Prof. A. M. Mayer's Researches in Acoustics. 429 



complete interference of the vibrations issuing from their mouths 

 is obtained, and the only sound that reaches the ear is the 

 faint sound given by the fork's action on the air outside the 

 angle included by the mouths of the resonators. If in these 

 circumstances we close the mouth of either resonator with a 

 piece of cardboard, the open resonator will strongly reinforce the 

 sound of the fork. If we now also cover the mouth of the latter 

 resonator with a piece of cardboard we shall again have silence. 

 Also, if we substitute, for one of the pieces of cardboard, a slip 

 of stout glazed note-paper, the same result is obtained. But 

 if we replace the piece of note-paper by a similar piece of French 

 tracing-paper, a faint sound issues from the resonator so covered, 

 because the tracing-paper is sufficiently permeable to sonorous 

 vibrations to permit the resonator to slightly reinforce the sound 

 of the fork. This reinforcement becomes greater if we sub- 

 stitute for the tracing-paper a piece of tissue paper, such as is 

 used in printed books to cover steel engravings; and a yet 

 greater reinforcement is produced when we put in the place of 

 the tissue paper a piece of the soft, loosely woven paper which 

 is used by French instrument-makers for the inner wrapping of 

 their packed wares. I thus obtained a graded series of sub- 

 stances, more and more permeable to sonorous vibrations. 



I again obtained neutralization by interference, with the 

 mouths of the resonators open, and then screened the mouth of 

 one of them with a bat's-wing coal-gas flame. The vibrations 

 issuing from the resonators were now no longer neutralized, but 

 the vibrations from the uncovered resonator had a great ascen- 

 dancy over the other, so that a strong sound issued from it. I 

 now tried to destroy this superiority by screening its mouth 

 successively with the graded series of paper screens. The loose, 

 soft paper was not equal to it ; nor was the tissue paper ; but the 

 tracing-paper just equalled the effect of the gas-flame in guard- 

 ing the mouth of the resonator from the entrance of sonorous 

 vibrations. On lowering the gas-flame, so that its top luminous 

 border was just below the mouth of the resonator, and there- 

 fore only a sheet of heated air ascended across the latter, the 

 balance of the tissue paper against the hot gases and vapour re- 

 mained unimpaired. Thus it appears that the reflecting-power 

 of a sheet of coal-gas flame or of a sheet of the heated carbonic 

 acid and vapour of water just above it, exactly equals, in the 

 above described circumstances, the reflecting power of tracing- 

 paper. 



I have also found that the passage of a sheet of cold coal-gas 

 across the mouth of the resonator was sufficient to destroy the 

 balance of the interference, and caused a faint sound to issue 

 from the other resonater ; a similar effect, and nearly equal in 



