JOSEPH FOURIER. 165 



origiually (I cite his own words) the earth did not differ from the sun in 

 any other respect than in being smaller. Upon this hyi)othesis, then, it 

 ought to be considered as an extinct sun. 



Leibnitz conferred upon this hypothesis the honor of appropriating it 

 to himself. He attempted to deduce from it the mode of formation of 

 the different solid envelopes of which the earth consists. Buffon, also, 

 imijarted to it the weight of his eloquent authority. According to that 

 great naturalist, the planets of our system are merely portions of the 

 sun, which the shock of a comet had detached from it some tens of thou- 

 sands of years ago. 



In support of this igneous origin of the earth, Mairan and Bufibu 

 cited already the high temperature of deep mines, and, among others, 

 those of the mines of Giromagny. It appears evident that if the earth 

 was formerly incandescent, we should not fail to meet in the interior 

 strata — that is to say, in those which ought to have cooled last — traces 

 of their primitive temperature. The observer who, ui)on penetrating 

 into the interior of the earth, did notlind an increasing heat, might then 

 consider himself amply authorized to reject the hyi)othetical conceptions 

 of Descartes, of Mairan, of Leibnitz, and of Bufibn. But has the con- 

 verse jiroposition the same certainty ? Would not the torrents of heat, 

 which the suu has continued incessantly to launch for so many ages, 

 have diffused themselves into the mass of the earth, so as to produce 

 there a temperature increasing with the depth f This is a question of 

 high importance. Certain easily satisfied minds conscientiously sup- 

 posed that they had solved it, when they stated that the idea of a con- 

 stant temperature was by far the most natural; but woe to the sciences 

 if they thus included vague considerations, which escape all criticism, 

 among the motives for admitting and rejecting facts and theories ! Fon- 

 tenelle, gentlemen, would have traced their horoscope in these words, 

 so well adapted for humbling our pride, and the truth of which the his- 

 tory of discoveries reveals in a thousand places: "When a thing may 

 be in two different ways, it is almost always that which appears at first ' 

 the least natural." 



Whatever importance these reflections may possess, I hasten to add 

 that, instead of the arguments of his predecessors, which have no real 

 value, Fourier has substituted proofs, demonstrations; and we know 

 what meaning such terms convey to the Academy of Sciences. 



In all places of the earth, as soon as we descend to a certain depth, 

 the thermometer no longer experiences either diurnal or annual varia- 

 tion. It marks the same degree, and the same fraction of a degree, from 

 day to day, and from year to year. Such is the fact: what says theory!? 



Let us suppose, for a moment, that the earth has constantly received 

 all its heat from the sun. Descend into its mass to a sufficient depth, 

 and you will find, with Fourier, by the aid of calculation, a constant 

 temperature for each day of the year. You will recognize further, that 

 this solar temperature of the inferior strata varies from one climate to 



