Page images
PDF
EPUB

Mechanism of Worlds.

87

for ever, would cause an infinite space, void of everything except the moving body; or if centres of equivalent energy were placed at equal distances, they would remain in equilibrium for ever. How, then, are the vast curves of the planetary bodies obtained? To represent it mentally, fill an apparent vacuity with an ethereal medium-a species of matter-countless lines radiate from the centre to every side, and along every line this medium presents resistance, so that the exact line on which matter sets out, drawn by attraction to the centre, cannot be continued, but becomes a curve; and a curve the more complex in proportion as the energies are more numerous and varied. Apply this to nebular condensation, and to precipitation of diffused matter into flocculi of denser matter. As the matter moves by gravitational energy, the direction would be, first of all, in a straight line; but the direction being continually influenced by surrounding bodies, themselves in relative movement, rotation and revolution would then be set up, and conditions analogous to those of the sun and solar system be established. The sun, our earth, the other planets, had their own concentrations, say of nebulous ring, gaseous spheroid, liquid spheroid, and spheroid externally solidified. If we regard this energy as merely mechanical, it possesses nothing directive; no more produces a planet than a poem ; neither explains the energy of gravity which brings all bodies together, nor that of repulsion which tears them asunder. The pressing and pushing about of matter are not the equivalents of all phenomena; but, as magnetism, heat, light, are held to be different modes of some one universal energy, we regard the attraction and repulsion of matter as manifestations of a mysterious Power,-a Power which, Herbert Spencer says, "transcends intuition and is beyond imagination."

Continue the mechanical investigation:1. The Earth-The form of the earth is a spheroid or ellipsoid. It is thus accounted for. A detached fluid mass, if at rest in space, would assume, by gravitation of its particles, the form of a sphere. When it began to rotate on its axis it would become flattened at the poles, and bulge at the equator. This bulge is now about 1 in 300, or something like

thirteen miles of extra matter all round. When separated and aggregated from the nebula, out of which was produced the solar system, our earth probably exceeded 400,000 miles in diameter, and the mass took about 29 days to rotate. The earth's oblateness is proof that it was modelled when in a yielding or plastic condition, before the crust had been formed, when an incandescent globe rolling through space, with all the water and other vapourisable matter in a gaseous state. Sir Isaac Newton calculated that “a comet formed of iron would absorb so much heat in its near approach to the sun as to require 5000 years to cool." The time required for the cooling of the earth from its molten condition to a habitable state would, according to Bischof's experiments on basalt, be 350,000,000 years. The computations vary from about 100,000,000 to about 1,000,000,000 years. The waters having attained a state fit for the support of life, the vital period began, and it is calculated from the rise of temperature (taken over the whole earth's surface), at an average of one degree for 100 feet of descent, that about 10,000,000 years ago the surface of the earth had just consolidated, or was about to consolidate, and that, in some thousands of years after that, the heat "would not interfere very greatly with the growth of plants; so from this point of view we are led to a limit of something like 10,000,000 years as the utmost we can give to geologists for their speculations as to the history even of their lowest order of fossils." 1 Some able men assert-" The physical data of this argument are unreliable. It is quite possible that no sensible temperature from within has been felt in the sea and in the soils since the commencement of the laying down of the lowest fossiliferous rocks."

2. The Planetary System.-Not only are the planets shaped like the earth, they move in the same direction round the sun, and in nearly the same plane, with rotation in the same direction as their orbital motion. The orbits, both of planets and satellites, nearly coincide, and differ but little from circles. These peculiarities are considered to be remaining indications of a previous state, in which the whole planetary system formed one connected mass, with uniform rotatory motion,

"Recent Advances in Physical Science," p. 167: P. G. Tait, M. A.

The Connected Mass.

89

and support the hypothesis that the star system was formed out of a nebulous mass by the mutual attractions of its parts: every one, star and sun, in his own order, receiving definite form, size, function, to become a dwelling-place for life; or, in some other way equally good, to show forth the glory of God. Our own system, according to Sir William Herschel, might "be said to be one that has fewer marks of profound antiquity on it than the rest." He stated in the same paper, 1785" Our system, after numbers of ages, may very possibly become divided, so as to give rise to a stratum of two or three hundred nebulæ, for it would not be difficult to point to so many beginnings or gathering clusters in it."

3. The Connected Mass.-The nebulous mass, out of which our system was formed, not only filled all the space now occupied by the system, but extended vastly further than the limits of the most distant planet. Comets, crowds of shooting stars, and the zodiacal light exhibit traces of dispersed matter such as existed in that old condition; matter, scattered as powder, but moving, and ignited like those nebulous patches which now shine in the far-off regions of the firmament.

