"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. There was nothing but solid stone. "Looks kind o' desperate," he told himself, "but I have simply got to find an opening." Again he prodded the place over without result. "It's no use," he finally said to himself. "If I am to get out of here, help will have to come from somewhere outside. But how can it?" He sat down on the stairs and thought deeply. "If it is as I think," he mused, "these stairs have been used recently. The very fact that Ambrosio is prowling around here is proof that Strong must have been here at some time. But where is Strong?" He slid down several steps and threw the light of his torch across to the opposite stairs. "Bang!" went something that sounded like a falling stone. Billie sprang to his feet. "Click! Bang!" and down the opposite stairway streamed a ray of light. Billie shut off his electric torch and waited. Then on the steps there appeared a foot, then another, until a whole human body was in sight. Then "click, snap!" and the light disappeared. "By George!" exclaimed Billie under his breath, "there's a man on the stairs right opposite to me. Who on earth can it be?" His first inclination was to turn on the light, but on second thought he decided not to. "I'll let him set the copy," thought Billie. "It's a mighty good thing to be able to spring the surprise." He drew his automatic and stood ready for any emergency. It was still as a tomb. He could hear his own heart beat and he could also hear the heavy breathing of the other man. "Sounds like he had been running and was just catching his breath," was Billie's mental comment. After some minutes the man's breathing became more quiet and Billie heard him slowly descending the stairs. "This won't do," thought Billie. "He probably has some kind of a light, and if he gets on the landing and I'm up here, I'll be like a man up a tree." The lad sat down and slowly slid toward the bottom. Being only a few steps from the landing, Billie was down first. He crowded to the right and listened. The other was now nearly on the landing. Now he was on the landing, hugging the wall on the side opposite Billie. Now they passed each other, or rather the man passed Billie, for Billie stood perfectly still. It seemed as though he must hear Billie's breathing, but if he heard anything he must have thought it his own echo, never dreaming that he was not alone in the passage. Now Billie could hear him ascending the stairs leading to Santiago's house, and his hope rose high. "He must know how to open the floor," thought Billie. "I'll be right there when he does."