appearance was due to accident. He acted as a sort of night
watchman, making a tour of the buildings, but he entered the shed where
the Mars was because, that day, he had left his knife in there, and
wanted to get it. Only for that he would not have gone in. When he
entered he surprised the four men.

Of course he attacked them at once, and they sprang at him. Then
ensued a terrific fight. Eradicate, arising to doctor his mule, as he
had said, heard the noise, and saw what was going on. He gave the
alarm.

"Well, Ned, any luck?" asked Tom, as his chum came in.

"No, they got away, Tom. I had a lot of your men out helping me search
the grounds, but it wasn't of much use."

"Particularly if you depended on some of my men," said Tom bitterly.

"What do you mean?"

"I mean that the place is filled with spies, Ned! But we will sift them
out in the morning. This has been a lucky night for me. It was touch
and go. Now, then, Koku, take these fellows and lock them up somewhere
until morning. Ned, you and I will remain on guard here the rest of the
night."

"I'm with you, Tom."

"Will you be a bit easy on us, considering what we told you?" asked
Kurdy.

"I'll do the best I can," said Tom, gently, making no promises.

The two captives were put in secure quarters, and the rest of the night
passed quietly. During the fight in the airship shed some machinery and
tools had been broken, but no great amount of damage was done. Tom and
Ned passed the remaining hours of darkness there.

A further search was made in the morning for the two conspirators who
had escaped, but no trace of them was found. Tom then realized why
Feldman was so anxious to be placed in the aeroplane department--it was
in order that he might have easier access to the Mars.

A technical charge was made against the two prisoners, sufficient to
hold them for some time. Then Tom devoted a day to weeding out the
suspected foreigners in his place. All the new men were discharged,
though some protested against this action.

"Probably I am hitting some of the innocent in punishing those who, if
they had the chance, would become guilty," Tom said to his chum, "but
it cannot be helped--I can't afford to take any chances."

The Mars was being put in shape for her first flight. The guns, fitted
with the recoil shock absorbers, were mounted, and Lieutenant Marbury
had returned to go aloft in the big aerial warship. He congratulated
Tom on discovering at lea