NO. 8.] REMARKS ON THE EARTH'S CRUST. 83 



We may convince ourselves of the correctness of the result arrived at 

 above, by examining the course of the lines of force through the earth's crust. 

 If the masses in the earth's crust had been so distributed that the density 

 at equal distances from the surface had been constant, the lines of force 

 would simply have run straight in along the radii to the places in question. 

 This, however, is not the case, even under the simplified conditions under 

 which we are here considering the matter. The density at a certain depth is 

 not the same beneath the continents as beneath the oceans. The lines of 

 force, in consequence of this, will be deflected from their straight course 

 during their passage inwards towards the inner nucleus. This deflection will 

 take place, as we may easily convince ourselves, when the lines pass through 

 a space in which the masses are unevenly distributed in directions at right 

 angles to them, and in such a manner that they will turn aside from those 

 places where the density of mass is least, towards those where it is greatest. 

 Instead of following the lines of force on their inward course from the surface 

 of the earth through the earth's crust to the inner nucleus, it will be pre- 

 ferable to follow them in the opposite direction, as we may assume that the 

 lines of force that reach the surface of the nucleus, are evenly distributed over 

 it. We must recollect, however, in thus following the lines of force backwards 

 in the opposite direction to their course, that their deflection will also be in 

 the opposite direction to that mentioned above, so that the lines we are follow- 

 ing will turn away from the places where the density is greatest towards 

 those where it is least. 



We will now assume the surface of the inner nucleus to be divided into 

 a number of equal surface-elements, and that through each element there 

 runs a line of force. If the masses had been evenly distributed throughout the 

 earth's crust, these lines, as already mentioned, would have been produced 

 through it as right lines, so that equal surfaces of the outer surface would 

 be intersected by an equal number of lines of force all over the earth. The 

 masses, however, as we know, are unevenly distributed, and on an average 

 the density at greater depths than that of the bottom of the ocean is somewhat 

 less beneath the continents than beneath the oceans. The consequence of 

 this is that the lines of force rising from the surface of the inner nucleus along 

 the border of the base of the continents, must converge under the latter, so that 

 the space under the continents will be more and more filled with lines of 



