Popular Science Monthly/Volume 9/October 1876/Modern Philosophers on the Probable Age of the World
|MODERN PHILOSOPHERS ON THE PROBABLE AGE OF THE WORLD.|
A SHORT time ago Sir William Thomson took occasion, at a meeting of the Geological Society of Glasgow, to make a somewhat startling statement. He said that the tendency of British popular geology was, at the time he spoke, in direct opposition to the principles of natural philosophy.
So strong an opinion expressed by the man who is, perhaps, foremost in this country in applied mathematics and natural science, naturally attracted great attention, and it is not too much to say that in the six years which have since elapsed a very great change has taken place in the views of those best able to form an opinion on the subject of Sir William Thomson's animadversions.
Whether or not we are correct in saying that such a change has actually taken place in educated public opinion, it is the object of this paper to show; but we may at least affirm at the outset, without fear of contradiction, that a very smart conflict has been raging on the subject in the scientific world. The opposing forces are the geologists and the mathematicians. There has been hard hitting on both sides, and no quarter given. Of late the mathematicians have brought up their reserve, a contingent of natural philosophers, who have done good service. The latest intelligence from the seat of war speaks of a suspension of hostilities. The mathematicians will make no concessions, but the geologists appear likely to abate somewhat of their high demands. There is even some talk of an amalgamation of the opposing armies. In plain English, there has been a dispute as to the age of the world. Geologists declared that the centuries of its duration could only be denoted by an array of figures so large as to paralyze the reasoning faculties and convey no definite impression to the mind. Other branches of science have shown cause for attributing to the solar system a limit of duration, vast indeed, but not absolutely inconceivable.
To those whose interest in such matters is literary rather than scientific, the progress of such a controversy is often very entertaining. It is true that the actual battles take place in places beyond our ken, generally at meetings of scientific societies, where the orators have it all their own way and confound their adversaries—till the opposition society meets. But though the philosophers retire for fighting purposes, and do battle in the clouds with weapons, phrases, and formulæ, that we cannot understand, they always come down again to earth to proclaim their victories or palliate their defeats. Once they come down, and we catch them with pens in their hands, the outsiders have their turn.
It is not, however, in the great books of Darwin, Huxley, Lyell, Helmholtz, Tait, or Thomson, that we may seek food for amusement. In these works every thought is in full dress and every phrase decorous. But there is another sort of literature in which we see the great men, so to speak, with their coats off. The "Proceedings" of the learned societies where the real fighting goes on are full of entertainment. Students of human nature need no further proof that, though every man may not be a philosopher, every philosopher is certainly a man. With what frank enjoyment they fight! With what irony—what sarcasm they annihilate their foes! It must, however, be confessed that sarcasm is not, as a rule, the strong point of the learned. The editor of a northern newspaper of our acquaintance was one day speaking in terms of praise of his sub-editor: "The brilliancy of yon young man," said he, "is surprising; the facility with which he jokes amazes me. I, myself, am in the habit of joking, but I joke with difficulty." We have observed the same peculiarity among other learned persons. They joke, but not with ease.
Most of the books which we have prefixed to this paper contain their authors' thoughts polished ad unguem. It would not be fair to judge of the opinions of the scientific persons we quote by any other standard than that which they have themselves carefully prepared; but yet we cannot refrain from entertaining a preference for the rough-and-ready, hard-hitting pamphlets, lectures, "proceedings," inaugural addresses, and the like, from which, almost without exception, these works have been compiled. For example, Mr. Croll's work on "Climate and Time" is everything which a scientific work should be that requires deep research and laborious thought, combined with the boldest generalization; but it is a digest of some five or six and thirty papers contributed to scientific magazines and periodicals during several years. Mr. Croll gives a list of his papers at the end of his volume. But though it is most convenient to see the whole before us at a glance, and to have them all under our hand or on the library-shelf, yet we acknowledge that while thinking over Mr. Croll's volume, for the purposes of this review, we found ourselves again and again going back to the pages of the Reader and the Philosophical Magazine, in which we first made acquaintance with them. It may be prejudice in favor of old acquaintances, but we liked them better before. Digressions, perhaps, are cut out; some little rash speculation quietly withdrawn; some hit at an opponent suppressed; but they do not always command the same ready assent, or appear so interesting as they did in their old form.
These remarks do not apply to Prof. Tait. His lectures now before us, from their nature, belong to the class of composition for which we avow our predilection. They were delivered extempore to a scientific audience, and printed from short-hand notes. They lose nothing of their vigor, to use an expression of Lord Macaulay, by translation out of English into Johnsonese. We are allowed to seize the thought in the making, and, if it loses anything in grace, the loss is more than counterbalanced by power.
Those who wish thoroughly to understand the subject of this paper should study Prof. Tait's lectures on theof energy, and the transformation of one sort of energy into another. Matthew Arnold's phrase, "let the mind play freely round" any set of facts of which you may become possessed, often recurs to the mind on reading these papers. There is a rugged strength about Prof. Tait's extempore addresses, which taken together with their encyclopedic range, and the grim humor in which the professor delights, makes them very fascinating. They have another advantage. Men not professionally scientific find themselves constantly at a loss how to keep up with the rapid advance which has characterized recent years. One has hardly mastered a theory when it becomes obsolete. But in Prof. Tait we have a reporter of the very newest and freshest additions to scientific thought in England and on the Continent, with the additional advantage of annotations and explanations by one of the most trustworthy guides of our time.
We propose to discuss the books and papers whose titles are prefixed to this article, in so far as they throw fresh light on the probable length of time during which the solar system may be supposed to have existed. It is but in recent times that any materials have been amassed for forming an opinion on the subject. Before the end of the last century geology hardly existed as a science; an inquiry as to the age of the world would have been unhesitatingly answered by the assertion that the earth was created in six days, 4,004 years before the birth of Christ. Though further research has shown that the sacred text bears no such interpretation, those copies of the "Authorized Version of the Bible" which are enriched with notes and marginal references still keep up the formal assertion.
A story is told in Brydone's "Tour in Sicily" which will serve to recall the state of public opinion on the subject of chronology at the end of the last century. The Canonico Recupero, a Sicilian priest, was Brydone's guide when he explored Mount Etna. Recupero (who afterward wrote a history of his native mountain) told the traveler that he had been vastly embarrassed by the discovery that many strata of lava, each covered deeply with earth, overlaid each other on the mountain-side. "Moses," said he, "hangs like a dead weight upon me, for I have not the conscience to make the mountain so young as that prophet makes the world." "The bishop," adds Brydone, "who is strenuously orthodox—for it is an excellent see—has warned him to be on his guard, and not to pretend to be a better historian than Moses."
The worthy Bishop of Catania was not alone in his views. Nearer home it was the generally-received opinion that to doubt the literal accuracy of the chronology supposed to be involved in the Mosaic account was a grave impiety. The poet Cowper, mildest of men, became fiercely satirical under the provocation of geology. Though few people read "The Task" nowadays, the lines will no doubt be remembered:
". . . . Some drill and bore
The solid earth, and from the strata there
Extract a register by which we learn
That He who made it, and revealed its date
To Moses, was mistaken in its age."
Fortunately, it is no longer considered impious to try and "extract a register" from the earth. Those who were inclined to be afraid that the Mosaic record would be discredited have long since laid aside their fears. It has been found that, far from being upset by scientific inquiry, the Bible account of the Creation accords in a very remarkable manner with modern discoveries; and long before Max Müller put the feeling into words, it was felt that only "by treating our own sacred books with neither more nor less mercy than the sacred books of other nations, they could retain their position and influence."
