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Show 128 ISLAND LIFE. [rART I. . t the temperate and even surplus heated water of the trdopws t.o . flows from the poles to . h'l col er wa ei to the polar regwns, w I e . An immense quantity of ameliorate the heat of the tropics. t' g water and the vapour . I d . n evapora m ' sun-heat IS a s~ use up I b the aerial currents to distant thus produced IS conveyed dy d 'nto rain it gives up much countn.e s wh ere, on b em· oCl' con ense I ' . of this h~at to the eart~ and att~osphere. heat is well exhibited Th wer of water m carrymg away . d e po mall hiCI'h temperature of and eserts by the fact of the abn~r Y 1 ol . bile the still more power-f d ountnes genera Y , w . . and o very ry 0 . fl f ov•m g a•u may b e appreciated ' by considenng ful m uence o m · heating a tightly-closed .re f n our northern sun m the eu.ects o eve . above the temperature produced by tl:e glass house to far h the free air and abundance of vertical sun of the equatofir ;" l ~refl ence Were it not for the · t th · r bene Cia m u · moisture exer. ei f the sun's heat carried away by air and large proportw~ o ld become uninhabitable furnaces-as t the tropics wou . h 1 ;:u~~ indeed any part of the earth where the sun shone bng t y tbrouCI'hout a summer's day. . h weo see t h ere1co re, that the excess of heat denved from t c s·u n at any p l a'c e canno t be stored up to an impor. tant adm outn t o. vnngd to the won d erf u l dispersinCoI' agency of air an . wa er ' d an houCI'h some heat does penetrate the ground and IS store . up ~h o this is so little in proportion to the whole amount received, anedr et,h e l arger part of it is so soon given out from thed sdu rfa. cea l· s that any surplus heat that may be thus ~reserve ~nno oanyee rs u' mmer rar ely or never remains in suffiCient quha ntittby to f£ t the temperature of the succeeding summer, so t at ere ~IS neco sueh thi'ng as an accumulation of earth-heat fdro m yet ar to B t though heat cannot, cold can be store up o an ~~~~st u:li:nited amount, owing to the peculiar property watet: possesses of becominCoI' solid at a moderately. low temperta ture '· d th·s is a sub1ect of the very greatest Importance o our ainn uairs -t1h e whole" question of the possr· bi' I·r ty of g 1a c!·a 1 e~o cb. s anqd wya rm p. en.o d s depending on it-we must consrder It m some detail. · th Effects of Snow on Olimate.-Lct us then exam~ne. ~ v;~~ different effects produced by water falling as a hqmd m I' HAP, VIJI.) TUE CAUSES OF GLACIAL EPOCHS. 129 form of rain, or as a solid in the form of snow, although the two may not differ from each other more than two or three degrees in temperature. The rain, however much of it may fall, runs off rapidly into streams and rivers, and soon reaches the ocean. If cold it cools the air and the earth somewhat while passing through or over them, but produces no permanent effect on temperature, because a few hours of sunshine restore to 'the air or the surface-soil all the heat they had lost. But if snow falls for a long time, the effect, as we all know, is very different, because it has no rnob,ility. It remains where it fell and becomes compacted into a mass, and it then keeps the earth below it and the air above, at or near the freezing-point till it .is all melted. If the quantity is great it may take days or weeks to melt; and if snow continues falling it goes on accumulating all over the surface of a country (which water cannot do), and may thus form such a mass that the warmth of the whole succeeding summer may not be able to melt it. It then produces -perpetual snow, such as we find above a certain altitude on all the great mountains of the globe ; and when this takes place cold is rendered permanent, no amount of sun-heat warming the air or the earth much above the freezing-point. This is illustrated by the often-quoted fact that at 80° N. Lat., Captain Scoresby had the pitch melted on one side of his ship by the heat of the sun, while water was freezing on the other side owing to the coldness of the air. The quantity of heat required to melt ice .or snow is very great, as we all know by experience of the .long time masses of snow will remain unmelted even in warm weather. We shall however be better able to appreciate the great effect this has upon climate, by a few figures ·showing .what this amount really is. In order to melt one cu hie foot of ice, as much heat is required as would heat a oubic foot of water from the freezingpoint to 176° F., or two cubic feet to 88° F. To melt a layer of ice a foot thirk will therefore use up as much heat as would raise a layer of water two feet thick to the temperature of 88° F.; and the effect becomes still more easily understood if we estimate it as applied to air, for to melt a layer of ice only 1 i inches thick would require as much heat as would raise a K |
