404 BEATS. 



cavity correspond, therefore, to one of the larynx- 

 sound: that is, every fourth image of the former coin- 

 cides with one of the latter, or every fourth tongue is 

 longer than the three intermediate ones. 



The images produced by A (as in /ate), /, and the 

 complex vowel- sounds of the diphthongs are again dif- 

 ferent from those which have been described; their 

 images are more complicated and not so intelligible as 

 those produced by 00, 0, and A, because in the former 

 the larynx-sound is accompanied by two sounds which in- 

 fluence the form of the image. Hence the series does 

 not consist of distinct and separate tongues of flame, but 

 of peaks and hollows which render the outline of the 

 whole somewhat indistinct. 



37. Beats. Concord. Discord. It has already been 

 mentioned in art. 34 that if two notes which are of nearly 

 the same pitch, so that the ear can perceive little or no 

 difference between them, are sounded together, the in- 

 tensity of the combined sound is regularly increased and 

 decreased. If the two primary sounds have different in- 

 tensities, or are produced by different instruments, these 

 variations in intensity of the combined sound, or ' beats/ 

 are not so perceptible as when the quality of the two 

 sounds is the same. In the latter case the decrease of 

 sound which takes place between each succeeding beat is 

 sometimes so perfect that a ' rest ' is produced; that is, 

 the sound ceases altogether for an instant. If the two 

 sounding bodies commence to vibrate precisely at the 

 same instant thus, for example, if they produce simul- 

 taneously a condensation of the air the effect of each 

 sound taken singly will clearly be increased consider- 

 ably by the effect of the other, and the sound of both 



