the Pianoforte Bridges. 117 



heavily spun with copper, and tuned to B n giving 31 vbns/sec. 

 It was struck at J of its length from the fixed bridge by a 

 pianoforte hammer. The force of the blow was made the 

 same in all cases by a small weight being allowed to drop 

 from a constant height on the key working the hammer. 

 The exposure was made 3 seconds after the string was struck. 



No. 1 PL I. shows the optical lever mounted to record the 

 horizontal vibrations of the bridge. 



No. 2 the same lever mounted to record the vertical 

 vibrations. In both photographs L is the cross-bar carrying 

 B a thick brass plate. Two legs of the optical lever T rest 

 in a hole and slot respectively in the plate B. M is the 

 mirror and S the straw. The latter, together with a small 

 wooden base and a needle, acts as the third leg of the lever 

 and forms the connexion with the bridge A. K is the sound- 

 board. The three steel strings W were replaced in the expe- 

 riment described by one steel string heavily spun with copper. 



No. 3 shows the curve for the vibration of the bridge at 

 right-angles to the string. 



No. 4 shows the curve for the vertical vibration parallel 

 to the string. 



The amplitude of No. 1 is about 5*3 times as great as that 

 of No. 2. Although considerably greater the difference is 

 not so much as the experiment with the weights described 

 above would lead us to suppose. It will be noticed that the 

 octave is prominent in No. 3, and in No. 4 we get only the 

 octave as we should expect. 



Returning to the consideration of the vibration of the 

 sound-board as set up in the manner described in (1), we 

 saw that the bridge acting as a lever upon the sound-board 

 would set the latter in vibration with its two halves in 

 opposite phase. 



The optical lever gives us a means of testing this. Two 

 levers were set up one on each side of the free bridge and 

 about 7*5 cm. distant from it. A mirror on each lever 

 threw images of the slit on the film, and a simultaneous 

 photograph of the vibrations at these two points on the 

 sound-board were thus obtained. 



Nos. 5 and 6 show this. Three steel strings, No. 18 gauge, 

 137 cm. long and tuned to Aft 116, were used. It will be 

 seen that the phase of the two is the same, and this shows 

 that the sound-board is not set in vibration as described in (1). 



Soundless Vibrations of a String. 



An interesting little experiment which may be performed 

 on any pianoforte may be here described. Place on the 



