314 CELESTIAL DYNAMICS. 



thousand years' duration during which these counteracting 

 influences will appear to be equal. Within this period a si- 

 dereal day attains its shortest length, and the velocity of the 

 earth's rotation its maximum circumstances which, accord- 

 ing to mathematical analysis, would tend to lengthen the du- 

 ration of this period of the earth's existence. 



The historical times of mankind are, according to La- 

 place's calculation, to be placed in this period. Whether we 

 are at the present moment still near its commencement, its 

 middle, or are approaching its conclusion, is a question which 

 cannot be solved by our present data, and must be left to fu- 

 ture generations. 



The continual cooling of the earth cannot be without an 

 influence on the temperature of its surface, and consequently 

 on the climate ; scientific men, led by Buffon, in fact, have 

 advanced the supposition that the loss of heat sustained by 

 our globe must at some time render it an unfit habitation for 

 organic life. Such an apprehension has evidently no founda- 

 tion, for the warmth of the earth's surface is even now much 

 more dependent on the rays of the sun than on the heat which 

 reaches us from the interior. According to Pouillet's meas- 

 urements, mentioned in Chapter III., the earth receives 8000 

 cubic miles of heat a day from the sun, whereas the heat 

 which reaches the surface from the earth's interior may be 

 estimated at 200 cubic miles per diem. The heat therefore 

 obtained from the latter source every day is but small in com- 

 parison to the diurnal heat received from the sun. 



If we imagine the solar radiation to be constant, and the 

 heat we receive from the store in the interior of the earth to 

 be cut off, we should have as a consequence various changes 

 in the physical constitution of the surface of our globe. The 

 temperature of hot springs would gradually sink down to the 

 mean temperature of the earth's crust, volcanic eruptions 

 would cease, earthquakes would no longer be felt, and the 

 temperature of the water of the ocean would be sensibly al- 



