being
that we almost always obtain a clear view of the detail on its surface.
Indeed, it is only to be expected from the kinetic theory that Mars
could not retain much of an atmosphere, as the force of gravity at its
surface is less than one-half of what we experience upon the earth. It
should here be mentioned that recent researches with the spectroscope
seem to show that, whatever atmosphere there may be upon Mars, its
density at the surface of the planet cannot be more than the one-fourth
part of the density of the air at the surface of the earth. Professor
Lowell, indeed, thinks it may be more rarefied than that upon our
highest mountain-tops.

Seen with the naked eye, Mars appears of a red colour. Viewed in the
telescope, its surface is found to be in general of a ruddy hue, varied
here and there with darker patches of a bluish-green colour. These
markings are permanent, and were supposed by the early telescopic
observers to imply a distribution of the planet's surface into land and
water, the ruddy portions being considered as continental areas (perhaps
sandy deserts), and the bluish-green as seas. The similarity to our
earth thus suggested was further heightened by the fact that broad white
caps, situated at the poles, were seen to vary with the planet's
seasons, diminishing greatly in extent during the Martian summer (the
southern cap in 1894 even disappearing altogether), and developing again
in the Martian winter.[18] Readers of Oliver Wendell Holmes will no
doubt recollect that poet's striking lines:--

"The snows that glittered on the disc of Mars
Have melted, and the planet's fiery orb
Rolls in the crimson summer of its year."

A state of things so strongly analogous to what we experience here,
naturally fired the imaginations of men, and caused them to look on Mars
as a world like ours, only upon a much smaller scale. Being smaller, it
was concluded to have cooled quicker, and to be now long past its prime;
and its "inhabitants" were, therefore, pictured as at a later stage of
development than the inhabitants of our earth.

Notwithstanding the strong temptation to assume that the whiteness of
the Martian polar caps is due to fallen snow, such a solution is,
however, by no means so simple as it looks. The deposition of water in
the form of snow, or even of hoar frost, would at least imply that the
atmosphere of Mars should now and then display traces of aqueous vapour,
which it does not appear to do.[