SUSPENSION BainnE. It is. however, given here. as it describes a mode of constructing centres applicable as well to stone as to iron arches:— " The duty assigned me," says Mr. Telford, " being to consider, and report respecting a bridge across the Menai, I shall confine myself to this object. Admitting the impor tance of the eonummication to justify acting on a large scale, I not only consider the constructing a practicable, but that two situations are remarkably favourable. It is scarcely necessary to observe, that one of these situations is at the Swillev rocks, and the other at Ynvs-v-Moch. These two being so evidently the best, the only question that can arise is, to which of them the preference ought to be given.
" From the appendix to the second report to the llolyhead roads and harbour, it appears that a considerable number of small coasting vessels, viz., from 10 to 100 tons, navigate the Menai, and that there have been a few from 100 to 150 tons. By statements from the principal ship-builders in the river, made in the year 1800, to the committee for improving the port of London, it also appears that vessels of 150 tons, when they have all on end, are only 58 feet in height above the water-line; and fit-titer, that even ships of 300 tons, with their top-gallant masts struck, are nearly the same height ; these in the Menai are extreme cases, and, if pros ided for, ought, as to navigation, to satisfy every reasonable person; it may, indeed, rather he a question whether the height should not be limited to vessels under 100 tons, by which the ex pense of a bridge would be considerably diminished.
" In the plans I have formed, provision is made for admit ting vessels of 150 tons to pass with all on end ; that is, in one design preserving 00 feet, and in the other 100 feet, be tween the line of high-waterand the lower side of the soffit of the arch. The first design is adapted for passing across the three rocks, named the Swillev, Benlass, and Ynys-well dog, which, by their shape and position, are singularly suitable. To embrace the situation most perfectly,1 have divided the space into three openings of 260 feet, and two of 100 feet each. making piers each 30 feet in thickness. Over the three large openings, the arches are made of cast-iron ; over the smaller spaces, in order to add N1 eight and stability to the piers, semicircular arches of stone are introduced; but over these, as well as the larger openings, the spandrels, roadway, and railing. are constructed of cast-iron. In this
way the navigation is not impeded, because the piers, stand ing near the outer edges, are guards for preventing vessels striking upon the rocks ; while the whole structure presents very little obstruction to tin wind. From the extremity of the abutments, after building rubble walls above the level of the tideway, I propose carrying embankments until the roadway reaches the natural ground. The annexed drawing will sufficiently explain the nature of the design. I propose the bridge to be 3e. feet in breadth; and, from minute cal culations made from detailed drawings, I find the expense of executing the whole in a perfect manner amounts to X158.654.
The other design is for the narrower strait, called Ynys y-Moch. Here the situation is particularly favourable fur constructing a bridge of one arch, and making that 500 feet span, leaves the navigation as free as at present. In this I have made the height 100 feet in the clear at high water spring tides; and I propose this bridge to be 40 feet in breadth. Estimating from drawings, as already described, I find the expense to be X1e'7,:331, or £31,3e3 less than the former. From leaving the channel uniinpeded, it is certainly the most perfect scheme of passing the Menai ; and would, in Inv opinion, be attended with the. least inconveni ence and in the execution.
In order to render this evident, I have made a drawing, to show in what manner the centering or frame, for an arch of this magnitude, may be constructed. Hitherto, the centering has been made by placing supports, and work ing from below : but in the case of the :Menai, from the nature of the bottom of the channel, the depth at low and the great rise and of the tides. this would be very difficult, if not impracticable. 1 therefore propose changing the mode, and working entirely from above, that is to say, instead of supporting, I mean to suspend the center ing. By inspecting the drawing, the general principle of this will be readily conceived.