Life of Robert Stephenson by William Pole: Chapter III (Volume 2)

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CHAPTER III. (Volume 2). The Britannia Bridge.

THIS celebrated structure - the Britannia Bridge - has for its object to carry the line of the Chester and Holyhead Railway, the main artery of communication between the English and Irish capitals, across the Straits of Menai, which separate the island of Anglesea from the mainland of North Wales,

The port of Holyhead, lying at the western extremity of the island, and forming the nearest point of land to Kingstown Harbour, has always been considered the most eligible place of departure for the passage across the Irish Channel, when certainty and speed of transit have been concerned. Liverpool has, it is true, carried on hitherto, and will doubtless continue to carry on, a large trade with the Irish capital by direct steamers, but it appears certain that in this case, as in the traffic with the Continent, that route must always be considered of the most importance which involves the least exposure to the perils and comparatively slow navigation of the sea.

Long before railways were thought of, the great Holyhead Trunk Road had made the fame of the engineer who constructed it, Thomas Telford; and this work presented, in one or two points of its course, difficulties so analogous to some of those which were vanquished in the Britannia Bridge, that they may be treated as common to the works of both engineers.

The island of Anglesea is separated from the mainland of Carnarvonshire by a narrow Strait, deeply sunk below the general level of the land, and with rocky and precipitous banks on either side. The length of the Strait is about 11| miles, its width of water-way varies from about 1,000 feet to three-quarters of a mile, and the average height of the shores on each side is above 100 feet.

For a long time the land traffic to Holyhead had been made to descend the bank, cross by a ferry, and ascend again on the other side; but the inconvenience, loss of time, and often positive danger of this passage, prompted at a very early period efforts to establish a permanent roadway across the ravine. Bridges of timber or stone, embankments with drawbridges for the passage of vessels, and tunnels, had all been suggested; and as early as 1785 a petition was presented to Parliament for the means of carrying into effect one of these schemes; but the measure had not at that time assumed such an importance as to warrant the necessary large expenditure. When, however, Ireland was united to Great Britain, in 1801, the intercourse between the two kingdoms rapidly increased; the inconvenience and danger to travellers were naturally and justly complained of; and the attention of Government became seriously directed to the provision of a remedy.

They directed the late Mr. Rennie to survey the Strait, and he prepared four designs of bridges for crossing at different sites, the chief features of all being large iron arches, in some cases as much as 450 feet span, and 150 feet above the water.

Local opposition, however, and a disinclination to provide the large sum required, caused the postponement of the matter till 1810, when a parliamentary committee was appointed to enquire into the state of the roads from Shrewsbury and Chester to Holyhead, After taking much evidence, this committee reported that the whole subject, including the bridge, required further professional investigation; and in consequence of their report, the Lords of the Treasury, in May 1810, instructed Mr. Telford to make an accurate survey of the roads, to report on their improvement generally, and to consider the best mode of passing the Straits. He proposed two plans of bridges—one, a single cast-iron arch of 500 feet span, and 100 feet high; the other, a series of iron and stone arches of smaller size—preferring, however, the former. These proposals were investigated in 1811 by another parliamentary committee, who strongly recommended the execution of the large iron bridge. The great dimensions of opening could not be dispensed with. The Straits, though tortuous and rocky, and of difficult navigation, were yet constantly used by large ships, on account of their sheltered situation, and the saving which they afforded of about 60 miles extra journey round the exposed and dangerous coast of the island; and hence it was absolutely prohibited that any fixed structure should be thrown across, except of such width and at such height as would allow the passage of large vessels underneath without inconvenience or danger.

Notwithstanding the approval of the parliamentary committee, still nothing was done, till a circumstance that occurred elsewhere gave a new turn to the design. In 1814 Mr. Telford was engaged in investigating the possibility of throwing a bridge across the Mersey at Runcorn, and finding the ordinary plan unavailable, he had proposed a large bridge on the suspension principle, which about that time was being brought into notice by Captain Brown. In 1815 a parliamentary commission was appointed to carry into effect the various improvements required in the Holyhead roads, Mr. Telford being appointed their engineer; and, after the general road works had proceeded for about two years, the enquiry arose whether the suspension principle might not be advantageously applied to the crossing of the Straits. Mr. Telford therefore again directed his attention to the subject, and early in 1818 submitted a report, design, and estimate, so strongly in favour of the suspension plan, that it was at once sanctioned by Government, and the works were put in hand in the latter part of the same year.

This resulted in the well-known magnificent suspension bridge, which, while it carries the road over the chasm at a convenient level, offers an uninterrupted water passage of nearly 550 feet wide and 120 feet high at high- water, dimensions sufficient to allow the largest ships using the Straits to pass under in full sail. Considering how little experience had been gained at that time in the use of iron for bridge construction, this bridge, so novel and daring in design and so successful and elegant in execution, has conferred lasting and well-merited fame on the engineer to whom its erection is due.

Telford’s bridge was opened in 1826, but in a few years after that time the new system of communication began to supersede the ordinary roads. The metropolis of England was soon brought into railway connection with the great commercial port of Liverpool, and public attention began to be directed to a similar improvement of the communication with Ireland. A railway was projected for the land part of the journey; but, before its direction could be decided on, a question arose as to the merits of Holyhead as a point of departure, compared with another port on the main land, somewhat further south, named Port Dynllaen. Each of these had its advocates as a packet station, and various investigations were entered upon, and reports made, both by civil engineers and naval officers.

These discussions ended in a decision in favour of Holyhead, which led to the adoption of a line of railway to that port from Chester, to be connected by a branch with the Birmingham and Liverpool Railway at Crewe. This line, called the Chester and Holyhead Railway, was first surveyed by Mr. George Stephenson about 1838, but was subsequently taken up and carried into execution by his son.

The Act was obtained (with a certain hiatus which will be hereafter referred to) in 1844, and the railway was opened for its entire length, including the passage across the Britannia Bridge, in 1850.

