t a comparatively rapid rate. Kepler
long ago maintained that "comets die," and this actually appears to be
the case. The ordinary periodic ones, such, for instance, as Encke's
Comet, are very faint, and becoming fainter at each return. Certain of
these comets have, indeed, failed altogether to reappear. It is notable
that the members of Jupiter's comet family are not very conspicuous
objects. They have small tails, and even in some cases have none at all.
The family, too, does not contain many members, and yet one cannot but
suppose that Jupiter, on account of his great mass, has had many
opportunities for making captures adown the ages.

Of the two theories to which allusion has above been made, that of
Bredikhine has been worked out so carefully, and with such a show of
plausibility, that it here calls for a detailed description. It appears
besides to explain the phenomena of comets' tails so much more
satisfactorily than that of Arrhenius, that astronomers are inclined to
accept it the more readily of the two. According to Bredikhine's theory
the electrical repulsive force, which he assumes for the purposes of his
argument, will drive the minutest particles of the comet in a direction
away from the sun much more readily than the gravitative action of that
body will pull them towards it. This may be compared to the ease with
which fine dust may be blown upwards, although the earth's gravitation
is acting upon it all the time.

The researches of Bredikhine, which began seriously with his
investigation of Coggia's Comet of 1874, led him to classify the tails
of comets in _three types_. Presuming that the repulsive force emanating
from the sun did not vary, he came to the conclusion that the different
forms assumed by cometary tails must be ascribed to the special action
of this force upon the various elements which happen to be present in
the comet. The tails which he classes as of the first type, are those
which are long and straight and point directly away from the sun.
Examples of such tails are found in the comets of 1811, 1843, and 1861.
Tails of this kind, he thinks, are in all probability formed of
_hydrogen_. His second type comprises those which are pointed away from
the sun, but at the same time are considerably curved, as was seen in
the comets of Donati and Coggia. These tails are formed of _hydrocarbon
gas_. The third type of tail is short, brush-like, and strongly bent,
and is formed of the _vapour of ir