rest before entering the pump, and consequently lose the value of the momentum acquired when entering the vacuous space. In the Worthington Condenser, however, the construction and arrangement are such that the momentum of the steam and water is conserved, and this force is utilised to assist the pump in its work.

"In the Worthington Condenser the air set free by the condensation of the steam is intermingled with the water. The pump has the same regularity of motion that is characteristic of Worthington pumping machinery; in fact, it acts as

M

B

Fig. 543.-WORthington Independent Jet CONDENSER. SECTIONAL VIEW.

a water-pump, although the water is aërated. There is here a great distinction between the Worthington Condenser pumping aerated water and an air-pump pumping air and water unmixed. In the latter case the air-pump has a varying and irregular duty to perform, and the inevitable result is an irregular and slamming movement."

The surface condenser consists of a chamber formed with a tube plate at each end, into which small tubes, usually of brass, are inserted. A circulating pump forces cold water through these tubes, while the steam conducted into the condenser passes around the outside of the tubes, thus becoming condensed.

The water from condensation and also air within the chamber, which if allowed to accumulate would gradually destroy the vacuum, are removed by the air-pump. The jet condenser is generally the most convenient form of condensing apparatus for underground engines. It is easily applied, effective in use and occupies less room than a surface condenser. When worked by the main engine, it is only necessary to have an extension of the bed-plate. Where space is available an independent jet condenser may be most usefully applied, as after a stoppage of the engine, the air-pump can be started to work and get a good vacuum ready for the main engine.

COMPOUND STEAM-CYLINDER, EXPANSIVE, CONDENSING ENGINES, WORKING DOUBLE-ACTING PISTON-PUMP.-In Fig. 544 is shown an arrangement of piston pump in which the low-pressure cylinder is in front of the high-pressure cylinder, with a distance-piece between. The pump is behind the high-pressure cylinder, and the steam-cylinders and the water-cylinders are in a line. The illustration does not show a condenser.

The "Davidson" Compound Piston-Pump-An elevation of this engine is shown in Fig. 545. The two steam-cylinders adjoin, the back end of the low-pressure forming the front end of the high-pressure cylinder. The steam and water-cylinders are connected and held together by two distance-bars, as shown in the illustration, one over and the other under the piston-rod. A condenser is not shown.

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The Cornish" Compound Piston-Pump.-This pump is of the double-acting bucket type, with leather or hydroderma packings, and fitted with Evans's patent rotative disc-valves. The peculiarity of this engine is in its compounding arrangement, whereby one steam-cylinder, one steam-chest, and one slide-valve only are employed, thus avoiding the loss of steam which attends the use of connecting pipes between the cylinders, also in the ports and clearances.

The engine is made by Messrs. Jos. Evans & Sons, Wolverhampton, suitable for heads up to 300 feet and steam-pressures up to 120 lbs. per square inch.

An engine with a high-pressure cylinder of 14 inches and a low-pressure cylinder of 24 inches in diameter, and 10-inch diameter water-cylinder, and 12 inches length of stroke, has a capacity of 20,400 gallons of water per hour raised 290 feet with 60 lbs. pressure of steam when driven at a piston speed of 100 feet per minute. Under the same conditions of steam-pressure and piston-speed an engine with a high-pressure cylinder of 19 inches and a low-pressure cylinder of 32 inches in diameter, and 14-inch diameter water-cylinder and 12 inches length of stroke, has a capacity of 39,980 gallons of water per hour raised 265 feet.

COMPOUND STEAM-CYLINDER, EXPANSIVE, CONDENSING ENGINES, WORKING DOUBLE-ACTING RAM-PUMP.-Davey's" Differential" Compound Plunger-Pump. -Mr. Davey's engine is made in different forms by Messrs. Hathorn, Davey & Co., Leeds. All are called "Differential" on account of the very ingenious and valuable arrangement of the valve gear for proportioning the supply of steam to the varying pressures of the working load.

The differential engine exists in three distinct types, viz., the single-cylinder, the compound and the triple-expansion engine, the last, admitting of being worked with very high-pressure steam (200 lbs.) and high degrees of expansion, is capable of realising the greatest economy of fuel. The chief peculiarity in the invention is the simple manner in which the engine is made perfectly safe in working under all conditions of load, automatically and instantly varying its supply of steam with every minute increase or decrease of resistance, the distribution of steam being such that the pumping is performed without shock, even when the pressures suddenly and greatly vary.

There are two forms of single-cylinder engines; the Differential Steam Pump and the Single-Cylinder Differential Pumping Engine, both for use underground. These have been already noticed. There are three forms of compound-cylinder

engines: Vertical and Horizontal Engines, used at the surface for actuating pump-rods, and the Horizontal Engine, applied underground: see Fig. 546. Before describing the last-named engine farther reference will be made to the distinguishing feature of all these engines-the Differential Valve Gear.

"The main principle of the gear is that the valves of the engine have a motion the resultant of two other motions, the first an independent constant motion, and the other the motion of the engine, which may be termed a dependent variable motion. Any erratic movement of the engine resulting from a change of load, or other cause, produces a corresponding variation in the distribution of steam through the resultant motion of the valves."

The principle of the gear as applied to engines having drop-valves and to existing Cornish engines has been explained; see Fig. 380, p. 394.

The Differential valve gear is further illustrated in Figures (547 to 550) which represent the gear as applied to the compound engine. "The diagrams are not drawn to scale, but are intended to show clearly the action of the gear. The main slide-valve, G, is actuated by the piston-rod, through a lever, H, working on a fixed centre, which reduces the motion to the required extent and reverses its direction. The valve-spindle is not coupled direct to this lever, but to an intermediate lever, L, which is jointed to the first lever, H, at one end; the other end, M, is jointed to the piston-rod of a small subsidiary steam-cylinder, J, which has a motion independent of the engine-cylinder; its slide-valve, I, being actuated by a third lever, N, coupled at one end to the intermediate lever, L, and moving on a fixed centre, P, at the other end. The motion of the piston in the subsidiary cylinder, J, is controlled by a cataract cylinder, K, on the same piston-rod, by which the motion of this piston is made uniform throughout the stroke; and the regulating plug, Q, can be adjusted to give any desired time for the stroke.

"The intermediate lever, L, has not any fixed centre of motion, its outer end, M, being jointed to the piston-rod of the subsidiary cylinder, J and the main valve, G, consequently receives a differential motion compounded of the separate motions given to the two ends of the lever L. If this lever had a fixed centre of motion at the outer end, M, the steam would be cut off in the engine-cylinder at a constant point in each stroke, on the closing of the slide-valve by the motion derived from the engine piston-rod; but, inasmuch as the centre of motion at the outer end, M, of the lever shifts in the opposite direction with the movement of the subsidiary piston J, the position of the cut-off point is shifted, and depends upon the position of the subsidiary piston at the moment when the slide-valve closes.