DAMS AND RESERVOIRS. 762 DAMS AND RESERVOIRS. at the top, and 14 feet 3 inches long, each weigh- ing 263 pounds when wet. The needles are set in pkioe by means of a derrick on a boat, and are lifted out by means of a cliain 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. Wicket or Shutter Dams have been devel- oped from simple gates or shutters working on a horizontal axis near their tops, which have been used in Holland for centuries. These simple shutters have been elaborated until they are now made of short lengtlis, which may be re- volved on their horizontal axis, then lowered so as to rest flatwise on the sill of the dam. They are operated from a bridge placed above them, which can be lowered much the same as the frames of the needle dams just described. In 1880 two wicket dams were completed on the Great Kanawha River by the United States Gov- ernment, and others have since been built on that stream. One, completed in 1892, has a pass 248 feet long; besides a weir 316 feet long and a lock 55 feet wide. The pass is closed with 62 Chanoine wooden wickets 3 feet 9 inches long and 14 feet high, with a three-inch space between them, which may be closed by means of a tim- ber, if desired. The first of a series of movable dams in the Ohio River, at Davis Island, near Pittsburg, was built in 1878-85, and also has Chanoine wickets. There is one pass 716 feet long, another 1223 feet long, besides a fixed dam 456 feet wide, several weirs, and .a lock 110 feet wide and 600 feet long between the gates. In 1899 a second Chanoine wicket dam was being built across the Ohio River 25 miles below Pitts- burg. Drum D.ms are modifications of the wicket or shutter dam. One of these, named after its inventor, Capt. H. M. Chittenden, may be de- scribed, roughly, as being shaped like one-sixth of a cj'linder. Yhen lowered by revolving on a horizontal axis at the centre of the cylinder, it drops into the chamber, leaving one radial side of the cylinder flush with the sill. The sections are raised by the force of the water acting from beneath. Be.rtr.p D.ms 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 flat 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 dam of this tvpe was built on the Lehigh River, in 1818 or 1819, by Josiah ATiite and Erskine Hazard, managers of the Lehigh Navigation Company, to secure slack water for shipping anthracite coal. Lentil 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 LTnited States engineer ofllcers in the Beattyville Dam, across the Kentucky River. Since 1886 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 IMajor A. O. 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 }^etcs, 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 niasonr.y structure of some height, permit- ting the upper gate to slide down the upper face of the masonry. This dam diiTers 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 descrijition of this structure is given in Engineering yews (New York) for March 24 and ^lay 26. 1899. A general review of mov- able dams, by B. F. Thomas, is given in- 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 1884 and 1887. Cofferd.ms (q.v.) are employed to exclude water from foundations and other classes of work while under construction. See Founda- tions. failures of dam.s. The Bradford earth dam, Sheflield, England, failed in March. 1864. This dam, built to supply water and furnish power to the city of Sheflield, was about 90 feet in height, 13 feet wide, and 1250 feet long, with slopes of 2^, to 1. The dam, except for a puddle wall extending from end to end and 60 feet into the giound. was of earth loosely dumped from carts. Cast-iron outlet jiipes about 500 feet in length, surroimded by clay puddle, extended through the base of the dam. While the reservoir was being filled for the first time, a leak suddenly appeared, and enlarged so rapidly that in 30 minutes the reser- voir had emptied itself. The flood reached Sheffield at midnight, without warning, causing great destruction of property and the loss of 238 lives. In the oflScial inquiry made as to the cause of the failure, it was claimed that in a work of such great magnitude the outlet pipes .should not have been placed through the dam itself. The failure of the IMill River Dam at Wil- liamsburg, Mass., in 1874. was a conspicuous example of improper construction which resulted in complete saturation of the embankment. No engineer had been employed in constructing the work, and no proper means used for consolidat- ing the embankment. One morning, when the water was 4 feet from the top of the dam, masses of earth were observed to slide from the outer slope of the embankment. In 20 minutes the reservoir was emptied of 100,000,000 cubic
