"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.
"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—"
"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.
"What was the matter down there?"
"Was anyone hurt?"
"Don't forget to look at those pressure gauges!"
"Bless my ham sandwich!"
Thus came the cries from those aboard the captive Mars. Ned, Lieutenant Marbury and Tom had called out in the order named. And, of course, I do not need to tell you what remark Mr. Damon made. Tom glanced toward where Ned and the government man stood, and saw that they had made notes of the pressure recorded on the recoil checks directly after the guns were fired. Mr. Damon, blessing innumerable objects under his breath, was looking over the side of the rail to discover the cause of the commotion and cries of warning from below.
"I don't believe it was anything serious, Tom," said the odd man. "No one seems to be hurt." "Look at Eradicate!" suddenly exclaimed Ned.
"And his mule! I guess that's what the trouble was, Tom!"
They looked to where the young bank employee pointed, and saw the old colored man, seated on the seat of his ramshackle wagon, doing his best to pull down to a walk the big galloping mule, which was dragging the vehicle around in a circle.
"Whoa, dere!" Eradicate was shouting, as he pulled on the lines. "Whoa, dere! Dat's jest laik yo', Boomerang, t' run when dere ain't no call fo' it, nohow! Ef I done wanted yo' t' git a move on, yo'd lay down 'side de road an' go to sleep. Whoa, now!"
But the noise of the shots had evidently frightened the long-eared animal, and he was in no mood for stopping, now that he had once started. It was not until some of the workmen ran out from the group where they had gathered to watch Tom's test, and got in front of Boomerang, that they succeeded in bringing him to a halt.
Eradicate climbed slowly down from the seat, and limped around until he stood in front of his pet.
"Yo'—yo're a nice one, ain't yo'?" he demanded in sarcastic tones. "Yo' done enough runnin' in a few minutes fo' a week ob Sundays, an' now I won't be able t' git a move out ob ye! I'se ashamed ob yo', dat's what I is! Puffickly ashamed ob yo'. Go 'long, now, an' yo' won't git no oats dish yeah day! No sah!" and, highly indignant, Eradicate led the now slowly-ambling mule off to the stable.
"I won't shoot again until you have him shut up, Rad!" laughed Tom. "I didn't know you were so close when I set off those guns."
"Dat's all right, Massa Tom," was the reply. "I done called t' you t' wait, but yo' didn't heah me, I 'spects. But it doan't mattah, now. Shoot all yo' laik, Boomerang won't run any mo' dis week. He done runned his laigs off now. Shoot away!"
But Tom was not quite ready to do this. He wanted to see what effect the first shots had had on his aerial warship, and to learn whether or not the newly devised recoil check had done what was expected of it.
"No more shooting right away," called the young inventor. "I want to see how we made out with the first round. How did she check up, Ned?"
"Fine, as far as I can tell."
"Yes, indeed," added Lieutenant Marbury. "The recoil was hardly noticeable, though, of course, with the full battery of guns in use, it might be more so."
"I hope not," answered Tom. "I haven't used the full strength of the recoil check yet. I can tune it up more, and when I do, and when I have it attached to all the guns, big and little, I think we'll do the trick. But now for a harder test."
The rest of that day was spent in trying out the guns, firing them with practice and service charges, though none of the shells used contained projectiles. It would not have been possible to shoot these, with the Mars held in place in the midst of Tom's factory buildings.
"Well, is she a success, Tom?" asked Ned, when the experimenting was over for the time being.
"I think I can say so—yes," was the answer, with a questioning look at the officer.
"Indeed it is—a great success! We must give the Newton shock absorber due credit."
Ned blushed with pleasure.
"It was only my suggestion," he said. "Tom worked it all out."
"But I needed the suggestion to start with," the young inventor replied.
"Of course something may develop when you take your craft high in the air, and discharge the guns there," said the lieutenant. "In a rarefied atmosphere the recoil check may not be as effective as at the earth's surface. But, in such case doubtless, you can increase the strength of the springs and the hydrostatic valves."
