Lozcering the arch is finished, the centre, having served its purpose, is removed. In the case of arches of small size, the centres are constructed with pairs of wedge-shaped blocks of hard wood, which Are inserted between the cap-piece of the supporting posts and the chord of the framework. By driving out these tapering- blocks the centre is gradually lowered, and the arch is left to support itself. The centre may then be taken apart and removed. As there will always be some set tling of the arch more or less, according to its size and weight and the materials of which it is built, the centres are usually made a trifle higher than the finished arch is intended to be when it shall have taken its per manent set. Furthermore, as the sudden removal of the support from a heavy structure like an arch of considerable size might cause a distortion of its shape by displacement, and even endanger its stability, it is import ant that the centres be lowered very slowly. For arches built of brick this precaution should on no account be neglected, since this building material possesses much less strength to resist a crushing stress than granite, sand stone, or limestone. On account of the danger to be feared from this cause, screws mav be substituted for the wedges for lowering the centres.
An ingenious artifice for removing centres—which is not only very simple, but is also in every way satisfactory for the purpose—consists in the use of sand-boxes. In this plan the posts or standards supporting the frames rest in short cylindrical boxes filled with sand. When it is desired to strike the centre, wooden plugs near the bottom of these boxes are with drawn, and a small quantity of the sand is allowed to flow out; the plugs are then replaced, and this operation may be repeated frequently until the arch stands free. This method is greatly to be preferred for large arches, since it avoids violent jarring of the structure by the knocking out of wedges and affords a means of lowering the centre with great steadiness and regularity. This plan was adopted in the construction of the arch of the Pont d'Alina, in Paris (1S36), referred to below. Trautwine speaks of it as being well worthy of adoption for all spans exceeding 6o feet. It may be of interest to state, also on the same authority, that in the case of railroad-cuttings crossed by bridges " the earth under the arch has been made to serve as a centre by dressing its surface to the proper curve and then embedding- in it curved timbers a few feet apart and extending- them from abutment to abutment for supporting the close plank lagging." Scafoldings.—In addition to the centres just described, in important works of this kind a temporary scaffolding- of convenient height is erected along the face and transverse diameter of the intended structure, on which cranes or travellers provided with a windlass (/5/. 39, figs. 1o, II) are
moved to and fro upon overhead rails. These labor-saving devices are employed for raising and transporting the blocks of masonry and other building materials needed from the g-round-surface to the points where they are required as the work prog,resses.
Sonic/rot/a,- Arthes.—The largest masonry semicircular arch in the world is that over Cabin John Creek, on the Washington Aqueduct, built by General Meigs, U. S. A. This arch has a span of 22o feet and a rise of 37!4 feet; the crown of the arch is IOI feet above the water. The arch-stones have a depth of 4 feet 2 inches at the crown and of 6 feet 2 inches at the springers. The arch of the Grosvenor Bridge, over the Dee, at Chester, is the largest masonry arch in England. This is also semicir cular, with a span of 2oo feet and a rise of 42 feet. The arch-stones are 4y, feet deep at the crown and 7 feet deep at the springers. The semicir cular central arch of the Ballochinyle Viaduct, of the Glasgow South western Railway, in Scotland, has a span of 179";;,„1 feet. Following in the order of the width of span collies the viaduct at Nogent-sur-Marne, with nearly semicircular arches of 164.2 feet span.
Fla/ notable flat arches are those of the Maidenhead Viaduct, with arch-radius of 169.1 feet and a rise of 27.7 feet (r : 6.1); the Ladenhurg-Neckar Bridge, with arch-radius of 95.5 feet and a rise of 1,3.6 feet (r : 7); and the bridge built by Perronet at Pontoise, which, with a radius of 164.2 feet, has a rise of only 12.5 feet (1 : 13.5).
Mica/ earliest bridge with flat elliptical arch is the handsome Santa Trinita at Florence, built in 1566 (16/. 4o, fig. 7). Other notable structures are, the London Bridge, elliptical arch, 152 feet; the bridge over the Aar at Berne, 15o.5 feet; the Gloucester Bridge, over the Severn, i5o feet; the Kleinwohnsdorff, on the Saxou-Silesian Railroad, 149.7 feet; the Dora Riparia, at Turin, 148 feet; the Pont d'Alina, at Paris, 14r.4 feet. The last-named is an elliptical arch built of rubble laid in cement. The earliest specimen of the basket-handle arch (a three-centred low-crowned arch) is the bridge of Chatellerault, bu.ilt in i6o9.
Poinica' Gothic style of architecture, which arose about the ninth century and soon spread over Europe, g-aye rise to the use of the pointed arch in bridge-construction. Of bridges with pointed arches, the greater number was built prior to the close of the eleventh century. In the Middle Ages the building and maintenance of bridges were considered acts of piety, and in the twelfth and thirteenth centuries a religious fra ternity styling-, itself the " Brothers of the Bridge was consecrated to these objects. The bridges of Avig,non, St. Esprit, and La Guillotiere at Lyons, among, others, were built by them.