"Mission completed," Tom said grimly. "No need for them to stick around
if they'd already snagged the missile."

Bud scowled at the thought. "Oh, no! That mustn't be true!"

Tom plopped down at his desk, frowning. "Bud, I've been itching to get
to work on a non-detectable sub, like the one that attacked us. But
maybe it would be smarter to get a line on Mirov's pals first."

"You mean down in the South Atlantic?"

Tom nodded. "I'd sure like to know if they found that missile."

"You and I both, pal!" Bud agreed. "Hey! We could use the electronic
hydrolungs for scouting around!" he added eagerly.

"I intend to," Tom said. "But we'll need speed to cover the area. So
first I want to add an ion drive to our equipment."

"Ion drive? For underwater?" Bud, who was familiar with ion propulsion
for spaceships, wrinkled his brow in a puzzled frown.

"A goofy idea just occurred to me, but I think it may work out," Tom
replied. He seized a pencil and began explaining what he had in mind.

The drive unit would take water into itself, separate the ionized
molecules, and expose them to an electric field. Thus a stream of water
would be forced out. This procedure, in turn, would set up a siphoning
action through a central tube--in effect, creating a small but powerful
water-jet motor.

"We'll be human submarines!" Bud exclaimed.

By the time Bud left the laboratory half an hour later, Tom had already
plunged into work on his newest invention. The idea was simple enough in
itself, Tom felt. The main problem would be the design job--laying out a
compact, lightweight unit which a swimmer could easily carry on his
back.

Fascinated, the young inventor worked late into the evening, stopping
only in response to a telephone plea from Mrs. Swift. By midmorning the
next day, Tom had assembled a pilot model of his ion-drive jet. In
appearance, it was a slender metal cylinder, two feet long, with an
inner concentric tube projecting at each end.

Tom had ordered a tank set up in his laboratory to test the unit. The
tank was filled chest-deep with water, and the ion drive was mounted on
a unitrack running the length of it. Tom set up his control board
alongside, with the main power switch within easy reach. The drive unit
was connected to the board by a suspended cable.

"Boy, this'll be like playing with a speedboat in a bathtub!" Tom
thought with a chuckle as he changed into swim trunks.

He climbed into the tank and slid the