s visitors,
travelling from out the depths of space, became visible in our skies, it
was regarded with apprehension and dread as betokening the occurrence
of calamities and direful events among the nations of the Earth.

The word comet is derived from the Greek {kome}, signifying
'hair,' to which the hazy, luminous appearance of those objects bears
some resemblance. A comet consists of a bright central part called the
_nucleus_; this is surrounded by layers of nebulous matter called the
_coma_, and both combined form the _head_, from which a long appendage
extends called the _tail_. The nucleus and tail are not essential parts
of a comet, for many have been observed in which both have been wanting.
The tail is frequently very conspicuous, and presents considerable
diversity both as regards its appearance and length. In some comets it
is entirely absent, and in others it has been observed to stretch over
an arc of sixty or seventy degrees, indicating a length of 100 to 150
million miles. Sometimes it is straight, and at other times it is curved
at the extremity; it has been observed bifurcated into two branches;
and, on rare occasions, comets have been seen with two or more tails.
The tail of a comet is always directed away from the Sun; it increases
in size as the comet approaches the orb, and diminishes as it recedes
from him. This depends upon the degree of heat to which the comet is
exposed, which has the effect of driving off or evaporating some of the
matter composing the head. During the time the comet is travelling round
the Sun there is a continuous emission of this highly attenuated matter,
which is visible as the tail, but when the comet begins to recede from
the orb and reaches cooler regions of space the tail diminishes in size
as the temperature becomes reduced, and ultimately it disappears.

The appearance of a comet in the sky is often sudden and unexpected, and
one of those erratic wanderers may become visible at any time and in any
part of the heavens. It was remarked by Kepler that there are as many
comets in the sky as there are fishes in the ocean. This may or may not
be true, for they only become visible when they approach the Sun, and
the time during which they remain so does not usually exceed a few weeks
or months. Ancient astronomers were much perplexed with the motions of
comets, which appeared to be much more irregular than those of other
celestial bodies and unconformed to any known laws. Tyc