306 MR. FARADAY ON A PECULIAR CLASS OF ACOUSTICAL FIGURES. 
trated by membranes. A piece of parchment was stretched and tightly tied, 
whilst moist, over the aperture of a funnel five or six inches in diameter ; a 
small hole was made in the middle, and a horse-hair passed through it, but 
with a knot at the extremity that it might thereby be retained. Upon fixing 
the funnel in an upright position, and after applying a little powdered resin to 
the thumbs and fore-fingers, drawing them upward over the horse-hair, the 
membrane was thrown into vibration with more or less force at pleasure. By 
supporting the funnel on a ring, passing the horse-hair in the opposite direc- 
tion through the hole in the membrane, and drawing the fingers over it down- 
wards, the direction in which the force was applied could be varied accord- 
ing to circumstances. 
23. When lycopodium or light powders were sprinkled upon this surface, 
the rapidity with which they ran to the centre, the cloud formed there, the 
involving heaps, and many other circumstances, could be observed very ad- 
vantageously. 
24. The currents which I have considered as existing upon the surface of 
the plate, membranes, &c. from the quiescent parts towards the centres or 
lines of vibration (9), arise necessarily from the mechanical action of that 
surface upon the air. As any particular part of the surface moves upwards 
in the course of its vibration, it propels the air and communicates a certain 
degree of force to it, perpendicular or nearly so to the vibrating surface ; as it 
returns, in the course of its vibration, it recedes from the air so projected, and 
the latter consequently tends to return into the partial vacuum thus formed. 
But as of two neighbouring portions of air, that over the part of the plate 
nearest to the centre of oscillation has had more projectile force communi- 
cated to it than the other, because the part of the plate urging it was moving 
with greater velocity, and through a greater space, so it is in a more unfavour- 
able condition for its immediate return, and the other, i. e. the portion next to 
it towards the quiescent line, presses into its place. This effect is still further 
favoured, because the portion of air thus displaced is urged from similar causes 
at the same moment into the place left vacant by the air still nearer the centre 
of oscillation ; so that each time the plate recedes from the air, an advance of 
the air immediately above it is made from the quiescent towards the vibrating 
parts of the plates. 
