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."

"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.

"We are convinced in Washington," went on Lieutenant Marbury, when he, Tom and Ned were seated in the private office, "that foreign spies are at work against you and against our government."

"Why against me?" asked Tom, in wonder.

"Because of the inventions you have perfected and turned over to Uncle Sam--notably the giant cannon, which rivals anything foreign European powers have, and the great searchlight, which proved so effective against the border smugglers. The success of those two alone, to say nothing of your submarine, has not only made foreign nations jealous, but they fear you--and us," the officer went on.

"Well, if they only take it out in fear--"

"But they won't!" interrupted the officer--"They are seeking to destroy those inventions. More than once, of late, we have nipped a plot just in time."

"Have they really tried to damage the big gun?" asked Tom, referring to one he had built and set up at Panama.

"They have. And now this fire proves that they are taking other measures--they are working directly against you."

"Why, I wonder?"

"Either to prevent you from making further inventions, or to stop you from completing your latest--the aerial warship."

"But I didn't know the foreign governments knew about that," Tom exclaimed. "It was a secret."

"Few secrets are safe from foreign Spies," declared Lieutenant Marbury. "They have a great ferreting-out system on the other side. We are just beginning to appreciate it. But our own men have not been idle."

"Have they really learned anything?" Tom asked. "Nothing definite enough to warrant us in acting," was the answer of the government man. "But we know enough to let us see that the plot is far-reaching."

"Are the French in it?" asked Ned impulsively.

"The French! Why do you ask that?"

"Tell him about Eradicate, and the man who wanted to buy the mule, Tom," suggested Ned,

Thereupon the young inventor mentioned the story told by Eradicate. He also brought out the fire-bomb, and explained his theory as to how it had operated to set the red shed ablaze.

"I think you are right," said Lieutenant Marbury. "And, as regards the French, I might say they are not the only nation banded to obtain our secrets--yours and the government's!"

"But I thought the French and the English were friendly toward us!" Ned exclaimed.

"So they are, in a certain measure," the officer went on. "And Russia is, too. But, in all foreign countries there are two parties, the war party, as it might be called, and the peace element.

"But I might add that it is neither France, England, nor Russia that we must fear. It is a certain other great nation, which at present I will not name."

"And you think spies set this fire?"

"I certainly do."

"But what measures shall I adopt against this plot?" Tom asked.

"We will talk that over," said Lieutenant Marbury. "But, before I go into details, I want to give you another warning. You must be very careful about--"

A sudden knock on the door interrupted the speaker.