50 



the vibration, and which therefore may admitof the settlement there 

 of light powders, while heavier powders can be stationary only at 

 the points of absolute rest. 



This explanation not appearing to the author to be satisfactory, 

 he made a great number of experiments, which are detailed at length 

 in the present paper, showing that the immediate cause of these 

 motions exists in the surrounding medium, and is to be found in 

 the currents arising from the mechanical action of the plate, while 

 vibrating upon that portion of the medium which is in contact with 

 the plate. These currents are directed from the quiescent lines 

 towards those parts where the oscillation is the greatest, and meet- 

 ing from opposite sides at these central points, thence proceed per- 

 pendicularly from the vibrating surface to a certain distance ; and 

 finally, receding from each other, return again in a direction to- 

 wards the nodal lines. The combination of these motions consti- 

 tutes vortices carrying with them any light particles which may lie 

 in the way of the currents. While in motion, the powders sustained 

 by these vortices appear in the form of clouds, the particles of which 

 have among themselves an intestine motion of revolution, rising in 

 the centre of the heap, and rolling down again on the outer sides. 

 The powders are collected in the same situations on the vibrating 

 plate, although the plate may be considerably inclined to the hori- 

 zon, and remain there even when the inclination is so great as to 

 prevent grains of sand from resting on the nodal lines. A piece of 

 gold leaf laid upon the plate was raised up in the form of a blister 

 at that part which corresponded with the centre of the clouds, even 

 to the height of one-twelfth of an inch. 



On attaching small pieces of card to different parts of the surface 

 of the vibrating plate, the currents of air are modified in various 

 ways, as shown by the different positions of the clouds, and the pro- 

 duction of partial accumulations of the powders. When a tuning- 

 fork is made to vibrate, and a little powder of lycopodium is 

 sprinkled over it ? the powder collects into clouds on the middle of 

 the upper surface, and also forms heaps along its sides, exhibiting 

 in a striking manner the intestine revolution of their particles. These 

 effects are also well illustrated by vibrating membranes ; for which 

 purpose a piece of parchment was stretched, and tied while moist 

 over the mouth of a funnel, and made to vibrate by means of a horse- 

 hair, having a knot at the end, passed through a hole in the centre 

 of the parchment ; the hair being drawn between the finger and 

 thumb, to which a little powdered rosin was previously applied. 

 The phenomena were still more conspicuous when the parchment 

 was made to vibrate under a glass plate held near it. When the 

 interval between the membrane and the glass plate was very small, 

 the whole of the powder was sometimes blown out at the edge, in 

 consequence of the vibrating membrane acting as a bellows. 



Reasoning from the theory which the author had framed in ex- 

 planation of these phenomena, he conceived that if the currents 

 were weakened bj placing the apparatus in rarefied air, they would 

 no longer be capable of sustaining the light powders, which would 



