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of high water, would have appeared rather heavy, as the greater part of the soffit, at this period, would have been under water,it gave the bridge a lighter appearance during the epochs of high water; and, second, as the obstruction to the free flow of the water from the spandrels would be very considerable at the same periods, the funnel form given to the soffit at the heads partially remedied this inconvenience.

The axis of the roadway, the cornice, and all the corresponding architectural lines were made horizontal, a feature in bridge architecture which the reputation of Perronet has since rendered classical; and to obtain which points truly essential conditions have in some more recent structures been sacrificed.

The abutments are 32 feet thick at the springing lines, and the piers but 13.8 feet at the same point, giving an example of judicious boldness combined with adequate strength, on scientific principles, which had been partially lost sight of by preceding engineers in designing this part of bridges.

The centres of the Neuilly bridge were designed upon the faulty principle of concentric polygonal frames. Perronet was aware of the inconveniences of this combination, and in no part of the construction of the bridge than in this was more sagacious forethought displayed by him, in providing for foreseen contingencies, nor greater resources and skill in remedying those which could not have been anticipated. An oversight, rather more serious in its consequences, was committed in widening the natural water-way of the river where the bridge was erected; the effect of this has been a gradual deposition near the bridge, and an obstruction of the navigable channels.

The bridge of Neuilly is a noble monument of the genius and practical skill of its engineer. The style of its architecture, both as a whole and in its several parts, is imposing and in the best taste.

(B) This bridge was built after the designs of Perronet. Seduced by a thorough knowledge of the capabilities of his art, the engineer was led, in planning this structure, into the error of sacrificing apparent strength, for the purpose of producing great boldness and lightness of design. This he effected by placing very flat segment arches upon piers formed of four columns; the two, forming the starlings, being united to the two adjacent by a connecting wall, an interval being left between the two centre columns. The diameters of the columns are 9.6 feet, with the same interval between them.

The engineer who constructed the bridge, apprehensive apparently for its safety, introduced into the courses of the piers and of the arches a large quantity of iron ties and cramping pieces, a measure of precaution which, if necessary, ought to have con

demned the original designs, although supported by the high authority of Perronet, and caused others to be substituted for them.

(C) This bridge, now designated as the Pont de l'Ecole Militaire, from its locality, and the bridge of Rouen, are built upon nearly the same designs. The former is a model of architectural taste and of skilful workmanship. Its horizontal architectural lines, its fine cornice, copied from that of the temple of Mars the Avenger, and the sculptured wreath on its spandrels, form a whole of singular beauty.

(D) This bridge, designated when first built as the Strand Bridge, is worthy of the great metropolis in which it is placed. The engineer, influenced perhaps by other examples of the same character in the vicinity of this structure, has placed small colthe starlings, which support recesses with seats for foot-passengers, and has thus, in no inconsiderable degree, deprived the bridge of that imposing character which its massiveness, and the excellent material of which it is built, could not otherwise have failed to produce.

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(E) This fine elliptical arch is, in some respects, built in imitation of the Neuilly bridge, with a funnel-shaped soffit. Its general architectural effect is heavy, and its mere ornamental parts are in questionable taste. The details of its construction are alike monuments of the eminent professional skill, and of the truthfulness of character of the great engineer who planned and superintended it. In his narrative of the work, Mr. Telford takes blame to himself for oversights and unanticipated results, in which the scrupulous care that he conscientiously brought to every undertaking committed to him is unwittingly thrown into bolder relief, by the very confession of his failures; and a lesson of instruction is conveyed, more pregnant with important consequences to the advancement of his profession than the recording of hundreds of successful instances only could have furnished.

(F) This noble work of Sir John Rennie must ever rank among the master-pieces of bridge architecture, in every point by which this class of structures should be distinguished. For boldness, strength, simplicity, massiveness without heaviness, and a happy adaptation of design to the locality, it stands unrivalled. The beauty which is generally recognised in a level bridge has, in this, been judiciously sacrificed to a well-judged economy; and the artificial approaches have thus been accommodated to the existing, by decreasing the dimensions of the arches from the centre to the two extremities. The square plain buttresses, which rise above the starlings and support the recesses for seats, are of farther obvious utility in strengthening the head-walls, which, at these points, are of considerable height; and they also

produce, in this case, a not unpleasing architectural effect, in separating the unequal arches, without impairing the unity of the general design.

(G) This is the boldest single arch of stone now standing, and is a splendid example of architectural design and skilful workmanship. The soffit of the arch is made slightly funnel-shaped, which gives the bridge an air of almost too great boldness. The cornice, which is copied from the same model as that of the bridge of Jena; the convex cylindrical-shaped wing-walls, which give an approach of 144 feet between the parapets; with the other architectural accessories, have made this bridge a model of good taste for imitation under like circumstances. From the omission of a usual architectural member, there is perhaps a slight feeling of nakedness produced on the mind of the rigid connoisseur in art, on first seeing this structure, and its beauty is in some degree marred by this want.

The abutments of this bridge are 40 feet thick at the foundations, and, besides the wing-walls, are strengthened by two counterforts 20 feet long and 10 feet wide.

(H) The span of this arch is the widest on record. For architectural effect this bridge presents but little to the eye that is commendable; for this the engineer who superintended it is hardly responsible, except so far as, from professional sympathy and respect for a deceased member of the profession, he was led to adopt the designs of another. The abutments form a continuation of the arch; and the other details of the construction throughout exhibit that thorough acquaintance with their art for which the Hartleys, father and son, are well known to the profession.

