EVAPORATION FROM BARE SOIL 107 



found in fine-grained soils, such as constitute alluvial deposits, 

 rather than in those rich in clay. 



Soils which have a permanent supply of water four feet 

 below the surface are naturally in a position to secure 

 exceptional advantages from capillary action. Such a cir- 

 cumstance is of course uncommon ; it should result in a high 

 condition of fertility. 



Evaporation from Bare Soil. When water is converted into 

 vapour, there is always a disappearance of heat, which. performs 

 the work of separating the molecules of water, and lifting 

 them as vapour. Without the presence of available heat, 

 no evaporation can take place. By heat we do not mean 

 temperature. On a cloudy hot summer's day there may 

 possibly be no evaporation, while on a cold winter's day 

 evaporation may be active. If the atmosphere surrounding 

 the moist surface is saturated with water vapour, no evapora- 

 tion will occur without a further increment of heat, such 

 in fact as might be afforded by sunshine. If, however, the 

 atmosphere is only partially saturated, evaporation of water 

 will take place however low the temperature ; and the tem- 

 perature of the water and air will fall till the atmosphere 

 becomes saturated, when evaporation will stop. If the un- 

 saturated atmosphere is continually renewed, as in the case 

 of a drying wind, evaporation will continue ; the heat demanded 

 for the formation of vapour being supplied by the cooling of 

 the air and water. The popular statement that evaporation 

 produces cold, is thus quite true. If one pint of water is 

 evaporated from 97 pints, the remaining 96 pints will have 

 fallen 10 F. in temperature, or an equivalent amount of 

 heat must have been supplied by surrounding bodies. 



The best way of approaching the subject of evaporation 



