Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/232

Rh 218 409feet.1 More recently, in returning to this question, Dr C‘roll reinarks "that the removal of two miles of ice from the Antarctic continent [and at present the mass of ice there is probably thicker than that] would displace the centre of gravity 190 feet, and the formation of a mass of ice equal to the oiie-lialf of this, on the Arctic regions, would carry the centre of gravity 95 feet farther, giving in all a total displacement of 285 feet, thus producing a rise of level at the north pole of 285 feet, and in the latitude of Edinburgh of 234 feet.” A very considerable additional displacement would arise from the increment of water to the mass of the ocean by the melting of the ice. Supposing half of the two miles of Antarctic ice to be replaced by an ice—cap of similar extent and one mile thick in the northern hemisphere, the other half being melted into water and increasing the mass of the ocean, Dr Croll estimates that from this source an extra 200 feet of rise would take place in the general ocean level, so that there would be a rise of 485 feet at the north pole, and 434- feet in the latitude of Edinburgh? There must thus have been an alternate sub- inergence and emergence of the low polar lands due to the alternate shifting of the centre of gravity. 7. I nﬂuence of the Is'art/t’s Jfovements upon C'limate.—— Although the treatment of this subject involves a reference to questions which must be discussed in their proper place in subsequent parts of this article, it will be most appro )I‘l- ately inserted here as a distinct and most important braiich of the astronomical relations of geology. In later pages it will be shown that the climate of the earth has under one many great vicissitudes during geological history,£for example, that a warm and genial temperature has once prevailed in arctic latitudes, while at another time snow and ice extended far down into the heart of Europe. Of this latter change, which took place within a comparatively recent geological period, the traces still remain remarkably fresh, and have excited great interest and discussion. It is known as the Glacial Period or Ice Age. But we now knowl tt-pa: (tl1€:I‘dS1ln1llal‘ 1)E;l'1((1)fl.St0f glreat cold probably precer e 1 a wi e y separa e in erva s. Various theories have been proposed in explanation of such striking variations in climate. Some of these have appealed to a change in the ositioii of the eartli’s axis relatively to the mass of the plzihet (p. 5216). Others have been based on the_notion that the earth may have passed through hot and cold regions of space. Others, again, have called in the effects of terrestrial changes, such as the dis- tribution of land and sea, on the assumption that elevation of land about the poles must cool the temperature of the globe, while elevation round the equator would raise it. But as the changes of temperature have affected vast areas of the earth’s surface, while there is a total absence of all proof of any such enormous vicissitudes in physical geography as would be required, and since there is accumu- lating_proof in favour of periodic alternations of climate, theilﬁ 11: a grogziiiigl Cl§)[lVlC]li1l0n that no mere local changes con ave su ee, ut t at secular variations in climate must lie assigned to some general and probably recurring cause. By degrees geologists accustomed themselves to the belief that the cold of the glacial period was not due to mere terrestrial changes, but was to be explained somehow as the result of cosmical causes. _ Among the various suggestions, one deserves careful consideration—ehange in the eccen- 1 Croll, in Reader for 2d Sept. 1865, and Phil. .‘|[.r/._r/., April 1866 ; Heath, Phil. l‘|[a_r/., April 1869; Pratt, Phil. .I!rI.g., March 1866; Fisher, Reader, 10th February 1866. 3Croll, Geol. .'|[ag., new series, i. (1874), p. 347; C'lz'nwte and Time, chaps. xxiii. and xxiv. 3 In Lyell's Principles of Geology the doctrine of the influence of geographical changes is maintained. GEOLOGY [L COSMICAL. tricity of the earth's orbit. Sir John llerschelt pointed out many years ago that the direct effect of a high condition of eccentricity is to produce an unusually cold winter followed by a correspondingly hot summer on the hemi- sphere whose winter occurs in aplielion, while an equable condition of climate will at the same time prevail on the opposite hemisphere. But both hemispheres must receive precisely the same amount of solar heat, because the deficiency of heat resulting from the siui’s greater distance during one part of the year is exactly compensated by the greater length of that season. Sir John llerschcl even considered that the direct effects of eccentricity must thus be nearly neutralized.