drive unit to one end of its
track. Then Tom metered out power slowly. With a gentle _whoosh_, the
ion-drive unit whizzed along the unitrack to the other end of the tank.

"Not bad," Tom muttered, a pleased grin on his face. "Now I'll rev it up
a little."

He slid the drive unit back to starting position, then opened the switch
wider. He had just started across the tank himself when suddenly he
became powerless to move.

Tom was pinned helplessly against the wall of the tank by the powerful
water-jet exhaust! And the control switch was beyond his reach!

CHAPTER VII

PORPOISE TAG

"Good grief! I'm trapped!" Tom squirmed desperately in a vain attempt to
free himself.

The ion-drive unit had hurtled to the far end of the tank at the first
flick of power. But its exhaust tube was still jetting out a current of
water with stunning force. Tom could feel the near-crushing pressure
against his chest, even the full length of the tank away!

"H-h-help!" Tom gasped.

Moments dragged by with agonizing slowness. Tom felt as if his last
ounce of breath were being squeezed out by the viselike pressure.

Suddenly a gravelly Western voice reached him, singing "Home on the
Range." It drew closer, swelling into a foghorn drone as the lab door
swung open.

"Good old Chow!" Tom thought. "Thank heavens!"

The grizzled, bowlegged cook ambled cheerfully into the laboratory,
pushing a lunch cart. But, to Tom's dismay, he cast only a passing
glance at the figure in the tank.

[Illustration: _Tom squirmed desperately to free himself_]

"Soup's on, son!" Chow announced loudly. He began to ladle out a bowl of
oyster stew from a steaming pot. Evidently he had not realized the young
inventor's dilemma!

"Extra good today too, if I do say so myself!" the old Texan went on,
setting out the rest of the lunch. "Well, come on, buckaroo! Break away
from them chores an' dive in! Brand my cactus salad, if there's one
thing that riles a cook--"

Summoning all his strength, Tom croaked out weakly, "Chow!... Get help!"

At the strange sound of Tom's voice, Chow jerked around. His eyes bugged
out at the look on the young inventor's face. Then he dashed to the
public-address outlet on the wall and switched on the mike.

"Help! Help!" Chow yelled. "Tom Jr.'s trapped in his lab!"

The roly-poly chef was quivering in panic. He dashed across the room and
paced helplessly about the tank. Within moments, excited men were
crowding into