[Transcriber's note: This article has been extracted and prepared from _The Geographical Journal_, v. 56, 1920.] THE EXPLORATION OF TIBESTI, ERDI, BORKOU, AND ENNEDI IN 1912-1917: A Mission entrusted to the Author by the French Institute Lieut.-Colonel Jean Tilho, Gold Medallist of the R.G.S. 1919 _Read at the Meeting of the Society, 19 January 1920. Map following p. 160._ [_Note: The names in the text are spelled in accordance with the manuscript of Colonel Tilho, a few of the principal names—as Chad—in their English form, but the greater number in the French transliteration of Arabic. On the accompanying map the names are transliterated according to the G.S.G.S. rules for transposing from the French to the British system. The retention of the French spelling in the text has the double advantage of familiarizing the student with the two systems, and of preserving in some degree the character of the lecture, which was delivered in French._—ED. _G.J._] =1. Object of the Mission.= BEFORE I begin my lecture, allow me to express once more, in your presence, my heartfelt gratitude to the Council of the Royal Geographical Society for the high recompense accorded me on the occasion of my last journey in Central Africa. It is of this journey, its chief incidents, and most important results, that I am about to have the honour of giving some account. Let me first of all explain to you, in a few words, what, from a geographical point of view, was the object of my expedition. Explorations in Central Africa, made during the second half of the nineteenth century and in the beginning of the twentieth, had left unsolved a very interesting problem: it had been noticed that the level of vast stretches of desert, several hundred miles north-east of Lake Chad, were considerably lower than that of the lake—the difference amounting in some places to 260 feet; besides this, a wide continuous trench, offering the appearance of an old valley—the Bahr El Ghazal—led from the lake to this low-lying ground, and seemed to stretch far away to the north-east, between the mountain groups of Tibesti and Ennedi. On proceeding towards the north-east, an increasing analogy is to be noticed between the malacological fauna of the Chad basin and that of the Nile. Besides which there had been found recently, in the waters of the Chad, a shrimp till then only found in the Nile basin—the _Palæmon Niloticus_, Roux. In short, all these signs appeared to confirm the supposition that the basin of the Chad was not a closed basin, but belonged to that of the Nile, and was a former affluent of the old river on whose banks had sprung up and flourished one of the most brilliant and ancient civilizations of the world.