very thick lining of the best fire-resisting material. At the bottom of the converter is a chamber called a "tuyère-box," from which a number of "tuyères" made of baked fire-clay pass upward through the lining of the vessel.

The "converter" is hung upon "trunnions" (much as cannon are), through one of which the blast is conveyed to the "tuyère-box"; these trunnions rest Fig. 57.—Section of a Bessemer Converter. in "bearings" which, in some of the earlier converters, were on the top of tall iron stands (the base of one of such stands is seen in Fig. 57) firmly bolted to foundations of masonry; but in more modern constructions these bearings are supported by iron girders sustained by columns, or masonry piers are carried up of sufficient height to receive them. The air blast being carried through the hollow trunnion permits the turning of the vessel upon its bearings without interrupting the blast by so doing; this turning is in modern practice generally effected by some adaptation of hydraulic machinery, and occasionally by "worm" or "spur" gearing as shown in Fig. 58. The pressure of blast used was at first four to five pounds, but this was soon increased to eight, and, although there are still a few converters blown at this pressure, in the larger establishments a blast of twenty to twenty-five pounds is usually employed. Why such pressures are used, involving very heavy and expensive machinery and an excessive amount of power, is not evident, as no better or cheaper steel can be made thereby.

The operations for making steel are as follows: The converter, after having been newly lined, is thoroughly heated by means of a coke fire built therein, and urged by a blast through the tuyères; when the lining is sufficiently hot, the converter is emptied, sufficient blast being used to blow out all the dust, etc. The vessel is