" The duty assigned me being to consider, and report respecting a bridge across the Menai, I shall confine my self to this object. Admitting the importance of the communication to justify acting on a large scale, I not only consider the constructing a bridge practicable, but that two situations arc remarkably favourable. It is scarcely necessary to observe, that one of these situa tions is at the Swilley rocks, and the other at Ynys-y Moch. These two being so evidently the best, the only question that can arise is, to which of them the prefer (nice ought to be given.
" From the appendix to the second report to the Ho lyhead roads and harbour, it appears, that a consider able number of small coasting-vessels, viz. from 16 to 100 tons, navigate the .Menai, and that there have been a few from 100 to 150 tons. By statements from the principal shipbuilders in the river, made in the year 1800, to the Committee for improving the Port of Lon don, it also appears, that vessels of 150 tons, when they have all on end, are only 88 feet in height above the wa ter-line ; and further, that even ships of 300 tons, with their top-gallant-masts struck, arc nearly the same height : these, in the Menai, are extreme cases, and, if provided for, ought, as to navigation, to satisfy every reasonable person ; it may, indeed, rather be a question, whether the height should not be limited to vessels un der 100 tons, by which the expense of a bridge would be considerably diminished.
" In the plans I have formed, provision is made for admitting vessels of 150 tons to pass with all on end ; that is, in one design preserving 90 feet, and in the other 100 feet between the line of high water and the lower side of the soffit of the arch. The first design is adapt ed for passing across the three rocks, named the Swil Icy, Benlass, and Ynys-well-dog ; which by their shape and position, are singularly suitable. To embrace the situation most perfectly, I 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 arc made of cast iron ; over the smaller spaces, in order to add weight and stability to the piers, semicircular arches of stone are introdu ced ; 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, be cause the piers standing near the outer edges, are guards for preventing vessels striking upon the rocks ; while the whole structure presents very little obstruction to the wind. From the extremity of the abutments, after
building rubble walls above the level of the tide-way, 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 32 feet in breadth ; and, from minute calculations made from detailed drawings, I find the ex pence of executing the whole, in a perfect manner, amounts to 158,6541.
" The other design is for the narrower strait, called Vnys-y-Moch. Ilere the situation is particularly fa vourable for 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 expence to be L.127,331, or L.31,323 less than the former. From leaving the whole channel unimpeded, it is certainly the most perfect scheme of passing the Menai ; and it would, in my opinion, be attended with the least inconvenience and risk in the execution.
" In order to render this evident, I have made a draw ing, (see Plate C.) to shew in what manner the center ing or frame, for an arch of this magnitude, may be con structed. Hitherto, the centering has been made by placing supports and working from below ; but in the case of the Menai, from the nature of the bottom of the channel, the depth at low water, and the great rise and rapidity of the tides, this would be very difficult, if not impracticable. I therefore propose changing the made, and working entirely from above, that is to say, instead of supporting, I mean to suspend the centering. By inspecting the drawing, the general principle of this will be readily conceived.
" I propose, in the first place, to build the masonry of the abutments as far as the lines AB, CD, and in the particular manner shewn in the section. Having carri ed up the masonry to the level of the roadway, I pro pose upon the top of the abutments to construct as many frames as there are to be ribs in the centre ; and of at least an equal breadth with the top of each rib. These frames to be about 50 feet high above the top of the ma sonry, and to be rendered perfectly firm and secure. That this can be done, is so evident, I avoid entering into details respecting the mode. These frames are for the purpose of receiving strong blocks or rollers and chains, and to be acted upon by windlasses or other powers.