It has been already mentioned that four of the roof-trusses vary from the rest on account of the greater load they have to sustain. The depth of these exceptional trusses is six feet, and their length seventy-two feet, or the width of the main avenue, which they bridge over. The principle of their construction is similar to that employed in the lighter trusses; but the arrangement of the parts is somewhat modified. The top rail consists of two pieces of [L section] iron, placed, as before, back to back; but they are further connected on the top by a flat piece [double L sections with flat]. The lower rail is formed by two flat bars placed upright [parallel flat bars], 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. [Illustration: 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.