Transcriber’s Notes:

The cover image was created by the transcriber and is placed in the public domain.

THE EVOLUTION OF WORLDS

THE MACMILLAN COMPANY
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Saturn—photographed at the Lowell Observatory
by Mr. E. C. Slipher. September, 1909.

THE EVOLUTION OF WORLDS

BY

PERCIVAL LOWELL, A.B., LL.D.

AUTHOR OF “MARS AND ITS CANALS,” “MARS AS THE ABODE OF LIFE,” ETC.

DIRECTOR OF THE OBSERVATORY AT FLAGSTAFF, ARIZONA; NON-RESIDENT
PROFESSOR OF ASTRONOMY AT THE MASSACHUSETTS INSTITUTE OF
TECHNOLOGY; FELLOW OF THE AMERICAN ACADEMY OF ARTS AND SCIENCES;
MEMBRE DE LA SOCIÉTÉ ASTRONOMIQUE DE FRANCE; MEMBER OF THE
ASTRONOMICAL AND ASTROPHYSICAL SOCIETY OF AMERICA; MITGLIED
DER ASTRONOMISCHE GESELLSCHAFT; MEMBRE DE LA SOCIÉTÉ
BELGE D’ASTRONOMIE; HONORARY MEMBER OF THE SOCIEDAD
ASTRONOMICA DE MEXICO; JANSSEN MEDALLIST OF THE
SOCIÉTÉ ASTRONOMIQUE DE FRANCE, 1904, FOR
RESEARCHES ON MARS; MEDALLIST OF THE
SOCIEDAD ASTRONOMICA DE MEXICO FOR
STUDIES ON MARS, 1908

ILLUSTRATED

New York
THE MACMILLAN COMPANY
1909

All rights reserved

Copyright, 1909,

By THE MACMILLAN COMPANY.

Set up and electrotyped. Published December, 1909.

Norwood Press
J. S. Cushing Co.—Berwick & Smith Co.
Norwood, Mass., U.S.A.

TO
THE PRESIDENT OF THE
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
TO MY COLLEAGUES THERE
AND TO ITS STUDENT BODY
TO WHOSE INTEREST AND ATTENTION THESE
LECTURES ARE INDEBTED
THEY ARE APPRECIATIVELY INSCRIBED

“Si je n’étais pas devenu général en chef et l’instrument du sort d’un grand people, j’aurais couru les bureaux et les salons pour me mettre dans la dépendance de qui que ce fût, en qualité de ministre ou d’ambassadeur? Non, non! je me serais jeté dans l’étude des sciences exactes. J’aurais fait mon chemin dans la route des Galilée, des Newton. Et puisque j’ai réussi constamment dans mes grandes entreprises, eh bien, je me serais hautement distingué aussi par des travaux scientifiques. J’aurais laissé le souvenir de belles découvertes. Aucune autre gloire n’aurait pu tenter mon ambition.”

—Napoleon Iᴱᴿ, quoted by Arago.

The substance of the following pages was written and presented in a university course of lectures before the Massachusetts Institute of Technology—in February and March of this year. The kind interest with which the lectures were received, not only by the students and professional bodies, but by the public, was followed by an immediate request from The Macmillan Company to issue them in book form, and as such they now appear.

PERCIVAL LOWELL.

Boston, Mass., May 29, 1909.

CONTENTS

LIST OF ILLUSTRATIONS

THE EVOLUTION OF WORLDS

CHAPTER I
BIRTH OF A SOLAR SYSTEM

ASTRONOMY is usually thought of as the study of the bodies visible in the sky. And such it largely is when the present state of the universe alone is considered. But when we attempt to peer into its past and to foresee its future, we find ourselves facing a new side of the heavens—the contemplation of the invisible there. For in the evolution of worlds not simply must the processes be followed by the mind’s eye, so short the span of human life, but they begin and end in what we cannot see. What the solar system sprang from, and what it will eventually become, is alike matter devoid of light. Out of darkness into darkness again: such are the bourns of cosmic action.

The stars are suns; past, present, or potential. Each of those diamond points we mark studding the heavens on a winter’s night are globes comparable with, and in many cases greatly excelling, our own ruler of the day. The telescope discloses myriads more. Yet these self-confessed denizens of space form but a fraction of its occupants. Quite as near, and perhaps much nearer, are orbs of which most of us have no suspicion. Unimpressing our senses and therefore ignored by our minds, bodies people it which, except for rare occurrences, remain forever invisible. For dark stars in countless numbers course hither and thither throughout the universe at speeds as stupendous as the lucent ones themselves.

Had we no other knowledge of them, reasoning would suffice to demonstrate their existence. It is the logic of unlimited subtraction. Every self-shining star is continually giving out light and heat. Now such an expenditure cannot go on forever, as the source of its replenishing by contraction, accretion, or disintegration is finite. Long to our measures of time as the process may last, it must eventually have an end and the star finally become a cold dark body, pursuing as before its course, but in itself inert and dead; an orb grown orbéd, in the old French sense. So it must remain unless some cosmic catastrophe rekindle it to life. The chance of such occurrence in a given time compared with the duration of the star’s light-emitting career will determine the number of dark stars relative to the lucent ones. The chance is undoubtedly small, and the number of dark bodies in space proportionally large. Reasoning, then, informs us first that such bodies must exist all about us, and second that their multitude must be great.

Valid as this reasoning is, however, we are not left to inference for our knowledge of them. There is a certain star amid the polar constellations known as Algol,—el Ghoul, the Arabs called it, or The Dæmon. The name shows they noticed how it winked its eye and recognized something sarcastically sinister in its intent. For once in two days and twenty hours its light fades to one-third of its usual amount, remains thus for about twenty minutes, and then slowly regains its brightness. Seemingly unmoved itself, its steady blinking from the time man first observed it took on an uncanniness he felt. To untelescoped man it certainly seemed demoniacal, this punctual recurrent wink. Spectroscoped man has learnt its cause.

Goodricke in 1795 divined it, and research since has confirmed his keen intuition. Its loss of light is occasioned by the passing in front of it of a dark companion almost of its own size revolving about it in a close elliptic orbit. That this is the explanation of its strange behavior, the shift of its spectral lines makes certain, by showing that the bright star is receding from us at twenty-seven miles a second seventeen hours before the eclipse and coming towards us at about the same rate seventeen hours after it; its dark companion, therefore, doing the reverse.

Algol is no solitary specimen of a mind-seen invisible star. Many eclipsing binaries of the same class are now known; and considering that the phenomenon could not be disclosed unless the orbital plane of the pair traversed the observer’s eye, an unlikely chance in a fortuitous distribution, we perceive how many such in truth there must be which escape recognition for their tilt.

Algol and its dark companion,

as seen from the Earth,

as seen from above orbit.

But if dark stars exist in connection with lucent ones, there must be many more that travel alone. Our own Sun is an instance in embryo. If he live long enough, he will become such a solitary shrouded tramp in his old age. For he has no companion to betray him. The only way in which we could become cognizant of these wanderers would be by their chance collision with some other star, dark or lucent as the case might be. The impact of the catastrophe would generate so much light and heat that the previously dark body would be converted into a blazing sun and a new star make its advent in the sky.

Star births of the sort have actually been noted. Every now and then a new star suddenly appears in the firmament—a nova as it is technically called. These apparitions date from the dawn of astronomic history. The earliest chronicled is found in the Chinese Annals of 134 b.c. It shone out in Scorpio and was probably the new star which Pliny tells us incited Hipparchus, “The Father of Astronomy,” to make his celebrated catalogue of stars. From this time down we have recorded instances of like character.

One of the most famous was the “Pilgrim Star” of Tycho Brahe. That astronomer has left us a full account of it. “While I was living,” he tells us, “with my uncle in the monastery of Hearitzwadt, on quitting my chemical laboratory one evening, I raised my eyes to the well-known vault of heaven and observed, with indescribable astonishment, near the zenith, in Cassiopeia, a radiant fixed star of a magnitude never before seen. In my amazement I doubted the evidence of my senses. However, to convince myself that it was no illusion, and to have the testimony of others, I summoned my assistants from the laboratory and inquired of them, and of all the country people that passed by, if they also observed the star that had thus suddenly burst forth. I subsequently heard that in Germany wagoners and other common people first called the attention of astronomers to this great phenomenon in the heavens,—a circumstance which, as in the case of non-predicted comets, furnished fresh occasion for the usual raillery at the expense of the learned.”

The new star, he informs us, was just like all other fixed stars, but as bright as Venus at her brightest. Those gifted with keen sight could discern it in the daytime and even at noon. It soon began to wane. In December, 1572, it resembled Jupiter, and a year and three months later had sunk beyond recognition to the naked eye. It changed color as it did so, passing from white through yellow to red. In May, 1573, it returned to yellow (“the hue of Saturn,” he expressly states), and so remained till it disappeared from sight, scintillating strongly in proportion to its faintness.

Thirty-two years later another stranger appeared and was seen by Kepler, who wrote a paper about it entitled “The New Star in the Foot of the Serpent.” It shone out in the same sudden manner and faded in the same leisurely way.

Since 1860 there have been several such apparitions, and since 1876 it has been possible to study them with the spectroscope, which has immensely increased our knowledge of their constitution. Indeed, this instrument of research has really opened our eyes to what they are. Nova Cygni, in 1876, Nova Aurigæ, in 1892, and Nova Persei, in 1901, besides several others found by Mrs. Fleming on the Arequipa plates, were excellent examples, and all agreed in their main features, showing that novæ constitute a type of stars by themselves, whose appearing in the first place and whose behavior afterwards prove them to have started from like cause and to have pursued parallel lines of development.