Now observe two facts,-1, A large amount of light and heat sent out into space returns not again; so that the sun and stars are cooling. 2, The motion of large bodies in space is being gradually stopped by ethereal friction. A great part of the original mechanical force has already been converted into heat and dissipated, and, as to the stoppage of visible motion, every tide on the earth and every day's march through space bring us nearer to the end, not of our system only, but of all systems. The sun is hastening to his destiny, to that gathering of matter into one universal centre, that equilibrium in which, motion ceasing, there will be no light, no warmth, no life. The life of man, the existence of our sun and planets, are but as a ripple on the bosom of the eternal deep.

Work this out more definitely. The chief part of the primordial energy now belonging to our system is in the form of solar heat. This energy will not remain ours for ever; portions are continually radiating from it into infinite space.1 "The sun is so much colder that we may have our fires; he is also so much

1 "Aim and Progress of Physical Science:" Prof. Helmholtz.

colder that we may have our horse-racing and our Alpine climbing. It is, for example, certain that the sun has been chilled to an extent capable of being accurately expressed in numbers, in order to furnish the power which lifted this year a certain number of tourists from the Vale of Chamouni to the summit of Mont Blanc."1 We calculate that only the 454th part of the original mechanical force which worked the condensation of our system remains, the other has been converted into heat; every tide, though with incalculable slowness, diminishes the mechanical force of the earth; and our store of heat, though sufficient for an immeasurable time, is lessened by daily scattering into space. These are not lost as to the universe, but whither they go and into what they are formed no man knoweth. Despite this scattering, the earth's temperature during the historic 4000 years has not sensibly diminished, and a sufficient quantity of heat for the total emission of 2100 years would be generated by the combustion of only 1-10,000th part of the sun's diameter. So small a change our closest astronomical observations would hardly detect. The store of energy we possess is indeed immensely great, and the incessant emission of light and heat during the period of human history has not sensibly decreased it; still, the inexorable laws of mechanics indicate that it must finally be exhausted. If the universe is delivered up, as physicists say, to the undisturbed action of physical processes, all energy will pass into the form of heat, and all heat passing, by radiation and conduction, from the warmer bodies into bodies less warm an equilibrium of temperature will be established. From that time evermore the sun will have neither light nor heat, the universe will rest, the earth be dead. Our race may have a long, not an endless existence. Ere the end, thus delineated by science, Scripture declares that there will be a crisis-a judgment-the time of which is mercifully hidden.

The common method of measuring geological time is, by some, considered to be unreliable, because the thickness of stratified rocks belonging to any period is no indication as to length of time. Ten thousand feet may, under certain circumstances, be formed in as many years; but, in other conditions,

1 "Vitality:" Prof. Tyndall.

Measurements of Time.

91

require as many centuries. Palæontology may be accounted still worse as a guide, for species may not change at anything like a uniform rate. If, however, the great variations of climate are the result of changes in the eccentricity of the earth's orbit, and by changes in the direction of the polar axis from precession of the equinoxes, we have a means, but still open to doubt, of calculating when those variations occurred. According to the formula of M. Leverrier, there are three chief periods of high eccentricity, with a few subordinate maxima between them. About 2,650,000 years back the eccentricity was inferior. It then began to increase, and 50,000 years after, namely 2,600,000 years ago, it attained *066; 50,000 years after that it diminished to 0167, about its present value. It then began to increase, and in another 50,000 years, or 2,500,000 years ago, it approached almost the superior limit of 0721. Then diminishing, at 2,450,000 years ago, it was 025. These two maxima separated by a minimum, and a period of 200,000 years, form the first great period of eccentricity. Passing on more than a million and a half years, there is the second great period of three maxima, separated by two minima: the first maximum 950,000 years ago, the second at 850,000 years ago, the third at 750,000 years ago the whole occupying nearly 300,000 years. Passing on another million and a half years, that is, to a time about 800,000 years in the future, we come to the third great period of three maxima, at periods of 800,000, 900,000, and 1,000,000 years to come, which are separated also by two minima. These three great periods, two past and one future, are separated from one another by about 1,700,000 years; and seven times in the whole period the earth's orbit is more circular, four in the past and three in the future.

Unless the physical principles on which these eccentricities were calculated are erroneous, climate must have been greatly affected. For example, 850,000 years ago the heat of the sun at midwinter was 837 instead of 1000 as at present. Whether this value be a little too high or too low the effect on temperature must have been considerable. The glacial epoch, which so greatly troubles geologists, is not that extending from about 980,000 to about 720,000 years ago; but the one beginning

« EelmineJätka »