of valve gear designed particularly to meet one or the other of these conditions. Of these the link motion invented by Howe in 1843, and first used on Stephenson's engines, is perhaps the most familiar, it being the gear very generally used, either as originally designed or in some modified form, on locomotives and marine engines. Fig. 7 shows the Howe link, or, as it is more commonly called, the Stephenson reversing link valve gear. The two eccentrics E and E, are set on the crank shaft, S, at right angles to the crank, C. These eccentrics carry two rods r and r diverging from each other at a slight angle and with their forward ends connected by the slotted, curved link, L. The valve stem, V, has a block or pin which connects it to the link by means of a sliding fit in the slot. If the link be lifted so that the rod r lies nearly in line with V, then the valve receives its motion merely as if driven directly by the single eccentric E; if the link be lowered so that rod r, lies nearly in line with V, then the valve acts practically as if driven by the single eccentric E. Now eccentric E, being set for forward motion of the engine and eccentric E being set for backward motion, it will be seen that the movement of the link gives a ready means for reversing the engine. It does more than this, however, since by putting the link in intermediate positions between full forward and full backward gear the cut-off of the valve can be adjusted for expansive working of the steam. The train of levers v, w, x, y, and z in Fig. 7 shows the means of operating this link in an ordinary locomotive engine. The governor of a steam engine is a device whose function is to regulate the energy developed in accordance with the load propelled. If the admission and pressure of the steam be constant, the speed will vary as the load varies; with a light load the speed will be high and with a heavy load it will be slow. These fluctuations in speed are always undesirable and may easily become dangerous, as, for example, when the engine 'races' or 'runs away,' causing the fly wheel to burst from centrifugal force. Governors act to regulate the energy in two ways, first by cutting off steam from the boiler as the speed increases, which amounts to a reduction di bs in boiler pressure, and second by reducing the quantity of steam admitted into the cylinder. The first are called throttling governors and the second are called cut-off governors. Governors are also classified according to their method of action and according to their form. The most common form of governor is the fly-ball or pendulum governor, which may operate either by throttling or by cut-off action. The diagram (Fig. 8) shows a form of pendulum governor. The stem or shaft, S, is given rotary motion by means of a belt embracing the pulley, P. Keyed to the top of the shaft is the plate a, carrying, by means of the arms b and b1, the heavy balls c and c1, and hung to the arms b and b, are the arms d FIG. 8. and d, which connect with the plate e, which is free to slide up and down the shaft S. The balls and their connecting arms revolve with the shaft, and because of the familiar law of centrifugal force they tend to fly apart as the speed of rotation increases and to come closer together as this speed decreases. As the balls fly apart, due to increased speed, they lift the plate e, and this operates the train of levers x, y, and z in such a way as either to shut off steam from the boiler or from the cylinders. That is, as the speed increases the energy is decreased, and conversely as the speed decreases the energy is increased. In this way the energy is proportioned to the load at every instant, the exactness with which this proportion is maintained depending upon the sensitiveness of the governor. The transmission mechanism of a steam engine consists of the piston rod, the crosshead, and the connecting rod. The duty of the piston rod is to convey the energy developed by the piston outside of the cylinder so that it may be transmitted to the crank shaft or fly wheel. At its inner end the piston rod is attached rigidly to the piston at its centre and at its outer end it is rigidly attached to the crosshead. The rod passes out of the cylinder through an orifice in its front end, this orifice being so constructed that it is steam-tight. It is due largely to the fact that a circular orifice is more easily made steam-tight than any other form that the piston rod is universally cylindrical in form. The crosshead is the connecting link which permits the rectilinear motion of the piston rod to be transformed into the swaying motion of the connecting rod. The manner in which it accomplishes this is clearly shown by Fig. 9, which is a section through a familiar make of crosshead. The piston rod coming from the right is rigidly attached to the crosshead by a threaded connection, while the connecting rod is connected by means of a horizontal pin, so that it can swing up and