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



same instant that the centre of gravity of the rocker is, by the 

 momentum of the latter, moving in the opposite direction: 

 hence a very simple maintaining power, sufficient, whenever 

 the rocker continues to vibrate, to compensate for the loss of 

 force in each half of the vibration which would occur if the 

 rocker and lead were of the same temperature. Mr. Faraday 

 illustrated the sustaining force of the lateral motion of the points 

 of support, by placing a rocker on a piece of lead, and the latter 

 on a board. A pair of sugar-tongs was held tightly by the 

 bend against the edge of the board, so that the line from the 

 tongs towards the rocker was perpendicular to the axis of the 

 latter. On making the limbs of the sugar-tongs vibrate in the 

 manner of a tuning-fork, they communicated longitudinal vibra- 

 tions of equal duration and number to the board, and through 

 it to the lead and points supporting the rocker ; which latter 

 itself immediately acquired vibratory motion isochronous with 

 the vibrations of the tongs, and by successive blows upon the 

 lead produced sound ; upon removing the rocker, and repeating 

 the other parts of the experiment, no sound was produced. 



Experiments with other metals were then made. A piece of 

 curved silver plate being heated and placed on an iron triblet, 

 rocked and sang in the manner of the others ; this is an effect 

 which working silversmiths have long known. The superiority 

 of lead, as the cold metal, was referred to its great expansive 

 force by heat, combined with its deficient conducting power, 

 which is not a fifth of that of copper, silver, or gold ; so that 

 the heat accumulates much more at the point of contact in it, 

 than it could do in the latter metals. 



Mr. Trevelyan's paper had been read to the Royal Society 

 of Edinburgh, but is not yet published. Mr. Faraday stated 

 that Mr. Trevelyan had very liberally allowed him the use of 

 a written copy. 



On a Peculiar Class of Acoustical Figures ; and on certain 

 Forms assumed by groups of particles upon vibrating elastic 

 Surfaces*. [Read May 1% 183L] 



1. THE beautiful series of forms assumed by sand, filings, or 

 other grains, when lying upon vibrating plates, discovered and 



* Philosophical Transactions, 1831, p. 2,99. 



