OF GE OL O GICA L D YNA MICS. \ \ 9 



metals, 1 we shall probably soon be able to estimate, 

 without any very unsatisfactory degree of vague- 

 ness, a limit to the possible amount of heat in the 

 earth. With a view to putting together the data 

 required for this estimate, it is important to notice 

 that we have strong reason to believe the earth is 

 not a mere thin shell filled with melted material of 

 rock or metal, or both, as many French and a few 

 English geologists assume it to be ; but is solid 

 from surface to centre with the exception of com- 

 paratively small spaces still occupied by fluid lava, 

 or subjected occasionally to melting in volcanic 

 action. 2 We may therefore say it is not at all 

 probable that there is now within the earth a 

 hundred times as much heat as that which would 

 raise a quantity of average surface rock equal in 

 mass to the whole -earth, from zero to 200 cent., 

 since this would be certainly many times more 



1 A very simple plan would be to pour small quantities of melted 

 rock into hollows in blocks of cast iron, massive enough not to rise 

 more than a few degrees of temperature, by the communication of 

 heat from the melted rocks. 



2 "Rigidity of the Earth "(W. Thomson), Trans. R.S., 1862 ; and 

 Thomson and Tait's Natural Philosophy, 832-849. 



