"Fifty miles, at least; maybe more. It will probably take us another half-day to get to them." Finally the youths turned and rode back down the hill to tell their elders of the magnificent view. Fekmah particularly would be pleased, Joe thought. But the boys were not overly anxious to get back to the oasis at once. There were many other high sand dunes that they would like to ride over. "We won't stay much longer," said Bob. "Just ride around a bit." To their right was another high hill that might afford a view in another direction. The youths rode over to it and climbed the gradual side. Then, when they came to the top, they cried out in surprise and fear. In the distance appeared to be a whole regiment of galloping horsemen coming toward them! CHAPTER XVIII Savage Tribesmen For a moment the youths were taken completely aback in surprise. That they would see anything like this away out on the Sahara was not in the least expected. They stood for some time in sheer amazement and not a little fear. "An army coming at us!" muttered Bob, staring at the distant spectacle. "An army, yes. Must be five hundred cavalrymen." "But--but it can't be! It's impossible. What would soldiers be doing away out here on the desert? Something's seriously wrong somewhere. If just one of us should see such a thing it might indicate that the old brain wasn't working just right, but for you and I both----" "Come on," suggested Joe, giving his dromedary a slight kick. "Let's get out of here. I'm greatly worried." The youths turned their camels back to camp for a short distance; then they urged them on to a fast trot. They were not a little relieved when they finally reached the oasis, where they found their friends awaiting them. "Where have you been so long?" inquired Mr. Lewis, his face not a little serious. "We thought maybe something held you back," added Dr. Kirshner. "It did," replied Bob, trying to remain calm. The men sat up quickly, sensing that some misfortune had come upon the boys. "What was it?" demanded Mr. Holton tensely. "An army," Joe returned soberly. For a second there was silence. Then the men broke out in laughter. Evidently they thought the youths were joking. Even Fekmah joined in, his dark features drawn together in mirth. "Nothing to laugh at," said Joe, vexed because the men thought their experience funny. "It nearly scared Bob and me out of our wits." Mr. Holton grew more serious. "Come, now," he said. "Tell us what you mean." Joe told of seeing the phenomenon from the top of the hill, saying that there appeared to be at least five hundred horsemen coming toward them. When he had finished, the naturalists and Dr. Kirshner jumped up in wonder and not a little fear, but Fekmah only laughed. "W-what's humorous!" demanded Mr. Lewis, greatly perplexed. "Everything," said Fekmah, laughing still harder. "What the young men saw was only an illusion or mirage. There no army on Sahara. Only look like army."