Eddie was examining the intricate apparatus. "You knew of the first one's escape, didn't you?" he asked. "How did it happen?"

Shelton again became the enthusiastic scientist. "Here," he said, "I'll show you and you can judge for yourself." He strode to the gleaming figure of a seven-foot robot of startlingly human-like appearance.

Lina let forth an exclamation of repugnance and fear.

"No, Mr. Shelton," Eddie objected. "The same thing will occur again.

Then there will be three."

"We'll fix that, my boy." The scientist was removing cover plates from the hip joints of the mechanical man. "I'll disconnect the cables that feed the locomotors. He _can't_ walk then."

Eddie was still doubtful but dared offer no further objection, especially since Lina Shelton was watching in wide-eyed silence. He examined the monster and saw that it was quite similar in outside appearance to those supplied by Universal for heavy manual labor, excepting that this one was armed as were those used for prison guards. There were the same articulated limbs and the various clamps and hooks for lifting and heavy hauling; the tentacles for grasping; machine guns front and back. Under the helical headpiece that was the antenna this robot seemed to have two eyes--a new feature--but closer examination showed these to be the twin lenses of a stereoscopic motion picture camera. This robot, then, could see. Or at least it could record what the lenses saw for its masters.

"There," Shelton grunted when he had finished his tinkering, "he's paralyzed from the waist down. Let this one try and get away from us."

"Guns aren't loaded, are they?" Eddie asked.

"Lord, no! Never have any of them loaded. That _would_ be a fool stunt." Shelton had pulled the starting handle of a motor-generator and its rising whine accompanied his words.

The vacuum tubes of the transmitter glowed into life and the scientist manipulated the controls rapidly. Lina was watching the robot with fascinated awe. Its arms moved in obedience to the controls, tentacles waved and coiled; the humming of its internal mechanisms filled the room. The locomotion controls had no effect, as the scientist had predicted. Eddie drew a sigh of relief.

"Now, Vail, watch," Shelton exulted. "I'll show you what I was doing with the first one." He closed a switch that lighted another bank of vacuum tubes behind the control panel.

"You can make this one invisible?" Eddie asked incredulously.

"Certainly--from the waist up. This ought to be good."

"Mind telling me the principle?"

"Not at all--now. I've your promise of secrecy. It's a simple matter, Vail, really. Just a problem of wave motions--light. Invisible light; the ultra-violet, you know. My robots are built of specially alloyed metals which permit great freedom of molecular vibration. The insulating materials and even the gla.s.s of the camera lenses are possessed of the same property. Get it? I merely set up a wave motion in the atoms of the material that is in synchronism with the frequency of ultra-violet light, which is invisible to the human eye. All visible colors are absorbed, or more accurately, none are reflected excepting the ultra-violet. Perfect transparency is obtained since there is neither refraction nor diffraction of the visible colors. And there you are!"

Eddie stared at the upper half of the robot and saw that it was changing color as Shelton tuned the transmitted wave. Then suddenly it was gone. The entire upper portion of the mechanism had vanished; had just snuffed out like the flame of a candle. He could see down into the tops of the thing's hollow legs. Shelton laughed at him as he stretched forth his hand and hesitatingly felt for the invisible mid-section and upper body. It was there all right, unyielding and cold, that metal body. But no trace of it was visible to the eye. He drew back his fingers as if they had touched a hot stove. The thing was positively uncanny.

"Dad! Turn it off--please," Lina begged. "It's getting on my nerves.

Please!"

Obligingly, Shelton pulled the switch. "Now you'll see," he said to Eddie, "whether the same thing happens. Watch."

Mistily at first, the outlines of the monster's torso and arms came into view, semi-transparent but clouding rapidly to opacity. Then it glinted with the barely visible violet, a solid once more, rigid and motionless. It was a lifeless mechanism, for the source of its energy had been cut off. Eddie had an almost irresistible impulse to pinch himself.

Then he gasped audibly, as did Shelton, for the thing snuffed out of sight again without warning, and the hum of its many motors resumed.

There came a terrific clanking as it waved arms and tentacles and violently threshed with its upper body. But the visible portion, its legs, remained rooted to the floor of the laboratory. Lucky it was that the scientist had disconnected those wires; lucky too that the machine guns were empty of ammunition.

"There now--see?" Shelton's voice rose excitedly. "It's been no fault of mine. The power is off but it moves--it moves. What on earth do you suppose--"

Eddie's shout interrupted him. He had seen something at the window: a face pressed against the pane and contorted with unutterable malice.

