426 JOSEPH FOURIER. 



the temperature of the terrestrial strata may be attributed 

 solely to the action of the solar rays. 



This being established, the increase of heat which is 

 observed in all climates when we penetrate into the inte- 

 rior of the globe, is the manifest indication of an intrinsic 

 heat. The earth, as Descartes and Leibnitz maintained 

 it to be, but without being able to support their assertions 

 by any demonstrative reasoning, thanks to a combina- 

 tion of the observations of physical inquirers with the 

 analytical calculations of Fourier, is an encrusted sun, 

 the high temperature of which may be boldly invoked 

 every time that the explanation of ancient geological 

 phenomena will require it. 



After having established that there is in our earth an 

 inherent heat, a heat the source of which is not the sun, 

 and which, if we may judge of it by the rapid increase 

 which observation indicates, ought to be already suffi- 

 ciently intense at the depth of only seven or eight leagues 

 to hold in fusion all known substances, there arises the 

 question, what is its precise value at the surface of the 

 earth ; what weight are we to attach to it in the deter- 

 mination of terrestrial temperatures ; what part does it 

 play in the phenomena of life ? 



According to Mairan, Buffon, and Bailly, this part is 

 immense. For France, they estimate the heat which 

 escapes from the interior of the earth, at twenty-nine 

 times in summer, and four hundred times in winter, the 

 heat which comes to us from the sun. Thus, contrary to 

 general opinion, the heat of the body which illuminates 

 us would form only a very small part of that whose pro- 

 pitious influence we feel. 



This idea was developed with ability and great elo- 

 quence in the Memoirs of the Academy, in the Epoques 



