ICE AND GLACIERS 223 



renewed compression of the air, the same quantity of heat is 

 reproduced which had disappeared during expansion. Thus 

 if, for instance, south winds drive the warm air of the 

 Mediterranean towards the north, and compel it to ascend 

 along the great mountain-wall of the Alps, where the air, 

 in consequence of the diminished pressure, expands by about 

 half its volume, it thereby becomes very greatly cooled for a 

 mean height of 11,000 feet, by from 18 to 30 C, according 

 as it is moist or dry and it thereby deposits the greater 

 part of its moisture as rain or snow, If the same wind, 

 passing over to the north side of the mountains as Fohn- 

 wind, reaches the valleys and plains, it again becomes con- 

 densed, and is again heated. Thus the same current of air 

 which is warm in the plains, both on this side of the chain 

 and on the other, is bitterly cold on the heights, and can 

 th^re deposit snow, while in the plain we find it insupportably 

 hot. 



The lower temperature at greater heights, which is due 

 to both these causes, is, as we know, very marked on the 

 lower mountain chains of our neighbourhood. In central 

 Europe it amounts to about i C. for an ascent of 480 feet; 

 in winter it is less 1 for about 720 feet of ascent. In the 

 Alps the differences of temperature at great heights are 

 accordingly far more considerable, so that upon the higher 

 parts of their peaks and slopes the snow which has fallen in 

 winter no longer melts in summer. This line, above which 

 snow covers the ground throughout the entire year, is well 

 known as the snow-line; on the northern side of the Alps it is 

 about 8,000 feet high, on the southern side about 8,800 feet. 

 Above the snow-line it may on sunny days be very warm; 

 the unrestrained radiation of the sun, increased by the light 

 reflected from the snow, often becomes utterly unbearable, 

 so that the tourist of sedentary habits, apart from the daz- 

 zling of his eyes, which he must protect by dark spectacles 

 or by a veil, usually gets severely sunburnt in the face and 

 hands, the result of which is an inflammatory swelling of 

 the skin and great blisters on the surface. More pleasant 

 testimonies to the power of the sunshine are the vivid colours 

 and the powerful odour of the small Alpine flowers which 

 bloom in the sheltered rocky clefts among the snow-fields. 



