332 On the Moving Groups of Particles [1831. 



mental evidence, because I have thought on the one hand 

 that the authority of Savart should not be doubted on slight 

 grounds ; and on the other, that if by accident it be placed in 

 the wrong scale, the weight of evidence against it should be 

 such as fully to establish the truth and prevent a repetition of 

 the error by others. 



52. It must be evident that the phenomena of collection at 

 the centres or lines of greatest vibration are exhibited in their 

 purest form at those places which are surrounded by nodal 

 lines ; and that where the centre or place of vibration is at or 

 near to an edge, the effects must be very much modified by the 

 manner in which the air is there agitated. It is this influence, 

 which, in the square plates (6. 12) and other arrangements, 

 prevents the clouds being at the very edge of the glass. They 

 may be well illustrated by vibrating tin plates under water over 

 a white bottom, and sprinkling dark-coloured sand or filings 

 upon various parts of the plates. 



On the peculiar Arrangement and Motions of the Heaps formed 

 by Particles lying on vibrating Surfaces. 



53. The peculiar manner in which the fine powder upon a 

 vibrating surface is accumulated into little heaps, either hemi- 

 spherical or merely rounded, and larger or smaller in size, has 

 already been described (6. 28), as well also as the singular 

 motion which they possess, as long as the plate continues in 

 vibration. These heaps form on any part of the surface which 

 is in a vibratory state, and not merely under the clouds pro- 

 duced at the centres of vibration, although the particles of the 

 clouds always settle into similar heaps. They have a tendency, 

 as heaps, to proceed to the nodal or quiescent lines, but are 

 often swept away in powder by the currents already described 

 (6). When on a place of rest, they do not acquire the invol- 

 ving motion. When two or more are near together or touch, 

 they will frequently coalesce and form but one heap, which 

 quickly acquires a rounded outline. When in their most per- 

 fect and final form, they are always round. 



54. The moving heaps formed by lycopodium on large 

 stretched drawing-paper (28), are on so large a scale as to be 

 very proper for critical examination. The phenomena can be 

 exhibited also even by dry sand on such a membrane, the sand 



