The giant’s 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 least one plot in time.

“But there may be more,” he warned the young inventor.  “You are not done with them yet.”

The Mars was floated out of her hangar, and made ready for an ascent.  Tom, Ned, Lieutenant Marbury, Mr. Damon, and several workmen were to be the first passengers.  Tom was busy going over the various parts to see that nothing had been forgotten.