is attached at the other extremity to a cross head, which is so guided that it is compelled to move in a vertical line, and thus a side strain upon the rod which would produce friction and leakage, even were it not to cause actual bend- ing and fracture, is prevented. In fig. 3 the STEAM ENGINE 341 FIG. 3. Watt's Engine, 1784. ross head is guided by a parallel motion p m, an arrangement of rods of which one set vi- brate about centres, thus displacing the centres )f vibration of the other set just sufficiently to compensate the tendency of the latter to irow the cross head out of line by their sweep rough their own curved path. This com- msation permits the head of the piston rod be securely guided in the vertical line. In iter practice, a more common method of ob- taining a rectilinear motion of the cross head is to place guides at each end of it, by which its extremities are kept in the desired line of motion. The sliding friction of the cross- "icad gibs upon these guides is slight, and is kept within proper limits by lubrication. The cross head is connected with the work- ing beam, B, by links (usually a pair), and the beam, vibrating about the main centre, transfers the motion by means of the connect- ing rod, c r, to the crank attached to the main shaft or crank shaft, which carries the fly wheel or balance wheel, "W. In this figure the crank is not shown, the sun and planet wheels taking its place. Steam is conveyed to the engine by the steam pipe, at some convenient point in which a stop valve is usually placed. Sometimes this valve is omitted, a throttle valve alone being used, adapted to adjust the supply of steam. The latter is either a disk valve, adjusted by a screw (in which form it is known as a screw stop valve), or it is some variety of slide valve, opening and closing by sliding transversely across the opening through which steam passes. "Where the supply of steam is determined automatically, a governor, G, is attached, which when the speed of the engine tends to exceed the desired maximum closes the throttle valve, and when the speed falls too low opens it. In the figure, the gov- ernor consists of a pair of suspended balls caused to revolve by a belt, or by gearing con- necting the spindle with the shaft, which when speed rises are given a high velocity of revo- lution about the spindle carrying them, and, separating under the action of centrifugal force, move the lever I, and thus close the throttle valve. There are many varieties of governors. The "fly-ball governor," just described, is most common, but, though simple and quite well adapted to general purposes, it is not per- fectly isochronous; t. e., it does not compel the engine to keep the precise speed at which it is set to work. As the governor and valve are rigidly connected, there is but one speed to which the position of the valve and of the governor can be perfectly adapted under any one set of conditions of steam pressure and of load. The valve gear is the system of valves and of actuating mechanism- which distribute the steam as the engine passes through its cycles of motion. The steam valves admit steam alternately to each end of the steam cylinder, as the piston moves backward and forward, and the exhaust valves alternately open and close the passages or ports through which the steam escapes, after impelling the piston, into the condenser c in the condensing engine, or into the open air from a non-con- densing engine. These valves are moved au- tomatically by some part of the engine itself. In the kind of engine shown in fig. 3, and in pumping engines which have no crank and revolving shaft, the motion is obtained from a rod depending from the beam, projections on which rod strike the tappets t as they rise and fall. This rod is called th*e plug rod. In nearly all other engines, the valve gear is actuated by an eccentric, or disk attached to and revolving with the crank shaft. "While the piston is moving upward, the steam valve below and the exhaust valve above are open, the steam entering below to drive the piston up, while the steam which had produced the downward stroke escapes through the open exhaust valve at the top into the condenser. During the descent of the piston these con- ditions are reversed. The condenser maybe either a jet condenser, as shown in the fig- ure, or a surface condenser. Its office is to condense the steam ejected from the cylinder, and thus to create a vacuum, so removing the resisting pressure of the atmosphere from be- fore the piston. With the jet condenser, the steam issuing from the exhaust pipe of the engine is received in a closed vessel, where it is brought into contact with jets of cold water, and thus instantly condensed, and the vacuum so produced pervades the condenser, the ex- haust pipe, and the exhausted end of the cylinder. The water of condensation, the re-
