Mr. Rennie also introduced a practice of some novelty and importance in the centering upon which the arches of the bridge were built. He adopted the braced prin CENTERING OF ARCH, WATERLOO BRIDGE. [After E. Elcre.] ciple. The centres spanning the whole width of the arch were composed of eight ribs each, formed in one piece, resting upon the same number of solid wedges, a solid vat or tub impermeable to By least below low water mark. The foundations there rested upon timber piles from 20 to 22 feet long, driven into the solid bed of the river. Upon the heads of these piles half-timber planking was spiked, and on this the solid masonry was built-every stone being fitted, mortared, and laid with studious accuracy and precision. The whole work was done with such solidity that, after the lapse of fifty years, the foundations have not yielded by a straw's breadth at any point. supported by inclined tressels placed upon longitudinal bearers, firmly fixed to the offsets of the piers and abutments. At the intersecting point of the bearers or braces in each rib, there was a cast iron box with two holes or openings in it, so that the butt-ends rested firmly against the metal; and to prevent them from acting like so many wedges to tear the rib to pieces when the vertical weight of the arch began to act upon them, pieces of hard wood were driven firmly into the holes above described, to check the effect of the bearers or strutts of the ribs; and this arrangement proved completely successful. The eight ribs were firmly connected together by braces and ties, so as to form one compact frame, and the curve or form of the arch was accurately adjusted by means of transverse timbers, 12 inches wide and 6 inches thick, laid across the whole of the ribs, set out to the exact form of the curve by ordinates from the main or longitudinal axis of the ellipsis; and in proportion as the voussoirs or arch-stones were carried up from the adjoining piers, the weight which had been laid upon the top of the centre to keep it in equilibrium according to the form of the arch during construction, was gradually removed as it advanced towards completion. When the arch was about two-thirds completed, a small portion of it was closed with the centre, and the remaining part of each side was brought forward regularly by offsets to the crown until the whole was finished. Each key-stone was accurately fitted to its respective place, and the last portion of each, for the space of about eighteen inches, was driven home by a heavy wooden ram or pile-engine, so as almost to raise the crown of the arch from the centre. About ten days after the main arches had been completed, and the inverts and spandrel walls between them carried up to the proper height, the arches were gently slackened, to the extent of about two inches, so as to bring each to its bearing to a cer tain extent. This was effected by driving back the wedges upon which the ribs of the centres rested, by means of heavy wooden rams attached to them, so that they could swing backwards and forwards with great facility when any external force was applied to them; and this was done by ropes worked by handlabour. After the first striking or slackening, the arches were allowed to stand for ten days, when the wedges were driven back six inches further. After ten days more the wedges were driven back sufficiently to render the arch altogether clear of the centering. By this means the mortar was firmly imbedded into all the joints, and the arch came gradually to its ultimate bearing without any undue crushing. In order to ascertain whether any change of form took place, three straight lines were drawn in black chalk on the extreme face of the arch previous to commencing the operation of striking the centre, one horizontally in the centre of the voussoirs forming the crown, and two from the haunches of the arch, each intersecting the first line at about 25 feet on each side of the keystone; so that if there had been any derangement of the curve or irregular sinking, it would at once have been clearly apparent. After the centres had been removed, it was found that the sinking of the arches varied from 2 to 31 inches, which was as nearly as possible the allowance made by the engineer in designing the work; the whole plan being worked out with admirable precision and accuracy. The method of fixing and removing the centres was entirely new; being precisely the same as was afterwards followed by Mr. Robert Stephenson in fixing the wrought iron ribs of the Conway and Britannia bridges, that is, by constructing them complete on a platform adjacent to the river, and floating them between the piers on barges expressly contrived for the purpose. They were then raised into their proper places by four strong screws, 8 inches in diameter and 4 feet long, fixed in a strong cast iron box firmly bedded in the solid floor of the barge. The apparatus worked so well and smoothly, that the whole centre, consisting of eight ribs, each weighing about fifty tons, was usually placed within the week. The means employed by Mr. Rennie for forming his road upon the bridge were identical with those adopted by Mr. Macadam at Bristol some six years later. But the arrangement constituted so small a part of our engineer's contrivances, that, as in many other cases, he made no merit of it. When the clay puddle placed along the intended roadway was sufficiently hard, he spread a stratum of fine screened gravel or hoggins, which was carefully levelled and pressed down upon the clay. This was then covered over with a layer of equally broken flints, about the size of an egg; after which the whole was rolled close together, and in a short time formed an admirable "macadamized" road. Mr. Rennie had practised the same method of making roads over his bridges long before 1809; and he continued to adopt it in all his subsequent structures. The whole of the stone required for the bridge (excepting the balustrades, which were brought ready worked from Aberdeen) was hewn in some fields adjacent to the erection on the Surrey side. It was transported on to the work upon trucks drawn along railways, in the first instance over temporary bridges of wood; and it is a remarkable circumstance that nearly the whole of the material was drawn by one horse, called "Old Jack"-a most sensible animal, and a great favourite. His driver was, generally speaking, a steady and trustworthy man, though rather too fond of his dram before breakfast. As the railway along which the stone was drawn passed in front of the public-house door, the horse and truck were usually pulled up while Tom entered for his "morning." On one occasion the driver stayed so long that "Old Jack," becoming impatient, poked his head into the open door, and taking his master's coat-collar between his teeth, though in a gentle sort of manner, pulled him out from the midst of his companions, and thus forced him to resume the day's work. The bridge was opened with great ceremony by His Royal Highness the Prince Regent, attended by the Duke of Wellington and many other distinguished personages, on the 18th of June, 1817. It was originally named the Strand Bridge; but after that date the name was altered to that of "Waterloo," in honour of the |