into every room and recess of the city, and exhaust it into the
shaft of a quiescent volcano, in whose gaseous outflow any trace of our
activities is, of course, imperceptible. For obvious reasons no rockets
or combustion motors are used in the city proper."

* * * * *

Thus Captain Czuv explained to the Terrestrials his own mode of life,
and received from them in turn full information concerning Earthly life,
activity, and science. Long they talked, and it was almost time to slow
down for the journey's end when the Callistonian brought the conversation
back to their immediate concerns.

"My lieutenant and I were upon a mission of some importance, but it is
more important to take you to Callisto, for there may be many things
in which you can help us. Not in rays--we know all the vibrations you
have mentioned and several others. The enemy, however, is supreme
in that field, and until our scientists have succeeded in developing
ray-screens, such as are used by the hexans, it would be suicidal to
use rays at all. Such screens necessitate the projection of pure, yet
dirigible, forces--you do not have them upon your planet?"

"No, and so far as I know such screens are also unknown upon Mars and
Venus, with whose inhabitants we are friendly."

"The inhabitants of all the planets should be friendly; the solar
system should be linked together in intercourse for common advancement.
But that is not to be. The hexans will eventually triumph here, and a
Jovian system peopled by hexans will have no intercourse with any human
civilization save that of internecine war. We, of Callisto, have only
one hope--or is it really a hope? In the South Polar country of Jupiter,
there dwells a race of beings implacably hostile to the hexans. They
seem to invade the country of the hexans frequently, even though they
are apparently repulsed each time. Our emissaries to the South Polar
country, however, have never returned--those beings, whatever they
are, if not actively inimical, certainly are not friendly toward us."

"You know nothing of their nature?"

"Nothing, since our electrical instruments are not sufficiently
sensitive to give us more than a general idea of what is transpiring
there, and vision is practically useless in that eternal fog. We know,
however, that they are far advanced in science, and we are thankful
indeed that none of their frightful flying fortresses have been launched
against us. They