Nearly all the people now engaged in iron-works are supported by the improvements that have been made in the manufacture, _by machinery_, since 1788. Yes, wholly by the machinery; for before then the quantity made by the charcoal of wood had fallen off one-fourth in forty-five years. The wood for charcoal was becoming exhausted, and nothing but the powerful blast of a machine will make iron with coke. Without the aid of machinery the trade would have become extinct. The iron and the coal employed in making it would have remained useless in the mines. And now, having seen what is required to produce a "pig" of cast iron, let us return to the knife, whose course of manufacture we traced a little way. The lump of cast iron as it leaves the furnace has many processes to go through before it becomes fit for making a knife. It cannot be worked by the hammer, or sharpened to a cutting edge; and so it must be made into malleable iron,--into a kind of iron which, instead of melting in the fire, will soften, and admit of being hammered into shape, or united by the process of welding. The methods by which this is accomplished vary; but they in general consist in keeping the iron melted in a furnace, and stirring it with an iron rake, till the blast of air in the furnace burns the greater part of the carbon out of it. By this means it becomes tough; and, without cooling, is taken from the furnace and repeatedly beaten by large hammers, or squeezed through large rollers, until it becomes the bar-iron of which so much use is made in every art of life. [Illustration: Rolling bar-iron.]