DRAINAGE 



269 



as shown in Fig. 207. The grease trap is provided with a perforated 

 tray or baler, similar to that in a stable gulley trap, which intercepts 

 solids. 



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ton* 

 Ml 



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Fig. 207. 



The discharge from a cow-byre may be arranged to take place into 

 a sump (or two sumps in the case of a double byre) in which solid matter 

 settles. The sewer gases are shut off by an intercepting trap, such as 

 that shown in Fig. 208. 



In all the above cases the discharge into gulley, trap, or sump, should 

 take place freely in the open air. 



Any reasonable length of drain pipe should be ventilated, and for 

 that purpose fresh air inlets and foul ah' outlets should be provided. 

 These are shown in Fig. 207. The ventilation prevents the accumulation, 

 and consequent rise in pressure, of sewer gas. Were the pipes not 

 ventilated, the gas might force its way past the water seal into the 

 dwelling or stock house served by the drain. A fresh air inlet is pro- 

 vided with a light flap of mica which opens inwards only, and thus, while 

 allowiDg of the free ingress of fresh air, prevents the egress of sewer 

 gases. The mica flap and its containing box are carried on the top of 

 a length of iron pipe, which is supported by a 4£" by 3" wooden post 

 placed behind it. Sometimes, in place of this wooden post, the ventilating 

 pipe is supported in a block of concrete at its base. 



A foul air outlet consists simply of an iron pipe carried well above 



