“And that was—­what?” asked Tom.

“The existence of a well-laid plot, not only against our government, but against you!”

“And why have they singled me out?” Tom demanded.

“I might as well tell it from the beginning,” the officer went on.  “As long as you have not received any official warning from Washington you had better hear the whole story.  But are you sure you had no word?”

“Well, now, I won’t be so sure,” Tom confessed.  “I have been working very hard, the last two days, making some intricate calculations.  I have rather neglected my mail, to tell you the truth.

“And, come to think of it, there were several letters received with the Washington postmark.  But, I supposed they had to do with some of my patents, and I only casually glanced over them.  There was one letter, though, that I couldn’t make head or tail of.”

“Ha!  That was it!” cried the lieutenant.  “It was the warning in cipher or code.  I didn’t think they would neglect to send it to you.”

“But what good would it do me if I couldn’t read it?” asked Tom.

“You must also have received a method of deciphering the message,” the officer said.  “Probably you overlooked that.  The Secret Service men sent you the warning in code, so it would not be found out by the plotters, and, to make sure you could understand it, a method of translating the cipher was sent in a separate envelope.  It is too bad you missed it.”

“Yes, for I might have been on my guard,” agreed Tom.  “The red shed might not have burned, but, as it was, only slight damage was done.”

“Owing to the fact that Tom put the fire out with sand ballast from his dirigible!” cried Ned.  “You should have seen it!”

“I should have liked to be here,” the lieutenant spoke.  “But, if I were you, Tom Swift, I would take means to prevent a repetition of such things.”

“I shall,” Tom decided.  “But, if we want to talk, we had better go to my office, where we can be more private.  I don’t want the workmen to hear too much.”

Now that the firing was over, a number of Tom’s men from the shops had assembled around the cannon.  Most of them, the young inventor felt, could be trusted, but in so large a gathering one could never be sure.

“Did you come on from Washington yesterday?” asked Tom, as he, Ned and the officer strolled toward the shed where was housed the aerial warship.

“Yes, and I spent the night in New York.  I arrived in town a short time ago, and came right on out here.  At your house I was told you were over in the fields conducting experiments, so I came on here.”

“Glad you did,” Tom said.  “I’ll soon have something to show you, I hope.  But I am interested in hearing the details of this suspected plot.  Are you sure one exists?”

“Perfectly sure,” was the answer.  “We don’t know all the details yet, nor who are concerned in it, but we are working on the case.  The Secret Service has several agents in the field.