Automatic flush-tanks are constructed similar to a manhole, but special care should be taken to make them water-tight. The apparatus for providing the automatic discharge may operate either with or without moving parts, the latter being preferable as they require less attention and are not so liable to get out of order. An automatic flush-tank of the Miller type is shown in Fig. 37. It is a patented device manufactured by the Pacific Flush Tank Company. The small pipe at the left is a service connection to the water main. Water is allowed to flow continuously into the tank at such a rate as to fill it in the required interval between discharges. The tanks are discharged as nearly once a day as it is practicable to regulate them. The rate of flow into the tank is determined by trial and varies to some extent with the water pressure. The regulator shown in the figure is desirable as the continuous flow through the ordinary cock soon wears it away. Some waters will cause deposits to form in the small passages of the cocks or regulators, thus cutting off the flow. [Illustration: FIG. 37.—Automatic Flush-Tank. Pacific Flush Tank Co. ] The tank operates as follows: when the water rising in the tank reaches the bottom of the bell, air is trapped in the bell and prevented from escaping through the main trap by the water at _A_. As the water continues to rise in the tank the air in the bell is compressed, the water level at _A_ is driven down and water trickles from the siphon at _C_. The height of the water in the tank above the level of the water in the bell is equal at all times to the height of _C_ above the lowering position of _A_. When _A_ reaches the position of _B_ a small amount of air is released through the short leg of the trap and a corresponding volume of water enters the bell. The head of water above the bell then becomes greater than the head of water in the short leg of the trap, which results in the discharge of all of the air in the long leg of the trap and the rapid discharge of the water in the tank through the siphon. The discharge is continued until the siphonic action is broken by the admission of air when the water level in the tank is lowered to the bottom of the bell. The size of the siphons is fixed by the diameter of the leg of the siphon. Table 23 shows the capacity and size of sewers for which the different sizes of siphons are recommended by the manufacturers.[37] TABLE 23 SIZES OF SIPHONS TO BE USED WITH AUTOMATIC FLUSH-TANKS