28 RAILWAY CONSTRUCTION. 



19. In cases where bridges or viaducts are constructed wholly 

 or partially of timber, a sufficient factor of safety, depending on 

 the nature and quality of the timber, to be provided for. 



N.B. The heaviest engines, boiler trucks, or travelling 

 clones in use on railways afford a 'measure of the greatest moving 

 loads to which a bridge can be subjected. The above rules apply 

 equally to the main transverse girders and rail-bearers. 



20. It is desirable that viaducts should, as far as possible, be 

 wholly constructed of brick or stone, and in such cases they 

 must have parapet walls on each side, not under 4 feet 6 inches 

 in height above the rail level, and not less than 18 inches 

 thick. 



Where it is not practicable to construct the viaducts of brick 

 or stone, and iron or steel girders are made use of, it is con- 

 sidered best that in important viaducts the permanent way 

 should be laid between the main girders. In all cases sub- 

 stantial parapets, with a height of not less than 4 feet 6 inches 

 above rail-level must be provided by an addition to the 

 girders, unless the girders themselves are sufficiently high. 

 On important viaducts where the superstructure is of iron, 

 steel, or timber, substantial outside wheel-guards to be fixed 

 above the level of, and as close to the outer rails as possible, but 

 not so as to be liable to be struck by any part of an engine or 

 train running on the rails. 



In the construction of the abutments or piers which support 

 the girders of high bridges and viaducts, cast-iron columns of 

 small size must not be used. 



In all large structures a wind-pressure of 56 Ibs. per square 

 foot to be assumed for the purpose of calculation, which will be 

 based on the rules laid down in the report, dated 30th May, 

 1881, of the committee appointed by the Board of Trade to con- 

 sider the question of wind-pressure on railway structures. 



21. The upper surfaces of the wooden platforms of bridges 

 and viaducts to be protected from fire. 



22. All castings for use in railway structures to be, where 

 practicable, cast in a similar position to that which they are 

 intended to occupy when fixed. 



23. The joints of rails to be secured by means of fish-plates, 

 or by some other equally secure fastening. On main lines, and 

 lines where heavy traffic may be worked at high speed, the 

 chairs not to weigh less than 40 Ibs. ; but on branch Hues, or 



