all and Lord Kelvin stated (Chap. IV.). Thus the
objection to Kepler's immaterial vortices is met and overcome by our
conception of the Aether (Chap. IV.). Descartes, as Whewell points out,
asserted, "that a vacuum in any part of the universe is impossible. The
whole universe must be filled with matter, which must be divided into
equal angular parts. This matter being in motion, the parts are
necessarily grounded into a spherical form, and the corners thus rubbed
off, forming a second or subtle matter. There is besides a third kind of
matter, of parts more coarse and less fitted for motion. The first part
makes the luminous bodies as sun and stars, the second part is the
transparent substance of the skies, and the third part is the material
of opaque bodies as the earth, planets and comets. We may suppose that
the motion of these parts takes the form of revolving circular currents
or vortices. By this means the first matter will be collected to the
centre of each vortex, while the second or subtle matter surrounds it,
and by its centrifugal effect constitutes light. The planets are carried
round the sun by the motion of the vortex, each planet being at such
distance from the sun as to be in a part of the vortex suitable to its
solidity and mobility. The satellites are in like manner carried round
their ordinary planets by subordinate vortices."

It would almost seem from this quotation that we had adopted purely and
simply Descartes' and Kepler's ideas _in toto_, whereas the truth is
that the hypothesis of a rotating electro-magnetic Aether has been
arrived at by following Newton's own Rules of Philosophy, and by
discarding everything not in harmony with experience and experiment.

Further, Descartes was unable to give, or explain the ellipticity of the
orbits of planets, and had to assume that there were elliptic vortices.
When we come to deal with Kepler's Laws, and their physical
interpretation, the correct solution of this problem will be given from
a purely experimental and philosophical standpoint, and in a way and
manner never suggested by Descartes or any other believer in the theory
of vortices as then known and understood. Indeed there is no objection
to the theory of vortices, which cannot be satisfactorily explained by a
rotating electro-magnetic Aether, as we shall see when we deal with
Newton's Laws of Motion and Kepler's Laws.

Both Liebnitz and Huyghens were believers in the theory of vortices, and
the