-" As a like verdict was afterwards given by Arago, Humboldt, and others, gcologi.-ts were satisﬁed that no important change of climate could he attri- buted to change of eccentricity. It is to the luminous memoirs of Dr James (‘roll that geology is indebted for the first fruitful suggestion in this matter, and for the subsequent elaborate developnient of the whole subject of the physical causes on which climate depends. He has been so good as to draw up for this article the following suiiiuiary of his views (taken chiefly from his paper in the I’/u'I. J[u_r/. for February 1870). The reader will ﬁnd the subject fully worked out in Dr Croll’s work, C’/2'-mate and Time, 1875. “Assuniing the mean distance of the sun to he 92,400,000 miles, then when the eccentricity is at its superior limit, '07 775, the tlistziiicc of the siui from the earth, when the latter is in the aphclion of its orbit, is no less than 99, 584,100 miles, and when in the pcriliclioii it is only 85,215,900 miles. The earth is, therefore, 14,368,211!) miles farther from the sun in the former than in the latter positimi. The direct heat of the sun being inversely as the square of the dis- tance, it follows that the amount of heat rcc.cived by the earth in these two positions will be as 19 to 26. The present ccccntrii-ity being '016S, the eartli’s distance during our northern winter is 90,847,680 miles. Suppose now that, from the pi-m-cssioii of the cquinoxes, winter in our northern hemisphere should happen wln-n the earth is in the apliclion of its orbit, at the time that the orbit is at its greatest eccentricity; the earth would then be 8,736,420 miles farther from the sun in winter than it is at 1Il't‘FCX1t. 'l‘lic dii‘ect heat of the sim would therefore, during winter, be oiic—ﬁl’ili less and during summer onc—fifth greater than iiov. 'l'his enormous diﬁcience would necessarily affect the climate to a very great extent. Were the winters under these circumstances to occur when the earth was in the perihelion of its orbit, the carth would then be 14,368,200 milcsncarer the sun in winter than in suniincr. ln this case the difference between winter and summer in our latitiidcs would be -almost annihilated. But as the wintcis in the one hemi- sphere correspond with the summers in the other, it follows that while the one hemisphere would be enduring the greatest extremes of summer heat and winter cold, the other would be enjoying per- petual siimincr. . “ It is quite true that whatever may be the eec0nl1‘icity of the earth": orbit, the two hemispheres must recc.ive equal quantitii s of heat pi-i annuni; for pi'oxiiiiity to the sun is exactly conipcnsatcd by the cffcet of swifter motion. The total amount of heat ]'L'('t'1'L'Ll from the sun between the two equinoxcs is therefore the same in both halves of the year, whatever the eccentricity of the earth's orbit may be. For example, vliatevcrcxtralicat the southern hemisphere may at present receive per day from the sim during its summer months, owing to greater proximity to the sini, is exactly compensated by a corresponding loss arising from the shortness of the season ; and, on the other hand, whatever deﬁciency of heat we in the northern hemisphere may at present have per day during our summcr_ hall'- year, in consequence of the earth's distance from the sun, is also exactly compensated by a corresponding length of season. “It is well known, however, that these simple changes in the sun's summer and winter distances would not alone produce a glacial epoch, and that physicists, confining their attention to the pui-i-ly astro- nomical effects, were perfectly correct in aflirming that no increase of eccentricity of the earth's orbit could account for that cpm-h. lint the important fact was overlooked that, although the. glacial epoch could not result directly from an increase of eccentricity, it might nevertheless do so indirectly from physical agents that were brought into operation-as a result of an increase of eccentricity. 'l‘lic follow- ing is an outline of what these pliysical agents were, how they were brought into operation, and the way in which they may lia'c led to the glacial epoch. ‘ Trrms. (Icnl. Snc., vol. iii. p. 293 (2drs£Trics). "’ (.'r.zbi'ncl C'ycIuptnli'a., sec. 315; Outlines of Astronom._1/, see. 368.