t; and the high
temperature of the much larger and still liquid mass of the sun is
apparent to everybody. Not till it begins to scum over will it be
perceptibly cooler.

[Illustration: FIG. 81.--Saturn.]

Many things are now known concerning heat which were not known to
Laplace (in the above paragraph they are only hinted at), and these
confirm and strengthen the general features of his hypothesis in a
striking way; so do the most recent telescopic discoveries. But fresh
possibilities have now occurred to us, tidal phenomena are seen to have
an influence then wholly unsuspected, and it will be in a modified and
amplified form that the philosopher of next century will still hold to
the main features of this famous old Nebular Hypothesis respecting the
origin of the sun and planets--the Evolution of the solar system.

NOTES TO LECTURE XII

The subject of stellar astronomy was first opened up by Sir William
Herschel, the greatest observing astronomer.

_Frederick William Herschel_ was born in Hanover in 1738, and brought up
as a musician. Came to England in 1756. First saw a telescope in 1773.
Made a great many himself, and began a survey of the heavens. His sister
Caroline, born in 1750, came to England in 1772, and became his devoted
assistant to the end of his life. Uranus discovered in 1781. Music
finally abandoned next year, and the 40-foot telescope begun. Discovered
two moons of Saturn and two of Uranus. Reviewed, described, and gauged
all the visible heavens. Discovered and catalogued 2,500 nebulae and 806
double stars. Speculated concerning the Milky Way, the nebulosity of
stars, the origin and growth of solar systems. Discovered that the stars
were in motion, not fixed, and that the sun as one of them was
journeying towards a point in the constellation Hercules. Died in 1822,
eighty-four years old. Caroline Herschel discovered eight comets, and
lived on to the age of ninety-eight.

LECTURE XII

HERSCHEL AND THE MOTION OF THE FIXED STARS

We may admit, I think, that, with a few notable exceptions, the work of
the great men we have been recently considering was rather to complete
and round off the work of Newton, than to strike out new and original
lines.

This was the whole tendency of eighteenth century astronomy. It appeared
to be getting into an adult and uninteresting stage, wherein everything
could be calculated and predicted. Labour and ingenuity, and a severe
mathematical trainin