"You mean it was a trick of nature, like the more common mirages of lakes on the desert?" asked Dr. Kirshner with great interest. "Yes," the Arab answered. "Caused by the bending of the rays of light when they strike the hot sand." "Well, that's a new one on me!" confessed Bob. "I was aware of the fact that mirages of lakes are common, but that I should see an army----" It was now rapidly becoming dark. The explorers thought it best to sleep all through the night and not wait for the moon, for they greatly needed the rest. "Tomorrow morning I'd like to see that mirage that you boys thought was an army," said Mr. Holton, when they prepared to retire. "And I, too," put in Dr. Kirshner. "As it isn't out of our way, we can all ride over there." "It'll be a good chance to take some motion pictures," said Bob. "A scene as unusual as that is sure to attract the curiosity of an audience." Tishmak informed them that they would be out of this short sand stretch early the next morning. Then they would come into the Ahaggar Mountains, the real home of the mysterious Tuaregs. "And I expect to begin my work in this region," announced Dr. Kirshner. "Perhaps if I put legend and history together, I can locate something that will prove of great value to the world of archæology. I have in mind at present the tomb of a great king who reigned in those mountains many thousands of years ago. He is said to be an ancestor of the Berbers, who are related to the Tuaregs. When we come to the many Tuareg villages, I intend to make inquiries as to their ancient legends." They were up early the next morning, anticipating the exploration of the mountains that lay ahead of them. But in order to get to the Ahaggars, it would be necessary to continue for a short distance over the sand dunes. After breakfast they rode over to the distant hill to get a view of the mirage seen by the boys the day before. Sure enough, the army of horsemen appeared to be riding toward them, and the details were rather plain. Mr. Lewis shook his head in bewilderment. "Sure is strange," he muttered. "Why should the horses and the riders be so clearly defined? I can easily understand the mirage of a lake, but this sure gets my goat." They stood for some time staring at the distant spectacle, Bob and Joe taking motion pictures. Finally they rode on up the hill to catch a glimpse of the Ahaggars. "I rather think that peak not Illiman but Oudane," said Fekmah to the youths, in answer to their question asking the name of the distant high mountain. "Mount Oudane very high, and much nearer than Mount Illiman." More movies were taken by the youths. Then they rode down the opposite side of the dune in the direction of the mountains.