p 99
concealed from our observation, and must always remain so, unless new and
unexpected disturbing causes come into play. These cosmical relations
involuntarily remind us of nearly similar conditions in the intellectual
world, where, in the domain of deep research into the mysteries and the
primeval creative forces of nature, there are regions similarly turned away
from us, and apparently unattainable, of which only a narrow margin has
revealed itself, for thousands of years, to the human mind, appearing, from
time to time, either glimmering in true or delusive light. We have hitherto
considered the primary planets, their satellites, and the concentric rings
which belong to one, at least, of the outermost planets, as products of
tangential force, and as closely connected together by mutual attraction; it
therefore now only remains for us to speak of the unnumbered host of
'comets' which constitute a portion of the cosmical bodies revolving in
independent orbits round the Sun. If we assume an equable distribution of
their orbits, and the limits of their perihelia, or greatest proximities to
the Sun, and the possibility of their remaining invisible to the inhabitants
of the Earth, and base our estimates on the rules of the calculus of
probabilities, we shall obtain as the result an amount of myriads perfectly
astonishing. Kepler, with his usual animation of expression, said that
there were more comets in the regions of space than fishes in the depths of
the ocean. As yet, however, there are scarcely one hundred and fifty whose
paths have been calculated, if we may assume at six or seven hundred the
number of comets whose appearance and passage through known constellations
have been ascertained by more or less precise observations. While the
so-called classical nations of the West, the Greeks and Romans, although
they may occasionally have indicated the position in which a comet first
appeared, never afford any information regarding its apparent path, the
copious literature of the Chinese (who observed nature carefully, and
recorded with accuracy what they saw) contains circumstantial notices of the
constellations through which each comet was observed to pass. These notices
go back to more than five hundred years before the Christian era, and many
of them are still found to be of value in astronomical observations.*
[footnote] *The first comets of whose orbits we have any knowledge, and
which were calculated from Chinese observations, are those of 240 (under
Gordian II.), 539 (under Justinian), 565, 568, 574, 837, 1337, and 1385.
See John Russell Hind, in Schum., 'Astron. Nachr.', 1843, No. 498. While
the comet of 837 (which, according to Du Sejour, continued during
twenty-four hours within a distance of 2,000,000 miles from the Earth)
terrified Louis I. of France to that degree that he busied himself in
building churches and founding monastic establishments, in the hope of
appeasing the evils threatened by its appearance, the Chinese astronomers
made observations on the path of this cosmical body, whose tail extended
over a space of 60 degrees, appearing sometimes single and sometimes
multiple. The first comet that has been calculated solely from European
observations was that of 1456, known as Halley's comet, from the belief
long, but erroneously, entertained that the period when it was first
observed by that astronomer was its first and only well-attested appearance.
See Arago, in the 'Annuaire', 1836, p. 204, and Langier, 'Comptes Rendus
des Seances de l'Acad.', 1843, t. xvi., 1006.
p 100
Although comets have a smaller mass than any other cosmical bodies -- being,
according to our present knowledge, probably not equal to 1/5000th part of
the Earth's mass -- yet they occupy the largest space, as their tails in
several instances extend over many millions of miles. The cone of luminous
vapor which radiates from them has been found, in some cases (as in 1680 and
1811), to equal the length of the Earth's distance from the Sun, forming a
line that intersects both the orbits of Venus and Mercury. It is even
probable that the vapor of the tails of comets mingled with our atmosphere
in the years 1819 and 1823.
Comets exhibit such diversities of form, which appear rather to appertain to
the individual than the class, that a description of one of these "wandering
light-clouds," as they were already called by Xenophanes and Theon of
Alexandria, contemporaries of Pappus, can only be applied with caution to
another. The faintest telescopic comets are generally devoid of visible
tails, and resemble Herschel's nebulous stars. They appear like circular
nebulae of faintly-glimmering vapor, with the light concentrted toward the
middle. This is the most simple type; but it can not, however, be regarded
as rudimentary, since it might equally be the type of an older cosmical
body, exhausted by exhalation. In the larger comets we may distinguish both
the so-called "head" or "nucleus," and the single or multiple tail, which is
characteristically denominated by the Chinese astronomers "the brush"
('sui'). The nucleus generally presents no definite outline, although, in a
few rare cases, it appears like a star of the first or second magnitude, and
has even been seen in bright sunshine;* as,
p 101
for instance, in the large comets of 1402, 1532, 1577, 1744, and 1843.
