220 CURRENTS IN THE AIR. 



the one above the other, or the one in opposition 

 to the other. Air moves with equal ease in all 

 directions, whatever they may be, if the impelling 

 force tends that way. So that there are often 

 many currents, moving in different directions and 

 with different velocities, within a very small space. 

 These give rise to innumerable compound motions, 

 the 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 cir- 

 cular, 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 circular motion of the whirlwind are 

 seldom of equal strength, the whirling follows pro- 

 gressively the motion of the more powerful ; and as 

 winds, more especially land winds, where the sur- 

 face 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 succes- 

 sive seconds. The first whirlwind is often taken in 

 another circulation as it moves along, and thus it 

 is made to describe 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 



