18. Centres, which are certain arrangements of timber framing, used to support the brick or stone work of arches when these are in course of construction, and are therefore purely temporary structures, and are taken down after the brick or stone arch has firmly settled. The taking down of the centres from beneath the brick or stone work is called striking the arch, and to aid this a certain arrangement is made use of, which will be presently explained. In fig. 272 a simple form of centre for a semicircular arch is shown ', in this the lines a b, c d show the side walls terminating the width of opening, e c, which is to be finished with a semicircular arch at top. When the walls are at the height of the line a c, which is the springing line of the arch, the "centreing" or "centre" is erected. The upper part of the arrangement of timbers which is to support the arch is framed in a way more or less complicated, according to the width of the opening, e c, in the span of the arch. In fig. 272 this part is simple, being formed of two planks, f g, the outer edges of which are cut to the circle of the arch, this being described from the point h in the line a c. These two pieces butt at their upper termination at i, and are nailed at their lower end j, as in fig. 273, and k, to a cross-piece 11. In some cases this cross-piece is omitted, the ends j and k simply resting on the pieces m and n, which run across the walls in the direction of their thickness, and at right angles to the piece b b. In the arrangement shown, the pieces or "centre" proper, f g and b b, are supported by the cross-pieces or cushion timbers m and n. These are again supported by the upright posts o and p, the lower ends of which rest upon the ground, in the case of arches being built on the ground floor of a building, and upon a sill in the wall in the case of an arch being built on an upper storey. To prevent the feet of the posts penetrating the ground or soil, they rest upon a piece of timber or sill, as a a, in fig. 275. The cross-pieces, m and n, pass through, as above stated, the opening across the thickness of the wall, a b, b c, and are terminated at the opposite or inside face of the wall, supposing the side, as seen in drawing, to be the outside of the wall. The inner end of the cross-pieces, m and n, support an arrangement of timber precisely similar to that shown in J f i, i g k, and 11, This is illustrated in fig. 274, which is a side or edge elevation of the centreing and wall - the wall, a b, a b, being that lettered also as a b in fig. 274. In fig. 274, c c, d d indicate the parts corresponding to j f i, i g k in fig. 273 - c c being that at the outer, d d that at the inner face of wall; m m, in fig. 274, is the cross-piece, m, in fig. 273; o o, the post corresponding to m and o in fig. 273. The two pieces, c c, d d, fig. 274, support or carry cross-pieces, q r s, these uniting the two sides, cc,dd, of the centre proper. These pieces are either placed close to each other, as at r s, fig. 273, forming a platform or floor, so to say, in which the bricks or stones forming the arch are laid in course of building; or the pieces may be laid, each being separated from its neighbour by a short space, as shown at q q, in fig. 273. The interspaces may be less than the breadth of a brick in small arches; or in the case of arches of wider span, and where stone is used, may be much wider. These cross-pieces are termed bolster pieces. The arrangement for "striking the centres" is shown at t w, v w, in figs. 273 and 274. In this double wedges are employed, the large end of one of the wedges, as t in fig. 274, being placed at the small end of the other wedge, as w. When the building of the arch is completed, the centre is not removed at once, but the whole allowed to remain for a length of time, longer or shorter according to circumstances. As the brickwork or stonework of the arch gradually settles, the wedges are gradually driven out or loosed, thus allowing the cross-pieces m and n, and the upper part of the centre, to drop gradually. When the settlement is completed, the wedges are driven clean out, and the centreing wholly removed. In some cases the wedges are used at the lower part of the posts, as at m, in fig. 275. In this fig. two other forms of centres are illustrated; in the one to the right a central post d is used; an angular piece, e; and a strut or brace, f; the space above e is filled in with a piece of plank, g g, the outer edge of which is cut to the arch. The centre to the left is for a pointed arch. In both of these, in place of two vertical posts, as o and p in fig. 273, three are used, the third, as h h, being placed in the centre of the cross beam, i i. In some cases where this central post is used, a diagonal strut, shown by the dotted lines j, fig. 275, is used on both sides of h h; and, in some cases, the side posts, as k k, l l, are dispensed with, and the diagonal struts, as j, used in place of them. In fig. 275, m m m are the cross-pieces corresponding to m and n in fig. 273. In fig. 276 is illustrated a form of centre used for a "segmental" arch; in fig. 277 one for a "scheme" arch. (For a description of the various forms of arches see Volume on Brickwork and Masonry, Advanced Series). In fig. 276, a a, the upright posts; b b, cross-pieces, corresponding to m and n, fig. 273; cc, sill, corresponding to l l, fig. 273; d, the filling-in piece, carrying the "bolster piece," c c. In fig. 277, the same letters indicate the same parts as above, but there is no filling-in piece, as d d, the brick being laid upon planks, d d, carried by the piece c c. In both of these the striking wedges are placed against the wall at the foot of posts a a, or as in fig. 275. In fig. 278 we give the upper part a b the centre line, of an open built centre for a bridge; and in fig. 279 the lower part.
In fig. 279a we give a diagram showing an arrangement for a centre keeping the water-way quite free all points of supports from the ground being done away with. The simplest form of timber bridge may be described as a bearer a a, fig. 280, thrown across the opening to be crossed; protection being afforded by the railing, constructed of uprights b b, cross or diagonal struts c c, and hand-rail d d; the flooring being made of simple planking e e, f f, these constitute the elements of a simple bridge in crossing narrow spans.
If the span is increased the beams may be trussed with iron rods, as g g g (see illustrations of trussed beams further on), or the beam may be trussed, king-post fashion as on the right-hand side of a b in fig. 283, or queen post as on the left-hand side; and for still larger spans the beam may be built and curved (see further on for illustrations of built beams). Figs. 280 and 281 are part elevations of a bridge or gangway, and fig. 282 the cross elevation showing roadway and hand-rails.