It has been found that more water pours into the ocean through the Congo's mouth, which is six miles wide, than from all the other rivers in Africa together. It is second among the world's rivers, and the dark detritus it carries to the Atlantic has been distinctly traced on the ocean bed for six hundred miles from the land. Some geographers still believed thirty years ago that all the waters of its upper basin might be tributary to the Nile. Map-makers have been kept very busy recording discoveries on the Congo. About one hundred explorers, some of them missionaries and many employees of the Congo Free State, have mapped the whole basin along its water-courses, and discovered the ultimate source of its main stream. Our ideas of the hydrography of this great basin have been revolutionized since Stanley, second only to Livingstone among the great African explorers, in 1877 revealed the course of the main river. On his map, for example, he showed the southern tributaries as probably flowing nearly due north; but all except one of these rivers rise in the east and flow far to the west. When Wissmann was sent to the Upper Kassai to follow it to the Congo, he was greatly surprised to find himself floating westward week after week. When he reached the Congo a steamboat was waiting for him at Equatorville, two hundred miles further up the river, where he was expected to emerge. Schweinfurth believed the Welle Makua flowed north to Lake Chad on the edge of the Sahara; seventeen years later, after six or seven explorers had tried to solve the problem, the river was found to be the upper part of the Mobangi tributary of the Congo, larger than any rivers of Europe, excepting the Volga and Danube. While Stanley was for five years planting his stations on the Congo, he knew nothing of this great tributary, 1,500 miles long, whose mouth was hidden by a cluster of islands which his steamers repeatedly passed. Missionary Grenfell, on his little steamer, was ascending the Congo one day, when accidentally he got into the mouth of the Mobangi and went on for one hundred miles before he discovered that he had left the main river. Few explorers have unwittingly stumbled upon so rich a geographical prize.