The supply of this river is chiefly derived from the drainage of the basin through which it runs and from the large tributaries which enter it downstream of the point where it issues from the Abyssinian hills. The Tsana lake has but a small influence upon its supply at any period of the year. It is at its lowest in May, when its discharge at times shrinks to nothing. It begins to rise in June and attains its maximum about the end of August. Its discharge in a year of good flood is as much as 10,000 metres cube per second, and it seems probable that in a year of exceptional flood 12,000 metres cube may pass Khartoum. In September it falls very rapidly, and during the winter months rarely discharges more than from 200 to 400 metres cube per second. The Khartoum gauges prove that a higher reading is recorded for a given flood discharge when the river is falling than is the case when the river is rising. This is probably due to the filling of the valley between Khartoum and the Shabluka Pass. 8. THE ATBARA. The first water from this river reaches the Nile in the last week of June, and the maximum is usually reached in the last days of August, or in the first week of September. The Atbara generally attains its maximum before the full flood from Khartoum has arrived at the junction of the two rivers. After the maximum has been reached, the fall of the Atbara is rapid, and by the end of the year the river reverts to its summer state of a series of pools. The maximum discharge of the Atbara, measured in 1903, was 3,088 metres cube per second, but this is probably surpassed in a year of very high flood. 9. THE NILE NORTH OF KHARTOUM. The discharges of 1903 record a maximum of 10,500 metres cube per second in an average year. If to this be added the volume of the Atbara, a total of nearly 14,000 cubic metres per second is reached. As in 1903 the levels at both Halfa and at Cairo did not pass those of a very ordinary flood supply, it would seem probable that in very high flood a volume of quite 16,000 metres cube per second must pass Berber. In conclusion it may be stated with confidence that the White Nile contributes practically nothing to the flood which reaches Egypt. This is entirely derived from the Blue Nile and from the Atbara. On the other hand, the supply passing Aswan during the spring and early summer is due, almost entirely, to the water of the great lakes brought down by the White Nile. The following are the water-slopes of the two rivers, as worked out from the discharges:— Bahr El Jebel— At Wadelai 1/27000 dry season. At Bor 1/12100 flood season. At 830 kilometres from Lake Albert 1/22500 flood season. At Hellet-el-Nuer 1/40000 flood season. At „ „ 1/16500 dry season.