“We will talk that over,” said Lieutenant Marbury.  “But, before I go into details, I want to give you another warning.  You must be very careful about—­”

A sudden knock on the door interrupted the speaker.

CHAPTER IX THE RECOIL CHECK

“Who is that?” asked Ned Newton, with a quick glance at his chum.

“I don’t know,” Tom answered.  “I left orders we weren’t to be disturbed unless it was something important.”

“May be something has happened,” suggested the navy officer, “another fire, perhaps, or a—­”

“It isn’t a fire,” Tom answered.  “The automatic alarm would be ringing before this in that case.”

The knock was repeated.  Tom went softly to the door and opened it quickly, to disclose, standing in the corridor, one of the messengers 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.