PHYSICAL INTER-RELATIONS IN SOILS 39 



the direction of the wind. The laws controlling evapora- 

 tion are definite. 



The heat apparently disappearing in the evaporation of 

 one pound of water would raise the temperature of 966.6 

 pounds of water one degree F. 



57. A concrete example. An acre-foot of dry loam 

 soil weighs about 2000 tons. Moisture is continually 

 passing by evaporation from the surface into the atmos- 

 phere. Normally, from uncropped, uncultivated sur- 

 faces, the rate of this evaporation may amount to 5 to 10 

 tons an acre in 24 hours. Some condition or mismanage- 

 ment may readily increase the rate of loss by 2 tons every 

 24 hours. It is not infrequently increased five times this 

 amount. 



In the evaporation of this extra 2 tons of water, enough 

 heat is used to raise the temperature of (966.6 X 2 =) 

 1933.2 tons of water one degree. The amount of heat 

 that would raise the temperature of 1933.2 tons of water 

 one degree would raise that of five times that number of 

 tons of dry loam one degree. 1933.2 tons X 5 = 9666.0 

 tons. 



The heat that would raise the temperature of 9666 tons 

 of loam one degree would raise that of an acre-foot of 

 loam, weighing 2000 tons, (fU<H) 4.8333, and it would 

 raise the acre-foot of loam, with its best capillary water 

 content, one-half of 4.833, or 2.416. 



The same heat applied to the upper six inches of the 

 acre-foot would raise its temperature (2.416 X 2 = ) 

 4.832 F. ; applied to the upper four inches of the acre- 

 foot, it would raise its temperature (2.416 X 3 = ) 

 7.248 F. Since the upper soil, being less compact than 

 the lower soils, weighs less to an acre-inch than do the 

 lower soils, the warming of the upper six inches and the 



