The earlier inventions in the textile industries, and the general application of steam to manufacture and to the transport services, have played the most dramatic part in the industrial revolution of the last hundred years. But it should be borne in mind that it is far from being true that the great forces of invention have spent themselves, and that we have come to an era of small increments in the growth of productive power. On the contrary, within this last generation a number of discoveries have taken place in almost all the chief industrial arts, in the opening up of new supplies of raw material, and in the improvement of industrial organisation, which have registered enormous advances of productive power. In the United States, where the advance has been most marked, it is estimated that in the fifteen or twenty years preceding 1886 the gain of machinery, as measured by "displacement of the muscular labour," amounts to more than one-third, taking the aggregate of manufactures into account. In many manufactures the introduction of steam-driven machinery and the factory system belongs to this generation. The substitution of machinery for hand labour in boot-making signifies a gain of 80 per cent. for some classes of goods, 50 per cent. for others. In the silk manufacture there has been a gain of 50 per cent., in furniture some 30 per cent., while in many minor processes, such as wood-planing, tin cans, wall-papers, soap, patent leather, etc., the improvement of mechanical productiveness per labourer is measured as a rise of from 50 to 300 per cent. or more. The gain is, however, by no means confined to an extension of "power" into processes formerly performed by human muscle and skill. Still more significant is the increased mechanical efficiency in the foundational industries. In the manufacture of agricultural implements the increase is put down at from 50 to 70 per cent., in the manufacture of machines and machinery from 25 to 40 per cent., while "in the production of metals and metallic goods long-established firms testify that machinery has decreased manual labour 33-1/3 per cent." The increase in the productive power of cotton mills is far greater than this. From 1870 to 1884 the make of pig-iron rose 131 per cent. in Great Britain and 237 per cent. in the rest of the world.[151] "In building vessels an approximate idea of the relative labour displacement is given as 4 or 5 to 1--that is, four or five times the amount of labour can be performed to-day by the use of machinery in a given time that could be done under old hand methods."[152]