One of the most remarkable examples of it is presented in the scapements of watches and clocks. In this case, however, it can scarcely be said with strict propriety that it is the rotation of the scapement-wheel (266.) which _communicates_ the vibration to the balance-wheel or pendulum. That vibration is produced in the one case by the peculiar nature of the spiral spring fixed upon the axis of the balance-wheel, and in the other case by the gravity of the pendulum. The force of the scapement-wheel only _maintains_ the vibration, and prevents its decay by friction and atmospheric resistance. Nevertheless, between the two parts thus moving there exists a mechanical connection, which is generally brought within the class of contrivances now under consideration. A beam vibrating on an axis, and driven by the piston of a steam-engine, or any other power, may communicate rotary motion to an axis by a connector and a crank. This apparatus has been already described in (311.). Every steam-engine which works by a beam affords an example of this. The working beam is generally placed over the engine, the piston rod being attached to one end of it, while the connecting rod unites the other end with the crank. In boat-engines, however, this position would be inconvenient, requiring more room than could easily be spared. The piston rod, in these cases, is, therefore, connected with the end of the beam by long rods, and the beam is placed beside and below the engine. The use of a fly-wheel here would also be objectionable. The effect of the dead points explained in (311.) is avoided without the aid of a fly, by placing two cranks upon the revolving axle, and working them by two pistons. The cranks are so placed that when either is at its dead point, the other is in its most favourable position. A wheel A, _fig. 169._, armed with wipers, acting upon a sledge-hammer B, fixed upon a centre or axle C, will, by a continued rotatory motion, give the hammer the reciprocating motion necessary for the purposes to which it is applied. The manner in which this acts must be evident on inspecting the figure. The treddle of the lathe furnishes an obvious example of a vibrating circular motion producing a continued circular one. The treddle acts upon a crank, which gives motion to the principal wheel, in the same manner as already described in reference to the working beam and crank in the steam-engine.