MR. FARADAY ON A PECULIAR CLASS OF ACOUSTICAL FIGURES. 305 
perpendicular to it elsewhere, and the powder gradually forms a line along 
the middle of the plate ; it is only by continuing the experiment for some 
time that it gathers up into a heap or cloud equidistant from the nodal lines. 
But upon fixing card walls upon this plate, as in fig. 7, the Fi 7 
course of the powder within the cards was directly parallel 
to them and to the edge, instead of being perpendicular, 
and also directly towards the centre of oscillation. To prove that it was not as 
a weight that the card acted, but as an obstacle to the currents of air formed, it 
was not moved from its place, but bent flat down outwards, and then the fine 
powder resumed the courses it took upon the plate when without the cards. 
Upon raising the cards the first effect was reproduced. 
19. The lycopodium sprinkled over the extremities of such a plate proceeds 
towards places equidistant from the sides and near the ends, as at a fig. 8 ; but 
on cementing a piece of paper to the edge, so as to 
form a wall about one quarter or one third of an inch 
high, b, the powder immediately moved up to it, and 
retained this new place. In a longer narrow plate, similarly arranged, the 
powder could be made to pass to either edge, or to the middle, according as 
paper interceptors to the currents of air were applied. 
20. Plates of tin, four or five inches long, and from an inch to two inches 
wide, fixed firmly at one end in a horizontal position, and vibrated by apply- 
ing the fingers, show the progress of the air and the light powders well. The 
vibrations are of comparatively enormous extent, and the appearances are con- 
sequently more instructive. 
21. If a tuning-fork be vibrated, then held horizontally with the broad sur- 
face of one leg uppermost, and a little lycopodium be sprinkled upon it, the 
collection of the powder in a cloud along the middle, and the formation of the 
involving heaps also in a line along the middle of the vibrating steel bar, may 
be beautifully observed. But if a piece of paper be attached by wax to the 
side of the limb, so as to form a fence projecting above it, as in the former ex- 
periments (19), then the powder will take up its place close to the paper ; and 
if pieces of paper be attached on different parts of the same leg, the powder 
will go to the different sides, in the different parts, at the same time. 
22. The effects under consideration are exceedingly well shown and illus- 
2 r 2 
Fig. 8. 
Fig- 7. 
