MR. FARADAY ON A PECULIAR CLASS OF ACOUSTICAL FIGURES. 303 
11. M. Savart seems to consider that the reason why the powder gathers 
together at the centres of oscillation is, “ that the amplitude of the oscillations 
being very great, the middle of each of those centres (of vibration) is the only 
place where the plate remains nearly plane and horizontal, and where, conse- 
quently, the powder may reunite, whilst the surface being inclined to the right 
or left of this point, the parcels of powder cannot stop there.” But the inclina- 
tion thus purposely given to the plate, was very many times that which any part 
acquires by vibration in a horizontal position, and consequently proves that the 
horizontality of any part of the plate is not the cause of the powder collecting 
there, although it may be favourable to its remaining there when collected. 
12. Guided by the idea of what ought to happen, supposing the cause now 
assigned were the true one, the following amongst many other experiments 
were made. A piece of card about an inch long and a quarter of an inch wide 
was fixed by a little soft cement on the face of the plate near one edge, the 
plate held as before at the middle, lycopodium or fine silica strewed upon it, 
and the bow applied at the middle of another edge ; the powder immediately 
Fig. 1. 
Fig. 2. 
advanced close to the card, and the place of the cloud was much 
nearer to the edge than before. Fig. 1 represents the arrange- 
ment ; the diagonal lines being those which sand would have 
formed, the line at the top a representing the place of the card, &lj 
and the X to the right the place where the bow was applied. 
On applying a second piece of card as at b , the powder seemed 
indifferent to it or nearly so, and ultimately collected as in the first figure : 
c represents the place of the cloud when no card is present. 
13. Pieces of card were then fixed on the glass in the three 
angular forms represented in fig. 2 ; upon vibrating the plate 
the fine powder always went into the angle, notwithstanding its 
difference of position in the three experiments, but perfectly in 
accordance with the idea of currents intercepted more or less by 
the card. When two pieces of card were fixed on the plate as 
in fig. 3. a, the powder proceeded into the angle but not to the 
edge of the glass, remaining about ^th of an inch from it ; but b 
on closing up that opening, as at b, the powder went quite up 
into the corner. 
Fig. 3. 
2 R 
MDCCCXXXI. 
