CONSTRUCTION OF THE BUILDING.
55
the upper portion of the connecting-piece extends, has a groove sunk upon itssurface; a groove corresponding to it in width is also sunk upon the projectionof the connecting-piece (C, fig. 4), and a small piece of iron is introduced betweenthe two. This iron acts as a key or dowel, and prevents the two surfaces slidingupon one another.
On the upper and lower part of the connecting-piece, between these projections (thereby <■„>,-which serve to retain the girders in their places, are cast holes, corresponding with coTumnsj 11 *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 arrangementwould enable any number of columns and connecting-pieces to be attached to oneanother, so as to make up one long length.
In order to make sure that the shaft thus composed of alternate columns and serving to strivenconnecting-pieces, should be capable of maintaining itself in a perfectly vertical ofduimte 18tinKposition, it was necessary that the whole of the surfaces of contact should bewrought 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 aperfectly true plane. When the number of these columns, base-plates, and connect- All tearing facesing pieces is taken into consideration, it may be easily imagined that the labour pUned “ lathes 'entailed by this apparently simple necessity could scarcely have been performedin any workshops but those provided with extraordinary facilities and resources.
In the connecting pieces of the 24-feet bay we are now describing, projectionsare 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-feetavenue or court on the other side.
The girders, which are attached, as above described, to the connecting pieces, 'file girders sup-serve to support the gallery floor. As, in the construction of this floor, it had Soor” s sallerybeen determined to bring the accumulation of pressure on the girders, uponpoints at 8 feet intervals, it became necessary, in arranging the form of thegirders, to concentrate strength at those points. The vertical lines of the girderhave, therefore, been arranged to occur at intervals of 8 feet, connecting the topand 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 connectthe junction of these standards with the top and bottom tables, and the principalparts 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