Hearing.
What has been said as to the two senses most continually employed and most important may illustrate the way in which I should exercise the other senses. Sight and touch deal alike with bodies at rest and bodies in motion. But as only the vibration of the air can arouse the sense of hearing, noise or sound can be made only by a body in motion. If everything were at rest, we could not hear at all. At night, when we move only as we choose, we have nothing to fear except from other bodies in motion. We therefore need quick ears to judge from our sensations whether the body causing them is large or small, distant or near, and whether its motion is violent or slight. The air, when in agitation, is subject to reverberations which reflect it back, produce echoes, and repeat the sensation, making the sonorous body heard elsewhere than where it really is. In a plain or valley, if you put your ear to the ground, you can hear the voices of men and the sound of horses' hoofs much farther than when standing upright. As we have compared sight with touch, let us also compare it with hearing, and consider which of the two impressions, leaving the same body at the same time, soonest reaches its organ. When we see the flash of a cannon there is still time to avoid the shot; but as soon as we hear the sound there is not time; the ball has struck. We can estimate the distance of thunder by the interval between the flash and the thunderbolt. Make the child understand such experiments; try those that are within his own power, and discover others by inference. But it would be better he should know nothing about these things than that you should tell him all he is to know about them.
We have an organ that corresponds to that of hearing, that is, the voice. Sight has nothing like this, for though we can produce sounds, we cannot give off colors. We have therefore fuller means of cultivating hearing, by exercising its active and passive organs upon one another.