. The lower rail is formed by two flat bars placed upright
, and these are riveted at the ends to standards of cast-iron, which, however, are considerably heavier in construction than those before described; and they have also in the centre, at (a) two slots, or sinkings, into which the ends of two of the diagonal bars are riveted. The whole length is then divided into three equal parts, each 24 feet long, by strong CAST-iron standards at (b) the ends of which are riveted between the rails, and these spaces are again subdivided into three eight-feet lengths by WROUGHT-iron standards at (c c). The top of each standard is next connected with the foot of the next but one to it by diagonal flat bars, which, together with the short pieces fastened into the slots at (a), complete the figure of the whole, forming a kind of trellis-work, two diamonds in depth. In the diagram only half the length of the girder is shown.
DIAGRAM OF ONE-HALF OF 72-FEET GIRDER.
The dimensions of the different bars of iron in this piece of construction are proportional to the amount of strain they have to bear. The two heavier out of the four trusses just described weighed when completed eight tons each, and the other two, which are of rather lighter construction, six tons each.
The riveting together of the wrought-iron trusses was performed on horizontal supports, on which the curve that they were to be made to was marked out. The bars having been previously cut to the requisite lengths, and punched and drilled with holes for the rivets, were laid out on the stages in the proper forms with the cast-iron standards, which were temporarily kept in place by bolts passed through some of the rivet-holes. The whole framework was then riveted up with red-hot rivets supplied from small portable furnaces, several sets of men being employed upon each truss, by which means as many as sixteen were completed in one day. The whole of the trusses, three hundred and seventy-two in number, required for the building were put together on the ground, and several ingenious mechanical contrivances were made use of to facilitate and hasten the work. To form some idea of the amount of labour that had to be performed, it may be mentioned that each of the trusses forty-eight feet in length, or the smallest, is held together by more than fifty rivets, requiring more than twice that number of holes to be made in bars of iron varying in thickness from a quarter of an inch upwards. About 25,000 rivets were thus required for the whole of the work.
HE holes for the rivets were made partly by drilling and partly by punching. In the machine used for the former the bar to be bored was laid upon a flat surface forming part of the solid cast-iron stand of the machinery; the drilling-point worked vertically, and could be moved in that direction to suit the different thicknesses of iron brought under its operation. It was suspended at one end of a lever, with a counterpoise at the other. This lever was also connected by a rod and crank, with another near the ground, one end of which was formed into a tread to be worked by the foot. The workman, when he had arranged the iron in the right position under the drill, pressed his foot upon the tread; thus raising the counterpoise end of the upper lever, and pressing the point of the drill, which was of a spear-head form, down upon the iron. Underneath the iron to be drilled was placed a piece of wood to protect the point of the drill when it had passed through the iron. It was also necessary to moisten the iron during the operation, in order to keep the drill-point cool. Three men were required to attend to this work, which was not so rapid as the other method of making the holes by punching.