316 
MR. FARADAY ON THE MOVING GROUPS OF PARTICLES 
paper (28), are on so large a scale as to be very proper for critical examina- 
tion. The phenomena can be exhibited also even by dry sand on such a mem- 
brane, the sand being in large quantity and the vibrations slow. When the 
surface is thickly covered by sand from a sieve, and the paper tapped with the 
finger, the manner in which the sand draws up into moving heaps is very 
beautiful. 
55. When a single heap is examined, which is conveniently done by holding 
a vibrating tuning-fork in a horizontal position, and dropping some lycopodium 
upon it, it will be seen that the particles of the heap rise up at the centre, over- 
flow, fall down upon all sides, and disappear at the bottom, apparently pro- 
ceeding inwards ; and this evolving and involving motion continues until the 
vibrations have become very weak. 
56. That the medium in which the experiment is made has an important 
influence, is shown by the circumstance of heavy particles, such as filings, ex- 
hibiting all these peculiarities when they are placed upon surfaces vibrating in 
water (39) ; the heaps being even higher at the centre than a heap of equal 
diameter formed of light powder in the air. In water, too, they are formed 
indifferently upon any part of the plate or membrane which is in a vibratory 
state. They do not tend to the quiescent lines ; but that is merely from the 
great force of the currents formed in water as already described (38), and the 
power with which they urge obstacles to the place of greatest vibration. 
5 7- If a glass plate be supported and vibrated (6), its surface having been 
covered with sand enough to hide the plate, and water enough to moisten and 
flow over the sand, the sand will draw together in heaps, and these will ex- 
hibit the peculiar and characteristic motion of the particles in a very striking 
manner. 
58. The aggregation and motion of these heaps, either in air or other fluids, 
is a very simple consequence of the mechanical impulse communicated to them 
by the joint action of the vibrating surface and the surrounding medium. Thus 
in air, when, in the course of a vibration, the part of a plate under a heap 
rises, it communicates a propelling force upwards to that heap, mingled as it 
is with air, greater than that communicated to the surrounding atmosphere, 
because of the superior specific gravity of the former ; upon receding from the 
heap, therefore, in performing the other half of its vibration, it forms a partial 
