.^-
^■^ N^ '^
-^o.
^ rf-^.- '^
v.^
. N c ^ ■/
.^^ %
<-/^ ^ tf 1
v>^2,;^^
FLOWERS OF THE SKY
<
jrtr;
'^^OHti^Oj^
FLOWERS OF THE SKY
By RICHARD Afl>ROCTOR
AUTHOR OF "the EXPANSE OF HEAVEN," THE INFINITIES AROUND US,' "the UNIVfiRSE OF STARS," " THE SUN," " THE MOON," ■^ ETC., ETC.
WITH FIFTY' FOUR IIIUSTRATIONS
A. C. ARMSTRONG AND SON
714, BROADWAY
All rights reserved
i ^ -
QBsi
187?
TRANSFBB O. 0, PUBLIC LIBBABir SEPT. lO, 1940
i t ;
r
o
*' spirit of nature ! here, "In this interminable wilderness Of worlds, at whose immensity Even soaring fancy staggers, Here is thy fitting temple.
Yet not the lightest leaf . That quivers to the passing breeze Is less instinct with thee." — Shelley.
CONTENTS.
PAGB
I. LIGHT ,-,---- I
II. SPACE - - - - - - 17
III. THE INFINITELY MINUTE - - - - 31
IV. THE MYSTERY OF GRAVITY - - - 43
V. THE END OF MANY WORLDS - - " 5^
VI. THE AURORA BOREALIS - - - - 9I
VII. THE LUNAR HALO - - - - - I r4
VIII. MOONLIGHT - - - - - 125
IX. THE PLANET MARS - - - - 149
X. THE PLANET JUPITER - - - - I9I
XI. THE RINGED PLANET SATURN - - - 21 5
XII. FANCIED FIGURES AMONG THE STARS ^ - 236
XIII. TRANSITS OF VENUS - - - -273
\^
I.
LIGHT,
** Vriiat soul was his, when, from the naked top Of some bold headland, he beheld the smi Rise up and bathe the world in light ! He looked — Ocean and earth, the solid frame of earth And ocean's liquid mass, beneath him lay In gladness and deep joy.''
E live in a mighty ocean whose waves are ever rushing hither and thither, always according to law, with velocity inconceiv- able, almost immeasurable. These waves lave the shore of that island of space which is our home, travelling to it from remotest regions, and making known to us all that we know of what lies outside our small abode. We call these waves, or rather their effects, by the name of Light. We recognise in light —
*' offspring of Heav'n's first-bom And of th' Eternal co-eternal beam '*—
the antecedent of all else that exists in the universe ;
I
2 LIGHT.
or^ as Sir John Herschel said, '^ the superior in point of rank and conception to all other products or results of creative power in the physical world. It is light which alone can give, and does give us, the assurance of a uniform and all-pervading energy — a mechanism almost beyond conception, complex, minute, and powerful, by which that influence, or rather that movement, is pro- pagated. Our evidence of the existence of gravitation fails us beyond the region of the double stars, or leaves at best only a presumption amounting to moral convic- tion in Its favour. But the argument for a unity of design and action afforded by light stands unweakened by distance, and is co-extensive with the universe itself"
What, then, is light ? What is that mysterious move- ment of some essence pervading all space, whereby, from remotest depths, news is brought to us, after journeys lasting many years, though space is traversed at a rate exceeding more than ten million times the velocity of the swiftest express train?
Light is in reality the result of undulations in what is called the ether of space, a perfectly transparent, almost perfectly elastic medium, occupying not only void space, but flowing as freely through the densest solids as the summer breeze flows through the forest trees. The waves of light cannot in this way pass through solid or liquid, or even aerial bodies, but either they are
LIGHT, 3
sooner or later brought to rest, or else they are more or less gradually deflected; just as the waves which traverse the ocean come to their end, or are deflected, when they meet the shore or shallows near the shore.
All light, however, has its real origin, not in the ethereal ocean itself, but in the movements of the minute particles of which all forms of matter known to us are composed. A tiny atom, far too small to be per- ceived with a microscope, even though one should be made ten thousand times more powerful than any yet constructed, when set in rapid vibration, raises minute waves in the ethereal ocean, just as a small body, vibrat- ing on the surface of a sheet of water, would generate waves there. And as the water-waves would travel radially away from the place of their birth, so do the light-waves generated by the vibrations of one of the atoms composing a luminous body radiate forth in all directions through the ethereal ocean until, encountering some obstacle, they are sent (reduced in size) in a new direction.
In some luminous bodies there are atoms vibrating in many different periods (all very small) so as to cause light-waves of many different kinds to proceed from the body. In other cases the atoms all vibrate at one rate, or at two or three or some definite num- ber of rates, so that only light-waves of certain kinds
4 LlGIlT.
proceed from the body. But in all cases these light- waves only cause us to see the body when they flow in through the pupil of the eye, and falling upon the retina (or the choroid membrane, or whatever part of the eye it may be which finally receives the waves), convey to the optic nerve, and thence to the brain, the information that such and such a body, so coloured, so shaped, so moving, exists towards that direction from which the light-waves seem to come. The body so seen, as we call it, may be the original source of light, or may be a body on which light has been reflected to us.
It is in this way that we receive information from light- waves. It will be conceived how minute they must be, how perfectly they must retain their separate character, multitudinous though they are, in traversing the ether (even when that ether is clogged by the gross matter of our ordinary air), if we remember how through the tiny eye-pupil we often receive light-waves telling us of all the details, all the varieties of colour and brightness, all the movements in a rich landscape.
Even more startling are the thoughts suggested by a view of the starlit heavens. From hundreds of suns at once the light-waves which have traversed varying but all enormous distances pour in upon the small circle of the eye-pupil, waves of many kinds coming in together from each sun. The waves which thus reach the eye from one
LIGHT, 5
bright star have been but a few years upon their journey; all that time they have been traversing an ocean swept in every part by untold millions of other waves, and yet they arrive as perfect in order and regularity as rollers which have traversed a wide sea pour in upon a level shore. From another star, as bright as the first, they have been years in travelling; from some among the fainter stars, hundreds, perhaps thousands of years. Yet still they flow on, each order of waves in perfect uni- formity as when they first left their parent sun.
But even this is not all. Among the waves which reach the eye many, nay, most, are so small that ordinary vision cannot perceive their action. Take, however, a telescope, and so gather them together as to intensify this action, and they are rendered perceptible, just as the unnoticed heaving of ocean becomes a manifest wave-motion when it reaches a regularly narrowing inlet. Thus, from stars so remote that their light has required thousands, or, even in some cases, perhaps, hundreds of thousands of years in reaching us, the light-waves flow steadily in upon us. So small are these waves, that the breadth of from forty to sixty thousand of them would occupy but a single inch. Through every point in space waves from all the hundred millions of stars are at all times simul- taneously rushing at the rate of one hundred and eighty- five thousand miles in every second of time : yet they
6 LIGHT.
travel on altogether undisturbed, and each tells its story as distinctly as though the ether had conveyed no other message, and that message but for a short distance.
It would be difficult to say which thought, considered in its real significance, is more striking, — the thought of what is done for us by light regarded as a terrestrial phe- nomenon, or the thought of what light is doing, and has done, in presenting to us a view of the starlit heavens.
When the sun rises in splendour above the eastern horizon, tinting the sky with varied colours, lighting up the clouds which till then have been but dark patches on the heavens, bringing out the colours of hill and dale, rock and river, fields and woods, the heart gladdens at the spectacle. A pleasing melancholy falls on us as the light fades away at eventide, tint after tint vanishing, until 'at length the gloom of night enshrouds alL The full splendour of mid-day, the chastened splendour of a moonlit night, and the glory of the heavens when "all the stars shine, and the shepherd gladdens in his heart," stir the soul in like manner ; and it might seem to many that to analyse these glories, to explain their scientific meaning, would be to rob the mind of the pleasure it had before found in such scenes. Many would be dis- posed to think that a purer enjoyment is expressed by Augustine than, any student of science could find in the wonders of light, in those words in which he expresses
1
LIGHT. 9
his sense of the loveliness of fair forms and brilliant colours. '' For light, queen of the colours," he says, '' bathing all I can look upon, from morning till evening, let me go where I will, will still keep gliding by me in unnumbered guises, and soothe me whilst I am busy at other things, and am thinking nothing of her.'' But the sensuous pleasure afforded by light is enhanced, while a purer and higher enjoyment is superadded, when the real meaning of the display is understood. As the astronomer sees in the sun a more glorious object than the sun of the poet, recognising in imagination not only the visible splendour of that orb, but the mighty energy with which it is swaying the motions of a scheme of circHng w^orlds, the wondrous activities at work throughout its entire frame, the inconceivable tumult which must prevail in that seemingly silent globe, so the glories of light, rightly understood, are far more impressive than as they appeal simply to the senses.
<ronsider, for instance, the real meaning of sunrise. The orb seemingly rising above the horizon, but, in reality, at rest, is the source of all the glory which is spreading over the fair face of earth. The atoms of that remote body, vibrating with intensest activity, send forth in all directions ethereal waves, and of these relatively but a very few, about one in two thousand millions, fall upon our earth, producing the phenomena of sunlight.
lo LIGHT,
They have been little more than eight minutes on the road, but in that short time they have traversed more than 90,000,000 of miles. Were they to fall directly upon our earth, we should see few of the splendours which attend the uprising of the sun. The deep air clothing our earth receives the onward rushing waves, and reflects them in all directions. To use Biot's simile, " The air is a sort of briUiant veil, which multiplies and diversifies the sunlight by an infinity of repercussions." Nor is the wonder of the scene, or its effect in filling the mind with solemn and poetic thoughts, diminished — on the contrary, it is enhanced — by the recollection that the gradually growing glory of day is brought about by the slow turning of the mighty earth, —
* * that spinning sleeps On her soft axle, as she paces even, And bears us soft with the smooth air along."
But if this is true of a scene of terrestrial splendour, how much more fully may it be said of the glories of the heavens? No poet, if unaware of the real meaning of modern discoveries respecting the celestial bodies, can be moved by the starlit depths as the astronomer is, at least the astronomer whose study of science is not limited to mere observation and calculation. Hundreds of bright points of light sparkling, and sometimes varying strangely in colour, form, no doubt, a beautiful scene \
LIGHT, II
but the scene is not less beautiful, and certainly it is far more impressive, when we remember that every one of these points of light is a sun, mighty in attractive energy like ours, its whole surface glowing with fiery heat, and every particle of its substance constantly in motion, if not always in the fierce rush of cosmic hurricanes, yet with the ceaseless vibrations which generate the ethereal light-waves telling us of the star's existence.
There is one strange thought connected with the motion of light-waves through the ether of space which has not, I think, received the attention it deserves.
Every one knows that when we look at the heavens we do not see the celestial bodies where they are, but where they were^ and again, not where they were at any one moment of time, but some where they were a short time ago, others where they were very long ago. But it is not so generally known, or remembered by those who do know it, that if light were not so active as it is the result would be that utterly incorrect pictures of the celestial depths would continually be presented to us. As matters actually are no orb in space can appear very far from its true place. We see the sun, for instance, at any moment, not where he is, but w^here he w^as (or rather towards the direction in which he lay) about eight minutes before. But as the real velocity of the earth, and therefore the apparent velocity of the sun, amounts only to about
12 LIGHT,
eighteen miles per second, the sun is only thrown about 9000 miles out of his true position, which is but about the ninetieth part of his diameter : so that we see the sun very nearly in his right place. Now it might seem that a star whose light takes, say, twenty years in reaching us, must be seen very far from its true place, supposing the star to be travelling along very quickly j and, in one sense, this is true. If such a star is moving at the rate of fifty miles per second, athwart the Hne of sight, it will be out of place by so considerable a distance as 315,000,000,000 of miles. Yet the star will appear very nearly in its true position, simply because, at the star's enormous distance from us, even the great distance just named is reduced to a very small apparent amount. Such a star w^ould, in fact, be displaced by only about the thirtieth part of the sun's or moon's apparent diame- ters, or by about a fifteenth part of the distance separat- ing the middle star of the Great Bear's tail from its small companion, sometimes called Jack by the Middle Horse. Thus the stellar heavens present very truly to us the positions of the stars; for such athwart motion as I have just imagined would be very much larger than the motion of far the greater number of the stars. But we only thus see the heavens truly pictured because of the enormous velocity with which light travels. If light swept along only at the rate of a hundred miles in a
!!illil!!lilii!!ll!illl!ii!i||!i
m
LIGHT. 15
second (a velocity still far beyond our powers of concep- tion), there would be no believing what we should see, for every star, and our own sun, and all the planets, and even our o^vn companion planet, the moon, would be thrown in appearance very far from their true positions. If they were all shifted in position by the same amount and in the same direction the picture would still be true, in a sense, just as we see a true picture of an object at the bottom of a clear lake, though the picture is dis- placed by the refractive action of the water on the rays of light. But, in the imagined case, the sun, and moon, and planets, and stars would be shifted by different amounts and in different ways, simply because they are moving at different rates and in different directions. The scene presented to us would have been utterly untrue. Astronomy as a science could probably have had no existence in such a case. Assuredly it could have had no existence until students of the heavenly bodies had learned to accept as the first axiom of their science the doctrine that ^^ Seeing is not believing."
A strange thought truly, that so active are the orbs peopling space, so s\viftly do they rush onwards upon their orbits, that light, carrying its message at a rate exceeding six thousand times the velocity of the swiftest express train, woiild be utterly unable to give a true
16 LIGHT,
account of the position and movements of the celestial bodies. Fortunately light gives a true record, because the qualities of the cosmic ether are such that the mes- sage of light is transmitted hundreds of times more swiftly than the swiftest bodies in the universe travel onwards upon their orbits around each other or in space.
11.
SPACE
^LTHOUGH astronomy tells us in clearest words of the vast depths of space which surround our earth on all sides, we are not thereby enabled to realize their enormous extension. It is not merely that the unknown depths beyond the range of our most powerful telescopes are inconceivable, but that the parts of space which we can examine are on too large a scale for us to conceive their real dimensions. It is hardly going too far to say that our powers of actual conception are limited to the extent of space over which the eye seems to range in the daytime. Of course in the daytime, at least in clear weather, there is one direction in which the eyesight ranges over a distance of many millions of miles, — namely, where we see the sun. But the sense of sight is not cognisant of that enormous distance, and simply presents the sun to
1 8 SPACE.
US as a bright disc in the sky, or perhaps rather nearer to us than the sky. Even the distance of the sky itself is under-estimated. A portion of the Ught we receive from the sky on a clear day comes from parts of the atmos- phere distant more than thirty or forty miles from us ; but the eye does not recognise the fact. The blue sky seems a little farther off than the clouds, but not much ; the light clouds of summer seem a little but not much farther off than the heavier clouds of a mnter sky; a cloud-covered winter sky seems a little farther off than heavy rain-clouds. The actual varieties of distance among clouds of various kinds are not much more clearly discerned than the actual varieties of distance among the heavenly bodies. The estimate formed of the distance of a cloud-covered sky overhead probably amounts to little more than a mile, and it is very doubt- ful whether the mind presents the remotest depths of a blue sky overhead at more than two miles. Towards the horizon the distance seems greater, and probably on a cloudy day the sky near the horizon is unconsciously regarded as at a distance of about five miles, while blue sky near the horizon may be regarded as lying at a distance of six or seven miles, the arch of a blue sky seeming to be far more deeply curved than that of a cloud-covered sky.
It is to distances such as these that the mind uncon-
SPACE. 19
sciously refers the celestial bodies. We know that the moon is about 2,000 miles in diameter, but the mind refuses to present her to us as other than a round disc much smaller than those other objects in sight which occupy a much larger portion of the field of vision. The sun cannot be conceived to exceed the moon enormously in size, seeing that he appears no larger; and all the multitude of stars are judged by the sight to be mere bright points of light in reaUty as they appear to be.
How, then, can we hope to appreciate the vastness of space whereof astronomy tells us ? To the student of science attempting to conceive the immensities of whose existence he is assured, the same lesson might be taught in parable which the child of St. Augustine's vision taught the Numidian theologian. As reasonably might an infant hope to pour the waters of ocean into a hollow, scooped with his tiny fingers in the sand, as man to picture in his narrow mind the length and breadth and depth of the abysses of space in which our earth is lost.
Yet, as a picture of a great mansion may be so drawn on a small scrap of paper as to convey just ideas of its proportions, so may the great truths which astronomy has taught us about the depths of space be so presented that just conceptions may be formed of the proportions of at least those parts of the universe which lie within
20 SPACE.
the range of scientific vision, though it would be hopeless to attempt to conceive their real dimensions.
Thus, when we learn that a globe as large as our earth, suspended beside the moon, would seem to have a diameter exceeding hers nearly four times, so that the globe wo uld cover a space in the heavens about thirteen times as large as the moon covers, we form a just con- ception of the size of the moon as compared with the earth, though tlie mind cannot conceive such a body as the moon or the earth really is. When, in turn, we are told that if a globe as large as the earth, but glowing as brightly as the sun, were set beside the sun, it would look a mere point of light, we not only learn to picture rightly to our- selves how largely the sun exceeds the earth, but also how enormous must be the real distance of the sun.
Another step leads us to a standpoint whence we can form a correct estimate of the vast' distance of the fixed stars j for we learn that so enormous is the distance of even the nearest fixed star, that the tremendous space separating the earth from that star sinks in turn into the merest point, insomuch that if a globe as bright as the sun had the earth's orbit as a close fitting girdle, then this glorious orb (with a diameter of some 184,000,000 of miles) would look very much smaller than such a globe as our earth would look at the sun's distance — would, in factj occupy but about one-fortieth part of the
SPACE. 21
Space in the sky which she, though she would then look a mere point, would occupy if viewed from that distance.
But there is a way of viev/ing the immensities of space which, though not aiding us indeed to conceive them, enables the mind to picture their proportions better than any other. The dimensions of the earth's path around the sun sink into insignificance beside those of the outermost planets ; but these in their turn dwindle into nothingness beside those of some among the comets. From the paths of these comets, if only sentient and reasoning beings could trace out in a comet's company those mighty orbits, and could have for the duration of their existence not the brief span of time which measures the longest human life, but many circuits of their comet home around the same ruling orb (as we live during many circuits of our globe around the sun), the dimensions of the star-depths, which even to scientific insight are all but immeasurable, would be directly discernible. Not only would the proportions of that mighty system be perceived, whose fruits and blossoms are suns and worlds, but even the gradually changing arrangement of its parts could be discerned.
Some comets, indeed, as I pointed out in an essay on comets several months ago (see Expanse of Heaven, p. 149), do not travel around the sun, but flit from sun to sun on journeys lasting millions of years, paying
22 SPACE.
each sun^ but a single visit. A being inhabiting such a comet, and having these interstellar journeys as the years of his existence, so, that he could live through many of them, would have a wonderful insight into the economy of the stellar system. If his powers of con- ception as far exceeded ours as the range of his travels and the duration of his existence, he would be able to recognise the proportions of a large part of the stellar universe as clearly as we recognise the proportions of the solar system.
But leaving these wonderful wanderers, whose journeys are as far beyond our powers of conception as the im- mensity of the regions of star-strewn space^ we may find, among the comets belonging to the sun's domain, bodies whose range ot travel would give their inhabitants far clearer views of the architecture of the heavens than even the profoundest terrestrial astronomer can possibly obtain.
Such a comet as Halley's (fig. 3) for instance, though one of comparatively limited range in space, yet travels so far from the sun that, from the extreme part of its path, it sees the stars displaced nearly twenty times as much (owing to its own change of position) as they are from the earth on opposite sides of her comparatively narrow orbit. And the length of this comet's year, if it indi- cated the length of the lives of all creatures travelling
r
SPACE.
