542 



BRIDGE. 



Practice. 



Over the 

 Mcnai. 



PJUTI C. 



There is great simplicity, and much of correct 

 principle in this design : 1. The springing plates be- 

 ing placed in the direction of the radius, and the 

 abutments receding to produce a space behind the 

 ribs equal to that between the upright pillars. 2. The 

 ribs being composed of two pieces, and one joint on- 

 ly : and, 3. Wrought-iron being wholly excluded. 

 But we regret still observing the varying dimensions 

 of the parts of the ribs ; and that the supporting 

 pillars are still placed perpendicularly ; and which, 

 as the arch has more curvature, has still a worse effect 

 than at Boston. 



In the course of his employment as engineer to the 

 Board of Parliamentary Commissioners for making 

 Roads and constructing Bridges in the Highlands 

 of Scotland, Mr Telford has lately made a design 

 for a cast-iron bridge now constructing upon an arm 

 of the sea which divides the county of Sutherland 

 from that of Ross, at a part where several of these 

 roads unite. In this bridge, the defects noticed in 

 the former works of this sort appear to be avoid- 

 ed. See Plate C. The arch is 150 feet span ; it 

 rises 20 feet, it is 16 feet in width, and has four 

 ribs. In the abutments, not only are the springing- 

 plates laid in the direction of the radius, but this 

 line is continued up to the roadway. The springing- 

 plates are each 16 feet in length, 3 feet in breadth, 

 and 4 inches in thickness, with sockets and shouldfr- 

 pieces to receive the ribs. In each plate are three 

 apertures, three feet in length, and 18 inches in 

 width. Each of the ribs, for the conveniency of 

 distant sea-carriage, is composed of five pieces, three 

 feet in depth in the direction of the radius, and two 

 and a half inches in thickness. There are triangular 

 apertures in the ribs, formed by pieces in the direc- 

 tion of the radius, and diagonals between them ; but 

 every part is of equal dimensions. At every joining 

 of the pieces of the ribs, a cast-iron grating passes 

 quite across the arch ; upon these are joggles or 

 shoulderings to receive the ends of the ribs : the ribs 

 have also Haunches, which are fixed to the grat- 

 ings with cast-iron screw-bolts. Each rib is pre- 

 served in a vertical plane, by covering the whole 

 with grated, flanched- plates, properly secured to- 

 gether, and to the top of the ribs, by cast-iron 

 screws and pins. In the spandrels, instead of circles 

 or upright pillars, lozenge, or rather triangular forms 

 are introduced, each cast in one frame, with a joggle 

 at its upper and lower extremities, which pass into 

 the sockets formed on the top of the ribs, and in the 

 bearers of the roadway. Where the lozenges meet 

 in the middle of their height, each has a square notch 

 to receive a cast-iron tie, which passes from each 

 side, and meets in the middle of the breadth of the 

 arch, where they are secured by forelocks. Next to 

 the abutments, in order to suit the inclined face of 

 the masonry, there are half lozenges. By means of 

 these lozenge or triangular forms, the points of pres- 

 sure are preserved in the direction of the radius. 

 The covering-plates, in order to preserve a sufficient 

 degree of strength, and lessen the weight, are, in- 

 stead of solid, made of a reticulated shape ; the a- 

 pertures widen below, to leave the matter between 

 them a narrow edge; and contract upwards, so as to 

 prevent the matter of the roadway from falling 



through. This disposition of the iron work, espe- 

 cially in the spandrels, also greatly improves the 

 general appearance. 



In a printed report of a committee of the House 

 of Commons, of the last session, we find some new 

 information respecting centering for an iron bridge, 

 which, as it promises to form a new a?ra in bridge- 

 building, we are happy in being enabled to lay it be- 

 fore our readers. 



This subject has been brought under discussion in 

 the course of investigating the most effectual mode 

 of improving the mail-roads from Holyhead through 

 North Wales. The island of Anglesea is divided 

 from Caernarvonshire by the celebrated strait or arm 

 of the sea named the Menai, through which the tide 

 flows with great velocity ; and, from local circum- 

 stances, in a very peculiar manner. This renders 

 the navigation difficult; and it has always been a 

 formidable obstacle 1:1 the before-mentioned commu- 

 nication. It has hitherto been crossed by a ferry- 

 boat at Bangor ; but the inconvenience and risk at- 

 tending this mode, has led to speculations of im- 

 provement for half a century past ; wooden bridges, 

 and embankments, with draw-bridges, have been al- 

 ternately proposed and abandoned. From a report 

 of the House of Commons, of June 1810, it appears, 

 that Mr Rennie the engineer, had given plans and 

 estimates for bridges at this place in 1802, and had 

 been called on to revise them in 1810. His plans, 

 which appear in the last-mentioned report, are, 

 1st, One arch of cast-iron, 450 feet span, over the 

 narrowest part of the strait, at a projecting rock 

 named Ynys-y-Moch : and 2d, Another upon the 

 Swilley Rocks, consisting of three cast-iron arches, 

 each 350 feet span. The expence of that at Ynys-y- 

 Moch is estimated at L. 259,140, and of that at the 

 Swilley, L. 290,147. He prefers the latter, be- 

 cause he says, " On account of the great span of 

 the arch at Ynys-y-Moch, and the difficulty and 

 hazard there will be in constructing a centre to span 

 the whole breadth of the channel at low water, with- 

 out any convenient means of supporting it in the 

 middle, on account of the depth of water and rapi- 

 dity of the tide, or of getting any assistance from 

 vessels moored in the channel to put it up ; I will 

 not say it is impracticable, but I think it too hazard- 

 ous to be recommended." And again, in the same 

 report : " I should be little inclined to undertake 

 the building a bridge at Ynys y-Moch." 



But from the report of June 181 1, it appears, that 

 in May 1810, Mr Telford was instructed by the 

 Lords of the Treasury, to survey, and report upon the 

 best method of improving the lines of communication 

 between Holyhead and .Shrewsbury, and also be- 

 tween Holyhead and Chester ; and to consider, and 

 give plans for passing the Menai. In the aforesaid 

 report (of 1811) we have his plans and estimate. 

 His explanations we shall give in his own words. 



" The duty assigned me being to consider, and re- 

 port respecting a bridge across the Menai, I shall 

 confine myself to this object. Admitting the im- 

 portance of the communication tojustify acting on a 

 large scale, I not only consider the constructing a 

 bridge practicable, but that two situations are re- 

 markably favourable. It is scarcely necessary to ob- 



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