of each of those centres (of vibration) is the only place where the plate remains nearly plane and horizontal, and where, consequently, 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 inclination 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;
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the powder immediately advanced close to the card, and the place of the cloud was much a nearer to the edge than before. Fig. 1 represents the arrangement; the diagonal lines being those which sand would have formed, the line at the top a representing the place of the card, and the × to the right 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.
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13. Pieces of card were then fixed on the glass in the three angular forms represented in fig. 2; upon vibrating the plate, the line 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 ⅛th of an inch from it; but on closing up that opening, as at b, the powder went quite up into the corner.
