At first the jolting of the camels was disagreeable, but in a little while the boys became accustomed to it, and tried to believe they were enjoying themselves. The camels of easiest motion are selected for riding purposes, and the youths were fortunate in their animals, which had been chosen by Abdul. Frank had a very tall and powerful camel, while Fred was on one little more than two-thirds the size of Frank's. The two animals were friendly to each other, but not to their riders, and the boys soon abandoned the attempt to establish social relations with their beasts of burden. "You may possibly be on good terms with them after a while," said Abdul, "but you must be patient. You could hardly expect it under a fortnight, and we ought to be in Aboo Hamed before that time. Most camels hate Europeans instinctively, but are docile with Arabs, just as mules are said to dislike white men and prefer the society of negroes." [Illustration: A MIRAGE IN THE DESERT.] On the second day of their journey Frank happened to look ahead, and to his great surprise saw a beautiful lake, surrounded by groves of trees. He shouted to Frank, and then to the Doctor and Abdul, who were riding just behind them. Abdul quickened the pace of his camel, and was soon at Frank's side. "You told us there was no water on the road," said Frank, "and there is a lake right before us. I suppose it is filled with salt-water, and therefore doesn't count." "Worse than salt-water," responded Doctor Bronson, who joined them. "It isn't water at all, but a mirage." "Of course it is," Frank exclaimed, with a laugh; "I ought to have known better than be deceived by it." "You are not the first to be deceived by it," said Abdul, "and it has been the cause of many deaths on this very route. Men have wandered from the road, confident that they were on the borders of a lake or river, and have fallen exhausted to die on the ground. Colonel Long, in his account of his travels in Central Africa, tells of a regiment on its march from Korosko across this desert. The men saw these lakes of water formed by the mirage; deceived by the illusion and maddened by thirst, they broke from the ranks, in spite of the protests of their guide, and went in search of water. They found their error too late, as most of them perished of exhaustion and thirst." "Look at the graves along the road, and the bleaching bones of the camels," said the Doctor, "and you will understand the perils of the journey over the desert." It was as Doctor Bronson suggested; the way was marked by thousands of skeletons of the faithful beasts of burden that had fallen in the service of their masters; and here and there, at painfully frequent intervals, were the graves of men who had perished of thirst or of the excessive heat. Even if no other landmarks existed, these would be sufficient to indicate the road.