23
along with it, would suggest a power of patiently watch- ing the progi^ess of changes lasting not a few of our years only, but for cen- turies. Seventy-five or seventy-six years elapse between each return of this comet to the sun's neighbourhood, and one who should have lived during sixty or seventy circuits of this body around its mighty orbit would have been able to watch the rush of stars, with their ve- locities of many miles per second, until visible
displacements had Fig. 3— Halley's Comet of 1835.
taken place in their positions.
This, however, is as nothing compared with the mighty range in space and the enormous period of the orbit of the great comet of the year 181 1 (fig. 4). This comet is, on the whole, the most remarkable ever known. It was visible for nearly seventeen months, and though it did not approach the sun within 100,000,000 miles, and was therefore not subject to that violence of action
24 SPACE,
which has caused enormous tails to be thrown out from Comets which have come within a few million miles of him, or even within less than a quarter of his own diameter, it flourished forth a tail 120,000,000 of miles in length. Its orbit has, according to the calculations of the astronomer Argelander, a space exceeding the earth's distance from the sun 211 times, and thus surpassing even the mighty distance of Neptune fully seven times. It occupies in circuiting this mighty path no less than 3065 of our years (with a possible error either way of about forty-three years). So that, according to Bible chronology, this comet's last appearance probably oc- curred during the rule of the judge Tola, son of Puah, son of Dodo, over the children of Israel, though it may have occurred during the rule of his predecessor Abime- lech, or during that of his successor Jair.'^' During one half of the enormous interval between that time and 181 1 the comet was rushing outwards into space,
* It might be suggested that the appearance of this blazing comet among the stars drove the more superstitious of the Israelites at that time to the worship of star-gods, as we read how, during the judg- ship of Jair, they ** served Baalim, and Ashtaroth, and the gods of Syria, and the gods of Moab, and the gods of the Philistines, and forsook the Lord and served not Him.'* To a people like the Jews, who seem to have been in continual danger of returning to the Sabaistic worship of their Chaldean ancestors, the appearance of a blazing comet may have been a frequent occasion of backsliding.
1^
Fig. 4.— Comet of iSit.
SPACE. £7
reaching the remotest part of its path somewhere about the year 278 (a.d.), and from that time to i8ri it was on its return journey. It is strange to think, however, that though the remotest part of its path lay 211 times farther from the sun than the earth's orbit, yet even this mighty path, requiring more than 3000 years for a single circuit, cannot be said to have carried the comet into the star- depths. If the earth were to shift its position by the same enormous amount the nearest fixed star would have its apparent position changed only by about an eighth part of the apparent diameter of the sun or moon, or by about one-quarter of the distance separating the middle star of the Bear's tail from its close companion.
But this fact of itself is most strikingly suggestive of the vast distance of the stars. For consider what it means. Imagine the middle star of the Bear's tail to be the really nearest of all the stars instead of lying probably twenty or thirty times farther away. Conceive a comet belonging to that sun after making its nearest approach to it to travel away upon an orbit requiring 3000 years for each circuit. Then (supposing that star equal to our sun in mass), the comet, though rushing away from its sun ^\ith inconceivable velocity during 1500 years, would, at the end of that vast period, seem to be no farther away than one-fourth of the distance separating the sun from its near companion. Look at the middle star of the Bear's
28
SPACE.
tail on any clear night, and on its small satellite, remem- bering this fact, and the awful immensity of the star depths are strongly impressed upon the mind. But the observer must not fail to remember that the star really is many times more remote than we have here for a
Fig. 5— Six-tailed Comet of 1744.
moment supposed, and that such a comet's range of travel would be proportionately reduced. Moreover, many among the stars are, doubtless, hundreds, even thousands, of times still farther away. Let us turn lastly to the amazing comet of the year
SPACE. 29
1744, pictured, at the time, as shown in fig. 5 (though probably the drawing is greatly exaggerated). We find that though it had the longest period of any which has ever been assigned to a comet as the result of actual mathematical calculation, yet its range in space would scarcely suffice to change the position of the stars in such sort that the aspect of the familiar constellations would be materially altered. Euler, the eminent mathe- matician, calculated for this comet a period of 122,683 years, which would correspond, I find, to a distance of recession equal to 2469 times the distance of the earth from the sun, or about eighty times the distance of Neptune, Yet this is but little more than twelve timcJ the greatest distance of the comet of 181 1. Probably the actual range of such an orbit from the middle star of the Bear's tail would be equal in appearance to the range described above on the supposition that the star is no farther from us than the nearest known star (Alpha Centauri). That is, such a comet, if it could be seen and watched during a period of about 122,000 years, would seem to recede from the star to a distance equal to about one-fourth the space separating it from its close companion, and then to return to the point of nearest approach to its ruling sun.
Such are the immensities of star-strewn space ! The journey of a comet receding from the sun with incon-
30 SPACE,
ceivable velocity during hundreds of thousands of years carries it but so small a distance from him compared with the distance of the nearest star as scarcely to change the appearance of the celestial landscape ; and yet the distances separating the sun from the nearest of his fellow suns are but as hair-breadths to leagues when compared with the proportions of the scheme of suns to which he belongs. These distances, though so mighty that by comparison with them the inconceivable dimen- sions of our own earth sink into utter nothingness, do not bring us even to the threshold of the outermost court of that region of space to which the scrutiny of our telescopes extends. Yet the whole of that region is but an atom in the infinity of space.
III.
CF THE INFINITELY MINUTE.
\ HEN I speak of the infinitely minute, I use the word infinitely not in its absolute sense, but relatively. Actual infinity of minute- ness is as utterly beyond our conceptions as actual infinity of vastness. But we may speak of what is very much less than the least object of which our senses can make us directly conscious as for us infinitely minute. Among the greatest wonders science has to deal with are those relating to bodies and movements thus beyond the direct ken of our senses. There is a universe within the universe which our senses reveal to us, — a universe whose structure is so fine that the minutest particle which the microscope can reveal to us is, by comparison, like one of the suns which people our universe compared with the unseen particles constituting matter.
It is a strange thought that the objects constituting
32 OF THE INFINITELY MINUTE.
our universe, so long regarded by man as the only uni- verse, are in a sense pervaded by the materials of an utterly different universe, — which yet is as essential to our very existence as what we commonly call matter. Wc cannot live without light and heat, for instance, and again, light and heat affect matter as we know it; but they thus exist and affect such matter by means only of a form of matter unlike any which we can conceive. It is certain tliat if absolute vacancy separated our earth from the sun, even by the narrowest imaginable gap, his heat and light could never reach us. They could on more pass that vacant space than tlic wave-motion of water can cross a space where water itself is wanting. It is because of relations such as these that it has been said, and justly, that matter is the less important half of the material constituting the physical universe.
Our knowledge of this universe within our universe has been obtained within comparatively recent years. Men were unwilling or at least they spoke and thought as if they were unwilling, to believe that the universe of matter which they had so long recognised was de- pendent on another universe for its chief if not all its properties. They regarded heat as some sort of sub- stance, which might, with more delicate means than they possessed, admit of being dealt with as chemists had dealt with the gases. The sun was full of this fluid, this
OF THE INFINITELY MINUTE. 33
phlogiston, as it was called. Light, in so far as it could be distinguished from heat, was another fluid ; electri- city was another. These were the imponderables, or un- weighable substances of last century's science, — not as with us, the effects of modes of motion taking place in a universe which, though material, is yet not made of matter such as we know, or even such as we can at present conceive.
This is the greatest of all human scientific marvels, — ■ the greatest because it includes all others. We know of a universe which is as infinite in extent, and doubtless in duration, as our own universe ; which pervades all forms of matter : and yet we know of this universe only in- directly ; by the effects of movements taking place within it, not by any perception of these movements themselves. Waves are ever beating upon the shores of our material universe, and constantly changing the form and condition of the coast line, but the waves themselves are unseen. We only know of their existence through the changes wrought by them.
We speak of the ether of space, and of waves travers- ing it, as though the ether were simply some fluid very much more attenuated than the rarest gas, even in a so- called vacuum. But in reaUty, so soon as we attempt to apply to the movements taking place in such an ether the mechanical considerations which ^ufifice for the
3
34 OF THE INFINITELY MINUTE,
motions of all ordinary forms of matter, we perceive that it must of necessity be utterly unlike any kind of sub- stance known to us. For instance, we find that though it is like a gas in being elastic, its elasticity is infinite compared with that of any material gas. Again, it is like a solid in retaining each of its particles always very near to a fixed position ; but again, no solid we know of can be compared with it for a moment as respects this kind of rigidity. It is at once infinitely elastic and infinitely rigid. We cannot, for example, explain the phenomena of light unless we suppose the elasticity of the ether at least 800,000,000,000 times greater than the elasticity of air at the sea-level ; and yet, as Sir J. Herschel long since pointed out, every phenomenon of light points strongly to the conclusion that none of the particles of the ether can be " supposed capable of inter- changing places, or of bodily transfer to any measurable distance from their own special and assigned localities in the universe. Again, how are we to explain the con- tinuance of the ether in its present condition, when we recognise the fact that a gas of similar elastic power would expand in all directions with irresistible force, diminishing correspondingly in density ; yet the ether of space remains always, so far as we can judge, absolutely unchanged in position. Its characteristics certainly re- mained unchanged. Light travels at the same rate now
OF THE INFINITELY MINUTE. 35
as it did last year, last century, a million years ago. The ether, then, that bears it has presumably remained un- changed. If it were gaseous, and bounded on all sides by vacuum, it would expand with inconceivable velocity. To suppose it infinite in extent is to get rid of the diffi- culty perfectly ; but only by introducing a difficulty far greater."'*
A wonderful feature of the infinitely tenuous ether is, that while its ultimate particles must be inconceiv- ably more minute than the ultimate atoms of ordinary matter, the movements taking place in it are trans- mitted with enormous velocities. The structure of our universe is on a grander scale ; its least atom may com- prise millions of millions of the largest component
* I do not say we can in any way avoid this far greater difficulty. Our own material universe cannot even be conceived as limited in any way save by void space of infinite extent ; and it is as impossible for us to conceive an infinite void as to conceive the infinite exten- sion of matter. Some modern mathematicians, indeed, assert that space is not necessarily infinite, but they accompany the assertion (very justly) with the admission that we cannot possibly conceive any boundary to space ; and as one of the things they ask mathematicians to admit is the possibility that a straight line indefinitely produced both ways will at length re-enter into itself, while another is the possibility that in other parts of the universe two and two may make three or five, they are not likely, I conceive, to persuade most mathematicians (profoundly mathematical though they are them- selves) that the mystery of infinity has been as yet entirely ex- pounded.
36 OF THE INFINITEL V MINUTE,
portions of that infinitely tenuous ether. But amid that ether motions arc transmitted with velocities trans- cending all but infinitely those which take place among the particles of matter composing the universe in which we ^^ live and move and have our being." The planets, immense aggregates of matter such as we know it, sweep onwards upon their immense orbits, traversing many thousands of miles in an hour; but light and heat sweep along the ether of space, and by virtue of motions taking place within that ether at the rate of many tens of thousands of miles per second. The suns which people space rush onwards with mightier momentum, but less swiftly than the planets in their orbits. Comets attain the greatest velocities of all the bodies that science deals with, rushing sometimes, in their periastral swoop, with a velocity of hundreds of miles per second, — though yet in mid-space the comets of widest orbital range lag slowly enough, insomuch that some of those which, when nearest our sun, travel at the rate of two or three hundred miles per second, move more slowly when very far from him than many of our rivers. Taking even the swiftest rush of a comet within the solar domain, we find that light speeds along five hundred times more quickly, — so that if we represent the velocity of light by that of an express train (reducing light's velocity in scale to ^bout
OF THE INFIMTELV MEVUTE, 37
one-io,ooo,oooth part of its real value), the velocity of the most swiftly-moving comet would be represented by that of a walk at the rate of one-eighth of a mile per hour, — a very slow walk indeed.
It is not only amid the depths of space that these wonderfully swift motions take place in the ethereal universe. As I have said, that universe pervades ours throughout its entire extent. The densest of our solids is as freely traversed by the ether as a forest by the summer breeze. As the foliage of a thick forest may prevent the passage of fierce winds, so may a solid body prevent the passage of light-waves — though all solid bodies, as we know, do not prevent, and some scarcely even modify, the passage of light. But sub- stances which prevent the passage of light are yet found capable of transmitting ethereal motions of similar velocity. According to Wheatstone's experi- ments electricity travels at the rate of more than 200,000 miles per second along stout copper wire. Fizeau's experiments gave a lower speed ; but they did not negative Wheiatstone's, the conditions not being the same. Can anything be more wonderful than the thought of the transmission of electricity with this enor- mous velocity? What really happens we do not know. Perhaps if we were told what really takes place be- tween and among the particles of the wire, we should
38 OF THE INFINITEL V MINUTE.
find ourselves utterly unable to conceive it — for, as we have seen, the properties of the ether, and, there- fore, the processes taking place in the ethereal universe, are probably unlike any within our experience. But this w^e know — a certain condition of the molecules of the wire is transmitted, by virtue of the ethereal medium pervading the wire, at a rate so enormous that, if the wire itself could move at that rate, the force required to bring its mass to rest would suffice to generate enough heat to turn many times as much metal into the vaporous state.
Nay, even as regards the energy of their action on the matter of our universe, these movements in the ethereal universe enormously exceed the forces we are accus- tomed to regard as most powerful. The effects pro- duced by gravity, for instance, are almost evanescent compared with those produced by heat. The sun's rays poured on a piece of metal for a few minutes pro- duce motions in every one of the ultimate particles of the metal. Each particle vibrates with inconceivable rapidity (referring to the rate at which the vibrations succeed each other), and with great actual velocity of motion. Summing up the energy thus pervading the piece of metal, we find that it incalculably exceeds the energy represented by the velocity which the sun's attraction would communicate in the same interval to
OF THE INFINITELY MINUTE, 39
that piece of metal, supposed to be entirely under its influence at the earth's distance from the sun.
Or take another instance. "Think for a moment," say the authors of the " Unseen Universe/' " of the fundamental experiments in electricity and magnetism, known to men for far more than 2000 years, — the lift- ing of light bodies in general by rubbed amber and of iron filings by a loadstone. To produce the same effect by gravitation-attraction, — at least, if the attract- ing body had the moderate dimensions of a hand- specimen of amber or loadstone, — we should require it to be of so dense a material as to weigh, at the very least, 1,000,000,000 pounds, instead of (as usual) a mere fraction of a pound. Hence it is at once obvious that the imposing nature of the force of gravity, as usually compared with other attractive forces, is due, not to its superior qualitative magni- tude, but to the enormous masses of the bodies which exercise it.''
We may put this illustration in another form. When we place a powerful magnet near a piece of iron, say at a distance of one inch, and the magnet lifts that piece of iron by virtue of its attractive power, a contest has been waged, if one may so speak, between the attractive powers of the small magnet and of the mighty earth, and the magnet has conquered the earth. Now the
43 OF THE INFINITE L Y MINUTE.
magnet has been much nearer than the earth to the piece of iron, for we know that the earth's attractive influence has been the same as though the entire mass of the earth were gathered at its centre, say 4000 miles from the piece of iron. A distance of 4000 miles con- tains 4000 times 1760 times thirty-six inches, or, roughly, 250 millions of inches. (This is in truth very near the true number of inches in the earth's radius, inso- much that many suppose the inch to have been ori- ginally taken as the 500,000,000th part of the earth's diameter. A British inch is about one-5oo,ooo,oooth part of the polar diameter of the earth.) Since attraction diminishes as the square of the distances increases, and vice versa, it follows that if the earth's entire mass could act on the piece of iron, at a distance of one inch, the attraction would exceed that actually exerted by the earth 250 million times 250 million times, or 62,500 milHons of milHons of times. In this degree, then, the earth is at a disadvantage compared with the magnet as respects distance. And one-62, 500,000,000,000,000th part of the earth's mass would be capable of attracting the piece of iron as strongly as the earth actually attracts it, if that fraction of the earth's mass could exert its pull from a distance of only one inch. But a 62,500,000,000,000,000th part of the earth would be an enormous mass. It v/ould weigh about 97,500 tons.
OP THE INFINITELY MINUTE. 41
or some 218 millions of pounds. Thus a magnet which a child can lift exerts a greater attraction on the piece of iron at the same distance than a mass at least 1000 million times its weight could ex^rt by its gravity only.
In fact we see from this illustration that gravity, though it produces effects so tremendous, though it sways the moon round the earth, the earth and all the other planets around the sun, and urges the sun and his fellow- suns through space, is, after all, but a puny force in itself A child can lift his own weight against the attraction of the mighty earth ; and by combined strength as many children as would have a weight equal to the earth's would easily bear a weight exceeding the earth's, if the force could be wholly and directly applied to such work.^
The attraction of gravity must, however, be regarded
* Of course the reader will understand that when I here speak of the earth's weight, I mean simply the pressure which would be exerted by the quantity of matter contained in the earth, if each portion were only subjected to an attractive force equal to that of gravity at the earth's surface. The actual force with which the earth is drawn in any direction, as a weight at the earth's surface is drawn downwards, depends on the distance and mass of the attracting body as well as on the mass of the earth ; and strictly speaking, we ought not to say that the earth weighs so many millions of tons, but that she contains so many million times as much matter as a mass which at her surface weighs a ton.
42 OF THE INFINITELY MINUTE.
as only one manifestation of the energies of the in- finitely minute. It is in this sense well worthy of careful study. I propose to present in a future paper some of the strange thoughts which are suggested by the action of this wonderful force, the range of whose activity is seemingly co-extensive with the material universe.
IV.
THE MYSTERY OF GRAVITY.
'^HE law of gravity, or of the mutual attrac- tion of masses of matter upon each other, accounts so perfectly for all the observed motions of the heavenly bodies, that we are apt to regard Newton's discovery of the great law as though it had finally solved the mystery of these motions. Many accept the verdict given by the poet Pope in the famous epitaph which he suggested for Newton, —
** Nature and Nature's laws lay hid in night : God said, Let Newton he I and all was Light.''
But Newton, who probably knew as much about his work as Pope, was of another opinion. Every one knows how he compared himself to a child who had picked up a few shells on the shore, while the ocean of truth lay unexplored before him. He has, however, spoken definitely of the great discovery which has
44 THE MYSTERY OF GRAVITY,
rendered his name illustrious, in terms which show that he did not find that all was light. Among the questions which he specially would have had answered, amongst the secrets of nature concealed beneath the ocean of truth, the mystery of gravity was probably the chief. When Newton asked of the Ocean of Truth what Mrs. Hemans later said, and in another sense, of the natural sea —
*' What hidest thou in thy treasure-caves and cells, Thou hollow-sounding and mysterious main ? "
he had in his thoughts the very power w^hich he is commonly supposed to have explained, but which was in truth for him, more than for any man that had ever lived, the mystery of mysteries.
