th the revolution of their
planet about the sun, and with the exception of the sun itself--which
they could see by making a journey to the opposite hemisphere--all the
members of the solar system would pass in succession through their
mid-heaven, and two of them would present themselves with a magnificence
of planetary display unknown on the earth. Venus, when in opposition
under the most favorable circumstances, is scarcely more than 24,000,000
miles from Mercury, and, showing herself at such times with a fully
illuminated disk--as, owing to her position within the orbit of the
earth, she never can do when at her least distance from us--she must be
a phenomenon of unparalleled beauty, at least four times brighter than
we ever see her, and capable, of course, of casting a strong shadow.

The earth, also, is a splendid star in the midnight sky of Mercury, and
the moon may be visible to the naked eye as a little attendant circling
about its brilliant master. The outer planets are slightly less
conspicuous than they are to us, owing to increase of distance.

The revolution of the heavens as seen from the night side of Mercury is
quite different in period from that which we are accustomed to, although
the apparent motion is in the same direction, viz., from east to west.
The same constellations remain above the horizon for weeks at a time,
slowly moving westward, with the planets drifting yet more slowly, but
at different rates, among them; the nearer planets, Venus and the earth,
showing the most decided tendency to loiter behind the stars.

On the side where eternal sunlight shines the sky of Mercury contains no
stars. Forever the pitiless blaze of day; forever,

"All in a hot and copper sky
The bloody sun at noon."

As it is difficult to understand how water can exist on the night
hemisphere, except in the shape of perpetual snow and ice, so it is
hard to imagine that on the day hemisphere water can ever be
precipitated from the vaporous form. In truth, there can be very little
water on Mercury even in the form of vapor, else the spectroscope would
have given unquestionable evidence of its presence. Those who think that
Mercury is entirely waterless and almost, if not quite, airless may be
right. In these respects it would then resemble the moon, and, according
to some observers, it possesses another characteristic lunar feature in
the roughening of its surface by what seem to be innumerable volcanic
craters