DRAINAGE. 19 



varied from six to ten degrees, equivalent to an eleva- 

 tion of from 1900 to 2500 feet. A rainfall of two inches 

 on an acre of land would weigh two hundred tons, and 

 require over twenty tons of coal to evaporate it; at least 

 one-third of this amount of heat would be abstracted 

 from the soil, the balance being furnished from the atmos- 

 phere. In a wet time an acre of some soils will contain 

 one thousand spare hogsheads of water, which the sooner 

 it is got rid of the better for the soil and the crops growing 

 in it. Such an amount of surplus water in the soil, if 

 taken from it by evaporation alone, would take over a 

 month, as the rate of evaporation per acre on a mid- 

 summer's day is about twenty-five hogsheads. So that the 

 necessity of artificial under-drainage is very apparent on 

 soils where the substratum does not afford a natural 

 outlet. 



The advantages of under-draining are that it makes the 

 soil warmer by decreasing the evaporation, and so makes 

 it earlier; it prevents injuries from drought because a 

 larger amount of moisture is contained in the soil, owing 

 to the greater depth to which it is absorbed; it causes the 

 manure in the soil to decompose more rapidly, for manure 

 decomposes very slowly when saturated with water, and 

 the water draining through the soil takes down with it the 

 various gases it has absorbed from the atmosphere, all of 

 which are elements of plant food. 



It often requires much study and consideration to lay 

 out a plan of drainage, as this has to be decided according 

 to circumstances, which are seldom exactly alike in any 

 two localities; the relative position of the site in regard to 

 higher and lower levels of the land adjoining, the undula- 

 tions of the land composing the site, the means of getting 

 rid of the drainage water, and the character of the subsoil 

 all being elements which must enter into the calculation. 



