to refute their charges. But even as he was
summoning words, refutation utter and complete came from the air.

A low, humming vibration sounded, grew in volume till it filled the
room--and as suddenly ceased: The light of midday faded to twilight.

"_The disc!_" gasped Kendrick, rushing to the west windows.

They followed, tense with awe. And there, between earth and sun, its
myriad towers and spires refracting a weird radiance, hovered that
vast flying city.

"My God!" muttered a famous general, staring as though he had seen a
ghost.

A great statesman opened his lips, but no words came.

"Appalling! Incredible!" burst from others of that stunned assemblage.

* * * * *

Their comments were cut short by a broadcast voice, thin and clear,
tremendously amplified, a voice Kendrick recognized at once as that of
Cor.

"People of America!" it said. "We of the planet Vada have come to
conquer your country. You will be given forty-eight hours to lay down
your arms. If complete surrender has not been made by high noon, two
days from now, New York will be destroyed."

The voice ceased. The humming recommenced--waned in volume till it
died away. Twilight turned once more to midday.

Peering fixedly through the west windows of the laboratory, the little
assemblage saw the disc swallowed up in the clear blue sky.

Then they turned, faced one another gravely.

Outside, on the streets, confusion reigned. In newspaper plants,
presses were whirling. In telegraph and cable offices, keys were
ticking. From radio towers, waves were speeding.

Within an hour, the nation and the world knew of this planetary
invader and its staggering ultimatum.

Naturally, the government at Washington refused to meet these shameful
terms. Military and naval forces were rushed to the threatened
metropolis. The Atlantic Fleet steamed up from Hampton Roads under
forced draught and assembled in the outer harbor. Thousands of planes
gathered at Mitchell Field and other nearby aerodromes.

* * * * *

But where was the enemy? He must be miles up in space, Kendrick knew,
as he toiled feverishly in the laboratory over his experiment after a
sleepless night. For had that flying city been nearer earth, it could
not have maintained invisibility without that peculiar humming
vibration.

Scout planes urged on by impatient squadron commanders, climbed till
they reached their ceilings, sear