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Page 37
_The Great Comet of_ 1843 passed nearer the sun than any known
body. It almost grazed the sun. If it ever returns, it will be in
A.D. 2373.
_Donati's Comet of_ 1858.--This was one of the most magnificent
of modern times. During the first three months it showed no tail,
but from August to October it had developed one forty degrees in
length. Its period is about two thousand years. Every reader remembers
the comet of the summer of 1875.
_Encke's Comet._--This comet has become famous for its supposed
confirmation of the theory that space was filled with a substance
infinitely tenuous, which resisted the passage of this gaseous
body in an appreciable degree, and in long ages would so retard
the motion of all the planets that gravitation would draw them
all one by one into the sun. We must not be misled by the term
retardation to suppose it means behind time, for a retarded body
is before time. If its velocity is diminished, the attraction of
the sun causes it to take a smaller orbit, and smaller orbits mean
increased speed--hence the supposed retardation would shorten its
periodic time. This comet was thought to be retarded two and a
half hours at each revolution. If it was, it would not prove the
existence of the resisting medium. Other causes, unknown to us,
might account for it. Subsequent and more exact calculations fail
to find any retardations in at least two revolutions between 1865
and [Page 131] 1871. Indications point to a retardation of one and a
half hours both before and since. But such discrepancy of result
proves nothing concerning a resisting medium, but rather is an
argument against its existence. Besides, Faye's comet, in four
revolutions of seven years each, shows no sign of retardation.
The truth may be this, that a kind of atmosphere exists around the
sun, perhaps revealed by the zodiacal light, that reaches beyond
where Encke's comet dips inside the orbit of Mercury, and thus
retards this body, but does not reach beyond the orbit of Mars,
where Faye's comet wheels and withdraws.
_Of what do Comets consist?_
The unsolved problems pertaining to comets are very numerous and
exceedingly delicate. Whence come they? Why did they not contract to
centres of nebul�? Are there regions where attractions are balanced,
and matter is left to contract on itself, till the movements of
suns and planets adds or diminishes attractive force on one side,
and so allows them to be drawn slowly toward one planet, and its
sun, or another? There is ground for thinking that the comet of
1866 and its train of meteors, visible to us in November, was thus
drawn into our system by the planet Uranus. Indeed, Leverrier has
conjecturally fixed upon the date of A.D. 128 as the time when it
occurred; but another and closer observation of its next return,
in 1899, will be needed to give confirmation to the opinion. Our
sun's authority extends at least half-way to the nearest fixed star,
one hundred thousand times farther than the orbit of the earth.
Meteoric and cometary matter lying [Page 132] there, in a spherical
shell about the solar system, balanced between the attraction of
different suns, finally feels the power that determines its destiny
toward our sun. It would take 167,000,000 years to come thence to
our system.
The conditions of matter with which we are acquainted do not cover
all the ground presented by these mysterious visitors. We know
a gas sixteen times as light as air, but hydrogen is vastly too
heavy and dense; for we see the faintest star through thousands of
miles of cometary matter; we know that water may become cloudy vapor,
but a little of it obscures the vision. Into what more ethereal,
and we might almost say spiritual, forms matter may be changed we
cannot tell. But if we conceive comets to be only gas, it would
expand indefinitely in the realms of space, where there is no force
of compression but its own. We might say that comets are composed
of small separate masses of matter, hundreds of miles apart; and,
looking through thousands of miles of them, we see light enough
reflected from them all to seem continuous. Doubtless that is sometimes
the case. But the spectroscope shows another state of things: it
reveals in some of these comets an incandescent gas--usually some
of the combinations of carbon. The conclusion, then, naturally is
that there are both gas and small masses of matter, each with an
orbit of its own nearly parallel to those of all the others, and
that they afford some attraction to hold the mass of intermingled and
confluent gas together. Our best judgment, then, is that the nucleus
is composed of separate bodies, or matter in a liquid condition,
capable of being vaporized by the heat of the sun, and driven off,
[Page 133] as steam from a locomotive, into a tail. Indications of
this are found in the fact that tails grow smaller at successive
returns, as the matter capable of such vaporization becomes
condensed. In some instances, as in that of the comet of 1843, the
head was diminished by the manufacture of a tail. On the other hand,
Professor Peirce showed that the nucleus of the comets of 1680,
1843, and 1858 must have had a tenacity equal to steel, to prevent
being pulled apart by the tidal forces caused by its terrible
perihelion sweep around the sun.
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