Mr. Telford submitted alternative plans for a bridge over the Strait: one at the Swilly Rock, consisting of three cast iron arches of 260 feet span, with a stone arch of 100 feet span between each two iron ones, to resist their lateral thrust; and another at Ynys-y-moch, to which he himself attached the preference, consisting of a single cast iron arch of 500 feet span, the crown of the arch to be 100 feet above high water of spring tides, and the breadth of the roadway to be 40 feet.
The principal objection taken to this plan by engineers generally, was the supposed difficulty of erecting a proper centering to support the arch during construction; and the mode by which Mr. Telford proposed to overcome this may be cited in illustration of his ready ingenuity in overcoming difficulties. He proposed to suspend the centering from above instead of supporting it from below in the usual manner—a contrivance afterwards revived by another very skilful engineer, the late Mr. Brunel. Frames, 50 feet high, were to be erected on the top of the abutments, and on these, strong blocks, or rollers and chains, were to be fixed, by means of which, and by the aid of windlasses and other mechanical powers, each separate piece of centering was to be raised into, and suspended in, its proper place. Mr. Telford regarded this method of constructing centres as applicable to stone as well as to iron arches; and indeed it is applicable, as Mr. Brunel held, to the building of the arch itself.*[1]
[Image] Proposed Plan of Suspended Centering
Mr. Telford anticipated that, if the method recommended by him were successfully adopted on the large scale proposed at Menai, all difficulties with regard to carrying bridges over deep ravines would be done away with, and a new era in bridge-building begun. For this and other reasons—but chiefly because of the much greater durability of a cast iron bridge compared with the suspension bridge afterwards adopted—it is matter of regret that he was not permitted to carry out this novel and grand design. It was, however, again objected by mariners that the bridge would seriously affect, if not destroy, the navigation of the Strait; and this plan, like Mr. Rennie's, was eventually rejected.
Several years passed, and during the interval Mr. Telford was consulted as to the construction of a bridge over Runcorn Gap on the Mersey, above Liverpool. As the river was there about 1200 feet wide, and much used for purposes of navigation, a bridge of the ordinary construction was found inapplicable. But as he was required to furnish a plan of the most suitable structure, he proceeded to consider how the difficulties of the case were to be met. The only practicable plan, he thought, was a bridge constructed on the principle of suspension. Expedients of this kind had long been employed in India and America, where wide rivers were crossed by means of bridges formed of ropes and chains; and even in this country a suspension bridge, though of a very rude kind, had long been in use near Middleton on the Tees, where, by means of two common chains stretched across the river, upon which a footway of boards was laid, the colliers were enabled to pass from their cottages to the colliery on the opposite bank.
Captain (afterwards Sir Samuel) Brown took out a patent for forming suspension bridges in 1817; but it appears that Telford's attention had been directed to the subject before this time, as he was first consulted respecting the Runcorn Bridge in the year 1814, when he proceeded to make an elaborate series of experiments on the tenacity of wrought iron bars, with the object of employing this material in his proposed structure. After he had made upwards of two hundred tests of malleable iron of various qualities, he proceeded to prepare his design of a bridge, which consisted of a central opening of 1000 feet span, and two side openings of 500 feet each, supported by pyramids of masonry placed near the low-water lines. The roadway was to be 30 feet wide, divided into one central footway and two distinct carriageways of 12 feet each. At the same time he prepared and submitted a model of the central opening, which satisfactorily stood the various strains which were applied to it. This Runcorn design of 1814 was of a very magnificent character, perhaps superior even to that of the Menai Suspension Bridge, afterwards erected; but unhappily the means were not forthcoming to carry it into effect. The publication of his plan and report had, however, the effect of directing public attention to the construction of bridges on the suspension principle; and many were shortly after designed and erected by Telford and other engineers in different parts of the kingdom.
Mr. Telford continued to be consulted by the Commissioners of the Holyhead Roads as to the completion of the last and most important link in the line of communication between London and Holyhead, by bridging the Straits of Menai; and at one of their meetings in 1815, shortly after the publication of his Runcorn design, the inquiry was made whether a bridge upon the same principle was not applicable in this particular case. The engineer was instructed again to examine the Straits and submit a suitable plan and estimate, which he proceeded to do in the early part of 1818. The site selected by him as the most favourable was that which had been previously fixed upon for the projected cast iron bridge, namely at Ynys-y-moch—the shores there being bold and rocky, affording easy access and excellent foundations, while by spanning the entire channel between the low-water lines, and the roadway being kept uniformly 100 feet above the highest water at spring tide, the whole of the navigable waterway would be left entirely uninterrupted. The distance between the centres of the supporting pyramids was proposed to be of the then unprecedented width of 550 feet, and the height of the pyramids 53 feet above the level of the roadway. The main chains were to be sixteen in number, with a deflection of 37 feet, each composed of thirty-six bars of half-inch-square iron, so placed as to give a square of six on each side, making the whole chain about four inches in diameter, welded together for their whole length, secured by bucklings, and braced round with iron wire; while the ends of these great chains were to be secured by a mass of masonry built over stone arches between each end of the supporting piers and the adjoining shore. Four of the arches were to be on the Anglesea, and three on the Caernarvonshire side, each of them of 52 feet 6 inches span. The roadway was to be divided, as in the Runcorn design with a carriage way 12 feet wide on each side, and a footpath of 4 feet in the middle. Mr. Telford's plan was supported by Mr. Rennie and other engineers of eminence; and the Select Committee of the House of Commons, being satisfied as to its practicability, recommended Parliament to pass a Bill and to make a grant of money to enable the work to be carried into effect.
[Image] Outline of Menai Bridge
The necessary Act passed in the session of 1819, and Mr. Telford immediately proceeded to Bangor to make preparations for beginning the works. The first proceeding was to blast off the inequalities of the surface of the rock called Ynys-y-moch, situated on the western or Holyhead side of the Strait, at that time accessible only at low water. The object was to form an even surface upon it for the foundation of the west main pier. It used to be at this point, where the Strait was narrowest, that horned cattle were driven down, preparatory to swimming them across the channel to the Caernarvon side, when the tide was weak and at its lowest ebb. The cattle were, nevertheless, often carried away, the current being too strong for the animals to contend against it.
At the same time, a landing-quay was erected on Ynys-y-moch, which was connected with the shore by an embankment carrying lines of railway. Along these, horses drew the sledges laden with stone required for the work; the material being brought in barges from the quarries opened at Penmon Point, on the north-eastern extremity of the Isle of Anglesea, a little to the westward of the northern opening of the Strait. When the surface of the rock had been levelled and the causeway completed, the first stone of the main pier was laid by Mr. W.A. Provis, the resident engineer, on the 10th of August, 1819; but not the slightest ceremony was observed on the occasion.