paths. Each of them must, therefore, have been near
the giant planet at some moment during their past history. Similarly the
other periodic comets of longer period approach near to the orbits of
either Saturn, Uranus, or Neptune, the last-mentioned planet being
probably responsible for the periodicity of Halley's comet. We have,
indeed, on more than one occasion, actually witnessed the violent
disturbance of a cometary orbit. The most interesting case is that of
Lexell's comet. In 1770 the French astronomer Messier (who devoted
himself with great success to the discovery of comets) detected a comet
for which Lexell computed the orbit, and found an ellipse with a period
of five years and some months. Yet the comet had never been seen before,
nor did it ever come back again. Long afterwards it was found, from most
laborious investigations by Burckhardt and Le Verrier, that the comet
had moved in a totally different orbit previous to 1767. But at the
beginning of the year 1767 it happened to come so close to Jupiter that
the powerful attraction of this planet forced it into a new orbit, with
a period of five and a half years. It passed the perihelion on the 13th
August, 1770, and again in 1776, but in the latter year it was not
conveniently situated for being seen from the earth. In the summer of
1779 the comet was again in the neighbourhood of Jupiter, and was thrown
out of its elliptic orbit, so that we have never seen it since, or,
perhaps, it would be safer to say that we have not with certainty
identified Lexell's comet with any comet observed since then. We are
also, in the case of several other periodic comets, able to fix in a
similar way the date when they started on their journeys in their
present elliptic orbits.

Such is a brief outline of the principal facts known with regard to
these interesting but perplexing bodies. We must be content with the
recital of what we know, rather than hazard guesses about matters beyond
our reach. We see that they are obedient to the great laws of
gravitation, and afford a striking illustration of their truth. We have
seen how modern science has dissipated the superstition with which, in
earlier ages, the advent of a comet was regarded. We no longer regard
such a body as a sign of impending calamity; we may rather look upon it
as an interesting and a beautiful visitor, which comes to please us and
to instruct us, but never to threaten or to destroy.

CHAPTER XVII.

SHO