290. A centre is a timber frame, or set of frames, for supporting the arch-stones of a bridge during the construction of the arch.

The qualities of a good centre consist in its being sufficient to sustain the weight or pressure of the arch-stones, without change of form while the work proceeds. It should be capable of being easily and safely removed, and so designed that it may be erected at a comparatively small expense.

In navigable rivers, where space must be left for the passage of vessels, and in deep and rapid rivers, where it is difficult to establish intermediate supports, and where there is a liability to sudden floods, the centre should span the whole width of the archway, or be framed so as to leave a considerable portion unoccupied. In such cases some skill is required to make the centre an effectual support for the arch-stones, particularly when the arch is large.

But in narrow rivers, and in those where the above-mentioned inconveniences do not interfere with the work, the framing may be erected upon horizontal tie-beams, supported in several places by piles, or by frames fixed in the bed of the river, in which case the construction is comparatively easy.

In large arches, when the arch-stones are laid to a considerable height, they often force the centre out of form by causing it to rise at the crown, and it is sometimes necessary to load the centre at the crown to prevent such rising; but this is a very imperfect remedy. Notwithstanding that the subject has been considered by so many eminent men, their works are not much calculated to instruct the carpenter how to avoid this difficulty; indeed the object of most of them seems to have been exclusively to calculate the strength of a centre already designed, instead of showing the principles on which it ought to bo contrived; and even in calculating the strength they are imperfect guides, because they have not attempted to find what forces would derange a centre, but only the load that it would support without fracture.*

Smeaton, in his designs for centres, always contrived his scantlings to suit the general sizes of timber obtainable in the market, with the view of saving labour, and of leaving the timber in as useful a state as possible when done with.† This practice is a good one, and ought to be imitated, as a centre or a scaffold is not like a permanent work. Nevertheless, some knowledge of the principle of disposing the parts is necessary, and also of estimating the degree of stiffness required; otherwise more timber may be wasted than if it had been cut to the proper scantling at the first, as all timber that has been used in a centre is more or less injured.

291. Centres are composed of several separate vertical frames or trusses called "ribs," connected together by horizontal ties, and stiffened by braces. When the ribs have to span the whole width of the archway, the offsets of the stonework afford a most substantial abutment for the support of the centre. The ribs of centres are generally placed from 4 to 6 feet apart, according to their strength and the pressure they have to support. In general there is one under each of the external rings of arch-stones, and the space between is equally divided by the intermediate ribs.

* See Pitot on * Centres,' Mem. de 1'Acad., 1726: Couplet on ditto, 'Mem. de PAcad.,' 1729. Frezier's ' Coupe des Pierres, tome iii., p. 408. † Reports, vol. iii., p. 236.

A bridge of three arches will require two centres, one of five arches requires three centres, etc.