It may be well to consider the very words of the great pliilosopher, so far at least as our more diffuse language can present the concise expressions of the original Latin:
*^ Hitherto we have explained," he says, "the pheno- mena of the heavens and of our sea by the power of gravity, but have not yet assigned the cause of this power. This is certain " (we must hearken attentively here, for when a man like Newton speaks of aught as certain, we have sure ground to go upon), — " this is cer- tain, that it must proceed from a cause that penetrates to the very centres of the sun and planets, without suffer- ing the least diminution of its forces ; that operates, not
THE MYSTERY OF GRAVITY, 45
according to the quantity of surfaces of particles on which it acts (as mechanical causes usually do), but according to the quantity of the solid matter which they contain, and propagates its virtue on all sides to immense distances, decreasing always as the squares of the dis- tances. Gravitation towards the sun is made up of the gravitations towards the several particles of which the body of the sun is composed, and in receding from the sun decreases accurately as the square of the distances as far as the path of Saturn . . . . , nay, and even to the remotest parts of the paths of comets .... But hitherto I have not been able to discover the cause of those properties of gravity from phenomena; and I frame no hypotheses : * for, whatever is not deduced from phenomena is to be called an hypothesis; and
* The words of Newton, ** Hypotheses non fingo," have been often quoted in such sort as to give an entirely incorrect idea of his real opinion as to the relation between theoretical and practical science. As too commonly understood, they would, in fact, make his discovery of gravitation a great exception to his own rule. They must be taken in connection with his definition of a hypothesis, as ** whatsoever is not deduced from phenomena." It is a part of true science, nay, it is the highest office of the student of science to deduce theories from phenomena. Such research stands as high above the simple observation of phenomena as architecture standi above brick-making or stone- cutting. But to frame hypotheses as the old Greeks did, trusting to the power of the understanding in- dependently of the observation of phenomena, is to make bricks without straw and to build with them upon the sand.
46 THE MYSTER Y OF GRA VITY.
hypotheses, whether metaphysical or physical, whether of occult qualities or mechanical, have no place in ex- perimental philosophy. ... To us it is enough that gravity does really exist, and act according to the laws which we have explained, and abundantly serves to account for all the motions of the celestial bodies and of our sea.''
^' Hitherto I have not been able to discover the cause of the properties of gravity." Such is the simple state ment of the man who discovered those properties.
And now let us inquire a little into this law of gravity, not with the hope of explaining this great mystery of nature, — though, for my own part, I believe that the time is not far distant when the progress of discovery will enable man to make this approach towards the mys- tery of mysteries, — but in order to recognise the real nature of the mystery, which is a very different thing from explaining it.
In the first place the study of gravity brings us at once to the consideration of the infinitely minute, — at least of what is for us practically infinite in its minuteness. If we consider the above quotation attentively, we perceive that this quality of gravity was recognised by Newton. " It is not the quantity of the surfaces of particleSy^ he says, '' but the quantity of solid matter which they con- tain," that gives to gravity its power. Gravity resides in
THE MYSTERY OF GRAVITY. 47
the ultimate particles of matter. We cannot conceive of matter so divided, no matter how finely, that non-gravi- tating particles could be separated from gravitating par- ticles. Without entering into the question what atoms are, we perceive that these ultimate constituents of matter must contain, each according to the quantity of matter in it, the gravitating energy. Only, observe how incongruously we are compelled to speak. (It is always so when we deal with the infinite, Vv^hether the infinitely great or the infinitely minute.) We are speaking of atoms as the ultimate constituents of matter, and yet we are compelled, in describing their gravitating energy, to speak of the quantity of matter contained in each atom, — in other words, we speak in the same breath of an atom as not admitting of being divided or diminished, and of its containing matter by q.uantity, that is, by more or less. May we not, however, reasonably accept both views? The reasoning is sound by which science has proved that, so far as our material universe is concerned, there is a limit beyond which the division of matter can. not be supposed to go, — insom^uch that Sir W. Thomson has indicated the actual limits of size of the atoms com- posing matter. Yet, passing in imagination beyond the bounds of our visible universe, and so entering into the next order of universe below it (in scale of construction), — the ether of space, — the atoms of our universe may be
48 THE MYSTERY OF GRAVITY,
infinitely divisible in that universe, may be, in fact, com- pared with its particles, as the suns and worlds of our universe are to our atoms and molecules.
But while gravity thus draws us to the contemplation of the infinitely minute, it also leads us to the considera- tion of what is for us the infinitely vast.
Newton was only able to speak confidently of the action of gravity at the distance of Saturi), the remotest planet knov/n in his day. He did, indeed, refer to the comets as probably obeying, even in the remotest parts of their paths, the force of the sun's gravity; but he could not be certain on that point, because in his time no comet had been proved to travel back to the sun after receding to the remotest portion of its track. We now know not only that the sun's attraction extends to the farthest parts of the solar system, having thus a domain in space nearly thirty times larger than the sphere of Saturn, but we perceive that many among the stars exert a similar force ; for around them travel other stars even as the planets travel around the sun. Thus we know that gravity is exerted in regions lying hundreds of thousands of times farther from the sun than Saturn is. We have, indeed, every reason to believe, not only that star unto star extendeth this mysterious attractive influence, but that the least particle in the inmost depths of sun or world exerts in full force on each particle, even
The mystery of gravity. 49
of suns lying millions of times beyond the range of the most powerful telescope yet constructed by man, the full energy corresponding (i.) to the (quantity of matter in itself and such particle, and (ii.) to the distance separat- ing each from each.
This is amazing enough; but there is something more perplexing and mysterious in gravity even than this. Not only does gravity lead us to consider the infinitely minute in space on the one hand, and the infinitely vast in space on the other, but also it leads us to consider the infinitely minute and the infinitely vast in time also, and this in such a way as to suggest a diffi- culty which, as yet, no man has been able to solve.
Light travels, as we know, with a velocity so enormous, that, by comparison with it, all the velocities we are familiar with seem absolutely as rest. But gravity acts so quickly that even the velocity of light becomes as rest by comparison with the velocity of the propagation of gravity. Laplace had occasion, now nearly a century ago, to inquire whether a certain change in the moon's motion, by which she seemed to be gradually hastening her motion round the earth, might not be caused by the circumstance that gravity requires time for its action to te propagated over great distances. He found that if the whole of that change had to be explained in this way, which would be giving to gravity the slowest admis-
4
so THE MYSTERY OE GRAVITY.
sible rate of transmission, the velocity with which gravity is propagated would be eight million times greater than the velocity of light. If, on the other hand, that change in the moon^s motion could be satisfactorily explained in some other way, then the velocity of gravity must be at least 16,000,000 times greater than the velocity of light. He himself soon after discovered what was in his day regarded as a complete explanation of the hastening of the moon's motion ; and though in our own time Adams of Cambridge has shewn that only half the hastening can be accounted for by Laplace's reasoning, the general explanation of the remaining half is that it is not a real hastening of the moon, but is only an apparent hastening caused by the gradual slowing of the earth's rate of turning on her axis. This makes the day by which we measure the moon's motion seem longer (very slightly, however)."*^ Supposing, however, half the moon's hasten- ing were left unexplained, and that the non-instantaneous transmission of gravity were the only way of accounting for it, even then it would be certain that gravity is pro- pagated at a rate exceeding 12,000,000 times the velocity of light.
Indeed, at present, owing to the more exact observa^
* The point is explained in a paper called * ' Our Chief Timepiece Losing Time,** in the first series of my ** Light Science for Leisure Hours."
THE MYSTERY OF GRAVITY, 51
tions available, and the greater range of time over which they extend, it may safely be said that the rate of propa- gation of gravity is far greater than this. It is even held by some that gravity acts instantaneously over any dis- tance, however vast.
Although I cannot here indicate the exact nature of the reasoning by which the enormous rapidity of the action of gravity is inferred, I must briefly indicate the general argument, that the reader may not suppose the matter to be merely speculative. Suppose that the action of gravity were propagated at the same rate as light.
M
;\ ^ ---
Fig. 6.
Then the earth would feel the pull of the sun eight minutes or so after she had been in the place where the sun began to exert that particular pull. The direction of the pull then would not be that of the straight line connecting the earth and sun at the moment when the pull was felt, but that of the straight line connecting the sun and the earth eight minutes or so before. For in- stance, when the earth is at Ep fig. 6, the sun at S would begin to exert a pull in the line Ei S, but the earth would only feel this pull when she got to Eo, her place eight minutes later, when it would act upon her in the direc-
52 THE MVSTER V OF GkA VI T\^.
tion E^ F, parallel to E^ S. Now this pull, E2 F, may be divided into two parts, one along E^ S, pulling the earth towards the sun S, the other along E2 T in the earth's course, hastening her therefore. But the maintenance by the earth of the same constant track depends entirely on the action of gravity sunwards. If there is any action in addition, hastening the earth, then she w^ill not keep her course," but will travel in a constantly widening path? —or, in a sort of spiral, very slowly retreating from the sun, but retreating constantly. The change of distance would not be measurable in millions of years ; but the increase in the length of the year would^ before long, be observable. Because there is no such increase, astro- nomers feel well assured that gravity is not only propa- gated more swiftly than light, but many times, even, as we have seen, many millions of times, more swiftly.
It is then in an infinitely minute time that the action of gravity traverses all ordinary distances. The earth's
* In the popular, but incorrect way of speaking, the balance between the centrifugal and the centripetal force will no longer be maintained : the increase of velocity will give the centrifugal force the advantage, and it will slowly draw the body away from the centre. In reality there is no centrifugal y2?;r(?, the only force acting on the earth in her course round the sun being the sun's attraction upon her, which, however, must keep bending her course from the straight line, if she is to maintain her distance. In the case above imagined it would not bend her course actively enough.
THE MYSTERY OF GRAVITY. 53
pull on the moon takes less than the 50,000,000th part of a second in reaching the moon, — and the particles constituting the mass of the earth act on ourselves, and on all the objects which lie near the earth's surface, in far less than the io,ooo,oooth part even of this utterly minute time-interval.
Yet age after age has passed during which this infinitely active force has been at work without dimi- nution, and age after age will continue to pass without any change in its activity. For millions of millions of aeons it has lasted and will last, so permanent is it; while its operation is felt simultaneously at points millions of millions of star-distances apart What infinities of distance has this wonderful attractive force traversed !
But even these considerations do not present the greatest of the marvels of gravity. It is wonderful, in- deed, to consider a form of attraction possessed by the infinitely minute, and exerted over the infinitely vast, operating in portions of time immeasurably small, and extending its operations throughout time infinite. But the mystery of mysteries is not here. The marvel of marvels is this, that, so far as we can perceive, the force of gravity is exerted without any material connection with the objects moved by it. Matter seems to act where it is not, to use the phraseology of the schools.
54
THE MYSTERY OF GRAVITY.
Of this " action at a distance," Newton himself said, that it is inconceivable, that in point of fact it is impossible. " No man," he said, " who has, in philosophical matters, a competent faculty of thinking," can "for a moment believe that a body can act through a vacuum, without the intervention of anything else by or through which the force may be conveyed from one body to another." Yet this is precisely what gravity seems to do. The ether occupies, indeed, all space; but there is nothing at present known to us by which we can understand how the either can transmit the force of gravity. The power of the ether in the rapid transmission of undulations seems to attain its limit in the propagation of light and heat and electricity at the rate of nearly 200,000 miles per second. How the ether can act so as to serve as a medium of communication between bodies at all dis- tances, transmitting impressions 10,000,000 times faster, at least, than light travels, nothing at present known to us enables us to say. I have, in a lecture which I gave in America upon the mysteries of the universe, indicated a way in which gravity may be conceived to be generated and transmitted ; and I may hereafter describe the con- ception (based partly on the views of Le Sage). But it is only a conception. There is no phenomenon (except the very form of attraction which has to be explained) tending to show that the conception is correct And
THE MYSTERY OF GRAVITY. 55
even if it be accepted, it brings us face to face with only greater marvels.
At present, however, let this simply be said in con- clusion— that the apparent action of gravity at a distance is, of all physical wonders, the greatest yet known to man. If we accept the opinion of Newton, which, in- deed, seems to me indisputable, that matter cannot act through a vacuum, then we must admit the existence of properties, as yet unthought of, in the ether of space, or in some still more subtle universe permeating that ether. If, on the other hand, we accept the belief that matter can act at a distance, then is there no miracle, either of those believed in by mankind generally, or of those more generally rejected, which exceeds in marvellousness this wonder of all the wonders of physical science.
V,
777^ END OF MANY WORLDS.
SIGN has recently appeared in the heavens which has been interpreted in a way sug- gesting that many worlds like our own have undergone a terrible catastrophe, every living creature upon them being consumed as by fire. I propose briefly to consider some of the thoughts suggested by this strange event.
It is difficult when we look at the star-lit heavens, suggestive as they are of solemn peace, to conceive the stupendous energy, the fierce uproar and tumult, of which even the faintest visible star in reality tells us. Pythagoras spoke of the harmony of the celestial spheres, which we are only prevented from hearing by its continuity. " There's not the smallest orb which thou b^holdest," said the science of the middle ages,
THE END OF MANY WORLDS. 57
** Eut in his motion like an angel sings, Still quiring to the young-eyed cherubim."
The science of our own time tells us a still stranger story. There's not the smallest orb which thou beholdest, she says, but in his motion throbs like a mighty heart, still pulsating life to the worlds which circle round it. But while our powers of vision are limited to the narrow range of our present telescopes, we cannot watch the action of these great centres of energy, nor can w^e hope that the uproar of those remote fires will ever reach mortal ears, though to the mind's ear clear and distinct. It is no longer a mere fancy that each star is a sun. Science has made this an assured fact, which no astronomer thinks of doubting. We know that in certain general respects each star resembles our sun. Each is glowing like our sun with an intense heat. Around each, as around our sun, are the vapours of many elements. In each the fires are maintained, as they are maintained in our sun, in some way which may be partly mechanical, partly chemical, but w^hich certainly does not in the least resemble combustion. We know that in each star processes resembling in violence those taking place in our own sun must be continually in progress, and that such processes must be accom- panied by a noise and tumult compared with which all th^ forms of uproar known upon our earth are a^
58 THE END OF MANY WORLDS.
absolute silence. The crash of the thunderbolt, the bellowing of the volcano, the awful groaning of the earthquake, the roar of the hurricane, the reverberat- ing peals of loudest thunder, any of these, or all combined, are as nothing compared with the tumult raging over every square mile, every square yard, of the surface of each one among the stars.
If we remember this when we hear of stars varying in brightness, we shall perceive that the least change which could be recognised from our remote stand-point must represent an accession or falling off of energy correspond- ing to far more than all the energies existing on our earth, or indeed on all the members of the solar system taken together. Astronomers recognise our sun as in one sense a variable star; for we can hardly suppose that he shines with the same degree of brilliancy when many spots mark his surface as when he is quite free from spots ; and astronomers know that these changes in the sun's condition correspond to wonderful changes in his activity. When spots are most numerous, the coloured flames rage with fierce energy over his whole globe, metallic vapours are shot forth from below his visible surface with velocities of many miles per second. Whereas, when he has no spots, the coloured flames sink down from their former height of tens of thousands of miles, t*ll they are but a few thousand miles in height \
THE END OF MANY WORLDS. 59
while metallic vapours are seldom emitted, and never to the same height, or with the same velocity, as w^hen the spots are most numerous. But though the sun thus varies in condition, and probably in his total brightness, we cannot suppose that such variations could be recog- nised from the distance of even the nearest among the fixed stars. ' What, then, must be the nature of changes taking place in a star, that we, at our enormous dist- ance, should be able to recognise them ! We may well believe that the entire aspect of such a star must be changed to the inhabitants, if such there are, of worlds circling around them.
If, however, the changes taking place in stars, whose variations of brightness can just be recognised, must be amazing, how stupendous must be the changes affecting a star w^hich alternates from brightness to invisibility, like Mira, the Star Wonderful, in the constellation of the Whale ! how destructive those affecting a star Uke Eta, of the ship Argo, which has varied from the fourth magnitude to a lustre nearly equalling that of Sirius, and thence to the lowest limit of visibility, in the course of the last hundred years !
Even these changes, however, though justly regarded as among the chief w^onders and mysteries of the star- depths, seem in turn to sink into nothingness by com- parison with the sudden appearance of a new star, as
6o THE END OE MANY WORLDS,
interpreted by modern scientific observations. Of old, when a new star appeared, it was thought for awhile to be a fresh creation ; a new sun set in the centre of a new system of worlds, — a thought which was not then so startling as in our own times it would be reckoned. When the new star was seen slowly to die out until at last it became invisible, men were content to regard it as a sign set in the heavens for a special purpose. Nor did they find much difficulty in associating such a phenome- non with some event of importance occurring during its continuance, or soon after the new star had died out. Such were the explanations offered respecting the ex- ceedingly bright star which made its appearance in the constellation Cassiopeia in the year 1572. The place in which it appeared is shown in fig. 7. It must have sprung into its full glory in a very short time, for Tycho Brahe, the celebrated astronomer, tells us that, returning on November i, 1572, from his laboratory to his dwelling- house, he saw the new star, which he was certain had not been visible an hour before, shining more brightly than any before seen. It surpassed all the stars in the heavens in brilliancy, and even Jupiter when that planet is at its brightest. Only Venus at her brightest was superior to fhe new star. For three weeks it shone with full lustre, after which it began slowly to decline. Being situated in a part of the heavens always above the horiz;on (for
THE END OF MAKry WORLDS,
t\
European observatories), the star's entire history could be followed. It remained for sixteen months steadfast in its position like the other stars. As it decreased in size it varied in colour. ''At first," says an old writei; *' its light was white and extremely bright ; it then be-
:^:^ltra^::
'CKf-Wf'Wv>m
^ig. 7. — Cassiopeia ; showing where a new star appeared in the year 1572.
came yellowish ; afterwards of a ruddy colour like Mars j and finished with a pale, livid white, resembling the colour of Saturn."
In passing it may be remarked that there are reasons for expecting the return of Tycho Brahe's star in the
62 • THE END OE MANY WORLDS,
course of a few years. For other new stars have been recorded as seen in the same part of the heavens in the 5^ears 945 and 1264, and though the interval from to 1264 (ox 319 years) exceeds by 11 years the interval from 1264 to 1572 (or 308 years), yet the difference is but small by comparison with either entire interval; and we may not unreasonably believe that the three new stars seen in Cassiopeia have been only three apparitions of one and the same star, which shines out, with superior lustre, for a few months, once in a period averaging about 313 years. It seems to me not at all unlikely that, some time during the next twenty years, astronomers will have an opportunity of examining, with the tele- scope and spectroscope, a star which last appeared before either instrument had been invented.
Already facts are known respecting the so-called new stars which will not permit us to accept the explanations of old so readily offered and admitted, simply because so little was certainly known.
In the year 1866 a star appeared suddenly in the constellation of the Northern Crow^n, where no star had before been visible to the naked eye. It was a little below the arc of stars forming the celestial coronet*
* Its place is indicated in my School Atlas, as well as (of course) in my Library Atlas, from the latter of which the small maps illus- rating the present article have been pricked off. The new star is
THE END OP MAA'Y WORLDS, 63
It shone as a second magnitude star when first seen, but very rapidly diminished in lustre. It increased our knowledge in two important respects.
First, on examining Argelander's charts of the northern heavens, the new star was found to have been observed and charted as a tenth magnitude star, that is, four magnitudes below the lowest limit of naked eye vision. It was not, then, a new sun, though it might still truly be called a new star, in this sense, that it was a new member of the set of stars which adorn our skies as seen by ordinary vision.
In the second place, the star was subject to the searching scrutiny of spectroscopic analysis, with results of a most interesting character.
The reader is no doubt aware that when the light of a star is analysed into its component colours by the instrument called the spectroscope, it is found that all the colours of the rainbow are present, as in the case of solar light, but (also in the sun's case) not all the tints of these colours. Certain dark lines athwart the rainbow- tinted streak, called the spectrum of the star, indicate the
marked T in the Crown (Map VIII.), and must not be confounded with the star r, as in Roscoe's Treatise on Spectral Analysis, and in some astronomical works. The star r is a well known fifth magnitude star, which has shone with no perceptible increase or diminution of splendour since Bayer's time certainly, and probably for thousands of years before.
