M. Arago's Historical Eloge of Joseph Fouriei: 231 



heat, not derived from the sun, and which, if we may judge 

 by the rapid increase shewn by observations, ought to be 

 powerful enough, at the trifling depth of from fifteen to twenty 

 miles, to fuse all known substances, the next question is to de- 

 termine what is its exact force at the surface of the earth ? what 

 amount should be attributed to it in estimating terrestrial tem- 

 peratures ? and what influence it exercises on the phenomena 

 of life ? According to Mairan, Buffon^ and Bailly^ this in- 

 fluence is immense. They estimate that, in France, the heat 

 disengaged from the interior of the earth is in summer 29 

 times, and in winter 400 times greater than what we receive 

 from the sun. Thus, contrary to the general opinion, the heat 

 of the heavenly body which gives us light, would only form a 

 very small portion of that, whose benign influence we experience. 



This idea was ingeniously and very eloquently developed in 

 the Memoirs of the Academy ; in Buffoii's " Epoques de la Na- 

 ture ,•"" and in Baillifs letters to Voltaire on the origin of tJw 

 Scierices, and on the Atlantide. But the ingenious romance 

 of which it served as the basis, was dispersed like a shadow be- 

 fore the light of mathematics. 



Fourier, having discovered that the excess of the total tem- 

 perature of the terrestrial surface, above that which would re- 

 sult from the mere action of the solar rays, has a necessary 

 and determinate relation to the increase of temperature at dif- 

 ferent depths, was able to deduce from the experimental 

 amount of this increase, a numerical determination of the ex- 

 cess in question. This excess is the thermometrical effect pro- 

 duced by the central heat at the surface ; now, in place of the 

 large numbers adopted by Mairan, Bailli/, and Biiff'an, what 

 did our fellow-member find it ? The thirtieth of a centigrade 

 degree ; not more. 



The surface of the globe, which, originally, was probably in- 

 candescent, was thus cooled during the course of ages so as 

 scarcely to retain any perceptible traces of its original tempe- 

 rature. Nevertheless, at great depths, the original heat is still 

 excessive. Time will make a considerable alteration on the 

 internal temperature ; but at the surface (and the phenomena 

 at the surface are the only ones which can modify or aflect the 

 existence of living beings), all the changes are very nearly ac- 



