" whispered the voice. "It is the guard. Go away from the window."

The boys jumped down from the table, and as they did so, Bert stumbled
and fell heavily against the wall. When he recovered his balance they
heard a strange grinding sound like a heavy door creaking on rusty
hinges. The boys listened in wonder.

"Gee, but this is a creepy old place," said Bert, as the noise
continued. "Now, what do you suppose that is?"

"It sounds as if it came from the wall there. Let's investigate."

They moved nervously over to the stone wall that separated their prison
room from that of Miss Juanita. The noise seemed nearer and more
distinct, but they could see nothing that might cause it. Still the
strange sound continued. In the semi-darkness they watched in wonderment
the blank face of the wall from which the sound seemed to proceed.
Suddenly Harry seized Bert by the arm.

"Look!" he whispered in a tense voice. He pointed to a large stone in
about the centre of the wall. "Doesn't it move?"

The stone to which Harry referred was larger than any other, being three
feet square, and placed about waist-high from the floor. Bert watched
intently. It seemed to him that he could see a slight trembling movement
and then an almost imperceptible jump as the hand of an electric clock
advances with a jerk. The face of the stone, too, seemed to be out of
line with the others.

They advanced closer, and Harry passed his hand cautiously under the
stone. Unquestionably it had moved, either by accident or design. The
upper edge projected into the room beyond the line of the wall at least
an inch and the lower edge receded in the same way. As Harry's hand
rested on the stone he felt it tremble and jump and the upper edge
advanced another quarter of an inch into the room.

"That stone is revolving on a horizontal axis," said Harry, confidently,
after his inspection. "Now the question is: How and why?"

"It seems uncomfortably like the times of the inquisition," said Bert,
shuddering.

"Oh, pshaw, don't you see that wall separates us from the cell of Miss
Juanita, and the Spaniards would have nothing to do with opening this
passage?"

"Do you think she is doing it, then?"

"No, for had she known of the stone she would have mentioned it when I
asked her if there was any chance of escape from her prison. It has come
about through an accident, I feel sure, but how? Of course there must be
some secret spring that works it, but where is it