
+sunward side. Among the particles of hydro-carbon, and perhaps solid
+carbon in the state of fine dust, which are thus set free there will
+be many whose size is within the critical limit which enables the
+light-waves from the sun to drive them away. Clouds of such particles,
+then, will stream off behind the advancing comet, producing the
+appearance of a tail. This accounts for the fact that the tails of
+comets are always directed away from the sun, and it also explains the
+varying forms of the tails and the extraordinary changes that they
+undergo. The speed of the particles driven before the light-waves must
+depend upon their size and weight, the lightest of a given size
+traveling the most swiftly. By accretion certain particles might grow,
+thus losing velocity and producing the appearance of bunches in the
+tail, such as have been observed. The hypothesis also falls in with
+the researches of Bredichin, who has divided the tails of comets into
+three principal classes -- viz.: (1) Those which appear as long,
+straight rays; (2) Those which have the form of curved plumes or
+scimitars; (3) Those which are short, brushy, and curved sharply
+backward along the comet's path. In the first type he calculates the
+repulsive force at from twelve to fifteen times the force of gravity;
+in the second at from two to four times; and in the third at about one
+and a half times. The straight tails he ascribes to hydrogen because
+the hydrogen atom is the lightest known; the sword-shaped tails to
+hydro-carbons; and the stumpy tails to vaporized iron. It will be seen
+that, if the force driving off the tails is that which Arrhenius
+assumes it to be, the forms of those appendages would accord with
+those that Bredichin's theory calls for. At the same time we have an
+explanation of the multiple tails with which some comets have adorned
+themselves. The comet of 1744, for instance, had at one time no less
+than seven tails spread in a wide curved brush behind it. Donati's
+comet of 1858 also had at least two tails, the principal one
+sword-shaped and the other long, narrow, and as straight as a rule.
+According to Bredichin, the straight tail must have been composed of
+hydrogen, and the other of some form of hydro-carbon whose atoms are
+heavier than those of hydrogen, and, consequently, when swept away by
+the storm of light-waves, followed a curvature depending upon the
+resultant of the forces operating upon them. The seven tails of the
+comet of 1744 presented a kind of diagram graphically exhibiting its
+complex composition, and, if we knew a little more about the
+constituents of a comet, we might be able to say from the amount of
+curvature of the different tails just what were the seven substances
+of which that comet consisted.
+
+If these theories seem to the reader fantastic, at any rate they are
+no more fantastic than the phenomena that they seek to explain.
+
+Meteors, Fire-Balls, and Meteorites
+
+One of the most terrorizing spectacles with which the heavens have
+ever caused the hearts of men to quake occurred on the night of
+November 13, 1833. On that night North America, which faced the storm,
+was under a continual rain of fire from about ten o'clock in the
+evening until daybreak.
+
+The fragments of a comet had struck the earth.
+
+But the meaning of what had happened was not discovered until long
+afterward. To the astronomers who, with astonishment not less than
+that of other people, watched the wonderful scene, it was an
+unparalleled ``shower of meteors.'' They did not then suspect that
+those meteors had once formed the head of a comet. Light dawned when,
+a year later, Prof. Denison Olmsted, of Yale College, demonstrated
+that the meteors had all moved in parallel orbits around the sun, and
+that these orbits intersected that of the earth at the point where our
+planet happened to be on the memorable night of November 13th.
+Professor Olmsted even went so far as to suggest that the cloud of
+meteors that had encountered the earth might form a diffuse comet; but
+full recognition of the fact that they were cometary debris came
+later, as the result of further investigation. The key to the secret
+was plainly displayed in the spectacle itself, and was noticed without
+being understood by thousands of the terror-stricken beholders. It was
+an umbrella of fire that had opened overhead and covered the heavens;
+in other words, the meteors all radiated from a particular point in
+the constellation Leo, and, being countless as the snowflakes in a
+winter tempest, they ribbed the sky with fiery streaks. Professor
+Olmsted showed that the radiation of the meteors from a fixed point
+was an effect of perspective, and in itself a proof that they were
+moving in parallel paths when they encountered the earth. The fact was
+noted that there had been a similar, but incomparably less brilliant,
+display of meteors on the same day of November, 1832, and it was
+rightly concluded that these had belonged to the same stream, although
+the true relationship of the phenomena was not immediately
+apprehended. Olmsted ascribed to the meteors a revolution about the
+sun once in every six months, bringing them to the intersection of
+their orbit with that of the earth every November 13th; but later
+investigators found that the real period was about thirty-three and
+one-quarter years, so that the great displays were due three times in
+a century, and their return was confidently predicted for the year
+1866. The appearance of the meteors in 1832, a year before the great
+display, was ascribed to the great length of the stream which they
+formed in space -- so great that they required more than two years to
+cross the earth's orbit. In 1832 the earth had encountered a
+relatively rare part of the stream, but in 1833, on returning to the
+crossing-place, it found there the richest part of the stream pouring
+across its orbit. This explanation also proved to be correct, and the
+predicted return in 1866 was duly witnessed, although the display was
+much less brilliant than in 1833. It was followed by another in 1867.
+
+In the mean time Olmsted's idea of a cometary relationship of the
+meteors was demonstrated to be correct by the researches of
+Schiaparelli and others, who showed that not only the November
+meteors, but those of August, which are seen more or less abundantly
+every year, traveled in the tracks of well-known comets, and had
+undoubtedly an identical origin with those comets. In other words the
+comets and the meteor-swarms were both remnants of original masses
+which had probably been split up by the action of the sun, or of some
+planet to which they had made close approaches. The annual periodicity
+of the August meteors was ascribed to the fact that the separation had
+taken place so long ago that the meteors had become distributed all
+around the orbit, in consequence of which the earth encountered some
+of them every year when it arrived at the crossing-point. Then
+Leverrier showed that the original comet associated with the November
+meteors was probably brought into the system by the influence of the
+planet Uranus in the year 126 of the Christian era. Afterward
+Alexander Herschel identified the tracks of no less than seventy-six
+meteor-swarms (most of them inconspicuous) with those of comets. The
+still more recent researches of Mr W. F. Denning make it probable that
+there are no meteors which do not belong to a flock or system probably
+formed by the disintegration of a cometary mass; even the apparently
+sporadic ones which shoot across the sky, ``lost souls in the night,''
+being members of flocks which have become so widely scattered that the
+earth sometimes takes weeks to pass through the region of space where
+their paths lie.
+
+The November meteors should have exhibited another pair of spectacles
+in 1899 and 1900, and their failure to do so caused at first much
+disappointment, until it was made plain that a good reason existed for
+their absence. It was found that after their last appearance, in 1867,
+they had been disturbed in their movements by the planets Jupiter and
+Saturn, whose attractions had so shifted the position of their orbit
+that it no longer intersected that of the earth, as it did before.
+Whether another planetary interference will sometime bring the
+principal mass of the November meteors back to the former point of
+intersection with the earth's orbit is a question for the future to
+decide. It would seem that there may be several parallel streams of
+the November meteors, and that some of them, like those of August, are
+distributed entirely around the orbit, so that every mid-November we
+see a few of them.
+
+We come now to a very remarkable example of the disintegration of a
+comet and the formation of a meteor-stream. In 1826 Biela, of
+Josephstadt, Austria, discovered a comet to which his name was given.
+Calculation showed that it had an orbital period of about six and a
+half years, belonging to Jupiter's ``family.'' On one of its returns,
+in 1846, it astonished its watchers by suddenly splitting in two. The
+two comets thus formed out of one separated to a distance of about one
+hundred and sixty thousand miles, and then raced side by side,
+sometimes with a curious ligature connecting them, like Siamese twins,
+until they disappeared together in interplanetary space. In 1852 they
+came back, still nearly side by side, but now the distance between
+them had increased to a million and a quarter of miles. After that, at
+every recurrence of their period, astronomers looked for them in vain,
+until 1872, when an amazing thing happened. On the night of November
+28th, when the earth was crossing the plane of the orbit of the
+missing comet, a brilliant shower of meteors burst from the northern
+sky, traveling nearly in the track which the comet should have
+pursued. The astronomers were electrified. Klinkerfues, of Goettingen,
+telegraphed to Pogson, of Madras: ``Biela touched earth; search near
+Theta Centauri.'' Pogson searched in the place indicated and saw a
+cometary mass retreating into the southern heavens, where it was soon
+swallowed from sight!
+
+Since then the Biela meteors have been among the recognized periodic
+spectacles of the sky, and few if any doubt that they represent a
+portion of the missing comet whose disintegration began with the
+separation into two parts in 1846. The comet itself has never since
+been seen. The first display of these meteors, sometimes called the
+``Andromedes,'' because they radiate from the constellation Andromeda,
+was remarkable for the great brilliancy of many of the fire-balls that
+shot among the shower of smaller sparks, some of which were described
+as equaling the full moon in size. None of them is known to have
+reached the earth, but during the display of the same meteors in 1885
+a meteoric mass fell at Mazapil in Northern Mexico (it is now in the
+Museum at Vienna), which many have thought may actually be a piece of
+the original comet of Biela. This brings us to the second branch of
+our subject.
+
+More rare than meteors or falling stars, and more startling, except
+that they never appear in showers, are the huge balls of fire which
+occasionally dart through the sky, lighting up the landscapes beneath
+with their glare, leaving trains of sparks behind them, often
+producing peals of thunder when they explode, and in many cases
+falling upon the earth and burying themselves from a few inches to
+several feet in the soil, from which, more than once, they have been
+picked up while yet hot and fuming. These balls are sometimes called
+bolides. They are not really round in shape, although they often look
+so while traversing the sky, but their forms are fragmentary, and
+occasionally fantastic. It has been supposed that their origin is
+different from that of the true meteors; it has even been conjectured
+that they may have originated from the giant volcanoes of the moon or
+have been shot out from the sun during some of the tremendous
+explosions that accompany the formation of eruptive prominences. By
+the same reasoning some of them might be supposed to have come from
+some distant star. Others have conjectured that they are wanderers in
+space, of unknown origin, which the earth encounters as it journeys
+on, and Lord Kelvin made a suggestion which has become classic because
+of its imaginative reach -- viz., that the first germs of life may
+have been brought to the earth by one of these bodies, ``a fragment of
+an exploded world.''
+
+It is a singular fact that astronomers and scientific men in general
+were among the last to admit the possibility of solid masses falling
+from the sky. The people had believed in the reality of such phenomena
+from the earliest times, but the savants shook their heads and talked
+of superstition. This was the less surprising because no
+scientifically authenticated instance of such an occurrence was known,
+and the stones popularly believed to have fallen from the sky had
+become the objects of worship or superstitious reverence, a fact not
+calculated to recommend them to scientific credence. The celebrated
+``black stone'' suspended in the Kaaba at Mecca is one of these
+reputed gifts from heaven; the ``Palladium'' of ancient Troy was
+another; and a stone which fell near Ensisheim, in Germany, was placed
+in a church as an object to be religiously venerated. Many legends of
+falling stones existed in antiquity, some of them curiously
+transfigured by the imagination, like the ``Lion of the
+Peloponnesus,'' which was said to have sprung down from the sky upon
+the Isthmus of Corinth. But near the beginning of the nineteenth
+century, in 1803, a veritable shower of falling stones occurred at
+L'Aigle, in Northern France, and this time astronomers took note of
+the phenomenon and scientifically investigated it. Thousands of the
+strange projectiles came from the sky on this occasion, and were
+scattered over a wide area of country, and some buildings were hit.
+Four years later another shower of stones occurred at Weston, Conn.,
+numbering thousands of individuals. The local alarm created in both
+cases was great, as well it might be, for what could be more
+intimidating than to find the blue vault of heaven suddenly hurling
+solid missiles at the homes of men? After these occurrences it was
+impossible for the most skeptical to doubt any longer, and the regular
+study of ``aerolites,'' or ``meteorites,'' began.
+
+One of the first things recognized was the fact that fire-balls are
+solid meteorites in flight, and not gaseous exhalations in the air, as
+some had assumed. They burn in the air during their flight, and
+sometimes, perhaps, are entirely consumed before reaching the ground.
+Their velocity before entering the earth's atmosphere is equal to that
+of the planets in their orbits -- viz., from twenty to thirty miles
+per second -- a fact which proves that the sun is the seat of the
+central force governing them. Their burning in the air is not
+difficult to explain; it is the heat of friction which so quickly
+brings them to incandescence. Calculation shows that a body moving
+through the air at a velocity of about a mile per second will be
+brought, superficially, to the temperature of ``red heat'' by friction
+with the atmosphere. If its velocity is twenty miles per second the
+temperature will become thousands of degrees. This is the state of
+affairs with a meteorite rushing into the earth's atmosphere; its
+surface is liquefied within a few seconds after the friction begins to
+act, and the melted and vaporized portion of its mass is swept
+backward, forming the train of sparks that follows every great
+fire-ball. However, there is one phenomenon connected with the trains
+of meteorites which has never been satisfactorily explained: they
+often persist for long periods of time, drifting and turning with the
+wind, but not ceasing to glow with a phosphorescent luminosity. The
+question is, Whence comes this light? It must be light without heat,
+since the fine dust or vapor of which the train can only consist would
+not retain sufficient heat to render it luminous for so long a time.
+An extremely remarkable incident of this kind occurred on February 22,
+1909, when an immense fire-ball that passed over southern England left
+a train that remained visible during two hours, assuming many curious
+shapes as it was drifted about by currents in the air.
+
+But notwithstanding the enormous velocity with which meteorites enter
+the air they are soon slowed down to comparatively moderate speed, so
+that when they disappear they are usually traveling not faster than a
+mile a second. The courses of many have been traced by observers
+situated along their track at various points, and thus a knowledge has
+been obtained of their height above the ground during their flight and
+of the length of their visible courses. They generally appear at an
+elevation of eighty or a hundred miles, and are seldom visible after
+having descended to within five miles of the ground, unless the
+observer happens to be near the striking-point, when he may actually
+witness the fall. Frequently they burst while high in the air and
+their fragments are scattered like shrapnel over the surface of the
+ground, sometimes covering an area of several square miles, but of
+course not thickly; different fragments of the same meteorite may
+reach the ground at points several miles apart. The observed length of
+their courses in the atmosphere varies from fifty to five hundred
+miles. If they continued a long time in flight after entering the air,
+even the largest of them would probably be consumed to the last scrap,
+but their fiery career is so short on account of their great speed
+that the heat does not have time to penetrate very deeply, and some
+that have been picked up immediately after their fall have been found
+cold as ice within. Their size after reaching the ground is variable
+within wide limits; some are known which weigh several tons, but the
+great majority weigh only a few pounds and many only a few ounces.
+
+Meteorites are of two kinds: stony meteorites and iron meteorites. The
+former outnumber the latter twenty to one; but many stone meteorites
+contain grains of iron. Nickel is commonly found in iron meteorites,
+so that it might be said that that redoubtable alloy nickel-steel is
+of cosmical invention. Some twenty-five chemical elements have been
+found in meteorites, including carbon and the ``sun-metal,'' helium.
+The presence of the latter is certainly highly suggestive in
+connection with the question of the origin of meteorites. The iron
+meteorites, besides metallic iron and nickel, of which they are almost
+entirely composed, contain hydrogen, helium, and carbonic oxide, and
+about the only imaginable way in which these gases could have become
+absorbed in the iron would be through the immersion of the latter
+while in a molten or vaporized state in a hot and dense atmosphere
+composed of them, a condition which we know to exist only in the
+envelopes of the sun and the stars.
+
+The existence of carbon in the Canyon Diablo iron meteorites is
+attended by a circumstance of the most singular character -- a very
+``fairy tale of science.'' In some cases the carbon has become
+diamond! These meteoric diamonds are very small; nevertheless, they
+are true diamonds, resembling in many ways the little black gems
+produced by Moissan's method with the aid of the electric furnace. The
+fact that they are found embedded in these iron meteorites is another
+argument in favor of the hypothesis of the solar or stellar origin of
+the latter. To appreciate this it is necessary to recall the way in
+which Moissan made his diamonds. It was by a combination of the
+effects of great heat, great pressure, and sudden or rapid superficial
+cooling on a mass of iron containing carbon. When he finally broke
+open his iron he found it a pudding stuffed with miniature black
+diamonds. When a fragment of the Canyon Diablo meteoric iron was
+polished in Philadelphia over fifteen years ago it cut the emery-wheel
+to pieces, and examination showed that the damage had been effected by
+microscopic diamonds peppered through the mass. How were those
+diamonds formed? If the sun or Sirius was the laboratory that prepared
+them, we can get a glimpse at the process of their formation. There is
+plenty of heat, plenty of pressure, and an abundance of vaporized iron
+in the sun and the stars. When a great solar eruption takes place,
+masses of iron which have absorbed carbon may be shot out with a
+velocity which forbids their return. Plunged into the frightful cold
+of space, their surfaces are quickly cooled, as Moissan cooled his
+prepared iron by throwing it into water, and thus the requisite stress
+is set up within, and, as the iron solidifies, the included carbon
+crystallizes into diamonds. Whether this explanation has a germ of
+truth in it or not, at any rate it is evident that iron meteorites
+were not created in the form in which they come to us; they must once
+have been parts of immeasurably more massive bodies than themselves.
+
+The fall of meteorites offers an appreciable, though numerically
+insignificant, peril to the inhabitants of the earth. Historical
+records show perhaps three or four instances of people being killed by
+these bodies. But for the protection afforded by the atmosphere, which
+acts as a very effective shield, the danger would doubtless be very
+much greater. In the absence of an atmosphere not only would more
+meteorites reach the ground, but their striking force would be
+incomparably greater, since, as we have seen, the larger part of their
+original velocity is destroyed by the resistance of the air. A
+meteorite weighing many tons and striking the earth with a velocity of
+twenty or thirty miles per second, would probably cause frightful
+havoc.
+
+It is a singular fact that recent investigations seem to have proved
+that an event of this kind actually happened in North America --
+perhaps not longer than a thousand or two thousand years ago. The
+scene of the supposed catastrophe is in northern central Arizona, at
+Coon Butte, where there is a nearly circular crater in the middle of a
+circular elevation or small mountain. The crater is somewhat over four
+thousand feet in diameter, and the surrounding rim, formed of upturned
+strata and ejected rock fragments, rises at its highest point one
+hundred and sixty feet above the plain. The crater is about six
+hundred feet in depth -- that is, from the rim to the visible floor or
+bottom of the crater. There is no evidence that volcanic action has
+ever taken place in the immediate neighborhood of Coon Butte. The rock
+in which the crater has been made is composed of horizontal sandstone
+and limestone strata. Between three hundred and four hundred million
+tons of rock fragments have been detached, and a large portion hurled
+by some cause out of the crater. These fragments lie concentrically
+distributed around the crater, and in large measure form the elevation
+known as Coon Butte. The region has been famous for nearly twenty
+years on account of the masses of meteoric iron found scattered about
+and known as the ``Canyon Diablo'' meteorites. It was one of these
+masses, which consist of nickel-iron containing a small quantity of
+platinum, and of which in all some ten tons have been recovered for
+sale to the various collectors throughout the world, that as before
+mentioned destroyed the grinding-tool at Philadelphia through the
+cutting power of its embedded diamonds. These meteoric irons are
+scattered about the crater-hill, in concentric distribution, to a
+maximum distance of about five miles. When the suggestion was first
+made in 1896 that a monster meteorite might have created by its fall
+this singular lone crater in stratified rocks, it was greeted with
+incredulous smiles; but since then the matter has assumed a different
+aspect. The Standard Iron Company, formed by Messrs. D. M. Barringer,
+B. C. Tilghman, E. J. Bennitt, and S. J. Holsinger, having become, in
+1903, the owner of this freak of nature, sunk shafts and bored holes
+to a great depth in the interior of the crater, and also trenched the
+slopes of the mountain, and the result of their investigations has
+proved that the meteoric hypothesis of origin is correct. (See the
+papers published in the Proceedings of the Academy of Natural Sciences
+of Philadelphia, December, 1905, wherein it is proved that the United
+States Geological Survey was wrong in believing this crater to have
+been due to a steam explosion. Since that date there has been
+discovered a great amount of additional confirmatory proof). Material
+of unmistakably meteoric origin was found by means of the drills,
+mixed with crushed rock, to a depth of six hundred to seven hundred
+feet below the floor of the crater, and a great deal of it has been
+found admixed with the ejected rock fragments on the outer slopes of
+the mountain, absolutely proving synchronism between the two events,
+the formation of this great crater and the falling of the meteoric
+iron out of the sky. The drill located in the bottom of the crater was
+sent, in a number of cases, much deeper (over one thousand feet) into
+unaltered horizontal red sandstone strata, but no meteoric material
+was found below this depth (seven hundred feet, or between eleven and
+twelve hundred feet below the level of the surrounding plain), which
+has been assumed as being about the limit of penetration. It is not
+possible to sink a shaft at present, owing to the water which has
+drained into the crater, and which forms, with the finely pulverized
+sandstone, a very troublesome quicksand encountered at about two
+hundred feet below the visible floor of the crater. As soon as this
+water is removed by pumping it will be easy to explore the depths of
+the crater by means of shafts and drifts. The rock strata (sandstone
+and limestone) of which the walls consist present every appearance of
+having been violently upturned by a huge body penetrating the earth
+like a cannon-ball. The general aspect of the crater strikingly
+resembles the impression made by a steel projectile shot into an
+armor-plate. Mr Tilghman has estimated that a meteorite about five
+hundred feet in diameter and moving with a velocity of about five
+miles per second would have made just such a perforation upon striking
+rocks of the character of those found at this place. There was some
+fusion of the colliding masses, and the heat produced some steam from
+the small amount of water in the rocks. As a result there has been
+found at depth a considerable amount of fused quartz (original
+sandstone), and with it innumerable particles or sparks of fused
+nickel-iron (original meteorite). A projectile of that size
+penetrating eleven to twelve hundred feet into the rocky shell of the
+globe must have produced a shock which was perceptible several hundred
+miles away.
+
+The great velocity ascribed to the supposed meteorite at the moment of
+striking could be accounted for by the fact that it probably plunged
+nearly vertically downward, for it formed a circular crater in the
+rocky crust of the earth. In that case it would have been less
+retarded by the resistance of the atmosphere than are meteorites which
+enter the air at a lower angle and shoot ahead hundreds of miles until
+friction has nearly destroyed their original motion when they drop
+upon the earth. Some meteoric masses of great size, such as Peary's
+iron meteorite found at Cape York, Greenland, and the almost equally
+large mass discovered at Bacubirito, Mexico, appear to have penetrated
+but slightly on striking the earth. This may be explained by supposing
+that they pursued a long, horizontal course through the air before
+falling. The result would be that, their original velocity having been
+practically destroyed, they would drop to the ground with a velocity
+nearly corresponding to that which gravity would impart within the
+perpendicular distance of their final fall. A
+six-hundred-and-sixty-pound meteorite, which fell at Knyahinya,
+Hungary, striking at an angle of 27 deg. from the vertical, penetrated the
+ground to a depth of eleven feet.
+
+It has been remarked that the Coon Butte meteorite may have fallen not
+longer ago than a few thousand years. This is based upon the fact that
+the geological indications favor the supposition that the event did
+not occur more than five thousand years ago, while on the other hand
+the rings of growth in the cedar-trees growing on the slopes of the
+crater show that they have existed there about seven hundred years.
+Prof. William H. Pickering has recently correlated this with an
+ancient chronicle which states that at Cairo, Egypt, in the year 1029,
+``many stars passed with a great noise.'' He remarks that Cairo is
+about 100 deg., by great circle, from Coon Butte, so that if the meteorite
+that made the crater was a member of a flock of similar bodies which
+encountered the earth moving in parallel lines, some of them might
+have traversed the sky tangent to the earth's surface at Cairo. That
+the spectacle spoken of in the chronicle was caused by meteorites he
+deems exceedingly probable because of what is said about ``a great
+noise;'' meteorites are the only celestial phenomena attended with
+perceptible sounds. Professor Pickering conjectures that this supposed
+flock of great meteorites may have formed the nucleus of a comet which
+struck the earth, and he finds confirmation of the idea in the fact
+that out of the ten largest meteorites known, no less than seven were
+found within nine hundred miles of Coon Butte. It would be interesting
+if we could trace back the history of that comet, and find out what
+malicious planet caught it up in its innocent wanderings and hurled it
+with so true an aim at the earth! This remarkable crater is one of the
+most interesting places in the world, for there is absolutely no
+record of such a mass, possibly an iron-headed comet, from outer space
+having come into collision with our earth. The results of the future
+exploration of the depths of the crater will be awaited with much
+interest.
+
+The Wrecking of the Moon
+
+There are sympathetic moods under whose influence one gazes with a
+certain poignant tenderness at the worn face of the moon; that little
+``fossil world'' (the child of our mother earth, too) bears such
+terrible scars of its brief convulsive life that a sense of pity is
+awakened by the sight. The moon is the wonder-land of the telescope.
+Those towering mountains, whose ``proud aspiring peaks'' cast
+silhouettes of shadow that seem drawn with india-ink; those vast
+plains, enchained with gentle winding hills and bordered with giant
+ranges; those oval ``oceans,'' where one looks expectant for the flash
+of wind-whipped waves; those enchanting ``bays'' and recesses at the
+seaward feet of the Alps; those broad straits passing between guardian
+heights incomparably mightier than Gibraltar; those locket-like
+valleys as secluded among their mountains as the Vale of Cashmere;
+those colossal craters that make us smile at the pretensions of
+Vesuvius, Etna, and Cotopaxi; those strange white ways which pass with
+the unconcern of Roman roads across mountain, gorge, and valley -- all
+these give the beholder an irresistible impression that it is truly a
+world into which he is looking, a world akin to ours, and yet no more
+like our world than Pompeii is like Naples. Its air, its waters, its
+clouds, its life are gone, and only a skeleton remains -- a mute but
+eloquent witness to a cosmical tragedy without parallel in the range
+of human knowledge.
+
+One cannot but regret that the moon, if it ever was the seat of
+intelligent life, has not remained so until our time. Think what the
+consequences would have been if this other world at our very door had
+been found to be both habitable and inhabited! We talk rather airily
+of communicating with Mars by signals; but Mars never approaches
+nearer than 35,000,000 miles, while the moon when nearest is only a
+little more than 220,000 miles away. Given an effective magnifying
+power of five thousand diameters, which will perhaps be possible at
+the mountain observatories as telescopes improve, and we should be
+able to bring the moon within an apparent distance of about forty
+miles, while the corresponding distance for Mars would be more than
+seven thousand miles. But even with existing telescopic powers we can
+see details on the moon no larger than some artificial constructions
+on the earth. St Peter's at Rome, with the Vatican palace and the
+great piazza, if existing on the moon, would unquestionably be
+recognizable as something else than a freak of nature. Large cities,
+with their radiating lines of communication, would at once betray
+their real character. Cultivated tracts, and the changes produced by
+the interference of intelligent beings, would be clearly recognizable.
+The electric illumination of a large town at night would probably be
+markedly visible. Gleams of reflected sunlight would come to us from
+the surfaces of the lakes and oceans, and a huge ``liner'' traversing
+a lunar sea could probably be followed by its trail of smoke. As to
+communications by ``wireless'' signals, which certain enthusiasts have
+thought of in connection with Mars, in the case of the moon they
+should be a relatively simple matter, and the feat might actually be
+accomplished. Think what a literature would grow up about the moon if
+it were a living world! Its very differences from the earth would only
+accentuate its interest for us. Night and day on the moon are each two
+weeks in length; how interesting it would be to watch the manner in
+which the lunarians dealt with such a situation as that. Lunar and
+terrestrial history would keep step with each other, and we should
+record them both. Truly one might well wish to have a neighbor world
+to study; one would feel so much the less alone in space.
+
+It is not impossible that the moon did at one time have inhabitants of
+some kind. But, if so, they vanished with the disappearance of its
+atmosphere and seas, or with the advent of its cataclysmic age. At the
+best, its career as a living world must have been brief. If the water
+and air were gradually absorbed, as some have conjectured, by its
+cooling interior rocks, its surface might, nevertheless, have retained
+them for long ages; but if, as others think, their disappearance was
+due to the escape of their gaseous molecules in consequence of the
+inability of the relatively small lunar gravitation to retain them,
+then the final catastrophe must have been as swift as it was
+inevitable. Accepting Darwin's hypothesis, that the moon was separated
+from the earth by tidal action while both were yet plastic or
+nebulous, we may reasonably conclude that it began its career with a
+good supply of both water and air, but did not possess sufficient mass
+to hold them permanently. Yet it may have retained them long enough
+for life to develop in many forms upon its surface; in fact, there are
+so many indications that air and water have not always been lacking to
+the lunar world that we are driven to invent theories to explain both
+their former presence and their present absence.
+
+But whatever the former condition of the moon may have been, its
+existing appearance gives it a resistless fascination, and it bears so
+clearly the story of a vast catastrophe sculptured on its rocky face
+that the thoughtful observer cannot look upon it without a feeling of
+awe. The gigantic character of the lunar features impresses the
+beholder not less than the universality of the play of destructive
+forces which they attest. Let us make a few comparisons. Take the
+lunar crater called ``Tycho'', which is a typical example of its kind.
+In the telescope Tycho appears as a perfect ring surrounding a
+circular depression, in the center of which rises a group of
+mountains. Its superficial resemblance to some terrestrial volcanic
+craters is very striking. Vesuvius, seen from a point vertically
+above, would no doubt look something like that (the resemblance would
+have been greater when the Monte del Cavallo formed a more complete
+circuit about the crater cone). But compare the dimensions. The
+remains of the outer crater ring of Vesuvius are perhaps half a mile
+in diameter, while the active crater itself is only two or three
+hundred feet across at the most; Tycho has a diameter of fifty-four
+miles! The group of relatively insignificant peaks in the center of
+the crater floor of Tycho is far more massive than the entire mountain
+that we call Vesuvius. The largest known volcanic crater on the earth,
+Aso San, in Japan, has a diameter of seven miles; it would take sixty
+craters like Aso San to equal Tycho in area! And Tycho, though one of
+the most perfect, is by no means the largest crater on the moon.
+Another, called ``Theophilus,'' has a diameter of sixty-four miles,
+and is eighteen thousand feet deep. There are hundreds from ten to
+forty miles in diameter, and thousands from one to ten miles. They are
+so numerous in many places that they break into one another, like the
+cells of a crushed honeycomb.
+
+The lunar craters differ from those of the earth more fundamentally
+than in the matter of mere size; they are not situated on the tops of
+mountains. If they were, and if all the proportions were the same, a
+crater like Tycho might crown a conical peak fifty or one hundred
+miles high! Instead of being cavities in the summits of mountains, the
+lunar craters are rather gigantic sink-holes whose bottoms in many
+cases lie two or three miles below the general surface of the lunar
+world. Around their rims the rocks are piled up to a height of from a
+few hundred to two or three thousand feet, with a comparatively gentle
+inclination, but on the inner side they fall away in gigantic broken
+precipices which make the dizzy cliffs of the Matterhorn seem but
+``lover's leaps.'' Down they drop, ridge below ridge, crag under crag,
+tottering wall beneath wall, until, in a crater named ``Newton,'' near
+the south lunar pole, they attain a depth where the rays of the sun
+never reach. Nothing more frightful than the spectacle which many of
+these terrible chasms present can be pictured by the imagination. As
+the lazy lunar day slowly advances, the sunshine, unmitigated by
+clouds or atmospheric veil of any kind, creeps across their rims and
+begins to descend the opposite walls. Presently it strikes the ragged
+crest of a ridge which had lain hidden in such darkness as we never
+know on the earth, and runs along it like a line of kindling fire.
+Rocky pinnacles and needles shoot up into the sunlight out of the
+black depths. Down sinks the line of light, mile after mile, and
+continually new precipices and cliffs are brought into view, until at
+last the vast floor is attained and begins to be illuminated. In the
+meanwhile the sun's rays, darting across the gulf, have touched the
+summits of the central peaks, twenty or thirty miles from the crater's
+inmost edge, and they immediately kindle and blaze like huge stars
+amid the darkness. So profound are some of these awful craters that
+days pass before the sun has risen high enough above them to chase the
+last shadows from their depths.
+
+Although several long ranges of mountains resembling those of the
+earth exist on the moon, the great majority of its elevations assume
+the crateriform aspect. Sometimes, instead of a crater, we find an
+immense mountain ring whose form and aspect hardly suggest volcanic
+action. But everywhere the true craters are in evidence, even on the
+sea-beds, although they attain their greatest number and size on those
+parts of the moon -- covering sixty per cent of its visible surface --
+which are distinctly mountainous in character and which constitute its
+most brilliant portions. Broadly speaking, the southwestern half of
+the moon is the most mountainous and broken, and the northeastern half
+the least so. Right down through the center, from pole to pole, runs a
+wonderful line of craters and crateriform valleys of a magnitude
+stupendous even for the moon. Another similar line follows the western
+edge. Three or four ``seas'' are thrust between these mountainous
+belts. By the effects of ``libration'' parts of the opposite
+hemisphere of the moon which is turned away from the earth are from
+time to time brought into view, and their aspect indicates that that
+hemisphere resembles in its surface features the one which faces the
+earth. There are many things about the craters which seem to give some
+warrant for the hypothesis which has been particularly urged by Mr G.
+K. Gilbert, that they were formed by the impact of meteors; but there
+are also many things which militate against that idea, and, upon the
+whole, the volcanic theory of their origin is to be preferred.
+
+The enormous size of the lunar volcanoes is not so difficult to
+account for when we remember how slight is the force of lunar gravity
+as compared with that of the earth. With equal size and density,
+bodies on the moon weigh only one-sixth as much as on the earth.
+Impelled by the same force, a projectile that would go ten miles on
+the earth would go sixty miles on the moon. A lunar giant thirty-five
+feet tall would weigh no more than an ordinary son of Adam weighs on
+his greater planet. To shoot a body from the earth so that it would
+not drop back again, we should have to start it with a velocity of
+seven miles per second; a mile and a half per second would serve on
+the moon. It is by no means difficult to believe, then, that a lunar
+volcano might form a crater ring eight or ten times broader than the
+greatest to be found on the earth, especially when we reflect that in
+addition to the relatively slight force of gravity, the materials of
+the lunar crust are probably lighter than those of our terrestrial
+rocks.
+
+For similar reasons it seems not impossible that the theory mentioned
+in a former chapter -- that some of the meteorites that have fallen
+upon the earth originated from the lunar volcanoes -- is well founded.
+This would apply especially to the stony meteorites, for it is hardly
+to be supposed that the moon, at least in its superficial parts,
+contains much iron. It is surely a scene most strange that is thus
+presented to the mind's eye -- that little attendant of the earth's
+(the moon has only one-fiftieth of the volume, and only one-eightieth
+of the mass of the earth) firing great stones back at its parent
+planet! And what can have been the cause of this furious outbreak of
+volcanic forces on the moon? Evidently it was but a passing stage in
+its history; it had enjoyed more quiet times before. As it cooled down
+from the plastic state in which it parted from the earth, it became
+incrusted after the normal manner of a planet, and then oceans were
+formed, its atmosphere being sufficiently dense to prevent the water
+from evaporating and the would-be oceans from disappearing continually
+in mist. This, if any, must have been the period of life in the lunar
+world. As we look upon the vestiges of that ancient world buried in
+the wreck that now covers so much of its surface, it is difficult to
+restrain the imagination from picturing the scenes which were once
+presented there; and, in such a case, should the imagination be
+fettered? We give it free rein in terrestrial life, and it rewards us
+with some of our greatest intellectual pleasures. The wonderful
+landscapes of the moon offer it an ideal field with just enough
+half-hidden suggestions of facts to stimulate its powers.
+
+The great plains of the Mare Imbrium and the Mare Serenitatis (the
+``Sea of Showers'' and the ``Sea of Serenity''), bordered in part by
+lofty mountain ranges precisely like terrestrial mountains, scalloped
+along their shores with beautiful bays curving back into the adjoining
+highlands, and united by a great strait passing between the nearly
+abutting ends of the ``Lunar Apennines'' and the ``Lunar Caucasus,''
+offer the elements of a scene of world beauty such as it would be
+difficult to match upon our planet. Look at the finely modulated
+bottom of the ancient sea in Mr Ritchey's exquisite photograph of the
+western part of the Mare Serenitatis, where one seems to see the play
+of the watery currents heaping the ocean sands in waving lines, making
+shallows, bars, and deeps for the mariner to avoid or seek, and
+affording a playground for the creatures of the main. What geologist
+would not wish to try his hammer on those rocks with their stony pages
+of fossilized history? There is in us an instinct which forbids us to
+think that there was never any life there. If we could visit the moon,
+there is not among us a person so prosaic and unimaginative that he
+would not, the very first thing, begin to search for traces of its
+inhabitants. We would look for them in the deposits on the sea
+bottoms; we would examine the shores wherever the configuration seemed
+favorable for harbors and the sites of maritime cities -- forgetting
+that it may be a little ridiculous to ascribe to the ancient lunarians
+the same ideas that have governed the development of our race; we
+would search through the valleys and along the seeming courses of
+vanished streams; we would explore the mountains, not the terrible
+craters, but the pinnacled chains that recall our own Alps and
+Rockies; seeking everywhere some vestige of the transforming presence
+of intelligent life. Perhaps we should find such traces, and perhaps,
+with all our searching, we should find nothing to suggest that life
+had ever existed amid that universal ruin.
+
+Look again at the border of the ``Sea of Serenity'' -- what a name for
+such a scene! -- and observe how it has been rent with almost
+inconceivable violence, the wall of the colossal crater Posidonius
+dropping vertically upon the ancient shore and obliterating it, while
+its giant neighbor, Le Monnier, opens a yawning mouth as if to swallow
+the sea itself. A scene like this makes one question whether, after
+all, those may not be right who have imagined that the so-called sea
+bottoms are really vast plains of frozen lava which gushed up in
+floods so extensive that even the mighty volcanoes were half drowned
+in the fiery sea. This suggestion becomes even stronger when we turn
+to another of the photographs of Mr Ritchey's wonderful series,
+showing a part of the Mare Tranquilitatis (``Sea of Tranquility''!).
+Notice how near the center of the picture the outline of a huge ring
+with radiating ridges shows through the sea bottom; a fossil volcano
+submerged in a petrified ocean! This is by no means the only instance
+in which a buried world shows itself under the great lunar plains.
+Yet, as the newer craters in the sea itself prove, the volcanic
+activity survived this other catastrophe, or broke out again
+subsequently, bringing more ruin to pile upon ruin.
+
+Yet notwithstanding the evidence which we have just been considering
+in support of the hypothesis that the ``seas'' are lava floods,
+Messrs. Loewy and Puiseux, the selenographers of the Paris
+Observatory, are convinced that these great plains bear characteristic
+marks of the former presence of immense bodies of water. In that case
+we should be forced to conclude that the later oceans of the moon lay
+upon vast sheets of solidified lava; and thus the catastrophe of the
+lunar world assumes a double aspect, the earliest oceans being
+swallowed up in molten floods issuing from the interior, while the
+lands were reduced to chaos by a universal eruption of tremendous
+volcanoes; and then a period of comparative quiet followed, during
+which new seas were formed, and new life perhaps began to flourish in
+the lunar world, only to end in another cataclysm, which finally put a
+term to the existence of the moon as a life-supporting world.
+
+Suppose we examine two more of Mr Ritchey's illuminating photographs,
+and, first, the one showing the crater Theophilus and its
+surroundings. We have spoken of Theophilus before, citing the facts
+that it is sixty-four miles in diameter and eighteen thousand feet
+deep. It will be noticed that it has two brother giants -- Cyrillus
+the nearer, and Catharina the more distant; but Theophilus is plainly
+the youngest of the trio. Centuries, and perhaps thousands of years,
+must have elapsed between the periods of their upheaval, for the two
+older craters are partly filled with debris, while it is manifest at a
+glance that when the south eastern wall of Theophilus was formed, it
+broke away and destroyed a part of the more ancient ring of Cyrillus.
+There is no more tremendous scene on the moon than this; viewed with a
+powerful telescope, it is absolutely appalling.
+
+The next photograph shows, if possible, a still wilder region. It is
+the part of the moon lying between Tycho and the south pole. Tycho is
+seen in the lower left-hand part of the picture. To the right, at the
+edge of the illuminated portion of the moon, are the crater-rings,
+Longomontanus and Wilhelm I, the former being the larger. Between them
+are to be seen the ruins of two or three more ancient craters which,
+together with portions of the walls of Wilhelm I and Longomontanus,
+have been honeycombed with smaller craters. The vast crateriform
+depression above the center of the picture is Clavius, an unrivaled
+wonder of lunar scenery, a hundred and forty-two miles in its greatest
+length, while its whole immense floor has sunk two miles below the
+general surface of the moon outside the ring. The monstrous
+shadow-filled cavity above Clavius toward the right is Blancanus,
+whose aspect here gives a good idea of the appearance of these chasms
+when only their rims are in the sunlight. But observe the
+indescribable savagery of the entire scene. It looks as though the
+spirit of destruction had gone mad in this spot. The mighty craters
+have broken forth one after another, each rending its predecessor; and
+when their work was finished, a minor but yet tremendous outbreak
+occurred, and the face of the moon was gored and punctured with
+thousands of smaller craters. These relatively small craters (small,
+however, only in a lunar sense, for many of them would appear gigantic
+on the earth) recall once more the theory of meteoric impact. It does
+not seem impossible that some of them may have been formed by such an
+agency.
+
+One would not wish for our planet such a fate as that which has
+overtaken the moon, but we cannot be absolutely sure that something of
+the kind may not be in store for it. We really know nothing of the
+ultimate causes of volcanic activity, and some have suggested that the
+internal energies of the earth may be accumulating instead of dying
+out, and may never yet have exhibited their utmost destructive power.
+Perhaps the best assurance that we can find that the earth will escape
+the catastrophe that has overtaken its satellite is to be found in the
+relatively great force of its gravitation. The moon has been the
+victim of its weakness; given equal forces, and the earth would be the
+better able to withstand them. It is significant, in connection with
+these considerations, that the little planet Mercury, which seems also
+to have parted with its air and water, shows to the telescope some
+indications that it is pitted with craters resembling those that have
+torn to pieces the face of the moon.
+
+Upon the whole, after studying the dreadful lunar landscapes, one
+cannot feel a very enthusiastic sympathy with those who are seeking
+indications of the continued existence of some kind of life on the
+moon; such a world is better without inhabitants. It has met its fate;
+let it go! Fortunately, it is not so near that it cannot hide its
+scars and appear beautiful -- except when curiosity impels us to look
+with the penetrating eyes of the astronomer.
+
+The Great Mars Problem
+
+Let any thoughtful person who is acquainted with the general facts of
+astronomy look up at the heavens some night when they appear in their
+greatest splendor, and ask himself what is the strongest impression
+that they make upon his mind. He may not find it easy to frame an
+answer, but when he has succeeded it will probably be to the effect
+that the stars give him an impression of the universality of
+intelligence; they make him feel, as the sun and the moon cannot do,
+that his world is not alone; that all this was not made simply to form
+a gorgeous canopy over the tents of men. If he is of a devout turn of
+mind, he thinks, as he gazes into those fathomless deeps and among
+those bewildering hosts, of the infinite multitude of created beings
+that the Almighty has taken under his care. The narrow ideas of the
+old geocentric theology, which made the earth God's especial
+footstool, and man his only rational creature, fall away from him like
+a veil that had obscured his vision; they are impossible in the
+presence of what he sees above. Thus the natural tendency, in the
+light of modern progress, is to regard the universe as everywhere
+filled with life.
+
+But science, which is responsible for this broadening of men's
+thoughts concerning the universality of life, itself proceeds to set
+limits. Of spiritual existences it pretends to know nothing, but as to
+physical beings, it declares that it can only entertain the
+supposition of their existence where it finds evidence of an
+environment suited to their needs, and such environment may not
+everywhere exist. Science, though repelled by the antiquated
+theological conception of the supreme isolation of man among created
+beings, regards with complacency the probability that there are
+regions in the universe where no organic life exists, stars which
+shine upon no inhabited worlds, and planets which nourish no animate
+creatures. The astronomical view of the universe is that it consists
+of matter in every stage of evolution: some nebulous and chaotic; some
+just condensing into stars (suns) of every magnitude and order; some
+shaped into finished solar bodies surrounded by dependent planets;
+some forming stars that perhaps have no planets, and will have none;
+some constituting suns that are already aging, and will soon lose
+their radiant energy and disappear; and some aggregated into masses
+that long ago became inert, cold, and rayless, and that can only be
+revivified by means about which we can form conjectures, but of which
+we actually know nothing.
+
+As with the stars, so with the planets, which are the satellites of
+stars. All investigations unite to tell us that the planets are not
+all in the same state of development. As some are large and some
+small, so some are, in an evolutionary sense, young, and some old. As
+they depend upon the suns around which they revolve for their light,
+heat, and other forms of radiant energy, so their condition varies
+with their distance from those suns. Many may never arrive at a state
+suitable for the maintenance of life upon their surfaces; some which
+are not at present in such a state may attain it later; and the forms
+of life themselves may vary with the peculiar environment that
+different planets afford. Thus we see that we are not scientifically
+justified in affirming that life is ubiquitous, although we are thus
+justified in saying that it must be, in a general sense, universal. We
+might liken the universe to a garden known to contain every variety of
+plant. If on entering it we see no flowers, we examine the species
+before us and find that they are not of those which bloom at this
+particular season, or perhaps they are such as never bear flowers. Yet
+we feel no doubt that we shall find flowers somewhere in the garden,
+because there are species which bloom at this season, and the garden
+contains all varieties.
+
+While it is tacitly assumed that there are planets revolving around
+other stars than the sun, it would be impossible for us to see them
+with any telescope yet invented, and no instrument now in the
+possession of astronomers could assure us of their existence; so the
+only planetary system of which we have visual knowledge is our own.
+Excluding the asteroids, which could not from any point of view be
+considered as habitable, we have in the solar system eight planets of
+various sizes and situated at various distances from the sun. Of these
+eight we know that one, the earth, is inhabited. The question, then,
+arises: Are there any of the others which are inhabited or habitable?
+Since it is our intention to discuss the habitability of only one of
+the seven to which the question applies, the rest may be dismissed in
+a few words. The smallest of them, and the nearest to the sun, is
+Mercury, which is regarded as uninhabitable because it has no
+perceptible supply of water and air, and because, owing to the
+extraordinary eccentricity of its orbit, it is subjected to excessive
+and very rapid alterations in the amount of solar heat and light
+poured upon its surface, such alterations being inconsistent with the
+supposition that it can support living beings. Even its average
+temperature is more than six and a half times that prevailing on the
+earth! Another circumstance which militates against its habitability
+is that, according to the results of the best telescopic studies, it
+always keeps the same face toward the sun, so that one half of the
+planet is perpetually exposed to the fierce solar rays, and the other
+half faces the unmitigated cold of open space. Venus, the next in
+distance from the sun, is almost the exact twin of the earth in size,
+and many arguments may be urged in favor of its habitability, although
+it is suspected of possessing the same peculiarity as Mercury, in
+always keeping the same side sunward. Unfortunately its atmosphere
+appears to be so dense that no permanent markings on its surface are
+certainly visible, and the question of its actual condition must, for
+the present, be left in abeyance. Mars, the first planet more distant
+from the sun than the earth, is the special subject of this chapter,
+and will be described and discussed a few lines further on. Jupiter,
+Saturn, Uranus, and Neptune, the four giant planets, all more distant
+than Mars, and each more distant than the other in the order named,
+are all regarded as uninhabitable because none of them appears to
+possess any degree of solidity. They may have solid or liquid nuclei,
+but exteriorly they seem to be mere balls of cloud. Of course, one can
+imagine what he pleases about the existence of creatures suited to the
+physical constitution of such planets as these, but they must be
+excluded from the category of habitable worlds in the ordinary sense
+of the term. We go back, then, to Mars.
+
+It will be best to begin with a description of the planet. Mars is
+4230 miles in diameter; its surface is not much more than one-quarter
+as extensive as that of the earth (.285). Its mean distance from the
+sun is 141,500,000 miles, 48,500,000 miles greater than that of the
+earth. Since radiant energy varies inversely as the square of
+distance, Mars receives less than half as much solar light and heat as
+the earth gets. Mars' year (period of revolution round the sun) is 687
+days. Its mean density is 71 per cent of the earth's, and the force of
+gravity on its surface is 38 per cent of that on the surface of the
+earth; i.e., a body weighing one hundred pounds on the earth would, if
+transported to Mars, weigh but thirty-eight pounds. The inclination of
+its equator to the plane of its orbit differs very little from that of
+the earth's equator, and its axial rotation occupies 24 hours 37
+minutes. so that the length of day and night, and the extent of the
+seasonal changes on Mars, are almost precisely the same as on the
+earth. But owing to the greater length of its year, the seasons of
+Mars, while occurring in the same order, are almost twice as long as
+ours. The surface of the planet is manifestly solid, like that of our
+globe, and the telescope reveals many permanent markings on it,
+recalling the appearance of a globe on which geographical features
+have been represented in reddish and dusky tints. Around the poles are
+plainly to be seen rounded white areas, which vary in extent with the
+Martian seasons, nearly vanishing in summer and extending widely in
+winter. The most recent spectroscopic determinations indicate that
+Mars has an atmosphere perhaps as dense as that to be found on our
+loftiest mountain peaks, and there is a perceptible amount of watery
+vapor in this atmosphere. The surface of the planet appears to be
+remarkably level, and it has no mountain ranges. No evidences of
+volcanic action have been discovered on Mars. The dusky and reddish
+areas were regarded by the early observers as respectively seas and
+lands, but at present it is not believed that there are any bodies of
+water on the planet. There has never been much doubt expressed that
+the white areas about the poles represent snow.
+
+It will be seen from this brief description that many remarkable
+resemblances exist between Mars and the earth, and there is nothing
+wonderful in the fact that the question of the habitability of the
+former has become one of extreme and wide-spread interest, giving rise
+to the most diverse views, to many extraordinary speculations, and
+sometimes to regrettably heated controversy. The first champion of the
+habitability of Mars was Sir William Herschel, although even before
+his time the idea had been suggested. He was convinced by the
+revelations of his telescopes, continually increasing in power, that
+Mars was more like the earth than any other planet. He could not
+resist the testimony of the polar snows, whose suggestive conduct was
+in such striking accord with what occurs upon the earth. Gradually, as
+telescopes improved and observers increased in number, the principal
+features of the planet were disclosed and charted, and ``areography,''
+as the geography of Mars was called, took its place among the
+recognized branches of astronomical study. But it was not before 1877
+that a fundamentally new discovery in areography gave a truly
+sensational turn to speculation about life on ``the red planet.'' In
+that year Mars made one of its nearest approaches to the earth, and
+was so situated in its orbit that it could be observed to great
+advantage from the northern hemisphere of the earth. The celebrated
+Italian astronomer, Schiaparelli, took advantage of this opportunity
+to make a trigonometrical survey of the surface of Mars -- as coolly
+and confidently as if he were not taking his sights across a
+thirty-five-million-mile gulf of empty space -- and in the course of
+this survey he was astonished to perceive that the reddish areas, then
+called continents, were crossed in many directions by narrow, dusky
+lines, to which he gave the suggestive name of ``canals.'' Thus a kind
+of firebrand was cast into the field of astronomical speculation,
+which has ever since produced disputes that have sometimes approached
+the violence of political faction. At first the accuracy of
+Schiaparelli's observations was contested; it required a powerful
+telescope, and the most excellent ``seeing,'' to render the
+enigmatical lines visible at all, and many searchers were unable to
+detect them. But Schiaparelli continued his studies in the serene sky
+of Italy, and produced charts of the gridironed face of Mars
+containing so much astonishing detail that one had either to reject
+them in toto or to confess that Schiaparelli was right. As subsequent
+favorable oppositions of Mars occurred, other observers began to see
+the ``canals'' and to confirm the substantial accuracy of the Italian
+astronomer's work, and finally few were found who would venture to
+affirm that the ``canals'' did not exist, whatever their meaning might
+be.
+
+When Schiaparelli began his observations it was generally believed, as
+we have said, that the dusky areas on Mars were seas, and since
+Schiaparelli thought that the ``canals'' invariably began and ended at
+the shores of the ``seas,'' the appropriateness of the title given to
+the lines seemed apparent. Their artificial character was immediately
+assumed by many, because they were too straight and too suggestively
+geometrical in their arrangement to permit the conclusion that they
+were natural watercourses. A most surprising circumstance noted by
+Schiaparelli was that the ``canals'' made their appearance after the
+melting of the polar snow in the corresponding hemisphere had begun,
+and that they grew darker, longer, and more numerous in proportion as
+the polar liquidation proceeded; another very puzzling observation was
+that many of them became double as the season advanced; close beside
+an already existing ``canal,'' and in perfect parallelism with it,
+another would gradually make its appearance. That these phenomena
+actually existed and were not illusions was proved by later
+observations, and today they are seen whenever Mars is favorably
+situated for observation.
+
+In the closing decade of the nineteenth century, Mr Percival Lowell
+took up the work where Schiaparelli had virtually dropped it, and soon
+added a great number of ``canals'' to those previously known, so that
+in his charts the surface of the wonderful little planet appears
+covered as with a spider's web, the dusky lines criss-crossing in
+every direction, with conspicuous knots wherever a number of them come
+together. Mr Lowell has demonstrated that the areas originally called
+seas, and thus named on the earlier charts, are not bodies of water,
+whatever else they may be. He has also found that the mysterious lines
+do not, as Schiaparelli supposed, begin and end at the edges of the
+dusky regions, but often continue on across them, reaching in some
+cases far up into the polar regions. But Schiaparelli was right in his
+observation that the appearance of the ``canals'' is synchronous with
+the gradual disappearance of the polar snows, and this fact has become
+the basis of the most extraordinary theory that the subject of life in
+other worlds has ever given birth to.
+
+Now, the effect of such discoveries, as we have related, depends upon
+the type of mind to whose attention they are called. Many are content
+to accept them as strange and inexplicable at present, and to wait for
+further light upon them; others insist upon an immediate inquiry
+concerning their probable nature and meaning. Such an inquiry can only
+be based upon inference proceeding from analogy. Mars, say Mr Lowell
+and those who are of his opinion, is manifestly a solidly incrusted
+planet like the earth; it has an atmosphere, though one of great
+rarity; it has water vapor, as the snows in themselves prove; it has
+the alternation of day and night, and a succession of seasons closely
+resembling those of the earth; its surface is suggestively divided
+into regions of contrasting colors and appearance, and upon that
+surface we see an immense number of lines geometrically arranged, with
+a system of symmetrical intersections where the lines expand into
+circular and oval areas -- and all connected with the annual melting
+of the polar snows in a way which irresistibly suggests the
+interference of intelligence directed to a definite end. Why, with so
+many concurrent circumstances to support the hypothesis, should we not
+regard Mars as an inhabited globe?
+
+But the differences between Mars and the earth are in many ways as
+striking as their resemblances. Mars is relatively small; it gets less
+than half as much light and heat as we receive; its atmosphere is so
+rare that it would be distressing to us, even if we could survive in
+it at all; it has no lakes, rivers, or seas; its surface is an endless
+prairie. and its ``canals'' are phenomena utterly unlike anything on
+the earth. Yet it is precisely upon these divergences between the
+earth and Mars, this repudiation of terrestrial standards, that the
+theory of ``life on Mars,'' for which Mr Lowell is mainly responsible,
+is based. Because Mars is smaller than the earth, we are told it must
+necessarily be more advanced in planetary evolution, the underlying
+cause of which is the gradual cooling and contraction of the planet's
+mass. Mars has parted with its internal heat more rapidly than the
+earth; consequently its waters and its atmosphere have been mostly
+withdrawn by chemical combinations, but enough of both yet remain to
+render life still possible on its surface. As the globe of Mars is
+evolutionally older than that of the earth, so its forms of organic
+life may be proportionally further advanced, and its inhabitants may
+have attained a degree of cultivated intelligence much superior to
+what at present exists upon the earth. Understanding the nature and
+the causes of the desiccation of their planet, and possessing
+engineering science and capabilities far in advance of ours, they may
+be conceived to have grappled with the stupendous problem of keeping
+their world in a habitable condition as long as possible. Supposing
+them to have become accustomed to live in their rarefied atmosphere (a
+thing not inconceivable, since men can live for a time at least in air
+hardly less rare), the most pressing problem for them is that of a
+water-supply, without which plant life cannot exist, while animal life
+in turn depends for its existence upon vegetation. The only direction
+in which they can seek water is that of the polar regions, where it is
+alternately condensed into snow and released in the liquid form by the
+effect of the seasonal changes. It is, then, to the annual melting of
+the polar snow-fields that the Martian engineers are supposed to have
+recourse in supplying the needs of their planet, and thus providing
+the means of prolonging their own existence. It is imagined that they
+have for this purpose constructed a stupendous system of irrigation
+extending over the temperate and equatorial regions of the planet. The
+``canals'' represent the lines of irrigation, but the narrow streaks
+that we see are not the canals themselves, but the irrigated bands
+covered by them. Their dark hue, and their gradual appearance after
+the polar melting has begun, are due to the growth of vegetation
+stimulated by the water. The rounded areas visible where several
+``canals'' meet and cross are called by Mr Lowell ``oases.'' These are
+supposed to be the principal centers of population and industry. It
+must be confessed that some of them, with their complicated systems of
+radiating lines, appear to answer very well to such a theory. No
+attempt to explain them by analogy with natural phenomena on the earth
+has proved successful.
+
+But a great difficulty yet remains: How to explain the seemingly
+miraculous powers of the supposed engineers? Here recourse is had once
+more to the relative smallness of the planet. We have remarked that
+the force of gravity on Mars is only thirty-eight per cent of that on
+the earth. A steam-shovel driven by a certain horse-power would be
+nearly three times as effective there as here. A man of our stature on
+Mars would find his effective strength increased in the same
+proportion. But just because of the slight force of gravity there, a
+Martian might attain to the traditional stature of Goliath without
+finding his own weight an encumbrance to his activity, while at the
+same time his huge muscles would come into unimpeded play, enabling
+him single-handed to perform labors that would be impossible to a
+whole gang of terrestrial workmen. The effective powers of huge
+machines would be increased in the same way; and to all this must be
+added the fact that the mean density of the materials of which Mars is
+composed is much less than that of the constituents of the earth.
+Combining all these considerations, it becomes much less difficult to
+conceive that public works might be successfully undertaken on Mars
+which would be hopelessly beyond the limits of human accomplishment.
+
+Certain other difficulties have also to be met; as, for instance, the
+relative coldness of the climate of Mars. At its distance it gets
+considerably less than half as much light and heat as we receive. In
+addition to this, the rarity of its atmosphere would naturally be
+expected to decrease the effective temperature at the planet's
+surface, since an atmosphere acts somewhat like the glass cover of a
+hot-house in retaining the solar heat which has penetrated it. It has
+been calculated that, unless there are mitigating circumstances of
+which we know nothing, the average temperature at the surface of Mars
+must be far below the freezing-point of water. To this it is replied
+that the possible mitigating circumstances spoken of evidently exist
+in fact, because we can see that the watery vapor condenses into snow
+around the poles in winter, but melts again when summer comes. The
+mitigating agent may be supposed to exist in the atmosphere where the
+presence of certain gases would completely alter the temperature
+gradients.
+
+It might also be objected that it is inconceivable that the Martian
+engineers, however great may be their physical powers, and however
+gigantic the mechanical energies under their control, could force
+water in large quantities from the poles to the equator. This is an
+achievement that measures up to the cosmical standard. It is admitted
+by the champions of the theory that the difficulty is a formidable
+one; but they call attention to the singular fact that on Mars there
+can be found no chains of mountains, and it is even doubtful if ranges
+of hills exist there. The entire surface of the planet appears to be
+almost ``as smooth as a billiard ball,'' and even the broad regions
+which were once supposed to be seas apparently lie at practically the
+same level as the other parts, since the ``canals'' in many cases run
+uninterruptedly across them. Lowell's idea is that these sombre areas
+may be expanses of vegetation covering ground of a more or less marshy
+character, for while the largest of them appear to be permanent, there
+are some which vary coincidently with the variations of the canals.
+
+As to the kind of machinery employed to force the water from the
+poles, it has been conjectured that it may have taken the form of a
+gigantic system of pumps and conduits; and since the Martians are
+assumed to be so far in advance of us in their mastery of scientific
+principles, the hypothesis will at least not be harmed by supposing
+that they have learned to harness forces of nature whose very
+existence in a manageable form is yet unrecognized on the earth. If we
+wish to let the imagination loose, we may conjecture that they have
+conquered the secret of those intra-atomic forces whose resistless
+energy is beginning to become evident to us, but the possibility of
+whose utilization remains a dream, the fulfillment of which nobody
+dares to predict.
+
+Such, in very brief form, is the celebrated theory of Mars as an
+inhabited world. It certainly captivates the imagination, and if we
+believe it to represent the facts, we cannot but watch with the
+deepest sympathy this gallant struggle of an intellectual race to
+preserve its planet from the effects of advancing age and death. We
+may, indeed, wonder whether our own humanity, confronted by such a
+calamity, could be counted on to meet the emergency with equal
+stoutness of heart and inexhaustibleness of resource. Up to the
+present time we certainly have shown no capacity to confront Nature
+toe to toe, and to seize her by the shoulders and turn her round when
+she refuses to go our way. If we could get into wireless telephonic
+communication with the Martians we might learn from their own lips the
+secret of their more than ``Roman recovery.''
+
+The Riddle of the Asteroids
+
+Between the orbits of Mars and Jupiter revolves the most remarkable
+system of little bodies with which we are acquainted -- the Asteroids,
+or Minor Planets. Some six hundred are now known, and they may
+actually number thousands. They form virtually a ring about the sun.
+The most striking general fact about them is that they occupy the
+place in the sky which should be occupied, according to Bode's Law, by
+a single large planet. This fact, as we shall see, has led to the
+invention of one of the most extraordinary theories in astronomy --
+viz., that of the explosion of a world!
+
+Bode's Law, so-called, is only an empiric formula, but until the
+discovery of Neptune it accorded so well with the distances of the
+planets that astronomers were disposed to look upon it as really
+representing some underlying principle of planetary distribution. They
+were puzzled by the absence of a planet in the space between Mars and
+Jupiter, where the ``law'' demanded that there should be one, and an
+association of astronomers was formed to search for it. There was a
+decided sensation when, in 1801, Piazzi, of Palermo, announced that he
+had found a little planet which apparently occupied the place in the
+system which belonged to the missing body. He named it Ceres, and it
+was the first of the Asteroids. The next year Olbers, of Bremen, while
+looking for Ceres with his telescope, stumbled upon another small
+planet which he named Pallas. Immediately he was inspired with the
+idea that these two planets were fragments of a larger one which had
+formerly occupied the vacant place in the planetary ranks, and he
+predicted that others would be found by searching in the neighborhood
+of the intersection of the orbits of the two already discovered. This
+bold prediction was brilliantly fulfilled by the finding of two more
+-- Juno in 1804, and Vesta in 1807. Olbers would seem to have been led
+to the invention of his hypothesis of a planetary explosion by the
+faith which astronomers at that time had in Bode's Law. They appear to
+have thought that several planets revolving in the gap where the
+``law'' called for but one could only be accounted for upon the theory
+that the original one had been broken up to form the several.
+Gravitation demanded that the remnants of a planet blown to pieces, no
+matter how their orbits might otherwise differ, should all return at
+stated periods to the point where the explosion had occurred; hence
+Olbers' prediction that any asteroids that might subsequently be
+discovered would be found to have a common point of orbital
+intersection. And curiously enough all of the first asteroids found
+practically answered to this requirement. Olbers' theory seemed to be
+established.
+
+After the first four, no more asteroids were found until 1845, when
+one was discovered; then, in 1847, three more were added to the list;
+and after that searchers began to pick them up with such rapidity that
+by the close of the century hundreds were known, and it had become
+almost impossible to keep track of them. The first four are by far the
+largest members of the group, but their actual sizes remained unknown
+until less than twenty years ago. It was long supposed that Vesta was
+the largest, because it shines more brightly than any of the others;
+but finally, in 1895, Barnard, with the Lick telescope, definitely
+measured their diameters, and proved to everybody's surprise that
+Ceres is really the chief, and Vesta only the third in rank. His
+measures are as follows: Ceres, 477 miles; Pallas, 304 miles; Vesta,
+239 miles; and Juno, 120 miles. They differ greatly in the reflective
+power of their surfaces, a fact of much significance in connection
+with the question of their origin. Vesta is, surface for surface,
+rather more than three times as brilliant as Ceres, whence the
+original mistake about its magnitude.
+
+Nowadays new asteroids are found frequently by photography, but
+physically they are most insignificant bodies, their average diameter
+probably not exceeding twenty miles, and some are believed not to
+exceed ten. On a planet only ten miles in diameter, assuming the same
+mean density as the earth's, which is undoubtedly too much, the force
+of gravity would be so slight that an average man would not weigh more
+than three ounces, and could jump off into space whenever he liked.
+
+Although the asteroids all revolve around the sun in the same
+direction as that pursued by the major planets, their orbits are
+inclined at a great variety of angles to the general plane of the
+planetary system, and some of them are very eccentric -- almost as
+much so as the orbits of many of the periodic comets. It has even been
+conjectured that the two tiny moons of Mars and the four smaller
+satellites of Jupiter may be asteroids gone astray and captured by
+those planets. Two of the asteroids are exceedingly remarkable for the
+shapes and positions of their orbits; these are Eros, discovered in
+1898, and T. G., 1906, found eight years later. The latter has a mean
+distance from the sun slightly greater than that of Jupiter, while the
+mean distance of Eros is less than that of Mars. The orbit of Eros is
+so eccentric that at times it approaches within 15,000,000 miles of
+the earth, nearer than any other regular member of the solar system
+except the moon, thus affording an unrivaled means of measuring the
+solar parallax. But for our present purpose the chief interest of Eros
+lies in its extraordinary changes of light.
+
+These changes, although irregular, have been observed and photographed
+many times, and there seems to be no doubt of their reality. Their
+significance consists in their possible connection with the form of
+the little planet, whose diameter is generally estimated at not more
+than twenty miles. Von Oppolzer found, in 1901, that Eros lost
+three-fourths of its brilliancy once in every two hours and
+thirty-eight minutes. Other observers have found slightly different
+periods of variability, but none as long as three hours. The most
+interesting interpretation that has been offered of this phenomenon is
+that it is due to a great irregularity of figure, recalling at once
+Olbers' hypothesis. According to some, Eros may be double, the two
+bodies composing it revolving around each other at very close
+quarters; but a more striking, and it may be said probable, suggestion
+is that Eros has a form not unlike that of a dumb-bell, or hour-glass,
+turning rapidly end over end so that the area of illuminated surface
+presented to our eyes continually changes, reaching at certain times a
+minimum when the amount of light that it reflects toward the earth is
+reduced to a quarter of its maximum value. Various other bizarre
+shapes have been ascribed to Eros, such, for instance, as that of a
+flat stone revolving about one of its longer axes, so that sometimes
+we see its face and sometimes its edge.
+
+All of these explanations proceed upon the assumption that Eros cannot
+have a simple globular figure like that of a typical planet, a figure
+which is prescribed by the law of gravitation, but that its shape is
+what may be called accidental; in a word, it is a fragment, for it
+seems impossible to believe that a body formed in interplanetary
+space, either through nebular condensation or through the aggregation
+of particles drawn together by their mutual attractions, should not be
+practically spherical in shape. Nor is Eros the only asteroid that
+gives evidence by variations of brilliancy that there is something
+abnormal in its constitution; several others present the same
+phenomenon in varying degrees. Even Vesta was regarded by Olbers as
+sufficiently variable in its light to warrant the conclusion that it
+was an angular mass instead of a globe. Some of the smaller ones show
+very notable variations, and all in short periods, of three or four
+hours, suggesting that in turning about one of their axes they present
+a surface of variable extent toward the sun and the earth.
+
+The theory which some have preferred -- that the variability of light
+is due to the differences of reflective power on different parts of
+the surface -- would, if accepted, be hardly less suggestive of the
+origin of these little bodies by the breaking up of a larger one,
+because the most natural explanation of such differences would seem to
+be that they arose from variations in the roughness or smoothness of
+the reflecting surface, which would be characteristic of fragmentary
+bodies. In the case of a large planet alternating expanses of land and
+water, or of vegetation and desert, would produce a notable variation
+in the amount of reflection, but on bodies of the size of the
+asteroids neither water nor vegetation could exist, and an atmosphere
+would be equally impossible.
+
+One of the strongest objections to Olbers' hypothesis is that only a
+few of the first asteroids discovered travel in orbits which
+measurably satisfy the requirement that they should all intersect at
+the point where the explosion occurred. To this it was at first
+replied that the perturbations of the asteroidal orbits, by the
+attractions of the major planets, would soon displace them in such a
+manner that they would cease to intersect. One of the first
+investigations undertaken by the late Prof. Simon Newcomb was directed
+to the solution of this question, and he arrived at the conclusion
+that the planetary perturbations could not explain the actual
+situation of the asteroidal orbits. But afterward it was pointed out
+that the difficulty could be avoided by supposing that not one but a
+series of explosions had produced the asteroids as they now are. After
+the primary disruption the fragments themselves, according to this
+suggestion, may have exploded, and then the resulting orbits would be
+as ``tangled'' as the heart could wish. This has so far rehabilitated
+the explosion theory that it has never been entirely abandoned, and
+the evidence which we have just cited of the probably abnormal shapes
+of Eros and other asteroids has lately given it renewed life. It is a
+subject that needs a thorough rediscussion.
+
+We must not fail to mention, however, that there is a rival hypothesis
+which commends itself to many astronomers -- viz., that the asteroids
+were formed out of a relatively scant ring of matter, situated between
+Mars and Jupiter and resembling in composition the immensely more
+massive rings from which, according to Laplace's hypothesis, the
+planets were born. It is held by the supporters of this theory that
+the attraction of the giant Jupiter was sufficient to prevent the
+small, nebulous ring that gave birth to the asteroids from condensing
+like the others into a single planet.
+
+But if we accept the explosion theory, with its corollary that minor
+explosions followed the principal one, we have still an unanswered
+question before us: What caused the explosions? The idea of a world
+blowing up is too Titanic to be shocking; it rather amuses the
+imagination than seriously impresses it; in a word, it seems
+essentially chimerical. We can by no appeal to experience form a
+mental picture of such an occurrence. Even the moon did not blow up
+when it was wrecked by volcanoes. The explosive nebulae and new stars
+are far away in space, and suggest no connection with such a
+catastrophe as the bursting of a planet into hundreds of pieces. We
+cannot conceive of a great globe thousands of miles in diameter
+resembling a pellet of gunpowder only awaiting the touch of a match to
+cause its sudden disruption. Somehow the thought of human agency
+obtrudes itself in connection with the word ``explosion,'' and we
+smile at the idea that giant powder or nitro-glycerine could blow up a
+planet. Yet it would only need enough of them to do it.
+
+After all, we may deceive ourselves in thinking, as we are apt to do,
+that explosive energies lock themselves up only in small masses of
+matter. There are many causes producing explosions in nature, every
+volcanic eruption manifests the activity of some of them. Think of the
+giant power of confined steam; if enough steam could be suddenly
+generated in the center of the earth by a downpour of all the waters
+of the oceans, what might not the consequences be for our globe? In a
+smaller globe, and it has never been estimated that the original
+asteroid was even as large as the moon, such a catastrophe would,
+perhaps, be more easily conceivable; but since we are compelled in
+this case to assume that there was a series of successive explosions,
+steam would hardly answer the purpose; it would be more reasonable to
+suppose that the cause of the explosion was some kind of chemical
+reaction, or something affecting the atoms composing the exploding
+body. Here Dr Gustav Le Bon comes to our aid with a most startling
+suggestion, based on his theory of the dissipation of intra-atomic
+energy. It will be best to quote him at some length from his book on
+The Evolution of Forces.
+
+``It does not seem at first sight,'' says Doctor Le Bon,
+
+  very comprehensible that worlds which appear more and more stable
+  as they cool could become so unstable as to afterward dissociate
+  entirely. To explain this phenomenon, we will inquire whether
+  astronomical observations do not allow us to witness this
+  dissociation.
+
+  We know that the stability of a body in motion, such as a top or a
+  bicycle, ceases to be possible when its velocity of rotation
+  descends below a certain limit. Once this limit is reached it loses
+  its stability and falls to the ground. Prof. J. J. Thomson even
+  interprets radio-activity in this manner, and points out that when
+  the speed of the elements composing the atoms descends below a
+  certain limit they become unstable and tend to lose their
+  equilibria. There would result from this a commencement of
+  dissociation, with diminution of their potential energy and a
+  corresponding increase of their kinetic energy sufficient to launch
+  into space the products of intra-atomic disintegration.
+
+  It must not be forgotten that the atom being an enormous reservoir
+  of energy is by this very fact comparable with explosive bodies.
+  These last remain inert so long as their internal equilibria are
+  undisturbed. So soon as some cause or other modifies these, they
+  explode and smash everything around them after being themselves
+  broken to pieces.
+
+  Atoms, therefore, which grow old in consequence of the diminution
+  of a part of their intra-atomic energy gradually lose their
+  stability. A moment, then, arrives when this stability is so weak
+  that the matter disappears by a sort of explosion more or less
+  rapid. The bodies of the radium group offer an image of this
+  phenomenon -- a rather faint image, however, because the atoms of
+  this body have only reached a period of instability when the
+  dissociation is rather slow. It probably precedes another and more
+  rapid period of dissociation capable of producing their final
+  explosion. Bodies such as radium, thorium, etc., represent, no
+  doubt, a state of old age at which all bodies must some day arrive,
+  and which they already begin to manifest in our universe, since all
+  matter is slightly radio-active. It would suffice for the
+  dissociation to be fairly general and fairly rapid for an explosion
+  to occur in a world where it was manifested.
+
+  These theoretical considerations find a solid support in the sudden
+  appearances and disappearances of stars. The explosions of a world
+  which produce them reveal to us, perhaps, how the universes perish
+  when they become old.
+
+  As astronomical observations show the relative frequency of these
+  rapid destructions, we may ask ourselves whether the end of a
+  universe by a sudden explosion after a long period of old age does
+  not represent its most general ending.
+
+Here, perhaps, it will be well to stop, since, entrancing as the
+subject may be, we know very little about it, and Doctor Le Bon's
+theory affords a limitless field for the reader's imagination.
+  _________________________________________________________________
+
+A printed version of this book is available from Sattre Press
+(http://csky.sattre-press.com). It includes extensive annotations, a
+new introduction and all the original photographs and diagrams.
+
+
+
+
+
+
+
+
+
+
+End of Project Gutenberg's Curiosities of the Sky, by Garrett Serviss
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+The Project Gutenberg EBook of Curiosities of the Sky, by Garrett Serviss
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+Title: Curiosities of the Sky
+
+Author: Garrett Serviss
+
+Release Date: October, 2004  [EBook #6630]
+[Yes, we are more than one year ahead of schedule]
+[This file was first posted on January 6, 2003]
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+Edition: 10
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+Language: English
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+
+*** START OF THE PROJECT GUTENBERG EBOOK, CURIOSITIES OF THE SKY ***
+
+
+
+
+
+                         Curiosities of the Sky
+
+by Garrett Serviss
+
+Curiosities of the Sky was first published in 1909 and the text is in
+the public domain. The transcription was done by William McClain
+(info@sattre-press.com), 2002.
+
+A printed version of this book is available from Sattre Press
+(http://csky.sattre-press.com). It includes extensive annotations, a
+new introduction and all the original photographs and diagrams.
