ing. The region round any attracting body where work has to be done to carry mass from one point to another is known as a field of force. The potential at any point in such a field of force is the amount of work required to bring up a unit mass from infinity to that point. It is to be noted that nothing is here said as to the path by which this unit mass, is brought up, for the work is the same whatever the path, provided the attraction obeys the law of inverse squares. This quantity was first introduced into physics by Laplace in his Mécanique Céleste; but the name, potential, is due to George Green (1828). The enormous importance of this quantity in electrical science will be appreciated best by reading some such treatment as J. J. Thomson's Elements of Electricity and Magnetism.

Poto'mac, a river of the United States, formed hy two branches which rise in the Allegheny Mountains in West Virginia and unite 15 miles southwest of Cumberland, Md., from which the river flows southeasterly for 400 miles and empties into Chesapeake Bay, after forming an estuary 100 miles long and from two to seven wide. A few miles above Washington it forms a cataract 35 feet high. Between this and Westport it falls more than 1,000 feet. The scenery in this portion of its course is wild and beautiful, especially where it passes through the Blue Ridge at Harper's Ferry. The Potomac forms a great part of the boundary line between Virginia and Maryland.

Potosi (po'tô-se1), a city of Bolivia, South America, nearly 50 miles southwest of Chu-quisaca (Sucre). It is built on the side of the Cerro de Potosi, 13,000 feet above the sea, and is one of the loftiest inhabited places on earth. The streets are too steep or narrow for wagons or carriages, and fully half of the town is composed of tottering and ruined buildings, uninhabited and desolateV There are considerable changes in the climate, but it generally is very cold, owing to the snow on the surrounding mountains, which scarcely ever melts. Yet Potosi is one of the principal commercial towns of South America, the industry of the place being limited to silvermining. As the country near Potosi is unproductive, all supplies have to be brought from a distance. The town was founded in 1545, and in 1611 had 160,000 inhabitants, but its population does not exceed 23,450. The department of Potosi is rich in minerals and cattle, and has an area of 48,900 square miles, with a population of


Potpourri (po'poo'rê'), a mixture of sweet-scented flowers, dried and placed in a vase with a perforated lid, that their odor may be diffused through the room in which they are placed. But it also signifies a dish of different viands, and corresponds in this sense to the hotch-potch of Scotland and the olla podrida of Spain. In music the name is

applied to a selection of pieces strung together without much system or arrangement — a sort of medley.

Pots'dam, the chief town of the Prussian province of Brandenburg and second residence town of the royal family of Prussia, is situated on an island beside Havel River, 16 miles by rail from Berlin. It is a handsome city, with broad streets, public gardens and fine squares, and in the neighborhood are more than half a dozen royal palaces. The manufactories include sugar, chemicals, harness, silk, waxcloth and beer. Flower-gardening, especially of violets, is an active industry. Alexander von Humboldt (a. v.) was a native of the place. Potsdam owes its creation as a town to Frederick William, the Great Elector, and to Frederick II. Prior to that period it was a fishing village on the site of an ancient Slav settlement. Population 61,414.

Pot'stone', a mineral composed of a mixture of talc, mica and chlorite. It is soft and easily cut when dug, but becomes hard after exposure to the air. It is made into pots and other utensils, which are cleaned by the fire. It was well-known to the ancients, and Pliny describes the manner of making vessels of it. It is quarried in Moravia, Norway, Sweden, Greenland and other countries.

Pot'tawat'tomies (signifying fire-makers), a tribe of North American Indians, belonging to the Algonquin stock. The early French settlers established a mission among them at Green Bay, Wis., and to this day many are Roman Catholics. They sided with the English during the Revolution and in the War of 1812, and afterwards settled in Kansas, where one band of over 400 now live in houses and cultivate the ground. Another band, 500 in number, is on a reservation in the same state, under the care of the Society of Friends.

Pot'ter, Paul, the greatest animal-painter of the Dutch school, was born at Enkhuizen in 1625, and was the pupil of his father, a landscape-painter, with whom he came to Amsterdam m 1631. He also was an excellent etcher and was so precocious that his best-etched pieces, The Herdsman and The Shepherd, were finished when he was only 18. He established himself at The Hague in 1649, but in 1653 he returned to Amsterdam. He died there in 1654, untimely. His Dairy Farm, measuring 48^ by 192 inches, was sold in London in 1890 for $30,450, or $65 the square inch.

Pot'tery, all objects of baked clay. This art is of high antiquity, being practiced among various races in prehistoric times. We find mention of earthenware in the Mosaic writings. The Greeks had important potteries at Samos, Athens and Corinth, and attained great perfection as regards form and ornamentation. Demaratus a Greek, the father of Tarquinius Priscus,