Few railways have exceeded this line, either in public importance or in engineering interest. The natural difficulties have been great, and a series of engineering works of almost unrivalled magnitude characterise its whole length of 84.5 miles. It emerges from Chester through a tunnel, and passes over a viaduct of 45 arches to the bridge by which it crosses the River Dee. From thence it follows the embanked channel of this river and its estuary, and farther on the shore of the Irish sea, having here and there important works, until it is stopped by the bold headlands of the Great and Little Orme’s Head. It then leaves the coast, and, passing through the narrow valley that separates these headlands from the main land, crosses the River Conway, beneath the castle walls, by a wrought-iron tubular bridge of 400 feet in one span. Passing through the town and under the walls by a short tunnel, it again reaches the coast at the Conway Marshes, and continues its course along the shore through the greenstone and basaltic promontories of Penmaen Bach, and Penmaen Mawr, the terminating spurs of the Snowdon range, which it passes by two tunnels cut in the solid rock. Beyond these it is carried for some distance along the beach, partly on a viaduct of cast-iron. The sea walls and defences on the one hand along this exposed coast are all on a large scale; whilst on the other side of the line, a timber construction, similar to the avalanche galleries on the Alpine roads, protects the line from the debris rolling down from the lofty and almost overhanging precipices above. The road again turns inland to Bangor, and thence rises continually to a proper level for crossing the Straits. In this space it passes through a very rough country. The River Ogwen is crossed by a viaduct 246 yards in length, and the Cogyn by one of 132 yards long and 57 feet high; and three ridges of hills are perforated by tunnels, 440, 920, and 726 yards in length respectively, through hard primitive and trap rocks. In Anglesey the road passes over a marsh, and through a tunnel 550 yards long, and enters Holyhead by partly making use of an embankment previously constructed by the commissioners for the turnpike road.

When the Bill for the line was presented to Parliament in 1843-4, the chief engineering work involved in it was the bridge over the River Conway. The passage of the Menai Straits was proposed to be effected by permanently appropriating to the railway one of the two roadways of Telford’s great suspension bridge. As the strength of this bridge, however, was deemed inadequate for the safe transit of heavy locomotive engines, it was intended to convey the trains across, in a divided state, if necessary, by means of horse power, another locomotive being in readiness on the opposite side—the passage of engines being thus entirely obviated. The Commissioners of Woods and Forests, however, refused to allow a permanent appropriation of the half of the bridge in this way, and as the expense to be incurred was inconsistent with the idea of a temporary expedient, the Railway Company were driven to abandon this part of their plan, and to propose an independent bridge for their line. The Bill was accordingly passed with a hiatus of five miles at this part, to give time for the arrangement of the plans.

The directors at once instructed Mr. Robert Stephenson, who had then become their engineer, to select a suitable place for crossing; and, after studying the subject well, he decided on a site about a mile to the west oi Telford’s bridge. The tide-way is here somewhat contracted ; but the feature which principally determined the choice was the existence of a rock or island in the middle of the stream, called THE BRITANNIA BOCK ; and from this, and not, as is often supposed, from any allegorical allusion, the bridge takes its name.

As the rock gave the opportunity of building a large pier, and so dividing the span into two parts, it was proposed to construct the bridge of two cast-iron arches, each 350 feet span, with a versed sine of 50 feet, the roadway being 105 feet above the level of high-water at spring-tides. The difficult problem of erecting these gigantic arches, in a situation where no centering or scaffolding would have been possible, was proposed to be solved by Mr. Stephenson in a very ingenious manner, and the Company prepared, at the end of 1844, a bill based upon this plan to go before Parliament the ensuing session.

As soon, however, as it became known what kind of a bridge it was proposed to build, a storm of opposition arose from the parties interested in the Straits, on the ground that such massive constructions would seriously interfere with the navigation. In March 1845, the Admiralty, in whom the guardianship of the navigation was vested, instructed three eminent engineers to examine the site and to report on the proposed plan; and as they stated that, in their opinion, the cast-iron bridge was ineligible, and that a clear passage of at least 100 feet high throughout the whole span should be insisted on, the proposal was abandoned.

Mr. Stephenson had already anticipated and prepared for this decision. He had fallen back upon the idea of the suspension bridge, and had begun to consider whether it was not possible to stiffen the platform so effectually as to make it available for the passage of railway trains at high velocities. His attention was directed to a suspension bridge at Montrose, where great stiffness had been afforded by a judicious system of trussing ; and, carrying out this idea further, he conceived that sufficient strength might be obtained by the combination of the suspension chains with deep trellis trussing, having vertical sides, with cross bearing frames at top and bottom; the roadway being thus surrounded on all sides by strongly trussed framework. But as this idea was dwelt upon, difficulties arose about the material in which this trussed framework should be made. Timber was deemed inadmissible by reason of its perishable nature, and the danger from fire; and Mr. Stephenson, reverting to the design he had made for a small bridge in wrought-iron in 1841, was led to consider the application of this material, by substituting for the vertical wooden trellis trussing, and the top and bottom cross beams, wrought- iron plates riveted together with angle-iron. The form which the idea then assumed was, consequently, that of a huge wrought-iron rectangular tube, so large that railway trains might pass through it, with suspension chains on each side.

The conception having reached this stage, only a httle farther careful consideration was necessary to arrive at the idea that such a tube would, if properly designed, serve the purposes of a beam or girder. The top and bottom of the tube, which it was intended to compose of thick wrought-iron plates, would evidently correspond with the top and bottom flanges of a common cast-iron girder, and might be made to perform their duties and take their strains; and having reference to this consideration, Mr. Stephenson began now to regard the tubular platform as a beam, comprising in itself the main element of its supporting power, and to which the chains were merely auxiliaries. Rough calculations were made, which though necessarily very imperfect, gave confidence in the feasibility of the design; and Mr. Stephenson’s reliance on it was further strengthened by some practical examples brought to his notice of the great strength shown by large iron vessels accidentally placed under circumstances of peculiar strain and trial. Mr. Stephenson, fortified by these facts, even went so far as to propose to dispense with the auxiliary chains altogether.

