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

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CHAPTER II (Volume 2). IRON BRIDGES.

BRIDGES have always formed important works in the practice of the civil engineer, and there are scarcely any eminent members of the profession whose names are not associated with structures of this kind, of greater or less magnitude. Mr. Stephenson, in the course of his large railway practice, must have erected vast numbers, but his name is pre-eminently connected with three bridges, so important in their objects and so bold in their design, that they have acquired for their author a world-wide fame. These are the Britannia Bridge, carrying the Chester and Holyhead Railway over the Straits of Menai, in North Wales; the High Level Bridge, for road and railway, across the Tyne at Newcastle ; and the great Victoria Bridge over the St. Lawrence in Canada. It is proposed to give a brief account of each of these structures; but the description of their peculiarities will be simplified by making some general preliminary remarks on bridge building in iron—a material which now enters so largely into the practice of the modern engineer. This will also give the opportunity of referring to some other bridge works of Mr. Stephenson’s, which afford interesting and instructive subjects for comment.

In this task it happens that we have an aid peculiarly appropriate and useful. The subject is one in which Mr. Stephenson took so great an interest, that he was induced, in the midst of his heavy professional engagements, to write the article ‘ Iron Bridges ’ for the eighth edition of the ‘ Encyclopedia Britannica.’ The essay is, of course, limited in its scope, and it is somewhat damaged by typographical errors ; but still it treats the subject in its historical, theoretical, and practical bearings in a very clear and able manner, and with as much fullness as the space at the author’s disposal would permit. We are fortunate therefore in being enabled to base our account of this subject on data of his own compiling, and frequently to give his own words.

‘ The exclusive use of iron in the construction of bridges is of modern date, though no other material is so peculiarly adapted to such a purpose ; its use was, however, long delayed, not so much because its advantages were not appreciated, as from the great cost, and even impossibility, of obtaining iron in large masses. It is now most extensively employed in bridge construction, and though in elegance or durability it cannot compete with stone, where the span is moderate, yet there are numberless cases where its adoption has been the means of solving many of the great problems of modern engineering. Its use has more especially become an absolute necessity in railway-bridge construction, where headway is so frequently of paramount importance, and where rapidity of execution is often a more necessary consideration than even economy or durability; while the defective foundations that have so often to be contended with, render the lightness, the independent strength, and the pliable character of iron of the utmost value for such structures.’

The early attempts at forming a platform or bridge over an open space must have been by means of beams; that is, by pieces of timber, or some other material sufficiently long to stretch from one side to the other, and strong enough to carry the load requited. But both the length and strength of such contrivances must have been naturally very limited ; and as the building arts advanced, a most important step was gained in the invention of the arch, by which a far greater span could be covered and a far greater weight supported, than by the simple beam. The arch is of very ancient date, having been traced back in Egyptian antiquities to many centuries before the Christian era. The Romans took advantage of it with great zeal and skill for the erection of bridges in vast numbers, and of large size and great constructive merit; and it has been used for the same purpose in all succeeding times down to the present day, wherever masonry has been the material employed for the structure. Yet it is a remarkable fact, that the development of the use of the more modern material, iron, for the purpose of bridge building, has just reversed this order of things. Iron bridges were first made by imitating the more advanced form of structure, the masonry arch, and for a long period none but arched bridges were constructed in iron; but with the increase of knowledge in the use of the material, and with the attainment of greater skill in its manufacture, the design of the structures reverted to the primitive form of the beam, in which by far the great majority of iron bridges, including those of the most colossal dimensions, are now made.

The history of iron bridges commences in the 16th century, when such structures were first proposed in some Italian works. In 1719 the subject was again revived by Dr. Desaguhers, the well-known mechanical philosopher, but nothing hke an attempt at construction was made till 1755, when an iron bridge was proposed and partly manufactured at Lyons ; but the design was subsequently abandoned from motives of economy, and a timber bridge was substituted.

The credit of erecting the first iron bridge belongs to this country, the work having been done immediately after the time when, by the impulse given to the iron manufacture by smelting with coke, cast iron had superseded timber in numerous details of mechanical construction. This bridge, erected in 1779, was a cast-iron semi-circular arch of 100 feet span across the Severn at Coalbrookdale, a work which still stands, and which, considering that the manipulation of the material was then completely in its infancy, evinces a boldness and skill highly creditable to its designers.

Shortly after this, some propositions for an extension of the principle were made by French engineers, but not one was carried into execution.

In 1794 two small iron bridges were erected in Germany, but the principle became much further developed in England before it was taken up in earnest in any other country. The celebrated Mr. Telford was one of the first to take advantage of the new material, and in 1796 he erected an iron bridge with a single arch of 130 feet span, also over the Severn, a little below Shrewsbury. In the same year, however, was finished a much larger iron bridge over the Wear near Sunderland, which Mr. Stephenson characterises as one of the boldest examples of arch construction in existence. It is also very remarkable in its paternity and history, its author being no other than the well-known Tom Paine, of sceptical and republican notoriety. This singular being, having a great aptitude for mechanics, proposed in 1790 to construct cast-iron arches, in what was then a novel manner, namely, of framed open panels in the form of voussoirs; and with characteristic energy he put his views to the test by making an experimental arch of 8S.[ feet span, which was exhibited at Paddington, and was completely successful. It happened that in the same year a committee was appointed for investigating the inconvenient and dangerous state of the ancient ferry in the middle of the harbour at Wearmouth; and as it was decided that a bridge should be built of cast iron, the ideas of Paine were adopted in its design, and part of the ironwork of his experimental arch was used in its construction. The span of this bridge, which is in one segmental arch, is no less than 236 feet, only 4 feet less than the centre arch of Southwark Bridge, the largest in existence, and yet it contains only about one-fifth the weight of iron!

If (says Mr. Stephenson) we are to consider Paine as its author, his daring in engineering certainly does full justice to the fervour of his political career ; for, successful as the result has undoubtedly proved, want of experience and consequent ignorance of the risk could have alone induced so bold an experiment; and we are led rather to wonder at, than to admire, a structure which, as regards its proportions and its small quantity of material, will probably remain unrivalled.

To complete the singular history of this bridge, it was sold in 1816, by a lottery, for £30,000, £3000 more than its original cost twenty years before; and certain alterations to it which will be described hereafter, formed the last work of Robert Stephenson’s engineering career.

In 1801 a remarkable design was given in by Messrs. Telford and Douglas for replacing London Bridge by a single cast-iron arch of 600 feet span; and the works were even put in hand; but the scheme was afterwards abandoned on account of the great and inconvenient rise that would be required in the approaches.

