CHAP. 1. 3. OP CLOUDS. 11 



changes its climate, arrives in a colder air, and 

 is thereby decomposed and thrown into a state 

 of visible cloud. The simple attraction of ag- 

 geration may perhaps cause the watery particles 

 to collect in a mass,* while their being similiarly 

 electrified may render them mutually repulsive, 

 and prevent their uniting to become rain. The 

 cumulus preserves its plane base, because it 

 floats on the vapour plane, or at that precise 

 elevation at which the air has as much water in 

 solution as from its quantum of heat and press- 

 ure from above it is able to contain. Whether 

 the conical form of this cloud is to be attributed 

 to the attraction of aggregation alone, or 

 whether something particular in its electric 

 state may also be concerned, has never, I think, 

 been determined. The variation of its figure, 

 according to different states of weather, seems to 

 favour the latter supposition. 



The cumulus, then, may either evaporate, 

 change into the other modifications, or, by in- 

 osculating with any of them differently electri- 

 fied, may form the cumulostratus, and ultimately 

 the nimbus, hereafter to be described. 



* This, however, is doubtful, as I mention in the account 

 of the stratus. 



