1831.] On a Peculiar Class of Acoustical Figures. S25 



centre of vibration. When the vibrations were strong, these 

 assumed a revolving motion, rolling towards the centre at the 

 part in contact with the membrane, and from it at the part 

 nearest the glass ; thus illustrating in the clearest manner the 

 double currents caged up between the glass and the mem- 

 brane. The effect was well shown by carbonate of magnesia. 



30. Sometimes, when the plate was held down very close and 

 tight, and the vibrations were few and large, the powder was 

 all blown out at the edge ; for then the whole arrangement 

 acted as a bellows ; and as the entering air travelled with 

 much less velocity than the expelled air, and as the forces of 

 the currents are as the squares of the velocity, the issuing air 

 carried the powder more forcibly than the air which passed in, 

 and finally threw it out. 



31. A thin plate of mica laid loosely upon the vibrating 

 membrane showed the rotating concentric lines exceedingly 

 well. 



32. From these experiments on plates and surfaces vibrating 

 in air, it appears that the forms assumed by the determination 

 of light powders towards the places of most intense vibration, 

 depend, not upon any secondary mode of division, or upon any 

 immediate and peculiar action of the plate, but upon the 

 currents of air necessarily formed over its surface, in conse- 

 quence of the extra-mechanical action of one part beyond 

 another. In this point of view the nature of the medium in 

 which those currents were formed ought to have great influ- 

 ence over the phenomena ; for the only reason why silica as 

 sand should pass towards the quiescent lines, whilst the same 

 silica as fine powder went from them, is, that in its first form 

 the particles are thrown up so high by the vibrations as to be 

 above the currents, and that if they were not thus thrown out 

 of their reach they would be too heavy to be governed by 

 them ; whilst in the second form they are not thrown out of 

 the lower current, except near the principal place of oscillation, 

 and are so light as to be carried by it in whatever direction it 

 may proceed. 



33. In the exhausted receiver of the air-pump, therefore, the 

 phenomena ought not to occur as in air ; for as the force of 

 the currents would be there excessively weakened, the light 

 powders ought to assume the part of heavier grains in the air. 