+_________________________________________________________________
+
+Preface
+
+What Froude says of history is true also of astronomy: it is the most
+impressive where it transcends explanation. It is not the mathematics
+of astronomy, but the wonder and the mystery that seize upon the
+imagination. The calculation of an eclipse owes all its prestige to
+the sublimity of its data; the operation, in itself, requires no more
+mental effort than the preparation of a railway time-table.
+
+The dominion which astronomy has always held over the minds of men is
+akin to that of poetry; when the former becomes merely instructive and
+the latter purely didactic, both lose their power over the
+imagination. Astronomy is known as the oldest of the sciences, and it
+will be the longest-lived because it will always have arcana that have
+not been penetrated.
+
+Some of the things described in this book are little known to the
+average reader, while others are well known; but all possess the
+fascination of whatever is strange, marvelous, obscure, or mysterious
+-- magnified, in this case, by the portentous scale of the phenomena.
+
+The idea of the author is to tell about these things in plain
+language, but with as much scientific accuracy as plain language will
+permit, showing the wonder that is in them without getting away from
+the facts. Most of them have hitherto been discussed only in technical
+form, and in treatises that the general public seldom sees and never
+reads.
+
+Among the topics touched upon are:
+  * The strange unfixedness of the ``fixed stars,'' the vast
+    migrations of the suns and worlds constituting the universe.
+  * The slow passing out of existence of those collocations of stars
+    which for thousands of years have formed famous
+    ``constellations,'' preserving the memory of mythological heroes
+    and heroines, and perhaps of otherwise unrecorded history.
+  * The tendency of stars to assemble in immense clouds, swarms, and
+    clusters.
+  * The existence in some of the richest regions of the universe of
+    absolutely black, starless gaps, deeps, or holes, as if one were
+    looking out of a window into the murkiest night.
+  * The marvelous phenomena of new, or temporary, stars, which appear
+    as suddenly as conflagrations, and often turn into something else
+    as eccentric as themselves.
+  * The amazing forms of the ``whirlpool,'' ``spiral,'' ``pinwheel,''
+    and ``lace,'' or ``tress,'' nebul�.
+  * The strange surroundings of the sun, only seen in particular
+    circumstances, but evidently playing a constant part in the daily
+    phenomena of the solar system.
+  * The mystery of the Zodiacal Light and the Gegenschein.
+  * The extraordinary transformations undergone by comets and their
+    tails.
+  * The prodigies of meteorites and masses of stone and metal fallen
+    from the sky.
+  * The cataclysms that have wrecked the moon.
+  * The problem of life and intelligence on the planet Mars.
+  * The problematical origin and fate of the asteroids.
+  * The strange phenomena of the auroral lights.
+
+An attempt has been made to develop these topics in an orderly way,
+showing their connection, so that the reader may obtain a broad
+general view of the chief mysteries and problems of astronomy, and an
+idea of the immense field of discovery which still lies, almost
+unexplored, before it.
+
+The Windows of Absolute Night
+
+To most minds mystery is more fascinating than science. But when
+science itself leads straight up to the borders of mystery and there
+comes to a dead stop, saying, ``At present I can no longer see my
+way,'' the force of the charm is redoubled. On the other hand, the
+illimitable is no less potent in mystery than the invisible, whence
+the dramatic effect of Keats' ``stout Cortez'' staring at the
+boundless Pacific while all his men look at each other with a wild
+surmise, ``silent upon a peak in Darien.'' It is with similar feelings
+that the astronomer regards certain places where from the peaks of the
+universe his vision seems to range out into endless empty space. He
+sees there the shore of his little isthmus, and, beyond, unexplored
+immensity.
+
+The name, ``coal-sacks,'' given to these strange voids is hardly
+descriptive. Rather they produce upon the mind the effect of blank
+windows in a lonely house on a pitch-dark night, which, when looked at
+from the brilliant interior, become appalling in their rayless murk.
+Infinity seems to acquire a new meaning in the presence of these black
+openings in the sky, for as one continues to gaze it loses its purely
+metaphysical quality and becomes a kind of entity, like the ocean. The
+observer is conscious that he can actually see the beginning of its
+ebon depths, in which the visible universe appears to float like an
+enchanted island, resplendent within with lights and life and gorgeous
+spectacles, and encircled with screens of crowded stars, but with its
+dazzling vistas ending at the fathomless sea of pure darkness which
+encloses all.
+
+The Galaxy, or Milky Way, surrounds the borders of our island in space
+like a stellar garland, and when openings appear in it they are, by
+contrast, far more impressive than the general darkness of the
+interstellar expanse seen in other directions. Yet even that expanse
+is not everywhere equally dark, for it contains gloomy deeps
+discernable with careful watching. Here, too, contrast plays an
+important part, though less striking than within the galactic region.
+Some of Sir William Herschel's observations appear to indicate an
+association between these tenebrious spots and neighboring star clouds
+and nebul�. It is an illuminating bit of astronomical history that
+when he was sweeping the then virgin heavens with his great telescopes
+he was accustomed to say to his sister who, note-book in hand, waited
+at his side to take down his words, fresh with the inspiration of
+discovery: ``Prepare to write; the nebul� are coming; here space is
+vacant.''
+
+The most famous of the ``coal-sacks,'' and the first to be brought to
+general attention before astronomers had awakened to the significance
+of such things, lies adjacent to the ``Southern Cross,'' and is truly
+an amazing phenomenon. It is not alone the conspicuousness of this
+celestial vacancy, opening suddenly in the midst of one of the richest
+parts of the Galaxy, that has given it its fame, but quite as much the
+superstitious awe with which it was regarded by the early explorers of
+the South Seas. To them, as well as to those who listened in rapt
+wonder to their tales, the ``Coal-sack'' seemed to possess some occult
+connection with the mystic ``Cross.'' In the eyes of the sailors it
+was not a vacancy so much as a sable reality in the sky, and as,
+shuddering, they stared at it, they piously crossed themselves. It was
+another of the magical wonders of the unknown South, and as such it
+formed the basis of many a ``wild surmise'' and many a sea-dog's yarn.
+Scientific investigation has not diminished its prestige, and today no
+traveler in the southern hemisphere is indifferent to its fascinating
+strangeness, while some find it the most impressive spectacle of the
+antarctic heavens.
+
+All around, up to the very edge of the yawning gap, the sheen of the
+Milky Way is surpassingly glorious; but there, as if in obedience to
+an almighty edict, everything vanishes. A single faint star is visible
+within the opening, producing a curious effect upon the sensitive
+spectator, like the sight of a tiny islet in the midst of a black,
+motionless, waveless tarn. The dimensions of the lagoon of darkness,
+which is oval or pear-shaped, are eight degrees by five, so that it
+occupies a space in the sky about one hundred and thirty times greater
+than the area of the full moon. It attracts attention as soon as the
+eye is directed toward the quarter where it exists, and by virtue of
+the rarity of such phenomena it appears a far greater wonder than the
+drifts of stars that are heaped around it. Now that observatories are
+multiplying in the southern hemisphere, the great austral
+``Coal-sack'' will, no doubt, receive attention proportioned to its
+importance as one of the most significant features of the sky. Already
+at the Sydney Observatory photographs have shown that the southern
+portion of this Dead Sea of Space is not quite ``bottomless,''
+although its northern part defies the longest sounding lines of the
+astronomer.
+
+There is a similar, but less perfect, ``coal-sack'' in the northern
+hemisphere, in the constellation of ``The Swan,'' which, strange to
+say, also contains a well-marked figure of a cross outlined by stars.
+This gap lies near the top of the cross-shaped figure. It is best seen
+by averted vision, which brings out the contrast with the Milky Way,
+which is quite brilliant around it. It does not, however, exercise the
+same weird attraction upon the eye as the southern ``Coal-sack,'' for
+instead of looking like an absolute void in the sky, it rather appears
+as if a canopy of dark gauze had been drawn over the stars. We shall
+see the possible significance of this appearance later.
+
+Just above the southern horizon of our northern middle latitudes, in
+summer, where the Milky Way breaks up into vast sheets of nebulous
+luminosity, lying over and between the constellations Scorpio and
+Sagittarius, there is a remarkable assemblage of ``coal-sacks,''
+though none is of great size. One of them, near a conspicuous
+star-cluster in Scorpio, M80, is interesting for having been the first
+of these strange objects noted by Herschel. Probably it was its
+nearness to M80 which suggested to his mind the apparent connection of
+such vacancies with star-clusters which we have already mentioned.
+
+But the most marvelous of the ``coal-sacks'' are those that have been
+found by photography in Sagittarius. One of Barnard's earliest and
+most excellent photographs includes two of them, both in the
+star-cluster M8. The larger, which is roughly rectangular in outline,
+contains one little star, and its smaller neighbor is lune-shaped --
+surely a most singular form for such an object. Both are associated
+with curious dark lanes running through the clustered stars like
+trails in the woods. Along the borders of these lanes the stars are
+ranked in parallel rows, and what may be called the bottoms of the
+lanes are not entirely dark, but pebbled with faint stellar points.
+One of them which skirts the two dark gaps and traverses the cluster
+along its greatest diameter is edged with lines of stars, recalling
+the alignment of the trees bordering a French highway. This road of
+stars cannot be less than many billions of miles in length!
+
+All about the cluster the bed of the Galaxy is strangely disturbed,
+and in places nearly denuded, as if its contents had been raked away
+to form the immense stack and the smaller accumulations of stars
+around it. The well-known ``Trifid Nebula'' is also included in the
+field of the photograph, which covers a truly marvelous region, so
+intricate in its mingling of nebul�, star-clusters, star-swarms,
+star-streams, and dark vacancies that no description can do it
+justice. Yet, chaotic as it appears, there is an unmistakable
+suggestion of unity about it, impressing the beholder with the idea
+that all the different parts are in some way connected, and have not
+been fortuitously thrown together. Miss Agnes M. Clerke made the
+striking remark that the dusky lanes in M8 are exemplified on the
+largest scale in the great rift dividing the Milky Way, from Cygnus in
+the northern hemisphere all the way to the ``Cross'' in the southern.
+Similar lanes are found in many other clusters, and they are generally
+associated with flanking rows of stars, resembling in their
+arrangement the thick-set houses and villas along the roadways that
+traverse the approaches to a great city.
+
+But to return to the black gaps. Are they really windows in the
+star-walls of the universe? Some of them look rather as if they had
+been made by a shell fired through a luminous target, allowing the eye
+to range through the hole into the void space beyond. If science is
+discretely silent about these things, what can the more venturesome
+and less responsible imagination suggest? Would a huge ``runaway
+sun,'' like Arcturus, for instance, make such an opening if it should
+pass like a projectile through the Milky Way? It is at least a
+stimulating inquiry. Being probably many thousands of times more
+massive than the galactic stars, such a stellar missile would not be
+stopped by them, though its direction of flight might be altered. It
+would drag the small stars lying close to its course out of their
+spheres, but the ultimate tendency of its attraction would be to sweep
+them round in its wake, thus producing rather a star-swarm than a
+vacancy. Those that were very close to it might be swept away in its
+rush and become its satellites, careering away with it in its flight
+into outer space; but those that were farther off, and they would, of
+course, greatly outnumber the nearer ones, would tend inward from all
+sides toward the line of flight, as dust and leaves collect behind a
+speeding motor (though the forces operating would be different), and
+would fill up the hole, if hole it were. A swarm thus collected should
+be rounded in outline and bordered with a relatively barren ring from
+which the stars had been ``sucked'' away. In a general sense the M8
+cluster answers to this description, but even if we undertook to
+account for its existence by a supposition like the above, the black
+gaps would remain unexplained, unless one could make a further draft
+on the imagination and suggest that the stars had been thrown into a
+vast eddy, or system of eddies, whose vortices appear as dark holes.
+Only a maelstrom-like motion could keep such a funnel open, for
+without regard to the impulse derived from the projectile, the proper
+motions of the stars themselves would tend to fill it. Perhaps some
+other cause of the whirling motion may be found. As we shall see when
+we come to the spiral nebul�, gyratory movements are exceedingly
+prevalent throughout the universe, and the structure of the Milky Way
+is everywhere suggestive of them. But this is hazardous sport even for
+the imagination -- to play with suns as if they were but thistle-down
+in the wind or corks in a mill-race.
+
+Another question arises: What is the thickness of the hedge of stars
+through which the holes penetrate? Is the depth of the openings
+proportionate to their width? In other words, is the Milky Way round
+in section like a rope, or flat and thin like a ribbon? The answer is
+not obvious, for we have little or no information concerning the
+relative distances of the faint galactic stars. It would be easier,
+certainly, to conceive of openings in a thin belt than in a massive
+ring, for in the first case they would resemble mere rifts and breaks,
+while in the second they would be like wells or bore-holes. Then, too,
+the fact that the Milky Way is not a continuous body but is made up of
+stars whose actual distances apart is great, offers another quandary;
+persistent and sharply bordered apertures in such an assemblage are a
+priori as improbable, if not impossible, as straight, narrow holes
+running through a swarm of bees.
+
+The difficulty of these questions indicates one of the reasons why it
+has been suggested that the seeming gaps, or many of them, are not
+openings at all, but opaque screens cutting off the light from stars
+behind them. That this is quite possible in some cases is shown by
+Barnard's later photographs, particularly those of the singular region
+around the star Rho Ophiuchi. Here are to be seen somber lanes and
+patches, apparently forming a connected system which covers an immense
+space, and which their discoverer thinks may constitute a ``dark
+nebula.'' This seems at first a startling suggestion; but, after all,
+why should their not be dark nebul� as well as visible ones? In truth,
+it has troubled some astronomers to explain the luminosity of the
+bright nebul�, since it is not to be supposed that matter in so
+diffuse a state can be incandescent through heat, and phosphorescent
+light is in itself a mystery. The supposition is also in accord with
+what we know of the existence of dark solid bodies in space. Many
+bright stars are accompanied by obscure companions, sometimes as
+massive as themselves; the planets are non-luminous; the same is true
+of meteors before they plunge into the atmosphere and become heated by
+friction; and many plausible reasons have been found for believing
+that space contains as many obscure as shining bodies of great size.
+It is not so difficult, after all, then, to believe that there are
+immense collections of shadowy gases and meteoric dust whose presence
+is only manifested when they intercept the light coming from shining
+bodies behind them.
+
+This would account for the apparent extinguishment of light in open
+space, which is indicated by the falling off in relative number of
+telescopic stars below the tenth magnitude. Even as things are, the
+amount of light coming to us from stars too faint to be seen with the
+naked eye is so great that the statement of it generally surprises
+persons who are unfamiliar with the inner facts of astronomy. It has
+been calculated that on a clear night the total starlight from the
+entire celestial sphere amounts to one-sixtieth of the light of the
+full moon; but of this less than one-twenty-fifth is due to stars
+separately distinguished by the eye. If there were no obscuring medium
+in space, it is probable that the amount of starlight would be
+noticeably and perhaps enormously increased.
+
+But while it seems certain that some of the obscure spots in the Milky
+Way are due to the presence of ``dark nebul�,'' or concealing veils of
+one kind or another, it is equally certain that there are many which
+are true apertures, however they may have been formed, and by whatever
+forces they may be maintained. These, then, are veritable windows of
+the Galaxy, and when looking out of them one is face to face with the
+great mystery of infinite space. There the known universe visibly
+ends, but manifestly space itself does not end there. It is not within
+the power of thought to conceive an end to space, for the instant we
+think of a terminal point or line the mind leaps forward to the
+beyond. There must be space outside as well as inside. Eternity of
+time and infinity of space are ideas that the intellect cannot fully
+grasp, but neither can it grasp the idea of a limitation to either
+space or time. The metaphysical conceptions of hypergeometry, or
+fourth-dimensional space, do not aid us.
+
+Having, then, discovered that the universe is a thing contained in
+something indefinitely greater than itself; having looked out of its
+windows and found only the gloom of starless night outside -- what
+conclusions are we to draw concerning the beyond? It seems as empty as
+a vacuum, but is it really so? If it be, then our universe is a single
+atom astray in the infinite; it is the only island in an ocean without
+shores; it is the one oasis in an illimitable desert. Then the Milky
+Way, with its wide-flung garland of stars, is afloat like a tiny
+smoke-wreath amid a horror of immeasurable vacancy, or it is an
+evanescent and solitary ring of sparkling froth cast up for a moment
+on the viewless billows of immensity. From such conclusions the mind
+instinctively shrinks. It prefers to think that there is something
+beyond, though we cannot see it. Even the universe could not bear to
+be alone -- a Crusoe lost in the Cosmos! As the inhabitants of the
+most elegant ch�teau, with its gardens, parks, and crowds of
+attendants, would die of loneliness if they did not know that they
+have neighbors, though not seen, and that a living world of indefinite
+extent surrounds them, so we, when we perceive that the universe has
+limits, wish to feel that it is not solitary; that beyond the hedges
+and the hills there are other centers of life and activity. Could
+anything be more terrible than the thought of an isolated universe?
+The greater the being, the greater the aversion to seclusion. Only the
+infinite satisfies; in that alone the mind finds rest.
+
+We are driven, then, to believe that the universal night which
+envelopes us is not tenantless; that as we stare out of the
+star-framed windows of the Galaxy and see nothing but uniform
+blackness, the fault is with our eyes or is due to an obscuring
+medium. Since our universe is limited in extent, there must be other
+universes beyond it on all sides. Perhaps if we could carry our
+telescopes to the verge of the great ``Coal-sack'' near the ``Cross,''
+being then on the frontier of our starry system, we could discern,
+sparkling afar off in the vast night, some of the outer galaxies. They
+may be grander than ours, just as many of the suns surrounding us are
+immensely greater than ours. If we could take our stand somewhere in
+the midst of immensity and, with vision of infinite reach, look about
+us, we should perhaps see a countless number of stellar systems, amid
+which ours would be unnoticeable, like a single star among the
+multitude glittering in the terrestial sky on a clear night. Some
+might be in the form of a wreath, like our own; some might be
+globular, like the great star-clusters in Hercules and Centaurus; some
+might be glittering circles, or disks, or rings within rings. If we
+could enter them we should probably find a vast variety of
+composition, including elements unknown to terrestrial chemistry; for
+while the visible universe appears to contain few if any substances
+not existing on the earth or in the sun, we have no warrant to assume
+that others may not exist in infinite space.
+
+And how as to gravitation? We do not know that gravitation acts beyond
+the visible universe, but it is reasonable to suppose that it does. At
+any rate, if we let go its sustaining hand we are lost, and can only
+wander hopelessly in our speculations, like children astray. If the
+empire of gravitation is infinite, then the various outer systems must
+have some, though measuring by our standards an imperceptible,
+attractive influence upon each other, for gravitation never lets go
+its hold, however great the space over which it is required to act.
+Just as the stars about us are all in motion, so the starry systems
+beyond our sight may be in motion, and our system as a whole may be
+moving in concert with them. If this be so, then after interminable
+ages the aspect of the entire system of systems must change, its
+various members assuming new positions with respect to one another. In
+the course of time we may even suppose that our universe will approach
+relatively close to one of the others; and then, if men are yet living
+on the earth, they may glimpse through the openings which reveal
+nothing to us now, the lights of another nearing star system, like the
+signals of a strange squadron, bringing them the assurance (which can
+be but an inference at present) that the ocean of space has other
+argosies venturing on its limitless expanse.
+
+There remains the question of the luminiferous ether by whose agency
+the waves of light are borne through space. The ether is as mysterious
+as gravitation. With regard to ether we only infer its existence from
+the effects which we ascribe to it. Evidently the ether must extend as
+far as the most distant visible stars. But does it continue on
+indefinitely in outer space? If it does, then the invisibility of the
+other systems must be due to their distance diminishing the quantity
+of light that comes from them below the limit of perceptibility, or to
+the interposition of absorbing media; if it does not, then the reason
+why we cannot see them is owing to the absence of a means of
+conveyance for the light waves, as the lack of an interplanetary
+atmosphere prevents us from hearing the thunder of sun-spots. (It is
+interesting to recall that Mr Edison was once credited with the
+intention to construct a gigantic microphone which should render the
+roar of sun-spots audible by transforming the electric vibrations into
+sound-waves). On this supposition each starry system would be
+enveloped in its own globule of ether, and no light could cross from
+one to another. But the probability is that both the ether and
+gravitation are ubiquitous, and that all the stellar systems are
+immersed in the former like clouds of phosphorescent organisms in the
+sea.
+
+So astronomy carries the mind from height to greater height. Men were
+long in accepting the proofs of the relative insignificance of the
+earth; they were more quickly convinced of the comparative littleness
+of the solar system; and now the evidence assails their reason that
+what they had regarded as the universe is only one mote gleaming in
+the sunbeams of Infinity.
+
+Star-Clouds, Star-Clusters, and Star-Streams
+
+In the preceding chapter we have seen something of the strangely
+complicated structure of the Galaxy, or Milky Way. We now proceed to
+study more comprehensively that garlanded ``Pathway of the Gods.''
+
+Judged by the eye alone, the Milky Way is one of the most delicately
+beautiful phenomena in the entire realm of nature -- a shimmer of
+silvery gauze stretched across the sky; but studied in the light of
+its revelations, it is the most stupendous object presented to human
+ken. Let us consider, first, its appearance to ordinary vision. Its
+apparent position in the sky shifts according to the season. On a
+serene, cloudless summer evening, in the absence of the moon, whose
+light obscures it, one sees the Galaxy spanning the heavens from north
+to southeast of the zenith like a phosphorescent arch. In early spring
+it forms a similar but, upon the whole, less brilliant arch west of
+the zenith. Between spring and summer it lies like a long, faint,
+twilight band along the northern horizon. At the beginning of winter
+it again forms an arch, this time spanning the sky from east to west,
+a little north of the zenith. These are its positions as viewed from
+the mean latitude of the United States. Even the beginner in
+star-gazing does not have to watch it throughout the year in order to
+be convinced that it is, in reality, a great circle, extending
+entirely around the celestial sphere. We appear to be situated near
+its center, but its periphery is evidently far away in the depths of
+space.
+
+Although to the casual observer it seems but a delicate scarf of
+light, brighter in some places than in others, but hazy and indefinite
+at the best, such is not its appearance to those who study it with
+care. They perceive that it is an organic whole, though marvelously
+complex in detail. The telescope shows that it consists of stars too
+faint and small through excess of distance to be separately visible.
+Of the hundred million suns which some estimates have fixed as the
+probable population of the starry universe, the vast majority (at
+least thirty to one) are included in this strange belt of misty light.
+But they are not uniformly distributed in it; on the contrary, they
+are arrayed in clusters, knots, bunches, clouds, and streams. The
+appearance is somewhat as if the Galaxy consisted of innumerable
+swarms of silver-winged bees, more or less intermixed, some massed
+together, some crossing the paths of others, but all governed by a
+single purpose which leads them to encircle the region of space in
+which we are situated.
+
+From the beginning of the systematic study of the heavens, the fact
+has been recognized that the form of the Milky Way denotes the scheme
+of the sidereal system. At first it was thought that the shape of the
+system was that of a vast round disk, flat like a cheese, and filled
+with stars, our sun and his relatively few neighbors being placed near
+the center. According to this view, the galactic belt was an effect of
+perspective; for when looking in the direction of the plane of the
+disk, the eye ranged through an immense extension of stars which
+blended into a glimmering blur, surrounding us like a ring; while when
+looking out from the sides of the disk we saw but few stars, and in
+those directions the heavens appeared relatively blank. Finally it was
+recognized that this theory did not correspond with the observed
+appearances, and it became evident that the Milky Way was not a mere
+effect of perspective, but an actual band of enormously distant stars,
+forming a circle about the sphere, the central opening of the ring
+(containing many scattered stars) being many times broader than the
+width of the ring itself. Our sun is one of the scattered stars in the
+central opening.
+
+As already remarked, the ring of the Galaxy is very irregular, and in
+places it is partly broken. With its sinuous outline, its pendant
+sprays, its graceful and accordant curves, its bunching of masses, its
+occasional interstices, and the manifest order of a general plan
+governing the jumble of its details, it bears a remarkable resemblance
+to a garland -- a fact which appears the more wonderful when we recall
+its composition. That an elm-tree should trace the lines of beauty
+with its leafy and pendulous branches does not surprise us; but we can
+only gaze with growing amazement when we behold a hundred million suns
+imitating the form of a chaplet! And then we have to remember that
+this form furnishes the ground-plan of the universe.
+
+As an indication of the extraordinary speculations to which the
+mystery of the Milky Way has given rise, a theory recently (1909)
+proposed by Prof. George C. Comstock may be mentioned. Starting with
+the data (first) that the number of stars increases as the Milky Way
+is approached, and reaches a maximum in its plane, while on the other
+hand the number of nebul� is greatest outside the Milky Way and
+increases with distance from it, and (second) that the Milky Way,
+although a complete ring, is broad and diffuse on one side through
+one-half its course -- that half alone containing nebul� -- and
+relatively narrow and well defined on the opposite side, the author of
+this singular speculation avers that these facts can best be explained
+by supposing that the invisible universe consists of two
+interpenetrating parts, one of which is a chaos of indefinite extent,
+strewn with stars and nebulous dust, and the other a long, broad but
+comparatively thin cluster of stars, including the sun as one of its
+central members. This flat star-cluster is conceived to be moving
+edgewise through the chaos, and, according to Professor Comstock, it
+acts after the manner of a snow-plough sweeping away the cosmic dust
+and piling it on either hand above and below the plane of the moving
+cluster. It thus forms a transparent rift, through which we see
+farther and command a view of more stars than through the intensified
+dust-clouds on either hand. This rift is the Milky Way. The dust
+thrown aside toward the poles of the Milky Way is the substance of the
+nebul� which abound there. Ahead, where the front of the star-plough
+is clearing the way, the chaos is nearer at hand, and consequently
+there the rift subtends a broader angle, and is filled with primordial
+dust, which, having been annexed by the vanguard of the star-swarm,
+forms the nebul� seen only in that part of the Milky Way. But behind,
+the rift appears narrow because there we look farther away between
+dust-clouds produced ages ago by the front of the plough, and no
+scattered dust remains in that part of the rift.
+
+In quoting an outline of this strikingly original theory the present
+writer should not be understood as assenting to it. That it appears
+bizarre is not, in itself, a reason for rejecting it, when we are
+dealing with so problematical and enigmatical a subject as the Milky
+Way; but the serious objection is that the theory does not
+sufficiently accord with the observed phenomena. There is too much
+evidence that the Milky Way is an organic system, however fantastic
+its form, to permit the belief that it can only be a rift in chaotic
+clouds. As with every organism, we find that its parts are more or
+less clearly repeated in its ensemble. Among all the strange things
+that the Milky Way contains there is nothing so extraordinary as
+itself. Every astronomer must many times have found himself marveling
+at it in those comparatively rare nights when it shows all its beauty
+and all its strangeness. In its great broken rifts, divisions, and
+spirals are found the gigantic prototypes of similar forms in its
+star-clouds and clusters. As we have said, it determines the general
+shape of the whole sidereal system. Some of the brightest stars in the
+sky appear to hang like jewels suspended at the ends of tassels
+dropped from the Galaxy. Among these pendants are the Pleiades and the
+Hyades. Orion, too, the ``Mighty Hunter,'' is caught in ``a loop of
+light'' thrown out from it. The majority of the great first-magnitude
+stars seem related to it, as if they formed an inner ring inclined at
+an angle of some twenty degrees to its plane. Many of the long curves
+that set off from it on both sides are accompanied by corresponding
+curves of lucid stars. In a word, it offers every appearance of
+structural connection with the entire starry system. That the universe
+should have assumed the form of a wreath is certainly a matter for
+astonishment; but it would have been still more astonishing if it had
+been a cube, a rhomboid, or a dodecahedron, for then we should have
+had to suppose that something resembling the forces that shape
+crystals had acted upon the stars, and the difficulty of explaining
+the universe by the laws of gravitation would have been increased.
+
+From the Milky Way as a whole we pass to the vast clouds, swarms, and
+clusters of stars of which it is made up. It may be, as some
+astronomers hold, that most of the galactic stars are much smaller
+than the sun, so that their faintness is not due entirely to the
+effect of distance. Still, their intrinsic brilliance attests their
+solar character, and considering their remoteness, which has been
+estimated at not less than ten thousand to twenty thousand light-years
+(a light-year is equal to nearly six thousand thousand million miles)
+their actual masses cannot be extremely small. The minutest of them
+are entitled to be regarded as real suns, and they vary enormously in
+magnitude. The effects of their attractions upon one another can only
+be inferred from their clustering, because their relative movements
+are not apparent on account of the brevity of the observations that we
+can make. But imagine a being for whom a million years would be but as
+a flitting moment; to him the Milky Way would appear in a state of
+ceaseless agitation -- swirling with ``a fury of whirlpool motion.''
+
+The cloud-like aspect of large parts of the Galaxy must always have
+attracted attention, even from naked-eye observers, but the true
+star-clouds were first satisfactorily represented in Barnard's
+photographs. The resemblance to actual clouds is often startling. Some
+are close-packed and dense, like cumuli; some are wispy or mottled,
+like cirri. The rifts and modulations, as well as the general
+outlines, are the same as those of clouds of vapor or dust, and one
+notices also the characteristic thinning out at the edges. But we must
+beware of supposing that the component suns are thickly crowded as the
+particles forming an ordinary cloud. They look, indeed, as if they
+were matted together, because of the irradiation of light, but in
+reality millions and billions of miles separate each star from its
+neighbors. Nevertheless they form real assemblages, whose members are
+far more closely related to one another than is our sun to the stars
+around him, and if we were in the Milky Way the aspect of the
+nocturnal sky would be marvelously different from its present
+appearance.
+
+Stellar clouds are characteristic of the Galaxy and are not found
+beyond its borders, except in the ``Magellanic Clouds'' of the
+southern hemisphere, which resemble detached portions of the Milky
+Way. These singular objects form as striking a peculiarity of the
+austral heavens as does the great ``Coal-sack'' described in Chapter
+1. But it is their isolation that makes them so remarkable, for their
+composition is essentially galactic, and if they were included within
+its boundaries they would not appear more wonderful than many other
+parts of the Milky Way. Placed where they are, they look like masses
+fallen from the great stellar arch. They are full of nebul� and
+star-clusters, and show striking evidences of spiral movement.
+
+Star-swarms, which are also characteristic features of the Galaxy,
+differ from star-clouds very much in the way that their name would
+imply -- i.e., their component stars are so arranged, even when they
+are countless in number, that the idea of an exceedingly numerous
+assemblage rather than that of a cloud is impressed on the observer's
+mind. In a star-swarm the separate members are distinguishable because
+they are either larger or nearer than the stars composing a ``cloud.''
+A splendid example of a true star-swarm is furnished by Chi Persei, in
+that part of the Milky Way which runs between the constellations
+Perseus and Cassiopeia. This swarm is much coarser than many others,
+and can be seen by the naked eye. In a small telescope it appears
+double, as if the suns composing it had divided into two parties which
+keep on their way side by side, with some commingling of their members
+where the skirts of the two companies come in contact.
+
+Smaller than either star-clouds or star-swarms, and differing from
+both in their organization, are star-clusters. These, unlike the
+others, are found outside as well as inside the Milky Way, although
+they are more numerous inside its boundaries than elsewhere. The term
+star-cluster is sometimes applied, though improperly, to assemblages
+which are rather groups, such, for instance, as the Pleiades. In their
+most characteristic aspect star-clusters are of a globular shape --
+globes of suns! A famous example of a globular star-cluster, but one
+not included in the Milky Way, is the ``Great Cluster in Hercules.''
+This is barely visible to the naked eye, but a small telescope shows
+its character, and in a large one it presents a marvelous spectacle.
+Photographs of such clusters are, perhaps, less effective than those
+of star-clouds, because the central condensation of stars in them is
+so great that their light becomes blended in an indistinguishable
+blur. The beautiful effect of the incessant play of infinitesimal rays
+over the apparently compact surface of the cluster, as if it were a
+globe of the finest frosted silver shining in an electric beam, is
+also lost in a photograph. Still, even to the eye looking directly at
+the cluster through a powerful telescope, the central part of the
+wonderful congregation seems almost a solid mass in which the stars
+are packed like the ice crystals in a snowball.
+
+The same question rises to the lips of every observer: How can they
+possibly have been brought into such a situation? The marvel does not
+grow less when we know that, instead of being closely compacted, the
+stars of the cluster are probably separated by millions of miles; for
+we know that their distances apart are slight as compared with their
+remoteness from the Earth. Sir William Herschel estimated their number
+to be about fourteen thousand, but in fact they are uncountable. If we
+could view them from a point just within the edge of the assemblage,
+they would offer the appearance of a hollow hemisphere emblazoned with
+stars of astonishing brilliancy; the near-by ones unparalleled in
+splendor by any celestial object known to us, while the more distant
+ones would resemble ordinary stars. An inhabitant of the cluster would
+not know, except by a process of ratiocination, that he was dwelling
+in a globular assemblage of suns; only from a point far outside would
+their spherical arrangement become evident to the eye. Imagine
+fourteen-thousand fire-balloons with an approach to regularity in a
+spherical space -- say, ten miles in diameter; there would be an
+average of less than thirty in every cubic mile, and it would be
+necessary to go to a considerable distance in order to see them as a
+globular aggregation; yet from a point sufficiently far away they
+would blend into a glowing ball.
+
+Photographs show even better than the best telescopic views that the
+great cluster is surrounded with a multitude of dispersed stars,
+suggestively arrayed in more or less curving lines, which radiate from
+the principle mass, with which their connection is manifest. These
+stars, situated outside the central sphere, look somewhat like vagrant
+bees buzzing round a dense swarm where the queen bee is sitting. Yet
+while there is so much to suggest the operation of central forces,
+bringing and keeping the members of the cluster together, the
+attentive observer is also impressed with the idea that the whole
+wonderful phenomenon may be the result of explosion. As soon as this
+thought seizes the mind, confirmation of it seems to be found in the
+appearance of the outlying stars, which could be as readily explained
+by the supposition that they have been blown apart as that they have
+flocked together toward a center. The probable fact that the stars
+constituting the cluster are very much smaller than our sun might be
+regarded as favoring the hypothesis of an explosion. Of their real
+size we know nothing, but, on the basis of an uncertain estimate of
+their parallax, it has been calculated that they may average
+forty-five thousand miles in diameter -- something more than half the
+diameter of the planet Jupiter. Assuming the same mean density,
+fourteen thousand such stars might have been formed by the explosion
+of a body about twice the size of the sun. This recalls the theory of
+Olbers, which has never been altogether abandoned or disproved, that
+the Asteroids were formed by the explosion of a planet circulating
+between the orbits of Mars and Jupiter. The Asteroids, whatever their
+manner of origin, form a ring around the sun; but, of course, the
+explosion of a great independent body, not originally revolving about
+a superior center of gravitational force, would not result in the
+formation of a ring of small bodies, but rather of a dispersed mass of
+them. But back of any speculation of this kind lies the problem, at
+present insoluble: How could the explosion be produced? (See the
+question of explosions in Chapters 6 and 14).
+
+Then, on the other hand, we have the observation of Herschel, since
+abundantly confirmed, that space is unusually vacant in the immediate
+neighborhood of condensed star-clusters and nebul�, which, as far as
+it goes, might be taken as an indication that the assembled stars had
+been drawn together by their mutual attractions, and that the tendency
+to aggregation is still bringing new members toward the cluster. But
+in that case there must have been an original condensation of stars at
+that point in space. This could probably have been produced by the
+coagulation of a great nebula into stellar nuclei, a process which
+seems now to be taking place in the Orion Nebula.
+
+A yet more remarkable globular star-cluster exists in the southern
+hemisphere, Omega Centauri. In this case the central condensation of
+stars presents an almost uniform blaze of light. Like the Hercules
+cluster, that in Centaurus is surrounded with stars scattered over a
+broad field and showing an appearance of radial arrangement. In fact,
+except for its greater richness, Omega Centauri is an exact duplicate
+of its northern rival. Each appears to an imaginative spectator as a
+veritable ``city of suns.'' Mathematics shrinks from the task of
+disentangling the maze of motions in such an assemblage. It would seem
+that the chance of collisions is not to be neglected, and this idea
+finds a certain degree of confirmation in the appearance of
+``temporary stars'' which have more than once blazed out in, or close
+by, globular star-clusters.
+
+This leads up to the notable fact, first established by Professor
+Bailey a few years ago, that such clusters are populous with variable
+stars. Omega Centauri and the Hercules cluster are especially
+remarkable in this respect. The variables found in them are all of
+short period and the changes of light show a noteworthy tendency to
+uniformity. The first thought is that these phenomena must be due to
+collisions among the crowded stars, but, if so, the encounters cannot
+be between the stars themselves, but probably between stars and meteor
+swarms revolving around them. Such periodic collisions might go on for
+ages without the meteors being exhausted by incorporation with the
+stars. This explanation appears all the more probable because one
+would naturally expect that flocks of meteors would abound in a close
+aggregation of stars. It is also consistent with Perrine's discovery
+-- that the globular star clusters are powdered with minute stars
+strewn thickly among the brighter ones.
+
+In speaking of Professor Comstock's extraordinary theory of the Milky
+Way, the fact was mentioned that, broadly speaking, the nebul� are
+less numerous in the galactic belt than in the comparatively open
+spaces on either side of it, but that they are, nevertheless, abundant
+in the broader half of the Milky Way which he designates as the front
+of the gigantic ``plough'' supposed to be forcing its way through the
+enveloping chaos. In and around the Sagittarius region the
+intermingling of nebul� and galactic star clouds and clusters is
+particularly remarkable. That there is a causal connection no
+thoughtful person can doubt. We are unable to get away from the
+evidence that a nebula is like a seed-ground from which stars spring
+forth; or we may say that nebul� resemble clouds in whose bosom
+raindrops are forming. The wonderful aspect of the admixtures of
+nebul� and star-clusters in Sagittarius has been described in Chapter
+1. We now come to a still more extraordinary phenomenon of this kind
+-- the Pleiades nebul�.
+
+The group of the Pleiades, although lying outside the main course of
+the Galaxy, is connected with it by a faint loop, and is the scene of
+the most remarkable association of stars and nebulous matter known in
+the visible universe. The naked eye is unaware of the existence of
+nebul� in the Pleiades, or, at the best, merely suspects that there is
+something of the kind there; and even the most powerful telescopes are
+far from revealing the full wonder of the spectacle; but in
+photographs which have been exposed for many hours consecutively, in
+order to accumulate the impression of the actinic rays, the revelation
+is stunning. The principle stars are seen surrounded by, and, as it
+were, drowned in, dense nebulous clouds of an unparalleled kind. The
+forms assumed by these clouds seem at first sight inexplicable. They
+look like fleeces, or perhaps more like splashes and daubs of luminous
+paint dashed carelessly from a brush. But closer inspection shows that
+they are, to a large extent, woven out of innumerable threads of filmy
+texture, and there are many indications of spiral tendencies. Each of
+the bright stars of the group -- Alcyone, Merope, Maia, Electra,
+Taygeta, Atlas -- is the focus of a dense fog (totally invisible,
+remember, alike to the naked eye and to the telescope), and these
+particular stars are veiled from sight behind the strange mists.
+Running in all directions across the relatively open spaces are
+nebulous wisps and streaks of the most curious forms. On some of the
+nebular lines, which are either straight throughout, or if they change
+direction do so at an angle, little stars are strung like beads. In
+one case seven or eight stars are thus aligned, and, as if to
+emphasize their dependence upon the chain which connects them, when it
+makes a slight bend the file of stars turns the same way. Many other
+star rows in the group suggest by their arrangement that they, too,
+were once strung upon similar threads which have now disappeared,
+leaving the stars spaced along their ancient tracks. We seem forced to
+the conclusion that there was a time when the Pleiades were embedded
+in a vast nebula resembling that of Orion, and that the cloud has now
+become so rare by gradual condensation into stars that the merest
+trace of it remains, and this would probably have escaped detection
+but for the remarkable actinic power of the radiant matter of which it
+consists. The richness of many of these faint nebulous masses in
+ultra-violet radiations, which are those that specifically affect the
+photographic plate, is the cause of the marvelous revelatory power of
+celestial photography. So the veritable unseen universe, as
+distinguished from the ``unseen universe'' of metaphysical
+speculation, is shown to us.
+
+A different kind of association between stars and nebul� is shown in
+some surprising photographic objects in the constellation Cygnus,
+where long, wispy nebul�, billions of miles in length, some of them
+looking like tresses streaming in a breeze, lie amid fields of stars
+which seem related to them. But the relation is of a most singular
+kind, for notwithstanding the delicate structure of the long nebul�
+they appear to act as barriers, causing the stars to heap themselves
+on one side. The stars are two, three, or four times as numerous on
+one side of the nebul� as on the other. These nebul�, as far as
+appearance goes, might be likened to rail fences, or thin hedges,
+against which the wind is driving drifts of powdery snow, which, while
+scattered plentifully all around, tends to bank itself on the leeward
+side of the obstruction. The imagination is at a loss to account for
+these extraordinary phenomena; yet there they are, faithfully giving
+us their images whenever the photographic plate is exposed to their
+radiations.
+
+Thus the more we see of the universe with improved methods of
+observation, and the more we invent aids to human senses, each
+enabling us to penetrate a little deeper into the unseen, the greater
+becomes the mystery. The telescope carried us far, photography is
+carrying us still farther; but what as yet unimagined instrument will
+take us to the bottom, the top, and the end? And then, what hitherto
+untried power of thought will enable us to comprehend the meaning of
+it all?
+
+Stellar Migrations
+
+To the untrained eye the stars and the planets are not
+distinguishable. It is customary to call them all alike ``stars.'' But
+since the planets more or less rapidly change their places in the sky,
+in consequence of their revolution about the sun, while the stars
+proper seem to remain always in the same relative positions, the
+latter are spoken of as ``fixed stars.'' In the beginnings of
+astronomy it was not known that the ``fixed stars'' had any motion
+independent of their apparent annual revolution with the whole sky
+about the earth as a seeming center. Now, however, we know that the
+term ``fixed stars'' is paradoxical, for there is not a single really
+fixed object in the whole celestial sphere. The apparent fixity in the
+positions of the stars is due to their immense distance, combined with
+the shortness of the time during which we are able to observe them. It
+is like viewing the plume of smoke issuing from a steamer, hull down,
+at sea: if one does not continue to watch it for a long time it
+appears to be motionless, although in reality it may be traveling at
+great speed across the line of sight. Even the planets seem fixed in
+position if one watches them for a single night only, and the more
+distant ones do not sensibly change their places, except after many
+nights of observation. Neptune, for instance, moves but little more
+than two degrees in the course of an entire year, and in a month its
+change of place is only about one-third of the diameter of the full
+moon.
+
+Yet, fixed as they seem, the stars are actually moving with a speed in
+comparison with which, in some cases, the planets might almost be said
+to stand fast in their tracks. Jupiter's speed in his orbit is about
+eight miles per second, Neptune's is less than three and one-half
+miles, and the earth's is about eighteen and one-half miles; while
+there are ``fixed stars'' which move two hundred or three hundred
+miles per second. They do not all, however, move with so great a
+velocity, for some appear to travel no faster than the planets. But in
+all cases, notwithstanding their real speed, long-continued and
+exceedingly careful observations are required to demonstrate that they
+are moving at all. No more overwhelming impression of the frightful
+depths of space in which the stars are buried can be obtained than by
+reflecting upon the fact that a star whose actual motion across the
+line of sight amounts to two hundred miles per second does not change
+its apparent place in the sky, in the course of a thousand years,
+sufficiently to be noticed by the casual observer of the heavens!
+
+There is one vast difference between the motions of the stars and
+those of the planets to which attention should be at once called: the
+planets, being under the control of a central force emanating from
+their immediate master, the sun, all move in the same direction and in
+orbits concentric about the sun; the stars, on the other hand, move in
+every conceivable direction and have no apparent center of motion, for
+all efforts to discover such a center have failed. At one time, when
+theology had finally to accept the facts of science, a grandiose
+conception arose in some pious minds, according to which the Throne of
+God was situated at the exact center of His Creation, and, seated
+there, He watched the magnificent spectacle of the starry systems
+obediently revolving around Him. Astronomical discoveries and
+speculations seemed for a time to afford some warrant for this view,
+which was, moreover, an acceptable substitute for the abandoned
+geocentric theory in minds that could only conceive of God as a
+superhuman artificer, constantly admiring his own work. No longer ago
+than the middle of the nineteenth century a German astronomer,
+Maedler, believed that he had actually found the location of the
+center about which the stellar universe revolved. He placed it in the
+group of the Pleiades, and upon his authority an extraordinary
+imaginative picture was sometimes drawn of the star Alcyone, the
+brightest of the Pleiades, as the very seat of the Almighty. This idea
+even seemed to gain a kind of traditional support from the mystic
+significance, without known historical origin, which has for many
+ages, and among widely separated peoples, been attached to the
+remarkable group of which Alcyone is the chief. But since Maedler's
+time it has been demonstrated that the Pleiades cannot be the center
+of revolution of the universe, and, as already remarked, all attempts
+to find or fix such a center have proved abortive. Yet so powerful was
+the hold that the theory took upon the popular imagination, that even
+today astronomers are often asked if Alcyone is not the probable site
+of ``Jerusalem the Golden.''
+
+If there were a discoverable center of predominant gravitative power,
+to which the motions of all the stars could be referred, those motions
+would appear less mysterious, and we should then be able to conclude
+that the universe was, as a whole, a prototype of the subsidiary
+systems of which it is composed. We should look simply to the law of
+gravitation for an explanation, and, naturally, the center would be
+placed within the opening enclosed by the Milky Way. If it were there
+the Milky Way itself should exhibit signs of revolution about it, like
+a wheel turning upon its hub. No theory of the star motions as a whole
+could stand which failed to take account of the Milky Way as the basis
+of all. But the very form of that divided wreath of stars forbids the
+assumption of its revolution about a center. Even if it could be
+conceived as a wheel having no material center it would not have the
+form which it actually presents. As was shown in Chapter 2, there is
+abundant evidence of motion in the Milky Way; but it is not motion of
+the system as a whole, but motion affecting its separate parts.
+Instead of all moving one way, the galactic stars, as far as their
+movements can be inferred, are governed by local influences and
+conditions. They appear to travel crosswise and in contrary
+directions, and perhaps they eddy around foci where great numbers have
+assembled; but of a universal revolution involving the entire mass we
+have no evidence.
+
+Most of our knowledge of star motions, called ``proper motions,''
+relates to individual stars and to a few groups which happen to be so
+near that the effects of their movements are measurable. In some cases
+the motion is so rapid (not in appearance, but in reality) that the
+chief difficulty is to imagine how it can have been imparted, and what
+will eventually become of the ``runaways.'' Without a collision, or a
+series of very close approaches to great gravitational centers, a star
+traveling through space at the rate of two hundred or three hundred
+miles per second could not be arrested or turned into an orbit which
+would keep it forever flying within the limits of the visible
+universe. A famous example of these speeding stars is ``1830
+Groombridge,'' a star of only the sixth magnitude, and consequently
+just visible to the naked eye, whose motion across the line of sight
+is so rapid that it moves upon the face of the sky a distance equal to
+the apparent diameter of the moon every 280 years. The distance of
+this star is at least 200,000,000,000,000 miles, and may be two or
+three times greater, so that its actual speed cannot be less than two
+hundred, and may be as much as four hundred, miles per second. It
+could be turned into a new course by a close approach to a great sun,
+but it could only be stopped by collision, head-on, with a body of
+enormous mass. Barring such accidents it must, as far as we can see,
+keep on until it has traversed our stellar system, whence in may
+escape and pass out into space beyond, to join, perhaps, one of those
+other universes of which we have spoken. Arcturus, one of the greatest
+suns in the universe, is also a runaway, whose speed of flight has
+been estimated all the way from fifty to two hundred miles per second.
+Arcturus, we have every reason to believe, possesses hundreds of times
+the mass of our sun -- think, then, of the prodigious momentum that
+its motion implies! Sirius moves more moderately, its motion across
+the line of sight amounting to only ten miles per second, but it is at
+the same time approaching the sun at about the same speed, its actual
+velocity in space being the resultant of the two displacements.
+
+What has been said about the motion of Sirius brings us to another
+aspect of this subject. The fact is, that in every case of stellar
+motion the displacement that we observe represents only a part of the
+actual movement of the star concerned. There are stars whose motion
+carries them straight toward or straight away from the earth, and such
+stars, of course, show no cross motion. But the vast majority are
+traveling in paths inclined from a perpendicular to our line of sight.
+Taken as a whole, the stars may be said to be flying about like the
+molecules in a mass of gas. The discovery of the radial component in
+the movements of the stars is due to the spectroscope. If a star is
+approaching, its spectral lines are shifted toward the violet end of
+the spectrum by an amount depending upon the velocity of approach; if
+it is receding, the lines are correspondingly shifted toward the red
+end. Spectroscopic observation, then, combined with micrometric
+measurements of the cross motion, enables us to detect the real
+movement of the star in space. Sometimes it happens that a star's
+radial movement is periodically reversed; first it approaches, and
+then it recedes. This indicates that it is revolving around a near-by
+companion, which is often invisible, and superposed upon this motion
+is that of the two stars concerned, which together may be approaching
+or receding or traveling across the line of sight. Thus the
+complications involved in the stellar motions are often exceedingly
+great and puzzling.
+
+Yet another source of complication exists in the movement of our own
+star, the sun. There is no more difficult problem in astronomy than
+that of disentangling the effects of the solar motion from those of
+the motions of the other stars. But the problem, difficult as it is,
+has been solved, and upon its solution depends our knowledge of the
+speed and direction of the movement of the solar system through space,
+for of course the sun carries its planets with it. One element of the
+solution is found in the fact that, as a result of perspective, the
+stars toward which we are going appear to move apart toward all points
+of the compass, while those behind appear to close up together. Then
+the spectroscopic principle already mentioned is invoked for studying
+the shift of the lines, which is toward the violet in the stars ahead
+of us and toward the red in those that we are leaving behind. Of
+course the effects of the independent motions of the stars must be
+carefully excluded. The result of the studies devoted to this subject
+is to show that we are traveling at a speed of twelve to fifteen miles
+per second in a northerly direction, toward the border of the
+constellations Hercules and Lyra. A curious fact is that the more
+recent estimates show that the direction is not very much out of a
+straight line drawn from the sun to the star Vega, one of the most
+magnificent suns in the heavens. But it should not be inferred from
+this that Vega is drawing us on; it is too distant for its gravitation
+to have such an effect.
+
+Many unaccustomed thoughts are suggested by this mighty voyage of the
+solar system. Whence have we come, and whither do we go? Every year of
+our lives we advance at least 375,000,000 miles. Since the traditional
+time of Adam the sun has led his planets through the wastes of space
+no less than 225,000,000,000 miles, or more than 2400 times the
+distance that separates him from the earth. Go back in imagination to
+the geologic ages, and try to comprehend the distance over which the
+earth has flown. Where was our little planet when it emerged out of
+the clouds of chaos? Where was the sun when his ``thunder march''
+began? What strange constellations shone down upon our globe when its
+masters of life were the monstrous beasts of the ``Age of Reptiles''?
+A million years is not much of a span of time in geologic reckoning,
+yet a million years ago the earth was farther from its present place
+in space than any of the stars with a measurable parallax are now. It
+was more than seven times as far as Sirius, nearly fourteen times as
+far as Alpha Centauri, three times as far as Vega, and twice as far as
+Arcturus. But some geologists demand two hundred, three hundred, even
+one thousand million years to enable them to account for the
+evolutionary development of the earth and its inhabitants. In a
+thousand million years the earth would have traveled farther than from
+the remotest conceivable depths of the Milky Way!
+
+Other curious reflections arise when we think of the form of the
+earth's track as it follows the lead of the sun, in a journey which
+has neither known beginning nor conceivable end. There are probably
+many minds which have found a kind of consolation in the thought that
+every year the globe returns to the same place, on the same side of
+the sun. This idea may have an occult connection with our traditional
+regard for anniversaries. When that period of the year returns at
+which any great event in our lives has occurred we have the feeling
+that the earth, in its annual round, has, in a manner, brought us back
+to the scene of that event. We think of the earth's orbit as a
+well-worn path which we traverse many times in the course of a
+lifetime. It seems familiar to us, and we grow to have a sort of
+attachment to it. The sun we are accustomed to regard as a fixed
+center in space, like the mill or pump around which the harnessed
+patient mule makes his endless circuits. But the real fact is that the
+earth never returns to the place in space where it has once quitted.
+In consequence of the motion of the sun carrying the earth and the
+other planets along, the track pursued by our globe is a vast spiral
+in space continually developing and never returning upon its course.
+It is probable that the tracks of the sun and the others stars are
+also irregular, and possibly spiral, although, as far as can be at
+present determined, they appear to be practically straight. Every
+star, wherever it may be situated, is attracted by its fellow-stars
+from many sides at once, and although the force is minimized by
+distance, yet in the course of many ages its effects must become
+manifest.
+
+Looked at from another side, is there not something immensely
+stimulating and pleasing to the imagination in the idea of so
+stupendous a journey, which makes all of us the greatest of travelers?
+In the course of a long life a man is transported through space thirty
+thousand million miles; Halley's Comet does not travel one-quarter as
+far in making one of its immense circuits. And there are adventures on
+this voyage of which we are just beginning to learn to take account.
+Space is full of strange things, and the earth must encounter some of
+them as it advances through the unknown. Many singular speculations
+have been indulged in by astronomers concerning the possible effects
+upon the earth of the varying state of the space that it traverses.
+Even the alternation of hot and glacial periods has sometimes been
+ascribed to this source. When tropical life flourished around the
+poles, as the remains in the rocks assure us, the needed high
+temperature may, it has been thought, have been derived from the
+presence of the earth in a warm region of space. Then, too, there is a
+certain interest for us in the thought of what our familiar planet has
+passed through. We cannot but admire it for its long journeying as we
+admire the traveler who comes to us from remote and unexplored lands,
+or as we gaze with a glow of interest upon the first locomotive that
+has crossed a continent, or a ship that has visited the Arctic or
+Antarctic regions. If we may trust the indications of the present
+course, the earth, piloted by the sun, has come from the Milky Way in
+the far south and may eventually rejoin that mighty band of stars in
+the far north.
+
+While the stars in general appear to travel independently of one
+another, except when they are combined in binary or trinary systems,
+there are notable exceptions to this rule. In some quarters of the sky
+we behold veritable migrations of entire groups of stars whose members
+are too widely separated to show any indications of revolution about a
+common center of gravity. This leads us back again to the wonderful
+group of the Pleiades. All of the principle stars composing that group
+are traveling in virtually parallel lines. Whatever force set them
+going evidently acted upon all alike. This might be explained by the
+assumption that when the original projective force acted upon them
+they were more closely united than they are at present, and that in
+drifting apart they have not lost the impulse of the primal motion. Or
+it may be supposed that they are carried along by some current in
+space, although it would be exceedingly difficult, in the present
+state of our knowledge, to explain the nature of such a current. Yet
+the theory of a current has been proposed. As to an attractive center
+around which they might revolve, none has been found. Another instance
+of similar ``star-drift'' is furnished by five of the seven stars
+constituting the figure of the ``Great Dipper.'' In this case the
+stars concerned are separated very widely, the two extreme ones by not
+less than fifteen degrees, so that the idea of a common motion would
+never have been suggested by their aspect in the sky; and the case
+becomes the more remarkable from the fact that among and between them
+there are other stars, some of the same magnitude, which do not share
+their motion, but are traveling in other directions. Still other
+examples of the same phenomenon are found in other parts of the sky.
+Of course, in the case of compact star-clusters, it is assumed that
+all the members share a like motion of translation through space, and
+the same is probably true of dense star-swarms and star-clouds.
+
+The whole question of star-drift has lately assumed a new phase, in
+consequence of the investigations of Kapteyn, Dyson, and Eddington on
+the ``systematic motions of the stars.'' This research will, it is
+hoped, lead to an understanding of the general law governing the
+movements of the whole body of stars constituting the visible
+universe. Taking about eleven hundred stars whose proper motions have
+been ascertained with an approach to certainty, and which are
+distributed in all parts of the sky, it has been shown that there
+exists an apparent double drift, in two independent streams, moving in
+different and nearly opposed directions. The apex of the motion of
+what is called ``Stream I'' is situated, according to Professor
+Kapteyn, in right ascension 85�, declination south 11�, which places
+it just south of the constellation Orion; while the apex of ``Stream
+II'' is in right ascension 260�, declination south 48�, placing it in
+the constellation Ara, south of Scorpio. The two apices differ very
+nearly 180� in right ascension and about 120� in declination. The
+discovery of these vast star-streams, if they really exist, is one of
+the most extraordinary in modern astronomy. It offers the correlation
+of stellar movements needed as the basis of a theory of those
+movements, but it seems far from revealing a physical cause for them.
+As projected against the celestial sphere the stars forming the two
+opposite streams appear intermingled, some obeying one tendency and
+some the other. As Professor Dyson has said, the hypothesis of this
+double movement is of a revolutionary character, and calls for further
+investigation. Indeed, it seems at first glance not less surprising
+than would be the observation that in a snow-storm the flakes over our
+heads were divided into two parties and driving across each other's
+course in nearly opposite directions, as if urged by interpenetrating
+winds.
+
+But whatever explanation may eventually be found for the motions of
+the stars, the knowledge of the existence of those motions must always
+afford a new charm to the contemplative observer of the heavens, for
+they impart a sense of life to the starry system that would otherwise
+be lacking. A stagnant universe, with every star fixed immovably in
+its place, would not content the imagination or satisfy our longing
+for ceaseless activity. The majestic grandeur of the evolutions of the
+celestial hosts, the inconceivable vastness of the fields of space in
+which they are executed, the countless numbers, the immeasurable
+distances, the involved convolutions, the flocking and the scattering,
+the interpenetrating marches and countermarches, the strange community
+of impulsion affecting stars that are wide apart in space and causing
+them to traverse the general movement about them like aides and
+despatch-bearers on a battle-field -- all these arouse an intensity of
+interest which is heightened by the mystery behind them.
+
+The Passing of the Constellations
+
+From a historical and picturesque point of view, one of the most
+striking results of the motions of the stars described in the last
+chapter is their effect upon the forms of the constellations, which
+have been watched and admired by mankind from a period so early that
+the date of their invention is now unknown. The constellations are
+formed by chance combinations of conspicuous stars, like figures in a
+kaleidoscope, and if our lives were commensurate with the �ons of
+cosmic existence we should perceive that the kaleidoscope of the
+heavens was ceaselessly turning and throwing the stars into new
+symmetries. Even if the stars stood fast, the motion of the solar
+system would gradually alter the configurations, as the elements of a
+landscape dissolve and recombine in fresh groupings with the
+traveler's progress amid them. But with the stars themselves all in
+motion at various speeds and in many directions, the changes occur
+more rapidly. Of course, ``rapid'' is here understood in a relative
+sense; the wheel of human history to an eye accustomed to the majestic
+progression of the universe would appear to revolve with the velocity
+of a whirling dynamo. Only the deliberation of geological movements
+can be contrasted with the evolution and devolution of the
+constellations.
+
+And yet this secular fluctuation of the constellation figures is not
+without keen interest for the meditative observer. It is another
+reminder of the swift mutability of terrestial affairs. To the passing
+glance, which is all that we can bestow upon these figures, they
+appear so immutable that they have been called into service to form
+the most lasting records of ancient thought and imagination that we
+possess. In the forms of the constellations, the most beautiful, and,
+in imaginative quality, the finest, mythology that the world has ever
+known has been perpetuated. Yet, in a broad sense, this scroll of
+human thought imprinted on the heavens is as evanescent as the summer
+clouds. Although more enduring than parchment, tombs, pyramids, and
+temples, it is as far as they from truly eternizing the memory of what
+man has fancied and done.
+
+Before studying the effects that the motions of the stars have had and
+will have upon the constellations, it is worth while to consider a
+little further the importance of the stellar pictures as archives of
+history. To emphasize the importance of these effects it is only
+necessary to recall that the constellations register the oldest
+traditions of our race. In the history of primeval religions they are
+the most valuable of documents. Leaving out of account for the moment
+the more familiar mythology of the Greeks, based on something older
+yet, we may refer for illustration to that of the mysterious Maya race
+of America. At Izamal, in Yucatan, says Mr Stansbury Hagar, is a group
+of ruins perched, after the Mexican and Central-American plan, on the
+summits of pyramidal mounds which mark the site of an ancient
+theogonic center of the Mayas. Here the temples all evidently refer to
+a cult based upon the constellations as symbols. The figures and the
+names, of course, were not the same as those that we have derived from
+our Aryan ancestors, but the star groups were the same or nearly so.
+For instance, the loftiest of the temples at Izamal was connected with
+the sign of the constellation known to us as Cancer, marking the place
+of the sun at the summer solstice, at which period the sun was
+supposed to descend at noon like a great bird of fire and consume the
+offerings left upon the altar. Our Scorpio was known to the Mayas as a
+sign of the ``Death God.'' Our Libra, the ``Balance,'' with which the
+idea of a divine weighing out of justice has always been connected,
+seems to be identical with the Mayan constellation Teoyaotlatohua,
+with which was associated a temple where dwelt the priests whose
+special business it was to administer justice and to foretell the
+future by means of information obtained from the spirits of the dead.
+Orion, the ``Hunter'' of our celestial mythology, was among the Mayas
+a ``Warrior,'' while Sagittarius and others of our constellations were
+known to them (under different names, of course), and all were endowed
+with a religious symbolism. And the same star figures, having the same
+significance, were familiar to the Peruvians, as shown by the temples
+at Cuzco. Thus the imagination of ancient America sought in the
+constellations symbols of the unchanging gods.
+
+But, in fact, there is no nation and no people that has not recognized
+the constellations, and at one period or another in its history
+employed them in some symbolic or representative capacity. As handled
+by the Greeks from prehistoric times, the constellation myths became
+the very soul of poetry. The imagination of that wonderful race
+idealized the principal star groups so effectively that the figures
+and traditions thus attached to them have, for civilized mankind,
+displaced all others, just as Greek art in its highest forms stands
+without parallel and eclipses every rival. The Romans translated no
+heroes and heroines of the mythical period of their history to the
+sky, and the deified C�sars never entered that lofty company, but the
+heavens are filled with the early myths of the Greeks. Herakles
+nightly resumes his mighty labors in the stars; Zeus, in the form of
+the white ``Bull,'' Taurus, bears the fair Europa on his back through
+the celestial waves; Andromeda stretches forth her shackled arms in
+the star-gemmed ether, beseeching aid; and Perseus, in a blaze of
+diamond armor, revives his heroic deeds amid sparkling clouds of
+stellar dust. There, too, sits Queen Cassiopeia in her dazzling chair,
+while the Great King, Cepheus, towers gigantic over the pole.
+Professor Young has significantly remarked that a great number of the
+constellations are connected in some way or other with the Argonautic
+Expedition -- that strangely fascinating legend of earliest Greek
+story which has never lost its charm for mankind. In view of all this,
+we may well congratulate ourselves that the constellations will
+outlast our time and the time of countless generations to follow us;
+and yet they are very far from being eternal. Let us now study some of
+the effects of the stellar motions upon them.
+
+We begin with the familiar figure of the ``Great Dipper.'' He who has
+not drunk inspiration from its celestial bowl is not yet admitted to
+the circle of Olympus. This figure is made up of seven conspicuous
+stars in the constellation Ursa Major, the ``Greater Bear.'' The
+handle of the ``Dipper'' corresponds to the tail of the imaginary
+``Bear,'' and the bowl lies upon his flank. In fact, the figure of a
+dipper is so evident and that of a bear so unevident, that to most
+persons the ``Great Dipper'' is the only part of the constellation
+that is recognizable. Of the seven stars mentioned, six are of nearly
+equal brightness, ranking as of the second magnitude, while the
+seventh is of only the third magnitude. The difference is very
+striking, since every increase of one magnitude involves an increase
+of two-and-a-half times in brightness. There appears to be little
+doubt that the faint star, which is situated at the junction of the
+bowl and the handle, is a variable of long period, since three hundred
+years ago it was as bright as its companions. But however that may be,
+its relative faintness at the present time interferes but little with
+the perfection of the ``Dipper's'' figure. In order the more readily
+to understand the changes which are taking place, it will be well to
+mention both the names and the Greek letters which are attached to the
+seven stars. Beginning at the star in the upper outer edge of the rim
+of the bowl and running in regular order round the bottom and then out
+to the end of the handle, the names and letters are as follows: Dubhe
+({\alpha}), Merak ({\beta}), Phaed ({\gamma}), Megrez ({\delta}),
+Alioth ({\epsilon}), Mizar ({\zeta}), and Benetnasch ({\eta}). Megrez
+is the faint star already mentioned at the junction of the bowl and
+handle, and Mizar, in the middle of the handle, has a close, naked-eye
+companion which is named Alcor. The Arabs called this singular pair of
+stars ``The Horse and Rider.'' Merak and Duhbe are called ``The
+Pointers,'' because an imaginary line drawn northward through them
+indicates the Pole Star.
+
+Now it has been found that five of these stars -- viz., Merak, Phaed,
+Megrez, Alioth, and Mizar (with its comrade) -- are moving with
+practically the same speed in an easterly direction, while the other
+two, Dubhe and Benetnasch, are simultaneously moving westward, the
+motions of Benetnasch being apparently more rapid. The consequence of
+these opposed motions is, of course, that the figure of the ``Dipper''
+cannot always have existed and will not continue to exist. In the
+accompanying diagrams it has been thought interesting to show the
+relative positions of these seven stars, as seen from the point which
+the earth now occupies, both in the past and in the future. Arrows
+attached to the stars in the figure representing the present
+appearance of the ``Dipper'' indicate the directions of the motions
+and the distances over which they will carry the stars in a period of
+about five hundred centuries. The time, no doubt, seems long, but
+remember the vast stretch of ages through which the earth has passed,
+and then reflect that no reason is apparent why our globe should not
+continue to be a scene of animation for ten thousand centuries yet to
+come. The fact that the little star Alcor placed so close to Mizar
+should accompany the latter in its flight is not surprising, but that
+two of the principal stars of the group should be found moving in a
+direction directly opposed to that pursued by the other five is
+surprising in the highest degree; and it recalls the strange theory of
+a double drift affecting all the stars, to which attention was called
+in the preceding chapter. It would appear that Benetnasch and Dubhe
+belong to one ``current,'' and Merak, Phaed, Megrez, Alioth, and Mizar
+to the other. As far as is known, the motion of the seven stars are
+not shared by the smaller stars scattered about them, but on the
+theory of currents there should be such a community of motion, and
+further investigation may reveal it.
+
+From the ``Great Dipper'' we turn to a constellation hardly less
+conspicuous and situated at an equal distance from the pole on the
+other side -- Cassiopeia. This famous star-group commemorating the
+romantic Queen of Ethiopia whose vain boasting of her beauty was
+punished by the exposure of her daughter Andromeda to the ``Sea
+Monster,'' is well-marked by five stars which form an irregular letter
+``W'' with its open side toward the pole. Three of these stars are
+usually ranked as of the second magnitude, and two of the third; but
+to ordinary observation they appear of nearly equal brightness, and
+present a very striking picture. They mark out the chair and a part of
+the figure of the beautiful queen. Beginning at the right-hand, or
+western, end of the ``W,'' their Greek letter designations are: Beta
+({\beta}), Alpha ({\alpha}), Gamma ({\gamma}), Delta ({\delta}), and
+Epsilon ({\epsilon}). Four of them, Beta, Alpha, Delta, and Epsilon
+are traveling eastwardly at various speeds, while the fifth, Gamma,
+moves in a westerly direction. The motion of Beta is more rapid than
+that of any of the others. It should be said, however, that no little
+uncertainty attaches to the estimates of the rate of motion of stars
+which are not going very rapidly, and different observers often vary
+considerably in their results.
+
+In the beautiful ``Northern Crown,'' one of the most perfect and
+charming of all the figures to be found in the stars, the alternate
+combining and scattering effects of the stellar motions are shown by
+comparing the appearance which the constellation must have had five
+hundred centuries ago with that which it has at present and that which
+it will have in the future. The seven principle stars of the asterism,
+forming a surprisingly perfect coronet, have movements in three
+directions at right angles to one another. That in these circumstances
+they should ever have arrived at positions giving them so striking an
+appearance of definite association is certainly surprising; from its
+aspect one would have expected to find a community of movement
+governing the brilliants of the ``Crown,'' but instead of that we find
+evidence that they will inevitably drift apart and the beautiful
+figure will dissolve.
+
+A similar fate awaits such asterisms as the ``Northern Cross'' in
+Cygnus; the ``Crow'' (Corvus), which stands on the back of the great
+``Sea Serpent,'' Hydra, and pecks at his scales; ``Job's Coffin''
+(Delphinus); the ``Great Square of Pegasus''; the ``Twins'' (Gemini);
+the beautiful ``Sickle'' in Leo; and the exquisite group of the Hyades
+in Taurus. In the case of the Hyades, two controlling movements are
+manifest: one, affecting five of the stars which form the well-known
+figure of a letter ``V,'' is directed northerly; the other, which
+controls the direction of two stars, has an easterly trend. The chief
+star of the group, Aldebaran, one of the finest of all stars both for
+its brilliance and its color, is the most affected by the easterly
+motion. In time it will drift entirely out of connection with its
+present neighbors. Although the Hyades do not form so compact a group
+as the Pleiades in the same constellation, yet their appearance of
+relationship is sufficient to awaken a feeling of surprise over the
+fact that, as with the stars of the ``Dipper,'' their association is
+only temporary or apparent.
+
+The great figure of Orion appears to be more lasting, not because its
+stars are physically connected, but because of their great distance,
+which renders their movements too deliberate to be exactly
+ascertained. Two of the greatest of its stars, Betelgeuse and Rigel,
+possess, as far as has been ascertained, no perceptible motion across
+the line of sight, but there is a little movement perceptible in the
+``Belt.'' At the present time this consists of an almost perfect
+straight line, a row of second-magnitude stars about equally spaced
+and of the most striking beauty. In the course of time, however, the
+two right-hand stars, Mintaka and Alnilam (how fine are these Arabic
+star names!) will approach each other and form a naked-eye double, but
+the third, Alnita, will drift away eastward, so that the ``Belt'' will
+no longer exist.
+
+For one more example, let us go to the southern hemisphere, whose most
+celebrated constellation, the ``Southern Cross,'' has found a place in
+all modern literatures, although it has no claim to consideration on
+account of association with ancient legends. This most attractive
+asterism, which has never ceased to fascinate the imagination of
+Christendom since it was first devoutly described by the early
+explorers of the South, is but a passing collocation of brilliant
+stars. Yet even in its transfigurations it has been for hundreds of
+centuries, and will continue to be for hundreds of centuries to come,
+a most striking object in the sky. Our figures show its appearance in
+three successive phases: first, as it was fifty thousand years ago
+(viewed from the earth's present location); second, as it is in our
+day; and, third, as it will be an equal time in the future. The
+nearness of these bright stars to one another -- the length of the
+longer beam of the ``Cross'' is only six degrees -- makes this group
+very noticeable, whatever the arrangement of its components may be.
+The largest star, at the base of the ``Cross,'' is of the first
+magnitude, two of the others are of the second magnitude, and the
+fourth is of the third. Other stars, not represented in the figures,
+increase the effect of a celestial blazonry, although they do not help
+the resemblance to a cross.
+
+But since the motion of the solar system itself will, in the course of
+so long a period as fifty thousand years, produce a great change in
+the perspective of the heavens as seen from the earth, by carrying us
+nearly nineteen trillion miles from our present place, why, it may be
+asked, seek to represent future appearances of the constellations
+which we could not hope to see, even if we could survive so long? The
+answer is: Because these things aid the mind to form a picture of the
+effects of the mobility of the starry universe. Only by showing the
+changes from some definite point of view can we arrive at a due
+comprehension of them. The constellations are more or less familiar to
+everybody, so that impending changes of their forms must at once
+strike the eye and the imagination, and make clearer the significance
+of the movements of the stars. If the future history of mankind is to
+resemble its past and if our race is destined to survive yet a million
+years, then our remote descendents will see a ``new heavens'' if not a
+``new earth,'' and will have to invent novel constellations to
+perpetuate their legends and mythologies.
+
+If our knowledge of the relative distances of the stars were more
+complete, it would be an interesting exercise in celestial geometry to
+project the constellations probably visible to the inhabitants of
+worlds revolving around some of the other suns of space. Our sun is
+too insignificant for us to think that he can make a conspicuous
+appearance among them, except, perhaps, in a few cases. As seen, for
+instance, from the nearest known star, Alpha Centauri, the sun would
+appear of the average first magnitude, and consequently from that
+standpoint he might be the gem of some little constellation which had
+no Sirius, or Arcturus, or Vega to eclipse him with its superior
+splendor. But from the distance of the vast majority of the stars the
+sun would probably be invisible to the naked eye, and as seen from
+nearer systems could only rank as a fifth or sixth magnitude star,
+unnoticed and unknown except by the star-charting astronomer.
+
+Conflagrations in the Heavens
+
+Suppose it were possible for the world to take fire and burn up -- as
+some pessimists think that it will do when the Divine wrath shall have
+sufficiently accumulated against it -- nobody out of our own little
+corner of space would ever be aware of the catastrophe! With all their
+telescopes, the astronomers living in the golden light of Arcturus or
+the diamond blaze of Canopus would be unable to detect the least
+glimmer of the conflagration that had destroyed the seat of Adam and
+his descendents, just as now they are totally ignorant of its
+existence.
+
+But at least fifteen times in the course of recorded history men
+looking out from the earth have beheld in the remote depths of space
+great outbursts of fiery light, some of them more splendidly luminous
+than anything else in the firmament except the sun! If they were
+conflagrations, how many million worlds like ours were required to
+feed their blaze?
+
+It is probable that ``temporary'' or ``new'' stars, as these wonderful
+apparitions are called, really are conflagrations; not in the sense of
+a bonfire or a burning house or city, but in that of a sudden eruption
+of inconceivable heat and light, such as would result from the
+stripping off the shell of an encrusted sun or the crashing together
+of two mighty orbs flying through space with a hundred times the
+velocity of the swiftest cannon-shot.
+
+Temporary stars are the rarest and most erratic of astronomical
+phenomena. The earliest records relating to them are not very clear,
+and we cannot in every instance be certain that it was one of these
+appearances that the ignorant and superstitious old chroniclers are
+trying to describe. The first temporary star that we are absolutely
+sure of appeared in 1572, and is known as ``Tycho's Star,'' because
+the celebrated Danish astronomer (whose remains, with his
+gold-and-silver artificial nose -- made necessary by a duel -- still
+intact, were disinterred and reburied in 1901) was the first to
+perceive it in the sky, and the most assiduous and successful in his
+studies of it. As the first fully accredited representative of its
+class, this new star made its entry upon the scene with becoming
+�clat. It is characteristic of these phenomena that they burst into
+view with amazing suddenness, and, of course, entirely unexpectedly.
+Tycho's star appeared in the constellation Cassiopeia, near a now
+well-known and much-watched little star named Kappa, on the evening of
+November 11, 1572. The story has often been repeated, but it never
+loses interest, how Tycho, going home that evening, saw people in the
+street pointing and staring at the sky directly over their heads, and
+following the direction of their hands and eyes he was astonished to
+see, near the zenith, an unknown star of surpassing brilliance. It
+outshone the planet Jupiter, and was therefore far brighter than the
+first magnitude. There was not another star in the heavens that could
+be compared with it in splendor. Tycho was not in all respects free
+from the superstitions of his time -- and who is? -- but he had the
+true scientific instinct, and immediately he began to study the
+stranger, and to record with the greatest care every change in its
+aspect. First he determined as well as he could with the imperfect
+instruments of his day, many of which he himself had invented, the
+precise location of the phenomena in the sky. Then he followed the
+changes that it underwent. At first it brightened until its light
+equaled or exceeded that of the planet Venus at her brightest, a
+statement which will be appreciated at its full value by anyone who
+has ever watched Venus when she plays her dazzling r�le of ``Evening
+Star,'' flaring like an arc light in the sunset sky. It even became so
+brilliant as to be visible in full daylight, since, its position being
+circumpolar, it never set in the latitude of Northern Europe. Finally
+it began to fade, turning red as it did so, and in March, 1574, it
+disappeared from Tycho's searching gaze, and has never been seen again
+from that day to this. None of the astronomers of the time could make
+anything of it. They had not yet as many bases of speculation as we
+possess today.
