Rocking sets children to sleep better than absolute rest; there is indeed scarcely anything at that age, which gives more pleasure than to be gently lifted up and down; the manner of playing which their nurses use with children, and the weighing and swinging used afterwards by themselves as a favorite amusement, evince this very sufficiently.

Most people must have observed the sort of sense they have had on being swiftly drawn in an easy coach on a smooth turf, with gradual ascents and declivities.

This will give a better idea of the beautiful, and point out its probable cause better, than almost anything else.

On the contrary, when one is hurried over a rough, rocky, broken road, the pain felt by these sudden inequalities shows why similar sights, feelings, and sounds, are so contrary to beauty: and with regard to the feeling, it is exactly the same in its effect, or very nearly the same, whether, for instance, I move my hand along the surface of a body of a certain shape, or whether such a body is moved along my hand.

But to bring this analogy of the senses home to the eye; if a body presented to that sense has such a waving surface, that the rays of light reflected from it are in a continual insensible deviation from the strongest to the weakest (which is always the case in a surface gradually unequal), it must be exactly similar in its effects on the eye and touch; upon the one of which it operates directly, on the other indirectly.