The Life of Thomas Telford Part 18

*[7] Evidence of William Waterhouse before the Select Committee, 10th March, 1819.

CHAPTER XII.

THE MENAI AND CONWAY BRIDGES.

[Image] Map of Menai Strait [Ordnance Survey]

So long as the dangerous Straits of Menai had to be crossed in an open ferry-boat, the communication between London and Holyhead was necessarily considered incomplete. While the roads through North Wales were so dangerous as to deter travellers between England and Ireland from using that route, the completion of the remaining link of communication across the Straits was of comparatively little importance. But when those roads had, by the application of much capital, skill, and labour, been rendered so safe and convenient that the mail and stage coaches could run over them at the rate of from eight to ten miles an hour, the bridging of the Straits became a measure of urgent public necessity. The increased traffic by this route so much increased the quant.i.ty of pa.s.sengers and luggage, that the open boats were often dangerously overloaded; and serious accidents, attended with loss of life and property, came to be of frequent occurrence.

The erection of a bridge over the Straits had long been matter of speculation amongst engineers. As early as 1776, Mr. Golborne proposed his plan of an embankment with a bridge in the middle of it; and a few years later, in 1785, Mr. Nichols proposed a wooden viaduct, furnished with drawbridges at Cadnant Island. Later still, Mr. Rennie proposed his design of a cast iron bridge. But none of these plans were carried out, and the whole subject remained in abeyance until the year 1810, when a commission was appointed to inquire and report as to the state of the roads between Shrewsbury, Chester, and Holyhead. The result was, that Mr. Telford was called upon to report as to the most effectual method of bridging the Menai Strait, and thus completing the communication with the port of embarkation for Ireland.

[Image] Telford's proposed Cast Iron Bridge

Mr. Telford submitted alternative plans for a bridge over the Strait: one at the Sw.i.l.l.y Rock, consisting of three cast iron arches of 260 feet span, with a stone arch of 100 feet span between each two iron ones, to resist their lateral thrust; and another at Ynys-y-moch, to which he himself attached the preference, consisting of a single cast iron arch of 500 feet span, the crown of the arch to be 100 feet above high water of spring tides, and the breadth of the roadway to be 40 feet.

The princ.i.p.al objection taken to this plan by engineers generally, was the supposed difficulty of erecting a proper centering to support the arch during construction; and the mode by which Mr. Telford proposed to overcome this may be cited in ill.u.s.tration of his ready ingenuity in overcoming difficulties. He proposed to suspend the centering from above instead of supporting it from below in the usual manner--a contrivance afterwards revived by another very skilful engineer, the late Mr. Brunel. Frames, 50 feet high, were to be erected on the top of the abutments, and on these, strong blocks, or rollers and chains, were to be fixed, by means of which, and by the aid of windla.s.ses and other mechanical powers, each separate piece of centering was to be raised into, and suspended in, its proper place. Mr. Telford regarded this method of constructing centres as applicable to stone as well as to iron arches; and indeed it is applicable, as Mr. Brunel held, to the building of the arch itself.*[1]

[Image] Proposed Plan of Suspended Centering

Mr. Telford antic.i.p.ated that, if the method recommended by him were successfully adopted on the large scale proposed at Menai, all difficulties with regard to carrying bridges over deep ravines would be done away with, and a new era in bridge-building begun.

For this and other reasons--but chiefly because of the much greater durability of a cast iron bridge compared with the suspension bridge afterwards adopted--it is matter of regret that he was not permitted to carry out this novel and grand design. It was, however, again objected by mariners that the bridge would seriously affect, if not destroy, the navigation of the Strait; and this plan, like Mr. Rennie's, was eventually rejected.

Several years pa.s.sed, and during the interval Mr. Telford was consulted as to the construction of a bridge over Runcorn Gap on the Mersey, above Liverpool. As the river was there about 1200 feet wide, and much used for purposes of navigation, a bridge of the ordinary construction was found inapplicable. But as he was required to furnish a plan of the most suitable structure, he proceeded to consider how the difficulties of the case were to be met.

