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to adopt a curved axis, and of the want of workmen sufficiently conversant with the application of working drawings of a rather complicated character, by placing full centre cylindrical arches upon piers with a trapezoidal horizontal section. This structure, with the exception of some minor details in rather questionable taste, as the slight iron parapet railing, for example, presents an imposing aspect, and does great credit to the intelligence and skill of the engineer at the time of its construction, but recently launched in a new career. The fine single arch, known as the Carrolton Viaduct, on the Baltimore and Ohio railroad, is also highly creditable to the science and skill of the engineer and mechanics under whom it was raised. One of the largest bridges in the United States, designed and partly executed: in stone, is the Potomac Aqueduct at Georgetown, where the Chesapeake and Ohio canal intersects the Potomac river. This work, to which a wooden superstructure has been made, was built under the superintendence of Captain Turnbull of the U. S. Topographical Engineers.

595. The following table contains a summary of the principal details of some of the more noted stone bridges of Europe:

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596. 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, etc., 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 framework 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 coffer-work 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 with-in 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 framework 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 superstructure.

To give greater security to foundations, they may be surrounded 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, besides the loose stone, a grating of heavy timber, placed between and enclosing the piling, may be used 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.

III.

WOODEN BRIDGES.

597. Abutments. The abutments and piers of wooden bridges 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.

598. Wooden abutments may be formed by constructing what is termed a crib-work, which consists of large pieces of square timber laid horizontally upon each other, to form the upright or sloping faces of the abutment. These pieces are halved into each other at the angles, and are otherwise firmly connected together by diagonal ties and iron bolts. The space enclosed by the crib-work, which is usually built up in the manner just described, only on three sides, is filled with earth carefully rammed, or with dry stone, as circumstances may seem to require.

A wooden abutment of a more economical construction may be made, by partly imbedding large beams of timber placed in a vertical or an inclined position, at intervals of a few feet from each other, and forming a facing of thick plank to sustain the earth behind the abutment. Wooden piers may also be made according to either of the methods here laid down, and be filled with loose stone, to give them sufficient stability to resist the forces to which they may be exposed; but the method is clumsy, and inferior, under every point of view, to stone piers, or to the methods which are about to be explained.

599. The simplest arrangement of a wooden pier consists (Fig. 146) in driving heavy square or round piles in a single row, placing them from two to four feet apart. These upright pieces are sawed off level, and connected at top by a horizontal beam, termed a cap, which is either mortised to receive a tenon made in each upright, or else is fastened to the uprights by bolts or pins. Other pieces, which are notched and bolted in pairs on the sides of the uprights, are placed in an inclined or diagonal position, to brace the whole system firmly. The several uprights of the pier are placed in the direction of the thread of the current. If thought necessary, two horizontal beams, arranged like the diagonal pieces, may be added to the system just below the lowest water-level. In a pier of this kind, the place of the starlings is supplied by two in

clined beams on the same line with the uprights, which are termed fender-beams.

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Fig. 146-Elevation of a wooden pier.

a, a, piles of substructure.

b, b, capping of piles arranged to receive the ends of the uprights c, c, which support the string-pieces i, 1.

d, upper fender beam,

e, lower fender beam.

1, horizontal ties bolted in pairs on the uprights.

9, 9, diagonal braces bolted in pairs on the uprights.

A capping of the uprights placed under the string pieces,

A, roadway.

B, parapet.

600. A strong objection to the system just described, arises from the difficulty of replacing the uprights when in a state of decay. To remedy this defect, it has been proposed to

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drive large piles in the positions to be occupied by the uprights (Fig. 147), to connect these piles below the low-water level by four horizontal beams, firmly fastened to the heads of the piles, which are sawed off at a proper height to receive the

horizontal beams. The two top beams have large square mortises to receive the ends of the uprights, which rest on those of the piles. The rest of the system may be constructed as in the former case. By this arrangement the uprights, when decayed, can be readily replaced, and they rest on a solid substructure not subject to decay; shorter timber also can be used for the piers than when the uprights are driven into the bed of the stream.

601. In deep water, and especially in a rapid current, a single row of piles might prove insufficient to give stability to the uprights; and it has therefore been proposed to give a sufficient spread to the substructure to admit of bracing the uprights by struts on the two sides. To effect this, three piles (Fig. 148) should be driven for each upright; one just under its position, and the other two on each side of this, on a line perpendicular to that of the pier. The distance between the three piles will depend on the inclination and length that it may be deemed necessary to give the struts. The heads of the three piles are sawed off level, and connected by two horizontal clamping pieces below the lowest water.

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A square mortise is left in these two pieces, over the middle pile, to receive the uprights. The uprights are fastened together at the bottom by two clamping pieces, which rest on those that clamp the heads of the piles, and are rendered firmer by the two struts.

602. In localities where piles cannot be driven, the uprights of the piers may be secured to the bottom by means of a grating, arranged in a suitable manner to receive the ends of the uprights. The bed, on which the grating is to rest, having been suitably prepared, it is floated to its position, and sunk either before or after the uprights are fastened to it, as may be found most convenient. The grating is retained in its place by loose stone. As a farther security for the piers, the uprights may be covered by a sheathing of boards, and the spaces between the sheathing be filled in with gravel.

603. As wooden piers are not of a suitable form to resist

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