Bascule n. : In mechanics an apparatus on the principle of the seesaw, in which one end rises as the other falls.

Example:
What is the world's longest Bascule bridge?

Taxonomy
bascule bridge, bridges, bascule bridges, Tower Bridge, double leaf, counterweight, trunnion bascule bridge, free encyclopedia, the bridge, span, Scherzer rolling lift, movable bridge, History, Pedestrian, bridge type, Span range, Design effort, leaf.

Defination from the web:
bascule bridge is a moveable bridge with a counterweight that continuously balances the span, or "leaf," throughout the entire upward swing in ... The Blagoveshchensky Bridge across the Neva River in Saint Petersburg was the first large bascule bridge, opened in 1850. ...