o cautiously."

Just then the sailors, who had had to pack up and carry the camping-out
necessaries, appeared at the edge, and waited there watching the little
party as they slowly descended toward the shimmering pool, threading
their way in and out among the blocks of lava and pumice which lay in
their road.

Sir John led, with Jack close beside him, and the doctor and Ned
followed a little way behind, to their right. But they had not
descended a hundred yards before Sir John stopped short.

"No farther!" he said. "The heat is getting intense, and overpowering
gases are escaping from the ashes. We must go back, Jack."

"I suppose so," said the lad unwillingly. "We don't see the pool any
the better for being here either. Oh, look at that!"

There was no need to call attention, for all were startled by a sudden
report, and a glow of heat swept past their faces as a huge fountain of
fire suddenly played up some sixty or seventy feet like a geyser, and
fell back with a heavy splash, lower and lower, still playing till there
was only a slight eminence, as if bubbling in the middle of the pool.
Then it was perfectly level again, and a cloud of white smoke floated
away.

"That would have been grand by night," shouted the doctor.

"It was grand now," replied Sir John.

"Well, I think we had better turn back," said the doctor. "There is no
doubt about its being molten fire below here, for the heat gets fiercer.
Look."

He had been resting on the climbing pole he brought up with him, and
found that the end had gone down a couple of feet, while as he drew it
out the point was charred and smoking.

This induced Sir John and Jack to do the same, and theirs were burnt as
well.

"Yes, get back at once!" shouted Sir John in startled tones. "Quick,
all of you; our weight is acting upon the ashes, and they are gliding
down with us."

"Hi! look out below, gen'lemen," shouted Lenny from the edge, "that
there's bending like thin ice."

The warnings were none too soon, for as the pair turned sharply and
began to climb back, it was quite plain that though the blocks of stone
about lay or half floated upon the ash-covered surface, any further
weight was sufficient to produce a change, and before they had taken
many steps, one huge mass not twenty yards from Sir John was seen to be
sinking slowly, then faster and faster, and disappeared through the
ashes, which changed rapidly to a shimmering fluid, and sent forth a
te