Page:Popular Science Monthly Volume 47.djvu/315

Rh surface, we shall probably not live long enough to see our prediction proved false. The deepest mines, therefore, far from reaching 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 similar 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