256

HISTORY OF ENGINEERING.

Book I.

believe, the first to use dredging machines, which he applied in this instance to prepare theearth on which the pier was to be built, and sunk a large barge filled with stones, sur-rounding it with piles and a jetty. A kind of chest was then sunk containing courses ofstone cramped together, which were rendered more secure by long guarding piles, and thespace between the walls was filled with rubble and puzzolana, then used for the first timein Paris .

The foundation was loaded by a weight much greater than what it would have to sustainafter the bridge was built, and as at the end of six months’ trial it only indicated a com-pression of three-fourths of an inch, which was attributed to the contraction of the mortar,the pier and two collateral arches were carried up in perfect security. In the formerwere deposited all the inscriptions and medals.

The bridge consists of five elliptical arches, from 68 feet 9 inches to 77 feet 2 inches span :the breadth is 55 feet 9 inches: the thickness of the arches 5 feet 6 inches. They arearranged with more regularity than in any of the preceding bridges of Paris . The twoentrances are widened by forming over half of the last arches recesses, supported on trompes,which greatly facilitates the passage of carriages. The river is narrower at this point thanat any other, consequently the current has a greatet depth and rapidity, and much of thebed is every year carried away, to prevent the evil results of which, materials are con-tinually thrown in.

The cost of this erection was 742,000 livres.

Bridge of Blois , on the Loire , was the first built after the establishment of the Pontset Chaussees. It was begun in 1720 by Pitrou on Gabriel’s designs. It consistsof 11 elliptical arches, from 54 feet 9 inches to 86 feet 4 inches span. The starlings arein the shape of an equilateral triangle up the stream, and a semi-hexagon down it. Thethree first piers are 16 feet thick, the two centre 17 feet, and the two others 24 feet6 inches; and these doubtless were intended for abutments in case some of the archesshould be carried away. The other piers, however, appear thick enough to resist thepressure. The slope of the paving is about 1 in 200: it is too great: the water risesto the key of the lesser arches, while in the middle arches a considerable space remainsuseless. This bridge appearing after its construction not to give sufficient water-way tothe Loire , a new channel for the water has been opened above.

Bridge of Compeigne , on the Oise , was built in 1733 by Hupeau, engineer of the Pontset Chaussees. It consists of three elliptical arches rising one-third: two of them are 70feet 3 inches in span, and the other 76 feet 9 inches. It appears that in this bridge starlingswere used for the first time, whose plan is a triangle formed by two arcs, each equal to one-sixth of the circumference.

Bridge of Tetes, on the Durance, was built, in 1732, by Ilenriana, a military engineer, toconnect the road between Brandon and Tetes. The arch is very nearly semicircular, andhas a span of 124 feet 8 inches. The voussoirs are alternately 4 feet 9 inches and 5 feet4 inches in length. The breadth in the middle is only 16 feet; but it is widened towardsthe entrance, and a considerable talus is given to the abutments, which no doubt adds tothe stability of the edifice; and we have many other examples, still it appears more ad-vantageously applied to timber than to stone bridges.

Bridge of Cravant, on the Yonne . This was built by M. Advyne in 1760, and has threeelliptical arches, rising a third, from 57 feet 5 inches to 64 feet span. The thickness of thepiers is 12 feet 9 inches.

Fig. 261. CRAVANT BRIDGE.

Bridge of Charities on the Moselle, built in 1740, consists of ten semicircular arches 64feet, and two lesser semicircular arches 34 feet 2 inches span. As the stream does not risehigher than 7 feet 5 inches, the water-way is evidently too great. The lesser arches areseparated from the rest of the bridge by massive piers 128 feet thick. The starlings are ofsquared stone, but the arches and spandrils of rusticated rubble.

Bridge of Tout, on the Moselle, built in 1754 by Gourdain, has seven elliptical archesfrom 48 feet to 54 feet 6 inches span: it is constructed of squared stone.