THE SOMERSETSHIRE COAL CANAL CAISSON LOCK Hugh Torrens

BIAS JOURNAL No 8 1975 THE SOMERSETSHIRE COAL CANAL CAISSON LOCK Hugh Torrens A. INTRODUCTION Herald for 27 June 1795, as follows: My curiosity about the Caisson lock constructed on the "This ingenious contrivance consists of a wood cistern, Somerset Coal Canal during 1796-1797 was first aroused having two square apertures with a slide door to each, at through an interest in the life and work of William Smith the respective levels of the upper and lower Canals; In this (1769-1839), geologist and civil engineer, and long reservoir (being always full of water) is immersed a Caisson, regarded as the father of English geology. He was employed or hollow vessel; having a door water-tight at each end, for as surveyor and engineer by the Canal Company from 1793 the purpose of receiving and enclosing a boat; this Caisson to 1799 when he was dismissed (Eyles, 1969). His subse- is ballasted, to be specifically of the same gravity as water, quent career might have been rather different had it not consequently will descend or ascend in its surrounding been for the Caisson as it seems that Smith's dismissal was medium (with or without a load) with the greatest connected with its failure. facility. Each end of the Caisson, when brought in contact with the square apertures of the reservoir, is also water- Canal lifts in general have been described by Tew (1953) tight; this is accomplished by an inverted valve, discharging and Weaver (1965). The Caisson lock was the invention of the water from the space between the slide-door of the Robert Weldon and it has been described in detail by reservoir and the door of the Caisson, the pressure instantly Buchanan (1969) and Clew (1970, p. 164-166). Weldon then fixes the Caisson to the apertures, therefore the slide- came from Lichfield which was until 1781 the home of door may be opened with much ease, and of course that of Charles Darwin's amazing grandfather Erasmus Darwin the Caisson. The boat may then be received for ascension, (1731-1802). Erasmus can be regarded as the innovator the doors re-shut, and the great difficulty of releasing the of the vertical canal lift and entered in his commonplace Caisson from the aperture (against which it was so power- book (1777) a description of the principle on which the fully pressed) obviated, by opening another valve to refill lift functioned. "Let a wooden box be constructed so the space just mentioned; it consequently follows, the large as to receive a loaded boat. Let the box be joined to pressure on the Caisson will be as before, equal on all its the end of the upper canal and then the boat is admitted, sides - by discharging a small quantity of water from the and the doors of admission secured again. The box with Caisson by means of a cock into the lower Canal, it the boat in it, being balanced on wheels or levers is let becomes lighter, and instantly ascends to the upper down and becomes part of the inferior lock". (King-Hele, aperture; the same process with the valves completes the 1968, p. 129). design which is truly philosophical". Darwin envisaged counterweights rather than complete The dimensions of the Stone Cistern as built were submersion in a water-filled cistern, but the similarity in 81 feet long design is enough to suggest Weldon might have got the 20 feet wide (maximum) basic idea from Darwin (Hadfield, 1969, p.265). 61½ feet deep (not 88 feet as often stated) Weldon's age at his death in 1805 is unknown but his The dimensions of the Wooden Caisson were will (Lichfield Joint Record Office) which was proved 80 feet long 2 January 1806 shows that his father was then still alive. 10½ feet wide His brother James to whom he willed his tools and 11½ feet high engineering models died on November 7 1841 (Will also lodged at Lichfield JRO). With these figures it is possible (Bath Herald, 9 June 1798). to reconcile the possibility that Weldon as an adult could have known Erasmus Darwin in Lichfield before the The depth of the lift achieved by the Caisson lock was 46 latter's departure in 1781. But nothing futher can be said feet. The summit level of the Canal was about 240 feet until Weldon's age is known. It is interesting that Weldon's above sea level. Thus the level in the exit tunnel would be name still appears as a subscriber to William Smith's great at about 195 feet, ie. on a level with lock 8. The total drop geological map of 1815. of the Combe Hay flight was 134 ft. (not 154ft as Clew (1970, p.64) says) so that three caissons if all of equal drop Weldon's particular lift was patented in 1792 and was one would have been required at Combe Hay (see p. 9). of several such lifts proposed about this time. Dr James Anderson's lift using two counterbalanced caissons (Clew At first the lift worked well, and John Farey (1806, p.315) 1970, p.28) was at first considered by the SCC (Anderson recorded that William Smith was one of those who passed 1796) but its limited capacity favoured Weldon's. Weldon's through it during successful trials which took place during contrivance is best described from contemporary notices. 