180 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



such distances from the resonators that 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, 

 Ave close the mouth of either resonator with a piece of card- 

 board, 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 card-board, we shall again have si- 

 lence. Also, if we substitute for one of the pieces of card- 

 board a slip of stout glazed note-paper, the same result is ob- 

 tained. But if we replace the piece of note-paper by a sim- 

 ilar piece of French tracing-paper, a faint sound issues from 

 the resonator so covered, because the tracing-paper is suffi- 

 ciently permeable to sonorous vibrations to permit the reso- 

 nator to reinforce the sound of the fork. This reinforcement 

 becomes greater if we substitute 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 pro- 

 duced when we put in the place of the tissue-paper a piece of 

 the soft, loosely woven paper which is used by French instru- 

 ment-makers for the inner wrapping of their packed wares. 

 Professor Mayer thus obtained a graded series of substances 

 more and more permeable to sonorous vibrations. 



Again, when both resonators with their mouths uncovered 

 produced interference, he screened the mouth of one of them 

 with a bat's- wing coal-gas flame. The vibrations issuing 

 from the resonators were no longer neutralized, but the vi- 

 brations from the uncovered resonator had a great ascend- 

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

 Now if we can shield the mouth of the open resonator so that 

 silence is exactly produced, we will have screened both reso- 

 nators with substances equally reflecting and equally perme- 

 able to sound. On using his graded series of screens, Professor 

 Mayer found that the tracing-paper just equaled in its re- 

 flecting power the gas flame. On lowering the position of 

 the gas flame, so that its top luminous border was just below 

 the mouth of the resonator and therefore only a sheet of 

 heated air ascended between the latter and the fork the bal- 

 ance of the tracing-paper against the hot gases and vapor re- 

 mained unimpaired. 