When once the plunge was made, it was soon found, as might have, been expected, that the fault was not in the oracle, but in the interpretation; and it is very remarkable in how many and unexpected directions the testimony of Moses has been strengthened by the criticism, not always friendly, which it has received. Of course, when the anciently-accepted date of the Creation was proved to be incorrect, and chronology was, as it were, thrown open to the public, there was nothing to prevent philosophers from allowing the freest scope to their imagination. In proportion as the six thousand years formerly assigned as the age of created matter was too small, the reaction of opinion claimed for it an antiquity which workers in other branches of physics feel it impossible to concede; and at the present moment there is among scientific men a revolt against the extreme views of the geologists. The latter affirmed with truth that creation in six solar days was demonstrably untrue, not because God could not create the world at a stroke, but because the world bears ample evidence that he did not so exercise his power. It was inconsistent alike with reasoning from probability or the investigation of facts. In all the operations of Nature as they unfolded themselves before our eyes God worked by law—by the process of slow development—by means beautifully simple, and involving no violence and no haste, yet irresistible. There was abundant evidence that these causes had been at work for thousands—perhaps millions—of years before the date of the supposed miracle. Beginning from the present age, the time was calculated that each development would require, till the united ages of all amounted to the enormous sum of three hundred millions of years.
Modern English geology holds that all geological changes have been effected by agents now in operation, and that those agents have been working silently at the same rate in all past time; that the great changes of the earth's crust were produced, not by great convulsions and cataclysms of Nature, but by the ordinary agencies of rain, snow, frost, ice, and chemical action. It teaches that the rocky face of our globe has been carved into hill and dale, and ultimately worn down to the sea-level, not only once or twice, but many times over during past ages; that the principal strata of the rocks—hundreds, and even thousands, of feet thick—have been formed on ocean-floor-beds by the slow decay of marine creatures and matter held in solution by the waves; that every part of the earth has been many times submerged, and has again been lifted into the air. This slow rising and sinking of the ground is an axiom of the geological creed. We are told that it is now going on, and that there are large areas of subsidence and of elevation on the surface of the globe. But when we consider the slow rate at which that oscillation is now proceeding, and argue back from the known to the unknown, we are landed in conclusions as to the length of time required for geological changes which the opponents of the theory declare to be absolutely inadmissible.
Sir William Thomson, Prof. Tait, and Mr. Croll, argue the question as one of geological dynamics. They find reason, in recent discoveries of science, to assert that the sun and the earth, from their physical condition, cannot possibly have existed for the enormous length of time supposed. Playfair, the founder of what is called the Uniformitarian school of geology, declares, on the other hand, that in the existing order of things there is no evidence either of a beginning or of an end. "In the planetary motions," he says, "where geometry has carried the eye so far both into the future and the past, we discover no mark either of the commencement or the termination of the present order. The author of Nature has not given laws to the universe, which, like the institutions of men, carry in themselves the elements of their own destruction." This was a bold assertion: it was adopted with very little limitation by Sir Charles Lyell and the later geologists—his disciples and contemporaries. Indeed, if they admitted any limitations at all, they placed the origin of the world so many hundreds of millions of years ago that the figures convey no practical idea to the mind, and amount in effect almost to what a distinguished geologist calls "eternity a parte ante."
The principal grounds upon which scientific opinion has recently declared itself in favor of limited periods for the duration of the solar system are based, first, on the belief that the earth is cooling—if not rapidly—at such a rate as to make it impossible that it should have existed, for very many millions of years; secondly, because there is reason to believe that the earth is not now rotating on her axis with the same rapidity as in former ages, and that, as her shape would have been different if, at the time she was in a molten state, she had been rotating more rapidly than now, she has not been rotating so long as has been supposed; thirdly, because the sun is parting with caloric at such a rate as to make it certain that he could not have continued to radiate heat at the same rate for more than a few millions of years; and lastly, because the changes in the earth's crust, stupendous and varied as they are, could have been, and probably were, accomplished in the course of much shorter periods than popular geology has hitherto considered possible.
It will, of course, be understood that any inquiry as to the date of creation must necessarily have relation only to the solar system—the sun, that is, and the planets which accompany the earth in its orbit round the central luminary.
The investigation is of necessity thus narrowed, because we have not, and cannot expect to have, any definite information as to the age of the rest of the visible universe. The stars are forever beyond our ken. If the spectroscope can bring intelligence of their component elements, it is as much as we can hope to attain; for their immeasurable distance effectually removes them from investigation. No action of gravity emanating from those distant luminaries affects the internal economy of the solar system. In the vast eternity of space the sun and his attendant satellites are altogether alone.
It is difficult to gaze upon the thousands of stars that brighten the night with their radiance, and yet realize our entire isolation. The solar system, with the radius of its orbit stretching from the sun to farthest Neptune, is but a point in a vast solitude. No star is nearer to us than 200 millions of millions of miles.
It is difficult, in dealing with such enormous numbers, to retain a definite impression on the mind. Our powers of conception are fitted rather to the wants of common life than to a complete survey of the universe.
Perhaps an intelligent may be substituted for a merely formal assent to these numbers, if they are considered on a greatly-diminished scale. Consider the figures on the scale of one mile to 100,000,000. On that scale the sun's distance from the earth will be represented by nearly one mile. Let the sun be represented by a globe on the top of St. Paul's Cathedral, and the earth by a little ball on the top of the clock-tower of the Houses of Parliament. The interior planets would revolve round St. Paul's as a centre; Mercury, at the distance of St. Clement's Church in the Strand; Venus, at the distance of St. Martin's Church, Trafalgar Square; Mars would be at Lambeth Bridge; Jupiter, at Walham Green; Saturn, in the middle of Richmond Park; Uranus, a little nearer the centre than Slough; Neptune, a couple of miles short of Reading. The outermost planet of the solar system, then, would on this scale revolve in an orbit comprising London and its neighborhood as far as Stevenage on the north, Chelmsford and Rochester on the east, and Horsham on the south.
On that same scale the nearest fixed star would be nearly as far away as the moon is in the actual heavens.
This inconceivable remoteness shows that the sun and his satellites lie apart in space. They form one whole, interdependent on each other, but completely removed, as regards their internal economy, from the influence of any attraction outside.
There are reasons for concluding that the system, thus organized and isolated, was brought into existence by one continuous act of creative energy, and that, however long the period over which the process may have been spread, the whole solar system forms part of one creation; and though it has been sometimes thought that the earth was made by itself, and that the sun was introduced from outside space, or created where he is at a different time, the evidence is strong against such a supposition.
In the first place, the orbits of all the planets are nearly in one plane, and describe very nearly concentric circles. If, when they received the original impulse which sent them revolving round the sun, any of them had been started with a little more original velocity, such planets would revolve in orbits more elongated. If, therefore, they had been the result of several distinct acts of creation, instead of being parts of one and the same act of creation, their orbits would probably have been so many ovals, narrow and wide in all degrees, and intersecting and interfering with each other in all directions. Yet if this want of harmony had existed, even to a small degree, it would have been sufficient to destroy the existing species of living creatures, and cause to disappear all security for the stability of the solar system. If the earth's orbit were much more eccentric than it is, all living creatures would die, for the extremes of heat and cold at different periods of the year would be fatal to life. If the orbit of Jupiter were as eccentric as that of Mercury, the attraction of the larger planet would cause the smaller to change their approximately circular orbits into very long ellipses; such would be the disturbance that they would fall into the sun or fly off into remote space. The moon would approach nearer and nearer to the earth with every revolution; the year would change its character; violent heat would succeed to violent cold; the planets would come nearer and nearer; we should see them portentous in size and aspect, glaring and disappearing at uncertain intervals; tides, like deluges, would sweep over whole continents; and, finally, the fall of the moon or one of the planets to the earth would result in the absolute annihilation of both of them.
