Chap. VI.

FRANCE.

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removed by first taking out the irons at the ends, cables were attached to the rings of thechains, the bolts which held the upright pieces were withdrawn, and the stays inside wereremoved ; the water then entered ; the upright timbers, which have been described as dove-tailed at their ends, were driven downwards, and the wedges at their side being released,were drawn up by hand ; these were all taken out in succession, and the sides drawn out bymeans of three crabs placed in a barge, after which they were towed away to be applied tothe bottom of another caissoon for the next piers.

To relieve the centres after the arches were turned, Pe Cessart adopted the plan of cuttingaway a portion of the ends of the braces ; he removed the wood by making as it were threemortices, and leaving the whole centre supported upon two tenons at its ends, 2 inches inwidth only; these were afterwards cut away, and the whole dropped in a body.

Bridge of Tours , on the Loire , begun 1755 by Bayeux , is the longest bridge in France ,except those of St. Esprit and La Guillotiere, over the Rhone at Lyons.

It was finished in 1762, and consists of fifteen elliptical arches, rising a third, and 80 feetspan. The piers are 16 feet thick; the breadth of the bridge is-48 feet; it was foundedpartly by cofferdams, and partly by caissoons ; the length and water-way appear great whencompared with the neighbouring bridges; several accidents have nevertheless occurred.

The bottom is a sand bank from G to 10 feet thick, under which, and from 19 to 23 feetbelow low water, is a bed of tufa, in which the piles penetrate about a foot. This foundationappeared sufficiently secure ; the piles of one pier have, however, yielded to the superin-cumbent weight, and it has sunk about 3 feet, and gone over about as much. The archeswere demolished, and the pier removed, as well as another construction on the old foundation ;this accident is attributed to the bad quality of the timber of the piles, which bad remaineda long time underground, and were partly rotten.

A thaw then occasioned the sinking of three other piers, the ice formed a kind of harabove the bridge, and the water running rapidly under the arches on one side only carriedaway the sand between the piles, and laid them bare, and four arches fell in. The recon-struction of the piers was very difficult, the ruins of the first pier were removed with greatlabour and expense, and the foundations were consolidated and rendered secure ; the secondpier was still more difficult, the piles having gone over 3 feet on one side, and it was pro-posed to suppress the three last arches, which would have given a greater water-way thanwas necessary ; this plan was therefore not adopted, and a method was suggested for es-tablishing a cofferdam on the platform of a caissoon, which projected 4 feet 3 inchesbeyond the masonry, and the latter faees were thus restored. The interior was then to beemptied, and the courses and bottom would be easily removed, it being impossible to ar-range a cofferdam in the usual manner, from the great depth, and the ruins of the arches.

This method did not entirely succeed, because the piles had yielded unequally ; those onthe outside having resisted more than the others, the bottom was broken and the pier hadpassed through it. These injuries rendered the total exhaustion of the cofferdam im-possible ; the remaining courses were raised piecemeal by multiplying the machines andpumps, and the bottom of the caissoon being entirely destroyed, it was removed by thirty-six chains attached to different machines, worked by ninety-six men, who raised about91} tons at once; it was brought ashore by 150 casks and two boats.

Bridge of MouUns, on the Alder, was begun in 1756, and finished in 1764, under thedirection of M. de Regemorte, and consists of 13 elliptical arches, 64 feet span. The piersare 11 feet 8 inches thick ; the breadth is 42 feet 8 inches. The construction of this bridgewas very difficult; three bridges had been erected in the same situation in 35 years ;two, of stone, had been successively carried away. The last was by Mansard , itconsisted of three great elliptical arches supported by thick piers, which only gave a water-way of 377 feet. The length was 830 feet. The fall of these bridges was attributableboth to defects in their construction and want of width, a circumstance wliieh the nature ofthe bottom rendered very dangerous. The l>ottom is a coarse sand of great depth, into wliiehit is difficult to force piles of from 10 to 12 feet,although the Hoods frequently tear it up toa depth of 16 or 20 feet.

M. de Regemorte perceived the necessity for increasing the water-way considerably, inorder to diminish the velocity of the current, and the sequel has proved that what he gaveit was far from being too much. In 1790 the water rose to within 3 feet of the crown ofthe arch, and it was thought that if the right bank had not given way, it would have runsome risk. The increase of width did not remove all fear for the safety of the bridge ; itwas thought that the least obstacle in one arch might occasion the soil to he carried awayfrom under the others ; to avoid this a framework was constructed under the bridge, 6 feet6 inches thick, having the upper surface 3 feet 3 inches below the level of the water : thebreadth is 111 feet 6 inches. This precaution set all anxiety at rest. There are few caseswhere the arrangements have been made so perfectly in accordance with the natural circum-stances of the place, or have been worked out in so intelligent a manner. The details ofthe construction have been published by M. de Regemorte.

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