228 INTERNAL HEAT OF THE EARTH. SECT. XXV. 



otherwise have, and must eventually increase it, but by a 

 quantity so evanescent that it is hardly possible to conceive a 

 time when a change will become perceptible. 



The temperature of space being so low as 239 Fahrenheit, 

 it becomes a matter of no small interest to ascertain whether the 

 earth may not be ultimately reduced by radiation to the tempera- 

 ture of the surrounding medium ; what the sources of heat are ; 

 and whether they be sufficient to compensate the loss, and to 

 maintain the earth in a state fit for the support of animal and 

 vegetable life in time to come. All observations that have been 

 made under the surface of the ground concur in proving that there 

 is a stratum at the depth of from 40 to 100 feet throughout the 

 whole earth where the temperature is invariable at all times and 

 seasons, and which differs but little from the mean annual tem- 

 perature of the country above. According to M. Boussingault, 

 that stratum at the equator is at the depth of little more than 

 a foot in places sheltered from the direct rays of the sun ; but 

 in our climates it is at a much greater depth. In the course of 

 more than half a century the temperature of the earth at the depth 

 of 90 feet, in the cellars of the Observatory at Paris, has never 

 been above or below 53 of Fahrenheit's thermometer, which is 

 only 2 above the mean annual temperature at Paris. This 

 zone, unaffected by the sun's rays from above, or by the internal 

 heat from below, serves as an origin whence the effects of the 

 external heat are estimated on one side, and the internal tem- 

 perature of the globe on the other. 



As early as the year 1740 M. Gensanne discovered in the lead- 

 mines of Giromagny, in the Vosges mountains, three leagues from 

 P>e'fort, that the heat of the ground increases with the depth below 

 the zone of constant temperature. A vast number of observations 

 have been made since that time, in the mines of Europe and 

 America, by MM. Saussure, Daubnisson, Humboldt, Cordier, Fox, 

 Eeich, and others, which agree, without an exception, in proving 

 that the temperature of the earth becomes higher in descending 

 towards its centre. The greatest depth that has been attained is 

 in the silver-mine of Guanaxato, in Mexico, where M.de Humboldt 

 found a temperature of 98 at the depth of 285 fathoms, the 

 mean annual temperature of the country being only 61. Next 

 to that is the Dalcoath copper-mine, in Cornwall, where Mr. 

 Fox's thermometer stood at 68 in a hole in the rock at the 



