If the end of the wire spring shown in Fig. 317 is struck the first few turns of the spring will be compressed. Since the spring possesses elasticity, the turns will move forward a little and compress those ahead, these will press the next in turn and so on. Thus a _compression_ wave will move to the end of the spring, where it will be reflected and return. Consider the turns of the spring as they move toward the end. On account of their _inertia_ they will continue moving until they have separated from each other _more_ than at first, before returning to their usual position. This condition of a greater separation of the turns of the spring than usual is called a _rarefaction_. It moves along the spring following the wave of compression. The condensation and rarefaction are considered as together forming a complete wave. Since the turns of wire move back and forth in a direction parallel to that in which the wave is traveling, these waves are called _longitudinal_. [Illustration: FIG. 318.--Longitudinal waves (1) in a spring, (2) in air, and (3) graphic representation showing wave length, condensations, and rarefactions.] =324. The transmission of a sound by the air= may be understood by comparing it with the process by which a _wave is transmitted by a wire spring_. Consider a light spring (Fig. 318, 1) attached at the end of a vibrating tuning fork, _K_, and also to a diaphragm, _D_. Each vibration of the fork will first compress and then separate the coils of the spring. These impulses will be transmitted by the spring as described in Art. 315, and cause the diaphragm to vibrate _at the same rate_ as the tuning fork. The diaphragm will then give out a sound similar to that of the tuning fork. Suppose that the spring is replaced by air, and the diaphragm, by the ear of a person, _E_, (Fig. 318, 2.) when the prong of the fork moves toward the ear it starts a compression and when it moves back a rarefaction. The fork continues vibrating and these impulses move onward like those in the spring at a speed of about 1120 ft. in a second. They strike the diaphragm of the ear causing it to move back and forth or to vibrate at the same rate as the tuning fork, just as in the case of the diaphragm attached to the spring.