438 RAILWAY FROM CHESTER TO HOLYHEAD. [PART II. CHAPTER XVIII. * CHESTER AND HOLYHEAD RAILWAY-MENAI AND CONWAY BRIDGES. We have now to describe briefly another great undertaking, begun by George Stephenson, and taken up and completed by his son, in the course of which the latter carried out some of his greatest works-we mean the Chester and Holyhead Railway, completing the railway connection with Dublin, as the Newcastle and Berwick line completed the connection with Edinburg. It will thus be seen how closely Telford was followed by the Stephensons in perfecting the highways of their respective epochs; the former by means of turnpike roads, and the latter by means of railways. George Stephenson surveyed a line from Chester to Holyhead in 1838, and at the same time reported on the line through North Wales to Port Dynallen, as proposed by the Irish Railway Commissioners. His advice was strongly in favor of adopting the line to Holyhead, as less costly and presenting better gradients. A public meeting was held at Chester in January, 1839, in support of the latter measure, at which he was present to give explanations. Mr. Uniacke, the mayor, in opening the proceedings, observed that it clearly appeared that the rival line through Shrewsbury was quite impracticable. Mr. Stephenson, he added, was present in the room, ready to answer any questions which might be put to him on the subject; and "it would be better that he should be asked questions than required to make a speech; for, though a very good engineer, he was a bad speaker." One of the questions then put to Mr. Stephenson related to the mode by which he proposed to haul the passenger-carriages over the Menai Suspension Bridge by horse-power; and he was asked whether he knew the pressure the bridge was capable of sustaining. His answer was that "he had not yet made any calcula-. tions, but he proposed getting data which would enable him to arrive at an accurate calculation of the actual strain upon the bridge during the late gale. He had, however, no hesitation in CHAP. XVIII.] MENAI SUSPENSION BRIDGE. 439 saying that it was more than twenty times as much as the strain of a train of carriages and a locomotive engine. The only reason why he proposed to convey the carriages over by horses was in order that he might, by distributing the weight, not increase the wavy motion. All the train would be on at once, but distributed. This he thought better than passing them linked together, by a locomotive engine." It will thus be observed that the practicability of throwing a rigid railroad bridge across the Straits had not yet been completed. The Dublin Chamber of Commerce passed resolutions in favor of Stephenson's line after hearing his explanations of its essential features. The project, after undergoing much discussion, was at length embodied in an act passed in 1844, and the work was brought to a successful completion by his son, with several important modifications, including the grand original feature of the tubular bridges across the Menai Straits and the estuary of the Conway. Excepting these great works, the construction of this line presented no unusual features, though the remarkable terrace cut for the accommodation of the railway under the steep slope of Penmaen Mawr is worthy of a passing notice. About midway between Conway and Bangor, Penmaen Mawr forms a bold and almost precipitous headland, at the base of which, in rough weather, the ocean dashes with great fury. There was not space enough between the mountain and the strand for the passage of the railway; hence in some places the rock had to be blasted to form a terrace, and in others sea walls had to be built up to the proper level, on which to form an embankment of sufficient width to enable the road to be laid. A tunnel of 10% chains in length was cut through the headland itself; and on its east and west sides the line was formed by a terrace cut out of the cliff, and by embankments protected by sea walls, the terrace being three times interrupted by embankments in its course of about a mile and a quarter. The road lies so close under the steep mountain face that it was even found necessary at certain places to protect it against possible accidents from falling stones, by means of a covered way. The terrace on the east side of the headland was, however, in some measure, protected against the roll of the sea by the mass of stone run out from the tunnel, which formed a deep shingle-bank in front of the wall. 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. 442 THE FIRST PROPOSED BRIDGE. [PART II. 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 |