To explain this we cite the following illustration: Two bodies, each having a mass of 4 pounds, and one inch apart, are attracted toward each other, so they touch. If one has twice the mass of the other, the smaller will draw the larger only one-quarter of an inch, and the large one will draw the other three-quarters of an inch, thus confirming the law that two bodies will attract each other in proportion to their mass. Suppose, now, that these balls are placed two inches apart,--that is, twice the distance. As each is, we shall say, four pounds in weight, the square of each would be 16. This does not mean that there would be sixteen times the attraction, but, as the law says, inversely as the square of the distance, so that at two inches there is only one-sixteenth the attraction as at one inch. If the cord of one of the balls should be cut, it would fall to the earth, for the reason that the attractive force of the great mass of the earth is so much greater than the force of attraction in its companion ball. INDESTRUCTIBILITY OF GRAVITATION.--Gravity cannot be produced or destroyed. It acts between all parts of bodies equally; the force being proportioned to their mass. It is not affected by any intervening substance; and is transmitted instantaneously, whatever the distance may be. While, therefore, it is impossible to divest matter of this property, there are two conditions which neutralize its effect. The first of these is position. Let us take two balls, one solid and the other hollow, but of the same mass, or density. If the cavity of the one is large enough to receive the other, it is obvious that while gravity is still present the lines of attraction being equal at all points, and radially, there can be no pull which moves them together. DISTANCE REDUCES GRAVITATIONAL PULL.--Or the balls may be such distance apart that the attractive force ceases. At the center of the earth an object would not weigh anything. A pound of iron and an ounce of wood, one sixteen times the mass of the other, would be the same,--absolutely without weight. If the object should be far away in space it would not be influenced by the earth's gravity; so it will be understood that position plays an important part in the attraction of mass for mass. HOW MOTION ANTAGONIZES GRAVITY.--The second way to neutralize gravity, is by motion. A ball thrown upwardly, antagonizes the force of gravity during the period of its ascent. In like manner, when an object is projected horizontally, while its mass is still the same, its weight is less. Motion is that which is constantly combating the action of gravity. A body moving in a circle must be acted upon by two forces, one which tends to draw it inwardly, and the other which seeks to throw it outwardly. The former is called centripetal, and the latter centrifugal motion. Gravity, therefore, represents centripetal, and motion centrifugal force.