The space below the first "purlin" or plate at the springing of the arch, down to the level of the lead-flat beneath it, is fitted with louvre-frames for ventilation. The diagonal bracing between the main ribs has been already alluded to. Each set consists of four wrought-iron rods three quarters of an inch in diameter, having eyes at one end, by means of which they are secured with bolts, passing through the thickness of the ribs; in the centre they meet in a cast-iron ring, on the inner side of which the ends are screwed up with nuts. The semicircular ends of the transept are filled in with tracery, formed by radiating timbers, strutted apart with short pieces placed in concentric rings. The circular heads of the openings are formed by iron castings screwed into their places, and the eye from which the radiating lines of the tracery proceed is also formed by solid iron castings bolted together. On the outer face the ribs of the tracery are moulded, and on the inner side glazed sashes are fixed, filling in the openings. The lead-flat, twenty-four feet wide, extending the whole length of the transept, on either side of the semicircular roof, is constructed in a similar manner to the floor of the galleries, by under-trussing two pairs of joists in each bay. In the width of the lead-flat roof a horizontal truss is formed by flat bars of iron fixed in the direction of the diagonal of the 24-feet square bays, to resist any possible thrust or tendency of the ends of the ribs to open outwards at the springing. The Facework. The external inclosures of the building, on the levels of the different storeys, require but little description in detail beyond that already given. The sash-bars dividing the sashes of the upper tiers are grooved for glass similarly to those used in the roof, and were cut out by the same machinery. The glass was put in after they were framed together, so that it was necessary to arrange the ends of the bars that it could be slipped in at one end. As the bars of these sashes were of slight dimensions and considerable length, they were strengthened by wrought-iron rods passed through the sash-frame and the bars, and screwed up at the ends, causing the whole to work together. The sashes are held in their position by small cast-iron clips, which are bolted on to the columns; and as the surface presented to the wind by the upright sides of the building is of such considerable extent, wooden bridges are fixed against the sashes on the inside, by small cast-iron shoes bolted to the columns; and at the internal angles, where the wind would exert its greatest force, these bridges are further strengthened by wrought-iron rods half an inch in diameter, pressing against the back of them, which is grooved for the purpose, and screwed up at each end in the cast-iron shoes. In this manner a connected chain of resistance to any external pressure is established round the whole circuit of the building. [Illustration: THE LOUVRE-FRAME.]