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'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."