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Issue 99 - The Remarkable Mr. Rennie

Scotland Magazine Issue 99
June 2018

 

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The Remarkable Mr. Rennie

John Hannavy explores the achievements of another great Scottish engineer

 

Every day, thousands of people cross over the 200-year-old bridge that carries the A699 across the Tweed in the Borders town of Kelso. Most don’t give it a second glance. Yet, this elegant bridge, which was built between 1800-1803, is one of many surviving structures across Britain designed by the eminent Scottish engineer John Rennie the Elder.

While both James Watt (See: Scotland Magazine #92) and Thomas Telford (See: Scotland Magazine #98) are known to most people, John Rennie’s name has always seemed to be slightly in the background. Nevertheless, his impact on the transport infrastructure of Britain is equally important. We were taught about Telford and Watt at school, but I had not heard Rennie’s name mentioned until I went to Manchester as a student in the early 1960s, while studying photographic technology.

Wandering around the city with my camera - it was my first time spent outside Scotland - I came across the then-derelict Rochdale Canal and became fascinated by it. In those days, there was none of the informative signage that abounds today, so my curiosity about who had built it, and when, drew me to the city’s wonderful Central Library. There I read about a few of John Rennie’s many achievements. Rennie was the youngest child of James, a brewer and tenant farmer, and Jean Rennie, and was born at Phantassie Farm near East Linton, East Lothian, in June 1761. John was only five years of age when his father died, so he was raised by his mother and elder brother, George.

From a very early age the boy developed a fascination for anything mechanical. Luckily, almost on his doorstep were several very mechanical sites: Preston Mill and Houston Mill on the River North Tyne. Rennie’s farmer brother would probably have been a customer to one, or maybe both, of them.

In charge of the Houston Mill, which is now a private house, during Rennie’s childhood was another eminent Scottish engineer: the millwright Andrew Meikle, who is best known for inventing (or perhaps just improving) the threshing machine around 1786. Meikle had, in the early 1770s, also designed a revolutionary system of slatted sails for windmills, which hugely improved their efficiency and allowed them to be adjusted mechanically in changing wind conditions. Houston Mill, however, like Preston Mill nearby, was water-powered. Preston Mill was maintained by Meikle for several years, perhaps even assisted by Rennie, and today has been beautifully preserved by the National Trust for Scotland. It is open to the public.

Meikle took the young Rennie under his wing in 1773 and taught him the basics of mechanics, especially with regard to mills. A millwright in those days was usually someone involved with the construction, operation and maintenance of corn mills. However, as the 18th Century drew to a close and industrialisation accelerated, millwrights were in demand in much larger, industrial mills. It seemed like an ideal career and, at the age of just 18, John Rennie set himself up in business as a millwright, apparently managing to balance the demands of his growing enterprise while pursuing academic studies at Edinburgh University.

After graduating with a degree in Natural Philosophy and Practical Sciences, in 1783 Rennie embarked on a tour of some of the industrial centres of England. One of the places he visited was Matthew Boulton’s Soho Manufactory, which had been established in Birmingham eight years earlier. There he met James Watt, who was developing a large condensing steam engine that was to be installed in London’s Albion Mills, which were being constructed at the time by Boulton. Watt must have been impressed by what he saw in Rennie, for the following year he offered him a job.

Boulton and Watt were, at the time, assembling condensing steam engines of their own design but not yet manufacturing all the components themselves - they would not bring the whole manufacturing process under their own roof until 1795. Rennie’s role seems to have been supervising the installation of the first of these steam giants. Amongst the mills in which Rennie installed steam engines were London’s Albion Mills, the world’s first steam-powered corn mill.

Despite this early commitment to mill machinery, Rennie’s career as a millwright was short-lived and by 1790 he had been appointed as Surveyor to the Kennet & Avon Canal Company. This is just one early example of his many achievements as a canal and bridge builder.

As well as redesigning part of the canal’s route, which runs from Bristol to Reading, Rennie’s major contribution was the replacement of the horse-drawn railway up Caen Hill near Devizes with a magnificent flight of 16 locks, which raise the canal up the steep hill. The Caen Hill lock system is actually three groups of locks raising the canal 237 feet (72m) in just under two miles (3.2km).

The project, which was completed in 1810, required the creation of large lagoons to one side of 15 of the locks in order to store the huge amounts of water needed to operate them and also to provide ‘parking’ for boats waiting to enter the next lock up or down.

Further along the canal, at Crofton, a steampowered pumping station was installed to feed water to one of the highest stretches of the waterway and, given his earlier time with James Watt, it is no surprise that the steam engine to power it was ordered from Boulton & Watt. It was operational by 1812 and is still running today, albeit as a tourist attraction - electric pumps now raise the water.

The Kennet & Avon was not Rennie’s first canal project. His Crinan Canal was opened in 1801, construction work having started on it in 1794. Three years earlier, in 1791, he had surveyed the route of the Rochdale Canal in Lancashire, his first major involvement in canal design, and would fill the roles of both surveyor and engineer on that project. Construction work on the Rochdale Canal and the Crinan Canal started within months of each other, but while the 32-mile Rochdale Canal would take almost 10 years to complete, the comparatively short nine miles between Ardrishaig and Crinan took seven.

But Rennie’s involvement with plans to make a cut across the Kintyre peninsula had started long before that. In 1781, when just 20 years old, he had been part of a team involved in surveying possible routes to avoid the long sail around Kintyre. It took four years for the Parliament to approve the formation of a company to develop the project and a further 16 years before the canal opened.

Two routes had been considered, the first being from Ardrishaig to Port Righ (now known as Crinan) and the other longer route to Duntrune on the north side of Crinan Loch. Had the longer route to Duntrune been chosen, the canal would have had to traverse Moine Mhor, the unstable mossy landscape on either side of the River Add.

The planned duration of the project was a mere three years, but such estimates proved wildly optimistic. Even the chosen route, along the edge of the moss, was fraught with construction difficulties. Indeed, the whole building project took more than twice as long as expected, with hard rocks to be cut through for the descent down into Crinan, and soft peaty bogs to be stabilised between Dunardry and Islandadd Bridge. As the canal was built before the age of steam, the towpath had to withstand the constant pounding of horses’ hooves.

The canal was still incomplete when it eventually opened to traffic and the increasing amount of coastal shipping in those first years of operation only served to exacerbate the problems. The rubble masonry construction of the canal sides and towpaths collapsed in several locations, the American oak timbers of the lock gates rotted prematurely and had to be replaced, and by 1805 a completely new stretch of canal had to be constructed between Cairnbaan and Oakfield, so severe had been the erosion of the original section. It was 1809 before the project was deemed ‘complete’.

It was not Rennie who was charged with sorting those problems out, however. That job went to Thomas Telford, who was already creating cuts between the lochs in the Great Glen, which would become the Caledonian Canal. Both canals have been in constant use since their completion, though today they are used mainly by pleasure craft. 

 

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