He had some difficulty in inducing the Coalbrookdale iron-masters, who undertook the casting of the girders, to depart from the plan of the earlier structure; but he persisted in his design, which was eventually carried out. It consisted of a single arch of 130 feet span, the segment of a very large circle, calculated to resist the tendency of the abutments to slide inwards, which had been a defect of the Coalbrookdale bridge; the flat arch being itself sustained and strengthened by an outer ribbed one on each side, springing lower than the former and also rising higher, somewhat after the manner of timber-trussing. Although the span of the new bridge was 30 feet wider than the Coalbrookdale bridge, it contained less than half the quantity of iron; Buildwas bridge containing 173, whereas the other contained 378 tons. The new structure was, besides, extremely elegant in form; and when the centres were struck, the arch and abutments stood perfectly firm, and have remained so to this day. But the ingenious design of this bridge will be better explained by the following representation than by any description in words.*[4] 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."*[5] 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 uses of cast iron in canal construction became more obvious with every year's successive experience; and Telford was accustomed to introduce it in many cases where formerly only timber or stone had been used. On the Ellesmere, and afterwards on the Caledonial Canal, he adopted cast iron lock-gates, which were found to answer well, being more durable than timber, and not liable like it to shrink and expand with alternate dryness and wet. The turnbridges which he applied to his canals, in place of the old drawbridges, were also of cast iron; and in some cases even the locks were of the same material. Thus, on a part of the Ellesmere Canal opposite Beeston Castle, in Cheshire, where a couple of locks, together rising 17 feet, having been built on a stratum of quicksand, were repeatedly undermined, the idea of constructing the entire locks of cast iron was suggested; and this unusual application of the new material was accomplished with entirely satisfactory results.
But Telford's principal employment of cast iron was in the construction of road bridges, in which he proved himself a master. His experience in these structures had become very extensive. During the time that he held the office of surveyor to the county of Salop, he erected no fewer than forty-two, five of which were of iron. Indeed, his success in iron bridge-building so much emboldened him, that in 1801, when Old London Bridge had become so rickety and inconvenient that it was found necessary to take steps to rebuild or remove it, he proposed the daring plan of a cast iron bridge of a single arch of not less than 600 feet span, the segment of a circle l450 feet in diameter. In preparing this design we find that he was associated with a Mr. Douglas, to whom many allusions are made in his private letters.*[6] The design of this bridge seems to have arisen out of a larger project for the improvement of the port of London. In a private letter of Telford's, dated the 13th May, 1800, he says:
"I have twice attended the Select Committee on the Fort of London, Lord Hawkesbury, Chairman. The subject has now been agitated for four years, and might have been so for many more, if Mr. Pitt had not taken the business out of the hands of the General Committee, and got it referred to a Select Committee. Last year they recommended that a system of docks should be formed in a large bend of the river opposite Greenwich, called the Isle of Dogs, with a canal across the neck of the bend. This part of the contemplated improvements is already commenced, and is proceeding as rapidly as the nature of the work will admit. It will contain ship docks for large vessels, such as East and West Indiamen, whose draught of water is considerable.
"There are now two other propositions under consideration. One is to form another system of docks at Wapping, and the other to take down London Bridge, rebuild it of such dimensions as to admit of ships of 200 tons passing under it, and form a new pool for ships of such burden between London and Blackfriars Bridges, with a set of regular wharves on each side of the river. This is with the view of saving lighterage and plunderage, and bringing the great mass of commerce so much nearer to the heart of the City. This last part of the plan has been taken up in a great measure from some statements I made while in London last year, and I have been called before the Committee to explain. I had previously prepared a set of plans and estimates for the purpose of showing how the idea might be carried out; and thus a considerable degree of interest has been excited on the subject. It is as yet, however, very uncertain how far the plans will be carried out. It is certainly a matter of great national importance to render the Port of London as perfect as possible."*[7]
Later in the same year he writes that his plans and propositions have been approved and recommended to be carried out, and he expects to have the execution of them. "If they will provide the ways and means," says he, "and give me elbow-room, I see my way as plainly as mending the brig at the auld burn." In November, 1801, he states that his view of London Bridge, as proposed by him, has been published, and much admired. On the l4th of April, 1802, he writes, "I have got into mighty favour with the Royal folks. I have received notes written by order of the King, the Prince of Wales, Duke of York, and Duke of Kent, about the bridge print, and in future it is to be dedicated to the King."
The bridge in question was one of the boldest of Telford's designs. He proposed by his one arch to provide a clear headway of 65 feet above high water. The arch was to consist of seven cast iron ribs, in segments as large as possible, and they were to be connected by diagonal cross-bracing, disposed in such a manner that any part of the ribs and braces could be taken out and replaced without injury to the stability of the bridge or interruption to the traffic over it. The roadway was to be 90 feet wide at the abutments and 45 feet in the centre; the width of the arch being gradually contracted towards the crown in order to lighten the weight of the structure. The bridge was to contain 6500 tons of iron, and the cost of the whole was to be 262,289L.
[Image] Telford's proposed One-arched Bridge over the Thames.
The originality of the design was greatly admired, though there were many who received with incredulity the proposal to bridge the Thames by a single arch, and it was sarcastically said of Telford that he might as well think of "setting the Thames on fire." Before any outlay was incurred in building the bridge, the design was submitted to the consideration of the most eminent scientific and practical men of the day; after which evidence was taken at great length before a Select Committee which sat on the subject. Among those examined on the occasion were the venerable James Watt of Birmingham, Mr. John Rennie, Professor Button of Woolwich, Professors Playfair and Robison of Edinburgh, Mr. Jessop, Mr.Southern, and Dr. Maskelyne. Their evidence will still be found interesting as indicating the state at which constructive science had at that time arrived in England.*[8] There was a considerable diversity of opinion among the witnesses, as might have been expected; for experience was as yet very limited as to the resistance of cast iron to extension and compression. Some of them anticipated immense difficulty in casting pieces of metal of the necessary size and exactness, so as to secure that the radiated joints should be all straight and bearing. Others laid down certain ingenious theories of the arch, which did not quite square with the plan proposed by the engineer. But, as was candidly observed by Professor Playfair in concluding his report—"It is not from theoretical men that the most valuable information in such a case as the present is to be expected. When a mechanical arrangement becomes in a certain degree complicated, it baffles the efforts of the geometer, and refuses to submit to even the most approved methods of investigation. This holds good particularly of bridges, where the principles of mechanics, aided by all the resources of the higher geometry, have not yet gone further than to determine the equilibrium of a set of smooth wedges acting on one another by pressure only, and in such circumstances as, except in a philosophical experiment, can hardly ever be realised. It is, therefore, from men educated in the school of daily practice and experience, and who to a knowledge of general principles have added, from the habits of their profession, a certain feeling of the justness or insufficiency of any mechanical contrivance, that the soundest opinions on a matter of this kind can be obtained."
It would appear that the Committee came to the general conclusion that the construction of the proposed bridge was practicable and safe; for the river was contracted to the requisite width, and the preliminary works were actually begun. Mr. Stephenson says the design was eventually abandoned, owing more immediately to the difficulty of constructing the approaches with such a head way, which would have involved the formation of extensive inclined planes from the adjoining streets, and thereby led to serious inconvenience, and the depreciation of much valuable property on both sides of the river.*[9] Telford's noble design of his great iron bridge over the Thames, together with his proposed embankment of the river, being thus definitely abandoned, he fell back upon his ordinary business as an architect and engineer, in the course of which he designed and erected several stone bridges of considerable magnitude and importance.