The only practicable plan, he thought, was a bridge constructed on the principle of suspension. Expedients of this kind had long been employed in India and America, where wide rivers were crossed by means of bridges formed of ropes and chains; and even in this country a suspension bridge, though of a very rude kind, had long been in use near Middleton on the Tees, where, by means of two common chains stretched across the river, upon which a footway of boards was laid, the colliers were enabled to pa.s.s from their cottages to the colliery on the opposite bank.

Captain (afterwards Sir Samuel) Brown took out a patent for forming suspension bridges in 1817; but it appears that Telford's attention had been directed to the subject before this time, as he was first consulted respecting the Runcorn Bridge in the year 1814, when he proceeded to make an elaborate series of experiments on the tenacity of wrought iron bars, with the object of employing this material in his proposed structure. After he had made upwards of two hundred tests of malleable iron of various qualities, he proceeded to prepare his design of a bridge, which consisted of a central opening of 1000 feet span, and two side openings of 500 feet each, supported by pyramids of masonry placed near the low-water lines. The roadway was to be 30 feet wide, divided into one central footway and two distinct carriageways of 12 feet each.

At the same time he prepared and submitted a model of the central opening, which satisfactorily stood the various strains which were applied to it. This Runcorn design of 1814 was of a very magnificent character, perhaps superior even to that of the Menai Suspension Bridge, afterwards erected; but unhappily the means were not forthcoming to carry it into effect. The publication of his plan and report had, however, the effect of directing public attention to the construction of bridges on the suspension principle; and many were shortly after designed and erected by Telford and other engineers in different parts of the kingdom.

Mr. Telford continued to be consulted by the Commissioners of the Holyhead Roads as to the completion of the last and most important link in the line of communication between London and Holyhead, by bridging the Straits of Menai; and at one of their meetings in 1815, shortly after the publication of his Runcorn design, the inquiry was made whether a bridge upon the same principle was not applicable in this particular case. The engineer was instructed again to examine the Straits and submit a suitable plan and estimate, which he proceeded to do in the early part of 1818.

The site selected by him as the most favourable was that which had been previously fixed upon for the projected cast iron bridge, namely at Ynys-y-moch--the sh.o.r.es there being bold and rocky, affording easy access and excellent foundations, while by spanning the entire channel between the low-water lines, and the roadway being kept uniformly 100 feet above the highest water at spring tide, the whole of the navigable waterway would be left entirely uninterrupted. The distance between the centres of the supporting pyramids was proposed to be of the then unprecedented width of 550 feet, and the height of the pyramids 53 feet above the level of the roadway. The main chains were to be sixteen in number, with a deflection of 37 feet, each composed of thirty-six bars of half-inch-square iron, so placed as to give a square of six on each side, making the whole chain about four inches in diameter, welded together for their whole length, secured by bucklings, and braced round with iron wire; while the ends of these great chains were to be secured by a ma.s.s of masonry built over stone arches between each end of the supporting piers and the adjoining sh.o.r.e. Four of the arches were to be on the Anglesea, and three on the Caernarvons.h.i.+re side, each of them of 52 feet 6 inches span.

The roadway was to be divided, as in the Runcorn design with a carriage way 12 feet wide on each side, and a footpath of 4 feet in the middle. Mr. Telford's plan was supported by Mr. Rennie and other engineers of eminence; and the Select Committee of the House of Commons, being satisfied as to its practicability, recommended Parliament to pa.s.s a Bill and to make a grant of money to enable the work to be carried into effect.

[Image] Outline of Menai Bridge

The necessary Act pa.s.sed in the session of 1819, and Mr. Telford immediately proceeded to Bangor to make preparations for beginning the works. The first proceeding was to blast off the inequalities of the surface of the rock called Ynys-y-moch, situated on the western or Holyhead side of the Strait, at that time accessible only at low water. The object was to form an even surface upon it for the foundation of the west main pier. It used to be at this point, where the Strait was narrowest, that horned cattle were driven down, preparatory to swimming them across the channel to the Caernarvon side, when the tide was weak and at its lowest ebb. The cattle were, nevertheless, often carried away, the current being too strong for the animals to contend against it.

