Bridgewater Canal, which has now been removed. The structure is adjacent to, and upstream of, the Barton Road Swing Bridge. Both bridges are operated from a brick control tower on an island in the centre of the ship canal. When in the open position, the aqueduct and road bridge line up along the length of the island, allowing ships to traverse each side. To avoid the risk of collision, the aqueduct is opened half an hour before traffic on the Manchester Ship Canal is scheduled to pass. Turning mechanism The turning mechanism built into the central island consists of a race plate embedded in granite blocks. Sixty-four tapered cast iron rollers sat on top of the race plate, held in position by a spider ring. On top of that an upper race plate supports the aqueduct and its circular gear rack, which was powered by a hydraulic engine manufactured by Sir W. G. Armstrong Mitchell of Newcastle. To reduce the pressure on the turning mechanism, a hydraulic press was installed in the pivot. When water was admitted to the press it took up to half the weight. So successful was this system of hydraulic assistance that Leader Williams retrofitted it to several road swing bridges being built over the Ship Canal including the ones at Stockton Heath and Knutsford Road in Warrington. The weight of the structure meant that the design was pushing the limits of the possible. This became clear when the sixty-four 35.5 cm (14 inches) mean diameter hollow cast-iron rollers started to deform. By 1927 the structure had dropped by 8.89 cm ( inches). In 1928 the iron rollers were replaced with steel and since then the bridge has dropped by only 2.4mm (3/32-inch). Once the iron rollers were replaced the hydraulic press assistance was dispensed with. Hydraulic power was originally supplied by steam from two Lancashire boilers housed in a pumping station on the Eccles bank of the ship canal; a service culvert beneath the bed of the canal conveyed the water under pressure to the control tower on the island. In 1939 the original hydraulic engines were replaced by a pair of radial three-cylinder engines manufactured by the Hydraulic Engineering Company of Chester, and the following year a power house was built on the island to house two electrically driven pumps. The old steam pumping station was demolished after the Second World War. See also