COMETARY CHANGES.

19

lisions among themselves, and often again collected together and solidified.1

In January 1866, the astronomers were looking for two little comets, moving in distinct but similar orbits. which intersected the orbit of the earth. In spite of the strictest watching the comets were not detected, and it is believed that the gravitating influence of some other body, probably the November meteor stream, has thrown them into new orbits.2 With regard to their past history also, it is known that the two comets were formerly one, having been discovered by M. Biela in 1826, and watched in its successive revolutions round the sun, which it accomplished in 6 years. Such a diversion from the old route is known to have taken place with the comet of 1770, which was found by Lexell to revolve in a moderate ellipse in the period of about five years, and whose return was predicted by him accordingly. The comet got entangled among the satellites of Jupiter, and changed its small orbit for a longer one, which keeps it out of sight, and occupies it twenty years or so to traverse. This new phase of its history is only one of a series; for the comet had never been seen previous to the year 1770, and Laplace found that it was the action of Jupiter which then brought it into view.

When comets experience no such tremendous perturbation, they are subject to a more regular and slower change, which threatens to bring their career to an end. Biela's comet, previous to disappearing, was found to be gradually approaching the sun, its period diminishing in the course of each revolution. Encke's comet, which 1 Lockyer's Elementary Astronomy.

2 The meteors of November 27, 1872, were, it is believed, a portion of one of these comets.

goes round the sun in 3 years, now performs the journey in three days less than it did eighty years ago. It will, therefore, probably fall ultimately into the sun, should it not first be dissipated altogether; and the same fate may be anticipated for most of the other comets of the system. This acceleration of a comet, says Sir J. Herschel, is evidently the effect which would be experienced from a very rare ethereal medium pervading the regions in which it moves; for such resistance, by diminishing its actual velocity, would diminish also its centrifugal force, and thus give the sun more power over it to draw it nearer.1

While we have this glimpse of the probable future of comets, we are not altogether without light in regard to their previous history. The credit of discovering the apparent identity between comets and meteor shoals ist due principally to Signor Schiaparelli, who finds reason to believe that they did not form part of our system. when that was first constituted, but are wandering nebulæ picked up by our sun.2

Even the planets, which have more right to be regarded as the true children of the sun, are not children who never grow older nor experience any changes. The rings of Saturn, it is now believed, cannot be solid, nor continuously fluid, but must be composed of myriads of satellites moving round the planet in their several orbits, with different velocities according to their respective distances. It is even thought that they frequently

1 Treatise on Astron., Cabinet Cyclopedia.

2 Mr Wm. Lassell: Presidential Address to Royal Astronomical Society, February 9, 1872.

3 On the Stability of Saturn's Rings, Prof. Clerk Maxwell. Saturn and its System, Mr R. Proctor.

PLANETARY CHANGES.

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16

come into collision with one another, and that the orbits alter; the general result being, that the rings are widening, and new ones forming nearer the planet. Our own Earth, in its character as one of the planets, and independently of the aerolites which fall upon it, is not without some slight change, which only needs the lapse of time to make it considerable. The moon, by attracting the surface of the earth more than its centre, heaps up the waters of the ocean under her, and, by attracting the centre more than the further side, causes them to bulge out on that side also; and these waters are held in these positions while the solid earth rotates or slips round beneath them. The tidal undulation, indeed, moves forward in its effort to keep under the moving moon, but not so fast as the earth rotates; and so the piled-up water comes to act as a break upon the earth, tending to hinder it from slipping round. It has been contended by Professor Helmholtz that inappreciable as may be the effect of this friction within known periods of time, it must be slowly diminishing the earth's rotatory motion. The extreme effect to be anticipated from this process is an extension of the earth's day to the length of a lunation.

The resistance of the ethereal medium before referred to must tell at last upon the planets as well as the less solid comets; indeed, certain astronomers contend that it even now shows its effects in the relative nearness to one another of the orbits of the older planets. If, then, retardation is going on, there must come a time, no matter how remote, when the slowly diminishing orbit of the earth will end in the sun, the collision reducing the earth's substance to a gaseous state. This event will be followed at intervals by

the similar dissolution of every other planet of the solar system.

And what of the sun itself, the source of all the life and energy of this world? Can it continue to give off an undiminished amount of light and heat through all future time? Can it continue to raise huge masses of water every day from the sea to the skies, and lift every year endless vegetation from the earth; set breeze and hurricane in motion; perform, in fine, the great bulk of the endless labour of this world and of other worlds, and never lose its energy? No; for force is a definite, measurable thing, and its absolute quantity never changes; the force known to us in solar radiations is the changed form of some other force of which the sun is the seat; and by the gradual dissipation of these radiations into space, this other force is being slowly exhausted. The central fire cannot glow for ever, unless the supply of force be replenished, any more than an ordinary fire can be kept in without fuel. Perpetual motion is a delusion; no finite construction of physical materials can continue to do work for an infinite time; the heat of the sun is just as surely limited in its power of doing work as a given number of tons of coal in the furnace of a steam-engine. Probably, therefore, there will come a time when the sun, having previously swallowed up all the planets, will roll on its solitary way through space a cold black ball.1

Here, again, science indicates the past as well as the future: the present contains the record of the one as

1 See Tyndall's translation of Helmholtz's paper in the Philosophical Magazine, supplement to vol. xi., fourth series. See also the famous article on the "Origin of Species" in the North British Review, June 1867.

cause.

ORIGIN OF PLANETS.

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well as the prophecy of the other, if we know how to interpret its characters. As Mr Grove says: Saturn's ring may help us to a knowledge of how our solar system developed itself; for it as surely contains that history as the rock contains the record of its own formation.1 The planets all move nearly in one plane, the plane to which they approximate being that of the sun's equator; the motion of the sun on its axis, of the planets on theirs, of the planets round the sun, and the moons round the planets, are, with scarcely an exception, in one direction—namely, from west to east. These facts of uniformity, Laplace calculates, tell against the supposition of separate accidental causes by a probability of four millions of millions to one, and point to one general Kant, Laplace, Sir W. Herschel, and others, have offered an explanation in the Nebular Hypothesis, according to which the matter of our solar system once existed in a diffused gaseous condition, from which it became more concentrated through the operation of gravitation. The aggregating spheroid, dissipating its heat, would acquire an unlikeness of density and temperature between its interior and exterior; this unlikeness would become more marked; from time to time "rings" of matter would be left behind, and eventually these would break up and concentrate into planets. This hypothesis fell into discredit, because every time a telescope larger than any formerly used was turned to the heavens, some fresh nebula was found to be a cluster of stars, so distant that telescopes of inferior power had failed to "resolve" it. But recent discoveries with the spectroscope have shown that some of these cloud-like patches are true nebulæ; that is to say, masses of glow1 Grove's Discourse on Continuity.