v.] EVAPORATION COOLS THE LAND 131 



it will be seen that the wind break, by checking evapora- 

 tion, maintained the soil temperature more than 2° 

 above that of the open ground. Sufficient attention is 

 not given in practice to the value of even slight wind 

 breaks for checking evaporation and so raising the 

 temperature of the soil in early spring. The raisers of 

 specially early vegetables, radishes in particular, on a 

 strip of light land close to the sea in Kent are, however, 

 in the habit of breaking the sweep of wind across their 

 fields by erecting temporary fences of lightly thatched 

 hurdles. 



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 



