Meantime heavy rains have been falling about Lado and on the Sobat. Towards April 15 the river begins to rise, the effect of which is felt at Khartoum about May 20, and at Assouan about June 10. A most curious phenomenon accompanies this preliminary increase—the appearance of the ‘green’ water. It used to be thought that this ‘green’ water proceeded from the sudd region. During the previous months the swamps in that country have been lying isolated and stagnant under the burning tropical sun; their waters have become polluted with decaying vegetable matter. When once more the rising river overtops its banks, this fœtid water was supposed to be swept out into the stream, and finally make its appearance in Egypt. But a closer examination of the facts, which has only been possible since the Soudan was re-opened, has caused this view to be abandoned. The ‘green water’ is caused by the presence of an innumerable number of microscopic algæ, which give it a very offensive taste and smell. So far as can be ascertained, their origin is in the tributaries of the Sobat above Nasser. The rains in April carry them out into the White Nile, and thence they pass down to Nubia and Egypt. Under a hot sun and in clear water they increase with amazing rapidity, and sometimes they form a column 250 to 500 miles long. These weeds go on growing, drying, and decaying, until the arrival of the turbid flood-water, which at once puts an end to the whole process, as they cannot survive except in clear water. Horrible as the ‘green’ water is, its appearance is hailed with delight by the Egyptian, for he knows that it is the forerunner of the rushing waters of the real flood-time, and the first sign of the coming close of the water-famine. By September 1 the Nile at Lado has reached its maximum of 1,600 cubic metres. In the valley of the Sobat the rains last till November, with the result that by September 15 or 20 the White Nile at Khartoum has attained its maximum discharge of 4,500 cubic metres per second. Meanwhile great things have been happening on the Blue Nile and the Atbara. About July 5 the Blue Nile begins to rise, and the flood comes down with considerable rapidity till it reaches its maximum of 5,500 cubic metres per second on August 25. The famous ‘red’ water reaches Assouan on July 15, and is seen ten days later at Cairo. The flood on the Atbara would begin at nearly the same time as on the Blue Nile, but for the fact that it spends a month in saturating its own dry bed and the adjoining country. Once it begins, however, about July 5, it comes very rapidly, and about August 20 reaches its maximum, which is usually some 3,400 cubic metres per second, but occasionally amounts to as much as 4,900.