Bascule Bridge. Rotherhithe, London. Uk Stock Photography

During the heydey of the London Docks and shipping trade Bascule bridges were in regular use. Two survive in Rotherhithe but are now static. A bascule bridge sometimes referred to as a drawbridge is a moveable bridge with a counterweight that continuously balances a span, or `leaf`, throughout its upward swing to provide clearance for boat traffic. It may be single- or double-leafed.The name comes from the French term for balance scale, which employs the same principle. Bascule bridges are the most common type of movable span because they open quickly and require relatively little energy to operate, while providing the possibility for unlimited vertical clearance for marine traffic.