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 soft of notion--"

Mary made a sudden gesture of silence.

"Hark!" she whispered to Tom, "I heard someone mention your name then. Listen!"