During the first half of the nineteenth century, when the European nations had been roused first to protest against and then to abolish the slave trade, a great deal of valuable work was done to increase the knowledge of Africa, especially by Englishmen. In 1823 W. Oudney, who was sent out as consul to Bornu, and H. Clapperton penetrated to Lake Chad, previously unvisited by any white man, and Clapperton afterwards explored the flourishing civilization of Bornu and Hausaland. He died on a second journey undertaken to open up trade with the Sultan of Sokoto; but in 1830 his servant on this last journey, Richard Lander, succeeded in solving one of the many problems which were beginning to present themselves to students of the continent--the question of the outlet of the Niger. He started from the Guinea coast, and followed the course of the river from Bussa to its mouth in canoes. Much good work was done also by A. G. Laing, who was the first European to visit Timbuktu (in 1826), but was killed there. R. Caillié, who succeeded in reaching the city in 1828, was the first to return from it in safety, which he did by crossing the Sahara to Tangier. H. Barth, who had already travelled all through North Africa, and had ascended the Nile to Wady Halfa, started in 1850 on a trading mission under the auspices of the British Government to the states of Central Africa; both his companions died, but Barth carried through alone a brilliant journey, in the course of which he travelled from Lake Chad to Timbuktu, and studied minutely many of the ancient civilized states of the region. Thus the geography of Senegal and the Niger had been largely cleared up by 1850. The great questions which remained untouched were those of the sources of the Nile and the Congo. By 1850 Abyssinia and the greater tributaries of the Nile were pretty well known, through the work of naturalists and travellers, the efforts of Austrian missionaries, who had established stations down to Gondokoro on the Nile, and the interest taken in exploration by Mahomed Ali, ruler of Egypt under the Turks. But the seventy miles of rapids above Gondokoro, and the fierce people of the Bari tribe who lived there, had prevented the acquisition of any but the most meagre knowledge of the upper reaches of the Nile beyond that point. The problem was now approached from a different direction. For some time missionaries had been working at Zanzibar, finding the natives there more tractable than in Abyssinia, the original field of their labours; and two of their number, L. Krapf and T. Redmann, had seen and sent home accounts of snow-mountains under the Equator--Kilimanjaro and Kenya--and also of the great lakes, which they imagined to be all parts of an enormous inland sea. Since the annexation of Aden by the British Government in 1839, moreover, officers of the English army stationed there had shown an interest in the exploration of the African coast opposite them; and in 1854 R. F.