Page:Africa by Élisée Reclus, Volume 3.djvu/417

LAKE TSAD. ' 349 farther west. For the same reason several other towns, such as ^gigmi in the north-west corner, have had to be rebuilt farther inland. While the water is thus advancing westwards, it is retiring on the opposite side, where the Bahr-el-Ghazal, although at a lower level than the Tsad, has been gradually drying up. This watercourse was long supposed to be a tributary of the lake, until Nachtigal's surveys confirmed the original statement of Denham and Clapperton, that it is really an old emissary, which is even still occasionally flooded. At the time of Nachtigal's visit, the current penetrated some 50 miles into the Bahr-el-Ghazal, which according to the local tradition, only ceased to be a regular affluent about the second half of the last century. Some infiltration probably still goes on below the surface, where brackish water can always be found at depths of from 4 to 6 or 7 feet. According to Nachtigal's preliminary survey, the Bahr-el-Ghazal flowed first east, then north-east for about 300 miles to the Bodele depression, at the foot of the Borku escarpments. West of this point occurs another broad depression, that of Egay, also at a lower level than Tsad, and separated from the Bahr-el- Ghazal by a barrier of dunes. Here the sandhills, all disposed in the direction from north-east to south-west, generally move with considerable rapidity under the action of the regular trade- winds. Where the original lacustrine bed is not concealed by these sands, it is found strewn with the remains of fish in such numbers and so well preserved that a naturalist might here conveniently study the ichthyology of the Tsad basin. At present there are neither cultivated tracts nor permanent settlements in this region, where, however, Nachtigal discovered the remains of a city, and where the Senusiya missionaries have announced their intention of founding an establishment near the copious Galakka springs, on the route between Bodele and Borku. Climate. — Flora. — Fauna. The climate of Bornu is much more equable than that of the Sahara, the difference of temperature being much less perceptible between day and night, and scarcely exceeding 17° F. between the hottest and coldest months. According to Denham the mean for the year at Kuka is 82"^, falling to 75° in December, and rising to 91° in April. Throughout most of the j^ear the trade winds prevail, flowing sometimes from the north-east, at others parallel with the equator. The rainfall increases generally in the direction from north to south, and from east to west, and is consequently much heavier in Bornu than in Wadai, in the Shari basin than in Kanem, and heaviest in the Mandara uplands, where the wet season lasts seven full months, and sometimes even more. In Bornu the corresponding period begins towards the end of May, and is over at the end of September, here the mean annual rainfall being certainly more than 40 inches. The remaining eight months are divided into a dry and a hot season, the former following, the latter preceding, the rains, and the transition between all these periods being everywhere very abrupt.