=325. Technical history of the steam-engine.=—The steam-engine has been in practical use in Europe since about 1700. The earliest engines, however, seem ludicrously crude now, and could be used only for pumping water. Progress was slow until the last part of the eighteenth century, when James Watt introduced improvements (separate condenser, double-acting piston, use of cut-off, etc.), which greatly increased the efficiency of the engine, and caused a gradual extension of its use from mining to manufactures. The introduction of the non-condensing, high-pressure engine about 1800 prepared the way for the use of steam on railroads. The compound engine, in which the steam passes through two or more cylinders before it is allowed to escape, was invented about the same time, though it was not brought into general use until about 1850. Since then improvements in details of the engine and in the form of boilers have enhanced still further the efficiency of steam power, until it now produces about two thirds of the work possible under ideal conditions. Practical engineers expect now no rapid progress or startling changes. Some measure of the progress achieved is furnished by the fact that Watt’s engines required ten pounds of coal an hour for each horse-power, the engines of the next generation required five, while the best modern engines require but one and a half, or, in rare cases, one. The previous paragraph referred to the reciprocating engine in which the piston moves constantly forward and back. Since rotary motion is the form in which the power is commonly transmitted and applied, it would be desirable to get the motion in this form originally, and many attempts have been made to make rotary engines. Success has been attained in the case of the steam turbine, in which jets of steam strike against the blades of a turbine wheel, and cause it to revolve. Originally applied by Dr. De Laval of Sweden to operate the centrifugal cream separator which he had devised, it has come into common use for the generation of electricity and for the propulsion of ships. While the steam turbine has proved its efficiency for special purposes it is less adaptable than the old form of reciprocating engine, and still leaves to that the larger part of the field.