KEPLER'S LAWS.


When Gilbert of Colchester, in his "New Philosophy," founded on his
researches in magnetism, was dealing with tides, he did not suggest that
the moon attracted the water, but that "subterranean spirits and
humours, rising in sympathy with the moon, cause the sea also to rise
and flow to the shores and up rivers". It appears that an idea,
presented in some such way as this, was more readily received than a
plain statement. This so-called philosophical method was, in fact, very
generally applied, and Kepler, who shared Galileo's admiration for
Gilbert's work, adopted it in his own attempt to extend the idea of
magnetic attraction to the planets. The general idea of "gravity"
opposed the hypothesis of the rotation of the earth on the ground that
loose objects would fly off: moreover, the latest refinements of the old
system of planetary motions necessitated their orbits being described
about a mere empty point. Kepler very strongly combated these notions,
pointing out the absurdity of the conclusions to which they tended, and
proceeded in set terms to describe his own theory.

"Every corporeal substance, so far forth as it is corporeal, has a
natural fitness for resting in every place where it may be situated by
itself beyond the sphere of influence of a body cognate with it. Gravity
is a mutual affection between cognate bodies towards union or
conjunction (similar in kind to the magnetic virtue), so that the earth
attracts a stone much rather than the stone seeks the earth. Heavy
bodies (if we begin by assuming the earth to be in the centre of the
world) are not carried to the centre of the world in its quality of
centre of the world, but as to the centre of a cognate round body,