In Astronomy you may learn that the movements of the planets around the sun and the moons around their planets--their regular and constant relative positions--are caused by the force of Gravitation. If it were not for this attraction by the Sun, the planets would fly out into space, like a stone from a sling. The Attraction of Gravitation acts on the planets just as does the string of the whirling sling that keeps the stone from flying away during the whirling until the string is released. Some astronomers think that our sun revolves around some greater sun, and this again around a greater, and so on to infinity. If this be so, then the Attraction of Gravitation is that which holds them all in their orbits and places in spite of their motion. And in Physics, you may learn that this same Attraction of Gravitation prevents the people and objects on the surface of the earth from flying off into space. And that it holds the portions of the earth together, preventing them from flying apart. And, remember this, for it is important--the Attraction of the Earth, great and powerful as it is, is nothing more than the _combined_ attractive power of its constituent molecules, or atoms, or parts. The centre of the Earth is the Centre of the Attraction, because it is the centre of the aggregation of its Particles. It must not be supposed that the Earth simply attracts "downward," that is, toward its centre. On the contrary, large masses of earth--large mountains, for instance--exert a certain degree of Attraction of Gravitation, and experiments have shown that a "plumb" is slightly deflected by reason of the proximity of a large mountain. And the reason that bodies "lose weight" as they descend from the surface of the earth is because they leave "above" them a certain large portion of the molecules composing the earth, which mass of molecules exert an attraction proportionate to their mass, which attraction balances the attraction of the mass of earth "beneath them." Science teaches that if the earth were hollow in the centre, the weight there would be Zero, or nothing at all, and that a body would float in the space at the centre of the earth just as does a balloon in the air, the reason thereof being that the attraction would be equalized--equal attraction from every direction, counterbalancing each other. Considering the earth's radius to be 4000 miles, a body that weighed 100 pounds on the surface would weigh but 75 pounds at the depth of 1000 miles; but 50 pounds at a depth of 2000 miles; but 25 pounds at a depth of 3000 miles; and Nothing, or Zero, at a depth of 4000 miles, which would be the Centre of the Earth. This, of course, supposes that the Substance of which the earth is composed is of uniform density from surface to centre.