v.] EVAPORATION COOLS THE LAND 149 



Even the stones upon the surface of the land help. 

 In the Journal of the Royal Agricultural Society for 

 1856, an experiment is described in which the flints 

 were picked off the surface of one plot of ground and 

 scattered over an adjoining plot, with the result that the 

 plot with double its usual allowance of stones was three 

 or four days earlier to harvest than the rest of the field, 

 while the plot without stones was a week later still. It 

 will always be noticed how the grass upon a field coated 

 with dung starts earlier into growth, because the loose 

 manure acts as a mulch and protects the soil from the 

 cooling due to evaporation. 



Land which is protected from evaporation, and to 

 some extent from radiation, by a layer of vegetation, is 

 always both warmer and less subject to fluctuations of 

 temperature than bare soil. 



The warming up of a well-tilled surface soil is 

 increased by the fact that the conduction of heat into 

 the soil below is much checked by a loose condition. A 

 solid body' will always conduct heat far better than 

 the same substance in the state of powder, and the 

 more compressed the powder is the better it will conduct, 

 simply because there are more points of contact. Hence 

 a rolled and tightened soil will conduct the heat it 

 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. 12) show the 

 hourly temperatures of the drained and undrained por- 

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

 /. R. Ag. Soc, 1844, 142), at depths of 7 inches and 13 

 inches respectively ; the sudden rise of temperature 



