314 MR. FARADAY ON A PECULIAR CLASS OF ACOUSTICAL FIGURES. 
being- deeper at the accumulations than elsewhere ; and they were also rendered 
beautifully evident by making- the experiment in sunshine, or by putting a 
candle beneath the plate, and placing a screen on the opposite side to receive 
the images formed at the focal distance. 
46. When the vibration of the plate ceased, the oil gradually flowed back 
until of uniform depth. On renewing the vibration, the accumulations were 
re-formed, the phenomena of accumulation occurring with as much certainty 
and beauty as if lycopodium powder had been used. 
4~. To remove every doubt of the fluid passing from the quiescent to the 
agitated parts, centres of vibration were used, nearly surrounded by nodal lines. 
A square plate, fig. 1 1, being held at c, and the bow applied at X , 
gave with sand, nodal lines, resembling those in the figure. Then 
clearing off the sand, putting oil in its place, and producing the 
same mode of vibration as before, the oil accumulated at a and 
b, forming two heaps or lenses as in the former experiment (45). 
48. The experiment made with water on the under surface (44) was now 
repeated with oil, the round plate being used (45). The hanging drop of oil 
rose up as the water did before, but the lateral diffusion was soon limited ; for 
lenses were formed at the centres of vibration just as when the oil was upon 
the upper surface, and, as far as could be ascertained by general examination, 
of the same form and power. On stopping the vibration, the oil gathered again 
into hanging drops; and on renewing it, it was again disposed in the lens-like 
accumulations. 
49. With white of egg the same observable accumulation at the centres of 
vibration could be produced. . 
50. Hence it is evident that when a surface vibrating normally, is covered 
with a layer of liquid, that liquid is determined from the quiescent to the 
vibrating parts, producing accumulation at the latter places ; and that this 
accumulation is limited, so that if purposely rendered too great by gravity or 
other means, it will quickly be diminished by the vibrations until the depth 
of fluid at any one part has a certain and constant relation to the velocity there 
and to the depth elsewhere. 
51. From the accumulated evidence which these experiments afford, I think 
there can remain no doubt of the cause of the collection of fine powders at the 
Fig. 11 . 
