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
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© 2015 Paul Spradbery
Figure
84.3: From this angle, the right-hand edge appears to be unsupported.
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© 2015 Paul Spradbery
Figure
84.4: Floating in mid-air? Not as perilous as it appears.
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© 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
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© 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