16G JOSEPH FOURIER. 



another; tliiit in each country, finally, it ought to be always the same, 

 so long as we do not descend to depths which are too great relatively 

 to the earth's radius. 



Well, the phenomena of nature stand in manifest contradiction to this 

 result. The observations made iu a multitude of mines, observations 

 of the temperature of hot springs coming from difierent depths, have 

 all given an increase of one degree of the centigrade for every twenty 

 or thirty meters of depth. Thus, there was some inaccuracy in the hy- 

 pothesis which we were discussing ujjon the footsteps of our colleague. 

 It is not true that 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 interior of the globe is the mani- 

 fest indication of an intrinsic heat. The earth, as Descartes and Leib- 

 nitz maintained it to be, but without being able to support their asser- 

 tions by any demonstrative reasoning, — thanks to a combination of the 

 observations of physical inquirers with the analytical calculations of 

 Fourier, — is an uicrusted 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 tliat tlun-e is iu our earth an inherent heat — 

 a heat the source of which is not the sun, and whicli, if we may judge 

 of it by the rapid increase which observation indicates, ought to be 

 already sufticiently intense at the depth of only seven or eight leagues 

 to hold in fusiou all known substances— there arises the question, what 

 is its precise value at the surface of the earth ; Avhat weight are we t^ 

 attach to it in the determination of terrestrial temperatures ; what part 

 does it i)lay in the phenomena of life 1 



According to INIairan, Buftbn, 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 whicli illuminates us would form only a 

 very snuiU part of that whose proi)itious inlluence we feel. 



This idea was developed with ability and great eloquence iu the 

 Memoirs of the Academy, in the Epoqnes snr la Xaiure of Buffon, in the 

 letters from Badly to Voltaire njwn the Origin of the Sciences and upon the 

 Atlantide. But the ingenious romance to which it has served as a base 

 has vanished like a shadow before the torch of mathematical science. 



Fourier having discovered that the excess of tlu? aggregate temper- 

 ature of the earth's surface above that which would result from the sole 

 action of the solar rays has a determinate relation to the increase of 

 temperature at ditferent depths, succeeded in deducing from the exper- 

 imental value of this increase a numerical determination of the excess 

 in question. This excess is tlie thermometric effect which the solar heat 

 produces at the surface. Now, instead of the large numbers adopted by 