At the same time, a landing-quay was erected on Ynys-y-moch, which was connected with the sh.o.r.e by an embankment carrying lines of railway. Along these, horses drew the sledges laden with stone required for the work; the material being brought in barges from the quarries opened at Penmon Point, on the north-eastern extremity of the Isle of Anglesea, a little to the westward of the northern opening of the Strait. When the surface of the rock had been levelled and the causeway completed, the first stone of the main pier was laid by Mr. W.A. Provis, the resident engineer, on the 10th of August, 1819; but not the slightest ceremony was observed on the occasion.

Later in the autumn, preparations were made for proceeding with the foundations of the eastern main pier on the Bangor side of the Strait. After excavating the beach to a depth of 7 feet, a solid ma.s.s of rock was reached, which served the purpose of an immoveable foundation for the pier. At the same, time workshops were erected; builders, artisans, and labourers were brought together from distant quarters; vessels and barges were purchased or built for the special purpose of the work; a quay was constructed at Penmon Point for loading the stones for the piers; and all the requisite preliminary arrangements were made for proceeding with the building operations in the ensuing spring.

A careful specification of the masonry work was drawn up, and the contract was let to Messrs. Stapleton and Hall; but as they did not proceed satisfactorily, and desired to be released from the contract, it was relet on the same terms to Mr. John Wilson, one of Mr. Telford's princ.i.p.al contractors for mason work on the Caledonian Ca.n.a.l.

The building operations were begun with great vigour early in 1820.

The three arches on the Caernarvons.h.i.+re side and the four on the Anglesea side were first proceeded with. They are of immense magnitude, and occupied four years in construction, having been finished late in the autumn of 1824. These piers are 65 feet in height from high-water line to the springing of the arches, the span of each being 52 feet 6 inches. The work of the main piers also made satisfactory progress, and the masonry proceeded so rapidly that stones could scarcely be got from the quarries in sufficient quant.i.ty to keep the builders at work. By the end of June about three hundred men were employed.

The two princ.i.p.al piers, each 153 feet in height, upon which the main chains of the bridge were to be suspended, were built with great care and under rigorous inspection. In these, as indeed in most of the masonry of the bridge, Mr. Telford adopted the same practice which he had employed in his previous bridge structures, that of leaving large void s.p.a.ces, commencing above high water mark and continuing them up perpendicularly nearly to the level of the roadway. "I have elsewhere expressed my conviction," he says, when referring to the mode of constructing these piers, "that one of the most important improvements which I have been able to introduce into masonry consists in the preference of cross-walls to rubble, in the structure of a pier, or any other edifice requiring strength.

Every stone and joint in such walls is open to inspection in the progress of the work, and even afterwards, if necessary; but a solid filling of rubble conceals itself, and may be little better than a heap of rubbish confined by side walls." The walls of these main piers were built from within as well as from without all the way up, and the inside was as carefully and closely cemented with mortar as the external face. Thus the whole pier was bound firmly together, and the utmost strength given, while the weight of the superstructure upon the lower parts of the work was reduced to its minimum.

[Image] Section of Main Pier

Over the main piers, the small arches intended for the roadways were constructed, each being 15 feet to the springing of the arch, and 9 feet wide. Upon these arches the masonry was carried upwards, in a tapering form, to a height of 53 feet above the level of the road. As these piers were to carry the immense weight of the suspension chains, great pains were taken with their construction, and all the stones, from top to bottom, were firmly bound together with iron dowels to prevent the possibility of their being separated or bulged by the immense pressure they had to withstand.

