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"These conclusions, drawn from a consideration of the necessary order of cooling and consolidation, according to Bischoff's results on the relative specific gravity of solid and melted rocks, are in perfect accordance with §§ 832-849, on the present condition of the earth's interior; that it is not, as commonly supposed, all liquid within a solid crust from thirty to one hundred miles thick, but is, on the whole, more rigid than a solid globe of glass of the same diameter, and probably than one of steel.”

The investigation here alluded to seems to me decisive against the doctrine of the earth's central fluidity, and carries to a further point the conclusion of Mr. Hopkins, thirty years ago, from the phenomena of nutation and precession. It accords with my own inference from an hypothesis wholly distinct. But while I think that Sir William has disproved the notion of the central fluidity of the earth, and justly rejects the notion of geological uniformity for many hundred millions of years, I wholly dispute the soundness of his doctrine, that the date of the formation of the crust can be defined by "Fourier's Theorems on conducted heat, or that the waste of solar heat is in constant excess over the fresh supply. In fact, the doctrine of uniformity would be equally untrue, whether the light and heat of the sun have increased or diminished sensibly in the course of a million years.

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IV. THE TRANSLATION THEORY.

18. Another view has been suggested by Poisson, to account for past changes in the earth's climate, and warm and glacial periods,—the earth's translation through hotter and colder regions of space. This does not need to detain us long, as there seem to be very simple and decisive reasons against it. Mr. Croll has thus given them briefly and clearly in a few words.

"This is not a very satisfactory hypothesis. . . . Space is not a substance which can possibly be either hot or cold. If we adopt this hypothesis, we must assume that the earth, during hot periods, was in the vicinity of some other great source of heat and light beside the sun. But the proximity of a mass of such magnitude as would be able to affect to any great extent the earth's climate, would, by its gravity, seriously disarrange the mechanism of the solar system. If it had ever, in a former period, come into the vicinity of such a mass, the orbits of the planets ought to afford evidence of it. But again, to account for a cold period, like the glacial epochs, we must assume the earth to have come near a cold body. And recent discoveries with regard to interglacial periods are wholly irreconcilable with this theory."

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19. But while this translation theory of Poisson is both vague and inadequate, and wanting in direct evidence, the fact of the movement of our system in space is a strong reason against the uniformity assumed by many geologists to have lasted through many millions of years. The rate of the sun's motion in space is held to be 150 millions of miles a year. This would carry it as far as a Centauri, the nearest star whose parallax is determined, in 140,000 years. The direction prolonged backward has its apex only 25° from Sirius, the brightest of all the stars, and of which the light has been reckoned to be 60 times greater than that of the sun. parallax is of a second. It has been lately inferred from the spectroscope that we are receding from Sirius at the rate of 25 miles a second, or 800 millions in a year, so as to traverse the whole distance in 100,000 years. And since we cannot tell whether the earlier motion may not have varied so far in its direction, we can have no assurance that all the elements of our system may not have been altered by the proximity of Sirius only one hundred thousand years ago. All estimates of solar force and the earth's inclination and excentricity which go back beyond this limit must remain highly uncertain on this ground alone, and are beyond the range of assured and certain science.

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20. Two other theories may be also dismissed in few words. First, that of an altered axis of rotation, so that the north and south poles of the diurnal rotation were at places considerably remote from those which they now occupy. But this is rendered all but impossible by the spheroidal shape of the earth. At any time, after the crust had once hardened and taken a spheroidal form, revolution on any axis, not adjacent to the present one, must have been mechanically impossible. Any secondary change of surface by the uprising of a mountain-chain might produce an increased nutation and a kind of waddling motion around the true axis, but it could not alter the place of that axis, or produce any sensible effect. on the climate of any main parts of the surface.

21. Another theory of the same kind is Sir C. Lyell's transposition theory. He supposes that the mean temperature would be raised if the land were mainly in the torrid zone, and be lowered if it were grouped around the poles. Mr. Croll argues that the effect would be diametrically opposite, and that the contour of the surface most favourable to the warmth of the earth is when the water is in all the middle part, and the land only at the poles. Now it is difficult to reason out all the consequences as to the mean temperature of the whole surface. The mere fact that two such opposite

views have been held suggests a doubt whether either can rest on sure scientific grounds. The one thing which seems clear and certain is, that a structure like that of our globe with two main oceans extending almost unbroken from the south to the north pole, over three-fourths of the whole surface, is the arrangement most favourable to a mitigation of fierce. extremes, and to fit our world for human habitation. At the same time, since the glacial epoch belongs to a stage of geology when the outlines of land and water were nearly the same as now, it is perfectly clear that no difference in their relative arrangement can serve to account for a much lower or a much warmer temperature than has obtained in the known historical period of the world.

V.-VARIED INCLINATION THEORY.

22. Another theory of a more definite kind is advocated by Lieut.-Colonel Drayson, in his work entitled "The Cause, Date and Duration of the Last Glacial Epoch of Geology." He places the period of maximum glaciation 13,700 years before Christ, or 15,500 years ago. Such a view, if it were established, would plainly be much more reconcilable with the Bible chronology for the date of man's appearance on the earth than the opinions just examined. But I believe that it rests on a fundamental mistake which it is not difficult to place in a clear light. Mr. Croll remarks on it as follows:

"The theory is beset by a twofold objection. First, it can be shown from celestial mechanics that the variations in the obliquity must always have been so small that they could not affect the climatic condition of the globe. Secondly, even admitting that the obliquity could change to an indefinite extent, it can be shown that no increase or decrease, howeve great, could possibly account for the glacial epoch, or a warm temperate condition in the polar regions."

