Though Borkou is more than 300 miles south of the Tropic of Cancer, and very low-lying (650 feet above sea-level), the heat is really excessive only for six or seven months of the year, from mid-March to mid-October. During our observations, extending over three years, the maxima registered in the hot season never exceeded 117° Fahr., but temperatures of 110° to 115° were frequent. During the cool season, from December to February, the minima sometimes fall below 50° Fahr. without ever getting down to freezing-point. The dryness of the air is very noticeable from November to June, when a difference of more than two to one may regularly be observed between the simultaneous indications of the dry and wet thermometers: for instance, when the former stands at 44° C. the second often reads less than 20°. On the other hand, in August and September, under the influence of the winds blowing from the Atlantic Ocean, the air becomes very damp and the heat grows stifling. In spite of its excessive heat, the climate of Borkou is comparatively healthy; very relaxing during the hot and damp season, it is extremely pleasant in the months corresponding to our autumn and winter. During my stay, lasting from 1913 to 1917, none of my European fellow-workers had any serious illness, and my black troops, though kept hard at work in the shape of arduous reconnoitring and escort duty, and with barely enough to eat, showed a percentage of sickness and deaths below the average of the other garrisons throughout the Chad Territory. _Population and Commerce._—The population of Borkou consists of nomads, the Tedas and the Nakazzas—the great nobles of the desert—and of a sedentary tribe, the Dozzas, who are only half noble, for want of the few camels whose possession would enable them to take a share in the profitable plundering raids in the desert. There is also a third category of inhabitants, the Kamajas, half serfs, half slaves, whose duty it is to attend to the gardens and the plantations of palms, and who are profoundly despised by the other two categories. The total population of Borkou would not appear to exceed some ten thousand souls, distributed among a score of more or less flourishing palm plantations.