In connexion with the fixing of the girders, it may be desirable to mention the provision that was made for the expansion and contraction of the iron, which in so great a length as that of the building might have otherwise produced results prejudicial to its stability. Between the projections cast on to the connecting-pieces and those projecting from the ends of the girders which they were made to clip, sufficient space was left for the introduction of oak keys, by driving in which the girder was fixed in its place, whilst the compressibility of the wood left sufficient play for the expansion of the metal. In describing the girders, it was mentioned that in the upper and lower flat flanges small sinkings were cast near the ends. Corresponding with these sinkings, a notch was left in the projection which came out from the connecting-piece; and when the girder was put into its place, iron wedges were driven in between the notch and the sinking, by which means any lateral motion of the girder was prevented. It was a great advantage to have the means of fixing the girders of so simple a nature, as any arrangement presenting the least complication, or requiring great nicety, would have materially retarded the progress of the work. The wrought-iron trusses were held by the connecting-pieces in a similar manner to the cast-iron girders; but, as an additional security, bolts were passed through holes provided in the standards at the ends, and through the connecting-pieces, where they were screwed up with nuts. The raising and fixing of the extra-strong roof-trusses crossing the main avenue near the side of the transept required particular care, from their great weight; the heaviest being, as we have before mentioned, no less than eight tons. These trusses were the first that were fixed across the central avenue, and about 150 men were engaged in the hoisting of each one. They are secured to the columns by four strong bolts passing through the end-standards. In order to provide additional support for the great weight brought upon the last-mentioned trusses by the transept roof, extra columns were introduced underneath them. These were built up in storeys corresponding with those of the other columns, with which they were connected, at the levels of the girders, by bolts and straps. A cast-iron shoe, fixed on the top of the columns, provided a bearing for the ends of the truss. The columns just described project slightly into the main avenue from the line of the other columns; and this is the only instance in the interior of the building of the iron columns occurring at a less distance than twenty-four feet apart. Glazing the Roof.