, and was evidently constructed from some
gas-impregnated materials. On this tarmac was a flight of shining
airplanes, ready to take off. There were the odd, ovoid figures of the
aviators in their silken overalls. More figures appeared, running out
from the buildings. It was clear that the sudden raid had taken them
all by surprise.

Luke Evans yelled and pointed. "We've got them now, sir!" Dick heard
above the whine of the helicopter engine. "We've--"

But of a sudden the old man's voice died away, though his mouth was
still moving.

Dick leaned out of his cockpit and fired a single red Very light, the
signal for the attack. And from each plane of his flight, beneath him,
a bomb slid from its rack and went hurtling down upon the gang below,
while the airplanes circled and hovered, each taking up its station.

* * * * *

Dick was too late. By a whole minute he had missed his chance. He
realized that immediately, for before the red light had flared from
his pistol, the hostile planes were in the air. He had flown too low,
and given the alarm.

It meant a fight now, instead of a mad dog destruction, and Dick did
not underestimate the power of the enemy. But he felt a thrill of
furious satisfaction at the prospect of battle. From every plane the
bombs were falling. Underneath, ruin and destruction, and leaping
flames--and yet darkness, save for the phosphorescent outlines of the
buildings.

And the lines of these were broken, converging into strange
criss-crosses of luminosity, as the beams fell in shapeless heaps.
Dark fire, sweeping through the headquarters of the Invisible Emperor,
a veritable hell for those below! A taste of the hell that they had
made for others!

Then a strange phenomenon obtruded itself upon Dick's notice. _Nothing
was audible!_ The bombs were falling, but they were falling silently.
No sound came up from beneath. And, except for the throbbing of his
engine, Dick would have thought it had stopped. He could no longer
hear it.

That terrific holocaust of death and destruction was inaudible.
Skimming the upper reach of the air, high above that wall of darkness,
Dick saw old Luke Evans pick up his end of the speaking-tube, and
mechanically followed suit. He could see the old man's lips moving.
But he heard nothing!

And now another phenomenon was borne in on his notice. His flight were
perhaps five hundred feet beneath him, hovering a little above the
barrag