THE EARTH'S OCEAN OF VAPOR. 91 



off vapor into the air varies with the following 

 circumstances : 



1. "With the amount of surface exposed. 

 Evaporation takes place only at the surface; 



consequently, the greater the surface, the greater 

 the rapidity of evaporation. When wet clothes 

 are hung out to dry, they are so opened or spread 

 out that the air can act on them from all sides. 

 A pound of water placed in an open shallow dish, 

 and exposed to the air, will evaporate much more 

 rapidly than the same quantity would if placed in 

 an open, narrow-necked bottle. 



For the same reason, an equal quantity of water 

 will evaporate still more rapidly when sprinkled on 

 the surface of a sheet hung out in the air to dry. 



2. On the temperature of the air. 



The capacity of a given volume of air for 

 water in a state of vapor rapidly increases with 

 its temperature. A cubic foot of dry air at the 

 temperature of melting ice, or 32 degrees Fahren- 

 heit, when saturated, holds a little more than half 

 a grain of vapor. It then being saturated can 

 hold no more water in an invisible state. In- 

 crease its temperature, however, to 212 degrees 

 Fahrenheit, and it can hold twenty grains, or 

 about forty times as much as it formerly held. 



