"Can't go wrong," said the captain, "if we keep within touch of the
sea."

"Hark! hark! What's that?" cried Jack.

There was no need for him to speak, for every one had stopped short, and
was listening intently to the echoes which ran reverberating along a
valley, after what seemed to have been the firing of a heavy gun.

CHAPTER TWENTY.

FLOATING BLACKS.

"Is that Mr Bartlett firing one of the yacht's guns for a recall?" said
Sir John.

"No, sir; they could not make a noise like that."

"It could not be thunder," said Jack.

"Oh yes, it could," said the captain. "I've heard short sharp cracks
like that often out here, but I don't think that was thunder."

"Must have been," cried the doctor. "Why, I saw the flash. There!
Look!"

A bright light suddenly appeared from somewhere inland, followed at a
few seconds' interval by a heavy detonation, exactly like the firing of
a great gun.

"Now what do you say, Captain Bradleigh?" cried Sir John.

"That it is what I thought at first, sir. The mountain yonder is firing
a shot or two. If we had been out at sea, I dare say we should have
seen a great red-hot stone flying up and falling back."

"Then there is going to be an eruption," cried Jack in excited tones.

"That does not follow at all. Some of these volcanoes do no more
perhaps than make a rumbling, and send up a few red-hot stones now and
then. Forward now, gentlemen. Close up, my lads, and follow two and
two."

The mountain, if the captain was right, made no farther sign, and now
began the most interesting part of the journey. With the exception of
having to be careful not to stumble over the blocks of coral limestone
which lay here and there in their road, it was easy walking in spite of
the darkness, while this latter was modified by the brilliant stars
overhead, the dazzling scintillations of the fireflies, which flittered
out whenever any of the bushes which fringed the sands were approached--
and the soft, luminous, oil-like appearance at the edge of the lake.

But the sand was soft, and it seemed to Jack as if they would never
reach the boat.

In the darkness Edward edged up close to his young master, and
whispered--

"Tired, Mr Jack?"

"Dreadfully."

"Makes one's legs feel as if they were made of cast lead."

"Or stones," said Jack.

"Well, p'r'aps you're right, sir. Stone is more like it. Let me carry
your gun, sir. Seems to get heavier every step, don't it?"

"Y