Speedy invention

Remembering the Combe Hay caisson lock


The Somerset Coal Canal was built around 1800. It transported thousands of tons of the lucrative black stuff from Somerset collieries to the Kennet & Avon Canal, then eastwards to London. The canal had to be built speedily, which proved a bit of a challenge when a flight of 22 locks was proposed to negotiate the sloping terrain around Combe Hay. A young engineer named Robert Weldon believed he had the perfect alternative…

The problem
While conventional lock flights get the job done of transporting narrowboats up and and down inclines, they require huge volumes of water, they’re expensive to construct and are time consuming to navigate. During this peak of the Industrial Revolution time was money, so a speedier and cheaper alternative to a lock flight was desired.

The solution
A young engineer from Sunderland named Robert Weldon had already patented the design of a new kind of boat lift – the “Hydrostatick Caisson Lock”. He’d demoed a half-scale model of it on the Shropshire Canal in 1792, and it must have caught the eyes of the canal committee, as they enlisted him to build a full-size one for them at once. In fact, they asked him to build three, to replace the 22 locks originally proposed.

How the lock worked
Weldon’s Hydrostatick Caisson Lock comprised a large water-filled chamber containing a watertight caisson or ‘submarine’ box that would carry the narrowboat from the upper canal to the lower canal, or vice versa. The caisson was wound up and down the chamber via rods, utilising a clever combination of hydrostatic pressure and water displacement to keep the caisson watertight during transit.

Teething problems
Weldon managed to complete one lock at Combe Hay but preliminary trials found huge flaws in the operation of the mechanism, largely due to the unstable quantity of water inside the caisson. Also, geological movement in the hillside and soaking of the surrounding earth due to leakages caused distortion in the machine. Despite the risks, investors and members of the committee took a ride in the lift, which became stuck halfway down and the passengers were rescued on the brink of suffocation.

Back to Plan A
Suffice to say, Weldon’s lock proved a flop, further construction of a second and third one was abandoned and plans reverted back to the original proposal of a flight of locks. Once completed in 1805, the canal transported mighty loads of coal along its 10-mile stretch for many years, but competition from the railways saw its closure around 1900.

The Somerset Coal Canal Society are currently restoring the disused canal, including the lock flight, and ongoing excavations have been taking place to locate the exact site of the first caisson lock. Discover more at coalcanal.org.