down. The top and bottom of the cross FIG. 9. SECTION OF CROSSHEAD. head are planed smooth and fit a groove or track in the upper and lower guides. These guides are a part of the engine frame, and serve to prevent any vibration of the outer end of the piston rod due to the swaying motion of the connecting rod. The connecting rod is commonly a rectangular or cylindrical bar having at each end a circular bearing, one to embrace the crosshead pin and the other to embrace the crank pin. Fig. 10 shows the construction described. The articles CRANK and FLY WHEEL describe the construction and functions of these parts of the engine's mechanism, and they will not be mentioned further. In concluding this section reference may be made to Fig. 11 as showing in a very plain manner the character and relation of the several structural details which have been described individually. In this engraving the different parts are designated by letters as follows: The engine frame, A; the cylinder, B; the piston rod, C; the crosshead, D; the connecting rod, E; the crank, F; the fly wheel, G; and the governor, H. The first practical use of importance to which FIG. 10. mill engines are not infrequent. A horizontal simple engine of comparatively small size which can be used for dynamo driving or other high-speed work, is shown by Fig. 11. A third type of stationary engine is the hoisting engine, which in its smaller sizes combines a vertical steam boiler and a duplex horizontal or vertical engine in one machine. Such engines do not have a fly wheel, but connect directly with a crank shaft which drives the drum upon which the hoisting rope is wound. Hoisting engines of larger size have separate boilers and CONNECTING ROD. Top and side views. steam engines were put was the pumping of water, and the pumping engine still remains one of the principal forms of the steam engine. The various types of pumping engines are described in the article on PUMPS AND PUMPING MACHINERY. The next important use of the stationary steam engine was for driving the machinery of factories, mills, and workshops, and such establishments still consume an enor mous aggregate of steam-engine power. The often operate as many as eight separate drums. The largest sizes of hoisting engines are those used in raising ore from deep mine shafts. These mine hoists have capacities of from 2000 to 5000 horse power. Like the smaller sizes, they are either duplex vertical or duplex horizontal engines. A duplex engine consists of a right-hand and a left-hand engine, both of which couple to the same crank shaft. They are to be distinguished from cross-compound engines, which have a modern form of mill engine is the horizontal direct-acting fly wheel engine, in which the power is taken from the fly wheel and transmitted to shafting by means of belts. (See BELTS and POWER, TRANSMISSION OF.) For mill engines of large size present practice favors compound engines; simple engines are used when the unit of power which is required is small. Generally tandem compound and cross compound engines are preferred to engines using steam with three or four expansions, although multiple-expansion similar appearance structurally, by the impor tant fact that each half of the machine is distinct from the other half so far as the use of the steam is concerned. In direct-acting mine hoists the drum or drums are mounted directly on the crank shaft; in general hoists the crank shaft drives a separate drum shaft by means of gearing. A fourth form of stationary engine is the rolling-mill engine, used for driving the trains of rolls in rolling mills. (See ROLLING MILL.) This is usually a horizontal simple engine of VERTICAL STEAM ENGINES DIRECT CONNECTED TO ELECTRIC GENERATORS 1. 6,000 Horse-power Poppet Valve Westinghouse Cross-compound Engine. 2. 5,000 Horse-power Westinghouse-Corliss Cross-compound Engine. large size and especially sturdy construction. The largest stationary steam engines now used in any form are those employed for driving the generators of electric power plants. These large machines are almost universally of the inverted vertical direct-acting type, illustrated in the accompanying plate. A sixth important form of steam engine is the steam-driven air compressor described in the article AIR COMPRESSORS. Engines other than stationary fall into one of two great classes, viz., locomotive engines for railways and marine engines for ship propulsion. The traction engine is essentially a locomotive engine designed to run on common roads, and the portable engine is practically a stationary engine and boiler plant of small size mounted on wheels so that it may be hauled from place to place. The growth and construction of the locomotive engine are described in the article LOCOMOTIVE. Marine engines fall into two separate classes. For paddle-wheel boats the beam engine and the inclined engine are universally employed. For screw-propelled vessels the inverted vertical direct-acting