agricultural districts, broken here and there by large forests, and in three places the river breaks through mountain ranges in narrow and rocky defiles. The third or uppermost of these, which is 140 miles above Bhamo and 1168 miles from the sea, is the head of navigation for boats and small steamers. Below this the river is wide and deep, and the current not too swift, even for sailboats. Steamers ply regularly to Bhamo, and the river is an important artery of commerce for the interior of Burma, though it has now been supplemented by a railroad running parallel with its valley from Rangoon to Bhamo, crossing the river at Mandalay. For 100 miles from the sea extensive embankments have been built to protect the low delta-plain from inundations. For a recent description of the river and the country on its banks, consult Mowbray, “A Sail Down on the Irrawaddy,” in the Scottish Geographical Magazine, vol. xvii. (Edinburgh, 1901).
IRREDENTISM (from irredentist, It. irredentista, from Italia irredenta (unredeemed Italy), irredenta being from Lat. in-, not + redemptus, p.p. of redimere, to redeem, from red-, back again + emere, to buy, take). A popular movement which originated in Italy after 1878, having for its object the recovery of what its adherents call ‘unredeemed Italy,’ Italia Irredenta, meaning all that territory at present belonging to Austria, Switzerland, France, and England having an Italian-speaking population, but no longer forming part of Italy. Under this description would come Southern Tyrol, Trieste, Görz, Istria, and Dalmatia, the Swiss Canton of Ticino, Corsica, Nice, and Malta. To all but the most exalted patriots the chimerical nature of the Irredentist programme is apparent, inasmuch as its execution would involve the surrender by Switzerland of territory it has held for more than three hundred years, by Austria of its only seaport, Trieste, and by England of its great base in the Central Mediterranean, Malta. Irredentism has ceased to be an influential factor save with the younger element in Italian politics. It received a severe setback in the formation of the Triple Alliance embracing Austria, the special foe against whom the Irredentist movement was aimed. See Political Parties, paragraph on Italy.
IRREDUCIBLE CASE. See Cubic Equation.
IRREFRAGABLE DOCTOR. See Alexander of Hales.
IRREGULARS (ML. irregularis, not according to rule, from Lat. in-, not + regularis, relating to rule, from regula, rule, from regere, to rule). A military term generally applied to partisan troops assisting the regular establishment. They are, as a rule, only partially trained and equipped, as, for instance, the Franc-tireurs (q.v.) in 1871; some of the Cossack (q.v.) regiments of Russia; the Bashi-Bazouks (q.v.) of Turkey; and the armies of the feudatory chiefs and rulers of India. See Armies.
IRRIGATION (Lat. irrigatio, from irrigare, to irrigate, from in, in + rigare, to moisten; connected with Goth, rign, AS. regn, OHG. regan, Ger. Regen, Eng. rain, and probably with Gk. βρέχειν, brechein, to wet). In agriculture, the method of increasing the productiveness of soils by an artificial supply of water. The practice of irrigation is very ancient. There is evidence to show that works for the storage and distribution of irrigation water were constructed in Egypt as early as B.C. 2000. Extensive works, intended for irrigation on a large scale, existed in times of remote antiquity also in Assyria, Mesopotamia, Persia, India, Ceylon, China, and other parts of the earth, as well as in Peru and Mexico in the Western Hemisphere. Remains of ancient irrigation works are found in the Southwestern United States (New Mexico and Arizona). In all of these regions irrigation is necessary for successful agriculture, because the rainfall is insufficient for the needs of ordinary crops. Irrigation is also required even in humid regions for crops, such as rice and cranberries, which require a large amount of water.
The area of the earth's surface over which the rainfall is deficient (less than 20 inches or 500 millimeters), and irrigation consequently a necessity for successful agriculture, is very extensive. In addition to this vast area in which, as a rule, agriculture is impossible without irrigation, there are considerable areas in the so-called humid regions in which the irregularity of the rainfall makes irrigation profitable. The extent to which the arid lands can be reclaimed depends upon the water-supply available for irrigation. It has been estimated that there is sufficient water to irrigate only about one-fifth of the arid region of the United States, or from 150,000,000 to 200,000,000 acres. Of this irrigable area probably less than 10,000,000 acres have already been reclaimed. In Europe irrigation prevails chiefly in the south, where it was introduced by the Romans. It is most extensively and systematically practiced in Lombardy. Spain, and the south of France, but exists to some extent in other parts of Europe. Wilson estimates the irrigated area in Italy to be about 3,070,000 acres, in Spain 500,000 acres, in France 400,000 acres. Nowhere is irrigation practiced on so large a scale as in India, and the irrigation systems of that country are being rapidly extended by the British Government. According to Wilson, the irrigated area in India is about 25,000,000 acres. Egypt follows with 6,000,000 acres, although irrigation works now in process of construction will vastly increase this area. Only at a comparatively recent date has irrigation been introduced into Australia, but it is rapidly extending there. The same is true in a measure of South Africa. The practice of irrigation has declined, or entirely disappeared, in many of those regions where it prevailed most extensively in remote antiquity.
Water-Supply for Irrigation. Water for irrigation is derived from (1) natural streams, springs and lakes; (2) wells; and (3) storage of storm waters. Occasionally also the sewage water of towns is used for irrigating purposes. The simplest and most common method of securing water for irrigation is to divert it from streams by means of a dam and a ditch or canal running along the sides of the valley of the stream, at a less grade than that of the stream itself. When the fall of the valley is great the canal can be readily led off to such a distance that extensive areas may thus be supplied with water. When the fall of the stream is slight, however, the canal may wind along close to the stream for miles before any extensive areas lie
