pin; and in the compound engine it is carried on the inside of one of the fly-wheels, the valve being operated by a disc eccentric. The two latter governors are of the inertia-centrifugal type, similar to that represented in Fig. 43. The form used on the compound engines is shown in Fig. 51. It consists of a single weighted bar (AB) pivoted on a pin (P) fixed to the hub of the wheel. A spring (S) is attached to the wheel with an adjusting nut at D and to the weighted bar at C. The centre of gravity of the bar being at G, increase in speed tends to make this point move away from the centre of the main shaft (M), and causes the bar to swing on its

pivot in a direction opposite to that of the arrow indicating the rotation of the wheel. The eccentric E carried by this bar is thus brought more nearly concentric with the shaft, and its angular advance is also increased so that the travel of the valve is less and the cut-off occurs earlier, the position shown being for latest cut-off. A sudden increase in speed will cause the bar AB, on account of its inertia, to lag behind the wheel, thereby producing the same effect momentarily as that resulting from centrifugal force-in fact the former may be said to anticipate the latter. The motion of the bar is limited by a stop at F and by the eccentric coming in contact with the shaft at E. Other facts con

cerning this general type of governor have already been given in connection with Fig. 43.

The "standard" engine is made in 13 sizes, ranging from 5 to 250 H.P., the speeds being respectively 500 and 250 revolutions per minute. The "junior" engine is built in 7 sizes, from 5 to 75 H.P., and speeds of 400 to 330 revolutions per minute. The compound (single-acting) engine is made in 9 sizes, from 35 to 250 H.P., and speeds of 375 to 250 r.p.m. (Fig. 52).

Willans Engine. A high-speed type widely used in England, where it originated, and built also by the Bradley Manufacturing Company of Pittsburg, is the Willans central-valve engine. It possesses several unusual features, being in the first place single acting; that is to say, the steam pressure is exerted only on one side of the piston. The governor is of the simple throttle-valve form, the cut-off being set at a certain point and not controlled. by the governor, whereas practically all American engines, whether high or low speed, are provided with an automatic cutoff. Another peculiarity is the location of the valve inside of the piston-rod, which is made hollow for the purpose. In spite of these apparent anomalies, the engine has been successful, and seems to possess advantages, the most prominent of which are -great compactness and economy in floor-space; avoidance of lost motion and knocking, owing to the fact that the steam pressure is always exerted in one direction; automatic and perfect lubrication of bearings and other working-parts, obtained by inclosing them in a chamber partly filled with oil; and, finally, high speed, being from 350 to 500 revolutions per minute. The economy of this type is very good, especially when its moderate size and very high speed are considered. It is made simple, compound, and triple-expansion; but usually only the two latter, with two sets of cylinders, are used in electric lighting.

The construction is shown in Fig. 53, which represents a pair of compound engines acting upon the same shaft. Each of the two sets of pistons is connected to its corresponding crank by a pair of connecting-rods, with a space between, containing an eccentric, forged directly upon the crank-pin.

Piston-valves are used, moving inside of a hollow piston-rod, which passes completely through the line of pistons, and through

FLY-WHEEL, FACED TO SERVE AS COUPLING

the ends of the cylinders. The reason for placing the eccentric on the crank-pin, and not on the shaft as usual, is the fact that the valve-face (i.e., the inside surface of the hollow piston-rod)

Fig. 53. Willans Central-valve Compound Engine.

moves with the pistons. Consequently the valve-motion required is relative to the pistons; and is obtained by mounting the eccentric on the crank-pin, which, like the piston-rod, moves with the pistons. Though its lead is different from that of an ordinary

eccentric, its effect upon the movement of the valves is practically the same. The action of the steam is shown in Fig. 54. It passes from the steam-chest into the hollow piston-rod, then out through ports in the latter into the high-pressure cylinder; on the return stroke flowing into the low-pressure cylinder.

The Case Engine is an example of a very high-speed engine adapted to being directly connected to dynamos for small electric plants. Fig. 54 shows a skeleton view of the internal arrange

ment and working-parts of this engine, which are inclosed in a cast-iron box. The cylinder is of the oscillating type, being capable of rocking in its casing, which permits the piston-rod to be directly connected to the crank-pin, and saves weight in the moving-ports by eliminating the connection-rod and cross-head.

The cut-off valve is of the plug type, balanced and made with a slight taper, so that it can be kept tight. Its only duty is to define the point of cut-off; the admission, release, and exhaust closure being controlled by the rocking of the main cylinder, which thus performs the valve-action. The governor is of the usual shaft type, except that it does not vary the throw of the valve but rotates the eccentric on the shaft, thereby changing the lead. General experience with these engines proves that they regulate well, and any slight change in speed can be offset by compound winding on the dynamo.

They are made in six sizes from 2.5 to 25 H.P. inclusive, with speeds from 900 to 550 r.p.m., being built in the pedestal form (Fig. 54); also in the bracket and hanger types, the two latter are mounted on the wall or ceiling where this may be desirable. For direct connection the pedestal form is bolted on a cast-iron base, to which the dynamo is also bolted, the two being connected by a flange coupling substituted for the wheel on the right in Fig. 54. In addition to the examples described other similar designs of high-speed engines are employed for driving electric generators, such as the Buckeye, Payne, Ames, Watertown, Skinner, McEwen, Chandler & Taylor, Atlas, Fitchburg, Buffalo Forge, Noye, and Reeves types.

Standardization of High-speed Engines and Dynamos.-The numerous sizes and styles of engines and generators of the various manufacturers resulted in great confusion, each builder being obliged to have many patterns to fit the different machines on the market. To avoid this trouble a committee of the American Society of Mechanical Engineers has made certain recommendations, which may be found in full in the Proceedings for 1901 and in the Electrical World of Dec. 7, 1901. The standard capacities, speeds, shaft diameters, and principal dimensions recommended are as shown in the table on page 178.

Medium-speed Engines. Besides the types of high-speed engines just described, a third class of engines was defined (p. 156) to be those which are a compromise between the low-speed Corliss and the high-speed Ball engine, for example. These are vertical or horizontal and cross-compound or tandem-compound, the speed being ordinarily between 125 and 200 r.p.m.