nd the sulphur is wonderful. These will do."

A tapping noise followed from about fifty feet away; then the fall of a
piece or two of stone, followed by a low hissing sound.

"Hear the steam escaping, Mark?" said the captain. "Ah, that's a good
bit, as far as I can see. Come, major."

There was no answer.

"O'Halloran!" cried the captain, and his voice went echoing away into
the distance, the name being partly repeated far in, as if whispered,
mockingly by some strange denizen of the cavern.

"Major O'Halloran!" shouted Mark excitedly. "What's that?"

"What, my lad?" cried the captain.

"That curious choking sour smell. Ah!"

"Back, boy, for your life!" cried the captain, snatching at his son's
arm and half dragging him towards where the cave was open to the sky.
"Are you all right?"

"Yes, yes, father," panted Mark, who was coughing violently. "Is--is--
Oh, father! the major."

The captain had taken a handkerchief from his pocket and loosely doubled
it, and this he tied over his mouth and nostrils.

"Hold my gun, Mark," he whispered; and then hoarsely, as if to himself,
"I can't leave him like that, come what may."

He paused for a moment to breathe hard and thoroughly inflate his lungs,
and then, regardless of the risk of falling, he ran rapidly in, while
Mark stood horror-stricken listening to his retiring footsteps.

His next act saved the lives of the two men.

"Small!--Widgeon!" he cried. "Here, quick!"

The two men ran to his side, ready to help.

"My father has gone in to help the major. As soon--as he comes--near
enough--go and help."

The men stood listening; and then, as they heard the coming steps, made
a dart in, but returned.

"You can't breathe. It chokes you," cried Billy Widgeon.

"Take a long mouthful, my lad, and hold your breath," growled the
boatswain. "Ha, he's down! Come on!"

CHAPTER THIRTY SIX.

HOW MARK AND BILLY WIDGEON WENT WRONG.

Mark did as the others did; inflated his lungs and rushed into the
darkness, till they nearly fell over the captain; and then how it was
done the lad hardly knew, but the two insensible men were dragged out to
where there was pure air to breathe, and the rescuers sank down beside
them, panting and exhausted. "Too late!" groaned Mark.

"Not we, my lad," growled Small. "I know. It's bad gas."

"It's the sulphur," cried Mark piteously.

"Well, aren't that bad gas? I know. They're just the same as if they
was drowned,