either of those at the poles of the earth.

"As the Martian winters must be fully as severe as ours," said
Cortlandt, "on account of their length, the planet's distance
from the sun, and the twenty-seven and a half degrees inclination
of its axis, we can account for the smallness of its ice-caps
only by the fact that its oceans cover but one fourth of its
surface instead of three quarters, as on the earth, and there is
consequently a smaller evaporation and rain and snow-fall."

They were too much interested to think of sleeping that night,
and so, after dining comfortably returned to their observatory.
When within four million miles of Mars the Callisto began to
swerve perceptibly, its curve, as when near the moon beginning
with a spiral. They swung on unconcernedly, however, knowing
they could check their approach at any time. Soon Mars appeared
to have a diameter ten times as great as that of the moon, and
promised shortly to occupy almost one side of their sky.

"We must be on the lookout for the satellites," said Cortlandt;
"a collision with either would be worse than a wreck on a desert
island."

They therefore turned their glasses in the direction of the
satellites.

"Until Prof. Hall, at Washington, discovered the two satellites
in 1877," he continued, "Mars was supposed to be without moons.
The outer one, Deimos, is but six miles in diameter, and revolves
about its primary in thirty hours and eighteen minutes, at a