The two glided about in the translucent blue depths, keeping in close range of each other. The sea was alive with shimmering fish of every hue, darting among the coral. Suddenly, as Tom veered around to rejoin Mel, his eyes widened in horror.

A vicious-looking hammerhead shark was zeroing in, directly behind his friend!

"Look out!" Tom yelled over his microphone.

CHAPTER XVIII

SMILEY THE SEA COW

Mel turned in the nick of time. The monster shark was bearing down on him like an undersea express train. Overcoming a moment of panic, Mel gunned his ion drive to dodge the attack.

As Tom watched in agonized suspense, he saw the shark's jaws open and shut in a lightning snap at Mel's outstretched arm. Its razor-sharp teeth missed their target by inches!

Mel's gasp of relief was audible over Tom's earphones. "Let's get out of here!" he cried, arrowing away from the man-killer.

Suddenly Tom realized the full extent of their peril. A long, sweeping coral reef, which extended above water, lay between them and the _Sea Hound_. Unless they could round the reef in time, the shark had them trapped!

"Quick! This way!" Tom exclaimed.

The shark was moving at blinding speed. As if sensing the boys' plan of escape, it launched itself in a wide curving sweep to cut them off.

"We can't make it!" Tom gasped. "We'll have to fight!"

Both swimmers were armed with skin diver's knives as a precaution. The two maneuvered to meet the killer's onslaught.

This time its broad nightmarish head was aiming straight at Tom. He jetted off to the right, but the monster veered instantly. Its lashing tail gave Mel a stunning blow.

As the shark's jaws gaped for a bite, Tom zoomed underneath the man-eater and slashed its belly with his knife.

The shark, maddened, thrashed the water in a frenzy. Tom moved like lightning to dodge a deadly blow from its bony tail. Again and again they felt the horrifying brush of the killer's fins or armor-tough hide.

By this time, Mel had revived. Repeatedly the two boys dived to jab and slash at the shark's soft underbelly.

Both were nearly exhausted when the monster at last went limp and floated slowly up toward the surface. Pale with shock and fright, Tom and Mel jetted back to the _Sea Hound_.

Zimby c.o.x was startled by their faces when they clambered aboard and ripped off their masks. "What happened to you two?"

Tom told him. "Good night!" Zimby cried out.

After resting, Tom and Mel dived in again. This time luck was with them. In less than twenty minutes they sighted a small porpoise.

"Think we can lure it back toward the _Sea Hound_?" Mel queried.

"We'll try," Tom replied.

The creature with the bottle-shaped snout was as friendly and playful as most of its fellow dolphins. Too playful, Tom concluded, after vainly trying to tease it into chasing them. Instead of following, it would "tag" Tom or Mel quickly, then swim away, evidently expecting to be chased in turn!

"I give up!" Mel snorted in disgust.

Tom grinned and bobbed to the surface. He waved his hand several times in a prearranged signal. Zimby at last spotted him and brought the _Sea Hound_ to the scene.

Raising his mask, Tom called, "Let's have the net!"

Zimby lowered a nylon net and some pieces of fish to the two swimmers as they came alongside. With the food as bait they tried to lure the porpoise to the seacopter. But just as they thought they had it, the monster would scoot off.

"It's just laughing at us!" Mel fumed.

At last, after winning its confidence with several bits of fish, the boys succeeded in snaring the porpoise. Tom clambered onto the _Sea Hound_'s deck and helped Zimby haul their catch aboard. "Quacking"

reproachfully, it was lowered through the hatch and placed in the tank, which was then pumped full of salt water.

As soon as the _Sea Hound_ arrived at Fearing, Tom phoned Chow Winkler at Enterprises and asked him to fly out to the base.

"Pardner, how'd you like to ride herd on this critter and gentle it down for me?" Tom asked, when he showed Chow the porpoise.

It had been transferred to a huge, gla.s.s-paneled tank which had been set up just outside Tom's Fearing Island laboratory during his flight to the Florida Keys.

"Reckon I kin try makin' friends with it," Chow declared.

The porpoise stared morosely at Chow. The kindly old Texan's heart was touched by the odd creature. To his delight, it soon responded to his friendly overtures and quickly recovered its good nature. By the next morning the porpoise was playing catch with Chow, or else swimming over to have its back scratched. The cook named it Smiley.

"She's kind of a sea cow," he told Tom, "and you got to talk to my Smiley like any cow!" Tom grinned and refrained from explaining to Chow that a real "sea cow" was a walrus.

Meanwhile, the young inventor was busy with his own experiments. By means of a microphone placed in the tank, he made exact recordings of Smiley's "talk." Using Mel Flagler as a subject, Tom also tape-recorded the sound of a skin diver propelled through the water by ion drive.

The next step was to compare the sound pattern of the tapes. Tom filtered out the difference in the two sounds with the correlation calculator unit of one of his quality a.n.a.lyzer sonars.

"Uh-huh. So you got the difference betwixt Smiley's talk an' the noise Mel made," muttered Chow as he watched the jagged lines of light flashing on the pulse-check oscilloscope. "Now what're you fixin' to do with it?"

"This will be fed into the diver's sonar along with his own noise output," Tom said, "to make him sound like a porpoise."

Chow howled. "That I've got to hear!"

The young inventor worked feverishly throughout the day and into the next, perfecting his new "porpoise sonar." Using microelectronic components, he was able to reduce all the units to amazingly small size.

Next, Tom began tailoring himself a completely new skin-diving suit.

Mask, ion-drive jet, and the various hydrolung units were molded into the plastic, with no loose wires or tubes showing.

Monday morning he was ready to try the outfit. The sonarscope with its tiny viewing screen was strapped to his left forearm. Another small unit was fastened to the inside of his wrist, with four plungers in finger-tip reach.

"What in tarnation's that?" Chow asked.

"Simplified controls," Tom explained. "One's for breathing adjustment, one's for the density unit, one is my ion-drive 'throttle,' and this last is for the sonar pulse--which will duplicate the porpoise sound."

The suit worked perfectly in a tank test. Chow was amazed as he listened to Tom gliding about, via an underwater microphone.

"If that don't beat all!" he declared. "Can't tell the difference 'twixt you an' Smiley!"