rendered us much other valuable aid throughout this little work, and thus furnished another proof of their known liberality in acquainting others with the useful and often costly results of their own extended experience.

It is seldom that the invention of works of new design and skilful mechanical arrangement is due entirely to one mind, any more than their construction is due to one pair of hands: hence great difficulty arises in assigning to each contributor his fair share of merit in their production. It must, however, be admitted, that to Mr. Robert Stephenson alone we are in this instance indebted for the original suggestion; and, with this admission, we have endeavoured to avoid any attempt to judge of the precise claims of the two eminent men whose joint labours have produced the Conway and the Britannia Tubular Bridges. That these great works owe their design and construction to these joint labours is clearly evident, and, we respectfully submit, amply sufficient to justify the record of the two names of ROBERT STEPHENSON and WILLIAM FAIRBAIRN in an honourable and enduring association.

In order to give a glimpse at the experience which had been had in Iron Bridge-building prior to the use of the malleable material, and to show the defects which this was designed to obviate, a brief sketch of the history of Iron Bridges is prefixed. This is followed by a notice of former applications of malleable iron, with the view of bringing up the sketch to the period at which tubular girders were first used. The description of the works of Telford upon the Holyhead Road is introduced on account of the generally interesting character of those works, and the absence of any account of them within the reach of ordinary readers. While exalting the names and works of our own time, wo can readily afford to acknowledge the claims of those of a preceding age.

TUBULAR, GIRDER, AND OTHER

IRON BRIDGES.

SECTION I.

Sketch of the History of Iron Bridges-Cast-Iron Arched BridgesCast-Iron Girder Bridges-Cast-Iron Compound Girder Bridges, trussed with Malleable-Iron Bars.

THE employment of iron as a material in the construction of bridges is of comparatively modern date. Seventy years have scarcely elapsed since the first iron bridge was constructed in England over the river Severn, and near to Coalbrook Dale. This bridge was built by Darby, and consisted of five ribs of cast iron, supporting perpendicular spandril pieces of the same material, and upon which the roadway is carried. The arched ribs are nearly semicircular, having a span of 100 feet, and a rise or versed sine of 45 feet. The arches spring at a height of 10 feet above low-water level, and the clear height up to the soffit of the arches is therefore 55 feet. At the time of its construction this bridge must have been duly regarded as a bold and successful work, and its form is well adapted to the high banks of the Severn at the place where it crosses. The design appears to have originated with Mr. Pritchard, an architect, of Eyton Turret, in Shropshire, who, in the year 1773, suggested the practicability of constructing large iron arches, capable of admitting navigation beneath them.

In the year 1787, Thomas Paine, the political writer, presented to the Academy of Sciences at Paris a model of an

B

iron bridge which he had invented; and during the greater part of the following year he resided at Rotherham, in Yorkshire, where a bridge, said to have been chiefly of wrought iron, was constructed under his direction by Messrs. Walker, the celebrated iron-founders of that place. This pattern bridge was exhibited in London, and intended for erection in America, but it was subsequently taken to pieces at Rotherham.

In 1790, Mr. Rowland Burdon designed a cast-iron arch for the river Weir at Sunderland, and, in 1792, obtained an Act of Parliament for erecting such a structure. Mr. Burdon's peculiar plan of construction, for which he obtained a patent, September 18, 1795, consisted in "a certain mode or manner of making, uniting, and applying cast-iron blocks, to be substituted in lieu of keystones, in the construction of arches." In this way the patentee proposed to retain the common form and principles of the old stone arch. The Sunderland bridge, as constructed according to this invention, consists of six ribs, 200 feet in span, and having a rise * of 30 feet. The total height from low-water level to the soffit of the arch is nearly 100 feet, and the whole structure is distinguished by peculiar elegance and boldness of design. The six ribs forming the arch are placed parallel to each other, and at a distance of 6 feet apart. Each rib consists of 105 separate blocks or castings, 5 feet in depth, connected together with bars and cotters of malleable iron. The ribs are braced together with cast-iron tubular braces and struts. The spandrils are filled in with cast-iron circles, meeting at their peripheries, and supporting the roadway, which is formed upon a strong timber frame, planked over, and covered with a mixture of chalk and tar, upon which a layer of marl-limestone and gravel is laid. The width of the bridge is 30 feet, and the abutments are of stone, founded on rock, and are 24 feet thick, and from 37 to 42 feet wide. The iron-work was executed at the foundry of Messrs. Walker, at Rotherham, and consists of 214 tons of cast and 40 tons of malleable iron. Mr. Thomas Wilson, of Bishop