Another reason for supposing that the solar system is the result of one separate act of creation is, that all parts of it are subject to one uniform law—that of gravitation. By that law every particle of matter attracts every other particle with a force directly proportionate to its mass. This force varies as the inverse square of the distance: that is, if the attractive force of a given mass at one mile were called 1, at two miles it would be 2 2 4, or ¼ of 1, and so on. This law of the inverse square, as it is called, is but the mathematical expression of a property which has been imposed upon matter by the Creator. It is no inherent quality, so far as we know. It is quite conceivable that the central law might have been different from what it is. There is no reason why the mathematical fact should be what it is except the will of the Being who imposed the law. Any other proportion could equally well be expressed mathematically, and its results calculated. As an instance of what would occur if any other proportion than the inverse square were substituted as the attractive force of gravity, suppose, at distances 1, 2, 3, the attractive force had varied as 1, 2, 3, instead of the squares of those numbers. Under such a law any number of planets might revolve in the most regular and orderly manner. But under this law the weight of bodies at the earth's surface would cease to exist; nothing would fall or weigh downward. The greater action of the distant sun and planets would exactly neutralize the attractive force of the earth. A ball thrown from the hand, however gently, would immediately become a satellite of the earth, and would for the future accompany its course, revolving about it in the space of one year. All terrestrial things would float about with no principle of coherence or stability—they would obey the general law of the system, but would acknowledge no particular relation to the earth. It is obvious that such a change would be subversive of the entire structure and economy of the world. From these and similar considerations, it follows that, although other laws are conceivable under which a solar system might exist, the solar system, such as we know it, could only exist under the actual laws which have been imposed upon its motions. And this seems entirely to exclude the idea that the various bodies of the system could have been created at different times or brought together from different parts of infinite space. We may then safely conclude that the solar system is absolutely isolated in space, and is collectively the result, of one act of creation. To the solar system, therefore, our inquiry is exclusively confined.
Although the received chronology of the world has for ages rested upon the supposed authority of the Bible, the sacred text really says nothing at all upon the subject. But, though the assertions which were so long made upon its supposed authority are not really contained in the Pentateuch, it is curious to observe how exactly the words of Moses appear to fit the most recent discoveries of science. No one has supposed that we were intended to learn science from the Bible; it is, therefore, an unexpected advantage to find that its short but pregnant sentences directly support the interpretation put by modern research upon the hieroglyphics of Nature. Moses teaches, just as modern science teaches, that the starry heavens existed far back in past duration, before the creation of the earth. He describes in majestic words the "emptiness" of chaos, and the condition of affairs from which light arose. He describes the formation of the sun, and its gradual condensation into a "light-holder" to give light upon the earth, in terms that almost seem to anticipate Herschel and Laplace. Far from assigning any date to the Creation, he is content to refer it to "former duration." No date is either mentioned or implied.
The so-called chronology was derived from two lists, one extending from Adam to Noah, the other from Noah to Abraham. These lists purport to give the direct line of descent from father to son, and the age of each individual member of the genealogy at the time when the next in succession was born. As Adam was supposed to have been created six days after the commencement of the Creation, it was simple work to add up the sum and fix the age of the world. As long as the progress of physical science showed no necessity for supposing a lengthened period to elapse between the creation of the world and the creation of man, it was taken for granted, almost without discussion, that when God had created the heavens and the earth in the beginning, he at once set about the work of arranging them for the use of man; that he distributed this work over six ordinary days, and at the close of the sixth day introduced our first parent on the scene.
Nowadays, all divines, English and foreign, agree that the word employed by Moses, and translated in our Bible by "the beginning," expresses duration or time previous to creation. Reshith, the Hebrew word for beginning, is in the original used without the definite article. The article was expressly omitted in order to exclude the application of the word to the order of creation, and to make it signify previous duration or previous eternity. The words of Moses, then, "In former duration God created the heavens and the earth," may mean millions of years just as easily as one. A few verses later, describing the second day of creation, Moses declares that God made the firmament and called it heaven. It is plain from this that the heavens of the first day's creation are different from the heavens of the second day; the difference of time proves a difference of subject. The heavens of the first verse were made in former duration, before the moving of the Spirit, before the creation of light; the heavens of the second day were made after the earth and after light.
Another statement made by Moses is an extraordinary anticipation of the most recent cosmological doctrines. "The earth was desolation and emptiness and darkness upon the face of the raging deep, and the Spirit of God brooding upon the face of the waters." It is now hardly doubtful that the earth was a molten sphere, over which hung, in a dense vapor, all the water which now lies upon its surface. As the crust cooled, the aqueous vapor that surrounded it became condensed into water and rested on the surface of the land. The conflicts between the waters and the fiery heat, as the crust of the earth was broken, fell in, or was upheaved, are well described by the words of Moses, "The earth was desolation and emptiness." It is curious that the great facts of the submersion of the earth and its condition of emptiness should have been thus exactly described by Moses.
We are then told that God said, "Let there be light, and there was light." Celsus, Voltaire, and a writer in "Essays and Reviews," have found it strange that' there should have been light before the creation of the sun; but, according to the theory of cosmogony now almost universally received, the earth did in fact exist before the condensation of the sun. Light there would be, from the gradually-condensing mass of nebulous and incandescent matter which occupied the whole space now circumscribed by the orbit of the earth. If Moses had wished to describe the modern doctrine concerning light, he could not have done so more happily. The sun is not called "ór," light, but Maór, a place of light, just what modern science has discovered it to be. If light be not matter, but vibrations of luminiferous ether, no words could more precisely explain what must have occurred when God set in motion the undulations which produced light, and said, "Let light be." The account given of the creation of the sun very closely anticipated modern science: "Let there be light-holders in the firmament of heaven, and let them be for lightholders in the firmament of heaven to give light upon the earth. . . and the stars." When the sun began to give his light, then, for the first time, the earth's fellow-planets, the stars, began to reflect his brilliance, and became luminaries also.
"Vestiges of Creation" was one of the first books which fairly awakened public interest in the debatable land which lies between that which is certainly known to science and that which must always defy inquiry. Before the appearance of that remarkable book, the theory that the sun and its attendant planets were produced by the condensation of a vast nebula was but little known to the unscientific world. The idea was originally entertained by Sir William Herschel, and affords one of the greatest proofs of his commanding genius. It was afterward elaborated by Laplace; but that great astronomer was himself distrustful of it, and, while he expounded the mechanical laws by which the proposed explanation could be supported, he was careful to speak of it only as an hypothesis. As time goes on, it seems probable that the saying of Arago will be accepted, and that the views of Laplace will be universally acknowledged to be "those only which, by their grandeur, their coherence, and their character, can be truly considered to form a physical cosmogony."
But, though Laplace is thus credited by Arago with the origination of this grand conception, he was not its author. Sir William Herschel gave the earliest sketch of the theory. His views w r ere expressed with so much precision, that one cannot help feeling a little jealousy for the prior right of discovery of the English astronomer. Herschel so plainly preceded Laplace, that it seems hard that Laplace should have the credit of it. Herschel began to search after nebulae in 1779, and soon formed a catalogue comprising an enormous number of them. By degrees it dawned upon his mind that the differences he observed in them were systematic, and at length occurred the magnificent intuition that the nebulæ are stars in process of formation.
They lie in enormous numbers in every part of the heavens, and apparently in every stage of progressive development. The slow growth of worlds, extending over ages of time, cannot, of course, be watched by any single observer. No more can a single tree among the trees of a forest be so observed. But a forest contains specimens of saplings, young trees, trees of vigorous growth, and trees in decay. In like manner the heavens contain specimens of worlds in the making, from the chaotic mass of vapory matter which forms the first stage of cosmical existence to the perfect, self-luminous star. Herschel arranged them in classes showing this gradual development, and he declares that each class is so nearly allied to the next, that they do not differ so much as would the annual description of a human figure, if it were given from the birth of a child till he comes to be a man in his prime. His catalogue arranges the objects he has actually observed somewhat in the following fashion: first, patches of extensive diffused nebulosity; "milky nebulosity," with condensation; round nebulæ; nebulæ with a nucleus; and soon till he reaches stellar nebulæ, nearly approaching the appearance of stars.
