These four fine metal structures designed by a London firm and built in the Cockerill works were installed between 1888 and 1917 (and on UNESCO's World Heritage list since 1998). Barges enter two basins filled with water (one in the upper canal section, the other in the lower section) and fixed to huge pistons that form a sort of pair of hydraulic scales. Water added to the upper basin causes it to sink, while the lower basin rises (principle of communicating vessels).