void space. He compared universal gravitation to magnetism, and
speaks of the work of Gilbert of Colchester. (Gilbert's book, _De
Mundo Nostro Sublunari, Philosophia Nova_, Amstelodami, 1651,
containing similar views, was published forty-eight years after
Gilbert's death, and forty-two years after Kepler's book and
reference. His book _De Magnete_ was published in 1600.)

A few of Kepler's views on gravitation, extracted from the
Introduction to his _Astronomia Nova_, may now be mentioned:--

1. Every body at rest remains at rest if outside the attractive power
of other bodies.

2. Gravity is a property of masses mutually attracting in such manner
that the earth attracts a stone much more than a stone attracts the
earth.

3. Bodies are attracted to the earth's centre, not because it is the
centre of the universe, but because it is the centre of the attracting
particles of the earth.

4. If the earth be not round (but spheroidal?), then bodies at
different latitudes will not be attracted to its centre, but to
different points in the neighbourhood of that centre.

5. If the earth and moon were not retained in their orbits by vital
force (_aut alia aligua aequipollenti_), the earth and moon would come
together.

6. If the earth were to cease to attract its waters, the oceans would