Compared with the earth it is so diminutive that it looks as if
it had been cut out on the pattern of a satellite rather than that of an
independent planet. Its diameter, 3,000 miles, only exceeds the moon's
by less than one half, while both Jupiter and Saturn, among their
remarkable collections of moons, have each at least one that is
considerably larger than the planet Mercury. But, insignificant though
it be in size, it holds the place of honor, nearest to the sun.
It was formerly thought that Mercury possessed a mass greatly in excess
of that which its size would seem to imply, and some estimates, based
upon the apparent effect of its attraction on comets, made it equal in
mean density to lead, or even to the metal mercury. This led to curious
speculations concerning its probable metallic composition, and the
possible existence of vast quantities of such heavy elements as gold in
the frame of the planet. But more recent, and probably more correct,
computations place Mercury third in the order of density among the
members of the solar system, the earth ranking as first and Venus as
second. Mercury's density is now believed to be less than the earth's in
the ratio of 85 to 100. Accepting this estimate, we find that the force
of gravity upon the surface of Mercury is only one third as great as
upon the surface of the earth--i.e., a body weighing 300 pounds on the
earth would weigh only 100 pounds on Mercury.
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