EARLY the next morning the youths were up getting ready for the long journey into the unknown. They had all their possessions packed when Dr. Rander came with mules and provisions.

He was introduced by Bob and Joe to the other members of the expedition, who, particularly Mr. Holton and Mr. Lewis, recognized him as a capable explorer.

The youths made arrangements for Karl Sutman to meet them in a town called Pasaje, at the end of a sufficient time. The aviator was to have his monoplane ready to take the youths to the locality occupied by other divisions of the expedition.

“Now do be careful and don’t take any chances,” warned Mr. Holton, after additional boxes of food had been strapped on the backs of other mules. “Remember, slow traveling with safety is far better than rapid going with danger.”

“We’ll be all right,” Joe assured him, as the mules were being placed in line. “And Karl will fly us to your locality in due time. Don’t forget that you are to be careful too.”

With fond farewells, Bob and Joe and the old man drove the mules toward the rim of mountains that skirted the eastern horizon. They rounded a high hill and lost sight of their relatives and friends.

For some time the youths were silent with their thoughts. Who knew whether they would ever see those dear ones again? Even at best, there were untold hardships and dangers in the mountains that lay before them. Would they be able to meet any crisis?

It was some time before the boys resumed their natural peace of mind. But when they did, they were eager to take in all the sights of this wonderland.

There was a wide trail that led eastward from Cuzco. Over this the pack train went at a slow but steady gait that promised to eat up the miles sooner than it might be thought possible.

“Wonder if we’ll see any big game?” remarked Bob, as he and Joe walked near the rear of the pack train. “I’d like especially to bag one of those white condors Dad was talking about. You think there are any?”

“Possibly,” replied Joe. “But if there are, it isn’t likely that we’ll see one.”

The three adventurers followed a well-beaten path to the town of Puquiura, which they found nothing more than a group of native mud huts.

“Not much life here,” observed Joe, as the caravan of pack animals passed on through the village.

“I suppose this is typical of all the towns in these mountains,” came from Bob. “Just a bunch of dirty mud dwellings.”

Led by Dr. Rander, the Americans wound around a narrow trail that reached steadily upward. They were making fairly good time, and if nothing prevented, they expected to arrive at a much larger town before noon.

“I think I’ll try riding my mule,” announced Bob, who, along with his friends, had been walking beside the mounts.

“Better watch out,” cautioned Joe. “Those little animals are treacherous sometimes.”

Bob called to Dr. Rander to wait for him. Then, pulling his mount out of the line, he placed his foot in the stirrup and threw his leg across the sturdy little mule’s back.

But just then something happened. The animal wheeled about, and, throwing its hind feet high in the air, it leaped forward with a snort of resentment.

“Help!” cried the amused and yet worried Bob.

“What do you want me to do?” inquired Joe, taking in the scene with interest.

“Grab hold of his tail! Do anything!” Despite his serious predicament, Bob could not help laughing, although he was angered.

“Get hold of his tail, huh? Not much.” Joe intended to derive as much amusement as possible from his friend’s plight.

All joking was cast aside a moment later when, at an unexpected moment, the mule gave a quick turn to the left and threw Bob to the ground. The youth caught the fall with his arm, and so escaped injury, but his anger was as strong as ever.

“I’ll fix you, you——”

Bob did not finish the words, for at that moment the mule leaped forward and galloped off at a rapid pace.

“After him!” Bob cried, dashing ahead as fast as his legs would carry him.

A more amusing sight could hardly be found. Joe’s laughter mingled with the sound of rapidly moving hoofs, and even old Dr. Rander joined in the merriment.

“Think he’ll catch him?” queried Joe, as pursuer and pursued vanished behind a heavy cloud of dust.

“I believe so,” the old man returned, straining his eyes to make out the figures ahead. “The mule will soon tire of such fast running. He isn’t used to it.”

Dr. Rander was right. Five minutes later Bob appeared from around a hill leading the now calm animal. There was a smile of triumph on the youth’s face as he faced his friends.

“Now that everything has worked out all right, suppose we forget that anything happened,” grinned Bob, as he placed the mule back in the line.

“You going to try riding him again?” asked Joe with a laugh.

“Not on your life. I value my hide too much for that.”

They set forward, heading for the distant high peaks, which were always visible.

An hour of steady climbing brought them to a high plateau, which was bordered by mountains. From this elevation the explorers could command a good view of Cuzco, which seemed but a miniature city in the distance.

“I don’t feel so well,” groaned Bob, whose face was becoming pale. “I can’t get my breath without wheezing. And my stomach seems out of order.”

Dr. Rander happened to be near when Bob complained, and lost no time in attending to the youth.

“You probably have soroche, or mountain sickness,” he said, noting that Bob’s pulse was unusually rapid. “Do you think you can keep on to the next town?”

“Sorry, but I’m afraid I’ll have to lie down somewhere.” The stricken Bob was visibly becoming worse with every minute.

“Wonder if I’ll have another touch of mountain sickness?” mused Joe, as his friend stretched out on a blanket that Dr. Rander had spread on the hard ground.

There was nothing to do but wait for the youth to recover. The old man explained that often patients remained ill for several days, and that there was a possibility of Bob’s sickness being lengthy.

In view of this, they thought it best to make camp and prepare to stay as long as necessary. There was no use making arrangements to continue the journey until Bob’s condition improved.

“Here, take this pill.” Dr. Rander held a little white tablet and a cup of water. “It will make you well sooner than anything else.”

But it was not soon enough for Bob. All the remainder of that day he moaned on with a splitting headache and terrible nausea. It was worse, he said, than sea sickness, of which he had experienced a touch on his first ocean voyage.

The next morning, although still weak, Bob was greatly improved. The ill effects had gone, and once more he had an ambition again to get to the trail.

But Dr. Rander protested.

“You are not strong enough yet,” he said. “We’ll wait till noon and see if you’re improved sufficiently by that time.”

By the time the sun was directly overhead, Bob was his old natural self again. He was overly anxious to make up for lost time.

Lunch over, the explorers again took to the trail, driving the staunch little mules along at a rapid pace.

“Now lead me to those secrets of the Andes!” said Bob, as Cuzco faded from view.

At length the adventurers came to another town, which Dr. Rander called Cameras. They would much rather have encircled the settlement, but as there was no other trail, they passed on through.

“Let’s leave the mules here near the edge of town,” suggested Joe. “I’d like to go back to that little store that we just passed. Might be able to get something we can use cheap.”

“I’ll go with you,” said Bob.

Dr. Rander announced that he would stay with the mules and catch a short rest. He cautioned the boys not to stay too long.

The store that Joe referred to was nothing more than an adobe hut filled with curios of the Andes. They purchased a few articles as souvenirs and started back to the edge of town, where the old man was waiting.

Suddenly there came the sound of rough voices, and a moment later a dozen shots rent the air.