Lord Rayleigh's Acoustical Observations. 345 



servation. In the spring of last year the apparent caprice was 

 traced to the ear, which very rapidly becomes deaf to sounds 

 of high pitch and moderate intensity. A bird-call was mounted 

 in connexion with a loaded gas-bag and a water-manometer, 

 by which means the pressure could be maintained constant for 

 a considerable time. When the ear is placed at a moderate 

 distance from the instrument, a disagreeable sound is heard at 

 first, but after a short interval, usually not exceeding three or 

 four seconds, fades away and disappears altogether. A very 

 short intermission suffices for at any rate a partial recovery 

 of the power of hearing. A pretty rapid passage of the hand, 

 screening the ear for a fraction of a second, allows the sound 

 to be heard again. During his visit to Cambridge in March 

 1881, I had the pleasure of showing this experiment to Prof. 

 Helmholtz. 



The uniformity of the sound in the physical sense may be 

 demonstrated with a sensitive flame, which remains uniformly 

 affected so long as the pressure indicated by the manometer 

 does not vary. The sensitive flame may also be employed to 

 determine the wave-length of the sound, in the manner de- 

 scribed in the Philosophical Magazine for March 1879, p. 154. 

 In the case of two bird-calls blown with a pressure of about 

 2y of water, the wave-lengths were found to be respectively 

 l // *304 and l //# 28. The method was found to work easily and 

 with considerable accuracy, almost identical results being ob- 

 tained from observations of the loops, where the flame is most 

 affected, and from the nodes, where it is least affected. 



By modifying the pressures with pinch-cocks, the two notes 

 could be brought into unison. Although both bird-calls were 

 blown from the same gas-bag, it was not possible to keep the 

 beats slow for more than a few seconds at a time ; but that 

 period was quite sufficient for the effects of the beats to mani- 

 fest themselves in a striking manner by the behaviour of the 

 flame. In repeating these experiments, it may be necessary 

 to bear in mind that many people cannot hear these high notes 

 at all, even at first. With a shorter wave-length of about ^", 

 as determined by the flame, I was myself quite unable to hear 

 any sound from the situation of the flame. A slight hissing 

 was perceived when the ear was brought up close to the source; 

 but it is probable that this was not the part of the sound that 

 agitated the flame. 



Sensitive Flames. 

 In the chapter devoted to this subject in TyndalPs ' Sound ' 

 (third edition, p. 231) the accomplished author remarks : — 

 *! An essential condition to entire success in these experiments 



