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 mechanical 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
