1831.] On a Peculiar Class of Acoustical Figures. 317 



condition ; for the powder of each parcel continues to rise up 

 at the centre and flow down on every side to the bottom, 

 where it enters the mass to ascend at the centre again, until 

 the plate has nearly ceased to vibrate. If the plate be made 

 to vibrate strongly, these parcels are immediately broken up, 

 being thrown into the air, and form clouds, which settle down 

 as before ; but if the plate be made to vibrate in a smaller 

 degree, by a more moderate application of the bow, the little 

 hemispherical parcels are thrown into commotion without being 

 sensibly separated from the plate and often slowly travel to- 

 wards the quiescent lines. When one or more of them have 

 thus receded from the place over which the clouds are always 

 formed, and a powerful application of the bow is made, suf- 

 ficient to raise the clouds, it will be seen that these heaps 

 rapidly diminish, the particles of which they are composed 

 being swept away from them, and passing back in a current 

 over the glass to the clouds under formation, which ultimately 

 settles as before into the same four groups of heaps. These 

 effects may be repeated any number of times, and it is evident 

 that the four parts into which the plate may be considered as 

 divided by the diagonal lines are repetitions of one effect. 



7. The form of the little heaps, and the involved motion 

 they acquire, are no part of the phenomena under considera- 

 tion at present. They depend upon the adhesion of the par- 

 ticles to each other and to the plate, combined with the action 

 of the air or surrounding medium, and will be resumed here- 

 after (53). The point in question is the manner in which fine 

 particles do not merely remain at the centres of oscillation, or 

 places of greatest agitation, but are actually driven towards 

 them, and that with so much the more force as the vibrations 

 are more powerful. 



8. That the agitated substance should be in very fine 

 powder, or very light, appears to be the only condition ne- 

 cessary for success ; fine scrapings from a common quill, even 

 when the eighth of an inch in length or more, will show the 

 effect. Chemically pure and finely divided silica rivals lyco- 

 podium in the beauty of its arrangement at the vibrating parts 

 of the plate, although the same substance in sand or heavy 

 particles proceeds to the lines of rest. Peroxide of tin, red 

 lead, vermilion, sulphate of baryta, and other heavy powders 



