Bascule bridges operate through the use of counterweights which continuously balance the bridge throughout its upward swing.
I believe that the Bascule Bridge at Stanley Dock is a Scherzer Rolling Lift type where the bridge deck not only lifts but rolls back to allow clear unobstructed clearance the full width of the waterway.
This type of brige was designed by William Scherzer (1858 - 1893) who was an American engineer who filed the patent for this design in May 1893 a few months before his death at the age of 35.
The patent was granted in December 1894 and after his death his work was continued by his brother Albert who started the Scherzer Rolling Lift Bridge Company.