If B is removed to the distance of two thousand miles from A, the sum total of the attractive force which A would exert upon B would be diminished by the square of the distance, because B would intercept just one fourth of the number of rays proceeding from A compared with the number which it intercepts when the two bodies are only one thousand miles apart; and the rays which B does not intercept would pass along through the realms of space, until they encountered some other body, on which they would exert a force that would follow the same law of diminution. In the diagram, the two bodies A and B may be single particles of matter or collections of particles; they are represented as cubes; but the law of direct action of the attracting force and the law of its diminution would be the same if the bodies were spheres or oblongs. The power of attraction which bodies exert upon each other resides in every individual particle of matter composing the body, and the attraction which that body exerts upon another body is the sum total of the attractions which proceed from all the particles composing the mass and which impinge upon that other body. [Illustration] In the diagram the two bodies A and B are supposed to be of the same mass. If, as in the case of the sun and the earth, one of the bodies is of far greater mass than the other, then the attraction of the sun for the earth is the same as the attraction of the earth for the sun, because the action is mutual; but the sun, being the greater mass, tends, by reason of its correspondingly greater inertia, to remain comparatively stationary, or, in other words, it has a greater resistance to being pulled out of its normal position, while the earth, having less inertia, is more easily deflected from its straight course in which its momentum tends to carry it, and so travels in an orbit around the sun, the resisting or centrifugal pull of the earth, due to its inertia, exactly balancing the inward pull due to the mutual attraction. I understand that, besides the law of universal gravitation, there are two fundamental laws of motion. By one of these laws, if a body be set in motion and be acted on by no other than the projectile force, it will move forward in a straight line and with a uniform velocity forever. But by another law, if the moving body is acted on by another force than that which originally projected it in a straight line, it will deviate from that line in the direction of that other force and in proportion to it. If A, the earth, liable to be drawn toward B, the sun, by their mutual attraction, was originally projected into space, at a certain distance from the sun, by a force which would carry it on in a straight line, it would be acted on by two forces: the projectile force would cause it to move in a straight line; the force of the mutual attraction would cause it to deviate from that line in the direction of the sun. The result would be that the earth