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Adrian Tuddenham, Member, on 8 January 2002

This very interesting talk attracted an audience of over 50 who were treated to a technically advanced presentation of a computer slide show, including the sounds made by a 150-year-old Boulton &Watt pumping engine.

In 1792 the owners of the mines in the Somerset coal field, which was located around Paulton and Radstock extending towards Dunkerton, had a problem. The South Wales mines could transport coal by boats over 50 miles to Bath more cheaply than they could carry it the 12 miles by road from Radstock. They decided to build a canal and used the most advanced technology of their day.

The route was from Paulton along the Cam valley to Dundas, where it would join the recently proposed Kennet & Avon(K&A) canal.

The fall over the approx. 10 miles of this route is 135 ft. The Cam brook was an inadequate source of water above Camerton to keep the canal filled, and the mills along it had water rights. Each barge traversing the series of locks (22 of them each 6 ft. deep) with a 25 ton load of coal caused 85 tons of

water to be discharged into the Cam below the locks. So the canal was designed with all 22 locks in one flight near Combe Hay and a pumping engine to raise water from the Cam there or possibly re-circulate the water - the first canal to entirely depend on pumping.

Whilst collecting information on other canals the owners learnt about Robert Weldon's Caisson Lock, three of which could replace the 22 conventional locks. Its big advantage was that it wasted no water - but it had only been tried in a one-third scale prototype. In spite of this they bravely decided to adopt the design - as `cutting edge' technology in its day as Sydney Opera House roof or the Dome.

Each lock was 80 ft long and 60 ft deep and contained a closed wooden box which could take a barge. (see `The Combe Hay Caisson Lock', Tuddenham A & Brown D, BRLSI Procs. 3, 40 (1999)). This box moved up and down in the 60ft deep pool of water, which never left the lock. The box suffered damage from an over-loaded bolt shearing on its first trial, was repaired, and then was demonstrated to the Prince Regent. The committee decided to take a ride in a barge through the lock - and the tube jammed! They just got out before they suffocated, but that was the end of caisson locks. It seems possible that the jam was caused because the walls of the lock moved when it had been emptied for repairs due to external water pressure from ground water.

The caisson lock was temporarily replaced with an inclined plane whilst 22 locks and a pumping engine were built to the latest design with metal plate clad wooden gates. It got into operation in 1805 and by 1838 it cost £5,700 p.a. to run, had an income of £17,000 p.a. and paid a dividend of £10,000. By 1858 it was carrying 165,000 tons of coal a year, 552 tons a day in 22 boat loads, using 1.06 tons of coal for the pumping engine per day.

The proposed canal to Radstock along the Wellow valley was quickly replaced by a tramway, which transferred the coal to barges on the canal section near Midford, and later a railway which took it all the way to Bath. At Midford there was a weighing machine that could weigh a barge, replacing the `Gauging Rod' which measured the freeboard height above water level to determine the weight of coal in the barge.

In the latter part of the 19th century canals suffered from competition by railways; the coal seams were being worked out; the owners could not find alternative loads or destinations; the income no longer covered the expenses; no depreciation had been allowed for in the accounts so there was no money for replacement of equipment; and the arrogance of the proprietors on losing their monopoly drove trade to a more expensive alternative - rail.

Later the railway owners took the same course when they came under pressure from road haulage, but in addition they lost skilled management and workers, and suffered meddling by politicians. The story is often no different today.

Donald Lovell

Did you know...


There are 1500 stuffed birds missing from our collections. 80 are left. They probably all rotted, insufficient mercury and arsenic compounds being used to preserve them. The remainders are probably quite toxic!

Curatorial Curiosities