AT Karl’s dread discovery the others uttered exclamations of alarm and astonishment.

“What could have caused it?” asked Bob grimly.

The aviator shook his head.

“Can’t say,” he returned. “Maybe something pierced it while we were in Cuba. Could have made a small leak that let out a little at a time. Or the storm could have done it.”

“Good thing we were able to make it across the Caribbean,” remarked Mr. Holton. “If it had been much larger, perhaps we wouldn’t be here now to find it out.”

The short-lived tropical twilight was upon them, with a promise of darkness being only a few seconds off.

“Suppose we put the tent up while we can see to do it,” suggested Bob. “Then we can attend to the leak in the morning. There’s some solder in the provision compartment, and we can put some of it on now to keep the remainder of the gas from running out.”

The others thought this good advice. While the youths and the naturalists made camp, Karl Sutman applied a heavy coat of liquid solder over the cut in the gasoline tank.

“I guess we’re in a mess,” the aviator said disgustedly. “We’ve used up all the gasoline in the spare tank, and now we haven’t enough to take us twenty-five miles. We could have flown to BogotÁ easily if it hadn’t been for that leak.”

“Bet there isn’t a gasoline station within fifty miles of here,” groaned Joe, glancing at the rugged country that was on all sides of them.

Darkness overtook the explorers before they had completed making camp. They were forced to turn on the lights of the monoplane until they could gather sufficient twigs for a fire.

When finally a roaring blaze illuminated the sky, they turned to complete making the camp.

As a precaution, this was made on a spot several hundred feet from the monoplane. This would do away with the danger of an explosion, for the intense heat from the fire might easily have ignited the remaining gas in the tank.

“Now to get a meal,” said Bob, edging closer to the blaze to escape the chill of the tropical night.

A delicious spread of food was prepared, all eating heartily. The eventful day had stimulated their appetites highly.

“I suppose there’s no use worrying,” grunted Karl, stretching out before the fire. “We’ll find a way out somehow. If we can’t do anything else, we can all hike to a town and carry back enough gas to carry us a short distance. Then we can hike to another town, and do the same thing over again.”

“Do these towns around here have gas, though?” came from Joe. Despite Karl’s expression of hope, he feared the worst.

“That we don’t know,” Mr. Wallace said. “It may be there hasn’t been an internal-combustion engine in this region for years, if at all.”

A rapidly growing exhaustion made the explorers for the time being forget their cares and curl up in the tent, after having heaped the fire high with fresh fuel. They had not thought it necessary to stand guard, as there was probably nothing in this region that would bother them.

The next morning Karl got out a map of South America and spread it out on the tail of the machine.

“Here we are about twenty miles inland,” he said. “The nearest town appears to be about fifteen miles from here. Luckily it’s south, and we won’t have to go much off our course.”

“Think we can get gasoline there?” queried Joe.

“Probably not,” Karl answered. “But if we have to we can take a train to Cartagena—that’s a city not far from here on the coast. Of course they have gas there.”

They climbed into the monoplane, which, with a roar, rolled over the high grass and headed south. Karl kept the machine going at as slow a speed as possible, for he desired to use every ounce of fuel to advantage. But even then they made the short trip to the little town in but a few minutes.

“Here we are, right near the town.” Karl climbed out of the cockpit after having made a perfect landing.

Scarcely had the explorers stepped to the ground when they caught sight of a score or more natives running toward them. It was a motley crowd that surrounded the Americans a few seconds later.

Surprise, bewilderment, amazement were displayed on the faces of the Colombians. The monoplane they viewed with a certain awe that was almost childish in its sincerity.

As soon as the jabbering had abated somewhat, Mr. Holton addressed them in Spanish, asking if it might be possible to procure gasoline for the airplane.

The faces of some were expressionless, but a few shook their heads.

“We do not use gasoline here,” one man said in the native tongue. “There are no great birds like this”—pointing to the monoplane—“in our land. And we have no carriages that are not drawn by animals.”

Mr. Holton then asked if it might be possible to get gasoline in Cartagena, the city on the coast.

Strange to say, the people did not know. Evidently they had never been to that place, although it was less than fifty miles distant.

“Well, then,” began Karl, “I suppose one of us will have to take a train to Cartagena. Whoever goes can take a gasoline can with him and get it filled. Then he can return on the next train.” The Americans could not help laughing at this, however necessary it might have been. The idea of boarding a train for a fifty-mile journey merely to get a can filled with gas seemed provoking.

“What a predicament!” roared Bob, catching hold of the monoplane in order to hold his balance.

“I suppose we ought to take this more seriously,” said Karl, who was also laughing. “But somehow it all seems humorous to me.”

At sight of the Americans laughing, the crowd of natives looked about sullenly. No doubt they thought the newcomers were making fun of them. Finally one man stepped up to Bob, and, with a sneer, uttered something in the native language.

The youth could only catch a word or two, but it was enough to make him glare at the man in anger.

“Be careful, Bob,” warned his father. “There are too many of them for us to get into a scrap.”

“Aw, I could lick them all with one hand!” snarled the youth, his eyes resting fearlessly on first one and then another of the men.

He was able to control his temper, however, and as the Colombians made no further move, he turned to Karl Sutman.

“Why can’t I make that train trip?” he asked. “I’ll pay my own fare. Really I’ll enjoy it.”

“All right,” came from Mr. Wallace. “And I’ll go with you. It will take two to carry the gas can when it’s full.”

“Be careful,” warned Mr. Holton. “We won’t be surprised if you’re gone a day or two.”

At the railroad station, which was little more than a mud hut, they found that a train would arrive in less than three hours. They thought it best to remain near the depot, for the schedule might not be accurate.

The train finally came, but, much to their disgust, the two gas seekers were informed by the conductor that they would arrive in Cartagena no sooner than four hours later.

At last they started moving and slowly left the station behind. The little crowd that had assembled to see the train off waved a farewell as it disappeared around a curve.

Bob and the naturalist gazed intently out of the window at the barren country they were passing through. Only at intervals could they make out an adobe house.

They had gone perhaps an hour when they were startled by a sudden commotion at the head of the train. Bob was looking out of the window trying to make out what was going on when he suddenly felt the train come to an abrupt stop.

Wondering what was meant, he and Mr. Wallace had started toward the front of the coach when they were interrupted by a cry that echoed through the train.

“We’re being robbed!” exclaimed Mr. Wallace, hurrying back to the seat. “There’s a gang holding up the train!”