180 JOURNEY TO CENTRAL AFRICA.

ably of a deeper hue. It is seen on a gravelly as well as a sandy surface, and often fills with shining pools the slight depressions in the soil at the bases of the hills. Where it extends to the horizon there is no apparent line, and it then becomes an inlet of the sky, as if the walls of heaven were melting down and flowing in upon the earth. Sometimes a whole mountain chain is lifted from the horizon and hung in the air, with its reflected image joined to it, base to base. I frequently saw, during the forenoon, lakes of sparkling blue water, apparently not a quarter of a mile distant. The waves ripple in the wind; tall reeds and water-plants grow on the margin, and the Desert rocks behind cast their shadows on the surface. It is impossible to believe it a delusion. You advance nearer, and suddenly, you know not how, the lake vanishes. There is a grayish film over the spot, but before you have decided whether the film is in the air or in your eyes, that too disappears, and you see only the naked sand. What you took to be reeds and water-plants probably shows itself as a streak of dark gravel.

The most probable explanation of the mirage which I could think of, was, that it was actually a reflection of the sky upon a stratum of heated air, next the sand.

I found the Desert life not only endurable but very agreeable. No matter how warm it might be at mid-day, the nights were always fresh and cool, and the wind blew strong from the north-west, during the greater part of the time. The .temperature varied from 50°— 55° at 6 a. m. to 80°— 85° at 2 p. ai.

The extremes were 47° and 100°. So great a change of temperature every day was not so unpleasant as might be supposed. In my case, Nature seemed to make a special provision in order to keep the balance right. During the hot hours of