thus be seen that issues of the most momentous character hang on
the _time-keeping_ of comets; for plainly all must in some degree suffer
the same kind of hindrance as Encke's, if the cause of that hindrance be
the one suggested. None of its congeners, however, show any trace of
similar symptoms. True, the late Professor Oppolzer announced,[248] in
1880, that a comet, first seen by Pons in 1819, and rediscovered by
Winnecke in 1858, having a period of 2,052 days (5.6 years), was
accelerated at each revolution precisely in the manner required by
Encke's theory. But M. von Haerdtl's subsequent investigation, the
materials for which included numerous observations of the body in
question at its return to the sun in 1886, decisively negatived the
presence of any such effect.[249] Moreover, the researches of Von Asten
and Backlund[250] into the movements of Encke's comet revealed a
perplexing circumstance. They confirmed Encke's results for the period
covered by them, but exhibited the acceleration as having _suddenly
diminished_ by nearly one-half in 1868. The reality and permanence of
this change were fully established by observations of the ensuing return
in March, 1885. Some physical alteration of the retarded body seems
indicated; but visual evidence countenances no such assumption. In
aspect the comet is no less thin and diffuse than in 1795 or in 1848.

The character of the supposed resistance in inter-planetary space has,
it may be remarked, been often misapprehended. What Encke stipulated for
was not a medium equally diffused throughout the visible universe, such
as the ethereal vehicle of the vibrations of light, but a rare fluid,
rapidly increasing in density towards the sun.[251] This cannot be a
solar atmosphere, since it is mathematically certain, as Laplace has
shown,[252] that no envelope partaking of the sun's axial rotation can
extend farther from his surface than nine-tenths of the mean distance of
Mercury; while physical evidence assures us that the _actual_ depth of
the solar atmosphere bears a very minute proportion to the _possible_
depth theoretically assigned to it. That matter, however, not
atmospheric in its nature--that is, neither forming one body with the
sun nor altogether aeriform--exists in its neighbourhood, can admit of
no reasonable doubt. The great lens-shaped mass of the zodiacal light,
stretching out at times far beyond the earth's orbit, may indeed be
regarded as an extension of the coro