the laboratory.

Mr. Swift, among the first to arrive, took in the situation at a glance.
He dashed to the control board and slammed shut the main switch, thus
cutting off power to the ion-drive jet.

"Whew! Th-thanks, Dad!" Tom's chest was heaving as he gulped in air to
relieve his tortured lungs.

Tom Sr. helped him climb out of the tank.

"B-b-brand my rhubarb rockets," Chow stuttered. "What in tarnation
happened?"

"Guess I gunned my new skin-diving jet a bit too hard," Tom said
sheepishly. "It was almost a K.O. for me!"

Mr. Swift asked Tom about the invention. After explaining how it worked,
Tom added with a grin, "Maybe you'd better hang around, Dad, until I
install some sort of density-control gadget for my hydrolung. Then I can
go up or down, or stay at any level easily."

Such a device, Tom felt, might prove to be a lifesaver if he should ever
become trapped under water--perhaps far from help.

The elder scientist chuckled and threw an arm around Tom's shoulders.
"I'd say you could design something like that with your eyes shut, son!"

Warmed by his father's appreciation, Tom set to work improving his
diving apparatus.

An hour later Bud came bursting into the laboratory. "Hey! What's this I
hear about your getting hammerlocked by a water jet?" the husky young
pilot asked. He had been on a test flight and just returned.

Tom laughed good-naturedly. "Nothing serious. In fact, I felt pretty
silly," he told his chum. "I souped up our ion-drive gizmo a bit too
much."

Bud picked up the slender metal cylindrical assembly from the workbench.
"This it?" he asked, his curiosity immediately aroused.

Tom nodded and demonstrated the device in the test tank.

Bud whistled with glee. "Boy! With this rig, we can scoot around like a
pair of barracudas!" he exclaimed. "What about that other thing you're
working on?" Bud pointed to a small electronic chassis on the workbench,
studded with a tangle of transistors, diodes, and condensers.

"It's a density-control device," Tom explained. "A substitute for
ballast tanks, you might say. It'll enable us to rise or sink to any
depth at will, simply by varying our underwater density."

Tom said the device would be carried in a small case, hooked to the
diver's belt, with a single tuning-knob control. The "throttle" or speed
control for the ion drive would be housed in the same unit.

"I can't wait to try out the new diving gear," Bud said excitedly.

By four o'