"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, Mass a 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