.

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. If we would still believe that Mercury is a habitable globe we
must depend entirely upon the imagination for pictures of creatures able
to endure its extremes of heat and cold, of light and darkness, of
instability, swift vicissitude, and violent contrast.

In the next chapter we shall study a more peaceful and even-going world,
yet one of great brilliancy, which possesses some remarkable
resemblances to the earth, as well as some surprising divergences from
it.

CHAPTER III

VENUS, THE TWIN OF THE EARTH

We come now to a planet which seems, at the first glance, to afford a
far more promising outlook than Mercury does for the presence of organic
life forms bearing some resemblance to those of the earth. One of the
strongest arguments for regarding Venus as a world much like ours is
based upon its remarkable similarity to the earth in size and mass,
because thus we are assured that the force of gravity is practically the
same upon the two planets, and the force of gravity governs numberless
physical phenomena of essential importance to both animal and vegetable
life. The mean diameter of the earth is 7,918 miles; that of Venus is
7,700 miles. The difference is so slight that if the two planets were
suspended side by side in the sky, at suc