miles distance had seen the mighty bulk of the Kaxorian
plane. At once they had dropped to the ground and then, by telephone
lines, had sent their report to far off Sonor.

In moments the interior of the _Solarite_ became a scene of swift
purposeful activity. All day the Terrestrians had been able to do so
little in preparation for the conflict they knew must come, the battle
for two worlds. They had wanted action, but they had no weapons except
their invisibility and the atomic hydrogen. It would not sink a plane.
It would only break open its armor, and they hoped, paralyze its crew.
And on this alone they must pin their hopes.

VI

Arcot lifted the _Solarite_ at once high into the air, and started
toward the point on the border, where the plane had been seen crossing.
In a short time Wade relieved him at the controls while he dressed.

They had been flying on in silence for about an hour, when suddenly Wade
made out in the distance the great bulk of the plane, against the dull
gray of the clouds, a mile or so above them. It seemed some monstrous
black bat flying there against the sky, but down to the sensitive
microphone on the side of the _Solarite_ came the drone of the hundred
mighty propellers as the great plane forged swiftly along.

Just how rapidly these giants moved, Arcot had not appreciated until he
attempted to overtake this one. It was going over a mile a second now--a
speed that demanded only that it move its own length in about
five-eights of a second! It made this tremendous speed by streamlining
and through sheer power.

The _Solarite_ hovered high above the dark ship at length, the roar of
the terrific air blast from its propellers below coming up to them as a
mighty wave of sound that made their own craft tremble! The hundred
gigantic propellers roaring below, however, would distribute their gas
perfectly.

"We're going invisible," Arcot exclaimed. "Look out!" There was a click
as the switch shut, and the _Solarite_ was as transparent as the air
above it. Arcot drove his ship swiftly, above and ahead of the mighty
colossus, then released the gas. There was a low hiss from the power
room, barely detectable despite the vacuum that shut them off from the
roar of the Kaxorian plane. The microphone had long since been
disconnected. Out of the gas vent streamed a cloud of purplish gas,
becoming faintly visible as it left the influence of the invisibility
apparatus, but only to those who kn