CHAPTER VIII. Death of a World

DEATH OF A WORLD

Everything around us on this Earth we see is subject to one inevitable cycle of birth, growth, decay. Nothing that begins but comes at last to end. Not less is this true of the Earth as a whole and of each of its sister planets. Though our own lives are too brief even to mark the slow nearing to that eventual goal, the past history of the Earth written in its rocks and the present aspects of the several planets that circle similarly round the Sun alike assure us of the course of aging as certainly as if time, with all it brings about, passed in one long procession before our very eyes.

Death is a distressing thing to contemplate under any circumstances, and not less so to a philosopher when that of a whole world is concerned. To think that this fair globe with all it has brought forth must lapse in time to nothingness; that the generations of men shall cease to be, their very records obliterated, is something to strike a chill into the heart of the most callous and numb endeavor at its core. That æons must roll away before that final day is to the mind of the far-seeing no consolation for the end. Not only that we shall pass, but that everything to show we ever were shall perish too, seems an extinction too overpowering for words.

But vain regret avails not to change the universe’s course. What is concerns us and what will be too. From facing it we cannot turn away. We may alleviate its poignancy by the thought that our interest is after all remote, affecting chiefly descendants we shall never know, and commend to ourselves the altruistic example so nobly set us by doctors of medicine who, on the demise of others at which—and possibly to which—they have themselves assisted, show a fortitude not easily surpassed, a fortitude extending even to their bills. If they can act thus unshaken at sight of their contemporaries, we should not fall behind them in heroism toward posterity.

Having in our last chapter run the gantlet of the geologists, we are in some sort fortified to face death—in a world—in this. The more so that we have some millenniums of respite before the execution of the decree. By the death of a planet we may designate that stage when all change on its surface, save disintegration, ceases. For then all we know as life in its manifold manifestations is at an end. To this it may come by many paths. For a planet, like a man, is exposed to death from a variety of untoward events.

Of these the one least likely to occur is death by accident. This, celestially speaking, is anything which may happen to the solar system from without, and is of the nature of an unforeseen catastrophe. Our Sun might, as we remarked, be run into. For so far as we know at present the stars are moving among themselves without any too careful regard for one another. The swarm may be circling a central Sun as André states, but the individual stars behave more like the random particles of a gas with licensed freedom to collide; whereas we may liken the members of the solar system to molecules in the solid state held to a centre from which they can never greatly depart. Their motions thus afford a sense of security lacking in the universe at large.

Such an accident, a collision actual or virtual with another sun, would probably occur with some dark star; of which we sketched the ultimate results in our first chapter. The immediate ones would be of a most disastrous kind. For prefatory to the new birth would be the dissolution to make such resurrection possible. Destruction might come direct, or indirectly through the Sun. For though the Sun would be the tramp’s objective point, we might inadvertently find ourselves in the way. The choice would be purely academic; between being powdered, or deorbited and burnt up.

So remote is this contingency that it need cause us no immediate alarm, as I carefully pointed out. But so strong is the instinct of self-preservation and so pleasurable the sensation of spreading appalling news, that the press of America, and incidentally Europe, took fire, with the result, so I have been written, that by the time the pictured catastrophe reached the Pacific “it had assumed the dimensions of a first magnitude fact.”

This is the first way in which our world may come by its death. It is possible, but unlikely. For our Earth, long before that, is morally certain to perish otherwise.

The second mode is one, incident to the very constitution of our solar system. It follows as a direct outcome of that system’s mechanical evolution, and may be properly designated, therefore, as due to natural causes. It might be diagnosed as death by paralysis. For such it resembles in human beings, palsy of individual movement afflicting a planet instead of a man.

Tidal friction is the slow undermining cause; a force which is constantly at work in the action of every body in the universe upon every other. As we previously explained, the pull of one mass upon another is inevitably differential. Not only is the second drawn in its entirety toward the first, falling literally as it circles round, but the nearer parts are drawn more than the centre and the centre more than those farthest away. We may liken the result to a stretched rotating rubber ball, with, however, one important difference,—that each layer is more or less free to shear over the others. The bulge, solicited by the rotation to keep up, by the disturber to lag behind, is torn two ways, and the friction acts as a break upon the body’s rotation, tending first to turn it over if it be rotating backward and then to slow it down till the body presents the same face in perpetuity to its primary. The tides are the bulge, not simply those superficial ones which we observe in our oceans, and know to be so strong, but substantial ones of the whole body which we must conceive thus as egg-shaped through the action that goes on—the long diameter of the egg pointing somewhat ahead of the line joining its centre to the distorting mass. All the bodies in the solar system are thus really egg-shaped, though the deformation is so slight as to escape detection observationally. The knowledge is an instance of how much more perceptive the brain is than the eye. For we are certain of the fact, and yet to see it with our present means is impossible, and may long remain so.

Two concomitant symptoms follow the friction of the tidal ansæ: a shift of the plane in which the rotation takes place, and a loss of speed in the spin itself. The first tends to bring the plane of rotation down to the orbital plane, with rotation and revolution in the same sense. This effect takes place quicker than the other, and in consequence different stages may be noted in the creeping paralysis by which the body is finally overcome. Loss of seasons characterizes the first. For the coincidence of the two planes means invariability in the Sun’s declination throughout the year for a given latitude. This reduces all its days to one dead level in which summer and winter, spring and autumn, are always and everywhere the same. There is thus a return at the end of the planet’s career to an uneventful condition reminiscent of its start; a senility in planets comparable to second childhood in man.

In large planets this outgrowing of seasons occurs before they have any, while the planet is yet cloud-wrapped. Such planets know nothing of some attributes of youth, like those unfortunate men who never were boys; just as reversely the meteorites are boys that never grew up. For if the planet be large, the action of the tidal forces is proportionately more powerful; while on the other hand the self-aging of the planet is greatly prolonged, and thus it may come about that the former process outstrips the latter to the missing of seasons entirely. This is sure to be the case with Jupiter, as the equator has already got down to within 3° of the orbit, and threatens to be the case with Saturn. These bodies, then, when they shall have put off their swaddling clothes of cloud, will wake to climates without seasons; globes where conditions are always the same on the same belts of latitude, and on which these alter progressively from equator to pole. Variety other than diurnal is thus excluded from their surfaces and from their skies. For the Sun and stars will rise always the same, in punctual obedience only to the slowly shifting year.

The next stage of deprivation is the parting with the day. Although the day disappears, the result is too much day or too little, depending on where you choose to consider yourself upon the afflicted orb. For tidal friction proceeds to lengthen the twenty-four or other hours first to weeks, then months, then years, and at last to infinity; thus bringing the sun to a stock-still on the meridian, to flood one side of the world with perpetual day and plunge the other in eternal night.

Which of these two hemispheres would be the worse abode, is matter of personal predilection; dust or glacier, deserts both. Everlasting unshielded noon would cause a wind circulation from all points of the enlightened periphery to the centre, whence a funnel-shaped current would rise to overflow back into the antipodes, thence to return by the horizon again. As the night side would be several hundred degrees at least colder than the noon one, all the moisture would be evaporated on the sunlit hemisphere, to be carried round and deposited as ice on the other, there to stay. Life would be either toasted or frappé. A Sahara backed by polar regions would be the obverse and the reverse of the shield.

VENUS—DRAWINGS BY DR. LOWELL SHOWING AGREEMENT AT DIFFERENT DISTANCES.]

