He did not dare to look down into the awful void beneath him—vast and empty as eternity itself. Keeping his eyes steadily fixed on the bulging bag, Nat climbed by sheer force of will power till he was up to the network that encased the bag.

Right here began the most difficult and terrifying part of his task. Hanging desperately above the immensity beneath him, he had to make his way to the upper part of the bag. He did not dare to think of what he was doing. The very notion of it made him feel sick and dizzy. The lad just climbed, fixing his mind on the thought of reaching and opening the valve.

Somehow—to this day Nat couldn’t tell you how—he clambered round under the bulge of the bag and began the easier task of making his way up the tightly rounded sides to the top of the great cylindrical gas container. As the professor had surmised, ice had formed on the outside of the bag, and made Nat’s endeavor ten times more hazardous and difficult. This ice had clogged the valve ropes, and Nat saw that the only thing to do was, as he had made up his mind, to climb on till he reached the top of the bag.

The possibilities of a slip were awful, and Nat no more dared think about them than he had about the chances of his slipping when he was hanging between earth and sky under the lower part of the bag. He resolutely dismissed them from his mind.

But the physical difficulties of the lad’s self-imposed task were almost overwhelming. There was a sharp pain in his chest, and his limbs felt as if they had leaden weights attached to them. Suddenly a warm stream of something Nat knew to be blood, gushed from his nose; but still he worked his way upward, climbing amidst the network meshes like a sailor on ratlines.

Once or twice he was compelled to pause from sheer exhaustion, and, clinging on with might and main, to spread himself flat on the surface of the gas bag to rest.

If Nat had not been a clean-lived lad all his life, and had not been a hater of smoking and bad company, he would never have been able to endure this ordeal; but somehow, his young vitality won out, and at last he could reach out a hand and touch the valve.

Bracing himself against the rigging, he tugged with all his might. But the condensed moisture had formed ice on the valve, and it stuck.

Nat felt a childish rage take possession of him. Raising his fists, he beat and tore at the valve, while tears of physical weakness and exhaustion streamed down his cheeks.

“I will get you open! I will! I will!” he cried again and again.

But even his frame gave way at last, and suddenly his eyes grew dim and he felt as if a sword had been plunged through and through him.

As everything grew black, Nat, with a last effort of consciousness, clutched at something to save himself from being plunged backward into space.

He caught it, or thought he did, and then his senses went out from him with a vivid flash and a terrible roaring in his ears like the sound of a hundred waterfalls.

Half an hour later, or at ten o’clock, Joe Hartley opened his eyes. At first he hardly knew what had befallen him; but in a few seconds his recollection came back with a rush. He remembered that the Discoverer had seemed doomed, recalled Nat’s plunge through the door and how he had tried to follow his chum, but had fallen, overcome by exhaustion, at the door.

But now all the chill was out of the air, bright sunlight streamed through the pilot-house ports, and the professor and Mr. Tubbs, both of whom had collapsed on the floor, were sitting up looking about them rather bewilderedly. The professor was the first to speak.

“A miracle has happened,” he declared. “The Discoverer is out of danger.”

“The barograph shows twenty-five hundred feet,” announced Joe, who had been studying that instrument.

“Where are the others?” asked Mr. Tubbs, rising rather weakly to his feet.

As if in answer to his question, Ding-dong Bell appeared in the doorway between the pilot-house and the main cabin.

“Where’s Nat?” he demanded.

“Isn’t he out there with you?” asked Joe, with a quick leap of his heart.

“No. The only person out there is Matco. He’s so scared that he’s under the ber-ber-bunk.”

“Where is the lad?” demanded the professor earnestly, with a note of anxiety in his voice.

Mr. Tubbs, who had been struggling with his dim memory of events preceding his collapse, spoke:

“I recall it now,” he said. “Nat said he was going to get that valve open”—he paused—“somehow.”

“And you let him go?” demanded the professor.

“I—I didn’t mean to,” stammered the repentant Mr. Tubbs, “but I was so nearly on the verge of caving in, that I couldn’t carry out my resolve.”

“Search the craft thoroughly,” ordered the professor, lines of anxiety showing in his face, “there was only one way to open that valve.”

They looked their questions.

“And that was by climbing around the gas bag and opening it by hand.”

“Good gracious!” exclaimed Joe. “And Nat dared do such a thing!”

“He must have, and succeeded, too,” said the professor in a curiously tense voice, “the opening of that valve was the only thing that would result in our having dropped to a supportable region of the air.”

“But we are dropping no longer.”

The exclamation came from Mr. Tubbs.

“No. The automatic cut-off arrangement would have closed the valve when we had reached a warmer belt of atmosphere,” explained the professor, “but don’t let us lose time talking here. Scatter through the Discoverer and make a thorough search. He may have dropped unconscious somewhere.”

The anxiety with which the search was conducted may be imagined. The Discoverer was allowed to drift lazily along while they sought some trace of the missing lad, but the search resulted in nothing.

“There is only one conclusion to be reached,” said the professor in a solemn voice, “poor Nat paid the penalty of his bravery with his life. He——”

The man of science broke off, unable to command his voice, and at the same instant came a cry from above them—a hail from out of the air, it seemed:

“Hello, people!”

“Good heavens! It’s Nat!” fairly shouted the professor, as Nat, whose feet were alone visible round the bulge of the gas bag, clambered nimbly down and dropped from the rigging, beside them.

In his excess of joy, the professor flung his arms around Nat’s neck, much to the lad’s embarrassment, while the rest fairly fought for a chance to grasp his hand. In intervals of joy making, Nat told his story, part of which we are familiar with.

It seemed that when he swooned on the swaying balloon top he instinctively clutched at the first thing his hand encountered, which was one of the valve ropes. The valve, already loosened by his pounding on it, yielded to the sudden pressure upon it and jerked open. At least, this was the only explanation Nat could furnish of the fortunate occurrence.

When he came to himself he said he saw that the Discoverer was at a reasonable height, and manipulating the cords he again closed the valve. He was too weak to attempt the descent at once, but lay outstretched on the top of the gas bag, regaining his strength. All this time he suffered with a dreadful fear that his friends below might have succumbed to the awful rigors of the upper air. With an apprehensive heart he at last began the climb down and he concluded:

“You may imagine how delighted I was to hear your voices, even if the professor was preaching my funeral sermon.”

The boys broke out into wild yells of enthusiasm.

“Three cheers for Nat Trevor, the bravest boy on earth!” shouted Joe Hartley.

The shouts rang out oddly in the thin atmosphere of mid-air, but they relieved the boys’ feelings. As they died out, Matco appeared at the door of the cabin, and gazed at the scene a moment. Then seeing that Nat was the idol of the moment the Indian ran nimbly along the swaying deck and throwing himself on his knees, placed Nat’s foot on his head.

It was the last straw.

“Say, fellows!” cried Nat with a red face, “that’s about all of this hero business. Let’s have some breakfast and get the engine going.”

And so, what might have been a tragedy, ended in one of the merriest meals ever enjoyed by aerial travelers.

By noon the Discoverer, none the worse for her involuntary flight into the icy realms of space, was able to resume her voyage over the desolate peaks and abysses of unknown depths, above which the adventurers were now soaring.