into which the cranks dip at each stroke. The pistons are of the trunk form, being open at the bottom, and are made sufficiently long (about 1 times their diameter), to serve as crossheads, since the connecting-rods are directly attached to them. In the "standard" type the valve-chest is in the form of a third cylinder, which is in a slightly oblique position, and is often mistaken for a working cylinder. In the "junior" and compound engines the valve-chest is at right angles to the cylinders across the top. A single piston-valve is used in all of these engines, the valve-seat being a removable bushing in which the ports are milled.

A

Fig. 54. Principle of Inertia Governor.

The governor is of the shaft type, and consists of two weights controlled by springs. In the "standard" engine this governor is located on the shaft between the two cranks, and actuates the valve directly; in the "junior" engine it is located on the outside of the engine in one of the two flywheels; and in the compound engine it is carried in a special governor wheel, which takes the form of an inclosed case filled with oil. The governor weights are so pivoted that they act by inertia, as well as by centrifugal force, the object being to make the engine regulate more quickly for changes in load. Assume, for example, that the engine is running at normal speed, and that the weight A, pivoted at B, is in a certain position, as represented in Fig. 54, where its centrifugal force is balanced by the tension of a spring not shown in the figure. If, now, the load is suddenly reduced, the fly-wheel will run ahead of the weight A, so to speak, thereby swinging it, in a very small fraction of a revolution, into the position indicated by dotted lines, the rotation being in a left-hand direction. If load is suddenly added, the converse action will take place. Extreme quickness of regulation is not so important in electric lighting as in electric-railway work, since changes of load are more gradual. But promptness in this respect is very desirable for incandescent lamps, as the least change of speed affects the light, even though it only lasts for one or two revolutions, while an ordinary centrifugal governor is adjusting itself.

The "standard" engine is made in 13 sizes, ranging from 5 to 250 horse-power, the speeds being respectively 500 and 250 revolutions per minute. The "junior" engine is built in 7 sizes, from 5 to 75 horse-power, and speeds of 400 to 330 revolutions per minute. The Westinghouse Machine Company stated on Jan. 1, 1893, that they had sold 3,600 of the former and 1,700 of the latter type; nevertheless, these engines do not have the reputation of being particularly economical in steam consumption. They are

employed where compactness, inclosed working-parts, and high speeds are desirable.

The compound Westinghouse engine is claimed to be of high economy, not only at full load, but also for variable loads. This engine is made in 11 sizes, from 35 to 700 horse-power, with speeds of 375 to 220 revolutions per minute (Fig. 55).

The Brotherhood Engine is a type used quite extensively in England and sometimes in the United States. It is somewhat similar to the Westinghouse engine in general character; but it has three working cylinders, and these are arranged radially with respect to the shaft, at 120° from each other. Each cylinder is single acting, and the engine runs at a very high speed.

Willans Engine. Another type of high-speed engine which is already widely used in England where it originated, and is now being introduced into this country, is the Willans central-valve engine. This engine is very unusual in its construction; indeed, it might be said that many of its features are directly antagonistic to generally accepted ideas. The engine is single acting; that is to say, the steam pressure is exerted on only 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 cut-off when used in electric lighting. Another striking peculiarity of the engine 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 very successful, and seems to possess many 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, which enables the engine to be directly coupled to the dynamo, securing a still further saving of space. The economy of this engine seems to be high, a steam consumption as low as 12.74 lbs. of steam per horse-power per hour having been obtained with a condensing-engine of this type of only 30 horse-power at 400 revolutions per minute, which is certainly a remarkable result. It is difficult to understand how very high economy can be secured with a throttlevalve governor, unless the engine is kept working at or near full load. An explanation of this apparent anomaly is given on page 133.

The engine is made simple, compound, and triple-expansion; but usually only the two latter kinds, with two sets of cylinders, are used in electric lighting.

The construction of the Willans engine is shown in Fig. 56, which represents a pair of compound engines connected to the same shaft. The two sets of pistons are each connected to their corresponding cranks by a pair of connecting-rods, with a space

between, which contains an eccentric, forged directly upon the crank-pin. The connecting-rods work at the top upon two hardened steel pins, so supported that the pressure of the rods exerts no twisting strain upon them; the eccentric-rod plays in the space between the former.

Piston-valves are used, moving inside of a hollow piston-rod R, which passes completely through the line of pistons, and through the ends of the cylinders. The reason that the eccentric is placed on the crank-pin, and not on the shaft as usual, is that the valveface (i.e., the inside surface of the hollow piston-rod) moves with the pistons. Consequently the valve-motion required is a motion. relative to the pistons; and this is obtained by mounting the eccentric on the crank-pin, which, like the piston-rod, moves up and down with the pistons. Though its lead is set differently from that of an ordinary eccentric, its effect upon the movement of the valves is practically the same.

The steam passes from the steam-chest into the hollow pistonrod, which moves steam-tight through the gland in the cylinder top. It then passes out through ports 4 in the piston-rod, into the high-pressure cylinder; and on the return stroke it leaves the high-pressure cylinder by the ports 5, and enters the low-pressure cylinder by the ports 6. In each case its entrance and exit are controlled by the valves V1, V2, etc., working in the hollow piston

rod.

The governor is of the simple throttle-valve type, the action of which is clearly shown in Fig. 56.

A test made by Siemens Brothers of London, under the superintendence of Professor Kennedy,* of a Siemens dynamo (type H. B. 27-40) directly coupled to a Willans compound engine (type III.), gave the following results: