PRIMER OF PRIMERS. 193 



From every water surface of the earth vapor is 

 almost constantly passing into the atmosphere. 

 This vapor diffuses through the air over such 

 water surfaces, and is carried hy the winds to 

 different regions of the earth. 



The heat which turns water into vapor disap- 

 pears, or becomes what is commonly called latent 

 heat. When such vapor is sufficiently chilled and 

 falls as rain, snow, or other form of precipitation, 

 the latent heat becomes sensible and warms the 

 surrounding air. 



The rapidity with which water evaporates or 

 passes into the air as vapor varies with the follow- 

 ing circumstances : 



1. The extent of the surface exposed. 



2. The temperature of the air. 



3. The quantity of vapor already in the air. 



4. The pressure. 



The vapor which passes into the air exerts a 

 considerable influence in moderating the extreme 

 temperatures that would otherwise exist in the 

 equatorial and polar regions of the earth, in the 

 following ways, viz. : 



1. By effecting an interchange between the 

 excessive heat of the equatorial regions and the 

 excessive cold of the polar regions, 

 i n 17 



