"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 and how and by whom has it been operated?"
"Hal, I believe I did it," whispered Bert, excitedly.
"What on earth do you mean?"