1831.] On a Peculiar Class of Acoustical Figures. 319 



of each of those centres (of vibration) is the only place where 

 the plate remains nearly plane and horizontal, and where, con- 

 sequently, 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 ; 



Fig. 1. 



the powder immediately advanced close to the 

 card, and the place of the cloud was much 

 nearer to the edge than before. Fig. 1 repre- 

 sents 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 x to the right place where the bow was 

 applied. On applying a second piece of card, 

 as at 6, 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, notwith- 

 standing 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 on closing up that opening, as at b, the powder 

 went quite up into the corner. 



Fig. 2. 