Thus the matter stood at the beginning of April 1845, when the reports to the Admiralty put an end, as Mr. Stephenson had anticipated, to the scheme of a cast-iron arch-bridge. The forethought and prudence with which he had prepared for this contingency, strikingly illustrate an element in his character, which was prominent through his whole professional career. Though he had strong confidence in his own views, when they were the result of sound reasoning and careful consideration, he never trusted with too sanguine an expectation to the favourable result of uncertain chances. He never undertook a doubtful course, without previously having determined a way of escape if it turned out contrary to his expectations ; and to this admirable prudence is due, without doubt, much of the success which attended his professional labours.

The extinguishing of a favourite scheme, for such the arch-bridge was, would have damped the ardour of many men; but no sooner had it occurred than Mr. Stephenson announced to the directors of the railway that he was prepared to carry out a bridge of such a kind as would comply witli the Admiralty conditions ; and, after he had explained his views, they—not, however, without some misgivings—gave him their confidence and authorised him to lay his designs before Parliament.

The Bill came before the Committee of the House of Commons early in May. Mr. Stephenson’s proposals, given on the first day, were received with much evident incredulity, and the Committee desired further evidence, and especially that of the Inspector-General of Railways, General Pasley, before they could pass the Bill authorising the erection of such a bridge as that which he had proposed. The Inspector concurred in the soundness of the idea, but most decidedly objected to the removal of the chains; and Mr. Stephenson, though he still expressed G 2 confidence in the sufficiency of the tube alone, thought it expedient to defer to this opinion, and to acquiesce in their retention. This satisfied the Committee, and the Bill in due course became law, by receiving the Royal assent, the 30th of June 1845.

It was now necessary to take steps in earnest for designing the tube. The calculations already made had been very rough: for such constructions being entirely novel, no experimental data were in existence of any use for practical purposes. No wrought-iron beam of any magnitude had ever been made or designed at all, and though the general properties of the material had been to some extent ascertained in suspension bridges, iron ships, and other wrought-iron constructions, the way to apply it in the best manner, so as to render its strength available in forming a large girder, was quite unknown. It was not the mere arrangement of the materials to resist the transverse strains which formed the difficulty of the problem. It was rather the practical design of any such structure at all—.the difficulty of obtaining the iron in the forms required, or of adapting such forms as were obtainable to new purposes— and of devising a beam, not merely strong enough for its ultimate use as a bridge, but of sufficient independent rigidity for keeping its form when erected, and for sustaining the complicated and trying processes connected with its first construction, its floatation, its conveyance to the site, and its elevation and fixing in place.

Mr. Stephenson, therefore, considering the magnitude of the matter at stake, at once decided on supplying the want of data by a series of experiments oh a large scale. before committing himself to the design for the tube. His own knowledge of the properties and manufacture of iron was very considerable, having been engaged from his youth up so actively and prominently in the manufactory at Newcastle; but, with the unassuming modesty of true merit, he did not think fit to rely on himself alone, for he felt that, considering the responsibility which he had publicly assumed, he would be doing injustice to the Board of Directors, who had placed such confidence in him, if he did not avail himself of all the practical and scientific aid within his reach. He accordingly entrusted the performance of the experiments to Mr. William Fairbairn, of Manchester, whose practical experience he estimated very highly, and with whom he had consulted on the subject previously to the parliamentary investigation. A short time afterwards, also, at Mr. Fairbairn’s suggestion, he engaged the assistance of the late Mr. Eaton Hodgkinson, whose valuable contributions to engineering science, more especially in regard to iron structures, had attracted much notice in the profession.

The experiments, which were designed and proceeded with under Mr. Stephenson’s personal superintendence, were not at first specific in their object. It was necessary rather to determine what kind of information was required, than to pursue any definite course—to ascertain generally in what manner tubes might be expected to fail, and to what extent their strength might be modified by form. The first idea of the tube was a rectangular section, but this was afterwards thought objectionable, and attention was directed to the circular or elliptical form. Model tubes of these sections were accordingly made and carefully tested; but they failed in strength, and, after due discussion and consideration of the experiments, it was decided that these shapes were ineligible, and the original rectangular form was reverted to. Trial tubes of this shape proved more satisfactory, and, in February 1846, Mr. Stephenson was able to report to the half-yearly meeting the general conclusion that had been arrived at. He stated that, after carefully studying the results as they developed themselves, he had satisfied himself that the wrought-iron tube was the most efficient as well as the most economical description of structure that could be devised for crossing the Straits—that the form of the tube must be rectangular— that the general disposition of the material had been determined—and that the only problem remaining was that of the necessary strength to be given: that apparently greater strength was required than had been at first proposed; but to establish the formulae of calculation more positively, as well as to settle doubtful points regarding the use of the chains, it was desirable to carry the experimental researches still further.

It was in this preliminary series of investigations that the remarkable and unexpected fact was brought to fight that the power of wrought-iron to resist compression was much less than to resist tension, being the reverse of that which held in cast-iron. This discovery had not only an important bearing on the design for the tube, but it has since formed a valuable datum in regard to the engineering use of the material generally.

Another point having important influence on the subsequent design was also brought out for the first time. It was found that in all the earliest trials of thin tubes the top part, which was exposed to a compressive strain, failed not by the direct crushing of the material, but by the buckling or collapse of the plates. This was a new fact altogether, and one which had never been taken into account in any of the formulae for strength previously employed. It indeed annihilated at once their practical utility; and, prominent as it became in subsequent experiments, it threatened temporarily even to frustrate the consummation of Mr. Stephenson’s design. It was, therefore, at once treated as the most important object of investigation. In some of the elliptical tubes a sort of cell or fin was introduced; but as this form was just then abandoned, the same difficulty arose with the rectangular tubes, the tops of which, when formed of thin flat plates, buckled up under the pressure. At length corrugations were made in the plates, which were found to add much to the stiffness; and this led to the formation of the top in a series of tubes or cells, which, while they gave the necessary rigidity, offered great facilities for the manufacture, as well as convenient access to aU parts of the material; an object which had been always prominent in Mr Stephenson’s mind.

