declared Tom. "If there's a hidden enemy aboard, I'll starve him out."

Accordingly, a guard, under the direction of Koku, was posted about the big shed, but nothing came of it. No stranger was observed to sneak out of the ship, after it had been deserted by the crew. The mystery seemed deeper than ever.

It took nearly a week to repair the big motor, and, during this time, Tom put some improvements on the airship, and added the finishing touches.

He was getting it ready for the final government test, for the authorities in Washington had sent word that they would have Captain Warner, in addition to Lieutenant Marbury, make the final inspection and write a report.

Meanwhile several little things occurred to annoy Tom. He was besieged with applications from new men who wanted to work, and many of these men seemed to be foreigners. Tom was sure they were either spies of some European nations, or the agents of spies, and they got no further than the outer gate.

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.

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, they were seized.

Against such odds little could be done, though our friends did not give up without a struggle.

"What does this mean?" angrily demanded Tom Swift. "Who are you? What are you doing aboard my craft? Who are--"

His words were lost in smothered tones, for one of his a.s.sailants put a heavy cloth over his mouth, and tied it there, gagging him. Another man, with a quick motion, whipped a rope about Tom's hands and feet, and he was soon securely bound.

In like manner the others were treated, and, despite the struggles of Mr. Damon, the two government men and Ned, they were soon put in a position where they could do nothing--helplessly bound, and laid on a bench in the main cabin, staring blankly up at the ceiling. Each one was gagged so effectively that he could not utter more than a faint moan.

Of the riot of thoughts that ran through the heads of each one, I leave you to imagine.

What did it all mean? Where had the strange men come from? What did they mean by thus a.s.saulting Tom and his companions? And what had happened to the others of the crew--Koku, Jerry Mound, the engineer, and George Ventor, the a.s.sistant pilot?

These were only a few of the questions Tom asked himself, as he lay there, bound and helpless. Doubtless Mr. Damon and the others were asking themselves similar questions.

One thing was certain--whatever the stowaways, as Koku had called them, had done, they had not neglected the Mars, for she was running along at about the same speed, though in what direction Tom could not tell. He strained to get a view of the compa.s.s on the forward wall of the cabin, but he could not see it.

It had been a rough-and-tumble fight, by which our friends were made prisoners, but no one seemed to have been seriously, or even slightly, hurt. The invaders, under the leadership of the Frenchman, were rather ruffled, but that was all.

Pantingly they stood in line, surveying their captives, while the man with the mustache and imperial smiled in a rather superior fashion at the row of bound ones. He spoke in his own tongue to the men, who, with the exception of one, filed out, going, as Tom and the others could note, to the engine-room in the rear.

"I hope I have not had to hurt any of you," the Frenchman observed, with sarcastic politeness. "I regret the necessity that caused me to do this, but, believe me, it was unavoidable."

He spoke with some accent, and Tom at once decided this was the same man who had once approached Eradicate. He also recognized him as the man he had seen in the woods the day of the outing.

"He's one of the foreign spies," thought Tom "and he's got us and the ship, too. They were too many for us!"

Tom's anxiety to speak, to hold some converse with the captor, was so obvious that the Frenchman said:

"I am going to treat you as well as I can under the circ.u.mstances. You and your other friends, who are also made prisoners, will be allowed to be together, and then you can talk to your hearts' content."

The other man, who had remained with the evident ringleader of the stowaways, asked a question, in French, and he used the name La Foy.

"Ah!" thought Tom. "This is the leader of the gang that attacked Koku in the shop that night. They have been waiting their chance, and now they have made good. But where did they come from? Could they have boarded us from some other airship?"

Yet, as Tom asked himself that question, he knew it could hardly have been possible. The men must have been in hiding on his own craft, they must have been, as Koku had cried out--stowaways--and have come out at a preconcerted signal to overpower the aviators.

"If you will but have patience a little longer," went on La Foy, for that was evidently the name of the leader, "you will all be together.

We are just considering where best to put you so that you will not suffer too much. It is quite a problem to deal with so many prisoners, but we have no choice."

The two Frenchmen conversed rapidly in their own language for a few minutes, and then there came into the cabin another of the men who had helped overpower Tom and his friends. What he told La Foy seemed to give that individual satisfaction, for he smiled.

"We are going to put you all together in the largest storeroom, which is partly empty," La Foy said. "There you will be given food and drink, and treated as well as possible under the circ.u.mstances. You will also be unbound, and may converse among yourselves. I need hardly point out," he went on, "that calling for help will be useless. We are a mile or so in the air, and have no intention of descending," and he smiled mockingly.