RADIATION OF HEAT 173 



We have already seen (p. 113) that keeping a few inches 

 of the surface soil in a loose pulverulent condition by frequent 

 cultivation is an effective method of conserving the moisture 

 of the soil beneath during the heat of summer. One result 

 of this cultivation is to diminish somewhat the temperature 

 of the underlying soil, and this diminution of temperature 

 is in itself favourable to the preservation of water. King 

 (Wisconsin loth Report, 190) records observations on the 

 temperature of the soil made in a maize field during the 

 month of July, in ground cultivated ij and 3 inches deep. 

 In both the third and fourth foot beneath the surface, the 

 average temperature observed in the day time was i-i F. 

 less where the deepest cultivation was maintained. In later 

 experiments (Wisconsin nth Report, 283) he shows that 

 the daily variation of temperature at a depth of i ft. was 

 slightly less where the deeper cultivation was adopted. 



Radiation of Heat. Several facts relating to the radiation 

 of heat by soil have been already noticed (p. 162), and we have 

 seen that the radiation is independent of the colour of the soil. 

 Ahr (Forsch. der Agrikulturphysik, xvii. 397) has investigated 

 the relative radiating power of various soil constituents. The 

 substance was placed in a thin layer, on a cube of warm 

 water, maintained at a constant temperature, and the heat 

 radiated by the substance measured by means of a thermo- 

 pile and galvanometer. When in a perfectly dry condition, 

 the various mineral constituents of soil proved to be better 

 radiators of heat than the organic constituents. The mineral 

 constituents showed no great difference among themselves ; 

 quartz sand was the best radiator. 



Water has a greater capacity for radiating heat than any 

 other soil constituent, it even somewhat exceeds soot in this 



