turn out. It's mighty lucky you thought of that,
for I sure was up against it, as the boys say."

"It just seemed to come to me," spoke Ned, "seeing how easily the door
closed."

"If the thing works I'll give you due credit for it," promised Tom.
"Now, I've got to figure out how much force a modified hydrostatic
valve check like that will take up, and how much recoil my biggest gun
will have."

"Then you're going to put several guns on the Mars?" asked Ned.

"Yes, four quick-firers, at least, two on each side, and heavier guns
at the bow and stern, to throw explosive shells in a horizontal or
upward direction. For a downward direction we won't need any guns, we
can simply drop the bombs, or shells, from a release clutch."

"Drop them on other air craft?" Ned wanted to know.

"Well, if it's necessary, yes. Though I guess there won't be much
chance of doing that to a rival aeroplane or dirigible. But in flying
over cities or forts, explosive bombs can be dropped very nicely. For
use in attacking other air craft I am going to depend on my lateral
fire, from the guns mounted on either beam, and in the bow and stern."

"You speak as though you, yourself, were going into a battle of the
air," said Ned.

"No, I don't believe I'll go that far," Tom replied. "Though, if the
government wants my craft, I may have to go aloft and fire shots at
targets for them to show them how things work.

"Please don't think that I am in favor of war, Ned," went on Tom
earnestly. "I hate it, and I wish the time would come when all nations
would disarm. But if the other countries are laying themselves out to
have aerial battleships, it is time the United States did also. We must
not be left behind, especially in view of what is taking place in
Europe."

"I suppose that's right," agreed Ned. "Have you any of your guns ready?"

"Yes, all but the mounting of them on the supports aboard the Mars. I
haven't dared do that yet, and fire them, until I provided some means
of taking up the recoil. Now I'm going to get right to work on that
problem."

There was considerable detailed figuring and computation work ahead of
Tom Swift, and I will not weary you by going into the details of higher
mathematics. Even Ned lost interest after the start of the problem,
though he was interested when Tom took down the door-check and began
measuring the amount of force it would take up, computing it on scales
and spring balances.

Once this had been done,