Woolf introduced the compound engine somewhat widely about 1814 as a pumping engine in the mines of Cornwall. But here it met a strong competitor in the high-pressure single-cylinder engine of Trevithick, which had the advantage of greater simplicity in construction. Woolf's engine fell into comparative disuse, and the single-cylinder type took a form which, under the name of the Cornish pumping-engine, was for many years famous for its great economy of fuel. In this engine the cylinder was set under one end of a beam, from the other end of which hung a heavy rod which operated a pump at the foot of the shaft. Steam was ad mitted above the piston for a short portion of the stroke, thereby raising the pump-rod, and was allowed to expand for the remain der. Then an equilibrium valve, connecting the space above and below the piston, as in fig. 3, was opened, and the pump-rod descended, doing work in the pump and raising the engine piston. The large mass which had to be started and stopped at each stroke served by its inertia to counterbalance the unequal pressure of the steam, for the ascending rods stored up energy of motion in the early part of the stroke, when the steam pressure was greatest, and gave out energy in the later part, when the pressure was much lowered by expansion. The frequency of the stroke was controlled by a device called a cataract, consisting of a small plunger pump, in which the plunger, raised at each stroke by the engine, was allowed to descend more or less slowly by the escape of fluid below it through an adjustable orifice, and in its descent liberated catches which held the steam and exhaust valves from opening. A similar device controlled the equilibrium valve, and could be set to give a pause at the end of the piston's down-stroke, so that the pump-cylinder might have time to become filled.
The final revival of the compound engine did not occur until about the middle of the 19th century, and then several agencies combined to effect it. In 1845 John M'Naught introduced a plan of improving beam engines of the original Watt type, by adding a high-pressure cylinder whose piston acted on the beam between the centre and the fly-wheel end. Steam of higher pressure than had formerly been used, after doing work in the new cylinder, passed into the old or low-pressure cylinder, where it was further expanded. Many engines whose power was proving insufficient for the extended machinery they had to drive were "M'Naughted" in this way, and after conversion were found not only to yield more power but to show a marked economy of fuel. The com pound form was selected by William Pole for the pumping engines of Lambeth and other waterworks about 1850; in 1854 John Elder began to use it in marine engines; in 1857 E. A. Cowper added a steam-jacketed intermediate reservoir for steam between the high- and low-pressure cylinders, which made it unnecessary for the stroke of the low-pressure piston to be just beginning when that of the other piston was just ending. As facilities increased for the use of high-pressure steam, compound expansion came into more general use, its advantage becoming more conspicuous with every increase in boiler pressure. In marine practice, where economy of fuel was from the first an obviously important factor in design, the principle of compound expansion was extended by the introduction of triple and even quadruple expansion engines.
Application to Locomotives and Steamboats.—The adap tation of the steam engine to railways, begun by Trevithick, be came a success in the hands of George Stephenson, whose engine, the "Rocket," when tried along with others in 1829, distanced its competitors. The principal features of the "Rocket" were an im proved steam-blast for urging the combustion of coal and a boiler (suggested by Henry Booth) in which a large heating surface was given by the use of many small tubes through which the hot gases passed. Further, the cylinders, instead of being vertical as in earlier locomotives, were set at a slope, which was after wards altered to a position more nearly horizontal. To these fea tures there was added later the "link motion," a contrivance which enabled the engine to be easily reversed, and the amount of ex pansion to be readily varied. In the hands of George Stephenson and his son Robert the locomotive took a form which in the main is retained by the far heavier locomotives (q.v.) now in use.
The first practical steamboat was the tug "Charlotte Dundas," built by William Symington, and tried in the Forth and Clyde canal in 1802. A Watt double-acting condensing engine, placed horizontally, acted directly by a connecting-rod on the crank of a shaft at the stern, which carried a revolving paddle-wheel. The trial was successful, but steam towing was abandoned for fear of injuring the banks of the canal. Ten years later Henry Bell built the "Comet," with side paddle-wheels, which ran as a passenger steamer on the Clyde; but an earlier inventor to follow up Syming ton's success was the American, Robert Fulton, who, after unsuc cessful experiments on the Seine, fitted a steamer on the Hudson in 1807 with engines made to his designs by Boulton and Watt, and brought steam navigation for the first time to commercial success.
Rise in Steam Pressure and in Piston Speed.—With im provements in the details of design and construction it gradually became practicable to use higher steam pressures and higher pis ton speeds, and consequently to obtain not only greater efficiency, but also a greater amount of power from engines of given bulk. The triple expansion engine, introduced by A. C. Kirk in 1874, did not come into general use until after 1881. It became the normal type of marine engine, with pressures ranging, as a rule, from 150 to 200 lb. per sq. inch, piston speeds generally of 500 or 600 ft. per minute, but sometimes as high as goo or 1,000, and coal consumption of about 1 lb. per hour per indicated horse power. It continues to be very largely used in steamships which are not driven by turbines. In some instances quadruple expan sion has been preferred, with somewhat higher pressures, but when the pressure is much raised the tendency is to abandon the piston type in favour of the steam turbine. This is true both in mar ine and in land practice. The gigantic concentration of steam power that is found in a great steamship or in a power station has been made practicable by the turbine. The selection of Parsons turbines in 1907 as the motive engines of the Cunarders "Lusi tania" and "Mauretania" constituted a new departure in steam engineering. It was then a novelty to develop some 70,00o horse power in the engine room of a single ship. Many of the turbines in power stations now exceed this figure.