Favourable Conditions. 209 periodical flooding they get detached from the bed of the depression and raised to the surface. Then they drift away before the high winds and reach the Nile in various states of development. During our detention at Fashoda, strong north winds had constantly prevailed, and under their action all the floating masses along the northern bends of the river would be inevitably driven into the main stream.

" In a word, the favourable conditions for the development of the sudd in the Upper Nile basin are :— "1. A rise of the flood-waters above the normal level in order to bring the backwaters into free communication with the river, and at the same time detach and raise the floating masses to the surface.

" 2. Favourable winds to further detach and drive these masses into the river, where they either drift harmlessly with the currents, or else coalesce together into formidable barriers. At the same time, the winds, as is obvious, may have a contrary effect, arresting as well as propelling the masses, breaking up as well as building up the barriers ; hence the constant changes that these formations are subject to through the shifting of the winds, as well as from the varying character of the periodica,! floods.

"3. The growth of those innumerable little plants, which spring up in still and sheltered waters, and then drift away to enlarge or render more compact the tangle of the floating masses. Such are, for instance, the Azolla, Pistia, Aldrovandua, Lemna, Ottelia, Ultricularia, Ceratophyllum, Potainogeton, Naias, Lagarosiphon, and others.

"But even under the most favourable conditions a protracted damming up of the Nile is of relatively rare occurrence, as, however deep the barrier may be, the current always flows underneath. In the opinion of longexperienced Arab boatmen, the prevailing relations in the forties and fifties were much the same as at present, though in those years the obstructions were never so compact as some of those occurring in recent times. The best known were those of 1863, at the time of Miss Tinne's visit; of 1870 and 1871, by which Sir Samuel Baker's expedition was delayed, and which was ultimately cleared away by the vigorous action of Ismail Eyub Pasha in 1874 ; of 1878, at my first excursion, when the Nile rose (1767) p