By the early part of the eighteenth century the iron industry, after exhausting the timber supplies of Sussex, had disappeared from that county; but it flourished in Shropshire, where it found both fuel and iron-stone in the Forest {190}of Dean, while the Severn provided water-power and inland navigation. The industry was also carried on in Staffordshire; and here, in the reign of James I., some important experiments were made in the direction of using coal instead of wood in the manufacture of iron; but this idea was not fully developed until Abraham Darby had shown, in 1735, how coke, in combination with a powerful blast, could be substituted for wood. What is regarded as the real turning-point in the iron industry followed in 1760, when Dr Roebuck built, at the Carron works, his new type of blast furnace, in which coke was to be used. An impetus was thus given to the industry, and an impetus it certainly needed, inasmuch as the production of iron in the United Kingdom had sunk in 1740 to 17,350 tons. Then, in 1783, Henry Cort, of Gosport, patented his process for converting pig-iron into malleable iron through the operation of "puddling" in a common air-furnace consuming coal, and in 1784 he patented a further process for turning malleable iron into bars by means of rollers instead of forge hammers. These further inventions were of much service; but the greatest advance of all followed on the application of steam to iron-making, as one of the many results of James Watt's achievements. Steam enabled the manufacturers to get a far more powerful blast in the new furnaces, at a consumption of about one-third less of coal, than had been possible in the process of smelting carried on with the help of water-power. The use, also, of coal instead of timber for fuel, and of steam-power in place of water-power, made the iron-masters independent both of the forests and of the rivers of southern England, and led to the further expansion of the iron industry being transferred to such districts as Staffordshire, the north-east coast, Scotland and South Wales, where the now all-important coal could be obtained no less readily than the iron-ore. So the migration of some of the greatest of our national industries from south to north, begun by the streams on Yorkshire hills, was completed by the steam-engine of James Watt.