308 The Uev. O. Fisher on Mr. Mallet's 



R=20,890,000 feet, say 21 x 10 6 (about 4000 miles), 



R-r = 400 miles = ^ 



= 21xl0 5 feet, 

 # = 32 feet, 

 p = 2'6, 



d= 0-0006 = ~ inch 



= Xq"5 foot - 



Substituting the above values, we get for the work of descent 

 of the crust 4851 x 10 16 ; and dividing this by Joule's equivalent 

 (772), we obtain for the corresponding number of units of heat 



6283 x 10 13 . 



It does not appear that Mr. Mallet has directly expressed the 

 annual amount of volcanic energy io terms of units of heat; but 

 he considers that he " has shown that its total amount cannot 

 exceed a small fraction of the entire heat dissipated annually, 

 being only j~~ thereof "*% We have therefore first to express 

 in units the heat annually dissipated. 



Now 777 cubic miles =1144x 10 11 cubic feet (about). And 

 each cubic foot of ice requires 8265 units of heat to melt it 

 (Addition, p. 4), or say 8 x 10 3 . Hence the number of units of 

 heat annually lost by the globe is 



1144 x 10 11 x 8 x 103 = 9152 x 10 14 . 



And it has been just shown that the entire work of subsidence 

 of a crust 400 miles thick is equivalent to 



628 xlO 14 units of heat. 



The latter is therefore -$&$ °f the former, or about T j. 



Mr. Mallet reckons the annual volcanic energy at ^g-^ of the 

 whole heat dissipated ; whence we obtain approximately, 



Volcanic energy : the entire energy of subsidence : : 1 : 100. 

 So that, if the nature of the materials by their resistance to 

 motion could allow P (the horizontal pressure) to accumulate to 

 a 'sufficiently large value, and if no work were done upon the 

 nucleus by the descent of the crust, there is no doubt that there 

 would be enough and to spare in the annual subsidence to 

 account for the required amount of energy as he has estimated 

 it. Much would depend, however, upon the degree of such 

 resistance. For instance, if the rocks were fluid there would 



■ * Addition to the paper on Volcanic Euergj-, par. 1, Phil. Trans, vol. 

 clxv. pt. 1. 