Iron bridges now began to be generally adopted. In 1802 a large arch of 180 feet span was erected over the Thames at Staines, by the same engineers and on the same plan as the Sunderland Bridge. The abutments were of insufficient strength, and the bridge subsequently required supporting; but it remained till the erection of the handsome stone structure on a neighbouring site by Messrs. Rennie in 1832, when it was taken down.

Iron bridges soon afterwards found their way into France. The Pont du Louvre, erected in 1803, was the first, and this was followed in succeeding years by the Pont d’Austerlitz and others, in Paris and in other parts of the country. In England they began to multiply fast. In 1816 Vauxhall Bridge was opened, and three years afterwards Southwark Bridge, which Mr. Stephenson declares stands confessedly unrivalled as an example of the cast-iron arch bridge, whether as regards its colossal proportions, its architectural effect, or the general simplicity and massive character of its details. The central arch is 240 feet span, the largest in the world; and the two side arches are each 210 feet. The bridge cost £800,000, and the quantity of iron in it is nearly 6000 tons.

At that time, therefore, the first form of iron bridge, that of the arch, may be considered as having arrived at its full development. Great numbers have been constructed in this and other countries, but there is nothing connected with them that needs further notice here.

Meanwhile, in the forty years that had elapsed since the first introduction of the iron bridge, great advancement had taken place in the manufacture of iron generally, and particularly in that of wrought iron. Cast iron had been eminently suitable for the arch bridge, as being specially adapted for resisting the strain to which arches were exposed, namely, direct compression or crushing. But to the use both of this form and of this material there was a necessary limit, from the massive construction and consequent great weight necessary, when employed for very large spans.

Wrought iron, by its greater tenacity and less liability to fracture, offered an extension of the limit of size possible for iron bridges. It was distinguished from cast iron by its property of withstanding a great tensile strain with a light weight of material, a property very valuable for bridge construction; the only problem being so to design the structure as to bring the material under this kind of strain. Hence arose a new construction of bridge, altogether differing from the arch, namely, the suspension bridge-, the principle of which was, that the strain of the load was thrown upon a chain, keeping it in a constant state of tension, instead of upon an arch, in a constant state of compression.

The idea of forming a communication between opposite shores of a ravine or river, by suspending ropes across it, and attaching a roadway thereto, is of great antiquity, bridges of this description having existed in China, and indeed among less civilized nations, from time immemorial.

But the suspension bridge formed of iron chains, to the construction of which the resources of modern art and science have been applied, is of comparatively recent date. The first of which there is any account in England, or indeed in Europe, was a small foot bridge erected in 1741 across the Tees, near Middleton, in Durham, for the use of the miners. It was 70 feet long and 60 feet above the river, the roadway being 2 feet wide, of planking, with a handrail on each side; but no further particulars are recorded as to its details, and probably it was a very primitive affair.

In the commencement of the present century, however, the subject was taken up by an energetic naval officer. Captain Samuel Brown, who appears to have had a knowledge of iron-work and of general construction worthy of an engineer of the present day. It was he who first introduced into the naval service the use of chain cables ; and for this and his other services to the country, among which his part in the introduction and improvement of the suspension bridge was not the least important, he was knighted by the Queen in 1838.

Captain Browm’s great improvement was the adoption of chains made of long iron bars instead of common link cables, which had been used up to that time. In 1813 he constructed a large model of a bridge on this plan, and in July 1817 he took out a patent for his invention.

The first bridge actually erected by him, and indeed the first suspension bridge ever constructed of much engineering pretensions, was the Union Bridge across the Tweed, five miles above Berwick, which was begun in August 1819 and opened in July 1820. It was a large bridge, being 450 feet span and having 30 feet deflection of chain. The roadway was 18 feet wide, and the chains were composed of link bars 15 feet long.

Captain Brown in following years also erected several other large structures on the same principle, among which, perhaps, the one best known is the chain pier at Brighton, opened in 1823.

In the meantime Mr. Telford was led to see the advantage of the suspension principle. About 1814 he had been requested to report on the practicability of forming a bridge at Runcorn over the Mersey. The plan of bar chains does not seem to have been then known to him, but he entered into a series of investigations and experiments to test the capabilities of the suspension principle generally, and its adaptability for the object he had in view. The strength of iron was not at that time so well determined as it is now, and one of Mr. Telford’s objects was to ascertain, by direct experiment, the tensile power of the material when applied in the form of a suspended chain.

Nothing came of this proposition at the time, but Mr. Telford treasured up the knowledge he had gained, and a few years afterwards he had the opportunity of bringing it into practical application.

In 1819 he commenced the celebrated bridge over the Menai Straits, which magnificent work would, if he had done nothing else, have itself sufficed to render his name immortal. It was finished and opened in 1826, and its success made the principle popular; suspension bridges of large magnitude having since become very common.

About the year 1830, therefore, there were two great classes of iron bridges in use ; the cast-iron arch bridge, heavy and of limited size, but rigid and strong ; and the wrought-iron suspension bridge, lighter and much more capable of large expansion, but slender and less steady. Each kind had been brought to a state of great perfection, but the number of bridges built was not great, and the completion of a single large structure of the kind was an event in history.

But now arrived an epoch in civil engineering, which at once enlarged tenfold its sphere of action, and gave the application of iron for bridge purposes an entirely new direction. This was the introduction of railways.

Hitherto, bridges had been applied generally to common roads ; if the arch was adopted, inclined approaches were of small importance ; and in determining the rise of his arch, the engineer selected any headway he thought proper, while every other consideration was likewise made subsidiary to the problem of constructing the bridge itself. If the suspension bridge was chosen for the purpose, the passing load was light, and its speed of transit could be easily reduced to suit the comparatively unstable nature of the structure.

But on the introduction of railways, hundreds of roads, rivers, and valleys had at once to be spanned with bridges perfectly level, and of a strength and rigidity sufficient to allow the dashing across of the ponderous and swift locomotive, instead of the light coach or the quiet team. Moreover, a series of new conditions arose for these bridges, which complicated the problem still more. Their time of construction was an important element; so was economy of their first cost; while every conceivable difficulty arose from their limited headway, their bad foundations, their oblique directions, their gigantic dimensions, and the necessity of bridging over navigable waters or crowded thoroughfares without interfering with the traffic upon them. The number of bridges required also became something quite unprecedented; Mr. Stephenson estimated that up to 1856 at least twenty-five thousand railway bridges must have been built in the United Kingdom alone.

The simple arch of masonry or brickwork was applied wherever it was practicable, but in many situations it was inapplicable, and the engineer, to whom the use of iron was now becoming every day more familiar, naturally turned to this material to supply the desideratum.

