208 M. Arago on the Thermometrical State 



incrusted star, whose high temperature may be boldly invoked, 

 ■whenever the explanation of geological phenomena shall require 

 it! 



III. Is there any means of determining for how many centuries the 

 Earth has been cooling down 9 



In the mathematical theory of heat, there are many results 

 which the present state of science allows us to pursue to their 

 numerical applications. On the other hand, there are problems, 

 whose solution have as yet been expressed only by general ana- 

 lytic formulae. Among these latter there is a formula, which is 

 intended to estimate the value of the cooling of the globe during 

 the period of a hundred years, and in which the number of 

 centuries since the commencement of the cooling process bears 

 a prominent part, assuming withal, that at this epoch a uniform 

 temperature had pervaded the whole mass. 



If the number of centuries were given, we could, from this for- 

 mula, deduce the numerical value of the globe's loss of heat 

 every hundred years ; and inversely, from the quantity of the 

 centenary refrigeration, when once known, we might, without 

 difficulty, calculate the epoch at which refrigeration commenced. 

 The question, then, so keenly disputed, concerning the antiquity 

 of our globe, comprehending its period of incandescence, is thus 

 brought to the determination of a thermometrical variation ; 

 which, moreover, on account of its extreme minuteness, must 

 still be reserved for ages to come! 



IV. In two thousand years^ the general temperature of the mass of 

 the Earth has not varied the tenth part of a degree. The 

 demonstration of this proposition is derived from the orbit of the 

 moon. 

 We have already admitted that the earth was at one time in- 

 candescent, and that its solid covering has been formed by re- 

 frigeration. It has moreover been proved that its heat, even at 

 moderate depths, is still enormous. From this last circumstance, 

 it follows that it must be still cooling. The only doubt that can 

 be entertained is as to the extent. As, then, the title of this 

 chapter indicates, we deduce, from the path of the moon, the 

 proof, that in 2000 years, the mean temperature of the earth — 



