to be on hand to help his chum,
if necessary.

The Mars was nearing completion. The repaired motor was better than
ever, and everything was in shape for the final test. Mr. Damon was
persuaded to go along, and Koku was to be taken, as well as the two
government officials.

The night before the trip the guards about the airship shed were
doubled, and Tom made two visits to the place before midnight. But
there was no alarm.

Consequently, when the Mars started off on her final test, it was
thought that all danger from the spies was over.

"She certainly is a beauty," said Captain Warner, as the big craft shot
upward. "I shall be interested in seeing how she stands gun fire,
though."

"Oh, she'll stand it," declared Lieutenant Marbury. The trip was to
consume several days of continuous flying, to test the engines. A large
supply of food and ammunition was aboard.

It was after supper of the first day out, and our friends were seated
in the main cabin laying out a program for the next day, when sudden
yells came from a part of the motor cabin devoted to storage. Koku, who
had been sent to get out a barrel of oil, was heard to shout.

"What's up?" asked Tom, starting to his feet. He was answered almost at
once by more yells.

"Oh, Master! Come quickly!" cried the giant. "There are many men here.
There are stowaways aboard!"

CHAPTER XXI

PRISONERS

For a moment, after hearing Koku's reply, neither Tom nor his friends
spoke. Then Ned, in a dazed sort of way, repeated:

"Stowaways!"

"Bless my--" began Mr. Damon, but that was as far as he got.

From the engine compartment, back of the amidship cabin, came a sound
of cries and heavy blows. The yells of Koku could be heard above those
of the others.

Then the door of the cabin where Tom Swift and his friends were was
suddenly burst open, and seven or eight men threw themselves within.
They were led by a man with a small, dark mustache and a little tuft of
whiskers on his chin--an imperial. He looked the typical Frenchman, and
his words, snapped out, bore out that belief.

What he said was in French, as Tom understood, though he knew little of
that language. Also, what the Frenchman said produced an immediate
result, for the men following him sprang at our friends with
overwhelming fierceness.

Before Tom, Ned, Captain Warner, Mr. Damon or Lieutenant Marbury could
grasp any weapon with which to defend themselves, had their intentions
been to do so