in physics, a kind of tremulous motion or vibration observable in a liquid, whereby it alternately rises and falls like the waves of the sea.

This undulatory motion, if the liquid be smooth and at rest, is propagated in concentric circles, as most people have observed upon throwing a stone or other matter upon the surface of a stagnant water, or even upon touching the surface of the water lightly with the finger or the like. The reason of these circular inductions is, that by touching the surface with your finger, there is produced a depression of the water in the place of contact. By this depression the subjacent parts are moved successively out of their place, and the other adjacent parts thrust upwards, which lying successively on the descending liquid, follow it; and thus the parts of the liquid are alternately raised and depressed, and that circularly. When a stone is thrown into the liquid, the reciprocal vibrations are more conspicuous.

Undulatory motion is likewise applied to a motion in the air, whereby its parts are agitated after the like manner as waves in the sea; as is supposed to be the case when the string of a musical instrument is struck. This undulatory motion of the air, is supposed the matter or cause of sound.