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Show 140 ISLAND LIFE. (PAllT I,' doubt warm the vapour in the atmosphere, but this heat would be carried off to other parts of the earth, while a considerable portion of the whole would be lost in space. It must also be remembered that an enormous quantity of heat is usell up in melting snow and ice, without raising its temper~ture ; each cubic foot of ice requiring as much beat to melt It as would raise nearly six cubic feet of water 30° F. It has, however, been ar<rued that because when water is frozen it evolves just as much0 beat as it requires to melt it again, there is no loss of heat on the whole; and as this is adduced as a valid argument over and over again in every criticism of Dr. Croll's theory, it may be well to consider it a little more closel!' In the act of freezing no. doubt water gives up some of Its heat t o the surrounding air; but that ni?· still 1·emains below ~he fr eezing point or free2ling would not take place. The heat hberated by freezing is, therefore, what may be termed low-grade heatheat inca·pable of melting snow or ice; while the heat absorbed while ice or snow is melting is high-grade heat, such as is capable of melting snow and supporting vegetable growth. Moreover, the low-grade heat liberated in the formation of snow is usually liberated high up in the atmosphere, where it may be carried off by winds to more southern latitudes, while the heat absorbed in melting the surface of snow and ice is absorbed close to the earth and is thus prevented from warming the lower atmosphere, which is in contact with vegetation. The two phenomena, therefore, by no means counterbalance or counteract each other, as it is so constantly and superficially asserted that they do. Effect of Clmt;ds and Fog in cutting off the S~m' s heat.-Anothcr very important cause of diminution of heat during summer in a glaciated country would be the intervention of clouds and fogs, which would reflect or absorb a large proportion of the sun-heat and prevent it reaching the surface of the earth; and such a cloudy atmosphere would be a necessary result of large areas of hi<rh land covered with snow and ice. That such a 0 prevalence of fogs and cloud is an actual fact in all icc-clad countries has been shown by Dr. Croll most conclusively, and he has further shown that the existence of perpetual snow often C'll AP, \'lll .] TilE CAUSES OF GLACIAL EfOCIIS. 141 ~epends upon it. S~uth Georgia in the latitu~ of Yorkshire Is almost, .and ~and wiCh Land in the latitude of the north of Scotland, IS enti~ely covered with perpetual snow . yet in their summer the sun th 'll' ' . . IS ree mi IOn miles nearer the earth than it iS m our summer, and the heat actuall . d f h • c Y receive rom t e sun ~nust be suffiCient to raise the temperature 20o -.:.., h' h th th 1 · d · L' • Ig er an dm' · e same atit. u es in the northern h em1. sp h ere, were the con-ltiOns equal-I~stead of which their summer temperature is probably full 20 lower. The chief cause of this can only be that the heat of the sun does not reach the surface of th tl . d th t h . . h e ear 1 , an . a t IS IS t e fact is testified by all A n t arc t.'1 0 voyagers. Darwin notes the cloudy sky and constant moisture of the souther~ part of Chile, and in his remarks on the climate and producti.ons ~f the. Antarctic islands be says: "In the Southern Ocean the wmter IS not so excessively cold, ·but the summer is far less hot (than in the north),for ·th~ clouded sky seldom allows the sun to warm the ocean, itself a bad absorbent of heat . and hence the mean temperature of the year, which regulate~ the Rzo ne of ·p erpetually congealed under soil ' is low·" s·I r J ames ~ss, Lieutenant Wilkes, and other Antarctic voyagers speak of the snow-s. torms, the. absence of sunshine , and the f reez·m g tempera. tu.r e m the hmg·ht of summer·' and D r. C ro 11 sh ows that this IS a constant phenomenon accompanying the presence of large masses of ice in every part of the world.l · . In reply to the objections of a recent critic Dr. Croll has g1ven a new proof of this imp~rtant fact by comparing the known amo_nnt of snow-fall with the equally well-known meltin<r power of d1~e?t s~n-heat in different latitudes. He says : "Tb: annua~ preCipitatiOn on Greenland in the form of snow and rain, ~ccordmg to. Dr. Rink, amounts to only twelve inches, and two m. ches of this h. e considers is never melted , but I·s carn·e d away m the form of web~rgs. The quantity of heat received at the equator from sunnse to sunset, if none were cut off by the atmosphere, would melt 3 ~3 inches of ice or 100 J!eet · • • · ' jl In a year. The quantity received between latitude 60° and 80°, which is 1 For .numer~us details and illustrations see the paper- " On Ocean .Cu.rren 1 ts m r.elabo~ to the Physical Theory of Secular Changes of Climate .. , -m t 1e Plnlosoph~cal ~llfagazine, 1870. |