While the problems of Central Africa were thus being elucidated, the exploration of the nine great rivers which form the Bahr-el-Ghazal, the chief tributary of the Nile, was attracting much attention, the most valuable work being done by Georg Schweinfürth, who mapped the river with high accuracy. The Nile above Gondokoro was also surveyed by English officers; the Sahara and the Sudan were widely explored by G. Rohlfs and G. Nachtigal between 1860 and 1875, and the connection of the river Welle with the Congo system was established by W. Junker, who also travelled in the Sudan from 1875 to 1886. The British during this period were pushing their way northward from South Africa, and emigrants were flocking to the goldfields and diamond mines. The French have spread their explorations over the Sahara and their territories in the Sudan and West Africa. From the date of Stanley’s return African history underwent a rapid change. The period of driving the main lines of exploration through the continent was over, and a new era began in which branch lines could be laid down, and colonial expansion could take place along them. The continent is now partitioned among the European Powers, and has been more or less provisionally mapped; but it will take long years of survey and scientific investigation before its features, its resources, its peoples, and its possibilities are adequately known. CHAPTER XII. THE NINETEENTH CENTURY AND AFTER: ASIA AND AUSTRALIA The beginning of the nineteenth century was signalized by the initiation of the great trigonometrical survey of India, and the first half-century was a period of much important geographical and anthropological work within that empire, but to no great extent beyond its boundaries, though in 1808 a mission penetrated to the sources of the Ganges, and Baluchistan and Afghanistan were in some part explored by officials of the East India Company. But the physical problems of the heart of the continent were left to a later period--those, for instance, concerned with the trans-Himalayan region (as viewed from India), including Tibet, eastward that region so important in the hydrography of the continent, where the river systems of China and Burma take their rise, northward the deserts of Mongolia and Turkestan, westward the nodal mountain-region of the Pamirs, and the area which long concealed the sources of the Brahmaputra (Tsanpo) and the rivers of Punjab. In spite of the endeavours of the Tibetans to hold inviolate the secrets of their land--in great measure successful so far as their capital, Lhasa, was concerned--the Indian native surveyors, such as Nain Singh, Krishna, and Ugyen Gyatso, were able to penetrate the country, and even Lhasa itself; their work covered the period 1863–82. And the last quarter of the century provides a wonderful record of continuous exploration in Tibet, as will appear from the mere quotation of names and dates--P. Bonvalot and Prince Henry of Orléans, 1886–87; W. W.