FLORA. 291 mouth of the gorges, where the cliffs spring directly from the river bed. But however narrow und exposed this river vulley may be, its appearance is none the less charming to travellers coining from the arid desert, where the only fluid obtainable is the brackish water of the wells, and where the horizon is bounded by the eternal rocks and sands. On approaching the river the Arabs perceive its vicinity by the moistness of the air, and they press forward with joyful cries of " Allah be praised ! we feel the Nile ! " The Nubian desert is one of those whose temperature varies the greatest between the heat of the day and the cold of the night. Although these regions are traversed by isothermal lines of 79° F. and 81° F., and although the thermo- meter frequently exceeds 104° F., nevertheless travellers often shiver with the cold before sunrise. The cause of this is the excessive drjTiess of the atmosphere, which causes the heat to radiate into space during the night; the north wind, which blows nearly constantly, also contributes to this fall of temperature after sunset. The moisture of the air is so slight that it rarely ever falls in dew on the Nubian deserts. The bodies of animals which have died on the journey dry up without becoming decomposed, the flesh gradually crumbling into dust beneath the hard and extended skin without emitting the least odour. Although the bodies of those who die during the journey are scarcely covered with u few inches of sand, they would easily pass unnoticed were it not for an upright stone placed over them by some pious hand. The purity of the dry desert air explains its perfect healthiness, not only for the native Nubian but also for the foreigner. No sana- torium could be preferred to an encampment far from the exhalations of the moist plain, at least by those who, like the Arabs, are careful to clothe them- selves in such a way as to be unaffected by the abrupt changes in the temperature of night and day. The Eg3'ptian plague has never penetrated into Nubia, and ophthalmia, so dreaded in the regions of the lower Nile, is unknown above the cataracts of Wady-IIalfa, in spite of the glare reflected from the polished rocks and the glittering surface of the river. But in the Nubian regions where the inundations of the Nile stretch far into the plains, leaving stagnant pools here and there, malignant fevers are very common and frequently terminate fatally. The majority of the natives do not draw their drinking water directly from the river, but sink wells some distance off into which the water filters through the sand, and they leave it exposed to the sun for some time before using it. They are also careful not to follow the example of the Turks, and build their towns on the river bank ; their villages stand on the steppe or on the edge of the desert, beyond the zone of the marsh fevers. Flora. A land of transition in its climate, Nubia also presents transitional forms in its fauna and flora. The baobab is no longer found in the plains to the north of Eordof^n and the advanced Abyssinian nmges. The deleb-palm, which predomi- nates in the region of the two Nilcs, is no longer met with north of the confluence; the southern variety of palm which is the true dum-polm, advances farther towards
