The last grand improvement was the rendering the engine double acting, by causing the pressure of the steam to operate during the ascent as well as the descent of the piston; and to obtain a rotatory motion from the rectilinear movement of the piston. This may be considered the crowning improvement; from being limited almost exclusively to the purpose of lifting water, the engine became universally applicable as a prime mover, and its utility was increased a hundred-fold.

But it was not merely to the principles upon which the action of the engines depended that Mr. Watt confined his care; every part of the engine occupied his attention, and received improvement at his hands.

To give a detailed account of all these improvements as they rapidly succeeded each other, would far exceed our limits, we therefore refer the reader to the accompanying cut, which represents the engine in its matured state, with most of the material improvements embodied.

a the cylinder: c the stuffing-box, through which the piston rod passes. The stuffing-box consists of a short tube, cast upon the cylinder cover with a flanch and screw-holes round its upper edge; the interior is supplied with some soft substance (hemp or cotton) to surround the piston rod; and upon the cover of the box is formed a ring or gland which fits into the box, and being screwed down upon the stuffing, presses it closely round the piston rod, and thereby prevents the escape of the steam from the cylinder. d is the working beam, resting upon its centre e, and connected at one end to the piston rod, at the other to the barf. To the cylinder 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 lever 1 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 descent of the piston rod (through the stuffing box) by its attachments to the beam, is explained in the article Parallel Motion, and need not here be repeated.

Mr. Watt had employed the fly-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 Crank) could have been appropriated to this purpose; but a patent was at that time in force for the exclusive adaptation of it by another; and as some dispute had arisen upon the subject, Watt was compelled to resort to some other mode, by which a similar 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 Sun-and-Planet wheels, the action of which is as follows: - the bar / (an inflexible rod) is attached at one end to the working beam, and at the other to h, a toothed wheel that can revolve upon its axis; o is likewise a toothed wheel fixed to the fly. As the beam rises the planet wheel h is drawn up on the circumference of the sun 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 previously 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. Clarke 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, u u, above the point of junction, the latter being fixed by a moveable joint to a slider, w, moving freely on the vertical rod s. The horizontal lever w H has a fulcrum, and raises or lowers another lever H Z, which is attached to a valve inside the steam-pipe at Z. On the pulley is a cord q proceeding from the fly-wheel; by this means a rotary motion is given to the vertical rod, and the balls by their centrifugal force (vide article) rising outwards, draw downwards the slider; which movement raises the opposite end of the horizontal lever H, 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 impulse 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