"Yes, I counted on that," Tom explained. "I shall have to work out that formula, though, and be ready for it. But, on the whole, I am pretty well satisfied."
"And indeed you may well feel that way," commented the government official.
The Mars was hauled back into the shed, and the roof slid shut over the craft. Much yet remained to do on it, but now that Tom was sure the important item of armament was taken care of, he could devote his entire time to the finishing touches.
As his plant was working on several other pieces of machinery, some of it for the United States Government, and some designed for his own use, Tom found himself obliged to hire several new hands. An advertisement in a New York newspaper brought a large number of replies, and for a day or two Tom was kept busy sifting out the least desirable, and arranging to see those whose answers showed they knew something of the business requirements.
Meanwhile Lieutenant Marbury remained as Tom's guest, and was helpful in making suggestions that would enable the young inventor to meet the government's requirements.
"I'd like, also, to get on the track of those spies who, I am sure, wish to do you harm," said the lieutenant, "but clues seem to be scarce around here."
"They are, indeed," agreed Tom. "I guess the way in which we handled that fire in the red shed sort of discouraged them."
Lieutenant Marbury shook his head.
"They're not so easily discouraged as that," he remarked. "And, with the situation in Europe growing more acute every day, I am afraid some of those foreigners will take desperate measures to gain their ends."
"What particular ends do you mean?"
"Well, I think they will either try to so injure you that you will not be able to finish this aerial warship, or they will damage the craft itself, steal your plans, or damage some of your other inventions."
"But what object would they have in doing such a thing?" Tom wanted to know. "How would that help France, Germany or Russia, to do me an injury?"
"They are seeking to strike at the United States through you," was the answer. "They don't want Uncle Sam to have such formidable weapons as your great searchlight, the giant cannon, or this new warship of the clouds."
"But why not, as long as the United States does not intend to go to war with any of the foreign nations?" Tom inquired.
"No, it is true we do not intend to go to war with any of the conflicting European nations," admitted Lieutenant Marbury, "but you have no idea how jealous each of those foreign nations is of all the others. Each one fears that the United States will cease to be neutral, and will aid one or the other."
"Oh, so that's it?" exclaimed Tom.
"Yes, each nation, which may, at a moments notice, be drawn into a war with one or more rival nations, fears that we may throw in our lot with its enemies."
"And, to prevent that, they want to destroy some of my inventions?" asked Tom.
"That's the way I believe it will work out. So you must be careful, especially since you have taken on so many new men."
"That's so," agreed the young inventor. "I have had to engage more strangers than ever before, for I am anxious to get the Mars finished and give it a good test. And, now that you have mentioned it, there are some of those men of whom I am a bit suspicious."
"Have they done anything to make you feel that way?" asked the lieutenant.
"Well, not exactly; it is more their bearing, and the manner in which they go about the works. I must keep my eye on them, for it takes only a few discontented men to spoil a whole shop full. I will be on my guard."
"And not only about your new airship and other inventions," said the officer, "but about yourself, personally. Will you do that?"
"Yes, though I don't imagine anything like that will happen."
"Well, be on your guard, at all events," warned Lieutenant Marbury.
As Tom had said, he had been obliged to hire a number of new men. Some of these were machinists who had worked for him, or his father, on previous occasions, and, when tasks were few, had been dismissed, to go to other shops. These men, Tom felt sure, could be relied upon.
But there were a number of others, from New York, and other large cities, of whom Tom was not so sure.
"You have more foreigners than I ever knew you to hire before, Tom," his father said to him one day, coming back from a tour of the shops.
"Yes, I have quite a number," Tom admitted. "But they are all good workmen. They stood the test."
"Yes, some of them are too good," observed the older inventor. "I saw one of them making up a small motor the other day, and he was winding the armature a new way. I spoke to him about it, and he tried to prove that his way was an improvement on yours. Why, he'd have had it short-circuited in no time if I hadn't stopped him."
"Is that so?" asked Tom. "That is news to me. I must look into this."
"Are any of the new men employed on the Mars?" Mr. Swift asked.
