366 Scientific Proceedings, Royal Dublin Society, 



NOTE ON COMBINATION-TONES 



IN CONNEXION WITH THE FOREGOING PAPER. 



By Professor W. F. Barrett, f.r.s. 



In an afternoon discourse that I was invited to deliver before the 

 Royal Dublin Society so long ago as 1869, 1 showed that a sensitive 

 flame, when responding to a note of definite pitch, entered into a 

 state of synchronous vibration ; in fact, the flame itself sometimes 

 yielded an audible musical note of the same pitch as that which 

 excited it. 1 It so happened that this occurred when preparing for 

 a lecture on "Sensitive Jets" which I recently delivered before the 

 Society in February, 1903. The gas issued at low pressure from 

 a v-shaped orifice (about one-sixteenth inch diameter) in a glass 

 tube, similar to that which I originally described in 1867 ; and the 

 flame was sensitive through a considerable range, responding to 

 notes of comparatively low pitch. When tuning-forks giving 512 

 and 384 vibrations per second were successively sounded, the flame 

 vigorously responded ; and the variation in its rate of vibration 

 was clearly seen in a moving mirror. But when both forks were 

 sounded simultaneously, the flame uttered a note much lower than 

 either fork, and the moving mirror showed its lower rate of vibra- 

 tion. My friend and former student, Mr. Belas, who was assisting 

 me at the time, and who has a remarkably fine musical ear, at 



1 See also my Papers on Sensitive Flames and Jets of Air in the " Phil. Mag." for 

 March and April, 1867. A plate showing the appearance of a sensitive flame when 

 viewed in a moving mirror, and its state of vibration thus revealed when responding 

 to notes of different pitch, is given in an article I published in the " Popular Science 

 Keview " for April, 1867. In that article I remark : " Hence a sensitive flame is the 

 analogue of a resonant column of air ; both are caused to vibrate by a given note, and 

 when the pitch of the note accords with the normal rate of vibration of the flame or 

 air, each responds with energy to that note, so that bringing the flame to the point at 

 which it is sensitive to a particular note is like adjusting the length of a column of 

 air till it responds to a certain tuning-fork." A sensitive jet, however, differs from a 

 resonant air-column inasmuch as it is in a state of unstable equilibrium when sensitive — 

 the air or gas or water then issuing at a certain critical velocity. 



