CURRENTS IN THE AIR. 213 



causes of which it is impossible to separate, or in 

 any way to understand, unless when they produce 

 some specific effect upon visible substances. 



When different currents set obliquely against each 

 other they produce a double motion, one circular, 

 and the other progressive. The circular motion is 

 a whirlwind, and it may have any degree of force, 

 from that which just twists the finest blades of grass, 

 or stirs the lightest dust on land, or dimples the 

 water with faint revolving circles, to that which 

 twists up trees by the roots, or wrenches off their 

 boughs, and raises them in the air, or wrenches the 

 masts of ships, or twists up the sea itself in water- 

 spouts. As the two winds which produce the cir- 

 cular motion of the whirlwind are seldom of equal 

 strength, the whirling follows progressively the 

 motion of the more powerful ; and as winds, more 

 especially land winds, where the surface is much 

 varied, blow in gusts, the centre of the whirlwind, 

 whether it be shown by a column of dust on land 

 or a column of water at sea, is very seldom a straight 

 line, or the same curve for two successive seconds. 

 The first whirlwind is often taken in another circu- 

 lation as it moves along, and thus it is made to de- 

 scribe circles in its progress. The same thing may 

 be observed in the water : a little revolving dimple 

 often floats down the stream, till it is taken in the 

 eddy of a reach, and there it will keep whirling for 

 many revolutions before its own motion be overrun 

 by that of the eddy. 



Many of the whirling motions of the air never 

 reach the surface of the earth ; and so the only 

 means that we have of judging of them are the 

 clouds. These are often in very wonderful commo- 

 tion ; and especially before thunder-storms, it is no 

 unusual thing to see them moving in twenty differ- 

 ent directions at different rates, while some are 

 whirling round horizontally, others tumbling in a 

 vertical manner, and others again moving backwards 



