Ill] TEMPERATURE 31 



through moist soil; there is less lag in transmission 

 and hence less accumulation of heat in the top layers. 

 Dry soils are therefore hotter at the surface in sunny 

 weather, but moist soils are the more uniform in 

 temperature. April showers do much to warm the 

 subsoil by setting up heat communication with the 

 surface and enabling the warmth of the sunshine to 

 travel down ; on the other hand, winter rains cool the 

 subsoil by letting out the heat stored up there. 

 Another important factor is the difference in specific 

 heats: water requires five times as much heat to 

 raise its temperature through 1° as does dry soil, so 

 that a given quantity of sunshine is less able to raise 

 the temperature of a wet than of a dry soil. 



Besides these factors concerned in the warming 

 of the soil there is another that has the effect of 

 cooling it. Heat is required for the evaporation of 

 water, and in consequence the soil is cooled when any 

 of its moisture dries off. 



Several devices may be adopted in warming the 

 soil. The heat received may be concentrated on 

 certain parts of the soil by laying it up in ridges 

 running E. and W. and therefore facing S. The 

 amount retained (for some is always reflected back 

 into space and lost) is increased by dressing with 

 a layer of black material, soot being the most con- 

 venient. Tlie effectiveness of the heat received is 

 increased by draining away any excess of water. 