[footnote] *Arago, 'Annuaire', 1832, p. 209, 211. The phenomenon of the
tail of a comet being visible in bright sunshine, which is recorded of the
comet of 1402, occurred again in the case of the large comet of 1843, whose
nucleus and tail were seen in North America on the 28th of February
(according to the testimony of J. G. Clarke, of Portland, state of Maine),
between 1 and 3 o'clock in the afternoon.(a) The distance of the very dense
nucleus from the sun's light admitted of being measured with much exactness.
The nucleus and tail appeared like a very pure white cloud, a darker space
intervening between the tail and the nucleus. ('Amer. Journ. of Science',
vol. xiv., No. 1, p. 229.)
[footnote] (a) [The translator was at New Bedford, Massachusetts, U.S., on
the 28th February, 1843, and distinctly saw the comet, between 1 and 2 in
the afternoon. The sky at the time was intensely blue, and the sun shining
with a dazzling brightness unknown in European climates.] -- Tr
This latter circumstance indicates, in particular individuals, a denser
mass, capable of reflecting light with greater intensity. Even in
Herschel's large telescope, only two comets, that discovered in Sicily in
1807, and the splendid one of 1811, exhibited well-defined disks;* the one
at an angle of 1 second, and the other at 0.77 seconds, whence the true
diameters are assumed to be 536 and 428 miles.
[footnote] *'Phil. Trans.' for 1808, Part ii., p. 155, and for 1812, Part
i., p. 118. The diameters found by Herschel for the nuclei were 538 and 428
English miles. For the magnitudes of the comets of 1798 and 1805, see
Arago, 'Annuaire', 1832, p. 203.
The diameters of the less well-defined nuclei of the comets of 1798 and 1805
did not appear to exceed 24 or 28 miles.
In several comets that have been investigated with great care, especially in
the above-named one of 1811, which continued visible for so long a period,
the nucleus and its nebulous envelope were entirely separated from the tail
by a darker space. The intensity of light in the nucleus of comets does not
augment toward the center in any uniform degree, brightly shining zones
being in many cases separated by concentric nebulous envelopes. The tails
sometimes appear single, sometimes, although more rarely, double; and in the
comets of 1807 and 1843 the branches were of different lengths; in one
instance (1744) the tail had six branches, the whole forming an angle of 60
degrees. The tails have been sometimes straight, sometimes curved, either
toward both sides, or toward the side appearing to us as the exterior (as in
1811), or convex toward the direction in which the comet is moving (as in
that of 1618); and sometimes the tail has even appeared like a flame in
motion. The tails are always turned away from the sun, so that their line
of prolongation passes through its center; a fact which, according to Edward
Biot, was noticed by the Chinese astronomers as early as 837, but was first
generally made known in Europe by Fracastoro and Peter Apian in the
sixteenth century. These emanations may be regarded as conoidal envelopes
of greater of less thickness,
p 102
and, considered in this manner, they furnish a simple explanation of many of
the remarkable optical phenomena already spoken of.
Comets are not only characteristically different in form, some being
entirely without a visible tail, while others have a tail of immense length
(as in the instance of the comet of 1618, whose tail measured 104 degrees),
but we also see the same comets undergoing successive and rapidly-changing
processes of configuration. These variations of form have been most
accurately and admirably described in the comet of 1744, by Hensius, at St.
Petersburg, and in Halley's comet, on its last reappearance in 1835, by
Bessel, at Konigsberg. A more or less well-defined tuft of rays emanated
from that part of the nucleus which was turned toward the Sun; and the rays
being bent backward, formed a part of the tail. The nucleus of Halley's
comet; with its emanations, presented the appearance of a burning rocket,
the end of which was turned sideways by the force of the wind. The rays
issuing from the head were seen by Arago and myself, at the Observatory at
Paris, to assume very different forms on successive nights.*
[footnote] *Arago, 'Des Changements physiques de la Comete de Halley du
15-23 Oct., 1835. 'Annuaire', 1836, p. 218, 221. The ordinary direction of
the emanations was noticed even in Nero's time. "Comae radios solis
effugiunt." -- Seneca, 'Nat. Quaest.', vii., 20.
