to THE IMPROVEMENT OF RIVERS. 



strain of xaoo Ibs. per square inch; the latter are about 8 feet long, 3$ inches thick 

 at the bottom and a} inches at the top. The width of 12 inches was adopted to save 

 leakage, and has proved very satisfactory, as with an extreme low-water discharge of 

 48 cubic feet per second, and a head of 12 feet 2 inches on the pass, the total leakage 

 through the dam was only 9 cubic feet per second. 



Under a full load the needles show a considerable deflection, but do not take any 

 permanent set, and the only cause of breakage has been the presence of knots or other 

 defects in the timber. They are handled partly by men and partly by a small derrick-boat 

 with an engine, and give little trouble in maneuvering. Those on the weir are placed 

 by hand, and if any have to be put in under a full head, as in regulating, their tops are 

 held against the escape-bar and the ends plunged in the water as before described. In 

 this case a rope is generally placed over the lower ends and used as a check against 

 the force of the water. The pass needles are placed by the derrick-boat, and can be put 

 in without great difficulty with a head of 3 or 4 feet. If the dam is raised during 

 the dry season a hook attached to a line running to the engine on the maneuvering 

 boat is used to press the needles all together, thus closing any spaces. 



The regulation is done principally by repoussing, or holding up stream about 1 2 

 inches the heads of alternate needles, thus allowing the water to escape between. 

 Next to the pier, on the weir side, a space of 1 2 feet is left, provided with two rows of 

 needles, one above the other, supported by a lower and an upper escape-bar, and this 

 also provides a convenient means of regulation and of passing debris. Several needles 

 6 inches wide have been substituted for 12 -inch ones on the weir to facilitate regula- 

 tion, being more easily removed. Lastly, in good weather, the water is allowed to flow 

 over the tops of the needles to a depth of 6 or 8 inches, along the entire crest. 



A method of placing the pass needles has also been used which afforded good 

 results, although the use of the derrick-boat eventually proved simpler. In it the 

 needles were placed on hinged shelves about a foot above the water, one needle being 

 placed every few feet against the sill to serve as a guide. The shelves were kept 

 from turning by triggers attached to a line, and when all was ready the triggers were 

 released, and the shelves revolved, allowing the needles to drop into the water all together. 



The removal of the needles is effected by attaching the up-stream side of each one 

 to a chain, allowing about 2 feet of slack between, and then pulling it from the boat, 

 which is moored about 100 feet above the dam. By this means all the weir needles 

 can be removed under a full head in a very short time, but the operation is always 

 done slowly, so as to retain the pool at about the proper level. The trestles are also pro- 

 vided with swinging escape-bars, but as the needles become bruised against the trestles 

 in falling, owing to their unusual size, this method of release has been rarely used. 



On swift rises drift in large quantities comes down the river, which has once or 

 twice threatened to cause damage to the trestles. To guard against this as far as 

 possible a rudder boom several hundred feet long is kept afloat above the lock, 

 stretching at a very flat angle across the river. This holds the drift until enough of 

 the dam has been lowered to remove the danger. 





