CONVICT LABOR FOR ROAD WORK. 
99 
age. This action is explained by reference to figure 5(b). When a 
considerable body of water rushes down through the pipe A, it forms 
a suction, and if the pipe is made air-tight, this suction is often 
sufficient to prevent enough water remaining in the trap to form a 
seal, thus leaving an opening for the passage of foul gases, as in 
figure 5. By connecting the upper bend of the trap with the outside 
air by means of the vent pipe V, figure 5(a), the suction will be 
stopped and the water in the pipe A will not fall below the level of the 
outlet at b. 1 
Sewage Purification and Disposal. 
In temporary camps provided with running water, it will be satis- 
factory to drain the used water and sewage into a cesspool or pit, 
Fig. 5.— Explanation of the action of the U-trap and vent-pipe. 
located if possible at a considerably lower elevation than the camp 
quarters, and at least 300 feet away from the well or water-source. 
The size of the cesspool will depend on the population of the camp 
and the character of the soil. In an impervious soil the dimensions 
of the pit should be large, else it will fill up rapidly and another will 
have to be dug. When the soil is light and porous enough to per- 
mit the liquid contents of the pool to run off through it, the filling 
up will be less rapid and the dimensions of the pool may be smaller. 
Cesspools always should be covered to confine the objectionable 
odors; and this may be done very simply by means of beams cov- 
Kidder, "Architects' and Builders' Pocketbook," p. 1327. 
