for the hand-gear. Watt introduced pair- and chain-closure by degrees into the machine. Thus, for instance, the force-closed beam-chains became the imperfect but still kinematically far more complete "parallel motion." Even to our own time the venerable pumping machinery used in our mines remains partly in the fetters of force-closure; it is only very lately that direct-acting steam pumping-engines have begun to dispute its position.
Fig. 176.
The well-known "Sun and Planet" wheels of Watt give us an interesting illustration of the course of the change from the one kind of closure to the other. The form in which Watt originally put the mechanism was not the familiar one of Fig. 175, but the entirely different one shown in Fig. 176.[1] In order to maintain
- ↑ See Muirhead's Inventions of James Watt, Vol. III. p. 50; also Bourne's Treatise on the Steam Engine, pp. 20-21.
