For raising the roof-trusses of seventy-two feet span over the main avenue a somewhat different method was employed. A single mast or derrick, more than seventy feet high, was placed in the centre of the avenue, and steadied in an upright position by guide-ropes spreading from the top in various directions. Near its summit the hoisting-tackle was firmly lashed on. The trusses to be hoisted were brought from the places where they had been put together, and placed across the main avenue at the points where they were to be fixed. Two ends of a stout chain were passed round the upper portion of the truss, at points dividing its length into about three equal parts. To this chain the hoisting-tackle was attached, guide-ropes being further fastened to each end of the truss to steady it in its ascent. In order to stiffen the truss horizontally, struts were attached at the centre projecting on each side, and held in their place by tie-rods attached to the upper part of the truss, and forming a triangle on each side. Before the truss, therefore, could bend in a horizontal direction, the attachment of these tie-rods must have given way. Six horses drew out the end of the fall-rope, and in the course of a very few minutes the truss was hoisted to its giddy height, and each end slipped in between the projections made in the connecting-pieces to receive it.
The animated scene presented by these operations was highly interesting from the number of men employed, both on the ground and for fixing the trusses in their position aloft, and from the rapid progress so many hands made. Each gang of men was managed by a foreman, who was obliged to issue his orders through a speaking-trumpet, to enable his voice to be heard in the din caused by the other works going on around. Besides the two large gangs of men engaged in the hoisting of the trusses, other smaller gangs were at work at different points getting up the columns and girders. In one part, the roofing of which was completed as early as practicable, a crowd of carpenters were preparing the Paxton's gutters and other portions of the work. In another place, as soon as a sufficient space could be roofed over and a temporary floor laid, various parts of the machinery we have already described were fitted up and worked by portable steam-engines. Of these there were three in different parts: one drove the machinery for finishing the sash-bars, gutters, ridges, &c.; another worked the drilling, punching, and other machinery connected with the iron-work; and a third was used for working circular saws.
Of the number of trusses that were hoisted as above described, in only one instance (and that the first) was the result otherwise than perfectly successful. The first truss was raised by its ends, instead of from the centre; but that method was afterwards abandoned, from the difficulty of maintaining the truss in an upright position during its ascent; which was important, as, if it turned on its side, its lateral strength was not sufficient to prevent it from bending, which would have destroyed the joints of the work.
One of the tall masts was worked on each side of the transept, from the centre to the ends of the building, being maintained constantly in an upright position, while traversing from point to point, by alternate slackening and hauling up of the ropes which steadied it; and it was curious to witness the motion of these tall giants, as they slowly progressed from one point to another, in the performance of their important office. Stout planks were laid along the ground, upon which the foot of the mast was forced forward by crowbars and levers; the planks served also to distribute the weight, which would otherwise have sunk the end into the ground. As many as seven trusses were hoisted in one day by each derrick, which had therefore to travel a distance of 168 feet.
So careful were the men, under the direction of the manager (to whom was intrusted the active superintendence of the whole erection of the building), that no accident of importance occurred in these difficult operations.
HOISTING THE 72-FEET TRUSSES.
N 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.