DOWN the steep slope Joe went, powerless to catch himself. There were no shrubs or stalks to grip, no hard soil to cling to.

His mind was in a whirl. Where would he land? How far had he yet to go? He comforted himself in the thought that there were no rocks or tree trunks to dash against, but if the soil at the bottom of the hill happened to be hard, his doom might be sealed. Already he could feel the jar and pain of suddenly coming to an abrupt stop on unyielding ground.

Down, down he went, past the pack camels that were still slowly climbing the steep side of the mound, lucky enough to have been missed by the falling camel.

Joe caught a momentary glimpse of his own dromedary, which was also rolling rapidly down.

Suddenly he felt a severe jar and gradually came to a stop in the soft sand at the foot of the dune.

A moment later he glanced up fearfully, as he saw his dromedary coming down at him with lightning rapidity.

He tried to rise and dash to one side, but his feet failed to respond to his efforts. They had been numbed by the jar of striking the ground.

The falling camel was almost upon him now. Something must be done at once!

Summoning all his strength, he threw his body to the right of the oncoming animal. It was an act of desperation, and he felt sure that it would fail. But a moment later there was a rush, the sound of swishing sand, and the animal whisked past, stirring up a dense cloud of fine soil.

Joe gave a sigh of relief and for a brief period sank back down on the sand, as though waiting for his strength to be restored. Then he was up, looking for the fallen camel.

It had come to a stop in a drift of loose sand and appeared to be uninjured. But it was snorting and kicking furiously, its anger visibly heightened to a high degree.

Joe ran over to it and attempted to give aid, but the furious dromedary kicked at him in a rage.

Mr. Lewis and Bob came running down, finding footholds with difficulty.

“Wait!” cautioned the naturalist. “Don’t hurry the camel. Let him take his time. His fit of anger will be over in a few minutes,” and then added: “That was some fall! We were afraid you’d be hurt seriously. No bones broken, are there?”

“I guess not,” Joe replied. “Just shaken up a bit. I’m afraid the dromedary is hurt more than I. Don’t see what could prevent it.”

“You may be right,” his father said solemnly. “We’ll find out in a minute.”

Before long the animal’s anger lessened, and it tried to rise. But its legs refused to move, despite the frantic efforts.

Mr. Lewis’s face took on an expression of anxiety. He bent down and felt of the struggling legs. Then he straightened up and shook his head hopelessly.

“Broken,” he muttered in a tense voice. “Two legs are broken.”

“What!” cried Mr. Holton, who, along with the others, had managed to scramble down the hill.

“Look for yourselves,” Mr. Lewis said gravely. “The camel’s weight was too great to come continually against the rather fragile limbs, and the hind ones snapped in two.”

The explorers’ faces were greatly sobered by the dread discovery. Now there was but one thing to do. The unfortunate beast must be left to die a slow and torturing death. There was no other way out, for the legs could not be set, and the beast could not be taken on.

Mr. Holton unstrapped his rifle from his shoulder.

“I’m going to put an end to it,” he said with a sudden decision. “It seems only humane, for otherwise death will come slowly and horribly.”

He walked over to a small elevation along the side of the dune, while his friends moved away to a safe distance.

After a moment of careful aim, he pulled the trigger.

The high-velocity bullet sped true, penetrating the brain.

There was but one spasmodic movement; then the camel rolled over, dead.

For some time the explorers were silent. It was not pleasant to see the useful dromedary come to such an end. Finally Joe began the ascent of the dune.

“It’ll seem strange without that camel,” he said. “How will I manage to have a mount?”

“You’ll have to take one of the pack animals,” returned Dr. Kirshner. “We’ll divide the supplies among the others and get along some way. If we do the right kind of scheming, I don’t believe the other camels will notice the extra load.”

It was difficult work climbing the treacherous slope of the dune. Many times their feet would scarcely find a foothold when the sand would give way, making it necessary for them to sprawl out or clutch one another in order to prevent rolling down.

At last they reached the top and began dividing the provisions among nine of the ten pack dromedaries. It required over an hour to complete the task, for it was difficult to find places for all their belongings.

“Now you may find it hard to get used to the strange camel,” said Mr. Holton, as Joe straddled the kneeling animal. “He may act up and threaten to throw you, but stick to it. It won’t take long to show him you mean business.”

“He’s been broken in some,” remarked Fekmah. “Though it may been good time since he had rider.”

Much to their surprise the dromedary responded to Joe’s commands at once, resigning himself over to his new master.

“And am I glad!” the youth exclaimed, as the little caravan prepared to resume the journey.

“That’s rather an exceptional piece of luck,” smiled Mr. Holton. “Usually it takes a good while for a camel to get used to its new rider.”

The explorers continued the journey, although they realized that they could cover but a few more miles before dark. Already the sun was beginning to sink behind the distant hills, tinting the dunes many colors.

“Didn’t get as far today as we did yesterday,” observed Bob, as the boys waited for Tishmak to find a suitable stopping place.

The guide found the place he wanted between two large dunes, which were spread out over a large amount of ground.

