PERCOLATION. 101 



fact, that during the conversion of every pound of 

 it into vapor, as much heat is consumed and lost 

 as would be produced by burning two or three 

 ounces of coal ; and when you think that an ordi- 

 nary rain-fall amounts to three thousand tons per 

 acre every year, you can easily perceive that the loss 

 of heat by the evaporation of a comparatively in- 

 considerable portion of this must involve a great 

 cooling of the land." 



Although some evaporation will take place even 

 upon drained land, yet, if we can convey off the most 

 of the surplus water after it has passed through 

 the substance of the soil and has deposited its fer- 

 tility, just that amount of heat is gained which 

 would have been required for its evaporation. 



The drained field will be in readiness to till in 

 the spring ten days before that which is undrained, 

 and vegetation upon it will not become checked so 

 suddenly in the fall, but will be hastened to maturity 

 during the entire summer by the increased degree 

 of its heat. The difference in the temperature of 

 the two soils is often from ten to fifteen degrees 

 Fahrenheit. 



3. B}/ percolating through its substance. The same 

 writer again says : " Let us now consider what 

 water does by percolation ; and its effects we can do 

 little more than enumerate. They are briefly these: 

 It carries the temperature of the air into the soil, a 

 thing, the possible injury of which in the autumn 

 9* 



