310 MR. FARADAY ON A PECULIAR CLASS OF ACOUSTICAL FIGURES. 
hausting the receiver to twenty-eight inches, and vibrating the plate, the silica 
went from the middle towards the nodal line or place of rest, performing exactly 
the part of sand in air. It did not move at the edges of the plate, and as the 
apparatus was inconvenient and broke during the experiment, the follow- 
ing arrangement was adopted in its place. 
35. The mouth of a funnel was covered (22) with a well-stretched piece of 
fine parchment, and then fixed on a stand with the membrane horizontal ; the 
horse-hair was passed loosely through a hole in a cork, fixed in a metallic tube 
on the top of the air-pump receiver ; the tube above the cork was filled to the 
depth of half an inch with pomatum, and another perforated cork put over 
that ; a cup was formed on the top of the second cork, which was filled with 
water. In this way the horse-hair passed first through pomatum and then 
water, and by giving a little pressure and rotatory motion to the upper cork 
during the time that the horse-hair was used to throw the membrane into 
vibration, it was easy to keep the pomatum below perfectly in contact with the 
hair, and even to make it exude upwards into the water above. Thus no pos- 
sibility of the entrance of air by and along the horse-hair could exist, and the 
tightness of all the other and fixed parts of the apparatus was ascertained by the 
ordinary mode of examination. A little paper shelf was placed in the receiver 
under the cork to catch any portion of pomatum that might be forced through 
by the pressure, and prevent its falling on to the membrane. 
36. This arrangement succeeded : when the receiver was full of air, the lyco- 
podium gathered at the centre of the membrane with great facility and readi- 
ness, exhibiting the cloud, the currents, and the involving heaps. Upon ex- 
hausting the receiver until the barometrical gauge was at twenty-eight inches, 
the lycopodium, instead of collecting at the centre, passed across the mem- 
brane towards one side which was a little lower than the other. It passed by 
the middle just as it did over any other part ; and when the force of the vibra- 
tions was much increased, although the powder was more agitated at the 
middle than elsewhere, it did not collect there, but went towards the edges or 
quiescent parts. Upon allowing air to enter until the barometer stood at 
twenty-six inches, and repeating the experiments, the effect was nearly the same. 
When the vibrations were very strong, there were faint appearances of a cloud, 
consisting of the very finest particles, collecting at the centre of vibration ; 