Of the two kinds of iron structures then in vogue, one, the suspension bridge, was, from its want of stability, quite out of the question ; but the other, the iron arch, was favourable in certain situations, where its well understood qualities of rigidity and strength warranted its adoption. Some of the most elegant and efficient railway bridges have been erected on this plan. The London and Birmingham and other early railways have several of this kind, and it is still used in situations where appearance is of importance, as the arch bridge may generally be made a handsomer structure than any other rigid form.

It was found, however, that the cast-iron arch bridge, from its great weight, and the small span of which it was capable within reasonable limits of cost, was but of comparatively limited application to railway requirements: hence it became necessary to discover some other kind of iron structure more generally suitable, and happily this was found by reverting to the earliest form of all, the primitive straight beam. This would seem, no doubt, a retrograde step from the elaborate and elegant structures on which so much scientific investigation and mechanical skill had been bestowed; but the retrogression was only apparent, for no sooner had the beam been established as the normal model for railway bridges, than the attention of scientific and practical men was at once called to its development, and under this stimulus it soon outgrew its original simple form and dimensions. Improvements and extensions of the principle were gradually introduced, and the simple beam is now scarcely to be recognised as the parent of the many magnificent structures which, far exceeding the largest arches in dimensions, have become our most prominent monuments of engineering enterprise and skill. Still, however, we cannot fail to be struck with the curious reverse order of progress in the history of iron bridges, when we find that the appliances resulting from ages of improvement have been rejected, to adopt a principle identical with the earliest attempt of the uncivilized savage.

It may be well here to explain the principal points of difference between the three different systems of bridges above referred to, and to show on what grounds one of them alone has proved so specially applicable to railway purposes. When any structure is employed for carrying weight over an open space, the laws of mechanics require that the vertical forces due to the gravity of the load should produce strains or thrusts in other directions more nearly approaching to the horizontal. In an arch, the essence of which is that it should curve downwards on each side from the crown, a compressive strain is produced along the whole line of the curve, which, operating at its extremities, tends to thrust the abutments outwards ; and this thrust must be efficiently resisted by massive solidity and strength of the abutments, to keep the bridge in equilibrium.

In a suspension bridge, this effect is reversed. The essence of the structure is that the suspending chain must curve upwards on each side from the centre, and must sustain along its whole length a tensile strain, which tends to draw the ends inwards; and this is usually provided against by securing the ends of the chain firmly into the ground on each side.

Now the beam, is a sort of compound of these two principles. It is usually straight, neither turning downwards at the ends hke the arch, nor upwards like the suspension chain; but it comprehends within itself the characters of both as regards the strains upon it; for the effect of the load is to divide, in principle, the beam into two longitudinal parts throughout its whole length ; the upper part bearing a horizontal strain of compression, hke the arch, and the lower a horizontal tensile strain, like the chain ; these two strains being, moreover, as an essential condition of the equilibrium, equal and contrary to each other. Hence the strength of a beam is entirely self-contained, and aU its horizontal forces are perfectly self-equilibrated ; the consequence of which is, that no resisting power whatever is required at the abutments or ends, further than is necessary to support the vertical pressure of the beam and its load, a condition capable of the simplest and easiest application. From these principles it will be seen that the advantages of the beam, or girder, as it is also called, for railway bridges consist in five great properties.

1. It supersedes the chain by its firmness and rigidity, being subject only to a shght deflection under its load, which is of no practical disadvantage.

2. Compared with the arch it has the great advantage of straightness, not requiring to be curved downwards at the ends, and so not only making a level road above, but also leaving a uniform height of headway underneath, which is often a vital necessity. It will be seen hereafter that it was this condition that determined the use of a beam for the Britannia Bridge.

3. As the beam requires no preparations in the abutments for resisting any horizontal or oblique thrust, the construction of these parts of the bridge, particularly as regards their foundations, is rendered very much simpler, more expeditious, and less costly.

4. The ironwork required for a beam is generally very much less in weight than for an arch of the same span and strength.

5. The self-contained strength of the beam, and its capability of being fixed in many cases without scaffolding, much facilitate its erection; not only as saving cost and time, but also in avoiding interference with navigation or traffic below.

The earliest iron beams of which any account is preserved were used by Messrs. Boulton & Watt in building a cotton mill at Manchester in the year 1800; and as soon as confidence became established in the material, and the improvements in the manufacture of iron enabled large castings of this description to be made with tolerable certainty as to quality, and at reasonable price, iron beams soon began to supersede the use of timber for many building purposes, as being much less liable to decay, or to destruction by fire.

In 1822 Tredgold wrote his celebrated ‘Practical Essay on the Strength of Cast-Iron,’ the principal object of which was to define the theoretical laws that governed the construction of iron beams, and to put them into such a shape as should be useful to the practical mechanic and builder. Two or three years afterwards cast-iron girders of 50 feet span, the largest then constructed, were erected by Mr. Eastrick at the British Museum.

It was natural that the first engineer who had railways of any importance to make, should first find out the applicability of the beam to railway-bridge construction; and accordingly the first bridges of this kind were erected by George Stephenson about the year 1830 on the Manchester and Liverpool Railway.

The girders used for this purpose were made entirely of cast-iron, in fact, were simple cast-iron beams, similar to those before used for other purposes; but as the object they had to serve soon became much more important, and the spans required much larger, more attention was called to the principles of their construction both from a theoretical and practical point of view.

The theoretical part was taken up by Mr. Eaton Hodgkinson soon after the erection of the first girder bridges, and he corrected some errors that had been entertained as to beams of cast-iron, and established greatly improved rules for their proportions, by which their strength was much increased and their cost greatly reduced.

In a practical point of view the attention of engineers was soon drawn to the uncertainty and weakness of cast- iron, when exposed to a tensile strain in the lower flange of the girder. The proper function of cast-iron had been developed in the arch, namely, to withstand compression; for a strain in the contrary direction it was peculiarly unfitted, not only by its want of cohesive strength, but still more from the almost inevitable existence, in all large castings, of hidden flaws and defects. Little benefit was obtained by increase of thickness, for the treacherous character of the material increased rapidly with the mass in which it was cast; and the difficulty of uniting cast-iron rendered impracticable the attempt to build up such girders of separate castings, so that the new girder-bridges were limited in their dimensions to very moderate spans.

In order to meet this difficulty the girders were in some cases made double, so as to diminish the dangerous influence of possible unsoundness; but still an obvious necessity arose for some new combinations of the material which should meet the desired end with greater aptitude.

