NOISE AND MUSIC 279 



must not only impart short, quick shocks to the air, but 

 must impart these shocks with unerring regularity and 

 strength. A flickering light irritates the eye; a flickering 

 sound or noise irritates the ear ; both are painful because of 

 the sudden and abrupt changes in effect which they cause, 

 the former on the eye, the latter on the ear. 



The only thing essential for the production of a musical 

 sound is that the waves which reach the ear shall be rapid 

 and regular ; it is immaterial how these 

 waves are produced. If a toothed wheel 

 is mounted and slowly rotated, and a stiff 

 card is held against the teeth of the wheel, 

 a distinct tap is heard every time the card 

 strikes the wheel. But if the wheel is ro- 

 tated rapidly, the ear ceases to hear the 

 various taps and recognizes a deep contin- 

 uous musical tone. The blending of the 



. , . . , , . , . , FIG. 176. A rotating 



individual taps, occurring at regular inter- disk, 



vals, has produced a sustained musical 

 tone. A similar result is obtained if a card is drawn slowly 

 and then rapidly over the teeth of a comb. 



That musical tones are due to a succession of regularly 

 timed impulses is shown most clearly by means of a rotating 

 disk on which are cut two sets of holes, the outer set equally 

 spaced, and the inner set unequally spaced (Fig. 176). 



If, while the disk is rotating rapidly, a tube is held over 

 the outside row and air is blown through the tube, a sus- 

 tained musical tone will be heard. If, however, the tube is 

 held, during the rotation of the disk, over the inner row of 

 unequally spaced holes, the musical tone disappears, and a 

 series of noises take its place. In the first case, the separate 

 puffs of air followed each other regularly and blended into 

 one tone ; in the second case, the separate puffs of air followed 



