The secular cooling that must someday overtake our planet has
already gone far indeed with our neighbour.  Its physical condition is
still largely a mystery, but we know now that even in its equatorial
region the midday temperature barely approaches that of our coldest
winter.  Its air is much more attenuated than ours, its oceans have
shrunk until they cover but a third of its surface, and as its slow
seasons change huge snowcaps gather and melt about either pole and
periodically inundate its temperate zones.  That last stage of
exhaustion, which to us is still incredibly remote, has become a
present-day problem for the inhabitants of Mars.  The immediate
pressure of necessity has brightened their intellects, enlarged their
powers, and hardened their hearts.  And looking across space with
instruments, and intelligences such as we have scarcely dreamed of,
they see, at its nearest distance only 35,000,000 of miles sunward of
them, a morning star of hope, our own warmer planet, green with
vegetation and grey with water, with a cloudy atmosphere eloquent of
fertility, with glimpses through its drifting cloud wisps of broad
stretches of populous country and narrow, navy-crowded seas.