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.

While exploratory enterprises have been centred largely in tropical Africa, no part of the continent has been neglected.  We now know that large areas of the Sahara are underlaid by waters which need only be brought to the surface to cover the desert around them with verdure; that most of the rain falling on the south slopes of the Atlas Mountains sinks into the earth to impermeable strata of rock, along which it makes its way far out into the desert; that where the surface is depressed so that these waters come near to it, there are wells for the refreshment of the camel caravans, and oases, blooming islands of green, in the sterile wastes; and that artesian wells bring inexhaustible supplies of water within reach, so that millions of date palms have been planted along the northern edge of the desert in southern Algiers and Tunis, making these regions the largest sources of the world’s supply of dates.

It has also been discovered why there are very large areas of dry or desert lands in Africa.  The Sahara and the southwest of Africa are deserts because the prevailing winds, the carriers of moisture, blow towards the sea instead of away from it, and consequently are always dry.  The winds from the Indian Ocean crossing the highlands of Abyssinia are wrung nearly dry while passing the mountains, and so Somaliland and the lowlands to the south of Abyssinia are parched.