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LEFT IBON STONE 197 IRRIGATION IRON STONE, a "stone" or mineral into the composition of which iron largeiy enters. IRONTON, a city and county-seat of Lawrence co., 0.; on the Ohio river, and the Cincinnati, Hamilton, and Dayton, Detroit, Toledo and Ironton and the Nor- folk and Western railroads; 127 miles S, of Columbus. It contains a Memorial Hall, Masonic and Odd Fellows' Halls, city hospital, Briggs Public Library, Kelley and Riverview Parks, electric street railroad and lighting plants. Holly system of waterworks, 3 National banks, and manufactories of fire brick, iron and steel, nails, and other commodities. The city is the mining, manufacturing, and farming trade center for a region within a radius of 30 miles, and is at the base of a range of lofty hills containing large deposits of iron ore and bituminous coal. Pop. (1910) 13,147; (1920) 14,007. IRONWOOD, a name given to various trees from the quality of their timber. The ironwood or hop-hornbeam of Amer- ica (Ostrya virginica), order Cupuliferse, is a tree with a trunk not exceeding six inches in diameter, with very hard wood, so heavy that it sinks in water, and foliage resembling that of birch. The species of the genus Sideroxylon, known as ironwood, are natives of the tropics and also of New Zealand, the Cape, etc. The S. inerme, or smooth ironwood of the cape, has long been cultivated in the greenhouses of Europe. Diospyros Eh- enum (the ebony) is also named iron wood, as is the Metrosideros -vera of Java. IRONWOOD, a city in Gogebic co., Mich., on the Chicago and Northwestern and the Minneapolis, St. Paul and Sault Ste. Marie railroads; 39 miles E. of Ashland, Wis. It is the trade center for the famous Gogebic iron region. Here is the Norrie Mine, once the largest pro- ducer of iron ore in the world, and there are also other large mines. The city has a National bank, trolley line con- necting with Hurley and Gile, Wis., weekly newspapers. Pop. (1910) 12,- 821; (1920) 15,739. IRONY, a kind of ridicule which ex- poses errors or faults by seeming to ap- prove, adopt, or defend them. The So- cratic irony is employed in argument when one speaker affects to take the positions of the other for granted, in order adroitly to lead him into self-con- tradiction or obvious absurdity. Veiled sarcasm is a brief modern definition of irony. IROQUOIS (ir'6-kwois), a confedera- tion of Indians formerly occupying the W. and central portion of New York State, consisting at first of five tribes, the Oneidas, Mohawks, Cayugas, Sene- cas, and Onondagas. In 1712 the Tusca- roras were admitted to the league, which adopted the title of the "Six Nations," and is so known to history. The Iroquois were at once the most powerful and the most enlightened; they lived in villages and pursued agriculture. In the Revolu- tionary War they were allies of the Eng- lish, but in 1779 they wei-e attacked by General Sullivan, and greatly injured. The Iroquois present the curious anom- aly among Indian peoples of steadily increasing in numbers since 1812. Most of them have been removed to various reservations farther W. IRRATIONAL, in mathematics any quantity which cannot be exactly ex- pressed by an integral number, or by a vulgar fraction: thus V 2 is an irra- tional quantity, because we cannot write for it either an integral number or a vulgar fraction; we may, however, ap- proximate to it as closely as may be de- sired. In general, every indicated root of an imperfect power of the degree indi- cated is irrational. Such quantities are often called surds. IRRIGATION, the process of water- ing or moistening land by ditches or other artificial means. It is probably the earliest application of science to agri- culture. During the last half of the 19th century the lands watered from the an- cient irrigation works of India were more than trebled by the completion of the Ganges canal system, the largest and costliest in the world, and by the more systematic and eff'ective operation of works previously built. The land re- claimed and the value of the products of irrigation in the historic Nile valley have been largely augmented with the completion of the great dam at Assuan, at a cost of $24,000,000. Since the con- struction of the first ditches in Utah by the Mormon pioneers, irrigation has marvelously extended the domain of en- terprise and civilization in all parts of the world. It has wholly changed the appearance and prospects of the W. third of the United States. It promises to make the Northwest Provinces of Canada one of the grain fields of the British em- pire. The irrigated farms of the Aus- tralian provinces of Victoria and New South Wales already rival, and will in time surpass, in value the livestock in- dustries which were first established on the arid plains of that continent. With- in the past quarter of a century irriga- tion has made the Hawaiian Islands one of the chief sources of supply for the