There is great simplicity, and much of correct prin ciple in this design : 1. The springing-plates being 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 only : and, 3. Wrought-iron being wholly excluded. But we regret still observing the varying dimensions of the pans of the ribs ; and that the supporting pillars are still placed perpendicularly ; and which, as the arch has more cur vature, 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 avoided. 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 spring ing-plates are each 16 feet in length, 3 feet in breadth, and 4 inches in thickness, with sockets and shoulder pieces to receive the ribs. In each plate are three aper tures, 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 direction of the radius, and dia gonals between them ; but every part is of equal di mensions. 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 gratings with cast-iron screw-bolts. Each rib is preserved in a vertical plane, by covering the whole with grated, flaunched-plates, properly secured toge ther, 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 intro duced, each cast in one frame, with a joggle at its up per 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 se cured 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 pressure are preserved in the di rection of the radius. The covering-plates, in order to preserve a sufficient degree of strength, and lessen the weight, are, instead of solid, made of a reticulated shape ; the apertures 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, especially
in the spandrels, also greatly improves the general ap pearance.
In a printed report of a committee of the House of Commons, of the last session, we find sonic new in formation respecting centering for an iron bridge, which, as it promises to form a new xra in bridge building, we are happy in being enabled to lay it before 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 Caer narvonshire by the celebrated strait or arm of the sea named the through which the tide flows with great velocity ; and, from local circumstances, in a very peculiar manner. This renders the navigation difficult ; and it has always been a formidable obstacle in the be fore-mentioned communication. It has hitherto been crossed by a ferry-boat at Bangor; but the inconve nience and risk attending this mode, has led to specula tions of improvement for half a century past ; wooden bridges, and embankments, with draw-bridges, have been alternately 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 esti mates for bridges at this place in 1802, and had been called on to revise them in 1810. His plans, which ap pear 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, because 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, without any con venient means of supporting it in the middle, on account of the depth of water and rapidity of the tide, or of get ting any assistance from vessels moored in the channel to put it up ; I will not say it is impracticable, but I think it too hazardous to be recommended." And again, in the same report : I should be little inclined to un dertake the building a bridge at Ynys-y-Moch." But from the report of June 1311, it appears, that in May 1810, Mr Telford was instructed by the Lords of the Treasury to survey, and report upon the best me thod of improving the lines of communication between Holyhead and Shrewsbury, and also between Holyhead and Chester ; and to consider, and give plans for pass ing the Menai. In the aforesaid report (of 1811) we have his plans and estimate. His explanations we shall give in his own words.