COMPARATIVE FUSIBILITY OF FOUNDRY METALS. 325 The comparative test cupola seen on page 241 is not an expensive affair, and is such as might often be a valuable adjunct to the laboratory of metallurgists, blast furnaces, foundries, etc., besides being useful for the production of repairs for breakdowns, etc., and then again for small castings, which it may be desirable to make of two separate grades of metal. It will be well to state that, where only one kind of iron is desired to be melted the center blast can be closed and the iron made to run to one tap hole by having one slanting bed as in regular cupola practice. In designing the cupola (Fig. 56) I arranged fora center blast, besides having outside tuyeres on the plan shown. This permits the greatest possible uniformity of combustion throughout the area of the cupola and affords every opportunity of regulation should the heat, from any cause, be greater in one portion than in another. This regulation is secured by diminishing or increasing the volume of blast by valves attached to branch pipes, not shown, leading to the tuyere openings A A, B B, and E. It may be asked, How is it possible to know when there is perfect uniformity of heat all over the area of the cupola? This is indicated by the color of the flame emanating from the open top of the cupola. If any difference should exist there on cither side, the eye will dete.ct it as quickly as the steel maker can note changes taking place in a Bessemer converter by means of the spectroscope. In operating this cupola the sand bed is put in with two slanting bottoms, as seen at H H, thus preventing either metal, as it comes down, from mingling with the other. The center tuyere has three pieces of Vn -inch round iron laid over its opening, as seen at M, highest heat, with the relative gradations of others, down to that most readily fused.the bottoms. Our «Mj)pnrheii.sion as to loss of iron through slai;" was r> ">'•• I I O/..