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.

(D) The bridge of Austerlitz is constructed on the same principle as the two last, and produces a light and pleasing architectural effect. Each curved rib consists of 21 voussoir-shaped panels, about 4 feet in depth. The spandrel-fillings present the appearance of a continuation of the curved rib outwards, to form a support for the roadway-bearers. The piers are terminated at the springing lines of the curved rib, and are at this point 13 feet thick; the roadway above them being supported by the ribs continued up to its level. The roadway is on a level, the roadwaybearers and flooring being of timber.

(E) In this structure the curved rib is formed of solid panels. The spandrel-fillings consist of vertical shafts united by cross pieces. The piers are built up to support the roadway-bearers; they are 13 feet thick at the springing line. The entire width of the bridge is 36 feet, the carriage-way occupying 25 feet.

(F) In this bold structure, the width of each of the two extreme bays is 210 feet. The curved rib is composed of thirteen solid panels, each of which is 23 inches thick, and has a rim, or flanch around it about 4 inches broad. The rib is 6 feet deep at the crown and 8 feet at the spring. The spandrel-filling is composed of lozenge-shaped panels with vertical joints; they are secured to the back of the curved rib and support the roadway-plates. The curved ribs are connected by tie-plates inserted between the joints of the voussoirs; and they are braced by feathered diagonal braces. The piers are 24 feet thick at the springing line, and are built up to the level of the roadway-plates. The width of the carriage-way is 25 feet, and that of each of the footpaths 7 feet.

(G) This bridge presents a very light and elegant appearance; the panels of the curved rib being cast with open curvilinear spaces, which divide the panel into several rectangular-shaped figures, with solid sides and diagonals. Each rib consists of twelve panels. The depth of the ribs is 3 feet. The thickness of the two exterior ribs is 2 inches, that of the four interior 2 inches. The ribs are connected by grated tie-plates between the panel-joints, and they abut against springing plates which are 3 feet wide and 4 inches thick. The roadway-bearers and road-plates are of cast iron. The spandrel-filling is composed of lozenge-shaped panels, the sides of the lozenges being feathered, and tapering from the middle to the extremities. The ribs of the bridge-frame are connected and braced in the usual manner. The road-bearers are laid lengthwise upon the ribs, to which they are firmly secured, and they are covered with iron road-plates, upon which the road-covering rests. The free roadspace is 24 feet.

(H) In this structure, (Figs. 138, 139,) the engineer has de

parted from the usual form of a circular segment arch, and adopted an elliptical segment. The following summary descrip

the thickness of the piers, where smallest, being 10 feet. The arches adjoining the centre are 95 feet span each, and rise 13 feet 3 inches. The piers, taken as before, are each 9 feet 6 inches thick. The extreme arches are each 81 feet span, and rise 10 feet 6 inches. The abutments are, in their smallest dimensions, 13 feet thick, forming at the back a strong arch abutting against the return-walls to resist the horizontal thrust. The ends of the piers are semicircular, having a curvilinear batter on the sides and ends formed with a radius of 35 feet, and extending upward from the level of high water to the springing course, and downward to the level of the water at the lowest ebb. The front of the abutments have a corresponding batter.

"The roadway is 24 feet wide, supported by 5 cast-iron equidistant ribs. Each rib is 2 feet 6 inches in depth at the springing, and 2 feet at the apex, by 2 inches thick, with a top and botton flange of 6 inches wide by 2 inches thick, and is cast in 5 pieces; their joints (which are flanged for the purpose) are connected by screw-pins with tie-plates equal in length to the width of the roadway, and in depth and thickness to the ribs; between these meeting-plates the ribs are connected by strong feathered crosses, or diagonal braces, with screw-pins passing through their flanges and the main ribs. The springing-plates are 3 inches thick, with raised grooves to receive the ends of the ribs, which have double shoulders. These plates are sunk flush into the springing course of the piers and abutments, which, with the cordon and springing course, are of granite. The pierstandards and spandrel-fillings are feathered castings, connected transversely by diagonal braces and wrought-iron bars passing through cast-iron pipes, with bearing-shoulders for the several parts to abut against. The roadway-bearers are 7 inches in depth by 1 thick, with a proportional top and bottom flange; they are fastened to the pier-standards by screw-pins through sliding mortises, whereby a due provision is made for either expansion or contraction of the metal; the roadway-plates are of an inch thick by 3 feet wide, connected by flanges and screwpins, and project 1 foot over the outer roadway-bearers, thus forming a cornice the whole length of the bridge.

"The adoption of these forms for the piers and arches, in unison with the plan of finishing the piers above the springing course with cast iron instead of masonry, has, as I had hoped, given a degree of uniform lightness combined with strength to the general effect, unobtainable by the usual form of straight-sided piers carried to the height of the roadway, with flat segments of a circle for the arches."

(I) The curved ribs of this bridge are tubular, the cross section of the tube being an ellipse, the transverse axis of which is

2 feet 6 inches, and the conjugate about 1 foot 4 inches. Each rib consists of eleven pieces, which are shaped and connected as described under the head of Framing. The spandrel-fillings are formed of contiguous cast-iron rings which rest upon the ribs, and support the longitudinal roadway-bearers. The ribs are tied and braced nearly in the usual manner. The flooring upon which the road-covering is laid is of timber. The piers are built up to receive the roadway-bearers.

The system of M. Polonceau presents a very light and elegant form of cast-iron bridge. The inventor claims for it more economy than by the ordinary combinations, and also more lightness combined with adequate strength. It has been objected to this system that it is defective in rigidity; this the inventor seems disposed to regard as an advantage, and has preferred the spandrel-filling of rings partly on this account, because their elasticity is favorable to a gradual yielding and restoration of form in the parts.

611. Effects of Temperature on stone and cast-iron Bridges. The action of variations of temperature upon masses of masonry, particularly in the coping, has already been noticed. The effect of the same action upon the equilibrium of arches was first observed by M. Vicat in the stone bridge built by him at Souillac, in the joints of which periodical changes were found to take place, not only from the ranges of temperature between the seasons, but even daily. Similar phenomena were also very accurately noted by Mr. George Rennie in a stone bridge at Staines.

From these recorded observations the fact is conclusively established, that the joints of stone bridges, both in the arches and spandrels, are periodically affected by this action, which must consequently at times throw an increased amount of pressure upon the abutments, but without, under ordinary circumstances, any danger to the permanent stability of the structure.

When iron was first proposed to be employed for bridges, objections were brought against it on the ground of the effect of changes of temperature upon this metal. The failure in the abutments of the iron bridge at Staines was imputed to this cause, and like objections were seriously urged against other structures about to be erected in England. To put this matter at rest, observations were very carefully made by Sir John Rennie upon the arches of Southwark bridge, built by his father. From these experiments it appears that the mean rise of the centre arch at the crown was about th of an inch for each degree of Fahr., or 1.25 inches for 50° Fahr. The change of form and increase of pressure arising from this cause do not appear to have affected in any sensible degree the permanent stability either of this structure, or of any of a like character in Europe.