29 LINE OF PERPETUAL SNOW. [SECT. xxvi. 



on the northern than on the southern side of the Himalaya 

 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 Sand- 

 wich Land, between the 54th and 58th degrees of latitude, 

 perpetual snow and ice extend 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, 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 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 obstruc- 

 tions opposed to them by the islands and continents about 

 the north pole. Icebergs seldom float farther to the south 

 than the Azores ; whereas those that come from the south 

 pole descend as far as the Cape of Good Hope, and occasion 

 a continual absorption of heat in melting. 



The influence of mountain-chains does not wholly depend 

 upon the line of perpetual congelation. They attract and 

 condense the vapours floating in the air, and send them 

 down in torrents of rain. They radiate heat into the atmo- 

 sphere at a lower elevation, and increase the temperature of 

 the valleys by the reflection of the sun's rays, and by the 

 shelter they afford against prevailing winds. But, on the 

 contrary, one of the most general and powerful causes of 

 cold arising from the vicinity of mountains is the freezing 

 currents of wind which rush from their lofty peaks along the 

 rapid declivities, chilling the surrounding valleys : such is 

 the cutting north wind called the bise in Switzerland. 



