ed cheerily. "Feeling yourself again--eh? Well, that's
good. And now we'll put an end to this nonsense."

"I certainly smell sulphur," said Venning; "and what is that blue
streak there?" He took a step towards the smooth cone. "It is
sulphur!" he cried. "See, it's burning."

Mr. Hume stepped to his side, and saw the unmistakable blue flame
given off by burning sulphur, while a whiff of the fumes made him
choke.

"You're right; it's a mass of sulphur. The burning wad front the
cartridge must have set it alight." He sliced off the burning patch
with his knife. "We don't want to be fumigated, or to die of
suffocation. Now, if you feel strong enough, we'll explore the
cave."

"Is it safe? I mean, are there any chasms?"

"Smooth as a floor. Keep close by me."

They examined the cavern carefully by means of the strange lantern
filled with fungus, and Mr. Hume halted by the second exit.

"This is where they enter," he said, "and I think our best plan will
be to build a fire in the mouth. We should then have the advantage
over them, as we should see them once they came into the
reflection."

They set about collecting wood, when Venning had a thought.

"Which way does the draught set in the tunnel--away from the cavern
or into it?"

"Why?"

"Because, if the current of air blows away from us, we can easily
keep them out."

"It blows from the cave into the tunnel. I found that out before."

"Then we have got them, whoever they are. Make the fire in the
passage, pile up blocks of this sulphur on the inner side, and the
wind will carry the fumes down into the tunnel."

"A splendid plan," said Mr. Hume; and very soon it was carded out, a
couple of shots being fired into the dark passage as a warning to
the enemy to keep off. As the flames caught the sulphur, a thick
smoke rolled away. "That will stop them; and now we can wait in
peace till the morning."

The rest of the night passed for them in peace as far as their
assailants were concerned, but the chilling damp of the vaults got
into their bones, and Venning was pinched and shivering when the
first ray of sunshine struck slanting down through the mist-laden
atmosphere, bringing with it a message of hope from the bright outer
world.

CHAPTER XXIII

THROUGH THE VAULTS

They shared the goats'-milk remaining in the calabash, and at once
entered the first exit, that was to lead them, as they ardently
hoped, into the warmth and light of the day. Venning w