Fourier's Theory of Heat. 



605 



trifling, and almost imperceptible ; that, however, which the earth 

 experiences is much less still. In fact, the strata of the mineral 

 shell are not composed of iron, but of substances which offer much 

 less facility for the transmission of heat. Now, the heating of 

 the ground is (for the same level of temperature in the direction 

 of the depth) directly proportioned to this facility ; whence it 

 follows that if, as is very likely, the substances of which the upper 

 envelope of the earth is composed, conduct eight times less heat 

 than iron, the excess of heat communicated by the internal fire will 

 only be the 32nd part of a centesimal degree, a quantity quite 

 insignificant. 



When we examine attentively, and according to known principles, 

 all the observations relative to the figure of the earth, we cannot 

 doubt, that this planet received at its origin, a very elevated tem- 

 perature. On the other hand, thermometrical observations shew 

 us, that the present distribution of heat in the terrestrial envelope 

 is that which would have occurred, if the globe had been first very 

 hot and then progressively cooled, till it reached the state in which 

 we now find it. The agreement of these two kinds of observations 

 furnishes, as we may perceive, the strongest argument for the 

 igneous origin of our planet. But, as we have just now seen, this 

 central fire, the existence of which can scarcely now be contested, 

 produces only imperceptible modifications on the surface of the 

 ground. 



As every thing proves, that the other planetary bodies have the 

 same origin as the earth, we cannot doubt that the same results are 

 applicable to them, which have been obtained with regard to our 

 globe. 



In applying this conclusion, mathematically proved, to all the 

 planetary bodies, we find that in each the focus of heat, although 

 still burning in the interior, is without any perceptible influence 

 upon the temperature of the surface, whence it results that among 

 all, the heat of the superficies must depend almost exclusively 

 upon their distance from the sun, and the manner in which they 

 present the different parts of their surface to the rays of that 

 orb, as well as on the state of the superficies; the presence or 

 absence, in particular, of an atmosphere, or of a great quantity of 