+
+Tycho's star has achieved a romantic reputation by being fancifully
+identified with the ``Star of Bethlehem,'' said to have led the
+wondering Magi from their eastern deserts to the cradle-manger of the
+Savior in Palestine. Many attempts have been made to connect this
+traditional ``star'' with some known phenomenon of the heavens, and
+none seems more idle than this. Yet it persistently survives, and no
+astronomer is free from eager questions about it addressed by people
+whose imagination has been excited by the legend. It is only necessary
+to say that the supposition of a connection between the phenomenon of
+the Magi and Tycho's star is without any scientific foundation. It was
+originally based on an unwarranted assumption that the star of Tycho
+was a variable of long period, appearing once every three hundred and
+fifteen years, or thereabout. If that were true there would have been
+an apparition somewhere near the traditional date of the birth of
+Christ, a date which is itself uncertain. But even the data on which
+the assumption was based are inconsistent with the theory. Certain
+monkish records speak of something wonderful appearing in the sky in
+the years 1264 and 945, and these were taken to have been outbursts of
+Tycho's star. Investigation shows that the records more probably refer
+to comets, but even if the objects seen were temporary stars, their
+dates do not suit the hypothesis; from 945 to 1264 there is a gap of
+319 years, and from 1264 to 1572 one of only 308 years; moreover 337
+years have now (1909) elapsed since Tycho saw the last glimmer of his
+star. Upon a variability so irregular and uncertain as that, even if
+we felt sure that it existed, no conclusion could be found concerning
+an apparition occurring 2000 years ago.
+
+In the year 1600 (the year in which Giordano Bruno was burned at the
+stake for teaching that there is more than one physical world), a
+temporary star of the third magnitude broke out in the constellation
+Cygnus, and curiously enough, considering the rarity of such
+phenomena, only four years later another surprisingly brilliant one
+appeared in the constellation Ophiuchus. This is often called
+``Kepler's star,'' because the great German astronomer devoted to it
+the same attention that Tycho had given to the earlier phenomenon. It,
+too, like Tycho's, was at first the brightest object in the stellar
+heavens, although it seems never to have quite equaled its famous
+predecessor in splendor. It disappeared after a year, also turning of
+a red color as it became more faint. We shall see the significance of
+this as we go on. Some of Kepler's contemporaries suggested that the
+outburst of this star was due to a meeting of atoms in space, and idea
+bearing a striking resemblance to the modern theory of ``astronomical
+collisions.''
+
+In 1670, 1848, and 1860 temporary stars made their appearance, but
+none of them was of great brilliance. In 1866 one of the second
+magnitude broke forth in the ``Northern Crown'' and awoke much
+interest, because by that time the spectroscope had begun to be
+employed in studying the composition of the stars, and Huggins
+demonstrated that the new star consisted largely of incandescent
+hydrogen. But this star, apparently unlike the others mentioned, was
+not absolutely new. Before its outburst it had shown as a star of the
+ninth magnitude (entirely invisible, of course, to the naked eye), and
+after about six weeks it faded to its original condition in which it
+has ever since remained. In 1876 a temporary star appeared in the
+constellation Cygnus, and attained at one time the brightness of the
+second magnitude. Its spectrum and its behavior resembled those of its
+immediate predecessor. In 1885, astronomers were surprised to see a
+sixth-magnitude star glimmering in the midst of the hazy cloud of the
+great Andromeda Nebula. It soon absolutely disappeared. Its spectrum
+was remarkable for being ``continuous,'' like that of the nebula
+itself. A continuous spectrum is supposed to represent a body, or a
+mass, which is either solid or liquid, or composed of gas under great
+pressure. In January, 1892, a new star was suddenly seen in the
+constellation Auriga. It never rose much above the fourth magnitude,
+but it showed a peculiar spectrum containing both bright and dark
+lines of hydrogen.
+
+But a bewildering surprise was now in store; the world was to behold
+at the opening of the twentieth century such a celestial spectacle as
+had not been on view since the times of Tycho and Kepler. Before
+daylight on the morning of February 22, 1901, the Rev. Doctor
+Anderson, of Edinburgh, an amateur astronomer, who had also been the
+first to see the new star in Auriga, beheld a strange object in the
+constellation Perseus not far from the celebrated variable star Algol.
+He recognized its character at once, and immediately telegraphed the
+news, which awoke the startled attention of astronomers all over the
+world. When first seen the new star was no brighter than Algol (less
+than the second magnitude), but within twenty-four hours it was
+ablaze, outshining even the brilliant Capella, and far surpassing the
+first magnitude. At the spot in the sky where it appeared nothing
+whatever was visible on the night before its coming. This is known
+with certainty because a photograph had been made of that very region
+on February 21, and this photograph showed everything down to the
+twelfth magnitude, but not a trace of the stranger which burst into
+view between the 21st and the 22nd like the explosion of a rocket.
+
+Upon one who knew the stars the apparition of this intruder in a
+well-known constellation had the effect of a sudden invasion. The new
+star was not far west of the zenith in the early evening, and in that
+position showed to the best advantage. To see Capella, the hitherto
+unchallenged ruler of that quarter of the sky, abased by comparison
+with this stranger of alien aspect, for there was always an unfamiliar
+look about the ``nova,'' was decidedly disconcerting. It seemed to
+portend the beginning of a revolution in the heavens. One could
+understand what the effect of such an apparition must have been in the
+superstitious times of Tycho. The star of Tycho had burst forth on the
+northern border of the Milky Way; this one was on its southern border,
+some forty-five degrees farther east.
+
+Astronomers were well-prepared this time for the scientific study of
+the new star, both astronomical photography and spectroscopy having
+been perfected, and the results of their investigations were
+calculated to increase the wonder with which the phenomenon was
+regarded. The star remained at its brightest only a few days; then,
+like a veritable conflagration, it began to languish; and, like the
+reflection of a dying fire, as it sank it began to glow with the red
+color of embers. But its changes were spasmodic; once about every
+three days it flared up only to die away again. During these
+fluctuations its light varied alternately in the ratio of one to six.
+Finally it took a permanent downward course, and after a few months
+the naked eye could no longer perceive it; but it remained visible
+with telescopes, gradually fading until it had sunk to the ninth
+magnitude. Then another astonishing change happened: in August
+photographs taken at the Yerkes Observatory and at Heidelberg showed
+that the ``nova'' was surrounded by a spiral nebula! The nebula had
+not been there before, and no one could doubt that it represented a
+phase of the same catastrophe that had produced the outburst of the
+new star. At one time the star seemed virtually to have disappeared,
+as if all its substance had been expanded into the nebulous cloud, but
+always there remained a stellar nucleus about which the misty spiral
+spread wider and ever wider, like a wave expanding around a center of
+disturbance. The nebula too showed a variability of brightness, and
+four condensations which formed in it seemed to have a motion of
+revolution about the star. As time went on the nebula continued to
+expand at a rate which was computed to be not less than twenty
+thousand miles per second! And now the star itself, showing
+indications of having turned into a nebula, behaved in a most erratic
+manner, giving rise to the suspicion that it was about to burst out
+again. But this did not occur, and at length it sunk into a state of
+lethargy from which it has to the present time not recovered. But the
+nebulous spiral has disappeared, and the entire phenomena as it now
+(1909) exists consists of a faint nebulous star of less than the ninth
+magnitude.
+
+The wonderful transformations just described had been forecast in
+advance of the discovery of the nebulous spiral encircling the star by
+the spectroscopic study of the latter. At first there was no
+suggestion of a nebular constitution, but within a month or two
+characteristic nebular lines began to appear, and in less than six
+months the whole spectrum had been transformed to the nebular type. In
+the mean time the shifting of the spectral lines indicated a
+complication of rapid motions in several directions simultaneously.
+These motions were estimated to amount to from one hundred to five
+hundred miles per second.
+
+The human mind is so constituted that it feels forced to seek an
+explanation of so marvelous a phenomenon as this, even in the absence
+of the data needed for a sound conclusion. The most natural
+hypothesis, perhaps, is that of a collision. Such a catastrophe could
+certainly happen. It has been shown, for instance, that in infinity of
+time the earth is sure to be hit by a comet; in the same way it may be
+asserted that, if no time limit is fixed, the sun is certain to run
+against some obstacle in space, either another star, or a dense meteor
+swarm, or one of the dark bodies which there is every reason to
+believe abound around us. The consequences of such a collision are
+easy to foretell, provided that we know the masses and the velocities
+of the colliding bodies. In a preceding chapter we have discussed the
+motions of the sun and stars, and have seen that they are so swift
+that an encounter between any two of them could not but be disastrous.
+But this is not all; for as soon as two stars approached within a few
+million miles their speed would be enormously increased by their
+reciprocal attractions and, if their motion was directed radially with
+respect to their centers, they would come together with a crash that
+would reduce them both to nebulous clouds. It is true that the chances
+of such a ``head-on'' collision are relatively very small; two stars
+approaching each other would most probably fall into closed orbits
+around their common center of gravity. If there were a collision it
+would most likely be a grazing one instead of a direct front-to-front
+encounter. But even a close approach, without any actual collision,
+would probably prove disastrous, owing to the tidal influence of each
+of the bodies on the other. Suns, in consequence of their enormous
+masses and dimensions and the peculiarities of their constitution, are
+exceedingly dangerous to one another at close quarters. Propinquity
+awakes in them a mutually destructive tendency. Consisting of matter
+in the gaseous, or perhaps, in some cases, liquid, state, their tidal
+pull upon each other if brought close together might burst them
+asunder, and the photospheric envelope being destroyed the internal
+incandescent mass would gush out, bringing fiery death to any planets
+that were revolving near. Without regard to the resulting disturbance
+of the earth's orbit, the close approach of a great star to the sun
+would be in the highest degree perilous to us. But this is a danger
+which may properly be regarded as indefinitely remote, since, at our
+present location in space, we are certainly far from every star except
+the sun, and we may feel confident that no great invisible body is
+near, for if there were one we should be aware of its presence from
+the effects of its attraction. As to dark nebul� which may possibly
+lie in the track that the solar system is pursuing at the rate of
+375,000,000 miles per year, that is another question -- and they, too,
+could be dangerous!
+
+This brings us directly back to ``Nova Persei,'' for among the many
+suggestions offered to explain its outburst, as well as those of other
+temporary stars, one of the most fruitful is that of a collision
+between a star and a vast invisible nebula. Professor Seeliger, of
+Munich, first proposed this theory, but it afterward underwent some
+modifications from others. Stated in a general form, the idea is that
+a huge dark body, perhaps an extinguished sun, encountered in its
+progress through space a widespread flock of small meteors forming a
+dark nebula. As it plunged into the swarm the friction of the
+innumerable collisions with the meteors heated its surface to
+incandescence, and being of vast size it then became visible to us as
+a new star. Meanwhile the motion of the body through the nebula, and
+its rotation upon itself, set up a gyration in the blazing atmosphere
+formed around it by the vaporized meteors; and as this atmosphere
+spread wider, under the laws of gyratory motion a rotation in the
+opposite direction began in the inflamed meteoric cloud outside the
+central part of the vortex. Thus the spectral lines were caused to
+show motion in opposite directions, a part of the incandescent mass
+approaching the earth simultaneously with the retreat of another part.
+So the curious spectroscopic observations before mentioned were
+explained. This theory might also account for the appearance of the
+nebulous spiral first seen some six months after the original
+outburst. The sequent changes in the spectrum of the ``nova'' are
+accounted for by this theory on the assumption, reasonable enough in
+itself, that at first the invading body would be enveloped in a
+vaporized atmosphere of relatively slight depth, producing by its
+absorption the fine dark lines first observed; but that as time went
+on and the incessant collisions continued, the blazing atmosphere
+would become very deep and extensive, whereupon the appearance of the
+spectral lines would change, and bright lines due to the light of the
+incandescent meteors surrounding the nucleus at a great distance would
+take the place of the original dark ones. The vortex of meteors once
+formed would protect the flying body within from further immediate
+collisions, the latter now occurring mainly among the meteors
+themselves, and then the central blaze would die down, and the
+original splendor of the phenomenon would fade.
+
+But the theories about Nova Persei have been almost as numerous as the
+astronomers who have speculated about it. One of the most startling of
+them assumed that the outburst was caused by the running amuck of a
+dark star which had encountered another star surrounded with planets,
+the renewed outbreaks of light after the principal one had faded being
+due to the successive running down of the unfortunate planets! Yet
+another hypothesis is based on what we have already said of the tidal
+influence that two close approaching suns would have upon each other.
+Supposing two such bodies which had become encrusted, but remained
+incandescent and fluid within, to approach within almost striking
+distance; they would whirl each other about their common center of
+gravity, and at the same time their shells would burst under the tidal
+strain, and their glowing nuclei being disclosed would produce a great
+outburst of light. Applying this theory to a ``nova,'' like that of
+1866 in the ``Northern Crown,'' which had been visible as a small star
+before the outbreak, and which afterward resumed its former aspect, we
+should have to assume that a yet shining sun had been approached by a
+dark body whose attraction temporarily burst open its photosphere. It
+might be supposed that in this case the dark body was too far advanced
+in cooling to suffer the same fate from the tidal pull of its victim.
+But a close approach of that kind would be expected to result in the
+formation of a binary system, with orbits of great eccentricity,
+perhaps, and after the lapse of a certain time the outburst should be
+renewed by another approximation of the two bodies. A temporary star
+of that kind would rather be ranked as a variable.
+
+The celebrated French astronomer, Janssen, had a different theory of
+Nova Persei, and of temporary stars in general. According to his idea,
+such phenomena might be the result of chemical changes taking place in
+a sun without interference by, or collision with, another body.
+Janssen was engaged for many years in trying to discover evidence of
+the existence of oxygen in the sun, and he constructed his observatory
+on the summit of Mount Blanc specially to pursue that research. He
+believed that oxygen must surely exist in the sun since we find so
+many other familiar elements included in the constitution of the solar
+globe, and as he was unable to discover satisfactory evidence of its
+presence he assumed that it existed in a form unknown on the earth. If
+it were normally in the sun's chromosphere, or coronal atmosphere, he
+said, it would combine with the hydrogen which we know is there and
+form an obscuring envelope of water vapor. It exists, then, in a
+special state, uncombined with hydrogen; but let the temperature of
+the sun sink to a critical point and the oxygen will assume its normal
+properties and combine with the hydrogen, producing a mighty outburst
+of light and heat. This, Janssen thought, might explain the phenomena
+of the temporary stars. It would also, he suggested, account for their
+brief career, because the combination of the elements would be quickly
+accomplished, and then the resulting water vapor would form an
+atmosphere cutting off the radiation from the star within.
+
+This theory may be said to have a livelier human interest than some of
+the others, since, according to it, the sun may carry in its very
+constitution a menace to mankind; one does not like to think of it
+being suddenly transformed into a gigantic laboratory for the
+explosive combination of oxygen and hydrogen! But while Janssen's
+theory might do for some temporary stars, it is inadequate to explain
+all the phenomena of Nova Persei, and particularly the appearance of
+the great spiral nebula that seemed to exhale from the heart of the
+star. Upon the whole, the theory of an encounter between a star and a
+dark nebula seems best to fit the observations. By that hypothesis the
+expanding billow of light surrounding the core of the conflagration is
+very well accounted for, and the spectroscopic peculiarities are also
+explained.
+
+Dr Gustov Le Bon offers a yet more alarming theory, suggesting that
+temporary stars are the result of atomic explosion; but we shall touch
+upon this more fully in Chapter 14.
+
+Twice in the course of this discussion we have called attention to the
+change of color invariably undergone by temporary stars in the later
+stages of their career. This was conspicuous with Nova Persei which
+glowed more and more redly as it faded, until the nebulous light began
+to overpower that of the stellar nucleus. Nothing could be more
+suggestive of the dying out of a great fire. Moreover, change of color
+from white to red is characteristic of all variable stars of long
+period, such as ``Mira'' in Cetus. It is also characteristic of stars
+believed to be in the later stages of evolution, and consequently
+approaching extinction, like Antares and Betelgeuse, and still more
+notably certain small stars which ``gleam like rubies in the field of
+the telescope.'' These last appear to be suns in the closing period of
+existence as self-luminous bodies. Between the white stars, such as
+Sirius and Rigel, and the red stars, such as Aldebaran and Alpha
+Herculis, there is a progressive series of colors from golden yellow
+through orange to deep red. The change is believed to be due to the
+increase of absorbing vapors in the stellar atmosphere as the body
+cools down. In the case of ordinary stars these changes no doubt
+occupy many millions of years, which represent the average duration of
+solar life; but the temporary stars run through similar changes in a
+few months: they resemble ephemeral insects -- born in the morning and
+doomed to perish with the going down of the sun.
+
+Explosive and Whirling Nebul�
+
+One of the most surprising triumphs of celestial photography was
+Professor Keeler's discovery, in 1899, that the great majority of the
+nebul� have a distinctly spiral form. This form, previously known in
+Lord Rosse's great ``Whirlpool Nebula,'' had been supposed to be
+exceptional; now the photographs, far excelling telescopic views in
+the revelation of nebular forms, showed the spiral to be the typical
+shape. Indeed, it is a question whether all nebul� are not to some
+extent spiral. The extreme importance of this discovery is shown in
+the effect that it has had upon hitherto prevailing views of solar and
+planetary evolution. For more than three-quarters of a century
+Laplace's celebrated hypothesis of the manner of origin of the solar
+system from a rotating and contracting nebula surrounding the sun had
+guided speculation on that subject, and had been tentatively extended
+to cover the evolution of systems in general. The apparent forms of
+some of the nebul� which the telescope had revealed were regarded, and
+by some are still regarded, as giving visual evidence in favor of this
+theory. There is a ``ring nebula'' in Lyra with a central star, and a
+``planetary nebula'' in Gemini bearing no little resemblance to the
+planet Saturn with its rings, both of which appear to be practical
+realizations of Laplace's idea, and the elliptical rings surrounding
+the central condensation of the Andromeda Nebula may be cited for the
+same kind of proof.
+
+But since Keeler's discovery there has been a decided turning away of
+speculation another way. The form of the spiral nebul� seems to be
+entirely inconsistent with the theory of an originally globular or
+disk-shaped nebula condensing around a sun and throwing or leaving off
+rings, to be subsequently shaped into planets. Some astronomers,
+indeed, now reject Laplace's hypothesis in toto, preferring to think
+that even our solar system originated from a spiral nebula. Since the
+spiral type prevails among the existing nebul�, we must make any
+mechanical theory of the development of stars and planetary systems
+from them accord with the requirements which that form imposes. A
+glance at the extraordinary variations upon the spiral which Professor
+Keeler's photographs reveal is sufficient to convince one of the
+difficulty of the task of basing a general theory upon them. In truth,
+it is much easier to criticize Laplace's hypothesis than to invent a
+satisfactory substitute for it. If the spiral nebul� seem to oppose it
+there are other nebul� which appear to support it, and it may be that
+no one fixed theory can account for all the forms of stellar evolution
+in the universe. Our particular planetary system may have originated
+very much as the great French mathematician supposed, while others
+have undergone, or are now undergoing, a different process of
+development. There is always a too strong tendency to regard an
+important new discovery and the theories and speculations based upon
+it as revolutionizing knowledge, and displacing or overthrowing
+everything that went before. Upon the plea that ``Laplace only made a
+guess'' more recent guesses have been driven to extremes and treated
+by injudicious exponents as ``the solid facts at last.''
+
+Before considering more recent theories than Laplace's, let us see
+what the nature of the photographic revelations is. The vast celestial
+maelstrom discovered by Lord Rosse in the ``Hunting Dogs'' may be
+taken as the leading type of the spiral nebul�, although there are
+less conspicuous objects of the kind which, perhaps, better illustrate
+some of their peculiarities. Lord Rosse's nebula appears far more
+wonderful in the photographs than in his drawings made with the aid of
+his giant reflecting telescope at Parsonstown, for the photographic
+plate records details that no telescope is capable of showing. Suppose
+we look at the photograph of this object as any person of common sense
+would look at any great and strange natural phenomenon. What is the
+first thing that strikes the mind? It is certainly the appearance of
+violent whirling motion. One would say that the whole glowing mass had
+been spun about with tremendous velocity, or that it had been set
+rotating so rapidly that it had become the victim of ``centrifugal
+force,'' one huge fragment having broken loose and started to gyrate
+off into space. Closer inspection shows that in addition to the
+principal focus there are various smaller condensations scattered
+through the mass. These are conspicuous in the spirals. Some of them
+are stellar points, and but for the significance of their location we
+might suppose them to be stars which happen to lie in a line between
+us and the nebula. But when we observe how many of them follow most
+faithfully the curves of the spirals we cannot but conclude that they
+form an essential part of the phenomenon; it is not possible to
+believe that their presence in such situations is merely fortuitous.
+One of the outer spirals has at least a dozen of these star-like
+points strung upon it; some of them sharp, small, and distinct, others
+more blurred and nebulous, suggesting different stages of
+condensation. Even the part which seems to have been flung loose from
+the main mass has, in addition to its central condensation, at least
+one stellar point gleaming in the half-vanished spire attached to it.
+Some of the more distant stars scattered around the ``whirlpool'' look
+as if they too had been shot out of the mighty vortex, afterward
+condensing into unmistakable solar bodies. There are at least two
+curved rows of minute stars a little beyond the periphery of the
+luminous whirl which clearly follow lines concentric with those of the
+nebulous spirals. Such facts are simply dumbfounding for anyone who
+will bestow sufficient thought upon them, for these are suns, though
+they may be small ones; and what a birth is that for a sun!
+
+Look now again at the glowing spirals. We observe that hardly have
+they left the central mass before they begin to coagulate. In some
+places they have a ``ropy'' aspect; or they are like peascods filled
+with growing seeds, which eventually will become stars. The great
+focus itself shows a similar tendency, especially around its
+circumference. The sense that it imparts of a tremendous shattering
+force at work is overwhelming. There is probably more matter in that
+whirling and bursting nebula than would suffice to make a hundred
+solar systems! It must be confessed at once that there is no
+confirmation of the Laplacean hypothesis here; but what hypothesis
+will fit the facts? There is one which it has been claimed does so,
+but we shall come to that later. In the meanwhile, as a preparation,
+fix in the memory the appearance of that second spiral mass spinning
+beside its master which seems to have spurned it away.
+
+For a second example of the spiral nebul� look at the one in the
+constellation Triangulum. God, how hath the imagination of puny man
+failed to comprehend Thee! Here is creation through destruction with a
+vengeance! The spiral form of the nebula is unmistakable, but it is
+half obliterated amid the turmoil of flying masses hurled away on all
+sides with tornadic fury. The focus itself is splitting asunder under
+the intolerable strain, and in a little while, as time is reckoned in
+the Cosmos, it will be gyrating into stars. And then look at the
+cyclonic rain of already finished stars whirling round the outskirts
+of the storm. Observe how scores of them are yet involved in the
+fading streams of the nebulous spirals; see how they have been thrown
+into vast loops and curves, of a beauty that half redeems the terror
+of the spectacle enclosed within their lines -- like iridescent cirri
+hovering about the edges of a hurricane. And so again are suns born!
+
+Let us turn to the exquisite spiral in Ursa Major; how different its
+aspect from that of the other! One would say that if the terrific coil
+in Triangulum has all but destroyed itself in its fury, this one on
+the contrary has just begun its self-demolition. As one gazes one
+seems to see in it the smooth, swift, accelerating motion that
+precedes catastrophe. The central part is still intact, dense, and
+uniform in texture. How graceful are the spirals that smoothly rise
+from its oval rim and, gemmed with little stars, wind off into the
+darkness until they have become as delicate as threads of gossamer!
+But at bottom the story told here is the same -- creation by gyration!
+
+Compare with the above the curious mass in Cetus. Here the plane of
+the whirling nebula nearly coincides with our line of sight and we see
+the object at a low angle. It is far advanced and torn to shreds, and
+if we could look at it perpendicularly to its plane it is evident that
+it would closely resemble the spectacle in Triangulum.
+
+Then take the famous Andromeda Nebula (see Frontispiece), which is so
+vast that notwithstanding its immense distance even the naked eye
+perceives it as an enigmatical wisp in the sky. Its image on the
+sensitive plate is the masterpiece of astronomical photography; for
+wild, incomprehensible beauty there is nothing that can be compared
+with it. Here, if anywhere, we look upon the spectacle of creation in
+one of its earliest stages. The Andromeda Nebula is apparently less
+advanced toward transformation into stellar bodies than is that in
+Triangulum. The immense crowd of stars sprinkled over it and its
+neighborhood seem in the main to lie this side of the nebula, and
+consequently to have no connection with it. But incipient stars (in
+some places clusters of them) are seen in the nebulous rings, while
+one or two huge masses seem to give promise of transformation into
+stellar bodies of unusual magnitude. I say ``rings'' because although
+the loops encompassing the Andromeda Nebula have been called spirals
+by those who wish utterly to demolish Laplace's hypothesis, yet they
+are not manifestly such, as can be seen on comparing them with the
+undoubted spirals of the Lord Rosse Nebula. They look quite as much
+like circles or ellipses seen at an angle of, say, fifteen or twenty
+degrees to their plane. If they are truly elliptical they accord
+fairly well with Laplace's idea, except that the scale of magnitude is
+stupendous, and if the Andromeda Nebula is to become a solar system it
+will surpass ours in grandeur beyond all possibility of comparison.
+
+There is one circumstance connected with the spiral nebul�, and
+conspicuous in the Andromeda Nebula on account of its brightness,
+which makes the question of their origin still more puzzling; they all
+show continuous spectra, which, as we have before remarked, indicate
+that the mass from which the light comes is either solid or liquid, or
+a gas under heavy pressure. Thus nebul� fall into two classes: the
+``white'' nebul�, giving a continuous spectrum; and the ``green''
+nebul� whose spectra are distinctly gaseous. The Andromeda Nebula is
+the great representative of the former class and the Orion Nebula of
+the latter. The spectrum of the Andromeda Nebula has been interpreted
+to mean that it consists not of luminous gas, but of a flock of stars
+so distant that they are separately indistinguishable even with
+powerful telescopes, just as the component stars of the Milky Way are
+indistinguishable with the naked eye; and upon this has been based the
+suggestion that what we see in Andromeda is an outer universe whose
+stars form a series of elliptical garlands surrounding a central mass
+of amazing richness. But this idea is unacceptable if for no other
+reason than that, as just said, all the spiral nebul� possess the same
+kind of spectrum, and probably no one would be disposed to regard them
+all as outer universes. As we shall see later, the peculiarity of the
+spectra of the spiral nebul� is appealed to in support of a modern
+substitute for Laplace's hypothesis.
+
+Finally, without having by any means exhausted the variety exhibited
+by the spiral nebul�, let us turn to the great representative of the
+other species, the Orion Nebula. In some ways this is even more
+marvelous than the others. The early drawings with the telescope
+failed to convey an adequate conception either of its sublimity or of
+its complication of structure. It exists in a nebulous region of
+space, since photographs show that nearly the whole constellation is
+interwoven with faintly luminous coils. To behold the entry of the
+great nebula into the field even of a small telescope is a startling
+experience which never loses its novelty. As shown by the photographs,
+it is an inscrutable chaos of perfectly amazing extent, where spiral
+bands, radiating streaks, dense masses, and dark yawning gaps are
+strangely intermingled without apparent order. In one place four
+conspicuous little stars, better seen in a telescope than in the
+photograph on account of the blurring produced by over-exposure, are
+suggestively situated in the midst of a dark opening, and no observer
+has ever felt any doubt that these stars have been formed from the
+substance of the surrounding nebula. There are many other stars
+scattered over its expanse which manifestly owe their origin to the
+same source. But compare the general appearance of this nebula with
+the others that we have studied, and remark the difference. If the
+unmistakably spiral nebul� resemble bursting fly-wheels or grindstones
+from whose perimeters torrents of sparks are flying, the Orion Nebula
+rather recalls the aspect of a cloud of smoke and fragments produced
+by the explosion of a shell. This idea is enforced by the look of the
+outer portion farthest from the bright half of the nebula, where
+sharply edged clouds with dark spaces behind seem to be billowing away
+as if driven by a wind blowing from the center.
+
+Next let us consider what scientific speculation has done in the
+effort to explain these mysteries. Laplace's hypothesis can certainly
+find no standing ground either in the Orion Nebula or in those of a
+spiral configuration, whatever may be its situation with respect to
+the grand Nebula of Andromeda, or the ``ring'' and ``planetary''
+nebul�. Some other hypothesis more consonant with the appearances must
+be found. Among the many that have been proposed the most elaborate is
+the ``Planetesimal Hypothesis'' of Professors Chamberlin and Moulton.
+It is to be remarked that it applies to the spiral nebul�
+distinctively, and not to an apparently chaotic mass of gas like the
+vast luminous cloud in Orion. The gist of the theory is that these
+curious objects are probably the result of close approaches to each
+other of two independent suns, reminding us of what was said on this
+subject when we were dealing with temporary stars. Of the previous
+history of these appulsing suns the theory gives us no account; they
+are simply supposed to arrive within what may be called an effective
+tide-producing distance, and then the drama begins. Some of the
+probable consequences of such an approach have been noticed in Chapter
+5; let us now consider them a little more in detail.
+
+Tides always go in couples; if there is a tide on one side of a globe
+there will be a corresponding tide on the other side. The cause is to
+be found in the law that the force of gravitation varies inversely as
+the square of the distance; the attraction on the nearest surface of
+the body exercised by another body is greater than on its center, and
+greater yet than on its opposite surface. If two great globes attract
+each other, each tends to draw the other out into an ellipsoidal
+figure; they must be more rigid than steel to resist this -- and even
+then they cannot altogether resist. If they are liquid or gaseous they
+will yield readily to the force of distortion, the amount of which
+will depend upon their distance apart, for the nearer they are the
+greater becomes the tidal strain. If they are encrusted without and
+liquid or gaseous in the interior, the internal mass will strive to
+assume the figure demanded by the tidal force, and will, if it can,
+burst the restraining envelope. Now this is virtually the predicament
+of the body we call a sun when in the immediate presence of another
+body of similarly great mass. Such a body is presumably gaseous
+throughout, the component gases being held in a state of rigidity by
+the compression produced by the tremendous gravitational force of
+their own aggregate mass. At the surface such a body is enveloped in a
+shell of relatively cool matter. Now suppose a great attracting body,
+such as another sun, to approach near enough for the difference in its
+attraction on the two opposite sides of the body and on its center to
+become very great; the consequence will be a tidal deformation of the
+whole body, and it will lengthen out along the line of the
+gravitational pull and draw in at the sides, and if its shell offers
+considerable resistance, but not enough to exercise a complete
+restraint, it will be violently burst apart, or blown to atoms, and
+the internal mass will leap out on the two opposite sides in great
+fiery spouts. In the case of a sun further advanced in cooling than
+ours the interior might be composed of molten matter while the
+exterior crust had become rigid like the shell of an egg; then the
+force of the ``tidal explosion'' produced by the appulse of another
+sun would be more violent in consequence of the greater resistance
+overcome. Such, then, is the mechanism of the first phase in the
+history of a spiral nebula according to the Planetesimal Hypothesis.
+Two suns, perhaps extinguished ones, have drawn near together, and an
+explosive outburst has occured in one or both. The second phase calls
+for a more agile exercise of the imagination.
+
+To simplify the case, let us suppose that only one of the tugging suns
+is seriously affected by the strain. Its vast wings produced by the
+outburst are twisted into spirals by their rotation and the contending
+attractions exercised upon them, as the two suns, like battleships in
+desperate conflict, curve round each other, concentrating their
+destructive energies. Then immense quantities of d�bris are scattered
+about in which eddies are created, and finally, as the sun that caused
+the damage goes on its way, leaving its victim to repair its injuries
+as it may, the dispersed matter cools, condenses, and turns into
+streams of solid particles circling in elliptical paths about their
+parent sun. These particles, or fragments, are the ``planetesimals''
+of the theory. In consequence of the inevitable intersection of the
+orbits of the planetesimals, nodes are formed where the flying
+particles meet, and at these nodes large masses are gradually
+accumulated. The larger the mass the greater its attraction, and at
+last the nodal points become the nuclei of great aggregations from
+which planets are shaped.
+
+This, in very brief form, is the Planetesimal Hypothesis which we are
+asked to substitute for that based on Laplace's suggestion as an
+explanation of the mode of origin of the solar system; and the
+phenomena of the spiral nebul� are appealed to as offering evident
+support to the new hypothesis. We are reminded that they are
+elliptical in outline, which accords with the hypothesis; that their
+spectra are not gaseous, which shows that they may be composed of
+solid particles like the planetesimals; and that their central masses
+present an oval form, which is what would result from the tidal
+effects, as just described. We also remember that some of them, like
+the Lord Rosse and the Andromeda nebul�, are visually double, and in
+these cases we might suppose that the two masses represent the
+tide-burst suns that ventured into too close proximity. It may be
+added that the authors of the theory do not insist upon the appulse of
+two suns as the only way in which the planetesimals may have
+originated, but it is the only supposition that has been worked out.
+
+But serious questions remain. It needs, for instance, but a glance at
+the Triangulum monster to convince the observer that it cannot be a
+solar system which is being evolved there, but rather a swarm of
+stars. Many of the detached masses are too vast to admit of the
+supposition that they are to be transformed into planets, in our sense
+of planets, and the distances of the stars which appear to have been
+originally ejected from the focal masses are too great to allow us to
+liken the assemblage that they form to a solar system. Then, too, no
+nodes such as the hypothesis calls for are visible. Moreover, in most
+of the spiral nebul� the appearances favor the view that the
+supposititious encountering suns have not separated and gone each
+rejoicing on its way, after having inflicted the maximum possible
+damage on its opponent, but that, on the contrary, they remain in
+close association like two wrestlers who cannot escape from each
+other's grasp. And this is exactly what the law of gravitation
+demands; stars cannot approach one another with impunity, with regard
+either to their physical make-up or their future independence of
+movement. The theory undertakes to avoid this difficulty by assuming
+that in the case of our system the approach of the foreign body to the
+sun was not a close one -- just close enough to produce the tidal
+extrusion of the relatively insignificant quantity of matter needed to
+form the planets. But even then the effect of the appulse would be to
+change the direction of flight, both of the sun and of its visitor,
+and there is no known star in the sky which can be selected as the
+sun's probable partner in their ancient pas deux. That there are
+unconquered difficulties in Laplace's hypothesis no one would deny,
+but in simplicity of conception it is incomparably more satisfactory,
+and with proper modifications could probably be made more consonant
+with existing facts in our solar system than that which is offered to
+replace it. Even as an explanation of the spiral nebul�, not as solar
+systems in process of formation, but as the birthplaces of stellar
+clusters, the Planetesimal Hypothesis would be open to many
+objections. Granting its assumptions, it has undoubtedly a strong
+mathematical framework, but the trouble is not with the mathematics
+but with the assumptions. Laplace was one of the ablest mathematicians
+that ever lived, but he had never seen a spiral nebula; if he had, he
+might have invented a hypothesis to suit its phenomena. His actual
+hypothesis was intended only for our solar system, and he left it in
+the form of a ``note'' for the consideration of his successors, with
+the hope that they might be able to discover the full truth, which he
+confessed was hidden from him. It cannot be said that that truth has
+yet been found, and when it is found the chances are that intuition
+and not logic will have led to it.
+
+The spiral nebul�, then, remain among the greatest riddles of the
+universe, while the gaseous nebul�, like that of Orion, are no less
+mysterious, although it seems impossible to doubt that both forms give
+birth to stars. It is but natural to look to them for light on the
+question of the origin of our planetary system; but we should not
+forget that the scale of the phenomena in the two cases is vastly
+different, and the forces in operation may be equally different. A
+hill may have been built up by a glacier, while a mountain may be the
+product of volcanic forces or of the upheaval of the strata of the
+planet.
+
+The Banners of the Sun
+
+As all the world knows, the sun, a blinding globe pouring forth an
+inconceivable quantity of light and heat, whose daily passage through
+the sky is caused by the earth's rotation on its axis, constitutes the
+most important phenomenon of terrestial existence. Viewed with a dark
+glass to take off the glare, or with a telescope, its rim is seen to
+be a sharp and smooth circle, and nothing but dark sky is visible
+around it. Except for the interference of the moon, we should probably
+never have known that there is any more of the sun than our eyes
+ordinarily see.
+
+But when an eclipse of the sun occurs, caused by the interposition of
+the opaque globe of the moon, we see its immediate surroundings, which
+in some respects are more wonderful than the glowing central orb.
+These surroundings, although not in the sense in which we apply the
+term to the gaseous envelope of the earth, may be called the sun's
+atmosphere. They consist of two very different parts -- first, the red
+``prominences,'' which resemble tongues of flame ascending thousands
+of miles above the sun's surface; and, second, the ``corona,'' which
+extends to distances of millions of miles from the sun, and shines
+with a soft, glowing light. The two combined, when well seen, make a
+spectacle without parallel among the marvels of the sky. Although many
+attempts have been made to render the corona visible when there is no
+eclipse, all have failed, and it is to the moon alone that we owe its
+revelation. To cover the sun's disk with a circular screen will not
+answer the purpose because of the illumination of the air all about
+the observer. When the moon hides the sun, on the other hand, the
+sunlight is withdrawn from a great cylinder of air extending to the
+top of the atmosphere and spreading many miles around the observer.
+There is then no glare to interfere with the spectacle, and the corona
+appears in all its surprising beauty. The prominences, however,
+although they were discovered during an eclipse, can now, with the aid
+of the spectroscope, be seen at any time. But the prominences are
+rarely large enough to be noticed by the naked eye, while the
+streamers of the corona, stretching far away in space, like ghostly
+banners blown out from the black circle of the obscuring moon, attract
+every eye, and to this weird apparition much of the fear inspired by
+eclipses has been due. But if the corona has been a cause of terror in
+the past it has become a source of growing knowledge in our time.
+
+The story of the first scientific observation of the corona and the
+prominences is thrillingly interesting, and in fact dramatic. The
+observation was made during the eclipse of 1842, which fortunately was
+visible all over Central and Southern Europe so that scores of
+astronomers saw it. The interest centers in what happened at Pavia in
+Northern Italy, where the English astronomer Francis Baily had set up
+his telescope. The eclipse had begun and Bailey was busy at his
+telescope when, to quote his own words in the account which he wrote
+for the Memoirs of the Royal Astronomical Society:
+
+  I was astounded by a tremendous burst of applause from the streets
+  below, and at the same moment was electrified by the sight of one
+  of the most brilliant and splendid phenomena that can well be
+  imagined; for at that instant the dark body of the moon was
+  suddenly surrounded with a corona, or kind of bright glory, similar
+  in shape and magnitude to that which painters draw round the heads
+  of saints...
+
+  Pavia contains many thousand inhabitants, the major part of whom
+  were at this early hour walking about the streets and squares or
+  looking out of windows in order to witness this long-talked-of
+  phenomenon; and when the total obscuration took place, which was
+  instantaneous, there was a universal shout from every observer
+  which ``made the welkin ring,'' and for the moment withdrew my
+  attention from the object with which I was immediately occupied. I
+  had, indeed, expected the appearance of a luminous circle round the
+  moon during the time of total obscurity; but I did not expect, from
+  any of the accounts of preceding eclipses that I had read, to
+  witness so magnificent an exhibition as that which took place...
+
+  Splendid and astonishing, however, as this remarkable phenomenon
+  really was, and although it could not fail to call forth the
+  admiration and applause of every beholder, yet I must confess that
+  there was at the same time something in its singular and wonderful
+  appearance that was appalling...
+
+  But the most remarkable circumstance attending the phenomenon was
+  the appearance of three large protuberances apparently emanating
+  from the circumference of the moon, but evidently forming a portion
+  of the corona. They had the appearance of mountains of a prodigious
+  elevation; their color was red tinged with lilac or purple; perhaps
+  the color of the peach-blossom would more nearly represent it. They
+  somewhat resembled the tops of the snowy Alpine mountains when
+  colored by the rising or the setting sun. They resembled the Alpine
+  mountains in another respect, inasmuch as their light was perfectly
+  steady, and had none of that flickering or sparkling motion so
+  visible in other parts of the corona...
+
+  The whole of these protuberances were visible even to the last
+  moment of total obscuration, and when the first ray of light was
+  admitted from the sun they vanished, with the corona, altogether,
+  and daylight was instantly restored.
+
+I have quoted nearly all of this remarkable description not alone for
+its intrinsic interest, but because it is the best depiction that can
+be found of the general phenomena of a total solar eclipse. Still, not
+every such eclipse offers an equally magnificent spectacle. The
+eclipses of 1900 and 1905, for instance, which were seen by the
+writer, the first in South Carolina and the second in Spain, fell far
+short of that described by Bailey in splendor and impressiveness. Of
+course, something must be allowed for the effect of surprise; Bailey
+had not expected to see what was so suddenly disclosed to him. But
+both in 1900 and 1905 the amount of scattered light in the sky was
+sufficient in itself to make the corona appear faint, and there were
+no very conspicuous prominences visible. Yet on both occasions there
+was manifest among the spectators that mingling of admiration and awe
+of which Bailey speaks. The South Carolinians gave a cheer and the
+ladies waved their handkerchiefs when the corona, ineffably delicate
+of form and texture, melted into sight and then in two minutes melted
+away again. The Spaniards, crowded on the citadel hill of Burgos, with
+their king and his royal retinue in their midst, broke out with a
+great clapping of hands as the awaited spectacle unfolded itself in
+the sky; and on both occasions, before the applause began, after an
+awed silence a low murmur ran through the crowds. At Burgos it is said
+many made the sign of the cross.
+
+It was not long before Bailey's idea that the prominences were a part
+of the corona was abandoned, and it was perceived that the two
+phenomena were to a great extent independent. At the eclipse of 1868,
+which the astronomers, aroused by the wonderful scene of 1842, and
+eager to test the powers of the newly invented spectroscope, flocked
+to India to witness, Janssen conceived the idea of employing the
+spectroscope to render the prominences visible when there was no
+eclipse. He succeeded the very next day, and these phenomena have been
+studied in that way ever since.
+
+There are recognized two kinds of prominences -- the ``erruptive'' and
+the ``quiescent.'' The latter, which are cloud-like in form, may be
+seen almost anywhere along the edge of the sun; but the former, which
+often shoot up as if hurled from mighty volcanoes, appear to be
+associated with sun-spots, and appear only above the zones where spots
+abound. Either of them, when seen in projection against the brilliant
+solar disk, appears white, not red, as against a background of sky.
+The quiescent prominences, whose elevation is often from forty
+thousand to sixty thousand miles, consist, as the spectroscope shows,
+mainly of hydrogen and helium. The latter, it will be remembered, is
+an element which was known to be in the sun many years before the
+discovery that it also exists in small quantities on the earth. A fact
+which may have a significance which we cannot at present see is that
+the emanation from radium gradually and spontaneously changes into
+helium, an alchemistical feat of nature that has opened many curious
+vistas to speculative thinkers. The eruptive prominences, which do not
+spread horizontally like the others, but ascend with marvelous
+velocity to elevations of half a million miles or more, are apparently
+composed largely of metallic vapors -- i.e. metals which are usually
+solid on the earth, but which at solar temperatures are kept in a
+volatilized state. The velocity of their ascent occasionally amounts
+to three hundred or four hundred miles per second. It is known from
+mathematical considerations that the gravitation of the sun would not
+be able to bring back any body that started from its surface with a
+velocity exceeding three hundred and eighty-three miles per second; so
+it is evident that some of the matter hurled forth in eruptive
+prominences may escape from solar control and go speeding out into
+space, cooling and condensing into solid masses. There seems to be no
+reason why some of the projectiles from the sun might not reach the
+planets. Here, then, we have on a relatively small scale, explosions
+recalling those which it has been imagined may be the originating
+cause of some of the sudden phenomena of the stellar heavens.
+
+Of the sun-spots it is not our intention here specifically to speak,
+but they evidently have an intimate connection with eruptive
+prominences, as well as some relation, not yet fully understood, with
+the corona. Of the real cause of sun-spots we know virtually nothing,
+but recent studies by Professor Hale and others have revealed a
+strange state of things in the clouds of metallic vapors floating
+above them and their surroundings. Evidences of a cyclonic tendency
+have been found, and Professor Hale has proved that sun-spots are
+strong magnetic fields, and consist of columns of ionized vapors
+rotating in opposite directions in the two hemispheres. A fact which
+may have the greatest significance is that titanium and vanadium have
+been found both in sun-spots and in the remarkable variable Mira Ceti,
+a star which every eleven months, or thereabout, flames up with great
+brilliancy and then sinks back to invisibility with the naked eye. It
+has been suggested that sun-spots are indications of the beginning of
+a process in the sun which will be intensified until it falls into the
+state of such a star as Mira. Stars very far advanced in evolution,
+without showing variability, also exhibit similar spectra; so that
+there is much reason for regarding sunspots as emblems of advancing
+age.
+
+The association of the corona with sun-spots is less evident than that
+of the eruptive prominences; still such an association exists, for the
+form and extent of the corona vary with the sun-spot period of which
+we shall presently speak. The constitution of the corona remains to be
+discovered. It is evidently in part gaseous, but it also probably
+contains matter in the form of dust and small meteors. It includes one
+substance altogether mysterious -- ``coronium.'' There are reasons for
+thinking that this may be the lightest of all the elements, and
+Professor Young, its discoverer, said that it was ``absolutely unique
+in nature; utterly distinct from any other known form of matter,
+terrestial, solar, or cosmical.'' The enormous extent of the corona is
+one of its riddles. Since the development of the curious subject of
+the ``pressure of light'' it has been proposed to account for the
+sustentation of the corona by supposing that it is borne upon the
+billows of light continually poured out from the sun. Experiment has
+proved, what mathematical considerations had previously pointed out as
+probable, that the waves of light exert a pressure or driving force,
+which becomes evident in its effects if the body acted upon is
+sufficiently small. In that case the light pressure will prevail over
+the attraction of gravitation, and propel the attenuated matter away
+from the sun in the teeth of its attraction. The earth itself would be
+driven away if, instead of consisting of a solid globe of immense
+aggregate mass, it were a cloud of microscopic particles. The reason
+is that the pressure varies in proportion to the surface of the body
+acted upon, while the gravitational attraction is proportional to the
+volume, or the total amount of matter in the body. But the surface of
+any body depends upon the square of its diameter, while the volume
+depends upon the cube of the diameter. If, for instance, the diameter
+is represented by 4, the surface will be proportional to 4 � 4, or 16,
+and the volume to 4 � 4 � 4, or 64; but if the diameter is taken as 2,
+the surface will be 2 � 2, or 4, and the volume 2 � 2 � 2, or 8. Now,
+the ratio of 4 to 8 is twice as great as that of 16 to 64. If the
+diameter is still further decreased, the ratio of the surface to the
+volume will proportionally grow larger; in other words, the pressure
+will gain upon the attraction, and whatever their original ratio may
+have been, a time will come, if the diminution of size continues, when
+the pressure will become more effective than the attraction, and the
+body will be driven away. Supposing the particles of the corona to be
+below the critical size for the attraction of a mass like that of the
+sun to control them, they would be driven off into the surrounding
+space and appear around the sun like the clouds of dust around a mill.
+We shall return to this subject in connection with the Zodiacal Light,
+the Aurora, and Comets.
+
+On the other hand, there are parts of the corona which suggest by
+their forms the play of electric or magnetic forces. This is
+beautifully shown in some of the photographs that have been made of
+the corona during recent eclipses. Take, for instance, that of the
+eclipse of 1900. The sheaves of light emanating from the poles look
+precisely like the ``lines of force'' surrounding the poles of a
+magnet. It will be noticed in this photograph that the corona appears
+to consist of two portions: one comprising the polar rays just spoken
+of, and the other consisting of the broader, longer, and less-defined
+masses of light extending out from the equatorial and middle-latitude
+zones. Yet even in this more diffuse part of the phenomenon one can
+detect the presence of submerged curves bearing more or less
+resemblance to those about the poles. Just what part electricity or
+electro-magnetism plays in the mechanism of the solar radiation it is
+impossible to say, but on the assumption that it is a very important
+part is based the hypothesis that there exists a direct solar
+influence not only upon the magnetism, but upon the weather of the
+earth. This hypothesis has been under discussion for half a century,
+and still we do not know just how much truth it represents. It is
+certain that the outbreak of great disturbances on the sun,
+accompanied by the formation of sun-spots and the upshooting of
+eruptive prominences (phenomena which we should naturally expect to be
+attended by action), have been instantly followed by corresponding
+``magnetic storms'' on the earth and brilliant displays of the auroral
+lights. There have been occasions when the influence has manifested
+itself in the most startling ways, a great solar outburst being
+followed by a mysterious gripping of the cable and telegraph systems
+of the world, as if an invisible and irresistible hand had seized
+them. Messages are abruptly cut off, sparks leap from the telegraph
+instruments, and the entire earth seems to have been thrown into a
+magnetic flurry. These occurrences affect the mind with a deep
+impression of the dependence of our planet on the sun, such as we do
+not derive from the more familiar action of the sunlight on the growth
+of plants and other phenomena of life depending on solar influences.
+
+Perhaps the theory of solar magnetic influence upon the weather is
+best known in connection with the ``sun-spot cycle.'' This, at any
+rate, is, as already remarked, closely associated with the corona. Its
+existence was discovered in 1843 by the German astronomer Schwabe. It
+is a period of variable length, averaging about eleven years, during
+which the number of spots visible on the sun first increases to a
+maximum, then diminishes to a minimum, and finally increases again to
+a maximum. For unknown reasons the period is sometimes two or three
+years longer than the average and sometimes as much shorter.
+Nevertheless, the phenomena always recur in the same order. Starting,
+for instance, with a time when the observer can find few or no spots,
+they gradually increase in number and size until a maximum, in both
+senses, is reached, during which the spots are often of enormous size
+and exceedingly active. After two or three years they begin to
+diminish in number, magnitude, and activity until they almost or quite
+disappear. A strange fact is that when a new period opens, the spots
+appear first in high northern and southern latitudes, far from the
+solar equator, and as the period advances they not only increase in
+number and size, but break out nearer and nearer to the equator, the
+last spots of a vanishing period sometimes lingering in the equatorial
+region after the advance-guard of its successor has made its
+appearance in the high latitudes. Spots are never seen on the equator
+nor near the poles. It was not very long after the discovery of the
+sun-spot cycle that the curious observation was made that a striking
+coincidence existed between the period of the sun-spots and another
+period affecting the general magnetic condition of the earth. When a
+curved line representing the varying number of sun-spots was compared
+with another curve showing the variations in the magnetic state of the
+earth the two were seen to be in almost exact accord, a rise in one
+curve corresponding to a rise in the other, and a fall to a fall.
+Continued observation has proved that this is a real coincidence and
+not an accidental one, so that the connection, although as yet
+unexplained, is accepted as established. But does the influence extend
+further, and directly affect the weather and the seasons as well as
+the magnetic elements of the earth? A final answer to this question
+cannot yet be given, for the evidence is contradictory, and the
+interpretations put upon it depend largely on the predilections of the
+judges.
+
+But, in a broad sense, the sun-spots and the phenomena connected with
+them must have a relation to terrestial meteorology, for they prove
+the sun to be a variable star. Reference was made, a few lines above,
+to the resemblance of the spectra of sun-spots to those of certain
+stars which seem to be failing through age. This in itself is
+extremely suggestive; but if this resemblance had never been
+discovered, we should have been justified in regarding the sun as
+variable in its output of energy; and not only variable, but probably
+increasingly so. The very inequalities in the sun-spot cycle are
+suspicious. When the sun is most spotted its total light may be
+reduced by one-thousandth part, although it is by no means certain
+that its outgiving of thermal radiations is then reduced. A loss of
+one-thousandth of its luminosity would correspond to a decrease of
+.0025 of a stellar magnitude, considering the sun as a star viewed
+from distant space. So slight a change would not be perceptible; but
+it is not alone sun-spots which obscure the solar surface, its entire
+globe is enveloped with an obscuring veil. When studied with a
+powerful telescope the sun's surface is seen to be thickly mottled
+with relatively obscure specks, so numerous that it has been estimated
+that they cut off from one-tenth to one-twentieth of the light that we
+should receive from it if the whole surface were as brilliant as its
+brightest parts. The condition of other stars warrants the conclusion
+that this obscuring envelope is the product of a process of
+refrigeration which will gradually make the sun more and more variable
+until its history ends in extinction. Looking backward, we see a time
+when the sun must have been more brilliant than it is now. At that
+time it probably shone with the blinding white splendor of such stars
+as Sirius, Spica, and Vega; now it resembles the relatively dull
+Procyon; in time it will turn ruddy and fall into the closing cycle
+represented by Antares. Considering that once it must have been more
+radiantly powerful than at present, one is tempted to wonder if that
+could have been the time when tropical life flourished within the
+earth's polar circles, sustained by a vivific energy in the sun which
+it has now lost.
+
+The corona, as we have said, varies with the sun-spot cycle. When the
+spots are abundant and active the corona rises strong above the
+spotted zones, forming immense beams or streamers, which on one
+occasion, at least, had an observed length of ten million miles. At
+the time of a spot minimum the corona is less brilliant and has a
+different outline. It is then that the curved polar rays are most
+conspicuous. Thus the vast banners of the sun, shaken out in the
+eclipse, are signals to tell of its varying state, but it will
+probably be long before we can read correctly their messages.
+
+The Zodiacal Light Mystery
+
+There is a singular phenomenon in the sky -- one of the most puzzling
+of all -- which has long arrested the attention of astronomers,
+defying their efforts at explanation, but which probably not one in a
+hundred, and possibly not one in a thousand, of the readers of this
+book has ever seen. Yet its name is often spoken, and it is a
+conspicuous object if one knows when and where to look for it, and
+when well seen it exhibits a mystical beauty which at the same time
+charms and awes the beholder. It is called ``The Zodiacal Light,''
+because it lies within the broad circle of the Zodiac, marking the
+sun's apparent annual path through the stars. What it is nobody has
+yet been able to find out with certainty, and books on astronomy
+usually speak of it with singular reserve. But it has given rise to
+many remarkable theories, and a true explanation of it would probably
+throw light on a great many other celestial mysteries. The Milky Way
+is a more wonderful object to look upon, but its nature can be
+comprehended, while there is a sort of uncanniness about the Zodiacal
+Light which immediately impresses one upon seeing it, for its part in
+the great scheme of extra-terrestrial affairs is not evident.
+
+If you are out-of-doors soon after sunset -- say, on an evening late
+in the month of February -- you may perceive, just after the angry
+flush of the dying winter's day has faded from the sky, a pale ghostly
+presence rising above the place where the sun went down. The writer
+remembers from boyhood the first time it was pointed out to him and
+the unearthly impression that it made, so that he afterward avoided
+being out alone at night, fearful of seeing the spectral thing again.
+The phenomenon brightens slowly with the fading of the twilight, and
+soon distinctly assumes the shape of an elongated pyramid of pearly
+light, leaning toward the south if the place of observation is in the
+northern hemisphere. It does not impress the observer at all in the
+same manner as the Milky Way; that looks far off and is clearly among
+the stars, but the Zodiacal Light seems closer at hand, as if it were
+something more intimately concerning the earth. To all it immediately
+suggests a connection, also, with the sunken sun. If the night is
+clear and the moon absent (and if you are in the country, for city
+lights ruin the spectacles of the sky), you will be able to watch the
+apparition for a long time. You will observe that the light is
+brightest near the horizon, gradually fading as the pyramidal beam
+mounts higher, but in favorable circumstances it may be traced nearly
+to the meridian south of the zenith, where its apex at last vanishes
+in the starlight. It continues visible during the evenings of March
+and part of April, after which, ordinarily, it is seen no more, or if
+seen is relatively faint and unimpressive. But when autumn comes it
+appears again, this time not like a wraith hovering above the westward
+tomb of the day-god, but rather like a spirit of the morning
+announcing his reincarnation in the east.
+
+The reason why the Zodiacal Light is best seen in our latitudes at the
+periods just mentioned is because at those times the Zodiac is more
+nearly perpendicular to the horizon, first in the west and then in the
+east; and, since the phenomenon is confined within the borders of the
+Zodiac, it cannot be favorably placed for observation when the
+zodiacal plane is but slightly inclined to the horizon. Its faint
+light requires the contrast of a background of dark sky in order to be
+readily perceptible. But within the tropics, where the Zodiac is
+always at a favorable angle, the mysterious light is more constantly
+visible. Nearly all observant travelers in the equatorial regions have
+taken particular note of this phenomenon, for being so much more
+conspicuous there than in the temperate zones it at once catches the
+eye and holds the attention as a novelty. Humboldt mentions it many
+times in his works, for his genius was always attracted by things out
+of the ordinary and difficult of explanation, and he made many careful
+observations on its shape, its brilliancy, and its variations; for
+there can be no doubt that it does vary, and sometimes to an
+astonishing degree. It is said that it once remained practically
+invisible in Europe for several years in succession. During a trip to
+South Africa in 1909 an English astronomer, Mr E. W. Maunder, found a
+remarkable difference between the appearance of the Zodiacal Light on
+his going and coming voyages. In fact, when crossing the equator going
+south he did not see it at all; but on returning he had, on March 6th,
+when one degree south of the equator, a memorable view of it.
+
+  It was a bright, clear night, and the Zodiacal Light was
+  extraordinarily brilliant -- brighter than he had ever seen it
+  before. The Milky Way was not to be compared with it. The brightest
+  part extended 75� from the sun. There was a faint and much narrower
+  extension which they could just make out beyond the Pleiades along
+  the ecliptic, but the greater part of the Zodiacal Light showed as
+  a broad truncated column, and it did not appear nearly as conical
+  as he had before seen it.
+
+When out of the brief twilight of intertropical lands, where the sun
+drops vertically to the horizon and night rushes on like a wave of
+darkness, the Zodiacal Light shoots to the very zenith, its color is
+described as a golden tint, entirely different from the silvery sheen
+of the Milky Way. If I may venture again to refer to personal
+experiences and impressions, I will recall a view of the Zodiacal
+Light from the summit of the cone of Mt Etna in the autumn of the year
+1896 (more briefly described in Astronomy with the Naked Eye). There
+are few lofty mountains so favorably placed as Etna for observations
+of this kind. It was once resorted to by Prof. George E. Hale, in an
+attempt to see the solar corona without an eclipse. Rising directly
+from sea-level to an elevation of nearly eleven thousand feet, the
+observer on its summit at night finds himself, as it were, lost in the
+midst of the sky. But for the black flanks of the great cone on which
+he stands he might fancy himself to be in a balloon. On the occasion
+to which I refer the world beneath was virtually invisible in the
+moonless night. The blaze of the constellations overhead was
+astonishingly brilliant, yet amid all their magnificence my attention
+was immediately drawn to a great tapering light that sprang from the
+place on the horizon where the sun would rise later, and that seemed
+to be blown out over the stars like a long, luminous veil. It was the
+finest view of the Zodiacal light that I had ever enjoyed -- thrilling
+in its strangeness -- but I was almost disheartened by the
+indifference of my guide, to whom it was only a light and nothing
+more. If he had no science, he had less poetry -- rather a remarkable
+thing, I thought, for a child of his clime. The Light appeared to me
+to be distinctly brighter than the visible part of the Milky Way which
+included the brilliant stretches in Auriga and Perseus, and its color,
+if one may speak of color in connection with such an object, seemed
+richer than that of the galactic band; but I did not think of it as
+yellow, although Humboldt has described it as resembling a golden
+curtain drawn over the stars, and Du Chaillu in Equatorial Africa
+found it of a bright yellow color. It may vary in color as in
+conspicuousness. The fascination of that extraordinary sight has never
+faded from my memory. I turned to regard it again and again, although
+I had never seen the stellar heavens so brilliant, and it was one of
+the last things I looked for when the morning glow began softly to
+mount in the east, and Sicily and the Mediterranean slowly emerged
+from the profound shadow beneath us.
+
+The Zodiacal Light seems never to have attracted from astronomers in
+general the amount of careful attention that it deserves; perhaps
+because so little can really be made of it as far as explanation is
+concerned. I have referred to the restraint that scientific writers
+apparently feel in speaking of it. The grounds for speculation that it
+affords may be too scanty to lead to long discussions, yet it piques
+curiosity, and as we shall see in a moment has finally led to a most
+interesting theory. Once it was the subject of an elaborate series of
+studies which carried the observer all round the world. That was in
+1845--46, during the United States Exploring Expedition that visited
+the then little known Japan. The chaplain of the fleet, the Rev. Mr
+Jones, went out prepared to study the mysterious light in all its
+phases. He saw it from many latitudes on both sides of the equator,
+and the imagination cannot but follow him with keen interest in his
+world-circling tour, keeping his eyes every night fixed upon the
+phantasm overhead, whose position shifted with that of the hidden sun.
+He demonstrated that the flow extends at times completely across the
+celestial dome, although it is relatively faint directly behind the
+earth. On his return the government published a large volume of his
+observations, in which he undertook to show that the phenomenon was
+due to the reflection of sunlight from a ring of meteoric bodies
+encircling the earth. But, after all, this elaborate investigation
+settled nothing.
+
+Prof. E. E. Barnard has more recently devoted much attention to the
+Zodiacal Light, as well as to a strange attendant phenomenon called
+the ``Gegenschein,'' or Counterglow, because it always appears at that
+point in the sky which is exactly opposite the sun. The Gegenschein is
+an extremely elusive phenomenon, suitable only for eyes that have been
+specially trained to see it. Professor Newcomb has cautiously remarked
+that
+
+  it is said that in that point of the heavens directly opposite the
+  sun there is an elliptical patch of light... This phenomenon is so
+  difficult to account for that its existence is sometimes doubted;
+  yet the testimony in its favor is difficult to set aside.
+
+It certainly cannot be set aside at all since the observations of
+Barnard. I recall an attempt to see it under his guidance during a
+visit to Mount Hamilton, when he was occupied there with the Lick
+telescope. Of course, both the Gegenschein and the Zodiacal Light are
+too diffuse to be studied with telescopes, which, so to speak, magnify
+them out of existence. They can only be successfully studied with the
+naked eye, since every faintest glimmer that they afford must be
+utilized. This is especially true of the Gegenschein. At Mount
+Hamilton, Mr Barnard pointed out to me its location with reference to
+certain stars, but with all my gazing I could not be sure that I saw
+it. To him, on the contrary, it was obvious; he had studied it for
+months, and was able to indicate its shape, its boundaries, its
+diameter, and the declination of its center with regard to the
+ecliptic. There is not, of course, the shadow of a doubt of the
+existence of the Gegenschein, and yet I question if one person in a
+million has ever seen or ever will see it. The Zodiacal Light, on the
+other hand, is plain enough, provided that the time and the
+circumstances of the observation are properly chosen.
+
+In the attempts to explain the Zodiacal Light, the favorite hypothesis
+has been that it is an appendage of the sun -- perhaps simply an
+extension of the corona in the plane of the ecliptic, which is not
+very far from coinciding with that of the sun's equator. This idea is
+quite a natural one, because of the evident relation of the light to
+the position of the sun. The vast extension of the equatorial wings of
+the corona in 1878 gave apparent support to this hypothesis; if the
+substance of the corona could extend ten million miles from the sun,
+why might it not extend even one hundred million, gradually fading out
+beyond the orbit of the earth? A variation of this hypothesis assumes
+that the reflection is due to swarms of meteors circling about the
+sun, in the plane of its equator, all the way from its immediate
+neighborhood to a distance exceeding that of the earth. But in neither
+form is the hypothesis satisfactory; there is nothing in the
+appearance of the corona to indicate that it extends even as far as
+the planet Mercury, while as to meteors, the orbits of the known
+swarms do not accord with the hypothesis, and we have no reason to
+believe that clouds of others exist traveling in the part of space
+where they would have to be in order to answer the requirements of the
+theory. The extension of the corona in 1878 did not resemble in its
+texture the Zodiacal Light.
+
+Now, it has so often happened in the history of science that an
+important discovery in one branch has thrown unexpected but most
+welcome light upon some pending problem in some other branch, that a
+strong argument might be based upon that fact alone against the too
+exclusive devotion of many investigators to the narrow lines of their
+own particular specialty; and the Zodiacal Light affords a case in
+point, when it is considered in connection with recent discoveries in
+chemistry and physics. From the fact that atoms are compound bodies
+made up of corpuscles at least a thousand times smaller than the
+smallest known atom -- a fact which astounded most men of science when
+it was announced a few years ago -- a new hypothesis has been
+developed concerning the nature of the Zodiacal Light (as well as
+other astronomical riddles), and this hypothesis comes not from an
+astronomer, but from a chemist and physicist, the Swede, Svante
+Arrhenius. In considering an outline of this new hypothesis we need
+neither accept nor reject it; it is a case rather for suspension of
+judgment.
+
+To begin with, it carries us back to the ``pressure of light''
+mentioned in the preceding chapter. The manner in which this pressure
+is believed generally to act was there sufficiently explained, and it
+only remains to see how it is theoretically extended to the particles
+of matter supposed to constitute the Zodiacal Light. We know that
+corpuscles, or ``fragments of atoms'' negatively electrified, are
+discharged from hot bodies. Streams of these ``ions'' pour from many
+flames and from molten metals; and the impact of the cathode and
+ultra-violet rays causes them to gush even from cold bodies. In the
+vast laboratory of the sun it is but reasonable to suppose that
+similar processes are taking place. ``As a very hot metal emits these
+corpuscles,'' says Prof. J. J. Thomson, ``it does not seem an
+improbable hypothesis that they are emitted by that very hot body, the
+sun.'' Let it be assumed, then, that the sun does emit them; what
+happens next? Negatively charged corpuscles, it is known, serve as
+nuclei to which particles of matter in the ordinary state are
+attracted, and it is probable that those emitted from the sun
+immediately pick up loads in this manner and so grow in bulk. If they
+grow large enough the gravitation of the sun draws them back, and they
+produce a negative charge in the solar atmosphere. But it is probable
+that many of the particles do not attain the critical size which,
+according to the principles before explained, would enable the
+gravitation of the sun to retain them in opposition to the pressure of
+the waves of light, and with these particles the light pressure is
+dominant. Clouds of them may be supposed to be continually swept away
+from the sun into surrounding space, moving mostly in or near the
+plane of the solar equator, where the greatest activity, as indicated
+by sunspots and related phenomena, is taking place. As they pass
+outward into space many of them encounter the earth. If the earth,
+like the moon, had no atmosphere the particles would impinge directly
+on its surface, giving it a negative electric charge. But the presence
+of the atmosphere changes all that, for the first of the flying
+particles that encounter it impart to it their negative electricity,
+and then, since like electric charges repel like, the storm of
+particles following will be sheered off from the earth, and will
+stream around it in a maze of hyperbolic paths. Those that continue on
+into space beyond the earth may be expected to continue picking up
+wandering particles of matter until their bulk has become so great
+that the solar attraction prevails again over the light pressure
+acting upon them, and they turn again sunward. Passing the earth on
+their return they will increase the amount of dust-clouds careering
+round it; and these will be further increased by the action of the
+ultra-violet rays of the sunlight causing particles to shoot radially
+away from the earth when the negative charge of the upper atmosphere
+has reached a certain amount, which particles, although starting
+sunward, will be swept back to the earth with the oncoming streams. As
+the final result of all this accumulation of flying and gyrating
+particles in the earth's neighborhood, we are told that the latter
+must be transformed into the semblance of a gigantic solid-headed
+comet provided with streaming tails, the longest of them stretching
+away from the direction of the sun, while another shorter one extends
+toward the sun. This shorter tail is due to the particles that we have
+just spoken of as being driven sunward from the earth by the action of
+ultra-violet light. No doubt this whole subject is too technical for
+popular statement; but at any rate the general reader can understand
+the picturesque side of the theory, for its advocates assure us that
+if we were on the moon we would doubtless be able to see the
+comet-like tails of the earth, and then we could appreciate the part
+that they play in producing the phenomenon of the Zodiacal Light.
+
+That the Light as we see it could be produced by the reflection of
+sunlight from swarms of particles careering round the earth in the
+manner supposed by Arrhenius' hypothesis is evident enough; and it
+will be observed that the new theory, after all, is only another
+variant of the older one which attributes the Zodiacal Light to an
+extension of the solar corona. But it differs from the older theory in
+offering an explanation of the manner in which the extension is
+effected, and it differentiates between the corona proper and the
+streams of negative particles shot away from the sun. In its details
+the hypothesis of Arrhenius also affords an explanation of many
+peculiarities of the Zodiacal Light, such as that it is confined to
+the neighborhood of the ecliptic, and that it is stronger on the side
+of the earth which is just turning away from a position under the sun
+than on the other side; but it would carry us beyond our limits to go
+into these particulars. The Gegenschein, according to this theory, is
+a part of the same phenomenon as the Zodiacal Light, for by the laws
+of perspective it is evident that the reflection from the streams of
+particles situated at a point directly opposite to the sun would be at
+a maximum, and this is the place which the Gegenschein occupies. Apart
+from its geometrical relations to the position of the sun, the
+variability of the Zodiacal Light appears to affirm its solar
+dependence, and this too would be accounted for by Arrhenius'
+hypothesis better than by the old theory of coronal extension. The
+amount of corpuscular discharge from the sun must naturally be
+governed by the state of relative activity or inactivity of the
+latter, and this could not but be reflected in the varying splendor of
+the Zodiacal Light. But much more extended study than has yet been
+given to the subject will be required before we can feel that we know
+with reasonable certainty what this mysterious phenomenon really is.
+By the hypothesis of Arrhenius every planet that has an atmosphere
+must have a Zodiacal Light attending it, but the phenomenon is too
+faint for us to be able to see it in the case, for instance, of Venus,
+whose atmosphere is very abundant. The moon has no corresponding
+``comet's tail'' because, as already explained, of the lack of a lunar
+atmosphere to repel the streams by becoming itself electrified; but if
+there were a lunar Zodiacal Light, no doubt we could see it because of
+the relative nearness of our satellite.
+
+Marvels of the Aurora
+
+One of the most vivid recollections of my early boyhood is that of
+seeing my father return hastily into the house one evening and call
+out to the family: ``Come outside and look at the sky!'' Ours was a
+country house situated on a commanding site, and as we all emerged
+from the doorway we were dumbfounded to see the heavens filled with
+pale flames which ran licking and quivering over the stars. Instantly
+there sprang into my terrified mind the recollection of an awful
+description of ``the Day of Judgment'' (the Dies Ir�), which I had
+heard with much perturbation of spirit in the Dutch Reformed church
+from the lips of a tall, dark-browed, dreadfully-in-earnest preacher
+of the old-fashioned type. My heart literally sank at sight of the
+spectacle, for it recalled the preacher's very words; it was just as
+he had said it would be, and it needed the assured bearing of my
+elders finally to convince me that
+
+  That Day of Wrath, O dreadful day,
+  When Heaven and Earth shall pass away,
+  As David and the Sibyl say
+
+had not actually come upon us. And even the older members of the
+household were not untouched with misgivings when menacing spots of
+crimson appeared, breaking out now here, now there, in the shuddering
+sky. Toward the north the spectacle was appalling. A huge arch spanned
+an unnaturally dark segment resting on the horizon, and above this
+arch sprang up beams and streamers in a state of incessant agitation,
+sometimes shooting up to the zenith with a velocity that took one's
+breath, and sometimes suddenly falling into long ranks, and marching,
+marching, marching, like an endless phalanx of fiery specters, and
+moving, as I remember, always from east to west. The absolute silence
+with which these mysterious evolutions were performed and the
+quavering reflections which were thrown upon the ground increased the
+awfulness of the exhibition. Occasionally enormous curtains of lambent
+flame rolled and unrolled with a majestic motion, or were shaken to
+and fro as if by a mighty, noiseless wind. At times, too, a sudden
+billowing rush would be made toward the zenith, and for a minute the
+sky overhead would glow so brightly that the stars seemed to have been
+consumed. The spectacle continued with varying intensity for hours.
+
+This exhibition occurred in Central New York, a latitude in which the
+Aurora Borealis is seldom seen with so much splendor. I remember
+another similar one seen from the city of New York in November, 1882.
+On this last occasion some observers saw a great upright beam of light
+which majestically moved across the heavens, stalking like an
+apparition in the midst of the auroral pageant, of whose general
+movements it seemed to be independent, maintaining always its upright
+posture, and following a magnetic parallel from east to west. This
+mysterious beam was seen by no less than twenty-six observers in
+different parts of the country, and a comparison of their observations
+led to a curious calculation indicating that the apparition was about
+one hundred and thirty-three miles tall and moved at the speed of ten
+miles per second!
+
+But, as everybody knows, it is in the Arctic regions that the Aurora,
+or the ``Northern Lights,'' can best be seen. There, in the long polar
+night, when for months together the sun does not rise, the strange
+coruscations in the sky often afford a kind of spectral daylight in
+unison with the weird scenery of the world of ice. The pages in the
+narratives of Arctic exploration that are devoted to descriptions of
+the wonderful effects of the Northern Lights are second to none that
+man has ever penned in their fascination. The lights, as I have
+already intimated, display astonishing colors, particularly shades of
+red and green, as they flit from place to place in the sky. The
+discovery that the magnetic needle is affected by the Aurora,
+quivering and darting about in a state of extraordinary excitement
+when the lights are playing in the sky, only added to the mystery of
+the phenomenon until its electro-magnetic nature had been established.
+This became evident as soon as it was known that the focus of the
+displays was the magnetic pole; and when the far South was visited the
+Aurora Australis was found, having its center at the South Magnetic
+Pole. Then, if not before, it was clear that the earth was a great
+globular magnet, having its poles of opposite magnetism, and that the
+auroral lights, whatever their precise cause might be, were
+manifestations of the magnetic activity of our planet. After the
+invention of magnetic telegraphy it was found that whenever a great
+Aurora occurred the telegraph lines were interrupted in their
+operation, and the ocean cables ceased to work. Such a phenomenon is
+called a ``magnetic storm.''
+
+The interest excited by the Aurora in scientific circles was greatly
+stimulated when, in the last half of the nineteenth century, it was
+discovered that it is a phenomenon intimately associated with
+disturbances on the sun. The ancient ``Zurich Chronicles,'' extending
+from the year 1000 to the year 1800, in which both sun-spots visible
+to the naked eye and great displays of the auroral lights were
+recorded, first set Rudolf Wolf on the track of this discovery. The
+first notable proof of the suspected connection was furnished with
+dramatic emphasis by an occurrence which happened on September 1,
+1859. Near noon on that day two intensely brilliant points suddenly
+broke out in a group of sun-spots which were under observation by Mr
+R. C. Carrington at his observatory at Redhill, England. The points
+remained visible for not more than five minutes, during which interval
+they moved thirty-five thousand miles across the solar disk. Mr R.
+Hodgson happened to see the same phenomenon at his observatory at
+Highgate, and thus all possibility of deception was removed. But
+neither of the startled observers could have anticipated what was to
+follow, and, indeed, it was an occurrence which has never been
+precisely duplicated. I quote the eloquent account given by Miss
+Clerke in her History of Astronomy During the Nineteenth Century.
