the Crust of the Earth. 



8^1 



if anything, it will be somewhat more dense. Hence the super- 

 incumbent mass will have its whole effect, and the least thick- 

 ness must be determined from the condition that the superin- 

 cumbent weight is not sufficient to break it through. 



Let E F A B C be a meridian section of the- continent of India 

 and of the ocean ; A the foot of the mountains ; F, Cape Comorin 

 supposed on the same level, that is, the sea-level O F A M C. 

 Let AM = iv; Aa = t, 'Bb = t', the thicknesses of the crust at A 

 and B; BM = A. Let C be the cohesive force of the rock, or 

 the length of rock of a unit section the weight of which equals 

 the cohesive force on a unit of surface of the joints A a, Bb, Cc. 

 Now if the mass A c sink, it will do so by opening at A, C, b ; 

 and it will be prevented sinking thus by the cohesive force of 

 the parts of the crust at the joints. Let the unit of mass be 

 that whose transverse section is a unit of surface. The moment 

 of the weight of AB M about the fulcrum a = w x ^hx§iv = Itv^h. 

 And the moments of the cohesive force at the joints A a, B b, to 

 turn A i in the opposite dii-ection about « = C x |/^ and C x ^t'^, 



.'.^w^.h=lC{t^ + t'% or C = |^,= (|)'| nearly. 



Now A=4 miles for the Himalayas, and iv=100 miles. Hence 

 200 



t=- 



miles. 



-V/3C 



Mr. Airy states that the crust could not exert such a cohesive 

 force as to make C so much even as one-fifth of a mile. In this 



case even the thickness t 



=200^ 



= 260 miles. 



If the same formula be applied to the ocean south of Cape 

 Comorin, 1^ = 1500 miles suppose, /i= 2 miles; and therefore 

 ifC = |, 



=1 Vf = 



2700 miles; 



or it would require this thickness to produce sufficient cohesive 

 force to resist the tendency of tlie lava to break up the ocean- 

 bed, in consequence of the deficiency of weight. If the thick- 



Z2 



