Spinning along at a height the barograph showed to be 1,500 feet, was an exhilarating experience. The slight feeling of apprehension which the Motor Rangers had felt when they set out on their novel cruise, soon wore off, and was replaced by a buoyant sensation.

“Well, Master Nat, what do you think of it?” inquired the professor, emerging from the cabin and coming “aft” to where Nat was standing by the smoothly running motor.

“It’s glorious,” replied Nat enthusiastically. “I had no idea, though, that it was possible to get used to it so soon.”

“Well, a craft of this kind is vastly different from an aeroplane,” commented the man of science. “It is my belief that the aerial trans-Atlantic liner of the future will be a dirigible.”

“I wouldn’t mind undertaking the trip in the Discoverer,” declared Nat, with glowing eyes and cheeks.

“What speed are we making?” inquired Joe Hartley.

“About forty miles an hour,” said the professor; “but you can tell the exact speed by stepping into the pilot-house and examining the instruments.”

The lads followed his advice, and found that the speed recorder registered a shade more than the professor had assumed. Mr. Tubbs had the wheel, and was gazing straight ahead, like a steamboat pilot.

The pilot-house of the Discoverer, in fact, was not unlike that of a steamer, although much smaller, of course. The registers and indicators, too, that were fastened to the walls, or rather the framework of the Discoverer’s “hull,” were totally unlike any that the lads had seen before.

Joe Hartley, who had been appointed chief cook and bottle washer, soon left, to begin his preparations for lunch. But Nat lingered on, fascinated. Joe’s meal proved an excellent one, and the fact that they were so high above the earth did not affect the boys’ appetites in the least. In fact, Ding-dong Bell observed that he had never felt so hungry in all his life before.

After the meal was concluded, the motors of the craft were slowed down a bit, so as to economize on gasolene as much as possible. The fact that the westerly wind had increased made it possible to slow the engine down and still make good progress.

“I wonder what they think of us down below there?” said Joe, as he stood by Nat’s side, leaning over the forward deck-rail and watching the dwarfed figures of the inhabitants of a village above which they were passing, scurrying to and fro like ants.

“I guess they must think we are some sort of demoniacal bird,” grinned Nat. “Hark!”

Faintly, very faintly, borne to their ears, came the sound of church bells ringing furiously.

“They must be going to hold services in our honor,” hazarded Joe.

“More likely they are going to pray that we don’t harm them,” responded Nat. “According to the professor, the people of this country are a very ignorant lot.”

By afternoon the Discoverer was flying above rugged country. The foothills of the great Andean range had been reached, and they were in Bolivia. It gave the boys a thrill to think that they were actually at last in the hoped-for vicinity of the lost city of the mysterious old Incas.

As the sun grew lower, the great altitude to which they had attained struck them with a sharp sense of chilliness.

“This part of the world ought to be called Chile,” observed Joe, as he and the professor and Nat stood on the forward deck just below the pilot-house.

“If you will come into the cabin and see what I have in that big chest, we can possibly get over that difficulty,” said the professor, with a smile.

The lads accompanied him within and found that the chest referred to contained a variety of warm clothing.

“I knew that the late afternoons and nights on the Andean heights were bitterly cold,” said the professor, as the boys selected some garments, not forgetting a coat-sweater for Ding-dong. “I therefore took the precaution to be prepared to meet them.”

It was not long after this that the professor addressed a few words to Mr. Tubbs, and the Discoverer began to drop. Then came a sudden signal to Ding-dong to slow up his engines. This being done, the lateral planes of the dirigible, which have not yet been mentioned, were inclined at an angle that brought her to earth with an easy, gliding motion.

“Are you going to land for the night?” asked Nat, who had watched the maneuvers with interest.

“Such is my intention,” said the professor. “It is too late in the day to get any observations now, and I don’t fancy traveling at night in this region. We might blunder miles off our course.”

The boys agreed that this was so, and then gave their full attention to what was going forward. Immediately beneath them was a charming, park-like savannah, set in the midst of dense forests of gigantic trees, from whose branches hung great twisted creepers, looking not unlike big snakes.

It formed an ideal landing spot for the big dirigible, which, in a few moments after the descending planes had been set, grazed the ground and then settled. Instantly the professor shouted an order for the anchoring process to begin.

The boys had been drilled in this before the voyage was started, and fell to work with a will on their task. By running the propeller slowly, with the descending planes set at a sharp angle, the Discoverer’s body was naturally held against the ground.

Nat and Joe leaped off on opposite sides, both armed with sledges. With these heavy hammers they drove sharp, barbed steel stakes into the ground till they were almost as firm as rocks. Each stake had a ring at its top through which ropes were rapidly looped. The ends were then led back on board and secured. This was done so that in case of a sudden attack the great aircraft could be released by those on board. Of course, in such an event, the stakes would have to be left behind, but as an extra supply was carried, this would not be such a serious matter.

Ten minutes after she nestled to the ground, the Discoverer was secured as snugly as a vessel at her wharf. The engine was shut off and the various necessary adjustments of the controls and apparatus of the pilot-house made. This done, the entire party stepped “ashore” for the first time in many hours.

“We will sleep on board, but cook our supper here,” decided the professor.

This plan just suited the boys, and they scattered in all directions to obtain firewood for the encampment. While they were doing this, Mr. Tubbs set about the task of getting the needed utensils from on board the cloud cruiser. He had been busily engaged on it for some time when the professor looked up from some calculations he was making on the back of an old envelope.

“It appears to me those boys are a long time gone,” he said. “I hope they are all right.”

“Oh, they are all right,” spoke the moving-picture artist easily. “They took the rifles with them, and agreed that in case of any danger or difficulty befalling them, they would fire three times.”

“In that case——” began the professor.

But he halted with an abrupt exclamation of consternation. Mr. Tubbs’s face likewise took on a perturbed look at the interruption.

From the forest, to their right, three shots, fired in rapid succession, had resounded.