inder's front side gave him the clue: from it, obviously, had come
the gas which had strangled the crews of the dirigibles, and the
covering over nose and mouth was a novel gas mask. The material they
were made of could, obviously, be rendered invisible--a virtue not
possessed by ordinary inorganic substances. Invisible death from an
invisible container, carried by an invisible man!

"Yess, dead," hissed Istafiev, probing the motionless, naked body. "He
just got here, told what had happened, and died. He was hurt too badly
to think of taking off the gas cylinder or putting on a coat. Well, it
makes no difference.... Here, Grigory, take off the mask and cylinder
and bury him. And you, Kashtanov, look well at this."

From the table, he picked up a large white piece of cardboard and
tapped it meaningly. There were two broad lines on it, running side by
side through other smaller lines and shaded patches, and there was
also a thick black arrow pointing to one particular place on it.

The chart was easy to understand. Chris Travers recognized it
immediately, and his heart seemed to stop for a moment as he did.

Their first step had been the dirigibles: their second was a blow
which paled the other into insignificance. And Chris told himself
desperately:

"It can't go through! It can't!"

The lines on the cardboard were a detailed map of the Panama Canal;
and the black arrow pointed unerringly to its most vulnerable,
unguarded and vital point, the Gatun Spillway, which, if wrecked,
would put the whole intricate Canal hopelessly out of commission.

* * * * *

Istafiev was speaking again, in low, terse tones, oblivious of the
desperate resolve forming in Chris's brain.

"Only one of the dirigibles had been destroyed. Well, it iss too bad,
but not fatal to the plan. The ZX-1 can hamper our country's
operations when she strikes, but if the ZX-2 were also in action, they
would be hampered much more--perhaps fatally. It iss not serious. So
we go ahead. Now, Kashtanov, for the last time, the scheme of wrecking
Gatun Spillway iss this:

"Note, here, the small golf course. That iss your landing space. You
know its location: a mile, perhaps, from Gatun Dam and the spillway.
At night, there iss no one near it or on it. You drop down to the golf
course from seven thousand feet: the helicopter motors are muffled,
and no one will hear you come. Some of the stretches of the course are
secluded and