PENMAEN MAWR. [By Percival Skelton, after his original Drawing.]

The part of the work which lies to the westward of the headland penetrated by the tunnel was exposed to the full force of the sea, and the formation of the road at that point was attended with great difficulty. While the sea wall was still in progress, its strength was severely tried by a strong northwesterly gale which blew in October, 1846, accompanied with a spring tide of 17 feet. On the following morning it was found that a large portion of the rubble was irreparably injured, and 200 yards of the wall were then replaced by an open viaduct, with the piers placed edgeways to the sea, the openings between them being spanned by ten cast-iron girders 42 feet long. This accident farther induced the engineer to alter the contour of the sea wall, so

CHAP. XVIII.] CROSSING OF THE MENAI STRAIT.

441

that it should present a diminished resistance to the force of the

waves.

But the sea repeated its assaults, and made farther havoc with the work, entailing heavy expenses and a complete reorganization of the contract. Increased solidity was then given to the masonry, and the face of the wall underwent farther change. At some points outworks were constructed, and piles were driven into the beach about 15 feet from the base of the wall for the purpose of protecting its foundations and breaking the force of the waves. The work was at length finished after about three years' anxious labor; but Mr. Stephenson confessed that if a long tunnel had been made in the first instance through the solid rock of Penmaen Mawr, a saving of from £25,000 to £30,000 would have been effected. He also said he had arrived at the conclusion that in railway works engineers should endeavor as far as possible to avoid the necessity of contending with the sea;* but if he were ever again compelled to go within its reach, he would adopt, instead of retaining walls, an open viaduct, placing all the piers edgeways to the force of the sea, and allowing the waves to break upon a natural slope of beach. He was ready enough to admit the errors he had committed in the original design of this work; but he said he had always gained more information from studying the causes of failures and endeavoring to surmount them, than he had done from easily-won successes. While many. of the latter had been forgotten, the former were indelibly fixed in his memory.

But by far the greatest difficulty which Robert Stephenson had to encounter in executing this railway was in carrying it across the Straits of Menai and the estuary of the Conway, where, like his predecessor Telford, when forming his high road through North Wales, he was under the necessity of resorting to new and altogether untried methods of bridge construction. At Menai, the waters of the Irish Sea are perpetually vibrating along the precipitous shores of the Strait, rising and falling from 20 to 25

*The simple fact that in a heavy storm the force of impact of the waves is from one and a half to two tons per square foot, must necessarily dictate the greatest possible caution in approaching so formidable an element. Mr. R. Stevenson (Edinburg) registered a force of three tons per square foot at Skerryvore during a gale in the Atlantic, when the waves were supposed to run twenty feet high.

feet at each successive tide, the width and depth of the channel being such as to render it available for navigation by the largest ships. The problem was to throw a bridge across this wide chasm -a bridge of unusual span and dimensions-of such strength as to be capable of bearing the heaviest loads at high speeds, and of such a uniform height throughout as not in any way to interfere with the navigation of the Strait. From an early period Mr. Stephenson had fixed upon the spot where the Britannia Rock occurs, nearly in the middle of the channel, as the most eligible point for crossing, the water width from shore to shore at high water being there about 1100 feet.

The engineer's first idea was to construct the bridge of two

cast-iron arches of 350 feet span each. There was no novelty in this idea; for, as early as the year 1801, Mr. Rennie prepared a design of a cast-iron bridge across the Strait at the Swilly Rocks, the great centre arch of which was to be 450 feet span; and at a later period, in 1810, Telford submitted a design of a similar bridge at Inys-y-Moch, with a single cast-iron arch of 500 feet. But the same objections which led to the rejection of Rennie's and Telford's designs proved fatal to Robert Stephenson's, and his iron-arched railway bridge was rejected by the Admiralty. The navigation of the Strait was under no circumstances to be interfered with; and even the erection of scaffolding from below, to support the bridge during construction, was not to be permitted. The idea of a suspension bridge was dismissed as inapplicable, a degree of rigidity and strength greater than could be secured by any

CHAP. XVIII.] PLAN OF SUSPENDED CENTERING.

