Brief History of the Caisson Lock at Combe Hay, nr Bath.

Brief History of the Caisson Lock.

Please note: This is a very exciting topic. This caisson was 'one of the wonders of the waterway world". The exact location and structure are still a mystery , but new evidence is constantly coming to light. Comments between the various parties can become a little heated!

I expect this to surface again soon as a new body of evidence is mounted to support the view that the site of the caisson was actually at the pumping engine site (the long shaft being sunk through the bottom of this 'cistern').

If you wish to read this topic first please scroll down to "Possible Sites", or, use this link: link

For visitors who have navigated directly to this site, may I suggest that you look first at the various diagrams and potted history of the site first?
Please go to: History of the SCC

The original idea was to cut a 3/4 mile tunnel at Combe Hay to link the 2 sections of the canal.
The decision to place the canal at a higher level, so as to avoid a long tunnel, was probably also what made it possible to concentrate the whole drop at Rowley Bottom (at Combe Hay) and that, in turn, made caissons a viable proposition.

Robert Weldon (b:? to d:1810) had a demo. model of the caisson lock on the Shropshire Canal (near Oaken Gates) in working order in Sept. 1792. (The patent is believed to date from June 1792).This obviously impressed the SCC committee.
However it seems likely that, being a native of Lichfield, he came across the writings of Charles Darwins' famous Grandfather - Erasmus - another native of this town. Erasmus published, in 1777, details of a wooden box, capable of receiving a boat which could be raised or lowered between the upper and lower levels of canals (Not in a water-filled well though)

Tha caisson was described as follows by the Bath Herald for 27 June 1795, as follows:

"This ingenious contrivance consists of a wood cistern, having two square apertures with a slide door to each, at the respective levels of the upper and lower Canals; In this reservoir (being always full of water) is immersed a Caisson, or hollow vessel; having a door water-tight at each end, for the purpose of receiving and enclosing a boat; this Caisson is ballasted, to be specifically of the same gravity as water, consequently will descend or ascend in its surrounding medium (with or without a load) with the greatest facility. Each end of the Caisson, when brought in contact with the square apertures of the reservoir, is also water- tight; this is accomplished by an inverted valve, discharging the water from the space between the slide-door of the reservoir and the door of the Caisson, the pressure instantly then fixes the Caisson to the apertures, therefore the slide- door may be opened with much ease, and of course that of the Caisson. The boat may then be received for ascension, the doors re-shut, and the great difficulty of releasing the Caisson from the aperture (against which it was so power- fully pressed) obviated, by opening another valve to refill the space just mentioned; it consequently follows, the pressure on the Caisson will be as before, equal on all its sides - by discharging a small quantity of water from the Caisson by means of a cock into the lower Canal, it becomes lighter, and instantly ascends to the upper aperture; the same process with the valves completes the design which is truly philosophical"

From "The Daily Telegraph".

In Jan.1796 adverts appeared in the local (Bath) press for masons to build a caisson lock at Combe Hay.

Dimensions were:
Chamber: 66 ft deep;width 20ft in middle, tapering to 11ft 6in at each end.
Length 88ft; Lift 46ft.
Brick lined(Clew). However Torrens states that it was built with 'freestone' (Local inferior oolitic limestone; this is probably correct).

Long-boat turn-around-time: about 7 mins.

Trials:
1st trial Feb. 1798. partly broke ... a failure.
2nd trial June 1798 worked.
3rd trial April 1799 worked.
4th trial 12 days later a spectacular and successful demo. in front of HRH The Prince of Wales!
5th trial 10 days later, over 60 ladies and gentlemen took "rides" in the caisson.
6th trial May 1799. Bulging brick/stone-work caused the wooden caisson to stick.

Another £1 000 was more than the SCC could justify as the work had cost £4 582 so far. It was to be replaced by an inclined plane.
As already stated, the probable reason for this was that the whole of the cistern (well) had been cut in the Fullers Earth Measures - a rock formation that swells like cat litter when wet. William Smith from Tuckingmill (on the canal near Midford) the "father of English Geology", surely must have known of this problem, especially as he had visited other areas of England such as the Sapperton Tunnel in 1798 and 1794 which had exactly the same problems and for exactly the same reasons. It may be that this was the reason for him eventually being dismissed from the SCC on the 5th of June 1799 (especially as he had sided, according to a Mrs Eyres - (quoted by Torrens), with the anti-caisson lobby). He should have spoken up sooner! He had also taken advantage of the SCC in building his house on the banks of the SCC at Tuckingmill where, ironically, there later was a Fullers Earth Mill. (The house that stands here has a plaque to Smith but this is the wrong house! (It is the next house))

Other candidates for Weldon's failure include the poor Tarras cement-mortar that was used. However this being partly based on the Bath Blue Lias limestones which had an excellent reputation, not only in use in canals but mines as well, would appear to contradict this notion.

