"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 himself among your men."

"Oh, he was an American, was he?" laughed Donald. "Then I believe we can put our finger on him with ease. But the man we have in mind always carried with him an ape."

"Then it may not be the same," replied Don Esteban, "for this one had no ape with him either time."

"It certainly does look like a mystery," was Adrian's comment.

"Where is the vault from which the box was taken?"

"On the other side of the patio."

"May we examine it?"

"Certainly. I'll show it to you now."

Don Esteban led the way downstairs and across the patio. Opening the rear door of the bank, he escorted them within and closed the door.

Pa.s.sing behind the counter, he opened the iron door of the vault, disclosing within a good-sized chamber, in the rear of which was set the great steel safe, locked with a time lock.

"There," explained Don Esteban, pointing to an upper shelf, "is where the box stood."

"Oh, it was not a large box, then!" said Adrian.

"Oh, no! Not more than eight or nine inches cube."

The boy examined the vault carefully by the light of a gas jet.

"There seems no way that any one could have entered except by the door," said Donald.

"None whatever!"

"You are sure there is no opening in the ceiling?"

"Absolutely."

Don Esteban led the boys out and locked the door of the vault.

"Well," declared Donald as they came out into the patio after finishing their inspection, not only of the vault but of the rest of the office, "I guess we had better go home and study over the matter a little. I should not be surprised if we could put our hands upon the mountebank with very little trouble; but I feel sure he had nothing to do with the disappearance of the box."

"I wish I could feel that way," said Adrian after they were out of hearing of Don Esteban. "I have never quite trusted Strong.

There is something strange about him."

"Yes, he is a bit queer; but how on earth could he have stolen the box if he did not even go behind the counter?"

"He couldn't; but still I mistrust him."

"I wonder where Ambrosio was all this time?" mused Billie.

"Tied up at home, most likely. An American among Americans would hardly feel like traveling around with a hand organ and a monkey," was Don's emphatic reply.

Upon arriving at Santiago's residence they were told that Strong had been there earlier in the evening and seemed much disappointed at not finding them at home.

"Did he leave any word?" asked Billie of the portero.

"None, senor; but Donna Lucia would like to see you in the library."

"What is the matter? Is Santiago worse?"

"No, senor. He is sleeping quietly. I could not say what she wants, but she seemed considerably disturbed."

"Better go up alone, Billie," said Donald. "If we are needed, you can call us. We'll wait here in the patio for a few minutes before we turn in."

Billie ran up the stairs and tapped at the library door. He was immediately admitted by Lucia and the door was closed behind him.