25 December 2010

Pontcysyllte Aqueduct

The Pontcysyllte Aqueduct, designed by Thomas Telford and built under the supervision of canal engineer William Jessop, is the highest navigable aqueduct in the world. It is also the longest aqueduct in Britain, a Grade I listed monument, and a World Heritage Site. It carries the Llangollen Canal at a height of 126 feet over the River Dee, between Froncysyllte (of Male Voice Choir fame) at its southern end and Trevor at its northern.

18 hollow piers, built of finely dressed sandstone, with narrow joints of lime mortar into which was mixed ox blood, support arched cast-iron ribs, four for each of the 19 spans of 53 feet. Each rib, the internal pairs pierced and the external pairs solid, is made from three voussoirs. Atop these arches sits a cast-iron trough, 1,007 feet long, 11 feet 10 inches wide, and 5 feet 3 inches deep.

The plates of the trough's bed are not affixed to the arches, but lugs fit either side of the ribs to keep all steady. The trough's sides are formed of bolted flanged plates of cast iron, also voussoirs, but only so as to form a decorative feature. The joints between the plates were bedded with Welsh flannel, white lead and iron particles.

Commenced in July 1795, the aqueduct was opened only ten years later, having cost just £47,000 (about £3 million at today's prices - fantastically cheap). When construction was completed the trough was filled with water and left for six months to test for leaks. Not a drop was lost, and the aqueduct opened in November 1805.

A towpath is mounted to one side of the trough, with a railing along its length. There are mounting holes on the other side, but no railing was ever fitted. The water level is just six inches below the top edge of the trough. As steeplejack Fred Dibnah put it: "... even for somebody like me with a head for heights, it can be a bit unnerving, because it feels as though the barge you are on is going to float right off the edge." From this height and in the snow, even the sewerage farm below looks attractive.

But how does one pronounce Pontcysyllte? The BBC's Pronunciation Unit recommends "pont-kussuhl-tay."

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