98 OJf THE PHYSIOLOGICAL CAUSES OF 



solely by the sympathetic vibration of the higher strings, 

 which correspond with the upper partial tones of the tone 

 sung. 



In this experiment the tones of numerous strings are 

 excited by a tone proceeding from a single source, the 

 human voice, which produces a motion of the air, equi- 

 valent in form, and therefore in quality, to that of this 

 single tone itself. 



We have hitherto spoken only of compositions of waves 

 of different lengths. We will now compound waves of 

 the same length which are moving in the same direction. 

 The result will be entirely different, according as the 

 elevations of one coincide with those of the other (in 

 which case elevations of double the height and depres- 

 sions of double the depth are produced), or the elevations 

 of one fall on the depressions of the other. If both 

 waves have the same height, so that the elevations of one 

 exactly fit into the depressions of the other, both eleva- 

 tions and depressions will vanish in the second case, and 

 the two waves will mutually destroy each other. Simi- 

 larly two waves of sound, as well as two waves of water, 

 may mutually destroy each other, when the condensations 

 of one coincide with the rarefactions of the other. This 

 remarkable phenomenon wherein sound is silenced by a 

 precisely similar sound, is called the interference of 

 sounds. 



This is easily proved by means of the siren already 

 described. On placing the upper box so that the puffs of 

 air may proceed simultaneously from the rows of twelve 

 holes in each wind chest, their effect is reinforced, and 



have all the same fundamental tone, and the upper partials only differ in 



intensity. For female voices the pitches T^-r^—^ a' to c" are favourable 



for all vowels. This is a fundamental experiment for the theory of vowel 

 sounds, and should be repeated by all who are interested in speech. — Tb. 



