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HYDRAULIC POWER APPLIED TO BRIDGES.
feet long, and weighs more than 1200 tons. In this bridge,instead of the weight resting upon live rollers, an hydraulicpress is applied to the centre; it has a pressure of about900 tons upon the ram, which relieves the pressure upon therollers to that extent. The rollers and roller path, how-ever, are sufficiently strong to carry the whole weight of thebridge, supposing any accident were to happen to the centrepress. The central press being always open to the accumulatorpressure, a permanent relief is afforded without any waste ofpower.
Hydraulic power was first applied to drawbridges in 1853to work the bridge over the river Tovey, on the South WalesEailway near Carmarthen ; and at the Sunderland Docks anhydraulic drawbridge was constructed about the same time.In the application of hydraulic power to drawbridges, the firstOperation consists in lifting the bridge sufficiently high to enableit to roll back over the permanent way in the rear. The liftingpresses then act directly under the main girders of the bridge,and, as the tail-end is heavier than the nose-end, the nose-endof the bridge is first raised against the roller bearings, and then,when the back-end is raised to its proper level, the bridge ishauled back by means of hydraulic reciprocating engines.These aet upon rollers, either attached to the bridge itself (inwhich case the rollers run upon a roller path fixed to themasonry) or upon rollers attached to the head of the liftingram, when the roller path is attached to the bridge itself. Anoticeable bridge of this type has recently been constructedat the Kattendyk entrance to the Antwerp Docks.
Hydraulic power has also been applied on the “ Bascule ” (orold lifting drawbridge) System, both single and double leaf. Abridge of this character occurs over one of the dock entrancesat Liverpool. Sometiines the dip of the “ Bascule ” bridge iscounterbalanced by the tail-end of the bridge. In some casesthe bridge is hinged on the quay level and is lifted bodily back,leaving the passage of the quay-way perfectly free and uninter-rupted for passengers. There is a Bascule bridge at York in