d no sign of clouds or obscurations, and indeed no
indication of any atmospheric envelope; the surface of Mercury is
colourless, "a geography in black and white."

We may assert that, there is a strong _a priori_ probability in favour
of the reality of Schiaparelli's discovery. Mercury, being one of the
planets devoid of a moon, will be solely influenced by the sun in so far
as tidal phenomena are concerned. Owing, moreover, to the proximity of
Mercury to the sun, the solar tides on that planet possess an especial
vehemence. As the tendency of tides is to make Mercury present a
constant face to the sun, there need be little hesitation in accepting
testimony that tides have wrought exactly the result that we know they
were competent to perform.

Here we take leave of the planet Mercury--an interesting and beautiful
object, which stimulates our intellectual curiosity, while at the same
time it eludes our attempts to make a closer acquaintance. There is,
however, one point of attainable knowledge which we must mention in
conclusion. It is a difficult, but not by any means an impossible, task
to weigh Mercury in the celestial balance, and determine his mass in
comparison with the other globes of our system. This is a delicate
operation, but it leads us through some of the most interesting paths of
astronomical discovery. The weight of the planet, as recently determined
by Von Asten, is about one twenty-fourth part of the weight of the
earth, but the result is more uncertain than the determinations of the
mass of any of the other larger planets.

CHAPTER VIII.

VENUS.

Interest attaching to this Planet--The Unexpectedness of its
Appearance--The Evening Star--Visibility in Daylight--Lighted only
by the Sun--The Phases of Venus--Why the Crescent is not Visible to
the Unaided Eye--Variations in the Apparent Size of the Planet--The
Rotation of Venus--Resemblance of Venus to the Earth--The Transit
of Venus--Why of such Especial Interest--The Scale of the Solar
System--Orbits of the Earth and Venus not in the same
Plane--Recurrence of the Transits in Pairs--Appearance of Venus in
Transit--Transits of 1874 and 1882--The Early Transits of 1631 and
1639--The Observations of Horrocks and Crabtree--The Announcement
of Halley--How the Track of the Planet differs from Different
Places--Illustrations of Parallax--Voyage to Otaheite--The Result
of Encke--Probabl