tared. Submarines?

Steve saw his look of bewilderment. "It happens that our new
atomic-powered submarines are conducting manuevers in this area. Does
that help?"

It did! Light slowly dawned. "Then these were scanning our subs! But I
still don't see why it would be any problem to find them. The subs must
have equipment that will tell when sonar beams hit them."

"They do. And that's a big part of the story."

Steve sipped his coffee for a moment. "These sonar devices are a new
type, and very cleverly designed. They don't send out a continuous beam.
Instead, they operate in bursts, in a random pattern. They might send
out a beam twice in a minute, or wait an hour between bursts. The beam
is a powerful one. It's effective for an extraordinary distance."

"The wail, of course, was the beam operating," Zircon interjected. "You
didn't hear the beam itself, since that's ultrasonic. But you did hear
the mechanical vibration of the brass ball. It had a kind of
sub-harmonic effect that was audible."

"That's right," Steve agreed. "Anyway, there were several different
stations, in different locations. Some were on islands, some on fishing
boats. Since they operated only in short bursts in a random pattern, the
subs--and the special teams we sent out--were never able to get a
bearing that meant much."

"They must be self-recording," Rick said thoughtfully, "otherwise the
enemy couldn't get the information out of them."

"They are. Whatever echo they get makes a tracing inside the box they're
mounted on."

Scotty objected, "But what kind of information is it? How can anyone
tell anything about the subs from such recordings?"

"By putting all the recordings together and running a rather complex
analysis. The analysis will give speed, depth of operation,
maneuverability--if the spies are lucky to have beams operating at the
right time--and number of torpedoes fired, with the same information on
the torps. That's enough information to make it worth an enemy's while."

"I'll say!" Scotty turned to Zircon. "And what were you doing,
Professor?"

"I'm afraid I arrived on the scene too late to do much good," Zircon
boomed. "However, I believe we can be useful in preventing such
occurrences in the future. I have an idea for an improvement in our
scanners that will allow a fix to be made on such beams."

There was a pause when dessert arrived. The boys savored excellent apple
pie smothered in a generous helping of ship-made