CONSTRUCTION OP THE BUILDING.

57

The floor, which is supported by these girders, consists of cross-beams, so The gallery doorunder-trussed with iron rods, shoes, and struts, as to distribute the whole weight g)rderl° nthat may be brought upon the floor pretty equally upon the eight points atwhich the ends of the beams rest upon the girders. Joists, stretching from theiron girders to the beams, and from one of the beams to the other, form thesupports for a floor which is not more than inch thick, but is at once amplystrengthened, and rendered impervious to the passage of dust, by the insertion, ina groove cut in the edge of each floor-board, of iron-hooping, forming a tongue.

A railing, designed by Mr. Owen JONES, surmounted by a mahogany handrail,adds at once to the utility and the beauty of the gallery.

The columns which rise at the gallery level are 16 feet 7^ inches long, and The columns sc-are surmounted by connecting pieces, similar in all respects to those occurring androeff! 11 ' 51 "' 3beneath. To these connecting pieces are attached, transversely in one directionand longitudinally in two, cast-iron girders of similar form and scantlingto those we have described; their office being to maintain perfectly true andrigid, the vertical shafts which carry the eye upward in one unbroken line fromthe ground to the roof which they serve to support.

As the strength of an iron column practically depends upon its length being b r

limited, far more than upon its substance, the value of dividing the whole lengthof the shafts reaching from the gallery to the roof into two parts by theseconnecting-pieces, and thus reducing the length of the columns one-half, must be First tu*.readily appreciated.

Above the second tier of girders rise columns of the same length as those last second tier,mentioned, and on them again are placed connecting pieces, to which the girderssupporting the roof are attached. These girders correspond with those support-ing the galleries, and exactly resemble those forming the tier immediately beneaththem, in every respect except their thicknesses. The whole of the girders on theupper tier have been proved in the building to a strain of nine tons.

By extending the area of our observations, we shall be enabled to include allthe varieties of trusses employed to support the flat roofing over the whole extent Varieties of roofof the building. It may be well, therefore, to consider that our original limita-tion to 24 feet square has been enlarged by the addition of a space of 72 feet by24 feet, being a compartment of the roof over the nave; and of an area of thesame width by 48 feet, being a portion of the roof over the avenue which extendsfrom east to west, beyond the aisle on each side of the nave.

As we have stated that the latter of these portions of the building (the 48-feet The 48 -ft. trussesavenue) rises to a height of two stories only from the ground, it will be manifestthat its roof-trusses must be attached at the level of the girders which serve tostiffen the main shafts of the nave, namely, at a height of 44 feet from the ground.

These 48-feet roof-trusses are attached to connecting pieces in a similar mode tothat already described for the girders, with the exception that their vertical posi-tion is maintained by bolts passing through their standards and through thecolumn, instead of by the system of keys as in the 24-feet girders.

In fig. 6 a representation is given of the principal parts of one of these trusses, of

which, it will be seen, is constructed for the most part of wrought iron ; thefew portions which are of cast iron acting only under compression. Thesetrusses follow the general principle of division into 8-feet compartments ;and, consequently, the cast-iron struts or standards (A A A A, fig. 6) occur inpositions corresponding with those in the gallery-girders already described.