The publication of Mr. Stephenson’s report on these preliminary experiments, which was accompanied by others from Mr. Fairbairn and Mr. Hodgkinson, formed an important epoch in the history of the bridge. Public attention was now for the first time drawn to the subject, and the directors of the Company were relieved from some anxiety by the more definite details submitted to them. But still the necessity for further experiments was obvious. Everybody had some doubts and fears to suggest—dismal warnings came in on all hands, suggesting every imaginable apprehension. The necessity for chains was still advocated, not only by General Pasley, but by Mr. Hodgkinson himself. Many doubted the efficiency of riveting to unite such a mass of plates; some foretold the most fatal oscillation and vibration from passing trains, sufficient even to destroy the sides of the structure ; while others asserted the insufficiency of the lateral strength to resist the wind. In fact, with few exceptions, scientific men generally either remained neutral or ominously shook their heads and hoped for the best, and even the most sanguine waited for further experimental investigation. All this was so discouraging, that Mr. Stephenson, strong as his faith was in his own plans, could not avoid appearing at times disheartened, when he withdrew from the turmoil of his metropolitan parliamentary duties to deliberate on the weighty difficulties he had to encounter in his gigantic undertaking in the distant hills of North Wales.

At this time, too, another serious matter presented itself. The preliminary considerations, discussions, and experiments summed up in Mr, Stephenson’s report had occupied much longer time than had been anticipated; but the work on the other portions of the line had been steadily progressing, and it became evident that the Britannia and Conway bridges would be ultimately the chief cause of delay in the completion of the hue. Hence the directors became impatient that Mr. Stephenson should sufficiently mature his plans to allow of the commencement of the masonry; and, while they did not hesitate to sanction the continuance of such further experiments as he might deem advisable, they, with a confidence in his proposals which few shared with them, entreated him without delay to commence operations simultaneously at both sites, and to complete his designs as he proceeded. This gratifying resolution added considerably to his anxiety, as he wished first to complete the smaller structure—the Conway Bridge, in order to obtain for the larger one the benefit of any experience it would afford. The plans of the masonry were however at once prepared. They were ready for contract by the middle of March 1846, and the first stone of the Britannia Bridge was laid April 10 in that year.

The further experiments which were needed for the completion of the design of the tubes, were of two kinds. In the first place it was considered necessary to make a model tube, very much larger than any of the previous ones, and representing more nearly the principles of the structure itself; with a view of putting it to every possible test, and by constant correction of its weak points of arriving gradually at the best form and proportions possible. And, secondly, it was found that, in order to give the power of reasoning from this model up to the structure itself, many more experimental data were necessary than were yet in existence, as to the qualities of the materials and the workmanship proposed to be used, and the influence of strains upon them.

These latter specific enquiries were undertaken by Mr. Hodgkinson. They consisted of careful and elaborate experiments and deductions on the compression, flexure, and crushing of materials and manufactured compound structures under direct pressure— on the extension and tensile strength of materials—on riveting—on the shearing of iron exposed to transverse strain— and on the transverse strength of beams and tubes of various kinds. They were, it is true, more particularly aimed at the question then pending; but they form a mass of general information of the most useful description, and probably, as a whole, unrivalled in extent and value.

The large model was constructed at Mr. Fairbairn’s works at Millwall, near London, in order that it might be tested under Mr. Stephenson’s more immediate inspection. The proportions having been thoroughly discussed, it was commenced in April 1846, and completed in July, and the experiments upon it were immediately put in hand. It was rectangular in shape, with a top composed of one row of cells, and its dimensions were determined in reference to the requirements for the Britannia Bridge, every dimension being one-sixth of the eventual magnitude then thought necessary. Thus the Britannia tube being 450 feet long in the clear, the length of the model between the bearings was 75 feet—the depth 4 feet 6 inches—and the width 2 feet 8 inches: forming a large bridge-girder of itself. The weight was between five and six tons. It was supported at each end on a pier, and weights were hung on the centre till it gave way. In the first experiment it broke with 30.25 tons by the rending asunder of the bottom plates. These were then repaired and strengthened, when it bore 43 tons, giving way at the sides, which were then strengthened in turn. Next the bottom gave way again several times, each time having larger dimensions; and so the trials and alterations were continued until at length a proportion was arrived at which proved to be about equally strong all over. As thus perfected, the tube bore 86 tons, or 2^ times that of the first trial, although in the strengthening only one ton of iron had been added—such being the effect of a judicious application of the material.

The experiments on this model directly proved what at first had appeared problematical, namely, that with such extensive horizontal developement of the top and bottom flanges, the whole of their sectional area would act effectually in resisting extension or compression throughout the entire width. In fact, when the model beam was broken, the tearing asunder of the bottom plates actually commenced at about the middle of the tube, and not at the outside edges—showing thus that the principles of simple girders were directly applicable to this construction also.

The experiments on the large model were continued till April 1847 ; but in the meantime the designs for the tube had not been neglected. During the first half of the year 1846 a great number of tentative drawings, models, and calculations were made; and although many of these attempts were necessarily discarded, as clearer views resulted from increased experimental information, yet some of the designs thus sketched out remarkably anticipated the ultimate plan. In July, when the experiments on the large model were commenced, a design for the great tubes had been made out in considerable detail. This was gradually improved as further information was obtained; and more perfect drawings were completed in the beginning of November. These, however, were further modified from time to time, the most important change being in the arrangement of the cells of the top, effected in February 1847, in accordance with certain principles resulting from the enquiries of Mr. Hodgkinson. In March the correct lists of the plates were made out, and the first complete working drawings for the tubes were finished, although still further improvements were introduced as the work went on.

