102 SOME REMARKS ON SEWERAGE SYSTEMS. 



sewer. The author inclines to the latter method, because 

 during the process of storing the sewage there is a diminu- 

 tion of the velocity and necessarily a depesition, which is 

 not likely to be all washed away when the sewage is 

 allowed to go. 



The sewage is sometimes stored by self-acting gates, 

 which open when it reaches a certain height, or by sluices 

 or penstocks placed in the manholes and lifted by the 

 flushing men. There are several ingenious forms of self- 

 acting flushing apparatus ; the most notable and, in my 

 opinion, the most reliable, being Field's self-acting siphon. 

 I have repeatedly tested these, and found them, when 

 properly fixed, unfailing. The whole secret of success lies 

 in the insertion of an annular ring, through which the 

 smallest dribble of liquid is sufficient to set the siphon in 

 action. It is interesting to notice how thoroughly these 

 siphons do their work when used in tanks. The sewers 

 through which the flush has passed look as if they had been 

 clean swept with a brush. In order to render flushing 

 efiicient, it should take place often, and before any deposit 

 has time to harden. 



The velocity of discharge of the siphon will be pro- 

 portional to the nett length of the discharge leg less the 

 depth of the surface of the water in tank below the top of 

 the leg. As the tank empties this will therefore vary. 

 Taking a 5|" discharge leg 3' 6" long, and reaching 10|" 

 below the bottom of tank, and using the theoretic formula 

 for falling bodies, V= \/2gs, the initial theoretical V will 

 be 15 feet per second, and the final F7*5 feet per second; 

 average, 11*25 feet per second, or 675 feet per minute. 

 This does not allow for friction of the air and in the pipe ; 

 but as the action of the siphon tends to form a vacuum, 

 the former will be nil, and the length of pipe is so small 



