CENTRE, in building. is a combination of timber-beams, so disposed as to form a frame, the convex side of which, hen boarded over, corresponds to the concavity of an arch; or the wooden mould, used for turning an arch of stone or brick during the time of erection.
Centre of a cylindric or eylindroidal arch, which rises more than the breadth of a plank, is a number of boards supported transversely by one or more vertical frames, or trusses. as the length of the cylinder, or that of its axis, limy require.
Centre for a groined arch upon a rectangular plan is thus constructed: Make the centre for one of the cylinders, or eylindroids, viz. that of the greatest diameter, when there is a difference, as if there had been no other cylinder crossing it; find out the places on the surface of this cylindric or evlindroidal centre, where the surface of the transverse vault would intersect : fix the whole ribs on the cross vault. and parts of ribs on the surface of the vault already completed, observing to keep the outer edge of these ribs the thickness of the boards wi bin the intended surface of the intrados of the arch; when this transverse vault is boarded over, the hoards will intersect the lines drawn on the first centre, and the surfaces of the boards of each vault will form the true surface of the groined centre, on which the stone or brick arch is to be turned.
The frames or trusses which support the boarding are frequently called ribs ; and the short ribs which are fixed to the boarding, and made to range with the whole ribs, are cal led jock-ribs.
Under the word STONE-BRIDGE, &c., the theory and construction of arches will be described ; in the present article we propose to show how the arch-stones arc supported till the arch is completed ; and the most commodious and least expensive manner in which this can be accomplished.
The proper construction of such supports, or the best mode of framing the centres for large works, has always been considered so important a subject, that it has occupied the attention, and exercised the talents. of the most eminent engineers and architects. The principal object to be kept in view is, to fix the various parts of the centering in such a manner, as to support, without change of shape, the weight of the materials that are to come upon them, the whole progress of the work, from the springing of the arch to the fixing of the key-stone. This object has not always
been sufficiently attended to by the professional men, either of this, or of other countries ; for in many instances it has been ascertained that the centres of bridges, from the injudi cious principles of their construction, have changed their shape considerably, or entirely failed, before the arch was complete ; and in consequence of change of shape only, the arches built upon them have varied, both in form and strength, from the intention of the engineer. In the large works of this kind, however, erected in Great Britain, our best engineers have constructed their centres on principles calculated to sup port every weight, and resist every strain to which they might be exposed.
The qualities of a good centre," says "consist in its being a sufficient support for the weight or pressure of the a•ch-stones, any sensible change of form throughout the progress of the work, from the springing of the arch to the fixing of the key-stone. It should be capable of being easily and safely removed, and designed so that it may be erected at a comparatively small expense." The centre of a large vault. as that of a bridge, is con structed of trusses disposed equidistantly in vertical parallel planes, and hoarded over so that the convexity of the boarding may coincide with the intended internal intrados of the arch. Tin: distance of the ribs may be disposed at from three to eight feet, according to the strength of the boarding and weight of the arch. In very large works, a bridging is laid for every course of arch-stones, with blo•kings between, to 1 keep them at regular distances. The ring-stones do not Ialways rest upon these bridgings ; planks being sometimes put between, that they may be cut away afterwards, to separate the centre and the intrados from each other, in order to ascertain whether there are any settlements, to repair the damages. and put the arch in a state of equilibrium.