ICE AND GLACIERS. . 109 



is moist ; the mass of air is itself heated thereby, and 

 only radiates slowly into space the heat which has been 

 gained. The expenditure of heat is thus retarded as com- 

 pared with the supply, and a certain store of heat is 

 retained along the whole surface of the earth. But on 

 high mountains the protective coating of the atmosphere 

 is far thinner — the radiated heat of the ground can escape 

 thence more freely into space ; there, accordingly, the 

 accumulated store of heat and the temperatm'e 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 ex- 

 pands, part of its store of heat disappears ; it becomes 

 cooler, if it cannot acquire fresh heat from without. 

 Conversely, by renewed compression of the air, the same 

 quantity of heat is reproduced which had disappeared du- 

 ring 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 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; 



