VII.] COLD SOILS. 69 



the heat is now entirely used in converting water 

 into steam, or evaporating it. Water which has to be 

 evaporated from the soil, requires heat to accomplish 

 the work, just as if that water were boiled away in a 

 vessel, upon a fire of coal. 



139. It has been calculated, that the water which 

 has to be evaporated from an acre of undrained land 

 in the course of a year, may be taken as equal to the 

 work of from 200 to 300 tons of coal. The sun's 

 rays which fall upon such land do not warm undrained 

 land, and thereby encourage vegetation, for they have 

 in the first place to dry the land. Every warm 

 breeze which passes over it, instead of favouring plant 

 life, is chilled by the evaporation of water. In fact 

 the heat which ought to be used for stimulating the 

 growing crop, is very largely employed in drying the 

 land. Such lands are consequently recognized as 

 "cold. 11 



140. Drained land is warmer than undrained land 

 for another reason. It is well known that, generally 

 speaking, warm water is lighter than cold water, and 

 will rise to the top of any vessel in which it is con- 

 tained. Suppose a warm breeze passes over land 

 containing stagnant water, the heat which is not 

 employed in evaporating the water will warm it 

 slightly, but the water thus warmed will remain at the 

 top, and the heat will not be communicated to the 

 soil beneath. On the other hand, suppose a cold 

 breeze to pass over the undrained land, this will chill 

 the water at the top, but the water thus cooled will 

 immediately sink into the soil, and be replaced by the 

 warmer water from beneath. Thus it robs the soil of 

 a part of its heat. In land in which a proper circula- 

 tion of water is secured, the warm rain, instead of 

 remaining at the top and losing its heat uselessly, 

 penetrates through the soil, and warms it. The influ- 

 ence of this increase of temperature upon the produc- 

 tiveness of the land is very striking. The harvest 



