166 TERRESTRIAL PHENOMENA, 



most brilliant application of the long knowledge we possess 

 of the movements of the heavens to the thermic condition of 

 our planet. The velocity of the Earth's rotation depends on 

 her volume ; and since, therefore, by the gradual cooling of the 

 mass from the effects of radiation the axis of rotation would 

 become shorter, such decrease of temperature would be 

 accompanied by increased velocity of rotation and dimi- 

 nished length of day. Now the comparison of the secular 

 inequalities in the Moon's motion with eclipses observed by 

 the ancients, shews that since the time of Hipparchus, or 

 during an interval of two thousand years, the length of the 

 day has certainly not been diminished by one -hundredth 

 part of a second : we know, therefore, that the mean tem- 

 perature of the Earth has not altered, during that period, so 

 much as the }- part of a Centigrade degree, or the -g-^-g- of 

 a degree of Fahrenheit. ( 14 ) 



This invariability of form presupposes also great inva- 

 fiability in the distribution of density in the interior of the 

 globe. Such transference of matter as is effected by the 

 action of our present volcanoes, the eruption of ferruginous 

 lavas, and the filling up of previously empty fissures and 

 cavities with dense mineral masses, are therefore to be 

 regarded merely as inconsiderable superficial phenomena, 

 wholly insignificant when considered in relation to the 

 dimensions of the Earth. 



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

 respect to its cause and distribution, almost exclusively from 

 the results of Fourier's admirable investigations. Poisson 

 doubted the uninterrupted increase of the Earth's tempera- 

 ture from the surface to the center ; he believed that its heat 

 had penetrated from without, and that the temperature of 



