58 



VETERINARY HYGIENE 



ping in of the lip constitutes the seal, and the depth of the seal 

 is measured by the distance to which the lip dips into the water. 

 If the pipe is not sufficiently bent, or in other words if the lip does 

 not dip sufficiently into the water, a seal too shallow for safety is 

 formed as it may be broken with very little reduction in the volume 

 of water as is indicated in the figure. 



Definition of a Good Trap. A satisfactory trap must effect- 

 ually prevent sewer gases under ordinary circumstances from pass- 

 ing up the inlet pipes. It must be self-cleansing, so that a moderate 

 influx of water should carry all the trap water before it and leave 

 no sediment behind. It must be of simple structure, have no mov- 

 able parts and no sharp angles or corners. The internal diameter of 



w<n 

 fete 



FIG. 19. A Dip-Stone or Mason's Trap showing the accumulation of sedi- 

 ment. The back passage of sewer gas over the top of the dip-stone is 

 indicated by the arrows. 



a trap should be the same throughout, except in the case of some 

 special forms. It must have a square base so that it can be set firm 

 in the ground without there being any risk of tilting, with conse- 

 quent reduction in the depth of water in the seal. The inlets and 

 outlets must be capable of forming perfect joints. An air inlet 

 hole for ventilation must be provided on the proximal or house side, 

 and a hand hole on the distal side for the purpose of removing any 

 possible chokage. 



Traps should be so laid as to be readily accessible. The depth 

 of seal should be such as to obviate any ordinary risk of reduction 

 to danger point by evaporation or waving ; a suitable depth is from 

 2 to 3 inches. The trap should contain just sufficient water, and 

 no more, to guarantee a safe seal. If the depth of seal is too great, 

 or too much water is retained in the body of the trap, there is a 



