Monday, June 29, 2015

Stream In The Sky

In 1795, two British civil engineers initiated an audacious project in northeast Wales. Two centuries later, their creation became a World Heritage Site. Thomas Telford (1745-1814) and William Jessop (1757-1834) designed and built the Pontcysyllte Aqueduct, which carries the Llangollen Canal across the valley of the River Dee (Figure 84.1). More than 300 metres in length, it is the longest aqueduct in Britain; and its cast-iron trough stands, on masonry pillars, 39 metres above the river, making it also the tallest.


Figure 84.1: Further information can be found at

Crown Copyright © 2013 Visit Wales

Last weekend, I took my sons for a long-overdue visit. First, I looked up a long-established firm called ‘Jones The Boats’, whose tree-shaded docks provide a pleasant view from a waterside café. There are passenger narrowboats for hire, catering for 45-minute trips across the aqueduct and back, or more leisurely dinner or party cruises (http://www.canaltrip.co.uk).

With my two young helmsmen perched on the small bow deck, we chugged along the canal (Figure 84.2) which was, in places, only marginally wider than the boat. When the aqueduct itself came into view after a few minutes, I could see that on the left-hand – sorry, port – side was a narrow towpath with nothing separating it from the water’s edge. On the starboard side, there was no barrier at all (Figure 84.3). The water level was only a few inches below the trough rim, and my sons could, and did, lean out and gaze over the edge of the giant bathtub and down into the abyss (Figure 84.4).


Figure 84.2: View from the bow

Copyright © 2015 Paul Spradbery


Figure 84.3: From this angle, the right-hand edge appears to be unsupported.

Copyright © 2015 Paul Spradbery


Figure 84.4: Floating in mid-air? Not as perilous as it appears.

Copyright © 2015 Paul Spradbery

The views were, naturally, spectacular. To the left, peeking through distant woodland and blending perfectly with it, was a Victorian viaduct carrying the Chester-to-Shrewsbury railway. On the (unobscured) right, running beneath us, was the shallow river, a few old stone cottages with characteristic Welsh slate roofs, and much untamed woodland. I could tell that the boys were more than impressed, not least because both maintained a reverent silence as we crossed the sky.

In my experience, having been fortunate enough to have visited more than fifty different countries, this unique trip was admirable for a multitude of reasons. I suppose it was inevitable that, being a scientist, I would find the aqueduct’s functionality as impressive as its aesthetics. Here’s why. An aqueduct differs from a railway viaduct in more than the obvious way. A viaduct, although built on similarly strong stone pillars, experiences variable loading stresses as a train passes across it. Stresses are greatest on those pillars directly beneath the train’s centre of gravity. One of the elegant advantages of an aqueduct is that such stresses remain more or less constant throughout the entire span. Archimedes’ principle states that the boat’s mass on the bridge displaces an equal mass of water off it. Ergo, the combined weight of vessel and water act with unvarying downward force equally on all load-bearing pillars at all times. (Archimedes was more than a mere genius.)

The ‘far’ side of the aqueduct was just as pleasing to the eye. As the waterway gradually broadened out, we turned left in front of a steep, tree-filled terrace, in the middle of which was the bright yellow Aqueduct Inn (Figure 84.5). There, next to a wooden footbridge, we managed to turn the boat through 360 degrees and make the return crossing.


Figure 84.5: The ‘other’ side

Copyright © 2015 Paul Spradbery

Having opened in 1805, it was fitting that on its bicentenary, the Pontcysyllte Aqueduct was formally nominated for UNESCO World Heritage Site status. This jewel of the Welsh countryside was, deservedly, inscribed on the World Heritage List in 2009.

Many thanks to the articulate young female student for answering all my questions.

Copyright © 2015 Paul Spradbery

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