bascule bridge

Century Dictionary and Cyclopedia

1. n. A drawbridge arranged with a counterpoise, so that, as the floor of the bridge is raised, the counterpoise descends into a pit prepared for it: the commonest form of medieval drawbridge. See balance-bridge.

Wiktionary

1. n. A movable bridge consisting of one or two opening leafs equipped with balancing counterweights that ease and speedy up their operation.

GNU Webster's 1913

1. n. a counterpoise or balanced drawbridge, which is opened by sinking the counterpoise and thus lifting the footway into the air.