The general opinion of engineers appears to be, that the east: iron arch is the best form for an iron bridge, when it can be selected without regard to expense, or to the height above the river or road which is to be crossed. For low bridges the bowstring girder is also strongly recommended. Lattice bridges appear to be of' doubtful merit.
We conic now to the description of that new mode of con strnetion introduced by Mr. Stephenson and his able coad jutors, and perfected in the magnificent structures to which the publie attention his been so long directed, the Conway and Ilritannia tubular bridg. s :— The application of iron in this form of bridge consists in the tiv•ting together boiler-plates as in iro.i ship-building, combined in various wag s with east-iron. low girders are thus formed, which are either made so large as to admit of the road and earriages passing through them, as in the bridges above described. or else these tube girders arc made on a smaller scale, and employed in the same manner as the ordinary east-iron girders, to sustain transverse joists which carry the road. The first kind is applicable to enormous spans, those of the two bridges above-mentioned being 400 and 462 feet respectively. The second kind are said to be cheaper and more elastic than other forms, for spans that exceed 40 feet. These methods appear to possess and to promise many advan tages, but they are of such recent introduction, that little experience has yet been acquired of their pow ers to resist the various actions of sudden changes of temperature, and other causes of deterioration. The eminent engineers examined before the commissioners, however, very generally expressed opinions favourable to the tubular form of bridge.
Before proceeding to describe the Britannia and Conway Bridges, it may be desirable to give a slight sketch of the circumstances which led to their erection : In the session of1S44, a -Joint Stock Company proposed to parliament to undertake the construction of a railway from Chester to Holyhead, in eontirmation of lines already made. The bill introduced for this purpose contemplated the use of Telford's celebrated suspension bridge over the Menai straits, as the means of conveying across the railway, carriages and waggons, divested of the ponderous locomotive engine. The Commissioners of Woods and Forests, however, who had the jurisdiction of the bridge, would at first only consent to this proposal as a temporary measure, and afterwards objected to its being made use of at all. The company not being pre
pared with any plan tor a permanent bridge, were obliged to content themselves with an act of parliament which gave them power to construct a railway from Chester to 'Bangor, :mud through the Isle of Angle-ea to Holyhead, leaving an hiatus of five miles at the :Menai straits. The means of filling up this break was reserved for future consideration, and for a permanent bridge to be sanctioned by the act of a subsequent session. In the autumn, Mr. Robert Stephenson, the engineer of the company. prepared plans for a east-iron bridge of two arches, each having a span or clear width of 350 feet. The height of each arch was to have been 100 feet at the crown, and the total cost of the bridge would have been .r250,000. The Britannia rock, about a mile to the west of the suspension bridge, was selected as the most eligible point for (Tossing the straits ; but in consequence of the rapid current, and other circumstances of difficulty which exhibited themselves at this point, added to the necessity of interfering as little as possible with the navigation of the straits, Mr. Stephenson proposed to build the arches of this bridge, not upon supports from beneath, as is usually clone, but by stringing on, as it a series of iron blocks or vertebrte from each pier, until they should meet in the middle, in a way somewhat similar which had been proposed in former years by Mr. Telford and Sir J. Brunel. The rail way company made their application to parliament in the session of 1845, in order to obtain power to construct this bridge. The company, however, themselves strongly opposed, not only by parties interested in the navigation of the straits, but by others NS 110Se interest lay in driving them to Port Dynllaen. An appeal was then made to the Lords Commissioners of the Admiralty. as conservators of the navi gation. and after a lengthened inquiry, the bridge proposed by Mm', Robert Stephenson received their lordships' veto, and a form of structure was prescribed, w high should leave a clear opening of not less than 450 feet over each of the two navi gable channels on each side of time Britannia rock, with a height of 105 feet above high-water over 370 feet of that opening, the remaining SO feet being allowed to have less height. so as to admit of the construction of an melt of that form. should it be desired by the company.