gh. Now we'll try another shot."

The echoes of the first had hardly died away before the second gun was
ready for the test. That, too, was satisfactory, and then the smaller
ones were operated. These were not quite so satisfactory, as the recoil
developed was larger, in proportion to their size, than Tom had figured.

"But I can easily put a larger hydrostatic check on them," he said.
"Now, we'll fire by batteries, and see what the total is."

Then began a perfect bombardment of the distant hillside, service
charges being used, and explosive shells sent out so that dirt,
stones and gravel flew in all directions. Danger signs and flags had
been posted, and a cordon of Tom's men kept spectators away from the
hill, so no one would be in the danger zone.

The young inventor was busy making some calculations after the last of
the firing had been completed. Koku was packing up the unfired shells,
and Mr. Damon was blessing his ear-drums, and the pieces of cotton he
had stuffed in to protect them, when a tall, erect man was observed
strolling over the fields in the direction of the guns.

"Somebody's coming, Tom," warned Ned.

"Yes, and a stranger, too," observed Tom. "I wonder if that can be
Eradicate's Frenchman?"

But a look at the stranger's face disproved that surmise. He had a
frank and pleasant countenance, obviously American.

"I beg your pardon," he began, addressing everyone in general, "but I
am looking for Tom Swift. I was told he was here."

"I am Tom Swift," replied our hero.

"Ah! Well, I am Lieutenant Marbury, with whom you had some
correspondence recently about--"

"Oh, yes, Lieutenant Marbury, of the United States Navy," interrupted
Tom. "I'm glad to see you," he went on, holding out his hand. "We are
just completing some tests with the guns. You called, I presume, in
reference to my aerial warship?"

"That is it--yes. Have you it ready for a trial flight?"

"Well, almost. It can be made ready in a few hours. You see, I have
been delayed. There was a fire in the plant."

"A fire!" exclaimed the officer in surprise. "How was that? We heard
nothing of it in Washington."

"No, I kept it rather quiet," Tom explained. "We had reason to suspect
that it was a fire purposely set, in a shed where I kept a quantity of
explosives."

"Ha!" exclaimed Lieutenant Marbury. "This fits in with what I have
heard. And did you not receive warning?" he asked Tom.

"Warning? No. Of what?"

"Of foreign spies!