The first important contrivance springing out of the necessities of railway bridges was a modification of the cast-iron arch. The chief obstacle to the use of the ordinary arch was the practical difficulty of meeting the thrust at the abutments, and of obtaining the requisite stability in the foundations; a difficulty much enhanced by the diminution of rise or versed sine which the use of iron allowed, giving a consequent augmentation of the thrust, and a more unmanageable direction of its action. To meet this difficulty, in cases where headway was not of importance, the device was hit upon of connecting the two ends of the arch together by a wrought- iron tie rod, which, by taking upon itself a horizontal tension, deprived the ends of the arch of the tendency to thrust outwards, and so relieved the abutments of all except vertical pressure. The structure thus became essentially a girder, as it contained within itself the perfect equilibration of all its horizontal strains; and as the form resembled that of a bow, having the tie rod for a string, it was called the Bowstring Girder.

Another advantage followed from this construction. It was soon found that by suspending the tie rod strongly from the arch, it might be made to carry the rails at a lower level; the depth of the girder being thus above the roadway instead of below it; by which the attainment of one of the greatest and most troublesome requirements of railway bridges, namely headway underneath, was greatly facilitated.

The earliest railway bridge on this plan was designed by Mr. Robert Stephenson in 1834, and erected in 1835 or 1836, to carry the London and Birmingham Railway over the Grand Junction Canal near Weedon. This kind of structure has since been much used, and the finest example of it is the High Level Bridge at Newcastle-on- Tyne, of which a more complete account will be given hereafter.

But this construction was expensive and cumbrous ; and attention became again turned towards the improvement of the simple cast-iron girder. The most prominent defect of this consisted, as already stated, in the weakness of the lower flange; and the most natural attempt to remedy the evil was by strengthening it with wrought- iron rods, so arranged as to take the tension upon themselves, and thus relieve the more defective cast metal from the tensile strain which it was so little able to bear. The wrought-iron rods were attached by screws at each extremity of the girder to its upper flange, and at the centre were brought down below the bottom flanges, being then tightened up by the screws to such a degree of tension as might be thought desirable. The girder was thus a compound one, of cast and wrought-iron together, and from the peculiar trussing up of the wrought-iron rods it was called the Trussed Girder. Such a beam, if made with due attention to the strains, was evidently less liable to accident than the simple casting, and was capable of application to much larger spans.

The first trussed compound girder, of 60 feet span, was erected about 1839 by Mr. Bidder, in conjunction with Mr. Stephenson, for carrying the Cambridge branch of the Great Eastern Railway (then called the Northern and Eastern) over the River Lea near Tottenham; others followed on the same and other fines, one of the best known being that over the Minories, on the Blackwall Railway. The plan was beginning to be somewhat extensively adopted in railway practice when an occurrence took place which at once checked its use, and which, from Mr. Stephenson’s connection with it, must be noticed at some length. This was the memorable and fatal accident that occurred through the failure of the bridge at Chester, in May 1847.

The Chester and Holyhead Railway crosses the River Dee immediately after leaving Chester, and from the Chester station to a little beyond the crossing the fine is also used, under an agreement, by the Chester and Shrewsbury Company, who, after running over this portion of railway, diverge to the westward by a line of their own.

The Dee Bridge, forming part of the works of the Holyhead line, was designed by Mr. Robert Stephenson, their engineer. The width of the river at this point is about 250 feet, and the railway is elevated nearly 40 feet above low water. The bridge was originally intended to consist of five brick arches, for which the piling was actually commenced; but apprehensions as to the foundations caused the engineer to change his design, and to substitute a bridge of iron girders, altering the number of openings from five to three, and increasing their spans accordingly. The bridge was considerably askew, forming an angle of 51° with the river, or 39° with the perpendicular crossing fine ; and the length of the girders was 98 feet clear span. There were four main girders to each span, twelve in all.

The girders were on the principle above described, i. e. cast-iron trussed with wrought-iron tension bars. Each was made in three lengths, bolted together, and was 3 feet 9 inches deep, or about one-twenty-sixth of the span, having flanges at the top and bottom. The trussing or tension rods, placed on each side, formed a chain of three long links; the middle link horizontal, and placed about the level of the bottom of the cast-iron girder; the two outside links rising up obliquely towards their ends, which stood at a height of about four feet above the top of the girder, and were bolted to large shoulders or bosses, projecting upwards above its top edge. The lower parts of these chains were caused, by means of screws, to press upwards against the cast-iron girders, and so to afford it support by suspension, the links forming essentially suspension chains.

The girders were probably designed in 1845 or early in 1846. In September of the latter year one fine of the bridge was passable; on October 20 it was examined and approved by the Government inspector, and, immediately afterwards it was opened for traffic, not by the Holyhead Company, to whom it belonged, but by the Shrewsbury Company, who were the first ready to use the bridge for public traffic. The Holyhead line was not opened till some time after the accident. From the time of the opening, the bridge was constantly used, not only for Shrewsbury passengers, but also for heavy trains of materials for both lines; but up to the day of the accident, May 24, 1847, nothing occurred to attract attention. It happened that about this time one of the Great Western bridges had been burnt down by cinders from an engine, and alarmed by this disaster, the authorities of the Chester Railway had laid down on the Dee Bridge about 18 tons of broken stone as a protection to its wooden platform. This was done on the afternoon of the day in question, and the first train that traversed the bridge afterwards was the fatal one. Leaving Chester, the engine passed safely over the first and centre openings, but when it arrived about the middle of the third opening, the left-hand or southern girder broke into three pieces, and the carriages fell into the river, at 36 feet below. Five people were killed, and all in the train more or less injured, except the driver; the engine, which ran on beyond the fracture, being the only vehicle that remained on the fine.

This accident made naturally a great sensation, not only from the gravity of the casualty, but from the importance of the consequences to railway engineering. It was felt that the bridge was upon its trial, and as it was soon found that nothing was defective in the manufacture oi the ironwork or the quality of the material, the investigation became directed to the principle of the girder, and to the question whether the strength of beams of this description could be depended on.

The enquiry before the coroner was a very lengthened one. A great deal of engineering evidence was brought forward; two referees, Mr. James Walker, civil engineer, and Captain Simmons, E.E., being also appointed by the Government to investigate the matter.

Mr. Stephenson was naturally looked to for his opinion, which he gave in a report addressed to the Railway Directors, and subsequently enlarged upon in oral evidence before the coroner. He stated that a few hours before the accident, on his way to Bangor, he had narrowly inspected every part of the bridge, and saw nothing to indicate weakness or imperfection. He confidently concluded that every part was firm and sufficient, a conclusion in which he conceived he was justified by the fact of the Chester and Shrewsbury traffic having been uninterruptedly carried on from October to May. He had examined carefully the appearances after the accident, and could arrive at no other conclusion than that the fracture of the girder arose, not from inability to support the weight, but from a violent blow given by the tender, which he conceived to have got off the rails, probably firom the fracture of one of the wheels, while passing the bridge. Mr. Stephenson had full confidence in the proper strength of the bridge, in which he was confirmed by an extensive experience in the combination and use of similar structures, tried under circumstances that demonstrated their capabilities to meet all the ordinary contingencies of railway traffic. An objection had been made that the wrought- iron tension rods did not act well in concert with the rigid cast-iron girder; but he had well considered this, and had had experiments made which had satisfied him there was no force in the objection. If the tension rods were properly screwed up, they would bear the whole strain from the weight passing over the bridge, and would thus take the place of the cast-iron girder, and that was what he sought most to rely upon. He did not maintain that the two principles could be brought into strict union at one and the same time, but he urged that they might mutually aid each other. Mr. Stephenson added that he had erected, in twenty years, more iron bridges than any other member of the profession, being more partial to them, and this was the first failure he had had, large or small.