As a typical case we may review the history of Nova Aurigæ. On February 1, 1892, an anonymous post-card was received by Dr. Copeland of the Royal Observatory, Edinburgh, that read as follows: “Nova in Aurigæ. In Milky Way, about 2° south of χ Aurigæ, preceding 26 Aurigæ. Fifth magnitude slightly brighter than χ.” The observatory staff at once looked for the nova and easily found it with an opera glass. They then examined it through a prism placed before their 24-inch reflector and found its spectrum. It proved to be that of a “blaze star.”

Dr. Thomas D. Anderson turned out to be the writer of the anonymous post-card—his name modestly self-obliterated by the nova’s light. He had detected the star on January 24, but had only verified it as a new one on the 31st. Harvard College Observatory then looked up its archived plates. The plates showed that it had appeared sometime between December 1 and 10. Its maximum had been attained on December 20, after which it declined, to record apparently another maximum on February 3 of the 3.5 magnitude. From this time its light steadily waned till on April 1 it was only of the 16th magnitude or ¹/₁₀₀₀₀₀ of what it had been. In August it brightened again and then waned once more.

Meanwhile its spectrum underwent equally strange fluctuations. At first it exhibited the bright lines characteristic of the flaming red solar prominences, the calcium, hydrogen, and helium lines flanked by their dark correlatives upon a continuous background, showing that both glowing and cooler gases were here concerned. The sodium lines, too, appeared, like those that come out in comets as they approach the furnace of the Sun. An outburst such as occurs in miniature in the solar chromosphere or outermost gaseous layer of the Sun was here going on upon a gigantic scale. A veritable spectral chaos next supervened, staying until the star had practically faded away. Then, on its reappearance, in August, Holden, Schaeberle, and Campbell discovered to their surprise not what had been at all, but something utterly new: the soberly bright lines only of a nebula. Finally, ten years later, January, 1902, Campbell found its spectrum had become continuous, the body having reverted to the condition of a star.

Now how are we to interpret these grandiose vicissitudes, visually and spectrally revealed? That we witnessed some great catastrophe is clear. The sudden increase of light of many thousand fold from invisibility to prominence shows that a tremendous cataclysm occurred. The bright lines in the spectrum confirm it and imply that vast upheavals like those that shake the Sun were there in progress, but on so stupendous a scale that, if for no other reason, we must dismiss the idea that explosions alone can possibly be concerned. The dark correlatives of the bright lines have been interpreted as indicating that two bodies were concerned, each travelling at velocities of hundreds of miles a second. But in Nova Aurigæ shiftings of the spectral lines implying six bodies at least were recorded, if such be attributed to motion in the line of sight, and Vogel was minded to throw in a few planets as well—as Miss Clerke pithily puts it. There is not room for so many on the stage of the cosmic drama. Other causes, as we now know, may also displace the spectral lines. Great pressure has been shown to do it, thanks to the labors of Humphreys and Mohler at Baltimore. “Anomalous refraction” may do it, as Professor Julius of Utrecht has found out. Finally, changes of density may produce it, as Michelson has discovered. To these causes we may confidently ascribe most of the shiftings in the stellar spectrum, for just such forces must be there at work.

Mr. Monck suggested the idea that new stars are the result of old dark stars rushing through gaseous fields in space and rendered luminous by the encounter. Seeliger revived and developed this idea, which in certain cases is undoubtedly the truth. Probably this occurred to the new star of 1885 which suddenly blazed out almost in the centre of the great nebula in Andromeda. It behaved like a typical nova and in due course faded to indistinguishability. Something like it happened, too, in the nova of 1860, which suddenly flared up in the star cluster 80 Messier, outdoing in lustre the cluster itself, and then, too, faded away.

But just as psychology teaches us that not only do we cry because we are sorrowful, but that we are sorrowful because we cry, so while a nova may be made by a nebula, no less may a nebula be made by a star.

Let us see how this might be brought about and what sign manuals it would present. Suppose that the two bodies actually grazed. Then the disruption would affect the star’s cuticle, first raising the outer parts, consisting rather of carbon than of the metals, since that substance is the lighter, to intense heat and the gases about it at the same time. The glowing carbon would be intensely bright, and at first its light would overpower that from the gases, and not till its great glow had partially subsided would theirs be seen. Then the gases, hydrogen, helium, and so forth, would make themselves evident. Finally only the most tenuous ones, those peculiar to a nebula, would remain visible. After which the more solid particles due to the disruption would fall together and light up again by their individual collisions. Much the same would result if without striking the stars passed close.

Spectrum of Nova Persei. (F. Ellerman, 40 in. Yerkes.)

Now to put this theory to the proof. In the early morning of the 22d of February, 1901, Dr. Anderson, the discoverer of Nova Aurigæ, perceived that Algol had a neighbor, a star as bright as itself, which had never been there before. Within twenty-four hours of its detection the newcomer rivalled Capella, and shortly after took rank as the premier star of the northern hemisphere. Its spectrum on the 22d was found at Harvard College Observatory to be like that of Rigel, a continuous one crossed by some thirty faint dark lines. On the 24th, however, so soon as it began to wane, the bright lines of hydrogen were conspicuous with their dark correlatives, just as they had been with Nova Aurigæ and other novæ. At the same time each particular spectral line proved a law unto itself, some shifted more than others, thus negativing motion as their only cause and indicating change of pressure or density as concerned concomitants of the affair. Blue emissions like those of Wolf-Rayet stars next made their appearance; then a band, found by Wright at the Lick to characterize nebulæ, shone out, and finally in July the change to a nebular spectrum stood complete.

Then came what is the most suggestive feature in the whole event. On August 22 and 23 Dr. Wolf at Königstahl took with his then new Bruce objective some long exposure plates of the nova, and on them found, to his surprise, wisps of nebulous matter to the southeast of the star. On September 20 Ritchey, with a two-foot mirror of his own constructing exposed for four hours, brought the whole formation to light. It turned out to be a spiral nebula encircling and apparently emanating from the star. Its connection with the nova was patent. But there was more to come. Later plates taken at the Lick on November 7 disclosed the startling fact that the nebula was visibly expanding, uncoiling outward from the star. A plate by Ritchey on November 13 confirmed this, and still later plates by him in December, January, and February showed the motion to be progressive. At the same time the star showed no parallax, and the speed of the motion seemed thus to be indicated as enormous. Kapteyn suggested to account for it that appearance, not reality, was here concerned; that the nebula had always existed, and was only shown up by the light from the conflagration travelling outward from the nova at the rate of one hundred and eighty-six thousand miles a second. This would make the catastrophe to have occurred as far back as the time of James I, of which the news more truthful but less timely than that of the morning papers had only just reached us.

December 14, 1901.

January 7 and 9, 1902.

1902. February 8, 1902.

The Moving Nebula surrounding Nova Persei—after Ritchey.

But a little of that simple reasoning by which Zadig recovered the lost horses of the Sultan, and which from its unaccustomedness in the affairs of men got him suspected of having stolen them and very nearly caused his death, will show the untenableness of this idea and help us to a solution. In the first place we note that the star holds the very centre of the nebular stage, a remarkable prominence if the star has no creative right to the position. Then the same knots and patches of the nebulous configuration are visible in all the photographs, in the same relative positions, turned through corresponding angles as one will see for himself, all having moved symmetrically from one date to another. At the truly marvellous mimicry implied if different objects were concerned common sense instinctively shies, and very properly, as the chances against it are millions to one. Clearly it was not a mere matter of ethereal motion, but a very material motion of matter, which was here concerned. Something corpuscular emanating from the nova spread outward into space.

Clinching this conclusion is the result of a search by Perrine for traces of the nebula on earlier plates. For on one taken by him on March 29 (1901) he found the process already started in two close coils, its conception thus clearly dating from the time of the star’s outburst. In Nova Persei, then, we actually witnessed a spiral nebula evolved from a disrupted star.

What was this ejectum and what drove it forth? Professor Very regarded it as composed of corpuscles such as give rise to cathode rays discharged from the star under the stress of light pressure or electric repulsion. But I think we may see in it something simpler still; to wit, gaseous molecules driven off by light pressure alone—the smoke, as one may say, of the catastrophe—akin exactly to the constituents of comet’s tails. The mere light of the conflagration pushed the hydrogen molecules away. This would explain their presence and their exceeding hurry at the same time. They were started on their travels by domestic jars and kept going by the vivid after-effects of that infelicity.

The fairly steady rate of regression from the nova observed may be explained by the observed decrease in the light of the repellent source. Such combined with the retarding effect of gravity might make the regression equable. This is the more explanatory as the speed was certainly much less than that of light, though greatly exceeding any possible from the direct disruption. At the same time both the bright and the dark lines of hydrogen seen in the spectrum stand accounted for; the colliding molecules, at their starting on their travels from the star, shining through their sparser fellows farther out. An interesting biograph of the levity of light!

Nova Persei thus introduces us at its birth to one of a class of most interesting objects comparatively recently discovered and of most pregnant import,—the spiral nebulæ.

Great Nebula in Orion—after Ritchey.

Great Nebula in Andromeda—after Ritchey.

Nebula M. 100 Comæ—after Roberts.

