By the following ingenious mechanism an alternate or vibrating force may be made to communicate a circular motion continually in the same direction. Let A B, _fig. 170._, be an axis receiving an alternate motion from some force applied to it, such as a swinging weight. Two ratchet wheels (253.) _m_ and _n_ are fixed on this axle, their teeth being inclined in opposite directions. Two toothed wheels C and D are likewise placed upon it, but so arranged that they turn upon the axle with a little friction. These wheels carry two catches _p_, _q_, which fall into the teeth of the ratchet wheels _m_, _n_, but fall on opposite sides conformably to the inclination of the teeth already mentioned. The effect of these catches is, that if the axis be made to revolve in one direction, one of the two toothed wheels is always compelled (by the catch _against_ which the motion is directed) to revolve with it, while the other is permitted to remain stationary in obedience to any force sufficiently great to overcome its friction with the axle on which it is placed. The wheels C and D are both engaged by bevelled teeth (263.) with the wheel E. According to this arrangement, in whichever direction the axis A B is made to revolve, the wheel E will continually turn in the same direction, and, therefore, if the axle A B be made to turn alternately in the one direction and the other, the wheel E will not change the direction of its motion. Let us suppose that the axle A B is turned against the catch _p_. The wheel C will then be made to turn with the axle. This will drive the wheel E in the same direction. The teeth on the opposite side of the wheel E being engaged with those of the wheel D, the latter will be turned upon the axle, the friction, which alone resists its motion in that direction, being overcome. Let the motion of the axle A B be now reversed. Since the teeth of the ratchet wheel _n_ are moved against the catch _q_, the wheel D will be compelled to revolve with the axle. The wheel E will be driven in the same direction as before, and the wheel C will be moved on the axle A B, and in a contrary direction to the motion of the axle, the friction being overcome by the force of the wheel E. Thus, while the axle A B is turned alternately in the one direction and the other, the wheel E is constantly moved in the same direction. It is evident that the direction in which the wheel E moves may be reversed by changing the position of the ratchet wheels and catches.