like gravel. Even so, there must be collisions absorbing the energy of
rotation, and tending to make the rings eventually fall into the
planet. The slower motion of the external parts has been proved by the
spectroscope in Keeler's hands, 1895.

Saturn has perhaps received more than its share of attention owing to
these rings. This led to other discoveries. Huyghens in 1655, and
J. D. Cassini in 1671, discovered the sixth and eighth satellites
(Titan and Japetus). Cassini lost his satellite, and in searching for
it found Rhea (the fifth) in 1672, besides his old friend, whom he
lost again. He added the third and fourth in 1684 (Tethys and
Dione). The first and second (Mimas and Encelades) were added by
Herschel in 1789, and the seventh (Hyperion) simultaneously by Lassel
and Bond in 1848. The ninth (Phoebe) was found on photographs, by
Pickering in 1898, with retrograde motion; and he has lately added a
tenth.

The occasional disappearance of Cassini's Japetus was found on
investigation to be due to the same causes as that of Jupiter's fourth
satellite, and proves that it always turns the same face to the
planet.

_Uranus and Neptune_.--The splendid discoveries of Uranus and two
satellites by Sir William Herschel in 1787, and of Neptune by Adams
and Le Verrier in 1846, have been already described. Lassel added two
more satellites to Uranus in 1851, and found Neptune's satellite in
1846. All of the satellites of Uranus have retrograde motion, and
their orbits are inclined about 80 degrees to the ecliptic.

The spectroscope has shown the existence of an absorbing atmosphere on