220 



SEWER DESIGN 



ference of water-level in the two legs of the trap equals the 

 difference of the levels between the water in the tank and the 

 water within the bell, it will be seen that the column of water 



FIG. 64. 



in the short discharge leg has practically the same depth as the 

 head of water in the tank above the level at which it stands in 

 the bell. The two columns of water therefore counterbalance 

 each other at a certain fixed depth in the tank. As soon as. 

 this depth is increased by a further supply, however small, 



