320 On a Peculiar Class of Acoustical Figures. [1831. 



14. Upon fixing two pieces of card on the plate as at 

 c, fig. 3, the powder between them collected in the middle 

 very nearly as if no card had been present ; Fi g- 3. 



but that on the outside of the cards gathered 

 close up against them, being able to proceed 

 so far in its way to the middle, but no further. 



15. In all these experiments the sound was 

 very little lowered, the form of the cross 

 was not changed, and the light powders col- 

 lected on the other three portions of the plate, exactly as if no 

 card walls had been applied on the fourth ; so that no reason 

 appears for supposing that the mode in which the plate 

 vibrated was altered, but the powders seem to have been 

 carried forward by currents which could be opposed or di- 

 rected at pleasure by the card stops. 



16. A piece of gold-leaf being laid upon the plate, so that it 

 did not overlap the edge, fig. 4, the current Fig 4 



of air towards the centre of vibration was 



beautifully shown ; for, by its force, the air 



crept in under the gold-leaf on all sides, and 



raised it up into the form of a blister ; that 



part of the gold-leaf corresponding to the 



centre of the locality of the cloud, when light 



powder was used, being frequently a sixteenth or twelfth of 



an inch from the glass. Lycopodium or other fine powder, 



sprinkled round the edge of the gold-leaf, was carried in by 



the entering air, and accumulated underneath. 



17. When silica was placed on the edge of 

 another glass plate, or upon a book, or block 

 of wood, and the edge of the vibrating plate 

 brought as nearly as possible to the edge of 

 the former, fig. 5, part of the silica was al- 

 ways driven on to the vibrating plate, and 

 collected in the usual place ; as if in the 

 midst of all the agitation of the air in the 

 neighbourhood of the two edges, there was 

 still a current towards the centre of vibration, even from bodies 

 not themselves vibrating. 



18. When a long glass plate^is supported by bridges or 

 strings at the two nodal lines represented in fig. 6, and made 



Fig 5. 



