TEMPERATURE OF SUBSOIL 183 



Mean weekly Temperatures of Soil, Pekin, 1870 



Depth 0-549 ra. Depth 1-090 m. Depth 1-646 m. 



February 22 ... 0.21 C. ... 3.14 C. ... 5-84 C. 



March 1 1.43 ... 3-34 ... 5-54 



8 3.24 ... 4.17 ... 5.86 



15 4.23 ... 4-93 ... 6-30 



It is evident at a glance that the temperature of the interme- 

 diate layer of soil is rising during the period under observation ; 

 it is equally evident that the rise is not due to the heat from 

 above, as the soil above is cooler than the intermediate layer 

 during the whole of the period. The rise is clearly due to an 

 accession of heat from below. The difference of temperature 

 is in fact greater between the intermediate layer and the 

 warmer soil below, than between it and the cooler soil above, 

 and the movement of the temperature in the intermediate 

 layer is determined by the greater of these two forces. The 

 popular idea that a frozen soil often thaws from beneath as 

 spring approaches is thus perfectly correct. 



The mean temperature of a soil is not far from that of the 

 air above it. The greatest disturbance to this general rule 

 is occasioned by snow. A covering of snow in winter time 

 greatly diminishes the cooling of the soil in severe climates. 

 In parts of Russia the soil has a mean annual temperature 

 9 F. in excess of the air as a consequence of this protection 

 during winter. 



In temperate climates the mean temperature of the sub- 

 soil increases slowly with increasing depth. At Edinburgh, 

 as a mean of 40 years' observation, the mean annual tem- 

 peratures at various depths have been as follows : 



Depth 3-2 feet ... 46-34 F. 



64 ... 46.60 



12.8 ... 46.99 



25-6 ... 47.25 



