ood, was vacant. The box was gone!"

"Gone?" exclaimed all the boys in unison. "Do you mean stolen?"

"So it would appear."

"How could it have been done?" asked Adrian.

"I cannot say; but the strange thing about the whole matter
is that in place of the box, there lay upon the shelf an
envelope--yellow with age, upon which was written in ink that had
scarcely faded the words: 'Montezuma's Mine.'"

"Well, what do you think of that?" queried Billie, looking at the
others in amazement.

"I don't think," laughed Adrian. "It's up to you to do the
thinking."

"Is there no clue whatever?" asked Donald.

"Not that could be really called a clue. The only suspicious
thing that has happened to-day at all, was that a mountebank came
into our bank----"

"A mountebank!" from all.

"Yes."

"Did he have an ape with him?"

"No! He was quite alone. He did not come in to make merry, but to
get a bill changed. While he was there he was observed to
scrutinize the place very closely."

"But he did not go into your vault?"

"No! He took his change and went peaceably out."

"Then, why should you suspect him?" insisted Donald, casting a
knowing glance at the other boys.

"Because, an hour later, he came in again and said that one of
the bills we gave him as change was a counterfeit."

"Was it?"

"No, it was not, although it was an old issue. The teller who
waited upon him had no recollection of ever having seen the bill
before, but rather than have a scene, we gave him another bill
for it."

"How large a bill was it?" asked Adrian.

"Only a peso"--that is a dollar--"and it seemed hardly worth
talking about; but you'd have thought it was a hundred."

"Perhaps it seemed a large amount to him," ventured Billie.

"Perhaps," admitted Don Esteban. "But be that as it may, I should
like to see the man again, and especially would I like to know
where he got that old dollar."

"Why?"

"Because it may have come out of that box."

"Well, yes," said Donald, with a shake of his head, "it might
have; but how could the mountebank have gotten the box?"

"That is the mystery," was Don Esteban's reply.

"And how do you wish us to help you?" asked Adrian.

"Why," explained the banker, "I asked Gen. Funston to find the
mountebank for me. He said you boys would do better than any one
else."

"But why us? Why not a Mexican policeman?"

"Because the mountebank was an American. He may even have been a
soldier and have hidden