181 Mr. J. Croll on Ocean-currents in relation to the 



summers are colder than our winters. Cover India with an ice- 

 sheet, and its summers would be colder than those of England. 



Second. Another cause of the cooling effect is that the rays 

 which fall on snow and ice are to a considerable extent reflected 

 back into space. But those that are not reflected, but absorbed, 

 do not raise the temperature, for they disappear in the mecha- 

 nical work of melting the ice. The latent heat of ice is about 

 142° ¥.; consequently in the melting of every pound of ice a 

 quantity of heat sufficient to raise one pound of water 142° dis- 

 appears, and is completely lost, so far as temperature is con- 

 cerned. This quantity of heat is consumed, not in raising the 

 temperature of the ice, but in the mechanical work of tearing the 

 molecules separate against the forces of cohesion binding them 

 together into the solid form. No matter what the intensity of 

 the sun's heat may be, the surface of the ground will remain per- 

 manently at 32° so long as the snow and ice continue unmelted. 



Third. Snow and ice lower the temperature by chilling the 

 air and condensing the vapour into thick fogs. The great 

 strength of the sun's rays during summer, due to his nearness at 

 that season, would, in the first place, tend to produce an increased 

 amount of evaporation. But the presence of snow-clad moun- 

 tains and an icy sea would chill the atmosphere and condense the 

 vapour into thick fogs. The thick fogs and cloudy sky would 

 effectually prevent the sun's rays from reaching the earth, and 

 the consequence would be that the snow would remain unmelted 

 during the entire summer. In fact we have this very condition 

 of things exemplified in some of the islands of the Southern 

 Ocean at the present day. Sandwich Land, which is in the same 

 parallel of latitude as the north of Scotland, is covered with ice 

 and snow the entire summer ; and in the island of South 

 Georgia, which is in the same parallel as the centre of Eng- 

 land, the perpetual snow descends to the very sea-beach. The 

 following is Captain Cook's description of this dismal place: — 

 "We thought it very extraordinary," he says, "that an island 

 between the latitude of 54° and 55° should, in the very height 

 of summer, be almost wholly covered with frozen snow, in some 

 places many fathoms deep The head of the bay was ter- 

 minated by ice-cliffs of considerable height ; pieces of which were 

 continually breaking off, which made a noise like a cannon. Nor 

 were the interior parts of the country less horrible. The savage 

 rocks raised their lofty summits till lost in the clouds, and valleys 

 were covered with seemingly perpetual snow. Not a tree nor a 

 shrub of any size were to be seen. The only signs of vegetation 

 were a strong-bladed grass growing in tufts, wild burnet, and a 



plant-like moss seen on the rocks We are inclined to think 



that the interior parts, on account of their elevation, never enjoy 



