Except for purely ornamental work and a few minor details of the operating machinery of drawbridges, cast iron will not be allowed to be used in the superstructure of any bridge, trestle, viaduct, or elevated railroad, cast steel being employed wherever important castings are necessary.

ROLLED STEEL.

All steel shall be manufactured by either the acid or basic open-hearth process, and must be uniform in character for each specified kind. Any attempt to substitute Bessemer or any other steel for the open-hearth product will be considered as a violation of the contract and a good and sufficient reason for cancelling the same.

All plates shall be rolled from slabs. These slabs shall be made by a separate operation, by rolling an ingot and cutting off the scrap. The original ingot shall have at least twice the cross-sectional area of the slab, and the latter shall be at least six times as thick as the plate.

All finished material coming from the mills must be free from seams, flaws, or cracks, and must have a clean, smooth finish.

COMPOSITION OF ROLLED STEEL.

The greatest allowable percentages of certain principal ingredients of the various kinds of rolled steel shall be as given in the following table: