§ 5 The Waves of Sound
How are we to picture to ourselves the condition of the air through which this musical sound is passing? Imagine one of the prongs of the vibrating fork swiftly advancing; it compresses the air immediately in front of it, and when it retreats it leaves a partial vacuum behind, the process being repeated by every subsequent advance and retreat. The whole function of the tuning-fork is to carve the air into these condensations and rarefactions, and they, as they are formed, propagate themselves in succession through the air. A condensation with its associated rarefaction constitutes, as already stated, a sonorous wave. In water the length of a wave is measured from crest to crest; while, in the case of sound, the wave-length is the distance between two successive condensations. The condensation of the sound-wave corresponds to the crest, while the rarefaction of the sound-wave corresponds to the sinus, or depression, of the water-wave. Let the dark spaces, a, b, c, d, Fig. 25, represent the condensations, and the light ones, a′, b′, c′, d′, the rarefactions of the waves issuing from the fork a b: the wave-length would then be measured from a to b, from b to c, or from c to d.