330 Lord Rayleigh : Acoustical Notes. 



But as the fork vibrated the aperture was in part uncovered, 

 and that twice during each complete vibration of the fork, 

 so that the pitch of the instrument, considered as a siren, 

 was 256. The fork was driven electrically from another 

 interrupter-fork o£ the same pitch situated outside. Fitted 

 to the aperture externally was a resonating tube whose length 

 could be adjusted to give the maximum effect. One of the 

 objects was to be able to vary the resonance without dis- 

 turbing the maintenance or the pitch of the siren. The 

 pressure employed was sometimes as low as 2'5 cm. of 

 mercury, and the consumption of wind about 3^- litres per 

 second, corresponding to *015 horse-power. At this rate of 

 working the pumps could be kept going by hand, or rather 

 by legs, for a moderate length of time. 



One unexpected effect presented itself, which seems worthy 

 of record. As has been mentioned, the intention had been 

 to keep the fork in motion electrically. But it was found 

 that, at any rate after being once started, it remained in 

 vigorous vibration under the action of the ivind alone, 

 although the electric connexion was cut off. It will be 

 observed that this case is altogether different from that of a 

 reed, where the tongue approaches and recedes from the 

 aperture normally. It is more analogous to the geolian harp, 

 where, as I have formerly shown *, the vibration is executed 

 in a plane perpendicular to the direction of the wind. So 

 far as I am aware, no adequate mechanical explanation of 

 this singular behaviour has been given. 



Stroboscopic Speed Regidation. 



The stroboscopic method f has often been employed for 

 testing and regulating the speed of revolving shafts. I used 

 it extensively in my determinations of absolute electrical 

 units J, referring the speed of revolving coils to the frequency 

 of vibration of an electro-magnetically maintained fork. 

 But I doubt whether even now the convenience of this method 

 for general purposes is appreciated. A few years ago the 

 late Mr. Gordon drew for me upon card rows of alternate 

 black and white " teeth " from 20 to 40. Photographs 

 from this upon flexible paper could be mounted upon a 

 revolving shaft so as to form reentrant circles of teeth for 



* Phil. Mag. vii. p. 149 879) ; Scientific Papers, i. p. 413. 

 t Plateau (1836) ; Topler, Phil. Mag. Jan. 1867. 

 % See for example Proc. Roy. Soc. xxxii. p. 104 (1881) ; Scientific 

 Papers, ii. p. 8. 



