SCT. XXVI. LINE OF PERPETUAL SNOW. 255 



proportion becomes latent, and less of it sensible. A 

 portion of air at the surface of the earth whose temper- 

 ature is 70 of Fahrenheit, if carried to the height of 

 two miles and a half, would expand so much that its tem- 

 perature would be reduced 50 ; and in the ethereal 

 regions the temperature is 90 below the point of con- 

 gelation. 



The height at which snow lies perpetually decreases 

 from the equator to the poles, and is higher in summer 

 than in winter ; but it varies from many circumstances. 

 Snow rarely falls when the cold is intense and the at- 

 mosphere dry. Extensive forests produce moisture by 

 their evaporation ; and high table-lands, on the contrary, 

 dry and warm the ah*. In the Cordilleras of the Andes, 

 plains of only twenty-five square leagues raise the tem- 

 perature as much as 3 or 4 above what is found at the 

 same altitude on the rapid declivity of a mountain, con- 

 sequently the line of perpetual snow varies according as 

 one or other of these causes prevails. Aspect in gen- 

 eral has also a great influence ; yet, according to M. 

 Jacquemont, the line of perpetual snow is much higher 

 on the northern than on the southern side of the Hima- 

 laya mountains. On the whole, it appears that the mean 

 height between the tropics at which the snow lies per- 

 petually is about 15,207 feet above the level of the sea ; 

 whereas snow does not cover the ground continually at 

 the level of the ocean till near the north pole. In the 

 southern hemisphere, however, the cold is greater than 

 in the northern. In Sandwich Land, between the 54th 

 and 58th degrees of latitude, perpetual snow and ice ex- 

 tend to the sea-beach ; and in the island of St. George's, 

 in the 53rd degree of south latitude, which corresponds 

 with the latitude of the central counties of England, per- 

 petual snow descends even to the level of the ocean. It 

 has been shown that this excess of cold in the southern 

 hemisphere cannot be attributed to the winter being 

 longer than ours by 7| days. It is probably owing to 

 the ice being more extensive at the south than the north 

 pole, and to the open sea surrounding it, which permits 

 the icebergs to descend to a lower latitude by 10 than 

 they do in the northern hemisphere, on account of the 

 numerous obstructions opposed to them by the islands 