64 The end of Many worlds.
presence of absorbing vapours in the star's atmosphere. This general statement is true of every fixed star, though the dark lines of some stars differ in number and position from the dark lines of others, showing that other absorb- ing vapours are present. In the case of the new star in the Crown, the usual stellar spectrum was shewn, — a rainbow-tinted streak crossed by a number of dark lines. But besides these, there were seen four very bright lines, — lines so bright that the rainbow-tinted streak appeared as a dark background. The meaning of this is well understood by spectroscopists. It signifies that besides '"0 . the vapours which, being cooler than the star, absorbed
a portion of its light, and produced the dark lines, some vapours were present in the star's atmosphere which were a great deal hotter than the star, and so produced bright lines. Now two of the Imes corresponded in position with two of the well Jcnown lines of the gas hydrogen, showing that this was one of the gases which had been raised to an unusual degree of heat.
It was inferred that there had been some tremendous disturbance in that remote star, by which the hydrogen and some other vapours present in its atmosphere had been intensely heated. But astronomers were unable to decide whether the disturbance was of Jhe nature of a conflagration, the hydrogen actually burning, or whether the heat was occasioned in some other way, as by the
THE END OF MANY WORLDS. 65
downfall of some immense mass upon that remote sun. For burning hydrogen and glowing hydrogen, though either could give the observed bright lines, are very different things. In the former case a chemical change is taking place, as in the case of burning wood or coal ; the latter case resembles that of redhot iron, which is not burning itself (not changing into a different form as everything does which burns), though it will burn other things, — in the ordinary, and incorrect, use of the ex- pression.
The general belief was that there had been a downfall of matter on the star in the Crown, by which the whole globe of that sun had been excited to an intense degree of heat, especially at the surface^ near which lies the hydrogen atmosphere of the star.
I must leave, however, to the next part, the further Consideration of the strange thoughts suggested by the outburst of this star. I wish to use the small space remaining at present to indicate the place where another new star burst forth last" November, so that any readers of these pages who have telescopes may know where to look for a sun which is now dying out, but was shining a few weeks ago as a third magnitude star. Fig. 8 presents a portion of the well-known constellation Cygnus or the Swan. Any star atlas will indicate the place of the lettered stars shown in the figure. The
THE END OF MANY W0RL3S^
constellation itself does not show at all well at this season of the year/'' The part shown in the figure is close to the horizon, and directly under the pole-star, at
1
Fig. 8.— Part of Cygnus, showing the place of new star (November 24, 1876).
about half-past ten in the middle of February; but a little higher up, between north and north-east, at mid-
* This chapter was first published in February, 1877, when the Star was already invisible to the naked eye,
The end op many worlds, ej
night. Professor Schmidt, of the Athens Observatory, noticed a new star, in the place shown, on November 24th last. It must have shone out suddenly, for Schmidt had been observing in that region on the night of November 22 nd (the last preceding clear night). It has since gradually faded, until now a small telescope is required to show it, shining as a seventh magnitude star, with a well-marked orange tint.
We have now to consider the history of this star, and discuss the general questions suggested by the sudden blazing out of suns which had for many years, and probably for many centuries, shone continuously with a far feebler lustre. It is clear that we have good reason to be interested in these questions, seeing that, for aught we know, our sun may be one of those exposed to sudden great increase of lustre.
It seems certain, in the first place, that this star leapt very suddenly to its full splendour. Schmidt had been observing the same regions of the heavens only two evenings before, and is sure the star was not then shining visibly to the naked eye. Again, astronomy is now studied by so many persons, and so many more who are not students of astronomy are now well ac- quainted with the constellations, that it is very diffi- cult for a new star to shine many hours without being
6S THE END OF MANY WORLDS.
detected. For example, the new star in the Cro^vn, which appeared in May, 1866, though not so well placed for observation, was detected by many observers at widely distant stations within a few hours of each other. It is probable that the star acquired its full lustre in a few hours at the utmost, and quite possible that, had any one been watching the place where the star appeared, he would have heen able to see the star grow into full brightness by visible change of lustre, just as the lustre of a. revolving light in a distant light- house visibly waxes and wanes. It may be, of course, that the increase of the star from its ordinary lustre, up to the stage when first it was visible to the naked eye, occupied many days, or even many months or years ; but it seems more likely that as the later stages of increase were rapid, so also was the entire develop- ment of the new lustre. In that case, if there were inhabited worlds circling around that remote sun, they had but brief warning of the fate in store for them, as presently to be described.
Like the star in the northern Crown, the new star in Cygnus was subjected to the searching scrutiny of the spectroscope. The results, though similar in general respects, were even more interesting than in the case of the brighter new star. In the interval between 1866 and 1876 spectroscopic analysis has developed largely.
THE END CF MANY WORLDS. 69
It has thus become possible to analyse more completely the light even of faint stars than the light of bright stars could be analysed a decade of years since.
The spectrum of the new star as examined by ]M. Cornu, of the Paris Observatory, showed the bright lines of hydrogen, indicating the presence of enormous quantities of glo\ving hydrogen, in a state of intense heat. But beside these bright lines, others also could be seen. One of these was an orange -yellow line. It \nll be understood that the faint spectrum of a star cannot be so readily lengthened by increasing the dis- persion as a bright spectrum; for with too great dis- persion the light fades out altogether. And though this is not strictly the case with the bright lines, which are merely thrown farther apart by dispersion, yet still it remains true that one cannot deal with a star spectrum e\'en of bright lines as one can with the solar spectrum. So that ]M. Cornu was not able to determine whether the orange-yellow line belonged to sodium, or to that other substance, whatever it may be, which pro- duces the orange-yellow line seen in the spectrum of a solar prominence."'' Another bright line, green in
* It will be remembered by those familiar with the history of solar observation, that when the spectrum of the solar prominence was first observed, the orange-yellow bright line was supposed to be the well-known double sodium line. It is so near to this pair
70 THE END OF MANY WORLDS.
colour, agreed In position with a triple line belonging to the metal magnesium. Lastly, a bright yellowish- green line was seen, which is known to be present in the spectrum of the sun's corona and of the low-lying ruddy matter round the sun, called the sie7'ra by some, and by others (apparently unfamiliar with the Greek language) the chromosphere.
Now all this agrees very well with what had been noticed in the case of the star in the Northern Crown. For, unquestionably, if a sun increases so much in heat and lustre that the hydrogen outside it glows more brightly than the body of the star, then other matter outside that sun might also be expected to share the great increase of heat. We see that, outside our own sun, hydrogen, a certain unknown vapour of an orange yellow colour, magnesium, and another unknown vapour of greenish-yellow colour are present in enormous quantities ; and it seems, therefore, reasonable to be- lieve that other suns have these gases extending far out- side the rest of their substance. It is certain that, if our sun were caused to glow with far more than its present degree of heat, the gases whose increase of brightness would be most discernible from a distant
of lines, that while they are called D i and D 2, it has been called D 3 ; and in a spectroscope of small dispersive power the thre^ would be seen as one.
1
7 HE END OF MANY WORLDS. 71
Station (as a world circling around some remote star) would be just those gases which were glowing so re- splendently around the star in Cygnus last November — or rather at the time when that light which reached us last November set out from the remote star in the Swan.
When we view the outburst of that remote sun in this way the thoughts suggested are not altogether satisfactory. That sun shows far too much resem- blance to our own, and behaved, so far as can be judged, far too much as our own sun would behave if roused to many times its present degree of heat and splendour. When we hear of a railway accident it is a matter of special interest to us (if we travel much) to learn whether the conditions under which the accident took place resembled those under which the trains proceed by which we chiefly travel. When an express train suffers in such a way as to show some special danger arising from great velocity, we find our- selves to some degree concerned personally in the in- vestigation which follows, if we travel generally by quick trains. If a bridge breaks down, and we have often to traverse bridges in railway journeying, we are simi- larly concerned, especially if any of the 'bridges we have to cross resemble in structure the one which has given way. So c^ko of rnany other special forms of danger
72 THE END OF MANY WORLDS.
in railway travelling. Now, on the same principle, we cannot but regard with considerable interest the circumstance that, apparently, a catastrophe has taken place in the star in Cygnus, which has not only affected a sun resembling our own very closely in constitution, but has produced effects very closely corresponding to those which would affect our own sun if, through any cause, he were excited to many times his present degree of heat.
Let us pause a little to reflect upon the effects which would follow a great increase of the sun's lustre. A change in our own sun, such as affected the star in Cygnus, or that other star in the Northern Crown, would unquestionably destroy every living creature on the face of this earth ; nor could any even escape which may exist on the other planets of the solar system. The star in the Northern Crown shone out with more than 800 times its former lustre : the star in Cygnus with from 500 to many thousand times its former lustre, according as we take the highest possible estimate of its brightness before the catastrophe, or consider that it may have been very much fainter. Now, if our sun were to increase tenfold in brightness, all the higher forms of animal life and nearly all vegetable life would inevitably be destroyed on this earth. A few stubborn animalcules might survive, and, possibly, a few of the lowest forms of vegetation, but
THE END OF MANY WORLDS. 73
naught else. If the sun increased a hundredfold in lustre his heat would doubtless sterilise the whole earth. The same would happen in other planets. The heat falling on the remotest members of the solar system would not, indeed, be excessive according to our concep- tions. But if we regard Neptune, Uranus, Saturn, and Jupiter as the abode of life (which, for my own part, I consider altogether improbable), we cannot but suppose the orders of living creatures in each of these planets to be well fitted to exist under the conditions subsisting around them. If this is so — as who can for a moment doubt ? — a sudden enormous increase in the sun's heat, though not making the supply received by those planets much greater than, or even equal to, the supply which we receive from the sun, v/ould prove as fatal to living creatures there as to living creatures on our earth.
If, then, the sun increased in splendour as the stars have increased which the astronomers call new stars or temporary stars, there would be an end of life upon this earth j and nothing short of either the spontaneous development of life, or of the creation of various forms of life, could people our earth afresh. Science knows nothing of spontaneous generation, and believers in reve- lation reject the doctrine. Science knows nothing of the creation of living forms, but believers in revelation accept the doctrine. Certain it is that if our sun ever
74
undergoes the baptism of fire which has affected some few among his brother suns, one or other of these processes (if creation can be called a process) must come into operation, or else our earth and her companion worlds would for ever after remain absolutely devoid of life.
But if our sun, without suffering so great a change, underwent a change of less degree, it might well happen that though there would be enormous destruction of life upon the earth and other planets, some life (presumably the strongest and best) would survive. In that case, after a long period of time, the earth would again be well peopled, and it might even be that the various races of terrestrial creatures would be improved, by the desolation which the great solar conflagration had wrought.
It is somewhat curious, considering how little there is in the ordinary progress of events to suggest the idea, that most of the ancient systems of cosmogony recognised the periodical destruction of living creatures on the earth by fire as well as by water. Each form of destruction was supposed to be brought about by planetary influences. The Ecpyrosis, or destruction by fire, was effected when all the planets were in conjunction with Cancer; the Cataclysm, or destruction by flood, when all the planets were in conjunction with Capricorn. Each form of de- struction was supposed also to purify the human race. ** Towards the termination of each era," v>Tites Lyell,
THE END OF MANY WORLDS, 75
speaking of these old ideas, '' the gods could no longer bear with the wickedness of men, and a shock of the elements or a deluge overwhelmed them; after which calamity Astrea again descended on the earth, to renew the golden age." The Greeks undoubtedly borrowed all such doctrines from the Egyptians, who ^^ believed the world to be subject to occasional conflagrations and deluges, whereby the gods arrested the career of human wickedness, and purified the earth from guilt. After each regeneration mankind was in a state of virtue and happiness, from which they gradually degenerated again into vice and immorality.'*
Considering that we have every reason to believe the records of great floods to relate to events which actually occurred, however imperfectly remembered, it seems not unreasonable to believe that the tradition of great heats had its origin in observed phenomena. As neither or- dinary conflagi'ations nor volcanic outbursts would have suggested traditions of the kind, it would seem not im- possible that at certain times our sun may have acquired for a time unusual lustre and heat, causing great and widely spread destruction among all forms of animal and vegetable life.
This idea may possibly seem to many, especially at a first view, too wild to be entertained for a moment. Our sun shines^ so f^ir as appears to ordinary observation,
76 THE END OF MANY WORLDS.
with steadfast lustre from year to year, and also from age to age. If an occasional hot season suggests for a while to some that the sun has grown hotter, or a cool season that he has grown cooler, the restoration of cool or warmer weather, as the case may be, causes the thought to be quickly cast on one side that a change of either kind has taken place. Again, if we examine the historical records of past ages, we find little to suggest the idea, or even the possibility, that the sun in former times shone with greater splendour or with less than at present. The men of those days were formed like the men of our own day, and could not have supported any much greater de- gree of heat or of cold than men can support at present. Any sudden accession (or diminution) of solar light and heat, such as we are considering, would certainly have attracted marked attention, and have been recorded for the benefit of future ages. The geologic record, again, does, indeed, suggest variations in the sun's emission of heat as constituting one among the few available ex- planations of the existence of tropical forms of life in certain strata and of arctic forms in other strata. But even if this explanation be the true one, which is by no means established, such variations must of necessity have been slow, the condition of increased heat continuing for many ages in succession, and the like with the condition of diminished heat, We have no evidence, historical or
The end op many worlds, 77
geological, of the occurrence of any sudden accession of solar heat, followed by a quick return to the normal temperature, unless we find such evidence in the tradition prevalent among Egyptian, Indian, and Chinese cosmo- gonists, that at certain recurring epochs in the past our earth has undergone destruction and renovation by fire.
Yet, as I shall now show, it appears that the one only natural interpretation w^hich can be given of the outburst of a new or temporary sun indicates an event which might happen to our own sun, and an event which if it happened at all would happen periodically. Moreo^^er, while it will appear that there is no reason for fearing the possible occurrence (which would, in such case, be really the recurrence) of such a catastrophe in the case of our own sun as has affected the stars in the Crown and in Cygnus, there is no reason for rejecting as incredible the idea that catastrophes very serious in their character may have affected our sun ; and there is abundant reason for believing that small alterations in the sun's total emis- sion of light and heat take place very often, in some cases periodically ; in others— so far as we can yet judge — periodically.
Lastly, it will be seen that there is always a possibility that our own or any other sun may undergo precisely such a change as the stars in Cygnus and the Northern Crown. Some indeed, even among men of science
78 TH^ END OF MANY WORLDS,
(as the Abbe Moigno, for example) believe that it Was an event of this sort which St. Peter predicted when he wrote, that as the old world, being overflowed with water, perished, so '' the heavens and the earth which are now, by the same word are kept in store, reserved unto fire." According to that view, the day of destruc- tion will come " as a thief in the night ; in the which the heavens shall pass away with a great noise, and the elements shall melt with fervent heat, the earth also, and the works that are therein shall be burned up."
Let us consider how the sudden brightness of a new star may be explained.
I must confess that for my own part I do not attach much weight to the suggestion once made by Mn Huggins, that an actual conflagration had taken place in the case of the new star in the Northern Crown. It does not seem to me that any process of mere burning could account for the enormous accession of light and heat which that sun underwent.
Consider the case of our own sun. His heat is very far beyond that which would be given out by any matter known to us undergoing any known process of true com- bustion. That is to say, if a mass as large as the sun of any known substance were caused to burn, under any conditions we can imagine, the momentary emission of
1
The end oe man^ worlI^s, J9
heat by that mass would be very much less than the momentary emission of heat by the sun.
Now it is quite conceivable that by some great accession of combustible matter, some supply of fuel exceeding many times his entire mass, the sun's entire emission of heat might be very largely increased. But though such an idea is conceivable, it seems altogether far-fetched. The conception is, in fact, inadmissible as an explanation of the increase of heat of a temporary star, not because of the improbability of the sudden accession of so enormous a quantity of matter (though that improbability is very great), but because if so enor- mous a quantity of matter fell upon the sun, many times as much heat would be generated by the mechanical effect of the impact as by the combustion of the freshly received matter. So that even with the daring assump- tion here made, combustion would account for only a small portion of the increase of light and heat.
Huggins' idea was indeed somewhat different. He supposed that in consequence of some great internal convulsion of the sun in the Northern Crown a large volume of hydrogen and other gases was evolved from the interior, the hydrogen then by burning giving out the light corresponding to the bright lines. At the same time, the mass of the sun would be intensely heated by the surrounding mass of glowing hydrogen. When the
8o THE END OF MANY WORLDS,
liberation of gas from the interior ceased the flame would die out, and the sun's surface would gradually cool. But if we judge by the case of our own sun, the heat of the burning hydrogen would be nothing near so great as the heat of the glowing hydrogen already outside and within the visible globe of a sun.
On the whole it seems altogether more probable that the accession of splendour observed in the case of tem- porary stars is due to the downfall of enormous masses of matter upon the surface of these suns. It is, no doubt, well known to most of my readers that the down- fall of meteoric matter upon the surface of our own sun has been considered a sufficient explanation of the sun's entire emission of light and heat. The theory that the sun's heat and light a7^e thus excited has long since been abandoned; but not because the cause would be insufficient. It has been abundantly proved that a downfall of meteors, not sufficient in quantity to add appreciably to the sun's size in many thousands of years, would generate more heat and light than he emits in that time. The meteoric theory has been abandoned simply because it has been shown that no such down- fall is taking place.
The reason why meteoric impact would suffice to warm the sun to his present temperature if the meteoric showers were heavy, and to warm him far beyond his
THE END OF MANY WORLDS, 8i
present temperature if for a few days very heavy meteoric showers fell upon him, is simply that his attraction upon matter approaching him from without is capable of generating a tremendous velocity. We know that when a cannon-ball strikes a metal target, with a velocity perhaps of some 400 yards per second, great heat is excited, and there is a momentary flash of light. If the velocity were doubled, the quantity of heat would be doubled also. Conceive, then, the tremendous heat which would be excited if a cannon-ball could be caused to strike a target with a velocity exceeding that just named some 1500 times ! The ball and target would both be vaporised by the shock, if — which, however, could never happen — the target resisted the blow and brought the ball to rest. Now matter which reaches the sun from without, under the influence of his tremendous attraction, strikes his globe with a velocity 1500 times greater than that of a cannon ball striking a target at a distance of two or three hundred yards. The heat excited is, therefore, very intense ; and if meteors were showering at all times and in dense flights upon the sun's surface, we should require no other explanation of the sun's heat.
But it appears that meteoric systems are neither so numerous nor so rich as to account for the sun's uniform emission of heat, though occasional meteoric showers upon the sun may be heavy enough to increase appreci-
82 THE END OF MANY WORLDS,
ably the amount of heat he emits. It would seem, from experiments which have been made by Professor Piazzi Smyth, of the Edinburgh Observatory, and later by the Astronomer Royal at Greenwich, that from time to time the sun's emission of heat really is greater than usual. It seems not at all improbable that the increase is due to the occasional fall of large masses of meteors in great numbers upon the sun.
Again, it seems that such falls occur periodically, or rather that at regular intervals great meteoric streams pour upon the sun's surface. For instance, the periodic increase and decrease in the number of sun-spots is accompanied (so far as we can judge by the observations made at Edinburgh and Greenwich) by an accession and diminution of the solar heat ; and if the change is attributed to the passage of a meteoric stream athwart the sun, we should have to assign to such a stream a period of rather more than eleven years. This, from what we know about the association between meteors and comets, would correspond simply to the existence of a comet whose path intersects the sun's globe, and which is followed by a train of milHons of large meteoric masses, many of which are consumed at each passage of the rich portion of the train athwart the globe of the sun. This comet must of necessity be inconspicuous, since it has hitherto escaped detection.
