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<br />DISCHARGE MEASUREMENTS AT GAGING STATIONS <br /> <br />23 <br /> <br /> <br />Figure 39.--Gasoline:-powered cable cor. <br /> <br />plate. A plastic tube partly filled with colored <br />l1ntifreeze fitted in a groove between the grad- <br />uated circle and the plate is the protractor <br />index. A stainless-steel rod is attached 1-0 the <br />lower end of the plate to ride against the down- <br />stream side of the sounding cable. The pro- <br />tractor will measure vertical angles from - 250 <br />to +900. The cranes shown in figures 41, 42 <br />are equipped with protractors at tbe outer end <br />of the boom. <br />Bridge boards may be used with an A-pack <br />or A-55 sounding reel and weights up to 50 <br />pounds. A bridge board is usually a plank abont <br />6-8 feet, long with a sheave at one end O\'er <br />which the meter cable passes and a reel seat <br />near the other end. The board is placed on the <br />brid!1:e rail so that t,he force exerted by the <br /> <br />sounding weight suspended from the reel cable <br />is counterbalanced by the weight of the sound- <br />ing reel. (See fig. 43.) The bridge board may be <br />hinged near the middle to let one end be placed <br />on the sidewalk or roadway. <br />:VI any special arrangements for measuring <br />from bridges have been devised to suit a par- <br />ticular purpose. Truck-mounted cranes are <br />often used for measuring from bridges over <br />larger rivers (see fig. 44). Monorail stream- <br />gaging cars have been developed for large <br />rivers. The car is suspended from the sub- <br />structure of bridges by means of I-beams. The <br />car is attached to the I-beam tracks by trolleys <br />and is propelled by a forklift motor having a <br />wheel in contact with the bottom of the beam. <br />The drive mechanism and sounding equipment <br />