es 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