In 1843 when Lord Rosse’s giant speculum, six feet across, was turned upon the sky, a nebula was brought to light which was unlike any ever before seen. It was neither irregular like the great nebula in Orion nor round like the so called planetary nebulæ,—the two great classes at that time known,—but exhibited a striking spiral structure. It proved the forerunner of a remarkable revelation. For the specimen thus disclosed has turned out to typify not only the most interesting form of those heavenly wreaths of light, but by far the commonest as well. As telescopic and especially photographic means improved, the number of such objects detected steadily increased until about thirteen years ago Keeler by his systematic discoveries of them came to the conclusion that a spiral structure pervaded the great majority of all the nebulæ visible. Their relative universality was outdone only by the invariability of their form. For they all represent spirals of one type: two coiled arms radiating diametrically from a central nucleus and dilating outward. Even nebulæ not originally supposed spiral have disclosed on better revelation the dominant form. Thus the great nebula in Andromeda formerly thought lens-shaped proves to be a huge spiral coiled in a plane not many degrees inclined to the plane of sight.

Nebula ♅ I. 226 Ursæ Majoris—after Roberts.

As should happen if the spirals are unrelated, left-handed and right-handed ones are about equally common. In Dr. Roberts’ great collection of those in which the structure is distinctly discernible, nine are right-handed, ten left-handed, showing that they partake of the ambidextrous impartiality of space.

Nebula ♅ V. 24 Comæ—after Roberts.

Showing globular structure.

Lastly the spirals are evidently thicker near the centre, thinning out at the edge, and when the central nucleus is pronounced, it seems to have a certain globularity not shared by the arms, and more or less detached from them. This appears in those cases where they are shown us edgewise, and it has been thought perceptible in the great nebula of Andromeda. The difficulty in establishing the phenomenon comes from the impossibility of both features showing at their best together. For the globularity to come out well, the spiral must be presented to us nearly in the plane of sight; for the spirality, in a plane at right angles to it.

Much may be learnt by pondering on these peculiarities. The widespread character of the phenomenon points to some universal law. We are here clearly confronted by the embodiment of a great cosmic principle, causing the helices it is for us to uncoil. It is a problem in mechanics.

In the first place, a spiral structure denotes action on the face of it. It implies a rotation combined with motion out or in. We are familiar with the fact in the sparks of pin-wheel pyrotechnics. Any rotating fluid urged by an outward or an inward impulse must take the spiral form. A common example occurs in the water let out of a basin through a hole in the centre when we draw out the plug. Here the force is inward, and because the bowl and orifice are not perfectly symmetric, a rotation is set up in the water trying to escape, and the two combine to give us a beautiful conchoidal swirl. In this case the particles seek the centre, but the same general shape is assumed when they seek to leave it.

Another point to be noticed is that a spiral nebula could not develop of itself and subsist. To continue it must have outside help. For if it were due to internal explosive action in the pristine body, each ejectum must return to the point it started from, or else depart forever into space, for the orbit it would describe must either be closed or unclosed. If the former, it would revisit its starting-point; if the latter, it would never return. Explosion, therefore, of itself could not have produced the forms we see, unless they be ephemeral apparitions, a supposition their presence throughout the heavens seems effectually to exclude.

Nebula M. 101 Ursæ Majoris—after Ritchey.

The form of the spiral nebulæ proclaims their motion, but one of its particular features discloses more. For it implies the past cause which set this motion going. A distinctive detail of these spirals, which so far as we know is shared by all of them, are the two arms which leave the centre from diametrically opposite sides. This indicates that the outward driving force acted only in two places, the one the antipodes of the other. Now what kind of force is capable of this peculiar effect? If we think of the matter, we shall realize that tidal action would produce just this result. We see it daily in the case of the Moon; when it is high tide in the open ocean hereabouts, it is high tide also at the opposite end of the Earth. The reason is that the tideraising body pulls the fluid nearest it more strongly than it pulls the Earth as a whole, and pulls the Earth as a whole more than it pulls the fluid at the opposite extremity.

Suppose, now, a stranger to approach a body in space near enough; it will inevitably raise tides in the other’s mass, and if the approach be very close, the tides will be so great as to tear the body in pieces along the line due to their action; that is, parts of the body will be separated from the main mass in two antipodal directions. This is precisely what we see in the spiral nebula. Nor is there any other action that we know of which would thus handle the body. If it were to disintegrate under increased speed of rotation due to contraction upon itself, parts of its periphery should be shed continually and a pin-wheel of matter, not a two-armed spiral, be thrown off. If explosion were the disintegrating cause, disruption would occur unsymmetrically in one or more directions, not symmetrically as here.

REPRESENTATIVE STELLAR SPECTRA

Photographed, in 1907 and 1908, by V. M. SLIPHER, at LOWELL OBSERVATORY
Flagstaff, Arizona, with prism spectrograph.

As the stranger passed on, his effect would diminish until his attraction no longer overbalanced that of the body for its disrupted portions. These might then be controlled and forced to move in elliptic orbits about the mass of which they had originally made part. Thence would come into being a solar system, the knots in the nebula going to form the planets that were to be.

Before proceeding to what proof we have that it actually did occur in this way we may pause to consider some consequences of what we have already learned. Thus what brought about the beginning of the system may also compass its end. If one random encounter took place in the past, a second is as likely to occur in the future. Another celestial body may any day run into the Sun, and it is to a dark body that we must look for such destruction, because they are so much more numerous in space.

That any of the lucent stars, the stars commonly so called, could collide with the Sun, or come near enough to amount to the same thing, is demonstrably impossible for æons of years. But this is far from the case for a dark star. Such a body might well be within a hundredth of the distance of the nearest of our known neighbors, Alpha Centauri, at the present moment without our being aware of it at all. Our senses could only be cognizant of its proximity by the borrowed light it reflected from our own Sun. Dark in itself, our own head-lights alone would show it up when close upon us. It would loom out of the void thus suddenly before the crash.

We can calculate how much warning we should have of the coming catastrophe. The Sun with its retinue is speeding through space at the rate of eleven miles a second toward a point near the bright star Vega. Since the tramp would probably also be in motion with a speed comparable with our own, it might hit us coming from any point in space, the likelihood depending upon the direction and amount of its own speed. So that at the present moment such a body may be in any part of the sky. But the chances are greatest if it be coming from the direction toward which the sun is travelling, since it would then be approaching us head on. If it were travelling itself as fast as the Sun, its relative speed of approach would be twenty-two miles a second.

The previousness of the warning would depend upon the stranger’s size. The warning would be long according as the stranger was large. Let us assume it the mass of the Sun, a most probable supposition. Being dark, it must have cooled to a solid, and its density therefore be much greater than the Sun’s, probably something like eight times as great, giving it a diameter about half his or four hundred and thirty thousand miles. Its apparent brightness would depend both upon its distance and upon its intrinsic brightness or albedo, and this last would itself vary according to its distance from the Sun. While it was still in the depths of space and its atmosphere lay inert, owing to the cold there, its intrinsic brightness might be that of the Moon or Mercury. As its own rotation would greatly affect the speed with which its sunward side was warmed, we can form no exact idea of the law of its increase in light. That the augmentation would be great we see from the behavior of comets as they approach the great hearth of our solar system. But we are not called upon to evaluate the question to that nicety. We shall assume, therefore, that its brilliancy would be only that of the Moon, remembering that the last stages of its fateful journey would be much more resplendently set off.

With these data we can find how long it would be visible before the collision occurred. As a very small telescopic star it would undoubtedly escape detection. It is not likely that the stranger would be noticed simply from its appearance until it had attained the eleventh magnitude. It would then be one hundred and forty-nine astronomical units from the Sun or at five times the distance of Neptune. But its detection would come about not through the eye of the body, but through the eye of the mind. Long before it could have attracted man’s attention to itself directly its effects would have betrayed it. Previous, indeed, to its possible showing in any telescope the behavior of the outer planets of the system would have revealed its presence. The far plummet of man’s analysis would have sounded the cause of their disturbance and pointed out the point from which that disturbance came. Celestial mechanics would have foretold, as once the discovery of another planet, so now the end of the world. Unexplained perturbations in the motions of the planets, the far tremors of its coming, would have spoken to astronomers as the first heralding of the stranger and of the destruction it was about to bring. Neptune and Uranus would begin to deviate from their prescribed paths in a manner not to be accounted for except by the action of some new force. Their perturbations would resemble those caused by an unknown exterior planet, but with this difference that the period of the disturbance would be exactly that of the disturbed planet’s own period of revolution round the Sun.

Our exterior sentinels might fail thus to give us warning of the foreign body because of being at the time in the opposite parts of their orbits. We should then be first apprised of its coming by Saturn, which would give us less prefatory notice.

It would be some twenty-seven years from the time it entered the range of vision of our present telescopes before it rose to that of the unarmed eye. It would then have reached forty-nine astronomical units’ distance, or two-thirds as far again as Neptune. From here, however, its approach would be more rapid. Humanity by this time would have been made acquainted with its sinister intent from astronomic calculation, and would watch its slow gaining in conspicuousness with ever growing alarm. During the next three years it would have ominously increased to a first magnitude star, and two years and three months more have reached the distance of Jupiter and surpassed by far in lustre Venus at her brightest.

Meanwhile the disturbance occasioned not simply in the outer planets but in our own Earth would have become very alarming indeed. The seasons would have been already greatly changed, and the year itself lengthened, and all these changes fraught with danger to everything upon the Earth’s face would momentarily grow worse. In one hundred and forty-five days from the time it passed the distance of Jupiter it would reach the distance of the Earth. Coming from Vega, it would not hit the Earth or any of the outer planets, as the Sun’s way is inclined to the planetary planes by some sixty degrees, but the effects would be none the less marked for that. Day and night alone of our astronomic relations would remain. It would be like going mad and yet remaining conscious of the fact. Instead of following the Sun we should now in whole or part, according to the direction of its approach, obey the stranger. For nineteen more days this frightful chaos would continue; as like some comet glorified a thousand fold the tramp dropped silently upon the Sun. Toward the close of the nineteenth day the catastrophe would occur, and almost in merciful deliverance from the already chaotic cataclysm and the yet greater horror of its contemplation, we should know no more.

