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it was set up on a bowling green at Paddington (where is the bowling green now ?) and exhibited as a curiosity. Paine had intended it for the Schuylkill in America. But his means failed, and he ran away to Paris, then in the height of its revolutionary frenzy, to join the friends of liberty or to avoid his creditors. The friends of liberty, more formidable than his creditors, threw him into prison, and would have guillotined him if he had not contrived to escape. In the subsequent confusion of his affairs, the bridge was ultimately taken back by the manufacturers, Messrs. Walker, and supplied part of the materials for T. Wilson's great arch across the Wear near Sunderland. This work was completed in 1796, and was long regarded as a world's wonder; it has indeed no longer the merits of novelty and rarity, but it well deserves the praise bestowed on it by Robert Stephenson, who pronounced it to be a structure which, as regards its proportions and the

quantity of material employed, will remain unrivalled. Contemporaneously with the construction of the iron bridge at Sunderland, the second actually completed, Telford was engaged in erecting another of the same material, two miles above the first -at Buildwas, to replace an ancient stone structure which had been carried away by the Severn in a recent flood; and so rapid was the progress which engineering had made in less than twenty years, that although the span of his bridge was thirty feet wider than that of Pritchard's, it contained less than half the quantity of cast iron. Since those days there has sprung up another rival of the parent arch some miles lower down the stream at Coalport- where is really made the china which London chooses to call by the name of Coalbrookdale, while to complete the triumph of Tradition over Fact, the structure itself is known in the neighbourhood as the “Wooden bridge.'

The largest cast-iron bridge is that of Southwark, built by Rennie in 1815-19, the principal arch of which has a span of 140 feet; but since their first invention bridges of this material have multiplied so fast, that the enumeration of them would be tedious, and the skilfulness of their construction has ceased to excite wonder. Nor is it only where great spaces were to be traversed, that cast-iron was employed; it has frequently formed the material of bridges of ordinary construction. But never, perhaps, was a greater compliment paid to iron than when it was selected to form the arches of the new bridge at Westminster, in immediate juxtaposition with the Houses of Parliament. From a very early date Telford used it largely for the aqueducts of his canals, as also for lock-gates and other purposes connected with inland navigation : and in two instances where

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it was found a lock had been constructed on a stratum of quicksand, he lined the whole interior of the basin with cast iron.

For many years no satisfactory plan could be proposed for bridging over the Menai Strait. Rennie had sent in a magnificent design for a cast-iron bridge, to the centre arch of

a which he gave a span of 450 feet, but the cost was enormous. Long afterwards Telford sent in alternative plans for two cast-iron bridges, to be carried across at a lower level - but obstruction to navigation was apprehended, and nothing was decided. At last when Telford published his design for a suspension bridge across the Mersey, the Commissioners of the Holyhead Road instructed him to prepare a plan for effecting the desired communication on this new

principle. New, strictly speaking, the principle was not. In many parts of the world it might be seen exemplified in hanging bridges of rude construction and perishable materials, but it could not be applied to works of importance till the increased supply of iron afforded a material of the requisite strength and durability. And the difficulties of applying the principle of suspension to a structure so vast, and to a material so ponderous, were such as to entitle the man who overcame them to all the credit of invention. Telford felt the greatest anxiety as to the result, and spared no pains to ensure success. He made, we are told, an elaborate series of experiments to test the tenacity of wrought-iron bars (for wrought iron he ascertained to be the proper material for a suspension bridge), and fully aware of the difference of quality which even in those days distinguished the product of different districts, he finally bound his contractor to use none but the best Shropshire iron.*

The Menai bridge has been followed by similar works of equal and even greater magnitude in various parts of the world; and previously to its erection, the principle of suspension had much engaged the attention of our engineers. Captain Brown, who subsequently built the chain pier at Brighton, took out a patent for bridges on this plan in 1817. There is probably some variety in the methods employed by different engineers, there is certainly a considerable difference in the results. In no case, indeed, can the vibration, which is the great objection to this

Life of Telford - Smiles' Lives of the Engineers. We have placed Mr. Smiles' work on the list which heads this article, because we hare occasionally availed ourselves of the information it contains, and we are glad of the opportunity to recommend it to the reader's attention. But the work is not yet complete, and we hope it will eventually embrace the great achievements of the Stephensons and the Brunels. VOL. CXVI. NO. CCXXXV.

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principle of construction, be wholly overcome; but in slighter works it is very perceptible, and its consequences are very serious. The Broughton bridge near Manchester gave way beneath the measured tread of a party of sixty men in marching order. In France several suspension bridges are said to have fallen. The great bridge at Angers, which had been built by the same engineers who constructed the bridge at Fribourg, gave way under the combined strain of a gale of wind and the passage of between four and five hundred troops. Troops in France are ordered to break ranks' in passing over these structures; but in this case the order was disobeyed for the purpose of military display, and the result was fatal.

When it was first designed to connect Eastern and Western Prussia by a permanent link of communication at Cologne, the Government in the first instance accepted a plan for a chain suspension bridge; but the flexibility of a bridge constructed on this principle rendered it unfit for the support of railway traffic, and the Chevalier Bunsen, then Prussian Minister in England, was able to report to his Government that bridges had been constructed in this country on a much larger scale than would be necessary at Cologne. The Britannia and Conway bridges had been recently opened, and were daily thronged with wondering visitors. On this representation, a Commission was sent over to inspect these new structures, and to make a report, and Mr. Fairbairn was invited to send in plans for a bridge on the same principle. The tubular bridge was, however, rejected, but the Commission did not revert to the original design of a suspension bridge; a modification of the lattice' bridge, a later invention, was ultimately adopted, and the result is one of the noblest works of the kind upon the continent of Europe.

