“WHAT is it?” asked Bob, slow to catch sight of the object.

“Gazelles,” returned his father. “A small herd of them.”

In a moment the agile animals came running out in full view of all, their beautiful coats showing clearly in the bright sunlight.

The creatures were about a hundred yards away, not any too near for an accurate shot, but the naturalists were much in desire of several as specimens.

Their long-distance rifles were equipped with telescopes, making it much easier, according to the naturalists, to sight an object at a hundred yards or more. Bob’s gun, however, did not have this convenience.

Mr. Lewis prepared to take aim.

“You take the animal to the extreme right, Howard,” he said to Mr. Holton. “Bob, you pick the one in the middle, and I’ll try to get one at the left. Tom,” he said to Dr. Kirshner, “suppose you count to three. When you say three we’ll all fire together. Let’s go, now.”

“One, two, THREE.”

Bang! Three rifles spoke as one.

Mr. Holton cried out in delight.

“Hurrah!” he said. “Got all of the ones we aimed at. Good work.”

The report of the guns caused great disorder in the herd, the graceful creatures leaping about at a speed that did their thin, muscular legs full justice. Before long they had disappeared behind the sand dunes.

“Now we’ll ride over and see what the ones killed look like,” said Mr. Lewis.

As the dead gazelles were not in a direction that would take the explorers off their steady course, the pack camels were driven along also.

When they reached the spot, Joe cried out in amazement at sight of the gazelles’ sleek skin. It was of a yellowish brown color, and the abdomen was as white as snow. The legs were long and thin, narrowing down to a finely shaped hoof.

“As beautiful an animal as one can find,” declared Mr. Lewis, who with Mr. Holton had been everywhere and knew what he was talking about.

“Two males and a female,” observed Bob’s father, noting the curved horns of the former.

“Look at the eyes,” said Joe. “They’re almost jet black.”

The naturalists began at once the task of removing the skins, for it was best not to leave them on too long under such a sun.

As soon as this was completed they were placed in a solution of alum that had been prepared by Dr. Kirshner under the naturalists’ directions.

“In our opinion, that’s the best preservative there is,” remarked Mr. Holton, “although there are many others in use.”

It required the better part of two hours to complete the task. The sun was just beginning to sink behind the distant mounds when they were ready to continue the journey.

“Be dark before long,” predicted Fekmah. “Then we will have to stop and wait for the moon. If you have not before traveled on desert under moon, you have much to see.”

They plodded continually on, Bob and Joe occasionally taking motion pictures. As the heat gradually became less intense, the youths felt a thrill of exultation run through their veins. Now there was nothing to worry about, no fierce sun to sweat under, no cares or anxieties to occupy their minds. Life—life on the boundless Sahara—seemed great.

“The folks back home are sure missing something,” smiled Bob. “Though I suppose a lot of them wouldn’t care for doings of this kind.”

Slowly the darkness came. At last, when the little caravan was in a notch between two unusually high dunes, Tishmak called a halt and conversed with Fekmah. Then the latter turned to the Americans.

“Tishmak says it is not thing to go on through darkness,” he told them. “Might come on to something bad. We’ll wait here for moon to come out, then go on.”

“Now’s a good chance to get our suppers,” said Dr. Kirshner, commanding his dromedary to lower.

By the aid of flashlights a limited but satisfying meal was prepared on paper, and the adventurers all ate heartily.

“Too bad there’s nothing for the camels,” said Mr. Lewis.

“No need of worrying about them,” returned Fekmah. “They can have no food or drink for good many days. Soon we come to region of thin vegetation. Then they eat.”

After the meal the explorers stretched out on the now cool sand, glad of a chance to rest their tired limbs. The constant jolting of the dromedaries had contributed much toward fatiguing them.

“Now’s the time when a coat comes in handy,” said Bob, reaching into his bag on the camel’s back.

“Wait till dawn, if you want to see real cold weather,” came from Dr. Kirshner. “Isn’t that right, Fekmah?”

The Arab nodded.

“Temperature often gets down to thirty-five degrees,” he said.

Before long the moon came out in full splendor, flooding the vast expanse with enchanting light and creating a scene beautiful beyond description. The sand looked white all about, broken only by an occasional ripple caused by the wind. The brilliant moon was surrounded by a large white circle, which seemed to throw silver darts on the dunes. In the distance were groups of luminous clouds, whose ragged edges were transparent. It was a wonderful sight, and the explorers sat for some time as though transfixed.

