Belas — Water-jets, and the Effect of Sound theneon. 361 



membrane itself, vibrating in one of its natural periods, and 

 perhaps assisted by the resonance of the hollow glass cjdinder 

 over which it was stretched. 



To test this I made an experiment as follows : — The jet — 

 other arrangements remaining the same — was allowed to flow on 

 to the diaphragm of a Bell's telephone - receiver, the coil and 

 magnet of which were protected from the water by paraffin wax. 

 This was connected in series with a couple of dry cells, and a 

 similar receiver. On applying the tuning-forks separately to the 

 support of the jet, the notes were heard very distinctly by an 

 observer listening in the telephone outside the room ; and when 

 the two tuning-forks were used together, the change to the 

 difference-tone was most marked. I have frequently heard this 

 tone myself, by mounting the two tuning-forks on their 

 respective sounding-boxes and bowing them strongly, but others 

 to whom I referred seemed unable to distinguish it. In the 

 telephone, however, the note was unmistakable, as the generating 

 tones disappeared entirely. This result seemed to me to exclude 

 any possibility of the note being due to resonance, or to the parti- 

 cular membrane used. I tried membranes of various sizes, some 

 of thin rubber. The result obtained was the same in all cases, 

 but the quality of tone varied, that given by the parchment 

 being more sonorous, while the rubber emitted penetrating and 

 " brassy " notes. 



Later on, by listening carefully to the impacts of the drops 

 upon the bottom of the sink or a wooden board, I was able to 

 hear the difference-tone. 



I next photographed the jet when falling under the influence 

 of the lower tuning-fork (384), and found the distances between 

 the centres of successive large drops in the photograph to be very 

 uniform, and equal to 2 cms. (See fig. i.) 



In a similar way, I found the distances between the drops 

 when the higher tuning-fork was used to be 1:5 cms. (See 

 fig. ii.) 



The ratio of these distances is 4 : 3 — inversely as the correspond- 

 ing periods of the tuning-forks. I next took photographs of the jet 

 when sounding the difference-tone. The results were at first most 

 disappointing. The jet did not seem to possess any regularity. 

 The drops were large, of irregular outline, and collisions seemed to 



2 G2 



