when the piston has arrived at the bottom of the cylinder, as represented in fig. 34., the valves c and s', that is, the upper exhausting-valve and the lower steam-valve, are opened, and the valves s and c', that is, the upper steam-valve and the *lower exhausting-valve, are closed.

If these valves, as has been here supposed, be opened and closed at the moments at which the piston reaches the top and bottom of the cylinder, it is evident that they may be all worked by a single lever connected with them by proper mechanism. When the piston arrives at the top of the cylinder, this lever would be made to open the valves s and c', and at the same time to close the valves s' and c; and when it arrives at the bottom of the cylinder, it would be made to close the valves s and c', and to open the valves s' and c.

If, however, it be desired to cut off the steam before the arrival of the piston at the termination of its stroke, whether upwards or downwards, then the steam-valves must be closed before the arrival of the piston at the end of its stroke; and as the exhausting-valve ought to be left open until the stroke is completed, these valves ought to be moved at different times. In that case separate levers should be provided for the different valves. We shall, however, return again to the subject of the valves which regulate the admission of steam to the cylinder and its escape to the condenser.

(117.) It will be remembered that in the single-acting engine the process of condensation was suspended while the piston ascended in the cylinder, and therefore the play of the jet of cold water in the condenser was stopped during this interval. In the double-acting engine, however, the flow of steam from the cylinder to the condenser is continued, whether the piston ascends or descends, and therefore a constant condensation of steam must be produced. The condensing jet, therefore, does not in this case, as in the former, play with intervals of intermission. A constant jet of cold water must be maintained in the condenser.

It will presently appear that in the double-acting engine applied to manufactures, the motion of the piston was subject

admitted to the cylinder was subject to a corresponding change. The quantity of steam, therefore, drawn into the condenser was subject to variation, and required a considerable change in the quantity of cold water admitted through the jet to condense it. To regulate this, the valve or cock by which the water was admitted into the condenser was worked in the double-acting engine by a lever furnished with an index, by which the quantity of condensing water admitted into the condenser could be regulated. This index played upon a graduated arch, by which the engine-man was enabled to regulate the supply.

CHAP. VIII.

FLY-WHEEL.

METHODS OF CONNECTING THE PISTON-ROD AND BEAM IN THE DOUBLE-
ACTING ENGINE. RACK AND SECTOR. PARALLEL MOTION. CON-
NECTING ROD AND CRANK.
THROTTLE-VALVE.-
GOVERNOR CONSTRUCTION AND OPERATION OF THE DOUBLE-ACTING
ENGINE-ECCENTRIC. COCKS AND VALVES. SINGLE-CLACK VALVE.
DOUBLE-CLACK VALVE.
SLIDE VALVES.-

CONICAL VALVES.

MURRAY'S SLIDES. THE D VALVE. - SEAWARD'S SLIDES.

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,

and playing upon the arch head of the beam. In the doubleacting 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 pistonrod 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.

Fig. 35.

Watt first proposed to effect this by attaching to the end of the piston-rod a straight rack, faced with teeth, which should work in corresponding teeth raised on the arch head of the beam, as represented in fig. 35. If his improved steam engines required no further precision of operation and construction than the atmospheric engines, this might have been sufficient; but in these engines it was indispensably necessary that the piston-rod should be guided with a smooth and even motion through the stuffing-box in the top of the cylinder, otherwise any shake or irregularity would cause it to work loose in the stuffing-box, and either to admit the air, or to let the steam escape. Under these circumstances, the motion of

the rack and toothed arch head were inadmissible, since it was impossible by such means to impart to the piston-rod that smooth and equable motion which was requisite. Another contrivance which occurred to Watt was, to attach to the top of the piston-rod a bar, which should extend above the beam, and to use two chains or straps, one extending from the top of the bar to the lower end of the arch head, and the other from the bottom of the bar to the upper end of the arch head. By such means the latter strap would pull the beam down when the piston would descend, and the former would pull the beam up when the piston would ascend. These contrivances, however, were superseded by the celebrated mechanism since called the Parallel Motion, one of the most ingenious mechanical combinations connected with the history of the steam engine.

(119.) It will be observed that the object was to connect by some inflexible means the end of the piston-rod with the extremity of the beam, and so to contrive the mechanism, that while the end of the beam would move alternately up and down in part of a circle, the end of the piston-rod connected with the beam should move up and down in a straight line. If the end of the piston-rod were fastened upon the end of the beam by a pivot without any other connection, it is evident that, being moved up and down in the arch of a circle, it would be drawn to the left and the right alternately, and would consequently either be broken or bent, or would work loose in the stuffingbox. Instead of connecting the end of the rod immediately with the end of the beam by a pivot, Watt proposed to connect them by certain moveable rods, so arranged that, as the end of the beam would move up and down in the circular arch, the rods would so accommodate themselves to that motion, that the end connected with the piston-rod should not be disturbed from its rectilinear course.

To explain the principle of the mechanism called the parallel motion, let us suppose that op (fig. 36.) is a rod or lever moveable on a centre o, and that the end P of this rod shall move through a circular arch P P′ P" P'" in a vertical plane, and let its play be limited by two stops s, which shall prevent its ascent above the point P, and its descent below