1831.] On a Peculiar Class of Acoustical Figures. 



of two neighbouring portions of air, that over the part of the 

 plate nearest to the centre of oscillation has had more pro- 

 jectile force communicated 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 unfavourable 

 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. 



25. It will be evident that this current is highly favourable 

 for the transference 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 currents ; 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 



