It was not till after many explanations and discus sions, that 11Ir Labalye satisfied the commissioners, ap pointed by parliament, of the facility, economy, and se curity to be derived from laying the foundations of the piers in caissons or chests, instead of placing them upon piles in the ancient manner, cut off about the level of low water ; or using bane rdeaux or coffrcdams, form ed around the foundations, and pumping the water from the inside, as had been performed in more modern times. This beautifully simple mode was, however, adopted, and the first stone of this great fabric was laid by the Earl of Pembroke on the 29th January 1739. During the same year, the commissioners directed Mr Labalye to prepare a design for a superstructure of stone, which he did, and it was approved of and adopted on the 31st January 1740: A liberal arrangement having been made with Mr King respecting his contract for the wooden superstructure, he immediately designed, for the stone arches, those excellent centres, which have ever since served as a model for works of a similar kind in England.
The works were carried on with great dispatch and success; the centre of the last arch was struck on the 25th July 1747, and, on the 14th November, the roads and streets were finished. A circumstance, however, took place, which prevented the bridge from being, at that time, opened to the public. The workmen em ployed to get gravel out of the bed of the river to cover the roadway of the bridge, finding some very suitable near the third pier, on the western side of the centre arch, they excavated considerably lower than the foun dation, and too near it; the gravel then run from under the platform, and the pier sunk so much as to render it necessary to take down the two arches which rested upon it. The securing the foundation, rebuilding the pier and two arches, and replacing the parapets, pave ments, and roadway, was completed, and the bridge opened to the public on the 18th Nov. 1750. This bridge consists of 13 large and two small arches; their forms are semicircular; the middle one is 76 feet span, and the breadth over the parapets 44 feet. See Plate LXXXVI.
About 10 years after the completion of Westminster bridge, another was begun to be erected between it and London bridge, now well known by the name of Black friars. The design was made by Robert Mylne It consists of nine arches of an elliptical form; the middle one is 100 feet span, and the breadth across the bridge is 43 feet 6 inches. Mr Mylne benefited by the exam ple of Labalye, and built the piers in caissons; but pro bably alarmed by the sinking of one of the piers at \Vestminster, he drove piles in the spaces upon which the bottoms of the caissons are placed.. His arches be ing of wider span and of an elliptical form, his piers of proportionally less thickness, and having less masonry over the top of the arches, this bridge has a much lighter appearance than that of \Vestminster. It has
been doubted, whether the slender detached Ionic co lumns are a proper accompaniment to such a work, and whether the divisions of the lengths of the rusti cated headers of the arches are any improvement. His centres are evidently a copy of those used at West minster.
The general style of this bridge bespeaks a mind em boldened by the success of his predecessor, to advance, though very cautiously, a step further in the practice of bridge-building. It is a work of great merit, and will not suffer by a comparison with any other constructed in the same age. It was begun in 1760, and completed in tin years. See Plate LXXXVI.
At the same time that this noble work was carrying on at Blackfriars, a very fine bridge was constructing upon the river Tay, at the town of Perth in Scotland. It consists of nine arches ; the middle one is 77 feet span, the width across is 26 feet, and the total length is 906 feet. It was designed and executed under the direction of Mr Smeaton, between 1760 and 1771. About the same time, and under the direction of the same gentleman, a bridge of no inconsiderable magni tude was built over the river Tweed at Coldstream, con sisting of 5 arches, the middle one being 64 feet span ; and also a bridge of seven arches over the river North Esk, near Montrose.
Previous to forming the plan of that magnificent ex tension of the city of Edinburgh, known by the name of the New Town, it was necessary to form a commodious communication with the central part of the High Street of the Old Town. This was accomplished by construct ing a bridge over the deep valley called the North Loch. This bridge consists of three arches, each about 72 feet span, and two small arches each 20 feet span ; the height from the present surface of the ground to the springing of the arches on the piers, is 17ft. 6in.; the arches being semicircular, rise 36 feet, the archstones are 21t. 9in., and from the top of the archstones to the top of the parapets, is 9ft. 9in., making the whole height, from the surface of the ground to the top of the parapet over the middle arch, 65 feet; the breadth across the soffit of the arches is 42ft. 3in. From the arches to the banks on each side of the valley, the spaces are occupied by coach houses, stables, bee. formed under the road way. The outline of the cornice and parapet, contra ry to usual practice, is a curved line, bending down wards. This, in viewing the elevation, gives the ap pearance of the middle part of the bridge having sunk. Excepting the arches, which are fine and well exe cuted, no part of the design claims much commenda tion; and we cannot help regretting, that the architect Mr Mylne, has failed in rendering this structure a suit able feature to the singularly fine situation which it oc cupies.