122 POPULAR LECTURES AND ADDRESSES. 



the knowledge of thermodynamics, which the 

 subsequent experiments of Davy, Rumford, and 

 Joule supplied, to lead to a thoroughly definite 

 explanation of all that is known regarding the 

 present actions and temperatures of the earth, and 

 of the sun, and other heavenly bodies. And if 

 Carnot's theory had been before him, he assuredly 

 would not have forestalled Hutton in the chimera 

 of " a reproductive operation, by which a ruined 

 constitution may be repaired." l ' 



37. Now the whole amount of potential energy 

 exhausted in the coming together of the earth's 

 materials, from infinite mutual distances (that is to 

 say, from distances many times greater than the 

 present diameter of the earth) to their present 

 relative positions, is easily estimated with great 

 accuracy with the knowledge we possess of the 

 earth's average density. If the density were uni- 

 form from surface to centre, the amount of poten- 

 tial energy in question would be equal to the work 



1 Sec the account of Kant's Cosmogony given by Professor Huxley, 

 in his "Address" of Feb. 19, 1869, to the London Geological 

 Society. 



