Let A B C D E F, _fig. 150._, be a _crank_, which is a double winch ((252.) and _fig. 89._), by which an axle, A B E F, is to be turned. Attached to the middle of C D by a joint is a rod, which is connected with a beam, worked with an alternate motion on a centre, like the brake of a pump, and driven by any constant power, such as a steam-engine. The bar C D is to be carried with a circular motion round the axis A E. Let the machine, viewed in the direction A B E F of the axis, be conceived to be represented in _fig. 151._, where A represents the centre round which the motion is to be produced, and G the point where the connecting rod G H is attached to the arm of the crank. The circle through which G is to be urged by the rod is represented by the dotted line. In the position represented in _fig. 151._, the rod acting in the direction H G has its full power to turn the crank G A round the centre A. As the crank comes into the position represented in _fig. 152._, this power is diminished, and when the point G comes immediately below A, as in _fig. 153._, the force in the direction H G has no effect in turning the crank round A, but, on the contrary, is entirely expended in pulling the crank in the direction A G, and, therefore, only acts upon the pivots or gudgeons which support the axle. At this crisis of the motion, therefore, the whole effective energy of the power is annihilated. After the crank has passed to the position represented in _fig. 154._, the direction of the force which acts upon the connecting rod is changed, and now the crank is drawn upward in the direction G H. In this position the moving force has some efficacy to produce rotation round A, which efficacy continually increases until the crank attains the position shown in _fig. 155._, when its power is greatest. Passing from this position its efficacy is continually diminished, until the point G comes immediately above the axis A, _fig. 156._ Here again the power loses all its efficacy to turn the axle. The force in the direction G H or H G can obviously produce no other effect than a strain upon the pivots or gudgeons.