Victorian illustration to download showing a picture of a large ship in the New Thames Graving Docks (floating dry docks). Men are on the quay beside the ship getting ready to work on her. These new graving docks (1850s) were like a huge hydraulic lift. Cast iron columns and girders were sunk under water. A pontoon was lowered on top of the girders, a ship was brought over the pontoon, and the whole structure was raised hydraulically so that the ship was above water and men could work on her from the pontoon. As ships got bigger, the cost of constructing traditional dry docks to accommodate them grew too high – hence this invention.