Mr. Locke and Mr. Vignoles supported the opinion of Mr. Stephenson that the fracture arose by a blow, and that the girder was sufficiently strong. Mr. Locke did not, however, like iron bridges, preferring those of brick or stone.

Mr. Robertson, the engineer of the Chester and Shrewsbury line, reported to his Directors his conclusion that the girder broke in the middle from its weakness to resist the strain, increased by the laying on of the extra ballast immediately before.

The referees appointed by Government made their report on June 15, 1847. After stating the facts and describing the bridge, they considered the strain of the girder and the action of its parts, the effect of temperature, of oscillation, &c., and summed up their opinion, that though the bridge was of sufficient strength if the cast and wrought-iron were supposed to act together, each taking its equal proportion of the strain, yet neither, separately, was sufficient for perfect stability; and that there was great difficulty in ensuring the joint action. They did not agree in Mr. Stephenson’s view that the fracture was caused by a blow.

This report was communicated to the coroner’s jury the last day of their sitting, and seems to have guided them in their decision. They gave a verdict, through Mr. E. Walker, their foreman, of accidental death; adding, however, that they were of opinion the girder broke from being made of a strength insufficient to bear the pressure of quick trains passing over it; that they considered the remainder of the bridge unsafe ; and that for the security of the public, they recommended a Government enquiry as to the safety of such bridges in general.

The propriety of this verdict was questioned at the time, but it must be recollected that at that period the nature of the strains in compound girders was very little understood, and therefore we may be quite prepared to admit that the girder may have been imperfect in design without in the least disparaging Mr, Stephenson’s credit as an engineer.

Indeed, we cannot offer a better description of the defects of this kind of girder than is given by Mr. Stephenson himself in his Essay of 1859. He says :—

The determination of the strength of such girders is a diffi- cult task. They are, in fact, compound girders, formed by combining the truss with the simple girder, the upper flange doing duty as a compression bar in both systems, and being thus subjected to two independent strains. It is evident, therefore, that if the upper flange is simply proportioned to its duty, as the top flange of the simple girder, it will be of insufficient strength for its additional duties as part of the truss. It has been argued that from the perfect union of the top flange with the vertical rib, a considerable portion of the whole girder might be taken as forming part of the truss. It is, however, evidently impossible by calculation to say how far such assistance may be relied on; and a still greater objection exists in the fact that such girders consist of two systems, the ultimate deflections of which are utterly different; the girder, for instance, may be broken before the truss attains half its ultimate deflection or has done half its duty. The objection to this girder is common to all girders in which two independent systems are attempted to be blended; and, as a general principle, all such arrangements should be avoided.

It is useless (adds Mr. Stephenson) to say more on the subject of this form of girder, as since the adoption of wrought-iron for girders they have been entirely superseded; they were designed when no other means existed of obtaining iron girders of great span, and the melancholy accident which occurred at Chester is the only existing instance of their failure.

Mr. Stephenson, in his evidence before the Iron Railway Structure Commission, further explained the objection to the design of these girders, which, in the more advanced state of our present knowledge, is clearly perceived to be the want of a due provision for withstanding the inward thrust of the ends of the wrought-iron ties. The bolts to which the tie bars (which acted, in fact, as suspension chains) were attached, were elevated some four feet above the general level of the top of the cast-iron girder, and no direct solid member for resisting the compressive strain existed between them. The cast-iron girder itself, being of such a small depth in proportion to the length, was very weak, and, as Mr. Stephenson stated, the principal reliance was on the wrought-iron bars; but when the heavy strain came upon these, tending, as in a suspension bridge, to draw their ends inwards, nothing existed sufficiently strong to keep them apart, and consequently acting with a strong leverage and in a most trying manner upon the top flange of the girder, they compressed it beyond its strength, and broke it through. That this was the true explanation of the failure is now clear from the form of the two fractures, which (although this does not seem to have been noticed at th6 time) are identically of the description peculiar to the case where the upper flange of a beam is broken by a compressive strain beyond its resisting power.

Mr. Stephenson on discovering this defect at once took measures to provide against it in other girders on this plan, by adding properly shaped compression pieces of cast-iron to the top of every girder, so as to fill in, solidly and strongly, the space formerly open between the ends of the ties ; and the bridges thus strengthened have never shown any signs of failure.

The Dee Bridge was altered by having inclined struts, bearing against the masonry, placed under each girder, so as to afford support in the middle ; and other bridges, made about the same time, were strengthened in like manner.

The last large girders on this principle were some of 96 feet span, made under Mr. Stephenson’s directions, in 1847, for the Florence and Leghorn Railway, crossing the Arno, and in these the proper improvements were introduced in the original design.

The matter, however, did not stop here. The Government Commissioners of Railways, on receiving the report of the two engineers to whom they referred the investigation of the accident, became alarmed about the iron bridges used on railways generally. On June 23, 1847, they addressed a circular to the secretaries of the different companies, requesting a return to be made of the iron bridges on all fines then working or constructing, giving their dimensions and particulars of their construction; and expressing a hope that wherever the security of such structures was at all doubtful, the companies would take measures to add to their stability, and in the meantime would direct the speed of the trains to be reduced in passing. A few days afterwards they published a minute, to the effect that they repudiated all responsibility for the strength of iron structures which had been inspected and passed by their officers, inasmuch as these gentlemen had only the opportunity of a superficial observation, and no sufficient control over the design.

Another matter of public interest followed. The Railway Commissioners, acting on the suggestion of the coroner’s jury, passed a minute calling the attention of the Government to the uncertainty which existed respecting the conditions to be complied with in employing iron in engineering works, and in particular to bridges which had to be traversed by loads of extraordinary weight with great velocity. They had reason to believe, they said, that much difference of opinion existed among the most eminent engineers of the time as to the proper form and dimensions to be given to railway girders of iron for bridge purposes; and they considered it desirable that the subject should be thoroughly investigated by a Commission, to be composed of scientific men and practical engineers, who should be appointed by Government, and should be requested to arrive at such principles, and to form such rules, as might enable the engineer and the mechanic to apply the metal with confidence in their respective spheres.

