The Sun.
London, England, 1858-10-12 00:00:00

THE C'O2IIET.

(From Humboldt's "Cosmos," trsnslatcd by Mrs. &thine)
We have now to notice the unnumbered host of
comets. If we estimate according to the rules of the
calculus of probabilities, the equable distribution of
the paths of these bodies, the limits of their peri-
helia, and the possibility of their remaining invisible
to the inhabitants of the earth, their possible num-
bers will be an anima of myriads astonishing to
the imagination. Kepler, with his usual liveliness
of expression, said, even in his day, " there are
More comets in space than fishes in the ocean." As
yet, however, we: hardly possess as many as 150
calculated paths of comets ; hut we have notices
more or less precise of the appearance of 600 or 700
of such'bodies; and of their passage through known
constellations.. Whilst the classic nations of the
West,the Greeks and-the Romans, do,. indeed, Some-
times mention the plaCe in the heavens where a
cometrwas first -seen, hilt never afford us informa-
tiousespecting its appasent path, the literature of
the Chinese (who observed nature diligently, and
carefully recorded everything) supplies us with cir-
cumstantial notices of the comets seen by them, and
of the constellations which each passed through.
These notices extend back to more than five centu-
ries before the ChriStian era, and many of them are
still found 'useful in astronomy: • The paths of the
four first comets which it has been possible to in-
vestigate have been computed from Chinese obser-
vations. They are those of 240 (in the reign of Gor-
dian, III:), of 539, (in that of Justinian) of 565, and
837. According to Du Sejour, the last of these must
have been 'for 24 hours 2,000,000 geographical miles
only from the earth. It appeared during the reign
of Louis-le -Debonnaire (the son: of Charlemagne),
and was thonght to prognosticate disasters which
that' monarch sought to avert by founding, conven-
tnal establishinents, .whild the 'Chinese astiono-
.mers- were soientifically engaged in tracing its
path. Its tail' was 60 degrees in, length, and. ap-
peared'sometiiiibs simple and sometimes divided.
The, first comet computed exch*ively from
observations -is ~laat- of :1456, or Halley's comet, so'
named on the occasion which .was long, though
erroneously;'- supposed to hai'e been its , first well
ascertained appearance:: ..13f all planetary..bodies,
'comets--though their mean mass is probably much
less than the fivoothousandth part-of that or the
earth—are -those NV.hick occupy the greatest space,
their wide-spreading. tails often. _extending Over
many Millions of miles. ,The cone of light reflect-
ing gaseous matters Which radiates from Them has
been found in "some instances,. as in 1680 and 1811,
to equal in length the distance of the_ earth from
the sun, or that of a line including, the orbits of the
two planets, 'Venus and Mercury.
,It is mien' prob-
able that the 'emanations from the comets of 1811
and 1823 mixed with our atmosphere.
Cornets show their diversities ef form, diversities
belonging rather to the indMdual than to the class,
that the desoription given. of one of these " wander-
ing light-clouds". (as they were called by Xeno-
phanes,"and" Theon of Alexandria; contemporaries
of Pappus), can only be applied with much
caution to another. The fainter telescopic comets
are for the most part without tails, and resemble
Herschel'S nebulous star. Their appearance is that
of circular nebulm shininm with a faint light con-
centrated towards the centre.. This is the simplest
type,,,but not, on that account, a rudimen-
tary type, for it might equally be the
type 'of - a Cosinical body 'grown Old and
exhausted by exhalation.: We cmi distinguish
in the larger comets, the " head," or " nucleus,"
and the " tail," either single or Multiple, whieli the
Chinese astronomers more 'characteristiCally deno-
minate by the word " brush" (sui). The nucleus
usually presents no definite outline, although in
some rare instances it appears-like a star of the first
,or second magnitude; and in the larger comets of
1402, 1532, 1577, 1744, and 1843; has even been
clearly seen in bright sunshine. As the tail of the
comet of 1402 was seen in bright sunshine, so also
-in the recent great comet of '1843 the nucieui and
tail were visible on the 28th of Feb. in North Ame-
rica, between one and three o'clock in the after-
noon. (J. G. Clarke, of Portland, in the State of
Maine.) The distance of the very dense nucleus
from the sun's could be measured with great
exactness. The nucleus-and tail, appeared as a very
pure white cloud, between the tail and the nucleus
there was one darker part. The latter circumstance
appears to indicate, in particular individuals, a
denser mass, capable of reflecting light with greater
intensity. Even in Herschel's large telescope, only
two comets ShoWed'well-defined. discs, viz., the comet
discovered 'in Sicily in 1807, and the-fine comet of
1811, the one, having-an angle of 1 sec., mid the
other of 0 sec. 77, whence he inferred, their true
diameters to be respectively 534 and 4N mlies.
The measurement of the less. well defined nuclei
of the comets of 1798 and 1805 •gave. diameters of
only 24 to 28 miles. In several comets which
were examined with great care, and particularly
in the above-namdd comet of 1811 which was so
long in sight,, the nucleus and its nebulous envelope
were entirely separated from the tail by a darker
space. The intensity of light in the nucleus of a
, comet does not- increase in a uniform manner to-
wards the centre, but bright zones alternate with
concentric nebulous envelopes, which are less lumi-
nous. The tail- sometimes appears single, more
rarely double, and in two instances, in the comets
of 1807 and
.1843, the two branches were of very
different length ; in the comet of 1744 the tail had
six, branches, the two exterior diverging at an angle
of 60 deg. The tails have been sometimes straight
sometimes curved ; in the latter case either concave
*towards both sides, or, as in 1618, convex towards
the direction in ich the comet is moving ; and
sometimes the tail even appears inflected like a
flame in motion. The tails are always turned from
-the sun, and so directed that the prolongation of
the axis would pass through the centre of that body
—this circumstance, according to Edouard Biot,
was remarked by the Chinese astronomers as early
as 837, but was first clearly.stated in Europe by
Fracastoro and Peter Apian in the 16th century.
We may conceive that these emanations form cono-
idal envelopes of greater or less thickness, which
would furnish a very simple explanation of several
of the remarkable optical phwnomena above men-
tioned.

