318 On a Peculiar Class of Acoustical Figures. [1831. 



when highly attenuated, collect also at the vibrating parts. 

 Hence it is evident that the nature of the powder has nothing 

 to do with its collection at the centres of agitation, provided it 

 be dry and fine. 



9. The cause of these effects appeared to me, from the first, 

 to exist in the medium within which the vibrating plate and 

 powder were placed, and every experiment which I have made, 

 together with all those in M. Savart's paper, either strongly 

 confirm, or agree with this view. When a plate is made to 

 vibrate (2), currents (24) are established in the air lying upon 

 the surface of the plate, which pass from the quiescent lines 

 towards the centres or lines of vibration, that is, towards those 

 parts of the plates where the excursions are greatest, and then 

 proceeding outwards from the plate to a greater or smaller di- 

 stance, return towards the quiescent lines. The rapidity of 

 these currents, the distance to which they rise from the plate 

 at the centre of oscillation, or any other part, the blending of 

 the progressing and returning air, their power of carrying light 

 or heavy particles, and with more or less rapidity or force, are 

 dependent upon the intensity or force of the vibrations, the 

 medium in which the vibrating plate is placed, the vicinity of 

 the centre of vibration to the limit or edge of the plate, and 

 other circumstances, which a simple experiment or two will im- 

 mediately show, must exert much influence on the phenomena. 



10. So strong and powerful are these currents, that when 

 the vibrations were energetic, the plate might be inclined 

 5, 6, or 8 to the horizon, and yet the gathering clouds retain 

 their places. As the vibrations diminished in force, the little 

 heaps formed from the cloud descended the hill; but on 

 strengthening the vibrations they melted away, the particles 

 ascending the inclined plane on those sides proceeding up- 

 wards, and passing again to the cloud. This took place when 

 neither sand nor filings could rest on the quiescent or nodal 

 lines. Nothing could remain upon the plate except those 

 particles which were so fine as to be governed by the currents, 

 which (if they exist at all) it is evident would exist in whatever 

 situation the plate was placed. 



11. M. Savart seems to consider that the reason why the 

 powder gathers together at the centres of oscillation is, " that 

 the amplitude of the oscillations being very great, the middle 



