_]

The following day Harry proposed a trip over to the cave which George
had discovered at the time he found the big air pocket that shot water
and spray out into the ocean.

"I am interested," he said, "in seeing the air pocket George spoke
about, and we might as well take our vacation to-day."

"As the distance is not great we need not bother about taking our
luncheon along." And off they started, with the Professor bringing up
the rear.

The course was first to Observation Hill, where they had erected a new
and a larger pole than the one which had so mysteriously disappeared two
months before. On every such visit it was the duty of the one who made
the trip to scan the horizon in every direction.

It must not be inferred that because the boys were engaged in work which
was all-absorbing that they had no thoughts of home, and had given up
all hopes of a final rescue. If they could only let the people at home
know they were alive and happy--that is, in learning the secrets of
nature and in the exciting exploring trips, they would be satisfied.

They had no time to think of these things when they were at work, but in
the night their thoughts often wandered back again to their homes and
friends. Could they be blamed for that homesick feeling which came over
them?

"Now lead the way, George; we want to see some more of your mysteries.
Isn't it fortunate that the tide is out? It will give us a good chance
to investigate."

The path which George had taken was farther to the right, but as he was
in a hurry to get down as quickly as possible he followed a course,
which was much steeper, with Harry and the Professor close on his heels.

When the bottom was reached there was no sign of a pocket, or a cave, or
anything of that kind. George was very much annoyed. He could not be
mistaken in the position, as it was directly to the right of Observation
Hill, and not three hundred feet from the spot where Harry had landed on
his first trip to the island.

"It seems to me, George, we are too low down. At high tide this place is
all covered with water. It must he higher up in the cliff sides."

Harry scrambled up again part way, and shortly afterwards cried out: "I
have found something here; come up at once."

He was distant not more than twenty feet above them, but so rugged were
the cliffs that the opening was entirely hidden from below.

[Illustration: _Fig. 18. Cave Entrance._]

"This is an example of the corrosi