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16 WATER-MOTION [L § 9.
position of wave-motion, and should be carefully mastered and remembered. In the figure employed to demonstrate it, a wave has been only roughly plotted out by a small number of spots, and its movements estimated at but a few arbitrarily chosen instants. By increasing the numbers of these two elements, however, we might make an indefinitely near approach to continuity both of form and of motion.
frequently seen on the surface of water, is by no means confined to fluid bodies. When a carpet is being shaken, bulging forms exactly like water-waves axe seen running along it. A flexible string, one end of which is tied to a fixed point and the other held in the hand, exhibits the same phenomenon when the loose end is sharply jerked aside. It has accordingly been found convenient to extend the term wave in order to designate as ' wave-motion' any movement coming under the definition just given. We proceed to an instance in which the individual particles, instead of describing circular orbits move to-and-fro, or vibrate, in straight lines perpendicular to the direction of wave-propagation.
Before applying to this case the method followed in Fig. 5 it will be desirable to define what is meant