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.
"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 airship 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.