which the sound passes being the air. A sounding body in the middle of a room, for instance, must send out shells of sound as it were, in all directions, because people above, below, and all round it, would hear the sound. Replace the stone by a tuning-fork. To one prong of this fasten a mirror, and on this mirror throw a powerful beam of light. When this tuning-fork is bowed, and a sound is heard, the light thrown by the attached mirror shows the fork to be vibrating, and when the tuning-fork is moved we get an appearance on the screen which reminds us of the rope; or we may use the fork as shown in Fig. 3, and obtain a wavy record on a blackened cylinder.
Experiment shows that we have at one time a sphere of compression—that is to say, the air is packed closely together; and, again, a sphere
Fig. 5.—Propagation of Sound-Waves along a Cylinder.
of rarefaction, when the particles of air are torn farther apart than they are in the other position. The state of things, then, that travels in the case of sound, is a state of compression and rarefaction of the air. Hence, the particle of air travels differently from the particle of water; it moves backward and forward in a straight line in the direction in which the sound is propagated.
Fig. 6.—Sound-Waves. Particles of air, a, b, c, d, e, are in position 1 at rest. The remaining positions show how they are situated at successive instants, when a continuous series of impulses reaches them from the left. In position 2, e. g., only one particle has begun its oscillation; in position 3, only two; while, in position 6, all are in motion.
