OF THE ARCHES.
The centres being placed, and properly secured, the setting the archstones is proceeded with. The masonry of the piers and abutments, near to the springing, is carefully adjusted ; and it is usual, immediately under the commencement of the curvature, to lay a capping, string, or cordon : this, by having a small projection, covers any trifling inaccuracy which may have taken place in setting out or carrying up the abutments and piers. if the courses have hitherto been worked with horizontal beds, the upper bed of this capping course is sometimes made to suit the radius of the arch. After the general form and dimensions of the arch have been determined, the form and dimensions of the archstoncs is of the first importance, for it is upon them that the great feature of bridge building depends. The nature of the different forms into which they have been, may, or ought to be made, have already been fully considered when treating of the principles of bridges ; we shall therefore now only simply state various instances where different sizes have been practised in great works.
From the foregoing examples it wilrbe seen, that the French have uniformly used very deep archstones, for their outside or headers are many of them more than those here described for the body of the arch; and this circumstance, no doubt, joined to their wide mortar joints, led, in some measure, to the enormous sinking of their arches. In Westminster and Blackfriars, the archstones are equally deep; but by the dimensions of those in Tongueland bridge, near Kirkcudbright, viz. 3 feet 6 inches, it may be observed that no such depth is neces sary; for here in an arch of 118 feet span, erected over a river, where, besides about 10 feet depth at low water, the tides rose above 16 feet, which of course rendered it somewhat difficult to fix and preserve the centering ; yet no sinking took place to open the joints, and the whole arch has ever since remained stable and perfect. Also in Dunkcld bridge, the arch of 90 feet span has archstones 3.2 deep, which are also quite sufficient.
No doubt the mode of managing the spandrels in both cases should also be taken into account, but we arc per suaded, that archstones are, in general, made much deep er than necessary ; and on account of the unnecessary weight and expense incurred by this practice, we consi der it our duty to draw the particular attention of the young engineer to this point. He may, in the instance of Llanrwst bridge, observe, that our British Palladio (Inigo Jones) has long ago shown us a bold example, which has stood the test of 174 years. In Llanrwst bridge, the middle arch of 58 feet span, 17 feet rise, and 14 feet in width across the soffit, the archstones arc only 18 inches deep. We have carefully examined this arch and can assure the reader, that it is not from the pecu liar excellence of the workmanship it derives any advan tage. The archstones vary in thickness from 5 to 16 inches ; many of them are 8 and 9 inches. There are, in some instances, two headers to answer one course of common archstones, and in others two courses of arch stones to answer one header. The masonry of the span drels is very irregularly laid; and as a complete proof of the stability of the middle arch, from a defect in the foundation of the west abutment, one of the side arches fell, and was rebuilt in 1703, but the others remained un injured. The piers are 10 feet in thickness, and the arch springs about 3 feet above the bed of the river. The road over the bridge is certainly too steep, and thereby no doubt lessens the weight upon the side arthes; but we are convinced, that the road might be made with a declivity of l in 24, and the same degree of lightness preserved, by constructing the spandrels hol low, in the way which will hereafter be described, in stead of filling them up solid as they are at present. We are more particular in describing this bridge, be cause of the authority it derives from the celebrity of its designer, and the length of time it has endured.