Sr., a strongly-fortified town of France, capital of an arrondissement of the same name in the department of Pas-de-Calais, stands on the River Aa, at the mouth of the Canal Neufossé, 25 miles by railway S.E. of Calais. It is built partly on the declivity of a hill, and partly on low marshy land at its foot. It is surrounded by fortifications about 2½ miles in circumference, and is further defended by several strong and extensive outworks; but its chief strength consists in its being surrounded by marshes, and standing on the Aa, by means of which three-fourths of its circuit may be protected by water. The principal streets are broad and regular, and the town is generally well built; but the houses have a dull and gloomy appearance, being chiefly built of yellow or gray bricks. There are numerous public fountains and several fine promenades. The cathedral is a very fine building in the Gothic style, completed about the middle of the sixteenth century. The abbey church of St Bertin, at one time the finest Gothic edifice in French Flanders, is now in ruins from the effects of the revolution of 1830. The college, town-hall, arsenal, prison, and theatre, are the principal public buildings. There are several convents and hospitals, courts of justice and commerce, a public library of about 20,000 volumes, and an English college for the education of British Roman Catholics. Woollen cloth, paper, beer, brandy, and leather are among the principal manufactures; and an active trade is carried on in corn, wine, flax, wool, &c. Pop. (1856) 19,796.