re
going to join their mothers and their aunt for the summer vacation.
What the boys were going to do after the encampment came to an end,
had not yet been decided.

Sunday was passed quietly by the Rover boys and their chums. Religious
services were held in the open air, and were attended by nearly all of
the cadets. In the afternoon Fred and Jack took a walk, accompanied by
Gif and Spouter, the twins remaining behind to write some letters.

The walk took the cadets to the foot of the cliff nearest to their
camp, and in rounding this close to the water front they made a
somewhat interesting discovery. They came to quite an opening among
the rocks, and, going inside, found themselves in a regular cavern,
ten feet high in some places and half again as broad. In the rear was
a smaller opening, leading downward and filled at the bay level with
water.

"Who would think there would be a cave like this around here!"
remarked Jack.

"Good place to come to in case of a storm," said Spouter. "A fellow
could play Robinson Crusoe if he wanted to."

By the aid of some matches the cadets examined the cavern, but found
nothing in it of value. At some time in the past birds had nested
there, but that was all. They were just ready to leave when Jack
suddenly put up his hand.

"Listen!" he said. "Am I mistaken, or do I hear something?"

All did as he commanded, and from a great distance a faint tapping
reached their ears. Then came a series of muffled explosions and a
clanking as of chains.

"That's something like the noise Randy said he heard when he was on
sentry duty," remarked Fred. "Don't you remember how worked up he was
over it?"

"What do you suppose it is, and where do you suppose it comes from?"
put in Gif.

No one could answer those questions. All was now silent, but presently
they heard another series of explosions, and then the tapping
continued steadily for several minutes. Then, however, the sounds died
away.

"That's got me guessing," declared Jack, after the crowd had left the
cavern. "We'll have to tell Randy about this, and maybe we had better
tell Captain Dale, too."

Monday proved an exceedingly sultry day. The thermometer went so high
that drills and exercises in the sun were all curtailed.

"Looks to me as if this was a weather breeder," remarked the young
captain to the others.

"Well, I don't care what it does, if only it cools off," grumbled
Randy. "Why, I feel as if I was living in a bake