one sometimes sees, now in the West, in the
lingering shimmer of the twilight, now in the East, when the tender
roseate dawn announces the advent of a clear day, a small star of the
first magnitude which remains but a very short time above the horizon,
and then plunges back into the flaming sun. This is Mercury, the agile
and active messenger of Olympus, the god of eloquence, of medicine, of
commerce, and of thieves. One only sees him furtively, from time to
time, at the periods of his greatest elongations, either after the
setting or before the rising of the radiant orb, when he presents the
aspect of a somewhat reddish star.

This planet, like the others, shines only by the reflection of the Sun
whose illumination he receives, and as he is in close juxtaposition with
it, his light is bright enough, though his volume is inconsiderable. He
is smaller than the Earth. His revolution round the Sun being
accomplished in about three months, he passes rapidly, in a month and a
half, from one side to the other of the orb of day, and is alternately a
morning and an evening star. The ancients originally regarded it as two
separate planets; but with attentive observation, they soon perceived
its identity. In our somewhat foggy climates, it can only be discovered
once or twice a year, and then only by looking for it according to the
indications given in the astronomic almanacs.

[Illustration: FIG. 32.--Orbits of the four Planets nearest to the Sun.]

Mercury courses round the Sun at a distance of 57,000,000 kilometers
(35,000,000 miles), and accomplishes his revolution in 87 days, 23
hours, 15 minutes; _i.e._, 2 months, 27 days, 23 hours, or a little less
than three of our months. If the conditions of life are the same there
as here, the existence of the Mercurians must be four times as short as
our own. A youth of twenty, awaking to the promise of the life he is
just beginning in this world, is an octogenarian in Mercury. There the
fair sex would indeed be justified in bewailing the transitory nature of
life, and might regret the years that pass too quickly away. Perhaps,
however, they are more philosophic than with us.

[Illustration: FIG. 33.--Orbits of the four Planets farthest from the
Sun.]

The orbit of Mercury, which of course is within that of the Earth, is
not circular, but elliptical, and very eccentric, so elongated that at
certain times of the year this planet is extremely remote from the solar
focus, and recei