Caisson lock

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Operation of caisson lock
Contemporary engraving of the lock at Coombe Hay

A 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 designed primarily as a water-saving measure, and also to minimise construction costs compared with other engineering solutions of the time. In use it was capable of replacing up to seven conventional locks.[1] Other design benefits were speed of boat descent/ascent, and only a little loss of water when operating compared with a conventional boat lock.


It was first demonstrated at Oakengates on the now lost Shropshire Canal in 1792, where its inventor, Robert Weldon (b:?1754 to 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.[2] He patented his invention as the 'Hydrostatick Caisson Lock'. The full-sized box, or "trunk",[2] 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 50 feet (15 m) of water i.e. about 3,000 lbf/ft² (150 kPa) gauge pressure at the bottom of the chamber.[3]

The proprietors of the Kennet and Avon Canal Company had inspected Weldon's device and recommended it to the adjoining Somerset Coal Canal for use at Combe Hay on their new line, to overcome water supply problems there. (The Somerset Coal Canal led directly to the Kennet and Avon and it was in the latter company's interest that the new route be opened as quickly as possible.) Three such locks were proposed, each to be 80 ft (24.3 m)long and 60 ft (18.2 m) deep and containing a closed wooden box which could take the boat. This box moved up and down in the 60 ft (18.2 m) deep pool of water, which never left the lock.

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.[4][5][6]

Method of operation[edit]

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. Instead, a masonry chamber ("cistern") was built with walls higher than the water level in the top pound and itself filled completely with water, so that even at its upper position the box remained below the surface.[8] A vertically sliding door sealed the caisson from the top pound and kept the water in.

The mechanism was operated from the top level. For a descent, the box was first wound into its upper position using a double rack-and-pinion mechanism, then drawn tightly against the frame of the opening using a ratchet mounted on the top of the wall. The outer door was then drawn up with another rack-and-pinion. At this point the water levels in the top pound and inside the box would have been roughly equal, but as the inner door – the box door – swung outwards horizontally (like a normal single lock gate) it would not open if the outer level was to any extent higher. A small equalising cock was therefore provided. The door was opened, the boat was floated in, the doors closed and the ratchet released. Because the entering boat would displace its weight of water back into the pound, the total weight of the box was always the same and no great endeavour was needed to wind it up and down. However, the operators could release a little water into the box to assist the descent. Water pressure against the outward-opening doors kept them firmly closed and watertight.

At the lower position the process was reversed. Here the water pressure was strong enough to press the box tightly into position against the exit opening. Another rack and pinion (again operated from above) lifted the outer gate, the levels were equalised again, the inner door on the box was swung open and the boat floated out. Apart from the inevitable leakage, no water had been used in the process.

Dimensions as built[edit]

  • height: 20 metres (66 ft)
  • width: from 3 to 6 metres (9.8 to 19.7 ft)
  • length: 27 metres (89 ft)
  • toothed rack: 14 metres (46 ft)
  • rotation: approximately 7 minutes


  • No 1: February 1798: cracks
  • No 2: June 1798: success
  • No 3: April 1799: success
  • No 4: April 1799: success, in the presence of the Prince Regent
  • No 5: April 1799: success, transport of 60 passengers
  • No 6: May 1799: box jammed by a projecting stone


The May 1799 test, above, occurred when a party of investors was aboard the vessel and they nearly suffocated before they could be freed. Work on the second lock was suspended (the third lock had not been started) and early in the following year an inclined plane, to carry boats’ cargoes in wheeled tubs, was built instead. Eventually a flight of nineteen locks on a longer alignment up the slope was constructed, with a Boulton & Watt Steam Pumping Station, capable of lifting 5,000 tons of water in 12 hours, used to recirculate the water.[9]

Other installations[edit]

In about 1817, the Regents Canal Company built one of these locks at the site of the present-day Camden Lock, north London. Here the motivation was, again, water supply problems. The change in level is much lower than that at Combe Hay. Soon, they too substituted conventional locks.[10] No commercially successful example has ever been built.

See also[edit]


  1. ^ "Levels at Rowley Bottom". The Somersetshire Coal Canal Society. Retrieved 2013-09-06. 
  2. ^ a b Robert Weldon, quoted in Billingsley, John (1795). "Robert Weldon's Hydrostatick or Caisson-Lock". General view of the agriculture of the county of Somerset (1798 ed.). London: Charles Dilly. pp. 316–318. OCLC 614002204. 
  3. ^ "The Combe Hay Caisson Lock". Bath Royal Literary and Scientific Institution. Retrieved 2006-10-08. 
  4. ^ "History of the Somersetshire Coal Canal". The Somersetshire Coal Canal (Society). Retrieved 2006-10-08. 
  5. ^ "The Somerset Coal Canal". Bath Royal Literary and Scientific Institution. Retrieved 2006-10-06. 
  6. ^ "History of the Caisson Lock On the Somersetshire Coal Canal". The Somersetshire Coal Canal (Society). Retrieved 2006-10-06. 
  7. ^ Oxford English Dictionary, Second Edition 1989, Oxford University Press.
  8. ^ See diagram.
  9. ^ Russell, Ronald (1971): Lost Canals of England and Wales. David and Charles, Newton Abbot, England. ISBN 0-7153-5417-5
  10. ^ Faulkner, Alan (2005): The Regent’s Canal: London’s Hidden Waterway. Waterways World Ltd. ISBN 1-870002-59-8.
  • Clew, Kenneth R (1977): Somersetshire Coal Canal and Railways. David and Charles, Newton Abbot, UK. ISBN 0-7153-4792-6.
  • Uhlemann, Hans-Joachim (2002): "Canal Lifts and Inclines of the World" Internat, Horsham, UK. ISBN 0-9543181-1-0.

External links[edit]