^
THE END OF MANY WORLDS, 83
In fact, its head and nucleus must long since have been entirely destroyed. Only the meteoric train, far more widely scattered, remains, simply because at each passage past the sun, though many are captured, far greater numbers get safely past.
I am careful to remind the reader that though I have, for convenience, used the indicative mood in describing these matters, I am in reality presenting merely a theory. It may be that the solar spots and the accessions of heat are produced in some other way. But I must admit I find strong reasons for regarding as probable the general theory, that the alternations of solar activity (not the solar activity itself be it noted) are excited from without. And since we know, as a matter of fact, that meteors exist in enormous numbers within the solar system, and that they aggregate with rapidly increasing density in the sun's neighbourhood, we must believe that they fall upon the sun in enormous numbers. We also perceive that the supply cannot be uniform, but must vary greatly from time to time j while what we know about the periodicity of meteoric showers on our own earth suggests the belief, we may almost say the certainty, that there must be periodic downfalls of very heavy meteoric showers upon the sun's surface. A\'e have, then, strong probability in favour of the belief that events may occur which, ij
84 THE END OF MANY WORLDS.
they occurred, might be expected, with a high degree of probability, to produce effects resembling those actually observed, — viz., the production of a heat more intense than usual, accompanied by signs of great disturbance like the sun-spots. It does, therefore, seem at least not improb- able that these accessions of heat and these signs of great disturbance really are brought about in the way supposed.
A further argument in favour of the meteoric origin of solar alternations of heat is to be found in the fact that, on one occasion at least, a solar phenomenon, corre- sponding precisely to what we should expect to see. if great meteoric masses fell upon the sun, has been followed by precisely the same signs of terrestrial disturbance which accompany and follow the formation of great solar spots. I refer to the remarkable occurrence wit- nessed by Carrington and Hodgson (at different obser- vatories) in September, 1859, when two intensely bright points of light were seen travelling beside each other at the rate of about 120 miles per second along a short arc of the sun's surface, — an arc only equal in length to some four-and-a-half times the diameter of our earth.
On that occasion the emission of solar heat may or may not have been increased in an appreciable degree for several minutes. My own belief is that it must have been ; but we certainly have no means of proving that it was. What we do l^now certainly is, that on that day all
THE END OE MANY WORLDS. 85
the phenomena which usually accompany the existence of many and large sun-spots showed themselves with exaggerated intensity. The magnetic needle was greatly disturbed, auroras displayed their coloured streamers in both hemispheres, telegraphic communication was inter- rupted, and everything tended to show that a disturbance of the same general character as that which produces sun-spots, but much more active while it lasted, had affected the sun. It seems, then, altogether reasonable to infer that sun-spots are due to the same cause as the disturbance which then occurred. So that if we con- clude, with most astronomers competent to form an opinion, that the disturbance witnessed by Carrington and Hodgson was due to the downfall of two very large meteoric masses upon the sun, it would follow that sun- spots are due to more wide-spread meteoric showers, not consisting of masses so large.
The reader will long since have guessed, no doubt, to what all this tends. If the periodical variations of the sun's surface are due to meteoric and cometic systems whose orbits intersect the sun's globe, their periods being short (that is, lasting but a few years), it may well be that more important meteoric and cometic systems inter- secting the sun's globe exist, which have much longer periods. When next one of these makes its passage athwart the sun, far more important solar disturbances
86 THE END OP MANY WORLDS.
may take place than those which occur when the regularly recurring systems salute the sun. Two or three times in the history of science comets have approached very close to the surface of the sun, as in 1680, and again in 1843, but without actually impinging upon it. Very slight changes in the motions of those comets, owing to the disturbing influences of the planets, would cause their very nuclei to strike the sun, and their meteoric trains to pour afterwards in a full stream upon him for many days, or even for many months and years in succession.
Now I do not think our sun would necessarily suffer very much from any of these known comets. They may long since have parted with the greater quantity of their substance. But it is quite possible that even one of those well-known comets of the solar system might cause very serious outbursts of solar heat and light ; and it is certainly not only possible but extremely probable that other comets, such as have visited the solar system on paths fortunately not bringing them near to the sun, w^ould have worked much mischief had their paths been diiferently situated.
We know that Newton held this opinion. He con- sidered the real danger from comets to reside, not in the possibility that one might strike our earth, but in the possibility that one, falling upon the sun, might excite that orb to a degree of heat so intense that
The end of many Worlds, ^7
all life on this earth would be destroyed. It is true that, in Newton's time, physical laws were not so well understood as at present, and a considerable portion of Newton's reasoning was consequently inexact. But nothing which is now known opposes itself to the belief which Newton adopted on this subject. On the contrary, whereas Newton only recognised the danger arising from the consumption of a comet as fuel for the sun, we now recognise a far more serious danger, from the force of meteoric impact, and the heat excited as the thermal equivalent of the destroyed velocities. Of this part of the danger Newton had no clear conception, the relations between mechanical energy and heat not having been established until quite recent times.
It appears to me, however, that the danger in the case of our own sun — or may we not say our danger ? — arises only from the possibility that some one of the comets which visit us from the star-depths may make straight for the sun; and this danger is exceedingly small. Almost certainly a comet which, leaving the domain of another sun, falls under the attractive influence of our own, would approach him on a path passing many millions of miles from his surface. The chances against a more direct approach are so great that they may be regarded as, to all intents and purposes, over- whelming. A comet nii^ht visit us from the star-depth
M THE END OF MANY WORLDS,
on a destructive course, just as a single black ball might be drawn at the first trial from a bag containing a million white balls and only that single black one. But the danger is exceedingly small.
We see, indeed, that other suns have suffered in this way, assuming cometic downfall to be the true cause of stellar outbursts. There are so many millions of suns^ however, in the region of space to which telescopic survey extends that the occurrence of ten or twelve such outbursts in the course of four or five centuries need not be regarded as implying any serious danger. Moreover, all the suns which have thus suffered lie within a par- ticular region of the heavens,— viz., in the Milky Way, and in that half of the Milky Way which is most irregu- lar, one may almost say raggedy in structure. (With one exception — the star in the Northern Crown, which, never- theless, lies on a faint outlying streamer of the Milky Way not discernible to ordinary vision.) If then our sun belongs to this region of space, the danger for him and for us is somewhat greater than my previous argument would indicate. For, in that case, we must compare the number of outbursts, not with the total number of stars within telescopic range, but with the number of those stars which lie within this particular region of space. On the other hand, if our sun does not lie within that region of space, the danger for him and for us is very much
THE END OF MANY WORLDS. 89
less ; for instead of a certain small number of accidents among his fellow suns, there have been no such accidents, only accidents affecting other suns which must be dif- ferently classed.
The case may be compared to the estimation of the dangers, let us say, of travelling by ocean steamships on a particular route. If we take the total number of accidents, for instance, to steamships travelling between England and the United States, we should estimate the risk of the journey as very small, the number of passengers who have lost their lives being very small compared with the number who have made the journey. But even this small risk is diminished if we estimate the danger for a passenger by Cunard steam- ships, simply because no passenger has yet lost his life through accident to one of these Cunard vessels.
So in the case of our sun, the danger of an outburst such as has affected the stars in the Northern Crown and Cygnus is small enough when we estimate it by com- paring the number of such accidents with the total number of stars, but vanishes almost into nothingness when we note that no insulated star like our sun seems hitherto to have undergone one of these tre- mendous catastrophes.
But as regards the fate of worlds circling round suns which have suffered in this way, we can form but one
9(5 fHE END OF MANY WORLDS.
opinion. Beyond all doubt, if such worlds existed and were inhabited when their central orb blazed forth with many hundred times its former lustre, all life must have perished from their surface. We may believe, as many do, that no conditions are too unlike those we are familiar with on earth to render life impossible ; that the creatures subsisting in a world exposed to the most fiery heat or to the most intense cold are adapted as perfectly to the conditions under which they subsist as we are to the circumstances of terrestrial life. But even adopting this view, though it seems to accord ill with what we know of our own earth, — where life ceases towards the polar and over large tracts of the equatorial regions, — we could not believe that creatures thus adapted to the conditions prevailing around them could endure an entire change of those conditions. With the accessions of heat in the stars in Cygnus and the Crown, such change must inevitably have taken place. Therefore, as I think, we must regard the catastrophes affecting those remote suns as assuredly involving " The End of many Worlds.^'
iV^/^.— What is stated in the latter portion of this chapter applies now only to the star in the Northern Crown ; for the star in Cygnus has not faded into a small star, but into a small nebula ! For the further,-,history of this star, the reader is referred to my forthcoming treatise entitled, '* Pleasant Ways in Science."
VL
THE AURORA BORE ALTS.
^MONG the objects in view, when the recent Polar expedition was fitted out, was the hope that during the winter of 1875-76 the scien- tific observers who accompanied the expedition might be able to study the Aurora Borealis under unusually favour- able conditions. This hope was, as most of my readers doubtless know, disappointed. Few auroras were seen, and those seen were not remarkable either for brilliancy or for beauty of colour. Yet in the very disappointment of the hope which had been entertained on this subject there was very significant evidence respecting the aurora, as will presently be shown. The quiescence, at that time, of the forces which produce the auroral streamers had its meaning, and a very strange one.
The aurora is one of those phenomena of nature which are characterized by exceeding beauty, and sometimes by
92 THE AURORA BOREALIS.
an imposing grandeur, but are unaccompanied by any danger, and indeed, so far as can be determined, by any influence whatever upon the conditions which affect our well-being. Comparing the aurora with a phenomenon akin to it in origin — lightning — we find in this respect the most marked contrast. Both phenomena are caused by electrical discharges ; both are exceedingly beautiful. It is doubtful which is the more imposing so far as visible effects are concerned. When the auroral crown is fully formed, and the vault of heaven is covered with the auroral banners, waving hither and thither silently, now fading from view, anon glowing with more intense splendour, the mind is not less impressed with a sense of the wondrous powers which surround us than when, as the forked lightnings leap from the thundercloud, the whole heavens glow with violet light, and then sink suddenly into darkness. The solemn stillness of the auroral display is as impressive in its kind as the crashing peal of the thunderbolt. But there is a striking contrast between the feelings with which we regard the safe splendours of the aurora and the terrible glory of the lightning flash. One display we contemplate with the calmness engendered by absolute security; the other —no matter how little the fear of death may affect the reason — cannot be regarded without exciting the con- sciousness of danger. We witness in safety, so far as
THE AURORA BOREALIS. 93
itself IS concerned, the flash whose light illuminates the cloud masses above and around us, but for aught we know it may be the last we shall ever see, since no man killed by lightning ever saw the flash which brought his death.
I do not purpose to consider here at any length those facts respecting the aurora which properly find their place in text-books of science, but those only which are less commonly dealt with, and seem at once most suggestive and most perplexing.
The reader is no doubt aware that auroras, or polar streamers, as they are sometimes called, are appearances seen not around the true poles of the earth, but around the magnetic poles, which lie very far away from those geographical poles which our arctic and antarctic sea- men have in vain attempted to reach. We in England, though much nearer to the north pole than the inhabi- tants of Canada, see far fewer auroras than they do, and those we see are far less splendid, simply because we are farther away from the northern magnetic pole. This will be seen from the accompanying pair of maps (from my " Elementary Physical Geography "), showing where the northern and southern magnetic poles lie. Again, you ^^ill see from the northern map, that from England the northern magnetic pole lies towards the west of due north. That is why when we see a fully developed
94
THE AURORA BOREALIS.
auroral arch in this country its crown Hes towards the west of north (almost midway between north and north- west). I may have occasion at another time to consider the curious changes which affect the actual position of
i
Fig. 9-— The Northern Magnetic Meridians and Lines of Equal Dip.
\!iit magnetic poles and lines ; in this place I merely note that what is now said respecting them only refers to the present time.
The formation of auroral streamers around the mag- netic poles of the earth shows that these light? ^re due
THE AURORA BORE ALTS. 95
to electrical discharges, just as the general magnetic phenomena of the earth indicate the existence of electri- cal currents. The earth, in fact, with its envelope of air, moist and dense near the surface, rare and dry above
Fig. 10. — The Southern Magnetic Meridians and Lines of Equal Dip.
may be regarded as an enormous magnetic instrument, a core surrounded by conducting matter, in which elec- trical currents pass whenever the condition of the earth's magnetism changes. The discharges of electricity, though only visible at night, take place in reality in the
96 THE AURORA BOREALIS.
daytime also. According to their extent and position, varying with the varying conditions under which they take place, their aspect changes. Moreover, from different parts of the earth the appearance of the aurora is different. From low latitudes (I speak now of magnetic latitudes as indicated by the closed curves around the magnetic poles in the maps), the auroral arch is seen towards the north in our hemisphere, towards the south in the other hemisphere. From points nearer the mag- netic pole it is seen overhead, and when that pole is approached still nearer, the crown of the arch is seen on the side remote from the pole, — that is, towards the south in our hemisphere, towards the north in the southern hemisphere.
Remembering that the aurora is due to electrical dis- charges in the upper regions of the air, it is interesting to learn what are the appearances presented by the aurora at places where the auroral arch is high above the hori- zon,— these being, in fact, places nearly tinder the auroral arch. M. Ch. Martins, who observed a great number of auroras at Spitzbergen in 1839, thus writes (as trans- lated by Mr. Glaisher) respecting them : " At times they are simple diffused gleams or luminous patches ; at others, quivering rays of pure white which run across the sky, starting from the horizon as if an invisible pencil were being drawn ever the celestial vault ; at times it
THE AURORA BOREALIS. 97
stops in its course, the incomplete rays do not reach the zenith, but the aurora continues at some other point j a bouquet of rays darts forth, spreads out into a fan, then becomes pale, and dies out. At other times long golden draperies float above the head of the spectator, and take a thousand folds and undulations as if agitated by the wind. They appear to be but at a slight elevation in the atmosphere, and it seems strange that the rustling of the folds as they double back on each other is not audible. Generally, a luminous bow is seen in the north ; a black segment separates it from the horizon, the dark colour forming a contrast with the pure white or bright red of the bow, which darts forth rays, extends, becomes divided, and soon presents the appearance of a luminous fan, which fills the northern sky, and mounts nearly to the zenith, where the rays, uniting, form a crown, which in its turn darts forth luminous jets in all directions. The sky then looks like a cupola of fire ; the blue, the green, the yellow, the red, and the white vibrate in the palpi- tating rays of the aurora. But this brilliant spectacle lasts only a few minutes ; the crown first ceases to emit luminous jets, and then gradually dies out; a diffused light fills the sky ; here and there a few luminous patches, resembling light clouds, open and close with incredible rapidity, like a heart that is beating fast. They soon get pale in their turn, everything fades away and becomes
7
98 THE AURORA BOREALIS,
confused, the aurora seems to be in its death-throes ; the stars, which its light had obscured, shine with a renewed brightness ; and the long polar night, sombre and pro- found, again assumes its sway over the icy solitudes of earth and ocean."
The association between auroral phenomena and those of terrestial magnetism has long been placed beyond a doubt. Wargentin in 1750 first established the fact, which had been previously noted, however, by Halley and Celsius. But the extension of the relation to phenomena occurring outside the earth — very far away from the earth — ^belongs to recent times.
The first point to be noticed, as showing that the aurora depends partly on extra-terrestial circumstances, is the fact that the frequency of its appearance varies greatly from time to time. It is said that the aurora was hardly ever seen in England during the seventeenth century, though the northern magnetic pole was then much nearer to England than it is at present. Halley states that before the great aurora of 17 16 none had been seen (or at least recorded) in England for more than eighty years, and no remarkable aurora since 1574. In the records of the Paris Academy of Sciences no aurora is mentioned between 1666 and 17 16. At Berlin one was recorded in 1707 as a very unusual phenomenon; and the one seen at Bologna in 1723 was described as the first
THE AURORA BOREALIS. 99
which had ever been seen there. Celsius, who described in 1733 no less than three hundred and sixteen observa- tions of the aurora in Sweden between 1706 and 1732, states that the oldest inhabitants of Upsala considered the phenomenon as a great rarity before 1 7 1 6 . Anderson, of Hamburg, states that in Iceland the frequent occur- rence of auroras between 17 16 and 1732 was regarded with great astonishment. In the sixteenth century, how- ■ ever, they had been frequent.
Here, then, we seem to find the evidence of some cause external to the earth, as producing auroras, or at least as tending to make their occurrence more or less frequent. The earth has remained to all appearance unchanged in general respects during the last three centuries, yet in the sixteenth her magnetic poles have been frequently surrounded by auroral streamers ; during the seventeenth these streamers have been seldom seen ; during the last two-thirds of the seventeenth century auroras have again been frequent ; and during the present century they have occurred sometimes frequently during several years in succession, at others very seldom.
Let us inquire a little more closely into the circum- stances attending auroral displays, in order to ascertair what external cause it is which thus influences their occurrence,
Connected as auroras are with the phenomena of
roo THE AURORA BOREALIS.
terrestrial magnetism, we may expect to find some help in our inquiry from the study of these phenomena.
Now it appears certain that magnetic phenomena are partly influenced by changes in the sun's condition. We may well believe that they are in the main due to the sun's ordinary action, but the peculiarities which affect them seem to depend on changes in the sun's action. It is found that the daily oscillation of the magnetic needle corresponds with the diurnal change in the position of the sun owing to the earth's rotation. An annual change affecting that oscillation depends on the varying distance of the sun as the year proceeds. The daily change is not only greater than the annual, but is characterized by irregularities, when the face of the sun shows the greatest number of spots. It was found by General Sabine, says Mr. Balfour Stewart, ^^ that the aggregate value of mag- netic disturbances at Toronto attained a maximum in 1848, nor was he slow to remark that this was also Schwabe's period of maximum sun-spots. It was after- wards found, by observations made at Kew, that 1859 (another of Schwabe's years) was also a year of maximum magnetic disturbance. . . . There is also some reason to believe that on one occasion our luminary was caught in the very act. On the first of September, 1869, ^wo astronomers, Carrington and Hodgson, were independ- ently observing the sun's disc, which exhibited at that
THE AURORA BOREAUS. lOl
time a veiy large spot, when, about a quarter past eleven, they noticed a very bright star of light suddenly break out over the spot and move with great velocity across the sun's surface. On Mr. Carrington sending afterwards to Kew Observatory, at which place the position of the magnet is recorded continuously by photography, it was found that a magnetic disturbance had broken out at the very moment when this singular appearance had been observed." The dip of the. magnetic needle^ its- deflec- tion from the north, the inferiority of its directive force, were all three simultaneously and abruptly altered, and continued so for many hours.
Nor are we left in any doubt as to the connection between such well-marked disturbances of the magnetic nee-dle. While the needle was thus violently displaced, vivid auroras occurred over the greater part of both the northern and southern (magnetic) hemispheres. They were seen in latitudes where usually auroras are as in- frequent as rain in Peru, — at Rome, in the West Indies, even within eighteen degrees of the equator.
The disturbance of the earth's electrical condition was well shown in other ways. Mr. C. V. Walker, the tele- graphist, found that strong electrical currents affected the various telegraphic lines throughout England. These currents changed in direction every two or three minutes. In many places it was impossible to send telegraphic
102 THE AUItORA BOREALtS,
messages. In America some of the signalmen received severe electric shocks. '^ At a station in Norway," says Sir J. Herschel, '^ the telegraphic apparatus was set fire to ; and at Boston, in North America, a flame of fire followed the pen of Bain's electric telegraph (which writes down the message upon chemically prepared paper)."