There is little doubt that this gradual growth and constant improvement of the designs conduced much to their perfection ; and at a much later period, when wrought-iron girders had been greatly developed by the experience of subsequent years, and the talent of engineers had given rise to numberless elegant and ingenious practical combinations in bridge construction, Mr. Stephenson declared that he found it difficult to conceive any better means than those adopted of solving the problem.

While the design of the tubes was thus being considered, another question of scarcely less importance had also called for investigation, namely, the means by which they were to be placed in their position. For it scarcely need be remarked that the immense size and weight of the tubes, and the peculiarities of the situation, put them completely out of the range of all ordinary experience.

Many suggestions for this purpose were made and discussed at various times. An early idea of Mr. Stephenson’s, when the cast-iron arch-bridge was proposed for Conway, was to float it to its place on pontoons; and the merits and difficulties of this plan had been fully discussed. When the arch was superseded by the tube in both localities, this mode of placing the tube was again considered, as the form which the bridge had assumed was evidently favourable for such an operation ; and it was accordingly proposed to construct the tubes on the beach, and to float them to their ultimate position. This presupposed sufficient strength in the bridge independently of chains; but Mr. Stephenson, at that time, considered that the insurance afforded by chains against any accident from unforeseen causes would be a consideration of vital importance; and he did not, in that stage of his experience, feel justified in throwing away such a security. He therefore determined on availing himself of these auxiliary suspension chains, in the first instance, for supporting a temporary platform or scaffolding, along which the tubes constructed on the land could be rolled into their places. This plan was maturely considered; the designs for the platform,—which would of itself have been a large suspension bridge—were prepared; and much attention was bestowed on the manner of making the chains available as additional means of security to the tube, after their temporary office as scaffolding had been performed.

As, however, the progress of the design in the early part of 1846 gave more confidence in the self-supporting power of the tubes, and as the completed estimates for the suspension platform, with the then high price of wrought-iron, were very large, the subject was again discussed ; and in July, Mr. Clark, who had accidentally obtained what he considered a good practical suggestion of a mode of raising the tube, urged upon Mr. Stephenson, with Mr. Fairbairn’s assistance, a renewal of the floatation scheme. The subject was carefully and candidly reconsidered by Mr. Stephenson, and ultimately the chains were abandoned; and it was decided to put the tubes together upon the shore of the Straits; to float them to their site on pontoons ; and to raise them to the required high level by hydraulic power; and this was the plan carried into practice.

Meantime it became urgent that arrangements should be made for the manufacture of the tubes, which the directors decided to put out to contract, reserving to themselves, however, the right to purchase the iron, and to supply it to the makers of the tubes at a fixed price per ton. In July 1846, the plates were contracted for by seven of the principal iron makers in the midland iron district; and shortly afterwards negotiations were commenced with several manufacturers for the construction of the tubes, but it was May 1847, before the arrangements were finally concluded. The first stipulation had been that the makers should construct the work at their own manufactories, in large sections, to be delivered on the shore of the Straits, and there put together; but as this plan involved difficulty, it was afterwards decided that the manufacture should be entirely done on the shore. On this understanding the contract for one large tube was given to Messrs. Garforth, of Manchester, and for the other seven to Mr. Charles Mare, of Blackwall. The site for the construction of the tubes had been determined some time previously. It was necessary that the making of the four large tubes should proceed simultaneously, and the clearing and preparation of the four places where they were to be made was a work of considerable difficulty and labour. Large and strong platforms of timber had to be laid down to build the tubes upon: these were occupied, and the ironwork began to get into shape by July 1847, and the first rivet for putting the tubes together was inserted on the 10th of August following.

The first of the large tubes was finished, and the wood platform was removed from beneath it by the 4th May 1849, leaving the weight of the tube supported on its two ends. This had, however, been anticipated by the Conway tube, finished in the January previous; and as the latter was in reality the first practical test of the great experiment, the anxiety of all concerned was intense to see the result as the timbers were gradually cut away. A deflection of 2 or 3 feet had been predicted, and many high authorities had affirmed that the tube could not support its own weight; while others foretold the buckling of the top, the distortion of the sides, the crushing of the extremities, and all sorts of failures. These forebodings were set at rest, and all fears at an end, when the platform was cleared away, and the tube took its own weight with just about the calculated deflection, and without the slightest appearance of undue strain or damage in any part.

The second tube was finished a few weeks after the first, the third in October 1849, and the fourth in February 1850.

Two other operations yet remained, each as gigantic and novel as the construction of the tubes, and attended with as much anxiety ; namely, their removal by floating from the shore where they were constructed to the site of the bridge, and the hoisting of them up to their required level.

In the Conway Bridge these operations had been undertaken by the contractors ; but, in the more important case of the Britannia Bridge, Mr. Stephenson preferred that they should be done immediately under his own direction. The arrangements for them had accordingly occupied his attention during the latter part of the year 1848, and to facilitate the study a model was made of considerable size, with real water, on which the whole operation could be imitated; so that by constant rehearsals of the process on this miniature pool the plans for floating were matured. Each tube was to be floated on eight pontoons, introduced in cuttings in the rock under the tube, and which, on a certain day, were to be emptied and allowed to rise by the flowing tide till they lifted the tube off its bearings, and took its weight upon themselves. They were then to be hauled out into the stream, in order that it might float them and their burden to the bridge, being carefully guided and controlled by hawsers attached to the shore on either side. This was to be done near high-flood, so that the tube might arrive at the bridge about the turn of the tide at still water, when its ends were to be lodged upon shelves prepared for the purpose at the foot of each tower, and the pontoons floated away. The difficulties of this operation consisted in the magnitude of the moving mass—the great number of departments and of hands entering into the process—the short time it had to be done in (only about one hour and a half)— the great velocity of the tide (about six miles an hour)— and the terrible consequences that might ensue if the operation should fail, and the floating mass become unmanageable under the swift and powerful ebb-tide. Mr. Stephenson’s energy, prudence, and foresight were here again admirably displayed. He devoted untiring attention to the organising and teaching of a large body of persons, many hundreds in number, who were to be engaged in the task, directed by his own staff of assistants; and as the work involved operations of a nature new to engineers, he obtained the aid of a large body of sailors and nautical men, under the command of Captain Claxton, R.N., who had acquired much reputation for his successful exertions in rescuing the Great Britain, stranded at Dundrum Bay. And further than this, impressed as Mr. Stephenson was by the immense responsibility of the operation, he invited two of his most eminent brethren in the profession—now, alas! like him, departed from this earthly scene of their labours— Mr. Brunel and Mr. Locke, to give him the benefit of their assistance, a trait of professional good feeling which did him infinite honour. This aid, we need hardly say, was cheerfully afforded, both gentlemen being at his side the whole time.