The evidence grows irresistible as we read, that in these wonderful objects we are gazing at works in process of formation as they lie plastic under the creative hand of the Almighty. Nor is it possible to withhold the inference—thus probably was the world we live in, and the solar system of which we form a part, evolved out of chaos.
The labors of Laplace commenced where Herschel ended. Herschel described what he saw. Laplace showed by mathematics how the known laws of gravitation could form, and probably did form, from such partially-condensed mass of matter an entire planetary system.
It is supposed that a film of vaporous matter filled up the space which is now bounded by the orbit of the outermost planet of our system. To the eye of an observer, if such there were, in a distant star such a vapor would appear like one of the numerous nebulæ which are everywhere visible in the heavens.
Laplace supposed that this nebula, extending beyond what is now the orbit of Neptune, possessed a rotatory motion round its centre of gravity, and that the parts of it which were situated at the limits where the centrifugal force exactly counterbalanced the attractive force of the central nucleus were abandoned by the central mass. Thus, as the nucleus became more and more dense under the action of gravity, were formed a succession of rings concentric with and revolving round the centre of gravity. Each ring would break up into masses which would be endued with motions of rotation, and would in consequence assume a spheroidal form. These masses formed the various planets, which, in their turn condensing, cast off in some instances their outlying rings, as the central mass had done, and thus formed the moons or satellites which accompany the planets. As each planet was in turn cast off, the central mass contracted itself within the orbit of that last formed, till, after casting off Mercury, it gathered with immense energy round its own centre and formed the sun.
Laplace's mechanical explanation does not rest only on theory. It has been experimentally shown that matter under certain conditions would exhibit phenomena similar in many important particulars to those which Laplace was led by mathematical considerations to suppose. Prof. Plateau, several years ago, tried the experiment of pouring olive-oil into alcohol and water, mixed in such proportions as exactly to equal the density of the oil. The oil thus became a liquid mass relieved from the operation of gravity, and free to take any exterior form which might be imposed by such forces as might be brought to bear upon it. The oil instantly took the form of a globe by virtue of molecular attraction. Prof. Plateau then introduced a wire into the globe of oil in such a manner as to form for it a vertical axis. The wire had on it a little disk coincident with the centre of the globular mass, and by turning the axis the oil was made to revolve. The sphere soon flattened at the poles and bulged out at the equator, thus producing on a small scale an effect which is admitted to have taken place in the planets. The experiment has since been several times repeated. When the rotation becomes very rapid, the figure becomes more oblately spheroidal, then hollows out above and below round the axis of rotation, stretches out horizontally until finally the outside layer of oil abandons the mass and becomes transformed into a perfectly regular ring. After a little while the ring of oil, losing its own motion, gathers itself once more into a sphere. As often as the experiment is repeated the ring thrown off immediately takes the globular form. These are seen to assume at the instant of their formation a movement of rotation upon themselves, which takes place in the same direction as that of the ring. Moreover, as the ring at the instant of its rupture had still a remainder of velocity, the spheres to which it has given birth tend to fly off at a tangent; but, as on the other side, the disk, turning in the alcoholic liquor, has impressed on the liquor a movement of rotation, the spheres are carried along and revolve for some time round the disk. Those which revolve at the same time upon themselves "present the curious spectacle of planets revolving at the same time on themselves and in their orbit." Another curious result is almost always exhibited in this experiment. Besides three or four large spheres into which the ring resolves itself, there are almost always two or three very small ones which may thus be compared to satellites. The experiment presents, therefore, an image in miniature of the formation of the planets, according to the hypothesis of Laplace, by the rupture of the cosmical rings attributable to the condensation of the solar atmosphere.
Modern discoveries carry the matter on much further. Recent investigations into the doctrine of the conservation of energy have shown the generation of cosmical heat. The amount of force comprised in the universe, like the amount of matter contained in it, is a fixed quantity, and to it nothing can either be added or taken away. It is, therefore, constantly undergoing change from one form to another. If it ceases in one form it is not destroyed, it is converted. The blow of a hammer on an anvil sets a certain amount of energy in motion. The anvil stops the blow, but the force changes into heat. Hammer a nail, and it will burn your fingers. Apply a brake to a wheel, and you will stop the motion, but the force will be changed, into heat, which will burn you if you touch the brake. Measure the hammered nail, and you find that it has expanded by the vibration of its particles; heat it still more, and the particles will overcome the attraction of cohesion and revolve about each other, that is, they will become molten; heat them still more, and they will assume the vaporous or gaseous form. Now, seeing that motion was convertible into heat, and heat into motion, it became an object of inquiry what was the exact relation between the two. Dr. Mayer, in Germany, and Dr. Joule, in England, set themselves to the solution of this problem. By various experiments it was demonstrated that, every form of motion being convertible into heat, the amount of heat generated by a given motion may be calculated. If the particles of a vast vaporous mass were brought into collision from the effect of their mutual attraction, intense heat would ensue. The amount of caloric generated by the arrest of the converging motion of a nebula like the solar system would be sufficient to fuse the whole into one mass and store up a reserve of solar heat for millions of years.
Such, then, is the most probable conjecture respecting the origin of our system. We now turn to consider the grounds on which attempts have been made to fix the probable date of its creation. It will be convenient to examine the views of modern geologists on the subject, and the objections, based on recent results of physical science, which natural philosophers have adduced against their speculations.
The great representative, in late years, of British geology, is the late Sir Charles Lyell. But a few months before his death he published the new edition of his "Principles of Geology," the title of which we have placed at the head of this paper. While he lived he bestowed upon the correction of his works unwearied labor. Edition after edition was called for, and in each whole passages—sometimes whole chapters—were remodeled. A quotation from one of the earlier editions may not improbably be searched for in vain in those which subsequently left his hands; and there are not wanting instances in which an opinion, contested by competent adversaries, was quietly dropped without any formal parade. His judgment was always open to appeal, and his clear and manly intellect acknowledged no finality in matters of opinion; therefore, on matters which we know to have been brought before him, with their accompanying evidence, we may consider ourselves as possessing his final verdict. It would not be fair when quoting, as we must do, comments unfavorable to some of the conclusions at which Sir Charles Lyell arrived, to refrain from acknowledging the care with which his opinions were formed, and the candor with which they were surrendered if ever his better judgment considered them untenable. For instance, as head of the Uniformitarian school, he was exceedingly anxious that the evidence for his favorite doctrine should be duly and impartially weighed. With this view he advocated, in his "Principles of Geology," "an earnest and patient endeavor to reconcile the indications of former change with the evidences of gradual mutations now in progress."
Upon this remark Dr. Whewell fell with merciless severity: "We know nothing," says he, "of causes; we only know effects. Why then should we make a merit of cramping our speculations by such assumptions? Whether the causes of change do act uniformly; whether they oscillate only within narrow limits; whether their intensity in former times was nearly the same as it is now: these are precisely the questions which we wish science to answer us impartially and truly. Where, then, is the wisdom of 'an earnest and patient endeavor' to secure an affirmative reply?"
This was rough handling of a pet theory, or, rather, of an argument in favor of a pet theory; but that Sir Charles Lyell felt its force is shown by the fact that no trace of the appeal attacked by Whewell appears in such later editions of the "Principles" as we have consulted.
As another instance of the same spirit, the following remark was made by Dr. Hooker, the President of the Royal Society, when addressing the British Association at Norwich. He was speaking of the progress made in public estimation by the theories of Mr. Darwin. "Sir Charles Lyell," he says, "having devoted whole chapters of the first edition of his 'Principles' to establishing the doctrine of special creations, abandons it in the tenth edition. I know no brighter example of heroism, of its kind, than this, of an author thus abandoning late in life a theory which he had for forty years regarded as one of the foundation-stones of a work that had given him the highest position attainable among contemporary scientific writers."
