The periodical rise in the Nile, recurring as regularly as the revolutions of the heavenly bodies, necessarily remained an unsolved mystery to the ancients, for until the discovery of the tropical regions, with their mountainous lakes and deluging rains, it was impossible to learn the occasion of this increase. It is now known that the Blue Nile, flowing out of the mountainous parts of Abyssinia, is the sole cause of the periodic overflow of the Nile. Without the tropical rains of the Ethiopian tablelands, there would be no great rise nor any fertilising deposits. Without the White Nile, which runs steadily from the perennial reservoirs of the great Central African lakes, the Lower Nile would assume the character of an intermittent wady, such as the neighbouring Khor Baraka, periodically flushed by the discharge of the torrential downpours from Abyssinia. Though there is a periodical increase in the flow of the upper waters of the White Nile, yet the effect of this, lower down, is minimised by the dense quantities of vegetable drift, which, combining with the forest of aquatic growth, forms those vast barriers, known by the name of _sudd_, which not only arrest navigation but are able to dam up large bodies of water. The sudd, it is supposed, stopped the advance of the Roman centurions who were sent up the Nile in the days of Nero. Sir Samuel Baker was the one who first pointed out the great disadvantage of allowing the vegetable matter to accumulate, both to merchants and to those who were employed to suppress the slave-trade. In the year 1863 the two branches of the White Nile were blocked above their junction at Lake No. Once blocked, the accumulation rapidly increased from the stoppage of outlet, forming the innumerable floating islands which at this part of the Nile customarily float down-stream. A marsh of vast extent had been formed, and to all appearance, as Baker narrates, the White Nile had disappeared. Baker cut through fifty miles of the sudd, and urged the khedive to reopen the Nile. The work was successfully undertaken by Ishmail Ayub Pasha, and the White Nile became clear for large vessels when Gordon reached Khartum in 1874. It is practically impossible to keep the central provinces of the Nile open to civilisation unless the course of the Nile is free. Yet in 1878 the obstruction had been renewed, and during the occupation of these provinces by the rebel dervishes under the Mahdi and the califa the Nile was completely blocked, as formerly, at Lake No. The alarming failure of the Nile flood in 1899--1900 was generally attributed to this blockade, and in 1899 fifty thousand dollars was placed at the disposal of the governor-general for reopening the White Nile by removing the vast accumulation of sudd which blocked the Bahr-el-Jebel from Lake No almost as far as Shambeh. The work was started under the direction of Sir William Garstin in 1899. In 1900 the greater part of the sudd had been