582. The practice of bridge building is now generally the same throughout the civilized world. In France, the method of laying the foundations by caissons has, in most of their later works, been preferred by her engineers to that of coffer-dams; and in the superstructure of their bridges the French engineers have generally filled in, between the arches and the roadway, with solid material. In some of these bridges, as in that of Bordeaux, where apprehension was felt for the stability of the piling, a mixed masonry of stone and brick was used, and the roadway was supported by a system of light-groined arches of brick. Among the recent French bridges, presenting some interesting features in their construction, may be cited that of Souillac over the Dordogne. The river at this place having a torrent-like character, and the bed being of lime-stone rock with a very uneven surface, and occasional deep fissures filled with sand and gravel, the obstacle to using either the caisson, or the ordinary coffer-dam for the foundations, was very great. The engineer, M. Vicat, so well known by his researches upon mortar, &c., devised, to obviate these

difficulties, the plan of enclosing the area of each pier by a cofferwork accurately fitted to the surface of the bed, and of filling this with beton to form a bed for the foundation courses. This he effected, by first forming a frame-work of heavy timber, so arranged that thick sheeting-piles could be driven close to the bottom, between its horizontal pieces, and form a well-jointed vessel to contain the semi-fluid material for the bed. After this cofferwork was placed, the loose sand and gravel was scooped from the bottom, the asperities of the surface levelled, and the fissures were voided, and refilled with fragments of a soft stone, which it was found could be more compactly settled, by ramming, in the fissures, than a looser and rounder material like gravel. On this prepared surface, the bed of beton, which was from 12 to 15 feet in thickness, was gradually raised, by successive layers, to within a few feet of the low-water level, and the stone superstructure then laid upon it, by using an ordinary coffer-dam that rested on the frame-work around the bed. In this bridge, as in that of Bordeaux, a provisional trial-weight, greater than the permanent load, was laid upon the bed, before commencing the superstruc

ture.

To give greater security to their foundations, the French usually

surround them with a mass of loose stone blocks thrown in and allowed to find their own bed. Where piles are used and project some height above the bottom, they, in some cases, use, besides the loose stone, a grating of heavy timber, which lies between and encloses the piling, to give it greater stiffness and prevent outward spreading. In streams of a torrent character, where the bed is liable to be worn away, or shifted, an artificial covering, or apron of stone laid in mortar, has, in some cases, been used, both under the arches and above and below the bridge, as far as the bed seemed to require this protection. At the bridge of Bordeaux loose stone was spread over the river-bed between the piers, and it has been found to answer perfectly the object of the engineer, the blocks having, in a few years, become united into a firm mass by the clayey sediment of the river deposited in their interstices. At the elegant cast-iron bridge, built over the Lary near Plymouth, resort was had to a similar plan for securing the bed, which is of shifting sand. The engineer, Mr. Rendel, here laid, in the first place, a bed of compact clay upon the sand bed between the piers, and imbedded in it loose stone. This method, which for its economy is worthy of note, has fully answered the expectations of the engineer.

The English engineers have greatly improved the method of centring, and, in their boldest arches, any settling approaching that which the French engineers usually counted upon, on striking their centres, would now be regarded as an evidence of great de

fect in the design, or of very unskilful workmanship. They have generally, in their recent bridges, supported their roadway either upon flat stones, resting on light walls built parallel to the heads, or else upon light cylindrical arches laid upon piers having the same direction. In the preparation for laying the beds of their foundations, they have generally preferred the coffer-dam to any other plan, although in many localities the most expensive, on account of the greater facility and security offered by it for carrying on the work. They have not, until recently, made as extensive an application of beton as the French for hydraulic purposes, and, from having mostly used what is known as concrete among their architects, have met with some signal failures in its employment for these purposes.

WOODEN BRIDGES.

583. A wooden bridge consists of three essential parts: 1st, the abutments and piers which form the points of support for the bridge frame; 2d, the bridge frame which supports the superstructure between the piers and abutment; 3d, the superstructure, consisting of the roadway, parapets, roofing, &c.

584. The abutments and piers may be either of stone, or of timber. Stone supports are preferable to those of timber, both on account of the superior durability of stone, and of its offering more security than frames of timber against the accidents to which the piers of bridges are liable from freshets, ice, &c.

585. The forms, dimensions, and construction of stone abutments and piers for wooden bridges will depend, like those for stone bridges, upon local circumstances, and the kind of bridgeframe adopted. If the bridge-frame is so arranged that no lateral thrust is received from it by the piers, the dimensions of the latter should be regulated to support the weight of the bridge-frame and its superstructure, and to resist any action arising from accidental causes, as freshets, ice, &c. The forms and dimensions of the abutments, under the like circumstances, will be mainly regulated by the pressure upon them from the embankments of the approaches.

586. If the bridge-frame is of a form that exerts a lateral pressure, the dimensions of the abutments and piers must be suitably adapted to resist this action, and secure the supports from being overturned. Abutment-piers may be used with advantage in this case, as offering more security to the structure than simple piers, when a frame between any two supports may require to be taken out for repairs. The starlings should in all cases be carried above the line of the highest water-level, and the portion of the pier above this line, which supports the roadway bearers, may be built with plane faces and ends.

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