1798 and 1799. Perhaps on June 4 1798, when a "scientific It was first suggested to the Canal Company proprietors as correspondent" reported to the Bath Herald (9 June 1798): a means of changing levels at Combe Hay in 1795, and a model was submitted in June. lt was described in the Bath "It is a pleasure to reflect that the hydrostatical contrivance Page No. 4 BIAS JOURNAL No 8 1975 for conveying of boats from upper to lower levels and vice the Caisson was constructed and may thus have realised the versa, is completed, and may now be esteemed one of the cause of the troubles. greatest discoveries of the age; every scrupulous objection to the practability of its operation being removed. In addition one must also remember that the technology of cement and especially water resistant cement was not very On the 4th of June, in the presence of a large body of advanced at this time. Ironically the Bath and West of spectators, this stupendous machine underwent a complete England Agricultural Society, many of whose members were trial. A little delay was occasioned by the inadvertency of actively involved in the Somerset Coal Canal, were advertis- the workmen in not striking off a useless plank from the ing throughout this period (1788-1801) one of their prizes upper door frame of the lock, and a similar omission "for Improvements in Mechanics" as follows: "6. Cements below, but was too momentary to deserve notice. We can for Cisterns. To the person who shall invent a cheap and ' scarcely describe the satisfaction this trial afforded; the effectual composition that shall completely answer the end facility and exactness of its ascent and descent were such of foreign Terras in the cementing or lining of brick or stone as to encourage several gentlemen of this city, some of cisterns so as to hold water perfectly well, either under or whom are officially engaged in the Canal, with others above ground; Plate value Five guineas". This must suggest who volunteered, entered the boat, and descended to the that the Caisson Cistern mortar used was of the tarras bottom of the cistern, (an immersion of 60 feet) which was mortar type - a mixture of slaked lime and siliceous particularly gratifying to the Committee, and the ingenious pozzolanic earth. In 1802 this premium was no longer inventor, Mr Weldon. offered "because no reward the Society could offer would operate as any incitement". But in the Secretary's copy of The simplicity of this wonderful machine is alone sufficient the 1801 printed Rules and Orders of the Society (Bath to give it a decided preference to all others; but it has one Univ. Lib.) a member of the premiums committee has property that no other can have; and that is (where the written ironically "Cheap and effect[ive] method already tonnage is downward) of raising for every ton of goods a known. Omit [premium] ". This may refer to James ton of water, from the lower to the upper level, which in all Parkers so called Roman cement patented in June 1796 cases is desirable, but in some so essential as not to be (Hudson, 1972, p.46-48) which was probably too late for navigable without it". it to be used for the Caisson Cistern masonry. But practical problems arose, not with the design of Weldon's Tarras mortar mentioned above was introduced into the machine but, with leakage of the cistern. On April 13, 1799, British Isles by John Smeaton (1724-1792), the engineer the Bath Herald reported that a boat could enter, descend who built the third Eddystone lighthouse from 1756 to and leave the Caisson in less than 10 minutes controlled by 1759. For this he used as mortar a mixture of well burnt a boy of 12. But "The cistern which contains the Caisson is Blue Lias limestones from Aberthaw in South Wales and so leaky that it is impossible to continue any trials for a pozzolana from Civita Vecchia in Italy. Before settling on longer period than three or four hours at a time, arising Aberthaw Blue Lias Smeaton had experimented with Blue from some deficiency in the masonry, which it is feared a Lias from the Bath area as a constituent of his water setting new cistern alone can remedy".

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