496* PROFESSOR FORBES ON THE DIMINUTION OF TEMPERATURE. 



In considering subterranean temperature at small depths (which, as I 

 have observed in the body of the paper, follows a law similar to that of the 

 annual-temperature-curves in the atmosphere), the approximation obtained from 

 the two first terms of Eq. (3) is quite sufficient, and indeed the variation may be 

 confined to the second alone. In this case, C may have a value of 180°, or even 

 more (the only limit being the sensible extinction of the annual variation, which 

 will probably prevent it from ever attaining the magnitude of a whole circum- 

 ference) the actual opposition of the epochs of maximum and minimum tempera- 

 ture being observed in most soils at a depth of about 30 feet. 



