But even in that instant of deadly peril, Ding-dong did not lose his presence of mind, or, perhaps, instinct of self-preservation would be a better phrase.

As he felt himself lose his balance, he clung to the network of the rail, and hung there head downward between the sky and the earth for one instant. But that brief molecule of time was enough for Joe and Nat to secure his feet, as they flashed over the rail, and drag him back on board.

“Go to the cabin, sir,” ordered the professor, who was white and shaky, as, indeed, were the others.

There was no gainsaying his words, but Ding-dong, as usual, had to say something. He was the most unperturbed person on board, in fact.

“I d-d-d-d-didn’t do it on p-p-purpose, you know,” he remarked, as he walked off.

“Good heavens!” exclaimed the professor, leaning against the rail, “what trouble is that boy going to get into next?”

The stuttering lad’s narrow escape had so unnerved them all that there was no answer.

“Well?” said the professor at length, as if seeking a reply to his question.

“Don’t ask me, sir,” gasped out Nat. “I haven’t got my breath back yet.”

It was, perhaps, half an hour later when the entire craft was electrified by a cry from Joe.

“Nat was right! It is a lake!”

No need to ask to what he referred. The professor ordered the Discoverer sent higher, so as to give them a larger horizon, or, rather, a bird’s-eye view.

As the craft rose upward in obedience to her planes, they saw beneath them, but still at some little distance, what Nat has since declared was the most wonderful sight he has ever seen or hopes to see.

Rimmed by bare, gaunt mountains, inhospitable and bleak, lay a small lake, set like a turquoise in dull gold. In the midst of this lake was an island, and on this island, even at that height, they could perceive, were buildings rising in terraced formation. At the extreme summit of the island, which rose to a peak, was something that flashed and glowed in the sunlight almost blindingly.

“It’s the golden dome of the lost city!” gasped Nat.

“Say, Nat,” said Joe in rather a shaky voice, laying one hand on Nat’s arm.

“What is it, Joe?” asked Nat, without taking his eyes off the wonderful sight before him.

“Nothing; only—only I feel a bit scared,” was Joe’s quavering confession.

“You may well feel awe-stricken,” said the professor, whose eyes were gleaming, “ours are the first eyes to behold that island since the mysterious catastrophe that wiped out the race that inhabited it, occurred.”

There came a sudden voice at their elbows.

“L-l-l-looks like C-C-C-C-Coney I-I-Island.”

It was the incorrigible Ding-dong, who had taken advantage of the excitement to slip out of his place of involuntary confinement.

But, in the general interest in all that was occurring, no attention was paid to him. In the midst of the eager talk, and still more eager scrutiny of the island, old Matco, who had come out upon the deck and had stood silently gazing at the lost city, uttered a sharp cry.

Then, raising his hands above his head and fixing his eyes upon the sun, he began muttering what seemed to be a prayer.

This done, he turned to the professor and poured out a rapid flood of eager, emphatic words in his corrupt Spanish. So fast did he speak that the professor had difficulty in following him. But by paying close attention he managed to make out the old man’s meaning.

“What does he say?” asked Mr. Tubbs, as the old Indian ceased his torrent of words, and leaned back, looking quite exhausted.

“Why, it’s like fiction,” said the professor. “The old man says that we are fulfilling a tradition of his race which says that one day winged men from the sky would discover the city.”

“Well, that’s a good omen,” said Nat.

“W-w-w-whatever that may be,” sputtered Ding-dong. “Guess you mean n-n-no men.”

But the professor paid no attention to the irrepressible youth. Instead, he assumed rather a grave look.

“Why, I’m not quite so sure that it is a good augury,” he said slowly. “The old man says that the prophecy or tradition goes on to say that the wrath of the long-dead Incas shall be visited on the violators of their hidden city, and that a terrible end will overtake the sky men who invade it.”

As the professor talked the old Indian fixed his eyes on him as if he realized what he was saying. As the man of science concluded, he nodded solemnly, as if indorsing all that had been told.

“Oh, well,” said Nat, “we are not going to turn back for the sake of an old Indian ghost story.”

“Of course not,” said the professor; “but I thought if any of you are superstitiously inclined, I would warn you.”

“I guess it would take more than talk like that to turn us back now,” said Joe. “I’d face a legion of spooks to investigate that place.”

The others agreed with him. Indeed, as the Discoverer grew nearer, the marvels of the lost city grew more and more awe-inspiring.

What had appeared in the distance to be a mere huddle of terraced buildings, were now seen to be stately palaces, some of them with trees still growing amidst them. The buildings rose in this form till they reached their climax at the great gold-plated dome that capped the summit of the wonderful isle.

The walls, so far as could be seen, were white, but profusely ornamented with barbaric magnificence.

Not a little of the mystic effect of the island was gained from the precipitous and rugged cliffs of the mountains that walled the lake.

“However do you suppose a lake came to be in such a situation?” wondered Nat, addressing the professor.

“In my opinion,” said the scientist, “that lake is what was once the crater of a volcano, more enormous than any yet known.”

“And what we thought were separate mountains were once only part of the summit of that volcano?” asked Nat wonderingly.

“I think we would be correct in assuming so. In many parts of the world the craters of extinct volcanoes are found to be filled with water, just as this one is.”

“The water must be of immense depth,” said Joe.

“In some cases it has been impossible to touch bottom, even with the longest lines and the most perfect sounding apparatus,” was the astonishing reply.

“But how does an island come to be in the middle of such a deep lake?” was what Mr. Tubbs wanted to know.