Among eminent persons holding the geological opinions to which the name of Catastrophism has been given, the name of the late Master of Trinity must occupy a foremost place. The words in which he avows his opinion are remarkable, not only for their exquisite beauty, but because they have a peculiar significance as almost the last utterance of a great man. The passage which follows occurs in the third of a series of sermons preached in the University Church at Cambridge, in 1827. But it is curious to learn, from his "Memoirs," published this year, that he again used the same words in his college chapel just before his death:
"Let us not deceive ourselves. Indefinite duration and gradual decay are not the destiny of this universe. It will not find its termination only in the imperceptible crumbling of its materials, or the clogging of its wheels. It steals not calmly and slowly to its end. No ages of long and deepening twilight shall gradually bring the last setting of the sun—no mountains sinking under the decrepitude of years, or weary rivers ceasing to rejoice in their courses, shall prepare men for the abolition of this earth. No placid euthanasia shall silently lead on the dissolution of the natural world. But the trumpet shall sound—the struggle shall come—this goodly frame of things shall be rent and crushed by the arm of its omnipotent Maker. It shall expire in the throes and agonies of some fierce convulsion; and the same hand which plucked the elements from the dark and troubled slumbers of chaos shall cast them into their tomb, pushing them aside that they may no longer stand between his face and the creatures whom he shall come to judge."
Holding these opinions, and believing as Prof. Whewell did that the upheavals and subsidence of strata which characterize the earth's crust were produced suddenly, and by violent agencies, the school to which he belonged were little likely to attempt to fix a date for the creation of the world. To their minds the facts of geology gave no evidence as to time. It is, therefore, to Sir Charles Lyell and his followers that we must turn for an estimate of duration drawn from the "testimony of the rocks."
It is impossible to deny that periods of very vast duration must have elapsed while the changes took place of which we see the traces. If, for instance, we search below the sand on English shores, we find, perhaps, a bed of earth with shells and bones; under that, a bed of peat; under that, one of blue silt; under that, a buried forest, with the trees upright and rooted; under that, another layer of blue silt, full of roots and vegetable fibre; perhaps, under that, again, another old land-surface, with trees again growing in it; and, under all, the main bottom clay of the district. In any place where bowlder clay crops out at the surface—in Cheshire or Lancashire, along Leith shore near Edinburgh, or along the coast of Scarborough—it will be found stuffed full of bits of different kinds of stone, the great majority of which have nothing to do with the rock on which the clay happens to lie, but have come from places many miles away. On examining the pebbles, they will prove to be rounded, scratched, and grooved, in such fashion as to show that at some period they have been subjected to a grinding force of immense violence. Among the pebbles in the clay, and on plains far away from mountains, are found great rocks of many tons in weight. They were carried on the backs of icebergs, which, at some time, covered the now temperate regions of the earth, and were dropped by the melting ice either in the shape of pebbles, as moraines of ancient glaciers, or as bowlders stranded when the icebergs melted in the lowlands.
Such evidence points to vast periods of more than arctic winter, which must have endured for many thousand years. But in close juxtaposition with these glacial shells and pebbles lie remains which tell of tropical climates that alternated with the dreary ages of ice. Fossil plants and the remains of animals prove that all Northern Europe was once warmer than it is now; that England bore the flora and fauna of the torrid zones. Underneath London there lies four or five hundred feet of clay. It is not ice clay; it belongs to a later geological formation, and was, in fact, the delta of a great tropical river. The shells in this clay are tropical—nautili, cones, fruits, and seeds of nipa palms, now found only at Indian river-mouths; anona-seeds, gourd-seeds, acacia fruits; the bones, too, of crocodiles and turtles; of large mammals allied to the Indian tapir, and the water-hog of the Cape. All this shows that there was once, where London stands, a tropical climate, and a tropic river running into the sea. We find in it the remains of animals which existed before the Ice age. The mammoth, or woolly elephant, the woolly rhinoceros, the cavelion, the cave-hear, the reindeer, and the musk-ox, inhabited Britain till the ice drove them south. When the climate became tolerable again, the mammoth and rhinoceros, the bison and the lion, reoccupied our lowlands; and the hippopotamus from Africa and Spain wandered over the plains where now the English Channel flows, and pastured side by side with animals which have long since retreated to Norway and Canada.
When the ages necessary for all these changes is allowed for, we have not, even yet, got beyond the latest period into which the history of the globe has been divided. Under the tertiary deposits lies the chalk, a thousand feet in thickness, which is composed of the shells of minute animals, which must have been deposited age after age at the bottom of a deep and still ocean, far out of reach of winds, tides, or currents. Recent dredgings in ocean-depths have proved beyond a doubt that the greater part of the Atlantic Sea floor is now being covered by a similar deposit. It must have taken ages to form, and, if the geologists are right in their estimate of the slow rate of upheaval, many more ages to become elevated above the ocean-bed where it lay. Not only once, but many times, the chalk was alternately above and beneath the waves. It is separated by comparatively thin and partial deposits of sand and clays, which show that it has been at many different points in succession a sea-shore cliff. The chalk is not flat, as it must have been at the sea-bottom; it is eaten out into holes by the erosion of the sea-waves, and upon it lie flints, beds of shore-shingle, beds of oysters lying as they grew, water-shells standing as they lived, and the remains of trees. Yet, again, there lie upon the chalk sands, such as those of Aldershot and Farnham, containing in their lower strata remains of tropical life, which disappeared as the climate became gradually colder and colder, and the age of ice once more set in. Everywhere about the Ascot Moors the sands have been ploughed by the shore-ice in winter, as they lay awash in the shallow sea, and over them is spread in many places a thin sheet of ice-borne gravel. All this happened between the date of the bowlder clay and that of the New Red Sandstone on which it rests.
We need not follow the geologist through the lower systems which overlie the metamorphic rock. The Oolite contains remains of plants and animals now extinct, the most remarkable being huge reptiles; the Triassic has fossils like the Oolite; and the Permian has remains like those in the coal on which it rests. Then follow the coal-measures, the fossil remnants of tropical vegetation; the Old Red Sandstone, with fossils principally of fishes and shells; the Silurian, in which are found the earliest forms of life; and, lastly, the hard and crystalline rocks, devoid of fossils, which are supposed to be the earliest constituent mass of our planet.
Sir Charles Lyell and his followers allege that the rate at which species of animals change is tolerably uniform. The fossils of one age differ but little from those of ages immediately preceding and following it. We must go back, he says, to a period when the marine shells differ as a whole from those now existing to form one complete period. Counting back in stages measured by changes of fossils, we have four such stages in the tertiary formations above the chalk.
Lyell saw reason to believe, on evidence which we shall presently examine, that the age of ice commenced about a million of years ago. The place of this age of ice among the series of fossil-changes is easily marked, and so he concludes that each of his four periods above the chalk "would lay claim to twenty millions of years." We must allow Sir Charles to work up to his stupendous conclusion in his own words:
"The antecedent Cretaceous, Jurassic, and Triassic formations would yield us three more epochs of equal importance to the three Tertiary periods before enumerated, and a fourth may be reckoned by including the Permian epoch with the gap which separates it from the Trias. In these eight periods we may add, continuing our retrospective survey, four more, namely, the Carboniferous, Devonian, Silurian, and Cambrian; so that we should have twelve in all, without reckoning the antecedent Laurentian formations which are older than the Cambrian. . . . If each, therefore, of the twelve periods represents twenty millions of years on the principles above explained, we should have a total of two hundred and forty millions for the entire series of years which have elapsed since the beginning of the Cambrian period."
