"Secondly, In engines that are to be worked wholly or partially by condensation of steam, the steam is to be condensed in vessels distinct from the steam-vessels or cylinders, although occasionally communicating with them ; these vessels I call condensers ; and, whilst the engines are working, these condensers ought at least to be kept as cold as the air in the neighbourhood of the engines, by application of water or other cold bodies.
"Thirdly, Whatever air or other elastic vapour is not condensed by the cold of the condenser, and may impede the working of the engine, is to be drawn out of the steam-vessels or condensers by means of pumps, wrought by the engines themselves, or otherwise.
"Fourthly, I intend in many cases to employ the expansive force of steam to press on the pistons, or whatever may be used instead of them, in the same manner in which the pressure of the atmosphere is now employed in common fire-engines. In cases where cold water cannot be had in plenty, the engines may be wrought by this force i of steam only, by discharging the steam into the air after it has done its office." The "common fire-engine" alluded to was the steam engine of Newcomen. Highly important as Watt's first inventions were, they resulted for a time in the production of nothing more than a greatly improved engine of the Newcomen type, much less waste ful of fuel, able to make faster strokes, but still only suitable for pumping, still single-acting, with steam admitted during the whole stroke, the piston, as before, pulling the beam by a chain work ing on a circular arc. The condenser was generally worked by in jection, but Watt has left a model of a surface condenser made up of small tubes, in every essential respect like the condensers now used.
Fig. 3 is an example of the Watt pumping engine of this period. It should be noticed that, although the top of the cylinder is closed, and steam has access to the upper side of the piston, this is done only to keep the cylinder and piston warm. The engine is still single-acting; the steam on the upper side merely plays the part which was played in Newcomen's engine by the atmosphere ; and it is the lower end of the cylinder alone that is ever put in communication with the condenser. There are three valves : the "steam" valve a, the "equilibrium" valve b, and the "exhaust" valve c. At the beginning of the down-stroke c is opened to pro duce a vacuum below the piston and a is opened to admit steam above it. At the end of the down-stroke a and c are shut and b
is opened. This puts the two sides in equilibrium and allows the piston to be pulled up by the pump-rod P, which is heavy enough to serve as a counterpoise. C is the condenser, and A is the air pump, which discharges into the hot well H, whence the supply of the feed-pump F is drawn.
In a second patent (1781) Watt describes the "sun-and-planet" wheels and other methods of making the engine give continuous revolving motion to a shaft provided with a flywheel. The crank and connecting-rod—already a familiar mechanical device from its use on the treadle of a lathe—would have been the natural means of doing this, but its application to the steam engine in a particular manner had been made the subject of a patent by James Pickard, and Watt, rather than make terms with Pickard, whom he regarded as a plagiarist of his own ideas, made use of his sun-and-planet motion until the patent on the crank expired. The reciprocating motion of earlier forms had served only for pumping; by making the steam engine drive a revolving shaft Watt opened up for it many other channels of usefulness. The engine was still single-acting; the connecting-rod was attached to the far end of the beam, and that carried a counterpoise which served to raise the piston when steam was admitted below it.
In 1782 Watt patented two further improvements of the first importance, both of which he had invented some years before.
One was the use of double action, that is to say, the application of steam and vacuum to each side of the piston alternately. The other (invented as early as 1769) was the use of steam expan sively, in other words the plan, essential to economy of fuel, of stopping the admission of steam when the piston had made only a part of its stroke, and allowing the rest of the stroke to be per formed by the expansion of the steam already in the cylinder. To let the piston push as well as pull the end of the beam Watt de vised his so-called parallel motion, an arrangement of links con necting the piston-rod head with the beam in such a way as to guide the rod to move in a very nearly straight line. He further added a throttle valve, for regulating the rate of admission of steam, and a centrifugal governor, in the form of a double coni cal pendulum, which controlled the speed by a throttle-valve.