"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 bra.s.s 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 windla.s.ses. "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, Ma.s.sa 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.

CHAPTER X

THE NEW MEN

"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, Ma.s.sa 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 sh.e.l.ls 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.