“A man!” exclaimed the amazed professor. “Why, how in the world did he come here?”

“I don’t know,” said Nat; “but there he is.”

“He must have caught the rope when the Discoverer shot upward,” suggested Joe. “Maybe he thought he could stop us.”

“He’s all wer-wer-wound up in the rope,” announced Ding-dong, who had been peering over the side during this dialogue. “His eyes are closed, and he seems half-dead from fright.”

“Let us drag him on board at once,” said the professor.

The boys lay flat, while the winch was started up until the man’s head was on a level with the under part of the substructure. Then three pairs of strong young arms reached down and dragged their involuntary passenger over the side.

“He’s an Indian!” cried Joe, as the man being dragged into safety from his precarious position proved to be a squat, black-haired little brown man, clad in a garment of rough fibre, and with one of the peculiar ornaments Nat had already noticed, thrust through his under lip.

All this time the Indian had kept his eyes tight closed, and had not uttered a word. Now, however, he opened his eyes, and threw himself down flat on his face on the Discoverer’s deck. There he groveled in an attitude of the most complete humility.

“He thinks we are sky gods, or demons of some sort,” declared the professor, reading the man’s consternation aright.

“I don’t much blame him,” said Nat, with a smile, “that ride through the air at the end of the rope must have been the most terrifying experience of his young life.”

“Young life,” scoffed Joe, “he must be sixty at least.”

“Well, that is young sometimes,” said the professor, who owned to that age himself, although he was as active as most men half his age.

Suddenly the Indian began to speak, but without raising his head. He poured out a flood of words. For an instant, they thought he was speaking his native dialect, but all at once the professor understood.

“He’s talking Spanish,” he said, “and imploring us to spare his life. Just as I thought, he thinks we are beings from another world.”

“Well, if I were in his fix I’d be inclined to think so myself,” said Joe.

But the professor began putting rapid questions, at the same time raising the man’s head and showing him by signs that they meant no harm to him. Little by little the Indian seemed to recover his courage. But he was sorely shaken by his adventure, and explained that when the ropes began to drag over the ground he had seized them to stop the dirigible, and had become entangled in them.

“Why did your tribe attack us?” asked the professor.

“We thought you were human beings,” was the response. “But now we know otherwise.”

He would have cast himself on his face again, but the professor raised him and spoke encouragingly to him.

“Maybe if you’d give him something to eat he’d feel better,” suggested Joe, practically.

“That might be a good idea, and it will show him that we mean him no harm,” said the professor.

The Indian, who said his name was Matco, was taken to the cabin, the sight of which, with its comfortable furnishings and strange scientific instruments, filled him with fresh terror. But little by little he regained his self-possession to a degree, and ate what he was given with zest.

The crew of the Discoverer joined him at the meal, of which they stood in need, Joe relieving Mr. Tubbs at the helm. The stout lad had taken a few lessons in steering before from Mr. Tubbs, and found that it was not as difficult as he had supposed it would be. But then, Joe had had plenty of experience at the wheels of both automobiles and boats.

But after all, the selection of a green hand at the wheel proved somewhat disastrous. The sun arose while they were still talking to the Indian, and Mr. Tubbs was hearing details of the strange manner in which the man had boarded the airship.

In that rarefied air the rays of the luminary of day soon warm the air, and, as a consequence, the gas within the Discoverer’s bag began to expand very rapidly. Those in the cabin, of course, did not notice that the craft was rising rapidly, and Joe did not give a glance at the barograph, it not occurring to him to do so.

All at once he gazed over the front of the pilot-house and looked down below. What he saw almost made him utter a cry. The Discoverer was at a tremendous height, and appeared to be rising more and more rapidly.

Joe, in a sudden panic, twitched a lever, and the next instant the craft shot skyward at breathtaking speed. The boy had set the wrong lever and had adjusted the planes to a rising angle.

Before the professor, who had felt the craft rear upward, could reach the pilot-house, the dirigible had shot up five hundred feet or more. Behind the professor came the others, except Matco, who was sent into a fresh paroxysm of fright by the strange and sudden upward leap of the airship.

“Good heavens!” cried the professor, as he jerked over the descending lever, “we have risen to a height of more than eight thousand feet.”

As he spoke they suddenly noticed that the air had grown bitterly chill.

“Just like Joe to make a break like that,” said Nat, with a good-natured laugh that took the sting out of his speech; “we’d better get down to earth once more as quickly as possible. It’s too cold to be comfortable up here.”

“We’ll soon drop now,” said Mr. Tubbs confidently.

But as the minutes passed and it grew colder, his face became grave.

“We’re rising,” cried the professor, glancing at the barograph.

“That’s right,” cried Nat. “What can be the matter?”

“Have you got the descending planes set at their sharpest angle?” demanded the professor.

“Yes,” was the response, “but they seem to have no effect on her at all.”

The professor thought a moment.

“We shall have to pull the escape valve and let out some gas,” he said. “The rising sun has warmed the air till the expansion of gas has made the bag too buoyant for the planes to have any effect on it.”

“Won’t that waste the gas?” asked Joe.

“Yes, but we will have to do it. Mr. Tubbs, pull the escape valve, please,” said the professor, whose nose was red and whose teeth were beginning to chatter.

“It’s snowing!” cried Nat suddenly.

The air was filled with flying flakes, and the Discoverer seemed to be soaring through a wonderful white void. But it was no time for admiring such effects.

Reaching above his head, Mr. Tubbs gave the cord that worked the escape valve situated on the top of the big bag, a sharp tug.

Then he gave it another and another, with no results.

“It’s stuck fast!” he said, the words coming out shrilly from his blue, frozen lips.

A look of dismay spread over the professor’s face.

“Nonsense,” he said. “It can’t be.”

“But it is, I tell you.”

“Let me try it.”

The professor gave a hard tug, but still the cord did not budge.

“Give me a hand here,” he said to Nat, and together they tugged.

Suddenly, and without the least warning, the cord broke off short in their hands, and they fell sprawling on the floor. To his astonishment, when Nat tried to rise, he found the task difficult. Breathing seemed to be a labor, and his limbs felt like lead. The professor had actually to be helped to his feet, and then staggered, with one hand over his heart, to the helmsman’s settee, on which he sank, breathing with a queer, whistling sound.

“What on earth has happened?” demanded Joe, who like the others, felt strangely oppressed and heavy. His head ached as if it would burst.

“The—the cord must have frozen to the sides of the bag,” gasped out the professor. “The change to this awful altitude turned the night moisture accumulated on the gas bag’s sides to ice. I fear we are doomed, unless——”

He paused, panting and gasping.

“Unless what?” demanded Nat, forcing the words out.

“Unless we can get that valve open.”

“And if we can’t?”

“Then we must drift higher and higher till we perish of cold, or the bag explodes and we are precipitated to the earth.”