214 THUNDER STORMS. 



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 be- 

 fore 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 mass of the atmosphere, in all that part of its 

 height which the masses of cloud occupy ; and it is 

 frequently, generally indeed, 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 decep- 

 tive, however, than the apparent lightness and clean- 

 liness of these white curls. Theirs is the region 

 of atmospheric sensibility ; and their 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 agitators in the whole, unless there be 

 some cause in the surface of the earth, such as one 

 place scorched to almost absolute dryness, while 

 another retains its average degree of moisture. A 

 large city, a barren moor, or an arid down may, in 

 very hot weather, which has been long continued, 

 be the means of producing those motions in the air, 

 the result of which is a thunder-storm ; but thunder- 

 storms that have that origin are generally very local, 

 and of short duration. 



If the storm has its origin in the upper regions 

 of the air, its primary cause must be at a greater 

 distance, and consequently more powerful, as it can 

 propagate its action through a greater volume of 

 air. The storm itself is therefore more widely ex- 

 tended, and of longer duration ; and indeed it gen- 

 erally brings a change of the weather. In those 



