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 fi-om 

 the two first terms of Eq, (3) is quite suf&cient, 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 

 wiU 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. 