The Commission was appointed by Royal Warrant on August 27, 1847, and consisted of Lord Wrottesley, Professor Willis, Captain James, R.E., Mr. George Rennie, Mr. (afterwards Sir) William Cubitt, and Mr. Eaton Hodgkinson, with Captain Douglas Gallon, R.E., for secretary. They spent nearly a year in examining witnesses, making theoretical investigations, trying experiments, and collecting a great mass of information on the subject, which was afterwards published in a Blue Book of 435 pages, accompanied by a large collection of lithographed plans.

Mr. Stephenson was one of the principal witnesses. He gave much information as to the nature and properties of iron—the construction of girder-bridges, particularly those of the kind used over the Dee— the effect on them of passing trains, &c. &c.; but he strongly impressed upon the Commissioners that any attempt to introduce restrictive legislative enactments in regard to the use of iron in railway structures would be highly inexpedient, and would act prejudicially on professional enterprise and skill. ‘ My opinion,’ said he, ‘ is rather strong that a collection of facts of all kinds is highly desirable, in reference to the shape of girders ; but I am convinced that the Commissioners will have infinite difficulty in laying down anything like rules. I cannot conceive myself being tied down in executing such a line, for instance, as the Holyhead, or the London and Birmingham. I cannot conceive myself going on successfully, and being tied down by preconceived rules, or hmitations as to the extent to which cast-iron should be used, and the forms that it should be used in. I think a collection of facts and observations wordd be most valuable ; but if you attempt to draw conclusions from those facts, and confine engineers, even in a limited way, to those conclusions, I am quite sure that it will tend to hamper the profession very much.’

In addition to his oral evidence, Mr. Stephenson furnished a valuable statement of the experiments on iron undertaken at his direction for the High Level Bridge at Newcastle; and also brought up before the Commission two of his chief assistants, namely, Mr. Edwin Clark, who gave a full account of the Britannia and Conway Bridges ; and Mr. Charles Heard Wild, who described other large girders made under Mr. Stephenson’s direction, Mr. Fairbairn and Mr. Hodgkinson also gave full accounts of the comprehensive experiments conducted for the great tubular bridges, so that Mr. Stephenson’s opinions and works may be fairly said to have formed the largest and most important part of the information collected by the Commissioners.

Many other witnesses were examined, skilled in the engineering of ironwork, among whom were Mr. Brunel, Mr. (now Sir) Charles Fox, Mr. Locke, Mr. Rastrick, and Mr. Charles May; and much information was also collected in the form of written statements. Professor Willis, aided by Professor Stokes of Cambridge, contributed an elaborate theoretical paper on the deflection of beams under moving loads ; and a comprehensive series of experiments was tried by the Commission on various points coming within the scope of their enquiry.

The Report of the Commissioners was presented to Her Majesty at the end of July 1848. They considered that bridges should be made somewhat stronger to meet the additional strains from moving loads; and they recommended that the greatest load should in no case exceed one-sixth of the stationary weight which would break the beam when laid on the centre. They also pointed out that weight is an advantage in enabling a structure to resist concussions. As to designs of iron railway bridges, they merely stated the facts and opinions laid before them by engineers. They testified to the careful and scientific manner in which the forms and proportions of the great tubes of the Conway and Britannia Bridges had been elaborated. They thought that wrought-iron plate girders generally appeared to possess and to promise many advantages. They found engineers to be for the most part favourably disposed towards them; but as no experience had yet been acquired of their powers to resist the various actions of sudden changes of temperature, vibrations, and other causes of deterioration, they were unable to express any opinion upon them. With regard to trussed cast-iron bridges, like that of the Lee, they found that difficulties arose from the different expansions and elongations of the two metals, and considered that the greatest skill and caution were necessary to ensure the safe employment of such combinations. They also stated that there existed a great want of uniformity in practice in many most important matters relating to railway engineering, which showed how imperfect and deficient it yet was in its leading principles (a reproach which unfortunately is almost as applicable in 1864 as it was in 1848); but considering that the attention of engineers had been sufficiently awakened to the necessity of providing a superabundant strength in railway structures, and also considering the great importance of leaving the genius of scientific men unfettered for the development of a subject so novel and so rapidly progressive as the construction of railways, they concurred in Mr. Stephenson’s opinion that any legislative enactments with respect to the forms and proportions of the iron structures employed therein would be highly inexpedient.

After the completion of the London and Birmingham Railway, notwithstanding the progress of iron roads in all directions, no important step seems to have been made in the improvement of iron bridges, until the epoch of the Britannia Bridge, the erection of which initiated a complete revolution in this branch of engineering science. As an account of this great structure will be given hereafter, it is only necessary to notice here the effect which it had upon bridge construction in general.

About 1845, when the experimental investigations commenced, the only forms of iron bridges used for railway purposes were the cast-iron arch, the simple cast-iron girder, the trussed compound girder, and the bowstring girder. In all these cast-iron had been the principal element, very little attention having been paid to wrought-iron as a material for girders, although its use had become common and was well understood for suspension chains.

Wrought-iron had indeed been used by Smeaton, in conjunction with wood, to form beams, by bolting an iron plate between two half balks of timber. The plate, being set vertically, contributed important strength to carry the load, while the wood furnished the necessary lateral stiffness. This kind of beam was, from its peculiar construction, called the ‘ flitch ’ or ‘ sandwich’ girder, and it was used subsequently—- in about 1839 or 40—in forming beams of thirty or forty feet span on the Cambridge branch of the Eastern Counties Railway. Wrought-iron beams, of analogous shape to those of 1841-45.] cast-iron, had also been constructed for iron ships, and other purposes, and Mr. Stephenson had himself used them, about 1841, in a small bridge on the abovementioned Cambridge line ; but these were probably the only instances in existence of the use of wrought-iron in railway bridges. Very little was known as to the proper application of the material. The principles of its strength when applied to girders were quite undetermined; no such thing as a constructed beam of any scientific pretensions or any large span had been imagined ; and even the process of connecting wrought-iron plates together by riveting was scarcely known beyond the boiler and iron shipping trade.