Many of my readers will doubtless remember the auroras of May 13, 1869, and October 24, 1870, both of which occurred when the sun's surface was marked by many spots, and both of which were accompanied by remarkable disturbance of the earth's magnetism.
It may, then, fairly be assumed that the occurrence of auroras depends in some way, directly or indirectly, on the condition of the sun. But what the real nature of that connection may be is not to be easily determined. It Is clear that the eleven-year-period of sun-spots is not the only, or even the chief period affecting auroras, for we have seen that sometimes for a full century, or even more, very few auroras are seen. It is not by any means certain that the connection between the sun's condition and the occurrence of auroras is of the nature of cause and effect ; quite probably sun-spots and auroras depend on some common cause as yet undetected, — and possibly never to be detected by man.
Regarding the auroral streamers as terrestrial lights only, but in some sense like the light reflected by planets
THE AURORA B0REAL13. IO3
in having their real source in the sun, we can no longer speak, as Humboldt was wont to do, of our planet possessing a power of emitting light of its own. Yet his manner of dealing with auroral light still possesses interest for us, especially in relation to the question whether these polar lights are emitted by other planets and may possibly be discerned from our earth. "It results from the phenomena of the aurora," said Humboldt, " that the earth is endowed with the pro- perty of emitting a light distinct from that of the sun. The intensity of this light is rather greater than that of the moon in its first quarter. It is at times, as on January 7, 1831, strong enough to admit of one's read- ing printed characters without difficulty. This light of the earth, the emission of which towards the poles is almost continuous " (this, however, is not strictly the case), " reminds us of the light of Venus, the part of which not lighted by the sun often glimmers with a dim phosphorescent light. Other planets may also possess a light evolved out of their own substance."
I would venture, however, to express strong doubts as to the possibility of discerning, either on Venus or on any other planet, the auroral gleams which may very probably illuminate at times their nocturnal skies. It must be remembered that the aurora, when at its brightest and covering a large part of the sky, orly
104 THE AURORA BOREALIS,
gives about as much light as the moon in her first quarter, — that is, as one half of a disc so small that 180,000 such discs would not equal the entire sky. The luminosity of the aurora is then in reality very small ; probably far less than that of the earth's sur- face when illuminated by the full moon. A distant hill on which the rays of the full moon are falling seems strongly illuminated, and yet its light is teally so faint that we could scarcely discern it at all save for the favouring effect of contrast. We know this, because we often see portions of the moon's surface which are illuminated by earthshine (when we see what is called the old moon in the new moon's arms), and these portions are quite faint by comparison with the rest of the moon; yet earthshine exceeds moonshine at least twelve times, and probably more nearly twenty times in splendour.
The glimmering phosphorescent light, supposed to have been seen on parts of Venus not lighted by the moon, is a phenomenon about which experienced tele- scopists are somewhat doubtful, though Webb speaks of the appearance as remarkably well attested, quoting, amongst others, the following cases. In 17 15, Derham, in his " Astro-Theology," says that " the sphericity or rotundity is manifest in our moon, yea, and in Venus, too, in whose greatest falcations " (t.e,, when they appear
THE AURORA BOREALIS. 105
as crescents) '^the dark parts of their globes may be perceived, exhibiting themselves under the appearance of a dull and rusty colour." In 1806, the phenomenon displayed itself beautifully to Harding three times and to Schroter once within five weeks. *' Guthrie and others noticed it a few years ago, with small reflectors, in Scotland; Purchas, at Ross, in England; De Vico and Palomba, many times in Italy." Winnecke re- cords a similar observation, though very faint, 187 1, Sep- tember 25, a little before noon. Van Hahn also says he saw it repeatedly, by day as well as by night, and \\ith several instruments ; he was, however, an inferior observer. The dark side is sometimes described as grey, sometimes as reddish; The phenomenon has, on the other hand, been looked for specially, on several occa- sions, by practised observers, using very fine instru- ments, who have failed to recognise any trace of it.
One of the most remarkable observations ever made on Venus must here be mentioned. Madler states that on one occasion, when he was observing the planet, he saw a number of brushes of light diverging from the circular side (/>., the outside of the planet's crescent), lasting as long as the planet could be seen that even- ing, and remaining unchanged when he changed the position of the telescopic eye-piece, or used a different one. " He attempts no explanation/' says Webb, ** but
I06 THE AURORA BOREALtS.
%
thinks it could not have been an optical illusion. This is certainly possible^ but it is an instructive instance of the oversights which may be incidental even to great philosophers, that it never seems to have occurred to him to try another telescope ! " It cannot be doubted that the evidence would have been greatly strengthened had he changed telescope as well as eye-piece ; though it is not readily to be explained how a known telescope, frequently used as well before as after this strange appearance was seen, could for one evening only have played so strange a trick as Madler's must have done, if what he saw was merely an instrumental illusion.
However, whether we have telescopic evidence or not respecting auroral lights surrounding the polar regions of other planets, we can have very little doubt that some among the planets, if not all of them, resemble our earth in this as in so many other respects. The aurora is a cosmical phenomenon, not one peculiar to cur own earth. It is not, indeed, altogether certain that our sun himself may not be girt round by mighty auroral streamers, and that the light of these may not constitute a noteworthy portion of the corona of glory seen around him during the time of total eclipse.
This view, indeed, although it has not been definitely entertained as I have here expressed it, has been sug- gested by reasoning which led others to suppose that the
THE AURORA BOREAU^. lo?
coloured prominences around the sun may be auroras. Perceiving the nature of the connection between terres- trial magnetism and auroras, Balfour Stewart reasoned that we may extend our inquiries and ask, " If the sun's action is able to create a terrestrial aurora, why may he not also create an aurora in his own atmosphere ? " It occurred independently to General Sabine, Prof. Challis, and himself, that the red flames visible during a total solar eclipse ^^may be solar aurorae.^' We now know that the solar flames are not aurorae, nor, properly speaking, flames at all, but great masses of glowing vapour. It is not, however, by any means so clear that the solar corona is not auroral in its nature. The following reasoning, applied by Balfour Stewart to the sun's prominences, applies with much greater force to the corona. After mentioning the height (from 70,000 to 80,000) which some prominences attain, he proceeds, *' Considering the gravity of the sun, we are naturally unwilling to suppose that there can be any considerable amount of atmosphere at such a distance from his surface; and we are therefore induced to seek for an explanation of these red flames amongst those pheno- mena which require the smallest possible amount of atmosphere for their manifestation. Now the experi- ments of Mr. Gassiot and the observed height of the terrestrial aurora alike convince us that this meteor
lo8 THE AURORA BORE A LIS.
will answer our requirements best. And besides this, the curved appearance of these red flames, and their high actinic power, in virtue of which one of them, not visible to the eye, was photographed by Mr. De la Rue, are bonds of union between these and terrestrial aurorae."
All this and much more may be said of the solar corona. Its streamers extend not 70,000 or 80,000 miles, but 700,000 or 800,000 miles , from the sur- face of the sun, where the pressure liiust be far smaller than near the summits of even the loftiest prominences. They are curved and striated, like those of the aurora, whereas the shapes of the prominences bear only a distant resemblance to auroral streamers. They possess a high actinic (/>., photographic) power, as is shewn by the readiness with which, during the total eclipse of December, 187 1, they were photographed, no less than six well-defined negatives being taken both by Col. Tennant, at Ootacamund, and by Mr. Davis, at Baikal, during the brief continuance (only a few minutes) of total obscuration. In every respect the solar corona accords far better than do the solar coloured prominences with the appearance we should expect to recognise in solar auroras.
In particular, it has always seemed to me that the curved, especially the doubly curved, streamers of the
THE AURORA BO RE A LIS, 109
corona can only be well explained by regarding the corona as in the main an auroral phenomenon. If mighty currents prevailed in the higher regions of a rare atmosphere, extending hundreds of thousands of miles from the sun's surface, appearances such as these curved streamers would undoubtedly be explained. But no one who considers the effect of the sun's tremen- dous attractive power on such an atmosphere can fail to perceive that, according to the known laws connecting gaseous pressure and density, the density of that atmos- phere would be enormously great, even at a very great distance from the sun's surface, if the curved streamers really were caused by atmospheric currents. We know, on the contrary, from the behaviour of comets which have passed very near to the sun, that the atmosphere above his visible surface must be very rare indeed.
It must not be understood, however, that I regard the corona as simply a great solar aurora. It is certain that the whole region filled by the corona is occupied by immense numbers of scattered meteors, and extremely probable that large quantities of cometic matter exist within the same region. Vaporous masses may also be there, circling independently around the sun. But that this region is illuminated constantly by auroral light, varying greatly in intensity and position, seems very strongly indicated by all that we know about the
no THE AURORA BOREALIS,
corona, as seen during different total eclipses of the sun.
If we so viewed the solar corona, and found our earth, therefore, in this respect resembling the great central orb of the solar system, we could not but regard as ex- tremely probable the theory that other planets also resemble the central body in this respect. We might then picture to ourselves every orb in the solar system carrying onward its faintly luminous crowns of boreal and austral light, not shining with constant lustre, or in the same constant position, but at one time leaping in coloured steamers to a great distance from the body they adorned, and anon sinking down and growing fainter and fainter, or occasionally disappearing alto- gether. Then, when some great disturbance affected the central sun, and caused his auroral banners to shine out more brilliantly and to attain a greater extension, sud- denly the auroral streamers of all the planets would leap out into new light and life, playing around the northern and southern magnetic poles of those orbs, even as electric brushes play around the positive and negative electrodes of a Geissler's tube. " Suddenly " at least so far as each planet is concerned, but not suddenly throughout the whole system. For the magnetic in- fluences, like the light and heat of the sun, require time for their transmission. Yet, so rapidly do they
THE AURORA BO RE A LIS, iii
travel that, in a few hours, the auroral illumination would extend from the central sun to the outermost Hmits of his system.
It remains that I should make a few remarks on the evidence which that wonderful instrument of research, the spectroscope, has afforded respecting the light of the aurora.
Angstrom was the first to observe the spectrum of the aurora borealis. He found that the greater part of the auroral light, as observed in 1867, was of one colour, yellow, but three faint bands of green and greenish blue colour were also seen. The aurora of April 15, 1869, was seen under very favourable conditions in America. Prof. Winlock, observing it at New York, found its spectrum to consist of five bright lines, of which the brightest was the yellow line just mentioned. One of the others seems to agree very nearly, if not exactly, in position with a green line, which is the most conspicuous feature of the spectrum of the solar corona. During the aurora of October 6, 1869, Flogel noticed the strong yellow line and a faint green band. Schmidt, on April 5, 1870, made a similar observation. He saw the strong yellow line, and from it there extended to- wards the violet end of the spectrum a faint greenish band, which, however, at times showed three defined lines, fainter, than the yellow line.
II* THE AURORA BORE A LIS.
It was not till the magnificent aurora of October 24, 25, 1870, that any red lines were seen in the spectrum of an aurora. On that occasion the background of auroral light was ruddy, and on the ruddy background there were seen three deep red streamers very well defined. The ruddy streamers, on the night of October 25, converged towards the auroral crown, which was on that occasion singularly well seen. Forster of Berlin failed to see any red line or band despite the marked ruddiness of the auroral light. But Capron at Guild- ford saw a faint line in the red part of the spectrum ; and Elger at Bedford observed a red band in the light of the red streamers, the band disappearing, however, when the spectroscope was directed on the white rays of the aurora.
As yet the auroral spectrum has not been interpreted. It is not a spectrum which can be (at present) artificially produced. We understand the spectrum of the sun and stars, because spectra of the same order can be pro- duced in our laboratories. The spectra of the planets, so far as they differ from the spectrum of reflected sun- light in showing signs of the absorptive action of the planetary atmosphere, have been similarly interpreted. So also the spectra of the coloured solar prominences are understood, while those of nebulae and comets, though not as yet thoroughly explained, have been
THE AURORA BOREALIS. 113
partly interpreted, because of their partial agreement with the known spectra of earthly elements. But as yet neither the spectrum of the aurora nor that of the solar corona has been explained. The reason probably IS, that the conditions under which the light of the aurora as of the corona is formed are not such as have been or perhaps can be attained or even approached in laboratory experiments.
VII.
THE LUNAR HALO.
^^^kT^m HERE are some phenomena of nature which ^^^^^' suggest false ideas. For instance, when we
look at the broad expanse of ocean on a moonlit night, and see a path of glory on its surface, directed towards the moon's place, we seem to be assured by the sense of sight that that broad track is illuminated while the waters all around are dark. A little considera- tion, however, assures us that the impression is a false one, that in this case seeing is not believing. The moon's rays really illumine the whole surface which lies before us, and we fail to receive light from other parts than the track below the moon, liot because they receive no light, but because the light which they receive is not reflected towards us. An observer^ stationed a mile or two towards the right or towards the left of our station, sees a different track of light, while the part which seems bright to us seems dark to him.
THE LUNAR HALO, ii5
The rainbow is another phenomenon of this decep- tive kind. We seem to see an arch of many colours sus- pended in the air, — and when we learn that it is due to the presence of drops of water in the air, we are apt to infer that where we see the red arch there are drops lit up with red light, where the yellow, green, or violet arch, that the drops are aglow with yellow, green, or violet light. But in reality this is not so ; the same drops which seem green to us will seem red to another ob- server, violet to another, and to yet other observers will show none of the prismatic colours, but only the dull grey colour of the cloud on which the rainbow is seen. We have here a pretty emblem of the varied aspects which events of the same real nature present to different persons, or according to the dilTerent circumstances under which the same person may see them. One shall see events in rosy tints, or with the freshness of spring hues, or with the melancholy symbolled by the
• deeper indigo (as when
The heavy- skirted evening droops with frost)—
while to others the same events shall show only the ordinary tints of common-place Hfe.
The lunar halo is one of the phenomena thus decep- tive to the view. We see all around the moon a circle or arc of light, nearly white, though sometimes faint
lid THE LVNAk itALO.
%
tints of colour can be perceived in it, while the space within the circle seems manifestly darker than the space outside. The appearance of the halo as seen under favourable conditions is shown in fig. it, on the next page. In this country the dark space round the moon is not generally so well seen as in countries where the air is clearer. But this is in reality the characteristic feature of the halo, as its name shows. For the name is derived from a Greek word signifying threshing-floor (the old threshing-floors being round), and thus naturally describes a round space relatively clear, surrounded on all sides by a ring of aggregated matter.
We seem in looking at the lunar halo, then, to see the moon at the centre of a dark space, surrounded by a ring of bright particles, outside which again are particles not quite so brightly illuminated as those forming the ring, but more brightly than those within the ring.
But in reality this impression, which, so far as the sense of sight is concerned, seems forced upon the mind, is entirely erroneous. There is no real distinction between the space which looks dark all round the moon, the space beyond which does not look dark, and the ring between the two spaces which looks bright. These are all equally illuminated by the moon, in the same sense, at least, that we say the surface of a moonlit sea is all equally illuminated, neglecting slight differences which
THE LUNAR HALO. uy
do not concern the point we are specially dealing with. Precisely as the path of light on the ocean is not a real path of illumination, bounded on either side by
-Lunar Halo
dark spaces, so the ring of light round the moon is not a real ring of light, bounded on one side by a less bright region, and within by a dark space. Although my object in these essays is not specially to
Il8 THE LUNAR HALO,
deal with scientific matters, but rather with the thoughts (much more important in my beUef) which they suggest —so that, in deaUng with my present subject, I wish rather to call attention to the manifold ways in which our senses may deceive us unless their evidence is carefully cross-examined — yet it may be worth while to notice how the particular illusion here considered has deceived even the scientific elect
It had been noticed by Tyndall, in certain experi- ments, that a very sensitive measurer of heat, when placed under the moon's rays, gathered together by a powerful condenser, seemed to indicate cooling rather than heat- ing, as we should expect. On this a French student of science pointed to the darkening under the moon where the lunar halo is seen as evidence that our satellite possesses a certain power of clearing away vaporous matter from the air. " On pent dire, ^^ he said, speaking of the dark space within the halo, " que la tune ouvre alors tme porie par laquelle s'echappe le calorique que r action solai7'e a em7nagasine dans les couches infirieicres,^^ " One may say,'' that is, '' that the moon then opens a door through which the heat escapes, which the sun's action has stored up in the lower layers " (of the air). It will be manifest, if we remember that a lunar halo can often be seen at the same time from stations hundreds of miles apart^ that there can be no such opening of clear air,
THE LUNAR HALO. 119
For the cloud layer in which the halo is formea is but a few miles above the observer; and therefore, if one observer saw a circular opening in this layer, with the moon at its centre, another, a hundred miles from him, would see the space in a very different direction. The moon would not only not be at the centre of the space for this second observer, but would not be visible through the space at all. Moreover, the space could not possibly seem round to both observers ; if it seemed round to one, it would look like a very flat oval of darkness (almost a mere line) to the other.
The real explanation of the lunar halo is very different. When you see such a halo, you may be certain that there is, high up in the air, a layer of light feathery cloud — the cirrus cloud, as it is called — composed of tiny crystals of ice. These crystals, as we know from those which in winter sometimes fall (not as snow, but as little ice-stars),, have all a definite shape. They are in fact little prisms of ice, with angles like those of an equilateral triangle. These little prisms deflect the light which falls upon them, just as one of the drops of a chandelier deflects any light which falls upon it. If you hold a prism-drop of a chandeher between the eye and a light, you will see that the prism looks dark ; it is really lit up, but it sends the light away in such a direction that the eye receives none. Now move it gradually away from the line of
120 THE LUNAR HALO.
sight to the Hght, and at a certain distance it appears full of light ; or, to speak more correctly, it sends the light it receives directly towards your eye. Beyond that posi- tion it again looks dark, but not so dark as when it was nearly between the eye and the light.
The little crystals of ice perform the same part with respect to the moon, when we see a lunar halo. Those between us and the moon, or within a certain distance from the line of sight to the moon, are, in reality, lit up by the moon's rays ; but they send off those rays in such directions that we do not receive the light. Thus, all the space lying towards the moon, and for a certain distance all round, looks dark. But, at a certain distance, these little crystals send us light. If we could see them separately, they would seem to be full of light. That is the distance where ice-crystals of their known shape act most favourably in deflecting light, — that is, send off most for all the varying positions (not places) they can be in. At greater distances, a small proportion send us light. Thus, at that distance we have a ring of light, and outside the ring we have a gradual falling off in the quantity of light.
But the reader will be apt, perhaps, to say, How can all this be proved ? No one has ever been among the ice-crystals of the feathery clouds when they are per- forming this work. When Coxwell and Glaisher made
THE LUNAR HALO. 121
their highest ascent, the feather-clouds seemed almost as high above them as ever. Nor, if any one could reach those clouds, could he see the ice-crystals at their work. Yet there are few points about which science is more certainly assured than about this explanation of the halo. For we know the shape constantly assumed by ice-crystals; we know according to what precise law ice bends rays of light falling upon it ; hence we can calculate quite cer- tainly where, if ice-crystals make the halo, its rings should be seen. And the halo has the precise position thus cal- culated from the known laws of optics, and the known facts about ice and ice-crystals. The diameter of the halo should be, and is, about eighty times the apparent diameter of the moon, or somewhat less than half the arc which separates the point overhead from the horizon.
There is, however, yet stronger evidence. Haloes form around the sua as well as round the moon,— in fact, more frequently. Solar haloes have so much more light in them that we can recognise varieties of tint. Now, it •follows from the laws of optics that, for the red part of the sun's light, the halo ring should have a smaller diameter than the halo ring for the violet part, inter- mediate colours having their corresponding intermedi- ate halo rings. Thus, the halo ring, as a whole, should * be rainbow-tinted, red on the inside, then orange, yellow.