Jane Austen and her father visited the site in 1801, so obviously it had not yet been filled-in.

The Rev. Warner also visited the site in 1801 ..."but the bason (sic) of which alone remains".

There were plans for another 2-3 such caissons. Certainly the Rev. Warner talks of the building of another caisson lock being suspended, so obviously at least 2 were (partly) built, one (perhaps) marked by a former flag pole, the other 180 feet away & (perhaps) marked by a chestnut tree atop an artificial mound.
These 2 markers are important clues it is thought in the location of the positions of the 2 caissons. (See later).

"...(the caisson) may then be easily raised or lowered at pleasure by means of racks and pinions, or chains and pulleys ... from one level to the other...." (lines 12-14 from the patent). This is at variance with the account from Clew, where in the passage describing label 'A', more emphasis appears to be placed on increasing the mass of the caisson by admitting water to the caisson, in order to lower it.

The last 3 lines of the patent do however leave room for the above method as a subsidiary method of (fine) control, ... "...and the machine is further regulated by means of a pump, an air pipe, and other apparatus, to be applied as occasion may require". (My emphasis). I have not encountered details before of such a "pump". (See later)

However 6 years previous to this (a long time methinks?) the same Canal Company had...
"... launched a competition for the 'best means of raising and lowering heavy weights from one navigation to another".

"The contest was won by Henry Williams and James Loudon, and by 1792 tub-boats were being hauled on rails up and down the steep slope, by teams of men and horses. This was the famous "Hay Inclined Plane". Boats entered and left the water, carried on wheeled cradles. By the year end a steam engine at the top of the incline had been installed, allowing full operation of the incline by a four-man team. It is 305m (333 yd) long and has a vertical rise of over 61m (66 yd) equivalent to twenty-seven canal locks. A pair of five-ton tub-boats could be passed in four minutes whereas it would have taken at least three hours to raise a vessel through conventional locks. The inclined plane worked for just over a century, its last recorded operation was in 1894, and its formal closure in 1907. "

It may thus seem strange that the proprietors of the SCC came to favour Weldon's relatively unproven invention, and, even after the failure of Weldon's caisson, approved another version of the inclined plane that obviously was not half as successful as the Hay version!
In these next paragraphs it should be noted that IE3 has problems reading the French! Netscape appears much better!

A Frenchman called Dutens also wrote extensively on the caissons etc in England. This source may well have been overlooked by previous historians. (Mike Clarke reports that the pages of the book had not been cut!). Dutens quoted extensively from both Reess (sic) and Chapman. (Full references are in Clew (ibid)). However it is obvious from his writings that he "visited the canal near Bath"

In a passage he appears to have written himself, and it has to be admitted that it is somewhat ambiguous, Dutens might be suggesting that the caisson had two doors (at each end):

..."les deux portes du caisson et du puits Ètaient ouvertes pour laisser passer le bateau".

(The 2 doors of the caisson and pit (or well) were opened to let the boat through).

However it could be read that the caisson and exit doorway on the side of the pit each had one door - making the pair. Probably this is what he meant...

Also he states that the building of a second caisson was suspended and that the mechanism for the raising and lowering this caisson was much simpler (than in the first).

"Quant ý la seconde Ècluse ý flotteur qui est restÈe sans exÈcution, bien que le mÈcanisme en dut paraÓtre d'une application beaucoup plus facile."

(As for the second floating lock this has remained without use, although the mechanism of it would appear to be a much simpler implementation).

As a footnote Dutens relates that a French Engineer (M. Sganzin) observed that the first caisson mechanism at Combe Hay was soon modified and was thus very similar to another planned caisson lock mechanism for the canal at Ruabon (near Wrexham in Wales). (My emphasis).
..."(il) fait observer qu'il fut modifÈ et corrigÈ peu de temps aprËs son exÈcution. La modification qu'indique M. Sganzin, semble rapprocher beaucoup ce systËme de celui de l'Ècluse de Ruabon,..."

There are three interpretations of these passages:

(1) the first lock used the rack and pinion gear to raise and lower the caisson whilst the second made more use of adjusting the mass of the caisson by water.
(2) Dutens is describing another, cradle-caisson lock, patented by Rowland & Pickering, in another part of the country.
(3) The second (Combe Hay) lock was different from the first and was much more like that of Rowland & Pickering's design. This is most unlikely. However see later!.

