oat. "Waters"
one of his crew. The mystery of the photograph. Information
that others of the ill-fated _Investigator_ were on the
island. Reasons why certain tribes sacrificed white captives.
A new expedition planned. Determine to go overland. Making new
guns. Ammunition. The boys invite Ralph and Tom to visit the
cave. The surprise of the boys at the skeletons and the
treasure. Exploring the cave. A terrific roar. Alarmed.
Determine to investigate. Finding the Professor and John. The
surprise party.

XVII. THE WONDERFUL PORTABLE FORT

The boys ask John about his wonderful cave. The charted
treasure caves. Seeing the treasure in the cave on the hill
where the boys were rescued. An occupied cave. The medicine
men. The two entrances and the cross-shaped interior. How the
hoards were acquired. Piracy on the high seas. The gold and
silver of the world. The precious metals taken to Europe by
the Spaniards. Rushing work on the preparations. The gun
barrels. Chief showing the boys how to make and use the bows.
The disappearance of chief. The invention of a portable fort.
How it was made. Stocking the wagon. Experimenting with the
fort. Necessity as the mother of invention. The improvements
in the fort. A new suggestion. Using the fort as a raft.

XVIII. TRAILING A WARRING PARTY OF NATIVES

Weight and gravity. Acting in all directions. Proving the law
of universal gravitation. Drilling with the raft equipment.
Grinding barley flour. Making sleeping mattresses. The bustle
of final preparations. The good-by to their herd of yaks. The
march to the falls. John discovers a log in the drift and a
rope. The dense forest. Crossing the river to the south.
Finding a camp fire with fresh bones. Numerous traces of
inhabitants. A glowing fire. Following the trail. Trying to
catch them before night. Efforts to capture one as a means of
opening communication. Sighting the camp. Hurried
consultation. Surrounding the camp of the natives.

LIST OF ILLUSTRATIONS

"He poised his spear as he raised himself out of the water"
Frontispiece

"The main body of the savages broke for cover, but several,
more venturesome than the rest, sought to carry away
the chief"

"'Come out into the light,' and Harry and George each put