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.
E have now traced the erection of the building up to the level of the roof, in which it will be readily conceived the operation of glazing was one of extreme difficulty, there being no scaffolding to aid the workmen in conducting their operations. When the glazing was first commenced a light scaffolding was suspended from the rafters; but this was found to be too tedious and troublesome a method of proceeding for so large an extent of roofing. It was, moreover, of great importance that some means should be devised for completing this part of the construction independently of the weather; a matter of some moment, when it is remembered that the work had to be done in the winter, when in our climate such operations are liable to be very much impeded by heavy rain. The arrangements made to meet this difficulty, as well as some others for carrying on the works, are very clearly described in a paper by Mr. Digby Wyatt, read at the Institution of Civil Engineers, on the 14th January, 1851, from which we quote some passages, by permission, for the benefit of our readers.
With reference to the means employed for glazing the roof he says: "To effect this purpose, a travelling stage was devised by Mr. Fox, which superseded the necessity of any scaffolding for glazing, and by means of seventy-six of these machines nearly the whole of the work has been executed. The stage was about eight feet square, and rested on four small wheels travelling in the Paxton's gutters. It thus embraced a width of one bay of eight feet of the roof, with one ridge and two sloping sides. Each bay in width required, therefore, a separate stage."
"Each stage was occupied by two workmen, and was covered by an awning of canvass stretched over hoops, to protect them in bad weather, and was further provided with a box on each side to contain a supply of glass. The sash-bars and other materials were piled upon the stage itself, the centre of the platform being left open for the convenience of hoisting up materials, for which purpose there was a small iron arm with a single block pulley."
