for the subsequent demonstration of the permanent
stability of the planetary system by Laplace and
Lagrange.

It was Euler also who demonstrated that within
certain fixed limits the eccentricities and places of the
aphelia of Saturn and Jupiter are subject to constant
variation, and he calculated that after a lapse of about
thirty thousand years the elements of the orbits of
these two planets recover their original values.



II

THE PROGRESS OF MODERN ASTRONOMY

A NEW epoch in astronomy begins with the work
of William Herschel, the Hanoverian, whom England
made hers by adoption. He was a man with a
positive genius for sidereal discovery. At first a mere
amateur in astronomy, he snatched time from his
duties as music-teacher to grind him a telescopic mirror,
and began gazing at the stars. Not content with
his first telescope, he made another and another, and
he had such genius for the work that he soon possessed
a better instrument than was ever made before. His
patience in grinding the curved reflective surface was
monumental. Sometimes for sixteen hours together
he must walk steadily about the mirror, polishing it,