ied his prediction with the statement that, owing to these unknown
possibilities, his calculations might be a month wrong one way or the
other. Clairaut made this memorable communication to the Academy of
Sciences on the 14th of November, 1758. The attention of astronomers was
immediately quickened to see whether the visitor, who last appeared
seventy-six years previously, was about to return. Night after night the
heavens were scanned. On Christmas Day in 1758 the comet was first
detected, and it passed closest to the sun about midnight on the 12th of
March, just a month earlier than the time announced by Clairaut, but
still within the limits of error which he had assigned as being
possible.

The verification of this prediction was a further confirmation of the
theory of gravitation. Since then, Halley's comet has returned once
again, in 1835, in circumstances somewhat similar to those just
narrated. Further historical research has also succeeded in identifying
Halley's comet with numerous memorable apparitions of comets in former
times. It has even been shown that a splendid object, which appeared
eleven years before the commencement of the Christian era, was merely
Halley's comet in one of its former returns. Among the most celebrated
visits of this body was that of 1066, when the apparition attracted
universal attention. A picture of the comet on this occasion forms a
quaint feature in the Bayeux Tapestry. The next return of Halley's comet
is expected about the year 1910.

There are now several comets known which revolve in elliptic paths, and
are, accordingly, entitled to be termed periodic. These objects are
chiefly telescopic, and are thus in strong contrast to the splendid
comet of Halley. Most of the other periodic comets have periods much
shorter than that of Halley. Of these objects, by far the most
celebrated is that known as Encke's comet, which merits our careful
attention.

The object to which we refer has had a striking career during which it
has provided many illustrations of the law of gravitation. We are not
here concerned with the prosaic routine of a mere planetary orbit. A
planet is mainly subordinated to the compelling sway of the sun's
gravitation. It is also to some slight extent affected by the
attractions which it experiences from the other planets. Mathematicians
have long been accustomed to anticipate the movements of these globes by
actual calculation. They know how the place of the plan