Wren, Halley, and many others. All of these men felt a repugnance to
accept the idea of a force acting across the empty void of space.
Descartes (1596-1650) proposed an ethereal medium whirling round the
sun with the planets, and having local whirls revolving with the
satellites. As Delambre and Grant have said, this fiction only
retarded the progress of pure science. It had no sort of relation to
the more modern, but equally misleading, "nebular hypothesis." While
many were talking and guessing, a giant mind was needed at this stage
to make things clear.



7. SIR ISAAC NEWTON--LAW OF UNIVERSAL GRAVITATION.


We now reach the period which is the culminating point of interest in
the history of dynamical astronomy. Isaac Newton was born in
1642. Pemberton states that Newton, having quitted Cambridge to avoid
the plague, was residing at Wolsthorpe, in Lincolnshire, where he had
been born; that he was sitting one day in the garden, reflecting upon
the force which prevents a planet from flying off at a tangent and
which draws it to the sun, and upon the force which draws the moon to
the earth; and that he saw in the case of the planets that the sun's
force must clearly be unequal at different distances, for the pull out
of the tangential line in a minute is less for Jupiter than for
Mars. He then saw that the pull of the earth on the moon would be less
than for a nearer object. It is said that while thus meditating he saw
an apple fall from a tree to the ground, and that this fact suggested
the questions: Is the force that pulled that apple from the tree the
same as the force which draws the moon to the earth? Does the