The caisson lock is a type of canal lock in which a narrowboat is floated into a sealed watertight box and raised or lowered between two different canal water levels. It was invented in the late 18th century as a solution to the problem posed by the excessive demand for water when conventional locks were used to raise and lower canal boats through large height differences. Such locks, each of which would only raise and lower boats through small height differences of a few feet, would not suffice when large height differences had to be tackled nor when water was in short supply. The caisson (or ) was thought to be one solution, although it transpired that the technology of the day was not capable of achieving this type of construction economically. It was designed primarily as a water-saving measure, and also was an attempt to minimise construction costs compared with other engineering solutions of the time. In use it was capable of replacing up to seven conventional locks. Other design benefits were speed of boat descent/ascent, and only a little loss of water when operating compared with a conventional boat lock. History The caisson lock was first demonstrated at Oakengates on a now lost section of the Shropshire Canal in 1792, where its inventor, Robert Weldon (b:?1754, d:1810) built a half-scale model. He claimed that his design would solve the problem of water supply in dry seasons or at greater elevations, be cheaper than building aqueducts or tunnels, and be quicker to operate than the number of surface locks his design could replace. He patented his invention as the 'Hydrostatick Caisson Lock'. The full-sized box, or "trunk", would probably have displaced about 270 tonnes and weighed about 170 tonnes, including the water in it, so about 100 tonnes of ballast would have been needed to give neutral buoyancy. The box would have needed to be strong enough to withstand the pressure of of water i.e. about gauge pressure at the bottom of the chamber. The first lock was completed in 1797 under Weldon's supervision. The device was demonstrated to the Prince Regent (later George IV), but was found to suffer from various engineering problems, possibly caused by the soft fuller's earth rock stratum in the area. Method of operation The system depended on the submerged, sealed box (the "caisson", from the French for "large chest") being heavily ballasted to