clock Tom had the apparatus perfected, and turned it over to
Arv Hanson for fast duplication.

"We'll give it a shakedown tomorrow morning," he told Bud.

The duplicates of the ion drive and density control were ready and
waiting when the boys arrived at the plant next day. They immediately
flew to Fearing Island and embarked in a motor launch, with Zimby Cox
again at the helm.

This time they cruised out to deeper water. Tom and Bud donned flippers
and belt, and helped each other strap on his ion-drive jet.

"_Down_ we go, into the wilds of sharks!" Bud chortled lustily. "Watch
your step, Tom."

"Just make sure you come up again in one piece," Zimby said with a grin.
"Also, don't get carried away with that ion squirt gun and take off on a
round-the-world underwater cruise."

"Who knows?" Tom joked. Adjusting his face mask, he plunged over the
side. Bud followed.

Down they glided into the sea-green wilderness. Leveling off in sight of
the ocean floor, they tried their drive jets. The effect was thrilling!
_Zip ... Whoosh!_ They darted to and fro like human torpedoes.

Then Tom twirled the control knob of his density unit. Immediately he
bobbed upward like a cork. A reverse twirl sent him plummeting toward
the bottom again. Bud, watching with wide-eyed excitement, began
experimenting on his own.

Soon the boys were engaging in all sorts of underwater acrobatics.
Presently Bud felt a nudge in the back that sent him hurtling a dozen
yards through the water.

"Snuck up on me, eh, pal?" he thought with a chuckle. "Okay, Tom old
boy, here's where the undersea terror strikes back!"

Swooping around to return the compliment, Bud gulped in surprise.
Instead of his chum, he found himself face to face with a bottle-nosed
dolphin!

"Good night!" Bud thought. "A porpoise! So you're the joker who nudged
me!"

With a playful toss of its comical-looking snout, the porpoise swam off,
as if inviting Bud to join in the fun and games. A whole school of the
creatures cavorted into view.

"Okay! If you want to play!" Chuckling, Bud darted in pursuit, whacked
the porpoise that had nudged him, and jetted off again. The porpoise
gave chase, whistling and grunting audibly.

Tom joined in the fun, and soon a rollicking game of underwater tag was
in full swing. The dolphins seemed as playful and mischievous as small
children.

Twenty minutes later the boys surfaced and hauled themselves aboard.
Both tore off their masks and