The wrought-iron girders, or trusses, are partly forty-eight and partly seventy-two feet long, to span the avenues of those respective widths; the principle of the construction is the same in each. The top rail (if it may be so called) of the truss is formed with two pieces of [L section] iron placed back to back [double L sections], and the bottom rail with two flat bars [parallel flat bars], the total depth being three feet; at the ends these bars are riveted on to cast-iron standards, and the intermediate distance is divided into eight-feet lengths by other cast-iron standards, to which the bars are also riveted, and thus a framework of rectangles is formed. In the trusses forty-eight feet span there are, therefore, six such divisions in the length, and nine in those of seventy-two feet span. These are then divided in the direction of ONE of the diagonals by a flat bar passing between and riveted to those forming the top and bottom rails. This completes the constructional part of the truss; but to render the appearance more uniform with that of the cast-iron girders, a flat bar of wood (shown by the dotted lines) is made to form the other diagonal of the rectangles. [Illustration: DIAGRAM OF 48-FEET GIRDER.] The trusses for a span of seventy-two feet are cambered or bent upwards about ten inches, which both adds to their strength and improves the appearance. The form and arrangement of these roof-trusses may be clearly traced in several of the views of the interior which are presented to the reader. The weight, when completed, of each of the trusses of seventy-two feet span is about thirty-five cwt., and of those of forty-eight feet span about thirteen cwt.