The most important point in the execution of the details of the bridge, where the engineer had no past experience to guide him, was in the designing and fixing of the wrought iron work. Mr. Telford had continued his experiments as to the tenacity of bar iron, until he had obtained several hundred distinct tests; and at length, after the most mature delilberation, the patterns and dimensions were finally arranged by him, and the contract for the manufacture of the whole was let to Mr. Hazeldean, of Shrewsbury, in the year 1820. The iron was to be of the best Shrops.h.i.+re, drawn at Upton forge, and finished and proved at the works, under the inspection of a person appointed by the engineer.

[Image] Cut showing fixing of the chains in the rock

The mode by which the land ends of these enormous suspension chains were rooted to the solid ground on either side of the Strait, was remarkably ingenious and effective. Three oblique tunnels were made by blasting the rock on the Anglesea side; they were each about six feet in diameter, the excavations being carried down an inclined plane to the depth of about twenty yards. A considerable width of rock lay between each tunnel, but at the bottom they were all united by a connecting horizontal avenue or cavern, sufficiently capacious to enable the workmen to fix the strong iron frames, composed princ.i.p.ally of thick flat cast iron plates, which were engrafted deeply into the rock, and strongly bound together by the iron work pa.s.sing along the horizontal avenue; so that, if the iron held, the chains could only yield by tearing up the whole ma.s.s of solid rock under which they were thus firmly bound.

A similar method of anchoring the main chains was adopted on the Caernarvons.h.i.+re side. A thick bank of earth had there to be cut through, and a solid ma.s.s of masonry built in its place, the rock being situated at a greater distance from the main pier; involving a greater length of suspending chain, and a disproportion in the catenary or chord line on that side of the bridge. The excavation and masonry thereby rendered necessary proved a work of vast labour, and its execution occupied a considerable time; but by the beginning of the year 1825 the suspension pyramids, the land piers and arches, and the rock tunnels, had all been completed, and the main chains were firmly secured in them; the work being sufficiently advanced to enable the suspending of the chains to be proceeded with. This was by far the most difficult and anxious part of the undertaking.

With the same careful forethought and provision for every contingency which had distinguished the engineer's procedure in the course of the work, he had made frequent experiments to ascertain the actual power which would be required to raise the main chains to their proper curvature. A valley lay convenient for the purpose, a little to the west of the bridge on the Anglesea side.

Fifty-seven of the intended vertical suspending rods, each nearly ten feet long and an inch square, having been fastened together, a piece of chain was attached to one end to make the chord line 570 feet in length; and experiments having been made and comparisons drawn, Mr. Telford ascertained that the absolute weight of one of the main chains of the bridge between the points of suspension was 23 1/2 tons, requiring a strain of 39 1/2 tons to raise it to its proper curvature. On this calculation the necessary apparatus required for the hoisting was prepared. The mode of action finally determined on for lifting the main chains, and fixing them into their places, was to build the central portion of each upon a raft 450 feet long and 6 feet wide, then to float it to the site of the bridge, and lift it into its place by capstans and proper tackle.

At length all was ready for hoisting the first great chain, and about the middle of April, 1825, Mr. Telford left London for Bangor to superintend the operations. An immense a.s.semblage collected to witness the sight; greater in number than any that had been collected in the same place since the men of Anglesea, in their war-paint, rus.h.i.+ng down to the beach, had shrieked defiance across the Straits at their Roman invaders on the Caernarvon sh.o.r.e.

Numerous boats arrayed in gay colours glided along the waters; the day--the 26th of April--being bright, calm, and in every way propitious.

At half-past two, about an hour before high water, the raft bearing the main chain was cast off from near Treborth Mill, on the Caernarvon side. Towed by four boats, it began gradually to move from the sh.o.r.e, and with the a.s.sistance of the tide, which caught it at its further end, it swung slowly and majestically round to its position between the main piers, where it was moored. One end of the chain was then bolted to that which hung down the face of the Caernarvon pier; whilst the other was attached to ropes connected with strong capstans fixed on the Anglesea side, the ropes pa.s.sing by means of blocks over the top of the pyramid of the Anglesea pier. The capstans for hauling in the ropes bearing the main chain, were two in number, manned by about 150 labourers. When all was ready, the signal was given to "Go along!" A Band of fifers struck up a lively tune; the capstans were instantly in motion, and the men stepped round in a steady trot. All went well. The ropes gradually coiled in. As the strain increased, the pace slackened a little; but "Heave away, now she comes!" was sung out. Round went the men, and steadily and safely rose the ponderous chain.

