parts were of a dark greenish gray hue; these were supposed to be
seas and oceans. Other parts had a bright, warm tint; these were
supposed to be the continents. During the last twenty years much
has been learned as to how this planet looks, and the details of
its surface have been mapped by several observers, using the best
telescopes under the most favorable conditions of air and climate.
And yet it must be confessed that the result of this labor is not
altogether satisfactory. It seems certain that the so-called seas
are really land and not water. When it comes to comparing Mars
with the earth, we cannot be certain of more than a single point
of resemblance. This is that during the Martian winter a white
cap, as of snow, is formed over the pole, which partially melts
away during the summer. The conclusion that there are oceans whose
evaporation forms clouds which give rise to this snow seems
plausible. But the telescope shows no clouds, and nothing to make
it certain that there is an atmosphere to sustain them. There is
no certainty that the white deposit is what we call snow; perhaps
it is not formed of water at all. The most careful studies of the
surface of this planet, under the best conditions, are those made
at the Lowell Observatory at Flagstaff, Arizona. Especially
wonderful is the system of so-called canals, first seen by
Schiaparelli, but mapped in great detail at Flagstaff. But the
nature and meaning of these mysterious lines are still to be
discovered. The result is that the question of the real nature of
the surface of Mars and of what we should see around us could we
land upon it and travel over it are still among the unsolved
problems of astronomy.

If this is the case with the nearest planets that we can study,
how is it with more distant ones? Jupiter is the only one of these