MR. FARADAY ON A PECULIAR CLASS OF ACOUSTICAL FIGURES. 301 
covered, will also show the principal mode of division of the plate. Most of 
the powder which remains upon the plate will, however, be collected in four 
parcels ; one placed near to each edge of the plate, and evidently towards the 
place of greatest agitation. Whilst the plate is vibrating (and consequently 
sounding) strongly, these parcels will each form a rather diffuse cloud, moving 
rapidly within itself ; but as the vibration diminishes, these clouds will first 
contract considerably in bulk, and then settle down into four groups, each 
consisting of one, two, or more hemispherical parcels (53), which are in an 
extraordinary 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 com- 
motion without being sensibly separated from the plate, and often slowly travel 
towards 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 appli- 
cation of the bow is made, sufficient 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 
cloud 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 consideration at present. They depend upon 
the adhesion of the particles to each other and to the plate, combined with the 
action of the air or surrounding medium, and will be resumed hereafter (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 veiy light, 
appears to be the only condition necessary for success ; fine scrapings from a 
