NO. 8.] REMARKS ON THE EARTH'S CRUST. 65 



that the density, Q , in the firm earth's crust beneath, changes on an average 

 in one and the same manner downwards. We may assume that the case is similar 

 with regard to the density, >,, beneath the portions of continent considered. 

 Q and j are functions of the distance from the earth's surface, and according to 

 the above, the average value along a vertical must be greater for Q than for 

 , in the outer spherical shell, of which we will call the thickness h lt when 

 we have ft, = R E i . If we now compare a portion of the earth's crust 

 belonging to the continental lowlands with a piece belonging to the ocean, 

 the result of the equation for the flux of force is that 



.-. * 



\R*da> (0, tfdh + brt \ 



which means that the difference between the mass of the two pieces above 

 equal elements of the inner spherical surface equals zero. 



Here R = R l -\- h = R h t -\- h; substituting this and expanding, we 

 obtain, after division by knR\ dw, 



I A, 

 ( Ql - 



*,-*, 



ht-h, h,-h, 



h,-h, 



The radius of the earth must here be regarded as great, not only in 

 proportion to the depth of the sea, h 2 , which we may take on an average 

 as 3.5 km., but also in proportion to the thicknes of the shell, /& t . The fourth 

 term in the above equation must therefore be exceedingly small, and even 

 the third term can only have a small value, as the two integrals in it have 

 opposite signs. The first two terms can consequently only have a value that 

 scarcely differs from 0; moreover it is easy to show that their sum must be 

 less than 0. If we inquire into the pressure that the separate parts of the 

 earth's crust each exert upon the interior nucleus on account of the attraction 

 of the latter, we shall find that, taking Af, as the mass of the interior nucleus, 

 it will amount for every surface element, R\ du, beneath the continents, to 



