190 CULTIVATION. 



If we wish to satisfy ourselves that this is pi^acti- 

 cally correct, we have only to prepare two boxes of 

 finely pulverized soil one, five or six inches deep, 

 and the other fifteen or twenty inches deep and 

 place them in the sun at mid-day in summer. The 

 thinner soil will be completely dried, while the deeper 

 one, though it may have been dried in an oven at 

 first, will soon accumulate a larcre amount of water 



/ o 



on those particles which, being lower and more 

 sheltered from the sun's heat than the particles of 

 the thin soil, are made cooler. 



With an open condition of subsoil, then, such as 

 may be secured by under-draining, we fortify our- 

 selves against drought. 



2. Under-draining admits an increased supply of 

 atmospheric fertilizers, because it secures a change 

 of air in the soil. This change is produced when 

 ever the soil becomes filled with water, and then 

 dried ; when the air above the earth is in rapid mo- 

 tion, and when the comparative temperature of the 

 upper and lower soils changes. It causes new quan- 

 tities of the ammonia and carbonic acid which it 

 contains to be presented to the absorbent parts of 

 the soil. 



3. Under-draining warms the lower parts of the 

 soil, because the deposit of moisture (1) is necessarily 

 accompanied by an abstraction of heat from the at- 

 mospheric vapor, and because heat is withdrawn 

 from the whole amount of air circulating through 

 the cooler soil. 



When rain falls on the parched surface soil, it roba 



