286 SEWAGE AND ITS PURIFICATION 



diverted to the next filter, and after a certain time, the contents 

 of the full filter are discharged by lines of drain pipe into a 

 stoneware main collector running into the discharge valve box. 

 Both the admission and discharge valves are suspended by valve 

 rods from a lever, which is pivoted on a bearing between the 

 admission valve chamber and the discharge well. At one end 

 of this lever is a counterweight, so adjusted as to hold the 

 admission valve open and the discharge valve down on its seat. 

 The gearing used later, while similar to that at Barrhead, 

 embodies several improvements, chiefly stated as follows : 



1. The discharge well and admission valve chamber are 

 made in cast-iron and mounted on a bed plate of the same 

 material so as to be entirely independent of the walls. 



2. The mechanical details of the gear have been greatly 

 simplified. 



3. Before each filter is filled, the tank effluent is held back 

 for a period of from one to two hours, the quantity so 

 accumulated filling the filters in a much shorter time than if 

 it had been allowed to flow continuously. 



This obviates Mansergh's criticism of the automatic gear, 

 that the decreased flow in the night fills one filter so slowly 

 that the corresponding one in the resting-fuU stage remains 

 charged so long as to seriously interfere with its aerobic action. 

 The machinery works without loss of head, so that its employ- 

 ment does not increase the fall which the filters would require 

 if used without gear. 



At Lake Forest, Illinois, and Wauwatosa, Wisconsin, the 

 apparatus used for dosing the sand filters consists of a float 

 which lifts a cannon ball in one of a set of hollow wooden columns 

 arranged in series ; at a certain height the ball rolls through a 

 trough from one column to the next, its passage striking a catch 

 which opens an air-valve attached to one of a series of bell 

 syphons discharging in rotation on the sand filters. 



Various forms of automatic gears are given in English patents 

 2647 of igoo; 5834, 10346 and 11368 of 1901 ; 9247 of 1902. 



We have seen that in the intermittent or holding-up system 

 the main work is done almost anaerobically in ''resting full," 

 and aerobically in " resting empty." The filling drives out 

 carbonic acid and other gases formed by decomposition, the 

 emptying renews the air in the filter bed. The first and last 

 stages of the cycle can be shortened so as to leave more time 

 for the fermentations, but (i) the entrance must not be so rapid 



