If a spring be compressed between two equal bodies, it will throw them off with equal velocities. If they are unequal, the velocity of the smaller body will be greater than that of the larger, and in proportion to its smallness. For this reason, when a ball issues from a cannon, though the cannon and the ball are equally acted upon by the elastic or expansive force of the gases set free by lighting the powder, the gun is moved but very little because the force is diffused through so large a mass, while the ball being so much smaller moves with great velocity. When a volcano throws stones from its crater the earth may be compared to the cannon, the stones to the ball, and the explosive materials throwing the stones to the exploding powder projecting the ball. As the cannon is moved as much as the ball, so is the earth moved as much as the stones, the only reason that it does not move as far and as rapidly as the stones being that the force is diffused through so large a bulk. These examples illustrate very well the relation of action and reaction, for whenever there is an action of one body upon another it is as if a spring were between the two bodies, acting equally upon both. When a man jumps from the ground it is as if a spring were compressed between him and the earth, and this expanding moves the earth exactly as much as it does the man. He really kicks the earth away from him. The motion of the earth is not obvious because it is diffused through so large a mass. The case is parallel to that of the ball and cannon. The same force is exerted upon the man and the earth, but the man, like the ball, moves the most, and in proportion to his comparative smallness. So when a bird hops from the ground, the earth moves as really as the bird. If the bird hop from a twig, you perceive that the twig is moved by the pressing down of the bird as it rises. When it starts from the ground it exerts the same downward pressure, and moves the earth as really as in the other case it did the twig.