earth! Another circumstance which militates against its habitability
is that, according to the results of the best telescopic studies, it
always keeps the same face toward the sun, so that one half of the
planet is perpetually exposed to the fierce solar rays, and the other
half faces the unmitigated cold of open space. Venus, the next in
distance from the sun, is almost the exact twin of the earth in size,
and many arguments may be urged in favor of its habitability, although
it is suspected of possessing the same peculiarity as Mercury, in
always keeping the same side sunward. Unfortunately its atmosphere
appears to be so dense that no permanent markings on its surface are
certainly visible, and the question of its actual condition must, for
the present, be left in abeyance. Mars, the first planet more distant
from the sun than the earth, is the special subject of this chapter,
and will be described and discussed a few lines further on. Jupiter,
Saturn, Uranus, and Neptune, the four giant planets, all more distant
than Mars, and each more distant than the other in the order named,
are all regarded as uninhabitable because none of them appears to
possess any degree of solidity. They may have solid or liquid nuclei,
but exteriorly they seem to be mere balls of cloud. Of course, one can
imagine what he pleases about the existence of creatures suited to the
physical constitution of such planets as these, but they must be
excluded from the category of habitable worlds in the ordinary sense
of the term. We go back, then, to Mars.

It will be best to begin with a description of the planet. Mars is
4230 miles in diameter; its surface is not much more than one-quarter
as extensive as that of the earth (.285). Its mean distance from the
sun is 141,500,000 miles, 48,500,000 miles greater than that of the
earth. Since radiant energy varies inversely as the square of
distance, Mars receives less than half as much solar light and heat as