588 MR. WHEATSTONE ON SOME EXPERIMENTS TO MEASURE THE VELOCITY 



while the wheel to which the motion was first commupicated was turned round once, 

 had there been no retardation to have been taken into consideration arising from the 

 slipping of the bands. M is a small Leyden jar, the inner coating of which is to be 

 constantly supplied, through the chain N, with electricity, either positive or negative, 

 from a machine ; the bent wire proceeding from the inner coating of the jar is in 

 immediate contact with the fixed discharger O P, and the spontaneous discharge of 

 the jar is to be regulated by varying the distance between the two balls. The wire 1 

 in connexion with the outer coating of the jar, and the wire 6 attached to the knob 

 of the brass frame, are continued to the similarly numbered wires of the spark-board. 

 When the jar is fully charged, and the arm Q, revolving with the axle, is brought 

 opposite the knob of the discharger, the discharge of electricity, or disturbance of 

 electric equilibrium, passes through the entire circuit, and the three sparks appear 

 perfectly simultaneous to the eye. When the face of the mirror is level with and 

 turned towards the spark-board, and is so adjusted as to form an angle of 45° with 

 the horizontal plane, the eye looking directly downwards sees the reflected images 

 of the three sparks. The plane glass or lens R is for the purpose of preventing the 

 eye approaching too near the mirror, and for accommodating the vision of long- or 

 short-sighted observers. The arm Q is so placed that the circuit may be completed 

 when the mirror is in the position just described ; the other arm serves merely as a 

 counterpoise. To obviate the inaccuracy which would result from discharges taking 

 place when the arm is in different positions with respect to the knob of the dis- 

 charger, a plate of mica, S, is interposed, having a very small horizontal slit exactly 

 opposite the axis of the discharger ; this fixes within narrow limits the occurrence 

 of the discharge, and, with whatever rapidity the mirror moves, the sparks are gene- 

 rally within the field of view. 



It was a point of essential importance to determine the angular velocity of the axle 

 carrying the mirror. No confidence could be placed in the result obtained by calcu- 

 lating the train of wheels, as in such rapid motion many retarding causes might 

 operate and render the calculation uncertain : it was necessary, therefore, to devise a 

 means independent of these sources of error, and which should immediately indicate 

 the ultimate velocity. Nothing appeared more likely to eflfect this purpose than to 

 attach a small syren to the instrument, the plate of which should be carried round 

 by the axle of the mirror. T is a small hollow box an inch in diameter, into which 

 wind was conveyed through a tube placed to the aperture u. On the face of this box 

 a number of equidistant apertures were arranged in a circle, and a disc moving 

 before it having the same number of apertures, periodically intercepted the issuing 

 current, and produced a sound corresponding to the frequency of the impulses. It 

 is obvious that the number of revolutions would be ascertained by dividing the num- 

 ber of vibrations in a second, corresponding to the sound, by the number of apertures. 

 I at first employed ten apertures : when the motion was slow, the sound could be 

 easily determined ; but on augmenting the velocity it became inappreciable. I then 



