The charge of charcoal being determined upon such principles, it is added by measure, and always in equal quantities, while the proportion of ore and flux is made to vary, not only by a gradual increase at the beginning of the operation, but according to the working of the furnace. The manner in which the furnace is working can be inferred, even before its products are ascertained, by the appearance of the flame at the trundle-head, and at the tymp, by the manner in which the charge descends, and more surely still, by the appearance of the scoriæ. By a strict attention to these circumstances the proportion of the charge of ore may be regulated. A fortnight usually elapses from the time of the first charge until it reaches a regular state of working, and variations will occur even after that period, in consequence of the greater or less moisture of the combustible and minerals, the continual wearing away of the sides of the furnace, the variations in the state of the atmosphere, and in the play of the blowing machines, the greater or less attention of the workmen, and numerous other accidental circumstances. The mode of proceeding when coke is the fuel employed, rests upon the same principles, but the dimensions of furnace that are best suited to the different combustibles are different. As a general principle, the height of furnaces must depend upon the force of the blast and the density of the fuel. If the fuel be dense, and the blowing machine weak, the furnace must not have a great height; and even if the blast can be made strong, too high a furnace is disadvantageous for light charcoal. Coke, on the other hand, may be used in furnaces of greater height than any species of charcoal, provided the blast be of sufficient power. So long as the imperfect bellows were used in blowing, the height of the furnace was limited wholly by their action. More powerful apparatus in the form of cylinders, analogous in form and arrangement to those of steam-engines, and like them, either single or double acting, have now been introduced; the intensity of the blast is in them only limited by the moving power, which is applied to them, and when this is the steam engine, it may be said, that no limit can arise from the want of blast. We may, therefore, at the present day, regulate the height of furnaces by the nature of the fuel that is consumed in them.