It was very evident from Strong's actions that he was taking in every word of the conversation, which was in English and in an unusually loud voice. He pretended to eat, then leaned over and said something to his companion, and a minute later arose from the table and hastily quitted the place.

But the boys were not to be fooled. They sprang from the table and hastily followed, Donald throwing a greenback to the cashier which more than doubly paid the bill.

Their sudden action did not escape the soldier, who, attributing it to another cause, also hastily quitted the cantina.

As the boys emerged into the street, they saw Strong hastening away in the direction of the custom house.

"Going to lose himself in the crowd," said Donald. "Don't let him get out of your sight."

"What's the matter?" asked the soldier, running to catch up with the boys. "Ugly greasers?"

"No," replied Donald, "we're shadowing a man who is wanted by Gen. Funston. That's him," pointing; "keep your eye on him."

"I thought you chaps had something on," laughed the soldier. "You can bet on me!"

Now that they approached nearer the center of the business portion of the city, there were more people on the street; but they were so near their quarry that they easily kept him in sight. Only once had he glanced back, but that was enough to convince him that he was followed.

"He's headed for somewhere," said Adrian. "He doesn't expect to get away from us simply by walking."

"Right," agreed Donald. "There he goes into the post office."

"Yes," from Adrian, "and it has two doors. You stop at the first one and I'll run to the other."

He started on a run and the soldier followed suit.

The sight of a man running and a soldier following, quickly attracted attention and pedestrians began to stop and see what the matter might be. In less than a minute a crowd had collected, among them several soldiers, who quickly brought Adrian to a halt.

"What's the matter?" asked one. Then as the soldier came up: "What are you chasing him for?"

"I'm not chasing him," laughed the seeming pursuer. "He and I are after another chap."

The delay occasioned by this interference was not great, but it was sufficient to allow Strong to escape, had it not been for Donald. He had seen the crowd gathering and, realizing what would happen, ran around the other way, just in time to see Strong disappearing around the corner of the street on which the bank was located.

Madly he dashed down the street and turned the corner in time to see Strong enter the big gate leading into the patio of Don Esteban's residence.

Putting on more steam, a couple of minutes later Donald also dashed into the patio; but Strong was nowhere to be seen.

"He must be in the bank," muttered the boy, and he quickly entered the door.

But Strong had disappeared as completely as though the earth had opened and swallowed him up.

CHAPTER x.x.x.

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 pa.s.sage, 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.

"n.o.body 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 pa.s.sage."

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.

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.