SANITATION 



57 



or other building. While the principle has remained, and is likely 

 to remain the same, the details of the construction of traps have 

 undergone great modification and improvement and in place of the 

 MTV insanitary and dangerous fittings of former days, which were 

 nothing less than hidden cesspools of the worst type, there are in 

 u^e to-day traps of simple design which effectually do their work 

 with the minimum of defects. 



If a length of pipe be bent as in figure 18 and placed hori- 

 zontally or on a slight gradient it will be obvious that if water flows 

 through it on account of 

 the gradient given to the 

 line of piping and the 

 trap, a body of water will 

 remain in the bent portion 

 after the influx has ceased, 

 provided, of course, that 

 the gradient be not too 

 steep. As the upper part 

 of the pipe has been bent 

 correspondingly with the 

 lower, or invert, it will be 

 plain that the bent-in por- 

 tion will project below the 

 level of the outlet and con- 

 sequently will dip into the 

 body of water which is 

 retained. So long as 

 there is water standing 

 well above the lowest 

 point of the bent-in por- 

 tion of the pipe, no gases 

 can find their way up 

 the drain without passing 

 through the retained 

 water which they do not do unless they accumulate under great 

 pressure and force their way through. If the pipe or trap is not 

 frequently flushed the stagnant water may, of course, absorb the 

 gases from the sewer side and pass them through. Under ordinary 

 conditions, however, this body of water forms a " seal " which shuts 

 off the drain on its distal or sewer side from the proximal or house 

 side. 



The reliability of a trap depends in the first place upon the depth 

 to which the in-bent portion or lip dips into the water. This dip- 



FIG. 18. Three sections of Simple Syphon 

 Traps. The top figure shows a satis- 

 factory seal A-B, the next figure shows a 

 seal that is too shallow, and the bottom 

 section a pipe that is not sufficiently bent 

 to form a seal. 



