of Sir Charles Lyell. Those extraordinary changes of climate are, according to his theory, attributed to differences in the distribution of land and water. Sir Charles concludes that, were the land all collected round the poles, while the equatorial zones were occupied by the ocean, the general temperature would be lowered to an extent that would account for the glacial epoch. And, on the other hand, were the land all collected along the equator, while the polar regions were covered with sea, this would raise the temperature of the globe to an enormous extent. It will be shown in subsequent chapters that this theory does not duly take into account the prodigious influence exerted on climate by means of the heat conveyed from equatorial to temperate and polar regions by means of ocean currents. In Chapters II. and III. I have endeavoured to prove (1), that were it not for the heat conveyed from equatorial to temperate and polar regions by this means, the thermal condition of the globe would be totally different from what it is at present; and (2), that the effect of placing all the land along the equator would be diametrically the opposite of that which Sir Charles supposes.

But supposing that difference in the distribution of land and water would produce the effects attributed to it, nevertheless it would not account for those extraordinary changes of climate which have occurred during geological epochs. Take, for example, the glacial epoch. Geologists almost all agree that little or no change has taken place in the relative distribution of sea and land since that epoch. All our main continents and islands not only existed then as they do now, but every year is adding to the amount of evidence which goes to show that so recent, geologically considered, is the glacial epoch that the very contour of the surface was pretty much the same then as it is at the present day. But this is not all; for even should we assume (1), that a difference in the distribution of sea and land would produce the effects referred to, and (2), that we had good geological evidence to show that at a very recent period a form of distribution existed which would produce the necessary

glacial conditions, still the glacial epoch would not be explained, for the phenomena of warm inter-glacial periods would completely upset the theory.

Geological Climate depending on Astronomical Causes.--For a good many years past an impression has been gradually gaining ground amongst geologists that the glacial epoch, as well as the extraordinary condition of climate which prevailed in arctic regions during the Miocene and other periods, must some way or other have resulted from a cosmical cause; but all seemed at a loss to conjecture what that cause could possibly be. It was apparent that the cosmical cause must be sought for in the relations of our earth to the sun; but a change in the obliquity of the ecliptic and the eccentricity of the earth's orbit are the only changes from which any sensible effect on climate could possibly be expected to result. It was shown, however, by Laplace that the change of obliquity was confined within so narrow limits that it has scarcely ever been appealed to as a cause seriously affecting climate. The only remaining cause to which appeal could be made was the change in the eccentricity of the earth's orbit-precession of the equinoxes without eccentricity producing, of course, no effect whatever on climate. Upwards of forty years ago Sir John Herschel and a few other astronomers directed their attention to the consideration of this cause, but the result arrived at was adverse to the supposition that change of eccentricity could greatly affect the climate of our globe.

As some misapprehension seems to prevail with reference to this, I would take the liberty of briefly adverting to the history of the matter,-referring the reader to the Appendix for fuller details.

About the beginning of the century some writers attributed the lower temperature of the southern hemisphere to the fact that the sun remains about seven days less on that hemisphere than on the northern; their view being that the southern hemisphere on this account receives seven days less heat than the northern. Sir Charles Lyell, in the first edition of his “Prin

ciples," published in 1830, refers to this as a cause which might produce some slight effect on climate. Sir Charles's remarks seem to have directed Sir John Herschel's attention to the subject, for in the latter part of the same year he read a paper before the Geological Society on the astronomical causes which may influence geological phenomena, in which, after pointing out the mistake into which Sir Charles had been led in concluding that the southern hemisphere receives less heat than the northern, he considers the question as to whether geological climate could be influenced by changes in the eccentricity of the earth's orbit. He did not appear at the time to have been aware of the conclusions arrived at by Lagrange regarding the superior limit of the eccentricity of the earth's orbit; but he came to the conclusion that possibly the climate of our globe may have been affected by variations in the eccentricity of its orbit. "An amount of variation," he says, "which we need not hesitate to admit (at least provisionally) as a possible one, may be productive of considerable diversity of climate, and may operate during great periods of time either to mitigate or to exaggerate the difference of winter and summer temperatures, so as to produce alternately in the same latitude of either hemisphere a perpetual spring, or the extreme vicissitudes of a burning summer and a rigorous winter."

