Two planes hanging from her rack," he muttered, half to himself and
half to the officers standing around him. "Both Navy. Say, they're
dropping off! Not coming this way, either. Going northeast. Fast, too.
Can't see 'em any more.... Those men getting up from the _Saratoga_?
Good. We'll find out something soon. Here she comes!"

Closer and closer roared the dirigible. Two planes from the _Saratoga_
were swooping up to enter her rack, but the other two planes that
shortly before had been suspended from it were gone--already vanished
into the northeast.

"Don't understand this at all!" said the Admiral of the Black, or
Pacific, Fleet of the United States Navy.

* * * * *

Things had broken well, Chris Travers considered. He had only wounded
the invisible raider; but, luckily, had wounded him badly, so that,
evidently, just one object was in the man's mind: to get back to where
he came from, to where he could find help. He seemed oblivious of the
scout that was following behind at the full speed of its mighty rotary
motor, following him to his base, wherever it was.

"Just as well I didn't kill him," Chris muttered.

The rush of wind had cleared his brain; his faculties were steady and
normal. Not so with the man in the plane he pursued. It was flying
crazily, but clinging to one course, nevertheless--into the northeast,
towards land, some two hundred and fifty miles over the horizon.

The great silver shape of the ZX-1, barren, now, of life, dropped
away, speeding ever due west; the hazy dots and blur of smoke which
denoted the motionless Black Fleet vanished. But Chris was in contact
with the fleet's flagship once more, through the compact
radio-telephone set of his scout. As he flew, his eyes fixed steadily
on the plane ahead, he was rapping into the microphone the story of
what had happened. He told of the invisibility of the strange
marauder, of how accurately he had judged the time of his raids; of
how he, Chris, had managed to prevent the destruction of the ZX-1.

"He uses a tremendously expansive gas resembling carbon monoxide," he
went on. "It seeps into every cranny of the dirigible, killing
everything. The crews got no warning; they didn't know what was
happening; couldn't see him! Well, I managed to wound him on the ZX-1.
He beat it. I'm following him. If he lasts out, he'll go to where he
came from, and we'll find out who's in back of all this. Let you know
where his base