1831.] On a Peculiar Class of Acoustical Figures. 327 



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 possibility 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 lycopodium gathered at the centre of the mem- 

 brane with great facility and readiness, exhibiting the cloud, 

 the currents, and the involving heaps. Upon exhausting the 

 receiver, until the barometrical gauge was at twenty-eight 

 inches, the lycopodium, instead of collecting at the centre, 

 passed across the membrane, 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 vibrations 

 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 re- 

 peating 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; but no sensible accumulation of the 

 powder took place. At twenty-four inches of the barometer 

 the accumulation at the centre began to appear, and there was 

 a sensible, though very slight effect visible of the return of the 

 powder from the edges. At twenty-two inches these effects 

 were stronger ; and when the barometer was at twenty inches, 

 the currents of air within the receiver had force enough to 

 cause the collection of the principal part of the lycopodium at 

 the centre of vibration. Upon again, however, restoring the 

 exhaustion to twenty-eight inches, all the effects were repro- 

 duced as at first, and the lycopodium again proceeded to the 

 lower or the quiescent parts of the membrane. These alternate 

 effects were obtained several times in succession before the 

 apparatus was dismounted. 