+
+  This unique phenomenon seemed as if specially designed to
+  accentuate the inference of a sympathetic relation between the
+  earth and the sun. From August 28 to September 4, 1859, a magnetic
+  storm of unparalleled intensity, extent, and duration was in
+  progress over the entire globe. Telegraphic communication was
+  everywhere interrupted -- except, indeed, that it was in some cases
+  found practicable to work the lines without batteries by the agency
+  of the earth-currents alone; sparks issued from the wires; gorgeous
+  auroras draped the skies in solemn crimson over both hemispheres,
+  and even in the tropics; the magnetic needle lost all trace of
+  continuity in its movements and darted to and fro as if stricken
+  with inexplicable panic. The coincidence was even closer. At the
+  very instant of the solar outburst witnessed by Carrington and
+  Hodgson the photographic apparatus at Kew registered a marked
+  disturbance of all the three magnetic elements; while shortly after
+  the ensuing midnight the electric agitation culminated, thrilling
+  the whole earth with subtle vibrations, and lighting up the
+  atmosphere from pole to pole with coruscating splendors which
+  perhaps dimly recall the times when our ancient planet itself shone
+  as a star.
+
+If this amazing occurrence stood alone, and as I have already said it
+has never been exactly duplicated, doubt might be felt concerning some
+of the inferences drawn from it; but in varying forms it has been
+repeated many times, so that now hardly anyone questions the reality
+of the assumed connection between solar outbursts and magnetic storms
+accompanied by auroral displays on the earth. It is true that the late
+Lord Kelvin raised difficulties in the way of the hypothesis of a
+direct magnetic action of the sun upon the earth, because it seemed to
+him that an inadmissible quantity of energy was demanded to account
+for such action. But no calculation like that which he made is final,
+since all calculations depend upon the validity of the data; and no
+authority is unshakable in science, because no man can possess
+omniscience. It was Lord Kelvin who, but a few years before the thing
+was actually accomplished, declared that aerial navigation was an
+impracticable dream, and demonstrated its impracticability by
+calculation. However the connection may be brought about, it is as
+certain as evidence can make it that solar outbursts are coincident
+with terrestial magnetic disturbances, and coincident in such a way as
+to make the inference of a causal connection irresistible. The sun is
+only a little more than a hundred times its own diameter away from the
+earth. Why, then, with the subtle connection between them afforded by
+the ether which conveys to us the blinding solar light and the
+life-sustaining solar heat, should it be so difficult to believe that
+the sun's enormous electric energies find a way to us also? No doubt
+the impulse coming from the sun acts upon the earth after the manner
+of a touch upon a trigger, releasing energies which are already stored
+up in our planet.
+
+But besides the evidence afforded by such occurrences as have been
+related of an intimate connection between solar outbreaks and
+terrestial magnetic flurries, attended by magnificent auroral
+displays, there is another line of proof pointing in the same
+direction. Thus, it is known that the sun-spot period, as remarked in
+a preceding chapter, coincides in a most remarkable manner with the
+periodic fluctuations in the magnetic state of the earth. This
+coincidence runs into the most astonishing details. For instance, when
+the sun-spot period shortens, the auroral period shortens to precisely
+the same extent; as the short sun-spot periods usually bring the most
+intense outbreaks of solar activity, so the corresponding short
+auroral periods are attended by the most violent magnetic storms; a
+secular period of about two hundred and twenty-two years affecting
+sun-spots is said to have its auroral duplicate; a shorter period of
+fifty-five and a half years, which some observers believe that they
+have discovered appears also to be common to the two phenomena; and
+yet another ``superposed'' period of about thirty-five years, which
+some investigators aver exists, affects sun-spots and aurora alike. In
+short, the coincidences are so numerous and significant that one would
+have to throw the doctrine of probability to the winds in order to be
+able to reject the conclusion to which they so plainly lead.
+
+But still the question recurs: How is the influence transmitted? Here
+Arrhenius comes once more with his hypothesis of negative corpuscles,
+or ions, driven away from the sun by light-pressure -- a hypothesis
+which seems to explain so many things -- and offers it also as an
+explanation of the way in which the sun creates the Aurora. He would
+give the Aurora the same lineage with the Zodiacal Light. To
+understand the application of this theory we must first recall the
+fact that the earth is a great magnet having its two opposite poles of
+magnetism, one near the Arctic and the other near the Antarctic
+Circle. Like all magnets, the earth is surrounded with ``lines of
+force,'' which, after the manner of the curved rays we saw in the
+photograph of a solar eclipse, start from a pole, rising at first
+nearly vertically, then bend gradually over, passing high above the
+equator, and finally descending in converging sheaves to the opposite
+pole. Now the axis of the earth is so placed in space that it lies at
+nearly a right angle to the direction of the sun, and as the streams
+of negatively charged particles come pouring on from the sun (see the
+last preceding chapter), they arrive in the greatest numbers over the
+earth's equatorial regions. There they encounter the lines of magnetic
+force at the place where the latter have their greatest elevation
+above the earth, and where their direction is horizontal to the
+earth's surface. Obeying a law which has been demonstrated in the
+laboratory, the particles then follow the lines of force toward the
+poles. While they are above the equatorial regions they do not become
+luminescent, because at the great elevation that they there occupy
+there is virtually no atmosphere; but as they pass on toward the north
+and the south they begin to descend with the lines of force, curving
+down to meet at the poles; and, encountering a part of the atmosphere
+comparable in density with what remains in an exhausted Crookes tube,
+they produce a glow of cathode rays. This glow is conceived to
+represent the Aurora, which may consequently be likened to a gigantic
+exhibition of vacuum-tube lights. Anybody who recalls his student days
+in the college laboratory and who has witnessed a display of Northern
+Lights will at once recognize the resemblance between them in colors,
+forms, and behavior. This resemblance had often been noted before
+Arrhenius elaborated his hypothesis.
+
+Without intending to treat his interesting theory as more than a
+possibly correct explanation of the phenomena of the Aurora, we may
+call attention to some apparently confirmatory facts. One of the most
+striking of these relates to a seasonal variation in the average
+number of auror�. It has been observed that there are more in March
+and September than at any other time of the year, and fewer in June
+and December; moreover (and this is a delicate test as applied to the
+theory), they are slightly rarer in June than in December. Now all
+these facts seem to find a ready explanation in the hypothesis of
+Arrhenius, thus: (1) The particles issuing from the sun are supposed
+to come principally from the regions whose excitement is indicated by
+the presence of sun-spots (which accords with Hale's observation that
+sun-spots are columns of ionized vapors), and these regions have a
+definite location on either side of the solar equator, seldom
+approaching it nearer than within 5� or 10� north or south, and never
+extending much beyond 35� toward either pole; (2) The equator of the
+sun is inclined about 7� to the plane of the earth's orbit, from which
+it results that twice in a year -- viz., in June and December -- the
+earth is directly over the solar equator, and twice a year -- viz., in
+March and September -- when it is farthest north or south of the solar
+equator, it is over the inner edge of the sun-spot belts. Since the
+corpuscles must be supposed to be propelled radially from the sun, few
+will reach the earth when the latter is over the solar equator in June
+and December, but when it is over, or nearly over, the spot belts, in
+March and September, it will be in the line of fire of the more active
+parts of the solar surface, and relatively rich streams of particles
+will reach it. This, as will be seen from what has been said above, is
+in strict accord with the observed variations in the frequency of
+auror�. Even the fact that somewhat fewer auror� are seen in June than
+in December also finds its explanation in the known fact that the
+earth is about three million miles nearer the sun in the winter than
+in the summer, and the number of particles reaching it will vary, like
+the intensity of light, inversely as the square of the distance. These
+coincidences are certainly very striking, and they have a cumulative
+force. If we accept the theory, it would appear that we ought to
+congratulate ourselves that the inclination of the sun's equator is so
+slight, for as things stand the earth is never directly over the most
+active regions of the sun-spots, and consequently never suffers from
+the maximum bombardment of charged particles of which the sun is
+capable. Incessant auroral displays, with their undulating draperies,
+flitting colors, and marching columns might not be objectionable from
+the point of view of picturesqueness, but one magnetic storm of
+extreme intensity following closely upon the heels of another, for
+months on end, crazing the magnetic needle and continually putting the
+telegraph and cable lines out of commission, to say nothing of their
+effect upon ``wireless telegraphy'', would hardly add to the charms of
+terrestrial existence.
+
+One or two other curious points in connection with Arrhenius'
+hypothesis may be mentioned. First, the number of auror�, according to
+his explanation, ought to be greatest in the daytime, when the face of
+the earth on the sunward side is directly exposed to the atomic
+bombardment. Of course visual observation can give us no information
+about this, since the light of the Aurora is never sufficiently
+intense to be visible in the presence of daylight, but the records of
+the magnetic observatories can be, and have been, appealed to for
+information, and they indicate that the facts actually accord with the
+theory. Behind the veil of sunlight in the middle of the afternoon,
+there is good reason to believe, auroral exhibitions often take place
+which would eclipse in magnificence those seen at night if we could
+behold them. Observation shows, too, that auror� are more frequent
+before than after midnight, which is just what we should expect if
+they originate in the way that Arrhenius supposes. Second, the theory
+offers an explanation of the alleged fact that the formation of clouds
+in the upper air is more frequent in years when auror� are most
+abundant, because clouds are the result of the condensation of
+moisture upon floating particles in the atmosphere (in an absolutely
+dustless atmosphere there would be no clouds), and it has been proved
+that negative ions like those supposed to come from the sun play a
+master part in the phenomena of cloud formation.
+
+Yet another singular fact, almost mystical in its suggestions, may be
+mentioned. It seems that the dance of the auroral lights occurs most
+frequently during the absence of the moon from the hemisphere in which
+they appear, and that they flee, in greater part, to the opposite
+hemisphere when the moon's revolution in an orbit considerably
+inclined to the earth's equator brings her into that where they have
+been performing. Arrhenius himself discovered this curious relation of
+auroral frequency to the position of the moon north or south of the
+equator, and he explains it in this way. The moon, like the earth, is
+exposed to the influx of the ions from the sun; but having no
+atmosphere, or almost none, to interfere with them, they descend
+directly upon her surface and charge her with an electric negative
+potential to a very high degree. In consequence of this she affects
+the electric state of the upper parts of the earth's atmosphere where
+they lie most directly beneath her, and thus prevents, to a large
+extent, the negative discharges to which the appearance of the Aurora
+is due. And so ``the extravagant and erring spirit'' of the Aurora
+avoids the moon as Hamlet's ghost fled at the voice of the cock
+announcing the awakening of the god of day.
+
+There are even other apparent confirmations of the hypothesis, but we
+need not go into them. We shall, however, find one more application of
+it in the next chapter, for it appears to be a kind of cure-all for
+astronomical troubles; at any rate it offers a conceivable solution of
+the question, How does the sun manage to transmit its electric
+influence to the earth? And this solution is so grandiose in
+conception, and so novel in the mental pictures that it offers, that
+its acceptance would not in the least detract from the impression that
+the Aurora makes upon the imagination.
+
+Strange Adventures of Comets
+
+The fears and legends of ancient times before Science was born, and
+the superstitions of the Dark Ages, sedulously cultivated for
+theological purposes by monks and priests, have so colored our ideas
+of the influence that comets have had upon the human mind that many
+readers may be surprised to learn that it was the apparition of a
+wonderful comet, that of 1843, which led to the foundation of our
+greatest astronomical institution, the Harvard College Observatory. No
+doubt the comet superstition existed half a century ago, as, indeed,
+it exists yet today, but in this case the marvelous spectacle in the
+sky proved less effective in inspiring terror than in awakening a
+desire for knowledge. Even in the sixteenth century the views that
+enlightened minds took of comets tended powerfully to inspire popular
+confidence in science, and Halley's prediction, after seeing and
+studying the motion of the comet which appeared in 1682, that it would
+prove to be a regular member of the sun's family and would be seen
+returning after a period of about seventy-six years, together with the
+fulfillment of that prediction, produced a revulsion from the
+superstitious notions which had so long prevailed.
+
+Then the facts were made plain that comets are subject to the law of
+gravitation equally with the planets; that there are many which
+regularly return to the neighborhood of the sun (perihelion); and that
+these travel in orbits differing from those of the planets only in
+their greater eccentricity, although they have the peculiarity that
+they do not, like the planets, all go round the sun in the same
+direction, and do not keep within the general plane of the planetary
+system, but traverse it sometimes from above and sometimes from below.
+Other comets, including most of the ``great'' ones, appear to travel
+in parabolic or, in a few cases, hyperbolic orbits, which, not being
+closed curves, never bring them back again. But it is not certain that
+these orbits may not be extremely eccentric ellipses, and that after
+the lapse of hundreds, or thousands, of years the comets that follow
+them may not reappear. The question is an interesting one, because if
+all orbits are really ellipses, then all comets must be permanent
+members of the solar system, while in the contrary case many of them
+are simply visitors, seen once and never to be seen again. The
+hypothesis that comets are originally interlopers might seem to derive
+some support from the fact that the certainly periodic ones are
+associated, in groups, with the great outer planets, whose attraction
+appears to have served as a trap for them by turning them into
+elliptical orbits and thus making them prisoners in the solar system.
+Jupiter, owing to his great mass and his commanding situation in the
+system, is the chief ``comet-catcher;'' but he catches them not for
+himself, but for the sun. Yet if comets do come originally from
+without the borders of the planetary system, it does not, by any
+means, follow that they were wanderers at large in space before they
+yielded to the overmastering attraction of the sun. Investigation of
+the known cometary orbits, combined with theoretical considerations,
+has led some astronomers to the conclusion that as the sun travels
+onward through space he ``picks up en route'' cometary masses which,
+without belonging strictly to his empire, are borne along in the same
+vast ``cosmical current'' that carries the solar system.
+
+But while no intelligent person any longer thinks that the appearance
+of a great comet is a token from the heavenly powers of the
+approaching death of a mighty ruler, or the outbreak of a devastating
+war, or the infliction of a terrible plague upon wicked mankind,
+science itself has discovered mysteries about comets which are not
+less fascinating because they are more intellectual than the
+irrational fancies that they have displaced. To bring the subject
+properly before the mind, let us see what the principal phenomena
+connected with a comet are.
+
+At the present day comets are ordinarily ``picked up'' with the
+telescope or the photographic plate before any one except their
+discoverer is aware of their existence, and usually they remain so
+insignificant in appearance that only astronomers ever see them. Yet
+so great is the prestige of the word ``comet'' that the discovery of
+one of these inconspicuous wanderers, and its subsequent movements,
+become items of the day's news which everybody reads with the feeling,
+perhaps, that at least he knows what is going on in the universe even
+if he doesn't understand it. But a truly great comet presents quite a
+different proposition. It, too, is apt to be detected coming out of
+the depths of space before the world at large can get a glimpse of it,
+but as it approaches the sun its aspect undergoes a marvelous change.
+Agitated apparently by solar influence, it throws out a long streaming
+tail of nebulous light, directed away from the sun and looking as if
+blown out like a pennon by a powerful wind. Whatever may be the
+position of the comet with regard to the sun, as it circles round him
+it continually keeps its tail on the off side. This, as we shall soon
+see, is a fact of capital importance in relation to the probable
+nature of comets' tails. Almost at the same time that the formation of
+the tail is observed a remarkable change takes place in the comet's
+head, which, by the way, is invariably and not merely occasionally its
+most important part. On approaching the sun the head usually
+contracts. Coincidently with this contraction a nucleus generally
+makes its appearance. This is a bright, star-like point in the head,
+and it probably represents the totality of solid matter that the comet
+possesses. But it is regarded as extremely unlikely that even the
+nucleus consists of a uniformly solid mass. If it were such, comets
+would be far more formidable visitors when they pass near the planets
+than they have been found to be. The diameter of the nucleus may vary
+from a few hundred up to several thousand miles; the heads, on the
+average, are from twenty-five thousand to one hundred thousand miles
+in diameter, although a few have greatly exceeded these dimensions;
+that of the comet of 1811, one of the most stupendous ever seen, was a
+million and a quarter miles in diameter! As to the tails, not
+withstanding their enormous length -- some have been more than a
+hundred million miles long -- there is reason to believe that they are
+of extreme tenuity, ``as rare as vacuum.'' The smallest stars have
+been seen shining through their most brilliant portions with
+undiminished luster.
+
+After the nucleus has been formed it begins to throw out bright jets
+directed toward the sun. A stream, and sometimes several streams, of
+light also project sunward from the nucleus, occasionally appearing
+like a stunted tail directed oppositely to the real tail. Symmetrical
+envelopes which, seen in section, appear as half circles or parabolas,
+rise sunward from the nucleus, forming a concentric series. The ends
+of these stream backward into the tail, to which they seem to supply
+material. Ordinarily the formation of these ejections and envelopes is
+attended by intense agitation of the nucleus, which twists and turns,
+swinging and gyrating with an appearance of the greatest violence.
+Sometimes the nucleus is seen to break up into several parts. The
+entire heads of some comets have been split asunder in passing close
+around the sun; The comet of 1882 retreated into space after its
+perihelion passage with five heads instead of the one that it had
+originally, and each of these heads had its own tail!
+
+The possession of the spectroscope has enabled astronomers during
+later years to study the chemical composition of comets by analyzing
+their light. At first the only substances thus discovered in them were
+hydro-carbon compounds, due evidently to the gaseous envelopes in
+which some combination of hydrogen with carbon existed. Behind this
+gaseous spectrum was found a faint continuous spectrum ascribed to the
+nucleus, which apparently both reflects the sunlight and gives forth
+the light of a glowing solid or liquid. Subsequently sodium and iron
+lines were found in cometary spectra. The presence of iron would seem
+to indicate that some of these bodies may be much more massive than
+observations on their attractive effects have indicated. In some
+recent comets, such as Morehouse's, in 1908, several lines have been
+found, the origin of which is unknown.
+
+Without going back of the nineteenth century we may find records of
+some of the most extraordinary comets that man has ever looked upon.
+In 1811, still spoken of as ``the year of the comet,'' because of the
+wonderful vintage ascribed to the skyey visitor, a comet shaped like a
+gigantic sword amazed the whole world, and, as it remained visible for
+seventeen months, was regarded by superstitious persons as a symbol of
+the fearful happenings of Napoleon's Russian campaign. This comet, the
+extraordinary size of whose head, greatly exceeding that of the sun
+itself, has already been mentioned, was also remarkable for exhibiting
+so great a brilliancy without approaching even to the earth's distance
+from the sun. But there was once a comet (and only once -- in the year
+1729) which never got nearer to the sun than four times the distance
+of the earth and yet appeared as a formidable object in the sky. As
+Professor Young has remarked, ``it must have been an enormous comet to
+be visible from such a distance.'' And we are to remember that there
+were no great telescopes in the year 1729. That comet affects the
+imagination like a phantom of space peering into the solar system,
+displaying its enormous train afar off (which, if it had approached as
+near as other comets, would probably have become the celestial wonder
+of all human memory), and then turning away and vanishing in the
+depths of immensity.
+
+In 1843 a comet appeared which was so brilliant that it could be seen
+in broad day close beside the sun! This was the first authenticated
+instance of that kind, but the occurrence was to be repeated, as we
+shall see in a moment, less than forty years later.
+
+The splendid comet of 1858, usually called Donati's, is remembered by
+many persons yet living. It was, perhaps, both as seen by the naked
+eye and with the telescope, the most beautiful comet of which we have
+any record. It too marked a rich vintage year, still remembered in the
+vineyards of France, where there is a popular belief that a great
+comet ripens the grape and imparts to the wine a flavor not attainable
+by the mere skill of the cultivator. There are ``comet wines,''
+carefully treasured in certain cellars, and brought forth only when
+their owner wishes to treat his guests to a sip from paradise.
+
+The year 1861 saw another very remarkable comet, of an aspect
+strangely vast and diffuse, which is believed to have swept the earth
+with its immense tail when it passed between us and the sun on the
+night of June 30th, an event which produced no other known effect than
+the appearance of an unwonted amount of scattered light in the sky.
+
+The next very notable comet was the ``Great Southern Comet'' of 1880,
+which was not seen from the northern hemisphere. It mimicked the
+aspect of the famous comet of 1843, and to the great surprise of
+astronomers appeared to be traveling in the same path. This proved to
+be the rising of the curtain for an astronomical sensation
+unparalleled in its kind; for two years later another brilliant comet
+appeared, first in the southern hemisphere, and it too followed the
+same track. The startling suggestion was now made that this comet was
+identical with those of 1843 and 1880, its return having been hastened
+by the resistance experienced in passing twice through the coronal
+envelope, and there were some who thought that it would now swing
+swiftly round and then plunge straight into the sun, with consequences
+that might be disastrous to us on account of the ``flash of heat''
+that would be produced by the impact. Nervous people were frightened,
+but observation soon proved that the danger was imaginary, for
+although the comet almost grazed the sun, and must have rushed through
+two or three million miles of the coronal region, no retardation of
+its immense velocity was perceptible, and it finally passed away in a
+damaged condition, as before remarked, and has never since appeared.
+
+Then the probable truth was perceived -- viz., that the three comets
+(1843, 1880, and 1882) were not one identical body, but three separate
+ones all traveling in the same orbit. It was found, too, that a comet
+seen in 1668 bore similar insignia of relationship. The natural
+inference was that these four bodies had once formed a single mass
+which had been split apart by the disruptive action of the sun.
+Strength was lent to this hypothesis by the fact that the comet of
+1882 was apparently torn asunder during its perihelion passage,
+retreating into space in a dissevered state. But Prof. George Forbes
+has a theory that the splitting of the original cometary mass was
+effected by an unknown planet, probably greater than Jupiter, situated
+at a hundred times the earth's distance from the sun, and revolving in
+a period of a thousand years. He supposes that the original comet was
+not that of 1668, but one seen in 1556, which has since been
+``missing,'' and that its disruption occurred from an encounter with
+the supposititious planet about the year 1700. Truly from every point
+of view comets are the most extraordinary of adventurers!
+
+The comet of 1882 was likewise remarkable for being visible, like its
+predecessor of 1843, in full daylight in close proximity to the sun.
+The story of its detection when almost in contact with the solar disk
+is dramatic. It had been discovered in the southern hemisphere only a
+couple of weeks before its perihelion, which occurred on September
+17th, and on the forenoon of that day it was seen by Doctor Common in
+England, and by Doctor Elkin and Mr Finlay at the Cape of Good Hope,
+almost touching the sun. It looked like a dazzling white bird with
+outspread wings. The southern observers watched it go right into the
+sun, when it instantly disappeared. What had happened was that the
+comet in passing its perihelion point had swung exactly between the
+earth and the sun. On the following morning it was seen from all parts
+of the world close by the sun on the opposite side, and it remained
+thus visible for three days, gradually receding from the solar disk.
+It then became visible for northern observers in the morning sky
+before sunrise, brandishing a portentous sword-shaped tail which, if
+it had been in the evening sky, would have excited the wonder of
+hundreds of millions, but situated where it was, comparatively few
+ever saw it.
+
+The application of photography to the study of comets has revealed
+many curious details which might otherwise have escaped detection, or
+at best have remained subject to doubt. It has in particular shown not
+only the precise form of the tails, but the remarkable vicissitudes
+that they undergo. Professor Barnard's photographs of Brooks' comet in
+1893 suggested, by the extraordinary changes in the form of the tail
+which they revealed, that the comet was encountering a series of
+obstructions in space which bent and twisted its tail into fantastic
+shapes. The reader will observe the strange form into which the tail
+was thrown on the night of October 21st. A cloud of meteors through
+which the comet was passing might have produced such deformations of
+its tail. In the photograph of Daniels' comet of 1907, a curious
+striping of the tail will be noticed. The short bright streaks seen in
+the photograph, it may be explained, are the images of stars which are
+drawn out into lines in consequence of the fact that the photographic
+telescope was adjusted to follow the motion of the comet while the
+stars remained at rest.
+
+But the adventures of comets are not confined to possible encounters
+with unknown obstacles. We have referred to the fact that the great
+planets, and especially Jupiter, frequently interfere with the motions
+of comets. This interference is not limited to the original alteration
+of their orbits from possible parabolas to ellipses, but is sometimes
+exercised again and again, turning the bewildered comets into
+elliptical paths of all degrees of eccentricity. A famous example of
+this kind of planetary horse-play is furnished by the story of
+Lexell's missing comet. This comet was first seen in 1770.
+Investigation showed that it was moving in an orbit which should bring
+it back to perihelion every five and a half years; yet it had never
+been seen before and, although often searched for, has never been seen
+since. Laplace and Leverrier proved mathematically that in 1767 it had
+approached so close to Jupiter as to be involved among the orbits of
+his satellites. What its track had been before is not known, but on
+that occasion the giant planet seized the interloper, threw it into a
+short elliptic orbit and sent it, like an arrested vagrant, to receive
+sentence at the bar of the sun. On this journey it passed within less
+than 1,500,000 miles of the earth. The form of orbit which Jupiter had
+impressed required, as we have said, its return in about five and a
+half years; but soon after 1770 it had the misfortune a second time to
+encounter Jupiter at close range, and he, as if dissatisfied with the
+leniency of the sun, or indignant at the stranger's familiarity,
+seized the comet and hurled it out of the system, or at any rate so
+far away that it has never since been able to rejoin the family circle
+that basks in the immediate rays of the solar hearth. Nor is this the
+only instance in which Jupiter has dealt summarily with small comets
+that have approached him with too little deference.
+
+The function which Jupiter so conspicuously fulfills as master of the
+hounds to the sun is worth considering a little more in detail. To
+change the figure, imagine the sun in its voyage through space to be
+like a majestic battleship surrounded by its scouts. Small vessels
+(the comets, as they are overhauled by the squadron, are taken in
+charge by the scouts, with Jupiter for their chief, and are forced to
+accompany the fleet, but not all are impressed. If a strange comet
+undertakes to run across Jupiter's bows the latter brings it to, and
+makes prize of it by throwing it into a relatively small ellipse with
+the sun for its focus. Thenceforth, unless, as happened to the unhappy
+comet of Lexell, it encounters Jupiter again in such a way as to be
+diverted by him into a more distant orbit, it can never get away.
+About thirty comets are now known to have thus been captured by the
+great planet, and they are called ``Jupiter's Comet Family.'' But, on
+the other hand, if a wandering comet crosses the wake of the chief
+planetary scout the latter simply drives it away by accelerating its
+motion and compels it to steer off into open space. The transformation
+of comets into meteors will be considered in the next chapter, but
+here, in passing, mention may be made of the strange fate of one
+member of Jupiter's family, Biela's comet, which, having become over
+bold in its advances to its captor, was, after a few revolutions in is
+impressed orbit, torn to pieces and turned into a flock of meteors.
+
+And now let us return to the mystery of comets' tails. That we are
+fully justified in speaking of the tails of comets as mysterious is
+proved by the declaration of Sir John Herschel, who averred, in so
+many words, that ``there is some profound secret and mystery of nature
+concerned in this phenomenon,'' and this profound secret and mystery
+has not yet been altogether cleared up. Nevertheless, the
+all-explaining hypothesis of Arrhenius offers us once more a certain
+amount of aid. Comets' tails, Arrhenius assures us, are but another
+result of the pressure of light. The reader will recall the
+applications of this theory to the Zodiacal Light and the Aurora. In
+the form in which we now have to deal with it, the supposition is made
+that as a comet approaches the sun eruptions of vapor, due to the
+solar heat, occur in its nucleus. These are naturally most active on
+the side which is directly exposed to the sun, whence the appearance
+of the immense glowing envelopes that surround the nucleus on the
+sunward side. Among the particles of hydro-carbon, and perhaps solid
+carbon in the state of fine dust, which are thus set free there will
+be many whose size is within the critical limit which enables the
+light-waves from the sun to drive them away. Clouds of such particles,
+then, will stream off behind the advancing comet, producing the
+appearance of a tail. This accounts for the fact that the tails of
+comets are always directed away from the sun, and it also explains the
+varying forms of the tails and the extraordinary changes that they
+undergo. The speed of the particles driven before the light-waves must
+depend upon their size and weight, the lightest of a given size
+traveling the most swiftly. By accretion certain particles might grow,
+thus losing velocity and producing the appearance of bunches in the
+tail, such as have been observed. The hypothesis also falls in with
+the researches of Bredichin, who has divided the tails of comets into
+three principal classes -- viz.: (1) Those which appear as long,
+straight rays; (2) Those which have the form of curved plumes or
+scimitars; (3) Those which are short, brushy, and curved sharply
+backward along the comet's path. In the first type he calculates the
+repulsive force at from twelve to fifteen times the force of gravity;
+in the second at from two to four times; and in the third at about one
+and a half times. The straight tails he ascribes to hydrogen because
+the hydrogen atom is the lightest known; the sword-shaped tails to
+hydro-carbons; and the stumpy tails to vaporized iron. It will be seen
+that, if the force driving off the tails is that which Arrhenius
+assumes it to be, the forms of those appendages would accord with
+those that Bredichin's theory calls for. At the same time we have an
+explanation of the multiple tails with which some comets have adorned
+themselves. The comet of 1744, for instance, had at one time no less
+than seven tails spread in a wide curved brush behind it. Donati's
+comet of 1858 also had at least two tails, the principal one
+sword-shaped and the other long, narrow, and as straight as a rule.
+According to Bredichin, the straight tail must have been composed of
+hydrogen, and the other of some form of hydro-carbon whose atoms are
+heavier than those of hydrogen, and, consequently, when swept away by
+the storm of light-waves, followed a curvature depending upon the
+resultant of the forces operating upon them. The seven tails of the
+comet of 1744 presented a kind of diagram graphically exhibiting its
+complex composition, and, if we knew a little more about the
+constituents of a comet, we might be able to say from the amount of
+curvature of the different tails just what were the seven substances
+of which that comet consisted.
+
+If these theories seem to the reader fantastic, at any rate they are
+no more fantastic than the phenomena that they seek to explain.
+
+Meteors, Fire-Balls, and Meteorites
+
+One of the most terrorizing spectacles with which the heavens have
+ever caused the hearts of men to quake occurred on the night of
+November 13, 1833. On that night North America, which faced the storm,
+was under a continual rain of fire from about ten o'clock in the
+evening until daybreak.
+
+The fragments of a comet had struck the earth.
+
+But the meaning of what had happened was not discovered until long
+afterward. To the astronomers who, with astonishment not less than
+that of other people, watched the wonderful scene, it was an
+unparalleled ``shower of meteors.'' They did not then suspect that
+those meteors had once formed the head of a comet. Light dawned when,
+a year later, Prof. Denison Olmsted, of Yale College, demonstrated
+that the meteors had all moved in parallel orbits around the sun, and
+that these orbits intersected that of the earth at the point where our
+planet happened to be on the memorable night of November 13th.
+Professor Olmsted even went so far as to suggest that the cloud of
+meteors that had encountered the earth might form a diffuse comet; but
+full recognition of the fact that they were cometary d�bris came
+later, as the result of further investigation. The key to the secret
+was plainly displayed in the spectacle itself, and was noticed without
+being understood by thousands of the terror-stricken beholders. It was
+an umbrella of fire that had opened overhead and covered the heavens;
+in other words, the meteors all radiated from a particular point in
+the constellation Leo, and, being countless as the snowflakes in a
+winter tempest, they ribbed the sky with fiery streaks. Professor
+Olmsted showed that the radiation of the meteors from a fixed point
+was an effect of perspective, and in itself a proof that they were
+moving in parallel paths when they encountered the earth. The fact was
+noted that there had been a similar, but incomparably less brilliant,
+display of meteors on the same day of November, 1832, and it was
+rightly concluded that these had belonged to the same stream, although
+the true relationship of the phenomena was not immediately
+apprehended. Olmsted ascribed to the meteors a revolution about the
+sun once in every six months, bringing them to the intersection of
+their orbit with that of the earth every November 13th; but later
+investigators found that the real period was about thirty-three and
+one-quarter years, so that the great displays were due three times in
+a century, and their return was confidently predicted for the year
+1866. The appearance of the meteors in 1832, a year before the great
+display, was ascribed to the great length of the stream which they
+formed in space -- so great that they required more than two years to
+cross the earth's orbit. In 1832 the earth had encountered a
+relatively rare part of the stream, but in 1833, on returning to the
+crossing-place, it found there the richest part of the stream pouring
+across its orbit. This explanation also proved to be correct, and the
+predicted return in 1866 was duly witnessed, although the display was
+much less brilliant than in 1833. It was followed by another in 1867.
+
+In the mean time Olmsted's idea of a cometary relationship of the
+meteors was demonstrated to be correct by the researches of
+Schiaparelli and others, who showed that not only the November
+meteors, but those of August, which are seen more or less abundantly
+every year, traveled in the tracks of well-known comets, and had
+undoubtedly an identical origin with those comets. In other words the
+comets and the meteor-swarms were both remnants of original masses
+which had probably been split up by the action of the sun, or of some
+planet to which they had made close approaches. The annual periodicity
+of the August meteors was ascribed to the fact that the separation had
+taken place so long ago that the meteors had become distributed all
+around the orbit, in consequence of which the earth encountered some
+of them every year when it arrived at the crossing-point. Then
+Leverrier showed that the original comet associated with the November
+meteors was probably brought into the system by the influence of the
+planet Uranus in the year 126 of the Christian era. Afterward
+Alexander Herschel identified the tracks of no less than seventy-six
+meteor-swarms (most of them inconspicuous) with those of comets. The
+still more recent researches of Mr W. F. Denning make it probable that
+there are no meteors which do not belong to a flock or system probably
+formed by the disintegration of a cometary mass; even the apparently
+sporadic ones which shoot across the sky, ``lost souls in the night,''
+being members of flocks which have become so widely scattered that the
+earth sometimes takes weeks to pass through the region of space where
+their paths lie.
+
+The November meteors should have exhibited another pair of spectacles
+in 1899 and 1900, and their failure to do so caused at first much
+disappointment, until it was made plain that a good reason existed for
+their absence. It was found that after their last appearance, in 1867,
+they had been disturbed in their movements by the planets Jupiter and
+Saturn, whose attractions had so shifted the position of their orbit
+that it no longer intersected that of the earth, as it did before.
+Whether another planetary interference will sometime bring the
+principal mass of the November meteors back to the former point of
+intersection with the earth's orbit is a question for the future to
+decide. It would seem that there may be several parallel streams of
+the November meteors, and that some of them, like those of August, are
+distributed entirely around the orbit, so that every mid-November we
+see a few of them.
+
+We come now to a very remarkable example of the disintegration of a
+comet and the formation of a meteor-stream. In 1826 Biela, of
+Josephstadt, Austria, discovered a comet to which his name was given.
+Calculation showed that it had an orbital period of about six and a
+half years, belonging to Jupiter's ``family.'' On one of its returns,
+in 1846, it astonished its watchers by suddenly splitting in two. The
+two comets thus formed out of one separated to a distance of about one
+hundred and sixty thousand miles, and then raced side by side,
+sometimes with a curious ligature connecting them, like Siamese twins,
+until they disappeared together in interplanetary space. In 1852 they
+came back, still nearly side by side, but now the distance between
+them had increased to a million and a quarter of miles. After that, at
+every recurrence of their period, astronomers looked for them in vain,
+until 1872, when an amazing thing happened. On the night of November
+28th, when the earth was crossing the plane of the orbit of the
+missing comet, a brilliant shower of meteors burst from the northern
+sky, traveling nearly in the track which the comet should have
+pursued. The astronomers were electrified. Klinkerfues, of G�ttingen,
+telegraphed to Pogson, of Madras: ``Biela touched earth; search near
+Theta Centauri.'' Pogson searched in the place indicated and saw a
+cometary mass retreating into the southern heavens, where it was soon
+swallowed from sight!
+
+Since then the Biela meteors have been among the recognized periodic
+spectacles of the sky, and few if any doubt that they represent a
+portion of the missing comet whose disintegration began with the
+separation into two parts in 1846. The comet itself has never since
+been seen. The first display of these meteors, sometimes called the
+``Andromedes,'' because they radiate from the constellation Andromeda,
+was remarkable for the great brilliancy of many of the fire-balls that
+shot among the shower of smaller sparks, some of which were described
+as equaling the full moon in size. None of them is known to have
+reached the earth, but during the display of the same meteors in 1885
+a meteoric mass fell at Mazapil in Northern Mexico (it is now in the
+Museum at Vienna), which many have thought may actually be a piece of
+the original comet of Biela. This brings us to the second branch of
+our subject.
+
+More rare than meteors or falling stars, and more startling, except
+that they never appear in showers, are the huge balls of fire which
+occasionally dart through the sky, lighting up the landscapes beneath
+with their glare, leaving trains of sparks behind them, often
+producing peals of thunder when they explode, and in many cases
+falling upon the earth and burying themselves from a few inches to
+several feet in the soil, from which, more than once, they have been
+picked up while yet hot and fuming. These balls are sometimes called
+bolides. They are not really round in shape, although they often look
+so while traversing the sky, but their forms are fragmentary, and
+occasionally fantastic. It has been supposed that their origin is
+different from that of the true meteors; it has even been conjectured
+that they may have originated from the giant volcanoes of the moon or
+have been shot out from the sun during some of the tremendous
+explosions that accompany the formation of eruptive prominences. By
+the same reasoning some of them might be supposed to have come from
+some distant star. Others have conjectured that they are wanderers in
+space, of unknown origin, which the earth encounters as it journeys
+on, and Lord Kelvin made a suggestion which has become classic because
+of its imaginative reach -- viz., that the first germs of life may
+have been brought to the earth by one of these bodies, ``a fragment of
+an exploded world.''
+
+It is a singular fact that astronomers and scientific men in general
+were among the last to admit the possibility of solid masses falling
+from the sky. The people had believed in the reality of such phenomena
+from the earliest times, but the savants shook their heads and talked
+of superstition. This was the less surprising because no
+scientifically authenticated instance of such an occurrence was known,
+and the stones popularly believed to have fallen from the sky had
+become the objects of worship or superstitious reverence, a fact not
+calculated to recommend them to scientific credence. The celebrated
+``black stone'' suspended in the Kaaba at Mecca is one of these
+reputed gifts from heaven; the ``Palladium'' of ancient Troy was
+another; and a stone which fell near Ensisheim, in Germany, was placed
+in a church as an object to be religiously venerated. Many legends of
+falling stones existed in antiquity, some of them curiously
+transfigured by the imagination, like the ``Lion of the
+Peloponnesus,'' which was said to have sprung down from the sky upon
+the Isthmus of Corinth. But near the beginning of the nineteenth
+century, in 1803, a veritable shower of falling stones occurred at
+L'Aigle, in Northern France, and this time astronomers took note of
+the phenomenon and scientifically investigated it. Thousands of the
+strange projectiles came from the sky on this occasion, and were
+scattered over a wide area of country, and some buildings were hit.
+Four years later another shower of stones occurred at Weston, Conn.,
+numbering thousands of individuals. The local alarm created in both
+cases was great, as well it might be, for what could be more
+intimidating than to find the blue vault of heaven suddenly hurling
+solid missiles at the homes of men? After these occurrences it was
+impossible for the most skeptical to doubt any longer, and the regular
+study of ``aerolites,'' or ``meteorites,'' began.
+
+One of the first things recognized was the fact that fire-balls are
+solid meteorites in flight, and not gaseous exhalations in the air, as
+some had assumed. They burn in the air during their flight, and
+sometimes, perhaps, are entirely consumed before reaching the ground.
+Their velocity before entering the earth's atmosphere is equal to that
+of the planets in their orbits -- viz., from twenty to thirty miles
+per second -- a fact which proves that the sun is the seat of the
+central force governing them. Their burning in the air is not
+difficult to explain; it is the heat of friction which so quickly
+brings them to incandescence. Calculation shows that a body moving
+through the air at a velocity of about a mile per second will be
+brought, superficially, to the temperature of ``red heat'' by friction
+with the atmosphere. If its velocity is twenty miles per second the
+temperature will become thousands of degrees. This is the state of
+affairs with a meteorite rushing into the earth's atmosphere; its
+surface is liquefied within a few seconds after the friction begins to
+act, and the melted and vaporized portion of its mass is swept
+backward, forming the train of sparks that follows every great
+fire-ball. However, there is one phenomenon connected with the trains
+of meteorites which has never been satisfactorily explained: they
+often persist for long periods of time, drifting and turning with the
+wind, but not ceasing to glow with a phosphorescent luminosity. The
+question is, Whence comes this light? It must be light without heat,
+since the fine dust or vapor of which the train can only consist would
+not retain sufficient heat to render it luminous for so long a time.
+An extremely remarkable incident of this kind occurred on February 22,
+1909, when an immense fire-ball that passed over southern England left
+a train that remained visible during two hours, assuming many curious
+shapes as it was drifted about by currents in the air.
+
+But notwithstanding the enormous velocity with which meteorites enter
+the air they are soon slowed down to comparatively moderate speed, so
+that when they disappear they are usually traveling not faster than a
+mile a second. The courses of many have been traced by observers
+situated along their track at various points, and thus a knowledge has
+been obtained of their height above the ground during their flight and
+of the length of their visible courses. They generally appear at an
+elevation of eighty or a hundred miles, and are seldom visible after
+having descended to within five miles of the ground, unless the
+observer happens to be near the striking-point, when he may actually
+witness the fall. Frequently they burst while high in the air and
+their fragments are scattered like shrapnel over the surface of the
+ground, sometimes covering an area of several square miles, but of
+course not thickly; different fragments of the same meteorite may
+reach the ground at points several miles apart. The observed length of
+their courses in the atmosphere varies from fifty to five hundred
+miles. If they continued a long time in flight after entering the air,
+even the largest of them would probably be consumed to the last scrap,
+but their fiery career is so short on account of their great speed
+that the heat does not have time to penetrate very deeply, and some
+that have been picked up immediately after their fall have been found
+cold as ice within. Their size after reaching the ground is variable
+within wide limits; some are known which weigh several tons, but the
+great majority weigh only a few pounds and many only a few ounces.
+
+Meteorites are of two kinds: stony meteorites and iron meteorites. The
+former outnumber the latter twenty to one; but many stone meteorites
+contain grains of iron. Nickel is commonly found in iron meteorites,
+so that it might be said that that redoubtable alloy nickel-steel is
+of cosmical invention. Some twenty-five chemical elements have been
+found in meteorites, including carbon and the ``sun-metal,'' helium.
+The presence of the latter is certainly highly suggestive in
+connection with the question of the origin of meteorites. The iron
+meteorites, besides metallic iron and nickel, of which they are almost
+entirely composed, contain hydrogen, helium, and carbonic oxide, and
+about the only imaginable way in which these gases could have become
+absorbed in the iron would be through the immersion of the latter
+while in a molten or vaporized state in a hot and dense atmosphere
+composed of them, a condition which we know to exist only in the
+envelopes of the sun and the stars.
+
+The existence of carbon in the Canyon Diablo iron meteorites is
+attended by a circumstance of the most singular character -- a very
+``fairy tale of science.'' In some cases the carbon has become
+diamond! These meteoric diamonds are very small; nevertheless, they
+are true diamonds, resembling in many ways the little black gems
+produced by Moissan's method with the aid of the electric furnace. The
+fact that they are found embedded in these iron meteorites is another
+argument in favor of the hypothesis of the solar or stellar origin of
+the latter. To appreciate this it is necessary to recall the way in
+which Moissan made his diamonds. It was by a combination of the
+effects of great heat, great pressure, and sudden or rapid superficial
+cooling on a mass of iron containing carbon. When he finally broke
+open his iron he found it a pudding stuffed with miniature black
+diamonds. When a fragment of the Canyon Diablo meteoric iron was
+polished in Philadelphia over fifteen years ago it cut the emery-wheel
+to pieces, and examination showed that the damage had been effected by
+microscopic diamonds peppered through the mass. How were those
+diamonds formed? If the sun or Sirius was the laboratory that prepared
+them, we can get a glimpse at the process of their formation. There is
+plenty of heat, plenty of pressure, and an abundance of vaporized iron
+in the sun and the stars. When a great solar eruption takes place,
+masses of iron which have absorbed carbon may be shot out with a
+velocity which forbids their return. Plunged into the frightful cold
+of space, their surfaces are quickly cooled, as Moissan cooled his
+prepared iron by throwing it into water, and thus the requisite stress
+is set up within, and, as the iron solidifies, the included carbon
+crystallizes into diamonds. Whether this explanation has a germ of
+truth in it or not, at any rate it is evident that iron meteorites
+were not created in the form in which they come to us; they must once
+have been parts of immeasurably more massive bodies than themselves.
+
+The fall of meteorites offers an appreciable, though numerically
+insignificant, peril to the inhabitants of the earth. Historical
+records show perhaps three or four instances of people being killed by
+these bodies. But for the protection afforded by the atmosphere, which
+acts as a very effective shield, the danger would doubtless be very
+much greater. In the absence of an atmosphere not only would more
+meteorites reach the ground, but their striking force would be
+incomparably greater, since, as we have seen, the larger part of their
+original velocity is destroyed by the resistance of the air. A
+meteorite weighing many tons and striking the earth with a velocity of
+twenty or thirty miles per second, would probably cause frightful
+havoc.
+
+It is a singular fact that recent investigations seem to have proved
+that an event of this kind actually happened in North America --
+perhaps not longer than a thousand or two thousand years ago. The
+scene of the supposed catastrophe is in northern central Arizona, at
+Coon Butte, where there is a nearly circular crater in the middle of a
+circular elevation or small mountain. The crater is somewhat over four
+thousand feet in diameter, and the surrounding rim, formed of upturned
+strata and ejected rock fragments, rises at its highest point one
+hundred and sixty feet above the plain. The crater is about six
+hundred feet in depth -- that is, from the rim to the visible floor or
+bottom of the crater. There is no evidence that volcanic action has
+ever taken place in the immediate neighborhood of Coon Butte. The rock
+in which the crater has been made is composed of horizontal sandstone
+and limestone strata. Between three hundred and four hundred million
+tons of rock fragments have been detached, and a large portion hurled
+by some cause out of the crater. These fragments lie concentrically
+distributed around the crater, and in large measure form the elevation
+known as Coon Butte. The region has been famous for nearly twenty
+years on account of the masses of meteoric iron found scattered about
+and known as the ``Canyon Diablo'' meteorites. It was one of these
+masses, which consist of nickel-iron containing a small quantity of
+platinum, and of which in all some ten tons have been recovered for
+sale to the various collectors throughout the world, that as before
+mentioned destroyed the grinding-tool at Philadelphia through the
+cutting power of its embedded diamonds. These meteoric irons are
+scattered about the crater-hill, in concentric distribution, to a
+maximum distance of about five miles. When the suggestion was first
+made in 1896 that a monster meteorite might have created by its fall
+this singular lone crater in stratified rocks, it was greeted with
+incredulous smiles; but since then the matter has assumed a different
+aspect. The Standard Iron Company, formed by Messrs. D. M. Barringer,
+B. C. Tilghman, E. J. Bennitt, and S. J. Holsinger, having become, in
+1903, the owner of this freak of nature, sunk shafts and bored holes
+to a great depth in the interior of the crater, and also trenched the
+slopes of the mountain, and the result of their investigations has
+proved that the meteoric hypothesis of origin is correct. (See the
+papers published in the Proceedings of the Academy of Natural Sciences
+of Philadelphia, December, 1905, wherein it is proved that the United
+States Geological Survey was wrong in believing this crater to have
+been due to a steam explosion. Since that date there has been
+discovered a great amount of additional confirmatory proof). Material
+of unmistakably meteoric origin was found by means of the drills,
+mixed with crushed rock, to a depth of six hundred to seven hundred
+feet below the floor of the crater, and a great deal of it has been
+found admixed with the ejected rock fragments on the outer slopes of
+the mountain, absolutely proving synchronism between the two events,
+the formation of this great crater and the falling of the meteoric
+iron out of the sky. The drill located in the bottom of the crater was
+sent, in a number of cases, much deeper (over one thousand feet) into
+unaltered horizontal red sandstone strata, but no meteoric material
+was found below this depth (seven hundred feet, or between eleven and
+twelve hundred feet below the level of the surrounding plain), which
+has been assumed as being about the limit of penetration. It is not
+possible to sink a shaft at present, owing to the water which has
+drained into the crater, and which forms, with the finely pulverized
+sandstone, a very troublesome quicksand encountered at about two
+hundred feet below the visible floor of the crater. As soon as this
+water is removed by pumping it will be easy to explore the depths of
+the crater by means of shafts and drifts. The rock strata (sandstone
+and limestone) of which the walls consist present every appearance of
+having been violently upturned by a huge body penetrating the earth
+like a cannon-ball. The general aspect of the crater strikingly
+resembles the impression made by a steel projectile shot into an
+armor-plate. Mr Tilghman has estimated that a meteorite about five
+hundred feet in diameter and moving with a velocity of about five
+miles per second would have made just such a perforation upon striking
+rocks of the character of those found at this place. There was some
+fusion of the colliding masses, and the heat produced some steam from
+the small amount of water in the rocks. As a result there has been
+found at depth a considerable amount of fused quartz (original
+sandstone), and with it innumerable particles or sparks of fused
+nickel-iron (original meteorite). A projectile of that size
+penetrating eleven to twelve hundred feet into the rocky shell of the
+globe must have produced a shock which was perceptible several hundred
+miles away.
+
+The great velocity ascribed to the supposed meteorite at the moment of
+striking could be accounted for by the fact that it probably plunged
+nearly vertically downward, for it formed a circular crater in the
+rocky crust of the earth. In that case it would have been less
+retarded by the resistance of the atmosphere than are meteorites which
+enter the air at a lower angle and shoot ahead hundreds of miles until
+friction has nearly destroyed their original motion when they drop
+upon the earth. Some meteoric masses of great size, such as Peary's
+iron meteorite found at Cape York, Greenland, and the almost equally
+large mass discovered at Bacubirito, Mexico, appear to have penetrated
+but slightly on striking the earth. This may be explained by supposing
+that they pursued a long, horizontal course through the air before
+falling. The result would be that, their original velocity having been
+practically destroyed, they would drop to the ground with a velocity
+nearly corresponding to that which gravity would impart within the
+perpendicular distance of their final fall. A
+six-hundred-and-sixty-pound meteorite, which fell at Knyahinya,
+Hungary, striking at an angle of 27� from the vertical, penetrated the
+ground to a depth of eleven feet.
+
+It has been remarked that the Coon Butte meteorite may have fallen not
+longer ago than a few thousand years. This is based upon the fact that
+the geological indications favor the supposition that the event did
+not occur more than five thousand years ago, while on the other hand
+the rings of growth in the cedar-trees growing on the slopes of the
+crater show that they have existed there about seven hundred years.
+Prof. William H. Pickering has recently correlated this with an
+ancient chronicle which states that at Cairo, Egypt, in the year 1029,
+``many stars passed with a great noise.'' He remarks that Cairo is
+about 100�, by great circle, from Coon Butte, so that if the meteorite
+that made the crater was a member of a flock of similar bodies which
+encountered the earth moving in parallel lines, some of them might
+have traversed the sky tangent to the earth's surface at Cairo. That
+the spectacle spoken of in the chronicle was caused by meteorites he
+deems exceedingly probable because of what is said about ``a great
+noise;'' meteorites are the only celestial phenomena attended with
+perceptible sounds. Professor Pickering conjectures that this supposed
+flock of great meteorites may have formed the nucleus of a comet which
+struck the earth, and he finds confirmation of the idea in the fact
+that out of the ten largest meteorites known, no less than seven were
+found within nine hundred miles of Coon Butte. It would be interesting
+if we could trace back the history of that comet, and find out what
+malicious planet caught it up in its innocent wanderings and hurled it
+with so true an aim at the earth! This remarkable crater is one of the
+most interesting places in the world, for there is absolutely no
+record of such a mass, possibly an iron-headed comet, from outer space
+having come into collision with our earth. The results of the future
+exploration of the depths of the crater will be awaited with much
+interest.
+
+The Wrecking of the Moon
+
+There are sympathetic moods under whose influence one gazes with a
+certain poignant tenderness at the worn face of the moon; that little
+``fossil world'' (the child of our mother earth, too) bears such
+terrible scars of its brief convulsive life that a sense of pity is
+awakened by the sight. The moon is the wonder-land of the telescope.
+Those towering mountains, whose ``proud aspiring peaks'' cast
+silhouettes of shadow that seem drawn with india-ink; those vast
+plains, enchained with gentle winding hills and bordered with giant
+ranges; those oval ``oceans,'' where one looks expectant for the flash
+of wind-whipped waves; those enchanting ``bays'' and recesses at the
+seaward feet of the Alps; those broad straits passing between guardian
+heights incomparably mightier than Gibraltar; those locket-like
+valleys as secluded among their mountains as the Vale of Cashmere;
+those colossal craters that make us smile at the pretensions of
+Vesuvius, Etna, and Cotopaxi; those strange white ways which pass with
+the unconcern of Roman roads across mountain, gorge, and valley -- all
+these give the beholder an irresistible impression that it is truly a
+world into which he is looking, a world akin to ours, and yet no more
+like our world than Pompeii is like Naples. Its air, its waters, its
+clouds, its life are gone, and only a skeleton remains -- a mute but
+eloquent witness to a cosmical tragedy without parallel in the range
+of human knowledge.
+
+One cannot but regret that the moon, if it ever was the seat of
+intelligent life, has not remained so until our time. Think what the
+consequences would have been if this other world at our very door had
+been found to be both habitable and inhabited! We talk rather airily
+of communicating with Mars by signals; but Mars never approaches
+nearer than 35,000,000 miles, while the moon when nearest is only a
+little more than 220,000 miles away. Given an effective magnifying
+power of five thousand diameters, which will perhaps be possible at
+the mountain observatories as telescopes improve, and we should be
+able to bring the moon within an apparent distance of about forty
+miles, while the corresponding distance for Mars would be more than
+seven thousand miles. But even with existing telescopic powers we can
+see details on the moon no larger than some artificial constructions
+on the earth. St Peter's at Rome, with the Vatican palace and the
+great piazza, if existing on the moon, would unquestionably be
+recognizable as something else than a freak of nature. Large cities,
+with their radiating lines of communication, would at once betray
+their real character. Cultivated tracts, and the changes produced by
+the interference of intelligent beings, would be clearly recognizable.
+The electric illumination of a large town at night would probably be
+markedly visible. Gleams of reflected sunlight would come to us from
+the surfaces of the lakes and oceans, and a huge ``liner'' traversing
+a lunar sea could probably be followed by its trail of smoke. As to
+communications by ``wireless'' signals, which certain enthusiasts have
+thought of in connection with Mars, in the case of the moon they
+should be a relatively simple matter, and the feat might actually be
+accomplished. Think what a literature would grow up about the moon if
+it were a living world! Its very differences from the earth would only
+accentuate its interest for us. Night and day on the moon are each two
+weeks in length; how interesting it would be to watch the manner in
+which the lunarians dealt with such a situation as that. Lunar and
+terrestrial history would keep step with each other, and we should
+record them both. Truly one might well wish to have a neighbor world
+to study; one would feel so much the less alone in space.
+
+It is not impossible that the moon did at one time have inhabitants of
+some kind. But, if so, they vanished with the disappearance of its
+atmosphere and seas, or with the advent of its cataclysmic age. At the
+best, its career as a living world must have been brief. If the water
+and air were gradually absorbed, as some have conjectured, by its
+cooling interior rocks, its surface might, nevertheless, have retained
+them for long ages; but if, as others think, their disappearance was
+due to the escape of their gaseous molecules in consequence of the
+inability of the relatively small lunar gravitation to retain them,
+then the final catastrophe must have been as swift as it was
+inevitable. Accepting Darwin's hypothesis, that the moon was separated
+from the earth by tidal action while both were yet plastic or
+nebulous, we may reasonably conclude that it began its career with a
+good supply of both water and air, but did not possess sufficient mass
+to hold them permanently. Yet it may have retained them long enough
+for life to develop in many forms upon its surface; in fact, there are
+so many indications that air and water have not always been lacking to
+the lunar world that we are driven to invent theories to explain both
+their former presence and their present absence.
+
+But whatever the former condition of the moon may have been, its
+existing appearance gives it a resistless fascination, and it bears so
+clearly the story of a vast catastrophe sculptured on its rocky face
+that the thoughtful observer cannot look upon it without a feeling of
+awe. The gigantic character of the lunar features impresses the
+beholder not less than the universality of the play of destructive
+forces which they attest. Let us make a few comparisons. Take the
+lunar crater called ``Tycho'', which is a typical example of its kind.
+In the telescope Tycho appears as a perfect ring surrounding a
+circular depression, in the center of which rises a group of
+mountains. Its superficial resemblance to some terrestrial volcanic
+craters is very striking. Vesuvius, seen from a point vertically
+above, would no doubt look something like that (the resemblance would
+have been greater when the Monte del Cavallo formed a more complete
+circuit about the crater cone). But compare the dimensions. The
+remains of the outer crater ring of Vesuvius are perhaps half a mile
+in diameter, while the active crater itself is only two or three
+hundred feet across at the most; Tycho has a diameter of fifty-four
+miles! The group of relatively insignificant peaks in the center of
+the crater floor of Tycho is far more massive than the entire mountain
+that we call Vesuvius. The largest known volcanic crater on the earth,
+Aso San, in Japan, has a diameter of seven miles; it would take sixty
+craters like Aso San to equal Tycho in area! And Tycho, though one of
+the most perfect, is by no means the largest crater on the moon.
+Another, called ``Theophilus,'' has a diameter of sixty-four miles,
+and is eighteen thousand feet deep. There are hundreds from ten to
+forty miles in diameter, and thousands from one to ten miles. They are
+so numerous in many places that they break into one another, like the
+cells of a crushed honeycomb.
+
+The lunar craters differ from those of the earth more fundamentally
+than in the matter of mere size; they are not situated on the tops of
+mountains. If they were, and if all the proportions were the same, a
+crater like Tycho might crown a conical peak fifty or one hundred
+miles high! Instead of being cavities in the summits of mountains, the
+lunar craters are rather gigantic sink-holes whose bottoms in many
+cases lie two or three miles below the general surface of the lunar
+world. Around their rims the rocks are piled up to a height of from a
+few hundred to two or three thousand feet, with a comparatively gentle
+inclination, but on the inner side they fall away in gigantic broken
+precipices which make the dizzy cliffs of the Matterhorn seem but
+``lover's leaps.'' Down they drop, ridge below ridge, crag under crag,
+tottering wall beneath wall, until, in a crater named ``Newton,'' near
+the south lunar pole, they attain a depth where the rays of the sun
+never reach. Nothing more frightful than the spectacle which many of
+these terrible chasms present can be pictured by the imagination. As
+the lazy lunar day slowly advances, the sunshine, unmitigated by
+clouds or atmospheric veil of any kind, creeps across their rims and
+begins to descend the opposite walls. Presently it strikes the ragged
+crest of a ridge which had lain hidden in such darkness as we never
+know on the earth, and runs along it like a line of kindling fire.
+Rocky pinnacles and needles shoot up into the sunlight out of the
+black depths. Down sinks the line of light, mile after mile, and
+continually new precipices and cliffs are brought into view, until at
+last the vast floor is attained and begins to be illuminated. In the
+meanwhile the sun's rays, darting across the gulf, have touched the
+summits of the central peaks, twenty or thirty miles from the crater's
+inmost edge, and they immediately kindle and blaze like huge stars
+amid the darkness. So profound are some of these awful craters that
+days pass before the sun has risen high enough above them to chase the
+last shadows from their depths.
+
+Although several long ranges of mountains resembling those of the
+earth exist on the moon, the great majority of its elevations assume
+the crateriform aspect. Sometimes, instead of a crater, we find an
+immense mountain ring whose form and aspect hardly suggest volcanic
+action. But everywhere the true craters are in evidence, even on the
+sea-beds, although they attain their greatest number and size on those
+parts of the moon -- covering sixty per cent of its visible surface --
+which are distinctly mountainous in character and which constitute its
+most brilliant portions. Broadly speaking, the southwestern half of
+the moon is the most mountainous and broken, and the northeastern half
+the least so. Right down through the center, from pole to pole, runs a
+wonderful line of craters and crateriform valleys of a magnitude
+stupendous even for the moon. Another similar line follows the western
+edge. Three or four ``seas'' are thrust between these mountainous
+belts. By the effects of ``libration'' parts of the opposite
+hemisphere of the moon which is turned away from the earth are from
+time to time brought into view, and their aspect indicates that that
+hemisphere resembles in its surface features the one which faces the
+earth. There are many things about the craters which seem to give some
+warrant for the hypothesis which has been particularly urged by Mr G.
+K. Gilbert, that they were formed by the impact of meteors; but there
+are also many things which militate against that idea, and, upon the
+whole, the volcanic theory of their origin is to be preferred.
+
+The enormous size of the lunar volcanoes is not so difficult to
+account for when we remember how slight is the force of lunar gravity
+as compared with that of the earth. With equal size and density,
+bodies on the moon weigh only one-sixth as much as on the earth.
+Impelled by the same force, a projectile that would go ten miles on
+the earth would go sixty miles on the moon. A lunar giant thirty-five
+feet tall would weigh no more than an ordinary son of Adam weighs on
+his greater planet. To shoot a body from the earth so that it would
+not drop back again, we should have to start it with a velocity of
+seven miles per second; a mile and a half per second would serve on
+the moon. It is by no means difficult to believe, then, that a lunar
+volcano might form a crater ring eight or ten times broader than the
+greatest to be found on the earth, especially when we reflect that in
+addition to the relatively slight force of gravity, the materials of
+the lunar crust are probably lighter than those of our terrestrial
+rocks.
+
+For similar reasons it seems not impossible that the theory mentioned
+in a former chapter -- that some of the meteorites that have fallen
+upon the earth originated from the lunar volcanoes -- is well founded.
+This would apply especially to the stony meteorites, for it is hardly
+to be supposed that the moon, at least in its superficial parts,
+contains much iron. It is surely a scene most strange that is thus
+presented to the mind's eye -- that little attendant of the earth's
+(the moon has only one-fiftieth of the volume, and only one-eightieth
+of the mass of the earth) firing great stones back at its parent
+planet! And what can have been the cause of this furious outbreak of
+volcanic forces on the moon? Evidently it was but a passing stage in
+its history; it had enjoyed more quiet times before. As it cooled down
+from the plastic state in which it parted from the earth, it became
+incrusted after the normal manner of a planet, and then oceans were
+formed, its atmosphere being sufficiently dense to prevent the water
+from evaporating and the would-be oceans from disappearing continually
+in mist. This, if any, must have been the period of life in the lunar
+world. As we look upon the vestiges of that ancient world buried in
+the wreck that now covers so much of its surface, it is difficult to
+restrain the imagination from picturing the scenes which were once
+presented there; and, in such a case, should the imagination be
+fettered? We give it free rein in terrestrial life, and it rewards us
+with some of our greatest intellectual pleasures. The wonderful
+landscapes of the moon offer it an ideal field with just enough
+half-hidden suggestions of facts to stimulate its powers.
+
+The great plains of the Mare Imbrium and the Mare Serenitatis (the
+``Sea of Showers'' and the ``Sea of Serenity''), bordered in part by
+lofty mountain ranges precisely like terrestrial mountains, scalloped
+along their shores with beautiful bays curving back into the adjoining
+highlands, and united by a great strait passing between the nearly
+abutting ends of the ``Lunar Apennines'' and the ``Lunar Caucasus,''
+offer the elements of a scene of world beauty such as it would be
+difficult to match upon our planet. Look at the finely modulated
+bottom of the ancient sea in Mr Ritchey's exquisite photograph of the
+western part of the Mare Serenitatis, where one seems to see the play
+of the watery currents heaping the ocean sands in waving lines, making
+shallows, bars, and deeps for the mariner to avoid or seek, and
+affording a playground for the creatures of the main. What geologist
+would not wish to try his hammer on those rocks with their stony pages
+of fossilized history? There is in us an instinct which forbids us to
+think that there was never any life there. If we could visit the moon,
+there is not among us a person so prosaic and unimaginative that he
+would not, the very first thing, begin to search for traces of its
+inhabitants. We would look for them in the deposits on the sea
+bottoms; we would examine the shores wherever the configuration seemed
+favorable for harbors and the sites of maritime cities -- forgetting
+that it may be a little ridiculous to ascribe to the ancient lunarians
+the same ideas that have governed the development of our race; we
+would search through the valleys and along the seeming courses of
+vanished streams; we would explore the mountains, not the terrible
+craters, but the pinnacled chains that recall our own Alps and
+Rockies; seeking everywhere some vestige of the transforming presence
+of intelligent life. Perhaps we should find such traces, and perhaps,
+with all our searching, we should find nothing to suggest that life
+had ever existed amid that universal ruin.
+
+Look again at the border of the ``Sea of Serenity'' -- what a name for
+such a scene! -- and observe how it has been rent with almost
+inconceivable violence, the wall of the colossal crater Posidonius
+dropping vertically upon the ancient shore and obliterating it, while
+its giant neighbor, Le Monnier, opens a yawning mouth as if to swallow
+the sea itself. A scene like this makes one question whether, after
+all, those may not be right who have imagined that the so-called sea
+bottoms are really vast plains of frozen lava which gushed up in
+floods so extensive that even the mighty volcanoes were half drowned
+in the fiery sea. This suggestion becomes even stronger when we turn
+to another of the photographs of Mr Ritchey's wonderful series,
+showing a part of the Mare Tranquilitatis (``Sea of Tranquility''!).
+Notice how near the center of the picture the outline of a huge ring
+with radiating ridges shows through the sea bottom; a fossil volcano
+submerged in a petrified ocean! This is by no means the only instance
+in which a buried world shows itself under the great lunar plains.
+Yet, as the newer craters in the sea itself prove, the volcanic
+activity survived this other catastrophe, or broke out again
+subsequently, bringing more ruin to pile upon ruin.
+
+Yet notwithstanding the evidence which we have just been considering
+in support of the hypothesis that the ``seas'' are lava floods,
+Messrs. Loewy and Puiseux, the selenographers of the Paris
+Observatory, are convinced that these great plains bear characteristic
+marks of the former presence of immense bodies of water. In that case
+we should be forced to conclude that the later oceans of the moon lay
+upon vast sheets of solidified lava; and thus the catastrophe of the
+lunar world assumes a double aspect, the earliest oceans being
+swallowed up in molten floods issuing from the interior, while the
+lands were reduced to chaos by a universal eruption of tremendous
+volcanoes; and then a period of comparative quiet followed, during
+which new seas were formed, and new life perhaps began to flourish in
+the lunar world, only to end in another cataclysm, which finally put a
+term to the existence of the moon as a life-supporting world.
+
+Suppose we examine two more of Mr Ritchey's illuminating photographs,
+and, first, the one showing the crater Theophilus and its
+surroundings. We have spoken of Theophilus before, citing the facts
+that it is sixty-four miles in diameter and eighteen thousand feet
+deep. It will be noticed that it has two brother giants -- Cyrillus
+the nearer, and Catharina the more distant; but Theophilus is plainly
+the youngest of the trio. Centuries, and perhaps thousands of years,
+must have elapsed between the periods of their upheaval, for the two
+older craters are partly filled with d�bris, while it is manifest at a
+glance that when the south eastern wall of Theophilus was formed, it
+broke away and destroyed a part of the more ancient ring of Cyrillus.
+There is no more tremendous scene on the moon than this; viewed with a
+powerful telescope, it is absolutely appalling.
+
+The next photograph shows, if possible, a still wilder region. It is
+the part of the moon lying between Tycho and the south pole. Tycho is
+seen in the lower left-hand part of the picture. To the right, at the
+edge of the illuminated portion of the moon, are the crater-rings,
+Longomontanus and Wilhelm I, the former being the larger. Between them
+are to be seen the ruins of two or three more ancient craters which,
+together with portions of the walls of Wilhelm I and Longomontanus,
+have been honeycombed with smaller craters. The vast crateriform
+depression above the center of the picture is Clavius, an unrivaled
+wonder of lunar scenery, a hundred and forty-two miles in its greatest
+length, while its whole immense floor has sunk two miles below the
+general surface of the moon outside the ring. The monstrous
+shadow-filled cavity above Clavius toward the right is Blancanus,
+whose aspect here gives a good idea of the appearance of these chasms
+when only their rims are in the sunlight. But observe the
+indescribable savagery of the entire scene. It looks as though the
+spirit of destruction had gone mad in this spot. The mighty craters
+have broken forth one after another, each rending its predecessor; and
+when their work was finished, a minor but yet tremendous outbreak
+occurred, and the face of the moon was gored and punctured with
+thousands of smaller craters. These relatively small craters (small,
+however, only in a lunar sense, for many of them would appear gigantic
+on the earth) recall once more the theory of meteoric impact. It does
+not seem impossible that some of them may have been formed by such an
+agency.
+
+One would not wish for our planet such a fate as that which has
+overtaken the moon, but we cannot be absolutely sure that something of
+the kind may not be in store for it. We really know nothing of the
+ultimate causes of volcanic activity, and some have suggested that the
+internal energies of the earth may be accumulating instead of dying
+out, and may never yet have exhibited their utmost destructive power.
+Perhaps the best assurance that we can find that the earth will escape
+the catastrophe that has overtaken its satellite is to be found in the
+relatively great force of its gravitation. The moon has been the
+victim of its weakness; given equal forces, and the earth would be the
+better able to withstand them. It is significant, in connection with
+these considerations, that the little planet Mercury, which seems also
+to have parted with its air and water, shows to the telescope some
+indications that it is pitted with craters resembling those that have
+torn to pieces the face of the moon.
+
+Upon the whole, after studying the dreadful lunar landscapes, one
+cannot feel a very enthusiastic sympathy with those who are seeking
+indications of the continued existence of some kind of life on the
+moon; such a world is better without inhabitants. It has met its fate;
+let it go! Fortunately, it is not so near that it cannot hide its
+scars and appear beautiful -- except when curiosity impels us to look
+with the penetrating eyes of the astronomer.
+
+The Great Mars Problem
+
+Let any thoughtful person who is acquainted with the general facts of
+astronomy look up at the heavens some night when they appear in their
+greatest splendor, and ask himself what is the strongest impression
+that they make upon his mind. He may not find it easy to frame an
+answer, but when he has succeeded it will probably be to the effect
+that the stars give him an impression of the universality of
+intelligence; they make him feel, as the sun and the moon cannot do,
+that his world is not alone; that all this was not made simply to form
+a gorgeous canopy over the tents of men. If he is of a devout turn of
+mind, he thinks, as he gazes into those fathomless deeps and among
+those bewildering hosts, of the infinite multitude of created beings
+that the Almighty has taken under his care. The narrow ideas of the
+old geocentric theology, which made the earth God's especial
+footstool, and man his only rational creature, fall away from him like
+a veil that had obscured his vision; they are impossible in the
+presence of what he sees above. Thus the natural tendency, in the
+light of modern progress, is to regard the universe as everywhere
+filled with life.
+
+But science, which is responsible for this broadening of men's
+thoughts concerning the universality of life, itself proceeds to set
+limits. Of spiritual existences it pretends to know nothing, but as to
+physical beings, it declares that it can only entertain the
+supposition of their existence where it finds evidence of an
+environment suited to their needs, and such environment may not
+everywhere exist. Science, though repelled by the antiquated
+theological conception of the supreme isolation of man among created
+beings, regards with complacency the probability that there are
+regions in the universe where no organic life exists, stars which
+shine upon no inhabited worlds, and planets which nourish no animate
+creatures. The astronomical view of the universe is that it consists
+of matter in every stage of evolution: some nebulous and chaotic; some
+just condensing into stars (suns) of every magnitude and order; some
+shaped into finished solar bodies surrounded by dependent planets;
+some forming stars that perhaps have no planets, and will have none;
+some constituting suns that are already aging, and will soon lose
+their radiant energy and disappear; and some aggregated into masses
+that long ago became inert, cold, and rayless, and that can only be
+revivified by means about which we can form conjectures, but of which
+we actually know nothing.
+
+As with the stars, so with the planets, which are the satellites of
+stars. All investigations unite to tell us that the planets are not
+all in the same state of development. As some are large and some
+small, so some are, in an evolutionary sense, young, and some old. As
+they depend upon the suns around which they revolve for their light,
+heat, and other forms of radiant energy, so their condition varies
+with their distance from those suns. Many may never arrive at a state
+suitable for the maintenance of life upon their surfaces; some which
+are not at present in such a state may attain it later; and the forms
+of life themselves may vary with the peculiar environment that
+different planets afford. Thus we see that we are not scientifically
+justified in affirming that life is ubiquitous, although we are thus
+justified in saying that it must be, in a general sense, universal. We
+might liken the universe to a garden known to contain every variety of
+plant. If on entering it we see no flowers, we examine the species
+before us and find that they are not of those which bloom at this
+particular season, or perhaps they are such as never bear flowers. Yet
+we feel no doubt that we shall find flowers somewhere in the garden,
+because there are species which bloom at this season, and the garden
+contains all varieties.
+
+While it is tacitly assumed that there are planets revolving around
+other stars than the sun, it would be impossible for us to see them
+with any telescope yet invented, and no instrument now in the
+possession of astronomers could assure us of their existence; so the
+only planetary system of which we have visual knowledge is our own.
+Excluding the asteroids, which could not from any point of view be
+considered as habitable, we have in the solar system eight planets of
+various sizes and situated at various distances from the sun. Of these
+eight we know that one, the earth, is inhabited. The question, then,
+arises: Are there any of the others which are inhabited or habitable?
+Since it is our intention to discuss the habitability of only one of
+the seven to which the question applies, the rest may be dismissed in
+a few words. The smallest of them, and the nearest to the sun, is
+Mercury, which is regarded as uninhabitable because it has no
+perceptible supply of water and air, and because, owing to the
+extraordinary eccentricity of its orbit, it is subjected to excessive
+and very rapid alterations in the amount of solar heat and light
+poured upon its surface, such alterations being inconsistent with the
+supposition that it can support living beings. Even its average
+temperature is more than six and a half times that prevailing on the
+earth! Another circumstance which militates against its habitability
+is that, according to the results of the best telescopic studies, it
+always keeps the same face toward the sun, so that one half of the
+planet is perpetually exposed to the fierce solar rays, and the other
+half faces the unmitigated cold of open space. Venus, the next in
+distance from the sun, is almost the exact twin of the earth in size,
+and many arguments may be urged in favor of its habitability, although
+it is suspected of possessing the same peculiarity as Mercury, in
+always keeping the same side sunward. Unfortunately its atmosphere
+appears to be so dense that no permanent markings on its surface are
+certainly visible, and the question of its actual condition must, for
+the present, be left in abeyance. Mars, the first planet more distant
+from the sun than the earth, is the special subject of this chapter,
+and will be described and discussed a few lines further on. Jupiter,
+Saturn, Uranus, and Neptune, the four giant planets, all more distant
+than Mars, and each more distant than the other in the order named,
+are all regarded as uninhabitable because none of them appears to
+possess any degree of solidity. They may have solid or liquid nuclei,
+but exteriorly they seem to be mere balls of cloud. Of course, one can
+imagine what he pleases about the existence of creatures suited to the
+physical constitution of such planets as these, but they must be
+excluded from the category of habitable worlds in the ordinary sense
+of the term. We go back, then, to Mars.
