teracted by those of the earth and its
satellite, the moon. He is just floating around in space, doing no work
whatsoever to maintain his own position. He has temporarily assumed the
role of a second satellite to us and is revolving around us at a
definite period that was calculated by Lindquist. The gravitational pull
of the moon keeps him from falling to the earth and that of the earth
keeps him from approaching the moon. The resultant of the set of forces
is what determines his orbit and the disturbance in the normal balance
is what has been observed by the astronomers who reported changes in the
tides and in the moon's orbit."

* * * * *

"But Lindquist's figures prove that the vessel or fleet of the enemy
must be of tremendous size to produce such discrepancies,
infinitesimally small though they might seem. We have a big fellow with
whom to deal, but we know where to find him now."

"How can he work from a fixed position to make his attacks on the earth
at such widely separated points?" I asked.

"It isn't a fixed position in the first place, and besides the earth
rotates once in twenty-four hours, while the moon travels around the
earth once in about twenty-eight days. But, even so, the widespread
destruction could not be accounted for. He must send out scouting
parties or something of that sort. That is one of the things we are to
learn when we get out there. We'll have some fun, Jack."

"Will the _Pioneer_ be ready?" I asked. Evidently I was to go.

"She will, with the exception of the acceleration neutralizers. But I'm
having some heavily-cushioned and elastic supports made that will, I
believe, save us from injury. And I guess we can stand the discomfort
for once."

"Yes," I agreed, "in such a cause, I, for one, am willing to go through
anything to help keep this overwhelming disaster from our good old
world."

"Jack," he whispered, "we must prevent it. We've got to!"

Then he was gone, and I watched him for a moment as he dashed headlong
from one task to another. He was a whirlwind of energy once more.

* * * * *

Forty-three hours and twenty minutes had passed since the receipt of the
enemy's ultimatum. The last bolt was being tightened in the remodeled
_Pioneer_, and Secretary Simler and his staff were on hand to witness
the take-off of the vessel on which the hopes of the world were pinned.
The news of our attempt had been spread by