228 Mr. Wheatstone on the 



materially depend ; one of the most important of these is, the 

 proper dimensions of the volume of air contained within the 

 sides ; the laws of these resonant cavities have occupied the 

 attention of Savart, but the obvious use of the bars placed 

 within these cavities to divide the mass of air, and thus to 

 enable it to vibrate more readily in separate portions, seems to 

 have escaped his notice. 



4. 



In the piano-forte, the guitar, &c., the ends of the strings 

 are not in immediate contact with the sounding-board, but they 

 rest on bars of wood, which are called bridges, through which 

 the vibrations are communicated to the board. In these in- 

 struments the bridge is usually about half an inch in height, 

 and in the violoncello does not exceed three inches. To as- 

 certain how far the distance might be extended between the 

 string and the sounding-board of a piano-forte without injury 

 to the tone, I substituted a glass rod five feet in length for the 

 bridge, and by placing at its end a string stretched on a steel 

 bow, I found that the sound of the string was as distinctly 

 audible as when it was immediately in contact with the board ; 

 a tuning-fork placed at the end of the rod gave the same 

 result. These experiments, which were the first I made on the 

 subject, and which suggested all the subsequent ones, have 

 been repeated in the theatre of the Royal Institution on a 

 larger scale. A series of connected deal rods, forty feet in 

 length, was suspended so as to extend, in a straight line, ob- 

 liquely from an open window of the cupola, to within a short 

 distance of the floor of the room ; on the upper end of this 

 conductor, an assistant placed the stem of a vibrating tuning 

 fork ; when no sounding-board was placed at the lower extre- 

 mity of the conductor, no sound was heard, but it became 

 powerfully audible the instant the communication was made : 

 this experiment was repeated with different acute and grave 

 toned tuning-forks, employed both in combination and in suc- 

 cession. 



Tuning-forks are the most convenient instruments for making 

 experiments on the transmission of sound, because their vibra- 



