The strength of every purlin has been duly calculated, according to its relative position in the arch ; thus, the three uppermost purlins, having the greatest strain on them, are each 131 inches in depth by 41 in width ; while the four lower purlins on each side, having gradually less strain to bear, diminish regularly in depth to 9 inches, all having the same width as those at the crown ; whereas, the lowest one on each side of the arch, being nearly horizontal, is increased to a scantling of S inches by inches.
Between each pair of main ribs are two intermediate ribs, or, as called in ordinary roofing, common rafters, 4 inches deep by 3 inches wide. On the top of these are gutter boards, in two thicknesses, as those described for the main ribs. The gutters, each 5 inches wide, are formed by two splayed fillets, let into and nailed securely to the upper side of the boards. These fillets also serve as abutments for the skylight bars, which are rather larger in section and size, as •those for the skylights in other parts of the building. Each bar is nailed at the lower end to the fillet. and at the upper end to the ridge-piece, which is formed of three pieces of fir, one above another, the lower section being 4 inches wide by 11 thick ; the middle piece, 3 inches by liths inch ; and the upper piece, 2 inches by liths inch, the latter having a groove on each side to receive the glass. The three thicknesses are necessary, in order to suit the curvature of the arch. The lower end of each ridge-piece is carried down to, and rests on, the lead flat. Condensation-gutters are formed in this roof, as in the roofs of the aisles and avenues, but by a differ - ent method. A sloping fillet is nailed on to each side of the gutter-board, and continued from the springing of the arch on one side, to the corresponding point on the other. With a view to retain the ridges in their places, wrought-iron rods • of* inch diameter, extend from the purlins to the under side of the ridges. In order thoroughly to carry off the rain-water from every part of the roof, all the skylight bars are fixed diagonally from the ridge to the gutter, and the water col lected in each curved gutter, is carried into a sloping trough at bottom, and thus discharged on to the lead flat, which is sloped towards the water-heads at tops of the hollow columns.
Looking at the roof from the lead flats, the whole has a herring-bone appearance.
Temporary ladders, fixed to suit the arched roof, were used for fixing the ridge-pieces, skylight bars, &c.; but, in order to facilitate and expedite the glazing of this roof, travelling scaffolds were used, which could be raised and lowered at pleasure, by means of ropes and pulleys, and by the power of four men working a crab engine, placed on the lead flat contiguous. Each travelling scaffold is formed with sides and ends, and has boxes, in convenient positions, for the glass, putty, and tools ; the whole running on small friction rollers, suited to the tops of the ridge-pieces, which serve as rails : without such a contrivance, the glazing of the roof of the transept would have been almost an endless job.
The vertical supports for the roof consist of cast-iron columns placed one above another in three tiers, sixteen in each tier on either side. Above the top pier are fixed the trussed girders, 3 feet in depth, spanning from column to column, which are each 24 feet from centre to centre ; and across the intersecting lines of the middle arch, are two double trussed girders, 6 feet in depth, and corresponding in length with three spaces of 24 feet each ; except under the columns of the south-east angle of the transept, the whole are placed on broad base plates, which rest on concrete Ibun dations. In the exceptional case, the foundation consists of a solid brick pier, built in cement. At the top of each column which supports the root, is a cast-iron socket, 4 feet 4 inches in height. The use of these sockets is to receive what may be termed the vertical legs or supports of the ribs, and which form parts of the ribs as framed together on the ground. In order to resist the lateral thrust of the roof, a strong gangway, 24 feet wide, is formed on each side of the transept, which may be considered as an abutment, being constructed of strongly-framed and braced flooring, supported by trussed girders. The main ribs have their bearing immediately above the columns, the intermediate ones on the girders between the columns.