He was led along the passage, past the guards, and up the stairs
again. The top door opened upon vacancy; it closed, and vanished. Dick
felt the rugs beneath his feet, but he was to all appearances standing
on a square of bare earth in the middle of a prairie.

"Come!" barked the guard again, and Dick accompanied him, trailing his
silver chain. Behind came Von Kettler.

"Here are steps!" said the guard, after they had proceeded a short
distance.

Dick stumbled against the lowest step of an invisible flight. The
breeze was cut off, showing that they had entered a building.
Underneath was a large oval of bare ground. Dick found a handrail and
groped his way up around a spiral staircase, four flights of it.

"Here is a room!"

* * * * *

Dick saw that widening edge of door again. The room inside was
perfectly visible, though it seemed to be supported upon air. It was a
spheroid, of huge size, with a number of large windows set into the
walls, and it was filled with machinery. About a dozen workmen in
blue blouses were moving to and fro, attending to what appeared to be
a number of enormous dynamos, but there were other apparatus of whose
significance Dick was ignorant. The dynamos were whirring with intense
velocity, but not the slightest sound was audible.

Von Kettler stepped to a switch attached to a stanchion of white
metal, surmounted by a huge opaque glass dome, and threw it over.
Instantly the hum and whir of machinery became audible, the sound of
footsteps, the voices of the workmen, and the creak of boards beneath
their feet.

"You see, we have discovered the means of destroying sound waves as
well as shadows, and it was a much simpler feat," said Von Kettler
with a sneer. "Tell them that when you get back to Washington, Yankee
pig. Also you might be interested to know that most of your bombs fell
on camouflaged structures that we had erected with the intention of
deceiving you."

He gestured to Dick to precede him, and halted him at a plain round
iron pipe or rod that rose up through the floor and passed through the
roof. It was surrounded by a mesh of fine wire. Attached to it were
various gauges, with dials showing red and black numbers.

"This is perhaps our greatest achievement, swine," remarked Von
Kettler, affably. "You shall see its operations from above." He
pointed to a narrow spiral staircase rising at the far end of the
room. "It is the practical applicati