The experimental investigations, however, which were conducted for the Britannia and Conway Bridges threw quite a new light on the subject. From the time of the abandonment of the large arch which Mr. Stephenson at first proposed, it had become evident that cast-iron could not be applied, and that wrought-iron was the only material from which any real success could be expected; and it was therefore to the investigation of the properties of this material, and the best manner of using it, that the experimental enquiries were directed. They had the effect of thoroughly developing the powers of wrought- iron, of making known its peculiar properties, of rendering its use perfectly amenable to theoretical calculation, and of proving in the most conclusive manner its special applicability to girders for bridges of any magnitude. And they further showed the practicability of building up or constructing, in that material, girders of almost any strength and size, by only exercising a skilful and careful attention to the details of the design, based on a correct scientific knowledge of the nature and distribution of the mechanical forces acting throughout the structure. And it is worthy of remark how thorough and how perfect these investigations were; for although nothing was known of wrought-iron girders before they were undertaken, and although since their date wrought-iron girders have come into very general use under the greatest variety of forms, and in preference to all other systems of iron-bridge construction, yet nothing essentially new or important has been added to our knowledge of the principles of their construction beyond what was developed in these investigations, the records of which comprise indeed almost the whole useful information, theoretical or practical, we possess on the subject.

To this date, then, may be referred the first use of wrought-iron girders for bridge construction—the greatest step made in iron bridges since their original introduction; as giving them not only the capability of application to larger spans, but making them cheaper, more secure, lighter, more convenient of erection, and clearer in the headway; advantages almost incalculable for railway purposes.

The experiments were commenced in the middle of 1845, and early in the next year so much progress had been made as to lead to the proposition of hollow plate iron girders for railway bridges of considerable span. In July 1846 Mr. Stephenson gave instructions for a bridge on this principle, but with a cast-iron top, for a road at Chalk Farm, crossing over the North Western Railway. This bridge was sixty feet span; it was completed in March 1847, and was the first actual application of hollow wrought-iron girders to the construction of bridges.

Meantime Mr. Fairbairn, who had previously made drawings for a bridge of this kind, foreseeing that the use of hollow plate girders would be considerably extended, proposed to Mr. Stephenson to take out a patent for their application. To this Mr. Stephenson consented; but being averse to his own name appearing in the patent, he refused to accept any share of the profit, though he consented to pay half the expenses of obtaining the patent. It was taken out October 8, 1846.

Mr. Fairbairn soon began to put the plan into operation, and in July 1847 completed bridges on this principle at Blackburn and Bolton in Lancashire, which answered well. The further progress of the designs for the large Britannia and Conway tubes, and their ultimate success, gave to the engineering world more complete confidence in the use of wrought-iron for bridge girders, and a few years more made its application universal.

It would be foreign to the object of this work to follow out in detail the wide and rapid progress of the art of iron bridge building after the epoch we have been considering; it will suffice to give a general view of the state it has now attained, and to describe briefly some of the numerous varieties which have sprung up in the construction.

Iron bridges may now be divided into three great classes — namely, Iron Arch bridges, Suspension bridges, and Iron Girder bridges.

The first two of these have already been sufficiently described, and we may therefore confine our attention to the third or Girder class, which is a very large one, and may be subdivided into several species somewhat as follows:—

1. Solid beams.
2. Trussed cast-iron girders.
3. Bowstring girders.
4. Simple T-shaped girders.
5. Tubular or hollow plate girders.
6. Triangular framed girders.
7. Lattice girders.
8. Rigid suspension girders.

A beam or girder is distinguished from other means of bridging space by containing within itself the double horizontal strains, the top part of the beam being under compression and the lower part under tension. Hence every beam may be considered as consisting of three distinct parts, each of which has its own special office to perform; first the top member, which has to resist crushing; secondly, the bottom member, which has to resist being torn asunder; and thirdly, the vertical part, which has to connect these two together, and to combine the beam into one structural whole.

In the Solid Beam no distinction is made between the functions of the three parts, the top and bottom merging insensibly into the vertical connecting part. This kind of beam is represented by a stone lintel or a wooden floor joist, the form being never used in iron, on account of its wasteful distribution of material.

The Trussed Cast-iron Girder, which has already been sufficiently described, is no longer in use.

Of the Bowstring Girder, with the bow in cast-iron, we have also given an account; but we may add here, that as soon as wrought-iron came into use, large bowstring bridges were made entirely of this material. Among these maybe instanced one of 200 feet span, built by Mr. Brunel in 1849, to carry a branch of the Great Western Railway over the Thames near Windsor; and two by Messrs. Fox and Henderson, carrying the North London Railway over the Commercial Road and the Regent's Canal at Stepney, about 165 feet span, built in 1848.

The I-shaped Girder was one of the earliest used in cast-iron. In it, economy of construction is aimed at by accumulating metal in the shape of flanges at the top and bottom, and connecting them by a vertical rib in the middle of their width, so as to give the whole section the shape of the letter I. This form was also early imitated in wrought-iron beams, and is still one of the simplest and best that can be used when the span is small.

As soon, however, as very moderate Emits are exceeded, difficulties arise of a practical nature, in consequence of the distortion of form to which the simple I-shaped girder would be liable, if of great length and supported only on two distant and Emited bearing surfaces, more especially as expansion and contraction prevent any rigid attachment even on these.

The Tubular or hollow construction of Wrought-iron Plate Girder was a step in advance to meet this difficulty. It differs from the I-shaped Girder only in that the top and bottom members are connected by two vertical plate ribs, one on each side, instead of a single one in the middle, so that the whole forms a sort of hollow tube, which is a stiffer and otherwise superior construction for large spans. Indeed, this plan may be carried to so large a size that, as in the Britannia and Conway Bridges, the engine and train may pass along inside the tube.

In the Triangular-framed Girder, the vertical rib, connecting the top and bottom members, instead of being composed of plates, is formed of a series of frames of a triangular shape, fastened to the top and bottom with large bolts. This form was first tried about 1850, at the London Bridge Station of the South Eastern Railway, and has since been a great deal adopted, with much success.

The largest bridge on this plan is one of 240 feet span, erected in 1852 on the Great Northern Railway over a branch of the Trent near Newark. A structure also very remarkable is a viaduct at Crumlin in Monmouthshire, erected in 1857. It consists of a series of seven triangular-framed girders, each 150 feet span, crossing a wide valley at an altitude of 200 feet, and supported by piers of framed ironwork, of great lightness of construction. The singular appearance of this structure can scarcely be imagined without seeing it, and it is certainly one of the engineering curiosities of Great Britain.

In the Lattice Girder, the vertical plates are replaced by a number of bars crossing each other so as to form a lattice-work, the strength of these bars being proportioned, by known rules, according to their places in the girder. The lattice principle was early used to a great extent for timber bridges, particularly in America. One of the largest, as also one of the earliest structures on this principle in iron, was the bridge or viaduct erected in 1855 on the line of the Dublin and Belfast Railway over the river Boyne near Drogheda. It consists of three spans, the centre 264 feet, and the sides 139 feet each, the height above high water being 90 feet.

