MR. FARADAY ON A PECULIAR CLASS OF ACOUSTICAL FIGURES. 307 
25. It will be evident that this current is highly favourable for the trans- 
ference of light powders towards the centre of vibration. Whilst the air is 
forced forward, the advance of the plate against the particles holds them tight ; 
but when the plate recedes, and the current exists, the particles are at that 
moment left unsupported except by the air, and are free to move with it. 
26. The air which is thus thrown forward at and towards the centre of 
oscillation, must tend by the forces concerned to return towards the quiescent 
lines, forming a current in the opposite direction to the first, and blending more 
or less with it. I endeavoured, in various ways, to make the extent of this 
system of currents visible. In the experiment already referred to, where gold- 
leaf was placed over the centre of oscillation (16), the upward current at the 
most powerful part was able to raise the leaf about one tenth of an inch from 
the plate. The higher the sounds with the same plate or membrane, i. e. the 
greater the number of vibrations, the less extensive must be the series of cur- 
rents ; the slower the vibrations, or the more extensive the excursion of the 
parts from increased force applied, the greater the extent of disturbance. 
With glass plates (2. 12) the cloud is higher and larger as the vibrations are 
stronger, but still not so extensive as they are upon the stretched membrane 
(22), where the cloud may frequently be seen rising up in the middle and 
flowing over towards the sides. 
27- When the membrane stretched upon the funnel (22) was made to vibrate 
by the horse-hair proceeding downwards, and a large glass tube, as a cylindri- 
cal lamp-glass, was brought near to the centre of vibration, no evidence of a 
current entirely through the lamp-glass could be perceived ; but still the most 
striking proofs were obtained of the existence of carrying currents by the effects 
upon the light powder, for it flew more rapidly under the edge, and tended to 
collect towards the axis of the tube ; it could even be diverted somewhat from 
its course towards the centre of oscillation. A piece of upright paper, held with 
its edge equally near, did not produce the same effect; but immediately that it 
was rolled into a tube, it did. When the glass chimney was suspended very 
carefully, and at but a small distance from the membrane, the powder often 
collected at the edge, and revolved there ; a complicated action between the 
currents and the space under the thickness of the glass taking place, but still 
tending to show the influence of the air in arranging and disposing the powders. 
