lination 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 distance of fourteen
thousand six hundred miles. As it takes but little longer to complete
a revolution than Mars does to rotate on its axis, it remains in the
Martial sky one hundred and thirty-two hours between rising and
setting, passing through all the phases from new moon to full and back
again four times; that is, it swings four times around Mars before
going below the horizon. It is one of the smallest bodies discovered
with a telescope. The inner one, Phobos, is considerably larger,
having a diameter of about twenty miles. It is but twenty-seven
hundred miles from Mars's surface, and completes its revolution in
seven hours and thirty-eight minutes, which is shorter than any other
known period, Jupiter's nearest moon being the next, with eleven hours
and fifty-nine minutes. It thus revolves in less than a third of the
time Mars takes to rotate, and must consequently rise in the west and
set in the east, as it is continually running ahead of the surface of
the planet, though the sun and all the other stars rise and set on Mars
in the same way as on the earth."

When about fifteen thousand miles from Mars, they sighted Deimos
directly ahead, and saw that they should pass on its left--i. e.,
behind--for