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Show 1.'52 ISLAND LIFK (PART I. fl · f the surface of the from producing any effect, by re ectwn rom .c h' 1 ll . . f 1 d and 1orr w 1c 1 wou l snow and by the mtervent10n o c ou s o cr' carry muc h of tl1 e h eat tlley received to othe. r re0 10US. I. t follows t h at ]. 0 ,0 00 years hence ' when our wm.t er occudr s m h l . (' t d f w 1· ap e wn Ins ea o , as no , 11 perihelion) ' there w1ll be pro uced a co ld er c1 1. ma t e, m. d epen dently of any change of land and sea, of heights of mountains, or the force of currents: . , B u t 1'f th1' s I·S t rue, then the reverse change, brmgmg th. e . su. n b ac k m· t o exac tly the sa rne position with r.e g. ard to us .a s. 1t IS m now (a 11 geograp h1. ca1 and physical cond.1 t1ons remammg . un-changed), would certainly bring back agam our present m1lder climate. The change either way would not proba?ly be very great, but it might be sufficient to bring the snow line ~own to 3,000 feet in Scotland, so that all the high~r mountams had their tops covered with perpetual snow. Th1s perpetual snow, down to a fixed line, would be kept up by the ne~essary suppl.y of snow fallinrr during autumn, winter, and spnng, and this would, as we h:ve seen, depend mainly on the increased length and greatly increased cold of the winter. As both the duration and the cold of winter decreased the amount of snow would certainly decrease, and of this lesser quantity of snow a larger proportion would be melted by the longer, though somewhat cooler summer. This would follow because the total amount of sun-heat received during the summer would be the same as before, while it would act on a less quantity of snow; there would thus be a smaller surface to reflect the heat, and a smaller condensing area to produce fogs, while the diminished intensity of the sun would produce a less dense canopy of clouds, which have been shown to be of prime importance in checking the melting of snow by the sun. We have considered this case, for simplicity of reasoning, on the supposition that all geographical and physical causes remained unchanged. But if an alteration of the climate of the whole north temperate and Arctic zones occurred, as here indicated, this would certainly affect both the winds and currents, in the manner already explained (seep. 137), _so as to react upon "climate and increase the differences produced by phases of precession. How far that effect would be again increased by corresponding but opposite changes in tho southern CliAP. VII!.] TilE CAUSES OF GLACIAL EPOCHS. 153 hemisphere it is impossible to say. It may be that existinrr ?eographical and physical conditions are there such potent agei1t~ m produc1~g a state of glaciation that no change in the phases of processiOn would materially affect it. Still, as the climate of the who~e so~th.ern hemisphere is dominated by the great mass of 1ce w1thm the Antarctic circle, it seems probable that if the winter were shorter and the summer longer the quantity of ice would slightly diminish; and this would again react on the northern climate as already fully explained. The essential p1·inciple of Climatal change restated.-The preceding discussion has been somewhat lengthy, owing to the varied nature of the facts and arguments adduced, and the extreme complexity of the subject. But if, as I venture to hope, tho principle here laid down is a sound one, it will be of the greatest assistance in clearing away some of the many difficulties that beset the whole question of geological climates. This principle is, briefly, that the great features of climate are determined by a combination of causes, of which geographical conditions and the degree of excentricity of the earth's orbit :1.re by far the most important; that when these combine to produce a severe glacial epoch, the changing phases of precession every 10,500 years have very little, if any, effect on the character of the climate, as mild or glacial, though it may modify the seasons; but when the excentricity becomes moderate and the resulting climate less severe, then the changing phases of preces:sion bring about a considerable alteration, and even a partial reversal of the climate. Tho reason of this may perhaps be made clearer by considering the stability of either very cold or very mild conditions, and the comparatjve instabjli ty of an intermediate state of climate. Wher~ a country is largely covered with ice, we may look upon it as possessing the accumulated or stored-up cold of a long series of preceding winters; and however much heat is poured upon it, its temperature ca.nnot be raised above the freezing point till that store of cold is got rid of-that is, till the ice is all melted. But the ice itself, when extensive, tends to its own preservation, even under the influence of heat; for the chilled atmosphere becomes filled with fog, and this keeps |