I could not conjecture.

On the fifth night I undressed and lay down as usual. It was quite
dark in the cell, and the only sound that reached me was the periodical
"All's well!" of the sentry stationed at the end of the corridor. For
a long time I lay puzzling over the strange situation, but at length
dropped into a light sleep.

Suddenly I was awakened by a queer sensation, and sat up in bed. It
was too dark to see anything, but I felt that some one was creeping
stealthily across the floor. Presently I heard a faint sound, and knew
that the object, whatever it might be, was approaching nearer. At the
side of the bed it stopped, and a muffled voice whispered, "Senor, are
you awake?"

"Yes," said I. "Who's there?"

"A friend of the silver key. Dress quickly and come with me; the way
is open."

"Where is the sentry?" I asked.

"Gagged and insensible," replied the voice. "Quick, while there is yet
time."

Perhaps it was rather venturesome thus to trust myself in the hands of
an unknown man, but I slipped on my clothes, and keeping touch of his
arm, accompanied him into the dimly-lighted corridor.

Turning to the left, we glided along close to the wall. At the end of
this passage the body of the sentry lay on the ground, while near at
hand crouched an Indian, keeping watch.

This man joined us, and my guide immediately led the way into an empty
room, the door of which was open. As soon as we were inside he closed
it softly.

"Keep close to me," he whispered, and then said something to an unseen
person in a patois I did not understand.

Presently he stopped, and I could just distinguish the figure of a
third man, who, grasping my hand, whispered, "The silver key has
unlocked the door, senor."

Before I could recover from my astonishment--for the man who spoke was
the sick jailer--my guide let himself down through a trap-door, and
called to me to follow. I found myself on a flight of steep steps in a
kind of shaft, very narrow, and so foul that breathing was difficult.
At the bottom was a fair-sized chamber, with a lofty roof--at least I
judged it so by the greater purity of the air--and here the guide
stopped until his companion caught up with us. The jailer, to my
surprise, had remained in the fort, but there was no time for
explanation.

The exit from the chamber was by means of an aperture so low that we
had to lie flat on the ground, and so narrow that even I found it hard
work to wriggle