Lord Rayleigh's Acoustical Observations. 189 



transverse direction. I usually placed the resonator's mouth 

 uppermost, so that the jets were horizontal. 



Good results were obtained at a pitch of 256. When two 

 forks of about this pitch, and slightly out of tune with one 

 another, were allowed to sound simultaneously, the evolutions 

 of the smoke-jet in correspondence with the audible beats 

 were very remarkable. By gradually raising the pressure at 

 which the smoke is supplied, in the manner usual in these 

 experiments, a high degree of sensitiveness may be attained, 

 either with a drawn-out glass nozzle or with the steatite pin- 

 hole burner used by Tyndall. In some cases (even at pitch 

 256) the combination of jet and resonator proved almost as 

 sensitive to sound as the ear itself. 



The behaviour of the sensitive jet does not depend upon 

 the smoke-particles, whose office is merely to render the effects 

 more easily visible. I have repeated these observations with- 

 out smoke by simply causing air-jets from the same nozzles 

 to impinge upon the flame of a candle placed at a suitable 

 distance. In such cases, as has been pointed out by Tyndall, 

 the flame acts merely as an indicator of the condition of the 

 otherwise invisible jet. Even without a resonator the sensi- 

 tiveness of such jets to hissing sounds may be taken advantage 

 of to form a pretty experiment. 



The combination of jet, resonator, and flame shows some- 

 times a tendency to speak on its own account ; but I did 

 not succeed in getting a well-sustained sound. Such as it is, 

 the effect probably corresponds to one observed by Savart and 

 Plateau with water-jets breaking up under the operation of 

 the capillary tension, and when resolved into drops impinging 

 upon a solid obstacle, such as the bottom of a sink, in mecha- 

 nical connexion with the nozzle from which the jet originally 

 issues. In virtue of the connexion, any regular cycle in the 

 mode of disintegration is able, as it were, to propagate itself. 



The increased and more discriminating sensitiveness ob- 

 tainable by use of resonators is turned to account in the 

 arrangement of flame described in the Proceedings of the 

 Cambridge Philosophical Society for November 8, 1880. 

 (See ' Nature,' December 16, 1880.) 



In this case the resonator takes the form of a tube, one of 

 whose ends opens in the gas-chamber close to the nozzle. The 

 other end is closed by a cork, whose position can be adjusted 

 so as to vary the pitch. I see from my note-book that, on the 

 evening of Dec. 4, 1879, I found the flame nearly as sensitive 

 as the ear to vibrations of frequency 512 ; but I have not 

 always been equally successful in subsequent attempts to 

 recover this degree of delicacy. 



