(D) The bridge of Austerlitz is constructed on the same prinDe 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 con tinued up to its level. The roadway is on a level, the roadwaynearers 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. 38, 139,) the engineer has de

and

parted from the usual form of a circular segment aici adopted an elliptical segment. The following summary des..p

Fig. 138-Represents a longitudi-
nal section through a pier and its
cast iron standard of Lary bridge,
showing the connection of the
cast-iron framing and the stone
piers.

A, upper portion of pier.
B, standard.

C, panel of the curved rib.
D, lozenge spandrel-filling.

tion is extracted from the engineer's published account of this work:-"The arrangement of the design differs materially from other works of a similar nature: first, in the masonry of the piers

finishing at the springing course of the arches; secondly, in the curvilinear forms of the piers and abutments; and thirdly, in the employment of elliptical arches.

"The centre arch is 100 feet span, and rises 14 feet 6 inches;

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 bottom 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. Το 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.

SUSPENSION BRIDGES.

612. The use of flexible materials, as cordage and the like, tc form a roadway over chasms, and narrow water-courses, dates from a very early period; and structures of this character were probably among the first rude attempts of ingenuity, before the arts of the carpenter and mason were sufficiently advanced to be made subservient to the same ends. The idea of a suspended roadway, in its simplest form, is one that would naturally present itself to the mind, and its consequent construction would demand only obvious means and but little mechanical contrivance; but the step from this stage to the one in which such structures are now found, supposes a very advanced state both of science and of its application to the industrial arts, and we accordingly find that bridge architecture, under every other guise, was brought to a high degree of perfection before the suspension bridge, as this structure is now understood, was attempted.

With the exception of some isolated cases which, but in the material employed, differed little from the first rude structures, no recorded attempt had been made to reduce to systematic rules the means of suspending a roadway now in use, until about the year 1801, when a patent was taken out in this country for the purpose, by Mr. Finlay, in which the manner of hanging the chain supports, and suspending the roadway from it, are specifically laid down, differing, in no very material point, from the practice of the present day in this branch of bridge architecture. Since then, a number of structures of this character have been erected both in the United States and in Europe, and, in some instances, valleys and water-courses have been spanned by them under circumstances which would have baffled the engineer's art in the employment of any other means.

A suspension bridge consists of the supports, termed piers, from which the suspension chains are hung; of the anchoring masses, termed the abutments, to which the ends of the suspension chains are attached; of the suspension chains, termed the main chains, from which the roadway is suspended; of the vertical rods, or chains, termed the suspending-chains, &c., which connect the roadway with the main chains; and of the roadway.

613. As the general principles upon which flexible supports for structures should be arranged have already been laid down under the head of Framing, nothing more will be requisite, under the present head, than to add those modifications of the applications of these principles called for by the character of the structures in question.

614. Bays. The natural water-way may be divided into any number of equal-sized bays, depending on local circumstances,