t 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.

"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 supp