now
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.

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