This opinion, however, was unfortunately to a great extent nullified by the statement which shortly afterwards appeared in his "Treatise on Astronomy," and also in the "Outlines of Astronomy," to the effect that the elliptic form of the earth's orbit has but a very trifling influence in producing variation of temperature corresponding to the sun's distance; the reason being that whatever may be the ellipticity of the orbit, it follows that equal amounts of heat are received from the sun in passing over equal angles round it, in whatever part of the ellipse those angles may be situated. Those angles will of course be described in unequal times, but the greater proximity of the sun exactly compensates for the more rapid description, and thus an equilibrium of heat is maintained. The sun, for example, is

much nearer the earth when he is over the southern hemisphere than he is when over the northern; but the southern hemisphere does not on this account receive more heat than the northern; for, owing to the greater velocity of the earth when nearest the sun, the sun does not remain so long on the southern hemisphere as he does on the northern. These two effects so exactly counterbalance each other that, whatever be the extent of the eccentricity, the total amount of heat reaching both hemispheres is the same. And he considered that this beautiful compensating principle would protect the climate of our globe from being seriously affected by an increase in the eccentricity of its orbit, unless the extent of that increase was very great.

"Were it not," he says, "for this, the eccentricity of the orbit would materially influence the transition of seasons. The fluctuation of distance amounts to nearly 1-30th of its mean quantity, and consequently the fluctuation in the sun's direct heating power to double this, or 1-15th of the whole. Now the perihelion of the orbit is situated nearly at the place of the northern winter solstice; so that, were it not for the compensation we have just described, the effect would be to exaggerate the difference of summer and winter in the southern hemisphere, and to moderate it in the northern; thus producing a more violent alternation of climate in the one hemisphere, and an approach to perpetual spring in the other. As it is, however, no such inequality subsists, but an equal and impartial distribution of heat and light is accorded to both."* Herschel's opinion was shortly afterwards adopted and advo cated by Arago† and by Humboldt.‡

Arago, for example, states that so little is the climate of our globe affected by the eccentricity of its orbit, that even were the orbit to become as eccentric as that of the planet Pallas (that is, as great as 0.24), "still this would not alter in

. "Treatise on Astronomy," 315; "Outlines," § 368.

† Annuaire for 1834, p. 199. Edin. New Phil. Journ., April, 1834, p. 224. "Cosmos," vol. iv. p. 459 (Bohn's Edition). "Physical Description of the Heavens," p. 336.

any appreciable manner the mean thermometrical state of the globe."

This idea, supported by these great authorities, got possession of the public mind; and ever since it has been almost universally regarded as settled that the great changes of climate indicated by geological phenomena could not have resulted from any change in the relation of the earth to the sun.

There is, however, one effect that was not regarded as compensated. The total amount of heat received by the earth is inversely proportional to the minor axis of its orbit; and it follows, therefore, that the greater the eccentricity, the greater is the total amount of heat received by the earth. On this account it was concluded that an increase of eccentricity would tend to a certain extent to produce a warmer climate.

All those conclusions to which I refer, arrived at by astronomers, are perfectly legitimate so far as the direct effects of eccentricity are concerned; and it was quite natural, and, in fact, proper to conclude that there was nothing in the mere increase of eccentricity that could produce a glacial epoch. How unnatural would it have been to have concluded that an increase in the quantity of heat received from the sun should lower the temperature, and cover the country with snow and ice! Neither would excessively cold winters, followed by excessively hot summers, produce a glacial epoch. To assert, therefore, that the purely astronomical causes could produce such an effect would be simply absurd.

Important Consideration overlooked.- The important fact, however, was overlooked that, although the glacial epoch could not result directly from an increase of eccentricity, it might nevertheless do so indirectly. Although an increase of eccentricity could have no direct tendency to lower the temperature and cover our country with ice, yet it might bring into operation physical agents which would produce this effect.

If, instead of endeavouring to trace a direct connection between a high condition of eccentricity and a glacial condition of climate, we turn our attention to the consideration of what