MOVABLE DAMS arc those which can be lowered Cr raised at will, according to the stage of water in the river. They are generally aids to naviga tion, placed at stretches where there are shallows or rapids, but where permanent structures to raise the water-level might do damage by causing floods at times of high water. They may also be used on any ordinary dam, or on waste weirs. It is more COMMOB, however, to call movable de vices connected with ordinary dams flashboards or floodgates. Flashboards are generally com paratively low and are of fragile construction, or have supports designed to give way in time of freshet. There are a great variety of movable dams, but they fall more or less closely into three groups—needle, wicket or shutter, and bear ,rata. The latter have some decided points of superiority, being raised and lowered by the force of the water itself, on its being turned under or discharged from chambers beneath or within the dam. Needle dams were developed in France about the close of the eighteenth cen tury. They are an outgrowth of the earlier French and English needle dams and consist of horizontal beams, or stop planks. dropped into grooves built in the two abutments of a pass through the dam. These beams could be lifted out at times of high water. Later, to facilitate handling, they were set vertically, or nearly so, resting against a sill below and a beam above. A chain was finally substituted for this beam to make greater lengths of dam feasible. In this way movable dams 40 feet wide were developed on the Yonne, in France. In 1834 M. Poitine in creased the width of one of these dams, or passes, to 72 feet by substituting iron bars for the chains. The bars were short and were supported by means of vertical, iron frames, placed at right angles to the length of the dam. To throw down
the dam, it was only necessary to remove the needles one by one, detach the horizontal bars, then lower the frames into recesses in the top of the masonry portion of the dam. The needle dams were somewhat modified subsequently and used in various parts of France, in Belgium. in Germany, and in the United States. The first needle dam in this country was built in 1891-97 by the United States Government across the Big Sandy River at Louisa, Ky. The whole improve ment includes a lock 52 feet wide, a navigable pass 130 feet long, and an overflow weir 140 feet long. The sill of the pass is 13 feet and that of the weir is 7 feet below the normal height of water in the pool, and the sill of the pass is one foot below low-water mark of recent years. The steel frames supporting the pass needles are four feet between centres. The horizontal bars connecting the frames and supporting the upper ends of the needles are hinged at one end and hooked at the other. The frames have a sheet iron floor, forming a foot-bridge, which falls with them, and are connected by a chain. The pass needles are of white pine, 12 inches wide. inches thick at the bottom, and inches thick at the top, and 14 feet 3 inches long, each weigh ing 263 pounds when wet. The needles are set in place by means of a derrick on a boat, and are lifted out by means of a chain passing through irons fastened to their top, operated by an engine. The frames are raised or lowered from one of the abutments to the pass by means of a chain crab.