304 MR. FARADAY ON A PECULIAR CLASS OF ACOUSTICAL FIGURES. 
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 ; 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 collected 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 directed at pleasure by the card stops. 
16. A piece of gold-leaf being laid upon the plate, so that it Fig. 4. 
did not overlap the edge, fig. 4, the current 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 always 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. 
1 8. When a long glass plate is supported by bridges or strings at the two 
nodal lines represented in fig. 6, and made to vibrate, the lycopodium 
collects in three divisions ; that between the nodal lines 
does not proceed at once into a line equidistant from the 
nodal lines and parallel to them, but advances from the 
edges of the plate towards the middle by paths, which are a little curved and 
oblique to the edges where they occur near the nodal lines, but are almost 
Fig. 5. 
