ntered the planet
Jupiter, made two revolutions round the sun, in the second of which it
was observed, then again encountered the planet, to be thrown out of its
orbit into one which did not admit of determination. The comet was never
again found.

A general conclusion which seems to follow from these conditions, and is
justified by observations, so far as the latter go, is that comets are
not to be regarded as permanent bodies like the planets, but that the
conglomerations of matter which compose them are undergoing a process of
gradual dissipation in space. This process is especially rapid in the
case of the fainter periodic comets. It was first strikingly brought out
in the case of Biela's comet. This object was discovered in 1772, was
observed to be periodic after several revolutions had been made, and was
observed with a fair degree of regularity at different returns until
1852. At the previous apparition it was found to have separated into two
masses, and in 1852 these masses were so widely separated that they
might be considered as forming two comets. Notwithstanding careful
search at times and places when the comet was due, no trace of it has
since been seen. An examination of the table of periodic comets given at
the end of this article will show that the same thing is probably true
of several other comets, especially Brorsen's and Tempel's, which have
each made several revolutions since last observed, and have been sought
for in vain.

In view of the seemingly inevitable dissipation of comets in the course
of ages, and of the actually observed changes of their orbits by the
attraction of Jupiter, the question arises whether the orbits of all
comets of short period may not have been determined by the attraction of
the planets, especially of Jupiter. In this case the orbit would, for a
period of several centuries, have continued to nearly intersect that of
the planet. We find, as a matter of fact, that several periodic comets
either pass near Jupiter or have their aphelia in the neighbourhood of
the orbit of Jupiter. The approach, however, is not sufficiently close
to have led to the change unless in former times the proximity of the
orbits was much greater than it is now. As the orbits of all the bodies
of the solar system are subject to a slow secular change of their form
and position, this may only show that it must have been thousands of
years since the comet became one of short period. The two cases of mos