The full effect of all these changes was not felt until the nineteenth century, but their importance in this period can be measured by the imports of raw cotton. In the forty-three years, 1741-1784, the annual imports rose from 4,000 to 28,000 bales, while in the sixteen years following they increased to 150,000 bales (1800). =247. Slower development of the woolen manufacture.=—No such rapidity of development as this can be traced in the woolen manufacture, for it had long been England’s mainstay, and changed more slowly partly because it was so firmly established. Little by little, however, the spinning-wheel was displaced by the jenny, and other sources of power than the human body were utilized. As in the case of cotton, power weaving was not important until after 1800; but the manufacture of worsteds (in which the fibers are longer than in woolens, and are kept parallel) was greatly helped by a second invention of Cartwright, for wool-combing by machinery. =248. Development of the iron industry with the use of pit-coal.=—The only other industry of this period which our space allows us to treat is that of iron. Until the eighteenth century iron was made almost entirely by smelting with charcoal, the primitive process which can be traced back to prehistoric times. A ton of iron required two loads of charcoal, and a load of charcoal two loads of wood, so that the industry depended largely on the wood supply, and was carried on at petty forges scattered through England, but established mainly in the South. A large proportion of the English iron supply was imported from Sweden. Coal, as we use the word, called then pit-coal or sea-coal, had for centuries been mined for domestic use, but had no importance in manufacture. Various men tried to smelt iron by coal or coke, but their experiments had no practical result till about 1760, when blast furnaces using coal were successfully established, and the industry began a period of rapid development, furthered about 1790 by the application of steam-power to the blast. Henry Cort invented in this period the processes by which pig was changed to malleable iron in a coal-puddling furnace, and the malleable iron was worked into bars by rollers instead of by the slow action of forge hammers. The production of iron had increased fourfold (17,000 to 68,000 tons) in the period 1740-1788, and in the period of eight years following nearly as much again was produced (125,000 tons in 1796). The industries depending on iron passed into a new stage, and the large export of iron and steel in 1800 is explained.