Eighty millions since the lower tertiary formation, one hundred and sixty millions since the formation of the coal-measures, and two hundred and forty millions since the beginning of the Cambrian period! And beyond that inconceivable antiquity lie the whole range of the primary rocks which contain no fossils.
Mr Darwin assigns to the world even a greater age. "In all probability," he says, "a far longer period than three hundred millions of years has elapsed since the latter part of the secondary period." Other geologists exceed even this estimate. Mr. Jukes, for instance, after referring to this passage, in which Mr. Darwin has given an estimate of the length of time necessary for wearing down the space between the North and South Downs, declares it is just as likely that the time which actually elapsed since the first commencement of the erosion, till it was nearly as complete as it now is, was really a hundred times greater than his estimate, "or thirty thousand millions of years!"
To any one but a professed geologist, it would almost seem as if these ideas of geological periods had been framed on the principle which guided Mr. Montague Tigg in fixing the capital of the Anglo-Bengalee Disinterested Loan and Life-insurance Company. "What," asked the secretary, "will be the paid-up capital according to the next prospectus?" "A figure of two," says Mr. Tigg, "and as many naughts after it as the printer can get into the same line."
It is hard for imagination to compass the meaning of a million, and, when that number is multiplied by hundreds, the effort is altogether beyond us. But we need not dwell on this consideration; we turn at once to the practical comments made by physical science on these and such-like opinions. The first is founded on the secular cooling of the earth.
If a red-hot ball be taken from a furnace, it begins at once to part with heat at a certain definite rate. As it becomes colder it cools more and more slowly. From the known laws of heat it is quite possible roughly to approximate to the period during which the earth has been habitable for animals and plants such as we now find upon it. Whenever a body is hotter at one part than at another, the tendency of heat is to flow from the hotter body to the colder. As the earth's crust is warmer as we go farther down, there must be a steady increase of heat from the surface to the centre, and the earth is even now losing heat at a perfectly measurable rate; therefore it is possible to calculate what was the distribution of heat a hundred thousand or a thousand thousand years ago, supposing the present natural laws to have been then in existence. According to these data, about ten millions of years ago the surface of the earth had just consolidated, or was just about to consolidate; and in the course of comparatively few thousand years after that time the surface had become so moderately warm as to be fitted for the existence of life such as we know it. If we attempt to trace the state of affairs back for a hundred millions, instead of ten millions of years, we should find that the earth (if it then existed at all) must have been liquid, and at a high white heat, so as to be utterly incompatible with the existence of life of any kind with which we are acquainted.
The next argument, namely, that founded on the earth's retardation by the tidal wave, is more recondite, and the theory that there is such a retardation at all is quite of recent date. Theoretical reasons connected with mechanics caused it to be adopted, and its establishment depends on the most refined astronomical investigation.
It is one of the peculiarities of time-measurement that, from the nature of things, no two periods of time can be compared directly one with another. The standards by which we measure time are less and less precise as we recede farther into the past. To-day we have as the standard unit of duration the interval between two successive transits of a star over the cross-wires of a fixed observatory-telescope. This measure has been considered until lately as absolutely fixed and invariable. And so it is for all practical purposes; the sidereal time of any heavenly body passing the meridian on a given day in 1880 may be ascertained from the "Nautical Almanac" to-day, and it will be found true within one-hundredth of a second. But that throws no light on the question, What is the absolute length of an hour or a second? They are both definite fractions of a day; and a day is a revolution of the earth on its axis; no artificial measurement of such an interval can prove whether the interval itself remains from age to age unchanged. To quote Humboldt as a sure guide to the received opinions of scientific men thirty years ago, "The comparison of the secular inequalities in the moon's motion, with eclipses observed by Hipparchus, or during an interval of two thousand years, shows conclusively the length of the day has certainly not been diminished by one-hundredth part of a second."
The assertion is derived from Laplace, and even now is mentioned as an unquestioned fact in the most recent astronomical text-books. Halley, it is true, in 1695, discovered that the average velocity with which the moon revolves round the earth had apparently been increasing from year to year, and this acceleration remained unexplained during more than a century. Halley compared the records of the most ancient lunar eclipses of the Chaldean astronomers with those of modern times. He likewise compared both sets of observations with those of the Arabian astronomers of the eighth and ninth centuries. The result was an unexplained discrepancy, which set all theory at defiance for a century or more. It appeared that the moon's mean motion increases at the rate of eleven seconds in a century; and that quantity, small in itself, becomes considerable by accumulation during a succession of ages. In 2,500 years the moon is before her calculated place by l½°—enough to make a very material difference in place of visibility of a solar eclipse. Laplace at last, as Sir John Herschel says, stepped in to rescue physical astronomy from its reproach, by pointing out the real cause of the phenomenon. Laplace accounted for the apparent acceleration by showing that the motion of the earth in her orbit was disturbed by the other planets, in a manner before insufficiently appreciated, and the explanation was accepted for many years as complete and satisfactory. The acceleration was calculated to the utmost point of precision attainable in mathematics by MM. Damoiseau and Plana. Using the formulas of Laplace, and the numbers deduced from them, it was found that the circumstances and places of ancient eclipses, as recorded by historians, were brought into strict accordance with the times and circumstances as they ought to have been if the theory were true. Laplace's explanation rests upon the fact that for many thousands of years past the orbit of the earth has been tending more and more to a perfect circle—that is, the minor axis is increasing while the major axis remains unchanged. The result is, that the average distance of the moon from the sun is greater than it was in past ages. But in proportion as the moon is released from the sun's influence she revolves faster round the earth.
When it was seen how completely the difficulties in ancient observations were explained away by the calculations of Laplace, all doubt was considered to be at an end, and astronomers supposed that the whole truth was known. But, in 1853, it occurred to Prof. Adams to recalculate Laplace's investigations, and the result was the detection of a material error, which vitiated the whole series of observations. The results of Prof. Adams's calculations were submitted to the Royal Society in a paper, the explanatory part of which is very short indeed, occupying but a couple of pages of the "Proceedings." The brief statement is followed by a corroborative sea of high mathematics, into which we have no intention of asking the reader to plunge. The result, roughly stated, was to halve the amount of acceleration calculated by Laplace, and thus to leave half of the acceleration of the moon necessary for his explanation of ancient eclipses to be found in some other way. Astronomers were now in a condition almost as bad as that from which they had been rescued by Laplace.
Adams communicated his final result to M. Delaunay, one of the great French mathematicians; and it seems to have been during the investigations which that astronomer undertook to verify the calculations of Adams that it occurred to him to inquire whether our measure of time itself remains unchanged? in other words, whether the earth itself may not be rotating more slowly, instead of the moon more quickly, than in by-gone ages? It is plain that the moon will appear to be moving more quickly round the earth, if the earth itself—which is furnishing the standard by which the moon's revolution is to be measured—is rotating more and more slowly from age to age.
Newton laid it down in his first law of motion that motion unresisted remains uniform forever; and he gave as an instance of constant motion, unaffected by any external causes, this very rotation of the earth about its axis. But M. Delaunay remembered that Kant had pointed out the resistance which the earth must incur from the tide-wave, and had even approximately calculated its amount. The tidal wave is lifted up toward the moon, and on the side of the earth opposite the moon; so that, as Prof. Tait puts it, the earth has always to revolve within a friction-brake. Adams adopted this theory of tidal friction; and, in conjunction with Prof. Tait and Sir William Thomson, assigned twenty-two seconds per century as the error by which the earth would, in the course of a century, get behind a thoroughly-perfect clock (if such a machine were possible).
