182 



RELATIONS OF SOIL TO HEAT 



the soil immediately over the thermometers is kept free from 



vegetation. 



TABLE XXX 



MEAN MONTHLY TEMPERATURES OF AIR AND SOIL, 

 PENNSYLVANIA, 1893 



The temperature of the subsoil is affected not only by the 

 varying temperature of the soil above it, it is affected equally 

 by the less varying temperature of the soil beneath it ; the 

 temperature of every particle of soil is in fact the mean of 

 the temperatures of the particles surrounding it. Towards 

 the end of winter, the heating of the subsoil from below is 

 often very apparent. To perceive this we must study the 

 temperature of a layer of subsoil equidistant from layers above 

 and below it, the temperatures of which are also determined. 

 An excellent opportunity of doing this is afforded by the 

 observations made at Pekin by H. Fritsche (Repertorium fur 

 Meteorologie, ii). 



