424 Irrigation and Drainage 



temperature. Stated in another way, the amount of 

 sunshine which will warm a given weight of water 

 10 F. will raise the temperature of an equal weight 

 of dry sand 52.38 F., clay 44.58 and humus 22.6. 

 It is plain, therefore, that very wet soils must warm 

 in the sun more slowly because the wate'r which they 

 contain tends to hold the temperature down. 



The chief cause, however, which makes a wet, 

 undrained soil colder than the better drained one, is 

 the cooling effect which results from the more rapid 

 evaporation of water from the wetter soil surface. 

 When the bulb of one of two similar thermometers 

 is covered with a jacket of muslin moistened with 

 pure water, and the two are swung side by side in 

 a dry air, it will often be observed that the bulb bear- 

 ing the moist cloth will have its temperature lowered 

 as much as 20 F. by the cooling effect of evaporating 

 water. So, too, when water evaporates from any sur- 

 face, no matter what, its temperature is lowered in 

 proportion to the rate at which evaporation is taking 

 place. The teakettle boiling over the hot fire has 

 its temperature constantly held down to 212 by the 

 rapid evaporation of water, although the heat of the 

 fire playing upon it is very many degrees hotter. 



It is the same way with a wet soil through which 

 water is continually brought to the surface as rapidly 

 as it can be evaporated in the heat of the sunshine. 

 The loss of the water in this way necessarily holds 

 the temperature down, and the lower the more rapidly 

 the evaporation takes place. The following table* 



*The Soil, p. 227. 



