146 RESONANCE. SECT. XVII. 



will resound if a body vibrating the natural note of the 

 cavity be placed opposite to its orifice, and be large 

 enough to cover it ; or at least to set a large portion of 

 the adjacent air in motion. For the sound will be alter- 

 nately reflected by the bottom of the cavity and the un- 

 dulating body at its mouth. The first impulse of the 

 undulating substance will be reflected by the bottom of 

 the cavity, and then by the undulating body, in time to 

 combine with the second new impulse. This reinforced 

 sound will also be twice reflected in time to conspire 

 with the third new impulse ; and as the same process 

 will be repeated on every new impulse, each will com- 

 bine with all its echoes to reinforce the sound pro- 

 digiously. Professor Wheatstone, to whose ingenuity 

 we are indebted for so much new and valuable informa- 

 tion on the theory of sound, has given some veiy striking 

 instances of resonance. If one of the branches of a vi- 

 brating 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 alteration in the 

 length of the column of air renders it no longer fit per- 

 fectly to reciprocate the sound of the fork. This exper- 

 iment 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 em- 

 bouchure entirely uncovered. He has also shown, by 

 the following experiment, that any one among several 

 simultaneous 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. 



