METHODS OF CONNECTING THE PISTON-ROD AND BEAM IN THE DOUBLE-ACTING ENGINE. — RACK AND SECTOR. — PARALLEL MOTION. — CONNECTING ROD AND CRANK. — FLY-WHEEL. — THROTTLE-VALVE. — GOVERNOR. — CONSTRUCTION AND OPERATION OF THE DOUBLE-ACTING ENGINE. — ECCENTRIC. — COCKS AND VALVES. — SINGLE-CLACK VALVE. — DOUBLE-CLACK VALVE. — CONICAL VALVES. — SLIDE VALVES. — MURRAY'S SLIDES. — THE D VALVE. — SEAWARD'S SLIDES. — SINGLE COCK. — FOUR-WAY COCK. — PISTONS. — COMMON HEMP-PACKED PISTON. — WOOLFE'S PISTON. — METALLIC PISTONS. — CARTWRIGHT'S ENGINE. — CARTWRIGHT'S PISTON. — BARTON'S PISTON. (118.) In the single-acting engine, the force of the piston acted on the beam only during its descent; and this force was transmitted from the piston to the beam, as we have seen, by a flexible chain, extended from the end of the piston-rod, [Pg194] and playing upon the arch head of the beam. In the double-acting engine, however, the force of the steam pressing the piston upwards must likewise be transmitted to the beam, so as to drive the latter upwards while the piston ascends. This action could not be accomplished by a chain connecting the piston with the arch head of the beam. Where the mechanical action to be transmitted is a _pull_, and not a _push_, a flexible chain, cord, or strap, is sufficient; but if a _push_ or _thrust_ is required to be transmitted, then the flexibility of the medium of mechanical communication afforded by a chain renders it inapplicable. In the double-acting engine, during the descent, the piston-rod still pulls the beam down; and so far a chain connecting the piston-rod with the beam would be sufficient to transmit the action of the one to the other; but in the ascent, the beam no longer pulls up the piston-rod, but is pushed up by it. A chain from the piston-rod to the arch head, as described in the single-acting engine, would fail to transmit this force. If such a chain were used with the double engine, where there is no counterweight on the opposite end of the beam, the consequence would be, that in the ascent of the piston the chain would slacken, and the beam would still remain depressed. It is therefore necessary that some other mechanical connection be contrived between the piston-rod and the beam, of such a nature that in the _descent_ the piston-rod may _pull_ the beam down, and may _push_ it up in the _ascent_. [Illustration: _Fig._ 35.]