The floating of the two Conway tubes in March and October 1848, had served as useful preliminary trials, from which much valuable experience had been gained, and which enabled Mr. Stephenson well to mature his plans. Preparations were made for floating the first Britannia tube on the 19th of June 1849, but in consequence of the fracture of a capstan at the commencement, it was postponed to the next day, when it was successfully performed—the lodging of the tube upon the shelves of the towers being greeted by cannon from the shore, and the hearty cheers of many thousands of spectators, whose sympathy and anxiety had been indicated by the unbroken silence with which the whole operation had been observed.

The tube lay across the water, out of reach of the tide, during the remainder of June and the whole of July, while the machinery for raising it to its proper height was fitted in the towers. This apparatus consisted of huge hydraulic presses, placed at the tops of the towers on each side, from which strong chains hung down to the tube. By working these presses, the tube was raised six feet at a time, the ends sliding up in recesses or vertical grooves, which were built up under the ends of the tube as fast as it rose, timber packing being further inserted, so that in case of fracture of any of the suspending machinery the tube would not have far to fall.

On the 10th of August the raising was commenced, and it proceeded slowly till the 17th, when one of the press cylinders burst, allowing the end of the tube to fall 8 or 9 inches on to the packing below—which, slight as the fall was, caused some damage. By the 1st of October the press was again ready: the raising steadily proceeded, and on the 13th the tube safely attained its final elevation.

The second tube was floated on December 6th, and it was in its place by January 7th, 1850. This was in a fine with the first one, and, as the two short or land tubes corresponding were already completed, it only required the four lengths to be joined in order to efiect a passage across the Straits. These junctions proceeded night and day, and were completed and the rails laid by March 4th. The next day Mr. Stephenson and some friends passed through on a locomotive, followed by an enormous train of forty-five coal wagons and carriages, containing seven hundred passengers, and drawn by three engines. On the same day the last rivet was formally put into the tube by Mr. Stephenson and the contractor, and the passage of the Menai Straits by the Chester and Holyhead Railway became an accomplished fact. On the 15th the bridge was passed by the Government Inspector, and on the 18th it was opened for public traffic. It was worked as a single line for some time. The third tube was floated on June 10th, an operation which the concurrence of several accidents made the most hazardous of all; and it was raised July 11th. The fourth and last tube was floated July 25th, and placed in position on August 12th. The last piece of scaffolding was removed on October 11th; and on October 19th, 1850, the bridge was completed and opened for the double line.

The description of the bridge need only be very brief, as full particulars and views have been made so accessible by publication. It will be confined to an enumeration of such prominent points in the structure as may best illustrate its novelty and magnitude.

The nature of the ravine over which the bridge forms a passage has been described, and the peculiar conditions of navigation of the Straits, have already been alluded to as having influenced the general design. The water-way was about 1,000 feet wide, with a rock in the middle, so that by building a tower of sufficient height on each side, and one on the rock, this space was divided into two equal spans. But as the shelving shore on each bank gave a considerable increase of width at the required level of the roadway, some mode was necessary for filling in this additional space. The simplest plan would have been to build it up from the ground with arches of masonry, as Telford had done in the Menai Suspension Bridge; but Mr. Stephenson resolved to make use of these side spaces to effect an important object in regard to the large tubes, namely, to diminish the strains upon them by making them parts of a continuous long beam, instead of leaving each a single isolated span. It is well-known that when a beam extends continuously over several openings ?—as, for example, in a floor-joist—the strain is much less than when each span is covered by an independent beam of the same size. This, therefore, was the principle which Mr. Stephenson put in practice in this case. He threw the abutments, or land terminations of the bridge, high up the rocks on each side, and filled in the land spans with shorter tubes, so that the bridge became one of four spans -—two large ones in the middle, flanked by a small one on each side. As regarded the bridge itself, these smaller land tubes were not required at all: they merely acted, so to speak, as counterpoises for the large tubes, converting them into continuous long beams, and their overhanging weight serving to relieve the centre parts of a portion of their strain. This application of the principle of continuity is a good example of Mr. Stephenson’s excellent intuitive practical perception of mechanics. The general fact was, indeed, known, and its explanation had been investigated in mathematical works; but it was not till long after the erection of the Britannia Bridge that it was brought prominently before the notice of the engineering profession, or applied to iron bridges generally, with any view to the advantages afforded by it.

It does not appear to have formed any important part in the preliminary experiments, or even to have been the subject of any recorded calculations. In all probability its application was dictated almost entirely by Mr. Stephenson’s practical judgment, and the test of elaborate mathematical analysis subsequently applied to the work shows how sound and accurate this judgment was.

The span of each of the long tubes is 460 feet clear of the towers—that of each of the short or land tubes, 230 feet. A separate line of tubes is provided for each line of railway, with a small space between them, but both resting on the same towers. The four land tubes were constructed in situ, upon scaffolding built temporarily for the purpose.