"No, not yet, but I shall have to shift some there from other work I think, in order to get finished on time."
"Well, they will bear watching I think," his father said.
"Why, have you seen anything—do you—" began the young man, for Mr. Swift had not been told of the suspicions of the lieutenant.
"Oh, it isn't anything special," the older inventor went on. "Only I wouldn't let a man I didn't know much about get too much knowledge of my latest invention."
"I won't, Dad. Thanks for telling me. This latest craft is sure going to be a beauty."
"Then you think it will work, Tom?"
"I'm sure of it, Dad!"
Mr. Swift shook his head in doubt.
Tom Swift pondered long and intently over what his father had said to him. He sat for several minutes in his private office, after the aged inventor had passed out, reviewing in his mind the talk just finished.
"I wonder," said Tom slowly, "if any of the new men could have obtained work here for the purpose of furthering that plot the lieutenant suspects? I wonder if that could be true?"
And the more Tom thought of it, the more he was convinced that such a thing was at least possible.
"I must make a close inspection, and weed out any suspicious characters," he decided, "though I need every man I have working now, to get the Mars finished in time. Yes, I must look into this."
Tom had reached a point in his work where he could leave much to his helpers. He had several good foremen, and, with his father to take general supervision over more important details, the young inventor had more time to himself. Of course he did not lay too many burdens on his father's shoulders since Mr. Swift's health was not of the best.
But Tom's latest idea, the aerial warship, was so well on toward completion that his presence was not needed in that shop more than two or three times a day.
"When I'm not there I'll go about in the other shops, and sort of size up the situation," he decided. "I may be able to get a line on some of those plotters, if there are any here."
Lieutenant Marbury had departed for a time, to look after some personal matters, but he was to return inside of a week, when it was hoped to give the aerial warship its first real test in flight, and under some of the conditions that it would meet with in actual warfare.
As Tom was about to leave his office, to put into effect his new resolution to make a casual inspection of the other shops, he met Koku, the giant, coming in. Koku's hands and face were black with oil and machine filings.
"Well, what have you been doing?" Tom wanted to know. "Did you have an accident?" For Koku had no knowledge of machinery, and could not even be trusted to tighten up a simple nut by himself. But if some one stood near him, and directed him how to apply his enormous strength, Koku could do more than several machines.
"No accident, Master," he replied. "I help man lift that hammer-hammer thing that pounds so. It get stuck!"
"What, the hammer of the drop forger?" cried Tom. "Was that out of order again?"
"Him stuck," explained Koku simply.
There was an automatic trip-hammer in one of the shops, used for pounding out drop forgings, and this hammer seemed to take especial delight in getting out of order. Very often it jammed, or "stuck," as Koku described it, and if the hammer could not be forced back on the channel or upright guide-plates, it meant that it must be taken apart, and valuable time lost. Once Koku had been near when the hammer got out of order, and while the workmen were preparing to dismantle it, the giant seized the big block of steel, and with a heave of his mighty shoulders forced it back on the guides.
"And is that what you did this time?" asked Tom.
"Yes, Master. Me fix hammer," Koku answered. "I get dirty, I no care. Man say I no can fix. I show him I can!"
"What man said that?"
"Man who run hammer. Ha! I lift him by one finger! He say he no like to work on hammer. He want to work on airship. I tell him I tell you, maybe you give him job—he baby! Koku can work hammer. Me fix it when it get stuck."
"Well, maybe you know what you're talking about, but I don't," said Tom, with a pleasant smile at his big helper. "Come on, Koku, we'll go see what it all means."
"Koku work hammer, maybe?" asked the giant hope fully.
"Well, I'll see," half promised Tom. "If it's going to get out of gear all the while it might pay me to keep you at it so you could get it back in place whenever it kicked up a fuss, and so save time. I'll see about it."
Koku led the way to the shop where the triphammer was installed. It was working perfectly now, as Tom could tell by the thundering blows it struck. The man operating it looked up as Tom approached, and, at a gesture from the young inventor, shut off the power.
