Shortly after the battle with the condors, the professor announced that, inasmuch as they were passing above a favorable landing place, he intended to make a landing. The spot selected was an open space beside a fairly large river, the glint of which could be plainly seen like a glittering ribbon beneath them.

Preparations for a landing were at once begun, and the Discoverer commenced nestling down toward the earth. The professor announced that the first task of the evening would be to replenish the supply of gas in the bag from the hydrogen tanks.

The anchorage was made without a hitch, and the Discoverer moored as securely as before; but in view of their experience of the night before, the travelers decided to have everything ready to “slip and run” in case the unpleasant experience was repeated.

As soon as the dirigible was secured, the task of adding to her depleted gas supply was begun. Two of the cylinders were dragged from their resting place and deposited on the ground, while the filling tube was made ready.

The Discoverer was anchored almost on the banks of the stream, a rapid one, with a rocky bottom and steep banks. While the others were working about the Discoverer, Ding-dong Bell set himself to examining the gas cylinders.

They were about ten feet long and very slender in proportion to their length. They were heavy, too, as the tremendous pressure within them made it necessary to construct them of the thickest and strongest steel,—the very finest grade obtainable, in fact.

Ding-dong, with his natural curiosity, started lifting one, and found that to raise one end was all he could manage, and that only by dint of puffing and blowing.

Joe Hartley, looking around from his work on the filling tube at which he was assisting Nat and the professor, noticed what his chum was up to.

“Say, put that down! You’re not strong enough to lift it,” he jeered. “Those things aren’t for kids to monkey with.”

“They’re not, eh?” puffed Ding-dong valiantly, “I’ll soon show you.”

With a supreme effort he managed to raise the cylinder and move it a short distance.

“Here, stop that!” shouted the professor as he espied what the boy was doing. “Don’t you know those things are dangerous unless handled carefully? They’ll go off like a bomb under a sudden shock.”

“That one must have got a sudden shock when it saw Ding-dong,” scoffed Joe. “Most people do.”

It was too much for Ding-dong. He set down the cylinder and made a jump toward his tormentor. In doing so, his foot struck the cylinder which, as it happened, was only just balanced on the steepish slope leading down to the precipitous river bank.

The gas container began rolling downward. The professor gave a shout.

“Stop it! Stop it! Don’t let it fall over the river bank or——”

Before he could complete the sentence, Ding-dong was valiantly off after the rolling cylinder. He grasped it, but its weight and the velocity it had attained, caused it to evade him, and while he fell sprawling in an effort to regain his balance, the cylinder bounded on toward the brink of the steep river bank.

“Down on your faces! Down on your faces! Everybody!” fairly roared the professor.

They all obeyed blindly, not sensing the utility of the order, but realizing its urgency in the tones of the professor’s voice.

The cylinder gave a leap as it struck a stone, and then bounded over the edge of the river bank.

Bo-oo-oo-oo-m!

An explosion that shook the ground followed almost instantly. From the bed of the river a geyser of mud and water and rocks spouted up, showering everything for a radius of several yards. The explosion the professor had dreaded had taken place; but, by a miracle, no one was hurt. No doubt the fact that the detonation took place below the river bank accounted for this fact.

But the lecture that Ding-dong received! And he admitted that he deserved it.

“If you ever catch me mo-mo-monkeying with that h-h-high-diddle-diddle g-g-g-gas again you can ber-ber-ber-blow me up with it,” he declared.

“That ‘high-diddle-diddle gas,’ as you call it, is much too precious for that,” said the professor with a laugh he could not restrain, “but I shall adopt other measures.”

The boys had a good opportunity then to see the destructive force stored in one of those innocent-looking cylinders. Peering over the river bank they could see that a great hole had been blown in its bed, and rocks riven and split in every direction.

“It’s as explosive as dynamite,” exclaimed Nat.

“It is, indeed,” said the professor. “The condition of that river bed gives mute evidence of that.”

“Just think what would happen if a spark should ever enter that gas bag of ours,” said Nat, with a slight shudder.

“We wouldn’t be able to think,” said Joe succinctly.

“Come, let us get back to work,” suggested the professor, “roll that gas cylinder closer to the filler tube and we will make the connections.”

Gingerly enough, as you may imagine, the lads rolled the cylinder toward the end of the filler tube, which now lay extended on the ground. The end of the tube was fitted with a union, which, in turn, was screwed on to the nozzle of the gas cylinder. Then the professor turned on the vapor, of whose power they had just had such a striking example.

