Bascule
A device operating like a balance or see-saw, especially an arrangement of a movable bridge, by which the rising floor or section is counterbalanced by a weight.
A bridge with a movable section hinged about a horizontal axis and counterbalanced by a weight.

London’s Tower Bridge has two bascules …

From the French bascule, formerly bacule, a see-saw; from battre to beat, to bump, or bas low, down + cul the posterior.
The word appears to have been first used in English in 1678.