engine is almost universal. (See STEAM NAVIGATION.) For a discussion of the theory of steam engines and heat engines in general, see STEAM and THERMODYNAMICS. For descriptions of special applications of steam engines, see AUTOMOBILE; FIRE-ENGINE; BLOWING-MACHINES. BIBLIOGRAPHY. For an account of the development of the steam engine, see Thurston, Manual of the Steam Engine (New York, 1892). Among the best theoretical and descriptive works are: Heck, The Steam Engine (New York, 1905); Seaton, A Manual of Marine Engineering (ib., 1895); Peabody, Valve Gears for Steam Engines (ib., 1892); Whitham, Steam Engine Design (ib., 1902); Pipper, Steam Engine History and Practice (London, 1906). STEAMER DUCK, LOGGERHEAD, or RACEHORSE. A very large duck (Tachyeres cinerus), numerous about the southern extremity of South America, so called on account of its peculiarity of rowing itself along the surface of the water at great speed. This is said to be due to the remarkable fact that this bird loses its power of flight when it reaches maturity. STEAM HAMMER. See HAMMER. STEAM HEATING. See HEATING AND VENTILATION. STEAM NAVIGATION. The Spaniards assert that as early as 1543 Blasco de Garay made an attempt to propel a vessel by steam in the harbor of Barcelona. In the absence of direct proof of the fact this may well be doubted. At the time mentioned the most advanced scientists in Europe had not yet begun seriously to consider steam as a source of power. The assertion is also made that Denis Papin (q.v.) in 1707 propelled a boat by steam on the River Fulda. Papin invented the safety valve and a single-acting steam cylinder pump, and made various improvements in steam pumps, but it does not appear that he ever built what might be called a steam engine. The boat which has been mentioned and which is frequently referred to had some sort of paddle wheels, but they were operated by the crew and not driven by steam power. In 1729 Dr. John Allen took out a patent in England for a method of propelling a boat by means of forcing water out of the stern with steam or other pressure. In 1736 the rather vague ideas of Allen were improved upon by Jonathan Hulls, a clockmaker of Campden, Eng HULLS' BOAT. (From an old drawing.) land, and he was granted a patent for mechanism to propel a boat by steam power. Like Allen, he apparently made no serious attempts to put his ideas into practice. In 1752 the French Academy of Sciences awarded a prize to the distinguished physicist Daniel Bernoulli for an essay on the manner of propelling boats without wind. In addition to other suggestions he proposed the use of the screw propeller. Up to this time successful steam navigation was impossible because a practical steam engine did not exist. This deficiency was supplied by Watt, who took out his first patent in 1769, but the engines contemplated were really singleacting pumps. In 1782, however, Watt brought out the double-acting engine, and developed the principle of expansive working by cutting off the steam at a suitable point instead of allowing it to follow full stroke. All the conditions for the propulsion of vessels by steam were now in peared. In 1783 the Marquis de Jouffroy built existence and experimental boats rapidly apone which was tried at Lyons, and it is said to veloped into a form for practical use the Revohave been successful; but before it could be de lution overtook and ruined him. At the same time John Fitch, James Rumsey, and Oliver Evans were experimenting in America. Rumsey's boats, like the proposed vessel of Dr. Allen, were fitted with jet propellers, whereby a stream of water was discharged by a steam-driven pump. His first boat was tried in Virginia in 1784 and a second, which attained a speed of 4 knots, was completed in 1786. He died in London in 1792, just previous to the trial of a new boat built from his plans. Fitch's boats were fitted with various types of propelling machinery -with paddle wheels in 1785 and afterwards with long paddles which were given motion similar to that of the paddle of an Indian canoe. In 1790 one of Fitch's boats attained a speed of 7 knots, and afterwards was used on the Delaware to carry passengers. In 1793 Fitch went to France; in 1796, after returning to America, he built a small screw steamboat, but the exact measure of success that he attained is uncertain. Evans experimented with various peculiar types of steamboats, one of which was fitted with a rude screw and wheels with which to run on shore. In England Joseph Bramah obtained a patent in 1785 for propelling vessels by means of "a wheel with inclined Fans or Wings similar to the fly of a Smoke-jack or the vertical sails of a windmill." A patent for a similar invention was issued to William Lyttleton in 1784 and to Edward Shorter in 1800. In 1791 John Stevens of Hoboken, N. J., patented a multitubular steam boiler, and he soon after began experiments with steam propulsion of boats, in which he was assisted by the |