"Been having trouble here?" asked Tom, noting that the workman was one of the new hands he had hired.
"Yes, sir, a little," was the respectful answer. "This hammer goes on a strike every now and then, and gets jammed. Your giant there forced it back into place, which is more than I could do with a big bar for a lever. He sure has some muscle."
"Yes," agreed Tom, "he's pretty strong. But what's this you said about wanting to give up this job, and go on the airship construction."
The man turned red under his coat of grime.
"I didn't intend him to repeat that to you, Mr. Swift," he said. "I was a little put out at the way this hammer worked. I lose so much time at it that I said I'd like to be transferred to the airship department. I've worked in one before. But I'm not making a kick," he added quickly. "Work is too scarce for that."
"I understand," said Tom. "I have been thinking of making a change. Koku seems to like this hammer, and knows how to get it in order once it gets off the guides. You say you have had experience in airship construction?"
"Yes, sir. I've worked on the engines, and on the planes."
"Know anything about dirigible balloons?"
"Yes, I've worked on them, too, but the engineering part is my specialty. I'm a little out of my element on a trip-hammer."
"I see. Well, perhaps I'll give you a trial. Meanwhile you might break Koku in on operating this machine. If I transfer you I'll put him on this hammer."
"Thank you, Mr. Swift! I'll show him all I know about it. Oh, there goes the hammer again!" he exclaimed, for, as he started it up, as Tom turned away, the big piece of steel once more jammed on the channel-plates.
"Me fix!" exclaimed the giant eagerly, anxious for a chance to exhibit his great strength.
"Wait a minute!" exclaimed Tom. "I want to get a look at that machine."
He inspected it carefully before he signaled for Koku to force the hammer back into place. But, if Tom saw anything suspicious, he said nothing. There was, however, a queer look on his face as he turned aside, and he murmured to himself, as he walked away:
"So you want to be transferred to the airship department, do you? Well, we'll see about that. We'll see."
Tom had more problems to solve than those of making an aerial warship that would be acceptable to the United States Government.
Ned Newton called on his chum that evening. The two talked of many things, gradually veering around to the subject uppermost in Tom's mind—his new aircraft.
"You're thinking too much of that." Ned warned him. "You're as bad as the time you went for your first flight."
"I suppose I am," admitted Tom. "But the success of the Mars means a whole lot to me. And that's something I nearly forgot. I've got to go out to the shop now. Want to come along, Ned?"
"Sure, though I tell you that you're working too hard—burning the electric light at both ends."
"This is just something simple," Tom said. "It won't take long."
He went out, followed by his chum.
"But this isn't the way to the airship shed," objected the young bank clerk, as he noted in which direction Tom was leading him.
"I know it isn't," Tom replied. "But I want to look at one of the trip-hammers in the forge shop when none of the men is around. I've been having a little trouble there."
"Trouble!" exclaimed his chum. "Has that plot Lieutenant Marbury spoke of developed?"
"Not exactly. This is something else," and Tom told of the trouble with the big hammer.
"I had an idea," the young inventor said, "that the man at the machine let it get out of order purposely, so I'd change him. I want to see if my suspicions are correct."
Tom carefully inspected the hammer by the light of a powerful portable electric lamp Ned held.
"Ha! There it is!" Tom suddenly exclaimed.
"Something wrong?" Ned inquired.
"Yes. This is what's been throwing the hammer off the guides all the while," and Tom pulled out a small steel bolt that had been slipped into an oil hole. A certain amount of vibration, he explained to Ned, would rattle the bolt out so that it would force the hammer to one side, throwing it off the channel-plates, and rendering it useless for the time being.
"A foxy trick," commented Tom. "No wonder the machine got out of kilter so easily."
"Do you think it was done purposely?"
"Well, I'm not going to say. But I'm going to watch that man. He wants to be transferred to the airship department. He put this in the hammer, perhaps, to have an excuse for a change. Well, I'll give it to him."
"You don't mean that you'd take a fellow like that and put him to work on your new aerial warship, do you, Tom?"
