his hole leads to the centre of the earth?" asked Mark.
"I've read somewhere, that the earth is hollow."

"Some scientists believe it," commented the professor. "This looks like
a big enough hole to lead clear through to China. Hark, you can hear the
roar of the water now."

They listened, and the wind brought to them the sound of the sea pouring
down into the unfathomable depths.

"Let's throw a big rock down," suggested Jack. "Maybe we can hear it
strike bottom."

With the aid of Mark he cast a big boulder down into the depths. They
listened intently, but not the slightest sound echoed back.

"I guess the bottom is too far away for you to hear the stone land,"
said the professor.

CHAPTER XVI

THE GHOST AGAIN

They spent some time looking down into the hole. The masses of vapor, or
clouds, rolled and swirled hundreds of feet below them, but never broke
sufficiently to allow of a clear sight.

"Well, we had better go back to the ship," remarked the professor. "We
must continue our voyage."

They were soon on the _Porpoise_ and steaming away from the strange
island, the inventor deciding to get far off out of the influence of the
terrible waterfall.

The night was, as usual, spent with the ship slowly steaming ahead on
the surface of the water. It was getting on toward twelve o'clock and
Washington had the watch. He was to be relieved by Jack.

The latter had been awakened by the alarm clock at the head of his bed,
which time-piece he had set to arouse him so that he might take
Washington's place. Jack was just getting the sleep from his eyes by a
vigorous rubbing when he heard a loud yell.

"Land a' Massy!" cried Washington from the deck above. "I's goin' t' die
suah! De ghostess am after me ag'in!"

Without waiting to dress, Jack sprang up the ladder and was soon out on
the deck. He saw Washington kneeling down in front of the conning tower
door while, at the after end of the deck, was a mysterious white object;
the same strange shape that had been observed before.

"I'm going to solve this puzzle!" exclaimed Jack to himself as he made a
dive toward the object in white. "This ghost business will have to
stop!"

But, unfortunately for his plans, his foot slipped on the smooth steel
deck, and he went down in a heap. When he got up the ghost was nowhere
to be seen.

Washington, however, was still kneeling down and praying to be spared
from the attack of the midnight visitor. Jack limped ove