The moments that followed were filled with a tenser excitement than any of the lads had ever known before. After the first frightened flurry of the alarmed creatures of the forest, a deep silence prevailed. It lasted for possibly fifteen minutes, and then the professor decided not to test their nerves to the breaking point.

“Turn on the searchlight!” came the breathed command.

A sharp click followed, as the light, which was supplied by current from the storage battery, was switched on.

A dazzling white pencil of light swept all about the Discoverer. Its brilliancy pierced the night like a saber, and illumined the solemn trees and the open savannah all about.

At almost precisely the same instant, a chorus of ferocious yells and cries broke out, and from all sides there rushed on the aerial adventurers a horde of short-statured Indians. The searchlight showed them to be wild-looking men, clothed in a single garment, their heads covered with straight black hair. Through their lower lips most of them had thrust a triangular bit of white stone with a sharp point. This added to their fantastic appearance.

Nat noted that one of them, larger in stature than the rest, seemed to be the leader. He also saw, with an unpleasant thrill, that they carried long blow pipes. It was through these pipes, the professor had said, that the poisoned arrows were discharged.

Rope in hand, ready to slip at the word of command, Nat stood his ground. On the opposite side of the framework Joe was likewise waiting. Neither boy budged an inch, and Ding-dong stood steady as a rock at his engines.

So suddenly had it all happened, in fact, that neither boy could regard it for an instant as more than a dream.

Suddenly something struck the metal framework by Nat’s head with a sharp ping!

It was an arrow, and so close had it come to the lad that he had caught its whistling sound as it sped past his ear.

“Phew! This is warm work, with a vengeance,” he muttered.

He saw the Indians give a sudden concerted onrush, yelling like maniacs.

“Keep the searchlight in their eyes. It dazzles them!” called the professor.

Then came another command.

“Let go your ropes!”

Nat and Joe instantly dropped their ropes and seized up rifles.

“Don’t fire!” cried the professor sharply. “We don’t want to injure them if we can help it.”

The great dirigible swayed for an instant and then began to rise.

“Turn on your power!” shouted the professor.

The bell for “full speed ahead” rang sharply out. At the same instant the propeller began to whir.

As it did so, several Indians, who, in their onrush on the dirigible, had clambered upon it, were thrown off in all directions. They rolled over and over, like so many footballs. This made the others pause an instant, and in that instant the dirigible rose from the ground.

But the chill night air had condensed the gas, and she rose slowly. Before more than five feet had been gained in her upward rise, the Indians recovered from their amazement and charged like a pack of furies.

“Flat on your faces!” shouted the professor, as a shower of arrows pinged and pattered in the framework of the craft.

They obeyed the command, and then Nat saw the queer gun brought into use. The professor raised it to his shoulder and pulled the trigger.

Instantly a stream of colored balls, like those that issue from a Roman candle, poured from the bell-like muzzle. But almost simultaneously with their discharge, they burst with sharp reports, and the whole air became impregnated with a black, all-obscuring smoke as thick as a London fog.

The dense clouds spread on every side, completely obscuring the dirigible from the view of the Indians below. Higher and higher she rose, while below her the dense smoke veiled everything like a curtain. Nat caught a whiff of the vapor, and it made him cough and choke.

“I’ll bet those Indians aren’t enjoying it,” he thought to himself. “So that was what that queer gun was.”

In a few moments they were high above the tree-tops, and the professor ordered the lights turned on. A switch was pushed over by Mr. Tubbs in the pilot-house, and the Discoverer blazed out with incandescents like an illuminated battleship. For a few seconds nothing much was done but to exchange congratulations. No one was hurt, and not an arrow had pierced the gas bag. This was accounted for by the fact that the Indians, not understanding how vulnerable that part of the craft was, had confined their volleys to the occupants of the lower structure.

“A most fortunate escape,” declared the professor, but suddenly he clapped his hand to his head.

“My hat!” he cried wildly, “I’ve lost another hat.”

“Here it is!” cried Joe, picking up the article of headgear.

He held it up, transfixed by an arrow. The missile had penetrated it and whisked it from the professor’s head without touching him.

“I wouldn’t have lost that for worlds,” said the professor, thanking Joe, and removing the arrow very gingerly.

“One scratch from that arrow would result in death,” he said, in explanation of his extreme care.

He held it out for the boys’ inspection. It had a stone head, discolored by some whitish matter at the tip. The shank of it was about two feet long, with some sort of cloth wrapped around the end to make it fit the blowpipe tightly.

“What kind of poison do they use?” asked Joe.

“An infusion of the St. Ignatius plant, from the beans of which strychnine, our deadliest narcotic, is obtained,” was the response.

“We’d better make a thorough search for any other arrows,” suggested Nat.

“I think so,” agreed the professor; “they are not the sort of things to have lying about.”

A search of the Discoverer’s lower structure resulted in the finding of a dozen or more of the deadly missiles. These were all thrown off into the air at once.

“And now,” said the professor, planting his hat firmly on his head, “I suppose you are anxious to know something about that queer gun I used.”

A chorus of assent greeted this remark.

“Well, it’s a weapon called the Fog-maker, and was invented by a friend of mine especially for use in aerial warfare, or for protecting a small vessel from hostile aeroplanes,” said the professor. “As you saw, it works perfectly, throwing out a thick cloud of dark, acrid smoke, which is heavier than the atmosphere. While it has no permanent bad results, yet it renders those who breathe it insensible for a time.”

“It is indeed an effective weapon,” declared Nat; “can we see one of the projectiles?”

The professor took up the gun and slid open a small space in the stock. Then, pressing a metal button, he caused two round black objects, about the size of small oranges, to roll out into his hand.

“The magazine holds ten of these,” he said. “They are made of glass and filled with chemicals.”

“What kind of chemicals?” asked Joe.

“Ah! That is the secret of the inventor,” was the reply, “nobody but he himself knows what they contain; but that they are effective, you must admit. He told me that the old ‘stink-pots’ that Chinese pirates used to use gave him the idea. If ever there is a war in the air, I think that the nation equipped with this invention will have a powerful implement of havoc.”

“I should think so,” said Nat; “one whiff of it was quite enough for me.”

All this time, by the professor’s directions, the dirigible had been swung in wide circles at an altitude of about fifteen hundred feet. So interested had they all been in the professor’s description of the novel aeroplane gun, and in the other matters that had occupied their attention, that the big air cruiser had not yet been “tidied up.”

This was the next task to demand their attention. Joe set to work to hoist up and coil the rope which had been cast loose when the hasty ascent was made. But he hadn’t given it more than a couple of tugs before he uttered a shout that brought the others, except Mr. Tubbs, who was at the helm, running along the substructure to his side.

“What’s up now?” demanded Nat.

“Why, either this rope has caught in something below, or there’s something heavy attached to it,” was the astonishing response.

“Impossible for it to have caught,” declared the professor, “we are now fifteen hundred feet or more above the surface of the earth, and the rope is not more than a hundred feet long, at the most.”

“Well, feel it yourself,” responded Joe.

Nat gave the rope a tug. As Joe had said, there was clearly something heavy attached to the end of it. But what could it be?

“We’ll soon see,” said the professor. “Master Joe, attach another length of rope to it, and then have Master Bell switch power on the electric winch.”

This was done, and the powerful winch began to revolve, winding the rope on its barrel. As the rope began to grow shorter, the boys peered over the edge of the substructure in an effort to make out what could be at the end of it. The glow of light spread by the illuminated craft soon showed them.

“It’s a man!” shouted Nat in a thunderstruck voice, as the figure of a human being, clinging desperately to the rope, was brought into view.