303. The frames or principal supports of a centre should be placed upon double wedges, or sometimes they may be placed upon blocks with wedge-formed steps cut in them; and when the centre is to be eased, the wedges, or wedge-formed pieces, are driven back so far as to suffer the centre to descend regularly, as described for the centre of Gloucester Bridge. (Art. 299.) This operation should be very gradually performed; in order that the arch, in taking its proper bearing, may not acquire any sensible degree of velocity; as it would be dangerous to let it settle too rapidly.

In small centres the wedges are driven back with mauls: men being stationed at each pair of wedges for that purpose. But in larger works a beam is mounted, as a battering-ram, to drive back the wedge-formed blocks. Before driving back the wedges a good precaution is to mark them, so that it may be easy to ascertain when they are regularly driven. As soon as the arch is completed, the centre should be eased a little, in order that the arch-stones may take their proper bearings before the mortar becomes hard, otherwise the joints will crack on the arch settling.

* 'Morning Chronicle' of August 27, 1819.

It is a great advantage in striking centres to have the power of allowing them to rest at any period of the operation; and in this, as well as in other respects, the methods of lowering and easing centres practised in Britain, are superior to those sometimes adopted by the French engineers. The French method consists in destroying, by little and little, the ends of the principal supports; a work of difficulty as well as of danger, and which cannot be done with so much regularity as with wedges.

A method of striking centres which is considered superior to any of the foregoing was adopted by H. Bouziat at the Bridge of Austerlitz in Paris, in 1854, and subsequently for several bridges erected in India. It consisted of iron cylinders about 12 inches diameter and 12 inches high, open at both ends, placed in a vertical position on a wooden platform, which formed the lower striking plate of the centre. The cylinders were prevented from slipping by being placed on circular disks of wood 3/4 of an inch thick nailed to the platform, and fitting the interior diameter of the cylinders, which were filled to within 2 inches of the top with fine dried sand. Into the space on the top of each cylinder was fitted a solid block or piston of wood about 10 inches high, making a total height of 20 inches.

The whole apparatus was then introduced under the centres in lieu of wedges.

The method of striking consisted of letting out the sand gradually through four holes about 3/4 inch diameter, which had been drilled in the sides of the cylinder, near the base, and stopped with corks.

In adopting this method care should be taken not to let the sand get wet during the construction of the arch, as it would prevent its running out freely.

The centres of Mylne's Bridge at Blackfriars and those of Waterloo Bridge were placed upon blocks, with wedgeformed steps cut in them; as is shown in Fig. 2, Plate XXXVI. Another method consists in forming the steps on beams that reach across the whole width of the bridge, passing between the feet of the trussed frames and the posts that support them. In Fig. 1, Plate XXXVII., the centres are supposed to be done in this manner. The frames being thus placed upon continued wedges, the centre may be struck without it being necessary to have workmen underneath, which is less dangerous, and can be done with fewer men. In consequence of nine men having been killed in removing the centre of a military work, Mr. Richard Williams proposed a similar method to the above, which was used with success at Chatham in 1807.*