DRAINAGE. 95 



1. // greatly lessens the evil effects of drought. During the 

 hottest weather there is a great amount of water in the atmos- 

 phere which has been evaporated from the earth by heat, and 

 which is held hy heat., in the form of vapor. When this vapor 

 comes in contact with bodies sufficiently cooler than itself, they 

 take away its heat, and the vapor contracts to tl^e liquid form 

 (condenses) and is at once deposited as dew on the surface of the 

 cooler substance. At night, after a hot summer day, the earth 

 is much cooler than the air, and consequently, as it absorbs heat 

 from the atmosphere and from the watery vapor contained in the 

 air, dew is deposited. The familiar example of a cold pitcher, 

 which seems to sweat in hot weather, while it is only absorbing 

 heat from the air, and causing the vapor of the air to be deposited 

 in a liquid form, is an illustration of the action of this law of 

 condensation. In like manner, a knife-blade condenses dew from 

 the breath, by depriving the moisture in the breath of its heat, 

 and thus causing it to assume the liquid form. 



So, when the water is removed from the soil, the spaces between 

 its particles (which, before drainage, had been filled with water) 

 are occupied by air, and, to a greater or less extent — owing to 

 the motion of the air above the surface caused by winds, and 

 to the effect of changes of temperature below the surface — this 

 air is constantly changing, and that which enters from above, 

 charged with vapor, gives up its heat and therefore its moisture, 

 both of which are absorbed by the lower and cooler soil. In 

 consequence of this action — especially where the surface of the 

 soil is kept in a loose condition, so as to admit air freely — 

 drained lands withstand drought better than those which are 

 undrained. 



2. // enables the soil to receive a larger supply of the fertilizing 

 gases of the atmosphere^ (^carbonic acid and ammonia.) The air 

 always contains more or less of these gases, which, with water, 

 are the chief sources of the materials of which plants are made. 

 When the water which fills the spaces between the particles of 

 the soil is drawn off, air enters and takes its place, and the car- 

 bonic acid and ammonia are absorbed, ready to be taken ap by the 



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