302. The principal beams of a centre should always abut end to end when it is possible. A very good method, where timbers meet at an angle, is to let them abut into a socket of cast iron, as in the centre of Waterloo Bridge. (See Fig. 2, Plate XXXVI.) The timbers should intersect one another as little as possible, as every joining increases the danger of settlement, and halving the timbers together destroys nearly half their strength. The pieces which tend towards the centre, and those which perform a similar office to the king post of a roof, should be notched upon the framing; they should be in pairs, that is, one on each side of the frame, and well bolted together. Most of the braces may also be applied in the same manner with advantage. The braces marked a a, in Fig. 2, Plate XXXVII., are supposed to be done in this way.

Ties should be continued across the frames in different places, particularly where many timbers meet; and diagonal braces between the ribs are also necessary, to secure them from lateral motion.

A sound and firm centre is a most desirable thing in the construction of an arch, as the motion of a weak one destroys the cohesion of the cementing materials. Sometimes centres have completely failed from inattention to these principles. In erecting a large arch over the river Derwent, on the line of the Glossop and Sheffield road, as they were proceeding to lay the key-stone, the centre gave way, and fell with a tremendous crash into the river. Several lives were lost.*