122 THE LUNAR HALO,
green, blue, indigo, and violet ; and these colours are shown (under favourable conditions) in this order.
The student looking out for haloes, solar or lunar, must be careful not to confound them with solar and lunar coronas, that is, not the corona of astronomy, but rings of light around the sun and moon, much smaller than the true halo rings. What I have said above about the size of the true halo will suffice to prevent such a mistake. Coronas are not nearly so easily, though they have been quite as thoroughly, explained by science, as haloes.
It is singular to observe how utterly unlike the in- terpretation of the halo by science is from the natural interpretation. The observer would say. There surely is a dark space all round the moon, and round that a ring of light, — I see these things, and seeing is believing. Science says there is no dark space, and there is no ring of light; while the eye of science perceives something where the lunar halo shines which ordinary vision cannot recognise. Up yonder, many miles above the earth, science sees millions of crystals of ice, carried hither and thither — so light are they — by every movement of the air. Science sees these ice crystals deflecting the rays of moonlight, sifting the red rays from the orange, and these from the yellow, yellow from green, green from blue, blue from indigo^ and indigo from violet, Science.
THE LVXAR HALO, 123
in fine, perceives processes taking place in those higher regions of air compared with which the most delicate analyses of the laboratory are utterly coarse and im- perfect.
There is a purer and nobler poetry in the lunar halo as thus understood than in its mere visible phenomena, attractive and beautiful though these are. Idle indeed is the fear that the interpretation of this special mystery of nature will leave the number of nature's mysteries diminished by one. On the contrary, for the one mys- tery explained many deeper mysteries are suggested. The phenomena discernible by the sense of sight are explained, but only by bringing into the range of a purer and more piercing vision phenomena infinitely more wonderful. If one could see through some amazing extension of \isual power, or if even the imagination could adequately picture, the rush of light waves of all orders of length upon the line of crystal breakers, their deflection in all directions, their separation into their various orders of wave-length ; if one could perceive the actual illumination of the ice-crystals, even where they seem dark to us, and the continual fluctuations of the troubled sea of ether between the crystal breakers and the earth below, — the scene would infinitely transcend in interest and mystery, the picture would be infinitely more suggestive of solemn thoughts, than the scene — beautiful
124 THE LUNAR HALO,
though it doubtless is— presented by the halo-girt moon to ordinary vision. Truly they know little of the real meaning of science who regard it as depriving natural phenomena of their effect on the imagination, as robbing Nature of her poetic influence.
VIII.
MOONLIGHT.
iHE light of the moon and the changes of the moon were probably the first phenomena which led men to study the motions of the heavenly bodies. In our times, when most men live where artificial illumination is used at night, we can scarcely appreciate the full value of moonlight to men who cannot obtain artificial light. Especially must moonlight have been valuable to the class of men among whom, according to all traditions, the first astronomers appeared. The tiller of the soil might fare tolerably well without nocturnal light, though even he, — as indeed the familiar designation of the harvest- moon shows us, — finds special value, sometimes, in moonlight. But to the shepherd moonlight and its changes must have been of extreme importance as he watched his herds and flocks by night We can under-
126 Moonlight.
stand how carefully he would note the change from the new moon to the time when throughout the whole night, or at least of the darkest hours, the full moon illuminated the hills and valleys over v/hich his watch extended, and thence to the tim_e when the sickle of the fast waning moon shone but for a short time before the rising of the sun. To him, naturally, the lunar month, and its sub- division, the week, would be the chief measure of time. He would observe-— or rather he could not help observ- ing— the passage of the moon around the zodiacal band, some twenty moon-breadths wide, which is the lunar road- way among the stars. These would be the first purely astronomical observations made by man; so that we learn without surprise that before the present division of the zodiac was adopted the old Chaldean astronomers (as well as the Indian, Persian, Egyptian, and Chinese astronomers, who still follow the practice) divided the zodiac into 28 lunar mansions, each mansion corre- sponding nearly to one day's motion of the moon among the stars.
It is easy to understand how the first rough obser- vations of moonlight and its changes taught men the true nature of the moon, as an opaque globe circling round the earth, and borrowing her light from the sun. They perceived, first, that the moon was only full when she was opposite the sun, shining at her highest in the
MOONLIGHT. iij
soiith at midnight when the sun was at his lowest beneath the northern horizon. Before the time of full moon, they saw that more or less of the moon^s disc was illuminated as he was nearer or farther from the position opposite the sun, the illuminated side being towards the west — that is, towards the sun; while after full moon the same law was perceived in the amount of light, the illuminated side being still towards the sun, that is, towards the east. They could not fail to observe the horned moon sometimes in the daytime, with her horns turned directly from the sun, and showing as plainly, by her aspect, whence her light was derived, as does any terrestrial ball lit up either by a lamp or by the sun.
The explanation they gave was the explanation still given by astronomers. Let us briefly consider it. In doing so I propose to modify the ordinary text-book illustration which has always seemed to me ingeniously calculated (with its double set of diversely illuminated moons around the earth) to make a simple subject obscure.
In figi 12, let E represent the earth one half in dark* ness, the other half illuminated by the rays of the sun S, which should be supposed placed at a much greater distance to the left, — in fact, about five yards away from E. To preserve the right proportions, also, the sun ought to be much smaller and the earth a mere point.
125
MOONLIGHT.
I mention this to prevent the reader from adopting erroneous ideas as to the size of these bodies. In
reality it is quite im- possible to show in such figures the true proportions of the heavenly bodies and of their distances. Next let Ml, Mg, Mg, etc., represent the moon in different posi- tions along her circuit around the earth at E. Now, it is clear that when the moon is at Mj, her illumi- nated face is turned from the earth, E. She therefore cannot be seen ; and accord- ingly, in fig. 2, she is presented as a black disc at I to corres- pond with her invisibility when she is as at Mj. She passes on to M^ ; and now from E a part of her illumi- nated half can be seen towards the sun, which would be
Cm
MOONLIGHT,
129
towards the right, if we imagine an eye at E looking towards Mg. Her appearance then is as shown at 2, fig. 13. In any intermediate portion between M^ and Mg, the sickle of light is visible but narrower. We see also that all this time the moon's place on the sky cannot be far from the sun's place, for the line from E to Mg is not greatly inclined to the line from E to S. When the moon has got round to M3, the observer on the earth sees as much of the dark half as of the bright half of the moon, the bright half being seen, of course, towards the sun. Thus the moon appears as at 3, fig. 13. Again as to position, the moon is now a quarter of a circuit of the heavens from the sun, for the line from E to M3 is square to the line from E to S. We see similarly that when at M^ the moon ap- pears as shown at 4, fig 13, for now the observer at E sees as small a part of the moon's dark side as he had seen of her bright side when she was at Mg. When she is at M5 the observer at E sees her bright face only, the dark face being turned directly from him. She, therefore, appears as at 5, fig. 13. Also being now exactly
9
c
5'
a
130 MOONLIGHT.
opposite the sun, as we see from fig. 12, she is at her highest when the sun is at his lowest, or at midnight ; and at this time she rules the night as the sun rules the day.''' As the moon- passes on to M^., a portion of her dark half comes into view, the bright side being now towards the left, as we look at Mg from E, fig. 12. Her appearance, theiefore, is as shown at 5. When at M^ she is seen as at 7, half bright and half-dark, as when she was at M3, but the halves interchanged. At Mg she appears as at 8, and, lastly, at M^ she is again undiscernible.
The ancient Chaldean astronomers could have litde doubt as to the validity of this explanation. In fact,
"^ It has been thought by some that, in the beginning, the moon was always opposite the sun, thus always ruhng the night. IMilton thus understood the account given in the first book of Genesis. For he says, —
Less bright the morn, But opposite in levell'd west was set His mirror, with full face, borrowing her light From him"; for other light she needed none In that aspect ; and still that distance keeps Till night, then in the east her turn she shines, Revolv'd on Heav'n's great axle.
It was only as a consequence of Adam's transgression that he con- ceives the angels sought to punish the human race by altering the movements of the celestial bodies —
To the blank moon Her office they prescribe —
It is hardly necessary to say, perhaps, that this interpretation is not bcientifically admi sible.
MOONLIGHT. 131
while it is the explanation obviously suggested by ob- served facts, one cannot see how any other could have occurred to them.
But if they had had any doubts for a while, the occur- rence of eclipses would soon have removed those doubts. They must early haye noticed that at times the full moon became first partly obscured, then either wholly disappeared or changed in colour to a deep coppery red, and after a while reappeared. Sometimes the darkening was less complete, so that at the time of greatest dark- ness a portion of the moon seemed eaten out, though not by a well defined or black shadow. These pheno- mena, they would find, occurred only at the time @f full moon. And if they were closely observant, they would find that these eclipses of the moon only occurred when the full moon was on or near the great circle round the stellar heavens, which they had learned to be the sun's track. They could hardly fail to infer that these darken- ings of the moon were caused by the earth's shadow, near which the moon must always pass when she is full, and through which she must sometimes pass more or less fully; in fact, whenever, at the time of full, she is on 01 near the plane in which the earth travels round the sun. Solar eclipses would probably be observed later. For though a total eclipse of the sun is a much more striking phenomenon than a total eclipse of the moon, yet the
132 MOONLIGHT.
latter are far more common. A partial eclipse of the sun may readily pass unnoticed, unless the sun's rays are so mitigated by haze or mist that it is possible to look at his disc without pain. Whenever solar eclipses came to be noted, and we know from the Chaldean discovery of the great eclipse period, called the Saros, that they were observed at least two thousand years before the Christian era, the fact that the moon is an opaque body circling round the earth, and much nearer to the earth than the sun is, must be regarded as demonstrated. Not only would eclipses of the sun be observed to occur only when the moon was passing between the earth and the sun, but in an eclipse of the sun, whether total or partial, the round black body cutting off the sun^s light wholly or partially would be seen to have the familiar dimensions of the lunar orb.
Leaving solar and lunar eclipses for description on another occasion, I will now proceed to consider a peculiarity of moonlight which must very early have attracted attention, — I mean the phenomenon called the harvest-moon.
The moon circuits the heavens in a path but slightly inclined to that of the sun, called the ecliptic, and for our present purpose we may speak of the moon as travelling in the ecHptic. Now we know that during the winter half of the year the sun is south of the equator,
MOONLIGHT. 133
the circle of the heavenly sphere which passes through the east and west points of the horizon, and has its plane square to the polar axis of the heavens. During the other or summer half of the year he is north of the equator. In the former case the sun is above the horizon less than half the twenty-four hours, day being so much shorter as the sun is farther south of the equator; whereas in the latter case the sun is above the horizon more than twelve hours, day being so much the longer as the sun Is farther north of the equator. Precisely similar changes affect the moon, only, instead of taking place in a year (the time in which the sun circuits the stellar heavens), they occur in what is called a sidereal month, the time in which the moon completes her circuit of the stellar heavens. For about a fortnight the moon is above the horizon longer than she is below the horizon, while during the next fortnight she is below the horizon longer than she is above the horizon. Now clearly when the length of what we may call the moon's diurnal path (meaning her path above the horizon) is lengthening most, the time of her rising on successive nights must change least. She comes to the south later and later each successive night by about 50^ minutes, because she is always travelling towards the east at such a rate as to complete one circuit in about four weeks ; and losing thus one day in 28, she losses about 50^
t34 MOONUGllT.
miuutes per day. If the inter\-al between her rising and her arri\ ing to the south were always the same, she would rise 50^ minutes later night after night But if the interval is lengthening, say by 10 minutes per niglit, she would of course rise only 40! minutes later ; if tlie inter\al is lengthening 20 minutes i)er night, she would rise only 30 J minutes later, and so forth. But the lunar diurnal arc /i^ lengthening all the time she is passing from her position farthest south of the equator to her position farthest north, just in the same way as the solar day is lengthening from midwinter to midsummer, only to a much greater degree. And as the solar day lengthens fastest at spring when the sun crosses the equator from south to north, so the time the moon is above tlie horizon lengthens most, day by day, when the moon is crossing the equator from south to north. It lengtliens, then^ from an hour to an hour and 20 minutes in one day, that is, the inten^l between moon-rise and moon- setting increases from 30 to 40 minutes. At this time, then, whencN'er it happens in each lunar month, the moon^s time of rising changes least: instead of the moon rising night after night 50J minutes later, the actual difference \^es only from 10 to 20 minutes.
Now if this happens at a time when the moon is not nearly full, it is not specially noticed, because the moon^s light is not then specially useful But if it
MOONLK.irr. 135
happens when the moon is nearly full, it is noticed, l)ccaii.se her light is tlieri so useful. A moon nearly full, afterwards quite full, and tlien for a day or two si ill nearly full, rising nigliL after night at nein-ly the same time, remaining also night after night longer ahovc tiie hori/on, manifestly ser\'es man for the time being in the most convenient way ])ossible. Hut it is clear that as the full moon is oi)|)OS!lc the sun, and as to fulfil the condition described we have se(.'n that she must be crossing the ecjuator from sr)uth to nr)rth, the sun, opjxjsite to her, must be at the i)arl of his j>ath where: he crosses the equator fiom north U) sr)Utli. In other words, the time of year must be the autunmal equinox. Thus the moon which comes to '' full " nearest to Sep- tember 22 or 23 will behave in the c onsenient way described. At this time, moreover, when she rises night after night nearly at the same time, the nights are lengthening fastest while the time the moon is abo\'e the hori/on is lengthening still more, and therefore, in all respects, the moon is then doing her best, so to si)eak, to illuminate the nights. At this season the mo(;n is called the harvest-moon, from the assistance she some- times renders to harvesters.
The moon which is full nearest to Sei>tember 22—23 may precede or follow thait date. In the former case only can it properly be called a harvest-moon. In the
136 Moonlight.
latter it is sometimes called the hunter's moon. The full moon occurring nearest to harvest time will always par- take more or less of the qualities of a full moon occur- ring at the autumnal equinox: and similarly of a full moon following the autumnal equinox. So that, in almost every year, there may be said to be a harvest-moon and a hunter's moon. But, of course, it will very often happen that in any particular agricultural district the harvest has to be gathered in during the wrong half of the lunar month, that is, during the last and first, instead of the second and third quarters.
The reader must not fall into the mistake of supposing, as I have seen sometimes stated in text-books of astro- nomy, that we are more favoured in this respect than the inhabitants of the southern hemisphere. It is quite true that the same full moon shines on us as on our friends in New Zealand, Australia, and Cape Colony, and also that our autumn is their spring, and their spring our autumn. But the full moon we have in autumn behaves in the southern hemisphere not as with us, but as our spring full moon behaves; and the full moon of our spring, which is their autumn, behaves with them as our autumn moon behaves with us. It is, therefore, for them a harvest-moon if it occur before the equinox, and a hunter's moon if it occur after the equinox. A very little consideration will show why this is. In fact if, in
MOONLIGHT. 137
the explanation given above, the words north and south be interchanged, and March 21 — 22 written for Septem- ber 22 — 23, the explanation will be precisely that which I should have given respecting the harvest (or March) moon of the southern hemisphere, if I had been writing for southern readers.
Having thus considered the moon as a light-giver, both in respect of her monthly changes and of that yearly change which causes her services to be most use- ful in harvest time, let us consider what science tells us of the orb which thus usefully reflects to us the solar rays.
The moon is a globe about 2159I miles in diameter, travelling round the earth at a mean distance of 238,818 miles. Her path round the earth is not, however, a circle, but an ellipse, which itself is constantly varying in shape. The average eccentricity of the moon's path is such that her greatest and least distances, as she circuits round it, are 251,953 miles and 225,683 miles respectively ; but when it is most eccentric, her greatest and least distances are 252,948 miles and 221,593 miles respectively; while, when it is least eccentric, they are respectively 250,324 miles and 227,312 miles. The earth's surface exceeds the moon's nearly 13I times, the actual number of square miles in the moon's sur-
138 MOONLIGHT,
face amounting to 14,600,000. This is nearly equal to Europe and Africa together, or, more nearly still, to North and South America together, without their islands. In volume our earth exceeds the moon rather more than 49 J times: or, more nearly, if the earth's volume be represented by 10,000, the moon's will be represented by 209. The materials of the moon's globe are either lighter or (more probably) they are less closely compacted than those forming our earth,— for, according to the best modern estimates, the earth exceeds the moon in mass nearly Z\\ times. Assun>| ing as the most probable value of the earth's mean density about 5^% times the density of water, the moon's| mean density is equal to 3jVo times that of waterJ Gravity at her surface is accordingly much less than at' the surface of the earth ; a quantity of matter weighing six pounds at the surface of the earth would weigh almost exactly one pound at the surface of the moon.
The moon circuits once round the earth in 2 yd. yh. 43m. 11.5s. This is the time in which, viewed from the earth, she seems to complete one circuit round the stellar heavens, and is therefore called a sidereal month. But as the earth is all the time travelling the same way round the sun, the lunar month is longer. Thus, suppose S (fig. 14) to be the sun, E the earth at thj beginning of a lunar month, M^ M^ M3 M^ the moon's
MOONLIGHT, 139
path, and M^ the moon's place on the line joining E and S. If the earth remained at rest while the moon went round the path M^ M3, then after completing one circuit the moon would again be at M^ on the line joining E and S, or it would be new moon again. But the earth is moving onwards along the arc EE' of her circuit round the sun. So that wlicn the moon has
Fig. 14. — Explaining the difierence between a sidereal lunar month and a common lunar month or lunation.
completed one circuit she is at M4 (E^m^ drawn parallel to EMj) and has still to travel some distance before she gets round to M' on the line joining S and E'. The lunation, or interval between successive new moons, has an average duration of 29d. i2h. 44m. 38s., exceeding a sidereal month by 2d. sh.
It would not, however, be correct to regard the earth
140 MOONLIGHT,
as the true centre of the moon's motion. The moon is in reality a planet circling round the sun, but largely perturbed by the attraction of its companion planet the earth. If the moon's path in the course of a year were carefully drawn to scale, or, better, were modelled by means of a fine wire, it would scarcely be distinguish- able from a similar picture or model of the earth's path round the sun. Or thus, the entire width of the moon's track is about 477,636 miles, while the diameter of the orbit along which she and the earth both travel is nearly 104,000,000 miles, or 385 times as great. If we draw then a circle Zt^^ inches in diameter to represent the earth's path round the sun, somewhat eccentrically placed, and the circular line is i-iooth of an Inch wide, the moon's track would be fairly represented by a curve touching alternately the inside and the outside edge of this circular line, at equidistant points dividing the circle into about 24I parts.
Regarding the moon as a planet, she may be said to have a year, and seasons, and day and night, as the earth has, but very unlike our seasons and days. Her axis is inclined only i^ degrees from uprightness to her path, whereas our earth's axis is inclined 23^ degrees. The sun's range of mid-day altitude is in fact not quite equal to the range of our sun in mid-day height, from four days before to four days after either
MOONLIGHT, 141
spring or autumn. The lunar day lasts a lunar month, daytime and nighc-time each lasting rather more than a fortnight. The lunar year of seasons is not, as is commonly stated, the same in length as ours. She goes round the sun in the same time, so that her side- real year is the same as ours ; but owing to the sway- ing round of her axis her year of seasons or tropical year is shorter. Our tropical year is also shorter than the sidereal year, but very little shorter, because the earth's axis sways round once only in 25,868 years. The moon's axis sways round once in i8f years, and accord- ingly the year of seasons is much more effectively shortened. It lasts, in fact, only 346d. i4h. 34m. of our time; and contains only ii| lunar days. So that I cannot altogether agree with Sir W. Herschel's state- ment, that ^^the moon's situation with respect to the sun is much like that of our earth, and by a rotation on its axis it enjoys an agreeable variety of seasons, and of day and night."