"Yes, I think I will, Ned. You see, I look at it this way: I haven't any real proof against him now. He could only laugh at me if I accused him. But you've heard the proverb about giving a calf rope enough and he'll hang himself, haven't you?"
"I think I have."
"Well, I'm going to give this fellow a little rope. I'll transfer him, as he asks, and I'll keep a close watch on him."
"But won't it be risky?"
"Perhaps, but no more so than leaving him in here to work mischief. If he is hatching a plot, the sooner it's over with the better I shall like it. I don't like a shot to hang fire. I'm warned now, and I'll be ready for him. I have a line on whom to suspect. This is the first clue," and Tom held up the incriminating bolt.
"I think you're taking too big a risk, Tom," his chum said. "Why not discharge the man?"
"Because that might only smooth things over for a time. If this plot is being laid the sooner it comes to a head, and breaks, the better. Have it done, short, sharp and quick, is my motto. Yes, I'll shift him in the morning. Oh, but I wish it was all over, and the Mars was accepted by Uncle Sam!" and Tom put his hand to his head with a tired gesture.
"Say, old man!" exclaimed Ned, "what you want is a day off, and I'm going to see that you get it. You need a little vacation."
"Perhaps I do," assented Tom wearily.
"Then you'll have it!" cried Ned. "There's going to be a little picnic to-morrow. Why can't you go with Mary Nestor? She'd like you to take her, I'm sure. Her cousin, Helen Randall, is on from New York, and she wants to go, also."
"How do you know?" asked Tom quickly.
"Because she said so," laughed Ned. "I was over to the house to call. I have met Helen before, and I suggested that you and I would take the two girls, and have a day off. You'll come, won't you?"
"Well, I don't know," spoke Tom slowly. "I ought to—"
"Nonsense! Give up work for one day!" urged Ned. "Come along. It'll do you good—get the cobwebs out of your head."
"All right, I'll go," assented Tom, after a moment's thought.
The next day, having instructed his father and the foremen to look well to the various shops, and having seen that the work on the new aerial warship was progressing favorably, Tom left for a day's outing with his chum and the two girls.
The picnic was held in a grove that surrounded a small lake, and after luncheon the four friends went for a ride in a launch Tom hired. They went to the upper end of the lake, in rather a pretty but lonesome locality.
"Tom, you look tired," said Mary. "I'm sure you've been working too hard!"
"Why, I'm not working any harder than usual," Tom insisted.
"Yes, he is, too!" declared Ned, "and he's running more chances, too."
"Chances?" repeated Mary.
"Oh, that's all bosh!" laughed Tom. "Come on, let's go ashore and walk."
"That suits me," spoke Ned. Helen and Mary assented, and soon the four young persons were strolling through the shady wood.
After a bit the couples became separated, and Tom found himself walking beside Mary in a woodland path. The girl glanced at her companion's face, and ventured:
"A penny for your thoughts, Tom."
"They're worth more than that," he replied gallantly. "I was thinking of—you."
"Oh, how nicely you say it!" she laughed. "But I know better! You're puzzling over some problem. Tell me, what did Ned mean when he hinted at danger? Is there any, Tom?"
"None at all," he assured her. "It's just a sort of notion—"
Mary made a sudden gesture of silence.
"Hark!" she whispered to Tom, "I heard someone mention your name then. Listen!"
Mary Nestor spoke with such earnestness, and her action in catching hold of Tom's arm to enjoin silence was so pronounced that, though he had at first regarded the matter in the light of a joke, he soon thought otherwise. He glanced from the girl's face to the dense underbrush on either side of the woodland path.
"What is it, Mary?" he asked in a whisper.
"I don't just know. I heard whispering, and thought it was the rustling of the leaves of the trees. Then someone spoke your name quite loudly. Didn't you hear it?"
Tom shook his head in negation.
"It may be Ned and his friend," he whispered, his lips close to Mary's ear.
"I think not," was her answer. "Listen; there it is again."
