But some strangers did manage to sneak into the works, though they were quickly detected and sent about their business.  Also, once or twice, small fires were discovered in outbuildings, but they were soon extinguished with little damage.  Extra vigilance was the watchword.

“And yet, with all my precautions, they may get me, or damage something,” declared Tom.  “It is very annoying!”

“It is,” agreed Ned, “and we must be doubly on the lookout.”

So impressed was Ned with the necessity for caution that he arranged to take his vacation at this time, so as 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.