346 Lord Rayleigh's Acoustical Observations. 



disclosed itself in the following manner. I was operating on 

 two fishtail flames, one of which jumped to a whistle while 

 the other did not. The gas of the non-sensitive flame was 

 turned off, additional pressure being thereby thrown upon the 

 other flame. It flared, and its cock was turned so as to lower 

 the flame ; but it now proved non-sensitive, however close it 

 might be brought to the point of flaring. The narrow orifice 

 of the half-turned cock interfered with the action of the sound. 

 When the gas was fully turned on, the flame being lowered by- 

 opening the cock of the other burner, it became again sensi- 

 tive. Up to this time a great number of burners had been 

 tried, but with many of them the action was nil. Acting, 

 however, upon the hint conveyed by this observation, the 

 cocks which fed the flames were more widely opened, and our 

 most refractory burners thus rendered sensitive." In the 

 abstract of a Royal-Institution lecture (Phil. Mag. Feb. 1867) 

 a rather more definite view is expressed : — " Those who wish 

 to repeat these experiments would do well to bear in mind, as 

 an essential condition of complete success, that a free way 

 should be open for the transmission of the vibrations from the 

 flame, backwards, through the gas-pipe which feeds it. The 

 orifices of the stopcocks near the flame ought to be as wide as 

 possible." 



During the preparation of some lectures on Sound in the 

 spring of last year, it occurred to me that light would probably 

 be thrown upon these interesting effects by introducing a 

 manometer on a lateral branch near the flame. In the path of 

 the gas there were inserted two stopcocks, one only a little 

 way behind the manometer-junction, the other separated from 

 it by a long length of india-rubber tubing. When the first cock 

 was fully open, and the flame was brought near the flaring- 

 point by adjustment of the distant cock, the sensitiveness to 

 external sounds was great, and the manometer indicated a pres- 

 sure of ten inches of water. But when the distant cock stood 

 fully open and the adjustment was effected at the other, high 

 sensitiveness could not be attained; and the reason was obvious, 

 because the flame flared without external excitation while the 

 pressure was still an inch short of that which had been borne 

 without, flinching in the former arrangement. On opening 

 again the neighbouring cock to its full extent, and adjusting 

 the distant one until the pressure at the manometer measured 

 nine inches, the flame was found comparatively insensitive. 



It appears, therefore, that the cause of the prejudicial action 

 of partially opened stopcocks in the neighbourhood of the 

 flame is not so much that they render the flame insensitive as 

 that they induce premature flaring. There are two ways in 



