“Some one’s listening,” whispered Ned.

Tom nodded.  The next moment he drew his heavy automatic revolver and remarked in loud tones: 

“My gun needs cleaning.  I’m going to empty it through the tent where that bulge is—­look out, Ned.”

The bulge against the canvas disappeared as if by magic, and the sound of some one crawling or creeping away could be heard outside.  Tom laughed.

“You see how it is,” he said.  “We can’t even think aloud.”

“Bless my collar button; who was it?” asked Mr. Damon.

“Some of Delazes’s men—­or himself,” replied the young inventor.  “But I guess I scared him.”

“Maybe it was Andy Foger,” suggested Ned with a smile.

“No, I guess we’ve lost track of him and his father,” spoke Tom.  “I’ve kept watch of the back trail as much as I could, and haven’t seen them following us.  Of course they may pick up our trail later and come here, and they may join forces with the Mexicans.  But I don’t know that they can bother us, once we’re off in the balloon.”

To Tom’s disappointment, the next day proved stormy, a heavy rain falling, so it was impossible to test the balloon with the gas.  The camp was a disconsolate and dreary place, and even Eradicate, usually so jolly, was cross and out of sorts.

For three days the rain kept up, and Tom and Ned thought they would never see the last of it, but on the fourth morning the sun shone, wet garments and shoes were dried out, tents were opened to the warm wind and everyone was in better spirits.  Tom and his chum at once set about making gas for the big bag, their operations being closely watched by the Mexicans.

As I have explained before, Tom had the secret of making a very powerful gas from comparatively simple ingredients, and the machinery for this was not complicated.  So powerful was it that the bag of the dirigible balloon did not need to be as large as usual, a distinct saving in space.

In a short time the bag began to distend and then the balloon took shape and form.  The bag was of the usual cigar shape, divided into many compartments so that the puncture of one would not empty out all the vapor.

Below the bag was a car or cabin made of light wood.  It was all enclosed and contained besides the motor, storage tanks for gasolene, oil and other things, sleeping berths, a tiny kitchen, a pilot house, and a room to be used for a living apartment.  Everything was very compact, and there was not half the room there was in some of Tom Swift’s other airships.  But then the party did not expect to make long voyages.

They could take along a good supply of canned and also compressed food, much of which was in tablet or capsule form, and of course they would take their weapons, and ammunition.

“And I hope you’ll leave room for plenty of gold,” said Ned in a whisper to Tom, as they completed arrangements for the gas test.