27* RAILWAY CONSTRUCTION. 



suitable distances, and 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. 



The successful lighting by day of a large roofed-in station 

 will depend principally upon an appropriate distribution of the 

 glazed portions. With a large span, and the glass skylights 

 placed near the apex, the side lines and platforms will be much 

 less efficiently lighted than those near the centre ; and again, if 

 the glazed parts are only at the sides, then the centre portion 

 will be rather in the shade. Where possible it is better to place 

 the glazed portions and slated portions alternately, so as to 

 obtain a more uniform light all over the centre area, somewhat 

 similar to the arrangement shown in Fig. 406. 



Roofs over passenger platforms at roadside stations are made 

 in many types, the arrangement depending in a great measure 

 upon the width of platform to be covered. In many of the 

 earlier stations the roof was extended across from side to side, 

 and included the lines of rails as well as the UP and DOWN 

 platforms, a system which was not only costly, but had the 