[Image] Cut of Bridge, showing state of Suspension Chain

The tide had by this time turned, and bearing upon the side of the raft, now getting freer of its load, the current floated it away from under the middle of the chain still resting on it, and it swung easily off into the water. Until this moment a breath less silence pervaded the watching mult.i.tude; and nothing was heard among the working party on the Anglesea side but the steady tramp of the men at the capstans, the shrill music of the fife, and the occasional order to "Hold on!" or "Go along!" But no sooner was the raft seen floating away, and the great chain safely swinging in the air, than a tremendous cheer burst forth along both sides of the Straits.

The rest of the work was only a matter of time. The most anxious moment had pa.s.sed. In an hour and thirty-five minutes after the commencement of the hoisting, the chain was raised to its proper curvature, and fastened to the land portion of it which had been previously placed over the top of the Anglesea pyramid. Mr. Telford ascended to the point of fastening, and satisfied himself that a continuous and safe connection had been formed from the Caernarvon fastening on the rock to that on Anglesea. The announcement of the fact was followed by loud and prolonged cheering from the workmen, echoed by the spectators, and extending along the Straits on both sides, until it seemed to die away along the sh.o.r.es in the distance.

Three foolhardy workmen, excited by the day's proceedings, had the temerity to scramble along the upper surface of the chain--which was only nine inches wide and formed a curvature of 590 feet--from one side of the Strait to the other!*[2] Far different were the feelings of the engineer who had planned this magnificent work.

Its failure had been predicted; and, like Brindley's Barton Viaduct, it had been freely spoken of as a "castle in the air." Telford had, it is true, most carefully tested every part by repeated experiment, and so conclusively proved the sufficiency of the iron chains to bear the immense weight they would have to support, that he was thoroughly convinced as to the soundness of his principles of construction, and satisfied that, if rightly manufactured and properly put together, the chains would hold, and that the piers would sustain them. Still there was necessarily an element of uncertainty in the undertaking. It was the largest structure of the kind that had ever been attempted. There was the contingency of a flaw in the iron; some possible scamping in the manufacture; some little point which, in the multiplicity of details to be attended to, he might have overlooked, or which his subordinates might have neglected. It was, indeed, impossible but that he should feel intensely anxious as to the result of the day's operations. Mr. Telford afterwards stated to a friend, only a few months before his death, that for some time previous to the opening of the bridge, his anxiety was so great that he could scarcely sleep; and that a continuance of that condition must have very soon completely undermined his health. We are not, therefore, surprised to learn that when his friends rushed to congratulate him on the result of the first day's experiment, which decisively proved the strength and solidity of the bridge, they should have found the engineer on his knees engaged in prayer. A vast load had been taken off his mind; the perilous enterprise of the day had been accomplished without loss of life; and his spontaneous act was thankfulness and grat.i.tude.

[Image] Menai Bridge

The suspension of the remaining fifteen chains was accomplished without difficulty. The last was raised and fixed on the 9th of July, 1825, when the entire line was completed. On fixing the final bolt, a band of music descended from the top of the suspension pier on the Anglesea side to a scaffolding erected over the centre of the curved part of the chains, and played the National Anthem amidst the cheering of many thousand persons a.s.sembled along the sh.o.r.es of the Strait: while the workmen marched in procession along the bridge, on which a temporary platform had been laid, and the St. David steam-packet of Chester pa.s.sed under the chains towards the Smithy Rocks and back again, thus re-opening the navigation of the Strait.

