It is therefore probable that for the last 96,000,000 years the rate of increase of temperature under ground has gradually diminished from 1/10 to about 1/50 of a degree Fahrenheit per foot, and that the thickness of the crust through which any stated degree of cooling has been experienced has gradually increased during that period from 1/5 of its present thickness to what it now really is."[150]
We freely admit our inability to sit in judgment on the validity of Sir William Thomson's conclusions. There are eminent geologists who entertain the opinion that the secular cooling of the earth has proceeded with much greater rapidity. It is, however, sufficient for our purpose that the most distinguished physicists of the day are agreed in teaching that the existing terrestrial economy had a beginning.
There are other terrestrial changes which engage the attention of the geologist, and which force upon him the conclusion that the existing terrestrial order had a beginning and must have an end. The surface of the earth has at intervals undergone great changes in the disposition of its land and water. That which is now dry land was once the ocean-bed, and the ocean waves now roll and murmur over what was once dry land. Sudden, or comparatively sudden, catastrophes have extinguished the then existing creations, and the earth has been repeopled by new orders of life. Changes are now in progress which are gradually reducing the populous regions of the earth to the condition of the Sahara of Africa and the Desert of Arabia. Upper and Lower Mesopotamia, the seat of the ancient monarchies of Chaldæa, Assyria, and Babylonia, now present "vast tracts of arid plain—yellow, parched, and sapless—which have now become a bare and uninhabited desert." That ancient continent drained by the Colorado, once as fertile as the Valley of the Mississippi, is now the Great American Desert. "Every freshet burdens the streams with a load of sediment; and the Mississippi bears daily to the Gulf material sufficient for a cotton plantation. From the slopes of the Alleghanies and the Rocky Mountains, from the broad acres over which the Mississippi and the Ohio reach their silver fingers to filch from the land, the sediments are stolen and carried away to the sea. The Western States are slowly traveling toward the Gulf. The hills are melting, and even the mountain cliffs are lowering under the ceaseless conflict with storm and frost. The summits of the Alleghanies have come down 3000 feet from their original altitudes. Give time enough, and the inequalities of the land will disappear. The ocean will be filled, and again assert a triumph over the continents which in the beginning were wrested from his dominion." Thus by the storms of heaven, the erosion of the atmosphere, the blasting power of frost, the gnawing of the tidal wave, the mountains are being leveled, and the rocks and soils carried onward by the rivers to fill up the basin of the sea. The headlong rush of the avalanche, the murmuring of the brook, the roaring of the sea, the voice of the storm—all proclaim, "The things which are seen are temporal!"—"The existing order of things had a beginning and must come to an end!"[151]
Astronomical Palætiology reduces all celestial phenomena to the same great law of finite duration. It teaches that planets, stars, systems, have their birth, their process of formation, their maturity, and their slow, protracted decay. The ephemeron perishes in an hour, man endures his three-score years and ten; continents and islands have their ages and æons; the stars of heaven are not exempt from this universal law of change and decay. According to the Nebular Hypothesis, the formation of this our system of sun, planets, and satellites was a process of the same kind as that which is still going forward in the heavens. One after another, nebulæ condense into separate masses, which begin to revolve about each other in obedience to dynamical laws, and form systems of which our system is a matured example. The present aspect of this planetary system is, however, but a passing phase in the history of its fleeting life. Our planet was once a self-luminous orb; it has now become opaque, and shines only with a borrowed light. The moon is probably in a state of total refrigeration; its lunar air and lunar seas have been changed by intensity of cold into the solid form.[152] The sun itself is radiating heat into space in quantities incomparably greater than it receives, and, as Helmholtz affirms, "the inexorable laws of mechanics show that its store of heat must be finally exhausted."[153] The planets in their motions encounter resistance from the interstellar ether; they must, therefore, necessarily move in shorter and shorter orbits, and at last fall into the sun. Thus the Nebular Hypothesis, combined with the doctrine of a resisting medium, teaches us that the solar system is wending its way, through successive changes, from a past of vaporous unity to a future of consolidated reunion. "It was once all nebula; it will, if left to physical agencies alone, collapse into an extinguished and exhausted sun."
