This increase of height adds to the mass of the contained combustible,--additional air is therefore required for effecting its complete inflammation, and the joint effect is, that a much higher temperature is generated. By this, the earthy matters either contained in the ores, forming portions of the combustible, or added as _fluxes_, are rendered fusible at a less expense of oxide of iron; the carburet formed, becomes more fluid, and the product is more likely to assume the character of grey pig-iron. Charcoal, as in the other processes, was the fuel originally employed, and is still principally used in most countries. But coal deprived of its volatile parts, and charred or converted into coke, has been substituted in some regions, as will hereafter be stated. Each of these combustibles requires a furnace of appropriate character, and demands a difference in the mode of management. A blast-furnace is a hollow chamber enveloped, generally speaking, in a mass of masonry, of the form of a truncated pyramid. The chamber is composed essentially of three parts; the upper has the figure of a truncated cone, whose greatest base is lowest: this may be called the body of the furnace; the middle portion has also the figure of a truncated cone, whose greater base is uppermost, and is common to it and the upper portion: this contraction is called the _boshes_ of the furnace; the lower position is called the hearth, and is usually enclosed on three sides by walls of refractory substances, on the fourth it is bounded by two stones, one serving as a lintel, which is called the tymp, the other resting on the foundation, and known by the name of the _dam_. Such at least is the shape of the blast furnaces in common use, and which will suffice for our present purpose.