Estimating 75 gallons daily of sewerage for each inhabitant of the house and four persons to a family, the septic tank should be large enough to hold 600 gallons, three hundred gallons in each compartment, which would require a tank about four feet in width and six feet in length and four feet in depth. These figures embrace more cubic feet of tank than necessary to meet the foregoing requirements. It is a good plan to leave a margin of safety. [Illustration: Figure 239.--Septic Tank, a double antiseptic process for purifying sewerage.] It is usual to lay a vitrified sewer, four inches in diameter, from below the bottom of the cellar to the septic tank, giving it a fall of one-eighth inch in ten feet. The sewer enters the tank at the top of the standing liquid and delivers the fresh sewerage from the house through an elbow and a leg of pipe that reaches to within about six inches of the bottom of the tank. The reason for this is to admit fresh sewerage without disturbing the scum on the surface of the liquid in the tank. The scum is a protection for the bacteria. It helps them to carry on their work of destruction. The same principle applies to the second compartment. The liquid from the first compartment is carried over into the second compartment by means of a bent pipe in the form of a siphon which fills up gradually and empties automatically when the liquid in the first compartment rises to a certain level. The discharging siphon leg should be the shortest. The liquid from the second compartment is discharged into the drain in the same manner. There are special valves made for the final discharge, but they are not necessary. The bottom of the tank is dug deep enough to hold sewerage from two to four feet in depth. The top surface of the liquid in the tank is held down to a level of at least six inches below the bottom of the cellar. So there is no possible chance of the house sewer filling and backing up towards the house. Usually the vitrified sewer pipe is four inches in diameter, the septic tank siphons for a small tank are three inches in diameter and the final discharge pipe is three inches in diameter, with a rapid fall for the first ten feet after leaving the tank. Septic tanks should be made of concrete, waterproofed on the inside to prevent the possibility of seepage. Septic tank tops are made of reinforced concrete with manhole openings. Also the manhole covers are made of reinforced concrete, either beveled to fit the openings or made considerably larger than the opening, so that they sit down flat on the top surface of the tank. These covers are always deep enough down in the ground so that when covered over the earth holds them in place.