308 MR. FARADAY ON A PECULIAR CLASS OF ACOUSTICAL FIGURES. 
28. A sheet of drawing-paper was stretched tightly over a frame so as to 
form a tense elastic surface nearly three feet by two feet in extent. Upon 
placing this in a horizontal position, throwing a spoonful of lycopodium upon 
it, and striking it smartly below with the fingers, the phenomena of collection 
at the centre of vibration, and of moving heaps, could be obtained upon a mag- 
nificent scale. When the lycopodium was uniformly spread over the surface, 
and any part of the paper slightly tapped by the hand, the lycopodium at any 
place chosen could be drawn together merely by holding the lamp-glass over 
it. It will be unnecessary to enter into the detail of the various actions com- 
bining to produce these effects ; it is sufficiently evident, from the mode in which 
they may be varied, that they depend upon currents of air. 
29. A very interesting set of effects occurred when the stretched parchment 
upon the funnel (22) was vibrated under plates ; the horse-hair was directed 
downwards, and the membrane, after being sprinkled over with light powder, 
was covered by a plate of glass resting upon the edge of the funnel ; upon 
throwing the membrane into a vibratory state, the powder collected with much 
greater rapidity than without the plate ; and instead of forming the semi-glo- 
bular moving heaps, it formed linear arrangements, all concentric to the centre 
of vibration. When the vibrations were strong, these assumed a revolving 
motion, rolling towards the centre at the part in contact with the membrane, 
and from it at the part nearest the glass ; thus illustrating in the clearest man- 
ner the double currents caged up between the glass and the membrane. The 
effect was well shown by carbonate of magnesia. 
30. Sometimes when the plate was held down very close and tight, and the 
vibrations were few and large, the powder was all blown out at the edge ; for 
then the whole arrangement acted as a bellows ; and as the entering air tra- 
velled with much less velocity than the expelled air, and as the forces of the 
currents are as the squares of the velocity, the issuing air carried the powder 
more forcibly than the air which passed in, and finally threw it out. 
3 1 . A thin plate of mica laid loosely upon the vibrating membrane showed 
the rotating concentric lines exceedingly well. 
32. From these experiments on plates and surfaces vibrating in air, it 
appears that the forms assumed by the determination of light powders towards 
the places of most intense vibration, depend, not upon any secondary mode of 
