geographer; while its rocks and their contents, its
volcanoes and its earthquakes, are to be studied by the geologists and
the physicists.

CHAPTER X.

MARS.

Our nearer Neighbours in the Heavens--Surface of Mars can be
Examined in the Telescope--Remarkable Orbit of Mars--Resemblance of
Mars to a Star--Meaning of Opposition--The Eccentricity of the
Orbit of Mars--Different Oppositions of Mars--Apparent Movements of
the Planet--Effect of the Earth's Movement--Measurement of the
Distance of Mars--Theoretical Investigation of the Sun's
Distance--Drawings of the Planet--Is there Snow on Mars?--The
Rotation of the Planet--Gravitation on Mars--Has Mars any
Satellites?--Prof. Asaph Hall's great Discovery--The Revolutions of
the Satellites--Deimos and Phobos--"Gulliver's Travels."

The special relation in which we stand to one planet of our system has
necessitated a somewhat different treatment of that globe from the
treatment appropriate to the others. We discussed Mercury and Venus as
distant objects known chiefly by telescopic research, and by
calculations of which astronomical observations were the foundation. Our
knowledge of the earth is of a different character, and attained in a
different way. Yet it was necessary for symmetry that we should discuss
the earth after the planet Venus, in order to give to the earth its true
position in the solar system. But now that the earth has been passed in
our outward progress from the sun, we come to the planet Mars; and here
again we resume, though in a somewhat modified form, the methods that
were appropriate to Venus and to Mercury.

Venus and Mars have, from one point of view, quite peculiar claims on
our attention. They are our nearest planetary neighbours, on either
side. We may naturally expect to learn more of them than of the other
planets farther off. In the case of Venus, however, this anticipation
can hardly be realised, for, as we have already pointed out, its dense
atmosphere prevents us from making a satisfactory telescopic
examination. When we turn to our other planetary neighbour, Mars, we are
enabled to learn a good deal with regard to his appearance. Indeed, with
the exception of the moon, we are better acquainted with the details of
the surface of Mars than with those of any other celestial body.

This beautiful planet offers many features for consideration besides
those presented by its physical str