The reader may deem the picture a fancy sketch which possibly may not appeal to him. Nevertheless, it not only is possible, but one which has overtaken our nearest of neighbors. To this pass the Mater Amorum, Venus herself, has already been brought. She betrays it by the wrinkles which modern observation has revealed upon her face. Innocent critics, with a gallantry one would hardly have credited them,—which shows how one may wrong even the humblest of creatures,—have denied the existence of these marks of age, on the chivalrous a priori assumption that it could not possibly be true because never seen before. Their negation, in naïve ignorance of the facts, partakes the logic of the gallant captain, who, when asked by a lady to guess her age, replied: “’Pon my word, I haven’t the slightest idea,” hastily adding, “But you don’t look it!” Less commendable than this conventional nescience, but unfortunately more to the point, is the evidence of prying scientific curiosity. Shrewdly divined as much as detected by Schiaparelli, made more certain by the crow’s-feet disclosed at Flagstaff, and corroborated by the testimony of the spectroscope there, her isochronism of rotation and revolution lies beyond a doubt. Attraction to her lord has conquered at last her who was the cynosure of all. Venus, in her old age, stares forever at the Sun, and we all know how ill an aging beauty can support a garish light.

Mercury has been brought to a like pass. This was evident even before the facts came out about Venus, for Venus, true to her instincts, shields herself with a veil of air which largely baffles man’s too curious gaze. Mercury, on the other hand, offers no objection to observation. When looked for at the proper time, his markings are quite distinct, dark, broken lines suggesting cracks. Schiaparelli, again, was the first to perceive the true state of the case, and his observations were independently confirmed and extended at Flagstaff in 1896. In so doing the latter disclosed a very interesting fact. It was evident that the markings held in general a definite fixed position upon the illuminated part of the disk, showing that the planet kept the same face always to the Sun. But systematic observation, continued day after day for weeks, disclosed a curious shift, which, though slight, was unmistakable. Upon thought the cause suggested itself, and on being subjected to calculation proved equal to such accounting. In this singular systematic sway stood revealed the libration in longitude caused by the eccentricity of the planet’s orbit.

_Effect of Libration

Rotation 88 days._]

Mercury revolves about the Sun in an ellipse more eccentric than that of any other principal planet. At times he is half as far off again from him as he is at others. When near, he travels faster than when far. For both reasons, nearness and speed, his angular revolution about the Sun varies greatly from point to point according to where he finds himself in his orbit. His rotation, however, is necessarily uniform. For even the Sun has no power at once to change the enormous moment of momentum of his axial spin. In consequence, at times his angular velocity of revolution gains on his rotation, at other times loses, both coming out together at the end of a complete Mercurial year. The result is a superb rhythmic oscillation, a true mercurial pendulum compensated by celestial laws to perfect isochronism of swing.

The outward sign of this shows in the movement of the markings. To observers in space like ourselves, the planet seems to sway his head as he travels along his orbit. For weeks he turns his face, as shown by the markings on it, more and more over to the left; then turns it back again as far over to the right. It is as if he were looking furtively around as he hastens over his planetary path.

Venus, of course, is equally subject to this law of distraction, but owing to the almost perfect circularity of her orbit she is less visibly affected. In fact, it is not possible to detect her lapse from a fixed regard to the Sun. At most it is no more than a glance out of the corner of her eyes—her slight deviation from perfect rectitude of demeanor. Knowledge of the laws governing such action alone permits us to recognize its occurrence.

Mercury and Venus are the only planets as yet that turn a constant face to their overruling lord. The reason for this appears when one goes into the matter analytically. The tidal force is not the direct pull of the Sun on a particle of the body, but the difference in the pulls upon a particle at the centre and one at the circumference. Being differential, it depends directly upon the radius of the distorted body and inversely upon the third power of its distance away. As the space through which the force acts is proportional to the force itself, the effect is as the squares of the quantities mentioned, or, inversely, as the sixth power of the distance and as the square of the body’s radius. The result thus proves greatest on the planets nearest to the Sun, and diminishes rapidly as we pass outward from him. If, then, the solar force had had time enough to produce its effects, it would be first in Mercury and then in Venus that it should be seen. And this is precisely where we observe it.

The Moon presents us a well-known case of such filial regard, resulting in permanent incompetency of action on its own account. It turns always the same face to us, following us about with the mute attention of a dog to its master. Here again the libration may be detected, for no dog but makes excursions on the road. This case differs from those of Mercury and Venus in that the body to which the regard is paid is not also the dispenser of light and warmth. In consequence, though the side of the Moon with which we are presented remains always the same, we do not always see it; the light creeping over it with the progress of the lunation, from new to full. On this account the worst that happens to our Moon in its old age is that its day becomes its month.

Our Moon is not peculiar in having its day and its month the same. On the contrary, it is now the rule with satellites thus to protract their days. So far as we can observe, all the large satellites of Jupiter turn the same face to him; those of Saturn pay him a like regard; while about those of Uranus and Neptune we are too far off to tell. Their direct respect for their primary, with only secondary recognition of the Sun, keeps them from the full consequences of their fatal yielding to attraction. It is bad enough to have the day half a month long, but worse to have one that never ends, or, still worse, perpetual night.

In our diagnosis of the cause of death in planets, we now pass from paralysis to heart failure. For so we may speak of the next affection which ends in their taking off, since it is due to want of circulation and lack of breath. It comes of a planet’s losing first its oceans and then its air.

To understand how this distressing condition comes about, we must consider one of the interesting scientific legacies of the nineteenth century to the twentieth: the kinetic theory of gases.

The kinetic theory of gases supposes them to be made up of minute particles all alike, which are perfectly elastic and are travelling hither and thither at great speeds in practically straight lines. In consequence, these are forever colliding among themselves, giving and taking velocities with bewildering rapidity, resulting in a state of confusion calculated to drive a computer mad. Somebody has likened a quiet bit of air to a boiler full of furious bees madly bent on getting out. The simile flatters the bees. To follow the vicissitudes of any one molecule in this hurly-burly would be out of the question; still more, it would seem, that of all of them at once. Yet no less Herculean a task confronts us. To find out about their motions, we are therefore driven to what is called the statistical method of inquiry,—which is simply a branch of the doctrine of probabilities. It is the method by which we learn how many people are going to catch cold in Boston next week when we know nothing about the people, or about colds, or about catching them. At first sight it might seem as if we could never discover anything in this hopelessly ignorant way, and as if we had almost better call in a doctor. But in the multitude of colds—not of counsellors—lies wisdom. So in other things not hygienic. As you cannot possibly divine, for instance, what each boy in town is going to do during the year, nor what is his make of mind, how can you say whether he will accidentally discharge a firearm and shoot his playmate or not! And yet if you take all the boys of Boston, you can predict to a nicety how many will thus let off a gun and “not know that it was loaded.”

In this only genuine method of prophecy, complete ignorance of all the actual facts, we are able without knowing anything whatever about each of the molecules to predicate a good deal about them all. To begin with, the pressure a gas exerts upon the sides of a vessel containing it must be the bombardment the sides receive from the little molecules; and the heating due this rain of blows, or the temperature to which the vessel is raised, must measure their energy of translation. On this supposition it is found that the laws of Avogadro and of Boyle are perfectly accounted for, besides many more properties of gases which the theory explains, and as nothing yet has been encountered seriously contradicting it, we may consider it as almost as surely correct as the theory of gravitation. To three great geniuses of the last century we owe this remarkable discovery—Clausius, Clerk Maxwell, and Boltzmann.

By determining the density of a gas at a given temperature and under a given pressure, we can find by the statistical method the average speed of its molecules. It depends on the most probable distribution of their energy. For hydrogen at the temperature of melting ice, and under atmospheric pressure, this speed proves to be a little over a mile a second—a speed, curiously enough, which is to that of light almost exactly as centimetres to miles. But some of the molecules are going at speeds much above the mean; fewer and fewer as the speed gets higher. Just how many there are for any assigned speed, we can calculate by the same ingenious application of unknown quantities.

These speeds have been found for a temperature of freezing, and as the speed varies as the square root of the absolute temperature, we might suppose that when an adventurous or lucky molecule arrived at practically the limit of the atmosphere, where the cold is intense, it would become numbly sluggish. But let us consider this. When we enclose a gas in a cooler vessel, the molecules bombard the sides more than they are bombarded back. In consequence, they lose energy; as we say, are cooled. But in free air if a molecule be fortunate enough to elude its neighbors, there is nothing to take away its motion but the ether through radiation, and this is a very slow process. Thus the escaping fugitive must arrive at the confines of the air with the speed it had at its last encounter. We reach, then, this result: In space there is no such thing as temperature; temperature being simply the aggregate effect of molecular temperament. The reason we should consider it uncommonly cold up there is that fewer molecules would strike us. Quantity, therefore, in our estimation replaces quality,—a possible substitution which also accounts for some reputations, literary or otherwise. The only forces which could affect this lonely molecule would be the heating by the Sun, the repellent force of light, and gravity.

