BRIDGE. 



543 



Cilice, serve, that one of these situations is at the Swilley 



""'' rocks, ami the otluM- at Yny;. y-Mocli. These two 



being so evidently tin- best, th< - 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 

 Holyhead roads and harbour, it appears, that a con- 

 sidcrabli- number of small coasting-vessels, vi/. from 

 16 to 100 tons, navigate the Menai, and that there 

 have been a few from 100 to 150 tons. By stub'- 

 ments from the principal shipbuilders in the river, 

 made in the year 1800, to the Committee for im- 

 proving the Port of London, it also appears, that 

 vessels of 150 tons, when they have all on end, are 

 only 88 feet in height above the water-line ; and 

 further, that even ships of .'500 tons, with their top- 

 gallant-masts struck, are nearly the same height : 

 these, in the Menai, are extreme cases, and, if pro- 

 vided for, ought, as to navigation, to satisfy every 

 reasonable person ; it may, indeed, rather be a ques- 

 tion, whether the height should not be limited to 

 vessels under 100 tons, by which the expence of a 

 bridge would be considerably diminished. 



" In the plans I hare formed, provision is made 

 for admitting vessels of 150 tons to pass with all on 

 eud ; 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 adapted for passing across the three 

 rocks, named the Swilley, Bcnlass, and Ynys-well- 

 dog ; which, by their shape and position, are singu- 

 larly suitable. To embrace the situation most per- 

 fectly, 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 are made of cast iron ; over the 

 smaller spaces, in order to add weight and stability 

 to the piers, semicircular arches of stone are intro- 

 duced ; but over these, as well as the larger open- 

 ings, the spandrels, roadway, and railing, are con- 

 structed of cast-iron. In tins way the navigation is 

 not impeded, because the piers standing near the out- 

 er edges, are guards for preventing vessels striking 

 upon the rocks ; while the whole structure presents 

 very little obstruction to the wind. From the ex- 

 tremity 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 ex- 

 plain the nature of the design. I propose the bridge 

 to be 32 feet in breadth ; and, from minute calcula- 

 tions made from detailed drawings, I find the expence 

 of executing the whole, in a perfect manner, amounts 

 to L.I 58,65k 



" The other design is for the narrower strait, 

 called Ynys-y-Moch. Here the situation is particu- 

 larly favourable for constucting a bridge of one arch ; 

 and making that 500 feet span, leaves the naviga- 

 tion 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 de- 

 scribed, 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 



t scheme of passing the Menai ; and it would, Practice. 



in my opinion, be attended with the least inconvcni- ' / 



md risk in the execution. 



" In order to render this evident, I have made a 

 drawing, (see Plate C.) to shew in what manner 

 the centering or frame, for an arch of this mag- 

 nitude, majr be constructed. Hitherto, the cen- 

 tering 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 water, and ihc great rise and rapidity of the tu; 

 this would be very difficult, if not impracticable, f 

 therefore propose changing the mode, and working 

 entirely from above, that is to say, instead of sup- 

 porting, I mean to suspend the centering. By in- 

 specting the drawing, the general principle of t 

 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 

 carried up the masonry to the level ot the roadway, 

 I propose upon the top of the abutments to construct 

 as many frames as there are to be ribs in the center ; 

 and of at least an equal breadth with the top of each 

 rib. These frames to be about ~>0 feet high above the 

 top of the masonry ; and to be rendered perfectly 

 firm and secure. That this can be done, is so evi- 

 dent, 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. 



' I next proceed to construct the center itself : it 

 is proposed to be made of deal bulk, and to consist 

 of four separate ribs ; each rib being a continuation 

 of timber frames live feet in width at the top and 

 bottom, varying in depth from 25 feet near the abut- 

 ments to 7 feet 6 inches at the middle or crown. 

 Next to the face of the abutments, one set of frames, 

 about 50 in length, can, by means of temporary scaf- 

 folding, and iron chain bars from the beforementioned 

 frames, be readily constructed, and fixed upon the 

 offsets of the abutments, and to horizontal iron ties 

 laid in the masonry for this purpose. A set of these 

 frames, (four in number,) having been fixed against 

 the face of each abutment ; they are to be secured 

 together by cross and diagonal braces, and there being 

 only spaces of 6 feet 8 inches left between the ribs, 

 (of which these frames are the commencement,) they 

 are to be covered with planking, and the whole con- 

 verted into a platform 50 feet by 40. By the nature 

 of the framing, and being secured by horizontal and 

 suspending bars, I presume every person accustomed 

 to practical operations will admit, that these plat- 

 forms may be rendered perfectly firm and secure. 



" The second portion of the centre frames, hav- 

 ing been previously prepared and fitted in the carpen- 

 ter's yard, are brought, in separate pieces, through. 

 passages purposely left in the masonry, to the be- 

 fore-mentioned platforms. They are here put together, 

 and each frame raised by the suspending bars and 

 other means, so that the end which is to be joined to 

 the frame already fixed, shall rest upon a smallmoveable 

 carriage. It is then to be pushed forward, perhaps 

 upon an iron rail road, until the strong iron forks, 

 which are fixed on its edge, shall fall upon a round 



