Chap. XXIIT.

DRAWBRIDGES.

1391

several of the loopholes through which these levers passed may yet be traced round theruins of our castles.

Rolling Bridges are sometimes substituted for those which swing, the pulleys and wheelsbeing so contrived that a considerable length may be operated upon without danger.

I'i “3LL£

Fig. 2307. ROLLING BRIDGE.

Drawbridge , with a platform composed of three principal timbers, is made to rise bypulling up a lever on each side, which has a pivot that works against the under side of eachbearer: when it is re*quired to be let downand form a defence, theframe in which the plat-form turns upon its pivotsis first drawn up bychains; the platform isthen swung round, andhangs upright upon thepivots, the longest enddescending into the ditch.

The dotted circle indi-cates the various positionsof the platform.

The bridge called ponssubductarius was hungat one end by a hinge,whilst the other was ele-vated by some such simplecontrivance as the ba-lance or plyers ; whenraised or elevated, it pre-vented the passage of anyone across the ditch ormoat it crossed. In manyparts of the continent thebridges are still made todraw backwards and forwards, so as to prevent or afford a passage across the water,whenever it is desired. The flying-bridge, or pons ductarius, was generally formed in adifferent manner, leather boats, casks, or metal pontoons, being laid on the river andcovered with a platform of timber : they are sometimes made by attaching or hangingseveral platforms to boats, then dropping them one to the other, the boats being pre-viously anchored or moored in the middle of the river : flying-bridges are occasionally madeby laying one platform over another, in such a manner that they can be pushed forwardsupon wheels or rollers.

Fig. 2308.