"Action and re-action are equal and opposite, or, to every action there is always an equal and contrary re-action." This law is also conformable to experience; for, by experiment, it has been proved to hold good for electric and magnetic action. As MacLaurin points out, the Third Law of Motion may be extended to all sorts of powers that take place in Nature, and belongs to attraction and repulsion of all kinds, and must not be considered as being arbitrarily introduced by Newton. The mutual action between any two bodies has, therefore, a double action. Thus a piece of stretched string must be conceived as pulling at both ends; the pull at the one end being exactly equal and opposite to the pull on the other end. A magnet will attract a piece of iron with a certain force, but it is equally true that the iron attracts the magnet with an exactly equal and opposite force. We might even extend the application of this Third Law to a falling stone in its relation to the earth. Thus, if a stone is dropped from a high altitude to the surface of the earth, although the motion seems to be all in one direction, yet if the Third Law holds good, then the earth is attracted by the stone in exactly an equal, but opposite direction, to that in which the earth attracts the stone. As, however, the mass of the earth is very great compared with that of the stone, it follows that the velocity of the stone compared with the velocity of the earth, must be very much greater, in order that the forces shall be equal. The application of this Third Rule of Motion to planetary and celestial phenomena is therefore philosophical, in that its conception agrees with experience and observation. Thus, while it is true that the sun attracts each of the planets in his system, it is equally true that the planets, in their turn, attract the sun with an exactly equal and opposite force. But the velocity of motion induced by the earth's attractive power upon the sun, would be less than the velocity of motion induced by the sun's attractive power upon the earth, although the two forces would be equal and opposite, simply because force, being a compound quantity, is dependent upon the mass of a body as well as upon its velocity. Not only, however, is it true that the sun and all the planets jointly attract each other, but it is equally true that the planets attract each other also, with an exactly equal and opposite effect. Indeed, as Gravitation is universal, it has to be conceived that there are no two bodies existing, but what the Third Law of Motion equally applies to those two bodies; so that equality of action and re-action is as universal as the Law of Gravitation itself.