The great Konigsberg astronomer concluded from many measurements, and from
theoretical considerations, "that the cone of light issuing from the comet
deviated considerably both to the right and the left of the true direction
of the Sun, but that it always returned to that direction, and passed over
to the opposite side, so that both the cone of light and the body of the
comet from whence it emanated experienced a rotatory, or, rather, a
vibratory motion in the plane of the orbit." He finds that "the attractive
force exercised by the Sun on heavy bodies is inadequate to explain such
vibrations, and is of opinion that they indicate a polar force, which turns
one semi-diameter of the comet toward the Sun, and strives to turn the
opposite side away from that luminary. The magnetic polarity possessed by
the Earth may present some analogy to this, and, should the Sun have an
opposite polarity, an influence might be manifested, resulting in the
precession of the equinoxes." This is not the place to enter more fully
upon the grounds on which explanations of this subject have been based; but
observations so remarkable,* and views of so exalted
p 103
a character, regarding the most wonderful class of the cosmical bodies
belonging to our solar system, ought not to be entirely passed over in this
sketch of a general picture of nature.
[footnote] *Bessel, in Schumacher, 'Astr. Nachr.', 1836, No. 300-302, s.
188, 192, 197, 200, 202, und 230. Also in Schumacher, 'Jahrb.', 1837, s.
149, 168. William Herschel, in his observations on the beautiful comet of
1811, believed that he had discovered evidences of the rotation of the
nucleus and tail ('Phil. Trans.' for 1812, Part i., p. 140). Dunlop, at
Paramatta thought the same with reference to the third comet of 1825.
Although, as a rule, the tails of comets increase in magnitude and
brilliancy in the vicinity of the sun, and are directed away from that
central body, yet the comet of 1823 offered the remarkable example of two
tails, one of which was turned toward the sun, and the other away from it,
forming with each other an angle of 160 degrees. Modifications of polarity
and the unequal manner of its distribution, and of the direction in which it
is conducted, may in this rare instance have occasioned a double, unchecked,
continuous emanation of nebulous matter.*
[footnote] *Bessel, in 'Astr. Nachr.', 1836, No. 302, s. 231. Schum,
'Jahrb.', 1837 s. 175. See, also Lehmann, 'Ueber Cometenschweife' (On the
Tails of Comets), in Bode, 'Astron. Jahrb. fur' 1826, s. 168.
Aristotle, in his 'Natural Philosophy', makes these emanations the means of
bringing the phenomena of comets into a singular connection with the
existence of the Milky Way. According to his views, the innumerable
quantity of stars which compose this starry zone give out a self-luminous,
incandescent matter. The nebulous belt which separates the different
portions of the vault of heaven was therefore regarded by the Stagirite as a
large comet, the substance of which was incessantly being renewed.*
[footnote] *Aristot., 'Meteor.', i., 8, 11-14, und 19-21 (ed. Ideler, t.
i., p. 32-34). Biese, 'Phil. des Aristoteles', bd. ii., s. 86. Since
Aristotle exercised so great an influence throughout the whole of the Middle
Ages, it is very much to be regretted that he was so averse to those grander
views of the elder Pythagoreans, which inculcated ideas so nearly
approximating to truth respecting the structure of the universe. He asserts
that comets are transitory meteors belonging to our atmosphere in the very
book in which he cites the opinion of the Pythagorean school, according to
which these cosmical bodies are supposed to be planets having long periods
of revolution. (Aristot., i., 6, 2.) This Pythagorean doctrine, which,
according to the testimony of Apollonius Myndius, was still more ancient,
having originated with the Chaldeans, passed over to the Romans, who in this
instance, as was their usual practice, were merely the copiers of others.
The Myndian philosopher describes the path of comets as directed toward the
upper and remote regions of heaven. Hence Seneca says, in his 'Nat.