When the moon is examined with a telescope her surface is seen to be marked by many irregularities. There are large dark regions which were formerly thought to be seas, but are now know to be land- surfaces. Some of these regions are singularly level, and have been thought to be old sea- bottoms. Moun- tains and mountain ranges are anothej important feature
142 MOONLIGHT.
of the moon's surface. Some, like our Rocky Mountains and Andes, form long continuous chains ; others form elevated plateaus whence ridges extend in various direc- tions. A very striking form is that of narrow ridges little raised above the general level, but reaching over enormous areas of the moon's globe. It is a system of this kind, radiating from a great lunai crater called Tycho, which gives to small photographs of the moon the appearance of a peeled orange. They are supposed to indicate the action of tremendous forces of upheaval, in past ages, bursting open portions of the moon's crust.
But the most characteristic of all the lunar features are the crater mountains, which exist on a scale not only much larger relatively to the moon's globe than the scale on which terrestrial craters are formed, but much larger absolutely. They are also far more nume- rous. Some parts of the moon's surface, especially in the bright south-western quarter of her face, are literally crowded with craters of various dimensions.
There are few signs of the former emission of lava from the lunar craters. Within some of them recent changes have been suspected. A remarkable instance is that of the crater Linne, marked in Madler's map as a deep, well-walled crater, some four miles in diameter. At present only a small crater can be seen in its place. The surrounding region is rather conspicuously bright.
MOONLIGHT. 143
It is not necessary to infer that there has been any volcanic disturbance, however. Far more probably the walls have been thrown dovv'n through the long-continued action of that alternate expansion and contraction, which must affect the moon's crust as the long fortnightly day proceeds, and then the equally long lunar night.
There are many well-marked valleys on the moon, besides clefts and ravines. The features called rilles are among the most perplexing objects on the moon's surface. Webb, in his charming and most useful little book, "Celestial Objects for Common Telescopes," thus de- scribes them : '^ These most singular furrows pass chiefly through levels, intersect craters (proving a more recent date), reappear beyond obstructing mountains, as though carried through by a tunnel, and commence and termi- nate with little reference to any conspicuous feature of the neighbourhood. The idea of artificial formation is negatived by their magnitude ; they have been more probably referred to cracks in a shrinking surface." Some observations would seem to show that they have been formed from rows of closely-adjacent small craters. Faults, 2i\^o, or closed cracks where the surface is higher on one side than on the other, have been recognised from the careful study of the shadows on the moon's disc.
From measurements of the shadows of lunar moun-
144 MOONLIGHT,
tains, it appears that their average height is about five miles. In comparing this elevation with that assigned to terrestrial mountains, it must be remembered that these are measured from the sea-level ; if the average height of terrestrial mountains were determined with reference to the sea-bottom it would be far greater. Still, even taking this circumstance into account, the average height of the lunar mountains bears a far greater ratio to the diameter of the globe on which they stand than the average height of our mountains to the earth's diameter.
Several circumstances agree in showing that the moon's atmosphere must be exceedingly rare. The shadows of lunar mountains are either actually black or nearly so. When the moon hides the sun in total eclipse, no sign can be seen of any refractive effort exerted on the sun's rays. When a star is hidden (or occulted) by the moon, the star vanishes in an instant and reappears with equal suddenness. It is certain from these phenomena that the moon has either no air, or air exceedingly tenuous. It is equally clear that she has no Avater, for if she had we should undoubtedly be able to recognise the occasional formation or dissipation of mist and vapour over parts of the moon's surface. No signs of such phenomena have ever been observed. The moon is certainly at present a waterless globe, so far at least as her surface is concerned.
It has been thought that though there is no water and
%
hiO ON LIGHT, 145
very little air on the side of the moon turned towards the earth, there may be both water and air on the farther unseen side. The theory has been long since given up, but the reasoning on which it depends is worth noting. Owing to the strange circumstance that the moon rotates on her axis in the same time in which she revolves round the earth, she always presents the same face towards the earth, or very nearly so. If her axis were exactly square to the path in which she circuits the earth, and if she revolved at a uniform rate, we should have exactly the same side constantly turned towards us. But as the axis is incHned about 6|" from uprightness to the path round the earth (which, be it remembered, is not in the same plane as the path round the sun, but inclined 5° 8' to it), the northern and southern parts of the moon are alternately swayed over by about 6|° into view. This apparent swaying is called a libration, and the libration just described is called the libration in latitude. Again, as the moon does not travel at a uniform rate round the earth, but faster than her mean rate when nearer to us, and slower when farther from us, she alter- nately gains and loses in her motion of revolution as compared with her motion of rotation, by a quantity varying between 5° and 7f°, to which varying extent the parts east and west of her mean disc are alternately swayed into view. This is called the libration in
10
146
Moonlight,
longitude. Thus we see, beyond the edge of the 7nean half turned towards us, a considerable fringe of the other half. If a globe, as PAP'B, fig. 15, were divided into two
^\r, 15.— lUus lafng lunar librat'on.
4
halves to represent the farther and nearer halves of the moon, and held so that that dividing circle were seen as PEP' in the figure, then Ppep'P' would represent the part brought into view at different times by the apparent
MOONLIGHT. 147
swaying described above ; while Vpep'Y* would represent the parts swayed out of view. The regions thus alter- nately in view and out of view have their greatest breadth, not at the poles or east and west, but at mMm and m'M'm', where the two librations act together. The narrow fringe bordering these regions is that brought into or out of view by changes in the place of the observer on earth, due to the earth's rotation. It is called the parallactic fringe, any change in the apparent position of a heavenly body, or part of one, on account of the earth's rotation, being termed parallax.
Lastly, let us return to the consideration of moonlight, as depending on the condition of the moon's surface. To one who observes the moon as seen on the sky, her light appears white ; but it must not be supposed that she is a white body. Careful estimates of the quantity of light she reflects show that she is more nearly black than white, though in reality she is neither one nor the other. It has been said, and truly, that if the surface of the moon were covered with black velvet she would still appear white ; for even black velvet reflects some light, and whatever Ught the moon reflected would show her by contrast with the blackness of the sky, as a luminous body or white. It follows from the observations made by Zollner that if the moon's surface were covered with white snow she could give us about 4^ times as much
14^ MOONLIGHT.
light as she actually does. If she were covered with white paper she would give more than 4 times as much light as she does. If she had a surface of white sand stone her light would be nearly half as great again as it is. She gives rather more light than she would if her sur^ face consisted entirely of weathered grey sandstone, .or of clay marl, and more than twice as much light as she would give if her surface were of moist earth, or dark grey syenite. As some parts of her surface are obviously much brighter than others, we must infer that some parts shine with much more, and others with much less, brightness than weathered grey sandstone. Probably some parts are much brighter than white sandstone, and some much darker than dark grey syenite. From the degree in which her lustre changes with her changing aspect, Zollner infers that her mountains have an average slope of about fifty- two degrees.
IX
THE PLANET MARS.
^Y^i^VERY one who notices the stars at all, — and who that thinks and can see does not ? — must have observed during the autumn of 1877 two bright stars in the southern heavens. One of these shone with a lustre which but for its ruddy hue would have caused the star to be taken for the planet Jupiter; the other shone with a somewhat yellowish light, and was much fainter, though surpassing most of the fixed stars in brightness. The former was the planet Mars, the latter the ringed planet Saturn. The motions of these two stars with respect to each other and to the neighbouring stars were sufficiently conspicuous to attract attention. During October these stars attracted still more attention, because they drew nearer and nearer together, to all appearance, until on November 4th they were at their nearest, when the distance separat-
ISO THE PLANET MARS. d|
ing them was about one-third the apparent diameter of the moon, so that in a telescope showing at one view the whole disc of the moon, Mars and Saturn on the night of November 4th appeared like a splendid double star, the primary a fine red orb, the* companion a smaller body, but attended by a splendid ring system and com- panion moons.
It was strange when we looked at these two stars, the yellow one apparently much smaller than the brighter, and the pair see^^ingly close together, to consider how thoroughly the reality differed from these appearances. The fainter and seemingly the smaller of the two stars was in reality some four thousand times larger than the brighter, and had, among eight orbs attending upon it, one nearly as large as the ruddy planet which as actu- ally seen so completely outshone Saturn himself. Again, instead of being near each other, those two bodies were in reahty separated by a distance exceeding some sixteen times that which separated us from the nearer of the two.
I propose now to consider some of the more interest- ing characteristics of these two planets, presenting specially those features which mark Saturn as the representative of one family of bodies, and Mars as the representative of another and an entirely different family.
It will be well to consider Mars first ; for although, as will presently be seen, Saturn came earlier of the two
X
THE PLAXET MARS, 153
to the portion of his path where he was most favourably seen, there was nothing specially remarkable about the approach of Saturn on that occasion, whereas Mars in the year 1877 made a nearer approach to the earth than he has for thirty-two years past, or will for some forty-seven years to come.
In the first place, let us note the apparent paths on which the two planets have been and are now travelling.
Fig. 16 presents that part of the zodiac along which lay the apparent paths of Mars and Saturn in 1877. The stars marked with Greek letters belong to the constellation Aquarius, or the Water-Bearer (his jar is formed by the stars in the upper right-hand corner of the picture), — with a single exception, the star marked k, which, with those close to it not lettered, belongs to the constellation Pisces, or the Fishes. Thus the loops traversed by the two planets in 1877 both fell in the constellation of the Water-Bearer ; but, as will be seen from the symbols on the ecliptic, these loops lie in the zodiacal sign Pisces, which begins at k and ends at T. The signs have long since passed away, in fact, from the constellations to which they originally belonged.
It will be noticed that Mars described a wide loop ranging to a considerable distance from the ecliptic (or sun"s track). Saturn, on the other hand, travelled on a
154 THE PLANET MARS.
narrow and shorter loop lying much nearer to the ecliptic, his whole track, except just where he was turning, — his stationary points, — lying nearly parallel to the ecliptic. It may be well to mention the reason of this well-marked difference. Mars does not in reality range even quite so widely from the plane of the ecliptic as Saturn does. Nay, his path is even less inclined to the ecliptic. (This may sound like repetition, but the inclination of a planet's path to the ecliptic is one thing, the range of the planet north and south of the ecliptic, in miles, is another. Mercury, for example, has of all planets the path most inclined to the ecliptic, but Mercury never attains any- thing like the same distance from the plane of the ecliptic which is attained by the remote planet Uranus, whose path is of all others the least inclined to the plane of the ecliptic. In fact, none of the planets, except Venus and Mars, have so small a range from the ecliptic in actual distance as Mercury has.) The reason why the range of Mars from the ecliptic appeared so much greater than that of Saturn, in 1877, is similar to the reason why Mars, though much smaller than Saturn, largely outshone him. Mars looked larger because he was nearer, his loop looked larger because his real path was nearer. For the same reason that a hut close by seems to stand higher above the horizon than a palace at a distance, or a mountain yet further away, so the displacement of Mars from the
THE PLANET MARS, 155
ecliptic plane appeared greater than that of Saturn, though in reality much less.
Let us consider how the paths of these planets are really situated. I know of no better way of showing this than by drawing the paths of the two families of planets separately. It is in fact utterly impossible to give an accurate yet clear view of the solar system in a single picture ; and the student may take it for granted that every drawing or plate in which this has ever been attempted is from one cause or another misleading.
In figs. 17 and 18 the shape and position of the plane- tary paths are correctly shown. Very little description is necessary, but it may be mentioned that on each orbit the point nearest to the sun is indicated by the initial letter of the planet, while the point farthest from the sun is indicated by the same letter accented. The places where each path crosses the plane of the earth's — which is sup- posed to be the plane of the paper — are marked 9> and ^, the former sign marking where the planet in travelling round in the direction shown by the arrows crosses the plane of the earth's path from below upwards, while the latter marks the place where the planet in travelling round crosses the plane of the earth's path from above down- wards.
Fig. 1 7 shows the paths of the inner family of planets of which our earth is a member. Fig. 18 showsi:he outer
156
THE PLANET MARS,
family of planets, and inside of it the ring of small planets called asteroids. Inside that ring, again, we see the paths of the inner family of planets ; but they
Fig- T7. — The paths of ^Mercury, Venus, the Earth, and Mars, around the Sun.
appear on a very small scale indeed. In fact, the scales appended to the two figures show that a length which represents 50,000,000 miles in fig. 17, represents
THE PLANET MARS.
157
1,000,000,000 miles in fig. i8; or, in other words, the scale of fig. 18 is only one-twentieth of the scale of fig. 17. On the scale of fig. 17 the sun would be fairly
Fig. iS. — The paths of Jupiter, Saturn, Uranus, and Neptune, around the rin^ of small planets.
represented by an ordinary pin-hole ; on the scale of fig. 18 the sun would be scarcely visible. The dots round the orbits show the planets' places at intervals of 10
158 THE PLANET MARS'.
days in fig. 17, and of 1000 days in fig. 18, starting always from the left side of orbit (on horizontal line through sun).
Now looking at fig. 18 and noting how small is the distance of the path of Mars from the earth's path, com- pared with the distance of Saturn's path, we understand why Saturn, despite his far superior size, shines far less brightly in our skies than Mars does. In fact, in Octo- ber, 1877, the Earth and Mars were on the parts of their tracks which lay nearest together, that is, the parts occu- pying the lower right-hand corner of fig. 17 ; and turning to fig. 18, we perceive that the distance separating the two paths here is very small indeed compared with Saturn's distance.
So that, when we looked at Mars and Saturn as they shone in conjoined splendour in our skies, in 1877, we saw in the bright orb of Mars the planet whose track lies nearest to us in that direction, whereas in looking at Saturn the range of view passed athwart the track of Mars, through the ring of asteroids, and past the orbit of Jupiter, before entering the wide and barren region which separates the orbits of the two giant oiembers of the solar system.
We study Mars under much more favourable con- ditions than either Jupiter or Saturn. And yet, at a first view, the telescopic aspect of this interesting planet is exceedingly disappointing. Galileo, who quite easily
THE PLANET MAkS. tj^
discovered the moons of Jupiter with his largest telescope, could barely detect with it the fact that Mars is not quite round at all times, but is seen sometimes in the shape of the moon two or three days before or after full. "' I dare not affirm." he wrote on December 30, 16 10, to his friend Castelli, '' that I can observe the phases of Mars ; } et, unless I mistake, I think I already perceive that he is not perfectly round." But even in a large telescope one can see very little except under very favourable con- ditions. It has only been by long and careful study, and piecing together the information obtained at various times, that astronomers have obtained a knowledge of the facts which appear in our text-books of astronomy. The possessor of a telescope who should expect, on turning the instrument towards Mars, to perceive what he has read in descriptions of the planet, would be con- siderably disappointed.
First noticed among the features of the planet were two white spots of light occupying the northern and southern parts of his disc. These are now known to be regions of snow and ice, like those which surround the poles of our own earth. But how different the reality must be from what we seem to see in the telescope ! These two tiny white specks represent hundreds of thou- sands of square miles covered over with great masses of snow and ice, which doubtless are moved by disturbing
!6o TilE PLANET MARS.
forces similar to those which make our arctic regions for the most part impassable even for the most daring of our seamen.
The snow-caps of Mars change in size as the planet circuits round the sun, completing his year of seasons (which lasts 687 of our days). They are largest in the winter of Mars, smallest in the Martian summer ; so that, as it is winter for one hemisphere when it is summer for the other, one of the snow-caps is larger than the other at the winter and summer seasons. In the same way, our arctic snows extend more widely during our winter, while the antarctic snows then retreat ; w^hereas, during our summer, when it is winter in the southern hemisphere, the antarctic snows advance and our arctic snows retreat.
But we have still to learn why these white spots are known to be masses of snow. They might well from analogy be considered to be snows, since they behave like the snows of our polar regions. Yet that would be very different from proving them to be snow masses. I shall now show how this has been done, and afterwards describe the lands and seas of the planet, and give a short account of the recent interesting discovery of two moons attending on the planet which Tennyson had called the "moonless Mars.''
Even before the poles of Mars had been discovered,
THE PLAXET MAR3. l6i
observers had perceived that the planet has marks upon its surface. Cassini, in 1666, at Paris, found by observing these spots that the planet turns on its axis once in about twenty-four hours forty minutes. In the same year Dr. Hooke observed Mars. He was in doubt whether the planet turned once round or twice round in about twenty- four hours ; for with his imperfect telescope two opposite faces of the planet seemed so much alike that he was doubtful whether they really were two different faces or the same. Fortunately he published two pictures of the planet, taken on the same night in March, 1666, and we have been able to keep such good count of Mars's turning on his axis, that we know exactly how many times he has turned since that distant time. However, at present, we need not further consider the turning motion of Mars, but rather what the telescope has shown us about him. Only, let it be remembered that he has a day of about twenty-four hours thirty-seven minutes, and is in this respect much like our earth.
Maraldi, Cassini's nephew, early in the last century observed several spots on Mars, and, in particular, one somewhat triangular dark spot, which was one of Hooke's markings, but more clearly seen by Maraldi. About this time it was seen that the darker markings have a some- what greenish colour; and towards the end of last century, or, more exactly, about a hundred years ago,
II
i62 THE PLANET MARS.
the idea was maintained by Sir W. Herschel that the dark-greenish markings are seas, while the lighter parts of Mars, to which the planet owes its somewhat ruddy colour, are lands. Sir W. Herschel also was the first to show that Mars, like our earth, has seasons. It had been supposed by Cassini, Maraldi, and others, that the axis of Mars is upright to the level of the path in which he travels. Of course, if this were so, the light of the sun would always fall on the planet in the same way ; for the sun is in that level. But the axis, like that of our own earth, is bowed considerably from uprightness ; so that at one part of his year the sun's rays fall' more fully on his northern regions, and his southern regions are correspondingly turned away from the sun ; then it is summer in his northern regions, winter in his southera At the opposite season the reverse holds, and then winter prevails over his northern and summer over his southern regions. Midway between these two seasons, the sun's rays are equably distributed over both hemispheres of Mars, and then the days and nights are equal, and it is spring in that hemisphere which is passing from winter to summer, and autumn in the other hemisphere which is passing from summer to winter. All these changes are precisely like those which take place in the case of our own earth. Only, the year of Mars, and therefore his seasons, are longer. He takes 687 days in travelling
>
TBE PLANET MARS. 165
round the sun, giving nearly 172 days, or more than five and a half of our months, for each season.
Figs. 19, 20, and 21 are three views of Mars, drawn by Mr. Nathaniel Green, an excellent observer, who has paid special attention to this planet. Fig. 19 shows a faintly-marked sea running north and south (the upper part of the picture being the south, because that is the way in which the telescope used by astronomers inverts objects.) This is one of the markings which deceived Hooke. This picture was drawn on May 30, 1873, at half-past seven in the evening. The second picture was drawn two days earlier, at eight in the evening ; but it shows the planet as it would have looked on May 30 at about a quarter past nine in the evening, by which time the sea running north and south had been carried over to the right and lost to view. But another north and south sea had come into view on the right. The third picture shows a view taken three hours later, or at eleven on May 28, when the planet appeared precisely as he would have appeared at a quarter past eleven in the early morning of May 31, had weather then per- mitted Mr. Green to continue his observations. You see in it the great north and south sea which Maraldi had noticed, the other of those two which had deceived Hooke.
It will be seen from these drawings, which, be it remembered^ w^ere taken at the telescope, that it i§
i66 THE I'LANET MARS,
possible from a great number of such drawings to