Now the speed which gravity on the Earth can control is 6.9 miles a second. It can impart this to a body falling freely to it from infinite space, and can therefore annul it on the way up, and no more. If, then, any of the molecules reach the outer boundary of the air going at more than this speed, they will pass beyond the Earth’s power to restrain. They will become little rovers in space on their own account, and dart off on interstellar travels of their own. This extension of the kinetic theory and of the consequent voyages of the molecules is due to Dr. Johnstone Stoney, who has since, humorously enough, tried to stop the very balls he set rolling. First thoughts are usually the best, after all.

As among the molecules some are already travelling at speeds in excess of this critical velocity, molecules must constantly be attaining to this emancipation, and thus be leaving the Earth for good. In consequence there is a steady drain upon its gaseous covering. Furthermore, as we know from comets’ tails, the repellent power of the light-waves, what we may call the levity of light, much exceeds upon such volatile vagrants the heat excitement or even the gravity of the Sun, so that we arrive at this interesting conclusion—their escape is best effected under cover of the night.

Again, the heavier the gas, the less its molecular speed at a given temperature, because its kinetic energy which measures that temperature is one-half the molecule’s mass into the square of its speed. Thus their ponderosity prevents as many of them from following their more agile cousins of a different constitution. So that the lighter gases are sooner gone. Water-vapor leaves before oxygen. Nor is there any escape from this escape of the gases. It may take excessively long, but go they must until a solitary individual who happens to have had the wrong end of the last collision is alone left hopelessly behind.

Another factor also is concerned. The smaller the planet, the lower the utmost velocity it can control, and the quicker, therefore, it must lose its atmosphere. For a greater number of molecules must at every instant reach the releasing speed. Thus those bodies that are little shall, perforce, have less to cover themselves withal.

Now this inevitable depletion of their atmospheric envelopes, the aspects of the various planets strikingly attest. They do so in most exemplary fashion, according to law. The larger, the major planets, as we have already remarked, have a perfect plethora of atmosphere, more than we at least know what to do with in the way of cataloguing yet. The medium-sized, like our own Earth, have a very comfortable amount; Mars, an uncomfortable one, as we consider, and the smallest none at all. All the smaller bodies of our system are thus painfully deprived so far as we can discover. We are certain of it in the case of our Moon and Mercury, the only ones we can see well enough to be sure. In further evidence it has been shown at the Yerkes and at Flagstaff that no perceptible effect of air betrays itself in the spectroscopic imprint of the rings of Saturn, those tiny satellites of his, and very recently a spectrogram of Ganymede, Jupiter’s third moon, made at Flagstaff for the purpose by Mr. E. C. Slipher has proved equally void of atmospheric hint.

With the loss of water and of air, all possibility of development departs. Not only must every organism die, but even the inorganic can no longer change its state. In the extinction thus not only of inhabitants but of the habitat that made them possible, occurs a curious inversion of the order we are familiar with in the life history of organisms. In planets it is the grandchildren that die first, then the children, and lastly their surviving parent. And this is not accidental, but inevitably consequent upon their respective origins. For the offspring, as we may spell it with a hyphen, of any cosmic mass is of necessity smaller than that from which it issued. Being smaller, it must age quicker. In the natural order of events, then, its end must be reached first.

Such has been the course taken, or still taking, by the bodies of our solar family. The latest generation has already succumbed to this ebbing of vitality with time. Every one of the satellites of the planets—those of Neptune, Uranus, Saturn, Jupiter, and our own Moon—is practically dead; born so the smaller which never were alive. Our own Moon carries its decrepitude on its face. To all intents and purposes its life is past; and that it had at one time a very fiery existence, the great lunar craters amply testify. It is now, for all its flooding with radiance our winter nights, the lifeless statue of its former self.

The same inevitable end, in default of others, is now overtaking the planetary group. Its approach is stamped on the face of Mars. There we see a world dying of exhaustion. The signs of it are legible in the markings we descry. How long before its work is done, we ignore. But that it is a matter of time only, our study of the laws of the inexorable lead us to conclude. Mars has been spared the fate of Mercury and Venus to perish by this other form of planetary death.

Last in our enumeration of the causes by which the end of a world may be brought about, because the last to occur in order of time, is the extinction of the Sun itself. Certain to come and conclude the solar system’s history as the abode of life, if all the others should by any chance fail to precede it, it fittingly forms the climax, grand in its very quietude, of all that went before.

By the same physical laws that caused our Earth once to be hot, the Sun shines to-day. Only its greater size has given it a life and a brilliancy denied to smaller orbs. The falling together of the scattered particles of which it is composed, caused, and still is causing, the dazzling splendor it emits. And so long as it remains gaseous, its temperature must increase, in spite of its lavish expenditure of heat, as Homer Lane discovered forty years ago.

But the Sun’s store of heat, immense as it is to-day, and continued as it is bound to be for untold æons by means of contraction of its globe upon itself, and possibly by other causes, must some day give out. From its present gaseous condition it must gradually but eventually contract to a solid one, and this in turn radiate all its heat into space. Slowly its lustre must dim as it becomes incapable of replenishing its supply of motive power by further shrinkage in size. Fitfully, probably, like Mira Ceti to-day, it will show temporary bursts of splendor as if striving to regain the brightness it had lost, only to sink after each effort into more and more impotent senility. At last some day must come, if we may talk of days at all when the great event occurs when all days shall be blotted out, that the last flicker shall grow extinct in the orb that for so long has made the hearth of the whole system. For, presciently enough, the Latin word focus means hearth, and the body which includes within it the focus about which all the planets revolve also constitutes the hearth from which they all are lighted and warmed.

When this ultimate moment arrives and the last spark of solar energy goes out, the Sun will have reverted once more to what it was when the cataclysm of the foretime stranger awoke it into activity. It will again be the dark body it was when our peering into the past first descries it down the far vista of unrecorded time. Ghostlike it will travel through space, unknown, unheralded, till another collision shall cause it to take a place again among the bright company of heaven. Thus, in our account of the career of a solar system, we began by seeing with the mind’s eye a dark body travelling incognito in space, and a dark body we find ourselves again contemplating at the end.

In this kaleidoscopic biograph of the solar system’s life, each picture dissolves into its successor by the falling together of its parts to fresh adjustments of stability, as in that instrument of pleasure which so witched our childish wonder in early youth. Just as when a combination had proved so pretty, once gone, to our sorrow no turning of the handle could ever bring it back, so in the march of worlds no retrace is possible of steps that once are past. Inexorable permutations lead from one state to the next, till the last of all be reached.

Yet, unlike our childhood’s toy, reasoning can conjure up beside the present picture far vistas of what preceded it and of what is yet to come. Hidden from thought only by the distraction of the day, as the universe to sight lies hid by the day’s overpowering glare, both come out on its withdrawal till we wonder we never gazed before. Our own surroundings shut out the glories that lie beyond. Our veil of atmosphere cloaks them from our view. But wait, as an astronomer, till the Sun sinks behind the hills and his gorgeous gold of parting fades to amber amid the tender tapestry of trees. The very air takes on a meaning which the flood of day had swamped. Seen itself, no longer imperfectly seen through, it wakes to semi-sentient existence, a spirit come to life aloft to shield us from the too immediate vacancy of space. The perfumes of the soil, the trees, the flowers, steal out to it, as the twilight glow itself exhales to heaven. In the hushed quiet of the gloaming Earth holds her breath, prescient of a revelation to come.

