230 M. Arago's Historical Eloge of Joseph Fourier. 



nierable occasions in the history of discoveries. " When a 

 thing may exist in two ways, it ahnost always does so in that 

 which at first seemed to us the least natural." 



Whatever be the importance of these reflections, I hasten to 

 add that, instead of the weak arguments of his predecessors, 

 Fourier substituted proofs and demonstrations ; and it is well 

 known what these terms mean in the Academy of Sciences. 



In all parts of the earth, on descending to a certain depth, 

 the thermometer no longer experiences any diurnal or annual 

 variation. It marks the same degree and the same fraction of 

 a degree, during the whole year, and during a continuance of 

 years. Such is the fact : what says theory ? 



Suppose, for an instant, that the earth has constantly re- 

 ceived all the heat of the sun. Penetrate sufficiently into its 

 mass, and you will find, along with Fourier, by calculation, a 

 constant temperature for all periods of the year. You will 

 also find that this solar temperature of the lower beds varies in 

 different climates, and finally, that it ought to be always the 

 same in each country, provided you do not descend very low 

 in comparison to the radius of the earth. Well ! natural phe- 

 nomena are in manifest contradiction to this result. The ob- 

 servations made in a number of mines, and the observations 

 on the temperature of the water of spouting fountains rising 

 from different depths, have all given an increase of one degree 

 centigrade (P.S F.), for twenty or thirty metres (60 to 90 feet) 

 of depth. Thus, there was something incorrect in the hypo- 

 thesis which we discussed by following the steps of our colleague. 

 It is not true that the phenomena of temperature in terrestrial 

 beds can be attributed to the mere action of the solar rays. 

 This being proved, the increase of heat observable in all cli- 

 mates on penetrating into the interior of the globe, is a distinct 

 indication of heat proper to the globe. The Earth, as Descartes 

 and LeibJiitz alleged, but without being able to bring forward 

 any convincing argument, is at length shewn to be — thanks to 

 the numerous observations of natural philosophers and the 

 analytical calculations of Fourier — an encrusted Sun, whose 

 high temperature may be boldly appealed to, as often as it 

 is required for the explanation of ancient geological pheno- 

 mena. 



After having proved that our earth possesses an original 



