100 EVAPORATIOlSr. 



tion exerted upon a wet subsoil. The power of this 

 agency varies widely in different soils. In those 

 of a porous character, it is far less potent than in a 

 close firm clay or loam ; because, in the former, the 

 interstices are greater, and the attraction is over- 

 come by the gravity of the water. This principle 

 should be borne in mind when we investigate the 

 required depth of the drain ; as one of four feet, in 

 a retentive soil, Avill thoroughly drain less depth of 

 earth than if placed at the same distance below the 

 surface in a loose sand or gravel. 



As there are three sources from which the soil is 

 supplied with moisture, so there are also three ways 

 by which it is deprived of it. 



1. By running off its surface. In this way it is 

 unproductive of benefit, as the fertility which it 

 contains is lost. It fails to comminute and disinte- 

 grate the soil by percolating through its particles. 

 If the warm showers gain admittance to the earth 

 in the spring, they raise its temperature, and vege- 

 table growth commences earlier. 



2. By evaporation. As by perspiration the body 

 of an animal is kept cool, so the soil in evaporation 

 parts with a great amount of heat, which would 

 otherwise be retained to encourage the growth of 

 the plants. A writer in the Gardeners' Chronicle, 

 of England, observes, that " the principal result of 

 the loss of water by evaporation is indicated by the 



