Pontcysyllte Aqueduct Wrexham County Borough, Wales

The Pontcysyllte Aqueduct (full name in Welsh: Traphont Ddŵr Pontcysyllte) is a navigable aqueduct that carries the Llangollen Canal over the valley of the River Dee in Wrexham County Borough in north east Wales. Completed in 1805, it is the longest and highest aqueduct in Britain, a Grade I Listed Building and a World Heritage Site. When the bridge was built it linked the villages of Froncysyllte, at the southern end of the bridge in the Cysyllte township of Llangollen parish (from where it takes its name), and Trevor (Trefor in Welsh), at the northern end of the bridge in the Trefor Isaf township of Llangollen parish.

Both townships were later transferred to Wrexham County Borough following local government reorganisation. The name is in the Welsh language and means "Cysyllte Bridge". For most of its existence it was known as Pont y Cysyllte ("Bridge of Cysyllte"). Other translations such as "Bridge of the Junction" or "The Bridge that links" are modern, and incorrect, inventions, from the literal English translation of cysyllte being "junctions" or "links", as the township of Cysyllte existed for centuries before the bridge was built. The aqueduct, built by Thomas Telford and William Jessop, is 307 m long, 3.4 m wide and 1.60 m deep. It consists of a cast iron trough supported 38 m above the river on iron arched ribs carried on nineteen hollow masonry piers (pillars). Each span is 16 m wide. The mortar used lime, water and ox blood. The iron castings for the trough were produced at the nearby Plas Kynaston Foundry, Cefn Mawr, which was built by the Shrewsbury ironfounder and millwright William Hazledine in the hope of gaining the contract. The rib castings may have been made at Hazledine's original works at Coleham, near Shrewsbury. The trough was made from flanged plates of cast iron, bolted together, with the joints bedded with Welsh flannel and a mixture of white lead and iron particles from boring waste. The plates are not rectangular but shaped as voussoirs, similar to those of a stone arch. There is no structural significance to their shape: it is a decorative feature only, following the lines of the stiffening plates in the castings beneath. The supporting arches, four for each span, are in the form of cast iron ribs, each cast as three voussoirs with external arches cast with an un-pierced web to give greater strength, at the cost of extra weight. Using cast iron in this way, in the same manner as the stone arch it supersedes, makes use of the material's strength in compression. They also give an impression of greater solidity than would be the case were the webs pierced. This impression is enhanced by the arrangement of strips of thicker stiffening incorporated into the castings, arranged in the manner of joints between voussoirs. Cast plates are laid transversely to form the bed of the canal trough. The trough is not fastened to the arches, but lugs are cast into the plates to fit over the rib arches to prevent movement. It was left for six months with water inside to check it was watertight. A feature of a canal aqueduct, in contrast with a road or railway viaduct, is that the vertical loading stresses are virtually constant. According to Archimedes' principle, the mass (weight) of a boat and its cargo on the bridge, pushes an equal mass of water off the bridge Every five years the ends of the aqueduct are closed and a plug in one of the highest spans is opened to drain the canal water into the River Dee below, for inspection and maintenance of the trough. The aqueduct and surrounding lands were submitted to the "tentative list" of properties being considered for UNESCO World Heritage Site status in 1999. The length of canal from Rhoswiel, Shropshire to the Horseshoe Falls including the main Pontcysyllte Aqueduct structure as well as the older Chirk Aqueduct, were visited by assessors from UNESCO during October 2008, to analyse and confirm the site management and authenticity. The aqueduct was inscribed by UNESCO on the World Heritage List on 27 June 2009.

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