tion, as it moves
through the air. Metals have not the same density on all sides and this
is particularly true of molded balls. As a result, when projected from
the gun, the heaviest side has a tendency to divert the ball and make it
more or less erratic in its motion, and, therefore, inaccurate. The
spiral motion has the effect of minimizing this difficulty. The cavity
formed at the rear of the projectile was devised particularly to cause
the thin lip of the bullet to be driven into the grooves formed in the
gun barrel, and by that means the boring motion was transmitted to the
bullet."

"But as we have no means of rifling our guns, there will be no necessity
of putting the cavity in the rear end of our bullets."

"We must have the cavity there, by all means."

"What for?"

"Simply because we do not want the bullet to turn around and travel end
over end after it leaves the gun."

"How does the cavity prevent this?"

"You have probably forgotten that a body travels through the air with
its heaviest end foremost. When a cavity is made it is lighter at that
end. Without the cavity, if the forward end is pointed, it will, on
leaving the gun, turn around and go through the air with the blunt end
foremost."

The molds were made, as directed, of a hard brass composition, and when
they were ready to cast them the Professor cautioned against making any
castings with the molds in any position except upright, so that any
inequality in the density of the metal would not form itself on the side
of the cast article.

[Illustration: _Fig. 11. THE BULLET_]

Quite a time had now elapsed since the last exploration of the cave
beyond Observation Hill. The Professor had spoken about it on several
occasions. For some reason he was intensely interested in doing that. In
fact, he appeared to be more concerned about that than any other of the
unknown things about the island.

The boys could not understand this peculiarity. He had never been
questioned on the subject directly, but it was evident he had a reason
for this predominating wish to continue the exploration.

George was just as much interested, but, as the sequel will show, for an
entirely different reason. Ever restless, and always willing to
undertake anything which promised to delve into hidden things, he
approached the Professor one day with the suggestion about the cave.

"I think we ought to take one day off and go to the cave."

The Professor was interested at