1888.] on Diffraction of Sound. 191 



corresponding to the odd multiples, and remains quiescent at the 

 nodes corresponding to the even multiples. This result is especially 

 remarkable, because the ear, when substituted for the flame, behaves 

 in the exactly opposite manner, being excited at the nodes and 

 not at the loops. The experiment may bo tried with the aid of a 

 tube, one end of which is placed in the ear, while the other is 

 held close to the burner. It is then found the ear is excited the 

 most when the flame is excited least, and vice versa. The result of 

 the experiment shows, moreover, that the manner in which the flame 

 is disintegrated under the action of sound is not, as might be expected, 

 symmetrical in regard to the axis of the flame. If it were symmetrical, 

 it would be most affected by the symmetrical cause, namely, the varia- 

 tion of pressure. The fact being that it is most excited at the loop, 

 where there is the greatest vibratory velocity, shows that the method 

 of disintegration is unsymmetrical, the velocity being a directed quan- 

 tity. In that respect the theory of these flames is different from the 

 theory of the water-jets investigated by Savart, which resolve them- 

 selves into detached drops under the influence of sonorous vibration. 

 The analogy fails at this point, and it has been pressed too far by 

 some experimenters on the subject. Another simple proof of the 

 correctness of the result of our experiment is that it makes all the 

 difference which way the burner is turned in respect of the direction 

 in v^hich the sound-waves are impinging upon it. If the phenomenon 

 were symmetrical, it would make no difference if the flame were 

 turred round upon its vertical axis. But we find that it does make 

 a difference. This is the way in which I was using the flame, and 

 you see that it is flaring strongly. If I now turn the burner round 

 through a right angle, the flame stops flaring. I have done nothing 

 more than turn the burner round and the flame with it, showing that 

 the sound-waves may impinge in one direction with great effect, and 

 in another direction with no effect. The sensitiveness occurs again 

 wh3n the burner is turned through another right angle ; after three 

 right angles there is another place of no effect ; and after a complete 

 revolution of the flame the original sensitiveness recurs. So that if 

 tlie flame were stationary, and the sound-waves came, say, firom the 

 n^rth or south, the phenomena would be exhibited ; but if they came 

 fiom the east or west, the flame would make no response. 



This is of convenience in experimenting, because, by turning the 

 birner round, I make the flame almost insensitive to a sound, and I 

 am now free to show the effect of any sound that may be brought to 

 it in the perpendicular direction. I am going to use a very small 

 reflector — a small piece of looking-glass. Wood would do as well ; 

 but looking-glass facilitates the adjustment, because my assistant, by 

 se3ing the reflection, will be able to tell me when I am holding it in 

 the best position. Now, the sound is being reflected from the bit of 

 glass, and is causing the flame to flare, though the same sound, travel- 

 ling a shorter distance and impinging in another direction, is incom- 

 petent to produce the result (Fig. 1). 



