nearly ten o'clock
It is due , in all likelihood , to the rapid radiation of surface heat . When the cold air has robbed sand and rock of the temperature acquired from the broiling sun , the atmosphere clears , and the desert reveals itself again in the gloomy monotone of night . It may reasonably be supposed , that the excess of humidity which caused the remarkable mirage of the afternoon helped to continue the `` black hour , '' as the Arabs term it , far beyond its ordinary limits . Hence it was nearly ten o'clock when Royson quitted the camp on his self-imposed task . To all outward semblance , he differed not a jot from the two Arabs who accompanied him . A burnous and hood covered his khaki riding costume . He bestrode a powerful camel nearly eight feet high .