110 Lord Rayleigh and Dr. A. Schuster. [May 5, 



and mirror when the coil was spinning with circuit open. At low 

 speeds the result was fairly satisfactory, but at six or more revolutions 

 per second a violent disturbance set in. This could not be attributed 

 to the direct action of wind, as the case surrounding the suspended 

 parts was nearly air-tight, except at the top. It was noticed by 

 Mr. Darwin that even at low speeds a disturbance was caused at every 

 stroke of the bell. This observation pointed to mechanical tremor, 

 communicated through the frame, as the cause of the difficulty, and 

 the next step was to support the case surrounding the suspended parts 

 independently. A rough trial indicated some improvement, but at this 

 point the experiments had to be laid aside for a time. 



From the fact that the disturbance in question was produced by 

 the slightest touch (as by a tap of the finger nail), upon the box, 

 while the upper parts of the tube could be shaken with impunity, it 

 appeared that it must depend upon a reaction between the air in- 

 cluded in the box and the mirror. It is known that a flat body tends 

 to set itself across the direction of any steady current of the fluid in 

 which it is immersed, and we may fairly suppose that an effect of the 

 same character will follow from an alternating current. At the 

 moment of the tap upon the box the air inside is made to move past the 

 mirror, and probably executes several vibrations. While these vibra- 

 tions last, the mirror is subject to a twisting force tending to set it at 

 right angles to the direction of vibration. The whole action being 

 over in a time very small compared with that of the free vibrations of 

 the magnet and mirror, the observed effect is as if an impulse had 

 been given to the suspended parts. 



In order to illustrate this effect I contrived the following experi- 

 ment.* A small disk of paper, about the size of a sixpence, was hung 

 by a fine silk fibre across the mouth of a resonator of pitch 128. 

 When a sound of this pitch is excited in the neighbourhood, there is 

 a powerful rush of air in and out of the resonator, and the disk sets 

 it§elf promptly across the passage. A fork of pitch 128 may be held 

 near the resonator, but it is better to use a second resonator at a little 

 distance in order to avoid any possible disturbance due to the neigh- 

 bourhood of the vibrating prongs. The experiment, though rather 

 less striking, was also successful with forks and resonators of 

 pitch 256. 



It will be convenient here to describe the method adopted for regu- 

 lating and determining the speed of rotation, which has proved 

 thoroughly satisfactory. In the experiments of the Committee a 

 governor was employed, and the speed was determined by means of 

 the bell already referred to. This bell received a stroke every 100 

 revolutions, and the times were taken with a chronometer. In this 



* " Proc. Camb. Phil. Soc," Nov. 8, 1880. 



