ABOUT VOLCANOS ANP EARTHQUAKES. 9 



warmth, the same temperature, as it is called : and that 

 always and everywhere is the same, or nearly the same, 

 as the average warmth of the climate of the place. 

 Forty or fifty feet deep in the ground, a thermometer 

 here, in this spot,* would always mark the same degree, 

 49 that is, or seventeen degrees above the freezing 

 point. Under the equator, at the same depth, it always 

 stands at 84, which is our hot summer heat, but which 

 there is the average heat of the whole year. And this is 

 so everywhere. Just at the surface, or a few inches 

 below it, the ground is warm in the daytime, cool at 

 night : at two or three feet deep the difference of day 

 and night is hardly perceptible, but that of summer and 

 winter is considerable. But at forty or fifty feet this 

 difference also disappears, and you find a perfectly fixed, 

 uniform degree of warmth, day and night ; summer and 

 winter ; year after year. 



(13.) But when we go deeper, as, for instance, down 

 into mines or coal-pits, this one broad and general fact is 

 always observed, everywhere, in all countries, in all 

 latitudes, in all climates, wherever there are mines, or 

 deep subterranean caves, the deeper you go, the hotter 

 the earth is found to be. In one and the same mine, 

 each particular depth has its own particular degree of 

 heat, which never varies : but the lower always the hotter ; 

 and that not by a trifling, but what may well be called 

 an astonishingly rapid rate of increase, about a degree of 

 the thermometer additional warmth for every 90 feet of 

 additional depth, which is about 58 per mile ! so that, 

 * At Hawkhurst in Kent. 



