The rotary engine is gradually coming into use for various special purposes, where small power is called for, and where economy of fuel is not important; but it has never yet competed, and may perhaps never in the future compete, with the reciprocating-piston engine where large engines are required, or where even moderate economy of fuel is essential. This form of engine has assumed so little importance, in fact, in the application of the steam-engine, that comparatively little is known of its history. Watt invented a rotary engine, and Yule many years afterward (1836) constructed such engines at Glasgow. Lamb patented another in 1842, Behrens still another in 1847. Napier, Hall, Massey, Holly, La France, and others, have built engines of this class in later times. Nearly all consist either of cams rotating in gear, as in those above sketched, or of a piston set radially in a cylinder of small diameter, which turns on its axis within a much larger cylinder set eccentrically, the piston, as the former turns, sliding in and out of the smaller cylinder as its outer edge slides in contact with the inner surface of the larger. In some forms of rotary engine, a piston revolves on a central shaft, and a sliding abutment in the external cylinder serves to separate the steam from the exhaust side and to confine the steam expanding while doing work. Nearly all of these combinations are also used as pumps. Fire-engines, made by the best-known American builders of engines, with reciprocating engines and pumps, such as are in general use in the United States, have become standard in general plan and arrangement of details. These are probably the best illustrations of extreme lightness, combined with strength of parts and working power, which have ever been produced in any branch of mechanical engineering. By using a small boiler crowded with heating-surface, very carefully proportioned and arranged, and with small water-spaces; by adopting steel for running-gear and working parts wherever possible; by working at high piston-speed and with high steam-pressure; by selecting fuel with extreme care--by all these expedients, the steam fire-engine has been brought, in this country, to a state of efficiency far superior to anything seen elsewhere. Steam is raised with wonderful promptness, even from cold water, and water is thrown from the nozzle at the end of long lines of hose to great distances. But this combination of lightness with power is only attained at the expense of a certain regularity of action which can only be secured by greater water and steam capacity in the boiler. The small quantity of water contained within the boiler makes it necessary to give constant attention to the feed, and the tendency, almost invariably observed, to serious foaming and priming not only compels unintermitted care while running, but even introduces an element of danger which is not to be despised, even though the machine be in