WHIRLWINDS. 221 



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 com- 

 motion; and especially before thunder-storms, it 

 is no unusual thing to see them moving in twenty 

 different directions at different rates, while some 

 are whirling round horizontally, others tumbling 

 in a vertical manner, and others again moving 

 backwards and forwards between the larger masses, 

 as if inviting them to come together. 



Those currents and commotions are always most 

 conspicuous when the clouds are congregating 

 before thunder storms ; and when they appear in 

 several masses of strata, the one above the other, 

 there are as many currents of air of different 

 temperatures, moving in different directions, and 

 mingling together. In these cases there is often 

 no general motion of the nlass of the atmosphere, 

 in all that part of its height which the masses of 

 cloud occupy ; and it is frequently, generally in- 

 deed, a dead calm on the surface of the ground, 

 while the motionless state of the thin white curl- 

 clouds that appear through the openings, shows 

 that there is not much apparent agitation in the 

 upper air. Nothing is more deceptive, however, 

 than the apparent lightness and cleanliness of these 

 white curls. Their' s is the region of atmospheric 

 sensibility ; and then* great height diminishes to 

 our view both their magnitudes and their motions : 

 and, though they appear to be above the gathering 

 storm, the probability is that they are the real agi- 

 tators in the whole, unless there be some cause in 

 u 3 