Quaest.', vii., 17: "Cometes non est species falsa, sed proprium sidus
sicut solis et lunae: altiora mundi secat et tunc demum apparet quum in
imum cursum sui venit;" and again (at vii., 27), "Cometes aternos esse et
sortis ejusdem, cujus caetera (sidera), etiamsi faciem illis non habent
similem." Pliny (ii., 25) also refers to Apollonius Myndius, when he says,
"Sunt qui et haec sidera perpetua esse credant suoque ambitu ire, sed non
nisi relicta a sole cerni."
p 104
The occulation of the fixed stars by the nucleus of a comet, or by its
innermost vaporous envelopes, might throw some light on the physical
character of these wonderful bodies; but we are unfortunately deficient in
observations by which we may be assured* that the occulation was perfectly
central; for, as it has already been observed, the parts of the envelope
contiguous to the nucleus are alternately composed of layers of dense or
very attenuated vapor.
[footnote] *Olbers, in 'Astr. Nachr.', 1828, s. 157, 184. Arago, 'De la
Constitution physique des Cometes; Annuaire de' 1832, p. 203, 208. The
ancients were struck by the phenomenon that it was possible to see through
comets as through a flame. The earliest evidence to be met with of stars
having been seen through comets is that of Democritus (Aristot., 'Meteor.',
i., 6, 11), and the statement leads Aristotle to make the not unimportant
remark, that he himself had observed the occulation of one of the stars of
Gemini by Jupiter. Seneca only speaks decidedly of the transparence of the
tail of comets. "We may see," says he, "stars through a comet as through a
cloud ('Nat. Quaest.', vii., 18); but we can ony see through the rays of the
tail, and not through the body of the comet itself: 'non in ea parte qua
sidus ipsum est spissi et solidi ignis, sed qua rarus splendor occurrit et
in crines dispergitur. Per intervalla ignium, non er ipsos, vides" (vii.,
26). The last remark is unnecessary, since, as Galileo observed in the
'Saggiatore (Lettera a Monsignor Cesarini', 1619), we can certainly see
through a flame when it is not of too great a thickness'.
On the other hand the carefully conducted measurements of Bessel prove,
beyond all doubt, that on the 29th of September, 1835, the light of a star
of the tenth magnitude, which was then at a distance of 7".78 from the
central point of the head of Halley's comet, passed through very dense
nebulous matter, without experiencing any deflection during its passage.*
[footnote] *Bessel, in the 'Astron. Nachr.', 1836, No. 301, s. 204, 206.
Struve, in 'Recueil des Mem. de l'Acad. de St. Peterab.', 1836, p. 140, 143,
and 'Astr. Nachr.', 1836, No. 303, s. 238, writes as follows: "At Dorpat
the star was in conjunction only 2".2 from the brightest point of the comet.
The star remained continually visible, and its light was not perceptibly
diminished, while the nucleus of the comet seemed to be almost extinguished
before the radiance of the small star of the ninth or tenth magnitude."
If such an absence of refracting power must be ascribed to the nucleus of a
comet, we can scarcely regard the matter composing comets as a gaseous
fluid. The question here arises whether this absence of refracting power
may not be owing to the extreme tenuity of the fluid; or does the comet
consist of separated particles, constituting a cosmical stratum of clouds,
which, like the clouds of our atmosphere, that exercise no influence on the
p 105
zenith distance of the stars, does not affect the ray of light passing
through it? In the passage of a comet over a star, a more or less
considerable diminution of light has often been observed; but this has been
justly ascribed to the brightness of the ground from which the star seems to
stand forth during the passage of the comet.
The most important and decisive observations that we possess on the nature
and the light of comets are due to Arago's polarization experiments. His
polariscope instructs us regarding the physical constitution of the Sun and
comets, indicating whether a ray that reaches us from a distance of many
millions of miles transmits light directly or by reflection; and if the
former, whther the source of light is a solid, a liquid, or a gaseous body.
