It was thoroughly characteristic of Mr. Stephenson, and of the caution with which he proceeded in every step of
this great undertaking—probing every inch of the ground before he set down his foot upon it—that he should, early in 1856, (sic) have appointed his able assistant, Mr. Edwin Clark, to scrutinise carefully the results of every experiment, and subject them to a separate and independent analysis before finally deciding upon the form or dimensions of the structure, or upon any mode of procedure connected with it. At length Mr. Stephenson became satisfied that the use of auxiliary chains was unnecessary, and that the tubular bridge might be made of such strength as to be entirely self-supporting.
While these important discussions were in progress, measures were taken to proceed with the masonry of the bridges simultaneously at Conway and the Menai Straits. The foundation-stone of the Britannia Bridge was laid on the 10th April, 1846; and on the 12th May following that of the Conway Bridge was laid. Suitable platforms and workshops were also erected for proceeding with the punching, fitting, and riveting of the tubes; and when these operations were in full progress, the neighbourhood of the Conway and Britannia Bridges presented scenes of extraordinary bustle and industry. About 1500 men were employed on the Britannia Bridge alone, and they mostly lived upon the ground in wooden cottages erected for the occasion. The iron plates were brought in ship-loads from Liverpool, Anglesey marble from Penmon, and red sandstone from Runcorn, in Cheshire, as wind and tide, and shipping and convenience, might determine. There was an unremitting clank of hammers, grinding of machinery, and blasting of rock, going on from morning till night. In fitting the Britannia tubes together, not less than 2,000,000 of bolts were riveted, weighing some 900 tons.
The Britannia Bridge consists of two independent continuous tubular beams, each 1511 feet in length, and each weighing 4680 tons, independent of the cast-iron frames inserted at their bearings on the masonry of the towers. These immense beams are supported at five places, namely,
on the abutments and on three towers, the central of which is known as the Great Britannia Tower, 230 feet high, built on a rock in the middle of the Strait. The side towers are 18 feet less in height than the central one, and the abutment 35 feet lower than the side towers. The design of the masonry is such as to accord with the form of the tubes, being somewhat of an Egyptian character, massive and gigantic rather than beautiful, but bearing the unmistakable impress of power.
The bridge has four spans,—two of 460 feet over the water, and two of 230 feet over the land. The weight of the larger spans, at the points where the tubes repose on the masonry, is not less than 1587 tons. On the centre tower the tubes rest solid; but on the land towers and abutments they lie on roller-beds, so as to allow of expansion and contraction. The road within each tube is 15 feet wide, and the height varies from 23 feet at the ends to 30 feet at the centre. To give an idea of the vast size of the tubes by comparison with other structures, it may be mentioned that each length constituting the main spans is twice as long as London Monument is high; and if it could be set on end in St. Paul’s Churchyard, it would reach nearly 100 feet above the cross.
The Conway Bridge is, in most respects, similar to the Britannia, consisting of two tubes, of 400 feet span, placed side by side, each weighing 1180 tons. The principle adopted in the construction of the tubes, and the mode of floating and raising them, were nearly the same as at the Britannia Bridge, though the general arrangement of the plates is in many respects different.
It was determined to construct the shorter outer tubes of the Britannia Bridge on scaffoldings in the positions in which they were permanently to remain, and to erect the larger tubes upon wooden platforms at high-water-mark on the Caernarvon shore, from whence they were to be floated in pontoons.
The floating of the tubes on pontoons, from the places
where they had been constructed, to the recesses in the masonry of the towers, up which they were to be hoisted to the positions they were permanently to occupy, was an anxious and exciting operation. The first part of this process was performed at Conway, where Mr. Stephenson directed it in person, assisted by Captain Claxton, Mr. Brunel, and other engineering friends. On the 6th March, 1848, the pontoons bearing the first great tube of the up-line were floated round quietly and majestically into their place between the towers in about twenty minutes. Unfortunately, one of the sets of pontoons had become slightly slued by the stream, by which the Conway end of the tube was prevented from being brought home; and five anxious days to all concerned intervened before it could be set in its place. In the mean time, the presses and raising machinery had been fitted in the towers above, and the lifting process was begun on the 8th April, when the immense mass was raised 8 feet, at the rate of about 2 inches a minute. On the 16th, the tube had been raised and finally lowered into its permanent bed; the rails were laid along it; and, on the 18th, Mr. Stephenson passed through with the first locomotive. The second tube was proceeded with on the removal of the first from the platform, and was completed and floated in seven months. The rapidity with which this second tube was constructed was in no small degree owing to the Jacquard punching-machine, contrived for the purpose by Mr. Roberts of Manchester. This tube was finally fixed in its permanent bed on the 2nd of January, 1849.