443

bridge erected on the principle of suspension being considered an indispensable condition of the proposed structure.

Mr. Stephenson next considered the expediency of erecting a bridge by means of suspended centering, after the ingenious method proposed by Telford in 1810,* by which the arching was to be carried out by placing equal and corresponding voussoirs on opposite sides of the pier at the same time, tying them together by horizontal tie-bolts. The arching, thus extended outward from each pier and held in equilibrium, would have been connected at the crown with the extremity of the arch advanced in like manner from the adjoining pier. It was, however, found that this method of construction was not applicable at the crossing of the Conway, and it was eventually abandoned. Various other plans were suggested; but the whole question remained unsettled even down to the time when the company went before Parliament in 1844 for power to construct the proposed bridges. No existing kind of structure seemed to be capable of bearing the severe extension to which rigid bridges of the necessary spans would be subjected, and some new expedient of engineering therefore be

came necessary.

Mr. Stephenson was then led to reconsider a design which he had made in 1841 for a road bridge over the River Lea at Ware, with a span of 50 feet, the conditions only admitting of a platform 18 or 20 inches thick. For this purpose a wrought-iron platform was devised, consisting of a series of simple cells, formed of boiler-plates riveted together with angle-iron. The bridge was not, however, carried out after this design, but was made of separate wrought-iron girders composed of riveted plates.† Recurring to his first idea of this bridge, the engineer thought that a stiff platform might be constructed, with sides of strongly-trussed frame-work of wrought iron, braced together at top and bottom with plates of like material riveted together with angle-iron, after a method adopted by Mr. Rendel in stiffening the suspension

*See "Lives of the Engineers," vol. ii., p. 445. It appears that Mr. Fairbairn' suggested this idea in his letter to Mr. Stephenson, dated the 3d of June, 1845, accompanied by a drawing. See his "Account of the Construction of the Britannia and Conway Tubular Bridges," etc. London, 1849.

+ Robert Stephenson's narrative of the early history of the design, in Edwin Clark's "Britannia and Conway Tubular Bridges," vol. i., p. 25, London, 1850.

bridge at Montrose with wooden trellis-work a few years before; and that such platform might be suspended by strong chains on either side to give it increased security. "It was now," says Mr. Stephenson, "that I came to regard the tubular platform as a beam, and that the chains should be looked upon as auxiliaries." It appeared to him, nevertheless, that without a system of diagonal struts inside, which of course would have prevented the passage of trains through it, this kind of structure was ill suited for maintaining its form, and would be very liable to become lozengeshaped. Besides, the rectangular figure was deemed objectionable, from the large surface which it presented to the wind.

It then occurred to him that circular or elliptical tubes might better answer the intended purpose; and in March, 1845, he gave instructions to two of his assistants to prepare drawings of such a structure, the tubes being made with a double thickness of plate at top and bottom. The results of the calculations made as to the strength of such a tube were considered so satisfactory, that Mr. Stephenson says he determined to fall back upon a bridge of this description on the rejection of his design of the two cast-iron arches by the Parliamentary Committee. Indeed, it became evident that a tubular wrought-iron beam was the only structure which combined the necessary strength and stability for a railway, with the conditions deemed essential for the protection of the navigation:

"I stood," says Mr. Stephenson," on the verge of a responsibility from which, I confess, I had nearly shrunk. The construction of a tubular beam of such gigantic dimensions, on a platform elevated and supported by chains at such a height, did at first present itself as a difficulty of a very formidable nature. Reflection, however, satisfied me that the principles upon which the idea was founded were nothing more than an extension of those daily in use in the profession of the engineer. The method, moreover, of calculating the strength of the structure which I had adopted was of the simplest and most elementary character; and whatever might be the form of the tube, the principle on which the calculations were founded was equally applicable, and could not fail to lead to equally accurate results."*

*Robert Stephenson's narrative in Clark's "Britannia and Conway Tubular Bridges," vol. i., p. 27.