recognised by him as identical with many previous comets. This comet,
called after Encke, has showed in each of its returns an inexplicable
reduction of mean distance, which led to the assertion of a resisting
medium in space until a better explanation could be found.[3]

Since that date fourteen comets have been found with elliptic orbits,
whose aphelion distances are all about the same as Jupiter's mean
distance; and six have an aphelion distance about ten per cent,
greater than Neptune's mean distance. Other comets are similarly
associated with the planets Saturn and Uranus.

The physical transformations of comets are among the most wonderful of
unexplained phenomena in the heavens. But, for physical astronomers,
the greatest interest attaches to the reduction of radius vector of
Encke's comet, the splitting of Biela's comet into two comets in 1846,
and the somewhat similar behaviour of other comets. It must be noted,
however, that comets have a sensible size, that all their parts cannot
travel in exactly the same orbit under the sun's gravitation, and that
their mass is not sufficient to retain the parts together very
forcibly; also that the inevitable collision of particles, or else
fluid friction, is absorbing energy, and so reducing the comet's
velocity.

In 1770 Lexell discovered a comet which, as was afterwards proved by
investigations of Lexell, Burchardt, and Laplace, had in 1767 been
deflected by Jupiter out of an orbit in which it was invisible from
the earth into an orbit with a period of 5-1/2 years, enabling it to be
seen. In 1779 it again approached Jupiter closer than some of his
satellites, and was sent off in another orbit, never to be again
recognised.