had? Had those devils learned to apply the gas to
the surfaces of airplanes? There was no reason why they should not
have done so.

But surely the utmost ingenuity of man had not contrived to render a
modern plane, with its metalwork and machinery, absolutely
transparent?

* * * * *

And, again, how was it possible to have silenced the sound of engines,
the whir of a propeller, so that there should be no auditory
indication whatever of a plane's presence?

Dick looked all about him. Nothing was in the air--he could have sworn
it. He replaced the soaring lever and banked in a close circle, his
glance piercing the night. No, there was nothing.

Crash! Boom! The plane rocked violently, tossing upon gusts of air. A
huge, gaping hole of blackness had suddenly appeared in the middle of
the White House lawn. The tents were flat upon the ground. Through the
rising smoke clouds Dick saw tongues of flame.

No shell that, but a bomb, and dropped from the skies less than five
hundred feet from where Dick hovered. Yet there was nothing visible in
the skies save the round orb of the moon.

A rush of wind past Dick's face! One of the vanes of the helicopter
crumpled and fluttered away into the night. Dick needed no further
persuasion. The dead soldier had not lied.

Von Kettler had begun the fulfillment of his threat!

CHAPTER V

_The Enemy Strikes_

As Dick's airship veered and side-slipped, he kicked hard on the left
rudder and brought the nose around. Furiously he sprayed the air with
a leaden hail from his quick-firer. He heard a rush of wind go past
him, and realized that his unseen antagonist had all but rammed him.

Yet nothing was visible at all, save the moon and the empty sky. He
had heard the rush of the prop-wash, but he had seen nothing, heard
nothing else. Incredible as it seemed, the pilot was flying a plane
that had attained not merely invisibility but complete absence of all
sound.

Dick side-slipped down, pancaked, and crashed. He emerged from a plane
wrecked beyond hope of early repair, yet luckily with no injury beyond
a few minor bruises. He rushed toward the hangar, to encounter a bevy
of scared mechanics.

"Another plane! Rev one up quick!" he shouted.

Planes were already being wheeled out, pilots in flying suits and
goggles were striding beside them. Dick ordered one of them away,
stepped into his plane, and in a moment was in the air again.

In the minut