(1790.)

No course of reasoning could be more simple, more exact, more profound,
and more beautiful than this which follows:

"As it has been shown that the spherical figure of a cluster is
owing to the action of central powers, it follows that those
clusters which, _caeteris paribus_, are the most complete in this
figure, must have been the longest exposed to the action of these
causes. Thus the maturity of a sidereal system may be judged from
the disposition of the component parts.

"Hence planetary nebulae may be looked on as very aged. Though we
cannot see any individual nebula pass through all its stages of
life, we can select particular ones in each peculiar stage." (1789.)

There is something almost grandiose and majestic in his statement of
the ultimate destiny of the Galaxy:

"To him the fates were known
Of orbs dim hovering on the skirts of space."

"--Since the stars of the Milky Way are permanently exposed to the
action of a power whereby they are irresistibly drawn into groups,
we may be certain that from mere clustering stars they will be
gradually compressed, through successive stages of accumulation,
till they come up to what may be called the ripening period of the
globular form, and total insulation; from which it is evident that
the Milky Way must be finally broken up and cease to be a stratum of
scattered stars.

"The state into which the incessant action of the clustering power
has brought it at present, is a kind of chronometer that may be used
to measure the time of its past and future existence; and although
we do not know the rate of going of this mysterious chronometer, it
is nevertheless certain that since the breaking up of the Milky Way
affords a proof that it cannot last forever, it equally bears
witness that its past duration cannot be admitted to be infinite."
(1814.)

HERSCHEL'S relations with his cotemporaries were usually of the most
pleasant character, though seldom intimate. This peace was broken but by
one unpleasant occurrence. In the _Philosophical Transactions_ for 1792,
SCHROETER had communicated a series of observations made with one of
HERSCHEL'S own telescopes on the atmospheres of _Venus_, the Moon, etc.
It was not only an account of phenomena which had been seen; it was
accompanied by measures, and the computations based on these led to