and how and by whom
has it been operated?"

"Hal, I believe I did it," whispered Bert, excitedly.

"What on earth do you mean?"

"You know when I jumped down from the table I fell against the wall. It
was immediately after that we heard the creaking."

"Thunder, you are right. You must have touched the spring."

"I think so. Let's look for it."

The boys carefully examined the wall near the place where Bert had
stumbled, and to the left of the revolving stone they found a small,
diamond-shaped stone that to the casual observer would appear to have
been set in the wall to fill in the broken corner of one of the larger
stones. Upon close inspection they found that it was set loosely in the
wall without mortar. They dared not touch it for fear it might stop the
invisible machinery that it had evidently set in motion.

Slowly the stone continued its unsteady revolution, until at the end of
about five minutes the creaking stopped, there was a clicking sound as
if a cog had settled into place, and all movement ceased. The big slab,
which was six inches thick, had now obtained a horizontal position,
leaving an opening above and below into the next room, or cell. The axis
upon which the stone revolved was a little above the centre, so that the
lower opening was nearly eighteen inches high.

The boys peered through into the darkness of the next cell.

"Miss Juanita," called Harry, softly. "May we come in? Perhaps this
scheme of opening walls may continue through to the outside world."

CHAPTER XV

THE EXECUTION AT DAWN

"Who is there?" came a frightened voice from the farthest corner of the
room.

"It's the American boys who were talking with you at the window,"
answered Harry, reassuringly. "We are friends. Do not fear."

"Oh," came in a gasp of relief. "I thought they were about to inflict
some new horror upon me. What have you done?"

"We do not quite know ourselves. In some way we touched a secret spring
that rolled over this stone and formed a passage between these two
cells. It is just possible that there may be another one. May we come in
and look?"

"Yes, yes, come in. Oh, perhaps it is true--perhaps we shall be able to
escape from this horrible place."

"Do not hope for too much. It was only a chance thought of mine.
However, we better see."

The boys climbed through the opening without difficulty and found
themselves in a room exactly similar to the one they had left, except
that it wa