“What does this mean?” demanded Dr. Kirshner, looking from one to the other, his black eyes snapping with anger.

“You are going to die!” was the grim answer from an evil-looking, flat-nosed fellow. He moved forward a step or two.

Bob did not understand the man but sensed that something sinister was to take place. He noticed the look of anger and anxiety on the scientist’s face.

With a sudden movement he drew out his automatic, at the same time stepping back several feet. His action was so quick that the Arabs were taken by surprise and stood for several moments trying to grasp the true meaning of it all.

“Now get back!” he commanded, flashing the shining pistol in their faces. “Take away their knives,” he said to Dr. Kirshner. “I’ve got them covered. Tell them I’ll shoot the first man who makes a forward move.”

The scientist did as directed and found that, beyond a vicious-looking knife, they were unarmed.

“You stay here and guard them,” said Bob. “I’m going in and see what’s happened to Fekmah.”

He moved over to the door and opened it. Holding the pistol in readiness, he walked slowly in the room. One glance told him that no one was in sight. But there was a door leading into a large alcove, and it was possible that he could find someone there. Perhaps the Arabs were in hiding, having sensed that they were in danger.

Tiptoeing as quietly as possible, Bob made his way to the closed door. He stood for several moments wondering what to do next. Then he decided to make a bold move.

Clutching his tiny automatic tightly, he took hold of the knob and with a sudden twist threw open the door.

The next instant he shrank back, for a tall Arab was almost upon him. The man held a vicious-looking knife in his uplifted right hand and was apparently ready to open the door when Bob did so first.

During the next few seconds Bob’s brain was in a whirl. He had half a mind to shoot the fellow outright and take no chances. But a moment later he thought better of it. He did not want to be guilty of murdering even this cruel Arab. If he had been as good a shot with a pistol as he was with a rifle, he would have shot the knife from the fellow’s hand. But as it was, he knew this could not be done.

Suddenly the Arab lowered the knife and, scowling cruelly, he rushed at the youth furiously.

Realizing that he must act quickly, Bob aimed at the man’s leg and fired. But his hand was unsteady from the terrific strain, and the bullet missed and lodged itself in the wall.

With a hoarse bellow the Arab came forward with terrific force, muttering angrily. He had the knife in readiness now, and was about to plunge it into the youth’s body when something unexpected happened.

Seeing a good chance, Bob pushed his foot forward with all his strength, bringing the man down with a thud that resounded through the little room. The Arab was taken completely unawares, and the knife went sliding across the floor.

The next instant he was on his feet, and, showing his black teeth wickedly, he darted toward his young enemy, his large hands ready to grip anything they might rest on.

Now was the time to act, Bob thought. Leaping forward, he sent his fist crashing into the Arab’s nose with all the strength in his powerful young body.

The man reeled, backed up, and then fell against the wall. Another victim of the youth’s boxing ability had gone to his fate.

Perspiration was dripping from the boy’s brow. His leg ached from the twist he gave it in bringing the Arab to the floor. Every muscle in his body seemed fatigued. For a brief moment a sensation of nothingness crept over him, and he felt slightly numb.

With a start he regained his composure and, with one glance at the still limp Arab, he dashed through the door, the shining automatic still in his possession. One thought stood out in his mind above all others. He must find Fekmah.

No one was in the alcove, but another door led out into a sort of hallway, and casting aside all thoughts of personal danger, Bob passed through it.

The next instant he found himself in a narrow passageway, with no windows or other means to admit light. The semidarkness was tantalizing, overcoming, but the boy went bravely on. Occasionally he stopped to listen. But no sound reached his ears. Could it be possible that Fekmah had been…

The thought was not a pleasant one, and Bob dismissed it from mind. Here was a door. Perhaps this would reveal something.

As quietly as possible he turned the latch and peeped in the room.

In the darkness he could make out no one at first. Then his eyes almost burst from his head as he saw, lying on the floor in the corner, tightly bound and gagged – Fekmah!

For a moment Bob stood spellbound. Then he rushed over to the unfortunate man.

“Fekmah,” he cackled gently and then bent over and felt of the Arab’s heart. It was still beating, and with a swift motion of a small knife Bob cut the ropes that bound the man. Then he removed the gag and stood back to see if his friend had been injured.

The Arab’s eyes opened, and he sat up with a start. A moment later the look of fear vanished from his face as he saw Bob.

“Praise be to Allah!” he murmured, getting feebly to his feet.

“Are you hurt?” the youth asked.

“No – not yet,” was the grim reply. “But the evil-doers were going to come back and kill me if I not tell where to find treasure. It is time now that they come. Let us go in haste, or they will stab us.”

