Caisson lock

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.

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.

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.

[4][5][6] The system depended on the submerged, sealed box (the "caisson", from the French for "large chest"[7]) being heavily ballasted to achieve neutral buoyancy, so it was never possible in ordinary operation to lift it to water level to allow a descending boat to float in.

Here the motivation was, principally, water supply problems but also to effect a quicker passage of vessels, as those going in opposite directions could pass in the lock.

[11] Congreve's patent then envisaged that having achieved "absolute equipoise" (neutral buoyancy) a rack-and-pinion, removable weights or a small winch could overcome inertia and move the caissons.

In a practical demonstration at Hampstead Road Congreve later elected to deploy an air compressor releasing into one caisson, estimating that one man could achieve raising and lowering in three minutes—the company's enabling act did not permit the installation of stationary steam engines.