NO. 8.] REMARKS ON THE EARTH'S CRUST. 85 



of force are obliged to turn back towards the oceans again, und that 

 more rapidly than they had turned in under the continent. On account of 

 the inconsiderable depth of the oceans as compared with the thickness of the 

 earth's crust, however, this turning back is not so complete as to cause a 

 regular distribution of the lines of force over the outer surface of the earth. 

 The lines of force will therefore be crowded rather more closely together on 

 the continents, along their boundary or towards the coast-line; while immedi- 

 ately outside this, on the ocean, they will lie farther from one another than 

 the normal distance. An endeavour to illustrate these conditions is made in 

 the accompanying figure. 



If we now consider a tube of force issuing from the outer surface, and 

 terminating in one of the surface-elements into which we have imagined the 

 surface of the inner nucleus to be divided, it is evident, since the lines of 

 force, according to the above, lie somewhat closer together than normally 

 just within the coast-line of a continent, that this tube of force will cut 

 off from the free surface an element somewhat smaller than it would have 

 been had the lines of force been normally distributed over the free surface, 

 and the acceleration been normal in consequence. The reverse will be the 

 case if the tube of force intersects the free surface somewhat beyond the 

 coast-line out on the ocean. According to our explanation on page 69, the direct 

 consequence of this will be that the acceleration must be rather greater than 

 normal on the continents in the neighbourhood of the coast-line, and somewhat 

 less than normal out on the ocean a little beyond the shore, as we have 

 demonstrated above. 



We have assumed above that on an average there are equally large 

 masses over equal elements of the surface of the inner nucleus. If we imagine 

 that there is an equilibrium of pressure upon this from the outer crust, the 

 result arrived at above will not be altered in any degree worth mentioning. 

 In order to satisfy this new condition, we need only, as regards the continents, 

 add at sea-level a stratum of rock about 50 m. thick, to the masses we have 

 considered above. 



As regards the oceanic islands, the increased attraction on them is easily 

 explained, whether we imagine the masses of which they are formed to repre- 

 sent actual surplus mass, or assume which seems the more reasonable 



