ENDNOTE TO GRACEFULNESS.
55 A parallel fact, further elucidating this, is supplied by a locomotive engine. On looking at the driving wheel, there will be found, besides the boss to which the connecting rod is attached, a corresponding mass of metal on the opposite side of the wheel, and equidistant from the centre; or, if the engine be one having inside cylinders, then, on looking between the spokes of the driving-wheel, it will be seen that against each crank is a block of iron, similar to it in size, but projecting from the axle in the reverse direction. Evidently, being placed on opposite sides of the centre of motion, each crank and its counterbalance move in opposite directions relatively to the axle; and by so doing, neutralize each other’s perturbing effects, and permit a smooth rotation. This relationship which exists between the motions of the counterbalance and the crank, is analogous to that which exists between the motions of the arms and legs in walking; and in the early days of railway-locomotion, before these counterbalance weights were used, locomotive driving-wheels were subject to violent oscillations, analogous to those jerkings of the shoulders which arise when we walk fast without moving our arms.