With the wood gathered by the young Motor Rangers, Mr. Tubbs soon had a roaring fire going. By sundown it was so cold that they were glad to huddle close to the cheerful blaze, which was for purposes of warmth only, the cooking being done on the denatured alcohol stove belonging to the galley of the Discoverer.

It was an odd meal, but one the boys enjoyed thoroughly. Mr. Tubbs was as good a hand at cooking as he was at anything else, and as a supply of fresh meat had been brought along, they had a capital meal, helped out with choice canned vegetables and even, to celebrate their first night in the land of their search, a generous portion each of plum pudding. It was canned, of course, but quite palatable, or so the boys appeared to find it.

After supper the professor gave the lads an interesting sketch of the country they were in, and finished up with an account of the old Incas, one of whose lost cities they had come to find.

Among other things of interest he told them concerning the lost race, was that they are believed to have been sun worshippers. At any rate, in one of the ruined cities which has been located in Peru, circular temples with the walls embellished with pictures of the sun have been found. Other facts concerning the vanished civilization of the Incas must ever remain a mystery, said the man of science.

For instance, at the remains discovered in Peru, a huge rock, shaped like a gigantic dome, was found. Traces of gold were discernible on its surface, and it is believed that at one time the whole great, monolithic mass was completely plated with this costly metal.

“Other strange features of these ruins,” went on the professor, “are dungeon-like chambers which are believed to have been used in cereomonies of initiation, and great baths fed by subterranean rivers, which are still flowing as they did in the days of the Incas.”

“Do you think we shall find such things?” asked Nat, his eyes aglow at the prospect.

“You mean, do I think we shall find the lost city?” corrected the professor, with a smile. “Well, Master Nat, I don’t doubt that if we find the city we shall also find such things. It is rumored that the lost city we are in search of is in even better preservation than the famous ruins of Peru itself.”

“I wish you would tell us some more about that sacred dome with all the gold on it,” said Joe.

“I’ll tell you all I know,” said the professor. “It is believed then, that the sacred dome was the place where Manco Capac, an Inca deity, descended to the earth. To this day the natives approach the spot with the utmost awe and reverence.

“According to their ideas, no bird would alight up, or animal approach it. All but priests were forbidden to come even within sight of the rock, although it is hard to know how this could be prevented, as it is of immense size. At ordinary times its gold plating was covered by a veil of the finest and most costly materials, and this was never removed, except on great religious festivals.”

“It must have been a fine sight to see that great golden rock glittering in the sun,” said Nat thoughtfully.

“It must, indeed,” agreed the professor. “There was also a Temple of the Moon, and a vast Temple of the Sun, as well as other buildings whose purposes are veiled in mystery, and must ever be. One thing is certain, though, human life must have been as cheap as water, for it is estimated that many thousands of slaves’ lives were sacrificed in building the city of which only ruins now remain.”

“It reminds one of Egypt,” said Nat.

“So travelers have observed,” rejoined the professor; “after all, the history of civilization repeats itself.”

“Has much treasure been discovered there?” inquired the practical Joe.

“Quite a good deal, yes,” was the reply; “but the Spaniards took an immense quantity of it, and to-day there is little left. However, from time to time a valuable find is made, I am informed.”

“And the city we are in search of—do the same conditions exist there?” inquired Nat.

“Very probably. According to tradition, the fierce and warlike Indians kept the Spaniards away from the spot,” was the reply.

“I hope so,” spoke Joe, in whose mind visions of vast treasures and strange, massive buildings were already rising. As for the others, perhaps they, too, even the professor, were also weaving castles in cloudland. At any rate, they were silent for a time, brooding over the great mystery to whose heart they hoped to penetrate ere long.

But the period of silence was not of lengthy duration. Mr. Tubbs, who possessed a good tenor voice, volunteered to sing a song.

“Is there anything he can’t do?” thought Nat.

