Chai*. VI.
FRANCE.
281
The centres for the arches were composed of fifteen pair of principals, each of whichformed as it were a single voussoir ; the pieces were put together on two boats united andraised in a body by means of crabs placed on the scaffold of the piers. By this simple andeconomical process the whole centre of an arch 86 feet 11 inches in span was placed in threedays.
Before the arches were commenced the piers were proved by loading each with a weightof 3924 tons, in the form of a pyramid composed of stone blocks and rubble; the weight ofthe arches was also diminished by lightening the internal mass of the tympanum andevery portion that was not necessary for the stability of the extrados, by constructinggalleries the lengthways of the bridge ; these again were traversed breadthways by vaultsof the same form and span. These precautions effected a diminution of 3280 cubic feetin the weight borne by the piers; greater facilities were thus afforded for examining theinterior of the bridge, ascertaining if any filtrations or settlements had taken place, andmaking any requisite repairs.
Squared stone and brick were both employed in the masonry ; the archivolts are entirelyof freestone, and they are united by rows of voussoirs in horizontal lines, so as to formcaissoons filled with brick; the heads are relieved by openings, in order to facilitate thepassage of the various substances carried down by the rapid currents of floods, which,without great precaution, might seriously injure the arches. The Garonne rises occasionallyto very great heights; in 1770, the increase was more than 23 feet above low water; thisrequired an extraordinary elevation to be given to the bridge, which, added to the ne-cessity of uniting two banks sometimes 3 feet under water, have prevented the causewayfrom being made entirely on the level, which is only maintained over the seven middlearches and half of the two next; the remaining portion has a fail of about one ineighty.
Quay walls, 574 feet long, are retained on each side of the abutments, at the ex-tremities of which steps descend to the river. The river forming too great an elbow just atthe bridge a dyke has been raised on the right bank above bridge, of rubble work, 16,404feet long; in some places 46 feet high, and more than 98 feet base. Its effects were suchthat in a few months the bar called “ la Manufacture” was entirely removed. The bed ofthe Garonne was deepened on the left bank, and the property on the right bank increasedby a clayey deposit of 100 hectares in surface, of which several portions are now coveredwith vegetation and plantations, and some are under cultivation.
Since 1820, a diving-bell has been used for any repairs that might be required in therubble work, and its general stability has thus been satisfactorily ascertained.
Bridge of Libourne , on the Dordogne , consists of nine semicircular arches, each of 65 feet7 inches span, resting on eight piers 12 feet 6 inches thick at the springing. All that hasbeen said of the bridge of Bordeaux , both as to form and system of construction, applies tothat of Libourne . The piling, the frame, the rubble work, the caissoons, the centres, thevoussoirs, the mixture of brick and stone, the voiding of the upper mass of the piers, thedouble slope from the middle, and the architectural decoration, were projected and executedon the same principles. The roadway on this bridge like that at Bordeaux is formed by abrick arch carrying masonry laid in hydraulic mortar, covered with a dressing of brokenstones.
The footways at Bordeaux are paved with small pebbles of different colours laid in con-crete, forming lozenged-shape compartments. Those of Libourne are paved with bricklaid flat in mortar. Each entrance of both bridges has two lodges, one for receiving toll,the other for the police. Their architecture is simple; those at Bordeaux are ornamentedwith a porch formed by two pilasters and two columns.
The foundation piles at Bordeaux and Libourne were shod, for the first time, with conicalcast-iron shoes, with a wrought axis in the centre; this method has since been employed inseveral great hydraulic operations in France , on account of the resistance it affords, and itsgreat economy.
The two bridges were built from the designs and under the direction of M. C. Des-champs, inspector-general of the Ponts et Chaussees.
The Pont du Louvre , or des Arts, was the first iron bridge constructed in France underthe auspices of M. de Cessart, inspector-general of the Ponts et Chaussees, and its executionwas confided to M. Didon. It is composed of nine arches, each measuring between thepiers 56 feet 8 inches, the piers being 6 feet 6 inches in width.
The arches are composed of five ribs, placed about 6 feet 8 inches apart; the ribs are cast-iron, 6 inches deep, and about 3 inches wide, formed in two thicknesses. The chord of thearch is 50 feet 8 inches, and its versed sine 12 feet.
When iron was first applied to bridges in France , its properties were by no meansunderstood by the engineers; they, in general, attributed to it a greater strength than itpossessed, and consequently the first constructions in this material were very faulty anddefective: what had been executed in England they greatly admired, and were anxious toimitate, but the cost of iron being much higher in France , they adopted too strict an