Royal Institution of Great Britain. 121 



of the rocker which is descending, having a greater space to fall 

 through than that which is passed over by the mere force of its 

 momentum during its previous rise. A curious consequence of this 

 action is, that the force which really lifts the rocker is on one side 

 of the centre of gravity, whilst the rising side of the rocker itself is 

 on the other. 



This, however, is not the only maintaining cause or mechanical 

 force generated by the alternate expansion and contraction of the 

 lead. If the vertical direction of the forces be put out of considera- 

 tion for a time, and the two points of support be examined, it will 

 be found that whilst the rocker is quiescent, both points (with their 

 neighbouring parts) being heated, will expand and compress the 

 lateral portions of the lead, until the tension of the latter is equal to 

 their own. When one side of the rocker is raised, the point that it 

 rested upon is instantly cooled, and therefore contracts ; but as the 

 neighbouring parts retain their tension, they move towards the 

 contracting part, the other point of support moving with the rest. 

 When the rocker returns in its oscillation, it reheats and re-expands 

 the first point of support, whilst the second, now out of contact, is 

 cooled and contracted, and the first point, therefore, moves towards 

 the second. A necessary consequence of this mutual relation of the 

 points is, that the one under process of heating is always moving 

 towards the other which is under process of cooling ; and, conse- 

 quently, towards a perpendicular from the centre of gravity ; but 

 as it is at the same time the supporting point to the rocker, that 

 supporting point is, by irresistible impulse, carried in a direction 

 under and towards the line passing from the centre of gravity 

 towards the earth, at the same instant that the centre of gravity of 

 the rocker is, by the momentum of the latter, moving in the oppo- 

 site 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 illus- 

 trated 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 vibrations of equal duration and 

 number to the board, and through it to the lead and points sup- 

 porting the rocker ; which latter itself immediately acquired vibra- 

 tory 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 work- 



