2. _Machinery of many sorts has long been used, but the "age of machinery" begins with the eighteenth century._ Inventions, new machines, and new processes, though not frequent, were not unknown in the Middle Ages; but no one class of machines took possession of a whole field of industry and gave rise to a great economic problem by the displacing of labor. The great industrial changes in the Middle Ages generally grew out of political changes, or of changes of routes of trade whereby large industries were disturbed, or of changes in the use of land through new methods and the bringing into use of land in other places. The industrial changes in England at the end of the eighteenth century on the contrary were due mainly to great mechanical inventions. The development of the textile machines for cotton and wool spinning and weaving mark the beginning of the movement. Here for the first time were inventions in such numbers, of such a nature, and under such conditions, that they were rapidly and widely applied, affecting the lives of a great number of workers. The steam-engine at the same time opened up the long line of mechanical inventions by which wood and iron are shaped and wrought, and the iron industry underwent notable developments. Since that time, have taken place in all Western countries that rapid expansion in the use of machines and those notable changes in industrial organization which distinguish our era from all others. [Sidenote: Increased use of power] 3. _Machinery is applicable in very different degrees to the different processes and industries._ Machinery can save much labor in some directions, little or none in others. It is especially adapted to the application of power. In the United States, in 1870, in manufactures alone, two and one third million horse-power were used; in 1900, eleven and one third million, the increase being five-fold. It is said that in the world, in 1870, three and one half million horse-power was furnished by stationary engines, ten millions by locomotives. Probably to-day the total is four-fold as great. [Sidenote: Machines can best be used in manufactures] Machinery is applicable with especial advantage to industries that change the form of materials easily transported and widely used. There must be a large output to justify the use of machinery. In 1840 a man's work in spinning cotton was three hundred and twenty times as effective as in 1769, in 1855 it was seven hundred times; and though the rate of improvement is diminishing, to-day the productivity of such labor is still greater. Similar examples are found in the manufacture of shoes, and in all varieties of wood- and iron-work. Machinery is most applicable where there is a compact plant; not so easily where the power has to be distributed over a wide area, unless a special track can be provided. [Sidenote: Not to so great an extent in agriculture]