Then as the half-light deepens, the universe appears. One by one the company of heaven stand forth to human sight. Venus first in all her glory brightens amid the dying splendor of the west, growing in lustre as her setting fades. From mid-heaven the Moon lets fall a sheen of silvery light, the ghostly mantle of her ghostlike self, over the silent Earth. Eastward Jupiter, like some great lantern of the system’s central sweep, swings upward from the twilight bow to take possession of the night. Beyond lies Saturn, or Uranus perchance dim with distance, measuring still greater span. All in order in their several place the noble cortège of the Sun is exposed to view, seen now by the courtesy of his withdrawal, backgrounded against the immensity of space. Great worlds, these separate attendants, and yet as nothings in the void where stare the silent stars, huge suns themselves with retinues unseen, so vast the distances ’twixt us and them.

No less a revelation awaits the opening of the shutters of the mind. If night discloses glimpses of the great beyond, knowledge invests it with a meaning unfolding and extending as acquaintance grows. Sight is human; insight seems divine. To know those points of light for other worlds themselves, worlds the telescope approaches as the years advance, while study reconstructs their past and visions forth their future, is to be made free of the heritage of heaven. Time opens to us as space expands. We stand upon the Earth, but in the sky, a vital portion not only of our globe, but of all of which it, too, forms part. To feel it is to enter upon another life; and if to realization of its beauty, its grandeur, and its sublimity of thought these chapters of its history have proved in any wise the portal, they have not been penned in vain.

NOTES

1 METEOR ORBITS

If the space of the solar system be equally filled with meteors throughout, or if they diminish as one goes out from the Sun according to any rational law, their average speed of encounter with the Earth would be nearly parabolic.

If they were travelling in orbits like those of the short-period comets, that is with their aphelia at Jupiter’s orbit and their perihelia at or within the Earth’s, their major axes would lie between 6.2 and 5.2. If we suppose their perihelion distances to be equally distributed according to distance, we have for the mean a major axis of 5.7. Their velocity, then, at the point where they cross the Earth’s track would be given by

2 1 v² = µ(——— - ——— ), 1 2.85

in which µ = 18.5² in miles per second = 342.25, whence v = 23.76 in miles per second.

Suppose them to be approaching the Earth indifferently from all directions.

At sunset the zenith faces the Earth’s quit; at sunrise the Earth’s goal. Let θ be the real angle of the meteor’s approach reckoned from the Earth’s quit; θ₁ the apparent angle due to compounding the meteor’s velocity-direction with that of the Earth. Then those approaching it at any angle 0 less than that which makes θ₁ = 90° will be visible at sunset; those at a greater angle, at sunrise. The angle 01 is given by the relation,

a cos θ₁ = + ——— , x

in which a is the Earth’s velocity, x the meteor’s, and θ₁ is reckoned from the Earth’s quit.

The portion of the celestial dome covered at sunset is, therefore,

⌠θ₁ ⌠360° │ │ sin θ·dθ·dφ, ⌡0 ⌡0

where φ is the azimuth,

⌠180° ⌠360° that at sunrise, │ │ sin θ·dθ·dφ. ⌡θ₁ ⌡0

If the meteors have direct motion only, θ can never exceed 90°, and the limits become,

⌠θ₁ ⌠360° for sunset, │ │ sin θ·dθ·dφ, ⌡0 ⌡0

⌠90° ⌠360° and for sunrise, │ │ sin θ·dθ·dφ. ⌡θ₁ ⌡0

The mean inclination at sunset is

⌠θ₁ ⌠360° │ │ θ₁·sin θ·dθ·dφ, ⌡0 ⌡0 ⸻⸻⸻⸻⸻⸻⸻⸻⸻ , ⌠θ₁ ⌠360° │ │ sin θ·dθ·dφ, ⌡0 ⌡0

in which θ₁ must be expressed in terms of θ, etc.

From this it appears that the relative number of bodies, travelling in all directions and at parabolic speed, which the Earth would encounter at sunrise and sunset respectively would be:—

sunrise 5.8 sunset 1.0

and with the speed of the short-period comets,

sunrise 8.0 sunset 1.0

If, however, the bodies were all moving in the same sense as the Earth, i.e. direct, the ratios would be:—

========+=========+==============+============================ |PARABOLIC|SPEED OF SHORT|SPEED OF ACTUAL SHORT-PERIOD | SPEED |PERIOD COMETS | COMETS ABOUT JUPITER --------+---------+--------------+---------------------------- Sunrise | 2.4 | 3.5 | 3.3 Sunset | 1.0 | 1.0 | 1.0 ========+=========+==============+============================

As the actual number encountered is between 2 and 3 to 1, we see that the greater part must be travelling in the same sense as the Earth, since they come indifferently at all altitudes from the plane of her orbit.

2 DENSITIES OF THE PLANETS

The densities of the principal planets, so far as we can determine them at present, the density of water being unity, are:—

Mercury 3.65 Venus 5.36 Earth 5.53 Moon 3.32 Mars 3.93 ———— mean 4.36 Jupiter 1.33 Saturn 0.72 Uranus 1.22 Neptune 1.11 ———— mean 1.09 Sun 1.38

The second decimal place is not to be considered as anything but an indication.

3 VARIATION IN SPECTROSCOPIC SHIFT

In the case of a body reflecting light, the shift differs from that for a body emitting it. If the planet be on the further side of the Sun, the approaching rim advances both toward the Sun and toward the Earth, thus doubling the shift. The receding rim recedes in like manner. At elongation the rims approach or recede with regard to the Earth, but not the Sun, and the shift is single as for emission. At inferior conjunction rotational approach to the Earth implies rotational recession from the Sun, and the two effects cancel.

4 ON THE PLANETS’ ORBITAL TILTS

The tilts of the plane of rotation of the Sun and of the orbits of the several planets to the dynamical plane of the system tabulated are:—

Sun 7° Mercury 6° 14′ Venus 2° 4′ Earth 1° 41′ Mars 1° 38′ Asteroids various Jupiter 20′ Saturn 56′ Uranus 1° 2′ Neptune 43′

where, in the determination of that plane, the latest values of the masses of the planets and the rotations of the Sun, Jupiter, and Saturn have been taken into account.

These tilts suggest something, doubtless, but it is by no means clear what it is they suggest. They are just as compatible with a giving off from a slowly condensing nebula as with an origin by shock. The greater inclinations of Mercury and Venus may be due to their late birth from the central mass without the necessity of a cataclysm, the rotation of that central mass out of the general plane being caused by the consensus of the motions of the particles from which it was formed. The accordance of the larger planetary masses with the dynamical plane of the system would necessarily result from their great aggregations. So that this, too, is quite possible without shock.

5 PLANETS AND THEIR SATELLITE SYSTEMS

If we compute the speeds of satellites about their primaries in the solar system and compare them with the velocities in their orbits of the planets themselves, a striking parallelism stands displayed between the several systems. This is shown in the following table of them:

==============+============================+===========+============ | | PARABOLIC | | MEAN SPEED, | SPEED AT | RATIO SPEED | MILES A SECOND | ORBIT | SAT. ABOUT +------------+---------------+-----------+ PRIMARY TO | of Primary | of Satellite | Miles a | PLANET’S | in Orbit | about Primary | second | SPEED | V | v | | IN ORBIT --------------+------------+---------------+-----------+------------ Jupiter | 8.1 | | 11.5 | Sat. 1 | | 10.7 | | 1.32 2 | | 8.5 | | 1.05 3 | | 6.7 | | 0.83 4 | | 5.1 | | 0.63 Saturn | 6.0 | | 8.5 | 1 | | 9.0 | | 1.50 2 | | 7.9 | | 1.31 3 | | 8.2 | | 1.36 4 | | 6.3 | | 1.05 5 | | 5.3 | | 0.89 6 | | 3.5 | | 0.59 8 | | 2.0 | | 0.34 Uranus | 4.2 | | 5.9 | 1 | | 3.5 | | 0.82 2 | | 2.9 | | 0.70 3 | | 2.3 | | 0.54 4 | | 2.0 | | 0.47 Neptune | 3.4 | | 4.8 | 1 | | 2.7 | | 0.81 ==============+============+===============+===========+============

The relations here disclosed are too systematic to be the result of chance.

