The Blue Nile is the true parent of the land of Egypt. The deposits of its muddy waters have made Egypt. The Atbara has added its quota, but the Blue Nile is incomparably the chief contributor; fed by the timely and plentiful rains of southern and south-eastern Abyssinia, it contributes 65 per cent of the waters which pass Assuân. The furthest sources are those of the Abai, which, after a course of 110 kilometres falls into Lake Tsana. This lake has an area of 3,000 square kilometres and lies about 1,760 metres above sea level. The Blue Nile leaves it at its south-eastern corner and hurries down to the Sudan, fed by numerous Abyssinian rivers. At Rosaires, after a course of 750 kilometres, it has fallen 1,260 metres; and below the Rosaires cataract enters the plain country south of Khartoum. For its remaining 615 kilometres on to Khartoum, where it meets the White Nile, it is navigable for the greater part of the year. North of Sennaar it is fed by the Dinder and Rahad rivers. Between Khartoum and El Damer, on a length of 320 kilometres, the Nile has its even passage broken by the 6th cataract at Shabluka. At El Damer the Nile receives the Atbara as a right hand tributary. The Atbara is a very muddy torrent fed by the rains of north-eastern Abyssinia. It runs for 4 months per annum and is dry for 8 months. Rising within a few kilometres of Lake Tsana, it falls 1500 metres in its first 300 kilometres, and is then joined by the Salaama, and, 100 kilometres lower down, by the Settit river. After the junction with the Settit, the Atbara flows for 480 kilometres and joins the Nile at El Damer, contributing a fair quantity of water and a very considerable quantity of Nile mud to the river. From the Atbara junction to the sea, the Nile has a course of 2,700 kilometres. In its first length of 1480 kilometres to Assuân it traverses the 5th and 4th cataracts between Berber and Dongola, the 3rd and 2nd cataracts between Dongola and Wady Halfa, and the 1st cataract at Assuân. All these cataracts are navigable in flood, but not so in summer. From Assuân to the Barrage at the head of the Delta north of Cairo, the Nile has a length of 970 kilometres and traverses Egypt without a cataract or interruption of any kind. At the Barrage, the Nile divides into the Rosetta and Damietta branches, and after a further course of about 240 kilometres in either branch, flows into the Mediterranean sea. Its greatest length from the sources of the Kagera river to the sea is 6350 kilometres, constituting it one of the longest rivers in the world. 4. =The Slopes and velocities of the Nile in its different reaches.=--Table 2 of Appendix B and Plate II comprise all the information available under this head which I have been able to collect. For the slopes I have adopted the following data: