POTATO FLY 327 POTOMAC, ABMY OF are attacked and decay, either in a moist manner, attended by a disagreeable odor, or by a drying up of the tissue. The potato disease first appeared in the United States. In 1845-1847 it caused the failure of the potato crop in Ireland, producing famine. It has never since completely disappeared. When it is prevalent, the potatoes should be pow- dered with flowers of sulphur before be- ing planted. POTATO FLY {Anthomyia tuberosa), a dipterous insect of the same genus with the radish fly, cabbage fly, turnip fly. In its perfect state it is very like the house fly. The potato-frog fly {Euteryx solani, Curtis) and the caterpillar of the death's-head moth {Achercmtia atropos, Linn.) feed on the leaves and stems of potatoes, but rarely do serious damage. POTEMKIN, GREGORY ALEXAN- DROVITCH, a Russian general, a favor- ite of the Empress Catharine II.; bom in September, 1736; descended from an ancient Polish family, and early trained to the military profession. From 1776 till his death, he exercised a boundless sway over the destinies of the empire. In 1783 he suppressed the khanate of the Crimea, and annexed it to Russia. In 1787, being desirous of expelling the Turks from Europe, he stirred up a new war, in the course of which he took Oczakoff (1788). In the following year he took Bender, but as the finances of Russia were now exhausted Catharine was desirous of peace. Potemkin, how- ever, resolved on conquering Constanti- nople, and went to St, Petersburg to win over the empress to his side (March, 1791) ; but during his absence Catharine sent plenary powers to Prince Repnin, who signed a treaty of peace. Potemkin died in Nicolaieff, Oct. 16, 1791. POTENTIAL, in electricity, a term holding the same relation to electricity that level does to gravity. The potential of the earth is taken at zero. Potential in physics is the sum of each mass-ele- ment of the attracting body divided by the distance of that element from the attracted point. Also, capable of being exerted, though not acting at the particu- lar moment. POTENTIAL MOOD, that form of a verb which is used to express power, pos- sibility, liberty, or necessity of an action or of being; as, he may go, you should write. POTENTILLA, cinquefoil, the typical genus of Potentillidse. Flowers white or yellow, rarely red; calyx, five, rarely four lobed, with as many small bracts; petals, five, rarely four: stylo, short. lateral, or nearly terminal; achenes, many, minute, on a small, dry receptacle. Chiefly from the N. temperate and Arctic zones. Known species, 120. POTENZA, a tov^Ti of southern Italy; in a valley of the Apennines; 103 miles E. by S. of Naples. It is surrounded by a wall, has a fine cathedral, and disused fortifications. Potenza was shaken by earthquakes in 1273, 1694, 1812, and 1857. Pop. (1917) 17,938. POTI, a seaport of Russian Caucasus; at the mouth of the Rion river, on the E. shore of the Black Sea, 200 miles W. of Tiflis. Here maize and manganese are shipped. Poti was seized by Russia in 1828. POT METAL, a cheap alloy for faucets; etc.; composed of copper, 10; lead, 6-8. (2) A kind of cast-iron suit- able for casting hollow ware. (3) A species of stained glass, the colors of which are incorporated with the glass while the latter is in a state of fusion in the pot. POTOCKI, an ancient Polish family, taking its name from the castle of Potok, and still holding possessions in Galicia and the Ukraine. Among its most dis- tinguished members was Count Ignatius, Grand Marshal of Lithuania before the downfall of Poland, and a fellow-patriot of Kosciusko, born in 1751. In 1791 he took refuge in Saxony, returning, how- ever, to share in the last struggle for in- dependence. He was some time in the prisons of St. Petersburg and Warsaw, and died in Vienna in 1809. POTOMAC, a river of the United States, formed by two branches which rise in the Allegheny Mountains in West Virginia, and unite 15 miles S. E. of Cumberland, Md., from which point the river flows in a generally S. E. course 400 miles, and falls into Chesapeake Bay, after forming an estuary nearly 100 miles long, and from 2% to 7 miles wide. The largest ships can ascend to Washing- ton. A few miles above Washington the river forms a cataract 35 feet high; and between there and Westport it falls more than 1,000 feet. The scenery in this portion of its course is wild and beautiful, especially where it breaks through the Blue Ridge at Harper's Ferry. Its principal affluents are the Shenandoah, Cacapon, and Monocacy. The Potomac forms the greater part of the boundary between Virginia and Maryland. POTOMAC, ARMY OF THE, a di- vision of the United States army during the Civil War, which operated in the E. section of the country. It was organized
