" I next proceed to construct the centre itself : it is proposed to be made of deal bulk, and to consist of four separate ribs ; each rib being a continuation of timber frames five feet in width at the top and bottom, varying in depth from 25 feet near the abutments to 7 feet 6 in ches at the middle or crown. Next to the face of the abutments, one set of frames, about 50 in length, can, by means of temporary scaffolding, and iron chain bars from the before-mentioned frames, be readily construct ed, 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 are to be secured together by cross and diagonal braces, and there being only spaces of 6 feet 8 inches left between the ribs, (of which these frames are the commencement,) they are to be covered with planking, and the whole con verted into a ylatform 50 feet by 40. By the nature of the framing, and being secured by horizontal and sus pending bars, I presume every person accustomed to practical operations will admit, that these platforms 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, are brought, in separate pieces. through passages purposely left in the masonry, to the before-mentioned platforms. They arc here put together, and each frame raised by the suspending bars and other means, so that the end which is to be joined to the frame already fixed, shall rest upon a small moveable carriage. It is then to be pushed forward, perhaps upon an iron rail road, un til the strong iron forks, which are fixed on its edge, shall fall 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 forks, and the aforesaid second portion of the frame work is suffered to descend to its intended position, by means of the suspending 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 conceive, that even without the aid of the suspending bars, that this second portion of the centering would be 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 80,0001bs. and that the powers of suspension are as the sections ; conse quently, a bar 4 inches square, will suspend 180,000lbs.;
but the whole weight of this portion of the rib, includ ing the weight of the suspending bar, is only about 30,0001bs• or one-sixth of the weight that might safely be suspended ; and as I propose two suspending chain bars to each portion of rib, if they had the whole to sup port, 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.
" Having thus provided for the second portion of the centering, 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 centering ; and being then braced to gether, 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 work, which, it is un derstood, has been previously fitted, can be put together with tne utmost correctness and facility ; the communi cation 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 ra dius. It is proposed in the breadth of the bridge (i. e. 40 feet) to have nine ribs, each cast in 23 pieces, and these connected by a cross grated plate, nearly in the same manner as in the great aqueduct of Ponteysylte, over the valley of the Dee, near Llangollen. The fixa tion of the several ribs in a vertical plane, appearing (alter the abutments) to be the most important object in iron bridges, I propose to accomplish this by cover ing the several parts or ribs, as they are progressively fixed, with grated or reticulated and !launched 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.
" The ribs being thus fixed, covered, and connected together, the great feature of the bridge is completed. And as, from accurate experiments made and communi cated to Inc by my friend, the late "William Reynolds of Cualbrookdale, it requires 448,0001bs. to crush a cube of one quarter of an inch of cast-iron, of the quality named guti-metal, it is clear, while the ribs are kept in their true position, that the strength provided is more than ample.