SOME REMAEKS ON SEWERAGE SYSTEMS. 103 



that a 5 per cent, reduction will probably cover the latter. 

 This gives an average theoretical velocity of about G41 feet 

 per minute, and an average discharge of about 105"8 cubic 

 feet per minute, which entering a 9' sewer laid at a gradient 

 of 1 in 100, would fill it nearly full, and maintain a velocity 

 of 240 feet per minute, sufficient to remove broken stones 

 and road detritus. 



In practice, however, this discharge is not quite so great, 

 as it takes an appreciable time after the water begins to 

 flow over the lip of the siphon before it bursts into full 

 action ; and again, the velocity is largely checked by the 

 weir into which the siphon dips. 



In a practical observation on a 1,000 gallon tank, erected 

 by the author at Redruth, with a 5|" discharge leg, and a 

 nett quantity of 154 cubic feet of water within reach of the 

 siphon, the time taken to discharge from the first dribble 

 over the annular ring was 3J minutes, giving an average 

 discharge of 44 cubic feet per minute. When at the height 

 the flush three-quarters filled a 9" pipe laid at a gradient 

 of 1 in 40, which would give a discharge of about 120 cubic 

 feet per minute. 



In most sewer systems of the present day, but not all, ven- 

 tilators are in use, so as to give free communication between 

 the external air and the sewer. These ventilators have long 

 been a bone of contention. At one time they were almost uni- 

 versally in the shape of gratings at the level of the streets ; 

 but in many places they are now carried up above the noses 

 of the public in pipes or shafts. In Carlisle the sewers are 

 connected with many of the factory chimneys, which form 

 powerful upcast shafts. It must be remembered, however, 

 that, owing to friction in the sewer, the area of their in- 

 fluence is not so large as we should theoretically suppose. 



A very promising, though apparently expensive form of 



