_The Inferior Planets_.--When the telescope was invented, the phases
of Venus attracted much attention; but the brightness of this planet,
and her proximity to the sun, as with Mercury also, seemed to be a bar
to the discovery of markings by which the axis and period of rotation
could be fixed. Cassini gave the rotation as twenty-three hours, by
observing a bright spot on her surface. Shroter made it 23h. 21m. 19s.
This value was supported by others. In 1890 Schiaparelli[2] announced
that Venus rotates, like our moon, once in one of her revolutions, and
always directs the same face to the sun. This property has also been
ascribed to Mercury; but in neither case has the evidence been
generally accepted. Twenty-four hours is probably about the period of
rotation for each of these planets.

Several observers have claimed to have seen a planet within the orbit
of Mercury, either in transit over the sun's surface or during an
eclipse. It has even been named _Vulcan_. These announcements would
have received little attention but for the fact that the motion of
Mercury has irregularities which have not been accounted for by known
planets; and Le Verrier[3] has stated that an intra-Mercurial planet
or ring of asteroids would account for the unexplained part of the
motion of the line of apses of Mercury's orbit amounting to 38" per
century.

_Mars_.--The first study of the appearance of Mars by Miraldi led him
to believe that there were changes proceeding in the two white caps
which are seen at the planet's poles. W. Herschel attributed these
caps to ice and snow, and the dates of his observations indicated a
melting of these ice-caps in the Martian summer.