His apparatus was used at the Paris Observatory in examining the light of
Capella and that of the great comet of 1819. The latter showed polarized,
and therefore reflected light, while the fixed star, as was to be expected,
appeared to be a self-luminous sun.*
[footnote] *On the 3d of July, 1819, Arago made the first attempt to
analyze the light of comets by polarization, on the evening of the sudden
appearance of the great comet. I was present at the Paris Observatory, and
was fully convinced, as were also Matthieu and the late Bouvard of the
dissimilarity in the intensity of the light seen in the polariscope, when
the instrument received cometary light. When it received light from
Capella, which was near the comet, and at an equal altitude, the images were
of equal intensity. On the reappearance of Halley's comet in 1835, the
instrument was altered so as to give, according to Arago's chromatic
polarization, two images of complementary colors (green and red). ('Annales
de Chimie', t. xiii., p. 108; 'Annuaire', 1832, p. 216.) "We must conclude
from these observations," says Arago, "that the cometary light was not
entirely composed of rays having the properties of direct light, there being
light which was reflected specularly or polarized, that is, coming from the
sun. It can not be stated with absolute certainty that comets shine only
with borrowed light, for bodies, in becoming self-luminous, do not, on that
account, lose the power of reflecting foreign light."
The existance of polarized cometary light announced itself not only by the
inequality of the images, but was proved with greater certainty on the
reappearance of Halley's comet, in the year 1835, by the more striking
contrast of the complementary colors, deduced from the laws of chromatic
polarization discovered by Arago in 1811. These beautiful experiments still
leave it undecided whether, in addition to this reflected solar light,
comets may not have light of their own. Even in the case of the planets,
as, for instance, in Venus, an evolution of independent light seems very
probable.
The variable intensity of light in comets can not always be
p 106
explained by the position of their orbits and their distance from the Sun.
It would seem to indicate, in some individuals, the existence of an inherent
process of condensation, and an increased or diminished capacity of
reflecting borrowed light. In the comet of 1618, and in that which has a
period of three years, it was observed first by Hevelius that the nucleus of
the comet diminished at its perihelion and enlarged at its aphelion, a fact
which, after remaining long unheeded, was again noticed by the talented
astronomer Valz at Nismes. The regularity of the change of volume,
according to the different degrees of distance from the Sun, appears very
striking. The physical explanation of the phenomenon can not, however, be
sought in the condensed layers of cosmical vapor occurring in the vicinity
of the Sun, since it is difficult to imagine the nebulous envelope of the
nucleus of the comet to be vesicular and impervious to the other.*
[footnote] *Arago, in the 'Annuaire', 1832, p. 217-220. Sir John Herschel,
'Astron.', 488.
The dissimilar eccentricity of the orbits of comets has, in recent times
(1819), in the most brilliant manner enriched our knowledge of the solar
system. Encke has discovered the existence of a comet of so short a period
of revolution that it remains entirely within the limits of our planetary
system, attaining its aphelion between the orbits of the smaller planets and
that of Jupiter. Its eccentricity must be assumed at 0.845, that of Juno
(which has the greatest eccentricity of any of the planets) being 0.255.
Encke's comet has several times, although with difficulty, been observed by
the naked eye, as in Europe in 1819, and according to Rumker, in New Holland
in 1822. Its period of revolution is about 3 1/3d years; but, from a
careful comparison of the epochs of its return to its perihelion, the
remarkable fact has been discovered that these periods have diminished in
the most regular manner between the years 1786 and 1838, the diminution
amounting, in the course of 52 years, to about 1 3/10th days. The attempt
to bring into unison the results of observation and calculation in the
investigation of all the planetary disturbances, with the view of explaining
this phenomenon, has led to the adoption of the very probable hypothesis
that there exists dispersed in space a vaporous substance capable of acting
as a resisting medium. This matter diminished the tangential force, and
with it the major axis of the comet's orbit. The value of the constant of
the resistance appears to be somewhat different before and after the
perihelion; and this may, perhaps, be ascribed
p 107
to the altered form of the small nebulous star in the vicinity of the Sun,
and to the action of the unequal density of the strata of cosmical ether.*
[footnote] *Encke, in the 'Astronomiche Nachrichten', 1843, No. 489, s.
130-132.
These facts, and the investigations to which they have led, belong to the
most interesting results of modern astronomy. Encke's comet has been the
means of leading astronomers to a more exact investigation of Jupiter's mass
(a most important point with reference to the calculation of perturbations);
and, more recently, the course of this comet has obtained for us the first
determination, although only an approximative one, of a smaller mass for
Mercury.