.

"It's just as I expected," he muttered. "If I can get
through this floor, I shall find myself in Don Esteban's
residence--somewhere. But just where? That's the question."

He pushed upon the stone above his head, but it refused to move.
Then he held up his torch and examined the ceiling carefully.
Whatever the method by which the stones could be moved, it was
carefully concealed.

Much chagrined, Billie at length decided to retrace his steps and
await the coming of his companions. He reached the landing and
crossed over to the steps by which he had descended.

Glancing up, he uttered an exclamation of surprise, for there at
the opening and peering down the stairs was Ambrosio, the ape.

With a cry of recognition the simian started to descend the
stairs, but at a noise from above he hesitated and then sprang
back and out of sight.

An instant later the opening was closed and Billie was left in
the darkness, except for the light of his torch.

CHAPTER XXIX.

THE MYSTERY DEEPENS.

While Billie was making his discoveries Donald and Adrian had
also been busy.

When they left the house, at Donald's suggestion, they went first
to the bank and told Don Esteban about the disappearance of the
box and what the maid had seen. He was much disappointed until it
was explained to him that they also had another clue.

"We shall spend our time in the slums," said Donald, "while our
chum keeps watch at the house. We feel sure that between us we
shall solve the mystery."

"I hope so," was Don Esteban's response. "Do you think I might be
able to see this man Santiago?"

"I'm afraid not, sir! The doctor wants him to sleep as long as
he can. We will let you know later in the day."

Leaving Don Esteban, the boys started for the plaza, intending to
play the part of sightseers and visit every place in which a
mountebank might reasonably expect to go. They felt certain that
Strong would keep away from the more aristocratic places.

Keeping their eyes open and ever on the alert, they wandered
about the streets and into many public places, but up to eleven
o'clock had made no discovery. Then they entered a cantina for
breakfast, purposely choosing one that was little frequented by
Americans.

Seating themselves at a table in one corner where they could see
without being seen, the boys ordered a hearty breakfast and then
turned their attention to the others in the place.

At the table nearest them were three men o