+
+It will be best to begin with a description of the planet. Mars is
+4230 miles in diameter; its surface is not much more than one-quarter
+as extensive as that of the earth (.285). Its mean distance from the
+sun is 141,500,000 miles, 48,500,000 miles greater than that of the
+earth. Since radiant energy varies inversely as the square of
+distance, Mars receives less than half as much solar light and heat as
+the earth gets. Mars' year (period of revolution round the sun) is 687
+days. Its mean density is 71 per cent of the earth's, and the force of
+gravity on its surface is 38 per cent of that on the surface of the
+earth; i.e., a body weighing one hundred pounds on the earth would, if
+transported to Mars, weigh but thirty-eight pounds. The inclination of
+its equator to the plane of its orbit differs very little from that of
+the earth's equator, and its axial rotation occupies 24 hours 37
+minutes. so that the length of day and night, and the extent of the
+seasonal changes on Mars, are almost precisely the same as on the
+earth. But owing to the greater length of its year, the seasons of
+Mars, while occurring in the same order, are almost twice as long as
+ours. The surface of the planet is manifestly solid, like that of our
+globe, and the telescope reveals many permanent markings on it,
+recalling the appearance of a globe on which geographical features
+have been represented in reddish and dusky tints. Around the poles are
+plainly to be seen rounded white areas, which vary in extent with the
+Martian seasons, nearly vanishing in summer and extending widely in
+winter. The most recent spectroscopic determinations indicate that
+Mars has an atmosphere perhaps as dense as that to be found on our
+loftiest mountain peaks, and there is a perceptible amount of watery
+vapor in this atmosphere. The surface of the planet appears to be
+remarkably level, and it has no mountain ranges. No evidences of
+volcanic action have been discovered on Mars. The dusky and reddish
+areas were regarded by the early observers as respectively seas and
+lands, but at present it is not believed that there are any bodies of
+water on the planet. There has never been much doubt expressed that
+the white areas about the poles represent snow.
+
+It will be seen from this brief description that many remarkable
+resemblances exist between Mars and the earth, and there is nothing
+wonderful in the fact that the question of the habitability of the
+former has become one of extreme and wide-spread interest, giving rise
+to the most diverse views, to many extraordinary speculations, and
+sometimes to regrettably heated controversy. The first champion of the
+habitability of Mars was Sir William Herschel, although even before
+his time the idea had been suggested. He was convinced by the
+revelations of his telescopes, continually increasing in power, that
+Mars was more like the earth than any other planet. He could not
+resist the testimony of the polar snows, whose suggestive conduct was
+in such striking accord with what occurs upon the earth. Gradually, as
+telescopes improved and observers increased in number, the principal
+features of the planet were disclosed and charted, and ``areography,''
+as the geography of Mars was called, took its place among the
+recognized branches of astronomical study. But it was not before 1877
+that a fundamentally new discovery in areography gave a truly
+sensational turn to speculation about life on ``the red planet.'' In
+that year Mars made one of its nearest approaches to the earth, and
+was so situated in its orbit that it could be observed to great
+advantage from the northern hemisphere of the earth. The celebrated
+Italian astronomer, Schiaparelli, took advantage of this opportunity
+to make a trigonometrical survey of the surface of Mars -- as coolly
+and confidently as if he were not taking his sights across a
+thirty-five-million-mile gulf of empty space -- and in the course of
+this survey he was astonished to perceive that the reddish areas, then
+called continents, were crossed in many directions by narrow, dusky
+lines, to which he gave the suggestive name of ``canals.'' Thus a kind
+of firebrand was cast into the field of astronomical speculation,
+which has ever since produced disputes that have sometimes approached
+the violence of political faction. At first the accuracy of
+Schiaparelli's observations was contested; it required a powerful
+telescope, and the most excellent ``seeing,'' to render the
+enigmatical lines visible at all, and many searchers were unable to
+detect them. But Schiaparelli continued his studies in the serene sky
+of Italy, and produced charts of the gridironed face of Mars
+containing so much astonishing detail that one had either to reject
+them in toto or to confess that Schiaparelli was right. As subsequent
+favorable oppositions of Mars occurred, other observers began to see
+the ``canals'' and to confirm the substantial accuracy of the Italian
+astronomer's work, and finally few were found who would venture to
+affirm that the ``canals'' did not exist, whatever their meaning might
+be.
+
+When Schiaparelli began his observations it was generally believed, as
+we have said, that the dusky areas on Mars were seas, and since
+Schiaparelli thought that the ``canals'' invariably began and ended at
+the shores of the ``seas,'' the appropriateness of the title given to
+the lines seemed apparent. Their artificial character was immediately
+assumed by many, because they were too straight and too suggestively
+geometrical in their arrangement to permit the conclusion that they
+were natural watercourses. A most surprising circumstance noted by
+Schiaparelli was that the ``canals'' made their appearance after the
+melting of the polar snow in the corresponding hemisphere had begun,
+and that they grew darker, longer, and more numerous in proportion as
+the polar liquidation proceeded; another very puzzling observation was
+that many of them became double as the season advanced; close beside
+an already existing ``canal,'' and in perfect parallelism with it,
+another would gradually make its appearance. That these phenomena
+actually existed and were not illusions was proved by later
+observations, and today they are seen whenever Mars is favorably
+situated for observation.
+
+In the closing decade of the nineteenth century, Mr Percival Lowell
+took up the work where Schiaparelli had virtually dropped it, and soon
+added a great number of ``canals'' to those previously known, so that
+in his charts the surface of the wonderful little planet appears
+covered as with a spider's web, the dusky lines criss-crossing in
+every direction, with conspicuous knots wherever a number of them come
+together. Mr Lowell has demonstrated that the areas originally called
+seas, and thus named on the earlier charts, are not bodies of water,
+whatever else they may be. He has also found that the mysterious lines
+do not, as Schiaparelli supposed, begin and end at the edges of the
+dusky regions, but often continue on across them, reaching in some
+cases far up into the polar regions. But Schiaparelli was right in his
+observation that the appearance of the ``canals'' is synchronous with
+the gradual disappearance of the polar snows, and this fact has become
+the basis of the most extraordinary theory that the subject of life in
+other worlds has ever given birth to.
+
+Now, the effect of such discoveries, as we have related, depends upon
+the type of mind to whose attention they are called. Many are content
+to accept them as strange and inexplicable at present, and to wait for
+further light upon them; others insist upon an immediate inquiry
+concerning their probable nature and meaning. Such an inquiry can only
+be based upon inference proceeding from analogy. Mars, say Mr Lowell
+and those who are of his opinion, is manifestly a solidly incrusted
+planet like the earth; it has an atmosphere, though one of great
+rarity; it has water vapor, as the snows in themselves prove; it has
+the alternation of day and night, and a succession of seasons closely
+resembling those of the earth; its surface is suggestively divided
+into regions of contrasting colors and appearance, and upon that
+surface we see an immense number of lines geometrically arranged, with
+a system of symmetrical intersections where the lines expand into
+circular and oval areas -- and all connected with the annual melting
+of the polar snows in a way which irresistibly suggests the
+interference of intelligence directed to a definite end. Why, with so
+many concurrent circumstances to support the hypothesis, should we not
+regard Mars as an inhabited globe?
+
+But the differences between Mars and the earth are in many ways as
+striking as their resemblances. Mars is relatively small; it gets less
+than half as much light and heat as we receive; its atmosphere is so
+rare that it would be distressing to us, even if we could survive in
+it at all; it has no lakes, rivers, or seas; its surface is an endless
+prairie. and its ``canals'' are phenomena utterly unlike anything on
+the earth. Yet it is precisely upon these divergences between the
+earth and Mars, this repudiation of terrestrial standards, that the
+theory of ``life on Mars,'' for which Mr Lowell is mainly responsible,
+is based. Because Mars is smaller than the earth, we are told it must
+necessarily be more advanced in planetary evolution, the underlying
+cause of which is the gradual cooling and contraction of the planet's
+mass. Mars has parted with its internal heat more rapidly than the
+earth; consequently its waters and its atmosphere have been mostly
+withdrawn by chemical combinations, but enough of both yet remain to
+render life still possible on its surface. As the globe of Mars is
+evolutionally older than that of the earth, so its forms of organic
+life may be proportionally further advanced, and its inhabitants may
+have attained a degree of cultivated intelligence much superior to
+what at present exists upon the earth. Understanding the nature and
+the causes of the desiccation of their planet, and possessing
+engineering science and capabilities far in advance of ours, they may
+be conceived to have grappled with the stupendous problem of keeping
+their world in a habitable condition as long as possible. Supposing
+them to have become accustomed to live in their rarefied atmosphere (a
+thing not inconceivable, since men can live for a time at least in air
+hardly less rare), the most pressing problem for them is that of a
+water-supply, without which plant life cannot exist, while animal life
+in turn depends for its existence upon vegetation. The only direction
+in which they can seek water is that of the polar regions, where it is
+alternately condensed into snow and released in the liquid form by the
+effect of the seasonal changes. It is, then, to the annual melting of
+the polar snow-fields that the Martian engineers are supposed to have
+recourse in supplying the needs of their planet, and thus providing
+the means of prolonging their own existence. It is imagined that they
+have for this purpose constructed a stupendous system of irrigation
+extending over the temperate and equatorial regions of the planet. The
+``canals'' represent the lines of irrigation, but the narrow streaks
+that we see are not the canals themselves, but the irrigated bands
+covered by them. Their dark hue, and their gradual appearance after
+the polar melting has begun, are due to the growth of vegetation
+stimulated by the water. The rounded areas visible where several
+``canals'' meet and cross are called by Mr Lowell ``oases.'' These are
+supposed to be the principal centers of population and industry. It
+must be confessed that some of them, with their complicated systems of
+radiating lines, appear to answer very well to such a theory. No
+attempt to explain them by analogy with natural phenomena on the earth
+has proved successful.
+
+But a great difficulty yet remains: How to explain the seemingly
+miraculous powers of the supposed engineers? Here recourse is had once
+more to the relative smallness of the planet. We have remarked that
+the force of gravity on Mars is only thirty-eight per cent of that on
+the earth. A steam-shovel driven by a certain horse-power would be
+nearly three times as effective there as here. A man of our stature on
+Mars would find his effective strength increased in the same
+proportion. But just because of the slight force of gravity there, a
+Martian might attain to the traditional stature of Goliath without
+finding his own weight an encumbrance to his activity, while at the
+same time his huge muscles would come into unimpeded play, enabling
+him single-handed to perform labors that would be impossible to a
+whole gang of terrestrial workmen. The effective powers of huge
+machines would be increased in the same way; and to all this must be
+added the fact that the mean density of the materials of which Mars is
+composed is much less than that of the constituents of the earth.
+Combining all these considerations, it becomes much less difficult to
+conceive that public works might be successfully undertaken on Mars
+which would be hopelessly beyond the limits of human accomplishment.
+
+Certain other difficulties have also to be met; as, for instance, the
+relative coldness of the climate of Mars. At its distance it gets
+considerably less than half as much light and heat as we receive. In
+addition to this, the rarity of its atmosphere would naturally be
+expected to decrease the effective temperature at the planet's
+surface, since an atmosphere acts somewhat like the glass cover of a
+hot-house in retaining the solar heat which has penetrated it. It has
+been calculated that, unless there are mitigating circumstances of
+which we know nothing, the average temperature at the surface of Mars
+must be far below the freezing-point of water. To this it is replied
+that the possible mitigating circumstances spoken of evidently exist
+in fact, because we can see that the watery vapor condenses into snow
+around the poles in winter, but melts again when summer comes. The
+mitigating agent may be supposed to exist in the atmosphere where the
+presence of certain gases would completely alter the temperature
+gradients.
+
+It might also be objected that it is inconceivable that the Martian
+engineers, however great may be their physical powers, and however
+gigantic the mechanical energies under their control, could force
+water in large quantities from the poles to the equator. This is an
+achievement that measures up to the cosmical standard. It is admitted
+by the champions of the theory that the difficulty is a formidable
+one; but they call attention to the singular fact that on Mars there
+can be found no chains of mountains, and it is even doubtful if ranges
+of hills exist there. The entire surface of the planet appears to be
+almost ``as smooth as a billiard ball,'' and even the broad regions
+which were once supposed to be seas apparently lie at practically the
+same level as the other parts, since the ``canals'' in many cases run
+uninterruptedly across them. Lowell's idea is that these sombre areas
+may be expanses of vegetation covering ground of a more or less marshy
+character, for while the largest of them appear to be permanent, there
+are some which vary coincidently with the variations of the canals.
+
+As to the kind of machinery employed to force the water from the
+poles, it has been conjectured that it may have taken the form of a
+gigantic system of pumps and conduits; and since the Martians are
+assumed to be so far in advance of us in their mastery of scientific
+principles, the hypothesis will at least not be harmed by supposing
+that they have learned to harness forces of nature whose very
+existence in a manageable form is yet unrecognized on the earth. If we
+wish to let the imagination loose, we may conjecture that they have
+conquered the secret of those intra-atomic forces whose resistless
+energy is beginning to become evident to us, but the possibility of
+whose utilization remains a dream, the fulfillment of which nobody
+dares to predict.
+
+Such, in very brief form, is the celebrated theory of Mars as an
+inhabited world. It certainly captivates the imagination, and if we
+believe it to represent the facts, we cannot but watch with the
+deepest sympathy this gallant struggle of an intellectual race to
+preserve its planet from the effects of advancing age and death. We
+may, indeed, wonder whether our own humanity, confronted by such a
+calamity, could be counted on to meet the emergency with equal
+stoutness of heart and inexhaustibleness of resource. Up to the
+present time we certainly have shown no capacity to confront Nature
+toe to toe, and to seize her by the shoulders and turn her round when
+she refuses to go our way. If we could get into wireless telephonic
+communication with the Martians we might learn from their own lips the
+secret of their more than ``Roman recovery.''
+
+The Riddle of the Asteroids
+
+Between the orbits of Mars and Jupiter revolves the most remarkable
+system of little bodies with which we are acquainted -- the Asteroids,
+or Minor Planets. Some six hundred are now known, and they may
+actually number thousands. They form virtually a ring about the sun.
+The most striking general fact about them is that they occupy the
+place in the sky which should be occupied, according to Bode's Law, by
+a single large planet. This fact, as we shall see, has led to the
+invention of one of the most extraordinary theories in astronomy --
+viz., that of the explosion of a world!
+
+Bode's Law, so-called, is only an empiric formula, but until the
+discovery of Neptune it accorded so well with the distances of the
+planets that astronomers were disposed to look upon it as really
+representing some underlying principle of planetary distribution. They
+were puzzled by the absence of a planet in the space between Mars and
+Jupiter, where the ``law'' demanded that there should be one, and an
+association of astronomers was formed to search for it. There was a
+decided sensation when, in 1801, Piazzi, of Palermo, announced that he
+had found a little planet which apparently occupied the place in the
+system which belonged to the missing body. He named it Ceres, and it
+was the first of the Asteroids. The next year Olbers, of Bremen, while
+looking for Ceres with his telescope, stumbled upon another small
+planet which he named Pallas. Immediately he was inspired with the
+idea that these two planets were fragments of a larger one which had
+formerly occupied the vacant place in the planetary ranks, and he
+predicted that others would be found by searching in the neighborhood
+of the intersection of the orbits of the two already discovered. This
+bold prediction was brilliantly fulfilled by the finding of two more
+-- Juno in 1804, and Vesta in 1807. Olbers would seem to have been led
+to the invention of his hypothesis of a planetary explosion by the
+faith which astronomers at that time had in Bode's Law. They appear to
+have thought that several planets revolving in the gap where the
+``law'' called for but one could only be accounted for upon the theory
+that the original one had been broken up to form the several.
+Gravitation demanded that the remnants of a planet blown to pieces, no
+matter how their orbits might otherwise differ, should all return at
+stated periods to the point where the explosion had occurred; hence
+Olbers' prediction that any asteroids that might subsequently be
+discovered would be found to have a common point of orbital
+intersection. And curiously enough all of the first asteroids found
+practically answered to this requirement. Olbers' theory seemed to be
+established.
+
+After the first four, no more asteroids were found until 1845, when
+one was discovered; then, in 1847, three more were added to the list;
+and after that searchers began to pick them up with such rapidity that
+by the close of the century hundreds were known, and it had become
+almost impossible to keep track of them. The first four are by far the
+largest members of the group, but their actual sizes remained unknown
+until less than twenty years ago. It was long supposed that Vesta was
+the largest, because it shines more brightly than any of the others;
+but finally, in 1895, Barnard, with the Lick telescope, definitely
+measured their diameters, and proved to everybody's surprise that
+Ceres is really the chief, and Vesta only the third in rank. His
+measures are as follows: Ceres, 477 miles; Pallas, 304 miles; Vesta,
+239 miles; and Juno, 120 miles. They differ greatly in the reflective
+power of their surfaces, a fact of much significance in connection
+with the question of their origin. Vesta is, surface for surface,
+rather more than three times as brilliant as Ceres, whence the
+original mistake about its magnitude.
+
+Nowadays new asteroids are found frequently by photography, but
+physically they are most insignificant bodies, their average diameter
+probably not exceeding twenty miles, and some are believed not to
+exceed ten. On a planet only ten miles in diameter, assuming the same
+mean density as the earth's, which is undoubtedly too much, the force
+of gravity would be so slight that an average man would not weigh more
+than three ounces, and could jump off into space whenever he liked.
+
+Although the asteroids all revolve around the sun in the same
+direction as that pursued by the major planets, their orbits are
+inclined at a great variety of angles to the general plane of the
+planetary system, and some of them are very eccentric -- almost as
+much so as the orbits of many of the periodic comets. It has even been
+conjectured that the two tiny moons of Mars and the four smaller
+satellites of Jupiter may be asteroids gone astray and captured by
+those planets. Two of the asteroids are exceedingly remarkable for the
+shapes and positions of their orbits; these are Eros, discovered in
+1898, and T. G., 1906, found eight years later. The latter has a mean
+distance from the sun slightly greater than that of Jupiter, while the
+mean distance of Eros is less than that of Mars. The orbit of Eros is
+so eccentric that at times it approaches within 15,000,000 miles of
+the earth, nearer than any other regular member of the solar system
+except the moon, thus affording an unrivaled means of measuring the
+solar parallax. But for our present purpose the chief interest of Eros
+lies in its extraordinary changes of light.
+
+These changes, although irregular, have been observed and photographed
+many times, and there seems to be no doubt of their reality. Their
+significance consists in their possible connection with the form of
+the little planet, whose diameter is generally estimated at not more
+than twenty miles. Von Oppolzer found, in 1901, that Eros lost
+three-fourths of its brilliancy once in every two hours and
+thirty-eight minutes. Other observers have found slightly different
+periods of variability, but none as long as three hours. The most
+interesting interpretation that has been offered of this phenomenon is
+that it is due to a great irregularity of figure, recalling at once
+Olbers' hypothesis. According to some, Eros may be double, the two
+bodies composing it revolving around each other at very close
+quarters; but a more striking, and it may be said probable, suggestion
+is that Eros has a form not unlike that of a dumb-bell, or hour-glass,
+turning rapidly end over end so that the area of illuminated surface
+presented to our eyes continually changes, reaching at certain times a
+minimum when the amount of light that it reflects toward the earth is
+reduced to a quarter of its maximum value. Various other bizarre
+shapes have been ascribed to Eros, such, for instance, as that of a
+flat stone revolving about one of its longer axes, so that sometimes
+we see its face and sometimes its edge.
+
+All of these explanations proceed upon the assumption that Eros cannot
+have a simple globular figure like that of a typical planet, a figure
+which is prescribed by the law of gravitation, but that its shape is
+what may be called accidental; in a word, it is a fragment, for it
+seems impossible to believe that a body formed in interplanetary
+space, either through nebular condensation or through the aggregation
+of particles drawn together by their mutual attractions, should not be
+practically spherical in shape. Nor is Eros the only asteroid that
+gives evidence by variations of brilliancy that there is something
+abnormal in its constitution; several others present the same
+phenomenon in varying degrees. Even Vesta was regarded by Olbers as
+sufficiently variable in its light to warrant the conclusion that it
+was an angular mass instead of a globe. Some of the smaller ones show
+very notable variations, and all in short periods, of three or four
+hours, suggesting that in turning about one of their axes they present
+a surface of variable extent toward the sun and the earth.
+
+The theory which some have preferred -- that the variability of light
+is due to the differences of reflective power on different parts of
+the surface -- would, if accepted, be hardly less suggestive of the
+origin of these little bodies by the breaking up of a larger one,
+because the most natural explanation of such differences would seem to
+be that they arose from variations in the roughness or smoothness of
+the reflecting surface, which would be characteristic of fragmentary
+bodies. In the case of a large planet alternating expanses of land and
+water, or of vegetation and desert, would produce a notable variation
+in the amount of reflection, but on bodies of the size of the
+asteroids neither water nor vegetation could exist, and an atmosphere
+would be equally impossible.
+
+One of the strongest objections to Olbers' hypothesis is that only a
+few of the first asteroids discovered travel in orbits which
+measurably satisfy the requirement that they should all intersect at
+the point where the explosion occurred. To this it was at first
+replied that the perturbations of the asteroidal orbits, by the
+attractions of the major planets, would soon displace them in such a
+manner that they would cease to intersect. One of the first
+investigations undertaken by the late Prof. Simon Newcomb was directed
+to the solution of this question, and he arrived at the conclusion
+that the planetary perturbations could not explain the actual
+situation of the asteroidal orbits. But afterward it was pointed out
+that the difficulty could be avoided by supposing that not one but a
+series of explosions had produced the asteroids as they now are. After
+the primary disruption the fragments themselves, according to this
+suggestion, may have exploded, and then the resulting orbits would be
+as ``tangled'' as the heart could wish. This has so far rehabilitated
+the explosion theory that it has never been entirely abandoned, and
+the evidence which we have just cited of the probably abnormal shapes
+of Eros and other asteroids has lately given it renewed life. It is a
+subject that needs a thorough rediscussion.
+
+We must not fail to mention, however, that there is a rival hypothesis
+which commends itself to many astronomers -- viz., that the asteroids
+were formed out of a relatively scant ring of matter, situated between
+Mars and Jupiter and resembling in composition the immensely more
+massive rings from which, according to Laplace's hypothesis, the
+planets were born. It is held by the supporters of this theory that
+the attraction of the giant Jupiter was sufficient to prevent the
+small, nebulous ring that gave birth to the asteroids from condensing
+like the others into a single planet.
+
+But if we accept the explosion theory, with its corollary that minor
+explosions followed the principal one, we have still an unanswered
+question before us: What caused the explosions? The idea of a world
+blowing up is too Titanic to be shocking; it rather amuses the
+imagination than seriously impresses it; in a word, it seems
+essentially chimerical. We can by no appeal to experience form a
+mental picture of such an occurrence. Even the moon did not blow up
+when it was wrecked by volcanoes. The explosive nebul� and new stars
+are far away in space, and suggest no connection with such a
+catastrophe as the bursting of a planet into hundreds of pieces. We
+cannot conceive of a great globe thousands of miles in diameter
+resembling a pellet of gunpowder only awaiting the touch of a match to
+cause its sudden disruption. Somehow the thought of human agency
+obtrudes itself in connection with the word ``explosion,'' and we
+smile at the idea that giant powder or nitro-glycerine could blow up a
+planet. Yet it would only need enough of them to do it.
+
+After all, we may deceive ourselves in thinking, as we are apt to do,
+that explosive energies lock themselves up only in small masses of
+matter. There are many causes producing explosions in nature, every
+volcanic eruption manifests the activity of some of them. Think of the
+giant power of confined steam; if enough steam could be suddenly
+generated in the center of the earth by a downpour of all the waters
+of the oceans, what might not the consequences be for our globe? In a
+smaller globe, and it has never been estimated that the original
+asteroid was even as large as the moon, such a catastrophe would,
+perhaps, be more easily conceivable; but since we are compelled in
+this case to assume that there was a series of successive explosions,
+steam would hardly answer the purpose; it would be more reasonable to
+suppose that the cause of the explosion was some kind of chemical
+reaction, or something affecting the atoms composing the exploding
+body. Here Dr Gustav Le Bon comes to our aid with a most startling
+suggestion, based on his theory of the dissipation of intra-atomic
+energy. It will be best to quote him at some length from his book on
+The Evolution of Forces.
+
+``It does not seem at first sight,'' says Doctor Le Bon,
+
+  very comprehensible that worlds which appear more and more stable
+  as they cool could become so unstable as to afterward dissociate
+  entirely. To explain this phenomenon, we will inquire whether
+  astronomical observations do not allow us to witness this
+  dissociation.
+
+  We know that the stability of a body in motion, such as a top or a
+  bicycle, ceases to be possible when its velocity of rotation
+  descends below a certain limit. Once this limit is reached it loses
+  its stability and falls to the ground. Prof. J. J. Thomson even
+  interprets radio-activity in this manner, and points out that when
+  the speed of the elements composing the atoms descends below a
+  certain limit they become unstable and tend to lose their
+  equilibria. There would result from this a commencement of
+  dissociation, with diminution of their potential energy and a
+  corresponding increase of their kinetic energy sufficient to launch
+  into space the products of intra-atomic disintegration.
+
+  It must not be forgotten that the atom being an enormous reservoir
+  of energy is by this very fact comparable with explosive bodies.
+  These last remain inert so long as their internal equilibria are
+  undisturbed. So soon as some cause or other modifies these, they
+  explode and smash everything around them after being themselves
+  broken to pieces.
+
+  Atoms, therefore, which grow old in consequence of the diminution
+  of a part of their intra-atomic energy gradually lose their
+  stability. A moment, then, arrives when this stability is so weak
+  that the matter disappears by a sort of explosion more or less
+  rapid. The bodies of the radium group offer an image of this
+  phenomenon -- a rather faint image, however, because the atoms of
+  this body have only reached a period of instability when the
+  dissociation is rather slow. It probably precedes another and more
+  rapid period of dissociation capable of producing their final
+  explosion. Bodies such as radium, thorium, etc., represent, no
+  doubt, a state of old age at which all bodies must some day arrive,
+  and which they already begin to manifest in our universe, since all
+  matter is slightly radio-active. It would suffice for the
+  dissociation to be fairly general and fairly rapid for an explosion
+  to occur in a world where it was manifested.
+
+  These theoretical considerations find a solid support in the sudden
+  appearances and disappearances of stars. The explosions of a world
+  which produce them reveal to us, perhaps, how the universes perish
+  when they become old.
+
+  As astronomical observations show the relative frequency of these
+  rapid destructions, we may ask ourselves whether the end of a
+  universe by a sudden explosion after a long period of old age does
+  not represent its most general ending.
+
+Here, perhaps, it will be well to stop, since, entrancing as the
+subject may be, we know very little about it, and Doctor Le Bon's
+theory affords a limitless field for the reader's imagination.
+  _________________________________________________________________
+
+A printed version of this book is available from Sattre Press
+(http://csky.sattre-press.com). It includes extensive annotations, a
+new introduction and all the original photographs and diagrams.
+
+
+
+
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+	
+<h1>The Project Gutenberg EBook of
+<br>Curiosities of the Sky, by Garrett Serviss</h1>
+<pre>
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+Title: Curiosities of the Sky
+
+Author: Garrett Serviss
+
+Release Date: October, 2004  [EBook #6630]
+[Yes, we are more than one year ahead of schedule]
+[This file was first posted on January 6, 2003]
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+Edition: 10
+
+Language: English
+
+Character set encoding: ASCII
+
+*** START OF THE PROJECT GUTENBERG EBOOK, CURIOSITIES OF THE SKY ***
+</pre>
+
+    <h1>Curiosities of the Sky</h1>
+
+    <h2>by Garrett Serviss</h2>
+    <cite>Curiosities of the Sky</cite> was first published in 1909
+    and the text is in the public domain. The transcription was
+    done by <a href="mailto:info@sattre-press.com">William
+    McClain</a>, 2002. 
+
+    <p>A printed version of this book is available from <a href=
+    "http://csky.sattre-press.com">Sattre Press</a>. It includes
+    extensive annotations, a new introduction and all the original
+    photographs and diagrams.</p>
+    <hr>
+    <strong>Preface</strong> 
+
+    <p>What Froude says of history is true also of astronomy: it is
+    the most impressive where it transcends explanation. It is not
+    the mathematics of astronomy, but the wonder and the mystery
+    that seize upon the imagination. The calculation of an eclipse
+    owes all its prestige to the sublimity of its data; the
+    operation, in itself, requires no more mental effort than the
+    preparation of a railway time-table.</p>
+
+    <p>The dominion which astronomy has always held over the minds
+    of men is akin to that of poetry; when the former becomes
+    merely instructive and the latter purely didactic, both lose
+    their power over the imagination. Astronomy is known as the
+    oldest of the sciences, and it will be the longest-lived
+    because it will always have arcana that have not been
+    penetrated.</p>
+
+    <p>Some of the things described in this book are little known
+    to the average reader, while others are well known; but all
+    possess the fascination of whatever is strange, marvelous,
+    obscure, or mysterious -- magnified, in this case, by the
+    portentous scale of the phenomena.</p>
+
+    <p>The idea of the author is to tell about these things in
+    plain language, but with as much scientific accuracy as plain
+    language will permit, showing the wonder that is in them
+    without getting away from the facts. Most of them have hitherto
+    been discussed only in technical form, and in treatises that
+    the general public seldom sees and never reads.</p>
+
+    <p>Among the topics touched upon are:</p>
+
+    <ul>
+      <li>The strange unfixedness of the ``fixed stars,'' the vast
+      migrations of the suns and worlds constituting the
+      universe.</li>
+
+      <li>The slow passing out of existence of those collocations
+      of stars which for thousands of years have formed famous
+      ``constellations,'' preserving the memory of mythological
+      heroes and heroines, and perhaps of otherwise unrecorded
+      history.</li>
+
+      <li>The tendency of stars to assemble in immense clouds,
+      swarms, and clusters.</li>
+
+      <li>The existence in some of the richest regions of the
+      universe of absolutely black, starless gaps, deeps, or holes,
+      as if one were looking out of a window into the murkiest
+      night.</li>
+
+      <li>The marvelous phenomena of new, or temporary, stars,
+      which appear as suddenly as conflagrations, and often turn
+      into something else as eccentric as themselves.</li>
+
+      <li>The amazing forms of the ``whirlpool,'' ``spiral,''
+      ``pinwheel,'' and ``lace,'' or ``tress,'' nebul&aelig;.</li>
+
+      <li>The strange surroundings of the sun, only seen in
+      particular circumstances, but evidently playing a constant
+      part in the daily phenomena of the solar system.</li>
+
+      <li>The mystery of the Zodiacal Light and the
+      Gegenschein.</li>
+
+      <li>The extraordinary transformations undergone by comets and
+      their tails.</li>
+
+      <li>The prodigies of meteorites and masses of stone and metal
+      fallen from the sky.</li>
+
+      <li>The cataclysms that have wrecked the moon.</li>
+
+      <li>The problem of life and intelligence on the planet
+      Mars.</li>
+
+      <li>The problematical origin and fate of the asteroids.</li>
+
+      <li>The strange phenomena of the auroral lights.</li>
+    </ul>
+
+    <p>An attempt has been made to develop these topics in an
+    orderly way, showing their connection, so that the reader may
+    obtain a broad general view of the chief mysteries and problems
+    of astronomy, and an idea of the immense field of discovery
+    which still lies, almost unexplored, before it.</p>
+
+    <p><strong>The Windows of Absolute Night</strong></p>
+
+    <p>To most minds mystery is more fascinating than science. But
+    when science itself leads straight up to the borders of mystery
+    and there comes to a dead stop, saying, ``At present I can no
+    longer see my way,'' the force of the charm is redoubled. On
+    the other hand, the illimitable is no less potent in mystery
+    than the invisible, whence the dramatic effect of Keats'
+    ``stout Cortez'' staring at the boundless Pacific while all his
+    men look at each other with a wild surmise, ``silent upon a
+    peak in Darien.'' It is with similar feelings that the
+    astronomer regards certain places where from the peaks of the
+    universe his vision seems to range out into endless empty
+    space. He sees there the shore of his little isthmus, and,
+    beyond, unexplored immensity.</p>
+
+    <p>The name, ``coal-sacks,'' given to these strange voids is
+    hardly descriptive. Rather they produce upon the mind the
+    effect of blank windows in a lonely house on a pitch-dark
+    night, which, when looked at from the brilliant interior,
+    become appalling in their rayless murk. Infinity seems to
+    acquire a new meaning in the presence of these black openings
+    in the sky, for as one continues to gaze it loses its purely
+    metaphysical quality and becomes a kind of entity, like the
+    ocean. The observer is conscious that he can actually
+    <em>see</em> the beginning of its ebon depths, in which the
+    visible universe appears to float like an enchanted island,
+    resplendent within with lights and life and gorgeous
+    spectacles, and encircled with screens of crowded stars, but
+    with its dazzling vistas ending at the fathomless sea of pure
+    darkness which encloses all.</p>
+
+    <p>The Galaxy, or Milky Way, surrounds the borders of our
+    island in space like a stellar garland, and when openings
+    appear in it they are, by contrast, far more impressive than
+    the general darkness of the interstellar expanse seen in other
+    directions. Yet even that expanse is not everywhere equally
+    dark, for it contains gloomy deeps discernable with careful
+    watching. Here, too, contrast plays an important part, though
+    less striking than within the galactic region. Some of Sir
+    William Herschel's observations appear to indicate an
+    association between these tenebrious spots and neighboring star
+    clouds and nebul&aelig;. It is an illuminating bit of
+    astronomical history that when he was sweeping the then virgin
+    heavens with his great telescopes he was accustomed to say to
+    his sister who, note-book in hand, waited at his side to take
+    down his words, fresh with the inspiration of discovery:
+    ``Prepare to write; the nebul&aelig; are coming; here space is
+    vacant.''</p>
+
+    <p>The most famous of the ``coal-sacks,'' and the first to be
+    brought to general attention before astronomers had awakened to
+    the significance of such things, lies adjacent to the
+    ``Southern Cross,'' and is truly an amazing phenomenon. It is
+    not alone the conspicuousness of this celestial vacancy,
+    opening suddenly in the midst of one of the richest parts of
+    the Galaxy, that has given it its fame, but quite as much the
+    superstitious awe with which it was regarded by the early
+    explorers of the South Seas. To them, as well as to those who
+    listened in rapt wonder to their tales, the ``Coal-sack''
+    seemed to possess some occult connection with the mystic
+    ``Cross.'' In the eyes of the sailors it was not a vacancy so
+    much as a sable reality in the sky, and as, shuddering, they
+    stared at it, they piously crossed themselves. It was another
+    of the magical wonders of the unknown South, and as such it
+    formed the basis of many a ``wild surmise'' and many a
+    sea-dog's yarn. Scientific investigation has not diminished its
+    prestige, and today no traveler in the southern hemisphere is
+    indifferent to its fascinating strangeness, while some find it
+    the most impressive spectacle of the antarctic heavens.</p>
+
+    <p>All around, up to the very edge of the yawning gap, the
+    sheen of the Milky Way is surpassingly glorious; but there, as
+    if in obedience to an almighty edict, everything vanishes. A
+    single faint star is visible within the opening, producing a
+    curious effect upon the sensitive spectator, like the sight of
+    a tiny islet in the midst of a black, motionless, waveless
+    tarn. The dimensions of the lagoon of darkness, which is oval
+    or pear-shaped, are eight degrees by five, so that it occupies
+    a space in the sky about one hundred and thirty times greater
+    than the area of the full moon. It attracts attention as soon
+    as the eye is directed toward the quarter where it exists, and
+    by virtue of the rarity of such phenomena it appears a far
+    greater wonder than the drifts of stars that are heaped around
+    it. Now that observatories are multiplying in the southern
+    hemisphere, the great austral ``Coal-sack'' will, no doubt,
+    receive attention proportioned to its importance as one of the
+    most significant features of the sky. Already at the Sydney
+    Observatory photographs have shown that the southern portion of
+    this Dead Sea of Space is not quite ``bottomless,'' although
+    its northern part defies the longest sounding lines of the
+    astronomer.</p>
+
+    <p>There is a similar, but less perfect, ``coal-sack'' in the
+    northern hemisphere, in the constellation of ``The Swan,''
+    which, strange to say, also contains a well-marked figure of a
+    cross outlined by stars. This gap lies near the top of the
+    cross-shaped figure. It is best seen by averted vision, which
+    brings out the contrast with the Milky Way, which is quite
+    brilliant around it. It does not, however, exercise the same
+    weird attraction upon the eye as the southern ``Coal-sack,''
+    for instead of looking like an absolute void in the sky, it
+    rather appears as if a canopy of dark gauze had been drawn over
+    the stars. We shall see the possible significance of this
+    appearance later.</p>
+
+    <p>Just above the southern horizon of our northern middle
+    latitudes, in summer, where the Milky Way breaks up into vast
+    sheets of nebulous luminosity, lying over and between the
+    constellations Scorpio and Sagittarius, there is a remarkable
+    assemblage of ``coal-sacks,'' though none is of great size. One
+    of them, near a conspicuous star-cluster in Scorpio, M80, is
+    interesting for having been the first of these strange objects
+    noted by Herschel. Probably it was its nearness to M80 which
+    suggested to his mind the apparent connection of such vacancies
+    with star-clusters which we have already mentioned.</p>
+
+    <p>But the most marvelous of the ``coal-sacks'' are those that
+    have been found by photography in Sagittarius. One of Barnard's
+    earliest and most excellent photographs includes two of them,
+    both in the star-cluster M8. The larger, which is roughly
+    rectangular in outline, contains one little star, and its
+    smaller neighbor is lune-shaped -- surely a most singular form
+    for such an object. Both are associated with curious dark lanes
+    running through the clustered stars like trails in the woods.
+    Along the borders of these lanes the stars are ranked in
+    parallel rows, and what may be called the bottoms of the lanes
+    are not entirely dark, but pebbled with faint stellar points.
+    One of them which skirts the two dark gaps and traverses the
+    cluster along its greatest diameter is edged with lines of
+    stars, recalling the alignment of the trees bordering a French
+    highway. This <em>road of stars</em> cannot be less than many
+    billions of miles in length!</p>
+
+    <p>All about the cluster the bed of the Galaxy is strangely
+    disturbed, and in places nearly denuded, as if its contents had
+    been raked away to form the immense stack and the smaller
+    accumulations of stars around it. The well-known ``Trifid
+    Nebula'' is also included in the field of the photograph, which
+    covers a truly marvelous region, so intricate in its mingling
+    of nebul&aelig;, star-clusters, star-swarms, star-streams, and
+    dark vacancies that no description can do it justice. Yet,
+    chaotic as it appears, there is an unmistakable suggestion of
+    unity about it, impressing the beholder with the idea that all
+    the different parts are in some way connected, and have not
+    been fortuitously thrown together. Miss Agnes M. Clerke made
+    the striking remark that the dusky lanes in M8 are exemplified
+    on the largest scale in the great rift dividing the Milky Way,
+    from Cygnus in the northern hemisphere all the way to the
+    ``Cross'' in the southern. Similar lanes are found in many
+    other clusters, and they are generally associated with flanking
+    rows of stars, resembling in their arrangement the thick-set
+    houses and villas along the roadways that traverse the
+    approaches to a great city.</p>
+
+    <p>But to return to the black gaps. Are they really windows in
+    the star-walls of the universe? Some of them look rather as if
+    they had been made by a shell fired through a luminous target,
+    allowing the eye to range through the hole into the void space
+    beyond. If science is discretely silent about these things,
+    what can the more venturesome and less responsible imagination
+    suggest? Would a huge ``runaway sun,'' like Arcturus, for
+    instance, make such an opening if it should pass like a
+    projectile through the Milky Way? It is at least a stimulating
+    inquiry. Being probably many thousands of times more massive
+    than the galactic stars, such a stellar missile would not be
+    stopped by them, though its direction of flight might be
+    altered. It would drag the small stars lying close to its
+    course out of their spheres, but the ultimate tendency of its
+    attraction would be to sweep them round in its wake, thus
+    producing rather a star-swarm than a vacancy. Those that were
+    very close to it might be swept away in its rush and become its
+    satellites, careering away with it in its flight into outer
+    space; but those that were farther off, and they would, of
+    course, greatly outnumber the nearer ones, would tend inward
+    from all sides toward the line of flight, as dust and leaves
+    collect behind a speeding motor (though the forces operating
+    would be different), and would fill up the hole, if hole it
+    were. A swarm thus collected should be rounded in outline and
+    bordered with a relatively barren ring from which the stars had
+    been ``sucked'' away. In a general sense the M8 cluster answers
+    to this description, but even if we undertook to account for
+    its existence by a supposition like the above, the black gaps
+    would remain unexplained, unless one could make a further draft
+    on the imagination and suggest that the stars had been thrown
+    into a vast eddy, or system of eddies, whose vortices appear as
+    dark holes. Only a maelstrom-like motion could keep such a
+    funnel open, for without regard to the impulse derived from the
+    projectile, the proper motions of the stars themselves would
+    tend to fill it. Perhaps some other cause of the whirling
+    motion may be found. As we shall see when we come to the spiral
+    nebul&aelig;, gyratory movements are exceedingly prevalent
+    throughout the universe, and the structure of the Milky Way is
+    everywhere suggestive of them. But this is hazardous sport even
+    for the imagination -- to play with <em>suns</em> as if they
+    were but thistle-down in the wind or corks in a mill-race.</p>
+
+    <p>Another question arises: What is the thickness of the hedge
+    of stars through which the holes penetrate? Is the depth of the
+    openings proportionate to their width? In other words, is the
+    Milky Way round in section like a rope, or flat and thin like a
+    ribbon? The answer is not obvious, for we have little or no
+    information concerning the relative distances of the faint
+    galactic stars. It would be easier, certainly, to conceive of
+    openings in a thin belt than in a massive ring, for in the
+    first case they would resemble mere rifts and breaks, while in
+    the second they would be like wells or bore-holes. Then, too,
+    the fact that the Milky Way is not a <em>continuous</em> body
+    but is made up of stars whose actual distances apart is great,
+    offers another quandary; persistent and sharply bordered
+    apertures in such an assemblage are <em>a priori</em> as
+    improbable, if not impossible, as straight, narrow holes
+    running through a swarm of bees.</p>
+
+    <p>The difficulty of these questions indicates one of the
+    reasons why it has been suggested that the seeming gaps, or
+    many of them, are not openings at all, but opaque screens
+    cutting off the light from stars behind them. That this is
+    quite possible in some cases is shown by Barnard's later
+    photographs, particularly those of the singular region around
+    the star Rho Ophiuchi. Here are to be seen somber lanes and
+    patches, apparently forming a connected system which covers an
+    immense space, and which their discoverer thinks may constitute
+    a ``dark nebula.'' This seems at first a startling suggestion;
+    but, after all, why should their not be dark nebul&aelig; as
+    well as visible ones? In truth, it has troubled some
+    astronomers to explain the luminosity of the bright
+    nebul&aelig;, since it is not to be supposed that matter in so
+    diffuse a state can be incandescent through heat, and
+    phosphorescent light is in itself a mystery. The supposition is
+    also in accord with what we know of the existence of dark solid
+    bodies in space. Many bright stars are accompanied by obscure
+    companions, sometimes as massive as themselves; the planets are
+    non-luminous; the same is true of meteors before they plunge
+    into the atmosphere and become heated by friction; and many
+    plausible reasons have been found for believing that space
+    contains as many obscure as shining bodies of great size. It is
+    not so difficult, after all, then, to believe that there are
+    immense collections of shadowy gases and meteoric dust whose
+    presence is only manifested when they intercept the light
+    coming from shining bodies behind them.</p>
+
+    <p>This would account for the apparent extinguishment of light
+    in open space, which is indicated by the falling off in
+    relative number of telescopic stars below the tenth magnitude.
+    Even as things are, the amount of light coming to us from stars
+    too faint to be seen with the naked eye is so great that the
+    statement of it generally surprises persons who are unfamiliar
+    with the inner facts of astronomy. It has been calculated that
+    on a clear night the total starlight from the entire celestial
+    sphere amounts to one-sixtieth of the light of the full moon;
+    but of this less than one-twenty-fifth is due to stars
+    separately distinguished by the eye. If there were no obscuring
+    medium in space, it is probable that the amount of starlight
+    would be noticeably and perhaps enormously increased.</p>
+
+    <p>But while it seems certain that some of the obscure spots in
+    the Milky Way are due to the presence of ``dark nebul&aelig;,''
+    or concealing veils of one kind or another, it is equally
+    certain that there are many which are true apertures, however
+    they may have been formed, and by whatever forces they may be
+    maintained. These, then, are veritable windows of the Galaxy,
+    and when looking out of them one is face to face with the great
+    mystery of infinite space. <em>There</em> the known universe
+    visibly ends, but manifestly space itself does not end there.
+    It is not within the power of thought to conceive an end to
+    space, for the instant we think of a terminal point or line the
+    mind leaps forward to the <em>beyond.</em> There must be space
+    outside as well as inside. Eternity of time and infinity of
+    space are ideas that the intellect cannot fully grasp, but
+    neither can it grasp the idea of a limitation to either space
+    or time. The metaphysical conceptions of hypergeometry, or
+    fourth-dimensional space, do not aid us.</p>
+
+    <p>Having, then, discovered that the universe is a thing
+    <em>contained</em> in something indefinitely greater than
+    itself; having looked out of its windows and found only the
+    gloom of starless night outside -- what conclusions are we to
+    draw concerning the beyond? It <em>seems</em> as empty as a
+    vacuum, but is it really so? If it be, then our universe is a
+    single atom astray in the infinite; it is the only island in an
+    ocean without shores; it is the one oasis in an illimitable
+    desert. Then the Milky Way, with its wide-flung garland of
+    stars, is afloat like a tiny smoke-wreath amid a horror of
+    immeasurable vacancy, or it is an evanescent and solitary ring
+    of sparkling froth cast up for a moment on the viewless billows
+    of immensity. From such conclusions the mind instinctively
+    shrinks. It prefers to think that there is <em>something</em>
+    beyond, though we cannot see it. Even the universe could not
+    bear to be alone -- a Crusoe lost in the Cosmos! As the
+    inhabitants of the most elegant ch&acirc;teau, with its
+    gardens, parks, and crowds of attendants, would die of
+    loneliness if they did not know that they have neighbors,
+    though not seen, and that a living world of indefinite extent
+    surrounds them, so we, when we perceive that the universe has
+    limits, wish to feel that it is not solitary; that beyond the
+    hedges and the hills there are other centers of life and
+    activity. Could anything be more terrible than the thought of
+    an <em>isolated universe?</em> The greater the being, the
+    greater the aversion to seclusion. Only the infinite satisfies;
+    in that alone the mind finds rest.</p>
+
+    <p>We are driven, then, to believe that the universal night
+    which envelopes us is not tenantless; that as we stare out of
+    the star-framed windows of the Galaxy and see nothing but
+    uniform blackness, the fault is with our eyes or is due to an
+    obscuring medium. Since <em>our</em> universe is limited in
+    extent, there must be <em>other</em> universes beyond it on all
+    sides. Perhaps if we could carry our telescopes to the verge of
+    the great ``Coal-sack'' near the ``Cross,'' being then on the
+    frontier of our starry system, we could discern, sparkling afar
+    off in the vast night, some of the outer galaxies. They may be
+    grander than ours, just as many of the suns surrounding us are
+    immensely greater than ours. If we could take our stand
+    somewhere in the midst of immensity and, with vision of
+    infinite reach, look about us, we should perhaps see a
+    countless number of stellar systems, amid which ours would be
+    unnoticeable, like a single star among the multitude glittering
+    in the terrestial sky on a clear night. Some might be in the
+    form of a wreath, like our own; some might be globular, like
+    the great star-clusters in Hercules and Centaurus; some might
+    be glittering circles, or disks, or rings within rings. If we
+    could enter them we should probably find a vast variety of
+    composition, including elements unknown to terrestrial
+    chemistry; for while the <em>visible</em> universe appears to
+    contain few if any substances not existing on the earth or in
+    the sun, we have no warrant to assume that others may not exist
+    in infinite space.</p>
+
+    <p>And how as to gravitation? We do not <em>know</em> that
+    gravitation acts beyond the visible universe, but it is
+    reasonable to suppose that it does. At any rate, if we let go
+    <em>its</em> sustaining hand we are lost, and can only wander
+    hopelessly in our speculations, like children astray. If the
+    empire of gravitation is infinite, then the various outer
+    systems must have <em>some,</em> though measuring by our
+    standards an imperceptible, attractive influence upon each
+    other, for gravitation never lets go its hold, however great
+    the space over which it is required to act. Just as the stars
+    about us are all in motion, so the starry systems beyond our
+    sight may be in motion, and our system as a whole may be moving
+    in concert with them. If this be so, then after interminable
+    ages the aspect of the entire system of systems must change,
+    its various members assuming new positions with respect to one
+    another. In the course of time we may even suppose that our
+    universe will approach relatively close to one of the others;
+    and then, if men are yet living on the earth, they may glimpse
+    through the openings which reveal nothing to us now, the lights
+    of another nearing star system, like the signals of a strange
+    squadron, bringing them the assurance (which can be but an
+    inference at present) that the ocean of space has other
+    argosies venturing on its limitless expanse.</p>
+
+    <p>There remains the question of the luminiferous ether by
+    whose agency the waves of light are borne through space. The
+    ether is as mysterious as gravitation. With regard to ether we
+    only infer its existence from the effects which we ascribe to
+    it. Evidently the ether must extend as far as the most distant
+    visible stars. But does it continue on indefinitely in outer
+    space? If it does, then the invisibility of the other systems
+    must be due to their distance diminishing the quantity of light
+    that comes from them below the limit of perceptibility, or to
+    the interposition of absorbing media; if it does not, then the
+    reason why we cannot see them is owing to the absence of a
+    means of conveyance for the light waves, as the lack of an
+    interplanetary atmosphere prevents us from hearing the thunder
+    of sun-spots. (It is interesting to recall that Mr Edison was
+    once credited with the intention to construct a gigantic
+    microphone which should render the roar of sun-spots audible by
+    transforming the electric vibrations into sound-waves). On this
+    supposition each starry system would be enveloped in its own
+    globule of ether, and no light could cross from one to another.
+    But the probability is that both the ether and gravitation are
+    ubiquitous, and that all the stellar systems are immersed in
+    the former like clouds of phosphorescent organisms in the
+    sea.</p>
+
+    <p>So astronomy carries the mind from height to greater height.
+    Men were long in accepting the proofs of the relative
+    insignificance of the earth; they were more quickly convinced
+    of the comparative littleness of the solar system; and now the
+    evidence assails their reason that what they had regarded as
+    <em>the</em> universe is only one mote gleaming in the sunbeams
+    of Infinity.</p>
+
+    <p><strong>Star-Clouds, Star-Clusters, and
+    Star-Streams</strong></p>
+
+    <p>In the preceding chapter we have seen something of the
+    strangely complicated structure of the Galaxy, or Milky Way. We
+    now proceed to study more comprehensively that garlanded
+    ``Pathway of the Gods.''</p>
+
+    <p>Judged by the eye alone, the Milky Way is one of the most
+    delicately beautiful phenomena in the entire realm of nature --
+    a shimmer of silvery gauze stretched across the sky; but
+    studied in the light of its revelations, it is the most
+    stupendous object presented to human ken. Let us consider,
+    first, its appearance to ordinary vision. Its apparent position
+    in the sky shifts according to the season. On a serene,
+    cloudless summer evening, in the absence of the moon, whose
+    light obscures it, one sees the Galaxy spanning the heavens
+    from north to southeast of the zenith like a phosphorescent
+    arch. In early spring it forms a similar but, upon the whole,
+    less brilliant arch west of the zenith. Between spring and
+    summer it lies like a long, faint, twilight band along the
+    northern horizon. At the beginning of winter it again forms an
+    arch, this time spanning the sky from east to west, a little
+    north of the zenith. These are its positions as viewed from the
+    mean latitude of the United States. Even the beginner in
+    star-gazing does not have to watch it throughout the year in
+    order to be convinced that it is, in reality, a great circle,
+    extending entirely around the celestial sphere. We appear to be
+    situated near its center, but its periphery is evidently far
+    away in the depths of space.</p>
+
+    <p>Although to the casual observer it seems but a delicate
+    scarf of light, brighter in some places than in others, but
+    hazy and indefinite at the best, such is not its appearance to
+    those who study it with care. They perceive that it is an
+    organic whole, though marvelously complex in detail. The
+    telescope shows that it consists of stars too faint and small
+    through excess of distance to be separately visible. Of the
+    hundred million suns which some estimates have fixed as the
+    probable population of the starry universe, the vast majority
+    (at least thirty to one) are included in this strange belt of
+    misty light. But they are not uniformly distributed in it; on
+    the contrary, they are arrayed in clusters, knots, bunches,
+    clouds, and streams. The appearance is somewhat as if the
+    Galaxy consisted of innumerable swarms of silver-winged bees,
+    more or less intermixed, some massed together, some crossing
+    the paths of others, but all governed by a single purpose which
+    leads them to encircle the region of space in which we are
+    situated.</p>
+
+    <p>From the beginning of the systematic study of the heavens,
+    the fact has been recognized that the form of the Milky Way
+    denotes the scheme of the sidereal system. At first it was
+    thought that the shape of the system was that of a vast round
+    disk, flat like a cheese, and filled with stars, our sun and
+    his relatively few neighbors being placed near the center.
+    According to this view, the galactic belt was an effect of
+    perspective; for when looking in the direction of the plane of
+    the disk, the eye ranged through an immense extension of stars
+    which blended into a glimmering blur, surrounding us like a
+    ring; while when looking out from the sides of the disk we saw
+    but few stars, and in those directions the heavens appeared
+    relatively blank. Finally it was recognized that this theory
+    did not correspond with the observed appearances, and it became
+    evident that the Milky Way was not a mere effect of
+    perspective, but an actual band of enormously distant stars,
+    forming a circle about the sphere, the central opening of the
+    ring (containing many scattered stars) being many times broader
+    than the width of the ring itself. Our sun is one of the
+    scattered stars in the central opening.</p>
+
+    <p>As already remarked, the ring of the Galaxy is very
+    irregular, and in places it is partly broken. With its sinuous
+    outline, its pendant sprays, its graceful and accordant curves,
+    its bunching of masses, its occasional interstices, and the
+    manifest order of a general plan governing the jumble of its
+    details, it bears a remarkable resemblance to a garland -- a
+    fact which appears the more wonderful when we recall its
+    composition. That an elm-tree should trace the lines of beauty
+    with its leafy and pendulous branches does not surprise us; but
+    we can only gaze with growing amazement when we behold <em>a
+    hundred million suns imitating the form of a chaplet!</em> And
+    then we have to remember that this form furnishes the
+    ground-plan of the universe.</p>
+
+    <p>As an indication of the extraordinary speculations to which
+    the mystery of the Milky Way has given rise, a theory recently
+    (1909) proposed by Prof. George C. Comstock may be mentioned.
+    Starting with the data (first) that the number of stars
+    increases as the Milky Way is approached, and reaches a maximum
+    in its plane, while on the other hand the number of
+    nebul&aelig; is greatest outside the Milky Way and increases
+    with distance from it, and (second) that the Milky Way,
+    although a complete ring, is broad and diffuse on one side
+    through one-half its course -- that half alone containing
+    nebul&aelig; -- and relatively narrow and well defined on the
+    opposite side, the author of this singular speculation avers
+    that these facts can best be explained by supposing that the
+    invisible universe consists of two interpenetrating parts, one
+    of which is a chaos of indefinite extent, strewn with stars and
+    nebulous dust, and the other a long, broad but comparatively
+    thin cluster of stars, including the sun as one of its central
+    members. This flat star-cluster is conceived to be moving
+    edgewise through the chaos, and, according to Professor
+    Comstock, it acts after the manner of a snow-plough sweeping
+    away the cosmic dust and piling it on either hand above and
+    below the plane of the moving cluster. It thus forms a
+    transparent rift, through which we see farther and command a
+    view of more stars than through the intensified dust-clouds on
+    either hand. This rift is the Milky Way. The dust thrown aside
+    toward the poles of the Milky Way is the substance of the
+    nebul&aelig; which abound there. Ahead, where the front of the
+    star-plough is clearing the way, the chaos is nearer at hand,
+    and consequently there the rift subtends a broader angle, and
+    is filled with primordial dust, which, having been annexed by
+    the vanguard of the star-swarm, forms the nebul&aelig; seen
+    only in that part of the Milky Way. But behind, the rift
+    appears narrow because there we look farther away between
+    dust-clouds produced ages ago by the front of the plough, and
+    no scattered dust remains in that part of the rift.</p>
+
+    <p>In quoting an outline of this strikingly original theory the
+    present writer should not be understood as assenting to it.
+    That it appears bizarre is not, in itself, a reason for
+    rejecting it, when we are dealing with so problematical and
+    enigmatical a subject as the Milky Way; but the serious
+    objection is that the theory does not sufficiently accord with
+    the observed phenomena. There is too much evidence that the
+    Milky Way is an organic system, however fantastic its form, to
+    permit the belief that it can only be a rift in chaotic clouds.
+    As with every organism, we find that its parts are more or less
+    clearly repeated in its ensemble. Among all the strange things
+    that the Milky Way contains there is nothing so extraordinary
+    as itself. Every astronomer must many times have found himself
+    marveling at it in those comparatively rare nights when it
+    shows all its beauty and all its strangeness. In its great
+    broken rifts, divisions, and spirals are found the gigantic
+    prototypes of similar forms in its star-clouds and clusters. As
+    we have said, it determines the general shape of the whole
+    sidereal system. Some of the brightest stars in the sky appear
+    to hang like jewels suspended at the ends of tassels dropped
+    from the Galaxy. Among these pendants are the Pleiades and the
+    Hyades. Orion, too, the ``Mighty Hunter,'' is caught in ``a
+    loop of light'' thrown out from it. The majority of the great
+    first-magnitude stars seem related to it, as if they formed an
+    inner ring inclined at an angle of some twenty degrees to its
+    plane. Many of the long curves that set off from it on both
+    sides are accompanied by corresponding curves of lucid stars.
+    In a word, it offers every appearance of structural connection
+    with the entire starry system. That the universe should have
+    assumed the form of a wreath is certainly a matter for
+    astonishment; but it would have been still more astonishing if
+    it had been a cube, a rhomboid, or a dodecahedron, for then we
+    should have had to suppose that something resembling the forces
+    that shape crystals had acted upon the stars, and the
+    difficulty of explaining the universe by the laws of
+    gravitation would have been increased.</p>
+
+    <p>From the Milky Way as a whole we pass to the vast clouds,
+    swarms, and clusters of stars of which it is made up. It may
+    be, as some astronomers hold, that most of the galactic stars
+    are much smaller than the sun, so that their faintness is not
+    due entirely to the effect of distance. Still, their intrinsic
+    brilliance attests their solar character, and considering their
+    remoteness, which has been estimated at not less than ten
+    thousand to twenty thousand light-years (a light-year is equal
+    to nearly six thousand thousand million miles) their actual
+    masses cannot be extremely small. The minutest of them are
+    entitled to be regarded as real suns, and they vary enormously
+    in magnitude. The effects of their attractions upon one another
+    can only be inferred from their clustering, because their
+    relative movements are not apparent on account of the brevity
+    of the observations that we can make. But imagine a being for
+    whom a million years would be but as a flitting moment; to him
+    the Milky Way would appear in a state of ceaseless agitation --
+    swirling with ``a fury of whirlpool motion.''</p>
+
+    <p>The cloud-like aspect of large parts of the Galaxy must
+    always have attracted attention, even from naked-eye observers,
+    but the true star-clouds were first satisfactorily represented
+    in Barnard's photographs. The resemblance to actual clouds is
+    often startling. Some are close-packed and dense, like cumuli;
+    some are wispy or mottled, like cirri. The rifts and
+    modulations, as well as the general outlines, are the same as
+    those of clouds of vapor or dust, and one notices also the
+    characteristic thinning out at the edges. But we must beware of
+    supposing that the component suns are thickly crowded as the
+    particles forming an ordinary cloud. They <em>look,</em>
+    indeed, as if they were matted together, because of the
+    irradiation of light, but in reality millions and billions of
+    miles separate each star from its neighbors. Nevertheless they
+    form real assemblages, whose members are far more closely
+    related to one another than is our sun to the stars around him,
+    and if we were in the Milky Way the aspect of the nocturnal sky
+    would be marvelously different from its present appearance.</p>
+
+    <p>Stellar clouds are characteristic of the Galaxy and are not
+    found beyond its borders, except in the ``Magellanic Clouds''
+    of the southern hemisphere, which resemble detached portions of
+    the Milky Way. These singular objects form as striking a
+    peculiarity of the austral heavens as does the great
+    ``Coal-sack'' described in Chapter 1. But it is their isolation
+    that makes them so remarkable, for their composition is
+    essentially galactic, and if they were included within its
+    boundaries they would not appear more wonderful than many other
+    parts of the Milky Way. Placed where they are, they look like
+    masses fallen from the great stellar arch. They are full of
+    nebul&aelig; and star-clusters, and show striking evidences of
+    spiral movement.</p>
+
+    <p>Star-swarms, which are also characteristic features of the
+    Galaxy, differ from star-clouds very much in the way that their
+    name would imply -- <em>i.e.,</em> their component stars are so
+    arranged, even when they are countless in number, that the idea
+    of an exceedingly numerous assemblage rather than that of a
+    cloud is impressed on the observer's mind. In a star-swarm the
+    separate members are distinguishable because they are either
+    larger or nearer than the stars composing a ``cloud.'' A
+    splendid example of a true star-swarm is furnished by Chi
+    Persei, in that part of the Milky Way which runs between the
+    constellations Perseus and Cassiopeia. This swarm is much
+    coarser than many others, and can be seen by the naked eye. In
+    a small telescope it appears double, as if the suns composing
+    it had divided into two parties which keep on their way side by
+    side, with some commingling of their members where the skirts
+    of the two companies come in contact.</p>
+
+    <p>Smaller than either star-clouds or star-swarms, and
+    differing from both in their organization, are star-clusters.
+    These, unlike the others, are found outside as well as inside
+    the Milky Way, although they are more numerous inside its
+    boundaries than elsewhere. The term star-cluster is sometimes
+    applied, though improperly, to assemblages which are rather
+    groups, such, for instance, as the Pleiades. In their most
+    characteristic aspect star-clusters are of a globular shape --
+    globes of suns! A famous example of a globular star-cluster,
+    but one not included in the Milky Way, is the ``Great Cluster
+    in Hercules.'' This is barely visible to the naked eye, but a
+    small telescope shows its character, and in a large one it
+    presents a marvelous spectacle. Photographs of such clusters
+    are, perhaps, less effective than those of star-clouds, because
+    the central condensation of stars in them is so great that
+    their light becomes blended in an indistinguishable blur. The
+    beautiful effect of the incessant play of infinitesimal rays
+    over the apparently compact surface of the cluster, as if it
+    were a globe of the finest frosted silver shining in an
+    electric beam, is also lost in a photograph. Still, even to the
+    eye looking directly at the cluster through a powerful
+    telescope, the central part of the wonderful congregation seems
+    almost a solid mass in which the stars are packed like the ice
+    crystals in a snowball.</p>
+
+    <p>The same question rises to the lips of every observer: How
+    can they possibly have been brought into such a situation? The
+    marvel does not grow less when we know that, instead of being
+    closely compacted, the stars of the cluster are probably
+    separated by millions of miles; for we know that their
+    distances apart are slight as compared with their remoteness
+    from the Earth. Sir William Herschel estimated their number to
+    be about fourteen thousand, but in fact they are uncountable.
+    If we could view them from a point just within the edge of the
+    assemblage, they would offer the appearance of a hollow
+    hemisphere emblazoned with stars of astonishing brilliancy; the
+    near-by ones unparalleled in splendor by any celestial object
+    known to us, while the more distant ones would resemble
+    ordinary stars. An inhabitant of the cluster would not know,
+    except by a process of ratiocination, that he was dwelling in a
+    globular assemblage of suns; only from a point far outside
+    would their spherical arrangement become evident to the eye.
+    Imagine fourteen-thousand fire-balloons with an approach to
+    regularity in a spherical space -- say, ten miles in diameter;
+    there would be an average of less than thirty in every cubic
+    mile, and it would be necessary to go to a considerable
+    distance in order to see them as a globular aggregation; yet
+    from a point sufficiently far away they would blend into a
+    glowing ball.</p>
+
+    <p>Photographs show even better than the best telescopic views
+    that the great cluster is surrounded with a multitude of
+    dispersed stars, suggestively arrayed in more or less curving
+    lines, which radiate from the principle mass, with which their
+    connection is manifest. These stars, situated outside the
+    central sphere, look somewhat like vagrant bees buzzing round a
+    dense swarm where the queen bee is sitting. Yet while there is
+    so much to suggest the operation of central forces, bringing
+    and keeping the members of the cluster together, the attentive
+    observer is also impressed with the idea that the whole
+    wonderful phenomenon may be <em>the result of explosion.</em>
+    As soon as this thought seizes the mind, confirmation of it
+    seems to be found in the appearance of the outlying stars,
+    which could be as readily explained by the supposition that
+    they have been blown apart as that they have flocked together
+    toward a center. The probable fact that the stars constituting
+    the cluster are very much smaller than our sun might be
+    regarded as favoring the hypothesis of an explosion. Of their
+    real size we know nothing, but, on the basis of an uncertain
+    estimate of their parallax, it has been calculated that they
+    may average forty-five thousand miles in diameter -- something
+    more than half the diameter of the planet Jupiter. Assuming the
+    same mean density, fourteen thousand such stars might have been
+    formed by the explosion of a body about twice the size of the
+    sun. This recalls the theory of Olbers, which has never been
+    altogether abandoned or disproved, that the Asteroids were
+    formed by the explosion of a planet circulating between the
+    orbits of Mars and Jupiter. The Asteroids, whatever their
+    manner of origin, form a ring around the sun; but, of course,
+    the explosion of a great independent body, not originally
+    revolving about a superior center of gravitational force, would
+    not result in the formation of a ring of small bodies, but
+    rather of a dispersed mass of them. But back of any speculation
+    of this kind lies the problem, at present insoluble: How could
+    the explosion be produced? (See the question of explosions in
+    Chapters 6 and 14).</p>
+
+    <p>Then, on the other hand, we have the observation of
+    Herschel, since abundantly confirmed, that space is unusually
+    vacant in the immediate neighborhood of condensed star-clusters
+    and nebul&aelig;, which, as far as it goes, might be taken as
+    an indication that the assembled stars had been drawn together
+    by their mutual attractions, and that the tendency to
+    aggregation is still bringing new members toward the cluster.
+    But in that case there must have been an original condensation
+    of stars at that point in space. This could probably have been
+    produced by the coagulation of a great nebula into stellar
+    nuclei, a process which seems now to be taking place in the
+    Orion Nebula.</p>
+
+    <p>A yet more remarkable globular star-cluster exists in the
+    southern hemisphere, Omega Centauri. In this case the central
+    condensation of stars presents an almost uniform blaze of
+    light. Like the Hercules cluster, that in Centaurus is
+    surrounded with stars scattered over a broad field and showing
+    an appearance of radial arrangement. In fact, except for its
+    greater richness, Omega Centauri is an exact duplicate of its
+    northern rival. Each appears to an imaginative spectator as a
+    veritable ``city of suns.'' Mathematics shrinks from the task
+    of disentangling the maze of motions in such an assemblage. It
+    would seem that the chance of collisions is not to be
+    neglected, and this idea finds a certain degree of confirmation
+    in the appearance of ``temporary stars'' which have more than
+    once blazed out in, or close by, globular star-clusters.</p>
+
+    <p>This leads up to the notable fact, first established by
+    Professor Bailey a few years ago, that such clusters are
+    populous with variable stars. Omega Centauri and the Hercules
+    cluster are especially remarkable in this respect. The
+    variables found in them are all of short period and the changes
+    of light show a noteworthy tendency to uniformity. The first
+    thought is that these phenomena must be due to collisions among
+    the crowded stars, but, if so, the encounters cannot be between
+    the stars themselves, but probably between stars and meteor
+    swarms revolving around them. Such periodic collisions might go
+    on for ages without the meteors being exhausted by
+    incorporation with the stars. This explanation appears all the
+    more probable because one would naturally expect that flocks of
+    meteors would abound in a close aggregation of stars. It is
+    also consistent with Perrine's discovery -- that the globular
+    star clusters are powdered with minute stars strewn thickly
+    among the brighter ones.</p>
+
+    <p>In speaking of Professor Comstock's extraordinary theory of
+    the Milky Way, the fact was mentioned that, broadly speaking,
+    the nebul&aelig; are less numerous in the galactic belt than in
+    the comparatively open spaces on either side of it, but that
+    they are, nevertheless, abundant in the broader half of the
+    Milky Way which he designates as the front of the gigantic
+    ``plough'' supposed to be forcing its way through the
+    enveloping chaos. In and around the Sagittarius region the
+    intermingling of nebul&aelig; and galactic star clouds and
+    clusters is particularly remarkable. That there is a causal
+    connection no thoughtful person can doubt. We are unable to get
+    away from the evidence that a nebula is like a seed-ground from
+    which stars spring forth; or we may say that nebul&aelig;
+    resemble clouds in whose bosom raindrops are forming. The
+    wonderful aspect of the admixtures of nebul&aelig; and
+    star-clusters in Sagittarius has been described in Chapter 1.
+    We now come to a still more extraordinary phenomenon of this
+    kind -- the Pleiades nebul&aelig;.</p>
+
+    <p>The group of the Pleiades, although lying outside the main
+    course of the Galaxy, is connected with it by a faint loop, and
+    is the scene of the most remarkable association of stars and
+    nebulous matter known in the visible universe. The naked eye is
+    unaware of the existence of nebul&aelig; in the Pleiades, or,
+    at the best, merely suspects that there is something of the
+    kind there; and even the most powerful telescopes are far from
+    revealing the full wonder of the spectacle; but in photographs
+    which have been exposed for many hours consecutively, in order
+    to accumulate the impression of the actinic rays, the
+    revelation is stunning. The principle stars are seen surrounded
+    by, and, as it were, <em>drowned in,</em> dense nebulous clouds
+    of an unparalleled kind. The forms assumed by these clouds seem
+    at first sight inexplicable. They look like fleeces, or perhaps
+    more like splashes and daubs of luminous paint dashed
+    carelessly from a brush. But closer inspection shows that they
+    are, to a large extent, <em>woven</em> out of innumerable
+    threads of filmy texture, and there are many indications of
+    spiral tendencies. Each of the bright stars of the group --
+    Alcyone, Merope, Maia, Electra, Taygeta, Atlas -- is the focus
+    of a dense fog (totally invisible, remember, alike to the naked
+    eye and to the telescope), and these particular stars are
+    veiled from sight behind the strange mists. Running in all
+    directions across the relatively open spaces are nebulous wisps
+    and streaks of the most curious forms. On some of the nebular
+    lines, which are either straight throughout, or if they change
+    direction do so at an angle, little stars are strung like
+    beads. In one case seven or eight stars are thus aligned, and,
+    as if to emphasize their dependence upon the chain which
+    connects them, when it makes a slight bend the file of stars
+    turns the same way. Many other star rows in the group suggest
+    by their arrangement that they, too, were once strung upon
+    similar threads which have now disappeared, leaving the stars
+    spaced along their ancient tracks. We seem forced to the
+    conclusion that there was a time when the Pleiades were
+    embedded in a vast nebula resembling that of Orion, and that
+    the cloud has now become so rare by gradual condensation into
+    stars that the merest trace of it remains, and this would
+    probably have escaped detection but for the remarkable actinic
+    power of the radiant matter of which it consists. The richness
+    of many of these faint nebulous masses in ultra-violet
+    radiations, which are those that specifically affect the
+    photographic plate, is the cause of the marvelous revelatory
+    power of celestial photography. So the veritable unseen
+    universe, as distinguished from the ``unseen universe'' of
+    metaphysical speculation, is shown to us.</p>
+
+    <p>A different kind of association between stars and
+    nebul&aelig; is shown in some surprising photographic objects
+    in the constellation Cygnus, where long, wispy nebul&aelig;,
+    billions of miles in length, some of them looking like tresses
+    streaming in a breeze, lie amid fields of stars which seem
+    related to them. But the relation is of a most singular kind,
+    for notwithstanding the delicate structure of the long
+    nebul&aelig; they appear to act as barriers, causing the stars
+    to heap themselves on one side. The stars are two, three, or
+    four times as numerous on one side of the nebul&aelig; as on
+    the other. These nebul&aelig;, as far as appearance goes, might
+    be likened to rail fences, or thin hedges, against which the
+    wind is driving drifts of powdery snow, which, while scattered
+    plentifully all around, tends to bank itself on the leeward
+    side of the obstruction. The imagination is at a loss to
+    account for these extraordinary phenomena; yet there they are,
+    faithfully giving us their images whenever the photographic
+    plate is exposed to their radiations.</p>
+
+    <p>Thus the more we see of the universe with improved methods
+    of observation, and the more we invent aids to human senses,
+    each enabling us to penetrate a little deeper into the unseen,
+    the greater becomes the mystery. The telescope carried us far,
+    photography is carrying us still farther; but what as yet
+    unimagined instrument will take us to the bottom, the top, and
+    the end? And then, what hitherto untried power of thought will
+    enable us to comprehend the meaning of it all?</p>
+
+    <p><strong>Stellar Migrations</strong></p>
+
+    <p>To the untrained eye the stars and the planets are not
+    distinguishable. It is customary to call them all alike
+    ``stars.'' But since the planets more or less rapidly change
+    their places in the sky, in consequence of their revolution
+    about the sun, while the stars proper seem to remain always in
+    the same relative positions, the latter are spoken of as
+    ``fixed stars.'' In the beginnings of astronomy it was not
+    known that the ``fixed stars'' had any motion independent of
+    their apparent annual revolution with the whole sky about the
+    earth as a seeming center. Now, however, we know that the term
+    ``fixed stars'' is paradoxical, for there is not a single
+    really fixed object in the whole celestial sphere. The apparent
+    fixity in the positions of the stars is due to their immense
+    distance, combined with the shortness of the time during which
+    we are able to observe them. It is like viewing the plume of
+    smoke issuing from a steamer, hull down, at sea: if one does
+    not continue to watch it for a long time it appears to be
+    motionless, although in reality it may be traveling at great
+    speed across the line of sight. Even the planets seem fixed in
+    position if one watches them for a single night only, and the
+    more distant ones do not sensibly change their places, except
+    after many nights of observation. Neptune, for instance, moves
+    but little more than two degrees in the course of an entire
+    year, and in a month its change of place is only about
+    one-third of the diameter of the full moon.</p>
+
+    <p>Yet, fixed as they seem, the stars are actually moving with
+    a speed in comparison with which, in some cases, the planets
+    might almost be said to stand fast in their tracks. Jupiter's
+    speed in his orbit is about eight miles per second, Neptune's
+    is less than three and one-half miles, and the earth's is about
+    eighteen and one-half miles; while there are ``fixed stars''
+    which move two hundred or three hundred miles per second. They
+    do not all, however, move with so great a velocity, for some
+    appear to travel no faster than the planets. But in all cases,
+    notwithstanding their real speed, long-continued and
+    exceedingly careful observations are required to demonstrate
+    that they are moving at all. No more overwhelming impression of
+    the frightful depths of space in which the stars are buried can
+    be obtained than by reflecting upon the fact that a star whose
+    actual motion across the line of sight amounts to two hundred
+    miles per second does not change its apparent place in the sky,
+    in the course of a thousand years, sufficiently to be noticed
+    by the casual observer of the heavens!</p>
+
+    <p>There is one vast difference between the motions of the
+    stars and those of the planets to which attention should be at
+    once called: the planets, being under the control of a central
+    force emanating from their immediate master, the sun, all move
+    in the same direction and in orbits concentric about the sun;
+    the stars, on the other hand, move in every conceivable
+    direction and have no apparent center of motion, for all
+    efforts to discover such a center have failed. At one time,
+    when theology had finally to accept the facts of science, a
+    grandiose conception arose in some pious minds, according to
+    which the Throne of God was situated at the exact center of His
+    Creation, and, seated there, He watched the magnificent
+    spectacle of the starry systems obediently revolving around
+    Him. Astronomical discoveries and speculations seemed for a
+    time to afford some warrant for this view, which was, moreover,
+    an acceptable substitute for the abandoned geocentric theory in
+    minds that could only conceive of God as a superhuman
+    artificer, constantly admiring his own work. No longer ago than
+    the middle of the nineteenth century a German astronomer,
+    Maedler, believed that he had actually found the location of
+    the center about which the stellar universe revolved. He placed
+    it in the group of the Pleiades, and upon his authority an
+    extraordinary imaginative picture was sometimes drawn of the
+    star Alcyone, the brightest of the Pleiades, as the very seat
+    of the Almighty. This idea even seemed to gain a kind of
+    traditional support from the mystic significance, without known
+    historical origin, which has for many ages, and among widely
+    separated peoples, been attached to the remarkable group of
+    which Alcyone is the chief. But since Maedler's time it has
+    been demonstrated that the Pleiades cannot be the center of
+    revolution of the universe, and, as already remarked, all
+    attempts to find or fix such a center have proved abortive. Yet
+    so powerful was the hold that the theory took upon the popular
+    imagination, that even today astronomers are often asked if
+    Alcyone is not the probable site of ``Jerusalem the
+    Golden.''</p>
+
+    <p>If there were a discoverable center of predominant
+    gravitative power, to which the motions of all the stars could
+    be referred, those motions would appear less mysterious, and we
+    should then be able to conclude that the universe was, as a
+    whole, a prototype of the subsidiary systems of which it is
+    composed. We should look simply to the law of gravitation for
+    an explanation, and, naturally, the center would be placed
+    within the opening enclosed by the Milky Way. If it were there
+    the Milky Way itself should exhibit signs of revolution about
+    it, like a wheel turning upon its hub. No theory of the star
+    motions as a whole could stand which failed to take account of
+    the Milky Way as the basis of all. But the very form of that
+    divided wreath of stars forbids the assumption of its
+    revolution about a center. Even if it could be conceived as a
+    wheel having no material center it would not have the form
+    which it actually presents. As was shown in Chapter 2, there is
+    abundant evidence of motion in the Milky Way; but it is not
+    motion of the system as a whole, but motion affecting its
+    separate parts. Instead of all moving one way, the galactic
+    stars, as far as their movements can be inferred, are governed
+    by local influences and conditions. They appear to travel
+    crosswise and in contrary directions, and perhaps they eddy
+    around foci where great numbers have assembled; but of a
+    universal revolution involving the entire mass we have no
+    evidence.</p>
+
+    <p>Most of our knowledge of star motions, called ``proper
+    motions,'' relates to individual stars and to a few groups
+    which happen to be so near that the effects of their movements
+    are measurable. In some cases the motion is so rapid (not in
+    appearance, but in reality) that the chief difficulty is to
+    imagine how it can have been imparted, and what will eventually
+    become of the ``runaways.'' Without a collision, or a series of
+    very close approaches to great gravitational centers, a star
+    traveling through space at the rate of two hundred or three
+    hundred miles per second could not be arrested or turned into
+    an orbit which would keep it forever flying within the limits
+    of the visible universe. A famous example of these speeding
+    stars is ``1830 Groombridge,'' a star of only the sixth
+    magnitude, and consequently just visible to the naked eye,
+    whose motion across the line of sight is so rapid that it moves
+    upon the face of the sky a distance equal to the apparent
+    diameter of the moon every 280 years. The distance of this star
+    is at least 200,000,000,000,000 miles, and may be two or three
+    times greater, so that its actual speed cannot be less than two
+    hundred, and may be as much as four hundred, miles per second.