Distinctly then, Tom heard, from some opening in the screen of bushes, his own name spoken. "Did you hear it?" asked Mary, barely forming the words with her lips. But Tom could read their motion.
"Yes," he nodded. Then, motioning to Mary to remain where she was, he stepped forward, taking care to tread only on grassy places where there were no little twigs or branches to break and betray his presence. He was working his way toward the sound of the unseen voice.
There was a sudden movement in the bushes, just beyond the spot Tom was making for. He halted quickly and peered ahead. Mary, too, was looking on anxiously.
Tom saw the forms of two men, partially concealed by bushes, walking away from him. The men took no pains to conceal their movements, so Tom was emboldened to advance with less caution. He hurried to where he could get a good view, and, at the sight of one of the men, he uttered an exclamation.
"What is it?" asked Mary, who was now at his side. She had seen that Tom had thrown aside caution, and she had come up to join him.
"That man—I know him!" the young inventor exclaimed. "It is Feldman—the one who wanted to be changed from the trip-hammer to the airship department. But who is that with him?"
As Tom spoke the other turned, and at the sight of his face Mary Nestor said:
"He looks like a Frenchman, with that little mustache and imperial."
"So he is!" exclaimed Tom, in a hoarse whisper. "He must be the Frenchman that Eradicate spoke about. I wonder what this can mean? I didn't know Feldman had left the shop."
"You may know what you're talking about, but I don't, Tom," said Mary, with a smile at her companion. "Are they friends of yours?"
"Hardly," spoke the young inventor dryly. "That one, Feldman, is one of my workmen. He had charge of a drop-forge press and trip-hammer that—"
"Spare me the details, Tom!" interrupted Mary. "You know I don't understand a thing about machinery. The wireless you erected on Earthquake Island was as much as I could comprehend."
"Well, a trip-hammer isn't as complicated as that," spoke Tom, with a laugh, as he noticed that the two men were far enough away so they could not hear him. "What I was going to say was, that one of those men works in our shops. The other I don't know, but I agree with you that he does look like a Frenchman, and old Eradicate had a meeting with a man whom he described as being of that nationality."
"And you say they are not friends of yours?"
"I have no reason to believe they are."
"Then they must be enemies!" exclaimed Mary with quick intuition. "Oh, Tom, you will be careful, won't you?"
"Of course I will, little girl," he said, a note of fondness creeping into his voice, as he covered the small hand with his own large one. "But there is no danger."
"Then why were these men discussing you?"
"I don't know that they were, Mary."
"They mentioned your name."
"Well, that may be. Probably one of them, Feldman, who works for me, was speaking to his companion about the chance for a position. My father and I employ a number of men, you know."
"Well, I suppose it is all right, Tom, and I surely hope it is. But you will be careful, won't you? And you look more worried than you used to. Has anything gone wrong?"
"Not a thing, little girl. Everything is going fine. My new aerial warship will soon make a trial flight, and I'd be pleased to have you as a passenger."
"Would you really, Tom?"
"Of course. Consider that you have the first invitation."
"That's awfully nice of you. But you do look worried, Tom. Has anything troubled you?"
"No, not much. Everything is going all right now. We did have a little trouble at a fire in one of my buildings—"
"A fire! Oh, Tom! You never told me!"
"Well, it didn't amount to much—the only suspicious fact about it was that it seemed to have been of incendiary origin."
Mary seemed much alarmed, and again begged Tom to be on his guard, which he promised to do. Had Mary known the warnings uttered by Lieutenant Marbury she might have had more occasion for worry.
"Do you suppose that hammer man of yours came to these woods to meet that Frenchman and talk about you, Tom?" asked his companion, when the two men had strolled out of sight, and the young people were on their way back to the launch.
"Well, it's possible. I have been warned that foreign spies are trying to get hold of some of my patents, and also to hamper the government in the use of some others I have sold. But they'll have their own troubles to get away with anything. The works are pretty well guarded, and you forget I have the giant, Koku, who is almost a personal bodyguard."
