Now, suppose the piston to be at the top of the cylinder, the cylinder below it being filled with steam, which has just pressed it up. Let the upper steam valve A´, and the lower exhausting-valve D´ be opened, and the other two valves closed. The steam which fills the cylinder below the piston will immediately pass through the valve D´ into the condenser, and a vacuum will be produced below the piston. At the same time, steam is admitted from the steam-pipe through the valve A´ above the piston, and its pressure will force the piston to the bottom of the cylinder. On the arrival of the piston at the bottom of the cylinder, the upper steam valve A´, and lower exhausting-valve D´, are closed; and the lower steam valve C´, and upper exhausting-valve B´ are opened. The steam which fills the cylinder above the piston now passes off through B´ into the condenser, and leaves a vacuum above the piston. At the same time, steam from the boiler is admitted through the lower steam valve C´, below the piston, so that it will press the piston to the top of the cylinder; and so the process is continued.
It appears, therefore, that the upper steam valve, and the lower exhausting-valve, must be opened together, on the arrival of the piston at the top of the cylinder. To effect this, one lever, E´, is made to communicate by jointed rods with both these valves, and this lever is moved by a pin placed on the piston-rod of the air-pump; and such a position may be given to this pin as to produce the desired effect exactly at the proper moment of time. In like manner, another lever, F´, communicates by jointed rods with the upper exhausting valve and lower steam valve, so as to open them and close them together; and this lever, in like manner, is worked by a pin on the piston-rod of the air-pump.
(61.) This method of connecting the valves, and working them, has been superseded by another, for which Mr. Murray of Leeds obtained a patent, which was, however, set aside by Messrs. Bolton and Watt, who showed that they had previously practised it. This method is represented in [figs. 18, 19]. The stems of the valves are perpendicular, and move in steam-tight sockets in the top of the valve-boxes. The stem of the upper steam valve A is a tube through which the stem of the upper exhausting-valve B passes, and in which it moves steam-tight; both these stems moving steam-tight through the top of the valve-box. The lower steam valve C, and exhausting-valve D, are similarly circumstanced; the stem of the former being a tube through which the stem of the latter passes. The stems of the upper steam valve and lower exhausting-valve are then connected by a rod, E; and those of the upper exhausting-valve and lower steam valve by another rod, F. These rods, therefore, are capable of moving the valves in pairs, when elevated and depressed. The motion which works the valves is, however, not communicated by the rod of the air-pump, but is received from the axis of the fly-wheel. This axis works an apparatus called an eccentric; the principle which regulates the motion of this may be thus explained:—
D E ([figs. 20, 21].) is a circular metallic ring, the inner surface of which is perfectly smooth. This ring is connected with a shaft, F B, which communicates motion to the valves by levers which are attached to it at B. A circular metallic plate is fitted in the ring so as to be capable of turning within it, the surfaces of the ring and plate which are in contact being smooth and lubricated with oil or grease. This circular plate revolves, but not on its centre. It turns on an axis C, at some distance from its centre A; the effect of which, evidently, is that the ring within which it is turned is moved alternately in opposite directions, and through a space equal to twice the distance (C A) of the axis of the circular plate from the common centre of it and the ring. The eccentric in its two extreme positions is represented in [figs. 20, 21]. The plate and ring D E are placed on the axis of the fly-wheel, or on the axis of some other wheel which is worked by the fly-wheel. So that the motion of continued rotation in the fly-wheel is thus made to produce an alternate motion in a straight line in the shaft F B. This rod is made to communicate by levers with the rods E and F ([figs. 18, 19].), which work the valves in such a manner, that, when the eccentric is in the position [fig. 20]., one pair of valves are opened, and the other pair closed; and when it is brought to the position [fig. 21]., the other pair are opened and the former closed and so on. It is by means of such an apparatus as this that the valves are worked almost universally at present.
The piston being supposed to be at the top of the cylinder ([fig. 18].), and the rod E raised, the valves A and D are opened, and B and C closed. The steam enters from the steam-pipe at an aperture immediately above the valve A, and, passing through the open valve, enters the cylinder above the piston. At the same time, the steam which is below the piston, and which has just pressed it up, flows through the open valve D, and through a tube immediately under it to the condenser. A vacuum being thus produced below the piston, and steam pressure acting above it, it descends; and when it arrives at the bottom of the cylinder ([fig. 19].) the rod F is drawn down, and the valves A and D fall into their seats, and at the same time the rod F is raised, and the valves B and C are opened. Steam is now admitted through an aperture above the valve C, and passes below the piston, while the steam above it passes through the open valve B into a tube immediately under it, which leads to the condenser. A vacuum being thus produced above the piston, and steam pressure acting below it, the piston ascends, and thus the alternate ascent and descent is continued by the motion communicated to the rods E F from the fly-wheel.
Pl. V.
WATT'S DOUBLE-ACTING STEAM ENGINE.
Pl. VI.
Drawn by the Author. Engr. by Peter Maverick