“I am perfectly serious,” resumed Professor Grigg solemnly.

“The idea was such a new one that I admit it staggered me a bit,” explained Nat hastily.

“Suppose you summon your friends, and I will explain in more detail,” rejoined the professor.

Joe, who was polishing up the brass work and putting things to rights generally on the storm-battered craft, was nothing loath to obey Nat’s summons to the bridge. Ding-dong Bell announced that his engines were in good running order and could be left to themselves for a time. So it was not long before they all, including Mr. Tubbs, were grouped in interested attitudes about the man of science.

“As Mr. Tubbs knows,” said the professor, “it was our original plan to resume our voyage on the Tropic Bird, following our observations and picture making at the volcanic islands. Our destination was to be the coast of Chile. From there we were to go in search of a lost Inca city, which is described in documents recently discovered.”

“G-g-g-g-g-gee wer-w-w-w-whiz!” sputtered Ding-dong.

“Hush!” admonished Nat, who could hardly attend to his steering for interest. As for Joe Hartley, his eyes fairly bulged in his head.

“A lost Inca city,” he murmured. “Sounds good to me.”

“Is nothing known of the location of the place?” inquired Nat.

“Not except in a general way,” was the reply. “It is known to be situated on an island in the midst of a lake high up on an Andean plateau in Bolivia.”

“Like the one on Lake Titicaca in Peru,” said Nat.

“Ah, you have read of that?” said the professor approvingly. “Yes, from the documents which came into the possession of the institute as the gift of a traveler in Chile, it is probable that the ruins which I am commissioned to search for are very similar in character to those you have mentioned.”

“How are they to be reached?” asked Joe.

The professor smiled.

“From what we have been able to learn,” he said, “earthquakes have destroyed the roads formerly used, and there is no way of reaching the lake by land——”

“Then—then——” stammered Ding-dong helplessly.

“One must fly to them,” said the professor as calmly as if he were in a class-room. “Thanks to modern science, I believe it may be possible at last to obtain pictures and priceless relics of that forgotten civilization.”

“But where are you going to get an airship?” asked Nat, when he had recovered his breath.

As for Joe and Ding-dong, they regarded the professor in silent amazement. Mr. George Washington Tubbs merely grinned. Clearly, the idea was no startling novelty to him.

“That has been arranged for,” rejoined the professor. “A dirigible balloon of the most modern type is already at Santa Rosa, a small town on the Chilian coast. Before leaving the States, I took some lessons in operating such a craft; but really, that was hardly necessary, as Mr. Tubbs is a fairly expert operator of dirigibles, and has a knowledge of their construction and machinery.”

“Then all that you will have to do, when you reach this town, is to get the dirigible ready and then start the search for the lost city?” inquired Nat eagerly.

“That is all. It should not take long, either. The machine is packed in numbered sections. For security it has been labeled ‘Merchandise,’ and is in charge of the American consular agent, who alone knows what the boxes really contain.”

“Excuse me for saying so,” stuttered Joe; “but it sounds like—like a wonderful fairy tale.”

“It is one,” said the professor smilingly, “a fairy tale which, with the aid of you boys, I hope to make true.”

“With our assistance?” echoed Nat in an astonished tone.

“Yes. I really believe that it was Providence that threw me in the path of you boys. You are exactly the type of self-reliant, clever young Americans that I need for assistants in the work. Are you willing to charter the Nomad to me, land me on the South American coast, instead of in California, and give me your services, for a substantial compensation?”

“I—I beg your pardon,” Nat managed to choke out, “but the idea is so entirely new to us that I think we shall have to hold a consultation first.”

“Take your time,” said the professor airily; “take your time. It is characteristic of me to arrive at quick decisions, as Mr. Tubbs knows, and I don’t mind telling you that I shall be very disappointed if you don’t see your way to accommodate me. We are now almost on a straight course for the coast of South America. If, on the other hand, we landed in Santa Barbara, I should have to take steamer from San Francisco to South America, and I might arrive too late.”

“Why?” demanded Nat. “Is there any one else in search of the lost city?”

“My colleagues fear so,” was the rejoinder. “The documents passed through many hands before they reached scientific ones, and the treasures of the lost city, if they come up to all accounts, are enough to tempt any one to search for them for their intrinsic value alone.”

“Have you any idea who the men are who may prove your rivals?” asked Nat.

“I have—yes. But I do not wish to discuss that phase of the matter any more just now. Suppose you and your friends hold your consultation and then notify me of its result?”

“Very well,” agreed Nat.

Leaving the wheel in charge of the rubicund-headed Mr. Tubbs, who was a capable steersman—indeed, there didn’t seem to be much he couldn’t do—the boys withdrew to Ding-dong’s domain—to wit, the engine room.

They were below for about fifteen minutes.

When they reappeared, Nat’s face bore a radiant expression. He walked straight up to the scientist, who was gazing at the sea with an abstracted look as he studied the various forms of life that were visible in the clear water.

“Well?” he asked, facing around, clearly anxious for “the verdict.”

“Well,” repeated Nat with a smile, which was strangely at variance with his words, “I regret to report that we cannot undertake the commission you proposed——”

“What! You cannot? But I——”

“That is,” continued Nat, “for any compensation. But we will agree to land you and your companion at the port you desire, and further than that, we will, from that time, place ourselves under your orders in the hunt for the lost city.”

As Nat spoke these words, the dignified man of science actually capered about, and snapped his bony fingers in huge delight.

As for Mr. Tubbs, he gave a wild “Hurr-oo!” of delight.

“Hurrah for the Grigg’s expedition!” he cried.

“Three cheers!” ordered Nat, and they were given with a will. The echoes were still ringing out, when Nat gave a sharp exclamation, and pointed to the eastward.

“A strange sail!” he cried, as they all turned eager eyes on the distant speck of canvas.