Page:Popular Science Monthly Volume 17.djvu/312

298 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 perfectly understood. As early as the seventeenth century Father Kircher mentions the subterranean heat that was felt at the bottom of mines. Boerhaave and Boyle also make mention of observations concerning 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 thermometer, 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 hundred metres, and proved that the temperature increases at the rate of one degree to nineteen metres. Toward the end of the century Horace 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 suffice to cite the most important.

Cordier, in his celebrated "Essay on the Temperature of the Interior 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 (Nièvre). The average of his compilations is 1° to 25 metres. From these investigations 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 temperature, 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