Just how Weldon's device was to be like that of the said Rowland & Pickerings' is not made clear. It is evident from Dutens that he believed that their design was superior to that of Weldon's, and would almost certainly have worked. I, too, believe it was superior. However, accepting the 'classic' reason for the demise of Weldon's lock - that of building it in Fullers Earth, - R&P's design would have foundered for the same reason!

"As to the second floating lock which was not constructed, even though its mechanism must have seemed much easier, I shall content myself with reporting from Reess (sic) (who states that) Messrs Rowland and Pickering took out a patent on the 18th of March 1794 for the invention of a barrel vault- or cradle- mobile lock, which, supported by props borne by a large plunger (diver), was able to be raised or lowered, so as to connect, according to need, with the level of the upper or lower reach whenever boats are ready to enter the airlock". (Translated from the French).
(The boat entered an open-top "caisson" with doors at each end. The enormous mass was counterbalanced by a giant watertight box in water. There were struts that connected the two together (the "caisson" or cradle being directly atop of the watertight box ("plunger or diver")). The cradle, when it descended, did so in an air-filled shaft. There were clever devices (spirals of weights and/or weights in conical barrels) to counter the apparent loss in mass as the struts sank into the water.)
Very rough sketch - my interpretation

Other interesting snippets from the few pages I have are:

William Smith (not unreasonably) visited the Combe Hay site.
(Torrens dates this as 1798 and 9)

Dutens states that Reess (sic) gives more detail on ..."which pumps are used by the boatmen who are shut away in the caisson aboard their boat in order to empty or to bring in the necessary volume of water in order to lighten or load the boat; and how the air pipes work so as to prevent accidents which would result from a lack of air in a situation where some circumstances would hold the caisson and the men below the water for too long." (translated form the French).

(One wonders whether the caisson was thus served by a suitable flexible (leather?) pipe from its roof to the atmosphere on the higher level). No such mention is made of this sensible precaution in Weldon's patent.

Dutens observed the inclined plane.

Dutens states not just that the walls of the chamber bulged but that they collapsed, and that this had led to the total abandonment of the workings ..."even though they had got the diver/plunger to work!" (My emphasis). The plunger/diver was a term reserved for R&P's airtight box. Surely, if it had been Weldon's design, he would have used the word "caisson". Again this reinforces the possiblity that the first caisson lock of Weldon's was modified to accommodate refinements from Rowland and Pickering's design. Very interesting, but hard to believe, especially as, if fully implemented, it would have necessitated the well to be twice as deep! Perhaps only minor alterations were made to Weldon's first design and that this explains the forementioned differences in the accounts of how the effective mass of the caisson was changed to raise or lower it. (Pulleys/weights/ water)

If any more information comes to light I would be pleased to pass it on.

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Possible Sites for the Caisson:

Abstract.

On the branch canal leading to Engine Wood.

Excavations by BIAS many years ago at the site showed lots of old brickwork but it was soon realised that this was the site of the pumping station!.. ( and thus not the caisson). Besides, the 'new cut' i.e. the branch canal or Top Reach, was built in 1801 - that is , according to Torrens, after the closure of the caisson.

However ... New Evidence:
- THE TOP REACH
  The top reach (i.e. the cut to the former pumping station) would not have needed to have been built when the caisson was in operation because the canal coming from Paulton had not got beyond Dunkerton at that time. There surely would only have been a short length of trial canal to test out the caisson.
  This negates the above paragraph.
- The canal ("top reach') now existing at the pumping engine site is full width (14ft+) for some distance. As the engine's consumption of coal would have been about 1 boatload per fortnight, a narrow reach would have sufficed because two boats would never need to pass.
  At Crofton, there isn't even a feeder canal, the engineman had to fetch the coal up in a wheelbarrow from the main line below. Why would a full sized canal reach have been built to Engine Wood at a time when the S.C.C. was strapped for cash? Or did it already exist because it was part of the caisson experiment?
- Sutcliffe's Estimates for the cost of replacing the inclined plane with locks:
  "Cutting a Draw from the tail of the last Lock to the Engine pit,two feet wide, three feet high, and arching it over where necessary - £564 10 0
  Sinking the Engine pit 30 [yards and] walling it where necessary, and one of the Caisson pits will serve for it, as far as it is sunk." £65. 0 0
  
