Water-closet flushing cisterns are of three main types-the valve, the siphon, and the tipper. A fourth kind, which may be called the pneumatic, may be used with high-pressure supplies.

In "combination" closets the cisterns are fixed immediately above the seats, and may be either of valve or siphon type, but with specially large outlets. In some valve cisterns of this kind the outlet is 4 in. in diameter, and in siphon cisterns 2 3/4 in. Three-gallon cisterns ought to be used where possible.

Siphonic closets must be fitted with special cisterns, adapted for the several closets, and designed so that the full flush will be discharged whether the handle is continuously held or immediately released. Valve cisterns with floating valves may be used, or suitable siphon cisterns. The capacity ought to be that recommended by the maker of the closet. Some makers say that their siphonic closets will work "perfectly" with a 2-gallon flush, but at the same time they "strongly" recommend a 3-gallon cistern "where permitted by the water companies." This strong recommendation seems to throw some doubt on the perfection of the action with a 2-gallon flush, and the larger cistern ought therefore to be used.

Fig. 115. Single-valve Cistern of Wood lined with Lead.

1. Valve Cisterns.- The simplest form of cistern contains a single valve. Fig. 115 shows a wooden cistern of this kind lined with lead. When the lever at A is depressed, the valve B is raised from its seating, and the water continues to flow down the flush-pipe as long as the lever is depressed or until the cistern is empty. In order to prevent a continuous flow of water, the flushing lever in this example is attached to the lever of the ball tap by the link C, so that the ball is raised and the water shut off as long as the flushing lever is depressed at A. An arrangement of this sort is demanded by many water companies. The inlet is shown at D and the overflow at E.

Double-valve cisterns consist of two chambers, usually formed by means of a longitudinal partition; in the first chamber the inlet and ball-cock are placed, and also a valve fitted on a small compartment communicating with the second chamber, in which the flushing valve is placed. The two valves are so connected to the flushing lever that one is closed while the other is open. Fig. 116 shows such a cistern fitted with an after-flush box, for use with a valve-closet. When the cistern is being filled the valve A is open, and the water passes through the opening into the small compartment B, and thence into the second chamber of the cistern, until both are filled to the level at which the ball-valve is closed. When the flushing lever at C is depressed it closes the small valve A, and thus closes the communication between the two chambers, and at the same time opens the flushing valve D, so that the water in the second chamber pours down into the after-flush box E and the flush-pipe F. On releasing the lever, the valve D is closed and the valve A opened, and the water stored in the first chamber passes into the second, thus lowering the ball and admitting a fresh supply. A small hole G in the flush-pipe, near the bottom of the after-flush box, allows the water in this to drain away to the basin after the valve D is closed; an air-pipe carried up from the box to the top of the cistern supplies the necessary air.

Fig. 116. Cast-iron Double-valve Cistern with After-flush Chamber.

Sometimes the two chambers are above each other, the lower containing the amount allowed for flushing. In cisterns of this kind a standpipe passes upwards from the top of the lower chamber through the upper chamber to a point above the water-level, and the spindle attached to the flushing-valve is carried down the stand-pipe, which also serves as an air-pipe for the lower chamber.

The upper chamber is sometimes large enough to hold six or more gallons, an arrangement which is advantageous where the water-supply is intermittent or at a low pressure.

It is obvious that if the flushing lever of an ordinary single or double-valve cistern is released too soon, only a portion of the water will be discharged, and although this may be sufficient to clear the basin, it may be utterly inadequate to flush the soil through the closet-trap and drain. To remedy this defect a float is, in some cisterns, attached to the flushing-valve, so that this does not return to its seating as soon as the handle is released; this device ensures the discharge of practically the full flush every time the handle is pulled.

The chief advantage of valve-cisterns is that they are less noisy than the common siphon-cisterns, which are objectionable on account of the loud gurgling and sucking sound caused by the inrush of air as the water reaches the foot of the short leg of the siphon. The double-valve cistern has the further advantage of being more silently filled, especially if a pipe is carried from the outlet of the ball-valve to the bottom of the first chamber, as the end of this will be always below water. Another advantage of the valve-cistern is that it can be operated at any stage of the filling; this is often a great convenience, as the first flush does not invariably cleanse the basin. Siphon-cisterns cannot be discharged till the water has reached a certain level, and in many cases several minutes must elapse before a second flush can be given.

2. Siphon Cisterns. - The water in these is discharged by siphonic action, and the principal difference between the various kinds lies in the method of starting this action. Four methods are in general use-the valve, the dome, the plunger, and the displacer. In the valve-siphon cistern (fig. 117), the depression of the flushing lever A raises the valve B and admits water into the long leg of the siphon; on releasing the lever the valve closes, and the greater portion of the remaining water is drawn up the short leg C and siphoned out. If, on the other hand, the lever A is held down, the water continues to flow through the open valve B (exactly as in an ordinary single-valve cistern), but the discharge is less rapid and less complete than when the siphon is in action. In the illustration the cistern is of cast-iron and the siphon of brass; in cheaper cisterns the siphon is of cast-iron. A link connects the flushing lever and the ball lever, so that the latter is held up and the water shut off as long as the end A of the flushing lever is depressed. An objection often raised to this type of cistern is that the valve may get out of order; on the other hand the valve is noiseless in action and starts the siphonage most effectually.