CHAPTER VI. 



NEEDLE DAMS. 



History. While the bear-trap was earlier in use on the Lehigh River, yet the 

 pioneer of movable dams on navigable rivers was the needle dam, invented by M. 

 Poiree in 1834, and first constructed at Basseville, France. It is called a needle dam 

 because the wall which holds and supports the water is made of needles or wooden 

 spars ranged side by side across the river. 



Modern needle dams are constructed throughout on almost the same principles 

 us this first dam. The curtain of the dam is formed of the needles, which are of a size 

 suited to the head of water, yet rarely larger than one man can control. Their bot- 

 toms rest against a sill, their tops against bars known as escape-bars, which are in 

 turn supported by trestles turning on hinges on the floor and lowered behind the sill 

 when not in use. A walkway, hinged to the trestles, or of loose planks, provides access 

 for maneuvering when the dam is up. 



In the Basseville dam the trestles were placed 3^ feet (one meter) apart, and 

 weighed 242 Ibs., with a height of 6 feet, the lift being only 3^ feet. The distance 

 between the trestles is now made about 4 feet, only one example being known at present 

 where it has been exceeded that of the Louisa dam in the United States, where the 

 trestles on the weir are 8 feet apart. There appears no reason, however, against using 

 wider spans where proper appliances are provided for maneuvering, and as a precau- 

 tion against drift this would be very desirable. 



Maneuvers of Trestles. All the trestles are connected, either by chains made 

 in separate lengths and fastened to the heads of adjacent trestles, or by one long chain 

 running through the heads, and fastened to them at the proper intervals by clamps or 

 by latches, and worked by a winch on the masonry. The latter method is the one almost 

 universally adopted in the modern needle dam, and by its means 2 to 6 trestles can be 

 lifted at the same time. When it is desired to raise the dam the first trestle is raised 

 up and connected to the masonry by the escape-bar and by the floor, and the same 

 process is followed with the succeeding trestles until all are in place. Where a continuous 

 chain is used it is released from each trestle in turn, as the latter becomes vertical, by 

 a man on the foot-bridge ; where separate chains are used a portable winch usually is 

 carried from one trestle to the next. After all the framework is in place the needles 



are put in. sometimes from a boat, but usually from the foot-bridge. 



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