Other Worlds | Page 7

all let us endeavor to
obtain a clear idea of the actual size and mass of this strange little
planet. 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.
This is an important matter, because not only the weight of bodies, but
the density of the atmosphere and even the nature of its gaseous
constituents, are affected by the force of gravity, and if we could
journey from world to world, in our bodily form, it would make a great
difference to us to find gravity considerably greater or less upon other
planets than it is upon our own. This alone might suffice to render some
of the planets impossible places of abode for us, unless a decided
change were effected in our present physical organization.

One of the first questions that we should ask about a foreign world to
which we proposed to pay a visit, would relate to its atmosphere. We
should wish to know in advance if it had air and water, and in what
proportions and quantities. However its own peculiar inhabitants might
be supposed able to dispense with these things, to us their presence
would be essential, and if we did not find them, even a planet that
blazed with gold and diamonds only waiting to be seized would remain
perfectly safe from our invasion. Now, in the case of Mercury, some
doubt on this point exists.
Messrs. Huggins, Vogel, and others have believed that they found
spectroscopic proof of the existence of both air and the vapor of water
on Mercury. But the necessary observations are of a very delicate
nature, and difficult to make, and some astronomers doubt whether we
possess sufficient proof that Mercury has an atmosphere. At any rate,
its atmosphere is very rare as compared with the earth's, but we need
not, on that account, conclude that Mercury is lifeless. Possibly, in
view of certain other peculiarities soon to be explained, a rare
atmosphere would be decidedly advantageous.
Being much nearer the sun than the earth is, Mercury can be seen by us
only in the same quarter of the sky where the sun itself appears. As it
revolves in its orbit about the sun it is visible, alternately, in the
evening for a short time after sunset and in the morning for a short time
before sunrise, but it can never be seen, as the outer planets are seen, in
the mid-heaven or late at night. When seen low in the twilight, at
evening or morning, it glows with the brilliance of a bright
first-magnitude star, and is a beautiful object, though few casual
watchers of the stars ever catch sight of it. When it is nearest the earth
and is about to pass between the earth and the sun, it temporarily
disappears in the glare of the sunlight; and likewise, when it it is
farthest from the earth and passing around in its orbit on the opposite
side of the sun, it is concealed by the blinding solar rays. Consequently,
except with the instruments of an observatory, which are able to show
it in broad day, Mercury is never visible save during the comparatively
brief periods of time when it is near its greatest apparent distance east
or west from the sun.

The nearer a planet is to the sun the more rapidly it