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.”

“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.

“Ahaggars very strange,” said Fekmah to the Americans, as they rode in a group at the back of their pack camels. “There are high cliffs, tall needle-like peaks, deep caves. There are canyons, ravines, underground passageways. We see much, and we too be in great danger.”

“Danger?” Joe looked up in some surprise.

“Yes. Very great danger. Wild Tuaregs roam about, and when on a raid, think only of robbing travelers. Then, too, we be in region where the two thieves who stole my map are. They perhaps be waiting for us and shoot us quick without giving warning. Many other dangers we might see.”

Fekmah sobered the Americans a little. They had not anticipated any great peril, although they knew the two thieves might, should they have arrived at the hidden riches first, give them trouble.

“But we’ll come out all right,” predicted Bob, again becoming cheerful. “We’ll show those fellows that we’re capable of attending to any crisis.”

A little farther on they reached the wall of rock that had previously shut out the view of the mountains. It stretched many miles to their right and left, but there were numerous breaks that afforded openings into the country beyond.

They had barely reached the other side of the wall-like formation when Joe caught sight of a group of tents quite a distance to the east. He motioned for his friends to look in that direction.

“Probably Arabs,” pronounced Fekmah, after Tishmak had chattered rapidly for a moment. “They nomads, who wander about the desert taking their flock of goats with them.”

“Suppose we go over and see them,” suggested Mr. Holton. “Perhaps they can give us a description of the country ahead of us. There may be many more wells than we think, and it will do us no harm to know of them.”

The others were in favor of carrying out Mr. Holton’s move. But Fekmah warned them to be on the lookout for treachery.

“They probably not do us harm, but can never tell,” he said, as the dromedaries were turned in the direction of the tents.

They reached the encampment in a very short time and were about to look up some of the Arabs when a savage growl made them wheel around in surprise and fear.

“Look!” cried Joe, laying his hand on his rifle.

Two large, savage dogs were making toward them with all fury, showing their terrible teeth in anger. The enraged creatures were probably owned by the Arabs in the tents and were acting as guards against all marauders.

The foremost dog was almost upon Mr. Lewis’s camel. In another moment the beast would sink its teeth in the dromedary’s throat.

Displaying the quickness of a cat, the naturalist unslung his rifle, took hasty aim, and fired.

The report of the gun was followed by a longdrawn howl from the huge dog.

“Quick!” cried Bob. “The other dog!”

The second beast was rushing forward angrily.

Mr. Lewis again took aim. The others, trusting in his marksmanship, made no move to get their rifles.

Click! There was no report this time. His magazine was empty!

Mr. Holton tried vainly to get his rifle out in time. Something must be done at once, for the savage dog would be at the camels in but a moment.

Suddenly Joe leaped from his camel directly in the path of the oncoming animal. The dog stopped for a second, then rushed at the youth with terrible ferocity.

“It’s now or never!” Joe thought and brought the butt of his rifle down with all his strength on the dog’s head.

There was a cry of pain, and the next moment the beast rolled over in a dazed condition. At last the terrible enemies had been overcome.

“Great work, Joe!” praised Mr. Holton. “We weren’t expecting to see you act so quickly.”

“I didn’t know whether I could hit him at the right time or not,” the youth said, wiping the perspiration from his brow. “But I thought I’d take a chance. It——”

He stopped fearfully as a rifle shot rang out. Another report followed the first, and Tishmak fell from his dromedary.

“Back!” cried Dr. Kirshner. “It’s the Arabs shooting from the tents. Hurry or we’ll all be hit!”

Tishmak was rapidly picked up and placed on his camel, and then the explorers retreated behind a formation of rock near the high wall of stone that was to their right.

“You look after Tishmak,” said Mr. Holton to Dr. Kirshner. “Meanwhile we’ll keep these Arabs away. We certainly aroused their tempers when we put those dogs out.”

A volley of shots came from the Arabs’ tents, and the Americans at once answered with their own rifles. Wherever a shot was heard, Mr. Holton directed his friends to fire at the spot.

Suddenly Mr. Lewis caught a glimpse of a large one-armed Arab who emerged into full view to send a bullet at his white enemies. Without hesitation the naturalist fired, bringing the man down with a thud.

“Look!” cried Bob. “They’re backing up. That fellow you shot must have been the leader.”

“Does seem that way,” agreed Mr. Lewis. “But we must remain on guard. These are treacherous characters.”

Only an occasional shot rang out. Then finally there was silence.

“Now we’ll see how Tishmak is,” said Mr. Holton, leaving his position at the end of the rocky crag.

They found that Dr. Kirshner had bound and treated the wound, which was in the left arm. The Arab seemed in high spirits, despite the fact that he was evidently in pain.

“It doesn’t appear serious,” said the archÆologist. “With the right kind of attention it will probably be all right in a few days.”

“Lucky that he wasn’t killed, or that more of us weren’t hit,” remarked Mr. Lewis gravely. “The Sahara is a dangerous place for explorers.”

They waited several minutes for any more rifle shots from the Arabs, but none came. Finally Mr. Holton mounted his dromedary.

“Let’s get on our way,” he suggested. “I don’t think there’s any danger now. The Arabs have retreated to a distance beyond their tents, and I believe they’ll stay there awhile.”

Tishmak was helped on his camel. Then, when the others had also mounted, they rode off.

They were now rapidly leaving the region of low sand dunes behind. Rocks of all sizes and shapes became more numerous, and vegetation was more abundant. There were, however, stretches of coarse sand plains, which were now and then dotted with boulders.

Suddenly, as they ascended a long low hill, Bob and Joe cried out in delight and pointed to something a half-mile or so away.

“A lake!” exclaimed Joe happily. “A lake of water!”