the upper portion of the connecting-piece extends, has a groove sunk upon its surface; a groove corresponding to it in width is also sunk upon the projection of the connecting-piece (C, fig. 4), and a small piece of iron is introduced between the two. This iron acts as a key or dowel, and prevents the two surfaces sliding upon one another.

nected with

On the upper and lower part of the connecting-piece, between these projections (thereby conwhich serve to retain the girders in their places, are cast holes, corresponding with columns), those at the top of the lower columns, and at the bottom of the upper columns; through which bolts being inserted, nuts (BB, fig. 4), fastened to those bolts, secure the columns and connecting-pieces together. A similar arrangement would enable any number of columns and connecting-pieces to be attached to one another, so as to make up one long length.

shafts, consisting of alternate columns and connecting-pieces.

In order to make sure that the shaft thus composed of alternate columns and Serving to stiffen connecting-pieces, should be capable of maintaining itself in a perfectly vertical position, it was necessary that the whole of the surfaces of contact should be wrought perfectly true and flat. Every column and every connecting piece had, therefore, to be placed in a lathe ; and the bed or surface at each end faced to a perfectly true plane. When the number of these columns, base-plates, and connect- All bearing faces ing pieces is taken into consideration, it may be easily imagined that the labour planed in lathes. entailed by this apparently simple necessity could scarcely have been performed in any workshops but those provided with extraordinary facilities and resources.

In the connecting pieces of the 24-feet bay we are now describing, projections are cast upon three sides, so that girders may be attached in three directions; and thus extend in every direction except that towards the nave, and the 48-feet avenue or court on the other side.

porting

floor;

The girders, which are attached, as above described, to the connecting pieces, The girders supserve to support the gallery floor. As, in the construction of this floor, it had ring gallery been determined to bring the accumulation of pressure on the girders, upon points at 8 feet intervals, it became necessary, in arranging the form of the girders, to concentrate strength at those points. The vertical lines of the girder have, therefore, been arranged to occur at intervals of 8 feet, connecting the top and bottom tables; on the good proportion of which, to the load to be supported, and to one another, the main strength of the whole depends. Diagonal lines connect the junction of these standards with the top and bottom tables, and the principal parts of the girder present the form shown in Fig. 5.

The girders are 3 feet deep, and the sectional area of the top table, (A, fig. 5,) Their details; which is of the T form, equals 5.31 inches, and that of the bottom, (B, fig. 5,) which is of a similar shape, but inverted, equals 7.64 inches. The areas of

Their sufficiency

to support a
"dead weight,"

and a moving load.

How tried :

By workmen ;

By soldiers marching

By rolling round

the diagonal struts or standards, and ties, average 3.50 inches. The breakingweight of the girders is calculated, and has been proved by various experiments, to amount to not less than 30 tons. Every one of the gallery-girders which has been used has been proved upon the ground to a strain of 15 tons; and, in exceptional cases, where it has appeared reasonable to expect that an accumulation of weight would have to be borne, their dimensions of thickness have been increased, and the amount of proof has amounted to no less than 22 tons.

A few simple figures will clearly exhibit the sufficiency of these girders to support the loads that are likely to be brought upon them. A bay of galleryfloor, measuring 24 feet by 24 feet, contains 576 square feet; and it has been found by experiment, that it is impossible to load any surface with men to an amount equal to one hundred-weight per foot superficial. Assuming, then, 576 cwts., or say 30 tons, to be by any possibility accumulated upon such a bay of gallery-floor, the load will be distributed over four girders, any two of which have been found to be fully competent to support the load.

In thus estimating the sufficiency of the girders, the load they might possibly be called on to support has been considered only as what is called "dead weight," or load to which no momentum of any kind had been imparted. In order, then, to test them under the action of a moving weight as well, a series of experiments was instituted. A perfect bay of gallery, 24 feet square, was constructed, with connecting pieces, girders, flooring, &c., complete. Its surface was first crowded with the contractors' workmen, as tight as they could be packed. The men were then set to walk over it, run over it, and, finally, to jump upon it with all their force.