+    It could be turned into a new course by a close approach to a
+    great sun, but it could only be stopped by collision, head-on,
+    with a body of enormous mass. Barring such accidents it must,
+    as far as we can see, keep on until it has traversed our
+    stellar system, whence in may escape and pass out into space
+    beyond, to join, perhaps, one of those other universes of which
+    we have spoken. Arcturus, one of the greatest suns in the
+    universe, is also a runaway, whose speed of flight has been
+    estimated all the way from fifty to two hundred miles per
+    second. Arcturus, we have every reason to believe, possesses
+    hundreds of times the mass of our sun -- think, then, of the
+    prodigious momentum that its motion implies! Sirius moves more
+    moderately, its motion across the line of sight amounting to
+    only ten miles per second, but it is at the same time
+    approaching the sun at about the same speed, its actual
+    velocity in space being the resultant of the two
+    displacements.</p>
+
+    <p>What has been said about the motion of Sirius brings us to
+    another aspect of this subject. The fact is, that in every case
+    of stellar motion the displacement that we observe represents
+    only a part of the actual movement of the star concerned. There
+    are stars whose motion carries them straight toward or straight
+    away from the earth, and such stars, of course, show no cross
+    motion. But the vast majority are traveling in paths inclined
+    from a perpendicular to our line of sight. Taken as a whole,
+    the stars may be said to be flying about like the molecules in
+    a mass of gas. The discovery of the radial component in the
+    movements of the stars is due to the spectroscope. If a star is
+    approaching, its spectral lines are shifted toward the violet
+    end of the spectrum by an amount depending upon the velocity of
+    approach; if it is receding, the lines are correspondingly
+    shifted toward the red end. Spectroscopic observation, then,
+    combined with micrometric measurements of the cross motion,
+    enables us to detect the real movement of the star in space.
+    Sometimes it happens that a star's radial movement is
+    periodically reversed; first it approaches, and then it
+    recedes. This indicates that it is revolving around a near-by
+    companion, which is often invisible, and superposed upon this
+    motion is that of the two stars concerned, which together may
+    be approaching or receding or traveling across the line of
+    sight. Thus the complications involved in the stellar motions
+    are often exceedingly great and puzzling.</p>
+
+    <p>Yet another source of complication exists in the movement of
+    our own star, the sun. There is no more difficult problem in
+    astronomy than that of disentangling the effects of the solar
+    motion from those of the motions of the other stars. But the
+    problem, difficult as it is, has been solved, and upon its
+    solution depends our knowledge of the speed and direction of
+    the movement of the solar system through space, for of course
+    the sun carries its planets with it. One element of the
+    solution is found in the fact that, as a result of perspective,
+    the stars toward which we are going appear to move apart toward
+    all points of the compass, while those behind appear to close
+    up together. Then the spectroscopic principle already mentioned
+    is invoked for studying the shift of the lines, which is toward
+    the violet in the stars ahead of us and toward the red in those
+    that we are leaving behind. Of course the effects of the
+    independent motions of the stars must be carefully excluded.
+    The result of the studies devoted to this subject is to show
+    that we are traveling at a speed of twelve to fifteen miles per
+    second in a northerly direction, toward the border of the
+    constellations Hercules and Lyra. A curious fact is that the
+    more recent estimates show that the direction is not very much
+    out of a straight line drawn from the sun to the star Vega, one
+    of the most magnificent suns in the heavens. But it should not
+    be inferred from this that Vega is drawing us on; it is too
+    distant for its gravitation to have such an effect.</p>
+
+    <p>Many unaccustomed thoughts are suggested by this mighty
+    voyage of the solar system. Whence have we come, and whither do
+    we go? Every year of our lives we advance at least 375,000,000
+    miles. Since the traditional time of Adam the sun has led his
+    planets through the wastes of space no less than
+    225,000,000,000 miles, or more than 2400 times the distance
+    that separates him from the earth. Go back in imagination to
+    the geologic ages, and try to comprehend the distance over
+    which the earth has flown. Where was our little planet when it
+    emerged out of the clouds of chaos? Where was the sun when his
+    ``thunder march'' began? What strange constellations shone down
+    upon our globe when its masters of life were the monstrous
+    beasts of the ``Age of Reptiles''? A million years is not much
+    of a span of time in geologic reckoning, yet a million years
+    ago the earth was farther from its present place in space than
+    any of the stars with a measurable parallax are now. It was
+    more than seven times as far as Sirius, nearly fourteen times
+    as far as Alpha Centauri, three times as far as Vega, and twice
+    as far as Arcturus. But some geologists demand two hundred,
+    three hundred, even one thousand million years to enable them
+    to account for the evolutionary development of the earth and
+    its inhabitants. In a thousand million years the earth would
+    have traveled farther than from the remotest conceivable depths
+    of the Milky Way!</p>
+
+    <p>Other curious reflections arise when we think of the form of
+    the earth's track as it follows the lead of the sun, in a
+    journey which has neither known beginning nor conceivable end.
+    There are probably many minds which have found a kind of
+    consolation in the thought that every year the globe returns to
+    the same place, on the same side of the sun. This idea may have
+    an occult connection with our traditional regard for
+    anniversaries. When that period of the year returns at which
+    any great event in our lives has occurred we have the feeling
+    that the earth, in its annual round, has, in a manner, brought
+    us back to the scene of that event. We think of the earth's
+    orbit as a well-worn path which we traverse many times in the
+    course of a lifetime. It seems familiar to us, and we grow to
+    have a sort of attachment to it. The sun we are accustomed to
+    regard as a fixed center in space, like the mill or pump around
+    which the harnessed patient mule makes his endless circuits.
+    But the real fact is that the earth never returns to the place
+    in space where it has once quitted. In consequence of the
+    motion of the sun carrying the earth and the other planets
+    along, the track pursued by our globe is a vast spiral in space
+    continually developing and never returning upon its course. It
+    is probable that the tracks of the sun and the others stars are
+    also irregular, and possibly spiral, although, as far as can be
+    at present determined, they appear to be practically straight.
+    Every star, wherever it may be situated, is attracted by its
+    fellow-stars from many sides at once, and although the force is
+    minimized by distance, yet in the course of many ages its
+    effects must become manifest.</p>
+
+    <p>Looked at from another side, is there not something
+    immensely stimulating and pleasing to the imagination in the
+    idea of so stupendous a journey, which makes all of us the
+    greatest of travelers? In the course of a long life a man is
+    transported through space thirty thousand million miles;
+    Halley's Comet does not travel one-quarter as far in making one
+    of its immense circuits. And there are adventures on this
+    voyage of which we are just beginning to learn to take account.
+    Space is full of strange things, and the earth must encounter
+    some of them as it advances through the unknown. Many singular
+    speculations have been indulged in by astronomers concerning
+    the possible effects upon the earth of the varying state of the
+    space that it traverses. Even the alternation of hot and
+    glacial periods has sometimes been ascribed to this source.
+    When tropical life flourished around the poles, as the remains
+    in the rocks assure us, the needed high temperature may, it has
+    been thought, have been derived from the presence of the earth
+    in a warm region of space. Then, too, there is a certain
+    interest for us in the thought of what our familiar planet has
+    passed through. We cannot but admire it for its long journeying
+    as we admire the traveler who comes to us from remote and
+    unexplored lands, or as we gaze with a glow of interest upon
+    the first locomotive that has crossed a continent, or a ship
+    that has visited the Arctic or Antarctic regions. If we may
+    trust the indications of the present course, the earth, piloted
+    by the sun, has come from the Milky Way in the far south and
+    may eventually rejoin that mighty band of stars in the far
+    north.</p>
+
+    <p>While the stars in general appear to travel independently of
+    one another, except when they are combined in binary or trinary
+    systems, there are notable exceptions to this rule. In some
+    quarters of the sky we behold veritable migrations of entire
+    groups of stars whose members are too widely separated to show
+    any indications of revolution about a common center of gravity.
+    This leads us back again to the wonderful group of the
+    Pleiades. All of the principle stars composing that group are
+    traveling in virtually parallel lines. Whatever force set them
+    going evidently acted upon all alike. This might be explained
+    by the assumption that when the original projective force acted
+    upon them they were more closely united than they are at
+    present, and that in drifting apart they have not lost the
+    impulse of the primal motion. Or it may be supposed that they
+    are carried along by some current in space, although it would
+    be exceedingly difficult, in the present state of our
+    knowledge, to explain the nature of such a current. Yet the
+    theory of a current has been proposed. As to an attractive
+    center around which they might revolve, none has been found.
+    Another instance of similar ``star-drift'' is furnished by five
+    of the seven stars constituting the figure of the ``Great
+    Dipper.'' In this case the stars concerned are separated very
+    widely, the two extreme ones by not less than fifteen degrees,
+    so that the idea of a common motion would never have been
+    suggested by their aspect in the sky; and the case becomes the
+    more remarkable from the fact that among and between them there
+    are other stars, some of the same magnitude, which do not share
+    their motion, but are traveling in other directions. Still
+    other examples of the same phenomenon are found in other parts
+    of the sky. Of course, in the case of compact star-clusters, it
+    is assumed that all the members share a like motion of
+    translation through space, and the same is probably true of
+    dense star-swarms and star-clouds.</p>
+
+    <p>The whole question of star-drift has lately assumed a new
+    phase, in consequence of the investigations of Kapteyn, Dyson,
+    and Eddington on the ``systematic motions of the stars.'' This
+    research will, it is hoped, lead to an understanding of the
+    general law governing the movements of the whole body of stars
+    constituting the visible universe. Taking about eleven hundred
+    stars whose proper motions have been ascertained with an
+    approach to certainty, and which are distributed in all parts
+    of the sky, it has been shown that there exists an apparent
+    double drift, in two independent streams, moving in different
+    and nearly opposed directions. The apex of the motion of what
+    is called ``Stream I'' is situated, according to Professor
+    Kapteyn, in right ascension 85&deg;, declination south 11&deg;,
+    which places it just south of the constellation Orion; while
+    the apex of ``Stream II'' is in right ascension 260&deg;,
+    declination south 48&deg;, placing it in the constellation Ara,
+    south of Scorpio. The two apices differ very nearly 180&deg; in
+    right ascension and about 120&deg; in declination. The
+    discovery of these vast star-streams, if they really exist, is
+    one of the most extraordinary in modern astronomy. It offers
+    the correlation of stellar movements needed as the basis of a
+    theory of those movements, but it seems far from revealing a
+    physical cause for them. As projected against the celestial
+    sphere the stars forming the two opposite streams appear
+    intermingled, some obeying one tendency and some the other. As
+    Professor Dyson has said, the hypothesis of this double
+    movement is of a revolutionary character, and calls for further
+    investigation. Indeed, it seems at first glance not less
+    surprising than would be the observation that in a snow-storm
+    the flakes over our heads were divided into two parties and
+    driving across each other's course in nearly opposite
+    directions, as if urged by interpenetrating winds.</p>
+
+    <p>But whatever explanation may eventually be found for the
+    motions of the stars, the knowledge of the existence of those
+    motions must always afford a new charm to the contemplative
+    observer of the heavens, for they impart a sense of life to the
+    starry system that would otherwise be lacking. A stagnant
+    universe, with every star fixed immovably in its place, would
+    not content the imagination or satisfy our longing for
+    ceaseless activity. The majestic grandeur of the evolutions of
+    the celestial hosts, the inconceivable vastness of the fields
+    of space in which they are executed, the countless numbers, the
+    immeasurable distances, the involved convolutions, the flocking
+    and the scattering, the interpenetrating marches and
+    countermarches, the strange community of impulsion affecting
+    stars that are wide apart in space and causing them to traverse
+    the general movement about them like aides and despatch-bearers
+    on a battle-field -- all these arouse an intensity of interest
+    which is heightened by the mystery behind them.</p>
+
+    <p><strong>The Passing of the Constellations</strong></p>
+
+    <p>From a historical and picturesque point of view, one of the
+    most striking results of the motions of the stars described in
+    the last chapter is their effect upon the forms of the
+    constellations, which have been watched and admired by mankind
+    from a period so early that the date of their invention is now
+    unknown. The constellations are formed by chance combinations
+    of conspicuous stars, like figures in a kaleidoscope, and if
+    our lives were commensurate with the &aelig;ons of cosmic
+    existence we should perceive that the kaleidoscope of the
+    heavens was ceaselessly turning and throwing the stars into new
+    symmetries. Even if the stars stood fast, the motion of the
+    solar system would gradually alter the configurations, as the
+    elements of a landscape dissolve and recombine in fresh
+    groupings with the traveler's progress amid them. But with the
+    stars themselves all in motion at various speeds and in many
+    directions, the changes occur more rapidly. Of course,
+    ``rapid'' is here understood in a relative sense; the wheel of
+    human history to an eye accustomed to the majestic progression
+    of the universe would appear to revolve with the velocity of a
+    whirling dynamo. Only the deliberation of geological movements
+    can be contrasted with the evolution and devolution of the
+    constellations.</p>
+
+    <p>And yet this secular fluctuation of the constellation
+    figures is not without keen interest for the meditative
+    observer. It is another reminder of the swift mutability of
+    terrestial affairs. To the passing glance, which is all that we
+    can bestow upon these figures, they appear so immutable that
+    they have been called into service to form the most lasting
+    records of ancient thought and imagination that we possess. In
+    the forms of the constellations, the most beautiful, and, in
+    imaginative quality, the finest, mythology that the world has
+    ever known has been perpetuated. Yet, in a broad sense, this
+    scroll of human thought imprinted on the heavens is as
+    evanescent as the summer clouds. Although more enduring than
+    parchment, tombs, pyramids, and temples, it is as far as they
+    from truly eternizing the memory of what man has fancied and
+    done.</p>
+
+    <p>Before studying the effects that the motions of the stars
+    have had and will have upon the constellations, it is worth
+    while to consider a little further the importance of the
+    stellar pictures as archives of history. To emphasize the
+    importance of these effects it is only necessary to recall that
+    the constellations register the oldest traditions of our race.
+    In the history of primeval religions they are the most valuable
+    of documents. Leaving out of account for the moment the more
+    familiar mythology of the Greeks, based on something older yet,
+    we may refer for illustration to that of the mysterious Maya
+    race of America. At Izamal, in Yucatan, says Mr Stansbury
+    Hagar, is a group of ruins perched, after the Mexican and
+    Central-American plan, on the summits of pyramidal mounds which
+    mark the site of an ancient theogonic center of the Mayas. Here
+    the temples all evidently refer to a cult based upon the
+    constellations as symbols. The figures and the names, of
+    course, were not the same as those that we have derived from
+    our Aryan ancestors, but the star groups were the same or
+    nearly so. For instance, the loftiest of the temples at Izamal
+    was connected with the sign of the constellation known to us as
+    Cancer, marking the place of the sun at the summer solstice, at
+    which period the sun was supposed to descend at noon like a
+    great bird of fire and consume the offerings left upon the
+    altar. Our Scorpio was known to the Mayas as a sign of the
+    ``Death God.'' Our Libra, the ``Balance,'' with which the idea
+    of a divine weighing out of justice has always been connected,
+    seems to be identical with the Mayan constellation
+    Teoyaotlatohua, with which was associated a temple where dwelt
+    the priests whose special business it was to administer justice
+    and to foretell the future by means of information obtained
+    from the spirits of the dead. Orion, the ``Hunter'' of our
+    celestial mythology, was among the Mayas a ``Warrior,'' while
+    Sagittarius and others of our constellations were known to them
+    (under different names, of course), and all were endowed with a
+    religious symbolism. And the same star figures, having the same
+    significance, were familiar to the Peruvians, as shown by the
+    temples at Cuzco. Thus the imagination of ancient America
+    sought in the constellations symbols of the unchanging
+    gods.</p>
+
+    <p>But, in fact, there is no nation and no people that has not
+    recognized the constellations, and at one period or another in
+    its history employed them in some symbolic or representative
+    capacity. As handled by the Greeks from prehistoric times, the
+    constellation myths became the very soul of poetry. The
+    imagination of that wonderful race idealized the principal star
+    groups so effectively that the figures and traditions thus
+    attached to them have, for civilized mankind, displaced all
+    others, just as Greek art in its highest forms stands without
+    parallel and eclipses every rival. The Romans translated no
+    heroes and heroines of the mythical period of their history to
+    the sky, and the deified C&aelig;sars never entered that lofty
+    company, but the heavens are filled with the early myths of the
+    Greeks. Herakles nightly resumes his mighty labors in the
+    stars; Zeus, in the form of the white ``Bull,'' Taurus, bears
+    the fair Europa on his back through the celestial waves;
+    Andromeda stretches forth her shackled arms in the star-gemmed
+    ether, beseeching aid; and Perseus, in a blaze of diamond
+    armor, revives his heroic deeds amid sparkling clouds of
+    stellar dust. There, too, sits Queen Cassiopeia in her dazzling
+    chair, while the Great King, Cepheus, towers gigantic over the
+    pole. Professor Young has significantly remarked that a great
+    number of the constellations are connected in some way or other
+    with the Argonautic Expedition -- that strangely fascinating
+    legend of earliest Greek story which has never lost its charm
+    for mankind. In view of all this, we may well congratulate
+    ourselves that the constellations will outlast our time and the
+    time of countless generations to follow us; and yet they are
+    very far from being eternal. Let us now study some of the
+    effects of the stellar motions upon them.</p>
+
+    <p>We begin with the familiar figure of the ``Great Dipper.''
+    He who has not drunk inspiration from its celestial bowl is not
+    yet admitted to the circle of Olympus. This figure is made up
+    of seven conspicuous stars in the constellation Ursa Major, the
+    ``Greater Bear.'' The handle of the ``Dipper'' corresponds to
+    the tail of the imaginary ``Bear,'' and the bowl lies upon his
+    flank. In fact, the figure of a dipper is so evident and that
+    of a bear so unevident, that to most persons the ``Great
+    Dipper'' is the only part of the constellation that is
+    recognizable. Of the seven stars mentioned, six are of nearly
+    equal brightness, ranking as of the second magnitude, while the
+    seventh is of only the third magnitude. The difference is very
+    striking, since every increase of one magnitude involves an
+    increase of two-and-a-half times in brightness. There appears
+    to be little doubt that the faint star, which is situated at
+    the junction of the bowl and the handle, is a variable of long
+    period, since three hundred years ago it was as bright as its
+    companions. But however that may be, its relative faintness at
+    the present time interferes but little with the perfection of
+    the ``Dipper's'' figure. In order the more readily to
+    understand the changes which are taking place, it will be well
+    to mention both the names and the Greek letters which are
+    attached to the seven stars. Beginning at the star in the upper
+    outer edge of the rim of the bowl and running in regular order
+    round the bottom and then out to the end of the handle, the
+    names and letters are as follows: Dubhe ({\alpha}), Merak
+    ({\beta}), Phaed ({\gamma}), Megrez ({\delta}), Alioth
+    ({\epsilon}), Mizar ({\zeta}), and Benetnasch ({\eta}). Megrez
+    is the faint star already mentioned at the junction of the bowl
+    and handle, and Mizar, in the middle of the handle, has a
+    close, naked-eye companion which is named Alcor. The Arabs
+    called this singular pair of stars ``The Horse and Rider.''
+    Merak and Duhbe are called ``The Pointers,'' because an
+    imaginary line drawn northward through them indicates the Pole
+    Star.</p>
+
+    <p>Now it has been found that five of these stars --
+    <em>viz.,</em> Merak, Phaed, Megrez, Alioth, and Mizar (with
+    its comrade) -- are moving with practically the same speed in
+    an easterly direction, while the other two, Dubhe and
+    Benetnasch, are simultaneously moving westward, the motions of
+    Benetnasch being apparently more rapid. The consequence of
+    these opposed motions is, of course, that the figure of the
+    ``Dipper'' cannot always have existed and will not continue to
+    exist. In the accompanying diagrams it has been thought
+    interesting to show the relative positions of these seven
+    stars, as seen from the point which the earth now occupies,
+    both in the past and in the future. Arrows attached to the
+    stars in the figure representing the present appearance of the
+    ``Dipper'' indicate the directions of the motions and the
+    distances over which they will carry the stars in a period of
+    about five hundred centuries. The time, no doubt, seems long,
+    but remember the vast stretch of ages through which the earth
+    has passed, and then reflect that no reason is apparent why our
+    globe should not continue to be a scene of animation for ten
+    thousand centuries yet to come. The fact that the little star
+    Alcor placed so close to Mizar should accompany the latter in
+    its flight is not surprising, but that two of the principal
+    stars of the group should be found moving in a direction
+    directly opposed to that pursued by the other five is
+    surprising in the highest degree; and it recalls the strange
+    theory of a double drift affecting all the stars, to which
+    attention was called in the preceding chapter. It would appear
+    that Benetnasch and Dubhe belong to one ``current,'' and Merak,
+    Phaed, Megrez, Alioth, and Mizar to the other. As far as is
+    known, the motion of the seven stars are not shared by the
+    smaller stars scattered about them, but on the theory of
+    currents there should be such a community of motion, and
+    further investigation may reveal it.</p>
+
+    <p>From the ``Great Dipper'' we turn to a constellation hardly
+    less conspicuous and situated at an equal distance from the
+    pole on the other side -- Cassiopeia. This famous star-group
+    commemorating the romantic Queen of Ethiopia whose vain
+    boasting of her beauty was punished by the exposure of her
+    daughter Andromeda to the ``Sea Monster,'' is well-marked by
+    five stars which form an irregular letter ``W'' with its open
+    side toward the pole. Three of these stars are usually ranked
+    as of the second magnitude, and two of the third; but to
+    ordinary observation they appear of nearly equal brightness,
+    and present a very striking picture. They mark out the chair
+    and a part of the figure of the beautiful queen. Beginning at
+    the right-hand, or western, end of the ``W,'' their Greek
+    letter designations are: Beta ({\beta}), Alpha ({\alpha}),
+    Gamma ({\gamma}), Delta ({\delta}), and Epsilon ({\epsilon}).
+    Four of them, Beta, Alpha, Delta, and Epsilon are traveling
+    eastwardly at various speeds, while the fifth, Gamma, moves in
+    a westerly direction. The motion of Beta is more rapid than
+    that of any of the others. It should be said, however, that no
+    little uncertainty attaches to the estimates of the rate of
+    motion of stars which are not going very rapidly, and different
+    observers often vary considerably in their results.</p>
+
+    <p>In the beautiful ``Northern Crown,'' one of the most perfect
+    and charming of all the figures to be found in the stars, the
+    alternate combining and scattering effects of the stellar
+    motions are shown by comparing the appearance which the
+    constellation must have had five hundred centuries ago with
+    that which it has at present and that which it will have in the
+    future. The seven principle stars of the asterism, forming a
+    surprisingly perfect coronet, have movements in three
+    directions at right angles to one another. That in these
+    circumstances they should ever have arrived at positions giving
+    them so striking an appearance of definite association is
+    certainly surprising; from its aspect one would have expected
+    to find a community of movement governing the brilliants of the
+    ``Crown,'' but instead of that we find evidence that they will
+    inevitably drift apart and the beautiful figure will
+    dissolve.</p>
+
+    <p>A similar fate awaits such asterisms as the ``Northern
+    Cross'' in Cygnus; the ``Crow'' (Corvus), which stands on the
+    back of the great ``Sea Serpent,'' Hydra, and pecks at his
+    scales; ``Job's Coffin'' (Delphinus); the ``Great Square of
+    Pegasus''; the ``Twins'' (Gemini); the beautiful ``Sickle'' in
+    Leo; and the exquisite group of the Hyades in Taurus. In the
+    case of the Hyades, two controlling movements are manifest:
+    one, affecting five of the stars which form the well-known
+    figure of a letter ``V,'' is directed northerly; the other,
+    which controls the direction of two stars, has an easterly
+    trend. The chief star of the group, Aldebaran, one of the
+    finest of all stars both for its brilliance and its color, is
+    the most affected by the easterly motion. In time it will drift
+    entirely out of connection with its present neighbors. Although
+    the Hyades do not form so compact a group as the Pleiades in
+    the same constellation, yet their appearance of relationship is
+    sufficient to awaken a feeling of surprise over the fact that,
+    as with the stars of the ``Dipper,'' their association is only
+    temporary or apparent.</p>
+
+    <p>The great figure of Orion appears to be more lasting, not
+    because its stars are physically connected, but because of
+    their great distance, which renders their movements too
+    deliberate to be exactly ascertained. Two of the greatest of
+    its stars, Betelgeuse and Rigel, possess, as far as has been
+    ascertained, no perceptible motion across the line of sight,
+    but there is a little movement perceptible in the ``Belt.'' At
+    the present time this consists of an almost perfect straight
+    line, a row of second-magnitude stars about equally spaced and
+    of the most striking beauty. In the course of time, however,
+    the two right-hand stars, Mintaka and Alnilam (how fine are
+    these Arabic star names!) will approach each other and form a
+    naked-eye double, but the third, Alnita, will drift away
+    eastward, so that the ``Belt'' will no longer exist.</p>
+
+    <p>For one more example, let us go to the southern hemisphere,
+    whose most celebrated constellation, the ``Southern Cross,''
+    has found a place in all modern literatures, although it has no
+    claim to consideration on account of association with ancient
+    legends. This most attractive asterism, which has never ceased
+    to fascinate the imagination of Christendom since it was first
+    devoutly described by the early explorers of the South, is but
+    a passing collocation of brilliant stars. Yet even in its
+    transfigurations it has been for hundreds of centuries, and
+    will continue to be for hundreds of centuries to come, a most
+    striking object in the sky. Our figures show its appearance in
+    three successive phases: first, as it was fifty thousand years
+    ago (viewed from the earth's present location); second, as it
+    is in our day; and, third, as it will be an equal time in the
+    future. The nearness of these bright stars to one another --
+    the length of the longer beam of the ``Cross'' is only six
+    degrees -- makes this group very noticeable, whatever the
+    arrangement of its components may be. The largest star, at the
+    base of the ``Cross,'' is of the first magnitude, two of the
+    others are of the second magnitude, and the fourth is of the
+    third. Other stars, not represented in the figures, increase
+    the effect of a celestial blazonry, although they do not help
+    the resemblance to a cross.</p>
+
+    <p>But since the motion of the solar system itself will, in the
+    course of so long a period as fifty thousand years, produce a
+    great change in the perspective of the heavens as seen from the
+    earth, by carrying us nearly nineteen trillion miles from our
+    present place, why, it may be asked, seek to represent future
+    appearances of the constellations which we could not hope to
+    see, even if we could survive so long? The answer is: Because
+    these things aid the mind to form a picture of the effects of
+    the mobility of the starry universe. Only by showing the
+    changes from some definite point of view can we arrive at a due
+    comprehension of them. The constellations are more or less
+    familiar to everybody, so that impending changes of their forms
+    must at once strike the eye and the imagination, and make
+    clearer the significance of the movements of the stars. If the
+    future history of mankind is to resemble its past and if our
+    race is destined to survive yet a million years, then our
+    remote descendents will see a ``new heavens'' if not a ``new
+    earth,'' and will have to invent novel constellations to
+    perpetuate their legends and mythologies.</p>
+
+    <p>If our knowledge of the relative distances of the stars were
+    more complete, it would be an interesting exercise in celestial
+    geometry to project the constellations probably visible to the
+    inhabitants of worlds revolving around some of the other suns
+    of space. Our sun is too insignificant for us to think that he
+    can make a conspicuous appearance among them, except, perhaps,
+    in a few cases. As seen, for instance, from the nearest known
+    star, Alpha Centauri, the sun would appear of the average first
+    magnitude, and consequently from that standpoint he might be
+    the gem of some little constellation which had no Sirius, or
+    Arcturus, or Vega to eclipse him with its superior splendor.
+    But from the distance of the vast majority of the stars the sun
+    would probably be invisible to the naked eye, and as seen from
+    nearer systems could only rank as a fifth or sixth magnitude
+    star, unnoticed and unknown except by the star-charting
+    astronomer.</p>
+
+    <p><strong>Conflagrations in the Heavens</strong></p>
+
+    <p>Suppose it were possible for the world to take fire and burn
+    up -- as some pessimists think that it will do when the Divine
+    wrath shall have sufficiently accumulated against it -- nobody
+    out of our own little corner of space would ever be aware of
+    the catastrophe! With all their telescopes, the astronomers
+    living in the golden light of Arcturus or the diamond blaze of
+    Canopus would be unable to detect the least glimmer of the
+    conflagration that had destroyed the seat of Adam and his
+    descendents, just as now they are totally ignorant of its
+    existence.</p>
+
+    <p>But at least fifteen times in the course of recorded history
+    men looking out from the earth have beheld in the remote depths
+    of space great outbursts of fiery light, some of them more
+    splendidly luminous than anything else in the firmament except
+    the sun! If <em>they</em> were conflagrations, how many million
+    worlds like ours were required to feed their blaze?</p>
+
+    <p>It is probable that ``temporary'' or ``new'' stars, as these
+    wonderful apparitions are called, really are conflagrations;
+    not in the sense of a bonfire or a burning house or city, but
+    in that of a sudden eruption of inconceivable heat and light,
+    such as would result from the stripping off the shell of an
+    encrusted sun or the crashing together of two mighty orbs
+    flying through space with a hundred times the velocity of the
+    swiftest cannon-shot.</p>
+
+    <p>Temporary stars are the rarest and most erratic of
+    astronomical phenomena. The earliest records relating to them
+    are not very clear, and we cannot in every instance be certain
+    that it was one of these appearances that the ignorant and
+    superstitious old chroniclers are trying to describe. The first
+    temporary star that we are absolutely sure of appeared in 1572,
+    and is known as ``Tycho's Star,'' because the celebrated Danish
+    astronomer (whose remains, with his gold-and-silver artificial
+    nose -- made necessary by a duel -- still intact, were
+    disinterred and reburied in 1901) was the first to perceive it
+    in the sky, and the most assiduous and successful in his
+    studies of it. As the first fully accredited representative of
+    its class, this new star made its entry upon the scene with
+    becoming <em>&eacute;clat.</em> It is characteristic of these
+    phenomena that they burst into view with amazing suddenness,
+    and, of course, entirely unexpectedly. Tycho's star appeared in
+    the constellation Cassiopeia, near a now well-known and
+    much-watched little star named Kappa, on the evening of
+    November 11, 1572. The story has often been repeated, but it
+    never loses interest, how Tycho, going home that evening, saw
+    people in the street pointing and staring at the sky directly
+    over their heads, and following the direction of their hands
+    and eyes he was astonished to see, near the zenith, an unknown
+    star of surpassing brilliance. It outshone the planet Jupiter,
+    and was therefore far brighter than the first magnitude. There
+    was not another star in the heavens that could be compared with
+    it in splendor. Tycho was not in all respects free from the
+    superstitions of his time -- and who is? -- but he had the true
+    scientific instinct, and immediately he began to study the
+    stranger, and to record with the greatest care every change in
+    its aspect. First he determined as well as he could with the
+    imperfect instruments of his day, many of which he himself had
+    invented, the precise location of the phenomena in the sky.
+    Then he followed the changes that it underwent. At first it
+    brightened until its light equaled or exceeded that of the
+    planet Venus at her brightest, a statement which will be
+    appreciated at its full value by anyone who has ever watched
+    Venus when she plays her dazzling r&ocirc;le of ``Evening
+    Star,'' flaring like an arc light in the sunset sky. It even
+    became so brilliant as to be visible in full daylight, since,
+    its position being circumpolar, it never set in the latitude of
+    Northern Europe. Finally it began to fade, turning red as it
+    did so, and in March, 1574, it disappeared from Tycho's
+    searching gaze, and has never been seen again from that day to
+    this. None of the astronomers of the time could make anything
+    of it. They had not yet as many bases of speculation as we
+    possess today.</p>
+
+    <p>Tycho's star has achieved a romantic reputation by being
+    fancifully identified with the ``Star of Bethlehem,'' said to
+    have led the wondering Magi from their eastern deserts to the
+    cradle-manger of the Savior in Palestine. Many attempts have
+    been made to connect this traditional ``star'' with some known
+    phenomenon of the heavens, and none seems more idle than this.
+    Yet it persistently survives, and no astronomer is free from
+    eager questions about it addressed by people whose imagination
+    has been excited by the legend. It is only necessary to say
+    that the supposition of a connection between the phenomenon of
+    the Magi and Tycho's star is without any scientific foundation.
+    It was originally based on an unwarranted assumption that the
+    star of Tycho was a variable of long period, appearing once
+    every three hundred and fifteen years, or thereabout. If that
+    were true there would have been an apparition somewhere near
+    the traditional date of the birth of Christ, a date which is
+    itself uncertain. But even the data on which the assumption was
+    based are inconsistent with the theory. Certain monkish records
+    speak of something wonderful appearing in the sky in the years
+    1264 and 945, and these were taken to have been outbursts of
+    Tycho's star. Investigation shows that the records more
+    probably refer to comets, but even if the objects seen were
+    temporary stars, their dates do not suit the hypothesis; from
+    945 to 1264 there is a gap of 319 years, and from 1264 to 1572
+    one of only 308 years; moreover 337 years have now (1909)
+    elapsed since Tycho saw the last glimmer of his star. Upon a
+    variability so irregular and uncertain as that, even if we felt
+    sure that it existed, no conclusion could be found concerning
+    an apparition occurring 2000 years ago.</p>
+
+    <p>In the year 1600 (the year in which Giordano Bruno was
+    burned at the stake for teaching that there is more than one
+    physical world), a temporary star of the third magnitude broke
+    out in the constellation Cygnus, and curiously enough,
+    considering the rarity of such phenomena, only four years later
+    another surprisingly brilliant one appeared in the
+    constellation Ophiuchus. This is often called ``Kepler's
+    star,'' because the great German astronomer devoted to it the
+    same attention that Tycho had given to the earlier phenomenon.
+    It, too, like Tycho's, was at first the brightest object in the
+    stellar heavens, although it seems never to have quite equaled
+    its famous predecessor in splendor. It disappeared after a
+    year, also turning of a red color as it became more faint. We
+    shall see the significance of this as we go on. Some of
+    Kepler's contemporaries suggested that the outburst of this
+    star was due to a meeting of atoms in space, and idea bearing a
+    striking resemblance to the modern theory of ``astronomical
+    collisions.''</p>
+
+    <p>In 1670, 1848, and 1860 temporary stars made their
+    appearance, but none of them was of great brilliance. In 1866
+    one of the second magnitude broke forth in the ``Northern
+    Crown'' and awoke much interest, because by that time the
+    spectroscope had begun to be employed in studying the
+    composition of the stars, and Huggins demonstrated that the new
+    star consisted largely of incandescent hydrogen. But this star,
+    apparently unlike the others mentioned, was not absolutely new.
+    Before its outburst it had shown as a star of the ninth
+    magnitude (entirely invisible, of course, to the naked eye),
+    and after about six weeks it faded to its original condition in
+    which it has ever since remained. In 1876 a temporary star
+    appeared in the constellation Cygnus, and attained at one time
+    the brightness of the second magnitude. Its spectrum and its
+    behavior resembled those of its immediate predecessor. In 1885,
+    astronomers were surprised to see a sixth-magnitude star
+    glimmering in the midst of the hazy cloud of the great
+    Andromeda Nebula. It soon absolutely disappeared. Its spectrum
+    was remarkable for being ``continuous,'' like that of the
+    nebula itself. A continuous spectrum is supposed to represent a
+    body, or a mass, which is either solid or liquid, or composed
+    of gas under great pressure. In January, 1892, a new star was
+    suddenly seen in the constellation Auriga. It never rose much
+    above the fourth magnitude, but it showed a peculiar spectrum
+    containing both bright and dark lines of hydrogen.</p>
+
+    <p>But a bewildering surprise was now in store; the world was
+    to behold at the opening of the twentieth century such a
+    celestial spectacle as had not been on view since the times of
+    Tycho and Kepler. Before daylight on the morning of February
+    22, 1901, the Rev. Doctor Anderson, of Edinburgh, an amateur
+    astronomer, who had also been the first to see the new star in
+    Auriga, beheld a strange object in the constellation Perseus
+    not far from the celebrated variable star Algol. He recognized
+    its character at once, and immediately telegraphed the news,
+    which awoke the startled attention of astronomers all over the
+    world. When first seen the new star was no brighter than Algol
+    (less than the second magnitude), but within twenty-four hours
+    it was ablaze, outshining even the brilliant Capella, and far
+    surpassing the first magnitude. At the spot in the sky where it
+    appeared nothing whatever was visible on the night before its
+    coming. This is known with certainty because a photograph had
+    been made of that very region on February 21, and this
+    photograph showed everything down to the twelfth magnitude, but
+    not a trace of the stranger which burst into view between the
+    21st and the 22nd like the explosion of a rocket.</p>
+
+    <p>Upon one who knew the stars the apparition of this intruder
+    in a well-known constellation had the effect of a sudden
+    invasion. The new star was not far west of the zenith in the
+    early evening, and in that position showed to the best
+    advantage. To see Capella, the hitherto unchallenged ruler of
+    that quarter of the sky, abased by comparison with this
+    stranger of alien aspect, for there was always an unfamiliar
+    look about the ``nova,'' was decidedly disconcerting. It seemed
+    to portend the beginning of a revolution in the heavens. One
+    could understand what the effect of such an apparition must
+    have been in the superstitious times of Tycho. The star of
+    Tycho had burst forth on the northern border of the Milky Way;
+    this one was on its southern border, some forty-five degrees
+    farther east.</p>
+
+    <p>Astronomers were well-prepared this time for the scientific
+    study of the new star, both astronomical photography and
+    spectroscopy having been perfected, and the results of their
+    investigations were calculated to increase the wonder with
+    which the phenomenon was regarded. The star remained at its
+    brightest only a few days; then, like a veritable
+    conflagration, it began to languish; and, like the reflection
+    of a dying fire, as it sank it began to glow with the red color
+    of embers. But its changes were spasmodic; once about every
+    three days it flared up only to die away again. During these
+    fluctuations its light varied alternately in the ratio of one
+    to six. Finally it took a permanent downward course, and after
+    a few months the naked eye could no longer perceive it; but it
+    remained visible with telescopes, gradually fading until it had
+    sunk to the ninth magnitude. Then another astonishing change
+    happened: in August photographs taken at the Yerkes Observatory
+    and at Heidelberg showed that the ``nova'' was <em>surrounded
+    by a spiral nebula!</em> The nebula had not been there before,
+    and no one could doubt that it represented a phase of the same
+    catastrophe that had produced the outburst of the new star. At
+    one time the star seemed virtually to have disappeared, as if
+    all its substance had been expanded into the nebulous cloud,
+    but always there remained a stellar nucleus about which the
+    misty spiral spread wider and ever wider, like a wave expanding
+    around a center of disturbance. The nebula too showed a
+    variability of brightness, and four condensations which formed
+    in it seemed to have a motion of revolution about the star. As
+    time went on the nebula continued to expand at a rate which was
+    computed to be not less than twenty thousand miles per second!
+    And now the star itself, showing indications of having turned
+    into a nebula, behaved in a most erratic manner, giving rise to
+    the suspicion that it was about to burst out again. But this
+    did not occur, and at length it sunk into a state of lethargy
+    from which it has to the present time not recovered. But the
+    nebulous spiral has disappeared, and the entire phenomena as it
+    now (1909) exists consists of a faint nebulous star of less
+    than the ninth magnitude.</p>
+
+    <p>The wonderful transformations just described had been
+    forecast in advance of the discovery of the nebulous spiral
+    encircling the star by the spectroscopic study of the latter.
+    At first there was no suggestion of a nebular constitution, but
+    within a month or two characteristic nebular lines began to
+    appear, and in less than six months the whole spectrum had been
+    transformed to the nebular type. In the mean time the shifting
+    of the spectral lines indicated a complication of rapid motions
+    in several directions simultaneously. These motions were
+    estimated to amount to from one hundred to five hundred miles
+    per second.</p>
+
+    <p>The human mind is so constituted that it feels forced to
+    seek an explanation of so marvelous a phenomenon as this, even
+    in the absence of the data needed for a sound conclusion. The
+    most natural hypothesis, perhaps, is that of a collision. Such
+    a catastrophe could certainly happen. It has been shown, for
+    instance, that in infinity of time the earth is sure to be hit
+    by a comet; in the same way it may be asserted that, if no time
+    limit is fixed, the sun is certain to run against some obstacle
+    in space, either another star, or a dense meteor swarm, or one
+    of the dark bodies which there is every reason to believe
+    abound around us. The consequences of such a collision are easy
+    to foretell, provided that we know the masses and the
+    velocities of the colliding bodies. In a preceding chapter we
+    have discussed the motions of the sun and stars, and have seen
+    that they are so swift that an encounter between any two of
+    them could not but be disastrous. But this is not all; for as
+    soon as two stars approached within a few million miles their
+    speed would be enormously increased by their reciprocal
+    attractions and, if their motion was directed radially with
+    respect to their centers, they would come together with a crash
+    that would reduce them both to nebulous clouds. It is true that
+    the chances of such a ``head-on'' collision are relatively very
+    small; two stars approaching each other would most probably
+    fall into closed orbits around their common center of gravity.
+    If there were a collision it would most likely be a grazing one
+    instead of a direct front-to-front encounter. But even a close
+    approach, without any actual collision, would probably prove
+    disastrous, owing to the tidal influence of each of the bodies
+    on the other. Suns, in consequence of their enormous masses and
+    dimensions and the peculiarities of their constitution, are
+    exceedingly dangerous to one another at close quarters.
+    Propinquity awakes in them a mutually destructive tendency.
+    Consisting of matter in the gaseous, or perhaps, in some cases,
+    liquid, state, their tidal pull upon each other if brought
+    close together might burst them asunder, and the photospheric
+    envelope being destroyed the internal incandescent mass would
+    gush out, bringing fiery death to any planets that were
+    revolving near. Without regard to the resulting disturbance of
+    the earth's orbit, the close approach of a great star to the
+    sun would be in the highest degree perilous to us. But this is
+    a danger which may properly be regarded as indefinitely remote,
+    since, at our present location in space, we are certainly far
+    from every star except the sun, and we may feel confident that
+    no great invisible body is near, for if there were one we
+    should be aware of its presence from the effects of its
+    attraction. As to dark nebul&aelig; which may possibly lie in
+    the track that the solar system is pursuing at the rate of
+    375,000,000 miles per year, that is another question -- and
+    they, too, could be dangerous!</p>
+
+    <p>This brings us directly back to ``Nova Persei,'' for among
+    the many suggestions offered to explain its outburst, as well
+    as those of other temporary stars, one of the most fruitful is
+    that of a collision between a star and a vast invisible nebula.
+    Professor Seeliger, of Munich, first proposed this theory, but
+    it afterward underwent some modifications from others. Stated
+    in a general form, the idea is that a huge dark body, perhaps
+    an extinguished sun, encountered in its progress through space
+    a widespread flock of small meteors forming a dark nebula. As
+    it plunged into the swarm the friction of the innumerable
+    collisions with the meteors heated its surface to
+    incandescence, and being of vast size it then became visible to
+    us as a new star. Meanwhile the motion of the body through the
+    nebula, and its rotation upon itself, set up a gyration in the
+    blazing atmosphere formed around it by the vaporized meteors;
+    and as this atmosphere spread wider, under the laws of gyratory
+    motion a rotation in the opposite direction began in the
+    inflamed meteoric cloud outside the central part of the vortex.
+    Thus the spectral lines were caused to show motion in opposite
+    directions, a part of the incandescent mass approaching the
+    earth simultaneously with the retreat of another part. So the
+    curious spectroscopic observations before mentioned were
+    explained. This theory might also account for the appearance of
+    the nebulous spiral first seen some six months after the
+    original outburst. The sequent changes in the spectrum of the
+    ``nova'' are accounted for by this theory on the assumption,
+    reasonable enough in itself, that at first the invading body
+    would be enveloped in a vaporized atmosphere of relatively
+    slight depth, producing by its absorption the fine dark lines
+    first observed; but that as time went on and the incessant
+    collisions continued, the blazing atmosphere would become very
+    deep and extensive, whereupon the appearance of the spectral
+    lines would change, and bright lines due to the light of the
+    incandescent meteors surrounding the nucleus at a great
+    distance would take the place of the original dark ones. The
+    vortex of meteors once formed would protect the flying body
+    within from further immediate collisions, the latter now
+    occurring mainly among the meteors themselves, and then the
+    central blaze would die down, and the original splendor of the
+    phenomenon would fade.</p>
+
+    <p>But the theories about Nova Persei have been almost as
+    numerous as the astronomers who have speculated about it. One
+    of the most startling of them assumed that the outburst was
+    caused by the running amuck of a dark star which had
+    encountered another star surrounded with planets, the renewed
+    outbreaks of light after the principal one had faded being due
+    to the successive running down of the unfortunate planets! Yet
+    another hypothesis is based on what we have already said of the
+    tidal influence that two close approaching suns would have upon
+    each other. Supposing two such bodies which had become
+    encrusted, but remained incandescent and fluid within, to
+    approach within almost striking distance; they would whirl each
+    other about their common center of gravity, and at the same
+    time their shells would burst under the tidal strain, and their
+    glowing nuclei being disclosed would produce a great outburst
+    of light. Applying this theory to a ``nova,'' like that of 1866
+    in the ``Northern Crown,'' which had been visible as a small
+    star before the outbreak, and which afterward resumed its
+    former aspect, we should have to assume that a yet shining sun
+    had been approached by a dark body whose attraction temporarily
+    burst open its photosphere. It might be supposed that in this
+    case the dark body was too far advanced in cooling to suffer
+    the same fate from the tidal pull of its victim. But a close
+    approach of that kind would be expected to result in the
+    formation of a binary system, with orbits of great
+    eccentricity, perhaps, and after the lapse of a certain time
+    the outburst should be renewed by another approximation of the
+    two bodies. A temporary star of that kind would rather be
+    ranked as a variable.</p>
+
+    <p>The celebrated French astronomer, Janssen, had a different
+    theory of Nova Persei, and of temporary stars in general.
+    According to his idea, such phenomena might be the result of
+    chemical changes taking place in a sun without interference by,
+    or collision with, another body. Janssen was engaged for many
+    years in trying to discover evidence of the existence of oxygen
+    in the sun, and he constructed his observatory on the summit of
+    Mount Blanc specially to pursue that research. He believed that
+    oxygen must surely exist in the sun since we find so many other
+    familiar elements included in the constitution of the solar
+    globe, and as he was unable to discover satisfactory evidence
+    of its presence he assumed that it existed in a form unknown on
+    the earth. If it were normally in the sun's chromosphere, or
+    coronal atmosphere, he said, it would combine with the hydrogen
+    which we know is there and form an obscuring envelope of water
+    vapor. It exists, then, in a special state, uncombined with
+    hydrogen; but let the temperature of the sun sink to a critical
+    point and the oxygen will assume its normal properties and
+    combine with the hydrogen, producing a mighty outburst of light
+    and heat. This, Janssen thought, might explain the phenomena of
+    the temporary stars. It would also, he suggested, account for
+    their brief career, because the combination of the elements
+    would be quickly accomplished, and then the resulting water
+    vapor would form an atmosphere cutting off the radiation from
+    the star within.</p>
+
+    <p>This theory may be said to have a livelier human interest
+    than some of the others, since, according to it, the sun may
+    carry in its very constitution a menace to mankind; one does
+    not like to think of it being suddenly transformed into a
+    gigantic laboratory for the explosive combination of oxygen and
+    hydrogen! But while Janssen's theory might do for some
+    temporary stars, it is inadequate to explain all the phenomena
+    of Nova Persei, and particularly the appearance of the great
+    spiral nebula that seemed to exhale from the heart of the star.
+    Upon the whole, the theory of an encounter between a star and a
+    dark nebula seems best to fit the observations. By that
+    hypothesis the expanding billow of light surrounding the core
+    of the conflagration is very well accounted for, and the
+    spectroscopic peculiarities are also explained.</p>
+
+    <p>Dr Gustov Le Bon offers a yet more alarming theory,
+    suggesting that temporary stars are the result of <em>atomic
+    explosion;</em> but we shall touch upon this more fully in
+    Chapter 14.</p>
+
+    <p>Twice in the course of this discussion we have called
+    attention to the change of color invariably undergone by
+    temporary stars in the later stages of their career. This was
+    conspicuous with Nova Persei which glowed more and more redly
+    as it faded, until the nebulous light began to overpower that
+    of the stellar nucleus. Nothing could be more suggestive of the
+    dying out of a great fire. Moreover, change of color from white
+    to red is characteristic of all variable stars of long period,
+    such as ``Mira'' in Cetus. It is also characteristic of stars
+    believed to be in the later stages of evolution, and
+    consequently approaching extinction, like Antares and
+    Betelgeuse, and still more notably certain small stars which
+    ``gleam like rubies in the field of the telescope.'' These last
+    appear to be suns in the closing period of existence as
+    self-luminous bodies. Between the white stars, such as Sirius
+    and Rigel, and the red stars, such as Aldebaran and Alpha
+    Herculis, there is a progressive series of colors from golden
+    yellow through orange to deep red. The change is believed to be
+    due to the increase of absorbing vapors in the stellar
+    atmosphere as the body cools down. In the case of ordinary
+    stars these changes no doubt occupy many millions of years,
+    which represent the average duration of solar life; but the
+    temporary stars run through similar changes in a few months:
+    they resemble ephemeral insects -- born in the morning and
+    doomed to perish with the going down of the sun.</p>
+
+    <p><strong>Explosive and Whirling Nebul&aelig;</strong></p>
+
+    <p>One of the most surprising triumphs of celestial photography
+    was Professor Keeler's discovery, in 1899, that the great
+    majority of the nebul&aelig; have a distinctly spiral form.
+    This form, previously known in Lord Rosse's great ``Whirlpool
+    Nebula,'' had been supposed to be exceptional; now the
+    photographs, far excelling telescopic views in the revelation
+    of nebular forms, showed the spiral to be the typical shape.
+    Indeed, it is a question whether all nebul&aelig; are not to
+    some extent spiral. The extreme importance of this discovery is
+    shown in the effect that it has had upon hitherto prevailing
+    views of solar and planetary evolution. For more than
+    three-quarters of a century Laplace's celebrated hypothesis of
+    the manner of origin of the solar system from a rotating and
+    contracting nebula surrounding the sun had guided speculation
+    on that subject, and had been tentatively extended to cover the
+    evolution of systems in general. The apparent forms of some of
+    the nebul&aelig; which the telescope had revealed were
+    regarded, and by some are still regarded, as giving visual
+    evidence in favor of this theory. There is a ``ring nebula'' in
+    Lyra with a central star, and a ``planetary nebula'' in Gemini
+    bearing no little resemblance to the planet Saturn with its
+    rings, both of which appear to be practical realizations of
+    Laplace's idea, and the elliptical rings surrounding the
+    central condensation of the Andromeda Nebula may be cited for
+    the same kind of proof.</p>
+
+    <p>But since Keeler's discovery there has been a decided
+    turning away of speculation another way. The form of the spiral
+    nebul&aelig; seems to be entirely inconsistent with the theory
+    of an originally globular or disk-shaped nebula condensing
+    around a sun and throwing or leaving off rings, to be
+    subsequently shaped into planets. Some astronomers, indeed, now
+    reject Laplace's hypothesis <em>in toto,</em> preferring to
+    think that even our solar system originated from a spiral
+    nebula. Since the spiral type prevails among the existing
+    nebul&aelig;, we must make any mechanical theory of the
+    development of stars and planetary systems from them accord
+    with the requirements which that form imposes. A glance at the
+    extraordinary variations upon the spiral which Professor
+    Keeler's photographs reveal is sufficient to convince one of
+    the difficulty of the task of basing a general theory upon
+    them. In truth, it is much easier to criticize Laplace's
+    hypothesis than to invent a satisfactory substitute for it. If
+    the spiral nebul&aelig; seem to oppose it there are other
+    nebul&aelig; which appear to support it, and it may be that no
+    one fixed theory can account for all the forms of stellar
+    evolution in the universe. Our particular planetary system may
+    have originated very much as the great French mathematician
+    supposed, while others have undergone, or are now undergoing, a
+    different process of development. There is always a too strong
+    tendency to regard an important new discovery and the theories
+    and speculations based upon it as revolutionizing knowledge,
+    and displacing or overthrowing everything that went before.
+    Upon the plea that ``Laplace only made a guess'' more recent
+    guesses have been driven to extremes and treated by injudicious
+    exponents as ``the solid facts at last.''</p>
+
+    <p>Before considering more recent theories than Laplace's, let
+    us see what the nature of the photographic revelations is. The
+    vast celestial maelstrom discovered by Lord Rosse in the
+    ``Hunting Dogs'' may be taken as the leading type of the spiral
+    nebul&aelig;, although there are less conspicuous objects of
+    the kind which, perhaps, better illustrate some of their
+    peculiarities. Lord Rosse's nebula appears far more wonderful
+    in the photographs than in his drawings made with the aid of
+    his giant reflecting telescope at Parsonstown, for the
+    photographic plate records details that no telescope is capable
+    of showing. Suppose we look at the photograph of this object as
+    any person of common sense would look at any great and strange
+    natural phenomenon. What is the first thing that strikes the
+    mind? It is certainly the appearance of violent whirling
+    motion. One would say that the whole glowing mass had been spun
+    about with tremendous velocity, or that it had been set
+    rotating so rapidly that it had become the victim of
+    ``centrifugal force,'' one huge fragment having broken loose
+    and started to gyrate off into space. Closer inspection shows
+    that in addition to the principal focus there are various
+    smaller condensations scattered through the mass. These are
+    conspicuous in the spirals. Some of them are stellar points,
+    and but for the significance of their location we might suppose
+    them to be stars which happen to lie in a line between us and
+    the nebula. But when we observe how many of them follow most
+    faithfully the curves of the spirals we cannot but conclude
+    that they form an essential part of the phenomenon; it is not
+    possible to believe that their presence in such situations is
+    merely fortuitous. One of the outer spirals has at least a
+    dozen of these star-like points strung upon it; some of them
+    sharp, small, and distinct, others more blurred and nebulous,
+    suggesting different stages of condensation. Even the part
+    which seems to have been flung loose from the main mass has, in
+    addition to its central condensation, at least one stellar
+    point gleaming in the half-vanished spire attached to it. Some
+    of the more distant stars scattered around the ``whirlpool''
+    look as if they too had been shot out of the mighty vortex,
+    afterward condensing into unmistakable solar bodies. There are
+    at least two curved rows of minute stars a little beyond the
+    periphery of the luminous whirl which clearly follow lines
+    concentric with those of the nebulous spirals. Such facts are
+    simply dumbfounding for anyone who will bestow sufficient
+    thought upon them, for these are <em>suns,</em> though they may
+    be small ones; and what a birth is that for a sun!</p>
+
+    <p>Look now again at the glowing spirals. We observe that
+    hardly have they left the central mass before they begin to
+    coagulate. In some places they have a ``ropy'' aspect; or they
+    are like peascods filled with growing seeds, which eventually
+    will become stars. The great focus itself shows a similar
+    tendency, especially around its circumference. The sense that
+    it imparts of a tremendous shattering force at work is
+    overwhelming. There is probably more matter in that whirling
+    and bursting nebula than would suffice to make a hundred solar
+    systems! It must be confessed at once that there is no
+    confirmation of the Laplacean hypothesis here; but what
+    hypothesis will fit the facts? There is one which it has been
+    claimed does so, but we shall come to that later. In the
+    meanwhile, as a preparation, fix in the memory the appearance
+    of that second spiral mass spinning beside its master which
+    seems to have spurned it away.</p>
+
+    <p>For a second example of the spiral nebul&aelig; look at the
+    one in the constellation Triangulum. <em>God, how hath the
+    imagination of puny man failed to comprehend Thee!</em> Here is
+    creation through destruction with a vengeance! The spiral form
+    of the nebula is unmistakable, but it is half obliterated amid
+    the turmoil of flying masses hurled away on all sides with
+    tornadic fury. The focus itself is splitting asunder under the
+    intolerable strain, and in a little while, as time is reckoned
+    in the Cosmos, it will be gyrating into stars. And then look at
+    the cyclonic rain of already finished stars whirling round the
+    outskirts of the storm. Observe how scores of them are yet
+    involved in the fading streams of the nebulous spirals; see how
+    they have been thrown into vast loops and curves, of a beauty
+    that half redeems the terror of the spectacle enclosed within
+    their lines -- like iridescent cirri hovering about the edges
+    of a hurricane. And so again are suns born!</p>
+
+    <p>Let us turn to the exquisite spiral in Ursa Major; how
+    different its aspect from that of the other! One would say that
+    if the terrific coil in Triangulum has all but destroyed itself
+    in its fury, this one on the contrary has just begun its
+    self-demolition. As one gazes one seems to see in it the
+    smooth, swift, accelerating motion that precedes catastrophe.
+    The central part is still intact, dense, and uniform in
+    texture. How graceful are the spirals that smoothly rise from
+    its oval rim and, gemmed with little stars, wind off into the
+    darkness until they have become as delicate as threads of
+    gossamer! But at bottom the story told here is the same --
+    creation by gyration!</p>
+
+    <p>Compare with the above the curious mass in Cetus. Here the
+    plane of the whirling nebula nearly coincides with our line of
+    sight and we see the object at a low angle. It is far advanced
+    and torn to shreds, and if we could look at it perpendicularly
+    to its plane it is evident that it would closely resemble the
+    spectacle in Triangulum.</p>
+
+    <p>Then take the famous Andromeda Nebula (see Frontispiece),
+    which is so vast that notwithstanding its immense distance even
+    the naked eye perceives it as an enigmatical wisp in the sky.
+    Its image on the sensitive plate is the masterpiece of
+    astronomical photography; for wild, incomprehensible beauty
+    there is nothing that can be compared with it. Here, if
+    anywhere, we look upon the spectacle of creation in one of its
+    earliest stages. The Andromeda Nebula is apparently less
+    advanced toward transformation into stellar bodies than is that
+    in Triangulum. The immense crowd of stars sprinkled over it and
+    its neighborhood seem in the main to lie this side of the
+    nebula, and consequently to have no connection with it. But
+    incipient stars (in some places clusters of them) are seen in
+    the nebulous rings, while one or two huge masses seem to give
+    promise of transformation into stellar bodies of unusual
+    magnitude. I say ``rings'' because although the loops
+    encompassing the Andromeda Nebula have been called spirals by
+    those who wish utterly to demolish Laplace's hypothesis, yet
+    they are not manifestly such, as can be seen on comparing them
+    with the undoubted spirals of the Lord Rosse Nebula. They look
+    quite as much like circles or ellipses seen at an angle of,
+    say, fifteen or twenty degrees to their plane. If they are
+    truly elliptical they accord fairly well with Laplace's idea,
+    except that the scale of magnitude is stupendous, and if the
+    Andromeda Nebula is to become a solar system it will surpass
+    ours in grandeur beyond all possibility of comparison.</p>
+
+    <p>There is one circumstance connected with the spiral
+    nebul&aelig;, and conspicuous in the Andromeda Nebula on
+    account of its brightness, which makes the question of their
+    origin still more puzzling; they all show continuous spectra,
+    which, as we have before remarked, indicate that the mass from
+    which the light comes is either solid or liquid, or a gas under
+    heavy pressure. Thus nebul&aelig; fall into two classes: the
+    ``white'' nebul&aelig;, giving a continuous spectrum; and the
+    ``green'' nebul&aelig; whose spectra are distinctly gaseous.
+    The Andromeda Nebula is the great representative of the former
+    class and the Orion Nebula of the latter. The spectrum of the
+    Andromeda Nebula has been interpreted to mean that it consists
+    not of luminous gas, but of a flock of stars so distant that
+    they are separately indistinguishable even with powerful
+    telescopes, just as the component stars of the Milky Way are
+    indistinguishable with the naked eye; and upon this has been
+    based the suggestion that what we see in Andromeda is an outer
+    universe whose stars form a series of elliptical garlands
+    surrounding a central mass of amazing richness. But this idea
+    is unacceptable if for no other reason than that, as just said,
+    all the spiral nebul&aelig; possess the same kind of spectrum,
+    and probably no one would be disposed to regard them all as
+    outer universes. As we shall see later, the peculiarity of the
+    spectra of the spiral nebul&aelig; is appealed to in support of
+    a modern substitute for Laplace's hypothesis.</p>
+
+    <p>Finally, without having by any means exhausted the variety
+    exhibited by the spiral nebul&aelig;, let us turn to the great
+    representative of the other species, the Orion Nebula. In some
+    ways this is even more marvelous than the others. The early
+    drawings with the telescope failed to convey an adequate
+    conception either of its sublimity or of its complication of
+    structure. It exists in a nebulous region of space, since
+    photographs show that nearly the whole constellation is
+    interwoven with faintly luminous coils. To behold the entry of
+    the great nebula into the field even of a small telescope is a
+    startling experience which never loses its novelty. As shown by
+    the photographs, it is an inscrutable chaos of perfectly
+    amazing extent, where spiral bands, radiating streaks, dense
+    masses, and dark yawning gaps are strangely intermingled
+    without apparent order. In one place four conspicuous little
+    stars, better seen in a telescope than in the photograph on
+    account of the blurring produced by over-exposure, are
+    suggestively situated in the midst of a dark opening, and no
+    observer has ever felt any doubt that these stars have been
+    formed from the substance of the surrounding nebula. There are
+    many other stars scattered over its expanse which manifestly
+    owe their origin to the same source. But compare the general
+    appearance of this nebula with the others that we have studied,
+    and remark the difference. If the unmistakably spiral
+    nebul&aelig; resemble bursting fly-wheels or grindstones from
+    whose perimeters torrents of sparks are flying, the Orion
+    Nebula rather recalls the aspect of a cloud of smoke and
+    fragments produced by the explosion of a shell. This idea is
+    enforced by the look of the outer portion farthest from the
+    bright half of the nebula, where sharply edged clouds with dark
+    spaces behind seem to be billowing away as if driven by a wind
+    blowing from the center.</p>
+
+    <p>Next let us consider what scientific speculation has done in
+    the effort to explain these mysteries. Laplace's hypothesis can
+    certainly find no standing ground either in the Orion Nebula or
+    in those of a spiral configuration, whatever may be its
+    situation with respect to the grand Nebula of Andromeda, or the
+    ``ring'' and ``planetary'' nebul&aelig;. Some other hypothesis
+    more consonant with the appearances must be found. Among the
+    many that have been proposed the most elaborate is the
+    ``Planetesimal Hypothesis'' of Professors Chamberlin and
+    Moulton. It is to be remarked that it applies to the spiral
+    nebul&aelig; distinctively, and not to an apparently chaotic
+    mass of gas like the vast luminous cloud in Orion. The gist of
+    the theory is that these curious objects are probably the
+    result of close approaches to each other of two independent
+    suns, reminding us of what was said on this subject when we
+    were dealing with temporary stars. Of the previous history of
+    these appulsing suns the theory gives us no account; they are
+    simply supposed to arrive within what may be called an
+    effective tide-producing distance, and then the drama begins.
+    Some of the probable consequences of such an approach have been
+    noticed in Chapter 5; let us now consider them a little more in
+    detail.</p>
+
+    <p>Tides always go in couples; if there is a tide on one side
+    of a globe there will be a corresponding tide on the other
+    side. The cause is to be found in the law that the force of
+    gravitation varies inversely as the square of the distance; the
+    attraction on the nearest surface of the body exercised by
+    another body is greater than on its center, and greater yet
+    than on its opposite surface. If two great globes attract each
+    other, each tends to draw the other out into an ellipsoidal
+    figure; they must be more rigid than steel to resist this --
+    and even then they cannot altogether resist. If they are liquid
+    or gaseous they will yield readily to the force of distortion,
+    the amount of which will depend upon their distance apart, for
+    the nearer they are the greater becomes the tidal strain. If
+    they are encrusted without and liquid or gaseous in the
+    interior, the internal mass will strive to assume the figure
+    demanded by the tidal force, and will, if it can, burst the
+    restraining envelope. Now this is virtually the predicament of
+    the body we call a sun when in the immediate presence of
+    another body of similarly great mass. Such a body is presumably
+    gaseous throughout, the component gases being held in a state
+    of rigidity by the compression produced by the tremendous
+    gravitational force of their own aggregate mass. At the surface
+    such a body is enveloped in a shell of relatively cool matter.
+    Now suppose a great attracting body, such as another sun, to
+    approach near enough for the difference in its attraction on
+    the two opposite sides of the body and on its center to become
+    very great; the consequence will be a tidal deformation of the
+    whole body, and it will lengthen out along the line of the
+    gravitational pull and draw in at the sides, and if its shell
+    offers considerable resistance, but not enough to exercise a
+    complete restraint, it will be violently burst apart, or blown
+    to atoms, and the internal mass will leap out on the two
+    opposite sides in great fiery spouts. In the case of a sun
+    further advanced in cooling than ours the interior might be
+    composed of molten matter while the exterior crust had become
+    rigid like the shell of an egg; then the force of the ``tidal
+    explosion'' produced by the appulse of another sun would be
+    more violent in consequence of the greater resistance overcome.
+    Such, then, is the mechanism of the first phase in the history
+    of a spiral nebula according to the Planetesimal Hypothesis.
+    Two suns, perhaps extinguished ones, have drawn near together,
+    and an explosive outburst has occured in one or both. The
+    second phase calls for a more agile exercise of the
+    imagination.</p>
+
+    <p>To simplify the case, let us suppose that only one of the
+    tugging suns is seriously affected by the strain. Its vast
+    wings produced by the outburst are twisted into spirals by
+    their rotation and the contending attractions exercised upon
+    them, as the two suns, like battleships in desperate conflict,
+    curve round each other, concentrating their destructive
+    energies. Then immense quantities of d&eacute;bris are
+    scattered about in which eddies are created, and finally, as
+    the sun that caused the damage goes on its way, leaving its
+    victim to repair its injuries as it may, the dispersed matter
+    cools, condenses, and turns into streams of solid particles
+    circling in elliptical paths about their parent sun. These
+    particles, or fragments, are the ``planetesimals'' of the
+    theory. In consequence of the inevitable intersection of the
+    orbits of the planetesimals, nodes are formed where the flying
+    particles meet, and at these nodes large masses are gradually
+    accumulated. The larger the mass the greater its attraction,
+    and at last the nodal points become the nuclei of great
+    aggregations from which planets are shaped.</p>
+
+    <p>This, in very brief form, is the Planetesimal Hypothesis
+    which we are asked to substitute for that based on Laplace's
+    suggestion as an explanation of the mode of origin of the solar
+    system; and the phenomena of the spiral nebul&aelig; are
+    appealed to as offering evident support to the new hypothesis.
+    We are reminded that they are elliptical in outline, which
+    accords with the hypothesis; that their spectra are not
+    gaseous, which shows that they may be composed of solid
+    particles like the planetesimals; and that their central masses
+    present an oval form, which is what would result from the tidal
+    effects, as just described. We also remember that some of them,
+    like the Lord Rosse and the Andromeda nebul&aelig;, are
+    visually double, and in these cases we might suppose that the
+    two masses represent the tide-burst suns that ventured into too
+    close proximity. It may be added that the authors of the theory
+    do not insist upon the appulse of two suns as the <em>only</em>
+    way in which the planetesimals may have originated, but it is
+    the only supposition that has been worked out.</p>
+
+    <p>But serious questions remain. It needs, for instance, but a
+    glance at the Triangulum monster to convince the observer that
+    it cannot be a solar system which is being evolved there, but
+    rather a swarm of stars. Many of the detached masses are too
+    vast to admit of the supposition that they are to be
+    transformed into planets, in our sense of planets, and the
+    distances of the stars which appear to have been originally
+    ejected from the focal masses are too great to allow us to
+    liken the assemblage that they form to a solar system. Then,
+    too, no nodes such as the hypothesis calls for are visible.
+    Moreover, in most of the spiral nebul&aelig; the appearances
+    favor the view that the supposititious encountering suns have
+    not separated and gone each rejoicing on its way, after having
+    inflicted the maximum possible damage on its opponent, but
+    that, on the contrary, they remain in close association like
+    two wrestlers who cannot escape from each other's grasp. And
+    this is exactly what the law of gravitation demands; stars
+    cannot approach one another with impunity, with regard either
+    to their physical make-up or their future independence of
+    movement. The theory undertakes to avoid this difficulty by
+    assuming that in the case of our system the approach of the
+    foreign body to the sun was not a close one -- just close
+    enough to produce the tidal extrusion of the relatively
+    insignificant quantity of matter needed to form the planets.
+    But even then the effect of the appulse would be to change the
+    direction of flight, both of the sun and of its visitor, and
+    there is no known star in the sky which can be selected as the
+    sun's probable partner in their ancient <em>pas deux.</em> That
+    there are unconquered difficulties in Laplace's hypothesis no
+    one would deny, but in simplicity of conception it is
+    incomparably more satisfactory, and with proper modifications
+    could probably be made more consonant with existing facts in
+    our solar system than that which is offered to replace it. Even
+    as an explanation of the spiral nebul&aelig;, not as solar
+    systems in process of formation, but as the birthplaces of
+    stellar clusters, the Planetesimal Hypothesis would be open to
+    many objections. Granting its assumptions, it has undoubtedly a
+    strong mathematical framework, but the trouble is not with the
+    mathematics but with the assumptions. Laplace was one of the
+    ablest mathematicians that ever lived, but he had never seen a
+    spiral nebula; if he had, he might have invented a hypothesis
+    to suit its phenomena. His actual hypothesis was intended only
+    for our solar system, and he left it in the form of a ``note''
+    for the consideration of his successors, with the hope that
+    they might be able to discover the full truth, which he
+    confessed was hidden from him. It cannot be said that that
+    truth has yet been found, and when it is found the chances are
+    that intuition and not logic will have led to it.</p>
+
+    <p>The spiral nebul&aelig;, then, remain among the greatest
+    riddles of the universe, while the gaseous nebul&aelig;, like
+    that of Orion, are no less mysterious, although it seems
+    impossible to doubt that both forms give birth to stars. It is
+    but natural to look to them for light on the question of the
+    origin of our planetary system; but we should not forget that
+    the scale of the phenomena in the two cases is vastly
+    different, and the forces in operation may be equally
+    different. A hill may have been built up by a glacier, while a
+    mountain may be the product of volcanic forces or of the
+    upheaval of the strata of the planet.</p>
+
+    <p><strong>The Banners of the Sun</strong></p>
+
+    <p>As all the world knows, the sun, a blinding globe pouring
+    forth an inconceivable quantity of light and heat, whose daily
+    passage through the sky is caused by the earth's rotation on
+    its axis, constitutes the most important phenomenon of
+    terrestial existence. Viewed with a dark glass to take off the
+    glare, or with a telescope, its rim is seen to be a sharp and
+    smooth circle, and nothing but dark sky is visible around it.
+    Except for the interference of the moon, we should probably
+    never have known that there is any more of the sun than our
+    eyes ordinarily see.</p>
+
+    <p>But when an eclipse of the sun occurs, caused by the
+    interposition of the opaque globe of the moon, we see its
+    immediate surroundings, which in some respects are more
+    wonderful than the glowing central orb. These surroundings,
+    although not in the sense in which we apply the term to the
+    gaseous envelope of the earth, may be called the sun's
+    atmosphere. They consist of two very different parts -- first,
+    the red ``prominences,'' which resemble tongues of flame
+    ascending thousands of miles above the sun's surface; and,
+    second, the ``corona,'' which extends to distances of millions
+    of miles from the sun, and shines with a soft, glowing light.
+    The two combined, when well seen, make a spectacle without
+    parallel among the marvels of the sky. Although many attempts
+    have been made to render the corona visible when there is no
+    eclipse, all have failed, and it is to the moon alone that we
+    owe its revelation. To cover the sun's disk with a circular
+    screen will not answer the purpose because of the illumination
+    of the air all about the observer. When the moon hides the sun,
+    on the other hand, the sunlight is withdrawn from a great
+    cylinder of air extending to the top of the atmosphere and
+    spreading many miles around the observer. There is then no
+    glare to interfere with the spectacle, and the corona appears
+    in all its surprising beauty. The prominences, however,
+    although they were discovered during an eclipse, can now, with
+    the aid of the spectroscope, be seen at any time. But the
+    prominences are rarely large enough to be noticed by the naked
+    eye, while the streamers of the corona, stretching far away in
+    space, like ghostly banners blown out from the black circle of
+    the obscuring moon, attract every eye, and to this weird
+    apparition much of the fear inspired by eclipses has been due.
