298 THE POPULAR SCIENCE MONTHLY. 



The fact of an elevated temperature in the depths of the earth can 

 no longer be doubted, though the law according to which the heat 

 increases as we descend below the surface is still far from being per- 

 fectly understood. As early as the seventeenth century Father Kir- 

 cher mentions the subterranean heat that was felt at the bottom of 

 mines.* Boerhaave and Boyle also make mention of observations con- 

 cerning the heat existing in the center of the earth. Still, it was not 

 until 1740, nearly a century and a half after the invention of the ther- 

 mometer, that a serious attempt was made to measure this heat. This 

 was first done by Gensanne, director of the lead-mines of Giromagny 

 (Vosges), who lowered a thermometer to a depth exceeding four hun- 

 dred metres, and proved that the temperature increases at the rate of 

 one degree to nineteen metres. Toward the end of the century Hor- 

 ace de Saussure, desiring to ascertain whether the earth's proper heat 

 had any effect on the melting of glaciers, made a similar experiment 

 in the salt-mines of Bex, and found the rate of increase to be 1 to 37 

 metres. Many similar experiments have since been made ; it will suf- 

 fice to cite the most important. 



Cordier, in his celebrated " Essay on the Temperature of the Inte- 

 rior of the Earth," read at the Academy of Sciences in the year 1827, 

 compiled the results of his predecessors' researches in this matter and 

 those obtained by himself in certain mines. In the mines of Carmeaux 

 (Tarn) he found an increase of 1 to 36 metres, 1 to 19 metres in the 

 mines of Littry (Calvados), and 1 to 15 metres at Decize (Nievre). 

 The average of his compilations is 1 to 25 metres. From these inves- 

 tigations he concluded that at a depth of some hundreds of kilometres 

 the heat must be 100 of Wedgwood's pyrometer sufficient to fuse 

 lava. 



To arrive at trustworthy results, it is not enough to observe merely 

 the temperature of the air at the bottom of a mine, or that of the 

 water that penetrates the adits, but the thermometers should be placed 

 in cavities made in the natural rock, and left there a sufficient length 

 of time to allow them to acquire the temperature of the surrounding 

 medium. The currents in the air of mines lower the normal tempera- 

 ture, particularly by producing an evaporation of the moisture in the 

 rock, and it thus happens in some mines that the temperature of the 

 air is lower than that of the surface-air, as is the case in the Maestricht 

 quarries. The heat due to the presence of workmen modifies the 

 effect of this in a measure. It is estimated that in a gallery 4,650 

 metres long, and two metres high by one wide, the temperature will be 

 raised 1 by ten men, each furnished with his lamp. As regards the 

 water found in the adits, it is evident that they will not indicate the 

 mine's true temperature unless they remain in it for a considerable 

 time, for the water infiltrated from the surface, or coming from springs 

 at certain depths, may be either warmer or colder than the rocks 



* " Mundus Subterraneus," 1664, vol. ii. 



