136 THE TEMPERATURE OF THE SOIL [chap. 



and raise the temperature of the absorbing 

 soil. 



(2) By precipitation, as in the spring when warm 



rain enters the ground and brings with it a 

 considerable quantity of heat, or when aqueous 

 vapour in the air is condensed on the colder soil. 



(3) By conduction from the heated interior of the 



earth a small amount of heat reaches the 

 surface. 



(4) By the changes which result in the decay of the 



organic material of the soil, when as much heat 

 is developed as if the same material had been 

 burnt in a fire. 



The surface soil loses heat : 



(1) By radiation; like any other body possessing 



heat, the surface of the soil is always emitting 

 invisible radiant heat, which may, or may 

 not, be counterbalanced by the corresponding 

 radiations it is absorbing. 



(2) By conduction either to cooler layers of earth 



below or to cooler air above. 



(3) By the evaporation of the water contained in 



the soil ; at ordinary temperatures the evapora- 

 tion of 1 lb. of water would absorb enough 

 heat to lower the temperature of about 7500 lbs. 

 of soil by i F. 



The actual temperature attained by a given soil at 

 any time depends upon the relative effect of the heat- 

 ing and cooling actions set out above. 



Soil Temperatures. 



The accompanying curves (Fig. 9)^ show the 

 monthly mean temperatures of the soil at 6 inches, 3 



