514 



BRIDGE. 



P actice. iron bar, which forms the outc<c edge of the first, or 

 < v -* 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 suffer- 

 ed 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 

 t'.iis second portion of the centering would be sup- 

 ported ; but we will, for a moment, suppose, that it 

 is to be wholly suspended. It is known, by experi- 

 ment, that a bar of good malleable iron, one inch 

 square, will suspend S0,000lbs. and that the powers 

 of suspension are as the sections ; consequently, a 

 bar ]{ 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 

 .'iO.OCOlbs. or one-sixth of the weight that might safe- 

 ly be suspended ; and as I propose two suspending 

 chain bars to each portion of rib, if they had the 

 whole to support, they would only be exerting about 

 one-Uvelfth of their power ; and considering the 

 proportion of the weight which rests upon the abut- 

 ments, 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 be- 

 ing then braced together, and covered with plank- 

 ing 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 understood, has been previously fitted, can be 

 put together with the utmost correctness and faci- 

 lity ; the communication from the shores to the cen- 

 tre 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 pro- 

 posed in the breadth of the bridge ('. e. 40 feet) t 

 have nine ribs, each cast in 23 pieces, and these con- 

 nected by a cross grated plate, nearly in the same 

 manner as in the great aqueduct of Pontcysylte, over 

 the valley of the Dee, near Llangollen. The fixa- 

 tion of the several ribs in a vertical plane, appearing 

 (after the abutments) to be the most important ob- 

 ject 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 immoveable, and convert 

 the whole breadth of the bridge into one frame. Be- 

 sides thus securing the top, I propose also having 

 cross braces near the bottom of the ribs. 



" The ribs being thus fixed, covered, and con- 

 ted together, the great feature of the bridge is 



completed. And as, from accurate experiments 

 made and communicated to me by my friend, the 

 late William Reynolds of Coalbrookdale, it requires 

 448,000lbs. to crush a cube of one quarter of an 

 inch of cast iron, of the quality named gun-metal, 

 it is clear, while the ribs are kept in their true po- 

 sition, that the strength provided is more than ample. 

 " When advanced thus far, I propose, though 

 not to remove, yet to ease the timber centering by 

 having the feet of the centering ribs (which are sup- 

 ported by offsets in the masonry of the front of the 

 abutment,) placed upon proper wedges ; the rest of 

 the centering to be eased at the same time by means 

 of the chain bars. Thus the hitherto dangerous 

 operation of striking the centering, will be rendered 

 gradual and perfectly safe; inasmuch that this new 

 mode ot suspending centering, instead of supporting 

 it from below, may perhaps hereafter be adopted as 

 an improvement. Although the span of the arch is 

 unusually great, yet by using iron as a material, the 

 weight, upon the centre, when compared with large 

 stone arches, is very small. Taking the mere ring of 

 archstones in the centre arch of Blackfriars bridge, 

 at 156 X 43 X . r >, equal to 33,4.50 cubic feet of stone, 

 it amounts to 2.'23t> tons ; whereas the whole of the 

 iron work, in the main ribs, cross plates, and ties, 

 and grated covering plates, that is to say, all that is 

 lying on the centering at the time it is to be eased, 

 weighs only 1791 tons. It is true, that from the 

 flatness of the iron arch, if left unguarded, a great 

 proportion of this weight would rest upon the center- 

 ing ; but this is counterbalanced by the operation of 

 the iron ties in the abutments, and wholly command- 

 ed by the suspending chain bars. 



" When the maiu ribs have been completed, the 

 next step is to proceed with the iron supporters of 

 the roadway ; and these, instead of being constructed 

 in the form of circles, or that of perpendicular pillars, 

 as hitherto, are here a series of triangles, thus includ- 

 ing the true line of bearing. These triangles are, of 

 course, preserved in a vertical plane by cross ties and 

 braces. Iron bearers are supported by these triangles, 

 and upon the bearers are laid the covering plates un- 

 der the roadway, which, instead of being solid, are, 

 (in order to lessen the weight,) proposed to be reti- 

 culated. 



" If I have, throughout this very succinct descrip- 

 tion, made myself understood, it will, I think, be ad- 

 mitted, that the constructing a single arch across the 

 Menai, is not only a very practicable, but a very 

 simple operation ; atid that it is rendered so, chiefly 

 by adopting the mode of working from each abut- 

 ment, without at all interfering with the tideway. 



" In the case of the Swilley bridge, although the 

 arches are smaller, yet being placed on piers, situated 

 on rocks, surrounded by a rapid tide, the incon- 

 venience of carrying materials, and working, is greatly 

 increased ; and supposing the bridge part constructed, 

 an enormous expence has still to be incurred before 

 the roadway can be carried over the flat ground on 

 the Anglesea shore. Therefore, whether ceconomy, 

 facifity of performance, magnificence, or durability 

 be consulted, the bridge of one arch is, in my opinion, 

 infinitely preferable ; and it is no less so, if considered 

 in what regards the navigation." 



P.-ac 



