Kendrick's heart was beating fast as he turned to his instruments. How could he fail, with faith like that behind him?--love, even, perhaps! He mustn't fail--nor would he, if his theories were sound. * * * * * Addressing the assemblage, he explained briefly the complicated apparatus. "These towers," he said, pointing to four steel structures about ten feet high, arranged at the corners of a square roughly twenty feet across, "are miniature radio masts. The area enclosed by them, we will assume, is the city of New York. That metal disc suspended above the area represents the invader. It contains a miniature heat-generator such as I was experimenting with recently in the Arizona desert." He paused, threw a switch. Somewhere in the laboratory a dynamo began to whir. "I am now sending electro-magnetic waves from the four towers," he resumed. "But instead of broadcasting them in every direction. I am bending them in concave cathode of force over the city. You may picture this cathode as an invisible shield, if you choose, but it is more than that. It it a reflector. If my theories are right, the radio-energetic ray I am about to project upon it from my miniature disc will be flung back to its source as though it had been a ray of light falling on a mirror. The success of the experiment depends upon what the result will be." * * * * * Kendrick ceased, moved toward a rheostat. As he made ready to touch it, a breathless tension settled upon the assemblage. Upon the outcome of what was now to happen rested the fate of America--and the world. Calmly, though every fiber of his being was at breaking stress, the young scientist opened the rheostat. For an instant, the ray seared down--then, as it boomeranged back, the disc burst into flame, dissolved, disintegrated. A thin dust, like carbon, slowly settled to the laboratory floor. Cutting off the current from the radio towers, Kendrick faced them, a light of triumph in his tired eyes. "You see--it works," he said. They saw. Beyond a doubt, it worked! And what Kendrick saw, as his eyes met Marjorie's, made him forget his fatigue. * * * * * The rest was a mad scramble of preparation. Only a few brief hours remained, and much was to be done. The application of the principle that had just been demonstrated involved a hook-up from the Consolidated Electric laboratory with every broadcasting station in the metropolitan area, power being supplied by commandeering every generating plant within a radius of fifty miles. The city, moreover, had to be evacuated of all but the few brave hundreds who volunteered to stand by their posts at radio stations and generating plants. As for Kendrick, it was the busiest, most hectic morning he had ever experienced. Only the realization of a girl's love and a nation's trust enabled him to overcome the exhaustion of two sleepless nights.