18 Lord Rayleigh, On vibrations iqjon a suspended disc. [Nov. 8, 



The apparatus exhibited was made in Prof. Stuart's workshop. 

 An adjustment for directing the jet exactly up the middle of the 

 brass tube is found necessary, and some advantage is gained by 

 contracting the tube somewhat at the place of ignition. 



(2) On an effect of vibrations upon a suspended disc. By Lord 

 Rayleigh, M.A., F.R.S. 



In the British Association experiment for determining the unit 

 of electrical resistance, a magnet and mirror are enclosed in a 

 wooden box, attached to the lower end of a tube through which 

 the silk suspension fibre passes. Under these circumstances it is 

 found that the slightest tap with the finger-nail upon the box 

 deflects the mirror to an extraordinary degree. The disturbance 

 appears to be due to aerial vibrations within the box, acting upon 

 the mirror. We know that a flat body, like a mirror, tends to set 

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

 which it is immersed, and we may fairty 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 vibrations last, the mirror is subject to a twisting force tend- 

 ing to set it at right angles to the direction of the 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. 



The experiment shewn is intended to illustrate this effect. A 

 small disc of paper, about the size of a sixpence, is hung by a fine 

 silk fibre across the mouth of a resonator of pitch 128. When a 

 sound of this pitch is excited, there is a powerful rush of air in and 

 out of the resonator, and the disc sets itself promptly across the pas- 

 sage. 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 neighbourhood of the 

 vibrating prongs. 



(3) On an apparatus illustrating the movement of sound-ivaves 

 and water-waves. By Sedley Taylor, M.A., late Fellow of Trinity 

 College. 



This was an apparatus illustrating wave-motion. The arrange- 

 ment consists of sixteen toothed wheels of brass, centred along a 

 straight line upon a flat board, and connected by intermediate 

 pinions in such a manner that, when one of the end-wheels is set in 

 motion by means of a winch attached to it, all the others rotate with 

 the same velocity and in the same direction. Sixteen slender stems 