The song he chose was “Old Kentucky Home.” When he came to the chorus the boys’ voices blended with his in the plaintive cadences of the music. It was a strange sound to be ringing out in that primeval place, where perchance the foot of civilized man had never trod before.

But the singing was due to terminate abruptly. Nat, who had been gazing outside the circle of firelight, caused the breaking off of the concert.

He sprang to his feet and seized up a rifle, calling on the others to do the same.

“What is it, my boy?” asked the professor, “a wild beast?”

“No—that is, I don’t think so,” rejoined the boy, whose face was rather pale. “I’m almost certain that what I saw was the figure of a man crouching over yonder and watching us.”

Exclamations of consternation filled the air.

“Indians!” gasped Ding-dong Bell.

“It may have been nothing but a jaguar or a prowling puma,” said the professor. “Are you sure your eyes didn’t deceive you?” he inquired of Nat.

“As I said, there’s a bare chance I might have been mistaken,” rejoined the lad, “but I don’t think so. However, the instant that I looked, the figure vanished.”

“It’s very strange,” mused the professor, “and yet it may have been an Indian, little as I like to think of such a contingency. However, we will keep a sharp watch to-night, and be prepared to ‘slip our moorings’ at an instant’s notice.”

All agreed that this would be an excellent plan, and forthwith the knots on the mooring ropes were retied, so that one tug from those on board the Discoverer would release the craft and allow her to shoot upwards. Preparations for what all felt was not likely to prove a restful night, were then begun.

The first watch was assigned to Mr. Tubbs and Joe, and would last till midnight. The next one would be assumed by Nat and the professor. Ding-dong Bell, who was still nervous and rather pale from his experience of the afternoon, was to be allowed to slumber all through the night.

He protested loudly against this, demanding to take his share with the rest; but was obliged to be content with the promise that if any trouble occurred he would be routed out to assume charge of the engine. In spite of their apprehensions, Nat and the professor slept as soundly as Ding-dong. In fact, it did not seem to Nat that he had been asleep more than a few minutes when Mr. Tubbs aroused him to take his watch.

“All quiet,” was the rubicund-headed one’s response to the professor’s inquiry.

Hardly were the words out of his mouth before the silence of the night was broken by an almost unearthly yell.

“What’s that?” cried Nat, considerably startled.

“Nothing but a screaming monkey,” said Mr. Tubbs. “I’ve heard them in Brazil often.”

“But they don’t cry out at night unless they are disturbed,” said the professor decidedly.

“You think some one is in the woods?” asked Nat.

“I don’t know about a human being. But the fact that you are almost certain that you saw a man prowling about last night, makes it look suspicious.”

“It may be only a panther,” said Mr. Tubbs.

“Possibly. Let us hope that is the case, but in the meantime prepare for trouble; then, if it comes, we can meet it. Master Joe, rouse out Master Bell. Nat, I wish you’d bring me that peculiar-looking gun you were asking me about yesterday when you saw me place it on board.”

The gun referred to was a queer-looking weapon, with a mouth shaped like an old-fashioned blunderbuss. It had an immense barrel, and altogether was a very odd-looking weapon. Nat knew where it stood in the gun-rack and soon fetched it. The professor examined the lock and appeared to find everything satisfactory.

“What kind of a gun is that?” asked Nat, full of interest.

“I don’t want to say much about it till I see how it works,” said the professor. “It is the invention of a friend of mine. If we are attacked it will be a fine opportunity to test it.”

Nat would like to have asked more questions, but at that instant a chorus of cries and shrieks arose from the woods on every side. The cries were uttered by roosting birds and monkeys, which had been disturbed by some cause. What that cause was, the professor soon guessed.

“It’s the Caripunas,” he whispered; “almost beyond a doubt. Master Bell, stand by your engines. Tubbs, take up your position at the wheel and be ready to manipulate the searchlight. Master Nat and Master Joe will stand ready to slip the tie-ropes when the word is given.”