Chap. XXV.

ON STONE BRIDGES.

1521

Of the Construction of the Tiers and Abutments. —The material of which these are formed isgranite or hard stone cut into headers and stretchers, the length, breadth, and depth varyingaccording to circumstances; excess of length should be avoided in either, which wouldrender them liable to fracture. Each course of stone around the outside should be laidheader and stretcher alternately; the latter should be from 18 inches to 2 feet in breadth,and the header should occupy one-third of the whole face* and be 3 feet or 4 feet in length;their upright joints should be perfectly square, at least 1 foot in from the face, and in nopart more than 1 inch in width. The interior or filling-in stones should be of equal heightto those on the outside, their upright joints not more than 1 inch in width, and theyshould break joint at least 12 inches ; the bed and upright joints of all the courses shouldbe flushed in with proper mortar, as they are laid, and the outer joint, when compressed, notshow more than £ of an inch : the whole should be run with grout. In many places it wasformerly the custom to fill in the interior of the piers with rubble-work, a practice thatcannot be too much reprobated, as such constructions can never be depended on.

Raising the Centres is the next operation after the abutments and piers are terminated,and is usually performed from a scaffold or temporary wooden bridge, constructed for the.purpose. The centres rest on wedges placed upon the bearing timbers, by which they canbe eased at any time; the wedges are in separate pairs, crossing each other under eachframe, or are fixed upon a piece of timber, extending across -the whole width of the soffite°f the intended arch, and passing between all the centre frames and the supporting framesor beams; the wedges are sometimes formed or fixed upon the timber, which is placedlongitudinally under the foot of each rib of the centre, resting on the supporting frame :when it is requisite to ease the centre, either the wedges or the timbers on which they arefixed are driven along each other, by which the wedges are made to take up a new position,and the centre descends very gradually. Beams mounted like a battering-ram are used toeffect this purpose.

The Construction of the Arches is commenced after the centres are fixed, and the masonryof the piers and abutments is carefully adjusted. When the voussoirs begin to bear uponthe centres, the latter are liable to a partial change of form, which must be provided forby loading them in various parts as the works proceed. Great care should be taken tomake each course of arch-stones point in the direction of the radius, and to do this effec-tually their thickness should be marked upon the outer ribs, and their line of directionupon the lower part of the beams : the courses should be laid equally on each side ofthe centre, an excess of weight on either side being likely to produce an alterationm the form, and the top must be loaded until the arch is complete: the key-stoneshould occupy its proper place, but not be driven home until the whole is finished ; all theback and end joints of the entire arch are then wedged up with slate, run with mortar,a nd left some time to dry. The masonry of the spandrills are brought up to aboutone quarter of the height of the arch, after which the centre may be removed. The softitesof the arch and all its joints are then pared, cleaned, and pointed with mortar; for thispurpose Perronet contrived a hanging scaffold at the bridge of Orleans, which could

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Fig. 2786.

SCAFFOLD FOR THE BRIDGE OF ORLEANS.

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Fig. 2787. -