HEARING 405 



But if the sound produced by tuning-forks (and low-toned 

 stopped organ-pipes) be omitted from the list, no pure tones 

 reach our ears. The notes of flutes, fiddles, trumpets, pianos, 

 have each a certain " quality " characteristic of the instru- 

 ment. Even in a violin the G string has not the same timbre 

 as the D string. Owing to the elasticity of the substances 

 which originate and of the substances which transmit sound, 

 its pulsations are not simple to-and-fro movements, uninter- 

 rupted from beginning to end. Each pulsation is partially 

 broken at intervals ; and the quality of the sound depends upon 

 the number and relative accentuation of these partial inter- 

 ruptions. Sound travels through air at the rate of 1,100 feet 

 per second. This figure, divided by the number of vibrations 

 per second of a tone, gives the wave-length in air of a tone of 

 that particular pitch. For example, the middle C has a 

 vibratory rate of 256. Its wave-length is, therefore, somewhat 

 over 4 feet. The lowest tone of an organ has a wave-length 

 of 37 feet ; its highest of 3J inches. These figures give no infor- 

 mation, however, regarding the movement of the particles 

 which pass on the sound. When air is transmitting a note 

 say the middle C its separate molecules do not move through a 

 distance of 4 feet. Each molecule moves but a short distance, 

 varying with the loudness of the tone ; but the " wave " of 

 crowding runs straight forward from the piano-string to the 

 ear, the molecules at the end of each stage of 4 feet taking on 

 a backward movement, so that the crowding, so far as the mole- 

 cules of that particular section are concerned, returns to its 

 starting-point. Between the piano-string and the ear there is 

 a crowding and forward movement at 0, 4, 8, 12 ... feet ; a 

 spreading and backward movement at 2, 6, 10, 14 ... feet. 

 Most illustrations which are intended to aid the mind in 

 forming a definite picture of the transmission of sound are 

 liable to be misinterpreted, because they translate rectilinear 

 movements into waves. They represent the movements of the 

 string, and not the movements of the molecules of air between 

 the string and the ear ; but with the aid of the imagination 

 one may picture the positions of the particles in this path. 

 The pulse, we will suppose, has just reached the limit of 12 feet. 

 Half-way from its 8-foot halting place the molecules are again 

 crowded, although not so densely. One-third of the distance 

 from the same point there again appears a tendency to 



