334 On the Grouping of Particles, $c. [1831. 



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 accumulates above, whilst the base is 

 continually lessened by what is swept in underneath, the parti- 

 cles 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 relation 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 irregula- 

 rities in the powder or surface would determine a commence- 

 ment, 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 sufficiently shown by the expe- 

 riments made in the exhausted receiver (36). When the baro- 

 meter 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 mechanically 

 urged in a medium of less density than themselves. 



62. The conversion of these round heaps into linear concen- 

 tric involving parcels, in the experiment already described 



