The single-chamber tank without a siphon, shown in figure 4, is easy to build, inexpensive, and entirely satisfactory in most instances. In very tight soils or for large installations a siphon and sometimes two chambers are advisable. Size The tank should be large enough to retain the sewage at least 24 hours. The size should be determined by the largest number of persons that may live in the house, rather than by the number actually living there at the time the tank is built. The additional cost of a large tank over a small one is relatively little. If there is any question as to which of two sizes should be built, it is wise to choose the larger. The dimensions recommended in the table in figure 4 are based on an average production of 50 gallons of sewage per person per day. Unusually large quantities of sewage call for a tank of large capacity. In village and suburban homes where there is less food preparation than on farms and where the number of persons is more or less fixed, slightly smaller sizes will serve. In no case should the capacity of the tank below the flow line be less than 500 gallons. A tank length of two to three times the width should be maintained, and it is advisable to provide a depth of at least 4 feet below the flow line. Allow about 1 foot of "freeboard," or air space, above the flow line for the accumulation of gases. This space is generally vented through the soil stack of the house. A siphon (fig. 5) with a dosing chamber is not considered necessary for a farm septic tank except for large installations (1,000 gallons or more), for those in tight soils, and where the disposal field is limited. [Illustration]