178 RELATIONS OF SOIL TO HEAT 



greatest variation in temperature. According to Mayer, the 

 soils of highest average temperature are those which are dry, 

 dark, and specifically heavy, as those composed of black basalt 

 or dolerite, clay slate, and some sands. 



Temperature of Subsoil. The temperature of the subsoil 

 may be affected to a considerable depth by the conditions of 

 temperature prevailing at the surface. At the Observatory 

 at Bombay the soil has an average temperature of about 83 

 at a distance of n ft. beneath the surface. At Jakutsk, in 

 Eastern Siberia, an attempt was made to obtain water by 

 sinking a well, but at a distance of 382 ft. beneath the surface 

 the soil was still frozen. The mean temperature of the air 

 in this locality was about 15 F. In many parts of North 

 America and Siberia crops are annually raised in summer 

 time upon the thawed surface soil, the subsoil remaining 

 permanently frozen. 



In the tropics, where day and night are of nearly equal 

 length throughout the year, and summer and winter are 

 unknown, the variations in the temperature of the subsoil 

 are but small ; but in higher latitudes, where the seasons are 

 very different in character, the range of temperature in the 

 subsoil becomes very considerable. The range of variation 

 is in all cases greatest near the surface, and becomes smaller 

 and smaller at increasing depths. 



At a certain depth in every soil a point is reached at which 

 the variations of the temperature at the surface cease to be 

 felt. The daily variations of temperature caused by the 

 alternations of day and night are seldom perceived below 3 ft. 

 from the surface. The effects of summer and winter may be 

 ielt at a much greater depth, but a point is at last reached at 

 which the thermometer remains unchanged throughout the 



