316 The Rev. 0. Fisher on Mr. Mallet's 



Heat per square foot of vertical plane section of crust 



=0-06 x 100 units 



= 6 units per annum. 

 In the case above supposed, 



BB'-S 400 - 200 * 



400 

 ~3 P 



4 5 



= 1^ mile; 



so that the heat would scarcely be localized at all. 



In presence of the enormous pressures to which the interior of 

 the earth must be subject, if they be no greater than would arise 

 from fluid pressure alone, a compressing force due to a column 

 of 400 miles of crust at a like depth is very probably consi- 

 derably smaller than the true value, which, within the extreme 

 limit of about 2000 miles pressure, is determined solely by the 

 pressure the strata can sustain. It is extremely probable, there- 

 fore, that the heat per unit of vertical section might be consider- 

 ably above 6 units per annum. But even then we get but a very 

 small way towards what may reasonably be thought requisite to 

 supply volcanic energy, especially when we consider that it must 

 be distributed through the whole thickness of the weak section. 



The above estimates have been made upon the supposition 

 that the action between the crust and the nucleus is of the 

 nature of friction between solids. If, however, as is more likely, 

 the crust rests upon a fluid or viscous layer, the resistance to 

 lateral motion will be much smaller ; but we are not able to 

 guess what it will be ; so that we cannot a priori assign a value to 

 fjb. But a probable estimate may be arrived at from the consi- 

 deration that a great circle of the sphere cuts, on an average, at 

 least three lines of weakness, as appears upon inspection of a 

 map showing the lines of volcanic action. 



Hence 



BB' = 8000 miles, 

 whence 



Px-P 

 8000 



fM=2 , X _: A & 



10 ' 



