284 



SEWAGE AND ITS PURIFICATION 



filled, until the air contained in its attached dome E is transferred 

 by the pressure of liquid around it, as the bed fills, through a 

 trapped air-pipe to the interior of the feed, creating an air- 

 lock, and blocking the further passage of liquid so that the 

 latter rises to a higher level in the source of supply until the 

 inlet sluice to the next bed is reached. Where an automatic 

 discharge is also used (Figs. 33, 34), the liquid contents of the 

 first bed will in the meantime have been discharged through 

 syphon M, and the overdraw pipe K attached. A tap L delivers 



Fig. 33. — Section of Automatic Discharge by Adams' Syphon. 



-G^ 





B^ 



H?-,.™. 



PLFIN 



Fig. 34. — Plan. 



liquid to the syphon chamber, and the time occupied in the 

 filling of this chamber will be that for which the sewage is held 

 up on the bed in contact with the filtering material. The fact 

 that the feed and discharge apparatus are quite distinct, enables 

 users to fix any desired time of contact. The second bed thus 

 fills and its liquid in turn displaces air from dome R (Fig. 34) ; 

 thus air is transferred to the feed C, its added bulk forcing the 

 water seal, freeing the confined air, and again bringing on the 

 supply to the first bed. At N is a pipe dotted, through which 

 any accumulation in tank B may be drawn off by valve O. 

 As applied to continuous filters, the feed C discharging to 