Unless the universe is otherwise articulated than we have reason to suppose, such a catastrophe sometime seems certain. But we may bear ourselves with equanimity in its prospect for two mitigating details. One is that there is no sign whatever at the moment that any such stranger is near. The unaccounted-for errors in the planetary theories are not such as point to the advent of any tramp. Another is, that judged by any scale of time we know, the chance of such occurrence is immeasurably remote. Not only may each of us rest content in the thought that he will die from causes of his own choosing or neglect, but the Earth herself will cease to be a possible abode of life, and even the Sun will have become cold and dark and dead so long before that day arrives that when the final shock shall come, it will be quite ready for another resurrection.

CHAPTER II
EVIDENCE OF THE INITIAL CATASTROPHE
IN OUR OWN CASE

BY quite another class of dark bodies than those we contemplated in the last chapter is the immediate space about us tenanted. For that, too, is anything but the void our senses give us to understand. Could we rise a hundred miles above the Earth’s surface we should be highly sorry we came, for we should incontinently be killed by flying brickbats. Instead of masses of a sunlike size we should have to do with bits of matter on the average smaller than ourselves but hardly on that account innocuous, as they would strike us with fifteen hundred times the speed of an express train. Only in one respect are the two classes of erratics alike, both remain invisible till they are upon us. Even so, the cause of their visibility is different. The one is announced by the light it reflects, the other by the glow it gives out on its destruction. These last are the meteorites or shooting-stars. They are as well known to every one for their commonness as, fortunately, the first are rare. On any starlight night one need not tarry long before one of these visitants darts across the sky, a brilliant thread of fire gone almost ere it be descried.

Usually this is all of which one is made aware. Silent, ghostlike, the apparition comes and goes, and nothing more of it is either seen or heard. But sometimes there is a good deal more. Occasionally a large ball of flame shoots through the air, a detonation like distant thunder startles the ear, and a luminous train, persisting for several seconds, floats slowly away. Finally if one be fortunate to be near,—but not too near,—one or more masses of stone are seen to fall swiftly and bury themselves in the ground. These are meteorites: far wanderers come at last to rest in graves they have dug themselves.

A great revolution has taken place lately in our ideas concerning meteorites. Indeed, it was not so very long ago, since modern man admitted their astronomic character at all. He looked as askance at them as he did at fossils. It was the fall at Aigle, in Switzerland, April 26, 1803, that first opened men’s eyes to the fact that such falls actually occurred. It is more than a nine days’ wonder at times how long men, as well as puppies, can remain blind. To admit that stones fell from heaven, however, was not to see whence they came. Their paternity was imputed to nearly every body in the sky. They were at first supposed to have been ejected from earthly volcanic vents, then from volcanoes in the Moon. That they are of domestic manufacture is, however, negatived by the paths they severally pursue. Nor can they for like reason have been ejected from the Sun.

The Earth was not their birthplace. It is alien ground in which they lie at last and from which we transfer them to glass cases in our museums. This fact about their parentage they tell by the speed with which they enter our air. They become visible 100 miles up and explode at from 20 to 10, and their speed has been found to be from 10 to 40 miles a second, which is that of cosmic bodies moving in large elliptic orbits about the Sun,—a speed greater than the Earth could ever have imparted.

Four classes of such small celestial bodies tenant space where the planets move: sporadic shooting-stars, meteorites, meteor-streams, and comets. The discovery of the relation of each of these to the solar system and then to each other forms one of the latest chapters of astronomic history. For they turn out to be generically one.

It was long, however, before this was perceived. The first step was taken simultaneously by Professor Olmstead of Yale and Twining in 1833 from reasoning on the superb November meteor-shower of that year. All the shooting-stars, “thick as snowflakes in a storm,” had a common radiant from which they seemed to come. Thus they argued that the meteors must all be travelling in parallel lines along an orbit which the previous shower, of 1799, showed to be periodic. This was the first recognition of a meteor-swarm.

The next advance was when Schiaparelli, in 1862, pointed out the remarkable connection between meteor-swarms and comets. On calculation the August meteor-stream and the comet of 1862 proved to be pursuing exactly the same path. Soon other instances of like association were discovered, and we now know mathematically that meteor-streams can be, deductively that they must be, and observationally that they are, disintegrated comets. More than one comet has even been seen to split.

Then came the recognition that comets are not visitors from space, as Sir Isaac Newton and Laplace supposed, but part and parcel of our own solar system. Without going into the history of the subject, which includes Gauss, Schiaparelli, and finally Fabry’s great Memoir, much too little known, the proof can, I think, be made comprehensible without too much technique, thanks to the fact that the Sun is speeding through space at the rate of eleven miles a second.

Orbits described by bodies under the action of a central force are always conic sections, as Sir Isaac Newton proved. There are two classes of such curves: those which return into themselves, such as the circle and ellipse, and those which do not, the hyperbolæ. If a body travel in the first or closed class about the Sun, it is clearly a member of his family; if in the second, it is a visitor who bows to him only in passing and never returns. Which orbit it shall pursue depends at a given distance solely upon the speed of the body; if that speed be one the Sun can control, the body will move in an ellipse; if greater, in an hyperbola. Obviously the Sun can control just the speed he can impart. Now a comet entering the system from without would already possess a motion of its own which, when compounded with the solar-acquired speed, would make one greater than the Sun could master. Comets, therefore, if visitors from space, should all move in hyperbolæ. None for certain do; and only six out of four hundred even hint at it. Comets, then, are all members of the solar family, excentric ones, but not to be denied recognition of kinship for such behavior.

Still, admittance to the solar family circle was denied to meteorites and shooting-stars. Thus Professor Kirkwood, in 1861, had considered “that the motions of some luminous meteors (or cometoids, as perhaps they might be called) have been decidedly indicative of an origin beyond the limits of the solar system.” Here cometoid was an apt coinage, but when comets were later shown not to be of extra-solar origin, the reasoning carried luminous meteors in its train.[1] Finally Schiaparelli, in 1871, concluded an able Memoir on the subject with the decision that “a stellar origin for meteorites was the most likely and that meteorites were identifiable with shooting-stars.”[2] A pregnant remark this, though not exactly as the author thought, for instead of proving both interstellar, as he intended, both have proved to be solar bound.

It was Professor Newton, in 1889, who first showed that meteorites were pursuing, as a rule, small elliptic orbits about the Sun, and that their motion was direct. He, too, was the first to surmise that meteorites are but bigger shooting-stars.

Now, as to their connection. Of direct evidence we have little. A few meteors have been observed to come from the known radiants of shooting-stars. Two instances we have of the fall of meteorites during star showers. One in 1095, when the Saxon Chronicle tells us stars fell “so thickly that no man could count them, one of which struck the ground and when a bystander cast water upon it steam was raised with a great noise of boiling.” The second case was the fall of a siderite, eight pounds’ worth of nickel-iron, at Mazapil during the Andromede shower of 1885, which was by many supposed to be a part of the lost Biela comet. It contained graphite enough to pencil its own history, but unfortunately could not write. The direction from which it came was not recorded, and so the connection between it and the comet not made out.

The Radiant of a Meteoric Shower, showing also the Paths of Three Meteors which do not belong to this Shower—after Denning.

If our direct knowledge is thus scanty, reasoning affords surer ground for belief. For at this point there steps in a bit of news about the family relations of shooting-stars from a source hardly to have been anticipated. Indeed, it arose from the thought to examine a qualitative statement in Young’s “Astronomy” quantitatively. Mathematics is simply precise reasoning, applied usually to the discovery that a pet theory will not work. But sometimes it presents one with an unexpected find. This is what it did here.

It is an interesting fact of observation that more meteors are visible at six o’clock in the morning than at six o’clock at night in the proportion of 3 to 1. This seeming preference for early rising is due to no matutinality on the part of the meteors, but to the matin aspect then presented by the Earth combined with its orbital motion round the Sun. For at six in the morning the observer stands on the advancing side of the Earth, at the bow of the airship; at six at night he is at the stern. He, therefore, runs into the meteors at sunrise and slips away from them at sunset. He is pelted in the morning in consequence. Just as a pedestrian facing a storm gets wetter in front than behind.

So far the books. Now let us examine this quantitatively according to the direction in which the meteors themselves may be moving before the encounter. Suppose, in the first place, that they were travelling in every possible direction, with the average velocity of the most erratic members of the family, the great comets. On this supposition calculation shows that we ought to meet 5.8 times as many at six in the morning as at six at night. If their orbits were smaller than this, say, something like those of the asteroids, we should find 7.6 to 1 for the ratio.

Suppose, however, that they were all travelling in the same sense as the Earth, direct as it is called in contradistinction to retrograde, and let us calculate what proportion in that case we should meet at the two hours respectively. It turns out to be 2.4 to 1 for the parabolic ones, 3.3 to 1 for the smaller orbited, or almost precisely what observation shows to be the case [[see NOTE 1]]. Here, then, a bit of abstract reasoning has apprized us of a most interesting family fact; to wit, that the great majority of shooting-stars are travelling in the same orderly sense as ourselves. Furthermore, as some must be moving in smaller orbits than the mean, others must be journeying in greater; or, in other words, shooting-stars are scattered throughout the system. In short, these little bodies are tiny planets themselves, as truly planets as the asteroids,—asteroids of a general instead of a localized habit.

