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From Nantwich to Whitchurch the distance is 16 miles, and the rise 132 feet, involving nineteen locks; and from thence to Ellesmere, Chirk, Pont-Cysylltau, and the river Dee, 1 3/4 mile above Llangollen, the distance is 38 1/4 miles, and the rise 13 feet, involving only two locks.
But the ingenious design of this bridge will be better explained by the following representation than by any description in words.* The bridge at Buildwas, however, was not Telford's first employment of iron in bridge-building; for, the year before its erection, we find him writing to his friend at Langholm that he had recommended an iron aqueduct for the Shrewsbury Canal, "on a principle entirely new," and which he was "endeavouring to establish with regard to the application of iron."* This iron aqueduct had been cast and fixed; and it was found to effect so great a saving in masonry and earthwork, that he was afterwards induced to apply the same principle, as we have already seen, in different forms, in the magnificent aqueducts of Chirk and Pont-Cysylltau.
The other great aqueduct on the Ellesmere Canal, named Pont-Cysylltau, is of even greater dimensions, and a far more striking object in the landscape. Sir Walter Scott spoke of it to Southey as "the most impressive work of art he had ever seen." It is situated about four miles to the north of Chirk, at the crossing of the Dee, in the romantic vale of Llangollen.
At another time, when engaged upon the viaduct at Pont-Cysylltau, he snatched a few day's leisure to run through North Wales, of which he afterwards gave a glowing account to his correspondent. He passed by Cader Idris, Snowdon, and Penmaen Mawr. "Parts of the country we passed through," he says, "very much resemble the lofty green hills and woody vales of Eskdale.
He had a model made of part of the proposed aqueduct for Pont-Cysylltau, showing the piers, ribs, towing-path, and side railing, with a cast iron trough for the canal. The model being approved, the design was completed; the ironwork was ordered for the summit, and the masonry of the piers then proceeded.
One of these a cast iron bridge, at Galton, of 150 feet span has been much admired for its elegance, lightness, and economy of material. Several others of cast iron were constructed at different points, and at one place the canal itself is carried along on an aqueduct of the same material as at Pont-Cysylltau.
Mr Telford largely employed cast iron in the construction of this canal, using it in the locks and gates, as well as in an extensive aqueduct which it was necessary to construct over a deep ravine, after the plan pursued by him at, Pont-Cysylltau and other places. The last canal constructed by. Mr.
The latter part of the undertaking presented the greatest difficulties; as, in order to avoid the expense of constructing numerous locks, which would also involve serious delay and heavy expense in working the navigation, it became necessary to contrive means for carrying the canal on the same level from one side of the respective valleys of the Dee and the Ceriog to the other; and hence the magnificent aqueducts of Chirk and Pont-Cysylltau, characterised by Phillips as "among the boldest efforts of human invention in modem times."* The Chirk Aqueduct carries the canal across the valley of the Ceriog, between Chirk Castle and the village of that name.
He had in the meantime been carefully gathering experience from a variety of similar undertakings on which he was employed, and bringing his observations of the strength of materials and the different forms of construction to bear upon the plans under his consideration for the great aqueducts of Chirk and Pont-Cysylltau.
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