362 Scientific Proceedings, Royal Dublin Society. 



occur anywhere, showing a want of regularity in the separation 

 of the drops. While endeavouring to rectify matters by varying 

 the conditions slightly, I noticed that the difference-tone, though 

 pronounced enough, was not quite steady, but had a quiver in it. 

 On loading one of the tuning-forks very slightly, this was 

 improved, and ultimately the note became quite steady. "When 

 the jet was photographed again, the result shown in fig. in. was 

 obtained. To produce this difference-tone, the jet of water must 

 be sensitive to both tuning-forks separately, and exhibit the 

 characteristic shape of a jet when thus affected. But a jet, if 

 sensitive to vibrations of such frequencies as 384 and 512, is not 

 affected by one of 128. A single tuning-fork making 128 

 vibrations per second will not cause the membrane to emit that 

 note unless the pressure of water and the size of the orifice be 

 changed. How, then, is the effect produced? I think, by reason 

 of the amplitude of the periodic disturbance produced when the 

 two forks act together. 



To quote a passage from one of Lord Rayleigh's Papers, 1 

 " If the initial disturbances are small enough, that one is 

 ultimately preponderant for which the measure of instability is 

 greatest ... But a disturbance of less favourable wave-length 

 may gain the preponderance in case its magnitude be sufficient 

 to produce disintegration in a less time than that required by 

 the other disturbances present." 



Now, the periods of the tuning-forks used are in the ratio of 

 3 : 4, and by the time that one fork has completed three whole 

 vibrations, the other will have completed four ; and if the time 

 was reckoned from the instant that the two tuning-forks were in 

 the same phase, they will be so again at the expiration of this 

 time and of multiples of it. Now, their being in the same phase 

 means a maximum amplitude of the resultant vibration ; and this- 

 takes place, then, at intervals corresponding to three complete 

 vibrations of the lower fork — i.e., 128 times per second. "We 

 might expect, then, as this disturbance is mainly instrumental in 

 resolving the jet, that drops would appear in the figure, separated 

 by a distance corresponding to this period — i.e., 6 cms. 



This would likely be the case if the jet could be resolved by 



1 Collected Papers, Lord Rayleigh, vol. i., p. 390. 



