Mr. Willans-which is also interesting as having been the subject of exceptionally complete scientific investigationis seen in Fig. 165. It was studied as a simple, a compound, and a triple-expansion engine, being easily adapted to either system.'

As here shown, its three cylinders are placed in series and "tandem." The valves are on one rod, driven by a single eccentric on the crankpin, the rod being in the axis of the engine and the valves within the hollow piston-rod. Cut-off is effected by the passage of the ports into metallic rings in the ends of the cylinders, and is adjustable by hand or by the governor. Compression is effected in the separate cushion - chamber."

These engines are usually grouped in pairs, with cranks at right angles.

In some cases the arrangement of a pair of complete engines, of properly selected sizes, in such manner that

EXHAUST!

STEAM

OIL

FUNNEL

FIG. 165.-Central-Valve Engine.

either the exhaust of one may be used in the other, or steam may be taken direct from the boiler to either, is

1 The discussion of this paper is remarkably interesting:-" Transactions of the British Institute of Civil Engineers," March, 1888; 18871889; No. 2,306, vol. xciii.

' Ibid., vol. lxxxi, p. 166.

found advantageous. When less power is demanded, or when one is disabled, the available engine may then be used alone. Economy has been attained by this plan even when the two engines are placed at considerable distances apart, the precaution being taken to carefully guard against loss of heat between them.

The "cross-compound" type of Corliss engine is illustrated by the accompanying sketch of a pair designed by Mr. Reynolds and built by Allis & Company for the Namquit Mills. The cranks are set at right angles, and the receiver is placed beneath the floor. This is a less common variety than the "tandem" form, but is still often adopted.

The general arrangement and disposition of the parts of a triple-expansion engine as built by the Corliss Company is seen in Fig. 166. Here the low-pressure cylinder is divided, one of its two elements being coupled with the high-pressure cylinder on the right, and the twin with the intermediate cylinder on the left. The cranks are set at 90°. These engines have cylinders 20, 34, 36, and 36 inches diameter and 5 feet stroke of piston. All cylinders are completely steam-jacketed, heads included, and the steam is somewhat superheated. Jet-condensers are used. The capacity of the engine is 1,000 I. H. P. or more, and its "duty" is about 135,000,000 pounds; the fuel used, when of good quality, amounting on test to 1.44 pounds per horse-power per hour.

"Compounding" simple engines is often a very economical and profitable plan. The method depends mainly upon the design of the engine to be so altered. The common forms of stationary beam-engine are frequently improved by what is called "McNaughting," placing a new high-pressure cylinder beside the old and low-pressure cylinder and connecting it to the same beam.

The Steam-Turbine constitutes a class of steam-engine which, although the first invented, and familiar as a type to all engineers from the days of Hero the Younger, and

known to have a high theoretical and moderately high actual efficiency, has been only experimentally used until a very recent date. That of Hero has been illustrated in Fig. 3. Branca's engine (Fig. 6) was the first exemplar of another now common form of "steam-turbine," although the name is not precisely correct. This is illustrated in the re

cent device of La Val. The Atwater engine of about 1840 was of this type, and was said to be as economical as the engines of the time of equal power. Steam-turbines of the inward-flow type have been used by Gorman and others.'

The later "compound" steam-turbine has recently been somewhat extensively employed in the operation of dynamoelectric machinery. It consists of two sets of parallel-flow turbines set in twin scries on one shaft on either side the induction-pipe, thus balancing. The passages are gradually enlarged as the volume of the steam increases with its progressive expansion.

The turbines thus alternate with their guide-blades, and both the vanes and the blades are carefully proportioned and set to secure maximum attainable efficiency at the proposed speed of rotation, their pitches and depths being suitably varied.

The actual consumption of steam is found to be 75 to 25 pounds per electrical horse-power produced, and per hour, as steam-pressures rise from 60 to 150 pounds by gauge. The speed of rotation ranges from 5,000 or 10,000 revolutions per minute upward, according to size and steampressure; 18,000 and 20,000 being common speeds for the smaller sizes.

Dow's turbine is an inward-flow wheel with concentric sets of guides and vanes in series, and is said to have attained 35,000 revolutions per minute, working regularly at 25,000, consuming 55 pounds of steam per horse-power per hour.

1 Rankine, p. 538.

The theory of this type of machine is that familiar to the hydraulic engineer, and the speeds of orifice for maximum efficiency are well known to be infinite in the Hero class of turbine and approximately one half the final velocity of flow in the guide-blade turbine. Since these speeds are impracticable in their use, a certain loss of energy is thus inevitable. In compensation for this loss, in the steamturbine, is the fact that it is not subject to that fluctuation of temperature of parts exposed to contact with the steam which results in large wastes by cylinder-condensation in the common forms of steam-engine. A gain of from 25 to 50 per cent. in this item is to be counted upon.

The Dow turbine, as built for work in connection with the Howell torpedo, gives an average of about 11 horsepower in coming up to speed in regular working, at 60 pounds steam-pressure, and weighs from 100 to 150 pounds, or not far from 13 pounds per horse-power.' Its fly-wheel rim attains a speed of nearly 7 miles an hour at 10,000 revolutions per minute. The designer estimates its power at 150 pounds steam-pressure and the same speed at 40 horse-power, or 1 horse-power to 3.75 pounds weight, and states that this may be still further reduced to the extraordinary minimum of 2 pounds weight per horse-power, a figure well within the estimated allowable maximum for use in aëronautic work.

The steam-turbine of Parsons (Fig. 167) is an engine consisting of a series of turbines, the different pairs of guides and wheels being so placed that the fluid passes successively from one pair to the next. Of the two forms, radial and axial flow, only the latter have been used here. Two series of cylindrical turbines are used, arranged symmetrically to the right and left of the central steam-inlet, the exhaust taking place from the two ends. In this manner a balance is obtained, and the bearings are relieved of end-pressure. Oil is forced through the bearings by a pump. The bear

1 Electrical World, April 18, 1891.