to its absolute limit and
was stored in long steel cylinders in the form of a liquid of extremely
low temperature. These cylinders were at once transferred to a special
steel vault where the temperature was kept at a low enough point to
prevent expansion and consequent loss of the explosive, not to speak of
the danger of destroying the entire lot of us in its escape.

The generating apparatus of the _Pioneer_ was to be dispensed with for
this trip, since it was of no value outside the atmosphere where there
was no air from which to extract the elements necessary for the
production of the explosive. Instead, the entire supply of fuel for the
trip was to be carried aboard the vessel in the cylinders we were
engaged in filling. Hart had calculated that there was just sufficient
room to store fuel for a trip of about two hundred thousand miles from
the earth and a safe return. We hoped this would be enough.

* * * * *

On the scaffolding around the _Pioneer_ there were now so many workers
that it seemed they must forever be in one another's way. But the work
was progressing with extreme rapidity. Already there projected from her
blunt nose a slender rod of shining metal which was the projector of one
of the destructive rays whose generator and auxiliaries were being
installed under the supervision of the government experts. The force had
been trebled and was now working in shifts of two hours each, the pace
being so exhausting that highest efficiency was obtained by using these
short periods.

Additional rocket rubes were being installed, and the steel framework of
a bulge now showed on the hull, this bulge being an additional fuel
storage compartment that would provide a slight additional resistance
and consequently lower speed in the lower levels, but would prove little
hindrance in level six and none at all in outer space.

When Hart emerged from his office he appeared to be very tired, indeed,
but his face bore an expression of triumph that could not be mistaken.
He and this little scientist from Washington had evidently arrived at
some momentous conclusion regarding the enemy.

"Jack," he said, when he reached my bench during his first round of the
hanger, "celestial mechanics is a wonderful thing. I had a hunch, and
this astronomer chap has proved it correct with his mathematics. Our
friend the enemy is out there in space at a point where his own mass and
velocity are exactly coun