"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."