=Siphons.=--Reference has already been made to the vital importance of air in sewage filtration. If the spaces within a filter or soil are constantly filled with water, air is excluded, and the action of the filtering material is merely that of a mechanical strainer with its clogging tendency. The purpose of a siphon is twofold: (1) To secure intermittent discharge, thus allowing a considerable period of time for one dose to work off in the soil and for air to enter the soil spaces before another flush is received; (2) to secure distribution over a larger area and in a more even manner than where the sewage is allowed to dribble and produce the conditions of the old-fashioned sink drain--namely, a small area of water-logged ground. [Illustration: Fig. 22.--Two-chamber septic tank, simple and inexpensive, constructed of 24-inch vitrified sewer pipe, size suitable for 125 gallons of sewage daily (nominally 3 persons). _A_, House sewer; _B_, settling chamber, made of double =T= branch and one length of straight pipe, each 3 feet long and 2 feet in diameter, supported by 4 inches of concrete, all joints made water-tight; _C_, submerged outlet, consisting of a metal =T= slipped into the sewer-pipe branch; _D_, wire screen ¼-inch mesh; _E_, siphon chamber made of one =T= branch 3 feet long and 2 feet in diameter; _F_, siphon; _G_, 3-inch overflow; _H_, sewer to distribution field; _I_, tight cover with lifting ring; _J_, concrete protection around sewer-pipe hubs] [Illustration: Fig. 23.--Typical two-chamber concrete septic tank. (See table for dimensions and quantities for different sizes)] Three types of sewage siphon are shown in Figure 24. In all, the essential principle is the same: A column of air is entrapped between two columns of water; when the water in the chamber rises to a predetermined height, called the discharge line, the pressure forces out the confined air, destroying the balance and causing a rush of water through the siphon to the sewer. The entire operation is automatic and very simple. The siphons shown are commercial products made of cast-iron; they have few parts and none that move, and the whole construction is simple and durable. The table (fig. 24) lists stock sizes adapted to farm use. Manufacturers furnish full information for setting their siphons and putting them in operation. For example, take type 2, Figure 24: (1) Set siphon trap (=U=-shaped pipe) plumb, making E (height from floor to top of long leg) as specified; (2) fill siphon trap with water till it begins to run out at B; (3) place bell in position on top of long leg, and the siphon is ready for service. Do not fill vent pipe on side of bell. [Illustration: Fig. 24.--Three types of sewage siphon. The table gives dimensions for setting standard 3 and 4 inch siphons; also the appropriate size and grade of the sewer to carry the siphon discharge]