At that time it occurred to Watt that the condensation of the steam from the cylinder after it had done its work, might be made more perfect if a perpetual vacuum was maintained beneath the piston, while an alternate steam-pressure and vacuum were produced above it. (Fig. 396.)
Fig. 396.
"e e is the cylinder. j. The piston, a. The steam-pipe. b. The regulating or throttle valve, e. The eduction and equilibrium single valve, performing the functions of both. c. The upper, and f the under, portholes, by which passages only the steam can enter and pass away. d, j, g. The eduction-pipe by which the steam passes from above the piston during every returning stroke to the condenser, a perpetual exhaustion being maintained beneath it."—From Bourne on the Steam-engine.
Instead of obtaining a specific advantage the contrary occurred, and Watt was obliged in this case to return to the ponderous Newcomen counterweight to balance the difference in the vacuum above and below the piston, consequently this form of the cylinder and valves was abandoned. The juvenile reader will perceive in the above drawing that the superior arrangement of Watt's cylinder to that of Newcomen arises from the steam operating above and below the piston, and that the piston rod works air-tight in a stuffing box at the top of the cylinder. A most important improvement in the employment of steam as a motive power has been discovered in the mode of using it "expansively," by which the steam, at a pressure say of sixty pounds on the square inch, is admitted below the piston, and then cut off and allowed to expand and drive up the latter without the expenditure of any more fuel, and leaving, after lifting the piston to a height say of three feet, an average or mean power of thirty pounds on the square inch.
Returning to the eight-horse condensing engine, d is the steam cylinder surrounded by a case to prevent the steam cooling and to maintain in the cylinder the same, or nearly the same, temperature as that of the steam in the boiler, according to the condition of Art. I. of Watt's Patent, quoted at p. 425 of this book. The same outer case is apparent around the cylinder in Fig. 396; e, the piston, which, by stuffing with hemp or other proper material, fits the interior of the cylinder in the most accurate manner, and prevents the escape of steam by its sides: e is the piston rod attached to the parallel motion. This clockwork-like piece of mechanism has often been quoted as one of the masterpieces of Watt, and in its greatest perfection is called the complete parallel motion, and may be found in all the best land beam steam-engines. The object of the parallel motion is to cause the piston and pump rods to move always in straight lines, never deviating to either side. (Fig. 397.)
Fig. 397.
a b is half the beam, a being the main centre, b e. The main links connecting the piston-rod f with the end of the beam. g d. The air-pump links, from the centre of which the air-pump rod is suspended. c d and e d produce the parallelism, because c d is moveable only round the fixed centre c, whilst e d is not only moveable round the centre d, but the centre itself in the arc described by c d, and by this action e d corrects the distorting influence of its own radius. The dotted lines and letters above enable the observer to see the effect of the movement of the beam on the parallel motion.
In the eight horse-power engine shown in page picture, e is also attached to the piston e, which moves the beam f, and the other end of this beam, by the connecting rod g, gives motion to the heavy fly wheel g, by means of the crank h.