Berlin, the capital of Prussia and, since 1871, of the German Empire, the third largest city of Europe, lies on the banks of the River Spree, which flows through the center of the city from southeast to northwest. Its population is 2,070,695, and it covers an area whose circumference is nearly thirty-five miles. The city dates as a small fishing village as far back as the 13th century, but the beginning of its rapid growth and prosperity was not made until the Great Elector, Frederick William (1640–88), united the separate duchies of which Prussia is now formed, and made Berlin the capital of the new state. Among the fine buildings for which the city is noted are the royal palace, with 700 apartments, and other palaces; the royal library, with over 710,000 volumes and 15,000 manuscripts; the old and new museums, with their fine collections and art galleries; the national gallery; the arsenal; the royal theater; the opera house; the guardhouse; and the university. Throughout the city and in the parks are numerous statues of national heroes, and monuments, such as the great Column of Victory, 197 feet high. The street called from its double avenue of limes Unter den Linden is one of the finest streets in Europe. The University of Berlin is now one of the foremost in Germany. Founded in 1810, it has had many famous professors and scholars. It now numbers over 414 professors and lecturers with over 5,000 students, and as many more non-matriculated students. Besides the university, there are the Academy of Sciences, one of the most learned institutions in Germany; the military academy and academies of art and architecture; and the schools of mining, agriculture, artillery, engineering, music, etc. The Zoölogical and Botanical Gardens are also worthy of mention. The commerce of Berlin, carried on by the Spree, the canals and many railroads, with its manufactures, places it in the front rank of the mercantile cities of the continent. Grain, cattle, spirits and wool are the staples of trade. The exchange is daily visited by 3,500 persons. The main branches of industry are woolen weaving, calico printing and the manufacture of engines and other machinery, also of iron, steel and bronze wares, drapery goods and confections. Together with Leipsic, Berlin holds the first place among German cities in the publishing trade. Owing to the recent great strides in German manufactures and in the extension abroad of German colonies, the capital has become a still greater financial and imperial center.