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24                           M-ETALLURGY   OF   CAST   IRON.
and between charges, and at the same time reduce the weight of the iron in both the bed and charges, as, if the same weight of soft as of hard coke found best is used, the bed of fuel would be raised above that point best for rapid and economical melting. It is to be understood that this does not mean that a less weight of soft coke will be required throughout the whole heat. Reducing the weight of iron on the bed of coke and between the charges calls for a greater number of charges of coke, as well as of iron, and thus may cause as much or a greater weight of soft coke to run off a heat than if hard coke had been used. When using soft grades of coke and following the above suggestions, the rule of charging three pounds of iron to one of coke on the bed and ten to one-between the charges will often serve as a guide in decreasing the weight of iron to approximately correspond with the decrease in the weight of fuel that may be found best to adopt. This is assuming the height of tuyeres to be about eighteen inches above the bottom plate; with lower tuyeres three to five pounds of iron to one pound of .coke may often be charged on a bed of coke. For further information on charging, etc., of cupolas, see " American Foundry Practice " and " Moulder's Text Book."g coke in cupolas it is very important to note                  > \^f