Thus meteorites and shooting-stars are kin, and from the fact that they are pursuing orbits not very unlike our own we get our initial hint of a community of origin. Indeed, they are the little bricks out of which the whole structure of our solar system was built up. What we encounter to-day are the left-over fragments of what once was, the fraction that has not as yet been swept up by the larger bodies. And this is why these latter-day survivors move, as a rule, direct. To run counter to the consensus of trend is to be subjected to greater chance of extermination. Those that did so have already been weeded out.

The Mart Iron.

(Proc. Wash. Acad. of Sci.
vol. II. Plate VI.)

From the behavior of meteorites we proceed to scan their appearance. And here we notice some further telltale facts about them. Their conduct informed us of their relationship, their character bespeaks their parentage.

Most meteorites are stones, but one or two per cent are nearly pure iron mixed with nickel. When picked up, they are usually covered with a glossy thin black crust. This overcoat they have put on in coming through our air. Air-begotten, too, are the holes with which many of them are pitted. For entering our atmosphere with their speed in space is equivalent to immersing them suddenly in a blowpipe flame of several thousand degrees Fahrenheit. Thus their surface is burnt and fused to a cinder. Yet in spite of being warm to the touch their hearts are still cosmically cold. The Dhurmsala meteorite falling into moist earth was found an hour afterwards coated with frost. Agassiz likened it to the Chinese culinary chef d’œuvre “fried ice.” It is the cold of space, 200° or more Centigrade below zero, that they bear within, proof of their cosmic habitat.

That they are bits of a once larger mass is evident on their face. Their shape shows that they are not wholes but parts, while their constitution bespeaks them anything but elementary. Diagnosis of it yields perhaps their most interesting bit of news. For it shows their origin. Their autopsy proves them to contain thirty known elements, and not one that is new. The list includes all the substances most common on the Earth’s surface, which is suggestive; but, what is still more instructive, these are combined into minerals which largely differ from those with which we are superficially familiar. Professor Newton, whose specialty they were, has said: “In general they show no resemblance in their mechanical or mineralogical structure to the granitic and surface rocks of the Earth. One condition was certainly necessary in their formation, viz. the absence of free oxygen and of enough water to oxidize the iron.” Thus they are not of the Earth earthy; nor yet, poor little waifs, of the upper crust of any other body.

Section of Meteorite showing Widmannstättian Lines.

(Field Columbian Museum, Chicago.)

Meteorite, Toluca.
(Field Columbian Museum, Chicago.)

In them prove to be occluded gases, which can be got out by heating in the laboratory, and which must have got in when the meteorites were still subjected to great heat and pressure. For only thus could these gases have been absorbed. Both such heat and such pressure accuse some great solid body as origin of this flotsam of the sky. Fragments now, they owe to its disruption their present separate state. This parent mass must have been much larger and more massive than the Earth, as the grate amount of occluded hydrogen, sometimes one-third the volume at 500° C., of the meteorite seems to testify.

The two classes of meteorites, the stone and the iron, show this further by the very differences they exhibit between themselves. For both the amount and the proportions of the occluded gases in the two prove to be quite distinct. In the stones the quantity of gas is greater and the composition is diverse. In the stones carbonic acid gas is common, carbon monoxide rare; in the irons the ratio is just the other way. Thus Wright found in nine specimens of the iron meteorites:—

in ten of stone:—

The stones are much lighter than the iron, their specific gravities being as 3 to 7 or 8 for the metallic. The stones, therefore, came from a more superficial layer of the body torn apart than the iron, and the composition of their occluded gases bears this out. Those in the stones are such as we may conceive absorbed nearer the surface, those in the iron from regions deeper down.

Here, then, the meteorites tell us of another, an earlier, stage of our solar system’s history, one that mounts back to before even the nebula arose to which we owe our birth. For the large body to whose dismemberment the meteorites were due can have been no other than the one whose cataclysmic shattering produced that very nebula which was for us the origin of things. The meteorites, by continuing unchanged, link the present to that far-off past. And they tell us, too, that this body must have been dark. For solid, they inform us, it was, and solidity in a heavenly body means deficiency of light.

That such corroborative testimony to a cataclysmic origin is forthcoming in the sky we shall see by turning again to the spiral nebulæ.

Of the two classes of nebulæ which we contemplated in the last chapter, the amorphous and the structural, there is more to be said than we touched on then.

Nebula ♅ V. 14 Cygni—after Roberts.

Not only in look are the two quite unlike, but the spectroscope shows that the difference in appearance is associated with dissimilarity of character. For the spectrum of the amorphous proves to consist of a few bright lines, due to hydrogen and nebulium chiefly, in the green, whence the name green nebulæ. That of the spirals, on the other hand, is continuous, and therefore white. The great nebula in Andromeda was one of the first in which this was recognized; and the perception was pregnant, for no nebula defies resolution more determinedly than it. We may, therefore, infer that it is not made up of stars, certainly big enough for us to see. On the other hand, from the fact that its spectrum is continuous it must be solid or liquid. Young pointed out that this did not follow, because a gas under great pressure also gives a continuous spectrum. But he forgot that here no such pressure could exist. A nebula of compressed gas could not have an irregular form and would have, in the case of the Andromeda nebula, a mass so enormous as to preclude supposition. Continuity of spectrum here means discontinuity of mass. The spectral solidity of the nebula speaks of a status quo ante, not of a condition of condensation now going on.

Nebula N. G. C. 1499 Persei—after Roberts.

Nebula N. G. C. 6960 in Cygnus—after Ritchey.

Advanced spectroscopic means reveals that the spectra of these “white” nebulæ are not simply continuous. Thus that of the Andromeda nebula shows very faint dark lines crossing it, apparently accordant with those of the solar spectrum and faint bright ones falling near and probably coincident with those of the Wolf-Rayet stars, due to hydrogen, helium, and so forth. These later observations make practically certain what earlier ones permitted us just now only to infer: that it is not composed of stars, but of something subtler still; to wit, of meteorites. The reasoning is interesting, as showing that if one have hold of a true idea, the stars in their courses fight for him.

Nebula M. 51 Canum Venaticorum—after Ritchey.

Although Lockyer has long been of opinion that the nebulæ are composed of meteorites, the present argument differs from his. The way in which their spectra establish their constitution may be outlined as follows: the white nebulæ are from their structure evidently in process of evolution, and if they are in stable motion, as we suppose them to be, their parts are moving round their common centre of gravity. As the white nebulæ resist resolution as obstinately as the green, these parts must be not only solid but comminuted (composed of small particles). Now this would be the case were they flocks of meteorites such as we have seen composed our own system once upon a time. Though all are travelling round the centre of gravity of the flock, each is pursuing its own orbit slightly different from, and intersecting those of, its neighbors. Collisions between the meteors must therefore constantly occur, and the question is, are these shocks sufficient to cause light. Let us take our own system and consider two meteorites at our distance from the Sun, travelling in the same sense, the one in an ellipse, the other in a circle, with a major axis five per cent greater and meeting the other at aphelion. This would be no improper jostle for such heavenly bodies. If we calculate the speeds of both and deduct the elliptic from the circular, we shall have the relative speed of collision. It proves to be a half a mile a second or 30 times the speed of an express train. As such a train brought up suddenly against a stone wall would certainly elicit sparks, we see that a speed 30 times as great, whose energy is 900 times greater, is quite competent to a shock sufficient to make us see stars en masse. But, indeed, there must be collisions much more violent than this; both because the central mass is often much greater and because the orbits differ much more, and the effect would increase as the square of the speed. The heat thus generated would cause the meteorites to glow, and at the same time raise the temperature of the gases in and about them. Furthermore, the light would come to us through other non-affected portions of gas between us and the scene of the collision. Thus all three peculiarities of the spectra stand explained: we have a continuous background of light due to heated solid meteorites, the bright lines of glowing gases, and dark lines due to other gases not ignited, lying in our line of sight.

In addition we should perceive another result. Collisions would be both more numerous and more pronounced toward the centre of the nebula, for it must speedily grow denser toward its core owing to the falling in of meteorites, in consequence of shock. Being denser in the centre, the particles would there be thicker and be travelling at greater speed. The nebulæ, therefore, should be brightest at their centres, which is accordant with observation.

Thus from having offered themselves exemplars of the way in which our own system came into being, the white nebulæ assert their present constitution to be that from which we know our system sprang.

Another suggestive fact about the present members of our solar system which has something to say about a past collision is the densities of the different planets. The average density of the four inner planets, Mars, the Earth, Venus, and Mercury is nearly four times that of the four outer ones Neptune, Uranus, Saturn, and Jupiter[[see NOTE 2]]. The discrepancy is striking and cannot be explained by size, as the smallest are the most massive, and if all were primally of like constitution, should be the least compressed. Nor can it be explained simply by greater heat tending to expand them, for Neptune and Uranus show no signs of being very hot. The minor differences between members of each group are probably explicable in part by these two factors, mass and heat, but the great gulf between the two groups cannot so be spanned. We are then driven to the supposition that the materials composing the outer ones were originally lighter. Now this is precisely what should happen had all eight been formed by disruption of a previous body. For its cuticle would be its least dense portion, and on disruption would travel farthest away, not because of being lighter, but because of being on the outside. Parts coming from deeper down would remain near, and be denser intrinsically.

What the present densities of the planets enable us to infer of the cataclysm from which they came, a remarkable set of spectrograms taken not long ago by Dr. V. M. Slipher, at Flagstaff, seems to confirm.

