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



Again, in denser media than air, as in water for instance, there 

 was every reason to expect that the heavier powders, as sand 

 and filings, would perform the part of light powders in air, and 

 be carried from the quiescent to the vibrating parts. 



34. The experiments in the air-pump receiver were made in 

 two ways. A round plate of glass was supported on four narrow 

 cork legs upon a table, and then a thin glass rod with a rounded 

 end held perpendicularly upon the middle of the glass. By pass- 

 ing the moistened fingers longitudinally along this rod the plate 

 was thrown into a vibratory state ; the cork legs were then ad- 

 justed in the circular nodal line occurring with this mode of vibra- 

 tion ; and when their places were thus found they were perma- 

 nently fixed. The plate was then transferred into the receiver of 

 an air-pump, and the glass rod by which it was to be thrown into 

 vibration passed through collars in the upper part of the re- 

 ceiver, the entrance of air there being prevented by abundance 

 of pomatum. When fine silica was sprinkled upon the plate, 

 and the plate vibrated by the wet fingers applied to the rod, 

 the receiver not being exhausted, the fine powder travelled 

 from the nodal line, part collecting at the centre, and another 

 part in a circle, between the nodal line and the edge. Both 

 these situations were places of vibration, and exhibited them- 

 selves as such by the agitation of the powder. Upon again 

 sprinkling fine silica uniformly over the plate, exhausting 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 following 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 rotary motion to the upper 



