324f On a Peculiar Class of Acoustical Figures. [1831. 



made to vibrate by the horse-hair proceeding downwards, and 

 a large glass tube, as a cylindrical lamp-glass, was brought 

 near to the centre of vibration, no evidence of a current en- 

 tirely through the lamp-glass could be perceived ; but still the 

 most striking proofs were obtained of the existence of carrying 

 currents by the effects upon the light powder, for it flew more 

 rapidly under the edge, and tended to collect towards the axis 

 of the tube ; it could even be diverted somewhat from its course 

 towards the centre of oscillation. A piece of upright paper, 

 held with its edge equally near, did not produce the same 

 effect ; but immediately that it was rolled into a tube, it did. 

 When the glass chimney was suspended very carefully, and at 

 but a small distance from the membrane, the powder often 

 collected at the edge, and revolved there ; a complicated action 

 between the currents and the space under the thickness of the 

 glass taking place, but still tending to show the influence of 

 the air in arranging and disposing the powders. 



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 magnificent 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 combining 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 mem- 

 brane, 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-globular moving 

 heaps, it formed linear arrangements, all concentric to the 