“Treasure?” cried Bob. “You mean they wanted you to tell where the hidden riches in the desert are?”

“Yes. They were going torture me if I not tell. They are demons. Allah curse them!”

“But how did they know?” demanded Bob. “How did they find out about it? They’re not the same ones who wrecked the train, are they?”

Fekmah shook his head.

“How they find out, I not know,” he said.

Cautiously they made for the door and opened it. Satisfied that there was no one in sight, they walked through the hallway and into the alcove.

“Now we must be careful,” Bob warned. “I knocked a man out a while ago, and he may have come to by now.”

But when they reached the place, they found the fellow still limp.

At sight of the Arab, Fekmah looked at Bob with admiration. It was evident that the youth had gone beyond his expectations.

In a short time they reached the door into the first room and found that Dr. Kirshner was still guarding the Arabs. The scientist looked up in relief when he again saw Bob and Fekmah.

“I feared something had happened to you,” the scientist said. “I shouldn’t have allowed you to go in there,” he said to Bob. “But I see you’ve done your duty and brought back Fekmah.”

“But what shall we do now?” Bob asked. “Are there any officers to take charge of these crooks?”

“Yes,” the archæologist returned. “You and Fekmah stay here and guard these men and I will go after them. It is best not to take them down to the police station for fear of attracting too much attention. Other Arab friends might charge us in a large band.”

The next moment he was gone, after having promised that he would be back with officers in a few minutes.

“There ought to be some way to get the fellow I knocked out in here,” Bob thought, as he guarded the Arabs closely. “But I’d better not risk letting Fekmah have the gun. I’ll trust the fellow is still out of his senses when Dr. Kirshner and the officers arrive.”

The Arabs whom the youth was guarding were silent, but their evil faces suggested what they would do if given the chance.

Bob was in a position where he could also watch the door in the next room. He was taking no chances on an attack from behind.

“I bet Dad and the others are worried,” the boy thought. “We’ve been gone a long time.”

Inside of ten minutes Dr. Kirshner was back with three native policemen, who handled the crooks roughly. Each man was handcuffed and made to walk in front of the officers.

“Now,” said Dr. Kirshner, “suppose we get back to the hotel room. Our prolonged absence has probably caused a great deal of anxiety among our friends.”

“Yes,” Bob returned. “We – ” He stopped suddenly and then dashed into the next room for the crook whom he had previously knocked out.

But he was not surprised to find the man gone. An opened door revealed that he had escaped.

“No use looking for him,” the youth thought, as he retraced his footsteps back to his companions.

Bob was forced to explain to Dr. Kirshner, who was puzzled at the youth’s sudden dashing away. When he had finished, the scientist regarded him admiringly but warned him against taking unnecessary chances.

“Those fellows would think nothing of stabbing you to death,” he said, shaking his head gravely.

They went on up the street and arrived in a short time at the hotel in which were their friends. Up at Fekmah’s room they were given a hearty welcome.

“What kept you so long?” asked Mr. Holton, seeing at a glance that something was wrong.

Dr. Kirshner related the details of the previous happenings, laying stress on the pluck and bravery of Bob.

The naturalists and Joe listened closely, eager to get an account of everything. They did not seem unduly surprised, for at the start they felt that something was wrong. But that their friends would be in such grave danger was not in the least anticipated.

“It’s lucky that you’re here to tell it,” said Mr. Lewis, with a shaking of his head. “Such desperate characters know nothing else but to rob and kill.”

“But as it is,” smiled Joe, “you came out all right, and had an unusual experience at that.” Secretly he was sorry he had not taken part in it.

“And that is the second time Bob brought in criminals,” said Mr. Lewis, recalling an incident that happened the summer before.

“I’d rather you wouldn’t take such risks, though,” urged Mr. Holton. “If one taxes good fortune too much, he is sure to come to a tragic end some time.”

There was a short silence. Then Joe ventured to touch upon the matter that concerned them before the abrupt interruption of a short time ago.

“What will be our next move?” he inquired.

“Hmm,” hesitated Dr. Kirshner, “that is hard to say at present. I suppose, though, that the logical thing to do is wait and take a train to Wargla as soon as possible. When the track will be cleared is hard to say. It might be a day, or it might be several. I can see no other way of getting to Wargla, can you, Fekmah?”

“No,” replied the Arab. “Of course there are good roads part of way, and might ride automobile some of way, but train could get there sooner, even if we wait.”

“I guess you’re right,” agreed Joe, thoughtfully. “There’s no use wasting time trying to do a thing that can’t be done. Have you made inquiries as to about when we can expect the train to be ready?”

“Yes,” answered Dr. Kirshner. “The station agent said inside of two days, but it probably won’t be that soon.”

