24 SEWAGE AND ITS PURIFICATION 



in a V notch is as the square of the height from the vertex, and 

 the volume discharged is given by the formula 



Q = 0-305 H|, 



where Q = cubic feet per minute and H the height in inches 

 from the vertex of the notch up to still-water level. Thomson 

 found that when H = 12 inches the flow was 2*54 cubic feet 

 per second, so that by measuring H in feet, taking this figure 

 to the I power, and multiplying by 2*54, the flow in cubic feet 

 per second is obtained. 



The formula in general use for discharge over a weir is 

 C = 3*566 JW if the sewage above the weir is not in motion, or 

 3*566 VH^ + o*035V^H"^ if it is in motion, H being the height of 

 the surface of the sewage above the edge of the weir, V being 

 the velocity in feet per second of sewage approaching the weir, 

 and C being the cubic feet per second discharged over each 

 foot in width of the weir. 



In those cases in which a permanent record of the flow 

 through pipes is required, a meter like the Parkinson's low 

 pressure or the Venturi will, if read at fixed intervals, give the 

 quantity of sewage dealt with. 



The diagrams (Figs. 2 and 3) show various arrangements of the 

 Venturi meter in connection with sewage works. Fig. 2 shows 

 the meter in a closed pipe, running on so steep a gradiant that 

 when the flow is small the pipe would not run full. To obviate 

 this the tube has been placed in an inverted siphon. The 

 recorder is, where the pressure permits, of the water column 

 type, the result being given either by counter showing the 

 total quantity passed, or by diagram showing the rate of flow 

 at any time, or by a combined recorder giving both results. 

 Fig. 3 illustrates a method of installing the instrument in an 

 open channel. The tube is sunk below the low-water line to 

 insure its being full at all times, and a low concrete dam is built 

 across the channel, into which the mouth of the tube is placed. 

 It is possible to employ the Venturi meter in the same works 

 in both of the ways previously described, some of the meters 

 being connected to closed pipes and others being placed in 

 open channels. 



A new form of water-meter of the proportional displacement 

 type, known as the '* Premier " meter, is in use in the United 

 States. It consists of a large and a small Venturi tube merging 

 into one at their throats, and having a common down-stream 



