Although in the important classification of the planets into
two groups oi interior and exterior planets, the general char-
acters of absolute magnitude, density, flattening at the poles,
velocity of rotation, absence of moons, present themselves as
dependent upon the distances, i. e., from their semi-orbital
axes, this dependence can not be affirmed of each one of these
groups Up to the present time we are ignorant, as I have
already remarked uf -my internal necessity, any mechanical



104



COSMOS.
law of nature, which (like the beautiful law which connect*
the square of the periods of revolution with the cube of the
major axes) represents the above-named elements of the ordei
of succession of the individual planetary bodies of edch group
m their dependence upon the distances. Although the planet
which is nearest to the Sun (Mercury) is the densest, even
six or eight times denser than some of the exterior planets,
Jupiter, Saturn, Uranus, and Neptune, the order of succes-
sion, in the case of Venus, the Earth, and Mars, or Jupiter,
Saturn, and Uranus, is very irregular. The absolute mag-
nitudes do generally, as Kepler has already observed {Har-
monice Mundi, vol. iv., p. 194 ; Cosmos, vol. i., p. 93-97),
increase with the distances ; but this does not hold good
when the planets are considered individually. Mars is small-
er than the Earth, Uranus smaller than Saturn, Saturn small-
er than Jupiter, and succeeds immediately to a host of plan-
ets, which, on account of their smalbiess, are almost im-
measurable. It is true the period of rotation generally in-
creases with the distance from the Sun ; but it is, in the case
of Mars, slower than in that of the Earth, slower in Saturn
than in Jupiter.
