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Show 136 ISLAND LIFE. [rAm' 1. from the quantity of moisture they .bring with th~m ~hich will be con d ense d m. to snow b Y cominoo- mto cont.a.c t with the frozen surf ace. W e maY. there.i1!' ore expect the transitiOn from pedr petual snow to a luxuriant arctic vegetation· to be very a?rupt, epend- m. g as 't t few derrrees more or less m the summer I mus on a o . . . . h temperat ure of th e a1· r., and this 1s qu1te. 1n acco. rdance w1t the fact of corn ripening by the sides of ~lpme glac.Iers. . . Efficiency of Astronomical Causes 'bn produc~ng Glac~atwn.- H avm· g now co 11 ec ted a sufficient body of facts, .l et us endeavour t o ascer t am· w h a t would be the state to whiCh the northern h erm·s ph ere wou ld be reduced by a hioo- h degree of e. xcentricity and a winter in aphelion. When the glacial epoch IS supposed to have been at its maximum, about 210,000 years ago, the excentricitv was more than three times as great as it is now, and, according to Dr. Groll's calculations, the mid-winter temperatur: e of the northern hemisphere would have been lowered 36° F., while the winter half of the year would have been twenty-six days longer than the summer half. This would brino- the January mean temperature of England and Scotland alm~st down to zero or about 30° F. of frost, a winter climate corresponding to that of Labrador, or the coast of Greenland on the Arctic circle. But we must remember that the summer would be just as much hotter than it is now, and the problem to be solved is, whether the snow that fell in winter would accumulate to· such an extent that it would not be melted in summer, and so go on increasing year by year till it covered the whole of Scotland, Ireland, and Wales, and much of England. Dr. Croll and Dr. Geikie answer without hesitation that it would. Sir Charles Lyell maintained that it would only do so when o-eorrraphical conditions were fa.vourable ; while the late 0 0 Mr. Belt has argued, that excentricity alone would not produce the effect unless aided by increased obliquity of the ecliptic, which, by extending the width of the polar region!:!, would increase the duration and severity of the winter to such an extent that snow and ice would be formed in the Arctic and Antarctic regions at the same time whether tho winter were in perihelion or aphelion. The problem we have now to solve is a very difficult one, CHAP. Vlii.] THE CAUSES OF GLACIAL EPOCHS. 137 because we have no case at all parallel to it from which we can draw direct conclusions. It is, however, clear from the various considerations we have already adduced, that the increased cold of winter when the exeentricity was great and the sun in aphelion during that season, would not of itself produce a glacial epoch unless. the· amount of vapour supplied for condensation was also exceptionally great. The greatest quantity of snow falls in the Arctic regions in summer and autumn, and with us· the greatest quantity of rain falls in the autumnal months. It seems probable, then, that in all northern lands glaciation would commence when autumn occurr.ed in aphelion. All the rain which falls on our mountains at that season would then fall as snow, and, being further increased by the snow of winter, would form accumulations which the summer might not be able to melt. As time went on, and the aphelion occurred in winter, the perennial snow on the mountains would have accumulated to such an extent as to chill the spring and summer vapours, so that they too would fall as snow, and thus increase the amount of deposition; bat it is probable that this would never in our latitudes have been sufficient to produce glaciation, were it not for a series of climatal reactions which tend still further to increase the production of snow. Action of Meteorological Causes in intensifying Glaciation. -The trade-winds owe their existence to the great difference between the temperature of the equator and the poles, which causes a constant flow of air towards the equator. The strength of this flow depenus on the difference of temperature and the extent of the cooled and heated masses of air, and this effect is now greatest between the south pole and the equator, owing to the much greater accumulation of ice in the Antarctic tegions. The consequence is, that the south-east trades. are stronger than the north-east, the neutral zone or belt of calms between them not being on the equator but several degrees to the north of it. But just in proportion to the strength of the trad.e-wi:nds is the strength of the anti-trades, that is, the upper r.eturn current which carries the warm moisture-laden air of the tropics towards the poles, descending in the temperate z.oUB as west and south-west winus. These are now strongest in the southern |