racy and
methodical skill.

The Sun having been assigned that central position in the system which
his magnitude and importance claim as his due, the planets circling in
orbits around him have their motions described in a manner indicative of
the precise knowledge which Milton acquired of this theory. At this time
the law of gravitation was unknown, and, although the ellipticity of the
orbits of the planets had been discovered by Kepler, the nature of the
motive force which guided and retained them in their paths still
remained a mystery. It was believed that the planets were whirled round
the Sun, as if by the action of magnetic fibres; a mutual attractive
influence having been supposed to exist between them and the orb,
similar to that of the opposite poles of magnets.

Milton alludes to this theory in the following lines:--

They, as they move
Their starry dance in numbers that compute
Days, months, and years, towards his all-cheering lamp
Turn swift their various motions, or are turned
By his magnetic beam.--iii. 579-83.

An important advance upon this theory was made by Horrox, who, in his
study of celestial dynamics, attributed the curvilineal motion of the
planets to the influence of two forces, one projective, the other
attractive. He illustrated this by observing the path described by a
stone when thrown obliquely into the air. He perceived that its motion
was governed by the impulse imparted to it by the hand, and also by the
attractive force of the Earth. Under these two influences, the stone
describes a graceful curve, and in its descent falls at the same angle
at which it rose. Hence arises the general law: 'When two spheres are
mutually attracted, and if not prevented by foreign influences, their
straight paths are deflected into curves concave to each other, and
corresponding with one of the sections of a cone, according to the
velocity of the revolving body. If the velocity with which the revolving
body is impelled be equal to what it would acquire by falling through
half the radius of a circle described from the centre of deflection, its
orbit will be circular; but if it be less than that quantity, its path
becomes elliptical.'

Newton afterwards embraced this law in his great principle of
gravitation, and demonstrated that the force which guides and retains
the Earth and planets in their orbits resides in the Sun. By the orb's
attractive influence a pl