"1 propose, in the first place, to build the masonry of' the abutments as fir as the lines a B. C D. and in the particular manner shown in the section. llaving carried up the masonry to the level of the roadway, I propose upon the top of the abutments to construct as many frames as there are to be ribs in the centre ; and of at least an equal breadth with the top of each rib. These frames to be about 50 feet high above the top of the masonry ; and to be rendered perfectly firm and secure. That this can be dune, is so evident, I avoid entering into details respecting the mole. These frames are for the purpose of receiving strong blocks or rollers and chains, and to be acted upon by windlasses or other powers.
" I next proceed to construct the centre itself; it is pro posed to be made of deal baulk, and to consist of four separate ribs; each rib being a continuation of timber frames, 5 feet in width at the top and bottom, varying in depth from 25 feet near the abutments to 7 feet 6 inches at the middle or crown. Next to the face of the abutments, one set of FMB:ICS, about 50 in length, can. by means of temporary scaffolding, and iron chain bars from the before-mentioned frames, be readily constructed, and fixed upon the offsets of the abutments, and to horizontal iron ties laid in the masonry for this purpose. A set of these frames (four in number) having been fixed against the face of each abutment ; they arc to be secured together by cross and diagonal braces, and there being only spaces of 6 feet S inches left between the ribs, (of which these frames are the commencement,) they are to be covered with planking, and the whole converted into a platform. 50 feet by 40. By the nature of the framing, and its being secured by horizontal and suspending bars, I presume every per!-on accustomed to practical operations w ill admit that these plattbrms may be rendered perfectly firm and secure.
"The second portion of the centre frames, having been previously prepared and fitted in the carpenter's yard, arc brought, in separate pieces, through passages purposely left in the masonry, to the before-mentioned platfimans. They are here put together, and each frame raised by the suspending bars and other means. so that the end %%hid) is to be joined to the frame already fixed, shall rest upon a small movable carriage. It is then to be pushed forward, perhaps upon an iron railroad, until the strong iron forks, which are fixed on its edge, shall fidl upon a round iron bar, which forms the outer edge of the first, or abutment frames. When this has been done, strong iron bolts are put through eyes in the fiedis, and the afiwesaid second portion of the frame-work is suffered to descend to its intended position, by means of the suspend ing chain-bars, until it closes with the end of the previously fixed frame, like a rule joint. Admitting the first frames
were firmly fixed, and that the hinge part of this joint is sufficiently strong, and the joint itself 20 feet deep, I con ceive, that even without the aid of the suspending bars, this second portion of the centering would he supported ; but we will for a moment suppose, that it is to be wholly suspended. It is known, by experiment, that a bar of good malleable iron, one inch square, will suspend 80,0001b.. and that the powers of suspension are as the sections; consequently, a bar 11 int+ square, will suspend 180.000 lb.; but the whole weight of this portion of the rib, ineluiling the weight of the suspend ing bar. is only about 30.0001h., or one-sixth of th weight that might safely be suspended ; and as I propose two sus pending chain-bars to each portion of rib, if they had the whole to support, they would only be exerting about one twelfth of their power ; and considering the proportion of the weight which rests upon the abutments, they are equal also to support all the iron work of the bridge, and be still far within their power.
"flaying thus provided for the second portion of the cen tering a degree of security far beyond what can be required, similar operations are carried on from each abutment until the parts are joined in the middle, and form a complete cen tering ; and being then braced together, and covered with planking where necessary, the whole becomes one general platform or wooden bridge, to receive the iron-work.
"It is, I presume, needless to observe, that upon such a centering or platform, the iron-world, which, it is understood, has been previously fitted, can be put together with the utmost correctness and facility : the communication from the shores to the centre will be through the before-mentioned passages in the masonry. The form of the iron-work of the main ribs will be seen, by the drawing, to compose a system of triangles, preserving the principal points of bearing in the direction of the radius. It is proposed in the breadth of the bridge (i. e. 40 feet) to have nine ribs, each cast in twenty three pieces, and these connected by a eross-grated plate, nearly in the same manner as in the great aqueduct of Pont eysylte, over the valley of the Dee, near Llangollen. The fixation of the several ribs in a vertical plane, appearing (after the abutments) to be the most important object in iron bridges, I propose to accomplish this by covering the several parts or ribs, as they are progressively fixed, with grated, or reticulated and flannelled plates, across the top of the ribs. This would keep the tops of the ribs immovable, and convert the whole breadth of the bridge into one frame. Besides thus securing the top, I propose also having cross-braces near the bottom of the ribs.