CHAPTER VII.
IRON AND AND OTHER BRIDGES.
Shrewsbury being situated in the immediate neighbourhood of the Black Country, of which coal and iron are the principal products, Telford's attention was naturally directed, at a very early period, to the employment of cast iron in bridge-building. The strength as well as lightness of a bridge of this material, compared with one of stone and lime, is of great moment where headway is ofimportance, or the difficulties of defective foundations have to be encountered. The metal can be moulded in such precise forms and so accurately fitted together as to give to the arching the greatest possible rigidity; while it defies the destructive influences of time and atmospheric corrosion with nearly as much certainty as stone itself.
The Italians and French, who took the lead in engineering down almost to the end of last century, early detected the value of this material, and made several attempts to introduce it in bridge-building; but their efforts proved unsuccessful, chiefly because of the inability of the early founders to cast large masses of iron, and also because the metal was then more expensive than either stone or timber. The first actual attempt to build a cast iron bridge was made at Lyons in 1755, and it proceeded so far that one of the arches was put together in the builder's yard; but the project was abandoned as too costly, and timber was eventually used.
It was reserved for English manufacturers to triumph over the difficulties which had baffled the foreign iron-founders. Shortly after the above ineffectual attempt had been made, the construction of a bridge over the Severn near Broseley formed the subject of discussion among the adjoining owners. There had been a great increase in the coal, iron, brick, and pottery trades of the neighbourhood; and the old ferry between the opposite banks of the river was found altogether inadequate for the accommodation of the traffic. The necessity for a bridge had long been felt, and the project of constructing one was actively taken up in 1776 by Mr. Abraham Darby, the principal owner of the extensive iron works at Coalbrookdale. Mr. Pritchard, a Shrewsbury architect, prepared the design of a stone bridge of one arch, in which he proposed to introduce a key-stone of cast iron, occupying only a few feet at the crown of the arch. This plan was, however, given up as unsuitable; and another, with the entire arch of cast iron, was designed under the superintendence of Mr. Darby. The castings were made in the works at Coalbrookdale, and the bridge was erected at a point where the banks were of considerable height on both sides of the river. It was opened for traffic in 1779, and continues a most serviceable structure to this day, giving the name to the town of Ironbridge, which has sprung up in its immediate vicinity. The bridge consists of one semicircular arch, of 100 feet span, each of the great ribs consisting of two pieces only. Mr. Robert Stephenson has said of the structure—"If we consider that the manipulation of cast iron was then completely in its infancy, a bridge of such dimensions was doubtless a bold as well as an original undertaking, and the efficiency of the details is worthy of the boldness of the conception."*[1]
[Image] The first Iron Bridge, Coalbrookdale.
It is a curious circumstance that the next projector of an iron bridge—and that of a very bold design—was the celebrated, or rather the notorious, Tom Paine, whose political writings Telford had so much admired. The son of a decent Quaker of Thetford, who trained him to his own trade of a staymaker, Paine seems early to have contracted a dislike for the sect to which his father belonged. Arrived at manhood, he gave up staymaking to embrace the wild life of a privateersman, and served in two successive adventures. Leaving the sea, he became an exciseman, but retained his commission for only a year. Then he became an usher in a school, during which he studied mechanics and mathematics. Again appointed an exciseman, he was stationed at Lewes in Sussex, where he wrote poetry and acquired some local celebrity as a writer. He was accordingly selected by his brother excisemen to prepare their petition to Government for an increase of pay, *[2] — the document which he drew up procuring him introductions to Goldsmith and Franklin, and dismissal from his post. Franklin persuaded him to go to America; and there the quondam staymaker, privateersman, usher, poet, an a exciseman, took an active part in the revolutionary discussions of the time, besides holding the important office of Secretary to the Committee for Foreign Affairs. Paine afterwards settled for a time at Philadelphia, where he occupied himself with the study of mechanical philosophy, electricity, mineralogy, and the use of iron in bridge-building. In 1787, when a bridge over the Schnylkill was proposed, without any river piers, as the stream was apt to be choked with ice in the spring freshets, Paine boldly offered to build an iron bridge with a single arch of 400 feet span. In the course of the same year, he submitted his design of the proposed bridge to the Academy of Sciences at Paris; he also sent a copy of his plan to Sir Joseph Banks for submission to the Royal Society; and, encouraged by the favourable opinions of scientific men, he proceeded to Rotherham, in Yorkshire, to have his bridge cast.