Not only are different comets characterised by
great difference of form—some being entirely with-
out visible tails, others, as the third comet of 1618,
having a tail of 104 degrees in length—but we also
see the same comets pass through successive and
rapid variations. These changes have been well
and most accurately described in the comet of 1744,
by Heinsius, at Petersburg, and in Halley's comet,
on its last re-appearance, in 1835, by Bessel at
Konigsberg. A tuft of rays issued from that part
of the nucleus which was turned towards the sun.
These rays thus issuing were bent backwards, so as
to form part of the tail. The nucleus of Halley's
comet, with its emanations, presented the appear-
ance of a burning rocket, the train of Which was
deflected sideways by a current of air. The rays
issuing from the head of the comet were seen by
Arago and myself, at the Observatory at Paris, to
assume very different forms on successive nights.
The great Konigsberg astronomer concluded from
many measurements and from theoretical considera-
tions, " that the outstreaming cone of light deviated
notably both to the right and to the left of the
true direction towards the sun, but that it always
returned to that direction, and passed beyond it to
the opposite side.; so that both the cone of light,
and the body of the comet from which it issued,
were subject to a rotatory or rather vibratory mo-
tion, in the plane of the orbit." He finds " that the
-ordinary power of the attractive force of the sun
on gravitating bodies is not sufficient to explain
such vibrations, and is of opinion that they would
seem to indicate a polar force, which tends to
turn one of the semi-diameters of the comet towards
the sun, and the opposite semi-diameter from the
sun. The magnetic polarity possessed by
the earth may present something analo-
.
gous, and should the sun have the oppo-
site polarity, a resulting influence might manifest
itself in the precession of the equinoxes." This is
not the place for entering more at length into this
subject; but observations so remarkable,itis general
and s
important with reference to the most wonderful
class of comical bodies belonging to our solar system
ought not to be entirely passed over n this
view of nature.

Although, in the greater number of cases, the.
tails of comets increase in, magnitude and brilliancy