A scholar and explorer, Dr. Samuel Fergusson, accompanied by his manservant Joe and his friend professional hunter Richard "Dick" Kennedy, sets out to travel across the African continent - still not fully explored - with the help of a balloon filled with hydrogen. He has invented a mechanism that, by eliminating the need to release gas or throw ballast overboard to control his altitude, allows very long trips to be taken. This voyage is meant to link together the voyages of Sir Richard Burton and John Hanning Speke in East Africa with those of Heinrich Barth in the regions of the Sahara and Chad. The trip begins in Zanzibar on the east coast, and passes across Lake Victoria, Lake Chad, Agadez, Timbuktu, Djenne and Segou to St Louis in modern day Senegal on the west coast. The book describes the unknown interior of Africa near modern day Central African Republic as a desert, when it is actually savanna. Un savant et explorateur, le Dr Samuel Fergusson, accompagne de son serviteur Joe et son ami chasseur professionnel Richard "Dick" Kennedy, decident de survoler l'Afrique orientale, de Zanzibar aux sources du Nil - pas encore totalement exploree - a bord d'un ballon gonfle a l'hydrogene, le "Victoria." Il a invente un mecanisme qui, en eliminant la necessite de liberer du gaz ou de jeter de lest pour controler son altitude, permet de tres longs trajets a prendre. Ce voyage est destine a relier les voyages de Sir Richard Burton et John Hanning Speke en Afrique de l'Est avec celles de Heinrich Barth dans les regions du Sahara et du Tchad. Le voyage commence a Zanzibar sur la cote est, et passe a travers le lac Victoria, le lac Tchad, Agadez, Tombouctou, Djenne et Segou a St Louis dans le jour moderne Senegal sur la cote ouest. Le livre decrit les multiples et dangereuses aventures que vont vivre les trois hommes."