As I spoke, my hand loosened a piece of rock that jutted out and before I knew it there was a crash.

“Confound it, Walter,” exclaimed Kennedy.

Down the passageway the figure was now thoroughly on the alert, staring with his goggle-like eyes into the blackness in our direction.  It was not the roof above him that was unsafe.  He was watched, and he did not hesitate a minute to act.

He seized the bag and picked his way quickly through the passage as if thoroughly familiar with every turn of the walls and roughness of the floor.

We were discovered and if we were to accomplish anything, it was now or never.

Kennedy dashed forward and I followed close after him.

We were making much better time than our strange visitor and were gaining on him rapidly.  Nearer and nearer we came to him, for, in spite of his familiarity with the cavern he was hampered by the outlandish head-gear that he wore.

It was only another instant, when Kennedy would have laid his hands on him.

Suddenly he half turned, raised his arm and dashed something to the earth much as a child explodes a toy torpedo.  I fully expected that it was a bomb; but, as a moment later, I found that Kennedy and I were still unharmed, I knew that it must be some other product of this devilish genius.

The thickest and most impenetrable smoke seemed to pervade the narrow cavern!

“A Chinese smoke bomb!” sputtered and coughed Kennedy, as he retreated a minute, then with renewed vigor endeavored to penetrate the dense and opaque fumes.

We managed to go ahead still, but the intruder had exploded one after another of his peculiar bombs, always keeping ahead of the smoke which he created, and we found that under its cover he had made good his escape, probably reaching the entrance of the cave in the underbrush.

At the other end of the passageway, up in the living-room of the cottage, the draught had carried large quantities of the smoke.  Elaine, Aunt Tabby and Joshua coughing and choking, saw it, and opened a window, which seemed to cause a current of air to sweep through the whole length of the passageway and helped to clear away the fumes rapidly.

Long Sin, meanwhile, had started to work his way through the bushes to reach the waiting car, with Wu, then paused and listened.  Hearing no sound, he replaced the helmet which he had taken off.

Pursuit was now useless for us.  With revolvers drawn, we crept back along the passageway until we came again to the chamber itself.  There, on the floor, lay a bag of tools, opened, as though somebody had been working with them.

“Caught red-handed!” exclaimed Kennedy with great satisfaction.

He looked at the tools a minute and then at the electric drill, and finally an idea seemed to strike him.  He took up the drill and advanced toward the safe.  Then he turned on the current and applied the drill.