lmost irresistible--they have generators that burn arenak
as though it were so much paper, and a series of deadly frequencies
against which only a copper-driven ray screen is effective, and even
that does not stand up long."

"How come you lasted till now, then?" asked Seaton.

"They have nothing like the _Skylark_, and no knowledge of intra-atomic
energy. Therefore their space-ships are of the rocket type, and for that
reason they can cross only at the exact time of conjunction, or whatever
you call it--no, not conjunction, exactly, either, since the two planets
do not revolve around the same sun: but when they are closest together.
Our solar system is so complex, you know, that unless the trips are
timed exactly, to the hour, the vessels will not be able to land upon
Osnome, but will be drawn aside and be lost, if not actually drawn into
the vast central sun. Although it may not have occurred to you, a little
reflection will show that the inhabitants of all the central planets,
such as Osnome, must perforce be absolutely ignorant of astronomy, and
of all the wonders of outer space. Before your coming we knew nothing
beyond our own solar system, and very little of that. We knew of the
existence of only such of the closest planets as were brilliant enough
to be seen in our continuous sunlight, and they were few. Immediately
after your coming I gave your knowledge of astronomy to a group of our
foremost physicists and mathematicians, and they have been working
ceaselessly from space-ships--close enough so that observations could be
recalculated to Osnome, and yet far enough away to afford perfect
'seeing,' as you call it."

"But I don't know any more about astronomy than a pig does about
Sunday," protested Seaton.

"Your knowledge of details is, of course, incomplete," conceded Dunark,
"but the detailed knowledge of the best of your Earthly astronomers
would not help us a great deal, since we are so far removed from you in
space. You, however, have a very clear and solid knowledge of the
fundamentals of the science, and that is what we need, above all
things."

"Well, maybe you're right, at that. I do know the general theory of the
motions, and I studied some Celestial Mechanics. I'm awfully weak on
advanced theory, though, as you'll find out when you get that far."

"Perhaps--but since our enemies have no knowledge of astronomy whatever,
it is not surprising that their rocket-ships can be launched only at one
par