beam, they add but little if any more strength and stiffness than would be given by straight timbers reaching from the springing point of the arch timbers to their crown; and they are certainly

less efficacious in subserving their end than would be inclined struts, occupying a like position at bottom, and abutting against a straining beam, placed either under the centre part of the chord, where the locality would permit it, or under the centre portion of the plate. In localities where fine timber is less abundant than in those in which the most of Burr's bridges have been built, a judicious regard to economy would undoubtedly have suggested a selection of forms for the secondary parts of the frame, which would have prevented these parts from being as much cut to waste as the Figs. show they must have been in the example taken to illustrate this system.

(I) This structure, constructed under the superintendence of Moncure Robinson, Esq., is upon Town's plan. The width of the bays varies from 130 to 153 feet. It consists of two ribs, each of which is formed of a double lattice, with two chords at bottom and one at top. The roadway, for rails, rests on the top girders. The ribs are braced by vertical diagonal braces, and by horizontal diagonal braces between each pair of the top and bottom girders. The piers are of rustic work; they are 40 feet above the low-water level, and 4 feet thick at top. The example here selected for illustration (Fig. 136) is taken from another bridge, of nearly the same width of bay, erected subsequently so the Richmond bridge, by the same engineer, in which the top

chord also is doubled, as the former bridge was found to be rather weak.

(K) This bridge is constructed on Howe's plan. It consists (Fig. 1) of two ribs which are connected at top and bottom, in the usul manner, with cross ties and diagonal braces. The roadway flooring rests upon the cross girders at bottom. The bridge is not roofed, as is usually the case, the ribs being covered in on the sides and at top by a sheathing of boards, and the flooring-bcards by a metallic covering.

The bridges constructed according to Colonel Long's plan have been mostly applied to medium spans. In the printed description of the different improvements of this system patented by Colonel Long, he very judiciously introduces struts, which he terms arch braces, either below the top or the bottom string, as the locality may demand, for the purpose of preventing sagging, which must necessarily take place in time in all open-built beams of considerable span, if not strengthened in this way.

Fig. 137-Represents a side view of the truss and an end view of the pier, M; a cross section of the truss and side view of the pier, N; and a top view O, of the pier of the railroad bridge at Springfield.

A, inclined plane of the ice-breaker of the up-stream starling.

a, a, iron side-stays of the ribs anchored into the piers at top.

606. Bridges of cast iron admit of even greater boldness of design than those of timber, owing to the superiority, both in strength and durability, of the former over the latter material; and they may therefore be resorted to under circumstances very nearly the same in which a wooden structure would be suitable.

607. The abutments and piers of cast-iron bridges should be built of stone, as the corrosive action of salt water, or even of fresh water when impure, would in time render iron supports of this character insecure; and timber, when exposed to the same destructive agents, is still less durable than cast iron.

The forms and dimensions of the stone abutments and piers are regulated on the same principles as the like parts in wooden bridges with curved frames. The piers may be either built up high enough to receive the roadway-bearers, or else they may be terminated just above the springing plates of the bridge-frame,

and form supports for cast-iron standards upon which the roadwaybearers may be laid.

608. The curved ribs of cast-iron bridge-frames have undergone various modifications and improvements. In the earlier bridges, they were formed of several concentric arcs, or curved beams, placed at some distance asunder, and united by radial pieces; the spandrels being filled either by contiguous rings, or by vertical pieces of cast iron upon which the roadway bearers were laid.

In the next stage of progress towards improvement, the curved ribs were made less deep, and were each formed of several segments, or panels cast separately in one picce, each panel consisting of three concentric arcs connected by radial pieces, and having flanches, with other suitable arrangements, for connecting them firmly by wrought-iron keys, screw-bolts, &c.; the entire rib thus presenting the appearance of three concentric arcs connected by radial pieces. The spandrels were filled either with panels formed like those of the curved ribs, with iron rings, or with a lozenge-shaped reticulated combination. The ribs were connected by cast-iron plates and wrought-iron diagonal ties.

In the more recent structures, the ribs have been composed of voussoir-shaped panels, each formed of a solid thin plate with flanches around the edges; or else of a curved tubular rib, formed like those of Polonceau, or of Delafield, described under the head of Framing. The 'spandrel-filling is either a reticulated combination, or one of contiguous iron rings. The ribs are usually united by cast-iron tie-plates, and braced by diagonal ties of cast and wrought iron.

609. The roadway-bearers and flooring may be formed either of timber, or of cast iron. In the more recent structures in England, they have been made of the latter material; the roadwaybearers being cast of a suitable form for strength, and for their connection with the ribs; and the flooring-plates being of cast iron.

The roadway and footpaths, formed in the usual manner, rest upon the flooring-plates.

The parapet consists, in most cases, of a light combination of cast or wrought iron, in keeping with the general style of the

structure.

610. The English engineers have taken the lead in this branch of architecture, and, in their more recent structures, have carried it to a high degree of mechanical perfection and architectural elegance. Among the more celebrated cast-iron bridges in England, that of Coalbrookdale belongs to the first epoch above mentioned; those of Staines and Sunderland to the second; and to the third, the bridge of Southwark at London; that of Tewkes

bury over the Severn; that over the Lary near Plymouth, and a number of others in various parts of the United Kingdom.

The French engineers have not only followed the lead set them by the English, but have taken a new step, in the tubular-shaped ribs of M. Polonceau. The Pont des Arts at Paris, a very light bridge for foot-passengers only, and which is a combination of cast and wrought iron, belongs to their earliest essays in this line; the Pont d'Austerlitz, also at Paris, which is a combination similar to those of Staines and Sunderland, belongs to their second epoch; and the Pont du Carrousel, in the same city, built upon Polonceau's system, with several others on the same plan, belong to the last.

In the United States a commencement can hardly be said to have been made in this branch of bridge architecture; the bridge of eighty feet span, with tubular ribs, constructed by Major Delafield at Brownsville, stands almost alone, and is a step contemporary with that of Polonceau in France.

The following Table contains a summary description of some of the most noted European cast-iron bridges.

(A) This is the first cast-iron bridge erected in England. The curved rib is nearly a semicircle in shape, and is composed of three concentric arcs, which are connected at intervals by short columnar pieces, in the direction of the radii of the curve.

(B) This structure, which connects Wearmouth and Sunderland, has a remarkably bold appearance, both from its great span, and its height, which is 100 feet between the high water-level and the intrados of the arch at the crown. The entire rib presents the appearance of an open-built beam, composed of three concentric arcs united by radial pieces. The spandrel-filling is formed of contiguous iron rings, of increasing diameters from the crown to the springing line, which rest upon the back of the curved rib, and support the roadway-bearers.

(C) Staines bridge was designed on the same plan as Wearmouth; but from a defect in the strength of its abutments, they successively yielded to the horizontal thrust, which in so flat an arch was very considerable.