"Yes, but he can't be everywhere at once. Oh, you will be careful, won't you, Tom?"
"Yes, Mary, I will," promised the young inventor. "But don't say anything to Ned about what we just saw and heard."
"Why not?"
"Because he's been at me to hire a couple of detectives to watch over me, and this would give him another excuse. Just don't say anything, and I'll adopt all the precautions I think are needful."
"I will on condition that you do that."
"And I promise I will."
With that Mary had to be content. A little later they joined Ned and his friend, and soon they were moving swiftly down the lake in the launch.
"Well, hasn't it done you good to take a day off?" Ned demanded of his chum, when they were on their homeward way.
"Yes, I think it has," agreed Tom.
"You swung your thoughts into a new channel, didn't you?"
"Oh, yes, I found something new to think about," admitted the young inventor, with a quick look at Mary.
But, though Tom thus passed off lightly the little incident of the day, he gave it serious thought when he was alone.
"Those fellows were certainly talking about me," he reasoned. "I wonder what for? And Feldman left the shop without my knowledge. I'll have to look into that. I wonder if that Frenchy looking chap I saw was the one who tried to pump Eradicate? Another point to settle."
The last was easily disposed of, for, on reaching his shops that afternoon, Tom cross-questioned the colored man, and obtained a most accurate description of the odd foreigner. It tallied in every detail with the man Tom had seen in the woods.
"And now about Feldman," mused Tom, as he went to the foreman of the shop where the suspected man had been employed.
"Yes, Feldman asked for a day off," the foreman said in response to Tom's question. "He claimed his mother was sick, and he wanted to go to see her. I knew you wouldn't object, as we were not rushed in his department."
"Oh, that's all right," said Tom quickly. "Did he say where his mother lived?"
"Over Lafayette way."
"Humph!" murmured Tom. To himself he added: "Queer that he should be near Lake Loraine, in an opposite direction from Lafayette. This will bear an investigation."
The next day Tom made it his business to pass near the hammer that was so frequently out of order. He found Feldman busy instructing Koku in its operation. Tom resolved on a little strategy.
"How is it working, Feldman?" he asked.
"Very well, Mr. Swift. There doesn't seem to be any trouble at all, but it may happen any minute. Koku seems to take to it like a duck to water."
"Well, when he is ready to assume charge let me know."
"And then am I to go into the aeroplane shop?"
"I'll see. By the way, how is your mother?" he asked quickly, looking Feldman full in the face.
"She is much better. I took a day off yesterday to go to see her," the man replied quietly enough, and without sign of embarrassment.
"That's good. Let me see, she lives over near Lake Loraine, doesn't she?"
This time Feldman could not repress a start. But he covered it admirably by stooping over to pick up a tool that fell to the floor.
"No, my mother is in Lafayette," he said. "I don't know where Lake Loraine is."
"Oh," said Tom, as he turned aside to hide a smile. He was sure now he knew at least one of the plotters.
But Tom was not yet ready to show his hand. He wanted better evidence than any he yet possessed. It would take a little more time.
Work on the aerial warship was rushed, and it seemed likely that a trial flight could be made before the date set. Lieutenant Marbury sent word that he would be on hand when needed, and in some of the shops, where fittings for the Mars were being made, night and day shifts were working.
"Well, if everything goes well, we'll take her for a trial flight to-morrow," said Tom, coming in from the shops one evening.
"Guns and all?" asked Ned, who had come over to pay his chum a visit. Mr. Damon was also on hand, invoking occasional blessings.
"Guns and all," replied Tom.
Ned had a little vacation from the bank, and was to stay all night, as was Mr. Damon.
What time it was, save that it must be near midnight, Tom could not tell, but he was suddenly awakened by hearing yells from Eradicate:
"Massa Tom! Massa Tom!" yelled the excited colored man. "Git up! Git up! Suffin' turrible am happenin' in de balloon shop. Hurry! An' yo' stan' still, Boomerang, or I'll twist yo' tail, dat's what I will! Hurry, Massa Tom!"
Tom leaped out of bed.