EXPLANATION. 117 



at night, yet the whole is not thus dissipated, 

 for when once the ground is heated, it parts 

 with its heat very slowly, in consequence of its 

 bad conducting powers. Hence the earth retains 

 permanently a certain amount of heat in its 

 crust, which was originally derived from the 

 sun. At a depth varying from 40 to 100 feet 

 below the surface there is a stratum at which the 

 temperature appears to be invariable, and corre- 

 sponding with the mean annual temperature of 

 the surface. The diurnal variations of tem- 

 perature are not perceived below two or three 

 feet. At Paris the stratum of invariable tem- 

 perature is ninety feet below the surface; in the 

 tropics three or four feet ; in temperate regions 

 generally fifty to sixty feet. This constant 

 temperature is highly advantageous to both the 

 animal and vegetable creation. If the earth 

 were a metal ball, it would be so hot during the 

 day in summer, as to scorch all substances on 

 its surface, while during the night it would cool 

 down so rapidly as to freeze everything with the 

 severest cold. In a word, it would have no con- 

 stant temperature of its own, like that which it 

 now possesses. 



What we have to consider is, whether the 

 increasing heat felt in descending into the earth 

 is due to this cause alone ? Down to a certain 

 depth, which varies according to climate, the 



