THE MYSTERY SOLVED.
When the opening at the head of the stairs had closed upon Billie and he realized that he was shut in a subterranean passage, for a minute his heart sank within him.
He had tried to find an opening at the top of the opposite stairs and had failed, and he did not know that he would have any better success in trying to find a way to open the place through which he had descended.
"Nobody on earth knows that I am down here," he thought, "and with Santiago sick and maybe dying, no one on earth probably knows that there is such a passage."
But Billie was not the lad to sit down and cry. He had been in tight places before and he had an abiding faith in his own ability to do things. Therefore, he called up his courage and slowly mounted the stairs leading to Santiago's patio.
At the top of the stairs he found himself confronted by exactly the same condition as he had found on the opposite side.
"But there is this difference," muttered Billie. "I know that there must be some sort of a crack where that slab fell back."
He held the electric bull's-eye close to the stone and scrutinized every spot.
Not a single crack could he spy.
Then he took out his big jack-knife and prodded with it clear across the width of the stairway.