EVAPORATION AND CONDENSATION. 293 



cumbent air. Such a state of things generally 

 forebodes an approaching thunder storm. 



That the condensation of atmospheric vapour 

 is owing to the abstraction of its caloric by 

 colder currents of air, is evident from the fact, 

 that in the tropical ocean far from land, where the 

 trade wind blows steadily in one direction, and 

 where the temperature seldom varies more than 

 2 or 3 degrees, there is less rain than in the 

 vicinity of continents and large islands, where 

 currents of air of different temperatures fre- 

 quently meet. 



In the great desert of Sahara, there is scarcely 

 any rain, because the wind blowing over it is 

 generally in the same direction ; while the va- 

 pour transported by it from the ocean is still 

 farther rarefied by the intense heat reflected from 

 the scorching sands, where there are no moun- 

 tains to condense it. There are also long 

 droughts in Egypt, Palestine, New Holland, 

 and many other parts of the world where the 

 winds blow long in one direction, without en- 

 countering colder currents. During summer, in 

 the United States, the atmosphere is often so 

 much heated, that the vapour brought from the 

 ocean by southern winds is not condensed for 

 many weeks, and sometimes two months, but is 

 farther expanded, until it becomes saturated, 

 and meets with a current from the northern 



