between the two centre ribs, or, when there are two exterior ribs, between them.

617. In frames which exert a lateral pressure against the abutments and piers, the lowest points of the framework should be so placed as to be above the ordinary high-water level; and plates of some metal should be inserted at those points, both of the frame and of the supports, where the effect of the pressure might cause injury to the woody fibre. 618. The roadway usually consists of a simple flooring formed of cross joists, termed the roadway-bearers, or floorgirders, and flooring-boards, upon which a road-covering either of wood or stone is laid. A more common and better arrangement of the roadway, now in use, consists in laying longitudinal joists of smaller scantling upon the roadwaybearers, to support the flooring-boards. This method preserves more effectually than the other the roadway-bearers from moisture. Besides, in bridges which, from the position of the roadway, do not admit of vertical diagonal braces to stiffen the framework, the only means, in most cases, of effecting this object is in placing horizontal diagonal braces between each pair of roadway-bearers. For like reasons,

stone_road-coverings for wooden bridges are generally rejected, and one of plank used, which, for a horse-track, should be of two thicknesses, so that, in case of repairs, arising from the wear and tear of travel, the boards resting upon the flooring-joists may not require to be removed. The footpaths consist simply of a slight flooring of sufficient width, which is usually detached from and raised a few inches above the roadway surface.

619. When the bridge-frame is beneath the roadway, a distinct parapet will be requisite for the safety of passengers. This may be formed either of wood, of iron, or of the two combined. It is most generally made of timber, and consists of a hand and foot rail connected by upright posts and stiffened by diagonal braces. A wooden parapet, besides the security it gives to passengers, may be made to add both to the strength and stiffness of the bridge, by constructing it of timber of a suitable size, and connecting it firmly with the exterior ribs.

620. In bridge-frames in which the ribs are above the roadway, a timber sheathing of thin boards will be requisite on the sides, and a roof above, to protect the structure from the weather. The tie-beams of the roof-trusses may serve also as ties for the ribs at top, and may receive horizontal diagonal braces to stiffen the structure, like those of the roadway

bearers. The rafters, in the case in which there is no centre rib, and the bearing, or distance between the exterior ribs, is so great that the roadway-bearers require to be supported in the middle, may serve as points of support for suspension pieces of wood, or of iron, to which the middle point of the roadway-bearers may be attached.

621. The frame and other main timbers of a wooden bridge will not require to be coated with paint, or any like composition, to preserve them from decay when they are roofed and boarded in to keep them dry. When this is not the case, the ordinary preservatives against atmospheric action may be used for them. The under surface and joints of the planks of the roadway may be coated with bituminous mastic when used for a horse-track; in railroad bridges a metallic covering may be suitably used when the bridge is not traversed by

horses.

622. Wooden bridges can produce but little other architectural effect than that which naturally springs up in the mind of an educated spectator in regarding any judiciouslycontrived structure. When the roadway and parapet are above the bridge-frame, a very simple cornice may be formed by a proper combination of the roadway-timbers and flooring, which, with the parapet, will present not only a pleasing appearance to the eye, but will be of obvious utility in covering the parts beneath from the weather. In covered bridges, the most that can be done will be to paint them with a uniform coat of some subdued tint. At best, from their want of height as compared with their length, covered wooden bridges must, for the most part, be only unsightly, and also apparently insecure structures when looked at from such a point of view as to embrace all the parts in the field of vision; any attempt, therefore, to disguise their true character, and to give them by painting the appearance of houses, or of stone arches, while it must fail to deceive even the most ignorant, will only betray the bad taste of the architect to the more enlightened judge.

and

The art of erecting wooden bridges has been carried to great perfection in almost every part of the world where timber has, at any period, been the principal building material at the disposal of the architect; but iron at the present day is fast taking the place of wood in the more

bridges.

important 623. The following Table contains the principal dimensions of some of the most celebrated American and European wooden bridges:

624. 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.

625. 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.

626. 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 piece, 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 further on. 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 roadway-bearers 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 combina tion of cast or wrought iron, in keeping with the general style of the structure.

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

in

The French engineers have not only followed the lead set them by the English, but have taken a new step, 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 Polon ceau'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 :

628. Iron Arches. Cast-iron arches may be used for the same objects as those of timber. The frames for these purposes consist of several parallel ribs of uniform dimensions, which are cast into an arch form, the ribs being connected by horizontal ties, and stiffened by diagonal braces. The weight of the superstructure is transmitted to the curved ribs in a variety of ways; most usually by an open castiron beam, the lower part of which is so shaped as to rest upon the curved rib, and the upper part suitably formed for the object in view. These beams are also connected by ties, and stiffened by diagonal braces.

Each rib, except for narrow spans, is composed of several pieces, or segments, between each pair of which there is a