Chap. VIII.
451
*21 inches thick, laid header and stretcher alternately, and an average depth on the bed ofnot less than 3 feet C inches; the upper bed of each course to be dressed smooth and evenbefore the next course is commenced; the faces, beds, and joints of the exterior stone-work, as well as the setting, to be executed similar to that described for the abutmentsand piers.
“ The Centres. — There are to be four complete sets of centres, on which the arches are tobe turned; each set of centres is to consist of eight ribs properly braced together, and tobe executed according to the drawings; they are to be composed of the best Dantzic, Memel,Riga, Stettin fir timber, except the springing pieces, which are to be of elm, and the
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Fig. 453.
striking wedges are to be of oak of the best quality, entirely free from sap or wane, andcased on the upper and lower sides with fine sheet copper, one-tenth of an inch thick, and tobe well greased previous to being put in their places.
“ 'The iron work to be composed of the very best English iron, and to be executed asdescribed in the drawings; the supports or trussels for carrying the centre to be ot thebest fir timber of the quality above described; the trussels and centres when fixed tobe firm and strongly braced longitudinally and diagonally, so as to make them firm andsecure.
“ The covering of the centre for carrying the arch-stone is to be of good sound fir, halftimber, 7 inches thick, to be carefully laid, properly levelled, and firmly packed andwedged up to the curvature of the respective arches.
“ Arches. —The arches are to be of semi-ellipses. The centre arch to be 152 feet span inthe clear, and 29 feet 6 inches rise when finished ; the voussoirs, or arch-stones, to be 4 feet9 inches deep at the crown, and not less than 10 feet at the springing. The two archesnext the centre are to be 140 feet span each, and 27 feet 6 inches rise; the voussoirs atthe crown to be 4 feet 7 inches deep, increasing to not less than 9 feet at the springing; thetwo side arches to be 130 feet span each, and 24 feet 6 inches rise ; the voussoirs, or archstones, at the crown to be 4 feet 6 inches deep, increasing to not less than 8 feet 6 inches atthe swinging.
“ The whole of the arch-stones are to be headers, except where the engineer ^all allowstretchers to be used. They are all to be 18 inches thick at the intrados or inside of thecurve, and to increase in thickness to the extrados or outside of the curve, according to theradius of curvature at the respective places where they are to be used. Of these arch-stones none are to be less than 2 feet 6 inches wide, and none of them to overlap at thejoints in their respective courses less than 15 inches. The length of the arch-stone is to
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