348 Phenomena of Thunder [Book IV. 



which can never he perceived) are feen in a rapid mo- 

 tion, driving about in very uncertain directions under 

 it. While thefe clouds are agitated with the moft ra- 

 pid" motions, the rain generally falls in the greateft 

 plenty, and if the agitation is exceedingly great, it 

 commonly hails. 



' While the thunder cloud is fwelling^ and extend- 

 ing its branches over a large tract of country, the 

 Jightning is feen to dart from one part of it to another, 

 and often to illuminate its whole mafs. When the 

 cloud has acquired a fufficient extent, the lightning 

 flrikes, between the cloud and the earth, in two oppo- 

 fite places, the path of the lightning lying through the 

 whole body of the cloud and its branches. The longer 

 this lightning continues, the rarer the cloud grows, and 

 the lefs dark is its appearance, till at length it breaks 

 in different places, and difplays a clear fky.' 



It is the opinion of the fame author, that the clouds 

 ferve as conductors to convey the electric fluid from 

 thofe places of the earth which are overloaded with it, 

 to thofe which are exhaufted of it. 



To prove that the earth is often positively charged 

 with refpect to the clouds, in one part while it is nega- 

 tive in another, he adverts to the fall of great quantities 

 of fand, and other light fubftances, which are often 

 carried into the air, and fcattered uniformly over a 

 large tract of country, when there was no wind to 

 effect this phenomenon, and even when there was, 

 they have been carried againft the wind ; he therefore 

 fuppofes, that thefe light bodies are raifed by a large 

 quantity of electrical matter hTuing out of the earth. 



This comparatively rare phenomenon, he thinks, 

 exhibits both a perfect image and demonftratlon of 

 the manner in which the vapours of the atmofphere 

 are raifed to form thunder clouds. The lame electric 



rnatter 4 



