M. Arago's Historical Eloge of Joseph Fourier. 229 



According to this hypothesis, it is evident, that the polar re- 

 gions may have possessed, during very ancient epochs, a tem- 

 perature equal to that of the equatorial regions where elephants 

 live at present, although they were deprived for whole months 

 of the sight of the sun. 



It was not in explanation of the occurrence of elephants 

 in Siberia, that the idea of heat proper to the globe was for 

 the first time proposed. Some learned men had adopted it 

 before the discovery of any of these animals. Descartes thought 

 that originally (I quote his own expressions), the earth differed 

 in nothing from the sun, except that it was smaller. It would 

 thus be necessary to consider it as an extinct sun. Leibnitz did 

 this hypothesis the honour of appropriating it to himself. He 

 tried to deduce from it the mode of formation of the different 

 solid coatings of which our globe is composed. Buffbn also 

 gave it the influence of his eloquent authority. It is well 

 known that, according to this great naturalist, the planets of 

 our system are mere portions of the sun, which the stroke of a 

 comet had detached from it some thousands of years ago. 



In support of this igneous origin of our globe, Mairan and 

 Buffon had already cited the high temperatures of deep mines, 

 and, among others, that of the mines of Giromagny. It ap- 

 pears evident, that if the earth was formerly incandescent, we 

 could not fail to find in the internal beds, that is to say, in 

 those which must have been the last to cool, traces of their ori- 

 ginal temperature. The observer, who, on penetrating into 

 the earth, did not find the heat increasing, might consider him- 

 self fully authorized to reject the hypothetical ideas oi Descartes^ 

 Leibnitz^ Mairan, and Biiffbti. But is the reverse of this pro- 

 position equally certain ? May not the supplies of heat given 

 out by the sun for so many ages, have been distributed over 

 the earth, in such a manner, as to produce in it temperatures 

 increasing with the depth ? This is a vital question. Certain 

 minds easily satisfied, conscientiously believed that they had 

 solved it, by declaring, that the idea of a constant temperature 

 was by far the most natural ; but woe to the sciences if they 

 admitted such vague Considerations among the reasons for ad- 

 mitting or rejecting facts and theories ! Fontenelle would have 

 traced their horoscope in these words, so well fitted to humble 

 our pride, But whose truth, however, is developed on innu- 



