SNOW; hail. 255 



separated from the moist air, the clear crust of ice 

 was formed, which in almost all hailstones sur- 

 rounds a core of white within. Hailstones of 

 great size may, according to Dove's belief, be 

 kept up in the air a long time by the whirlwinds 

 which so often accompany storms of thunder and 

 hail. Hail-storms are almost peculiar to summer 

 and to the temperate zone, They occur at all 

 hours of the day, but chiefly in the afternoon. 

 Many places are much oftener visited by them 

 than others, and, where a hail-storm occurs, it 

 generally extends over but a small tract of coun- 

 try. Our knowledge of the laws of weather is as 

 yet too narrow to enable us to account for this 

 behaviour. 



On the crests of very high mountains — for 

 instance, on the Alps — single clouds are often 

 seen to hang for clays apparently motionless. 

 They are, however, in ceaseless motion, just as is 

 the moist air from which they are formed, as it 

 sweeps over the cold and perhaps snow-capped 

 peaks. With this air they travel on, and vanish 

 again as soon as they are out of reach of the cool- 

 ing influence; not, however, generally without 

 leaving behind a part of their moisture as a fall of 

 rain or snow. Thus the Alps are often, for many 

 days together, shrouded in dense clouds, from which 

 rain pours heavily every day, while over the warm 

 valley of the Po, notwithstanding the constant south 



