[22] Farey, Treatise on the Steam Engine, p. 122.
[23] Farey on the Steam Engine, p. 297.
DOUBLE-ACTING ENGINE.—CITY SAW-MILLS.
[CHAP. VIII.]
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.)
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.