196 THE IMPROVEMENT OF RIVERS. 



Balanced or Butterfly Valve. There are several types of valves in use, each of 

 which has its advantages and drawbacks. One of the most satisfactory is the bal- 

 anced or butterfly valve. When used in the gates they usually turn on a horizontal 

 shaft, and are worked by a lever and rack wheel from the top arm. They are easily 

 repaired and not expensive. Another type turns in a vertical plane and is worked by 

 a rod which runs through the valve and to the top of the gate, where a simple cranked 

 lever is used for operating. This valve is simpler in mechanism than the former, but 

 as the leverage obtainable is small, the valve also has to be made small. The following 

 sizes of gate valves with horizontal shafts have been taken from examples of success- 

 ful practice. 



For heads up to 8 feet, each valve was 4 feet 3 inches long by 2 feet 5 inches wide. 



For a head of 16 feet, each valve was 2 feet 6 inches long by 2 feet 3 inches wide. 



These dimensions are about the extreme limits of size which can be operated for 

 the respective heads by one man, and sizes for intermediate heads can be determined 

 accordingly. 



The valves may be of cast iron or of f -inch steel plate, stiffened. The latter material 

 is preferable, as it stands accidents better. 



Where balanced valves are placed in culverts they are usually made a little 

 higher than wide, and turn on vertical shafts which connect with the operating gear, 

 set in recesses in the coping. They may be of cast iron, or of J-inch steel plate stiffened 

 with angles or beams. Cast-iron valves of this class have not proved very satisfactory 

 in use, several cases having occurred where the castings have broken, sometimes 

 through a piece of drift jamming them, sometimes for no apparent cause except that 

 of the flaws inseparable from this material. They have also to be made much heavier 

 than a steel wicket. 



The shaft for a wicket of about 25 square feet of area should be 3 J or 4 inches in 

 diameter. If a cast wicket is used, the shaft should be forged square for the connec- 

 tion, not hexagonal, as with the latter shape the edges sometimes wear off, and the 

 grip is lost. It should be slightly smaller at the bottom of the wicket than at the top, 

 or wedge-shaped, and should only bear for about a foot at each end. This will permit 

 of its being withdrawn for repairs without much trouble. 



The wicket itself is set in a rectangular cast-iron frame, which is shaped to suit 

 the masonry and provided with ribs projecting about an inch, against which the edges 

 of the wicket close. These ribs should be provided with wooden strips bolted on, as 

 this permits of trimming to a closer fit than can be obtained with metal only. The 

 arms of the wicket may be made equal on each side of the shaft, or the arm which opens 

 inward may be made a little longer than the other, on the principle that as soon as the 

 opening begins the water will act on this additional length and assist in the maneuver. 

 While this principle is doubtless correct, it appears to be of little practical use, as the 

 violent action of the currents in the culverts is so uncertain in different cases that we 

 have seen unequal-armed wickets open automatically in one lock and close automati- 