In August the road platform was commenced, and in September the trussed bearing bars were all suspended. The road was constructed of timber in a substantial manner, the planking being spiked together, with layers of patent felt between the planks, and the carriage way being protected by oak guards placed seven feet and a half apart. Side railings were added; the toll-houses and approach-roads were completed by the end of the year; and the bridge was opened for public traffic on Monday, the 30th of January, 1826, when the London and Holyhead mailcoach pa.s.sed over it for the first time, followed by the Commissioners of the Holyhead roads, the engineer, several stage-coaches, and a mult.i.tude of private persons too numerous to mention.

We may briefly add a few facts as to the quant.i.ties of materials used, and the dimensions of this remarkable structure. The total weight of iron was 2187 tons, in 33,265 pieces. The total length of the bridge is 1710 feet, or nearly a third of a mile; the distance between the points of suspension of the main bridge being 579 feet.

The total sum expended by Government in its erection, including the embankment and about half a mile of new line of road on the Caernarvon side, together with the toll-houses, was 120,000L.

Notwithstanding the wonders of the Britannia Bridge subsequently erected by Robert Stephenson for the pa.s.sage across the same strait of the Chester and Holyhead Railway, the Menai Bridge of Telford is by far the most picturesque object. "Seen as I approached it," says Mr. Roscoe, "in the clear light of an autumnal sunset, which threw an autumnal splendour on the wide range of hills beyond, and the sweep of richly variegated groves and plantations which covered their base--the bright sun, the rocky picturesque foreground, villas, spires, and towers here and there enlivening the prospect-- the Menai Bridge appeared more like the work of some great magician than the mere result of man's skill and industry."

[Image] Conway Suspension Bridge

Shortly after the Menai Bridge was begun, it was determined by the Commissioners of the Holyhead road that a bridge of similar design should be built over the estuary of the Conway, immediately opposite the old castle at that place, and which had formerly been crossed by an open ferry boat. The first stone was laid on the 3rd of April, 1822, and the works having proceeded satisfactorily, the bridge and embankment approaching it were completed by the summer of 1826. But the operations being of the same kind as those connected with the larger structure above described, though of a much less difficult character, it is unnecessary to enter into any details as to the several stages of its construction. In this bridge the width between the centres of the supporting towers is 327 feet, and the height of the under side of the roadway above high water of spring tides only 15 feet. The heaviest work was an embankment as its eastern approach, 2015 feet in length and about 300 feet in width at its highest part.

It will be seen, from the view of the bridge given on the opposite page, that it is a highly picturesque structure, and combines, with the estuary which it crosses, and the ancient castle of Conway, in forming a landscape that is rarely equalled.

Footnotes for Chapter XII.

*[1] In an article in the 'Edinburgh Review,' No. exli., from the pen of Sir David Brewster, the writer observes:--"Mr. Telford's principle of suspending and laying down from above the centering of stone and iron bridges is, we think, a much more fertile one than even he himself supposed. With modifications, by no means considerable, and certainly practicable, it appears to us that the voussoirs or archstones might themselves be laid down from above, and suspended by an appropriate mechanism till the keystone was inserted. If we suppose the centering in Mr. Telford's plan to be of iron, this centering itself becomes an iron bridge, each rib of which is composed of ten pieces of fifty feet each; and by increasing the number of suspending chains, these separate pieces or voussoirs having been previously joined together, either temporarily or permanently, by cement or by clamps, might be laid into their place, and kept there by a single chain till the road was completed. The voussoirs, when united, might be suspended from a general chain across the archway, and a platform could be added to facilitate the operations." This is as nearly as possible the plan afterwards revived by Mr. Brunel, and for the originality of which, we believe, he has generally the credit, though it clearly belongs to Telford.

*[2] A correspondent informs us of a still more foolhardy exploit performed on the occasion. He says, "Having been present, as a boy from Bangor grammar school, on the 26th of April, when the first chain was carried across, an incident occurred which made no small impression on my mind at the time. After the chain had reached its position, a cobbler of the neighbourhood crawled to the centre of the curve, and there finished a pair of shoes; when, having completed his task, he returned in safety to the Caernarvon side!