"What is it?"
"The only reason we're starting this wall here is to keep well ahead
of the flow so we can work as long as possible, isn't it?"
"Yes, I suppose so. I never thought of trying anywhere else. The valley
would mean a much shorter dam, but if the flow isn't through it by now
it would be before we could get there--oh! Wait a minute!"
"Yes, sir. You can put the main switch anywhere in a D. C. circuit.
Where are the seismology stores we never had to use?"
Four minutes later the tractor set out from the _Albireo_, carrying
Rowson and Zaino. Six minutes after that it stopped at the base of the
ash cone which formed the north side of the valley from which the lava
was coming. They parked a quarter of the way around the cone's base
from the emerging flood and started to climb on foot, both carrying
burdens.
Forty-seven minutes later they returned empty-handed to the vehicle, to
find that it had been engulfed by the spreading liquid.
With noticeable haste they floundered through the loose ash a few
yards above the base until they had outdistanced the glowing menace,
descended and started back across the plain to where they knew the
ship to be, though she was invisible through the falling detritus.
Once they had to detour around a crack. Once they encountered one
which widened toward the chasm on their right, and they knew a detour
would be impossible. Leaping it seemed impossible, too, but they did
it. Thirty seconds after this, forty minutes after finding the tractor
destroyed, the landscape was bathed in a magnesium-white glare as the
two one-and-a-half kiloton charges planted just inside the crater rim
let go.
* * * * *
"Should we go back and see if it worked?" asked Zaino.
"What's the use? The only other charges we had were in the tractor.
Thank goodness they were nuclear instead of H. E. If it didn't work
we'd have more trouble to get back than we're having now."
"If it didn't work, is there any point in going back?"
"Stop quibbling and keep walking. Dr. Burkett, are you listening?"
"Yes, Captain."
"We're fresh out of tractors, but if you want to try it on foot you
might start a set of flow measures on the lava. Arnie wants to know
whether our landslide slid properly."
However, the two were able to tell for themselves before getting back
to the _Albireo_.
The flow didn't stop all at once, of course; but with the valley
feeding it blocked off by a pile of volcanic ash four hundred feet high
on one side, nearly fifty on the other and more than a quarter of a
mile long, its enthusiasm quickly subsided. It was thin, fluid stuff,
as Burkett had noted; but as it spread it cooled, and as it cooled it
thickened.
Six hours after the blast it had stopped with its nearest lobe almost a
mile from the ship, less than two feet thick at the edge.
When Mardikian's tractor arrived, Burkett was happily trying to analyze
samples of the flow, and less happily speculating on how long it would
be before the entire area would be blown off the planet. When Marini's
and Harmon's vehicles arrived, almost together, the specimens had been
loaded and everything stowed for acceleration. Sixty seconds after the
last person was aboard, the _Albireo_ left Mercury's surface at two
gravities.
The haste, it turned out, wasn't really necessary. She had been in
parking orbit nearly forty-five hours before the first of the giant
volcanoes reached its climax, and the one beside their former site was
not the first. It was the fourth.
"And that seems to be that," said Camille Burkett rather tritely as
they drifted a hundred miles above the little world's surface. "Just a
belt of white-hot calderas all around the planet. Pretty, if you like
symmetry."
"I like being able to see it from this distance," replied Zaino,
floating weightless beside her. "By the way, how much bonus should I
ask for getting that idea of putting the seismic charges to use after
all?"
"I wouldn't mention it. Any one of us might have thought of that. We
all knew about them."
"Anyone _might_ have. Let's speculate on how long it would have been
before anyone _did_."
"It's still not like the other idea, which involved your own specialty.
I still don't see what made you suppose that the gas pillar from the
volcano would be heavily charged enough to reflect your radio beam. How
did that idea strike you?"
* * * * *
Zaino thought back, and smiled a little as the picture of lightning
blazing around pillar, cloud and mountain rose before his eyes.
"You're not quite right," he said. "I was worried about it for a while,
but it didn't actually strike me."
It fell rather flat; Camille Burkett, Ph.D., had to have it explained
to her.