"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, Mn 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 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 ben 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--"

"Yes, he just happened to solve the problem for me!" interrupted Tom, as he told the story of the door-spring.

"A good idea!" commented Lieutenant Marbury.

Tom then briefly described the principle on which his aerial warship would work, explaining how the lifting gas would raise it, with its load of crew, guns and explosives, high into the air; how it could then be sent ahead, backward, to either side, or around in a circle, by means of the propellers and the rudders, and how it could be raised or lowered, either by rudders or by forcing more gas into the lifting bags, or by letting some of the vapor out.

And, while this was being done by the pilot or captain in charge, the crew could be manning the guns with which hostile airships would be attacked, and bombs dropped on the forts or battleships of the enemy.

"It seems very complete," observed the lieutenant. "I shall be glad when I can give it an official test."

"Which ought to be in about a week," Tom said. "Meanwhile I shall be glad if you will be my guest here."

And so that was arranged.

Leaving Ned and the lieutenant to entertain each other, Tom went to see the mechanics who had applied for places. He found them satisfactory and engaged them. One of them had worked for him before. The other was a stranger, but he had been employed in a large aeroplane factory, and brought good recommendations.

There followed busy days at the Swift plant, and work was pushed on the aerial warship. The hardest task was the mounting of the guns, and equipping them with the recoil check, without which it would be impossible to fire them with the craft sailing through the air.

But finally one of the big guns, and two of the smaller ones were in place, with the apparatus designed to reduce the recoil shock, and then Tom decided to have a test of the Mars.

"Up in the air, do you mean?" asked Ned, who was spending all his spare time with his chum.

"Well, a little way up in the air, at least," Tom answered. "I'll make a sort of captive balloon of my craft, and see how she behaves. I don't want to take too many chances with that new recoil check, though it seems to work perfectly in theory."

The day came when, for the first time, the Mars was to come out of the big shed where she had been constructed. The craft was not completed for a flight as yet, but could be made so in a few days, with rush work. The roof of the great shed slid back, and the big envelope containing the buoyant gas rose slowly upward. There was a cry of surprise from the many workmen in the yard, as they saw, most of them for the first time, the wonderful new craft. It did not go up very high, being held in place with anchor ropes.

The sun glistened on the bright brass and nickel parts, and glinted from the gleaming barrels of the quick-firing guns.

"That's enough!" Tom called to the men below, who were paying out the ropes from the windlasses. "Hold her there."

Tom, Ned, Lieutenant Marbury and Mr. Damon were aboard the captive Mars.

Looking about, to see that all was in readiness, Tom gave orders to load the guns, blank charges being used, of course.

The recoil apparatus was in place, and it now remained to see if it would do the work for which it was designed.

"All ready?" asked the young inventor.

"Bless my accident insurance policy!" exclaimed Mr. Damon. "I'm as ready as ever I shall be, Tom. Let 'em go!"

"Hold fast!" cried Tom, as he prepared to press the electrical switch which would set off the guns. Ned and Lieutenant Marbury stood near the indicators to notice how much of the recoil would be neutralized by the check apparatus.

"Here we go!" cried the young inventor, and, at the same moment, from down below on the ground, came a warning cry:

"Don't shoot, Massa Tom. Don't shoot! Mah mule, Boomerang--"

But Eradicate had spoken too late. Tom pressed the switch; there was a deafening crash, a spurt of flame, and then followed wild cries and confused shouts, while the echoes of the reports rolled about the hills surrounding Shopton.