In order further to observe the effects which would be produced by a load to which a uniform, instead of an irregular motion, had been conveyed, a number of soldiers of the corps of Royal Sappers and Miners were ordered to march over it, to run over it, and, finally, to mark time upon it in the most trying manner. The result of these experiments developed the correctness of the theory upon which the dimensions of the girders had been based, since not the slightest damage was done to the bay of gallery; and the fact was fully evidenced, that the quality of elasticity or springiness in the floor served to protect the girders from the effect of sudden shocks, and prevented the danger of the communication to them of the accumulating momentum, generated by the possible isochronous movements of a crowd.

Emboldened by the satisfactory result of these experiments, a yet more conshot "in situ;" clusive series was instituted. An apparatus was contrived by Mr. FIELD, the late President of the Institution of Civil Engineers, by means of which it was possible to draw, at a quick walking pace, over the whole of the galleries on which the public would have to tread, a number of 68-pounder shot, collected together so as to produce a uniform load of 100 lbs. per foot superficial. No damage whatever was produced by these rude tests, and they may be considered to have conclusively set at rest any doubts as to the sufficiency, in point of strength, of the gallery-floor, or of the girders which support it.

Who made by.

The whole of these girders are of cast-iron, and, together with the columns and similar castings, have been made in Staffordshire, at the foundries of the contractors, at the London Works, Smethwick, near Birmingham; at those of Messrs. A. and B. COCHRANE, of the Woodside Iron Works, Dudley; and at those of Messrs. JOBSON's, of Holly Hall, near the same town.

resting on the

The floor, which is supported by these girders, consists of cross-beams, so The gallery floor under-trussed with iron rods, shoes, and struts, as to distribute the whole weight girders. that may be brought upon the floor pretty equally upon the eight points at which the ends of the beams rest upon the girders. Joists, stretching from the iron girders to the beams, and from one of the beams to the other, form the supports for a floor which is not more than 1 inch thick, but is at once amply strengthened, and rendered impervious to the passage of dust, by the insertion, in a 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.

tween galleries

The columns which rise at the gallery level are 16 feet 7 inches long, and The columns beare surmounted by connecting pieces, similar in all respects to those occurring and roof. beneath. To these connecting pieces are attached, transversely in one direction and longitudinally in two, cast-iron girders of similar form and scantling to those we have described; their office being to maintain perfectly true and rigid, the vertical shafts which carry the eye upward in one unbroken line from the ground to the roof which they serve to support.

division into

As the strength of an iron column practically depends upon its length being Strengthened by limited, far more than upon its substance, the value of dividing the whole length of the shafts reaching from the gallery to the roof into two parts by these connecting-pieces, and thus reducing the length of the columns one-half, must be First tier. 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 girders supporting the roof are attached. These girders correspond with those supporting the galleries, and exactly resemble those forming the tier immediately beneath them, in every respect except their thicknesses. The whole of the girders on the upper tier have been proved in the building to a strain of nine tons.

trusses.

By extending the area of our observations, we shall be enabled to include all the varieties of trusses employed to support the flat roofing over the whole extent Varieties of roof of the building. It may be well, therefore, to consider that our original limitation to 24 feet square has been enlarged by the addition of a space of 72 feet by 24 feet, being a compartment of the roof over the nave; and of an area of the same width by 48 feet, being a portion of the roof over the avenue which extends from 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. trusses; avenue) rises to a height of two stories only from the ground, it will be manifest that its roof-trusses must be attached at the level of the girders which serve to stiffen 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 to that already described for the girders, with the exception that their vertical position is maintained by bolts passing through their standards and through the column, instead of by the system of keys as in the 24-feet girders.

construction.

In fig. 6 a representation is given of the principal parts of one of these trusses, Their details of which, it will be seen, is constructed for the most part of wrought iron; the few portions which are of cast iron acting only under compression. These trusses 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 in positions corresponding with those in the gallery-girders already described.