be necessary to keep the planets moving on their
endless round; the _primum mobile_ of Ptolemy had been stopped; an angel
was sometimes assigned to each planet to carry it round, but though a
widely diffused belief, this was a fantastic and not a serious
scientific one. Descartes's vortices seemed to do exactly what was
wanted.

It is true they had no connexion with the laws of Kepler. I doubt
whether he knew about the laws of Kepler; he had not much opinion of
other people's work; he read very little--found it easier to think. (He
travelled through Florence once when Galileo was at the height of his
renown without calling upon or seeing him.) In so far as the motion of a
planet was not circular, it had to be accounted for by the jostling and
crowding and distortion of the vortices.

Gravitation he explained by a settling down of bodies toward the centre
of each vortex; and cohesion by an absence of relative motion tending to
separate particles of matter. He "can imagine no stronger cement."

The vortices, as Descartes imagined them, are not now believed in. Are
we then to regard the system as absurd and wholly false? I do not see
how we can do this, when to this day philosophers are agreed in
believing space to be completely full of fluid, which fluid is certainly
capable of vortex motion, and perhaps everywhere does possess that
motion. True, the now imagined vortices are not the large whirls of
planetary size, they are rather infinitesimal whirls of less than atomic
dimensions; still a whirling fluid is believed in to this day, and many
are seeking to deduce all the properties of matter (rigidity,
elasticity, cohesion gravitation, and the rest) from it.

Further, although we talk glibly about gravitation and magnetism, and so
on, we do not really know what they are. Progress is being made, but we
do not yet properly know. Much, overwhelmingly much, remains to be
discovered, and it ill-behoves us to reject any well-founded and
long-held theory as utterly and intrinsically false and absurd. The more
one gets to know, the more one perceives a kernel of truth even in the
most singular statements; and scientific men have learned by experience
to be very careful how they lop off any branch of the tree of knowledge,
lest as they cut away the dead wood they lose also some green shoot,
some healthy bud of unperceived truth.

However, it may be admitted that the idea of a Cartesian vortex in
connexion with the solar syst