The Life of Thomas Telford Part 12

*[1] The Ellesmere Ca.n.a.l now pays about 4 per cent. dividend.

*[2] 'A General History of Inland Navigation, Foreign and Domestic,' &c. By J. Phillips. Fourth edition. London, 1803.

*[3] [Image] Section of Pier

Telford himself thus modestly describes the merit of this original contrivance: "Previously to this time such ca.n.a.l aqueducts had been uniformly made to retain the water necessary for navigation by means of puddled earth retained by masonry; and in order to obtain sufficient breadth for this superstructure, the masonry of the piers, abutments, and arches was of ma.s.sive strength; and after all this expense, and every imaginable precaution, the frosts, by swelling the moist puddle, frequently created fissures, which burst the masonry, and suffered the water to escape--nay, sometimes actually threw down the aqueducts; instances of this kind having occurred even in the works of the justly celebrated Brindley.

It was evident that the increased pressure of the puddled earth was the chief cause of such failures: I therefore had recourse to the following scheme in order to a void using it. The spandrels of the stone arches were constructed with longitudinal walls, instead of being filled in with earth (as at Kirkcudbright Bridge), and across these the ca.n.a.l bottom was formed by cast iron plates at each side, infixed in square stone masonry. These bottom plates had flanches on their edges, and were secured by nuts and screws at every juncture. The sides of the ca.n.a.l were made water-proof by ashlar masonry, backed with hard burnt bricks laid in Parker's cement, on the outside of which was rubble stone work, like the rest of the aqueduct. The towing path had a thin bed of clay under the gravel, and its outer edge was protected by an iron railing. The width of the water-way is 11 feet; of the masonry on each side, 5 feet 6 inches; and the depth of the water in the ca.n.a.l, 5 feet. By this mode of construction the quant.i.ty of masonry is much diminished, and the iron bottom plate forms a continuous tie, preventing the side-walls from separation by lateral pressure of the contained water."--'Life of Telford,' p. 40.

*[4] Letter to Mr. Andrew Little, Langholm, dated Shrewsbury, 13th March, 1795.

*[5] Matthew Davidson had been Telford's fellow workman at Langholm, and was reckoned an excellent mason. He died at Inverness, where he had a situation on the Caledonian Ca.n.a.l.

*[6] Mr. Hughes, C.E., in his 'Memoir of William Jessop,' published in 'Weale's Quarterly Papers on Engineering,' points out the bold and original idea here adopted, of constructing a water-tight trough of cast iron, in which the water of the ca.n.a.l was to be carried over the valleys, instead of an immense puddled trough, in accordance with the practice until that time in use; and he adds, "the immense importance of this improvement on the old practice is apt to be lost sight of at the present day by those who overlook the enormous size and strength of masonry which would have been required to support a puddled channel at the height of 120 feet."

Mr. Hughes, however, claims for Mr. Jessop the merit of having suggested the employment of iron, though, in our opinion, without sufficient reason.

Mr. Jessop was, no doubt, consulted by Mr. Telford on the subject; but the whole details of the design, as well as the suggestion of the use of iron (as admitted by Mr. Hughes himself), and the execution of the entire works, rested with the acting engineer.

This is borne out by the report published by the Company immediately after the formal opening of the Ca.n.a.l in 1805, in which they state: "Having now detailed the particulars relative to the Ca.n.a.l, and the circ.u.mstances of the concern, the committee, in concluding their report, think it but justice due to Mr. Telford to state that the works have been planned with great skill and science, and executed with much economy and stability, doing him, as well as those employed by him, infinite credit. (Signed) Bridgewater."

*[7] Letter to Mr. Andrew Little, Langholm, dated Shrewsbury, 16th Sept., 1794.

*[8] lbid.

*[9] Letter to Mr. Andrew Little, Langholm, dated Salop, 20th Aug., 1797.

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 princ.i.p.al 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 ma.s.ses 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 princ.i.p.al 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 circ.u.mstance 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 Suss.e.x, where he wrote poetry and acquired some local celebrity as a writer.

