necessary observations are of a very delicate
nature, and difficult to make, and some astronomers doubt whether we
possess sufficient proof that Mercury has an atmosphere. At any rate,
its atmosphere is very rare as compared with the earth's, but we need
not, on that account, conclude that Mercury is lifeless. Possibly, in
view of certain other peculiarities soon to be explained, a rare
atmosphere would be decidedly advantageous.

Being much nearer the sun than the earth is, Mercury can be seen by us
only in the same quarter of the sky where the sun itself appears. As it
revolves in its orbit about the sun it is visible, alternately, in the
evening for a short time after sunset and in the morning for a short
time before sunrise, but it can never be seen, as the outer planets are
seen, in the mid-heaven or late at night. When seen low in the twilight,
at evening or morning, it glows with the brilliance of a bright
first-magnitude star, and is a beautiful object, though few casual
watchers of the stars ever catch sight of it. When it is nearest the
earth and is about to pass between the earth and the sun, it temporarily
disappears in the glare of the sunlight; and likewise, when it it is
farthest from the earth and passing around in its orbit on the opposite
side of the sun, it is concealed by the blinding solar rays.
Consequently, except with the instruments of an observatory, which are
able to show it in broad day, Mercury is never visible save during the
comparatively brief periods of time when it is near its greatest
apparent distance east or west from the sun.

The nearer a planet is to the sun the more rapidly it is compelled to
move in its orbit, and Mercury, being the nearest to the sun of all the
planets, is by far the swiftest footed among them. But its velocity is
subject to remarkable variation, owing to the peculiar form of the orbit
in which the planet travels. This is more eccentric than the orbit of
any other planet, except some of the asteroids. The sun being situated
in one focus of the elliptical orbit, when Mercury is at perihelion, or
nearest to the sun, its distance from that body is 28,500,000 miles, but
when it is at aphelion, or farthest from the sun, its distance is
43,500,000 miles. The difference is no less than 14,000,000 miles! When
nearest the sun Mercury darts forward in its orbit at the rate of
twenty-nine miles in a second, while when farthest from the sun the
speed is reduced to twenty-