132 



THE TEMPERATURE OF THE SOIL [chap. 



receives more rapidly to the lower layers than one 

 which is loose and pulverulent. King has shown that, 

 despite the increased evaporation, there is always a 

 higher temperature below a rolled than an unrolled 

 surface. 



A few observations may be given showing the effect 

 of drainage in enabling the sun's heat to raise the 

 temperature of soil. The curves (Fig. n) show the 

 hourly temperatures of the drained and undrained por- 

 tions of a peat bog during two last days in June (Parkes, 

 /. R. Ag. Soc, 1844, I 4 2 )> at depths of 7 inches and 13 

 inches respectively; the sudden rise of temperature 

 between 3 and 4 P.M. on the second day was due to a 

 thunderstorm, during which heavy rain at a temperature 

 of j2>° F. was falling. 



The figures in the table below are derived from 

 observations made by Bailey-Denton in 1857 (/. R. 

 Ag. Soc, 1859, 273), on a stiff clay soil situated on the 

 Gault at Hinxworth, the drains being 4 feet deep and 

 25 feet apart in the drained part It is noteworthy 

 that the temperature of the air 9 inches above the 

 surface is higher for the drained than for the undrained 

 land, thus supplying further evidence of the cooling 

 effect ot evaporation. 



Mean Temperature °F. at 9 a.m. 