“Tishmak say we wait here and get supper,” said Fekmah, after a moment of conversation in the native tongue. “Then when moon come out, we go on and try to make up for lost time.”

“That suits me,” agreed Dr. Kirshner. “Almost anything would satisfy my appetite right now.”

The explorers dismounted and prepared the meal, which after the strenuous hours of constant traveling tasted delicious. Then they sat quietly in the semidarkness.

At last the moon came out, and the desert was again seen to take on a new appearance. The sand was even more beautiful than the night before, white as it was from the soft light.

Tishmak motioned for the adventurers to mount their dromedaries and get under way. If they expected to get over much more territory, it would be necessary to hurry.

“Here’s hoping we can come up to our hundred-mile minimum,” said Joe, urging his camel to trot faster.

It was nearly ten o’clock when they finally stopped for the night’s slumber. Fekmah was well satisfied with the distance they had made that day.

“Tomorrow we will come to Tudemait Plateau,” he said, after conversing with Tishmak. “It is a rocky region, with large black boulders.”

“Then we won’t see any more sand dunes?” asked Bob.

“No. The country will be rather level, but we will have to go many kilometers out of way to avoid rocks.”

“That’ll suit me,” put in Joe. “I’m tired of seeing nothing but sand.”

That night they trusted to good fortune and did not set a guard. Tishmak informed them that there was little danger of robbers in that region, and the alertness of the camels would be an added precaution.

“We are nearing the country of the Tuaregs,” explained Fekmah. “These natives are friendly, but drive away nomad robbers. We probably be safe tonight.”

And they were. The next morning they had completely regained their natural liveliness and pep, feeling thoroughly refreshed and ready for anything the day might bring.

Breakfast over, the long journey was resumed.

As Tishmak had predicted, the country gradually began to change. The sand dunes that had recently been high slowly became smaller and lower, and vegetation increased still more. Small rocks were everywhere, their color a dark gray or black. In the distance the sharp crags of the Tudemait Plateau could be seen.

As the explorers neared a low long mound, Tishmak’s dromedary suddenly halted and sniffed nervously. A moment later the animals of the other explorers also came to a stop.

“What’s this!” exclaimed Mr. Lewis, looking about wonderingly.

“Guess they heard something,” said Dr. Kirshner. “What it was, we’ll see in a moment. They have an uncanny power of detecting noises that we men can’t catch.”

Tishmak unstrapped his rifle and motioned for the others to do likewise.

“Wonder if it’s bandits,” muttered Joe, holding his gun in readiness.

The sound of camels’ footsteps now reached the adventurers’ ears. Then from behind a hill appeared a long trading caravan of Arabs, who also had their rifles in readiness.

Tishmak rushed forward, throwing his hands apart in a gesture of friendliness. He was followed by Fekmah and the others.

As soon as the Arabs saw that the whites meant no harm, they stopped and talked freely. Much to Dr. Kirshner’s delight, he could understand the men easily, their language differing greatly from that of Tishmak.

The Americans learned that the caravan was on its way to Wargla, loaded with a large supply of dried dates, grapes, and many other products of the desert oases. It was made up of over seven hundred camels, which were in a line several miles long.

“Look at the leader,” said Joe to his chum, referring to the head camel. “It’s decorated with about as many colors as there are.”

“Want to make a good showing,” smiled Bob. “From what I’ve heard, a caravan is judged by how well the first camel is engulfed by colors.”

Much as the explorers would have liked to talk with the Arabs, they did not stop long. Time was too valuable to be wasted, especially when they were literally running a race with the two thieves who had stolen Fekmah’s map.

But before they left, Bob and Joe took motion pictures of the caravan at several distances. Especially were the youths pleased by the friendly attitude of the Arabs when the camera was held before them.

“Doubt if they know what it’s all about,” smiled Bob. “But that doesn’t keep the pictures from being good.”

“Wouldn’t they be surprised if they could see themselves on the screen!” laughed Joe. “They’d probably think something supernatural was taking place.”

With a cheery farewell the American expedition left the Arabs at the head of the trading caravan and again took up the journey. But it required nearly a half-hour to pass the last of the pack camels, which were guarded at intervals of every fifty animals by an armed Arab.

“Seven hundred camels is a good many,” remarked Bob, as the youths rode at the rear of their own pack animals. “Their owner must have a good bit of money.”

“Many of those Arabs are wealthy, I guess,” said Joe. “Fekmah said some have as many as two thousand camels.”

As the explorers moved on, they saw other signs of small game. Occasionally Mr. Holton and Mr. Lewis displayed their skill with a rifle by shooting creatures from a great distance.

“Seems to be getting a little darker,” remarked Joe, as they descended into a little valley.

“Darker? Couldn’t be.”

Then, as Bob gazed ahead at the opposite slope, he uttered an exclamation of bewilderment.

“It is getting darker!” he cried. “Why—why, it’s impossible. Dark at nine in the morning! Something’s wrong somewhere!”

A second later Mr. Holton rode up to the boys anxiously.

“Get out your goggles!” he directed them. “We seem to be riding into a fog of black dust. Tishmak thinks we may have to stop for the day.”