In a wrought-iron or steel bridge, the greatest load which can be brought upon it, added to the weight of the superstructure, not to produce a greater strain per square inch on any part of the material than five tons where wrought-iron is used, or six tons and a half where steel is used. The engineer responsible for any steel structure to forward to the Board of Trade a certificate to the effect that the steel employed is either cast-steel, or steel made by some process of fusion, subsequently rolled or hammered, and of a quality possessing considerable toughness and ductility, together with a statement of all the tests to which it has been subjected. 19. In cases where bridges or viaducts are constructed wholly or 28 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 cranes 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 considered best that in important viaducts the permanent way should be laid between the main girders. In all cases substantial 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 lbs. 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 consider 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.