1831.] formed on Vibrating Elastic Surfaces. 333 



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 posi- 

 tion, and dropping some lycopodium upon it, it will be seen 

 that the particles of the heap rise up at the centre, overflow, 

 fall down upon all sides, and disappear at the bottom, appa- 

 rently proceeding 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, exhibiting 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. 



57. If a glass plate be supported and vibrated (6), its sur- 

 face 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 exhibit 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 mecha- 

 nical 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 spe- 

 cific gravity of the former ; upon receding from the heap, there- 

 fore, in performing the other half of its vibration, it forms a 

 partial vacuum, into which the air, round the heap, enters with 



