FORMED ON VIBRATING ELASTIC SURFACES. 
317 
vacuum, into which the air, round the heap, enters with more readiness than 
the heap itself ; and as it enters, carries in the powder at the bottom edge of 
the heap with it. This action is repeated at every vibration, and as they occur 
in such rapid succession that the eye cannot distinguish them, the centre part 
of the heap is continually progressing upwards ; and as the powder thus accu- 
mulates above, whilst the base is continually lessened by what is swept in under- 
neath, the particles necessarily fall over and roll down on every side. 
59. Although this statement is made upon the relation of the heap, as a 
mass, to the air surrounding it, yet it will be seen at once that the same rela- 
tion exists between any two parts of the heap at different distances from the 
centre ; for the one nearest the centre will be propelled upward with the greatest 
force, and the other will be in the most favourable state for occupying the 
partial vacuum left by the receding plate. 
60. This view of the effect will immediately account for all the appearances ; 
the circular form, the fusion together of two or more heaps, their involving 
motion, and their existence upon any vibrating part of the plate. The manner 
in which the neighbouring particles would be absorbed by the heaps is also 
evident; and as to their first formation, the slightest irregularities in the pow- 
der or surface would determine a commencement, which would then instantly 
favour the increase. 
61. It is quite true, that if the powder were coherent, that force alone would 
tend to produce the same effect, but only in a very feeble degree. This is suf- 
ficiently shown by the experiments made in the exhausted receiver (36). When 
the barometer of the air-pump was at twenty-eight inches, that in the air being 
about 29.2 inches, the heaps, or rather parcels, formed very beautifully over the 
whole surface of the membrane ; but they were very flat and extensive compared 
with the heaps in air, and the involving motion was very weak. As the air was 
admitted, the vibration being continued, the heaps rose in height, contracted 
in diameter, and moved more rapidly. Again, in the experiments with filings 
and sand in water, no cohesive action could assist in producing the effect ; it 
must have been entirely due to the manner in which the particles were me- 
chanically urged in a medium of less density than themselves. 
62. The conversion of these round heaps into linear concentric involving 
parcels, in the experiment already described (29. 31), when the membrane was 