Each line of tubes is connected throughout, forming one continuous tube 1,511 feet long, and weighing, with the permanent way, 5,270 tons. This long tube is securely fixed in the centre tower, but its bearings on the side towers and abutments are moveable, that it may expand and contract freely from changes of temperature.

The depth of the tube externally is 30 feet at the centre tower, diminishing to 23 feet at each end, so that while the bottom outline is straight, the top forms a portion of a curve. The internal clear height at the ends is 16 feet 4 inches. The breadth of the tube is about 14 feet, allowing room for a man to stand safely on the side during the passage of a train.

As each span of the tube had to bear its own weight between the supports before they were connected together, it was necessary, in the design, to treat each as a separate beam. The top and bottom members were the effective portions in resisting the strain, and in them, consequently, the largest amount of material was collected, being disposed in the shape of a series of square cells or flues, eight in the top and six in the bottom, of sufficient size to allow workmen to enter for the purpose of riveting, and also to cleanse and paint the interior. The sectional area of solid metal in the top, at the middle of the length of the large tube, is 648 square inches, of the bottom 585 square inches. This is reduced towards the ends.

The engraving, fig. 6, represents a section of the tube, and will give a general idea of its construction.

The sides are plain sheets of plates, stiffened by vertical ribs or pillars of T iron, within and without, and also by gussets or comer-pieces, filling up the angles on the inside. The sides increase very much in thickness towards the towers, and are strengthened at the end with massive cast-iron frames.

The entire weight of ironwork in the bridge is 11,468 tons—the rivets in the tubes number above two millions.

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The strength of the tubes has been well determined by several modes of calculation. Considering one of the large tubes as an independent beam, it is found that it would not break with less than about 5,000 tons equally distributed along its length. Now the tube itself weighs 1,550 tons, and adding to this the greatest moveable load that could possibly come upon it would make up lttle more than 2,000 tons, or two-fifths of its ultimate strength. But this is less favourable than the reality, inasmuch as the strength is nearly doubled by the continuity of the beam over the several spans.

The strain upon the metal at the middle of the length of the long tube would be about 5.25 tons per square inch, if considered as an independent beam, but is reduced to tons by the continuity.

Mr. Stephenson made, at a later period, some explanations of certain peculiarities in the construction of the tubes which necessary to the design.

The sides the whole weight, this weight might have been considerably reduced. But in the operation of floating, the tubes were liable to be supported at any point of their length, besides being subjected to chances of considerable dislocation, and to disasters which, on more than one occasion, did actually threaten their entire destruction. The stiffening frames and gussets, which in an ordinary girder would have only been necessary at the ends, became therefore requisite throughout the whole length; and even the top and bottom were considerably modified, as while overhanging the pontoons at each end to the extent of 70 feet, the top, instead of being in compression, was thrown into extension. Again, the tubes had to be raised by being suspended freely from four chains, requiring provisions for this support of a different character from that which they needed when laid on their permanent bed; and further, the variation in the strains when the four tubes were ultimately joined to form one continuous tube—parts before in tension being then thrown into compression, and vice versa— required a suitable arrangement of the material: the effect of all these provisions being to increase the quantity and modify the arrangement of the metal in the tube. In proportioning, therefore, the parts of a structure destined for such usage, the mere consideration of the strain to which, as an ordinary beam, it would be subjected, formed but a part of the problem, and no fair direct comparison can be made between the weight of this bridge and that of an ordinary beam.

Mr. Stephenson was of opinion that some misapprehension existed on the object and importance of the cells of which the top and bottom of the tube were composed, as well as on the choice of form of the tube; and he has given explanations to clear up both these points. He shows that to collect the necessary quantity of material of the top and bottom in single plates would have required the former to be 2.7 inches, and the latter 2.3 inches thick; and had such plates been procurable, nothing better could have been desired, and the cells would have been unnecessary.

At that time, however, it was impossible to procure plates of such a thickness, whose quality could be depended on; and the engineer in this, as in numberless other details, had to adopt what he could obtain. Now, the arrangement of the plates in cells is almost the only conceivable arrangement possible for getting the required section, allowing access, at the same time, to every part for construction and future maintenance. This alone led to their use in the bottom of the tube, where the form was quite indifferent. With respect to the top, however, it was of great importance, since thick plates could not be had, to ascertain the best form of cell for resistance to compression that could be devised with thin plates. A series of valuable experiments by Mr. Eaton Hodgkinson led to the rectangular cells actually used, not because such form presented any pecuhar advantage over any other, as some have imagined, but because these experiments demonstrated that cells of that magnitude and thickness were independent of form, and were crushed only by the actual crushing of the iron itself. Under these circumstances the square cells were used as the best practical method of obtaining the sectional area required.

Similar misapprehension has also existed as to the considerations which led to the rectangular form of the tubes themselves.

The result of direct experiments made with round, oval, and rectangular tubes— there being precisely the same section and weight of metal in all three— was that the tube the mediate, adoption. Its form however was not favourable for its practical construction, or for its connection with the suspension chains, which were originally intended to be used in the erection; and practical considerations in this case also dictated the use of the rectangular tube. It must also be remarked that the result of experiments made on oval, round, and rectangular wrought iron tubes, when reduced to the same depth and compared, was in favour of the rectangular form—although, within ordinary limits, the form was not proved to be a matter of great importance.

The centre or Britannia Rock tower is 230 feet high. The base is 60 feet by 50 feet, and the size at the level of the tube is 55 feet by 45. The pressure on the base is 16 tons per superficial foot.

The side towers are 18 feet lower than the Britannia tower ; the base of each is 60 feet by 37 feet; the size at the level of the tube 59.5 by 36.5 feet. The great height of the towers above the tubes was necessary for fixing the hydraulic presses which raised these ponderous masses into their places.

The shore abutments are 35 feet lower than the side towers.

The internal work of the towers and abutments is of Runcorn sandstone, with some brickwork. The exterior is faced with Anglesea marble, from quarries in the carboniferous limestone at Penmaen, the northern extremity of the island.