+    But if the corona has been a cause of terror in the past it has
+    become a source of growing knowledge in our time.</p>
+
+    <p>The story of the first scientific observation of the corona
+    and the prominences is thrillingly interesting, and in fact
+    dramatic. The observation was made during the eclipse of 1842,
+    which fortunately was visible all over Central and Southern
+    Europe so that scores of astronomers saw it. The interest
+    centers in what happened at Pavia in Northern Italy, where the
+    English astronomer Francis Baily had set up his telescope. The
+    eclipse had begun and Bailey was busy at his telescope when, to
+    quote his own words in the account which he wrote for the
+    <em>Memoirs of the Royal Astronomical Society:</em></p>
+
+    <blockquote>
+      I was astounded by a tremendous burst of applause from the
+      streets below, and at the same moment was electrified by the
+      sight of one of the most brilliant and splendid phenomena
+      that can well be imagined; for at that instant the dark body
+      of the moon was suddenly surrounded with a corona, or kind of
+      bright glory, similar in shape and magnitude to that which
+      painters draw round the heads of saints... 
+
+      <p>Pavia contains many thousand inhabitants, the major part
+      of whom were at this early hour walking about the streets and
+      squares or looking out of windows in order to witness this
+      long-talked-of phenomenon; and when the total obscuration
+      took place, which was <em>instantaneous,</em> there was a
+      universal shout from every observer which ``made the welkin
+      ring,'' and for the moment withdrew my attention from the
+      object with which I was immediately occupied. I had, indeed,
+      expected the appearance of a luminous circle round the moon
+      during the time of total obscurity; but I did not expect,
+      from any of the accounts of preceding eclipses that I had
+      read, to witness so magnificent an exhibition as that which
+      took place...</p>
+
+      <p>Splendid and astonishing, however, as this remarkable
+      phenomenon really was, and although it could not fail to call
+      forth the admiration and applause of every beholder, yet I
+      must confess that there was at the same time something in its
+      singular and wonderful appearance that was appalling...</p>
+
+      <p>But the most remarkable circumstance attending the
+      phenomenon was the appearance of <em>three large
+      protuberances</em> apparently emanating from the
+      circumference of the moon, but evidently forming a portion of
+      the corona. They had the appearance of mountains of a
+      prodigious elevation; their color was red tinged with lilac
+      or purple; perhaps the color of the peach-blossom would more
+      nearly represent it. They somewhat resembled the tops of the
+      snowy Alpine mountains when colored by the rising or the
+      setting sun. They resembled the Alpine mountains in another
+      respect, inasmuch as their light was perfectly steady, and
+      had none of that flickering or sparkling motion so visible in
+      other parts of the corona...</p>
+
+      <p>The whole of these protuberances were visible even to the
+      last moment of total obscuration, and when the first ray of
+      light was admitted from the sun they vanished, with the
+      corona, altogether, and daylight was instantly restored.</p>
+    </blockquote>
+
+    <p>I have quoted nearly all of this remarkable description not
+    alone for its intrinsic interest, but because it is the best
+    depiction that can be found of the general phenomena of a total
+    solar eclipse. Still, not every such eclipse offers an equally
+    magnificent spectacle. The eclipses of 1900 and 1905, for
+    instance, which were seen by the writer, the first in South
+    Carolina and the second in Spain, fell far short of that
+    described by Bailey in splendor and impressiveness. Of course,
+    something must be allowed for the effect of surprise; Bailey
+    had not expected to see what was so suddenly disclosed to him.
+    But both in 1900 and 1905 the amount of scattered light in the
+    sky was sufficient in itself to make the corona appear faint,
+    and there were no very conspicuous prominences visible. Yet on
+    both occasions there was manifest among the spectators that
+    mingling of admiration and awe of which Bailey speaks. The
+    South Carolinians gave a cheer and the ladies waved their
+    handkerchiefs when the corona, ineffably delicate of form and
+    texture, <em>melted</em> into sight and then in two minutes
+    melted away again. The Spaniards, crowded on the citadel hill
+    of Burgos, with their king and his royal retinue in their
+    midst, broke out with a great clapping of hands as the awaited
+    spectacle unfolded itself in the sky; and on both occasions,
+    before the applause began, after an awed silence a low murmur
+    ran through the crowds. At Burgos it is said many made the sign
+    of the cross.</p>
+
+    <p>It was not long before Bailey's idea that the prominences
+    were a part of the corona was abandoned, and it was perceived
+    that the two phenomena were to a great extent independent. At
+    the eclipse of 1868, which the astronomers, aroused by the
+    wonderful scene of 1842, and eager to test the powers of the
+    newly invented spectroscope, flocked to India to witness,
+    Janssen conceived the idea of employing the spectroscope to
+    render the prominences visible when there was no eclipse. He
+    succeeded the very next day, and these phenomena have been
+    studied in that way ever since.</p>
+
+    <p>There are recognized two kinds of prominences -- the
+    ``erruptive'' and the ``quiescent.'' The latter, which are
+    cloud-like in form, may be seen almost anywhere along the edge
+    of the sun; but the former, which often shoot up as if hurled
+    from mighty volcanoes, appear to be associated with sun-spots,
+    and appear only above the zones where spots abound. Either of
+    them, when seen in projection against the brilliant solar disk,
+    appears white, not red, as against a background of sky. The
+    quiescent prominences, whose elevation is often from forty
+    thousand to sixty thousand miles, consist, as the spectroscope
+    shows, mainly of hydrogen and helium. The latter, it will be
+    remembered, is an element which was known to be in the sun many
+    years before the discovery that it also exists in small
+    quantities on the earth. A fact which may have a significance
+    which we cannot at present see is that the emanation from
+    radium gradually and spontaneously changes into helium, an
+    alchemistical feat of nature that has opened many curious
+    vistas to speculative thinkers. The eruptive prominences, which
+    do not spread horizontally like the others, but ascend with
+    marvelous velocity to elevations of half a million miles or
+    more, are apparently composed largely of metallic vapors --
+    <em>i.e.</em> metals which are usually solid on the earth, but
+    which at solar temperatures are kept in a volatilized state.
+    The velocity of their ascent occasionally amounts to three
+    hundred or four hundred miles per second. It is known from
+    mathematical considerations that the gravitation of the sun
+    would not be able to bring back any body that started from its
+    surface with a velocity exceeding three hundred and
+    eighty-three miles per second; so it is evident that some of
+    the matter hurled forth in eruptive prominences may escape from
+    solar control and go speeding out into space, cooling and
+    condensing into solid masses. There seems to be no reason why
+    some of the projectiles from the sun might not reach the
+    planets. Here, then, we have on a relatively small scale,
+    <em>explosions</em> recalling those which it has been imagined
+    may be the originating cause of some of the sudden phenomena of
+    the stellar heavens.</p>
+
+    <p>Of the sun-spots it is not our intention here specifically
+    to speak, but they evidently have an intimate connection with
+    eruptive prominences, as well as some relation, not yet fully
+    understood, with the corona. Of the real cause of sun-spots we
+    know virtually nothing, but recent studies by Professor Hale
+    and others have revealed a strange state of things in the
+    clouds of metallic vapors floating above them and their
+    surroundings. Evidences of a cyclonic tendency have been found,
+    and Professor Hale has proved that sun-spots are strong
+    magnetic fields, and consist of columns of ionized vapors
+    rotating in opposite directions in the two hemispheres. A fact
+    which may have the greatest significance is that titanium and
+    vanadium have been found both in sun-spots and in the
+    remarkable variable Mira Ceti, a star which every eleven
+    months, or thereabout, flames up with great brilliancy and then
+    sinks back to invisibility with the naked eye. It has been
+    suggested that sun-spots are indications of the beginning of a
+    process in the sun which will be intensified until it falls
+    into the state of such a star as Mira. Stars very far advanced
+    in evolution, without showing variability, also exhibit similar
+    spectra; so that there is much reason for regarding sunspots as
+    emblems of advancing age.</p>
+
+    <p>The association of the corona with sun-spots is less evident
+    than that of the eruptive prominences; still such an
+    association exists, for the form and extent of the corona vary
+    with the sun-spot period of which we shall presently speak. The
+    constitution of the corona remains to be discovered. It is
+    evidently in part gaseous, but it also probably contains matter
+    in the form of dust and small meteors. It includes one
+    substance altogether mysterious -- ``coronium.'' There are
+    reasons for thinking that this may be the lightest of all the
+    elements, and Professor Young, its discoverer, said that it was
+    ``absolutely unique in nature; utterly distinct from any other
+    known form of matter, terrestial, solar, or cosmical.'' The
+    enormous extent of the corona is one of its riddles. Since the
+    development of the curious subject of the ``pressure of light''
+    it has been proposed to account for the sustentation of the
+    corona by supposing that it is borne upon the billows of light
+    continually poured out from the sun. Experiment has proved,
+    what mathematical considerations had previously pointed out as
+    probable, that the waves of light exert a pressure or driving
+    force, which becomes evident in its effects if the body acted
+    upon is sufficiently small. In that case the light pressure
+    will prevail over the attraction of gravitation, and propel the
+    attenuated matter away from the sun in the teeth of its
+    attraction. The earth itself would be driven away if, instead
+    of consisting of a solid globe of immense aggregate mass, it
+    were a cloud of microscopic particles. The reason is that the
+    pressure varies in proportion to the <em>surface</em> of the
+    body acted upon, while the gravitational attraction is
+    proportional to the <em>volume,</em> or the total amount of
+    matter in the body. But the surface of any body depends upon
+    the <em>square</em> of its diameter, while the volume depends
+    upon the <em>cube</em> of the diameter. If, for instance, the
+    diameter is represented by 4, the surface will be proportional
+    to 4 &times; 4, or 16, and the volume to 4 &times; 4 &times; 4,
+    or 64; but if the diameter is taken as 2, the surface will be 2
+    &times; 2, or 4, and the volume 2 &times; 2 &times; 2, or 8.
+    Now, the ratio of 4 to 8 is twice as great as that of 16 to 64.
+    If the diameter is still further decreased, the ratio of the
+    surface to the volume will proportionally grow larger; in other
+    words, the pressure will gain upon the attraction, and whatever
+    their original ratio may have been, a time will come, if the
+    diminution of size continues, when the pressure will become
+    more effective than the attraction, and the body will be driven
+    away. Supposing the particles of the corona to be below the
+    critical size for the attraction of a mass like that of the sun
+    to control them, they would be driven off into the surrounding
+    space and appear around the sun like the clouds of dust around
+    a mill. We shall return to this subject in connection with the
+    Zodiacal Light, the Aurora, and Comets.</p>
+
+    <p>On the other hand, there are parts of the corona which
+    suggest by their forms the play of electric or magnetic forces.
+    This is beautifully shown in some of the photographs that have
+    been made of the corona during recent eclipses. Take, for
+    instance, that of the eclipse of 1900. The sheaves of light
+    emanating from the poles look precisely like the ``lines of
+    force'' surrounding the poles of a magnet. It will be noticed
+    in this photograph that the corona appears to consist of two
+    portions: one comprising the polar rays just spoken of, and the
+    other consisting of the broader, longer, and less-defined
+    masses of light extending out from the equatorial and
+    middle-latitude zones. Yet even in this more diffuse part of
+    the phenomenon one can detect the presence of submerged curves
+    bearing more or less resemblance to those about the poles. Just
+    what part electricity or electro-magnetism plays in the
+    mechanism of the solar radiation it is impossible to say, but
+    on the assumption that it is a very important part is based the
+    hypothesis that there exists a direct solar influence not only
+    upon the magnetism, but upon the weather of the earth. This
+    hypothesis has been under discussion for half a century, and
+    still we do not know just how much truth it represents. It is
+    certain that the outbreak of great disturbances on the sun,
+    accompanied by the formation of sun-spots and the upshooting of
+    eruptive prominences (phenomena which we should naturally
+    expect to be attended by action), have been instantly followed
+    by corresponding ``magnetic storms'' on the earth and brilliant
+    displays of the auroral lights. There have been occasions when
+    the influence has manifested itself in the most startling ways,
+    a great solar outburst being followed by a mysterious gripping
+    of the cable and telegraph systems of the world, as if an
+    invisible and irresistible hand had seized them. Messages are
+    abruptly cut off, sparks leap from the telegraph instruments,
+    and the entire earth seems to have been thrown into a magnetic
+    flurry. These occurrences affect the mind with a deep
+    impression of the dependence of our planet on the sun, such as
+    we do not derive from the more familiar action of the sunlight
+    on the growth of plants and other phenomena of life depending
+    on solar influences.</p>
+
+    <p>Perhaps the theory of solar magnetic influence upon the
+    weather is best known in connection with the ``sun-spot
+    cycle.'' This, at any rate, is, as already remarked, closely
+    associated with the corona. Its existence was discovered in
+    1843 by the German astronomer Schwabe. It is a period of
+    variable length, averaging about eleven years, during which the
+    number of spots visible on the sun first increases to a
+    maximum, then diminishes to a minimum, and finally increases
+    again to a maximum. For unknown reasons the period is sometimes
+    two or three years longer than the average and sometimes as
+    much shorter. Nevertheless, the phenomena always recur in the
+    same order. Starting, for instance, with a time when the
+    observer can find few or no spots, they gradually increase in
+    number and size until a maximum, in both senses, is reached,
+    during which the spots are often of enormous size and
+    exceedingly active. After two or three years they begin to
+    diminish in number, magnitude, and activity until they almost
+    or quite disappear. A strange fact is that when a new period
+    opens, the spots appear first in high northern and southern
+    latitudes, far from the solar equator, and as the period
+    advances they not only increase in number and size, but break
+    out nearer and nearer to the equator, the last spots of a
+    vanishing period sometimes lingering in the equatorial region
+    after the advance-guard of its successor has made its
+    appearance in the high latitudes. Spots are never seen on the
+    equator nor near the poles. It was not very long after the
+    discovery of the sun-spot cycle that the curious observation
+    was made that a striking coincidence existed between the period
+    of the sun-spots and another period affecting the general
+    magnetic condition of the earth. When a curved line
+    representing the varying number of sun-spots was compared with
+    another curve showing the variations in the magnetic state of
+    the earth the two were seen to be in almost exact accord, a
+    rise in one curve corresponding to a rise in the other, and a
+    fall to a fall. Continued observation has proved that this is a
+    real coincidence and not an accidental one, so that the
+    connection, although as yet unexplained, is accepted as
+    established. But does the influence extend further, and
+    directly affect the weather and the seasons as well as the
+    magnetic elements of the earth? A final answer to this question
+    cannot yet be given, for the evidence is contradictory, and the
+    interpretations put upon it depend largely on the predilections
+    of the judges.</p>
+
+    <p>But, in a broad sense, the sun-spots and the phenomena
+    connected with them <em>must</em> have a relation to terrestial
+    meteorology, for they prove the sun to be a variable star.
+    Reference was made, a few lines above, to the resemblance of
+    the spectra of sun-spots to those of certain stars which seem
+    to be failing through age. This in itself is extremely
+    suggestive; but if this resemblance had never been discovered,
+    we should have been justified in regarding the sun as variable
+    in its output of energy; and not only variable, but probably
+    increasingly so. The very inequalities in the sun-spot cycle
+    are suspicious. When the sun is most spotted its total light
+    may be reduced by one-thousandth part, although it is by no
+    means certain that its outgiving of thermal radiations is then
+    reduced. A loss of one-thousandth of its luminosity would
+    correspond to a decrease of .0025 of a stellar magnitude,
+    considering the sun as a star viewed from distant space. So
+    slight a change would not be perceptible; but it is not alone
+    sun-spots which obscure the solar surface, its entire globe is
+    enveloped with an obscuring veil. When studied with a powerful
+    telescope the sun's surface is seen to be thickly mottled with
+    relatively obscure specks, so numerous that it has been
+    estimated that they cut off from one-tenth to one-twentieth of
+    the light that we should receive from it if the whole surface
+    were as brilliant as its brightest parts. The condition of
+    other stars warrants the conclusion that this obscuring
+    envelope is the product of a process of refrigeration which
+    will gradually make the sun more and more variable until its
+    history ends in extinction. Looking backward, we see a time
+    when the sun must have been more brilliant than it is now. At
+    that time it probably shone with the blinding white splendor of
+    such stars as Sirius, Spica, and Vega; now it resembles the
+    relatively dull Procyon; in time it will turn ruddy and fall
+    into the closing cycle represented by Antares. Considering that
+    once it must have been more radiantly powerful than at present,
+    one is tempted to wonder if that could have been the time when
+    tropical life flourished within the earth's polar circles,
+    sustained by a vivific energy in the sun which it has now
+    lost.</p>
+
+    <p>The corona, as we have said, varies with the sun-spot cycle.
+    When the spots are abundant and active the corona rises strong
+    above the spotted zones, forming immense beams or streamers,
+    which on one occasion, at least, had an observed length of
+    <em>ten million miles.</em> At the time of a spot minimum the
+    corona is less brilliant and has a different outline. It is
+    then that the curved polar rays are most conspicuous. Thus the
+    vast banners of the sun, shaken out in the eclipse, are signals
+    to tell of its varying state, but it will probably be long
+    before we can read correctly their messages.</p>
+
+    <p><strong>The Zodiacal Light Mystery</strong></p>
+
+    <p>There is a singular phenomenon in the sky -- one of the most
+    puzzling of all -- which has long arrested the attention of
+    astronomers, defying their efforts at explanation, but which
+    probably not one in a hundred, and possibly not one in a
+    thousand, of the readers of this book has ever seen. Yet its
+    name is often spoken, and it is a conspicuous object if one
+    knows when and where to look for it, and when well seen it
+    exhibits a mystical beauty which at the same time charms and
+    awes the beholder. It is called ``The Zodiacal Light,'' because
+    it lies within the broad circle of the Zodiac, marking the
+    sun's apparent annual path through the stars. What it is nobody
+    has yet been able to find out with certainty, and books on
+    astronomy usually speak of it with singular reserve. But it has
+    given rise to many remarkable theories, and a true explanation
+    of it would probably throw light on a great many other
+    celestial mysteries. The Milky Way is a more wonderful object
+    to look upon, but its nature can be comprehended, while there
+    is a sort of uncanniness about the Zodiacal Light which
+    immediately impresses one upon seeing it, for its part in the
+    great scheme of extra-terrestrial affairs is not evident.</p>
+
+    <p>If you are out-of-doors soon after sunset -- say, on an
+    evening late in the month of February -- you may perceive, just
+    after the angry flush of the dying winter's day has faded from
+    the sky, a pale ghostly presence rising above the place where
+    the sun went down. The writer remembers from boyhood the first
+    time it was pointed out to him and the unearthly impression
+    that it made, so that he afterward avoided being out alone at
+    night, fearful of seeing the spectral thing again. The
+    phenomenon brightens slowly with the fading of the twilight,
+    and soon distinctly assumes the shape of an elongated pyramid
+    of pearly light, leaning toward the south if the place of
+    observation is in the northern hemisphere. It does not impress
+    the observer at all in the same manner as the Milky Way; that
+    looks far off and is clearly among the stars, but the Zodiacal
+    Light seems closer at hand, as if it were something more
+    intimately concerning the earth. To all it immediately suggests
+    a connection, also, with the sunken sun. If the night is clear
+    and the moon absent (and if you are in the country, for city
+    lights ruin the spectacles of the sky), you will be able to
+    watch the apparition for a long time. You will observe that the
+    light is brightest near the horizon, gradually fading as the
+    pyramidal beam mounts higher, but in favorable circumstances it
+    may be traced nearly to the meridian south of the zenith, where
+    its apex at last vanishes in the starlight. It continues
+    visible during the evenings of March and part of April, after
+    which, ordinarily, it is seen no more, or if seen is relatively
+    faint and unimpressive. But when autumn comes it appears again,
+    this time not like a wraith hovering above the westward tomb of
+    the day-god, but rather like a spirit of the morning announcing
+    his reincarnation in the east.</p>
+
+    <p>The reason why the Zodiacal Light is best seen in our
+    latitudes at the periods just mentioned is because at those
+    times the Zodiac is more nearly perpendicular to the horizon,
+    first in the west and then in the east; and, since the
+    phenomenon is confined within the borders of the Zodiac, it
+    cannot be favorably placed for observation when the zodiacal
+    plane is but slightly inclined to the horizon. Its faint light
+    requires the contrast of a background of dark sky in order to
+    be readily perceptible. But within the tropics, where the
+    Zodiac is always at a favorable angle, the mysterious light is
+    more constantly visible. Nearly all observant travelers in the
+    equatorial regions have taken particular note of this
+    phenomenon, for being so much more conspicuous there than in
+    the temperate zones it at once catches the eye and holds the
+    attention as a novelty. Humboldt mentions it many times in his
+    works, for his genius was always attracted by things out of the
+    ordinary and difficult of explanation, and he made many careful
+    observations on its shape, its brilliancy, and its variations;
+    for there can be no doubt that it does vary, and sometimes to
+    an astonishing degree. It is said that it once remained
+    practically invisible in Europe for several years in
+    succession. During a trip to South Africa in 1909 an English
+    astronomer, Mr E. W. Maunder, found a remarkable difference
+    between the appearance of the Zodiacal Light on his going and
+    coming voyages. In fact, when crossing the equator going south
+    he did not see it at all; but on returning he had, on March
+    6th, when one degree south of the equator, a memorable view of
+    it.</p>
+
+    <blockquote>
+      It was a bright, clear night, and the Zodiacal Light was
+      extraordinarily brilliant -- brighter than he had ever seen
+      it before. The Milky Way was not to be compared with it. The
+      brightest part extended 75&deg; from the sun. There was a
+      faint and much narrower extension which they could just make
+      out beyond the Pleiades along the ecliptic, but the greater
+      part of the Zodiacal Light showed as a broad truncated
+      column, and it did not appear nearly as conical as he had
+      before seen it.
+    </blockquote>
+
+    <p>When out of the brief twilight of intertropical lands, where
+    the sun drops vertically to the horizon and night rushes on
+    like a wave of darkness, the Zodiacal Light shoots to the very
+    zenith, its color is described as a golden tint, entirely
+    different from the silvery sheen of the Milky Way. If I may
+    venture again to refer to personal experiences and impressions,
+    I will recall a view of the Zodiacal Light from the summit of
+    the cone of Mt Etna in the autumn of the year 1896 (more
+    briefly described in <em>Astronomy with the Naked Eye</em>).
+    There are few lofty mountains so favorably placed as Etna for
+    observations of this kind. It was once resorted to by Prof.
+    George E. Hale, in an attempt to see the solar corona without
+    an eclipse. Rising directly from sea-level to an elevation of
+    nearly eleven thousand feet, the observer on its summit at
+    night finds himself, as it were, lost in the midst of the sky.
+    But for the black flanks of the great cone on which he stands
+    he might fancy himself to be in a balloon. On the occasion to
+    which I refer the world beneath was virtually invisible in the
+    moonless night. The blaze of the constellations overhead was
+    astonishingly brilliant, yet amid all their magnificence my
+    attention was immediately drawn to a great tapering light that
+    sprang from the place on the horizon where the sun would rise
+    later, and that seemed to be blown out over the stars like a
+    long, luminous veil. It was the finest view of the Zodiacal
+    light that I had ever enjoyed -- thrilling in its strangeness
+    -- but I was almost disheartened by the indifference of my
+    guide, to whom it was only a light and nothing more. If he had
+    no science, he had less poetry -- rather a remarkable thing, I
+    thought, for a child of his clime. The Light appeared to me to
+    be distinctly brighter than the visible part of the Milky Way
+    which included the brilliant stretches in Auriga and Perseus,
+    and its color, if one may speak of color in connection with
+    such an object, seemed richer than that of the galactic band;
+    but I did not think of it as yellow, although Humboldt has
+    described it as resembling a golden curtain drawn over the
+    stars, and Du Chaillu in Equatorial Africa found it of a bright
+    yellow color. It may vary in color as in conspicuousness. The
+    fascination of that extraordinary sight has never faded from my
+    memory. I turned to regard it again and again, although I had
+    never seen the stellar heavens so brilliant, and it was one of
+    the last things I looked for when the morning glow began softly
+    to mount in the east, and Sicily and the Mediterranean slowly
+    emerged from the profound shadow beneath us.</p>
+
+    <p>The Zodiacal Light seems never to have attracted from
+    astronomers in general the amount of careful attention that it
+    deserves; perhaps because so little can really be made of it as
+    far as explanation is concerned. I have referred to the
+    restraint that scientific writers apparently feel in speaking
+    of it. The grounds for speculation that it affords may be too
+    scanty to lead to long discussions, yet it piques curiosity,
+    and as we shall see in a moment has finally led to a most
+    interesting theory. Once it was the subject of an elaborate
+    series of studies which carried the observer all round the
+    world. That was in 1845--46, during the United States Exploring
+    Expedition that visited the then little known Japan. The
+    chaplain of the fleet, the Rev. Mr Jones, went out prepared to
+    study the mysterious light in all its phases. He saw it from
+    many latitudes on both sides of the equator, and the
+    imagination cannot but follow him with keen interest in his
+    world-circling tour, keeping his eyes every night fixed upon
+    the phantasm overhead, whose position shifted with that of the
+    hidden sun. He demonstrated that the flow extends at times
+    completely across the celestial dome, although it is relatively
+    faint directly behind the earth. On his return the government
+    published a large volume of his observations, in which he
+    undertook to show that the phenomenon was due to the reflection
+    of sunlight from a ring of meteoric bodies encircling the
+    earth. But, after all, this elaborate investigation settled
+    nothing.</p>
+
+    <p>Prof. E. E. Barnard has more recently devoted much attention
+    to the Zodiacal Light, as well as to a strange attendant
+    phenomenon called the ``Gegenschein,'' or Counterglow, because
+    it always appears at that point in the sky which is exactly
+    opposite the sun. The Gegenschein is an extremely elusive
+    phenomenon, suitable only for eyes that have been specially
+    trained to see it. Professor Newcomb has cautiously remarked
+    that</p>
+
+    <blockquote>
+      it is said that in that point of the heavens directly
+      opposite the sun there is an elliptical patch of light...
+      This phenomenon is so difficult to account for that its
+      existence is sometimes doubted; yet the testimony in its
+      favor is difficult to set aside.
+    </blockquote>
+
+    <p>It certainly cannot be set aside at all since the
+    observations of Barnard. I recall an attempt to see it under
+    his guidance during a visit to Mount Hamilton, when he was
+    occupied there with the Lick telescope. Of course, both the
+    Gegenschein and the Zodiacal Light are too diffuse to be
+    studied with telescopes, which, so to speak, magnify them out
+    of existence. They can only be successfully studied with the
+    naked eye, since every faintest glimmer that they afford must
+    be utilized. This is especially true of the Gegenschein. At
+    Mount Hamilton, Mr Barnard pointed out to me its location with
+    reference to certain stars, but with all my gazing I could not
+    be sure that I saw it. To him, on the contrary, it was obvious;
+    he had studied it for months, and was able to indicate its
+    shape, its boundaries, its diameter, and the declination of its
+    center with regard to the ecliptic. There is not, of course,
+    the shadow of a doubt of the existence of the Gegenschein, and
+    yet I question if one person in a million has ever seen or ever
+    will see it. The Zodiacal Light, on the other hand, is plain
+    enough, provided that the time and the circumstances of the
+    observation are properly chosen.</p>
+
+    <p>In the attempts to explain the Zodiacal Light, the favorite
+    hypothesis has been that it is an appendage of the sun --
+    perhaps simply an extension of the corona in the plane of the
+    ecliptic, which is not very far from coinciding with that of
+    the sun's equator. This idea is quite a natural one, because of
+    the evident relation of the light to the position of the sun.
+    The vast extension of the equatorial wings of the corona in
+    1878 gave apparent support to this hypothesis; if the substance
+    of the corona could extend ten million miles from the sun, why
+    might it not extend even one hundred million, gradually fading
+    out beyond the orbit of the earth? A variation of this
+    hypothesis assumes that the reflection is due to swarms of
+    meteors circling about the sun, in the plane of its equator,
+    all the way from its immediate neighborhood to a distance
+    exceeding that of the earth. But in neither form is the
+    hypothesis satisfactory; there is nothing in the appearance of
+    the corona to indicate that it extends even as far as the
+    planet Mercury, while as to meteors, the orbits of the known
+    swarms do not accord with the hypothesis, and we have no reason
+    to believe that clouds of others exist traveling in the part of
+    space where they would have to be in order to answer the
+    requirements of the theory. The extension of the corona in 1878
+    did not resemble in its texture the Zodiacal Light.</p>
+
+    <p>Now, it has so often happened in the history of science that
+    an important discovery in one branch has thrown unexpected but
+    most welcome light upon some pending problem in some other
+    branch, that a strong argument might be based upon that fact
+    alone against the too exclusive devotion of many investigators
+    to the narrow lines of their own particular specialty; and the
+    Zodiacal Light affords a case in point, when it is considered
+    in connection with recent discoveries in chemistry and physics.
+    From the fact that atoms are compound bodies made up of
+    corpuscles at least a thousand times smaller than the smallest
+    known atom -- a fact which astounded most men of science when
+    it was announced a few years ago -- a new hypothesis has been
+    developed concerning the nature of the Zodiacal Light (as well
+    as other astronomical riddles), and this hypothesis comes not
+    from an astronomer, but from a chemist and physicist, the
+    Swede, Svante Arrhenius. In considering an outline of this new
+    hypothesis we need neither accept nor reject it; it is a case
+    rather for suspension of judgment.</p>
+
+    <p>To begin with, it carries us back to the ``pressure of
+    light'' mentioned in the preceding chapter. The manner in which
+    this pressure is believed generally to act was there
+    sufficiently explained, and it only remains to see how it is
+    theoretically extended to the particles of matter supposed to
+    constitute the Zodiacal Light. We know that corpuscles, or
+    ``fragments of atoms'' negatively electrified, are discharged
+    from hot bodies. Streams of these ``ions'' pour from many
+    flames and from molten metals; and the impact of the cathode
+    and ultra-violet rays causes them to gush even from cold
+    bodies. In the vast laboratory of the sun it is but reasonable
+    to suppose that similar processes are taking place. ``As a very
+    hot metal emits these corpuscles,'' says Prof. J. J. Thomson,
+    ``it does not seem an improbable hypothesis that they are
+    emitted by that very hot body, the sun.'' Let it be assumed,
+    then, that the sun does emit them; what happens next?
+    Negatively charged corpuscles, it is known, serve as nuclei to
+    which particles of matter in the ordinary state are attracted,
+    and it is probable that those emitted from the sun immediately
+    pick up loads in this manner and so grow in bulk. If they grow
+    large enough the gravitation of the sun draws them back, and
+    they produce a negative charge in the solar atmosphere. But it
+    is probable that many of the particles do not attain the
+    critical size which, according to the principles before
+    explained, would enable the gravitation of the sun to retain
+    them in opposition to the pressure of the waves of light, and
+    with these particles the light pressure is dominant. Clouds of
+    them may be supposed to be continually swept away from the sun
+    into surrounding space, moving mostly in or near the plane of
+    the solar equator, where the greatest activity, as indicated by
+    sunspots and related phenomena, is taking place. As they pass
+    outward into space many of them encounter the earth. If the
+    earth, like the moon, had no atmosphere the particles would
+    impinge directly on its surface, giving it a negative electric
+    charge. But the presence of the atmosphere changes all that,
+    for the first of the flying particles that encounter it impart
+    to it their negative electricity, and then, since like electric
+    charges repel like, the storm of particles following will be
+    sheered off from the earth, and will stream around it in a maze
+    of hyperbolic paths. Those that continue on into space beyond
+    the earth may be expected to continue picking up wandering
+    particles of matter until their bulk has become so great that
+    the solar attraction prevails again over the light pressure
+    acting upon them, and they turn again sunward. Passing the
+    earth on their return they will increase the amount of
+    dust-clouds careering round it; and these will be further
+    increased by the action of the ultra-violet rays of the
+    sunlight causing particles to shoot radially away from the
+    earth when the negative charge of the upper atmosphere has
+    reached a certain amount, which particles, although starting
+    sunward, will be swept back to the earth with the oncoming
+    streams. As the final result of all this accumulation of flying
+    and gyrating particles in the earth's neighborhood, we are told
+    that the latter must be transformed into the semblance of a
+    gigantic solid-headed comet provided with streaming tails, the
+    longest of them stretching away from the direction of the sun,
+    while another shorter one extends toward the sun. This shorter
+    tail is due to the particles that we have just spoken of as
+    being driven sunward from the earth by the action of
+    ultra-violet light. No doubt this whole subject is too
+    technical for popular statement; but at any rate the general
+    reader can understand the picturesque side of the theory, for
+    its advocates assure us that if we were on the moon we would
+    doubtless be able to see the comet-like tails of the earth, and
+    then we could appreciate the part that they play in producing
+    the phenomenon of the Zodiacal Light.</p>
+
+    <p>That the Light as we see it could be produced by the
+    reflection of sunlight from swarms of particles careering round
+    the earth in the manner supposed by Arrhenius' hypothesis is
+    evident enough; and it will be observed that the new theory,
+    after all, is only another variant of the older one which
+    attributes the Zodiacal Light to an extension of the solar
+    corona. But it differs from the older theory in offering an
+    explanation of the manner in which the extension is effected,
+    and it differentiates between the corona proper and the streams
+    of negative particles shot away from the sun. In its details
+    the hypothesis of Arrhenius also affords an explanation of many
+    peculiarities of the Zodiacal Light, such as that it is
+    confined to the neighborhood of the ecliptic, and that it is
+    stronger on the side of the earth which is just turning away
+    from a position under the sun than on the other side; but it
+    would carry us beyond our limits to go into these particulars.
+    The Gegenschein, according to this theory, is a part of the
+    same phenomenon as the Zodiacal Light, for by the laws of
+    perspective it is evident that the reflection from the streams
+    of particles situated at a point directly opposite to the sun
+    would be at a maximum, and this is the place which the
+    Gegenschein occupies. Apart from its geometrical relations to
+    the position of the sun, the variability of the Zodiacal Light
+    appears to affirm its solar dependence, and this too would be
+    accounted for by Arrhenius' hypothesis better than by the old
+    theory of coronal extension. The amount of corpuscular
+    discharge from the sun must naturally be governed by the state
+    of relative activity or inactivity of the latter, and this
+    could not but be reflected in the varying splendor of the
+    Zodiacal Light. But much more extended study than has yet been
+    given to the subject will be required before we can feel that
+    we know with reasonable certainty what this mysterious
+    phenomenon really is. By the hypothesis of Arrhenius every
+    planet that has an atmosphere must have a Zodiacal Light
+    attending it, but the phenomenon is too faint for us to be able
+    to see it in the case, for instance, of Venus, whose atmosphere
+    is very abundant. The moon has no corresponding ``comet's
+    tail'' because, as already explained, of the lack of a lunar
+    atmosphere to repel the streams by becoming itself electrified;
+    but if there were a lunar Zodiacal Light, no doubt we could see
+    it because of the relative nearness of our satellite.</p>
+
+    <p><strong>Marvels of the Aurora</strong></p>
+
+    <p>One of the most vivid recollections of my early boyhood is
+    that of seeing my father return hastily into the house one
+    evening and call out to the family: ``Come outside and look at
+    the sky!'' Ours was a country house situated on a commanding
+    site, and as we all emerged from the doorway we were
+    dumbfounded to see the heavens filled with pale flames which
+    ran licking and quivering over the stars. Instantly there
+    sprang into my terrified mind the recollection of an awful
+    description of ``the Day of Judgment'' (the <em>Dies
+    Ir&aelig;</em>), which I had heard with much perturbation of
+    spirit in the Dutch Reformed church from the lips of a tall,
+    dark-browed, dreadfully-in-earnest preacher of the
+    old-fashioned type. My heart literally sank at sight of the
+    spectacle, for it recalled the preacher's very words; it was
+    just as he had said it would be, and it needed the assured
+    bearing of my elders finally to convince me that</p>
+
+    <blockquote>
+      <br>
+       That Day of Wrath, O dreadful day,<br>
+       When Heaven and Earth shall pass away,<br>
+       As David and the Sibyl say<br>
+      <br>
+    </blockquote>
+
+    <p>had not actually come upon us. And even the older members of
+    the household were not untouched with misgivings when menacing
+    spots of crimson appeared, breaking out now here, now there, in
+    the shuddering sky. Toward the north the spectacle was
+    appalling. A huge arch spanned an unnaturally dark segment
+    resting on the horizon, and above this arch sprang up beams and
+    streamers in a state of incessant agitation, sometimes shooting
+    up to the zenith with a velocity that took one's breath, and
+    sometimes suddenly falling into long ranks, and <em>marching,
+    marching, marching,</em> like an endless phalanx of fiery
+    specters, and moving, as I remember, always from east to west.
+    The absolute silence with which these mysterious evolutions
+    were performed and the quavering reflections which were thrown
+    upon the ground increased the awfulness of the exhibition.
+    Occasionally enormous curtains of lambent flame rolled and
+    unrolled with a majestic motion, or were shaken to and fro as
+    if by a mighty, noiseless wind. At times, too, a sudden
+    billowing rush would be made toward the zenith, and for a
+    minute the sky overhead would glow so brightly that the stars
+    seemed to have been consumed. The spectacle continued with
+    varying intensity for hours.</p>
+
+    <p>This exhibition occurred in Central New York, a latitude in
+    which the Aurora Borealis is seldom seen with so much splendor.
+    I remember another similar one seen from the city of New York
+    in November, 1882. On this last occasion some observers saw a
+    great upright beam of light which majestically moved across the
+    heavens, stalking like an apparition in the midst of the
+    auroral pageant, of whose general movements it seemed to be
+    independent, maintaining always its upright posture, and
+    following a magnetic parallel from east to west. This
+    mysterious beam was seen by no less than twenty-six observers
+    in different parts of the country, and a comparison of their
+    observations led to a curious calculation indicating that the
+    apparition was about <em>one hundred and thirty-three miles
+    tall</em> and moved at the speed of ten miles per second!</p>
+
+    <p>But, as everybody knows, it is in the Arctic regions that
+    the Aurora, or the ``Northern Lights,'' can best be seen.
+    There, in the long polar night, when for months together the
+    sun does not rise, the strange coruscations in the sky often
+    afford a kind of spectral daylight in unison with the weird
+    scenery of the world of ice. The pages in the narratives of
+    Arctic exploration that are devoted to descriptions of the
+    wonderful effects of the Northern Lights are second to none
+    that man has ever penned in their fascination. The lights, as I
+    have already intimated, display astonishing colors,
+    particularly shades of red and green, as they flit from place
+    to place in the sky. The discovery that the magnetic needle is
+    affected by the Aurora, quivering and darting about in a state
+    of extraordinary excitement when the lights are playing in the
+    sky, only added to the mystery of the phenomenon until its
+    electro-magnetic nature had been established. This became
+    evident as soon as it was known that the focus of the displays
+    was the magnetic pole; and when the far South was visited the
+    Aurora Australis was found, having its center at the South
+    Magnetic Pole. Then, if not before, it was clear that the earth
+    was a great globular magnet, having its poles of opposite
+    magnetism, and that the auroral lights, whatever their precise
+    cause might be, were manifestations of the magnetic activity of
+    our planet. After the invention of magnetic telegraphy it was
+    found that whenever a great Aurora occurred the telegraph lines
+    were interrupted in their operation, and the ocean cables
+    ceased to work. Such a phenomenon is called a ``magnetic
+    storm.''</p>
+
+    <p>The interest excited by the Aurora in scientific circles was
+    greatly stimulated when, in the last half of the nineteenth
+    century, it was discovered that it is a phenomenon intimately
+    associated with disturbances on the sun. The ancient ``Zurich
+    Chronicles,'' extending from the year 1000 to the year 1800, in
+    which both sun-spots visible to the naked eye and great
+    displays of the auroral lights were recorded, first set Rudolf
+    Wolf on the track of this discovery. The first notable proof of
+    the suspected connection was furnished with dramatic emphasis
+    by an occurrence which happened on September 1, 1859. Near noon
+    on that day two intensely brilliant points suddenly broke out
+    in a group of sun-spots which were under observation by Mr R.
+    C. Carrington at his observatory at Redhill, England. The
+    points remained visible for not more than five minutes, during
+    which interval they moved <em>thirty-five thousand miles</em>
+    across the solar disk. Mr R. Hodgson happened to see the same
+    phenomenon at his observatory at Highgate, and thus all
+    possibility of deception was removed. But neither of the
+    startled observers could have anticipated what was to follow,
+    and, indeed, it was an occurrence which has never been
+    precisely duplicated. I quote the eloquent account given by
+    Miss Clerke in her <em>History of Astronomy During the
+    Nineteenth Century.</em></p>
+
+    <blockquote>
+      <p>This unique phenomenon seemed as if specially designed to
+      accentuate the inference of a sympathetic relation between
+      the earth and the sun. From August 28 to September 4, 1859, a
+      magnetic storm of unparalleled intensity, extent, and
+      duration was in progress over the entire globe. Telegraphic
+      communication was everywhere interrupted -- except, indeed,
+      that it was in some cases found practicable to work the lines
+      <em>without batteries</em> by the agency of the
+      earth-currents alone; sparks issued from the wires; gorgeous
+      auroras draped the skies in solemn crimson over both
+      hemispheres, and even in the tropics; the magnetic needle
+      lost all trace of continuity in its movements and darted to
+      and fro as if stricken with inexplicable panic. The
+      coincidence was even closer. <em>At the very instant</em> of
+      the solar outburst witnessed by Carrington and Hodgson the
+      photographic apparatus at Kew registered a marked disturbance
+      of all the three magnetic elements; while shortly after the
+      ensuing midnight the electric agitation culminated, thrilling
+      the whole earth with subtle vibrations, and lighting up the
+      atmosphere from pole to pole with coruscating splendors which
+      perhaps dimly recall the times when our ancient planet itself
+      shone as a star.</p>
+    </blockquote>
+
+    <p>If this amazing occurrence stood alone, and as I have
+    already said it has never been exactly duplicated, doubt might
+    be felt concerning some of the inferences drawn from it; but in
+    varying forms it has been repeated many times, so that now
+    hardly anyone questions the reality of the assumed connection
+    between solar outbursts and magnetic storms accompanied by
+    auroral displays on the earth. It is true that the late Lord
+    Kelvin raised difficulties in the way of the hypothesis of a
+    direct magnetic action of the sun upon the earth, because it
+    seemed to him that an inadmissible quantity of energy was
+    demanded to account for such action. But no calculation like
+    that which he made is final, since all calculations depend upon
+    the validity of the data; and no authority is unshakable in
+    science, because no man can possess omniscience. It was Lord
+    Kelvin who, but a few years before the thing was actually
+    accomplished, declared that aerial navigation was an
+    impracticable dream, and demonstrated its impracticability by
+    calculation. However the connection may be brought about, it is
+    as certain as evidence can make it that solar outbursts are
+    coincident with terrestial magnetic disturbances, and
+    coincident in such a way as to make the inference of a causal
+    connection irresistible. The sun is only a little more than a
+    hundred times its own diameter away from the earth. Why, then,
+    with the subtle connection between them afforded by the ether
+    which conveys to us the blinding solar light and the
+    life-sustaining solar heat, should it be so difficult to
+    believe that the sun's enormous electric energies find a way to
+    us also? No doubt the impulse coming from the sun acts upon the
+    earth after the manner of a touch upon a trigger, releasing
+    energies which are already stored up in our planet.</p>
+
+    <p>But besides the evidence afforded by such occurrences as
+    have been related of an intimate connection between solar
+    outbreaks and terrestial magnetic flurries, attended by
+    magnificent auroral displays, there is another line of proof
+    pointing in the same direction. Thus, it is known that the
+    sun-spot period, as remarked in a preceding chapter, coincides
+    in a most remarkable manner with the periodic fluctuations in
+    the magnetic state of the earth. This coincidence runs into the
+    most astonishing details. For instance, when the sun-spot
+    period shortens, the auroral period shortens to precisely the
+    same extent; as the short sun-spot periods usually bring the
+    most intense outbreaks of solar activity, so the corresponding
+    short auroral periods are attended by the most violent magnetic
+    storms; a secular period of about two hundred and twenty-two
+    years affecting sun-spots is said to have its auroral
+    duplicate; a shorter period of fifty-five and a half years,
+    which some observers believe that they have discovered appears
+    also to be common to the two phenomena; and yet another
+    ``superposed'' period of about thirty-five years, which some
+    investigators aver exists, affects sun-spots and aurora alike.
+    In short, the coincidences are so numerous and significant that
+    one would have to throw the doctrine of probability to the
+    winds in order to be able to reject the conclusion to which
+    they so plainly lead.</p>
+
+    <p>But still the question recurs: How is the influence
+    transmitted? Here Arrhenius comes once more with his hypothesis
+    of negative corpuscles, or ions, driven away from the sun by
+    light-pressure -- a hypothesis which seems to explain so many
+    things -- and offers it also as an explanation of the way in
+    which the sun creates the Aurora. He would give the Aurora the
+    same lineage with the Zodiacal Light. To understand the
+    application of this theory we must first recall the fact that
+    the earth is a great magnet having its two opposite poles of
+    magnetism, one near the Arctic and the other near the Antarctic
+    Circle. Like all magnets, the earth is surrounded with ``lines
+    of force,'' which, after the manner of the curved rays we saw
+    in the photograph of a solar eclipse, start from a pole, rising
+    at first nearly vertically, then bend gradually over, passing
+    high above the equator, and finally descending in converging
+    sheaves to the opposite pole. Now the axis of the earth is so
+    placed in space that it lies at nearly a right angle to the
+    direction of the sun, and as the streams of negatively charged
+    particles come pouring on from the sun (see the last preceding
+    chapter), they arrive in the greatest numbers over the earth's
+    equatorial regions. There they encounter the lines of magnetic
+    force at the place where the latter have their greatest
+    elevation above the earth, and where their direction is
+    horizontal to the earth's surface. Obeying a law which has been
+    demonstrated in the laboratory, the particles then follow the
+    lines of force toward the poles. While they are above the
+    equatorial regions they do not become luminescent, because at
+    the great elevation that they there occupy there is virtually
+    no atmosphere; but as they pass on toward the north and the
+    south they begin to descend with the lines of force, curving
+    down to meet at the poles; and, encountering a part of the
+    atmosphere comparable in density with what remains in an
+    exhausted Crookes tube, they produce a glow of cathode rays.
+    This glow is conceived to represent the Aurora, which may
+    consequently be likened to a gigantic exhibition of vacuum-tube
+    lights. Anybody who recalls his student days in the college
+    laboratory and who has witnessed a display of Northern Lights
+    will at once recognize the resemblance between them in colors,
+    forms, and behavior. This resemblance had often been noted
+    before Arrhenius elaborated his hypothesis.</p>
+
+    <p>Without intending to treat his interesting theory as more
+    than a possibly correct explanation of the phenomena of the
+    Aurora, we may call attention to some apparently confirmatory
+    facts. One of the most striking of these relates to a seasonal
+    variation in the average number of auror&aelig;. It has been
+    observed that there are more in March and September than at any
+    other time of the year, and fewer in June and December;
+    moreover (and this is a delicate test as applied to the
+    theory), they are slightly rarer in June than in December. Now
+    all these facts seem to find a ready explanation in the
+    hypothesis of Arrhenius, thus: (1) The particles issuing from
+    the sun are supposed to come principally from the regions whose
+    excitement is indicated by the presence of sun-spots (which
+    accords with Hale's observation that sun-spots are columns of
+    ionized vapors), and these regions have a definite location on
+    either side of the solar equator, seldom approaching it nearer
+    than within 5&deg; or 10&deg; north or south, and never
+    extending much beyond 35&deg; toward either pole; (2) The
+    equator of the sun is inclined about 7&deg; to the plane of the
+    earth's orbit, from which it results that twice in a year --
+    <em>viz.,</em> in June and December -- the earth is directly
+    over the solar equator, and twice a year -- <em>viz.,</em> in
+    March and September -- when it is farthest north or south of
+    the solar equator, it is over the inner edge of the sun-spot
+    belts. Since the corpuscles must be supposed to be propelled
+    radially from the sun, few will reach the earth when the latter
+    is over the solar equator in June and December, but when it is
+    over, or nearly over, the spot belts, in March and September,
+    it will be in the line of fire of the more active parts of the
+    solar surface, and relatively rich streams of particles will
+    reach it. This, as will be seen from what has been said above,
+    is in strict accord with the observed variations in the
+    frequency of auror&aelig;. Even the fact that somewhat fewer
+    auror&aelig; are seen in June than in December also finds its
+    explanation in the known fact that the earth is about three
+    million miles nearer the sun in the winter than in the summer,
+    and the number of particles reaching it will vary, like the
+    intensity of light, inversely as the square of the distance.
+    These coincidences are certainly very striking, and they have a
+    cumulative force. If we accept the theory, it would appear that
+    we ought to congratulate ourselves that the inclination of the
+    sun's equator is so slight, for as things stand the earth is
+    never directly over the most active regions of the sun-spots,
+    and consequently never suffers from the maximum bombardment of
+    charged particles of which the sun is capable. Incessant
+    auroral displays, with their undulating draperies, flitting
+    colors, and marching columns might not be objectionable from
+    the point of view of picturesqueness, but one magnetic storm of
+    extreme intensity following closely upon the heels of another,
+    for months on end, crazing the magnetic needle and continually
+    putting the telegraph and cable lines out of commission, to say
+    nothing of their effect upon ``wireless telegraphy'', would
+    hardly add to the charms of terrestrial existence.</p>
+
+    <p>One or two other curious points in connection with
+    Arrhenius' hypothesis may be mentioned. First, the number of
+    auror&aelig;, according to his explanation, ought to be
+    greatest in the daytime, when the face of the earth on the
+    sunward side is directly exposed to the atomic bombardment. Of
+    course visual observation can give us no information about
+    this, since the light of the Aurora is never sufficiently
+    intense to be visible in the presence of daylight, but the
+    records of the magnetic observatories can be, and have been,
+    appealed to for information, and they indicate that the facts
+    actually accord with the theory. Behind the veil of sunlight in
+    the middle of the afternoon, there is good reason to believe,
+    auroral exhibitions often take place which would eclipse in
+    magnificence those seen at night if we could behold them.
+    Observation shows, too, that auror&aelig; are more frequent
+    before than after midnight, which is just what we should expect
+    if they originate in the way that Arrhenius supposes. Second,
+    the theory offers an explanation of the alleged fact that the
+    formation of clouds in the upper air is more frequent in years
+    when auror&aelig; are most abundant, because clouds are the
+    result of the condensation of moisture upon floating particles
+    in the atmosphere (in an absolutely dustless atmosphere there
+    would be no clouds), and it has been proved that negative ions
+    like those supposed to come from the sun play a master part in
+    the phenomena of cloud formation.</p>
+
+    <p>Yet another singular fact, almost mystical in its
+    suggestions, may be mentioned. It seems that the dance of the
+    auroral lights occurs most frequently during the absence of the
+    moon from the hemisphere in which they appear, and that they
+    flee, in greater part, to the opposite hemisphere when the
+    moon's revolution in an orbit considerably inclined to the
+    earth's equator brings her into that where they have been
+    performing. Arrhenius himself discovered this curious relation
+    of auroral frequency to the position of the moon north or south
+    of the equator, and he explains it in this way. The moon, like
+    the earth, is exposed to the influx of the ions from the sun;
+    but having no atmosphere, or almost none, to interfere with
+    them, they descend directly upon her surface and charge her
+    with an electric negative potential to a very high degree. In
+    consequence of this she affects the electric state of the upper
+    parts of the earth's atmosphere where they lie most directly
+    beneath her, and thus prevents, to a large extent, the negative
+    discharges to which the appearance of the Aurora is due. And so
+    ``the extravagant and erring spirit'' of the Aurora avoids the
+    moon as Hamlet's ghost fled at the voice of the cock announcing
+    the awakening of the god of day.</p>
+
+    <p>There are even other apparent confirmations of the
+    hypothesis, but we need not go into them. We shall, however,
+    find one more application of it in the next chapter, for it
+    appears to be a kind of cure-all for astronomical troubles; at
+    any rate it offers a conceivable solution of the question, How
+    does the sun manage to transmit its electric influence to the
+    earth? And this solution is so grandiose in conception, and so
+    novel in the mental pictures that it offers, that its
+    acceptance would not in the least detract from the impression
+    that the Aurora makes upon the imagination.</p>
+
+    <p><strong>Strange Adventures of Comets</strong></p>
+
+    <p>The fears and legends of ancient times before Science was
+    born, and the superstitions of the Dark Ages, sedulously
+    cultivated for theological purposes by monks and priests, have
+    so colored our ideas of the influence that comets have had upon
+    the human mind that many readers may be surprised to learn that
+    it was the apparition of a wonderful comet, that of 1843, which
+    led to the foundation of our greatest astronomical institution,
+    the Harvard College Observatory. No doubt the comet
+    superstition existed half a century ago, as, indeed, it exists
+    yet today, but in this case the marvelous spectacle in the sky
+    proved less effective in inspiring terror than in awakening a
+    desire for knowledge. Even in the sixteenth century the views
+    that enlightened minds took of comets tended powerfully to
+    inspire popular confidence in science, and Halley's prediction,
+    after seeing and studying the motion of the comet which
+    appeared in 1682, that it would prove to be a regular member of
+    the sun's family and would be seen returning after a period of
+    about seventy-six years, together with the fulfillment of that
+    prediction, produced a revulsion from the superstitious notions
+    which had so long prevailed.</p>
+
+    <p>Then the facts were made plain that comets are subject to
+    the law of gravitation equally with the planets; that there are
+    many which regularly return to the neighborhood of the sun
+    (perihelion); and that these travel in orbits differing from
+    those of the planets only in their greater eccentricity,
+    although they have the peculiarity that they do not, like the
+    planets, all go round the sun in the same direction, and do not
+    keep within the general plane of the planetary system, but
+    traverse it sometimes from above and sometimes from below.
+    Other comets, including most of the ``great'' ones, appear to
+    travel in parabolic or, in a few cases, hyperbolic orbits,
+    which, not being closed curves, never bring them back again.
+    But it is not certain that these orbits may not be extremely
+    eccentric ellipses, and that after the lapse of hundreds, or
+    thousands, of years the comets that follow them may not
+    reappear. The question is an interesting one, because if all
+    orbits are really ellipses, then all comets must be permanent
+    members of the solar system, while in the contrary case many of
+    them are simply visitors, seen once and never to be seen again.
+    The hypothesis that comets are originally interlopers might
+    seem to derive some support from the fact that the certainly
+    periodic ones are associated, in groups, with the great outer
+    planets, whose attraction appears to have served as a trap for
+    them by turning them into elliptical orbits and thus making
+    them prisoners in the solar system. Jupiter, owing to his great
+    mass and his commanding situation in the system, is the chief
+    ``comet-catcher;'' but he catches them not for himself, but for
+    the sun. Yet if comets do come originally from without the
+    borders of the planetary system, it does not, by any means,
+    follow that they were wanderers at large in space before they
+    yielded to the overmastering attraction of the sun.
+    Investigation of the known cometary orbits, combined with
+    theoretical considerations, has led some astronomers to the
+    conclusion that as the sun travels onward through space he
+    ``picks up <em>en route</em>'' cometary masses which, without
+    belonging strictly to his empire, are borne along in the same
+    vast ``cosmical current'' that carries the solar system.</p>
+
+    <p>But while no intelligent person any longer thinks that the
+    appearance of a great comet is a token from the heavenly powers
+    of the approaching death of a mighty ruler, or the outbreak of
+    a devastating war, or the infliction of a terrible plague upon
+    wicked mankind, science itself has discovered mysteries about
+    comets which are not less fascinating because they are more
+    intellectual than the irrational fancies that they have
+    displaced. To bring the subject properly before the mind, let
+    us see what the principal phenomena connected with a comet
+    are.</p>
+
+    <p>At the present day comets are ordinarily ``picked up'' with
+    the telescope or the photographic plate before any one except
+    their discoverer is aware of their existence, and usually they
+    remain so insignificant in appearance that only astronomers
+    ever see them. Yet so great is the prestige of the word
+    ``comet'' that the discovery of one of these inconspicuous
+    wanderers, and its subsequent movements, become items of the
+    day's news which everybody reads with the feeling, perhaps,
+    that at least he knows what is going on in the universe even if
+    he doesn't understand it. But a truly great comet presents
+    quite a different proposition. It, too, is apt to be detected
+    coming out of the depths of space before the world at large can
+    get a glimpse of it, but as it approaches the sun its aspect
+    undergoes a marvelous change. Agitated apparently by solar
+    influence, it throws out a long streaming tail of nebulous
+    light, directed away from the sun and looking as if blown out
+    like a pennon by a powerful wind. Whatever may be the position
+    of the comet with regard to the sun, as it circles round him it
+    continually keeps its tail on the off side. This, as we shall
+    soon see, is a fact of capital importance in relation to the
+    probable nature of comets' tails. Almost at the same time that
+    the formation of the tail is observed a remarkable change takes
+    place in the comet's head, which, by the way, is invariably and
+    not merely occasionally its most important part. On approaching
+    the sun the head usually contracts. Coincidently with this
+    contraction a nucleus generally makes its appearance. This is a
+    bright, star-like point in the head, and it probably represents
+    the totality of solid matter that the comet possesses. But it
+    is regarded as extremely unlikely that even the nucleus
+    consists of a uniformly solid mass. If it were such, comets
+    would be far more formidable visitors when they pass near the
+    planets than they have been found to be. The diameter of the
+    nucleus may vary from a few hundred up to several thousand
+    miles; the heads, on the average, are from twenty-five thousand
+    to one hundred thousand miles in diameter, although a few have
+    greatly exceeded these dimensions; that of the comet of 1811,
+    one of the most stupendous ever seen, was a million and a
+    quarter miles in diameter! As to the tails, not withstanding
+    their enormous length -- some have been more than a hundred
+    million miles long -- there is reason to believe that they are
+    of extreme tenuity, ``as rare as vacuum.'' The smallest stars
+    have been seen shining through their most brilliant portions
+    with undiminished luster.</p>
+
+    <p>After the nucleus has been formed it begins to throw out
+    bright jets directed toward the sun. A stream, and sometimes
+    several streams, of light also project sunward from the
+    nucleus, occasionally appearing like a stunted tail directed
+    oppositely to the real tail. Symmetrical envelopes which, seen
+    in section, appear as half circles or parabolas, rise sunward
+    from the nucleus, forming a concentric series. The ends of
+    these stream backward into the tail, to which they seem to
+    supply material. Ordinarily the formation of these ejections
+    and envelopes is attended by intense agitation of the nucleus,
+    which twists and turns, swinging and gyrating with an
+    appearance of the greatest violence. Sometimes the nucleus is
+    seen to break up into several parts. The entire heads of some
+    comets have been split asunder in passing close around the sun;
+    The comet of 1882 retreated into space after its perihelion
+    passage with <em>five heads</em> instead of the one that it had
+    originally, and each of these heads had its own tail!</p>
+
+    <p>The possession of the spectroscope has enabled astronomers
+    during later years to study the chemical composition of comets
+    by analyzing their light. At first the only substances thus
+    discovered in them were hydro-carbon compounds, due evidently
+    to the gaseous envelopes in which some combination of hydrogen
+    with carbon existed. Behind this gaseous spectrum was found a
+    faint continuous spectrum ascribed to the nucleus, which
+    apparently both reflects the sunlight and gives forth the light
+    of a glowing solid or liquid. Subsequently sodium and iron
+    lines were found in cometary spectra. The presence of iron
+    would seem to indicate that some of these bodies may be much
+    more massive than observations on their attractive effects have
+    indicated. In some recent comets, such as Morehouse's, in 1908,
+    several lines have been found, the origin of which is
+    unknown.</p>
+
+    <p>Without going back of the nineteenth century we may find
+    records of some of the most extraordinary comets that man has
+    ever looked upon. In 1811, still spoken of as ``the year of the
+    comet,'' because of the wonderful vintage ascribed to the skyey
+    visitor, a comet shaped like a gigantic sword amazed the whole
+    world, and, as it remained visible for seventeen months, was
+    regarded by superstitious persons as a symbol of the fearful
+    happenings of Napoleon's Russian campaign. This comet, the
+    extraordinary size of whose head, greatly exceeding that of the
+    sun itself, has already been mentioned, was also remarkable for
+    exhibiting so great a brilliancy without approaching even to
+    the earth's distance from the sun. But there was once a comet
+    (and only once -- in the year 1729) which never got nearer to
+    the sun than four times the distance of the earth and yet
+    appeared as a formidable object in the sky. As Professor Young
+    has remarked, ``it must have been an enormous comet to be
+    visible from such a distance.'' And we are to remember that
+    there were no great telescopes in the year 1729. That comet
+    affects the imagination like a phantom of space peering into
+    the solar system, displaying its enormous train afar off
+    (which, if it had approached as near as other comets, would
+    probably have become <em>the</em> celestial wonder of all human
+    memory), and then turning away and vanishing in the depths of
+    immensity.</p>
+
+    <p>In 1843 a comet appeared which was so brilliant that it
+    could be seen in broad day close beside the sun! This was the
+    first authenticated instance of that kind, but the occurrence
+    was to be repeated, as we shall see in a moment, less than
+    forty years later.</p>
+
+    <p>The splendid comet of 1858, usually called Donati's, is
+    remembered by many persons yet living. It was, perhaps, both as
+    seen by the naked eye and with the telescope, the most
+    beautiful comet of which we have any record. It too marked a
+    rich vintage year, still remembered in the vineyards of France,
+    where there is a popular belief that a great comet ripens the
+    grape and imparts to the wine a flavor not attainable by the
+    mere skill of the cultivator. There are ``comet wines,''
+    carefully treasured in certain cellars, and brought forth only
+    when their owner wishes to treat his guests to a sip from
+    paradise.</p>
+
+    <p>The year 1861 saw another very remarkable comet, of an
+    aspect strangely vast and diffuse, which is believed to have
+    swept the earth with its immense tail when it passed between us
+    and the sun on the night of June 30th, an event which produced
+    no other known effect than the appearance of an unwonted amount
+    of scattered light in the sky.</p>
+
+    <p>The next very notable comet was the ``Great Southern Comet''
+    of 1880, which was not seen from the northern hemisphere. It
+    mimicked the aspect of the famous comet of 1843, and to the
+    great surprise of astronomers appeared to be traveling in the
+    same path. This proved to be the rising of the curtain for an
+    astronomical sensation unparalleled in its kind; for two years
+    later another brilliant comet appeared, first in the southern
+    hemisphere, <em>and it too followed the same track.</em> The
+    startling suggestion was now made that this comet was identical
+    with those of 1843 and 1880, its return having been hastened by
+    the resistance experienced in passing twice through the coronal
+    envelope, and there were some who thought that it would now
+    swing swiftly round and then plunge straight into the sun, with
+    consequences that might be disastrous to us on account of the
+    ``flash of heat'' that would be produced by the impact. Nervous
+    people were frightened, but observation soon proved that the
+    danger was imaginary, for although the comet almost grazed the
+    sun, and must have rushed through two or three million miles of
+    the coronal region, no retardation of its immense velocity was
+    perceptible, and it finally passed away in a damaged condition,
+    as before remarked, and has never since appeared.</p>
+
+    <p>Then the probable truth was perceived -- <em>viz.,</em> that
+    the three comets (1843, 1880, and 1882) were not one identical
+    body, but three separate ones all traveling in the same orbit.
+    It was found, too, that a comet seen in 1668 bore similar
+    insignia of relationship. The natural inference was that these
+    four bodies had once formed a single mass which had been split
+    apart by the disruptive action of the sun. Strength was lent to
+    this hypothesis by the fact that the comet of 1882 was
+    apparently torn asunder during its perihelion passage,
+    retreating into space in a dissevered state. But Prof. George
+    Forbes has a theory that the splitting of the original cometary
+    mass was effected by an unknown planet, probably greater than
+    Jupiter, situated at a hundred times the earth's distance from
+    the sun, and revolving in a period of a thousand years. He
+    supposes that the original comet was not that of 1668, but one
+    seen in 1556, which has since been ``missing,'' and that its
+    disruption occurred from an encounter with the supposititious
+    planet about the year 1700. Truly from every point of view
+    comets are the most extraordinary of adventurers!</p>
+
+    <p>The comet of 1882 was likewise remarkable for being visible,
+    like its predecessor of 1843, in full daylight in close
+    proximity to the sun. The story of its detection when almost in
+    contact with the solar disk is dramatic. It had been discovered
+    in the southern hemisphere only a couple of weeks before its
+    perihelion, which occurred on September 17th, and on the
+    forenoon of that day it was seen by Doctor Common in England,
+    and by Doctor Elkin and Mr Finlay at the Cape of Good Hope,
+    almost touching the sun. It looked like a dazzling white bird
+    with outspread wings. The southern observers watched it go
+    <em>right into the sun,</em> when it instantly disappeared.
+    What had happened was that the comet in passing its perihelion
+    point had swung exactly between the earth and the sun. On the
+    following morning it was seen from all parts of the world close
+    by the sun on the opposite side, and it remained thus visible
+    for three days, gradually receding from the solar disk. It then
+    became visible for northern observers in the morning sky before
+    sunrise, brandishing a portentous sword-shaped tail which, if
+    it had been in the evening sky, would have excited the wonder
+    of hundreds of millions, but situated where it was,
+    comparatively few ever saw it.</p>
+
+    <p>The application of photography to the study of comets has
+    revealed many curious details which might otherwise have
+    escaped detection, or at best have remained subject to doubt.
+    It has in particular shown not only the precise form of the
+    tails, but the remarkable vicissitudes that they undergo.
+    Professor Barnard's photographs of Brooks' comet in 1893
+    suggested, by the extraordinary changes in the form of the tail
+    which they revealed, that the comet was encountering a series
+    of obstructions in space which bent and twisted its tail into
+    fantastic shapes. The reader will observe the strange form into
+    which the tail was thrown on the night of October 21st. A cloud
+    of meteors through which the comet was passing might have
+    produced such deformations of its tail. In the photograph of
+    Daniels' comet of 1907, a curious striping of the tail will be
+    noticed. The short bright streaks seen in the photograph, it
+    may be explained, are the images of stars which are drawn out
+    into lines in consequence of the fact that the photographic
+    telescope was adjusted to follow the motion of the comet while
+    the stars remained at rest.</p>
+
+    <p>But the adventures of comets are not confined to possible
+    encounters with unknown obstacles. We have referred to the fact
+    that the great planets, and especially Jupiter, frequently
+    interfere with the motions of comets. This interference is not
+    limited to the original alteration of their orbits from
+    possible parabolas to ellipses, but is sometimes exercised
+    again and again, turning the bewildered comets into elliptical
+    paths of all degrees of eccentricity. A famous example of this
+    kind of planetary horse-play is furnished by the story of
+    Lexell's missing comet. This comet was first seen in 1770.
+    Investigation showed that it was moving in an orbit which
+    should bring it back to perihelion every five and a half years;
+    yet it had never been seen before and, although often searched
+    for, has never been seen since. Laplace and Leverrier proved
+    mathematically that in 1767 it had approached so close to
+    Jupiter as to be involved among the orbits of his satellites.
+    What its track had been before is not known, but on that
+    occasion the giant planet seized the interloper, threw it into
+    a short elliptic orbit and sent it, like an arrested vagrant,
+    to receive sentence at the bar of the sun. On this journey it
+    passed within less than 1,500,000 miles of the earth. The form
+    of orbit which Jupiter had impressed required, as we have said,
+    its return in about five and a half years; but soon after 1770
+    it had the misfortune a second time to encounter Jupiter at
+    close range, and he, as if dissatisfied with the leniency of
+    the sun, or indignant at the stranger's familiarity, seized the
+    comet and hurled it out of the system, or at any rate so far
+    away that it has never since been able to rejoin the family
+    circle that basks in the immediate rays of the solar hearth.
+    Nor is this the only instance in which Jupiter has dealt
+    summarily with small comets that have approached him with too
+    little deference.</p>
+
+    <p>The function which Jupiter so conspicuously fulfills as
+    master of the hounds to the sun is worth considering a little
+    more in detail. To change the figure, imagine the sun in its
+    voyage through space to be like a majestic battleship
+    surrounded by its scouts. Small vessels (the comets, as they
+    are overhauled by the squadron, are taken in charge by the
+    scouts, with Jupiter for their chief, and are forced to
+    accompany the fleet, but not all are impressed. If a strange
+    comet undertakes to run across Jupiter's bows the latter brings
+    it to, and makes prize of it by throwing it into a relatively
+    small ellipse with the sun for its focus. Thenceforth, unless,
+    as happened to the unhappy comet of Lexell, it encounters
+    Jupiter again in such a way as to be diverted by him into a
+    more distant orbit, it can never get away. About thirty comets
+    are now known to have thus been captured by the great planet,
+    and they are called ``Jupiter's Comet Family.'' But, on the
+    other hand, if a wandering comet crosses the wake of the chief
+    planetary scout the latter simply drives it away by
+    accelerating its motion and compels it to steer off into open
+    space. The transformation of comets into meteors will be
+    considered in the next chapter, but here, in passing, mention
+    may be made of the strange fate of one member of Jupiter's
+    family, Biela's comet, which, having become over bold in its
+    advances to its captor, was, after a few revolutions in is
+    impressed orbit, torn to pieces and turned into a flock of
+    meteors.</p>
+
+    <p>And now let us return to the mystery of comets' tails. That
+    we are fully justified in speaking of the tails of comets as
+    mysterious is proved by the declaration of Sir John Herschel,
+    who averred, in so many words, that ``there is some profound
+    secret and mystery of nature concerned in this phenomenon,''
+    and this profound secret and mystery has not yet been
+    altogether cleared up. Nevertheless, the all-explaining
+    hypothesis of Arrhenius offers us once more a certain amount of
+    aid. Comets' tails, Arrhenius assures us, are but another
+    result of the pressure of light. The reader will recall the
+    applications of this theory to the Zodiacal Light and the
+    Aurora. In the form in which we now have to deal with it, the
+    supposition is made that as a comet approaches the sun
+    eruptions of vapor, due to the solar heat, occur in its
+    nucleus. These are naturally most active on the side which is
+    directly exposed to the sun, whence the appearance of the
+    immense glowing envelopes that surround the nucleus on the
+    sunward side. Among the particles of hydro-carbon, and perhaps
+    solid carbon in the state of fine dust, which are thus set free
+    there will be many whose size is within the critical limit
+    which enables the light-waves from the sun to drive them away.
+    Clouds of such particles, then, will stream off behind the
+    advancing comet, producing the appearance of a tail. This
+    accounts for the fact that the tails of comets are always
+    directed away from the sun, and it also explains the varying
+    forms of the tails and the extraordinary changes that they
+    undergo. The speed of the particles driven before the
+    light-waves must depend upon their size and weight, the
+    lightest of a given size traveling the most swiftly. By
+    accretion certain particles might grow, thus losing velocity
+    and producing the appearance of bunches in the tail, such as
+    have been observed. The hypothesis also falls in with the
+    researches of Bredichin, who has divided the tails of comets
+    into three principal classes -- <em>viz.:</em> (1) Those which
+    appear as long, straight rays; (2) Those which have the form of
+    curved plumes or scimitars; (3) Those which are short, brushy,
+    and curved sharply backward along the comet's path. In the
+    first type he calculates the repulsive force at from twelve to
+    fifteen times the force of gravity; in the second at from two
+    to four times; and in the third at about one and a half times.
+    The straight tails he ascribes to hydrogen because the hydrogen
+    atom is the lightest known; the sword-shaped tails to
+    hydro-carbons; and the stumpy tails to vaporized iron. It will
+    be seen that, if the force driving off the tails is that which
+    Arrhenius assumes it to be, the forms of those appendages would
+    accord with those that Bredichin's theory calls for. At the
+    same time we have an explanation of the multiple tails with
+    which some comets have adorned themselves. The comet of 1744,
+    for instance, had at one time no less than seven tails spread
+    in a wide curved brush behind it. Donati's comet of 1858 also
+    had at least two tails, the principal one sword-shaped and the
+    other long, narrow, and as straight as a rule. According to
+    Bredichin, the straight tail must have been composed of
+    hydrogen, and the other of some form of hydro-carbon whose
+    atoms are heavier than those of hydrogen, and, consequently,
+    when swept away by the storm of light-waves, followed a
+    curvature depending upon the resultant of the forces operating
+    upon them. The seven tails of the comet of 1744 presented a
+    kind of diagram graphically exhibiting its complex composition,
+    and, if we knew a little more about the constituents of a
+    comet, we might be able to say from the amount of curvature of
+    the different tails just what were the seven substances of
+    which that comet consisted.</p>
+
+    <p>If these theories seem to the reader fantastic, at any rate
+    they are no more fantastic than the phenomena that they seek to
+    explain.</p>
+
+    <p><strong>Meteors, Fire-Balls, and Meteorites</strong></p>
+
+    <p>One of the most terrorizing spectacles with which the
+    heavens have ever caused the hearts of men to quake occurred on
+    the night of November 13, 1833. On that night North America,
+    which faced the storm, was under a continual rain of fire from
+    about ten o'clock in the evening until daybreak.</p>
+
+    <p><em>The fragments of a comet had struck the earth.</em></p>
+
+    <p>But the meaning of what had happened was not discovered
+    until long afterward. To the astronomers who, with astonishment
+    not less than that of other people, watched the wonderful
+    scene, it was an unparalleled ``shower of meteors.'' They did
+    not then suspect that those meteors had once formed the head of
+    a comet. Light dawned when, a year later, Prof. Denison
+    Olmsted, of Yale College, demonstrated that the meteors had all
+    moved in parallel orbits around the sun, and that these orbits
+    intersected that of the earth at the point where our planet
+    happened to be on the memorable night of November 13th.
