296 



THE ATMOSPHERE AND METEOEOLOGY. 



winds, similar to the circles which the water of a river describes 

 above a rock.* 



As to whirlwinds, properly so called, they are phenomena of 

 small importance compared to the cyclones ; but, like them, they are 

 due to the encounter of two, more or less considerable, masses of air, 

 which strike against each other obliquely. Still they do not turn in- 

 variably in one direction for each hemisphere, for they are not caused. 



Fig. 120.— Stonn in the Pyrenees ; after Lartigue. 



like the hurricanes, by the strife of two regular winds, but may arise 

 from the conflict of all the currents of air, either normal or variable, 

 which traverse the surface of the earth. Observers have seen in the 

 same regions whirlwinds which revolve from the north to the south, 

 some passing by the west, and others by the east. During a l^empest 

 there may even form on each side of the atmospheric current, as on the 

 shores of a fluvial current, a series of eddies revolving in the contrary 

 * Lartigue, Essai sur les Ouragans et Ics TempHes. 