Wearmouth, designed the architectural features of the bridge, and superintended its erection, which was completed within a period of three years, and at a total cost of £26,000, of which Mr. Burdon, the projector, subscribed £22,000. In October, 1816, the bridge was disposed of for a sum of £30,000 in a lottery, wherein there were 6,000 tickets and 150 prizes, varying in amount from £100 to £5,000 each. The confined situation of the site rendered it necessary to erec the bridge without interrupting the passage of ships with their rigging standing, and this was effected by a perpendicular scaffolding or framing resting upon piles in the middle of the river, and leaving a sufficient passage on each side for the vessels. The centre or transverse framing for supnorting the arch was fixed on this scaffolding, and answered its purpose satisfactorily. Some time after the centre was removed, the arch was found to have moved in a horizontal direction eastward, forming a curve of 12 to 18 inches versed sine. This unexpected circumstance, which, if unremedied, would doubtless have led to the destruction of the bridge, was very skilfully counteracted by introducing transverse and diagonal tie-bars and braces, aided by screws and wedges, by which the whole was ultimately restored to its original position, and permanently retained in a substantial state. On July 23, 1802, a patent was granted jointly to Thomas Wilson and Rowland Burdon, of Durham, for "methods of connecting the metallic patent blocks of the said R. Burdon for constructing arches."

Several iron bridges were subsequently erected by Telford, the first of which was that across the river Severn at Buildwas, in Shropshire, consisting of a single arch, 130 feet in span, and having a versed sine or rise of 27 feet. The arch consists of three ribs, placed at a distance of 9 feet apart, or 18 feet wide from out to out. These ribs are 3 feet 10 inches in depth, and connected transversely by tie-bars. The spandrils for supporting the roadway are formed of vertical bars of cast iron, and the abutments are of stone

"The two outer ribs consist of two segments of circles, each struck from different centres, the crown of one terminating immediately below the roadway, the other at the top of the parapet, so that the platform forming the roadway is both suspended and insistent; the object of this being, it is presumed, to increase the depth of the truss supporting the roadway, and thus add to the strength of the bridge but it was unnecessary, and does not appear to have been adopted in any of Telford's subsequent designs, which are namerous. ."* Rennie constructed an iron bridge over the Witham, at Boston, in Lincolnshire, which is remarkable for boldness of design and flatness, the rise being only 4 feet, and the span 100 feet. In construction, this bridge resembles the Sunderland bridge, but has an improved arrangement of transverse and diagonal braces, and vertical spandril pieces, instead of circular ones.

The largest iron-arch bridge yet constructed is that over the river Thames at London, and known as the Southwark bridge, which was designed and erected by the eminent Rennie. This splendid bridge, which was opened on March 25, 1819 (the first casting for it having been run on January 1, 1815), consists of three arches, all segments of the same circle, the centre arch being 240 feet in span, with a rise of 24 feet, and the two side arches being each 210 feet in span, with a rise of 18 feet 10 inches. The piers are 24 feet thick; the width of the roadway over the bridge is 28 feet; and the footways on either side are each 7-feet in width. Each arch consists of eight ribs, and each rib is formed of fifteen pieces, which are of such depth that the rib is 6 feet deep at the crown and 8 feet deep at the springing. The metal is 21⁄2 inches thick in the middle, and 4 inches at the top and bottom of the ribs. The ribs are connected transversely by cast-iron tie-braces of the same depth as the ribs, but open in the centre of each, and in the diagonal direction the ribs are

Sir J. Rennie's Address to the Institution of Civil Engineers, Session 1846.