urposes. 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 c