No one can hold Egypt securely unless he holds also the whole valley of the Nile. The sources of the river in hostile, or even in indifferent, lands must always be a grave cause of danger, or, at the best, anxiety. If tradition be correct, the Abyssinians on more than one occasion made use of their position on the Nile as a powerful lever in negotiations with Egypt. Vansleb, the Dutch seventeenth-century traveller already quoted,[6] is not perhaps perfectly trustworthy as to his facts, but at least he is evidence as to the tradition. He had in his possession, he says, a copy in Arabic of a letter written by David, King of Ethiopia (_i.e._, Abyssinia), surnamed Constantine, to ‘Abu Seid Barcuk, King of Egypt, in the year of martyrs 1193, in which he threatens in two distinct places to turn aside the river Nilus and hinder it from entering into Egypt,’ so as to cause all the inhabitants to perish with hunger, if he ‘continues to vex the Copties.’ Vansleb also says that ‘the King of Ethiopia hindered the current of Nilus and turned it out of Egypt in the days of Mostanser, one of the califfes of Egypt, which obliged him to send the Patriarch of the Copties with rich presents to the King of Ethiopia, to entreat him to take away the bank, which he had raised to turn aside the river. The King of Ethiopia having granted this request for the sake of the Patriarch, the river increased in one night 3 cubits, which was sufficient to water the fields.’ Doubtless the tradition is founded on fact; if the country in question had been occupied by a Power with any engineering skill, there is little doubt that this method of pressure would have been oftener adopted. The danger is by no means an imaginary one, and this is recognised in the latest treaty between the Government of the Soudan and Abyssinia, concluded May 15, 1902, in which it is laid down that no work shall be constructed across the Blue Nile, Lake Tsana, or the river Sobat, which shall arrest the flow of their waters into the Nile, except by mutual agreement.