"The ribs being thus fixed, covered, and connected toge ther, 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 Colebrook Dale, it requires 448,0001b. 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 position, that the strength pro vided 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 supported 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 •hain-bars. Thus the hitherto dan gerous operation of striking the centering, will be rendered gradual and perfectly safe; inasmech that this new mode of suspending centering, instead of supporting it from below, may perhaps hereafter he 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 areh-stones in the centre arch of Blackfriars' bridge, 156 x 43 X 5, equal to 33.450 cubic feet of stone, it amounts to 2.230 tons; whereas the whole of the iron work, in the main ribs, •ross-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 1,791 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, mlnInanded by the sus pending chain-Lars.
"When the main 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 including the true line of bearing. These
triangles are, of Course, preserved in a vertical plane by cross ties and braces. lam bearers are supported by these triangles, and upon the bearers are laid the covering plates under the roadway, which, instead of being solid, are (in order to lessen the weight) proposed to be reticulated.
if 1 have, throughout this very succinct description, made myself' understood, it will, I think. he admitted, that the con structing a single arch across the Menai, is not only a very practicable, but a very simple operation ; and that it is ren dered so, chiefly by adopting the mode of working each abutment, 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 ro•ks,surrounded by a rapid tide, the inconvenience of carrying materials, and working, is greatly increased ; and supposing the bridge part constructed, au enormous expense is still to be incurred before the roadway can be carried over the flat ground on the Anglesea shore. Therefore whether economy, facirity of pertbrmance, 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 naviga tion." See SUSPENSION BRIDGE.
A very handsome bridge was erected over the river Trent, in the county of Stafford, from the designs and under the direction of James Potter, Esq. The specification for this structure is so ably drawn, and describes so precisely every part of the works to be contracted for, that we think we shall do good service to the student, by transcribing it in exteuso, as a model for similar compositions.