And that was done while, under Tom’s direction, his men took the damaged motor apart, with a view to repairing it.

“What was it, Tom?” asked Ned, coming back to join his chum, after George Ventor, the assistant pilot, had taken charge of the wheel.

“I don’t exactly know, Ned,” was the answer.  “But I feel certain that some of my enemies came aboard here and worked this mischief.”

“Your enemies came aboard?”

“Yes, and they must be here now.  The placing of that copper bar proves it.”

“Then let’s make a search and find them, Tom.  It must be some of those foreign spies.”

“Just what I think.”

But a more careful search of the craft than the one Tom had casually made revealed the presence of no one.  All the crew and helpers were accounted for, and, as they had been in Tom’s service for some time, they were beyond suspicion.  Yet the fact remained that a seemingly human agency had acted to put the main motor out of commission.  Tom could not understand it.

“Well, it sure is queer,” observed Ned, as the search came to nothing.

“It’s worse than queer,” declared Tom, “it’s alarming!  I don’t know when I’ll be safe if we have ghosts aboard.”

“Ghosts?” repeated Ned.

“Well, when we can’t find out who put that bar in place I might as well admit it was a ghost,” spoke Tom.  “Certainly, if it was done by a man, he didn’t jump overboard after doing it, and he isn’t here now.  It sure is queer!”

Ned agreed with the last statement, at any rate.

In due time the Mars, having fought her way above the storm, came over Shopton, and then, the wind having somewhat died out, she fought her way down, and, after no little trouble, was housed in the hangar.

Tom cautioned his friends and workmen to say nothing to his father about the mysterious happening on board.

“I’ll just tell him we had a slight accident, and let it go at that,” Tom decided.  “No use in causing him worry.”

“But what are you going to do about it?” asked Ned.

“I’m going to keep careful watch over the aerial warship, at any rate,” 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.