Doubtless there is something very attractive about a large span roof, its bold outline stretching from side to side of a wide covered area imparts an imposing effect which cannot be claimed for smaller or more moderate spans; but where roofs are constructed for purely utilitarian purposes it becomes a question worthy of grave consideration whether a series of smaller spans would not provide the same practical benefits as would be obtained from one very large span. Upon referring to the typical sketch of a terminal station, Fig. 373, it will be seen that the total width from inside to inside of main walls is 240 feet. The lines and platforms are so arranged that by placing rows of columns at A, A, B, B, and C, C, the entire width may be divided out into four spans of 60 feet; or, if preferred, a row of columns at B, B may be adopted, resulting in two spans of 120 feet, or the entire width may be included in one large span of 240 feet. Any one of the three arrangements will provide an effectual roof-covering, and the selection must be decided by the cost or expediency. Another way to avoid the introduction of large span-roof principals, and to preserve the covered area free from intervening columns, is to erect strong truss-girders extending across at right angles from the main walls. These truss-girders are placed at suitable distances, and 272 carry simple roof-principals of convenient spans. In some cases the roof-principals are placed as shown in Figs. 407 and 408, and in others as in Fig. 409. In another system the roof-principals are incorporated with the main truss-girders, as in Fig. 410. With the above type of covering the truss-girders take the place of the arched wrought-iron girders and cast-iron columns, as illustrated in Fig. 406, but will be more costly, as may be gathered from the following brief comparison: Assuming the area to be covered as 480 feet long and 180 feet wide, then the width of 180 feet could be divided into three spans of 60 feet each, or one centre span of 65 feet, and two of 57 feet 6 inches if they would work in more conveniently. With columns at 48-foot centres longitudinally, the three-span arrangement would contain the following:--Twenty cast-iron columns in the two rows, or twenty-two columns if two columns are placed side by side at the extreme end; 960 lineal feet of light arched wrought-iron girders in twenty girders of 48 feet span. On the other hand, with the truss-girders placed at 40-foot centres to suit roof-principals resting on the tops of girders, as shown in Fig. 409, or to suit the arrangement shown in Fig. 407, there would be twelve heavy truss-girders, each of 180 feet span, making a total length of 2160 lineal feet of deep truss-girder work, exclusive of about another 60 lineal feet, which would be required for the bearings on the side walls.