62 POPULAR LECTURES AND ADDRESSES. 



coal in ancient times ; or has it passed off so long 

 ago that the strata are now not sensibly warmer on 

 account of it ? 



30. I shall conclude by simply referring to 

 calculations regarding the quantity of heat at pres- 

 ent conducted out from the interior of the earth, 

 which I have given in an article, entitled "The 

 ' Doctrine of Uniformity ' in Geology, briefly re- 

 futed ; " l and to analytical investigations regarding 

 antecedents of the present condition of underground 

 heat contained in a paper " On the Secular Cooling 

 of the Earth," - 1 appended to the volume on 

 Natural Philosophy by Professor Tait and my- 

 self, recently published. The first of these shows, 

 by mere calculation of the actual conduction, that 

 the present rate of increase of underground tem- 

 perature could not last for twenty or thirty thous- 

 and million years, without there being dissipated 

 out of the earth as much heat as would be given 

 off by a quantity of ordinary surface rock equal to 

 100 times the earth's mass, cooling from 100 cent. 



1 Proceeding A'. S. A., December, 1865, and p. 6, above. 

 - First rcpublished in the 7'rans. /.'. .S'. A'., 1862, published in 

 Mathematical and Physical Papers i Vol. iii., p. 295. 



