(121.)

By the contrivance which has been explained above, the force of the piston in ascending and descending would be conveyed to the working end of the beam; and the next problem which Watt had to solve was, to produce by the force exerted by the working end of the beam in ascending and descending a continuous motion of rotation. In the first instance he proposed to accomplish this by a crank placed upon the axle to which rotation was to be imparted, and driven by a rod connecting it with the working end of the beam. Let K ( fig. 38.) be the centre, to which motion is to be imparted by the working end H of the beam. On the axle K suppose a short lever K I to be fixed so that when K I is turned round the centre K, the axle must turn with it. Let an iron rod, the weight of which shall balance the piston and piston-rod at the other end of the beam, be connected by joints with the working end H of the beam, and the extremity I of the [Pg203] lever K I. As the end H of the beam is moved upwards and downwards, the lever K I will be turned round the centre K, taking successively the positions represented by faint lines in the figure; and thus a motion of continued rotation will be imparted to the axle K.

This simple and effectual expedient of producing a continued rotatory motion by a crank was abandoned by Watt, as already explained, by reason of a patent having been obtained upon information of his experiments surreptitiously procured. To avoid litigation, he therefore substituted for the crank the sun and planet wheel already described; but at the expiration of the patent, which restricted the use of the crank, the sun and planet wheel was discontinued in Watt's engine, and the crank restored.