“Never saw anything like it in my life!” exclaimed Mr. Holton, his breath almost taken away by the magnificent spectacle.

“Wonder if we’re dreaming?” mused Joe, also fascinated.

Fekmah and Tishmak had been out to witness such a spectacle before, but seemed to think no less of it.

“I guess here’s a case where familiarity doesn’t breed contempt,” laughed Mr. Lewis, his eyes on the Arabs.

Bob got out the movie camera, intent upon taking several scenes that would be “really different.”

“Moonlight always makes a good impression on a motion-picture audience,” remarked Joe. “And away out here on the Sahara—well, it’ll seem wonderful!”

Tishmak made signs that they move on, and the others were glad to do so. Since there was so much light, it would be easy to find their way.

They were now in a region of high sand hills, with still no rocks other than the small bright-colored pebbles that were scattered about. Vegetation began to be more numerous in the form of had, drinn, and other plants, which were occasionally eaten by the dromedaries.

“I wonder if Fekmah is sure he’s headed right?” said Bob, as he and his friend again took their places at the end of the caravan.

“He doesn’t seem to be having any trouble yet,” was the reply. “You remember he made another map from memory after the first one was stolen. Chances are he knows where he’s going.”

“If he doesn’t, we probably won’t find the hidden riches,” was Bob’s return.

They trudged on, up and down the sand hills, until about ten o’clock. Then Tishmak called a halt.

“We camp here for night,” announced Fekmah, after a short conference with the guide. “It not wise to go any farther tonight. We get a sleep; then start on in morning.”

“I was just thinking it’s about time,” laughed Dr. Kirshner.

Tishmak had purposely picked out a deep gulch between the mounds as the stopping place. This would be a fair precaution against the nomad bandits who were reputed to be in this region.

“Takes a guide to know what’s best,” remarked Bob, observing the high walls of sand all about them. “The country in this vicinity is so rough that a caravan wouldn’t come anywhere near it, especially later on, when it will be pitch dark.”

The explorers dismounted and got out the tent. The poles were put up, and in a short time everything was in readiness for the night’s sleep.

“Curl up tightly in your blankets,” advised Mr. Lewis, as he prepared to occupy a corner of the tent. “It will be very cold along toward dawn.”

“And will we sleep!” said Bob, throwing himself down in relaxation.

Bob’s prediction proved right. The boys enjoyed the deep slumber that only an explorer can experience. It was wholly unexpected that they be awakened at dawn by their elders.

“It’s a dirty trick!” grinned Joe, as his father urged him to fully awaken.

“I know,” the naturalist said sympathetically. “But we must be on our way before the sun gets so hot that traveling will be dangerous.”

But once up, it was easy to forget sleepiness and enjoy the early-morning air.

“Talk about cold!” cried Joe, shivering in spite of the fact that he was wrapped in a heavy coat. “I’ll bet the old mercury is down to thirty degrees.”

“Not quite that bad,” laughed Dr. Kirshner, holding up a thermometer. “It’s thirty-eight.”

“Even that is cold for the desert,” said Bob. “I always had the impression that it is hot all the time.”

“There are some places that way,” returned Mr. Lewis. “But certainly not the old Sahara.”

Mr. Lewis and Mr. Holton got out the provisions and began preparing the breakfast.

“And while they’re doing that, suppose we go up to the top of this mound,” suggested Bob to his chum. “It’s rather high. Maybe we can get a view of the country ahead of us.”

“Don’t be gone long, boys,” called Dr. Kirshner, as they made their way to the edge of the hill.

The sand under their feet was, in many places, loose and unstable, but they found sufficient hard spots to make safe footholds. It was not a little difficult to estimate where they could safely climb higher. Perspiration was dripping from them when they at last reached the top and turned their gaze toward the horizon.

A cry of astonishment came from Joe at the scene that was spread out before them. The dunes that stretched away in the distance were tinted with many colors, showing strangely in the early-morning light. Some were pink, others blue, still others green, while the ones farther away paled away into nothingness.

“That’s a new one on me,” remarked Bob, highly puzzled. “I never expected anything like this.”

“Nor I. Wonder what causes it?”

“Beyond me.”

They stood for some time gazing at the varicolored mounds. Then Joe turned about.

“Better get back to camp,” he suggested. “Breakfast may be ready.”

They were about to retrace their footsteps when they heard a rustling sound but a few feet away.

“What’s that?” muttered Bob, sensing that danger was at hand.

The youths glanced around—and then shrank back in horror!