86 ON THE PHYSIOLOGICAL CAUSES OF 



sound of the vowel is produced solely by the sympathetic vibra- 

 tion 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, equivalent 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 depressions of double the depth are produced), or the 

 elevations of one fall on the depi-essions 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 elevations and depres- 

 sions will vanish in the second case, and the two waves will 

 mutually destroy each other. Similarly 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 we obtain the fundamental tone of 



sharply sung, or called out, beside a piano of which the dampers have been 

 raised, that vowel will be echoed back. There is generally a sensible pause 

 before the echo is heard. Before repeating the experiment with a new vowel, 

 whether at the same or a different pitch, damp all the strings and then again 

 raise the dampers. The result can easily be made audible to a hundred persons 

 at once, and it is extremely interesting and instructive. It is peculiarly so if 

 different vowels be sung to the same pitch, so that they have all the same 

 fundamental tone, and the upper partials only differ in intensity. For female 



voices the pitches jgja J ^- a' to c" are favourable for all vowels. This is a 



fundamental experiment for the theory of vowel sounds, and should be re- 

 peated by all who are interested in speech. TR. 



