126 HEAT OF EARTH'S OUTER CRUST, 



For the same reason, the inner strata lose but 

 very slowly the heat which has once been imparted 

 to them ; and the temperature of the outer layers 

 is already on the rise, before the cooling can have 

 reached to any depth, but while it is still going 

 on within. Thus the daily variations of tempera- 

 ture, especially all rapid and irregular changes, are 

 scarcely perceptible at a few inches below the sur- 

 face, and are soon entirely lost as the depth 

 increases. However, the condition of the soil is 

 certainly not without influence in modifying these 

 processes. Thus, in dense and solid rocks, the heat 

 is usually conducted somewhat more quickly than 

 in light spongy soils ; in the latter, on the other 

 hand, the rain-water, and with it its temperature, 

 can penetrate to greater depths. 



The effect of the daily variations reaches in 

 Central Europe, on an average, to a depth of 

 about one foot and a half; at the depth of three 

 feet they are no more perceptible, — that is, pro- 

 longed alterations of the external temperature are 

 only felt at this depth after several days. At 

 four or five feet below the surface such changes 

 only take effect, as occur from week to week, 

 and still deeper, only those from month to month ; 

 till at last we reach the limits at which the tem- 

 perature is invariable from year to year. 



The experiments which have hitherto been 

 undertaken for the purpose of ascertaining this 



