German Empire in 1871. Polish social and academic organizations continued to be set up, including the Central Economic Society for the Grand Duchy of Poznań (1861) to promote modern agriculture, the Poznań Society of Friends of Learning (1875), and the People's Libraries Society (1880). In 1873–1875 a Polish theatre was built by public subscription (Teatr Polski, still functioning today). The authorities, however, continued efforts to Germanize the region, including through the activities of the Prussian Settlement Commission, founded in 1886. Germans accounted for 38% of the city's population (20,000 out of 53,000) in 1867 – by 1910 their number would rise to 50,000, though this represented a smaller percentage of the total population of the city (whose boundaries had significantly expanded in the meantime). As the population increased the proportion of Jews in the city also fell: from 20.5% in 1831 to 13.3% in 1867 and 3.6% in 1910 (a total of about 5,000). A gasworks was built on Grobla in 1853–1856, enabling the first gas streetlights to be installed in 1858 (this form of lighting would continue in use until the early 1960s). Grobla was also the site of the city's first modern waterworks (1866) and major electricity works (1904). Sewers began to be installed on a large scale at the end of the 19th century. To strengthen the city's defences, in the second stage of the Festung Posen scheme, an outer ring of forts was built to encircle the city. The nine original forts were built starting in 1876, and nine intermediate forts were built between them from 1887. Poznań was by now a major military post, headquarters of the German Fifth Army Corps. However much of the original inner ring of fortifications was now redundant and could be taken down, enabling the city to expand more freely, particularly to the west. Near the old Berlin Gate a series of imposing buildings was constructed, including the neo-Romanesque imperial palace (now called Zamek), completed in 1910, which lent the city the status of a royal residence. The other buildings included the opera house, a post office headquarters (Oberpostdirektion), the railway board headquarters, a Royal Academy, and the headquarters of the Settlement Commission, where teachers and clerics were trained. (The last two are now university buildings: Collegium Minus of Adam Mickiewicz University, and Collegium Maius, occupied mainly by Poznań Medical University.) Serious floods (reaching the Old Market) occurred in 1855,