“Well, boys, everything appears to be all right.”

It was morning in the large compound, or garden, adjoining the consul’s house, and our adventurers were grouped about an odd collection of articles that had formed the contents of several big packing cases.

“By the way,” put in Mr. Stowe, who had been an interested spectator of the unpacking of the cases, “I have news for you, Master Trevor.”

“What is it?” inquired Nat, whose shoulders still felt a bit stiff and sore, but was otherwise in fine fettle.

“The police say that they will arrest that man who struck you—to-morrow.”

“I thought so,” said Nat, with a laugh, as he caught a twinkle in the consul’s eyes; “I guess it will be one of those to-morrows that never come.”

“I’m afraid so,” said the consul. “There is little law in this country, and it’s a case of every one looking out for himself.”

After some more talk, in which all freely expressed their indignation against the rascals who had decoyed Nat, work on the erection of the dirigible was begun. It proceeded rapidly. By afternoon the lower framework of the craft was in position and bolted firmly in place. This part of the craft merits a somewhat detailed description. It was of an aluminum alloy, of great strength and lightness.

Amidships of the structure, which was shaped not unlike a long sleigh, was a canvas-enclosed cabin. The front part of this was fitted with round windows for the helmsman to see out of, and contained the wheel by which the great rudder was controlled. The various levers and handles for the management of the engine were also manipulated, like the rudder, from this “pilot-house,” as it may be called.

Just aft of the pilot-house the canvas-enclosed framework did duty as a dining, living and sleeping room, being fitted with swinging bunks, which, when not in use, folded up against the ceiling. A collapsible table and other furniture of the same character were also to be found in this chamber, as well as a denatured alcohol stove for cooking, and a complete outfit of plates, knives, forks, etc.

Behind the pilot-house came the heavy frames and stringers, destined to support the engine. This was a six-cylinder motor of one hundred horse power, which drove a big suction propeller attached to the front of the framework. Thus the dirigible was drawn, and not pushed, through the air. The propeller was ten feet from tip to tip, and formed of laminated wood covered with canvas stretched tightly upon the timber.

A sort of gangway, or path, extended from bow to stern of this framework, enabling the aerial navigators to walk to any part of the structure at will.

The entire frame was secured to the vast gas bag by numbered ropes, with steel cores to insure their stoutness. Relief valve-cords and gas controls all ran to the pilot-house, under which structure a steel tank, capable of holding two hundred gallons of gasolene, was suspended. A reserve supply of fuel was also carried, as well as lubricating oil, and what Joe Hartley called “a machine shop full of tools.”

There were other features of the craft, which will be described as occasion arises; but when we say that the Discoverer was, roughly, a hundred and fifty feet from stem to stern, one of the largest airships of her type, constructed in America, had a capacity of 150,000 cubic feet of gas and could lift 6,000 pounds, we have covered the main features of her construction. It may be added that the motor was of the four-cycle type, and, despite its high horse power, weighed but a trifle over 250 pounds. Aluminum alloy had been used freely in its construction.

By nightfall the engine was in place and firmly bolted to its foundation plates. A test showed it to be working perfectly. The cabin provision lockers were then stored with canned goods of all descriptions, and staples, such as flour, beans, bacon, corned beef and preserved butter. Tea was also carried, but no coffee. One feature of the cabin was the “armory.” This was a chest containing rifles and shotguns of the latest automatic type. It was an important feature of the Discoverer’s equipment, inasmuch as the adventurers expected to “live on the country” to a great extent, for Bolivia abounds in game.

All that remained to do then, was to inflate the great gas bag. The adjustment of this to the frame proved tedious work. But at last it was done, and the folds all carefully straightened out, in itself an arduous job. The whole party was pretty well tired out by this time, and work was discontinued for the day.

“In the morning,” said the professor, “we will inflate the bag, and then there will be nothing more to detain us.”

The boys gave a cheer. It seemed almost too good to be true—the idea that before many hours had passed they would be flying high above old Mother Earth in a cloud cruiser, that for completeness and effectiveness surpassed their wildest dreams.

Between four and five o’clock the next morning the lads were astir. After early coffee and some fruit and rolls, the task of inflating the great bag was begun. Huge wooden tanks full of iron filings and metal scrap had already been erected. Acid was now added to the filings and the tops clamped on. Then the inflation pipes, purifier and nozzles were adjusted.

A cheer broke from the boys as they saw the huge bag begin to swell like a live thing as the gas poured into it. By noon the professor announced the inflation as being sufficient. At that time, the great yellow bag was as tight as a drum almost, and the heat of the sun served to swell it still further. While the bag had been filling, the under frame of the dirigible had been weighted down by bags of sand. Otherwise it would have risen of its own volition.

The last things loaded on the framework were several cylinders of hydrogen gas at tremendous pressure. This was the reserve supply of the adventurers, and the tanks contained enough almost to refill the bag in case of necessity. A hasty lunch was consumed at the consul’s table, and Nat gave final instructions to the man who had been employed to take care of the Nomad during their absence.

This done, there was nothing else to wait for, and at one-thirty sharp, the professor gave a final look over things. Then he turned to Ding-dong Bell.

“You can take your place at the motor,” he said. “Mr. Tubbs, you will attend to the handling of the craft as we rise.”

The versatile Mr. Tubbs, whose moving picture apparatus was in readiness, paused to take a few pictures, and then mounted to his place in the pilot-house.

Nat and his chums bade good-bye to the consul, and then took their places. It was Nat and Joe’s task to attend to the throwing off of ballast as they arose.

“Good-bye and good fortune to you,” said the consul, as the great airship quivered and strained, as if anxious to be up.

The bags had been thrown off so rapidly that now the weight of only a few held her down. The professor took his place beside Mr. Tubbs. The consul’s wife waved a dainty handkerchief.

The departure had been kept a secret, but the sight of the great yellow bag’s outlines rising above the compound walls had attracted a crowd outside. A cheer arose as the Discoverer’s electric siren sounded a prolonged blast.

It was the signal for throwing off the remaining bags. Nat and Joe worked with a will. Suddenly the craft bounded upward, almost throwing them off. Hastily they cast off the final sacks, while Ding-dong, his face pale with excitement, stood by his engines.

Clang-clang! came from the gong at his elbow.

The lad’s hand shoved over the starting lever that gave the engines their first impulse by means of compressed air. Then he manipulated the sparking and gas controls.

The mighty propeller began to beat the air as the Discoverer soared buoyantly, and yet in stately fashion, high above the houses and tree-tops.

“Hurray! We’re off!” cried Nat, clambering along the runway as nimbly as a sailor.

Faster and faster the propeller revolved. The wind was blowing lightly out of the west, aiding the Discoverer on her flight toward the mountains.

Suddenly Ding-dong felt something fan the air past his ear. It was a bullet. At the same instant a report came from below. Somebody was shooting at the craft of the clouds. The others rushed out excitedly. They were just in time to see two figures struggling in the hands of several native policemen.

“It’s that rascal Lawless and his mate Durkee!” cried Nat. “Now I know why those voices seemed so familiar. It was those two ruffians who captured me the other night.”

“But how in the world did they get here?” asked Joe.

It was many days before that mystery was solved for the Motor Rangers, but in the meantime they at least had the satisfaction of seeing that the cowardly endeavor to injure the airship had resulted in their arrest.

But they gave little time to thinking of Lawless and his fellow ruffian. The land of mystery, of the lost city, of the unknown, lay before them.

With a fair wind and with perfectly working engines, the Discoverer drove forward at forty miles an hour, carrying the Motor Rangers on the strangest cruise of their eventful lives.