A plant for use all year round should have two chambers, one to secure settlement and septicization of the solids and the other to secure periodic discharge of the effluent by the use of an automatic sewage siphon. The first chamber is known as the settling chamber, the second as the siphon or dosing chamber. The siphon chamber is often omitted and the effluent is allowed to dribble away through subsurface tile, as illustrated in Figure 16. The latter procedure is not generally advised, but may be permissible where the land slopes sharply or has long periods of rest, as at summer houses and camps. The septic tanks shown in this bulletin are designed to satisfy the following conditions: 1. Water consumption of 40 gallons per person per day of 24 hours. 2. A detention period of about 24 hours; that is, the capacity of the settling chamber below the flow line is approximately equal to the quantity of sewage discharged from the house in 24 hours. 3. Where a siphon chamber is provided, its size is such that the dose of sewage shall be approximately equal to 20 gallons per person; that is, the capacity of the siphon chamber between the discharge and low-water lines is roughly equal to the quantity of sewage discharged in 12 hours. A simple one-chamber brick tank suitable for a household discharging 180 to 280 gallons of sewage daily is shown in Figure 21. A small two-chamber tank constructed of 24-inch vitrified pipe, suitable for a household discharging about 125 gallons of sewage daily, is shown in Figure 22. A typical two-chamber concrete tank is shown in Figure 23. Excepting the submerged outlet, all pipes within the tank and built into the masonry are cast-iron soil pipe with cast-iron fittings. Vitrified or concrete sewer pipe and specials are generally used, as they are frequently more readily obtainable and a slight saving in first cost may be effected. Cast iron is less liable to be broken in handling or after being set rigidly in masonry, and the joints are more easily made water-tight. The submerged outlet is midway of the depth of liquid in the settling chamber. The inside depth of the siphon chamber is the drawing depth of the siphon plus 1 foot 5 inches. The following table gives the principal dimensions with quantities of materials for four sizes of tank as illustrated in Figure 23: _Dimensions and quantities for septic tanks_