(122.) Whether the crank or the sun and planet wheel be used, there is still a difficulty in the maintenance of a regular motion of rotation. In the various positions which the crank and connecting rod assume throughout a complete revolution, there are two in which the moving power loses all influence in impelling the crank. These positions are those which the crank assumes when the piston is at the top and bottom of the [Pg204] cylinder, and is just about to change the direction of its motion. When the piston is at the bottom of the cylinder, the pivot I (_fig._ 38.), by which the connecting rod H I is attached to the end of the crank, is immediately over the axle K of the crank, and under the pivot H, which joins the upper end of the connecting rod with the beam. In fact, in this position the connecting rod and crank are in the same straight line, extending from the end of the beam to the axle of the crank. The steam, on entering the cylinder below the piston, and pressing it upwards, would produce a corresponding downward force on the connecting rod at H, which would be continued along the connecting rod and crank to the axle K. It is evident that such a force could have no tendency to turn the crank round, but would expend its whole energy in pressing the axle K downwards. The other position in which the power loses its effect upon the crank is when the piston is at the top of the cylinder. In this case, the working end of the beam will be at the lowest point of its play, and the crank-pin I will be immediately below the axle K; so that K will be placed immediately between H and I. When the steam presses on the top of the piston, it will expend its force in drawing the end H of the connecting rod upwards, by which the crank-pin I will likewise be drawn upwards. It is evident that this force can have no effect in turning the crank round, but will expend its whole energy in producing an upward strain on the axle K.