and the dirigible balloon which I'm going to take along."

"Pretty uncertain way to look for a buried city of gold," commented
Mr. Swift. "But I suppose even if you don't find it you'll have the
fun of searching for it, Tom."

"But we ARE going to find it!" the lad declared. "We'll get there,
you'll see!"

"But how are you going to know it when you see it?" asked his
father. "If it's underground even a balloon won't help you much."

"It's true it is underground," agreed Tom, "but there must be an
entrance to it somewhere, and I'm going to hunt for that entrance.
Mr. Illingway writes that the city is a very old one, and was built
underground by the priests of some people allied to the Aztecs. They
wanted a refuge in times of war and they also hid their valuables
there. They must have been rich to have so much gold, or else they
didn't value it as we do."

"That might be so," assented Mr. Swift. "But I still maintain, Tom,
that it's like looking for a needle in a haystack."

"Still, I'm going to have a try for it," asserted the lad. "If I can
once locate the plain of the big temple I'll be near the entrance to
the underground city."

"What is the 'plain of the big temple,' Tom?"

"Mr. Illingway writes," said the lad, again referring to the letter,