The outer inclosure, on the ground-floor, is formed by dividing each 24-feet bay between the columns into three 8-feet bays by half columns of wood, between which is placed boarding, held in its place by iron clips and bolts; a plinth, four feet high, is formed immediately above the floor by frames, filled with what are commonly called louvre-blades, which are hung on pivots, and of which a large number can be moved simultaneously for the admission of air; similar ventilating-frames, three feet deep, are introduced at the top of each storey round the entire circuit of the building, and by this means a ventilating-surface of no less than 40,800 square feet is obtained, or rather more than one acre.
PORTION OF THE LOWER STOREY OF THE PRINCIPAL ELEVATIONS.
Externally some light arches are inserted, and open panels form the inclosure for the upper louvre-frames. The details we have been describing may be readily traced in the engraving of a portion of the lower storey as seen from the outside. The exit doors occupy one of the 8-feet bays opening about six feet wide. The inclosure to the upper storeys closely resembles those of the ground-floor, but glazed sashes are substituted for the close boarding, and the plinth is omitted. Each storey is crowned externally with a cornice and cresting ornament, and over the columns posts are carried up, to which flagstaffs will be fixed.
To return to the interior. The whole of the floor is boarded; that below is laid with an interval of half an inch between the boards, to allow the passage of dust from the millions of feet by which it will be trod; the gallery floor, on the contrary, has iron tongues between the boards to prevent the dust from coming through on the heads of the visitors below.
The roof of the transept, which we have described as crossing the building about the centre of its length, differs from that of the other parts, its general form being semicircular instead of flat, and rising above the rest of the building so as to show the whole of the semicircle externally. This roof is supported by arched timber ribs placed twenty-four feet apart, or one over every column, which forms a socket, into which the foot of the rib is fitted and secured by iron straps. Between the ribs, timbers are fixed which carry minor ribs at a distance of eight feet apart, and upon these the ridge-and-furrow roofing is constructed in the manner that has been described for the flat roofing, but following the curve of the arched ribs. At the springing or foot of the arch on either side of the transept there is a range of louvre-frames to assist in the ventilation of the building, and on the top of the arch externally a narrow passage is formed to give access to the different parts of this roof. On the inner side of the arch diagonal tie-rods are introduced between the main ribs, which, while they serve to increase the strength of the construction by tying together all the parts from end to end, produce an agreeable play of lines forming a kind of network over the whole of the surface.
The ends of the transept are closed in with fan-like tracery, reminding the spectator of the magnificent wheel windows of our Gothic cathedrals; this elegant feature is not visible in our interior view, but will be seen in some of the exteriors.
There is, perhaps, no part of this interesting building in which the great size and singular lightness, almost airiness, of the construction are so strikingly displayed as in the TRANSEPT, inclosing as it does a row of fine old elm-trees, as if to protect them in their venerable age from the smoke of the thousands of chimneys that have been gradually forming a destructive circle around them.
The only portion of solid untransparent roofing in the whole of this building is formed on either side of the arched roof just described, where there is a lead flat twenty-four feet wide. This was partly required for a platform to serve for carrying on the works for the arched roof, and was also exceedingly useful in giving access to the other roofs on either side; it likewise afforded the opportunity of giving some additional strength at the springing of the arched ribs to resist any possible tendency they might have to spread outwards.
