s
employed about the shops.

"Well, what is it?" asked Tom a bit sharply.

"Oh, if you please, Mr. Swift," said the boy, "a man has applied for
work at the main office, and you know you left orders there that if any
machinists came along, we were to--"

"Oh, so I did," Tom exclaimed. "I had forgotten about that," he went on
to Lieutenant Marbury and Ned. "I am in need of helpers to rush through
the finishing touches on my aerial warship, and I left word, if any
applied, as they often do, coming here from other cities, that I wanted
to see them. How many are there?" Tom asked of the messenger.

"Two, this time. They both say they're good mechanics."

"That's what they all say," interposed Tom, with a smile. "But, though
they may be good mechanics in their own line, they need to have special
qualifications to work on airships. Tell them to wait, Rodney," Tom
went on to the lad, "and I'll see them presently."

As the boy went away, and Tom closed the door, he turned to Lieutenant
Marbury.

"You were about to give me another warning when that interruption came.
You might complete it now."

"Yes, it was another warning," spoke the officer, "and one I hope you
will heed. It concerns yourself, personally."

"Do you mean he is in danger?" asked Ned quickly.

"That's exactly what I do mean," was the prompt reply. "In danger of
personal injury, if not something worse."

Tom did not seem as alarmed as he might reasonably have been under the
circumstances.

"Danger, eh?" he repeated coolly. "On the part of whom?"

"That's just where I can't warn you," the officer replied. "I can only
give you that hint, and beg of you to be careful."

"Do you mean you are not allowed to tell?" asked Ned

"No, indeed; it isn't that!" the lieutenant hastened to assure the
young man. "I would gladly tell, if I knew. But this plot, like the
other one, directed against the inventions themselves, is so shrouded
in mystery that I cannot get to the bottom of it.

"Our Secret Service men have been working on it for some time, not only
in order to protect you, because of what you have done for the
government, but because Uncle Sam wishes to protect his own property,
especially the searchlight and the big cannon. But, though our agents
have worked hard, they have not been able to get any clues that would
put them on the right trail.

"So we can only warn you to be careful, and this I do in all
earnestness. That was part of my errand in c