132 



DRY-FARMING 



To understand tlie methods employed for check- 

 ing evaporation from the soil, it is necessary to review 

 briefly the conditions that determine the evapora- 

 tion of water into the air, and the manner in which 

 water moves in the soil. 



T lie formation of icater vapor 



Wlienever water is left freely exposed to the air, 

 it evaporates; that is, it passes into the gaseous 

 state and mixes with the gases of the air. Even 

 snow and ice give off water vapor, though in very 

 small quantities. The quantity of water vapor 