The orbits of all these satellites have no perceptible eccentricity independent of perturbation except Iapetus, of which the eccentricity is about .03.

In view of the various cosmogonies which have been advanced for the genesis of the solar system it is interesting to note what these speeds imply as to the effect upon the satellites of the impact of particles circulating in the interplanetary spaces at the time the system evolved. To simplify the question we shall suppose—which is sufficiently near the truth—that the planets move in circles, the interplanetary particles in orbits of any eccentricity.

Taking the Sun’s mass as unity, the distance R of any given planet from the Sun also as unity, let the planet’s mass be represented by M and the radius of its satellite’s orbit, supposed circular, as r. We have for the space velocity of the satellite on the sunward side of the planet, calling that of the planet in its orbit V and that of the satellite in its orbit round the planet v,

______ _____ V - v = √(1/R) - √M/r_.

For a particle, the semi-major axis of whose orbit is a₁ and which shall encounter the satellite, the velocity is

v₁ = (2/(R-r) - 1/a₁)^{½}.

That no effect shall be produced by the impact of these two bodies, their velocities must be equal, or

____ _____ __________________ √1/R - √M/r = √2/(R-r) - 1/a₁_

As R-r = a₁(1 + e) for the point of impact if the particle be wholly within the orbit of the planet and e the eccentricity of its orbit, we find

________________ e = 2 √MR/r - RM/r_ approx.

for the case of no action, the other terms being insensible for the satellites in the table, since in all r < R/400.

Supposing, now, the particles within the orbit of the planet to be equally distributed according to their major axes, then as the velocity of any one of them, taking R-r = R approx. as unity, is

v₁ = (2/1 - 1/a₁)^{½},

the mean velocity of all of those which may encounter the satellite is, at the point of collision,

⌠¹ │ ((2a₁ - 1)^{½} / a₁^{½})da₁ ⌡{½} ———————————————————————————————————— ⌠¹ │ da₁ ⌡{½}

┌¹ _ ___ ┐ = 2│ (2a₁² - a₁)^{½} - 1/√(2) log{(2a₁ - 1)^{½} + √2a₁ }│ └{½} ┘

= 0.754;

that is, just over three-quarters of the planet’s speed in its orbit.

If we suppose the particles to be equally distributed in space, we shall have more with a given major axis in proportion to that axis, and our integral will become

⌠¹ │ (2a₁ - 1)^{½}a₁^{½} da₁ ⌡{½} ———————————————————————————————— ⌠¹ │ a₁ da₁ ⌡{½}

₁ = 8/3 [{½} (4a₁-1)/8 (2a₁² - a₁)^{½} - (1/16√ 2 ) log[(2a₁² - a₁)^{½} + √ 2 · a₁_ - 1/(2√ 2 )]]

= 0.792 of the planet’s orbital speed.

The speed v, then, at which a satellite must be moving round the planet to have the same velocity as the average particle within the planet’s orbit, is

V - v₁ = v.

This velocity is, for the several planets:—

========+====================+================ | DISTRIBUTION OF | DISTRIBUTION OF | PARTICLES AS THEIR | PARTICLES EQUAL | MAJOR AXES | IN SPACE +--------------------+---------------- | Miles a second | Miles a second --------+--------------------+---------------- Jupiter | 2.0 | 1.6 Saturn | 1.5 | 1.2 Uranus | 1.0 | 0.9 Neptune | 0.8 | 0.7 ========+====================+================

If the satellite be moving in its orbit less fast than this, its space-speed will exceed that of the average particle; it will strike the particle at its own rear and be accelerated by the collision. If faster, the particle will strike it in front and retard it in its motion round its primary.

From the table it appears that all the large satellites of all the planets have an orbital speed round their primaries exceeding those in either column. In consequence, all of them must have been retarded during their formation by the impact of interplanetary particles and forced nearer their primaries than would otherwise have been the case; and this whether the particles were distributed more densely toward the Sun, as 1/a₁, or were equally strewn throughout.

For interplanetary particles whose orbits lie without the particular planet’s path the mean speed is the parabolic at the planet’s distance, given in the third column of the table. This is the case on either supposition of distribution. The orbital speed of the satellite which shall not be affected by collisions with them is, for the several planets:—

========+============== |MILES A SECOND --------+-------------- Jupiter | 3.4 Saturn | 2.5 Uranus | 1.7 Neptune | 1.4 ========+==============

All the satellites but Iapetus have orbital speeds exceeding this, and consequently are retarded also by these particles.

For particles crossing the orbit (2) the mean velocity would be practically parabolic, 1.4, even if the distribution were as 1/r′, r′ being the distance from the Sun. The effect would depend upon the angle of approach and in the mean give a greater velocity for the particle than for the satellite within the orbit, a less one without; retarding the satellite in both cases. Thus the total effect of all the particles encountering the large satellites is to retard them and to tend to make them hug their primary.

For retrograde satellites the velocities of impact with inside and outside particles moving direct are respectively:

=========+===========+========== | INSIDE | OUTSIDE ---------+-----------+--------- Jupiter | 2.0 + v | v + 3.4 Saturn | 1.5 + v | v + 2.5 Uranus | 1.0 + v | v + 1.7 Neptune | 0.8 + v | v + 1.4 =========+===========+=========

In both cases the impact tends to check the satellite.

Comparing with these the velocities of impact for direct satellites in a direct plenum:—

=========+===========+=========== | INSIDE | OUTSIDE ---------+-----------+----------- Jupiter | 2.0 - v | 3.4 - v Saturn | 1.5 - v | 2.5 - v Uranus | 1.0 - v | 1.7 - v Neptune | 0.8 - v | 1.4 - v =========+===========+===========

the signs being taken positive when the motion is direct, we see that retrograde satellites would be more arrested than direct ones with the same orbital speed round the primary.

In a plenum of direct moving particles, then, the force tending to stop the satellite and bring it down upon the planet is greater for retrograde satellites than for direct ones.

If, therefore, the positions of the satellites have been controlled by the impact of interplanetary particles, the retrograde satellites should be found nearer their planets than the direct ones.

6 ON THE INDUCED CIRCULARITY OF ORBITS THROUGH COLLISION

Since the moment of momentum is the velocity into the perpendicular upon its direction, in the time dt it is:—

vp dt = h dt = r²dΘ.

The whole moment of momentum from perihelion to perihelion is therefore:—

⌠360° │ r²dΘ = a²·(1-e²)²/1-e² ⌡₀

┌360° │ (-e sin Θ)/(1+e cos Θ) └₀ ____________ ┐ + 2/(1-e²)^{½} tan⁻¹ (√1-e)/(1+e_·tan (Θ/2))│ ┘

= 2πa² · (1 - e²)^{½},

which is twice the area of the ellipse.

The energy in the ellipse during an interval dt is

(½)mv²dt = (½)mµ(2/r - 1/a)dt,

from the well-known equation for the velocity in a focal conic. The integral of this for the whole ellipse is

⌠ᵀ ⌠360° │ (½)mv² dt = │ (½)(mµ/h)(2r - r²/a)dΘ ⌡₀ ⌡₀

= mµ^{½}πa^{½}.

Since

⌠ ⌠ │ rdΘ = │ (a · 1 - e²)/(1 + e cos Θ)dΘ ⌡ ⌡ _______________ = (2a· 1 -e²)/(1 -e²)^{½} tan⁻¹(√(1 -e)/(1 +e_)tan (Θ/2))

and ∫r² dΘ is given above.

By collision a part of this energy is lost, being converted into heat. The major axis, a, is, therefore, shortened. But from the expression 2πa² · (1-e²)^{½} for the moment of momentum we see that this is greatest when e is least. If, therefore, a is diminished, e must also be diminished, or the moment of momentum would be lessened, which is impossible.

