Lord Rayleigh's Acoustical Observations. 341 



paper slider was adjusted to the position of maximum effect. 

 This observation was repeated many times, the distance be- 

 tween marks fixed on the pipe and on the slider respectively 

 being recorded. The following numbers give the results, ex- 

 pressed in fiftieths of an inch: — 



31 



33 



30 



25 



31 



2b 



32 



31 



34 



29 



35 



28 



29 



30 





The extreme range being only one fifth of an inch, shows that 

 the observation is capable of considerable precision, corre- 

 sponding as it does to only about 2 vibrations per second out 

 of a total of 255. Finally, the slider was fixed at the mean of 

 the above-determined positions, and the natural note of the 

 pipe was then considered to be 255. The error in length was 

 probably less than ^ inch, and the error in pitch less than 

 half a vibration per second. 



The pipe was then blown from a well-regulated bellows : and 

 the beats were counted between its note and that of the standard 

 fork above referred to, the pressure being taken simultaneously 

 with a water-manometer. Three observers were found to be 

 ne jessary for accurate working — one to count the beats, rising 

 to the rate of ten per second, one to keep the bellows uniformly 

 supplied with wind, and one to observe the manometer. At 

 pressures between 4'2 inches and T53 inch the pitch of the 

 pipe was very well defined and considerably higher than the 

 natural note. Below 1 inch the pitch became somewhat un- 

 steady, and distinct fluctuations in the frequency of the beats 

 were perceived, while no corresponding variation of pressure 

 could be detected. At about *8 inch the pitch of the pipe 

 falls to unison with the natural note, and with further dimi- 

 nishing pressures becomes the graver of the two. Below *7 

 inch the unsteadiness is such as to preclude accurate estima- 

 tions of pitch. 



The results are embodied in the accompanying table, which 

 shows the correspondence of pitch and pressure. Instead of 

 the actual number of beats counted, which involves a reference 

 to the extraneous element of the pitch of the standard fork, 

 the number (greater by unity) is given which expresses the 

 excess in the frequency of vibration of the actual over that of 

 the natural note of the pipe. It will be seen that at practical 

 pressures the pitch is raised by the action of the wind, but that 

 this rule is not universal. 



Phil. Mag. S. 5. Vol. 13. No. 82. May 1882. 2D 