The astronomer who has been accustomed to regard every question relating to his favorite science as almost exclusively a problem in mathematics, will pronounce the above "a crude and adventurous" attempt on the part of the physicist to solve a problem which belongs to "the calculus of variations." Is the universe a Conservative or a Dissipative system? Under its present laws will it run on forever, or will these very laws in the end lead to its subversion? Will the mechanism of the heavens finally run down as surely as the weights of a clock run down to their lowest position, or are we authorized on scientific grounds to assert the permanent stability of the solar system? This question has been earnestly discussed by the most distinguished astronomers since the days of Newton. Until recently, the general conclusion—reached mainly on mathematical grounds—seems to have been that the universe is a thoroughly conservative system, and that the celestial machinery by a species of perpetual motion will run on forever. But must not all applied mathematical reasoning obtain its data from the exact observation of material facts? The mathematician must also be a good natural philosopher; he must lay his account with all the facts of the universe, otherwise his symbols have no contents, and his reasoning, however faultless in its processes, will be fallacious in its results. The discoveries of the present century respecting the correlation of the various forms of energy, the nature of the solar light and heat, the motions of comets, and especially the new doctrine of the "Dissipation of Energy," have introduced new elements into the great problem, which seem to indicate that gravitation is by no means the only force by which the motions of the heavenly bodies are influenced, and that causes are now in operation which are slowly but surely undermining the system. We now find, therefore, such high authorities as Whewell, Sir John Herschel, Sir William Thomson, Balfour Stewart, Prof. Maxwell, Dr. J. R. Mayer, Helmholtz, Tyndall, Littrow, Comte, Adolph Fick, asserting that the solar system is not a self-winding clock which may run forever, but that it is a dissipative system which must ultimately lose all motion, unless some power capable of controlling the laws of material nature interfere to preserve it. We have no more valid reason for concluding that the Deity intended the system should be eternal than that He intended the earthly life of man should be eternal.[154] A few general statements may assist the reader in appreciating the merits of the discussion.
It has been observed since the dawn of science that changes are taking place in the motions of the heavenly bodies. The eccentricity of the earth's orbit has been gradually diminishing from the earliest observations to the present time. The moon, also, has been moving faster and faster from the time of the first recorded eclipses, and is now in advance by about four times her own breadth of what her place would have been had she not been affected by these accelerations.[155] In a few thousand years she will be half a month ahead of the place she would be in if her month were to remain constant. The moon is, therefore, approaching closer and closer to the earth; and if these changes go on uninterruptedly, without any reaction or adjustment, sooner or later the final catastrophe must come, and the moon be precipitated on the body of the earth.
Toward the close of the last century, Laplace, in his great work, the "Méchanique Céleste," attempted by certain mathematical computations to show that, nevertheless, the solar system is stable and permanent. The planets, by their mutual attractions, produce perpetual perturbations in one another's movements. Laplace believed he could prove that these were periodic; they reach a maximum value and then diminish, oscillating between very narrow extremes. He therefore taught that the machine would go on by a kind of perpetual motion, without any winding up or adjustment from without; and, consequently, the eternal continuance of the solar system is insured.
All the investigations of Laplace, and the computations of Lagrange, proceeded on two assumptions: first, that the planets are moving in vacuo; and, secondly, that they are solid throughout their entire mass. The latter assumption is certainly in conflict with well-determined geological facts; and there is no à priori ground for assuming that the planetary spaces are void and empty. On the contrary, the general analogies of nature would lead us to the very opposite conclusion, and all attempts at producing a perfect vacuum have hitherto failed. Furthermore, the great body of modern physicists, and nearly all modern astronomers, hold that the celestial spaces are filled with a "material ether," which must by its very nature offer some resistance to planetary motion.
"Scientific men," says Mayer, "do not doubt the existence of such an ether." The presence of such "material ether—dense, elastic, and capable of motion—subject to and determined by mechanical laws,"[156] is demanded for the explanation of radiant heat, light, and actinism. No other theory ever proposed has so beautifully and completely accounted for all the facts. Its reality must be admitted, until the positions established by Huyghens, Young, Fresnel, Foucault, and Fiziau are shown to be untenable. All the prominent experimental physicists of the present day agree in teaching that light and heat are transmitted by vibrations or wave-like motions in a material medium universally diffused through space, and permeating all material bodies. Light and heat are the ceaseless thrill which the distant orbs collectively create in the ether, and which constitute what has been called the temperature of space. If the existence of such material medium as the assumed ether be denied, we can not account in any conceivable or rational manner for the transmission of light and heat from the sun. And now, if the space between the celestial bodies contain no other matter than that necessary for the transmission of light, "that alone," says Littrow, "is sufficient, in the course of time, to alter the motion of the planets, and the arrangements of the solar system itself; the fall of all the planets and comets into the sun, and the destruction of the present state of the solar system, must be the final result of this action."[157]
But it is further claimed by Helmholtz, Mayer, and Sir William Thomson that the phenomena presented by Encke's comet furnish "direct proof" of the existence of such resisting medium. The observations on this comet made during the past thirty or forty years show that the periods of its revolution are continually diminishing at the rate of 0.11° per revolution of nearly 3-1/3 years. In other words, the comet's mean distance from the sun is diminishing by slow and regular degrees. The solution which Encke himself proposed, and which Herschel informs us "is generally received,"[158] is that resistance is experienced from the medium in which the comet moves; such resistance diminishing its actual velocity and also its centrifugal force, thus giving the sun greater power to draw it nearer. It will, therefore, fall into the sun. A similar fate, says Helmholtz, threatens all the planets. "The analogies of nature, and the ascertained facts of physical science, forbid us to doubt that every star, and, indeed, every body of every kind moving in any part of space, has its relative motion impeded by the air, gas, vapor, medium, or whatever we call the substance occupying space immediately around them, just as the motion of a rifle-bullet is impeded by the resistance of the air."[159]