The spectrograms in question were made possible by his production of a new kind of plate. His object was to obtain one which should combine sufficient speed with great photographic extension of the spectrum into the red. For it is in the red end that the absorption lines due to the planets’ atmospheres chiefly lie. With the plates heretofore used it was impossible to go much beyond the yellow, the C line marking the Ultima Thule of attent. Not only was it advisable to get more particularity in the parts previously explored, but it was imperative to go beyond into parts as yet unknown. After several attempts he succeeded, the plates when exposed showing the spectra beyond even the A band. Of their wealth of depiction it is only necessary to say that in the spectrum of Neptune 130 lines and bands can easily be counted between the wave-lengths 4600 µµ, 7600 µµ. Of these, 31 belong to the planet, which compares with 6 found by Huggins, 10 by Vogel, and 9 by Keeler in the part of its spectrum they were able to obtain.

THE SPECTRA OF THE MAJOR PLANETS.

Photographed, in 1907, by V. M. Slipher, at THE LOWELL OBSERVATORY
Flagstaff, Arizona.

The result was a revelation. The plates exposed a host of lines never previously seen; lines that do not appear in the spectrum of the Sun, nor yet in the added spectrum of the atmosphere of the Earth, but are due to the planets’ own envelopes. But this was only the starting-point of their disclosures. When in this manner he had taken the color signatures of Jupiter, Saturn, Uranus, and Neptune, an orderly sequence in their respective absorption bands stood strikingly confessed. In other words, their atmospheres proved not only peculiar to themselves and unlike what we have on Earth, but progressively so according to a definite law. That law was distance from the Sun. When the spectra were arranged vertically in ordered orbital relation outward from the Sun, with that of the lunar for comparison on top, a surprising progression showed down the column in the strange bands, an increase in number and a progressive deepening in tint. The lunar, of course, gives us the Sun and our own air. All else must therefore be of the individual planet’s own. Beginning, then, with Jupiter, we note, besides the reënforcement of what we know to be the great water-vapor bands ‘a,’ several new ones, which show still darker in the spectrum of Saturn. The strongest of these is apparently not identifiable with a band in the spectra of Mira Ceti in spite of falling near it. Passing on to Uranus, we perceive these bands still more accentuated, and with them others, some strangers, some solar lines enhanced. Thus the hydrogen lines stand out as in the Sirian stars. All deepen in Neptune, while further newcomers appear.

Thus we are sure that free hydrogen exists in large quantities in the atmospheres of the two outermost planets and most so in the one farthest off. Helium, too, apparently is there, and other gases which in part may be those of long-period stars, decadent suns, in part substances we do not know.

From the fact that these bands are not present in the Sun and apparently in no type of stars, we may perhaps infer that the substances occasioning them are not elements but compounds to us unknown. And from the fact that free hydrogen exists there alongside of them, and apparently helium, too, we may further conclude that they are of a lighter order than can be retained by the Earth.

But now, we may ask, why should these lighter gases be found where they are? It cannot be in consequence simply of the kinetic theory of gases from which a corollary shows that the heaviest bodies would retain their gases longest, because the strange gases are not apportioned according to the sizes of their hosts. Jupiter, by all odds the biggest in mass, has the least, and Saturn, the next weightiest, the next in amount. Nor can title to such gaseous ownership be lodged in the planet’s present state. For though Jupiter is the hottest and Saturn the next so, the increased mass more than makes up in restraint what increased temperature adds in molecular volatility—as we perceive in the cases of the Sun and Earth.

No; their envelopes are increasingly strange because their internal constituents are different, and as hydrogen is most abundant in Neptune, the lightest of all the gases, it is inferable that this planet’s material is lighter. As distance from the Sun determines their atmospheric clothing, so distance decides upon their bodies, too. It was all a case of primogeniture. The light strange matter that constitutes them was so because it came from the outer part of the dismembered parent orb. Neptune the outermost, Uranus the next, then Saturn and Jupiter came in that order from the several successive layers of the pristine body, while the inner planets came from parts of it deeper down. The major planets were of the skin of the dismembered body, we of its lower flesh.

Very interesting the study of these curious spectral lines from the outer planets for themselves alone; even more so for what one would hardly have imagined: that they should actually tell us something of the genesis of our whole solar system. They corroborate in so far what the meteorites have to say.

That the meteorites are solid and, except for their experiences in coming through our air, bear no marks of external heat, is a fact which is itself significant. It seems to hint not at a crash as their occasioning but at disruptive tidal strains. The parent body appears to have been torn apart without much development of heat. Perhaps, then, we had no gloriously pyrotechnic birth, but a more modest coming into existence. But about this we must ourselves modestly be content to remain for the present in the dark.

Not the least important feature of the theory I have thus outlined is that it finishes out the round of evolution. It becomes a conception sapiens in se ipso totus, teres atque rotundus. To frame a theory that carries one back into the past, to leave one there hung up in heaven, is for inconclusiveness as bad as the ancient fabulous support of the world, which Atlas carried standing on an elephant upheld by a tortoise. What supported the tortoise we were not told. So here, if meteorites were our occasioning, we must account for the meteorites, starting from our present state. This the present presentation does.

Thus do the stones that fall from the sky inform us of two historic events in our solar system’s career. They tell us first and directly of a nebula made up of them, out of which the several planets were by agglomeration formed and of which material they are the last ungathered remains. And then they speak to us more remotely but with no less certainty of a time antedating that nebula itself, a time when the nebula’s constituents still lay enfolded in the womb of a former Sun.

Man’s interest in them hitherto has been, as with other things, chiefly proprietary. Greed of them has grown so keen that legal questions have been raised of the ownership of their finding, and our courts have solemnly declared them not “wild game” but “real estate,” and as such belonging to the owner of the land on which they fall.

But to the scientific eye their estate is something more than “real,” for theirs is the oldest real estate in the solar system. They were what they are now when the Earth we pride ourselves in owning was but a molten mass.

So that when in future you see these strange stones in rows upon a museum’s shelves, regard them not as rarities, in which each museum strives to outdo its neighbors by the quantity it can possess, but as rosetta stones telling us of an epoch in cosmic history long since passed away—of which they alone hold the key. Look at them as the literary do their books, for that which they contain, not as the bibliophile to whom a misprint copy outvalues a corrected one and by whom “uncuts” are the most prized of all.

CHAPTER III
THE INNER PLANETS

When we recall that the Ptolemaic system of the universe was once taught side by side with the Copernican at Harvard and at Yale, we are impressed, not so much with the age of our universities, as with the youth of modern astronomy and with the extraordinary vitality of old ideas. That the Ptolemaic system in its fundamental principle was antiquated at the start, the older Greeks having had juster conceptions, does not lessen our wonder at its tenacity. But the fact helps us to understand why so much fossil error holds its ground in many astronomic text-books to-day. That stale intellectual bread is deemed better for the digestion of the young, is one reason why it often seems to them so dry.

Orbits of the Inner Planets.

Before entering upon the problem of the genesis and career of a world, it is essential to have acquaintance with the data upon which our deductions are to rest. To set forth, therefore, what is known of the several planets of our solar system, is a necessary preliminary to any understanding of how they came to be or whither they are tending; and as our knowledge has been vitally affected by modern discoveries about them, it is imperative that this exposition of the facts should be as near as possible abreast of the research itself. I shall, therefore, give the reader in this chapter a bird’s-eye view of the present state of planetary astronomy, which he will find almost a different part of speech from what it was thirty years ago. It is not so much in our knowledge of their paths as of their persons that our acquaintance with the planets has been improved. And this knowledge it is which has made possible our study of their evolution as worlds.

Could we get a cosmic view of the solar system by leaving the world we live on for some suitable vantage-point in space, two attributes of it would impose themselves upon us—the general symmetry of the whole, and the impressively graded proportions of its particular parts.

Round a great central globular mass, the Sun, far exceeding in size any of his attendants, circle a series of bodies at distances from him quite vast, compared with their dimensions. These, his principal planets, are in their turn centres to satellite systems of like character, but on a correspondingly reduced scale. All of them travel substantially in one plane, a fact giving the system thus seen in its entirety a remarkably level appearance, as of an ideal surface passing through the centre of the Sun. Departing somewhat from this general uniformity in their directions of motion, and also deviating more from circularity in their paths, some much smaller bodies, a certain distance out, dart now up now down across it at different angles and from all the points of the compass, agreeing with the others only in having the centre of the Sun their seemingly never attained goal of endeavor. These bodies are the asteroids. Surrounding the whole, and even penetrating within its orderly precincts, a third class would be visible which might be described for size as cosmic dust, and for display as heavenly pyrotechnics. Coming from all parts of space indifferently they would seem to seek the Sun in almost straight lines, bow to him in circuit, and then depart whence they came. For in such long ellipses do they journey that these seem to be parabolas. These visitants are the comets and their associates the meteor-streams.

Although for purposes of discrimination we have labelled the several classes apart, an essential fact about the whole company is to be noted: that no hard and fast line can be drawn separating the several constituents from one another. In size the members of the one class merge insensibly into the other. Some of the planets are hardly larger than some of the satellites; some of the satellites than some of the asteroids; some of the asteroids than comets and shooting-stars. In path, too, we find every gradation from almost perfect circularity like the orbits of Io and Europa to the very threshold of where one step more would cease to leave the body a member of the Sun’s family by turning its ellipse into an hyperbola. Finally, in inclination we have every angle of departure from orthodox platitude to unconforming uprightness. This point, that heavenly bodies, like terrestrial ones, show all possible grades of indistinction, is kin to that specific generalization by which Darwin revolutionized zoölogy a generation ago. It is as fundamental to planets as to plants. For it shows that the whole solar system is evolutionarily one.

