ICE AND GLACIERS. 97 



store of heat and the temperature are far smaller than at lower 

 levels. 



To this must be added another property of air which acts 

 in the same direction. In a mass of air which expands, part of 

 its store of heat disappears; it becomes cooler, if it cannot 

 acquire fresh heat from without. Conversely, by renewed com- 

 pression 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 Eohn-wind, reaches the valleys and plains, it again becomes 

 condensed, 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 there 

 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 1 C. for an ascent of 480 feet; in winter it 

 is less 1 for about 720 feet of ascent. In the Alps the differ- 

 ences 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 tho- 

 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 



