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 fur