The Rigid Suspension Girder is placed in a separate class on account of its use by Mr. Brunel in two magnificent iron bridges of gigantic dimensions and economic construction over the Wye at Chepstow, and the Tamar at Saltash. The former, completed in 1853, has a single span of 300 feet, at a height of 46 feet above high water. The girder is, in fact, a rigid suspension bridge, the tension of the chains being resisted, not in the usual way, by anchorage in the ground at each end, but by a huge cylindrical wrought-iron strut or column, stretching across from side to side, at a height of 50 feet at the centre, above the roadway. The whole is well braced together, and it thus forms a colossal trussed girder.

The Saltash Bridge, with the viaduct which forms its approach, carries the Cornwall Railway over the estuary and valley of the Tamar near Plymouth. The whole structure comprises nineteen openings, and is 2,240 feet long; but the bridge itself, crossing the river, consists of two spans of 450 feet each, at a height of 100 feet above the water. The main girders, or trusses, are in principle analogous to those at Chepstow; but here the compression tubes, which resist the pull of the suspension chains, are curved instead of being straight, the rise of the tube being equal to the drop of the chains. The tube thus partakes of the nature of an arch, and in fact the whole girder is a kind of intermediate between the Chepstow truss and the old bowstring girder.

The centre pier was a work of considerable difficulty on account of the great depth of water. The substructure is a solid cylindrical pillar of granite, 35 feet in diameter, resting on a rock foundation 86 feet below high-water mark. It was built in a coffer dam or cylinder of plate iron sunk through the bed of the river till it rested on the rock below, and then emptied and kept clear of water partly by pumping and partly by compressed air, to allow of the construction of the granite column inside.

The bridge was commenced in 1853, and was opened in May 1859 by H.R.H. the late Prince Consort, by whose permission it was called the Royal Albert Bridge.

Subsequently to the erection of the great works in North Wales, Mr. Stephenson designed three other iron bridges of considerable magnitude. The first was over the Aire, on the York and North Midland Railway, erected in 1850. The span was 225 feet, and Mr. Stephenson adopted the tubular girder, similar to the Welsh bridges, the trains passing inside the tube; but in this bridge, the span being so much smaller, the cells were dispensed with, and the top and the bottom were formed of simple plates. The tubes were originally constructed of a tapering section, narrower at the top than the bottom, but after they were erected and the bridge was opened, the narrowness at the upper part was objected to by the Government Inspector, and the top plate had to be cut through longitudinally and widened in situ by the insertion of a strip of iron all along; a delicate and unprecedented operation, but which, under great care, was perfectly successful.

In 1855 Mr. Stephenson erected a large wrought-iron bridge on the Egyptian Raihvay over the Damietta branch of the Nile near Benha. It consists of ten spans or openings, each of 80 feet, except the two centre ones, which are 60 feet each, and are made to open, forming what is called a swing bridge, one of the largest hitherto attempted. The girders for this bridge were also tubular, but from their small size the roadway is carried upon the top of the tubes, and not in their interior. The total length of the swing beam is 157 feet; it is balanced at the middle of its length on a large central pier, so that when open to the navigation, a clear waterway of 60 feet is left on either side. Each half of the beam sustains its own weight as a cantilever 66 feet long.

The piers consist of wrought-iron cylinders, 7 feet in diameter below the level of low tide, and 5 feet diameter above that level. They were sunk by the pneumatic process to a depth of 33 feet below the bed of the river, through sod of a peculiarly shifting character, and were filled in with concrete. There are six of these cylinders in the central pier which supports the swing bridge, and the adjacent piers on either side of the centre have each four cylinders. Each of the remaining piers has two cylinders only. This plan of obtaining the foundation of piers by sinking large iron cylinders has been a most important modern advance in bridge-building. One of the latest examples is in the bridge carrying the Charing Cross Railway across the Thames.

The beams or tubes are 6 feet 6 inches deep, and 6 feet 6 inches wide at the bottom, tapering to 6 feet wide at the top. They rest at their ends on rollers working between planed surfaces, to admit of the motion caused by expansion and contraction. The tubes carry a single line of railway on their tops, the rails being laid on longitudinal sleepers; and there is also a roadway four feet wide on either side, supported by wrought-iron brackets bolted to the sides of the tube.

The revolving machinery for the swing part of the bridge consists of a turntable 19 feet diameter, running upon eighteen conical rollers, connected by what is called a ‘live ring.’ The whole of this machinery is most carefully fitted and susceptible of the most accurate adjustment. The bridge is turned by a capstan connected by gearing with the moving parts, and which can be worked with facility by two men.

In 1853 Mr. Stephenson took up the subject of the Great Victoria Bridge over the St. Lawrence in Canada, a work of immense magnitude, of which a separate notice is given in a subsequent chapter.

The last work of his life had to do with one of the earliest iron bridges ever erected, and one in which he had always taken particular interest. This was the strengthening, and indeed almost the entire reconstruction, of the celebrated bridge over the Wear at Sunderland, which has been already noticed in the early part of this chapter.

(See the PDF version for the diagram inserted here)

At the request of the authorities of the town Mr. Stephenson had several times examined this bridge, and expressed his conviction that its stability was extremely precarious; and as they concurred in this opinion, he was in the year 1857 requested to undertake its repair. He was then in failing health, and reluctant to burden himself with further work, and he accordingly entrusted the details of the operation to one of his earliest friends and assistants, Mr. G. H. Phipps, by whom it was carried to completion; Mr. Stephenson, however, giving his advice and opinion, and occasionally visiting the bridge during the progress of the works. The work consisted of the introduction of three new tubular arched ribs of wrought-iron between the original cast-iron ribs of the bridge; the latter being firmly bolted to the new girders, and thus being relieved of the chief part of their load.

The width of the bridge between the hand-railings was also increased from 32 feet to 41 feet 1.5 inch, and the road over the bridge and on the approaches was much improved, its inclination, or gradient, being lessened from 1 in 17 to 1 in 50.

One of the chief difficulties was the construction of a timber scaffolding, supported by pile work from the bed of the river, which should be sufficiently substantial to carry the weight of the arches, and should at the same time allow of the passage, with their masts standing, of the large amount of shipping frequenting the port; this scaffolding also formed a temporary bridge, both for carriage and foot traffic, during the progress of the alterations.

The work was let by contract, towards the end of 1857, to [B. C. Lawton| Mr. B. C. Lawton]], by whom it was satisfactorily completed, and the bridge was re-opened for traffic in the, summer of 1859, a few months prior to Mr. Stephenson’s death.

The accompanying plate contains diagrams of several large iron bridges all drawn to the same scale, from which a comparison of their respective magnitudes can be made. The two small figures are the Sunderland Bridge and the Southwark Bridge; the others represent the four principal bridges erected, by Mr. Stephenson, namely the Benha Bridge, the High Level Bridge, the Britannia Bridge, and the Great Victoria Bridge in Canada.

W. P.

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