The Phenomena of the Atmosphere. 87 



cold, the low countries in India to the east- 

 ward of it become hotter than Africa during the 

 summer, and the air is naturally drawn thence 

 to the eastward. From the same cause the trade 

 wind in the Indian ocean blows, from April to 

 October, in a north-east direction, contrary to 

 the general course of the trade wind in the open 

 sea in the same latitude ; but when the sun re- 

 tires behind the tropic of Capricorn, these north- 

 ern parts become cooler, and the general trade 

 wind assumes its natural direction. In the north- 

 ern tropic the monsoons depend upon similar 

 causes. 



In our climate the winds are more variable, 

 because the rarefactions which take place in the 

 air are here more partial, more frequent and sud- 

 den, than in the tropical regions. I have suf- 

 ficiently explained, that whatever dilates or rare- 

 fies the air in any part must produce a wind or 

 current of air towards that part. Among the 

 most pewerful causes of winds, therefore, we 

 must account the electricity of the atmosphere, 

 which (as will be explained hereafter) is the 

 cause of thunder and lightning. A thunder 

 storm, therefore, is commonly either preceded or 

 followed by a smart gale of wind. The rays of 

 the sun are also sometimes partially interrupted 

 by clouds or mists in particular places, conse- 

 quently the earth will be more strongly heated in 

 one part than another, in which case there will 

 always be a current of air from the colder to the 



