THE EARTH'S OCEAN OF VAPOR. 97 



land would be attended by the destruction of 

 every plant which a freezing temperature could 

 kill." 



The amount of vapor in the air of any country, 

 though dependent on the direction from which the 

 winds come, is also markedly influenced by the 

 nature of its surface. 



The presence of forests over any section of 

 country has the effect of decreasing the rapidity 

 with which the wet surface parts with or loses its 

 water by evaporation. This decrease in the rapid- 

 ity of evaporation is caused : 



(a.) Because the air over the forest is generally 

 moister than that over the open fields, and evapo- 

 ration takes place less rapidly in moist air. 



(b.) Because the ground in the forest is shielded 

 from the direct rays of the sun. 



(c.) Because the wet ground is protected from 

 the direct action of the wind. 



The presence of the forest, therefore, tends to 

 keep the air moist for a longer time, and to thus 

 prevent the occurrence of marked contrasts in the 

 humidity of the air. 



Some experiments made in France show that 

 the rapidity of evaporation is sixty-three per cent, 

 less in the forest than in the open fields. 



9 9 



