Then began a perfect bombardment of the distant hillside, service charges being used v, 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!” was the unexpected answer.  “I am sorry.  Some of our Secret Service men unearthed something of a plot against you, and I presumed you had been told to watch out.  If you had, the fire might not have occurred.  There must have been some error in Washington.  But let me tell you now, Tom Swift—­be on your guard!”

CHAPTER VIII A SUSPECTED PLOT

The officer’s words were so filled with meaning that Tom started.  Ned Newton, too, showed the effect he felt.

“Do you really mean that?” asked the young inventor, looking around to make sure his father was not present.  On account of Professor Swift’s weak heart, Tom wished to spare him all possible worry.

“I certainly do mean it,” insisted Lieutenant Marbury.  “And, while I am rather amazed at the news of the fire, for I did not think the plotters would be so bold as that, it is in line with what I expected, and what we suspected in Washington.”