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to have their contents ready, and to pour them in succession into the ingot mould.
Another way is to make the steel by the converter plan, wherein fused cast iron is decarbonised, by the blowing in of air, which burns off the carbon, leaving the contents of the vessel in the state of fluid wrought iron, and then to run in so much of a metal rich in carbon as shall give to the whole charge, the proportion required. The third mode, the open hearth, consists in melting the material in gas furnaces, and so proportioning the mixture, as to obtain the desired amount of carbonisation, ascertaining before tapping, what the condition really is, by means of test pieces.
For gun purposes, we may confine our attention to the first, and to the third of these three modes. The ingot is, in either case, cast vertically, and in order to obtain freedom from blow-holes and other imperfections, the upper part is cut off and is rejected. Commonly, if the weight of the forging is to be 30 tons, the ingot as cast may be from 45 to 50 tons, that is, only some 60 per cent, are used, while some 40 per cent, are rejected.
Now I need hardly say, that ingots of 50 tons, are a considerable time in solidifying, and during this time an effect takes place, which is a source of trouble, and of uncertainty. The carbon tends to separate out, and to come upwards, with the result, that even if the whole mass had when run, a perfectly uniform percentage of carbon throughout, by the time a large ingot has set, there will be found a decrease of carbon at the lower end, and an increase of carbon at the upper end. You must not think this a matter of slight importance, for assuming all other things to be equal, it may be said, roughly speaking, that unless in every thousand parts of the steel, there are present about two and a half parts of carbon, the metal will be too "low" or "mild," while if there be more than about four parts, the metal will be too "high" for the purposes of gun manufacture.
Allowance has to be made, therefore, in proportioning the