The total quantity of masonry in the bridge is nearly a million and a half cubic feet.

The design of the bridge, as regards its architectural character, must, considering the entire novelty of the form, and the colossal dimensions of the structure, have been an arduous thing to attempt. The object aimed at was the adoption of such a character as would best accord with the tubes, the external appearance of which is simply a representation of beams of gigantic proportions. With this view, a combination of the Egyptian and Grecian styles was thought the most appropriate, the former as applied to the general and more massive portions of the design, and the latter to the less ponderous parts and to the details generally. The colossal lions on the approaches, designed and executed by the late Mr. John Thomas, were intended as allegorical representations of the strength of the edifice and the boldness of the undertaking.

A colossal figure of Britannia was designed also by Mr, Thomas for the centre tower, but the great expense prevented its construction.

It is unfortunate that the bridge consists of an even number of spans, architectural beauty requiring an opening in the centre and not a pier. But the existence of the rock which determined the site of the bridge left no option on this point.

The total cost of the Britannia Bridge was a little over £600,000. The ironwork cost £375,000 or nearly £33 per ton—a very high price, no doubt; but it must be recollected that at the time these contracts were made iron was very dear, and the character of the work was new. At the present day there would be no difficulty in getting it for about half the sum.

The cost of the experiments was about £5,300.

Since the bridge has been in use the deflection has been carefully tested from time to time, and no perceptible increase has taken place. The painting has been attended to, and the tubes have been covered by a roof to shelter the ironwork from the rain. Mr. Stephenson continued to satisfy himself as to the condition of the bridge until near his death, and gave the opinion that he found it difficult to conceive that even the lapse of centuries could in any way affect such a structure.

It is to be hoped this opinion may be borne out by experience, and that the bridge may prove one of the most durable, as it certainly is one of the most remarkable, monuments of the science and enterprise of the present age.

A few words must be added relative to the Conway Bridge, which has been mentioned incidentally in the account of the larger structure. The difficulties here, also, were formidable. It was necessary for the railway to cross the Conway River, a large estuary running high up into the land. The average width is about three- quarters of a mile, but advantage had been taken by Mr. Telford of a rocky island intercepting the channel, to reduce the width to a much smaller space, which he spanned with a suspension bridge for the passage of the Holyhead road. There could be no doubt that the proper site for crossing with the railway must be close to that occupied by the road; but it was also evident that, on account of the great depth at this point, 63 feet at high water, and the fearful velocity with which the tide ran through it to fill the large expanse above, it would be impossible either to build any intermediate pier in the water way, or to fix any centring or scaffolding for the erection of the bridge. The span of the suspension bridge is 315 feet, but from the form of the rocks the least span that could be obtained alongside it was 400 feet, and thus the problem became, to erect a bridge of this width in one span and without any fixed scaffolding. It will be recollected that Mr. Stephenson’s first idea for the Menai Straits was to construct the bridge of large cast-iron arches, and it was proposed to treat the Conway river in a similar way, one colossal arch spanning the entire opening. The principal difficulty was with the erection; the ingenious plan which Mr. Stephenson had contrived for the Britannia Bridge was inapplicable here, and he proposed to build the arch upon pontoons, which, when the work was finished, were to be floated to the site, and made to deposit the entire structure at once upon its bearings.

When, however, the arch scheme was abandoned for the Menai Straits, it was also put aside for the Conway, as it soon became apparent that the problem was essentially so identical in the two cases, that any design adopted for the larger structure would, in all probabi- hty, be the most suitable for the smaller. Hence no further special attention was given to the Conway crossing till the general principles of the Britannia Bridge were settled, after which the two designs progressed simultaneously.

It was, however, at the Conway Bridge, as has been already stated, that the success of the great experiment was first put to the test. The contract for the tubes was let in October 1846 to Mr. Evans, who was already executing the masonry. It was this enterprising man who first proposed to construct the tubes entirely on the site, a plan afterwards adopted at the Britannia Bridge with so much advantage ; and in the case of the Conway he undertook, at his own risk, the arduous and perilous duty of floating the tubes and of erecting them complete in their places, which he accomplished very successfully. The contract price paid to him for the tubes fixed complete was only about £4 per ton more than was given for the tubes only at the larger structure. The first stone of the bridge was laid May 12, 1846, but the manufacture of the tubes was not commenced till March 1847 ; the first tube was tested in January 1848, floated to its place in March, and ultimately raised and in use for railway traffic in April, a rapidity of execution highly praiseworthy. The second tube was floated in October, and the bridge was opened for traffic on the double hue in December 1848.

There are two tubes, one for each line of railway; they are 400 feet long in the clear between the supports ; the external height in the middle of the length is 25 feet 6 inches, diminishing to 22 feet 6 inches at the ends. The height of the bottom of the tube above high-water line is 17 feet. The general design of the tube corresponds with that in the Britannia Bridge, but the arrangement of the material is somewhat modified, from the circumstance that the latter is designed to act as a continuous beam, whereas the former is an independent one. The tubes were constructed on the shore of the estuary above the bridge, floated down to the site on pontoons, and raised by hydraulic power, as in the Britannia Bridge.

The artistic design of the Conway tube -will probably be considered less successful than that of the Britannia Bridge; the situation is picturesque in the highest degree, and the elegant suspension bridge rather added to than diminished its beauty; but the same remark will hardly apply to the subsequent erection.

An attempt was made to give a style corresponding to that of the castle, but alterations subsequently introduced into the construction, and the omission of the ornamental parts to save expense, crippled the design ; and the circumstance of the tubular bridge not being parallel to, but considerably askew from the suspension bridge immediately alongside, is a sad eyesore.

The unfettered reign of private enterprise, which, under the dictatorship of the engineer, has of late so much prevailed in this country, has been no doubt a grand source of works of commercial utility, but it has doomed us to much bitter humiliation in matters of art and taste.

W. P.

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