"I think you are right," said Lieutenant Marbury. "And, as regards the French, I might say they are not the only nation banded to obtain our secrets--yours and the government's!"

"But I thought the French and the English were friendly toward us!" Ned exclaimed.

"So they are, in a certain measure," the officer went on. "And Russia is, too. But, in all foreign countries there are two parties, the war party, as it might be called, and the peace element.

"But I might add that it is neither France, England, nor Russia that we must fear. It is a certain other great nation, which at present I will not name."

"And you think spies set this fire?"

"I certainly do."

"But what measures shall I adopt against this plot?" Tom asked.

"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 circ.u.mstances.

"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 a.s.sure 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 coming here, though, of course, I am anxious to inspect the new aerial warship you have constructed. So watch out for two things--your inventions, and, more than all, your life!"

"Do you really think they would do me bodily harm?" Tom asked, a trifle skeptical.

"I certainly do. These foreign spies are desperate. If they cannot secure the use of these inventions to their own country, they are determined not to let this country have the benefit of them."

"Well, I'll be careful," Tom promised. "I'm no more anxious than anyone else to run my head into danger, and I certainly don't want any of my shops or inventions destroyed. The fire in the red shed was as close as I want anything to come."

"That's right!" agreed Ned. "And, if there's anything I can do, Tom, don't hesitate to call on me."

"All right, old man. I won't forget. And now, perhaps, you would like to see the Mars," he said to the lieutenant.

"I certainly would," was the ready answer. "But hadn't you better see those men who are waiting to find out about positions here?"

"There's no hurry about them," Tom said. "We have applicants every day, and it's earlier than the hour when I usually see them. They can wait.

Now I want your opinion on my new craft. But, you must remember that it is not yet completed, and only recently did I begin to solve the problem of mounting the guns. So be a little easy with your criticisms."

Followed by Ned and Lieutenant Marbury, Tom led the way into the big airship shed. There, swaying about at its moorings, was the immense aerial warship. To Ned's eyes it looked complete enough, but, when Tom pointed out the various parts, and explained to the government officer how it was going to work, Ned understood that considerable yet remained to be done on it.

Tom showed his official guest how a new system of elevation and depressing rudders had been adopted, how a new type of propeller was to be used and indicated several other improvements. The lower, or cabin, part of the aircraft could be entered by mounting a short ladder from the ground, and Tom took Ned and Lieutenant Marbury through the engine-room and other compartments of the Mars.

"It certainly is most complete," the officer observed. "And when you get the guns mounted I shall be glad to make an official test. You understand," he went on, to Tom, "that we are vitally interested in the guns, since we now have many aircraft that can be used purely for scouting purposes. What we want is something for offense, a veritable naval terror of the seas."

"I understand," Tom answered. "And I am going to begin work on mounting the guns at once. I am going to use the Newton recoil check," he added.

"Ned, here, is responsible for that."

"Is that so?" asked the lieutenant, as Tom clapped his chum on the back.

"Yes, that's his invention."

"Oh, it isn't anything of the sort," Ned objected. "I just--"