This connection is a neat and inexpensive condensing arrangement (not shown in the figure), and does away with the exhaust steam-pipe in the shaft.

With a pumping plant similar to that described, mines can be sunk to great depths, or flooded mines recovered with facility and security. For mines of extreme depth it has been found advisable to put pumping stations at every 500

to 600 feet, although this distance can be increased to 800 to 1,000 feet when desired. By this system of pumping in stages, which is similar in principle to the Cornish pump-lifts, the main engine in the lower station delivers to the next above (which is a duplicate of the same), and so on to the surface. Below the lowest pump station is placed the sinking pump, as referred to above. Generally these sinking-pumps are duplicated for convenience and security.

Fig, 561,-THE WORTHINGTON COMPOUND PLUNGER-PUMP (COMPOUND DUPI EX).

Non-Rotary Vertical Engines.-Compound steam-pumps of this class are not much used for mining purposes, being more suitable for boiler-feeding, factories, and general purposes at the surface than for underground pumping. A description of them, therefore, is unnecessary.

Triple-Expansion Steam-Pumps.-Very few triple-expansion engines of any type are used underground. Although increased economy can be obtained from one of these engines compared with a compound engine, it is questionable

whether it is judicious to go to the additional cost of the engine, and of the greater excavation space required underground in order to secure the more economical work. Besides the extra cost involved, there is greater complication of parts, and, therefore, increased liability to injury and disablement.

In order to get the full benefit of the higher range of expansion, an increased initial pressure of the steam is necessary; the pressures used vary from about 100 lbs. per square inch, which in ordinary cases may be taken as the minimum useful pressure, to about 200 lbs. per square inch, absolute. Triple-expansion engines are extensively used for marine and waterworks purposes, and to great advantage, very high duties being achieved. The steam consumption varies according to the design of the engine and the circumstances of its use, from about 1175 to 16 lbs. per indicated horse-power per hour, or, say (assuming the evaporative power of the coal to be 9 lbs. of water per pound of coal), from 13 to 18 lb. of coal.

Fig. 565 is an elevation of the Worthington triple