“What we call an island is probably the summit of another peak of the crater,” said the professor, “or it may have been formed, like those volcanic islands of which we have such a keen recollection, by the action of earth’s internal fires.”

The dirigible dropped lower. It was now almost directly above the lost city. It could be seen that surrounding the golden dome was a vast, semi-circular platform or courtyard of stone, with other stones set up perpendicularly around it.

“It is precisely like the arrangement of the Temple of the Sun in Peru,” said the professor.

“It will make a good place to land,” spoke the practical Joe.

“Doesn’t it seem almost like a sacrilege to bring a modern dirigible to earth in the very courtyard where the rites of ancient religion were practiced?” spoke Nat, who was an imaginative lad.

“Not at all,” said the professor, “and as for that ancient religion, if we had lived in the days when it flourished, I fancy we wouldn’t have liked it much. Like most ancient religions, it was a creed of bloodshed and violence. Human sacrifices may have been indulged in on those very stones we see beneath us.”

The boys agreed that this put quite another light on the matter, and the descent was made without further comment. The dirigible came to rest in the lost city of the Bolivian Andes at three o’clock in the afternoon. Mr. Tubbs was left to guard the Discoverer with old Matco, who refused to move one step through the silent, long-deserted streets. But the boys and the professor set out on a tour of exploration.

The streets, they found, were like those of mountainous cities in Europe, and consisted mostly of steps. It was one of the most uncanny feelings that any of them had ever experienced, this walking through a city of the dead. For, although the ancient places were mostly in ruins, from earthquakes the professor judged, the city yet seemed lifelike enough for some of the vanished race to turn a corner at any instant.

For some reason, the boys kept very close to each other and to the professor, showing no disposition to wander. They found that, as they approached the lake, the buildings grew poorer in character and were not carved or decorated like those closer to the temple. The remains of a splendid wharf remained, however, which set the boys to wondering what had become of the boats that must have once plied between the city and the shore.

This, in turn, suggested ruminations upon the means employed by the vanished race of reaching the lake, for to climb over the mountains was obviously impossible. The professor opined that, at some time, a tunnel must have existed. This set the boys crazy to try to find it, but the man of science declared that, in all probability, the tunnel, if it had ever existed, had been ruined by earthquakes long since.

They stood by the lake side for a time looking into its dark blue depths, and then began a return up the street, climbing the steps cut in the rock.

“Where’s all the treasure we were going to find?” asked Joe, as they climbed the steep causeway worn by the feet of a race long since passed out of existence.

“I don’t imagine we are likely to find much that is valuable,” said the professor. “My belief now is, that when the Spaniards came the inhabitants of this city concealed everything valuable in it in some place known only to themselves.”

“Maybe the lake bottom,” suggested Joe.

“That is not improbable. At any rate, I think we shall have to content ourselves with the glory of having discovered this wonderful place. It is far more perfect than the ruins of Peru are described as being.”

“What about taking that gold plating off the sacred dome?” said the practical-minded Joe.

“Not with my consent,” said the professor. “I would wish this city to be the Mecca of antiquarians from all over the world.”

“I agree with you,” said Nat. “It would be vandalism of the worst sort to strip that rock.”

“Oh, I was only joking,” said Joe, with a rather red face.

“Here’s a peculiar-looking building,” went on Joe, a few moments later, as they passed a tower-like structure, higher than the other buildings, and without windows.

“Let us survey it,” said the professor. “See, here is a door. It has fallen in, it is true, but I imagine we can squeeze through.”

By dint of getting on their hands and knees they managed to crawl under the richly carved and broken portal, Nat pausing to notice that the carvings seemed to be of various astronomical bodies.

Within the tower they found themselves standing at the bottom of a tall, narrow, perpendicular shaft. It was, in fact, like looking up a circular chimney. At the top was something which at first sight seemed to be a big glass lens; but the professor pronounced it to be pure crystal.

“This is the most amazing find yet!” he exclaimed with enthusiasm. “I believe that this tower formed a sort of rude telescope, through which different observations were carried on.”

He clasped his hands in scientific fervor. Indeed, they had seen enough that afternoon to turn the brain of the least imaginative man of science!

Nat informed the professor of the carvings he had noticed.

“That settles the matter,” said the professor enthusiastically. “Good heavens, what a find! It has long been a controversy between various scientific men as to whether or no the ancient races understood astronomy in the true sense. The finding of this rude telescope will go far toward—— Gracious! what was that?”

“What?” cried Nat, considerably startled.

“Why, a hand reached out and grasped my hat and——”

Before the professor could conclude his sentence the boys saw a small brown paw project from a ledge above him and whisk his unlucky hat from his head.

“It’s a monkey!” cried Nat.

“A lot of them!” exclaimed Joe.

“T-t-t-there they go,” cried Ding-dong, as a dozen or more apes of the prehensile tailed type rushed off amidst the ruins, chattering and squealing and tearing and clawing at the professor’s unlucky headgear.

“Just to think,” sighed the man of science with resignation, “that I came all this way, and we have made all these discoveries, and yet my ill-fortune with hats pursues me still.”

“I’d give several dozen hats to have seen what we’ve seen,” Nat reminded him.

“That is so! that is so!” Professor Grigg agreed; “but——”

“Look out!” cried Joe, behind him, suddenly.

The professor leaped back just as an ugly flat head, with a pair of malicious leaden eyes, protruded itself at his elbow from between the crevices. It was the head of an immense snake.

Without more ado the explorers made haste to get out of the astronomical tower.

“Exploring is certainly strenuous work,” commented Joe as they gained the open air.

“Yes; I don’t wish to do any more without a rifle,” agreed Nat.