THE BOWELS OF THE EARTH. 



33 



surface, we shall probably not live long enough to see our predic- 

 tion proved false. The deepest mines, therefore, far from reach- 

 ing the bowels of the earth, can not pierce so far in proportion 

 as does the mosquito into the human epidermis. And yet we are 

 not wholly without information concerning the deeper regions of 

 the earth. 



In the first place, man has succeeded in the weighing of the 

 earth as a whole. In accordance with the law of gravity, if two 

 balls of lead attached to elastic steel rods are placed close to each 

 other, they must attract each other with a force increasing with 

 their masses, but decreasing with the distance which separates 

 them. The steel rods will be very slightly bent toward each other 

 in consequence. But the same steel rods extended horizontally 

 will be far more strongly bent downward, owing to the attraction 

 of this great ball which we call the earth. If, then, we compare 

 the size, the distance apart, and the density of the two balls, and 

 the effect they produce, with the size of the earth, the distance of 

 its center, and the effect it produces, we may find the average 

 density and weight of the earth. We find that the earth weighs 

 much more than would a ball of granite of like size, but less than 

 a ball of iron. Its density is about halfway between the two, and 

 it is about twice as heavy as, on the average, are the rocks at the 

 surface. 



Not only do we know the average weight and density of the 

 earth, but we can form some idea as to how that density varies. 

 It must, of course, increase toward the center, as the surface rocks 

 are lighter than the average ; but we can be even more precise 

 than that. If we compare two tops of like mass which have simi- 

 lar conditions of support and are spinning away so as to make an 

 equal number of turns a minute, that one will wabble least whose 

 mass is farthest from the axis about which it turns. Therefore 

 a top is often made in the shape of a light upright axis upon 

 which it may turn, and this axis is connected by light spokes to a 

 wheel in the rim of which, as far as possible from the axis, the 

 mass is mainly collected, for we thus have the extra stability. 

 If we have two such tops of exactly the same shape and size and 

 weight, but the one having a wooden wheel spinning on an iron 

 axis, the other having the iron in the rim of the wheel and the 

 axis all wood, the latter will wabble least. Now, the earth is 

 spinning like a top, and the axis about which she spins connects 

 the north and south poles, and points at present nearly to the 

 north star. But this axis wabbles also, and has not always pointed 

 to the place to which it now points in the starry firmament. The 

 time has been (since Egyptian monuments were built) when the 

 pole star was other than the present one to which the lip of the 

 Dipper points, and quite possibly our remote descendants may 