A second point to be noticed in passing is that undue inclination and excessive eccentricity go together. The bodies that have their paths least circular have them, as a rule, the most atilt. And with these two qualities goes lack of size. It is the smallest bodies that deviate most from the general consensus of the system. With so much by way of generic preface, the pregnancy of which will become apparent as we proceed, we come now to particular consideration of its members in turn.

Nearest to the Sun of all the planets comes Mercury. So close is he to that luminary, and so far within the orbit of the earth, that he is not a very common object to the unaided eye. Copernicus is said never to have seen him, owing, doubtless, to the mists of the Vistula. By knowing when to look, however, he may be seen for a few days early in the spring in the west after sunset, or before sunrise in the east in autumn. He is then conspicuous, being about as bright as Capella, for which star or Arcturus he is easily mistaken by one not familiar with the constellations.

His mean distance from the Sun is thirty-six million miles, but so eccentric is his orbit, the most so of any of the principal planets, that he is at times half as far off again as at others. Even his orbital behavior is the least understood of any in the solar system. His orbit swings round at a rate which so far has defied analysis. It may be a case of reflected perturbation, one, that is, of which the indirect effect from another body becomes more perceptible than would be the direct effect on the body itself. As yet it baffles geometers.

As to his person, our ignorance until lately was profound. It is only recently that such fundamental facts about him as his size, his mass, and his density have been reached with any approach to precision. This was because he so closely hugs the Sun that observations upon his full, or nearly full, disk had never been attempted. When I say that his volume was not known to within a third of its amount, his mass not closer than one-half, while his received density was nearly double what we now have reason to suppose the fact, some idea of the depth of our nescience may be imagined. This, of course, did not prevent text-books from confidently misinstructing youth, or Nautical Almanacs from misguiding computers with figures that thus almost achieved immortality, so long had they passed current in spite of lacking that perfection which is usually assigned as its warrant.

Sulla Rotazione di Mercurio—Di G. V. Schiaparelli.

Schiaparelli first put astronomy on the right track. By attempting daylight observations of the planet, not toward night, but actually at midday, he made some remarkable discoveries, and though he did not detect the hitherto erroneous values of the volume, the mass, or the density, his method of observation paved the way for their ascertainment. What he sought, and found, was evidence of markings upon the disk by which the planet’s time of rotation might be determined. Up to then, Schroeter’s value of about twenty-four hours had been accepted, on very slender evidence indeed, and passed into all the books. But when the planet came to be observed by noon, very definite markings stood out on its face, which showed its rotation to take place, not in twenty-four hours, but in eighty-eight days. By a persistence equal to his able choice of observing time, he established this beyond dispute. He proved the revolutionizing fact that Mercury’s periods of rotation and of revolution were the same.

He detected, too, the evidence in the position of the markings of the planet’s great libratory swing due to the eccentricity of its orbit, a result as remarkable as a feat of observation as it was conclusive as a proof.

If Schiaparelli had never done any other astronomical work, this study of Mercury would have placed him as the first observer of his day. For the observations are so difficult that the planet not only baffled all his predecessors, but has foiled many since who are credited with being observers of eminence.

In 1896 the study of Mercury was taken up at the Lowell Observatory in Arizona along the same lines that had proved so successful with Schiaparelli, but without using his observations as guide. Indeed, his papers had not then been read there. The two conclusions were, therefore, independent of one another. The outcome was a complete corroboration and an extension of Schiaparelli’s work. We shall begin with the consideration of the most fundamental point. In the clear and steady air of Flagstaff, permitting of measurement of his disk up to within a few degrees of the Sun, Mercury was found to be much larger than previously thought.

Instead of a diameter of three thousand miles he proved to have one of thirty-four hundred, making his volume nearly half as large again as had been credited him. These measures bore intrinsic evidence of their trustworthiness in an interesting manner, and at the same time produced internal testimony that accounted for the smallness of previous determinations. Measures heretofore had been made, usually if not invariably, either when the planet transited the Sun or when it exhibited a pronounced phase. Now in both these cases the planet looks smaller than it is. In the first case this is due to irradiation, the surrounding disk of the Sun encroaching both to the eye and to the camera upon the silhouette of Mercury. And this inevitable effect had not been allowed for in the measures. In the second case the horns of the planet never seem to extend quite to their true position. This was rendered evident by the Flagstaff series of measures, which began when the planet was a half-moon and continued till it was almost full. As it did so, the values for the diameter steadily increased, even after irradiation was allowed for, although this against the brilliant background of the noonday sky must have been exceeding small, and tended in part to be diminished as the planet attained the full, because of its consequent nearing of the Sun. The measures thus explained themselves and vouched for their own accuracy.[3]

Then came a curious bit of unexpected proof to corroborate them. In his “Astronomical Constants,”[4] published but a short time before, Newcomb had detected a systematic error in the right ascensions of Mercury which he was not able to explain. By diligent mousing that eminent computer had discovered that Mercury was registered by observers too far from the Sun on whichever side of him it happened to be, and in proportion roughly not to its distance off but to the phase the planet exhibited. When the disk was a crescent the discrepancy between observation and theory was large, and thence decreased as the planet passed to the full. He suspected the cause, and would have found it had he not considered the diametral measures of the planet too well assured to permit of doubt. As it was, he neglected a factor which has vitiated almost all the observations made on the planets up to within a few years, the correction for irradiation. This was the case here. The received measures, beginning with Bradley and ending with Todd, had almost without exception been made in transit, and, as no regard had been paid to the contracting effect of irradiation, had been invalidated in consequence. The new method supplied almost exactly the amount needed to explain the right ascensions, a second of arc, and in precise accordance with the place which the discrepancy demanded.

About the mass there has been, and still is, great uncertainty. This is because it can only be found from the perturbing effect it has on Venus, the Earth, or Encke’s comet. Modern determinations, however, are smaller than the older ones; thus Backlund in 1894 got from the effect on Encke’s comet only one-half the mass that Encke had, fifty-three years before. Probably the most reliable information comes from Venus, which Tisserand found to give for Mercury ¹/₇₁₀₀₀₀₀ of the mass of the Sun, or ¹/₂₁ of the mass of the Earth. If we take ¹/₇₀₀₀₀₀₀ as the nearest round number, we find the planet’s density to be 0.66 that of the Earth.

MAP of MERCURY

LOWELL OBSERVATORY 1896-97

The same observations that disclosed at Flagstaff the planet’s size revealed a set of markings on his face so definite as to make the rotation period unmistakable. It takes place, as Schiaparelli found, in eighty-eight days, or the time of the planet’s revolution round the Sun. The markings disclosed the fact, as Schiaparelli had also discovered, in a most interesting manner, for the ellipticity of the planet’s orbit stood reflected in the swing of the markings across the face of the disk, a definiteness in the proof of a really surprising kind. What this means we shall see in a subsequent chapter when we take up the mechanical problem of the tides. Another result that issued from the positions of the markings was the determination of the planet’s pole. Except for the libration above noticed, the markings kept an invariable longitudinal position upon the illuminated disk, showing that the planet turned always the same face to the Sun; but latitudinally a difference was noticeable between their place in October-November, 1896, and in February-March, 1897, the latter being 4° farther north. Now this is just what the orbital position should have caused, if the pole stood vertically to it. Thus a difference of 4° from perpendicularity should have been discernible, had it existed,—a very small amount in such a determination. We may, therefore, conclude that the axis stands plumb to the orbit, and this is what theory demands.

The state of things this introduces to us upon that other world is to our ideas exceeding strange. It is not so much the slowness of the diurnal spin, eighty-eight times as long as our own, which is surprising, as the fact that this makes its day infinite in length. Two antipodal hemispheres divide the planet, the one of which frizzles under eternal sun, the other freezes amid everlasting night. The Sun does not, indeed, stand stock-still in the sky, but nods like some huge pendulum to and fro along a parallel of latitude. In consequence of libration the two great domains of day and night are sundered by a strip of debatable ground 23½° in breadth on either side, upon which the Sun alternately rises and sets. Here there is a true day, eighty-eight of our days in length from one sunrise to the next. But its day and night are not apportioned alike. The eastern strip has its daylight briefer than its starlight hours; the western has them longer. Nor are different portions of the strips similarly circumstanced in their sunward regard. Only the edge next perpetual day has anything approaching an equal distribution of sunlight and shade. The farther one just peeps at the Sun for a moment every eighty-eight days, and then sinks back again into obscurity.

The transition from day to night is equally instantaneous and profound. For little or no twilight here prolongs the light; since the air, if there be any at all, is too thin to bend it to service round the edge to illuminate the night. When the libratory Sun sets, darkness like a mantle falls swiftly over the face of the ground. No evidence of atmosphere has ever been perceived, and theory informs that it should be nearly, if not wholly, absent.

In consequence of the rigid uprightness of the planet’s axis, seasons do not exist. Their nearest simulacrum comes from the seeming dilatation of the Sun during half the year, and its apparent contraction during the other half. It expands so much between its January and its July as to receive more heat in the ratio of nine to four. A seasonless, dayless, and almost yearless planet, it is better to look at than to look from; but its study opens our eyes to the great diversity which even one of our nearest neighbors exhibits from what we take as matters of course on Earth.

That what we take offhand to be purely astronomic phenomena should turn out to be so essentially of the particular world, worldly, clarifies vision of what these really are, and how dependent on and interwoven with everyday life astronomy is. Or, we may consider it turned about and realize how purely astronomic relations, such abstract mechanical matters as rotations and revolutions, result in completely changing the very face and character of the globe concerned. Mercury to-day stares forever at the Sun. The markings we see have stereotyped this stare to its inevitable result. For they seem to mark a globe sun-cracked. At such a condition the curious crisscross of dark, irregular lines certainly hints, accentuated and perfected as it is by a bounding curve where the mean sunward side terminates to the enclosing them as by the carapace of a tortoise. Though they cannot probably be actual cracks, however much they may resemble such, yet they may well owe their existence to that fundamental cause.

