The steam-pipe, _a b d d e_, leads the steam from the boiler to the chambers, _b_ and _e_. The exhaust-pipe, _g g_, leads from _h_ and _i_ to the condenser. In the sketch, the upper steam and the lower exhaust valves, _b_ and _f_, are opened, and the steam-valve, _e_, and exhaust-valve, _c_, are closed, the piston being near the upper end of the cylinder and descending. _l_ represents the plug-frame, which carries tappets, 2 and 3, which engage the lever, _s_, at either end of its throw, and turn the shaft, _u_, thus opening and closing _c_ and _e_ simultaneously by means of the connecting-links, 13 and 14. A similar pair of tappets on the opposite side of the plug-rod move the valves, _b_ and _f_, by means of the rods, 10 and 11, the arm, _r_, when struck by those tappets, turning the shaft, _t_, and thus moving the arms to which those rods are attached. Counterbalance-weights, carried on the ends of the arms, 4 and 15, retain the valves on their seats when closed by the action of the tappets. When the piston nearly reaches the lower end of the cylinder, the tappet, 1, engages the arm, _r_, closing the steam-valve, _b_, and the next instant shutting the exhaust-valve, _f_. At the same time, the tappet, 3, by moving the arm, _s_, downward, opens the steam-valve, _e_, and the exhaust-valve, _c_. Steam now no longer issues from the steam-pipe into the space, _c_, and thence into the engine-cylinder (not shown in the sketch); but it now enters the engine through the valve, _e_, forcing the piston upwards. The exhaust is simultaneously made to occur at the upper end, the rejected steam passing from the engine into the space, _c_, and thence through _c_ and the pipe, _g_, into the condenser. This kind of valve-gear was subsequently greatly improved by Murdoch, Watt's ingenious and efficient foreman, but it is now entirely superseded on engines of this class by the eccentric, and the various forms of valve-gear driven by it. [Illustration: FIG. 33.--Watt's Half-Trunk Engine, 1784.] The "trunk-engine" was still another of the almost innumerable inventions of Watt. A half-trunk engine is described in his patent of 1784, as shown in the accompanying sketch (Fig. 33), in which _A_ is the cylinder, _B_ the piston, and _C_ its rod, encased in the half-trunk, _D_. The plug-rod, _G_, moves the single pair of valves by striking the catches, _E_ and _F_, as was usual with Watt's earlier engines. Watt's steam-hammer was patented at the same time. It is seen in Fig. 34, in which _A_ is the steam-cylinder and _B_ its rod, the engine being evidently of the form just described. It works a beam, _C C_, which in turn, by the rod, _M_, works the hammer-helve, _L J_, and the hammer, _L_. The beam, _F G_, is a spring, and the block, _N_, the anvil. [Illustration: FIG. 34.--The Watt Hammer, 1784.]