BEAR-TRAP DAMS or GATES. in their simplest form, consist of two leaves extending across the pass or opening to be closed, and so hinged to the sill of the dam on their outer edge that they form a triangle when in use and lie fiat on the sill of the dam when open; one leaf, when open, overlaps the others. The space within the tri angle is filled with water. On drawing out this water through suitable openings, the dam fills, and on admitting water beneath the leaves, when the dam is open. the leaves are raised slowly into position. Thus the dam is operated by the force of the water. The first clam of this type was built on the Lehigh River, in 1818 or 1819, by Josiah White and Erskine Hazard, managers of the Lehigh Navigation Company. to secure slack water for shipping anthracite coal. Until 1880 or later this type seems to have been used but little outside of Pennsylvania ; but in 1886 two bear-trap gates. each 60 feet long, were built by United States engineer officers in the Beattyville Dam, across the Kentucky River. Since 18S6 a number of other dams of this type' have been built in this country. A study of bear-trap dams was made for the United States Government by Capt. H. M. Chittenden and Major A. 0. Pow ell, beginning in 1892. (See Journal Asso ciation Engineering Societies, Philadelphia, for June. 1896; also an article by Captain Chitten den reviewing the whole subject, in Engineering Yea's, New York, February 7, 1895.) Various
modifications of the bear-trap dam have been made, including a hinge at the apex and another in the upper leaf, so in falling the dam falls over on itself, and there is no overlapping at the apex. The largest bear-trap dam yet built forms a part of the regulating works of the Chicago Drainage Canal (q.v.), where the canal discharges into the Des Plaines River. The pass closed by this dam is 160 feet long and 20 feet high, besides which there are 15 sluice gates, 30 feet wide and 20 feet high, working vertically between masonry piers. Eight of the gates were walled up, as the full capacity was not deemed necessary for some time. This bear trap dam is not for navigation, so it is mounted on a masonry structure of some height. permit ting the upper gate to slide down the upper face of the masonry. This dam differs from others of the same type not only in being of steel instead of wood, but in having various mechanical de vices to supplement its operation. A full illus trated description of this structure is given in Engineering News (New York) for _March 24 and Nay 20, 1899. A general review of mov able dams. by B. F. Thomas, is given fir the Transactions of the American Society of Civil Engineers (New York. 1888) ; also see Reports Chief of Engineers, United States Army (Wash ington, 1860 et seq.), particularly those for 18S4 and 1SS7.