velocity by the effect of resistance, it would be drawn proportionally nearer the sun, the tendency towards the centre being no longer sufficiently counteracted by that centrifugal force which arises from the body's velocity. And if the resistance were to continue to act, the body would be drawn perpetually nearer and nearer to the centre, and would describe its revolutions quicker and quicker, till at last it would reach the central body, and the system would cease to be a system.

This result is true, however small be the velocity lost by resistance; the only difference being, that when the resistance is small, the time requisite to extinguish the whole motion will be proportionally longer. In all cases the times which come under our consideration in problems of this kind are enormous to common apprehension. Thus Encke's comet, according to the results of the observations already made, will lose, in ten revolutions, or thirty-three years, less than 1-1000th of its velocity; and if this law were to continue, the velocity would not be reduced to one-half its present value in less than seven thousand revolutions, or twenty-three thousand years. If Jupiter were to lose one-millionth of his velocity in a million years (which, as has been seen, is far more than can be considered in any way probable), he would require seventy millions of years lose 1-1000th of the velocity; and a period seven hundred times as long to reduce the velocity to onehalf. These are periods of time which quite overwhelm the imagination; and it is not pretended that the calculations are made with any pretensions to

to

accuracy. But at the same time it is beyond doubt, that though the intervals of time thus assigned to these changes are highly vague and uncertain, the changes themselves must, sooner or later, take place, in consequence of the existence of the resisting medium. Since there is such a retarding force perpetually acting, however slight it be, it must in the end destroy all the celestial motions. It may be millions of millions of years before the earth's retardation may perceptibly affect the apparent motion of the sun; but still the day will come (if the same Providence which formed the system, should permit it to continue so long) when this cause will entirely change the length of our year and the course of our seasons, and finally stop the earth's motion round the sun altogther. The smallness of the resistance, however small we choose to suppose it, does. not allow us to escape this certainty. There is a resisting medium; and, therefore, the movements of the solar system cannot go on for ever. The moment such a fluid is ascertained to exist, the eternity of the movements of the planets becomes as impossible as a perpetual motion on the earth.

III. The vast periods which are brought under our consideration in tracing the effects of the resisting medium, harmonise with all that we learn of the constitution of the universe from other sources. Millions, and millions of millions of years are expressions that at first sight appear fitted only to overwhelm and confound all our powers of thought: and such numbers are no doubt beyond the limits of anything which we can distinctly conceive. But our powers of conception

are suited rather to the wants and uses of common life, than to a complete survey of the universe. It is in no way unlikely that the whole duration of the solar system should be a period immeasurably great in our eyes, though demonstrably finite. Such enormous numbers have been brought under our notice by all the advances we have made in our knowledge of nature. The smallness of the objects detected by the microscope and of their parts;-the multitude of the stars which the best telescopes of modern times have discovered in the sky; the duration assigned to the globe of the earth by geological investigation;-all these results require for their probable expression, numbers, which, so far as we see, are on the same gigantic scale as the number of years in which the solar system will become ⚫entirely deranged. Such calculations depend in some degree on our relation to the vast aggregate of the works of our Creator; and no person who is accustomed to meditate on these subjects will be surprised that the numbers which such an occasion requires should oppress our comprehension. No one who has dwelt on the thought of a universal Creator and Preserver, will be surprised to find the conviction forced upon the mind of every new train of speculation, that viewed in reference to Him, our space is a point, our time a moment, our millions a handful, our permanence a quick decay.

Our knowledge of the vast periods, both geological and astronomical, of which we have spoken, is most slight. It is, in fact, little more than that such periods exist; that the surface of the earth has, at wide intervals

of time, undergone great changes in the disposition of land and water, and in the forms of animal life; and that the motions of the heavenly bodies round the sun are affected, though with inconceivable slowness, by a force which must end by deranging them altogether. It would, therefore, be rash to endeavour to establish any analogy between the periods thus disclosed; but we may observe that they agree in this, that they reduce all things to the general rule of finite duration. As all the geological states of which we find evidence in the present state of the earth have had their termination, so also the astronomical conditions, under which the revolutions of the earth itself proceed, involve the necessity of a future cessation of these revolutions.

The contemplative person may well be struck by this universal law of the creation. We are in the habit sometimes of contrasting the transient destiny of man with the permanence of the forests, the mountains, the ocean, with the unwearied circuit of the sun. But this contrast is a delusion of our own imagination: the difference is after all but one of degree. The forest tree endures for its centuries and then decays; the mountains crumble and change, and perhaps subside in some convulsion of nature; the sea retires, and the shore ceases to resound with the "everlasting" voice of the ocean: such reflections have already crowded upon the mind of the geologist; and it now appears that the courses of the heavens themselves are not exempt from the universal law of decay; that not only the rocks and the mountains, but the sun and the moon have the

sentence "to end" stamped upon their foreheads. They enjoy no privilege beyond man except a longer respite. The ephemeron perishes in an hour; man endures for his threescore years and ten; an empire, a nation, numbers its centuries, it may be its thousands of years; the continents and islands which its dominion includes, have perhaps their date, as those which preceded them have had; and the very revolutions of the sky by which centuries are numbered will at last languish and stand still.

To dwell on the moral and religious reflections suggested by this train of thought is not to our present purpose; but we may observe that it introduces a homogeneity, so to speak, into the government of the universe. Perpetual change, perpetual progression, increase and diminution, appear to be the rules of the material world, and to prevail without exception. The smaller portions of matter which we have near us, and the larger, which appear as luminaries at a vast distance, different as they are in our mode of conceiving them, obey the same laws of motion; and these laws produce the same results: in both cases motion is perpetually destroyed, except it be repaired by some living power; in both cases the relative rest of the parts of a material system is the conclusion to which its motion tends.

IV. It may, perhaps, appear to some, that this acknowledgment of the tendency of the system to derangement through the action of a resisting medium is inconsistent with the argument which we have drawn, in a previous chapter, from the provisions for its