g 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