Rene Caillie, who started his journey to Timbuktu from Boke in 1827, did much to quicken French interest in the district, and from 1838 onward French naval officers, Bout-Willaumez and his successors, made detailed studies of the coast.
The middle Niger, however, reaches its maximum near Timbuktu only in January; in February and March it sinks slowly above the narrows of Tosaye, and more rapidly below them, the level being kept up by supplies from backwaters and lakes; and by April there is a decrease of about 5 ft.
Heinrich Barth (1851-1854) made known to Europe the course of the river from Timbuktu to Say.
The whole of the north of Africa from Egypt to Morocco has been mahommedanized, and Mahommedan influence is general and fairly strong from Timbuktu to Lake Chad and Wadai.
A still more important expedition was that of Lieutenant Hourst, who, starting from Timbuktu in January 1896, navigated the Niger from that point to its mouth, executing a careful survey of the river and the various obstructions to navigation.
In 1352 the restless man started for Central Africa, passing by the oases of the Sahara (where the houses were built of rock-salt, as Herodotus tells, and roofed with camel skins) to Timbuktu and Gogo on the Niger, a river which he calls the Nile, believing it to flow down into Egypt, an opinion maintained by some up to the date of Lander's discovery.
Another region so called is that part of the Sahara washed by the Atlantic. The name is also used to designate the territory under French jurisdiction west of Timbuktu and north of the Senegal.
After a journey into Spain he set out once more for Central Africa in 1352, and reached Timbuktu and the Niger, returning to Fez in 1353.
Chudeau, summing up the evidence available in 1909, set forth the hypothesis that the existing upper Niger and the existing lower Niger were distinct streams. According to this theory the upper Niger, somewhat above where Timbuktu now stands, went north and north-west and emptied into the Juf, which in the beginning of the quaternary age was a salt-water lake, the remnant of an arm of the sea which in the tertiary age covered the northern Sudan and southern Sahara as far east as Bilma.
The best known are Koroko, Kong and Bona, entrepots for the trade of the middle Niger, and Bontuku, on the caravan route to Sokoto and the meeting-place of the merchants from Kong and Timbuktu engaged in the kola-nut trade with Ashanti and the Gold Coast.