of Halley's comet have been calculated by J. R.
Hind, and more recently by P. H. Cowell and A. C. D. Crommelin of
Greenwich, the latter having carried the comet back to 87 B.C. with
certainty, and to 240 B.C. with fair probability. It was detected by Max
Wolf at Heidelberg on plates exposed on Sept. 11, 1909, and subsequently
on a Greenwich plate of Sept. 9.

The known comet of shortest period bears the name of J. F. Encke, the
astronomer who first investigated its orbit and showed its periodicity.
It was originally discovered in 1789, but its periodicity was not
recognized until 1818, after it had been observed at several returns.
This comet has given rise to a longer series of investigations than any
other, owing to Encke's result that the orbit was becoming smaller, and
the revolutions therefore accelerated, by some unknown cause, of which
the most plausible was a resisting medium surrounding the sun. As this
comet is almost the only one that passes within the orbit of Mercury, it
is quite possible that it alone would show the effect of such a medium.
Recent investigations of this subject have been made at the Pulkova
Observatory, first by F. E. von Asten and later by J. O. Backlund who,
in 1909, was awarded the Gold Medal of the Royal Astronomical Society
for his researches in this field. During some revolutions there was
evidence of a slight acceleration of the return, and during others there
was not.

The following is a list (compiled in 1909) of comets which are well
established as periodic, through having been observed at one or more
returns. In addition to what has already been said of several comets in
this list the following remarks may be made. Tuttle's comet was first
seen by P. F. A. Mechain in 1790, but was not recognized as periodic
until found by Tuttle in 1858, when the resemblance of the two orbits
led to the conclusion of the identity of the bodies, the period of which
was soon made evident by continued observations. The comets of Pons and
Olbers are remarkable for having an almost equal period. But their
orbits are otherwise totally different, so that there does not seem to
be any connexion between them. Brorsen's comet seems also to be
completely dissipated, not having been seen since 1879.

_List of Periodic Comets observed at more than one Return._

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|Designation.| 1st Perih. | Last Perih. | Perio