238 FARM MECHANICS 



in the form of a siphon which fills up gradually and 

 empties automatically when the liquid in the first com- 

 partment 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 bot- 

 tom 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, water- 

 proofed on the inside to prevent the possibility of seep- 

 age. 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 open- 

 ing, 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. 



In laying vitrified sewer it is absolutely necessary to 

 calk each joint with okum or lead, or okum reinforced 

 with cement. It is almost impossible to make a joint 

 tight with cement alone, although it can be done by an 

 expert. Each length of the sewer pipe should be given 

 a uniform grade. The vitrified sewer is trapped out- 

 side of the building with an ordinary S-trap ventilated, 



