"I mean that he still suspects that we are after gold, and he is going to do his best to get on our trail. Of course he can't follow us through the air, but he'll note in what direction we start and as soon as we are out of sight he and his men will hit the trail in the same direction."

"What, and leave the camp?" asked Ned.

"Yes, though they'll probably skip off with some of our supplies.

That's why I'm going to take along an unusually large supply. We may not come back to this camp at all. In fact, it won't be much use after Delazes and his crowd clean it out and leave."

"And you really think they'll do that, Tom?" asked his chum.

"I'm almost sure of it, from the way the Mexicans have been acting lately. Delazes has been hinting around trying to surprise me into saying which direction we're going to take. But I've been careful.

The sight of that golden image aroused him and his men. They're hungry for gold, and they'd do away with us in a minute if they thought they could find what we're looking for and get it without us. But our secret is ours yet, I'm glad to say. If only the balloon behaves we ought soon to be in the--"

"Hark!" exclaimed Ned, holding up a warning hand. They heard a rustling outside the tent, and one side bulged in, as if some one was leaning against it.

"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 b.u.t.ton; 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 s.p.a.ce.

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 airs.h.i.+ps. 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.

"I guess we can manage to store all that we can get out of the underground city," replied his chum. "I'm going to find a place for the big gold statue if we can manage to lift it."

"Say, we'll be millionaires all right!" exulted Ned.

Though much still remained to be done on the balloon, it was soon in shape for an efficient test, and that afternoon Tom, Ned and Mr.

Damon went up in it to the no small wonder, fear and delight of the Mexicans. Some, who had never seen an air craft before, fell on their knees and prayed. Others shouted, and when Tom started the motor, and showed how he could control his aircraft, there were yells of amazement.

"She'll do!" cried the young inventor, as he let out some gas and came down.

Thereupon followed busy days, stocking the airs.h.i.+p for the trip to discover the ruined temple. Food and supplies were put aboard, spare garments, all their weapons and ammunition, and then Tom paid Delazes and his men, giving them a month's wages in advance, for he told them to wait in camp that long.

"But they won't," the young inventor predicted to Ned.

There was nothing more to be done. All that they could do, to insure success had been completed. From now on they were in the hands of fate.

"All aboard!" cried Tom, as he motioned for Eradicate to take his place in the car. Mr. Damon and Ned followed, and then the young inventor himself. He shook hands with Delazes, though he did not like the man.

"Good bye," said Tom. "We may be back before the month is up. If we are not, go back to Tampico."

"Si, senor," answered the contractor, bowing mockingly.

Tom turned the lever that sent more gas into the bag. The balloon shot up. The young gold-seeker was about to throw on the motor, when Delazes waved his hand to the little party.

"Bon voyage!" he called. "I hope you will find the city of gold!"

"Bless my soul!" cried Mr. Damon. "He knows our secret!"

"He's only guessing at it," replied Tom calmly. "He's welcome to follow us--if he can."

Up shot the aircraft, the propellers whirling around like blades of light. Up and up, higher and higher, and then forward, while down below the Mexicans yelled and swung their hats.

Straight for the north Tom headed his craft, so as to throw the eagerly watching ones off the track. He intended to circle around and go west when out of sight.

And then the very thing Tom had predicted came to pa.s.s. The balloon was scarcely half a mile high when, as the young inventor looked down, he uttered a cry.

"See!" he said. "They're breaking camp to follow us."

And it was so. Riding along in one of the lightest ox carts was Delazes, his eyes fixed on the balloon overhead, while behind him came his followers.

"They're following us," said Tom, "but they're going to get sadly left."

In an hour Tom knew his balloon would not be visible to the Mexicans, and at the end of that time he pointed for the west. And then, flying low so as to use the trees as a screen, but going at good speed. Tom and his friends were well on their way to the city of gold.

"We must keep a good lookout down below," said Tom, when everything was in working order. "We don't want to fly over the plain of the ruined temple."

"We may in the night," suggested Ned.

"No night flying this time," said his chum. "We'll only move along daytimes. We'll camp at night."