SRCT. xxvi.] INTERNAL HEAT OF THE EARTH. 277 



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 temperature of the country 

 above. According to M. Boussingault, indeed, 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 caves 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 in- 

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

 of the external heat are estimated on one side, and the in- 

 ternal temperature of the globe on the other. 



As early as the year 1740, M. Gensanne discovered in the 

 lead mines of Geromagny, three leagues from Befort, that 

 the heat of the ground increases with the depth below the 

 zone of constant temperature. A vast number of observa- 

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

 and America, by MM. Saussure, Daubuisson, Humboldt, 

 Cordier, Fox, Reich, and others, which agree, without an 

 exception, in proving that the temperature of the earth be- 

 comes higher in descending towards its centre. The greatest 

 depth that has been attained is in the silver mine of 

 Guamaxato, in Mexico, where M. de Humboldt found a tem- 

 perature 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 

 depth of 230 fathoms, and at 82 in water at the depth of 



