"Listen, Mr. Shelton," Eddie interrupted. "If you'll think a little you'll realize that they'll do no such thing. Has any new and terrible engine of destruction ever accomplished that result? No--the enemy always finds a way of combating the new weapon and of devising another still more terrible. You've discovered a marvelous thing, but its value is quite problematical." "How can they ever combat a thing they cannot see?" "Easily. Why, I could devise a teleview attachment in two days that would make them visible. Photo-electric cells are capable of detecting ultra-violet light as you well know. Radium glows under its rays. Why not coat a teleview screen with some radio-active material?" * * * * * Shelton frowned thoughtfully. "You're right. Vail," he said, after a moment of silence; "absolutely right. It was only a dream." With dragging feet he walked to the transmitter, his expression grim in the realization of failure. He started the motor-generator with a gesture of finality. "What are you going to do?" Eddie asked fearfully. "Watch me! At least I can demonstrate another phase of the basic principle I have discovered." The motors of both robots whirred. "Don't!" Cadorna wailed. "For God's sake, don't blink 'em out!" Carlos cursed his chief for a coward. Shelton was talking rapidly as he manipulated the controls. Instead of building up the wave motion to the frequency of invisible light he was reducing it. Past the other end of the spectrum and into the infra-red. The heat ray! Both monsters were changing color as he marched them through the door and into the open. But now they glowed with a visible red that rapidly intensified to the dazzling whiteness of intense heat. Cadorna babbled in superstitious terror. Then, in an instant, both mechanisms were reduced to shapeless blobs of molten metal. Lina clapped her hands gleefully. Shelton looked up with enthusiasm once more shining in his face. "Vail, my boy," he said, "we can find some use for that in industry. Let the next war take care of itself." "You bet!" Eddie was lost in contemplation of the girl--the flush of pleasure that came at her father's words; the shining eyes. "Then you'll leave the old place down here?" she asked eagerly. "Yes, as soon as we get rid of these crooks and the other robot. Vail is to spend the rest of his vacation with us, too--if he will." Would he? Eddie gazed at the girl in rapt admiration and with an inward thrill over his astounding good fortune. Her eyes dropped before the intensity in his and her flush heightened. David Shelton was wiping his glasses and peering at them with an understanding smile. Good sport, Shelton--and in some ways as wise as they made them. Eddie waited breathlessly for the girl to speak. "Oh, that's wonderful, Dad," she approved; "and I'm sure that Mr. Vail will agree."