1 83 1 .] On Sounds from heated Metals. 3 1 3 



neighbouring portions of lead, the expansion ceases, and the 

 hill falls. When the rocker, therefore, is left free, the raised 

 side descends through a greater space than that through which 

 it was lifted, and also to a lower level than the other side ; in 

 consequence of which a momentum is given to it, which carries 

 its centre of gravity beyond the point to which it would pass 

 if there had been no alteration in the heights of the sustaining 

 points. It is this additional force which acts as a maintaining 

 power ; and recurs twice in each vibration, i. e. once on each 

 side. The force is gained by the whole rocker being lifted 

 bodily by the point on which it is for the time supported, and 

 comes into play by the side 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 mecha- 

 nical force generated by the alternate expansion and contraction 

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

 of consideration 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, consequently, 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 



