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.
But if we suppose Mercury to possess an atmosphere much rarer than that
of the earth, we may perceive therein a possible provision against the
excessive solar heat to which it is subjected, since, as we see on high
mountains, a light air permits a ready radiation of heat, which does not
become stored up as in a denser atmosphere.
As the sun pours its heat without cessation upon the day hemisphere the
warmed air must rise and flow off on all sides into the night
hemisphere, while cold air rushes in below, to take its place, from the
region of frost and darkness. The intermediate areas, which see the sun
part of the time, as explained above, are perhaps the scene of
contending winds and tempests, where the moisture, if there be any, is
precipitated, through the rapid cooling of the air, in whelming floods
and wild snow-storms driven by hurrying blasts from the realm of endless
night.
Enough seems now to have been said to indicate clearly the hopelessness
of looking for any analogies between Mercury and the earth which would
warrant the conclusion that the former planet is capable of supporting
inhabitants or forms of life resembling those that swarm upon the
latter.
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