What I have said about the use of the two senses whose use is most constant and most important, may serve as an example of how to train the rest. Sight and touch are applied to bodies at rest and bodies in motion, but as hearing is only affected by vibrations of the air, only a body in motion can make a noise or sound; if everything were at rest we should never hear. At night, when we ourselves only move as we choose, we have nothing to fear but moving bodies; hence we need a quick ear, and power to judge from the sensations experienced whether the body which causes them is large or small, far off or near, whether its movements are gentle or violent. When once the air is set in motion, it is subject to repercussions which produce echoes, these renew the sensations and make us hear a loud or penetrating sound in another quarter. If you put your ear to the ground you may hear the sound of men's voices or horses' feet in a plain or valley much further off than when you stand upright. As we have made a comparison between sight and touch, it will be as well to do the same for hearing, and to find out which of the two impressions starting simultaneously from a given body first reaches the sense-organ. When you see the flash of a cannon, you have still time to take cover; but when you hear the sound it is too late, the ball is close to you. One can reckon the distance of a thunderstorm by the interval between the lightning and the thunder. Let the child learn all these facts, let him learn those that are within his reach by experiment, and discover the rest by induction; but I would far rather he knew nothing at all about them, than that you should tell him. In the voice we have an organ answering to hearing; we have no such organ answering to sight, and we do not repeat colours as we repeat sounds. This supplies an additional means of cultivating the ear by practising the active and passive organs one with the other.