In this furnace the lime and clay, or earth of flint, unite, and form a sort of glass, which floats upon the surface. At the same time the carbon, or pure charcoal, of the fuel, with the assistance of the limestone, mixes with the stone, or ore, and melts the iron, which, being heavier than the other matters, runs down to the bottom of the furnace, and remains there till the workman lets it out by a hole made at the bottom of the furnace for that purpose, and plugged with sand. When the workman knows there is enough melted, or when the appointed time arrives, he displaces the plug of sand with an iron rod, and the melted iron runs out like water, and is conveyed into furrows made in sand, where it cools, and the pieces formed in the principal furrows are called "sows," and those in the furrows branching from them "pigs." We are now advanced a considerable way towards the production of a knife. We have the materials of a knife. We have the iron extracted out of the iron-ore. Before we trace the progress of a knife to its final polish, let us see what stupendous efforts of machinery have been required to produce the cast iron. In every part of the operation of making iron--in smelting the iron out of the ore; in moulding cast iron into those articles for which it is best adapted; in working malleable iron, and in applying it to use after it is made; nothing can be done without fire, and the fuel that is used in almost every stage of the business is coal. The coal trade and the iron trade are thus so intimately connected, so very much dependent upon each other, that neither of them could be carried on to any extent without the other. The coal-mines supply fuel, and the iron-works give mining tools, pumps, railroads, wheels, and steam-engines, in return. A little coal might be got without the iron engines, and a little iron might be made without coals, by the charcoal of wood. But the quantity of both would be trifling in comparison. The wonderful amount of the production of iron in Great Britain, and the cheapness of iron, as compared with the extent of capital required for its manufacture, arises from the fact that the coal-beds and the beds of iron-ore lie in juxta-position. The iron-stones alternate with the beds of coal in almost all our coal-fields; and thus the same mining undertakings furnish the ore out of which iron is made and the fuel by which it is smelted. If the coal were in the north, and the fuel in the south, the carriage of the one to the other would double the cost.