It should be mentioned as a proof of the excellent manner in which this noble structure has been executed, that on removing the centering, the greatest settlement did not exceed 1 7/8 inch, which is unprecedently small in arches of such great span. We shall now proceed to notice the New London bridge, erected by the present Mr. John Rennie, from the design of the late Mr. Rennie, his father, who died shortly before the work was commenced. According to the original plan, the new bridge was to have been built on the site of the old one, but the corporation of London decided that the bridge should be built 180 feet higher up the stream. The bridge was also made 6 feet wider, and the abutment arches carried 2 feet higher than in the original design. The foundations of the bridge were laid in coffer dams, and the first pile for the dam of the south pier was laid March 15, 1824. As the works proceeded, it was found expedient that two of the small arches of the old bridge, on each side, should be thrown into one, to compensate for the additional obstruction which the works occasion to the navigation of the river.

This was effected with such skill, that the usual traffic was never once interrupted during the operation, the heaviest carriages passing over the bridge, and vessels passing through it, with the same facility as before; whilst such was the promptitude and despatch used, that the whole operation only occupied six weeks. The engraving on page 267, with the following description, will explain the manner in which the alteration was effected. The roadway was first boarded in and taken up one half at a time, and a space cleared away for the reception of a transverse iron girder A. A set of transverse timber principals B B was then laid on to this girder from the extreme piers of the two arches, bestriding, as it were, the central pier, which was to be removed; and these, instead of being placed at intervals as in roofs, were all fixed and bolted close together from one side of the roadway to the other, forming one unbroken mass of timber. Above the girder there was inserted what may not be unaptly termed the brestsummer C, into which the purlins D D were mortised at intervals for the support of a substantial planking, on which the pavement was laid as before.

The strength of the truss-work was farther augmented by a number of counter-principals, as 1, 2, some of the struts being fixed close together, and others with an interval of one width between them. The bridge itself was nearly completed before the line of the approaches to it were decided upon; it was at length finally settled that the approach on the London side should commence at Cannon-street, Eastcheap, passing over Thames-street by means of a dry arch, and that on the Borough side the road should form an inclined plane, supported on a series of brick arches, commencing near St. Thomas's Hospital, with a large dry arch facing Tooleystreet. On the 1st of August, 1831, the bridge was opened to the public, the period occupied in its erection from the time of driving the first pile, being seven years, five months, and thirteen days. The engraving in the margin is an elevation of the new bridge; it is formed of five semi-elliptical arches, the least of which is larger than any other elliptical stone arch ever before erected.

The centre arch is 152 feet span, with a rise of 29 feet 6 inches above high water mark; the two arches next the centre are 140 feet in span, with a rise of 27 feet 6 inches; and the two abutment arches are 130 feet span, with a rise of 24 feet 6 inches; thus the clear water way at all times of the tide is 692 feet, which is above 60 feet more than the old bridge afforded at high water mark. The whole length of the bridge from the extremities of the abutments, is 928 feet; within the abutments, 782 feet; and the width between the parapets is 53 feet. The engraving represents the new bridge. Towards the close of the eighteenth century some bridges were erected, the arches of which were constructed entirely of cast iron. The honour of this invention is due to this country, and the first of these structures was the bridge over the Severn, at Colebrook Dale, erected by Mr. Darby, in 1779. Some years after (about 1790,) the celebrated Thomas Paine had one constructed at the same place, which he intended for America, and which was put together in a meadow near that place; but he not being able to pay for it, it was taken down, and part of the materials were employed in the construction of the Sunderland Bridge, erected by Mr. Burdon over the river Wear, in 1793.

Subsequently, numerous iron bridges have been erected: the principal ones are Build was (Colebrook Dale Company), 1795-6; Tame, Herefordshire (failed), 1795-6; Parret, at Bridgewater, 1796; Staines (twice failed), 1800; Tees, at Yarm (failed); Boston, in Lincolnshire; New River at Bristol (two); Vaux-hall Bridge, over the Thames; but the most celebrated of these structures is the magnificent one over the Thames, called the Southwark Bridge, designed and erected by the late Mr. Rennie. This noble bridge consists of three circular arches of cast iron, supported by piers of granite; the centre arch is of 250 feet span, and the side arches 210 feet each. The piers are 24 feet thick. The bridge is 718 feet long between the abutments, and 42 wide between the parapets. There is a dry arch over the road on the Southwark side. The work was begun in 1814, and completed in 1824. The annexed cut is a view of the centre arch.

Iron Suspension Bridges were in use in Europe at the time of Scamozzi, the architect, who mentions them in his work Del Idea Archi, 1615; but the knowledge requisite to determine the properties of this kind of bridge was not published till the time of Bernouilli. Suspension bridges are described as existing in various parts of Asia, Africa, and America, before this species of construction began to be practised in Europe. The first chain bridge erected in England is supposed to be that over the Tees, forming a communication between the counties of Durham and York. Since that time the number of these structures has been increasing, some of which, in size and magnificence, exceed any thing of the kind in the known world. Of these, the most celebrated is the one recently erected over the Straits of Menai, by Mr. Telford. The bridge consists of one opening of 560 feet between the points of suspension, and 100 feet in height between the high-water line and the under side of the road-way, which is horizontal.