He was accordingly selected by his brother excis.e.m.e.n to prepare their pet.i.tion to Government for an increase of pay, *[2] -- the doc.u.ment 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 Yorks.h.i.+re, 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 s.h.i.+pped 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 a.s.signees, but was liberated by the a.s.sistance 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 n.o.ble 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 furnis.h.i.+ng 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 quant.i.ty 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.

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 quant.i.ty 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 Ca.n.a.l, "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 ca.n.a.l 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 Ca.n.a.l, 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 ca.n.a.ls, 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 Ca.n.a.l opposite Beeston Castle, in Ches.h.i.+re, 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 princ.i.p.al 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 a.s.sociated 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 ca.n.a.l 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 s.h.i.+p 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 s.h.i.+ps of 200 tons pa.s.sing under it, and form a new pool for s.h.i.+ps 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 ma.s.s 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 b.u.t.ton 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 antic.i.p.ated 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 circ.u.mstances 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 n.o.ble 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.

In the spring of 1795, after a long continued fall of snow, a sudden thaw raised a heavy flood in the Severn, which carried away many bridges--amongst others one at Bewdley, in Worcesters.h.i.+re,-- when Telford was called upon to supply a design for a new structure.

At the same time, he was required to furnish a plan for a new bridge near the town of Bridgenorth; "in short," he wrote to his friend, "I have been at it night and day." So uniform a success had heretofore attended the execution of his designs, that his reputation as a bridge-builder was universally acknowledged.

"Last week," he says, "Davidson and I struck the centre of an arch of 76 feet span, and this is the third which has been thrown this summer, none of which have shrunk a quarter of an inch."

Bewdley Bridge is a handsome and substantial piece of masonry.

The streets on either side of it being on low ground, land arches were provided at both ends for the pa.s.sage of the flood waters; and as the Severn was navigable at the point crossed, it was considered necessary to allow considerably greater width in the river arches than had been the case in the former structure.

The arches were three in number--one of 60 feet span and two of 52 feet, the land arches being of 9 feet span. The works were proceeded with and the bridge was completed during the summer of 1798, Telford writing to his friend in December of that year-- "We have had a remarkably dry summer and autumn; after that an early fall of snow and some frost, followed by rain. The drought of the summer was unfavourable to our ca.n.a.l working; but it has enabled us to raise Bewdley Bridge as if by enchantment. We have thus built a magnificent bridge over the Severn in one season, which is no contemptible work for John Simpson*[10] and your humble servant, amidst so many other great undertakings. John Simpson is a treasure--a man of great talents and integrity. I met with him here by chance, employed and recommended him, and he has now under his charge all the works of any magnitude in this great and rich district."

[Image] Bewdley Bridge.

Another of our engineer's early stone bridges, which may be mentioned in this place, was erected by him in 1805, over the river Dee at Tongueland in the county of Kirkcudbright. It is a bold and picturesque bridge, situated in a lovely locality. The river is very deep at high water there, the tide rising 20 feet. As the banks were steep and rocky, the engineer determined to bridge the stream by a single arch of 112 feet span. The rise being considerable, high wingwalls and deep spandrels were requisite; but the weight of the structure was much lightened by the expedient which he adopted of perforating the wings, and building a number of longitudinal walls in the spandrels, instead of filling them with earth or inferior masonry, as had until then been the ordinary practice. The ends of these walls, connected and steadied by the insertion of tee-stones, were built so as to abut against the back of the arch-stones and the cross walls of each abutment. Thus great strength as well as lightness was secured, and a very graceful and at the same time substantial bridge was provided for the accommodation of the district.*[11]

[Image] Tongueland Bridge.

In his letters written about this time, Telford seems to have been very full of employment, which required him to travel about a great deal. "I have become," said he, "a very wandering being, and am scarcely ever two days in one place, unless detained by business, which, however, occupies my time very completely." At another time he says, "I am tossed about like a tennis ball: the other day I was in London, since that I have been in Liverpool, and in a few days I expect to be at Bristol. Such is my life; and to tell you the truth, I think it suits my disposition."

Another work on which Telford was engaged at this time was a project for supplying the town of Liverpool with water conveyed through pipes in the same manner as had long before been adopted in London. He was much struck by the activity and enterprise apparent in Liverpool compared with Bristol. "Liverpool," he said, "has taken firm root in the country by means of the ca.n.a.ls"