“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.