+    Professor Olmsted even went so far as to suggest that the cloud
+    of meteors that had encountered the earth might form a diffuse
+    comet; but full recognition of the fact that they were cometary
+    d&eacute;bris came later, as the result of further
+    investigation. The key to the secret was plainly displayed in
+    the spectacle itself, and was noticed without being understood
+    by thousands of the terror-stricken beholders. It was <em>an
+    umbrella of fire</em> that had opened overhead and covered the
+    heavens; in other words, the meteors all radiated from a
+    particular point in the constellation Leo, and, being countless
+    as the snowflakes in a winter tempest, they ribbed the sky with
+    fiery streaks. Professor Olmsted showed that the radiation of
+    the meteors from a fixed point was an effect of perspective,
+    and in itself a proof that they were moving in parallel paths
+    when they encountered the earth. The fact was noted that there
+    had been a similar, but incomparably less brilliant, display of
+    meteors on the same day of November, 1832, and it was rightly
+    concluded that these had belonged to the same stream, although
+    the true relationship of the phenomena was not immediately
+    apprehended. Olmsted ascribed to the meteors a revolution about
+    the sun once in every six months, bringing them to the
+    intersection of their orbit with that of the earth every
+    November 13th; but later investigators found that the real
+    period was about thirty-three and one-quarter years, so that
+    the great displays were due three times in a century, and their
+    return was confidently predicted for the year 1866. The
+    appearance of the meteors in 1832, a year before the great
+    display, was ascribed to the great length of the stream which
+    they formed in space -- so great that they required more than
+    two years to cross the earth's orbit. In 1832 the earth had
+    encountered a relatively rare part of the stream, but in 1833,
+    on returning to the crossing-place, it found there the richest
+    part of the stream pouring across its orbit. This explanation
+    also proved to be correct, and the predicted return in 1866 was
+    duly witnessed, although the display was much less brilliant
+    than in 1833. It was followed by another in 1867.</p>
+
+    <p>In the mean time Olmsted's idea of a cometary relationship
+    of the meteors was demonstrated to be correct by the researches
+    of Schiaparelli and others, who showed that not only the
+    November meteors, but those of August, which are seen more or
+    less abundantly every year, traveled in the tracks of
+    well-known comets, and had undoubtedly an identical origin with
+    those comets. In other words the comets and the meteor-swarms
+    were both remnants of original masses which had probably been
+    split up by the action of the sun, or of some planet to which
+    they had made close approaches. The annual periodicity of the
+    August meteors was ascribed to the fact that the separation had
+    taken place so long ago that the meteors had become distributed
+    all around the orbit, in consequence of which the earth
+    encountered some of them every year when it arrived at the
+    crossing-point. Then Leverrier showed that the original comet
+    associated with the November meteors was probably brought into
+    the system by the influence of the planet Uranus in the year
+    126 of the Christian era. Afterward Alexander Herschel
+    identified the tracks of no less than seventy-six meteor-swarms
+    (most of them inconspicuous) with those of comets. The still
+    more recent researches of Mr W. F. Denning make it probable
+    that there are no meteors which do not belong to a flock or
+    system probably formed by the disintegration of a cometary
+    mass; even the apparently sporadic ones which shoot across the
+    sky, ``lost souls in the night,'' being members of flocks which
+    have become so widely scattered that the earth sometimes takes
+    weeks to pass through the region of space where their paths
+    lie.</p>
+
+    <p>The November meteors should have exhibited another pair of
+    spectacles in 1899 and 1900, and their failure to do so caused
+    at first much disappointment, until it was made plain that a
+    good reason existed for their absence. It was found that after
+    their last appearance, in 1867, they had been disturbed in
+    their movements by the planets Jupiter and Saturn, whose
+    attractions had so shifted the position of their orbit that it
+    no longer intersected that of the earth, as it did before.
+    Whether another planetary interference will sometime bring the
+    principal mass of the November meteors back to the former point
+    of intersection with the earth's orbit is a question for the
+    future to decide. It would seem that there may be several
+    parallel streams of the November meteors, and that some of
+    them, like those of August, are distributed entirely around the
+    orbit, so that every mid-November we see a few of them.</p>
+
+    <p>We come now to a very remarkable example of the
+    disintegration of a comet and the formation of a meteor-stream.
+    In 1826 Biela, of Josephstadt, Austria, discovered a comet to
+    which his name was given. Calculation showed that it had an
+    orbital period of about six and a half years, belonging to
+    Jupiter's ``family.'' On one of its returns, in 1846, it
+    astonished its watchers by suddenly splitting in two. The two
+    comets thus formed out of one separated to a distance of about
+    one hundred and sixty thousand miles, and then raced side by
+    side, sometimes with a curious ligature connecting them, like
+    Siamese twins, until they disappeared together in
+    interplanetary space. In 1852 they came back, still nearly side
+    by side, but now the distance between them had increased to a
+    million and a quarter of miles. After that, at every recurrence
+    of their period, astronomers looked for them in vain, until
+    1872, when an amazing thing happened. On the night of November
+    28th, when the earth was crossing the plane of the orbit of the
+    missing comet, a brilliant shower of meteors burst from the
+    northern sky, traveling nearly in the track which the comet
+    should have pursued. The astronomers were electrified.
+    Klinkerfues, of G&ouml;ttingen, telegraphed to Pogson, of
+    Madras: <em>``Biela touched earth; search near Theta
+    Centauri.''</em> Pogson searched in the place indicated and saw
+    a cometary mass retreating into the southern heavens, where it
+    was soon swallowed from sight!</p>
+
+    <p>Since then the Biela meteors have been among the recognized
+    periodic spectacles of the sky, and few if any doubt that they
+    represent a portion of the missing comet whose disintegration
+    began with the separation into two parts in 1846. The comet
+    itself has never since been seen. The first display of these
+    meteors, sometimes called the ``Andromedes,'' because they
+    radiate from the constellation Andromeda, was remarkable for
+    the great brilliancy of many of the fire-balls that shot among
+    the shower of smaller sparks, some of which were described as
+    equaling the full moon in size. None of them is known to have
+    reached the earth, but during the display of the same meteors
+    in 1885 a meteoric mass fell at Mazapil in Northern Mexico (it
+    is now in the Museum at Vienna), which many have thought may
+    actually be a piece of the original comet of Biela. This brings
+    us to the second branch of our subject.</p>
+
+    <p>More rare than meteors or falling stars, and more startling,
+    except that they never appear in showers, are the huge balls of
+    fire which occasionally dart through the sky, lighting up the
+    landscapes beneath with their glare, leaving trains of sparks
+    behind them, often producing peals of thunder when they
+    explode, and in many cases falling upon the earth and burying
+    themselves from a few inches to several feet in the soil, from
+    which, more than once, they have been picked up while yet hot
+    and fuming. These balls are sometimes called bolides. They are
+    not really round in shape, although they often look so while
+    traversing the sky, but their forms are fragmentary, and
+    occasionally fantastic. It has been supposed that their origin
+    is different from that of the true meteors; it has even been
+    conjectured that they may have originated from the giant
+    volcanoes of the moon or have been shot out from the sun during
+    some of the tremendous explosions that accompany the formation
+    of eruptive prominences. By the same reasoning some of them
+    might be supposed to have come from some distant star. Others
+    have conjectured that they are wanderers in space, of unknown
+    origin, which the earth encounters as it journeys on, and Lord
+    Kelvin made a suggestion which has become classic because of
+    its imaginative reach -- <em>viz.,</em> that the first germs of
+    life may have been brought to the earth by one of these bodies,
+    ``a fragment of an exploded world.''</p>
+
+    <p>It is a singular fact that astronomers and scientific men in
+    general were among the last to admit the possibility of solid
+    masses falling from the sky. The people had believed in the
+    reality of such phenomena from the earliest times, but the
+    savants shook their heads and talked of superstition. This was
+    the less surprising because no scientifically authenticated
+    instance of such an occurrence was known, and the stones
+    popularly believed to have fallen from the sky had become the
+    objects of worship or superstitious reverence, a fact not
+    calculated to recommend them to scientific credence. The
+    celebrated ``black stone'' suspended in the Kaaba at Mecca is
+    one of these reputed gifts from heaven; the ``Palladium'' of
+    ancient Troy was another; and a stone which fell near
+    Ensisheim, in Germany, was placed in a church as an object to
+    be religiously venerated. Many legends of falling stones
+    existed in antiquity, some of them curiously transfigured by
+    the imagination, like the ``Lion of the Peloponnesus,'' which
+    was said to have sprung down from the sky upon the Isthmus of
+    Corinth. But near the beginning of the nineteenth century, in
+    1803, a veritable shower of falling stones occurred at L'Aigle,
+    in Northern France, and this time astronomers took note of the
+    phenomenon and scientifically investigated it. Thousands of the
+    strange projectiles came from the sky on this occasion, and
+    were scattered over a wide area of country, and some buildings
+    were hit. Four years later another shower of stones occurred at
+    Weston, Conn., numbering thousands of individuals. The local
+    alarm created in both cases was great, as well it might be, for
+    what could be more intimidating than to find the blue vault of
+    heaven suddenly hurling solid missiles at the homes of men?
+    After these occurrences it was impossible for the most
+    skeptical to doubt any longer, and the regular study of
+    ``aerolites,'' or ``meteorites,'' began.</p>
+
+    <p>One of the first things recognized was the fact that
+    fire-balls are solid meteorites in flight, and not gaseous
+    exhalations in the air, as some had assumed. They burn in the
+    air during their flight, and sometimes, perhaps, are entirely
+    consumed before reaching the ground. Their velocity before
+    entering the earth's atmosphere is equal to that of the planets
+    in their orbits -- <em>viz.,</em> from twenty to thirty miles
+    per second -- a fact which proves that the sun is the seat of
+    the central force governing them. Their burning in the air is
+    not difficult to explain; it is the heat of friction which so
+    quickly brings them to incandescence. Calculation shows that a
+    body moving through the air at a velocity of about a mile per
+    second will be brought, superficially, to the temperature of
+    ``red heat'' by friction with the atmosphere. If its velocity
+    is twenty miles per second the temperature will become
+    thousands of degrees. This is the state of affairs with a
+    meteorite rushing into the earth's atmosphere; its surface is
+    liquefied within a few seconds after the friction begins to
+    act, and the melted and vaporized portion of its mass is swept
+    backward, forming the train of sparks that follows every great
+    fire-ball. However, there is one phenomenon connected with the
+    trains of meteorites which has never been satisfactorily
+    explained: they often persist for long periods of time,
+    drifting and turning with the wind, but not ceasing to glow
+    with a phosphorescent luminosity. The question is, Whence comes
+    this light? It must be light without heat, since the fine dust
+    or vapor of which the train can only consist would not retain
+    sufficient heat to render it luminous for so long a time. An
+    extremely remarkable incident of this kind occurred on February
+    22, 1909, when an immense fire-ball that passed over southern
+    England left a train that remained visible during two hours,
+    assuming many curious shapes as it was drifted about by
+    currents in the air.</p>
+
+    <p>But notwithstanding the enormous velocity with which
+    meteorites enter the air they are soon slowed down to
+    comparatively moderate speed, so that when they disappear they
+    are usually traveling not faster than a mile a second. The
+    courses of many have been traced by observers situated along
+    their track at various points, and thus a knowledge has been
+    obtained of their height above the ground during their flight
+    and of the length of their visible courses. They generally
+    appear at an elevation of eighty or a hundred miles, and are
+    seldom visible after having descended to within five miles of
+    the ground, unless the observer happens to be near the
+    striking-point, when he may actually witness the fall.
+    Frequently they burst while high in the air and their fragments
+    are scattered like shrapnel over the surface of the ground,
+    sometimes covering an area of several square miles, but of
+    course not thickly; different fragments of the same meteorite
+    may reach the ground at points several miles apart. The
+    observed length of their courses in the atmosphere varies from
+    fifty to five hundred miles. If they continued a long time in
+    flight after entering the air, even the largest of them would
+    probably be consumed to the last scrap, but their fiery career
+    is so short on account of their great speed that the heat does
+    not have time to penetrate very deeply, and some that have been
+    picked up immediately after their fall have been found cold as
+    ice within. Their size after reaching the ground is variable
+    within wide limits; some are known which weigh several tons,
+    but the great majority weigh only a few pounds and many only a
+    few ounces.</p>
+
+    <p>Meteorites are of two kinds: <em>stony</em> meteorites and
+    <em>iron</em> meteorites. The former outnumber the latter
+    twenty to one; but many stone meteorites contain grains of
+    iron. Nickel is commonly found in iron meteorites, so that it
+    might be said that that redoubtable alloy nickel-steel is of
+    cosmical invention. Some twenty-five chemical elements have
+    been found in meteorites, including carbon and the
+    ``sun-metal,'' helium. The presence of the latter is certainly
+    highly suggestive in connection with the question of the origin
+    of meteorites. The iron meteorites, besides metallic iron and
+    nickel, of which they are almost entirely composed, contain
+    hydrogen, helium, and carbonic oxide, and about the only
+    imaginable way in which these gases could have become absorbed
+    in the iron would be through the immersion of the latter while
+    in a molten or vaporized state in a hot and dense atmosphere
+    composed of them, a condition which we know to exist only in
+    the envelopes of the sun and the stars.</p>
+
+    <p>The existence of carbon in the Canyon Diablo iron meteorites
+    is attended by a circumstance of the most singular character --
+    a very ``fairy tale of science.'' In some cases <em>the carbon
+    has become diamond!</em> These meteoric diamonds are very
+    small; nevertheless, they are true diamonds, resembling in many
+    ways the little black gems produced by Moissan's method with
+    the aid of the electric furnace. The fact that they are found
+    embedded in these iron meteorites is another argument in favor
+    of the hypothesis of the solar or stellar origin of the latter.
+    To appreciate this it is necessary to recall the way in which
+    Moissan made his diamonds. It was by a combination of the
+    effects of great heat, great pressure, and sudden or rapid
+    superficial cooling on a mass of iron containing carbon. When
+    he finally broke open his iron he found it a pudding stuffed
+    with miniature black diamonds. When a fragment of the Canyon
+    Diablo meteoric iron was polished in Philadelphia over fifteen
+    years ago it cut the emery-wheel to pieces, and examination
+    showed that the damage had been effected by microscopic
+    diamonds peppered through the mass. How were those diamonds
+    formed? If the sun or Sirius was the laboratory that prepared
+    them, we can get a glimpse at the process of their formation.
+    There is plenty of heat, plenty of pressure, and an abundance
+    of vaporized iron in the sun and the stars. When a great solar
+    eruption takes place, masses of iron which have absorbed carbon
+    may be shot out with a velocity which forbids their return.
+    Plunged into the frightful cold of space, their surfaces are
+    quickly cooled, as Moissan cooled his prepared iron by throwing
+    it into water, and thus the requisite stress is set up within,
+    and, as the iron solidifies, the included carbon crystallizes
+    into diamonds. Whether this explanation has a germ of truth in
+    it or not, at any rate it is evident that iron meteorites were
+    not created in the form in which they come to us; they must
+    once have been parts of immeasurably more massive bodies than
+    themselves.</p>
+
+    <p>The fall of meteorites offers an appreciable, though
+    numerically insignificant, peril to the inhabitants of the
+    earth. Historical records show perhaps three or four instances
+    of people being killed by these bodies. But for the protection
+    afforded by the atmosphere, which acts as a very effective
+    shield, the danger would doubtless be very much greater. In the
+    absence of an atmosphere not only would more meteorites reach
+    the ground, but their striking force would be incomparably
+    greater, since, as we have seen, the larger part of their
+    original velocity is destroyed by the resistance of the air. A
+    meteorite weighing many tons and striking the earth with a
+    velocity of twenty or thirty miles per second, would probably
+    cause frightful havoc.</p>
+
+    <p>It is a singular fact that recent investigations seem to
+    have proved that an event of this kind actually happened in
+    North America -- perhaps not longer than a thousand or two
+    thousand years ago. The scene of the supposed catastrophe is in
+    northern central Arizona, at Coon Butte, where there is a
+    nearly circular crater in the middle of a circular elevation or
+    small mountain. The crater is somewhat over four thousand feet
+    in diameter, and the surrounding rim, formed of upturned strata
+    and ejected rock fragments, rises at its highest point one
+    hundred and sixty feet above the plain. The crater is about six
+    hundred feet in depth -- that is, from the rim to the visible
+    floor or bottom of the crater. There is no evidence that
+    volcanic action has ever taken place in the immediate
+    neighborhood of Coon Butte. The rock in which the crater has
+    been made is composed of horizontal sandstone and limestone
+    strata. Between three hundred and four hundred million tons of
+    rock fragments have been detached, and a large portion hurled
+    by some cause out of the crater. These fragments lie
+    concentrically distributed around the crater, and in large
+    measure form the elevation known as Coon Butte. The region has
+    been famous for nearly twenty years on account of the masses of
+    meteoric iron found scattered about and known as the ``Canyon
+    Diablo'' meteorites. It was one of these masses, which consist
+    of nickel-iron containing a small quantity of platinum, and of
+    which in all some ten tons have been recovered for sale to the
+    various collectors throughout the world, that as before
+    mentioned destroyed the grinding-tool at Philadelphia through
+    the cutting power of its embedded diamonds. These meteoric
+    irons are scattered about the crater-hill, in concentric
+    distribution, to a maximum distance of about five miles. When
+    the suggestion was first made in 1896 that a monster meteorite
+    might have created by its fall this singular lone crater <em>in
+    stratified rocks,</em> it was greeted with incredulous smiles;
+    but since then the matter has assumed a different aspect. The
+    Standard Iron Company, formed by Messrs. D. M. Barringer, B. C.
+    Tilghman, E. J. Bennitt, and S. J. Holsinger, having become, in
+    1903, the owner of this freak of nature, sunk shafts and bored
+    holes to a great depth in the interior of the crater, and also
+    trenched the slopes of the mountain, and the result of their
+    investigations has proved that the meteoric hypothesis of
+    origin is correct. (See the papers published in the
+    <em>Proceedings of the Academy of Natural Sciences of
+    Philadelphia,</em> December, 1905, wherein it is proved that
+    the United States Geological Survey was wrong in believing this
+    crater to have been due to a steam explosion. Since that date
+    there has been discovered a great amount of additional
+    confirmatory proof). Material of unmistakably meteoric origin
+    was found by means of the drills, mixed with crushed rock, to a
+    depth of six hundred to seven hundred feet below the floor of
+    the crater, and a great deal of it has been found admixed with
+    the ejected rock fragments on the outer slopes of the mountain,
+    absolutely proving synchronism between the two events, the
+    formation of this great crater and the falling of the meteoric
+    iron out of the sky. The drill located in the bottom of the
+    crater was sent, in a number of cases, much deeper (over one
+    thousand feet) into unaltered horizontal red sandstone strata,
+    but no meteoric material was found below this depth (seven
+    hundred feet, or between eleven and twelve hundred feet below
+    the level of the surrounding plain), which has been assumed as
+    being about the limit of penetration. It is not possible to
+    sink a shaft at present, owing to the water which has drained
+    into the crater, and which forms, with the finely pulverized
+    sandstone, a very troublesome quicksand encountered at about
+    two hundred feet below the visible floor of the crater. As soon
+    as this water is removed by pumping it will be easy to explore
+    the depths of the crater by means of shafts and drifts. The
+    rock strata (sandstone and limestone) of which the walls
+    consist present every appearance of having been violently
+    upturned by a huge body penetrating the earth like a
+    cannon-ball. The general aspect of the crater strikingly
+    resembles the impression made by a steel projectile shot into
+    an armor-plate. Mr Tilghman has estimated that a meteorite
+    about five hundred feet in diameter and moving with a velocity
+    of about five miles per second would have made just such a
+    perforation upon striking rocks of the character of those found
+    at this place. There was some fusion of the colliding masses,
+    and the heat produced some steam from the small amount of water
+    in the rocks. As a result there has been found at depth a
+    considerable amount of fused quartz (original sandstone), and
+    with it innumerable particles or sparks of fused nickel-iron
+    (original meteorite). A projectile of that size penetrating
+    eleven to twelve hundred feet into the rocky shell of the globe
+    must have produced a shock which was perceptible several
+    hundred miles away.</p>
+
+    <p>The great velocity ascribed to the supposed meteorite at the
+    moment of striking could be accounted for by the fact that it
+    probably plunged nearly vertically downward, for it formed a
+    circular crater in the rocky crust of the earth. In that case
+    it would have been less retarded by the resistance of the
+    atmosphere than are meteorites which enter the air at a lower
+    angle and shoot ahead hundreds of miles until friction has
+    nearly destroyed their original motion when they drop upon the
+    earth. Some meteoric masses of great size, such as Peary's iron
+    meteorite found at Cape York, Greenland, and the almost equally
+    large mass discovered at Bacubirito, Mexico, appear to have
+    penetrated but slightly on striking the earth. This may be
+    explained by supposing that they pursued a long, horizontal
+    course through the air before falling. The result would be
+    that, their original velocity having been practically
+    destroyed, they would drop to the ground with a velocity nearly
+    corresponding to that which gravity would impart within the
+    perpendicular distance of their final fall. A
+    six-hundred-and-sixty-pound meteorite, which fell at Knyahinya,
+    Hungary, striking at an angle of 27&deg; from the vertical,
+    penetrated the ground to a depth of eleven feet.</p>
+
+    <p>It has been remarked that the Coon Butte meteorite may have
+    fallen not longer ago than a few thousand years. This is based
+    upon the fact that the geological indications favor the
+    supposition that the event did not occur more than five
+    thousand years ago, while on the other hand the rings of growth
+    in the cedar-trees growing on the slopes of the crater show
+    that they have existed there about seven hundred years. Prof.
+    William H. Pickering has recently correlated this with an
+    ancient chronicle which states that at Cairo, Egypt, in the
+    year 1029, ``many stars passed with a great noise.'' He remarks
+    that Cairo is about 100&deg;, by great circle, from Coon Butte,
+    so that if the meteorite that made the crater was a member of a
+    flock of similar bodies which encountered the earth moving in
+    parallel lines, some of them might have traversed the sky
+    tangent to the earth's surface at Cairo. That the spectacle
+    spoken of in the chronicle was caused by meteorites he deems
+    exceedingly probable because of what is said about ``a great
+    noise;'' meteorites are the only celestial phenomena attended
+    with perceptible sounds. Professor Pickering conjectures that
+    this supposed flock of great meteorites may have formed the
+    nucleus of a comet which struck the earth, and he finds
+    confirmation of the idea in the fact that out of the ten
+    largest meteorites known, no less than seven were found within
+    nine hundred miles of Coon Butte. It would be interesting if we
+    could trace back the history of that comet, and find out what
+    malicious planet caught it up in its innocent wanderings and
+    hurled it with so true an aim at the earth! This remarkable
+    crater is one of the most interesting places in the world, for
+    there is absolutely no record of such a mass, possibly an
+    iron-headed comet, from outer space having come into collision
+    with our earth. The results of the future exploration of the
+    depths of the crater will be awaited with much interest.</p>
+
+    <p><strong>The Wrecking of the Moon</strong></p>
+
+    <p>There are sympathetic moods under whose influence one gazes
+    with a certain poignant tenderness at the worn face of the
+    moon; that little ``fossil world'' (the child of our mother
+    earth, too) bears such terrible scars of its brief convulsive
+    life that a sense of pity is awakened by the sight. The moon is
+    the wonder-land of the telescope. Those towering mountains,
+    whose ``proud aspiring peaks'' cast silhouettes of shadow that
+    seem drawn with india-ink; those vast plains, enchained with
+    gentle winding hills and bordered with giant ranges; those oval
+    ``oceans,'' where one looks expectant for the flash of
+    wind-whipped waves; those enchanting ``bays'' and recesses at
+    the seaward feet of the Alps; those broad straits passing
+    between guardian heights incomparably mightier than Gibraltar;
+    those locket-like valleys as secluded among their mountains as
+    the Vale of Cashmere; those colossal craters that make us smile
+    at the pretensions of Vesuvius, Etna, and Cotopaxi; those
+    strange white ways which pass with the unconcern of Roman roads
+    across mountain, gorge, and valley -- all these give the
+    beholder an irresistible impression that it is truly a world
+    into which he is looking, a world akin to ours, and yet no more
+    like our world than Pompeii is like Naples. Its air, its
+    waters, its clouds, its life are gone, and only a skeleton
+    remains -- a mute but eloquent witness to a cosmical tragedy
+    without parallel in the range of human knowledge.</p>
+
+    <p>One cannot but regret that the moon, if it ever was the seat
+    of intelligent life, has not remained so until our time. Think
+    what the consequences would have been if this other world at
+    our very door had been found to be both habitable and
+    inhabited! We talk rather airily of communicating with Mars by
+    signals; but Mars never approaches nearer than 35,000,000
+    miles, while the moon when nearest is only a little more than
+    220,000 miles away. Given an effective magnifying power of five
+    thousand diameters, which will perhaps be possible at the
+    mountain observatories as telescopes improve, and we should be
+    able to bring the moon within an apparent distance of about
+    forty miles, while the corresponding distance for Mars would be
+    more than seven thousand miles. But even with existing
+    telescopic powers we can see details on the moon no larger than
+    some artificial constructions on the earth. St Peter's at Rome,
+    with the Vatican palace and the great piazza, if existing on
+    the moon, would unquestionably be recognizable as something
+    else than a freak of nature. Large cities, with their radiating
+    lines of communication, would at once betray their real
+    character. Cultivated tracts, and the changes produced by the
+    interference of intelligent beings, would be clearly
+    recognizable. The electric illumination of a large town at
+    night would probably be markedly visible. Gleams of reflected
+    sunlight would come to us from the surfaces of the lakes and
+    oceans, and a huge ``liner'' traversing a lunar sea could
+    probably be followed by its trail of smoke. As to
+    communications by ``wireless'' signals, which certain
+    enthusiasts have thought of in connection with Mars, in the
+    case of the moon they should be a relatively simple matter, and
+    the feat might actually be accomplished. Think what a
+    literature would grow up about the moon if it were a living
+    world! Its very differences from the earth would only
+    accentuate its interest for us. Night and day on the moon are
+    each two weeks in length; how interesting it would be to watch
+    the manner in which the lunarians dealt with such a situation
+    as that. Lunar and terrestrial history would keep step with
+    each other, and we should record them both. Truly one might
+    well wish to have a neighbor world to study; one would feel so
+    much the less alone in space.</p>
+
+    <p>It is not impossible that the moon did at one time have
+    inhabitants of some kind. But, if so, they vanished with the
+    disappearance of its atmosphere and seas, or with the advent of
+    its cataclysmic age. At the best, its career as a living world
+    must have been brief. If the water and air were gradually
+    absorbed, as some have conjectured, by its cooling interior
+    rocks, its surface might, nevertheless, have retained them for
+    long ages; but if, as others think, their disappearance was due
+    to the escape of their gaseous molecules in consequence of the
+    inability of the relatively small lunar gravitation to retain
+    them, then the final catastrophe must have been as swift as it
+    was inevitable. Accepting Darwin's hypothesis, that the moon
+    was separated from the earth by tidal action while both were
+    yet plastic or nebulous, we may reasonably conclude that it
+    began its career with a good supply of both water and air, but
+    did not possess sufficient mass to hold them permanently. Yet
+    it may have retained them long enough for life to develop in
+    many forms upon its surface; in fact, there are so many
+    indications that air and water have not always been lacking to
+    the lunar world that we are driven to invent theories to
+    explain both their former presence and their present
+    absence.</p>
+
+    <p>But whatever the former condition of the moon may have been,
+    its existing appearance gives it a resistless fascination, and
+    it bears so clearly the story of a vast catastrophe sculptured
+    on its rocky face that the thoughtful observer cannot look upon
+    it without a feeling of awe. The gigantic character of the
+    lunar features impresses the beholder not less than the
+    universality of the play of destructive forces which they
+    attest. Let us make a few comparisons. Take the lunar crater
+    called ``Tycho'', which is a typical example of its kind. In
+    the telescope Tycho appears as a perfect ring surrounding a
+    circular depression, in the center of which rises a group of
+    mountains. Its superficial resemblance to some terrestrial
+    volcanic craters is very striking. Vesuvius, seen from a point
+    vertically above, would no doubt look something like that (the
+    resemblance would have been greater when the Monte del Cavallo
+    formed a more complete circuit about the crater cone). But
+    compare the dimensions. The remains of the outer crater ring of
+    Vesuvius are perhaps half a mile in diameter, while the active
+    crater itself is only two or three hundred feet across at the
+    most; Tycho has a diameter of fifty-four miles! The group of
+    relatively insignificant peaks in the center of the crater
+    floor of Tycho is far more massive than the entire mountain
+    that we call Vesuvius. The largest known volcanic crater on the
+    earth, Aso San, in Japan, has a diameter of seven miles; it
+    would take <em>sixty</em> craters like Aso San to equal Tycho
+    in area! And Tycho, though one of the most perfect, is by no
+    means the largest crater on the moon. Another, called
+    ``Theophilus,'' has a diameter of sixty-four miles, and is
+    eighteen thousand feet deep. There are hundreds from ten to
+    forty miles in diameter, and thousands from one to ten miles.
+    They are so numerous in many places that they break into one
+    another, like the cells of a crushed honeycomb.</p>
+
+    <p>The lunar craters differ from those of the earth more
+    fundamentally than in the matter of mere size; <em>they are not
+    situated on the tops of mountains.</em> If they were, and if
+    all the proportions were the same, a crater like Tycho might
+    crown a conical peak fifty or one hundred miles high! Instead
+    of being cavities in the summits of mountains, the lunar
+    craters are rather gigantic sink-holes whose bottoms in many
+    cases lie two or three miles below the general surface of the
+    lunar world. Around their rims the rocks are piled up to a
+    height of from a few hundred to two or three thousand feet,
+    with a comparatively gentle inclination, but on the inner side
+    they fall away in gigantic broken precipices which make the
+    dizzy cliffs of the Matterhorn seem but ``lover's leaps.'' Down
+    they drop, ridge below ridge, crag under crag, tottering wall
+    beneath wall, until, in a crater named ``Newton,'' near the
+    south lunar pole, they attain a depth where the rays of the sun
+    never reach. Nothing more frightful than the spectacle which
+    many of these terrible chasms present can be pictured by the
+    imagination. As the lazy lunar day slowly advances, the
+    sunshine, unmitigated by clouds or atmospheric veil of any
+    kind, creeps across their rims and begins to descend the
+    opposite walls. Presently it strikes the ragged crest of a
+    ridge which had lain hidden in such darkness as we never know
+    on the earth, and runs along it like a line of kindling fire.
+    Rocky pinnacles and needles shoot up into the sunlight out of
+    the black depths. Down sinks the line of light, mile after
+    mile, and continually new precipices and cliffs are brought
+    into view, until at last the vast floor is attained and begins
+    to be illuminated. In the meanwhile the sun's rays, darting
+    across the gulf, have touched the summits of the central peaks,
+    twenty or thirty miles from the crater's inmost edge, and they
+    immediately kindle and blaze like huge stars amid the darkness.
+    So profound are some of these awful craters that days pass
+    before the sun has risen high enough above them to chase the
+    last shadows from their depths.</p>
+
+    <p>Although several long ranges of mountains resembling those
+    of the earth exist on the moon, the great majority of its
+    elevations assume the crateriform aspect. Sometimes, instead of
+    a crater, we find an immense mountain ring whose form and
+    aspect hardly suggest volcanic action. But everywhere the true
+    craters are in evidence, even on the sea-beds, although they
+    attain their greatest number and size on those parts of the
+    moon -- covering sixty per cent of its visible surface -- which
+    are distinctly mountainous in character and which constitute
+    its most brilliant portions. Broadly speaking, the southwestern
+    half of the moon is the most mountainous and broken, and the
+    northeastern half the least so. Right down through the center,
+    from pole to pole, runs a wonderful line of craters and
+    crateriform valleys of a magnitude stupendous even for the
+    moon. Another similar line follows the western edge. Three or
+    four ``seas'' are thrust between these mountainous belts. By
+    the effects of ``libration'' parts of the opposite hemisphere
+    of the moon which is turned away from the earth are from time
+    to time brought into view, and their aspect indicates that that
+    hemisphere resembles in its surface features the one which
+    faces the earth. There are many things about the craters which
+    seem to give some warrant for the hypothesis which has been
+    particularly urged by Mr G. K. Gilbert, that they were formed
+    by the impact of meteors; but there are also many things which
+    militate against that idea, and, upon the whole, the volcanic
+    theory of their origin is to be preferred.</p>
+
+    <p>The enormous size of the lunar volcanoes is not so difficult
+    to account for when we remember how slight is the force of
+    lunar gravity as compared with that of the earth. With equal
+    size and density, bodies on the moon weigh only one-sixth as
+    much as on the earth. Impelled by the same force, a projectile
+    that would go ten miles on the earth would go sixty miles on
+    the moon. A lunar giant thirty-five feet tall would weigh no
+    more than an ordinary son of Adam weighs on his greater planet.
+    To shoot a body from the earth so that it would not drop back
+    again, we should have to start it with a velocity of seven
+    miles per second; a mile and a half per second would serve on
+    the moon. It is by no means difficult to believe, then, that a
+    lunar volcano might form a crater ring eight or ten times
+    broader than the greatest to be found on the earth, especially
+    when we reflect that in addition to the relatively slight force
+    of gravity, the materials of the lunar crust are probably
+    lighter than those of our terrestrial rocks.</p>
+
+    <p>For similar reasons it seems not impossible that the theory
+    mentioned in a former chapter -- that some of the meteorites
+    that have fallen upon the earth originated from the lunar
+    volcanoes -- is well founded. This would apply especially to
+    the stony meteorites, for it is hardly to be supposed that the
+    moon, at least in its superficial parts, contains much iron. It
+    is surely a scene most strange that is thus presented to the
+    mind's eye -- that little attendant of the earth's (the moon
+    has only one-fiftieth of the volume, and only one-eightieth of
+    the mass of the earth) firing great stones back at its parent
+    planet! And what can have been the cause of this furious
+    outbreak of volcanic forces on the moon? Evidently it was but a
+    passing stage in its history; it had enjoyed more quiet times
+    before. As it cooled down from the plastic state in which it
+    parted from the earth, it became incrusted after the normal
+    manner of a planet, and then oceans were formed, its atmosphere
+    being sufficiently dense to prevent the water from evaporating
+    and the would-be oceans from disappearing continually in mist.
+    This, if any, must have been the period of life in the lunar
+    world. As we look upon the vestiges of that ancient world
+    buried in the wreck that now covers so much of its surface, it
+    is difficult to restrain the imagination from picturing the
+    scenes which were once presented there; and, in such a case,
+    should the imagination be fettered? We give it free rein in
+    terrestrial life, and it rewards us with some of our greatest
+    intellectual pleasures. The wonderful landscapes of the moon
+    offer it an ideal field with just enough half-hidden
+    suggestions of facts to stimulate its powers.</p>
+
+    <p>The great plains of the <em>Mare Imbrium</em> and the
+    <em>Mare Serenitatis</em> (the ``Sea of Showers'' and the ``Sea
+    of Serenity''), bordered in part by lofty mountain ranges
+    precisely like terrestrial mountains, scalloped along their
+    shores with beautiful bays curving back into the adjoining
+    highlands, and united by a great strait passing between the
+    nearly abutting ends of the ``Lunar Apennines'' and the ``Lunar
+    Caucasus,'' offer the elements of a scene of world beauty such
+    as it would be difficult to match upon our planet. Look at the
+    finely modulated bottom of the ancient sea in Mr Ritchey's
+    exquisite photograph of the western part of the <em>Mare
+    Serenitatis,</em> where one seems to see the play of the watery
+    currents heaping the ocean sands in waving lines, making
+    shallows, bars, and deeps for the mariner to avoid or seek, and
+    affording a playground for the creatures of the main. What
+    geologist would not wish to try his hammer on those rocks with
+    their stony pages of fossilized history? There is in us an
+    instinct which forbids us to think that there was never any
+    life there. If we could visit the moon, there is not among us a
+    person so prosaic and unimaginative that he would not, the very
+    first thing, begin to search for traces of its inhabitants. We
+    would look for them in the deposits on the sea bottoms; we
+    would examine the shores wherever the configuration seemed
+    favorable for harbors and the sites of maritime cities --
+    forgetting that it may be a little ridiculous to ascribe to the
+    ancient lunarians the same ideas that have governed the
+    development of our race; we would search through the valleys
+    and along the seeming courses of vanished streams; we would
+    explore the mountains, not the terrible craters, but the
+    pinnacled chains that recall our own Alps and Rockies; seeking
+    everywhere some vestige of the transforming presence of
+    intelligent life. Perhaps we should find such traces, and
+    perhaps, with all our searching, we should find nothing to
+    suggest that life had ever existed amid that universal
+    ruin.</p>
+
+    <p>Look again at the border of the ``Sea of Serenity'' -- what
+    a name for such a scene! -- and observe how it has been rent
+    with almost inconceivable violence, the wall of the colossal
+    crater Posidonius dropping vertically upon the ancient shore
+    and obliterating it, while its giant neighbor, Le Monnier,
+    opens a yawning mouth as if to swallow the sea itself. A scene
+    like this makes one question whether, after all, those may not
+    be right who have imagined that the so-called sea bottoms are
+    really vast plains of frozen lava which gushed up in floods so
+    extensive that even the mighty volcanoes were half drowned in
+    the fiery sea. This suggestion becomes even stronger when we
+    turn to another of the photographs of Mr Ritchey's wonderful
+    series, showing a part of the <em>Mare Tranquilitatis</em>
+    (``Sea of Tranquility''!). Notice how near the center of the
+    picture the outline of a huge ring with radiating ridges shows
+    through the sea bottom; a fossil volcano submerged in a
+    petrified ocean! This is by no means the only instance in which
+    a buried world shows itself under the great lunar plains. Yet,
+    as the newer craters in the sea itself prove, the volcanic
+    activity survived this other catastrophe, or broke out again
+    subsequently, bringing more ruin to pile upon ruin.</p>
+
+    <p>Yet notwithstanding the evidence which we have just been
+    considering in support of the hypothesis that the ``seas'' are
+    lava floods, Messrs. Loewy and Puiseux, the selenographers of
+    the Paris Observatory, are convinced that these great plains
+    bear characteristic marks of the former presence of immense
+    bodies of water. In that case we should be forced to conclude
+    that the later oceans of the moon lay upon vast sheets of
+    solidified lava; and thus the catastrophe of the lunar world
+    assumes a double aspect, the earliest oceans being swallowed up
+    in molten floods issuing from the interior, while the lands
+    were reduced to chaos by a universal eruption of tremendous
+    volcanoes; and then a period of comparative quiet followed,
+    during which new seas were formed, and new life perhaps began
+    to flourish in the lunar world, only to end in another
+    cataclysm, which finally put a term to the existence of the
+    moon as a life-supporting world.</p>
+
+    <p>Suppose we examine two more of Mr Ritchey's illuminating
+    photographs, and, first, the one showing the crater Theophilus
+    and its surroundings. We have spoken of Theophilus before,
+    citing the facts that it is sixty-four miles in diameter and
+    eighteen thousand feet deep. It will be noticed that it has two
+    brother giants -- Cyrillus the nearer, and Catharina the more
+    distant; but Theophilus is plainly the youngest of the trio.
+    Centuries, and perhaps thousands of years, must have elapsed
+    between the periods of their upheaval, for the two older
+    craters are partly filled with d&eacute;bris, while it is
+    manifest at a glance that when the south eastern wall of
+    Theophilus was formed, it broke away and destroyed a part of
+    the more ancient ring of Cyrillus. There is no more tremendous
+    scene on the moon than this; viewed with a powerful telescope,
+    it is absolutely appalling.</p>
+
+    <p>The next photograph shows, if possible, a still wilder
+    region. It is the part of the moon lying between Tycho and the
+    south pole. Tycho is seen in the lower left-hand part of the
+    picture. To the right, at the edge of the illuminated portion
+    of the moon, are the crater-rings, Longomontanus and Wilhelm I,
+    the former being the larger. Between them are to be seen the
+    ruins of two or three more ancient craters which, together with
+    portions of the walls of Wilhelm I and Longomontanus, have been
+    honeycombed with smaller craters. The vast crateriform
+    depression above the center of the picture is Clavius, an
+    unrivaled wonder of lunar scenery, a hundred and forty-two
+    miles in its greatest length, while its whole immense floor has
+    sunk two miles below the general surface of the moon outside
+    the ring. The monstrous shadow-filled cavity above Clavius
+    toward the right is Blancanus, whose aspect here gives a good
+    idea of the appearance of these chasms when only their rims are
+    in the sunlight. But observe the indescribable savagery of the
+    entire scene. It looks as though the spirit of destruction had
+    gone mad in this spot. The mighty craters have broken forth one
+    after another, each rending its predecessor; and when their
+    work was finished, a minor but yet tremendous outbreak
+    occurred, and the face of the moon was gored and punctured with
+    thousands of smaller craters. These relatively small craters
+    (small, however, only in a lunar sense, for many of them would
+    appear gigantic on the earth) recall once more the theory of
+    meteoric impact. It does not seem impossible that some of them
+    may have been formed by such an agency.</p>
+
+    <p>One would not wish for our planet such a fate as that which
+    has overtaken the moon, but we cannot be absolutely sure that
+    something of the kind may not be in store for it. We really
+    know nothing of the ultimate causes of volcanic activity, and
+    some have suggested that the internal energies of the earth may
+    be accumulating instead of dying out, and may never yet have
+    exhibited their utmost destructive power. Perhaps the best
+    assurance that we can find that the earth will escape the
+    catastrophe that has overtaken its satellite is to be found in
+    the relatively great force of its gravitation. The moon has
+    been the victim of its weakness; given equal forces, and the
+    earth would be the better able to withstand them. It is
+    significant, in connection with these considerations, that the
+    little planet Mercury, which seems also to have parted with its
+    air and water, shows to the telescope some indications that it
+    is pitted with craters resembling those that have torn to
+    pieces the face of the moon.</p>
+
+    <p>Upon the whole, after studying the dreadful lunar
+    landscapes, one cannot feel a very enthusiastic sympathy with
+    those who are seeking indications of the continued existence of
+    some kind of life on the moon; such a world is better without
+    inhabitants. It has met its fate; let it go! Fortunately, it is
+    not so near that it cannot hide its scars and appear beautiful
+    -- except when curiosity impels us to look with the penetrating
+    eyes of the astronomer.</p>
+
+    <p><strong>The Great Mars Problem</strong></p>
+
+    <p>Let any thoughtful person who is acquainted with the general
+    facts of astronomy look up at the heavens some night when they
+    appear in their greatest splendor, and ask himself what is the
+    strongest impression that they make upon his mind. He may not
+    find it easy to frame an answer, but when he has succeeded it
+    will probably be to the effect that the stars give him an
+    impression of the universality of intelligence; they make him
+    feel, as the sun and the moon cannot do, that his world is not
+    alone; that all this was not made simply to form a gorgeous
+    canopy over the tents of men. If he is of a devout turn of
+    mind, he thinks, as he gazes into those fathomless deeps and
+    among those bewildering hosts, of the infinite multitude of
+    created beings that the Almighty has taken under his care. The
+    narrow ideas of the old geocentric theology, which made the
+    earth God's especial footstool, and man his only rational
+    creature, fall away from him like a veil that had obscured his
+    vision; they are impossible in the presence of what he sees
+    above. Thus the natural tendency, in the light of modern
+    progress, is to regard the universe as everywhere filled with
+    life.</p>
+
+    <p>But science, which is responsible for this broadening of
+    men's thoughts concerning the universality of life, itself
+    proceeds to set limits. Of spiritual existences it pretends to
+    know nothing, but as to physical beings, it declares that it
+    can only entertain the supposition of their existence where it
+    finds evidence of an environment suited to their needs, and
+    such environment may not everywhere exist. Science, though
+    repelled by the antiquated theological conception of the
+    supreme isolation of man among created beings, regards with
+    complacency the probability that there are regions in the
+    universe where no organic life exists, stars which shine upon
+    no inhabited worlds, and planets which nourish no animate
+    creatures. The astronomical view of the universe is that it
+    consists of matter in every stage of evolution: some nebulous
+    and chaotic; some just condensing into stars (suns) of every
+    magnitude and order; some shaped into finished solar bodies
+    surrounded by dependent planets; some forming stars that
+    perhaps have no planets, and will have none; some constituting
+    suns that are already aging, and will soon lose their radiant
+    energy and disappear; and some aggregated into masses that long
+    ago became inert, cold, and rayless, and that can only be
+    revivified by means about which we can form conjectures, but of
+    which we actually know nothing.</p>
+
+    <p>As with the stars, so with the planets, which are the
+    satellites of stars. All investigations unite to tell us that
+    the planets are not all in the same state of development. As
+    some are large and some small, so some are, in an evolutionary
+    sense, young, and some old. As they depend upon the suns around
+    which they revolve for their light, heat, and other forms of
+    radiant energy, so their condition varies with their distance
+    from those suns. Many may never arrive at a state suitable for
+    the maintenance of life upon their surfaces; some which are not
+    at present in such a state may attain it later; and the forms
+    of life themselves may vary with the peculiar environment that
+    different planets afford. Thus we see that we are not
+    scientifically justified in affirming that life is ubiquitous,
+    although we are thus justified in saying that it must be, in a
+    general sense, universal. We might liken the universe to a
+    garden known to contain every variety of plant. If on entering
+    it we see no flowers, we examine the species before us and find
+    that they are not of those which bloom at this particular
+    season, or perhaps they are such as never bear flowers. Yet we
+    feel no doubt that we shall find flowers somewhere in the
+    garden, because there <em>are</em> species which bloom at this
+    season, and the garden contains <em>all</em> varieties.</p>
+
+    <p>While it is tacitly assumed that there are planets revolving
+    around other stars than the sun, it would be impossible for us
+    to see them with any telescope yet invented, and no instrument
+    now in the possession of astronomers could assure us of their
+    existence; so the only planetary system of which we have visual
+    knowledge is our own. Excluding the asteroids, which could not
+    from any point of view be considered as habitable, we have in
+    the solar system eight planets of various sizes and situated at
+    various distances from the sun. Of these eight we know that
+    one, the earth, is inhabited. The question, then, arises: Are
+    there any of the others which are inhabited or habitable? Since
+    it is our intention to discuss the habitability of only one of
+    the seven to which the question applies, the rest may be
+    dismissed in a few words. The smallest of them, and the nearest
+    to the sun, is Mercury, which is regarded as uninhabitable
+    because it has no perceptible supply of water and air, and
+    because, owing to the extraordinary eccentricity of its orbit,
+    it is subjected to excessive and very rapid alterations in the
+    amount of solar heat and light poured upon its surface, such
+    alterations being inconsistent with the supposition that it can
+    support living beings. Even its average temperature is more
+    than six and a half times that prevailing on the earth! Another
+    circumstance which militates against its habitability is that,
+    according to the results of the best telescopic studies, it
+    always keeps the same face toward the sun, so that one half of
+    the planet is perpetually exposed to the fierce solar rays, and
+    the other half faces the unmitigated cold of open space. Venus,
+    the next in distance from the sun, is almost the exact twin of
+    the earth in size, and many arguments may be urged in favor of
+    its habitability, although it is suspected of possessing the
+    same peculiarity as Mercury, in always keeping the same side
+    sunward. Unfortunately its atmosphere appears to be so dense
+    that no permanent markings on its surface are certainly
+    visible, and the question of its actual condition must, for the
+    present, be left in abeyance. Mars, the first planet more
+    distant from the sun than the earth, is the special subject of
+    this chapter, and will be described and discussed a few lines
+    further on. Jupiter, Saturn, Uranus, and Neptune, the four
+    giant planets, all more distant than Mars, and each more
+    distant than the other in the order named, are all regarded as
+    uninhabitable because none of them appears to possess any
+    degree of solidity. They may have solid or liquid nuclei, but
+    exteriorly they seem to be mere balls of cloud. Of course, one
+    can imagine what he pleases about the existence of creatures
+    suited to the physical constitution of such planets as these,
+    but they must be excluded from the category of habitable worlds
+    in the ordinary sense of the term. We go back, then, to
+    Mars.</p>
+
+    <p>It will be best to begin with a description of the planet.
+    Mars is 4230 miles in diameter; its surface is not much more
+    than one-quarter as extensive as that of the earth (.285). Its
+    mean distance from the sun is 141,500,000 miles, 48,500,000
+    miles greater than that of the earth. Since radiant energy
+    varies inversely as the square of distance, Mars receives less
+    than half as much solar light and heat as the earth gets. Mars'
+    year (period of revolution round the sun) is 687 days. Its mean
+    density is 71 per cent of the earth's, and the force of gravity
+    on its surface is 38 per cent of that on the surface of the
+    earth; <em>i.e.,</em> a body weighing one hundred pounds on the
+    earth would, if transported to Mars, weigh but thirty-eight
+    pounds. The inclination of its equator to the plane of its
+    orbit differs very little from that of the earth's equator, and
+    its axial rotation occupies 24 hours 37 minutes. so that the
+    length of day and night, and the extent of the seasonal changes
+    on Mars, are almost precisely the same as on the earth. But
+    owing to the greater length of its year, the seasons of Mars,
+    while occurring in the same order, are almost twice as long as
+    ours. The surface of the planet is manifestly solid, like that
+    of our globe, and the telescope reveals many permanent markings
+    on it, recalling the appearance of a globe on which
+    geographical features have been represented in reddish and
+    dusky tints. Around the poles are plainly to be seen rounded
+    white areas, which vary in extent with the Martian seasons,
+    nearly vanishing in summer and extending widely in winter. The
+    most recent spectroscopic determinations indicate that Mars has
+    an atmosphere perhaps as dense as that to be found on our
+    loftiest mountain peaks, and there is a perceptible amount of
+    watery vapor in this atmosphere. The surface of the planet
+    appears to be remarkably level, and it has no mountain ranges.
+    No evidences of volcanic action have been discovered on Mars.
+    The dusky and reddish areas were regarded by the early
+    observers as respectively seas and lands, but at present it is
+    not believed that there are any bodies of water on the planet.
+    There has never been much doubt expressed that the white areas
+    about the poles represent snow.</p>
+
+    <p>It will be seen from this brief description that many
+    remarkable resemblances exist between Mars and the earth, and
+    there is nothing wonderful in the fact that the question of the
+    habitability of the former has become one of extreme and
+    wide-spread interest, giving rise to the most diverse views, to
+    many extraordinary speculations, and sometimes to regrettably
+    heated controversy. The first champion of the habitability of
+    Mars was Sir William Herschel, although even before his time
+    the idea had been suggested. He was convinced by the
+    revelations of his telescopes, continually increasing in power,
+    that Mars was more like the earth than any other planet. He
+    could not resist the testimony of the polar snows, whose
+    suggestive conduct was in such striking accord with what occurs
+    upon the earth. Gradually, as telescopes improved and observers
+    increased in number, the principal features of the planet were
+    disclosed and charted, and ``areography,'' as the geography of
+    Mars was called, took its place among the recognized branches
+    of astronomical study. But it was not before 1877 that a
+    fundamentally new discovery in areography gave a truly
+    sensational turn to speculation about life on ``the red
+    planet.'' In that year Mars made one of its nearest approaches
+    to the earth, and was so situated in its orbit that it could be
+    observed to great advantage from the northern hemisphere of the
+    earth. The celebrated Italian astronomer, Schiaparelli, took
+    advantage of this opportunity to make a trigonometrical survey
+    of the surface of Mars -- as coolly and confidently as if he
+    were not taking his sights across a thirty-five-million-mile
+    gulf of empty space -- and in the course of this survey he was
+    astonished to perceive that the reddish areas, then called
+    continents, were crossed in many directions by narrow, dusky
+    lines, to which he gave the suggestive name of ``canals.'' Thus
+    a kind of firebrand was cast into the field of astronomical
+    speculation, which has ever since produced disputes that have
+    sometimes approached the violence of political faction. At
+    first the accuracy of Schiaparelli's observations was
+    contested; it required a powerful telescope, and the most
+    excellent ``seeing,'' to render the enigmatical lines visible
+    at all, and many searchers were unable to detect them. But
+    Schiaparelli continued his studies in the serene sky of Italy,
+    and produced charts of the gridironed face of Mars containing
+    so much astonishing detail that one had either to reject them
+    <em>in toto</em> or to confess that Schiaparelli was right. As
+    subsequent favorable oppositions of Mars occurred, other
+    observers began to see the ``canals'' and to confirm the
+    substantial accuracy of the Italian astronomer's work, and
+    finally few were found who would venture to affirm that the
+    ``canals'' did not exist, whatever their meaning might be.</p>
+
+    <p>When Schiaparelli began his observations it was generally
+    believed, as we have said, that the dusky areas on Mars were
+    seas, and since Schiaparelli thought that the ``canals''
+    invariably began and ended at the shores of the ``seas,'' the
+    appropriateness of the title given to the lines seemed
+    apparent. Their artificial character was immediately assumed by
+    many, because they were too straight and too suggestively
+    geometrical in their arrangement to permit the conclusion that
+    they were natural watercourses. A most surprising circumstance
+    noted by Schiaparelli was that the ``canals'' made their
+    appearance <em>after</em> the melting of the polar snow in the
+    corresponding hemisphere had begun, and that they grew darker,
+    longer, and more numerous in proportion as the polar
+    liquidation proceeded; another very puzzling observation was
+    that many of them became double as the season advanced; close
+    beside an already existing ``canal,'' and in perfect
+    parallelism with it, another would gradually make its
+    appearance. That these phenomena actually existed and were not
+    illusions was proved by later observations, and today they are
+    seen whenever Mars is favorably situated for observation.</p>
+
+    <p>In the closing decade of the nineteenth century, Mr Percival
+    Lowell took up the work where Schiaparelli had virtually
+    dropped it, and soon added a great number of ``canals'' to
+    those previously known, so that in his charts the surface of
+    the wonderful little planet appears covered as with a spider's
+    web, the dusky lines criss-crossing in every direction, with
+    conspicuous knots wherever a number of them come together. Mr
+    Lowell has demonstrated that the areas originally called seas,
+    and thus named on the earlier charts, are not bodies of water,
+    whatever else they may be. He has also found that the
+    mysterious lines do not, as Schiaparelli supposed, begin and
+    end at the edges of the dusky regions, but often continue on
+    across them, reaching in some cases far up into the polar
+    regions. But Schiaparelli was right in his observation that the
+    appearance of the ``canals'' is synchronous with the gradual
+    disappearance of the polar snows, and this fact has become the
+    basis of the most extraordinary theory that the subject of life
+    in other worlds has ever given birth to.</p>
+
+    <p>Now, the effect of such discoveries, as we have related,
+    depends upon the type of mind to whose attention they are
+    called. Many are content to accept them as strange and
+    inexplicable at present, and to wait for further light upon
+    them; others insist upon an immediate inquiry concerning their
+    probable nature and meaning. Such an inquiry can only be based
+    upon inference proceeding from analogy. Mars, say Mr Lowell and
+    those who are of his opinion, is manifestly a solidly incrusted
+    planet like the earth; it has an atmosphere, though one of
+    great rarity; it has water vapor, as the snows in themselves
+    prove; it has the alternation of day and night, and a
+    succession of seasons closely resembling those of the earth;
+    its surface is suggestively divided into regions of contrasting
+    colors and appearance, and upon that surface we see an immense
+    number of lines geometrically arranged, with a system of
+    symmetrical intersections where the lines expand into circular
+    and oval areas -- and all connected with the annual melting of
+    the polar snows in a way which irresistibly suggests the
+    interference of intelligence directed to a definite end. Why,
+    with so many concurrent circumstances to support the
+    hypothesis, should we not regard Mars as an inhabited
+    globe?</p>
+
+    <p>But the differences between Mars and the earth are in many
+    ways as striking as their resemblances. Mars is relatively
+    small; it gets less than half as much light and heat as we
+    receive; its atmosphere is so rare that it would be distressing
+    to us, even if we could survive in it at all; it has no lakes,
+    rivers, or seas; its surface is an endless prairie. and its
+    ``canals'' are phenomena utterly unlike anything on the earth.
+    Yet it is precisely upon these divergences between the earth
+    and Mars, this repudiation of terrestrial standards, that the
+    theory of ``life on Mars,'' for which Mr Lowell is mainly
+    responsible, is based. Because Mars is smaller than the earth,
+    we are told it must necessarily be more advanced in planetary
+    evolution, the underlying cause of which is the gradual cooling
+    and contraction of the planet's mass. Mars has parted with its
+    internal heat more rapidly than the earth; consequently its
+    waters and its atmosphere have been mostly withdrawn by
+    chemical combinations, but enough of both yet remain to render
+    life still possible on its surface. As the globe of Mars is
+    evolutionally older than that of the earth, so its forms of
+    organic life may be proportionally further advanced, and its
+    inhabitants may have attained a degree of cultivated
+    intelligence much superior to what at present exists upon the
+    earth. Understanding the nature and the causes of the
+    desiccation of their planet, and possessing engineering science
+    and capabilities far in advance of ours, they may be conceived
+    to have grappled with the stupendous problem of keeping their
+    world in a habitable condition as long as possible. Supposing
+    them to have become accustomed to live in their rarefied
+    atmosphere (a thing not inconceivable, since men can live for a
+    time at least in air hardly less rare), the most pressing
+    problem for them is that of a water-supply, without which plant
+    life cannot exist, while animal life in turn depends for its
+    existence upon vegetation. The only direction in which they can
+    seek water is that of the polar regions, where it is
+    alternately condensed into snow and released in the liquid form
+    by the effect of the seasonal changes. It is, then, to the
+    annual melting of the polar snow-fields that the Martian
+    engineers are supposed to have recourse in supplying the needs
+    of their planet, and thus providing the means of prolonging
+    their own existence. It is imagined that they have for this
+    purpose constructed a stupendous system of irrigation extending
+    over the temperate and equatorial regions of the planet. The
+    ``canals'' represent the lines of irrigation, but the narrow
+    streaks that we see are not the canals themselves, but the
+    irrigated bands covered by them. Their dark hue, and their
+    gradual appearance after the polar melting has begun, are due
+    to the growth of vegetation stimulated by the water. The
+    rounded areas visible where several ``canals'' meet and cross
+    are called by Mr Lowell ``oases.'' These are supposed to be the
+    principal centers of population and industry. It must be
+    confessed that some of them, with their complicated systems of
+    radiating lines, appear to answer very well to such a theory.
+    No attempt to explain them by analogy with natural phenomena on
+    the earth has proved successful.</p>
+
+    <p>But a great difficulty yet remains: How to explain the
+    seemingly miraculous powers of the supposed engineers? Here
+    recourse is had once more to the relative smallness of the
+    planet. We have remarked that the force of gravity on Mars is
+    only thirty-eight per cent of that on the earth. A steam-shovel
+    driven by a certain horse-power would be nearly three times as
+    effective there as here. A man of our stature on Mars would
+    find his effective strength increased in the same proportion.
+    But just because of the slight force of gravity there, a
+    Martian might attain to the traditional stature of Goliath
+    without finding his own weight an encumbrance to his activity,
+    while at the same time his huge muscles would come into
+    unimpeded play, enabling him single-handed to perform labors
+    that would be impossible to a whole gang of terrestrial
+    workmen. The effective powers of huge machines would be
+    increased in the same way; and to all this must be added the
+    fact that the mean density of the materials of which Mars is
+    composed is much less than that of the constituents of the
+    earth. Combining all these considerations, it becomes much less
+    difficult to conceive that public works might be successfully
+    undertaken on Mars which would be hopelessly beyond the limits
+    of human accomplishment.</p>
+
+    <p>Certain other difficulties have also to be met; as, for
+    instance, the relative coldness of the climate of Mars. At its
+    distance it gets considerably less than half as much light and
+    heat as we receive. In addition to this, the rarity of its
+    atmosphere would naturally be expected to decrease the
+    effective temperature at the planet's surface, since an
+    atmosphere acts somewhat like the glass cover of a hot-house in
+    retaining the solar heat which has penetrated it. It has been
+    calculated that, unless there are mitigating circumstances of
+    which we know nothing, the average temperature at the surface
+    of Mars must be far below the freezing-point of water. To this
+    it is replied that the possible mitigating circumstances spoken
+    of evidently exist in fact, because we can <em>see</em> that
+    the watery vapor condenses into snow around the poles in
+    winter, but melts again when summer comes. The mitigating agent
+    may be supposed to exist in the atmosphere where the presence
+    of certain gases would completely alter the temperature
+    gradients.</p>
+
+    <p>It might also be objected that it is inconceivable that the
+    Martian engineers, however great may be their physical powers,
+    and however gigantic the mechanical energies under their
+    control, could force water in large quantities from the poles
+    to the equator. This is an achievement that measures up to the
+    cosmical standard. It is admitted by the champions of the
+    theory that the difficulty is a formidable one; but they call
+    attention to the singular fact that on Mars there can be found
+    no chains of mountains, and it is even doubtful if ranges of
+    hills exist there. The entire surface of the planet appears to
+    be almost ``as smooth as a billiard ball,'' and even the broad
+    regions which were once supposed to be seas apparently lie at
+    practically the same level as the other parts, since the
+    ``canals'' in many cases run uninterruptedly across them.
+    Lowell's idea is that these sombre areas may be expanses of
+    vegetation covering ground of a more or less marshy character,
+    for while the largest of them appear to be permanent, there are
+    some which vary coincidently with the variations of the
+    canals.</p>
+
+    <p>As to the kind of machinery employed to force the water from
+    the poles, it has been conjectured that it may have taken the
+    form of a gigantic system of pumps and conduits; and since the
+    Martians are assumed to be so far in advance of us in their
+    mastery of scientific principles, the hypothesis will at least
+    not be harmed by supposing that they have learned to harness
+    forces of nature whose very existence in a manageable form is
+    yet unrecognized on the earth. If we wish to let the
+    imagination loose, we may conjecture that they have conquered
+    the secret of those intra-atomic forces whose resistless energy
+    is beginning to become evident to us, but the possibility of
+    whose utilization remains a dream, the fulfillment of which
+    nobody dares to predict.</p>
+
+    <p>Such, in very brief form, is the celebrated theory of Mars
+    as an inhabited world. It certainly captivates the imagination,
+    and if we believe it to represent the facts, we cannot but
+    watch with the deepest sympathy this gallant struggle of an
+    intellectual race to preserve its planet from the effects of
+    advancing age and death. We may, indeed, wonder whether our own
+    humanity, confronted by such a calamity, could be counted on to
+    meet the emergency with equal stoutness of heart and
+    inexhaustibleness of resource. Up to the present time we
+    certainly have shown no capacity to confront Nature toe to toe,
+    and to seize her by the shoulders and turn her round when she
+    refuses to go our way. If we could get into wireless telephonic
+    communication with the Martians we might learn from their own
+    lips the secret of their more than ``Roman recovery.''</p>
+
+    <p><strong>The Riddle of the Asteroids</strong></p>
+
+    <p>Between the orbits of Mars and Jupiter revolves the most
+    remarkable system of little bodies with which we are acquainted
+    -- the Asteroids, or Minor Planets. Some six hundred are now
+    known, and they may actually number thousands. They form
+    virtually a ring about the sun. The most striking general fact
+    about them is that they occupy the place in the sky which
+    should be occupied, according to Bode's Law, by a single large
+    planet. This fact, as we shall see, has led to the invention of
+    one of the most extraordinary theories in astronomy --
+    <em>viz.,</em> that of the explosion of a world!</p>
+
+    <p>Bode's Law, so-called, is only an empiric formula, but until
+    the discovery of Neptune it accorded so well with the distances
+    of the planets that astronomers were disposed to look upon it
+    as really representing some underlying principle of planetary
+    distribution. They were puzzled by the absence of a planet in
+    the space between Mars and Jupiter, where the ``law'' demanded
+    that there should be one, and an association of astronomers was
+    formed to search for it. There was a decided sensation when, in
+    1801, Piazzi, of Palermo, announced that he had found a little
+    planet which apparently occupied the place in the system which
+    belonged to the missing body. He named it Ceres, and it was the
+    first of the Asteroids. The next year Olbers, of Bremen, while
+    looking for Ceres with his telescope, stumbled upon another
+    small planet which he named Pallas. Immediately he was inspired
+    with the idea that these two planets were fragments of a larger
+    one which had formerly occupied the vacant place in the
+    planetary ranks, and he predicted that others would be found by
+    searching in the neighborhood of the intersection of the orbits
+    of the two already discovered. This bold prediction was
+    brilliantly fulfilled by the finding of two more -- Juno in
+    1804, and Vesta in 1807. Olbers would seem to have been led to
+    the invention of his hypothesis of a planetary explosion by the
+    faith which astronomers at that time had in Bode's Law. They
+    appear to have thought that several planets revolving in the
+    gap where the ``law'' called for but one could only be
+    accounted for upon the theory that the original <em>one</em>
+    had been broken up to form the several. Gravitation demanded
+    that the remnants of a planet blown to pieces, no matter how
+    their orbits might otherwise differ, should all return at
+    stated periods to the point where the explosion had occurred;
+    hence Olbers' prediction that any asteroids that might
+    subsequently be discovered would be found to have a common
+    point of orbital intersection. And curiously enough all of the
+    first asteroids found practically answered to this requirement.
+    Olbers' theory seemed to be established.</p>
+
+    <p>After the first four, no more asteroids were found until
+    1845, when one was discovered; then, in 1847, three more were
+    added to the list; and after that searchers began to pick them
+    up with such rapidity that by the close of the century hundreds
+    were known, and it had become almost impossible to keep track
+    of them. The first four are by far the largest members of the
+    group, but their actual sizes remained unknown until less than
+    twenty years ago. It was long supposed that Vesta was the
+    largest, because it shines more brightly than any of the
+    others; but finally, in 1895, Barnard, with the Lick telescope,
+    definitely measured their diameters, and proved to everybody's
+    surprise that Ceres is really the chief, and Vesta only the
+    third in rank. His measures are as follows: Ceres, 477 miles;
+    Pallas, 304 miles; Vesta, 239 miles; and Juno, 120 miles. They
+    differ greatly in the reflective power of their surfaces, a
+    fact of much significance in connection with the question of
+    their origin. Vesta is, surface for surface, rather more than
+    three times as brilliant as Ceres, whence the original mistake
+    about its magnitude.</p>
+
+    <p>Nowadays new asteroids are found frequently by photography,
+    but physically they are most insignificant bodies, their
+    average diameter probably not exceeding twenty miles, and some
+    are believed not to exceed ten. On a planet only ten miles in
+    diameter, assuming the same mean density as the earth's, which
+    is undoubtedly too much, the force of gravity would be so
+    slight that an average man would not weigh more than three
+    ounces, and could jump off into space whenever he liked.</p>
+
+    <p>Although the asteroids all revolve around the sun in the
+    same direction as that pursued by the major planets, their
+    orbits are inclined at a great variety of angles to the general
+    plane of the planetary system, and some of them are very
+    eccentric -- almost as much so as the orbits of many of the
+    periodic comets. It has even been conjectured that the two tiny
+    moons of Mars and the four smaller satellites of Jupiter may be
+    asteroids gone astray and captured by those planets. Two of the
+    asteroids are exceedingly remarkable for the shapes and
+    positions of their orbits; these are Eros, discovered in 1898,
+    and T. G., 1906, found eight years later. The latter has a mean
+    distance from the sun slightly greater than that of Jupiter,
+    while the mean distance of Eros is less than that of Mars. The
+    orbit of Eros is so eccentric that at times it approaches
+    within 15,000,000 miles of the earth, nearer than any other
+    regular member of the solar system except the moon, thus
+    affording an unrivaled means of measuring the solar parallax.
+    But for our present purpose the chief interest of Eros lies in
+    its extraordinary changes of light.</p>
+
+    <p>These changes, although irregular, have been observed and
+    photographed many times, and there seems to be no doubt of
+    their reality. Their significance consists in their possible
+    connection with the form of the little planet, whose diameter
+    is generally estimated at not more than twenty miles. Von
+    Oppolzer found, in 1901, that Eros lost three-fourths of its
+    brilliancy once in every two hours and thirty-eight minutes.
+    Other observers have found slightly different periods of
+    variability, but none as long as three hours. The most
+    interesting interpretation that has been offered of this
+    phenomenon is that it is due to a great irregularity of figure,
+    recalling at once Olbers' hypothesis. According to some, Eros
+    may be double, the two bodies composing it revolving around
+    each other at very close quarters; but a more striking, and it
+    may be said probable, suggestion is that Eros has a form not
+    unlike that of a dumb-bell, or hour-glass, turning rapidly end
+    over end so that the area of illuminated surface presented to
+    our eyes continually changes, reaching at certain times a
+    minimum when the amount of light that it reflects toward the
+    earth is reduced to a quarter of its maximum value. Various
+    other bizarre shapes have been ascribed to Eros, such, for
+    instance, as that of a flat stone revolving about one of its
+    longer axes, so that sometimes we see its face and sometimes
+    its edge.</p>
+
+    <p>All of these explanations proceed upon the assumption that
+    Eros cannot have a simple globular figure like that of a
+    typical planet, a figure which is prescribed by the law of
+    gravitation, but that its shape is what may be called
+    accidental; in a word, it is a <em>fragment,</em> for it seems
+    impossible to believe that a body formed in interplanetary
+    space, either through nebular condensation or through the
+    aggregation of particles drawn together by their mutual
+    attractions, should not be practically spherical in shape. Nor
+    is Eros the only asteroid that gives evidence by variations of
+    brilliancy that there is something abnormal in its
+    constitution; several others present the same phenomenon in
+    varying degrees. Even Vesta was regarded by Olbers as
+    sufficiently variable in its light to warrant the conclusion
+    that it was an angular mass instead of a globe. Some of the
+    smaller ones show very notable variations, and all in short
+    periods, of three or four hours, suggesting that in turning
+    about one of their axes they present a surface of variable
+    extent toward the sun and the earth.</p>
+
+    <p>The theory which some have preferred -- that the variability
+    of light is due to the differences of reflective power on
+    different parts of the surface -- would, if accepted, be hardly
+    less suggestive of the origin of these little bodies by the
+    breaking up of a larger one, because the most natural
+    explanation of such differences would seem to be that they
+    arose from variations in the roughness or smoothness of the
+    reflecting surface, which would be characteristic of
+    fragmentary bodies. In the case of a large planet alternating
+    expanses of land and water, or of vegetation and desert, would
+    produce a notable variation in the amount of reflection, but on
+    bodies of the size of the asteroids neither water nor
+    vegetation could exist, and an atmosphere would be equally
+    impossible.</p>
+
+    <p>One of the strongest objections to Olbers' hypothesis is
+    that only a few of the first asteroids discovered travel in
+    orbits which measurably satisfy the requirement that they
+    should all intersect at the point where the explosion occurred.
+    To this it was at first replied that the perturbations of the
+    asteroidal orbits, by the attractions of the major planets,
+    would soon displace them in such a manner that they would cease
+    to intersect. One of the first investigations undertaken by the
+    late Prof. Simon Newcomb was directed to the solution of this
+    question, and he arrived at the conclusion that the planetary
+    perturbations could not explain the actual situation of the
+    asteroidal orbits. But afterward it was pointed out that the
+    difficulty could be avoided by supposing that not one but a
+    series of explosions had produced the asteroids as they now
+    are. After the primary disruption the fragments themselves,
+    according to this suggestion, may have exploded, and then the
+    resulting orbits would be as ``tangled'' as the heart could
+    wish. This has so far rehabilitated the explosion theory that
+    it has never been entirely abandoned, and the evidence which we
+    have just cited of the probably abnormal shapes of Eros and
+    other asteroids has lately given it renewed life. It is a
+    subject that needs a thorough rediscussion.</p>
+
+    <p>We must not fail to mention, however, that there is a rival
+    hypothesis which commends itself to many astronomers --
+    <em>viz.,</em> that the asteroids were formed out of a
+    relatively scant ring of matter, situated between Mars and
+    Jupiter and resembling in composition the immensely more
+    massive rings from which, according to Laplace's hypothesis,
+    the planets were born. It is held by the supporters of this
+    theory that the attraction of the giant Jupiter was sufficient
+    to prevent the small, nebulous ring that gave birth to the
+    asteroids from condensing like the others into a single
+    planet.</p>
+
+    <p>But if we accept the explosion theory, with its corollary
+    that minor explosions followed the principal one, we have still
+    an unanswered question before us: What caused the explosions?
+    The idea of <em>a world blowing up</em> is too Titanic to be
+    shocking; it rather amuses the imagination than seriously
+    impresses it; in a word, it seems essentially chimerical. We
+    can by no appeal to experience form a mental picture of such an
+    occurrence. Even the moon did not blow up when it was wrecked
+    by volcanoes. The explosive nebul&aelig; and new stars are far
+    away in space, and suggest no connection with such a
+    catastrophe as the bursting of a planet into hundreds of
+    pieces. We cannot conceive of a great globe thousands of miles
+    in diameter resembling a pellet of gunpowder only awaiting the
+    touch of a match to cause its sudden disruption. Somehow the
+    thought of human agency obtrudes itself in connection with the
+    word ``explosion,'' and we smile at the idea that giant powder
+    or nitro-glycerine could blow up a planet. Yet it would only
+    need <em>enough</em> of them to do it.</p>
+
+    <p>After all, we may deceive ourselves in thinking, as we are
+    apt to do, that explosive energies lock themselves up only in
+    small masses of matter. There are many causes producing
+    explosions in nature, every volcanic eruption manifests the
+    activity of some of them. Think of the giant power of confined
+    steam; if enough steam could be suddenly generated in the
+    center of the earth by a downpour of all the waters of the
+    oceans, what might not the consequences be for our globe? In a
+    smaller globe, and it has never been estimated that the
+    original asteroid was even as large as the moon, such a
+    catastrophe would, perhaps, be more easily conceivable; but
+    since we are compelled in this case to assume that there was a
+    series of successive explosions, steam would hardly answer the
+    purpose; it would be more reasonable to suppose that the cause
+    of the explosion was some kind of chemical reaction, or
+    something affecting the atoms composing the exploding body.
+    Here Dr Gustav Le Bon comes to our aid with a most startling
+    suggestion, based on his theory of the dissipation of
+    intra-atomic energy. It will be best to quote him at some
+    length from his book on <em>The Evolution of Forces.</em></p>
+
+    <p>``It does not seem at first sight,'' says Doctor Le Bon,</p>
+
+    <blockquote>
+      very comprehensible that worlds which appear more and more
+      stable as they cool could become so unstable as to afterward
+      dissociate entirely. To explain this phenomenon, we will
+      inquire whether astronomical observations do not allow us to
+      witness this dissociation. 
+
+      <p>We know that the stability of a body in motion, such as a
+      top or a bicycle, ceases to be possible when its velocity of
+      rotation descends below a certain limit. Once this limit is
+      reached it loses its stability and falls to the ground. Prof.
+      J. J. Thomson even interprets radio-activity in this manner,
+      and points out that when the speed of the elements composing
+      the atoms descends below a certain limit they become unstable
+      and tend to lose their equilibria. There would result from
+      this a commencement of dissociation, with diminution of their
+      potential energy and a corresponding increase of their
+      kinetic energy sufficient to launch into space the products
+      of intra-atomic disintegration.</p>
+
+      <p>It must not be forgotten that the atom being an enormous
+      reservoir of energy is by this very fact comparable with
+      explosive bodies. These last remain inert so long as their
+      internal equilibria are undisturbed. So soon as some cause or
+      other modifies these, they explode and smash everything
+      around them after being themselves broken to pieces.</p>
+
+      <p>Atoms, therefore, which grow old in consequence of the
+      diminution of a part of their intra-atomic energy gradually
+      lose their stability. A moment, then, arrives when this
+      stability is so weak that the matter disappears by a sort of
+      explosion more or less rapid. The bodies of the radium group
+      offer an image of this phenomenon -- a rather faint image,
+      however, because the atoms of this body have only reached a
+      period of instability when the dissociation is rather slow.
+      It probably precedes another and more rapid period of
+      dissociation capable of producing their final explosion.
+      Bodies such as radium, thorium, etc., represent, no doubt, a
+      state of old age at which all bodies must some day arrive,
+      and which they already begin to manifest in our universe,
+      since all matter is slightly radio-active. It would suffice
+      for the dissociation to be fairly general and fairly rapid
+      for an explosion to occur in a world where it was
+      manifested.</p>
+
+      <p>These theoretical considerations find a solid support in
+      the sudden appearances and disappearances of stars. The
+      explosions of a world which produce them reveal to us,
+      perhaps, how the universes perish when they become old.</p>
+
+      <p>As astronomical observations show the relative frequency
+      of these rapid destructions, we may ask ourselves whether the
+      end of a universe by a sudden explosion after a long period
+      of old age does not represent its most general ending.</p>
+    </blockquote>
+
+    <p>Here, perhaps, it will be well to stop, since, entrancing as
+    the subject may be, we know very little about it, and Doctor Le
+    Bon's theory affords a limitless field for the reader's
+    imagination.</p>
+    <hr>
+
+    <p>A printed version of this book is available from <a href=
+    "http://csky.sattre-press.com">Sattre Press</a>. It includes
+    extensive annotations, a new introduction and all the original
+    photographs and diagrams.</p>
+
+<pre>
+*** END OF THE PROJECT GUTENBERG EBOOK, CURIOSITIES OF THE SKY ***
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