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MODERN STEAM PRACTICE.

placed across the pumps, in communication with the valve chests,having one vertical delivery pipe placed at the end.

THE CENTRIFUGAL PUMP.

Centrifugal pumps have been much used for pumping water outof works in course of construction, and are recommended for theirsimplicity and the ease with which they can be applied to almostany Situation. The pump, when placed in position, is driven by abeit from the fly wheel of a portable engine. This is a temporaryarrangement; but many centrifugal pumps have been fitted up ofa permanent kind, driven by wheel gearing. When the lift is ofmoderate height these pumps throw a vast body of water; and asthey are not so liable to get choked with foreign matter, they maybe used in many situations for pumping sewage with advantage.

Fig. 149 represents a pair of centrifugal pumps of the largest sizefor drainage purposes connected directly to the engines, a dass ofmachinery brought to great perfection by the Messrs. Gwynne & Co.of London . This description of pump is admirably adapted forworks of construction, water works, graving docks, &c., and moreespecially for drainage purposes, large tracts of land having beenreclaimed by its aid. Where low lifts only are required it far eclipsesthe ponderous pumping engine of the beam type.

The construction of this pump is very simple. The revolvingwheel or disc is formed of two concave plates, placed parallel withtheir concave surfaces towards each other. Two saucers, placedin corresponding positions, will give an idea of the arrangement.Between these discs is an arm or impeller, radiating from a boss orhollow axis, mounted on a shaft which works horizontally, vertically,or at any intermediate angle. This impeller, which regulates thedistance between the discs, varies in breadth; its narrowest part isat the outer edge of the discs, becoming gradually broader untilits edge intersects the inner surface of the openings for the suc-tion. Its breadth is varied in such a ratio that the areas of anysection cut from the wheel by the surfaces of circular cylinders,whose axes coincide with that of the shaft, shall be equal to suchother section at any distance from the centre; and these areas areso arranged in order that the column of water or other fluid enter-ing the wheel when in a state of revolution may have an uninter-rupted flow from the centre to the circumference, and that the