It will presently appear that in the double-acting engine applied to manufactures, the motion of the piston was subject to more or less variation of speed, and the quantity of steam [Pg192] 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.
[Illustration: HEATHFIELD HOUSE, NEAR BIRMINGHAM, THE RESIDENCE OF WATT.]
[20] These effects are explained in my Treatise on Heat; and they have lately been verified by experiments made with locomotive engines by M. de Pambour, who found that the steam raised from the boiler of a locomotive engine, under a pressure of above 50 lbs. per square inch, was in the state of common steam as it issued from the chimney at a very diminished pressure and at a lower pressure.
[21] It is strange that this absurdity has been repeatedly given as unquestionable fact in various encyclopædias, as well as in by far the greater number of treatises expressly on the subject.
[22] Farey, Treatise on the Steam Engine, p. 122.
[Illustration: DOUBLE-ACTING ENGINE.--CITY SAW-MILLS.]
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.