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 such a distance that their disks
resembled that of the full moon, the eye would notice no inequality
between them.
The mean density of Venus is about nine tenths of that of the earth, and
the force of gravity upon its surface is in the ratio of about 85 to 100
as compared to its force on the surface of the earth.
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