7 CAPTURE OF SATELLITES

See has recently shown (Astr. Nach. No. 4341-42) that a particle moving through a resisting medium under the attraction of two bodies revolving round one another in circles may eventually be captured by one of them though originally under the domination of both. The argument consists in introducing the effect of a resisting medium upon the motion in the space permitted by Jacobi’s integral, following Darwin’s examination of this space. In the actual case of nature the effect is much more complicated, and at present is not capable of exact solution for masses other than indefinitely small, even supposing circular orbits for the chief bodies. It may, however, explain the curious relation shown in the arrangement of the direct and retrograde movement of satellites.

INDEX

A Abnormality, the survival of original state, 144, 146. Absorption in spectrum, planetary, 52, 161. of Uranus, 118. of Jupiter, 152. of Saturn, 152. Achilles, 94. Adams, 119, 121. Adams, Mr. J. C., 123-126. Agassiz, 41. Airy, 121, 123. Albedo, of dark star, 27. of Mercury, 62, 73-75. of Venus, 73-75. of Moon, 75. of Jupiter, 104, 105. of Saturn, 109. of Uranus, 116. of Neptune, 168. of clouds, 195. Algol, 3. American Academy, 125. Amphibians, first record of, 188. Anderson, Dr. Thomas D., 8, 12. André, 215. Andromeda, great nebula in, 10, 20, 21. constitution disclosed by spectroscope, 45, 48. Apex of Sun’s way, 26. Arago, 121. Asteroids, 39, 60, 61, 94-102. domain of, 94. diminutive size, 94, 101. number, 94, 101. peculiar discovery of, 95-98. never formed part of a pristine whole, 98. where thickest, 98. formation of large planet from, prevented, 98, 99. mid-course between planets and comets, 100. shape of, 101, 102. mammoth meteorites, 102. mark transition between inner and outer planets, 102. Atmosphere, spectrographic study of, 53, 54, 161. Mercury deprived of, 71, 75, 232. reflecting power, 75. of Venus, 75. Moon deprived of, 75, 232. thin on Mars, 75, 91, 232. of Uranus, enormous, 117, 118, 232. of Neptune, vast, 118, 232. of Jupiter, 166, 232. depletion of, 231-233. none on Ganymede, 232, 233. of Saturn, 232. lacking in Saturn’s rings, 232. Avogadro, 228. Axes of planets, systematic righting of, 132. tilts accounted for, 146.

B Babinet, 147. Backland, 68. Ball, Sir Robert, 145. Barrande, M., 178. Belopolski, 87. Bessel, 120, 121. Blandet, M., 175, 176. Bode, 95, 119. Bode’s law, 96, 100, 119, 122, 126. Bolometer, 194. Bolton, Mr. Scriven, 103, 105, 106. Boltzmann, 228. Bose, 157. Bouvard, Alexis, 120, 121. Boyle, 228. Bradley, 68.

C Cambrian era, 178. Cambridge Observatory, 123. Campbell, 9. Carboniferous period, 179. Cassini, 76, 162. Celestial mechanics, 28, 94, 155. Ceres, 101. Challis, 123. Chemistry, indebted to the stars, 160. Clausius, 228. Clerke, Miss, 9, 164. Climate, advent of, 185. Clouds, none on Venus, 75. of Jupiter not ordered as ours, 107, 163, 167. Uranus wrapped in, 168. Neptune wrapped in, 168. Earth once wrapped in, 170, 171, 178. Collision of dark star with Sun, 25, 215. warning of, 26-29. disturbances previous to, 29, 30. rarity of event, 30. Collisions between meteorites of a flock, 11, 49. causing light, 49, 50. Columbus, 188. Comets, 33, 61. members of solar system, 34, 35. orbits of, 61, 100. Commensurability of orbital period, 99, 111. Congruities of solar system, 128-137. deviations from, 62, 100, 101, 130, 131, 141. specify mode of evolution, 137. Convection currents, 219. in atmosphere of Venus, 80. Copeland, Dr. 7. Copernican system, 58. Copernicus, 62. Cosmic action, 1, 22, 184. Croll, 196. Cuticle of star, effect of impact on, 11.

D Dana, 177, 186, 189. Dark stars, origin, 2. number, 2, 25. evidence of, 3-5. collision of, 10, 11. rendered visible, 26. Darwin, 62, 138, Notes 252. Day, lengthened to infinity, 70, 219. none on Venus, 83. Jovian, 163. first appreciation of, 186. coincides with month, on satellites, 225. Death of a planet, defined, 214. catastrophic cause, 215, 216. due to tidal retardation of rotation, 216-219. outcome of loss of oceans and air, 226, 233. caused by extinction of Sun itself, 234. Density, of dark star, 27. of planets, 51, Notes 243. of Mercury, 63, 64. of Venus, 90. of Jupiter, 103, 117. of Uranus, 115. Deserts, increase of, on Earth, 208-211. Devonian era, 187. Dhurmsala meteorite, 41. Diameter, of Mercury, 63, 64, 66, 67. of Venus, 90. of Earth, 90. of Mars, 91. of satellites of Mars, 92. of Jupiter, 103. of Uranus, 115-117. Dust, in atmosphere of Venus, 75.

E Earth, characteristics, not universal, 90, 91, 155. evolved from a nebula, 149. internal heat, 150. early surface temperature, 160, 169, 170. once cloud-wrapped, 170, 171, 178. solid surface formed, 171. hot seas of, 171, 172. self-sustained, 182. study of, within province of astronomy, 184. ceased to be self-centred, 187. Sun becomes dominant factor in organic life of, 190. Earth shine, 82. Eccentricity, orbital, of Mercury, 63, 65, 69, 222. of asteroids, erratic, 100, 101. of satellites, increases with distance from primary, 134. Eclipsing binaries, 3, 4. Ejectum from nova, 5, 16. rate of regression, 16. Elemental substances, 159. in Sun, 159. once in Earth, 160. discovery of, in stars, 161, 162. Ellipticity, of Jupiter, 103. of Saturn, 109. of Uranus, 115. Encke, 68. Energy, conservation of, 140, 150, 151. dissipation, 140-142. conditions for a minimum, 142. Eros, fluctuation of light of, gives evidence of form, 101, 102. Evolution, 153. white nebulæ in process of, 49. rounded out, 56. of solar family, 100. evidence of, in solar system, 117. manner of, lessens energy, 141. Evolution, chemical, 155, 173. universal, 156. temperature conducive to, 157, 158. attendant upon cooling, 158, 162. steps in, shown by spectroscope, 161. Evolution, physical, 155, 162. induced by cooling, 162.

F Fabry, 34. Fauna, 178, 179, 187. Faye, 175, 176. Flagstaff, Arizona, 52, 66, 68, 79, 83, 89, 92, 106, 110, 221, 232. clear and steady air of, 66, 86. Flamstead, 119. Fleming, Mrs., 7. Flemming, 120, 121. Flora, of paleologic times, 177. French Academy, 122.

G Galle, Dr., 122, 123, 125. Gases, peculiar to nebulæ, 11, 16. occluded in meteorites, 42, 43. in atmospheres of planets, 53-55. Gauss, 34, 96, 97. Geikie, 160, 177, 189. Geology, relation to astronomy, 173, 174, 183, 184. scope of, 174, 203. Geysers, avenues to earlier state, 160. Goodricke, 3.

H Hakluyt, 188. Harvard College Observatory, 8, 12. Heat, molecular motion, 150, 157, 230. the result of evolving, 153. the preface to higher evolution, 153, 156. laws governing amount of, 190. atmosphere keeps out, as well as stores, 191. effective, received from Sun, 192-194. invisible rays, 194. retained, 194-196. radiated, 194-196. Heat of condensation of Earth, accuses concourse of particles, 151. evaluated, 151, 152. sufficient for geologic phenomena, 152. Hector, 94. Helmholtz, 151. Hencke, 98. Herschel, Sir John, 122. Herschel, Sir William, 96, 114, 162. Hertha, periodic variability, 102. Hipparchus, 5. Holden, 9. Hubbard, Professor, 124. Huggins, 52. Humphreys, 10. Huntington, 209.

