The agencies which Chemistry reveals to us are ever at work causing motion among the particles of matter; and though they generally work in silence, they sometimes show themselves in tremendous explosions, and in convulsions of nature. Busy life is every where producing motion, more especially in the animal world. It gives to the myriads of animals, great and small, that swarm the earth not only the power of moving themselves, but also the power, to some extent, of moving the material world around them. 182. =Action and Reaction Equal.=--When any of the causes of motion act, the action is met by an opposite and equal reaction. If, for example, a blow be given, an equal blow is received in return. For this reason, if one in running hits his head against the head of another both are equally hurt. When a child knocks his head against a table, there is sound philosophy in the common saying that he has given the table as good a blow as he has received, though it may afford him no comfort. Many very interesting illustrations of this law of motion suggest themselves, of which I will give a few. [Illustration: Fig. 126.] A swimmer, pressing the water downward and backward with his hands and feet, is carried along forward and upward by the reaction of the water. And in this case, as in every other, the greater the action the greater is the reaction; in other words, the more strongly he presses with his hands and feet, the more rapidly is he borne along by the reaction of the water against the pressure. A boat advances in proportion to the force with which the oars press against the water. So the rapidity of a steamboat depends on the force with which the paddles drive the water astern. Birds rise in the air by the reaction of the air against their wings as they are pressed downward. A sky-rocket pursues its rapid flight because a large quantity of gaseous matter issues from its lower end, and, being resisted by the reaction of the air, by its pressure throws the rocket upward. So if a ship fire guns from the stern its advance will be accelerated, but if from the bow it will be retarded. When a broadside is fired the ship inclines to the other side. In Fig. 126 (p. 136) is represented the plan of Barker's mill. It consists of a cylinder, _c_, arranged in a frame in such a way that it can revolve on the point upon which it rests. Water runs into it by a tube, _p_, and escapes by the branches, _a_ and _d_. These are so arranged that the reaction upon the issuing water makes the cylinder revolve rapidly, causing the ends of the branches to whirl around as indicated by the dotted lines and arrows.