There are several other principles of constructing the ribs of eklitering ; one of these may be that of a large truss, spinininP: the whole opening, having its vertex supporting the siumnit of the arch, and its rafters, or principal braces. supporting other subordinate trusses which resist the pressure of the arch at other intermediate points.
Of this kind is that of the bridge of Orleans, by M. Hupcau, one of the boldest centres ever executed in Europe. Another princiHe is that of two independent trusses, one supporting the sides or haunches of the arch, and the other the crown. Of this construction Yeas the centering of the nave and tran septs if St. Peter's church, at Rome, by Michael Angelo. and two centres by Pitot. Another principle of centering is that if inscribed equilateral poly guns ; that is, the exterior beams, supporting the curve, are of equal lengths, and joined toge ther in the thrill of a polygon: another polygon is thrilled this. having its angles in the middle IX the sides of the thriller, and so on.;ilternately. until there are as many poly gons illseribed as will make the centering sufficiently strong or still: This mode or centering may lie of two kinds: one, when the angles are fixed at their junction to the sides of the last poly win with bolts; bridles, or double truss-pieces. being put over the angles to prevent a transverse strain at the sec tion of the timbers where the two pieces meet, and to support the Cline above, The other kind is, when the polygons act independently I if each other ; these polygons are brought into action by bridles, W hick support the curve, and act upon the angtilar 1 oints of each other's polygon. Of this kind were , the conic' of the bridges of Cravant, Nugent, Mavence, and y. constructed by Perrollet. Though these center ing= have been executed to very large spans, the last men tioned being 120 feet, their equilibrium is by no means so secure as when the angles of the inner polygon are fastened to that immediately preceding, as is evident from the infor mation given if' the erection of these bridges, by the ingenious architect who has favoured the world with a treatise on this subject.
Another principle of centering is that of Westminster and idges, London. They consist of a series of trusses, each supporting a point in the arch, the principal braces having their lower extremities abutting below, at each end of the centering, on the st•iking-plates, and at the upper end, upon apron-pieces, which are bolted to the curve that supports h•idgings tor binding the pieces which compose them together at their junction. Thi•re is one disadvantage under
which this mode labours ; that is, the frequent intersection of the 'principal braces with each other: they must either be halved one upon the other, otherm ise they must be discon tinued, and made in various lengths. Both these modes diminish their lateral strength, and consequently make them much more liable to buckle than when whole; but of the two, that of halving is to be preferred : as by the braces being in one leneth. there can be no sagging occasioned by interme diate joggles, and the braces may be rendered sufficiently seen le, laterally, by running straps longitudinally across the notched part on each side, bolting these straps to the braces.
1,ast I v. another mode of centering may he that of a number of quadrilateral frames abutting on each other, having their joints radiating to a centre, in the manner of the wedge-stones of an arch in nmsonry. These frames should all be resolved into triangles by one or two diagonals, according to the kind of strain, k•eping.in view that a piece, which is a tie in one is, in :he other diagonal of the same quadrilateral, a struts ; but if' the kind of strain on any frame lie not well ascertained, it would he better to place two diagonals, luilved upon each other. The frames are to he secured with keys or bolts, and by this precaution each frame will be rendered quite immovable.
The general principle of construction is a series of trian files, of which every two are connected by :t etiminon side.
.flute. I., Figure 1. Let ADCDEFG he the curve of an arch which requires a centre ; let the points A, 13, e, &c., be connected so as to fortn the equilateral A II c D Si 0, and join A c, c E, and E G ; the timbers thus disposed w ill form three triangles, which may be looked upon as so many solids, revolvable about the angular points A, c, E, G; sup pose now, that these are to be in equilibrinin, the smallest force on either side would throw it down, and therefore, without other connecting timbers, it would be unfit for the purpose of a centre.