I Ice Age, 196. not of orbital occasioning, 197-199. increased precipitation, the cause, 199, 200. a local affair, 200-202. Irradiation, affecting diameter of Mercury, 66, 68.

J Jacobi, Notes 252. Julius, Professor, 10. Juno, 101. Jupiter, 103-108. not solid, 104, 107. a semi-sun, 105, 108, 152, 166, 167. white spots of, 106. Jupiter, “great red spot” of, 164. time of rotation, 164. a vast uprush of heated vapor, 165, 166. Jupiter’s belts, secular progression, 104. rotate at different speeds, 104, 162, 163. color, 104. wisps across, 105, 106. bright ones, cloud, 163, 167. spectrographic study of, 166.

K Kapteyn, 14. Keeler, 19, 52, 110. Kepler, 6. Kinetic theory of gases, 226, 228. corollary of, 54. extension of, 230, 231. Kirkwood, Professor, 35.

L Lagrange, 94, 97. Lalande, 123, 124. Lane, Homer, 234. Langley, 191, 194. Laplace, 34, 110, 127, 129, 131, 132, 138, 139, 147, 152, 175. Laplacian cosmos, 129, 130. false congruities of, 131-133. annular genesis, disproved, 138, 139. original “fire-mist” of, impossible, 138. Lapparent, de, 173-176, 183, 189. Lemonnier, 115, 119. Leonard, Miss, 79. Leverrier, 119, 121-126. Lexell, 115. Libration in longitude, of Mercury, 65, 69, 70, 222, 223. causes true day, 70, 71. of Venus, inappreciable, 83, 223. of Moon, 224. Lick Observatory, 13, 14. Lockyer, 48. Lowell Observatory, 65, 74.

M Major planets, gaseous, 117. constitution of, differs from Sun or Earth, 161. types of early planetary stages, 162. self-centred and self-sustained, 168. Man, immanent, 159. Mars, polar caps, 198. canals in dark regions, 206, 207. dying of exhaustion, 234. Mass, of Mercury, 63, 64, 68. of Mars, 91. of Jupiter, 103. arrangement of, in solar system, 135-137, 148. Massachusetts Institute of Technology, 134, 184. Mauvais, 125. Maxwell, Clerk, 110, 113, 228. Mayer, 119, 151. Mendeléeff, 161. Mercury, 62-73. time of rotation and revolution the same, 65, 69. axis stands plumb to orbit, 70. turns same face to the Sun, 70, 72, 134, 221. surface markings, 72, 221. color, 72. Meteorites, 31, 35, 36. cosmic bodies, 32, 33. relation to shooting-stars, 36. members of solar system, 36. composition, 40-44, 55. fused by friction with atmosphere, 40. temperature, 41, 55. fragments of a dark body, 44. link past to present, 44, 56, 57, 130. Meteors, orbits of, 36, 39, Notes 241-243. visibility of, 38. Meteor-streams, 33, 61. first recognition of, 34. disintegrated comets, 34. Michelson, 10. Milham, Professor, 99. Mira Ceti, 235. Mohler, 10. Molecular speeds, gaseous, 228-231. critical velocity, 230, 231. Molecule, organic, power in its instability, 160. Moment of momentum, 140, Notes 250. cause of original, 130. Moment of momentum, conservation of, 140. applied to solar system, 141-143. Momentum, 140. Monck, Mr., 10. Moon, turns same face to Earth, 134, 208, 224, 225. once fiery, now dead, 233, 234. Mountains, none on Mars, 91. Müller, 73, 74, 104, 105, 116.

N Naval Observatory at Washington, 122. Nebulæ, origin of, 10, 11. amorphous, 18, 44. planetary, 18. spectrum of amorphous, 45. Nebulæ, spiral, 17-25, 44. evolved from disrupted stars, 10-15. relation to novæ, 14-16. corpuscular character of, 15, 16. knots and patches of, 15. most common, 19, 20. two-armed, 20, 25. central nucleus, globular, 21. not due to explosive action, 22, 23, 25. not caused by disintegration, 24, 25. cause of development, 24, 25. spectrum of, 45-48. composed of flocks of meteorites, 48, 49. constitution established by spectroscope, 49, 50. Nebular hypotheses, 173. Neologic times, clearing of sky in, 185. Neptune, 118. rotates backward, 118. owes discovery to mathematical triumph, 119-126. faint belts on, 168. further advanced than giant planets, 168. Newcomb, 67. Newton, Professor, 36, 42. Newton, Sir Isaac, 34. Nova Aurigæ, 7, 8, 12. history chronicled by its spectrum, 8, 9. Nova Cygni, 7. Novæ, 6, 7. origin 5, 10. first chronicled, 5. spectroscopic study of, 7. Nova Persei, 7. history of, 12-15.

O Oceans, none on Mars, 91. evaporation of, 204. basins of, on Moon, 204-208. basins of, on Mars, 206, 207. Olbers, 97. Olmstead, Professor, 33. Orbital distance, of Mercury, 62. of Venus, 73. of Mars, 91. of Eros, 94. of Saturn, 108. Orbital tilts, of asteroids, erratic, 100, 101. of satellites of Uranus, 116. of planets, substantially the same, 129-131, Notes 244. deviation from rule, by Mercury, 131. of satellites, increase with distance from primary, 133, 134. Orbits, determining factors, 35. rendered more circular by collisions, 141-143, Notes 250, 251. made more conformant to general plane by collisions, 141-143. Orion, great nebula in, 18.

P Paleologic times, much warmth and little light in, 172. fallacies in geologists’ expositions of, 174-176. climate continuous, 177, 186. seas warm, 177, 178. explained by cloud envelope, 178. corroboration of explanation, 187, 179. excessive rain in, 185, 186. passage into Neologic, essentially astronomic, 185. Pallas, 101. Parabolic speed at orbit, Notes 245. Patroclus, 94. Peirce, 110, 125, 126. Perrine, 15. Perrotin, 116. Perturbations, in motion of planets, heralding a catastrophe, 28, 30. reflected, 63. mass of planet determined by, 68. of asteroids by Jupiter, 98, 99. restrictive action of, 99. the fashioning force of planetary orbits, 99, 100. of rings of Saturn by satellites, 111, 112. of Uranus lead to discovery of Neptune, 121-126. Petersen, Dr., 123. Photometric determinations, 92, 93. background, the fundamental factor in, 92, 93. Piazzi, 96. Pilgrim Star, 5, 6. Planetary astronomy, advance in, 59, 60. Planetology, 203. defined, 173, 174. Planets, 61. knots in spiral nebulæ, 25, 139. developed by agglomeration, 143, 149, 151, 152. Pliny, 5. Plutonic rocks, 160. Pluvial eras, contemporaneous with glacial, 200. Polyp corals, in paleologic times, 186. Pristine motion of planetary particles, retrograde, 144. superfluous energy in, 145. unstable, 145. Ptolemaic system, 58.

R Refrigeration, tempered by loss of cloud, 196. Revolutions, of shooting-stars, 39. of asteroids, direct like planets, 100. planetary, in same sense, 129, 130. outermost satellites, retrograde, 132. of satellites explained, 146, 147, Notes 252. Ritchey, 14. Roberts, Dr., 20. Roche, Edouard, 110. Rosse, Lord, 17. Rotation of planets, 131, 132. systematic righting of axes, 132. initially, retrograde, 146. Rotation period, of Venus, spectrographically determined, 83, 85-90. of Mars, spectrographically determined, 88, 89. of Jupiter, spectrographically determined, 89. of Uranus, 116. Royal Observatory, Edinburgh, 7.

