846. The middle
region between Jupiter and Mars yielded the little planetoid Ceres
in 1801, Pallas in 1802, and one hundred and ninety others since.
The inner region between Mercury and the sun is of necessity full
of small meteoric bodies; the question is, are there any bodies
large enough to be seen?

The same great genius of Leverrier that gave us Neptune from the
observed perturbations of Uranus, pointed out perturbations in
Mercury that necessitated either a planet or a group of planetoids
between Mercury and the sun. Theoretical astronomers, aided by the
fact that no planet had certainly been seen, and that all asserted
discoveries of one had been by inexperienced observers, inclined
to the belief in a group, or that the disturbance was caused by
the matter reflecting the zodiacal light.

When the total eclipse of the sun occurred in 1878, [Page 138]
astronomers were determined that the question of the existence of an
intra-mercurial planet should be settled. Maps of all the stars in
the region of the sun were carefully studied, sections of the sky
about the sun were assigned to different observers, who should
attend to nothing but to look for a possible planet. It is now
conceded that Professor Watson, of Ann Arbor, actually saw the
sought-for body.

VULCAN.

The god of fire; its sign [Symbol], his hammer.

DISTANCE FROM THE SUN, 13,000,000 MILES. ORBITAL REVOLUTION, ABOUT
20 DAYS.

MERCURY.

The swift messenger of the gods; sign [Symbol], his caduceus.

DISTANCE FROM THE SUN, 35,750,000 MILES. DIAMETER, 2992 MILES.
ORBITAL REVOLUTION, 87.97 DAYS. ORBITAL VELOCITY, 1773 MILES PER
MINUTE. AXIAL REVOLUTION, 24H. 5M.

Mercury shines with a white light nearly as bright as Sirius; is
always near the horizon. When nearly between us and the sun, as
at D (Fig. 46, p. 113), its illuminated side nearly opposite to
us, we, looking from E, see only a thin crescent of its light.
When it is at its greatest angular distance from the sun, as A or
C, we see it illuminated like the half-moon. When it is beyond the
sun, as at E, we see its whole illuminated face like the full-moon.

The variation of its apparent size from the varying distance is
very striking. At its extreme distance from the earth it subtends
an angle of only five seconds; nearest to us, an angle of twelve
seconds. Its distance from the earth varies nearly as one to three,
and its apparent size in the inverse ratio.

[Page 139]
When Mercury c