It was the necessity of carrying roads at a dead level across wide spaces, so as to allow the greatest amount of head room below, and at the same time to impart to the connecting structure a degree of solidity capable of sustaining the force of a train at

a full speed,- it was, in short, the very need so conspicuously manifest at Cologne, that goaded R. Stevenson to the invention of the tubular girder and the tubular bridge. The tubular girder is a hollow rectangular beam, composed of four plates of wrought iron, of different strengths proportionate to the different strain on each. The tubular bridge is only the tubular girder expanded to such dimensions that the trains run in the inside of tubular beams, instead of running on roads supported by them; but the planes which form the top and bottom of the great tube are themselves tubular.

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For further explanation of this masterpiece of constructive skill, we must refer the reader to Mr. Fairbairn's interesting volume on the Britannia and Conway Bridges. Our business now is not with the mechanical contrivance of the engineer, but with his materials. Great inventions are usually followed by a host of others differing from them in detail and exhibiting more or less novelty of principle. Inflexible suspension bridges have been contrived by suspending the roadway beneath a large castiron arch. Various modifications of lattice bridges have been constructed, of which hitherto that of Cologne is the most considerable. But one of vast size is now in the course of construction for the Jumna. Bowstring bridges, in which the roadway takes the place of the string, have many advocates. The Saltash bridge, which carries the Cornish railway across the Tamar, is one of Mr. Brunel's most ingenious and imposing structures. But it would be endless to enumerate all the new plans of bridges which our rapidly extending railways have called forth, in almost capricious variety: we have only to note how largely iron enters into the composition of them all. Railway bridges must be calculated to resist forces very different from those which act on bridges designed for ordinary traffic; and it became important to ascertain the effect of violent concussions, and the passage of heavy bodies in rapid motion, in deflecting and fracturing the beams on which they are made to act; nor was it less needful to discover whether metal which has been exposed for a long period to concussions and vibrations undergoes any change in its cellular structure by which it becomes weakened. In 1849 a Commission, of which Lord Wrottesley was president, was appointed to inquire into these matters, with a view to dis

cover such principles, and to form such rules, as may enable the engineer and the mechanic to apply the metal with confidence." Their report is in the highest degree interesting and valuable. The general result is that a “superabundant strength is needed in

railway structures, but that the conditions of safety will be 'realised if the greatest load on a railway bridge does not exceed 'one-sixth of the weight which would break the beam when laid

on at rest in its centre.' Among many other useful practical suggestions the Committee recommend, that engineers in con'tracting for castings should stipulate for iron to bear a certain

weight instead of endeavouring to procure a certain mixture.' In the experiments which were made by the Commission for the purpose of testing the strength of different kinds of iron, it is gratifying to find what superior qualities they selected for trial: we fear it is long since similar metal has been actually employed in any railway structure.

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The experiments which Mr. Fairbairn conducted, in order to ascertain the strength of the materials to be employed in the tubular bridges, led him to the discovery, which he tells us he had not anticipated, that wrought iron answers better than cast iron for many of the purposes to which cast iron exclusively had hitherto been applied. The reader is doubtless aware that pig iron is the raw material of both wrought and cast iron ; but, while the former is brought to its perfection by repeated working, the latter is produced by merely once more making the metal fluid in the cupola furnace,' and then pouring it into a mould of the form required. Hence, as the process of manufacturing is so much less laborious, cast iron is proportionably cheaper than wrought; but it must not be supposed that these two forms of iron resemble each other in kind, and differ only in degree. For all practical purposes they are distinct metals:

Cast iron differs from wrought,' says Mr. Fairbairn, in its physical as well as its mechanical qualities. It is a hard rigid crystalline unmalleable substance. It possesses great powers of resistance to compression, but comparatively small resistance to that of extension, and from its low degree of ductility it undergoes but little elongation when acted on by a tensile force. On the contrary, wrought iron is a flexible malleable ductile substance, which presents great resistance to a force of extension, but a somewhat less resistance to a force of compression ; from its high degree of ductility it undergoes a considerable elongation when acted upon by a tensile force. And for a long time it was assumed that when applied to resist compression, it would crumple like leather.' (P. 47.)

Mr. Fairbairn gives a most interesting account of the experiments by which he disposed of the crumpled leather' theory. On the other hand, he gives excellent reasons why cast iron cannot be depended on. The unequal contraction of the metal which takes place when it is exposed to great variations of the temperature, causes it to snap. Moreover, the nature of the material is treacherous: 'all crystalline bodies are of a more • brittle and uncertain character than those which are of a • fibrous structure. Flaws and imperfections are of frequent occurrence in the casting, which cannot be discovered by the minutest inspection of the surface.

* Repeated instances have occurred wherein castings presenting every appearance of perfection have been found to contain the elements of destruction, either in concealed air bubbles, or in the infusion of scoriæ, which had been run into the moulds and skinned over by a smooth covering of apparently sound iron.'

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