372 VETERINAEY HYGIENE 



communicates with the interior of a drain. It is a con- 

 trivance "which holds water, and so arranged that the water 

 acts as a seal between the building and the drain ; of itself 

 it is insufficient protection against sewer air, hence the 

 reason for disconnection just discussed, but as a valuable 

 accessory it is indispensable. 



Traps without ventilation cannot be depended upon, for 

 the reason that the pressure of gas in the sewer pipe may 

 be sufficient to force the seal. Or the water in the trap 

 may absorb the sewer gas at one side and give it off at 

 the other. Further, evaporation may cause the water to 

 fall below the tongue of the trap, and thus open the passage 

 for sewer air. 



p, C ^ 



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rig. 157. — Buchan's Disconnecting Syphon. Tlie sewage enters at A and 

 escapes into the sewer pipe at B. I) is an access opening in case of an 

 obstruction. communicates with the outside air, and ensures tlie air in 

 the stable pipe being pure. 



Traps are of various kinds and their construction depends 

 upon where they are to be placed. Fig. 167 is a syphon trap ; 

 the arrows show the inlet and outlet, between which is 

 interposed a volume of water which should stand not less 

 than J inch above the highest level of the water in the 

 bend. The interior of a syphon must be smooth, the descent 

 should be sharp, the ascent more gradual, so as to offer no 

 impediment to the flow ; the fiat bottom ensures it being 

 laid level, for on no account must the outlet be higher than 

 the inlet. In Fig. 157, two openings besides inlet and outlet 

 are seen. This is a disconnecting syphon ; the opening near 

 to the entry is carried up to the level of the ground and is 

 covered by a grating (Fig. 158) ; it is an air inlet, and if 

 necessary a foul gas outlet. The opening near the outlet is 

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