THE EARTH'S OCEAN OF VAPOR. 95 



heated to sustain the present life of the earth, were 

 the water surfaces replaced hy land. 



Not only does a water surface heat more rapidly 

 than a land surface, but the vapor which arises 

 from it, locks up much of the heat in a form that 

 is sometimes popularly called latent heat. 



To change a pound of ice, at thirty-two degrees 

 Fahrenheit, into a pound of water, at thirty-two 

 degrees Fahrenheit, requires one hundred and 

 forty-two heat units, or one hundred and forty- 

 two times as much heat as is required to raise 

 the temperature of a pound of water one degree 

 Fahrenheit. To convert one pound of water at 

 sixty degrees Fahrenheit into vapor requires nearly 

 one thousand heat units, an amount of heat that 

 would be able to raise more than six pounds of 

 ice-cold water to the temperature of its boiling- 

 point. When the vapor is condensed and falls as 

 rain or snow, this heat reappears and raises the 

 temperature of the air. When, therefore, the ex- 

 cessive heat of the sun in the equatorial regions 

 falls on the extended water surfaces, much of the 

 heat is absorbed by the vapor, and the air is pre- 

 vented from growing too hot. This vapor is car- 

 ried by the winds to the polar regions, where it 

 gives up its heat to the air, and falls as rain or snow. 



