est footfall.

Dick sprang forward to the door, but his outstretched arms encountered
only emptiness. In spite of the Secretary of State's instructions, he
was almost minded to apprehend the man. If he could get him!

The corridor was empty. A guard of soldiers was at the entrance, but
they did not block the entrance. Even now Von Kettler might be passing
them! Why didn't his feet sound upon the floor? How could a bulky man
glide so smoothly?

Perhaps because Dick was undecided what to do, Von Kettler escaped
him. By the time he reached the guards he knew he had escaped.
Suddenly there came an unexpected denouement. Somewhere on the White
House lawn a guard challenged, fired. The snap of one of the silenced
automatics answered him.

When Dick and the guards reached the spot, the man was lying in a
crumpled heap.

"An airplane," he gasped. "Invisible airplane. I--bumped into it.
Men--in it. The damned dogs!"

He died. Dick stared around him. There was no sign of any airplane on
the lawn, nothing but the tents of the guards, white in the moonlight,
and the grim array of anti-aircraft guns that Dick had placed there.

But behind the White House, in hastily constructed hangars, were a
half-dozen of the latest pursuit airships--beautiful slim hulls,
heavily armored, with armored turrets containing each a quick-firer
with the new armor-piercing bullets. One of these ships, Dick's own,
was kept perpetually warmed and ready to take the air.

* * * * *

Dick raced across the lawn, yelled to the startled guard in charge.
The mechanics came running from their quarters. Almost by the time
Dick reached it the ship was ready.

He twirled the helicopter starter, and she roared and zoomed, taking
an angle of a hundred and twenty-five degrees upward off a runway of
twenty yards. Into the air she soared, into the moonlight, up like an
arrow for five hundred feet.

Dick pulled the soaring lever, and she hung there, buzzing like a bee
as her helicopters, counteracting the pull of gravity, held her
comparatively stable. He scanned the air all about him.

Washington lay below, her myriad lights gleaming. Immediately beneath
him Dick saw the guns and the tents of the soldiers, and the little
group that was removing the body of the murdered soldier on a
stretcher. But there were no signs of any hostile craft.

Had the murdered man really bumped into an invisible airship, or had
he only thought he