TI1K RIVER WITHOUT WATEK. 179 It crosses all tlie caravan routes in the desert, and is supposed to extend from the Nile to the Eed Sea. It may have been the outlet for the river, before its waters forced a passage through the primitive chains which cross its bed at Assouan and Kalabshee. A geological exploration of this part of Africa could not fail to produce very interesting results. Beyond the Balir bela 31a extends the broad central plateau of the Desert, fifteen hundred feet above the sea. It is a vast reach of yellow sand, dotted -with low, isolated hills, which in some places are based on large beds of light-gray sandstone of an unusually fine and even grain. Small towers of stone have been erected on the hills nearest the road, in order to guide the couriers who travel by night. Near one of them the guide pointed out the grave of a merchant, who had been murdered there two years previous, by his three slaves. The latter escaped into the Desert, but probably perished, as they were never heard of afterwards. In the smooth, loose sand, I had an opportunity of reviving my forgotten knowledge of trackography, and soon learned to distinguish the feet of hyenas, foxes, ostriches, lame camels and other animals. The guide assured me that there were devils in the Desert, but one only sees them when he travels alone.

On this plain the mirage, which first appeared in the Biban, presented itself under a variety of wonderful aspects. Thenceforth, I saw it every day, for hours together, and tried to deduce some rules from the character of its phenomena. It appears on all sides, except that directly opposite to the sun, but rarely before nine a. m. or after three p. m. The color of the apparent water is always precisely that of the sky, and this is a good test to distinguish it from real water, which is invari-