Neptune in 1873,
and is already hastening back to the warmth and light of the sun. It
will be looked for in 1911; and there is good hope of predicting,
long before it is seen, the time of its perihelion within a day.

_Biela's lost Comet._--This was a comet with a periodic time of
six years and eight months. It was observed in January, 1846, to
have separated into two parts of unequal brightness. The lesser
part grew for a month until it equalled the other, then became
smaller and disappeared, while the other was visible a month longer.
At disappearance the parts were 200,000 miles asunder. On its next
return, in 1852, the parts were 1,500,000 miles apart; sometimes
one was brighter and sometimes the other; which was the fragment
and which was the main body could not be recognized. They vanished
in September, 1852, and have never been seen since. Three revolutions
have been made since that time, but no [Page 130] trace of it could
be discovered. Probably the same influence that separated it into
parts, separated the particles till too thin and tenuous to be seen.
There is ground for believing that the earth passed through a part
of it, as before stated under the head of meteors.

_The Great Comet of_ 1843 passed nearer the sun than any known
body. It almost grazed the sun. If it ever returns, it will be in
A.D. 2373.

_Donati's Comet of_ 1858.--This was one of the most magnificent
of modern times. During the first three months it showed no tail,
but from August to October it had developed one forty degrees in
length. Its period is about two thousand years. Every reader remembers
the comet of the summer of 1875.

_Encke's Comet._--This comet has become famous for its supposed
confirmation of the theory that space was filled with a substance
infinitely tenuous, which resisted the passage of this gaseous
body in an appreciable degree, and in long ages would so retard
the motion of all the planets that gravitation would draw them
all one by one into the sun. We must not be misled by the term
retardation to suppose it means behind time, for a retarded body
is before time. If its velocity is diminished, the attraction of
the sun causes it to take a smaller orbit, and smaller orbits mean
increased speed--hence the supposed retardation would shorten its
periodic time. This comet was thought to be retarded two and a
half hours at each revolution. If it was, it would not prove the
existence of the resisting medium. Other c