In color the planet is ghastly white; of that wan hue that suggests a body from which all life has fled. Far whiter than Venus in point of fact, the rosy tint with which it sparkles in the sunset glow is all borrowed of the dying day and vanishes when the planet is looked at in the uncompromising light of noon. Seen close together once at Flagstaff it was possible directly to compare the two; when Mercury, although lit by the Sun two and a half times as brilliantly as Venus, was, surface for surface, more than twice as faint. Müller has found its intrinsic brightness about that of our Moon, which in some respects it resembles, though it apparently departs widely from any similarity in others. The bleached bones of a world; that is what Mercury seems to be.

Venus comes next in order outward from the Sun. To us her incomparable beauty is partly the result of propinquity: nearness to ourselves and nearness to the Sun. Relatively so close is she to both that she does not need the Sun’s withdrawal to appear, but may nearly always be seen in the daytime in clear air if one knows where to look for her. Situate about seven-tenths of our own distance from our common giver of light and heat, she gets about double the amount that falls to our lot, so that her surface is proportionately brilliantly illuminated. Being also relatively near us, she displays a correspondingly large surface.

But though part of her lustre is due to her position, a part is her own. Direct visual observation, as we remarked above, shows her intrinsic brightness to be more than five times that of Mercury, square mile to square mile of surface for the two. Now this has been determined very carefully photometrically by Müller at Potsdam. The result of his inquiry was to indicate that Mercury shines with 0.17 of absolute reflection, Venus with 0.92. So high a value has seemed to many astronomers impossible, because so far surpassing that which has tacitly been taken as the ne plus ultra of planetary brightness, that of cloud, 0.72.

Now, one of the direct outcomes of the study of Venus at the Lowell Observatory was an explanation of this seemingly incredible phenomenon. When the planet came to be critically examined there under conditions of seeing which permitted discovery, markings very faint, but nevertheless assurable, stood presented on the planet’s face. These markings, of which we shall have more to say in a moment, had this of pertinency to our present point, that they kept an invariable position to one another. They thus betrayed themselves to be surface features. Furthermore, their dimness was as invariable an attribute of them as their place. They were not obscured on some occasions and revealed at others, but stayed, so far as one might judge, permanently the same. They were thus neither clouds themselves nor subject to the caprice of cloud. The old idea that Venus was a cloud-wrapped planet and owed her splendor to this envelope, vanished literally into thin air.

It is precisely because she is not cloud-covered that her lustre is so great. She “clothes herself with light as with a garment” by a physical process of some interest. As becomes the Mother of the Loves, this is gauze of the most attenuated character, and yet a wonderful heightener of effect. For it consists solely of the atmosphere that compasses her about. It is well known that a substance when comminuted reflects much more light than when condensed into a solid state. Now an atmosphere is itself such a comminuted affair, and, furthermore, holds in suspension a variety of dust. This would particularly be the case with the atmosphere of Venus, as we shall have reason to see when we consider the conditions upon that planet made evident by study of its surface markings. To her atmosphere, then, she owes four-fifths or more of her brilliancy. And this stands corroborated by the low albedo of both Mercury and the Moon, which have no atmosphere, and by the intermediate lustre of Mars, which has some, but little.[5]

The rotation time of Venus, the determination, that is, of the planet’s day, is one of the fundamental astronomical acquisitions of recent years. For upon it turns our whole knowledge of the planet’s physical condition. More than this, it adds something which must be reckoned with in the framing of any cosmogony. It is not a question of academic accuracy merely, of a little more or a little less in actual duration, but one which carries in its train a completely new outlook on Venus and sheds a valuable side-light upon the history of our whole planetary system.

Unconsciously influenced, one is inclined to think, by terrestrial analogies, astronomers for more than a couple of centuries, ever since the time of the first Cassini in 1666, deemed the day of Venus to be just under twenty-four hours in length. So well attested was its determination, and so precisely figured to the minute, that it imposed itself upon text-books which stated it as an acquired fact down to the last second. Nevertheless, Schiaparelli was not so sure, and proceeded to look into the matter. He first looked for himself, and then looked up all the old observations. His chief observational departure was observing by day as near to noon as possible; because then the planet was highest, to say nothing of the taking off from its glare by the more brilliant sky. From certain dark markings around two bright spots near the southern cusp, of one of which spots the detection dates from the time of Schroeter, and from a long, dark streak stretching thence well down the disk, he convinced himself that no such period as twenty-four hours could possibly be correct, inasmuch as whenever he looked, the markings were always there. His notes read, “Same appearance as yesterday,” day after day, until he would really have saved ink and penmanship had he had the phrase cut into a die and stamped. He concluded that the rotation was at least six months long, and was probably synchronous with the planet’s time of revolution. This was in 1889. In 1895 he became still more sure, and showed how the older observations were really compatible with what he had found.

In 1896 the subject was taken up at Flagstaff. Very soon it became evident there that markings existed on the disk, most noticeable as fingerlike streaks pointing in from the terminator, faint but unmistakable from the identity of their successive presentation. Schroeter’s projection near the south cusp was also clearly discernible as well as two others, one in mid-terminator, one near the northern cusp. Schiaparelli’s dark markings also came out, developing into a sort of collar round the southern pole. Other spots and streaks also were discernible, and all proved permanent in place. By watching them assiduously it was possible to note that no change in position occurred in them, first through an interval of five hours, then through one of days, then of weeks. Care was taken to guard against illusion. It thus became evident that they bore always the same relation to the illuminated portion of the disk. This illuminated part, then, never changed. In other words, the planet turned always the same face to the Sun. The fact lay beyond a doubt, though of course not beyond a doubter.[6]

Venus. October, 1896—March, 1897—Drawings by Dr. Lowell.

Venus. April 12, 1909, 3h 26m—4h 22m—by Dr. Lowell.

The years that have passed since these observations were made have brought corroboration of them. Several observers at Flagstaff have seen and drawn them and added discoveries of their own, among whom are especially to be mentioned, of the observatory staff: Miss Leonard, Dr. Slipher, and Mr. E. C. Slipher.[7]

In character these markings were peculiar and distinctive. In addition to some of more ordinary character were a set of spokelike streaks which started from the planet’s periphery and ran inwards to a point not very distant from the centre. The spokes started well-defined and broad at the edge, dwindling and growing fainter as they proceeded, requiring the best of definition for their following to their central hub.

The peculiar symmetry thus displayed, a symmetry associated with the planet’s sunrise and sunset line, or, strictly speaking, what would be such did the Sun for Venus ever rise or set, would seem inexplicable, except for that very association. When we reflect, however, upon what this means, a very potent cause for them becomes apparent, so potent that surprise is turned into appreciation that nothing else could well exist. That Venus turns on her axis in the same time that she revolves about the Sun, in consequence of which she turns always the same face to him, must cause a state of things of which we can form but faint conception, from any earthly analogy. One face baked for countless æons, and still baking, backed by one chilled by everlasting night, while both are still surrounded by air, must produce indraughts from the cold to the hot side of tremendous power. A funnel-like rise must take place in the centre of the illuminated hemisphere, and the partial vacuum thus formed would be filled by air drawn from its periphery, which, in its turn, would draw from the regions of the night side. Such winds would sweep the surface as they entered, becoming less superficial as they advanced, and the marks of their inrush might well be discernible even at the distance we are off. Deltas of such inroad would thus seam the bounding circle of light and shade.

I
Showing convection currents in the planet’s atmosphere.

II
Showing shift in central barometric depression due
to rotation of the planet affecting the winds.

Venus.

Another result of the aërial circulation would be the removal of all moisture from the sunward face, and its depositing in the form of ice upon the night one. For the heated air would be able to carry much water in suspension, which, on cooling, after it had reached the dark hemisphere would unload it there. In the low temperature there prevailing, this moisture would all be frozen, and so largely estopped from return. This process continuing for ages would finally deplete one side of all its water to heap it up in the form of ice upon the other.

Now it is not a little odd that a phenomenon has been observed upon Venus which seems to display just this state of things. Many observers have noted an ashen light on the dark side of her disk. Some have tried to account for it as Earth shine, the same earth-reflected light that makes dimly visible the old moon in the new moon’s arms. But the Earth is too far away from Venus to permit of any such effect; nor is there any other body that could thus relieve its night. But if the night hemisphere of Venus be one vast polar sheet, we have there a substance able to mirror the stars to a ghostlike gleam which might be discernible even from our distant post.

Venus

Rotation 225 days.

Thus when we reason upon them we see that the peculiar markings of the planet lose their oddity, becoming the very pattern and prototype of what we should expect to view. Interpreted, they present us the picture of a plight more pitiable even than that of Mercury. For the nearly perfect circularity of Venus’ orbit prevents even that slight change from everlasting sameness which the libration of Mercury’s affords. To Venus the Sun stands substantially stock-still in the sky,—a fact which must prove highly reassuring to Ptolemaic astronomers there, if there be any still surviving from her past. No day, no seasons, practically no year, diversifies existence or records the flight of time. Monotony eternalized,—such is Venus’ lot.

What visual observations have thus discovered of the rotation time of Venus, with all that follows from it, the spectroscope at Flagstaff has confirmed. At Dr. Slipher’s hands, spectrograms of the planet have told the same tale as the markings. It was with special reference to this point that the spectrograph there was constructed, and the first object to which it was directed was Venus.[8]