  The depth of the engine pit is known to have been 135ft but they only estimated for sinking 90ft of that depth. The rest, 45ft, was the internal depth of the caisson; so it looks as though the pumping engine well was sunk through the floor of the caisson.
- The ledge below the present engine site is approx. 43-45 feet lower - just about the drop in Weldon's design and consistent with the view thatthe tunnel here would have been 'robbed-out' for stone and then allowed to collapse.
- SEISMIC EVIDENCE
  (Somewhat crude it is admitted)
  The boundary between the two distinct sounds from the geophone followed a curving path on the western side, exactly where the backing rock face would have been behind the wall of the caisson if the stonework had been robbed-out. This was approx. 80ft long i.e. the length of the caisson!
  
  On 29/9/99 Another survey was undertaken with an unbiased observer.
  The results can be seen at:Adrian's seismic survey -data.
  
  PS. On 2/10/99 I too went to the site and listened with Adrian. I can confirm the reduction in the bass reverbration as one crosses the 'west' wall. It is interesting to note, as Adrian says, that tree roots in this area often grow from walls. There are trees marking the north, south, east & west "walls". Those to the east and west 'bulge slightly from the mid-line - just like Weldon's design.
  However a danger here is that it way be the tree roots themselves that are contributing to this anomoly.
  
  Please visit this next site. I've taken pictures (Sept 1999) of this site and very crudely, have attempted to add various annotations. Pictures of the pump-house site
  (Jan 2000)
  Adrian writes:
  There is further news from the Engine Site. We have uncovered the capping of the disused well and found (with the geophone) that there is probably a solid connection a long way underground between the well and the engine foundations. There should be another work party on that site in the Spring.
- THE MAPS
  There are several maps which show the caisson (or all three caissons) in caisson field. When these were scanned for a presentation it was found that they all needed to be turned to get north at the top. - All by the same angle!
  "It appears they were all copied from the same original map which pre-dates the actual building of the caisson and so cannot be taken as evidence for its final construction site."
  However, as Mr. F Pole has observed (Oct 1999), an alternative explanation might be that all the cartographers used Magnetic North raher than True North.
- DESTRUCTION BY GEOLOGICAL PRESSURE
  If the caisson was in Caisson Field it would have to have been built at right angles to the contour lines so as to avoid an extensive and tortuous lower exit tunnel. This would need a right-angled turning basin built in the top canal to allow boats to sweep round into the caisson entrance. There is no sign that such a basin was excavated back into the hillside so it has always been assumed the canal was turned in such a way that the caisson stuck out over the gentle drop (where we excavated and found nothing). If this had been the case, the caisson would have been embanked for much of its height and length and would have been relatively immune to geological pressures. The fact that it leaked and started to collapse, virtually proves that it could not have been at Caisson Field.
- If it were at Engine Wood, it would have been built on the natural 'nose' of a projecting contour; this would give the shortest exit tunnel and cheapest structure, without any bends either top or bottom. It would also make it susceptible to geological pressure on the uphill side, where it was cut back into a rock face.
- THE SECOND CAISSON SITE
  The exit tunnel would have been 45ft below the upper canal level. There is a plateau about 43 - 45ft lower down the Engine Wood hillside. To cross the valley would then have needed a small aqueduct, similar to the one at Dunkerton Hollow. The next caisson (which was started and abandoned) would have had to have been further along that 45ft-below-summit-level contour. If the contour 'nose' theory is correct,we would expect to find it at another sharp turn in the contour.
  Quarter of a mile down the valley, at a 'nose' in the contour, is another excavation. It is now occupied by Glen Cottage but is marked on the map as 'old quarry'. The material it could have supplied as a quarry would only have been useable as backfill, if at all. The site is about the size of a caisson.
- THE PUMPING ENGINES
  "The pumping engines were planned as a pair but the first one to be built was on the eastern (downhill) side of the (obviously man-made) plateau. This does not make sense in ordinary engineering terms, because it would have needed a lot more preparatory work than if it had been on the western, uphill, side. However, if the remains of the caisson were being used as a foundation, the damaged uphill side would take time to stabilise after the water was removed and replaced with infill, so the downhill side would have to be the first side to be built on."
  
