" When advanced thus far, I propose, though not to removt, yet to ease the timber centering, by having the feet of the centering ribs (which are supported by off sets 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 cen tering, will be rendered gradual and perlectly safe ; in asmuch that this new mode of suspending centering, instead of supporting it from below, may perhaps here after 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 nacre ring of archstones in the centre arch of Black friars bridge, at 156 X43 X 5, equal to .33,450 cubic feet of stone, it amounts to 2,236 tons ; wher,..as 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 ly ing 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 centering ; but this is coun terbalanced by the operation of the iron ties in the abut ments, and wholly commanded by the suspending chain bars.
" When the main ribs have been completed, the next step is to proceed with the iron supporters of the road way ; 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, pre served 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 under the roadway, which, instead of being solid, are, (in order to lessen the weight,) proposed to be reticulated.
" If I have, throughout this very succinct description, made myself understood, it will, I think, be admitted, that the constructing a single arch across the Menai, is not only a very practicable, but a very simple opera tion; and that it is rendered so, chiefly by adopting the mode of working from each abutment, without at all in terfering 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 inconvenience of carrying materials, and working, is greatly increas ed ; and supposing the bridge part constructed, an enor mous expence has still to be incurred before the road way can he carried over the flat ground on the An glesea shore. Therefore, whether oeconomy, facility of
performance, magnificence, or durability be consulted, the bridge of one arch is, in my opinion, infinitely pre ferable ; and it is no less so, if considered in what re gards the navigation." This mode of constructing centres, applicable to stone as well as iron arches, being an original idea, and per fectly simple, and the effects of all its operations being more capable of correct demonstration than those of the former mode of supporting from below, we were glad of being enabled, while this article was in the press, to communicate the outlines of the scheme, as given by the engineer in his report to the lords of the treasury. If this should be successfully practised on so large a scale at the Menai, all difficulties with regard to car rying bridges over inaccessible ravines or turbulent streams, will, in future, be done away, and a new xra formed in bridge building.
We have only given a Plate of the centering, because the construction of the iron work is precisely the same in principle (though on a larger scale) as what has been adopted by the same engineer in Bonar bridge. See Plate C.
We have now given our readers a distinct view of the progress of the practice of constructing bridges with cast iron, as far as it has hitherto been carried. We are well convinced that much more will still be ac complished; and when we come, under the article IRON, to treat generally of its qualities and different modes of application, some light will be thrown upon the sub ject, which we expect will tend still further to enhance its importance in the great practical purposes of Although it is trusted the reader will, in this article, have found bridge building as fully discussed as he could expect in a work of this nature, yet if disposed to prosecute the subject more at length, he will be enabled to do so by consulting the following able authors, who have written on the subject either generally, or in des cribing particular works.
It is rather surprising, that although the Romans had, before the time of Vitruvius, introduced the use of the arch, not only in bridges and aqueducts, but in theatres and temples, yet lie has not been led to discuss, or even particularly to notice this principal feature in Roman architecture.
Leoni Baptisti Alberti first wrote upon bridges in 1481. Palladio followed about a century afterwards. Serlio and Scammozzi also treated the subject, and the latter gives useful rules respecting foundations. See also Ferrari on Arches, in the ?la. Sienn. vi. 143.