bascule
    n 1: a structure or device in which one end is counterbalanced
         by the other (on the principle of the seesaw)

Bascule \Bas"cule\ (b[a^]s"k[-u]l), n. [F., a seesaw.]
   In mechanics, an apparatus on the principle of the seesaw, in
   which one end rises as the other falls.
   [1913 Webster]

   Bascule bridge, a counterpoise or balanced drawbridge,
      which is opened by sinking the counterpoise and thus
      lifting the footway into the air.
      [1913 Webster]