only on some such hypothesis can the appearance and
apparent loss of irregular comets be explained.

ART. 113. _Short Period Comets and Long Period Comets._--We have seen in
the previous article, that some Comets revolve round the sun in closed
orbits of exceeding great eccentricity, and the return of these may be
calculated with certainty. There are about two dozen comets which revolve
around the sun, and which return at intervals lying between three years
and 76 years.

This class of comets may be divided into two kinds, which are known as
Short Period Comets and Long Period Comets respectively. The following
table gives a list of the chief of the Short Period Comets, together
with some particulars relating to time of revolution, etc.:--

COMETS. PERIOD OF PERIHELION APHELION
REVOLUTION. DISTANCE. DISTANCE.

Encke's ... 3-1/4 years. 32,000,000 miles. 387,000,000 miles.
De Vico's ... 5-1/2 " 110,000,000 " 475,000,000 "
Biela's ... 6-1/2 " 82,000,000 " 585,000,000 "
D'Arrest's ... 6-1/2 "
Faye's ... 7-1/2 " 192,000,000 " 603,000,000 "
Halley's ... 76-3/4 " 56,000,000 " 3,200,000,000 "

Encke's Comet was discovered by Professor Encke of Berlin, and named
after him. It revolves in an ellipse of great eccentricity, as proved by
the fact that when nearest to the sun, it is inside Mercury's orbit, but
when furthest away from the sun, it passes beyond the orbit of Mars,
reaching almost to the orbit of Jupiter. One of the most remarkable
facts about this comet is, that it has done more to establish the
existence of that resisting medium around the sun, whose existence we
have demonstrated, than any other comet. Encke found on its periodical
return that its mean distance was gradually getting less, and in order
to account for this, he supposed that it was due to the existence of a
resisting medium which enveloped the sun, and extended some distance
into space.

This conclusion has been supported in recent years by Von Asten, a
German mathematician, who has supported the theory of a resisting
medium. On this point Herschel writes in his _Outlines of Astronomy_,
Art. 577: "This is evidently the effect which would be produced by a
resistance experienced by the comet from a very rare aetherial medium
pervading the regions in which it moves; for suc