CELLARS AND MINES. 37 



winter frost; and the workmen employed in the 

 mines, find there from year to year a temperature 

 not the least affected by the variations of that at the 

 surface. Cassini remarked, as early as in 1671, that 

 the temperature in the deep cellars under the obser- 

 vatory in Paris was invariable. Towards the end of 

 the last century a thermometer was placed there in 

 a wide vessel filled with sand, at a depth of eighty- 

 six feet; and during fifty years the instrument 

 has shown an unchanged temperature of 11°* 7 C. 

 (53°*06 F.) A like constancy in the amount 

 of heat has been found in many other places, 

 though not by observations carried on for so long 

 a time. The part of the earth's crust, that can be 

 reached by the heat coming from without, has there- 

 fore for its lower limit a level, beneath which the 

 temperature at any given place is always the same, 

 and above which, on the contrary, the changes of 

 the outer temperature are more and more per- 

 ceptible the nearer the surface is approached. In 

 the temperate zone the changes due to the heat from 

 without vanish at a depth of about sixty-four feet ; 

 at a much less depth in the hotter regions of the 

 earth, and near the equator at a depth of three or 

 four feet. 



Below this limit of constant temperature ivefind, 

 in every part of the world, the heat steadily increas- 

 ing with the depth. 



This remarkable fact, although long known to 



