Ellesmere canal include two achievements which were on a scale then unparalleled in England and marked by great originality. The aqueducts over the valley of the Ceiriog at Chirk and over the Dee at Pont-Cysylltau have been pronounced by the chief English historian of inland navigation to be ‘among the boldest efforts of human invention in modern times.’ The originality of the conception carried out lay in both cases not so much in the magnitude of the aqueducts, unprecedented as this was, as in the construction of the bed in which the canal was carried over river and valley. A similar feat had been performed by Brindley, but he transported the water of the canal in a bed of puddled earth, and necessarily of a breadth which required the support of piers, abutments, and arches of the most massive masonry. In spite of this the frosts, by expanding the moist puddle, frequently produced fissures which burst the masonry, suffering the water to escape, and sometimes causing the overthrow of the aqueducts. For the bed of puddled earth Telford substituted a trough of cast-iron plates infixed in square stone masonry. Not only was the displacement produced by frosts averted, but there was a great saving in the size and strength of the masonry, an enormous amount of which would have been required to support a puddled channel at the height of the Chirk and Pont-Cysylltau aqueducts. The Chirk aqueduct consisted of ten arches of forty span each, carrying the canal 70 ft. above the level of the river over a valley 700 ft. wide, and forming a most picturesque object in a beautiful landscape. On a still larger scale was the Pont-Cysylltau aqueduct over the Dee four miles north of Chirk and in the vale of Llangollen; 121 ft. over the level of the river at low water the canal was carried in its cast-iron trough, with a water-way 11 ft. 10 in. wide, and nineteen arches extending to the length of 1,007 ft. The first stone of the Chirk aqueduct was laid on 17 June 1796, and it was completed in 1801. The first stone of the other great aqueduct was laid on 25 June 1795, and it was opened for traffic in 1805. Of this Pont-Cysylltau aqueduct Sir Walter Scott said to Southey that ‘it was the most impressive work of art which he had ever seen’ (Smiles, p. 159).
In 1800 Telford was in London giving evidence before a select committee of the House of Commons which was considering projects for the improvement of the port of London. One of these was the removal of the old London Bridge and the erection of a new one. While surveyor of public works for Shropshire Telford had had much experience in bridge-building. Of several iron bridges which he built in that county, the earliest, in 1795–8, was a very fine one over the Severn at Buildwas, about midway between Shrewsbury and Bridgnorth; it consisted of a single arch of 130 feet span. He now proposed to erect a new London Bridge of iron and of a single arch. The scheme was ridiculed by many, but, after listening to the evidence of experts, a parliamentary committee approved of it, and the preliminary works were, it seems, actually begun. The execution of the bold project was not proceeded with, on account, it is said, of difficulties connected with making the necessary approaches (ib. p. 181). But Telford's plan of the new bridge was published in 1801, and procured him favourable notice in high quarters, from the king and the Prince of Wales downwards.
Telford's skill and energies were now to be utilised for an object very dear to him, the improvement of his native country. At the beginning of the century, at the instance of his old friend Sir William Pulteney, who was governor of the British Fisheries Society, he inspected the harbours at their various stations on the northern and eastern coasts of Scotland, and drew up an instructive and suggestive report. Telford's name was now well known in London, but doubtless this report contributed to procure him in 1801 a commission from the government to undertake a far wider Scottish survey. This step was taken from considerations partly connected with national defence. There was no naval station anywhere on the Scottish coasts, and an old project was being revived to make the great glen of Scotland, which cuts it diagonally from the North Sea to the Atlantic, available as a water-way for ships of war as well as for traffic. The results of Telford's investigations were printed in an exhaustive report presented to parliament in 1803. Two bodies of commissioners were appointed to superintend and make provision for carrying out his recommendations, which included the construction of the Caledonian canal in the central glen already mentioned, and, what was still more urgently needed, extensive road-making and bridge-building in the highlands and northern counties of Scotland. Telford was appointed engineer of the Caledonian canal, the whole cost of which was to be defrayed by parliamentary grants. The expenditure on the road-making and bridge-building, to be planned by him, was to be met only partly by parliamentary grants, government supplying one half of the money required wherever the landowners were ready to contribute the other