“Let’s hope it will,” said Bob. “I’m anxious to get started into the desert. How long will it probably be before we reach our destination, once we get started on camelback?”

“That all depends,” replied the archæologist. “If we see fit to use dromedaries, or the one-hump camel, we will make much better time. They can travel from ninety to a hundred miles a day with ease, and if forced, can do much better than that.”

“But the jolting is most uncomfortable,” Mr. Lewis reminded him, and Fekmah nodded.

“Soon get used to it, though, don’t you?” asked Joe.

Fekmah laughed.

“No, never,” he said. “But not feel it so bad after many days of riding.”

“But getting back to your question,” resumed the archæologist, speaking to Joe, “if we have good luck we should cover the thousand miles to the Ahaggar Mountains inside of two weeks, considering, of course, stops at oases, small towns, and the like. Then how soon we can find the hidden riches would be a mere guess, because without the map Fekmah will probably have much more difficulty in recognizing various landmarks.”

“But you must remember that our main object was to work for the good of science,” Bob’s father said. “Mr. Lewis and I were to get specimens of any animal life that exists in the far reaches of the desert, and you were to look up the remains of ancient civilizations. Incidentally, though, we’ll search for the hidden riches.”

“And Joe and I were to take motion pictures when we get started on camelback,” said Bob proudly.

“Speaking of animal life,” began Mr. Lewis, “suppose in the morning we get out in the open country and see if we can collect any specimens. From what I’ve read and heard, I guess it is necessary to go at least as far as Arba before we would find a region that is worth hunting in.”

“I’m with you,” said Mr. Holton. “We’ll engage an automobile to take us the fourteen miles or so to Arba, and then foot it from there. It will be funny if we can’t bag a few worth-while specimens, at least.”

“Of course Joe and I can go, can’t we?” grinned Bob.

“You bet. And Dr. Kirshner and Fekmah, if they care to.”

But those persons announced that they would remain in Algiers and further discuss the coming expedition.

“Then too,” the archæologist remarked, “I’d like to keep posted on the progress made in getting the train ready for the journey to Wargla.”

It was well past noon, and the explorers now realized for the first time that they were extremely hungry. The strain of the eventful morning had stimulated their appetites to a high degree, and they felt they could devour almost anything in the way of food.

“Lead me to the café and watch what happens,” said Joe, making for the door.

The others followed, and although not as emotional they were no less hungry.

The meal tasted good, despite the fact that they were ignorant of its composition. Fekmah tried to explain one of the dishes, but the foodstuffs used in its preparation were unknown to the Americans, and they were as much in the dark after he had finished the explanation as before he had begun it.

“We’ll trust there isn’t anything repulsive in its make-up,” smiled Mr. Holton.

“At any rate the taste isn’t so bad,” came from Bob. “I’ve seen stuff much worse.”

“Referring to the delicacies of the South American Indians, I suppose,” laughed Mr. Lewis, and the boys nodded.

“We were forced to eat almost anything for a while,” reflected Joe. “Getting lost in the Brazilian jungle was a serious thing, and we realized it. Then when we got in with those savages, we accepted their offer of food at once, even though we weren’t sure it was all right.”

“That was a great adventure, boys,” remarked Mr. Holton, his thoughts going back to the countless days of river traveling in unknown South America. “The more I think of it the more convinced I am that we should be extremely proud of that exploit. Mr. Lewis and I have had many adventures in unknown lands while collecting specimens for the museum, but none surpasses that one.”

“I’d like to go there some time,” said Dr. Kirshner. “But so far, my work as an archæologist has not called me into that region. When it will, if ever, I do not know.”

After the bountiful meal the explorers took it easy in the hotel, and Fekmah returned to his room.

“Since we haven’t anything in particular to do, we might as well rest up for the hunting trip tomorrow,” said Mr. Lewis, as he and the others sat in the lobby of their hotel. “We’ve had a hard day of it and deserve to be idle for a while.”

The others agreed, and the remainder of the day was spent in quiet.

But early the next morning the boys and their fathers were busily preparing for the hunting expedition. By seven o’clock they had their rifles and other necessities in readiness, and with a farewell to Dr. Kirshner, who wished to talk over the Sahara journey with Fekmah, they left the hotel.

Mr. Lewis had earlier made arrangements for a car to take them to Arba, whence they would go on foot into the forests and desert. The automobile was parked in a small garage not far away, and they reached the place in but a few minutes. It was a low, dirty structure, with few windows.

A swarthy native came out to meet them and at once recognized Mr. Lewis. He led the way through a small door and into a room that served as the garage.

“Here we are,” said Joe’s father, as they rounded a corner.

He pointed to something that at once caused Bob and Joe to burst out with laughter.