THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOUKNAL OF SCIENCE. 



[FIFTH SERIES.] 



MARCH 1879. 



XXVII. Acoustical Observations. II. 

 By Lord Rayleigh, F.R.S.* 



Pure Tones from Sounding Flames. 



THE best approximation to a pure tone is doubtless that 

 given by a fork held over a suitably tuned air reso- 

 nator ; but unless the vibrations are maintained, the sound is 

 of but short duration, and varies in intensity throughout. On 

 the other hand the introduction of an electro-magnetic main- 

 tenance (as in Helmholtz's vowel experiments) somewhat 

 complicates the apparatus. For many purposes extreme 

 purity and constancy of pitch are not important ; and thus an 

 arrangement which shall be simple and easy to manage, even 

 though less perfect in its operation than a tuning fork, is still 

 a desideratum. 



During the last year I have often used with good effect air 

 resonators whose vibrations were maintained in a well-known 

 manner by hydrogen-flames. In the common form of the ex- 

 periment an open cylindrical tube is employed as resonator, 

 and gives a sound, usually of a highly compound character. 

 In order to obtain a pure tone, it is only necessary to replace 

 the tube by a resonator of different form, such as a rather wide- 

 mouthed bottle or jar ; but a difficulty then arises from the 

 progressive deterioration of the limited quantity of air in- 

 cluded. A better result is obtained from a tube with a cen- 

 tral expansion, such as a bulbous paraffin-lamp chimney, 

 which allows of a through draught, and yet departs sufficiently 



* Communicated by the Author. 

 PMl. Mag. S. 5. Yol. 7. No. 42. March 1879. N 



