HIGH-WAYS AND DRY-WAYS; OR, TIE BRITANNIA AND CONWAY TUBULAR BRIDGES CHAPTER I. THE PRINCIPLE UPON WHICH THE BRITANNIA AND CONWAY IN continuation of our sketch of the practical working of the London and North-Western Railway, we now offer to our readers a short descriptive outline of the aërial passages through which it is proposed by the Directors of the Chester and Holyhead Railway, that the public shall, without cuneiform sustentation, fly across the Menai Straits. We shall divide our subject into the following compart ments: 1. The principle upon which the Britannia Bridge is constructed. 2. The mode of its construction. 3. The floating of its tubes. 4. The manner in which they are subsequently raised. 5. Mr. Fairbairn's complaint that Mr. Robert Stephenson has deprived him of a considerable portion of the merit of the construction of the Conway and Britannia Bridges.' PRINCIPLE OF THE PROPOSED PASSAGE. In the construction of a railway from Chester to Holyhead, the great difficulty which its projectors had to contend with was to discover by what means, if any, long trains of passengers and of goods could, at undiminished speed, be safely transported across that great tidal chasm which separates Carnarvon from the island of Anglesey. To solve this important problem the Company's engineer was directed most carefully to reconnoitre the spot; and as the picture of a man struggling with adversity has always been deemed worthy of a moment's attention, we will endeavour to sketch a rough outline of the difficulties which one after another must have attracted Mr. Robert Stephenson's attention, as on the Anglesey side of the Menai Straits he stood in mute contemplation of the picturesque but powerful adversaries he was required to encounter. Immediately in his front, and gradually rising towards the clouds above him, were the lofty snow-capped mountains of Snowdon, along the sides of which, or through which, the future railroad, sometimes in bright sunshine and sometimes in utter darkness, was either to meander or to burrow. Beneath him were the deep Menai Straits, in length above 12 miles, through which, imprisoned between precipitous shores, the waters of the Irish Sea and of St. George's Channel are not only everlastingly vibrating backwards and forwards, but at the same time, and from the same causes, are progressively rising or falling from 20 to 25 feet with each successive tide, which, varying its period of high water every day, forms altogether an endless succession of aqueous changes. The point of the Straits which it was desired to cross—although broader than that about a mile distant, preoccupied by Mr. Telford's Suspension-bridge-was of course one of the narrowest that could be selected; in consequence of which the ebbing and flowing torrent rushes through it with such violence that, except where there is back-water, it is often impossible for a small boat to pull against it; besides which the gusts of wind which come over the tops, down the ravines, and round the sides of the neighbouring mountains, are so sudden, and occasionally so violent, that it is as dangerous to sail as it is difficult to row; in short, the wind and the water, sometimes playfully, and sometimes angrily, seem to vie with each other-like some of Shakspeare's fairies-in exhibiting before the stranger the utmost variety of fantastic changes which it is in the power of each to assume. But in addition to the petty annoyances which air, earth, and water could either separately or conjointly create, the main difficulty which Mr. Stephenson had to encounter was from a new but irresistible element in Nature, an 'orbis veteribus incognitus,' termed in modern philosophy The First Lord, or, generically, The Admiralty. The principal stipulation which the requirements of War, and the interests of Commerce, very reasonably imposed upon Science was, that the proposed passage across the Menai Straits should be constructed a good hundred feet above high-water level, to enable large vessels to sail beneath it; and as a codicil to this will it was moreover required that, in the construction of the said passage, neither scaffolding nor centering should be used—as they, it was explained, would obstruct the navigation of the Straits. Although the latter stipulation, namely that of constructing a large superstructure without foundation, was generally considered by engineers as amounting almost to a prohibition, Mr. Stephenson, after much writhing of mind, extricated himself from the difficulty by the design of a most magnificent bridge of two castiron arches, each of which commencing, or, as it is termed, springing, 50 feet above the water, was to be 450 feet broad and 100 feet high-the necessity for centering being very ingeniously dispensed with by connecting together the half arches on each side of the centre pier, so as to cause them to counterbalance each other like two boys quietly seated on the opposite ends of a plank supported only in the middle. This project, however, which on very competent authority has been termed 6 one of the most beautiful structures ever invented,' the Admiralty rejected, because the stipulated height of 100 feet would only be attained under the crown of the arch, instead of extending across the whole of the watercourse. It was also contended that such vast cast-iron arches would take the wind out of vessels' sails, and, as a further objection, that they would inevitably be much affected by alternations of temperature. Although this stern and unanticipated demand, that the passage throughout its whole length should be of the specified height, appeared to render success almost hopeless, it was evidently useless to oppose it. The man of science had neither the power nor the will to contend against men of war, and accordingly Mr. Stephenson felt that his best, and indeed only, course was like poor little Oliver Twist when brought before his parish guardians-'TO BOW to the BoarD;' and we beg leave to bow to it too, for, gnarled as were its requirements, and flat as were its refusals, it succeeded, at a cost to the Company to which we will subsequently refer, in effecting two great objects;-first, the maintenance for ever, for the purposes of War and Commerce, of an uninterrupted passage for vessels of all nations sailing through the Menai Straits; and secondly, the forcing an eminent engineer to seek until he found that which was required; in fact, just as a collision between a rough flint and a piece of highly-tempered steel elicits from the latter a spark which could not otherwise have appeared, so did the rugged stipulations of the Admiralty elicit from Science a most brilliant discovery, which possibly, and indeed probably, would never otherwise have come to light. But to return to the Anglesey shore of the Menai Straits. When Mr. Stephenson, after many weary hours of rumination in his London study, beheld vividly portrayed before him the physical difficulties with which he had to contend in the breadth and rapidity of the stream; when he estimated not only the ordinary violence of a gale of wind, but the paroxysms or squalls which in the chasm before him, occasionally,-like the Erle King terrifying the 'poor baby,'-convulsed even the tempest in its career; and lastly, when he reflected that, in constructing a passage so high above the water, he was to be allowed neither centerings, scaffoldings, nor arches, it occurred to him, almost as intuitively as a man when his house is on fire at once avails himself of the means left him for escape, that the only way in which he could effect his object was by constructing in some way or other, at the height required, a straight passage, which, on the principle of a common beam, would be firm enough to allow railway trains to pass and repass without oscillation, danger, or even the shadow of risk; and it of course followed that an aërial road of this description should be composed of the strongest and lightest material; that its form should be that best suited for averting the wind; and lastly, that no expense should be spared to protect the public from the awful catastrophe that would result from the rupture of this 'baseless fabric' during the passage over it of a train. |