Fig. 15 shows another type of valve which is supplied by some firms making sewer pipe and consists, as may be seen, of a light moving valve which is attached to a projection cast on the top of the vitrified tile pipe in such a way that the valve comes to an even seat on the bevelled end of the pipe. It is found that with pressure acting against the valve the thin metal of which it is composed is pressed against the pipe so that little, if any, water or sewage will escape. The valve can easily be opened by attaching a cord or chain to the ring at the lower edge of the valve, and when released the valve shuts automatically. This is a very cheap and convenient design, and answers every purpose for emptying tanks by hand. More elaborate structures of the same general type have been made, using cast iron as the metal, the stationary collar with the bevelled end being built into the masonry wall of the tank. This type of flap valve is faced with bronze, and the bearings or joints have bronze bushings. A satisfactory valve of this sort can be made at a local foundry and machine shop, but there is danger that the valve will not be water-tight. Fig. 16 shows such a valve with the metal seat which is intended to be bolted into the masonry of the tank wall. [Illustration: FIG. 15.—Flap Valve Attached to Length of Sewer Pipe.] Fig. 17 shows another form of this same sort of valve, taken from the catalogue of the Adams Hydraulic Company, and noteworthy because of the loose-link connection at the upper part of the valve, the object of this being to prevent the valve closing at the upper part without, at the same time, closing at the bottom. If the dosing tank is to work automatically and independently of human agency, an arrangement which is always preferable, there must be installed some mechanism which takes the place of the valve operated by hand. This mechanism is in almost every case a siphon which is put into action when the water level reaches a certain height, and which discharges rapidly until the water falls to a point when air is admitted to the inside of the siphon pipe, thereby interrupting the flow. [Illustration: FIG. 16.—Flap Valve with Metallic Seat Attached.]