It may be asked, If the earth's movement be diminishing gradually in rapidity, will it eventually stop altogether? No; if ever the earth shall so far yield to the action of the tidal wave as to rotate not more rapidly than the moon, she will present to the moon always the same part of her surface. Then the liquid protuberance directed toward the moon will no longer be a cause of delay, and the retardation will cease. This cessation of effect, owing to the cause having ceased, appears to have actually happened with regard to the moon herself. At some time the moon's crust, and, indeed, her whole substance, was in a molten state. Enormous tides must have been produced by the attraction of the earth in this viscous mass of molten rock, and the time of the moon's rotation must have been quickly compelled, by the friction, to become identical with the time of its revolution round the earth, and now, as is well known, the moon always presents to the earth the same side of her sphere.
It being thus established that there is retardation of the earth's motion, and the amount of retardation being calculated, it remains only to inquire how the fact affects the question of the world's age. We know that the flattening at the poles and bulging at the equator is the result of rotation; from the amount of retardation it can be calculated how fast the earth was rotating in by-gone ages. Two thousand millions of years ago she would, according to such calculation, have been revolving twice as fast as at present, and the amount of centrifugal force at the equator would have been four times as great as now. If the earth, subjected to such strong centrifugal force, had been liquid or even pasty, when it began to rotate, the equatorial protuberance would have been much greater than it is. It therefore follows that she was rotating at about the same rapidity as now, when she became solid, and as the rate of rotation is certainly diminishing, the epoch of solidification cannot be more than ten or twelve millions of years ago.
A third argument for restricted periods is founded on an examination of the question, How long can the sun be supposed to have kept the earth, by its radiation, in a state fit to support animal and vegetable life? Here, as might be expected, a wider range of opinion exists.
It will be conceded at once that the age of organic life upon the earth must, of necessity, be more recent than the age of the sun. The several theories as to the way in which the sun may have derived his heat may be put aside in favor of that of Helmholtz, viz., that the sun has been condensed from a nebulous mass, filling at least the entire space at present occupied by the whole solar system. The gravitation theory of Helmholtz is now generally admitted to be the only conceivable source of the sun's heat. The opinion that it can be obtained from combustion is not tenable for a moment. The amount of heat radiated is so enormous that, if the sun were a mass of burning coal, it would all be consumed bodily in 5,000 years! On the other hand, a pound of coal falling on the surface of the sun from an infinite distance would produce 6,000 times more heat from concussion than it would generate by its combustion. An idea of the amount of energy exerted by one pound weight falling into the sun will be conveyed by stating that it would be sufficient to hurl the Warrior, with all its stores, guns, and ammunition, over the top of Ben Nevis! But, if we accept gravitation as the source of energy, we accept a cause, the value of which can be mathematically determined with very considerable accuracy.
The amount of heat given off by radiation in a year is known; the total amount of work performed by gravitation in condensing a nebulous mass to an orb of the sun's present size is known. The result is, that the amount of heat thus produced by gravitation would suffice for about twenty millions and a quarter of years. This is on the assumption that the nebulous matter composing the sun was originally cold, and that heat was generated in it by the process of condensation only. It is, however, quite conceivable that the nebulous mass possessed a store of heat previous to condensation, and that the very reason why it existed in the gaseous condition was that its temperature was excessive. The particles composing it would have had a tendency, in virtue of gravitation, to approach one another if they had not been kept apart by the repulsive energy of heat; it is not, then, unreasonable to suppose that the attenuated and rarefied mass was vaporous by reason of heat, and began to condense only when its particles began to cool. By the known laws under which heated gases condense, the amount of heat originally possessed by the gas bears a definite and known proportion to the amount of heat generated by condensation; and, on the assumption that the analogy holds good in the case of the sun, which holds in the condensation of other heated gases, nearly fifty millions of years' heat must have been stored up in the mass as original temperature. This, added to the twenty and a quarter millions which resulted from gravitation, gives rather more than seventy millions of years' sun-heat.
As, however, this quantity gives the total amount of heat given out by the mass since it began to condense, the earth could not have had an independent existence till long after that time. The sun must have had time to condense from its outer limits as a nebula, to within the limit of the earth's orbit, before that separate existence could begin; for before then the earth must have formed part of the fiery mass of the sun. This calculation, like the others, falls short by nearly two hundred millions of years of the period estimated by Sir Charles Lyell for the commencement of life upon the earth.
But it would not be satisfactory to see a theory upset, if with the theory the means of accounting for observed facts were also destroyed. One great reason which weighs with geologists in assigning an almost incalculable age to the earth is, that among the fossils of the latest glacial epoch there are found the remains of tropical plants and animals, deposited in alternate strata with the remains of temperate climates, and this not once, but many times over. A hot climate prevailed at one time, and the earth became peopled with the flora and fauna appropriate to those conditions: after a lapse of many ages, the land subsided, and became the bed of the ocean; a vast period of upheaval then ensued, and dry land once more appeared: the climate gradually changed and ice set in: after ages more there was another slow subsidence, another equally slow upheaval, and another change of climate; and so on without end. Seeing the slow way in which the land sinks or is upheaved nowadays, it naturally appeared that no conceivable lapse of time could be enough to explain that which had obviously taken place.
Mr. Croll, however, has recently afforded an explanation at once beautiful, simple, and complete. About the facts to be accounted for there can be no doubt. The land has been many times under the sea, and the most violent changes of climate have succeeded one another. Mr. Croll's explanation is partly astronomical, and partly rests on geological dynamics. The beat of the sun is great in proportion to his distance from the earth. This distance is greater at one time of the year than another. The orbit of the earth is not quite circular, but its eccentricity Varies slowly from century to century. It is just now very small, and the summer of the northern hemisphere happens when the earth is at its greatest distance from the sun. Both these circumstances tend to produce in Europe a moderate climate. But the longitude of the perihelion, as this state of things is called, is constantly changing, and the line joining the solstices moves round the orbit in about twenty-one thousand years. It follows that every ten thousand years, or thereabouts, the winter of the northern hemisphere will occur when the earth is at its farthest from the sun; and, if at that time the earth's orbit is very eccentric, the two causes combined will produce a very severe climate. Eleven thousand years hence the northern hemisphere will be nearest to the sun in summer, and farthest from him in winter. Now, if, when that state of things occurred, the eccentricity of the earth's orbit happened to be very great—if the earth in winter-time was at a part of her orbit several millions of miles farther from, and in summer-time was very much nearer, the sun than she is now, the climate of the northern hemisphere would be very different from what it is.
One such period of great eccentricity occurred about two million five hundred thousand years ago. Fifty thousand years later there was another. Again, eight hundred and fifty thousand years ago there was a third, and two hundred thousand years ago a fourth. Those periods were characterized by cold such as we have no conception of. More than arctic winter lingered far on into the spring, and unmelted ice of one year accumulated through the next, till from the pole to the south of Scotland the earth was covered with a vast icecap, probably several miles in thickness.
Now, in Europe and America, wherever in fact any records are left of the glacial epoch, it is remarked that a general subsidence of the land followed closely on the appearance of the ice. This fact led certain geologists to conclude that there was some physical connection between the two phenomena, and Mr. Jamieson suggested to the Geological Society that the crust of the earth might have yielded under the enormous weight of the ice. Mr. Croll, however, gives a different explanation; and the more it is understood the more it appears to gain ground with those capable of forming an opinion. He says that the surface of the ocean always adjusts itself in relation to the earth's centre of gravity, no matter what the form of the earth happens to be. If a large portion of the water of the ocean were formed into solid ice, and placed round the north pole, its weight would naturally change the centre of gravity of the earth. The centre would be changed a little to the north of its former position. The water of the ocean would then forsake its old centre, and adjust itself with reference to the new. The surface of the ocean will therefore rise toward the north pole, and fall toward the south. The land will not sink under the sea, but, what amounts to the same thing, the sea will rise upon the land. The extent of submergence will be in proportion to the weight of the ice.
