nce Tom had kept a balloon, but which was now given over to
his several airships. In front of it was a big, level grassy
space, needed to enable the aircraft to get a "running start"
before they could mount into the clouds.

"See anything of Koku?" whispered Ned.

"No," answered Tom, in the same cautious voice. "I guess he must
be hiding--"

"There he goes now!" hissed Ned, pointing to a big figure that was
approaching the hangar. It was undoubtedly that of the giant, and
he could be seen, in the dim light, stalking cautiously along.

"I wonder where the uninvited guests are?" asked Tom.

"Probably in the airship shed," answered Ned. "Koku was after them
as soon as the alarm went off, and they couldn't have gotten away.
They must be inside there yet. But what can their game be?"

"It's hard to say," admitted Tom. "They may be trying to get
something belonging to me, or they may imagine they can pick up
some valuable secrets. Or they may--" He stopped suddenly, and
then exclaimed:

"Come on, Ned! They're after one of the airships! That's it! My
big biplane is all ready to start, and they can get it in motion
inside of a few seconds. Oh, why didn't I hurry?" he added,
bitterly.

But the hangar was still some distance away, and it would take two
or three minutes of running to reach it.

Meanwhile, and at the instant Tom had his thought of the possible
theft of his biggest aircraft, something happened.

The doors of the shed were suddenly thrown open, and the two boys
could see the large airship being wheeled out. The hazy light of
the moon behind the clouds shone on the expanse of white planes,
and on the fish-tail rudder, one of Tom's latest ideas.

"Hey, there!" cried Tom, warningly.

"Leave that alone!" yelled Ned.

"Koku! Koku!" shouted Tom, shrilly. "Get after those fellows!"

"Me get!" boomed out the giant, in his deep voice.

He had been standing near the entrance to the hangar, probably
waiting for developments, and watching for the arrival of Tom and
Ned. The big form was seen to leap forward, and then several dark
shadows swarmed from around the airship, and were seen to fling
themselves upon the giant.

"That's a fight!" cried Ned. "They're attacking him!"

"Koku can take care of himself!" murmured Tom. "But come on. I
don't see what their game is."

He understood a moment later, however, for while several of the
midnight visitors were engaged in a hand-to-hand tussle with the
giant ther