SANITATION 



55 



this may be provided with a raking arm as shown in the figure. 

 Access covers for fireclay fittings are luted down with a mixture 



. t" lime and cement. 



: i>^-: .''.;'.' ;.>':' VENT PIPE '; ..";-;."'; : ;jr. ,'r' "..',!; '."-] 





FIG. 17. Section of Manhole showing combination of inspection chamber, 

 disconnecting trap with clearing arm, iron steps and manhole cover. 



The Size of Pipes. If a certain volume of water flows at a 

 certain speed through a pipe having a diameter of, say, 4 inches, 

 the same volume of water will flow at a reduced speed through a 

 pipe set at the same gradient but having a greater diameter. The 

 volume of water will be spread over a wider area and the amount 

 of friction would therefore be increased with consequent retardation 

 of the flow. Not only is the flow reduced in speed, but the volume 

 is reduced in depth and the slower the speed and the shallower the 

 sewage the greater opportunity is there for solids to be left behind 

 in the pipes as the liquid passes on. If actual blockage does not 

 occur silting to a certain extent is certain to take place, with the 

 consequence that the stranded filth decomposes in the drain and fills 

 it with noxious sewer gases. Such filth is only removed when an 

 unusual flush takes place. The size of drain pipes should therefore 

 be as small as possible, allowance being made for the estimated 

 amount of " drainage " to be carried and for possible flooding 



