CITY WATER SYSTEMS 



495 



plished by means of the DRIP VALVE at the lowest point of the 

 hydrant. The drip valve is controlled by .a right-hand screw 

 on the lower end of the rod which controls the cutoff valve. 

 When the rod is turned clockwise closing the cutoff valve, it 

 opens the drip valve by raising it as shown in A, Fig. 302. 

 When the rod is turned counter-clockwise opening the cutoff 

 valve, it lowers the drip valve, thus preventing any loss of 

 water at the drip. To insure better drainage, the lower end 

 of the hydrant is usually set in a bed of small stone, broken 

 brick, or coarse gravel. 



580. The Water Pressure-gauge. Water pressure is usually 

 indicated by PRESSURE-GAUGE. The pressure-gauge consists 

 essentially of an elliptically shaped, thin-walled tube bent into 

 a nearly circular form (Fig. 303). 



WTien the pressure within the tube 

 increases, the tube tends to 

 straighten out. This motion is 

 transmitted to the pointer which 

 moves over the face of the dial. 

 The mechanism is so adjusted that 

 the instrument shows directly the 

 pressure per square inch. 



581. The Water Meter. While 

 the pressure-gauge is a necessity 

 at the pumping station indicating 

 to the engineer the exact pressure 

 on the city mains, the majority of 



consumers are more concerned with the water meter. The 

 pressure-gauge merely indicates the pressure under which the 

 water is kept; the water meter indicates the number of gallons 

 or cubic feet of water which flow through the pipes. Gener- 

 ally, the consumer pays for the amount of water consumed, no 

 account being taken of the pressure maintained. This is so, 

 notwithstanding the fact that it requires more work (see Art. 

 636, and Chap. X) and costs much more to pump the same 

 number of gallons of water into the city mains when a high 



FIG. 303. Pressure-gauge. 



