as the echoes died away. "How was it?"

"Couldn't have been better," declared Lieutenant Marbury. "There
wasn't the least shock of recoil. Tom Swift, you have solved the
problem, I do believe! Your aerial warship is a success!"

"I'm glad to hear you say so. There are one or two little things that
need changing, but I really think I have about what the United States
Government wants."

"I am, also, of that belief, Tom. If only--" The officer stopped
suddenly.

"Well?" asked Tom suggestively.

"I was going to say if only those foreign spies don't make trouble."

"I think we've seen the last of them," Tom declared. "Now we'll go on
with the tests."

More guns were fired, singly and in batteries, and in each case the
Mars stood the test perfectly. The double barrel had solved the recoil
problem.

For some little time longer they remained out over the sea, going
through some evolutions to test the rudder control, and then as their
present object had been accomplished Tom gave orders to head back to
Shopton, which place was reached in due time.

"Well, Tom, how was it?" asked Mr. Swift, for though his son had said
nothing to his friends about the prospective test, the aged inventor
knew about it.

"Successful, Dad, in every particular."

"That's good. I didn't think you could do it. But you did. I tell you
it isn't much that can get the best of a Swift!" exclaimed the aged man
proudly. "Oh, by the way, Tom, here's a telegram that came while you
were gone," and he handed his son the yellow envelope.

Tom ripped it open with a single gesture, and in a flash his eyes took
in the words. He read:

"Look out for spies during trial flights."

The message was signed with a name Tom did not recognize.

"Any bad news?" asked Mr. Swift.

"No--oh, no," replied Tom, as he crumpled up the paper and thrust it
into his pocket. "No bad news, Dad."

"Well, I'm glad to hear that," went on Mr. Swift. "I don't like
telegrams."

When Tom showed the message to Lieutenant Marbury, that official, after
one glance at the signature, said:

"Pierson, eh? Well, when he sends out a warning it generally means
something."

"Who's Pierson?" asked Tom.

"Head of the Secret Service department that has charge of this airship
matter. There must be something in the wind, Tom."

Extra precautions were taken about the shops. Strangers were not
permitted to enter, and all future work on the Mars was kept secret.
Nevertheless, To