MR. FARADAY ON A PECULIAR CLASS OF ACOUSTICAL FIGURES. 311 
but no sensible accumulation of the powder took place. At twenty-four inches 
of the barometer the accumulation at the centre began to appear, and there 
was a sensible, though very slight effect visible of the return of the powder 
from the edges. At twenty-two inches these effects were stronger ; and when 
the barometer was at twenty inches, the currents of air within the receiver 
had force enough to cause the collection of the principal part of the lycopo- 
dium at the centre of vibration. Upon again, however, restoring the exhaustion 
to twenty-eight inches, all the effects were reproduced as at first, and the lyco- 
podium again proceeded to the lower or the quiescent parts of the membrane. 
These alternate effects were obtained several times in succession before the 
apparatus was dismounted. 
37. In this form of experiment there were striking proofs of the existence of 
a current upwards from the middle of the membrane when vibrating in air, 
(24), and the extent of the system of currents (26) was partly indicated. The 
powder purposely collected at the middle by vibrations, when the receiver was 
full of air, was observed as to the height to which it was forced upwards by 
the vibrations ; and then the receiver being exhausted, the height to which the 
powder was thrown by similar vibrations was again observed. In the latter 
cases it was nothing like so great as in the former, the height not being two- 
thirds, and barely one-half, the first height. Had the powder been thrown up 
by mere propulsion, it should have risen far higher in vacuo than in air : but 
the reverse took place ; and the cause appears to be, that in air the current 
had force enough to carry the fine particles up to a height far beyond what the 
mere blow which they received from the vibrating membrane could effect. 
38. For the experiments in a denser medium than air, water was chosen. A 
circular plate of glass was supported upon four feet in a horizontal position, sur- 
rounded by two or three inches of water, and thrown into vibration by applying 
a glass rod perpendicular to the middle, as in the first experiment in vacuo 
(34) ; the feet were shifted until the arrangement gave a clear 3ound, and 
the moistened brass filings sprinkled upon the plate formed regular lines or 
figures. These lines were not however lines of rest, as they would have been 
in the air, but were the places of greatest vibration ; as was abundantly evi- 
dent from their being distant from that nodal line determined and indicated 
by the contact of the feet, and also from the violent agitation of the filings. 
2 s 
MDCCCXXXI. 
