The drill was of the very latest design and it went quickly through the steel.  But beyond that there was another thin steel partition.  This Kennedy tackled next.

The drill went through and he withdrew it.

Instantly the most penetrating and nauseous odor seemed to pervade everything.

Kennedy cried out.  But his warning was too late.  We staggered back, overcome by the escaping gas and fell to the ground.

. . . . . . .

Long Sin, with his oxygen helmet on again, had returned to the passageway and was now stealthily creeping back.

He came to the chamber and there discovered us lying on the ground, overcome.  He bent down and, to his great satisfaction, saw that we were really unconscious.

Quickly he moved over to the safe and pried open the last thin steel plate.

Inside was a small box.  He picked it up and tried to open it, but it was locked.  There was no time to work over it here, and he took it under his arm and started to leave.

He paused a moment to look at us, then took out a piece of paper and a pencil and on the paper wrote, “Thanks for your trouble.”  Beneath, it was signed by his special stamp—­the serpent’s head, mouth open and fangs showing.

Long Sin looked at us a moment, then a subtle smile seemed to spread over his face.  At last he had us in his power.

He drew out a long, wicked-looking Chinese knife and stuck it through the note.

Then he felt the edge of the knife.  It was keen.

. . . . . . .

In the sitting-room, Elaine, Aunt Tabby and Joshua had been listening intently at the fireplace but heard nothing.

They were now getting decidedly worried.  Finally, the fumes which we had released made their way to the room.  They were considerably diluted by fresh air by that time, but, although they were nauseous, were not sufficient to overcome any one.  Still, the smell was terrible.

“I can’t stand it any longer,” cried Elaine.  “I’m going down there to see what has become of them.”

Aunt Tabby and Joshua tried to stop her, but she broke away from them and went down the ladder.  Rusty leaped down after her.

Joshua tried to follow, but Aunt Tabby held him back.  He would have gone, too, if she had not managed to strike the spring and shut the door, closing up the passageway.

Joshua got angry then.  “You are making a coward of me,” he cried, beating on the panel with the butt of his gun and struggling to open it.

He seemed unable to fathom the secret.

Elaine was now making her way as rapidly as she could through the tunnel, with Rusty beside her.

. . . . . . .

It was just as Long Sin had raised his knife that the sound of her footsteps alarmed him.

He paused and leaped to his feet.