SECTION OP WCIR  . 69 2.* The earliest form of shutter weir, known as a bear-trap, intro- duced in the United States in 1818, and subsequently erected across the Marne in France, consists of two wooden gates, each turning on a horizontal axis laid across the apron, inclined towards one another and abutting together at an angle in the centre when the weir is closed; the up-stream one serves as the weir, and the down-stream one forms its support, and both fall flat upon the apron for opening the weir) . This weir is raised by admitting water under pressure beneath the gates through culverts in connexion with the upper pool; and is lowered by unfastening the raised gates and letting the water under them escape into the lower pool . This old form of bear-trap has been used for closing an opening 52 ft. wide to provide for the escape of drift at the Davis Rivers and Canals, p . 132 and plate iv. fig . 15 . Island weir across the Ohio . Improvements, however, in the bear-trap have been introduced in the United States, one of the best novel forms being shown in fig . 8, whereby the pass of a weir 8o ft. in width can be readily closed, opened or partially opened under a maximum head of i6 ft. by means of chains worked by a winch.' The shutter weir, introduced on the upper Seine about the middle of the 19th century and subsequently adopted for weirs across several rivers in France, Belgium and the United States, consists of a row of wooden or iron shutters turning on a horizontal axis a little above their centre of pressure, borne by an iron trestle at the back of each shutter, which is hinged to the apron of the weir, and supported when raised by an iron prop resting against an iron shoe fastened on the apron (fig . 9) .