412 STABILITY OF THE EARTIl's HEAT. 



CHAPTER velty of the means interpose, therefore, very narrow 

 xxyi^ii. ]irnits to our investigation regarding the temperature of 

 Intprnal lipat the air. It is quite otlierwise, liowever, witli the solu- 

 tion of the great problem of the internal heat of the 

 whole earth. As we may judge of uniformity of tem- 

 perature from the unaltered time of vil)ration of a pen- 

 dulum, so we may also learn from the unaltered rotatory 

 velocity of the earth the amount of stability in the meai' 

 temperature of our glol)e. Tiiis insight into the rel? • 

 tions between the length of the day and the heat of the 

 earth is the result of one of the most brilliant applica- 

 tions 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 



Relations of on its Volume ; and, since by the gradual cooling of the 



motion and , ,. . ' ... , , , 



lie.1t. mass by radiation, the axis of rotation would become 



shorter, the rotatory 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 since the time 



of Ilipparchus, that is, for full 2000 years, the length of 



the jlay 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, therefore, 



taking the extremest limits, diminished as much as 



l-30Gth of a degree of Fahrenheit. 



Invp.riiibiiity n rpj^jg invariability of form ])resupposes also a gi-eat 



invariiibiiity 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 only to be regarded as slight superficial 



phenomena affecting merely one portion of tiie carth'p 



crust, which, from tiieir smallness when compared to the 



earth's radius, become wholly insignificant." 



Such are some of the remarkable readings of nature, 