Then it was gone. With the shout of warning still in his throat, Eddie bounded through the door in pursuit of the intruder. Lina's cry of recognition followed him into the twilight. "Carlos!" she had called.

He saw a stocky figure slink around the corner of the laboratory and make for the underbrush beyond. In a flash he was after him. No, he thought grimly, Shelton hadn't any enemy clever enough to duplicate his transmitter! The h.e.l.l he didn't! Who the devil was this fellow Carlos anyway? He tore savagely at the impeding branches as he plunged deeper and deeper into the thicket.

It was a fruitless chase and Eddie soon retraced his steps to the laboratory. Swell mess he'd gotten himself into! His car was gone: probably wrapped around a tree by this time. And here was a situation that spelled real danger, a thing with which Shelton was utterly unable to cope. As a matter of fact, he was so impractical--such a visionary cuss, after the fashion of all geniuses--that he'd never be convinced of the seriousness of the matter until it was too late. What to do? The girl was a corker, though, and game as they made 'em. Just the sort a fellow could tie to....

Lina's firm clear voice came to him through the open door of the laboratory. "Dad," she was saying, "why don't you give it up? Let's go back to New York where it is safe for you and for me. Let the things go and forget about them. What do they amount to, after all? We've plenty of money and you already have earned enough fame to last the rest of your life. Come on now--please--for me."

"What do they amount to?" Shelton reiterated, his voice rising querulously. "Lina, it's the most tremendous thing I've ever done.

Think for a moment of what my robots could accomplish in the next war.

And there'll be a next war as sure as you're alive. Think of it! No sending of our young manhood into the b.l.o.o.d.y fields of battle; no manning of our air fleets with the cream of our youth; no bloodshed on our side whatsoever. Instead, these robots will fight the war. They'll fight other robots too, no doubt, but the property of invisibility will be an invincible weapon. It will be a war that will end war once and for all. You can't--"

"Nonsense, Father," the girl returned sharply. "You've let your enthusiasm run away with your judgment. See what's happened already?--someone's figured it out before you've even perfected the thing. An enemy of our country could do the same in wartime. Maybe it's a foreign spy who has done what's been done to-day."

Eddie walked into the laboratory. "Couldn't find him," he announced briefly.

"No difference," said Shelton. "He doesn't count in this. We called to you when you rushed out, but couldn't make you hear."

"Who is he?" Eddie asked shortly. What he had overheard made him more than ever impatient with the older man. So clever and yet so dense, Shelton was.

Lina avoided his gaze.

"Only Carlos--Carlos Savarino," said Shelton, carelessly, "a Chilean, I think. He worked for me for two months during the summer and I fired him for getting fresh with Lina. Good mechanic, but dumb as an ox. Had to tell him every little detail when he was doing something in the shop. I'd have saved time if I'd done it myself."

The girl looked at Eddie squarely now. She was flushing hotly. "And I horsewhipped him," she added.

"Yes," Shelton laughed; "it was rich. He sneaked away like a whipped puppy, and this is the first time we've seen him since."

Eddie whistled. "And you think he doesn't count in this?" he asked.

"Of course not. Too dumb, I tell you. Doesn't know the first principles of science. He thinks the only wave motion is that of the ocean." Shelton chuckled over his own jest.

"I wouldn't be too sure," Eddie snapped. "And I want to tell you something, Mr. Shelton. Through no fault of my own, I heard some of your conversation with Li--with your daughter, before I returned here.

I was puzzled over your reasons for working so absorbedly on this thing, but now I know them and I think you're wasting your time and keeping your daughter in needless danger."

"You dare talk to me like this!" Shelton roared.

"I do, sir, and you'll thank me later." Eddie returned the older man's glare with one equally savage.

Lina's gurgle of laughter broke the tension. "He's right, Dad, and you know it," she interposed. "Let him finish."

Eddie needed no such encouragement, though it warmed his heart. And Shelton listened respectfully when he continued, "I'm into this now, sir, and I intend to see it through to the end. I'll keep your secret, too, though I doubt if it'll ever be of much value to you. Know what I think? I think this Carlos is a d.a.m.n clever fellow instead of the a.s.s you took him to be. He probably just pretended he was ignorant of science. Why shouldn't he? That way he got a liberal education from you in the very things he wanted to find out. Since you tied the can to him he's had plenty of chances to build a duplicate of your control apparatus--with the aid of some foreign government, no doubt--and now they've stolen two of your machines to complete the job. Your secret already is out and in the very hands you've tried to keep it from."