  I am indebted to a SCC member, Adrian, for this exciting information. Most of the above detail has been taken from e'mail correspondence between the two of us. I believe he has put up an excellent case although I'm not quite convinced (yet) about the geological evidence. I also feel that a company that was willing to start work on the second and perhaps third, caisson when the first was still unproven, would have built the foundations at least, for the aqueduct about which we have no plans, accounts or financial estimates.
  Also (from Torrens) comes this piece of information:
  "In 13th Nov.1795 the Committee had advertised for...proposals... for undertaking about 30 yards of deep cutting and driving a tunnel 30 yards in length by 13ft high by 10 1/2 feet wide to an intended caisson lock." (my emphasis)
  The upper reach is much longer than 30 yards and is not in a deep cutting; nor is there a tunnel (unless of course it refers to the exit tunnel!)
  The Committee must therefore have known of the location of the caisson site and thus one would have supposed that the (?next) map of 1796 would be accurate.
  
  I also wonder whether a cash-sensitive committee would have countenanced the extra cost involved in taking the canal, via a short aqueduct, across a valley when there was no need.
  
  However, Adrian's final piece of evidence is rather good. He has scaled and overlain maps of the area with his curved wall and the pump plans. The effect is startling!
  See Adrian's overlays at Engine Wood
  
  Adrian also gives a plausible explanation for the deviation in the line of locks between locks 5 & 6.
  Previous texts have assumed that this strange 'kink' is because it needed to by-pass the caisson site. Adrian's explanation is the kink is a relic of the building of the locks here which needed to go under the inclined plane which was still operational. He also opines that if the caisson site was nearby, the locks ( lock 5), would NOT have been built close to it because of possible leakage/ subsidence into the former well.
  Please visit: Kinky link
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Continuation of the possible sites:
The 'old reservoir' above Caisson House This can be clearly seen in the following map which should be compared with another version in the body of the 'history' pages of this site.
Mitchell (1874) stated that " The flag-pole (see map) opposite Caisson House marks the site of the ill-fated Caisson"
This cannot be as the effective depth of the caisson and its distance from the entry point (approx. on the 240ft contour line (250ft is marked on the map) do not match. However the most likely explanation is that the flag-pole marked the entrance to the second caisson!
Clew originally thought that the site must be somewhere between the present remains of locks 5 and 6 in front of Caisson House. However he apparently now concedes that the chestnut tree marks the spot. (See map)

NEW

Writing in the November 1999 'Weigh House", Clew obviously still believes that the caisson is here. The nearby hillock which was once partly excavated by Mr R Bilby and his son (for a school project). ..."They ... found many rusty nails and numerous stone chippings which seemingly came from 'dressing' stone blocks for further use. We wondered why this was done, not realising that Ray Bilby had discovered part of the caisson site." (p10).
Not unreasonably, Clew states that probably much of the stone work was taken away and used in the construction of the locks, rather than in the construction of Caisson House. However it should be noted that this (sensible) idea is at variance with point 4 (below).

Torrens obviously believes, as with Clew, that the caisson is here, as he points to 4 different pieces of evidence, all pointing to the same spot:

1. The 1796 Map & notesSmith-Cary map shows the the course of the caisson to the SW of the late inclined plane.

2. The Geological Evidence. The caisson was cut in the (approx.) 45 feet of Fullers Earth (with its characteristic small oyster fossil of Praexogyra acuminata) in this area. From a comparison with the Geological Map ("Frome") one can see that this starts at approx. the summit of the canal here, that is the 240ft contour.
(The top wall must have been a few feet above this of course ..."so much higher than the upper canal as to contain a height of water just sufficient to cover the Caisson when opposite the upper level" (Farey 1806))

3. "Evidence from the 1810 map .. is crucial." (Torrens). The canal basin that probably served the top caisson can clearly be seen to the SW of Caisson House (CH on the map).

4. James Tunstall's "Rambles about Bath and its neighbourhood" (1876) mentions Combe Hay ("A lovely spot though art, Combehay" (sic)) and that the site of the caisson was near to Caisson House and marked with a chestnut tree. "The drop was 60feet and the walls are believed to be still perfect as when filled up".

References:

Torrens. H.(1975) "The Somersetshire Coal Canal Caisson Lock". (BIAS)
I am indebted to Simon Jones for a copy of this document.
Clew. K.R. "The Somersetshire Coal Canal and Railways".2nd ed. Pub.: Head. 1982.

Dutens. "Notice sur les Elevators".
Sorry no more info. on this important and, perhaps, previously untapped, source. I have asked for more detail.
A. Tuddenham ( countless e'mails!)

I am indebted to Jackie Dixon for the translation from the French.

If anyone can assist with re-reading the French to ensure the translation is accurate please contact me.

Please contact me by E'mail:rtj@enterprise.net or tel: (0114) 2630-797.

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