188 cui/rrvATiON. 



the finer comminution of the earthy parts of the 

 Boil. 



IG. It prevents, in a great measure, the evapora- 

 tion of water, and the consequent cooling of the soil. 



17. It admits fresh quantities of water from rains, 

 etc., wliich are always more or less imbued with the 

 fertilizing gases of the atmosphere, to be deposited 

 amoiig the absorbent parts of soil, and given up to 

 the demands of plants. 



IS. It prevents the formation of so hard a crust 

 on the surface of the soil as is customary on heavy 

 lands. 



1. Under-draining lessens the effect of drought, be- 

 cause it gives a better circulation of air in the soil 

 (it does so by making it more open). There is al- 

 ways the same amount of water in and about the 

 surface of the earth. In winter there is more in the 

 soil than in summer, while in summer, that which 

 has been dried out of the soil exists in the atmosphere 

 in the form of a vapor. It is held in the vapory 

 form by heat, wliich acts as hi^aces to keep it distend- 

 "ed. When vapor comes in contact with substances 

 sufficiently colder than itself, it gives up its lieat — 

 thus losing its braces — contracts, and becomes liquid 

 water. 



This may be observed in hundreds of common 

 operations. 



It is weU known that a cold pitcher in summer 



