148 HISTORY OF TIIERMOTICS. 



permanent state, be uniform in the same vertical line, as soon as we 

 get beyond the influence of the superficial oscillations of which AVI 

 have spoken ; and that, before the distribution of temperature reaches 

 this limit, it will decrease, not increase, in descending. It appeared 

 also, by the calculation, that this remaining existence of the primitive 

 heat in the interior of the earth's mass, "was quite consistent with the 

 absence of all perceptible traces of it at the surface; and that the 

 same state of things which produces an increase of one degree of heat 

 in descending forty yards, does not make the surface a quarter of a 

 degree hotter than it would otherwise be. Fourier was led also to 

 home conclusions, though necessarily very vague ones, respecting the 

 time which the earth must have taken to cool from a supposed original 

 state of incandescence to its present condition, which time it appeared 

 must have been very great ; and respecting the extent of the future 

 cooling of the surface, which it was shown must be insensible. Every- 

 thing tended to prove that, within the period which the history of the 

 human race embraces, no discoverable change of temperature had 

 taken place from the progress of this central cooling. Laplace further 

 calculated the effect 16 which any contraction of the globe of the earth 

 by cooling would produce on the length of the day. He had ahvad\ 

 shown, by astronomical reasoning, that the day had not become shorter 

 by l-200th of a second, since the time of Hipparchus ; and thus his 

 inferences agreed with those of Fourier. As far as regards the small- 

 ness of the perceptible effect due to the past changes of the earth's 

 temperature, there can be no doubt that all the curious conclusions just 

 stated are deduced in a manner quite satisfactory, from the fact of a 

 general increase of heat in descending below the surface of the earth ; 

 and thus our principles of speculative science have a bearing upon the 

 history of the past changes of the universe, and give us information 

 concerning the state of things in portions of time otherwise quite out 

 of our reach. 



4. Heat of the Planetary Spaces, In the same manner, this por- 

 tion of science is appealed to for information concerning parts of space 

 which are utterly inaccessible to observation. The doctrine of heat 

 leads to conclusions concerning the temperatures of the spaces which 

 surround the earth, and in which the planets of the solar system 

 revolve. In his Memoir, published in l^L'T, 17 Fourier states that he 

 conceives it to follow from his principles, that these planetary 



15 Conn, des Terns, 1823. ' 1T Him. last. torn. vii. y> 580 



