TEMPERATURE OF SUBSOIL 181 



surface has ceased. Thus the date of maximum temperature 

 in the subsoil occurs later and later in the year with increas- 

 ing depths, till at a certain point the maximum temperature 

 in the subsoil takes place at the same time as the minimum 

 temperature at the surface, and the seasons are reversed. At 

 still greater depths the maximum may occur yet later, till 

 its influence disappears altogether. The average dates of 

 maximum and minimum temperature at various depths of 

 subsoil at Greenwich and Edinburgh are as follows : 



Greenwich. Edinburgh. 



The passage of heat through the Edinburgh soil is evidently 

 slower than through that at Greenwich. 



In temperate climates, in the northern hemisphere, the sub- 

 soil at moderate depths will be cooler than the surface soil 

 from April to August, and warmer than the surface from 

 September to February or March. The precise date at which 

 the relation changes depends on the depth in question, and 

 on other local conditions. Towards the end of September, 

 and in March, a fairly uniform temperature will be found to 

 a considerable depth in the subsoil. The general relations of 

 the temperature of the subsoil at moderate depths to those 

 observed at the surface, and in the air, will be gathered from 

 the following table containing the observations made at the 

 Pennsylvania State College in 1893. The soil is described as 

 a compact loam for the first seven inches, the subsoil being a 

 stiff clay. The neighbouring soil is covered with turf, but 