With a hiss and a roar the gas poured through the filler tube into the bag, and several small wrinkles, which had developed in its upper surface, began to fill out. Two cylinders were emptied before the professor and Mr. Tubbs announced that the bag was full enough.

The evening passed off quietly. As before, the evening meal was eaten on the ground, and the adventurers utilized the cabin of the Discoverer for sleeping quarters. Old Matco, the Indian, shared the meal, but refused to sleep within the cabin. Instead, he rolled himself up outside, on the substructure, like an animal of some sort. He had the true aborigine’s dislike of sleeping under a roof. It savored to him of a trap possibly.

The old fellow, now that he had become used to aerial navigation, did not seem to object to it in the slightest. He rather appeared to like it, in fact, and took a childish delight in watching the various operations that went on on board. It appeared that he had no intention of detaching himself from the party as yet, and indeed, seemed to have the liveliest gratitude to them for rescuing him from his unpleasant position at the end of the swinging rope.

The professor was of the opinion that Mateo might prove useful to them, so no move was made to urge him to return to his tribe. Indeed, they were now in the country of another tribe of Indians altogether,—so Matco informed them,—a tribe as warlike and resentful of the intrusion of white men as his own. This was not encouraging news, but the adventurers resolved to make the best of it, and guard against surprises by keeping a good watch.

Nothing occurred during the first part of the night, and when Ding-dong and Joe came on duty at midnight the professor and Nat had nothing to report.

“Don’t forget that time you shot at the mule,” warned Nat, addressing himself to Ding-dong.

“Oh, no danger of my doing that again,” Ding-dong assured him; “b-b-b-b-besides, they d-d-don’t have mules in this p-p-part of the country.”

“That’s good logic, at all events,” laughed the professor, who had heard the story of how Ding-dong shot at a mule in mistake for an Indian the night the Motor Rangers camped in the petrified forest in the Sierras.

Ding-dong and Joe marched up and down for some time, without anything occurring to mar the quiet of the night. But on what was, perhaps, the stuttering lad’s twentieth parade around the dirigible, he heard a queer, inexplicable sort of noise coming from the river.

“Indians,” was his first thought. But then:

“That sounds like somebody snoring, and Indians who were coming to attack us wouldn’t announce their presence like that,” thought Ding-dong.

The snoring noise continued. Joe was on the other side of the dirigible, while Ding-dong was on the river end of it.

“It’s a good chance to distinguish myself,” thought the lad, “after the mess I made of that gas cylinder this afternoon. I’ll just creep down there and see what on earth that racket is.”

He began tiptoeing softly toward the river bank, while the grunting, snoring sound still continued.

“I do believe it’s some one asleep down there,” exclaimed the lad to himself. “Maybe I’ll make a prisoner and get even on Joe for laughing at me.”

His mind full of these visions of glory, Ding-dong at last reached the river bank. Behind him he could hear Joe softly calling, but he made no answer.

“I’m going to investigate this thing alone,” he said to himself.

Lying flat on his stomach Ding-dong peered cautiously over the bank. He could see the gleam of the water about ten feet below him and—what was that? Two dark figures, that appeared to have bulk of considerable size, moving about in the water? One was larger than the other, and it didn’t take the boy long to make out that whatever the mysterious objects were, they were not human beings.

“Wonder if they’re panthers?” thought the boy with a sudden chill. But then he recollected that panthers are not in the habit of prowling about in the river bottom.

“And I never heard of a panther grunting,” considered Ding-dong, “I guess I’ll just——”

But what Ding-dong had “just” made up his mind to do was never revealed. The bank at the point where he had been leaning over, was cut out beneath by the action of the river, and in scrutinizing the dark objects he had leaned rather far over.

Suddenly the bank caved in, and amidst a shower of gravel, rocks and small bushes, Ding-dong went rolling down into the river.

Splash!

He landed in a deep pool, which, luckily for him, was of sufficient depth for him to avoid injuring himself. Still clutching his rifle he rose to the surface, puffing and blowing, and scrambled out.

“Well, here’s a fix,” thought Ding-dong, “just like my luck. I’m always getting in bad.”

All this time he had quite forgotten about the two dark, moving objects, to whom he owed his present predicament. But their existence was rudely recalled to him as, out of the darkness, something rushed at him, snorting loudly and angrily, and advancing like an express locomotive.