NEEDLE DAMS. 219 



The removal of these "hook needles" is usually effected by a crab carried on a 

 truck, which raises the needle till it passes over the sill, when it swings on the support- 

 bar and in the current. It is then lifted up by hand from the footway and loaded on 

 to a car. A boat is only used in exceptional cases, and when the pools have reached 

 the same level. 



Another method of removal is that known as escaping. The escape-bar, which 

 in all earlier dams was a loose bar connected when in use to the trestles by claw ends or 

 by bolts, was later on hinged at one end so as to swing horizontally, the other end 

 resting against a post shaped so that on being turned it released the end of the bar and 

 allowed it to swing. The force of the water then carried the needles through the 

 opening and they were picked up below. This is known as the Kummer escapement, 

 from the name of its originator. The method has been largely adopted in Belgium 

 and elsewhere, although the fixed bar is still employed in certain cases. 



Another method, which has been in satisfactory use for some years and which 

 does not bruise the needles, as often happens in the method of escapement, is to attach 

 a long chain or rope to their up-stream sides and pull them away with a crab or an engine. 



Dam at Louisa, Ky. This dam, which was completed in 1896, on the Big Sandy 

 River, in Kentucky, was the first of the needle type in the United States, and presents 

 certain features which have proved to be an advance in some ways in the design and 

 operation of needle dams. The original project called for a fixed dam, but before its 

 completion strong opposition on the part of those connected with the river commerce 

 developed, as it was feared that the placing a stationary dam in a river carrying so 

 much sand would cause the pool to silt up, and impede rather than benefit navigation. 

 The plan was accordingly changed to that of a movable dam, and owing to the 

 small low- water discharge of the river (48 cubic feet per second), needles were 

 adopted. 



The pass is 130 feet long, with 13 feet of water on the sill and trestles 4 feet apart, 

 and the weir is 140 feet long, with 7 feet of water on the sill and trestles 8 feet 

 apart. 



As the river carries large quantities of sand it was desirable to have the recess 

 behind the sill as shallow as possible, to avoid the accumulation of deposit ~ over the 

 trestles. The latter are accordingly shaped like an inverted V without any axle, the 

 bracing, etc., being placed so that they lie down, one inside the other, instead of 

 one on top of the other, as is the usual way. By this means a height of sill of only 

 15 inches is needed. They are all raised by a continuous chain worked from the 

 masonry, and can be lifted or lowered, if necessary, six at one time. The chain rolls 

 on sprocket-wheels in the heads, and can be attached to or released from them at any 

 point by means of latches. 



The needles, both for the pass and for the weir, are made of white pine, 12 inches 

 wide, and weigh apiece 263 Ibs. and 80 Ibs., respectively. The former are about 14 

 feet long, 8 inches thick at the bottom and 4^ inches at the top, being designed for a 



