90 OUTLINES OF FORESTRY. 



IX. THE EARTH'S OCEAN OF VAPOR. 



FROM every water surface on the earth there is 

 almost constantly rising and passing into the air 

 an invisible form of water called vapor. 



Yapor is formed wherever water is sufficiently 

 heated under such circumstances that its particles 

 have freedom to expand, and thus occupy a greater 

 space. 



The waters of the earth are caused to pass into 

 the atmosphere as vapor mainly by the heat of the 

 sun. 



The vapor that passes into the air from the 

 ocean and other water surfaces spreads or diffuses 

 through the air, and is carried by the winds to 

 different parts of the earth's surface. The air di- 

 rectly over a water surface is, however, generally 

 moister than that over a land surface. 



When, by any cause, water vapor loses the heat 

 which caused it to become a vapor, it again be- 

 comes visible as dew, fog, cloud, or mist, or falls 

 as rain, hail, or snow. 



The rapidity with which water surfaces throw 



