Science for beginners . m the top of the hydrant toits bottom. A screw thread is cuton the rod opposite the point whereconnection is made to the citymain. The upper half of thethread is left-handed, the lowerhalf is right-handed. The upper,left-handed thread carries a left-handed burr; the lower, right-handed thread carries a right-handed burr. To these burrs areattached short rods or levers whichcontrol the valve. When the valve is to be opened, a wrenchapplied to the projecting portion of the rod at the top of the hy-drant is turned counter-clockwise. This revolving of the rodforces the uppe
Science for beginners . m the top of the hydrant toits bottom. A screw thread is cuton the rod opposite the point whereconnection is made to the citymain. The upper half of thethread is left-handed, the lowerhalf is right-handed. The upper,left-handed thread carries a left-handed burr; the lower, right-handed thread carries a right-handed burr. To these burrs areattached short rods or levers whichcontrol the valve. When the valve is to be opened, a wrenchapplied to the projecting portion of the rod at the top of the hy-drant is turned counter-clockwise. This revolving of the rodforces the upper burr upward and the lower burr levers spread as the burrs separate and the valve is drawnback away from its seat, thus permitting the water from themain to rush into the hydrant. 498. The Water Pressure-gage.—Water pressure is usu-ally indicated by pressure-gage. The pressure-gage con-sists essentially of an elliptically shaped, thin-walled tubebent into a nearly circular form (Fig. 252). When the pres-. Fig. 252.—Pressure-gage. 424 WATER SUPPLY AND SEWAGE DISPOSAL sure within the tube increases, the tubes tend to straightenout. This motion is transmitted to the pointer which movesover the face of the dial. The mechanism is so adjusted thatthe instrument shows directly the pressure per square The Water Meter.—While the pressure-gage is anecessity at the pumping station indicating to the engineerthe exact pressure on the city mains, the majority of con-sumers are more concerned with the water meter. The pres-sure-gage merely indicates the pressure under which thewater is kept; the water meter indicates the number of gal-lons or cubic feet of water which flow through the pipes.
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