1856] WRITINGS OP JOSEPH HENRY. 413 



vent an isolated echo, provided the distance be sufficiently 

 great and the sound sufficiently loud, yet that they do have 

 an important effect in stopping the resonance is evident from 

 theory and experiment. In a room 50 feet square in which 

 the resonance of a single intense sound continued six seconds, 

 when cases and other objects were placed around the wall its 

 continuance was reduced to two seconds. 



Fourthly, the duration of the resonance will depend upon 

 the nature of the material of the wall. A reflection always 

 takes place at the surface of a new medium, and the amount 

 of this will depend upon the elastic force or power to resist 

 compression and the density of the new medium. For ex- 

 ample, a wall of nitrogen, if such could be found, would 

 transmit nearly the whole of a wave of sound in air, and 

 reflect but a very small portion ; a partition of tissue-paper 

 would produce nearly the same effect. A polished wall of 

 steel however, of sufficient thickness to prevent yielding, 

 would reflect for practical purposes all the impulses through 

 the air which might fall upon it. The rebound of the wave 

 is caused, not by the oscillation of the wall, but by the elas- 

 ticity and mobility of the air. The striking of a single ray 

 of sound against a yielding board would probably increase 

 the loudness of the reverberation but not its continuance. 

 On this point a series of experiments was made by the use 

 of the tuning-fork. In this instrument the motion of the 

 foot and of the two prongs gives a sonorous vibration to the 

 air, which, if received upon another tuning-fork of precisely 

 the same size and form, would re-produce the same vibra- 

 tions. 



It is a fact well established by observation that when two 

 bodies are in perfect unison, and separated from each other 

 by a space filled with air, vibrations of the one will be taken 

 up by the other. From this consideration it is probable 

 that relatively the same effect ought to be produced in trans- 

 mitting immediately the vibration of a tuning-fork to a 

 reflecting body as to duration and intensity as in the case of 

 transmission through air. This conclusion is strengthened 

 by floating a flat piece of wood on water in a vessel standing 



