p 93
Such general considerations regarding certain characteristic properties
appertaining to whole groups, can not, however, be applied with equal
justice to the individual planets of every group, nor to the relations
between the distances of the revolving planets from the central body, and
their absolute size, density, period or rotation, eccentricity, and the
inclination of their orbits and the axes. We know as yet of no inherent
necessity, no mechanical natural law, similar to the one which teaches us
that the squares of the periodic times are proportional to the cubes of the
major axes, by which the above-named six elements of the planetary bodies
and the form of their orbit are made dependent either on one another, or on
their mean distance from the Sun. Mars is smaller than the Earth and Venus,
although further removed from the Sun than these last-named planets,
approaching most nearly in size to Mercury, the nearest planet to the Sun.
Saturn is smaller than Jupiter, and yet much larger than Uranus. The zone
of the telescopic planets, which have so inconsiderable a volume,
immediately procede Jupiter (the greatest in size of any of the planetary
bodies), if we consider them with regard to distance from the Sun; and yet
the disks of these small asteroids, which scarcely admit of measurement,
have an areal surface not much more than half that of France, Madagascar, or
Borneo.
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