thought, however, by a certain astronomer named Clairaut, that
the larger planets, as Saturn and Jupiter, might interfere with its
orbital motions, and after careful calculations a difference of 618 days
was allowed, which brought its anticipated reappearance down to April
1759. It actually reappeared in March of that year. Its next
reappearance was fixed to take place about November 1835. The comet
became visible on 5th August 1835, and continued to be seen till April
1836, when it again disappeared.

As the reappearance of the comet was calculated by the application of
the Newtonian Law of Gravitation, such a result only gave added
confirmation to the application of that law to cometary bodies.

Of the Long Period Comets there are several known. That of 1858 has a
period, it is thought, of 2000 years. The 1811 comet has a period of
3000 years, while that of 1844 has a period of over 10,000 years. All
these comets move in orbits of such great size that their return is
improbable. One of the characteristic features about Long Period Comets
is their great brilliancy and size.

The 1858 comet, known as Donati's Comet, was first seen by that
astronomer at Florence in June. It was invisible, however, to the naked
eye, as it only appeared through the telescope like a faint cloud of
light, gradually getting brighter and brighter. Toward the end of August
it began to show signs of developing a tail, and became visible to the
eye on August 29th. During September and October it greatly increased in
size and brilliancy, and was plainly visible in the western heavens.
After October 10th it was only visible in the southern hemisphere,
gradually decreasing in brightness. It was seen till March 1859, when it
disappeared, and will probably not return till the year 3858, as its
period of revolution is about 2000 years.

Donati's Comet passed between the earth and many stars, which could be
seen very distinctly through its tail. One of the stars was Arcturus,
and, though some of the densest parts of the comet passed over it, yet
the star could be seen all the time, thus conclusively proving that the
head and tail of a comet are only composed of gaseous matter, probably
condensed Aether, as suggested in Art. 111.

ART. 114. _Parts of a Comet._--A comet may be divided into three parts:
1st, Nucleus; 2nd, Head or Coma; and 3rd, Tail.

The nucleus is the central part of the head or coma, and is generally
the brightest part of the w