Chap. VI.

FRANCE.

279

Bridye of the Military School , formerly bridge of Jena, on the Seine at Paris , is situatedin the prolongation of the axis of the Military School and of the Champ de Mars . Theerection of a bridge in this situation was decided on in 1806,the arches of which were to have been of cast-iron ; but in1808, Lamande obtained permission to execute it on thepresent design, which is composed of five equal segmentalarches, 91 feet 10 inches span, and 10 feet 9 inches high.

The thickness of the arches is 4 feet 8 inches; their spring-ings are 20 feet 1 inch above the surface. The piers arc9 feet 10 inches thick, and have semicircular starlings; theywere founded by caissoons on piles cut off 5 feet 4 inchesbelow the surface, and 3 feet 9 inches apart. The abutmentsare formed of a mass 49 feet 2 inches thick, and 59 feet wide,

13 feet 1 inch high at the level of the springing, of roughstone, bonded horizontally and vertically. The width of thebridge is 46 feet, that of the road-way 30 feet 7 inches,and of each footway 7 feet 10 inches ; it is crowned by alevel cornice supported on consoles. There are at theentrance four pedestals for equestrian statues.

The arches were constructed on centres formed by threecourses of principal timbers, disposed according to the systemof M. Perronet, but strengthened by two rows of piles.

Tlie motion of the arch during the placing of the voussoirswas scarcely perceptible. The centres were easily struckby first removing the struts applied against the piers, andthen the intermediary piles. The total sinking of the archwas at most 6 inches.

The semicircular arch having its springing upon the hori-zontal line, or diameter, its load or weight acts perpendicular,or in the direction of its gravitation, and it has not that ten-dency to spread and exert a lateral thrust, and consequentlydoes not require such solid abutments, as that which is theportion of a circle or a segment; but the semicircle cannot bealways introduced, as its height would require that all the ap-proaches should be elevated, and where the banks of a river,as in the present instance, are low, it is not very practicable.

The segmental arch obviates this objection, and also canbe executed with less material, but the lateral pressure beingaugmented, more consideration is required for the strengthof the abutments; in an arch of this description, the thrustincreases as the angular measure of the length of the arcdiminishes. The arches of the bridge of Jena are verynearly those produced by the side of a hexagon, and itsportion of the circle: they seem to have been set out bystriking a curve round one side of an equilateral triangle,which is as flat a segment as has hitherto been introduced.

The horizontal thrust of such an arch requires a provisionnot only that it may bear its own weight, but also anywhich may be added to it; the piers may be made light,where the arches comprise only sixty degrees, as the weight iscarried to the extremities. In the five arches of this ex-ample the abutments received the greatest attention; theirthrust, it was considered, acted upon the two extremities ofthe bridge, where the masonry was sufficiently strong to resistit. The flatter the arch or the greater the segment, theless width is required to be given to the piers, but themasonry saved in the piers must be given at the abutments.

During the occupation of Paris by the foreign powers in1814, the Prussian army were desirous of destroying amonument consecrated to one of Napoleon ’s most brilliantvictories, and preparations were made for undermining thelower part of the piers; but this act of barbarism was coun-termanded, and the evidences of it have since been effaced.

The bridge of the Ecole Militaire combines both sim-plicity and elegance, and may be considered to possess thehighest degree of beauty that can be imparted to con-structions of this description.

t 4

Fig. 288.