However striking may be the extremely small density of all the
colossal planets, which are furthest removed from the Sun, we are yet unable
in this respect to recognize any regular succession.*
[footnote] *See Kepler, on the increasing density and volume of the planets
in proportion with their increase of distance from the Sun, which is
described as the densest of all the heavenly bodies; in the 'Epitome Astran.
Copern. in' vii. 'libros digesta', 1618-1622, p. 420. Leibnitz also
inclined to the opinions of Kepler and Otto von Guericke, that the planets
increase in volume in proportion to their increase of distance from the Sun.
See his letter to the Magdeburg Burgomaster (Mayence, 1671), in Leibnitz,
'Deutschen Schriften, herausg. von Guhrauer', th. i., 264.
Uranus appears to be denser than Saturn, even if we adopt the smaller mass,
1/24605, assumed by Lamont; and, notwithstanding the inconsiderable
difference of density observed in the innermost planetary group,* we find
both Venus and Mars less dense than the Earth, which lies between them.
[footnote] *On the arrangement of masses, see Encke, in Schum., 'Astr.
Nachr', 1843 Nr. 488, 114.
The time of rotation certainly diminishes with increasing solar distance,
but yet it is greater in Mars than in the Earth, and in Saturn than in
Jupiter.
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