formed, besides, a peculiarly delicate test of its motion.
The opportunity was thus afforded, by directly comparing the comet's
velocity before and after its critical plunge through the solar
surroundings, of ascertaining with approximate certainty whether any
considerable retardation had been experienced in the course of that
plunge. The answer distinctly given was that there had not. The computed
and observed places on both sides of the sun fitted harmoniously
together. The effect, if any were produced, was too small to be
perceptible.

This result is, in itself, a memorable one. It seems to give the _coup
de grace_ to Encke's theory--discredited, in addition, by Backlund's
investigation--of a resisting medium growing rapidly denser inwards. For
the perihelion distance of the comet of 1882, though somewhat greater
than that of its predecessors, was nevertheless extremely small. It
passed at less than 300,000 miles of the sun's surface. But the ethereal
substance long supposed to obstruct the movement of Encke's comet would
there be nearly 2,000 times denser than at the perihelion of the smaller
body, and must have exerted a conspicuous retarding influence. That none
such could be detected seems to argue that no such medium exists.

Further evidence of a decisive kind was not wanting on the question of
identity. The "Great September Comet" of 1882 was in no hurry to
withdraw itself from curious terrestrial scrutiny. It was discerned with
the naked eye at Cordoba as late as March 7, 1883, and still showed in
the field of the great equatoreal on June 1 as an "excessively faint
whiteness."[1319] It was then about 480 millions of miles from the
earth--a distance to which no other comet--not even excepting the
peculiar one of 1729--had been pursued.[1320] Moreover, an arc of 340
out of the entire 360 degrees of its circuit had been described under
the eyes of astronomers; so that its course came to be very well known.
That its movement is in a very eccentric ellipse, traversed in several
hundred years, was ascertained.[1321] The later inquiries of Dr.
Kreutz,[1322] completed in a volume published in 1901,[1323]
demonstrated the period to be of about 800 years, while that of its
predecessor in 1843 might possibly agree with it, but is much more
probably estimated at 512 years. The hypothesis that they, or any of the
comets associated with them, were returns of an individual body is
peremptorily excluded. They may all, howe