SECT. XXV. LINE OF PERPETUAL SNOW. 241 



would be reduced 50 ; and in the ethereal regions the tem- 

 perature is 239 below the zero point of Fahrenheit. 



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 atmosphere dry. Extensive forests 

 produce moisture by their evaporation ; and high table-lands, on 

 the contrary, dry and warm the air, because the air at great ele- 

 vations is too rare to absorb much of the sun's heat. In the 

 Cordilleras of the Andes, plains of only twenty-five square 

 leagues from their extent raise the temperature as much as 3 

 or 4 above what is found at the same altitude on the rapid de- 

 clivity of a mountain, consequently the line of perpetual snow 

 varies according as one or other of these causes prevails. Aspect 

 in general has also a great influence ; yet the line of perpetual 

 snow is much higher on the northern than on the southern side 

 of the Himalaya, partly because the air is nearly deprived of its 

 moisture by precipitation before it arrives at the northern side of 

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

 between the tropics at which the snow lies perpetually 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 extend 

 to the sea-level ; and in the island of S. Georgia, in the 53rd 

 degree of south latitude, which corresponds with the latitude of 

 the central counties of England, perpetual 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 7f days. It is probably owing to the 

 open sea surrounding the south pole, 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 and continents about the north pole. Ice- 

 bergs from the Arctic seas seldom float farther to the south than 

 the Azores ; whereas those that come from the south pole descend 

 to as low a latitude as that of the Cape of Good Hope. 



The influence of mountain-chains does not wholly depend upon 

 the line of perpetual congelation. They attract and condense 



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