Chap. XXIII.
DRAW BRIDGES*
1389
Drawbridges are another description of shifting bridges, and consist of a wooden platform,wide enough to allow of the passage of horses and wheel carriages, and extending in lengthfrom one side of the watercourse to the other, or from a jetty to a projecting pier onthe opposite side: whenever such bridges are adopted, the passage of the water is con-tracted as much as possible, and only sufficient room left for vessels to pass through; bythis means, the platform is rendered as small and light as possible, without sacrificing thenecessary strength : it is secured by pivots or large hinges at one of its ends to the jetty,so that it can be raised from its horizontal to a vertical position, which is easily effectedby chains attached to each of the corners of the side that is unhinged, and afterwards passedover pulleys, or hung to the ends of long levers, balanced on the top of stout posts, fixedfirmly, and braced in that position. Those formerly existing at Brussels were balancedby weights attached to chains passing over standards that stood immediately over thewalls of the canals: they were strongly braced, and firmly kept in their vertical position
Fig. 2302. DRAWBRIDGE , BRUSSE1.S.
by means of iron ties. That contrived by Perronet opened in the middle, to allow themasts to pass through; by a windlass mounted in a frame it was easily manoeuvred, and
raised or let down promptly.
Fig. 2303.
Balancing Bridges at Brussels , to work in a quadrant of iron : when in a positionto permit a passage over them, they are supported by struts, which have a bearing ona set-off in the masonry ; when tilted up, they are maintained in that position by a rackand pinion ; at Neuf Brisack the bridge is lifted by a weight over a pulley.