S Satellites, 61. of Mars, 92. of Saturn, 108, 112. of Uranus, 116. solid, 117. of Neptune, 118. turn same face to primaries, 134, 147, 148, 225. latest discoveries in regard to motions of, 146. origin of, 147. death of, before planet, 233. impact of interplanetary particles on, Notes 246-250. capture of, Notes 251, 252. Saturn, 108-114. belts of, 109, 168. inherent light, 109, 152. Saturn’s rings, 109-114. mechanical marvel of, not early appreciated, 110. discrete particles, 110, 135. knots upon, 110-113. not flat, but tores, 111-114. show devolution—not pristine state of solar system, 138, 139. once a congeries, 139. Schaeberle, 9. Schiaparelli, 34, 36, 64-66, 69, 76, 77, 221. Schroeter, 65, 77. Seasons, loss of, 71, 83, 217, 218. begin with clearing of sky, 185. fully developed, 189. See, Notes 251. Seeliger, 10. Shooting-stars, 33, 35. radiant of, 33, 36. members of solar system, 36-40. tiny planets, 39. Siderite, 36. Silurian era, 178. Sirona, periodic variability of, 102. Sky, cause of clearing, 187. Slipher, Dr. V. M., 52, 79, 83, 86, 88, 89, 117, 161, 166. Slipher, Mr. E. C., 79, 233. Solar constant, 191. Solar system, evolved from a dark star, 44. evidence of origin, 51, 130. characteristics of, 60-62. evolutionarily one, 62. gap in progression of orbital distances, 95-100. bodies of, egg-shaped, 217. Specific gravity, of stone and iron, 44. Spectroscope, 7, 84. Spectroscopic shift, 84. determining velocity, 3. in Nova Aurigæ, 9. produced by great pressure, 10, 13. produced by anomalous refraction, 10. produced by change of density, 10, 13. explained, 85. variation in, Notes 243, 244. Spectrum, of Nova Persei, 12, 13. nebular, 13, 16, 45-48. peculiarities of nebular, explained, 50. photographic extension of, 52, 117, 161. of major planets, 52, 53, 161. of belts of Jupiter, 166. Spiral structure, implies rotation combined with motion out or in, 22. Stability of a system, condition for, 140, 141. Stoney, Dr. Johnstone, 231. Struve, 109. Suess, 179. Sun, original slow rotation of the, 130. heat of, 234, 235. reversion to a dark star, 235, 236. Sun spots, 104, 166.

T Temperature, of Moon, 191, 192. of Mars, 192, 194, 196. defined, 230. no such thing as, in space, 230. Tercidina, periodic variability of, 102. Tertiary times, entrance of color with, 189, 190. Tidal action, 143-147, 216-218. causes loss of energy, 144. inoperative, 144, 145, 147. changes retrograde rotation of planet to direct, 145-147, 217. on satellites, 147. slows down spin, 148, 217. brings plane of rotation down to orbital plane, 217. lengthens day to infinity, 219. analytically expressed, 224. greatest on planets near Sun, 135, 224. Tidal action, disruptive, 130. exemplified by spiral nebulæ, 24, 25. hinted at, by meteorites, 55. theory corroborated by densities of planets, 51. theory corroborated by atmospheres of planets, 52-55. on comets, 139. cause of Saturn’s rings, 139. Tisserand, 68. Titius, 95. Todd, 68. Trees, deciduous, first appearance of, 189. Trilobites, blindness of, 178, 179. Twining, 33. Tycho Brahe, 5.

U Uranus, 114-118. history of discovery, 114, 115, 119. a ball of vapor, 115, 117. belts of, 115, 116, 168. tilt of axis to ecliptic, great, 115. spectroscopic revelations of, 117, 118. in an early amorphous state, 118. further advanced than the giant planets, 168.

V Velocity, of Mercury in orbit, 63. of satellites about primary, Notes 245. of major planets, in orbit, Notes 245. Venus, 73-90. surface markings, 74, 77, 79, 80, 83, 220, 221. brilliancy due to cloudless atmosphere, 75. importance of rotation period, 75, 76. turns same face to the Sun, 77-80, 134, 220, 221. ice on the night side, causes ashen light, 82. Very, Professor, 16, 191, 192, 194. Vesta, 101. Vogel, 52. Volcanoes, avenues to earlier state, 160. Von Zach, 96.

W Walker, Mr., 123, 124. Water, becoming more scarce, 203, 204, 211. lacking on Moon, 204. Water-vapor, in atmosphere of Jupiter, 53. in atmosphere of Mars, 91, 161. smaller planet has less hold on, 207. Williams, Mr. Stanley, 103. Witt, de, 94. Wolf, Dr., 13. Wolf, Max, 94. Wolf-Rayet stars, 13, 48. Wright, 13, 43.

Y Year, of Uranus, 116. Yerkes Observatory, 232. Young, 46.

PERCIVAL LOWELL’S

Mars and Its Canals

Illustrated, 8vo, $2.50 net

“The book makes fascinating reading and is intended for the average man of intelligence and scientific curiosity. It represents mature reflection, patient investigation and observation, and eleven years’ additional work and verification.... It is the work of a scientist who has found inspiration and joy in his work; it is full of enthusiasm, but the enthusiasm is not allowed to influence unduly a single conclusion.”—Chicago Evening Post.

“It seems impossible that Mr. Lowell can raise another girder more grandly impressive and expressive of the whole fabric or take another step in his scientific syllogism that will hold us any tighter in his logic. He has practically reached already his ‘Q. E. D.’ The thing is done, apparently, except for filling in the detail. But with his racy, epigrammatic brilliancy of style, his delicate, quiet humor, his daring scientific imagination—all held in check by instructive modesty of good breeding, gayly throwing to the winds all professional airs and mere rhetorical bounce—his course will be no doubt as charming to the end as it has been steadily illuminating even for the illuminati.”—Boston Transcript.

“Whether or not we choose to follow the author of this book to his ultimate inferences, he at least opens up a field of fascinating conjecture. The work is written in a style as popular as the precise enumeration of the ascertained facts permits, and if the narrative is not in all its details as entrancing as a novel, it nevertheless transports us into a region of superlatively romantic interest.”—New York Tribune.

“No doubt the highest living authority on Mars and things Martian is Prof. Percival Lowell, director of the observatory at Flagstaff, Arizona, an astronomical investigator and writer known over the entire world. Professor Lowell’s book, ‘Mars and Its Canals,’ is the final word, up to the present, on the planet and what we know of it.”—Review of Reviews.

PUBLISHED BY THE MACMILLAN COMPANY 64-66 Fifth Avenue, New York

PERCIVAL LOWELL’S

Mars as the Abode of Life

Illustrated, 8vo, $2.50 net

The book is based on a course of lectures delivered at the Lowell Institute in 1906, supplemented by the results of later observations. It is, in the large, the presentation of the results of the author’s research into the genesis and development of what we call a world; not the mere aggregating of matter, but the process by which that matter comes to be individual as we find it. He bridges with the new science of planetology the evolutionary gap between the nebular hypothesis and the Darwinian theory.

“It is not only as an astronomer but as a writer that Professor Lowell charms the reader in this work. The beguilement of the theme is well matched by the grace and literary finish of the style in which it is presented. The subject is one to beget enthusiasm in its advocates, and the author certainly is not devoid of it. The warmth and earnestness of the true lover of his theme shine through the entire work so that in its whole style and illustrations it is a charming production.”—St. Louis Globe Democrat.

“Mr. Lowell approaches the subject by outlining the now generally accepted theory of the formation of planets and the solar system. He describes the stages in the life history of a planet three of which are illustrated in the present state of the earth, Mars, and the moon. He tells what conditions we would expect to find on a planet in what we may call the Martian age, and proceeds to show how the facts revealed by observation square with the theories. The book is fascinatingly readable.”—The Outlook.

“So attractive are the style and the illustrations that the work will doubtless draw the attention of many new readers to its fascinating subject. Professor Lowell has fairly preëmpted that portion of the field of astronomy which interests the widest readers, for there is no doubt that speculation regarding the possibility of life on other planets than our own has a peculiar attraction for the average human mind.... For the convenience of the non-technical reader, the body of the book has been made as simple and understandable as possible.”—Philadelphia Press.

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