SECT. XVII. SPEAKING MACHINES. 151 



fleeted by the bottom of the cavity and the undulating body at 

 its mouth. The first impulse of the undulating substance will 

 be reflected by the bottom of the cavity, and then by the un- 

 dulating body, in time to combine with the second new impulse. 

 This reinforced sound will also be twice reflected in time to con- 

 spire with the third new impulse ; and, as the same process will 

 be repeated on every new impulse, each will combine with all 

 its echoes to reinforce the sound prodigiously. Professor Wheat- 

 stone, to whose ingenuity we are indebted for so much new and 

 valuable information on the theory of sound, has given some 

 very striking instances of resonance. If one of the branches of 

 a vibrating tuning-fork be brought near the embouchure of a 

 flute, the lateral apertures of which are stopped so as to render 

 it capable of producing the same sound as the fork, the feeble 

 and scarcely audible sound of the fork will be augmented by the 

 rich resonance of the column of air within the flute, and the tone 

 will be full and clear. The sound will be found greatly to 

 decrease by closing or opening another aperture ; for the altera- 

 tion in the length of the column of air renders it no longer fit 

 perfectly to reciprocate the sound of the fork. This experiment 

 may be made on a concert flute with a C tuning-fork. But 

 Professor Wheatstone observes, that in this case it is generally 

 necessary to finger the flute for B, because, when blown into 

 with the mouth, the under-lip partly covers the embouchure, 

 which renders the sound about a semitone flatter than it would 

 be were the embouchure entirely uncovered. He has also shown, 

 by the following experiment, that any one among several simul- 

 taneous sounds may be rendered separately audible. If two 

 bottles be selected, and tuned by filling them with such a quantity 

 of water as will render them unisonant with two tuning-forks 

 which differ in pitch, on bringing both of the vibrating tuning- 

 forks to the mouth of each bottle alternately, in each case that 

 sound only will be heard which is reciprocated by the unisonant 

 bottle. 



Several attempts have been made to imitate the articulation 

 of the letters of the alphabet. About the year 1779, MM. Krat- 

 zenstein of St. Petersburg, and Kempelen of Vienna, constructed 

 instruments which articulated many letters, words, and even 

 sentences. Mr. Willis of Cambridge has adapted cylindrical 

 tubes to a reed, whose length can be varied at pleasure by sliding 



