Theory of Volcanic Energy. 315 



ing P x , and also by increasing* P A ; that is, by lessening the 

 pressure which causes the yielding, and by increasing the pres- 

 sure at the place of yielding after the compression has taken place. 



Mr. Mallet has obtained experimentally the pressure neces- 

 sary to crush small cubes of rock of various kinds. These are 

 valuable results as far as they go ; but they do not tell us much 

 of the pressures which would be required to cause the strata of 

 the earth to yield at great depths. In his paper of July, p. 4, 

 he makes some estimates upon this point, which, for the sake of 

 illustration, we will adopt. 



Let us then take 434 x 10 4 lbs. as the force per square foot 

 which just causes a cube of rock to yield. Mr. Mallet thinks 

 that 2*14 times this will be the pressure requisite at 10 miles 

 depth and 4*28 times at 20 miles. Call the height in feet of a 

 column of rock which would just crush the cube^?. A cube of 

 1 foot of such rock weighs, say 200 lbs. Hence, if p be the 

 height which will just crush, 



^x 200=434 xlO 4 ; 

 .'. ^ = 217 xlO 5 feet. 



The pressure at 400 miles depth will therefore be that due to a 

 column of the height 



40 x 2-14 x 217 xlO 2 feet; 

 and dividing by the number of feet in a mile (5280), this gives 

 for the compressing force necessary to crush the section of the 

 crust throughout, P x = 35 miles of rock. 



It is obvious that this force is quite inadequate with the as- 

 sumed value of fj, to shear the crust over the nucleus. We must 

 therefore suppose Px to accumulate to a much greater degree. 



Let us, then, take it just sufficient to shear the crust, which it 

 has been seen is probably the pressure due to the thickness of 

 crust. It is clear that this compressing force will not be con- 

 fined to one depth alone, since it is supposed to arise from 

 the contraction of the nucleus and not of the crust. Hence 

 suppose P x = 400. But after the crust has been compressed, 

 it is reasonable to suppose that there will be still some pressure 

 at A which cannot well be less than the hydrostatic pressure 

 due to the depth ; call it, therefore, the mean of this for the 

 whole depth, or P A =200. Hence we have 



P x 2 - P A * = 160000 - 40000 = 120000. 

 Hence we have 



work in the supposed case 12 x 10 4 _ n . n ~ 

 work in limiting case 2 x 10^' 



And the heat will be in the same proportion, viz. : — - 



