of a disc to fit the pump-barrel, there may be, as in Figure 2 (pl. a pole or a plunger working in a stuffing-box, and operating by means of its volume rather than by reason of the space through which it passes.
Differential combination of the plunger with the valved piston is shown in Figure 3, which represents the special form used in the Berlin water-works for pumping to the filter the water to be clar ified. The suction-pipe, seen in the centre of the Figure, is here by reason of its great size given a form differing from that of the ordinary valved piston. Its valve, like the suction-valve below it, consists of two steps, each of which contains a horizontal annular grating for the support of the rubber " clacks" or valves proper. By the ascent of the double piston the entire quantity of water delivered by the forward find backward motion is sucked, and the quantity of water contained in the annular space formed by the plunger (which is hollow) and the barrel is simultaneously raised, while by the descent of the piston the portion of the mass of the water passing through the valved piston and not finding room in the annular space is lifted. This portion is exactly one-half the entire mass, so that an equal quantity of water is delivered by the ascent and by the descent. The discharge-valve or delivery-valve, seen at the right on the top of the Figure, has several clacks.
the single-acting pump, in which the water is raised on the upward stroke only and discharged upon the down ward stroke only, through the differential pump, in which it is raised upon only the upward stroke and discharged upon both the upward and the downward, there is a natural transition to the double-acting pump, in which the fluid is taken in and discharged upon each stroke. This is effected by providing each end of the barrel with an inlet-valve and also with a discharge-valve, so that while one end is taking in liquid the other is discharging it under pressure. In this case it is generally best to place the barrel horizontally. A good example is shown in Figure 4 (/5/. l 13).
—Thc pole- or plunger-pump is made double-acting by providing it at each end with an inlet- and with a discharge-valve, and by separating the barrel into two portions by all annular partition through which the plunger passes air-tight. Figure S
shows one of this type.
—One of the simplest forms of valve is the plain clack or clap pet, consisting of a flat piece of leather or of soft rubber somewhat larger than the aperture which it is to cover, and fastened at one side, so that it can readily open and close. It is best to weight the moving part with a disc of metal to ensure its seating promptly and remaining closed when it should be closed. A simpler valve consists of a ball which lies upon a circular aperture which may or may not be countersunk to afford a seat. A suitable cage or other restraining device prevents its being raised too far or carried out of its place. One form of valve in very common use consists of a metal disc which covers a circular aperture, its rising and falling being guided by a central stem or pin. Sometimes it is aided in seating by a spiral spring around its stem. There are numerous other forms and types of valve: many pumps have, instead of one valve for the suction and one for the discharge, several having a united area equal to or greater than that which would be given to a single valve.
The double-seat bell-shaped valve (A/. S3, fig.') is used for very large pumps, and for still larger ones there is employed the step-valve (pl.
fig. 3) with horizontal or obliquely-placed clacks, the lumber of steps increasing with the size of the pump. The gill-valve of the pump of the East London water-works 6) has nine such steps, and has the enor mous diameter of 31 feet.
the discharge of almost any reciprocating pump, and particularly that of one only single-acting, would be very irregular, there is added to reciprocating force-pumps, for the purpose of steadying the flow, what is known as an " air-chamber "—a spherical, cylindrical, or inverted-pear-shaped vessel in connection with the discharge-pipe (gene rally between the pump-barrel and the delivery-valve), which, by reason of the elasticity of the air within it, acts as a sort of cushion or spring to the pulsating liquid. Dranghting-pumps which have a quick piston speed are also supplied with a similar although smaller air-chamber, placed on the suction side and called the "suction-chamber." Both suction- and discharge-chambers may be seen on several of the pumps shown in the illustrations.