ect to find when we land. For instance, going to Mars
we must use an atmosphere more and more rarefied each day, until
gradually we become used to the thin air we expect to find there. Of
course, there is an especially designed barometer and thermometer,
capable of being read in the rear compartment, but exposed outside near
the rudder. The barometer will give us the pressure of the earthly
atmosphere as it becomes more and more rare with our ascent. It will
show us what pressure there is of the ether, which may vary
considerably, depending on our nearness to heavenly bodies. It will also
immediately indicate to us when we are entering any new atmosphere. When
we have arrived at Mars, we shall observe the exact pressure of the
Martian air, and then manufacture one of the same pressure inside, and
try breathing it before we venture out. The thermometer will give us the
temperature of the ether, will indicate the loss of heat as we leave the
sun, and will show us the Martian temperature before we venture into
it."

"But you have said the condensed air will be used to resist the outer
heat. This will certainly make it so hot it will be unfit to breathe," I
interposed.

"Ah, but you forget that the quick expansion of a gaslike air produces
cold. We shall regulate our temperature in that way. If it is becoming
too warm inside, the new measure of condensed air will be quickly
introduced into the partial vacuum, and its sudden expansion will
produce great cold, and freeze ice for us if we wish it. On the other
hand, if the compartments are already cold, we shall allow the condensed
air to enter very gradually, and its slow expansion will produce but
little cold. The question of heating the projectile is the most
difficult one I have found. We cannot have any fires, for there is no
way for the smoke to escape, and we cannot carry oxygen enough to keep
them burning. I have decided that we must depend on the heat arising
from outer friction and from absorption of the Sun's rays by our black
surface. When we are in ether where friction is very little, the
velocity will be all the greater, and I believe we shall always be warm
enough. You must remember, we shall not have the slightest suspicion of
a draught, and we must necessarily take along the warmest clothing for
use on Mars. Even then we probably cannot safely visit any but his
equatorial districts."

"This is the rudder, I suppose; but haven't you put it in wrong end
firs