324 Lord Rayleigh : Acoustical Notes. 



from the same source. When the reaction suffices, only one 

 note is sounded, and that is usually higher in pitch than the 

 notes of either pipe separately. It is proposed to record the 

 results of some observations of a similar character recently 

 made upon so-called singing flames, i. e. tubes caused to speak 

 by means of hydrogen flames. 



The tubes were of glass from the same length,- each 30 cm. 

 long and 16 mm. internal diameter. The hydrogen bottles 

 were also similar and were provided with burners formed 

 from 11 cm. lengths of glass drawn down at the upper ends. 

 Very small flames suffice. The tubes were held vertically and 

 so that their upper (and lower) ends were at the same level. 



When the distance between the tubes is considerable, say 

 30 cm., and draughts are avoided, fairly slow beats may be 

 obtained by suitable tuning, as by approach of the finger to 

 one end of that tube which vibrates the quicker. But when 

 the distance is reduced to perhaps 8 or 10 cm., a difficulty 

 begins to be experienced in producing slow beats. Either 

 they are rather quick or else, when the tuning does not allow 

 of that, they disappear altogether, the vibrations as it were 

 engaging. On the margin where beats still occur, their 

 character is peculiar. They appear unsym metrical, the swell 

 being protracted and the fall hurried. The phenomenon is 

 the same as that observed optically in the case of forks 

 (p. 323). When the tubes are as close as possible — they may 

 conveniently be tied together with string, — even moderately 

 slow beats are excluded. In this situation the sound is much 



Fisr. 3. 



attenuated, indicating that the phases of vibration are opposite,, 

 at any rate in the ideal case. The ideal case is, however, 

 rather difficult to attain. There should be complete cessation 

 of the principal tone in a resonator (C) whose mouth is held 



