176 COSMOS. 



of the air. It is quite otherwise, however, with the solution 

 of the great problem of the internal heat of the whole Earth. 

 As we may judge of uniformity of temperature from the unal- 

 tered time of vibration of a pendulum, so we may also learn, 

 from the unaltered rotatory velocity of the Earth, the amount 

 of stability in the mean temperature of our globe. This 

 msight into the relations between the length of the day and 

 the heat of the Earth is the result of one of the most brilliant 

 applications of the knowledge we had long possessed of the 

 movement of the heavens to the thermic condition of our 

 planet. The rotatory velocity of the Earth depends on its 

 volume ; and since, by the gTadual cooling of the mass by 

 radiation, the axis of rotation would become shorter, the rota- 

 tory velocity would necessarily increase, and the length of the 

 day diminish, with a decrease of the temperature. From the 

 comparison of the secular inequalities in the motions of the 

 Moon with the eclipses observed in ancient times, it follows 

 that, smc« the time of Hipparchus, that is, for full 2000 

 years, the length of the day has certainly not diminished by 

 the hundredth part of a second. The decrease of the mean 

 heat of the globe during a period of 2000 years has not, there- 

 fore, taking the extremest limits, diminished as much as a^gth 

 of a degree of Fahrenheit.* 



This invariability of form presupposes also a great invaria- 

 bility in the distribution of relations of density in the interior 

 of the globe. The translatory movements, which occasion 

 the eruptions of our present volcanoes and of ferruginous lava, 

 and the filling up of previously empty fissures and cavities 

 with dense masses of stone, are consequently dnly to be re- 

 garded as slight superficial phenomena afiecting merely one 

 portion of the Earth's crust, which, from their smallness 

 when compared to the Earth's radius, become wholly insig- 

 nificant. 



I have described the internal heat of our planet, both with 

 reference to its cause and distribution, almost solely from the 

 results of Fourier's admirable investigations. Poisson doubts 

 the fact of the uninterrupted increase of the Earth's heat 



* Laplace, Exp. dit, Syst. du Monde, p. 229 and 263 ; Micaniqu6 

 Celeste, t. v., p. 18 and 72. It should be remarked that the fraction 

 5^th of a degree of Fahrenheit of the mercurial thermometer, given in 

 the text as the limit of the stability of the Earth's temperature since 

 the days of Hipparchus, rests on the assumption that the dilatation of 

 the substances of which the Earth is composed is equal to that of glass. 

 that is to say, y^.^o-oth for 1°. Regarding this hypothesis, see Arago 

 in the Annuaire for 1834, p. 177-190. 



