THE NON-LIVING WORLD 159 



Such is the case when the trade winds, laden with 

 vapour from the Atlantic, begin to ascend the slopes of 

 the Andes, the summits of which even at the equa- 

 tor are clothed with perpetual snow at a height of 

 sixteen thousand feet. At such an altitude, and at so 

 low a temperature, the vapour assumes the solid form of 

 crystalline water i.e., snow. The limit at which such 

 snow can form itself on mountains called the snow-line 

 descends as we recede on either side from the equator. 

 In the Swiss Alps it is at about from 8500 to 8000 feet, 

 but descends to 5000 feet on the Norwegian mountains. 

 In the Arctic and Antarctic regions it gains, apart from 

 the influence of winds, the sea-level. The mere cold 

 of very great altitudes keeps lofty mountain-tops con- 

 stantly below 32 F. In the equatorial regions, heat 

 diminishes i for e^ery 333 feet of ascent, and therefore 

 above 15,700 feet it must freeze every night. But this 

 lowering of the snow-line does not by any means take 

 place with regularity, as the previously mentioned curva- 

 ture of isothermal lines would alone be enough to show. 

 Thus, Captain Cook found snow at the sea level in the 

 island of South Georgia between 54 and 55 of south lati- 

 tude. This is not more distant south from the equator 

 than Durham is distant to the north of it. The piling up 

 of snow by continual deposits on lofty mountains, causes 

 the mass to force its way slowly down the highest valleys, 

 through the action of gravity. Gradually solidifying, it 

 forms glaciers, and they accommodate themselves to the 

 various capacities of the depressions through which they 

 travel, by breaking themselves up. But their broken frag- 

 ments re-adhere with such speed that the mass often 

 seems as if it squeezed through narrows without be- 

 coming fractured. On their road, they score and furrow 

 the rocks they press against, and are powerful enough to 