23. This second objection, whether true or false, seems to me diametrically opposed to the reasoning of Mr. Croll in favour of his own hypothesis, when he would account for a glacial season by an increased excentricity, concurring with a northern winter solstice in aphelion. With regard to the total heat there is this slight difference, that a change of inclination leaves it quite unaltered, but an increased excentricity causes a small increase. So far the second is less suited than the first to account for a glacial period. But with regard to total winter temperature, the operation of the two causes is precisely of the same kind, and the relative effect in the ratio

of twice the excentricity to the sine of the inclination. Hence an increase of the inclination from 23° 28′ to 35° 56′ with the present excentricity would cause the same degree of inequality as an increase of the excentricity to 0747, its supposed amount 850,000 years ago. If glaciation would result, as Mr. Croll contends, from the latter combination, it must have done so from the other, and for the same reason. On the other hand, if a hotter summer undoes and reverses the effect of a colder winter with an increased obliquity, it must equally do so with an increased excentricity.

24. The real error of Lieut.-Col. Drayson's theory is its contradiction to the laws of physical astronomy. The pole of the equator, by precession, is receding 50′′ in longitude annually at a right angle to the pole of the ecliptic. But the obliquity is also slowly lessening, and the poles are coming nearer together. Lieut.-Col. Drayson finds that the two phenonema will be reconciled, and the observations of precession and polar distance satisfied from Tycho down to the present day, if we assume the pole of the equator to revolve round a point at 6° distance from the pole of the ecliptic. In this case, the nearest approach would be about five centuries hence, the period of revolution 31,840 years, and B.C. 13,600 the obliquity would have its maximum value, or 35° 26'. The excentricity, by Mr. Croll's table, would then be 01875, and the effect to produce inequality of heat at midwinter and midsummer, the same as with the present obliquity and an excentricity of 1095, or half as great again as the maximum in Mr. Croll's table.

25. But the mistake is here. The precession or backward motion of the pole of the equator, and the diminished obliquity or the motion of the pole of the ecliptic nearer to that of the equator depend on two wholly distinct causes. One is due to the action of the sun on the equatorial protuberance, and must be at right angles to the line which joins the two poles at the moment and in no other direction. The other is due to the disturbing action of the other planets on the earth's annual orbit. It does not make the pole of the equator move with reference to that of the ecliptic, but the reverse, that is, the pole of the ecliptic approaches to or recedes from that of the equator. Thus the earth's pole does not revolve round a fixed centre 6° away from the pole of the ecliptic, but round a pole itself moving in a small self-returning curve of definite limits. It moves in fact in a sort of cycloid of a rather complex kind, and not in a circle. No doubt a circle may be found, as Lieut.-Col. Drayson has proved, to satisfy the observations, which range over only four centuries. But this is a striking example of the danger of trusting to a purely

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empirical law beyond the limits of the observations from which it is deduced, even when it is much more scientific than a bare average. Lieut.-Col. Drayson's circle is an approximation of the second order, and will satisfy the observations of four centuries. much better than a simple average, which is of the first order only. But it will wholly mislead when carried beyond those limits; for the true curve of the earth's pole projected on the celestial sphere is not an excentric circle, but a kind of cycloid, or a circle of which the centre is ever moving, though within narrow limits. The pole of the equator does not move towards that of the ecliptic, but at right angles to the joining line, while the latter does approach to and recede from the pole of the equator. If the hypothesis were true, there is no reasonable doubt that it would involve the consequence of fierce extremes of summer heat and winter cold, over a large part of each hemisphere of the earth.

VI.—THE THEORY OF INCREASED EXCENTRICITY.

26. The most popular theory, at present, which offers a kind of geological chronology, is that of Mr. Croll, in his work entitled, "Climate and Time in their Geological Relations." It has been adopted by Mr. Geikie in his "Great Ice Age," by Sir C. Lyell, and apparently by many others, and has been developed, in a volume of five hundred pages, with great labour, research, and ingenuity. It professes to account for a recurrence of extremely cold or glacial periods by the coincidence of two astronomical elements,-an increased excentricity of the earth's orbit at certain past dates, and the position of the northern winter solstice near the aphelion. It is held, further, that when the southern winter solstice was in the aphelion, there would be a similar period of glaciation of the southern hemisphere. Mr. Croll has calculated the excentricity, by Leverrier's formulæ, at intervals of 50,000 years, for three millions of years of past, and one million of future time, and every 10,000 years for the last million only. He discovers two maxima, 850 and 210 thousand years ago, and identifies them with a Miocene and a Post-Pliocene Ice Period, assumed to be proved by modern geology. The first signs of man's presence on the earth are usually held to be either soon after, or else just before, the Boulder Drift, the second of these periods. The effect, then, of Mr. Croll's theory would be to place the entrance of man on our planet above two hundred thousand years ago. During this vast interval, thirty times greater

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