It is easy to see that glaciation would not be contemporaneous on both hemispheres. One hemisphere would be covered with ice and snow, while the other would be enjoying a perpetual spring. A glacial epoch resulting from the eccentricity of the earth's orbit would extend over a period of a hundred thousand years. But, for the reason given above, the glaciation would be transferred from one hemisphere to another every ten thousand years. A glacial epoch extending over a hundred thousand years would therefore be broken up into several warm periods. The warm period in one hemisphere would coincide with the cold one in the other, and there would be elevation of the land during the warm period and subsidence during the cold.
This cause would be quite sufficient to effect the alternate upheaval and depression. During the successive ages that each pole alternately was subjected to glaciation, the winter ice, unmelted by the brief summer, would accumulate till a cap many thousand feet thick formed at the pole, and would ultimately spread far down into what is now the temperate zone. If such an ice-cap were only equal in density to 1,000 feet of earth, accumulated, say, on the north side of the globe, the centre of gravity would be shifted 500 feet to the north; and as the ocean would accommodate itself to the centre there would be a subsidence at the north pole equal to 500 feet. But this is not all, for at the time the ice-sheet was forming on the northern hemisphere, a sheet of equal size would be melting on the southern. This would double the effect, and produce a total submergence of 1,000 feet at the north pole and a total elevation of 1,000 feet at the south pole.
It is clear that all the upheavals and submergences of land which have so impressed geologists with the immensity of time required for their execution can thus be accounted for within periods, stupendous indeed if compared to historical time, or even to the duration of man on the earth, but still conceivable by human imagination. The nightmare of subsidence and emergence need no longer oppress the geologist. He has only to remark surface-changes and see how far forces now at work are capable of effecting them, and, if so, how long they would take. The discovery of Mr. Croll upsets the whole scale of geological time. Sir Charles Lyell was quite right in saying that the earth could not have subsided and emerged from the sea half a dozen times, in less than a million of years, if it sank or rose in the leisurely manner which has characterized it in recent times: consequently he could not accept as "the glacial epoch" the most recent period of great eccentricity. He was obliged to go back to the next, which happened nearly a million years ago. Sir Charles Lyell's standard of measurement is the date of the age of ice. If, therefore, the age of ice is assigned to a period 200,000 years ago instead of a million years ago, the standard of Sir Charles Lyell is diminished by four-fifths; and, adapting his conclusions to the altered premises, we should have forty-eight millions of years instead of two hundred and forty millions for the age of the fossiliferous rocks.
This change of standard would agree very well with the fact that there are evidences in the Eocene and Miocene periods of ice ages antecedent to the last. These might well be referred to the former periods of high eccentricity.
Enormous as are the periods which have undoubtedly passed since the creation of the world, it need not startle us to be told that every succession of events of which we have any evidence may well have occurred within a manageable number of millions of years. Could we stand, as Mr. Croll says, upon the edge of a gorge a mile and a half in depth, that had been cut out of the solid rock by a tiny stream scarcely visible at the bottom of this fearful abyss, and were we informed that the little streamlet was able in one year to wear off only one-tenth of an inch of its rocky bed, what would be our conception of the prodigious length of time that it must have taken to excavate the gorge? We should certainly feel startled when on making the necessary calculations we found that the stream had performed this enormous amount of work in something less than a million years.
The absolute settlement of the question must ever be above our powers. For a few centuries only we have the comparative daylight of historical times; thence backward lies the rapidly-gathering twilight of tradition; beyond that, geological periods the duration of which can be only vaguely guessed at, and beyond all these, far back in past eternity, the epoch when Time began. The old belief, which limited the existence of the earth to less than seven thousand years, gave way once for all, almost within living memory. All men are now agreed that the six days of creation were periods of indefinite extent. They are not solar days—for evening happened and morning happened three times over before the sun was created. Not being days measured by the sun, we know not how many thousands of years they may have endured. The reaction was sudden and complete. Geology jumped to the conclusion that the past history of the world was without any limits that human imagination could conceive. But in quite recent years, as we have tried to show, the calm light of science has proved that the practical eternity of matter is not more tenable than the arbitrary limitation by which thought was formerly confined.
"I dare say," says Prof. Tait, "that many of you are acquainted with the speculations of Lyell and others, especially of Darwin, who tell us that even for a comparatively brief portion of recent geological history three hundred millions of years will not suffice! We say—so much the worse for geology as at present understood by its chief authorities, for.... physical considerations render it impossible that more than ten or fifteen millions of years can be granted."
Sir William Thomson is not so sweeping in his assertion: but then the nature of the problem before him did not require any such opinion at his hands. His argument aimed at disproving Playfair's assertion that neither the heavenly bodies nor the earth offered any evidence of a beginning, or any advance toward an end. If, therefore, Sir William Thomson was able to show that there was good evidence both of a beginning and an end, he was not concerned to speculate how long past time had existed, or when the end would come. His summing up is this:
"We must admit some limit. . . . Dynamical theory of the sun's heat renders it almost impossible that the earth's surface has been illuminated by the sun many times ten million years. And when finally we consider underground temperature we find ourselves driven to the conclusion that the existing state of things on the earth, life on the earth, and all geological history showing continuity of life, must be limited within some such period of past time as one hundred million years."
We have passed in rapid review the evidence upon which guesses, more or less plausible, as to the age of the world, have been founded. Whatever may be the opinion at which men will ultimately arrive, it cannot but be satisfactory to note from how many quarters and in how many ways Natural Science has in latter days cast light on the inquirer's path.—Quarterly Review.
- "Lectures on some Recent Advances in Physical Science." By Prof. P. G. Tait, Professor of Natural Philosophy in the University of Edinburgh. 1876.
- "On Geological Dynamics." By Sir William Thomson, LL.D., F.R.S. "Transactions of the Geological Society of Glasgow," 1869.
- "On Geological Time." By Sir William Thomson, LL.D. "Transactions of the Geological Society of Glasgow." 1868.
- "Sur le Ralentissement du Mouvement de Rotation de la Terre." Par M. Delaunay. Paris, 1866.
- "Climate and Time." By James Croll. "H. M. Geological Survey of Scotland." London, 1875.
- "Principles of Geology." By Sir Charles Lyell. Fourteenth edition. London, 1875.
- On the scale of 1 mile to 100,000,000 miles:
Miles. Miles. Mercury would be distant from the sun 0.35 Saturn 8.71 Venus 0.66 Uranus 17.52 The earth 0.91 Neptune 27.43 Mars 1.39 And a Centauri, the nearest fixed star 206,560.00 Jupiter 4.75
- Lyell, b. iv., p. 328, fourth edition.
- "History of the Inductive Sciences," b. viii., sec. 2, edition of 1857.
- "Sermons in the University Church at Cambridge, 18th February, 1827."
- "Origin of Species," edition of 1859, p. 287.
- "The 'Doctrine of Uniformity' in Geology briefly refuted." "Proceedings of the Royal Society, Edinburgh, December, 1865."
- "Cosmos," i., 161.
- June 16, 1853.
- To maintain the present rate of radiation it would require the combustion of 1,500 pounds of coal on every square foot of the sun's surface per hour.—Croll, 346.
- The velocity with which a body falling from an infinite distance would reach the sun would be equal to that which would be generated by a constant force equal to the weight of the body at the sun's surface operating through a space equal to the sun's radius. One pound would at the sun's surface weigh about twenty-eight pounds. Taking the sun's radius at 441,000 miles, the energy of a pound of matter falling into the sun from infinite space would equal that of a 28-pound weight descending upon the earth from an elevation of 441,000 miles, supposing the force of gravity to be as great at that elevation as it is at the earth's surface. It would amount to upward of 65,000,000,000 foot-pounds.
- The total amount radiated from the whole surface of the sun per annum is 8,340 x 1030 foot-pounds.—Croll, 346.