*[3] An American gentleman, named Whiteside, having advanced money to Paine on security of his property in the States, to enable the bridge to be completed, the castings were duly made, and shipped off to London, where they were put together and exhibited to the public on a bowling-green at Paddington. The bridge was there visited by a large number of persons, and was considered to be a highly creditable work. Suddenly Paine's attention was withdrawn from its further prosecution by the publication of Mr. Burke's celebrated 'Thoughts on the French Revolution,' which he undertook to answer. Whiteside having in the meantime become bankrupt, Paine was arrested by his assignees, but was liberated by the assistance of two other Americans, who became bound for him. Paine, however, was by this time carried away by the fervour of the French Revolution, having become a member of the National Convention, as representative for Calais. The "Friends of Man," whose cause he had espoused, treated him scurvily, imprisoning him in the Luxembourg, where he lay for eleven months. Escaped to America, we find him in 1803 presenting to the American Congress a memoir on the construction of Iron Bridges, accompanied by several models. It does not appear, however, that Paine ever succeeded in erecting an iron bridge. He was a restless, speculative, unhappy being; and it would have been well for his memory if, instead of penning shallow infidelity, he had devoted himself to his original idea of improving the communications of his adopted country. In the meantime, however, the bridge exhibited at Paddington had produced important results. The manufacturers agreed to take it back as part of their debt, and the materials were afterwards used in the construction of the noble bridge over the Wear at Sunderland, which was erected in 1796.
The project of constructing a bridge at this place, where the rocky banks of the Wear rise to a great height oh both sides of the river, is due to Rowland Burdon, Esq., of Castle Eden, under whom Mr. T. Wilson served as engineer in carrying out his design. The details differed in several important respects from the proposed bridge of Paine, Mr. Burdon introducing several new and original features, more particularly as regarded the framed iron panels radiating towards the centre in the form of voussoirs, for the purpose of resisting compression. Mr. Phipps, C.E., in a report prepared by him at the instance of the late Robert Stephenson, under whose superintendence the bridge was recently repaired, observes, with respect to the original design,—"We should probably make a fair division of the honour connected with this unique bridge, by conceding to Burdon all that belongs to a careful elaboration and improvement upon the designs of another, to the boldness of taking upon himself the great responsibility of applying. this idea at once on so magnificent a scale, and to his liberality and public spirit in furnishing the requisite funds [to the amount of 22,000L.]; but we must not deny to Paine the credit of conceiving the construction of iron bridges of far larger span than had been made before his time, or of the important examples both as models and large constructions which he caused to be made and publicly exhibited. In whatever shares the merit of this great work may be apportioned, it must be admitted to be one of the earliest and greatest triumphs of the art of bridge construction." Its span exceeded that of any arch then known, being 236 feet, with a rise of 34 feet, the springing commencing at 95 feet above the bed of the river; and its height was such as to allow vessels of 300 tons burden to sail underneath without striking their masts. Mr. Stephenson characterised the bridge as "a structure which, as regards its proportions and the small quantity of material employed in its construction, will probably remain unrivalled."
[Image] Wear Bridge, at Sunderland.
The same year in which Burdon's Bridge was erected at Sunderland, Telford was building his first iron bridge over the Severn at Buildwas, at a point about midway between Shrewsbury and Bridgenorth. An unusually high flood having swept away the old bridge in the Year 1795, he was called upon, as surveyor for the county, to supply the plan of a new one. Having carefully examined the bridge at Coalbrookdale, and appreciated its remarkable merits, he determined to build the proposed bridge at Buildwas of iron; and as the waters came down with great suddenness from the Welsh mountains, he further resolved to construct it of only one arch, so as to afford the largest possible water-way.