Fig. 23 shows three similar siphons installed with some auxiliary piping attached for the purpose of making the periodic discharge more positive. These small auxiliary pipes are so put together that there is an auxiliary siphon passing under the edge of the bells. When one siphon discharges, the auxiliary siphon of the corresponding large siphon is filled with water, and at the same time part of the water in the auxiliary siphon of the other is discharged, so that it will be the first to operate at the next filling. When the water is forced to the bottom of the small siphon, it is blown out through the vent pipe, and, the air following, the large siphon is started. [Illustration: FIG. 23.—Triple Alternating Siphons of the Miller Type.] [Illustration: FIG. 24.—Single “Merritt” Automatic Siphon.] Fig. 24 shows an automatic discharging siphon made by the Merritt Company, of Camden, N. J., and embodying a different principle. The main discharge pipe is built in the form of a “U” tube, the longer leg containing an auxiliary small air pipe, with a return bend at its lower end. When the chamber starts to fill, this small pipe bend or seal is filled with water, so that the rising water confines and compresses air in both the large and small “U” pipes. In time, and at any desired height, determined by changing the relative lengths of the parts of the small pipe siphon, the seal is broken and the air escaping draws air enough from the large pipe to start it in action. The method has an advantage in that it requires no deep excavation, and the mechanism can be set after the siphon chamber is built. [Illustration: FIG. 25.—Air-lock Siphon for Admitting and Releasing Sewage from each one of Four Beds in Regular Order.]