Mr. Fairbairn's tests were of the most elaborate and eventually conclusive character, bringing to light many new and important facts of great practical value. The due proportions and thicknesses of the top, bottom, and sides of the tubes were arrived at after a vast number of separate trials, one of the results of the experiments being the adoption of Mr. Fairbairn's invention of rectangular hollow cells in the top of the beam for the purpose of giving it the requisite degree of strength. About the end of August it was thought desirable to obtain the assistance of a mathematician, who should prepare a formula by which the strength of a full-sized tube might be calculated from the results of the experiments made with tubes of smaller dimensions. Professor Hodgkinson was accordingly called in, and he proceeded to verify and confirm the experiments which Mr. Fairbairn had made, and afterward reduced them to the required formulæ, though Mr. Fairbairn states that they did not appear in time to be of any practical service in proportioning the parts of the largest tubes.[96]
Mr. Stephenson's time was so much engrossed with his extensive engineering business that he was in a great measure precluded from devoting himself to the consideration of the practical details, which he felt were safe in the hands of Mr. Fairbairn—"a gentleman," as he stated to the Committee of the Commons, "whose experience was greater than that of any other man in England." The results of the experiments were communicated to him from time to time, and were regarded by him as exceedingly satisfactory. It would appear, however, that while Mr. Fairbairn urged the sufficient rigidity and strength of the tubes without the aid of chains, Mr. Stephenson had not quite made up his mind upon the point. Mr. Hodgkinson, also, was strongly inclined to retain them.[97] Mr. Fairbairn held that it was quite practicable to make the tubes "sufficiently strong to sustain not only their own weight, but, in addition to that load, 2000 tons equally distributed over the surface of the platform—a load ten times greater than they will ever be called upon to support."
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 his foot down upon it—that he should, early in 1846, have appointed his able assistant, Mr. Edwin Clark, to scrutinize carefully the results of every experiment, whether made by Mr. Fairbairn or Mr. Hodgkinson, 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. That great progress had been made by the two chief experimenters before the end of 1846 appears from the papers on the subject read by Messrs. Fairbairn and Hodgkinson before the British Association at Southampton in September of that year. In the course of the following month Mr. Stephenson had become 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.[98] While these important discussions were in progress, measures were taken to proceed with the masonry of the bridges simultaneously at Conway and the Menai Strait. The foundation-stone of the Britannia Bridge was laid by Mr. Frank Forster, the resident engineer, on the 10th of April, 1846; and on the 12th of May following that of the Conway Bridge was laid by Mr. A. M. Ross, resident engineer at that part of the works. 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 neighborhood of the Conway and Britannia Bridges presented scenes of extraordinary bustle and industry. On the 11th of July, 1847, Mr. Clark informed Mr. Stephenson that "the masonry gets on rapidly. The abutments on the Anglesea side resemble the foundations of a great city rather than of a single structure, and nothing appears to stand still here." 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, Anglesea 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 to 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 abutments 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 longer spans, at the points where the tubes repose on the masonry, is not less than 1587 tons. On the centre tower the tubes lie 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 Church-yard, it would reach nearly 100 feet above the cross.
CONSTRUCTION OF THE MAIN BRITANNIA TUBE ON THE STAGING.
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, was 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—in like manner as Rennie had floated into their places the centerings of his Waterloo and other bridges—and then raised into their proper places by means of hydraulic power, after a method originally suggested by Mr. Edwin Clark. The tubes of the Conway Bridge also were to be constructed on shore, and floated to their places on pontoons, as in the case of the main centre tubes of the Britannia Bridge.
