st
known period of any of these bodies, namely, 3-1/3 years. Encke
predicted that it would return in 1822. This duly occurred, the comet
passing at its nearest to the sun within three hours of the time
indicated; being thus the second instance of the fulfilment of a
prediction of the kind. A certain degree of irregularity which Encke's
Comet displays in the dates of its returns to the sun, has been supposed
to indicate that it passes in the course of its orbit through some
retarding medium, but no definite conclusions have so far been arrived
at in this matter.

A comet, which appeared in 1826, goes by the name of Biela's Comet,
because of its discovery by an Austrian military officer, Wilhelm von
Biela. This comet was found to have a period of between six and seven
years. Certain calculations made by Olbers showed that, at its return in
1832, it would pass _through the earth's orbit_. The announcement of
this gave rise to a panic; for people did not wait to inquire whether
the earth would be anywhere near that part of its orbit when the comet
passed. The panic, however, subsided when the French astronomer, Arago,
showed that at the moment in question the earth would be some 50
millions of miles away from the point indicated!

[Illustration: PLATE XVII. DONATI'S COMET

From a drawing made on October 9th, 1858, by G.P. Bond, of Harvard
College Observatory, U.S.A. A good illustration of Bredikhine's theory:
note the straight tails of his _first_ type, and the curved tail of his
_second_.

(Page 257)]

In 1846, shortly after one of its returns, Biela's Comet divided into
two portions. At its next appearance (1852) these portions had separated
to a distance of about 1-1/2 millions of miles from each other. This
comet, or rather its constituents, have never since been seen.

Perhaps the most remarkable comet of recent times was that of 1858,
known as Donati's, it having been discovered at Florence by the Italian
astronomer, G.B. Donati. This comet, a magnificent object, was visible
for more than three months with the naked eye. Its tail was then 54
millions of miles in length. It was found to revolve around the sun in a
period of over 2000 years, and to go out in its journey to about 5-1/2
times the distance of Neptune. Its motion is retrograde, that is to say,
in the contrary direction to the usual movement in the solar system. A
number of beautiful drawings of Donati's Comet were made by the American
astronomer, G.