There is also a cast-iron shoe receiving the other end of this beam, which is formed with sockets and flanges, so as to receive and connect together the king-post and tie-beam of the.above named semi-truss, and the longitudinal plate alluded to above, and the head of the great iron upright or pillar. The chief use of this iron shoe is to prevent the lateral compression of the timbers, and to prevent the natural shrinkage of the horizontal plate from letting down the roof.
The intermediate ribs (those which do not rest on the iron pillars) rest on another iron shoe, which is fitted on to the head of an upright piece that rests on the centre of the longi tudinal plate ; but the bearing is thrown on to the iron pillars by stout braces framed into the said upright piece and lon gitudinal plate.
It will thus be seen, that throughout the skeleton of this roof there is no case of any important timber resting with its end upon the side of any other piece, but that they all boar with an end-grain abutment, so that the cracks and failures arising from the necessary shrinkage, as well as from the lateral compression of the timbers, are avoided. This was a precau tion rendered necessary by the very large scantlings of the timbers, and by the discharge of the whole weight of roof and ceiling being thrown on so few points of support. The circular part of the roof is covered with copper laid on diagonal boarding, which is supported on rafters notched on to the great ribs, and running longitudinally. The roofs over the sides or galleries are slated, with a flat plaster ceil ing ; the ceiling of the circular part is deeply panelled, the main ribs forming the core for the cradling. The enrich ments are of papier macho, a material that was well adapted for the large architectural ornaments, from its lightness, and from the safety with which it could be screwed up to the timbers.
The longitudinal plate above alluded to consists of two pieces of fir, each l2 X 6, bolted together the bearings between the iron pillars are reduced by braces, which dis charge the bearing on to the iron pillar, and at the same time receive the wood cradling for the spandrels. Each end of this plate is let into the wall, and is received by a corbel of teak wood, on to which it is bolted and locked down.
A considerable improvement upon the system of Philibert de Lorene was effected by Colonel Einy, a French engineer, in the early part of the present century, by the employment of laminated ribs. The principal difference between the two consists in this, that in Colonel Emy's improvement the direction of the fibres of the wood coincides with the curva ture of the rib, and in consequence the joints are much less frequent, and the rib possesses greater elasticity, so as slightly to yield, rather than break, under any violent strain.
The alternate thicknesses break joint, as in D. Loaners roof, and all are securely bolted together.
The earliest roof constructed after this fashion was at Marac, near Bayonne, in 1S25. The span is 65 feet, and the main ribs are formed of planks bent round on templates to the proper curve, and kept together by iron straps, and also by the radiating struts which support the principals ; they arc in pairs, notched out so as to clip the rib between them. In this roof the entire weight of the roof is thrown on the walls at the feet of the ribs, which are considerably below the principals, so that the weight of the upper part of the walls serves to diminish the effect of any thrust against the lower part of the walls. A great saving of wall-material is hereby effected.
This principle has often been applied in the case of rail way bridges, and there is at present one erecting at the London terminus of the Great Northern Railway. In this instance the spandrels are of cast-iron.
Of somewhat similar construction is the roof over the transept of the Great Exhibition building. The principals, or main ribs, of which are placed at intervals of 24 feet from centre to centre, and are made up of three planks, two of 2 iuehes in thickness, and the middle one 4 inches thick, with a moulded piece on the under side 22_ inches thick, and two 11-inch planks at the top, each one inch thick, and nailed together to form the gutter-board, the whole being firmly connected together by wrought-iron bolts passing through belts of the same material, running at top and bottom of each rib. In order to form the ribs with the proper curvature, each plank, In inches wide, was cut on one edge to its proper segmental form, and the two complements cut off were nailed to the lower or straight edge of the flank ; the whole being put together, so as to breach joint throughout, the length of each section so cut being about 9 feet 4 inches. The preparation for the reception of the ends of the purlins was on this wise : the two wide planks were cut across, so as to form a cavity to admit the ends of the purlins, the parts of the planks thus separated being connected together by cast-iron plates. The openings thus made were filled in temporarily with wooden blocks, until the joiners were ready to fit in the ends of the purlins.