This will serve to exemplify the mode by which the steam contained in the cylinder becomes condensed in a separate vessel, and the improvements which that discovery of itself almost necessarily suggested ; namely, the enclosure of the cylinder to prevent the loss of heat which it imbibed from the steam ; the employment of an upper plate, c, by which the cold atmospheric air that fol lowed the descent of the piston might be excluded, and the elasticity of the steam substituted for atmospheric pressure ; and the piston made effectually air and steam-tight. The wood-cut on the following page (marked Watt's Engine) will, however, convey a more general idea of the engine, with some of the subsequent improvements and additions.
a the cylinder ; c the stuffing-box, through which the piston-rod passes. The lower part of this stuffing-box consists of a hoop, with a Ranch and screw holes ; the interior of this hoop is of greater capacity than the piston-rod, and supplied with some soft substance (hemp or cotton) to surround the piston rod , the upper part, or cover, of the stuffing-box, is less than the interior of the lower that contains the "stuffing;" and being screwed down upon it, presses the stuffing closely round the piston-rod, and thereby prevents the escape of the steam from the cylinder. d the working-beam, resting upon its centre e, and connected at one end to the piston rod, at the other to the heel To the cylin der a (which, in this modification, has no external jacket) is attached a tube, through which the steam is allowed to pass above and below the piston, through the pipe Z, connected with the boiler ; in this tube are placed valves, one above, and one below the point of junction with Z, which are moved by external levers. Now supposing the blowing-valve to have been opened, and the vacuum formed in the condenser in the manner before described, the steam rushing through the upper elbow of the tube upon the piston, forces it to descend to the bottom of the cylinder - by this action the is turned downwards, by means of tappets placed on the pump rod 4, and shuts that valve ; whilst 2, on a pipe behind that which we see, opens a passage to the condenser. The lever 3 is at the same time opened, admitting steam under the piston, which consequently ascends. 5 is a rod connected with the discharging pump attached to the condenser ; 6 a small pump, which supplies the boiler with heated water from the condenser.
The vertical dement of the piston rod (through the stuffing box) by its attach ments to the beam, is explained in the article PARALLEL Mosion, and need not here be repeated.
The wood-cut also exhibits some excellent contrivances, which, though applied oy Watt at a subsequent period, may here as well be described. He had em ployed the fiis-wheel, in order to equalize the motion of the piston in the cylinder (see article FLY) ; but as it became an important object to convert the alternate motion into a rotary one, he applied himself to the discovery of a ready means of effecting it. The crank (see article Caen') could have been priated to this purpose ;but a patent was at that time in force for the exclusive adaptation of it by another; and as some dispute bad arisen upon the subject, Watt was compelled to resort to some other mode, by which a similir effect might be achieved, without the invasion of another's privilege. This ended in
the construction of what is now known by the name of the Star and-Planet wheels, the action of which is as follows :—the bar f (an inflexible rod) is attached at one end to the working-beam, and at the other to h, a wheel that can revolve upon its axis ; o is likewise • toothed wheel fixed to the fly. As the beam rises the planet wheel h is drawn up on the circumference of the not wheel, and turns it round, causing the sun wheel to make two revolutions while the planet wheel travels once over its circumference; the momentum of the fly being sufficiently powerful to preserve the tendency of the machinery to revolve in the same direction during the change of motion in the piston, and to urge the planet wheel over the inactive points in its circuit, the continued rotary motion becomes at once effected, and with this advantage, that as the fly makes twice the number of revolutions it would make by the common crank, a lighter body of material composing the fly is required. There are, however, several disadvantages attending this mode of converting an alternate into a rotary motion, such as being more complex, expensive, and liable to derangement.
One of the last improvements made by Watt upon his engines was the application of a fine piece of mechanism which had been previowdy used for other objects—the governor or regulator. The invention has been ascribed, but Improperly, to Watt ; and although it is said that the notion of applying it to steam-engines was first suggested by a Mr. Clark. of Manchester, it does not appear that he ever carried it into practice. The governor s is composed of two balls, fixed each to a lever attached to other and shorter levers, w a, above the point of junction, the latter being fixed by a movable joint to a slider to, moving freely on the vertical rod s. The horizontal lever w H has a fulcrum, and raises er lowers another lever H Z, which is attached to a valve inside the steam-pipe at Z. On the pulley is a cord g proceeding from the flywheel ; by this means a rotary motion is given to the vertical rod, and the balls by their centrifugal force (vide article) ruing outwards, draw downwards the slider ; which movement raises the opposite end of the horizontal lever II, which acting on the lever connected to it, opens or shuts (as it may be adjusted) the valve Z inside the steam pipe, and diminishes or enlarges the area by which the steam flows into the cylinder. The fall of the balls when the motion decreases, reverses all these movements of course ; and by thus enlarging or contracting the steam way, and admitting more or less steam into the cylinder, the unpulse of the piston is rendered much more uniform. The valve in this part of the steam pipe is now called the throttle valve, and the regulating pendulum the governor.